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 continues. 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, CeO2-x and perovskites such as La1-xSrxFeOy. 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 La1-x SrxFeOy, exsolution of metals was essentially found to have no direct impact on CO2 electrolysis. In the context of catalyst coking on CeOx electrodes, new descriptors, such as the number of reduced sites (Ce3+), and the existence of metal carbonyl species "Ce3+ -- CO" have emerged.