P‑Doped Iron–Nickel Sulfide Nanosheet Arrays for Highly Efficient Overall Water Splitting

Iron–nickel sulfide ((Ni,Fe)₃S₂) 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)₃S₂ is still unsatisfactory. Herein, three-dimensional self-supported phosphorus-doped (Ni,Fe)₃S₂ nanosheet arrays on Ni foam (P-(Ni,Fe)₃S₂/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 (ΔG_H*/ΔG_H2O*), enhanced electrical conductivity, and increased electrochemical surface area. Accordingly, the optimal P-(Ni,Fe)₃S₂/NF exhibits relatively low overpotentials of 98 and 196 mV at 10 mA cm^–2 for HER and OER in 1 M KOH, respectively. Furthermore, an alkaline electrolyzer comprising the P-(Ni,Fe)₃S₂/NF electrodes needs a very low cell voltage of 1.54 V at 10 mA cm^–2 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.