The dark side of metal exsolution: a combined in situ surface spectroscopic and electrochemical study on perovskite-type cathodes for high-temperature CO₂ electrolysis

In solid oxide CO2 electrolysis cells, moderate activity and coking of the cathode are major issues that hinder commercialization of this important technology. It has been already shown that cathodes based on a mixed conducting oxide decorated with well-dispersed metal nanoparticles, which were grown via an exsolution process, are highly resilient to carbon deposition. Using perovskite-type oxides that contain reducible transition metals, such nanoparticles can be obtained in situ under sufficiently reducing conditions. However, the direct catalytic effect of exsolved metal nanoparticles on the CO2 splitting reaction has not yet been explored thoroughly (e.g. by employing well-defined model systems), thus, an in-depth understanding is still lacking. In this study, we aim at providing a crucial piece of insight into high-temperature electrochemical CO2 splitting on exsolution-decorated electrodes: we present the results of combined Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) and electrochemical measurements on three different ferrite perovskites, which were employed as thin film model electrodes. The investigated materials are: La0.6Ca0.4FeO3−δ (LCF), Nd0.6Ca0.4FeO3−δ (NCF), and Pr0.6Ca0.4FeO3−δ (PCF). The results obtained allow us to directly link the electrode's CO2 splitting activity to their surface chemistry. Especially, the electro-catalytic activity of the materials decorated with and without metallic iron nanoparticles was in focus. Our experiments reveal that in contrast to their beneficial role in H2O electrolysis, exsolved Fe0 metal particles deteriorate CO2 electrolysis activity. This behavior contrasts with expectations derived from earlier reports on porous samples, and is likely a consequence of the differences between the CO2 splitting and H2O splitting mechanism.