Bismuth substituted strontium cobalt perovskites for catalyzing oxygen evolution
In this study, we employ the strategy of substitution with more electronegative/acidic Asite ions in the cobalt perovskites to alter O 2p-band center, surface hydroxide affinity, and oxygen evolution reaction (OER) activity and stability in the basic electrolyte.Galvanostatically charged Bi0.2Sr0.8CoO3-δ (δ close to zero) was shown to exhibit record OER specific activity exceeding not only LaxSr1-xCoO3-δ but also charged SrCoO3-δ (δ close to zero), one of the most active oxide OER catalysts reported so far. The enhanced OER activity of charged Bi0.2Sr0.8CoO3-δ can be attributed to greater hydroxide affinity facilitating the deprotonation of surface bound intermediates due to the presence of strong Lewis acidic A-site Bi 3+ ions, while the high stability can result from lowered O 2p-band center relative to the Fermi level. This work provides a novel example in the rational design of highly active oxide catalysts for OER by leveraging the inductive effect.
INTRODUCTION<!>RESULTS AND DISCUSSION<!>EXPERIMENTAL PROCEDURES<!>SUPPLEMENTAL INFORMATION
<p>The distribution and on-demand use of electrical energy from sustainable resources, such as solar energy, requires storage technologies that are cost effective and involving earth-abundant elements. Solar energy can be stored in the form of chemical bonds by electrochemical water splitting to produce hydrogen, 1,2 or CO2 reduction to CO, methanol and hydrocarbons 3,4 as energy carriers, where oxygen evolution reaction (OER) is required to accompany above cathodic transformations. These technologies are advantageous to lithium ion batteries for large-scale storage as they have high gravimetric energy, 5,6 and chemical bonds used for energy storage involve elements (such as O, H, C) that are among the most abundant in the planet. In contrast, energy storage with current Li-ion batteries requires storing each electron using one transition metal ion, and thus powering the planet with this technology is limited by the availability of metals such as cobalt and nickel in the earth crust. 7 The efficiency of the storage technologies based on H2/CO/CH3OH/CnH2n+2 production is, however, severely limited to a significant extent by the sluggish reaction kinetics of OER, [8][9][10] which is catalyzed by precious metal catalysts such as RuO2 11,12 and IrO2. 11,13,14 The development of active OER electrocatalysts composed of earth-abundant elements 15 is therefore crucial for the large-scale implementation of these technologies. 16 Late first-row transition metal oxides [17][18][19][20][21][22] based on cobalt, 23,24 nickel [25][26][27][28] and iron 29,30 are reported to be the most active to catalyze OER in basic solution. Experimental and computational studies in the past decade have been focusing on elucidating OER mechanisms 8,9 and identifying activity and stability descriptors, [31][32][33] which can be used to design better catalysts. Perovskites (ABO3-δ) with immense structural, chemical and electronic flexibility associated with vast selections of A-site and B-site metal ions and oxygen deficiency 34 have been used to develop design principles of OER activity 33,34 and stability. 35,36 Recent work 37 has shown that lowering charge-transfer gap or increasing metal-oxygen covalency in perovskites can improve the OER kinetics, which is faciliated by first lowering the energetic barrier of electron transfer for the semiconducting oxides, reducing adsorption strength of oxygenated intermediates of OER 38 and then decreasing the energetic barriers 34,37 associated with OER steps on the surface of metallic oxides including the most active catalysts. Unfortunately, reducing the charge-transfer gap of these perovskites also lowers the Fermi level on the absolute energy scale, making it below the OER redox potential in the basic solution for the most active catalysts such as La0.5Sr0.5CoO3-δ. 37 This can generate more negatively charged surface and therefore result in weaker hydroxide affinity, 17,39,40 which can impede surface deprotonation during OER in basic solution (such as M-OHad + OH -→ M-Oad + H2O + e -). 