# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 5 Composite Cathodes
Air electrodes consisting of an oxide-ion-conducting phase and lanthanum manganite-based compositions as the electron-conducting component have been studied throughout the short history of PCFC cathode research. In one of the first such studies, [PERSON] et al. [173] employ a La\({}_{0.8}\)Sr\({}_{0.2}\)MnO\({}_{3-\delta}\)-La\({}_{0.5}\)Ce\({}_{0.5}\)O\({}_{1.75}\) (LSM82-50 LDC) composite on an La\({}_{0.99}\)Ca\({}_{0.01}\)NbO\({}_{4}\) thin electrolyte, and report an MPD of 0.065 W\(\cdot\)cm\({}^{-2}\) at 800 \({}^{\circ}\)C. [PERSON] et al. [54] studied LSM73, LSCM, and BSCF cathodes for a similar LaNbO\({}_{4}\)-based electrolyte, finding that the LSCM cathode was the most chemically and mechanically stable with the lowest electrode polarisation resistance. For composite electrodes with LSM82 and LaNbO\({}_{4}\), the lowest electrode polarisation-resistance value (R\({}_{\rm p}\) = 22 \(\Omega\cdot\)cm\({}^{2}\) at 750 \({}^{\circ}\)C) was achieved on mixing LSM82 and La\({}_{0.995}\)Ca\({}_{0.005}\)NbO\({}_{4}\) in a 50:50 vol % ratio. [PERSON] et al. [174] note that in such LSM82-based composites, the protonic phase should exhibit considerable H\({}^{+}\) conductivity, since proton transport through the electrode is rate-limiting. Similarly, LSM82 electrodes in conjunction with La\({}_{5.5}\)WO\({}_{12-\delta}\) electrolyte were improved substantially upon the addition of the electrolyte material into the LSM82 electrode, thereby increasing the TPB length [175]. The rate-limiting steps in the LSM82-La\({}_{5.5}\)WO\({}_{1.25-\delta}\) cathode were subsequently attributed to surface-related processes, which could be accelerated by impregnation with nanosized 20 SDC particles and by imposing a net current through the electrode [176]. Similarly, the reaction kinetics of LSM82-La\({}_{28-x}\)W\({}_{4+x}\)O\({}_{54+x/2}\) (x = 0.85) could be accelerated on the addition of Pt nanoparticles [177]. [PERSON] et al. [178] studied the effects of humidified on BaCe\({}_{0.85}\)Y\({}_{0.15}\)O\({}_{3-\delta}\) (BCY15)-based cells with LSM82-Ce\({}_{0.9}\)Gd\({}_{0.1}\)O\({}_{2-\delta}\) (10 GDC) electrodes, observing that reaction sites become occupied by water vapour, thereby increasing the charge-transfer resistance. Replacing the 10 GDC component with H\({}^{+}\)-conducting BCY15 increases the effective TPB and mitigates the effects of humidity. Recently, an La\({}_{0.5}\)Sr\({}_{0.5}\)MnO\({}_{3-\delta}\) (LSM55)-based cathode in conjunction with K\({}_{2}\)NiF\({}_{4}\)-type layered La\({}_{0.5}\)Sr\({}_{1.5}\)MnO\({}_{4+\delta}\) with interstitial oxygen transport was prepared by an in situ co-assembly technique, and exhibited both a much lower R\({}_{\rm p}\) than single-phase LSM55 on BCZY71 electrolyte and an exceptional MPD of 0.94 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C [179]  
LSM82-BCZY71 composites have also been employed as steam electrodes in electrolysis mode on BCZY71 electrolyte, in which the introduction of nanoparticles with catalytic activity, particularly Pr\({}_{6}\)O\({}_{11}\)-CeO\({}_{2}\), lowered the R\({}_{\rm p}\) toward water splitting and oxygen evolution [180]. A similar composition, (La\({}_{0.75}\)Sr\({}_{0.25}\))\({}_{0.95}\)Mn\({}_{0.5}\)Cr\({}_{0.5}\)O\({}_{3-\delta}\)-BaCe\({}_{0.5}\)Zr\({}_{0.3}\)Y\({}_{0.16}\)Zn\({}_{0.04}\)O\({}_{3-\delta}\), has also been employed for steam electrolysis with a current efficiency of 22% [181]. As discussed in the previous section, layered RP phases and layered perovskites have been associated with triple-conducting properties and excellent air-electrode performances. Unsurprisingly, in the past few years, studies of composites with layered phases have become much more common. One of the first examples employs Nd\({}_{1.95}\)NiO\({}_{4+\delta}\) and BCZYYb711, with the lowest R\({}_{\rm p}\) observed for 60 wt % of the nickelate at 0.43 \(\Omega\cdot\)cm\({}^{2}\) at 750 \({}^{\circ}\)C and a power density of 0.154 W\(\cdot\)cm\({}^{-2}\)[182]; the relatively low MPD is attributed to a high electrolyte resistance. [PERSON] et al.