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Membranes Structural Complexity at Different Scales

The structure of PEMs, in particular their phase-separated morphology at nm-scale, has been studied with a number of experimental techniques, including small- and wide-angle X-ray and neutron scattering, infrared and Raman spectra, time-dependent FTIR, NMR, electron microscopy, positron annihilation spectroscopy, scanning probe microscopy, and scanning electrochemical microscopy (SECM) (for a review of this literature see [31]). Structural studies of PEMs have mainly focused on Nafion. A thorough recent review on this particular membrane is provided in [32]. [Pg.19]

CrystalHnity in Nafion was probed by detailed wide-angle X-ray diffraction (WAXD) experiments [35]. A small degree of crystallinity ( 12%) was found in sulfonated 1100 EW Nafion. [Pg.19]

In spite of tremendous experimental activities aiming at imraveling structure and dynamics in fuel cell membranes, the quest for a universally accepted model continues. Primordially, the ambiguity of structural models stems from the random morphology of the membrane. Indeed, due to this [Pg.21]

At low water content the quasi-crystalline motif in the distribution of inverted micelles is strong the size of the micelle is smaller than the persistence length of the bundles of backbone-chains forming the membrane skeleton. A legitimate question arises then how can one build a quasicrystalline structure of inverted micelles (aqueous droplets supported by hydrated sidechains), if the sidechains are attached to the backbones In an attempt to answer this question, a more detailed morphological model of Nafion-type ionomers was suggested [31] a quasi-crystalline arrangement of units cells as depicted in Fig. 1. [Pg.22]

Indeed, if the channels evolve at the beginning as extremely narrow units (less than half nm radius of the narrowest part of the cathenoid) and remain narrow even in the mature state , it is clear why the activation energy of the proton mobility, entirely controlled by the necks, will depend, dramatically, on water content, as experimentally observed (see Fig. 2). The bottleneck of conductance would be the proton transfer through the aqueous necks, or the fluctuational formation of the neck itself (in the spirit of a conjectme made by Vishnyakov and Neimark [53]). Furthermore, the more flexible the sidechains are, the higher the proton mobility, since fluctuations of the chains will support the necks, reducing their surface tension, in addition to possible sidechain-fiuctuation promoted proton transport. [Pg.23]


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