Rational design of highly efficient and cost-effective bifunctional electrocatalyst for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is extremely desirable but still challenging for electrochemical water splitting. Herein, we report the synthesis of hybrid composites consisting of defective nickel quantum dots (Ni QD) encapsulated in N-doped carbon (NC) anchored on the surface of reduced graphene oxide (Ni QD@NC@rGO) through a low-temperature pyrolysis of rGO-wrapped two dimensional (2D) sheet-like nickel zeolite imidazolate framework. The as-fabricated Ni QD@NC@rGO catalysts only require overpotentials of 265 and 133 mV to deliver a current density of 10 mA cm(-2) for OER and HER in 1.0 M KOH, respectively. Remarkably, an alkaline electrolyzer constructed with Ni QD@NC@rGO catalyst for both anode and cathode can drive a current density of 10 mA cm(-2) at a low cell voltage of 1.563 V, superior to that of the Pt@CI lIrO(2)@C couple (1.614 V). The enhanced electrocatalytic performance of Ni QD@NC@rGO can be mainly attributed to its 2D hierarchically porous structure and the synergistic effect of the highly dispersed Ni QD, in-situ coupled thin layer NC and rGO, giving rise to a large surface active sites exposure, enhanced electron/ion transfer ability and optimal Gibbs free energy of adsorption.