# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 5 Composite Cathodes  
Composite cathodes consisting of two or more oxide phases with distinct properties have been successfully employed in SOFC research for a considerable number of years [126]. A typical composite consists of the fine-grained electrolyte material with high ionic conductivity mixed with a phase of high electronic conductivity to increase the TPB. The particle size in the porous mixture should be limited to sustain a high purity and the TPB length. It is unsurprising that composite cathodes have been widely adopted in PCFC research, given their ubiquity in oxide-ion-conducting SOFCs. Figure 1d shows a schematic diagram of a composite electrode for a PCFC in its simplest form, consisting of a phase with high electronic conductivity and a phase with high protonic conductivity. The reaction area may be extended considerably through mixed oxide-ionic-electronic conduction or protonic-electronic conduction or triple conduction in one or both components. The performance advantages offered by composite electrodes due to the more precise and ready tailoring of electrochemical and electrocatalytic requirements should be weighed against the greater complexity of the electrode system in terms of component thermal and chemical compatibility between the phases and with other cell components, in addition to greater processing difficulty. Cobalite perovskites, such as Sm\({}_{0.5}\)Sr\({}_{0.5}\)CoO\({}_{3-\delta}\), have been intensively researched as SOFC cathodes due to their extremely good mixed electron oxide-ion-conducting properties. Therefore, it is unsurprising that such phases have been readily adopted in composite electrodes for protonic ceramic cells. One of the first studies of multiple-component air electrodes for PCFCs constitutes a composite with SSC and the proton conductor BaCe\({}_{0.8}\)Sm\({}_{0.2}\)O\({}_{3-\delta}\) (BCS20) [127]. The interfacial R\({}_{\rm P}\) reached a minimum for approximately 60 wt % SSC (0.21 \(\Omega\cdot\)cm\({}^{2}\) at 700 \({}^{\circ}\)C and MPD of 0.24 W\(\cdot\)cm\({}^{-2}\)). SSC was then adopted with the more stable proton conductor, rendering an MPD of 0.528 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C with BaCe\({}_{0.5}\)Zr\({}_{0.3}\)Y\({}_{0.16}\)Zn\({}_{0.04}\)O\({}_{3-\delta}\) also as electrolyte [128]. [PERSON] et al. achieve a very similar MPD, 0.529 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C, employing an SSC-BaCe\({}_{0.8}\)Zr\({}_{0.1}\)Y\({}_{0.1}\)O\({}_{3-\delta}\) (BCZY81) composite cathode deposited on BZCY81 electrolyte by wet powder spraying [129]. BCZY71 electrolyte is replaced by BZY20 to mitigate the reaction between the two ceramic phases in a SSC-BZY20 electrode, which is co-fired in a one-step process with BCZY71 electrolyte, attaining a power density of 0.3 W\(\cdot\)cm\({}^{-2}\) at 600 \({}^{\circ}\)C [130]. By preparing a fibrous composite cathode of SSC with embedded BaCe\({}_{0.5}\)Zr\({}_{0.35}\)Y\({}_{0.15}\)O\({}_{3-\delta}\) particles using an electrospinningning process, [PERSON] et al. achieved a lower polarisation resistance of 0.185 \(\Omega\cdot\)cm\({}^{2}\) at 700 \({}^{\circ}\)C in comparison to a typical nanocomposite electrode (MPD was 0.64 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C) [131]. Further improvement was achieved by impregnating a Y-doped BaZrO\({}_{3}\) (BZY) electrolyte backbone with SSC nanoparticles, reaching an MPD of 0.6 W\(\cdot\)cm\({}^{-2}\) at 600 \({}^{\circ}\)C [132]. He et al. employed a composite SSC-BCS20 cathode to study cathode reaction models by impedance spectroscopy as a function of \(\mathrm{pO_{2}}\) and \(\mathrm{pH_{2}O}\)[133].