Evolution on Nonstoichiometric, Mixed Metal Oxides

Perspective Cite This: Chem. Mater. 2018, 30, 2860−2872

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Oxygen Sponges for Electrocatalysis: Oxygen Reduction/Evolution on Nonstoichiometric, Mixed Metal Oxides† Xiang-Kui Gu,‡ Samji Samira,‡ and Eranda Nikolla*

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Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, Michigan 48202, United States ABSTRACT: Electrocatalysis of oxygen reduction and evolution (ORR and OER) have become of significant importance due to their critical role in the performance of electrochemical energy conversion and storage devices, such as fuel cells, electrolyzers, and metal air batteries. While efficient ORR and OER have been reported using noble-metal based catalysts, their commercialization is cost prohibitive. In this Perspective, we discuss the potential of nonprecious metal based, mixed electronic−ionic conducting oxides (i.e., perovskites, double perovskites, and Ruddlesden−Popper (R-P) oxides) for efficient oxygen electrocatalysis at high and low temperatures. The nonstoichiometry of oxygen in these materials provides key catalytic properties that facilitate efficient ORR/OER electrocatalysis. We discuss the importance of surface structure and composition as critical parameters to understand and tune the ORR/OER activity of these oxides. We argue that techniques facilitating controlled synthesis and characterization of the surface structures are key at achieving a correlation between structure and activity of these materials. We make the case for combinatorial approaches involving quantum chemical calculations combined with detailed characterization, controlled synthesis, and testing as effective ways for developing the fundamental knowledge at the molecular level required to guide the design of efficient nonstoichiometric, mixed metal oxides for oxygen electrocatalysis. We conclude by summarizing current advances and devising future directions in this area.

1. INTRODUCTION High CO2 emissions and rapid depletion of fossil fuel resources have become contemporary challenges.1−3 Efficient alternative energy conversion and storage systems, such as fuel cells, electrolyzers, and metal−air batteries are at the forefront of sustainable energy conversion and storage to overcome some of these challenges. Oxygen electrocatalysis plays a key role in the performance of these systems.4,5 It mainly revolves around the catalysis of oxygen evolution during water splitting in the presence of electrons storing energy in chemical form in electrolyzers and subsequent recombination of O2 with H2 to form water in fuel cells, generating electrical energy. The sluggish, complex chemistry of oxygen in these systems leads to large overpotential losses that significantly affect the overall efficiency.6−12 Therefore, design of cost-effective catalytic materials that improve oxygen electro-kinetics could significantly contribute toward overcoming some of the current energy challenges. Nonstoichiometric mixed metal oxides with mixed ionic and electronic conducting properties, such as perovskites, double perovskites, and Ruddlesden−Popper (R-P) oxides have attracted significant attention as oxygen electrodes for catalyzing oxygen reduction and evolution reactions (ORR and OER) at high and low temperatures.9−20 Their potential has especially been explored for elevated temperature oxygen electrocatalysis in solid oxide fuel cells (SOFCs) and

electrolysis cells (SOECs) due to their enhanced ionic and electronic conducting properties at these conditions. ORR and OER at elevated temperatures are generally governed by the process of surface oxygen exchange.9,11,12,21−23 On these oxides, surface oxygen exchange activity highly depends on the surface structure (i.e., surface termination and orientation) and the chemical compositions of the A- and B-sites. It is generally suggested that the B-site transition metals are catalytically more active for this process as compared to the A-site rare earth and alkaline earth metals.9,11,24−27 Thus, control of the oxide structure with preferentially exposed B-site metal terminated surface has led to significant enhancement in surface oxygen exchange and, subsequently, ORR/OER kinetics.9−11 Proper ORR/OER activity optimization of these oxides through tuning their composition has proven fairly challenging due to the flexibility of the A- and B-site compositions and their various possible concentrations in these materials. Some effort toward this has been achieved using combined experimental and theoretical studies to develop descriptors that can guide the design of oxides with optimal activity. However, universality of these descriptors has been hindered by limited validation of these descriptors experimentally across the various oxides.11,18,28−30 Received: February 14, 2018 Revised: April 13, 2018 Published: April 13, 2018

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This Perspective is part of the Up-and-Coming series. © 2018 American Chemical Society

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DOI: 10.1021/acs.chemmater.8b00694 Chem. Mater. 2018, 30, 2860−2872

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Chemistry of Materials

Figure 1. Crystal structures of mixed electronic and ionic conducting oxides. (a) Simple cubic perovskite structure. (b) Double perovskites with twice the unit cell of a simple perovskite. (c) R-P oxides with alternating rocksalt and perovskite layers. The green, cyan, purple, and red spheres represent the A-site metal, A′ site metal, B-site metal, and O atoms, respectively.

electrocatalysis at low and high temperatures and devise future directions in this area.

