# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 4 Triple Proton Oxide Ion Electron Hole-Conducting Oxides
An A-site-deficient, layered perovskite, (PrBa\({}_{0.8}\)Ca\({}_{0.2}\))\({}_{0.95}\)Co\({}_{2}\)O\({}_{6-\delta}\), was developed as an oxygen electrode for a reversible protonic ceramic cell where current density reached \(-\)0.72 A\(\cdot\)cm\({}^{-2}\) at 1.3 V, and a peak power density of 0.540 W\(\cdot\)cm\({}^{-2}\) was obtained at 600 \({}^{\circ}\)C in electrolysis and fuel-cell mode, respectively, which were much higher values than the A-site stoichiometric analogue [105]. A-site deficiency is generally recognized to improve the chemical stability of perovskite oxides and may also increase the oxide-ion-vacancy concentration and thereby the electrochemical performance. [PERSON] et al. [106] have recently developed a double perovskite cathode, Sr\({}_{2}\)Sc\({}_{0.1}\)Nb\({}_{0.1}\)Co\({}_{1.5}\) Fe\({}_{0.3}\)O\({}_{6-\delta}\) (SSNCF), for a dual ion (oxide-ion and proton) SOFC, where triple conduction in the cathode is reported to lead to an enhanced electrochemical performance, 0.840 W\(\cdot\)cm\({}^{-2}\) at 650 \({}^{\circ}\)C on BCZYYb7111 electrolyte. The authors employed an oxide-ion blocking technique in combination with H\({}_{2}\) and O\({}_{2}\) permeability measurements to demonstrate proton and oxide-ion transport. Another report of high performance with a double-perovskitecathode concerns GdBaCuCoO\({}_{5+\delta}\) on a BCZY71 electrolyte with a power density of 0.480 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C [107]. The substitution of Co with Cu lowered electrical conductivity, but a better thermal compatibility with the electrolyte was achieved. [PERSON] et al. [108] reported a Pr\({}_{2}\)BaNiMnO\({}_{7-\delta}\) cathode that showed excellent compatibility with BCZYYb7111 electrolyte and generated a remarkable MPD of 1.07 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C with almost negligible degradation after 100 h.  
The origin of triple conduction in the double perovskites has been the focus of several studies. EIS as a function of pO\({}_{2}\) and pH\({}_{2}\)O was employed to indicate that, for PrBaCo\({}_{2}\)O\({}_{5+\delta}\), protons are involved in the electrochemical process [109]. Compositions with large oxygen-vacancy concentrations for water insertion provide the best cathode performance due to the delocalisation of water from the electrolyte/electrode interface to the electrolyte/gas interface. The substitution of Co with Fe lowers the oxygen-vacancy concentration and, hence, increases the polarisation resistance. [PERSON] et al. [110] investigated \({}^{18}\)O and \({}^{2}\)H diffusion in PBCO at 300 \({}^{\circ}\)C by secondary ion mass spectroscopy (SIMS), showing that mixed oxide-ionic-electronic conductivity is retained and that the presence of water gives rise to an enhancement of the surface exchange rate by a factor of 3 compared to pure oxygen. \({}^{2}\)H diffusion is associated with probable large-scale defects, such as pores, consisting of hydrated Ba compounds. The authors conclude that PBCO and GBCO act principally as MIEC materials in PCFC devices, although proton conductivity may be involved to a small extent. Recent work by [PERSON] et al. [111] also paints a complex picture as regards the location of protons and their role in the electrochemical reaction in double perovskites. The authors indicate that whereas the single-phase, double perovskite  
Figure 4: SEM cross-section images of a single cell based on a PBSCF cathode on BZCYYb4411 electrolyte (**a**); magnification image corresponding to PBSCF interlayer deposited by pulsed laser deposition (PLD); (**b**) electrochemical performance of the single cell at different temperatures (**c**); and polarization curves in both electrolysis and fuel cell modes (**d**). Copyright 2019 by Royal Society of Chemistry [102]. Gd\({}_{0.8}\)La\({}_{0.2}\)Ba\({}_{0.95}\)La\({}_{0.05}\)Co\({}_{2}\)O\({}_{6\cdot 8}\) does not hydrate, the impurity phase BaCo\({}_{0.8}\)Gd\({}_{0.2}\)O\({}_{3-\delta}\) absorbs a significant amount of water.