Creating new surface-exchange pathways on the misfit Ca-cobaltite electrode by the addition of an active interlayer

Among the most promising electrode choices for solid oxide cells, the misfit calcium cobaltite, with the nominal composition [Ca2CoO3-δ]0.62[CoO2] (C349), is gaining special relevance due to its ability to provide good performance in oxidizing conditions, and also for possessing a thermal expansion coefficient similar to that of the standard electrolytes. In this work, we investigate the electrochemical behavior of the misfit C349 with the addition of an active interlayer made of Ce0.8Pr0.2O2-δ (CPO, + 2 mol% Co), compared to the electrode mechanism of an interlayer-free C349 electrode. A combination of three different approaches for the distribution function of relaxation times analysis permits a careful analysis on the characteristic time constants associated with the surface-exchange processes, revealing the existence of parallel reaction paths at higher temperatures, where the pathway through the C349 electrode may take on an important role. Conversely, at lower temperatures, due to the poor ionic conductivity of the C349 compound, the interlayer pathway may become predominant, since the CPO + Co provides much higher ionic conductivity than the C349. These results highlight the competition between an increased electroactive area and the role of the surface-exchange on the CPO + Co interface, where the latter becomes more relevant on decreasing temperature.