Oxygen Evolution ReactionâThe Enigma in Water Electrolysis - ACS

Viewpoint Cite This: ACS Catal. 2018, 8, 9765−9774

pubs.acs.org/acscatalysis

Oxygen Evolution ReactionThe Enigma in Water Electrolysis Emiliana Fabbri*,† and Thomas J. Schmidt*,†,§ †

Electrochemistry Laboratory, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland Laboratory of Physical Chemistry, ETH Zurich, 8093 Zurich, Switzerland

ACS Catal. Downloaded from pubs.acs.org by 188.68.3.129 on 09/26/18. For personal use only.

§

I. THE OXYGEN EVOLUTION REACTION MECHANSIM: AN OLD BUT CURRENT ENIGMA In the course of replacing fossil-fuel-based energy technologies, the development of energy storage systems is crucial to mediate the variable nature of energy generation from renewable resources. Within this scenario, hydrogen production via water electrolysis represents a central technology for seasonal and decentralized energy storage.1,2 Even though the electrochemical splitting of water has been known since the 19th century when Paets van Troostwijk/ Deiman and Nicholson/Carlisle made one of the most exciting scientific discoveries ever made, unveiling that electricity can decompose water into hydrogen and oxygen (see Figure 1), the oxygen evolution reaction (OER), that is, the anodic reaction of water electrolyzers, is still an enigma.3 Neither the reaction mechanism nor the ideal catalyst in terms of activity and stability has been revealed so far. Therefore, still open questions and great challenges orbit around the OER. From the point of view of speeding up the widespread market penetration of water electrolyzers, the development of a highly active, stable, and inexpensive electrocatalyst is highly demanded.3 However, the design of optimal catalysts first calls for a better understanding of the electrochemical reaction mechanisms, particularly the one related to the anodic OER and for a rationalization of the reasons behind the often observed changes in the catalyst’s electrochemical activity (either as positive or negative trend) during operation. In this Viewpoint, we aim at increasing the awareness among the scientific community devoted to progresses in water electrolyzers of the very recent development made in the fundamental understanding of the OER, particularly focusing on the increasing consciousness that several processes actually underpin the evolution of oxygen from a metal oxide catalyst. Traditionally, the OER mechanism on metal oxides has been derived from that on metal catalysts, where the main parameter governing the reaction overpotential is the binding strength of oxygen (or oxygenated species/intermediates) on the catalyst surface following the Sabatier principle: the best catalyst in terms of displaying the minimum overpotential binds oxygen on its surface neither too strongly nor too weakly.4,5 Several reaction mechanisms having the metal centers as active sites have been proposed in the past.3 Figure 2 shows an example of the conventional OER mechanism, in the sense that it is likely the most widespread used to describe the OER, for the acidic and the alkaline environment, respectively.3,6,7 Typically, the difference in the theoretical OER overpotential among different catalysts has been correlated to a single descriptor following the above-mentioned Sabatier principle, namely, the oxygen adsorption energy on the catalyst surface4 and it has been postulated that scaling relations exist between © XXXX American Chemical Society

adsorption energies of the oxygenated intermediate binding species.5 According to this analysis, Co3O4 and RuO2 among binary oxides and LaNiO3 and SrCoO3 among perovskites present the lowest theoretical overpotentials due to their optimal trade-off between strong and weak binding energy of oxygen.5 This well-established understanding of the reaction mechanism and parameters regulating catalyst activity has been progressively challenged over the past couple of years by few experimental and theoretical observations pointing toward the possibility of a different reaction mechanism than the conventional one which foresees the participation of lattice oxygens in the OER process, the so-called lattice oxygen evolution LOER.8−11 The process involving the oxidation of the lattice oxygen (more precisely the lattice O2− anions) does not exclude that the binding energy of oxygenated intermediates is still a valid activity descriptor for the theoretical overpotential of metal oxide catalysts but definitively points toward the need of considering dynamic catalyst surfaces where the active sites are not limited to the metal centers. Furthermore, the participation of lattice oxygen in the OER will occur with different extents on individual catalysts, complicating the search of a single guiding parameter for the OER activity and necessitating a case-by-case study of the reaction mechanism. The participation of lattice oxygen is actually a common phenomenon in gas-phase catalytic reactions,12 but only recently has it been extensively considered as an alternative reaction pathway, and sometimes the most favorable one,11 in oxygen electrocatalysis. The first studies mentioning the possibility of a different reaction mechanism than the traditional one4 involving lattice oxygen dates back, to the best of our knowledge, to 1976, 1983, and 1987 studies by Damjanovic and Jovanovic, Bockris and Otagawa, and Wohlfahrt-Mehrens and Heitbaum, respectively.13−15 A couple of decades later, other manuscripts published between 2007 and 2013 have also mentioned that the surface oxygen from the metal oxide participates in the OER16,17 and pointed toward the importance of metal oxide surface hydroxylation for the OER.18,19 In 2015, Binninger at al.8 have demonstrated most assuredly by thermodynamic consideration that at equilibrium the OER, the metal oxide dissolution, and the lattice oxygen evolution reaction (LOER) are mutually linked by their chemical potentials, and thus, when the catalyst is brought out of equilibrium, if the OER takes place, both the metal dissolution and the LOER are triggered. Afterward an increasing number of publications has appeared in the literature supporting the occurrence of the LOER,10,20 Received: July 11, 2018 Published: September 12, 2018 9765

DOI: 10.1021/acscatal.8b02712 ACS Catal. 2018, 8, 9765−9774

Viewpoint

ACS Catalysis

Figure 1. On the left, William Nicholson and Anthony Carlisle observing water electrolysis. They took a small tube, mounted vertically, filled with water then sealed, into which at either end was inserted a platinum wire connected to one of the two terminals of a voltaic pile. As the tips of the wires were gradually advanced toward each other they observed that a stream of bubbles was produced from each tip, one found to be of oxygen, the other hydrogen. Figure supplied by Science Source. On the right, a scheme of a water electrolyzer where on the anodic electrode (gray spheres) water is oxidized to oxygen and on the cathodic electrode (green spheres) hydrogen evolution takes place.

Figure 2. Conventional OER mechanism involving proton electron transfers on the surface metal centers. A perovskite structure ABO3 has been used as representative catalyst where the orange, green, and red spheres represent the A-site cations, B-site cations, and the oxygens, respectively. For binary oxides, only the terminating BO layer can be considered.

particularly on metal oxide perovskite catalysts.9−11,21−24 However, at present not much correlation exists between the above-mentioned studies, and a coordination effort is needed to develop a novel and unified perspective on the OER process, which can trigger the development of advanced water-splitting metal oxide catalysts for new energy conversion and storage devices.

