Understanding Selectivity for the Electrochemical Reduction of Carbon Dioxide to Formic Acid and Carbon Monoxide on Metal Electrodes

Increases in energy demand and in chemical production, together with the rise in CO2 levels in the atmosphere, motivate the development of renewable energy sources. Electrochemical CO2 reduction to fuels and chemicals is an appealing alternative to traditional pathways to fuels and chemicals due to its intrinsic ability to couple to solar and wind energy sources. Formate (HCOO-) is a key chemical for many industries; however, greater understanding is needed regarding the mechanism and key intermediates for HCOO- production. This work reports a joint experimental and theoretical investigation of the electrochemical reduction of CO2 to HCOO- on polycrystalline Sn surfaces, which have been identified as promising catalysts for selectively producing HCOO-. Our results show that Sn electrodes produce HCOO-, carbon monoxide (CO), and hydrogen (H-2) across a range of potentials and that HCOO- production becomes favored at potentials more negative than -0.8 V vs RHE, reaching a maximum Faradaic efficiency of 70% at -0.9 V vs RHE. Scaling relations for Sn and other transition metals are examined using experimental current densities and density functional theory (DFT) binding energies. While *COOH was determined to be the key intermediate for CO production on metal surfaces, we suggest that it is unlikely to be the primary intermediate for HCOO- production. Instead, *OCHO is suggested to be the key intermediate for the CO2RR to HCOO- transformation, and Sn's optimal *OCHO binding energy supports its high selectivity for HCOO-. These results suggest that oxygen-bound intermediates are critical to understand the mechanism of CO2 reduction to HCOO- on metal surfaces.