Teflon-like backbones

In this type of membrane, as well as in similar products made by other manufacturers, the Teflon-like backbone is responsible of very high chemical resistance (due to the strong bond between carbon and fluorine), high hydrophobic characteristics, and good mechanical properties. The hydrophobic feature is useful to favor the expulsion of product water out of the cell, in order to prevent flooding phenomena, while the mechanical strength permit the production of very thin films (down to 50 pm). 

High-resolution NMR of some perfluoroionomer shows an unusual combination of a nonpolar. Teflon-like backbone, with polar and ionic side branches. Liu and Schmidt-Rohr [21] have obtained the first high-resolution NMR spectra of solid perfluorinated polymers by combining 28 kHz magic-angle spinning (MAS) with rotation-synchronized 19F pulses. Their NMR studies enable more detailed structural investigations of the nanometer-scale structure and dynamies of PTFE based ionomers. It has also been well established [1-21] that anions are tethered to the polymer backbone and cations (H", Na" ", Li ) are mobile and solvated by polar or ionic liquids within the nanoclusters of size 3-5 nanometers. 

The success of DuPont s Nafion spurred the development of other polymeric materials with similar chemical architecture. The most notable material developments have been the Dow experimental membrane (Dow Chemicals), Flemion (Asahi Glass), Aciplex (Asahi Kasei), as well as Hyflon Ion and its most recent modification Aquivion (SolviCore). In addition to excellent ionic conductivity, materials of the PFSA family, illustrated in Figure 2.2, exhibit exceptional stability and durability in highly corrosive acidic environments, owing to their Teflon-like backbone (Yang et al., 2008 Yoshitake and Watakabe, 2008). 

Teflon-like backbone is highly hydrophobic, the sulphonic acid at the end of the side chain is highly hydrophyllic. The hydrophyllic regions are created around the clusters of sulphonated side chains. This is why this kind of material absorbs relatively large amounts of water (in some cases up to 50% by weight). ions movement within well-hydrated regions makes these materials proton conductive. 

The ionic groups (-SO3H) form clusters in the chemically stable Teflon-like matrix (-CF2CF2-) [6]. These hydrophilic domains form micelles, surrounded by the hydrophobic fluorocarbon backbone and connected by small channels. The Nafion resin is prepared as a copolymer of tetrafluoroethylene and perfluoro-2-(fluorosul-fonylethoxy)propyl vinyl ether [1,7]. Unlike other resinsulfonic acids, e. g. sulfo-nated polystyrenes (Dowex-50, Amberlyst-15, Amberlite IR-112, etc.), Nafion and related perfluoroalkanesulfonic acids are stable in corrosive environments and the maximum operating temperature is up to 473K [8]. 

Many different types of fuel-cell membranes are currently in use in, e.g., solid-oxide fuel cells (SOFCs), molten-carbonate fuel cells (MCFCs), alkaline fuel eells (AFCs), phosphoric-acid fuel cells (PAFCs), and polymer-electrolyte membrane fuel cells (PEMFCs). One of the most widely used polymers in PEMFCs is Nalion, which is basically a fluorinated teflon-like hydrophobic polymer backbone with sulfonated hydrophilic side chains." Nafion and related sulfonic-add based polymers have the disadvantage that the polymer-conductivity is based on the presence of water and, thus, the operating temperature is limited to a temperature range of 80-100 °C. This constraint makes the water (and temperature) management of the fuel cell critical for its performance. Many computational studies and reviews have recently been pubhshed," and new types of polymers are proposed at any time, e.g. sulfonated aromatic polyarylenes," to meet these drawbacks. 

The Teflon-like molecular backbone gives these materials excellent long-term stability in both oxidative and reductive environments. A lifetime of over 60,000 h under fuel cell conditions has been achieved with commercial Nafion membranes. These membranes exhibit proton conductivity as high as 0.1 S cm" under fully... 

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