Symmetry or asymmetry: which one is the platform of nitrogen vacancies for alkaline hydrogen evolution

Conventional nitrogen vacancies with a symmetric coordination of metal cations (i.e., M₁-N_v-M₁) play a crucial role in tuning the local environment of the metal sites in metal nitrides and improving their electrochemical activity in the hydrogen evolution reaction (HER). However, the symmetric N_v sites, which feature a uniform charge distribution on adjacent metal sites, suffer from sluggish water dissociation kinetics and a poor capability for hydrogen desorption. Here, we fabricated Cr-doped and N_v-rich Co₄N nanorods grown on a Ni foam (Cr-Co₄N-N_v/NF) with asymmetric Cr-N_v-Co sites to effectively catalyze hydrogen evolution under alkaline conditions, with a low overpotential of 33 mV at a current density of 10 mA cm^-2 and a small Tafel slope of 37 mV dec^-1. The experimental characterizations and theoretical simulations collectively reveal that the construction of asymmetric Cr-N_v-Co sites gives rise to the upshift of the d-band center, thus promoting water adsorption and activation. Moreover, asymmetric N_v sites allow a balance between hydrogen adsorption and desorption, which avoids the limited desorption process over the symmetric Co-N_v-Co sites.