| Summary: | 碩士 === 國立臺灣科技大學 === 化學工程系 === 94 === The main target of this study is to develop a new polymer electrolyte membrane for low temperature direct methanol fuel cell as a possible alternative to the existing state-of-the art Nafion membrane which is having drawbacks such as high cost and poor ability in prevention of methanol crossover. Because of its low cost, film-fabrication ability, good chemical stability and high selectivity of water to alcohols, Poly (vinyl alcohol) was chosen to be the matrix of proton exchange membranes in this study. 4-Formyl-1,3-Benzenedisulfonic acid disodium salt (DSDSBA) was employed as a sulfonating agent to introduce SO3−H+ moieties into the membrane to increase proton conductivity, glutaraldehyde was utilized as the cross-linking agent to form a stable network structure with an improved mechanical property. Various analytical techniques were used to characterize the structure, physico-thermal properties, proton conductivity, water uptake, free water and bound water ratio as well as methanol permeability of the developed membranes. The variation of molecular structure during the film formation was also investigated.
From the in-situ IR spectrum results, it was indicated that the strength of hydrogen bonding between water / polymer chain, water / SO3H groups, and intra-molecular / inter-molecular were enhanced gradually during film formation at the proton type SPVA membrane system. However, the variation tendency of the hydrogen bonding was opposite to that of the sodium type. Furthermore, T1ρ measurement in NMR was illustrated that the CH-OH molecular chain of SPVA membrane was more flexible than that of PVA membrane. It was evidenced from TGA and TPD/MS results that the decomposition temperature of hydroxide groups in SPVA membrane was observed about 150~200℃, and that of the sulfonated groups were about 220~380℃. Moreover, the side chain-benzenz structure of SPVA membrane and the terminal backbone of SPVA were decomposed between 390 and 480℃, as well as most of the backbone of SPVA were decomposed above 480℃.
Among the modified membranes, the SPVA2-10m membrane showed the best proton conductivity of 4.07x10−2 S/cm at 25 oC in flooded state. The lowest methanol permeability of the SPVA18.2-20m membrane was measured to be 4.09x10−7 cm2/s. It was found that a significant improvement in methanol crossover could be achieved using the modified PVA membrane. On sum up these two properties, SPVA2-20m showed the best selectivity value of 4.81×104 which is about nine times higher than that of the Nafion membrane. The relationship between the characteristics and the proton conductivity as well as the methanol permeability for the developed membranes was also discussed in this study.
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