Low temperature ORR/OER electrocatalysis using nonstoichiometric mixed metal oxides has been mainly tested in alkaline environments due to the limited stability of these oxides in acidic media.31 Comparable ORR/OER performances of these oxides to those of cost-prohibitive, precious metal based catalysts (i.e., Pt, IrO2) have been reported.17,18,32−36 The ability to engineer sites with high activities for both OER and ORR as in the case of LaCoO3 and LaNiO3 provides an advantage over the State-of-the-art precious metal-based catalysts (i.e., Pt, Ir, Ru), which mainly exhibit specific ORR or OER activity.37 This could potentially aid in designing electrochemical systems that are regenerative or rechargeable in nature and can operate reversibly as fuel cells and electrolyzers. While the reported mechanisms for low-temperature ORR/ OER on these oxides are somewhat different from high temperature electrocatalysis, they are similarly governed by the interactions of the surface defects and the A/B-site metal atoms with oxygen-containing intermediates. Similar to high temperature electrocatalysis, B-site transition metals in these oxides are reported to be the most active toward ORR/OER via favorable interactions with oxygen-containing intermediates through B− O bond formation.17,35 Ni, Mn, and Co-based perovskites are generally found to be promising candidates for ORR and OER, but the underlying design principles that govern the overall activity of these oxides are still under debate.17,18,30,33,38,39 As in the case of high temperature oxygen electrocatalysis, systematic development of structure−performance relations has been hindered by the large window of possibilities of A- and B-site compositions and their distribution on the surface, making screening approaches very challenging.40 In this Perspective, we discuss the details of oxygen electrocatalysis using perovskites, double perovskites, and R− P oxides at high and low temperatures. The factors that govern their crystal structures and electronic/ionic conductivities are detailed and linked to their electrochemical ORR/OER performances. A rationale for the importance of controlling and characterizing their surface and nanostructure as key to tuning oxygen electrocatalysis and developing structure− performance relations that can guide their performance optimization is provided. The role of theoretical studies in advancing knowledge with respect to the fundamentals of the oxygen surface chemistry on these complex structures is also discussed in detail. We conclude by summarizing key features/ properties of these oxides that play a role in oxygen

2. PHYSICAL PROPERTIES 2.1. Crystal Structure. Perovskites, double perovskites, and R−P oxides exhibit general formulas of ABO3−δ, AA′B2O5+δ, and An+1BnO3n+1 (n = 1, 2, 3, ...), respectively. A and A′ represent rare earth and alkaline earth metals, while B represents the transition metal. Ideally, in all these oxides the smaller size B-site cations are coordinated by 6 oxygen anions to form octahedral BO6. The combination of 8 BO6 octahedra leads to the formation of a hole, which is occupied by the A-site cation coordinated by 12 oxygen anions. The crystal structures of these oxides are determined by the distribution of the A-/A′site metals as shown in Figure 1. Perovskites exhibit a cubic crystal structure where the rare earth and alkaline earth metals are randomly distributed in the A-site layer.13 Double perovskites are generally formed when the difference in size between the two A site cations is large, leading to the formation of a unit cell that is twice that of a simple cubic perovskite structure.23 Alternating distributions of rare earth and alkaline earth metals with the structure of AO−BO2−A′O−BO2 are found in double perovskites.13 In the case of R-P oxides, they are composed of distinct, alternating perovskite ((ABO3)n) and rock salt layers (AO).41 The crystal structures of these oxides are highly dependent on the chemical compositions of the A- and B-sites and synthesis/treatment conditions.13,16 These factors can induce phase transitions in these oxides, consequently effecting the surface structure, which is important for oxygen electrocatalysis.42−44 Phase transitions are created due to distortions of BO6 octahedra induced by the diverse size of the A and B site atoms in the structure and oxygen nonstoichiometry.45 In perovskites, distortion of the BO6 octahedral induced by B-site replacement can lead to phase transition from the ideal (undistorted) cubic Pm3̅m structure to a series of subgroup structures, such as P4/mbm and I4/mmm.46 Similarly, phase transitions in double perovskites as a function of composition, synthesis method, and temperature have been reported.42,43,47,48 Phase transition of double perovskites at elevated temperatures has been linked to thermally induced oxygen vacancy disordering and spin-state transitions of the Bsite cations.48 In the case of R-P oxides, phase transitions between Fmmm orthorhombic and I4/mmm tetragonal structure are reported as a function of temperature for the 2861


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Chemistry of Materials first series, favoring a Fmmm orthorhombic structure at room temperature, which transforms to I4/mmm tetragonal structure at temperatures higher than 150 °C.49 This phase transition has also been observed as a function of chemical composition in high-order R-P oxides.50 Oxygen nonstoichiometry in perovskites and double perovskites is dominated by oxygen deficiencies (oxygen vacancies) to facilitate charge neutrality due to the difference in the oxidation states of the mixed A- and B-site atoms in their structure. Oxygen vacancies are randomly distributed in perovskites, while they are localized in the rare earth metal oxide layers in double perovskites, since the smaller rare earth cations prefer to coordinate with fewer oxygens as compared to the larger alkaline earth cations.16 The oxygen vacancy content in double perovskites at room temperature is found to be linearly correlated to the size difference between the A-site cations.16,44 An increase in the amount of the oxygen vacancies is observed as a function of an increase in the size difference. When oxygen vacancy content approaches ∼0.5, oxygen vacancies tend to be ordered, lowering the crystal symmetry from tetragonal to orthorhombic through phase transition. Different from perovskites and double perovskites, generally RP oxides exhibit oxygen hyper-stoichiometry with excess oxygen localized in the interstitial sites of the AO layer.12 The concentration of these interstitial oxygen affects their crystal structures. For instance, a phase transition from Fmmm orthorhombic to I4/mmm tetragonal is observed with a decrease in the interstitial oxygen content.49 In summary, chemical composition and synthesis/treatment methods largely affect oxygen nonstoichiometry and induce distortions of BO6 octahedra in mixed metal oxides leading to structural transitions, which consequently affect the surface structure. An impact of the structural transitions on the electronic and ionic conductivities via altering the charge carrier concentrations and/or the geometries of the O−B−O bonds is also observed as we discuss in detail below. 2.2. Electronic and Ionic Conductivities. High electronic and ionic conductivities of nonstoichiometric oxides are key at minimizing ohmic losses in electrochemical systems. Electronic conduction in these oxides is shown to take place through the B-site network, where the electron hopping occurs from B(n−1)+ to Bn+ cations through the oxygen bridge.51−53 Changes in the oxidation state of the B-site metal are facilitated by the oxygen defect chemistry in these oxides. To enable electron hoping from B(n−1)+ to Bn+ cations, efficient charge transfer via the O− B−O bond is shown to be important54 and is highly affected by the geometry of this bond. Changes in the chemical composition of the oxide can affect this geometry. For example, it is generally reported that substituting La with Sr in the A-site enhances electronic conductivity due to Sr increasing electron− hole concentration and straightening of the O−B−O bond, leading to improved orbital overlapping between B-3d and O2p.55 In contrast, a decrease of this orbital overlap is reported for O−B−O bending, resulting in a decrease in electronic conductivity.56,57 For example, a decrease of the electronic conductivity in LnBaCo2O5+δ (Ln = La, Nd, Sm, Gd, and Y) is observed when the size of Ln3+ decreases from La to Y leading to an increase in oxygen vacancy content.57 The increase in oxygen vacancy content results in O−B−O bond bending, supported by the lowering of the crystal symmetry from tetragonal to orthorhombic. The nature of the B-site metal also affects electronic conductivity by inducing changes in the charge carrier content and/or O−B−O bond geometry. For

