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.