Characterization of porous La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ based cathode films for intermediate temperature solid oxide fuel cells. An electrochemical impedance study

The La_xSr_1-xCo_yFe_1-yO_3-δ family of mixed conducting materials shows high electron- and oxygen ion conductivity, together with an appreciable catalytic activity for dissociation of ambient oxygen. These properties are of importance for solid oxide fuel cells. In this family of compounds, La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF6428) has been well-studied, both fundamentally and in actual applications. The related composition, La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (LSCF6482) has received much less attention despite its higher electronic and ionic conductivity. Literature results show for this composition sometimes rather conflicting results. The finegrained (100-150 nm) porous LSCF6482 electrodes show at higher temperatures a low-frequency dispersion, in the frequency range of ∼0.01–10 Hz. This dispersion is the result of gas phase diffusion limitation (GDL) coupled to the redox behavior of the mixed conducting LSCF6482. Applying a dense, thin layer of LSCF6482 between electrolyte and porous electrode improves the electrode properties, as it removes the ‘bottle neck’ for charge transfer of surface adsorbed oxygen moieties. Mixing Gd-doped cerium oxide, Ce_0.9Gd_0.1O_1.95 (CGO) with LSCF6482 in a porous electrode structure improves the electrode properties significantly as CGO has apparently a better catalytic activity for oxygen dissociation. The mid-frequency capacitance, C_mid, is assigned to surface charge, i.e. adsorbed O_ad⁻ species. The introduction of CGO in the electrode appears to shift the dissociative adsorption of oxygen from the LSCF surface to the catalytically more active CGO surface. The significantly lower area specific resistance (ASR) is, however, strongly dominated by a larger GDL contribution at temperatures above ∼600 °C.