OER mechanism is inextricably linked to the understanding of the metal dissolution and the LOER mechanism, which represents for the electrochemistry community unquestionably a challenging task. Driven by the complexity of the oxygen evolution, the most common approach has been so far the assumption of a common reaction mechanism (for example the one shown in Figure 2), canalizing the efforts toward the identification of activity descriptors. Activity descriptors can help in the identification and prediction of the most OER active oxide catalysts, and thus, they could be defined as the main catalyst physicochemical property governing the OER activity.3 It must be shortly mentioned that the definition of OER activity and how it is determined is at present not homogenized in the community, if even not “chaotic”. OER activity is generally identified as current density at a fixed potential or the overpotential at a fixed current density; however, how the current normalization is carried out appears to be arbitrary in the literature as well as the selection of the potential at which the current density is determimed.5,31−36 OER currents are very often normalized by the geometric surface area of the electrode, which is actually providing very poor information since the activity is a function of the catalyst loading and its specific surface area. A reliable method to

II. ACTIVITY DESCRIPTORS: WHAT DO THEY REALLY INDICATE? Despite the large efforts dedicated to the understanding of the OER mechanism on the surface of oxide materials, the OER mechanism is actually not fully resolved and, both for the acidic and the alkaline environment, mechanisms including different reaction steps have been proposed in the course of the last decades.3,25 As mentioned above, the study of the OER mechanism becomes particularly complicated when other electrochemical reactions occurring on the surface of oxide catalysts concurrently to the conventional OER process6,7 are taken into account. Theoretical considerations and experimental evidence suggest that for many oxides the OER is accompanied by metal oxide dissolution and/or LOER.8,9,11,23,24,26−30 Therefore, an understanding of the 9766


Viewpoint

ACS Catalysis determine the electrochemical active surface area (ECSA) for metal oxides is presently still debated, and therefore the best practice is to normalize OER currents for the catalyst surface area (i.e., BET surface area) or the catalyst mass loading.3 We still recommend to follow our previous suggestion to compare OER activity at a mass-specific current density of 10 A/goxide.3 As mentioned above, not only the OER activity definition, but also how experimentally the activity is determined appears presently not consistent within the different related studies. Some best practices such as the use of (i) nonglass cells (for alkaline environments), (ii) an alkaline binder or a cation exchanged Nafion binder (for alkaline environments), (iii) a nonplatinum counter electrode to avoid contamination particularly for non-noble metal catalysts in thin film rotating disk electrode (RDE) measurements are very often not applied. Last but not least, to remove capacitive contributions to the overall measured current, OER activity should be determined by chronoamperometric measurements rather than linear scan or cyclic voltammetry. Moving back to OER activity descriptors, for binary oxides, the use of the oxygen binding energy as activity descriptor places RuO2 at the top of the volcano plot (with the lowest theoretical overpotential).4,5 In the study by Halck et al.,37 it has been shown that by doping RuO2 with Ni or Co, deviations from the volcano plot correlating the OER overpotential and the binding energies of oxygenated reaction intermediates can be achieved, which in turn means the breaking of the scaling relationships. These deviations have been explained by a different reaction mechanism involving the presence of a cocatalyst, which activates a proton donor− acceptor functionality on the surface site bridge originally inactive.37 A different conclusion has been reached by the study of the electrochemical activity and stability of Ni-doped IrO2 catalysts where a correlation between the Ni dissolution and the OER activity has been established.27 The Ni leaching from the rutile structure leads to a more active IrO2 due to the higher surface area rich in hydroxyl groups. The surface coverage of reactive surface hydroxyl groups has been identified as a suitable activity descriptor for this class of materials, confirmed also by operando X-ray absorption (XAS) measurements on high surface area IrO2.38 Surface leaching leading to superior activity than that of binary iridium oxide has been observed also for Ir-based perovskites, such as Ba2NdIrO639 or SrIrO3.40 The attractive possibility given by the perovskite family to synthesize a vast variety of oxide compositions particularly demands design principles able to proficiently and effectively predict the most OER active perovskite catalysts. In this context, extensive studies have been performed with the aim of discovering the key materials physicochemical property (or the different properties) governing the perovskite OER activity in alkaline environment,3,7,33−35,41−46 with the further purpose of improving known materials or predicting new active ones. The identification of a single activity descriptor that can capture all perovskite affinity toward the OER is obviously highly attractive but it might represent an unattainable target due to the very different nature of the known and potentially new perovskite structures. Indeed, a rather substantial variety of activity descriptors have been proposed so far for perovskite OER catalysts, such as, for example, the number of delectrons,41 the eg band filling of the transition-metal cations,43 the difference between the surface binding energies of O*and HO* reaction intermediates,7 the oxide formation energy,35

and the accumulation of the magnetic moment on the conduction plane atoms,34 suggesting that there is currently no consensus on the activity descriptors for perovskite catalysts and least of all on the reaction mechanism. Very recently it has been suggested that rather than a single descriptor, a network of descriptors can better describe the trend in the OER activity for a large variety of perovskite oxides.46 The main obstacle in the activity descriptor definition for perovskite catalysts is probably the fact that the surface of these complex oxides is prone to be modified under oxygen evolution conditions both due to cation leaching39,40,47 and participation of the lattice oxygen,21 which can lead to the formation of a surface layer completely different from the bulk oxide. Particularly, Fabbri et al.21 have recently unambiguously demonstrated that the electronic and local structure of active perovskite catalysts can change during operation, making thus many ex situ catalyst properties (i.e., properties measured on the as prepared catalyst) unreliable descriptors. One of the reasons of the observed changes in the electronic and local structure during OER has been related to the simultaneous occurrence of the classical oxygen evolution and the lattice evolution mechanism, respectively, which will be discussed in detail in the following.

