Electrical degradation of porous and dense LSM/YSZ interface

Electrochemical cells formed by the interface between dense and porous lanthanum strontium manganate (LSM) and yttria stabilized zirconia (YSZ) were submitted to annealing temperatures varying from 1373 K to 1673 K for 200 h and studied by Impedance Spectroscopy (IS) in order to investigate how the high annealing temperature can modify the contact between LSM/YSZ and to which extension these changes influence the electrical behavior of dense and porous LSM electrodes before and after the formation of insulating phases. Up to 1473 K the annealing process did not lead to substantial electrical behavior modifications at the LSM/YSZ interfaces for both porous and dense electrodes. IS measurements show two capacitive semicircles, the best fitting of impedance data brings to an equivalent circuit constituted by a serial combination of the electrolyte resistance and two parallel combinations of a resistance and a constant phase element, CPE. The higher frequency semicircles, HF, were attributed to the diffusion of oxide ions from the interface LSM/YSZ to the oxide ion vacancies located at the electrolyte surface. The semicircle at lower frequency, LF, will be ascribed to the oxygen species adsorption and diffusion in the LSM. At 1473 K the only changes recorded are related with the sinterization process of the porous electrodes. Over of 1473 K, the resistance contributions increased largely, especially for porous electrodes, and one additional semicircle was observed. This semicircle was associated to the oxygen diffusion process at the new insulating phases formed from YSZ and LSM solid state reactions. Porous and dense electrodes exhibited different rates for the degradation process. The porous electrode degraded faster than the dense one, probably because of the morphological effects as grain growth and their coalescence during annealing at higher temperatures.