Hydrocarbon ionomers

The Cluster on Network Model. Previous small angle x-ray (]L-J>) and neutron (4) scattering experiments clearly indicate that ionic clustering is present in "Nation". However, details of the arrangement of matter in these clusters cannot be obtained from these results alone. For hydrocarbon ionomers several different interpretations have been advanced as the cause of the SAXS maximum. These include a model of spherical clusters on a paracrystalline lattice proposed by Cooper et al. (16), the shell core model of Macknight et al. (17) and more recently a lamellar model (18). At present there is no consensus about which of these models best describes clustering in hydrocarbon ionomers. 

These findings are, more or less, applicable to the other aromatic hydrocarbon ionomers and useful to design the higher-order structure and properties of ionomer membranes. 

Low cost (final target for electric vehicle apphcations would be cheaper than US 10/m ) need to be taken into account. While a number of PEMs have been developed, no single membranes fulfill aU of these requirements. Currently, the most promising PEMs are perfluoro sulfonic acid (PFSA) ionomers. Another candidate second to the PFSA ionomers is non-fluorinated (or in some cases only slightly fluorinated) hydrocarbon ionomers. The aim of this chapter is to review the most recent progress on these two classes of ionomer membranes for low-temperature fuel cell applications... 

Block copolymers have been utilized in order for the hydrocarbon ionomer membranes to have well-developed and intercmmected ionic domains. The strategy seems to work in many ionomers of different molecular structure. The block... 

One of the significant differences between hydrocarbon ionomers and perfluoro-sulfonic acid polymers is add groups. The pKa value of benzenesulfonic acid (PhSOsH) is 2.5 and that of trifluoromethanesulfonic acid (CF3SO3H) is —13. The pKa value was estimated to be ca. 1 for sulfonated polyether ketone and ca. —6 for Nafion membranes [78]. Therefore, the effective proton concentration and proton mobility should be lower in the hydrocarbon ionomer membranes. Without appropriate molecular design such as multiblock copolymer and sulfonic acid clusters as mentioned above, hydrocarbon ionomer membranes lack well-developed ionic channels due to less pronounced hydrophilic and hydrophobic phase separation, which causes the lower proton conductivity at low humidity. 

Fig. 17.7 Structures of some hydrocarbon ionomers. Polymer 4 is from reference 30 and Polymers 5 and 6 are from reference [32]...

The studies mentioned above show that through control of the electronic and structural features of hydrocarbon ionomers, increased conductivity can be achieved. However, this is often at the expense of the mechanical stability of the polymer to the point where these materials cannot be used in fuel cells. One potential method of... 

FIGURE 17.8 Comparison between typical PFSA and hydrocarbon ionomer ionic conductivity as a function of relative humidity. 

For hydrocarbon membranes, the degraded products can be directly measured owing to the stabihty of the carbon radicals. Fenton-type reaction on hydrocarbon model compounds has been smdied (Hubner and Roduner 1999). Possible weak sites for HO attack have been identified for various hydrocarbon ionomer membranes. 

Miyatake, K., Watanabe, M. (2006) Emerging membrane materials for high temperature polymer electrolyte fuel cells Durable hydrocarbon ionomers. J. Mater. Chem. 16, 4465-4467. 

The disadvantages of PFSA membranes have stimulated many efforts to synthesize PEM based on fluorine-free hydrocarbon ionomer membranes as alternatives to... 

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