Emerging Materials in Heterogeneous Electrocatalysis Involving

Review www.acsami.org

Cite This: ACS Appl. Mater. Interfaces 2018, 10, 33737−33767

Emerging Materials in Heterogeneous Electrocatalysis Involving Oxygen for Energy Harvesting Moumita Rana,‡ Sanjit Mondal,† Lipipuspa Sahoo,† Kaustav Chatterjee,† Pitchiah E. Karthik,† and Ujjal K. Gautam*,† †

ACS Appl. Mater. Interfaces 2018.10:33737-33767. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/12/18. For personal use only.

Department of Chemical Sciences, Indian Institute of Science Education and Research-Mohali, Sector 81, Mohali, SAS Nagar, Punjab 140306, India ‡ IMDEA Materials Institute, C/Eric Kandel 2, Parque de Tecnogetafe, Getafe 28906, Spain S Supporting Information *

ABSTRACT: Water-based renewable energy cycle involved in water splitting, fuel cells, and metal-air batteries has been gaining increasing attention for sustainable generation and storage of energy. The major challenges in these technologies arise due to the poor kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reactions (OER), besides the high cost of the catalysts. Attempts to address these issues have led to the development of many novel and inexpensive catalysts as well as newer mechanistic insights, particularly so in the last three-four years when more catalysts have been investigated than ever before. With the growing emphasis on bifunctionality, that is, materials that can facilitate both reduction and evolution of oxygen, this review is intended to discuss all major families of ORR, OER, and bifunctional catalysts such as metals, alloys, oxides, other chalcogenides, pnictides, and metal-free materials developed during this period in a single platform, while also directing the readers to specific and detailed review articles dealing with each family. In addition, each section highlights the latest theoretical and experimental insights that may further improve ORR/OER performances. The bifunctional catalysts being sufficiently new, no consensus appears to have emerged about the efficiencies. Therefore, a statistical analysis of their performances by considering nearly all literature reports that have appeared in this period is presented. The current challenges in rational design of these catalysts as well as probable strategies to improve their performances are presented. KEYWORDS: bifunctional catalysts, electrocatalysis, heterogeneous catalysis, oxygen reduction reaction, oxygen evolution reaction 4e− in acidic media or 4OH− → O2 + 2H2O + 4e− in alkaline media) is the primary limiting factor, compromising device efficiencies.5,12 Therefore, significant efforts have been made to develop highly efficient electrocatalysts for ORR and OER. Benefiting from these findings, generation of bifunctional catalysts that can simultaneously perform ORR and OER has emerged as the latest research hotspot. Both molecular as well as heterogeneous catalysts have been found to be promising, even though recovery and recyclability of the molecular catalysts remains a bigger challenge.13−17 Heterogeneous catalysts are advantageous in this regard, despite suffering from loss of some catalytic activity under harsh electrochemical conditions.18 So far, carbon-supported Pt nanocrystals and precious metal oxides (e.g., IrO2 and RuO2) have been used as highly efficient benchmarking catalysts for ORR and OER, respectively.19,20 Thus, the bottleneck for commercialization of these technologies has been the high cost these catalysts, leading to an extensive search for economically affordable materials with sufficiently high activity and stability.21

1. INTRODUCTION The continuous rise in global energy demand is leading to an escalated consumption of fossil fuels and increase in greenhouse gases in the atmosphere. Therefore, the development of complementary energy resources is gaining attention.1−6 The target set by the Department Of Energy, United States, is to produce 80% of electricity from renewable resources by 2050.7 The major renewable energy cycles are based on water, carbon, and nitrogen, among which the water-energy cycle is considered to be the most promising one.1 In this cycle, energy harvested by solar cell, wind, and sea current can be stored in the form of chemical energy by the electrochemical splitting of water to produce hydrogen and oxygen. These molecules with high energy density can later be recombined by redox processes in a fuel cell for power generation.8 Water cycle also plays an important role in hybrid electrical devices such as regenerative fuel cells and metal-air batteries, where water splitting and oxygen reduction take place in one of the electrodes, respectively (Figure 1).9−11 Among the reactions involved in the water-energy cycle, the sluggish kinetics of oxygen reduction reaction (ORR, O2 + 4H+ + 4e− → 2H2O in acidic media or O2 + 2H2O + 4e− → 4OH− in alkaline media) and oxygen evolution reaction (OER, 2H2O → O2 + 4H+ + © 2018 American Chemical Society

Received: May 31, 2018 Accepted: September 17, 2018 Published: September 17, 2018 33737

DOI: 10.1021/acsami.8b09024 ACS Appl. Mater. Interfaces 2018, 10, 33737−33767

Review

ACS Applied Materials & Interfaces HO2 * + H+ + e− = H 2O + O*

(7)

O* + H+ + e− = *OH

(8)

+

−

2*OH + 2H + 2e = 2H 2O + 2*

Considering this mechanism, if the O−O bond in the adsorbed species (eq 7) is not broken, it results in the formation of H2O2, which can be further reduced to H2O or can be desorbed as the final product. It is desirable to have the O2 reduction occurring at potentials close to the thermodynamic ORR potential of 1.23 V of (vs normal hydrogen electrode (NHE)) with a satisfactory reaction rate. Kinetics of ORR is obstructed by the first electron transfer from the cathode to oxygen, the hydration of oxygen, and desorption of the intermediates. Therefore, estimation of the binding energy of the intermediates on catalyst surface is necessary. Initiated by pioneering work of Bockris on oxygen evolution reaction, researchers have performed extensive work in the last five decades or so and elucidated reaction mechanisms associated with OER considering both acidic or alkaline reaction conditions.25,26 Despite some disagreement, most of the proposed mechanisms are similar and include a surfaceadsorbed −OH or a surface-bound −O group.27,28 A general mechanism can be outlined in the following equations (eqs 10−14). The primary difference between the various pathways is the participation of a surface hydroperoxy group (−OOH), which is generated from a surface hydroxyl group (eqs 13 and 14 and eqs 18 and 19). In acidic conditions, OER occurs with the adsorption of H2O on the catalyst surface involving the steps described below (either following eqs 10−12 or eqs 10, 11, 13, and 14) or a combination of both the pathways):

Figure 1. Schematic showing the scope of ORR, OER, and bifunctional electrocatalysts in water cycle associated with renewable energy, where oxygen is either generated or consumed in one of the electrode processes. In an electrolyzer cell, water converts to O2 by splitting at the anode (OER), whereas in a fuel cell, oxygen gets reduced at the cathode forming water (ORR). The same reactions also occur in a metal-air battery during discharging and charging, respectively.

Understanding of the material surface, surface−reactant interaction, and associated reaction mechanism is necessary to overcome the challenges. In aqueous medium ORR occurs mainly through two pathways:(i) direct four-electron reduction pathway, from O2 to H2O, and (ii) two-electron reduction pathway forming hydrogen peroxide.22 The four-electron processes are preferred for fuel cell applications, while twoelectron pathway can also be useful for industrial production of H2O2. It was found that the four-electron reduction process can occur by two mechanisms: (i) dissociative mechanism and (ii) associative mechanism. In both, O2 first gets adsorbed on the catalyst surface (eqs 1 and 4). In the first case, the adsorbed O2 molecule splits to form adsorbed atomic oxygen, which is further reduced by two more electrons to form H2O. O2 + * = 2O*

(1)

O* + H+ + e− = *OH

(2)

*OH + H+ + e− = H 2O + *

(3)

+

O2 * + H + e = *OOH

(5)

*OOH + * = HO* + O*

(6)

(10)

*OH → O* + H+ + e−

(11) (12) +

*O + H 2O → *OOH + H + e

−

*OOH → * + O2 + H+ + e−

(13) (14)

However, in alkaline condition, OER occurs involving surface-adsorbed −OH groups: * + OH− → OH* + e−

(15)

−

*OH + OH → O* + H 2O + e

−

(16)

O* → * + 0.5O2

(17)

O* + OH− → *OOH + e−

(18)

*OOH + OH− → * + O2 + H 2O + e−

(19)

In a neutral medium, OER is expected to follow either of the mechanisms depending on the nature of the catalyst surface charge. Water molecules will be adsorbed on a negative surface, and hydroxyl group will be involved on positive surfaces. Even though investigations on ORR and OER date back to several decades, it gained momentum only recently.22 The last three-four years have been particularly significant when more catalysts have been investigated than ever before, and novel materials have been developed. In operando spectroscopic and other measurement techniques such as ambient pressure X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-

(4) −

* + H 2O → *OH + H+ + e−

O* → * + O2

Here * denotes the active sites on the catalyst surface and placed near the atom that is adsorbed. The affinity between such an active site and reactant species is crucial in determining the ease of the reaction. Density functional theory (DFT) studies have established that a Sabatier volcano relationship exists between the binding affinity of OH on the catalyst surface and its ORR activity.23,24 In dissociative ORR pathway, if the catalyst surface does not stabilize O2, H2O2 cannot be produced. Alternatively, in associative mechanism, the O−O bond does not cleave initially and instead O2 + * = O2 *

(9)

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ACS Applied Materials & Interfaces

families of materials, specifically, of oxides, hydroxides, chalcogenides, pnictides, and metal-free and heteroatomdoped carbon-based materials. It is not intended to provide an overview that is in-depth in nature but to create a singleplatform for the readers to access the important findings pertaining to ORR, OER, and bifunctionality in the last three− four years. We highlight the recently developed key strategies for improving ORR and OER functionality and also point out the few critical investigations that have led to in-depth understanding of the reaction mechanism or newer insights. In the case of bifunctional catalysts, because these are relatively new and difficult to compare, we took into consideration a statistically significant number of recent reports and compared their performances in terms of the most, the least, and mean activities for the different families of catalysts. In the concluding section, we point out the current challenges and opportunities in catalyst design and benchmarking the performances of catalysts that could help in creating more efficient materials in near future.

edge spectroscopy have been frequently employed to gain insights into the reaction mechanism and catalyst deactivation. In addition, several new families of bifunctional ORR-OER catalysts have also been discovered. Figure 2 represents the most widely investigated ORR and OER active as well as bifunctional materials.

2. MEASUREMENTS AND PERFORMANCES The performance of a catalyst for ORR and OER is usually evaluated using a three-electrode half-cell setup at room temperature, wherein the catalyst is loaded on the working electrode. Under the experimental conditions, the reactant species are adsorbed on the electrode surface thereby facilitating the electron transfer from the electrode to an intermediate species followed by the release of the product. The performance of a catalyst is examined by linearly varying the applied voltage and measuring the current (linear sweep voltammetry (LSV)). The efficiency of a catalyst depends on both thermodynamic and kinetic considerations and can be benchmarked by measuring several parameters of which halfwave potential (E1/2), overpotential (η), and Tafel slope are most widely used. In an ideal condition, an electrochemical reaction should take place at zero overpotential, that is, at equilibrium potential (Eeq). However, this does not happen due to activation barriers associated with electron transfer and intrinsic electrical resistance of the measurement setup. Thus, η can be considered as a measure of the excess applied potential (E) over the equilibrium potential (E − Eeq) required for the particular reaction. In case of OER, η is usually reported at a current density of 10 mA cm−2. The total current (i) in an electrochemical experiment originates from the anodic and the cathodic current (eq 20).33

Figure 2. Most widely investigated ORR, OER, and ORR-OER bifunctional electrocatalysts [C: carbon, CNT: carbon nanotube, CS: core−shell, DH: double hydroxide, G: graphene (many authors loosely refer to graphene oxide and reduced graphene oxide as graphene too) LDH: layered double hydroxide, NC: nanocrystal, TM: transition metal].

i = ianode + icathode

A number of critical review articles describing the progress in the fields of ORR and OER activities have been published recently.1,5,29−31 Most of these are devoted to developments in a single family of compounds, and some illustrate the emerging insights on the reaction mechanisms in-depth.1,32 Clearly, the underlying principles responsible for improved ORR and OER efficiencies in a material form the basis for development of bifunctional catalysts. However, the bifunctional catalysts have not been discussed in light of the developments in the field of ORR and OER. Besides, the literature efficiencies of the bifunctional catalysts appear to be widely and alarmingly varying, probably due to the need for tuning two functionalities simultaneously unlike in ORR and OER, and thus without setting any clear guidelines for further studies. In this review, we summarize the recent progress in the performances of the catalysts for oxygen reduction, oxygen evolution, and bifunctional ORR-OER reactions by considering nearly all important

(20)

The relationship between η and i can be expressed using the Butler−Volmer (B−V) equation (eq 21). i = i0(e αanF / RT − e−αcnF / RT )

(21)

where i0 is the exchange current, αa and αc denote transfer coefficients for the anodic and cathodic reactions, respectively, and n is the number of electrons involved. At high field conditions (high η > RT ), only one of the reactions dominate. αaF

Assuming that to be the anodic conditions and T = 300 K, the B−V equation can be rearranged to give the Tafel equation (eq 22). η= 33739

0.059 0.059 log i − log i0 αan αan

(22)