example, it is reported that the doping of Co-based oxides with Cr, Mn, Fe, Ni, and Cu decreases the electronic conductivity due to a decrease in the charge carrier content and/or increase in the bending of the O−B−O bonds depending on the nature of the dopant.55 Furthermore, since charge transfer occurs through the O−B−O bond, increasing the number of these bonds has been shown to enhance electronic conductivity in higher order series of layered R-P oxide structures (i.e., Lan+1NinO3n+1).58 Oxygen bulk diffusion in perovskites and double perovskites proceeds via an oxygen vacancy transport mechanism. Thus, the criteria discussed above for enhancing oxygen vacancy concentration (i.e., compositional changes) also improves ionic conductivity.59 Doping the A-site in these oxides with different valence state atoms (i.e., doping La-based perovskites with Sr) results in enhancement of ionic conductivity via an increase in oxygen defect concentration due to oxygen loss required to compensate for the difference in the valence state of A site atoms (Sr2+ vs La3+).55 For example, it is reported that the oxygen diffusion coefficient in La 1−xSrxCoO3−δ linearly increases with an increase in Sr doping, due to an increase in oxygen vacancy concentration.60 Differently from perovskites and double perovskites, oxygen diffusion in R-P oxides occurs through an interstitialcy mechanism involving interstitial oxygen sites.61 Therefore, in these oxides the concentration of interstitial sites is important for efficient oxygen ion transport. Similar to oxygen deficiencies, the interstitial oxygen concentration can also be affected by chemical composition and sample treatment methods.12 An anisotropic effect for oxygen diffusion in layered double perovskites and R-P oxides has been widely reported, and it is found that this diffusion is much more facile in the (a, b) plane as compared to that in that in the c-axis direction.42,62,63

3. SYNTHESIS AND CHARACTERIZATION OF THE SURFACE STRUCTURE Surface structure of nonstoichiometric mixed metal oxides is critical for oxygen electrocatalysis at high and low temperatures.9,10,12,64,65 Therefore, controlled synthesis and systematic characterization of the surface structure of these complex oxides is key at engineering their catalytic activity toward ORR/OER. Synthesis of these materials has been mainly dominated by solid state reaction, sol−gel, nitrate combustion, and coprecipitation methods, which provide limited control over their surface and nanostructure. Solid-state reaction methods involve annealing of metal oxides in appropriate proportions at very high temperatures (∼1000 °C) leading to bulky oxide particles with low surface area.42,66−69 Sol−gel based methods involve complexing metal nitrates with different agents such as ethylenediaminetetraacetic acid (EDTA)-citrates (Pechini method) in water leading to the formation of a gel.32,58,70−72 The gel is solidified by heating to form a foam/ash-like precursor, which is calcined at high temperatures to form the desired oxides. Coprecipitation approaches, which involve precipitation of metal salt solutions using titrating reagents (i.e., NaOH, tetramethylammonium hydroxide, NH3) to maintain homogeneity of the precursors prior to calcination, have been reported to enhance the surface area of the oxide particles.18,73,74 Control over the surface structure has been achieved using thin-film methods (such as pulsed layer deposition (PLD)), which involve directed growth of the oxide along a crystal plane on a single crystal substrate.65,75−79 While thin films are 2862


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(i.e., scanning electron microscopy (SEM) and transmission electron microscopy (TEM)) to determine morphology and particle size (Figure 2a).66 Surface area is generally measured using N2 physisorption.80 A significant challenge is the characterization of the surface structure, which is key to developing structure-performance relations for oxygen electrocatalysis on these oxides. High-angle angular dark field imaging using scanning transmission electron microscopes (HAADFSTEM) and selected area electron diffraction (SAED) patterns have been successfully implemented to determine the surface orientation of these materials (Figure 2b). In the case of the surface composition, X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) have been conventionally used.83,84 However, they accumulate data from multiple atomic layers, limiting the ability to obtain detail regarding the very top surface layer. Alternatively, surface sensitive techniques, such as low energy ion scattering (LEIS) spectroscopy, angle-resolved XPS, and atomic resolution scanning transmission electron microscopy (STEM) along with electron energy loss spectroscopy (EELS) have led to insightful compositional information on the outmost surface of these oxides.9,24,76,80,83,85,86 For instance, the feasibility of LEIS has been demonstrated in determining the local surface chemical environment of R-P oxides to understand the near surface and subsurface enrichment of AO and BO, respectively.24,76,83,86 Atomic resolution electron microscopy and spectroscopy have been usefully used by our group to characterize the very top surface layer of R-P oxide (La2NiO4+δ) nanostructures as illustrated in Figure 2.9,80 While insightful surface structure information on these oxides is obtained using high resolution HAADF-STEM and EELS, the instability of the oxide surface under a high energy electron beam is a challenge. Thus, a compromise between the energy of the beam and sensitivity is necessary to avoid artifacts due to restructuring of the surface using these techniques.9 Limitations with the ex-situ experimental surface characterization techniques have motivated the use of quantum chemical calculations to gain insights into the surface structure of these oxides. Calculations of the surface energies of possible surface facets have been reported to understand the thermodynamic preferential surface structures.87−89 For instance, theoretical studies have shown that (111), (001), and (100) surface facets of La2NiO4 are thermodynamically the most favorable and consequently the most dominant surface structures.88 This has been confirmed experimentally as shown in Figure 2. Theoretical surface energy calculations have also shed light on the stability of the AO/BO2 surface terminations in perovskites.89 Modeling the energetics of the surface structure of these materials under reaction conditions has also been attempted, due to lack of operando surface sensitive characterization techniques. For example, quantum chemical calculations have been used to determine the thermodynamically stable surface structures at relevant ORR/OER applied potentials and pH.90,91 While insightful, theoretical methods are limited by the use of static model systems which hinder systematic analysis of surface structure changes induced by electrochemical reaction conditions, solvent effects, and binding of adsorbates.24,86,90,91 Furthermore, the commonly used density functionals provide limited accuracy on the energetics of the transition metal oxides, due to challenges with treating their strongly correlated properties.92 Although this can be improved using hybrid density functionals and dynamical mean field theory,93,94 these methods are computationally demanding and expensive. Thus,

