The Preparation,Characterization And Applications Of Functional Cation Exchange Membranes

Ion exchange membranes (IEMs) exhibit various advantages during the preparation and application process, such as environmental friendly, local zero emission of harmful byproducts, and high efficiency and modularity, etc. Therefore, IEMs have attracted worldwide attention and have been widely applied in a number of separation processes as well as fuel cells (FCs) for energy conversion over the past decades. Although the application of IEMs is extending widely, many challenges such as reducing cost, durability targets, organic fouling remain prior to the further development of IEMs. Thus, there is still a great demand in designing and preparation of new materials for IEMs, because of most existing materials have side drawbacks that need innovation of new strategies. Compared with anion exchange membranes, cation exchange membranes deserve more attention because they played a central role in both membrane-based separation processes and energy storage and transformation devices. At present, the most commonly used cation exchange membranes are perfluorosulfonic acid polymers which were trademarked as NafionR. However, due to the expansive cost, high fuel diffusion and low oxidative stability of NafionR membranes, the development of alternative materials is urgent.In this dissertation, the author focused on the design and preparation of different types of cation exchange membranes and applied them into two environmental friendly IEM processes:diffusion dialysis (DD) and proton exchange membrane fuel cells (PEMFCs). The main contents can be summarized as followings:(1) Cation exchange hybrid membranes with multi-functional groups have been prepared via the blending of sulfonated poly(2,6-dimethyl-l,4-phenylene oxide)(SPPO) with poly(vinyl alcohol)(PVA), which is then crosslinked with multisilicon copolymer of poly(acrylic acid-co-y-methacryloxypropyl trimethoxy silane (poly(AA-co-y-MPS)). The membranes contain-SO3Na,-COOH and-OH groups have been applied in alkali recovery. The strong ion exchangeable-SO3Na groups are effective for the transport of Na+ions. While, the-COOH and-OH groups can accelerate the transport of OH-ions not only by enhancing the membrane hydrophilicity but also through weak interactions. Thus the multi-functional hybrid membranes showed higher dialysis coefficients for NaOH (0.0075-0.032m/h) than those of commercial membranes. Further, the incorporation of multisilicon copolymer has improved the thermal and chemical stabilities of the membrane. (2) A series of side-chain-type sulfonated poly(ether ketone/ether benzimidazole)s containing side-chain sulfonate groups and main-chain benzimidazole rings were successfully synthesized via benzimidazolization and acylation in a one-pot method. By varying the amount of sulfonate monomers, the ratio of sulfonate acid unit to benzimidazole base unit was easily controlled.’H NMR and IR spectra were used to characterize the polymer structure. The resulting membranes exhibited excellent mechanical properties (11-45MPa tensile stress and9-23%elongation at break) and good thermal stability. The proton conductivity of SPEKEBI-4was136mS cm-1at30℃, much higher than NafionR115membrane; while the methanol permeabilities of all the membranes were lower than NafionR115membrane. Moreover, the passive direct methanol fuel cells (DMFC) with SPEKEBI-2membrane exhibited a maximum power density of39.3mW cm-2at25℃, which shows great potential for application in polymer electrolyte membrane fuel cells.(3) y-(2,3-epoxypropoxy) propyltrimethoxysilane (KH-560), as a crosslinker, was incorporated in various proportions into side-chain-type sulfonated poly(ether ketone/ether benzimidazole) to make membranes with high ion exchange capacities and excellent performance for direct methanol fuel cells(DMFCs). Systematical measurements including FT-IR, SEM-EDS, XPS proved the complete disappearance of epoxy groups in KH-560and the existence of Si in the membranes. The resulting membranes showed increased mechanical strength and thermal stability than the unmodified sulfonated poly(ether ketone/ether benzimidazole) membrane in appropriate doping amount. Meanwhile, the methanol permeabilities were decreased by the cross-linking modification. In this way, the hybrid membranes showed higher selectivities than pristine SPEKEBI membrane.