Review

ACS Applied Materials & Interfaces The slope of the Tafel plot (η vs log i) is defined as [0.059/ αan] and is denoted in the form of millivolt per decade current. This is a kinetic parameter for that particular electrochemical reaction that is affected by electron transfer coefficient, rate constants, and electron transfer numbers during or before ratedetermining step (RDS); and though not straightforward, it can be used to derive information about the RDS. In those special cases where there is no significant difference in concentration of reactant in electrode surface and bulk, and where there is only one RDS, which is much slower than the rest of the elementary steps, Tafel slope can ideally be used to estimate the number of electrons transferred from the electrode to a reactive species in the RDS. However, such situations are expected to be extremely rare (the absolute rate theory of electron transfer by Marcus suggests that the same is impossible, because energy associated with reorientation of the medium prefers a stepwise electron transfer process), and thus oversimplified interpretation may lead to hypothesizing inaccurate reaction pathways. It may however be pointed out here that a number of researchers loosely assume a transfer coefficient of 0.5, while considering Tafel slopes for ORR and OER, and thereby arrive at an electron transfer number n = 1, 2, 3, or 4, when the slopes are found to be 120, 60, 40, or 30 mV/decade, respectively. Electron transfer coefficient is dependent on the surface coverage of the electrode with reaction intermediates, which in turn is dependent on the applied potential and therefore cannot be constant over the potential range the Tafel analysis is performed. Detailed discussions on this key issue of utilization of Tafel slope for analysis of electrochemical ORR and OER mechanisms may be found in several articles.34,35 A recent Technical Report prepared by IUPAC recommends that instead of nα, α alone be used in B−V and Tafel relations, giving α the status of an experimentally determined quantity that is devoid of any mechanistic conclusions.36 As dissolved oxygen is the reactive species in ORR, the electrochemical measurements are performed in an O2saturated electrolyte. In an OER experiment, if the generated oxygen must be further quantified by another technique, an inert atmosphere is used. The choice of the electrolyte is made on the basis of pH requirement, adsorption affinity of the electrolyte on the catalyst surface, the stability of the catalyst, and the catalyst binder. Thus, acidic media is preferred for the noble metal-based catalysts, since these tend to undergo surface oxidation in basic media. On the one hand, acids with larger anions, such as HClO4, are suitable electrolyte to yield higher current densities. On the other hand, HCl is not suitable due to the high polarizability of Cl− ion and its stronger affinity to the catalyst surface. Transition-metal oxides tend to leach out in acidic media, and therefore basic electrolytes are usually employed. Nafion, a common proton exchanger and catalyst binder, is not stable in basic media due to salt formation, and many catalysts have been tested in the binder-free form.37 Finally, it is important to use a suitable reference electrode compatible with the electrolyte for accurate and stable measurements. One may also note a fundamental difference in ORR and OER kinetics originating at the supply of reactants to the catalyst surface. On the one hand, ORR is a diffusion-limited process wherein diffusion of dissolved O2 to the electrode can limit the current. OER on the other hand is not diffusionlimited, as it involves water or OH− abundantly present near the electrode. Therefore, ORR LSV plots should be acquired in

same scan rate to compare the performance of various catalysts.

3. EMERGING MATERIALS FOR ORR Among various electrocatalytic processes involving oxygen, ORR has been more extensively studied in the past decade. A diverse range of materials has been explored as catalysts including noble metals, metal oxides, and carbon-based materials. Even though their performances have been constantly improving with design strategies, significantly aided by theoretical studies, further progress is needed in finding economically viable catalyst systems. Pt exhibits excellent ORR activity and has been considered as the benchmark ORR catalyst for polymer electrolyte membrane (PEM) fuel cells. Taking due consideration of its high activity but low earth abundance, the Department of Energy (DOE), United States, has estimated a minimum attainable current value of 0.44 A per gram of Pt in a full cell configuration, besides less than 40% activity loss after 30 000 voltage cycling.35 high stability for successful commercial applications. A number of approaches have been adapted to achieve these goals, which are broadly based on following strategies: (i) improving surface activity by manipulating shape and size of the Pt nanoparticles (1st generation), (ii) improving catalyst activity by alloying Pt with other noble (2nd generation) and non-noble metals (3rd generation), (iii) developing noble metal-free materials (4th generation), and (iv) developing metal-free materials (5th generation).38−40 We will discuss the most important design strategies for ORR catalysts in the following section. Note that engineering of bifunctional materials also involves ORR. However, most ORR catalysts exhibit poor OER activity. Therefore, different strategies must be adopted for them to be employed as bifunctional catalysts, which will be discussed in another section. 3.1. Metal-Based Nanostructures. Pt nanocrystals loaded on amorphous carbon support have been used for commercial fuel cells.23 Therefore, to develop the firstgeneration catalysts, it is crucial to understand the surface structure−ORR activity relationship, which was further inspired by careful analysis of the reactant−catalyst interactions using theoretical studies. With a DFT approach, Tripkovic et al. estimated the influence of shape and size of the nanoparticles on the ORR activity and showed that {111} crystal facet of Pt is the most active facet. Therefore, among the most usual shapes such as cube, tetrahedron, octahedron, truncated octahedron, etc., tetrahedron is expected to show highest ORR activity.41 In general, the catalytic activity enhances on high index planes due to high density of atomic steps, edges, and kinks.42 This was elegantly shown by Sun and co-workers by synthesizing nanocrystals with high index (730), (210), and (520) facets.43 Xia and co-workers have used Pt nanostructures with concave surfaces, specifically, nanoframes with rare {740} facets and nanocubes enclosed with {510}, {720}, and {830} facets that exhibited much higher activity toward ORR as compared to other highly efficient Pt nanostructures. The authors envision that the negative surface curvature can enhance the mass exchange rate as well.44,45 However, a better understanding of such high activity, theoretical investigations, examining the possibility of scaleup production, and long-term stability are necessary. In the family of simple nanostructured Pt, tetrahedron nanocrystals (Pt-NTd) with exposed (111) facets are expected 33740


Review


Figure 3. (a) TEM images of carbon-supported Pt NTd. (b−e) 1. Atomic model, 2. TEM image, and 3. FFT pattern of Pt NTd at different orientations. (f) LSV of Pt-NTd/C (red) and commercial Pt/C (black). (inset) The change in E1/2 after 5000 potential cycles. (g) Comparison of mass and specific activities of Pt-NTd/C with commercial Pt/C at 0.9 V. Reproduced with permission from ref 50. Copyright 2015 American Chemical Society. (h) TEM image of FePt NPs two atomic layers of Pt shell on C support. (insets) The high-resolution HAADF-STEM image of a particle. (i) ORR polarization curves of the particles in (h) and commercial Pt, initial (BOL) and after accelerated durability test (ADT) (5000 and 10 000 cycles at 60 °C in oxygen-saturated 0.1 M HClO4). Reproduced with permission from ref 63. Copyright 2018 American Chemical Society.

centering that can tune the tendency for OH chemisorption.24 With a single-crystalline Pt3Ni (111) surface as a proof of concept, Stamenkovic et al. showed that its specific activity can be enhanced by up to 10 times when compared with pure Pt(111) surface.51 Encouraged by many such findings, a number of alloy nanocrystals with different shapes have been generated.52−56 In one such study recently, Zhao et al. have prepared octahedral [email protected] core−shell nanocrystals with ultrathin PtNi alloy shells and showed that the core−shell materials exhibited fivefold enhanced ORR activity and only 10% loss after 6000 cycles compared to the commercial Pt/C, where the loss was 27%.57 The enhanced ORR activity was found to be dependent on the shell composition and showed the best activity when the ratio of Pt/Ni is 1.8. From another perspective, even though most catalyst surfaces are convex in nature, using a concave surface can be useful for restrained diffusion of reactants across the catalyst surface. A suitable interplay of strain and surface stabilizing agents has been exploited to create concave decahedra Pd@Pt core−shell structures with (100) and (211) facets that exhibited significant enhancement of catalytic performance arising from

to be the most active Pt catalysts, as compared to other shapes (e.g., nanotetrahedra, nanocube,39 nanowire,46,47 nanoparticle,48 mesoporous film,40 multioctahedra49) due to highly exposed {111} facets.41 However, Pt-NTds have rarely been experimentally realized in the case of pure Pt, instead forming octahedral nanocrystals with lower surface-to-volume ratios and hence higher stability. In an important recent report, some of us have been successful in synthesizing Pt-sub-10 nm size tetrahedral nanocrystals employing a simple one-step and highyield method for the first time (Figure 3a−e).50 Generation of secondary amine in situ in the synthesis chamber has stabilized these nanocrystals. As expected, these sub-10 nm Pttetrahedral nanocrystals supported on conducting carbon (Pt-NTd/C) has exhibited much higher mass activity than other shape-controlled Pt nanostructures, besides excellent durability (Figure 3f,g). It is further established that the ORR activity of the Pt-based materials can be improved by modifying the catalyst surfaces during alloy formation (M−Pt, where M can be one of the transition metals, e.g., V, Cr, Mn, Fe, Pd, Co, Cu, Ni, Mo, W, Re). Such enhancement arises due to appropriate d-band 33741


Review


Figure 4. (a) A schematic describing the synthesis of heteroatom-doped amorphous carbon from soya chunks. (b) Comparison of ORR LSV plots for SC-1000 and Pt/C in 0.1 M KOH. Reproduced with permission from ref 75. Copyright 2015 Elsevier. (c) Schematic representation of the role of Fe−Nx and Fe−Fe3C@C sites for the ORR, (d) ORR polarization curves of Pt/C and Fe−Fe3C@C/CNT catalysts. (inset) A representative TEM image of the sample. Reproduced with permission from ref 76. Copyright 2017 American Chemical Society.

Pt adatoms trapped at the high-energy edges.58 The ORR mass activity was found to be 6.6 times higher than that of the commercial Pt/C catalyst. Huang et al. prepared a set of transition-metal doped Pt alloy nanoparticles, M-Pt3Ni/C, where M was chosen from V, Cr, Mn, Fe, Co, Mo, W, and Re. Among them, Mo−Pt3Ni/C exhibited the best ORR performance with a mass activity of 6.98 A/mgPt, which is astoundingly 50-fold higher than commercial Pt/C.59 Yang et al. have reported the synthesis of Pt−Ag alloy nanocages of various compositions with an edge length of 18 nm and a wall thickness of 3 nm.60 These materials also exhibited excellent ORR activity and stability. The Pt56Ag44 nanocages showed 3.3 times higher specific activity over the commercial Pt/C catalyst and 52% retention of mass activity after 30 000 potential cycles. Here, a volcano-like mass activity behavior was observed with highest mass activity for Pt56Ag44 nanocages. With DFT in the same study, it was shown that PtAg is the most active catalyst due to its stable transition state favoring O2 dissociation. Pd-Based nanocrystals, unlike Pt-based ones, are known to undergo fast degradation in the acidic medium under electrochemical conditions. Therefore, despite being relatively inexpensive, these are not used for ORR. Of late, it has been shown that alloying Pd with other nonprecious metals can significantly improve their stability. Wang et al. synthesized Pd6CoCu ternary alloy nanocatalysts decorated with gold, which has shown 102 mV improved half-wave potential as compare to Pd/C in acidic medium.21 Introducing gold through galvanic replacement results in symmetric distribution of gold nanoparticles and charge transfer from Pd to Au causing decrease in d-electron density on Pd, which is responsible for showing outstanding stability. Fu et al. have synthesized ternary PdCuPt nanodendrites containing lower Pt loading. Transmission electron microscopy (TEM) observation confirmed the formation of well-defined dendritic nanostructures with average diameter of ∼7 nm. The onset potential for the ternary system is more positive (0.933 V) than the commercial Pt/C (0.908 V). The mass activity

measured 14 times higher than that of the commercial Pt/C catalyst, and after 5000 cycles, 70% mass activity was retained.61 In a recent study, using in situ scanning tunnelling microscopy and surface X-ray scattering Kobayashi et al. have shown that monolayer of Pt over Pt3Co(111) single crystal can result in 25 times higher ORR efficiency over Pt(111), originated from the positive polarization, induced by the Corich layer (98%) under Pt monolayer.62 Implying a similar concept, Sun and co-workers have demonstrated that sub-10 nm FePt nanoparticle core with atomically thin Pt shell could exhibit mass activity of 0.23 mA/g (as compared to 0.08 mA/ gPt for commercial Pt/C, TKK) in hydrogen fuel cell configuration with its negative loss even after 30 000 cycles shown in Figure 3h,i.63 Thus, both experimental and theoretical studies establish that Pt monolayer overcoated core−shell structures can be extremely efficient and stable ORR catalysts, and hence such studies ought to be extended to include other materials such as FeNi, FeCo, etc. Much is yet to be explored though, about surface structures on the overcoated samples and alloys, especially under operando conditions, and such experiments are expected to continue. While those mentioned above are some of the most interesting recent reports to us, Zhu et al. and Kulkarni et al. have reviewed the scope of such type of metal-based materials in great detail.30,64,65 3.2. Carbon-Based Materials. Despite high activity and stability of the noble-metal based catalysts, these are expensive. Therefore, several research groups have investigated the ORR activities of carbon allotropes in search of cheaper alternatives.66 They found that the catalytic activities of the allotropes are reasonable and probably can be further improved, even though they are inferior to commercial Pt/ C.67 Carbon materials show relatively high activity in alkaline medium, when their performance in acidic medium is negligible. Their poor efficiency in alkaline medium was partially addressed by incorporating low-electronegative heteroatoms (e.g., N, P, S, etc.) in the carbon network, which were expected to give rise to high electrophilicity in the 33742


