# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 5 Composite Cathodes
One of the earliest examples, by [PERSON] et al., reports superior performance of LSCF-\(\mathrm{BaCe_{0.9}Y_{0.1}O_{3-\delta}}\) (BCYb10) cathodes in a ratio of 50:50 wt % with an \(\mathrm{R_{p}}\) of 0.14 \(\Omega\)-\(\mathrm{cm^{2}}\) at 700 \({}^{\circ}\)C, and better fuel-cell performance in comparison to Pt [143]. [PERSON] et al. [144] studied the electrochemical performance of symmetrical cells with porous cathodes LSCF, LSCF-BCY10, and LSCF-BZY20 as a function of \(\mathrm{pO_{2}}\) and \(\mathrm{pH_{2}O}\), observing that the BCY-containing cathode gave the best performance due to extension of the TPB, whereas the poorest performance, exhibited by the LSCF-BZY20 electrode, was attributed to poor proton conductivity along the BZY particles; the ORR was signalled as the principal rate-determining step rather than proton transport. In another study employing electrospinning, \(\mathrm{BaCe_{0.5}Zr_{0.3}Y_{0.15}O_{3-\delta}}\)-embedded LSCF fibrous cathodes were prepared using an LSCF precursor gel containing the \(\mathrm{BaCe_{0.5}Zr_{0.35}Y_{0.15}O_{3-\delta}}\) nanopowder; an anode-supported single cell of this cathode on \(\mathrm{BaCe_{0.5}Zr_{0.35}Y_{0.15}O_{3-\delta}}\) electrolyte provided an MPD of 0.537 \(\Omega\)-\(\mathrm{cm^{-2}}\) at 700 \({}^{\circ}\)C [145]. The mixed cobaltite-ferrite series \(\mathrm{Ca_{x}Y_{1.5}Fe_{0.5}Co_{0.5}O_{3-\delta}}\) exhibits high conductivities (202 S-\(\mathrm{cm^{-1}}\) at 750 \({}^{\circ}\)C for x = 0.3) and excellent compatibility with BCZY71 electrolyte [146] (Figure 6). The power density produced by a \(\mathrm{Ca_{0.3}Y_{0.7}Fe_{0.5}Co_{0.5}O_{3-\delta}}\)-BCZY71 composite cathode on BCZY71 electrolyte was 0.798 W-\(\mathrm{cm^{-2}}\) at 750 \({}^{\circ}\)C [146]. [PERSON] et al. [147] undertook a performance comparison of cobaltite perovskite electrodes La\({}_{0.6}\)Sr\({}_{0.4}\)CoO\({}_{3-\delta}\) (LSC64), La\({}_{0.6}\)Ba\({}_{0.4}\)CoO\({}_{3-5}\), and LSCF with BCZYYb7111 proton-conducting phase in anode-supported cells with a BCZYYb7111 electrolyte. All three cathodes were chemically compatible with the electrolyte and exhibited stable PCFC performance, with the La\({}_{0.6}\)Ba\({}_{0.4}\)CoO\({}_{3-\delta}\)-BCZYYb7111 composite achieving the best power output (1.0 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C). As is the case with LSCF, the popular SOFC cathode material BSCF has also been adopted regularly as the mixed oxide-ionic-electron-conducting component of composite PCFC air electrodes. One of the first examples, in 2009, is a BSCF-BCZY71 composite prepared by a modified Pechini method, which provided an MPD of 0.42 W\(\cdot\)cm\({}^{-2}\) and a low R\({}_{\text{P}}\) of 0.1 \(\Omega\cdot\)cm\({}^{2}\) at 700 \({}^{\circ}\)C with a BCZY71 electrolyte [148]. [PERSON] et al. [149] then showed that a composite BSCF-BCY10 exhibited a lower ASR value (0.53 \(\Omega\cdot\)cm\({}^{2}\) at 600 \({}^{\circ}\)C) compared to pure BSCF. The same group subsequently fabricated a BSCF-BZCYYb1711 cathode deposited on a BCZYYb711 electrolyte by wet powder spraying, achieving an MPD of 0.422 W\(\cdot\)cm\({}^{-2}\) at 600 \({}^{\circ}\)C [150]. A reversible protonic ceramic cell was developed by [PERSON] and [PERSON] [151] employing a BCZY81-ZnO (5 mol%) electrolyte and BSCF-BCZY81 cathode. Although the electrochemical performance was promising in both fuel-cell and electrolyzer modes, the authors note that high temperature, accompanied with high water-vapor partial pressure, can accelerate degradation of the Ba-containing materials. Interestingly, K-doped BSCF, Ba\({}_{0.4}\)K\({}_{0.1}\)Sr\({}_{0.5}\)Co\({}_{0.8}\)Fe\({}_{0.2}\)O\({}_{3-\delta}\), exhibited a lower proton migration energy when compared with undoped-BSCF [152]. The composite Ba\({}_{0.4}\)K\({}_{0.1}\)Sr\({}_{0.5}\)Co\({}_{0.8}\)Fe\({}_{0.2}\)O\({}_{3-\delta}\)-BCZY71 provided an exceptionally high MPD of 1.275 W\(\cdot\)cm\({}^{2}\) at 750 \({}^{\circ}\)C, much higher than that obtained for pristine BSCF [152]. [PERSON] et al.