Nonstoichiometric Oxides as Low-Cost and Highly-Efficient Oxygen

Review pubs.acs.org/CR

Nonstoichiometric Oxides as Low-Cost and Highly-Efficient Oxygen Reduction/Evolution Catalysts for Low-Temperature Electrochemical Devices Dengjie Chen,†,⊥,∇ Chi Chen,†,⊥ Zarah Medina Baiyee,† Zongping Shao,‡,§ and Francesco Ciucci*,†,∥

Chem. Rev. Downloaded from pubs.acs.org by UNIV OF WINDSOR on 09/14/15. For personal use only.

†

Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ‡ State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing 210009, China § Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845, Australia ∥ Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China 5.1. 5.2. 5.3. 5.4.

MnOx Oxygen Catalysts Ceria-Related Oxygen Catalysts NiOx-Based Oxygen Catalysts Delithiated LiCoO2−Related Oxygen Catalysts 5.5. Summary 6. Benchmarking Oxygen Catalysts 6.1. Overview of the State-of-the-Art Oxygen Catalysts 6.2. ORR/OER Bifunctional Oxygen Catalysts 7. Design Principles 7.1. Descriptors from Molecular Orbital Theory 7.2. Descriptors from DFT Calculations 8. Mechanistic Understanding 9. Summaries and Research Directions 9.1. Perovskite-Type Oxygen Catalysts 9.2. Pyrochlore-Type Oxygen Catalysts 9.3. MnOx, CeOx, and NiOx Related Oxygen Catalysts 9.4. Catalyst Support 9.5. Electrode Architectures 9.6. Solid Electrolytes/Separators/Protectors 9.7. System 9.8. Advanced Characterizations 9.9. Transferring Current Materials 9.10. Materials Design with Reliable Measurement 10. Conclusions Author Information Corresponding Author Present Address Author Contributions Notes Biographies Acknowledgments Abbreviations References

CONTENTS 1. Introduction 1.1. Background 1.2. Oxygen Catalysts 1.3. Nonstoichiometric Oxides 2. ABO3‑δ Perovskite-Type Catalysts 2.1. General Introduction 2.2. Ba0.5Sr0.5Co0.8Fe0.2O3−δ Oxygen Catalysts 2.3. Effects of Cations 2.3.1. Effects of A-Site Cations 2.3.2. Effects of B-Site Cations 2.3.3. Effects of A′-Site Cations 2.3.4. Effects of B′-Site Cations 2.4. Extrinsic Strategies for Enhancing Performance and Stability 2.4.1. Controlling Phase Structure and Surface Activities 2.4.2. Nanostructured Materials 2.4.3. Crystallographic Orientations 2.4.4. Amorphous Phases 2.4.5. Composites 2.5. Summary 3. Layered-Perovskite-Type Catalysts 3.1. Ruddlesden−Popper Phases 3.2. Double Perovskites 3.3. Summary 4. Pyrochlore-Type Catalysts 4.1. Structure and Properties 4.2. Summary 5. Other Typical Nonstoichiometric Oxides © XXXX American Chemical Society

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Received: February 4, 2015

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DOI: 10.1021/acs.chemrev.5b00073 Chem. Rev. XXXX, XXX, XXX−XXX

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and an electrolyzer function (2H2O → 2H2 + O2) where H2 is stored. The alkaline membrane technology can also be incorporated into the regenerative fuel cell, as shown in Figure 1a. In the electrolyzer mode, hydrogen evolution reaction and

1. INTRODUCTION


1.1. Background

Developing efficient energy conversion devices that are in line with the conservation of the natural environment is a critical challenge for modern society. Fossil fuels such as oil, natural gas, and coal are unsustainable, depleting rapidly, and they are the primary contributor to global climate change, noxious gas emissions, and environmental deterioration. Modern society is gradually but inevitably evolving from a fossil fuel to a clean energy based economy. This is demonstrated by the increasing development and deployment of clean-energy solutions such as wind and solar energy technologies. The full deployment of renewable energy sources is, however, limited by several challenges, most significantly the intermittent nature of the energy sources, which introduce load security risks when incorporated into energy grids. A viable solution to this challenge is the integration of advanced electrochemical energy storage and conversion systems, to secure the intermittent renewable energy supply. Furthermore, the development of electrochemical energy storage and conversion systems is also crucial for portable technologies and transport. The advancement of electrochemical devices is therefore central to developing an environmentally conscious and reliable energy future.1−11 Great scientific and engineering efforts have been devoted to enhancing the activity and stability of various electrochemical energy storage and conversion devices, most prominently lithium-ion batteries, supercapacitors, low-temperature fuel cells (LTFCs)11−13 and metal−air batteries (MABs),14 with the primary focus on lowering the cost and improving stabilities to make the technologies commercially viable. These technologies share several electrochemical similarities, and although energy storage and conversion mechanisms differ, there exists significant overlap with many new concepts and techniques applied across the field.7 Sir William Grove is generally credited with inventing the first fuel cell in 1839;13 later, the application was prominently established in space engineering. Electrochemical energy generation via the fuel cell technology is a promising alternative to the combustion device. This is primarily due to their potential high efficiency, that is not limited by the Carnot cycle, and to their reduced environmental impact due to clean fuel sources and byproducts. However, the commercialization of the technology has been stalled by several limitations, most prominently the cost and relatively high operating temperature (150−200 °C). Both of these limitations are associated with the high overpotentials required for the redox reactions at the fuel cell electrodes. The electrodes primarily work as charge-transfer components, with the electrochemical redox reactions taking place at the electrode surface, generally associated with H2 oxidation and O2 reduction. Various types of fuel cells have been developed with intrinsic mechanisms to facilitate lowtemperature operation, known as LTFCs.13 These include several novel solutions, such as the alkaline membrane fuel cell and regenerative fuel cells. The alkaline membrane fuel cell has the potential to utilize nonprecious catalysts and low-cost membranes, offering a more economically viable solution.15,16 On the other hand, regenerative fuel cells, integrated with renewable energy conversion devices (e.g., solar cells), are particularly attractive for grid energy systems.17 The dual functionality regenerative fuel cell typically comprises a fuel cell function (2H2 + O2 → 2H2O) where electricity is produced

Figure 1. Schematic illustrations of a typical alkaline membrane regenerative fuel cell (a) and a typical regenerative Li−air battery (b). Various anion exchange membranes and proton exchange membranes for low-temperature fuel cells can be found elsewhere for reference.12,27,28 More configurations of Li−air batteries and reactions for Li−O2 couples can be found elsewhere.24,26,29−32

oxygen evolution reaction (OER) occur, while, in the fuel cell mode, hydrogen oxidation reaction and oxygen reduction reaction (ORR) take place.17 The MABs were first invented in 1868 and further developed in 1932.18 The MABs technology features two characteristics that are borrowed from both traditional batteries (a pure metal as the anode/negative electrode) and fuel cells (ambient air as the reactant). The preference of employing the 4-electron redox of oxygen in MABs facilitates a much higher gravimetric energy density compared to the state-of-the-art Li-ion batteries. 19 Several compositions of MABs have been developed, such as Li−air/O2 batteries (LABs) and Zn-air/O2 batteries (ZABs), where the LABs have demonstrated the highest specific energy density among batteries (3505 Wh kg−1 with nonaqueous electrolyte and 3582 Wh kg−1 with aqueous electrolyte).20 The first nonaqueous (organic polymer electrolyte) rechargeable LAB was successfully assembled in 1996 by Abraham and Jiang.21 Further developed by the Bruce group in 2006, the technology demonstrated an attractive discharge capacity of ∼600 mA h g−1 after 50 cycles at a rate of 70 mA g−1.22 However, the nonaqueous LAB has significant limitations B



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taken to reduce noble metal loading in the noble-metal-based catalysts using alloy or carbon support. However, progressive reductions in noble metal loadings have generally been offset by a simultaneous increase in cost. Furthermore, a large proportion of noble-metal catalysts face additional challenges associated with the catalysis of both ORR and OER because very few exhibit a strong activity in both. As a result, high overpotentials are required for either ORR or OER in charge/ discharge or regenerative operations, which leads to a large overall voltage drop. Further challenges for noble-metal catalysts (e.g., Pt/C) include the following: the loss of electrochemically active surface area, which is mainly ascribed to the corrosion of carbon supports, Ostwald ripening, agglomeration of metal nanoparticles, oxidation of noble metal under OER conditions, and dissolution of oxidized species (chemical reduced) in acid solution; and the segregation and dissolution of the non-noble component of a Pt bimetallic catalyst in an acidic electrolyte, leading to challenges in stability.51−56 Therefore, to overcome these challenges and to potentially develop stable bifunctional catalysts, a variety of electrocatalytically active non-noblemetal catalysts have been studied for potential MABs and LTFCs applications.57 The most prominent catalysts include carbon based materials,58−76 transition-metal oxides and composites/hybrids,22,77−89 and transition-metal macrocycles.90−94 We note that several recent publications have provided excellent reviews on related work.20,24,29,95−105 Rechargeable MABs and regenerative LTFCs require that the oxygen-catalytic electrode is used for both ORR and OER processes, unlike nonregenerative systems where only ORR is required at the electrode. Therefore, the activity for oxygen evolution is an additional challenge for rechargeable MABs and regenerative LTFCs. For example, in LABs with nonaqueous aprotic solvent electrolytes, the electrochemical decomposition of the discharge product (e.g., Li2O2) requires a large anodic overpotential even at moderate current densities and very high discharge capacities. As the vast majority of battery applications require rechargeability, the bifunctionality of oxygen catalysts for both ORR and OER is essential in developing MABs. The same also applies to regenerative LTFCs. It is important to note that regenerative LTFCs typically operate at higher current densities (on the order of 102 to 103 mA cm−2)11,12,106 than rechargeable MABs (on the order of 10−1 to 101 mA cm−2).24,107,108 With higher operating current densities, the overpotentials of ORR/OER are expected to be a more prominent issue in LTFCs than in MABs. Nevertheless, the high overpotentials for both technologies remain a significant challenge. In this context, the majority of oxygen nonstoichiometric mixed metal oxides, e.g., mixed transition-metal oxides with perovskite structure, pyrochlore, or related structures, are of particular interest due to their intrinsic bifunctional activities toward both ORR and OER. Furthermore, the roundtrip efficiency (roundtrip efficiency (%) = (energydischarge/energycharge) × 100% = (Vdischarge/Vcharge) × 100%) of nonstoichiometric oxides, e.g., perovskite-type catalysts, is superior to that of noble-metal catalysts.109,110 In addition, electrodes with nonstoichiometric oxide catalysts are potentially more cost-effective and stable than commercial products (e.g., Pt/C electrodes), and they are suitable for alkaline solution applications. As a result, the research focus of oxygen catalysts for MABs and LTFCs has recently shifted toward nonstoichiometric perovskite oxides.

associated with the electrolyte and the battery discharge products (e.g., Li2O2, shown in Figure 1b). It was found that the LAB battery deteriorates with operation due to the nonsolubility of the discharge products in the nonaqueous electrolyte, which results in gradual product growth and clogging of the porous air electrode.23 To overcome potential problems (air electrode clogging, volume expansion, and reduced electrical conductivity) associated with nonaqueous electrolytes,24 aqueous systems, especially alkaline solutions, can be employed as alternatives. The discharge products, (2Li + 1 /2O2 + H2O → 2LiOH), are soluble to a large extent in an alkaline solution electrolyte.25 Furthermore, more cost-effective nonprecious catalysts, e.g., nonstoichiometric oxides, can be effectively employed in batteries with alkaline solution electrolytes. Over the past decade, the majority of research into MABs technology has focused on nonaqueous aprotic solvent electrolytes.26 The chemistry/electrochemistry at the air electrode differs for different electrolytes, yet breakthroughs in the aqueous system may also be applied to nonaqueous systems, as there is significant overlap. Furthermore, this is also true for the catalysts in this review, as although the discussion is focused on alkaline solution environments, the catalysts can, in principle, be applied to the nonaqueous, solid-state, and hybrid systems. 1.2. Oxygen Catalysts

High specific-energy-density MABs and high-efficiency LTFCs have not yet been successfully commercialized in a widespread and cost-effective way.33 There are several possible reasons for this, including high manufacturing cost, low efficiency, and low operation stability in practical applications. All of these challenges can be attributed to the component materials. Among all the components in MABs and LTFCs, the oxygen catalyst of the air electrode is of crucial importance because it is one of the main contributors to the overall system performance and cost. The development of the oxygen-catalytic material is therefore critical to bringing these technologies to a competitive commercial standing (Figure 2).23,34,35

Figure 2. Main issues for MABs and LTFCs.

Thus far, materials based on noble metals and their alloys have been widely employed as oxygen catalysts for ORR/OER in MABs and LTFCs due to their electrocatalytic activities.35−50 However, the high cost of noble-metal oxides, metals, and their alloys (e.g., IrO2, RuO2, Pt, Au, Pt−Au alloy) is a significant limitation to the large-scale commercialization of these technologies. Tremendous efforts have, therefore, been underC


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Figure 3. Possible crystallographic defects in nonstoichiometric oxides (a) and nonstoichiometric oxides presented in this review (b).

expected that nonstoichiometric oxides with perovskite, Ruddlesden−Popper, pyrochlore, or other structures will likely become leading oxygen catalysts for low-cost and environmentally benign electrochemical devices. In addition to collating key experimental observations from this field, activity trends and insights from theoretical studies are also reviewed. The two forefront theoretical tools, namely molecular orbital theory and density functional theory (DFT) calculations, have been widely applied to gain insight and understanding into the origin of the catalytic activity of nonstoichiometric oxides, and to predict activity trends and improve the rational design of new materials. In molecular orbital theory, the most prominent descriptors of catalytic activity are related to the t2g and eg orbital fillings of the transition-metal cation. The transitionmetal cations are the active sites of catalysis; therefore, the t2g and eg orbital fillings are rational descriptors, as these relate to the bond strength between the active sites and the reaction intermediates. Hence, a medium level of the orbital filling is indicative of neither a too strong nor a too weak bond, which is beneficial for the overall reaction process. Based on this, the more general conduction band filling parameter has emerged as descriptor for material activity. Besides, DFT calculations are also gaining increasing attention in catalyst design, due to the ability of DFT to reproduce experimental characteristics and the development of descriptors to predict activity trends. A common feature that has shown good reproducibility by DFT calculations is the band structure of electrocatalysts, which can elucidate the electronic conductivity mechanism. Furthermore, the adsorption energies of the reaction intermediates for ORR and OER may be calculated using DFT, and employed to predict material activity, usually comparatively for a series of materials. In general, trends and insights from DFT calculations provide atomic level understanding of the electrocatalytic and conductivity mechanisms, and are emerging as a useful tool for the rational design and development of better oxide materials. Both theories will be detailed in section 7. In the final section, a summary is provided to highlight future research directions. It is important to note that while the majority of the electrode materials are assessed in alkaline solution and evaluated by assembling them into LABs, the key findings are, in principle, applicable to other MABs, LTFCs, and solar water-splitting devices.

The application of nonstoichiometric perovskite oxides as oxygen catalysts in MABs and LTFCs is still relatively new and has been rarely demonstrated in practical systems. The first application of perovskite oxides as oxygen catalysts appeared in the 1970s.111 Later, an extensive book edited by Trasatti introduced metallic oxides as electrodes in 1980, in which Tamura et al. summarized the electrochemical properties of perovskite oxides available at the time.112 After a period of stagnation, a surge in publications on nonstoichiometric oxides (especially perovskite oxides) has appeared in the last 5 years. These materials have already demonstrated great potentials on the laboratory scale, including high roundtrip efficiencies, acceptable stabilities, and high intrinsic activities while retaining a very low cost. Furthermore, oxygen-deficient perovskites can both be employed as oxygen storage materials under oxygenrich conditions and serve as oxygen suppliers under oxygendepleted conditions. This feature could be applied in MABs and LTFCs to, for example, effectively alleviate the insufficient oxygen supply during discharge. However, further improvements of the activities and stabilities are required before application in industry.113 In light of this considerable research progress of nonstoichiometric oxides as oxygen catalysts for MABs and LTFCs applications,114−119 a comprehensive overview of the recent developments and challenges is beneficial. This work aims to provide researchers a timely point of reference for this field, offering details on the design of nonstoichiometric oxides as oxygen catalysts and on related mechanistic understanding. This review highlights promising families of nonstoichiometric oxides: (i) perovskite oxides, with a general formula ABO3−δ, where A = rare-earth or alkaline-earth ion, e.g., Ca, Sr, Ba, or La, and B = transition-metal ion, e.g., Cr, Mn, Fe, Co, or Ni; (ii) the Ruddlesden−Popper series, with a general formula An+1BnO3n+1+δ (n ≥ 1), where A = rare-earth or alkaline-earth ion, e.g., La or Sr, and B = transition-metal ion, e.g., Ni or Fe; (iii) double perovskites AA′B2O5+δ, where A = rare-earth ion, A′ = alkaline-earth ion, and B = transition-metal ion, e.g., Co or Mn; or with the general formula of A2BB′O5+δ, where A = alkaline-earth ion, and B/B′ are transition-metal ions; (iv) pyrochlore oxides A2B2O6O′1−δ, where A is typically a rareearth ion or an element of Pb, Ti, or Bi, and B is typically a transition/post-transition-metal ion, e.g., Ru, Ir, or Sn; (v) other typical nonstoichiometric transition-metal oxides. Emphasis is placed on the recently developed ABO3−δ perovskitetype oxygen catalysts with improved ORR/OER activities and stabilities. Benefiting from their remarkable electrochemical activities and operation stabilities in alkaline solution, it is

1.3. Nonstoichiometric Oxides

In metal oxides, the defect-induced nonstoichiometry determines the electrochemical activities. Previous studies by Bockris and Otagawa associated the electrocatalytic activity with the ability of lattice oxygen loss and/or the formation of oxygen D


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vacancies.120 Furthermore, Trasatti noted that nonstoichiometry in oxides is the main factor affecting the electrocatalytic activity.121 The defect-induced disorders of nonstoichiometric oxides may take several forms, including vacancy (Schottky defect), interstitial, vacancy-interstitial pair (Frenkel defect), and substitutional defects with different-sized atoms (Figure 3a). Starting from the general formula ABO3 (not limited to the perovskite structure), A-site and O-site deficiencies or excesses can result in many variations of nonstoichiometric oxides, as shown in Figure 3b. This figure summarizes all the potential nonstoichiometric oxides involved in this review, e.g., oxygen deficient perovskite-like oxides, Ruddlesden−Popper series, pyrochlore-type oxides, etc., where the typical compositions are highlighted in the corresponding region. In keeping with the format of the original work, some materials with a nonstoichiometry may be presented here with a stoichiometric formula; for example, La2NiO4+δ may be written as La2NiO4.

devices leverage the oxygen vacancies or interstitial defects for oxygen transport and incorporation. Furthermore, the vacancies also provide oxygen storage capacity for oxygen sensor and storage applications. Perovskites can be engineered to fit a wide range of applications when more complex compositions are developed. Meanwhile, it has been suggested that oxygen nonstoichiometric perovskite oxides with the reversible oxygen release and uptake nature are also excellent bifunctional catalysts enabling high roundtrip efficiency in the MABs/LTFCs. This finding sparked a renaissance in the perovskite research. The application of perovskite materials as oxygen catalysts was demonstrated in the early 1970s by Meadowcroft, who studied LaCoO3 in alkaline solution.111 It should be noted that the majority of the reviewed perovskite materials are generally suited to alkaline environments and are prone to degradation in acidic environments, whereas noble-metal catalysts are suited to acidic environments. Perovskites have several key properties which facilitate their good bifunctional electrocatalytic activity for ORR/OER: flexibility in the oxidation states of transition metals leading to the formation of redox couples (e.g., Bm+/ Bm+1), defective structures for oxygen vacancy or excess, excellent oxygen anion mobility, and excellent oxygen exchange kinetics. Recent research has identified correlations (descriptors) between the electronic band structure and the electrochemical catalytic activities.113,115,116 These correlations have stimulated further interest in perovskite catalysts

2. ABO3‑δ PEROVSKITE-TYPE CATALYSTS 2.1. General Introduction

Perovskites were discovered by mineralogist Gustav Rose in 1839 and later named after the Russian mineralogist Lew A. Perowski.122 Subsequently, perovskite became the nomenclature for the series of compounds that have the same crystal structure as CaTiO3, which can be written in a general formula ABO 3 (Figure 4). Although many compounds, e.g.,

2.2. Ba0.5Sr0.5Co0.8Fe0.2O3−δ Oxygen Catalysts

Recently, a number of perovskites developed as cathode materials for SOFCs have also been successfully redeveloped as bifunctional catalysts in MABs.110,131−135 Among these perovskites, a mixed electronic and ionic conductor, Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ , has received notable focus. Ba0.5Sr0.5Co0.8Fe0.2O3−δ is well-known for its low area-specific resistances as a cathode material in intermediate-temperature SOFCs136 and high oxygen fluxes in oxygen transport membranes (OTMs),137 signifying its high oxygen catalytic activities. The high activities have been associated with its inherent capability to accommodate a high concentration of oxygen vacancies and to retain high anion mobility and oxygen exchange kinetics.137−140 However, significant challenges have also been identified for the application of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ in SOFCs and OTMs. While Ba 0.5Sr0.5Co0.8Fe0.2O 3−δ has a high catalytic activity, it deteriorates due to carbonate formation and lattice instability, which may lead to performance degradation. Therefore, the operation conditions should be optimized simultaneously for both stability and catalytic activity. In addition, the instability may be alleviated by doping/substituting high-valence and redox-inactive cations (e.g., Zr4+) on the B site, or by partially replacing Ba2+ with other large and polarizable cations, e.g., Bi3+. So far, Ba0.5Sr0.5Co0.8Fe0.2O3−δ has been extensively investigated in high-temperature devices, and several comprehensive reviews are available on this topic.141,142 While Ba0.5Sr0.5Co0.8Fe0.2O3−δ has been studied as a catalyst in hightemperature devices, only a few works have directly investigated this material as an oxygen catalyst for MABs and LTFCs.116,131,143−145 The general research trend of electrochemical technologies is moving toward low-temperature systems such as MABs and LTFCs due to the relatively mild experimental conditions and high commercialization potential.

