Interface engineering of Ni/NiO hierarchical heterojunction for oxygen evolution reaction: A combined experimental and theoretical study

• An interface engineering strategy is developed for synthesizing Ni/NiO heterojunction for OER; • Ni/NiO 150 exhibited superior OER performance than IrO 2 due to the interfacial effect; • Structure-activity correlation is revealed by synchrotron characterization and theoretical calculations. NiO is widely considered as one of most forceful noble-metal-free oxygen evolution reaction (OER) catalysts due to its high intrinsic catalytic activity and robust stability. However, the unsatisfactory conductivity has become a stumbling block for further improving its catalytic performance. Here, a Ni/NiO hierarchical heterojunction is synthesized by an interface engineering strategy. The developed strategy endows the electrocatalysts adequately exposed catalytic active sites, optimized kinetics, including rapid charge and mass transfer, bubble release, and reduced intermediate adsorption free energy. Therefore, the obtained Ni/NiO 150 catalyst displays an overpotential of 280 mV to reach a current density of 10 mA cm -2 and a small Tafel slope of 67 mV dec -1 . In addition, Ni/NiO 150 exhibits a negligible current attenuation after 25 h amperometric operation, indicating outstanding stability. Furthermore, the synchrotron X-ray adsorption spectroscopy results demonstrate that, the introduction of metal Ni phase could induce electronic interaction at the heterojunction interface, which plays a crucial role in the catalytic performance enhancement. Further, density functional theory calculations are carried out to reveal the mechanism of optimized conductivity and catalytic activity enhancement caused by interface effect. This work offers a generic interface engineering strategy for developing high-performance and cost-effective electrocatalysts toward OER, water splitting, and beyond.