Synergistic effect of a diatomic boron-doped layered two-dimensional MSi2N4 monolayer for an efficient electrochemical nitrogen reduction

Abstract

Developing efficient metal-free single-atom-catalysts (SACs) for the electrochemical nitrogen reduction reaction (eNRR) is of great significance, yet it remains a great challenge. By using first-principles calculations, we designed single B- and diatomic B-doped MSi₂N₄ monolayers (namely B@MSi₂N₄ and B₂@MSi₂N₄, respectively, M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) as electrocatalysts for the nitrogen reduction reaction. Our calculation results show that the substitutional B doping significantly reduces the band gap of the material, and thus promotes the charge transfer required for the eNRR. The filled sp³ orbitals of the doped B atoms can back-donate electrons to the π* orbitals of N₂ to form B-to-N π antibonding, weakening the NN bond. Compared with the single B atom site, the synergistic effect of adjacent B atoms largely promotes the activation of N₂. The detailed reaction free energy pathways show that one-B atom-doped MSi₂N₄ materials exhibit poor catalytic activity for N₂ reduction to NH₃. In contrary, the two adjacent B atom-doped MSi₂N₄ can effectively catalyze N₂ conversion into NH₃ while suppressing the HER, with a low limiting potential smaller than −0.3 V. In particular, for M = Ti, V, Nb, and Ta, all electrochemical elementary steps occurring on diatomic B sites for the eNRR are spontaneous through the distal mechanism. Furthermore, the eNRR process on B₂@MoSi₂N₄ is easily achieved, kinetically, by overcoming the low activation barrier less than 0.77 eV. This work provides a new strategy for the design of metal-free single atom-based catalysts.