Transitional metal phosphides with array-like structure grown on conductive support materials are promising bifunctional catalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).In this study,a method was developed to synthesize directly porous Ni2P nanosheet arrays and Ni2P nanoparticles onto nickel foam via a hydrothermal reaction followed by a phosphorization process.Mechanistic studies revealed that the allomorphs of Ni2P nanosheets and Ni2P nanoparticles in the array-like structure were formed via the Kirkendall effect and Ostwald ripening.A fully functional water electrolyzer containing Ni2P as electrodes for the OER and HER exhibited promising activity and stability.At 10 mA·cm-2,a Ni2P cell voltage of 1.63 V was obtained,which was only 0.05 V smaller than that found for Pt/C/NF‖RuO2/NF cell.The enhanced electrocatalytic performance resulted from the favorable porosity of the Ni2P arrays and the synergistic effect between Ni2P nanosheets and Ni2P nanoparticles.