Fuel Cell Using The New Cross-linked Proton Exchange Membranes

Wholly aromatic polymers are thought to be one of the more promising routes to high performance proton exchange membranes (PEMs) because of their lower cost, good thermal stability, mechanical properties and anticipated stability in the fuel cell environment. Sulfonated aromatic hydrocarbon polymers are usually used as PEMs for low temperature fuel cells, and phosphoric acid doped polybenzimidazole is usually used for high temperature fuel cells, all of the membranes have apparently merits but still have unacceptable disadvantages as the proton exchange membranes.The main problem existed in the sulfonated hydrocarbon PEMs is the oxidative stability (still poorer than Nafion) and low conducting performance at low moisture atmosphere. The high ion exchange capacity (IEC) of the membranes led to high conductivity but swelling too much in water, the balance between IEC, proton conductivity and mechanical stability of a PEM plays an important role on its comprehensive performance in a fuel cell. A high temperature fuel cell system, operational at up to 200oC based on the use of a phosphoric acid-doped polybenzimidazole membrane, is demonstrated. However, PBIs usually have rather poor solubility in organic solvents leading to poor processability; another problem is the poor mechanical strength at high PA doping levels (defined as the averaged mole number of PA per mole of PBI repeat unit) which is essential for achieving high proton conductivity. The commercial PBI, for example, becomes too weak at high doping levels of 13-16 to fabricate into membrane electrode assemblies (MEAs), and it is suggested that the doping levels should be controlled at 5-6 to balance the proton conductivity and the mechanical strength of membranes; the most intractable problem is that phosphoric acid will leak from the PBI membrane gradually, the product of cathode reaction is H2O, so phosphoric acid tends to move to the doped PBI membrane surface, result in decreasing the life of fuel cell largely.In this paper, series cross-linked membranes were designed and prepared to solve some of the problems mentioned above, and the properties of these cross-linked membranes were studied and discussed deeply, it may be good for the future researches. In chapter 2, a series of cross-linked sulfonated poly(sulfide sulfone) (SPSSF) membranes have been prepared via a polyphosphoric-acid-catalyzed condensation reaction at 180 oC for a period of time (1.5 - 5.0 h) and the resulting cross-linking bonds are the highly stable sulfonyl groups. The cross-linking density could be controlled by regulating the reaction time. Cross-linking caused significant enhancement in the mechanical properties and large reduction in both water uptake and methanol permeability. The SPSSF-60 membrane (the numeral 60 refers to the degree of sulfonation), for example, had a tensile strength increased from 16 to 27 MPa (wet membranes) after cross-linking for 5 h, while the water uptake substantially decreased from 320 to 58 wt% and the methanol permeability decreased from 1.9×10-6 to 2.7×10-7 cm2 s-1 (30 oC). Single cell test on hydrogen/oxygen revealed that the cross-linked SPSSF-50 (1.5 h) membrane displayed higher open circuit voltage (OCV, 1.02 V), higher maximum output power density (1.32 W cm-2) and significantly slower OCV decay rate than the uncross-linked SPSSF-40 membrane under the same operating conditions despite their similar ion exchange capacities. The cross-linked SPSSF-60 (5.0 h) and SPSSF-50 (1.5 h) membranes showed significantly better fuel cell performance than Nafion 212.In chapter 3, two diamines contained imidazole rings were synthesized successfully, named bis(2-(4-aminophenyl))bibenzimidazole (BAPBI) and 2,2'-dibenzimidazole benzidine (DBBz). The goal structures were confirmed by 1H NMR spectra and Element Analysis. Then a series of sulfonated copolyimides containing benzimidazole groups (SPIs) were synthesized by random copolymerization of 1, 4, 5, 8-naphthalenetetracarboxylic dianhydride (NTDA), 4,4'-bis(4-aminophenoxyl)biphenyl-3,3'-disulfonic acid (BAPBDS), 9,9-bis(4-aminophenyl)fuorene(BAPF)/1,3-bis(4-aminophenoxy)benzene (BAPBz) and BAPBI/DBBz in m-cresol in the presence of benzoic acid and triethylamine at 180 oC for 20h. Membranes with good mechanical properties were prepared by solution cast method. Proton exchange treatment resulted in ionic cross-linking and the membranes were further covalently cross-linked by treating them in polyphosphoric acid (PPA) at 180 oC for 10h. The swelling of the membranes in water was decreased effectively by covalently cross-linking. The covalently cross-linked membranes displayed slightly lower ion exchange capacities (IECs) and proton conductivities than the corresponding covalently uncross-linked ones because small part of the sulfonic acid groups had been consumed during the cross-linking process. Fenton's test (3% H2O2 + 3ppmFeSO4, 80oC) revealed that benzimidazole groups played an important role in the radical oxidative stability of the membranes, while the cooperative effect of benzimidazole groups and covalent cross-linking led to much more signicant enhancements in the radical oxidative stability of the membranes than each alone. The membrane N-BAPBDS/BAPF/BAPBI (3/1/1), for example, after covalent cross-linking could maintain membrane form more than 6h measurement (τ1: 380min,τ2: 680min), which was much longer than that (4h) before covalent cross-linking under the same conditions.During experiments it could be found that the SPIs contained imidazole rings have high PPA uptake during cross-linking process, at the same time, the PPA doped membranes still have good measurement stability and mechanical properties. Therefore, PPA doped SPIs may be another way to be studied as high temperature proton exchange membranes. For example, the PPA uptake of membrane N-BAPBDS/BAPBz/DBBz (3/1/1), C10h could reach to ~1200wt%, led to high proton conductivity 0.11 S/cm at 0% RH and 170oC, and mechanical properties of the membrane (PPA uptake=~1200wt%) were acceptable even at 150 oC, Max. stress was 2.1 MPa, elongation was 22.5%.In chapter 4, series of new polybenzimidazoles (PBIs) with pendant amino groups have been synthesized via condensation polymerization of 5-aminoisophthalic acid (APTA), isophthalic acid (iPTA) and 3,3'diaminobenzidine (DAB) in polyphosphoric acid at 190 oC for 20 h. The molar ratios between APTA and iPTA were controlled at 1:0, 2:1, 1:1 and 1:2, respectively, and the copolymerization reactions were carried out via both random and sequenced manners. The resulting polymers showed good solubility in some organic solvents such as dimethylsulfoxide (DMSO) and N, N-dimethylacetamide (DMAc). The pendant amino groups of the PBIs were utilized to react with two kinds of cross-linkers, 1, 3-dibromopropane and ethylene glycol diglycidyl ether, to yield various cross-linked membranes. The cross-linked membranes generally showed good mechanical properties even at high phosphoric acid (PA) doping levels, whereas the uncross-linked membranes highly swelled or even dissolved in PA. Fenton's test revealed that the cross-linked PBI membranes had excellent radical oxidative stability. The proton conductivities of the PA-doped cross-linked membranes increased with an increase in temperature and high proton conductivity up to 0.14 S/cm at 0% relative humidity at 170 oC was achieved. The membranes with high PA-doping levels, good mechanical properties and high proton conductivities have been successfully developed.