Sulfonimide polyphosphazene membran

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. 

Membranes with promising properties have been prepared from polyphosp-hazenes in combination with sulfonimide (polyphosphazene-based sulfonimide Chalkova et al., 2002) or with polyacrylonitrile (blended polyphosp-hazene/polyacrylonitrile Carter et al., 2002). Low methanol crossover was also seen with membranes prepared from poly(vinyl alcohol) that contained mordenite (a zeolite variety Libby et al., 2001). Various aspects of the work on composite membranes prepared from different polymers have been discussed in detail in a review by Savadogo (2004). 

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

The improved performance of the sulfonimide polyphosphazene MEA, as compared to that with Nafion 117, was probably the result of the lower areal resistance of the former (the areal resistance is defined as the ratio of membrane thickness to proton conductivity). While the conductivity of the polyphosphazene membrane was approximately 60% that of Nafion 117 (at room temperature), its thickness was half that of Nafion 117. If temperature does not alter significantly the thickness to conductivity ratio, then the areal resistance of the sulfonimide MEA is estimated to be 15-20% less than that of Nafion 117. It should also be noted that there was no long-term stability analysis of the sulfonimide membranes. Such tests are critical in evaluating new membranes for fuel cells. 

Blended membranes of the sulfonimide polyphosphazene and PVDF in a 15%I25% w/w ratio were cast from Al,Al-dimethylacetamide. No phase separation was observed, with the membrane water uptake reduced to 41 wt% from 119 wt%. It was surprising that the proton conductivity of blended membrane increased more than 20% to 0.060 S cm", even though PVDF is an inert polymer. 

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

Preliminary results with membranes based on sulfonimide-substituted polyphosphazenes (226) show a good proton conductivity and moderate swelling in water, depending on the degree of cross-linking. ... 

In a follow-up study, Lvov et al. [46] prepared membranes of good physicochemical characteristics from blends of sulfonimide functionalized polyphosphazene and PVDF. For example, a membrane prepared from poly-... 

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