Selective Electrochemical Production of Formate from Carbon Dioxide with Bismuth-Based Catalysts in an Aqueous Electrolyte

For the efficient electroconversion of CO2 to formate, CO and H2 evolution must be suppressed. Herein, carbon-supported BiOx nanoparticles (BiOx/C) were investigated as a potential candidate for CO2 reduction. In bicarbonate solutions, the BiOx/C catalysts exhibited a high Faradaic efficiency of 93.4% for formate from −1.37 to −1.70 V versus Ag/AgCl with a negligible amount of CO and H2. Stable partial current densities and high Faradaic efficiencies were also achieved in 0.5 M NaCl (12.5 mA cm–2 and 96.0%, respectively). The possible reaction pathways and kinetic parameters of formate formation were examined using systematic electrochemical methods, including Tafel, pH dependence, and in situ X-ray absorption near-edge structure analyses. From the results of these mechanistic studies, we propose that dual mechanisms are functional on the BiOx/C catalysts. Specifically, a two-electron and one-proton transfer reaction to adsorbed CO2 or a chemical proton transfer reaction to CO2– anion are the possible rate-determining steps (RDSs) at low potentials, whereas a one-electron transfer reaction to CO2 is the RDS at high potentials.