Multinuclear solid-state NMR of fuel cell materials
Orr, Simon Timothy (2010) Multinuclear solid-state NMR of fuel cell materials. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b2491643~S15
This thesis describes the application of multinuclear solid-state NMR to three materials systems: first, components of polymer-based proton-exchange fuel cells including the fluoropolymer membranes (Chapter 4) and the precious metal supported catalysts (Chapter 5); secondly, the formation of a complex bismuth niobium aluminoborosilicate glass-ceramic with novel dielectric properties (Chapter 6); finally, platinum (II) dialkyldithiophosphates which belong to a class of compounds (metal dialkyldithiophosphates) some of which are used in mineral separation processing (Chapter 7). A full investigation into the effects of different conditions during sample preparation and 19F NMR experiments on fluoropolymer membranes recommended unmilled preparation, dry storage and magic angle spinning below 24 kHz for the study of structural differences between membranes. The application of 19F NMR to a range of commercial and experimental fluoropolymer membranes revealed that the equivalent weight does not affect the mobility of the polymer molecules such that can be detected by this technique. Calculations of equivalent weight from 19F NMR differed with quoted values by up to 14%. Discrepancies were smallest in the short sidechain polymers, as low as 3%. The assignment of spectra was invariant with sidechain structure apart from a change in the number of ester links. The presence or absence of oxygen affected chemical shielding even around nuclei separated by several bonds. Differences in 1H linewidths between membranes could not be interpreted without the control and comparison of manufacturing techniques. It is desirable to remove the necessity for organic solvents in membrane casting. However, membranes cast from aqueous solution do not possess the same properties as those from propanol. It had been proposed that rapid drying of water cast membranes would result in a structure more similar to those from organic solvent. 1H NMR revealed that the opposite is the case, rapid drying makes the ordinarily more inhomogeneous aqueous membranes even more so. The application of both 19F and 1H NMR revealed that the monomolecular layers of fluoropolymer deposited on the surface of fuel cell catalysts to aid proton conductivity are categorically different in nature to the same materials in the bulk state. 19F NMR suggests a polymer structure either more disordered, greatly less mobile or both. 1H NMR displayed water environments that could not be reconciled to the standard model of rapid exchange between bulk water and water associated with acid groups. Spectral differences caused by solvent and polymer loading were discussed. The first complete and quantitative Fourier transformed 195Pt NMR spectra of platinum fuel cell catalysts, acquired using a field sweeping method, are analysed for deviation from the cubooctohedral particle model and surface oxidation. A combination of 11B, 27Al and 29Si studies of the BN1 ceramic system after different temperature heat treatments confirmed much of the previous work on phase evolution. However, it was shown that kyanite does not make up a significant proportion of the material until heat treatment reaches 1000 ºC and that aluminium impurities in bismuthbiobate crystals appear to increase with treatment temperature. The nature and abundance of glassy phases in the system are explored for the first time. Field sweep 195Pt NMR was employed to characterise the 195Pt chemical shift anisotropy of five platinum (II) dialkyldithiophosphates complexes. Additionally the 31P chemical shift anisotropies of two of the complexes, previously unpublished are presented.
|Item Type:||Thesis or Dissertation (PhD)|
|Subjects:||Q Science > QC Physics|
|Library of Congress Subject Headings (LCSH):||Nuclear magnetic resonance, Proton exchange membrane fuel cells, Glass-ceramics -- Analysis, Platinum compounds -- Analysis|
|Institution:||University of Warwick|
|Theses Department:||Department of Physics|
|Supervisor(s)/Advisor:||Smith, Mark E. ; Hanna, John V. ; Fisher, J. ; Thompsett, D.|
|Sponsors:||Engineering and Physical Sciences Research Council (EPSRC) ; Johnson Matthey Plc.|
|Extent:||xvi, 151 leaves : ill., charts|
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