Mathematical Modeling of CO2 Reduction to CO in Aqueous Electrolytes: I. Kinetic Study on Planar Silver and Gold Electrodes

Abstract

Experimental data for (and ) reduction to CO (and ) on flat gold and silver electrodes in and aqueous electrolytes and at room temperature are analyzed using a steady-state mathematical model. Rate constants and charge transfer coefficients for and reduction reactions are derived from the experimental data, assuming that the rate-determining steps for and reduction reactions are the formation of and radicals adsorbed at the electrode surface on both metal electrodes, respectively. It is found that reduction to CO is positively shifted by on gold as compared to silver, while hydrogen evolution is positively shifted by only . This explains why higher CO current efficiencies are obtained on gold ( for gold as compared to only 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 increase and concentration decrease at the electrode surface. Finally, an analysis of data for various 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.