Rational Design of Single-Atom Catalysts for Enhanced Electrocatalytic Nitrogen Reduction Reaction

Electrocatalytic reduction of N-2 to ammonia (eNRR) provides a sustainable alternative to an energy-intensive Haber-Bosch process. However, the poor activity and selectivity of the eNRR catalysts limit their large-scale applications. Recently, transition metals (TMs) doped graphitic carbon nitride-based single-atom catalysts (SACs) have emerged as a very promising class of catalysts. Inspired by their activity and selectivity for a range of reactions, using density functional theory we investigated TMs (3d, 4d, and 5d series) anchored on h-C4N3 as possible catalysts for eNRR. We employed a search scheme for finding an efficient TM SAC for eNRR, based on its optimum N-2 adsorption, N-2 protonation feasibility, and selectivity against HER. The optimum bond strength of TM-N bond is characterized by a change in the magnetic moment of the metal upon N-2 adsorption, and the charge transferred (Delta q) from TM to N-2 molecule. We also established the number of valence electrons (group number) of the TM as a potential descriptor to determine the feasibility of N-2 protonation, which ultimately decides the activity and selectivity of an eNRR catalyst. SACs with TMs belonging to the groups 5-7 are found to have the highest activity. Mo- and W-SACs emerge as the most suitable candidates after the third level of screening, which is based on the selectivity toward eNRR. The overpotentials for both Mo- and W-SACs are 0.02 and 0.33 V versus SHE, respectively. The thermodynamic analysis suggests that Mo-based SAC is the most active catalyst for eNRR with an ultralow overpotential and high faradaic efficiency for eNRR. The Mo-SAC mimics the naturally occurring nitrogenase enzyme, which also has a Mo atom as the active site. Our in-depth analysis provides a rational design of a new class of highly efficient catalysts for the electrochemical NRR under ambient conditions.