Elementary reaction modeling of CO2/H2O co-electrolysis cell considering effects of cathode thickness

Abstract

A one-dimensional elementary reaction model of CO2/H2O co-electrolysis in solid oxide electrolysis cell (SOEC) coupled with heterogeneous elementary reactions, electrochemical reactions, electrode microstructure, and the transport of mass and charge is developed in this paper. This model, validated with the experimental performance of H2O electrolysis, CO2 electrolysis and CO2/H2O co-electrolysis at 700 °C, is demonstrated to be a useful tool for understanding the intricate reaction and transport processes within SOEC electrode and the electrode structure design and optimization. The simulation results indicate that the heterogeneous reactions reach the equilibrium near the cathode outside surface, and the electrochemical reactions mainly occur in the electrode near the electrode–electrolyte interface. The main zone of electrochemical reactions is far enough from the main zone of heterogeneous reactions, so that the two kinds of reactions almost don't influence each other when the cathode is thick enough (e.g. 700 μm). While, as the cathode thickness reduces, the zones of electrochemical reactions and the non-equilibrium heterogeneous reactions overlap each other, and the electrochemical performance of CO2/H2O co-electrolysis is affected by the variations of elementary species concentrations of O(Ni) and (Ni) due to the heterogeneous reactions. The model successfully explains the experimental phenomenon that the polarization curve of CO2/H2O electrolysis lies between that of H2O and CO2 electrolysis in a cathode supported SOEC, but almost the same as that of H2O electrolysis in a electrolyte supported SOEC.