Influence of the Metal Support─Catalyst Contact on the Performance of NiO-Based O₂ Evolution Electrocatalysts

The performance of NiO-based electrocatalysts for the oxygen evolution reaction (OER) is strongly influenced by the interface between the metal support (current collector) and the catalyst layer, which modulates electronic properties and electrochemical activity. This study systematically investigates the solid-solid interface behavior of NiO thin films prepared by reactive magnetron sputtering on Pt, Au, and Ni, followed by electrochemical characterization. Stepwise NiO deposition and X-ray photoelectron spectroscopy reveal distinct band alignment and electronic structure differences at the metal-catalyst interface. NiO on Pt forms a Schottky barrier that facilitates charge separation but increases electron transfer resistance, resulting in the lowest current densities. NiO on Au exhibits superior OER performance, attributed to forming an interfacial NiAu oxide that enhances electrochemically active site density and modifies the electronic structure at the solid-solid and solid-liquid interfaces. High-resolution TEM reveals that NiO grains on Au adopt a metastable hexagonal structure along the [10-10] zone axis, deviating from the typical cubic phase. The absence of (001) and (010) reflections in FFT patterns suggests disrupted lattice periodicity caused by point or planar defects. These structural distortions may stabilize the hexagonal phase under strain and limited surface diffusion during deposition. NiO on Ni exhibits the lowest interfacial resistance and the highest current density, but is limited by a low electrochemically active surface area and sluggish kinetics, as reflected in a higher overpotential. Electrochemical impedance spectroscopy and band alignment confirm that interfacial electronic and structural modifications have a critical influence on catalytic activity. These findings highlight the importance of substrate selection and interface engineering, including defect-mediated metastability, in optimizing NiO-based OER anodes for next-generation alkaline water electrolysis systems.