III. LATTICE OXYGEN EVOLUTION REACTION: WHEN A CATALYST REALLY INVOLVES/EVOLVES ITSELF Traditionally the active sites of a metal oxide catalyst have been always considered the metal centers (see Figure 2), but it is actually possible to learn from the earlier and very fundamental papers on OER oxide catalysts that lattice oxygens can be also involved in the catalytic reaction. Amazingly, the revolutionary concept of an oxygen evolution reaction mechanism where lattice oxygens evolves during the reaction has been originally proposed as early as 1976 by Damjanovic and Janovic for PtO catalyst13 in a time where electrocatalysis was considered to be governed purely by surface processes (which, to mention in passing, is still the case today). According to their mechanism, oxygen anions are the active sites adsorbing OH* and directly participating in the oxygen evolution reaction. The oxygen vacancies formed by the lattice oxygen evolution are subsequently replenished in a parallel reaction by water dissociation products. Interestingly, the authors suggested that for this reaction mechanism, there is no need for a high coverage of reaction intermediates because it is the oxygen in the oxide lattice acting as reaction intermediate itself. The authors proposed the LOER as an alternative reaction mechanism to the traditional OER based on the isotope labeling measurements performed by Rozenthal and Veselevskii in 1956.13 The mechanism proposed by Damjanovic and Janovic13 was refined a few years later by Bockris and Otagawa14 who discussed particularly the case of LaNiO3, a perovskites catalyst presenting a large number of oxygen vacancies and loosely bound lattice oxygens atoms. According to them, in an alkaline environment surface lattice oxygen as hydroxide species can directly participate in the OER reacting with the hydroxide ions adsorbed on the metal centers to form H2O2, as shown in Figure 3. It should be noted that in such reaction mechanism, even though the lattice oxygen is involved, the active sites remain the metal atoms. However, similar to ref 13, Bockris and Otagawa14 suggested that in this reaction mechanism there is no need for high-surface OH− coverage to achieve low OER overpotential due to the participation of lattice hydroxide species. Furthermore, they also indicate that the oxygen vacancies formed during the 9767


Viewpoint

ACS Catalysis

with the initial assumption of Bockris and Otagawa14 who assumed the LOER mechanism particularly valid for LaNiO3 catalyst, a perovskite structure very rich in oxygen vacancies. LaNiO3 has been also widely investigated in an alkaline environment by Hardin et al.,18 suggesting a very similar reaction mechanism to the one proposed by Bockris and Otagawa shown in Figure 3,14 with the difference that the lattice hydroxide species participate in the formation of surface-adsorbed hydroperoxide, rather than in the formation of H2O2. Hardin et al.18 also claimed that for the LaNiO3, the rate-determining step is governed by the concentration of lattice hydroxide species that participate in the formation of the hydroperoxide species. Therefore, the higher the lattice hydroxide concentration the faster the rate at which the OER proceeds. Furthermore, the oxygen vacancies formed during this reaction mechanism are supposed to be rapidly replenished from the electrolyte resulting in a lack of hysteresis observed during OER testing. The same mechanism has been also proposed by Hardin et al.19 for Co and Mn based perovskites. As for the mechanism proposed by Bockris and Otagawa,14 also in this case19 the active sites are considered the transition metal atoms. A rigorous formulation of the occurrence of the LOER concurrently with the conventional OER mechanism has been proposed by Binninger et al. in 20158 (who also coined the term LOER) based on thermodynamic considerations. Considering the chemical equilibria of the classical OER reaction (eq 1), metal oxide dissolution (eq 2), and LOER (eq 3), it is relatively straightforward to see that the chemical potentials of all three reactions are interlinked and to conclude that at potentials above the oxygen evolution equilibrium potential reaction 2 and (3) also need to take place. Although, in eqs 1-3, a general metal oxide MOn in alkaline environment is considered, conceptually these equations are valid for all oxides in aqueous electroytes.8

Figure 3. OER mechanism proposed by Bockris and Otagawa14 involving lattice oxygen participation in the reaction. In the scheme readapted from ref 14, (V) and (M) denote an oxygen vacancy and an the metal atom, respectively. The OER is discussed for LaNiO3 catalyst in an alkaline environment.

oxygen evolution can be replenished by another OH − produced by the decomposition of H2O2 (see Figure 3). In 1987, Wohlfahrt-Mehrens and Heitbaum15 performed isotope labeling studies on RuO2 films showing that anodically generated O2 comes partially from the electrolyte (solution of H2SO4) and partially from lattice oxygen. They have not really postulated a LOER mechanism but rather hypothesized that in the conventional mechanisms,3,6,7 the active site S is not the simple metal center but rather RuO2, claiming in the conclusion of their manuscript that “in case of Ru (oxygen) it is evolved by the oxide”. A couple of decades later WohlfahrtMehrens and Heitbaum15 findings have been corroborated by the studies of Fierro et al.16 and Macounova et al.,17 which also showed by isotope labeling experiments in HClO4 electrolyte that lattice oxygen is evolved in the OER mechanism from IrO2 and Ru-based oxide catalysts, respectively. According to Fierro et al.,16 the amount of lattice oxygen involved in the LOER (denoted by the authors as “oxygen exchange reaction”) is in the order of 1% of the total IrO2 loading suggesting that only the outer surface layer of IrO2 participates in the reaction. Similar results have been obtained also performing labeling measurements on Co3O4 nanoparticles in alkaline environment; it was observed that the total number of oxygen atoms of the oxide participating in OER was 0.1−0.2% of the total oxide loading, corresponding to about 10−30% of the surface atoms.48 According to the study of Macounova et al.17 on Ru-based oxides, the mechanism where oxygen is evolved from water is dominant at lower overpotentials, while at high overpotentials, the latter mechanism coexists with the LOER. Furthermore, they determined that the amount of the lattice oxygens participating in the oxygen evolution depends on the local structure of the catalyst because the lattice oxygen in Ru0.9Ni0.1O2−δ is much more active that in RuO2. Interestingly, the former catalyst is supposed to possess a larger amount of oxygen vacancies than RuO2 for charge balancing the lower valence state of Ni compared with Ru. This finding is in line

conventional OER: 2OH−aq ↔

1 O2 + H 2O + 2e− 2

(1)

chemical dissolution of the metal oxide: M2n +On2 − + nH 2O 2n + ↔ Maq + 2nOH−aq 2n + LOER: M2n +On2 − ↔ M aq +

(2)

1 nO2 + 2ne− 2

(3)

However, reactions 1 and 3 are likely not only thermodynamically linked, but also, the kinetics of the two processes should be closely related. Indeed, the adsorbed oxygen species on the metal oxide surface from the electrolyte and from the oxygen ions in the metal oxide lattice surface can be fairly distinguishable for the process of the oxygen evolution and, thus, the OER (1) and LOER (2) might also share the same reaction steps.8 Figure 4 schematically illustrates the processes described in eqs 1−3. In this scenario, the surface of the metal oxide catalyst starts to “boil”, colorfully speaking, at the onset of OER and LOER. The metal cations resulting from the LOER (i) can be dissolved in the electrolyte and get oxidized to a higher valence state or (ii) it can recombine with hydroxide ions from the electrolyte. In the former case, electrode mass loss will be observed with eventual sample degradation, while in the latter case the metal cations formed by the LOER will form an oxyhydroxide layer closing the cycle and leading the catalyst superficial layer into a dynamic but 9768


Viewpoint

ACS Catalysis

Figure 4. Schematic representation of the OER/metal dissolution/ LOER process as proposed by Binninger et al.8 This cycle can lead to the formation of a 3-dimensional superficial layer between the metal oxide lattice and the electrolyte. Due to the LOER, the catalyst cations in the superficial hydrous layer can participate to the OER/LOER by recombination with aqueous hydroxide ions or they can dissolve in the electrolyte either with unchanged valence or, after an additional oxidation step, in a higher valence state. Oxygen anions can in principle diffuse from the bulk toward the surface, which is equivalent to oxygen vacancies diffusion in the opposite direction. The figure has been readapted from ref 8. A perovskite structure AMO3 has been used as representative catalyst where the orange, the green the red, and the violet spheres represent the A-site cations, M-site cations, the oxygens, and the oxygen vacancies, respectively. For binary oxides, only the terminating MO layer can be considered.

