الفهرس 
Chapter 1 : IntroductionOnly 14 pages are availabe for public view
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Abstract
Because of the growing demand for power, researchers go to great lengths to find new and useful resources. Polymer electrolyte membrane fuel cells (PEMFCs) are a common energy conversion system that can be used in a variety of applications. Because methanol is a high-energy-density fuel that is readily available, direct methanol fuel cells (DMFCs) are suitable for both fixed and portable equipment. The polymer electrolyte membrane must have a good proton conductor, a high dielectric constant, low methanol or gas permeability, a high chemical balance, and precise mechanical properties based on the fuel cell principle. Because of these limitations, only a few membrane types are acceptable for technological packages. Therefore, the main objective of this research is to characterize commercial Nafion 112 and Fumapem® under a variety of conditions, including electric field strength and relative humidity.
On Nafion 112 membranes, the influence of electric field intensities up to 140 MV/m at 90 oC was investigated. The electric field effect has no change on the chemical structure of the membranes, according to Fourier transform infrared (FTIR) spectra, however, WAXD data show that the electric field reduces the degree of crystallinity (from 39 to 30 %). It was found that, the water absorption increases from 7.4 to 15.5 %, resulting in a considerable rise in proton conductivity. It was observed that the proton conductivity was found to vary between 1.0x10-3 and 2.1x10-3 S/m depending on the electric field strength. The mechanical and thermal stability of the Nafion 112 membranes are improved by increasing the strength of the electric field. Furthermore, a strong linear relationship between Young’s modulus and crystallinity was obtained. The microstructure of Nafion 112 membranes with varying electric field intensities was also studied using PAL spectroscopy. With increasing electric field intensity from 0 to 140 MV/m, the hole volume size, which is estimated from the positron annihilation lifetime (PAL) parameter, increases from 0.135 to 0.163 nm3. Moreover, strong correlations were found between the hole volume size and both water uptake and proton conductivity, implying that free volume plays a significant role.
The effect of relative humidity (0–80%) on Fumapem® (F-950, F-1050, and F-14100, respectively, with ion exchange capacities (IECs) of 1.05, 0.95, and 0.71 meq/g) was investigated. With increasing the IEC, the chemical structure of the membranes remains unchanged, as confirmed by Fourier transform infrared (FTIR) spectroscopy. Moreover, as the IEC of the membranes increases, the concentration of water molecules and SO3--H3O+ groups increases, resulting in significant water absorption. With an increase in the IEC from 0.71 to 1.05 meq/g, the water uptake increases from 7 to 15 %, while the concentration of water molecules/SO3 group increases from 5.5 to 8. Furthermore, by increasing the membrane’s IEC, the proton conductivity is increased from 8x10-8 to 1.0x10-5 S/m. Moreover, the glass transition temperature (Tg) of the membrane rises as the IEC decreases, as evidenced by dynamic mechanical analysis (DMA). When compared to other membranes, the activation energies of decomposition temperature (Ea) deduced from thermalgravimetric analysis (TGA) data for the F-950 membrane are the lowest (Ea = 2.856 and 1.030 eV for the 1st and 2nd decomposition temperatures), implying that membranes with high IEC can be thermally unstable. The methanol permeability of the membrane increases as the IEC rises, indicating that more concentration of SO3 group prompts an increase in the penetration of the methanol through the membrane.
The ortho-positronium (o-Ps) lifetime, as determined by the PAL technique, decreases with increasing relative humidity from 0% to around 20%, while an increase in the o-Ps lifetime is clearly evident above 20% relative humidity for the three Fumapem® membranes. Based on the well-known relationship between the o-Ps lifetime and hole volume size, the average hole volume size was determined as the size of a sphere. Water molecules fill the hole volume size below 20% relative humidity, causing the free volume hole to shrink, and the plasticizing impact of water molecules causes the free volume hole to expand in the high humidity area. Furthermore, increasing water content at high relative humidity has a positive effect on the membrane’s proton conductivity. The current research demonstrates that the hole volume size and both methanol permeability and proton conductivity have good relationships. This strong correlation shows that the hole volume size influences a variety of hydrated Fumapem® membrane transport characteristics. The power density for the different Fumapem® membranes was found to be between 0.6 and 50 mW/cm2 based on the results of the single fuel cell test. The ion exchange capacity (IEC) of the Fumapem® membranes is raised dramatically, resulting in a large improvement in power density.