ABSTRACT

This chapter is devoted to the analysis of the local atomic structure of solid oxide fuel cell (SOFC) electrolyte and electrode materials. Among the experimental techniques able to provide information about the local structure of materials, the X-ray absorption spectroscopy has the peculiarity of element selectivity, allowing to investigate the chemical environment of specific atoms embedded in a matrix. Solid oxide electrolytes allow the conduction of ionic charge carriers between the electrodes of a SOFC device; the mechanism of ion conduction depends on the type of carrier, and it is finely tuned-or even only possible-if the solid matrix is suitably tailored by insertion of doping species that modify the local structure and the dynamics of the lattice. The oxygen

reduction and fuel oxidation processes are catalyzed at the respective electrodes; moreover, the cathode and anode layers must: (1) allow an efficient exchange of reactants with the environment, (2) convey the ionic species to/from the electrolyte and (3) ensure the conduction of the electrons involved in the reduction and oxidation processes. The chemical and physical stability of the interface with the electrolyte layer is another essential issue of electrode materials. The chapter begins with a section describing the basic theory underlying the X-ray absorption spectroscopy and the experimental set-ups used in fuel cell applications. The second section reports on case studies and perspectives of future development of fuel cell materials under the keynote of local structure. 4.1 X-Ray Absorption SpectroscopyX-Ray Absorption Spectroscopy (XAS) is an atomic absorption spectroscopy. In the following paragraph, we will see how it is used to gain insights on the local atomic structure of a single element in a solid oxide. Different acronyms are used in the literature to refer to XAS techniques. These are EXAFS-extended X-ray absorption fine structure, the oscillations of the XAS spectrum far from the absorption edge; XANES-X-ray absorption near-edge structure, the features of the XAS spectrum superimposed on the absorption edge; XAFS-X-ray absorption fine structure, comprising both XANES and EXAFS. Let us consider a monochromatic beam of hard X-rays (with energy 5-50 keV) interacting with a ceramic oxide (e.g., CeO2, or Y:BaZrO3). The X-ray interaction with matter in this case is dominated by photoelectric absorption: when the energy of the impinging photons is larger than the binding energy of a core electron, this is ejected to the continuum (photoelectron). The intensity of the We restrict here to the case that is most relevant for SOFCs, while XAS can be employed on glassy, liquid or gaseous samples, and is not restricted to hard X-rays. However, the data analysis scheme presented here is valid for hard X-rays only. Soft X-rays, used for the analysis of lighter atoms, involve completely different experimental setup and data analysis models: moreover, the information contained in the XAS spectra of lighter elements (for which the term NEXAFS, Near Edge XAFS, is often used) mainly concerns the electronic structure rather than the atomic structure.