Evolution

Review pubs.acs.org/CR

Spinels: Controlled Preparation, Oxygen Reduction/Evolution Reaction Application, and Beyond Qing Zhao,† Zhenhua Yan,† Chengcheng Chen,† and Jun Chen* Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Collaborative Innovation Center of Chemical Science and Engineering, College of Chemistry, Nankai University, Tianjin 300071, China ABSTRACT: Spinels with the formula of AB2O4 (where A and B are metal ions) and the properties of magnetism, optics, electricity, and catalysis have taken significant roles in applications of data storage, biotechnology, electronics, laser, sensor, conversion reaction, and energy storage/conversion, which largely depend on their precise structures and compositions. In this review, various spinels with controlled preparations and their applications in oxygen reduction/evolution reaction (ORR/OER) and beyond are summarized. First, the composition and structure of spinels are introduced. Then, recent advances in the preparation of spinels with solid-, solution-, and vapor-phase methods are summarized, and new methods are particularly highlighted. The physicochemical characteristics of spinels such as their compositions, structures, morphologies, defects, and substrates have been rationally regulated through various approaches. This regulation can yield spinels with improved ORR/OER catalytic activities, which can further accelerate the speed, prolong the life, and narrow the polarization of fuel cells, metal−air batteries, and water splitting devices. Finally, the magnetic, optical, electrical, and catalytic applications beyond the OER/ORR are also discussed. The future applications of spinels are considered to be closely related to environmental and energy issues, which will be aided by the development of new species with precise preparations and advanced characterizations.

CONTENTS 1. Introduction 2. Spinels 2.1. Composition 2.2. Structure 2.3. Comparison of Spinels and Other Catalysts 3. Preparation 3.1. Solid-Phase Method 3.1.1. High-Temperature Solid Phase 3.1.2. Flux Growth 3.1.3. Solid-Phase Combustion 3.1.4. Thermal Decomposition 3.1.5. Metal−Organic Framework Derivation 3.1.6. Pulsed Laser Deposition/Ablation 3.2. Solution-Phase Method 3.2.1. Sol−Gel 3.2.2. Hydrothermal/Solvothermal 3.2.3. Precipitation 3.2.4. Microemulsion 3.2.5. Microwave 3.2.6. Electrochemical 3.2.7. High-Temperature Solution Phase 3.3. Vapor-Phase Methods 3.3.1. Chemical Vapor Deposition 3.3.2. Spray Pyrolysis 3.3.3. Magnetron Sputtering 3.3.4. Plasma Method 3.4. Summary of Spinel Preparations 4. ORR/OER © 2017 American Chemical Society

4.1. Basic Introduction to the ORR 4.2. Basic Introduction to the OER 4.3. Evaluation of ORR/OER 4.3.1. Linear Sweep Voltammetry (LSV) Curves 4.3.2. Koutecky−Levich (K−L) Curves and Tafel Curves 4.3.3. Generation of HO2− and Electron Transferred Number in the ORR 4.3.4. Capacitance 4.3.5. Turnover Frequency (TOF) 4.3.6. Faradaic Efficiency 4.4. Reaction Pathways with Spinel Catalysts 4.4.1. ORR Pathways of Transition Metal Oxides 4.4.2. OER Pathways of Transition Metal Oxides 5. ORR/OER with Spinels 5.1. MCo2O4 5.1.1. Co3O4 5.1.2. NiCo2O4 5.1.3. MnCo2O4 5.1.4. Other Co-Based Spinels 5.2. MMn2O4 5.2.1. Mn3O4 5.2.2. CoMn2O4 5.2.3. Other Mn-Based Spinels 5.3. MFe2O4 5.3.1. Fe3O4 5.3.2. CoFe2O4

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Received: January 19, 2017 Published: July 26, 2017 10121

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Chemical Reviews 5.3.3. Other Fe-Based Spinels 5.4. Other Spinels 5.5. Summary of Strategies to Enhance ORR/OER Properties 6. Applications of the ORR/OER 6.1. Fuel Cells 6.1.1. Solid Oxide Fuel Cells 6.1.2. Microbial Fuel Cells 6.2. Metal−Air Batteries 6.2.1. Li−Air Batteries 6.2.2. Zn−Air Batteries 6.2.3. Mg/Al−Air Batteries 6.3. Water Splitting 6.3.1. Electrochemically Driven 6.3.2. Photochemically Driven 7. Properties and Applications Beyond the ORR/OER 7.1. Magnetism 7.2. Optics 7.3. Electricity 7.3.1. Supercapacitors 7.3.2. Batteries 7.4. Catalyzation Beyond ORR/OER 7.4.1. Gas Conversion 7.4.2. Liquid Conversion 7.4.3. Solid Conversion 8. Conclusions and Perspectives 8.1. Conclusions 8.2. Perspectives 8.2.1. Applications 8.2.2. Materials Preparation and Scalability of the Latter 8.2.3. Fundamental Scientific Challenges Author Information Corresponding Author ORCID Author Contributions Notes Biographies Acknowledgments List of Abbreviations References

Review

mostly related to ferrimagnetism and paramagnetism.8,9,16 The potential magnetism applications are in information technology (data storage),8 biotechnology (magnetic sensing),8,17,18 and the electronic industry (spintronic devices).19 Transparent or semitransparent spinels have exhibited the optical properties of electrochemical luminescence and photoluminescence10 as well as parasitic light absorption,20 demonstrating application prospects in laser devices21 and magneto-optical recording.22 The electrical characteristics of spinels have also allowed their applications to extend into energy storage fields, such as supercapacitors23−25 and metal−ion batteries.26,27 Most spinel oxides with alterable chemical valence have demonstrated redox capacitance. Meanwhile, spinel compounds have also been widely used as anodes and cathodes in Li-ion batteries. Recently, spinels have also been studied as electrode materials in Naion,28,29 Mg-ion,30 and Zn-ion batteries.31 The benefits of spinel compounds such as their controllable composition, structure, valence, and morphology have made them suitable as catalysts in various reactions. Spinel catalysts have been used to facilitate NOx reduction,15,32 CO oxidation,33 CO2 reduction,34 hydrogen evolution reaction (HER),35,36 ORR,14 OER,7 chemical looping combustion (CLC),37 NH3 oxidation,38 formaldehyde oxidation,39 methane combustion,40 alcohols oxidation,41,42 pmethoxytoluene oxidation,43 H2O2 decomposition,44 urea oxidation,45 NH4ClO4 decomposition,46cyanide and phenol oxidation,47 glucose oxidation,48 and methylene blue degradation.49 Among various catalytic reactions, ORR and OER with spinel catalysts have revealed fascinating prospects. ORR/OER is the key process of many energy conversion and storage devices such as fuel cells,50,51 metal−air batteries,52,53 and electrolyzers (water splitting).54−57 Solar and wind sustainable energy are generally known to benefit from their low price, environmental friendliness, and zero emissions in comparison with fossil fuels. However, their production demonstrates notable regional and time features, and they are thus referred to as “intermittent”. Thus, developing the above energy utilization devices is vitally important to permanently employ sustainable energy. However, these devices are hampered by the high over potential and sluggish kinetics ascribed to unsatisfactory ORR/OER processes.58 In general, the Pt, Pd, IrO2, and RuO2 noble metal catalysts have acceptable kinetics.59−63 However, the high cost and limited reserves of noble-metal-based catalysts have precluded these renewable energy technologies from largescale commercial applications. In comparison, the inexpensive spinel catalysts, whose properties can be adjusted by controlling their structure, composition, phase, valence, morphology, and defects, have demonstrated catalytic activities competitive with noble metals. With decades of research, both the preparation and applications of spinels have achieved greatly increased development. However, there has been no systematic summary of this hot topic in the last ten years. We here comprehensively review the recent representative important preparation and application of spinels (Figure 2). The review follows the following procedures. First, in light of the critical effects of spinel compositions and structures on their physicochemical properties, the basic characteristics of spinels will be introduced. Second, the preparation with parameter regulations of spinels is to be focused on. Extensive solid-, solution-, and vapor-phase methods will be covered. The traditional synthesis of spinels generally involves grinding a mixture of oxides, nitrates, or carbonates, followed by high-temperatures calcination for

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1. INTRODUCTION Spinels, mostly with the composition AB2O4 (where A and B are metal ions),1 have been initially employed as precious stones, such as red spinels containing Cr3+ and blue spinels containing Fe2+ and Zn2+ (Figure 1).2,3 However, the structures of spinels were little known until 1915. Bragg and Nishikawa confirmed that spinels generally have a composition formed of A−O tetrahedrons and B−O octahedrons.1,4 Spinels form a very large family, and they can contain one or more metal elements. Nearly all of the main group metals and transition metals have been observed in spinels.5 The traditional synthesis of spinels generally follows a high temperature solid-state route.6 In the past 20 years, many low-temperature synthesis methods have been developed to fabricate spinels with different sizes and morphologies.7 Because of their manifold compositions, electron configurations, and valence states, spinels have demonstrated intrinsic magnetic,8,9 optical,10 electrical,11−13 and catalytic properties.14,15 Many spinel compounds, especially those including Fe, Co, Cr, or Ni have demonstrated various magnetic properties, 10122

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Figure 1. (a) Natural mined sapphire−ruby including spinel structure. Grains range up to 1.5 cm in size. Reprinted with permission from ref 2. Copyright 2009 Taylor & Francis Publishing Group. (b) Polished pink spinel could be as jewelry. Reprinted with permission from ref 3. Copyright 1990 Springer Publishing Group.

Figure 2. Illustration of the structures, synthesis, strategies, and applications of spinels. 10123

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Table 1. Radius and Valence of Ions that Occupy the A, B, and X Sites66a A site

B site

X site

atom

valence

radius (Å)

atom

valence

radius (Å)

atom

valence

radius (Å)

Li (IV) Mg (IV) Ca (VI) Ba (VI) Mn (IV) Fe (IV) Co (IV) Ni (IV) Cu (II) Zn (IV) Cd (IV) Ti (IV) Ge (IV) Sn (IV)

+1 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +4 +4 +4

0.59 0.57 1.00 1.35 0.67hp 0.63hp 0.58hp 0.55 0.46 0.60 0.78 0.61 0.73 0.55

Mg (VI) Zn (VI) Al (VI) Cr (VI) Mn (VI) Fe (VI) Co (VI) Ni (VI) Ga (VI) In (VI)

+2 +2 +3 +3 +3 +3 +3 +3 +3 +3

0.72 0.74 0.54 0.62 0.58lp/0.65hp 0.55lp/0.65hp 0.55lp/0.61hp 0.56lp/0.60hp 0.62 0.80

F (VI) O (VI) S (VI) Se (VI) Te (VI) N (VI)

−1 −2 −2 −2 −2 −3

1.33 1.40 1.84 1.98 2.21 1.48

a

The radius data were adopted from the database of ionic radii: hosted by the Atomistic Simulation Group in the Materials Department of Imperial College. lp stands for the low spin and hp stands for high spin. The “()” after the atom is the coordination number

Table 2. Currently Reported Major Spinelsa LiCr2O4

LiMn2O4

LiFe2O4

LiCo2O4

LiNi2O4

MgAl2O4 CaAl2O4 BaAl2O4 MnAl2O4 FeAl2O4 CoAl2O4 NiAl2O4 CuAl2O4 ZnAl2O4

MgCr2O4 CaCr2O4 BaCr2O4 MnCr2O4 FeCr2O4 CoCr2O4 NiCr2O4 CuCr2O4 ZnCr2O4

MgMn2O4 CaMn2O4

MgCo2O4 CaCo2O4 BaCo2O4 MnCo2O4 FeCo2O4 CoCo2O4 NiCo2O4 CuCo2O4 ZnCo2O4 GeCo2O4

MgNi2O4 CaNi2O4 BaNi2O4 MnNi2O4 FeNi2O4 CoNi2O4 NiNi2O4 CuNi2O4 ZnNi2O4 GeNi2O4

MgGa2O4 CaGa2O4 BaGa2O4 MnGa2O4 FeGa2O4 CoGa2O4 NiGa2O4 CuGa2O4 ZnGa2O4

MgIn2O4 CaIn2O4 BaIn2O4 MnIn2O4 CoIn2O4 NiIn2O4 CuIn2O4 ZnIn2O4

γ-Al2O3 FeFe2S4 SiSi2N4

CdCr2S4 SnSn2N4

MnCr2S4 GeG2N4

MgFe2O4 CaFe2O4 BaFe2O4 MnFe2O4 FeFe2O4 CoFe2O4 NiFe2O4 CuFe2O4 ZnFe2O4 GeFe2O4 γ-Fe2O3 CuCo2S4 BeLiF4

CuCr2Se4 MoNa2F4

CdCr2Se4 MgK2(CN)4

CuCr2Te4 ZnK2(CN)4

CdK2(CN)4

MnMn2O4 FeMn2O4 CoMn2O4 NiMn2O4 CuMn2O4 ZnMn2O4

a

The spinels in this table include Li, Mg, Ca, Ba, Mn, Fe, Co, Ni, Cu, Zn and Ge in the A site; B site of Al, Cr, Mn, Fe, Co, Ni, Ga, and In in the B site; and O in the X site. Some other common spinel types (sulfide, selenide, telluride, nitride, fluoride and cyanide) are also listed.

prolonged periods to overcome the diffusion barriers. To replace such conventional ceramic preparation, new approaches have been developed in sol−gel processing, coprecipitation, and hydrothermal/solvothermal methods. These methods can proceed at moderate temperatures with controllable product particle size due to enhanced reaction kinetics. With the overall understanding of spinels, we will then focus on their ORR/OER properties. Co-based, Mn-based, and Febased spinels will be discussed systematically. Additionally, general strategies for designing effective spinel catalysts for ORR/OER will be summarized. To maximize their catalytic activity, various types of spinels with multifunctional features have been fabricated. The designed morphology, composition, structure, and defect features can optimize the electronic structure of spinels and enable better affinity with oxygencontaining groups. In addition, spinel/carbon hybrids with coupling effects, small particle sizes, superior dispersion, and electrical conductivity are also confirmed to be a factor in promoting the catalytic activities. The spinel catalysts of outstanding ORR/OER can be further applied to facilitate the reactions in fuel cells, metal−air batteries, and water splitting. For fuel cells, spinels are mainly applied as an ORR catalyst on the cathode sides. For metal−air batteries and water splitting, spinel

composites can be adopted as multifunctional catalysts. Furthermore, other magnetic, optical, electrical, catalytic properties, and applications will also be discussed. Future development is expected to focus on environmental and energy issues. From that point of view, research efforts should focus on the exploration of novel functional spinels with intrinsic conductivity, in situ characterization of the formation process of spinels, insight into the “structure & effect” relationship between spinels parameters and properties, and elevating the durability of catalytic activity in various devices.

2. SPINELS 2.1. Composition

Typical spinels are briefly described as AB2X4 (A = Li, Mn, Zn, Cd, Co, Cu, Ni, Mg, Fe, Ca, Ge, Ba, etc.; B = Al, Cr, Mn, Fe, Co, Ni, Ga, In, Mo, etc.; X = O, S, Se, Te, N, etc.), where metal A occupies the centers of tetrahedrally coordinated positions, metal B occupies the centers of octahedrally coordinated positions, and the anion (e.g., O2−) sits at the polyhedral vertexes (for normal spinels).64,65 Usually, the tetrahedral interstices are smaller than the octahedral interstices. Therefore, cations with smaller radii prefer to occupy the A sites, while larger cations prefers to occupy 10124

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Figure 3. Representative structures of (a) a normal spinel (MgAl2O4), (b) an inverse spinel (NiFe2O4), and (c) a complex spinel (CuAl2O4) in different styles and views. The green and purple polyhedra correspond to octahedral and tetrahedral metal occupation sites, respectively. Representative A, B, and O defect sites in spinel AB2O4 have been illustrated in panel a. (d) Normal spinel (MgAl2O4) with (111), (311), and (400) view directions.

the B sites. In the formula AB2X4, the anion X usually adopts −2 oxidation state. To maintain the valence equilibrium, cation A can be in the +2 or +4 oxidation state and the corresponding cation B can be in the +3 or +2 oxidation state. These possibilities can be expressed as A2+B3+2X2−4 and A4+B2+2X2−4. The valence and radius of common cations that can be accommodated in the tetrahedral and octahedral interstices are summarized in Table 1. Brik et al. have thoroughly examined the parameters of over 180 types of binary and ternary spinel compounds.5 Table 2 summarizes the most common spinels (only two metal elements) with A sites containing Li, Mg, Ca, Ba, Mn, Fe, Co, Ni, Cu, Zn, or Ge and B sites of Al, Cr, Mn, Fe, Co, Ni, Ga, or In. Additionally, some other common spinel forms (sulfide, selenide, telluride, nitride, fluorides, and cyanide) are also listed.

introduced. The ions before the parentheses are located in the tetrahedral sites, whereas the ions in the parentheses are located in the octahedral sites. λ = 0 is regarded as “normal” spinel (Figure 3a), λ = 1 is the “inverse” spinel (Figure 3b), and 0 < λ < 1 reflects the complex spinels (Figure 3c). In the typical normal spinel oxide MgAl2O4 (Figure 3a,d), the Mg2+ cations occupy the centers of the tetrahedral sites while Al3+ occupies the octahedral sites. The inverse spinels can be described as B(AB)X4. For example, in the inverse spinel NiFe2O4 (Figure 3b), half of the Fe3+ cations occupy the centers of the tetrahedral sites, whereas Ni2+ and the remaining half of the Fe3+ occupy the octahedral sites, which can be expressed as Fe(NiFe)O4. The complex spinels are defined as being intermediate between the normal and inverse spinels. One example is the complex spinel CuAl2O4 (Figure 3c), in which both Cu2+ and Al3+ cations partially occupy both the octahedral and tetrahedral sites (Cu1‑λAlλ(CuλAl2‑λ)O4). There are several factors that can determine the distribution of cations in spinels such as the mentioned cation radius, Coulomb interactions between the cations, and crystal field effects of the octahedral site preference energy (OSPE) of cations. According to crystal field theory, the OSPE is defined as the stable crystal

2.2. Structure

Different cation distributions exist in spinels, which means that the A and B cations distribute in different ratios in the tetrahedral and octahedral interstices, respectively.67−69 Depending on their situations of cation distribution, spinels can be classified into three types: normal, inverse, and complex spinels. To distinguish these spinels, a more accurate format of A1‑λBλ(AλB2‑λ)X4 is 10125

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field energy difference between the octahedral and the tetrahedral fields. A greater absolute OSPE value means that the cations will preferably occupy the octahedral interstices. As

may have cation vacancies (A and B vacancies) and O vacancies. For example, the spinel γ-Al2O3 displays Al vacancies in the B site (octahedral interstices), which can be expressed as (Al3+) (Al3+5/3V1/3)O4 (V corresponds to the vacancy). The oxygen vacancy level can be controlled by the oxygen partial pressure or the synthesis temperature.75 Nonintrinsic defects are often caused by external dopants such as substitutions of A and B sites with different cations. The cation-doped spinel oxides can be expressed as A1−xA′xB2‑yB′yO4 (0 ≤ x/y ≤ 1). These defects play a key role in electrochemical catalytic reactions. Furthermore, it is well accepted that creating oxygen vacancies can improve the electron conductivity of oxides.70,76

Table 3. Number of d Electrons, Crystal Field Stable Energy (CFSE), and Octahedral Site Preference Energy (OSPE) of 3d Transition Metal Ionsa CFSE (kJ mol−1)

a

ion

no. of 3d electrons

octahedral field

tetrahedral field

OSPE (kJ mol−1)

Sc3+ Ca2+ Ti4+ Ti3+ V3+ Cr3+ Cr2+ Mn3+ Mn2+ Fe3+ Fe2+ Co3+ Co2+ Ni2+ Cu2+ Zn2+ Ga3+ Ge3+

0 0 0 1 2 3 4 4 5 5 6 6 7 8 9 10 10 10

0 0 0 −87.4 −160.2 −224.7 −100.4 −135.6 0 0 −49.8 −188.3 −92.9 −122.2 −90.4 0 0 0

0 0 0 −58.6 −106.7 −66.9 −29.3 −40.2 0 0 −33.1 −108.8 −61.9 −36.0 −26.8 0 0 0

0 0 0 −28.8 −53.5 −157.8 −71.1 −95.4 0 0 −16.7 −79.5 −31.0 −86.2 −63.7 0 0 0

2.3. Comparison of Spinels and Other Catalysts

As shown in Table 4, spinels form an important class of transition metal oxides with appealing ORR/OER catalytic activity in alkaline aqueous and/or nonaqueous media. The advantages of using spinels as electrocatalysts for the ORR/OER include their desirable activity,78 widespread availability,79 low cost, easy synthesis,7 thermodynamic stability, low electrical resistance, and environmental friendliness.80,81 In the spinel structure, the coexistence of tetrahedral and octahedral sites provides multiple sites to accommodate different transition-metal cations with a wide range of valence states to form a large number of oxides. It is noted that perovskite oxides (ABO3) with versatile physical, chemical, and electronic properties are also alternative electrocatalysts.71 The elements in perovskite structures can be chosen in a flexible manner from various types and in different concentrations. By partially substituting cations at A and B sites, a variety of perovskite compounds can be obtained.92 Meanwhile, oxygen nonstoichiometric perovskites with reversible oxygen release and uptake are also good bifunctional catalysts.71 The flexibility in the oxidation states of transition metals leads to the formation of redox couples, defective structures for oxygen vacancies or excess, and suitable oxygen anion mobility and oxygen exchange kinetics, which facilitate their bifunctional electrocatalytic activity for the ORR/OER.71 In general, the synthesis temperature of perovskite oxides are relatively high (600−900 °C), and their activities for the ORR/OER are not as good as those of spinels.93 ORR catalysts with Pt-based precious metal/alloy have very high electrocatalytic activities.94 The single noble metal oxides such as IrO2 and RuO2 exhibit good OER activity.85 However, these precious catalysts rarely possess bifunctional electrocatalytic activity for both the ORR and OER. Their scarcity, high cost, and limited stability hinder their widespread application.

Adapted from ref 77.

summarized in Table 3, the absolute OPSE value of Mn3+ (95.2 kJ mol−1) is higher than that of Mn2+ (0 kJ mol−1), indicating that Mn3+ prefers to occupy the octahedral interstices while Mn2+ tends to occupy the tetrahedral interstices. As a result, Mn3O4 is determined to be a normal spinel with a formula of Mn2+(Mn3+)2O4. In contrast, the inverse spinel Fe3O4 can be expressed as Fe3+(Fe3+Fe2+)O4 because the absolute OPSE value of Fe2+ (16.7 kJ mol−1) is higher than that of Fe3+ (0 kJ mol−1). Defects in solid oxides are crucial to increase the activity of battery, electrocatalysis, and photocatalysis.31,70−74 In typical spinel oxides, the defects can be classified into two types: intrinsic and nonintrinsic defects. Intrinsic defects are found in atomic arrangements without any dopants, including vacancies (Schottky defects), vacancy−interstitial pairs (Frenkel defects), and interstitial defects. As shown in Figure 3a, the spinel AB2O4 Table 4. Characteristics of Spinels Compared to Other Materials

activity catalyst

representative examples

cost

synthesis

conductivity

spinels

CoMn2O47 Co3O414

low

easy

low

perovskites perovskites metals single metal oxides single metal oxides bimetals alloys M−N−C 2D materials MOFs

Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF)82 LaMnO3+δ83 Pt84 IrO2/RuO285,86 MnO287 Pt3Ni88 Fe−N−C89 N doped-reduced graphene oxide (N-rGO)90 Ni3(hexaiminotriphenylene)2 (Ni3(HITP)2)91

low low high high low high low high high

high temperature high temperature easy easy easy complex high temperature complex complex

moderate moderate high high low high high high high

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ORR

OER

stability

high

high

good in alkaline

moderate high high moderate moderate very high high high moderate

high low low high low low low low low

good in alkaline good in alkaline moderate moderate moderate good good moderate poor

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MnO2 and Fe2O3. The mixed powders were first calcined at 900 °C for 30 h and then crushed and further calcined at 1050 °C. Additionally, CoFe2O4 in which Ni and Mn or CuFe2O4 in which Al have replaced part of the Fe in the B sites;111,112 MnFe2O4 in which Al and Cr have replaced part of the Fe in the B sites;113 and FeCr2O4 in which Mg has replaced part of the Fe in the A sites114 have been synthesized. For certain compounds that face a high ion-diffusion energy barrier, the high temperature must also be accompanied by high pressure. Through pressing the raw materials at 5 GPa and calcining at 1250 °C, Huppertz et al. obtained Ga2.79O3.05N0.76 with defects.115 For certain ceramic materials, such as MgAl2O4, the calcining temperature is usually over 1500 °C.20,116 Intriguingly, some well-known layered compounds, such as LiCoO2, can form the spinel phase at low temperatures.117,118 Manthiram’s group synthesized LiCoO2 spinel by heating

Most other nonprecious simple single metal oxides show relatively low activity. Graphene, a one-atom-thick, crystalline carbon film, is an exemplary 2D material because of its unexpected properties including its ultrahigh room-temperature carrier mobility, quantum hall effect, ultrahigh specific surface area, high Young’s modulus, and excellent optical transparency and electrical conductivity.95 Nanocarbon-based carbon materials, especially those that are heteroatom-doped, with electroneutrality break and charge relocation, have attracted intense attention because of their economic viability, tunable surface chemistry, and fast electron transfer capacity.90 However, substantially increasing the active sites of heteroatom-decorated carbon materials remains challenging for practical ORR/OER applications.96 Metal−organic frameworks (MOFs) are compelling choices for electrocatalytic applications because their high surface areas maximize active site density and their tunable chemical structures afford tailor-made microenvironments for controllable reaction conditions within their pores.97 Despite their promising features, MOFs have rarely been used for electrocatalytic applications because they are typically electrical insulators and are unstable in electrolytes.91 Metal−N-decorated nanocarbon materials show superior ORR electrocatalytic activity compared to the Pt/C catalyst because of strong interactions between metal and N species in the carbon framework. However, the OER activity of these materials requires further improvement.89 Therefore, compared with other materials, the spinels posssess advantages of diverse compositions, bifunctional catalytic activity, and cost efficiency.

3. PREPARATION Nearly all approaches involving chemical and physical transformations can be utilized to prepare spinels. We divide them here into three broad categories: solid-phase methods, solutionphase methods, and vapor-phase methods. The solid-phase methods include high-temperature, flux growth, combustion, nitrate decomposition, MOF-derived, and pulsed laser methods. The solution-phase methods include sol−gel, hydrothermal/ solvothermal, precipitation, microemulsion, microwave, electrochemical, and high-temperature methods. The vapor-phase methods include spray pyrolysis, chemical vapor deposition (CVD), atomic layer deposition (ALD), magnetron sputtering, and plasma methods. With elaborate design preparation approaches, the compositions, structures, valence, defects, morphologies, and loading substrates of spinels have been rationally designed.

Figure 4. (a) X-ray diffraction (XRD) patterns of cubic spinel type LiCoO2 (blue curves, denoted as LT-LiCoO2). Layered LiCoO2 (red curves, denoted as HT-LiCoO2) and spinel Co3O4 (purple curves) were prepared for comparisons. The (440) reflection in spinel LT-LiCoO2 splits into (108) and (110) in the layered HT-LiCoO2. (b) Aberrationcorrected HAADF-scanning transmission electron microscopy (STEM) images of LT-LiCoO2, the inset is the fast Fourier transform’s image. (c) Higher magnification STEM images with the corresponding unit of LTLiCoO2. The purple spheres represent Co3+, and the green spheres represent Li+. Reproduced with permission from ref 118. Copyright 2014 Nature Publishing Group.

3.1. Solid-Phase Method

3.1.1. High-Temperature Solid Phase. The high-temperature solid-phase method is a simple way to prepare spinels. In general, spinel compounds can be prepared at high temperature with corresponding metals, metal oxides, metal halides, metal sulfates, metal hydroxides, or metal carbonates.98−107 For example, through calcining a mixture of LiOH (or Li2CO3) and MnO2 at a molar ratio of 1:2 at 400 °C for 10 h and then at 750 °C for 48 h, a cubic phase of LiMn2O4 can be obtained.108 Similarly, a cubic spinel of Cu1.28Mn1.82O4 was synthesized with CuO and MnO2 powders at high temperature.109 Moreover, spinels with A or B site substitution have also been prepared. For example, in the preparation of MgMnxFe2‑xO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0), the metal oxides MgO, MnO2, and α-Fe2O3 were selected as the raw materials.110 Then, the composition of MgMnxFe2‑xO4 could be tuned by controlling the mole ratios of

stoichiometric amounts of Co3O4 and Li2CO3 at 400 °C for 7 days (Figure 4a). Unlike the common spinel oxides with the AB2O4 composition, the prepared LiCoO2 (Li2Co2O4) displayed a lithiated spinel structure in which Li+ occupied the 16c octahedral sites and Co3+ occupied the 16d octahedral sites, belonging to the space group Fd3m ̅ (Figure 4b,c). In addition, the Li+ in the LiCoO2 can be partially removed by the oxidizer NO2BF4, forming compounds with spinel structures and a composition of Li0.5CoO2. Similarly, the spinel λ-MnO2 is also 10127

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a liquid molten salt method eliminates the impurities and grain boundaries often formed in samples such as LiNixMn2−xO4.142 3.1.3. Solid-Phase Combustion. The combustion method is a rapid synthesis method that utilizes exothermic reactions to speed the reaction rates; it is also referred to as self-propagating high-temperature synthesis, which was first proposed by Merzhanov et al. in the late 1960s.145,146 This approach is often adopted to acquire ferrite spinels, in which iron powders act as both fuel to release heat and as a component of the final spinel products. Ferrites such as MnFe2O4, ZnFe2O4, NiFe2O4, and CoFe2O4 have been created in this fashion. In addition, O2 and other compounds with strong oxidation properties are usually selected as oxidants. The oxygen pressure can influence the final product purity. For example, with iron as power and fuel, metal oxides (MnO/Mn3O4, NiO, ZnO, etc.) as raw materials, NaClO4 as the additional oxygen agent, and Fe2O3 as the thermal ballast/ moderator to tune the synthesis temperature, a series of spinel oxides can be fabricated;147 the reaction is listed below:

obtained after dissolving the Li ions from the structure of LiMn2O4, thus creating a totally A-site−absent spinel structure.102 The high-temperature solid-state method is useful in largescale applications. However, the diffusion processes of solid-state preparation are usually sluggish, requiring long reaction times and high temperatures. Ball milling is usually applied to reduce the size of reactants and increase the reaction rate.119−123 In addition, high-energy ball milling will also decrease the particle size of the final products and create oxygen vacancies.124 For example, NiFe2O4 micronanoparticles were initially ball milled for 2 h in ethyl alcohol before high-temperature calcination for 2 h at 1200 °C.125 The prepared NiFe2O4 exhibited a particle size of 200−500 nm. As a benefit of the high energy of ball milling, some products can even be formed without high-temperature calcination. Bid et al. prepared microstructural NiFe2O4 by ball milling the NiO and Fe2O3 raw materials.126 Nearly pure NiFe2O4 was formed after ball milling for 20 h. Defects in spinel structures can also be introduced through high-temperature solid-state methods.127−130 The preparation of LiMgyMn2‑yO4 is an example; the normal compound can be obtained from the raw materials of Li2CO3, MnCO3, and 4MgCO3·Mg(OH)2·3H2O by heating at 600 °C for 6 h and 750 °C for 3 days in air atmosphere.131 At a reduced oxygen pressure, however, oxygen defects were created.132 In addition, hydrogen treatment at high temperature also causes the absence of oxygen.133 Muhler and co-worker utilized a Co−Mn−Mg−Al mixed oxide as a catalyst to grow N-doped carbon nanotubes (CNTs) with pyridine as a raw material. Interestingly, after washing the CNTs with dilute HNO3 and heating them in air, the spinel oxides (originally used as the catalyst) enclosed inside or tightly located on the tip of the CNTs and with defects were synthesized.134 In addition to spinel oxides, other spinels are also available through the high-temperature solid-state method.135 To prepare spinel CuIr2S4, Cu, Ir, and sulfur powders were placed in an evacuated tube and then calcined for 8 days at 850 °C. The obtained powder was pressed into pellets and further calcined for a second 8 days at 850 °C to obtain CuIr2S4.6 CdCr2Se4135,136 and CuCr2Te4137 have been prepared with similar methods using Se and Te powders, respectively. Spinel nitrides such as Si3N4 were also obtained at the high pressure of 13−15 GPa and high temperature of 1800 °C with Si2N2(NH) and a-Si3N4 as raw materials.138 3.1.2. Flux Growth. To increase the rate of solid-state reaction, the flux growth method is applied. The flux growth method usually adopts one or more low-melting-point salts as the reaction medium. Scheel et al. reported large crystal growth with PbO-PbF-B2O3 molten salt in 1971.139 Typically, the reactants often exhibit some solubility in the molten salts, which turns the solid−solid reaction into a solid−liquid reaction, thus increasing the reaction rate and decreasing the reaction time.140 Inorganic salts such as Na2SO4,141 LiCl-KCl,142 PbO2−B2O3,143 and Na2B4O7144 have been applied as the reflux material. For example, employing melted PbO2 and B2O3 at a molar ratio of 1:2 as the reaction medium, ZnGa2O4 spinel was synthesized from ZnO and Ga2O3 at 1250 °C for 8 h.143 CdGa2O4 was obtained from CdO and Ga2O3 at 1100 °C for 8 h.143 CoCr2O4/ carbon nanosheets were obtained by first synthesizing the CoCr2(OH)x/OA(oleate) precursor with coprecipitation. This precursor was then mixed with Na2SO4 and heated at 700 °C for 3 h. After washing, CoCr 2 O 4 /carbon nanosheets were obtained.141 Unlike traditional solid-state synthesis, the use of

Fe + 0.5Fe2O3 + x AO + (1 − x)BO + 1.5/4NaClO4 = (A xB1 − x )Fe2O4 + (1.5/4)NaCl

(1)

3.1.4. Thermal Decomposition. The most widely used thermal decomposition method is nitrate decomposition and nitrate combustion.38,40,148−150 Nanoparticles are a major morphology prepared through nitrate decomposition.9 In this review, we classify nitrate decomposition into the solid-state reactions because the main component is in the solid state at the reaction temperature. The principles of metal nitrate decomposition are well-known in general chemistry. With reference to the metal activity series (K > Na > Mg > Al > Zn > Fe > Sn > Pb > H > Cu > Ag), the metal nitrates before Mg will form nitrites after decomposition. The metal nitrates between Mg and Cu will form metal oxides. Finally, the metal nitrates after Cu will decompose to pure metal. Spinels such as CoCr2‑xVxO4, CuxMn3‑xO4, and MnxCo3‑xO4 with various compositions can be synthesized by tuning the ratios of metal nitrates.151−153 In comparison with the solid-state combustion method that exploits the heat released by the thermite reaction, the lowtemperature nitrates combustion method utilizes the metal nitrates as oxidizing agents and organic compounds including urea,154−156 glycine,157−162 cellulose,8 and citric acid163 as fuels. A model reaction is listed below:155,164 (with the preparation of CoCr2O4 as an example, urea was used as fuel): 3Co(NO3)2 + 6Cr(NO3)3 → 3CoCr2O4 + 12N2 ↑ + 30O2 ↑

(2)

20CO(NH 2)2 + 30O2 → 20CO2 + 40H 2O + 20N2 (3)

Analogously, Co3O4−boron carbon nitride (BCN) composites were obtained with Co(NO3)2 as the Co source, B2O3 as the boron source, and glycine as both fuel and the N source.157 Three ferrospinels, CoFe2O4, NiFe2O4, and CuFe2O4, have been fabricated from the corresponding metal nitrates and glycine. The autoignition was set at 300 °C.158 The metal nitrate oxidizing agent and organic fuels usually dissolve together in aqueous solution or are ball mixed together to obtain well-mixed hybrids.159,162,165,166 The compounds prepared directly from nitrates usually require higher temperatures because of the endothermic nature of nitrate decomposition.167−169 As a benefit of the universality 10128

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Figure 5. (a) Schematic diagram of mesoporous spinels accompanied by crystal structures of as-prepared spinel oxides. (b) Scanning electron microscope (SEM) images and (c) TEM images of mesoporous Co3O4. Reproduced from ref 175. Copyright 2015 American Chemical Society.

