SOFC Anodes Based on LST–YSZ Composites and on Y0.04Ce0.48ZrM0.48O2

The properties of solid oxide fuel cell (SOFC) anode functional layers prepared by impregnation of 1 wt % Pd and 10 wt % ceria into porous scaffolds of either Y0.04Ce0.48Zr0.48O2 (CZY) or composites of La0.3Sr0.7TiO3 (LST) and yttria-stabilized zirconia (YSZ) were examined to determine whether these scaffold materials would have sufficient electronic and ionic conductivity. Laminated tapes were cofired to produce 50 m YSZ electrolytes and 50 μm scaffolds, supported on LSF–YSZ cathodes. The electronic conductivities of LST–YSZ composites were a function of the porosity and the weight fraction of LST but could be sufficient for use in thin functional layers. However, anodes made with LST–YSZ composites had higher nonohmic losses than cells made with YSZ scaffolds. With CZY scaffolds, some migration of Ce into the YSZ electrolyte was observed after cofiring. While CZY exhibited electronic conductivity, the loss in ionic conductivity compared to YSZ again resulted in higher nonohmic losses. The implications of these results for producing better ceramic anodes are discussed. Comments Reprinted from © The Electrochemical Society, Inc. 2008. 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, Volume 155, Issue 4, February 2008, pages B360-B366. This journal article is available at ScholarlyCommons: http://repository.upenn.edu/cbe_papers/111 SOFC Anodes Based on LST–YSZ Composites and on Y0.04Ce0.48Zr0.48O2 Guntae Kim, Michael D. Gross, Wensheng Wang,* John M. Vohs,* and Raymond J. Gorte* Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA Department of Chemical Engineering, Bucknell University, Lewisburg, Pennsylvania 17837, USA The properties of solid oxide fuel cell SOFC anode functional layers prepared by impregnation of 1 wt % Pd and 10 wt % ceria into porous scaffolds of either Y0.04Ce0.48Zr0.48O2 CZY or composites of La0.3Sr0.7TiO3 LST and yttria-stabilized zirconia YSZ were examined to determine whether these scaffold materials would have sufficient electronic and ionic conductivity. Laminated tapes were cofired to produce 50 m YSZ electrolytes and 50 m scaffolds, supported on LSF–YSZ cathodes. The electronic conductivities of LST–YSZ composites were a function of the porosity and the weight fraction of LST but could be sufficient for use in thin functional layers. However, anodes made with LST–YSZ composites had higher nonohmic losses than cells made with YSZ scaffolds. With CZY scaffolds, some migration of Ce into the YSZ electrolyte was observed after cofiring. While CZY exhibited electronic conductivity, the loss in ionic conductivity compared to YSZ again resulted in higher nonohmic losses. The implications of these results for producing better ceramic anodes are discussed. © 2008 The Electrochemical Society. DOI: 10.1149/1.2840473 All rights reserved. Manuscript submitted September 4, 2007; revised manuscript received January 7, 2008. Available electronically February 12, 2008. The development of high-performance ceramic anodes would provide significant advantages for solid oxide fuel cells SOFCs . For example, unlike the more commonly used Ni-based electrodes, ceramic anodes would not promote the formation of carbon fibers when exposed to hydrocarbon fuels. Furthermore, it is expected that ceramic electrodes would tolerate repeated oxidation and reduction cycles without fracturing. Because of these potential advantages, there have been a significant number of reports of SOFCs that use electronically conductive oxides as the main component of the anode. In general, the performance of ceramic anodes has been modest due to their low conductivities and catalytic activities compared to Ni. Therefore, much of the effort in this area has been directed toward identifying new materials, such as in the work that led to the recent discoveries of Sr2Mg1−xMnxMoO6− 11 and La0.75Sr0.25Cr0.5Mn0.5O3. 12 In our laboratory, the focus has been on developing ceramic anodes that have separate functional and conduction layers so that different materials may be used for conductivity and electrocatalysis. Because the electrochemical reactions occur only in the functional layer, the conduction layer can be essentially any porous material which has a high electronic conductivity. This concept has been demonstrated in a previous study in which the electrode performance was found to be essentially the same when the conduction layer was made from either Ag paste or a porous layer of La0.7Sr0.3TiO3 LST . 13 For the functional layer, we have focused on materials that are good oxidation catalysts e.g., transition-metaldoped ceria . The conductivity of the functional layer can be modest if this layer is thin. In our past work, electronic conductivity was achieved by impregnating relatively high loadings of ceria 40–60 wt % into a porous layer of yttria-stabilized zirconia YSZ . Even though the conductivity of the functional layer in the fresh cell was 0.02 S/cm in humidified H2 at 973 K, ohmic losses in the anode were acceptable in the freshly prepared cells because the functional layers were only 12 m thick. The total anode losses, including the ohmic contribution, for a cell with a functional layer made by impregnating 1 wt % Pd and 40 wt % ceria into a 12 m thick, porous YSZ, were 0.26 cm2 in humidified H2 at 973 K. 13 Unfortunately, long-time aging or heating to 1173 K caused the conductivity of the functional layer to decrease, apparently due to changes in the ceria film. The goal of the present work was to modify the functional layer by replacing the porous YSZ with materials that have electronic conductivity, with the goal of further enhancing the performance of the multilayered anode. Metal-doped ceria was still added to the porous layer by impregnation but only to provide catalytic activity. Two strategies were investigated for achieving conductivity. First, the porous YSZ was replaced with a porous mixture of LST and YSZ. LST–YSZ mixtures remain as separate phases, even after calcination at 1773 K, so that porous composites can have both electronic conductivity from the LST and ionic conductivity from the YSZ . This approach is similar to that used in two earlier studies, which employed porous mixtures of YSZ and Sr0.92Y0.08TiO3 YST as the backbone for the anode. LST was chosen for the present study because it has slightly higher conductivity than YST and is similarly unreactive with YSZ. The effects on conductivity of the LST–YSZ composition and of the composite porosity were also examined here. In the second approach, the porous YSZ was replaced with porous Y0.04Ce0.48Zr0.48O2 CZY . CZY was found to remain as a single-phase material after high-temperature calcination and exhibited electronic conductivity under reducing conditions due to the reduction of Ce+4. Because solid solutions of ceria and zirconia reduce more easily than pure ceria or ceria doped with other rare earths, such as Sm+3 or Y+3, it was expected that CZY would have good conductivity under reducing conditions. It will be shown that both approaches, preparing porous functional layers by impregnating catalysts into LST–YSZ composites or into CZY, can help stabilize the ohmic losses in cells. However, replacing YSZ with either an LST–YSZ composite or CZY results in a loss of ionic conductivity that decreases the overall performance of the electrode.