37 Therefore, tuning the affinity towards hydroxide ions on these highly active oxide surfaces (e.g. by phosphate functionalization) 41 represents new opportunities to further enhance the OER performance.</p><p>In this work, we explored the substitution of A-site ions with high electronegativity or Lewis acidity in the cobalt perovskites to maintain high Co-O covalency by the inductive effect, 28 and potentially increase hydroxide affinity on the surface by introducing highly Lewis acidic ions to facilitate OER kinetics. This argument is supported by the enhanced OER kinetics previously observed for LnNiO3 perovskite series upon the substitution of La 3+ with more electronegative Gd 3+ and Eu 3+ . 42 Herein we examine the OER activity of bismuth-substituted strontium cobalt perovskites, Bi0.2Sr0.8CoO3-δ, where the Bi 3+ substituents possess the highest Lewis acidity (i.e. highest electronegativity) among 2+/3+ A-site ions, 43 with a pKa value of 1.58 much lower than that of other commonly used Asite metals in cobalt-based perovskite oxides (e.g. pKa([Sr 2+ (H2O)n]) = 13.18, pKa([La 3+ (H2O)n]) = 9.06, pKa([Pr 3+ (H2O)n]) = 8.55). Remarkably, Bi0.2Sr0.8CoO3-δ that was galvanostatically charged to minimize oxygen vacancies was shown to exhibit an exceptionally low Tafel slope (~25 mV decade -1 ) and higher instrinic OER activity than La1-xSrxCoO3-δ and charged SrCoO3-δ (one of the most active catalysts reported to date) 38,44 at high current densities (> ~1 mA cm -2 oxide). The record intrinsic OER activity can be rationalized by stronger affinity towards hydroxide during surface deprotonation associated with more acidic Bi 3+ than La 3+ and Sr 2+ , and comparable Co-O covalency to SrCoO3-δ as shown from density functional theory (DFT) calculations. In addition, no amorphization or changes in the composition were observed for the surface of charged Bi0.2Sr0.8CoO3-δ after OER, with such high structural stability being attributed to the lowered O 2p-band center relative to the Fermi level of Bi0.2Sr0.8CoO3-δ than that of SrCoO3-δ. Tuning of the electronic structure and surface acid-base chemistry through inductive effect associated with rational metal substitution represents a new strategy for enhancing the activity and stability of OER catalysts.</p><!><p>Pristine Bi0.2Sr0.8CoO3-δ was synthesized through a conventional solid-state route starting from Bi2O3, CoO and SrCO3 precursors 45 as detailed in Supplemental Experimental Procedure. Higher levels of bismith subsitution were not explored due to previously reported solubility limit of ~20% 45 and the need to have high Co oxidation state (close to 4+) to achieve high OER activities. 46 Refinement of the powder X-ray diffraction (PXRD) pattern (Figure 1) confirmed a cubic structure with Pm-3m space group and a unit cell parameter of 3.896 Å (Table S1), which is comparable to that reported in the original study (~3.9 Å). 45 As previous neutron diffraction refinements and cobalt oxidation state (~3) based on iodometric titrations 45 revealed the oxygen vacancy content, δ ≈ 0.4, for pristine Bi0.2Sr0.8CoO3-δ, we estimated δ ≈ 0.4 for the oxygen deficiency for our assynthesized pristine Bi0.2Sr0.8CoO3-δ sample. This argument is further supported by the Co K-edge X-ray absorption spectroscopy (XAS). The cobalt oxidation state of ~2.96 for pristine Bi0.2Sr0.8CoO3-δ determined from the position of Co K-edge (Figure S1) is in qualitative agreement with the estimated oxygen vacancy level (δ ≈ 0.4).