Co-anion exchange prepared 2D structure Ni(Co,Fe)PS for efficient overall water electrolysis

Two-dimensional (2D) Ni alloy catalysts have attracted significant attention for alkaline overall water electrolysis owing to their facile oxidation state changes and long-term stability. However, their low hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performances owing to their non-optimized hydrogen, water, and hydroxide ion adsorption energies, compared to those of noble metal catalysts, have limited their further application. To address these issues, in this study, we synthesized a 2D-structured Ni(Co,Fe)PS using a co-ion (P and S) exchange method. The synthesized 2D-structured Ni(Co,Fe)PS utilized the averaging effect of the combination of sulfide, which exhibits high hydrogen and water adsorption energies, and phosphide, which exhibits a relatively low hydrogen and water adsorption energy. During a HER process, the synthesized NiCoPS exhibited a lower overpotential (61 mV) compared to NiCoP (75 mV) and NiCoS (66 mV) for the generation of a current density of 10 mA·cm−2, and a low Tafel slope value of 44.5 mV·dec-1. In addition, the charge transfer resistance of the NiCoPS catalyst (5 Ω) was significantly lower than those of NiCoP (12.1 Ω) and NiCoS (14.3 Ω). As an alkaline OER catalyst, NiFePS exhibited a lower overpotential (242 mV) for the generation of a current density of 10 mA·cm−2 compared to NiFeP (250 mV) and NiFeS (260 mV), and a remarkably low Tafel slope value of 38.0 mV·dec-1. The charge transfer resistance of NiFePS (1.5 Ω) was significantly lower than those of NiFeP (4.2 Ω) and NiFeS (4.9 Ω). An overall water splitting (OWS) catalyst system was fabricated by combining the NiCoPS and NiFePS catalysts, and OWS system required an overpotential of only 1.54 V for the generation of a current density of 10 mA·cm−2, which is significantly lower than those required by previously reported transition metal-based phosphide catalysts. This study presents a new method to develop highly efficient HER and OER catalysts via a simple co-ion exchange of 2D-structured Ni-alloy layered double hydroxides.