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

In this study, we have engineered the MXene supports to boost the single and homo double atoms of Fe, Ru, and Os for efficient NH3 production via electrochemical nitrogen reduction reaction (N2RR) using DFT calculations. We designed the different MXene surfaces which are composed of nine early transition metals [M2CO2 (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 N2RR, dissolution potential, and agglomeration energy. First, we found that the NH2 adsorption energy is the activity descriptor for representing the NH3 productivity and the density of state near the Fermi level of the single Ru atom is the important factor in determining N2RR 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 Ru2/Mo2CO2 catalyst showed the highest NH3 productivity with the high N2RR selectivity over hydrogen evolution reaction. In addition, the best Ru2/Mo2CO2 catalyst exhibited the intermediate density of state near the Fermi level, leading to the optimal descriptor value (NH2 adsorption strength) for NH3 production and in turn the reduction of overpotential for the electrochemical NH3 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 NH3 production at ambient conditions.