The development, electrochemical characterization and applications of sulphonated silica sol-gels

Aylward, Wanda Marie (2006) The development, electrochemical characterization and applications of sulphonated silica sol-gels. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The major goals of this research were to prepare novel proton conducting materials and to investigate their ion exchange and ion transport properties. -- The sol-gel process was used to synthesize silica ORMOSIL hydrogels with sulphonate functionality by incorporating 2(4-chlorosulfonylphenyl) ethyl-trichlorosilane into a tetramethyl orthosilicate sol-gel network. Gel modified carbon fiber paper working electrodes were prepared by suspending strips of carbon fiber paper into a sol solution. -- Cyclic voltammetry, chronoamperometry and chronocoulometry using Ru(NH₃)₆²⁺/³⁺ redox probes were used to investigate the ion exchange and ion transport properties within the hydrogels. It was found that the hydrogels studied in this work exhibited excellent ion exchange and ion transport properties and were very stable in electrochemical studies. -- The effects of electrolyte concentration and pH on these properties were studied. As well, sol-gel synthetic conditions such as sol-gel aging, sulphonate concentration, solvent and catalyst effects were also studied. The degree of interaction between the redox probes and sol-gel network was investigated by studying the ion exchange and transport properties of probes having different size and charge than Ru(NH₃)₆²⁺/³⁺. The sulphonated silica hydrogels were found to exhibit excellent permselectivity, allowing the positively charged redox probes to diffuse into their porous network but not a negatively charge probe. -- Electrochemical impedance spectroscopy was used to study the ionic conductivity of the hydrogels. It was found that incorporating the sulphonate functionality into the silica sol-gel greatly increased the ionic conductivity when compared to a tetramethyl orthosilicate sol-gel. The sulphonated silica sol-gels displayed rather large ionic conductivities, larger in fact than most reports in the literature for various other ormosil materials. -- Composites of a sulphonated silica sol-gel and polypyrrole (PPY/gel) were prepared to incorporate the electronic conductivity of the conducting polymer into the ionic conducting network of the sol-gel. Ion transport in these composites was investigated using cyclic voltammetry, impedance spectroscopy and X-Ray emission analysis. It was found in this work that reduction and reoxidation of the PPY/gel involved the uptake and expulsion of cations and anions. -- Another goal of this work was to modify a proton exchange membrane fuel cell catalyst with a sulphonated silica sol-gel. Composites consisting of a sulphonated silica sol-gel and Pt/C were prepared and preliminary investigations using these catalysts in hydrogen and direct methanol fuel cells are discussed.

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