effective model systems for controlled studies of oxygen electro-kinetics,15,64,65 they are limited by their low surface areas making their implementation in electrochemical devices challenging. From this aspect, synthesis methods that lead to nanostructured oxides with high surface area and controlled surface structure present promise in obtaining fundamental and applied knowledge on oxygen electro-kinetics in electrochemical devices. Recently, our group has reported on a solution-based reverse microemulsion method for synthesizing nanostructured, nonstoichiometric mixed metal oxides (through the example of R-P oxides) with controlled surface structure.9,80,81 A typical synthesis procedure involves a reaction of two different microemulsions, with one containing the metal cations and the other containing the precipitating reagent anions. Controlled surface structure during synthesis is achieved through tuning parameters, such as the surfactant/ cosurfactant concentration and mass ratio of surfactant to water, to facilitate control of the reverse micelles and, hence, the final product.12,80−82 For example, rod-like nanostructures of R-P oxides highly terminated by B-site metal oxide surfaces were achieved using a controlled surfactant to water mass ratio of 1.6 (Figure 2a−c). Increasing this ratio led to changes in the

Figure 2. Characterization of La2NiO4+δ nanostructures. (a) Bright field-transmission electron microscopy (BF-TEM) image of an individual La2NiO4+δ nanostructure with rod-like morphology. (b) Atomic resolution HAADF-STEM image of an La2NiO4+δ nanorod showing a (001) NiO termination, with the SAED pattern inset. DFT modeled surface is overlapped to understand the surface termination. (c) EELS spectrum of La2NiO4+δ nanorod with partly overlapped Ni (L2,3) and La (M4,5) edges. (d) TEM image of a polyhedral shaped La2NiO4+δ showing mixed surface termination. The data in (a) and (d), and (b) and (c) are reproduced with permissions from refs 80 and 9, respectively. Copyright 2014 Royal Society of Chemistry and 2015 American Chemical Society, respectively.

oxide nanostructure morphology to polyhedral, terminated by a mixture of irregularly distributed surface facets (Figure 2d).9,80 This is a first demonstration of using a wet-chemistry technique to synthesize nanostructures of nonstoichiometric mixed metal oxides with controlled surface termination that can be directly implemented in electrochemical devices to enhance ORR/OER activity. Commonly used techniques for characterizing nonstoichiometric mixed metal oxides include X-ray diffraction (XRD) to confirm the bulk crystal structures and electron microscopy 2863


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Figure 3. Activity descriptors for surface oxygen exchange on nonstoichiometric mixed metal oxides. (a) Surface oxygen exchange activity as a function of ionic conductivity on perovskites. (b) Surface oxygen exchange activity as a function of bulk oxygen 2-p band center on perovskites and double perovskites. BSCF: Ba 0.5 Sr 0.5 Co 0.75 Fe 0.25 O 3 ; SSC: Sm 0.5 Sr 0.5 CoO 3 ; PBCO: PrBaCo 2 O 6 ; GBCO: GdBaCo 2 O 6 ; LSCF: Ba0.625Sr0.375Co0.25Fe0.75O3; LSC: La0.75Sr0.25CoO3; LCO: LaCoO3; LSM: La0.75Sr0.25MnO3; LMO: LaMnO3. (c) Surface oxygen exchange rate as a function of O2 binding energy on a surface oxygen vacancy on R-P oxides. “A” represents A-site terminated surface. (d) Surface oxygen exchange activity as a function of the enthalpy of the oxygen vacancy formation (Vf) in binary oxide (MOx) or the B-site dopant ion (i.e., Hf4+, Zr4+, Ti4+, and Al3+) in doped La0.8Sr0.2CoO3 (LSC). (a), (b), (c), and (d) are reproduced with permission from refs 28, 29, 11, and 107, respectively. Copyright 2012 Materials Research Society, 2011 Royal Society of Chemistry, 2017 American Chemical Society, and 2016 Springer Nature, respectively.

