Theoretical design toward highly efficient single-atom catalysts for nitrogen reduction by regulating the "acceptance-donation" mechanism.

This work focuses on tuning the NRR activity by constructing SACs based on MXenes substrates and elucidating the intrinsic factors that regulate the "acceptance-donation" mechanism by first principles calculations. [Display omitted] • NRR performance can be tuned by constructing SACs based on defective Ti 2 CO 2 and Ti 2 NO 2 MXenes. • Number of unpaired electrons in d orbitals (N un-d) determines occupation of d orbitals, thus regulating the "acceptance-donation" mechanism. • Δ E (*NNH) and N iso-d can be considered as promising descriptors for screening and designing effective SACs. • Insightful guidance is provided for electrochemical reactions with high bond-order molecules and multi-electron steps, e.g., NRR, ORR and CO 2 RR. Single-atom catalysts (SACs) show great potential for driving electrochemical nitrogen reduction reaction (NRR). However, the lack of physico-chemical understanding of the ambiguous activation mechanism impedes the development of electrocatalysts. In this work, using first principles calculations, we tuned NRR activity and elucidated regulation mechanism of N 2 activation by constructing SACs based on defective Ti 2 CO 2 and Ti 2 NO 2 MXenes. Interestingly, Mo@Ti 2 CO 2 -v and W@Ti 2 CO 2 -v show excellent NRR activity and selectivity with low limiting potentials of –0.27 and –0.27 V at the first hydrogenation (*N 2 →*NNH) and the last hydrogenation (*NH 2 →*NH 3) steps via distal pathway, respectively. The intrinsic activity tendency can be related to adsorption energy of intermediate *NNH (Δ E (*NNH)). Furthermore, electronic structure analysis demonstrates that number of unpaired electrons in d orbitals (N un-d) determines occupation of d orbitals, thus regulating the "acceptance-donation" mechanism. Particularly, transition metals with N un-d of 5 and 4 show moderate "acceptance-donation" interaction, thus achieving encouraging NRR performance. Moreover, Δ E (*NNH) and N un-d can be considered as promising descriptors for screening and designing effective SACs. Finally, our investigation not only contributes to discovery of novel SACs toward NRR, but also provides insightful guidance for other electrochemical reactions with high bond-order molecules and multi-electron steps, such as ORR and CO 2 RR. [ABSTRACT FROM AUTHOR]