</p><p>We further reduced the oxygen deficiency in Bi0.2Sr0.8CoO3-δ by galvanostatic charging in O2-saturated 1 M KOH (Figure S2), as having high covalency for Co-O bonds associated with the presence of Co 4+ is needed to obtain high intrinsic OER activity. 37,38 Electrochemical oxygen intercalation into the perovskite structure can take place during galvanostatic charging to fill the oxygen vacancies in oxygen-deficient perovskites, 47,48 such as SrCoO3-δ, 38,49 upon slow oxidation in basic solutions. During charging, two voltage plateaus were observed for thin-film Bi0.2Sr0.8CoO3-δ deposited on glassy carbon (GC) electrodes, similar to those reported for La0.5Sr0.5CoO3-δ 38 and SrCoO3-δ 38,47 (plateaus more defined in the latter case, Figure S3), which can be attributed to the filling of oxygen vacancies (Bi0.2Sr0.8CoO3-δ + 2δOH -→ Bi0.2Sr0.8CoO3 + δH2O + 2δe -). These redox events were reflected by the two redox peaks centered at ~1.1 and ~1.3 V vs. reversible hydrogen electrode (RHE) in the cyclic voltammetry (CV) measurements (Figure 2). Similar redox features have been reported for other Co-based catalysts such as CoOOH (in 1 M KOH), 23 and LiCoO2 (in 0.1 M KOH), 50,51 which have been attributed to Co 2+ /Co 3+ and Co 3+ /Co 4+ redox couples, respectively. The oxygen intercalation into the bulk structure of Bi0.2Sr0.8CoO3-δ upon charging is supported by the reduction in the lattice parameter of the perovskite structure for Bi0.2Sr0.8CoO3-δ pellet electrodes (without carbon and binder) after galvanostatic charging as revealed by PXRD analysis (Figure 1), indicative of increased Co oxidation state associated with oxygen deficiency reduction. Charged Bi0.2Sr0.8CoO3-δ (with one electron passed per formula unit at a C-rate of C/50 or C/20, Figure S2) was found to have smaller unit cell parameters a = 3.835 Å (C/50 charging) and a = 3.838 Å (C/20 charging) than that of pristine Bi0.2Sr0.8CoO3-δ (a = 3.896 Å). Although the exact value of oxygen deficiency δ on the surface of charged Bi0.2Sr0.8CoO3-δ during OER is not known, it is proposed that δ value is close to zero, especially for the C/50 sample. This hypothesis is supported by the following observations. First, ~0.8 eper formula unit passed before the OER plateau at 1.4 V vs. RHE 38 (Figure S3), which would result in a fully stoichiometric perovskite structure based on the oxygen deficiency level in pristine Bi0.2Sr0.8CoO3-δ (δ ≈ 0.4). Second, the slower galvanostatic charging typically imparts smaller oxygen deficiency in Bi0.2Sr0.8CoO3-δ and other perovskites. 38 As the oxygen intercalation kinetics and oxygen ion transport in peroskites are slow, the surfaces of charged Bi0.2Sr0.8CoO3-δ can have even smaller δ than the bulk for given charging rates.</p><p>Third, C/50 galvanostatically charged Bi0.2Sr0.8CoO3-δ was found to have much greater OER activity than pristine Bi0.2Sr0.8CoO3-δ (Figure S3). This enhanced OER kinetics can be attributed to greater Co oxidation state on the surface, which is expected from the previously reported correlations 32,37,38,44,49 demonstrating that lowering the cobalt 3d band with respect to the oxygen 2p-band center (i.e. lowering the charge-transfer gap) by increasing the nominal oxidation state of cobalt is correlated with increasing specific OER activity of cobalt-based perovskites. Therefore, we will focus on the OER kinetics of C/50 charged Bi0.2Sr0.8CoO3-δ further in this work. galvanostatically charged at 6.1 mA g -1 for 20 hr (C/20), and at 2.4 mA g -1 for 50 hr (C/50). Patterns were indexed in a cubic unit cell with space group Pm-3m. All spectra were collected for powder samples (~0.3 g), in a continuous scan mode with a scan rate of 2 degrees per min, and a scan range from 15 ° to 65 °. Lattice parameters as assessed by PXRD are consistent with those reported previously. 