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

The Co-O covalency in perovskite oxide cobaltites such as La_1-SrCoO₃ is believed to impact the electrocatalytic activity during electrochemical water splitting at the anode where the oxygen evolution reaction (OER) takes place. 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 La_0.6Sr_0.4CoO₃ and undoped and less-active LaCoO₃. Ultrathin LaCoO₃ capping layers of 2-8 unit cells on La_0.6Sr_0.4CoO₃ show intermediate OER activity between La_0.6Sr_0.4CoO₃ and LaCoO₃ evidently caused by the increased surface Co-O covalency compared to single LaCoO₃ as detected by X-ray photoelectron spectroscopy. A Mott-Schottkyanalysis 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. The combined X-ray photoelectron spectroscopy and Mott-Schottky analysis 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.