Electrodeposited Nickel–Cobalt–Sulfide Catalyst for the Hydrogen Evolution Reaction

A novel Ni-Co-S-based material prepared by the potentiodynamic deposition from an aqueous solution containing Ni²⁺, Co²⁺, and thiourea is studied as an electrocatalyst for the hydrogen evolution reaction (HER) in a neutral phosphate solution. The composition of the catalyst and the HER activity are tuned by varying the ratio of the concentrations of Ni²⁺ and Co²⁺ ions in the electrolytes. Under optimized deposition conditions, the bimetallic Ni-Co-S exhibits higher electrocatalytic activity than its monometallic counterparts. The Ni-Co-S catalyst requires an overpotential of 150 mV for the HER onset, and 10 mA cm^-2 current density is obtained at 280 mV overpotential. The catalyst exhibits two different Tafel slopes (93 and 70 mV dec^-1) indicating two dissimilar mechanisms. It is proposed that the catalyst comprises two types of catalytic active sites, and they contribute selectively toward HER in different potential regions.