Fresh look at transport in perfluorosulfonate ionomers: ultramicroelectrode investigations of Nafion and the Dow ionomers

Abstract

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.

Electrochemical and Enzymatic Studies of Electron Transfer Mediation by Ferrocene Derivatives with Nafion-Glucose Oxidase Electrodes

“…The limiting value is of the same order of magnitude as the value determined for DiMeFc in Na®on-GOx ®lms [24]. The variation of the diffusion coef®cient with surface coverage is similar to that previously reported for the Na®on-TMAFc electrode [53]. Consequently, the increase of the amperometric current with loading cannot be accounted for by an increase of the diffusion coef®cient.…”

“…As noted above, the variation of the apparent diffusion coef®cient for DMAFc in Na®on-GOD ®lms as a function of its ®lm concentration follows that reported by Whiteley and Martin [53] for plain Na®on coatings. They proposed a cluster-channel model with a``bottleneck'' effect to interpret the concentration dependence.…”

“…The apparent formal potential of the DMAFc ao redox couple incorporated in the Na®on-GOx ®lm is about (0.37 V vs. SCE) and is slightly more positive than those previously reported for Na®on-coated electrode [52,53]. The scan rate dependence of the cyclic voltammetry for a PtyNa®on-GOx-DMAFc electrode is shown in Figure 1B.…”

“…The mechanism of charge transport for DMAFc redox species incorporated into Na®on ®lms on electrode surfaces has previously been investigated by many researchers [53,56,57]. In these ®lms, charge can be transferred across the polymer layer by either physical diffusion or electron hopping.…”

Solid-State Voltammetry — Analytical Prospects

“…The development of ultramicroelectrode techniques [52][53][54][55][56][57][58] has led to an improvement in the quality of solid-state voltammetric data as well as to new diagnostic and analytical possibilities [14][15][16][17]40,42,[59][60][61]. One of the most useful properties of ultramicroelectrodes is, in addition to their compact size and utility in characterization of small samples, that they are much less susceptible to distortion caused by uncompensated resistance because the small currents make ohmic drops negligible.…”