Fundamental understanding of water transport and morphology is critical for improving ion conductivity in polymer electrolyte membranes (PEMs). Herein, we present comprehensive water transport measurements comparing anion-exchange membranes (AEMs) based on ammonium-functionalized poly(phenylene oxide) and cation-exchange membranes (CEMs) based on sulfonated poly(ether sulfone). We investigate the influence of counter ions, alkyl side chain, and degree of functionalization on water transport in AEMs and CEMs using pulsed-field-gradient (PFG) NMR diffusometry. Water diffusion in both AEMs and CEMs exhibit specific trends as a function of water uptake (wt%), indicating morphological similarities across common chemical structures. Furthermore, restricted diffusion reveals micron-scale heterogeneity of the hydrophilic network in both CEMs and AEMs. We propose a model wherein the hydrophilic network in these membranes has micron-scale distributions of local nm-scale dead ends, leading to changes in tortuosity as a function of water content, counterion type, and polymer structure. We furthermore parse tortuosity into two regimes, corresponding to nm-to-bulk and µm-to-bulk ranges, which reveal the importance of multi-scale morphological structures that influence bulk transport. This study provides new insights into polymer membrane morphology from nm to µm scales with the ultimate goal of controlling polymeric materials for enhanced fuel cells and other separations applications / MS / Using clean energy in place of fossil fuels to reduce carbon dioxide emissions is one of the biggest challenges of the 21st century. Among emerging technologies, fuel cells (FCs) show tremendous potential to be a candidate for the energy of the future. An FC is “an electrochemical device that directly converts chemical energy into electrical energy” with the only byproduct being heat and water. The key component of an FC is a polymer-electrolyte membrane, which helps to separate electrons and fuel and allows ions to move through. The current commercial membranes, named cation-exchange membranes (CEMs), employ precious metals such as platinum (Pt) as a catalyst, significantly increasing the cost. Anion exchange membranes (AEMs) are another alternative currently being investigated to reduce the cost of FCs because they can employ cheaper catalysts such as nickel or silver. This thesis investigated the motion of water inside AEMs and CEMs, and proposed a model to explain how water transports in these membranes. The result of this study provides new insights into polymer membrane internal structure with the ultimate goal of controlling polymeric materials for enhanced fuel cells and other separations applications such as reverse-osmosis water purification.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/88849 |
| Date | 12 October 2017 |
| Creators | Thieu, Lam Mai |
| Contributors | Chemistry, Madsen, Louis A., Deck, Paul A., Dorn, Harry C. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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