The electrochemical reduction of CO₂ is very important for power to gas/fuel processes to allow storage of sustainably produced electric power in form of high energy density fuels. CO₂ reduction in solid oxide electrolysis cells (SOECs) is particularly promising owing to the high operating temperature above 700 °C, which lowers the thermodynamic and kinetic overpotentials that are required for electrolysis. Additionally, SOECs have the advantage that the oxygen vacancies that are required for the reduction reaction are continuously transported through the gas separating electrolyte to the CO₂ reducing electrode. Therefore, the CO₂ splitting reaction can run continuously. For this reaction, mixed ionic and electronic conducting electrodes promise some advantages over the currently mostly used Ni-YSZ cermet electrodes, e.g. stability in oxidizing atmosphere, a wide range of doping possibilities to increase the catalytic kinetics and higher resistance towards coking. The reaction mechanisms on the surface and their connection to defect concentrations are, however, scarcely investigated so far. Only few experiments were carried out on ceria^{1, 2}, while on perovskite-type materials, to the best of our knowledge, H₂O splitting was investigated so far^{3, 4}, while mechanistic studies of CO₂ splitting are missing. In order to fill this knowledge gap we present here, a mechanistic study of CO₂ electrolysis on mixed conducting perovskite-type electrodes in well-defined thin film geometry. In addition, we employed near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to monitor the concentrations of reaction intermediates. These measurements revealed the formation of a carbonate intermediate, which is present only upon cathodic polarization (i.e. under sufficiently reducing conditions). Moreover, the amount of adsorbates was found to depend on the vacancy concentration in the electrode material, which means that the easier reducible La_0.4Sr_0.6FeO_3-δ electrodes have a higher surface carbonate coverage than the more reduction tolerant La_0.8Sr_0.2CrO_3-δ based electrodes thus identifying vacant oxygen sites as the predominant adsorption sites for carbon dioxide. Since the corresponding surface species were only observed under during application of a sufficiently large voltage, an additional electron transfer appears to be necessary to form an intermediate carbonate radical. Furthermore, it was found that especially chromite-based perovskites show a high resilience against carbon deposition. Electrode degradation due to coking, which occurred during very strong cathodic polarization, could be easily removed by retracting the applied voltage without damaging the electrode, thus proving chromite-based electrodes to be remarkably stable in CO₂ electrolysis conditions.

References

1. Y. Yu, B. Mao, A. Geller, R. Chang, K. Gaskell, Z. Liu, and B. W. Eichhorn, Physical Chemistry Chemical Physics, 16 (23), 11633-11639 (2014).

2. Z. A. Feng, M. L. Machala, and W. C. Chueh, Physical Chemistry Chemical Physics, 17 (18), 12273-12281 (2015).

3. A. Nenning, A. Opitz, C. Rameshan, R. Rameshan, R. Blume, M. Hävecker, A. Knop-Gericke, G. Rupprechter, B. Klötzer, and J. Fleig, The Journal of Physical Chemistry C, (2015).

4. A. K. Opitz, A. Nenning, C. Rameshan, R. Rameshan, R. Blume, M. Hävecker, A. Knop‐Gericke, G. Rupprechter, J. Fleig, and B. Klötzer, Angewandte Chemie International Edition, (2014).