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Summary: In this study, we report a Janus- or twins-type honeycomb 3D porous nitrogen-doped carbon (NC) nanosheet array encapsulating ultrafine CoP/Co2P nanorods supported on Ti foil (CoP/Co2P@NC/Ti) as a self-supported electrode for efficient hydrogen evolution. The synthesis and formation mechanism of 3D porous NC nanosheet array assembled into a honeycomb layer with ultrafine CoP/Co2P single-crystal nanorods encapsulated is systematically presented. The CoP/Co2P@NC/Ti electrode exhibits low overpotentials (η10) of 31, 49, and 64 mV at a current density of −10 mA cm−2 in 0.5 M H2SO4, 1.0 KOH, and 1.0 M PBS, respectively, exceeding the overwhelming majority of the documented transition metal phosphide-based electrocatalysts. Density functional theory calculation reveals that the superior electrocatalytic performance for hydrogen evolution reaction could be ascribed to the strong coupling effects of the reactive facets of CoP and Co2P with the 3D porous NC nanosheet, making it exhibit a more thermo-neutral hydrogen adsorption free energy.