An Examination of SOFC Anode Functional Layers Based on Ceria in YSZ

The properties of solid oxide fuel cell (SOFC) anode functional layers prepared by impregnation of ceria and catalytic metals into porous yttria-stabilized zirconia (YSZ) have been examined for operation at 973 K. By varying the thickness of the functional layer, the conductivity of the ceria-YSZ composite was determined to be only 0.015–0.02 S/cm. The initial performance of anodes made with ceria loadings of 40 or 60 wt % were similar but the anodes with lower loadings lost conductivity above 1073 K due to sintering of the ceria. The addition of dopant levels of catalytic metals was found to be critical. The addition of 1 wt % Pd or Ni decreased the anode impedances in humidified H2 dramatically, while the improvement with 5 wt % Cu was significant but more modest. Pd doping also decreased the anode impedance in dry CH4 much more than did Cu doping; however, addition of either Pd or Cu led to similar improvements for operation in n-butane. Based on these results, suggestions are made for ways to improve SOFC anode functional layers. Disciplines Biochemical and Biomolecular Engineering | Chemical Engineering | Engineering Comments Suggested Citation: M.D. Gross, J.M. Vohs and R.J. Gorte. (2007). An Examination of SOFC Anode Functional Layers Based on Ceria in YSZ. Journal of The Electrochemical Society. 154(7) B694-B699. © The Electrochemical Society, Inc. 2007. 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 154, Issue 7, 2007, pages B694-B699. Publisher URL: http://scitation.aip.org/JES/ This journal article is available at ScholarlyCommons: http://repository.upenn.edu/cbe_papers/149 An Examination of SOFC Anode Functional Layers Based on Ceria in YSZ M. D. Gross, J. M. Vohs,* and R. J. Gorte* Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA The properties of solid oxide fuel cell SOFC anode functional layers prepared by impregnation of ceria and catalytic metals into porous yttria-stabilized zirconia YSZ have been examined for operation at 973 K. By varying the thickness of the functional layer, the conductivity of the ceria-YSZ composite was determined to be only 0.015–0.02 S/cm. The initial performance of anodes made with ceria loadings of 40 or 60 wt % were similar but the anodes with lower loadings lost conductivity above 1073 K due to sintering of the ceria. The addition of dopant levels of catalytic metals was found to be critical. The addition of 1 wt % Pd or Ni decreased the anode impedances in humidified H2 dramatically, while the improvement with 5 wt % Cu was significant but more modest. Pd doping also decreased the anode impedance in dry CH4 much more than did Cu doping; however, addition of either Pd or Cu led to similar improvements for operation in n-butane. Based on these results, suggestions are made for ways to improve SOFC anode functional layers. © 2007 The Electrochemical Society. DOI: 10.1149/1.2736647 All rights reserved. Manuscript submitted January 4, 2007; revised manuscript received March 8, 2007. Available electronically May 22, 2007. While the large majority of solid oxide fuel cells SOFCs have anodes made from ceramic-metallic cermet composites of Ni with yttria-stabilized zirconia YSZ , there has recently been significant interest in developing alternative anodes to avoid some of the important limitations of Ni-based electrodes. In particular, Ni composites are sensitive to sulfur, can form carbon deposits in the presence of hydrocarbon fuels, and are intolerant of oxidation cycles. Anodes based on conductive ceramics are especially attractive for replacing Ni cermets because it is anticipated that they should be insensitive to redox cycling and could exhibit high thermal stability. Furthermore, at least some conductive ceramics have demonstrated tolerance to sulfur and resistance to carbon formation in the presence of hydrocarbons. In most cases, however, ceramic anodes have also led to much higher overpotentials than are commonly achieved with Ni–YSZ cermets. The modest electrochemical performance of ceramic anodes is due to the difficulty of producing ceramics that have both high electronic and ionic conductivity at low P O2 , while also having good surface reactivity with H2 and other fuels. 6 We have recently reported that it is possible to obtain good ceramic anode performance, without having to develop new oxide compositions, through the use of separate functional and conduction layers. That study demonstrated that low anode overpotentials could be achieved if the materials used in the functional layer had sufficient catalytic activity, even if those materials had only modest electronic conductivity. Ohmic losses associated with poor electronic conductivity in the functional layer were kept low by making the functional layers very thin, on the order of 10 m. Current collection was accomplished primarily within a thicker conduction layer in which electrochemical activity was unnecessary, so that similar performance was achieved when using either Ag paste or porous La0.3Sr0.7TiO3 LST for electronic conduction. The functional layer in the previous study was made from 1 wt % Pd and 40 wt % ceria, impregnated into 65% porous YSZ. Pd-doped ceria was chosen in that study because ceria provides some electronic conductivity under reducing conditions and Pd/ceria is one of the most active catalysts known for hydrocarbon oxidation. In the present research, we examined the role of the functional layer in more detail. While we continued to prepare the functional layer by impregnation methods, we examined the role of functional layer thickness and composition more carefully, particularly looking toward avoiding the use of precious metals and application of ceramic anodes with other fuels. The results further demonstrate the promise of using the functional layer approach for developing ceramic anodes and point toward directions for further improvements.