45 The samples were quenched to room temperature after every heat treatment during the solid-state synthesis to minimize the presence of the minor secondary phase of Bi 2 Sr 2 Co 3 O y , 45,52 which could be detected by peaks between 24 ° and 30 ° on the PXRD spectra (with estimated volume fraction of less than 1.7%). Galvanostatic charging experiments were performed for pelletized Bi 0.2 Sr 0.8 CoO 3-δ powder (60 mg, 5 mm diameter pellet) attached to a graphene sheet  The OER kinetics of charged Bi0.2Sr0.8CoO3-δ (thin films on GC, C/50 charging in O2saturated 1 M KOH, Figure S3) were assessed in O2-saturated KOH solutions at different pHs through steady-state galvanostatic measurements (i.e. chronopotentiometry) instead of CV, as the contributions of current from electrochemical oxygen intercalation in bulk cannot be excluded from OER kinetic currents when using the CV measurements. 38 The specific activity was obtained by normalizing iR-corrected kinetic currents to the Brunauer-Emmett-Teller (BET) specific oxide surface area. The specific OER activity of charged Bi0.2Sr0.8CoO3-δ is much higher than those of LaCoO3, charged La0.5Sr0.5CoO3-δ and Pr0.5Ba0.5CoO3-δ 38 in both 0.1 M and 1 M KOH (Figures 3A and S4), which can be attributed to greater Co-O covalency associated with higher Co oxidation state 32,34,37,38,44 expected for charged Bi0.2Sr0.8CoO3-δ (δ close to zero for all these charged perovskites).</p><p>Remarkably, the specific OER activity of charged Bi0.2Sr0.8CoO3-δ was found to be comparable to charged SrCoO3-δ at low current densities, and much greater at high current densities due to an exceptionally low Tafel slope of ~25 mV decade -1 as compared to other oxide catalysts 20,25,33 (e.g. ~60 mV decade -1 for charged SrCoO3-δ). 38 Moreover, charged Bi0.2Sr0.8CoO3-δ exhibited pH-dependent OER activity on the RHE scale, where the specific activity increased with increasing pH (Figures 3B and S6). Similar pH-dependent OER kinetics have been reported for charged La0.5Sr0.5CoO3-δ, Pr0.5Ba0.5CoO3-δ and SrCoO3-δ, 38 which cannot be explained by the conventional OER mechanism 8,9,53,54 with four concerted proton-coupled electron transfer (PCET) steps. Instead, OER kinectics on these highly active and covalent oxides are limited by proton transfer, which is decoupled from electron transfer. 37 In contrast, less active LaCoO3 38 exhibits pH-independent OER activity on the RHE scale expected from the conventional concerted PCET pathways. 37 Notably, charged Bi0.2Sr0.8CoO3-δ exhibits OER currents one order of magnitude higher than those of fully charged SrCoO3-δ at 1.55 V vs. RHE for all pHs examined (Figure 3B). Possible physical origins for the record high OER activity are discussed below together with processes responsible for its pH-dependent OER activity on the RHE scale. The high OER activity of charged Bi0.2Sr0.8CoO3-δ exceeding that of charged SrCoO3δ can be attributed in part to having comparable Co-O covalency to SrCoO3-δ due to the inductive effect 28 introduced by substituting Sr 2+ with strongly Lewis acidic Bi 3+ (pKa for [Bi(H2O)n] 3+ ions is 1.58). 43 Despite the fact that having lower nominal Co oxidation state in charged Bi0.2Sr0.8CoO3-δ (3.8+) than charged SrCoO3-δ (4+) is expected to impart lower OER activity than SrCoO3-δ based on previous reported correlation, 32,37,38,44 Bi 3+ ions with much higher electronegativity can pull more electron density from cobalt than other divalent/trivalent A-site ions with lower electronegatively (e.g., Sr 2+ , La 3+ ), lowering the energy of cobalt 3d band (i.e. antibonding orbitals of cobalt-ligand bonds) 28 and thus leading to retained high Co-O covalency upon bismuth substitution (Figures 4A and 4B). This argument is supported by our computed density of states for fully stoichiometric Bi0.125Sr0.875CoO3 and Bi0.25Sr0.75CoO3 (Figure S9), where the energy difference between the Co 3d-band center and the O 2p-band center, a measure of the covalency for the cobaltoxygen bonds, 37,38 remains unchanged relative to stoichiometric SrCoO3 (Figure 4B).