Separating the Effects of Band Bending and Covalency in Hybrid Perovskite Oxide Electrocatalyst Bilayers for Water Electrolysis

Abstract

Testing single crystalline epitaxial thin films as model systems for metal oxide electrocatalysts in the oxygen evolution reaction (OER) enables differentiating the OER descriptors such as crystal facet orientation, surface chemical composition, electronic structure and band bending at the interface to the electrolyte [1,2,3]. We designed single crystalline perovskite oxide bilayer structures to systematically tune the surface electronic structure and the band bending at the electrode/electrolyte interface to understand their role in the OER.The Co−O covalency in perovskite oxide cobaltites such as La1 − x SrxCoO3 is believed to impact the electrocatalytic activity during the OER. Additionally, space charge layers through band bending at the interface to the electrolyte may affect the electron transfer into the electrode, complicating the analysis and identification of true OER activity descriptors. Here, we separate the influence of covalency and band bending in hybrid epitaxial bilayer structures of highly OER-active La0.6Sr0.4CoO3 and undoped and less-active LaCoO3. Ultrathin LaCoO3 capping layers of 2−8 unit cells on La0.6Sr0.4CoO3 show intermediate OER activity between La0.6Sr0.4CoO3 and LaCoO3 evidently caused by the increased surface Co−O covalency compared to single LaCoO3 as detected by X-ray photoelectron spectroscopy. A Mott−Schottky analysis revealed low flat band potentials for different LaCoO3 capping layer thicknesses, indicating that no limiting extended space charge layer exists under OER conditions as all catalyst bilayer films exhibited hole accumulation at the surface [1].The combined X-ray photoelectron spectroscopy and Mott−Schottky analysis of atomically defined bilayer structures thus enables us to differentiate between the influence of the covalency and intrinsic space charge layers, which are indistinguishable in a single physical or electrochemical characterization. Our results emphasize the prominent role of transition metal oxygen covalency in perovskite electrocatalysts and introduce a bilayer approach to fine-tune the surface electronic structure [1].[1] L. Heymann, M. L. Weber, M. Wohlgemuth, M. Risch, R. Dittmann, C. Baeumer, F. Gunkel (2022) Separating the Effects of Band Bending and Covalency in Hybrid Perovskite Oxide Electrocatalyst Bilayers for Water Electrolysis, ACS Appl. Mater. Interfaces[2] M. Wohlgemuth, M. L. Weber, L. Heymann, C. Baeumer, F. Gunkel (2022) Activity-Stability Relationships in Oxide Electrocatalysts for Water Electrolysis, Front. Chem.[3] M. L. Weber, G. Lole, A. Kormanyos, A. Schwiers, L. Heymann, F. D. Speck, T. Meyer, R. Dittmann, S. Cherevko, C. Jooss, C. Baeumer, F. Gunkel, (2022) Atomistic Insights into Activation and Degradation of La0.6Sr0.4CoO3-δ Electrocatalysts under Oxygen Evolution Conditions Figure 1