# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 4 Triple Proton Oxide Ion Electron Hole-Conducting Oxides
The same group also studied the effect of Sr content in the Pr\({}_{2-\delta}\)Sr\({}_{x}\)NiO\({}_{4+\delta}\) system on the oxygen reduction reaction and hydration, indicating that evidence for triple conduction exists in the Pr\({}_{2}\)NiO\({}_{4+\delta}\) phase but not in the Sr-doped composition [81]. Ruddlesden-Popper nickelates of the form A\({}_{2}\)NiO\({}_{4+\delta}\), and in particular the praseodymium analogue (Pr\({}_{2}\)NiO\({}_{4+\delta}\)), have continued to be the focus of study in protonic ceramic cells. A high performance of 0.82 W\(\cdot\)cm\({}^{-2}\) was recently obtained with a Pr\({}_{2}\)NiO\({}_{4+\delta}\) cathode on BaCe\({}_{0.55}\)Zr\({}_{0.3}\)Y\({}_{0.15}\)O\({}_{3-\delta}\) electrolyte at 650 \({}^{\circ}\)C, employing a microwave heat treatment to minimize the reaction between the cathode and electrolyte [82]. In electrolysis mode, a Pr\({}_{1.95}\)Ba\({}_{0.05}\)NiO\({}_{4+\delta}\) oxygen electrode was reported to provide an excellent hydrogen production rate of 19 mL\(\cdot\)min\({}^{-1}\) at 600 \({}^{\circ}\)C on a tubular BaCe\({}_{0.5}\)Zr\({}_{0.3}\)Dy\({}_{0.2}\)O\({}_{3-\delta}\) electrolyte [83]. Ba-doping in the nickelate series Nd\({}_{2-x}\)Ba\({}_{x}\)NiO\({}_{4+\delta}\) is documented to reduce diffusion of Ba from the electrolyte to the electrode [84], whereas anion doping with F in the series Nd\({}_{1.9}\)Ba\({}_{0.1}\)NiO\({}_{4+\delta}\)F\({}_{Y}\) improves the electrochemical performance in electrolysis mode [85]. A single cell with La\({}_{1.2}\)Sr\({}_{0.8}\)Ni\({}_{0.6}\)Fe\({}_{0.4}\)O\({}_{4+\delta}\) (LSNF) cathode based on a BCZY71 electrolyte achieved a maximum power density of 0.781 W\(\cdot\)cm\({}^{-2}\) with a low interfacial polarisation resistance of 0.078 \(\Omega\cdot\)cm\({}^{2}\) at 700 \({}^{\circ}\)C, demonstrating high long-term stability [86]. Another elevated power output was recently reported for Ca-doped La\({}_{2}\)NiO\({}_{4}\), with a composition of La\({}_{1.5}\)Ca\({}_{0.5}\)NiO\({}_{4+\delta}\), offering an MPD of 0.923 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C [87]. The effect of Co-doping in RP-type cathodes based on La\({}_{3}\)Ni\({}_{2}\)O\({}_{7}\) is explored by [PERSON] et al. [88], who indicate that Co on the Ni site improves activity toward the oxygen reduction reaction. In contrast to general trends, the substitution of Co for Fe in the LaSrCo\({}_{1.5}\)Fe\({}_{0.4}\)O\({}_{4+\delta}\) RP-type series was found to improve electrochemical performance [89]. The influence of the microstructure on the performance of RP-based cathodes has also been investigated. Nanofibers of La\({}_{2}\)NiO\({}_{4}\) and LaNi\({}_{0.6}\)Fe\({}_{0.4}\)O\({}_{3-\delta}\) were obtained by an electrospinningning technique and employed as a highly efficient cathode, reaching a power density of 0.551 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C [90]. An interesting nickelate with a different layered structure, in which lithiation is reported to improve conductivity and cathode activity in SOFCs, is Li\({}_{0.2}\)Ni\({}_{0.79}\)Co\({}_{0.2}\)Zn\({}_{0.01}\)O\({}_{2}\)[91]. [PERSON] and [PERSON] [92] explored the electrode properties of an LiNi\({}_{0.8}\)Co\({}_{0.2}\)O\({}_{2}\) cathode on BCZY71 electrolyte, which rendered an MPD of 0.410 W\(\cdot\)cm\({}^{-2}\) at 650 \({}^{\circ}\)C; TGA and electrochemical impedance spectroscopy (EIS) were employed to indicate the triple-conducting nature of the material in wet conditions. An improved electrode performance with increasing water content indicated a role of protons in BSCF as well as Pr\({}_{2}\)NiO\({}_{4+\delta}\)[93]. The first reports of using double perovskites in protonic ceramic cells appear around the same period as the RP nickelate phases. In 2008, GdBaCo\({}_{2}\)O\({}_{5+\delta}\) (GBCO) was employed on a BCZY71 electrolyte, reaching a power density of 0.266 W\(\cdot\)cm\({}^{-2}\) at 700 \({}^{\circ}\)C [94].