Understanding Selectivity of Carbon Dioxide Reduction to Carbon Monoxide and Formic Acid on Sn Electrodes

Increases in energy demand and chemical production, together with the rise in CO 2 levels in the atmosphere require new, renewable energy sources to be developed. Electrochemical CO 2 reduction to fuels and chemicals is an appealing alternative to traditional methods of producing energy and chemicals due to its simplicity and ability to implement with solar and wind energy sources. Sn has been identified as a promising catalyst for the CO 2 reduction reaction (CO 2 RR) to formate (HCOO - ), a key chemical for many industries; however, there is a lack of experimental data to corroborate the prevailing theories of the mechanism and key intermediates for HCOO - production. This work reports a joint experimental and theoretical investigation of the electrochemical reduction of CO 2 on polycrystalline Sn surfaces. 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 after -0.8V vs. RHE, reaching a maximum faradaic efficiency of 70%. 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, a strong correlation between *OCHO and HCOO - production is observed, suggesting that the key intermediate for the CO 2 RR to HCOO - . Sn’s optimal *OCHO binding energy explains its high selectivity for HCOO - . These results suggest that oxygen bound intermediates are critical to understand the mechanism of CO 2 reduction to HCOO - on metal surfaces.