Figure 6. (a) Schematic diagram of the generation of MxCo3‑xO4 (M = Co, Mn, Fe) porous nanocages from the MOFs of M3[Co(CN)6]2·nH2O (M = Co2+, Mn2+, Fe2+). (b) SEM and insert TEM images of MnxCo3‑xO4. Reproduced with permission from ref 201. Copyright 2015 The Royal Society of Chemistry. (c) Schematic diagram of the generation of Co3O4@N−C nanocomposites from a Co−I−MOF precursor. (d) TEM and inset highrevolution (HR) TEM images of Co3O4@N−C nanocomposites. Reproduced with permission from ref 202. Copyright 2014 The Royal Society of Chemistry.

synthesized by a similar process.170 In addition, adding a carbon source and a metal oxide source (Al2O3) into the reaction can also synthesize the hybrid spinel materials.172−175 Through painting or brushing metal nitrates onto inactive substrate such as Ti, Ni, fluorine-doped tin oxide (FTO), and

of metal nitrates, it is easy to dope other metal elements in spinel compounds with this method.170,171 For instance, Liu et al. reported Ag-doped Mn3O4 hybrids formed by a one-step nitrate decomposition with Mn(NO3)2 and AgNO3. The formed Ag particles were as small as 2−3 nm. Ru-doped Co3O4 was also 10129

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Figure 7. Preparation of Fe3O4 nanocrystals with PLD and PLA methods. (a) A schematic diagram demonstrating the effect of substrate temperature and chamber partial pressure. With a high substrate temperature or low partial pressure, the Bi2O3 will be lost during deposition, resulting in the formation of nanocrystals with high aspect ratios at high temperature and nanocrystals with a lattice network at low pressure. With low substrate temperature or high partial pressure, Bi2O3 remains as deposited and is removed during the final PLA process, resulting in the generation of uniform nanocrystals lacking an optimal shape at low temperature and nanocrystals with greater distances between each nanocrystal at high pressure. (b) The pressure−temperature diagram of nanocrystal morphologies. (c) A colored SEM image of Fe3O4 (red). (d) A 3D atomic force microscrope (AFM) image of Fe3O4 nanocrystals. Reproduced with permission from ref 208. Copyright 2012 American Institute of Physics.

CoCr2O4, and CoFe2O4) with hard template nanocasting technology (Figure 5).175 In addition to the decomposition of metal nitrates, metal carbonyls can also be directly transferred into spinels at lower temperatures. Chang et al. synthesized a Pd−Mn3O4 derived from the decomposition of Mn2(CO)10 (similarly, Co3O4 was obtained from the decomposition of Co4(CO)12195).196 Notably, the temperature was only 230 °C. Moreover, the Pd particle size was only approximately 4 nm. Recently, inorganic salts (NaCl and KCl) have been introduced in the preparation of spinels, enabling the possibility of preparing nanostructured spinel oxides by decomposition. For example, Fu et al. synthesized 3D porous MnCo2O4 spinel nanoparticles. NaCl was used as the templating agent, and glucose was applied as the carbon source.197 Qiao’s group also prepared mesoporous MnxCo3‑xO4‑y by introducing

stainless foam, spinel composites can be formed on functional substrates.43,176−182 Impregnating the metal nitrates into porous carbon and (La,Sr)MnO3-yttria with good dispersion results in spinel oxides in porous frameworks.183−190 Additionally, nanocasting technology with soft or hard templates such as silica xerogel and metal nitrates as raw material is used to prepare porous spinel nanoparticles.47,191−193 Their specific surface areas are up to 250 m2 g−1 (Mg-substituted Co3O4).194 In the example of the fabrication of Mn3O4/CMK-3 nanohybrids, mesoporous CMK-3 was first dispersed in a mixed solution of H2O and ethanol containing the Mn(NO)3. The solvents were vaporized by heating at 80 °C. After that, a heat treatment of 350 °C was used to fabricate the Mn3O4/CMK-3 composites.189 Furthermore, the removal of templates will lead to neatly mesoporous spinels. Gu et al. applied an evaporation-assisted impregnation approach to fabricate a series of spinels (Co3O4, CuCo2O4, 10130

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Figure 8. Patterns and crystal structure changes of LixNi0.5Mn1.5O4 prepared with sol−gel method. (a) Neutron diffraction patterns of LixNi0.5Mn1.5O4 with different Li ratios. A small amount of NiO phase exists (x = 1) and the others are single spinel Fd3̅m phase (x = 0.7, 0.5, 0.3, 0.1, and 0). (b) The magnified view of selected Bragg peaks. (c) The change of lattice parameters of LixNi0.5Mn1.5O4 as a function of x. (d) A schematic view of one LixNi0.5Mn1.5O4 unit cell with spinel cubic structure. x = 0 is the dotted line and x = 1 is the solid line, the unit cell expands from x = 1 to 0. The expansion is exaggerated 10-fold for better visualization. Reproduced from ref 253. Copyright 2015 American Chemical Society.

interaction of Ni3+−O2−−Ni2+ double exchange.19 The PLD and PLA methods can also be used to prepare spinel nanoparticles. For example, in the preparation of Fe3O4, a BiFeO3 precursor was initially deposited on SrTiO3 substrates. Then, the PLA method was applied to decompose the BiFeO3. The generated Bi2O3 was vaporized during the spinel Fe3O4 composite creation (Figure 7).208 The morphology prepared by PLD is controlled by the power of the pulse, the flow rate of the gas, and the temperature of the substrate.209

the inorganic KCl as a template. The as-synthesized compound exhibited a narrow pore distribution of 10 nm.198 3.1.5. Metal−Organic Framework Derivation. Recently, MOF-derived spinels have received increasing attention in energy storage and conversion applications. Ascribed to the porous structures of MOFs and organic ligands, MOFs can decompose to spinel oxides with carbon coatings.199,200 Ding’s group synthesized three MxCo3‑xO4 (M = Co2+, Mn2+, Fe2+) porous nanocages from the calcination of corresponding MOFs of M 3 [Co(CN)6 ]2·nH2 O (Figure 6a,b).201 Zhang et al. synthesized Co3O4/C nanoarrays on Ni substrates by calcining Co-MOFs first in an Ar atmosphere at 400 °C and then in an air atmosphere at 250 °C.25 Furthermore, Co3O4@N−C was also acquired from another Co-MOF (Figure 6c,d).202 It is fascinating that the flexible structures of MOFs allow different spinel oxides with various compositions and morphologies to be obtained by regulating the MOF metal ion and ligands. This method not only opens a new approach toward spinel fabrication but also broadens the application fields of MOFs. 3.1.6. Pulsed Laser Deposition/Ablation. Through applying high-energy lasers to bombard reactants, pulsed laser methods, which include pulsed laser deposition (PLD) and pulsed laser ablation (PLA), are rapid with short reaction times. The escaped reactants are deposited on the selected substrate with the formed production film.203 Generally, the PLD method is used to transfer bulk compounds into a thin film, simultaneously enriching the oxygen defects.204,205 For example, a CoFe2O4 film on a Si substrate was prepared employing a CoFe2O4 substrate with a KrF excimer laser and a fluence of 1.5 J cm−2.206 NiFe2O4/NiCo2O4 could also be obtained by the same method.207 Interestingly, the prepared films exhibit metallic behavior at low temperatures (T < 400 °C) and show insulating behaviors at temperatures higher than 400 °C, which is considered to be caused by the concentration of Ni3+ and the

3.2. Solution-Phase Method

3.2.1. Sol−Gel. Sol−gel is a mild method and is widely applied in preparing spinels with nanostructures (nanoparticles are the most common).210−220 Typically, metal salts are used as precursors. Citric acid,221−230 ethylenediamine tetraacetic acid,231−233 propionic acid,234−237 ethylene glycol,237−241 glycine,242 glacial acetic acid,243 P123,244,245 or resorcinol/ formaldehyde246 are usually applied as chelating agents. Glucose is often used as a pore-former and carbon resource. These reactants are first mixed uniformly in the solvent. Then, reactions such as hydrolysis and condensation will occur in the formed transparent sol system. Through the loss of the fluid solvent during the aging procedure, the sol will gradually turn into a gel. After a calcination process, the compounds are finally obtained. For example, in the preparation of MCr2O4 (M = Co, Cu, and Zn), different nitrates were applied as the precursors, and citric acid was used as the chelating agent. The solution was heated to 80 °C and stirred to form the gel. The gel was first burned at 180 °C and then calcined for 6 h at 700 °C to obtain MCr2O4.247 Some other technologies can be applied to assist the sol−gel method. For example, ultrasound-assisted sol−gel grants smaller particle sizes, and dip-coating methods enable the formation of nanoparticle films on smooth substrates.248 With nitrates as the raw materials, the formed gels can combust at low temperatures.249−251 In those cases, citric acid can be employed as the 10131

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Figure 9. Sol−gel library of Co3O4 prepared from the sol−gel methods. (a) Table 1 is the library prepared with the raw materials of Co(NO3)2. (b) Table 2 is the library prepared with CoCl2 as the raw material. The corresponding SEM images obtained from specimens 1, 2, and 7 (Table 1) and 3, 4, and 6 (Table 2). Reproduced from ref 255. Copyright 2014 American Chemical Society.

Figure 10. (a) Schematic diagram of the “copolymer-co-morphology” technique to synthesize the PBAs and the derived spinel oxides with controlled shapes. The final products demonstrate the morphologies of both precursors. Field-emission SEM (FESEM) images of (b) Mn0.6Fe1.2Co1.2O4 and (c) Mn1.4Fe0.4Co1.2O4, and (d) corresponding energy dispersive spectrometer (EDS) mapping of Mn1.4Fe0.4Co1.2O4. (e) HRTEM and (f) selected area electron diffraction (SAED) pattern of Mn1.4Fe0.4Co1.2O4. Reproduced with permission from ref 257. Copyright 2016 The Royal Society of Chemistry.

using the sol−gel method. Neutron diffraction patterns with Rietveld revision clearly demonstrate the crystal lattice changes with different x values (Figure 8a,b). As shown in Figure 8c,d, with the decrease in Li content, the crystal cells expand linearly. Simultaneously, the bond lengths of O−O, Mn/Ni−O, and Li− O also exhibit regular changes.253 The morphology of products has also been controlled by tuning the synthetic parameters.217,254,255 In the preparation of Co3O4, CoCl2·6H2O and Co(NO3)2·6H2O were selected as the precursor salts.255 Four alcohols, ethanol, 2-propanol, methanol, and ethylene glycol, were tested as the solvents. The concentration of the Co salts was set as 0.6 M. Four epoxides, propylene oxide, n-butyl-glycidyl ether, glycidol, and epichlor-

fuel. The combustion is able to accelerate the reaction and reduce energy consumption. Defects can be controlled by changing the temperature of calcination. In the preparation of spinel LiMn2O4, cationic defects were generated at the low temperature of 300 °C (Li1−xMn2‑xO4). When increasing the temperature to 820 °C, regular LiMn2O4 was obtained. In comparison, the oxygen defect spinel LiMn2O4‑x was generated between 820 and 920 °C. Continued heating (over 920 °C) caused the decomposition of LiMn2O4.252 Spinel components have been regulated by varying the ratios of raw materials. LixNi0.5Mn1.5O4 spinel materials with different Li levels (x = 1, 0.7, 0.5, 0.3, 0.1, and 0) have been acquired by 10132

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Figure 11. Morphological control of spinels by using the hydrothermal method. (a) SEM image of ZnFe2O4 octahedrons and (b) corresponding structure demonstration of ZnFe2O4 octahedrons enclosed by (111) facets. Reproduced with permission from ref 263. Copyright 2012 Tsinghua University Press and Springer. (c) The crystal structure of ZnCo2O4 with cubic spinel structures. (d) Corresponding SEM image. Reproduced with permission from ref 34. Copyright 2015 The Royal Society of Chemistry. (e) SEM image of macroporous NiCo2O4 sheets. (f) XRD pattern of NiCo2O4 with insertion of cubic spinel crystals. Reproduced with permission from ref 268. Copyright 2013 Wiley-VCH. (g) Schematic diagram of fabricating NiCo2O4 during its reaction process. The inset (h) is the SEM image of final NiCo2O4 core−shell microspheres. Reproduced with permission from ref 269. Copyright 2015 Elsevier.

ohydrin, were selected. The results showed that, first, the metal precursor and epoxide ratios had no macroscopic difference. Second, the salt precursors were vital in controlling the morphology of the aerogel. As displayed in Figure 9, the gels derived from Co(NO3)2 showed plated structures, while the compounds prepared from CoCl2 displayed nanoparticle structures. With triblock copolymer P123 as the chelating agent, the prepared spinels usually unfolded the mesoporous structure heaped up by the nanoparticles.244,245 The approach such as microwave has been used to assist the sol−gel method.256 Moreover, spinel morphologies have also been tailored with a template approach. Recently, Li et al. developed a “copolymer-comorphology” strategy to prepare mixed spinel oxides from the derivation of Prussian blue analogues (PBAs).257 As displayed in Figure 10, the PBAs with different morphologies were first prepared with FeCl2·4H2O, Mn(NO3)2 (CoCl2·6H2O or Zn(NO3)2·6H2O), K3[Co(CN)6], and polyvinylpyrrolidone (PVP) with a modified sol−gel method accompanied by a calcination process. After further calcining the PBAs, spinel oxides with various morphologies were acquired. 3.2.2. Hydrothermal/Solvothermal. The hydrothermal method is known as a high-pressure solution method, which can largely reduce the required reaction temperature.258 The outstanding capability offered by the hydrothermal method is morphology adjustment. Various morphologies have been fabricated by this approach. Spinels with regular morphology geometries contribute to the exposure of specified crystal facets.259−261 Co3O4 with a cubic morphology predominantly exposes the (100) surface, while the octahedral morphology predominantly exposes the (111)

surface.262 Nano single-crystal ZnFe2O4 octahedrons were synthesized by using ferrous sulfate, zinc acetate, and hydrazine hydrate as reactants (Figure 11a,b). Meanwhile, the octahedral structure is in accordance with fcc structures. The highly activated crystal facets of (311) grow quickly, which exposes the weak (111) crystal facets on the surface of the crystal.263 Similarly to the sol−gel method, the hydrothermal method can also produce nanoparticles.264−266 To fabricate well-dispersed nanoparticles, surfactants such as cetyltrimethylammonium bromide (CTAB) or ethyl alcohol have been added.267 For example, 20− 40 nm CuCr2O4 nanoparticles have been synthesized by using cetyl alcohol as a modifier.267 Spinels with 1D structures, including nanorods, nanowires, and nanoarrays, have been controlled with the hydrothermal method (Figure 11c,d).34,41,270 NiCo2S4 nanorods were prepared with Ni(OAc)2·4H2O as the Ni source, Co(OAc)2·4H2O as the Co source, and thiourea as the S source. The solvent was water/ ethylene glycol, and the temperature was 160 °C.271 In addition, 2D spinels have also been prepared, including squares, nanosheets, and nanodisks.272−274 Garg et al. rationally controlled hexagonal or square NiCo2O4 by using different metal salts and hydrolyzing agents.274 By using the metal nitrate as raw material and urea as the hydrolyzing agent, square sheets were obtained. The hexagonal shape was obtained with ethylenediamine salts and NaOH as the hydrolyzing agent. Calcination of some products will generate water vapor and CO2, causing porous structures (Figure 11e,f).268,275 Xu et al. synthesized porous CoGa2O4 sheets by using the hydrothermal method with porous colloidal NaGaO2 as templates.276 3D porous spinel oxides facilitate ion transport and the infiltration of 10133

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Figure 12. Spinel hybrids prepared by the hydrothermal method. (a) Schematic diagram of preparing NiCo2O4 nanowires on carbon fabric. (b and c) Corresponding SEM images. Reproduced from ref 282. Copyright 2016 American Chemical Society. (d) Schematic diagram of synthesizing the Co3O4−Mn3O4/(graphene oxide) GO composite. (e) TEM image and (f) HRTEM image of Co3O4−Mn3O4/GO composite. Reproduced with permission from ref 283. Copyright 2016 Elsevier. (g) Schematic illustration of synthesizing NiCo2O4/3D porous composite. (h) Corresponding SEM and (i) TEM images. Reproduced from ref 284. Copyright 2014 American Chemical Society. (j) Schematic illustration of preparing NiCo2O4 and Ni0.33Co0.67S2 nanowires on Ti foil. SEM images of (k) NiCo2O4 and (l) Ni0.33Co0.67S2. Reproduced with permission from ref 285. Copyright 2015 Wiley-VCH.

Various substrates including graphene (rGO),286−299 carbon cloth,282,300−302 CNTs,303−307 mesoporous carbon,284,308 N- or S-doped carbon,309−319 conductive polymers,320,321 Ni,273,322,323 Ti,285,324 and La0.8Sr0.2MnO3325 have been employed with in situ and ex situ technology in the hydrothermal method to load the spinels. The carbon materials and conductive substrates facilitate the electron transfer and stabilize the nanostructures.326−330 1D CNT substrates, carbon nanofibers, and knitted carbon cloth have been employed to support spinels.331 Various CNTs such as helical CNTs,303 multiwalled CNTs (MWCNTs),304,332 and oxidized CNTs have also been used.307 Notably, oxidative treatment or acid-functionalized CNTs are usually necessary to avoid agglomeration.305 In addition to CNTs, 1D carbon fiber is also a good selection. Moreover, the carbon fibers can assemble into carbon cloth, which can act as a flexible and free-standing substrate to support the spinels and then directly serve as electrodes for the ORR/OER or batteries.300−302 Nanoarray morphologies are commonly formed on carbon cloth (Figure 12a−c).282 Graphene or rGO is a widely used 2D substrate, which is beneficial to the dispersion of spinel oxides.292,295,296 Graphene is often synthesized by the reduction of graphene oxide (GO), which is acquired through a modified Hummers’ method.287 An in situ method with GO287,288,291,294 and an ex situ method with graphene286 have been utilized to load spinel oxides. The GO is easily dispersed in water and reduced under a

electrolytes, which is of vital importance for catalyst and battery applications.277−279 For example, in the preparation of CoFe2O4 hollow nanospheres, [(NH4)2Fe(SO4)2·6H2O] was used as an Fe source. [CoSO4·7H2O] was used as the Co source, and glucose was used as the pore-forming agent. These reactants were dissolved in water and transferred to a Teflon-lined stainless steel autoclave, which was heat-treated at 160 °C for 28 h at an inner high pressure and temperature environment. The obtained precipitates were further calcined at 550 °C for 2 h to remove the carbon cores to yield hollow nanospheres.280 Applying a hard template such as polystyrene particles, Wang et al. synthesized hollow porous NiCo 2 O 4 in the form of nanorods. 281 Analogously, Ni-doped CoFe2O4 hollow spheres were fabricated with soft template method in which glucose was used as the template and pore former.278 Peng et al. reported the controlled synthesis of spinel cobaltite with porous core−shell structures. The metal nitrite served as raw material, and the poly(ethylene glycol) 6000 (PEG 6000) was the dispersing agent. PEG 6000 can adsorb on the nuclei and play an important role in forming the porous core−shell structures. As shown in Figure 11g,h, the nanoparticles were initially obtained at the early reaction stage. Then, the nanoparticles were generally packed into bigger microspheres, after which the microspheres were gradually split into nanoplates. Finally, the core−shell structures were formed.269 10134

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Figure 13. Major factors influencing precipitation methods. SEM images of Co3O4 prepared from the precipitants of (a) NH3·H2O and (b) urea as precipitants. Reproduced with permission from ref 389. Copyright 2010 Elsevier B.V. SEM images of Mn3O4 prepared from solvents of (c) ethanol and (d) water. Reproduced with permission from ref 397. Copyright 2016 Elsevier. (e) Ratio of [Mn+]precipitate/[Mn+]introduced vs pH, Al3+ (square) and Zn2+ (circle). (f) Ratio of [Al3+]/[Zn2+] for coprecipitation materials prepared at various pH. The dashed line stands for the theoretical ratio. Reproduced with permission from ref 375. Copyright 2013 The Royal Society of Chemistry. (g) The area of the formation of core−ring NiCo2O4 in relation to the temperature and annealing time. (h) Schematic illustration of core−ring NiCo2O4 nanoplatelet formation. The inset is a TEM image of core−ring NiCo2O4. Reproduced with permission from ref 398. Copyright 2008 Wiley-VCH.

on Ti foil through a simple hydrothermal method with primary raw materials of Ni(NO3)2·6H2O, Co(NO3)2·6H2O and urea. Furthermore, this NiCo2O4 was transformed into Ni0.33Co0.67S2 nanowires under sulfur vapor (Figure 12j−l).285 In summary, hydrothermal methods enable spinels with various morphologies, creating spinels suitable for various applications after they are loaded on selected substrates. However, hydrothermal processes are only applicable for synthesizing compounds that are not sensitive to aqueous atmospheres. As an alternative, the solvothermal method is also suitable for obtaining spinels. Ethanol,334−339 ethylene glycol,340,341 isopropanol,342 PEG,343 tertiary butanol,344 and dimethylformamide (DMF)345,346 are general reaction solvents for this method. In some cases, the solvothermal method is milder and more rapid than a hydrothermal process. For instance, the postspinel CaMn2O4 can be synthesized by the hydrothermal method at ∼210 °C for over 70 h. In comparison, the solvothermal method with ethanol requires only 190 °C for 36 h.334 Interestingly, some spinels exhibit different crystallinities when formed by the hydrothermal and solvothermal methods. CoFe2O4 prepared hydrothermally exhibits high crystallinity, but amorphous CoFe2O4 was achieved with tertiary butanol solvothermally.344 The spinel morphology can be tuned by using different solvents. In the preparation of NiCo 2O 4 compounds, a solvent of water and DMF at a volume ratio of 2:1 resulted in nanoneedles, while replacing DMF with ethanol caused the formation of nanosheets.347 Yang et al. found that both the organic additives and temperature affected the morphology of CoxMn3−xO4. Nanorods were obtained with an oleylamine additive at a hydrothermal temperature of 120 °C. Porous microspheres were formed with triethanolamine additives at 180 °C. Nanocubes were generated at 180 °C without any additives.348

hydrothermal atmosphere with a common reductant (NaBH4, glucose).289 As mentioned above, thiourea is adopted as a S source to form Ni0.3Co2.7S4/rGO hybrids.288 Recently, Wang and co-workers prepared Mn3O4-decorated Co3O4 nanoparticles on GO (Figure 12d−f). The Co3O4/GO was initially prepared through a hydrothermal reaction. Then, the Co3O4/GO was further applied to synthesize the Co3O4−Mn3O4/GO hybrids. Interestingly, the Mn3O4 displayed epitaxial growth on the side of Co3O4.283 Adding a surfactant including PVP with coordination of the metal-ion surface contributes to the formation of 2D nanoplatelets. Moreover, the decomposition of PVP at high temperatures results in the formation of mesopores.293 3D carbon materials endowed with porous structures can release the volume changes of spinel electrodes in a battery field and simultaneously facilitate the transport of electron/ions (Figure 12g−i).284,308 Through the use of SiO2 spheres as a template, FeCo2O4/hollow graphene spheres were obtained.297 Recently, spinels loaded on N-doped carbon with further enhanced electron transfer abilities have received increasing attention. Common N sources include dicyandiamide,309 ammonia,298,312,314,315,333 urea,310 ammonium bicarbonate,318 and polypyrrole (PPy).317 For example, Li et al. reported a MnCo2O4/NCNT hybrid formed by adding NH3·H2O into the hydrothermal step. The nitrogen content in the final product was calculated as 1.09%, which is composed of amino N, graphitic N and pyrrolic N.311 Yan et al. acquired N/S dual-doped CoFe2O4/ 3D rGO composites by using thioacetamide (CH3CSNH2) as both the nitrogen and sulfur source.319 In addition to carbon substrates, unreactive metal substrates such as Ni foam273,322,323 and Ti foil285,324 can also properly support micro/nano structured spinel oxides. The flexible and porous structures provide sufficient area for spinel oxide deposition. Peng et al. synthesized NiCo2O4 nanowires grown 10135

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Co(NO3)2·6H2O aqueous solution followed by a rapid addition of NaOH. Moreover, they investigated the morphology and structure changes in relation to calcination temperature and time.398 As shown in Figure 13g, the spinel phase began to be generated above 180 °C. The core−ring nanoplatelets were formed between 200 and 400 °C within 2 h. Nanoplatelets without a core−ring structure were formed after 2 h. However, increasing the temperature caused the nanoplatelets to decompose (Figure 13h). Singh et al. found that the unit cell shrank with increasing calcination temperature.410 The length (a) of cubic spinel CoFe2O4 prepared at 100 °C is 8.420 Å and at 380 °C is 8.390 Å. The crystal size should increase with increasing temperature according to the Scherrer formula. Through controlling the raw materials, spinels with different components or other element dopings have been fabricated,396,411−414 including Ni-doped,415 K-doped,32 rare earth metal-doped (MnIn2O4, occupying the 16d site),10 and Ni, Co, Ge-doped.382 Chen et al. first fabricated NiCo2O4 nanoparticles by using Ni(NO3)2 and Co(NO3)2 with NaOH as the agent. Interestingly, after impregnation in KOH and Al(OH)3 solution, K-NiCo2O4 and Al-NiCo2O4 were also obtained. The surface area was considered to be tailored by the different metal dopings, and the K-doped spinel demonstrated the highest specific surface area.416 Meanwhile, some noble metals such as Pt have been loaded during deposition to increase the activities of catalysts. Kim et al. fabricated ∼1.6 nm Pt on spinel Mn3O4 by reducing Na2PtCl4·xH2O on MnO/Mn3O4 core−shell nanoparticles.417 Recently, our group developed an “oxidation−precipitation and insertion−crystallization” method to fabricate spinels with tunable phase and composition.79 The reaction mechanism is summarized below. (1) Oxidation−precipitation process:

3.2.3. Precipitation. The precipitation method usually mixes different contents in solution. After adding the precipitant, the precursor is obtained after filtering. Then, the precursors usually require calcination at high temperature to decompose and produce the spinels.16,349−360 For example, Co3O4 was prepared with CoSO4 as the Co source and H2C2O4 as the precipitant. The precipitate was initially formed with the composition of CoC2O4· 2H2O. The second reaction occurred at 150 °C corresponding to the loss of crystal water and the formation of CoC2O4. Finally, at 260−280 °C, the CoC2O4 further reacted with O2 to form the Co3O4 and CO2.361 The common precipitants used to prepare spinels are NaOH,46,362−369 KOH,370,371 Na2CO3,372,373 H2C2O4,374 ammonia (also used as the coordination agent),375−378 NaHCO3,379 sodium dodecyl sulfate,380 (NH4)2CO3 or NH4HCO3,381,382 tetramethylammonium hydroxide,383 and urea.384−386 The external force such as microwave can better disperse the precipitate.387 In addition to the precipitants, some oxidative and reductive agents are often added to obtain the final products.388,389 CuCo2O4 is obtained by using H2O2 as an oxidative agent.46 Meanwhile, templates such as Al2O3 can be involved in this method to fabricate mesoporous spinel oxides.390 Carbonate and oxalate precipitants also form porous structures because of the generation of CO 2 in the calcination process.391−393 Certain organic solvents such as ethylene glycol are often added as a stabilizer.394,395 Conductive substrates such as Ti film396 can be selected to load the deposition products onto. The physical and chemical characteristics of the obtained spinels are affected by the precipitants.399 Bo et al. synthesized NiCo2O4 nanoparticles by using Co(NO3)2·6H2O and Ni(NO3)2·6H2O. Different precipitants including NaOH, Na2CO3, and oxalic acid were tested.389 The product prepared with NaOH had the highest specific surface area and the smallest particle size. Li et al.389 found that Co3O4 prepared with a NH3·H2O precipitant formed homogeneous nanocubes (Figure 13a), while urea produced Co3O4 nanocubes accompanied by nanorods and nanoflakes (Figure 13b). In addition, the atmosphere also affects the final product morphologies. Yang and co-workers found that (Mn,Co)3O4 prepared in a pure O2 atmosphere formed nanoparticles. In comparison, the compounds prepared in a 5% O2/Ar atmosphere formed octahedral morphologies.400 This is because the concentration of oxygen influences the nucleation rates of spinels. High oxygen concentrations result in the fast nucleation of spinels, which results in the formation of nanoparticles. Meanwhile, the solvent also results in different morphologies. Liu et al. found that changing the solvent from ethanol to DMF resulted in smaller Mn3O4 particle sizes. Moreover, using water as the solvent promoted the generation of nanosquares (Figure 13c,d).397 The solution pH in the synthesis of the precursor also has a crucial role in the final spinel compositions.401−405 For instance, Kang et al. successfully obtained 10 nm Fe3O4 at a pH of 11−12 in 1996.406 CuxCo1−xCo2O4 was synthesized with K2CO3 as the precipitant at a pH of 9.1−9.4.407 CoxFe3‑xO4 nanoparticles were generated with FeCl3 and CoCl3 as raw materials. Then, NaOH was added dropwise until the pH was 11 to 12.408 Cornu et al. found that a pH of 8.4 was optimal for precipitating the final products of ZnAl2O4 (Figure 13e,f).375 Ramsundar successfully obtained Co3O4 nanorods at the pH of 7 with Co(NO3)2·6H2O and K2CO3.371 The calcination temperature and heating rate can also affect the spinel oxide morphology.398,409 Cui et al. synthesized core− ring NiCo2O4 nanoplatelets by using a Ni(NO3)2·6H2O and

2Mn 2 + + 12NH3H 2O + 2Co2 + + O2 + 2OH‐ → 2Co(NH3)6 3 + + 2MnOOH ↓ + 12H 2O

(4)

In this reaction, a Mn2+ source and NH3·H2O were added first, followed by a Co2+ source. If we reverse the order of the Co2+ and Mn2+ sources, the prior-generated sufficient Co(NH3)63+ will oxidize the MnOOH as below: 7MnOOH + 5Co(NH3)6 3 + + 5OH− → Mn 7O13 + 5Co(NH3)6 2 + + 6H 2O

(5)

(2) Insertion−crystallization process: Co(NH3)6 3 + + MnOOH → tetragonal Co xMn3 − xO4 (180°C)

(6)

Co(NH3)6 3 + + Mn 7O13 → cubic CoxMn3 − xO4

(180°C) (7)

This method achieved the goals of controlling both composition and phase. Cubic CoxMn3‑xO4 spinel oxides were obtained with inherited Mn7O13 structures. In comparison, the tetragonal Co xMn 3‑xO 4 spinel oxides inherited the low symmetric MnOOH structures. Six CoxMn3‑xO4 compounds with different phases and compositions have been prepared. Through careful design, spinels acquired from the precipitation method can be enriched with deficiencies and exhibit various morphologies.381,422,423 NiCo2O4 and NiMnxCo2−xO4−y flake-like morphologies have been fabricated with Ni(NO3)2, Co(NO3)2, and MnSO4 aqueous salts by adding an ammonia 10136

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Figure 14. Regulations of vacancies, composition, porosity, and specific surface area of spinels by using precipitation methods. NiCo2O4 enriched with oxygen vacancies. (a) TEM image of a NiCo2O4 nanosheet. The inset is a photograph of its colloidal dispersion with the Tyndall effect. (b) Corresponding AFM image. (c) Schematic illustration of topological transformation from Ni/Co hydroxide to NiCo2O4 with oxygen vacancies. Reproduced with permission from ref 418. Copyright 2015 Wiley-VCH. Co3O4 nanoarrays with different Ni doping. (d) Composition of Co3O4 with various Ni-doping levels. SEM images of (e) Co3O4, (f) NCO-1, and (g) NCO-2. Reproduced with permission from ref 419. Copyright 2010 WileyVCH. Porous NiCo2O4 hollow cages. (h) Schematic illustration of hollow mesoporous NiCo2O4 nanocages fabrication. Corresponding (i) SEM and (j) TEM (the inset is SAED) images. Reproduced with permission from ref 420. Copyright 2015 The Royal Society of Chemistry. Ni0.3Co2.7O4 with hierarchical structures and different specific surface area. (k) N2 adsorption−desorption isotherms of Ni0.3Co2.7O4. Corresponding SEM images of (l), (m), and (n) with different calcination temperatures. Reproduced with permission from ref 421. Copyright 2013 The Royal Society of Chemistry.