Figure 4. Crystal structure of a cubic perovskite oxide with a chemical formula of ABO3.

CH3NH3PbI3‑xClx,123 have a perovskite structure, this review will focus only on oxides and their applications in MABs and LTFCs. In an ideal cubic-symmetry structure, the A site is occupied by 12-fold coordinated cations with a large ionic radius, such as Ca, Sr, Ba or Ln (rare-earth element), and the B site is occupied by small transition-metal elements in 6-fold coordination to the oxygen anions. The elements in perovskite structures can be chosen in a flexible manner, from various types and with different concentrations. By partial substitution of cations at A and B sites, a variety of perovskite compounds can be obtained with the formula A1−xA′xB1−yB′yO3−δ. According to the Goldschmidt’s tolerance factor rule,124 around 90% of the metallic natural elements in the periodic table are stable in the perovskite structure.125 While perovskite oxides have a relatively simple structure, they have a multitude of physical, chemical, optical, and electronic properties. Some exhibit interesting properties, such as superconductivity,126 ferroelectricity,127 piezoelectricity,128 thermoelectricity,129 and (electro)catalysis.125,130 Many oxygen nonstoichiometric perovskite oxides are commonly used in high-temperature electrochemical devices, e.g., solid oxide fuel cells (SOFCs) and membranes. Oxygen nonstoichiometry occurs when the total charge of cations is below or above +6, leading to the creation of oxygen vacancies or interstitial oxygen in the lattice according to the electroneutrality in the bulk. High-temperature E



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Figure 5. (a) OER activity (potentials @ 50 μA cm−2ox) of some perovskite oxides represented by a volcano-type relationship as a function of egfilling of transition-metal B ions in ABO3. (b) Comparison of the OER specific activity between Ba0.5Sr0.5Co0.8Fe0.2O3−δ and IrO2 in O2-saturated 0.1 M KOH. Reprinted with permission from ref 116. Copyright 2011 American Association for the Advancement of Science. (c) ORR activity (Potentials @ 25 μA cm−2ox) as a function of eg-filling of transition-metal B ions in ABO3. Reprinted with permission from ref 115. Copyright 2011 Nature Publishing Group.

number of electrons involved in the ORR process to be ∼2.9 (Koutecky−Levich analysis), and between ∼2.8 and 3 (RRDE measurements). This indicated the parallel formation of OH− (4-electron transfer) and HO2− (2-electron transfer) species.144 The relatively poor performance of the pure Ba0.5Sr0.5Co0.8Fe0.2O3−δ electrode may be linked to the intrinsically low specific surface area (10 m2 g−1) of the material,144 a result of high-temperature calcination during synthesis.146 Moreover, Ba0.5Sr0.5Co0.8Fe0.2O3−δ itself has relatively low electronic conductivity.147 These results suggest that pristine Ba0.5Sr0.5Co0.8Fe0.2O3−δ alone is unsuitable for lowtemperature applications. It has been reported that Co3O4 nanocrystals deposited on the N-doped graphene exhibit unexpectedly high ORR/OER activities, in contrast to the limited electrocatalytic activity of the independent Co3O4 or graphene oxide.84 The unusual ORR/OER activities may be attributed to the synergistic effects between the two materials.84,148,149 Similarly, Ba0.5Sr0.5Co0.8Fe0.2O3−δ composites have also been studied to potentially improve the electrocatalytic activities via optimizing the electron transfer path, increasing the specific surface area, and introducing synergistic effects.116,131,143,150 A mixture of Ba0.5Sr0.5Co0.8Fe0.2O3−δ and conductive carbon was investigated to explore the possible beneficial interactions between the two materials. Not surprisingly, the addition of carbon led to a more positive onset potential for ORR, a higher ORR current, a reduced hydroperoxide concentration, and an

An overview of Ba0.5Sr0.5Co0.8Fe0.2O3−δ for low temperature applications is presented below. In 2011, Suntivich et al. demonstrated a volcano-like trend for OER activity versus eg orbital filling.116 This study systematically evaluated more than 10 perovskite materials, where Ba0.5Sr0.5Co0.8Fe0.2O3−δ, with an eg filling near 1, occupied the peak (Figure 5a). Furthermore, the authors observed that Ba0.5Sr0.5Co0.8Fe0.2O3−δ possessed high intrinsic OER activity (oxide surface area normalized current density) which was measured to be at least 1 order of magnitude higher than that of the state-of-the-art catalyst (IrO2 nanoparticles, ∼6 nm) in alkaline solution at the same potential (Figure 5b).116 The success of the eg orbital filling descriptor to predict OER activity is associated with the Sabatier principle. That is, the optimum bonding between catalysts and intermediates will maximize catalytic activity. Since the eg orbital is antibonding, a high eg occupation corresponds to a weak bonding, and a low occupation corresponds to a strong bonding. Therefore, an eg filling of approximately 1 produces the highest activity, as observed for Ba0.5Sr0.5Co0.8Fe0.2O3−δ. Later, the ORR activity and the reaction mechanism of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (without carbon support) were evaluated by Fabbri et al. in alkaline solution.144 Unfortunately, a large ORR overpotential and a large amount of hydroperoxide generation were observed. This finding suggests that ORR on pure Ba0.5Sr0.5Co0.8Fe0.2O3−δ does not follow the efficient 4electron transfer process. Parallel characterizations found the F



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Figure 6. (a) Illustration of the bifunctional catalyst: La0.3(Ba0.5Sr0.5)0.7Co0.8Fe0.2O3−δ (La0.3-5582), where the rhombohedral phase LaCoO3−δ grains are segregated on the surface of cubic Ba0.5Sr0.5Co0.8Fe0.2O3−δ based grains. (b and c) ORR (b) and OER activities (c) of 80 wt % La0.3-5582 on 20 wt % Ketjen Black (KB) composite and other typical catalysts for comparison. Reprinted with permission from ref 151. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Ba0.5Sr0.5Co0.8Fe0.2O3−δ: BSCF5582.

the carbon, and also a ligand effect. These findings may be extended to other similar oxygen catalysts characterized by high intrinsic activities to yield high specific activities. In addition, these findings also suggest that highly active oxygen catalysts may also be further improved by employing other conductive phases, such as carbon fibers, graphene sheets, carbon nanotubes, or even polymers. Recently, a facile approach, using tetraethoxysilane as a template, was proposed for the in situ synthesis of porous Ba0.5Sr0.5Co0.8Fe0.2O3−δ with specific surface areas of up to 32.1 m2 g−1, the highest reported specific surface area to date.150 The application of porous Ba0.5Sr0.5Co0.8Fe0.2O3−δ exhibited an OER mass activity up to 35.2 A g−1 at 1.63 V vs reversible hydrogen electrode (RHE). Similarly, the overpotential for OER of ball-milled Ba0.5Sr0.5Co0.8Fe0.2O3−δ was shown to be nearly identical to that of the state-of-the-art IrO2 at the same mass-normalized catalytic activity (10 A g−1), which is attributed to the increased surface area (3.9 m2 g−1).116 The bifunctional ORR/OER activities of Ba0.5Sr0.5Co0.8Fe0.2O3−δ were recently further improved by either cation doping or mixing. Jung et al. reported a novel structure based on La-doped Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ (La0.3(Ba0.5Sr0.5)0.7Co0.8Fe0.2O3−δ) (Figure 6a).151 In this structure the oxygen catalyst consists of rhombohedral LaCoO3 nanoparticles (∼10 nm) distributed on the surface of cubic Ba0.5Sr0.5Co0.8Fe0.2O3−δ grains. The resulting perovskite catalysts, which feature an increased surface area, exhibit ORR/ OER activities comparable to/better than those of the state-ofthe-art catalysts, such as RuO2 for ORR (Figure 6b) and IrO2 for OER (Figure 6c).151 This finding is surprising, as it had previously been reported that LaCoO3 has relatively poor activities when compared to Ba0.5Sr0.5Co0.8Fe0.2O3−δ; however, it must be noted that the morphology of LaCoO3 was different in their study.152 The Shao group suggested a universal and facile approach for the development of efficient bifunctional catalysts for both ORR and OER in alkaline solution.153 Using Ba0.5Sr0.5Co0.8Fe0.2O3−δ as an example, it was found that a

increased electron transfer (∼3.5) compared to the independent Ba0.5Sr0.5Co0.8Fe0.2O3−δ and acetylene black electrodes.143 Generally, the improved overall 4-electron transfer of perovskite + carbon composites can be attributed to the combined ability of carbon to initially catalyze the 2-electron O2 reduction to HO2− and perovskite oxides toward hydroperoxide electrochemical reduction to OH− and/or chemical decomposition of the hydroperoxide species to produce O2 and OH−. However, in the case of Ba0.5Sr0.5Co0.8Fe0.2O3−δ + acetylene black composites, Fabbri et al. excluded the occurrence of the sequential reaction steps (first reduction on acetylene black to HO2− and then HO2− to OH− on Ba0.5Sr0.5Co0.8Fe0.2O3−δ) for the improved activity. Although the HO2− disproportionation reaction catalyzed by Ba0.5Sr0.5Co0.8Fe0.2O3−δ was observed, the measured rate for this reaction was quite low. Instead, the electronic interaction between acetylene black and Ba0.5Sr0.5Co0.8Fe0.2O3−δ was suggested as the driver of the improved activity.143 Jin et al. also suggested a 4-electron transfer pathway for ORR in the Ba0.5Sr0.5Co0.8Fe0.2O3−δ + carbon composite, supported by an experimental observation of 3.5 electron transfer.131 The maximum diffusion-limiting current density of Ba0.5Sr0.5Co0.8Fe0.2O3−δ + carbon is close to that of the commercial Pt/C catalyst under the same testing conditions, which is higher than that of the urchin-like La0.8Sr0.2MnO3132 and Ba0.9Co0.5Fe0.4Nb0.1O3−δ133 catalysts. This behavior further supports that the electrocatalytic activity of Ba0.5Sr0.5Co0.8Fe0.2O3−δ + carbon is boosted by the synergistic interactions between BSCF and carbon.131,143 Furthermore, for OER activity, it is observed that the anodic current density of the Ba0.5Sr0.5Co0.8Fe0.2O3−δ + carbon electrode based on the disk geometric surface area was reported to be as high as 30.25 mA cm−2 at 2500 rpm and 1.0 V vs Ag/ AgCl in O2 saturated 0.1 M KOH solution, which is much higher than the case for an individual carbon electrode.131 The outstanding OER and ORR activities of the composite may be attributed to the high surface oxygen vacancy concentration for the Ba0.5Sr0.5Co0.8Fe0.2O3−δ, the relatively high conductivity of G


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La0.58Sr0.4FeO3 vs La0.6Sr0.4FeO3). Although a universal descriptor for specific dopants and dopant levels is difficult to derive, this work demonstrated that the presence of specific cations (e.g., La and Co) and their nonstoichiometry were strongly related to the OER activity. Although it is quite challenging to draw holistic conclusions to sort the best perovskite-type oxides as oxygen catalysts, it is still useful to summarize the possible effects of cation selections at length (species and doping levels), in order to deduce rational design principles. 2.3.1. Effects of A-Site Cations. The effects of the A-site cations on the ORR/OER activities of perovskite catalysts have been investigated in alkaline solution. The effects may be attributed to the differences in crystal structures, ionic radii/ binding energies, and electronic conductivities.155−157 For example, the ORR activities of the LnMnO3 (Ln = La, Pr, Nd, Sm, Gd, Dy, Yb, or Y) series were systematically assessed in an 8 M alkaline solution with similar surface areas of all oxides.155 The cathodic polarization results indicated that the ORR activities were highly dependent on the A-site element; specifically, the activities decreased with the decrease in the ionic radius of the A-site cation, Ln3+ (in decreasing order of La > Pr > Nd > Sm > Gd > Y > Dy > Yb), where LaMnO3−δ exhibited the highest current density. This trend may be explained by the change of the crystal structure, resulting from the adoption of various A-site cations. In perovskite oxides, both the tolerance factor and symmetry increase with the A-site cation radius. Therefore, La3+, with the largest ionic radius in the series, tends to form the highly symmetric cubic or rhombohedral phases, while orthorhombic or tetragonal phases are stabilized when the ionic radius of the A-site dopant is smaller.158 Indeed, the La3+ cation occupying the A site was characterized as a rhombohedral structure, and an orthorhombic structure was found for the Pr and Nd in the A site, a GdFeO3-type orthorhombic structure for Sm, Gd, and Dy, and a hexagonal structure for Yb and Y.155 To mitigate the structural influence of the A-site cation on the ORR/OER and stability, deposited thin films may be employed to control the phase structures. A-site cation substitution may also drive the rotation and distortion of the BO6 octahedra, and therefore also indirectly influence the ORR/OER activity. For example, if Mn takes the B site, the Mn−O bond length and the bending degree of Mn−O−Mn can be tuned by varying the A-site rareearth element. This modifies the catalytic activities, since ORR/ OER reactions are suggested to occur on the transition-metal ion (B-site) in perovskites.159,160 In addition to the indirect structural influence on the BO6 octahedra, the electronic conductivity is also affected by A-site cations. More specifically, the 4f electrons of the A-site cations contribute to the density of states around the Fermi level, and these states degenerate substantially. As a result, the 4f electrons are suggested to limit the electronic conductivity. As La3+ has no 4f electrons, the conductivity is expected to be high, which is consistent with above suggestions. However, while Y3+ also does not have 4f electrons, its ionic radius is small, which causes a large distortion of the electron conduction path, limiting the electronic conductivity.158 Indeed, it has been found that the general activity trend for these A-site cations is consistent with results for the resistivity of the same cation substitutions in LnxSr1−xCoO3−δ.157 This work showed that the resistivities varied by several orders of magnitude, with Ln3+ ranging from La, Pr, Nd, Sm, Gd, to Dy in LnxSr1−xCoO3−δ.157

composite material of Pt/C-Ba0.5Sr0.5Co0.8Fe0.2O3−δ, fabricated by simple ultrasonic mixing, showed better ORR activity than an independent Pt/C catalyst and better OER activity than an independent Ba 0.5Sr0.5Co0.8Fe0.2 O3−δ oxide. The Pt/C− Ba0.5Sr0.5Co0.8Fe0.2O3−δ (1:1) composite was characterized to be highly active toward both ORR and OER with a bifunctional performance of ΔE = 0.80 V (1.59 V @ 10 mA cm−2 and 0.79 V @ −3 mA cm−2). The above outlines the recent development and current challenges for the promising perovskite-type Ba0.5Sr0.5Co0.8Fe0.2O3−δ oxide as a catalyst for ORR/OER. In the following sections, detailed descriptions of several factors determining ORR/OER activities in various perovskite-type oxides are reviewed.


2.3. Effects of Cations

The structure and electrocatalytic activities of perovskite oxides can vary significantly with different A- or B-site cations. Furthermore, the electrocatalytic activity may also be significantly adjusted by the partial substitution of A- or Bsite cations. Considerable work on ORR/OER has been undertaken with many compositions, which can be generally described by the formula A1−xA′xB1−yB′yO3−δ. A schematic of the perovskite-type oxides with various possible compositions is shown in Figure 7.

Figure 7. Various possible doping strategies for ABO3 perovskite-type oxides.

The most common methods, aliovalent doping on the A site and/or easily reduced transitional metal doping on the B site, are employed to increase the ORR/OER electrocatalytic activities. These strategies generally aim at increasing the oxygen vacancy concentration and/or maintaining the redox couples. Although the stability and activity of oxygen catalysts can also be tuned via the perovskite composition, this typically results in a compromise between the two. The effects of A, A′, B, and B′ cations on the electrocatalytic activities and relevant properties are discussed in detail in the following sections. Very recently, a comparative investigation of perovskite oxides with 14 compositions was reported by the Schuhmann group.154 A descending order of OER activity was observed for the investigated compositions: La0.58Sr0.4Co0.2Fe0.8O3 > La0.76Sr0.2Co0.2Fe0.8O3 > La0.83Ca0.15Mn0.6Co0.4O3 > La0.6Sr0.4FeO3 > La 0.74 Sr 0.2 Co 0.2 Fe 0.8 O 3 > La 0.58 Sr 0.4 Co 0.2 Cu 0.1 Fe 0.7 O 3 > La0.75Sr0.2Mn0.9Co0.1O3 > La0.97Mn0.4Co0.3Cu0.3O3, while Pr0.65Sr0.3MnO3, La0.78Sr0.2Co0.2Fe0.8O3, La0.7Sr0.25Mn0.5Cr0.5O3, La0.78Sr0.2FeO3, La0.58Sr0.4FeO3, and La0.65Sr0.3MnO3 showed negligible catalytic activity toward OER. The results confirm the sensitivity of electrocatalytic activity to different cations (e.g., Co vs Cr) or even minor variations in stoichiometry (e.g., H



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Figure 8. ORR activities of (a) LaNiO3, LaCoO3, LaFeO3, LaMnO3, and LaCrO3 and (b) LaNi0.5Co0.5O3, LaNi0.5Fe0.5O3, LaNi0.5Mn0.5O3 and LaNi0.5Cr0.5O3. Reprinted with permission from ref 171. Copyright 2012 American Chemical Society.

influence the electrocatalytic activities of perovskite-type oxides, since the B site is generally regarded as the active site. The B site substituted perovskite series LaMO3 (M = Cr, Mn, Fe, Co, and Ni) have been widely studied, and they are generally characterized with high thermal stability, high oxygen mobility, and a large amount of redox couples. The last two factors may be the main contributors to the high activities of LaMO3. Recently, Sunarso et al. investigated the ORR activities of LaMO3 (M = Cr, Mn, Fe, Co, and Ni) perovskite oxides in alkaline solution by rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques.171 The results found a dependency on B-site cation for the ORR activities, with LaCoO3 exhibiting the largest diffusion-limited ORR current density, followed by LaMnO3, LaNiO3, LaFeO3, and LaCrO3 in descending order (Figure 8a). Furthermore, LaCoO3 also displayed the most positive ORR onset potential, yet this trend was not observed across the rest of perovskite oxides. Very recently, the electrocatalytic activities of LaMO3 (M = Mn, Fe, and Co) perovskite nanoparticles for both ORR and OER were studied by Zhu et al.172 In contrast, this work revealed LaMnO3 as the best catalyst for both ORR and OER (measured using the half-cell design), followed by LaCoO3, LaFeO3, and carbon black. However, the LaCoO3 perovskite electrode exhibited the best rechargeability and stability, while the LaMnO3 perovskite electrode degraded very quickly in ZABs.172 More recently, Hardin et al. developed a colloidal approach to yield phase pure perovskite oxides of the LaBO3 (B = Ni, Co, and Mn) series with comparable morphology and surface area.173 This study also identified Co substitution to be optimal, and found that LaCoO3 with N-doped carbon as support displayed the lowest total overpotential between the OER and ORR (1.64 V@ 10 mA cm−2 and 0.64 V@ −3 mA cm−2).173 Furthermore, their ORR activities for the same perovskite oxides with graphene as support (LaNiO3 > LaCoO3 ∼ LaMnO3) correlated well with the oxygen vacancy formation energies.174 A similar study by Wang et al. comparatively estimated the ORR mechanism on LaBO3 (B = Fe, Mn, and Cr) surfaces by first-principle calculations based on DFT using a hybrid-functional method.159 Here, a theoretical ORR overpotential was computed from free energy diagrams, and based on this, the ORR activity for this series followed a decreasing order: LaMnO3 > LaCrO3 > LaFeO3.159

