45844-G10
The Role of Catalysis in Next Generation Direct Hydrocarbon Solid Oxide Fuel Cell Anodes
Annual Report: The Role of Catalysis in Next Generation Direct Hydrocarbon Solid Oxide Fuel Cell Anodes
Grant Number: ACS/PRF 45844-G10
Annual Report for period ending August 31st 2008.
Prepared by: Dr. Steven McIntosh, Department of Chemical Engineering, University of Virginia. Charlottesville, VA 22904
Solid Oxide Fuel Cells (SOFCs) are considered one of the most promising candidates for future power generation. SOFC operate via oxygen anion transport from the cathode to the anode. This mechanism can allow SOFC to operate on a wide range of fuels, including conventional petroleum, diesel and natural gas and future bio-derived hydrocarbons. In addition, due to the high SOFC operating temperature (>700¼C), SOFC can employ low cost, sulfur and CO tolerant transition metal catalysts. The primary challenge in realizing these direct hydrocarbon SOFCs is the design and development of materials and catalysts for the SOFC anode. These anode materials must be catalytically active for hydrocarbon fuel oxidation while providing sufficient oxygen anion and electronic conductivity to facilitate transport of these species to/from the electrochemical reaction site.
This grant has enabled us to develop techniques for the fabrication and testing of both catalytic materials and SOFC. This work has resulted in two peer-review publications that were partially funded by the ACS/PRF program. These publications are attached. The student and faculty salary support provided by this funding has proven invaluable to the research detailed in these papers.
The first paper is a catalytic study of the methane oxidation activity and selectivity of La0.75Sr0.25CrxMn1-xO3-d (LSCM). LSCM is a promising material for the anode of SOFC; however, its catalytic properties have not been examined. We have utilized a pulse reactor system to characterize these properties as a function of lattice oxygen non-stoichiometry under SOFC anode conditions. This is the first work that seeks to relate catalytic performance to the oxygen stoichiometry of the anode material. We found that the rate of hydrocarbon oxidation and selectivity towards total oxidation products both decreased with increasing oxygen non-stoichiometry. This is relevant to the SOFC anode as our working hypothesis is that the catalyst non-stoichiometry will be set by a balance between the rate of oxygen supply through the electrolyte (fuel cell current density) and oxygen consumption by the fuel oxidation reaction.
The second publication was primarily authored by an undergraduate student, Michael Bruce, and formed the technical component of his senior thesis project for the B.S. degree in Chemical Engineering at the University of Virginia. Another undergraduate student, Yannick Kimmel, aided in this project by performing initial fuel cell testing. Graduate student Michael van den Bossche oversaw the day-to-day laboratory work of the undergraduates. In this paper we fabricated SOFC with LSCM-Cu-yttria stabilized zirconia (YSZ) anodes. The cell performance was measured in H2, CH4, and C4H10 fuel with H2 providing significantly higher performance than the hydrocarbon fuels. This indicates that the catalytic activity of the anode towards fuel oxidation was the rate-limiting step. Electrochemical impedance spectra (EIS) were performed as a function of current density through the cell – this is directly related to the rate of oxygen supply to the reaction sites. Significant changes in EIS spectra were suggested to be a result of changing oxygen stoichiometry, and hence changing catalytic activity, in the LSCM phase. Further analysis of the cell electrochemistry and anode product stream suggest that the anode reaction mechanism shifts towards total oxidation of hydrocarbon fuels at high current density. This is attributed to local decrease in LSCM oxygen non-stoichiometry due to the flux of oxygen into the material from the electrolyte.
Finally, new work is proceeding in two areas. Undergraduate student, Yannick Kimmel, is leading the first. He is utilizing polymer nanosphere templating to fabricate three dimensionally ordered inverse opal nanostructures of LSCM. University of Virginia energy research seed funding provides the equipment/materials support for this project. We are fabricating the polymethyl methacrylate nanospheres using an emulsion polymerization process. These are then self-assembled into a colloidal crystal and the gaps between nanospheres filled with an LSCM precursor solution. Upon firing, the polymer burns out while the ceramic forms in the voids to create the porous structure. Work is continuing to incorporate these nanostructures into the SOFC anode. The second new direction is being led by graduate student Maria Azimova and focuses upon proton conducting oxide materials. The goal is to extend the previous catalytic work on oxygen ion conducting oxides to proton conducting oxides. In particular, we seek to probe the role of proton transport in facilitating hydrogenation and dehydrogenation reactions.