very challenging, thus motivating the development of activity descriptors that can guide structure−activity optimization. To find the empirical descriptors for surface oxygen exchange on perovskites, correlations between surface oxygen exchange coefficients and bulk properties (i.e., electronic and ionic conductivities) have been investigated.28,97,98 The focus has been largely on bulk properties due to challenges with properly characterizing the surface structure, as discussed above. It is found that no direct relationship between the activity of the surface oxygen exchange and the electronic conductivity exists at elevated temperatures.28 This suggests that electronic conductivity of these oxides is not a limiting factor in this process at these temperatures. On the other hand, a correlation between the activity and the ionic conductivity is reported (Figure 3a, for perovskites), where an increase in ionic conductivity leads to an increase in activity.28 This is reasonable since both the oxygen exchange activity and the ionic conductivity are effected by the oxygen vacancy concentration. Using the electronic structure of bulk perovskites, the Fermi level of the oxide and the bulk O 2p-band center have been proposed as activity descriptors.29,99 As an example, for surface oxygen exchange on SrTi1−xFe xO 3−δ with different Fe concentrations, a correlation between the activity and the Fermi level with respect to the bottom edge of the conduction band is reported.99 From the correlation between the activity and the theoretically calculated bulk O 2p-band center of the perovskites and double perovskites (Figure 3b), it is found that the activity decreases gradually with a deeper O 2p-band center (larger difference between d-band center and Fermi level).29

development of more accurate and computationally effective theoretical methods is important to address these challenges. In summary, synthesis methods that allow for control over the surface structure of nonstoichiometric mixed metal oxides are important at developing structure−performance relations for oxygen electrocatalysis. A combination of experimental surface-sensitive characterization techniques along with theoretical calculations are necessary to obtain insights regarding the surface structure of these oxides. The prevalent challenge is the characterization of the surface structures of these catalysts under electrochemical conditions. Development of operando characterization techniques and advanced theoretical methods would move the field forward by facilitating better understanding of the oxide surface under reaction conditions, eventually aiding the development of robust structure− performance relations.

4. HIGH TEMPERATURE ORR AND OER High temperature ORR and OER are generally governed by the surface oxygen exchange process,21−23 which proceeds through gas-phase O2 adsorption, dissociation, and incorporation into the oxide lattice, followed by the association of lattice oxygens through oxygen evolution in the gas phase to close the catalytic cycle. The rate of this process as described by the surface oxygen coefficient (k) has been widely studied on perovskites, double perovskites, and R-P oxides with different chemical compositions to search for optimal electrocatalysts.12,28,60,95,96 The flexibility of the A- and B-site compositions and their various possible concentrations in these oxides make this search 2864


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In summary, the developed activity descriptors for surface oxygen exchange (which governs ORR/OER at elevated temperatures) on nonstoichiometric mixed metal oxides are mainly correlated to the oxygen vacancies. The use of bulk properties to describe catalytic behavior is often reported due to challenges with proper surface characterization of these oxides. While the indirect bulk properties can provide insights on the activity of some of these oxides, determining the link between surface structure properties and catalytic activity will be key for developing universal activity descriptors for oxygen electrocatalysis, due to the surface sensitive nature of ORR/ OER.

This is consistent with the deeper O 2p-band center resulting in a higher formation energy for oxygen vacancies, lowering the driving force for their formation. While in general insightful, these descriptors provide limited insights on the link between surface structure to catalytic activity of these materials. An activity descriptor for oxygen exchange that is directly linked to the surface structure of R-P oxides has been reported.9,11 Combined quantum chemical calculations and experimental studies have shown that the rates of surface oxygen exchange on R-P oxides are governed by a compromise between the energetics associated with O2 dissociation and oxygen vacancy formation. The binding energy of O2 on a surface oxygen vacancy is reported as an effective descriptor in predicting the effect of composition of the oxide on the energetics of these two steps, which consequently impacts ORR/OER activity (Figure 3c).11 The binding of O2 on a surface oxygen vacancy directly correlates to the oxygen surface chemistry, where the surface oxygen vacancy is suggested to be the active site for O2 activation.9,11,100 Oxygen on a surface vacancy binds to both the A- and B-site metals effectively capturing contributions from both the A- and the B-site compositions of R-P oxides on their activities. This descriptor predicts that B-site terminated surfaces are catalytically more active than A-site terminated surfaces, which is consistent with experimental observations.25,101−103 It is shown that substituting the B-site Ni in La2NiO4 with a metal with lower oxygen affinity, such as Cu, results in unfavorable energetics for O2 dissociation, lowering the catalytic activity. On the other hand, substituting Ni with a higher oxygen affinity metal, such as Fe, leads to unfavorable energetics for surface oxygen vacancy formation, consequently hindering catalytic activity. The optimal compromise between O2 dissociation and surface oxygen vacancy formation has been predicted for Co-doped La2NiO4, leading to the highest activity, consistent with experimental results.104 Stability is another key parameter that can affect the performance of these materials. Activity suppression has been widely reported due to A-site metal segregation on these oxides.26,27,83,85,86,105 Significant efforts have focused on minimizing this, for instance, through controlled synthesis methods.9,80 It has also been shown that reducing the concentration of surface oxygen vacancies can decrease the driving force for segregation, namely, the electrostatic attraction between the positively charged oxygen vacancy enriched at the surface and the negatively charged A-site cations.106 This has been achieved by incorporating less reducible cations into the B-site of La0.8Sr0.2CoO3 (i.e., Hf4+, Zr4+, Ti4+, and Al3+), successfully suppressing Sr segregation.107 A “volcano”-type relation is reported for the structural stability as a function of the enthalpy of the oxygen vacancy formation on the binary oxide of the cation dopant (Figure 3d), which can also be used to predict the activity of the surface oxygen exchange on these B-site doped perovskites. Figure 3d clearly shows that a modest reducibility of the doped cation is required to obtain the highest activity and stability. This is because low reducibility would significantly reduce the surface oxygen vacancy concentration, hindering oxygen dissociation and evolution. On the other hand, high reducibility would lead to a high surface oxygen vacancy concentration, which promotes A-site surface segregation and leads to structural instability. Thus, a balance between the oxygen vacancy concentration and structural stability is critical for the long-term stable performance of these materials.

