Preparation And Properties Of New Imidazolium Type Alkaline Anion Exchange Membranes

Recently alkaline anion exchange membrane fuel cells (AAEMFCs) have attracted much attention because they possess the advantages of conventional alkaline fuel cells (AFCs) and proton exchange membrane fuel cells (PEMFCs). In an AAEMFC system, a nonprecious metal may be used as the catalyst leading to drastic reduction in fuel cell cost because both the oxidation of a fuel and the reduction of oxygen are much easier in basic medium than in acidic medium. The problems associated with conventional AFCs such as carbonate salt precipitation (formed due to the reaction of alkali electrolyte with carbon dioxide in air) and electrolyte leakages are also resolved by using polymer anion exchange membranes to replace alkali electrolyte.Being one of the core components of an AAEMFCs, an anion exchange membrane (AEM) functions as the cathode/anode separator, hydroxide anion conductor and electronic insulator. From viewpoint of practical applications, it is highly desirable to develop AEMs with low cost, high hydroxide conductivity, low swelling ratio, excellent chemical stability and superior mechanical properties. In this thesis, various imidazolium-type AEMs are synthesized and their properties are studied. The main research contents are as follows:In chapter 3, first poly(1-vinylimidazole) (PVI) was prepared through radical polymerization of 1-vinylimidazole. Then, the PVI was quaternized with 1-bromohexane to yield poly(1-vinyl-3-hexylimidazolium bromide) (QPVI-Hx) with varied degree of quaternization (DQ). A poly(ether benzimidazole) (OBPI) was also synthesized via condensation polymerization of 4,4'-dicarboxyldiphenyl ether and 3,3'-diaminobenzidine in Eaton's reagent at 140 ?. It was found that the QPVI-Hx exhibited extremely poor film-forming ability. However, blending the QPVI-Hx with the OPBI in combination of covalent cross-linking using 1,6-hexamethylene dibromide as the cross-linker gave free-standing and ductile membranes. The membranes are transparent at low weight fraction of OPBI (?40%). The membranes with higher content of OPBI tend to have higher mechanical strength, lower swelling and lower anion conductivity. The highest hydroxide conductivity (16.7 mS cm-1,80?, in water) was achieved with the membrane OPBI/QPVI-0.1/0.9 due to its highest IEC. This membrane also exhibits good mechanical properties (tensile strength:33 MPa, elongation at break:199%) and moderate swelling ratio (in-plane:13.2%, through-plane:10.4%,60?).In chapter 4, the PVI was quaternized with benzyl bromide to yield poly (1-vinyl-3-benzylimidazolium bromide) (QPVI-Bz) with varied DQ. A polybenzimidazole with pendant amino groups (H2N-PBI) was also synthesized through condensation polymerization of 5-aminoisophthalic acid and 3,3'-diaminobenzidine in polyphosphoric acid at 190 ?. A series of cross-linked blend membranes were prepared from the QPVI-Bz and the H2N-PBI using 1,6-dibromohexane and bisphenol A type epoxy resin as the cross-linking agents. By changing the content of the QPVI-Bz, membranes with different IEC values were obtained. The incorporation of H2N-PBI significantly improved the mechanical properties of membrane due to the reinforce effect as well as the covalent cross-linking. All the prepared membranes are transparent even at high weight fraction of H2N-PBI (H2N-PBI/QPVI-0.5/0.5) indicating good miscibility of the two polymers. The highest hydroxide conductivity (23 mS cm-1,80?, in water) was achieved with the membrane H2N-PBI/QPVI-0.1/0.9 due to its highest IEC. The membrane H2N-PBI/QPVI-0.2/0.8 exhibits the best balanced performances, i.e.,it has a high tensile strength (33 MPa), a low swelling ratio (?8%,60?) and a high hydroxide conductivity (15.1 mS cm-1,80 ?, in water).