# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 3 Mixed Oxide-Ion-Electron-Conducting Cathodes
[46] achieved a power density of 0.753 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C with LSCF on a BaCe\({}_{0.95}\)Tb\({}_{0.05}\)O\({}_{3-\delta}\) electrolyte. In one of the first reports of tubular PCFCs, LSCF electrodes were employed in anode-supported cells on BCZY71 electrolyte with a power range of 0.079-0.189 W\(\cdot\)cm\({}^{-2}\) in the temperature range 700-850 \({}^{\circ}\)C [47]. In terms of single-cell experiments, a much improved performance was found with nanostructured LSCF cathode, again prepared by planetary bead-milling, in comparison to the unmilled LSCF phase [48]. An alternative strategy to improve the performance of LSCF involves the infiltration of BaCO\({}_{3}\) nanoparticles forming a synergistic catalyst, which reduced the polarisation resistance by nearly 75% from 1.123 to 0.293 O\(\cdot\)cm\({}^{2}\) at 700 \({}^{\circ}\)C, yielding a power density of 0.404 W\(\cdot\)cm\({}^{-2}\)[49]. The well-known SOFC cathode Ba\({}_{0.5}\)Sr\({}_{0.5}\)Co\({}_{0.8}\)Fe\({}_{0.2}\)O\({}_{3-\delta}\) (BSCF) has also regularly been used and compared as a PCFC air electrode [50; 51; 52]. A very high power density of up to 0.800 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C, and a stable performance in CO\({}_{2}\), with BaZr\({}_{0.4}\)Ce\({}_{0.45}\)Y\({}_{0.15}\)O\({}_{3-\delta}\) as the electrolyte has been reported (Figure 3) [53]. [PERSON] et al. compared the performance of BSCF, LSCF, and Pr\({}_{2}\)NiO\({}_{4-\delta}\) (a phase that is discussed further in the next section), reporting that the area-specific resistance (ASR) values of the electrode process were similar but that, at low temperature, BSCF appeared to be the most efficient of the three cathodes on BaCe\({}_{0.9}\)Y\({}_{0.1}\)O\({}_{3-\delta}\) (BCY10) electrolyte [51]. However, BSCF is reported to react with the LaNbO\({}_{4}\)-based electrolyte, whereas La\({}_{0.75}\)Sr\({}_{0.25}\)Cr\({}_{0.5}\)Mn\({}_{0.5}\)O\({}_{3-\delta}\) (LSCM) is chemically compatible with a lower ASR value for the electrode process [54]. Similar behavior was observed with the proton-conducting electrolyte La\({}_{6}\)WO\({}_{12}\), which is stable with La\({}_{0.7}\)Sr\({}_{0.3}\)MnO\({}_{3-\delta}\) (LSM73) and LSCM but reacts with BSCF [55]. In 2013, [PERSON] et al. [56] documented that the phase BaCo\({}_{0.4}\)Fe\({}_{0.4}\)Zr\({}_{0.2}\)O\({}_{3-\delta}\) (BCFZ) exhibits lower polarisation resistance, longer performance stability, and better compatibility with the BCZYYb7111 electrolyte than LSCF under the same conditions, achieving a power density of 0.225 W\(\cdot\)cm\({}^{-2}\) at 600 \({}^{\circ}\)C. Rare-earth doping in the related BaCo\({}_{0.4}\)Fe\({}_{0.5-\delta}\)Ce\({}_{0.1}\)Re\({}_{x}\)O\({}_{3-\delta}\) (Re = Y, Gd; x = 0.1) system has also been reported, with the Gd-doped phase exhibiting a promising peak power density of 0.504 W\(\cdot\)cm\({}^{-2}\) at 600 \({}^{\circ}\)C in a cell with BCZYYb7111 as the electrolyte [57]. [PERSON] et al. [58] assessed the reactivity of SOFC perovskite cathodes LaMO\({}_{3}\) (M = Mn, Fe, Co) and the layered phase La\({}_{2-\delta}\)Sr\({}_{x}\)NiO\({}_{4+\delta}\) (x = 0, 0.8) in contact with SrCoO\({}_{3}\)-based electrolyte, indicating that the reaction occurs readily on high-temperature sintering for the perovskite compositions due to Sr diffusion from the electrolyte to the cathode; however, the Ruddlesden-Popper-type oxide La\({}_{1.2}\)Sr\({}_{0.8}\)NiO\({}_{4+\delta}\) does not react. The chemical compatibility of the same perovskite compositions with BaZrO\({}_{3}\)-based electrolytes was better, with only minor interdiffusion of Ba and La occurring [59]. Nevertheless, the authors  
Figure 3: Microstructure of BSCF cathode on BZCY40 electrolyte (**a**), single-cell performance (**b**), and the corresponding Nyquist spectra at different water partial pressures (**c**). Copyright 2016 by Elsevier [53]. point out that thermal expansion mismatch should be controlled with careful choice of chemical substitutions.