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

Abstract

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: La_0.6Ca_0.4FeO_3−δ (LCF), Nd_0.6Ca_0.4FeO_3−δ (NCF), and Pr_0.6Ca_0.4FeO_3−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.