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A noble-metal-free electrocatalyst is fabricated via in situ formation of nanocomposite of nitrogen-doped graphene quantum dots (NGQDs) and Ni S nanosheets on the Ni foam (Ni S -NGQDs/NF). The resultant Ni S -NGQDs/NF can serve as an active, binder-free, and self-supported catalytic electrode for direct water splitting, which delivers a current density of 10 mA cm at an overpotential of 216 mV for oxygen evolution reaction and 218 mV for hydrogen evolution reaction in alkaline media. This bifunctional electrocatalyst enables the construction of an alkali electrolyzer with a low cell voltage of 1.58 V versus reversible hydrogen electrode (RHE) at 10 mA cm . The experimental results and theoretical calculations demonstrate that the synergistic effects of the constructed active interfaces are the key factor for excellent performance. The nanocomposite of NGQDs and Ni S nanosheets can be promising water splitting electrocatalyst for large-scale hydrogen generation or other energy storage and conversion applications.