Polyphosphazene hydrogen fuel cells

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. 

Fig. 15 Hydrogen fuel-cell performance curves with a sulfonimide polyphosphazene proton-exchange membrane at 22 °C (a) and 80 °C (b)...

One of the recent attempts to patent polyphosphazenes for fuel cells is an application filed by Honda Motor Co., Ltd., Tokyo on January 20, 2005 [52]. The invention deals with the fabrication of membranes composed of highly sidfonated polyalkylphenoxyphosphazenes, for possible use in a hydrogen/air or direct methanol fuel cell. Methods of synthesizing polyphosphazenes with an lEC as high as 4.9 mmol/g are described and the proton conductivity of the resulting films is presented. 

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. 

There is only one example in the literature of polyphosphazene performance in a proton-exchange membrane (PEM) hydrogen fuel ceU. Allcock and Lvov [45] tested a sulfonimide polyphosphazene membrane in a hy-drogen/oxygen fuel cell at room temperature and at 80 °C. The membrane-electrode-assembly (MEA) was fabricated from a 100 xm thick sulfonimide polyphosphazene membrane that was crosslinked with y-radiation (40 MRad). The polymer lEC was 0.99 mmol/g, with an equilibrium water swelling of 42%, and a proton conductivity of 0.058 S/cm. The anode and cathode were prepared from carbon-supported platinum (20% Pt on Vulcan XC-72R) at a Pt loading of 0.33 mg/cm. The electrodes were hot pressed onto the membrane at 65 °C and 400 psi for 30 s. As a reference, a Nafion 117 MEA was also prepared with the same electrode catalyst at a loading of 0.26 mg/cm for the anode and 0.48 mg/cm for the cathode. For Nafion, the electrodes were hot pressed at 125 °C and 1400 psi for 2 min. 

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