</p><p>Having high metal-ligand covalency for perovskite oxides is key to promote OER kinetics by enabling the surface deprotonation of reaction intermediates by hydroxide ions decoupled from electron transfer which limits the OER kinetics. 37 Moreover, the lattice oxygen sites of oxides on the surface can be possibly activated and participate in OER in addition to surface metal ions. 38 Fermi level. This trend can be rationalized by the energetics associated with the creation of oxygen vacancies in the perovskite structure (Figure S15). The calculation results of LaCoO 3 , La 0.5 Sr 0.5 CoO 3 , Pr 0.5 Ba 0.5 CoO 3 , SrCoO 3 (E vac only), and Ba 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 have been reported in our previous work. 38 The calculation results of La 0.75 Sr 0.25 CoO 3 have been reported in our previous work. 72 The oxygen vacancy formation energy for fully stoichiometric Bi 0.125 Sr 0.875 CoO 3 and Bi 0.25 Sr 0.75 CoO 3 was further projected following the linear correlation and with the computed oxygen 2p-band centers of these two compounds.</p><p>Employing Bi 3+ with high electronegativity or Lewis acidity can increase hydroxide affinity on the oxide surface, which can facilitate OER kinetics as the reaction kinetics on highly covalent oxides are limited by processes associated with the deprotonation of surface reaction intermediates. 37 Like other perovskites with high Co-O covalency, 37 charged Bi0.2Sr0.8CoO3-δ has the oxide Fermi level much lower than the thermodynamic OER redox potential energy in the basic solution (Figures 5A and 5B), which results in negatively charged oxide surfaces when equilibrated with the electrolyte. This negative charge on the surface of oxides can come from surface adsorption of hydroxide ions from the solution and/or redistribution of the electron density from the bulk to the surface. Specifically, high Co-O covalency and lowered charge-transfer gap increase bulk electron mobility leading to accumulation of the negative charges on the interface with electrolyte solution, making this mechanism favored over hydroxide adsorption, 37,39,40 thus imparting weaker hydroxide affinity to the oxide surface under equilibrium conditions. The presence of strongly electronegative Bi 3+ at the oxide surface of charged Bi0.2Sr0.8CoO3-δ can enhance affinity towards hydroxide ions as compared to SrCoO3-δ, increase ionicity of the O-H bonds, and therefore facilitate chemical/electrochemical surface deprotonation during OER in basic solution (such as M-OHad + OH -→ M-Oad + H2O + e -). 37 This argument is in agreement with the observed pH-dependent OER activity of charged Bi0.2Sr0.8CoO3-δ on the RHE scale (Figures 3B and S6), which can be rationalized by pathways involving acidbase equilibrium before the rate-limiting step, 55 and/or rate-limiting proton transfer decoupled from electron transfer. 37,38,56 Similar pH-dependent OER activity has been reported for Ni-Fe oxyhydroxides, which has been attributed to the non-concerted metal oxidation coupled with surface hydroxides deprotonation leading to formation of negatively charged intermediates. 57 Here we argue that increasing pH (i.e. increasing hydroxide ion concentration in the solution) would promote the deprotonation of surface species like M-OHad by OHvia mechanisms including either acid-base equilibrium before the rate-limiting step 55 or proton transfer decoupled from electron transfer as the ratelimiting step, 37 and/or electrochemical surface depronationation with non-integer electron transfer, [58][59][60] which would lead to increased OER activity on the RHE scale with increasing pH. Further support came from the A-site-independent OER kinetics for those less covalent LnCoO3 perovskites (Ln = La, Nd, Gd, Sm, Eu), where the OER kinetics are limited by electron transfer, 37 and having more Lewis acidic A-site ions did not induce any changes in the OER activity beyond experimental uncertainty (Figure S4). OHin the solution to form M-Oad species. 