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

Experimental data for CO2 (and H2O) reduction to CO (and H-2) 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 CO2 center dot- and H-center dot radicals adsorbed at the electrode surface on both metal electrodes, respectively. It is found that CO2 reduction to CO is positively shifted by similar to 370 mV on gold as compared to silver, while hydrogen evolution is positively shifted by only similar to 110 mV. This explains why higher CO current efficiencies are obtained on gold (similar to 90% for gold as compared to only similar to 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. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3502532] All rights reserved.