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The preferential site for oxygen isotope exchange was investigated on layered films of La 2−x Sr x CoO 4 ͑x = 0.5,1.0͒/La 0.6 Sr 0.4 CoO 3 prepared by pulsed laser deposition ͑PLD͒ on a polycrystalline Ce 0.9 Gd 0.1 O 1.95 . An isotope exchange experiment was performed at 773 K in 18 O-enriched oxygen of 0.2 ϫ 10 5 Pa. Three-dimensional imaging of secondary ion mass spectrometry ͑SIMS͒ was carried out to visualize the isotope distribution. The SIMS images suggested that there are fast oxygenincorporation paths along the hetero-phase interface of La 2−x Sr x CoO 4 /La 0.6 Sr 0.4 CoO 3 . The results have reproduced our former findings on the fast oxygen surface exchange with La 2−x Sr x CoO 4 /La 0.6 Sr 0.4 CoO 3 composite polycrystals.

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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. La 0.6 Sr 0.4 CoO 3-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 m • cm 2 at 600 • C, increasing to 1.9 • cm 2 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) 2 CoO 4±δ and Co 3 O 4 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 La 0.6 Sr 0.4 CoO 3-δ did not lead to further beneficial effects.