Figure 5. OER/LOER and dissolution/redeposition mechanism leading to the formation of a self-assembled active surface layer, rich in CoO(OH) and FeO(OH) on the surface of Ba0.5Sr0.5Co0.8Fe0.2O3‑δ perovskite.21 The LOER process leads to the direct evolution of the perovskite lattice oxygen, and it is accompanied by cation dissolution. Ba2+ and Sr2+ cations are highly soluble, and thus, they can easily leach out from the perovskite structure (see equation on the bottom). LOER also triggers dissolution of Co and Fe cations. However, being rather insoluble species, Co and Fe redeposition on the catalyst surface can take place, especially when near the electrode surface the cation concentration (due to the initial dissolution) becomes significant. Furthermore, the lattice oxygen consumed by the LOER can be replenished by OH− from the electrolyte. Therefore, a stable dynamic cycle is established, permitting the coexistence of a self-assembled active surface layer with the original BSCF perovskite structure. Figure readapted from ref 21.

stable state. It is interesting to note that both in the process suggested by Bockris and Otagawa14 and by Bininnger et al.,8 no change in the oxidation state of the transition metal is needed for the LOER to take place, differently from what was proposed by Hardin et al.18,19 Indeed, in the LOER, it is the oxidation of the lattice oxygen ions and not the oxidation of the cations to a higher valence state that triggers the reaction. The LOER proposed by Binninger et al.8 foresees dynamically stable oxides the formation of a hydroxide layer with reduced structural order and rather undefined composition regarding the metal:oxygen stoichiometry. Although never correlated before, it becomes clear now that the formation of a superficial amorphous, (oxy)hydroxide layer, experimentally observed in many studies on OER oxide catalysts, can be triggered not only by cycling the catalyst in a redox couple potential range, but also by the OER/LOER process.20,21,27,29,30,45,49,50 In particular, the growth of a CoO(OH) layer after the onset potential of the OER has been observed by Fabbri et al. 2 1 by performing operando XAS on Ba0.5Sr0.5Co0.8Fe0.2O3‑δ (BSCF) perovskite catalyst nanoparticles, which is a highly active catalyst toward the OER and a semiconductor materials showing a high amount of oxygen vacancies and high oxygen ion mobility.51 The formation of the self-assembled CoO(OH) and the great performance stability of the BSCF catalysts in alkaline environment have been explained by the reaction scheme depicted in Figure 5, where due to the occurrence of LOER, the superficial A-site cations of the perovskite dissolve in the electrolyte leaving a shortrange ordered (Co/Fe)O(OH) layer, which in principle can also undergo LOER. However, the limited solubility of the Bsite cations in the electrolyte promotes its redeposition on the catalyst surface and the evolved oxygen can be replenished by the surrounding OH− from the electrolyte.

By the operando XAS analysis of different Co-based catalysts, it was suggested that the more important the growth of this assembled hydroxide layer is, the higher the OER activity of the catalyst. Flexible structures such as BSCF can be seen as an ideal precursor catalyst for the LOER because their highly defective surface can facilitate oxygen exchange and dynamic self-reconstruction of the surface.21 Flexibility in the structure can be associated with oxygen vacancies concentration and ion mobility, and indeed, a direct correlation between oxygen vacancies content an OER activity has been revealed in ref 21. Oxygen evolution within the lattice of BSCF has also been observed by environmental TEM analysis.22 It has been suggested that optimal energy gaps between the O 2p-band centers and the Fermi levels of BSCF favors this singularity, which has been not observed for the Co or Mnbased perovskite, such as La0.5Sr0.5CoO3‑d, LaCoO3, and LaMnO3. The latter perovskite oxides show high energy gaps and, thus, low oxygen mobility and high formation energy for oxygen vacancies. Therefore, it appears from different studies that the presence of oxygen vacancies particularly for perovskite catalysts is crucial for the occurrence of the LOER.9,21,22,52 Revisiting the mechanism proposed in refs 18,19, Mefford et al.9 have investigated the perovskite series La1−xSrxCoO3‑d, and they have found that as the Sr content increases, the oxygen vacancy concentration and the oxygen diffusivity increase as well. Furthermore, a direct correlation between oxygen vacancies and oxygen diffusivity and the OER activity has been observed. However, while a correlation between oxygen vacancies and LOER is emerging from different studies,10,21,22 9769


Viewpoint

ACS Catalysis

Figure 6. Reaction mechanism for the OER including the participation of lattice oxygen in alkaline environment as proposed by (a) Mefford et al.9 Reprinted with permission from ref 9. Copyright 2016 Nature Publishing Group; (b) Grimaud et al.10 Reprinted with permission from ref 10. Copyright 2017 Springer Nature: Nature Chemistry; and (c) Yoo et al.11 Reprinted with permission from ref 11. Copyright 2018 American Chemical Society.

calculations and also very susceptible to the choice of parameters used for the calculation.56 DFT calculation on LaNiO3 (001) single crystals shows that the reaction mechanism comprising the direct evolution of lattice oxygens via the reversible formation of surface oxygen vacancies (the active site is considered to be the transition metal center, see Figure 6c) has lower reaction barriers than the traditional one based on adsorbate intermediates.24 Furthermore, the authors show a progressive transition from the traditional OER mechanism to the LOER mechanism with decreasing perovskite bulk stability and formation energy of oxygen vacancies. Interestingly, the authors suggested that inducing bulk oxygen vacancies by tuning the B site oxidation state could switch the OER mechanism from the traditional one to the LOER.24 Furthermore, the same group has also shown that employing a linear scaling relationship between reaction steps and constructing OER activity volcanos for the LOER as shown in Figure 6c and for the conventional OER mechanism, where the adsorbate is sequentially oxidized via OH* → O* → OOH* → O2(g), the lattice-oxygen-participating mechanism can lead to a minimization of the thermodynamic OER overpotential compared to the conventional mechanism (see Figure 7).11 The conventional OER mechanism is favorable for strongly binding perovskites such as LaCoO3, whereas the LOER is more favorable for weakly binding perovskites such as LaCuO3 as the formation of oxygen vacancies is more favorable.11,23 For moderate binding surfaces like in LaNiO3, both mechanisms are possible, but the lattice-oxygenparticipating mechanism remains the most favorable.11,23 The

