LAUSR

Modulating nitrogen‐rich nitrides with favorable electron structure to enhance hydrogen production activity is challenging due to thermodynamically unfavorable characteristics. Herein, an ultrathin heterojunction of metallic Co and nitrogen‐rich nitride (Co‐Mo5N6) is prepared through the ammonia annealing process as a robust electrocatalyst for hydrogen evolution reaction (HER). Density functional theory simulations and experiments reveal that the obtained Co‐Mo5N6 enables electron redistribution between the nitrogen‐rich phase and Co for more negative H2O adsorption energy, decreasing the subsequent energetic barrier of dissociation (0.05 eV) and optimizing H* absorption (ΔGH* = 0.1 eV). The structure is connected by nanosheet (≈1.2 nm) building blocks with abundant interstitial spaces, open and connective channels, and strong capillary forces, which accelerate mass transfer and electrical conductivity. The Co‐Mo5N6 exhibits excellent HER activity with an extremely low overpotential of 19 mV at 10 mA cm−2 and Tafel slope of 29.0 mV dec−1. Notably, the required overpotential is only 280 mV to achieve a high current density of 1000 mA cm−2 which is better than commercial Pt/C. This work not only improves the understanding of the catalytic activity and the electron redistribution of nitrogen‐rich nitrides, but also presents a new strategy to design other nitrogen‐rich metal nitrides (such as W2N3, Ta5N6).