solution dropwise.424 NiCo2O4 ultrathin nanosheets have been synthesized by adding Co(NO3)2·6H2O and Ni(NO3)2·6H2O in NH3·H2O solution to p-aminobenzoic acid with vigorous stirring. The solution was centrifuged and dried at 60 °C for 12 h. After that, the compound was obtained as ultrathin nanosheets with rich oxygen vacancies through heating in air at 300 °C. In comparison, compounds with poor vacancies were obtained at the same temperature in an O2 atmosphere (Figure 14a−c).418 Wu’s group applied an “ammonia evaporationinduced” growth method to fabricate NixCo3‑xO4 nanowire arrays on a Ti film. Co3O4 nanoarrays were initially synthesized. After introducing Ni(NO3)2 into the solution, the spinel NixCo3‑xO4 was also obtained. A greater content of Ni led to thicker and rougher nanowires (Figure 14d−g). Moreover, the conductivity and specific surface increased with the increase in Ni content. Additionally, the overall Ni/Co ratio determined by inductively coupled plasma-optical emission spectrometry (ICPOES) and the surface Ni/Co ratio determined by X-ray

photoelectron spectroscopy (XPS) differed from the ratios of the raw materials.419 Lu et al. also prepared Co3O4 nanowire arrays with Ni-substitution on Ni foam with a similar method.425,426 A hollow spinel form of NiCo2O4 was obtained by using solid Cu2O crystal as a template (Figure 14h−j). As mentioned above, oxalate and carbonate precursors will result in the formation of mesoporous structures.420 Lou’s group prepared a series of mesoporous Ni0.3Co2.7O4 hierarchical structures by slowly heating Ni0.1Co0.9C2O4·nH2O at a rate of 1 °C per min. All of the samples prepared at 400, 450, and 550 °C showed the hierarchical mesoporous structure. However, higher calcination temperatures led to the decrease in specific surface area because of the further agglomeration of nanoparticles (Figure 14l−n).421 Spinel hybrids have also been obtained with precipitation methods.44,427−432 Introducing the templates will generate pores after removing the templates.433 NiCo2O4 nanosheets on CNTs were acquired by dissolving Ni(NO3)2·6H2O and Co(NO3)2· 10137

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Figure 15. (a) Schematic illustration of microwave-assisted reflux equipment with (b) SEM and (c) TEM image of prepared Co3O4 with micro- and nanostructures. Reproduced from ref 442. Copyright 2008 American Chemical Society. (d) Schematic illustration of synthesizing Mn3O4/graphene composites with (e) and (f) corresponding SEM image. Reproduced with permission from ref 447. Copyright 2016 Elsevier.

structures have been obtained, ascribed to the generation of CO2 during the calcination process. 3.2.5. Microwave. Microwaves with wavelengths from 1 mm to 1 m will cause molecular motion with friction, generating the energy needed for the synthesis of spinels. Because of the high energy of microwaves, syntheses involving microwaves can greatly shorten reaction times, simplify preparation methods, and lower the reaction temperatures (even lower than 100 °C).256,442−448 For example, with a microwave-assisted reflux method, the precursor of Co3O4 could be obtained in only 15 min (Figure 15a−c).442 Mn3O4 nanoparticle-rGO have been fabricated at 90 °C (Figure 15d−f).447 Mn3O4 nanomaterial/3D graphene/single-walled CNT (SWCNT) hybrids448 and CuCo2O4/CuO particles24 have also been obtained via microwave-assisted reflux methods. NiCo2O4/C composites were obtained via a program-controlled microwave oven with 20 cycles of a heating treatment of only 5 s on and 5 s off.387 Moreover, the high energy of the microwaves resulted in betterdispersed products with large specific surface areas.444,449 Microwave methods are usually applied to assist other common methods.443,450−453 Co3O4 nanorods doped with some rare earth elements (Pr, Sm, and Tb) were prepared with a microwaveassisted coprecipitation method.450 Mesoporous Co3O4 nanoflakes were obtained with a microwave-assisted hydrothermal method.443 MnCo2O4 nanoparticles were prepared through a microwave-assisted solvothermal synthesis.454 MnCo2O4/C composites were formed with a microwave-assisted nitrate decomposition method.455 In this nitrate decomposition method, the metal nitrates were first mixed with Ketjenblack carbon. When the composite was treated with microwaves, it was ignited by the high-energy microwaves. Then, the burning heat caused the decomposition of nitrates to produce MnCo2O4/C hybrids. 3.2.6. Electrochemical. Electrochemical methods can be generally divided into electrodeposition and electrospinning. Electrodeposition is an inexpensive, convenient, and rapid

6H2O in water with PVP. Oxidized CNTs were also dispersed in this solution. After ultrasonication, NH3·H2O was added to adjust the pH to 9. Through drying at 70 °C and annealing in air at 200 °C, the final products were formed.434 Mn3O4/rGO hybrids were obtained via a one-step reaction with Mn(Ac)2· 4H2O and rGO in KOH−C2H5OH solution.435 Porous Co3O4 nanosheets were prepared with GO as sacrificial template.436 Dai’s group synthesized Co3O4 nanocrystal/graphene hybrids through hydrolysis and oxidation of Co(OAc)2 on GO sheets.14 Moreover, the addition of NH3·H2O could not only reduce the particle size of Co3O4 (from 12−25 nm to 4−8 nm) but also introduce N-dopants into the final products. They further synthesized MnCo2O4/graphene hybrids with N-doping. In addition, the X-ray near-edge spectra demonstrated that there were strong C−O and C−N bonds in the prepared N-rmGO (reduced mildly oxidized graphene oxide) nanosheets, reflecting the strong interactions between N-rmGO and spinel nanoparticles with the formation of C−O−metal bonds and C−N− metal bonds.299 Additionally, yeast has been used as a biocarbon source to fabricate CoFe2O4/biocarbon composites.429 Recently, CoMn2O4, ZnMn2O4, and CdMn2O4 spinels have been prepared at room temperature. The obtained spinels have smaller particle sizes and are enriched with deficiencies.437 3.2.4. Microemulsion. A microemulsion solution is obtained by mixing two kinds of immiscible solvents with the addition of a surfactant.438−440 This process takes advantage of its monodisperse and excellent interface, which usually results in spinels with one-dimensional features, such as nanorods.438,440,441 Typically, two types of solvents (water phase and oil phase) are used. For example, in the preparation of Mg2MnO4, MnCl2·4H2O, MgSO4, and (NH4)2C2O4·H2O dissolved in water were used as the water phase, and isooctane was used as the oil phase. CTAB and 1-butanol were applied as the surfactant and cosurfactant, respectively. The obtained precursor was further calcined between 500 and 800 °C to obtain Mg2MnO4.441 With metal oxalates as the precursors, porous 10138

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Figure 16. Electrochemical methods to prepare spinels. (a) Schematic illustration of the electrodeposition process. (b) Schematic diagram of preparing ZnCo2O4 and SEM images of prepared ZnCo2O4 film. Reproduced from ref 467. Copyright 2014 American Chemical Society. (c) Schematic illustration of preparing spinels with tube-in-tube, nanotube, and solid 1D structure. (d) TEM images of tube-in-tube CoMn2O4. Reproduced from ref 472. Copyright 2015 American Chemical Society.

method that can prepare spinels with different morphologies in a very short time.456−460 A typical electrodeposition experiment usually includes three electrodes. A reference electrode such as Hg/HgO or Ag/AgCl with working electrode forms a voltage loop to test the potential. A stable counter electrode such as Pt, Ti, or stainless steel with working electrode forms a current loop to test the current. A working electrode such as Ni foam or conductive glass is used to generate the electrochemical reaction and load the generated product.461,462 Generally, electrodeposition can be potentiostatic deposition or galvanostatic deposition.463,464 Various morphologies such as nanosheets and nanoparticles with different compositions have been synthesized.42,465,466 Both simple spinel oxides and complex spinel compounds have been fabricated by electrodeposition. For instance, with 0.01 M MnCl2·4H2O as the Mn source, 0.02 M Co(NO3)2·4H2O as the Co source, and 0.01 M KCl as the electrolyte, the synthesis of MnCo2O4 nanosheets precursor on conductive glass substrates only required 60 s.465 Kim electrodeposited ZnCo2O4 by tuning [OH−] in the electrolyte. As reported, the generation of [OH−] will lead to the Zn2+/Co2+ solution/deposition equilibrium shifting to the right, causing the codeposition of ZnO and Co(OH)2. Calcining these products with dissolved excess ZnO will generate ZnCo2O4 (Figure 16a,b).467 NO−3 + H 2O + 2e− → NO−2 + 2OH−

The crystallinity of products can also be controlled by the deposition temperature. Koza reported that electrodeposition temperatures between 50 and 90 °C granted amorphous Co3O4, while 103 °C yielded crystalline Co3O4.468 In addition to electrodeposition, there is also electrophoretic deposition (EPD), in which the electrically charged colloid particles with a zeta potential move in an appointed direction under the effect of an electric field, forming a uniform film on the selected substrate.469,470 Moreover, electropolymerization has been applied to coat polymers on spinel oxides.466 Combined with other methods such as plasma engraving, the prepared spinels are enriched with oxygen vacancies.471 Electrospinning is a method using a high voltage power to pulverize a polymer fluid.473,474 The polymer flow jet can move a certain distance and then solidify into fibers. The process of electrospinning was described in 1995.475 The prepared fiber usually generates porous and self-standing structures with nitrogen doping, which can facilitate their application as electrodes in battery and sensor fields.29,48,476 With a precursor solution containing Co(NO3)2·6H2O and Fe(NO3)3·9H2O salts, PVP polymer, and DMF solvent, Co(NO3)2/Fe(NO3)2/PVP fibers were initially synthesized by electrospinning. Then, these fibers were heated at 800 °C for 2 h to obtain CoFe2O4 compounds.477 Generally, the morphology and structures of spinel oxides can be controlled by varying the metal salt precursors and the ratio of salts/carbon source.478 Recently, Peng et al. found that the heating rate and precursor polymers could control the final morphology and composition (Figure 16c,d).472 For example, three kinds of CoMn2O4 with 1D structures, solid, tube, and tube-in-tube structures, have been synthesized by tuning the heating rate. In the prepared Co−Mn precursor nanofibers,

E θ = 0.01 V (8)

Zn 2 + + H 2O → ZnO ↓ +2H+

Co2 + + 2H 2O → Co(OH)2 ↓ +2H+

(9) (10) 10139

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Figure 17. (a) Synthesis summary of preparing CoxFe3−xO4 nanocubes with varying amounts of Co and fixed levels of other contents. (b) TEM and (c) corresponding EDS mapping of Co0.5Fe2.5O4. Reproduced from ref 486. Copyright 2016 American Chemical Society. (d−f) TEM and HRTEM of CuCr2S4 nanocrystals. (g) Crystal structure of CuCr2S4. Reproduced from ref 487. Copyright 2011 American Chemical Society. (h and i) TEM and HRTEM of CuCr2Se4 nanocrystals. Reproduced from ref 488. Copyright 2007 American Chemical Society. (j−l) TEM and HRTEM of CuCr2Te4 nanocrystals. Reproduced with permission from ref 489. Copyright 2012 The Royal Society of Chemistry.

Fe3O4 nanocrystals with diameters of 20 nm were synthesized. Later, by tuning the reaction conditions or using seed-mediated growth, the particle size was varied from 3 to 20 nm.482 Afterward, they also used epitaxial growth to synthesize dumbbell-like Pt−Fe3O4 nanoparticles.483 Additionally, complex spinel oxides such as CoFe2O4 and MnFe2O4 have also been obtained.484 Recently, MxFe3−xO4 (M = Mn, Co) spinels with different ratios of Mn to Fe and Co to Fe have been synthesized with similar sizes and shapes. Oleic acid was added as a size selection agent to narrow the size distribution of the compounds. According to the results of small-angle X-ray scattering, the size distribution was largely narrowed after selection. The standard deviations were 11.5% and 9.6% before and after size selection.485 Sathya reported cube-shaped CoxFe3‑xO4 nanocrystals, in which they could precisely control the size and cobalt stoichiometry (Figure 17a−c).486

polyacrylonitrile (PAN) acts as the core and PVP/metal acts as the shell. At the low heating rate of 1 °C min−1, the core PAN can gradually be eliminated with homogeneous mass loss of the PVP/metal shell, leading to the hollow tube structures. At a medium heating rate, the core eliminates more rapidly, while the PVP/metal forms a rigid layer to prevent the rapid contraction of the shell, leading to the generation of tube-in-tube structures. Finally, a relatively solid fiber is obtained at high heating rates because of the collapse of the shell. In addition, by varying the raw materials, tube-in-tube structures CoFe2O4, NiCo2O4, NiMn2O4, and ZnMn2O4 were also fabricated. 3.2.7. High-Temperature Solution Phase. High-temperature solution methods are suitable for synthesizing monodisperse nanocrystals.17,479−481 In 2002, Sun et al. prepared monodisperse Fe3O4 nanocrystals with this method.479 They used Fe (acetylacetonate) as the Fe source and phenyl ether as the solvent. In the presence of alcohol, oleylamine, and oleic acid, 10140

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Figure 18. Sonochemical reaction to prepare spinel MnxCo3−xO4 nanocrystals. (a) XRD patterns of prepared MnxCo3−xO4 samples. Selected TEM images of (b) Co3O4 and (c) Mn3O4. Reproduced with permission from ref 497. Copyright 2014 Elsevier.

the temperature up to 80 °C.499 Furthermore, Xiao et al. fabricated Mn3O4/CNT composites simply by adding manganese acetate into a mixture of CNTs in DMF at 80 °C.500 Co3O4/graphene composites have been prepared with Co(AC)2 and Na(BH)4 a GO solution.501 The NaBH4 reductant will generate H2, leading to porous structures in the spinel oxides.502 Our group has developed a “reduction−crystallization” approach to fabricate spinels.7 The reaction mechanism is summarized as follows:

“Hot injection” technology is also used in high-temperature solution reactions. To prepare Fe3S4, a sulfur/oleylamine solution was rapidly injected into a Fe(acetylacetonate)/ hexadecylamine solution at 300 °C.490 “Hot injection” technology ensures that the reaction occurs rapidly at high temperature, thus forming nanocrystal structures. With this technology, CuCo2S4,491 CuCr2S4 (Figure 17d−g),487 CuCr2Se4 (Figure 17h,i)488 and CuCr2Te4 (Figure 17j−l)489 spinels have also been synthesized. Lyubutin et al. applied a polyol-mediated process to synthesize Fe3S4 nanocrystals with high boiling polyalcohol solvents. The obtained Fe3S4 nanoparticles had an average size of approximately 9 nm.492 Zhang et al. prepared NiCo2S4 submicron spheres by using nickel acetate tetrahydrate (C4H6NiO4·4H2O), cobalt(II) chloride hexahydrate (CoCl2· 6H2O), and 3-mercapto-1,2-propanediol in the oleylamine solvent. The as-synthesized compounds exhibited entirely monodisperse spherical morphologies in which Ni, Co, and S were homogeneously distributed.493 Typically, prepared spinel nanocrystals are stored in organic solvents (n-hexane as an example). This means that the direct use of spinel nanocrystals as catalysts will lead to the agglomeration of nanoparticles. Thus, loading these nanocrystals on carbon materials such as graphene with high specific surface areas is crucial for practical applications.494,495 In addition, modifying the crystals with noble catalysts also increases the activities of the catalysts.496 In addition to the above liquid reaction, there are other methods to prepare spinels. Lee et al. used a sonochemical reaction to prepare MnCo3‑xO4. The raw materials were Co(OAC)2, Mn(OAC)2 and porous carbon. These raw materials were mixed in a solvent of DMF and distilled water. Then, the reaction was induced by high-intensity ultrasound for 2 h at 130 °C. After drying, MnCo3‑xO4 nanoparticles with different x values were formed (Figure 18a−c). The reaction is believed to proceed as follows: (1) formation of H2O2 by ultrasound, (2) oxidation of the divalent cation, and (3) coprecipitation of the divalent and trivalent cations.497 Meanwhile, low-temperature and even room-temperature solution methods have been developed to synthesize spinels. Bag et al. prepared Mn3O4/rGO nanohybrids directly through reducing KMnO4 by N2H4 in the presence of GO at 95 °C.498 (Co1−xMnx)3O4 (x = 0, 0.25, 0.33, 0.50, 0.66, 0.75, and 1) nanoparticles have been fabricated by coprecipitation method at

reduction process: 3MnO2 + BH−4 → Mn3O4 + BO2− + 2H 2 , ΔG 0 = −681 kJ/mol

(11)

crystallization process: xCo2 + + Mn3O4 → Cox Mn3 − xO4 + x Mn 2 +

(12)

Co2+ could replace Mn2+ because r(Co2+) < r(Mn2+), and they have compatible site preference energy. This preparation process occurs at room temperature. The obtained spinels are endowed with high specific surface areas and defect levels. Chowdhury et al. further prepared the conducting CoMn2O4−poly(3,4-ethylenedioxythiophene) (PEDOT) nanocomposites with this method.503 Xie and Lou cooperated and designed a general method to prepare complex spinel compounds with tube-in-tube structures. Using carbon nanofibers as a template, several tube-in-tube spinels including CuCo2O4, ZnCo2O4, CoMn2O4 and NiCo2O4 were fabricated.504 Zhuang’s group reported spinel-type Mn2AlO4 hexagonal nanosheets by treating a MnAl alloy with a NaOH solution.505 They initially fabricated an Al95Mn5 alloy with a melt-spinning technique. Afterward, the alloy was treated in NaOH solution repeatedly. Finally, the treated sample was further annealed at 450 °C to obtain Mn2AlO4. 3.3. Vapor-Phase Methods

3.3.1. Chemical Vapor Deposition. CVD methods use gaseous reactants to form a solid thin film on a solid substrate; these methods are usually used to improve product purity or to dope another constituent. One of the earliest examples of CVD at a large scale was in 1890.506 The easily vaporizable metal salts are often used as reactants (Figure 19a).507−509 For example, to 10141

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Figure 19. (a) Schematic diagram of CVD. (b) The relationship between temperature and the growth rate or Mn content for when preparing (Mn, Co)3O4 films. (c) The relationship between the Mn(thd)3 pulsing fraction and Mn content or growth rate of (Mn, Co)3O4 film. Panels b and c are reproduced from ref 75. Copyright 2010 American Chemical Society.

reactions such as oxygen reduction and ethylene glycol oxidation. Lapham et al. utilized electrostatic spray deposition with a high voltage electric field in the spray region to deposit NiCo2O4,524 which can be considered an improvement of the spray pyrolysis method. Many spinel electrode materials, such as octahedral LiNi0.5Mn1.5O4525 and LiMn2O4 microspheres,526 have been synthesized by spray pyrolysis. Kang’s group acquired spinel LiNi0.5Mn1.5O4 with yolk−shell structures by utilizing the spray pyrolysis technology with raw materials of LiNO3, Ni(NO3)2· 6H2O, and Mn(NO3)2·6H2O. Sucrose was added as the poreforming agent. The calcination temperature was set as 600, 700, and 750 °C. The formation and TEM images of the core−shell structures are shown in Figure 20b,c.527 3.3.3. Magnetron Sputtering. Magnetron sputtering was introduced in the 1970s; it is a fast physical vapor deposition method for the preparation of thin films that consist of nanoparticles.528−534 Argon ions ionized by collisions with electrons will first bombard the target materials. Then, the sputtering particles generated from the target will deposit on the substrate and form a thin film (Figure 21a). With Co metal as the target, a Co3O4 spinel film was fabricated with a working pressure of 20 mTorr and a power of 100 W under an Ar/O2 (3:1) atmosphere (Figure 21b−d).532 After post annealing in oxygen, the crystallinity was improved. NixCo(3−x)O4 films with different x values were also prepared with Ni−Co metal alloy targets with compositions of Ni1Co1 (Ni1.5Co1.5O4), Ni1Co2 (Ni1Co2O4), and Ni1Co3 (Ni0.75Co2.25O4).535 3.3.4. Plasma Method. Plasma is regarded as a fourth state that is different from solid, liquid, and gas; the term was initially coined by Langmuir and Tonks in 1929.536−538 By using oxygenplasma-assisted methods, reaction times will be shortened

prepare Co3‑xMnxO4 spinel, manganese (Mn(thd)3) (thd = 2,2,6,6-tetramethylhephtane-2,5-dionate) and cobalt (Co(acac)2) (acac = acetylacetonate) were first dissolved in ethanol. Then, this solution was used for pulsed-spray evaporation and transported to a chamber for deposition with N2/O2 flowing. The generated Co3‑xMnxO4 was deposited on the selected substrate.510 By regulating the molar ratio of Co(acac)3 and Fe(acac)3, CoxFe3−xO4 with different x values was fabricated.511 The ALD method, which was developed in the 1970s by Suntola el al.,512 shares many similarities with CVD. However, ALD can deposit compounds in a layer-by-layer fashion on the substrate, which enables adjustable thickness and composition by controlling the deposition time and pulsing fraction.513,514 With Mn(thd)3 and Co(thd)2 as precursors and ozone as the oxidant, (Mn,Co)3O4 with different ratios of Mn and Co was synthesized. After 1000 basic cycles, the thickness of (Mn,Co)3O4 was approximately 20 nm (Figure 19b,c).75 3.3.2. Spray Pyrolysis. Spray pyrolysis is an aerosol reaction with liquid reactants. In 1966, Chamberlin and co-workers applied this method to fabricate films for solar cells.515 The aerosol generated by the atomizer passes through a hightemperature furnace for reaction. The total reaction occurs rapidly with the generation of micronano structured products (Figure 20a). The aerosol speed, liquid reactant concentrations, gas pressure, and calcination temperature will influence the final products.516−518 For example, with a 0.3 M Co(NO3)2·6H2O aqueous solution as the reactant, Co3O4 could be prepared with an air pressure of 2−3 atm, a flow rate of 3−4 cm3/min, and a temperature of 350 °C.519 Usually, the spray pyrolysis products are directly deposited on conductive substrates such as gold,520 conductive glass,521 and Ti522,523 and applied as catalysts for 10142

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Figure 20. (a) Schematic illustration of the formation process of yolk−shell LiNi0.5Mn1.5O4 powders by spray pyrolysis. Corresponding (b) TEM and (c) EDS dot-mapping images of compounds that were post-treated at 750 °C. Reproduced with permission from ref 527. Copyright 2013 The Royal Society of Chemistry.

(Figure 22a).537,538 Spinel Mn3‑xCoO4 film was synthesized with this method. In a plasma oxygen environment, the Mn and Co flux were evaporated from the K cell at high temperature. The growth temperature was set as 300 °C.539 In addition, a Mn1.5Co1.5O4 coating was also prepared by a plasma spray process.540 The films grown on MgO (001) and MgO (011) revealed different peak distributions. For example, Mn3‑xCrxO4 on MgO (001) primarily showed (004) and (008) peaks (Figure 22b), while Mn3‑xCrxO4 on MgO (001) exhibited major peaks of (202) and (404). Furthermore, the lattice parameters could be changed by tuning the ratios of x in Mn3‑xCrxO4 (Figure 22c). In practical applications, two or more of these summarized approaches have been combined to fabricate the final spinel compounds. Liu et al. synthesized hierarchical ZnCo2O4@ NiCo 2 O 4 core−sheath nanowires by combining several approaches. First, the precursor nanofiber was synthesized by electrospinning, using raw materials of Zn(NO3)2 and Co(NO3)2. After calcining the products at 600 °C for 2 h, ZnCo2O4 nanowires were formed. Afterward, the ZnCo2O4@NiCo2O4 core−sheath nanowires were obtained by using a modified coprecipitation method. After annealing the precipitate at 350 °C for 2 h, the final ZnCo2O4@NiCo2O4 products were obtained (Figure 23a−e).541 Recently, Li and co-workers prepared CoFe2O4/C porous nanorod arrays by combining electrodeposition, hydrothermal, and MOF derivative decomposition methods. Polyaniline (PANI) was initially grown on Ni foam by electrodeposition (denoted as NF@PANI). Then, the NF@ PANI was used to adsorb Co2+ and Fe2+ and undergo hydrothermal reaction to form MOF-74-Co/Fe on the surface of the NF@PANI. After annealing this product at high

temperature in N2 atmosphere, the final CoFe2O4/C porous nanorod arrays on nickel foam were rationally synthesized (Figure 23f,g).542 3.4. Summary of Spinel Preparations

During the development of spinel synthesis, the approach trends have varied from the high-temperature solid phase to the lowtemperature liquid phase. Morphologies from the bulk to various micro/nano structures have been created. The obtained crystals have varied from the perfect to the defect-enriched. Increasing numbers of methods are being reported, and new spinels have been synthesized and predicted (Figure 24). With the rational design of composition, structures, defects, morphology, and loading substrates, spinels can be applied in various fields. Tables 5−7 summarize the primary methods reported to fabricate spinel compounds. Their overviews, characteristics (pros and cons, scalability), and example applications have been listed for comparison.

4. ORR/OER 4.1. Basic Introduction to the ORR

The ORR is a series of complex electrochemical reactions, which are proposed to involve multistep electron transfer processes and complicated oxygen-containing species. Despite decades of extensive research work, the mechanism of oxygen reduction remains poorly understood because of the involvement of four electrons and four protons to O2 and the cleavage of the double bond of O2. It is generally recognized that the ORR first involves O2 adsorption on the surface of an electrode. The adsorbed O2 obtains electrons and is reduced to H2O or OH−. This reaction is 10143

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Figure 21. (a) Schematic illustration of the magnetron sputtering method. (b) XRD patterns of Co3O4 prepared with the magnetron sputtering method. Corresponding SEM images of the (c) top view and (d) cross-sectional view of prepared Co3O4 films. Panels b−d are reprinted with permission from ref 532. Copyright 2005 Elsevier.

Figure 22. (a) Schematic illustration of the plasma method. XRD patterns of Mn3‑xCrxO4 prepared on (b) a MgO (001) substrate and (c) a MgO (011) substrate. The inset is the lattice parameters of Mn3‑xCrxO4 with different x values. Panels b and c are reprinted with permission from ref 536. Copyright 2011 AIP Publishing.

the vital process of fuel cells and metal−air batteries, determining their working efficiency. Typically, the ORR process can occur via two different routes, in one step known as the 4-electron reaction or in two steps with 2-plus-2 electrons per reaction. These reactions have been listed as follows:52,86

In acid aqueous systems (1) The 4-electron reaction process: O2 + 4H+ + 4e− → 2H 2O

E θ = 1.229 V

(13)

(2) The 2-plus-2 electrons reaction process: 10144

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Figure 23. (a) Schematic illustration of synthesizing ZnCo2O4@NiCo2O4 core−sheath nanowires. (b and c) SEM images of prepared ZnCo2O4@ NiCo2O4 core−sheath nanowires and (d) corresponding SEM mapping and (e) TEM image. Reproduced with permission from ref 541. Copyright 2015 Wiley-VCH. (f) Schematic illustration of synthesizing porous CoFe2O4/C nanorod arrays and corresponding (g) SEM image. Reproduced with permission from ref 542. Copyright 2016 Wiley-VCH.

Figure 24. Schematic illustration of the primary reported methods of preparing spinels. The emerging methods have been marked with a star.

O2 + 2H+ + 2e− → H 2O2

E θ = 0.67 V

H 2O2 + 2H+ + 2e− → 2H 2O

(14)

E θ = 1.77 V

(15)

or disproportionation 10145

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Table 5. Summary of Solid-Phase Methods for Preparing Spinels methods hightemperature solid-phasea flux grown solid-phase combustion thermal decomposition thermal decomposition thermal decomposition MOF derived

pulsed laser deposition a

overview

characteristics

highlight examples (applications)

manual mixing or mechanical mixing and annealing

simple raw materials, long reaction time, hightemperature

LiCoO2 (ORR/OER),117,118 CuIr2S4 (photoemission),6 Si3N4 (light-emitting diodes)138,543

transfer solid−solid to solid−liquid reaction using molten salts using exothermic reaction to accelerate the speed

relatively low temperature and short reaction time, high purity short reaction time

LiNixMn2‑xO4 (Li-ion batteries),142 ZnGa2O4 (transparent electronic conductor)143 ferrite spinels (AxB1‑xO4), AFe2O4 (water splitting)147

nitrate decomposition or nitrate combustion (nitrates as oxidizing agents to ignite organic fuels)a metal carbonyl decompositionb

nanoparticles, low temperature, metal-doped spinels very low temperature (∼200 °C)

CoCr2‑xVxO4 (methane conversion),40 Ag-doped Mn3O4 (ORR),170 CoFe2O4/CMK-3 (CO oxidation)175 Pd (4 nm)-Mn3O4 nanoparticles (ORR)196

decomposition with halide salt as template, washing halide salt (NaCl, KCl)b calcining MOFs in air or Ar atmosphereb

mesoporous structures, facile

3D porous MnCo2O4 nanoparticles (ORR)197

various micro/nano morphologies, small spinel sizes, carbon substrates, limited spinels short reaction time, spinel films or nanoparticles, enriched defects

Co3O4 yolk−shell nanocages (OER),200 Co3O4@Ndoped carbon (ORR)202

high-energy laser to bombard reactants, deposit and ablate the products

CoFe2O4 film (magnetic properties),206 Fe3O4 nanoparticles (magnetic properties)208

The method is easily scalable. bThe methods are new, emerging methods to prepare spinel compounds.

2H 2O2 → 2H 2O + O2 ↑

In acid aqueous electrolyte

(16)

In alkaline aqueous systems (1) The 4-electron reaction process: O2 + 2H 2O + 4e− → 4OH−

E θ = 0.401 V

2H 2O → O2 ↑ +4H+ + 4e−

4OH− → O2 ↑ + 2H 2O + 4e−

E θ = 0.065 V

HO2− + H 2O + 2e− → 3OH−

E θ = 0.867 V

(18)

In an aprotic electrolyte

(19)

O2 2 − → O2 ↑ +2e−

−

2HO2 → 2OH + O2 ↑

(20)

In an aprotic electrolyte O2 + e− → O2− −

−

O2 + e → O2

E = Eθ +

(21) 2−

E θ = 0.401 V

(24)

(25)

In nonstandard conditions, the equilibrium potential of oxygen electrode reactions is established according to the Nernst equation:

or disproportionation −

(23)

In alkaline aqueous systems

(17)

(2) The 2-plus-2 reaction process: O2 + H 2O + 2e− → HO2−

E θ = 1.229 V

RT aOx ln nF aRed

(26)

where R is the gas constant (8.314 J mol−1 K−1), T is the temperature in Kelvin, n is the number of electrons transferred, F is the Faraday constant (96485 C mol−1), and a is the activity of the oxidized and reduced species. Indeed, the reaction free energy ΔG of an electrochemical reaction depends critically on the choice of the reference electrode. If the reference is set to the SHE, defined as H2 (g) at p = 1 atm and T = 298 K with electrolyte pH 0, ΔG at fixed voltage will linearly depend on pH as RT/F·pH. This means that the thermodynamic driving force of the reaction varies with the electrolyte acidity. Other electrodes not involving proton exchange, such as Ag/AgCl or the saturated calomel electrode (SCE), give the same pH-dependent ΔG as SHE for electrochemical reactions. If the reversible hydrogen electrode (RHE) potential is instead chosen as reference, where at zero voltage the reaction H+ + e− ↔ 1/2H2 (g) is considered at equilibrium for p(H2) = 1 atm at every pH and temperature, ΔG becomes pH independent a fixed voltage. Therefore, when referenced to RHE, the thermodynamic driving force of the overall reaction does not depend on pH. The vast majority of results reported in the literature use pHdependent reference electrodes such as Ag/AgCl or SCE for these measurements. Using pH-dependent reference electrodes reduces the experimental complexity. The relationship between

(22)

Eθ is the standard reduction potential versus standard hydrogen electrode (SHE). From these reactions, we can see that, both in acid and in alkaline systems, the process of oxygen reduction can be divided into two scenarios. Typically, the initial process (4-electron reaction) involves the direct dissociation of an O−O bond, which requires more energy than the dissociation of H2O2 or HO2− (2plus-2 electrons reaction). This indicates that the 2-plus-2 reaction is more feasible, while the direct 4-electron reactions is more effective without generating the byproduct of HO2−. Peroxide species are corrosive and can cause premature degradation of the electrochemical cell.80 4.2. Basic Introduction to the OER

The OER can be regarded as the reverse of the ORR, which means losing electrons from H2O or OH− and generating O2. For practical application, the overpotential of OER is even more severe than that of ORR, which exists not only in aqueous electrolyte but also in aprotic electrolytes. For example, in Zn− air and Li−air batteries, this overpotential will result in a low round-trip efficiency. Even worse, the high charge overpotential will lead to the decomposition of the catalyst substrate and electrolyte, which is considered to be the major reason for battery failure. The OER process is summarized as follows: 10146

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10147

a

Al95Mn5 alloy formation, NaOH solution, dealloying and annealing

room-temperature, rapid, easily large-scale, nanocrystals, energy-saving, rich deficiencies

NaBH4 reduction, divalent cation-ion replacement due to site preference energy metal glycolate formation and calcination hexagonal nanosheets

tube-in-tube structures

dispersed nanoparticles

high-intensity ultrasound

electrospinningb organic metal salts, high boiling point solvent hot injection technologyb

fast, facile, low-temperature, free-standing, low crystallinity 1D morphologies, free-standing and flexible monodisperse nanocrystals, stored in organic solvent or loaded on substrate rapid hydrolysis at high temperature

252

LixNi0.5Mn1.5O4 (Li-ion batteries),253 Co3O4 (supercapacitors)255

highlight examples (applications)

Mn2AlO4 (ORR)505

CuCo2O4, ZnCo2O4, CoMn2O4, ZnMn2O4, NiCo2O4 (ORR, Li-ion batteries)504

MxMn3−xO4 (M is a divalent metal, ORR/OER, Zn−Air batteries)7

MnCo3‑xO4 nanoparticles (ORR)497

CuCr2S4 (magnetic),487 CuCr2Se4 (magnetic),488 CuCr2Te4 (magnetic)489

CoMn2O4 tube-in-tube, tube and solid structures (supercapacitor, Li-ion batteries)472 MxFe3−xO4 (M = Mn, Co) nanocrystals (theranostic applications)486

MnCo2O4 nanosheets (supercapacitor),465 NiCo2O4 nanosheets (water splitting)470

LiNi0.5Mn1.5O4 porous nanorods (Li-ion batteries)440 hydrotalcite such as Co3O4 (ORR),442 Mn3O4/RGO hybrids (ORR)447

cubic and tetragonal CoxMn3‑xO4 spinels (ORR/OER, Zn−Air batteries, Li−Air batteries)79

Co3O4 nanoarrays (OER)419

Co3O4 (OER),389 Mn3O4 (OER),397 ZnAl2O4 (luminescence),375 NiCo2O4 core−ring (OER),398 nanosheets (water splitting),418 nanocages (OER)420 and mesoporous (supercapacitors)421

Mn0.3Fe1.5Co1.2O4, Mn1.4Fe0.4Co1.2O4 (ORR/OER)257 ZnFe2O4 octahedrons (Li-ion batteries),263 ZnCo2O4 porous nanorods (CO2 reduction),34 NiCo2O4 macroporous sheets (ORR)268 and core−shell microspheres (Li−O2 batteries)269 NiCo2O4 nanowires on carbon fabric (Li−O2 batteries),282 Co3O4−Mn3O4 nanoparticles on GO (ORR),283 NiCo2O4 nanoparticles on 3D porous carbon (Li-ion batteries),284 Ni0.33Co0.67S2 nanowires on Ti foil (water splitting)285 CoFe2O4 3D N- and S-doped graphene (CH3CSNH2 as S and N source, ORR/OER)319

Li1+xMn2‑xO4,

The method is easily scalable. bThe methods are new, emerging methods to prepare spinel compounds.

carbon nanofiber templateb dealloying− annealingb

electrochemical high-temperature solution phase high-temperature solution phase sonochemical reactionb reduction− crystallizationa,b

electrochemical

microemulsion microwave

oxidation−precipitation and insertion− crystallizationa water phase and oil phase, surfactant microwave energy (molecular friction) microwave-assistedb electrodepositiona

various compositions and morphologies, spinel hybrids, rich oxygen deficiencies and easily large-scale NH3·H2O as precipitants and evaporated from substrate, nanoarrays, free-standing tunable phase and composition, low temperature, nanocrystals and easily large-scale 1D nanorods fast and low temperature (even below 100 °C)

controlled by precipitants, pH, solvents and calcination parametersa

precipitation

spinel/N(S)-doped carbon hybrids

N and S source participationb

“ammonia evaporation-induced” growthb

spinel hybrids with carbon materials, metal substrates, perovskites, etc.

