Single-Atom Low-Valent Alkaline-Earth-Metal Catalysts for Electrochemical Nitrogen Reduction with an Acceptance–Backdonation Mechanism

Single-atom catalysts (SACs) have drawn great attention in developing highly active and low-cost catalysts for electrocatalytic nitrogen reduction reaction (NRR) in ammonia synthesis, but the atomic metal centers are mainly limited to transition metals. Here, four stable alkaline-earth-metal (AEM)-based SACs are proposed by anchoring AEM on nitrogen-doped graphene nanoribbons, based on first-principles calculations. All SACs exhibit excellent NRR performance with competitive limiting potentials compared to stepped Ru (0001), and Ca-based SAC achieves optimal activity with a potential of −0.716 V. It is revealed that the low oxidation state of AEM is crucial for the activation of N2 through an acceptance–backdonation mechanism. The antibonding 2π* orbital of N2 can accept residual s electrons of low-valent AEM and backdonate electrons to the empty d orbitals of AEM, resulting in activation of N2 molecules. In particular, the activation degree of N2 and NRR activity is linearly associated with the charge states of AEMs. Our work reveals the underlying mechanism of AEMs for N2 activation and reduction and presents the potential of AEM SACs as efficient electrochemical NRR catalysts.