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 Pr0.1Ce0.9O2-∂ (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; BaxSr1-xTi1-yFeyO3-y/2+δ (BSTF) and PrxCe1-xO2-∂ (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 SrTi1-xFexO3-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. [↩]

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