Engineering Composite Oxide SOFC Anodes for Efficient Oxidation of Methane

Ceramic anodes for solid oxide fuel cells SOFCs were prepared by aqueous impregnation of nitrate salts to produce composites with 45 wt % La0.8Sr0.2Cr0.5Mn0.5O3 (LCSM)in a 65% porous yttria-stabilized zirconia (YSZ) scaffold. Scanning electron micrographs indicate that the LSCM coats the YSZ pores following calcination at 1473 K. Composites produced in this manner exhibit conductivities at 1073 K of approximately 1 S/cm in air and 0.1 S/cm in humidified H2. A SOFC with a composite anode composed of 45 wt % LSCM, 0.5 wt % Pd, and 5 wt % ceria exhibited maximum power densities at 1073 K of 1.1 and 0.71 W cm−2 in humidified (3% H2O) H2 and methane, respectively. Comments © 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 Electrochemical and Solid State Letters, Volume 11, Issue 2, 2008, pages B16-B19. Publisher URL: http://dx.doi.org/10.1149/1.2817809 This working paper is available at ScholarlyCommons: http://repository.upenn.edu/cbe_papers/103 Engineering Composite Oxide SOFC Anodes for Efficient Oxidation of Methane G. Kim, G. Corre, J. T. S. Irvine,* J. M. Vohs, and R. J. Gorte* Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA School of Chemistry, University of St. Andrews, Fife KY16 9ST, Scotland Ceramic anodes for solid oxide fuel cells SOFCs were prepared by aqueous impregnation of nitrate salts to produce composites with 45 wt % La0.8Sr0.2Cr0.5Mn0.5O3 LSCM in a 65% porous yttria-stabilized zirconia YSZ scaffold. Scanning electron micrographs indicate that the LSCM coats the YSZ pores following calcination at 1473 K. Composites produced in this manner exhibit conductivities at 1073 K of approximately 1 S/cm in air and 0.1 S/cm in humidified H2. A SOFC with a composite anode composed of 45 wt % LSCM, 0.5 wt % Pd, and 5 wt % ceria exhibited maximum power densities at 1073 K of 1.1 and 0.71 W cm−2 in humidified 3% H2O H2 and methane, respectively. © 2007 The Electrochemical Society. DOI: 10.1149/1.2817809 All rights reserved. Manuscript submitted October 16, 2007; revised manuscript received November 2, 2007. Available electronically November 30, 2007. Solid oxide fuel cells SOFCs offer uniquely scalable, highefficiency conversion of chemical-to-electrical energy. Present SOFC anodes based upon Ni cermets provide good electrochemical and catalytic performance but suffer from important limitations. While state-of-the-art, Ni-based anodes perform well in hydrogenbased fuels, the strong tendency of Ni to catalyze formation of carbon filaments makes SOFC operation with hydrocarbons other than methane and even methane must be co-fed with large amounts of steam impractical. Not only is the Ni surface deactivated by surface carbon formation, but filamentous carbon can lead to loss of the Ni by metal dusting and catastrophic fracture of Ni composites by expansion of the carbon filaments. Furthermore, even the bestengineered Ni-based electrodes have only limited tolerance to oxidation during start-up and shut-down cycles due to the expansion that occurs when Ni forms NiO. Conductive ceramics would offer an attractive alternative to Ni composites if comparable electrochemical performance could be achieved. Ceramic materials tend to be stable against carbon-fiber formation in hydrocarbon fuels and would be less sensitive to oxidation and reduction cycles. For operation with hydrocarbon fuels, the ability to oxidize the anode would be particularly useful, because this would allow periodic oxidation cycles to remove impurities brought in with the fuel or carbon deposits formed by gas-phase pyrolysis. Because many ceramics have high melting temperatures, excellent thermal stability is also anticipated. Our laboratories have investigated two different approaches for the development of ceramic anodes. The group at St. Andrews has been investigating oxides that show high conductivities under reducing conditions, an effort that has demonstrated the promising properties of Sr-doped LaCr0.5Mn0.5O3 LSCM . 9 However, the catalytic performance of LSCM-based electrodes is not comparable to Ni-based anodes at lower temperatures. The group at Penn has investigated ceramic anodes prepared by impregnation of electronically conductive and catalytic components into porous scaffolds. The best performance was achieved by preparing thin functional layers of porous yttria-stabilized zirconia YSZ , into which ceria and dopant levels of Pd were added for electronic conductivity and catalytic activity. While the initial performance of these anodes were excellent the anode impedance for one cell was estimated to be 0.26 cm2 at 973 K in humidified H2 11 , the conductivity achieved by impregnating ceria into the porous YSZ layer was not thermally stable. In the present study, we have combined these approaches by preparing LSCM-based electrodes using the porous-scaffold approach. We demonstrate that high performance can be achieved in this way. The electrodes are also redox tolerant and stable in dry methane.