Nafion microstructure

Structural evolution of Nafion microstructure as a function of water content. (From Gebel, G. 2000. Po/yraer 41 5829-5838.)... 

J. Wang and H. Wu, Highly selective biosensing of glucose utilizing a glucose oxidase + rhodium + Nafion biocatalytic-electrocatalytic-permselective surface microstructure. J. Electroanal. Chem. 395, 287-291 (1995). 

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. 

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... 

Porat, Z., Fryer, J. R., Huxham, M. and Rubenstein, 1.1995. Electron microscopy investieation of the microstructure of Nafion films. Journal of Physical Chemistry 99 4667- 71. 

Snapshots of the final microstructure in hydrated Nafion membrane at different water contents. Hydrophilic domains (water, hydronium, and side chains) are shown in gray, while hydrophobic domains are shown in black. 

Miura and Yoshida also investigated the changes in the microstructure of 1100 EW Nafion sulfonate membranes, in alkali, ammonium, and alkylammonium cation forms, that were induced by swelling in ethanol using DSC, dynamic mechanical analysis (DMA), SAXS, and electron probe microanalysis (EPMA). These studies were performed within the context of liquid pervaporation membranes that could potentially be used to separate ethanol from water... 

Figure 13. A few microstructural parameters for Nafion and sulfonated poly(arylene ether ketone)s,i as a function of the solvent (water and/or methanol) volume fraction Xy. (a) the internal hydrophobic/hydrophilic interface, and (b) the average hydrophobic/hydrophilic separation and the diameter of the solvated hydrophilic channels (pores).

Bauer, F., Willert-Porada, M. (2004). Microstructural charaxterization of Zr-phosphate-Nafion membranes for direct methanol fuel cell (DMFC) application. /. Membrane Science 233,141-149. 

Fig. 4 Schematic representation of the microstructures of Nafion and a sulfonated PEK (derived from SAXS experiments) illustrating the less pronounced hydrophobic/hydrophilic separation of the latter compared to that of the former. (From Ref. l)...

Kreuer et al. [25] investigated the membrane properties, including water sorption, transport (proton conductivity, electro-osmotic water drag and water diffusion), microstructure and viscoelasticity of the short-side-chain (SSC) perfluorosulfonic acid ionomers (PFSA, Dow 840 and Dow 1150) with different lEC-values. The data were compared to those for Nafion 117, and the implications for using such ionomers as separator materials in direct methanol and hydrogen fuel cells discussed. Tire major advantages of PFSA membranes were seen to be (i) a high proton conductivity. 

In conclusion, we have demonstrated how it is possible to build up quite a detailed picture of the microstructure of the ionic phase of neutralized Nafion membranes by exploiting the magnetic properties of iron ions, and obtaining Mossbauer spectra at low temperatures in an external magnetic field. Results common to the two Mossbauer cations (Fe2+ and Fe +) concerning the anomalous character of the aqueous phase can reasonably be expected to apply for any other cation. The tendency or Fe3+ ions to form pairs and larger groups is not shared by Fe2, and it may be a special feature of small tripositive ions. 

FIGURE 21.17 Schematic representation of the microstructures of (a) Nafion and (b) an s-PEEK illustrating the less pronounced hydro-phobic/hydrophilic separation of the latter compared to the forma-. (Reprinted from J. Membr. Sci., On the development of proton conducting polymer membranes for hydrogen and methanol fuel cells, 185, 2001, 29, Kreuer, K.D. et al. With permission from Elsevier Kreuer, K.D., J. Membr. ScL, 185,29,2001.)... 

A. Vishnyakov and A. V. Neimark, Molecular Dynamics Simulation of Microstructure and Molecular Mobilities in Swollen Nafion Membranes, Journal of Physical Chemistry B, 105, 9586 (2001). 

Following the above preparation protocol, Nafion -ZrP membranes have also been extensively characterised by Yang, Bocarsly et al. [72-74] and Willert-Porada [75,76], in particular with regard to fuel cell performance and microstructural properties. In these studies either commercial Nafion -117 [68,75] or -115 [72], or recast Nation [73] films have been used, Nafion being re-formed from alcoholic solution and thermally treated at 160 °C before use. The starting membranes in all studies are thus expected to be simi-... 

The main microstructural differences between Nafion and sPEEK, inferred from SAXS experiments, are displayed in Eig. 6.29 [1]. The water filled channels in sPEEK are narrower compared with those in Nafion, are less separated, highly branched, and with more dead-end channels. 

Dry PBI membranes a =0) have very low conductivities (doping levels 2a < 1-5, and moderate conductivities ( 3.0 and temperatures close to 200 °C. A small increment in humidity, by exposing the membrane to the ambient [408] results in an important increase in conductivity, particularly at high doping levels [418]. Proton conductivities close to that found for Nafion membranes (around 100 mS.cm ) are found for PBI membrane with 2a > 3.0 in high humidity conditions, but the solvent used to prepare the membrane by casting seems to have a great influence, probably due to the formation of different polymer microstructures. On the other hand, the temperatures at which PBI membranes reach such conductivity levels are above 150 °C. 

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