Although there is the technical possibility of ORR occurring at the air/cathode interface because of insignificant ionic conduction of pure electronic conductor cathode materials, the oxide ions produced fails to migrate to the anode for useful work. Thus, the practical region for the ORR to occur in such materials is a very narrow TPB. It is here that so-called mixed-ionic electronic conducting (MIEC) cathode materials assume crucial importance. The mixed conductivity of MIEC cathode materials lets the system to extend the otherwise narrow ORR region and thus increase the efficiency of the cell (Figure 2) . In this regard, the last decade has seen significant research activity in developing MIEC materials for SOFC cathodes based on perovskites, double perovskites, Ruddlesden-Popper phases, and other layered oxide materials.