Investigation on the Reaction Mechanism of Solid Oxide Co-Electrolysis with Different Inlet Mixtures Based on the Comparison of CO2 Electrolysis and H2O Electrolysis

The reaction mechanism of co-electrolysis on the solid oxide electrolysis cells (SOEC) was widely discussed and no consensus had ever been reached. The performance of CO2 electrolysis, H2O electrolysis and co-electrolysis was tested on an industrial-size cell with a conventional structure of Ni-YSZ|YSZ|GDC|LSCF. The co-electrolysis performance was inferior to that of H2O electrolysis but superior to that of CO2 electrolysis. Also, the total resistance (Rtot) of co-electrolysis first increased to a maximum and then began to descend with the increase of applied current, which was similar to the Rtot pattern of CO2 electrolysis but totally different from the monotonous increasing Rtot pattern of H2O electrolysis. Study on a button cell with the same structure as the industrial-size cell showed that the catalytic activity of fuel electrode for H2O electrolysis was 9 times higher than that for CO2 electrolysis and this huge difference might be the main cause of the different activation resistance (Ract) and thus different Rtot behavior of H2O electrolysis and CO2 electrolysis. The appearance of Rtot summit can be regarded as the signal of dominance of CO2 electrochemical reduction during co-electrolysis. Based on the position of Rtot summit and the impedance spectroscopy (EIS) results, the reaction mechanism of co-electrolysis was explored and it was found H2O electrolysis dominated at small current density and CO2 electrolysis gradually occurred and dominated with the increase of current density. Besides, H2 concentration in the inlet showed a more profound impact on the reaction mechanism of co-electrolysis compared to CO2 concentration.