The Effect of Composition of Nafion Deposition Solutions on the Diffusional Properties of the Films

The properties of Nafion films on electrodes were studied as a function of the water content in the deposition solutions (0.7% Nafion in mixed ethanol/water). The rate of loading and unloading (ie., ion-exchange) of the complex ion Ru(bpy)²₃⁺, loading into films which were thermally treated, and penetration of the anion Fe(CN)₆^4- were examined. All these measurements show a nearly linear dependence of the voltammetric peak currents (for the oxidation of the incorporated ions) on the percentage of water in the deposition solutions. These results lead to the conclusion that films that are cast from solutions of higher water content have more “open” structures that allow faster diffusion of ions. It is also suggested that Ru(bpy)²₃⁺ exists in the films in two forms: one is strongly bound to the sulfonic groups by ion-pairing, for which the mass-transport mechanism is primarily electron self-exchange (characterized by D_et). The other is bound to the polymer by weak electrostatic or hydrophobic interactions, for which mass transport proceeds mainly by physical diffusion (D₀), The two diffusion coefficients were determined separately in each kind of film, and they both show nearly linear dependence on the percent of H₂0 in the Nafion solutions from which the films are deposited.