PBA-derived spinels, shape control abundant micro/nano morphologies

mild and large scale, nanoparticles (common), composition regulable

characteristics

substrate participation

chelating agents, sol solution formation, gel formation after losing solvent, calcinationa copolymer-comorphologyb hydrothermal reactor, high-pressure

overview

precipitation

hydrothermal/ solvothermal precipitation

sol−gel hydrothermal/ solvothermal hydrothermal/ solvothermal

sol−gel

methods

Table 6. Summary of Liquid-Phase Methods for Preparing Spinels

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Table 7. Summary of Gas-Phase Methods for Preparing Spinels methods

overview

characteristics

highlight examples (applications)

chemical vapor deposition chemical vapor deposition spray pyrolysis

transfer gaseous reactant to solid thin film high-purity, thin film

CoxFe3−xO4 film (CO oxidation)511

ALD technologyb

fast, layer-by-layer deposition

MnCo2O4+δ (magnetic)75

aerosol reaction from liquid reactants

yolk−shell LiNi0.5Mn1.5O4 (Li-ion batteries)527

magnetron sputtering plasma method

bombard the target, sputter the particles, deposit the product high energy and oxygen-plasma-assistedb

fast, nanostructures, porous, low crystalline thin films composed of nanoparticles fast, film

a

Co3O4 film (Li-ion batteries),532 Ni1.5Co1.5O4, Ni1Co2O4, and Ni0.75Co2.25O4 (electrical and magnetic)535 Mn3‑xCrxO4 (Magnetic)536

The method is easily scalable. bThe methods are new, emerging methods to prepare spinel compounds.

compared to the benchmark. For instance, Pt/C is the state-ofthe-art for ORR;59,548 RuO2 or IrO2 are the best for OER.60 In most cases, this approach is impractical because the membrane electrode assembly fabrication and testing require special skills, specific equipment, and abundant materials. Rapid screening techniques are more suitable to characterizing the electrochemical behavior of catalysts at the lab scale. The rotating ringdisk electrode (RRDE) technique is a powerful tool to rapidly evaluate the activity and reaction pathways of oxygen reactions

pH-independent RHE and pH-dependent SHE can be described using the following expression:545,546 E RHE = ESHE + +

1/2 RT p(H 2) ln = ESHE nF a H+

2.303RT × pH = ESHE + 0.059 × pH F

(27)

where n = 1 and 2.303RT/nF is 0.059 V at 25 °C. Therefore, when SCE was used as the reference electrode, the measured potentials were converted to the RHE scale according to eq 27 as follows:547 θ E RHE = ESCE + 0.059 × pH + ESCE

(28)

EθSCE

where ERHE is the converted potential vs RHE and = 0.2415 V at 25 °C. ESCE is the experimentally measured potential against SCE reference. When tests are in 0.1 M KOH, the pH is approximately 13, so E RHE = ESCE + 0.059 × 13 + 0.2415 = ESCE + 1.008 V (29)

If a saturated Ag/AgCl electrode is used as the reference electrode in the same conditions, E RHE = EAg/AgCl + 0.059 × 13 + 0.1976 = EAg/AgCl + 0.9646 V

(30)

If a saturated Hg/HgO electrode is used as the reference electrode

Figure 25. Schematic diagram of a typical RRDE testing setup in a threeelectrode configuration. The disk electrode loaded with the catalyst layer is the working electrode; RE and CE are the abbreviations of the reference electrode and counter electrode, respectively.

E RHE = E Hg/HgO + 0.059 × 13 + 0.098 = E Hg/HgO + 0.865 V

(31)

for a given catalyst. The convective electrode system of RRDE consists of a coaxial disk, a ring-disk electrode, and a rotating shaft (Figure 25). This technique was first proposed by Stonehart and Ross at United Technologies Corp. (UTC) in 1976.549,550 Then, Gloaguen et al. developed the general technique to fabricate the thin film on the electrode in 1994.551 Since then, RRDE has been the most widely used technique to evaluate oxygen reaction catalysts. Catalyst powders are often dispersed in a water/alcohol mixture to form a uniform ink. Nafion has often been added as a binder to retain the catalysts on the disk electrode when the electrode is rotated. The Nafion content should be as low as possible to minimize the extra diffusion resistance of O2 and an IR drop caused by the Nafion film.550,552 As a general rule, the Nafion film should not exceed 0.2 μm in thickness when it is cast on top of the catalyst layer, or the content of solid Nafion should be less than 20 wt % in the catalyst film when it is mixed in the ink. Because of the low solubility of

For dilute solutions, eq 15 can be expressed directly in terms of concentrations (since the activity coefficients are close to unity). At higher concentrations, the true activities of the ions must be used. This complicates the use of the eq 15, since estimating the nonideal activities of ions generally requires experimental measurements. To obtain more reliable data in actual measurements, the potential of the reference electrode must be calibrated to RHE through cyclic voltammetry (CV).300 The calibration is measured in the same high-purity H2-saturated electrolyte with a Pt wire as the working electrode. The average of the two potentials at which the current crossed zero is taken to be the thermodynamic potential for the hydrogen electrode reaction. 4.3. Evaluation of ORR/OER

Ideally, newly developed ORR catalysts should be evaluated in a fuel cell or metal−air cell environment (OER catalysts should be evaluated in a metal−air cell or water splitting environment) and 10148

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backward reaction (OER) is negligible. Moreover, the adsorption process is more rapid than the electron transfer step. When the potentials are large negative, the ORR current is limited by convective-diffusional transport.555,556 The intrinsic activity (kinetic current without mass transfer effects) of the catalysts is more accurate for evaluating the ORR activities. At a constant potential, a larger current density promises better ORR properties. The kinetic current density can be obtained with the following equation:557

O2 in aqueous solutions, the electrodes are rotated to increase the mass transfer rates of O2 at the electrode surface, which is effective at eliminating the mass transfer effect during ORR activity measurements. During OER evaluation, rotating the electrode helps clear the O2 product.

JK = nFkC0 =

J × JL JL − J

(33)

This equation is applicative when the species have low diffusivity with large negative applied overpotential. In that case, the cathodic response corresponds to that value limited by the maximum rate of convective-diffusional mass transport. Meanwhile, the total electroactive surface area should be equal to the geometric area of the rotating disk surface.555 JL = 0.62nFC0(D0)2/3 ν−1/6ω1/2

(34)

The number of electrons transferred (n) and the electron transfer rate constant (k) can be obtained from the slope and intercept of K−L plots (J−1 vs ω−1/2) at various rotation speeds, respectively. The slope of the reciprocal plot is only a reliable measure of the total value of n for the sequential processes when there is a large negative applied overpotential. It is important to note that the K−L equation is based on smooth electrode surfaces under laminar flow hydrodynamics.553,558 Therefore, the quality of a given catalyst film can greatly impact the accuracy of the kinetic current calculation in the RDE measurements. A good catalyst film is thin, uniform, and smooth. Thick films lead to increased mass-transport resistance through the film and incomplete utilization of the catalyst. It is clear that irregularly built-up films (nonuniform coverage, very rough surface, etc.) must be avoided in RDE measurements because the K−L equation is not valid under those conditions.559 Consider the following hypothetical half-cell electrode reaction (cathode reaction):560

Figure 26. Typical ORR/OER polarization curves.

4.3.1. Linear Sweep Voltammetry (LSV) Curves. LSV curves provide some of the best visual evidence to evaluate ORR/ OER behavior. Figure 26 shows a typical ORR/OER polarization curve obtained with RRDE, in which some reaction kinetics performance indicators are illustrated. The onset potential (Eonset), which indicates the ORR or OER start, usually refers to the potential at 10 μA cm−2. The half-wave potential (E1/2) of ORR is defined as the potential at which the current reaches half of the diffusion-limiting current density. ηj is the overpotential under a specific current density, and jd is the diffusion-limiting current density.553 The Eonset, and ηj can be used to evaluate the difficulty of the oxygen reaction occurring. In general, a higher Eonset and E1/2 or a lower ηj facilitate the ORR, and a lower Eonset and ηj facilitate the OER. The jd value can be used to evaluate the speed of the ORR. A more negative current indicates a more rapid ORR speed. 4.3.2. Koutecky−Levich (K−L) Curves and Tafel Curves. For reactions that are controlled by both diffusion and kinetics at (rotating disk electrodes) RDEs, the total current density (j) of the reacting electroactive species is related to the speed of rotation of the electrode (ω) by what is commonly known as the K−L equation:554 1 1 1 1 1 = + = + J JK JL nFkC0 0.62nFC0(D0)2/3 ν ‐1/6ω1/2

(35)

O + ne ↔ R

The forward and backward reaction rates vf and vb (mol cm−2 s ), respectively, are as follows: −1

νf = κ f COS =

If nFA

(36)

νb = κbC R S =

Ib nFA

(37)

where kf and kb are the rate constants for the forward and the backward reactions (cm−1 s−1) and CO and CR are reactant and product concentrations (mol cm−3), respectively. In this model, S is incorporated into the reaction rate expression as a “geometrical factor”, which is a function of the catalyst layer microstructure (catalyst particle size, loading, and utilization) and is equal to the total “active” surface area (cm2). If and Ib are the forward (cathodic) and backward (anodic) current contributions to the total half-cell current in amperes or coulombs per second. A is the electrode nominal surface area (cm2), F is the Faraday constant (96 485 C/mol), and n is the number of electrons consumed in the electrode reaction. The net reaction rate is, therefore, the difference between the forward and backward reaction rates, i.e.

(32)

where Jk is the kinetic current density, JL is the diffusion-limiting current density, n is the overall number of electrons transferred, F is the Faraday constant, k is the electron transfer rate constant, C0 is the O2 concentration in the electrolyte, D0 is the diffusion coefficient of O2 in the electrolyte, ν is the viscosity of the electrolyte, and ω is the angular velocity in units of rad s−1. Equation 32 assumes electron transfer is the rate-determining step, which is first order in the adsorbed species, while the 10149

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Review

I If − b nFA nFA

ηc =

(38)

Rearranging Inet = If − Ib = nFAS(κ f CO − κbC R )

where Inet is the net half-cell current. Writing the rate constants as a function of the standard rate constant (k°) and the half-cell activation overpotential (η = E − Eeq) −αnFη RT

(40)

κb = κ °exp

(1 − α)nFη RT

(41)

where α is a charge transfer coefficient and E is the half-cell potential. Substituting eqs 40 and 41 into eq 39 and assuming no mass transfer effects (good convection) or that currents are kept at a sufficiently low level that the surface concentrations do not differ appreciably from the bulk values yields a “modified” Butler−Volmer (BV) equation:553,561,562 ⎡ ⎛ α nFη ⎞ ⎛ αcnFη ⎞⎤ ⎟ − exp⎜ − ⎟⎥ I = I0⎢exp⎜ a ⎝ RT ⎠⎦ ⎣ ⎝ RT ⎠

H 2O2 % = 200

(42)

(43)

⎛ αcnFηc ⎞ jc = −j0 exp⎜ − ⎟ ⎝ RT ⎠

(44)

n=4

RT RT ln j − ln j = a + b ln j nFαa nFαa 0

(47)

Id Id + Ir /N

(48)

where Id is the disk current (0, H2O2 electrooxidation to O2), and N is the current collection efficiency (N) of the ring electrode. N was determined from the reduction of K3Fe[CN]6. Equation 48 is only valid when the disk surface H2O2 to O2 does not exist.567 This equation indicates that, when the ORR is a total 4-electron transfer process, Ir = 0, the apparent number is 4. When the ORR is a pure 2-electron transfer process, Ir = IdN, eq 48 will give an apparent number of 2. As mentioned above, the 4electron reaction is more effective for ORR activities, in which there is almost no generation of HO2−. 4.3.4. Capacitance. Catalysts have the ability to act as capacitors and build-up charge at the film/electrolyte interface. Therefore, measuring the differential capacitance (Cdl) can be a useful tool to determine the electrochemically active surface area (ECSA), which can relate knowledge of the “surface” structure to the number of accessible cations. Two methods prevalent in the literature for obtaining Cdl values are CV and impedance spectroscopy.568,569 The capacitance of a material is measured by CV as follows: First, a nonfaradaic potential range is identified from CV in quiescent solution. This non-Faradaic region is typically a 0.1 V window around the open-circuit potential, and all measured current in this region is assumed to be due to double-layer charging. Based on this assumption Cdl =

where the subscripts “a” and “c” denote anodic and cathodic, respectively. The Tafel equations are the semilogarithmic forms of eqs 43 and 44. ηa =

Ir / N Id + Ir /N

and567

where I0 is the exchange current (the current in the absence of net electrolysis and at zero overpotential, in both the cathodic and anodic directions at the equilibrium potential). α is the charge transfer coefficient, R is the universal gas constant, and T is the absolute temperature. The BV equation is one of the most fundamental relationships in electrochemical kinetics. It describes how the current on an electrode depends on the electrode potential, considering that both a cathodic and an anodic reaction occur on the same electrode. The equation reflects the total currents from both reduction and oxidation reactions. This form of BV equation is valid when the electrode reaction is controlled by electrical charge transfer at the electrode (and not by the mass transfer between from the electrode surface and the bulk electrolyte).563 The BV equation is limited to describing chemically reversible electrocatalytic reactions. This equation implies that the smaller the value of I0, the more sluggish the kinetics; hence, a larger activation overpotential is required to achieve the desired net current. Due to the high irreversibility of both the ORR and OER, the generated oxygen reaction currents at higher overpotentials (|η| > 50 mV) would more appropriately be described as follows (by convention, the current density and overpotential associated with cathodic oxygen reduction are taken as negative and those with anodic oxygen evolution as positive):564

⎛ αanFηa ⎞ ja = j0 exp⎜ ⎟ ⎝ RT ⎠

(46)

Based on the Tafel equations, the Tafel slope of b is obtained from the Tafel curves drawn with ln(Jk) as the x axis, and the overpotential as the y axis. In cathodic regions, Tafel curves indicate reaction mechanisms as well as calculating n values. A b of −60 mV dec−1 corresponds to a pseudo two-electron reaction being the rate-determining step, whereas a b of −120 mV dec−1 suggests that the first electron reduction of oxygen is the ratedetermining step.565 The ORR consists of multiple steps that can each be described by j0 and Tafel slopes. 4.3.3. Generation of HO2− and Electron Transferred Number in the ORR. An RRDE contains a ring electrode and a disk electrode. The ORR occurs on the disk electrode, while the intermediate product will be thrown to the ring electrode. The ring electrode is set within the potential window above the diffusion-limiting value for the oxidation of intermediates such as H2O2 and hydrogen peroxide anion (HO2−) but below the Eonset of the oxidation of OH− to O2. From the current on the ring and disk electrode, the product of H2O2 and the electron transferred number can be calculated from the following reactions:550,566

(39)

κ f = κ °exp

RT RT ln j0 − ln j = a − b ln j nFαc nFαc

i dQ = dl dE υ

(49)

The Cdl is obtained from a plot of double-layer charging current (idl) vs the scan rate υ. Plotting idl as a function of υ yields a straight line with a slope equal to Cdl. The ECSA of the catalyst can be calculated by dividing Cdl by the specific capacitance of the sample.

(45) 10150

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4.3.5. Turnover Frequency (TOF). To accurately compare OER catalysts and ultimately design better catalysts, appropriate activity metrics are critical. A different activity metric for OER is the TOF,570 defined as mol O2 /s current/4F TOF = = mol active sites mol active sites

Mm + − O − O2 − + H 2O + e− → M(m − 1) + − O − HO− + OH−

M(m − 1) + − O − OH− + e− → Mm + − O2 − + OH− (58)

(50)

Mm + − O − O2 − + H 2O + e−

where F is the Faraday constant. The challenge with TOF is defining the active site. The simplest method is to take every metal cation as an “active site” and use the total film mass derived from quartz crystal microbalance or ICP measurements combined with the formula unit to determine the moles of metal in the film. This results in a total-metal TOF (TOFtm), which is essentially an average TOF for all the metals in the film. TOF calculation method is to base it on the number of metal cations that are electrochemically active. For instance, TOF can be obtained by integrating a redox wave or performing a capacitance measurement. 4.3.6. Faradaic Efficiency. Faradaic efficiency is a common metric for electrocatalytic systems. 569,571 Under alkaline conditions, the steady-state faradaic efficiency for water oxidation (following film redox) is essentially unity as there are no other species to oxidize and peroxide formation is unfavorable (note that for highly loaded films studied at low current densities, film redox can take a substantial time to complete). For other systems that have several possible byproducts or that use conductive carbon binders susceptible to oxidation, the faradaic efficiency is important to measure (e.g., using an oxygen sensor572 or mass spectrometry573 of the cell headspace).

→ M(m − 1) + − OH− + HO2−

where reactions 53 and 59 are the rate-determining steps. 4.4.2. OER Pathways of Transition Metal Oxides. The general OER mechanism in alkaline solution begins with the adsorption and discharge of the OH− anion at the catalyst surface of a metal site (M) to form adsorbed OH− species (60), followed by the reaction of OH− with the adsorbed OH species to produce H2O and adsorbed atomic O* and release an electron (61). Then, a OH− anion reacts with an adsorbed O* atom to form adsorbed OOH species (62). In addition, further reaction with additional OH− anions results in the formation of adsorbed O2 and H2O (63). Adsorbed O2 then desorbs in the last step of sequence (64), which is described as follows:229,579

(51) (52)

(53)

Mm + − O2 − + H 2O + e− → M(m − 1) + − OH− + OH− (54)

O2 + e → O2,ads

(61)

M − O* + OH− → M − OOH + e−

(62)

M − OOH + OH− → M − O2 + e− + H 2O

(63)

M − O2 → M + O2

(64)

Most of the MCo2O4 spinel types, in which the M = Co2+, Ni2+, Mn2+, Fe 2+, Cu2+, or Zn2+ ions occupy the tetrahedral sites and Co3+ ions are in the octahedral sites, have been widely studied for catalyzing ORR/OER. In this section, Co-based spinels are presented as examples, focusing on the most recently reported progress. 5.1.1. Co3O4. The normal spinel of Co3O4 is known to be a promising catalytic material and has received great interest since the 1980s.580,581 Recently, the electrochemical properties of spinel-type Co3O4 have been widely investigated in connection with oxygen electrocatalysis in alkaline media. Co3O4 benefits from its promising activity, low cost, simple preparation, and high stability. However, bulk Co3O4 alone shows relatively low

or with greater probability

−

M − OH + OH− → M − O* + e− + H 2O

5.1. MCo2O4

2M(m − 1) + − OH− + O2,ads− + e−

−

(60)

5. ORR/OER WITH SPINELS Nonprecious-metal-based ORR/OER electrocatalysts have been extensively studied in the recent spate of energy storage and conversion system research. Spinels, as one of the most promising nonprecious metal electrocatalysts for both the ORR and OER, have garnered increasing attention due to their outstanding catalytic activity and stability in alkaline solution in addition to their utilization of abundant, inexpensive raw materials. In this section, the electrochemical performances of various spinels and their catalyst design strategies will be thoroughly reviewed. Considering the critical effects of their chemical composition on their physicochemical properties, spinels can be categorized into B-site−based metallic species, such as Co-based MCo2O4, Mn-based MMn2O4, Fe-based MFe2O4, and other-based spinels.

2Mm + − O2 − + 2H 2O + 2e−

→ 2Mm + − O2 − + 2OH−

M + OH− → M − OH + e−

In this scheme, the rate-limiting step is hypothesized to be reaction 62.

4.4.1. ORR Pathways of Transition Metal Oxides. The ORR is a structure-sensitive reaction. Its pathways and mechanisms vary with the applied catalytic materials.52 Different configurations of oxygen adsorption exist on active sites of catalysts, depending on the crystallographic structure (surface geometry) and the binding energy.574−576 To create OH− species from metal oxides, the protonation of the surface oxygen ligand is charge-compensated by the reduction of a surface metal cation (M) such as Mn4+, Co3+, Fe3+, Ni3+, and so forth. The M−OH− species further interacts with O2 that adsorbs on oxide surfaces with O2 which adsorb on oxide surfaces with either end-on or side-on configurations.96 The ORR pathways on oxide surfaces would be

O2 + e− → O2,ads−

(59) 578

4.4. Reaction Pathways with Spinel Catalysts

→ 2M(m − 1) + − OH− + 2OH−

(57)

(55)

M(m − 1) + − OH− + O2,ads− → Mm + − O − O2 − + OH− (56) 10151

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Figure 27. (a) TEM image of Co3O4/N-rmGO hybrid composites. (b) Oxygen reduction polarization curves (Co3O4/rmGO, Co3O4/N-rmGO hybrids, and Pt/C) dispersed on carbon fiber paper (CFP) in in O2-saturated 1 M KOH electrolyte. (c) ORR and OER potential window of Co3O4/NrmGO hybrid, Co3O4 nanocrystal, and Pt/C catalysts dispersed on CFP in O2-saturated 0.1 M KOH. (d) Chronoamperometric responses of Co3O4/NrmGO hybrid and Pt/C−CFP electrodes maintained at 0.70 V in O2-saturated 0.1 M KOH. Reproduced with permission from ref 14. Copyright 2011 Nature Publishing Group.

Figure 28. (a) Synthetic scheme of the formation of Co3O4/NiCo2O4 double-shelled nanocages (Co3O4/NiCo2O4). (b) Corresponding TEM image and (c) electrocatalytic OER activity in comparison with Co3O4 nanocages. Reproduced from ref 589. Copyright 2015 American Chemical Society. (d) Synthetic scheme of Au@Co3O4 core−shell nanocrystals. (e) TEM image of Au@Co nanocrystals with a histogram of the size distribution (inset). (f) HRTEM image of a single Au@Co3O4 nanocrystals. (g) iR-corrected polarization curves of Au@Co3O4, Au+Co3O4, Co3O4, and Au catalysts in O2saturated 0.1 M KOH at a scan rate of 5 mV s−1 and a rotation speed of 2500 rpm. (h) Activity of different catalysts at an overpotential of 0.35 V. Reproduced with permission from ref 496. Copyright 2014 Wiley-VCH.

electrical conductivity and exhibits little catalytic activity. Formed by the direct nucleation/growth and anchoring of nanocrystals of

various shapes on GO sheets, Co3O4/GO hybrids show optimal electrical conductivity and stability due to the chemical coupling 10152

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Figure 29. (a) Schematic illustration of (Co)[Co2]O4, (Co)[FeCo]O4, and (Co)[Fe2]O4 spinels and their corresponding oxygen adsorptions and ORR activities. (b) LSVs of the above catalysts and corresponding JK (inset) at 0.85 V in O2-saturated 0.1 M KOH solution at 1600 rpm (c) The O−O bond length and adsorption energy of oxygen in the duel pattern (top) and triangle pattern (bottom) of (Co)[Co2]O4, (Co)[FeCo]O4, and (Co)[Fe2]O4. Reproduced with permission from ref 78. Copyright 2016 Wiley-VCH.

include controlling the composition and phase, designing micro/ nanostructures, and integrating conductive nanostructures. 5.1.1.1. Controlling the Composition and Phase. Incorporating other metal elements such as Li,329,583 La,256 Ni,584 Mg,188 and Cu153,534 into the Co3O4 lattice can tune its electrical structures, change the valence state of the surface oxide, and improve the specific surface area and porosity of the Co3O4, which is an effective way to improve the properties of electrocatalysts. Liu et al. synthesized Li-doped Co3O4 solid solution nanocrystals with average particle sizes of approximately 4 nm through direct nucleation and growth of the lithium−cobalt oxide on an acid-treated carbon black.329 The activity of these catalysts showed a typical volcano plot as a function of the Li content; the Li/Co atomic ratio of 5% displayed the best activity, which was 3.3 times higher than that of the undoped Co3O4/C. The improved electrocatalytic performance of the doped catalyst can be attributed to the markedly increased content of covalent OC−O−CoIII−O bonds formed at the interfaces between the Li-doped Co3O4 and the carbon support. Moreover, the covalent electron transfer from the CoIII species to the electronwithdrawing OC−O species through the OC−O−CoIII−

between the nanoparticles and GO substrates. In 2011, Dai’s group found that the direct nucleation and growth of ∼4−8 nm Co3O4 nanoparticles on N-doped (with carbon atoms covalently bonded to nitrogen-containing functional groups) rGO (NrGO) afforded intimate bonding and synergistic coupling effects, leading to significantly higher electrocatalytic ORR and OER activity than available from either Co3O4 or their physical mixture (Figure 27).14 Concurrently, Zhao’s group reported carbonsupported Co3O4 electrocatalysts with nanorods and spherical structures,582 in which the ORR catalytic activity of the Co3O4 was sensitive to the number and activity of the surface-exposed Co3+ ions, which could be tailored by the morphology. In particular, they demonstrated that a Co3O4 electrocatalyst with a nanorod structure (12 nm in length and 5.1 nm in diameter) exhibited a higher current density than that of a much more expensive palladium-based catalyst at the low potential region. These important discoveries stimulated research into using various Co3O4 as efficient ORR/OER electrocatalysts. In the past five years, substantial efforts have been made to further improve the performance of Co3O4. Strategies to enhance spinel activity 10153

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Figure 30. (a) Schematic of the preparation of the Ar-plasma-engraved Co3O4 with oxygen vacancies. (b) Corresponding SEM image. (c) The polarization profiles of the OER of Co3O4 (0 s) and Ar-plasma-engraved Co3O4 (120 s). Reproduced with permission from ref 471. Copyright 2016 Wiley-VCH. (d) Schematic illustration of mesoporous Mg−Co3O4 formation via Mg cation leaching. (e) TEM image of Mg−Co3O4. (f) Oxygen yield from chemical water oxidation with (NH4)2Ce(NO3)6 as the oxidant. Reproduced from ref 194. Copyright 2013 American Chemical Society.

ORR activity of all spinel structures, even exceeding the performance of the state-of-the-art commercial Pt/C by 42 mV in alkaline medium (Figure 29b). Density functional theory (DFT) calculations revealed that the enhancement of the intrinsic ORR activity of (Co)[FeCo]O4 originates from the socalled dissimilarity effect of the Fe and Co atoms at the octahedral sites, which modulates the adsorption energy and elongates the O−O bonds compared to those of the normal spinel (Figure 29c). This work reveals a new way to modulate the catalytic activity of spinels from a phase perspective. 5.1.1.2. Designing Micro/Nanostructures. Spinel Co3O4 alone suffers from inefficient catalytic activity due to selfaccumulation and intrinsically poor electrical conductivity. To address these challenges, micro/nanostructured materials with various morphologies (nanoparticles, 426,481,592−595 nanosheets,294,436,471,596−599 porous,191,194,436,443,577,596,600−603 nanorod,361,582,604−606 nanocubes,262,372,577,604,607,608 nanotubes,603 nanoflowers,609 etc.) have been designed. Wang et al.610 embedded Co@Co3O4 core@shell nanoparticles in ultrathin highly graphitized N-doped carbon, which exhibited excellent stability and good tolerance to methanol. Singh et al. decorated thermally reduced nitrogen-doped graphene with cubic, bluntedged cubic, and spherical Co3O4.594 The spherical Co3O4 nanoparticles supported on the reduced nitrogen-doped graphene catalyst demonstrated significant ORR activity. Catalytic activity is primarily a surface phenomenon. Materials with the same chemical compositions but different exposed facets show distinctly different performance, because of their different atomic arrangements and electronic structures. The effective exposure of catalytic planes may enhance the catalytic activity. Switzer’s group epitaxially deposited crystalline Co3O4 films directly from refluxing electrolyte on single-crystal Au(100), which opened up the possibility of studying the catalytic activity of different crystal planes exposed to the electrolyte.468 Xiao et al. reported the importance of crystal plane control to catalytic

O bonds could not only promote the oxidation of active sites CoIII into CoIV but also facilitate the surface hydroxide displacement, which could significantly contribute to the ORR kinetics. In addition to doping Co3O4 with other metals, hybrids with other materials such as Ag,585 Fe,586 Mg,188 RuO2,352 Mn3O4,587 NiCo2O4, (Figure 28a−c),588,589 CoFe2O4,323 MnCo2O4,310,360 C3N4,241,584,590 and X20CoCrWMo10−9 (a Co-based tool steel)591 have been proposed to further promote the ORR/ OER activity of Co3O4. Yeo and Bell reported that Co3O4 electrochemically grown on an Au electrode is more easily oxidized, benefiting from the highly electronegative Au substrate, and thus has enhanced OER activity.579 The activity of small amounts of cobalt oxide deposited on Au, Pt, Pd, Cu, and Co decreased monotonically in the order of Au > Pt > Pd > Cu > Co, paralleling the decreasing electronegativity of the substrate metal. Yan’s group synthesized monodisperse Au@Co3O4 core−shell nanocrystals, which were converted from monodisperse Au@Co core−shell nanocrystals with an overall diameter of 8 nm.496 At an overpotential of 0.35 V, the Au@Co3O4 core−shell nanocrystals had an OER current density 7 times higher than that of Co3O4 nanocrystals alone and an Au/Co3O4 nanocrystals mixture (Figure 28d−h). A stable overpotential of only 0.31 V was needed for Au@Co3O4 to achieve a current density of 10 A g−1 catalyst. This work indicates that the synergistic effect provided by the core−shell structure can significantly enhance the catalytic performance and that high-performance catalysts can be produced by rational design of the core−shell hybrid structure. Wei’s group tuned the phase of Co3O4 from normal to its inverse and then back to normal by adjusting the iron content in the synthesis (Figure 29a), which demonstrated that the ORR performance of the Co-based spinel for the catalysis of ORR could be significantly promoted by regulating the phase of the spinel.78 The inverse (Co)[FeCo]O4 spinel exhibited the highest 10154

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Figure 31. (a) Schematic of composite pomegranate-like Co3O4 nanocrystals with carbon. (b) ORR polarization profiles of Co3O4 nanocrystals embedded into N-doped, partially graphitized carbon framework and Pt/C catalysts at 900 rpm and (c) OER polarization profiles in comparison with Ir/ C and Pt/C catalysts. Reproduced with permission from ref 634. Copyright 2016 Wiley-VCH. (d) Schematic illustrating the preparation of the core− shell Co@Co3O4 nanoparticles encapsulated in CNT-grafted N-doped polyhedral carbon. LSVs of different catalysts during the (e) ORR and (f) OER at 1600 rpm. Reproduced with permission from ref 631. Copyright 2016 Wiley-VCH.

