Synthesis and characterisation of oxyanion-doped cobalt containing perovskites

Abstract

In this paper we report the incorporation of borate, silicate and phosphate into La0.6Sr0.4Co0.8Fe0.2O3−δ (LSCF) and Sr0.9Y0.1CoO3−δ (SYC) cathode materials for SOFCs. In the former, an increase in the electronic conductivity was observed, which can be correlated with electron doping due to the oxyanion doping favouring the introduction of oxide ion vacancies. The highest conductivity was observed for\ud La0.6Sr0.4Co0.76Fe0.19B0.05O3−δ, 1190 S·cm-1 at 700ºC, in comparison with 431 S·cm-1 for undoped La0.6Sr0.4Co0.8Fe0.2O3−δ at the same temperature. For Sr0.9Y0.1CoO3−δ series the conductivity suffers a decrease on doping, attributed to any effect of electron doping\ud being outweighed by the effect of partial disruption of the electronic conduction pathways by the oxyanion. In order to investigate the potential of these materials as SOFC cathodes, the chemical compatibility with Gd0.1Ce0.9O1.95 (CGO10) was investigated and no reaction was observed between RT and 1100 ºC for both series. Composites of these cathode materials with 50% CGO10 were examined on dense\ud CGO10 pellets and the area specific resistances (ASR) in symmetrical cells were determined. The ASR values, at 800ºC, were 0.20, 0.08 and 0.11 W·cm2 for\ud La0.6Sr0.4Co0.8Fe0.2O3−δ, La0.6Sr0.4Co0.76Fe0.19B0.05O3−δ and La0.6Sr0.4Co0.78Fe0.195Si0.025O3−δ, respectively. For the SYC materials, the oxyaniondoped compositions also showed an improvement in the ASR values with respect to the\ud parent compounds, despite the lower electronic conductivity in these cases. At 800ºC, the values obtained for Sr0.9Y0.1CoO3−δ, Sr0.9Y0.1Co0.975B0.025O3−δ,\ud Sr0.9Y0.1Co0.975Si0.025O3−δ and Sr0.9Y0.1Co0.975P0.025O3−δ, were 0.09, 0.05, 0.05 and 0.03 W·cm2, respectively. This observation may be due to an increase in ionic conductivity\ud due to oxyanion incorporation leading to the formation of oxide ion vacancies. In addition, the stability of these systems towards CO2 was studied. For La0.6Sr0.4Co0.8(1-\ud x)Fe0.2(1-x)MxO3−δ series, all compositions showed no evidence for reactivity with CO2 between RT and 1000 ºC. On the other hand, for the Sr0.9Y0.1Co1-xMxO3−δ series, some\ud reactivity was observed, although the CO2 stability was shown to be improved on oxyanion doping. Thus these results show that oxyanion doping can have a beneficial effect on the performance of perovskite cobaltite cathode materials