Nickel phosphide polymorphs with an active (001) surface as excellent catalysts for water splitting

Abstract

Since the emergence of hydrogen generation by water-splitting as a core renewable-energy technology, the development of related catalysts with high efficiency, long-term stability, and low cost has been vigorously pursued. We report the temperature-controlled synthesis of two nickel phosphide polymorphs, Ni₂P and Ni₅P₄, by phosphorization of Ni foil or foam using phosphine gas. The hexagonal phase Ni₂P nanowires and Ni₅P₄ nanosheets were grown on Ni substrates with vertical alignment, and uniformly exposed active (001) planes. The Ni₅P₄ nanosheets possess significant stacking faults along the [0001] direction. Both Ni₂P and Ni₅P₄ exhibit excellent electrocatalytic activity toward the hydrogen evolution reaction (HER). Their overpotential for 10 mA cm⁻² was 0.126 and 0.114 V, and the Tafel slope was 42 and 34 mV dec⁻¹ in 0.5 M H₂SO₄ electrolyte, respectively. A decrease in HER performance was observed for Ni₅P₄, but the change was negligible for Ni₂P. Strain mapping using a precession-assisted nanobeam electron diffraction technique showed that only Ni₅P₄ underwent degradation of basal (001) planes during HER, which explains the lower stability of catalytic activity. Furthermore, the Ni₂P nanowires demonstrated excellent catalytic activity toward overall water splitting, which could be attributed to the stable surface as well as the highly conductive crystal structures.