Phosphoric acid copolymer membranes

The state of the art of PBI-based polymer electrolyte membranes for their use in high-temperature fuel cells working at 150-200 °C has been reviewed [28]. Several PBI copolymers and related compounds have been investigated for this application. Besides phosphoric acid, many other strong inorganic acids have been used for impregnation. 

Copolymers with benzimidazole and benzoxazole units have been prepared and used as a polymer electrolyte material [30,11]. The polymer electrolyte material has both high proton conductivity and excellent mechanical properties even when it is obtained by in situ phosphoric acid doping. The polymer electrolyte material may substitute for the conventional phosphoric acid doped polybenzimidazole in a polymer electrolyte membrane fuel cell, particularly in a high-temperature polymer electrolyte membrane fuel cell. 

Figure 20. Van t HofF diagrams, ln(l/PH2o) versus VT, at various hydration levels derived from Fig. 19. Phosphoric acid data are from Ref. 101. All data correspond to 100% H3PO4 concentration both in the membrane and in free H3PO4 solution of Ref 101 Solid symbols for Copolymer IIso, open symbols for liquid H3PO4 and crossed open symbols for PBI/PPy(50)PSF.

Jeong et al. [69] synthesized acid-doped sulfonated poly(aryl ether benzimidazole) (s-PAEBI) copolymers for use in high-temperature fuel cells. The polymer was made in a direct polymerization (structure confirmed with NMR) and doped with phosphoric acid to levels of 0.7-5.7. The degree of sulfonation was varied from 0-60%. The copolymer s physicochemical properties were studied using AFM, TGA, and conductivity measurements. TGA runs showed good stability of the nonsulfonated, sulfonated, and acid-doped sulfonated membranes up to 450 °C, with a slow decline above this temperature. The conductivity depended on the doping level of the polymer. At 130 °C with no hiunidification, a polymer with a doping level of 5.7 had a conductivity of 7.3 X 10 S cm ... 

Various authors have prepared multilayer membranes from hydrogenated polymers, but few surveys exist involving such a system with fluoropolymers. Park et al. [192] prepared a trilayer membrane from a copolymer matrix composed of a poly(VDF-co-HFP) copolymer doped with a 5% Nafion /hydrated tungstcnic phosphoric acid solution. This constitutes the central thickness of the membrane (60 pm). Then, a 5% Nafion solution was deposited on each side to obtain a trilayer membrane of 70 pm. Finally, that membrane was pressed and laminated to obtain a composite membrane of 30 pm thickness. The Nafion layer deposited on both sides of the membrane improved the conductivity at the membrane/electrode interface and also limits the oxidation of hydrogen on the central polymer. Furthermore, Nafion acts as an adhesive to ensure better contact between the membrane and the electrode [192]. Such composite membrane allows to limit the methanol crossover for its use in DMFC, which is the main drawback of that standard polymer. 

Current research activities are focused on the identification of membrane materials and structures with high proton conductivity and low methanol permeability. Ultimately, one would tike membranes that work well in a DMFC at 10-20 M methanol, but the focus of most research is on much lower methanol feed concentrations (0.5 and 1.0 M). Since DMFCs are designed primarily for the portable power/electronics market, operating temperatures in the 25-80°C range are usually considered. Extensive data is available in the literature on new membrane materials with reduced methanol permeability, including sulfonated or phosphonated copolymers, phosphoric-acid-doped polybenzimidazole, and various blends and composites (see Table 29.6 for a listing of DMFC properties). 

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