Further electrocatalytic correlation with the A-site cation substitution has also been found for the oxygen nonstoichiometric CaMnO3−δ substituted with Ln (Ln = La, Nd, Sm, Gd, Y, and Ho) in alkaline solution.161−166 Again, La3+ substitution in La2x/3Ca1−xMnO3−δ demonstrated a relatively high electronic conductivity at room temperature, even with 50 to 60% porosity. The high electronic conductivity of these materials, without conductive additives, is attributed to the existence of oxygen nonstoichiometry.161,162 Among the La2x/3Ca1−xMnO3−δ series, the material with the substitution level of x = 0.1 showed a promising discharge capacity in 5% LiOH solution, which was comparable to that of MnO2 and graphite catalysts.161 In addition, an increase in current density achieved for the HoxCa1−xMnO3−δ catalysts, compared to CaMnO3, was attributed to the smaller particle size and the higher electrical conductivity.163 Similarly, the influence of A-site cations on the ORR activity was also observed in high-temperature SOFCs with solid electrolytes, yet the activity trend is different from that of the low-temperature case.167−169 Chen et al. comparatively investigated the effect of Ln3+ in LnBaFe2O5+δ (Ln = Lanthanides or Y) on the electrochemical performance for SOFCs. 167 This work employed the O 2 temperatureprogrammed desorption technique, and it was found that the deintercalation of oxygen was typically associated with the reduction of Fe4+ to Fe3+. Furthermore, as previously identified, this process was also strongly affected by the ionic radius of the A-site cation. It was found that the onset temperature increased monotonically from Ln = La to Gd, with the size of the Ln3+ cation decreasing and the bonding energy increasing.167 These results indicate that the A-site cations within perovskites play a critical role in determining the ORR activities, as the A-O/B−O interactions may shift the redox energies of the B-site cations,170 consistently with the observations outlined above. In summary, the electrocatalytic activities of perovskite catalysts vary with the A-site cations because of the differences in crystal structures, ionic radii/binding energies, and electronic conductivities. Generally, perovskite-type oxides with a high symmetry in crystal structure and a high electronic conductivity are reported to have relatively high electrocatalytic activities in alkaline solution. 2.3.2. Effects of B-Site Cations. In addition to A-site cations, B-site cations have also been shown to strongly I



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partial substitution of A-site cations. A-site partial substitution with low-valence metal ions generates additional oxygen vacancies and shifts a large proportion of the B-site transition-metal ions to unstable oxidation states (B3+/B4+ redox couple). This may result in enhanced oxygen mobility and kinetics, thereby increasing the electrocatalytic activities of perovskite-type oxides.181,182 For example, considering the typical perovskite oxide catalyst LaCoO3, doping the La site with Sr produces two typical defects, as represented in the following defect equations:183

Ca-doped LaMO3 (M = Cr, Mn, Fe, Co, and Ni) perovskite oxides have also received considerable attention due to their enhanced electrocatalytic activities, although the relationship between the material compositions and the electrocatalytic activity remains unclear. Among these, Ca-doped LaCoO3 has been extensively investigated and demonstrated to be one of the most promising bifunctional catalysts to replace noble metals in alkaline solutions. Recently, ORR activities of La0.6Ca0.4CoO3 and La0.6Ca0.4CoO3-carbon composite were investigated by the RRDE method; an ∼4 overall electron transfer number was observed for both materials in 1, 4, and 6 M KOH electrolytes.175 Further work has suggested that the enhanced ORR activities of La0.6Ca0.4CoO3−carbon composites may be ascribed to a synergistic effect (2 + 2-electron transfer pathway).175 The stability of this perovskite oxide was earlier investigated against Pr0.6Ca0.4MnO3 in alkaline solution. In this work, it was found that the La 0.6Ca 0.4 CoO3 structure decomposed largely into La(OH)3, and the performance showed an obvious decay with operation time, in contrast to the Pr0.6Ca0.4MnO3, which remained relatively stable.176 Other work also found that the stabilities for similar perovskites were influenced by B-site cation substitution and followed a descending order, La 0.1Ca 0.9 MnO 3 > La 0.6Ca 0.4 CoO 3 > LaNiO3.177 Given this trend and the stability investigations, it can be deduced that Co-free and Mn-based perovskite catalysts may be promising for applications as bifunctional electrodes in alkaline solution. However, for ZAB applications, the La0.6Ca0.4CoO3 perovskite demonstrated a good stability, with only minor structural changes for at least 1300 h. This was characterized by ex situ and in situ X-ray absorption spectroscopy and X-ray diffraction.178 Müller et al. demonstrated an even longer stability (3000 h) for La0.6Ca0.4CoO3 when combined with graphitized Vulcan XC 72.179 In addition, it has also been observed that the La0.8Sr0.2MnO3 material showed a low discharge capacity on the first cycle and experienced a rapid capacity fading among other investigated catalysts.34 These conflicting results may be attributed to the influence of catalyst morphology during preparation and operation and to the measurement methods employed. It should be noted that the trade-off between electrocatalytic activity and stability seems to be ubiquitous. To demonstrate the significant roles of the surface electronic structure, such as oxidation and spin state, on the OER activity and stability, the coordination of transition-metal ions within the perovskite-type and related layered structures was studied by the Shao-Horn group.180 This study found that while Mn3+ (LaMnO3+δ) in corner-sharing octahedra was the least active catalyst, Mn4+ (e.g., CaMnO3 and Ba6Mn5O16) showed slightly increased OER activities and Co-based perovskites exhibited the highest OER activity. This was attributed to the spin configuration and oxidation state of the Co ions, enabling easy adjustment for catalytic reactions.180 In summary, ORR/OER activities and stabilities vary with the transition metal. Generally, Co-based and Mn-based perovskite-type catalysts have the highest ORR/OER activities, while Fe-based and Cr-based materials are characterized by relatively low activities in alkaline solution. 2.3.3. Effects of A′-Site Cations. While A-site cations of perovskite oxides typically do not contribute to the electronic structure near the Fermi level, they may affect the B site or the crystal structure, thereby influencing the electrocatalytic activities. Given the significance of the A site, the electrocatalytic activities of perovskite oxides have been explored by

× 2SrO + 2La ×La + OO ⇔ 2Sr′La + V •• O + La 2O3(surface)

(1) × 2SrO + 2La ×La + 2CoCo +

1 O2 2

⇔ 2Sr′La + 2Co•Co + La 2O3(surface)

(2)

where Kröger−Vink notation is used, i.e. LaLa× (trivalent lanthanum ion located on a lanthanum site), Sr′La (divalent × strontium ion located on a lanthanum site), CoCo (trivalent • cobalt cation located on a cobalt site), CoCo (tetravalent cobalt cation located on a cobalt site), OO× (divalent oxygen anion located on an oxygen site), and V•• O (oxygen vacancy located on an oxygen site). With Sr doping, the oxygen vacancy concentration increases, as shown in eq 1, which leads to improved oxygen adsorption. Furthermore, as indicated in eq 2 an additional charge compensation mechanism may lead to the generation of holes, resulting in an increase in the electronic conductivity of the material. In addition to the defect mechanism, the addition of aliovalent element to the lattice also alters the Madelung energy and the oxide formation energy, which may lead to weaker oxygen bonds and thereby lower oxygen vacancy formation energy. This example demonstrates that rational doping with aliovalent elements to the A site can also lead to better overall performance. Alkaline-earth elements are normally selected as doping ions because of the close ionic radii to the lanthanides and their thermodynamic stability during high-temperature materials preparation. The ORR activities of Pr0.8A′0.2MnO3 with the substitution of the A′ site (Ca, Sr, or Ba) were systematically evaluated, among which Pr0.8Ca0.2MnO3 showed the highest current densities, closely followed by Pr0.8Sr0.2MnO3, whereas Ba substitution showed the worse performance, even lower than the undoped PrMnO3.176 In relation to the OER activities, Sr substitution showed better performance toward oxygen evolution than that of Ca substituted materials.176 The ORR/ OER activities of La0.6A′0.4Fe0.8Mn0.2O3 (A′ = Ca, Sr, Ba, and La) and LaxA′1−xCoO3 (A′ = Ca, Sr, and Ba) shared similar trends as Pr0.8A′0.2MnO3.101,157 Several families of perovskite compounds, e.g., La1−xSrxCoO3,184 Sm1−xSrxCoO3,185 and La1−xSrxMnO3,186 have also been extensively evaluated. For example, La1−xSrxMnO3 perovskites are more active than undoped LaMnO3 toward ORR in an alkaline 1 M KOH electrolyte.186 However, it should be noted that partial substitution of cations in the lattice can also lead to the change of surface area, electronic conductivity, and crystal structure. Lanthanides have long been serving as host ions in perovskite oxides, due to the large ionic radii and easy availability. Several combinations of elements, e.g., La and Sr, La and Ca, or Sm and Sr, have been selected as the A-site host and substitutional ions. Unlike A′ site substitution with aliovalent ions, A′ site substitution with isovalent ions, where charge compensation J



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showed that the electrocatalytic activity of Fe-doped La0.4Ca0.6Mn1−yFeyO3 deceased with increasing Fe content, which also indicates increased ORR activity for Mn ions over Fe ions in these perovskite-type oxides.190 In relation to the onset potential of ORR, this work found that the partial substitution of 50% Ni led to a more positive onset potential, with LaNi0.5Mn0.5O3 at the optimum. This indicated a synergistic electrocatalytic effect from the two transition-metal ions. However, although the onset potential was more positive for LaNi0.5Fe0.5O3, the largest diffusion-limited ORR current density of LaNi0.5Fe0.5O3 was much lower than that of undoped LaNiO3. Meanwhile, it was reported that the partial substitution of Ni ions in LaFeO3 resulted in an enhanced electrocatalytic activity, with a maximum activity for the LaFe0.25Ni0.75O3 structure.197 Hardin et al. hypothesized that a tendency of LaFe0.25Ni0.75O3 for peroxide disproportionation was responsible for the improved ORR activity compared to unsubstituted LaNiO3.173 The same possible explanation for the ORR/OER activities enhancement was also suggested for the substitution of Fe ion into La0.6Ca0.4CoO3−δ to form La0.6Ca0.4Co0.8Fe0.2O3−δ.198 Other than the synergistic effects of combining two different cations, the formation of certain oxidation states of cations may also influence electroactivities. It was suggested that the mixed oxidation valence states of both Fe and Ni, that is Fe3+ to Fe4+ and partial reduction of Ni3+ to Ni2+, as found by X-ray photoelectron spectroscopy and Mössbauer spectroscopy, may be responsible for the reported enhanced activities.197 Bivalent-state-Mg-doped perovskite oxides LaNi1−xMgxO3 (x = 0, 0.08, 0.15) were recently reported as oxygen catalysts, where the current densities of both ORR and OER demonstrated a linear increase with increase of Mg content at the B′ site, i.e. in the order of LaNi0.85Mg0.15O3 > LaNi0.92Mg0.08O3 > LaNiO3.199 This trend was associated with the suppression of Ni2+ formation resulting from the introduction of Mg2+ and ensures high ORR/OER activities of LaNiO3 by maintaining Ni3+ (eg = 1). The enhanced activities may also be linked to a change of morphology. A better dispersion and more effective mesoporous architecture design of the perovskite are beneficial for ORR/OER.200 For example, among the LaNixCo1−xO3 series, electrodes with LaNi0.25Co0.75O3−δ exhibited the best discharge capacity of 7720 mAh g−1 at 0.1 mA cm−2, and cyclability for 49 cycles while maintaining a moderate specific capacity of 1000 mAh g−1 under the limited capacity mode.200 In the series of La0.6Ca0.4Co0.8B′0.2O3 (B′ = Mn, Fe, Co, Ni, and Cu), the maximum bifunctional activity was achieved for B′ = Fe. Although it is believed that Fe is not as active as Mn, Co, or Ni, the maximum surface area (28 m2 g−1) was obtained by Fe doping.198 The effects of B-site cation doping, forming BB′, strongly depend on the selection of cations, and they can result in both positive and negative effects on the electrocatalytic activity. In the series of La0.8Sr0.2Co1−yB′yO3 (B′ = Cu, Ni, Fe, and Cr), the authors found that La0.8Sr0.2Co0.9Ni0.1O3 and La0.8Sr0.2Co0.9Fe0.1O3 showed the highest electrocatalytic activity in 1 M KOH solution, whereas the partial substitution of Co with Cu or Cr exhibited detrimental effects on the electrocatalytic activity.201 Doping with precious metals can also lead to synergistic effects that improve electrocatalytic activities (e.g., the incorporation of Ir into La0.6Ca0.4CoO3 to form La0.6Ca0.4Co0.8Ir0.2O3202 and Ru-doped La0.6Ca0.4CoO3

is not required, induces elastic strain effects due to the cation size mismatch. This strain effect may also lead to a change of the ORR/OER activity. Recently, alkaline-earth elements in both A and A′ sites have also gained increased attention131,133 after the successful development of the BaxSr1−xCoyFe1−yO3−δ series for SOFCs and OTMs.136,137 The B3+/B4+ couple may also be formed by partial substitution of the A site with monovalent cations (e.g., Na+, K+, or Rb+) or high-valence cations (e.g., Ce4+). Following this strategy, enhanced electrode performance of the La0.8Rb0.2MnO3 perovskite oxide as oxygen catalyst has been observed.187 Moreover, the substitution of the A′ site with high-valence ions has also resulted in improved electrocatalytic activities. For example, Meng et al. synthesized the LaFe0.5Mn0.5O3−CeO2 composite as an oxygen catalyst in LABs.188 It was reported that the specific capacity was effectively improved with LaFe0.5Mn0.5O3−CeO2 composites, where a capacity of ∼4700 mAh g−1 was achieved.188 The improved capacity was attributed to the ORR activity of CeO2 nanoparticles,189 the enhanced oxygen storage/release capability, and the increased conductivity with the incorporation of Ce into the A′ site of LaFe0.5Mn0.5O3. The doping level of A′-site cation has also been shown to strongly influence the electrocatalytic activity for perovskite oxides. There have been several studies that demonstrate this trend. For example, the Pr1−xCaxMnO3 series exhibited the greatest ORR activity at x = 0.4,176 and La1−xCaxFe0.8Mn0.2O3 obtained maximum anodic and cathodic current, also at x = 0.4.101 In a similar case, the ORR activity (based on polarization curves) of La1−xCaxMn0.9Fe0.1O3 increased with increasing the doping level, until x = 0.6, where La0.4Ca0.6Mn0.9Fe0.1O3 obtained the highest current density.190 On the other hand, La1−xCaxCoO3 exhibited enhanced electrode performance with an optimum at x = 0.4.191,192 Among the materials in the La1−xSrxMnO3 series, La0.4Sr0.6MnO3 exhibited the best ORR activity and also the largest surface area.186 The activity as a function of A′-site cation substitution level may also be related to the change of crystalline structures. It has been demonstrated that there is a progressive structural transition from pseudocubic LaMnO 3 to hexagonal SrMnO 3 in the La1−xSrxMnO3 (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) series, where the lattice symmetry is lowered with Sr content.186 Interestingly, a rearrangement from the hexagonal structure (at x ≤ 0.4) to the cubic (at 0.4 < x < 0.8) and back to the hexagonal structure at x > 0.8 was also reported within La1−xSrxCoO3 compositions.193 In summary, A-site doping with specific A′ ions can lead to either enhanced electronic conductivities or an increase in surface oxygen vacancies, which, in turn, benefits ORR/OER. In addition, the difference in charge between the dopants and the A-site ions favors the creation of redox couples, which also results in an enhanced activity toward ORR/OER. 2.3.4. Effects of B′-Site Cations. Another effective strategy for improving the oxygen catalyst activity leverages on the effects of combining two different cations at the B-site (BB′).194−196 Sunarso et al. investigated the effect of changing the B′-site cations on the ORR activities of LaNi0.5M0.5O3 (M = Cr, Mn, Fe, Co, and Ni) perovskite-type oxides in alkaline solution using RDE and RRDE techniques.171 This work found the ORR-limited current densities of the LaNi0.5M0.5O3 oxides in descending order: LaNi0.5Mn0.5O3 > LaNi0.5Cr0.5O3 > LaNi0.5Co0.5O3 > LaNi0.5Fe0.5O3 (Figure 8b). It was shown therein that Mn-doping demonstrated the best performance enhancement on LaNiO3 for ORR activities. Another work K


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Figure 9. (a) X-ray diffraction patterns of LaNiO3−δ perovskites with quenching temperatures of 400 °C, 600 °C, 800 °C, and room temperature (LN-400, LN-600, LN-800, and LN-RT) and (b) corresponding ORR activities. Reprinted with permission from ref 206. Copyright 2013 American Chemical Society.

