Surface amorphized nickel hydroxy sulphide for efficient hydrogen evolution reaction in alkaline medium

Non-oxide/hydroxide hydrogen evolution reaction (HER) catalysts undergo hydroxylation to a significant extent even under reductive condition when exposed to alkali. Actual role of such hydroxylation in alkaline HER electrocatalysis is not previously given any significance. In this study, we report an intriguing finding that nickel sulfide a well-known HER electrocatalyst when subjected to anodic potential cycling covering Ni²⁺and Ni³⁺redox couple led to accelerated hydroxylation accompanying surface amorphization. As a result, improved electrochemical surface, better HER kinetics, and better charge transfer were achieved that lowering the HER overpotential by 110 mV at 100 mA cm². This surface amorphized and hydroxylated nickel sulfide exhibited excellent stability upon both galvanostatic and potentiostatic electrolysis for over 50 h. Besides, it also showed a lower Tafel slope (120 mV dec^-1), higher relative ECSA in terms of 2C_dl (3.85 µF cm⁻²), and higher electrochemical accessibility for Ni sites (3.7 × 10¹⁷ cm⁻²) which further advocate the superiority of our way of improving HER activity of a non-oxide/hydroxide catalyst. Thus, this study open up new avenues for re-examining other non-oxide/hydroxide catalysts in alkaline HER for benefiting the energy and cost-efficient hydrogen generation.