61,62 Similar shifts for pre-OER redox peaks have been found for Ni-Fe oxyhydroxides (up to -60 mV pH -1 ), 57 RuO2 (101), ( 001), ( 001) surfaces (up to -27 mV pH -1 ) 63 and IrO2 (110) surface (~-7.5 mV pH -1 ), 64 which have been attributed to the oxide-water interface structure changes at different pHs 65 or non-integer number of electrons transferred during this step. [58][59][60] Here we relate this pH-dependent redox peak shift to the energetics of hydroxide affinity established recently. 37 With decreasing pH, the electron energy associated with OER redox decreases on the absolute energy scale and moves closer to the oxide Fermi level (Figure 5A), 37 which reduces the accumulation of negative charges on the oxide surface and results in stronger hydroxide affinity and more effective surface deprotonation associated with Co 3+ /Co 4+ redox (Figure 5B), in agreement with the negative redox peak shift with increasing pH (~-40 mV pH -1 ).</p><p>Lastly, the Tafel slope (~25 mV decade -1 ) for charged Bi0.2Sr0.8CoO3-δ is considerably lower than the Tafel slopes of other highly covalent Co-based perovskites such as charged La0.5Sr0.5CoO3-δ, Pr0.5Ba0.5CoO3-δ and SrCoO3-δ (~60 mV decade -1 ), 38 but comparable to those of perovskites that are known to amorphize upon leaching out A-site ions during OER (~40 mV decade -1 ) 66 and Ni-Fe oxyhydroxides (~30 mV decade -1 ). 67 Such a low Tafel slope has been attributed previously 68 to having a chemical deprotonation as the rate-limiting step for catalysts with high surface coverage of surface hydroxide (M-OHad).</p><p>Further in situ electrochemical diffraction 69 and spectroscopic 70,71 studies are needed to provide detailed insights into the surface species and reaction mechanisms as a function of potential. Neither structural transformation (e.g. amorphization) nor compositional changes (e.g. metal leaching) were detected for Bi0.2Sr0.8CoO3-δ after galvanostatic charging and OER.</p><p>We performed high-resolution transmission electron microscopy (HRTEM) imaging and energy-dispersive X-ray (EDX) elemental analysis (Figures 6 and S12) of Bi0.2Sr0.8CoO3-δ before and after galvanostatic charging (to ~1.4 V vs. RHE at C/50, Figure S3) and OER measurements. No surface amorphization of charged Bi0.2Sr0.8CoO3-δ after OER was detected from HRTEM analysis (Figure 6B), Raman spectrsocopy (Figure S13) and O Kedge XAS spectra (Figure S14), where the surface remained crystalline like the pristine Bi0.2Sr0.8CoO3-δ powder (Figure 6A). Such high surface stability of charged Bi0.2Sr0.8CoO3δ after OER is in contrast to the rapid surface amorphization and the formation of surface hydroxides comprised of edge-sharing octahedra clusters found for Ba0.5Sr0.5Co0.8Fe0.2O3δ and SrCo0.8Fe0.2O3-δ at OER potentials. 66 In addition, Bi0.2Sr0.8CoO3-δ after galvanostic charging and OER appeared to be more stable than SrCoO3-δ as few nanometers of amorphous layers were detected on SrCoO3-δ particles after charging and OER meaurements as reported recently. 38 Moreover, EDX elemental analysis (Figures 6C, 6D and S12) confirmed no A-site or B-site metal leaching after charging and OER, in contrast to the notable A-site metal leaching found in Ba0.5Sr0.5Co0.8Fe0.2O3-δ, 66 SrCo0.8Fe0.2O3-δ, 66 and SrxCa1-xFeO3-δ (x = 0, 0.5 or 1). 49 Both bulk and surface metal ratios between A-site (Bi and Sr) and B-site (Co) metals of Bi0.2Sr0.8CoO3-δ before and after charging and OER measurements were found to be close to 1:4:5 as expected from the nominal stoichiometry.