400, 600, and 800 °C to room temperature (Figure 9a).206 Compared to the rhombohedral structure, the ORR activities of cubic LaNiO3 perovskites dramatically increased in the alkaline solution (Figure 9b). It was suggested that the difference in Ni−O bond length contributed to this ORR/OER activity variance. In addition to the quenching approach, postannealing processes can also lead to significant differences in ORR/ OER activities by influencing not only oxygen vacancies, the oxidation states of the transition metal, but also radicals on the surface. Wu et al. investigated the effect of annealing temperatures on the ORR/OER activities of La0.6Ca0.4CoO3−δ.207 They proposed two mechanisms to explain the ORR/OER activity trends with treatment temperatures. First at annealing temperatures above 300 °C, a decrease in activity with reduced annealing temperature was explained by a reduction in oxygen vacancy concentration. Conversely, at annealing temperature below 300 °C, the enhanced ORR/OER activities were explained by the increased OH coverage and by fewer charged cations at the surface. In the low-temperature range, the ORR activity reached a maximum at 200 °C and the OER activity at 150 °C.207 As another example, Malkhandi et al. prepared high-surface-area La0.6Ca0.4CoO3−δ with two steps: precursors were decomposed at 350 °C first, and then the materials were annealed in air at various temperatures between 600 and 750 °C for 2 h.208 The area-specific electrocatalytic activity for ORR/OER, the oxidation state of cobalt, and the crystallite size increased with annealing temperatures, while the Tafel slope remained constant.208 This trend, i.e., higher oxidation states of cobalt improve activity toward ORR/OER, provides new insights into the role of the cobalt center. Further improvements to ORR/OER may be achieved by tuning the oxidation state of the transition metal via a simple postannealing process. A controllable thermal reduction approach on nonstoichiometric perovskite-type CaMnO3−δ was proposed by the Chen group.213 The oxygen content of pristine perovskite microspheres and nanoparticles was adjusted between δ = 0.5 and δ = 0. A strong correlation between electrocatalytic activities and oxygen vacancy content/average oxidation of Mn in the perovskite oxides was observed. Here, it was found that the oxygen-vacant CaMnO3−δ had a higher activity than the stoichiometric CaMnO3. Notably, CaMnO3−δ nanoparticles with δ = 0.25 showed an ORR activity close to that of the benchmark Pt/C catalyst. This high ORR activity was

with the composition of La0.6Ca0.4Co0.8Ru0.2O3203); however, the high cost of these elements has hindered this development. The above outlines the current trends and rational methods for partial B-site substitution in perovskite oxides, highlighting that a beneficial effect to the electrocatalytic activities can be obtained. This benefit can be attributed to many factors, including synergistic effects, morphology, and introduction of mixed valence states. However, due to the many compositions available, it is difficult to establish holistic trends, and detailed understanding is necessary in the future development. 2.4. Extrinsic Strategies for Enhancing Performance and Stability

Numerous perovskite-type oxides have shown promising electrocatalytic activities and relatively high stabilities as catalysts for ORR/OER. Other than modifying the electrocatalytic activities by adjusting cations (species and ratio), many chemical or engineering strategies may also boost the activity and enhance the stability. These methods may produce more active sites for ORR/OER and ideal pathways for the transportation of oxygen, electrolyte, and reactants. Previously demonstrated strategies for preparing enhanced perovskite-type oxygen catalysts are reviewed, since perovskites share common features and tuning perovskites with extrinsic methods is more challenging. It should be noted that the extrinsic strategies for enhancing the electrocatalytic activities are not limited to simple perovskites and can be, in principle, applied to other types of nonstoichiometric oxides as well. Therefore, for other nonstoichiometric oxides, these strategies will be briefly reviewed in the corresponding sections. 2.4.1. Controlling Phase Structure and Surface Activities. The temperature and atmosphere conditions employed during the preparation of the catalysts may alter the crystal structures, surface properties, and oxidation states/ oxygen vacancies.204−208 Various groups have suggested a link between crystal structures and electrocatalytic properties in perovskite.209−212 Sunarso et al. studied LaMO3 (M = Cr, Mn, Fe, Co, and Ni) and LaNi0.5M0.5O3 (M = Cr, Mn, Fe, Co, and Ni), and found that LaCoO3, LaNi0.5Co0.5O3, and LaNi0.5Fe0.5O3 exhibited the trigonal crystal structure with the space group R3̅c; LaNi0.5Mn0.5O3 and LaNi0.5Cr0.5O3 exhibited a cubic structure with the space group Pm3̅m; LaNiO3 contained 2 phases; LaFeO3 and LaCrO3 formed orthorhombic structures.171 Later, Zhou and Sunarso proposed a correlation between crystal structure and ORR/OER activities for LaNiO3−δ catalysts by simply quenching the samples from L



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Figure 10. Field emission scanning electron microscopy (a and b) and high-magnification transmission electron microscopy (c and d) images of the porous La0.75Sr0.25MnO3 nanotubes. Discharge/charge specific capacity (e), and Coulombic efficiency (f) of Li−O2 cells with porous La0.75Sr0.25MnO3 nanotube catalysts at a current density of 0.025 mA cm−2. Reprinted with permission from ref 110. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

ature electrochemical devices.215 To increase the surface areas, nanoengineering materials are typically employed. Nanostructured metal-free materials characterized to be efficient for ORR were discussed in a recent review, yet perovskite-type oxides were not covered.98 To increase the catalytic reaction active sites of perovskites, modified preparation methods, e.g., nanosized oxides prepared by thermal decomposition of maleate-based precursors,216 and sol−gel methods involving various precursors were suggested by the Singh group.217−219 The Yamazoe group prepared nanosized LaMnO3-based perovskite oxides by reverse micelle-based methods and reverse homogeneous precipitation methods. Nanosized catalysts prepared through the wet processes showed enhanced ORR activity compared to Pt loaded electrodes or catalysts prepared by conventional methods.220−223 Moreover, Malkhandi et al. adopted nanocrystalline La0.6Ca0.4CoO3 perovskite with a unique cellular internal structure as bifunctional catalysts for ORR/OER.208 The enhanced activities and stabilities were also observed in metal nanoparticles-decorated bifunctional catalysts.134,224 As aforementioned, utilizing perovskite-type nanoparticles as oxygen catalysts can enhance the electrocatalytic performance for ORR/OER, since perovskite oxides are often limited by their low active surface area/active sites. However, nanoparticle catalysts without pores limit the electrocatalytic reactions to only the external surface, thus limiting apparent activities. The porous structure may provide not only more catalytic active sites but also pathways for gas diffusion, electrolyte transfer, and reactants transportation. Therefore, there have been considerable efforts in recent years to develop a facile, effective, and scalable strategy for the preparation of highly active oxygen catalysts with porous nanostructures. The enhanced activities and stabilities were observed for the porous structures,225 hierarchical mesoporous structures,135 perovskite nanotubes formed by combining the electrospinning and heating methods110 or by combining sol−gel and anodic alumina

associated with the enhanced intrinsic electronic conductivity and oxygen activation. It was later supported by DFT calculations which found an increased O−O bond length of adsorbed oxygen molecules on CaMnO2.75.213 Kim et al. developed a new low-temperature method by treating CaMnO3 (Mn4+) with H2 5% + Ar 95% at 350 °C for 3 h to obtain oxygen nonstoichiometric perovskite Ca2Mn2O5 (Mn3+), which exhibited higher OER activities compared to CaMnO3 in alkaline solution.214 Furthermore, it was found that the oxygen nonstoichiometric Ca2Mn2O5 catalyzed the evolution of oxygen at ∼1.50 V and achieved an OER mass activity of 30.1 A g−1 at 1.70 V vs RHE. The enhanced performance of oxygen nonstoichiometric Ca2Mn2O5 may result from the high spin electron configuration on the manganese cation, and the easy adsorption and transport of OH− via oxygen vacancies. This section highlights the significance of the preparation conditions and the methods of controlling the structure and surface activities as a way to tune the activity of the perovskite oxide catalyst. 2.4.2. Nanostructured Materials. Although the intrinsic activity (activities per oxide specific surface area) of traditionally prepared perovskite compounds may be rather high,116 the main drawback of them lies in their substantially low mass activities. This is because perovskite-type oxides are typically prepared by ball milling (solid-state reaction), sol−gel, combustion synthesis, and so on,146 and the diameters of the particles prepared by these methods. The diameters of the particles prepared by these methods are generally on the order of micrometers (at least >100 nm), and the specific surface area is usually low (800 °C) calcination and long annealing processes required to form crystalline perovskite-type oxides. As a result, the surface catalyzed reactions are limited by a low specific surface area/ large particle size. Therefore, high specific surface areas and, especially, high pore diameters (volumes) are essential to the high discharge capacity/mass activity required for low-temperM



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Figure 11. Scanning electron microscopy images (a) Scanning electron microscopy and (b and c) transmission electron microscopy images of the core−corona bifunctional catalyst (CCBC) with the N-doped carbon nanotube (NCNT) on the surface of the core LaNiO3 particle. (d) Discharge and charge polarization curves of ZABs with Pt/C, CCBC-2 (optimized CCBC catalyst with respect to the amount of precursor solution utilized), and LaNiO3 as oxygen catalysts, and (e) cyclability of CCBC-2. Reprinted with permission from ref 109. Copyright 2012 American Chemical Society.

template technology,226 and urchin-like structures.132 A promising method utilized carbon black as a pore former and employed calcination at 650 °C in air to produce a welldispersed nanoporous La0.6Ca0.4CoO3 perovskite, which exhibited a high specific surface area (>210 m2 g−1) and high electrocatalytic activities.225 2.4.2.1. La1−xSrxMO3−δ (M = Co or Mn). Tuning the electrocatalytic activity and stability of La1−xSrxMnO3 has also been extended to nanostructured morphologies, including nanoparticles, urchin-like nanostructures, hollow-spheres, and nanotubes. For example, La1−xSrxMnO3 nanoparticles and carbon nanotubes synthesized in a single-step method have shown a better electronic contact and demonstrated better electrocatalytic activities than conventional La1−xSrxMnO3 particles (1 μm) supported on carbon black for oxygen reduction.227 Materials consisting of bamboo-like carbon nanotubes, with a conformal coating of nanostructured La0.67Sr0.33MnO3, may also be applied as potential oxygen catalysts.228 Materials with urchin-like nanostructures have been extensively adopted in energy storage and conversion applications,229−231 including pseudocapacitors and photoelectrochemical cells. Jin et al. prepared urchin-like La0.8Sr0.2MnO3 perovskite-type oxides with the specific surface area of 48 m2 g−1 for ORR/OER bifunctional catalyst applications.132 This micro-/nanostructured catalyst exhibited considerable activities for ORR/OER compared to the regularly prepared La0.8Sr0.2MnO3 catalyst. To further improve the electrocatalytic activities of the La0.8Sr0.2MnO3 catalyst, the same group developed hollow-spherical La0.8Sr0.2MnO3 to increase the active surface area.232 Other hollow-spherical materials have been reported to provide larger electrochemical

active surface area, improve the catalytic activity specific capacity, and show excellent cycling stability.233−237 Similarly, porous nanotubes possess both inner and external active sites for catalytic reactions. Therefore, this structure is also expected to enhance the electrocatalytic activities of perovskite-type oxides. Xu et al. prepared porous La0.75Sr0.25MnO3 nanotubes by combining the electrospinning and heating method, as shown in Figure 10a−d. 1 1 0 By applying porous La0.75Sr0.25MnO3 nanotubes as oxygen catalysts in LABs, rate capacities of above 9000−11000 mA h g−1 (Figure 10e), Coulombic efficiencies of ∼100% (Figure 10f), and stabilities for up to 124 cycles were achieved. The authors ascribed the improved performance to the hollow porous tubular structure, which not only provided more catalytic sites but also facilitated the transportation of electrons and reactants. Recently, a high specific capacity of LAB, over 11000 mA h g−1, was achieved by the Mai group with the adoption of the hierarchical mesoporous perovskite La0.5Sr0.5CoO2.91 nanowires as oxygen catalysts.135 The authors attributed the improvement to 4 key factors of the hierarchical mesoporous perovskite structure: (i) the large specific surface area of 96.78 m2 g−1, (ii) the disorder-piled pores formed by attached La0.5Sr0.5CoO2.91 nanorods, which is beneficial to the oxygen transport, (iii) the oxygen deficient nature of the perovskite, which is beneficial to the ORR and oxygen mobility, and (iv) greatly reduced selfaggregation of nanorods. Furthermore, the exceptionally high capacity of La0.5Sr0.5CoO2.91 nanowires was very close to the highest value ever reported in this field when hierarchically porous graphene was adopted as an oxygen catalyst.64 2.4.2.2. LaNiO3−δ. LaNiO3 has long been identified for high OER activity, with Otagawa and Bockris reporting a low Tafel N



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slope of ∼40 mV decade−1, in 1982, for the pure pellet perovskite.238 More recently, LaNiO3 was again identified for both high intrinsic OER and ORR activities by the Shao-Horn group (Figure 5a and c).115,116 As previously discussed, the specific surface area is a crucial characteristic for catalysts, as it has a direct correlation with the number of active sites and therefore activity. Methods to improve specific surface area, such as synthesizing nanocrystals and combining nanoparticles on high-surface-area supports, have been employed to improve the electrochemical performance. For example, porous LaNiO3 perovskite nanocubes prepared via a modified hydrothermal process239 were adopted as bifunctional catalysts for LABs. The nanocubes showed an enhanced capacity (3407 mAh g−1) and good cyclability (75 cycles), much higher than that of VX-72 carbon under similar conditions.240 Furthermore, Chen et al. demonstrated the promising application of this perovskite in ZABs, where the LaNiO3 particles were supported by N-doped carbon nanotubes comprising a core−corona-structured bifunctional catalyst (Figure 11a−c).109 The nanostructured design of the catalyst allowed the core and corona to catalyze ORR/OER with high activity (Figure 11d), efficiency, and improved cycling stabilities (Figure 11e), comparable to the state-of-the-art commercial oxygen catalysts. Besides surface area limitations, phase impurities (common during synthesis) may also lead to low mass activities. These impurities typically result from the disparities in hydrolysis rates for precursors with two or more metal elements and/or side reactions between carbon and precursors. Therefore, synthesis of phase-pure materials while maintaining a high specific surface area is important. Hardin et al. reported the successful synthesis of a phase-pure nanocrystalline LaNiO3 perovskite, which was characterized to be highly active for both OER and ORR with a bifunctional character of ΔE = 1.02 V (1.66 V @ 10 mA cm−2 and 0.64 V @ −3 mA cm−2 vs RHE) when supported on Ndoped carbon.241 The OER mass electrocatalytic activity of this LaNiO3 catalyst was nearly 3 times higher than that of 6 nm IrO2 with no hysteresis during the oxygen evolution process. This is a significant discovery, since even phase-pure LaNiO3 catalysts have a specific surface area nearly 7 times lower than that of IrO2. In addition to phase purity and high specific surface area (∼11 m2 g−1), the presence of lattice hydroxide species (surface hydroxylation) was also considered as a contributor to the high activity. 2.4.2.3. Other Nanostructured Perovskites. Han et al. have prepared perovskite-type CaMnO 3 , layered structured Ca2Mn3O8, and postspinel CaMn2O4 and CaMn3O6 through thermal decomposition of carbonate solid−solution precursors.242 These materials exhibited similar morphologies of porous microspheres with agglomerated nanoparticles. Han et al. demonstrated that the perovskite CaMnO3, with open tunnels and multivalence, exhibited the highest ORR activities, and CaMn3O6 and CaMnO3 showed higher OER activity.242 Later the Chen group reported the application of CaMnO3 with a porous nanostructure (Figure 12a−d) as a catalyst for rechargeable LABs, synthesized via a facile sol−gel method.243 This work showed that the CaMnO3/C electrodes exhibited improved performance with a lower overpotential (a discharge−recharge voltage gap of 0.98 V (CaMnO3/C) vs 1.60 V (pure carbon), Figure 12e), improved rate capability and stability (>80 cycles (CaMnO3/C) vs SrRuO3 (110) > SrRuO3 (001). Stability: SrRuO3 (001) > SrRuO3 (110) > SrRuO3 (111). Reprinted with permission from ref 257. Copyright 2014 Nature Publishing Group.

stable (110) surface, in alkaline solution.253 Studies have identified that the favorable crystal orientation is specific to the perovskite composition and slight perturbations can result in conflicting trends. This was demonstrated in separate studies of A-site doped LaCoO3 thin films. Using model electrodes with different crystallographic orientations, Lippert et al. observed that thin films of La0.6Ca0.4CoO3 oriented along the (001) orientation showed higher ORR/OER activity than films with (110) orientation.250 An opposite trend was observed for La0.8Sr0.2CoO3, where the (110)-oriented thin film was found to exhibit a higher ORR/OER activity than that of the (001) or (111) thin films.254 In the latter, the formation of more oxygen vacancies on the (110)-oriented La0.8Sr0.2CoO3 thin film was suggested as the mechanism yielding the superior performance.254 Interestingly, it has been demonstrated that the (001)oriented LaMnO3-based perovskite thin films have intrinsic activities comparable to those of high-surface-area powder-form LaMnO3 catalysts, suggesting the (001) facet surface may have the highest activity.255 It was also identified that the ORR activities reduced with decreasing thickness and that Mn3+ is the active valence state for ORR.255 Recently, the Shao-Horn group combined the advantages of two typical perovskite catalysts, La0.8Sr0.2MnO3−δ with high intrinsic ORR activity and Ba0.5Sr0.5Co0.8Fe0.2O3−δ with high OER activity, to develop a novel bifunctional catalyst using thin film techniques.256 The group fabricated Ba0.5Sr0.5Co0.8Fe0.2O3−δ-decorated (001)-oriented La0.8Sr0.2MnO3−δ thin-film electrodes and demonstrated this composite exhibited ORR activities surpassing those of La0.8Sr0.2MnO3−δ powders and OER activities comparable to those of Ba0.5Sr0.5Co0.8Fe0.2O3−δ powders. These results provide

inspiration for the design of highly active thin-film oxygen catalysts for ORR/OER by not only combining two specific materials, but also tuning the preferential orientation of welldefined surfaces. Although some important insights were offered from single crystalline thin films, few studies pointed out the relationship between stability and activity. Very recently, Chang et al. tried to elucidate the fundamental links between stability and activity of SrRuO3 single crystalline thin films. The films were prepared on Nb-doped SrTiO3 substrates by radiofrequency magnetron sputter deposition.257 The OER activities were found to follow a decreasing order, i.e. SrRuO3 (111) > SrRuO3 (110) > SrRuO3 (001) (Figure 14a). The film stabilities were also determined via evaluation of the change of film thickness (Figure 14b−d) and inductively coupled plasma mass spectroscopy (Figure 14e) of SrRuO3 thin films. It was found that the activity was inversely proportional to the stability of the Ru and Sr surface cations, as determined by inductively coupled plasma mass spectroscopy (Figure 14e). It was therefore proposed that the OER activity was controlled by the density of surface defects rather than the binding energy between the substrate and oxygenated species.257 Although significant, this finding is limited to the SrRuO3 structure and further work is required to determine a universal trend for perovskite catalysts. The above details the application of single crystalline thin films to investigate crystallographic-orientation effects on ORR/OER activity. This discussion exempts the complexities of activity evaluation and highlights the importance of orientations on the catalytic performance. At present, several promising materials have been proposed for high-temperature ORR from thin-film investigations.258−265 Conversely, only a Q



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hybridization.86,289,290 The covalent coupling between the perovskite-type oxide nanoparticles and carbon supports, such as carbon nanotubes and graphene, may also improve the electrocatalytic activities of perovskite-type catalysts by tuning of the electronic structure.291 This effect, however, may not be achieved by physical mixing alone. Hardin et al. investigated the role of the carbon support for perovskite oxides and found that the mass ORR/OER activities of perovskite oxides supported on N-doped carbon were greater than those supported on graphene, and they proposed that this enhancement is due to the introduction of peroxide disproportionation functionality via support interactions.173 In addition to the activity enhancement by the combination of carbon materials, a recent similar study for hybrids also suggests that carbon nanotubes were preferable over the graphene counterpart.87 Other than carbon, alternative supports for the air electrode have been investigated to overcome corrosion due to carbon oxidation. Yuasa et al. employed LaNiO3 as a support as it has been characterized to have high electronic conductivity and high ORR/OER activities.292 LaMnO3, another active catalyst for ORR, was successfully prepared onto the LaNiO3 support using a reverse micelle method. Excellent bifunctional ORR/ OER activities with the LaMnO3/LaNiO3 composite electrode in alkaline solution were achieved.292 However, a low specific surface area of the LaNiO3 support ( β- > γ-MnO2 for ORR in alkaline solution,350 α- ≅ δ- > γ- > β-MnO2 for CO oxidation351 and α- > amorphous - > β- > δ-MnO2 for both ORR and OER in alkaline solution.352 More generally, manganese oxides for ORR/OER can be considered as the nonstoichiometric form MnOx,353−358 which have demonstrated a high activity in CO oxidation.359,360 To stabilize the intermediate Mn3+/Mn4+ species (redox couple) or Mn3+ in the MnOx material and consequently enhance the activity, metal cation dopants have been employed.361,362 It has been reported that MnOx doped with the transition metals (Ni, Bi, and Cr) are slightly more active than those with Ca and Mg.362 In addition, it has been suggested that the presence of the doping metal cations may assist the transfer of O2,ads to Oads.361 Another way to stabilize the intermediate Mn3+/Mn4+ species in the manganese oxides is the facile heat treatment354,355 Recently, the Chen group reported a facile method of merely W



