Preparation And Properties Of Proton Exchange Membranes Based On Vinyl Addition Polynorbornene Derivatives

Direct methanol fuel cells (DMFCs) have been one of priority development of the energy technologies due to their high power density, environment friendly and security. The proton exchange membrane (PEM) is one of the key components determining the performance of the fuel cells. Nafion(?) is the current state-of-the-art PEM material, while several drawbacks of Nafion(?), such as high cost, high methanol permeability, low humidity and a major reduction in conductivity at high temperatures, has limited its industrial applications. These limitations have stimulated worldwide research activities for developing new materials with high performance as proton exchange membrane.Vinyl addition type polynorbornenes exhibit good membrane formation, high thermal stability, excellent methanol resistant, high glass transition temperature and good chemical stability. Because all this materials that have dual-ring structure, usually have high dimensional stability and excellent dielectric properties etc., the vinyl addition type polynorbornene is suitable for development as a direct methanol fuel cell proton exchange membrane material.The poly (2-butoxymethylenyl-5-norbornene-co-2-methylenyl acetate-5-norbornene) (PNBF1/NBF2-91,91 indicate the mole feed rate of NBF,:NBF2 is 9:1) was successfully synthesized via the vinyl addition copolymerization of 2-butoxymethylenyl-5-norbornene (NBF1, NB=norbornene, F1=-CH2-O-CH2CH2CH2CH3) and 2-methylenyl acetate-5-norbornene (NBF2, F2=-CH2-O-COCH3) by using bis-((β-ketonaphthylamino)nickel(Ⅱ)/B(C6F5)3 catalytic system. The poly (2-butoxymethylenyl-5-norbornene-co-2-methylol-5-norbornene) (PNBF1/ NBF3-91, F3=-CH2-OH) was obtained by the de-esterification of PNBFi/NBF2-91. Then PNBF1/NBF3-91 was crosslinked with 4,5-imidazole dicarboxylic acid (IDA) via the esterification of the-OH groups and the-COOH groups of IDA. Upon doping with phosphoric acid (H3PO4) to form imidazole-H3PO4 complexes to obtain the PNBF1/NBF3-91/IDA/H3PO4 (PNBIH) cross-linked membranes and the corresponding morphology, water uptake, proton conductivity, methanol permeability, thermal stability, as well as tensile properties were investigated. The results indicate that the crosslinked membranes showed higher proton conductivity at higher temperatures and lower methanol permeability after incorporation of more content of IDA and H3PO4. The PNBIH membranes showed lower methanol permeability from 0.25×10-6 to 0.4×10-6 cm2·s-1, which is order of magnitude lower than that of N115.The sulfonated functional polynorbornenes (SPNBF1/NBF3-xx) are successfully prepared via the sulfobutylation of the PNBF1/NBF3-xx (xx is the mole feed ratio of NBF1 and NBF2 is from 9:1 to 5:5) with 1,4-butane sultone, and the chemical structures of the polymers are confirmed by the 1H NMR and the FT-IR. Except for SPNBF1/NBF3-91, the rest SPNBF1/NBF3-xx can not be dissolved in common organic solvents. So the PEM membrane is only from SPNBF1/NBF3-91, With the purpose of improving the proton conductivity of SPNBF1/NBF3-91 membrane, a composite membrane with a proper content of PNBF1/NBF3-91 copolymer, IDA and H3PO4, has been prepared. The SPNBF1/NBF3-91 and its blend membranes have excellent methanol resistant and dimensional stability, and the methanol permeability of the SPNBF1/NBF3-91 and its blend membranes are 0.87×10-7 cm2 and 1.83×10-7 cm2·s-1, respectively. But SPNBF1/NBF3-91 membrane with so poor proton conductivity that can not obtained its DMFC performance. Compare with the N115 (0.559 V,16 mW·cm-2), open circuit voltages (0.483 V) of the blend membrane at 2 mol·L-1 methanol (70℃) exhibited only slightly lower, while its power density decreases sharply to 1.35 mW·cm-2.