Hetero-Interfaces at Nanoscaled (La,Sr)CoO3-δ Thin-Film Cathodes Enhancing Oxygen Surface-Exchange Properties

Miniature solid oxide fuel cells (μ-SOFCs) to be operated at temperatures between 400 and 600°C require custom-tailored electrolyte and electrode structures on the nanoscale. La0.6Sr0.4CoO3-δ (LSC) thin-film cathodes derived by metal organic deposition (MOD) and with a nanoscaled microstructure were previously presented and exhibited an extremely low area specific polarization resistance of 7 mΩ·cm2 at 600°C, increasing to 1.9 Ω·cm2 at 400°C. Naturally, grains and pores at the nanoscale are essential, but the decisive aspect to high-performance is presented here for the first time. In fact, our LSC thin-film cathodes exhibit enhanced oxygen surface-exchange properties, with a k* value up to 47 times better than that of bulks of (nominally) equal composition. The presence of secondary phases such as (La,Sr)2CoO4±δ and Co3O4 leads to this significant improvement, forming finely dispersed hetero-interfaces with LSC. Interesting enough, these secondary phases crystallize out during a low-temperature decomposition of the metal organic sol-gel film. Thermodynamic calculations revealed that low oxygen partial pressures must prevail for this, which occur during the oxidation of the organics. Deliberate stoichiometry variations with up to 10 % A-site (La, Sr) or B-site (Co) excess added to La0.6Sr0.4CoO3-δ did not lead to further beneficial effects.