Nitrate-induced and in situ electrochemical activation synthesis of oxygen deficiencies-rich nickel/nickel (oxy)hydroxide hybrid films for enhanced electrocatalytic water splitting.

Hydrogen produced by electrochemical water splitting offers a hopeful and renewable solution for addressing the global energy crisis; however, development of highly efficient non-noble-metal electrocatalysts remains a big challenge. Herein, we report a facile strategy to fabricate oxygen deficiencies-rich nickel/nickel (oxy)hydroxide hybrid films as efficient electrocatalysts for water splitting by in situ oxygen evolution reaction (OER) activation. Under OER conditions, the originally deposited Ni films from the ethaline-based deep eutectic solvent (DES) undergo a structural rearrangement with a phase transformation in the oxidation state from Ni(ii) to Ni(iii) at the surface. The change is coupled with an increase in oxygen deficiencies and a pronounced defective precursor is induced by the addition of nitrate ions, providing structural disordering and boosting the intrinsic activity of the catalyst, which strongly enhances the water splitting performance.