Sulfur-Doped Nickel Phosphide Nanoplates Arrays: A Monolithic Electrocatalyst for Efficient Hydrogen Evolution Reactions

Searching for cost-efficient electrocatalysts with high catalytic activity and stability for hydrogen generation by means of water electrolysis would make a great improvement on energy technologies field. Herein, we report high-performance hydrogen evolution reaction (HER) electrocatalysts based on sulfur-doped Ni₅P₄ nanoplate arrays grown on carbon paper (S-Ni₅P₄ NPA/CP). This ternary, robust, monolithic S-Ni₅P₄ NPA/CP exhibits remarkable performance for the HER compared to nickel phosphide and nickel sulfide catalysts. The S-Ni₅P₄ NPA/CP with ∼6% S presents the most promising behavior for water electrolysis applications. Specifically, it shows an onset potential of 6 mV, needing overpotentials (η) of 56 and 104 mV to attain current densities of 10 and 100 mA cm^-2 with a Tafel slope of 43.6 mV dec^-1. The turnover frequency of 6% S-Ni₅P₄ NPA/CP is about 0.11 s^-1 at overpotential of 100 mV, which is ca. 10 and 40 times that of Ni₅P₄ NPA/CP and NiS₂ NPA/CP, respectively. It also shows remarkable stability and durability in 0.5 M H₂SO₄ solution. The results indicate that S and P tune the electronic properties mutually and produce an active catalyst phase for the HER. Furthermore, the density functional theory calculations show that S-Ni₅P₄ NPA/CP exhibits only 0.04 eV of hydrogen adsorption free energy(Δ G_H^*), which is more suitable than Pt (∼-0.09 eV). We propose that the S-doping not only restrains the surface oxidation and dissolution of S-Ni₅P₄ NPA/CP in acid solution but also reduces the Δ G_H^*. We believe that our work will provide a new strategy to design transition metal phosphide composite materials for practical applications in catalysis and energy fields.