Tuning Crystalline Phase and Electrochemical Surface Area of Sulphurized Nickel Electrodes for Hydrogen Evolution Reaction

Nickel sulphides have emerged as promising electrocatalysts due to their cost-effectiveness, abundance, and excellent catalytic activity for the hydrogen evolution reaction (HER). Their performance is largely affected by their crystallographic properties, which are intertwined with their electronic structure, electrochemical surface area (ECSA), and catalytic activity. In parallel with a major research interest in nickel sulphide catalysts, there is also the need for a straightforward, simple synthesis approach, such as gas-phase sulphurization, that enables tuning of both crystallographic phases and ECSA. The present study systematically investigates the role of sulphurization process parameters, including the temperature of the sulphurization oven, H2S flow rate, and sulphurization time, in steering phase formation, crystallite growth, and electrochemical surface area, while establishing correlations with HER activity. Utilizing a controlled sulphurization oven with H2S, commercially available nickel electrodes were modified, leading to variations in nickel sulphide phase composition, crystallite size, and thickness. Our findings reveal a simultaneous phase composition and microstructure evolution in response to sulphurization process parameters. The highest HER activity was associated with a combination of Ni3S2 and Ni3S4 phases, which exhibited the highest ECSA among all tested samples, yielding a current density of -210 mA·cm-2 at -0.4 V vs RHE. This performance significantly surpasses that of the pristine micropillar-based Ni electrode (-18 mA·cm-2) and approaches that of the Pt-coated counterpart (-250 mA·cm-2) under the same conditions.