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.