Effects of sintering temperature on composition, microstructure and electrochemical performance of spray pyrolysed LSC thin film cathodes

Abstract

Thin nanoporous LSC (La0.6Sr0.4CoO3-δ) cathodes are deposited by spray pyrolysis onto gadolinium-doped ceria (GDC) electrolyte substrates, followed by sintering at 600°C, 800°C, and 1000°C. The investigation includes quantitative microstructure analysis, electrochemical characterization and application of Adler-Lane-Steele (ALS) model in order to extract intrinsic material properties and to explain the effects of variation in sintering temperature. A secondary gray phase (SGP) is detected, which consists of Sr and O and has a contrast in backscatter imaging intermediate between the pores and the LSC. SGP fills 66% of the mesopores in LSC sintered at 600 °C. With increasing sintering temperature the amount of SGP decreases until it disappears at 1000 °C. In this investigation we intend to understand the effect of SGP formation. For this purpose the influence of microstructural changes (i.e. active surface area) and variation of intrinsic material properties (exchange flux density) associated with SGP formation need to be quantified. The area specific resistance (ASR) of symmetrical LSC/GDC/LSC cells is measured between 400 and 600°C by impedance spectroscopy. ASR values as low as 0.13 Ω cm2 are obtained for samples sintered at 600°C, and 80 times higher for samples sintered at 1000 °C. These results indicate that the SGP is not blocking gas diffusion of O2 in the pores and therefore surface oxygen reduction reaction may take place over the entire LSC surface. Hence at low sintering temperatures a high specific surface area is obtained and the results indicate that formation of SGP does not bring a negative effect neither on the oxygen transport in 1-μm thin electrodes, nor on the oxygen reduction kinetics of LSC. An inverse correlation between the measured ASR values and the LSC-surface is obtained. The exchange neutral flux density, r0, is calculated using the ALS model, which results in r0-values in the range between 10-8 (600°C) and 10-9 mol/cm2/s (1000°C). Considering the formation of a secondary SrO-phase in a mass-balance for the entire sample also leads to the conclusion that there must be an increase of A-site deficiency and oxygen vacancies in LSC. In summary, it can be concluded that the variation of the ASR between LSC sintered at 600°C and 1000 °C is more strongly related to the difference in intrinsic material property, (r0 varies by a factor of 40) than the difference in surface area, (a varies by a factor of 2). For a controlled optimization of cathode performance it is necessary to consider all these different aspects (non-stoichiometric compositions, microstructure, secondary phase formation, intrinsic properties, sintering temperature).