# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 5 Composite Cathodes
It was found that the migration of protons to the TPBs and the surface diffusion of adsorbed O' species may be rate limiting in wet atmospheres, whereas in dry atmospheres, the rate-limiting steps are likely to be the reduction of \(\mathrm{O_{ad}}\) to \(\mathrm{O_{ad}}\)- and surface diffusion of \(\mathrm{O_{ad}}\)- A similar SSC-BCS20 electrode composition was used to evaluate the principal parameters for modelling PCFCs by numerical methods, emphasizing the underestimated role of hole conductivity in the electrolyte on simulations, and finding that surface diffusion is rate-limiting [134]. A combined modelling and experimental study of the composite SSC-\(\mathrm{Ce_{0.8}Sm_{0.2}O_{2-\delta}}\) (20 SDC) on BCZY71 electrolyte indicated that the surface diffusion of oxide ions is dominant in the reaction order [135]. In 2020, [PERSON] et al. [136] reported a \(\mathrm{SrCo_{0.8}Fe_{0.15}Zr_{0.05}O_{3-\delta}}\)-BCZYYb7111 composite cathode with high structural stability and \(\mathrm{CO_{2}}\) tolerance, which exhibited a low \(\mathrm{R_{p}}\) value of 0.07 \(\Omega\)-\(\mathrm{cm^{2}}\) at 700 \({}^{\circ}\)C and an MPD of 0.71 W-\(\mathrm{cm^{-2}}\) at 700 \({}^{\circ}\)C. A composite cathode composed of a single and a layered perovskite, \(\mathrm{Sm_{0.5}Sr_{0.5}CoO_{3-\delta}}\)-\(\mathrm{SmBaCo_{2}O_{5+\delta}}\), has recently been reported, showing a low \(\mathrm{R_{p}}\) (0.18 \(\Omega\)-\(\mathrm{cm^{2}}\) at 700 \({}^{\circ}\)C) and a remarkable MPD of 1.57 W-\(\mathrm{cm^{-2}}\) at 750 \({}^{\circ}\)C [137]. Other perovskite cobaltites employed as the MIEC phase in a composite electrode include \(\mathrm{LaCoO_{3}}\), which was infiltrated into a porous BZCY712 backbone, thereby avoiding detrimental cation interdiffusion, which may occur on high-temperature sintering; a low \(\mathrm{R_{p}}\) of 0.11 \(\Omega\)-\(\mathrm{cm^{2}}\) at 600 \({}^{\circ}\)C in air was attained [138]. A barium lanthanide cobaltite, \(\mathrm{Ba_{4}Sr_{2}Sm_{2}Co_{4}O_{15}}\), was used in a composite cathode with \(\mathrm{BaCe_{0.5}Pr_{0.3}Y_{0.2}O_{3-\delta}}\) proton conductor, achieving the best performance for a 30:70 wt % \(\mathrm{Ba_{4}Sr_{2}Sm_{2}Co_{4}O_{15}}\)/\(\mathrm{BaCe_{0.5}Pr_{0.3}Y_{0.2}O_{3-\delta}}\) ratio, with \(\mathrm{Ba_{4}Sr_{2}Sm_{2}Co_{4}O_{15}}\) serving as an active phase for the dissociative absorption of oxygen; an MPD of 0.197 W-\(\mathrm{cm^{-2}}\) at 600 \({}^{\circ}\)C and stability of \(\mathrm{Ba_{4}Sr_{2}Sm_{2}Co_{4}O_{15}}\) in 1% CO\({}_{2}\) in air was reported [139]. The cobaltite composite \(\mathrm{La_{0.5}Ba_{0.5}CoO_{3-\delta}}\)-\(\mathrm{BaZrO_{3}}\) was prepared by both a modified Pechini method, followed by resolution from the previously prepared, single-phase perovskite \(\mathrm{La_{0.3}Ba_{0.7}Zr_{0.4}Co_{0.6}O_{3-\delta}}\), and by direct calcination of the precursor, with the former process demonstrating an \(\mathrm{R_{p}}\) of 1.54 \(\Omega\)-\(\mathrm{cm^{2}}\) at 600 \({}^{\circ}\)C [140]. A similar synthesis technique was then employed for the fabrication of \(\mathrm{La_{1.5}Ba_{x}CoO_{3-\delta}}\)-\(\mathrm{BaZr_{0.9}Y_{0.1}O_{2.95}}\) electrodes with direct deposition of the single-phase precursor material on the \(\mathrm{BaZr_{0.9}Y_{0.1}O_{3-\delta}}\) (BZY10) electrolyte [141]; weight ratios of \(\mathrm{La_{1.5}Ba_{x}CoO_{3-\delta}}\):BZY10 of 60:40 and 80:20 were associated with \(\mathrm{R_{p}}\) values of 0.21 and 0.27 \(\Omega\)-\(\mathrm{cm^{2}}\) at 600 \({}^{\circ}\)C, respectively. The cobaltite perovskite \(\mathrm{SrCo_{0.7}Fe_{0.2}Zr_{0.1}O_{3-\delta}}\) (SCFZ) exhibited high stability in comparison to the Zr-free analogue [142]. A composite of SCFZ and BCZY71 on a BCZY71 electrolyte in fuel-cell reactor mode produced a power output of 0.129 W-\(\mathrm{cm^{-2}}\) and 91% selective ethylene yield at 21% ethane conversion at 700 \({}^{\circ}\)C [142]. Composite cathodes with LSCF as the primary electron-hole-conducting component have been employed in numerous studies.