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

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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, Ni 2 P and Ni 5 P 4 , by phosphorization of Ni foil or foam using phosphine gas. The hexagonal phase Ni 2 P nanowires and Ni 5 P 4 nanosheets were grown on Ni substrates with vertical alignment, and uniformly exposed active (001) planes. The Ni 5 P 4 nanosheets possess significant stacking faults along the [0001] direction. Both Ni 2 P and Ni 5 P 4 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 H 2 SO 4 electrolyte, respectively. A decrease in HER performance was observed for Ni 5 P 4 , but the change was negligible for Ni 2 P. Strain mapping using a precession-assisted nanobeam electron diffraction technique showed that only Ni 5 P 4 underwent degradation of basal (001) planes during HER, which explains the lower stability of catalytic activity. Furthermore, the Ni 2 P 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.

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