Investigation of factors enhancing electrochemical properties of the porous La0.6Sr0.4CoO3-δ-Ce0.9Gd0.1O1.95 composite electrode for solid oxide fuel cell

One method for enhancing the electrochemical performance of a solid oxide fuel cell (SOFC) cathode at low temperatures is to mix two oxides with dissimilar structures to form a composite electrode. To understand the enhancement factor of the composite electrode consisting of an ionic conducting oxide, Ce0.9Gd0.1O1.95 (GDC), and a mixed ionic and electronic conducting oxide, La0.6Sr0.4CoO3-delta (LSC), electrochemical measurements were performed as a function of composition ratio, temperature (673-1073 K), and oxygen partial pressure (p(O-2), 1-10(-4) bar). The area-specific conductivity (sigma(E)) that was obtained from the impedance spectra was enhanced at low temperature (T < 873 K) in the high p(O-2) region (1-10(-1) bar) for the samples that contained above 40 % of GDC. However, the enhancement was not significant at high temperatures (T > 873 K) under all measured p(O-2) conditions. Although some LSC particles were replaced by GDC, the enhancement of the chemical capacitance of the composite electrode was observed. This indicates that GDC particles function as ionic conducting pathways in the composite electrode. To understand the enhancement mechanism, the experimental data of sigma(E) were compared with the calculated results using a one-dimensional transmission-line model (1-D TLM) considering only the contributions of surface resistivity and ionic resistivity. Results indicate that there is a discrepancy between the measured result of sigma(E) and the calculated result. Several plausible reasons for the discrepancy were discussed, where the contribution of the triple phase boundary reaction resistivity could not be ignored in the calculation of sigma(E).