Mathematical Modeling of CO₂ Reduction to CO in Aqueous Electrolytes

Experimental data for CO₂ (and H₂O) reduction to CO (and H₂) on flat gold and silver electrodes in KHCO₃ and NaClO₄ aqueous electrolytes and at room temperature are analyzed using a steady-state mathematical model. Rate constants and charge transfer coefficients for CO₂ and H₂O reduction reactions are derived from the experimental data, assuming that the rate-determining steps for CO₂ and H₂O reduction reactions are the formation of CO^•-₂ and H^• radicals adsorbed at the electrode surface on both metal electrodes, respectively. It is found that CO₂ reduction to CO is positively shifted by ~370 mV on gold as compared to silver, while hydrogen evolution is positively shifted by only ~110 mV. This explains why higher CO current efficiencies are obtained on gold (~90% for gold as compared to only ~68% for silver in potassium bicarbonate). The current fade for CO evolution at low electrode potential is related to the current increase for hydrogen evolution, which yields a high pH increase and CO₂ concentration decrease at the electrode surface. Finally, an analysis of data for various CO₂ partial pressures in equilibrium with the electrolyte is performed, in which the effect of acid-base reactions coupled with the CO evolution reaction is accounted for.