Development Of New Proton Exchange Membranes Based On Polyimide And Poly(sulphide Sulphone) For Fuel Cells

As one of the core components of proton exchange membrane fuel cells(PEMFCs), proton exchange membranes(PEMs) have been extensively studied in the past decade. Current state-of-the art PEM is Dupont’s Nafion®, a typical perfluorinated polymer membrane, which possesses high proton conductivity and excellent chemical and electrochemical stability. However, the shortcomings such as high cost, low working temperature(< 90 oC) due to its low glass transition temperature and high fuel permeability seriously limit its applications in fuel cells. Therefore, researches in this area are mainly focused on the development of cost-effective and high performance hydrocarbon polymer membranes. Because of their strong dependence of proton conductivity on relative humidity, sulfonated hydrocarbon membranes are mainly used in medium-low-temperature(<110 oC) PEMFCs, while acid-doped polymer membranes, typically phosphoric acid doped polybenzimidazole(PBI) membranes are mainly used in high(150-200 oC) temperature PEMFCs. Although both of these two types of PEMs have been extensively studied and great progresses have been made, up to date few of them have been practically used in fuel cell industry for transportation because the performances of the PEMs cannot fully meet the requirements for practical applications of fuel cells.The main problems associated with sulfonated hydrocarbon membranes are as follows. First, there is a contradiction relationship between ion exchange capacity(IEC) and membrane stability. To achieve high proton conductivity, high IEC is generally required. However, too high IEC often causes excess swelling or even dissolution of the PEM in water leading to short operation lifetime of the fuel cells. Secondly, proton conductivity is closely dependent on relative humidity. At low relative humidities, proton conductivity becomes rather low because of lack of enough water molecules as carrier for proton transport. Thirdly, the chemical stability, in particular, the radical oxidative stability, of hydrocarbon membranes is generally rather poor which seriously affects the lifetime of the fuel cells. For phosphoric acid doped polybenzimidazole(PBI) membranes, high doping level which is essential to achieve high proton conductivity often causes poor mechanical strength making the membranes hard to handle. Phosphoric acid leaking is another problem associated with this type of membranes.To solve the above-mentioned problems, it is very crucial to systematically study the structure-property relationship of PEMs. This thesis mainly focuses on the synthesis, characterization and properties of various novel sulfonated polyimide membranes as well as a series of acid-doped poly(imide benzimidazole) membranes. In addition, a series of cross-linked blend membranes derived from sulfonated poly(sulfide sulfone) and a polybenzimidazole are also developed and their performances are evaluated. The main results are summarized as follows.1. A series of copolyimides containing benzimidazole groups(PIs) were synthesized by random copolymerization of biphenyl-4,4’-diylbis(oxo)-4,4’-bis(1,8-naphthalenedicarboxylic anhydride)(BPNDA), 2-(4-aminophenyl)-5-aminobenzimidazole(APABI) and 1,3-bis(4-aminophenoxy)benzene(BAPBZ) in m-cresol in the presence of benzoic acid and isoquinoline at 180 °C for 20 h. The PIs were further post-sulfonated with concentrated sulfuric acid to yield the benzimidazole-containing sulfonated polyimide(SPIs) with varied degrees of sulfonation. It was found that high degree of sulfonation could be achieved without significant polymer degradation by performing the post-sulfonation reaction at 50 oC for 24 h. The interaction between the basic benzimidazole groups and the sulfonic acid groups resulted in ionic cross-linking of the membranes(in their proton form). Further treatment of the SPI membranes in polyphosphoric acid(PPA) at 180 oC for 14 h yielded covalent cross-linking. In comparison with the non-covalently cross-linked SPIs membranes(ionic cross-linking only), the covalently cross-linked membranes(CSPIs) showed lower water uptake, lower swelling