current density of 10 mA cm−2 in 1 M KOH and a small Tafel slope of 59 mV dec−1. Wang’s group demonstrated a simple but efficient plasma-engraving strategy to produce Co3O4 nanosheets. Although less electrocatalyst remained, more active sites and better catalytic activity were realized by obtaining high surface area and abundant oxygen vacancies (Figure 30a−c).471 Porous nanoarchitectures are usually beneficial to the electrocatalytic activity of electrocatalyst due to their high specific surface area with short diffusion and transport path length for the reactants. Chen et al. synthesized mesoporous Co3O4 nanoflakes with an interconnected architecture; they exhibited excellent electrochemical performance both as anode materials in lithium-ion batteries and as catalysts in the OER.443 Jiao’s group fabricated a mesoporous cobalt oxide with an ultrahigh surface area (up to 250 m2 g−1) by using Mg substitution in mesoporous Co3O4 followed by a Mg-selective leaching process, resulting in a highly porous cobalt oxide with a significant amount of defects in the spinel structure (Figure 30d− f).194 The activated mesoporous cobalt oxide displayed high OER activities in both the visible-light-driven system and the chemical water oxidation system. 5.1.1.3. Loading on a Conductive Matrix. Highly conductively metals (Ti, Ni, and Au),612,613 and metal oxides (Tl2O3 and PbO2)614 were initially used as supporting materials. In recent years, carbon materials (ordinary carbon,25,327,615−617 carbon nanotubes,427,618−622 graphene, 501,611,623−625 and PPy626,627) have often been introduced into Co3O4 catalysts to increase their conductivity and structural stability for ORR/OER. Musiani et al. compared different metal-matrix and oxide-matrix composites containing Co3O4 as the dispersed phase and found that the overpotential for the OER varied in the order of Tl2O3matrix < PbO2 < Pb < Ni.614 Graphene has attracted tremendous research interest as an electrocatalyst substrate material due to its large surface area, high mechanical and chemical stability, and

activity and found that Co3O4 nanorods with more (110) exposed facets showed surprisingly high catalytic activity for lowtemperature CO oxidation due to the surface richness of active Co3+ sites.611 These results underscore the importance of morphological control in the design of highly efficient ORR catalysts. Su et al. synthesized single crystalline Co3O4 nanocrystals (nanocubes, pseudo octahedrons, nanosheets, hexagonal nanoplatelets, and nanolaminar material) with different exposed crystal planes and established an order of OER catalytic activity: (111) > (110) > (112) > (100).604 Chen et al. also found higher OER activity with greater exposure of the Co3O4 (111) surface and better stability with more (100) surfaces.262 Combining experimental and theoretical studies, Liu and co-workers further elucidated the exposed crystal plane−performance relationship of Co3O4 as a bifunctional electrocatalyst for rechargeable Li−O2 batteries.607 Co3O4 octahedrons with exposed (111) planes showed much better performance than did cubes with exposed (001) planes, which may be largely attributed to the richer Co2+ and more active sites on the (111) plane of Co3O4 octahedrons. DFT-based first-principles calculations further indicated that Co3O4 (111) has a lower activation barrier to O2 desorption in OER than Co3O4 (001). 2D nanostructures are of great interest due to their high surface area and rich edge sites, which are favorable for a wide variety of applications. Odedairo et al. interleaved Co3O4 nanosheets with graphene to develop a sheet-on-sheet heterostructured electrocatalyst for the ORR, whose electrocatalytic activity outperformed the state-of-the-art commercial Pt/C, with exceptional durability in alkaline solution.294 Paik’s group found that well-defined porous Co3O4 nanosheets effectively provided more exposed active sites, beneficial both for electrochemical reaction and to facilitate ion transportation across the sheets.436 The sheets showed good electrocatalytic activity for the OER, with an overpotential of 368 mV to drive the 10155

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Co@Co3O4 nanoparticles embedded in CNT-grafted N-doped carbon polyhedrons obtained by the pyrolysis of a cobalt MOF (ZIF-67; Figure 31d).631 The catalysts afford 0.85 V reversible overvoltage in 0.1 M KOH, surpassing Pt/C, IrO2, and RuO2, and thus ranking as one of the best nonprecious-metal electrocatalysts for reversible oxygen electrodes (Figure 31e,f). This type of N-doping metal−carbon-based MOF provides a promising model for the design of complex catalysts. Loading Co3O4 on a conductive matrix from renewable precursors is an environmentally friendly process. Fellinger’s group demonstrated a simple, versatile, and inexpensive synthetic route for the preparation of Co3O4-decorated porous carbon.641 The hybridization of Co3O4 with the blood-derived carbon resulted in improved activities toward not only the ORR but also the OER. This improved ORR activity can be ascribed to a synergistic catalytic effect due to the intimate contact between Co3O4 particles and the highly conductive, heteroatom-doped carbon support, which is mediated by cobalt−nitrogen or cobalt−phosphorus coordination sites. These heterojunctions should facilitate the electron transfer by preventing an accumulation of electron density within the Co3O4 particles. 5.1.1.4. Active Sites for OER/ORR. In spite of the wide employment of Co3O4 in ORR/OER, the roles of two of the geometrical cobalt ions in the OER have remained elusive. Lyons et al. systematically studied the CVs of Co surfaces.642 An anodic peak prior to the onset of oxygen evolution has been assigned to the oxidation of CoIII to CoIV. The efficiency of cobalt oxide as an OER catalyst may be enhanced by increasing the population of CoIV centers present at the oxide surface. Bell and Yeo achieved this goal by depositing a thin layer of cobalt oxide on the surface of a highly electronegative Au.579 The electrochemical activity enhancement is attributed to the increase in the surface CoIV population mediated by the Au support. Although the deposited cobalt oxide is present as Co3O4, it progressively oxidizes to CoOOH with increasing anodic potential. To find the catalytically active sites for the ORR and OER in the crystal structure of Co3O4, Kang’s group utilized a (100)-faceted Co3O4 cube enclosed by only a Co2+ site and a (112) facet that exposed a partial Co3+ site.643 They found that the Co3+ site played the crucial role of determining the adsorption properties of reactants, enabling high ORR and OER activity. Song et al. reported a onestep wet-chemical synthesis of Ni-/Mn-promoted mesoporous cobalt oxides through an inverse micelle process, revealing that the redox activity of Co3+ to Co4+ is crucial for OER performance, while the oxygen vacancies and surface area of the cobalt oxides dominated the ORR activities.244 Liu and co-workers further investigated the geometrical-site-dependent OER activity of the Co3O4 catalyst by replacing Co2+ in the tetrahedral site and Co3+ in the octahedral site with inactive Zn2+ and Al3+, respectively.229 With a thorough in operando analysis by electrochemical impedance spectroscopy and X-ray absorption spectroscopy, they revealed that Co2+ in the tetrahedral site is responsible for the formation of cobalt oxyhydroxide (CoOOH), which served as the active site for water oxidation. 5.1.2. NiCo2O4. NiCo2O4 has an inverse spinel structure with the space group of Fd3m ̅ , in which Ni occupies the 16d octahedral sites and Co is distributed in both 16d octahedral and 8a tetrahedral sites. Most studies believe that the mixed redox couples of the Ni (Ni3+/Ni2+) and Co (Co3+/Co2+) elements are present in the oxidation state of NiCo2O4.419 Its physicochemical properties are also related to its composition, metal ion oxidation states, crystalline structure, morphology, and electronic/ionic conductivity, which greatly rely on the synthetic conditions.

notable electrical conductivity. Suryanto et al. developed a bottom-up approach to layer-by-layer fabricate graphene/cobalt oxide composites as bifunctional electrocatalysts for both the OER and ORR.623 They obtained significantly improved physical stability and catalytic activities toward the OER and ORR compared to those from the Co3O4 catalysts alone. Heteroatom-doped carbon with synergistic catalytic effects can further improve the bulk properties and surface chemical properties. More recently, it was reported that N-doping into graphene led to superior ORR catalysts as the N atoms introduced into the graphene lattice were transferred the neighboring carbon atoms to “active regions” with enhanced electrocatalytic activity. Many studies have examined integration of Co3O4 nanoparticles and N-doped carbonaceous materials with good electrical conductivity. The types of N-doped carbon materials include carbon nanowebs,195,628 N-doped carbons,202,62 N-doped CNT,630−638 and graphene,14,594,639,640 with superior ORR and OER activities observed in all cases. Co3O4−PPy/GN was fabricated by graphite exfoliation and pyrrole polymerization occurring simultaneously by one-step in situ ball milling.626 The resultant catalysts showed efficient electrocatalytic performance for the ORR in an alkaline medium. Liu et al. proposed a Co3O4/N-doped Ketjenblack (Co3O4/NKB) composite as a high-performance catalyst for Al−air batteries.633 The synergistic effect between Co3O4 and N-KB gave the Co3O4/N-KB composite a much higher cathodic current, a much more positive half-wave potential and more electron transfer in comparison with Co3O4 or N-KB alone. Chen’s group designed and characterized a pomegranate-like electrocatalyst based on Co3O4 nanocrystals embedded in a nitrogen-doped, partially graphitized carbon framework (Figure 31a).634 The electrocatalysts demonstrate excellent catalytic activity for the ORR/OER and outstanding durability (Figure 31b,c). This could be attributed to the unique pomegranate composite architecture, where not only do the low-dimension nanocrystal seeds possess highly active sites for electrochemical reactions but the graphitized carbon shell and framework also significantly increase the electrical conductivity and structural stability. Moreover, the pomegranate-like structure efficiently prevents the metal oxide from self-accumulating and provides mass transfer pathways, which further supports the catalytic activity. Metal/carbon hybrid electrocatalysts, which are derived from Co-based MOFs such as ZIF-67 and ZIF-9 with a highly ordered 3D structure composed of a well-organized Co center and organic linkers, have attracted great attention in oxygen electrochemistry due to their numerous advantages: (1) metal nanoparticles can be stably encapsulated in the carbon matrix, (2) their high surface area and porous structure provide a fast channel for O2 and electrolyte diffusion, and (3) the N atom from the organic ligand doped into the porous carbon as well as the involvement of the Co-Nx moiety provides valid catalytic active sites. Carbonized ZIF-67 and ZIF-9 with Co-based compounds supported by graphitic carbon have been intensely studied for oxygen electrocatalysis. Zhang et al. developed a one-step thermal treatment synthesis of Co3O4@N−C core−shell nanocomposites using the MOF as a raw material.202 The hybrids displayed longer stability and higher methanol tolerance in the ORR than the commercial Pt/C catalysts in alkali solution. After that, Wang’s group successfully embedded a Co nanoparticle into N-doped CNT/porous carbon by pyrolyzing MOFencapsulated Co3O4.620 Muhler and co-workers reported a highly active bifunctional electrocatalyst comprising core−shell 10156

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Figure 32. (a) Schematic illustration of the adsorption of H2O on the NiCo2O4 spinel structure and the partial charge density of the oxygen vacancies. (b) Polarization curves of the different spinel oxides and (c) corresponding Tafel plots. Reproduced with permission from ref 418. Copyright 2015 Wiley-VCH.

Recently, NiCo2O4 has been particularly attractive in the ORR/ OER field due to its intrinsic high activity, corrosion resistance, and easy availability. However, its poor electrical conductivity and bulky morphology have limited its electrocatalytic capability. Many improved methods have been reported to enhance its performance, including tuning composition/structures, controlling valence, creating defects, designing morphology, and selecting carbon substrates. 5.1.2.1. Tuning Composition/Structures. Tuning composition/structures is an effective way to combine the advantages of compositions with different phases or functions. NiCo2O4− Ni0.33Co0.67S2 nanowires with complementary functional structure were developed through a hydrothermal method as a water splitting electrocatalyst. Because of the 1D nanowire morphology and enhanced charge transport capability of Ni0.33Co0.67S2, the Ni−Co-based hybrid displayed high efficient catalytic activity and stability with a current density of 5 mA cm−2 at a voltage of 1.65 V.285 Hierarchical ZnCo2O4@NiCo2O4 core−sheath nanowires were synthesized through facile electrospinning combined with a simple coprecipitation method. As ORR catalysts, the ZnCo2O4@NiCo2O4 catalyst showed better electrocatalytic performance than single ZnCo2O4 and NiCo2O4 in both onset potential and current density. The superior performance of this hybrid composition can be assigned to two features. First, the high electrical conductivity and rich redox chemical valences of

NiCo2O4 combined with the high electrochemical activity of ZnCo2O4. Second, the porous core−sheath structures offered a fast pathway for electrolyte/electron diffusion and more electroactive sites.541 Forming spinel composites with other structured materials (e.g., perovskites) can also enhance the catalytic activity via a synergistic effect. Shchukin synthesized macroporous NiCo2O4/ La0.65Sr0.3CoO3‑δ nanocrystallites for highly efficient OER. Because of the synergistic reaction between the La0.65Sr0.3CoO3‑δ and NiCo2O4, the composite displayed remarkable catalytic ability, with a potential step equal to 100 mV in an anodic direction.240 As mentioned, adding other noble metals can greatly increase active sites and induce complementary effects. Au/NiCo2O4 arrays were fabricated by Liu et al. for use as an OER catalyst. Benefiting from their ordered architecture and highly monodisperse Au, the hybrid electrodes, with facilitated transport of ions and electrons, highly efficiently utilize the active material for the OER. The optimized electrode delivered an improved OER activity, almost four times higher than that of Ir/ C (at 1.75 V), and a small Tafel slope (63 mV dec−1).324 5.1.2.2. Controlling Valence. Controlling valence can enhance the electronic/ionic conductivity of materials and improve their kinetics. Recently, Wu’s group fabricated NixCo3‑xO4 nanowire arrays via an ammonia-evaporationinduced growth method. By tuning the Ni-doping level, the ratio of Ni3+/Ni2+ could be controlled to enhance the surface 10157

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roughness, conductivity, and active site density. This significantly increased the OER performance of the composite, with a Tafel slope of 64 mV dec−1 and an exchange current density of 8.8 × 10−9 A cm−2.419 Liu and co-workers reported the preparation of Ni−Co oxide hierarchical nanosheets with Ni3+-induced formation of active NiOOH on the surface by a simple hydrothermal reaction. The large surface area offered by the 3D hierarchical nanostructure facilitated the formation of active sites. The incorporation of surface Ni3+ active sites greatly decreased the overpotential and promoted the catalytic reaction. Thus, this composite delivered improved OER performance with a low overpotential of 0.34 V at a current density of 10 mA cm−2 and a small Tafel slope of 51 mV dec−1.644 5.1.2.3. Creating Defects. Creating defects can produce more active sites and reduce the reaction energy barrier of materials. Xie’s group fabricated NiCo2O4 ultrathin nanosheets with rich oxygen vacancies via topochemical transformations from fewlayered Ni−Co hydroxides. Such vacancy-defect nanosheets exhibited excellent performance for electrocatalytic water oxidation, with a large current density of 285 mA cm−2 at 1.8 V and a small overpotential of 0.32 V. This excellent performance should be attributed to not only the enhanced active sites induced by oxygen vacancies but also the increased number of active sites created by the ultrathin nanosheet structure, which reduced the H2O adsorption energy and facilitated reactions on the surface (Figure 32a−c).418 Lin and co-workers prepared porous NiCo2O4 nanosheets with abundant surface oxygen vacancies through topochemical transformations. The hierarchically porous morphology and the introduction of abundant oxygen vacancies offered a large surface area with highly efficient mass transport during the electrochemical process. As a result, the tailored NiCo2O4 presented a low overpotential (0.22 V), high current density (32.5 mA cm−2 at 1.65 V), and excellent durability in alkaline electrolytes. 4 6 4 Additionally, FexNi1−xCo2O4 (0 ≤ x ≤ 1) compositions with metal vacancies were prepared by coprecipitation and tested as electrocatalysts for the OER in alkaline water electrolysis. The insertion of Fe dopants in A sites considerably impacted the electrocatalytic process. Fe0.1Ni0.9Co2O4 displayed the highest OER activity due to its increased acidity and oxygen absence.366 5.1.2.4. Designing Micro/Nanostructures. As with Co3O4, NiCo2O4-based spinels with micro/nanostructures have been prepared to improve their ORR/OER properties. To date, NiCo2O4 spinels have been successfully prepared as 1D nanoneedles347,645 or nanowires,300,419,646−650 2D nanoplatelets, 2 6 8 , 6 5 1 − 6 5 3 and 3D core−shell, 4 9 , 3 0 0 mesoporous183,293,420,654−656 or macroporous structures268,657,658 for ORR/OER applications. The morphologies with different dimensions have exhibited special electronic pathways and many active sites. Some NiCo2O4 compounds are rich in pores and surface area, such as hollow nanosponges502 and mesoporous nanocages.420 Lin’s group reported hierarchical NiCo2O4 hollow microcuboids obtained by a thermally driven conversion process. Benefiting from the unique 3D hierarchical hollow structures with large active surface area and smooth pathways, the OER was significantly promoted for water splitting (Figure 33a−d).343 Jin et al. prepared 1D NiCo2O4 nanowire arrays with mesoporous structure, oriented electronic/ionic conductivity, and high specific surface area, which had excellent catalytic activity for the ORR (close to the behavior of the Pt/C electrocatalyst).646 Furthermore, 2D core−ring NiCo2O4 nanoplatelets were designed by Lin’s group via a coprecipitation decomposition method. They had enriched Co in the inner core

Figure 33. (a) SEM and (b) TEM images of the NiCo2O4 hollow microcuboids. (c) Polarization curve of NiCo2O4 hollow microcuboids for OER. (d) Galvanostatic test of OER and HER using the NiCo2O4 hollow microcuboids in 1 M NaOH solution at current densities of +10 and −10 mA cm−2, respectively. Reproduced with permission from ref 343. Copyright 2016 Wiley-VCH.

of the nanoplatelet and Ni/Co of 1:2 at the edge of the ring and had excellent OER performance with an overpotential of 1.38 V at a current density of 100 mA cm−2.398 Additionally, Liu and coworkers fabricated complex 3D NiCo2O4 core−shell nanowires on a flexible conductive carbon cloth substrate. This special architecture displayed large anodic current and low onset overpotential for the OER, with an overpotential of 320 mV at a current density of 10 mA cm−2.300 5.1.2.5. Selecting Carbon Substrates. Different carbon substrates have been selected to support NiCo2O4 to improve the electrical conductivity and adsorption capability of the composite. 0D carbon nanoparticles,237,387 1D carbon nanotubes,307,434 2D rGO or N-doped rGO,659−662 and 3D porous carbon183,293,656 have been applied to support NiCo2O4. 1D carbons possess oriented electronic transport pathways. CNTdecorated hierarchical NiCo2O4 nanosheets have remarkable conductivity, high specific surface area of the CNTs, and synergistic effects between the oxygen-enriched groups and the metal ions. Thus, the hierarchical NiCo2O4/CNT composites showed superior OER catalytic properties with a more negative onset potential, smaller Tafel slope, and higher stability.434 Qiao’s group designed hierarchically porous N-doped rGONiCo2O4 hybrid paper as an advanced OER catalyst through a heterogeneous reaction method. The composites, with welldeveloped in- and out-of-plane pores, 3D conductive networks, and N-doped active sites, possessed dual active sites of marginal NiCo2O4 and the N (O)−metal (Ni or Co) bonds. Therefore, enhanced OER performance with the overpotential of 373 mV at 5 mA cm−2 and a Tafel slope of 156 mV dec−1 was observed.184 Bo et al. synthesized a NiCo2O4-diffused CMK-3 composite. Because of the high adsorption capability of CMK-3, the composite had good selectivity and high activation as an ORR catalyst in alkaline solution.183 5.1.3. MnCo2O4. Many cationic distributions of MnCo2O4 have been proposed, including Co 2 + [Co 2 + Mn 4+ ]O 4 , Co3+[Mn2+Co3+]O4, and Co2+[Mn3+xCo3+2−x]O4, with their formation dependent on the preparation method and calcination temperature. MnxCo3‑xO4 is commonly described as an inverse spinel in which the manganese cations show a preference for the 10158

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Figure 34. (a) TEM image of the MnCo2O4/N-rmGO hybrid with corresponding electron diffraction pattern. (b) ORR polarization curves of the different catalysts in O2-saturated 1 M KOH at a sweep rate of 5 mV s−1 at 1600 rpm. (c) OER performance of selected catalysts in 1 M KOH electrolyte. Reproduced from ref 299. Copyright 2012 American Chemical Society. (d) Synthetic scheme of the spinel Mn−Co oxide nanoparticles partially embedded in NCNTs. (e) LSV profiles of NCNTs from different synthesis conditions at 100 rpm and 5 mV s−1. (f) OER performance of the blank, NCNTs, NCNT-300, and NCNT-500 at 1 mV s−1. Reproduced from ref 134. Copyright 2014 American Chemical Society. (g) TEM image of the activated carbon-based defective carbon-supported MnCo2O4 nanoparticles (inset: histogram of the particle size distribution). (h) ORR polarization curves of the different catalysts tested at a scan rate of 10 mV s−1 and 1600 rpm in O2-saturated 0.1 M KOH solution. (i) Methanol tolerance test with 5% (by volume) methanol. Reproduced with permission from ref 338. Copyright 2014 Wiley-VCH.

IrO2 catalysts under identical experimental conditions. Doping a moderate content of Mn into the spinel framework led to improved electrical conductivity and strong oxidizing ability accompanied by high affinity between the OH− reactants and the catalyst surface, contributing to the smooth mass transport and endowing the catalyst with superior oxygen evolution activity. Kwon’s group synthesized manganese cobalt spinel oxide (MnxCo3‑xO4) nanoparticles and investigated their ORR activity in alkaline media.497 They concluded that the Co3+ and Mn3+ ions in the octahedral site undergo an internal redox reaction when x ≤ 2.0 to produce Co2+ and Mn4+ pairs, which increase the conductivity. The x = 0.4 cubic sample has the highest ORR activity and stability among the samples. The electronic states of catalytically active ions are also affected by the composition, mainly because Mn is larger than Co and the Mn3+ favors Jahn− Teller distortion.

octahedral sites. As a mixed-valent oxide, MnxCo3‑xO4 provides a unique opportunity to investigate the contribution of solid-state chemistry to electrocatalytic reactivity. Compounds with different physical, physicochemical and interfacial properties have been obtained. 5.1.3.1. Tuning Composition. The cations placed in B sites play an important role in assisting the chemisorption of O2 through their cationic d orbitals.520 The distribution of the cations among the different coordination sites depends strongly on the synthesis conditions and determines the textural and morphological characteristics of the electrode surface.152,243,358,359,523 Qiao’s group synthesized compositionadjustable spinel-type metal oxides MnxCo3‑xO4‑δ (x = 0.8− 1.4) by a rapid inorganic self-templating mechanism.198 The materials showed ultrahigh oxygen evolution activity and strong durability in alkaline solutions, superior to the performance of 10159

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Figure 35. (a) ORR and (b) OER activities (with an expanded region shown on the right) of Co3O4 and LT-Li1−xCoO2 for 0 ≤ x ≤ 0.5 in 0.1 M KOH solution at 1600 rpm. (c) Mechanism of OER on Co4O4 cubic units in 1LT-LiCoO2. (d) Qualitative one-electron energy drawing of Li1−xCoO2, showing the pinning of the Co3+/Co4+:3d energy with the top of the O2−:2p band. Reproduced with permission from ref 118. Copyright 2014 Nature Publishing Group.

(carbon,173,338,455 CNT,311,332,668 and graphene299) have also been used. Dai’s group developed a simple solvothermal method to prepare MnCo2O4/graphene hybrid material (Figure 34a).299 The hybrid showed more positive onset/peak potential for the ORR and a greater electron transfer number than the corresponding physical mixture of MnCo2O4 nanoparticles and N-doped graphene sheets. Moreover, the MnCo2O4/graphene not only exhibited higher activity than the Co3O4/graphene hybrid but also outperformed Pt/C in terms of ORR current density and stability for the same mass loading in alkaline solutions (Figure 34b,c). This suggests that the strongly coupled hybrid materials offer a promising strategy for advanced electrocatalysts. Muhler and co-workers developed a simple, easily scalable, and novel method for the synthesis of spinel Mn− Co oxide nanoparticles partially embedded in N-doped CNTs (NCNTs; Figure 34d).134 The nitrogen-functionalized carbon groups in NCNTs, which act as active sites for oxygen reduction, are conserved during the thermal oxidative treatment process. As a result of a synergistic effect between the nitrogen groups in the NCNTs and the spinel Mn−Co oxide particles, both the ORR and OER capabilities of the hybrid are tremendously enhanced (Figure 34e,f). Yao’s group introduced a low amount of MnCo2O4 spinel into an activated carbon-based defective carbon via a facile solvothermal method (Figure 34g).338 The ORR activity of this material was comparable to that of commercial Pt/ C (20 wt % Pt) in an alkaline medium, and the hybrid was much more stable than the Pt/C and was additionally free from methanol poisoning (Figure 34h,i).

5.1.3.2. Designing Micro/Nanostructures. The activity of ORR/OER catalysts has a direct relation with their morphology and surface area. A porous nanostructured MnxCo3‑xO4 catalyst can reduce the overpotential and enhance the activity by virtue of an abundance of active sites and high surface area. Ma et al. fabricated multiporous MnCo2O4 microspheres via a solvothermal method followed by pyrolysis of a carbonate precursor; the product demonstrated excellent bifunctional catalytic activity toward both the ORR and OER.663 Qiao’s group synthesized mesoporous MnCo2O4 with a large surface area up to 263 m2 g−1, numerous oxygen vacancy defects and high porosity by a novel template-free method.664 The synthesized electrocatalysts exhibited catalytic activity for the ORR comparable to that of Pt/C with much better stability and methanol tolerance. Fu et al. reported a spinel catalyst consisting of uniform MnCo2O4 nanoparticles cross-linked with 2D porous carbon nanosheets.197 The obtained porous MnCo2O4/C nanosheets presented the combined properties of an interconnected porous architecture and a large surface area (175.3 m2 g−1), as well as good electrical conductivity. As a result, the as-prepared porous nanosheet catalyst showed enhanced ORR activity. Other MnCo2O4 morphologies such as hollow nanocages,665 octahedrons,666 and monodisperse nanoparticles667 have also been reported for the ORR/OER. 5.1.3.3. Selecting Conductive Substrates. As with other MCo2O4 spinels, MnCo2O4/conductive substrate hybrids have been fabricated for the ORR/OER. For instance, surface modification of MnCo2O4 with conducting PPy improved catalytic activity for the ORR.169 In addition, carbon materials 10160

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Figure 36. Schematic illustration of synthesizing Mn3O4/CoSe2 nanohybrid (b) TEM image of Mn3O4/CoSe2 hybrid and the corresponding SAED pattern. (c) HRTEM image of a Mn3O4 nanoparticle and its neighboring CoSe2 support. (d) OER polarization profiles of bare GC, 20 wt % Pt/C, Mn3O4 NRs, pure CoSe2, and Mn3O4/CoSe2 electrode. (e) Tafel plots derived from the OER curves. Reproduced from ref 674. Copyright 2012 American Chemical Society.

5.1.4. Other Co-Based Spinels. Other Co-based spinels are acquired when other metal elements are employed to replace some of the Co2+ in the Co2+(Co2)3+O4 spinel structure with the “synergistic effect”. The resulting materials are heterogeneous catalysts known as binary spinel oxides, with compositions of MxCo3−xO4 (with M = Li, Fe, Cu, Zn, etc.). Copper cobaltite spinel with the general formula of CuxCo3‑xO4 is well-known for its catalytic activity toward oxygen evolution. Marsan’s group reported a study of the surface of CuxCo3‑xO4 electrodes for OER electrocatalysis in 1988.177 They observed that CuCo2O4 has better electrocatalytic performance than Co3O4 does.210 Chi’s group analyzed the cation distribution of CuxCo3‑xO4 according to the change in the copper content (x).411 When x increased from 0.7 to 0.9, more Cu2+ ions entered the octahedral sites of the spinel structure. Having Co3+ ions occupying the tetrahedral sites was confirmed to be beneficial to the OER activities. Ning et al. demonstrated that CuCo2O4 nanoparticles supported on N-doped rGO were highly efficient and durable ORR electrocatalysts in alkaline media. 669 Thiospinel CuCo2S4 was shown to be active for electrocatalytic ORR in 1975.670 Recently, CuCo2S4 nanoparticles have been synthesized and found to be highly active electrocatalysts for the OER under alkaline conditions.491 A covalently coupled FeCo2O4/hollow rGO sphere hybrid with a 3D architecture was fabricated by Yan et al.297 This hybrid exhibited comparable

ORR activity and superior OER activity compared to commercial Pt/C in 0.1 M KOH aqueous solution. The structure of ZnCo2O4 includes Co3+ in the octahedral sites and Zn2+ in the tetrahedral sites, similar to Co3O4. Therefore, Kim et al. prepared ZnCo2O4 and Co3O4 with comparable surface morphologies and thicknesses to compare their catalytic properties and examine whether Co2+ in Co3O4 is catalytically active for the OER.467 The results suggested that the Co2+ in Co3O4 is not catalytically critical for OER and that ZnCo2O4 can be an economical and environmentally benign replacement for Co3O4 as an OER catalyst in both 1 M KOH (pH 13.8) and 0.1 M phosphate buffer (pH 7) solutions. The use of a carbon support (CNT315,671,672 and rGO295) can further increase the dispersion and utilization of the active catalyst, thereby improving the ZnCo2O4 catalytic activity. Liu and coworkers successfully designed ZnCo2O4 quantum dots strongly coupled with NCNTs as an efficient OER/ORR electrocatalyst.673 Their outstanding activities are mainly attributed to the high substrate conductivity, the high valence state of cobalt, the abundant activity sites of N doping, and the high specific surface area. LiCoO2 has been intensively studied as a cathode material for lithium-ion batteries. Manthiram’s group successfully synthesized spinel-phase LiCoO2 at low temperature (Figure 4).118 After chemically delithiation, the LT-Li1−xCoO2 samples 10161

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Figure 37. (a) Schematic illustration of the preparation process of Mn3O4 nanoparticles with three different morphologies on N-doped graphene sheets. HRTEM images of (b) quasi-nanosphere Mn3O4 on N-doped graphene (NSNG), (c) quasi-cube Mn3O4 on NG (NCNG), and (d) quasinanoellipsoids Mn3O4 on NG (NENG). Reproduced with permission from ref 677. Copyright 2014 Wiley-VCH.

5.2. MMn2O4

exhibited combined high ORR and OER activities (Figure 35a,b). The high activities of these delithiated compositions are attributed to the Co4O4 cubane subunits and a pinning of the Co3+/4+. Then, they further compared the OER catalytic activities of the spinel-LiCoO2 and layered-LiCoO2 in alkaline medium to determine the species formed on the surface (Figure 35c,d).117 The results demonstrated that, although the two forms of LiCoO2 exhibited different specific and gravimetric activities, the surfaces of both materials became spinel (Co3O4)-like during the OER. Dismukes’s group demonstrated that cubic spinel LiCo2O4 formed after deintercalating partially labile Li+ from layered or cubic phase LiCoO2.148 The presence of [Co4O4]n+ cubane structural units in spinel LiCo2O4 provided lower oxidation potential to Co4+ and lower intercubane hole mobility. Therefore, the spinel phase showed excellent electrocatalytic efficiency for the OER.

