Diving deep into solid oxide-based CO2 electrolysis: Operando insights

CO₂ reduction to fuels using solid oxide electrodes is a promising approach due to high faradaic and energy efficiencies. CO₂ reducing electrodes (cathodes) form the central challenge in enabling solid oxide technology for CO₂ electrolysis. Typical cathodes can comprise of both oxides such as perovskites and metals such as Ni and Fe. Efforts at improving the activity, selectivity, and stability of the electrodes continue. Operando methods provide direct access to active sites during the reaction and provide valuable information such as the identity of catalytic material, nature of reaction intermediates, oxidation state of catalytic ions, etc. These methods have created a deeper mechanistic understanding, unravelled new performance indicators, and increasingly enabling a deep diagnostic based systematic development of catalysts and processes. This study summarises and analyses data from operando approaches to develop an understanding of CO₂ reduction mechanism on certain commonly studied electrodes. In particular, this review discusses CO₂ reduction mechanism on electrodes such as Ni-YSZ, CeO_2-x and perovskites such as La_1-xSr_xFeO_y. The CO₂ reduction on these surfaces essentially progresses on an oxide terminated surface via formation of a three coordinated carbon (carbonate type) intermediate formed at oxygen defect sites. Metal electrodes such as Ni-YSZ were found to oxidize in situ in presence of CO₂ and the reaction proceeded via oxide mediated mechanism. In electrodes such as La_1-x Sr_xFeO_y, exsolution of metals was essentially found to have no direct impact on CO₂ electrolysis. In the context of catalyst coking on CeO_x electrodes, new descriptors, such as the number of reduced sites (Ce³⁺), and the existence of metal carbonyl species “Ce³⁺ – CO” have emerged.