Understanding Anion, Water, and Methanol Transport in a Polyethylene-b-poly(vinylbenzyl trimethylammonium) Copolymer Anion-Exchange Membrane for Electrochemical Applications

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• 作者：Himanshu N. Sarode ; Yuan Yang ; Andrew R. Motz ; Yifan Li ; Daniel M. Knauss ; Soenke Seifert ; Andrew M. Herring
• 刊名：Journal of Physical Chemistry C
• 出版年：2017
• 出版时间：February 2, 2017
• 年：2017
• 卷：121
• 期：4
• 页码：2035-2045
• 全文大小：613K
• ISSN：1932-7455

文摘

Herein, we report the anion and water transport properties of an anion-exchange membrane (AEM) comprising a block copolymer of polyethylene and poly(vinylbenzyl trimethylammonium) (PE-b-PVBTMA) with an ion-exchange capacity (IEC) of 1.08 mequiv/g. The conductivity varied little among the anions CO₃^2–, HCO₃^–, and F^–, with a value of E_a ≈ 20 kJ/mol and a maximum fluoride conductivity of 34 mS/cm at 90 °C and 95% relative humidity. The Br^– conductivity showed a transition at 60 °C. Pulsed gradient stimulated spin echo nuclear magnetic resonance (PGSE NMR) experiments showed that water diffusion in this AEM is heterogeneous and is affected by the anion present, being fastest in the presence of F^–. We determined the methanol self-diffusion in this membrane and observed that it is lower than that in Nafion 117, because of the lower water uptake. This article reports the first measurements of ¹³C-labeled bicarbonate self-diffusion in an AEM using PGSE NMR spectrometry, which was found to be significantly slower than F^– self-diffusion. Back-calculation of the bicarbonate conductivity using the Nernst–Einstein equation gave a value that was significantly lower than the measured value, implying that bicarbonate transport involves OH^– in the transport mechanism. Fourier transform infrared spectroscopy, PGSE NMR spectrometry, and small-angle X-ray scattering (SAXS) indicated the presence of different types of waters present in the membrane at different length scales. The SAXS data indicated that there is a water-rich region within the hydrophilic domains of the polymer that has a temperature dependence in intensity at 95% relative humidity (RH).