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LaCoO3 or Nd3IrO7, enhancements to both the OER and ORR activities were observed for the composite catalysts.368 It was also reported that the hybrid catalyst MnOx-Co3O4/C had better electrocatalytic activity toward ORR in comparison to individual catalysts.369 In addition, Ag-MnOx composites have also been tried to enhance electrochemical activities, since Ag is also active for oxygen reduction and less expensive than Pt and Pd.370−373 However, studies have demonstrated lower catalytic activities than expected. Recently, Slanac and colleagues synthesized evenly dispersed nanosized Ag on nanoplatelets of MnOx with close proximity to each other and supported on carbon.374 The mass ORR activities were comparable to commercial Pd/C and Pt/C. The enhanced ORR activities may be attributed to the facilitation of the formation and disproportionation of HO2− with Ag-MnOx nanodomains and the ligand effects from the special electronic interaction at the Ag/MnOx interface.374

annealing pristine manganese oxide in Ar or O2 to prepare nonstoichiometric nanocrystalline MnO2.355 Here oxygen vacancies were compensated by the reduction of Mn4+ to Mn3+, generating redox couples. Both experiments and computations showed that this approach was beneficial for the ORR activity. Furthermore, the introduction of oxygen vacancies into MnO2 enabled a more positive ORR onset potential, a larger current density, and a lower peroxide formation rate during ORR. The DFT calculations revealed that the presence of oxygen vacancies enhanced the surface-oxygen interaction and reduced the kinetic barrier.355 By controllably annealing of the atomic layer deposited-MnOx, materials with good activities for both ORR and OER have also been produced.354 The MnO catalyst showed poor activity for the ORR but good activity for the OER, and the Mn2O3 was a good bifunctional catalyst.354 The large difference in ORR between the MnO and Mn2O3, but similar OER activity, is likely due to similar electrochemically induced surface structures at the high oxidative potentials. ́ Ramirez et al. demonstrated the effect of the MnOx crystallographic structure for OER activity.363 The work compared MnOx (Mn4+ and Mn3+), α-Mn2O3 (Mn3+) and Mn3O4 (Mn2+ and Mn3+) electrodeposited films for OER in 1 M KOH. An amorphous phase was found on top of all manganese oxide films after OER, which was associated with the OER activity enhancement. Further comparisons of onset potentials of OER revealed that the highest catalytic activity was enhanced by the presence of Mn3+ ions in a distorted lattice, a high concentration of oxygen vacancy, and various Mn−O distances.363 Mn oxides with the presence of Mn3+ have also shown promising bifunctional activities, similar to that of the best known precious-metal catalysts.364 The ORR catalytic activity of MnOx is also highly dependent on the morphology, surface area and availability of active sites. It was reported that ORR activities of α-MnO2 nanospheres and nanowires were better than those of the counterpart microparticles.350 Moreover, it was suggested that a nanoporous structure is suited to the design of gas diffusion electrodes, and the application of amorphous oxides as catalysts had more favorable characteristics. Nanoporous amorphous MnOx with high surface area and high defects density has been demonstrated to be efficient oxygen catalysts for ORR in alkaline solution.269 The Liu and Cho group also integrated amorphous MnOx nanowires with conductive KB carbon materials via a polyol method.365 The KB-carbon-supported amorphous MnOx nanowires were shown to improve the inherently low electronic conductivity of MnOx that usually limits the ORR activity. Also the cost of KB carbon is lower in comparison to carbon nanotubes or graphene.358,366 Indeed, a ZAB assembled with a-MnOx/KB carbon composite electrode exhibited a peak power density of ∼190 mW cm−2.365 This performance may be attributed to the high specific surface area of amorphous MnOx nanowires and a high concentration of defects, which potentially provided more active sites for oxygen adsorption and thus contributed to the improved catalytic activity for ORR in alkaline solution.365 Yang and coauthors suggested that the enhanced catalytic activity of the MnOx/ carbon composite may be ascribed to the high positive charge generated on the MnOx-carbon nanotube surface.358 Electrocatalytic activities of MnOx are usually limited by a low conductivity. Other than mixing with carbon to enhance the activities,367 methods of compositing with other conductive oxides have also been explored. By introducing either LaNiO3,

5.2. Ceria-Related Oxygen Catalysts

Fluorite cubic structured ceria/doped ceria or its composites are abundant and have therefore been broadly deployed in many energy conversion applications, such as highly active anode or electrolyte for SOFCs,136,375−378 water/CO2 splitting,379−381 oxygen sensing382 and oxygen storage.383,384 In ceria-based materials, the Ce4+/Ce3+ redox couple, Ce3+, and the surface oxygen vacancies, are believed to be the active sites for catalytic reactions.385 The inherent good storage, release and transport of oxygen in ceria, enable successful application of ceria-based materials as oxygen catalysts in LTFCs386−389 and LABs.189,390−392 However, their low conductivity and relatively low activity limit their widespread applications. For more information about the preparation, characterization and typical applications of nanostructured ceria-based materials, the readers are referred to a recent review.384 CeO2 has long been employed with precious metals or alloys for cocatalysts in LTFCs.346 Generally, CeO2-incorporated Pt/ C catalysts exhibit enhanced ORR activity compared to pure Pt/C.393 The enhancement of the activity from introducing CeO2 has been ascribed to several factors: the presence of the amorphous cerium oxide layer on Pt that inhibits the oxidation of the Pt surfaces,394 the increase of the local oxygen concentration thanks to its potential nonstoichiometry (CeO2 → CeO2−δ + (δ/2)O2, 0 ≤ δ ≤ 0.5),395 the increase in dispersion of nanoparticles, and the change of surface intermediate Ce3+/Ce4+ species.396−398 To understand the role of CeOx in enhancing ORR activities, in situ X-ray absorption fine structure measurements were carried out. The improvement can be attributed to the oxidation of Ce3+ to Ce4+, which suppressed Pt oxidation.388 A dramatic change of the microstructure of CeOx with Pt/C in an acidic condition was also observed due to the strong interaction between Pt and ceria, where the amount of ceria around the Pt particles was reduced and limited.399 Due to the porous and/or discontinuous structure of CeOx, the remaining ceria, however, was still sufficient to prevent the growth of Pt particles during sintering and to increase the triple-phase boundaries, both of which promote ORR.399 To improve the catalytic activity, substitution on the Ce4+ site of CeOx has been employed. It is proposed that substitution with the irreducible Zr4+ ion, reducible Ti4+ or Sn4+ may increase the reducibility of Ce4+ in the CeO2, and therefore produce more active sites for catalytic reactions. Kalubarme et al. investigated Zr-doped ceria and Al-doped ceria X



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low overpotential of 336 mV at 10 mA cm−2 and a Tafel slope of 30 mV dec−1, was associated with the in situ formation of Ni0.9Fe0.1OOH species.403 The Bell group observed unusually short Fe−O bond distances by operando X-ray absorption spectroscopy using high energy resolution fluorescence detection for Ni1−xFexOOH.414 This work identified a 500fold OER activity enhancement with mixed Ni1−xFexOOH over NiOOH and FeOOH.414 Boettcher et al. identified Fe impurity as the driver for the improvement of OER activity.415 Likewise, it was reported that the removal of Fe impurities led to a significant decrease of the OER activity of Ni(OH)2.404 However, the fundamental role of Fe in NiO x -based compounds on the OER activity was not fully understood. In an attempt to elucidate the impact of Fe on the OER activity, Boettcher et al. applied high-purity Ni(OH)2 precipitate as an Fe-absorbent to remove any trace Fe from KOH electrolyte.415 This work suggested that Fe incorporation increased the conductivity of the catalyst and induced a partial-charge transfer activation effect on Ni, but the increased conductivity was not critical for the enhanced activity.415 In addition, it has also been suggested that the incorporation of Fe may induce the formation of layered double hydroxide structure.408 It is envisaged that a full understanding of the role of Fe may strongly facilitate the design of highly efficient catalysts for NiOx-based compounds. In contrast to Fe, recent work on Co doped NiOx-based compounds has found conflicting trends. Indeed it was suggested for Co doped NiyCo1−yOx that there exists synergistic effects for OER catalysis.416 However, other work found a decreasing activity of NiyCo1−yOx thin films compared to NiOx films with increasing Co content.403 This work suggested that the in situ formation of highly efficient layered double hydroxide structure is suppressed with increasing Co content.403 It should be noted that these experiments were carried out in different conditions, therefore general conclusions are limited due to the differences in structural, compositional and electronic features of the materials. Boettcher et al. further supported this work by fabricating well-defined ultrathin film catalysts to accurately measure the OER activities.417 Their work suggested that the structural and chemical changes during OER was correlated to the OER activities in alkaline solution.417 In addition to the composition of materials, the microstructure can also play an important role in improving electrochemical activity.404,407,418,419 For example, Song et al. successfully fabricated single-layered double hydroxides for OER by liquid phase exfoliation.418 Compared with bulk layered double hydroxides and a commercial IrO2 catalyst, the exfoliated single-layer nanosheets exhibited significantly higher OER activity, associated with more active sites and improved electronic conductivity.418 One should note that the singlelayered double hydroxides showed outstanding stability too.418 Xu et al. achieved an increased mass specific activity by synthesizing a 3D flower-like morphology with a large surface area by hydrothermal methods with the assistance of microwave irradiation.419 On the other hand, although most OER catalysts are crystalline, high OER activities have also been obtained with amorphous mixed-metal oxides.266,420 For instance, amorphous Ni0.69Fe0.31Ox/C outperforms commercial Ir/C and exhibited an overpotential of 280 mV at 10 mA cm−2 with a Tafel slope of 30 mV dec−1 in 1 M KOH solution.420 As outlined above, oxygen catalysts with the layered double hydroxide structure have demonstrated superior electrocatalytic

as oxygen catalysts for LABs.390,392 Among Ce1−xZrxO2 with x = 0.1−0.5, Ce0.8Zr0.2O2 exhibited the maximum discharge specific capacity of 1620 mAh g−1.392 Unfortunately, this LAB proved unstable, with cell degradation observed after 5 cycles, yet the authors ascribed the instability to the carbonate electrolyte.400 Later, porous nanocrystalline Zr-doped ceria prepared by a urea-assisted solution combustion method was investigated with a noncarbonate electrolyte in the cell, and a stable capacity after 40 cycles was observed.391 The Zr-doped ceria also showed promising ORR/OER activities with a discharge capacity of 8435 mA h g−1 and a lower potential for Li2O2 oxidation than pure carbon.391 It was suggested that the formation of Ce3+ on the surface due to the Zr doping and the porous nanocrystalline structure were responsible for the promising ORR/OER activities in the LAB.391 Among Ce1−xAlxOy with x = 0.1−0.3, the highest discharge capacity of 1549 mAh g−1 was achieved when x = 0.2, less than the Zr doped LAB.390 In the experiments, this material was tested in LABs with carbonate electrolytes.390 The discharge product consisted mostly of lithium carbonate with only a small amount of lithium peroxide, suggesting that the properties of Al-doped ceria were similar to the Zr-doped ceria material and performance was affected by the electrolyte.391 Later work utilized the fast electron conduction provided by graphene and high electrocatalytic activity of Zr-doped ceria.401 The graphene/Zr-doped ceria nanoblend with only 10% loading of Zr-doped ceria on graphene, showed a 3-fold enhancement in the discharge capacities compared to bare graphene as oxygen catalysts in LABs.401 5.3. NiOx-Based Oxygen Catalysts

NiOx-based compounds are also promising oxygen catalysts in alkaline media since they are based on earth-abundant elements. The compounds include Fe-doped NiOx, Co-doped NiOx, NiOOH and their composites. Typically, first-row transition-metal oxides are considered to be poorly active due to their inherently too strong or too weak metal−oxygen bonding strength. Recent work has indicated that the formation of metal oxyhydroxide species can correlate with enhanced activity.402,403 Metal oxyhydroxides have a layered double hydroxide structure with large intersheet spacing. This structure ensures that the catalysis is not limited to the surface and allows electrocatalytical activity for nearly all metal atoms and the movement of electrons and hydroxide groups between the layers.403,404 Due to these properties, metal (oxy)hydroxides have been widely recognized as active materials in electrocatalytic reactions.403,405−409 Among 3d metal (oxy)hydroxide catalysts, one typical OER activity trend governed by the OHM2+δ bond strength (0 ≤ δ ≤ 1.5) was reported as Ni > Co > Fe > Mn.402 In particular, the β-NiOOH phase (Ni: ∼ 3) is considered to be more active for OER than the highly oxidized γ-NiOOH phase (Ni: ∼ 3.6 or 3.7).410 This suggests that lowering the average oxidation state of Ni is associated with the higher OER activity, where adding Fe has been suggested to achieve this.410 In contrast, Nocera et al. concluded that the γNiOOH phase is a more efficient OER catalyst characterized with the in situ X-ray absorption spectroscopy.405 There is consensus in the literature that the coexistence of Ni and Fe is beneficial for high OER activity.411−413 For example, Boettcher et al. found that the Ni0.9Fe0.1Ox shows quite similar OER activity to one of the best reported OER catalysts (Ba0.5Sr0.5Co0.8Fe0.2O3−δ) and much higher activity than IrO2.403 In this study, the high OER activity, exhibited by a Y


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temperature devices is their extremely low conductivity, limiting their ORR/OER activities. However, increasing the concentration of anion or cation defects is identified as a promising method to enhance both the electronic and ionic conductivity. Other methods to develop these materials include the introduction of more redox couples, the adoption of innovative synthesis methods, the use of hybrids via leveraging on the interactions between oxides and metal nanoparticles. Although some of the discussed nonstoichiometric oxides have been comprehensively investigated and developed to exhibit excellent electrocatalytic activities, comprehensive and in-depth studies on structure, composition, morphology and performance are still needed as to develop efficient compositions and to elucidate the electrochemical mechanisms. Future developments may also consider the scalability of preparation, cost of fabrication, and long-term stability during operation and environmental conditions.


activities, yet the active phases of catalysis are unclear. Recently, DFT calculations have been performed on selected surfaces of different NiOx catalysts to clarify this.421 This study suggested that overpotentials for OER had the following trend: Fe-doped β-NiOOH (0.26 V) < NiFe2O4 (0.42 V) < β-NiOOH (0.46 V) < Fe-doped γ-NiOOH (0.48 V) < γ-NiOOH (0.52 V) < Fe3O4 (0.70 V).421 A similar overpotential for the β-NiOOH and γNiOOH was observed, which supported that both phases were active. In addition, Fe-doped β-NiOOH was identified to be the most efficient OER catalyst, agreeing well with the experiment observations. Nevertheless, this study is limited to a single surface for each phase, i.e. β-NiOOH(011̅5), γ-NiOOH(101), Fe-doped β-NiOOH(0115̅ ), γ-NiOOH(101), NiFe2O4(001), and Fe3O4(001). Although these materials are promising candidates, theoretical studies on them are still scarce. An extension of both theoretical and experimental studies are desired to further identify advanced oxygen catalysts. 5.4. Delithiated LiCoO2−Related Oxygen Catalysts

6. BENCHMARKING OXYGEN CATALYSTS

Materials that have been intensively studied in Li-ion batteries or Na-ion batteries,422−426 e.g., LiCoO2/Li1−xCoO2, have also been investigated as oxygen catalysts, leveraging on existing techniques and literature.427,428 For example, the Manthiram group adopted LiCoO2/Li1−xCoO2 as catalysts for both OER and ORR.427 LiCoO2 was prepared with a spinel structure by adjusting the firing time and temperature during synthesis. This structure showed a higher specific OER activity than the stateof-the-art IrO2. Further optimization of the Li content in the lithium cobalt oxide produced chemically delithiated Li0.5CoO2 with the mixed oxidation states of Co (Co3+/Co4+). The delithiated Li0.5CoO2 exhibited both high ORR and OER activities and therefore could be used as a potential bifunctional catalyst for rechargeable MABs. The Co4O4 cubane in Li0.5CoO2 is similar to that found in CaMn4Ox,429 in which a pinning of the Co3+/Co4+ 3d energy with the top of the O2− 2p band was suggested as significant factor of the high electrocatalytic activities. Additional work by Lu et al. proposed an electrochemical tuning process to obtain delithiated LiCoO2, where the lithium ions were extracted from the stoichiometric LiCoO2 when charging to a high potential.430 Applying the same method, oxides including delithiated LiCo0.5Ni0.5O2, LiCo0.5Fe0.5O2, LiCo0.33Ni0.33Fe0.33O2 and LiCo0.33Ni0.33Mn0.33O2 exhibited improved OER activity. In p a r t i c u l a r , t h e e l e c t r o c h em i c a l l y d e l i t h i a t e d L i Co0.33Ni0.33Fe0.33O2 showed onset potentials of ∼1.47 and ∼1.525 V vs RHE at 10 mA cm−2, even better activities than commercial Ir/C catalyst.430 Overall, the spinel-type delithiated LiCoO2-related materials have exhibited high electrocatalytic activities for ORR/OER. The modification of the d electrons on the transition-metal ion without influencing the structure has been suggested to be explored systematically with other compositions to establish a correlation of the d-electron count to activities and to develop a better understanding of the mechanisms.427

6.1. Overview of the State-of-the-Art Oxygen Catalysts

In order to establish a clear overview of state-of-the-art oxygen catalysts, objective comparisons of their activities are required. However, comparisons across studies are challenging due to a lack of established standards; for example, different groups may utilize different instruments, measurement techniques, and even activity descriptions. In addition, oxygen catalysts may be deposited on a variety of different substrates, and evaluated at a range environmental conditions, including pH values, temperatures, and electrolyte solutions.431 Therefore, benchmarking the electrocatalytic activities of oxygen catalysts remains a critical challenge. When comparing the activities, attention must be paid to the units of interest. In general, the kinetic current for the evaluation of oxygen catalysts can be normalized against geometric area, mass, electrocatalytic active surface area, or the number of active sites of oxygen catalysts. This gives geometric activity, mass activity, specific activity, and turnover frequency (defined as the number of reaction products generated per active site and per unit time), respectively.432,433 For commercial applications, it is essential that geometric activity is maximized, moreover, for noble-metal (oxide) oxygen catalysts, the mass activity should also be maximized due to cost. Preparing catalysts with increased specific activity and high surface area can maximize both geometric activity and mass activity. Therefore, the specific activity is also an important intrinsic parameter for the catalyst. Since it is typically challenging to measure the electrochemically active surface area, flat and low-roughness thin-film electrodes may be employed. For this, thin films fabricated by pulsed laser deposition or molecular beam epitaxy, may be applied to ensure that the electrocatalytically active surface area is as close as possible to the geometric surface area. The Tafel plot is an alternative benchmark parameter. However, the information obtainable from the Tafel plot is limited, and the reaction mechanism, including the rate-limiting steps, cannot be determined reliably by Tafel plots alone. This is especially significant for reactions that involve multiple pathways as it is the case in ORR/OER. As an alternative, it has become common to compare oxygen catalysts using the current density at a given overpotential or conversely the overpotential required for a given current density. For example, the overpotential for a current density of 10 mA cm−2 (geometric

5.5. Summary

In this section, we have reviewed the recent progress of other typical nonstoichiometric oxides, e.g., MnOx, ceria-related, NiOx-based, and delithiated LiCoO2-related oxygen catalysts, and we have also highlighted their promising activities in ORR/ OER processes. As discussed, several elegant strategies to manipulate structure, morphology and compositions have been extensively adopted. At present, the most critical issue for the application of these materials as oxygen catalysts in lowZ



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Figure 22. (a) Overview of the state-of-the-art oxygen catalysts in alkaline solution for ORR. Data extracted from the literature. Pt/C-1,435 Pt/C2,364 Co3O4,84 LaCu0.5Mn0.5O3,435 CaMnO3,242 LaNiO3,241 LaNi0.75Fe0.25O3,173 LaTi0.65Fe0.35O3−δ,436 Ba0.5Sr0.5Co0.8Fe0.2O3−δ,439 LaCoO3,173 La0.5Sr0.5CoO3−δ,135 La0.58Sr0.4Co0.2Fe0.8O3,437 and La0.5Sr0.5Co0.8Fe0.2O3.291 (b) Overview of the state-of-the-art oxygen catalysts in alkaline solution for OER. Curves adapted from the reported data. (i, magenta and red lines) “simple” perovskite oxides: LaFeO3, LaCoO3, LaNiO3 LaCaCoO3−δ, and Ba0.5Sr0.5Co0.8Fe0.2O3−δ.116 (ii, center magenta line) double perovskite oxide: Pr0.5Ba0.5CoO3−δ.114 (iii, green lines) amorphous metal oxides prepared by photochemical metal−organic deposition: a-CoOx, a-NiOx, a-Fe50Ni50Ox, a-Fe50Co50Ox, and a-Fe33Co33Ni33Ox.266 (iv, blue lines) Delithiated oxides: LT-Li0.5CoO2 (synthesized at 400 °C) and Co3O4 (for comparison purpose);427 De-LiCoO2, De-LiCo0.5Ni0.5O2, De-LiCo0.5Fe0.5O2, and DeLiCo0.33Ni0.33Fe0.33O2.430 (v, yellow and yellow-green lines) Transition-metal oxides/layered oxyhydroxide: FeOx, CoOx, IrOx, and NiOx.403 ORR/ OER activities are normalized according to the geometric surface area unless otherwise stated. OER activities of some oxygen catalyst are normalized to the surface area of oxides (denoted by the symbol # in the plot).

