Bismuth Substituted Strontium Cobalt Perovskites for Catalyzing Oxygen Evolution

Abstract

Late first-row transition metal oxides, based on cobalt, nickel and iron [1] are reported to be the most active non-precious catalysts for oxygen evolution reaction (OER) in basic solution. Experimental and computational studies in the past decade have been focusing on elucidating OER mechanisms [2-3] and identifying activity and stability descriptors [3-6], where perovskites family (ABO3-δ) with immense structural, chemical and electronic flexibility associated with vast selections of A-site and B-site metal ions and oxygen deficiency [7] has been used to develop design principles of OER activity and stability. Recent works [6-8] have shown that lowering charge-transfer gap or increasing metal-oxygen covalency in perovskites can enhance the OER kinetics, by reducing the energetic barriers associated with electron transfer on the surface of metal oxides including the most active catalysts. However, reducing the charge-transfer gap 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 and rendering weaker surface hydroxide affinity [6]. Consequently, the OER kinetics on highly covalent/active metal oxides are limited by proton transfer. Moreover, increasing the covalency typically moves the O 2p band closer to the Fermi level, rendering less energy penalty for the creation of oxygen vacancies and thus leading to surface or bulk instability at OER potentials [5]. Here 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 [9], and tune the surface acid-base chemistry by introducing highly Lewis acidic A-site ions to facilitate OER kinetics. Bismuth-substituted strontium cobalt perovskite, Bi0.2Sr0.8CoO3-δ, was shown to exhibit record OER specific activity in alkaline solution, exceeding those of other Co-based perovskite oxides reported to date, including SrCoO3-δ [8], at high current densities (> 1 mA cm-2 oxide). In addition, neither structural or chemical changes have been found for Bi0.2Sr0.8CoO3-δ, indicative of greater structural stability than other highly covalent oxide catalysts, e.g. Ba0.5Sr0.5Co0.8Fe0.2O3-δ [4]. The enhanced OER kinetics and high surface stability can be attributed to the stronger affinity towards hydroxide ions to facilitate surface deprotonation due to the presence of strong Lewis acidic surface Bi3+ ions, and the lowered O 2p-band center relative to the Fermi level upon bismuth substitution into the perovskite structure, respectively. This work exemplifies a novel strategy to facilitate the OER kinetics of highly active oxide catalysts by leveraging the inductive effect associated with rational metal substitution to maintain high metal-oxygen covalency and strengthen hydroxide affinity without the expense of surface stability.