# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 5 Composite Cathodes
Nd(Ba\({}_{0.75}\)Ca\({}_{0.25}\))Co\({}_{1.5}\)Fe\({}_{0.4}\)Ni\({}_{0.1}\)O\({}_{5+\delta}\) was prepared as a composite with both BCZYYb711 and Ce\({}_{0.9}\)Gd\({}_{0.1}\)O\({}_{2-\delta}\) in the weight ratio 6:4, with the latter composite providing the better performance with a peak power density of 0.88 W\(\cdot\)cm\({}^{-2}\) on an anode-supported cell with  
Figure 7: Scanning electron micrograph of a protonic ceramic single cell based on nanoparticles of (Pr\({}_{0.9}\)La\({}_{0.1}\))\({}_{2}\)(Ni\({}_{0.74}\)Cu\({}_{0.21}\)Nb\({}_{0.05}\))O\({}_{4+\delta}\) infiltrated on a BCZY71 skeleton as cathode, a BCZY71 electrolyte, and Ni-BCZY71 anode (**a**); magnified image of the cathode composite based on infiltrated nanoparticles on BCZY71 (**b**); single-cell performance curves (**c**) and impedance spectra of the cell under open circuit conditions (**d**), under humidified hydrogen and air in the range 600–700 \({}^{\circ}\)C. Copyright 2017 by Elsevier [184]. BCZYYb7111 electrolyte [191]. Additionally, NdBa\({}_{0.5}\)Sr\({}_{0.5}\)Co\({}_{1.5}\)Fe\({}_{0.5}\)O\({}_{5+\delta}\) was studied as an electrode in an electrolyser based on BCZYYb711, reaching 3.16 A-cm\({}^{-2}\) at 1.3 V and 750 \({}^{\circ}\)C in H\({}_{2}\)[192]. The composite PrBaCo\({}_{2}\)O\({}_{5+\delta}\)-BCZYYb7111 was prepared by impregnating a BZCYYb1711 scaffold with PrBaCo\({}_{2}\)O\({}_{5+\delta}\) over five cycles. The optimal PBCO loading was ascertained at 36 wt % PBCO, leading to an MPD of 0.49 W-cm\({}^{-2}\) at 750 \({}^{\circ}\)C [193]. Ta-doped PrBaCo\({}_{2}\)O\({}_{5+\delta}\) cathodes showed improved chemical stability and higher resistance to CO\({}_{2}\)[194]. In addition, composite cathodes of PrBa\({}_{0.9}\)Ca\({}_{0.1}\)Co\({}_{2.8}\)Zn\({}_{x}\)O\({}_{5+\delta}\) and BCZYYB7111, as reported by [PERSON] et al. [195], showed a higher concentration of oxygen vacancies after Zn-doping of the double perovskite, which resulted in a higher power output of 0.87 W-cm\({}^{-2}\) for the x = 0.15 composition at 750 \({}^{\circ}\)C in comparison to 0.33 W-cm\({}^{-2}\) for the Zn-free composite. The triple-conducting, simple perovskite BaCo\({}_{0.4}\)Fe\({}_{0.4}\)Zr\({}_{0.1}\)Y\({}_{0.1}\)O\({}_{3-\delta}\), prepared with 10 at % Ba deficiency to improve chemical stability in CO\({}_{2}\)- and H\({}_{2}\)O-containing atmospheres, was employed in a composite with BCZY71 on a BCZY71 electrolyte [196]. Polarisation losses decreased on addition of the BCZY71 proton conductor as a component of the composite electrode, which was attributed to improved oxygen gas adsorption and dissociation, in addition to an enhanced diffusion of oxygen species; an MPD of 0.537 W-cm\({}^{-2}\) was  
Figure 8: SEM image of a protonic ceramic electrolysis cell based on a Pr\({}_{2}\)NiO\({}_{4}\)-BCZY62 composite as anode, BCZY62 as electrolyte, and Ni-BCZY62 as cathode (**a**); magnified anode component and corresponding anode–electrolyte interface (**b**); impedance spectra of the half cell in three-electrode configuration with Pr\({}_{2}\)NiO\({}_{4}\)–BCZY62 composite anode in air at 550 \({}^{\circ}\)C with different humidifications (**c**); current–voltage curves of the single cell in electrolysis mode, with dry H\({}_{2}\) fed to the cathode and 40% of wet air to the anode, in the range 550–700 \({}^{\circ}\)C (**d**). Copyright 2018 by Royal Society of Chemistry [186]. achieved with the composite at 700 \({}^{\circ}\)C. A cobalt-free, triple-conducting simple perovskite Ba\({}_{0.95}\)La\({}_{0.05}\)Fe\({}_{0.8}\)Zn\({}_{0.2}\)O\({}_{3-\delta}\), used in a composite with BCZYYb7111, reached a single-cell power density of 0.329 W\(\cdot\)cm\({}^{-2}\) at 750 \({}^{\circ}\)C [197]. Distribution of Relaxation Times (DRT) analyses of EIS results were employed to indicate that below 650 \({}^{\circ}\)C, the rate-limiting steps in the cathodic reaction are the incorporation of oxygen in the lattice and proton bulk diffusion to the TPBs. A limited number of studies have extended the composite concept to three-component phases. [PERSON] et al.