High Performance Polymer Electrolyte Membranes: Synthesis And Properties Studies

Direct methanol fuel cell (DMFC) is a new type of polymer electrolyte membranefuel cells (PEMFCs) derived from polymer electrolyte membrane fuel cells.Compared with the hydrogen/oxygen PEMFCs, by using methanol as the fuel,DMFCs may be the first mass-produced fuel cell varieties due to the advantages ofhigh fuel energy density, rich in resources, easy for storage and transportation, simplestructure and low cost.Polymer electrolyte membranes (PEMs) can be divided into two types: protonexchange membranes and anion exchange membranes. As the key component ofDMFCs, the inadequate over-all performance of PEMs is the main problem that themass-production of DMFCs faced. Until now, the general shortages of the developedPEMs are the poor resistance of methanol and the poor dimensional stability at highionic conductivity. Even Nafion, with perfluorinated sulfonated structure, cannot meetthe requirements of DMFCs. Therefore, developing high performance PEMs forDMFCs is the key to promote the commercialization process of DMFCs.Focused on the problems existed in the PEMs, the current studies are concentratedin two aspects: one is to develop alternative proton exchange membranes to Nafion,which should be methanol resistable, dimensional stable, proton conductivitycomparable and low cost; the other one is to develop anion exchange membranes withenough alkaline tolerance and comparable ionic conductivity, which can get rid of theuse of noble metal catalyst.Poly (aryl ether ketone)s (PAEKs) and poly (aryl ether sulfone)s (PAESs) are series of semi-crystalline polymer materials, which exhibit high thermal stability, chemicalresistance and mechanical properties. The modified PAEKs and PAESs are promisingpolymer electrolyte membrane materials, because they can inherit the advantages ofPAEKs and PAESs. However, as the proton exchange membrane materials, sulfonatedpoly (aryl ether ketone)s (SPAEKs) and sulfonated poly (aryl ether sulfone)s (SPAESs)usually show their drawbacks of poor methanol resistance and dimensional stability athigh operating temperature and high degree of sulfonation, while as the anionexchange membranes, PAEKs and PAESs usually suffer from low dimensionalstability and rapid decomposition of ion-conductive groups under alkaline condition.In order to solve the problems above and obtain PEMs proper for DMFCs, this thesisdesigned targeted methods by analyzing the characteristics of the above issues.First of all, for the proton exchange membrane materials, we used cross-linking asthe modified method.We synthesized a sulfonated poly (ether ether ketone) containing dipropenyl groupson the side chain (SDPEEK). In order to minimize the effect on the protonconductivity of the composite membranes, we incoporated phosphotungstic acid(PWA) into the cross-linked network composed of SDPEEK and γ-methacryloxypropyltrimethoxy silane (KH570), which could avoid the loss ofsulfonated groups while introduce the inorganic proton acid in. The results of thescanning electron microscopy and proton conductivity indicated that the sol-gelprocess can immobile the PWA particles and make them dispersed well in the matrix.By studied the swelling ratio and the selectivity of the composite membranes, it canbe observed obviously that the cross-linking enhanced the dimensional stability andselectivity effectively. For example, the swelling ratio of the composite membranes isonly a third of that of the pristine membrane at80oC.Since it's born, Nafion is always the only choice for the proton exchange membraneof proton exchange membrane fuel cells. Despite of the good membrane properties,Nafion can also be used as the ionomer of the catalyst layer. One of the most desirableproperties of an ionomer for use in the catalyst layer is high solubility in water orlow-boiling-point water-soluble solvents such as ethanol. The study above proved that cross-linking is an effective way to enhance the dimensional stability of themembranes. Therefore, in the fourth chapter, we increased the sulfonated degree ofSDPEEK to obtain a water-soluble SDPEEK. We prepared cross-linked membraneswith comparable high cross-linking degree through click reaction, using dithiols withdifferent length aliphatic chain as cross-linkers. The introducing of the flexiblecross-linkers improved the mechanical properties of the cross-linked membranes.Excessive cross-linking of SDPEEK-nH led to the lake of proton carriers by theinadequate absorbed water while excessively low cross-linking of SDPEEK-nS led tothe loss of mechanical properties in water. Thus, both of them showed lower protonconductivity than SDPEEK-nE with a proper cross-linking degree. As a result, thecross-linked membranes, using dithiols as cross-linkers, exhibited comparableproperties with Nafion. For example, at25oC, the proton conductivity were all above0.07S cm-1(Nafion117:0.076S cm-1) while the methanol permeability were all under4.13×10-7cm2s1(Nafion117:2.38×10-6cm2s1). What's more, the swelling ratioswere all less than10%at25oC which showed excellent dimensional stability of themembranes.Second, for anion exchange membrane materials, we prepared novel poly (arylerther sulfone)s based on the molecular design.We synthesized poly (aryl ether sulfone)s containing dipropenyl groups on the sidechain. We attached bromine atoms onto the benzyl groups using N-bromosuccinimideas the bromination agent, which provided enough activity both for the aromaticbenzene ring to attack the bromomethyl group in a Friedel–Crafts electrophilicsubstitution C-alkylation reaction and for the quantitatively convertion into thequaternary ammonium hydroxide moieties. Together with the formation of thecross-linked network, the formation of the conjugated πbond formed by phenyl,propenyl groups and-SO2-groups, preventing the occurrence of Hofmann eliminationmechanism, thereby assured the excellent alkaline tolerance and methanolpermeability. The self-cross-linked membranes showed the highest ionic conductivityand the lowest methanol permeability compared with the membranes reported in thereferences with similar methanol permeability and similar ionic conductivity, respectively.As well known, quaternary phosphonium hydroxide moieties are more stable thanquaternary ammonium hydroxide moieties under alkaline environment. Thus, in thesixth chapter, we prepared anion exchange membranes based on poly (aryl ethersulfone) containing quaternary phosphonium groups (QPPES). QPPESs showedexcellent alkaline tolerance: there were no weight losses observed in the QPPESs afterimmersing in30%KOH solution for a week, which can be attributed to theadvantageous stability of quaternary phosphonium groups and the chemical resistanceof the poly (aryl ether sulfone) backbones. The chemical structure of QPPES issimilar with that of SPES-nOH. The difference between them is the ion conductivegroups they used (quaternary phosphonium hydroxide moieties for QPPESs andquaternary ammonium hydroxide moieties for SPES-nOH). Although theion-conductive ability of quaternary phosphonium hydroxide moieties is a littleweaker than that of quaternary ammonium hydroxide moieties, QPPES-6showshigher ionic conductivity than SPES-2OH (0.0667S cm-1for QPPES-6,0.0455Scm-1for SPES-2OH).This phenomenon may caused by the formation of theself-cross-linked net work which consumed part of the ion-conductive groups inSPES-nOH membranes while limited the mobility of the polymer chain.