not many provide evidence with regard to an existing correlation between LOER and oxygen mobility/diffusion,9 even though oxygen diffusion and intercalation into oxides at room temperature and from a solid/liquid interface has been known for many years, showing oxygen diffusivities in the range of 10−15 to 10−9 cm2 s−1.9,10,52−54 On the basis of the observed correlation between oxygen vacancies/mobility and OER activity, Mefford et al.9 suggested a parallel reaction mechanism to the conventional one for perovskites in alkaline environment, which involves the participation of the lattice oxygen. In their reaction mechanism shown in Figure 6a, lattice oxygen reacts with adsorbed oxygen on the Co site to form OO- intermediates and leave an oxygen vacancy in the lattice. Differently, Grimaud et al.10 recently proposed a LOER mechanism where no redox chemistry of the metal centers is involved, but it rather foresees an anionic redox process as previously described by Binninger et al.8 (see eq 3). In particular, the full mechanism proposed by Grimaud et al.10 demonstrates oxygen adsorption on the superficial lattice oxygen and the formation of two oxygen vacancies around the metal center once molecular oxygen evolves from the lattice oxygen sites (see Figure 6b). Using isotope labeling experiments in an alkaline environment, the authors found that the OER on perovskites with high covalent character, such as La0.5Sr0.5CoO3‑d and SrCoO3‑d, proceeds both via the conventional mechanism and via the LOER. Interestingly, the more covalent the perovskite oxide, the higher the oxygen vacancy concentration.9 While for complex oxides there appears to be some convergence to accept the occurrence of LOER in parallel to the OER, for binary oxides like RuO2 or NiO, the community is more diverging. For instance, recent isotope labeling experiments on well crystalline RuO2 films, both in acidic and alkaline environment, detect no involvement of the lattice oxygen,55 contrary to the past findings.15,17 The authors suggested that for RuO2 only amorphous or nanocrystalline electrodes with undercoordinated edge sites might evolve lattice oxygen in the OER process.55 Recently, besides experimental studies, DFT calculation efforts have also been directed toward the investigation of the LOER mechanism. It must be acknowledged that for metal oxide catalysts, theoretical studies encounter several difficulties in providing atomic-level insights of OER catalysts. Indeed, while DFT calculations model OER catalysts as single crystals, mostly in a cubic structure, generally metal oxide catalysts are experimentally studied as polycrystalline samples with different crystalline structures. Furthermore, many of the OER catalysts belong to the class of strongly correlated oxides, whose electronic structure is extremely difficult to simulate by DFT

Figure 7. Sketch of the OER activity volcano that takes into account both conventional OER mechanism (black) and the lattice-oxygenparticipating OER mechanism11 (red). Readapted with permission from ref 11. Copyright 2018 American Chemical Society. 9770


Viewpoint

ACS Catalysis

Figure 8. Proposed LOER mechanism in alkaline environment where lattice oxygens represents the reactions sites, leading to lattice oxygen evolution and consequent formation of oxygen vacancies in the metal oxide lattice, which can be replenished by a final step by reacting with OH− in the electrolyte. Four electrons are overall exchanged in the LOER but decoupled proton electron steps take place. Step 2 and 3 are represented as formal reaction steps, but they would rather occur as a concerted process where H+ and O2 are released. The oxygen vacancy can be replenished by reaction with OH− (step 4), but also by H2O which can dissociate in hydroxide ion and a proton; the former fills an oxygen vacancy, while the latter can form a covalent bond with lattice oxygen. Proton incorporation via the dissociative adsorption of water has been widely discussed for proton conducting perovskites, such as BaZr1−xYxO3‑δ.57−59 In the figure, a perovskite structure AMO3 has been used as representative catalyst where the orange, the green, the red, and the violet spheres represent the A-site cations, M-site cations, the oxygens (O), and the oxygen vacancies (V), respectively. For binary oxides, only the terminating MO layer can be considered.

that the mechanism shown in Figure 6b might be not feasible for strongly and moderately binding surface because the diffusion barriers for O* and OH* to move from the transition metal to the lattice oxygen site are likely very high. The mechanism shown in Figure 6c is instead not feasible for weakly binding surface because it requires OH* to be on the transition metal to produce O2 and the diffusion barrier of OH* on the oxygen site to the transition metal can be high.11 In summary, even though the research community working on the OER is increasing more and more its awareness of the occurrence of the LOER and an unanimous consensus on the LOER reaction steps is still far to be reached. Furthermore, it must be noted that at present most of the reaction mechanisms involving the participation of lattice oxygen have been formulated for perovskite oxides and in alkaline environment, where the abundant presence of OH− allows replenishing the oxygen vacancies formed in the metal oxide lattice by the

authors also suggested that the LOER is likely to be more favorable when the A-site of the perovskite is more tolerant toward formation of vacancies in the AO layer (in order of tolerance Ba > Sr > La).11 Yoo et al.11 also compared the lattice-oxygen-participating mechanism proposed in their recent publications11,23 (Figure 6c) and the LOER mechanism suggested by Grimaud et al.10 (Figure 6b). In the mechanism in Figure 6b, an oxygen intermediate is adsorbed on a lattice oxygen site, and the Olatt− Oads desorb as O2 leading an oxygen vacancy on the lattice oxygen site. In the mechanism illustrated in Figure 6b, the active site is the transition metal of the perovskite, and a surface lattice oxygen shifts out of the surface plane to react with OH− adsorbed on the transition metal site to form an OO* intermediate and an oxygen vacancy. Comparing the adsorption energies of O* and OH* on the transition metal vs the oxygen site for different perovskites, Yoo et al.11 concluded 9771


ACS Catalysis

■

ACKNOWLEDGMENTS We gratefully acknowledge funding from Innosuisse and the Swiss Competence Center for Energy Research (SCCER) Heat & Electricity Storage. We thank Moritz J. Schmidt for his assistance to acquire “water splitting” images for the graphical abstract.

LOER process. Overall, two school of thought regarding the LOER can be identified at present: one considering the metal cations still the reaction centers where the redox chemistry takes place9,11,19 and one considering the lattice oxygen as the reaction site and placing the anionic redox process at the center for the LOER.8,10,21 The latter view of the LOER completely diverges from the conventional OER mechanism and can open new perspective in the development of oxide catalysts. Figure 8 proposes a possible reaction scheme for the LOER in alkaline environment based on anionic redox processes and decoupled proton electron transfers. However, at present no experimental evidence has been offered to support the suggested reaction steps for the LOER mechanisms proposed in Figure 69−11 or in Figure 8. Therefore, more theoretical calculations and advanced experimental investigation, such as time-resolved spectroscopy studies, are still needed to unravel the enigma of the oxygen evolution reaction. There is still much work ahead for the community, but it can be achieved one step at a time.

