Investigation of the Role of Sr and Development of Superior Sr- Doped Hexagonal BaCoO 3−δ Perovskite Bifunctional OER/ORR Catalysts in Alkaline Media Rakesh Mondal, Himanshu Ratnawat, Soham Mukherjee, Asha Gupta, and Preetam Singh* Cite This: Energy Fuels 2022, 36, 3219-3228 Read Online ACCESS Metrics & More Article Recommendations ABSTRACT: Superior electrocatalytic activity of catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) enhances the reversible energy storage efficiency of metal− air batteries and electrochemical water splitting performances to produce hydrogen. Sr incorporation in the BaCoO 3−δ lattice in the form of 2H-type Ba 1−x Sr x CoO 3−δ (0 ≤ x ≤ 0.5) perovskites enhances both ORR and OER activities. A relatively low overpotential of 395 mV at 10 mA/cm 2 , lower Tafel slope of 64.95 mV dec −1 , and good stability up to 500 cycles (10% reduction of current density and overpotential shift to a 0.04 V higher value) in a 0.1 M KOH electrolyte were obtained for the Ba 0.5 Sr 0.5 CoO 3−δ electrode. Incorporation of Sr in the BaCoO 3−δ lattice decreases the Co−O−Co bond angle that results in a superior orbital overlap between Co(3d) and O(2p) orbitals and a decrease in lattice parameters that generates lower surface oxygen separation pathways and a large number of active sites on the (011) planes, making Ba 0.5 Sr 0.5 CoO 3−δ a superior catalyst with increased OER/ORR activity. The formation of oxygen-vacant CoO 5 octahedra containing surface oxygen vacancies, the presence of Co 3+/4+ valence states, and the superior overlap between O(2p)- Co(3d) bands (covalency increases) result in a higher electronic conductivity, a lower flat band potential, and improved OER and ORR activities. The key highlight of this work is the matching of the onset potential with the calculated flat band (E fb ) potential from the Mott−Schottky plot. The Mott−Schottky plot was utilized to calculate the flat band potential (E fb ) that indicates the basic information about the electrochemical interface potential between the electrode and the electrolyte, and in the case of Ba 0.5 Sr 0.5 CoO 3−δ , it matches very well with the onset potential for the OER activity of the catalyst. ■ INTRODUCTION Without coupling to a reliable energy storage system with renewable energy solutions such as solar, wind, and tidal energy, the global energy demand cannot be fulfilled. Oxygen reduction and evolution reactions (ORR and OER) are cornerstones for renewable energy generation devices, high- temperature fuel cells, low-temperature metal−air batteries, and water splitting systems to produce hydrogen. 1−11 Well- known noble metal oxide-based ORR/OER catalysts such as IrO 2 and RuO 2 are costly, and continuous performance decay restricts their large-scale commercial application. 12 Recently, low-cost, earth-abundant transition metal oxides gained greater interest to be examined as oxygen electrocatalysts for energy conversion and storage devices. 13−22 Perovskites are of great importance because they exhibit greater cation ordering and order channels of oxygen vacancies, resulting in the fast mobility of oxygen ions that improves ORR and OER rates. ABO 3 perovskite is important to the structure for the study of the OER/ORR catalysis because of its superior electrical and electronic properties that can be tuned systematically by cation substitutions on both the A and B sites with different valence states and ionic sizes. 23−32 The electronic structure of transition metal cations governs the catalytic activity of many important reactions such as oxygen electrocatalysis for energy storage applications. 13−25 However, the superior transition metal (3d) and oxygen (2p) orbital overlap at the active sites, the separation of the surface oxygen species, and the pH dependence for the catalytic OER/ORR activity of the perovskites have been less explored. 23−25 The formation of oxygen vacancies is always accompanied by the change in the charge/ionic state and the electronic structure of transition metal ions in the perovskite materi- als. 23−32 The covalency between the transition metal 3d band Received: February 7, 2022 Revised: February 22, 2022 Published: March 7, 2022 Article pubs.acs.org/EF © 2022 American Chemical Society 3219 https://doi.org/10.1021/acs.energyfuels.2c00357 Energy Fuels 2022, 36, 3219−3228 Downloaded via UNIV OF SOUTH CAROLINA COLUMBIA on May 1, 2024 at 15:41:09 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.