Enhanced Catalysis of Electrochemical Overall Water Splitting in Alkaline Media by Fe Doping in Ni₃S₂ Nanosheet Arrays

The development of bifunctional electrocatalysts with high performance for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with earth-abundant elements is still a challenge in electrochemical water splitting technology. Herein, we fabricated a free-standing electrocatalyst in the form of vertically oriented Fe-doped Ni₃S₂ nanosheet array grown on three-dimensional (3D) Ni foam (Fe-Ni₃S₂/NF), which presented a high activity and durability for both HER and OER in alkaline media. On the basis of systematic experiments and calculation, the Fe-doping was evidenced to increase the electrochemical surface area, improve the water adsorption ability, and optimize the hydrogen adsorption energy of Ni₃S₂, which resulted in the enhancement of HER activity on Fe-Ni₃S₂/NF. Moreover, metal sites of Fe-Ni₃S₂/NF were proved to play a significant role in the HER process. During the catalysis of OER, the formation of Ni–Fe (oxy)­hydroxide was observed on the near-surface section of Fe-Ni₃S₂/NF, and the introduction of the Fe element dramatically enhanced the OER activity of Ni₃S₂. The overall water splitting electrolyzer assembled by Fe-Ni₃S₂/NF exhibited a low cell voltage (1.54 V @ 10 mA cm^–2) and a high durability in 1 M KOH. This work demonstrated a promising bifunctional electrocatalyst for water electrolysis in alkaline media with potential application in the future.