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Highly active and durable bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) play a pivotal role in overall water splitting.Herein,we demonstrate the construction of interface-strengthened CoP nanosheet array with Co2P nanoparticles as such an electrocatalyst through a facile hydrothermal reaction and the subsequent phosphorization process.The twodimensional (2D) nanosheets with thickness of ～55 nm expose a great number of active sites.The surface chemical state indicates that the strongly coupled CoP/Co2P electrocatalysts can adsorb or generate more targeted intermediates (e.g.OH-or OOH*) for both HER/OER.As a result,the CoP/Co2P electrocatalysts exhibit small overpotentials of 68 and 256mV to drive 10mAcm-2 for HER and OER,respectively,outperforming most of the recently reported Co-based electrocatalysts.Furthermore,an alkaline electrolyzer assembled by using CoP/Co2P as both cathode and anode can achieve a current density of 10mAcm 2 at a low voltage of 1.57 V and work continuously for over 58 h.This work provides a feasible structural design for transition metal phosphides electrocatalysts with efficient and stable overall water splitting.