Structural and Electrochemical Evolution of Nickel Sulfides During Alkaline Hydrogen Evolution Reaction

Nickel Sulfides have emerged as promising electrocatalysts for alkaline hydrogen evolution reaction (HER) due to their cost-effectiveness and high catalytic activity. While growing research has focused on the initial catalyst design, less attention has been paid to structural and electrochemical modifications during prolonged HER operation. Understanding these transformations is essential for developing more active and stable nickel sulphide-based HER catalysts. This study investigated the post-HER evolution of various nickel sulphide crystalline catalysts, including NiS, NiS2, Ni3S2, and Ni3S4, after prolonged cyclic voltammetry (CV) cycling and constant current polarization. Upon 500 CV sweeps, Raman spectroscopy confirmed structural phase transformation of all nickel Sulfides toward Ni3S2, i.e., the most HER-active phase, irrespective of their initial chemical composition. This electrochemical activation process led to an improvement in electrochemical surface area and charge–transfer properties. Moreover, the kinetic analysis indicated a shift in the rate-determining step from a Volmer-limited mechanism to a mixed Volmer-Heyrovsky pathway, contributing to enhanced HER kinetics. Sulfur leaching was identified as a key factor in this transformation, facilitating surface restructuring and exposure of active Ni sites to the electrolyte. Importantly, post-stability characterization confirmed that leaching occurs predominantly during initial activation and ceases thereafter, with no further structural changes over prolonged operation.