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L1.07 - Identifying, Visualizing and Modifying Reaction Pathways of Oxygen Reduction on Lanthanum Manganite (LSM) Model Electrodes 
April 22, 2014   11:00am - 11:15am

Sr-doped lanthanum manganite (LSM) is a widely used cathode material in commercially produced solid oxide fuel cells (SOFC). Despite being a poor ion conductor, LSM electrodes may reduce oxygen via different pathways: a path which includes surface diffusion of oxygen species (surface path) and a path based on oxygen bulk diffusion (bulk path). The relevance of each path can be expected to depend on geometry and microstructure, temperature, overpotential and partial pressure. However, separation of effective reaction rates on LSM cathodes into contributions of each path is experimentally nontrivial. Hence, a detailed knowledge of the rate limiting steps and their dependence on experimental parameters is still missing.In this contribution several different methods are employed to identify, visualize and modify the oxygen reduction paths of (La0.8Sr0.2)MnO3 and (La0.8Sr0.2)0.95MnO3 thin films and thin film microelectrodes deposited by pulsed laser deposition (PLD):i) LSM films on strontium titanium oxide (STO) and yttria-stabilized zirconia (YSZ) with differentmicrostructures, from epitaxial to fine columnar textured, were investigated by 18O tracer diffusion. Numerical analysis allowed to separate surface resistance and bulk diffusion properties of both, grains and grain boundaries.ii) Material parameters calculated from 18O depth profiles were compared to results gained from impedance spectroscopy measurements on microelectrodes.iii) 18O incorporation upon cathodic bias visualizes different reaction pathways and their changing contributions for microelectrodes under different measurement conditions.iv) Current-voltage studies on microelectrodes with variation of geometry, bias and oxygen partial pressure allows separating reaction pathways, identifying the rate limiting step and gives information on the current voltage characterization of different kinetic steps.The combination of these complementary tools under various operation conditions thus lead to a substantially improved understanding of the oxygen reduction kinetics on LSM thin films.

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