Nafion structure

None of the models address the question of how the main chains are packed, and details of crystallinity are neither factored into nor predicted by mathematical models of the structure and properties of Nafion. Chains packed in crystalline arrays are usually considered to be rigid within the context of certain properties for example, with regard to diffusion, crystallites are viewed as impenetrable obstacles. F NMR studies indicate otherwise. Molecular motions that do not significantly alter symmetry can in fact occur in polymer crystals. It would seem, for example, that the response of the Nafion structure to applied stress would depend on the flexibility of the polymer backbone, a certain fraction of which is incorporated in crystalline regions. On the other hand. Starkweather showed that the crystallinity and swelling of Nafion are not correlated. 

These structures are fictional in the sense that these sequences do not correspond to the actual statistical polymerization based on the comonomer reactivity ratio, although it was said that the results have significance with respect to Nafion structural optimization and guidance in the search for Nafion replacements. Also, the non-insignificant degree of crystallinity of Nafion was not accounted for in the model. 

Thus, the addition of silica and heteropolyacid modifies the relative ratio between crystalline and amorphous structure of ctist Nafion with respect to bare Ntifion. Yet, due to the low loading of silica and heteropolyacid in the membrane (3% silica, less than 1.5% PWA), it appears unlikely that the inorganic components are responsible of such significant modification in the Nafion structure. More probably, the observed changes have to be attributed to the final thermal treatment (160°C). 

Figure 15.10. Snapshots from full atomistic MD simulations showing microphase-separated structure of hydrated Nafion, at hydration levels of (a) /i = 4 and (b) h = 20. Spherical domains were cut out from the cubic MD simulation box. For visual clarity, three-dimensional Connolly surfaces were generated for the subsystem of hydrophobic atoms. The equilibrium Nafion structure consists of water-filled pores (diameter 40 A) that are connected by channels having diameter of order 10 A. This pore-channel nanostructure provides energetically favorable pathways through the nonpolar fluorocarbon matrix of the membrane for water and other mobile species.

The quartet is formed in Nafion/Cu(II) by the transformation of the quintet radical, with fliso = 22.5 G. A similar quintet (fliso = 24 G) was detected during the photolysis of perfluoroketones and assigned to radical 1 below. By analogy, radical 2 was proposed as responsible for the quintet detected in Nafion. Structure 2 implies that the unpaired electron is located on the carbon atom in the Nafion backbone that is linked to the pendant chain by loss of a fluorine atom. This structure is in agreement with the detection of fluorine anions during fuel cell operation. ... 

Rollet, A. L., Diat, O., and Gebel, G. 2002. A new insight into Nafion structure. 

However, Yakovlev et al. pointed out that the images of Nafion show strong effects of electron beam-induced damage. At elevated dose rates, the membrane is damaged even before the first image is collected. The low level of contrast in low-dose images is a good representation of Nafion structure. In contrast to the claims that... 

FIGURE 4-13 Structures of common polymeric coatings (a) Nafion, (b) polyvinyllferro-cene (c) polyvinylpyridine id) polypyrrole. 

A variety of ionomers have been described in the research literature, including copolymers of a) styrene with acrylic acid, b) ethyl acrylate with methacrylic acid, and (c) ethylene with methacrylic acid. A relatively recent development has been that of fluorinated sulfonate ionomers known as Nafions, a trade name of the Du Pont company. These ionomers have the general structure illustrated (10.1) and are used commercially as membranes. These ionomers are made by copolymerisation of the hydrocarbon or fluorocarbon monomers with minor amounts of the appropriate acid or ester. Copolymerisation is followed by either neutralisation or hydrolysis with a base, a process that may be carried out either in solution or in the melt. 

Fig. 1 shows SEM images for the surface of the untreated and the surface treated Nafion 112 membranes with ion dose density from lO to lO ions/cm at ion beam energy of 1 keV. With increasing the ion dose density, surface of the membrane was clearly roughened. Surfece of the membrane seemed to have a nodule-like structure at ion dose densities of lO and lO ... 

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]. 

But when the contents of Nafion ionomer was increased from 30 to 45 % to find out the better electrode structures, the Pt-Ru/SRaw, which had showed the lowest single cell performance, became the best electro-catalyst. By this result one can conclude that as long as the structure of the electrode can be optimized for the each of new electro-catalysts, the active metal size is a more important design parameter rather than inter-metal distances. Furthermore, when the electro-catalysts are designed, the principal parameters should be determined in the consideration of the electrode structures which affect on the electron conduction, gas permeability, proton conductivity, and so on. 

Figure 5 Chemical structure of Nafion (reproduced by permission of Elsevier from ref. 53).

Multilayered Structures with Metal Cations Based on LB Films ofPerfluorinated Sulfoacid Polymer (Nafion-wpe)... 

The reason for Nafion LB-film fabrication was the wish to obtain the highly ordered systems from perfluorinated ion exchange polymer with multilayered structure, where the ionic layers (conductors) would alternate with fluorocarbon polymer layers (insulators), and to investigate the properties of such films.74 This polymer contains a hydrophobic fluorocarbon polymeric chain and hydrophilic ionic groups, so it is sufficiently amphiphilic it has a comblike structure that makes it a suitable polymer for LB-film deposition. 

Repeated membrane failures during the early part of the 12-kW plant 500-hour DMMP run prevented effective control of water balance and levels of silver and organic material in the catholyte system. After laboratory-scale testing from October 11 to October 21,2001, AEA concluded that the failures resulted from foaming or pockmarks in the lattice structure of the PTFE support in the Nafion membrane and that the pockmarks formed only if the membrane came into contact with the cathode. 

GDE s may be interesting for synthesis cells as depolarized electrodes (e.g. [48]). A hydrogen-consuming anode will work at a low potential that avoids undesired anodic oxidations (e.g. no chlorine evolution in presence of chlorides). In order to reject an excess of the electrolyte from the GDE structure, a proton-conducting membrane (Nafion ) between the GDE and the electrolyte can be used ( Hydrina , De Nora Spa. [49]). 

Because the reaction in a CL requires three-phase boundaries (or interfaces) among Nafion (for proton transfer), platinum (for catalysis), and carbon (for electron transfer), as well as reacfanf, an optimized CL structure should balance electrochemical activity, gas transport capability, and effective wafer management. These goals are achieved through modeling simulations and experimental investigations, as well as the interplay between modeling and experimental validation. 

Passalacqua, E., Lufrano, F., Squadrito, G., Patti, A., and Giorgi, L. Nafion content in the catalyst layer of polymer electrolyte fuel cells Effects on structure and performance. Electrochimica Acta 2001 46 799-805. 

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. 

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

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