Effects of chemical environment on diffusivities within thin Nafion films as monitored from chronoamperometric responses of generator-collector double microband assemblies

摘要

The possibility of monitoring in situ the modifications of a polymeric thin film in relation to its chemical environment by means of measurement of diffusive properties and assessment of the volume available to diffusion is investigated. These measurements are performed electrochemically through the monitoring of the currents at paired-microband electrodes operated in a generator-collector mode. The experimental results reported here were primarily focused to assess the validity of the principle. This involved the evaluation of the effects of the chemical environment of the polymer on the diffusional cross-talk of Fe~(III) redox species incorporated into a Nafion micrometric film coating the electrode assembly. Modification of the chemical environment was performed by adding different amounts of organic compound such as methanol, ethanol, ethyleneglycol, propanol and dimethylformamide (DMF) from 0 to 20% v/v) into the 0.1 mol l~(-1) H_2SO_4 electrolyte in which the assembly is placed. An procedure that could be automated based on a theoretical diffusional model developed previously was used to analyze the generator-collector chronoamperometric responses. This allowed the determination of the concentration c~o and the diffusion coefficients of Fe~(III) and Fe~(II) species within Nafion films as well as the film thickness h. This showed a concentration dependence of the Fe~(III) diffusion coefficient suggesting that the rate of physical diffusion or iron centers governs mainly the charge transport within Nafion thin films under the conditions investigated. The presence of an organic compound in the surrounding electrolyte caused a decrease of the steady-state generator-collector current, the effect being the most significant for DMF. The magnitude of this decrease was observed to depend on the nature and the concentration of the organic compound added to the electrolyte. Detailed chronoamperometric analyses established that the current decrease was related principally to the decrease in diffusion coefficient and to a moderate change of the amount of iron species available to diffusional transport, without significant change of the film thickness. This suggested that the presence of organic compound modified the plasticization of the polymer matrix.