■

REFERENCES

(1) Babic, U.; Suermann, M.; Büchi, F. N.; Gubler, L.; Schmidt, T. J. Critical ReviewIdentifying Critical Gaps for Polymer Electrolyte Water Electrolysis Development. J. Electrochem. Soc. 2017, 164, F387−F399. (2) Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D. A Comprehensive Review on PEM Water Electrolysis. Int. J. Hydrogen Energy 2013, 38, 4901−4934. (3) Fabbri, E.; Habereder, A.; Waltar, K.; Kotz, R.; Schmidt, T. J. Developments and Perspectives of Oxide-based Catalysts for the Oxygen Evolution Reaction. Catal. Sci. Technol. 2014, 4, 3800−3821. (4) Rossmeisl, J.; Qu, Z. W.; Zhu, H.; Kroes, G. J.; Nørskov, J. K. Electrolysis of Water on Oxide Surfaces. J. Electroanal. Chem. 2007, 607, 83−89. (5) Man, I. C.; Su, H.-Y.; Calle-Vallejo, F.; Hansen, H. A.; Martínez, J. I.; Inoglu, N. G.; Kitchin, J.; Jaramillo, T. F.; Nørskov, J. K.; Rossmeisl, J. Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces. ChemCatChem 2011, 3, 1159−1165. (6) Rossmeisl, J.; Logadottir, A.; Nørskov, J. K. Electrolysis of Water on (Oxidized) Metal Surfaces. Chem. Phys. 2005, 319, 178−184. (7) Man, I. C.; Su, H. Y.; Calle-Vallejo, F.; Hansen, H. A.; Martinez, J. I.; Inoglu, N. G.; Kitchin, J.; Jaramillo, T. F.; Norskov, J. K.; Rossmeisl, J. Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces. ChemCatChem 2011, 3, 1159−1165. (8) Binninger, T.; Mohamed, R.; Waltar, K.; Fabbri, E.; Levecque, P.; Kötz, R.; Schmidt, T. J. Thermodynamic Explanation of the Universal Correlation Between Oxygen Evolution Activity and Corrosion of Oxide Catalysts. Sci. Rep. 2015, 5, 12167. (9) Mefford, J. T.; Rong, X.; Abakumov, A. M.; Hardin, W. G.; Dai, S.; Kolpak, A. M.; Johnston, K. P.; Stevenson, K. J. Water Electrolysis on La1−xSrxCoO3−δ Perovskite Electrocatalysts. Nat. Commun. 2016, 7, 11053. (10) Grimaud, A.; Diaz-Morales, O.; Han, B.; Hong, W. T.; Lee, Y.L.; Giordano, L.; Stoerzinger, K. A.; Koper, M. T. M.; Shao-Horn, Y. Activating Lattice Oxygen Redox Reactions in Metal Oxides to Catalyse Oxygen Evolution. Nat. Chem. 2017, 9, 457. (11) Yoo, J. S.; Rong, X.; Liu, Y.; Kolpak, A. M. Role of Lattice Oxygen Participation in Understanding Trends in the Oxygen Evolution Reaction on Perovskites. ACS Catal. 2018, 8, 4628−4636. (12) Védrine, J. Heterogeneous Catalysis on Metal Oxides. Catalysts 2017, 7 (11), 341. (13) Damjanovic, A.; Jovanovic, B. Anodic Oxide Films as Barriers to Charge Transfer in O 2 Evolution at Pt in Acid Solutions. J. Electrochem. Soc. 1976, 123, 374−381. (14) Bockris, J. O. M.; Otagawa, T. Mechanism of Oxygen Evolution on Perovskites. J. Phys. Chem. 1983, 87, 2960−2971. (15) Wohlfahrt-Mehrens, M.; Heitbaum, J. Oxygen evolution on Ru and RuO2 Electrodes Studied Using Isotope Labelling and On-line Mass Spectrometry. J. Electroanal. Chem. Interfacial Electrochem. 1987, 237, 251−260. (16) Fierro, S.; Nagel, T.; Baltruschat, H.; Comninellis, C. Investigation of the Oxygen Evolution Reaction on Ti/IrO2 Electrodes Using Isotope Labelling and On-line Mass Spectrometry. Electrochem. Commun. 2007, 9, 1969−1974. (17) Macounova, K.; Jirkovsky, J.; Makarova, M. V.; Franc, J.; Krtil, P. Oxygen Evolution on Ru1-xNixO2-y Nanocrystalline Electrodes. J. Solid State Electrochem. 2009, 13, 959−965. (18) Hardin, W. G.; Slanac, D. a.; Wang, X.; Dai, S.; Johnston, K. P.; Stevenson, K. J. Highly Active, Nonprecious Metal Perovskite Electrocatalysts for Bifunctional Metal−Air Battery Electrodes. J. Phys. Chem. Lett. 2013, 4, 1254−1259.

IV. SUMMARY AND OUTLOOK In summary, more and more consciousness of the complexity of the OER is emerging within the scientific community devoted to the development of catalyst materials for water electrolyzers. The occurrence of anionic redox processes, the so-called lattice oxygen evolution reaction (LOER), has been predicted already many decades ago but now more and more experimental evidence and theoretical calculations support this novel reaction pathway. The first step has been taken, but several questions still remain unanswered. First of all, there is the need to understand in detail the LOER reaction pathway, or if there is a single reaction pathway for the LOER. The competition between conventional OER and LOER, that is, between cation and anion redox chemistry on the surface of complex oxides, also represents a great challenge. Finally, it is really important to understand which are the physicochemical properties of the catalysts that promotes the LOER over the conventional OER and if the right strategy is to push catalyst development toward the optimization of the LOER thermodynamics and kinetics. Oxides with a large amount of oxygen vacancies and flexible structures (or poor crystallinity) seem to be the more promising actors to play in the LOER performance, but definitively systematic investigations coupled with the latest advanced operando techniques will play a major role in the proper understanding of the best catalysts for the LOER. Eventually, more insights into the formation of an active oxyhydroxide layer on the surface on metal oxide catalysts will be achieved together with the understanding of the real role of the metal oxide: a simple catalyst precursors or an active player in the OER/LOER under constant operation?

