# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 4 Triple Proton Oxide Ion Electron Hole-Conducting Oxides
The same authors subsequently achieved better performances of 0.382 and 0.533 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C on substituting Gd with Sm (SmBaCo\({}_{2}\)O\({}_{5+\delta}\)) [95] and then also partially substituting Ba with Sr (SmBa\({}_{0.5}\)Sr\({}_{0.5}\)Co\({}_{2}\)O\({}_{5+\delta}\)) [96], respectively. Early studies of double perovskites include that of [PERSON] et al. [97], who compared the performance of LaBaCuO\({}_{5+\delta}\) (M = Fe, Co) and similarly found that the better single-cell performance of the Co-containing phase (0.432 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C), in comparison to the Fe-containing analogue (0.327 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C), was attributable to the higher catalytic activity of the former. The term \"triple-conducting oxide\" comes to greater prominence soon after the studies of [PERSON] et al. [80; 81] when [PERSON] et al. reported an excellent power output of 1.61 W\(\cdot\)cm\({}^{-2}\) at 750 \({}^{\circ}\)C for a single cell with a BCZYYb7111 electrolyte and the layered double-perovskite cathode NdBa\({}_{0.5}\)Sr\({}_{0.5}\)Co\({}_{1.5}\)Fe\({}_{0.5}\)O\({}_{5+\delta}\) (NBSCF) [98]. [PERSON] et al. [99] studied a series of double perovskites BaGd\({}_{0.8}\)La\({}_{0.2}\)Co\({}_{2}\)O\({}_{5+\delta}\), BaGdCo\({}_{1.8}\)Fe\({}_{0.2}\)O\({}_{5+\delta}\), PrBaCo\({}_{2}\)O\({}_{5+\delta}\), and BaPrCo\({}_{1.4}\)Fe\({}_{0.6}\)O\({}_{5+\delta}\) on BCZY71 electrolyte, observing that polarisation resistances were lowest for the Fe-free materials. The electrode impedance responses were composed of two apparent contributions, which were attributed to charge transfer and diffusion or surface-related processes, and they were modeled with a system of two ionic and one electronic charge carriers [99]. [PERSON] et al. [100] employed molecular dynamics to study proton diffusion in GBCO and found that the predominant mechanism was proton transfer between neighbouring oxygens (Grothuss mechanism) but that vehicular migration via OH groups may also occur. The praseodymium-containing analogue of NBSCF, PrBa\({}_{0.5}\)Sr\({}_{0.5}\)Co\({}_{1.5}\)Fe\({}_{0.5}\)O\({}_{5+\delta}\) (PBSCF), was subsequently reported to proffer an exceptional power density (0.500 W\(\cdot\)cm\({}^{-2}\) at 500 \({}^{\circ}\)C) when employed as a dense interlayer between the electrolyte BaZr\({}_{0.4}\)Ce\({}_{0.4}\)Y\({}_{0.1}\)Y\({}_{0.1}\)O\({}_{3-\delta}\) (BCZYYb4411) and a cathode of the same material [101]. In the same work, the authors offer evidence of proton uptake in PBSCF by TGA and attribute the very low contact resistance between the electrolyte and cathode to the proton permeability of the dense PBSCF interlayer. The same group then demonstrated reversible operation of the NiO-BCZYYb4411/BCZYYb4411/PBSCF cell, which showed negligible degradation after 500 h of operation at 550 \({}^{\circ}\)C with a current density of \(-\)1.80 A\(\cdot\)cm\({}^{-2}\) at 600 \({}^{\circ}\)C at an operating voltage of 1.3 V (Figure 4) [102]. PBSCF has recently been employed as a cathode in a thin film PCFC in which each of the active components (anode functional layer, electrolyte, and cathode) was fabricated by a slurry spin-coating technique on a BaCe\({}_{0.6}\)Zr\({}_{0.2}\)Y\({}_{0.1}\)Y\({}_{0.1}\)O\({}_{3-\delta}\) anode support, reaching an MPD of 0.250-0.650 W\(\cdot\)cm\({}^{-2}\) at 500-600 \({}^{\circ}\)C [103]. The role of the effect of A-site cation ordering on the cathode performance and chemical stability in double perovskites is investigated for A-site cation-ordered LaBaCo\({}_{2}\)O\({}_{5+\delta}\) and -disordered La\({}_{0.5}\)Ba\({}_{0.5}\)CoO\({}_{3-\delta}\) by [PERSON] et al. [104], observing that A-site cation ordering leads to a higher oxygen-vacancy concentration, which explains the better electrochemical performance of LaBaCo\({}_{2}\)O\({}_{5+\delta}\) compared to the disordered phase.