Varying Mechanisms Dominate Formate Formation in Electrochemical CO2 Reduction on Cu, Au, and Pb: A Constant Potential DFT-Based Microkinetic Analysis

Abstract Electrochemical CO2 reduction offers a route to convert renewable electricity into fuels and chemicals while closing the anthropogenic carbon cycle. However, the technology struggles to surpass the lab scale as it is limited by low selectivity and energy efficiency. These challenges are less severe for the two-electron products, carbon monoxide and formate, making them attractive targets. Formate is of particular interest due to its potential as an energy carrier and its relatively high market value. Yet its formation mechanism remains debated, hindering rational catalyst design. Here, we combine grand canonical DFT, a hybrid explicit-implicit solvent model, and microkinetic simulations to elucidate the formate formation mechanism on Cu, Au, and Pb, and show that distinct pathways dominate on each metal. On Cu, formate formation proceeds via O-bound chemisorption of CO2 followed by protonation. Au exhibits negligible formate activity despite supporting different formate pathways depending on facet termination and applied potential, as these pathways remain kinetically unfavorable. Finally, Pb facilitates a direct outer-sphere-like process from physisorbed CO2 to formate, with high activity. The enhanced activity may originate from the distinct electronic structure of Pb, characterized by diffuse p-like states near the Fermi level, a feature shared by other formate-selective post-transition-metal catalysts. The variation in mechanisms for formate production likely underlies the difficulty of identifying universal activity descriptors for electrochemical CO2 and suggests that catalyst design might necessitate mechanism-specific descriptors.