Part I. Improvements in the rotation-rate step experiment for the evaluation of diffusion coefficients at rotating disk electrodes. Part II. Ion-pairing and electric field effects on electron hopping in the Nafion-tris(2,2'-bipyridine)osmium(3+/2+) system

Citation

Blauch, David N. (1991) Part I. Improvements in the rotation-rate step experiment for the evaluation of diffusion coefficients at rotating disk electrodes. Part II. Ion-pairing and electric field effects on electron hopping in the Nafion-tris(2,2'-bipyridine)osmium(3+/2+) system. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8683-j681. https://resolver.caltech.edu/CaltechETD:etd-06152007-081035

Abstract

Part I: An improved description of the current transient produced by an abrupt change in the rate of rotation of a rotating disk electrode has been obtained by the method of orthogonal collocation. The procedure provides a formula that accurately describes the expected current transient for at least 90% of its duration. If the final rotation rate is chosen to be ca. 58% of the initial rotation rate, the resulting current transient exhibits simple exponential decay, thereby facilitating data analysis. A simple offset in the time scale of the experiment proves effective in compensating for the effects of both hydrodynamic relaxation and imperfections in the response of the electrode rotator.

Part II: The high ionic content and low dielectric constant that prevail in the interior of many redox polymers are expected to promote ionic association between the polyelectrolyte and counterions. The present study is an attempt to evaluate the influence of ion-pairing interactions on charge propagation within polyelectrolyte films. The system under investigation consists of the Os(bpy)3(3+/2+) redox couple incorporated into Nafion, where ion-pairing between the osmium complex and pendant sulfonate groups is argued to be responsible for the irreversible retention of the complex within the film. The apparent diffusion coefficient characterizing the dynamics of electron propagation through the redox polymer exhibits a remarkably sudden increase as the film approaches electrostatic saturation with the Os(bpy)3(3+) complex. Existing models, even those taking into account the presence of electric fields within the film, do not account satisfactorily for the observed behavior of the apparent diffusion coefficients. The introduction of ion-pairing into the model for charge transport leads to predictions that are consistent with the observed behavior. Key ingredients in the successful model are the assumptions that the predominant forms of the Os(bpy)3(3+/2+) complex incorporated in Nafion are neutral aggregates resulting from the formation of triple or double ion-pairs and that the triply ion-paired Os(bpy)3(3+) species dissociates into a singly charged species containing the same number of sulfonate groups as the predominant form of the Os(bpy)3(2+) complex, thereby providing a low-energy pathway for electron self-exchange. The dissociation of the triply ion-paired Os(bpy)3(3+) complex provides a natural explanation for the steep increase in the apparent diffusion coefficient, i.e., the rate of electron propagation, as the concentration of the osmium complex comes close to saturation, because as saturation is approached the ion-pairing equilibrium shifts to favor the formation of the doubly ion-paired form of Os(bpy)3(3+) that is the best partner for accepting an electron from the doubly ion-paired Os(bpy)3(2+) complex. The inevitable presence of electric fields within the polyelectrolyte films also affects the observed behavior, especially as the concentration of the incorporated cation is increased.

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