Electrochemical reduction reaction of nitrogen (NRR) offers a promising pathway to produce ammonia (NH₃) from renewable energy. However, the development of such process has been hindered by the chemical inertness of N₂. It is recently proposed that hydrogen species formed on the surface of electrocatalysts can greatly enhance NRR. However, there is still a lack of atomic-level connection between the hydrogenation behavior of electrocatalysts and their NRR performance. Here, we report an atomistic understanding of the hydrogenation behavior of a highly twinned ZnSe (T-ZnSe) nanorod with a large density of surface atomic steps and the activation of N₂ molecules adsorbed on its surface. Our theoretical calculations and in situ infrared spectroscopic characterizations suggest that the atomic steps are essential for the hydrogenation of T-ZnSe, which greatly reduces its work function and efficiently activates adsorbed N₂ molecules. Moreover, the liquid-like and free water over T-ZnSe promotes its hydrogenation. As a result, T-ZnSe nanorods exhibit significantly enhanced Faradaic efficiency and NH₃ production rate compared with the pristine ZnSe nanorod. This work paves a promising way for engineering electrocatalysts for green and sustainable NH₃ production.