P-Doped Iron-Nickel Sulfide Nanosheet Arrays for Highly Efficient Overall Water Splitting.

Iron-nickel sulfide ((Ni,Fe)S) is one of the most promising bifunctional electrocatalysts for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline media because of their metallic conductivity and low cost. However, the reported HER activity of (Ni,Fe)S is still unsatisfactory. Herein, three-dimensional self-supported phosphorus-doped (Ni,Fe)S nanosheet arrays on Ni foam (P-(Ni,Fe)S/NF) are synthesized by a simple one-step simultaneous phosphorization and sulfuration route, which exhibits dramatically enhanced HER activity as well as drives remarkable OER activity. The incorporation of P significantly optimized the hydrogen/water absorption free energy (Δ/Δ), enhanced electrical conductivity, and increased electrochemical surface area. Accordingly, the optimal P-(Ni,Fe)S/NF exhibits relatively low overpotentials of 98 and 196 mV at 10 mA cm for HER and OER in 1 M KOH, respectively. Furthermore, an alkaline electrolyzer comprising the P-(Ni,Fe)S/NF electrodes needs a very low cell voltage of 1.54 V at 10 mA cm and exhibits long-term stability and outperforms most other state-of-the-art electrocatalysts. The reported electrocatalyst activation approach by anion doping can be adapted for other transition-metal chalcogenides for water electrolysis, offering great promise for future applications.