5. LOW TEMPERATURE ORR AND OER Extensive studies of ORR and OER on nonstoichiometric mixed metal oxides in alkaline media at low temperatures using hydrodynamic methods via rotating disc electrode (RDE) studies have been reported.17−19,37,74,108−110 However, limited universal methods to test the activities of these oxides consistently exist, leading to challenges in proper comparison between performances reported from different literature sources even on the same oxide catalysts. Some effort toward this has been achieved by extending the thin-film (∼1 μm) based approach used for Pt/C111,112 electrodes, to making thinfilms of mixed oxide catalyst and carbon on a conductive substrate to minimize reactant mass transport resistances and improve electronic conductivity.18,74,110 While this approach is widely used, challenges still remain in preparing consistent, uniform thin films. This is due to the complexity of the film, which in addition to the oxide, also contains a conductive support (i.e., carbon) and a binder to keep the film components intact.108,113−115 The complexity of the thin film also presents a challenge for appropriate normalization of the kinetic rates measured using RDE per active surface area, due to the fact that the geometric surface area of the film is not necessarily entirely electrochemically active for ORR/OER.35 Therefore, different approaches have been adopted for normalization of the rates in addition to the geometric surface area of the electrode,34,105,109 including (i) the surface area of the oxide catalyst measured either using gas adsorption measurements or particle size measurements from different microscopy techniques,17,18,33,116 (ii) electrochemical active surface area (ECSA) measured using double layer capacitance measurements, namely, the charging current measured using cyclic voltammetry at different scan rates in the non-Faradaic potential region,35,117,118 (iii) the number of active surface sites as determined by a combination of refined lattice parameters, Brunauer−Emmett−Teller (BET) surface area, and the roughness factor to obtain turnover frequency (TOF),40 and (iv) the mass of the active material (oxide catalyst) to gain an understanding into the mass activity, critical for practical applications.113,118 This long list of normalization approaches presents limitations by creating inconsistencies in defining the most appropriate method to use in normalizing the activity of these oxides. Normalization per ECSA and number of active sites has been reported to be the most appropriate for obtaining the intrinsic activity of electrodeposited oxide thin-film electrodes of the order of a few nanometers.35,119,120 This is facilitated by the ease of electron conduction through short distances120 in electrodeposited thin films and their roughness factor being close to unity. For bulky composite films consisting of the oxide catalyst and carbon, the surface area of the oxides provides a good first-order approximation of the specific activity given the inconsistencies 2865


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Chemistry of Materials that arise from the nonuniformity of the film per geometric area.17,18,20,74,116 This approach can be even more effective if oxide nanostructures with controlled surface composition are used such that one can relate geometric surface area to the concentration of active sites for ORR/OER.80 Although there are challenges associated with understanding the intrinsic activity of these oxides, consistencies in approaches used for activity analysis have led to some mechanistic insights and structure-performance trends for ORR/OER. It is suggested that ORR in alkaline media generally proceeds either through a direct 4e− transfer mechanism or consecutive 2e− transfer steps, where molecular oxygen is reduced to 4 OH− ions. Scheme 1 shows the reaction pathway in which molecular

important to note that the change in oxidation state has been mainly hypothesized based on kinetic modeling and molecular orbital theory with limited experimental evidence.18,37,122 Experimental kinetic insights have been hindered by many factors, including the artifacts in the polarization curves due to potential-dependent redox couples of the transition metals in the oxides.123 For example, in the case of LaMnO3, Ar-saturated CV’s show two major redox peaks attributed to the redox couple of Mn2O3 to Mn3O4 and Mn3O4 to Mn(OH)+ in the ORR potential regime, leading to artifacts in the ORR onset overpotential and challenges with proper analysis of the ORR kinetics.123 In the case of OER, two prominent 4e− pathways have been suggested: (i) adsorbate evolution mechanism (AEM) where 4 OH− ions undergo sequential bond cleavage and formation to form O2 in the gas phase30 (Scheme 2a) and (ii) lattice oxygen mechanism (LOM, Mars-Van Krevelen type mechanism) where lattice oxygen participates in oxygen evolution because of the higher thermodynamic driving force for lattice oxygen evolution as compared to AEM (Scheme 2b).33,39,119,124 LOM has been recently supported by experimental studies where systematic substitution of La with Sr in La1−xSrxCoO3−δ (0 ≤ x ≤ 1) led to an increase in the oxygen diffusivity, thereby creating oxygen defects, which facilitated favorable insertion and removal of lattice oxygen.33 The OER-LOM has also been linked to the structural instability of perovskites due to loss of lattice oxygen in the process (increase in oxygen vacancies).39,116 Similar to ORR, the adsorption of oxygenated species has been reported to lead to changes in the oxidation state of the surface transition metals (Bn+ to B(n+1)+) during AEM.17,33,40 However, no such changes in oxidation states of the transition metals have been predicted for LOM.33 This is mainly because, under an anodic potential regime, the driving force for vacancy formation, followed by exchange with the adsorbed atomic oxygen, leads to conservation of charge at the transition metal surface.33 To understand the chemical composition effect on the activity of ORR/OER on these oxides, studies varying the Aand B- site compositions have been widely reported.17,18,36,37,109,125,126 In the case of the A-site effects, a decrease in the activity of ORR is observed in LnMnO3 (Ln = La, Pr, Nd, Sm, Gd, Y, Dy, Yb) as the A-site metal changes from La to Yb.125 This has been linked to a decrease in the ionic

