The primary objectives of the work described in this paper were to evaluate the concentration and diffusion coefficient of O2 in a commercial perfluorosulfonate ionomer (Nafion) and to assess the effect of a film of this ionomer on the rate of O2 reduction at a substrate Pt electrode surface. Electrochemical experiments at Nafion film‐coated electrodes were used to obtain these data. A high temperature film‐casting procedure, which has been shown to yield high quality solution‐cast Nafion films, was used to prepare the Nafion‐coated electrodes. The Nafion film‐coated electrodes were equilibrated with and exposed to 0.7M H3PO4 during the electrochemical experiments; it was of particular interest to compare the diffusion, solubility, and kinetic data obtained at the film‐coated electrode to analogous data obtained at a bare electrode, exposed to this electrolyte. We have found that the diffusion coefficient is lower in Nafion than in the electrolyte solution but that the solubility and oxygen reduction rate are higher.

of the water was surprisingly low, 4 kJ/mol, when compared to reported values for structurally similar materials. For example, in cationic monolayer hydrates motional activation energies for intercalated water have values between 13 and 30 k J / m~l . I~, '~ This low value is indicative of the rapid translational mobility of the water within the interlayers. This mobility is present in spite of the strong hydrogen bonding which orients the water molecules and most likely occurs via a "jumping" type mechanism between adjacent H-bonding sites.Although a relaxation minimum cannot be determined from the relaxation time-temperature plot, Le., Figure 6, it lies above 370 K. Thus, a lower value for a room temperature diffusion coefficient, pT, may be calculated from the correlation times and assuming only two-dimensional translational motion according to20 DRT = d 2 / 2 rwhere d is the distance between "jumps" from one hydroxide group to the next (taken to be about 3 A) and T is the correlation time at room temperature. This yields DRT = 3 X IO-" m2/s, comparable to some of the diffusion coefficients described for monolayerIn view of the observed differences in the temperature behavior of the spectra, differences must exist in the relative diffusivity of interlayer water depending on the layer composition and hydrothermal treatment. However, because of the inaccuracies inherent in our estimation, we cannot measure these differences. The reported value of the diffusion coefficient should not be taken to be more that simply an order-of-magnitude estimate. More precise relaxation minima will be required for an accurate value of DRT.

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