MTL Annual Research Report 2013

Chemomechanics of Fuel-cell-related Materials

Solid oxide fuel cells (SOFCs) convert chemical energy directly into electrical energy with higher efficiency and lower emission than conventional energy systems, and SOFCs provide flexibility of fuel choice allowing to use a wide variety of fuels ranging from hydrogen and kerosene to gasified coal^[1],^[2]. During operation, many of the advanced oxides utilized in SOFCs experience significant changes in oxygen content, or oxygen nonstoichiometry, resulting in changes in volume and elastic properties termed chemomechanics^[3],^[4]. This lattice dilation known as chemical expansion is oxygen nonstoichiometry-induced, and large stress gradients can be developed 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 nonstoichiometry of thin film and bulk SOFC oxide materials. Thin films are of particular interest because they allow control of strain and increase the surface-to-volume ratio, which is particularly important for electrode performance. Furthermore, there is a trend towards the use of thinner structures such as µ-SOFCs^[6]. The chemomechanical properties are being investigated using high-temperature and atmosphere-controlled nanoindentation, high-temperature x-ray diffraction, dilatometry, impedance spectroscopy, optical transmittance 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 damage mechanism 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. [↩]