Synthesis And Properties Of Novel Proton Exchange Membranes For Fuel Cell Applications

A proton exchange membrane (PEM) is one of the key components of a fuel cell. From viewpoint of practical applications it is very crucial to develop cost-effective and high performance (high proton conductivity, high chemical stability, good mechanical strength, low swelling ratio, low fuel and oxygen crossover, etc.) PEMs. This thesis mainly focuses on the preparation, characterization and properties of three types of novel PEMs: the cross-linked poly(sulfide sulfone)(SPSSF)/polybenzimidazole blend membranes, the sulfonated hyperbranched polyimide membranes and the sulfonated polyimide membranes containing imidazole rings.In chapter2, two kinds of sulfonated poly(sulfide sulfone)s with the degrees of sulfonation of50%(SPSSF-50) and80%(SPSSF-80) and a polybenzimidazole with pendant amino groups (H2N-PBI) have been synthesized. The H2N-PBI was chemically modified with3-glycidyloxypropyltrimethoxysilane (KH-560) to yield a trimethoxysilyl-grafted polybenzimidazole ((CH3O)3Si-PBI). A series of covalently and ionically cross-linked sulfonated poly(sulfide sulfone)/polybenzimidazole blend membranes with good mechanical properties have been successfully prepared by casting the(CH3O)3Si-PBI and the SPSSF-80(Na+form) solution mixtures with varied ratios followed by hydrolysis-induced covalent cross-linking under acidic conditions. The Fenton’s test results revealed that the cross-linked blend membranes had much better radical oxidative stability than the pure SPSSF membranes. On the other hand, although part of the sulfonic acid groups of the SPSSF-80component in the blend membranes had been consumed due to the ionic cross-linking, these blend membranes still displayed fairly high proton conductivities in deionized water which are comparable to that of Nafion112. In addition, the synergic action of the covalent cross-linking and ionic cross-linking significantly suppressed membrane swelling.Chapter3mainly deals with the synthesis, characterization, membrane formation and properties of a new type of sulfonated hyperbranched polyimide (SHBPI). A new dianhydride monomer4,4’-(9-fluorenylidene)bis(4-phenoxy-1,8-naphthalic anhydride)(FBPNA) has been synthesized from4-bromo-1,8-naphthalenedicarboxylic anhydride and4,4’-(9-fluorenylidene)diphenol. An amine-terminated hyperbranched polyimide (HBPI) has been synthesized by condensation polymerization of the FBPNA (A2monomer) and tris(4-aminophenyl)amine (B3monomer) in m-cresol at a monomer molar ratio of1:1and a solid content of4w/v%. The resulting HBPI was sulfonated with concentrated sulfuric acid under different conditions. A series of sulfonated hyperbranched polyimide (SHBPI) membranes were prepared by casting the SHBPI solutions at80℃in the presence of a cross-linker (epoxy resin). All the SHBPI membranes showed high mechanical strength (50-60MPa) and good thermal stability. The IECs of the SHBPI membranes are in the range1.67-1.98meq/g. These SHBPI membranes showed low swelling ratios (in-plane:7-15%, through-plane:4-8%, in deionized water at80℃), high proton conductivity (0.054-0.098S/cm in deionized water at60℃), but poor oxidative stability which needs to be enhanced in the future.In chapter4, the effects of the chemical structure of two imidazole-group-containing diamines on the radical oxidative stability and hydrolytic stability of the relevant sulfonated polyimide (SPI) membranes have been investigated.1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTDA) was randomly copolymerized with4,4’-bis(4-aminophenoxy)biphenyl-3,3’-disulfonic acid (BAPBDS) and three diamine comonomers:2-(4-aminophenyl)-5-aminobenzimidazole (APABI),1,3-bis(4-aminophenoxy)-5-(2-benzimidazole)benzene (BAPBIB) to yield three kinds of SPIs:NTDA-BAPBDS/APABI(5/1)(main-chain-type), NTDA-BAPBDS/BAPBIB(4/1)(side-chain-type), and NTDA-BAPBDS/BAPB(3/1)(imidazole-group-free). It was found that the two imidazole-group-containing SPIs, showed significantly higher radical oxidative stability than the imidazole-group-free SPI. Water stability test revealed that the NTDA-BAPBDS/BAPBIB(4/1) was much more stable toward hydrolysis than the NTDA-BAPBDS/APABI(5/1) judging from the much lower viscosity loss (3%) of the former after being soaked in deionized water in an autoclave at140℃for24h than that (42%) of the latter under the same conditions. The poor hydrolytic stability of the NTDA-BAPBDS/APABI(5/1) is due to the decreased basicity of the amino groups of the APABI moiety resulting from the strong electron-withdrawing effect of the protonated imidazole groups, whereas such an electron-inductive effect was hindered by the ether bonds in the BAPBIB moiety.It is noted that these three kinds of SPI membranes displayed reasonably high proton conductivities (～0.1S/cm at60℃in deionized water).Besides the above-mentioned studies, some research works on a novel kind of solid polymer electrolyte (SPE) as separator for lithium batteries and a SPI membrane for fundamental study have also been done. In appendix1, poly(ethylene oxide)(PEO, Mn=1500Da)-segmented polysulfone copolymers (PSF-PEO) with varied PEO contents (35-50wt%) have been synthesized by condensation polymerization. The SPE membranes were prepared by casting the solution mixture of the PSF-PEO, lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) and succinonitrile (SN) with varied compositions. High lithium conductivity (1.14×10-3S/cm,80℃) and high mechanical strength (9.6MPa,80℃) SPE has been successfully achieved. In appendix2, a SPI has been synthesized via random copolymerization of NTDA, BAPBDS and BAPB. The SPI membrane was converted to its lithium form and subsequently underwent pulsed field gradient (PFG) NMR and conductivity measurements to investigate the variation of the diffusivity coefficient of lithium as a function of water content in the sample.