■

Viewpoint

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Emiliana Fabbri: 0000-0002-8627-6926 Thomas J. Schmidt: 0000-0002-1636-367X Notes

The authors declare no competing financial interest. 9772


Viewpoint

ACS Catalysis

for the Oxygen Evolution Reaction. ChemCatChem 2016, 8, 2968− 2974. (35) Calle-Vallejo, F.; Díaz-Morales, O. A.; Kolb, M. J.; Koper, M. T. M. Why Is Bulk Thermochemistry a Good Descriptor for the Electrocatalytic Activity of Transition Metal Oxides? ACS Catal. 2015, 5, 869−873. (36) Calle-Vallejo, F.; Inoglu, N. G.; Su, H. Y.; Martinez, J. I.; Man, I. C.; Koper, M. T. M.; Kitchin, J. R.; Rossmeisl, J. Number of Outer Electrons as Descriptor for Adsorption Processes on Transition Metals and Their Oxides. Chemical Science 2013, 4, 1245−1249. (37) Halck, N. B.; Petrykin, V.; Krtil, P.; Rossmeisl, J. Beyond the Volcano Limitations in Electrocatalysis - Oxygen Evolution Reaction. Phys. Chem. Chem. Phys. 2014, 16, 13682−13688. (38) Abbott, D. F.; Lebedev, D.; Waltar, K.; Povia, M.; Nachtegaal, M.; Fabbri, E.; Copéret, C.; Schmidt, T. J. Iridium Oxide for the Oxygen Evolution Reaction: Correlation between Particle Size, Morphology, and the Surface Hydroxo Layer from Operando XAS. Chem. Mater. 2016, 28, 6591−6604. (39) Diaz-Morales, O.; Raaijman, S.; Kortlever, R.; Kooyman, P. J.; Wezendonk, T.; Gascon, J.; Fu, W. T.; Koper, M. T. M. Iridium-based Double Perovskites for Efficient Water Oxidation in Acid Media. Nat. Commun. 2016, 7, 12363. (40) Seitz, L. C.; Hersbach, T. J. P.; Nordlund, D.; Jaramillo, T. F. Enhancement Effect of Noble Metals on Manganese Oxide for the Oxygen Evolution Reaction. J. Phys. Chem. Lett. 2015, 6, 4178−4183. (41) Bockris, J. O.; Otagawa, T. The Electrocatalysis of Oxygen Evolution on Perovskites. J. Electrochem. Soc. 1984, 131, 290−302. (42) Matsumoto, Y.; Yamada, S.; Nishida, T.; Sato, E. Oxygen Evolution on La1-Xsrxfe1-Ycoyo3 Series Oxides. J. Electrochem. Soc. 1980, 127, 2360−2364. (43) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383− 1385. (44) Fabbri, E.; Nachtegaal, M.; Cheng, X.; Schmidt, T. J. Superior Bifunctional Electrocatalytic Activity of Ba0.5Sr0.5Co0.8Fe0.2O3-δ/ Carbon Composite Electrodes: Insight into the Local Electronic Structure. Adv. Energy Mater. 2015, 5, 1402033. (45) Chang, S. H.; Danilovic, N.; Chang, K.-C.; Subbaraman, R.; Paulikas, A. P.; Fong, D. D.; Highland, M. J.; Baldo, P. M.; Stamenkovic, V. R.; Freeland, J. W.; Eastman, J. A.; Markovic, N. M. Functional Links Between Stability and Reactivity of Strontium Ruthenate Single Crystals During Oxygen Evolution. Nat. Commun. 2014, 5, 4191. (46) Cheng, X.; Fabbri, E.; Yamashita, Y.; Castelli, I.; Kim, B. J.; Uchida, M.; Haumont, R.; Puente Orench, I.; Schmidt, T. J. Oxygen Evolution Reaction on Perovskites: A Multi-Effect Descriptors Study Combining Experimental and Theoretical Methods. ACS Catal. 2018, 8, 9567. (47) Kim, B.-J.; Abbott, D. F.; Cheng, X.; Fabbri, E.; Nachtegaal, M.; Bozza, F.; Castelli, I. E.; Lebedev, D.; Schäublin, R.; Copéret, C.; Graule, T.; Marzari, N.; Schmidt, T. J. Unraveling Thermodynamics, Stability, and Oxygen Evolution Activity of Strontium Ruthenium Perovskite Oxide. ACS Catal. 2017, 7, 3245−3256. (48) Amin, H. M. A.; Baltruschat, H. How Many Surface Atoms in Co3O4 Take Part in Oxygen Evolution? Isotope Labeling Together with Differential Electrochemical Mass Apectrometry. Phys. Chem. Chem. Phys. 2017, 19, 25527−25536. (49) Calvillo, L.; Carraro, F.; Vozniuk, O.; Celorrio, V.; Nodari, L.; Russell, A. E.; Debellis, D.; Fermin, D.; Cavani, F.; Agnoli, S.; Granozzi, G. Insights Into the Durability of Co-Fe Spinel Oxygen Evolution Electrocatalysts Via Operando Studies of the Catalyst Structure. J. Mater. Chem. A 2018, 6, 7034−7041. (50) Favaro, M.; Yang, J.; Nappini, S.; Magnano, E.; Toma, F. M.; Crumlin, E. J.; Yano, J.; Sharp, I. D. Understanding the Oxygen Evolution Reaction Mechanism on CoOx using Operando AmbientPressure X-ray Photoelectron Spectroscopy. J. Am. Chem. Soc. 2017, 139, 8960−8970.