highlight the importance of both composition engineering and morphology optimization on the ORR activities. For further details, please refer to the original literature or the corresponding statements in the previous sections. We should note that the most active ORR oxygen catalysts are primarily based on noble metals, however their cost is a significant bottleneck. In this review, we have surveyed the promising nonstoichiometric oxides to consolidate the current studies of these materials for oxygen catalysis, with the hope of identifying guidelines and new understanding. We envisage that it is useful in the further development of nonstoichiometric oxide catalysts. As shown in Figure 22b, regardless of the activity parameter used, the reviewed nonstoichiometric oxides show promising OER activity, with relatively low overpotentials and Tafel slopes. Furthermore, several materials are competitive with or even better than noble-metal oxides. Moreover, effective perovskite oxides, i.e. Pr0.5Ba0.5CoO3−δ, Ba0.5Sr0.5Co0.8Fe0.2O3−δ, also show extremely promising OER activities when normalized to geometric area.438 It should be noted that the amorphous oxides showed the highest OER performance compared to other oxides, with amorphous CoOx and Fe33Co33Ni33Ox exhibiting the best performances in the series. However, stabilities of amorphous oxides should be further investigated for the development of advanced oxygen catalysts. In summary, most nonstoichiometric oxides show comparable or better OER activities than noble-metal oxide (IrO2) in alkaline solutions, while most nonstoichiometric oxides show slightly lower or comparable ORR activities when compared to the typical noble metals (Pt/C).

area) is the typical parameter of the evaluation of OER catalysts. This current density is approximately the expected current density for a 10% efficient solar-to-fuel device under 1 sun illumination.431,434 A list (not limited to the nonstoichiometric oxides) of stateof-the-art ORR/OER catalysts in alkaline solution is shown in Figure 22. Here, the direct comparison of these materials is difficult due to the aforementioned lack of standard. For ORR catalysts (Figure 22a), catalysts commonly exist in the form of hybrids or composites, therefore morphological difference may also play a significant role. Nitrogen doped graphene and carbon nanotubes are among the most widely used components of hybrids or composites. Four categories of materials are compared, presented with different colors. The first group, i.e. Pt/C and Co3O4/N-doped graphene, is included for reference since this contains the best ORR catalysts. In the second group (Mn-based materials), CaMnO3 shows a lower overpotential in comparison to LaCu0.5Mn0.5O3. This is likely due to the presence of less active Cu, which impedes the ORR activity. However, the morphology difference should be noted; porous CaMnO3 microspheres242 and microsized LaCu0.5Mn0.5O3 with low surface area (1.1 m2 g−1)435 were used in the two studies. Regarding the third group, one first notices that after adding Fe to LaNiO3, the activity is enhanced. Although Fe-doped LaNiO3 has low conductivity and reduced surface area, its ability to disproportionate peroxide is enhanced, leading to better performance.173 Second, by replacing Ni with Ti, the ORR activity is further improved. This is most likely linked to the enlarged surface area of LaTi0.65Fe0.35O3−δ obtained by nanoengineering the materials.436 In the last perovskite group, s h o w n in m a g e nt a , t h e e x c e l l e nt OER c a t a l ys t Ba0.5Sr0.5Co0.8Fe0.2O3−δ shows negligible ORR activity. For LaCoO3, after partial substitution of La with Sr, the ORR activity increases in conjunction with an increased electronic conductivity. The further increase in activity with La0.58Sr0.4Co0.2Fe0.8O3 is likely linked to the introduction of Fe−Nx/C into the perovskites, which in turn dramatically increases the ORR performance.437 Lastly, La0.5Sr0.5Co0.8Fe0.2O3 has the highest activity due to its enhanced surface area by nanoengineering.291 These studies

6.2. ORR/OER Bifunctional Oxygen Catalysts

For the rechargeable MABs and regenerative LTFCs, both ORR (discharge, fuel cell mode) and OER (charge, electrolyzer mode) are equally important. Therefore, developing bifunctional oxygen catalysts, which are effective toward both ORR and OER, is one of the most imperative challenges of this field. A common indicator for evaluating the bifunctional activity is the overpotential difference between ORR and OER defined as ΔE = EOER @ 10 mA cm−2 - EORR @ −3 mA cm−2. It should be noted that an ORR current density of −3 mA cm−2 is near the AA


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Table 1. Overview of Bifunctionality of State-of-the-Art Oxygen Catalystsa ORR potential (V) @ −3 mA cm−2

Oxygen catalyst LaNiO3−δ206

−0.40 vs Ag/AgCl

LaNiO3−δ206

−0.32 vs Ag/AgCl

LaNiO3241 LaNiO3440 LaNi0.8Fe0.2O3441

0.64 vs RHE ∼0.77 vs RHE ∼-0.40 vs saturated calomel electrode (SCE)

LaNi0.75Fe0.25O3173


LaTi0.65Fe0.35O3−δ436

0.67 vs RHE ∼0.72 vs RHE

LaCoO3173

0.64 vs RHE

OER potential (V) @ 10 mA cm−2

ΔE (V)

0.76 vs Ag/AgCl @ 5 mA cm‑2 0.69 vs Ag/AgCl @ 5 mA cm‑2 1.66 vs RHE

1.16

Traditional synthesis

1.01

Temperature shock from 800 °C

1.02

∼1.74 vs RHE ∼0.67 vs SCE

0.97 1.07

1.68 vs RHE

1.01

∼1.77 vs RHE

1.05

1.64 vs RHE

1.00

Nanostructured LaNiO3 Supported on nitrogen-doped carbon Intertwined core−corona structure Most active among LaNi1−xFexO3 (x = 0, 0.1, 0.2, 0.6) Nanostructured LaNiO0.75Fe0.25O3 Supported on nitrogen-doped carbon One-dimensional nanoparticles Embedded in the nitrogen-doped carbon nanorods Nanostructured LaCoO3 Supported on nitrogen-doped carbon Hierarchical mesoporous nanowires Ball milled perovskite oxide Incorporate nitrogen-doped carbon nanotubes into the perovskite nanoparticles Incorporate Fe−Nx/C moieties into perovskites Oxygenation at 950 °C for 48 h 10-nm-size LaCoO3−δ particles distributed on the surface Nanostructured oxide Supported nitrogen-doped reduced graphene oxide nanosheets Nanoparticles Co3O4 nanoparticle-modified MnO2 nanotube

La0.5Sr0.5CoO3−δ135 La0.6Sr0.4CoO3−δ442 La0.5Sr0.5Co0.8Fe0.2O3443

∼0.76 vs RHE ∼-0.26 vs Hg/HgO ∼-0.41 vs SCE

∼1.83 vs RHE ∼ 0.89 vs Hg/HgO ∼0.78 vs SCE

1.07 1.15 1.19

La0.58Sr0.4Co0.2Fe0.8O3437

∼0.77 vs RHE

∼1.68 vs RHE

0.91

Ba0.5Sr0.5Co0.8Fe0.2O3−δ439

∼0.61 vs RHE ∼-0.32 vs Hg/HgO

∼1.73 vs RHE ∼0.68 vs Hg/HgO

1.12 1.00

1.77 vs RHE 1.71 vs RHE

1.04 0.93

1.84 vs RHE ∼1.90 vs RHE @ 5 mA cm‑2 ∼0.73 vs SCE @ 5 mA cm‑2 1.61 vs RHE 1.62 vs RHE 1.62 vs RHE 2.02 vs RHE ∼1.75 vs RHE @ 5 mA cm‑2 0.75 vs SCE

1.64 1.62

La0.3(Ba0.5Sr0.5)0.7Co0.8Fe0.2O3−δ151 Mn oxide364 MnCoFeO4444

0.73 vs RHE 0.78 vs RHE

MnCoFeO4444 MnO2/Co3O4445

0.20 vs RHE ∼0.28 vs RHE

Co3O4446

∼-0.27 vs SCE

20 wt % Ir/C364 20 wt % RuO2/C444 20 wt % Ru/C364 20 wt % Pt/C364 Caron foams447

0.69 0.68 0.61 0.86 ∼0.82

Nitrogen-doped coaxial carbon nanocables448

∼-0.37 vs SCE

vs vs vs vs vs

RHE RHE RHE RHE RHE

Feature

1.00

Supported on graphene

0.92 0.94 1.01 1.16 0.93

Precious metal Precious-metal oxide Precious metal Precious metal N and P codoped Mesoporous Core−shell structure

1.12

a The data in italic style are extracted from the linear sweep voltammetry curves. The potentials vs the Ag/AgCl, Hg/HgO, and SCE reference electrodes could be converted to the RHE via the Nernst equation.

E RHE = EAg/AgCl + 0.059pH + EoAg/AgCl E RHE = E Hg/HgO + 0.059pH + E oHg/HgO o E RHE = ESCE + 0.059pH + ESCE

where ERHE is the converted potential vs RHE; EAg/AgCl, EHg/HgO, and ESCE are the experimental potentials measured against Ag/AgCl, Hg/HgO, and ° , EHg/HgO ° , and ESCE ° are the standard potential of Ag/AgCl, Hg/HgO, and SCE at 25 °C, respectively. SCE reference electrodes, respectively; EAg/AgCl

half-wave potential of the state-of-the-art ORR catalyst (e.g., Pt/C), while an OER current density of 10 mA cm−2 is approximately equivalent to the expected current density for an ideal solar cell device with 10% efficiency, under 1 sun illumination.431,434 However, comparison of different oxygen catalysts remains challenging as the ORR/OER overpotentials are sometimes reported for lower current densities, if the above are not achievable. Additional activity indicators include the

onset potential (ORR/OER), the diffusion-limiting current (ORR), the half-wave potential (ORR). Table 1 summarizes the state-of-the-art bifunctional catalysts based on the aforementioned relation where a few exceptions are noted. It should be noted that the experimental conditions, such as, catalyst loading, rotation speed, scan rate, O2-saturated or N2saturated electrolyte, and electrolyte concentration, may be not consistent across these studies. In addition, some studies did AB


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descriptors has evolved to the atomistic level, by leveraging on molecular orbital theory and DFT calculations.


not adopt capacitance- and IR-corrected ORR/OER activities. Although many factors may influence the evaluation of materials, it is clear that electrocatalytic activities for both ORR and OER of nonstoichiometric oxides (here perovskite oxides) are comparable to those of precious-metal-based catalysts and carbon-based materials. In summary, the above discussion has highlighted the promising activities for ORR/OER of nonstoichiometric oxides, but also brings attention to the limitations of objective comparison. It should also be noted that the activities presented in this section are not intrinsic, so it is problematic to correlate these to the atomic-level mechanisms. In sections 7 and 8, we review activity descriptors and mechanistic understanding, which may help the rational comparison and design of oxygen catalysts.

7.1. Descriptors from Molecular Orbital Theory

In order to devise descriptors for ORR/OER, it is essential to understand the electronic structure of the oxides. Although perovskites are ionic crystals, the bonds between the transition metal and oxygen usually exhibit both mixed electrostatic and covalent interactions. From the electrostatic perspective, the Madelung potential, defined as the Coulombic potential exerted on an ion site by all other ions, is significant to the perovskite molecular orbital structure. For the perovskite structure, this potential oppositely effects the anion and cation electron orbitals as shown in Figure 23.455 As cation electrons are

7. DESIGN PRINCIPLES Research on single and mixed oxides has increased greatly since the pioneering work of Beer on conductive oxides as electrocatalysts.449,450 However, due to the complexity and multiplicity of the parameters governing ORR/OER, it is difficult to establish unified design principles for effective oxide catalysts. This issue was first addressed by Hickling and Hill in 1947 in their work on metals.451 Although their OER overpotential measurements on a number of metals were reproducible, a general theory was not proposed until the contribution of Rütschi and Delahay.452 By assuming the initial discharge of water to form adsorbed hydroxyl radical at a metal site (M−OH) as the rate-determining step at high overpotentials, Rütschi and Delahay predicted that the overpotential decreased with the increase of OH adsorption strength to the metal surface. However, this trend was only qualitative because the bond strength calculated from different methods showed inconsistency. Later work for perovskite oxides revealed, instead, an inverse dependence between the activity and B− OH bond strength, suggesting the desorption of intermediate OH as the rate-determining step.120,453 Furthermore, Tseung et al. proposed a set of guidelines for oxide OER catalysts based on basic concepts of electrocatalysis: first the redox potential of the transition metal should be lower than or close to the oxygen electrode potential; second, the electronic conductivity of the oxide should be relatively high; third, as the oxides need to reduce oxygen, the surface should not adsorb oxygen too strongly.53 Later a pioneering study by Trasatti put forward several criteria for predicting OER activities of various oxides in a unified fashion.121 In particular, the volcano-shaped plot of OER overpotential against the standard oxidation enthalpy from lower state to higher state was introduced for simple oxides, agreeing well with the Sabatier principle. Other descriptors such as the pH of zero charge (pHzc) and gasphase isotopic oxygen exchange kinetics were also introduced.454 Recently, significant progress has been made in the application of nonstoichiometric oxides, specifically perovskite-type oxides, as oxygen catalysts for ORR and OER. The development of efficient oxygen catalysts for low-temperature applications remains challenging, and it is primarily associated with the lack of fundamental understanding of material design principles. It has been suggested that searching for highly active catalysts with suitable design principles can effectively accelerate the identification of efficient ORR/OER catalysts.115,116,159 Among the recent works, the search for

Figure 23. Formation of electronic state splitting from electrostatic potentials.

surrounded by negatively charged oxygen anions, this produces a repulsive Madelung potential and therefore increases the orbital energy. Whereas for anion electrons, an attractive Madelung potential decreases the orbital energy. This potential is responsible for the stability of the O2− in the perovskite lattice, even though the second electron affinity of O is positive. In addition, from this ionic model perspective, the electronic conductivity of the perovskites can be qualitatively analyzed from the electronic configurations. For example, SrTiO3 is predicted to be insulating since Sr2+, Ti4+ and O2− all have closed-shell electronic configurations. On the other hand, openshell electronic configurations may produce electronically conductive materials, which are beneficial to ORR/OER. The B-site transition metal in perovskites forms BO6 octahedra with O2− ligands, which leads to orbital splitting for the B-site metal d-band and modifies the material band structure. The electronic structure can be explained from first principle molecular orbital theory. For free ions, the approach of negatively charged O2− to the B-site metal cations leads to an increase in the total orbital energies B-site d electrons as rationalized by the Madelung potential and shown in Figure 23. If the negative charge is spherically symmetric, then the lifting of the d electron energies will be uniform. However, in the octahedral configuration, O2− approaches the B-site cation from certain directions. This results in a nonuniform energy splitting of the d electrons. Specifically, due to the symmetry of these orbitals, dz2−r2 and dx2−y2 form σ bonding and σ* antibonding states with the O 2p orbitals. It should be noted that the σ* antibonding states are indicated by eg. On the other hand, dxy, AC



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dyz and dxz interact with O 2p orbitals and form π bonding and π* antibonding states, noted as t2g. Typically the σ interactions are stronger than the π interactions due to larger electron overlap, resulting in higher energies for eg in comparison to t2g. As the A-site cation exhibits a higher energy state, it does not explicitly contribute to the valence or conduction bands near the Fermi level. These states are usually unoccupied and for this reason, the A-site cation is generally overlooked in band structure analysis. However, the electrostatic potential of the Asite cation significantly influences the p-d valence and conduction bands.455 Moreover, the size of the A-site cation has a decisive role in the structural distortion of perovskites from the ideal cubic phase, and hence the BO6 octahedra. Due to the mixed ionic covalent nature of the BO6 octahedra in perovskites, the metal d orbitals and oxygen p orbitals may hybridize and form continuous bands. Furthermore, the covalent mixing from the p-d electron overlap provides electron pathways between the B-site cation and O anion, which is primary for the material electronic conductivity. Therefore, the enhancement of the p-d hybridization is key and methods include, increasing the metal cation electronegativity and d orbital occupation. In general, increasing the metal cation electronegativity moves the metal d band toward the oxygen p band, increasing interactions and the ligand-field splitting. In the periodic table, the number of d electrons and the electronegativity increase from left to right across a row. For perovskites with early transition metal as B site, e.g., LaCrO3, the B-site cations are less electronegative than oxygen anion and therefore the metal d band is higher than the oxygen p band.456,457 As a result, these perovskite oxides usually exhibit insulating electronic structures, with localized d electrons occupying the highest valence states. However, if the d band is lowered, delocalized oxygen p electrons largely occupy the highest valence states, leading to semiconducting/metallic electronic structures. Guided by the molecular orbital theory, Bockris and Otagawa first identified the B-site d band electron occupation correlation to OER activity, for the perovskite series with Ni, Co, Fe, Mn, Cr, and V as B-site cations.120 The OER current density at a given overpotential was found to increase with the number of d electrons, following the order V < Cr < Mn < Fe < Co < Ni. This is also consistent with the electronegativity and ionization energy of the B-site cation, and essentially related to the bond strength between the reaction intermediates and the active site. In contrast to this linear descriptor, Dowden earlier proposed an M-shape dependence of the adsorption strength to the number of d electrons, with an enhancement of adsorption at d3 and d8 and minima at d0, d5, and d10.458 This descriptor distinguished the contribution from the eg and the t2g orbitals of the d electrons, considering the electron filling order of these two orbitals with increasing the number of d electrons. The eg and t2g occupancy are related to the crystal field stabilization energy, which was found important in deciding the activation energy of catalytic reactions.458,459 The eg/t2g occupancy is a result of the competition between the electron pairing energy and the crystal field splitting energy. Two electrons occupying the same orbital experience repulsive forces. If this repulsive Coulombic force overcomes the crystal field splitting energy, then the electron will fill the eg orbital after singly occupying the 3 t2g degenerate states, forming a high spin configuration. Otherwise, a low spin configuration is formed with low eg occupancy. Based on this, and as mentioned earlier, the stronger σ interactions of the eg states govern adsorption and

bonding, and are therefore better correlated to OER/ORR activities. Furthermore, this rationale also leads to the recent egfilling descriptor devised by the Shao-Horn group.115,116 The Shao-Horn group recently extended the above rationale and demonstrated that the ORR activity for perovskite-type oxides, with various A-site and B-site substitutions in alkaline solution, can be primarily correlated to surface electronic structure, e.g., transition-metal eg-filling and covalency.115,433 This work found that, for the σ*-orbital (eg) occupation, an egfilling of ∼1 could be correlated to the maximum activity. The optimum eg-filling was found for LaMnO3, LaCoO3, and LaNiO 3 perovskites. It was clearly demonstrated that La1−xCaxCrO3 with too low eg-filling of ∼0 and La1−xCaxFeO3 with too high eg‑filling of ∼2 exhibited a lower ORR activity. This reduction was attributed to the too strong (low filling) and too weak (high filling) transition-metal−oxygen covalent bonding, suggesting that a maximum activity is achieved at a moderate (neither too strong nor too weak) bond strength. Visualization of this trend is shown in Figure 5c, where a volcano-type correlation of eg-filling of B-site transition-metal cations in ABO3 with the ORR activity is observed. These materials exhibited a Tafel slope of ∼60 mV decade−1, and therefore, a voltage span of 0.25 V corresponds to an approximately 4 orders of magnitude increase of the intrinsic ORR activity. It was also suggested that, by increasing the covalency between the B-site transition-metal 3d and oxygen 2p orbitals, an even higher ORR activity for perovskite-type catalysts could be achieved. The eg occupation and metal− oxygen covalency were suggested to be the primary and secondary descriptors for designing efficient catalysts for ORR. Later, they continued this line of research of ORR descriptors and applied the same principle to predict OER activity. The group demonstrated that the OER activity follows similar design principles for most perovskite oxide materials.116 Indeed, a similar volcano-type correlation for eg-filling in relation to intrinsic OER activity was observed for the perovskite oxides tested (Figure 5a). This work undertook detailed characterization of the perovskite oxides studied, using RRDE and galvanostatic measurement, and suggested an exceptionally high OER activity for the Ba0.5Sr0.5Co0.8Fe0.2O3−δ perovskite material. Surprisingly, the measured intrinsic OER activity of Ba0.5Sr0.5Co0.8Fe0.2O3−δ was at least an order of magnitude higher than that of the state-of-the-art catalyst (IrO2 nanoparticles, ∼6 nm) in alkaline solution (Figure 5b). Furthermore, the same group suggested promising design strategies for highly active bifunctional catalysts for ORR/OER, by tuning the surface electronic structure in for perovskite oxides. The group effectively employed fundamental molecular orbital theory to determine a simple descriptor, the eg-filling parameter. This parameter serves as an indicator of the transition-metal−oxygen bonding, with under-filling suggesting insufficient desorption of intermediates due to the overly strong oxygen binding, and overfilling resulting in weak binding of intermediates and loss of oxygen activation. This descriptor also serves as a simple method to prescreen catalysts, based on the electronic configuration of the transition-metal cation, i.e. perovskite oxides, with B-site eg-filling close to 1, and for partially substituted B-site cations, the most active cation should be chosen. It should be noted that oxygen adsorption energies as computed by DFT on the perovskite series, LaBO3 (B = Ti, V, Cr, Mn, Fe, Co, Ni, and Cu, for increasing adsorption energy barrier from left to right), were found to be well described by AD