MMn2O4 is one of the most intriguing composite oxides because manganese has many advantages including low cost, high abundance, low toxicity, multiple valences, and a prominent Jahn−Teller effect. Manganese-based spinels have a range of versatile applications in areas such as lithium insertion electrodes, magnetic materials, and catalysts. The physicochemical properties of MMn2O4 are highly sensitive to its composition, structural parameters, and cation distribution and oxidation state, which greatly depend on the synthesis conditions. 5.2.1. Mn3O4. Mn3O4 is a normal spinel, in which the Mn2+ ions locate in tetrahedral voids and the Mn2+ ions locate in octahedral voids. 5.2.1.1. Tuning Composition. Yu’s group successfully grew and anchored Mn3O4 nanoparticles on CoSe2 nanobelts through a simple polyol reduction method.674 The resulting Mn3O4/ CoSe2 hybrid displayed good stability and excellent OER electrocatalytic activity (Figure 36). The synergistic effect at 10162

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Figure 38. Synthesis scheme of (a) cubic and (b) tetragonal spinel phases including oxidation−precipitation and crystallization process. (c) ORR performance of different catalyst electrodes in O2-saturated 0.1 M KOH solution. (d) Comparison of ORR and OER performances of different catalystmodified electrodes in 0.1 M KOH solution at 900 rpm. Reproduced with permission from ref 79. Copyright 2015 Nature Publishing Group.

potential-determining step of the ORR, which was thermodynamically more facile on the (001) surface than on the (101) surface. Guo et al. used a hydrothermal method to obtain 50 nm pure Mn3O4-phase octahedrons that also acted as an efficient OER electrocatalyst.261 Mn3O4 nanoparticles with different shapes grown on nitrogen-doped graphene have been fabricated for the ORR (Figure 37).677 The ellipsoidal Mn3O4 particles exhibited the best ORR activities. Numerous studies have claimed that the performance of Mn3O4 catalysts can be significantly improved by the underlying substrate. Xia’s group loaded Mn3O4 nanoparticles (∼10 nm) on the outer surface of CMK-3 rather than inside the mesopores.189 The ordered interconnected pores within the bulk of CMK-3 can provide effective gas diffusion channels and a sufficient threephase interface for the ORR. Dai’s group developed a novel ORR catalyst by nucleating and growing Mn3O4 nanoparticles on GO sheets interconnected by MWCNTs. The hybrid demonstrated improved activity and stability for the OER catalysis.678 Qiao’s group prepared Mn3O4 nanoparticles on nitrogen-doped graphene via a solvothermal process and investigated their application as an ORR catalyst for the first time.339 Xu and coworkers prepared monodisperse nanoparticles (∼4−6 nm) on rGO through a one-step solution method.435 The developed hybrid catalysts exhibited a strong synergistic effect toward the ORR. Bag et al. then demonstrated a facile route for the synthesis of hybrid Mn3O4/N-rGO, which showed pronounced electrocatalytic activity toward the ORR in alkaline solution.498 In addition, 3D and self-assembled Mn3O4 hierarchical networks grown on N-rGO have also been controllably fabricated via electrodeposition for ORR catalysis.456 It has been widely accepted that the defects and edges of graphene can serve as active sites in the electrocatalytic process. Therefore, introducing holey structures is an effective way to enhance catalytic properties. Lv et al. prepared a hybrid of holey graphene and Mn3O4 that demonstrated enhanced catalytic activity and efficiency toward the ORR because of increased active sites and strong interaction between holey graphene and Mn3O4.430

the metal/metal oxide junction occasionally leads to considerably improved catalytic performance, which cannot be achieved with unsupported catalysts. Kim et al. deposited Pt on the surface of Mn3O4 nanoparticles through a galvanic replacement process.417 The Pt nanocrystals were stably and homogeneously immobilized on the surface of the Mn3O4 nanoparticles. Compared with commercial Pt/C catalyst, the Pt/Mn3O4 nanocomposite showed enhanced specific activity and durability. Pt/Mn3O4/CNTs500 and rGO−Mn3O4−Pt370 nanocomposites were obtained with a similar method. The asprepared hybrid catalysts also displayed much enhanced electrocatalytic activity for the ORR. Li et al. further confirmed that, after Pt loading with Mn3O4, the binding energy values of Pt 4f and Mn 2p had a positive and negative shift, respectively.675 This would accelerate the formation of a Pt−OH bond in the active centers. Recently, highly dispersed Ag and Mn3O4 nanocrystals were covalently coupled with carbon black170 and were successfully synthesized. The Ag−Mn3O4 nanocomposite displayed remarkably improved electrocatalytic activity and longterm durability. In addition, other Mn3O4-based hybrid nanocomposite catalysts such as Mn3O4@CoMn2O4−CoxOy nanoparticles,676 Co3O4−Mn3O4/GO,283 Ni-doped Mn3O4,415 and Pd−Mn3O4/C452 were also synthesized to improve ORR or OER properties. 5.2.1.2. Designing Micro/Nanostructures and Conductive Substrates. Yu et al. reported a facile method to prepare Mn3O4 nano-octahedrons on Ni foam as an efficient 3D electrocatalyst for the OER.383 The Mn3O4/NF electrode exhibited excellent OER activity with a small overpotential of 287 mV to achieve a current density of 10 mA cm−2 in alkaline media. Sun’s group synthesized Mn3O4 nanocrystals with preferentially exposed (001) or (101) crystal planes by controlling the morphology.397 The results demonstrated that Mn 3O 4 nanoflakes with preferentially exposed (001) planes displayed ORR activity 1 order of magnitude higher than that of Mn3O4 nanorods with preferentially exposed (101) planes. Further DFT calculations suggested that the first electron transfer process was the 10163

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Figure 39. (a) LSV curves of ORR using different Co−Mn−O catalyst-modified rotating disk electrodes in O2-saturated alkaline electrolyte. (b) LSV curves of OER tested with different Co−Mn−O-modified electrodes. (c−f) First-principles investigation of surface oxygen adsorption on different sites of cubic and tetragonal spinel phases: geometries and binding energies of oxygen molecules (purple) on cobalt (cyan) and manganese (gray) defect sites. Red spheres represent lattice oxygen. (g−h) Corresponding density of states of the bare and O2-adsorbed spinels. Reproduced with permission from ref 7. Copyright 2011 Nature Publishing Group.

of CoMn2O4 spinels are needed. In general, the ORR/OER activity of CoMn2O4-based spinels is highly correlated with controllable parameters, including the Mn2+/Mn3+ or Co2+/Co3+ ratios and the vacancies. These parameters depend greatly on the preparation approaches. 5.2.2.1. Controlling the Phase and Composition. CoMn2O4 spinel can display both cubic and tetragonal phases. Moreover, the Co/Mn ratios in CoMn2O4 spinel composites are also tunable because of the variable valence of both Co (+2/+3) and Mn (+2/+3). In 2011, our group reported a facile and rapid method to prepare nanocrystalline spinel MxMn3−xO4 (M represents divalent metals such as Co, Mg, and Zn) under ambient conditions.7 The spinel MxMn3−xO4 were used as a bifunctional electrocatalyst for both the ORR and OER, where the cubic Co−Mn−O spinel outperformed the tetragonal phase in terms of intrinsic ORR catalytic activity, but the tetragonal spinel surpassed the cubic phase for OER, due to the dissimilar binding energies of oxygen adsorption on cobalt and manganese defect sites. Recently, we further developed an “oxidation− precipitation and crystallization” method to fabricate spinels with tunable phase and composition (Figure 38a,b).79 This method

Schuhmann and co-workers obtained Mn3O4 nanoparticles embedded in nitrogen-doped carbon (NC) by selective pyrolysis and subsequent mild calcination.679 Intimate interaction was observed between the metals and nitrogen. The exceptionally active bifunctional catalytic ability of the composite was due to the nitrogen-functionalized carbon groups in the NC. Particularly, pyridinic, graphitic, and pyrrolic groups acted as complementary ORR catalysts in addition to conferring conductivity to the Mn3O4 nanoparticles. 5.2.2. CoMn2O4. In normal CoMn2O4 spinel crystal, the oxygen anions form a cubic close-packed lattice, while the cations A2+ (Co2+) and B3+ (Mn3+) occupy the octahedral and tetrahedral sites in the lattice. Some oxygen atoms can shift from their original positions to the interspace of the other atoms or leave the crystal entirely, generating oxygen vacancies. In the early formation stages of CoxMn3‑xO4 spinel, high-temperature calcinations or prolonged preparations are always needed to overcome the diffusional barrier, and the obtained materials display poor properties with irregular shapes, large particle sizes, small specific surface areas, and low electrochemical reaction activities. Thus, the design and development of mild preparations 10164

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rGO composites revealed both higher ORR and OER activities.314 They were further applied as efficient catalysts in Zn−air batteries, demonstrating a high discharge platform, low charge platform, and long cycle stability. Liu et al. prepared a 3D nitrogen-doped graphene aerogel loaded with CoMn2O4 nanoparticles by a facile two-step hydrothermal method, and the nanohybrid material had excellent electrocatalytic activity for the ORR in alkaline solution.312 The performance may be attributed to the unique 3D crumpled porous nanostructure of the nitrogen-doped graphene aerogel with large specific area and fast electron transport and the synergistic covalent coupling between the CoMn2O4 nanoparticles and the aerogel. A novel hybrid composed of CoMn2O4 nanoparticles and a N, P dualdoped graphene aerogel was successfully fabricated.682 Electrochemical measurements revealed that the composite had high activity, good methanol tolerance, and excellent durability for the ORR, which can be ascribed to P doping and the synergistic effect between CoMn2O4 and the aerogel. 5.2.3. Other Mn-Based Spinels. Spinel-type LiMn2O4 is a highly studied material and has been used as a cathode in rechargeable batteries. It has a structure in which MnIII and MnIV ions occupy the octahedral B sites and Li ions sit in the tetrahedral A sites. 102 Meanwhile, spinel-type oxides LiMn2‑xMxO4 (M = 3d metal) have also been examined as catalysts in the ORR/OER. Li et al. first reported that LiMn2‑xCoxO4 delivered good electrocatalytic activity in 7 M KOH solution. They further synthesized three series of spineltype complex oxides of LiMn2‑xCoxO4, LiMn2‑xFexO4, and LiMn 2‑x Co x/2 Fe x/2 O 4 by an improved amorphous citric precursor method and evaluated their electrocatalytic performance in gas diffusion oxygen electrodes.683 Manthiram’s group investigated the OER activity of the spinel oxide LiMn1.5Ni0.5O4 with different morphologies (cubic, spherical, octahedral, and truncated octahedral) in alkaline solutions.402 The OER activity order is truncated octahedral < cubic < spherical < octahedral. They demonstrated that the catalytic activities of the oxide catalysts could be tuned and optimized by controlling the surface morphologies/planes via novel synthetic approaches. Dismukes and co-workers measured the electrocatalytic OER activity of a series of ternary spinel oxides derived from LiMn2O4 by either replacement at the tetrahedral A site or Co substitution at the octahedral B site.221 Progressive replacement of Mn(III/IV) by Co(III) at the B site of LiMn0.25Co1.75O4 yielded the highest OER activity within the spinel structure type. Liu et al. further synthesized N-rGO nanosheets modified with LiMn2O4 nanoparticles as an efficient catalyst for ORR in an aluminum−air battery.313 Our group reported a facile synthesis of 1D CaMn2O4 nanostructures and their applications as cheap and active electrocatalysts for the ORR.334 Marokite CaMn2O4 nanorods with a postspinel phase were prepared by a solvothermal route at mild temperatures. The as-prepared nanorods had preferentially exposed (023) planes on their surfaces. The nanorods exhibited considerable catalytic performance comparable to the counterpart Pt nanoparticles supported on carbon in alkaline electrolytes. Porous CaMn2O4 microspheres were also prepared as electrocatalysts through thermal decomposition of carbonate.392 Other Mn-based spinels such as Cu x Mn 3‑x O 4 151,431,684 NixAl1−xMn2O4,265 Cu1.4Mn1.6O4,367 and MgMn2O4441 have also been reported for the ORR or OER.

achieved the goals of controlling both composition and phase. Six CoxMn3‑xO4 compounds with different phases and compositions were prepared. The cubic CoMn2O4 showed the best ORR properties (Figure 38c), and its OER property was also remarkable (Figure 38d). This superior bifunctional ORR/ OER capability enabled a lower discharge−charge overpotential and a more stable voltage plateau on cycling than those of Pt/C. Thus, the cubic CoMn2O4/C composites revealed high catalytic activity for potential applications in rechargeable Zn−air and Li− air batteries. Hirai et al. systematically studied the tetragonal spinel oxide CoxMn3‑xO4 for the OER; its OER catalytic activity was dramatically improved by an increase in Co content and the suppression of Jahn−Teller distortion.413 5.2.2.2. Designing Micro/Nanostructures with Defects. CoMn2O4 has been designed with different micro/nanostructures for ORR/OER catalysis. The microstructures contribute to the overall spinel stability, while the nanostructures facilitate the active site exposure. Different methods have been applied to prepare CoMn2O4 spinels with different morphologies, including hollow spheres from the solvothermal method,381 nanocrystals from the reduction−crystallization method,7 hollow rods from electrospinning,472 nanodots with “hot injection” followed by the “heat up” route,494 and multilevel nanotubes from electrospinning.680 Compared with bulk CoMn2O 4, nanosized CoMn2O4 usually exhibits higher ORR/OER activities, ascribed to its higher specific surface area and increased number of active sites. The defects in spinel oxides have proved useful for improving oxygen adsorption, thus accelerating the ORR/OER process. In general, it is easier to generate defects in the spinels prepared at low temperature, especially at room temperature. For example, CoxMn3‑xO4 spinel oxides prepared at room temperature are endowed with high specific surface area and defects, which facilitate the ORR/OER process (Figure 39a,b). The ORR catalytic activity of spinels depends on the O2 adsorption strength on the catalyst surface (Figure 39c−f). The oxygen adsorption energy on cubic CoxMn3−xO4 with both Co (−2.42 eV) and Mn (−2.23 eV) defects allows easier adsorption than on the tetragonal phase (−0.26 eV on Co defects and −0.60 eV on Mn defects).7 In comparison with the tetragonal spinel, the cubic phase displays an increase in the adsorbed oxygen (Oads)-induced d-band, indicating a strengthened metal−O2 bond (Figure 39g,h). Furthermore, the investigated surfaces of both phases contain the same number of catalytic sites per surface unit cell, but the area of the cubic (113) unit cell is less. Therefore, for a given surface area, the number of available active sites on the cubic (113) surface will exceed that on the tetragonal (121) surface. Accordingly, this can be interpreted as the cubic Co− Mn−O spinel outperforming the tetragonal phase in intrinsic ORR catalytic activity. Similarly, regarding electrocatalytic OER, one can predict that the performance of the tetragonal spinel will surpass that of the cubic phase because the OER can be viewed as a reverse process of the ORR. These properties are advantageous to promote the catalysis of reversible electrochemical ORR/ OER. 5.2.2.3. Loading on Conductive Matrix. The common spinel composites usually display low conductivity, which does not facilitate electron transfer in the ORR/OER process. Thus, different conductive matrixes such as graphene,296,312,314,681,682 CNTs,346 and conductive polymers503 have been applied to improve the conductivity. A hybrid CoMn2O4/N-rGO material was prepared by Shanmugam et al. Compared with pure CoMn2O4 and CoMn2O4/rGO composites, the CoMn2O4/N10165

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Figure 40. (a) Synthesis of Fe3O4@Co9S8/rGO composites through two solvothermal steps. (b) Low-resolution and (c) high-resolution TEM images of Fe3O4@Co9S8/rGO composites and (d) the further enlarged square region from the marked area. (e) LSV profiles of CoSy/rGO-600, Fe3O4/rGO, Pt/C, RuO2, Fe3O4@Co9S8/rGO-1, Fe3O4@Co9S8/rGO-2, and Fe3O4@Co9S8/rGO-3 as the electrocatalysts in 1 M KOH solution. (f) Tafel slopes of the above catalysts. Reproduced with permission from ref 345. Copyright 2016 Wiley-VCH.

5.3. MFe2O4

also proved that the catalytic activity of the Pt nanoparticle catalyst could be maximized by controlling its interaction with Fe3O4 nanoparticles. Padilla et al. synthesized Fe3O4@Pt core− shell composites using three different stirring methods and evaluated their electrochemical activity for the ORR.686 Robinson et al. introduced a synthetic strategy to achieve uniform shell-like epitaxial growth of Pt on Fe3O4 nanoparticles and found that the Pt shell could protect the Fe3O4 cores from corrosion, thus ensuring catalyst stability.687 A highly active electrocatalyst, CNx-shell isolated Fe3O4 nanoparticles, was prepared by Yang et al.688 through pyrolysis of a mixture of lysine and Fe3O4−CNx. This catalyst displayed good catalytic

5.3.1. Fe3O4. Fe3O4 is a cubic inverse spinel structure with space group Fd3̅m.685 The tetrahedral A sites are occupied by Fe3+, whereas the twice as abundant octahedral B sites are randomly occupied by Fe2+ and Fe3+. 5.3.1.1. Tuning Composition. Sun’s group synthesized monodisperse dumbbell-like Pt−Fe3O4 nanoparticles by epitaxial growth of Fe onto Pt nanoparticles followed by Fe oxidation.483 The interaction between Pt and Fe3O4 resulted in a higher electron population on Pt, making it more active in ORR than the single component Pt nanoparticle catalyst. This work 10166

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Figure 41. (a) Schematic illustration of the fabrication process of 3D N-doped graphene aerogel-supported Fe3O4 (Fe3O4/N-GAs) catalyst. (b and c) SEM images of Fe3O4/N-GAs showing the 3D porous architecture and uniform distribution of Fe3O4 nanoparticles on the GAs surface. (d) HRTEM of Fe3O4/N-GAs showing the Fe3O4 wrapped by graphene layers. (e) Rotating disk electrode test of the ORR on different modified catalysts (Fe3O4/NGAs, Fe3O4/N-graphene nanosheets (GSs), Fe3O4/N-carbon black (CB)) in an O2-saturated 0.1 M KOH electrolyte at 1600 rpm; the ring current as a function of electrode potential (inset). (f) Electron transfer number of different modified catalysts as functions of the electrode potential. Reproduced from ref 317. Copyright 2012 American Chemical Society.

commonly used catalysts. Recently, Xia and co-workers demonstrated the use of atomic-layer-deposited Fe3O4 on metal/carbon nanospheres in the form of ternary core−shell arrays for efficient ORR electrocatalysis. The nanospheres exhibited enhanced electrocatalytic ORR properties with higher onset potential and catalytic current than other iron oxide-based counterparts.692 Their noticeable methanol tolerance and CO antipoisoning was acquired from the integrated architecture and was probably a synergistic coupling effect. Yang et al. first reported iron-based-nanoparticle−decorated Co9S8 in situ grown on an rGO surface (Fe3O4@Co9S8/rGO), which exhibited good durability and excellent OER electrochemical activity with a small overpotential of 0.34 V at the current density of 10 mA cm−2 (Figure 40).345

activity for the ORR, comparable to that of commercial Pt/C and superior in terms of tolerating the methanol in alkaline medium. Zhan et al. prepared a dispersion of Mn and Co cosubstituted Fe3O4 nanoparticles on N-rGO nanosheets by a hydrothermal method.298 This catalyst showed good bifunctionality for oxygen electrocatalysis. Nonstoichiometric nano Fe3O4 in an activated carbon−air cathode was prepared using a simple ultrasonic doping method to boost the charge transfer of the cathode and enhance the power performance in a microbial fuel cell (MFC).689 Fe3O4@NiFexOy core−shell structures exhibited high electrocatalytic activity for the OER in a carbonate electrolyte.690 Mondal et al. demonstrated sustainable production of Fe3O4/Fe-doped graphene nanosheets from an abundant seaweed resource.691 The high surface area and high conductivity of the nanosheets granted higher ORR activity than those of 10167

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Figure 42. (a) Synthetic process for the preparation of the CIO-1 (crystalline phase CoFe2O4) and CIO-2 (amorphous phase CoFe2O3.66) catalysts. TEM images of (b) CIO-1 and (c) CIO-2, with the corresponding HRTEM images and SAED (inset). (d) CV curves of CIO-1 and CIO-2 in 0.1 M KOH solution with scan rate of 6 mV s−1. (e) LSV of CIO-1, CIO-2, and 20 wt % Pt/Vulcan X-72 catalysts in O2-saturated 0.1 M KOH with a scan rate of 10 mV s−1 at 1600 rpm. Reproduced from ref 344. Copyright 2014 American Chemical Society.

supports such as carbon,66 NC,316,699 graphene,286,317,700,701 and conducting polymers386,702 have been used to maximize the electroactive surface area of catalysts and improve their catalytic activity and durability. Among these, graphene, a two-dimensional single-layer sheet of hexagonal carbon, has emerged as a new-generation catalyst support because of its excellent electrical conductivity, high surface area, good chemical and environmental stability, and strong adhesion to catalyst particles. Feng and co-workers317 successfully fabricated 3D monolithic Ndoped graphene aerogel-supported Fe3O4 nanoparticle hybrids, which showed excellent electrocatalytic activity for the ORR in alkaline electrolytes due to its 3D macroporous structure and high surface area (Figure 41). 5.3.2. CoFe2O4. Spinel cobalt ferrite (CoFe2O4) has a typical inverse spinel structure, (Fe3+)[Co2+Fe3+]O4, in which oneeighth of the tetrahedral sites (Td or A sites) are occupied by Fe3+ and one-half of the octahedral sites (Oh or B sites) are occupied by Co2+ and Fe3+. Such structures have shown good electrical conductivity due to the electron hopping between valence states of the cations at Oh sites and could provide desirable electrochemical activity due to their necessary surface redox active centers. Considering its unique structural and chemical stability, CoFe2O4 has received attention as an effective catalyst for the ORR and OER. Nevertheless, the spinel ORR activities are still insufficient to replace Pt. The primary strategies used to

Catalytic performance is known to have a notable structure dependence. Multiple studies have shown that yolk−shell nanostructures can play important roles in catalysis. Ye et al. reported a facile template-free route to prepare hierarchical Fe3O4−Co3O4 yolk−shell nanostructures.693 These highly porous architectures were assembled from Co3O4 flower-like shells and Fe3O4 spherical cores. The obtained yolk−shell nanostructures exhibited excellent electrocatalytic ORR performance. Similar core−shell nanocomposites such as Fe3O4@ Pt694,695 and Fe@Fe3O4696,697 have also been designed for ORR applications. Octahedral Fe3O4 crystals with a size of 200−300 nm have been synthesized by a facile electrochemical deposition method.463 The unique octahedral structure provided a high density of Fe2+ active sites and facilitated the ORR via a fourelectron pathway. Cao et al. prepared novel metal−polymeric framework-derived hierarchically porous carbon/Fe3O4 nanohybrids using a “spontaneous bubble-template” method by onestep carbonization.698 The as-prepared nanohybrids displayed a 3D interpenetrating morphology with well-distributed Fe3O4 nanoparticles that were coated with a carbon layer, allowing a remarkable ORR activity in phosphate buffer solution. 5.3.1.2. Loading on Conductive Matrix. Metal oxide Fe3O4 catalysts frequently suffer from dissolution and agglomeration in electrolytes during repeated operation, which can result in degradation. To overcome this obstacle, nanostructured catalyst 10168

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Figure 43. (a) LSV profiles, (b) Tafel plots, and (c) chronopotentiometry curves of (at 10 mA cm−2) NF@NC-CoFe2O4/C nanorod arrays (NRAs), NF@NC-CoFe2O4 powders, NF@NC-CoFe2O4/C powders, and NF@NC-IrO2 powders (loading mass of 1.03 mg cm−2) in 1 M KOH solution. (d) LSV profiles of NF@NC-CoFe2O4/C NRAs before and after a 60 h durability test at 100 mA cm−2. Reproduced with permission from ref 542. Copyright 2016 Wiley-VCH.

conducting surface to ensure fast electron transport and obtain good catalytic activity. For example, a sandwich-type composite electrode of PPy and CoFe2O4 nanoparticles was obtained by using a sequential electrodeposition method.466 This composite exhibited excellent ORR electrocatalytic reactivity and stability even in moderately acidic media. A hybrid of CoFe2 O4 nanoparticles coupled with CNTs (CoFe2O4/CNTs) displayed high electrocatalytic activity for both the ORR and OER with long-term stability in basic media, outperforming the CoFe2O4+CNT mixtures.304,337 As with other spinels, graphene is a commonly used matrix.287,290,292,318,319,336 A CoFe2O4/graphene nanohybrid fabricated via a hydrothermal process showed a significantly increased onset potential for the ORR at 0.82 V compared to that of a CoFe2O4+rGO mixture (0.74 V).287 The diffusion-limiting current density and the half-wave potential were comparable with those of Pt/C (20 wt % Pt). The Tafel slope of CoFe2O4/rGO (67 mV dec−1) was smaller than that of a CoFe2O4+rGO mixture (78 mV dec−1) and was close to that of Pt/C (69 mV dec−1). Recently, CoFe2O4 nanoparticles (∼30 nm) grown onto rod-like ordered mesoporous carbon were also demonstrated as an efficient ORR and OER bifunctional catalyst.308 Li’s group directly obtained a homogeneous CoFe2O4/C composite by one-step pyrolysis of bimetal−organic frameworks with the assistance of PANI films (Figure 43).542 This porous CoFe2O4/ C exhibited remarkable OER catalytic activity and superior longterm stability. This finding opened a new avenue for developing novel porous hybrid nanostructures derived from MOFs with controllable morphology and functionality for the next generation of electrocatalysts.

enhance CoFe2O4 activity include designing unique nano/micro structures, integrating conductive matrixes, and regulating composition by doping cations. 5.3.2.1. Designing Micro/Nanostructures. Unique nano/ microstructured electrode materials can afford many more active sites, facilitate kinetics, and decrease mass transport resistance. Both the ORR and OER stabilities of CoFe2O4 hollow nanospheres are better than those of solid CoFe2O4 nanospheres and commercial Pt/C, which is attributed to the hierarchical porous structure benefiting oxygen transportation and the formation of the triphase reactive activity sites.280 Electrospun CoFe2O4 nanofibers showed high OER catalytic activity with a low onset potential of 1.6 V (vs RHE).477 The excellent OER activity of the nanofibers benefits from their special 3D networks and well-developed in- and out-of-plane micro/meso/macropores. These lead to the decrease in mass transport resistance and the increased exposure of electrocatalytic active sites. Recently, amorphous CoFe2O3.66 nanoparticles were prepared using a large-scale and simple solvothermal process (Figure 42a−c).344 The amorphous catalyst had improved catalytic activity for both ORR and OER compared to the crystalline material (Figure 42d,e). The superior performance of the amorphous material could be explained by the combination of a higher surface area and the increased presence of Co3+ in the octahedral sites. Monodispersed porous CoFe2O4 nanospheres directly grown on rGO sheets were fabricated by a one-pot solvothermal method.341 This special structure exposed more active sites and facilitated the transport of O2 in the electrolyte when the hybrid was employed as an ORR/OER electrocatalyst. 5.3.2.2. Integrating Conductive Matrixes. Because of their semiconducting nature, spinel oxides are usually supported on a 10169

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5.3.2.3. Controlling the Composition. Substitution with foreign cations has been frequently used to modify the magnetic, electrical, and catalytic properties of metal oxides. For CoFe2O4 spinel, the partial replacement of Fe or Co with Ni/Mn/Cr has been studied.111,278,369,486,703 For example, CoFe2O4, CoFe1.7Ni0.3O4, and CoFe1.6Ni0.4O4 were synthesized by the standard ceramic powder method using Co3O4, Fe2O3, NiO, and MnO2.703 A comparison of their OER activities revealed that the introduction of Ni and Mn in the CoFe2O4 spinel structure led to an increase and decrease in the apparent current density, respectively. The overpotential was lowest for the electrodes containing Ni (0.26 V) and highest for those containing Mn (0.36 V). The activity increase could be attributed to the presence of the Co2+/Co3+ pair in the octahedral sites of the spinel structure. Recently, Ni-doped NixCo1−xFe2O4 (x = 0, 0.25, 0.5, 0.75) hollow nanospheres were synthesized by using a simple hydrothermal approach.278 For the ORR, Ni0.5Co0.5Fe2O4 showed the most positive onset and half-wave potentials and the highest diffusion-limited current density. For OER, Ni0.75Co0.5Fe2O4 exhibited the best catalytic activity, with a more negative onset potential, compared to the other asprepared Ni-doped samples. Based on the results from XPS and EIS analysis, the enhanced catalytic activities could be ascribed to the synergistic effect of the cation distribution ratio in the octahedral/tetrahedral sites, the oxygen vacancies on the surface, and the low charge transfer resistance of the spinel oxides. 5.3.3. Other Fe-Based Spinels. NiFe2O4 in principle exhibits enhanced catalytic activity toward both the ORR and OER because of the presence of the multivalent elements Ni3+/ Ni2+ and Fe3+/Fe2+. Compared with the reported Co- or Mnbased spinels, Fe- and Ni-containing NiFe2O4 is inexpensive, abundant, and environmentally benign for potential applications. Spinel-type ternary ferrites with a NiFe2−xCrxO4 (0 ≤ x ≤ 1) composition were synthesized by a precipitation method. The results indicated that their OER activity in alkaline solutions increased by substituting Cr from 0.2 to 1.0 mol in the spinel matrix.404 Later, another study indicated that substituting V from 0.25 to 1.0 mol for Fe in NiFe2O4 greatly promoted its OER electrocatalytic activity.349 Fukuzumi’s group first demonstrated NiFe2O4 as a superior catalyst for photocatalytic water oxidation. It had high catalytic activity as well as durability in photocatalytic water oxidation with Na2S2O8 and [Ru(bpy)3]2+.704 Yang and coworkers reported a controllable synthesis of a 3D-ordered mesoporous NiFe2O4 with a tunable pore size by using KIT-6 as the hard template.705 These materials delivered significantly enhanced rate capability, charge−discharge efficiency, and cyclability in a Li−O2 battery. Jadhav et al.384 also reported macroporous NiFe2O4 nanoparticles as electrocatalysts for lithium−oxygen batteries. Li et al. loaded NiFe2O4 nanoparticles on CNTs304,337 via a simple hydrothermal procedure for ORR and OER catalysis. Zhou’s group disclosed that NiFe2O4 had superior OER activity for the decomposition of Li2O2 using firstprinciples computations. Guided by the computations, they prepared NiFe 2 O 4 −CNTs and applied them to Li−O 2 batteries.706 The batteries with NiFe2O4−CNT air cathodes displayed lower charging overpotential and better cycling performance than did those with CNT air cathodes. MnFe2O4 is also a reasonable bifunctional catalyst. The effect of partial substitution of Cr on the electrocatalytic properties of MnFe2O4 toward OER in alkaline medium was studied. The partial substitution of Cr for Fe greatly enhanced the electrocatalytic activity of the oxide.369 Sun’s group synthesized

sub-10 nm MnFe2O4 nanoparticles and loaded them on a commercial carbon support.495 The supported nanoparticles were as efficient as commercial Pt in catalyzing the ORR in 0.1 M KOH solution (Figure 44). Khilari et al. demonstrated MnFe2O4

Figure 44. (a) TEM and (b) HRTEM images of 5 nm MnFe2O4 nanoparticles. (c) CVs of MnFe2O4 and Fe3O4 in O2-saturated 0.1 M KOH solution at a scan rate of 50 mV s−1. (d) ORR performance of MnFe2O4 nanoparticles and commercial Pt/C nanoparticles in O2saturated 0.1 M KOH solution at a rate of 10 mV s−1 and Tafel plots at low overpotentials (inset). Reproduced from ref 495. Copyright 2013 American Chemical Society.

spinel as a bifunctional catalyst in an MFC.321 Incorporating MnFe2O4 nanoparticles on Vulcan XC or PANI significantly improved their ORR catalytic activity. MnFe2O4 nanofiber-based films with 3D configurations were synthesized by electrospinning and subsequent thermal treatment processes. Additionally, CuFe2−xCrxO4 (0 ≤ x ≤ 1.0) nano spinels have been prepared by a precipitation method for OER electrocatalysis in alkaline solutions.403 Partially replacing the Fe with Mn has been reported to influence the OER electrocatalytic activity of the Li(1−0.5x)Fe(1.5x+1)Mn(1−x)O4 spinel system.707 Stahl’s group reported the inverse spinel NiFeAlO4 as a highly active oxygen evolution electrocatalyst. The activity was promoted by a redox-inert Al ion.160 Bein’s group prepared spinel ZnFe2O4 by ALD and used it as a photoabsorber material for light-driven water splitting.514 Using the optimized welldefined thin films as a model system, the ZnFe2O4 thin films in photoelectrochemical water oxidation achieved a lower photocurrent at the reversible potential than that of the benchmark αFe2O3 films. 5.4. Other Spinels

Singh et al. studied the electrocatalytic properties of spinel-type MMoO4 (M = Fe, Co, and Ni) electrodes for the OER in alkaline solutions.105 A strongly coupled CoCr2O4/carbon nanosheet nanocomposite was grown concurrently by a facile one-step molten salt calcination method.141 Compared with pure CoCr2O4, the catalyst coupled with carbon nanosheets exhibited better electrocatalytic activity, good durability, and lower overpotential at a current density of 10 mA cm−2. Spinel-type porous MnNi2O4 nanorods were prepared using a facile 10170

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Figure 45. Summary of strategies to prepare spinels with enhanced ORR/OER properties.

conductive substrates. The conductive substrates facilitate the electron conduction and promote the number of active sites. Meanwhile, the formation of metal−C bonds increases the catalyst stability. Moreover, N-doped carbon can further increase the electronic conductivity and durability (with the formation of metal−N bonds) (Figure 45). Through these regulation methods, elevated ORR and OER activities accompanied by large ECSAs will be obtained (Tables 8−10). In the ORR process, higher onset potential and half-wave potential and larger limited current will be achieved. In the OER process, lower overpotential, higher TOFs, and smaller Tafel slopes will be achieved. In addition, the catalyst stability will be largely enhanced, which can be of value in ORR/OER-controlled storage devices.

electrospinning and subsequent calcination approach as an efficient bifunctional catalyst for rechargeable Li−O2 batteries.254 High electrocatalytic activity and excellent methanol tolerance during the OER have been reported for hexagonal Mn2AlO4 nanosheets.505 Recently, Yan’s group demonstrated that lowcrystallinity mesoporous spinel CoGa2O4 displayed excellent bulk electrocatalytic stability and activity for the OER.276 5.5. Summary of Strategies to Enhance ORR/OER Properties

As reviewed, with their preparation approaches discussed, various spinels with rational morphologies and structures have been designed, which can greatly improve their ORR/OER properties. In summary, five strategies have been introduced. (1) Designing micro/nano morphologies. Spinels with 0D, 1D, 2D, and 3D structures can be synthesized with different morphologies. Notably, good catalysts most often possess hierarchical and porous structures and high specific surface areas. A hierarchical nature is favorable to maintaining the structure. Porous structures benefit electrolyte infiltration. High specific areas can expose more active sites. (2) Creating proper defects. A-site, B-site and O-site defects have been created to improve catalyst activities. The O-defects can optimize the electronic structures of spinels, improving the conductivity and the combination ability with oxygen. (3) Changing the composition of spinels. Compared with simple spinels, complex spinels can generate synergistic effects taking advantage of each component. Loading highly active noble metal nanoparticles on spinels can simultaneously enhance the number of active sites and reduce the catalyst loading required. (4) Regulating the phase/structures. There are usually different phases of the same spinel composition (for example, cubic and tetragonal). Different phases lead to different active lattice planes being available to O2, which adjusts the binding energy on different surfaces. (5) Loading on

6. APPLICATIONS OF THE ORR/OER 6.1. Fuel Cells

Fuel cells have attracted tremendous interest because of their great potential for renewable, environmentally friendly use in power generation in stationary, transport-related, and sustainable energy resources (Figure 46a−c).722,723 The first concept of a fuel cell was demonstrated in 1839 by William Grove,724 who discovered that a small current flowed through the circuit in the opposite direction when the current was switched off after the electrolysis of water. Then, the term “fuel cell” was first proposed in 1889 by Mond and Langer, in a report in which the cells were improved with a porous platinum black electrode structure and a porous nonconducting diaphragm. Although fuel cells were discovered over 170 years ago, their commercialization and largescale application remain limited due to the high cost of fuel cell technology.725−727 Recently, various types of fuel cells have been developed, mainly differing in their electrolyte, including alkaline 10171

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Table 8. ORR Performance Overview of Typical Reported Spinel Catalysts spinela

Eonset (V)b

Ehalf (V)c

JL (mA cm−2)d

Co3O4582 Co3O4202 Co3O4628 Co3O4610 Co3O4294 Co3O4594 Co3O4328 Co3O4585 Co3O4−Mn3O4283 MnCo2O4668 MnCo2O4197 MnCo2O4338 MnCo2O4665 Mn0.4Co2.6O4497 NiCo2O4708 NiCo2O4661 NiCo2O4654 NiCo2O4709 ZnCo2O4315 ZnCo2O4710 Li0.5Co2.5O4329 (Co)FeCoO478 CuCo2O4669 Fe3O4692 Fe3O4316 Fe3O4104 Fe3O4698 Fe3O4463 Fe3O4711 Fe3O4699 Fe3O4317 CoFe2O4344 CoFe2O4336 MnFe2O4495

0.87 0.91 0.90 0.86 0.93 0.90 0.91 0.91 0.93 0.86 0.95 0.88 0.92 0.89 0.86 0.89 0 0.95 0.94 0.88 0.85 0.92 0.98 0.92 0.93 0.93 0.97 0.72 0.90 0.92 0.91 0.85 0.80 0.91 0.90

0.82 0.68 0.77 0.76 0.83 0.76 0.80 0.79 0.79 0.75 0.77 0.80 0.72 0.77 0.73 0.65 0.86 0.87 0.78 0.62 0.83 0.87 0.86 0.79 0.80 0.82 0.53 0.64 0.81 0.77 0.65 0.76 0.81 0.81

