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Chemomechanics of Fuel-cell-related Materials

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

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. [↩]

Investigation of Fuel Cell Cathode Performance in Solid Oxide Fuel Cells – Application of Model Thin Film Structures

• Authors: J. J. Kim, D. Chen, N. Thompson, N. Perry, S. R. Bishop. H. L. Tuller
• Sponsorship: Department of Energy

Understanding the oxygen reduction reaction (ORR) kinetics at solid oxide fuel cell (SOFC) cathodes is considered critical for enhanced performance, especially at reduced operating temperatures. Although numerous research efforts have been directed toward understanding the electrode reaction mechanisms, they remain unsatisfactory, and their conclusions are controversial^[1],^[2]. The complete ORR at mixed conducting cathode thin films consists of oxygen adsorption, dissociation, charge-transfer, and incorporation steps. The kinetic parameters associated with the overall reaction such as the diffusion coefficient (D) and surface exchange coefficient (k) are strongly influenced by oxygen nonstoichiometry, ∂. Because many advanced oxides used in SOFC experience significant changes in ∂ during operation at elevated temperatures and under reducing/oxidizing conditions, the ability to diagnose a material’s behavior under operating conditions is important. Our group recently demonstrated that d in Pr_0.1Ce_0.9O_2-∂ (10 PCO) thin films was reliably derived by utilizing chemical capacitance extracted from electrochemical impedance spectroscopy (EIS) measurements^[3]. Furthermore, we introduced a non-contact optical means for in situ recording of transient redox kinetics as well as the equilibrium Pr oxidation state and, in turn, ∂ in 10 PCO thin films, by monitoring the change in absorption spectra upon change in pO₂ or temperature^[4],^[5]. In this project, we are investigating cathode kinetics and nonstoichiometry of two model cathode oxide thin film; Ba_xSr_1-xTi_1-yFe_yO_3-y/2+δ (BSTF) and Pr_xCe_1-xO_2-∂ (PCO) by utilizing impedance spectroscopy and in situ optical absorption spectroscopy as a function of temperature and pO₂. Changes in surface chemistry and their impact on electrode impedance are also being investigated by using atomic force microscopy, x-ray photoelectron spectroscopy and low-energy ion scattering spectroscopy.

1. S. B. Alder, “Factors governing oxygen reduction in solid oxide fuel cell cathodes,” Chem. Rev., vol. 104, pp. 4791-4843, Oct. 2004. [↩]
2. W. Jung and H. L. Tuller, “A new model describing solid oxide fuel cell cathode kinetics: model thin film SrTi_1-xFe_xO_3-d mixed conducting oxide – a case study,” Adv. Energy Mater., vol. 1, pp. 1184-1191,  Nov. 2011. [↩]
3. D. Chen, S. R. Bishop and H. L. Tuller, “Non-stoichiometry in oxide thin films: a chemical capacitance study of the praseodymium-cerium oxide system,” Adv. Funct. Mater., DOI: 10.1002/adfm.201202104 [↩]
4. J. J. Kim, S. R. Bishop, N. Thompson, Y. Kuru and H. L. Tuller, “Optically derived energy band gap states of Pr in ceria,” Solid State Ionics, vol. 225, pp. 198-200, Oct. 2012. [↩]
5. S. R. Bishop, J. J. Kim, N. Thompson and H. L. Tuller, “Probing redox kinetics in Pr doped ceria mixed ionic electronic conducting thin films by in situ optical absorption measurements”, ECS transactions, vol. 45, pp. 491-495, 2012. [↩]