Few-layered exfoliated black phosphorus (EBP) has attracted surging interest for electronics, optoelectronics, and catalysis. As compared to excellent progress in electronic and optoelectronic applications, very few reports are available for electrocatalysis by metal-free EBPs. Herein, we couple solution-processable ultrathin EBP nanosheets with higher Fermi level of N-doped graphene (NG) into a new metal-free 2D/2D heterostructure (EBP@NG) with well-designed interfaces and unique electronic configuration, as efficient and durable bifunctional catalysts toward hydrogen evolution and oxygen evolution reactions (HER/OER) for overall water splitting in alkaline media. By rational interface engineering, the synergy of EBP and NG is fully exploited, which not only improves the stability of EBP, but also effectively modulates electronic structures of each component to boost their intrinsic activities. Specifically, due to the lower Fermi level of EBP relative to NG, their electronic interaction induces directional interfacial electron transfer, which not only enriches the electron density over EBP and optimizes H adsorption/desorption to promote HER, but also introduces abundant positively charged carbon sites on NG and provides favorable formation of key OER intermediates (OOH*) to improve OER energetics. Thus, despite that pure EBP or NG alone has poor or negligible activity, EBP@NG achieves remarkably enhanced bifunctional HER/OER activities, along with an excellent durability. This endows an optimized electrolyzer using EBP@NG as anode and cathode with a low cell voltage of 1.54 V at 10 mA cm-2, which is smaller than that of the costly integrated Pt/C@RuO2 couple (1.60 V).