High-Entropy A-Site Disorder Enhances Oxygen Evolution Reaction Activity and Stability in Nickelate Thin Films

Rare-earth nickelates are among the most active oxide electrocatalysts for the oxygen evolution reaction (OER), owing to their tunable Ni-O covalency, near-unity eg occupancy, and sensitivity to A-site chemistry, oxygen stoichiometry, and epitaxial strain. Here, we use epitaxial A-site high-entropy (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)NiO3 thin films to reveal how epitaxial strain influences defect formation, Ni valence, electronic structure, and OER activity in a chemically disordered rare-earth (RE) sublattice. Tensile strain promotes Ruddlesden-Popper fault formation, interrupts the corner-sharing NiO6 network, and lowers the average Ni oxidation state. Although the occupied density of states near the Fermi level remains largely unchanged, O K-edge X-ray absorption spectroscopy reveals increased low-energy unoccupied states under tensile strain. First-principles calculations suggest that tensile strain shifts RE 4f-derived states toward the Fermi level, enhancing RE 4f-O 2p-Ni 3d hybridization. These coupled structural and electronic changes lead to earlier OER onset, stronger pre-catalytic Ni redox features, and higher activity than compressively strained films and representative single-A-site nickelates. The high-entropy nickelate also shows enhanced resistance to OER-induced structural degradation compared with LaNiO3. These results suggest that A-site configurational disorder and epitaxial strain can jointly tune defect formation and rare-earth-derived electronic states, providing a route for improving perovskite oxide electrocatalysts.