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Although great advances have been achieved in the field of electrocatalysis, the design of highly efficient and stable earth-abundant electrocatalysts for overall water splitting remains a significant challenge. Herein, we have successfully developed cost-efficient three dimensional (3D) highly open hierarchical catalysts with cobalt-molybdenum nanosheet arrays on nickel foam (NF) (denoted as CoMoO NSs@NF) and CoMoP NSs@NF via phosphorization for efficient overall water splitting. The optimized Co 5 Mo 1.0 O NSs@NF exhibits excellent OER activity with low overpotentials of 270 mV at 10 mA cm −2 and superior stability after 140 h and the optimized Co 5 Mo 1.0 P NSs@NF shows excellent HER activity with low overpotential of 173 mV at 10 mA cm −2 and superior stability after 1000 CV cycles. In addition to the enhanced electron migration and mass transfer benefited from the 3D structure, the XPS analysis shows that Mo plays a vital role on modulating the number of active sites on the surface, leading to the electrocatalytic enhancement. Significantly, the Co 5 Mo 1.0 O NSs@NF//Co 5 Mo 1.0 P NSs@NF couple showed highly efficient overall water electrolysis with a low potential of 1.68 V at 10 mA cm −2 in an alkaline electrolyzer as well as excellent stability after long-term water splitting, holding great potential for practical overall water splitting electrocatalysis. [ABSTRACT FROM AUTHOR]