Uncovering the Nature of Active Species of Nickel Phosphide Catalysts in High-Performance Electrochemical Overall Water Splitting

A systematic structural elucidation of the near-surface active species of the two remarkably active nickel phosphides Ni₁₂P₅ and Ni₂P on the basis of extensive analytical, microscopic, and spectroscopic investigations is reported. The latter can serve as complementary efficient electrocatalysts in the hydrogen (HER) versus oxygen evolution reaction (OER) in alkaline media. In the OER Ni₁₂P₅ shows enhanced performance over Ni₂P due to the higher concentration of nickel in this phase, which enables the formation of an amorphous NiOOH/Ni­(OH)₂ shell on a modified multiphase with a disordered phosphide/phosphite core. The situation is completely reversed in the HER, where Ni₂P displayed a significant improvement in electrocatalytic activity over Ni₁₂P₅ owing to a larger concentration of phosphide/phosphate species in the shell. Moreover, the efficiently combined use of the two nickel phosphide phases deposited on nickel foam in overall electrocatalytic water splitting is demonstrated by a strikingly low cell voltage and high stability with pronounced current density, and these catalysts could be an apt choice for applications in commercial alkaline water electrolysis.