Thermodynamic and Kinetic Studies of Redox Reactions within Polyelectrolyte Coatings on Electrodes

Citation

Tsou, Yu-Min (1985) Thermodynamic and Kinetic Studies of Redox Reactions within Polyelectrolyte Coatings on Electrodes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/c31b-xm06. https://resolver.caltech.edu/CaltechTHESIS:01112019-110100839

Abstract

The formal potentials of several redox couples incorporated in coatings of a perfluorocarboxylate on graphite electrodes are measured and compared with the formal potentials of the same couples in homogeneous solution. For every redox couple investigated, the difference observed agrees with that calculated from the Nernst equation with the independently measured ratio of incorporation coefficients of the two halves of the couple. The dependences of the shifts in formal potentials on the nature of the incorporated complex ion, the ionic strength, and the temperature are also determined. They indicate that the incorporation equilibrium is governed by electrostatic and hydrophobic interactions that act in opposite directions. The incorporation of most cations examined is driven by large increases in entropy which overcome the usually unfavorable enthalpy changes.

The mediated oxidation of ascorbate by Os(bpy)₃³⁺ incorporated in Nafion coatings on graphite electrodes is examined. Attempts to account for the magnitude of the mediated ascorbate oxidation currents in terms of current models and theories are only partially successful. As the ascorbate concentration is increased, the limiting oxidation currents at coated rotating disk electrodes do not increase as rapidly as expected on the basis of the models. Some possible reasons for this deviant behavior are suggested. The implications of the results on the general utility of Nafion-coated electrodes in electrocatalysis are presented.

The heterobinuclear complex [formula; see abstract in scanned thesis for details] is synthesized and used as a probe to gain the insight into the diffusional behavior of redox reactants within polymeric coatings. Its diffusion coefficient within Nafion coatings is measured chronocoulometrically in all three of its possible redox states (Ru^IIILFe^III, Ru^IIILFe^II, Ru^II LFe^II). The three oxidation states exhibit large differences in their diffusional rates. Possible origins of the differences and their concentration dependences that are considered include intermolecular electron self- exchange, single- file diffusion and electrostatic cross-linking. Both enhancement and depression of charge transport rates produced by electron transfer cross-reactions between the fully oxidized and fully reduced complex are observed and compared with those calculated on the basis of a simple model. Reasonable agreement is obtained. The important effect that intermolecular cross-exchange reactions can exert on the diffusive flux of reactants is demonstrated.

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