−3.9 −2.1 −4.7 −4.2 −5.8 −6.0 −4.3 −4.2 −5.8 −4.0 −4.8 −4.7 −5.2 −5.4 −2.9 −4.2 −6.2 −4.7 −3.6 −5.8 −3.0 −5.7 −3.3 −4.5 −6.0 −6.5 −8.6 −4.6 −5.5 −5.1 −4.5 −6.5 −3.5 −6.0

nanorod (1 M KOH) core−shell structure from MOF nanocrystals coupled with O- and N-doped carbon nanoweb Co@Co3O4 core@shell nanoparticles Co3O4 nanosheets interleaved with graphene nanoparticles on NG nanorod Ag/Co3O4 hybrid nanoparticles on GO microspheres on N-MWCNTs nanoparticles cross-linked with porous carbon nanosheets nanoparticles on defective AC mesoporous with oxygen vacancy nanoparticles porous NiO/NiCo2O4 nanotubes on N-graphene mesoporous nanoplates on 3D graphene foam nanoparticles on N-graphene (1 M KOH) on NCNTs hollow porous microspheres nanocrystals (1 M KOH) inverse spinel on N-graphene nanoparticles on N-rGO (1 M KOH) metal/carbon sphere arrays nanorods on NC nano-Fe3O4 grown on porous carbon N/Co dual-doped hierarchically porous carbon/Fe3O4 nanohybrids octahedral nanoparticles embedded into N-enriched carbons graphitic N-doped carbon-supported Fe3O4 nanoparticles 3D N-graphene aerogel-supported Fe3O4 nanoparticles amorphous supported on N-rGO monodisperse nanoparticles

MnFe2O4321 NiFe2O4335 Mn3O4712 Mn3O4678 Mn3O4530 Mn3O4339 Mn3O4498 Mn3O4456 Mn3O4677 Mn3O4713 Mn3O4432 Mn3O4448 CaMn2O4334 CaMn2O4392 CoMn2O4346

0.61 0.78 0.79 0.91 0.80 0.87 0.93 0.88 0.83 0.92 0.81 0.92 0.95 0.85 0.96

0.51 0.58 0.66 0.85 0.70 0.71 0.72 0.77 0.65 0.84 0.70 0.81 0.79 0.70 0.84

−5.2 −4.7 −4.0 −3.0 −6.0 −4.2 −2.2 −4.4 −4.5 −5.5 −5.2 −4.2 −4.3 −3.7 −5.8

CoxMn3‑xO47 Mn2AlO4505

0.92 0.76

0.78 0.59

−4.8

nanoparticle/PANI hybrid composite (0.1 M PBS) nanoparticles on graphene nanosheets nanoparticles on ionic liquid-modified graphene nanoparticles on GO sheets interconnected by MWCNTs supported on glassy carbon mesoporous nanoparticles on N-graphene Mn3O4 on N-rGO 3D network on N-graphene nanoparticles on N-graphene Ag/Mn3O4 On GO Mn3O4 nanowires/3D N-graphene/SWCNT nanorods microspheres spinel nanocrystals on poly diallyldimethylammonium chloridefunctionalized CNTs nanocrystalline hexagonal spinel nanosheets

feature

synthesis method precipitation MOF derived high-temperature high-temperature hydrothermal solvothermal solvothermal low-temperature hydrothermal high-temperature KCl template solvothermal low-temperature sonochemical electrospinning hydrothermal hydrothermal hydrothermal hydrothermal high-temperature hydrothermal hydrothermal hydrothermal ALD hydrothermal high-temperature Slid-phase MOF derived electrochemical ionic liquids as precursors solvothermal hydrothermal solvothermal solvothermal high-temperature solution phase hydrothermal solvothermal low-temperature low-temperature magnetron sputtering solvothermal low-temperature reduction electrochemical low-temperature low-temperature reduction precipitation microwave solvothermal precipitation solvothermal reduction−crystallization MnAl alloy treatment

a

The data are extracted from the LSV curves at an RDE rotation rate of 1600 rpm, the electrolyte is O2-saturated 0.1 M KOH unless otherwise stated, and all of the potentials are converted to vs RHE by the Nernst equation as introduced in section 4.2. bEonset is the onset potential of the ORR recorded at a current density of approximately −10 μA cm−2. cEhalf is the half-wave potential of the ORR recorded at a half of the diffusion-limited current density. dJL is the limited current density of the ORR.

membrane fuel cells.728 Nevertheless, the basic operating principle of all fuel cell types is similar (Figure 46d). Typically,

fuel cells, solid oxide fuel cells (SOFCs), MFCs, phosphoric acid fuel cells, molten carbonate fuel cells, and polymer electrolyte 10172

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Table 9. OER Performance Overview of Typical Reported Spinel Catalysts Eonset (V)b

E (V) @10 mA cm−2

Tafel (mV dec−1)

Co3O4496

1.52

1.60

60.0

0.1 M/GCE

Co3O4443 Co3O4618 Co3O4427 Co3O4200 Co3O4617 Co3O425 Co3O4714 Co3O4241 Co3O4715

1.52 1.51 1.56 1.52 1.54 1.52 1.55 1.42 1.50

1.61 1.62 1.58 1.64 1.60 1.53 1.72 1.50 1.57

48.0 65.0 68.0 84.9 82.0 81.0 61.0 76.0 71.0

1 M/GCE 0.1 M/GCE 1 M/GCE 0.1 M/GCE 0.5 M H2SO4/ carbon paper 1 M/Ni foam 0.1 M/GCE 1 M/GCE 1 M/GCE

Co3O4591

1.30

1.45

140.8

Co3O4593

1.52

1.60

58.1

0.1 M phosphate buffer solution (PBS)/grit 400 SiC sanding paper 1 M/FTO

Co3O4608

1.43

1.51

69.0

1 M/GCE

Co3O4603 Co3O4471

1.54 1.45

1.62 1.53

76.0 68.0

0.1 M/GCE 0.1 M/Ti foil

Co3O4 nanocubes supported on nitrogen-doped graphene mesoporous nanotubes nanosheets with oxygen vacancies

Co3O4@ CoxFe3‑XO4323 Co3O4/ NiCo2O4589 CuCo2S4491

1.52 1.53 1.46

1.54 1.57 1.64

53.0 88.0 115.0

1 M/Ni foam 1 M/Ni foam 1 M/GCE

hierarchical porous film double-shelled nanocages thiospinel nanoparticles

LiCoO2118

1.56

1.57

52.0

0.1 M/GCE

spinel-type lithium−cobalt oxide

LiCoO2148

1.59

1.66

48.0

0.1 M/GCE

cubic LiCoO2 (50 nm)

LiCoO2148

1.63

1.80

75.0

1 M PBS/GCE

cubic LiCoO2 (50 nm)

MnCo2O4716

1.48

1.52

55.0

0.1 M/GCE

MnCo2O4‑δ198 NiCo2O4184

1.52 1.54

1.58 1.69

85.0 156.0

0.1 M/GCE 0.1 M/GCE

NiCo2O4281

1.54

1.63

51.3

1 M/GCE

NiCo2O4647

1.52

1.69

90.0

1 M/FTO

418

1.55

1.60

30.0

1 M/GCE

NiCo2O4285 NiCo2O4502 NiCo2O4343 NiCo2O4717 NiCo2O4718 NiCo2O4719 ZnCo2O4467 LiMn0.25Co1.75O4221 Mn3O4674

1.56 1.50 1.46 1.46 1.50 1.50 1.50 1.58 1.50

1.60 1.59 1.52 1.48 1.55 1.57 1.62 1.68 1.68

60.0 64.4 53.0 38.8 76.0 63.0 46.0 60.0 49.0

1 M/GCE 0.1 M/GCE 1 M/GCE 1 M/Ni foam 0.1 M/GCE 0.1 M/1 M/GCE 1 M/GCE 0.1 M/GCE

MnCo2O4@C nanoparticle assembled nanowires with abundant surface oxygen vacancies hierarchically porous nitrogen-doped graphene/NiCo2O4 hybrid paper hierarchical hollow urchin-like NiCo2O4 nanomaterial porous crystalline NiCo2O4 nanowire arrays on a conductive electrode ultrathin nanosheets rich in oxygen deficiencies NiCo2O4/Ni0.33Co0.67S2 nanowires hollow nanosponges hierarchical hollow microcuboids NiFe/NiCo2O4 3D hierarchical porous ultrathin nanosheets and graphene self-supported nanoporous nanowires porous film nanocrystal Mn3O4/CoSe2 nanocomposite

Mn3O4261 Mn3O4383 Fe3O4345

1.58 1.47 1.50

1.52 1.55

71.5 86.0 54.5

0.1 M/GCE 1 M/NF 1 M/GCE

Fe3O4690

1.60

1.88

48.0

0.2 M carbonate/ITO

CoFe2O4290 CoFe2O4331

1.53 1.53

1.57 1.61

31.0 73.0

1 M/GCE 1 M/carbon paper

CoFe2O4542 CoFe2O4477

1.46 1.54

1.47 1.67

45.0 82.2

1 M/NF 0.1 M/GCE

spinel

NiCo2O4

a

featurec

KOH solution/substrate

10173

synthesis method

monodisperse Au@Co3O4 core−shell nanocrystals mesoporous Co3O4 nanoflakes on mildly oxidized MWCNTs CNTs-Au@Co3O4 hybrids yolk−shell nanocages carbon-coated nanoarrays hierarchically porous nanowire arrays amorphous surface layer Co3O4 embedded in tubular g-CN Co@Co3O4 core−shell particle encapsulated N-doped mesoporous carbon cage metal-ceramic composite of X20CoCrWMo10−9/Co3O4

high-temperature solution phase microwave hydrothermal precipitation MOF derived low-temperature MOF derived precipitation sol−gel MOF derived

thin film

nitrate decomposition hydrothermal

nano-octahedrons nano-octahedrons on Ni foam Fe3O4-decorated Co9S8 nanoparticles in situ grown on rGO Fe3O4@NiFexOy nanoparticles on rGO nanoparticle attachment on 3D carbon fiber papers porous hybrid nanorod arrays spinel nanofibers

electrochemical oxidation

self-template electrophoretic plasma engraving hydrothermal MOF derived high-temperature solution phase high-temperature solid phase nitrates decomposition nitrates decomposition MOFs derived KCl as templates nitrate decomposition hydrothermal hydrothermal precipitation hydrothermal low-temperature solvothermal electrodeposition low-temperature hydrothermal electrochemical sol−gel hydrothermal+ polyol reduction hydrothermal precipitation solvothermal high-temperature solution phase hydrothermal hydrothermal MOFs derived electrospinning

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Table 9. continued spinel Fe0.5Ni0.5Ox

a

199

Eonset (V)b

E (V) @10 mA cm−2

Tafel (mV dec−1)

1.60

1.82

72.0

featurec

KOH solution/substrate 0.1 M/GCE

synthesis method

hierarchical

MOFs derived

a

The data are extracted from the LSV curves at an RDE rotation rate of 1600 rpm, and the electrolyte is 0.1 M KOH unless otherwise stated. All of the potentials are converted to vs RHE by the Nernst equation as introduced in section 4.2. bEonset is the onset potential of the OER recorded at a current density of approximately −10 μA cm−2. cAbbreviations: GCE, glass carbon electrode; FTO, fluorine-doped tin oxide; ITO, indium tin oxide.

Table 10. ORR/OER Performance Overview of Typical Reported Bifunctional Spinel Catalysts EORR (V) @−3 mA cm−2

EOER (V) @ 10 mA cm−2

ΔEb (V)

feature

synthesis method

Co3O414 Co3O4333 Co3O4641 Co3O4629 Co3O4630 Co3O4195

0.82 0.59 0.82 0.77 0.77 0.80

1.54 1.68 1.62 1.68 1.55 1.64

0.72 1.09 0.80 0.91 0.78 0.84

on N-rGO on MWCNTs blood powder-derived heteroatom-doped porous carbon on N-doped Vulcan carbon on N-doped mesoporous carbon layer/MWCNT on N-doped carbon nanoweb

Co3O4631 Co3O4620 Co3O4634 Co3O4621 Co3O4−MnCo2O4310 FeCo2O4297 Li0.5CoO2118

0.78 0.75 0.84 0.73 0.77 0.75 0.64

1.65 1.61 1.68 1.70 1.68 1.71 1.58

0.87 0.86 0.84 0.97 0.91 0.96 0.94

core−shell Co@Co3O4 nanoparticles embedded in carbon nanotube/porous carbon pomegranate inspired Co3O4/MnO2−CNTs on N-rGO FeCo2O4/hollow graphene spheres lithiated spinel structure

MnCo2O4381 MnCo2O4364 MnCo2O4134

0.62 0.72 0.76

1.74 1.74 1.65

1.12 1.02 0.89

MnCo2O4667

0.73

1.76

1.03

MnCo2O4332 MnCo2O4231 MnCo2O4663 MnCo2O4300 NiCo2O4646 NiCo2O4293 NiCo2O4720

0.80 0.68 0.48 0.84 0.65 0.61 0.70

1.74 1.69 1.86 1.57 1.64 1.71 1.67

0.94 1.01 1.38 0.73 0.99 1.10 0.97

porous microspheres surface modification of MnCo2O4 with conducting PPy Mn−Co oxide nanoparticles partially embedded in NCNTs monodisperse MnCo2O4 nanoparticles on nitrogenenriched carbon nanofiber spinel/nanocarbon hybrid spinel powders multiporous MnCo2O4 microspheres covalent hybrid of spinel with graphene nanowire arrays mesoporous nanoplatelets on graphene hollow carbon-supported nanoparticles

precipitation hydrothermal thermal deposition hydrothermal MOF derived metal carbonyls decomposition MOF derived MOF derived hydrothermal+ pyrolysis gas-phase synthesis hydrothermal hydrothermal high-temperature solid phase precipitation precipitation high-temperature solid state solvothermal

ZnCo2O4673 CoMn2O4381 NiMn2O4721 Mn3O4@ CoMn2O4676

0.80 0.75 0.73 0.80

1.65 1.83 1.61 1.67

0.85 1.08 0.88 0.87

quantum dots dispersed on NCNTs porous microspheres ilmenite/spinel hybrid Mn3O4@CoMn2O4−CoxOy nanoparticles

CoFe2O4287 CoFe2O4308 CoFe2O4337 NiFe2O4305

0.72 0.71 0.66 0.54

1.69 1.56 1.69 1.56

0.97 0.85 1.03 1.02

on graphene nanoparticles on rod-like ordered mesoporous carbon coupled with CNTs nanoparticles cross-linked with MWCNTs

spinelsa

hydrothermal sol−gel solvothermal+ pyrolysis hydrothermal precipitation hydrothermal without surfactants or templates hydrothermal precipitation coprecipitation high-temperature solution phase hydrothermal hydrothermal solvothermal hydrothermal

a

The data are extracted from the LSV curves at a RDE rotation rate of 1600 rpm, the electrolyte is 0.1 M KOH unless otherwise stated, and all of the potentials are converted to vs RHE by the Nernst equation as introduced in section 4.2. bΔE is the potential gap of EOER and EORR.

at the anode, fuels, such as H2 are oxidized into protons and electrons. Concurrently, oxygen is reduced to oxide species at the cathode. Either protons or oxide ions are transported depending on the electrolyte and then react to form water while electrons travel an external circuit delivering electric power.71,449,729,730 We discuss here several applications of spinel oxides in SOFCs and MFCs. 6.1.1. Solid Oxide Fuel Cells. SOFCs use ceramic inorganic oxides as electrolytes (generally, yttria-stabilized zirconia, YSZ) rather than liquid electrolytes. These cells operate at high temperature, approximately 700−1000 °C. Generally, a mixture

of H2 and CO formed by internally reforming a practical hydrocarbon fuel is used as the reductant, and air is used as the oxidant, producing H2O and CO2 (Figure 47a).724,731 SOFCs possess several potential advantages because of their high operating temperature, including highly reversible electrode reactions, high tolerance to catalyst poisons, low internal resistance, long operating life, and sufficient utilization of highquality waste heat (for other uses). Therefore, this type of cell is well suited to large-scale power generation and industrial applications. However, a high working temperature is a doubleedged sword, as the critical cell components such as the 10174

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Figure 46. (a) Outdoor fuel-cell stack with internal humidiffier, electric heating, coolingribs, and single-cell. Reproduced with permission from ref 725. Copyright 2006, Elsevier. (b) Image of application of MFC as a viable power source to power a meteorological buoy. Reproduced with permission from ref 726. Copyright 2004, Elsevier. (c) Image of direct methanol fuel cell stack with external manifold type. Reproduced with permission from ref 727. Copyright 2012, Springer. (d) Summary of fuel cell types. The oxidation reaction occurs at the anode and involves the liberation of electrons, and the reduction reaction occurs at the cathode (for example, 2H2 → 4H++4e− and O2 + 4H+ + 4e− → H2O). Reproduced with permission from ref 729. Copyright 2001 Nature Publishing Group.

Figure 47. (a) Schematic diagram of the operating principles of SOFCs. (b) SEM image of the interface of a Cu1.4Mn1.6O4 cathode/Sc2O3 stabilized ZrO2 electrolyte. (c) I−V and corresponding power density curves of a Cu1.4Mn1.6O4-based SOFC at different temperature. Reproduced with permission from ref 163. Copyright 2016, Elsevier. (d) SEM image of the cross-section of Cu1.3Mn1.7O4 on ferritic stainless steel. (e) Area specific resistance values for uncoated and coated samples as a function of time at 750 °C. Reproduced with permission from ref 162. Copyright 2015, Elsevier.

electrodes, electrolytes, and interconnects require chemical and thermal compatibility at such high temperatures.106 This necessitates key materials possessing high structural stability, suitable thermal expansion, and chemical inertness with interconnects. Most spinels have considerable structural stability, remarkable oxidation resistance, high reaction activity, and suitable electrical conductivity at high temperatures. Specifically, Co- and Mn-based spinel oxides are regarded as promising candidates for SOFC cathodes or interconnects and have been widely investigated toward those ends.162,232,732

SOFC cathode materials generally require high electrical conductivities, electrocatalytic activity, and suitable thermal expansion coefficients (approximately 11−12.5 × 10−6 K−1).724 To enhance the electrical conductivities and electrocatalytic activities of spinels, doping to adjust the transition metal valence states and create crystal defects is a common strategy. Yang et al. prepared Cr-doped LaMnO3 electrodes for SOFCs via deposition of elemental Cr. The formation of (Cr, Mn)3O4type spinel induced the generation of Mn2+, which could enhance the cathode performance.99 Copper-doped MnCo2O4 greatly improved the oxidation state of the Mn cations in the spinel 10175

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Figure 48. (a) Operating principles of an MFC. Reproduced from ref 734. Copyright 2016, American Chemical Society. (b) Steady-state ORR polarization plots of different MnCo2O4 composites and Pt/C at 1600 rpm. (c) Polarization plots of MFCs (power density and voltage as a function of current density) with different air cathodes. Reproduced with permission from ref 320. Copyright 2014, Elsevier.

decreased the area specific resistance (17.4 mΩ cm2) after 200 h oxidation at 800 °C compared to uncoated substrates (36.1 mΩ cm2).733 In addition, Hosseini et al. introduced Cu into the spinel and developed a Cu1.3Mn1.7O4 coating (Figure 47d,e). It showed a high electrical conductivity of 140 S cm−1 and a low area specific resistance of 19.3 mΩ cm2 after 500 h.162 6.1.2. Microbial Fuel Cells. MFCs are a type of biological fuel cells that can transform chemical energy into electrical energy through electrochemical reactions. MFCs have recently drawn global research attention due to the promise of clean and renewable energy from various wastes and wastewaters via utilizing electrochemically active bacteria.734,735 Generally, MFCs consist of an anode, electrolyte, and cathode with external circuits (Figure 48a).734 The performance of MFCs strongly depends on the efficiency of the electrode, especially the cathode. Therefore, further improving the ORR kinetics at the cathode is one of the critical steps needed to promote the development of MFCs.736 Pt is a good cathode catalyst for the ORR and electron acceptance in MFCs but is too expensive. Therefore, several Ptfree spinel oxides using non-noble transition metals such as Co, Mn, and Fe have been significantly developed as alternatives because of their excellent ORR catalyst activity. Ge et al. utilized ortho-hexagon spinel nano-Co3O4 doped into active carbon as an MFC cathode. This spinel material offered a high micropore surface area and abundant active sites, displaying a maximum power density of 1500 mW m−2.327 Mn addition can achieve better cost effectiveness and structural stability.737 MnCo2O4 nanorods were in situ grown on highly conductive PPy by Pradhan’s group. The in situ MnCo2O4/PPy composite revealed high electrical conductivity and catalytic activity due to the synergistic effect of MnCo2O4 and PPy (Figure 48b,c).320 Subsequently, their group introduced ferrite into a Mn-based spinel and fabricated a bifunctional MnFe2O4/polyaniline electrode material for MFC cathodes. The synergy of the Fe3+ in MnFe2O4 with its capacitive nature greatly improved the extracellular electron transfer of the exoelectrogens. The MnFe2O4/polyaniline hybrid composite not only displayed excellent ORR activity but also an enhanced anode half-cell potential.321 Recently, research has been devoted to exploiting more inexpensive, efficient, and environmental friendly cathode catalyst materials such as ferric oxides. Gnana kumar et al. prepared an inverse spinel structured Fe3O4 combined with PEDOT and graphene. They applied this hybrid as an ORR catalyst for continuous electricity production in MFCs. The composites, with extended surface areas, high electrical

structure, which increased the power density peak of the cathode to 506 mW cm−2 at 800 °C.232 Furthermore, infiltrating spinels into the yttria stabilized zirconia (YSZ) solid electrolyte is also a helpful way to enhance the catalytic activity of the electrode. Liu et al. found that Mn1.5Co1.5O4 spinel infiltrated on YSZ exhibited outstanding electrocatalytic and chemical behavior as a SOFC cathode, with a polarization resistance of 0.70 Ω cm2 at 800 °C. Subsequently, an improved (Mn,Co)3O4 spinel composite with an ionic conducting phase delivered better electrochemical performance as an SOFC cathode and decreased the polarization resistance to 0.43 Ω cm2 at 800 °C.187 Recently, Zhang et al. fabricated cathodes of Co1.5Mn1.5O4 (20−30 nm) nanocrystals infiltrated into (La,Sr)MnO3−YSZ, which greatly reduced both ohmic resistance (0.136 Ω cm2 at 700 °C) and polarization resistance in the cells. This could be attributed to the high catalytic oxygen reduction activity of the nanosized Co1.5Mn1.5O4 coupled with the highly reversible Mn3+/Mn4+ redox.190 Preparing mixed-valent pairs in nonstoichiometric composites is also an effective method. Zhen et al. reported Cu1.4Mn1.6O4 spinel oxide (Figure 47b,c) and assessed it as a potential SOFC cathode. Because of the nonstoichiometric mixed Cu+/Cu2+ and Mn3+/Mn4+ couples, the electrodes exhibited superior electrochemical performance with polarization resistances of 0.143 Ω cm2 at 800 °C and a maximum power density of 1076 mW cm−2.163 SOFC interconnects provide electrical connections between single cells, physically separate the oxidant at the cathode and the fuel at the anode, and distributes gases to the electrodes.733 Recent studies have reported that chromia-forming ferritic stainless steels (e.g., type 430 alloy, Crofer 22 APU) are considered to be the most promising interconnect materials. Spinel oxides have frequently been used as protection layers on the surface of these chromia-forming stainless steels to prevent the SOFC cathodes being poisoned by migration of chromium. Stevenson’s group utilized Mn1.5Co1.5O4 spinel as a Crofer 22 APU protection layer, which not only significantly reduced the contact area specific resistance between the cathode and interconnect but also prevented the migration of Cr, which greatly improved the SOFC stability and electrochemical performance.106 To overcome poor chemical stability and unsuitable thermal expansion at high temperature, researchers have focused on Co-free spinel oxides. The cobalt-free spinels better tolerate high-temperature and long-term operation. Ebrahimifar and Zandrahimi found that Mn3O4 and MnFe2O4 mixed spinel coated on AISI 430 ferritic stainless steel could effectively limit the outward diffusion of Cr and the inward diffusion of oxygen anions. The use of these spinel coatings also 10176

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Figure 49. (a) Schematic diagram of metal−air batteries, in which the Li/Na/Zn−air batteries are rechargeable and most reported Mg/Al−air batteries are nonrechargeable. (b) Theoretical (blue) and practical (orange) specific energies density of various representative types of batteries and an H2−air fuel cell with their estimated driving ranges. The theoretical values are calculated based on the thermodynamics active materials (metal and oxygen), and the values for driving distances are based on the minimum specific energies of the batteries and scaled to the Nissan Leaf.52,749−752

performance for practical applications. This work set off a rapid renaissance of Li−air batteries.753 However, Li−air battery development has faltered for lack of low-cost and highperformance cathode catalysts. Recently, spinel-type oxides, especially Co3O4, CoMn2O4 and NiCo2O4, have been widely used as cathode catalysts.754 Kim and workmates reported immobilizing Co3O4 nanofibers on graphene nanoflakes to enhance their electrical conductivity and catalytic activity (Figure 50a,b). The tailored electrode exhibited a maximum first discharge capacity of 10 500 mA h g−1 and superior cyclability of 80 cycles with a limited capacity of 1000 mA h g−1.755 Additionally, Zhu et al. elucidated the catalytic mechanism of Co3O4 for the OER through DFT-based firstprinciples studies (Figure 50c,d). They revealed that the O-rich Co3O4 (111) has a high catalytic activity in reducing the overpotential and O2 desorption barrier, but the basic sites of the Co3O4 (110) surface induce Li2O2 decomposition into Li2O and a dangling Co−O bond.755 Notably, Chen and Ramakrishna prepared various desired spinel transition metal oxides with 1D nanostructures via electrospinning, including CoMn2O4 , NiCo2O4, CoFe2O4, NiMn2O4, and ZnMn2O4. Among these, CoMn2O4 (Figure 50e,f) was used as a cathode catalyst in Li−air cells, showing the low overpotential of 1.2 V and excellent cyclic stability of 110 cycles with a limited capacity of 1000 mA h g−1.472 Subsequently, they further investigated porous NiCo2O4 core− shell microspheres as highly efficient spinel catalysts in Li−air batteries. The hierarchical porous microspheres not only provided many electrocatalytic sites but also accelerated the flow of O2 and the infiltration of the electrolyte. The porous NiCo2O4 exhibited excellent electrochemical performance with a stable capacity and limited cycling at 1000 mA h g−1 even over 128 cycles and the overpotential of 1.23 V.284 Another helpful strategy to improve the catalytic activity of spinel oxides is to create nanosize effects. Zhang and co-workers synthesized ultrafine NiCo2O4 nanocatalysts (∼2 nm) through an electrochemical prelithiation process; they exhibited their initial highest capacity of 29 280 mAh g−1 and an enhanced cycling stability with a specific capacity of >1000 mA h g−1 for 100 full discharge/ charge cycles.282 In addition, CuCo2O4186 and ZnCo2O4275,185 with different structures and morphologies have also been synthesized for use as bifunctional electrocatalysts for rechargeable Li−O2 batteries with outstanding results.

conductivity, and abundant oxygen adsorption sites, exhibited excellent ORR kinetics and superior durability for 600 h.386 6.2. Metal−Air Batteries

Metal−air batteries feature an open cell structure (Figure 49a), which makes them similar to fuel cells.738,739 However, they generate electricity through a redox reaction between metal and oxygen in air. This feature uses the metals as “fuels” and allows the supply of the cathode active material (oxygen) continuously and almost infinitely from an external source (air).740,741 Compared with the conventional battery systems of primary batteries (Zn−Mn), rechargeable lead−acid, nickel−metal hydride (Ni−MH), and Li-ion batteries, metal−air batteries show a highest theoretical energy density, which makes them excellent candidates for next generation electrical vehicles (EVs) and hybrid electrical vehicles (HEVs; Figure 49b).52,332,476,742,743 Various metal−air batteries have been based on different metal anodes. Typically, Li−air batteries are the most attractive system because of their high theoretical energy density of 3458 Wh kg−1. For example, IBM used them in their “Battery 500 Project” to boost EV range to 500 miles or more.426,435,744,745 In addition, the primary Zn−air batteries have been commercialized as a hearing aid battery.165,314 Al−air and Mg−air batteries have also found military applications under the excitation of saline systems.742,746,747 Although the reaction mechanisms of these different batteries vary, the basic principles of all metal−air batteries to achieve high efficiency and reversibility involve accelerating the electrochemical ORR or OER.188,361,633 For the cathode in particular (i.e., air electrode), the major obstacles are high prices (noble metals), slow reaction rates, high overpotentials, and poor reversibility, which limit the application of rechargeable metal−air batteries.222,405 As an alternative, the spinel-type catalysts are one of the most attractive due to their low cost, facile preparation, structural stability, and highly efficient synergy with double transition metals. Among the spinel metal oxides, Co- and Mn-based materials have been extensively investigated as cathode catalysts because of their ORR and OER activity.296,748 6.2.1. Li−Air Batteries. In 1996, Abraham and Jiang first introduced a successful rechargeable Li−air battery, which was composed of a Li metal anode, a carbon composite cathode, and a Li+ conductive organic polymer electrolyte membrane.752 Later, Bruce and co-workers demonstrated sustainable cycling of a Li−air battery in 2006, showing promising electrochemical 10177

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Figure 50. (a) Initial discharge/charge curves of Co3O4 composites in a voltage window of 4.35−2.35 V at a current density of 200 mA g−1 and (b) corresponding cycle performances under a specific capacity limit of 1000 mA h g−1. Reproduced from ref 755. Copyright 2010 American Chemical Society. (c) Decreased overpotential and O2-desorption barrier in the OER of Li2O2 supported on transition-metal-doped Co3O4 (111) as a function of the ionization potential of doped transition-metal. (d) Schematic diagram for the discharge product (Li2O2) deposited on the electrocatalyst surface (Co3O4). Reproduced from ref 756. Copyright 2015 American Chemical Society. (e) Discharge/charge curves of CoMn2O4 nanofibers with a limited capacity of 1000 mA h g−1 at a current density of 200 mA g−1, and (f) corresponding cycle performances of the Li−O2 cell with the CoMn2O4 nanofibers electrode. Reproduced from ref 472. Copyright 2015 American Chemical Society. (g) Performance of rechargeable Zn−air cells based on c-CoMn2/C and Pt/C catalysts at a cycling rate of 10 mA cm−2 with a duration of 400 s per cycle. Inset schematically depicts the structure of assembled rechargeable Zn−air cells. Reproduced with permission from ref 79. Copyright 2015 Nature Publishing Group. (h) Discharge−charge profile of the rechargeable Zn− air battery with the pomegranate-like Co3O4 nanocomposite electrocatalyst. Reproduced with permission from ref 634. Copyright 2016 Wiley-VCH. (i) Zn−air battery performance with the typical deep discharge specific capacity at 20 mA cm−2. Reproduced with permission from ref 478. Copyright 2014 The Royal Society of Chemistry.

