Abstract
The objectives of this research are the preparation and characterizations of proton exchange membrane for use in direct methanol fuel cell by utilizing Nafion® and its composite membranes. There are four parts in this dissertation. The first part of this dissertation discusses the spatially enlarged organoclays prepared by using poly(oxyproplene)-backboned quaternary ammonium salts of various molecular weights Mw. 230, 400 and 2000 as the intercalating agents for Na+- montmorillonite. The modified MMT was utilized to improve the compatibility with Nafion®. Interaction of the modified MMT with Nafion® was studied by using X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS). The performance of the Nafion®/m-MMT composite membranes for direct methanol fuel cell (DMFCs) was evaluated in terms of methanol permeability, proton conductivity, and cell performance. The proton conductivity of Nafion®/MMT-POP400 composite membrane decreased slightly by adding MMT-POP400. The methanol permeability of the composite membrane decreased dramatically with the increasing of MMT-POP-diamine content in the composite membrane. The Nafion®/5 wt% MMT-POP400 membrane showed higher selectivity (c/p ratio increased 47 %) than that of recasting Nafion®. The current densities of the composite membrane containing 0 and 5 wt % MMT-POP400 contents were 51 and 56 mA/cm2, respectively, at a potential of 0.2 V. The combination of these effects led to a significant improvement of Nafion®/5 wt% MMT-POP400 composite membranes in the performance of the DMFCs. The second part of this dissertation describes the preparation of a novel functional organoclay using POP-backboned quaternary ammonium salts that contained sulfonic acid (-SO3H) to improve the performance of Nafion® membranes used in direct methanol fuel cells. POP-backboned oligomers that bear an organic sulfonic acid intercalating agent can be used to prepare the highly ordered silicates with basal spacing of up to 18 Å via a cation exchanging mechanism. Modified layered silicate clays were cast with Nafion®. The methanol permeability of the composite membrane declined as the MMT-POPD400-PS content increased. The MMT was functionalized using organic sulfonic acid to enhance proton conductivity. The proton conductivity of the composite membrane exceeded that of pristine Nafion®. The Nafion®/MMT-POPD400-PS composite membranes exhibit a higher selectivity (c/p ratio increased 182 %) than that of pristine Nafion®, because of the increased proton conductivity and decreased methanol permeability of the composite membranes. The composite membrane with 5 wt % MMT-POPD400-PS outperformed pristine Nafion®. The current densities of composite membranes containing 0 and 5 wt % MMT-POPD400-PS, were 51 and 95 mA/cm2, respectively, at a potential of 0.2 V. The third part of this dissertation illustrates the preparation of an organic sulfonic acid (-SO3H) by grafting sulfonic acid on the surface of mesoporous silica to improve proton conductivity. The proton conductivity of the composite membrane increased from 0.10 to 0.12 (S/cm) with M-SiO2-SO3H content increasing from 0 to 3 wt %. The increase in proton conductivity may be associated with the M-SiO2-SO3H, which contains more hydrophilic regions, in the form of sulfonic acid groups, than that of pristine Nafion®. The methanol permeability of the composite membrane decreased dramatically as the M-SiO2-SO3 content in the composite membrane increased. The methanol permeability of the composite membranes that contained 3 wt % M-SiO2-SO3H was 4.5×10-6 cm2/S, which was 30 % lower than that of pristine Nafion®. The Nafion®/3 wt% M-SiO2-SO3H membrane possesses a higher selectivity (c/p ratio increased 77 %) than recast Nafion®. The current densities of the composite membranes with 0, 1, 3 and 5 wt % M-SiO2-SO3H, were 51 and 66, 80 and 70 mA/cm2, respectively, at a potential of 0.2 V. The fourth part of this dissertation discusses a novel functional poly(propylene oxide)-backboned diamine of Mw 400 (abbreviated as D400) was grafted with sulfonic acid (abbreviated as D400-PS) to improve the performance of Nafion® membranes for use in direct methanol fuel cells (DMFCs). The interaction of the D400-PS with Nafion® was studied by Fourier transform infrared spectroscopy (FT-IR) and differential scanning calorimetry (DSC). The proton conductivity of the blend membrane was slightly reduced by rendering proton conductivity to D400 by functionalizing with an organic sulfonic acid. The methanol permeability of the blend membrane decreased with the increasing of D400-PS content. The methanol permeability of the blend Nafion®/D400-PS with the composition 3/1 (-SO3H/-NH2) was 1.02□10-6 cm2/S, which was reduced 50 % compared to that of pristine Nafion®. The Nafion®/ D400-PS (5/1, -SO3H/-NH2) membrane possesses a higher selectivity (related c/p ratio increased=48 %) than that of recast Nafion®. The current densities of the Nafion®/D400-PS blend membranes in the ratio 1/0 and 5/1 (-SO3H/-NH2), were 51 and 72 mA/cm2, respectively, at a potential of 0.2 V. Consequently, the blend Nafion®/D400-PS membranes significantly improved the single-cell performance of DMFC. The composite membranes prepared in this study exhibit decreased methanol permeability; the exceeded proton conductivity compared to pristine Nafion®. The combination of these effects substantially improved the properties of the composite membranes, which are appropriate for DMFC applications.