Effects of Electron Exchange on the Photochemical, Electrochemical and Electrocatalytic Responses of Polymer Modified Electrodes

A spectroelectrochemical method is presented which may be used to measure the concentrations of the two oxidation states of a redox couple in a polymer film. The method is applied to Ru(bpy)32+ in Nafion films on transparent tin oxide electrodes. Measurements of the emission of this complex in Nafion films on graphite and tin oxide electrodes are used to obtain information about the quenching of Ru(bpy)32+* by Ru(bpy)33+. Features of the luminescence which yield insight into the structure of the Nafion film are discussed. Experiments are described which bear on the question of when electron self-exchange reactions can contribute to the propagation of electrons through polymer films on electrodes. The results of experiments on the Co(bpy)32+/+ couple are discussed in terms of a theory derived by Dahms and Ruff et al. which describes the contribution of electron self-exchange reactions to diffusion in mixtures containing both halves of a redox couple. The propagation of charge through polymer films by electron exchange cross-reactions between different redox couples which are co-incorporated into a polymer film on an electrode is discussed. Two experimental examples of this phenomenon are presented. An electrocatalytic application of a polymer modified electrode is described. Cobalt tetraphenylporphyrin is incorporated into a Nafion film on a graphite electrode, and this porphyrin is used to catalyze the two-electron electroreduction of dioxygen. Enhanced catalytic currents are observed when an electron mediator (Ru(NH3)63+) is added to the film. This mediator serves to shuttle electrons between the electrode and the relatively immobile catalyst sites. The performance of electrocatalysts in polymer films is discussed with emphasis on ways to enhance stability without sacrificing efficiency.