</p><p>The high surface stability of charged Bi0.2Sr0.8CoO3-δ at OER potentials can be attributed to its lowered O 2p-band center with respect to the Fermi level (Figure 4C) in comparison to other highly active Co-based and Fe-based perovskites (such as SrCoO3-δ and Ba0.5Sr0.5Co0.8Fe0.2O3−δ). The computed oxygen 2p-band centers of Bi0.125Sr0.875CoO3 and Bi0.25Sr0.75CoO3 were moved down by ~0.10 eV and ~0.25 eV relative to the Fermi level when compared with stoichiometric SrCoO3 (Figures 4B and 4C), respectively, which can be attributed to the reduction of the nominal cobalt oxidation state (e.g. from +4 in SrCoO3 to +3.875 in Bi0.125Sr0.875CoO3 and +3.75 in Bi0.25Sr0.75CoO3) and thus the increase of cobalt 3d-band filling and the Fermi level (Figure 4A). Given that the O 2p-band center relative to the Fermi level can correlated to the formation enthalpy of oxygen vacancies in perovskite oxides (Figure 4C), 72 lower O 2p-band centers of Bi0.125Sr0.875CoO3 and Bi0.25Sr0.75CoO3 relative to the Fermi level would have higher energy penalty for the creation of oxygen vacancies in the perovskite structure than that of SrCoO3, indicative of greater structural stability (Figure S15).</p><p>In this study, we report the record intrinsic OER activity and high surface stability of the fully charged Bi0.2Sr0.8CoO3-δ (δ ≈ 0) perovskite oxide in alkaline electrolyte. We show that the inductive effect associated with rational bismuth substitution results in marked increase of the intrinsic OER catalytic activity, exceeding those of other Co-based perovskite oxides reported to date, including charged SrCoO3-δ, at high current densities (> ~1 mA cm -2 oxide). Although Bi 3+ subtitution (i.e. replacing Sr 2+ with Bi 3+ ) reduced the cobalt oxidation state and thus lowered the oxygen 2p band relative to the Fermi level, DFT computations show that high Co-O covalency maintained due to the inductive effect from Bi 3+ and is comparable with charged SrCoO3-δ. As the OER kinetics of highly covalent oxides are limited by the chemical or electrochemical deprotonation at the oxide surface due to weak hydroxide affinity, the high OER activity of charged Bi0.2Sr0.8CoO3-δ can be attributed to the enhanced affinity towards hydroxide ions due to the presence of strongly Lewis acidic A-site Bi 3+ ions on the perovskite surface. In addition, the observed pH dependence of the pre-OER redox peaks and OER activity on the RHE scale implies nonconcerted proton-electron transfer pathways and pH-dependent binding of intermediates, however further in-situ diffraction and spectroscopic studies are needed to provide further detailed insights into the surface species and rate-limiting step. Moreover, neither structural or chemical changes have been found for Bi0.2Sr0.8CoO3-δ after galvanostatic charging and OER, and the high surface stability can be attributed to higher energy penalty for the creation of oxygen vacancies due to the lower oxygen 2p band relative to the Fermi level.</p><p>Demonstrating how leveraging the inductive effect associated with metal substitution enhances the specific activity of highly OER-active cobalt-based perovskites through maintaining high metal-oxygen covalency and strengthening hydroxide affinity without the expense of surface stability, this work highlights new prospects for the rational design of novel catalysts for electrochemical energy coversion and storage.</p><!><p>For full details please refer to Supplemental Experimental Procedure.</p><!><p>Supplemental Information includes Supplemental Experimental Procedures, 16 figures, and 2 tables and can be found with this article online. Experimental Facilities at MIT, supported by the National Science Foundation under award number DMR-1419807.</p>