Review


Porous core−shell Fe3C embedded N-doped carbon nanofibers were synthesized by pyrolysis of poly(vinylidene fluoride) and FeCl3 mixture78 that simultaneously exhibited higher ORR activity than commercial Pt/C and stability under both alkaline and acidic media. The enhancement in ORR activity has been attributed to the electronic effects generated from Fe3C in core Fe/Fe3C and graphitic-N species in Ndoped carbon shell.79 In another interesting approach that may help simple and flexible device fabrication, Lu et al. prepared Co−N-containing CNTs arrays on carbon fiber paper by adopting solvothermal method. The N-doping has been done on CNT using melamine as the precursor. The materials were found to be highly superaerophilic80 with a strong interaction for both oxygen and air bubbles and exhibited very high ORR activities and stabilities in both acidic and basic media. In the carbide family, He et al. have developed stable threedimensional (3D) porous Fe−W−C electrocatalyst at a relatively moderate temperature of 750 °C,81 which contains a protective graphitic shell outside the active carbide cores substantially improving the durability of the electrocatalyst. In this context, it may be pointed out that the other two families of compounds that have attracted the attention of the researchers are the oxide and the transition-metal chalcogenides, in combination with carbon materials, the first ones due to their higher stability82 and the other ones due to higher activities.83 Carbon-based materials for ORR have also been reviewed with detailed insights elsewhere.84,85 On the basis of such examples, it is emerging that the introduction of transition-metal atoms in an N-doped carbon network, in particular, in a manner that the metal neighborhood resembles the naturally occurring oxygen reduction active sites, results in significant enhancement in the ORR activity.86,87 Moreover, importantly, these materials have been found to be ORR-active not only in basic medium, but also in acidic medium. In another such example, Huo et al. synthesized two-dimensional (2D) layered mesoporous transition metal−nitrogen-doped graphene (M = Fe, Co, and Ni) using metal nitrate, 4,4-bipyridine, and KIT-6/N-doped graphene as precursors. Among them, meso-Fe−N−C/N−G nanocomposite has exhibited the highest ORR activity (E1/2 = 0.85 in acidic medium and 0.89 V in alkaline medium) and durability both in acidic and alkaline media, comparable to the commercial Pt/C.88 Besides, these materials have been found to have a fair methanol tolerance behavior. The origin of their superior ORR activity has been attributed to high surface area, high electron transportation, presence of ultrafine Fe or FeOx nanocrystals distributed over the nanosheets, and their interaction with N in the graphene-sheets forming Fe−Nx species. A detailed X-ray absorption spectroscopic and DFT study by Zitolo et al. revealed that rather than their metallic form, insertion of non-noble transition-metal atoms in a multiple N-co-ordinating site (M−Nx), particularly in a porphyrin-like four-coordinated moiety, is responsible for their enhanced ORR activity.89 Similar conclusions have also been made by other groups, where it is becoming apparent that Fe is particularly electroactive in this embedded form.90 The origin of such enhanced ORR activity appears to be related to optimal O2 adsorption strength and facile electron transfer.91 Zitolo et al. also showed that under operando conditions in acidic medium (in 0.0 to 1.0 V range, vs reversible hydrogen electrode (RHE)) Fe-based moieties experience structural and electronic-state changes, while Co-based moieties do not, which originate from a strong Fe−O interaction leading to

neighboring C atoms. Since ORR is a diffusion-limited reaction, such centers in turn absorb O2 with high efficiency leading to improved ORR.68−70 Moreover, the activity of these catalysts depends on the extent and the nature of N-doping and the microstructure of the catalyst. Recently, it was found that inhibiting mass transfer rate can be useful for high activity of carbon materials in acidic medium. Titirici et al. prepared highly porous N-doped C materials with 1−14 nm pores, where the small pores limit mass transfer,71 and found its catalytic activity superior to even commercial Pt/C in both acidic and alkaline conditions. O- and S-doped carbon materials have also been known to exhibit ORR activity comparable to commercial Pt/C.72 To reduce the expenses further and utilize waste materials, biomasses have been extensively studied as a promising precursor for the synthesis of doped-carbon materials.73 For instance, human hair contains α-keratin with inherent C−N and C−S bonds. This has been transformed into S- and Ndoped high surface area carbon with high ORR activity.74 Because of the absence of metals, these exhibit excellent tolerance toward the methanol crossover. In a recent report, we used soya chunks as a precursor to synthesize N and S codoped high surface area carbon (1072 m2/g) with excellent ORR activity (Figure 4a,b).75 Being naturally rich in amino acids with inherent C−N bonds (arginine, leucine, lysine, phenyl alanine, and tryptophan) as well as C−S bonds (cysteine and methionine), we could derive N and S codoped amorphous carbon. With careful analysis, we showed that the presence of high concentration of pyridinic-N along with high surface area enhances the ORR activity in SC-1000 (pyrolyzed at 1000 °C) over other materials, pyrolyzed at lower temperatures of 900, 800, and 700 °C (SC-900, SC-800, and SC-700), respectively (Figure 4b). Importantly, these doped carbon materials have exhibited superior durability compared to commercial Pt/C and other carbon samples. While it appears difficult for carbon to match the catalytic properties of a metal having d-electrons, clearly carbon is advantageous due to exceptionally high surface area and methanol resistance, besides its suitable electrical conductivity. Recently it has been observed that ORR activity of the heteroatom-doped carbon materials could be further improved by introducing non-noble metal centers. The incorporation of transition metals such as Fe and Co to the N-doped graphenebased materials forms metal−nitrogen−carbon (M−N−C)based chemical entities that help to increase the ORR activity. Kim et al. demonstrated preparation of catalyst via the phase conversion reactions of Fe3O4 nanoparticles supported on carbon nanotubes (CNTs), resulting in selectively contained Fe−Nx and Fe−Fe3C@C sites, which were responsible for ORR. This kind of generation of selective active sites in ORR catalysts were not possible in existing high-temperature pyrolysis-based synthetic techniques. Fe−Nx sites were dominating the ORR via 4e− pathway, while Fe−Fe3C@C sites mainly promote 2e− reduction of oxygen followed by 2e− peroxide reduction. It was also observed that Fe−Nx active sites show higher activity compared to other sites as shown in Figure 4c,d.76 In another approach, Co−N moieties decorated graphene aerogel were obtained by hydrothermal selfassembly, freeze-drying, and then pyrolysis process. Polyaniline (PANI) was chosen as pore-forming agent and nitrogen source.77 This material exhibited high ORR activity with E1/2 = 0.73 V, high electron transfer, excellent stability, and, unlike the metal catalysts, high methanol tolerance in acidic medium. 33743


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ACS Applied Materials & Interfaces Table 1. Overview of Recently Developed Efficient ORR Catalystsa Sl No.

materials

E1/2 (V) vs RHE

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

Pt−FeNC nanoparticles Fe2N@N−porous C PdCuPtnanodendrites N-doped GO Pd@Pt3−4Loctahedra Fe−Fe3C@C porous Fe−W−C NC ultrafine jagged Pt NW Au in (Pd6CoCu) NC Pt−Ag alloy nanocages N-doped C microspheres Pt−Ni bimetallic NS MoS2−CMPs N-CNS Mo−Pt3Ni/C Pd@Pt decahedra [email protected] NC mesoporous N−doped C carbon (Fe−P) catalysts C nanotube/Fe3C NP ultrathin Pt nanowires meso-Pt-thin films N- and S-doped C sub-10 nm Pt (111) NTd Co−N-CNTs FePt/Pt nanoparticles PtCuCo ternary NC CuAg@Ag core−shell

0.827 V 0.803 V 0.864 V 0.791 V 0.891 V 0.87 V 0.727 V 0.935 0.856 V 0.889 V 0.529 V 0.95 V 0.84 V 0.768 V 0.971 V 0.908 V 0.956 V 0.815 V 0.803 V 0.861 V 0.813 V 0.866 V 0.79 V 0.873 V 0.73 V 0.945 V 0.93 V 0.85 V

MA (A/mg) 0.69 at 0.9 V 0.42 at 0.85 V

stability 91% 90% 70% 99%

after after after after

electrolyte

10 000 cycles 25 000 s at 0.58 V 5000 cycles 20 h at 0.56 V

0.70 at 0.9 V 4.8 at 0.90 V 13.6 at 0.90 V

NL in E1/2 after 5000 cycles 88% after 6000 cycles 25 mV change in E1/2 after 10 000 cycles

0.64 at 0.9 V NL in E1/2 after 3500 cycles 1.59 at 0.9 V

6.98 at 0.9 V 1.60 at 0.9 V 0.79 at 0.9 V

0.69 at 0.9 V 0.33 at 0.9 V 201 A/g at 0.9 V 0.70 at 0.9 V 1.56 at 0.9 V

NL in E1/2 after 5000 cycles 94.5% after 8000 cycles 48% after 10 000 cycles 90% after 6000 cycles 93.3% after 10 000 s at 0.70 V 81% after 25 000 s at 0.58 V 23 mV loss of E1/2 after 3000 cycles 90% after 3000 cycles after 10 000 cycles E1/2 decreases 40 mV 94.5% after 3000 s at 0.58 V ∼90% after 5000 cycles NL after 20 h 36 mV loss of E1/2 after 10 000 cycles 73% mass activity after 10 000 cycles 90% after 20 000 s at 0.66 V

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.5 0.1 0.1 0.1 0.1 0.1 0.1 0.1

M M M M M M M M M M M M M M M M M M M M M M M M M M M M

HClO4 KOH HClO4 KOH HClO4 KOH KOH HClO4 HClO4 HClO4 KOH HClO4 KOH KOH HClO4 HClO4 HClO4 KOH KOH KOH H2SO4 HClO4 KOH HClO4 KOH HClO4 HClO4 KOH

ref 107 108 61 72 109 76 81 110 21 60 71 111 83 112 59 58 57 113 114 115 47 40 75 50 80 63 116 117

a

Legend: NL = negligible loss.

medium.97 While these materials are yet to be investigated extensively to realize their full potential, the ORR activities of some more materials of this family will be discussed in the context of their ORR-OER bifunctional activity. 3.3. Emerging Insights on Performance. On the basis of the above discussions, we show some of the most active and stable ORR catalysts developed in the last couple of years in Table 1.While many active catalysts were prepared accidentally or intuitively, it is important to understand the reaction mechanisms occurring at the catalyst surface to improve the kinetics and stability of the catalysts. Therefore, an increasing number of investigations involve in situ spectroscopic experiments to derive detailed information about the reaction intermediates. A few key insights that could help rational design of superior catalysts are described below. With DFT approach, Nørskov et al. showed that Pt is the best metal for ORR due to easy adsorption of oxygen and electron transfer.23 Pt-based alloy materials are suggested to be even better due to the tailoring of the bond energies.24,98 Insertion of the transition metals in Pt leads to contraction of Pt−Pt bond distances and changes the d-electron count, which in turn decreases its affinity toward OH chemisorptions. In the case of the transition-metal oxides (perovskites and spinels), molecular oxygen replaces the surface hydroxide species followed by peroxide and oxide formation and hydroxide regeneration.99 Herein, the presence of a single electron in the eg orbital of the metal contributes to optimized B−O bond strength (B is represented from the common formula of ABO3 and AB2O4 for perovskite and spinel, respectively). The population of eg electrons at B site of perovskites and spinels is

efficient ORR. Kramm et al. developed a route to synthesize metal−nitrogen-doped carbon with exclusively packed with MN4 moieties, which exhibited an ORR onset of 0.88 V in 0.5 M H2SO4 medium. Their study also supports that rather than total metal content in the material, the onset potential and mass activity relies more on density of MN4 sites.91 Sa et al. showed that, instead of an extended carbon network resembling graphene, CNTs can also be used. These workers coated CNTs with a thin layer of porphyrinic carbon using a silica-assisted protective-layer approach that contained stabilized FeN4 sites. These materials exhibited a half-wave potential at 0.79 V in 0.1 M HClO4 electrolyte and negligible decrease in performance loss even after 10 000 potential cycles.92 This was further integrated into an acidic proton exchange membrane fuel cell, where a significant volumetric current density of 320 Acm−3 was recorded. Another highly stable isolated single-atom Fe/N-doped porous carbon catalyst was recently developed by Li and co-workers that exhibited excellent ORR performance with a half-wave potential of 0.90 V outperforming commercial Pt/C in alkaline medium.93 Among different precursors for the synthesis of such kind of materials, metal−organic frameworks are emerging as promising ones due to the presence of intrinsic M−N−C bonds.94,95 For example, lamellar metal−organic framework derived Fe− N−C materials having bimodal porosity has exhibited and E1/2 that is 17 mV more positive when compared with commercial Pt/C catalyst.96 Lai et al. reported on a metal−organic framework derived five-coordinated Fe−N−C rich material that has shown comparable ORR half-wave potential and current density to that of commercial Pt/C in acidic 33744


Review


or a fraction of the tetrahedral and octahedral sites, leading to normal or inverse spinels, respectively. The choice of cations can be extended to mostly transition metals (A: Mn, Cu, Fe, Ni, Co, Zn; B: Co, Fe, Mn, Ni) with varying oxidation states. These materials, in particular Co3O4, have gained ample interest due to suitable adsorption affinity for hydroxide anion.5,21,126−128 However, their poor conductivity and low surface area have been detrimental. Ni and Fe doping of Co3O4 were shown to further enhance electrical conductivity and better charge transport under electrochemical conditions.129 In an important recent finding, Xu et al. have shown that plasma treatment of even bare Co 3 O 4 can generate a high concentration of oxygen vacancies, leading to remarkably low OER overpotential of 300 mV for a current density of 10 mA cm−2 having Tafel slope of 68 mV/dec.130 Further to tailor surface area and conductivity as well as to enable easy diffusion of electrolytes to catalyst surface simultaneously, Gao et al. succeeded in fabricating NiCo2O4 microcuboids for OER having hierarchical hollow structures that provide efficient diffusion of the electrolyte (OH−), which exhibits excellent activity toward OER and hydrogen evolution reaction (HER).21 It is important to point out that the nature of the catalyst surface can undergo tremendous changes and become very different from the bulk material during OER. Under highly anodic conditions in the proximity of reactants, the spinel surface undergoes phase transformation and alters surface oxides to highly active oxyhydroxide species.131 Tung et al. showed that coating Co3O4 nanocubes with a thin CoO layer induces not only high efficiency with 430 mV overpotential at 10 mA cm−2 (Tafel slope 89 mV/dec) but also long-term stability of over 1000 h, one of the longest durations ever known for OER catalyst (Figure 5a,b).132 With in situ grazing angle X-ray diffractometry, they have shown that the CoO layer is extremely facile in developing an oxyhydroxide phase, which is the active species in OER otherwise.133 These oxyhydroxide active sites have inspired the investigations to develop layered hydroxide materials for OER, as discussed later. Spinel materials for electrocatalytic applications were extensively reviewed by Zhao et al. recently.134 Perovskites ABO3 (where A = alkaline-earth or rare-earth metals; B = transition metals and X = O) are structurally comparable to spinels, where the OER active transition metal occupies an octahedral site created by oxygen, and are long known for being effective in OER reactions.135 More importantly, there is significant scope to improve the efficiency of this family of materials by doping the A and B sites due to the controlled filling of eg electrons in the antibonding orbitals.135 For example, considering the case of the first-row transition-metal perovskites, LaMO3 (M = Ni, Co, Fe, Mn, Cr), the OER activity varies in the order of Ni > Co > Fe > Mn > Cr due to variation of electron population in the eg orbitals.136 Two characteristic features are important for high OER activity, namely, (a) eg electrons in surface cation should be near to unity and (b) high covalent character in transition metal−oxygen bond. Of nearly 10 perovskite materials, Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) was found to have the best OER activity, which aced the volcano plot (Figure 6a) exhibiting one order higher performance than that of the state-of-the-art iridium oxide catalysts.137 Recently, Guo et al. developed Ca0.9Yb0.1MnO3−δ by hydrogen treatment, where the perovskite attains an optimum Mn eg electron filling of ∼0.81 and exhibits higher electrical conductivity and OER

