# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 4 Triple Proton Oxide Ion Electron Hole-Conducting Oxides
However, the extent of proton absorption in double perovskites seems to be subtly dependent on composition. SIMS showed that the double perovskite BaGd\({}_{0.8}\)La\({}_{0.2}\)Co\({}_{2}\)O\({}_{6-\delta}\) (BGLC) incorporates protons in the bulk, whereas the simple perovskite Ba\({}_{0.5}\)La\({}_{0.5}\)CoO\({}_{3-\delta}\) (BLC) does not [112]. Nevertheless, a better performance as electrolyser with a SrZr\({}_{0.5}\)Ce\({}_{0.4}\)Y\({}_{0.1}\)O\({}_{3-\delta}\) electrolyte was observed for BLC. A recent study also adopts the double perovskite Ba\({}_{1-x}\)Gd\({}_{0.8}\)La\({}_{0.2+x}\)Co\({}_{2}\)O\({}_{6-\delta}\) as the steam anode in a tubular protonic ceramic electrolysis cell with a low polarisation resistance (<1 \(\Omega\cdot\)cm\({}^{2}\)) at 600 \({}^{\circ}\)C [113]. [PERSON] et al. [107] suggest that the focus should be shifted to studying the role of essolved secondary phases on promoting the electrochemical reaction or on the triple-conducting secondary phases themselves, such as BaCo\({}_{0.8}\)Gd\({}_{0.2}\)O\({}_{3-\delta}\). Recently, a simple cobaltite perovskite PrNi\({}_{0.5}\)Co\({}_{0.5}\)O\({}_{3-\delta}\) (PNC) associated with triple-conducting properties was processed with a nanofibre microstructure, leading to a very low polarisation resistance of 0.055 \(\Omega\cdot\)cm\({}^{2}\) at 500 \({}^{\circ}\)C and an excellent MPD of 0.611 W\(\cdot\)cm\({}^{-2}\) at 600 \({}^{\circ}\)C when operating on BCZYYb4411 electrolyte [114] (Figure 5). Another simple cobaltite perovskite developed in 2014, around the period that the first double perovskites were reported, is BaCo\({}_{0.4}\)Fe\({}_{0.4}\)Zr\({}_{0.1}\)Y\({}_{0.1}\)O\({}_{3-\delta}\) (BCFZY). This phase gave an excellent power output of 0.455 W\(\cdot\)cm\({}^{-2}\) at 500 \({}^{\circ}\)C due to its triple-conducting character [115]. The same material then demonstrated excellent performance as a cathode in SOFCs with an oxide-ion conducting Ce\({}_{0.8}\)Gd\({}_{0.2}\)O\({}_{2-\delta}\) electrolyte [116], providing an untypical example of transfer of technology from PCFC to SOFCs. BCZFY was chosen as the cathode in the development of a manufacturing cost model to estimate the production costs of PCFC stack technology using high-volume manufacturing processes [117]. As is the case for double perovskites [105], A-site deficiency in BCFZY increases oxygen deficiency, significantly improving oxygen diffusion and hydration kinetics: an MPD of 0.797 W\(\cdot\)cm\({}^{-2}\) was achieved at 650 \({}^{\circ}\)C for the composition Ba\({}_{0.9}\)Co\({}_{0.4}\)Fe\({}_{0.4}\)Zr\({}_{0.1}\)Y\({}_{0.1}\)O\({}_{3-\delta}\)[118]. B-site deficiency in BCZFY is also reported to enhance performance with a cell containing a Ba(Co\({}_{0.4}\)Fe\({}_{0.4}\)Zr\({}_{0.1}\)Y\({}_{0.1}\))\({}_{0.95}\)O\({}_{3-\delta}\) cathode, achieving a power output of 0.840 W\(\cdot\)cm\({}^{-2}\) at 650 \({}^{\circ}\)C on a thin-film, B-site-deficient Ba(Zr\({}_{0.1}\)Ce\({}_{0.7}\)Y\({}_{0.1}\)Y\({}_{0.1}\))\({}_{0.95}\)O\({}_{3-\delta}\) electrolyte [119]. On partially replacing Ba by Ca to obtain Ba\({}_{0.95}\)Ca\({}_{0.05}\)Co\({}_{0.4}\)Fe\({}_{0.4}\)Zr\({}_{0.1}\)Y\({}_{0.1}\)O\({}_{3-\delta}\), [PERSON] et al. [120] reduced superficial Ba segregation, improving phase stability and CO\({}_{2}\) tolerance, as well as the electrocatalytic performance. Recently, Ni-doped BaCo\({}_{0.4}\)Fe\({}_{0.4}\)Zr\({}_{0.1}\)Y\({}_{0.1}\)O\({}_{3-\delta}\) was also evaluated with a Ba(Ce\({}_{0.7}\)Zr\({}_{0.1}\)Y\({}_{0.1}\)Y\({}_{0.1}\))\({}_{0.95}\)O\({}_{3-\delta}\) electrolyte, showing improved oxygen mobility, bulk oxide-ion-proton conductivity, and surface exchange kinetics [121]. BCFZY is arrived at through Y-doping of the BaCo\({}_{0.4}\)Fe\({}_{0.4}\)Zr\({}_{0.2}\)O\({}_{3-\delta}\) cathode discussed in the previous section. [PERSON] et al. [122] offer a detailed study of proton uptake in BCFZ using thermogravimetry.