Hydrogen Evolution and Carbon Dioxide Reduction Pathways on Graphitic Carbon Nitride Decorated by Single Atoms of Transition Metals‡

The conversion of carbon dioxide (CO₂) into valuable products represents a promising strategy to mitigate CO₂ emissions and enable sustainable energy storage. However, the development of efficient and selective catalysts for the electrocatalytic reduction of CO₂ (CO₂RR) remains a significant challenge. In this study, we explore the performance of first-row transition metal single atoms anchored on g-C₃N₄ monolayers as potential catalysts for the CO₂RR. We employed density functional theory (DFT) calculations to investigate the hydrogen evolution reaction (HER) as a competing pathway to the CO₂RR. Only the candidates that suppress the HER are promising candidates for the selective CO₂RR. Our results indicate that Ni₁/C₃N₄ emerges as the most promising catalyst due to its relatively moderate-to-high overpotential for the HER and a favorable reaction pathway that favors CO production through the HCOO* intermediate. Despite some challenges, such as the strong Ni–CO interaction hindering CO desorption, Ni₁/C₃N₄ presents a viable route for CO₂RR. Mn and Co single atoms exhibit slighly lower overpotential toward HER, overcoming one of the main limitations to be active and selective for CO₂RR. Nevertheless, Co₁/C₃N₄ shows large energy barriers for CO hydrogenation and HCOOH production, while Mn₁/C₃N₄ opens the route for formic acid production. This work highlights the importance of evaluating the HER alongside the CO₂RR to identify catalysts with optimal selectivity and efficiency.