Co-electrolysis of CO2 and H2O in SOECs: advance in cathodes

Co-electrolysis of CO2 and H2O using high-temperature solid oxide electrolysis cells (SOECs) into valuable chemicals has attracted great attentions recently due to the high conversion and energy efficiency, which provides opportunities of reducing CO2 emission, mitigating global warming and storing intermittent renewable energies. A single SOEC typically consists of an ion conducting electrolyte, an anode and a cathode where the co-electrolysis reaction takes place. The high operating temperature and difficult activated carbon–oxygen double-bond of CO2 put forward strict requirements for SOEC cathode. Great efforts are being devoted to develop suitable cathode materials with high catalytic activity and excellent long-term stability for CO2/H2O electro-reduction. The so far cathode material development is the key point of this review and alternative strategies of high-performance cathode material preparation is proposed. Understanding the mechanism of CO2/H2O electro-reduction is beneficial to highly active cathode design and optimization. Thus the possible reaction mechanism is also discussed. Especially, a method in combination with electrochemical impedance spectroscopy (EIS) measurement, distribution functions of relaxation times (DRT) calculation, complex nonlinear least square (CNLS) fitting and operando ambient pressure X-ray photoelectron spectroscopy (APXPS) characterization is introduced to correctly disclose the reaction mechanism of CO2/H2O co-electrolysis. Finally, different reaction modes of the CO2/H2O coelectrolysis in SOECs are summarized to offer new strategies to enhance the CO2 conversion. Otherwise, developing SOECs operating at 300–600 °C can integrate the electrochemical reduction and the Fischer– Tropsch reaction to convert the CO2/H2O into more valuable chemicals, which will be a new research direction in the future.