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Figure 24. (a) OER activity trends plotted against the ΔG0O* − ΔG0HO* descriptor for perovskites. Reprinted with permission from ref 476. Copyright 2011 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim. (b) OH adsorption energies on metals (red), monoxides (blue), La/Y perovskites (green), and Sr/Ca/Ba perovskites (orange), as a function of the number of outer electrons. Reprinted with permission from ref 473. Copyright 2013 Royal Society of Chemistry.

both the eg and t2g occupancy.460 In fact, the occupancies of eg and t2g have contributions from both the occupied valence band states and the conduction band states, and it is the number of occupied conduction states that correlate with the activity trend. Therefore, the reactive states near the Fermi level could be a good descriptor.460

parameters as indicators to the material ORR/OER catalytic activity. Previous work has suggested the transition-metal dband center, for metal catalysts, as a descriptor to correlate with the catalytic performance.463,481 Inspired by the band-centerdetermined descriptor, in a recent work, the Morgan group identified the O p-band center as a descriptor.474 This descriptor was rationalized in terms of the rigid band model, whereby the addition of O into the system corresponds to the transition of electrons from the Fermi level to the O p-band (the opposite for the removal of O), and since O adsorption and removal is key to ORR, this serves as an effective descriptor. Specifically, this study suggested that a higher pband center relative to the Fermi level is correlated with higher ORR performance for SOFC cathode applications. This approach also confirmed the Ba0.5Sr0.5Co0.75Fe0.25O3 material for the best ORR activity (the specific atom fraction is chosen due to the limitation of computational supercell size). In addition, although outlined for high temperature ORR, this descriptor was found to be effective in describing lowtemperature OER,114 given the accurate prediction for the same optimum oxide for OER, consistently with the eg-filling model.116 Furthermore, the O p-band descriptor is consistent with predictions of surface oxygen vacancy formation energies.474 This study notably employed the Hubbard U approach, to model the highly localized d orbitals of the transition-metal B sites and, furthermore, found the d-band descriptor had a limited correlation to activity. However, in a different study, the surface oxygen vacancy formation energy was found to correlate with the d-band center in perovskites; that is, it decreased with the decrease of the surface d-band center of the (001) surfaces (d-band center: LaTiO3 > LaVO3 > LaCrO3 > LaMnO3 > LaFeO3 > LaCoO3 > LaNiO3 > LaCuO3 and SrTiO3 > SrVO3 > SrCrO3 > SrMnO3 > SrFeO3 > SrCoO3 > SrNiO3 > SrCuO3).482 In addition, the surface adsorption energy followed a reversed trend with respect to the d-band center.482 Besides, the strain effect was also discussed; in LaBO3, compressive strain was found to make the dissociation of O2 more favorable, while the tensile strain made it less favorable. In SrBO3, both strain types made the O2 adsorption easier but the difference was small.482 This study did not employ the Hubbard U approach, and this was suggested as the driver of the discrepancy between this study and the work by

7.2. Descriptors from DFT Calculations

In addition to the molecular orbital theory interpretation, DFT has also been widely used in the initial design of ORR and OER materials in this field. DFT is primarily employed to screen possible active materials,461−467 understand the ORR/OER mechanism,159,468−472 and identify descriptors for guiding materials selection.473−478 Increasingly accessible and economic computational resources are driving the growth of DFT as a predictive tool. However, for the DFT calculations involving transition-metal oxides, identifying an accurate method to account for the localized d-orbital electrons is often a challenge. A widely used approach is to employ a DFT+U approach by including the semiempirical Hubbard-U parameter. A previous study on the popular LaBO3 (B = Mn, Fe, Co, and Ni) perovskite series found that key activity properties, such as the surface oxygen vacancy formation and adsorption, were strongly influenced by this parameter.174 Therefore, determining an accurate parameter is essential, and several methods are suggested, including experimental oxidation and electronic and optical property fitting. However, for more complex doped perovskite A1−xA′xB1−yB′yO3−δ structures,479,480 or newly developed materials, fitting the U parameters for multiple cations requires simultaneous optimization or a lack of data exists. As a result, in several studies the U parameter is not included. Furthermore, recent work identified a strong dependency of the ORR energetics and potential-determining step (PDS) on the exchange correlation employed, and suggested that with a hybrid functional including nonlocal orbital-dependent Hatree-Fock methods, the experimental trends were most accurately modeled for the LaBO3 series (B = Mn, Cr, and Fe) in comparison to the generalized gradient approximation (GGA) and GGA+U approaches.159 The descriptor approach developed from DFT calculations potentially provides a fast and effective method to develop ORR/OER catalysts, bypassing experimental cost and time. The basics of this approach is to use easily obtainable DFT AE



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the Morgan group.474 As the Hubbard U approach serves to localize the d orbitals, using this model, the electrons behave less like metals. Therefore, in the DFT+U approach, the d-band model for the adsorption may not be valid, since the model is for the interactions between adsorbates and the delocalized metallic states. Studying OER by first principle periodic DFT calculations, the Rossmeisl group developed a single universal descriptor for assessing the OER activity on oxide surfaces.476 This descriptor related the overpotential for oxygen evolution and the free energy difference between O* and HO*, i.e. ΔG0O* − ΔG0H*. This was rationalized by considering the 4-step OER process, where the potential determining step is either HO* ⇔ O* + H+ + e− or O* + H2O(l) ⇔ HOO* + H+ + e−. The relative values of the free energies, ΔG0O*, ΔG0HO*, and ΔG0HOO*, can determine the reaction barrier, and therefore a descriptor of OER activity. Interestingly, a constant difference of 3.2 eV was found for the ΔG0HO* and ΔG0HOO*, and therefore, only two free energies are necessary to identify this descriptor. Incidentally, one should note that this is different from a previous descriptor using the O* binding energies.477 Their results found a similar volcano-type correlation for free energy difference with OER catalytic activity, where the perovskite materials, SrCoO3 and LaNiO3, lie at the top of the volcano plot, as shown in Figure 24a. The activity for all the perovskites studied followed the trend SrCoO3 > LaNiO3 > SrNiO3 > SrFeO3 > LaCoO3 > LaFeO3 > LaMnO3, consistently with the experimental observations in alkaline solution.120,483 Interestingly, further work found that the contribution of the Hubbard U term serves to shift the volcano in comparison to “simple” DFT calculations; that is, the data points on the left branch go uphill while those on the right branch go downhill.484 For example, with “simple” DFT, this descriptor predicted an activity trend of RhO2 > IrO2 > PtO2 > RuO2, which was corrected to IrO2 > RhO2 > RuO2 > PtO2 with the DFT+U approach. The DFT+U approach proved more consistent with experimental observations.485 A similar descriptor for ORR was proposed by the Jaramillo and Rossmeisl group.478 Their work pointed out that the free energy of adsorbed HO*, i.e. ΔG0HO*, could also be applied as a universal descriptor for ORR activities. Calle-Vallejo et al. also employed DFT techniques to identify a new descriptor for adsorption energies of the ORR/ OER intermediates, based on the number of outer electrons for transition metals (Sc to Cu) and their oxides.473 The impact of varying the transition-metal element, the valence state, and the coordination number of surface metal atoms was systematically investigated, relative to the adsorption energy. The investigation determined a correlation between the adsorption energies of the reaction intermediates and the outer electrons, following a simple linear trend, as shown in Figure 24b. This descriptor provided a simple yet effective approach to rationally select catalysts for ORR/OER. In summary, these descriptors recognize the importance of the adsorption energies of OER/ ORR intermediates and find correlations to the band structure and electronic configurations. In particular, while ΔG0O* − ΔG0HO* and ΔG0O* have been proposed as descriptors for predicting the activities, new trends may be discovered if alternative rate-determining or potential-determining steps are identified. In addition, the existing descriptors that are related to band structures have primarily used the d-band of the transition metal or the O p-band for OER of perovskites. It may be interesting to investigate how these two bands interact

within perovskites from DFT calculations and how the interaction may be correlated to the activities. The importance of oxygen vacancies on OER/ORR activity has been recognized in several studies.213,355 Furthermore, a very recent study linked the activity of oxygen vacancies to the oxygen exchange capacity of perovskites, where thermochemical oxygen separation using oxides was employed.486 This design principle was validated by the fact that SrCoO3, taking the peak of the volcano plot constructed by the free energy of the redox cycle versus the enthalpy of oxide reduction, exhibited the best performance. Recently, another descriptor was proposed by Deml et al. to predict the oxygen vacancy formation energy, a parameter closely linked to the performance of nonstoichiometric oxide catalysts.475 This descriptor considered both the oxide formation enthalpy and band gap energy, and found that the energy to form a single, neutral oxygen vacancy decreased with both parameters. Employing these parameters as a descriptor is logical, as the oxide formation enthalpy is associated with metal−oxygen binding strength, and the band gap energy is associated with oxygen vacancy electron redistribution. Therefore, both parameters drive the oxygen vacancy formation energy. Although accurate, the linearity of this descriptor was limited to materials from a single crystal structure group, in this case perovskites. Therefore, the group recently extended this study and developed a new descriptor model of single functional form, for 45 binary and ternary oxide groups including perovskites. The main development in the model was the inclusion of the midgap energy relative to the O 2p band center and the atomic electronegativities.487 These findings extended our understanding of the nature of oxygen vacancy formation in complex oxides and provided a fundamental method for predicting oxygen vacancy formation energies using purely intrinsic bulk properties. Although, as discussed above, several descriptors have been identified to predict trends in activity of oxides, due to the many approximations in computational modeling, direct consistency with experimental observations still remains challenging. Indeed, the complex environmental conditions, mechanisms, and kinetics of ORR/OER are, at present, limited or omitted from descriptor development, including pH, surface area, and temperature. Furthermore, as discussed previously, material properties are strongly dependent on fabrication method and environment. For example, Co3O4 was predicted to be more active than RuO2 from computations,476 whereas experimental observations suggested the opposite.483 Experimentally, Co3O4 shows nonstoichiometry by possessing excess oxygen in the lattice, and the crystallite size varies depending on the calcination temperature,454 which introduces variations to the measured activities. These aspects, however, are not considered in computations. Even so, some results obtained by using Co3O4 thin films showed high activities, matching the theoretical predictions.488 This example highlights an additional prominent challenge to developing universal descriptors.

8. MECHANISTIC UNDERSTANDING A holistic understanding of the rate-determining steps in ORR/ OER is of critical importance for the rational design of materials. Here, the processes of ORR/OER are discussed for alkaline media, as, in general, oxides are unstable in acidic solutions. Despite significant work undertaken in the application of nonstoichiometric oxides (specifically perovskite-type oxides) as ORR/OER catalysts, the exact oxygen AF


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reduction pathways on perovskites or perovskite/carbon remain unclear. It has previously been suggested that, in alkaline solutions, the O2 reduction to OH− typically follows several pathways (Figure 25): (i) a direct 4-electron transfer process, where O2 is


Figure 26. Role of perovskite and carbon within the perovskite/carbon composite during ORR in alkaline solution. The main concept in the drawing is according to ref 489.

in Figure 26). This suggestion is reasonable given the excellent catalytic ability of transition-metal cations for the decomposition of hydrogen peroxides.492,494−498 Furthermore, it has been shown that oxygen deficiency and/or redox couples in nonstoichiometric oxides favor the electrocatalytic reduction of HO2− to OH−.499 However, the perovskite oxide may also contribute to the O2 reduction to HO2− in the initial 2-electron transfer step of ORR. This is followed by the chemical or electrochemical reduction of HO2− species to OH− (shown on the right side in Figure 26). In addition, in some cases carbon materials themselves, especially functionalized nanotubes and graphene with efficiently catalytic sites, can catalyze both O2 reduction to HO2− and effective decomposition of HO2− . The direct 4-electron oxygen reduction mechanism has been associated with the catalyst sites of perovskite oxides only, even if carbon is added to the electrodes during the preparation.115,435 This is because the measured ORR activity is considered dominated by the oxide since the background contribution from carbon materials is negligible above ∼0.7 V vs RHE.115,435 The 4-electron direct ORR pathway of O2+2H2O+4e− → 4OH− consists of 4 steps (Figure 27a) for pure perovskite-type oxide catalysts in alkaline solution.115,170 The 4-step process involves (1) surface hydroxide displacement, (2) surface peroxide formation, (3) surface oxide formation and (4) surface hydroxide regeneration. The ratedetermining step of this mechanism has been suggested as either the surface hydroxide displacement (1) or the surface hydroxide regeneration (4). The mechanisms for OER have been studied for several decades,120,453,483,500−502 with most derived from the work by Bockris and Otagawa. Despite numerous studies on the OER, mechanistic models of the reaction steps based on atomic-level experiments are scarce. The primary reason for this is the difficulty in identifying the reaction intermediates in direct spectroscopic experiments.503 In general the overall mechanisms for oxides involve several consecutive interaction steps between the reactants and the oxide surfaces. In alkaline media, OER typically begins with OH− adsorption onto the oxide active sites, followed by elementary steps that differ according to different mechanisms, yet all involve the adsorption/ desorption of intermediates such as O*, HO*, and HOO*. The multiple OER paths suggested for electrode kinetics in alkaline solution were summarized in a recent review.504 An example OER path for perovskite oxides in alkaline media is given in Figure 27b,116 where 4 elementary steps are involved. This basic OER 4-step process is as follows (Figure 27b): (1) OH− is adsorbed onto the surface active site, which in the perovskite case is usually the B-site transition metal. Subsequent to this, H2O is formed, by the interaction of another OH− with the adsorbed, and produces O2− on the Bsite. A free electron is generated as a result of this reaction, increasing the valence state of the B-site transition metal. (2)

Figure 25. Proposed ORR pathways in alkaline solution.

reduced to OH− (①); (ii) an initial 2-electron transfer process, of O2 reduction to both peroxide intermediates HO2− and OH− (②); then depending on the intrinsic electrocatalytic properties of the catalyst, the intermediate is either reduced to OH− via a 2-electron transfer pathway (③), (iii) the HO2− diffuses away from the electrode (④), or (iv) it undergoes chemical disproportionation (⑤).143,489,490 The three distinct pathways consist of either the “direct pathway” (①) or the two “serial pathways” (②+③ or ②+⑤), and all ensure an overall 4 electrons transferred.143,148 Based on the standard pathway outlined above, the quantity of HO2− produced during ORR is indicative of the process followed. The direct 4-electron pathway (①) is optimum, and therefore, producing more HO2− indicates a selectivity for other indirect pathways and, hence, a lower catalyst efficiency for ORR. The quantity of HO2− produced during ORR can be determined using the RRDE technique, where the ring potential is poised to oxidize the HO2− formed in the diskreduction process. This technique has been employed in many studies to identify the preferred pathway for material development. Detailed information about the RRDE technique and the usage in the evaluation of oxygen catalysts can be found in a recent book.491 Recent studies have shown a limited ORR activity for pristine perovskites; therefore, composites with carbon are typically employed for perovskites and other nonstoichiometric material catalysts.490 Indeed, studies have demonstrated a dramatic increase of the ORR activity for persovskite with carbon composites.143,492 Therefore, as this is fast becoming a common method, it is important to identify the individual roles of perovskite and carbon in the perovskite/carbon composites during ORR.143,489,490,493 As shown in Figure 26, in addition to working as the electrical contact between perovskite particles and ensuring the adequate dispersion of the perovskite particles, carbon may also catalyze the 2-electron O2 reduction to HO2− in ORR (shown on the left side in Figure 26).489,492,494 As perovskite/carbon composite electrodes typically exhibit an approximate 4-electron transfer ORR process, it is suggested that the perovskite facilitates the HO2− decomposition and/or reduction (shown on the left side AG



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Figure 27. (a and b) Proposed (a) ORR and (b) OER pathways on perovskite-type oxide catalysts in alkaline solution. Reprinted with permission from ref 116. Copyright 2011 American Association for the Advancement of Science. (c) Another proposed OER cycle by utilizing lattice hydroxide. Reprinted with permission from ref 173. Copyright 2014 American Chemical Society.

the B-site adsorbed O2− reacts with another OH− and forms HO−2 , resulting in a reduction in the B-site valence and producing a free electron. (3) Another OH− from the solution interacts with HO−2 and forms H2O, leaving an O2− 2 adsorbed on the B site, increasing B-site valence state and producing a − free electron. (4) The B site adsorbed O2− 2 is replaced by OH , releasing one free electron, and O2 is produced. Step 2 (peroxide formation) and step 3 (extraction of proton) are generally considered to be the rate-limiting steps in the overall OER process. Another plausible pathway for OER involves surface lattice hydroxide species.173,241,453 It has been widely shown for metal oxides that the lattice oxygen may be employed as the oxygen source for gas phase catalytic reactions. This mechanism was demonstrated by isotope labeling studies of IrO2505 and RuO2,506 and was further proposed in the case of LaNiO3241 in the OER. Considering the contribution of surface lattice oxygen, another 4-step OER mechanism was proposed based on the original work of Bockris et al.,453 and shown in Figure 27c. In step 1, hydroxide radicals form on the surface. Then, in step 2, OH− from the alkaline media react with the hydroxide radicals on the surface to form H2O, and leave O2− at the catalyst surface. These two steps are similar to step 1 of Figure 27b. However, the reaction mechanism differs substantially in step 3, as shown in Figure 27c. In this mechanism, a surface lattice oxygen is transferred to the adsorbed OH− species to form HOO−, leaving surface oxygen vacancies. In the last step, the OH− from the alkaline solution combines with the oxygen vacancies and generates O2. This reaction mechanism assumes that oxygen vacancies play a key role and identifies that the disproportionation reaction could be the rate-limiting step for the OER with perovskite catalysts. Recently, Hardin et al. systematically studied the mechanisms of OER in LaBO3 series (B = Ni, Ni0.75Fe0.25,Co, Mn).173 This study found the worst

catalytic performance for the LaMnO3/N-doped carbon composite, with a large Tafel slope of 145 mV decade−1 compared to 42 to 51 mV decade−1 for others in the series. Furthermore, LaNi0.75Fe0.25O3, LaNiO3, and LaCoO3 experienced a similar response in the rate-limiting step, in which the OER increased faster at higher voltage, which was not observed for LaMnO3. The authors proposed that the difference was associated with the intrinsic ability of the material to disproportionate HOO−, and they concluded that LaMnO3 was limited by this elementary step. While these findings were inconsistent with the eg filling descriptor, they seem to correlate with the oxygen vacancy formation energies at the surface, suggesting the key role of lattice oxygen in the electrocatalysis.173 The majority of recently developed catalysts for OER typically include carbon as part of the electrode; however, only a handful of works study the influence of carbon on the OER activity. In perovskite/carbon composites, the perovskite oxide has been suggested to be responsible for the oxidation step of hydroxide anions, since carbon materials have limited OER activities.490 In addition, previous work has identified a negligible contribution of carbon toward the OER activity when the potential is below ∼1.65 V vs RHE.116 In spite of this, other recent studies have demonstrated an enhancement in OER activity from the carbon supports, in addition to improving conductivity.173,507,508 Nevertheless, the interaction between perovskite oxide and carbon in the perovskite/carbon composite that is responsible for the enhanced OER activity still remains unclear. The recent progress in understanding the ORR/OER mechanism has been aided by the rapid development of resources and simulation tools. As a result, the individual elementary step of ORR/OER at the electrode surface can be effectively modeled and elucidated by employing, for example, AH



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Figure 28. Standard free energy diagrams for an ideal catalyst (a and c) and LaMnO3 (b and d). Adapted with permission from ref 476. Copyright 2011 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim.

reaction proceeds spontaneously as shown in Figure 28c. However, for LaMnO3 the standard free energy of step 3 is much higher than the rest of the steps, indicating the formation of peroxide as the PDS. This is also indicated in Figure 28d for the case of an applied potential of U = 1.23 V. Here, although the overpotential for the full reaction is 0, step 3 still exhibits an energy barrier to form O* to HOO*. The ORR proceeds in the reverse direction, i.e. from the right to the left in the energy diagrams in Figure 28. For the ideal catalyst, the ORR is spontaneous at the Nernst potential. However, for LaMnO3, considerable potential barriers exist in the reaction steps from O* to HO* and from HO* to H2O. The computational hydrogen electrode model introduced by Nørskov et al. is commonly employed to approximate the electrode and chemical potentials when modeling the ORR/ OER reactions.477,510 As a result of the implicit inclusion of condition parameters, the PDS determined in one medium is, in principle, equivalent to others. Indeed, a further similar study of the OER mechanism on LaMnO3 surfaces identified a common PDS of the formation of HOO* from O*, across a range of pH (0−12) and electrode potentials (0.5−2 eV).512 This study also explored the stability of surface structures and stoichiometry as a function of the electrolyte pH and electrode potential, and it introduced a novel method to include cationleaching effects of the electrolyte/electrode interface. The inclusion of these effects elucidated the experimental observations of LaMnO3 and LaMnO3+δ activity for OER and ORR. This study suggests that electrolyte leaching of surface Mn cations under OER conditions leads to an LaMnO3 surface oxidation state similar to that of LaMnO3+δ, which justifies the similar experimentally observed OER activity. Furthermore, the similarity is unlikely to occur in an O-rich environment, which explains the different ORR activities. The extension of this method to other cation-sensitive surfaces, such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ, may be valuable in their developments.

