| As electrical energy can be obtained direcly by fuel cell, proton exchangemembrane(PEM) which is the key component of fuel cell has been studied and attracted moreand more attention. PEM plays the roles of transferring of protons and separating electrons.As PEMs are the heart part of fuel cells, basic requirements are high proton conductivity andlong time span. And also it requires good mechanical properties and low methanolpermeability when applied methanol as the fuel. Right now, the most famous commercialavailable PEM is Nafion. However, the high cost and the high methanol permeability limittheir large scale application at industrial and civil areas.As the most promising candidates, sulfonated aromatic polymers(PAEKs) have beenstudied for years. After being sulfonated, the PAEKs show high proton conductivities pluslower methonal permeabilities, no need to mention their excellent thermal and mechanicalstability. Now, the main-chain type sulfonated aromatic polymer is the most wildly studiedpolymer. But, there are lots of drawbacks of this kind of polymer. The main-chain typepolymer poseeses less hydrophilic/hydrophobic phase separation than the side-chain type one,which is unlike Nafion. When it comes to lower IEC, sulfonated acid groups in polymerhighly distributed which could not form “cluster” are not connected to each other, and turnout dead-ends. Generally speaking, researchers often prefer higher IEC polymers whichachieve higher proton conductivity. But in that case, the whole membrane suffers fromdimensional deformation(Some kinds of polymer are even dissolve in hot water). At the sametime, the mechanical properties are really poor. So, it is a big challenge for the researchers todevleope a new polymer with special structure, in which case the membrane would formphase separation just like Nafion. At the same time, it would be better for the membrane to possess high proton conductivity and keep good dimensional stability.The reason why Nafion possesses such high proton conductivity is that the sulfonatedgroups are located at the end of side chain which is more flexible and higher mobility. Andalso F which is a high nucleophilic element makes the sulfonated group more acidic. So, thekey to design a structure like Nafion would be grafting side chains onto the polymer bone. Upto now, there are two methods to get sulfonated aromatic polymer: First, synthesis sulfonatedmonomer and then the polymer is synthesized by nucleophilic substitution reaction: thesecond would be sulfonating the aromatic polymer after it has been polymerized. There areboth advantages and disadvantages for those methods. The objective of this study was todevelop and evaluate new type structure, performance, Nafion-like proton exchangemembranes, expecting to be operating in proton exchange membrane fuel cell.Firstly, with the goal of obtaing a novel side chain type sulfonic aromatic polymer,two new monomers have been synthesized:1,5-bis(4-fluorobenzoyl)-2,6-dimethoxynaphthalene (DMNF) and (4-methoxy)phenylhydroquinone (MHQ). And then, DMNF and MHQ were used to synthesize thepoly(arylene ether ketone) containing three methoxy groups(TMQNPAEK) per unit. Theconversion of the methoxyphenyl groups in TMQNPAEK to the phenolic hydroxyl groupswas accomplished via the demethylation reaction using BBr3as the reducing agent. Then,SQNPAEK-x with different butylsulfonate contents were synthesized through thenucleophilic ring-opening reaction by adjusting the amount of1,4-butanesultone, thetemperature and the reation time. The structures of all the monomers and all the polymershave been confirmed both by1H NMR and13C NMR spectra. We have characterized all theproperties of this series of SQNPAEK-x membrane, including thermal stability, mechanicalproperties, dimensional stability and methanol permeability ect. All the results showed thenaphthalene-based poly(acrylenen ether keone) possessed excellent performance. Mostimportant of all, this side-chain type membrane achieved high level proton conductivity,much higher than Nafion117at the same testing condition. At the same time, we alsoperformed TEM test. It clearly showed that this new type polymer came with the samestructure as Nafion. As the dimensional stability and the chemical stability would not meet therequirements of PEM, we need to modify this series of membrane. Crosslink would be a good solution to increase the chemical stability and decrease the swelling ratio. So, in next chapter,we synthesized (3,3’,5,5’-tetramethylbiphenyl) epoxy resin (TMBP) as a small molecularcrosslinker. By the reaction between epoxy group and hydroxyl group at190oC,SQNPAEK/TMBP-xx crosslinked membranes were obtained. As expected, the water uptakeand methanol permeabilities of SQNPAEK/TMBP-xx membranes decreased with increasingthe content of TMBP. Compared to Nafon117, the cross-linked membranes showedcomparable proton conductivities and much higher selectivities, which were in the range of5.18×105~1.46×106S s-1cm3. Other properties of the cross-linked membranes, such asmechanical properties, thermal properties were also investigated. TEM also showed that therewas obvious phase separation even it came to SQNPAEK/TMBP-15. All the results indicatedthat the cross-linked membranes based on SQNPAEKs and TMBP were promising candidatefor direct methanol fuel cells.In chapter five, still keeping the goal of seeking high overall performance of PEM,novel self-crosslinkable pendent poly(arylene ether ketone) SQNPAEK-AC-x for protonexchange membranes with high proton conductivity, excellent dimensional stability andextraordinary methanol resistance were synthesized successfully. The chemical structures ofthe self-crosslinkable polymers were analyzed by1H NMR. In this series crosslinkedmembrane, there is no need to add any catalyst or a separated cross-linker. The cross-linkedmembranes SC-SQNPAEK-x were obtained by thermal curing of the cross-linkable polymers.The water uptake and swelling ratio of the crosslinked membrane were greatly suppressed.And at the same time it still maintained relatively high proton conductivity. For instance, theSQNPAEK-2.5membrane had a conductivity of0.317S cm-1at80oC. After crosslinked, theconductivity of SC-SQNPAEK-2.5was0.220S cm-1. Although, the proton conductivitydecreased with increasing cross-linking moiety, the methanol permeability growth range waslarger than the conductivity. In this way, the crosslinked membranes can hold highconductivity at the same time with low methanol permeability. The superior performance ofthese cross-linked membranes demonstrated the potential application of these materials aselectrolytes for PEM fuel cells.In chapter six, sulfonated poly(arylene ether)(SPAEK-COOH-60) pendent carboxylicacid which acted as macrocrosslinker was synthesized. SQNPAEK/COOH-xs were obtained by Friedel-Craft reaction at160oC. The reaction happened between carboxylic acid groupand the nucleophilic phenyl rings in the polymer bones. After being cross-linked, wateruptake and swelling ratio of cross-linked membrane decreased from57.3%to18.7%, andfrom12.2%to3.03%, respectively. Furthermore, with the increase of crosslinking extent, themethanol permeabilities coefficient of the membranes gradually decreased from0.94×10-7cm2s-1to0.37×107cm2s-1. Without sacrificing proton conductivity, the crosslinkedmembranes show enhanced performance, higher than the proton conductivity of originalmembrane. The cross-linked membrane also exhibited enhanced chemical resistance andoxidative stability based on Fenton`s tests. No need to mention that after crosslinking, themechanical properties of the membranes also improved quite a lot. Thus, cross-linkedSQNPAEK/COOH-x membranes possessed the high proton conductivity and lower methanolpermeability methanol fuel cells implying its potential practical use in high-energy-densitydevices. |
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