Scheme 1. Reaction Pathways for ORR in Alkaline Media

oxygen is reduced to OH− ions by either a direct 4e− transfer or a sequential 2e− transfer involving the chemistry of hydroperoxo species. It is important to note that both pathways are limited by O2(g) activation and strong interactions of oxygen species with the surface that could potentially lead to surface passivation. At the surface of these oxides, the transition metal is generally coordinated by 5 oxygen atoms. Therefore, the oxygenated adsorbates complete the BO6 octahedra in these oxides facilitated by an electron transfer from the transition metal, resulting in a change in the oxidation state of the metal from Bn+ to B(n+1)+.121 This change in oxidation state has been hypothesized to occur mainly with the adsorption of O2− 2 and O2− intermediates in the direct 4e− pathway.18 On the other hand, for oxides that undergo pseudo 4e− transfer (sequential 2 e− steps), the oxidation state change happens at two adjacent transition metal sites with atomic oxygen adsorption.122 It is Scheme 2. Two Potential Pathways for OER on Perovskites33a

(a) Adsorbate evolution mechanism (AEM), where 4 OH− are oxidized to form O2(g). (b) Lattice oxygen mechanism (LOM), where lattice oxygen participates in the reaction leading to oxygen evolution.

a

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Figure 4. Activity descriptors for low temperature OER and ORR. (a) Activity “volcano” relating energy of an OER reaction step (ΔG0O − ΔG0OH) with the OER overpotential. (b) Activity “volcano” relating eg filling with the measured potential at 25 μA/cmoxide2 for ORR. (a) and (b) are reproduced with permission from refs 30 and 18, respectively. Copyright 2011 John Wiley and Sons and 2011 Springer Nature, respectively.

optimal eg filling of just below 1 (as in the case of LaNiO3 and LaMnO3+δ) is shown to lead to the highest activity for ORR (Figure 4b).18 This descriptor has also been extended to OER and an optimal eg filling of slightly above 1 leading to the highest OER activity as in the case of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and La0.5Ca0.5CoO3−δ.17 Although eg filling has been successful at predicting the ORR and OER activity of different oxide-based catalysts including double perovskites and R-P oxides,20,34,124 reports have challenged its universality.33,37,131 Significant gaps exist in reported activities among different perovskites with an optimal eg filling of ∼1. For example, Hardin et al. systematically studied the effect of the B-site metal on the activities for LaBO3 (B = Ni, Co, Mn and Ni0.75Fe0.25) with an optimal eg filling of ∼1 and showed significant differences in their ORR and OER activities, suggesting that this bulk property (eg filling) of the oxide might not be adequate to determine the surface adsorption chemistry.37 Furthermore, this descriptor also fails at explaining the activity dependence on pH, as well as the participation of the lattice oxygen, which remains critical for highly active OER catalysts.33,37,39,124,132 Long-term stability of these oxides for low temperature ORR/OER is also critical for their practical implementation in electrochemical systems. It has been shown that highly active OER oxide catalysts, such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ, undergo surface reconstruction and amorphization accompanied by ion leaching during electrochemical testing.116 Therefore, understanding the factors that govern stability of these oxides under alkaline electrochemical conditions is critical but challenging due to the numerous accessible redox couples for the transition metal in these oxides under relevant potentials for ORR/ OER.37,123 To address these challenges, the development of advanced operando surface-sensitive techniques coupled with controlled electrochemical studies is necessary to obtain an understanding of the factors that govern the long-term stability of these materials. In summary, the identification of the reaction mechanism is critical in understanding the activity and stability of these oxides under reaction conditions. The activity descriptors developed thus far, while insightful, provide several limitations. For example, bulk properties, such as eg filling, have failed at accurately predicating the catalytic activity of many of these oxides due to the potential effect of the surface structure under

radius of Ln3+, thereby suggesting that the activity is linearly correlated to the ionic radius.125 However, no mechanistic insights exist in explaining the correlation between the ionic radii and the measured activity. Transition metal composition effects in LaBO3 (B = Ni, Co, Fe, Mn, and Cr) on ORR have also been reported, showing that Ni, Mn, and Co result in the highest ORR activity and selectivity to the 4e− ORR process.18,127−129 This is due to favorable valence states of the under-coordinated surface transition metals for oxygen adsorption.38 Similarly, the effect of A- and B-site compositions in A1−xA′xBO3 (A = La, A′ = Sr, B = Ni, Co, Fe, Mn, Cr, and V) perovskites on the OER activity has been reported.126 It is shown that Ni and Co based perovskites exhibit the highest activity because of their intermediate spin state, leading to appropriate overlap between the molecular orbitals of cations and the oxygenated species.17,19,37,126 To develop structure−performance relations, descriptors have been developed that correlate activity of these oxides to the binding energies of ORR/OER intermediates or other bulk properties. It is suggested that the adsorption energy of any of the oxygen-based intermediates (i.e., O*, OH*, and OOH*) could be used as an activity descriptor for ORR/OER, since linear scaling relationships have been established between the adsorption energies of these species. For example, theoretical calculations show that the adsorption energies of intermediates, OH* and OOH*, scale linearly with that of O* over a large range of perovskites.30,130 It is shown that the bond-formation energies of the oxygenated species involved in the process and their differences are critical at predicting and tuning the activity across different oxides.30 For example, it is reported that the adsorption energy difference of O* and OH* (ΔG0O − ΔG0OH) is effective in predicting the OER activity of perovskites.30 This descriptor, when plotted against the theoretical overpotential, leads to an activity “volcano” as shown in Figure 4a. This implies that a compromise in the oxygen adsorption energetics leads to the highest OER activity, as in the case of SrCoO3 and LaNiO3, consistent with experimental observations.17,33 The ORR/OER activity trends of these oxides have also been linked to the number of d-band electrons and eg orbital filling of the B-site cation using molecular orbital theory.17,18 The eg band represents the filling of the d-electrons of the B-cation in the antibonding orbitals, which interact strongly with the O-2pσ orbitals, thus affecting the strength of the B−O bond. An 2867


