Nafion morphology

Evaluation of Nafion Morphology through 4545 Studies of Oriented Membranes... 

Unfortunately, the small-angle scattering techniques used in the investigations of Nafion morphology generally probe but a small region of reciprocal space and Fourier inversion methods of analyzing the... 

In these circumstances, when the microphase-separated morphology is clearly evidenced by numerous experiments, which can be however ambiguously interpreted, modeling and simulation attract much attention as a means to clarify and extend the concept of Nafion morphology and ionic conductivity. Such studies provide a valuable tool for a deeper understanding of the complex properties of water-containing Nafion systems at a fundamental level. 

Various simulation techniques, as applied to PEFCs, differing in the degree of coarseness and the basie equations solved have been reviewed [10-14]. In contrast to these reviews, in which the transport phenomena in ionomer membranes are discussed in detail, we will consider the problems related to the modeling of Nafion morphology, with the emphasis on the role of water content. 

Dorenbos, G. Suga, Y Simulation of equivalent weight dependence of Nafion morphologies and predicted trends regarding water diffusion. J. Membrane Sci. 330 (2009), pp. 5-20. 

At the opposite side of the timescale (in the range of seconds to minutes), the macroscopic diffusion coefficient of water in swollen Nafion membranes, as determined by the diffusion of tritiated water through the membrane, is lower by a factor of 10 compared to the local diffusion coefficient or the self-diffusion in bulk water. This high value integrates all the restricted motions, which shows that the Nafion morphology is favorable to obtain a high ionic conductivity [160]. One important issue is the identification of the typical... 

Litt, M. H., Reevaluation of NAFION morphology. Polym. Prepr. 2001, 38, 80-81. 

When considering the morphology of prepared electro-catalysts are different to each other especially to the commercial one, one can think that the structure of electrode which was optimized to the commercial catalyst may not be optimum. So, the for the better electrode structures was conducted by investigating the effect of NFP. Fig. 2 is a schematic of electrode which depicts the effect of Nafion content[9]. For the conventional electrocatalysts, the range of 30 35 % NFP is reported as optimum value[10]. 

The reason that the full coverage could not be achieved is considered that some site are not accessible by CO. It is not known, however, whether this stems from the morphology of the platinum or an effect of Nafion as a surface modifier. 

Two other important factors that control the conductivity of PEMs are polymer microstructure and morphology. Within this section, Nafion will serve as the prime example to describe how the formation of hydrophobic and hydrophilic domains relates to proton transport. The microstructures of a few PEMs will then be described to highlight the importance of this area upon proton conductivity. 

With increasing water content, the ionic domains swell from 40 to 50 A in diameter and the structure of fhe membrane is fhoughf to consist of spherical ionic domains joined by cylinders of wafer dispersed in fhe polymer matrix. Within this region of wafer confenf, proton conductivify steadily increases. At > 0.5, a morphological inversion occurs in which a connected network of aggregated polymer "rods" is now surrounded by water. This network continues to swell for X, = 0.5 —> 0.9 and fhe conductivify of fhe membrane approaches the values observed for Nafion solutions. 

Studies on morphology and conclusions about observed levels of proton conductivity have also been carried out on PEMs other than Nafion and sulfonated poly(ether ketone). These include studies in which phenomenological examinations of relationships between conductivity and observed microstructure were carried out upon polymer systems where acid content was varied but the basic chemical structure was kept constant. In addition, other systems allowed... 

For ETFE- -PSSA membranes with the same lEC, water uptake is higher than MeOH uptake of the membrane, but for Nation and S-SEBS membranes, MeOH uptake of membrane is always higher than water uptake. Chemical structure and morphology of membranes affect the solvent absorption. Nafion is considered to consist of ionic clusters that are separated from the polymer phase. For grafted polymers, heterogeneity exists to some extent due to the hydrophobic base polymer however, a regular clustered structure, as in the case of Nafion, has not been proposed for these materials. 

In comparing the diffusion behavior of these two membranes at low MeOH concentrations, MeOH in BPSH 40 membranes exhibits significantly higher diffusion coefficients than those in Nafion 117. This may be the result of differences in morphology. Tapping mode AFM measurements found that, for dry membranes, the domains for BPSH 40 appeared to be 10-25 nm in diameter for Nafion 117, the domains were smaller, approximately 4-10 nm. Although one would expect restricted diffusion in both cases, it is possible that the smaller domains limit diffusion to a greater extent. 

Gierke, T. D., Munn, G. E. and Wilson, F. C. 1981. The morphology in Nafion perfluorinated membrane products, as determined by wide-angle and small-angle x-ray studies. Journal of Polymer Science Polymer Physics 19 1687-1704. 

Lee, K., Ishihara, A., Mitsushima, S., Kamiya, N. and Ota, K. 2004. Effect of recast temperature on diffusion and dissolution of oxygen and morphological properties in recast Nafion. Journal of the Electrochemical Society 151 A639-A645. Vebrugge, M. R. 1989. Journal of the Electrochemical Society 136 417. 

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

Schematic depiction of the structural evolution of polymer electrolyte membranes. The primary chemical structure of the Nafion-type ionomer on the left with hydrophobic backbone, side chains, and acid head groups evolves into polymeric aggregates with complex interfacial structure (middle). Randomly interconnected phases of these aggregates and water-filled voids between them form the heterogeneous membrane morphology at the macroscopic scale (right).

The reduction of the long-range diffusivity, Di by a factor of four with respect to bulk water can be attributed to the random morphology of the nanoporous network (i.e., effects of connectivity and tortuosity of nanopores). For comparison, the water self-diffusion coefficient in Nafion measured by PFG-NMR is = 0.58 x 10 cm s at T = 15. Notice that PFG-NMR probes mobilities over length scales > 0.1 /rm. Comparison of QENS and PFG-NMR studies thus reveals that the local mobility of water in Nafion is almost bulk-like within the confined domains at the nanometer scale and that the effective water diffusivity decreases due to the channeling of water molecules through the network of randomly interconnected and tortuous water-filled domains. ... 

Figure 6.4 shows that long-range diff usivities of water in Nafion membranes measured by QENS, Di are equal to self-diff usivities determined by PFG-NMR, Dg, at A > 10. In well-hydrated membranes, the major geometric constraints for water mobility due to the phase-segregated, random network morphology of... 

As much as the nanophase segregated morphology of Nafion has been a controversial issue in the literature over several decades, the need for understanding the structure and distribution of wafer in PEMs has sfimulafed many efforfs in experimenf and theory. Major classifications of water in PEMs distinguish (1) surface and bulk wafer, (2) nonfreezable, freezable-bound, and free wafep and (3) wafer vapor or liquid water. Anofher fype of wafer offen discussed is that associated with hydrophobic regions. 

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