| Novel alternative clean energy sources are desired consideration of the shortage of fossil fuels as well as protection of the environment. Fuel cell, as a kind of energy device, is a good choice to solve the problems. There are many advantages for fuel cells including high efficiency, low pollution, more selective fuel sources and possibilities to use abundant non-Pt electrode catalysts. Therefore great efforts have been made to develop various fuel cells. Among different types of fuel cells, the operating circumstance of the anion exchange membrane (AEM) fuel cell is alkalescence because of the hydroxide ions conducting in the AEM. Therefore the working principle of the AEM based cell is different from that of the proton exchange membrane (PEM) fuel cell. Unlike the PEM fuel cells, the moving ions in the AEM electrolyte are hydroxide ions (OH"). AEM fuel cells have some advantages over PEM fuel cells due to the alkaline operation medium. For the cells with methanol as the fuel, the methanol oxidation rate is faster in alkali than that in acid. The immigration direction of the anion OH" opposes that of the methanol flux through the membrane, which leading to an intrinsic reduction in methanol permeability. From this point of view, the AEM-based direct methanol fuel cells (DMFCs) are superior than PEM based DMFCs. Moreover, using solid phase AEMs instead of the liquid alkali electrolyte could avoid the leakage of the alkaline solutions and thus an easy integration of the fuel cell system.As mentioned above, the performance of AEM is vital to AEM direct methanol fuel cells and AEM alkaline fuel. The generally used AEMs include quaternary ammonium, quaternary phosphonium, and tertiary sulfonium. The quaternary ammonium is among the widely used functional groups for anionic conduction due to such as easy quaternization for any practicable polymers. However one problem to quaternized electrolytes is that the contradictory effects of the degree of quaternization (DQ) ammonium on conductivity, tensile stress and methanol permeability. The higher DQ may result in the higher ion exchange capacity (IEC) and higher conductivity but worse tensile stress and methanol permeability. Contrarily, the lower DQ brings the AEM lower IEC and the worse conductivity while maintaining the good tensile stress and lower methanol permeability. Therefore, improvements on the AEMs are needed to make them more suitable for application in the fuel cells.In this thesis, a series of novel AEMs was fabricated and modified based on chitosan (CS) and polyvinyl alcohol (PVA).(2,3-epoxypropyl) trimethylammonium chloride (EPTMAC) was used as etherifying agent to introduce quaternary ammonium groups onto the structure of CS and PVA through electrophilic reactions, respectively. The produced quaternized chitosan (QCS) and quaternized PVA (QPVA) with different DQ were used for membrane preparation. The physicochemical properties relating to application in fuel cells were investigated for the prepared AEMs.This thesis contains seven chapters. Chapter1is an overview of the development, the categories and the technical modifications for the AEMs. Chapter2focuses on the apparatus, chemical reagent and experiment method etc.In chapter3, the experimental conditions of synthesis QCS were investigated. In order to solve the problems of worse tensile stress and large water swelling result from the higher DQ, a series of AEMs with different crosslinking degree were prepared based on the QCS (DQ=81.10±3.20%) using glutaraldehyde (GA) as the crosslinker. Characterizations of the membranes were made. When the GA content increased from0.04to0.27wt%, the tensile stress at break of the membrane was increased from8.95MPa to14.43MPa, the methanol permeability was decreased from7.70×10-5cm2S-1to8.50×10-6cm2S-1, respectively.Chapter4and5introduced the applications of semi-and full-interpenetrating network (IPN) technologies in modification of the AEMs, respectively. During the preparation of the QCS based AEMs, both the quaternization and crosslinking took place on the ammonium groups of the chitosan backbone. Therefore the QCS with a high DQ is normally needed in order to have a high conductivity. Nevertheless a poor mechanical property of the membrane may resulte from the higher DQ. Thus AEMs with IPN polymer networks were prepared to improve the tensile stress of the membrane consisting of QCS with high DQ. In this two chapters the AEMs with semi-interpenetrating network (semi-IPN-X, X represent the mass percent of PS in the membrane) composed of QCS and PS and full-interpenetrating network (QCSx-(PAM/PS)y,x, y represent the mass content of QCS and GA in membrane, respectively) composed of QCS and PAM/PS were prepared, respectively. The associated processes were performed using potassium persulfate (KPS) as an initiator and GA as a crosslinker. The DQ of QCS was75.10±2.70%for semi-IPN-X, and49.80±3.50%for QCSx-(PAM/PS)y, respectively. Compared with the pure crosslinked QCS membrane (semi-IPN-0), the stability in tensile stress of semi-IPN-21(PS=21wt%) in10mol L-1KOH solutions was increased significantly. However the conductivity at75℃was decreased from0.055S cm-1to0.027S cm-1. The similar results was also obtained to the (QCSx-(PAM/PS)y) membrane. Compared with QCS100-(PAM/PS)0membrane (pure crosslinked QCS membrane), the stability in tensile stress of QCS60-(PAM/PS)0.2in1mol L-1KOH solution was increased from30.1MPa to43.9MPa and25h to150h, respectively. However the stability in conductivity at80℃was decreased from0.0147S cm-1to0.0060S cm-1. It is well known that the conductivity of the fuel cell based on AEM is inferior to that based on PEM. One of the reasons for this is that the inherent mobility of OH-ions is lower than that of protons. Moreover, the IEC and conductivity of the AEM, which is based on quaternary ammonium, is mainly determined by the DQ of the polymer. Therefore the improvement on the conductivity of the AEMs is a challenge. In chapter6, we prepared a novel AEM by embedding positively charged PS particle fillers into QCS (DQ=35.02±2.70%) casting solution. The positive charges carried by the PS latex were supposed to benefit the alkaline doping and thus high anionic conductivity. Moreover, the presence of the pristine hydrophobic PS was expected to increase the resistance of the methanol permeation through the AEM. During the experiment the PS was synthesized by emulsion polymerizing styrene monomers containing a surfactant of cetyltrimethyl ammonium bromide (CTAB) and an initiator of KPS. The characterization results were indicated that the positive charges carried by the PS facilitated the composite membrane to have a high IEC without increasing the methanol permeation. The conductivities of the AEMs at a level of10-2S cm-1were achieved at80℃. The methanol permeability of the AEMs was in a range of2.70×10-6-3.30×10-7cm2s-1. The obtained AEMs were stable up to200℃in air atmosphere according to the data of thermal gravimetric analysis. The tensile stress at break of the AEMs was in a range of16.9024.90MPa under ambient atmosphere, and12.20-13.40MPa of which were retained after immersing the AEM in a10mol L-1KOH solution at room temperature for96h. It is well known that incorporating inorganic fillers into polymer membranes can alter and improve the physical and chemical properties of the polymer (such as thermal stability, mechanical strength and water retention) while retaining its important properties to enable operation in the fuel cell. Therefore two series of organic-inorganic composite membranes of QPVA (DQ=69.6%) and inorganic materials hydroxyapatite (HA), and of QCS (DQ=81.10±3.20%) and inorganic materials SiO2are fabricated and characterized in chapter7, respectively. Compared with the pure polymer membrane with the same DQ, the composite membranes QPVA/HA and QCS/SiO2exhibited high tensile stresses of60.10MPa and27.56MPa, and low methanol permeability of1.06×10"6cm2S-1and1.55×10-6cm2S-1, respectively. It is worthwhile to mention that the tensile stress and methanol permeability for pure QPVA is47.90MPa and6.93×10-6cm2S-1, respectively. And for pure QCS membrane is15.78MPa and7.55×10-6cm2S-1respectively. |
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