Abstract
The objectives of this dissertation are the preparation and characterizations of proton exchange membrane in fuel cell utilizing sulfonated poly(ether ether ketone) (SPEEK) and its blend membranes. A post- sulfonation method was used to enhance the proton conductivity of SPEEK. Both water uptake and solvent uptake were increasing with the increasing of the degree of sulfonation. In order to reduce the swelling of the membrane at high proton conductivity, various polymers were blended with SPEEK. There are four parts in this dissertation. The first part discusses the solubility parameter of SPEEK. SPEEK exhibited two solubility parameters, 26.4 and 35.7 J1/2 · cm-3/2, which was similar to that of Nafion®. Since Nafion® has two cohesive energy densities. The theoretical solubility parameter of SPEEK, 26.1 J1/2 · cm-3/2, has been determined using the van Krevelen’s method and was correlated to the experimental value. The theoretical volume fraction of SPEEK in the solvent was determined using the Flory’s equation. The trend of theoretical volume fraction of SPEEK was fit quite well with the experimental results when the solubility parameter of solvent was lower than 35 J1/2 · cm-3/2. The significant deviation of the experimental volume fraction of SPEEK in high solubility parameter was resulted from the presence of sulfonic acid group. SPEEK with sulfonated group in sodium form (SO3Na) exhibited the reduced solvent absorption in comparison with the one in acid form (SO3H). The second part of this dissertation describes the preparation of polymer blends of SPEEK and poly (ether sulfone) (PES). The investigation on water uptake, methanol uptake, permeability and proton conductivity has been conducted. The spin-lattice relaxation time (T1ρH) in the rotating frame of PES/SPEEK phase was obtained from the results of cross-polarization magic angle spinning (CP/MAS) solid state 13C NMR. SPEEK blended with PES result in increasing T1ρH, indicating the molecular motion of polymer chain was reduced. The glass transition temperature of the PES/SPEEK blend membranes were predicted by the Kwei equation. PES plays an important role in reducing water uptake, methanol uptake and methanol permeability while enhancing the thermal stability of the blend membrane, which shows the feasibility for direct methanol fuel cell. The third part of this dissertation was the investigation on the blend of SPEEK with nitrogen-containing polymers. The nitrogen-containing polymers used in this research were poly(vinylpyrrolidone) (PVP), poly(arylene ether benzonitrile) (BPCN), and Poly (amide imide) (PAI). The first sub-part is the SPEEK/PVP acid-base polymer blends, which was designed to reduce methanol uptake and to decrease methanol permeability while maintaining high proton conductivity. The acid-base interaction occurring on the sulfonic acid group and on the tertiary amide group was characterized by FT-IR and DMA. As the composition of PVP is lower than 20 wt. % in the blends, the acid-base interaction causes great reduction on methanol uptake and the methanol permeability, however, the proton conductivity is still high. In this work, membrane-electrode assemblies (MEAs) have been prepared for direct methanol fuel cell (DMFC) from both blend membrane and Nafion® 117. DMFC single cell performance was also evaluated. Results confirmed that SPEEK with the degree of sulfonation (DS) = 69% blended with PVP (Mn=1,300,000) at a ratio of 80/20 (w/w) exhibits higher open-circuit voltages (OCV), 0.73 V and lower polarization loss (0.15V, current density > 80 mA cm-2) than those of Nafion® 117 (0.15V, current density > 60 mA cm-2). The acid-base blend membrane will be suitable for DMFC application. The second sub-part was the SPEEK blended with poly(arylene ether benzonitrile), BPCN, which was synthesized using 2,6-dichlorobenzonitrile and biphenol. Two molecular weights of BPCN 1,641,060 and 185,976 g mole-1 were synthesized by controlling the stoichiometry of the monomers and were blended with SPEEK. The higher the molecular weight of the BPCN in the blends, the lower the degree of swelling can be obtained. At the same ion exchange capacity, the SPEEK blended with high molecular weight of BPCN resulted in the lower water uptake, low swelling, low lambda value, and low methanol permeability comparing to the one with low molecular weight. The molecular interaction between SPEEK and BPCN was studied by FT-IR. From the increasing shoulder peak of carbonyl of SPEEK and the slightly shifted peak of nitrile of BPCN, it was suggested that the molecular interaction between these two functional groups were existed. The glass transition temperature and thermal stability of SPEEK/BPCN blends was also discussed in this study. The third sub-part was the SPEEK blended with poly(amide imide), PAI, which was synthesized using 1,2,4-benzenetricarboxylic anhydride (BTBA) and 4,4’-methylenebis (phenyl isocyanate) (MBPI). SPEEK/PAI blend membranes were prepared and the properties were investigated by NMR, GPC, FT-IR and AFM. The chemical structures of PAI and SPEEK were characterized by using NMR and FT-IR. The adsorption of the SPEEK/PAI blend membrane of water or methanol solution was also characterized. The significant swelling of the blend membrane in concentrated methanol solution was explained by the solubility parameter. The water diffusion coefficient (DH2O) was related to the lambda value of the membrane. The SPEEK/PAI blend membrane had a lower proton conductivity and methanol permeability than that of Nafion® 117. Furthermore, the relative selectivity (proton conductivity divided by methanol permeability) of the SPEEK/PAI 70/30 w/w blend membrane was 3.46 x 104 S s cm-3, which is closed to that of Nafion® 117 (3.30 x 104 S s cm-3). The fourth part describes the amine salt modified colloidal silica blended with SPEEK. By means of the acid-base interaction between the modified silica and SPEEK, the swelling and methanol uptake of the membranes were reduced. Although the proton conductivity was decreased up 50 %, the 80 % reduction was found in the methanol permeability. The composite membrane with 30 % amine salt modified silica is suitable for further DMFC application.