Herein, a novel cadmium nitrite initiated mesoporous carbon shell encapsulated binary cobalt/cobalt phosphide (Co/Co2P@MC-(Cdx)) composites were first prepared by a facile solvothermal reaction following a phosphorization procedure. The Co/Co2P@MC-(Cd0.5), with molar ratio of Cd/Co = 0.5 in the initial feeding stock, exhibited overpotentials of as low as 99 mV at 10 mA cm(-2) and small Tafel slopes of 53 mV dec(-1) under 0.5 M H2SO4 conditions. Various techniques further demonstrated that the superior activity of Co/Co2P@MC-(Cd0.5) was attributed to the introduction of Cd initiator, which resulted in the mesoporisity on carbon shell and the modified electron state between the Co and Co2P. This novel Cd initiated mesoporous carbon encapsulated Co/Co2P composites hold promising potential for utilization in the hydrogen evolution from electrocatalytic water splitting. Graphic Abstract A novel cadmium nitrite initiated mesoporous carbon shell encapsulated binary cobalt/cobalt phosphide (Co/Co2P@MC-(Cdx)) composites were first prepared. The Co/Co2P@MC-(Cd0.5), with molar ratio of Cd/Co = 0.5 in the initial feeding stock, exhibited overpotentials of as low as 99 mV at 10 mA cm(-2) and small Tafel slopes of 53 mV dec(-1) under 0.5 M H2SO4 conditions. The superior activity of Co/Co2P@MC-(Cd0.5) was attributed to the introduction of Cd initiator, which resulted in the mesoporisity on carbon shell and the modified electron state between the Co and Co2P. [GRAPHICS] .