Mathematical modeling of CO2 reduction to CO in aqueous electrolytes

Abstract

A cell design for CO2 (and H2O) reduction to CO (and H2), similar to a proton-exchange-membrane fuel cell but with a silver or gold catalyst at the cathode and a pH buffer layer (aqueous KHCO3) between the cathode catalyst layer and the membrane, is evaluated. The cell can operate at a CO current density as high as ?135 mA/cm2 (on supported Au catalyst). The general framework for treating equilibrated reactions and equilibrated interfacial mass transfer in a multiphase medium is derived and used to set forth a mathematical model of the electrolysis cell. At low current density, the model accounts for the experimental data pretty well, using rate constant values obtained on flat Ag and Au electrodes. The model is further used to understand some of the cell features, such as the effect of CO2 partial pressure in the cathode gas channel and KHCO3 concentration in the buffer layer, the resistance increase and the CO efficiency decrease at high current density, and the decay in CO efficiency upon operation. Finally, the model is used to predict the behavior of a cell design based on a porous anion-exchange membrane instead of the aqueous buffer layer.