The thesis reports about a detailed investigation of the local structure and chemical disorder of a commercially available Pt-Co alloy nanocatalyst. This material, supported on high surface area carbon electrodes, is used in proton exchange membrane fuel cells (PEMFC) diminishing the amount of noble metal needed for those devices. Various state-of-the art material science techniques have been used to study microscopic properties of those nanomaterials, including XAS (X-ray Absorption Spectroscopy) ex-situ and in-situ (in an operating PEMFC), XRD (X-Ray Diffraction), high resolution TEM (Transmission Electron Microscopy), XRF (X-Ray Fluorescence). High-quality XAS spectra at the Co K-edge and Pt L3-edge were obtained using original procedures both at BM29 (European Synchrotron Radiation Facility, Grenoble) and XAFS beamline at ELETTRA (Trieste). XAS double-edge multiplescattering structural refinements of the Pt-Co spectra have been performed accounting for the reduction of the coordination numbers and degeneracy of three-atom configurations, resulting from the measured size distribution obtained by TEM and XRD. This robust approach for nanomaterial characterization, combining different techniques, can be in principle applied for structural refinements of any binary nanocrystalline functional system, including those for energy-related applications. In particular, XAS spectra are shown to be sensitive to the degree of chemical disorder in the alloy, here analyzed through an order parameter s related to the deviation from a chemically ordered crystalline compound (s=1). The initial nanocrystals are only partially ordered (s-0.6) and become more ordered as an effect of ageing when used as a catalyst in PEMFCs. Moreover, interatomic Pt-Pt and Pt-Co distances are found to be slightly longer for higher current densities and working time in operating PEMFCs, while distance variances (Ïƒ2) tend to reduce. The small amount of Co oxide initially present is found to disappear gradually in operating PEMFCs. We have also found, combining XRD, XAS, and XRF data, that the stoichiometry of the considered alloy changes from the initial value Pt-3Co (similar to that of the stable compound Pt3Co) to Pt-4Co during and after controlled cycles of usage within PEMFCs. No substantial aggregation processes increasing the sizes of the nanoparticles are observed for these alloys, in contrast with what happens pure Pt nanoparticles. The degradation of the PEMFC performances appears thus to be related to specific changes of the microscopic structural properties of the nanocatalysts, occurring under the typical operating conditions of the PEMFC. Correlations between structural properties and performances and possible consequences on the selection and optimization of the nanomaterials are briefly discussed.
|Titolo:||Study of the atomic structure and morphology of the Pt3Co nanocatalyst for applications in proton exchange membrane fuel cells (PEMFC).|
|Data di pubblicazione:||8-lug-2010|
|Appare nelle tipologie:||Tesi di dottorato (Pregresso)|