Broadband dielectric spectroscopic characterization of Nafion^® chemical degradation

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• 作者：David W. Rhoades ; Mohammad K. Hassan ; Shawn J. Osborn ; Robert B. Moore ; Kenneth A. Mauritz
• 关键词：Broadband dielectric spectroscopy ; Nafion ; Chemical degradation ; Distribution of relaxation times
• 刊名：Journal of Power Sources
• 出版年：2007
• 期刊代码：58_03787753
• 类别：ce
• 出版时间：11 October 2007
• 卷：172
• 期：1
• 页码：72-77
• 文件大小：571 K

摘要

H⁺-form Nafion 212 films were chemically degraded via Fenton's reagent and characteristic polymer chain motions were analyzed using broadband dielectric spectroscopy. The β-relaxation peak maximum frequency (f_max) shifts to higher temperatures upon degradation, reflecting slower chain motions. Perhaps this is due to an upward shift in average molecular weight caused by low molecular weight fragments being leached out of the membrane during the degradation experiment. Permittivity data for degraded and undegraded materials were fitted to the Havriliak–Negami equation. The quantities extracted from these fits were the relaxation time (τ) and parameters that reflect the breadth and asymmetry of the distribution of τ. These parameters were also used in the Vogel–Fulcher–Tammann–Hesse (VFTH) equation which was well-fitted to relaxation time versus temperature data. The increase in the Vogel temperature with degradation reflects more restricted chains. The distribution of relaxation time curve shifts to longer times, broadens, and is transformed from being bi- to tri-modal which reflects an increase in microstructural heterogeneity after degradation. It is concluded that dielectric spectroscopy can be a powerful tool in assessing the chemical degradation of Nafion membranes in the fuel cell environment.