Optimizing the electron donation and back-donation effect through the combination of d-block transition metal and s-block calcium atoms for efficient nitrogen fixation†

Abstract

Single transition metal (TM) atoms embedded into nitrogen-doped graphene (such as the M–N₄–C configuration) present promising potential for the electrochemical nitrogen reduction reaction (eNRR). However, the electron “acceptance–donation” effect between the d orbitals of most TM atoms and the frontier molecular orbitals of N₂ is limited for N₂ activation. Herein, combining the advantages of heavy alkaline-earth metal calcium (Ca) and TM atoms, we designed novel TM–Ca sites as potential dual-atomic catalysts (DACs) for efficient nitrogen fixation by means of first-principles calculations. Unlike the weak coupling of N₂ with single TM or Ca sites, the newly formed TM–Ca pairs demonstrated an improved electron acceptance of the unoccupied d orbitals of the metal center from the σ bonding orbitals of N₂ and promoted electron back-donation to the π* orbitals of N₂. Accordingly, owing to the synergistic effect between the TM and Ca atoms, the optimized electron “acceptance–donation” mechanism largely improved N₂ adsorption and activation as well as lowered the free-energy barrier of the potential-determining step, thus facilitating N₂–to–NH₃ conversion. Among the designed DACs, V–Ca and Cr–Ca pairs were found to be excellent eNRR catalysts, with a low limiting potential of −0.38 V. Furthermore, based on the intrinsic atomic properties alone, we developed an excellent descriptor to predict catalytic performance. This work provides a new framework for designing dual-atom electrocatalysts using s-block alkaline-earth metals.