Polyphosphazenes fuel cell membranes

Polyphosphazenes are an important class of inorganic macromolecule and have many commercial applications, e.g. fire retardants, elastomers, fuel cell membranes, biomedical applications. The scheme below shows synthetic routes to three polymers, (a) Outline how (NPCl2) is produced on a large scale starting from PCI5 and NH4CI. (b) What are the identities of the polymers A-C ... 

Carter, R., Wycisk, R., Yoo, H. and Pintauro, P. N. 2002. Blended polyphosphazene/polyacrylonitrile membranes for direct methanol fuel cells. Electrochemical and Solid-State Letters 5 A195-A197. 

Polyphosphazene-based PEMs are potentially attractive materials for both hydrogen/air and direct methanol fuel cells because of their reported chemical and thermal stability and due to the ease of chemically attaching various side chains for ion exchange sites and polymer cross-linking onto the — P=N— polymer backbone. Polyphosphazenes were explored originally for use as elastomers and later as solvent-free solid polymer electrolytes in lithium batteries, and subsequently for proton exchange membranes. 

Allcock et al. also have investigated the use of phosphonated polyphosphazenes as potential membrane materials for use in direct methanol fuel cells (Figure A2) Membranes were found to have lEC values between 1.17 and 1.43 mequiv/g and proton conductivities between 10 and 10 S/cm. Methanol diffusion coefficients for these membranes were found to be at least 12 times lower than that for Nafion 117 and 6 times lower than that for a cross-linked sulfonated polyphosphazene membrane. 

Polyphosphazenes block copolymers containing sulfonimide side groups, (V), were prepared by Allcock et al. (4) and used in membrane blends in fuel cells. 

Polyphosphazene has good chemical and thermal stability. Its polyphosphazene backbone is highly flexible. Various side chains can be introduced to this backbone readily. Cross-linking is needed in order to reduce the dimensional changes in the presence of methanol or water. The membranes have shown reasonable proton conductivity and low methanol crossover. However, an improvement in mechanical strength is needed for practical fuel cell applications. 

Diffusion coefficients for methanol (concentration range 1.0-5.0 M) in ion-exchange membranes of UV-crosslinked sulfonated poly(bis-3-methoxyphenoxyphosphazene) (134) have been reported to be much smaller than those in Nafion perfluorosulfonic ion-exchange membranes. Application of polyphosphazene-based membranes in methanol-based fuel cells has been reported. ... 

Nafion membranes, as discussed, have been intensively used for fuel cells because they show high proton conductivity and chemical stability, but their methanol permeability is too high. However, the critical aspect of Nafion is still its high cost. Several nonfluorinated membranes, with potentially lower costs, have been tested for fuel cells. Sulfonated PSF, sulfonated poly(ether ether ketone) (SPEEK), sulfo-nated polyphosphazene, and sulfonated polyamides (PA) with good performance for hydrogen fuel cells are described in several reports (Savadogo 1998 Zaidi et al. 2000 Guo et al. 1999 Vallejo et al. 1999). However, the methanol permeability in many cases is still relatively high. 

Zhou, X. Weston, 1. Chalkova, E. Hofmann, M.A. Ambler, C.M. Allock, H.R. and Lvov, S.N. 2003. High temperature transport properties of polyphosphazene membranes for direct methanol fuel cells. 48 2173-2180. 

Herrero and Acosta (80) investigated the microstmcture of poly(ethylene oxide)-poly[(octafluoropentoxy)(trifluoroethoxy)phosphazene] blends. Limited miscibility of both components was inferred, based on the observed shift of the components glass-transition temperatures. Wycisk and co-workers (81) prepared membranes from blends of sulfonated poly[bis(3-methylphenoxy)phosphazene] with polyimides, polyacrylonitrile, and Kynar FLEX PVDF. Morphology, electrochemical performance, and methanol permeabilities of the membranes were then evaluated as part of a program to investigate such blends in direct methanol fuel cells. The polymers were immiscible and a domain-type structure was observed. The best compatibility resulted when the tetrabutylammonium or sodium salt of the polyphosphazene was used (82). 

R. Wycisk and P. N. Pintauro, Polyphosphazene Membranes for Fuel Cells, in Fuel Cells II, Advances in Polymer Science, G. Scherer (Ed.), Springer-Verlag Berlin, Berlin, vol. 216, p. 157. 

Pintauro PN, Wycisk R (2004) Sulfonated polyphosphazene membranes for direct methanol fuel cells . Phosphazenes 591. 

Polyphosphazenes were used as solid polymer electrolytes for batteries, membranes for gas and liquid separations, and proton exchange membranes for fuel cells as well as for energy storage and energy generation applications. ... 

Keywords Condensation polymers Fuel cells Phosphorylation Polyelectrolytes Polyphosphazenes - Polysulfones Proton exchanging membranes... 

Polyphosphazene Membranes for Direct Methanol Fuel Cells (DMFCs). . 174... 

For a polyphosphazene to be used as the membrane material in a fuel cell, it must possess some kind of protogenic functionality, to allow the polymer to conduct protons. The most widely used protogenic group is the sulfonic acid moiety which can be incorporated into the macromolecular chain either at the polymer synthesis step (Fig. 4a) or later, during postsulfonation (Fig. 4b). While the postsulfonation reaction is generally easier to carry out, it is the direct synthesis of the functionahzed polyphosphazene that should give better control over the final material properties and minimize imwanted side... 

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