Partial sulfidation engineering of a Ni(OH)2/NiSx heterostructure towards sulfion oxidation-assisted seawater splitting

Abstract

Substituting the oxygen evolution reaction (OER) with the sulfion oxidation reaction (SOR) and further coupling with the hydrogen evolution reaction (HER) can significantly reduce the potential required for overall alkaline seawater electrolysis, thereby achieving low-energy hydrogen production. Here, the Ni(OH)₂ nanosheet array on nickel foam was first synthesized via hydrothermal treatment, followed by partial sulfidation engineering to obtain the bifunctional Ni(OH)₂/NiS_x heterostructure electrocatalysts. For the HER, only 180 mV overpotential is required to achieve 100 mA cm⁻², while for the SOR, just 0.315 V versus reversible hydrogen electrode (vs. RHE) is needed to reach the same current density. Furthermore, a two-electrode HER||SOR system was assembled utilizing the optimal catalyst as both the cathode and anode, and the cell voltage is sufficient to attain 100 mA cm⁻² at only 0.843 V, which is a decrease of 1.250 V compared to using the OER as the anode. Meanwhile, this coupling system demonstrates stable operation at 50 mA cm⁻² over a period of 36 h. The efficient HER/SOR bifunctional electrocatalytic performance of Ni(OH)₂/NiS_x is mainly attributable to the following factors: one is that the heterogeneous structure provides a rich hydroxide/sulfide heterogeneous interface, which effectively modulates the electronic structure of the electrode surface; another key factor is that the self-supporting nanosheet arrays improve the electrical conductivity and mass transfer of the catalysts.