Decoding CO2 Adsorption Modes and CO Yield Variation on TM-N4 Single-Atom Catalysts via Constant-Potential...

Transition metal-based TM-N4 single-atom catalysts (SACs) are promising materials for electrochemical CO2 reduction reactions (CO2RR) toward CO formation. In this study, we employed the constant-potential method to investigate CO2 adsorption and reduction on 10 TM-N4 SACs (TM = Sc-Zn), with a particular focus on Ni-N4. We first evaluated the CO2 adsorption mode on the TM active site as a function of the applied electrode potential, U. Landau free energy analysis revealed a transition in CO2 adsorption mode from physisorption (D∞h-D2h) to chemisorption (C2v-C2v) at a critical electrode potential, referred to as the "transorption" potential (Utrans). Walsh diagram and machine-learning analyses revealed significant π*-orbital splitting and an unusual σ back-donation governing Utrans. We then examined the CO2RR processes by considering both adsorption modes of CO2. The transition from CO2 physisorption to chemisorption modes leads to a shift in the rate-limiting step from CO2 hydrogenation to CO desorption, the emergence of new active sites for competing hydrogen evolution reactions, and a decrease in electron transfer rates at more negative potentials. Taken together, these observations align with the experimental CO yield profile under increasingly negative potentials, capturing key performance trends of rise, plateau, and decline. Notably, the calculated Utrans values are consistent with the applied potentials at which CO yield declines for five SACs for which experimental data are available (Mn, Fe, Ni, Cu, and Zn). Thus, this work establishes Utrans as a novel predictive parameter for CO yield performance and offers valuable guidance for the design of advanced SACs in future studies.