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will require the development of controlled benchmarking, synthesis/treatment techniques, and operando surface sensitive characterization techniques that facilitate development of consistent experimental activity trends. We suggest that synthesis of high surface-area nanostructured oxides with well-controlled surface structure can be used as a platform for developing surface structure−performance relations. The development of controlled experimental protocols along with insights from surface sensitive characterization techniques can facilitate a closer link with theoretical calculations, which when combined can effectively lead to the development of robust structure−activity relations for optimizing oxygen electrocatalysis on nonstoichiometric mixed metal oxides.

reaction conditions that cannot be captured accurately by bulk properties.37 Thus, systematic characterization of the surface structure is important for developing reliable universal activity descriptors that would enhance the overall accelerated discovery of highly active and stable nonstoichiometric oxides for ORR and OER. While characterization of the surface might be challenging given the diverse nature of commonly used spherical oxide particles (truncated polyhedrons), development of synthetic techniques that allow for controlled synthesis of nanostructures that preferentially expose certain surface facets can provide a platform for correlating surface structure to activity of these materials.

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6. CONCLUSIONS AND FUTURE DIRECTION Nonstoichiometric mixed metal oxides (i.e., perovskites, double perovskites, and R-P oxides) have shown promise as ORR/ OER electrocatalysts due to their mixed electronic and ionic conductivities. These properties are highly dependent on their crystal/surface structures and oxygen defects (oxygen vacancy and interstitial oxygen), which are affected by chemical composition and synthesis/treatment conditions. Electronic conductivity in these oxides originates from the electron hopping/charge transfer along the O−B−O bond. Thus, increasing the charge carrier concentration and/or strengthening the O−B−O bond facilitates electron conduction. In the case of ionic conductivity, enhancement in oxygen defects improves conductivity via providing more paths for oxygen diffusion. Chemical composition and surface structure play a critical role in tuning the ORR/OER activity of nonstoichiometric mixed metal oxides, by effecting the nature of the active sites for O2 dissociation and evolution. It is generally found for both high and low temperature electrocatalysis that the B-site terminated surfaces are catalytically more active for ORR/OER than the A-site terminated ones. The A-site composition impacts activity indirectly by effecting the oxygen defect concentration. For high-temperature ORR/OER, it is generally reported that B-site terminated Co containing oxides exhibit the best performance among these oxides due to their promising electronic/ionic conductivity and modest binding strengths of the intermediates. In the case of low-temperature ORR, although Mn, Co, and Ni based oxides have been shown to exhibit good performance, discrepancies still exist regarding the link between activity and B-site composition. In general, low temperature ORR/OER activity trends are convoluted with artifacts from lack of consistent approaches for testing the electrochemical activity of these oxides, due to complexities in electrode composition induced by their low electronic conductivity under these conditions. In addition, the flexibility of the B- and A-site compositions and their various possible combinations in these oxides make the search for optimal OER/ORR oxide electrocatalysts very challenging. Thus, performance descriptors developed based on the correlations between activity and specific oxide properties have been suggested to guide the design of optimal oxides for ORR/ OER. While important insights have been reported using activity descriptors that mainly correlate to bulk properties, in order to develop universal descriptors, a direct link between the surface structure and activity is critical. To achieve a link between the surface structure and the activity of these oxides, we suggest that approaches that combine theoretical calculations with well-controlled synthesis, detailed characterization, and testing are key. A step toward this

AUTHOR INFORMATION

Corresponding Author

*(E.N.) E-mail: [email protected]. ORCID

Eranda Nikolla: 0000-0002-8172-884X Author Contributions

‡ (X.-K.G. and S.S.) These authors contributed equally to this Perspective.

Notes

The authors declare no competing financial interest. Biographies Xiang-Kui Gu received his Ph.D. from Dalian Institute of Chemical Physics, Chinese Academy of Sciences. After as a postdoctoral appointment in the Department of Chemical Engineering at Purdue University, he joined Prof. Eranda Nikolla’s group as a postdoctoral scholar. His current research interests focus on utilizing state-of-the art computational methods to design efficient and selective heterogeneous catalysts for electrocatalysis and biomass conversion processes. Samji Samira is a second year Ph.D. candidate in Chemical Engineering working under the supervision of Professor Eranda Nikolla. He received his Bachelor’s and Master’s degree in Chemical Engineering from B.M.S. College of Engineering, India, and Carnegie Mellon University, Pittsburgh, in 2013 and 2015, respectively. He worked with Professor Andrew Gellman during his time at Carnegie Mellon on developing microkinetic models for hydrogenation catalysis. His research interests are oxygen electrocatalysis for applications in fuel cells and metal−air batteries. Eranda Nikolla is an associate professor in the Department of Chemical Engineering and Materials Science at Wayne State University. She received her Ph.D. in Chemical Engineering from the University of Michigan in 2009 under the supervision of Profs. Suljo Linic and Johannes Schwank, followed by a two-year postdoctoral work at California Institute of Technology with Prof. Mark E. Davis. Her research interests lie in the development of active and selective heterogeneous catalysts and electrocatalysts for chemical/electrochemical conversion processes. She is the recipient of a number of awards, including the National Science Foundation CAREER Award, the Department of Energy CAREER Award, 2016 Camille Dreyfus Teacher-Scholar Award, and the Young Scientist Award from the International Congress on Catalysis.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of the Department of Energy (DOE), Basic Energy Science (BES), Early Career Program, under Award No. DE-SC0014347, and the National Science Foundation (CBET-1434696). The authors acknowl2868


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edge the Camille and Henry Dreyfus Foundation and Wayne State University for additional support.

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Evolution on Nonstoichiometric, Mixed Metal Oxides

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