DFT calculations. A prominent example is the identification of the well-known Brønsted−Evans−Polanyi relations, which were determined from DFT calculations for O2 dissociation on transition-metal oxides, such as rutiles and perovskites.509 Using this approach, the OER mechanism is typically simplified into the following steps,476,477,510,511 H 2O(l) + * ⇔ HO* + H+ + e−

(3)

HO* ⇔ O* + H+ + e−

(4)

O* + H 2O ⇔ HOO* + H+ + e−

(5)

HOO* ⇔ * + O2 + H+ + e−

(6)

+

−

where H and e can be referred to the standard hydrogen electrode, i.e. at 298 K, 1 bar and 0 pH, the reaction free energy 1 /2H2 ⇔ H+ + e− is zero, and the chemical potential of (H+ + e−) can be related to the gaseous H2. The standard free energies of each reaction step, (3) to (6), defined as ΔG01, ΔG02, ΔG03, and ΔG04, are derived from the calculated ground state of the catalyst surface and adsorbed intermediates. The highest reaction free energy can therefore be indicative of the ratedetermining step, which is also referred to as the PDS. Note that the PDS mechanism is, in principle, applicable to the alkaline solution case, as all steps are influenced in the same way with the pH and potential U. Energy diagrams can be constructed for specific materials based on the computed reaction free energies, which clarify the ORR/OER mechanism. For example,476 the energy diagrams for the ideal catalyst in comparison with LaMnO3 at U = 0 V are shown in Figure 28a and Figure 28b, respectively, with the free energy of H2O as zero reference. For the ideal catalyst, all reaction steps have an equal free enecrgy of 1.23 eV at zero potential, so that when the electrode is biased by 1.23 V vs RHE, the Nernst potential of oxygen evolution, the free energy diagram becomes flat and the AI



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effective methods to maximize ORR/OER activity for MABs or LATCs include the following: tailoring the surface and bulk properties by cation or anion stoichiometry, optimizing the morphology, primarily by nanostructuring, modifying the surface electronic structure, and preparing composites/hybrids.

However, the significant sensitivity to pH and electrode potential also highlights the necessity for an atomistic approach to model surface/electrolyte interface, electrode potential, and the electrolyte environment. Other influences to ORR/OER beyond surface composition have also been identified. As outlined earlier, deposited thin films can be employed to provide well-defined geometries of oxygen catalyst materials. These thin film materials often exhibit different characteristics from bulk material, and these characteristics are largely attributed to the substrate/electode interface and lattice strain. A recent DFT study correlated the effect of surface strain with surface activity for oxygen adsorption and vacancy formation energies in LaBO3 and SrBO3, where B are the first row transition metals.482 This work found a correlation between surface strain and surface activity for oxygen adsorption and vacancy formation energies. This trend was rationalized to the strain induced changes to the Bsite and oxygen overlap, and consequent B-site d-band properties. However, the effects of strain were suggested as a tuning parameter for reactivity, as this study highlighted the dominant role of the B-site oxidation state and d-band filling. A recent miniperspective by the Rossmeisl group513 on first principle modeling of electrochemical electrode/electrolyte interfaces suggests that the electrochemical solid/liquid interface represents a present frontier in first principle modeling. The largely employed reaction intermediate free energy method focuses only on the adsorption of intermediates, with limited insight into the effects of the kinetics of individual reaction steps, the solvent kinetics, and indeed the electrode/electrolyte interface. Therefore, the development of current and new models to precisely incorporate these effects is essential to fully understand these reactions. Recently, several new methods to model the solvent media have been introduced, such as the water bilayer approach.514,515 In this approach, several layers of water are included in the surface model. This method enables the tuning of the electrode potential by introducing additional solvated protons into the water layer and excess electrons on the electrode surface. However, it still fails to capture the solvent fluctuations and prominently the fixed potential of the real electrochemical system. Methods to model the latter were the topic of a recent review: one approach identified was to determine the pH as a function of the electrode catalyst metal work function and the electrochemical potential of the protons and electrons (H+ + e−).516 The electrode potential is consequently set up by introducing additional H atoms into the system and by adjusting the electrolyte water dipole orientations. The employment of this method highlighted the pH effect on adsorbate coverage and water dipole orientation, which was expected to strongly influence the reaction barriers. This study was introduced for metal/solution interfaces and has already been extended to triple phase boundary studies of SOFCs.516 Therefore, the extension to oxide catalysts may be promising for more accurate surface screening, optimization, and mechanistic understanding.

9.1. Perovskite-Type Oxygen Catalysts

Perovskite-type oxides offer a diverse variety of material choices as oxygen catalysts for MABs and LTFCs. Among the oxygen nonstoichiometric perovskite-type oxides, Co-based catalysts are generally highly active yet unstable, especially in severe conditions, e.g., concentrated alkaline solution. Conversely, Febased and Cr-based catalysts are stable yet not very active. Furthermore, Mn-based and Ni-based catalysts perform better as practical oxygen catalysts with rather high activity and good stability. It should be remembered, however, that these trends are not conclusive and are highly dependent on the specific tailoring of the perovskite oxide compositions and structures. Usually, a balance between electrocatalytic activities and chemical stabilities may be reached through the rational design of catalysts, yet still far below the expectation. The extremely low specific surface areas of perovskite-type oxides are expected to be a significant limiting factor to their practical application. Therefore, the development of methods to enlarge the active sites is key. At present, it is difficult to obtain pure phase structures of perovskites with high specific surface areas, when materials are prepared at relatively low temperature or with novel methods.218,225 This is largely due to more elements with different characteristics in perovskites in comparison to simple metal oxides. The readers are referred to a critical review on the crystallization of ABO3 perovskites under hydrothermal or solvothermal conditions.334 9.2. Pyrochlore-Type Oxygen Catalysts

One drawback of pyrochlore-type oxides is the employment of toxic Pb or expensive Ru in some compositions. Therefore, replacing them with less toxic and less expensive elements, e.g., rare-earth metal or Sn, is a promising route to commercialization. The Ln2Sn2O7−δ (Ln = La, Pr, Nd, and Sm) series is identified as a promising candidate for ORR with an onset potential of ∼0.85 V vs RHE.517 Generally, pyrochlore-type oxygen catalysts require significant enhancement of their electrocatalytic activities to compete with other precious metals and metal oxides. 9.3. MnOx, CeOx, and NiOx Related Oxygen Catalysts

A relatively low conductivity of these catalysts is a critical issue limiting their ORR/OER activities. Increasing the concentration of anion or cation defects is a promising method to enhance both the electronic and ionic conductivity. Other directions, including partial substitution of cations, innovative synthesis, or adopting hybrids, may also be feasible to further develop these materials. 9.4. Catalyst Support

The support/substrate (e.g., carbon) is an indispensable component of nonstoichiometric oxide electrodes. Carbon support enables dispersion and has several additional advantages, including lightweight, good electronic conduction, low cost, and a porous structure. Carbon is, however, susceptible to electrochemical corrosion/oxidation under highly oxidative electrochemical potentials, accelerating the reduction of active sites. Furthermore, an additional disadvantage of carbon in nonaqueous LABs is that it is reactive with Li2O2 and produces an interfacial layer of Li2CO3 between carbon and

9. SUMMARIES AND RESEARCH DIRECTIONS This review of nonstoichiometric oxides as oxygen catalysts has shown that the ORR/OER activities and stabilities are not simply a consequence of oxygen nonstoichiometry and, instead, depend on many intrinsic and extrinsic factors. As highlighted throughout this review, the factors identified by the current literature are vast, and it is difficult to identify universal trends for nonstoichiometric oxide catalysts. Generally, the most AJ


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play a critical role in realizing highly stable and active batteries for practical applications in the near future.

Li2O2 on the charge (oxidation) cycle. This reaction is believed to be responsible for the large overpotential/low efficiency and drives the charging voltage to higher than 4 V.518,519 To overcome this drawback and to provide a low-cost and oxidation-resistant support/substrate, electronically conductive substitutes, including mesoporous silica, titanium oxide, boron carbide, Ni foam/mesh, titanium metal sheets, and stainless steel, have been explored.520−525

9.8. Advanced Characterizations

Other than preparing highly efficient oxygen catalysts with novel synthesis methods or with the adoption of novel compositions, advanced characterizations are also important to classify highly efficient materials and structures. In situ investigations may provide a fundamental understanding of ORR/OER which is beneficial to developing highly active catalysts for rechargeable LABs.410,540,544−547 The electrochemical strain microscopy technique may provide a direct visualization of the ORR/OER activation process on the scale of several nanometers and provide nanoscale understanding into local kinetics.548−550 In addition, a rapid identification of new oxygen catalysts with a fluorescence-based screening method has also been reported by Gerken et al.551,552 Furthermore, in operando and surface-sensitive techniques, e.g., synchrotron-based ambient pressure X-ray photoelectron spectroscopy, may promote the verification of active species to reveal the origin of the high electrocatalytic activities.385,414,553−556 For example, the importance of surface anion-redox chemistry in nonstoichiometric oxides has been proposed by Chueh et al.555 The accurate classification and improved understanding of the structures and kinetics using these techniques may finally boost the efficiency of MABs or related energy conversion and storage systems.


9.5. Electrode Architectures

Other than tailoring the physicochemical properties to enhance the intrinsic catalytic activities, it is also beneficial to apply novel synthesis strategies in order to develop nanostructured catalysts. As highlighted by the Chen group,119 desirable electrode architectures should possess hierarchical pores from micro- to meso- and macroscale for achieving effective gas transportation, liquid wetting, and solid accommodation. Developing electrode architecture with carbon- and binderfree methods is also a feasible way.526 9.6. Solid Electrolytes/Separators/Protectors

The ever-increasing demand for batteries with high power and energy densities motivates the development of Li-based battery technologies.527,528 Solid electrolytes have a promising potential, to improve the stability and safety of the nextgeneration high-energy batteries. The application of solid electrolytes is, however, currently limited by their relatively low conductivities. The recently developed Li1.35Ti1.75Al0.25P2.7Si0.3O12,529 Li1+x+yAlxTi2−xSiyP3−yO12,530,531 Li1+xAlyGe2−y(PO4)3,532 Li10GeP2S12, and related materials,533−535 and antiperovskites536 have extremely high ionic conductivities, comparable to or even higher than those of organic liquid electrolytes. This finding may, in turn, promote the development of the next generation Li-based batteries (e.g., LABs). However, several factors still remain challenging for their application, including the chemical stability with electrode, phase, and electrochemical stability of these materials.531,534,537

9.9. Transferring Current Materials

An efficient approach to discover new materials with high intrinsic activities is to tune the electronic structure of existing materials. Materials that have been intensively studied for cathode applications in Li-ion batteries and Na-ion batteries,422−426 e.g., Na0.44MnO2 and LiCoO2/Li1−xCoO2, have also been applied and tested as oxygen catalysts for MABs.427,428 Intelligent catalysts (e.g., LaFe0.57Co0.38Pd0.05O3557), with active precious nanoparticles anchored on the surface of perovskite-supporting frameworks, have been widely adopted as catalysts for automotive emissions control and high-temperature fuel cells.558 Due to the similar working principle, they may also be introduced in MABs and LTFCs.559 A recent study indicates that Pd3/4+ in LaFe0.95Pd0.05O3−δ and LaFe0.9Pd0.1O3−δ is even more active (84-fold higher based on mass activity) than Pd0 exsolved from the perovskite structure.560 Given this observation, it may be a promising strategy to tune noble metal (e.g., Pt, Ru, and Rh)-doping perovskite oxide, since the mass activity can be dramatically improved. Furthermore, similar to the concept of N-doped carbon nanotubes or graphene with enhanced electrocatalytic activities, improvements may be achieved by inserting nitrogen, fluorine, or chlorine into nonstoichiometric oxides.561−564

9.7. System

Despite a high energy density, MABs have a relatively low power density. Developments and elucidations of ORR/OER activities in the alkaline environment are advancing quickly, yet little is known about these ORR/OER mechanisms in the presence of Li+ ions. Besides, cations in the alkaline solution, e.g., Li+, Na+, K+, or Ba2+, may also influence the ORR/OER activities.538 Development in the understanding of the cation effect may further clarify the surface reaction on nonstoichiometric oxides in alkaline solution, leading to better system design. In addition, for the aprotic nonaqueous LABs, additional challenges are also required to improve energy storage and prevent Li2O2 from clogging and surface growth, and for better mechanistic understanding of the entire system.539,540 Several further challenges also remain, including developing an efficiently reversible electrochemical reaction, simplifying the cell configuration, and developing an efficient protective layer for the Li metal electrode in alkaline solution.24,32,531,541 Furthermore, as MABs typically operate at ambient conditions, the detrimental effect of atmospheric constituents of air, e.g., H2O and especially CO2, on the electrode and electrolytes should also be considered and investigated to enhance the battery lifetime.19,542,543 It is envisaged that the mutual development of nonstoichiometric oxides with other key system components will

9.10. Materials Design with Reliable Measurement

Until now, the discovery of the majority of the successful oxygen catalysts has been via an Edisonian approach. Despite recent work and progress in theoretical and computational methods to rationally design nonstoichiometric oxide catalysts, few novel catalysts have been suggested. Instead, most recently reported high-performance catalysts (e.g., LaNiO 3 , La1−xSrxCoO3, Ba0.5Sr0.5Co0.8Fe0.2O3−δ, and Pr0.5Ba0.5CoO3−δ) have already been shown to be highly active in many applications. The main progress in recent developed materials is attributed to the advanced synthesis and characterizations. AK


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RDE or RRDE measurements normally require catalysts supported with a conducting carbon/Nafion composite, which limits the reliability of intrinsic activity measurements, especially at high (OER) or low (ORR) potentials. Therefore, it is suggested that a combination of RDE or RRDE measurements with well-defined thin films (e.g., prepared by pulsed laser deposition), is an effective method to evaluate intrinsic electrochemical properties.256,490 With this configuration, the differences in porosity, surface areas, and roughness can be alleviated, and binders or conductive additives can be effectively avoided. A reliable measurement of the intrinsic activities is quite beneficial to the rational design of oxygen catalysts.

Zarah Medina Baiyee has worked in the UK photovoltaic and renewable energy industry for 3 years, subsequent to completing an M.Sc. at Durham University and a B.Sc. at King’s College London with first class honors in physics. Since returning to academia, she has worked on density function theory calculations for oxygen catalyst electrode materials, as part of Prof. Ciucci’s research group at the Hong Kong University of Science and Technology. She has been awarded the Hong Kong Ph.D. Fellowship to support her work. Zongping Shao earned his Ph.D. from Dalian Institute of Chemical Physics, Chinese Academy of Sciences, in May 2000. He then worked as a postdoc at Institut de Reserches sur la Catalyse, CNRS, France, for one and a half years (August 2000−February 2002). After that, he worked as a postdoc in Sossina M Haile’s group at California Institute of Technology, USA (from March 2002 until June 2005). He joined the College of Chemistry and Chemical Engineering in Nanjing University of Technology in July 2005, where he was promoted to professor. Since then he has been the director of Institute of New Energy Materials and Technology. Currently, he is the dean of College of Energy. His research interests include lithium-related batteries, supercapacitors, solid oxide fuel cells, oxygen permeable membranes, and low-temperature fuel cells. He has published over 300 journal papers with more than 8000 citations in total. He was selected by Thomson Reuters as one of the Highly Cited Researchers 2014 (Engineering Section). Currently, Prof. Shao also directs a research group at Curtin University, Australia.

10. CONCLUSIONS This review has highlighted the recent advances in the development of nonstoichiometric oxides, ranging from simple oxide, perovskite, layered perovskite, and pyrochlore, for ORR and OER in MABs and LTFCs. These catalysts are characterized to be low cost and earth-abundant, as well as possess relatively high activity and stability under operation conditions. It is expected that these catalysts will be essential to the future development of multiple technologies. For practical large-scale commercialization, the development of even more active and stable oxygen catalysts is essential and will be the central topic of advanced research. It is expected that the development of nonstoichiometric oxides, with the mutual development of system components, will lead to highly stable and efficient MABs and LTFCs in practical applications in the near future.

Francesco Ciucci is an assistant professor in Mechanical and Aerospace Engineering and in Chemical and Biomolecular Engineering at the Hong Kong University of Science and Technology. He received his Ph.D. from the California Institute of Technology, where he was supported by a Rotary Ambassadorial Scholarship and by a Bechtel Fellowship. He did postdoctoral work at the Institute of Scientific Computing at the University of Heidelberg, where he received a Heidelberg Graduate School Postdoctoral Fellowship and a Marie Curie Reintegration Grant from the European Union (2010−2013). Prior to that he obtained a dual M.Sc. degree in Applied Physics at Ecole Centrale Paris, France, and in Engineering at Politecnico di Milano, Italy. His current research interests include modeling electrochemical devices using continuum and atomistic simulations, applied statistics and uncertainty quantification, fuel cells, and lithiumbased batteries.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Address ∇

Department of Chemistry, Jinan University, Guangzhou 510632, China

Author Contributions ⊥

D.C. and C.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Francesco Ciucci gratefully acknowledges HKUST for providing start-up funds, and Francesco Ciucci acknowledges the Research Grants Council of Hong Kong for support through the projects DAG12EG06 and ECS 639713. Zarah Medina Baiyee acknowledges the support of the Hong Kong PhD Fellowship Scheme. Zongping Shao acknowledges the support from the Future Fellowships-Australian Research Council.

Biographies Dengjie Chen was born in 1985 in Zhejiang, China. He received his B.E. (2008) and Ph.D. (2013) from the College of Chemistry and Chemical Engineering at the Nanjing University of Technology, China, where he carried out his research on the development and optimization of perovskite-type materials for oxygen reduction reactions in solid oxide fuel cells under the supervision of Prof. Zongping Shao. After that, he joined Ciucci’s group at the Hong Kong University of Science and Technology as a Post-Doctoral Research Associate in 2013. His current research interests include metal−air batteries, fuel cells, and ceramic membrane technologies, in particular the development and utilization of thin film oxides in these technologies.

ABBREVIATIONS DFT density functional theory GGA generalized gradient approximation KB Ketjen Black LABs Li−air/O2 batteries LTFCs low-temperature fuel cells MABs metal-air batteries OER oxygen evolution reaction ORR oxygen reduction reaction OTMs oxygen transport membranes PDS potential-determining step RDE rotating disk electrode

Chi Chen was born in Hunan, China. He received his B.E. degree from Department of Thermal Science and Energy Engineering at University of Science and Technology of China in 2012. After that, he joined Ciucci’s group at Hong Kong University of Science and Technology, and he is now a Ph.D. candidate. His current research interests include molecular dynamics simulations and density functional theory calculations applied to oxide materials for fuel cells. AL


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reversible hydrogen electrode rotating ring-disk electrode saturated calomel electrode solid oxide fuel cells Zn−air/O2 batteries


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