A review on cathode materials for conventional and proton-conducting solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are energy conversion technologies known for their excellent efficiency and high energy density. However, the application of SOFCs is restrained by their high operating temperatures (800–1000 °C), which result in overall energy system degradation. Reducing the operating temperatures of SOFCs leads to a reduction in power density because of insufficient protonic–ionic conduction. Ionic conduction at the cathode component is related to polarization and area-specific resistance that varies following the selected material. To date, several types of cathode materials have been investigated, namely, pure electronic conductor, mixed protonic–electronic conductor, mixed ionic–electronic conductor (MIEC) and triple protonic–electronic–ionic conductor (THOEC). Amongst these conductors, MIEC and THOEC currently lead in research development and application in conventional and proton-conducting intermediate–low-temperature SOFCs, yet further studies need to be carried out to ensure the continuous improvement of these materials as SOFC cathode. In this review, an explanation on the different types of cathodes will be discussed, with emphasis on MIEC- and THOEC-based cathodes. This review also includes recent progress and challenges encountered for both materials in the SOFC environment.