# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 5 Composite Cathodes
[153] performed long-term tests using 11 different fuels, including hydrogen, methane, propane, and ethanol, in NiO-BZY20/BZY20/BCZYYb7111/BCFZY cells between 500 and 600 \({}^{\circ}\)C for over 6000 h, observing very low degradation and high resistance to sulphur poisoning. Optimisation of the microstructure of BCFZY cathode, using a low-temperature spray-pyrolysis deposition method, greatly decreased the polarisation resistance to values as low as 0.018 \(\Omega\cdot\)cm\({}^{2}\) at 700 \({}^{\circ}\)C [154]. Although cobalt-containing perovskites, such as LSCF and BSCF, are excellent MIEC components in composite cathodes, there is considerable advantage in replacing Co to mitigate high thermal expansion, carbonation and, to some extent, costs. Thus, ferrite- and manganite-based perovskites have been the focus of attention in composite electrodes as the lower oxide-ion conductivity than that offered by the cobaltite phases may be compensated somewhat by oxide-ion transport in the proton-conducting component of the composite electrode. Figure 6: SEM images of (**a**) a protonic ceramic fuel cell based on a Ca\({}_{0.3}\)Y\({}_{0.7}\)Fe\({}_{0.5}\)Co\({}_{0.5}\)O\({}_{3-\delta}\)–BCZY71 composite cathode over BCZY71 electrolyte, (**b**) cathode-electrolyte interface, and (**c**) anode-electrolyte interface; (**d**) performance of the single cell at different temperatures. Copyright 2019 by Elsevier [146]. [PERSON] et al. [155] employed a component cathode of BaCe\({}_{0.8}\)Sm\({}_{0.2}\)O\({}_{3-\delta}\) and La\({}_{0.7}\)Sr\({}_{0.3}\)FeO\({}_{3-\delta}\) (LSF73) on a BaCe\({}_{0.8}\)Sm\({}_{0.2}\)O\({}_{3-\delta}\) electrolyte, generating an MPD of 0.35 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C, in which the surface was modified by spray-coating to promote adherence of the cathode to the electrolyte. In another study, both composite anode and cathode were prepared by the infiltration of (La\({}_{0.7}\)Sr\({}_{0.3}\))V\({}_{0.9}\)O\({}_{3-\delta}\) and La\({}_{0.8}\)Sr\({}_{0.2}\)FeO\({}_{3-\delta}\), respectively, into the proton conductor BaCe\({}_{0.51}\)Zr\({}_{0.3}\)Y\({}_{0.15}\)Zn\({}_{0.04}\)O\({}_{3-\delta}\) as scaffold, and the cell performance was compared to that of YSZ and Ba(Ce\({}_{0.51}\)Zr\({}_{0.30}\)Y\({}_{0.15}\)Zn\({}_{0.04}\))O\({}_{3-\delta}\) as electrolyte [156]. Higher power densities were observed for the proton-conducting cell, which were attributable to lower electrode polarisation and lower ohmic resistance of the electrolyte. A comparison of composite electrodes composed of the same H\({}^{+}\)-conducting phase with La\({}_{0.8}\)Sr\({}_{0.2}\)FeO\({}_{3-\delta}\) (LSF82) or undoped LaFeO\({}_{3}\) (LF) revealed similar performances, implying that the greater oxide-ion conductivity of the LSF82 phase had a minimal effect on the electrode performance [157]. This led the authors to suggest that the role of the hydroxyl ion may be critical in generating oxide ions and water. In a follow-up study [158], a comparison of BaZr\({}_{0.75}\)Y\({}_{0.15}\)O\({}_{3-\delta}\), BaCe\({}_{0.75}\)Y\({}_{0.15}\)O\({}_{3-\delta}\), and BaCe\({}_{0.45}\)Zr\({}_{0.30}\)Y\({}_{0.15}\)O\({}_{3-\delta}\) scaffolds infiltrated with LSF found the best cathode performance for the BaZr\({}_{0.75}\)Y\({}_{0.15}\)O\({}_{3-\delta}\) and BaCe\({}_{0.45}\)Zr\({}_{0.30}\)Y\({}_{0.15}\)O\({}_{3-\delta}\) scaffolds. [PERSON] et al. [159] studied the influence of Zr\({}^{4+}\) doping on the Sr\({}_{2}\)Fe\({}_{1.5}\)Mo\({}_{0.5}\)O\({}_{6-\delta}\) perovskite, achieving improved proton diffusion kinetics, D\({}_{H,Chem}\) = 8.71\(\cdot\)10\({}^{-7}\) cm\({}^{2}\) s\({}^{-1}\), and a low polarisation resistance, 0.169 \(\Omega\cdot\)cm\({}^{2}\) (both at 700 \({}^{\circ}\)C) for Zr-doped material. Recently, [PERSON] et al. [160] reported a nanostructured Sr\({}_{2}\)Fe\({}_{1.5}\)Mo\({}_{0.5}\)O\({}_{6-\delta}\) composite cathode operating in electrolyser mode, with a performance of 0.21 A\(\cdot\)cm\({}^{-2}\) at 600 \({}^{\circ}\)C and a faradaic efficiency of 63.6%.