Investigation of the structure and dynamics of Nafion 117 with multi-quantum NMR and sodium substitution

• Main Authors: Tsung-che Lee, 李宗哲
• Other Authors: Shang-Wu Ding
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/43218014368014160636

碩士 === 國立中山大學 === 化學系研究所 === 104 === To better understand the structure, morphology, degradation and proton transport of Nafion, the mostly used proton exchange membrane in direct methanol fuel cells, a series of Nafion samples sodium-substituted Nafion samples were prepared where the proton on the sulfonic group was substituted with sodium by different degrees (0 - 1). Because 23Na is a spin-3/2 quadrupolar nucleus, it experiences the electric field gradient (EFG) at the nuclear position hence offering crucial structural and dynamic information of the material. Furthermore, the electric quadrupolar interaction facilitates multi-quantum transitions which can be utilized to provide additional structural and dynamic information. Therefore, with one spin species as the NMR probe, many spectra and relaxation rates can be acquired from a single sample. The 23Na zero-quantum (ZQ), single-quantum (SQ), double-quantum (DQ) and triple quantum (TQ) filtered spectra and relaxation rates enable us to measure the full EFG tensor as well as the chemical shift tensor of sodium spin in each sample. Based on this novel technique, supplemented by proton NMR, some interesting and valuable findings have been made as summarized on fully hydrated Nafion samples: (1) The proton spectra are rather narrow, indicating that the water in the samples is like free water; however, the 23Na spectra of all orders of quantum coherences (ZQ, SQ, DQ, TQ) all show clear broadening but different from typical solid state NMR powder spectrum, a direct evidence that the polymer matrix is neither like a single crystal not like a polycrystalline or powder, but more like a liquid crystal. (2) The more hydrogen is substituted by sodium, the broader the 23Na spectra, meaning less mobility and higher overall stability of Nafion. (3) While the 1H relaxation shows unappreciable trend with respect to substitution degree, the 23Na relaxation rates increases with substitution, consistent with the fact that protons in Nafion probe all kinds of microenvironments but sodium mostly reports the information on surfaces inside the material. Therefore, 1H and 23Na can provide supplementary information on Nafion. (4) The acidity inside Nafion pores and channels decreases with sodium substitution. (5) The values of EFG and chemical shift tensors as well as the relaxation rates enable us to estimate the size and shape of the nanopores and nanochannels inside Nafion, revealing important dynamic information such as the sulfonic group rotating around its symmetric axis with the rotational rate decreasing with the increase of sodium substitution. These findings are important for further understanding proton transport, methanol crossover, the mechanical properties of Naifon and how to improve its performance.