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

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 B-2@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 sp(3) orbitals of the doped B atoms can back-donate electrons to the pi* orbitals of N-2 to form B-to-N pi antibonding, weakening the N N bond. Compared with the single B atom site, the synergistic effect of adjacent B atoms largely promotes the activation of N-2. The detailed reaction free energy pathways show that one-B atom-doped MSi2N4 materials exhibit poor catalytic activity for N-2 reduction to NH3. In contrary, the two adjacent B atom-doped MSi2N4 can effectively catalyze N-2 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 B-2@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.