| Summary: | Layered <i>Ruddlesden–Popper</i> (RP) lanthanide nickelates, Ln<i><sub>n</sub></i><sub>+ 1</sub>Ni<i><sub>n</sub></i>O<sub>3<i>n</i> + 1</sub> (Ln = La, Pr, and Nd; <i>n</i> = 1, 2, and 3) have generated great interest as potential cathodes for proton conducting fuel cells (PCFCs). The high-order phase (<i>n</i> = 3) is especially intriguing, as it possesses the property of a high and metallic-type electronic conductivity that persists to low temperatures. To provide the additional requirement of high ionic conductivity, a composite electrode is here suggested, formed by a combination of La<sub>4</sub>Ni<sub>3</sub>O<sub>10 ± δ</sub> with the proton conducting phase BaCe<sub>0.9</sub>Y<sub>0.1</sub>O<sub>3-δ</sub> (40 vol%). Electrochemical impedance spectroscopy (EIS) is used to analyse this composite electrode in both wet (<i>p</i><sub>H2O</sub> ~ 10<sup>−2</sup> atm) and low humidity (<i>p</i><sub>H2O</sub> ~ 10<sup>−5</sup> atm) conditions in an O<sub>2</sub> atmosphere (400–550 °C). An extended analysis that first tests the stability of the impedance data through <i>Kramers-Kronig</i> and <i>Bayesian</i> Hilbert transform relations is outlined, that is subsequently complemented with the distribution function of relaxation times (DFRTs) methodology. In a final step, correction of the impedance data against the short-circuiting contribution from the electrolyte substrate is also performed. This work offers a detailed assessment of the La<sub>4</sub>Ni<sub>3</sub>O<sub>10 ± δ</sub> - BaCe<sub>0.9</sub>Y<sub>0.1</sub>O<sub>3-δ</sub> composite cathode, while providing a robust analysis methodology for other researchers working on the development of electrodes for PCFCs.
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