Electrochemical Modeling and Simulation of CO₂ Reduction in Solid Oxide Electrolysis Cells

Abstract

Mathematical models for electrochemical conversion of CO2 to CO in a solid oxide electrolysis cell (SOEC) are formulated. Model simulation has been carried out on a button cell having Ni-yttria stabilized zirconia (YSZ) composite as cathode, YSZ as electrolyte, and lanthanum strontium manganite (LSM) as anode material. The simulation results were used to estimate the overpotentials associated with the electrochemical reduction reactions of CO2 to CO and hence predict the overall performance of a SOEC under varying operating conditions. Overpotential includes the contribution from activation, concentration, and ohmic losses corresponding to cathode, anode, and electrolyte components of the SOEC. Fick’s model, the Butler–Volmer equation, and Ohm’s law are used to estimate the values of concentration, activation, and ohmic overpotential losses, respectively. Of the three, ohmic overpotential dominates over other losses throughout the operating voltages. The calculated value of ohmic overpotential loss is 0.19 V at an operating voltage of 1.2 V and current density of 0.8 A/cm2. The model is simulated for varying fuel gas compositions (CO2/CO = 99/01, 70/30, 50/50, and 30/70) at a temperature of 1,073 K, and corresponding current–voltage characteristics of the SOEC are predicted. As the CO2 content in fuel gas increases from 30% to 99%, the estimated current density is raised from −0.28 to −0.76 A/cm2 at an applied potential of 1.1 V, indicating the presence of CO2 in fuel and leading to improved performance of the SOEC. Similarly, at higher temperatures, part of the thermal energy contributes to activation of CO2 molecules; hence, the calculated current density at a particular applied voltage (1 V) is enhanced from −0.2 to −0.6 A/cm2 with a rise in temperature from 1,073 to 1,123 K of the SOEC system.