A self-supported NiCo₂O₄/Cu_xO nanoforest with electronically modulated interfaces as an efficient electrocatalyst for overall water splitting

Developing non-precious metal catalysts capable of both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in an alkaline medium is of great significance in the electrocatalytic water-splitting industry. Herein, a novel nanoforest-like NiCo₂O₄/Cu_xO/Cu heterojunction with excellent overall water splitting activity and long-term stability is synthesized through the facile substrate etching and hydrothermal crystallization processes. The nanotree-like NiCo₂O₄/Cu_xO supported on Cu foam achieves a current density of 10 mA cm⁻²at small overpotential (η₁₀= 92 mV for the HER;η₁₀= 213 mV for the OER), which is significantly ahead of the corresponding pristine catalyst NiCo₂O₄/Cu (η₁₀= 295 mV for the HER;η₁₀= 349 mV for the OER) and Cu_xO/Cu (η₁₀= 309 mV for the HER;η₁₀= 347 mV for the OER). In addition, NiCo₂O₄/Cu_xO/Cu can work stably for more than 125 h, and the stability is superior to the noble metal catalysts (Pt/C and RuO₂) at different potentials. The electrons transfer from Cu₂O to NiCo₂O₄with the formation of Cu-O bonds at the interface, making parts of the relatively inert Cu(i) into the active Cu(ii), introducing abundant new active sites. The hierarchical self-supported nanoforest-like structure enlarges the electrochemically active surface area and ensures the bonding strength of NiCo₂O₄/Cu_xO with the Cu skeleton, giving rise to excellent stability. The superb electrocatalytic performance and stability of NiCo₂O₄/Cu_xO/Cu makes it a promising bifunctional catalytic material for overall water splitting. This study provides new testimony of interface effects and the mechanism may inspire the extending design of more effective catalysts.