important, as during adsorption, O2 binds to B site with an end-on geometry when the eg electrons overlap with O-2pσ more strongly compared to the overlap between the t2g orbital with O-2pπ orbitals. In the case of pure carbon materials, the π-electrons are inert for ORR. It can however be improved by doping the carbon network by heteroatoms such as N or B, which introduces unbalanced spin density and polarization into the network, thereby facilitating adsorption and electron transfer to oxygen.100−103 Zhang et al. have predicted that the presence of pyridinic-N in carbon network facilitates O2 adsorption due to induced positive spin polarization and spin density in the active site.100 The activity of pyridinic−N was later inferred from the experimental studies by Guo et al.104 and Xing et al.81 As we discussed before, the ORR activity of heteroatomdoped materials could be improved by introducing metal centers in the carbon network. However, which of the metal or N site is active for ORR is still debatable.105 Recently, Kucernak and his co-workers demonstrated a route for estimating the ORR active sites in Fe-loaded N-doped C catalysts by means of nitrite adsorption followed by its reductive stripping.106 It was demonstrated that the ORR onset potential was significantly dropped in nitrite adsorbed catalyst and was regained to the unpoisoned one with the reductive stripping, which corroborates the fact that Fe center acts as the active center for oxygen reduction.

4. EMERGING MATERIALS FOR OXYGEN EVOLUTION REACTION The noble-metal oxides IrO2 and RuO2 are the most studied catalysts due to their excellent performance toward OER and are considered as the benchmarking OER electrocatalysts. However, electrochemical stability of these materials, Ru in particular, is low, limiting their utility in commercial devices. Their deterioration occurs by a mechanism where the Ru4+ and Ir2+ ions transform into [Ru8+]O4 and [Ir6+]O3 moieties and solubilize in aqueous medium.118,119 Recently it was shown that alloying and forming heterostructures can impart extra electrochemical stability as well as high activity in these materials.120,121 Karthik et al. demonstrated that high reactivity of anodized gold for the condensation of [Ir(OH)6]2− provides a simple procedure for the electroless deposition of IrOx NPs on an anodized Au electrode with high catalytic activity.122 Interestingly, DNA scaffold was also recently used to load IrO2 nanoparticles, where the synergistic interactions of catalyst and water with PO43− of DNA promoted its OER activity.123,124 Despite being the best choices, IrO2- and RuO2-based OER catalysts are too expensive for commercial applications and, thus, have encouraged an extensive search for alternative inexpensive catalysts. Nonprecious transition-metal oxides have gained much research interest because of their abundance and high catalytic activity. These are excellent candidates for OER due to their multivalent oxidation states. Their activities are highly dependent on the morphology, composition, oxidation state, the d-electron density of the transition-metal ions, and surface oxygen binding energy. The most notable materials recently investigated include spinels, perovskites,1 transitionmetal oxides, layered double hydroxides, and pnictides.125 The most notable findings in the past few years have been described in the following. 4.1. Spinel and Perovskite-Based Catalysts. The spinel structure (AB2O4, where A, B = metal) can be imagined as cubic array of O2− ions, where A2+ and B3+ cations occupy all 33745


Review


Figure 6. (a) The relation between the OER catalytic activity, defined by the overpotentials at 50 mA cm−2 of OER current, and the occupancy of the eg-symmetry electron of the transition metal (B in ABO3). Reproduced from ref 137. Copyright 2011 AAAS. (b) Schematic representation of the O p-band for transition-metal oxides and (c) potential at 0.5 mA cm−2 vs the O p-band center relative to EF (eV) of (Ln0.5Ba0.5)CoO3−δ with Ln = Pr, Sm, Gd, and Ho. The O pband center relative to the Fermi level was computed by DFT. Reproduced with permission from ref 140. Copyright 2015 Nature Publishing Group.

Figure 5. (a) Schematic illustration of structural transformation within single-crystal and polycrystalline Co3O4 showing the formation of more active/oxidized phases that generate structural stress and/or voids and weaken the stability of materials. (b) OER polarization curves of Co3O4@CoO SC, Co3O4 SC, IrO2, and RuO2 catalysts. Reproduced with permission from ref 132. Copyright 2015 Nature Publishing Group.

these oxides and hydroxides were envisioned to be good OER catalysts, since the oxyhydroxide intermediates formed on spinel surfaces during OER has similarities with their structure. Furthermore, oxygen defects in these oxides create unusual metal oxidation states that play the most crucial role in determining their efficiencies. Oxygen-deficient phases enable fast charge transfer along with optimal adsorption energies for OER intermediates.144 Gardner et al. examined two polymorphs of LiCoO2 under OER conditions to understand the above-mentioned mechanism in detail.145 They showed that both layered and cubic phases of LiCoO2 form same type of structure during catalysis, that is, the cubic spinel, LiCo2O4, though only the cubic LiCoO2 nanoparticles exhibited excellent OER performance. The activity of cubic LiCoO2 was ascribed to the existence of [Co4O4]n+ cubane structural units that provided not only a lesser oxidation potential to Co4+ formation but also decreased intercubane hole mobility, which was not observed in the layered case. Another interesting recent study showed that the formation of cubane can be made facile by electrochemical delithiation when the formation of cubane becomes facile.146 The overpotential of the LDH phase diminished from 510 to 390 mV after delithiation. Weng et al. reported a new layered Na1−xNiyFe1−yO2 double-oxide deposited on Ni foam exhibiting activity and stability surpassing those of IrO2 and RuO2.147

activity over the pristine sample.138 Similar observation was reported in the case of Fe-based perovskite also.139 In addition, when the oxygen p-band is close to the Fermi level of transition metals, OER activity shows marked enhancement (shown in Figure 6b,c).140 Diaz-Morales et al. synthesized Ir-based double perovskites (Ir-DP) having general formula A2BB′O6, where A is a large cation and B, B′ are smaller cations.141 It was inferred that 3D network of corner-sharing octahedron is an important prerequisite for the enhanced OER activity as well as high chemical stability of Ir-DPs under harsh anodic electrochemical conditions. With extensive details, Han et al. recently reviewed Fe-based perovskite materials for OER applications.142 4.2. LDH and Other Non-Noble Metal-Based Catalysts. Transition-metal oxides and hydroxides such as CoO, NiO, LiCoO2, Ni−Fe LDH, Co−Ni LDH, etc. offer fewer possibilities of tuning the electrical properties when compared with perovskites and spinels.5,143 However, the ease of preparation and nanostructure formation leading to high surface area make them attractive candidates for OER. Besides, 33746


Review


Figure 7. (a) Schematic illustration of layered double hydroxide structure and chemical components. Reproduced with permission from ref 150. Copyright 2015 Royal Society of Chemistry. (b) OER performance of exfoliated NiFe-, NiCo, and CoCo-NS (nanosheet) and nonexfoliated NiFe-, NiCo, and CoCo-B (bulk). Reproduced with permission from ref 143. Copyright 2014 Nature Publishing Group.

4.3. Other Transition-Metal Chalcogenides and Pnictides. Metal chalcogenides, layered transition-metal chalcogenides (TMCs), and pnictides have interesting electrochemical properties that depend on their elemental composition, crystal structure, size, and defects. Compared to the metal oxides with high OER activities, as discussed earlier, the heavier congeners of the chalcogen family exhibit lower band gap and higher electrical conductivity and thus are expected to exhibit further improved OER performance. The chalcogenides have been widely investigated for HER156,157 and ORR158,159 in the past due to their fast charge transfer ability and stability in both acidic as well as alkaline media. Recent studies have shown that the TMCs and pnictides also exhibit robust and comparable OER activities as that of the oxide materials.5 Interestingly though, as a few careful studies have pointed out, these compounds potentially undergo chemical transformation on the surface under the anodic conditions forming thin layers of oxygen-containing species such as oxide or hydroxide layers that are responsible for their high OER activities.160,161 Therefore, chalcogenides and pnictides appear to act as unique precursors, assisting in the facile in situ formation of OER active layers on the catalyst surface. Majority of the OER active chalcogenides and pnictides consists of Ni- and Co-based compounds, and even though the origin of their high activity is not clear, it is perhaps related to the ease of formation of oxide layers on the surface. For example, NiS nanosheet arrays supported on a stainless steel mesh have shown efficient OER activity with an overpotential of ∼300 mV at 10 mA cm−2 current density.162 In comparison to oxides, other metalchalcogen bonds are weaker and more covalent in nature. Besides, doping at the metal site becomes more convenient due to the larger size of the anions and metal polyhedra. Therefore, the catalytic activity can be conveniently tuned by controlling the metal-to-chalcogen ratio and by doping at the

The Na ion deintercalation resulted in decreased potential from 1.52 to 1.49 V at 10 mA cm−2 with a Tafel slope of 40 mV/dec Here, the increased partial oxidation state of Fe and Ni in deintercalated Na1−xNiyFe1−yO2 accelarate the OER kinetics. OER active LDH (Figure 7a) mostly includes cobalt and nickel-based compounds.148−150 Subbaraman et al. performed an extensive study on 3d transition-metal (Ni, Co, Fe, Mn) hydr(oxy)oxide and inferred a reactivity trend similar to that of perovskite systems following an order of Ni > Co > Fe > Mn, which is reverse to the M2+δ−OH bond strength.151 It appears that higher OER activity for NiOOH can be attributed to the optimum bonding strength between Ni and OH species.152 The OER activity of LDH mounted on 3D Ni foam results in further improvements due to better dispersion, faster mass transport, and synergism between the catalysts with the support. In this regard, Song et al. have demonstrated Ni−Fe LDH through exfoliation process, which exhibited an excellent OER activity having lower overpotential of 300 mV with Tafel slope of 40 mV dec−1 (Figure 7b).143 Interestingly, Klaus et al. demonstrated that the OER performance of Ni(OH)2 would increase in the presence of trace amount of Fe.153 Similarly, Ni improves the activity of CoOOH, while Mn does not.149 Their electrochemical impedance spectroscopic studies revealed that Ni incorporation improves the ability of the catalysts to stabilize surface intermediates, whereas Mn incorporation impedes intermediate stabilization. Kundu and co-workers have recently demonstrated petal-like 3D hierarchical array of β-Ni(OH)2 nanosheets and nanoburls activated by faceting.154 The formation of oxyhydroxide exhibited a lower overpotential 300 mV at a current value of 10 mA cm−2 and an exceptional turnover frequency (TOF) = 47.14 s−1(at 1.53 V vs RHE). For further information, a concise review article by Hunter et al. may be referred to.155 33747


Review


Figure 8. (a) TEM image of FeCoNiP nanostructures showing the hollow tubular structure. (right) STEM-HAADF image and corresponding elemental maps of C, Fe, Co, Ni, and P. (b) OER polarization curves of the various catalysts and (c) the corresponding overpotentials required to achieve anodic current densities of 10, 20, 50, and 100 mA cm−2. Reproduced with permission from ref 177. Copyright 2018 Royal Society of Chemistry.

metal site. For the first time Swesi et al. demonstrated a metalrich form of nickel subselenide, Ni3S2, as a potential candidate for OER in alkaline condition. The potential they observed was 1.53 V at 10 mA cm−2, with a stability performance of 18 h conducted by chronoamperometry.163 The performance can be further improved by faceting the catalyst particles and exposing a more suitable crystal facet. While demonstrating this, Zou and co-workers induced in situ growth of {2̅10} high-index faceted nanosheet of Ni3S2 arrays on nickel foam that exhibited ∼100% faradaic yield toward OER and remarkable catalytic stability for 200 h. The authors illustrated with the help of DFT calculations that orientation of the Ni atoms in this facet is significantly more favorable for OER as compared to conventional facets.164 Further by incorporating Ni in Co0.85Se, enhancement in the electrical conductivity was observed that resulted in better OER activity. Co0.85Se and (Ni,Co)0.85Se exhibited overpotential of 324 and 255 mV, respectively.165 The latter catalyst also showed durability for 24 h with a minimal increase in overpotential to 276 mV. In addition to the incorporation of Ni to Co0.85Se, intentional coating with layered double hydroxide on the surface of (Ni,Co)0.85Se by electrodeposition enhanced the OER activity with an overpotential to 216 mV with a Tafel slope of 77 mV/dec.165 The

scope of these materials for OER has been elegantly reviewed by Suen et al. and Kuang et al.5,166 Akin to metal chalcogenides, metal pnictides, especially metal nitrides and metal phosphides (M2N, M3N, M4N, MP, and M2P; M = Co, Ni) have also been extensively studied for OER activities.167−170 Here too, a thin layer of metal oxide or hydroxide is believed to be formed on the surface of these catalysts due to surface oxidation under alkaline conditions, which is responsible for OER activities.125,171 For example, Stern et al. have demonstrated for the first time a nickel pnictide ∼Ni2P undergoes surface oxidation under the electrochemical conditions and transforms to Ni2P/NiOx core−shell assembly, which led to an OER overpotential of 290 mV at 10 mA cm−2 and Tafel slope of 59 mV dec−1, besides long-term stability up to 10 h.172 Among phosphoruscontaining materials, the Co-based compounds such as CoP, Li2CoP2O7, Na2CoP2O7, NaCoPO4, and LiCoPO4 exhibited appreciable OER activity and durability comparable to nitrides.173,174 As example, Zhu et al. used a simple electrodeposition technique to fabricate arrays of mesoporous CoP nanorods on Ni foam, which was used as working electrode for OER.175 Hydrothermally synthesized CoP nanorods have been shown to exhibit consistent OER current density of 10 mA cm−2 at an overpotential of 320 mV and 33748


Review

ACS Applied Materials & Interfaces Table 2. Overview of Recently Developed Efficient OER Catalysts Sl no.

materials

EOER at 10 mA cm−2 vs RHE (V)

1. 2. 3.

