# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 5 Composite Cathodes
[183] made use of a thick, porous BZCY71 backbone, which was infiltrated with La\({}_{2}\)NiO\({}_{4+\delta}\) nanoparticles for hydrogen-flux measurements. Similarly, nanoparticles of (Pr\({}_{0.9}\)La\({}_{0.1}\))\({}_{2}\)(Ni\({}_{0.74}\)Cu\({}_{0.21}\)Nb\({}_{0.05}\))O\({}_{4+\delta}\) were infiltrated on the surface of a BCZY71 skeleton with a load of 46 wt % to achieve an MPD of 0.770 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C; the cell was operated without degradation for 200 h at 600 \({}^{\circ}\)C [184] (Figure 7). [PERSON] et al. [185] employed a Pr\({}_{1.9}\)Ba\({}_{0.1}\)NiO\({}_{4+\delta}\)-BaCe\({}_{0.5}\)Zr\({}_{0.3}\)Dy\({}_{0.2}\)O\({}_{3-\delta}\) composite cathode, which was deposited by tape calendaring and co-sintered in a single step with BaCe\({}_{0.5}\)Zr\({}_{0.3}\)Dy\({}_{0.2}\)O\({}_{3-\delta}\) electrolyte and Ni-BaCe\({}_{0.5}\)Zr\({}_{0.3}\)Dy\({}_{0.2}\)O\({}_{3-\delta}\) anode layers; cell performance was reported as 0.470 W\(\cdot\)cm\({}^{-2}\) at 600 \({}^{\circ}\)C. A similar composite of Pr\({}_{2}\)NiO\({}_{4}\)-BCZY62 was used in electrolysis mode in a thin film BCZY62-based single cell, achieving a current density of 0.977 A\(\cdot\)cm\({}^{-2}\) at an electrolysing potential of 1.3 V [186] (Figure 8). La\({}_{2}\)NiO\({}_{4+\delta}\) has also been used as the scaffold for infiltration with LaNi\({}_{0.6}\)Fe\({}_{0.4}\)O\({}_{3-\delta}\) (LNF) particles, with the cathode achieving an outstanding power output of 0.969 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C employing an LNF loading of 31 wt % and a BCZY71 electrolyte [187]. Recently, [PERSON] et al. [188] obtained an MPD of 0.17 W\(\cdot\)cm\({}^{-2}\) in Pr\({}_{1.7}\)Ca\({}_{0.3}\)NiO\({}_{4+\delta}\) composite cathodes with BaCe\({}_{0.8}\)Gd\({}_{0.19}\)Cu\({}_{0.01}\)O\({}_{3-\delta}\) as the composite phase and electrolyte; Ca doping is indicated to improve the chemical compatibility and electrochemical performance. [PERSON] et al. [189] employed a composite of the layered RP phase SrEu\({}_{2}\)Fe\({}_{1.8}\)Co\({}_{0.2}\)O\({}_{7-\delta}\) and BCZY71 as air electrode for reversible proton-conducting solid oxide cells, which showed no degradation over 135 h under both oxidising and reducing conditions; an MPD of 0.562 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C was reported in fuel-cell mode. Other triple-conducting oxides, which have been adopted in a composite architecture, include double perovskites of the type A\({}^{\prime}\)A\({}^{\prime}\)B\({}^{\prime}\)\({}^{\prime}\)O\({}_{5+\delta}\). In a study aimed at improving the durability of PCFCs, [PERSON] et al. [190] employed a composite cathode of NdBa\({}_{0.5}\)Sr\({}_{0.5}\)Co\({}_{1.5}\)Fe\({}_{0.5}\)O\({}_{5+\delta}\)-Ce\({}_{0.9}\)Nd\({}_{0.1}\)O\({}_{2-\delta}\) with a BCZYYb711 electrolyte, noting that the deleterious effects of water vapour on cathode degradation can be essentially avoided by a series of operational protocols, which simulate a cathode potential to zero during operation, thereby minimising the effect of high pO\({}_{2}\) and pH\({}_{2}\)O producing undesirable oxide phases on the cathode surface (Figure 9).