Ni-NiO Heterojunction: A Binder-Free Catalyst for Enhanced Oxygen Evolution Reaction

In this study, we applied a two-step electrochemical anodization process to produce highly porous nanostructured nickel suboxides. We then annealed these materials in different environments: air, Ar, and Ar/H₂. Annealing in a reductive environment (Ar/H₂) resulted in a Ni-NiO heterojunction with a high defect density, as confirmed by the Mott-Schottky analysis. Our results demonstrate that these defects and active sites significantly enhance the electrocatalytic activity for the oxygen evolution reaction (OER). Utilizing X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and electrochemical analysis, we demonstrate that the heterojunction system containing Ni-NiO, formed through annealing in an Ar/H₂ atmosphere, acts as a highly efficient electrocatalyst for the OER. This catalyst achieves an impressively low overpotential of 293 mV at 10 mA cm^-2, a Tafel slope of 74 mV dec^-1, and exhibits outstanding stability, maintaining performance over 1000 cycles. Notably, our most optimized NiO _<i>x</i> electrode outperforms the conventional reference RuO₂ electrode by a factor of 1.72. Our findings demonstrate the potential of binder-free Ni-NiO heterojunctions in developing high-performance electrocatalysts for alkaline electrolysis.