ratio and higher radical oxidative stability. Fenton test(80 °C, 3% H2O2 + 3 ppm Fe SO4) results showed that, although the covalently cross-linked membrane CSPI-1/1 exhibited three times longer Ï„1(125 min, time elapsed when the membrane started to dissolve in the Fenton’s reagent) than the non-covalently cross-linked membrane SPI-1/2(Ï„1 = 42 min) despite of their similar IEC values indicating much better radical oxidative stability of the former. The SPIs and CSPIs membranes exhibited proton conductivities in the range 0.07-0.30 S/cm(60 °C, in water) depending on the IEC levels, and among them the CSPI-1/1 and CSPI-1/2 showed high proton conductivities comparable to that of Nafion 112.2. Three types of copolyimides(random copolymers, multiblock copolymers and sequenced copolymers) were synthesized by copolymerization of 4,4’-bis(4-aminophenoxy)biphenyl-3,3’-disulfonic acid(BAPBDS), 1,4,5,8-naphthalenetetracarboxylic dianhydride(NTDA) and 1,12-dodecamethylenediamine(DDA). For the synthesis of the multiblock copolyimides, the anhydride-terminated polyimide oligomers which were prepared via condensation polymerization of DDA and excess NTDA were used as the hydrophobic block(averaged block length = 5, 10, 20), while the amine-terminated polyimide oligomers prepared from excess BAPBDS and NTDA were used as the hydrophilic block(averaged block length = 5, 7.5, 10, 15, 20, 30). Further polymerization of the hydrophobic oligomers with the hydrophilic oligomers yielded various multiblock copolyimides. For the synthesis of sequenced copolyimides, the anhydride-terminated hydrophobic oligomers were used instead of the monomer DDA to copolymerized with the BAPBDS and NTDA. X-ray diffraction patterns suggest that both the multiblock copolyimides and the sequenced copolyimides are crystalline polymers, whereas the random copolyimides are almost amorphous. The crystallinity of the multiblock copolyimide membranes and the sequenced copolyimide membranes resulted in significant suppression of membrane swelling in ‘in-plane’ direction, which should be favorable for improving fuel cell durability. Proton conductivity test results revealed that for the membranes with the same IEC the proton conductivity was in the order: multiblock copolyimide > sequenced copolyimide > random copolyimide at a given temperature and relative humidity. For example, the conductivity values measured at 60 oC in water are 0.136 S/cm for the multiblock copolyimide membrane B-X10Y15(here, X10 refers to the hydrophobic block with an averaged block length of 10, while Y15 refers to the hydrophilic block with an averaged block length of 15. Hereafter, the same nomenclature rule is applied to the other multiblock copolymers), 0.127 S/cm for the sequenced copolyimide membrane S2(hydrophobic block with an averaged block length of 10) and 0.100 S/cm for the random copolyimide membrane R1, respectively. The length of the hydrophilic/hydrophobic blocks also affects the conductivity of multiblock copolyimide membranes and the combination of the hydrophobic block length of 10 and the hydrophilic block length of 15 gives the highest proton conductivity. For example, the conductivity values measured at 60 oC in water are 0.12 S/cm for the B-X5Y7.5, 0.136 S/cm for the B-X10Y15 and 0.126 S/cm for the B-X15Y30, respectively. All the three types of copolyimide membranes showed high hydrolytic stability and good radical oxidative stability.3. A novel dianhydride monomer 9,9-fluorylidenebis(4,1-phenylene)bis(oxo)-4,4’-bis(1,8-naphthalenedicar boxylic anhydride)(FBPNA), a monoanhydride compound 4-phenoxy-1,8-naphthalic anhydride(PNA), and a triamine monomer tris(4-aminophenyl)amine(TAPA) were synthesized and their chemical structures were characterized by 1H NMR spectra and FT-IR spectra. A six-membered ring type of hyperbranched polyimide(HBPI) was synthesized by copolymerization of FBPNA and TAPA, using A2+B3 way. The resulting HBPI was further chemically modified on the basis of the reaction between the anhydride groups of the PNA and the terminal amino groups of the HBPI to give various hyperbranched polyimides(HBPI-PNAs) with varied contents of the residual amino groups. Post-sulfonation of the HBPI and