Abstract: Hydrogen evolution reaction (HER) and oxygen evolution/reduction reaction (OER/ORR) relying on high-performance and low-cost single-atom catalysts (SACs) driven by renewable energy sources offer a sustainable route to carbon-neutral chemicals and fuels. Herein, first-principles calculations were performed to investigate the catalytic HER/OER/ORR activity of a novel graphitic carbon nitride monolayer (g-C₇N₃) supported single transition metal (TM@g-C₇N₃). High stability as well as positively charged active site (TM-site) and desirable electrical conductivity lay the foundation for TM@g-C₇N₃ acting as efficient HER/OER/ORR electrocatalysts. We screened out the non-noble-metal Rh@g-C₇N₃ SAC exhibiting great potential as the trifunctional electrocatalysts for water splitting (ηHER = 0.06 V and η^OER = 0.46 V) and a metal-air battery (ηORR = 0.28 V) on both kinetic and thermodynamic scales, whereas the Ni@g-C₇N₃ can be served as a bifunctional OER/ORR catalyst with a low overpotential of 0.33 V/0.31 V, for both of which the high thermodynamic stability and oxidation barrier guarantee their outstanding performances at ambient conditions. The mechanism analysis indicates the filling of d-orbital electrons of TM-atom can play an important role in determining the value of an energy descriptor (ΔG_OH*), and the suitable ΔG_OH* values make for the TM@g-C₇N₃ candidates to possess favorable OER/ORR overpotential. Particularly, the Rh-d orbital of Rh@g-C₇N₃ is evidently hybridized with the OH*-p orbital, resulting in the lone electrons initially distributed in the antibonding orbital pairing up and occupying the downward bonding orbital, ensuring OH* can be adsorbed on Rh@g-C₇N₃ appropriately. Moreover, multiple-level descriptors including d-band center, COHP, N_d, and φ are used to reveal the origin of the electrocatalytic activity.

Key words: Single atom catalysts, Hydrogen evolution reaction, Oxygen evolution reaction, Oxygen reduction reaction, Monolayer g-C₇N₃