(666h) Understanding Selectivity Control in the Electrocatalytic Reduction of CO2 to Liquid Products in Gas-Fed Electrolyzers

The room-temperature electrochemical reduction of CO₂ to liquid products is a soaring carbon utilization technology with an environmental impact, offering a pathway to convert renewable energy into valuable C₁ (e.g., formic acid) and C₂₊ products (e.g., ethanol and n-propanol). Gas-fed flow electrolyzers, in which a gas diffusion layer is used to transport gaseous CO₂ into the electrode, have emerged as promising electrocatalytic reactors for large-scale applications, reaching competitive costs for C₁ products. Despite their increased use in recent years, several factors governing their performance have yet to be understood.

Here, Density Functional Theory (DFT) is used to understand how the CO₂ inlet concentration alters the HCOOH:CO product selectivity in a gas-fed flow cell electrolyzer, as experimentally observed at fixed current densities and varying flow rates using both SnO₂ and Bi₂O₃ electrodes. Computed binding free energies of formate and carboxyl intermediates, respectively used as descriptors of HCOOH and CO selectivity, confirm the experimentally observed preference for HCOOH production in both catalysts. Coverage effect analysis reveals how O-H lateral interactions stabilize the carboxyl intermediate, matching the observed decrease in the HCOOH:CO ratio at higher CO₂ concentrations. Moreover, half-cell in situ Raman studies show both electrodes reduce under the applied CO₂ reduction potentials, with Bi₂O₃ rapidly reducing to its metallic form, while partially reduced Sn_xO_y oxides form, which gradually disappear with increasing potential. To elucidate how different catalyst phases affect the product selectivity, the reaction free-energy pathways for HCOOH, CO, and H₂ production were computed on metallic Sn and Bi, and their respective oxides (Sn_xO_y and Bi₂O₃). In general, both metallic systems exhibited lower limiting potentials for HCOOH production and competing hydrogen evolution, matching diluted CO₂ feed experiments. Overall, this study reveals a product selectivity control with inlet concentrations in flow cell electrolyzers, with a strong agreement between theory and experiments.