New insights on the co-electrolysis of CO2 and H2 O through a solid oxide electrolyser operating at intermediate temperatures

A solid oxide electrolyser cell based on a Ni-yttria-stabilized zirconia (Ni-YSZ) supporting cathode, a double layer YSZ and Gadolinia-doped Ceria (GDC) electrolyte and a strontium doped lanthanum cobaltite anode (LSC) was investigated for the co-electrolysis of H2O and CO2 in the range of temperatures between 525 °C and 700 °C. The flow rate of the inlet stream was selected in order to reproduce practical operating conditions conventionally adopted for solid oxide electrolysis stacks. The flow of CO2 and H2O, was 2 times of that required by the faradaic reaction at high current densities in order to achieve an utilization close to 50%. A steady-state galvanostatic operating mode was selected in combination with the analysis of the effluents by gas-chromatography. Electrochemical studies also included electrochemical impedance spectroscopy (EIS) and polarization curves. The results showed an increase of CO content versus H2 in the formed syngas promoted by the increase of temperature. A small occurrence of methane was observed at 525 °C. The post operation analysis of the spent cell excluded the presence of carbon but it showed a fracture occurring in the cell at the interface cathode-electrolyte possibly as consequence of the re-oxidation of the cathode at the high operating temperatures. Such evidence suggests that an improvement of cell microstructure for high temperature solid oxide electrolysis is required.