OXYGEN-TRANSFER PROCESSES IN (LA,SR)MNO3 Y2O3-STABILIZED ZRO2 CATHODES - AN IMPEDANCE SPECTROSCOPY STUDY/

Impedance spectroscopy was used to study the oxygen reaction kinetics of La0.8Sr0.2MnO3 (LSM)-based electrodes on Y2O3-stabilized ZrO2 (YSZ) electrolytes. Three types of electrodes were studied: pure LSM, LSM-Y SZ composites, and LSM/LSM-YSZ bilayers. The electrodes were formed by spin coating and sintering on single-crystal YSZ substrates. Measurem ents were taken at temperatures ranging from 550 to 850 degrees C and oxygen partial pressures from 1 x 10(-3) An are whose resistance R-el had a high activation energy, E-a = 1.61+/-0.05 eV, and a weak oxygen partial pressure dependence, (P-O2)(-1/6), was observed for the LSM el ectrodes. A similar are was observed far LSM-YSZ electrodes, where R(e l)similar to(P-O2)(-0.29) and the activation energy was 1.49+/-0.02 eV . The combination of a high activation energy and a weak P-O2 dependen ce 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, bu t exhibited an additional are attributed to the resistance of YSZ grai n boundaries within the LSM-YSZ. At 850 degrees C and low P-O2, an add itional are was observed with size varying as (P-O2)(-0.80) for LSM an d (P-O2)(-0.57) for LSM-YSZ, suggesting that diffusion had become an a dditional rate limiting step. Bilayer LSM/LSM-YSZ electrodes yielded r esults intermediate between LSM and LSM-YSZ. The results showed that m ost of the improvement in electrode performance was achieved for a LSM -YSZ layer only approximate to 2 mu m thick. However, a decrease in th e grain-boundary resistance would produce much better performance in t hicker LSM-YSZ electrodes. (C) 1998 Published by Elsevier Science B.V. All rights reserved.