Haile - SOFCs

Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) operate at high temperatures, and, as a consequence, they can utilize hydrocarbon fuels, they provide for efficient catalysis, and they exhaust 'high quality' waste heat that can be used for additional power generation. High temperature operation, however, has generally been responsible for the high cost of SOFCs and for their undesirability for portable applications. Our efforts in solid oxide fuel cells span a number of activities. These include the use of ceria as an electrolyte for reduced temperature operation, the incorporation of ceria in fuel cell anodes for enhanced electrooxidation activity for complex hydrocarbon fuels, the development of alternative perovskite cathodes for high activity oxygen electroreduction, and the operation of fuel cells in so-called single chamber mode for portable power.

Ceria as a fuel cell electrolyte
Ceria has higher conductivity than the conventional SOFC electrolyte, yttria-stabilized zirconia (YZS) but has the drawback of measurable electronic conductivity, particularly at high temperatures. In addition, the grain boundaries in doped ceria exhibit extensive space charge effects, depending on the level of doping, the concentration of impurities, and the extent of dopant/impurity segregation induced by the processing. Often, the grain boundaries in acceptor doped are depleted of oxygen vacancies and enhanced in free electrons. As a consequence, they are resistive in terms of ion transport and conductive in terms of electron transport. These characteristics lead to a range of properties that can be manipulated via mictrostructural engineering and stoichiometry control.

Advanced cathodes for solid oxide fuel cell
A key obstacle to reduced-temperature operation of SOFCs has been the poor activity of traditional cathode materials for electrochemical reduction of oxygen. We have explored the activity of an alternative material, Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF) as a new cathode for reduced temperature SOFC operation. This perovskite has been selected because it has already been demonstrated to have exceptionally high oxygen diffusivity. Thin-film ceria electrolyte fuel cells using BSCF as the cathode exhibit high power densities -- over 1 W/cm2 at 600C when operated with humidified hydrogen as the fuel and air as the cathode. Moreover, BSCF has the unusual characterisitc of being highly inactive towards direct chemical oxidation of alkane fuels. As a consquence, it is ideally suited for single chamber fuel cells, in which premixed fuel and oxygen are supplied to a single electrode chamber. Our singl chamber fuel cells put out over 400 mW/cm2 when operated on propane and over 700 mW/cm2 when operated on methane.

Electrochemistry of solid oxide fuel cell anodes
Anodes in solid oxide fuel cells are charged with the task of electro-oxidation of hydrocarbon fuels. That is, oxygen ions must reaction with the hydrocarbon to generate water and carbon dioxide and release electrons. To achieve high fuel cell efficiency, one must only identify highly active anode materials, but also ensure the anode surface area is sufficiently high so as provide a great concentration of reaction sites. Here we implement CeO₂ based aerogels and inverse opals, that incorporate Ni as a second phase, to both ensure high surface area and provide a means for probing reaction mechanisms. Fabrication of these structures is carried out in collaboration with Prof. Bruce Dunn of UCLA.

Solid oxide fuel cells for portable power
Although SOFCs offer the advantage of fuel flexibility and high efficiency as compared polymer electrolyte membrane fuel cells, they have not been seriously considered for small portable power applications for a variety of reasons. These include the difficulty of maintaining a high temperature in a small device, and the mechanical stresses associated with thermal cycling of ceramic components. In this work we aim to address these issues by incorporating a 'single chamber fuel cell' into a 'Swiss roll' heat exchanger. The single chamber design eliminates many of the problems generated by thermal cycling, whereas the heat exchanger maintains the fuel cell at high temperatures while the exterior walls of the structure are close to ambient temperatures. Incorporation of the fuel cells into the Swiss roll heat exchangers is done in collaboration with Prof. Paul Ronney of USC, and modeling of the single chamber fuel cell electrichemical charactersitics is carrierd out in collaboration with Prof. David Goodwin (Caltech).

Acknowledgments ($$$)

Office of Naval Research; Department of Energy

Former: Defense Advanced Research Projects Agency (DARPA)

Selected Publications

W. Lai and S. M. Haile, "Impedance Spectroscopy as a Tool for Chemical and Electrochemical Analysis of Mixed Conductors: A Case Study of Ceria," J. Amer. Cer. Soc. 88, 2979-2997 (2005).

Z. P. Shao, S. M. Haile, J. M. Ahn, P.D. Ronney, Z. L. Zhan and S. A. Barnett, "A thermally self-sustained micro Solid-Oxide Fuel Cell stack with high power density," Nature 435, 795-798 (2005).

Z. Shao and S. M. Haile, "A High Performance Cathode for the Next Generation Solid-Oxide Fuel Cells," Nature 431, 170-173 (2004).

Z. Shao, C. Kwak, S. M. Haile, "Preparation and investigation of anode-supported thin-film fuel cells operated in a single chamber configuration," Solid State Ionics 175, 39-46 (2004)

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