Effect of gas compositions on co-electrolysis performance of solid oxide cells: Electromotive force and polarization resistance

• H 2 O CO 2 co-electrolysis performance of SOEC is experimentally investigated. • OCV in co-electrolysis is close to the theoretical EMF from H 2 O electrolysis. • A model based on mixed potential theory successfully reproduces OCVs in experiment. • Active sites in hydrogen electrode are mostly occupied by H 2 and H 2 O. • Reaction resistance in co-electrolysis is close to that in H 2 O electrolysis near OCV. Co-electrolysis performance of solid oxide cells is experimentally investigated with a focus on the electromotive force (EMF) and reaction resistance. Outlet gas composition is also analyzed to investigate the effect of the reverse-water-gas-shift (rWGS) reaction. The open-circuit voltage (OCV) of the cell is analyzed under various gas compositions with different reactant ratios. The mixed potential theory is employed to predict the EMF values based on the surface coverage ratio in the hydrogen electrode. The reaction resistance under co-electrolysis operation obtained from the equivalent circuit fitting analysis of the impedance spectra is compared with that under H 2 O and CO 2 electrolysis operations. The results obtained in this study indicate that, near the OCV condition, H 2 O electrolysis is the main reaction under co-electrolysis operation to determine the OCVs and reaction resistance, and the main conversion pathway of CO 2 into CO is the rWGS reaction.