Polyphosphazene membranes, DMFC

Studies of methanol permeation through proton-conducting polyphosphazene membranes show that polyphosphazene-based proton-exchange membranes have low methanol permeability and good proton conductivity [15], These membranes have been tested in the DMFCs and have shown good performance. 

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

Representative i-V curves for MEAs with four different polyphosphazene membranes are shown in Fig. 18b, along with a reference plot of a Nafion 117-based MEA. For these tests, the electrodes were hot pressed directly to the SPBPP/PBI membrane. DMFC performance with a 1.2 mmol/g lEC SPBPP membrane with 3 wt % PBI was essentially identical to that with Nafion 117,... 

FIGURE 7.29 DMFC performance with SPBPP/PBI blended membranes 60°C, 1.0 M methanol, and ambient pressure air at 500 seem, (a) Sandwiched MEA and (b) direct hot pressing of electrodes. Cross denotes the methanol crossover flnx (mol cm min ) at open circuit, relative to that in Nafion 117. (Reprinted with kind permission from Springer Science+Business Media Fuel Cells II, Polyphosphazene membranes for fuel cells, 2008, 157, Wycisk, R. and Pintauro, P.N./Scherer, G., ed.)... 

Wycisk, R., Lee, J.K., and Pintauro, P.N., Sulfonated polyphosphazene-polybenzimid-azole membranes for DMFCs, J. Electrochem. Soc., 152, A892, 2005. 

Wycisk R, Lee JK, Pintauro PN (2005) Sulfonated polyphosphazene-polybenzimidazole membranes for DMFCs. J Electrochem Soc 152 A892-A898... 

Two types of polyphosphazene (PPh) group such as sulfonated and phosphonated PPh have been developed recently for DMFC application (Gao and Sammes 1999 AUock et al. 2002 Zhou et al. 2003). Zhou et al. reported that the methanol crossover of the sulfonated membrane was about eight times lower than that of Nafion 117 at room tanperature even though the values were comparable at HO C (Zhou et al. 2003). The methanol permeability of the phosphonated PPh derivative was about 40 times lower than that of Nafion 117 at room temperature and about 9 times lower at 120 C. This is a significant improvement over the behavior of Nafion 117. Although the conductivities of both PPh membranes were lower than that of the Nafion 117 manbrane, they are still practical for DMFC applications since the selectivity of both sulfonated PPh (at temperature <85°C) and phosphonated PPh (at temperature 22°C-125°C) is higher than that of Nafion 117. 

Poiyphosphazene Biends Blends of sulfonated polyphosphazene, for example, sulfonated poly[bis(3-methylphenoxy)phosphazene] or poly[(bisphenoxy)phosphazene] (see Fig. 29.14) with either an inert organic polymer such as poly(vinylidene fluoride) (Pintauro and Wycisk, 2004 Wycisk et al., 2002) or polyacrylonitrile (Carter et al., 2002) or a reactive polymer (e.g., polybenzimidazole) (Wycisk et al., 2005) have been investigated. The resultant membranes had conductivities of 0.01-0.06 S/cm (in water at 25°C) and equilibrium water swelling from 20 to 60% (at 25°C). Blends of poly[(bisphe-noxy)phosphazene] and polybenzimidazole (where acid-base complexation occurred between the sulfonic acid and the imidazole nitrogen) exhibited good mechanical properties and low methanol permeability. MEAs with this membrane material outperformed Nafion 117 in a DMFC at 60°C with concentrated (5-10 M) methanol feeds. With 1.0 M methanol and 0.5 L/min ambient air at 60°C, the maximum power density was 97 mW/cm and the methanol crossover was 2.5 times lower than that with Nafion 117 (Wycisk et al., 2005). 

Nonfluorinated polymers such as sulfonated polyimides, polystyrene sulfonic acid, sulfonated poly(arylene ether sulfones), polyphosphazene, polybenzimidazole, sulfonatedpolysulfone, sulfonated poly(phthala-zinone ether ketone), and sulfonated poly(ether ether ketone) have been developed as a potential electrolyte membrane for DMFC [4,6,16-23]. 

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