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

For the efficient electroconversion of CO<sub>2</sub> to formate, CO and H<sub>2</sub> evolution must be suppressed. Herein, carbon-supported BiO<sub><i>x</i></sub> nanoparticles (BiO<sub><i>x</i></sub>/C) were investigated as a potential candidate for CO<sub>2</sub> reduction. In bicarbonate solutions, the BiO<sub><i>x</i></sub>/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 H<sub>2</sub>. Stable partial current densities and high Faradaic efficiencies were also achieved in 0.5 M NaCl (12.5 mA cm<sup>–2</sup> 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 <i>in situ</i> X-ray absorption near-edge structure analyses. From the results of these mechanistic studies, we propose that dual mechanisms are functional on the BiO<sub><i>x</i></sub>/C catalysts. Specifically, a two-electron and one-proton transfer reaction to adsorbed CO<sub>2</sub> or a chemical proton transfer reaction to CO<sub>2</sub><sup>–</sup> anion are the possible rate-determining steps (RDSs) at low potentials, whereas a one-electron transfer reaction to CO<sub>2</sub> is the RDS at high potentials.