IrO2 NP @ DNA IrO2/Au2O3 IrOx/SrIrO3 film

1.542 1.60 1.57

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

Fenton-treated gold surface BaNi0.83O2.5 La0.6Sr0.4Co0.6Fe0.4O3−δ Pr0.5Ba0.5CoO3−δ by Ca doping Ba0.5Sr0.5Co0.8Fe0.2O3−δ La0.95FeO3−δ Co3O4 (plasma engraved) CoFe2O4/C NRAs NiFe2O4 CuFe2O4 Na0.08Ni0.9Fe0.1O2 De-LiCoO2 Ni3Se2 CoMnP Ni−Fe LDH Ni-CoOOH Co-Pi NA/Ti Mn−Co oxyphosphide g-C3N4/Ti3C2 CoS-DNA hybrids black phosphorus Fe(PO3)2/Ni2P 3D foam cobalt−vanadium hydr(oxy)oxide CeO2 in NiO Mo & Fe modified Ni(OH)2/ NiOOH

1.8 1.57 1.619 1.67 1.6 1.63 1.53 1.47 1.67 1.64 1.49 1.62 1.53 1.56 1.53 1.516 1.68 1.55 1.65 1.58 1.6 1.407 1.445 1.60 1.51

stability

Tafel slope (mV dec−1)

900 min,a NL

32

30 hb at 10 mA cm−2, Δη = +20 mV

50 35

20 h,b NL >3 hb at 10 mA cm−2, NL 1000 cycles,c 86.4%

2000 cycles,c 87% 60 hb at 50 mA cm−2, NL

30 hb at 10 mA cm−2, NL 10 h,a NL 18 h,a NL 500 cycles,c Δη = +40 mV 25 hb at 10 mA cm−2, NL 20 h,a NL 8 h,a NL 5000 cycles,c Δη = +10 mV 1000 min,a NL 10 000 s,a NL 20 h,a NL 10 000 s,a 77% 50 h,a NL

62.5 73 48 68 45 98 94 40 57 122.0 61 40 36 187 52 74.6 61 72.9 51.9 44 118.7 47

electrolyte

ref

0.1 M NaOH 0.1 M NaOH 0.5 M H2SO4

123 122 121

0.1 M NaOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 1 M KOH 0.1 M KOH 0.1 M KOH 1.0 M KOH 0.1 M KOH 0.3 M KOH 1 M KOH 1 M KOH 0.1 M KOH 0.1 M PBA 1.0 M KOH 0.1 M KOH 1 M KOH 0.1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH

192 193 79 185 137 139 130 127 128 128 147 146 163 78 143 149 194 195 183 124 196 197 198 199 200

a

Chronoamperometry. bChronopotentiometry. cCV cycles; NL = negligible loss.

Tafel slope of 71 mV dec−1 in 1 M KOH for over 12 h.143 The surface oxidation was found to be quite severe in this case, giving rise to significantly crystalline cobalt oxide coating on the catalyst surface. By doping metal pnictides with another metal, further improvement in the OER activity has been shown.167 Recently, Xiao et al. fabricated a new family of Co-based bimetallic phosphide ultrathin nanosheets (CoM-P-NS, M = Ni, Mn, Cu, Zn) with homogeneous composition and unique porous architecture using ultrathin metal−organic framework nanosheets as precursor.176 They reported a remarkably low overpotential for OER ≈ 209 mV in an alkaline solution for the optimized nickel foam supported CoNi (20:1) phosphide with stability for 11 h. They reported a remarkably low overpotential for OER ≈ 209 mV in alkaline solution for the optimized nickel foam supported CoNi (20:1) phosphide with stability for 11 h. Overall, investigations on pnictides-mediated OER are limited in number when compared with oxides and chalcogenides, probably because the attribution of the activities becomes complicated due to oxidation of the surface. However, of late, increasing numbers of studies have been performed in the context of bifunctional catalysis, as discussed in the next section, since their high activity and stability cannot be neglected despite complex surface chemistry. Very recently, to investigate how the OER activity of transition-metal pnictide (TMPs) varies systematically with the catalyst composition, Liu and co-workers synthesized a series of mixed TMP nanoparticles supported on carbon nanofibers.177

They found that presence of a secondary transition metal in pnictides indeed significantly increase their OER activity in the order of FeP < NiP < CoP < FeNiP < FeCoP < CoNiP < FeCoNiP (Figure 8). In particular, the trimetallic pnictide FeCoNiP has exhibited one of the lowest OER overpotentials of 200 mV with TOF over 0.94 s−1 at the overpotential of 350 mV. The Tafel slopes of the mixed TMPs are found to be in the range of 60−120 mV/dec, suggesting that the adsorption of OH− on the TMP nanoparticles is probably the ratelimiting step. These materials have exhibited “no degradation” stability for 24 h. Overall, investigations on pnictides-mediated OER is limited in number when compared with oxides and chalcogenides, probably because the attribution of the activities becomes complicated due to oxidation of the surface. However, of late, increasing numbers of studies have been performed in the context of bifunctional catalysis, as discussed in the next section, since their high activity and stability cannot be neglected despite complex surface chemistry. 4.4. Carbonaceous Materials. The recent progress in using carbon-based materials as electrocatalysts has been remarkable. During the past decade carbon-based materials such as graphene, meso/microporous carbons, CNTs, and other nanocarbons have been found to be highly active for OER, just as in the case of ORR. These materials show promising OER activity due to huge surface area and excellent electrical conductivity. Moreover, here too, akin to ORR, the catalytic performances have been further enhanced by doping these with N, S, P, and O.178−180 33749


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Figure 9. (a) Schematic illustration of the AEM and LOM operating simultaneously in perovskite oxides. (b, c) Plots highlighting the differences in the reaction free energies of the four charge transfer steps in AEM and LOM, respectively, for lanthanum-based perovskites, which are plotted as functions of oxygen adsorption energy, ΔGO. The shaded region shows the theoretical OER activity volcano, and the dashed line indicates the equilibrium OER potential. The difference between the dashed line and the boundary of the activity volcano depicts the theoretical overpotential for OER. Reproduced with permission from ref 191. Copyright 2018 American Chemical Society.

coordination moiety in the g-C3N4 matrix is suggested to be responsible for such activity. Such investigations are few and very recent in nature. A more detailed understanding of these reaction active sites will be desirable and interesting. When we document here the most recent findings, reviews detailing the use of carbonaceous materials for OER have been published elsewhere.186,187 4.5. Emerging Insights on Performance. We have so far discussed some of the highly efficient OER catalysts. Table 2 lists the performance and stabilities of such catalysts reported of late. One cannot fail to notice in these reports that the attribution of high catalytic activities involves some experimental evidence and some arbitrariness, such as synergistic effect. However, there have been increasing numbers of dedicated studies elucidating the fine details of reaction mechanism, which should assist rational design of highly active OER catalysts. As in the case of ORR, the oxygen evolution reaction is also a four-electron process. Herein hydroxyl anion gets adsorbed on the catalyst surface, followed by electron transfer. Alternatively, two OH− adsorbed at two consecutive sites may undergo association to form H2O2, which may dissociate to O2 and H2O by disproportionation. In a recent study using an in situ spectrometry, Koper and coworkers showed the formation of various intermediates such as trihydroxide, peroxide, and oxide layers on Au surface at different applied potential, before the released molecular oxygen.28 To understand the OER mechanism on IrO2 surface, Casalongue et al. employed in situ ambient pressure XPS and showed that, under OER condition, the population of Ir4+ was reduced with the evolution of Ir5+.188 Such change in the oxidation state of surface Ir from +4 to +5 is coupled to a decrease in surface concentration of adsorbed hydroxyl species, supporting a peroxy species (OOH) mediated deprotonation mechanism. The OER activity of spinel and perovskite oxides also relies on the switchable oxidation state of the metal ions, where the

N-Doped carbon having pyridinic and quaternary N acted as the active sites for OER as revealed by Hashimoto and coworkers.181 Furthermore, these workers concluded that OER performance improves with the increasing amount of pyridinic and quaternary N sites on carbon. The findings gave strong grounds to establish that N species are the active sites for OER. Moreover, edge dangling bonds, surface functional groups, or defects in carbon materials contribute to electrocatalytic performance in alkaline medium. Conversely, metal-doped carbon materials show appreciable activity in acidic medium too. Recently Yang et al. deposited Co3O4 nanosheets on carbon paper (Co3O4−CP) through electroplating followed by a two-step calcination procedure.182 During this process, a layer of amorphous carbon formed in situ on the top of Co3O4. The authors postulated that, due to this layer, Co3O4−CP exhibited excellent OER activity in 0.5 M H2SO4 with excellent overpotential 370 mV and also the commendable stability for ∼90 h at100 mA cm−2 with a Tafel slope of 87 mV/dec. Because of high N content and excellent chemical stability along with graphene-like structures, g-C3N4 has also gained significant attention for electrocatalytic OER.183 Tahir et al. have developed strongly coupled hybrid cobalt hydroxide nanowires by adding graphitic carbon nitride nanosheets.184 They demonstrated the presence of merely 5% g-C3N4 resulted in coupled Co(OH)2@g-C3N4 nanosheets with an overall diameter of 110 nm and a coating C3N4 layer of 10 nm, which exhibits a remarkable overpotential of 320 mV as well as high stability up to 10 h having Tafel slope of 59 mV/dec. Subsequently it was found that, by adding tiny bit of transition metals in the form of nanoparticles, OER performance of carbon-based materials can be further improved. Very recently, for example, Zheng et al. fabricated M-C3N4/CNT (M = Cr, Mn, Fe, Co, Ni, Cu, Zn) and found that the cobalt analogue exhibited the best OER performance.185 Physicochemical and electrochemical characterization combined with DFT computation was employed to identify the active sites. A Co−N 33750


Review


Figure 10. (a) ORR and (b) OER activities of ZnCo2O4/N-CNT, Co3O4/N-CNT, and ZnCo2O4 and comparison with benchmark catalysts. (c) Schematic illustration of ORR and OER mechanism on the surface of ZnCo2O4/N-CNT. Reproduced with permission from ref 207. Copyright 2016 Wiley-VCH. (d) ORR and OER activities of pure CNTs, Cu@CNTs, Cu@NCNTs, Cu@NCNT/CoxOy composites, Pt/C, and IrO2 in 0.1 M aqueous KOH solution. (inset) A schematic diagram of a Cu@NCNT/CoxOy composite. Reproduced with permission from ref 211. Copyright 2017 Wiley-VCH.

(VO + *OH) + H 2O → (HO − site* + *OH) + H+ + e−

catalytic process can involve the lattice oxygen too. With in situ 18 O isotope labeling and online electrochemical mass spectrometry, Grimaud et al. in a very careful study showed the evolution of labeled molecular oxygen (34O2 and 36O2), where LaCoO3 and related perovskite was employed as active materials.189 The degree of covalency in the M−O bonds is important, since the number of surface oxide layers (few to few tens) participating in the reaction depends on this parameter. There is another similarity with the ORR reaction; that is, here too the presence of a single electron in the eg orbital of the metal ion at B site of perovskite has shown superior OER activity. Deng et al. reported on the presence of Cu3+ species as OER active site for the catalysts Cu, Cu2O, Cu(OH)2, and CuO using in situ Raman and X-ray absorption near-edge structure spectroscopy (XANES).190 From several other such in situ studies, it is emerging that a higher oxidation state of the metal ions acts as OER active center in the metal oxide catalysts. Unlike the case of metal ions, investigations on the role of the lattice oxygen (LO) in the OER mechanism is rather limited. In a very recent DFT study, the participation of LO and consequent variation in activity have been carefully examined by Kolpak and co-workers.191 They argued that the previous studies point to two competing OER mechanisms operating simultaneously on perovskites, specifically, adsorbate evolution mechanism (AEM) and lattice-oxygen participation mechanism (LOM), that can be clearly distinguished on the basis of participation of LO (Figure 9a). AEM can be represented by eqs 10, 11, 13, and 14, while they gets modified in LOM as follows: OH* → (VO + OO*) + H+ + e−

(25)

(HO − site* + *OH) → *OH + H+ + e−

(26)

where VO defines surface oxygen vacancy. The study showed that (1) AEM-based OER activity volcano is universal for all perovskites, while (2) LOM can induce higher OER activity depending on the A site cation, which is related to the different behavior of the reaction free energies of the four OER charge transfer steps and lowering of the thermodynamic overpotential (Figure 9b,c). Relying on the two-mechanism approach, the authors went on to predict new materials with potentially higher OER activity, which would be worth testing. Similar studies involving other OER active materials is expected to help develop superior electrocatalysts.