the HBPI-PNAs using sulfuric acid as the sulfonating reagent produced various sulfonated hyperbranched polyimides(SHBPI and SHBPI-PNAs). Under the optimal sulfonation conditions(50 oC, 24 h), the sulfonation reactions underwent smoothly without significant polymer degradation. The SHBPI and SHBPI-PNAs could be dissolved in most aprotic solvents such as dimethylsulfoxide(DMSO), N,N-dimethylacetamide(DMAC), N,N-dimethylformamide(DMF) and 1-methylpyrrolidone(NMP). Free-standing membranes(CSHBPI and CSHBPI-PNAs) with reasonably good mechanical strength have been successfully prepared by covalent cross-linking of the residual amino groups using bisphenol A epoxy resin(BADGE) as the cross-linker. The CSHBPI and CSHBPI-PNA membranes showed high heat resistance, good oxidative stability and high proton conductivity. The proton conductivity of the CSHBPI-PNA50 membrane(IEC = 2.21 meq/g) was 0.149 S/cm, which was higher than that of Nafion 112. All the membranes showed obvious anisotropic swelling behavior(in-plane direction > through-plane direction). Fenton test(80 °C, 3% H2O2 + 3 ppm Fe SO4) results showed that the Ï„1 values of CSHBPI and CSHBPI-PNAs were about 60 min indicating relatively good radical oxidative stability of the membranes. The CSHBPI-PNA50 membrane exhibited higher proton conductivities at low relative humidities than the SPI-1/1 reported in chapter 2 despite of their similar IEC values. This indicates that the hyperbranched architecture is likely favorable for improving the proton conductivity at low relative humidities.4. A series of covalently cross-linked blend membranes were prepared from a sulfonated poly(sulfide sulfone) with 80 % degree of sulfonation(SPSSF80) and a polybenzimidazole with pendant amino groups(H2N-PBI) using glycidyloxypropyltrimethoxysilane(KH-560) and bisphenol A diglycidyl ether(BADGE) as cross-linkers. The resulting cross-linked membranes showed increased tensile strength but slightly decreased elongation at break comparing to the plain SPSSF80. The radical oxidative stability of the blend membranes was significantly improved due to the synergic action of the covalent cross-linking and the presence of the PBI component. For example, the cross-linked membrane with the composition of SPSSF80/H2N-PBI/KH-560 = 7/1/3 started to break into pieces after being soaked in Fenton’s reagent for 98 min which is about four times longer than that(20 min) of the plain SPSSF50(degree of sulfonation = 50%). The covalent cross-linking is also essential to suppress membrane swelling and to enhance membrane water stability. The KH-560-cross-linked blend membranes tend to show higher proton conductivities at low relative humidities than the BADGE-cross-linked one due to the hydrophilic silica network in the former. At fully hydrated state, the cross-linked membranes generally showed high proton conductivities comparable to that of Nafion112.5. A series of poly(imide benzimidazole) random copolymers(PIBIs and m PIBIs) were synthesized by condensation copolymerization of BPNDA,(2-(4-aminophenyl)-5-aminobenzimidazole(APABI) or bis(2-(3-aminophenyl))bibenzimidazole(m BAPBI) and 4,4’-diaminodiphenyl ether(ODA) in m-cresol in the presence of benzoic acid and isoquinoline at 180 °C for 20 h. The resulting PIBIs and m PIBIs showed excellent thermo-oxidative as well as radical-oxidative resistance and, depending on the composition of the random copolymers, the PIBI and m PIBIs membranes could be readily doped in polyphosphoric acid(PPA) or in 85 wt% orthophosphoric acid under pressure at 180 °C to give acid uptakes as high as 780 wt% and anhydrous proton conductivity of up to 0.26 S cm-1 at elevated temperatures. The PIBI membrane with a 1:1 molar ratio between APABI:ODA(PIBI-1/1) and with an acid uptake of 300 wt % showed an elastic modulus of 0.1 GPa at 160 °C, which is an order of magnitude higher than that of the common polybenzimidazole membranes with similar acid contents. A preliminary H2/air fuel cell test at 180 °C showed a peak power density of 350 m W cm-2 of the fuel cell equipped with the phosphoric acid doped PIBI-1/1 membrane with a 300 wt% acid uptake, demonstrating the technical feasibility of the novel electrolyte materials.