# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 3 Mixed Oxide-Ion-Electron-Conducting Cathodes
The significant, deleterious effects of cobalt on stability in H\({}_{2}\)O and thermal expansion, as well as its limited availability and increasing cost due to greater use, have promoted the study of alternative Co-free materials. (La,Sr)FeO\({}_{3-\delta}\) has proved to be a popular choice as a Co-free cathode since [PERSON] et al. [60] showed that the overpotential of La\({}_{0.7}\)Sr\({}_{0.3}\)FeO\({}_{3-\delta}\) was smaller than that of both La\({}_{0.7}\)Sr\({}_{0.3}\)MnO\({}_{3-\delta}\) and La\({}_{0.7}\)Sr\({}_{0.3}\)CoO\({}_{3-\delta}\) using SrCe\({}_{0.95}\)Yb\({}_{0.05}\)O\({}_{3-\delta}\) as the electrolyte. [PERSON] et al. reported a power output of 0.538 W\(\cdot\)cm\({}^{-2}\) at 650 \({}^{\circ}\)C on BCZ71 electrolyte using cobalt-free SrFe\({}_{0.95}\)Nb\({}_{0.05}\)O\({}_{3-\delta}\) as the cathode [61]. A further La-free electrode with good performance is Ba\({}_{0.5}\)Sr\({}_{0.5}\)Fe\({}_{0.8}\)Zn\({}_{0.2}\)O\({}_{3-\delta}\)[62], with a power output of 0.486 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C on BCZY71 electrolyte. Nevertheless, the general tendency is for electrode performance to improve with the inclusion of Co in the cathode composition. A study of the substitution of Fe on the B-site in the series Nd\({}_{0.5}\)Ba\({}_{0.5}\)Fe\({}_{0.9}\)M\({}_{0.1}\)O\({}_{3-\delta}\) (M = Ni, Cu, Co) indicates that the introduction of cobalt leads to higher ionic conductivity and lower polarisation resistance [63]. The RBa(Co,M)\({}_{4}\)O\({}_{7}\) series (R = Y, Ca; M = Zn, Fe, Al) with tetrahedral corner-sharing (Co/M)O\({}_{4}\) units has also been studied as both the SOFC and PCFC cathode for exhibiting a high catalytic activity and low thermal-expansion coefficient compatible with typical electrolyte materials; the Zn-containing composition YBa\({}_{3}\)Co\({}_{3}\)ZnO\({}_{7}\) is most promising due to its high thermal stability [64]. Y\({}_{0.8}\)Ca\({}_{0.2}\)BaCo\({}_{4}\)O\({}_{7}\) exhibits a good electrochemical performance on BCZY71 electrolyte, achieving a maximum power density (MPD) of 0.472 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C [65]. The same composition was deposited by a tape-calendering method on BaCe\({}_{0.5}\)Zr\({}_{0.3}\)Y\({}_{0.2}\)O\({}_{3-\delta}\) (BCZY53) electrolyte, providing an MPD of 0.308 W\(\cdot\)cm\({}^{-2}\) at 725 \({}^{\circ}\)C [66]. Other cobaltite materials with layered misfit-type structures, consisting of alternating layers of a disordered AO-CoO-AO rock-salt-type layer and a Co-O CdI\({}_{2}\)-type layer stacked along the c-direction, have been of interest due to their good thermoelectric properties [67]. The Ca\({}_{3-x}\)La\({}_{x}\)Co\({}_{4}\)O\({}_{9+\delta}\) (x = 0, 0.3) series was reported to have high electrical conductivity (53 S\(\cdot\)cm\({}^{-1}\) at 600 \({}^{\circ}\)C, for x = 0.3) and promising electrocatalytic activity on BCY10 electrolyte [68]. The composition Ca\({}_{3}\)Co\({}_{4}\)O\({}_{9+\delta}\) was employed more recently in a reversible protonic ceramic cell based on a BCZY53 electrolyte, yielding a power density of \(\approx\)0.290 W\(\cdot\)cm\({}^{-2}\) in fuel-cell mode and an electrolysis current of \(\approx\)0.580 A\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C [69].