On the role of metal cation in MXene in boosting the catalytic activity of single/double atom toward electrochemical NH3 production

• Homo double Ru 2 /Mo 2 CO 2 was derived via activity, stability, and selectivity screening. • Elucidating the correlation between the d-band structure of metal cation constituting the MXene support and the electron density near the Fermi level of single and homo double atom catalyst. • The change in the activity of single and double atoms supported on the MXene surface originated from the difference in overlap ratio between their d orbital and the p orbital of the N atom in NH 2 adsorbate. In this study, we have engineered the MXene supports to boost the single and homo double atoms of Fe, Ru, and Os for efficient NH 3 production via electrochemical nitrogen reduction reaction (N 2 RR) using DFT calculations. We designed the different MXene surfaces which are composed of nine early transition metals [M 2 CO 2 (M = Cr, Hf, Mo, Nb, Ta, Ti, V, W, Zr)] and examined the activity/stability of single and homo double atoms by calculating the free energy diagram of N 2 RR, dissolution potential, and agglomeration energy. First, we found that the NH 2 adsorption energy is the activity descriptor for representing the NH 3 productivity and the density of state near the Fermi level of the single Ru atom is the important factor in determining N 2 RR activity. Next, our DFT calculation on the descriptor-based computational search for the novel MXene-based catalysts showed that among the chemically and electrochemically stable candidate catalysts, the homo double Ru 2 /Mo 2 CO 2 catalyst showed the highest NH 3 productivity with the high N 2 RR selectivity over hydrogen evolution reaction. In addition, the best Ru 2 /Mo 2 CO 2 catalyst exhibited the intermediate density of state near the Fermi level, leading to the optimal descriptor value (NH 2 adsorption strength) for NH 3 production and in turn the reduction of overpotential for the electrochemical NH 3 production. More fundamentally, we identified that the electron density near the Fermi level of these single or double atoms is closely correlated with the electron structure of the cationic metal atoms constituting MXene supports. Our study highlights the rational design of single and homo double atom catalysts by tuning the property of MXene supports, which provides insight into the key factors in enhancing NH 3 production at ambient conditions.