Bifunctional electrocatalysts based on transition-metal phosphides are appealing for overall water splitting owing to their excellent electrical conductivity, low cost, and high stability. However, these specials are often restricted by some serious drawbacks such as its relatively poor activity for oxygen evolution reaction (OER) and its manufacture, which usually requires one to add additional large numbers of P sources and, consequently, inevitably leads to the release of flammable and detrimental PH3. Herein, we show an effective avenue to overcome these issues. For the first time, the in situ topological transformation of PO43--intercalated NiFeAl-layered double hydroxide nanosheet arrays upon calcination under a H-2 atmosphere is developed to fabricate supported nickel phosphide without any additional P source. The resulting phase affords unique Ni2P@NiFeAlOx core-shell nanosheet arrays, which exhibit an excellent performance for OER and hydrogen evolution reaction in 1.0 M KOH, with low overpotentials of 210 and 105 mV at 10 mA cm(-2,) respectively. Impressively, it can also serve as both a cathode and an anode to drive water splitting in alkaline media, giving 10 and 100 mA cm(-2) at cell voltages of only 1.52 and 1.62 V, respectively. This value is better than the commercial criterion of the Pt/C//IrO2 counterpart and also ranks at the top level in all established bifunctional electrocatalysts. The outstanding performance of Ni2P@NiFeAlOx is mainly attributed to the synergistic effect from a highly dispersed Ni2P core and a thin NiFeAlOx shell, as well as the efficient mass transport of a hierarchical nanoarray framework.