Nickel phosphide polymorphs with an active (001) surface as excellent catalysts for water splitting
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  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.
Publication Source (Journal or Book title)
Jung, C., Park, K., Lee, Y., Kwak, I., Kwon, I., Kim, J., Seo, J., Ahn, J., & Park, J. (2019). Nickel phosphide polymorphs with an active (001) surface as excellent catalysts for water splitting. CrystEngComm, 21 (7), 1143-1149. https://doi.org/10.1039/c8ce01884g