# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 3 Mixed Oxide-Ion-Electron-Conducting Cathodes  
A more widely employed strategy is to adopt mixed oxide-ion/electron-hole conductors (MIECs) as air electrode for the proton-conducting electrolyte. Oxygen may then be adsorbed on the cathode surface, which is followed by dissociation and migration as oxide ions (Equations (8)-(10)) either on the surface (\"electrode surface path\") or through the cathode bulk (\"bulk-electrode path for oxide ions\"). However, the reaction sites are limited to the triple-phase boundaries on the electrolyte surface, where the diffused oxide ions meet protons (Figure 1b). The \"electrode surface path\" in these components is described by Equations (2)-(7), whereas the \"bulk-electrode path for oxide ions\" is described as follows:  
\[O_{2(g)} \rightarrow O_{2(ad)} \tag{16}\] \[O_{2(ad)} \rightarrow 2O_{(ad)}\] (17) \[O_{(ad)}+2\ e^{-} \rightarrow O_{(bulk)}^{2-}\] (18) \[O_{(bulk)}^{2-} \rightarrow O_{(TPB)}^{2-}\] (19) \[O_{(TPB)}^{2-}+2H^{+} \rightarrow H_{2}O_{(TPB)}\] (20) \[H_{2}O_{(TPB)} \rightarrow H_{2}O_{(g)}. \tag{21}\]  
Since the 1990s, there has been a general trend in SOFC research to replace Mn in the B site of the classical LaMnO\({}_{3}\)-based perovskite cathode with Co and/or Fe [37]. The higher diffusion of oxide ions and faster kinetics at the electrode/electrolyte interface than LaMnO\({}_{3}\)-based air electrodes make the Co/Fe-containing perovskites more favorable in the lower-temperature range where their higher thermal expansion is less problematic. Hence, the first studies involving cathodes for H\({}^{+}\)-conducting solid oxide cells frequently employed perovskites with Fe and/or Co- containing B-site cations. Nevertheless, [PERSON] et al. recently demonstrated a high uptake of protons in La\({}_{0.7}\)Sr\({}_{0.3}\)MnO\({}_{3-\delta}\) by using XAFS, TG, and hydration experiments [38]. One of the first documented attempts to replace Pt as the cathode in laboratory single cells was made by [PERSON] et al. [39], who experimented with a limited number of nickelate, manganite, and cobaltate cathodes, finding the best performance for La\({}_{0.6}\)Ba\({}_{0.4}\)CoO\({}_{3-\delta}\) (0.18 W\(\cdot\)cm\({}^{-2}\) at 1000 \({}^{\circ}\)C). Perovskites with Co as the principal B-site cation generally provide high power outputs. A competitive single-cell performance of 0.3 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C was reported in 2008 with the classical SOFC cathode La\({}_{0.5}\)Sr\({}_{0.5}\)CoO\({}_{3-\delta}\) on BaCe\({}_{0.7}\)Zr\({}_{0.1}\)Y\({}_{0.2}\)O\({}_{3-\delta}\) (BCZY71) electrolyte [40]. The performance of SrCo\({}_{0.9}\)Sb\({}_{0.1}\)O\({}_{3-\delta}\), which was previously reported as a good SOFC cathode [41], compared favourably to La\({}_{0.8}\)Sr\({}_{0.2}\)MnO\({}_{3-\delta}\) (LSM82), achieving an output of 0.259 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C on BCZY71 [42]. It was shown that Sm\({}_{0.5}\)Sr\({}_{0.5}\)CoO\({}_{3-\delta}\) (SSC) prepared by planetary bead-milling had improved performance over the material without such treatment, highlighting the importance of microstructure on electrode performance [43]. Similarly, an improvement in the performance of SrCo\({}_{0.9}\)Nb\({}_{0.1}\)O\({}_{3-\delta}\) is documented by the wet chemical method in comparison to solid-state reaction (0.348 W\(\cdot\)cm\({}^{-2}\) and 0.204 W\(\cdot\)cm\({}^{-2}\), respectively, at 700 \({}^{\circ}\)C) [44]. Cobaltites have also achieved good performances as air electrodes in protonic ceramic electrolysis cells (PCECs), with a cell composed of an anode of Ba\({}_{0.5}\)La\({}_{0.5}\)CoOand an electrolyte of Ba(Zr\({}_{0.5}\)Ce\({}_{0.4}\))\({}_{8/9}\)Y\({}_{0.2}\)O\({}_{3-\delta}\), reaching a hydrogen evolution rate of 127 mol\(\cdot\)cm\({}^{-2}\cdot\)min\({}^{-1}\) at 0.5 A\(\cdot\)cm\({}^{-2}\) and 600 \({}^{\circ}\)C [45]. Other authors have focused on Fe/Co-containing perovskites that are well known as SOFC cathodes, particularly La\({}_{0.6}\)Sr\({}_{0.4}\)Co\({}_{0.2}\)Fe\({}_{0.8}\)O\({}_{3-\delta}\) (LSCF). [PERSON] et al.