High Temperature Co-electrolysis of Carbon dioxide and Steam in a Solid Oxide Cell for Synthesis Gas Production

The utilisation of CO2 as a feedstock in the production of valuable products such as synthetic fuel is a promising pathway for mitigating its atmospheric concentration. A review of the high temperature co-electrolysis of CO2 and H2O in a solid oxide cell for syngas production has identified that further understanding of the co-electrolysis reaction mechanism is one of three key areas of development.
In this work, a co-electrolysis test facility was designed, developed and commissioned. Additionally, the performance of a NextCellTM electrolyte supported cell was investigated for CO2 electrolysis and CO2/H2O co-electrolysis with an aim to gain a better understanding of the reaction mechanism.
During CO2 electrolysis, an increase in cell area specific resistance was observed with increasing CO2 concentration. In addition, AC impedance spectra measurements showed a significant increase in polarisation resistance at the fuel electrode with increasing CO2/CO ratio. Short term durability studies carried out at -0.5 A/cm2, 850oC and fuel electrode compositions of 50% CO2, 25% CO and 25% N2 showed a sharp increase in cell voltage corresponding to a passivation rate of 120 mV/h in the first 5 hours of operation. This increase in cell voltage was caused by the adsorption of impurities to the Ni surface prompting partial blockage of the active Ni sites.
During CO2/H2O co-electrolysis, the exhaust gas compositions measured at open circuit voltage were ±2 mol % of the thermodynamic equilibrium compositions. AC impedance spectra measurements showed a slight increase in polarisation resistance at the fuel electrode with increasing CO2/H2O concentration. Direct current measurements showed a 21% increase in cell performance during CO2/H2O co-electrolysis compared to CO2 electrolysis. Furthermore, co-electrolysis durability studies carried out at -0.5 A/cm2 showed a significantly lower degradation rate of 1.3 mV/h over 44 hours of operation compared to CO2 electrolysis.