Substrate-Mediated Spreading and Phase Segregation at LSM-Zirconia Interfaces

Atomic force microscopy and electron microscopy with energy dispersive X-ray analysis was used to characterize changes in teh structure and composition of La0.8Sr0.2MnO3 (LSM) nanoparticles supported on single crystal YSZ (100) (yttria-stabilized zirconia) and SrTiO3(100) surfaces as a function of temperature and exposure to oxidizing and reducing environments. On YSZ(100), LSM particles were found to decompose into Mnand La-rich phases and spread over the surface upon calcination in air at temperatures above 1123 K. The Mn-rich phase was observed to have a higher mobility and spread more rapidly. In contrast to YSZ(100), on SrTiO3(100) the LSM particles underwent agglomeration via an Ostwald ripening mechanism upon calcination at temperatures above 1123 K, resulting in an increase in the particle size. Phase separation was not observed on this substrate. Disciplines Biochemical and Biomolecular Engineering | Chemical Engineering | Engineering Comments Suggested Citation: Kim, J.S., Lee, S., Gorte, R.J. and J.M. Vohs. (2010). "Substrate-Mediated Spreading and Phase Segregation at LSM-Zirconia Interfaces." Journal of the Electrochemical Society. Vol. 158 (2). pp. B79-B83. © The Electrochemical Society, Inc. 2010. All rights reserved. Except as provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The archival version of this work was published in Journal of the Electrochemical Society Publisher URL: http://scitation.aip.org/JES/ This journal article is available at ScholarlyCommons: http://repository.upenn.edu/cbe_papers/139 Substrate-Mediated Spreading and Phase Segregation at LSM-Zirconia Interfaces Ju-Sik Kim, Shiwoo Lee,* Raymond J. Gorte,* and John M. Vohs* Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA Atomic force microscopy and electron microscopy with energy dispersive X-ray analysis was used to characterize changes in the structure and composition of La0.8Sr0.2MnO3 LSM nanoparticles supported on single crystal YSZ 100 yttria-stabilized zirconia and SrTiO3 100 surfaces as a function of temperature and exposure to oxidizing and reducing environments. On YSZ 100 , LSM particles were found to decompose into Mnand La-rich phases and spread over the surface upon calcination in air at temperatures above 1123 K. The Mn-rich phase was observed to have a higher mobility and spread more rapidly. In contrast to YSZ 100 , on SrTiO3 100 the LSM particles underwent agglomeration via an Ostwald ripening mechanism upon calcination at temperatures above 1123 K, resulting in an increase in the particle size. Phase separation was not observed on this substrate. © 2010 The Electrochemical Society. DOI: 10.1149/1.3507286 All rights reserved. Manuscript submitted July 7, 2010; revised manuscript received October 5, 2010. Published November 30, 2010. Solid oxide fuel cells SOFC and solid oxide electrolysis cells SOE are promising technologies for the efficient interconversion of chemical and electrical energies. These devices typically use yttria-stabilized zirconia YSZ as an oxygen ion conducting electrolyte and operate at temperatures in excess of 923 K. The air electrodes in these systems are generally composed of a porous composite of the YSZ electrolyte and an electronic or mixed electronic/ionic conducting perovskite oxide with strontium-doped lanthanum manganate, La0.8Sr0.2MnO3 LSM , being the most common. The structural and chemical properties of the interfaces between the electronically conducting perovskite and the YSZ electrolyte are critical in determining the performance of the electrode. For LSM, which has very low ionic conductivity, this is especially true because the electrochemical reactions likely occur only at three-phase boundary TPB sites, i.e., the locus of sites at the exposed interface between the electrolyte, the LSM, and the gas phase. The performance of LSM–YSZ composite electrodes is, therefore, highly dependent on the local structure and composition at the interface, which can be a function of the synthesis and operating conditions. An important example of this is the observation that the area specific resistance ASR of LSM–YSZ electrodes often decreases dramatically when they are cathodically polarized. This phenomenon, which is referred to as activation polarization, is reversible upon anodic polarization or annealing in air at open circuit. A variety of mechanisms for activation polarization have been proposed including Sr segregation within the LSM and morphological changes in the LSM resulting from changes in PO2. 8,12,14,15 Understanding the effects that LSM–YSZ interactions have on the structure and performance of cathodes and the causes for activation polarization have motivated a number of previous studies of the dynamics of model LSM–YSZ interfaces. For example, our group has previously investigated how changes in sintering temperature and PO2 affect the structure of LSM nanoparticles supported on a YSZ 100 single crystal surface. In this work atomic force microscopy AFM images showed that the supported LSM nanoparticles spread over the YSZ 100 surface upon annealing in air at temperatures above 1273 K. The spreading was reversed, however, when the sample was annealed in H2 at 973 K, conditions that simulate high cathodic overpotentials. This treatment resulted in the reappearance of a distribution of LSM nanoparticles on the YSZ 100 surface. Based on these results it was argued that the decrease in the ASR of LSM–YSZ cathodes that occurs upon cathodic polarization is caused by the breakup of the dense LSM coating that is formed on the YSZ during the high-temperature synthesis steps, resulting in an increase in the TPB area. Several subsequent studies have provided additional evidence to support this mechanism. For example, la O’ et al. have reported that cathodic polarization caused a 65 nm thick, dense, LSM film on a planar YSZ support to break up into nanoparticles. A study from our laboratory has also shown that LSM–YSZ electrodes that consist of a high surface-area, LSM nanoparticle coating on the YSZ does not exhibit polarization activation. These electrodes were synthesized by infiltrating the LSM into a porous YSZ scaffold, thereby avoiding high-temperature 1273 K annealing steps that tend to produce a dense LSM film. While the results described above provide strong evidence that structural modification of the LSM plays an important role in the mechanism of activation polarization, there is also some evidence that ion segregation and/or phase separation may play a role. For example, Fister et al. used synchrotron-based total reflection X-ray fluorescence measurements to monitor the spatial variations in the composition of La0.7Sr0.3MnO3 001 -oriented single crystal thin films as a function of PO2 and observed segregation of Sr to the surface of the film at low PO2 10 −1 atm . This process was reversible and Sr-enrichment of the surface was eliminated upon the application of higher PO2. Backhaus-Ricoult et al. have also observed via in situ photoelectron spectroscopy that Mn ions spread away from the LSM–YSZ TPB and over the YSZ surface under cathodic polarization. This process was found to be reversed upon anodic polarization. These results suggest that decomposition of the LSM and the spreading of MnOx films on the YSZ surface may play a role in the polarization-dependent properties of the LSM. In this study we have further examined how temperature and PO2 affect the sintering behavior and the structure and composition of LSM particles in contact with YSZ. AFM and secondary electron microscopy SEM were used to characterize the structure of model LSM–YSZ 100 interfaces as a function of temperature and PO2. Of particular interest in this work was to evaluate whether the PO2-dependent morphological changes in YSZ-supported LSM particles observed in our previous studies are accompanied by changes in the composition of the YSZ and the possible role that the formation of MnOx films on the YSZ surface, as suggested by BackhausRicoult et al., may play in directing the observed morphological changes.