Universität Stuttgart
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Item Open Access Transport of protonic charge carriers in methyl-sulfonic-acid/water mixtures : a model for lowly hydrated sulfonic acid based ionomers(2008) Telfah, Ahmad D. S.; Majer, Guenter (PD Dr.)Polymer-electrolyte-membrane fuel cells (PEM-FCs) deliver high power density and offer the advantages of low weight and volume, compared to other fuel cell systems. State-of-the-art separator materials in PEM-FCs are sulfonic acid functionalized polymers, like the perfluorinated polymer Nafion. The suitability of proton-conducting materials as separators in a particular fuel-cell application is essentially dependent on its transport properties, durability, and reactivity. Thus, this thesis has focused on the protonic transport properties and the protonic charge formation. Owing to the complexity of polymeric proton-conducting materials, we have studied Methanesulfonic acid (MSA)-water as a suitable model system in order to understand the basic mechanisms of protonic charge formation and proton transport in such systems. In order to clarify whether structure diffusion plays a major role at low degree of hydration, we performed nuclear magnetic resonance (NMR) and electrical conductivity measurements on different sulfonic acid model systems, including MSA-water system. These studies have been extended over wide ranges of temperature and water content. Pulsed-field-gradient (PFG) NMR provides a direct access to the long-range diffusivity, while the spin-lattice-relaxation rate allows identifying local transport processes. It is important to note that due to the different chemical shift of protons forming C-H and O-H bonds, the diffusivities of both kinds of protons in MSA and MSA-water mixtures could be measured separately by PFG-NMR. Complementary to diffusion studies, AC-impedance spectroscopy has been applied to investigate charge transport. The dependence of the chemical shift on the water content of MSA-water systems, as observed by high-resolution 1H-NMR, provides information on the dissociation constant of the system, and thus on the protonic charge carrier density. If the charge carrier density is known, a comparison of conductivity and diffusion data, which are related via the Nernst-Einstein relation, permits analyzing the proton conduction mechanism. We report on comprehensive diffusion, spin-lattice relaxation and conductivity measurements on MSA at different degrees of hydration. The results indicate that the vehicle transport mechanism is the dominant proton conduction mechanism in MSA-water system and similar sulfonated systems at low degrees of hydration. MSA-water-alcohol system has been used as a complementary model system to investigate the effect of hydrophobicity of the real fuel cell polymer backbone. Data on high resolution 1H-NMR spectra and electrical conductivity have been obtained on MSA-water-[ethanol, isopropanol] as two different alcohols with different degree of hydrophobicity. The results suggest that the main effect of hydrophobic interactions on the proton conductivity is a reduction of the -SO3H dissociation. Proton conductivity data on examples of real sulfonated fuel cell membranes (Nafion-1100, Dow-858, and SPSO2-360) have been measured and compared to the conductivity of the MSA-water system. The protonic conductivity of these examples for sulfonated membranes have also been investigated at a water vapor pressure of p = 105 Pa at temperature from T = 100 to 160 °C.