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Chemomechanics of Fuel-cell-related Materials

• Authors: S. R. Bishop, J. J. Kim, M. Kuhn, H. L. Tuller, in collaboration with B. Yildiz, K. V. Vliet, S. Yip
• Sponsorship: Department of Energy

Solid oxide fuel cells (SOFCs) directly convert chemical to electrical energy with high efficiency and can operate using a wide variety of fuels ranging from hydrogen and kerosene to gasified coal ^{[1] [2]} . Many of the more advanced oxides utilized in SOFCs experience significant changes in oxygen content, or oxygen stoichiometry, during operation, resulting in changes in volume and elastic properties termed chemomechanics ^{[3] [4]} . This lattice dilation known as chemical expansion, analogous to temperature induced thermal expansion, is oxygen nonstoichiometry-induced and can cause large stress gradients across a SOFC stack, with potential for negative impact on device performance ^[5] . Therefore, a fundamental understanding regarding the coupling between solid-state electrochemistry and mechanical deformation is required for successful development of functionally superior and long-lived fuel cell systems. In this project, we are studying the chemical expansion coefficient, elastic properties, and oxygen stoichiometry of thin film and bulk SOFC oxide materials. Thin films are of particular interest since they allow control of strain and increase the surface-to-volume ratio, particularly important for electrode performance. Furthermore, there is a trend towards the use of thinner structures such as m-SOFCs ^[6] . The chemomechanical properties are being investigated using high temperature and atmosphere-controlled nanoindentation, high-resolution x-ray diffraction, dilatometry, impedance spectroscopy, and thermo-gravimetry techniques.

1. B. C. H. Steele and A. Heinzel, “Materials for fuel-cell technologies,” Nature, vol. 414, pp. 345-352, Nov 2001. [↩]
2. M. Mogensen and K. Kammer, “Conversion of hydrocarbons in solid oxide fuel cells,” Ann. Rev. Mater. Res., vol. 33, pp. 321-331, Aug 2003. [↩]
3. H. L. Tuller and S. R. Bishop, “Point defects in oxides: tailoring materials through defect engineering,” Ann. Rev. Mater. Res., vol. 41, pp. 369-398, Aug 2011. [↩]
4. Y. Kuru, S. R. Bishop, J. J. Kim, B. Yildiz, and H. L. Tuller, “Chemomechanical properties and microstructural stability of nanocrystalline Pr-doped ceria: An in situ X-ray diffraction investigation,” Solid State Ionics, vol. 193, pp.1-4, June 2011. [↩]
5. K. Sato, H. Omura, T. Hashida, K. Yashiro, H. Yugami, T. Kawada, and J. Mizusaki, “Tracking the onset of damagemechanism in ceria-based solid oxide fuel cells under simulated operating conditions,” Journal of Testing and Evaluation, vol. 34, pp. 246-250, May 2006. [↩]
6. A. Evans, A. Bieberle-Hütter, J. L. M. Rupp, and L. J. Gauckler, “Review on microfabricated micro-solid oxide fuel cell membranes,” Journal of Power Sources, vol. 194, pp. 119-129, Oct 2009. [↩]

Ionic Conduction Studies in TlBr Radiation Detector Materials

• Authors: S. R. Bishop, M. Kuhn, H. L. Tuller
• Sponsorship: Domestic Nuclear Detection Office, Radiation Monitoring Devices, Inc.

Detection of high-energy radiation (e.g., γ-rays) is key in nuclear non-proliferation strategies.  When a wide-band gap semiconductor detector intercepts a γ-ray, electron-hole pairs are formed, resulting in an increase in electrical conductivity. This change in conductivity is maximized if the conductivity in the non-illuminated (dark) state is very low. Current semiconductor technologies require cooling to very low temperatures, which adds to cost and reduces portability.  TlBr is an attractive detector material given its low room-temperature dark conductivity, as well as its high mass density, leading to higher radiation absorption.

In this project, we have characterized the dominant ionic conduction properties in TlBr using impedance spectroscopy.  Through doping techniques, we have determined that TlBr is primarily a Schottky-type ionic conductor, meaning that Tl and Br move through the material by vacancy motion.  These measurements have led us to predict a doping strategy to minimize the dark conductivity, and we are collaborating with a local company (RMD) to implement this technology as well as developing it further by studying new TlBr based material systems.  In addition, our newfound understanding of TlBr has led us to investigate novel device designs never before used in ionic conducting systems.