Different Performance and Mechanisms of CO2 Electrolysis with CO and H2 As Protective Gases in Solid Oxide Electrolysis Cell

The significance of converting CO2 into valuable feedstock is highly emphasized with the carbon neutrality vision. Generally, CO and H2 are usually applied as the protective gases during CO2 electrolysis in nickel based Solid Oxide Electrolysis Cell (SOEC). In this paper, the effects of H2 and CO as protective gases on the performance of a Ni-YSZ (yttria stabilization zirconia) based SOEC are studied. The electrode processes of the cell are investigated by the electrochemical impedance spectroscopy (EIS) and calculated distribution of relaxation times (DRT) results. The results show that CO makes little difference on the performance of the cell while the cell shows a superior performance with H2 as protective gas, indicating different mechanisms behind. The distinct discrepancies in characteristic frequency ranges of electrode processes with the CO2–CO and CO2–H2 mixtures show that the replacement of CO by H2 changes the reduction pathway of CO2 from electrochemical reduction to thermochemical reduction, and only steam is electrolyzed. However, the direct injection of steam deteriorates slightly the performance of SOEC. The effect of reverse water gas shift (RWGS) reaction on the electrochemical performance of SOEC is explored and its effect of delaying the concentration polarization and extending the limiting current density is confirmed. Furthermore, the unique pattern of flattened j-V curve at high applied current of CO2 electrolysis is repeatedly emphasized.