###### Abstract
In contrast to these lower order phases, the higher order La\({}_{4}\)Ni\({}_{3}\)O\({}_{10\pm\delta}\), \(n=3\), material shows a uniform, metallic-type, conductivity in air that increases monotonically with decreasing temperature, thus, providing the important advantage of a high level of electronic conductivity that persists across the complete temperature range, from high temperatures (e.g., 900 \({}^{\circ}\)C) to room temperature [16]. The stability of these RP phases was studied in detail, as a function of oxygen partial pressure (\(p_{\text{O2}}\)), by [PERSON] et al. using thermodynamic calculations [21]. In their work, the authors demonstrated that, despite the observance of a smooth dependence of conductivity on temperature in air, the stability of the La\({}_{4}\)Ni\({}_{3}\)O\({}_{10\pm\delta}\) compound faces an important limit towards less oxidising conditions, due to the release of oxygen and its decomposition into La\({}_{3}\)Ni\({}_{2}\)O\({}_{7\pm\delta}\) and NiO (Figure 1), as follows:  
\[3\text{La}_{4}\text{Ni}_{3}\text{O}_{10(\text{s})}\to 4\text{La}_{3}\text{Ni}_{2} \text{O}_{7(\text{s})}+\text{NiO}_{(\text{s})}+0.5\text{O}_{2(\text{g})} \tag{1}\]  
Of concern, this factor may become relevant upon applied cathodic polarisation due to a localised drop in the chemical potential of oxygen at the cathode/electrolyte interface upon fuel cell operation [7; 22]. Nonetheless, reference to Figure 1 shows that this \(p_{\text{O2}}\) decomposition limit can be improved upon decreasing the operation temperature to the intermediate range (i.e., below 600 \({}^{\circ}\)C), an operating window that concurs well with the temperature range of interest for PCFCs (400-600 \({}^{\circ}\)C). A preliminary example of the viability of the La\({}_{4}\)Ni\({}_{3}\)O\({}_{10\pm\delta}\) material as a PCFC cathode material was provided by the current authors [19], where the performance of La\({}_{4}\)Ni\({}_{3}\)O\({}_{10\pm\delta}\) and La\({}_{4}\)Ni\({}_{3}\)O\({}_{10\pm\delta}\)-BaCe\({}_{0.9}\)Y\({}_{0.1}\)O\({}_{3\cdot\delta}\) composite cathodes, deposited on a BaCe\({}_{0.9}\)Y\({}_{0.1}\)O\({}_{3\cdot\delta}\) (BCY10) proton-conducting electrolyte, was studied in the temperature range 350-550 \({}^{\circ}\)C in a wet O\({}_{2}\) atmosphere. The presence of the BCY10 composite phase was shown to be highly beneficial to lower the polarisation resistance, providing performance figures that were in line with some of the best PCFC cathodes currently reported [19]. Figure 1: Stability limits calculated for the La\({}_{4}\)Ni\({}_{3}\)O\({}_{10\pm\delta}\) (LNO) phase as a function of temperature, calculated from data from [21]. The typical \(p_{\text{O2}}\) values of O\({}_{2}\) and N\({}_{2}\) atmospheres are indicated as a reference. Despite this promise, the strong chemical terrecation of BCY10 with high levels of water vapour has been shown to severely compromise the long-term performance of this material [1; 23]. To avoid this limitation, a very recent work highlighted the extended applicability of yttrium-doped barium create, BaCe\({}_{0.9}\)Y\({}_{0.1}\)O\({}_{3-8}\) (BCY10), for operation in nominally dry conditions in the intermediate temperature range (350-600 \({}^{\circ}\)C) [24]. In such conditions, this composition was shown to be able to retain predominant proton conduction to very low values of water vapour partial pressure (\(p_{\text{H2O}}>10^{-4}\)-\(10^{-5}\) atm). This feature is highly important as the chemical stability of this material can also be retained at such low water vapour partial pressures [23]. Hence, we aim to extend our previous work to assess the continued usability of the La\({}_{4}\)Ni\({}_{3}\)O\({}_{10:\pm 5}\)-BaCe\({}_{0.9}\)Y\({}_{0.1}\)O\({}_{3-8}\) composite cathodes for operation under such low humidity contents (i.e., \(p_{\text{H2O}}\sim 10^{-5}\) atm) [23; 24].