# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 5 Composite Cathodes
Another way in which the cathodic reaction differs in PCFCs in comparison to SOFCs is the generation of steam in the cathode. [PERSON] et al. [161] explored the use of small amounts of alkali oxides as steam-generating catalysts, demonstrating that Li\({}_{2}\)O as catalyst significantly improves the cathodic performance in the range 400-600 \({}^{\circ}\)C. The enhancement was attributed to catalytic activity for the dissociative chemisorption and diffusion of reactants on the composite-electrode surface. [PERSON] et al. [162] compared a proton-conducting LSF73-BCZY71 and a proton-blocking LSF73-20 SDC cathode over a BCZY71 proton-conducting electrolyte, showing that although LSF73-BCZY71 performed better in polarisation measurements, single cells with the LSF73-20 SDC cathode displayed higher OSVs and lower polarisation resistances, achieving an MPD of 0.45 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C. The same group then optimised the LSF73-20 SDC cathode in terms of microstructure and electrolyte-interface characteristics, raising the MPD to 0.542 W\(\cdot\)cm\({}^{-2}\) at 650 \({}^{\circ}\)C [163]. The proton-blocking strategy is further explored with the composite La\({}_{2}\)NiO\({}_{4+8}\)-LaNi\({}_{0.6}\)Fe\({}_{0.4}\)O\({}_{3-\delta}\) electrode, with a 70\(\cdot\)30 wt % mixture showing the lowest ASR and an MPD of 0.590 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C on BCZY71 electrolyte [164]. Another ferrite-based, cobalt-free, composite cathode achieving a high power output is Pr\({}_{0.6}\)Sr\({}_{0.4}\)Cu\({}_{0.2}\)Fe\({}_{0.8}\)O\({}_{3-\delta}\)-20 SDC with MPD values of 0.45 and 0.55 W\(\cdot\)cm\({}^{-2}\) at 650 \({}^{\circ}\)C on BaCe\({}_{0.5}\)Zr\({}_{0.3}\)Y\({}_{0.2}\)O\({}_{3-\delta}\) and BCZY71 electrolytes, respectively [165]. Further improvement was obtained on single La\({}_{0.35}\)Pr\({}_{0.15}\)Sr\({}_{0.5}\)FeO\({}_{3-\delta}\) in the composite cathode, reaching a power density of 1.08 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C [166]. [PERSON] et al. [167] studied the ferrite phase Pr\({}_{0.58}\)Sr\({}_{0.4}\)Fe\({}_{0.8}\)Co\({}_{0.2}\)O\({}_{3-\delta}\) in conjunction with BCYb10, reporting the lowest polarisation resistance for a 50:50 vol % mixture. In the series of ferrite-based perovskites Nd\({}_{0.5}\)Ba\({}_{0.5}\)Fe\({}_{1.5}\)Ni\({}_{0.3-\delta}\) (0 \(\leq\) x \(\leq\) 0.15), the highest conductivity is observed for the x = 0.1 member [168]. A composite cathode of Nd\({}_{0.5}\)Ba\({}_{0.5}\)Fe\({}_{1.5}\)Ni\({}_{3}\)O\({}_{3-\delta}\)-BCZY71 on BCZY71 electrolyte exhibited a power density of 0.49 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C. The perovskite BaCe\({}_{0.4}\)Fe\({}_{0.4}\)Co\({}_{0.2}\)O\({}_{3-\delta}\) was prepared as a precursor powder by a modified Pechini method, from which two perovskite phases, referred to as a nanocomposite, were discernible on heat treatment at 900 \({}^{\circ}\)C [169]. A single cell of the cathode on BZCYYb17 electrolyte produced an MPD of 0.335 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C. A similar performance of 0.341 W\(\cdot\)cm\({}^{-2}\) and a low R\({}_{\rm P}\) of 0.1 \(\Omega\cdot\)cm\({}^{2}\) at 700 \({}^{\circ}\)C was achieved for the cobalt-free composite Sm\({}_{0.5}\)Sr\({}_{0.5}\)FeO\({}_{3-\delta}\)-BCZY71 on BCZY71 electrolyte [170]. One of the best performances of a ferrite-based cobalt-free electrode was reported for the series Ba\({}_{0.95}\)Ca\({}_{0.05}\)Fe\({}_{0.9}\)-xSn\({}_{x}\)Y\({}_{0.1}\)O\({}_{3-\delta}\) as a composite phase with SDC, in which Sn-doping was found to enhance electrochemical performance, with the x = 0.05 phase providing an MPD of 0.95 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C on BCZY71 electrolyte [171]. [PERSON] et al. [172] have recently reported on the ferrite-based series (SrBa)\({}_{1-x}\)Pr\({}_{x}\)(CuTi)\({}_{0.2}\)Fe\({}_{0.8}\)O\({}_{3-\delta}\), in which the best electrochemical performance is obtained for x = 0.4 in a composite with SDC, achieving an MPD of 0.925 W\(\cdot\)cm\({}^{-2}\) at 750 \({}^{\circ}\)C on BCZY71 electrolyte.