Understanding of CO2 Electrochemical Reduction Reaction Process via High Temperature Solid Oxide Electrolysers

The electrochemical reduction of CO₂ was studied via solid oxide electrolysis cell (SOEC), a type of device that can work reversely into a solid oxide fuel cell (SOFC) to generate electricity. Application of SOEC for CO₂ electrolysis possesses potential rewards both in energy and environmental aspects, as it offers a way to recycle CO₂ into chemicals and value-added fuels, which helps to reduce the accumulation of atmospheric CO₂ and realize the carbon neutral cycling of fuels. Secondly, SOEC techniques provide a means to utilise the intermittent renewable sources, such as wind, tide, etc., as energy input to store excess electricity in the form of H₂, CO and hydrocarbons and use these chemicals when necessary.  

Yet it is a challenging task to realize an efficient reduction of CO₂ by SOEC due to the non-polar nature of CO₂ fuels which are hard to be chemically absorbed and activated in high temperature range. The CO product from CO₂ reduction is also demanding for the choice of fuel electrode (i.e. cathode in SOEC and anode in SOFC) materials, which has been an issue for the CO/hydrocarbon-fuelled SOFCs. To date, the CO₂ electrolysis by SOEC is still at the starting point, and the mechanisms on the electrochemical reduction of CO₂through SOECs are not fully understood. Extensive efforts need to be dedicated to the material developments, mechanism study, and system designs etc.

Effort was made in our lab to find a highly performed, long-term durable cathode material for electrochemical reduction of CO₂ by SOEC and to obtain more understandings of the mechanism of CO₂ electrochemical reduction process. Different cathode materials were employed, including Ni-8 mol% yttria stabilised zirconia (YSZ) cermet, (La,Sr)(Cr,Mn)O₃ (LSCM)-YSZ composite, and LSCM-(Gd, Ce)O₂ (GDC) composite. Focus was casted on LSCM based cathodes which were found to be carbon-resistive, and the microstructure of LSCM-based cathodes was tuned to high performance and efficient CO₂ electrolysis by the strategy of applying a gradient composite cathode and of adopting wet impregnation as cell fabrication procedures. The electrochemical performance of CO₂ electrolysis was characterized in various CO₂-CO mixtures and applied potentials in 900-750^oC with the aid of impedance spectroscopy, on YSZ electrolyte supported three-electrode SOECs.

In this paper, impedance behaviour of the electrochemical reduction reaction of CO₂ from different cathode SOECs was correlated with the variations in gas composition, operating temperature and loading potential. As both charge transfer and surface adsorption/desorption equilibration and surface diffusion of activated species were found to be dominant processes taking place in the scope of cathodes under examination, with the later being significant in CO₂-rich fuels, areas of discussion will be the effects of operational conditions on the impedance arcs associated with these steps, and how the cathode microstructure impacts the impedance behaviour and the corresponding elementary steps from CO₂electrochemical reduction.

By introducing a LSCM-YSZ 30-70/LSCM-YSZ 60-40 graded cathode and by incorporating extra catalyst into LSCM-GDC composite, the cathode performance was greatly improved, and the surface activity was profoundly accelerated. However, the most effective way to promote cathode performance was to introduce the cathode components in separate steps via wet impregnation. A competitive CO₂ electrolysis performance to Ni-YSZ cermet was obtained from the GDC impregnated LSCM-YSZ cathode with 0.5% Pd extra catalyst, which also showed a comparable performance between SOEC and SOFC when operating in CO₂-CO 50-50 mixture.