Oxygen transfer processes in (La,Sr)MnO 3/Y 2O 3-stabilized ZrO 2 cathodes: an impedance spectroscopy study

Abstract

Impedance spectroscopy was used to study the oxygen reaction kinetics of La 0.8Sr 0.2MnO 3 (LSM)-based electrodes on Y 2O 3-stabilized ZrO 2 (YSZ) electrolytes. Three types of electrodes were studied: pure LSM, LSM–YSZ composites, and LSM/LSM–YSZ bilayers. The electrodes were formed by spin coating and sintering on single-crystal YSZ substrates. Measurements were taken at temperatures ranging from 550 to 850°C and oxygen partial pressures from 1×10 −3 to 1 atm. An arc whose resistance R el had a high activation energy, E a=1.61±0.05 eV, and a weak oxygen partial pressure dependence, ( P O 2 ) −1/6, was observed for the LSM electrodes. A similar arc was observed for LSM–YSZ electrodes, where R el∼( P O 2 ) −0.29 and the activation energy was 1.49±0.02 eV. The combination of a high activation energy and a weak P O 2 dependence was attributed to oxygen dissociation and adsorption rate-limiting steps for both types of electrodes. LSM–YSZ composite cathodes showed substantially lower overall interfacial resistance values than LSM, but exhibited an additional arc attributed to the resistance of YSZ grain boundaries within the LSM–YSZ. At 850°C and low P O 2 , an additional arc was observed with size varying as ( P O 2 ) −0.80 for LSM and ( P O 2 ) −0.57 for LSM–YSZ, suggesting that diffusion had become an additional rate limiting step. Bilayer LSM/LSM–YSZ electrodes yielded results intermediate between LSM and LSM–YSZ. The results showed that most of the improvement in electrode performance was achieved for a LSM–YSZ layer only ≈2 μm thick. However, a decrease in the grain-boundary resistance would produce much better performance in thicker LSM–YSZ electrodes.