# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 4 Triple Proton Oxide Ion Electron Hole-Conducting Oxides  
It has been well known for some time that in certain oxides, the transport species protons, oxide ions, and electrons (electron holes in oxidising conditions) may exist simultaneously. The advantage of triple-conducting oxides (TCOs) as electrode materials for the oxygen reduction reaction in PCFCs is that both protons from the electrolyte and oxide species adsorbed from the air may migrate through the bulk and over the surface of the cathode, extending the reaction area over the whole of the electrode. In this situation, the cathodic electrochemical reaction encompasses the parallel contributions from the \"electrode-surface path\", \"bulk-electrode path for protons\", and \"bulk-electrode path for oxide ions\" according to Equations (2)-(21), as shown in Figure 1c. Some of the most prominent triple-conducting oxides forming the basis of study for PCFC cathodes are layered perovskites, which fall into the class of either double perovskites A\({}_{2}\)B\({}_{2}\)O\({}_{5+\delta}\)[70] or Ruddlesden-Popper (RP) perovskite-related structures of general formula A\({}_{n+1}\)B\({}_{n}\)O\({}_{3n+1}\)[71]. Initial reports of such phases in the context of proton-conducting cells focus on compositions employed in oxide-ion-conducting electrolyte SOFC research as mixed oxide-ionic-electronic conductors, and they do not necessarily involve or consider three conducting species in the electrochemical oxidation reaction. Determination of the relations of proton, oxide-ion, and electron-hole partial conductivities requires detailed experiments, such as the concentration-cell method [72] or measurements in different oxygen and water-vapor partial pressures, followed by analysis based on defect chemistry relations [73; 74; 75]. Generally, these experiments are only successfully employed for a significant proton contribution, and partial-conductivity data are not available for the great majority of candidate materials. Instead, hydration experiments coupled with thermogravimetric analysis (TGA) have been used to indicate the triple-conducting nature of materials with potential as PCFC cathodes. In one of the first reports of layered perovskites as PCFC cathode, published in 2009, [PERSON] et al. adopted different architectures (CaTi\({}_{0.9}\)Fe\({}_{0.1}\)O\({}_{3-\delta}\), and the RP series La\({}_{2}\)NiO\({}_{4+\delta}\) and La\({}_{4}\)Ni\({}_{3}\)O\({}_{10}\)) as electrodes for LaNbO\({}_{4}\)-based electrolyte thin films [76]. The layered RP-type cathode was then employed on BCY10 electrolyte, providing a power density of 0.130 W\(\cdot\)cm\({}^{-2}\) at 650 \({}^{\circ}\)C [77]. [PERSON] et al. [78] studied a series of perovskite oxides AMO\({}_{3-\delta}\) (A = La, Ba, Sr; M = Mn, Fe, Co, Ni) and layered RP-type materials A\({}_{2}\)MO\({}_{4+\delta}\) (A = La, Nd, Pr or Sr; M = Ni), noting that the lowest electrode polarisation resistances are observed for Ba\({}_{0.5}\)Sr\({}_{0.5}\)Fe\({}_{0.8}\)Co\({}_{0.2}\)O\({}_{3-\delta}\) and Pr\({}_{2}\)NiO\({}_{4+\delta}\). [PERSON] and [PERSON] then reported the successful operation of a single cell over 1000 h with an Nd\({}_{2}\)NiO\({}_{4}\) layered cathode, although the power output was low (0.060 W\(\cdot\)cm\({}^{-2}\) at 600 \({}^{\circ}\)C) [79]. The first papers to explicitly associate the layered perovskites with triple-conducting behavior appeared in 2012 [80; 81]. [PERSON] et al. [80] investigated the hydration and electrochemical properties of LSCF, BSCF, PrBaCo\({}_{2}\)O\({}_{5+\delta}\) (PBCO) and La\({}_{2}\)NiO\({}_{4+\delta}\) by TGA and polarisation studies, indicating that proton transfer and water release appear to be the rate-determining steps for BSCF, PBCO, and Pr\({}_{2}\)NiO\({}_{4+\delta}\), but that no rate-determining step involving protons was found for LSCF.