A review on recent advances and trends in symmetrical electrodes for solid oxide cells

Symmetrical solid oxide cells (SSOCs) with identical air and fuel electrodes have gained significant scientific interest in the last decade because they offer several advantages over conventional cell configurations. Among other features, simpler fabrication, better chemical and thermo-mechanical compatibility between cell layers, and electrode reversibility, make them attractive for electricity generation, H2 production and CO2 electroreduction. This review offers an overview of the most recent advances in the field of SSOCs, paying special attention to the relationship between electrode composition, crystal structure and properties. With that aim, symmetrical electrodes are classified in four groups according to their redox stability, i.e. single phases, composites, electrodes with exsolved metal particles and those that suffer a drastic phase transformation under reducing conditions, known in the literature as quasi-symmetrical electrodes. Furthermore, an outlook of other cell configurations with increased scientific interest are also discussed, i.e. symmetrical protonic fuel cells (H–SSOCs) and solid oxide electrolyzers for CO2 electroreduction. With this overview in mind, the authors would like to highlight the challenge ahead of finding electrode materials that optimally work under both oxidizing and reducing conditions in terms of redox stability and electrochemical properties, and further conclude on the future development of SSOCs.