Ion-conducting polymer

A solid polymer ion-conducting membrane used in the center of the fuel cell membrane electrode assembly. 

Polymers. Ion implantation of polymers has resulted in substantial increases of electrical conductivity (140), surface hardness (141), and surface texturing (142). A four to five order of magnitude increase in the conductivity of polymers after implantation with 2 MeV Ar ions at dose levels ranging from 10 -10 ions/cm has been observed (140). The hardness of polycarbonate was increased to that of steel (141) when using 1 MeV Ar at dose levels between 10 -10 ions/cm. Conductivity, oxidation, and chemical resistance were also improved. Improvements in the adhesion of metallizations to Kapton and Teflon after implantation with argon has been noted (142). 

The first use of ionic liquids in free radical addition polymerization was as an extension to the doping of polymers with simple electrolytes for the preparation of ion-conducting polymers. Several groups have prepared polymers suitable for doping with ambient-temperature ionic liquids, with the aim of producing polymer electrolytes of high ionic conductance. Many of the prepared polymers are related to the ionic liquids employed for example, poly(l-butyl-4-vinylpyridinium bromide) and poly(l-ethyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide [38 1]. 

Table 1. Some imide ions and carbanions used in salts to enhance polymer electrolyte conductivity and reduce crystallinity...

Addition of both ion-conducting and inert ceramics enhances the conductivity of a polymer electrolyte. This increase is attributed to an increase in volume fraction of the amorphous phase [133-136]. No... 

Oxidized conducting polymers, or conducting polymer composites can be envisaged as stores of anions or cations, respectively. As stated earlier, these ions can be liberated, under a well-defined control, in a solution. This idea is being developed in order to store ions of pharma-... 

As illustrated in the previous sections, the electrochemical properties of conducting polymer films are strongly influenced by polymer-ion interactions. These interactions are in turn influenced by the nature of the solvent and the solvent content of the film. Consequently, the electrochemical behavior of conducting polymer films can be highly solvent dependent. Films can even become electrochemically inactive because of lack of solvation.114,197... 

In polymer electrolytes (even prevailingly crystalline), most of ions are transported via the mobile amorphous regions. The ion conduction should therefore be related to viscoelastic properties of the polymeric host and described by models analogous to that for ion transport in liquids. These include either the free volume model or the configurational entropy model . The former is based on the assumption that thermal fluctuations of the polymer skeleton open occasionally free volumes into which the ionic (or other) species can migrate. For classical liquid electrolytes, the free volume per molecule, vf, is defined as ... 

Another example of ion conducting polymer/ion/solvent systems are polyelectrolytes based on ion-exchange polymers, also called ionomers. The ionic conductivity of ion-exchange polymers is usually very low in the dry state, but increases abruptly by orders of magnitude upon addition of a... 

Ion-exchanger membranes with fixed ion-exchanger sites contain ion conductive polymers (ionomers) the properties of which have already been described on p. 128. These membranes are either homogeneous, consisting only of a polyelectrolyte that may be chemically bonded to an un-ionized polymer matrix, and heterogeneous, where the grains of polyelectrolyte are incorporated into an un-ionized polymer membrane. The electrochemical behaviour of these two groups does not differ substantially. 

The synthesis of a variety of lithium ion conducting borosiloxane polymers have been reported.51 The incorporation of the Lewis acidic boron and silicon into the polymer backbone was expected to facilitate their interaction with anions and thereby increase the T+ (transference numbers) of the resulting polymers.52... 

Ion-conductive properties of anion-trapping-type organoboron polymer electrolytes was evaluated after adding lithium salts (Fig. 3). In these systems, ionic conductivity of 3.05 X 10 s 5.22 X 10 6Scm 1 was observed at 50°C. This indicates... 

As described in previous sections (Sections VI and VII), macromolecular design of polymer/salt hybrids with a highly dissociable lithium borate unit proved to be a valuable approach for single-ion conductive polymers. To further improve the degree of lithium salt dissociation, we have designed a polymer/salt hybrid bearing the boron-stabilized imidoanion (BSI)38 (Fig. 10). 

A variety of organoboron polymer electrolytes were successfully prepared by hydroboration polymerization or dehydrocoupling polymerization. Investigations of the ion conductive properties of these polymers are summarized in Table 7. From this systematic study using defined organoboron polymers, it was clearly demonstrated that incorporation of organoboron anion receptors or lithium borate structures are fruitful approaches to improve the lithium transference number of an ion conductive matrix. 

Table 1 Ion-Conductive Properties of Organoboron Polymer Electrolytes ...

Design of organoboron polymer electrolytes will continue to have a great deal of potential based on the ability to tailor boron atoms. This is an attractive approach for single ion conductive materials. 

Ding, J., Chuy, C. and Holdcroft, S. 2001. A self-organized network of nanochannels enhances ion conductivity through polymer films. Chemistry of Materials 13 2231-2233. 

Allcock, H. R. and Wood, R. M. 2006. Design and synthesis of ion-conductive polvphosphazenes for fuel cell applications Review. Journal of Polymer Science Part B 44 2358-2368. 

---