V and considerable specific energy densities.7 Recently, we further exploited the phase-controllable spinel CoMn2O4/C composites via oxidation−precipitation and crystallization. The nanoscaled CoMn2O4 particles (Figure 50g) showed excellent ORR/OER performance, with a high energy density of ∼650 Wh kg−1 at 10 mA cm−2 and only an 8.5% voltage reduction after 155 cycles when used in Zn−air cells.79 Cho’s group anchored Mn3O4 on ionic liquid-modified reduced graphene oxide nanosheets via a facile solution-based growth mechanism and applied the composite to a Zn−air battery as an effective electrocatalyst for the ORR. The ionic liquid moiety increased the conductivity and the electrocatalytic activity.712 In addition, Chen’s group designed Co3O4 with a pomegranate-like architecture with abundant active sites, enhanced electron transfer, and strong synergistic coupling. Then, a single-cell Zn−air battery (Figure 50h) was fabricated using this catalyst and displayed virtually no voltage fading for either discharge or charge after 80 h of continuous operation.634 NiCo2O4 fibers have also been reported as highly efficient bifunctional catalysts

6.2.2. Zn−Air Batteries. In the early 1970s, Zn−air batteries had been successfully introduced for use in hearing aids to replace 2-electrode cells. By the mid 1980s, Zn−air batteries had become the standard for hearing aid applications.757 Subsequently, button batteries have been gradually developed toward Zn−air fuel batteries with increased electrochemical capacity, higher voltage, cell-to-cell performance consistency, and compatible battery pack systems.758 In a typical example, Lawrence Livermore National Laboratory developed rechargeable Zn−air batteries for more applications, including vehicles with a range of 250−350 miles before refueling.758 Currently, Zn−air batteries are also one of the most promising candidate for EVs or HEVs. As with Li−air batteries, many spinel-type oxides with excellent ORR/OER properties have been investigated as catalysts for Zn−air batteries.428 Our group reported the rapid room-temperature synthesis of nanocrystalline MnxCo3‑xO4 spinel and its applications to catalyze Zn−air batteries. The most active MnxCo3‑xO4−P electrode delivered a stable galvanostatic discharge voltage at 1.3 10178

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Figure 51. (a) Image of hydrogen fueling station, operated by BR Distribuidora. (b) Hybrid fuel cell bus. (c) Hydrogen fueling station, where a is a compressor, b are hydrogen storage cylinders, and c are fireproof walls. Reproduced with permission from ref 762. Copyright 2013, Elsevier.

efficient active sites with a working voltage plateau of 1.2 V in the Al−air battery.307 Liu et al. used spinel LiMn2O4 dispersed on Ndoped rGO as an efficient electrocatalyst for Al−air batteries. The batteries with LiMn2O4/N-rGO cathodes displayed a high energy density of 585 mA h g−1 and a sluggish potential drop.313

for Zn−air batteries, showing promising practical application value with working voltage plateaus of approximately 1.25 V and a discharge capacity of 580 mA h g−1 (Figure 50i).478 6.2.3. Mg/Al−Air Batteries. Primary Mg/Al−air batteries have been applied in the military due to their high power density, price advantage, and excellent availability. Recently, the Furukawa battery Co., Ltd. developed Mg−air fuel cells for emergency use. They begin generating power as soon as water is poured into them. They can generate electricity for up to 5 days with a capacity of 300 Wh.759 Additionally, Phinergy, an Israeli startup, has proposed an Al−air battery that consumes Al as a fuel. The batteries have exhibited a capability of powering an EV for up to 1000 miles. This battery must be refilled with water every 200 miles to replenish the electrolyte, and the “Al plate fuel” must be physically replaced after exhaustion.745 However, the development of rechargeable Mg/Al−air batteries is facing great obstacles and has had few breakthroughs. Because Al and Mg tend to erode in aqueous/nonaqueous electrolytes, anodic products (e.g., Al2O3, MgO, or Mg(OH)2) may block the porous electrode and cause premature battery performance degradation, and the electrochemical irreversibility of the discharge products (Al2O3 and MgO) is a serious barrier. Therefore, researchers have paid more attention to using catalysts with high ORR/OER properties, which could further promote the voltage and power density of Mg/Al−air batteries. Recently, Chou and co-workers developed ultrafine Mn3O4 nanowires with 3D graphene and SWCNTs for enhanced Mg−air batteries. Benefiting from the tailored composites with high electrical conductivity, large surface area, and abundant active sites, the battery has a high plateau voltage of 1.34 V and a long discharge time of approximately 70 h at 0.2 mA cm−2.448 Similarly, Zhang et al. employed a NiCo2O4/CNT hybrid to improve Al−air batteries. Because of the Ni cation incorporated into the spinel structure, the hybrid showed increased electrical conductivity and highly

6.3. Water Splitting

Pure hydrogen, without carbon species, has been considered the most promising energy carrier for a sustainable future fuel.54 Water splitting, when considering energy and environmental issues, is a clean and efficient technology to generate hydrogen with high purity. The water splitting reaction is an uphill reaction with the Gibbs free energy increased by 237 kJ mol−1, in which the energy is provided by electrical or optical energy.55 Therefore, water splitting can generally be divided into two types: electrochemical water splitting and photochemical water splitting. Electrochemical water splitting with OER/HER on the cathode has a long history of study.394,517 However, the success of photochemical water splitting has emerged in recent years with the development of solar energy technology.56 The applications of water splitting have been widely reported in news or scientific reports. ITM announced the sale of a 1 MW electrolyzer system with some additional equipment to ZEAG Energie AG.760 Air Liquide Company constructed the hydrogen station. This station has a total capacity of 260 kg day−1 which provided fuel to 5 Van Hool buses. More importantly, the electricity at the station is guaranteed to come from 100% renewable energy sources.761 In addition, the hydrogen fueling stations (Figure 51a−c) are operated by BR Distribuidora at EMTU/SP, Sao Bernardo do Campo. It can provide the daily fuel to hybrid fuel cell vehicles.762 6.3.1. Electrochemically Driven. Electrochemical water splitting is the decomposition of water into oxygen and hydrogen via an electric current being passed through water.153,276,324 In 1789, Deiman and Troostwijk discovered the gasification of 10179

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Figure 52. (a) Overall water splitting performance in a two-electrode system at current densities of 10 and 20 mA cm−1. The photos of NiCo2O4 electrode and their water splitting device (inset). Reproduced with permission from ref 343. Copyright 2016 Wiley-VCH. (b) LSV profile of the NiCo2O4−Ni0.33Co0.67S2 nanowire in a 1 M KOH water splitting system, photograph of the all-nanowire-based electrolyzer (inset). (c) Chronoamperometry of NiCo2O4−Ni0.33Co0.67S2 nanowire in a water splitting system at 1.65 V in 1 M KOH. Reproduced with permission from ref 285. Copyright 2015 Wiley-VCH. (d) Photos of the plating baths and measured chopped-illumination photocurrent densities as a function of bias potential. Reproduced from ref 57. Copyright 2016 American Chemical Society. (e) Electrolysis of membrane-electrode assemblies with Li2Co2O4 as catalyst films at 1.8 V (pH 7.2). Reproduced with permission from ref 211. Copyright 2012 Wiley-VCH. (f) Oxygen evolution profiles for the photochemical water oxidation with different CoFe2O4 samples in the presence of a 300 W Xe lamp at 20 °C. Reproduced from ref 344. Copyright 2014 American Chemical Society.

through morphological control, vacancy creation and recombination with other spinels or carbon. Lin and Zhang designed hierarchical NiCo2O4 hollow microcuboids with large active surfaces and abundant active species diffusion paths for overall water splitting. They exhibited (Figure 52a) excellent OER/HER activity with 10 and 20 mA cm−2 water-splitting current reached by applying just 1.65 and 1.74 V across the two electrodes, respectively.343 Xie’s group reported that NiCo2O4 ultrathin nanosheets with rich oxygen vacancies delivered a remarkable performance for electrocatalytic water oxidation with a large current density of 285 mA cm−2 at 1.8 V and a small overpotential of 0.32 V.418 Sulfurized spinel Ni−Co-based nanowires (Figure 52b,c) were fabricated as water splitting electrocatalysts by Zheng and co-workers. Benefiting from the unique homologous, highly efficient and all-nanowire-based system, the electrode showed a current density of 5 mA cm−2 at a voltage of 1.65 V.285 Qiao’s group designed hierarchically porous N-doped rGONiCo2O4 hybrid paper as an advanced OER catalyst for electrocatalytic water splitting. The composites with welldeveloped in- and out-of-plane pores, 3D conductive networks, and N-doping active sites possessed dual active sites of marginal NiCo2O4 and the N (O)−metal (Ni or Co) bonds. Thus, enhanced OER performance with an overpotential of 373 mV at 5 mA cm−2 and a Tafel slope of 156 mV dec−1 was obtained.184 6.3.2. Photochemically Driven. In 1972, Honda and Fujishima reported the first photoelectrochemical cell for water splitting using TiO2 as a photoelectrode.767 Subsequently, various types of cell systems and materials (Figure 52d) have been explored to improve the efficiency and reactivity. Currently, although hydrogen production by photochemical water splitting has been successfully realized on the laboratory scale, the

water in a Leyden jar by using electrostatic devices connected to a gold electrode. Subsequently, in 1800, after the invention of the voltaic battery by Volta, Nicholson and Carlisle used this battery for water electrolysis with great success.763 However, water electrolysis had not received much attention before Gramme invented the direct-current generator in 1869, which turned it into a relatively low-cost method to produce hydrogen.764 In 1939, the first electrolyzer, generating 10 000 N m3 h−1 of hydrogen, was installed. This capacity is still used in the largest production facilities in the world today. Great achievements have been made in finding effective ways to produce high-purity hydrogen, such as advanced catalysts and industrial devices.147,154,470,765,766 However, to render the overall process more cost and energy efficient, it is critical to develop the next generation of technology for electrochemical water splitting. Therefore, it is of interest to design efficient nonprecious metal water-splitting catalysts from abundant resources and with facile preparation, high electrolysis activity, and structural stability.226,587 Recently, Co-based complexes have been widely investigated as homogeneous molecular catalysts for the HER/OER. Liu’s group carefully elucidated the geometrical-site-dependent OER activity of Co3O4 through in situ electrochemical impedance and X-ray absorption spectroscopy analysis. The results show that the Co2+Td (Co2+ in the tetrahedral site) in Co3O4 is responsible for releasing electrons and promoting the affinity to oxygen ions on the catalyst surface to form CoOOH, which plays the main role in the OER. The Co3+Oh (Co3+ in the octahedral sites) tends to stably bond with −OH groups to assist in catalysis.229 In addition, many Ni−Co-based spinel oxides have been widely investigated as OER catalysts for electrochemical water splitting 10180

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Figure 53. Selected spinels with magnetic properties and applications. (a) Electron energy loss spectroscopy of prepared CoxFe3−xO4 nanocubes. (b) SEM images. (c) Magnetic properties of CoxFe3−xO4. Reproduced from ref 17. Copyright 2016 American Chemical Society. (d) H-T diagram of Mn2.5Co0.5O4 thin film. Reproduced with permission from ref 539. Copyright 2010 AIP publishing. (e) Magnetic properties of ZnFe2O4/RGO and (f) its therapeutic effects against cancer. Reproduced with permission from ref 291. Copyright 2014 The Royal Society of Chemistry.

process in aprotic systems (Li−air batteries) with spinel catalysts is still poorly understood. A more comprehensive understanding will promote the rational design of spinel catalysts. Moreover, the rapid development of water splitting increase the demand to explore bifunctional spinel catalysts that benefit both the OER and HER.

commercial viability of photochemical water splitting is limited due to the high cost of the raw materials used for the photoelectrodes and catalysts.201,768 Dismukes and Greenblatt reported spinel-like Li2Co2O4 (Figure 52e) for photocatalytic oxidation of water with a high TOF of 1.0 × 10−3 s−1 and good stability.211 Liu et al. further investigated the enhanced spineltype LiCoO2 created by chemical delithiation. This modification greatly improved the hole mobility and increased the water oxidation activity, showing the maximum oxygen evolution of 10.9 μmol m−2 and maximum TOF value of 9 × 10−3 μmol s−1 m−2 (2.5 times higher than the TOF for nanocrystalline Co3O4).214 Furthermore, Driess and Strasser developed CoFe2O4 spinel for photochemical water oxidation with low overpotential. The total amount of oxygen produced by CoFe2O4 (Figure 52f) was ∼60 mmol(O2) mol(cat)−1, and the anodic current density reached 400 mA mg−1 at a potential of 1.74 V in a 0.1 M KOH solution.344 Co-free spinel materials are also considered strong candidates for water splitting electrocatalysts owing to their better economic efficiency and stability. Stahl’s group, for example, systematically investigated the inverse spinel NiFeAlO4 and defined the unclear promoting effect of Al ions on Ni−Fe-based electrocatalysts.160 Hufnagel et al. introduced an n-type semiconducting spinel ZnFe2O4 thin film as a photoabsorber material for light-driven water splitting. These films, with lower photocurrent and slower surface charge recombination, exhibited an oxidation onset potential 200 mV over the cathodic potential in addition to high charge transfer efficiency.514 In summary, spinels have been applied in various applications for enhanced ORR/OER and OER/HER. For ORR applications in solid-state fuel cells, the spinels require both high ORR catalytic activity and thermal stability. For microbial cells, decent conductivity and compatibility with microorganisms are necessary. For metal−air batteries, developing porous 3D architectured spinels with high specific surface areas facilitates both the air transport and ORR/OER. However, the ORR/OER

7. PROPERTIES AND APPLICATIONS BEYOND THE ORR/OER 7.1. Magnetism

Magnetism properties include ferromagnetism, ferrimagnetism, paramagnetism, diamagnetism, and antiferromagnetism. Most studied spinel compounds with Fe, Co, Cr, or Ni-based compositions exhibit ferrimagnetism. For example, the spinel ferrites with a general composition of (M 1−x 2+ Fe 3+ )[Mx2+Fe2‑x3+]O4 (in which x stands for the inverse degree of the spinel and M = Co2+, Ni2+, Fe2+, Mg2+, Zn2+) usually exhibit ferrimagnetism.17,19,769 The A sites being exchanged by Fe3+ cations leads to the amount of magnetization, and their distribution decides the magnetic properties.770 The magnetic properties of spinels can be employed in the fields of information technology for data storage,8 nanobiotechnology for selfalignment with trypsin molecules and magnetic sensing,8,17,18,291 and electronics for spintronic devices.19 The saturation magnetization (MS) stands for the highest value magnetized by an external magnetic field, and the corresponding coercivity (HC) stands for the magnetic field that is required to reduce the magnetization of a material to zero after the magnetization. These two parameters are important values of ferro- or ferrimagnetic spinels and are influenced by crystallinity and oxygen vacancies. A large crystallite size with high crystallinity usually results in enhanced MS. Ounnunkad et al. fabricated a series of ferrimagnetic Cu0.5Co0.5Fe2O4 at a temperature range from 450 to 600 °C.8 With increasing calcination temperature, the crystallinity increased, with larger 10181

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Figure 54. Selected spinels with optical properties and application. (a) Excitation spectra and (b) emission spectra of Cr3+-doped MgAl2O4 spinel glass ceramics (a = 0.5 wt %, b = 1 wt %, c = 1.5 wt %, and d = 2.0 wt %. Reproduced with permission from ref 21. Copyright 2014 Elsevier B.V. Luminescence spectra of ZnAl2O4 at (c) 1200 and (d) 1350 °C. Reproduced with permission from ref 375. Copyright 2013 The Royal Society of Chemistry.

of 326 K.137 Gupta’s group prepared CuCr2Te4 nanocrystals by using a high-temperature solution-phase method, which had a TC of ∼330 K.489 The TC of CdCr2Se4 spinel is ∼130 K, and CdCr2Se4 orders magnetically below TC.135 The magnetic ordering is related to the Cr−Cr spin coupling and Cr−Se−Cr exchange interaction. Although the substitution of Cd2+ with Sb, In, and Sn ( CoCr 2 O 4 > MnCr 2 O 4 . 155,164 NO x conversion usually results in the generation of a strong global warming gas (N2O), which makes it vital to prepare catalysts that decompose N2O (reaction: N2O + e → N2 + O−, O− → 1/2 O2).15,32 K-doping Co−Mn−Al spinel-like oxides has been

shown to lower the decomposition temperature of N2O into N2 and O2.32 Kaczmarczyk et al. studied the activities of Mn3O4, Fe3O4, and Co3O4 nanoparticles, revealing that the Co3+/Co4+ couple of Co3O4 with a one-electron process controls the N2O decomposition, while the sesquioxide (γ-Fe2O3 and γ-Mn2O3) of Fe3O4 and Co3O4 controls the activities.15 These phenomena result in the higher activities of Co3O4 than those of Mn3O4 and Fe3O4. Amrousse et al. also found that nanosized Co-substituted Fe3O4 allows a low N2O decomposition temperature.408 7.4.1.2. CO Oxidation/CO2 Reduction. Like NOx, CO is also a toxic gas. The oxidation of CO (CO + 1/2 O2 → CO2) is important for CO removal full cells,154 treatment of automotive exhaust gas, air cleaning and sensing.175 Co3O4 with nanorod morphologies and mesoporous structures has been regarded as an efficient catalyst for CO oxidation.372 The oxidation of surface Co2+ is believed to cause the high activity of CO oxidation, and thus, Co−Fe based spinels have also received potential interest.175 In addition, a commercially available black pigment 10185

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methanol, ethanol (Ni−Co oxides42) and ethylene glycol (Ni− Co oxides,42 Ni0.3Co2.7O4522) oxidation are also accelerated by spinels. Ethanol steam reforming, generating CH4, CO, and H2 fuels, has been investigated with MgAl2O4-supported Ni/Co/Ge catalysts.382 The defect sites of MgAl2O4 provide the intrinsic activity, and Ni species on defect oxides facilitate the first step of ethanol reforming. Spinels (Co3O4 and CoMn2O4/rGO340,544) can also facilitate the oxidation of aromatic alcohols, which is important in the pharmaceutical, petrochemical, and agricultural industries. 7.4.2.2. Others. Lodowicks used CoMn2O4/Ti anodes to electrosynthesize p-methoxybenzaldehyde through the oxidation of p-methoxytoluene.43 NixCo3‑xO4/PPy150 and MFe2O4 (M = Fe, Co, Mn)/CNT44 spinels have been applied to decompose H2O2. The H2O2 electroreduction strength is considered to be in the order Fe3O4 > CoFe2O4 > MnFe2O4,44 which is caused by the metal(II) proportion on the compound surface, the different compositions, and the metal(II)/Fe(III) ratios. 7.4.3. Solid Conversion. Urea can be regarded as a promising hydrogen carrier with a sustainable supply. Periyasamy et al. studied NiMn2O4, Ni1.5Mn1.5O4, and MnNi2O4 as catalysts for urea oxidation.45 Among them, Ni1.5Mn1.5O4 showed the highest urea oxidation current density in alkaline media due to the three mixed phases. Mesoporous spinel NiCo2O4 was also studied as a highly efficient electrocatalyst for urea oxidation.391 Alizadeh-Gheshlaghi et al. demonstrated that nanosized CuCo2O4 had better activity than CuO and Co3O4 in lowering the decomposition temperature of NH4ClO4 (main component of many propellants).46 An electrode formed of Co3O4 by nanocasting showed better activity to oxidize cyanide and phenol than that deposited by thermal decomposition.47 Serving as a glucose oxidation electrocatalyst allows MnCo2O4 nanofibers to act as a nonenzymatic glucose sensor with a detection limit of 0.01 μM (Figure 56g−i).48 The core−ring NiCo2O4 with small particle sizes, high optical absorption, and very active internal electron transitions exhibits higher photocatalytic activity in degrading methylene blue than the common NiCo2O4 and TiO2 do.49

mainly consisting of Cu1.5Mn1.5O4 also showed complete CO conversion at 180 °C after heat treatment.33 In addition, CO can also be converted into CO2 and H2 through a water−gas shift reaction (CO + H2O → CO2 + H2).154 Considering the major greenhouse gas of CO2, the inverse process (CO2 reduction) is also receiving increasing interest. Approaches to convert CO2 into fuel such as photosplitting 34 and thermochemical splitting376,513 have been studied. Wang et al. fabricated porous ZnCo2O4 nanorods as a photocatalyst to convert CO2 and water into H2 and CO fuels.34 Nordhei et al. demonstrated that the oxygen deficiency in spinel ferrites (AFe2O4‑x, A = Ni, Co, Zn) cause higher activities than those of the bulk materials. Among them, NiFe2O4‑x displays the highest catalyst activities because of its increased ratio of octahedrally coordinated iron.353 Darwin has achieved the CO2 cycle by using CoFe2O4 and FeAl2O4 spinels (Figure 56a−c).513 7.4.1.3. Hydrogen Evolution Reaction. The HER reaction is one of the reactions in water splitting, which have been mentioned as a major method to generate H2 as clean energy fuel.55,56 As with the ORR and OER, its slow kinetics and high overpotential must be resolved. Many nanostructured spinel compounds or composites such as Co3O4@graphitic carbon nitride with tubular nanostructures,241 hierarchical NiCo2O4 hollow microcuboids,343 Co/CoO/Co3O4 nanoparticles@Ndoped C,635 and porous NiFe/NiCo2O4/Ni have been fabricated to accelerate the HER reaction. 7.4.1.4. Chemical Looping Combustion. The CLC technologies apply an oxygen-carrier to transform the traditional fuel combustion process into two separate gas−solid reactions. The CLC reaction achieves the goals of nonflaming combustion, limiting generation of NOx and gathering the CO2. The spinels with high thermal stability can be used as environmentally benign oxygen-carriers.401 For example, with metal oxide as the oxygen carrier, the CH4 chemical looping process with metal oxide (MeO) can be written as follows: (1) CH4 reduction, 4MeO + CH4 → 4Me + CO2 + 2H2O; (2) air oxidation, 4Me + 2O2 → 4MeO. Therefore, the net chemical reaction is 2O2+ CH4 → CO 2 + 2H 2 O. Various spinels such as MnFe 2 O 4 , 37 NiFe2O4,37,125,167,401 ZnFe2O4,37 CoFe2O4,37,167 CuFe2O4,37 and NiO/NiAl2O4161 have been used in the chemical looping process. 7.4.1.5. Others. In addition to the above-mentioned gas conversion, additional reactions have used spinel catalysts, such as NH3 oxidation with a CuCr2O4 catalyst;38 the oxidation of formaldehyde (HCHO + O2 → CO2 + H2O) with manganesesubstituted spinel ferrites, which notably decreased formaldehyde (toxic gas) oxidation temperature;39 and the combustion of methane catalyzed by CoCr2‑xVxO4 (x = 0− 0.20)40 and CoCr2O4.350 7.4.2. Liquid Conversion. 7.4.2.1. Alcohol Oxidation. Direct methanol fuel cells are promising candidates for energy conversion. However, the oxidation of methanol is sluggish with a high overpotential. Singh’s group found that nanosized spinel CuxCo3‑xO4 (x = 0, 0.3, and 1.0) could catalyze methanol oxidation.373 Increasing the content of Cu could enhance the methanol oxidation current densities. NiCo2O4 with urchin-like morphologies also demonstrated much higher methanol oxidation currents (Figure 56d−f).41 Furthermore, NiCo2O4rGO hybrids also demonstrated higher methanol oxidation current and lower onset potential than pure NiCo2O4 and rGO hybrids.653 Meanwhile, methanol steam reforming (CH3OH + H2O → CO2 + 3 H2) is also catalyzed by spinels (e.g., Cu1.5Mn1.5O4) to generate clean H2 energy.121 In addition to

8. CONCLUSIONS AND PERSPECTIVES 8.1. Conclusions

The development of spinels has gone from the employment of precious stones to the confirmation of crystal structures. Preparation approaches vary from traditional solid-state/hightemperature methods to various phases/temperatures with rational regulation. The morphologies can vary from the bulk with high crystallinity to abundant micro/nano morphologies with low crystallinity or amorphous characters and high defect levels. The applications of spinels have expanded from jewelry to the multifarious aspects of industry/daily life, becoming indispensable in modern society. The structures of spinels include normal spinel (MgAl2O4), inverse spinel (NiFe2O4), and complex spinel (CuAl2O4). In traditional normal spinels such as MgAl2O4, Mg2+ occupies the tetrahedral voids, Al3+ occupies the octahedral voids, and O2− occupies the polyhedral vertexes. The valences of A are +1, +2, or +3 and +4; B is +1, +2, or +3; and the O atom can be replaced with F (−1), CN (−1), S (−2), Se (−2), Te (−2), or N (−3). Defects can be at any of the A, B, or O sites. Spinels have been synthesized with solid-, liquid-, and vaporstate methods. The high-temperature solid-state method shows advantages when fabricating spinels with special compositions 10186

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Table 11. General Comparison of Traditional Spinels and Possible Future Developments spinels

traditional

future development

composition preparation

AB2X4, A and B are mainly transition metals from the fourth period elements (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) and the main group (Li, Mg, Ba, Ba, Al, Ga, In), X is focused on VI main group (O, S, Se, Te) high-temperature and energy or high-pressure process, uneven morphologies

application

data storage, spintronic devices, lasers, supercapacitors, Li-ion batteries, fuel cells

AB2X4, A and B can be extended to the fifth period elements (Y, Zr, Nb, Mo, Tc, Ru, Rh, Rd, Ag, Cd) and even the sixth. The inorganic−organic hybrid spinels are also promising Low-temperature and precise synthesis, controlled structure and morphologies, new methods (3D-printed preparation, etc.) nanobiotechnology, targeted therapy, luminescence, sensors, Na/ Mg/Zn-ion batteries, metal−air batteries, water splitting, CO2 reduction

Figure 57. Future development of spinel compounds, applications, preparations, and fundamental challenges. The development of spinels is supposed to focus on environment and energy issues, which includes using solar energy to reduce greenhouse gas emission (CO2 and NOx) and produce clear energy (H2 etc.) with highly efficient storage (batteries, fuel cells, etc.) and utilization (EV etc.). These applications depend on precise, grid-scale, lowtemperature preparation of spinels with various characters (porous, defect, etc.). Meanwhile, the fundamental scientific challenges such as creating new spinels with new structures and compositions and developing new methods and characterization techniques deserve continuous attentions.

morphologies. Of all these, the high-temperature solution-phase method excels for obtaining homogeneous spinel nanocrystals. Electrochemical technologies such as electrodeposition and electrospinning are advantageous for fabricating binder-free and free-standing spinel electrodes. The spinels derived from MOF decomposition and spray pyrolysis can easily be used for preparing spinel/carbon hybrids. Specifically, the decomposition of MOFs is of potential interest, as thousands of MOFs have

(for instance, CuIr2S4 and Si3N4). Flow growth, combustion, and nitrate decomposition methods have been utilized to decrease the temperature of solid-state reactions. The pulse laser, CVD, magnetron sputtering, and plasma method synthesis approaches with sophisticated instruments are often used to fabricate spinel films with uniform surfaces. Liquid-phase preparation methods (sol−gel, hydrothermal/solvothermal, and precipitation) are predominant in acquiring spinels with various micro/nano 10187

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(CO) into chemical fuels. In particular, the reduction of CO2 into useful fuels (e.g., HCOOH) can not only decrease CO2 emissions but also create renewable and clean energy. Third, spinels can act as electrodes or catalysts in rechargeable batteries. Specifically, the high-capacity metal−air batteries with spinels as catalysts and the very safe aqueous batteries with spinels as electrodes are of potential interest. Moreover, the bifunctional and multifunctional catalysts represent promising directions. 8.2.2. Materials Preparation and Scalability of the Latter. The nanostructured spinel oxides are generally accepted to be preferable for catalytic reactions. Newly prepared spinel compounds can focus on the following points. First, spinel fabrication processes exposing more active sites, which can mean forming ever smaller materials. For example, preparing 2D spinels that are several atomic layers thick or 0D spinel nanodots with preferably exposed crystalline planes would be interesting. Second, preparing spinels with stable structure and morphology is vital for commercial applications. Although nanostructured spinel catalysts have been demonstrated to be useful for elevating efficiency, maintaining the morphologies of the catalysts after long cycles has been a point of frustration. Toward this end, highly conductive carbon materials such as graphene and carbon nanotubes are promising substrates. The powerful physical or chemical interactions between the spinels and the substrates are beneficial to maintaining stability. Third, rationally designing spinels to precisely control their structure, composition, morphology, defects, and porosity. To achieve spinels with the above advantages, the fabrication process is critical. Considering grid-scale applications, future preparation processes should be facile and environmentally friendly. In that vein, developing low-temperature and even room-temperature approaches with rapid reaction processes has promise. 8.2.3. Fundamental Scientific Challenges. Most currently studied spinels are metal oxides. For future development, other spinels including metal sulfides, metal selenides, and metal nitrides deserve study. Exploring new inorganic−organic hybrid spinels (like perovskite hybrids) is also a new path toward widening the spinel family. The inorganic−organic perovskite hybrids such as CH3NH3PbI3 have demonstrated superb prospects in photovoltaic conversion. Either the A or B site of AB2O4 could also be replaced with some organic component (for example, replacing the A site with CH 3 NH 3 to form CH3NH3B2O4 would be interesting). In addition, most of the novel preparation methods can be used to fabricate spinel oxides, while the preparation of spinels such as CuIr2S4 and Si3N4 is still limited. Exploring facile methods to synthesize spinels beyond oxides remains challenging. In addition, exploring the in situ techniques to characterize the intermediate states of chemical reactions and tracing the reaction processes that are still less understood would be beneficial. The operando methods of representation will contribute to the deep understanding of spinels. These fundamental scientific challenges provide impetus for the exploration of new synthesis methods. Meanwhile, new characterization approaches and instruments should be introduced to better understand spinels. Additionally, regulating the intrinsic conductivity and electron structures of spinels remains complicated. To better catalyze the various reactions, it is necessary to deeply understand how the catalytic reaction occurs, where the active site is, what morphology is preferable, and which composition is better. Although the spinels with tailored

been reported. Simultaneously, the calcination derivatives of spinels can have such features as carbon loading, N-doping, micro/nano structured morphologies, and large surface area. By controllably preparing spinels with different characteristics, the ORR/OER properties of spinels have been carefully examined. The current spinels used as ORR/OER catalysts are focused within the Fe-, Ni-, Co- and Mn-based spinels. Although the spinels are inexpensive and widely abundant, the low conductivity of most spinels and the low levels of active sites of their bulk morphologies must be resolved. In consideration of these problems, regulating compositions and structures, designing micro/nano morphologies, loading onto conductive matrixes, creating defects, elevating the specific areas and creating porosity have all been used to accelerate the ORR/ OER. Among these, creating defects with facile low-temperature preparation process exhibits fascinating prospects. The outstanding ORR/OER properties of spinels can help catalyze the reactions of corresponding energy storage and conversion devices. In particular, SOFCs and MFCs cells have adopted spinels to accelerate the ORR process. The second type (rechargeable) of metal−air batteries such as Li−air and Zn−air apply spinels as catalysts to enhance the discharge capacity and narrow the discharge−charge gap both in aqueous electrolyte and in aprotic electrolyte. Furthermore, water splitting devices can also use spinels to catalyze water to produce clear hydrogen gas. Specially, applying solar energy to split water is a promising approach to convert solar energy to chemical energy. Good spinel catalysts can reduce the overall water splitting voltage and accelerate the water splitting rates. Recently, this area has seen enormous development. In addition, spinels with various compositions, valences and electron configurations have demonstrated intriguing magnetic, optical, electrical, and other catalytic characteristics. The spinels containing Fe, Co, Cr, or Ni usually display magnetic behavior, which can be widely used in information technology, biotechnology, and the electronic industry. The transparent or semitransparent spinels with optical properties have shown application aspects in laser devices and magneto-optical recording. Spinels are currently widely used in the energy storage field in supercapacitors and batteries. In addition to catalyzing the ORR/OER reactions, spinel catalysts have been used to facilitate toxic gas/liquid/solid redox reaction, CO2 reduction, fuel oxidation, and the HER. Through regulating the compositions, morphologies, valences, etc., these properties and applications have also been optimized. 8.2. Perspectives

The compositions, preparations, and applications of spinels have seen rapid development in recent decades (Table 11). Future work should continue to enrich the spinel family, developing new synthesis and characterization approaches and expanding the fields of application. 8.2.1. Applications. Considering the increasing energy and environmental issues, the applications of spinels should focus on the following points (Figure 57). First, using spinel compounds catalyzes the use of clean energy, partly or completely replacing noble metal catalysts. To this point, water splitting driven by effectively infinite solar energy provides a sustainable approach to produce H2. Therefore, developing highly efficient OER and HER spinel catalysts can facilitate this process. Moreover, the produced H2 can be further used in fuel cells. In that case, the spinel can be used to catalyze the ORR. Second, using spinel catalysts convert greenhouse gases (CO2 and NOx) and toxic gas 10188

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ACKNOWLEDGMENTS This work was supported by the National Programs for NanoKey Project (2016YFA0202500 and 2017YFA0206700), the National Natural Science Foundation of China (21231005 and 51231003), and the Ministry of Education (B12015 and IRT13R30).

parameters have been known to have crucial importance for catalytic activities, deep insights into the atomic level relationships such as the defects and active sites remain less investigated. Better knowledge of “structure−effect” relationships will guide the synthesis of spinels. In addition, most spinels are applicable in alkaline solutions; the stability of spinels in neutral and acidic systems remains challenging. A more comprehensive understanding of spinels will result in their sustainable growth in future daily life and industrial applications.

LIST OF ABBREVIATIONS ACAC acetylacetonate AFM atomic force microscrope ALD atomic layer deposition BCN boron carbon nitride BSCF Ba0.5Sr0.5Co0.8Fe0.2O3‑δ BV Butler−Volmer Cdl differential capacitance CE counter electrode CFP carbon fiber paper CFSE crystal field stable energy CLC chemical looping combustion CNTs carbon nanotubes CTAB cetyltrimethylammonium bromide CV cyclic voltammetry CVD chemical vapor deposition DFT density functional theory DMF dimethylformamide ECSA electrochemically active surface area ECL electrochemical luminescence EDS energy dispersive spectrometer EVs electrical vehicles FESEM field-emission scanning electron microscopy FTO fluorine-doped tin oxide GC glass carbon GCE glass carbon electrode GO graphene oxide HC coercivity HER hydrogen evolution reaction HEVs hybrid electrical vehicles HRTEM high revolution transmission electron microscopy ICP-OES inductively coupled plasma-optical emission spectrometry ITO indium tin oxide KB Ketjenblack K-L Koutecky−Levich LSV linear sweep voltammetry MFC microbial fuel cell MOFs metal−organic frameworks MS saturation magnetization MWCNTs multiwalled CNTs NC N-doped carbon NF Ni foam Ni3(HITP)2 Ni3(hexaiminotriphenylene)2 N-rGO N-doped reduced graphene oxide OER oxygen evolution reaction ORR oxygen reduction reaction Oh octahedral OSPE octahedral site preference energy PANI polyaniline PBAs Prussian blue analogues PBS phosphate buffer solution PEDOT poly(3,4-ethylenedioxythiophene) PEG poly(ethylene glycol) PL photoluminescence luminescence

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Jun Chen: 0000-0001-8604-9689 Author Contributions †

Q.Z., Z.Y., and C.C. contributed equally.

Notes

The authors declare no competing financial interest. Biographies Qing Zhao was born in Liaoning, China. He received his B.S. degree in Chemistry from Nankai University (2012). He then continued his Ph.D. study in Nankai Universty and joined the Key Laboratory of Advanced Energy Materials Chemistry under the supervision of Prof. Jun Chen. He received his Ph.D. in 2017 and now is a postdoc at Cornell University. His major is inorgnaic chemistry. His resarch interest focuses on the controlled preparation of micro/nanostructured inorgnic and hydrids material and their application in various energy stroage fields. Zhenhua Yan was born in Hunan, China. He received his M.S. degree in Chemistry and Chemical Engineering from Liaocheng University in 2014. As a Ph.D. candidate, he moved to Nankai University in 2015. His research focuses on the preparation of transition metal oxides and their applications as ORR/OER electrocatalysts in metal−air batteries. Chengcheng Chen was born in Hebei, China. He received his B.S. degree from North China Institute of Science and Technology in 2012. He joined Jun Chen’s group in Nankai University in 2014, and got his Ph.D. in June 2017. His research focuses on transition metal oxides and their applications as electrocatalysts and metal−air materials. Jun Chen obtained his B.Sc. and M.Sc. degrees from Nankai University in 1989 and 1992, respectively, and his Ph.D. from Wollongong University (Australia) in 1999. He held the NEDO fellowship at National Institute of AIST Kansai Center (Japan) from 1999 to 2002. He became a full Professor of Nankai University in 2002. He is a Cheung Kong Scholar Professor, Director of Key Laboratory of Advanced Energy Materials Chemistry (KLAEMC) (Ministry of Education) and Dean of College of Chemistry at Nankai University, Chairman of the Chinese Society of Electrochemistry, and a Fellow of the Royal Society of Chemistry. His research focuses mainly on nanomaterials chemistry and high-energy batteries. His achievements include the putting forward of “reduction oxidation conversion crystallization”, establishing the methods for structure and composition control of metal oxides/sulfides, and revealing the mechanism of energy storage and conversion of hydrogen, lithium, sodium, and magnesium. He has published 260 SCI journal papers with citations exceeding 20 000. With 10 edited books on energy chemistry and advanced batteries, he is also coinventors of 30 patents. He is an Associated Editor of Inorganic Chemistry Frontiers and Science China Materials and a member of the Editorial Board of ACS Energy Letters, ACS Sustainable Chemistry & Engineering, Nano Research, Solid State Sciences, and Journal of Energy Chemistry. 10189

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pulsed laser ablation pulsed laser deposition polypyrrole polyvinylpyrrolidone rotating disk electrodes reference electrode reversible hydrogen electrode rotating ring-disk electrode reduced graphene oxide reduced mildly oxidized graphene oxide selected area electron diffraction saturated calomel electrode scanning electron microscopy standard hydrogen electrode solid oxide fuel cells scanning transmission electron microscopy single-walled CNT Curie temperature tetrahedral 2,2,6,6-tetramethylhephtane-2,5-dionate turnover frequency X-ray photoelectron spectroscopy X-ray diffraction yttria-stabilized zirconia

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