(19) Hardin, W. G.; Mefford, J. T.; Slanac, D. A.; Patel, B. B.; Wang, X. Q.; Dai, S.; Zhao, X.; Ruoff, R. S.; Johnston, K. P.; Stevenson, K. J. Tuning the Electrocatalytic Activity of Perovskites through Active Site Variation and Support Interactions. Chem. Mater. 2014, 26, 3368− 3376. (20) Spöri, C.; Kwan, J. T. H.; Bonakdarpour, A.; Wilkinson, D. P.; Strasser, P. The Stability Challenges of Oxygen Evolving Catalysts: Towards a Common Fundamental Understanding and Mitigation of Catalyst Degradation. Angew. Chem., Int. Ed. 2017, 56, 5994−6021. (21) Fabbri, E.; Nachtegaal, M.; Binninger, T.; Cheng, X.; Kim, B.-J.; Durst, J.; Bozza, F.; Graule, T.; Schäublin, R.; Wiles, L.; Pertoso, M.; Danilovic, N.; Ayers, K. E.; Schmidt, T. J. Dynamic Surface Selfreconstruction Is the Key of Highly Active Perovskite Nanoelectrocatalysts for Water Splitting. Nat. Mater. 2017, 16, 925. (22) Han, B.; Stoerzinger, K. A.; Tileli, V.; Gamalski, A. D.; Stach, E. A.; Shao-Horn, Y. Nanoscale Structural Oscillations in Perovskite Oxides Induced by Oxygen Evolution. Nat. Mater. 2017, 16, 121. (23) Yoo, J. S.; Liu, Y.; Rong, X.; Kolpak, A. M. Electronic Origin and Kinetic Feasibility of the Lattice Oxygen Participation During the Oxygen Evolution Reaction on Perovskites. J. Phys. Chem. Lett. 2018, 9, 1473−1479. (24) Rong, X.; Parolin, J.; Kolpak, A. M. A Fundamental Relationship between Reaction Mechanism and Stability in Metal Oxide Catalysts for Oxygen Evolution. ACS Catal. 2016, 6, 1153− 1158. (25) Reier, T.; Nong, H. N.; Teschner, D.; Schlögl, R.; Strasser, P. Electrocatalytic Oxygen Evolution Reaction in Acidic Environments − Reaction Mechanisms and Catalysts. Adv. Energy Mater. 2017, 7, 1601275. (26) Chang, S. H.; Connell, J. G.; Danilovic, N.; Subbaraman, R.; Chang, K.-C.; Stamenkovic, V. R.; Markovic, N. M. Activity-stability Relationship in the Surface Electrochemistry of the Oxygen Evolution Reaction. Faraday Discuss. 2014, 176, 125−133. (27) Reier, T.; Pawolek, Z.; Cherevko, S.; Bruns, M.; Jones, T.; Teschner, D.; Selve, S.; Bergmann, A.; Nong, H. N.; Schlogl, R.; Mayrhofer, K. J. J.; Strasser, P. Molecular Insight in Structure and Activity of Highly Efficient, Low-Ir Ir-Ni Oxide Catalysts for Electrochemical Water Splitting (OER). J. Am. Chem. Soc. 2015, 137, 13031−13040. (28) Cherevko, S.; Zeradjanin, A. R.; Topalov, A. A.; Kulyk, N.; Katsounaros, I.; Mayrhofer, K. J. J. Dissolution of Noble Metals during Oxygen Evolution in Acidic Media. ChemCatChem 2014, 6, 2219−2223. (29) Risch, M.; Grimaud, A.; May, K. J.; Stoerzinger, K. A.; Chen, T. J.; Mansour, A. N.; Shao-Horn, Y. Structural Changes of Cobalt-Based Perovskites upon Water Oxidation Investigated by EXAFS. J. Phys. Chem. C 2013, 117, 8628−8635. (30) May, K. J.; Carlton, C. E.; Stoerzinger, K. A.; Risch, M.; Suntivich, J.; Lee, Y. L.; Grimaud, A.; Shao-Horn, Y. Influence of Oxygen Evolution During Water Oxidation on the Surface of Perovskite Oxide Catalysts. J. Phys. Chem. Lett. 2012, 3, 3264−3270. (31) Grimaud, A.; Carlton, C. E.; Risch, M.; Hong, W. T.; May, K. J.; Shao-Horn, Y. Oxygen Evolution Activity and Stability of Ba6Mn5O16, Sr4Mn2CoO9, and Sr6Co5O15: The Influence of Transition Metal Coordination. J. Phys. Chem. C 2013, 117, 25926− 25932. (32) Grimaud, A.; May, K. J.; Carlton, C. E.; Lee, Y. L.; Risch, M.; Hong, W. T.; Zhou, J. G.; Shao-Horn, Y. Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution. Nat. Commun. 2013, 4, Article No. 2439 DOI: 10.1038/ ncomms3439. (33) Cheng, X.; Fabbri, E.; Nachtegaal, M.; Castelli, I. E.; El Kazzi, M.; Haumont, R.; Marzari, N.; Schmidt, T. J. Oxygen Evolution Reaction on La1−xSrxCoO3 Perovskites: A Combined Experimental and Theoretical Study of Their Structural, Electronic, and Electrochemical Properties. Chem. Mater. 2015, 27, 7662−7672. (34) Lim, T.; Niemantsverdriet, J. W.; Gracia, J. Layered Antiferromagnetic Ordering in the Most Active Perovskite Catalysts 9773


Viewpoint

ACS Catalysis (51) Shao, Z.; Haile, S. M. A High-performance Cathode for the Next Generation of Solid-oxide Fuel Cells. Nature 2004, 431, 170. (52) Tahini, H. A.; Tan, X.; Schwingenschlogl, U.; Smith, S. C. In Operando Self-Healing of Perovskite Electrocatalysts: A Case Study of SrCoO3 for the Oxygen Evolution Reaction. Particle & Particle Systems Characterization 2017, 34, 1600280. (53) Grenier, J.-C.; Pouchard, M.; Wattiaux, A. Electrochemical Synthesis: Oxygen Intercalation. Curr. Opin. Solid State Mater. Sci. 1996, 1, 233−240. (54) Grenier, J. C.; Wattiaux, A.; Doumerc, J. P.; Dordor, P.; Fournes, L.; Chaminade, J. P.; Pouchard, M. Electrochemical Oxygen Intercalation Into Oxide Networks. J. Solid State Chem. 1992, 96, 20− 30. (55) Stoerzinger, K. A.; Diaz-Morales, O.; Kolb, M.; Rao, R. R.; Frydendal, R.; Qiao, L.; Wang, X. R.; Halck, N. B.; Rossmeisl, J.; Hansen, H. A.; Vegge, T.; Stephens, I. E. L.; Koper, M. T. M.; ShaoHorn, Y. Orientation-Dependent Oxygen Evolution on RuO2 without Lattice Exchange. ACS Energy Letters 2017, 2, 876−881. (56) Tripkovic, V.; Hansen, H. A.; Garcia-Lastra, J. M.; Vegge, T. Comparative DFT+U and HSE Study of the Oxygen Evolution Electrocatalysis on Perovskite Oxides. J. Phys. Chem. C 2018, 122, 1135−1147. (57) Fabbri, E; Bi, L.; Pergolesi, D.; Traversa, E. Towards the Next Generation of Solid Oxide Fuel Cells Operating Below 600 °C with Chemically Stable Proton-Conducting Electrolytes. Adv. Mater. 2012, 24, 195−208. (58) Fabbri, E.; Pergolesi, D.; Traversa, E. Materials Challenges Toward Proton-conducting Oxide Fuel Cells: A Critical Review. Chem. Soc. Rev. 2010, 39, 4355−4369. (59) Pergolesi, D.; Fabbri, E.; D’Epifanio, A.; Di Bartolomeo, E.; Tebano, A.; Sanna, S.; Licoccia, S.; Balestrino, G.; Traversa, E. High Proton Conduction in Grain-boundary-free Yttrium-doped Barium Zirconate Films Grown by Pulsed Laser Deposition. Nat. Mater. 2010, 9, 846.

9774


Oxygen Evolution ReactionâThe Enigma in Water Electrolysis - ACS

Recommend Documents

Oxygen Evolution ReactionâThe Enigma in Water Electrolysis - ACS