What governs the electrocatalytic N₂ reduction activity of sp-hybridized boron?

Electrocatalytic nitrogen reduction reactions (NRRs) have attracted intensive scientific attention, and boron in various forms has been shown promising for the activation of N₂. In this work, we assessed the NRR activities of sp-hybridized-B (sp-B) doped into graphynes (GYs) using first-principles calculations. Eight inequivalent sp-B sites on five graphynes were considered. We found that boron doping greatly modifies the electronic structures at the active sites. Both the geometric effects and electronic effects play vital roles in the adsorption of the intermediates. Some intermediates prefer the sp-B site while others are bonded to both the sp-B and sp-C sites, which leads to two descriptors: the adsorption energy of the end-on N₂ and the side-on N₂. The former correlates well with the p-band center of sp-B while the latter correlates well with both the p-band center of sp-C and the formation energy of sp-B-doped GYs. The activity map demonstrates that the limiting potentials of the reactions are very small (-0.57 V to -0.05 V for the eight GYs). The free energy diagrams reveal that the distal path is normally the most favorable pathway, and the reaction may be limited by the adsorption of N₂ when its binding free energy is higher than 0.26 eV. All eight B-doped GYs locate near the top of the activity volcano, suggesting that there are very promising candidates for the efficient NRR. This work provides a comprehensive understanding of the NRR activity of sp-B-doped GYs, and it should help guide the design of sp-B-doped catalysts.