Detailed Electrochemical Study on Nanoscaled La_0.6Sr_0.4CoO_3-δSOFC Thin-Film Cathodes in Dry, Humid and CO₂-Containing Atmospheres

High-performance La 0.6 Sr 0.4 CoO 3-δ SOFC thin-film cathodes ( ASR pol = 9 mΩ·cm 2 at 600°C) with a nanoscaled and nanoporous microstructure were investigated in detail by means of electrochemical impedances spectroscopy. The thin-film cathodes were derived by a wet chemical process and exhibit a film thickness of about 200 nm and an average grain and pore size in the sub 100 nm regime. Systematic electrochemical impedance measurements were conducted on symmetrical cells across a broad range of temperatures (400 ... 600°C) and oxygen atmospheres ( pO 2 = 0.01 ... 0.5 atm), followed by a detailed data analysis. Five independent cathode processes were identified. Four of the processes are thermally activated, whereas one process at low frequencies exhibits a slight deactivation with increasing temperature. Furthermore, the three processes in the low and mid frequency range are oxygen partial pressure dependent, whereas the two high frequency processes remain unaffected by pO 2 -variations. Based on these dependencies, an equivalent circuit model is presented and physical mechanisms such as gas and ionic diffusion, oxygen surface exchange and charge transfer are discussed as possible origins of the five loss processes. In addition, the influence of H 2 O(g) and CO 2 on the cathodic reaction in general and on the individual processes will be presented.