Mechanical acid Nafion

The most widely studied fuel-ceU membrane is DuPont s Nafion , a copolymer of tetrafluoroethylene and perfluoro(4-methyl-3,6-dioxa-7-octene-l-sulfonic acid). Nafion is the membrane material of choice for most proton-exchange membrane fuel cells that operate at a temperature <80 °C. While Nafion offers high conductivity combined with exceptional chemical and mechanical stability [3], it suffers from several critical drawbacks. When used in a direct methanol fuel cell, Nafion shows significant methanol leakage (crossover from the anode to the cathode) with the resultant reduction in fuel-ceU performance. To overcome this shortcoming the methanol concentration in the anode feed is usuaUy reduced to 0.5-2.0 M, which necessitates... 

In Nafion, the hydrophobic perfluorinated segments of the polymer are incompatible with the hydrophilic sulfonic acid groups and thus phase separation occurs. When exposed to water, the hydrophilic domains swell to provide channels for proton transport, whereas the hydrophobic domains provide mechanical integrity and, at least in the case of lower lEC samples. 

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. °... 

The situation for hydrated Nafion in the acid form, or as containing aqueous acids or strong bases, is more complex because protons and defect protons (i.e., OH ions), migrate according to a somewhat different mechanism. Proton transfer in either case occurs throughout and between clusters of hydrogen bonded water molecules to a degree that depends on the relative water content. 

Tant et al. reported a dynamic mechanical transition of around 100 °C (maximum in G") for acid form Nafion having 1140 EW. o Since this transition also appeared for the sulfonyl fluoride precursor, but at a much lower temperature ( 0 °C), they concluded that it involved main chain motions that are restricted by the conversion to the acid form. These motions were further restricted by the conversion to the Na " sulfonate form owing to strong ionic associations between the side chains. In contrast with the work of Kyu and Eisenberg, no transition appeared at 0 °C in addition to that at 100 °C. While the equivalent weights of the samples utilized by Eisenberg and Kyu and Tant et al. were not quite the same, the notable difference in matrix Tg assignment is cause for confusion. 

Miura and Yoshida noted a dynamic mechanical tan (5 peak at 120 °C at 1 Hz, labeled a, for dry 1100 EW acid form Nafion. These investigators assigned this transition to motions within the polar clusters because it was sensitive to cation type. 

Cable and Moore performed DMA (dynamic mechanical analysis) studies of various Nafion membranes including the acid form. ° A tan <3 peak with maximum at 110 °C, referred to as Tg , was seen, and there is a suggestion of a shoulder on the low temperature side that might arise from another mechanism. As this membrane was dried at only 60 °C, the possibility of residual water incorporation exists. Moore and Cable concluded that the a relaxation was due to chain motions within and/or near the ion-rich domains and that the ji relaxation was... 

In a related experimental study, Cirkel and Okada compared mechanical and electrical percolation that developed during the gelation of 3 1 (v/v) 2-propanol/ water solutions of Nafion 117 in the acid and Na+ forms.Attention should be paid to the particular manner in which these samples were prepared, as different conditions may yield different results. Also, caution should be applied in comparing these results with those of percolation studies using preformed films, such as that of Hsu et al. ... 

The rheological properties change behavior, relative to more dilute solutions, above cp = 0.2, where non-Newtonian behavior is then exhibited. The power law dependence of rj on cp is in harmony with the Zimm rather than the Rouse model, which suggests that hydrodynamic interactions between these polymers, in a mean field sense, are important. Electrical properties also begin to deviate for Nafion solutions above cp = 0.2, and mechanical percolation is essentially the same for the sodium and acid forms. 

Dupont de Nemours) or analogous perfluorosulphonic acid membranes have been the dominating choice. The structure of the repeat structure of the polymer fluorocarbon backbone and a side chain with sulphonic acid ends upon which Nafion is based is shown in Fig. 3.42 (the commercial product is sold with various thicknesses and dimensions denoted by a number code such as "Nafion-117", related to non-SI units). The membranes should have high protonic conductivity, low gas permeability and, of course, a suitable mechanical strength and low temperature sensitivity. 

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