5. EMERGING BIFUNCTIONAL ORR-OER CATALYSTS Development of efficient bifunctional catalysts for oxygen redox reaction is essential for improving the efficiency and durability of energy conversion devices such as regenerative fuel cells or metal-air batteries. Pt/C is considered as benchmark catalyst for ORR, while IrO2 and RuO2 are taken as benchmark catalyst for OER. However, neither of them are good for both OER/ORR bifunctional activity.201 Recently, many non-noble metal (e.g., Co, Fe, Ni, Mn, Cr)-based oxides, perovskites, spinels, and layered double hydroxides-based materials have been found to be highly active bifunctional electrocatalysts.166,202−205 Combining efficient individual OER and ORR catalyst can also serve as bifunctional behavior. Besides, metal-free doped carbon-based materials are also emerging as an attractive class of compounds in this area.205,206 Investigations on bifunctional catalysts are relatively newer as compared to ORR and OER catalysts, and the reports are

(23)

(VO + OO*) + H 2O → O2 + (VO + *OH) + H+ + e− (24) 33751


Review


Figure 11. (a) Comparison of OER activities of LaMnO3, CaMnO3, LaMn7O12, CaMn7O12, and RuO2. (b) Comparison of ring/disk current density for ORR of LaMnO3, CaMnO3, LaMn7O12, CaMn7O12, Pt/C. (c) Crystal structure LaMn7O12, shown by following colors: (orange polyhedra) MnO6 octahedra, (green square) MnO4 square, and (blue spheres) O atoms of the OH− adsorbates. (d) Schematic of OER mechanism on LaMn7O12. (inset, center) MnO4 square and MnO6 octahedra of AMn7O12 (A = La). Reproduced with permission ref 4. Copyright 2017 WileyVCH.

material has displayed 60 mV more positive onset potential compared to Cu@NCNTs for ORR and exhibits an overpotential of 370 mV at 10 mA cm−2 for OER (Figure 10d).211 Such strategies have been extended to many other spinel systems.61,213 Notably, cobalt oxide present on N-doped carbon-based material exhibits a series of Faradaic peaks in voltammogram as compared to the pristine samples, probably due to the formation of Co−N bond.214 Apart from surface area and conductivity, the ease of charge transfer between the catalyst surface and the reactant is the key to the facile bifunctional activity. From this perspective, enhanced ORR/OER activity of the perovskite can be related to the number of transition-metal eg electrons, the position of p-band center of oxygen, and the d-band center of transitionmetal ion with respect to Fermi energy level and the population of oxygen vacancies.2,215 Close proximity of O 2p-band center and Fermi level decreases the energy of oxygen vacancy formation facilitating the oxygen surface exchange coefficient and promotes OER/ORR activity.216 Increasing the number of oxygen vacancies favors both ORR and OER catalysis. For example, in the case of La1−xFeO3−δ, it was shown that, along with surface oxygen vacancy, formation of trace quantity of Fe4+ species having d-orbital filling (t2g3 eg1) facilitates the bifunctional behavior.139 Detailed investigations on the electronic configuration of perovskites (ABO3) revealed that electronic configuration of B site cation having more than one eg electron favors OER, while less than one promotes ORR.99,137,217,218 In a similar way, other perovskite oxides like Lan+1NinO3n+1, La0.8Sr0.2Mn1−xNixO3, and La0.5Sr0.5CoO3−x also exhibit high ORR/OER catalytic performances, where higher oxygen stoichiometry promotes higher oxidation state of cobalt due to increased hybridization between Co-3d and O-2p orbital.219−222 Yagi and co-workers compared the bifunctional behavior of simple (AMnO3) and quadruple (AMn7O12) perovskite, where A = Ca or La.4 They observed that the AMn7O12 exhibited bifunctional behavior, while AMnO3

scattered. In the following, we reviewed the most successful strategies to obtain efficient bifunctional catalysts. 5.1. Spinels and Perovskites. High bifunctional activity in spinels such as Co3O4 is originated from the mixed valences of cobalt (Co2+/Co3+), which facilitate the electron transfer through donor−acceptor interaction, thereby providing sites for reversible electron transfer in anodic OER and cathodic ORR.166 However, surface area and electrical conductivity of the metal oxides are usually low.207,208 Recently, several research groups have attempted to improve their surface area employing innovative approaches. In one such approach, Park et al. developed high surface area 3D ordered mesoporous Co3O4 using polystyrene beads as a template and observed significantly enhanced bifunctional activity.209 With 10 nm Co3−xMnxO4, Li et al. demonstrated that nanocrystallization of the material reduces the path length of the electrons and is a promising way to improve the conductivity and thereby catalytic activity.208 However, such approaches do not lead to significant improvements, as the nanoparticles tend to agglomerate on the electrode surface, and the interparticle junctions lead to high electrical resistance. Alternatively, mounting the catalyst particles on a conducting catalyst support such as CNT or graphene can also improve their activities.61,207,210−212 Liu et al. demonstrated this by integrating ZnCo2O4 quantum dots on conductive N-CNT that exhibited superior bifunctional behavior (Figure 10a− c).207 N doping makes carbon atom more electropositive and oxygen-philic, which promotes ORR. In such a report, Zhao et al. obtained Cu@N-CNT/CoxOy composites in which Cu is embedded inside the CNT and CoxOy decorated outside the CNT (as shown in the inset of Figure 10d).211 Detailed investigation using DFT calculations revealed that the work function (Φ) of CNT surface can be further reduced by encapsulating Cu metal inside CNT, making the surfaces more active toward ORR and favoring the adsorption of hydroxyl ion on the single-walled (SW) CNT in the vicinity of CoxOy. This 33752


Review

ACS Applied Materials & Interfaces displayed only ORR activity (as shown in Figure 11a,b). The authors attributed the exceptional OER activity of the quadruple perovskite (LaMn7O12) to its unique structure (Figure 11c) that allows an average Mn−Mn distance (∼3.2 Å) favoring O−O bond formation. The reason for enhanced ORR activity of LaMn7O12 has not clearly been understood. On the one hand, as seen in Figure 11d, the reaction intermediate superoxide O22− is easily formed due to optimum Mn−Mn distance facilitating OER kinetics. On the other hand, Yan et al. showed that doping of a large cation (e.g., Ir) at the B site of LaMnO3 (La0.8Sr0.2)1−xMn1−xIrxO3 (x = 0.5) leads to one of best perovskite-based bifunctional catalyst in terms of activity and durability.223 Herein, the creation of A site deficiency as well as doping of a larger cation at B site resulted in a larger lattice structure and reduced valence state of Mn, consequently weakening the bonding with adsorbed oxygenated intermediate, which eventually facilitated both ORR and OER kinetics. Cho and co-workers have synthesized a series of doped BSCF perovskite materials to impart bifunctional behavior.205,224 Nanocrystals of La-doped BSCF perovskite catalysts exhibit outstanding bifunctional performance due to high surface area and presence of La cation in A site, which increases the oxidation state of cobalt cation at the B site.205 Further improvement in the catalytic activity can be achieved by mixing the perovskite material with nitrogendoped carbon material, as the former is good for OER and later can improve the ORR activity.225−228 Similarly, Lee et al. demonstrated that addition of polypyrrole to perovskite oxide resulted in remarkable improvement in bifunctional behavior.225 Superior activity therein arises due to preferential adsorption of oxygen on polypyrrole forming charge transfer complex (Py+O2−) that hastens the O2 redox kinetics. Following this, very recently, it was found that the double perovskite NdBa0.5Sr0.5Co1.5Fe0.5O5+δ (NBSCF) incorporated in Ndoped reduced GO also exhibits excellent bifunctional behavior.229 NBSCF exhibited very low potential gap of 0.766 V between ORR and OER. DFT analysis inferred that oxygen p-band center moves away from the Fermi level of Nd after its doping in double perovskite, leading to high activity and stability. These findings have demonstrated that tuning the electronic structure of perovskites through different doping and combining with doped carbon can be a promising route toward imparting excellent bifunctionality. Note that Zhu et al. and Cai et al. have reviewed electrochemical performances of this family of materials recently.1,230 5.2. LDH and Other Metal Oxides. Apart from spinels and perovskites, other transition-metal oxides such as CoxOy, NixOy, MnxOy have also shown attractive bifunctional activity.231−233 MnxOy generally enhances ORR kinetics but has poor OER activity.234 Combining MnxOy with efficient OER catalysts such as RuO2 or CoxOy can improve their bifunctional behavior. On the one hand, recently, Yoon et al. reported a facile synthesis of double-walled RuO2/Mn2O3 composite fibers that exhibited efficient bifunctional activity.232 On the other hand, controlling the morphology to expose a suitable crystal facet that favors oxygen adsorption should result in further enhancement of the bifunctional behavior. With this motivation, single-crystal CoO nanorods (SC CoO NRs) prepared by Ling et al. exhibited impressive catalytic performance and stability for ORR/OER.144 These nanorods have desired oxygen vacancies generated by surface structure engineering, as shown in Figure 12a. Oxygen vacancies

Figure 12. (a) Schematic representation of surface engineering of SC CoO NRs. (b) Comparison of ORR activities of SC, PC CoO NRs, and Pt catalysts in 1 M KOH solution. (inset) Chronoamperometric test for ORR after methanol addition. (c) Specific activity of ORR at 0.6 V RHE. (d) Comparison of OER activities of SC, PC CoO NRs, and commercial RuO2 catalyst in 1 M KOH solution. (inset) Tafel plot for OER. (e) Specific activity of OER at 1.65 V RHE. Reproduced with permission from ref 144. Copyright 2016 Nature Publishing Group.

localized on the {111}-O facets improve the conductivity of the material as well as adsorption of ORR/OER intermediates, suggesting enhanced bifunctional catalytic activity. On the one hand, for ORR SC CoO NRs presented half-wave potential of 0.85 V, similar to that of Pt/C with specific kinetic current density greater than 7 times than that of polycrystalline PCCoO NRs (Figure 12b,c). On the other hand, 2.7-fold increase in specific current density than PC-CoO NRs was observed (Figure 12d,e). Employing another popular approach very recently, oxygen vacancies have been created by doping oxides with N to enhance oxyphilicity of the catalyst surface. Guo and 33753


Review


Figure 13. Comparison of bifunctional activity of SHG, GC, GS, Pt/C, and RuO2 nanoparticles. (inset) ΔE (Ej=10 − E1/2) of ORR and OER activities for various catalyst reported catalyst. Reproduced with permission from ref 250. Copyright 2017 Wiley-VCH.

workers245 first theoretically predicted that generation of Co4 N and Co−N−C as active ORR and OER site, respectively, on a composite material should facilitate the redox reactions.245 A proof-of-concept experiment revealed the same in the ORR and OER activity of Co4N, N-doped carbon fibers network, and carbon cloth (Co4N/CNW/CC). However, such examples are scarce, and further investigation is required for understanding and enhancing the catalytic activity of metal nitrides. Metal carbides are unique due to a small difference in electronegativity between the constituting atoms, which makes their Fermi level behave somewhat similar to noble metals. Kou et al. have developed a precise thermal-chlorination strategy for transformation of metal carbide to metal-selfdoped graphene.247 They observed that, among the various 2D metal carbides (MxCy, e.g., Cr3C2, Mo2C, NbC, and VC) that have been examined as bifunctional catalysts, Cr self-doped graphene displayed considerably enhanced bifunctional activity toward ORR/OER. Cr being less electronegative than carbon acquires slightly positive charge facilitating the adsorption of oxygen molecules. Incorporation of Cr creates defect in the graphene sheet, which enhances the catalytic activity. 5.4. Carbon-Based Materials. 5.4.1. Metal-Free Catalysts. As discussed in the earlier sections that heteroatomdoped carbon exhibits useful ORR and OER activities, one would envision them having excellent bifunctional activity too. Indeed investigations in the last couple of years have shown that a number of metal-free carbon-based materials have met such expectations. However, their performance depends mainly on the microstructures (therefore all efficient ORR and OER catalysts are not necessarily efficient bifunctional catalysts too), percentage and nature of heteroatom doping, and active surface area. Nitrogen is the most widely used heteroatom, while increasing number of heteroatoms by this precursor alone is outdated.248,249 Hu et al. synthesized N and S codoped graphitic sheet (SHG) as an active bifunctional catalyst that also catalyzes hydrogen evolution reaction. SHG exhibited ORR activity comparable to that of commercial Pt/C, retaining ∼93% stability up to 100 h, and it also displayed comparable OER activity as that of benchmarked RuO2 (as displayed in Figure 13).250 Authors have established that codoping of both N and S is responsible for exceptional high activity, as it generates positive C centers that facilitate oxygen adsorption. Shinde et al. have used phosphoric acid and methanesulfonic acid precursors for codoping P and S on carbon nitride sponges (P,S-CNS), respectively; it has shown excellent bifunctional activity such as

co-workers have demonstrated that nitrogenization of NiCo2O4 nanowires resulted in the formation of NiO/CoN porous interface NW arrays, giving rise to outstanding bifunctional activity.233 Herein, increased oxygen vacancies due to the incorporation of nitrogen resulted in decreased coordination of cobalt and coupling between NiO and CoN interface that facilitated bifunctional activity; this was experimentally verified using EXAFS and electron spin resonance (ESR) studies. Similar to the perovskite samples discussed earlier, it is emerging that combining the oxide materials with a heteroatom-doped (usually nitrogen) carbon network is a useful strategy to improve the electrocatalytic performance of low-dimensional material.235−238 5.3. Transition-Metal Chalcogenides, Pnictides, and Carbides. Because of their similarities in electronic structure with the metal oxides, metal chalcogenides and nitrides are also expected to exhibit appreciable bifunctional activity.239−243 However, low conductivity of these materials in pure form limits their performance. From this point of view, hybrid structure of TMCs with doped carbon material has been created that showed excellent bifunctionality.243 In such an approach, Tiwari et al. reported a simple strategy to synthesize stacked WS2 nanosheets on the surface of conductive CNTs, which effectively changes the charge density and spin density of carbon in CNT leading to enhanced bifunctional activity for ORR and OER.243 The proposed mechanism explains that H+ ions are connected to sulfur atoms via hydrodesulfurization process, while oxygen gets adsorbed on tungsten facilitating ORR. Metallic WC enhances the hydrodesulfurization by fast electron transfer from W to CNT. All these processes simultaneously add up to give promising catalytic activity toward ORR/OER. Similarly, hybrid structure of cobalt sulfide with N-doped carbon network (N−C) has also drawn a lot of attention due to its outstanding bifunctional activity for oxygen redox reaction.239−242 In these materials also Co−S acts as active site for both ORR and OER, and carbon improves the conductivity of the system. Metal nitrides are emerging as promising electrode materials for oxygen redox reactions.125,244,245 Chen et al. synthesized atomically dispersed Fe−Nx on N and S codecorated on carbon network, which exhibited superior bifunctional activity.246 High catalytic activity of this material can be ascribed to (i) N and S doping creating positive polarization on neighboring carbon center facilitating oxygen adsorption, (ii) atomically dispersed Fe−Nx species on carbon network provides more of active sites, (iii) heteroatom doping increases the conductivity of the system causing faster electron transfer from Fe to carbon network. Similarly, Zhang and co33754


