Screening of effective NRR electrocatalysts among the Si-based MSi 2 N 4 (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) monolayers

Recently, Science reported a novel centimeter-scale monolayer film of MoSi 2 N 4 synthesized by the chemical vapor deposition method ( Science , 2020, 369 , 670). Since Si atom has been recognized to be an active phase for N 2 fixation, in this paper, we systematically evaluate the catalytic performance in the electrochemical N 2 reduction reaction (NRR) on a series of MSi 2 N 4 (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) monolayers, by means of density functional theory calculations. It is found that the N vacancy, which should be firstly created to expose the active Si atoms, is more favorably formed on a pre-hydrogenated surface. Through a three-step screening, TiSi 2 N 4 and TaSi 2 N 4 monolayers are predicted to be highly potential and promising electrocatalysts because they can (i) successfully capture N 2 , (ii) effectively recover to the initial active states after a round of catalytic cycle without a strong surface reconstruction, and (iii) be stabilized at high temperatures. Moreover, thorough NRR mechanism investigations show that the NRR process proceeds via a Mars–van Krevelen mechanism, where the calculated limiting potentials are only −0.41 and −0.46 V for TiSi 2 N 4 and TaSi 2 N 4 , respectively.