Electrochemical properties of La 0 . 6 Sr 0 . 4 CoO 3 − δ thin fi lms investigated by complementary impedance spectroscopy and isotope exchange

a r t i c l e i n f o The oxygen exchange and diffusion properties of La 0.6 Sr 0.4 CoO 3 − δ thin films on yttria stabilized zirconia were analyzed by impedance spectroscopy and 18 O tracer experiments. The investigations were performed on the same thin film samples and at the same temperature (400 °C) in order to get complementary information by the two methods. Electrochemical impedance spectroscopy can reveal resistive and capacitive contributions of such systems, but an exact interpretation of the spectra of complex oxide electrodes is often difficult from impedance data alone. It is shown that additional isotope exchange depth profiling can significantly help interpreting impedance spectra by giving reliable information on the individual contribution and exact location of resistances (surface, electrode bulk, interface). The measurements also allowed quantitative comparison of electrode polarization resistances obtained by different methods. Mixed ionic and electronic conducting (MIEC) oxides are promising materials for electrochemical devices based on gas–solid interactions such as solid oxide fuel cells (SOFCs), gas sensors, or permeation membranes [1–4]. Several analytical methods exist to investigate the catalytic activity of MIEC electrodes towards the oxygen reduction reaction (ORR) with two of the most important approaches being elec-trochemical impedance spectroscopy (EIS) and 18 O isotope exchange depth profiling (IEDP). EIS yields information on resistive and capacitive contributions of MIEC electrodes on ionic conducting substrates. Many properties such as the catalytic activity of surfaces, oxygen non-stoichiometry, chemical diffusion, conductivities, transport reactions across solid|solid phase boundaries, or the formation of impurity phases can thus be indirectly probed. However, the correct interpretation of impedance spectra is crucial for the validity of the extracted parameters. This interpretation can be trivial for simple spectra [5,6] but in complex systems impedance analysis is often very difficult and far from being un-ambiguous. An equivalent circuit for mixed conductors was introduced in Ref. [7], but it is also restricted in its applicability, cf. Ref. [8]. Compared to EIS, oxygen isotope exchange and subsequent depth profiling [9] has the simpler methodology for data interpretation even though the experiment itself is more elaborate. There, the properties of oxygen exchange are tested by providing an isotopic tracer (e.g. 18 O) via the gas phase and establishing a time dependent concentration depth profile in a sample. The local tracer concentration is determined by secondary ion mass spectrometry (SIMS) and from the resulting depth profile, ion exchange and diffusion …