Transport in Nafion

In this Section, we report on transport properties generated from the non-reactive simulations in the first task and discuss work in progress on extracting transport properties from the reactive simulations in task II applied to PFSA membranes. 

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Liu, C. and Martin, G. R. 1990. Ion-transporting composite membranes III. Selectivity and rate of ion transport in Nafion-impregnated Gore-Tex membranes by a high-temperature solution casting method. Journal of the Electrochemical Society 137 3114-3120. 

Several attempts have been made to simulate transport in realistic fully atomistic MD simulations of water/Nafion mixtures. Vishnyakov and Neimark [72-74] investigated alkali transport in aqueous and methanolic solution (and in mixed solvents) in the presence of Nafion. They found indications for the existence of the fluctuative bridging mechanism. The group by Kokhlov and Khalatur has also performed extensive yet unpublished studies of simple ion transport in Nafion. Goddard and coworkers [75] compared structural and dynamical properties of two different copolymerisation patterns, in order to estimate the effect of statistical vs. regular copolymerisation of TFE with the sulfonated vinyl ether. 

Choi, P. Jalani, N.H. Datta, R. Thermodynamics and proton transport in Nafion II. Proton diffusion mechanisms and conductivity. J. Electrochem. Soc. 2005, 152 (3), E123-E130. 

Thus the model in Figure 3 is consistent with spectroscopic and diffusional results, but is certainly an oversimplified picture nevertheless. Other approaches to the modeling of transport in Nafion, such as the recent application of percolation theory by Hsu and co-workers (22), may yield further insight into the problem. 

The cluster separation (5.0 nm) is consistent with the SAXS experiments, and the cluster diameters (4.0 nm) are consistent with the results given in Tables 1-3. The significance of the cross-hatched area will be explained shortly. As we will demonstrate, this model of ionic clustering is very useful in describing ion transport in "Nafion". 

Summary. We have shown that ion transport in "Nafion" per-fluorinated membrane is controlled by percolation, which means that the connectivity of ion clusters is critical. This basically reflects the heterogeneous nature of a wet membrane. Although transport across a membrane is usually perceived as a one-dimensional process, our analysis suggests that it is distinctly three-dimensional in "Nafion". (Compare the experimental values of c and n with those listed in Table 7.) This is not totally unexpected since ion clusters are typically 5.0 nm, whereas a membrane is normally several mils thick. We have also uncovered an ionic insulator-to-conductor transition at 10 volume % of electrolyte uptake. Similar transitions are expected in other ion-containing polymers, and the Cluster-Network model may find useful application to ion transport in other ion containing polymers. Finally, our transport and current efficiency data are consistent with the Cluster-Network model, but not the conventional Donnan equilibrium. 

R.Y. Yeo, Ion clustering and proton transport in Nafion membranes and its applications as solid polymer electrolyte, J. Electrochem. Soc., 1983, 130, 533-538. 

C. Gavach, G. Pamboutzoglou, M. Nedyalkov, and G. Pourcelly, Ac Impedance Investigation of the Kinetics of Ion-Transport in Nafion Perfluorosulfo-nic Membranes, Journal of Membrane Science, 45, 37 (1989). 

Hwang, G. S. Kaviany, M. Gostick, J. T. Kientiz, B. Weber, A. Z. Kim, M. H., Role of Water States on Water Uptake and Proton Transport in Nafion Using Molecular Simulations and Bimodal Networks. Polymer 2011,52,2584-2593. 

The previous discussion suggests that hydraulic permeation is the dominant mode of water transport in Nafion at sufficiently large k, whereas a diffusive contribution to water transport will dominate at low k. This change in the prevailing mechanism of water transport with k could explain the peculiar transition in water concentration profiles through operating PEM observed in recent neutron scattering experiments. 

Jang, S. S., Molinero, V., Cagin, T, and Goddard, W. A. 2004. Nanophase-segregation and transport in Nafion 117 from molecular dynamics simulations Effect of monomeric sequence. 108(10), 3149-3157. 

For water transport in Nafion polymer membrane in a PEM fuel cell, the equation is given as... 

Fig. 3 The parameter for oxygen transport in Nafion film in contact with 0.05 mol...

Thompson, E.L., Capehart, T.W., Fuller, T.J., and Jome, J. (2006) Investigation of low-temperature proton transport in Nafion using direct current conductivity and differential scanning calorimetry, J. Electrochem. Soc., 153, A2351-A2362. 

Zhao and Benziger identified three contributions to the resistance to water transport in Nafion (1) the volume of the hydrophilic domains available for water transport (2) the dependence of water diffusivity on the concentration of water in the hydrophilic domains and (3) the Nafion/fluid interface [49]. They showed that at low water content in Nafion, the diffusion across the membrane is the rate-limiting step. But at high water activity, the combination of increased hydrophilic domain volume and increased coimectivity of the hydrophilic domains result in a large increase in the diffusion rate across the membrane, and interfacial transport across the Nafion/vapor interface becomes the rate-limiting step. 

Majsztrik PW et al (2007) Water sorption, desorption and transport in Nafion membranes. J Memb Sci 301 93-106... 

Hsu WY, Gierke TD (1982) Ion clustering and transport in Nafion perfluOTinated membranes. J Electrochem Soc 129(3) C121... 

Motupally et al. [18] studied water transport in Nafion membranes, and, by comparing the variation in literature values of water diffusion coef-... 

Choi P, Jalani N H and Datta R (2005) Thermodynamics and Proton Transport, Nafion, III. Proton Transport in Nafion/ Sulfated Zr02 Nanocomposite Membranes, J. Electrochem. Soc., 152, pp. A1548-A1554. 

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