Review

ACS Applied Materials & Interfaces Table 3. Overview of Recently Developed Efficient ORR/OER Bifunctional Catalysts No.

materials

E1/2,ORR vs RHE (V)

EOER at 10 mA cm−2 vs RHE (V)

Tafel slope for OER (mV dec−1)

EOER at 10 mA cm−2 − E1/2,ORR (V)

stability (ORR/OER)

ref 209

0.95 0.79

1000 cycles, ΔE = 28 mV at 0.1 mA cm−2 a/1000 cycles, ΔE = 13 mV at 10 mA cm−2 a 50 h, 91.5%, at 0.8 Vb 10 h, 90% at 0.5 Vb/10 h, 95% at 1.58 Vb

91.5

0.85

25 h, NL at 0.77 Vb/45 h, NL at 10 mA cm−2 c

213

1.629

77.2

0.825

266

0.79

1.662

79.65

0.877

20 000 s, 97.6% at 0.6 Vb/1000 s, ΔE = 0.1 V at 20 mA cm−2 c 10 000 s, 85.6%b/10000 s, 112%b

0.82 0.869

1.65 1.635

80 40

0.83 0.766

214 229

LSMI LO-NF-NCNTs SC-CoO-NRsd NiO/CoN PINWsd CoO@Co/NrGO NC-CoO/C NiFe-LDH − Fe−N−C N-GCNT/ FeCo-3 FeNO−CNTCNFF-800 Co0.85Se@NCd Co1−xS/N−S-G

0.78 0.772 0.85 0.68

1.67 1.74 1.56 1.53

103

0.89 0.968 0.71 0.85

22 h, ΔE = ∼3.7 mV at 10 mA cm−2 c 10 h, ΔE = ∼0.08 V at −3 mA cm−2 c/10 h, ΔE = ∼0.04 V at 5 mA cm−2 c 5 h at 1.56, 97.6%/5 h at 1.56 V, 90% 10 h, 83% at 0.8 Vb/11 h, 95.8% at 1.87 Vb 10 h, 97% at 0.60 Vb/ 10 000 cycles, NLa/30000 cycles, 92.44%a

223 227 144 233

0.81

1.65

0.84

5000 cycles, NLa/1000 cycles, NLa

235

0.793 0.793

1.592 1.539 ± 0.006

45.2

0.799 0.747 ± 0.009

231 267

0.92

1.73

99.5

0.81

0.87

1.66

63

0.786

0.82 0.862

1.55 1.601

75 63

0.73 0.739

Co4N/CNW/ CCd PPy/FeTCPP/ Co Ni3Fe/N−C sheets BNPC-1100 N-GRWd PS-CNS SHG S,N−Fe/N/C− CNT Fe−N-CIG C@Co-NGR

0.8

1.54

81

0.74

20 000s, 97% at 0.65 Vb/6000 s, 90% at 1.6 Vb 24 h, ΔE = +0.4 at −3 mA cm−2 c/24 h, NL at 4 mA cm−2 c 20 h, 87.58% at 0.3 Vb/20 h, ΔE = +0.06 V at 10 mA cm−2 c 55 000 s, ∼92% at 0.65 Vb/25000 s, ∼75% at 1.65 Vb 10 h, 82.5% at 0.67 Vb/12 h, 93.4% at 1.55 Vb 30 000 s, 80% at 0.682 Vb/200 cycles, ΔE = +20 mV at 10 mA cm−2 c 20 h, 98% at 0.5 Vb/20 h, 98% at 1.54 Vb

0.86

1.61

61

0.75

10 h, 95% at 0.86 Vb/10 h, ∼84% at 1.57 Vb

261

∼0.76

1.60

77

0.84

12 000s, 96% at 0.7 Vb/12000 s, 50% at 1.6 Vb

254

0.793 0.84 0.87 0.87 0.85

1.55 1.59 1.56 1.60 1.60

201 52 64 71 82

0.757 0.75 0.69 0.73 0.75

8000 cycles, 75%a/10 h, NL 12 h, 90% at 0.7 Vb/24 h, 91% at 1.59 Vb 8000 s, 97.4%b 100 h, ∼93%at −0.3 Vb/20 h, ∼91%at 1.55 Vb 10 000 cycles, ∼98%a/NA

268 205 251 250 246

0.84 0.83

1.67 1.68

96 98.8

0.80 0.85

1000 s, 96% at 0.7 Vb/1000 s, NL at 1.67 Vb 3000 cycles, NLa/NA

257 269

1

3DOM Co3O4

0.65

1.66

58

1.01

2 3

c-CoMn2/C ZnCo2O4/NCNT Co@Co3O4/ NC-1 NixCoyO4/ Co−NG Co3O4 /NCNTA Co3O4 /NRGOd NBSCF

0.83 0.87

∼1.78 1.66

70.6

0.80

1.65

0.804

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

44 35

208 207

210

258 259 226 239 245

CV cycles; electrolyte −0.1 M KOH. bChronoamperometry. cChronopotentiometry. d1 M KOH as e; NL = negligible loss.

a

η = 330 mV for OER and ORR E1/2 of 870 mV.251 Positive carbon sites adjacent to nitrogen and oxygen in these samples are active for both ORR and OER, while P and S promote its improved efficiency through modification of electronic structure. In general, heteroatom doping increases the conductivity of carbon framework, thereby improving the rate of charge transfer that results in enhanced catalytic activity.251,252 However, large surface area materials possessing a number of pores and dangling atoms tremendously increase the accessibility of the active sites that promote fast electron transfer as well as ion diffusion at electrochemical interfaces. In a theoretical investigation, Guo and co-workers demonstrated the bifunctional activity of P and N codoped graphene framework. The authors elucidated the P atom, being larger than C and N, always moves out of the planar surface in the

graphitic framework and occupies the edge sites. These P sites get readily oxidized in the presence of oxygen and hence become inactive toward ORR/OER. While in basal plane, P sites adjacent to N in a graphitic framework trigger the adjacent carbon sites for OER, the N-doped carbon sites are efficient for ORR.253 This was further supported by the experimental results, as this material showed superior ORR and OER activity and stability than the noble metal-based benchmarked catalysts such as Pt and Ru. Wang and coworkers demonstrated in situ exfoliation of carbon cloth using Ar plasma, which resulted in oxygen-functionalized graphene sheet (P-CC).252 The synergistic effect between defects formed by in situ exfoliation and O-doping enhances the catalytic activity. Note, however, that though the carbon-based materials possess excellent bifunctional activity comparable with transition-metal oxides, they suffer from poor stability. 33755


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Figure 14. Box-whisker plot comparing performances for various families of ORR-OER bifunctional catalysts in alkaline media, developed in the last 3−4 years. On the basis of a large number of literature reports, the maximum, minimum, median, and mean for each set of data set was plotted (see refs in Supporting Information). (upper) Comparison of the half-wave potential (E1/2) depicting the ORR activities. (lower) Representation of the overpotential corresponding to 10 mA cm−2 OER current. (■) The mean of the data set. For convenience, the corresponding values for the benchmark catalysts were plotted to the extreme right.

possibly FeNx developed during doping on the graphene surface. However, Fe−N−C exhibited inferior OER activity compared to other state-of-the-art materials. The sluggish OER kinetics is because of formation of semiconductive FeOOH species during OER, which reduces electron transfer rate. Hence doping with OER active material in Fe−N−C system can further enhance the efficiency. Many reports are available in the literature describing bimetallic nitrogen-doped carbon as an efficient material for oxygen redox reaction.258−261 Numerous studies found that cobalt doping in Fe−N−C system is highly beneficial to improve OER kinetics.261−263 Su et al. synthesized bimetal FeCo nanoparticles encapsulated on N-doped graphitic carbon nanotubes (N-GCNT) serves as an excellent oxygen redox reaction catalyst.258 Acid treatment of the material so as to remove the metal nanoparticle does not show any deterioration in catalytic activity implying pyridinic N−C structure as the key electroactive sites. However, FeCo nanoparticles are crucial for structuring of carbon nanotube. On the contrary, DFT calculation explained a synergistic effect present between FeCo nanoparticles and N-CNT helps in surface modification favoring both ORR and OER. Other studies revealed that addition of a bimetal to M−N−C system increases the amount of pyridine nitrogen and strong coupling

Further, a compiled and illustrative review article on carbonbased materials for OER/ORR performances by Chen et al. can also be referred to by readers.31 5.4.2. Metal-Atom-Loaded Carbonaceous Materials. Insertion of transition metals into carbon network was found to enhance the bifunctional activity by several folds than the undoped ones. NiFe-N/C and MnCo-N/C, Ni3Fe/N−C sheets, CoxSe@NC bimetallic nitrogen-doped carbon framework outperforms most other well-known bifunctional catalyst families with ΔE (EOER at10 mA cm‑2 − E1/2,ORR) in the range of ∼0.73−0.76 V.226,254 Enhanced OER activity can be attributed to the bimetallic center, while superior ORR performance can be related to the N-doped carbon. However, detailed investigation is required to appreciate the origin of bifunctional behavior in metal−nitrogen doped carbon (M−N−C) systems. M−N−C systems have been realized as promising materials for ORR overcoming commercial Pt/C due to their low cost, durability, and high catalytic activity.255,256 However, these materials have moderate OER activity. In a recent report, Hu et al. explained the bifunctionality of nanocarbon-intercalated graphene material doped with nitrogen and iron (Fe−NCIG).257 The catalytically active sites are N, Fe3O4, and 33756


Review


based support improves them toward highest activity. In case of the spinel family, for instance, the ORR activities of binary spinel members are at least 150 mV lower than the C supported one. Such distinct trends are observed within the perovskite, metal oxide, and metal hydroxide family as well. Furthermore, the activity of the pure compound improves significantly upon doping with another suitable metal. It appears that, with a greater number of different metal atoms within the crystal lattice of the catalyst, the better is the performance. Metal hydroxides generally are good for OER, but there are at least few instances where meaningful bifunctional behavior has also been observed. Among other materials, M−N−Cs exhibit the highest E1/5 value of greter than 0.9 V and, in average, marginally better ORR activity when compared with pnictide and metal carbide. Overall, the performances of the metal-free carbon-based materials appear better than the perovskites and metal hydroxides. In the case of OER performance, a trend is recognizable in moving from left to right of the figure, where the overpotential reaches a minimum in the family of TMC. According to our literature survey, the spread of OER activity was found to be the least in case of pure spinels and highest in case of TMCs. On a closer look at the first four families, it is evident that use of a carbon-based support enhances the OER activity, greatly at times, as in the case of ORR. LDH-based materials are known to exhibit excellent water oxidation ability in both metal-air batteries and standalone OER half-cell reactions, but the incorporation of carbon drastically decreases the overpotential median from ∼0.4 to ∼0.3 V. The activity of an efficient bifunctional catalyst is a trade-off between a good OER and ORR activities, so it is impertinent to improve the individual half-cell reactions independently. From Figure 14, it appears that carbon-supported metal hydroxides, M−N, and TMC families are more promising bifunctional catalysts. From the large difference in the applied potential of ORR and OER, it occurs that the same surface site cannot act as active site for both the reactions, and thus both sites ought to be independently engineered, if possible, to attain superior bifunctionality. This notion has been augmented from a recent observation by using in situ operando X-ray absorption spectroscopy (XAS) study, which shows that the ORR and OER reaction conditions stabilize markedly different oxidation states of the active metal.264 In the case of N-doped carbon, the ratio of pyridinic and quaternary N atom was found to be crucial in improving their efficiency.265 Since such investigations are relatively recent and sparse, and the origin of bifunctionality is expectedly more complex than ORR or OER activity alone, more detailed understanding of the nature of the catalyst surface would be necessary to make rational progress.

between metal nanoparticle and nitrogen-doped carbon considered as the prime factor for bifunctional activity.260 Yang et al. extensively studied the catalytically active sites for ORR, OER, and HER on FeCo−N−C species.261 By performing many control experiments such as monometal doping and thiocyanate poisoning of the metal site, it was found that Fe−N−C is the active site for both ORR and OER, while Co−N−C is the active site for HER. However, the origin of the bifunctional behavior of Fe−N−C site is still not wellunderstood. Enhanced OER activity is possibly due to the bimetallic center, while ORR performance can be related to the N-doped carbon as discussed earlier. 5.5. Emerging Insights on Performance. We have so far discussed some of the most interesting ORR/OER bifunctional catalysts that have emerged recently. In Table 3, we depict the most efficient catalysts reported in the last couple of years. However, while sorting the recent publications, one can easily notice that the performances of these catalysts vary widely among the different reports. Even for the closely related systems, their activities seem to differ vastly. This can be potentially confusing in screening an appropriate catalyst and thus evaluating their performances on an overall basis rather than being selective. In Figure 14, we plot the activity for various families of bifunctional catalysts in alkaline electrolyte using box-whisker plot and denote the maximum, minimum, median, and mean values for a range of data obtained from over 150 literature reports in last two-three years (references in Supporting Information). We neglected a few reports, where stability has not been examined. In the figure, we compare the E1/2 and η for ORR and OER, respectively. A higher value of E1/2 (i.e., a higher position for a catalyst in panel (i) in Figure 14) and smaller η (i.e., a lower position in penal (ii)) is a good indicator for a promising bifunctional catalyst. To assist the readers with a concise and systematic approach toward screening an effective material, we classified the catalysts into nine different families: spinel, perovskite, metal oxide, metal hydroxide, TMC, pnictide (M−N), carbide (M−C), M−N−C, and metal-free carbon. The same for the benchmarking catalysts are also marked. While the plot can be easily understood, we point out the key observations in the following. It is apparent from the figure that the same family may have widely varying activity as depicted by the spread in the highest and lowest activity values. It will be more useful to compare them on the basis of the median of the data set. The families can be further subdivided on the basis of the two crucial approaches, adopted to improve them, specifically, (i) doping a catalyst with other metals and (ii) combining them with carbon-based materials as catalyst supports. Investigations on the spinel and the perovskite families, in particular, are extensive as compared to the others. Therefore, the spinels are further divided into pure spinel, mixed spinel, and carbonsupported spinel. Similarly, the perovskite family consists of binary (pure), ternary (either A site or B site doped), and quaternary (both A and B sites doped) systems and their carbon-supported analogues. Investigations on metal oxides and hydroxides are limited and hence classified as with and without C support. Investigations on M−N, M−C, and M−N− C families are even more limited, and all such catalysts are analyzed together. In the case of ORR, one can observe that the activities of the best catalysts are approaching that of commercial Pt/C in alkaline medium, even surpassing them occasionally. Within a family, generally put, the pure compounds exhibit minimal activities, while using a carbon-

