Alkaline water electrolysis holds promise for large-scale hydrogen production, yet it encounters challenges like high voltageand limited stability at higher current densities, primarily due to inefficient electron transport kinetics. Herein, a novel cobalt-based metallic heterostructure (Co₃Mo₃N/Co₄N/Co) is designed for excellent water electrolysis. In operando Raman experiments reveal that the formationof the Co₃Mo₃N/Co₄N heterointerface boosts the free water adsorption and dissociation, increasing the available protons for subsequent hydrogen production. Furthermore, the altered electronic structureof the Co₃Mo₃N/Co₄N heterointerface optimizes ΔG_H of the nitrogen atoms at the interface.This synergistic effect between interfacial nitrogen atoms and metal phase cobalt creates highly efficient active sites for the hydrogen evolution reaction (HER), thereby enhancingthe overall HER performance. Additionally, the heterostructure exhibits a rapid OH^- adsorption rate, coupled with great adsorption strength, leading to improved oxygen evolution reaction (OER) performance. Crucially, the metallic heterojunction accelerates electron transport, expediting the afore-mentioned reaction steps and enhancing water splitting efficiency. The Co₃Mo₃N/Co₄N/Co electrocatalyst in the water electrolyzer delivers excellent performance, with a low 1.58 V cell voltage at 10 mA cm^-2, and maintains100% retention over 100 hours at 200 mA cm^-2, surpassing the Pt/C||RuO2 electrolzer.