6. CONCLUSIONS AND OUTLOOK The key to the success of the water-based renewable cycle energy was deemed to be overcoming the challenge of replacing the expensive benchmarking catalysts with inexpensive earth-abundant materials and improving the kinetics of the sluggish electrocatalytic reactions involving oxygen. Led by this, a large number of electroactive materials have been developed in the recent years. This review summarizes the most significant research findings on oxygen reduction and oxygen evolution reactions discovered recently, including the most efficient catalysts for ORR, OER, and bifunctional activities as well as the key mechanistic insights that may help 33757


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on the catalytic properties has rarely been probed. For example, Bao et al. have shown recently that poly(vinylpyrrolidone) (PVP)-free octahedral Pd@Pt3−4L/C catalyst has specific activity 50% higher than that of the PVPstabilized materials C.109 Besides, even though the smaller particles show high mass activity, there is a possibility of activity loss during prolonged usage due to changes in their shape or coalescence triggered by the presence of a large number of under-coordinated atoms. On the one hand, in regard to the production of catalyst materials, even though shape-controlled nanomaterials have shown promising activities, their utilization in large-scale device fabrication requires scalability of their synthesis procedure, which is rather nontrivial. On the other hand, catalyst-decorated carbon materials are equally facile and can be easily produced in large scale. However, these are mostly investigated in alkaline medium, which is undesirable due to the poor chemical stability of anion exchange membranes. Therefore, the budgetary focus of U.S. DOE (2016) has been shifted to development of noble metal-based ORR active materials usable in acidic electrolyte.7 This underlines the need to search for electrocatalysts that are inexpensive and yet exhibit high performance in acidic medium. Given the fact that the active catalytic sites are often different from the crystallographic sites (and many catalysts such as carbon based ones, etc., are barely crystalline), and identification of the active site involves indirect set of measurements, detailed theoretical investigations and developing models on reaction pathways would be extremely helpful in developing robust ORR-OER catalysts and designing experiments toward discerning reaction mechanisms. A few of the recent theoretical studies have been performed in this regard, though their numbers are rather limited.282−287 For example, Ag is preferable ORR catalyst over benchmark Pt due to their higher abundance. However, despite a lower onset potential, their performance is not at par with Pt due to the nonoptimal adsorption strength of the intermediates. The theoretical studies on Ag surfaces have been aimed at calculating the binding energies of the intermediates. However, these calculations do not take into account the neighboring surface-adsorbed species. Therefore, Liu and co-workers have recently examined the ORR mechanism on Ag(111) using a combined DFT and kinetic Monte Carlo method to mimic real electrodes.282 In the process, they identified an interesting new chemisorbed water (*H2O)-mediated 4 e− associative pathway. They showed that, despite a lower ORR onset, the current change is slower than Pt because, as the electrode potential increases, coverage of the Ag electrode by H2O and OH is significantly different than in Pt leading to lower current. Such studies clearly demonstrate the need to conclusively examine the reaction mechanisms on various electrode surfaces and also call for different approaches to enhance the reaction rates by focusing of adsorption energies of solvent, for example. Similarly, the electrocatalytic activities of bulk-immiscible alloys are poorly explored as compared to the bulk-miscible counterparts owing to the difficulties in their synthesis. Henkelman and co-workers have computationally screened Ag- and Au-based alloys for the ORR and further demonstrated experimentally that alloy nanocrystals of classically immiscible Au and Rh, that is, Au2Rh, indeed exhibit excellent ORR activity.288 Such reports should inspire development of novel synthetic strategies to realize these metastable phases at the nanoscale. Understanding function-

further improvement. It is apparent that a number of factors such as size, shape, composition, porosity, surface structure, crystal defect, surface strain, preparation methods, and posttreatment play crucial roles in determining the activity of a catalyst. This has given rise to vastly different efficiencies even for the closely related systems, of which many are indeed excellent and match benchmarked performances. Despite notable progress, there are several challenges yet to be overcome before viable utilization of water renewable energy cycle. The first would be to ascertain the actual reaction centers and to elucidate the mechanisms occurring on the surface of inexpensive catalysts in greater detail. There are considerable differences in opinion regarding the active catalytic centers in complex materials. As an example, Kim et al. proposed that ORR activity is generated from the compound containing only Fe−Fe3C@C sites,76 whereas Strickland et al. attributed the same activity to synergistic effect existing between Fe−Fe3C@ C sites and the Fe−Nx sites.270 It is particularly intriguing in the case of chalcogenides, nitrides, phosphides, and carbides, where a stronger metal−oxygen bond than the existing metal anion bond in the catalyst lattice is shown to form under the reaction condition that is responsible for oxygen evolution and besides, which must be cleaved to regenerate the catalyst surface. It appears that the catalyst surface undergoes oxidation forming an oxide and hydroxide layer that is the actual active center for the reaction. An interesting editorial by Prof. Jin of ACS Energy Letters correctly pointed out that the accurate identification of the catalytic center is necessary to claim a nonoxide earth-abundant catalyst as an efficient one.271 A multipronged approach using in situ spectroscopic studies, isotope labeling studies, and theoretical investigations will be highly beneficial. Second, even though some of the catalysts are extremely good, their stability under the harsh electrochemical conditions is an important concern. For sustainability, the renewable energy devices are expected to run for years without the need for a change in the catalysts, suggesting that long-term stability of the catalysts must be evaluated with priority. However, envisioning stability of these materials is still difficult due to the absence of any credible long duration stability test (at the moment, such tests are performed for a few hours or few thousand cycles at best as seen from the tables). Careful monitoring for prolonged period may lead to findings of profound interest for electrocatalysis or otherwise. Considering the carbon-based supports, even though these have been widely used in conjunction with most inorganic catalysts, these would eventually be exposed to high electrode potential during startup and shutdown conditions in a device, which could lead to strong corrosion.251 Particularly during fuel starvation, the support would potentially undergo carbon oxidation reaction, (COR, ECO2/C = 0.207 V vs NHE at T = 298 K), aggravated at defect sites.250,272 Therefore, due attention must be paid and alternatives must be developed.273−278 In addition, a considerable amount of catalyst active surface area gets masked while using a catalyst support, and self-supported catalysts could be beneficial for higher surface area.279,280 In this context, use of transition-metal foams where the surface is chemically converted to the corresponding catalyst (while the core provides higher conductivity) is noteworthy and ought to be investigated in greater detail.281 To control size and shape, surfactant and capping agents have been widely used. However, the effect of their presence 33758


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(7) Mai, T.; Sandor, D.; Wiser, R.; Schneider, T. Renewable Electricity Futures Study: Executive Summary. NREL/TP-6A20−52409-ES; National Renewable Energy Laboratory: Golden, CO, 2012; pp 2−3. (8) Das, V.; Padmanaban, S.; Venkitusamy, K.; Selvamuthukumaran, R.; Blaabjerg, F.; Siano, P. Recent Advances and Challenges of Fuel Cell Based Power System Architectures and Control − A Review. Renewable Sustainable Energy Rev. 2017, 73, 10−18. (9) Wang, Y.; Leung, D. Y. C.; Xuan, J.; Wang, H. A Review on Unitized Regenerative Fuel Cell Technologies, Part-A: Unitized Regenerative Proton Exchange Membrane Fuel Cells. Renewable Sustainable Energy Rev. 2016, 65, 961−977. (10) Truong, T. T.; Liu, Y.; Ren, Y.; Trahey, L.; Sun, Y. Morphological and Crystalline Evolution of Nanostructured MnO2 and Its Application in Lithium-Air Batteries. ACS Nano 2012, 6, 8067−8077. (11) Sumboja, A.; Ge, X.; Zong, Y.; Liu, Z. Progress in Development of Flexible Metal−Air Batteries. Funct. Mater. Lett. 2016, 9, 1630001. (12) Li, D.; Lv, H.; Kang, Y.; Markovic, N. M.; Stamenkovic, V. R. Progress in the Development of Oxygen Reduction Reaction Catalysts for Low-Temperature Fuel Cells. Annu. Rev. Chem. Biomol. Eng. 2016, 7, 509−532. (13) Oldacre, A. N.; Friedman, A. E.; Cook, T. R. A Self-Assembled Cofacial Cobalt Porphyrin Prism for Oxygen Reduction Catalysis. J. Am. Chem. Soc. 2017, 139, 1424−1427. (14) Costentin, C.; Robert, M.; Savéant, J.-M. Molecular Catal. Electrochem. Reactions. Curr. Opin. Electrochem. 2017, 2, 26−31. (15) Zoladek, S.; Rutkowska, I. A.; Blicharska, M.; Skorupska, K.; Kulesza, P. J. Enhancement of Oxygen Reduction at Co-Porphyrin Catalyst by Supporting onto Hybrid Multi-Layered Film of Polypyrrole and Polyoxometalate-Modified Gold Nanoparticles. J. Solid State Electrochem. 2016, 20, 1199−1208. (16) Nunes, M.; Fernandes, D. M.; Rocha, I. M.; Pereira, M. F. R.; Mbomekalle, I.-M.; De Oliveira, P.; Freire, C. Phosphomolybdate@ Carbon-Based Nanocomposites as Electrocatalysts for Oxygen Reduction Reaction. ChemistrySelect 2016, 1, 6257−6266. (17) Dong, R.; Zheng, Z.; Tranca, D. C.; Zhang, J.; Chandrasekhar, N.; Liu, S.; Zhuang, X.; Seifert, G.; Feng, X. Immobilizing Molecular Metal Dithiolene−Diamine Complexes on 2D Metal−Organic Frameworks for Electrocatalytic H2Production. Chem. - Eur. J. 2017, 23, 2255−2260. (18) Maganas, D.; Trunschke, A.; Schlögl, R.; Neese, F. A Unified View on Heterogeneous and Homogeneous Catalysts through a Combination of Spectroscopy and Quantum Chemistry. Faraday Discuss. 2016, 188, 181−197. (19) Jung, S.; McCrory, C. C. L.; Ferrer, I. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking Nanoparticulate Metal Oxide Electrocatalysts for the Alkaline Water Oxidation Reaction. J. Mater. Chem. A 2016, 4, 3068−3076. (20) Mamtani, K.; Singh, D.; Tian, J.; Millet, J. M. M.; Miller, J. T.; Co, A. C.; Ozkan, U. S. Evolution of N-Coordinated Iron−Carbon (FeNC) Catalysts and Their Oxygen Reduction (ORR) Performance in Acidic Media at Various Stages of Catalyst Synthesis: An Attempt at Benchmarking. Catal. Lett. 2016, 146, 1749−1770. (21) Gao, X.; Zhang, H.; Li, Q.; Yu, X.; Hong, Z.; Zhang, X.; Liang, C.; Lin, Z. Hierarchical NiCo2O4 hollow Microcuboids as Bifunctional Electrocatalysts for Overall Water-Splitting. Angew. Chem., Int. Ed. 2016, 55, 6290−6294. (22) Song, C.; Zhang, J. Electrocatalytic Oxygen Reduction Reaction. In PEM Fuel Cell Electrocatalysts and Catalyst Layers: Fundamentals and Applications; Zhang, J., Ed.; Springer: London, U.K., 2008; pp 89−134. (23) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jónsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 17886−17892. (24) Mukerjee, S. Role of Structural and Electronic Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction. J. Electrochem. Soc. 1995, 142, 1409.

ality of some of the most active catalysts such as poorly crystalline metal hydroxides and recently developed catalysts containing transition-metal atoms in N-doped carbon network is more challenging. In regard to the hydroxides, Goddard III and co-workers’ investigation using grand canonical quantum mechanics including kinetics approach underlines the need for uniform distribution of Fe and Ni in (Fe,Ni)OOH, since Fe can dramatically reduce the reaction overpotential, while neighboring Ni can efficiently catalyze O−O coupling.289 It is encouraging to observe an increase in detailed theoretical studies dedicated to screening of novel catalysts and elucidating reaction mechanisms that would be certain to inspire development of more improved catalysts. Finally, to access these catalysts in a practical sense, one must develop prototype devices and test the performance for long hours. With this, and considering the rapid progress that has been made in the past few years, a clear picture on the viability of water-cycle-based renewable devices should appear in the next few years.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b09024.

■

References used to prepare Figure 14 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Moumita Rana: 0000-0002-9348-4491 Ujjal K. Gautam: 0000-0002-0731-0429 Author Contributions

All authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.M. and K.C. thank IISER-Mohali for fellowship. L.S. thanks UGC for senior research fellowship. P.E.K. thanks SERB-India for DST-NPD fellowship.

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REFERENCES

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