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Effect of bulky cations on the function and stability of anion exchange membranes
Liu, Ye
Liu, Ye
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2016
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A major advantage of alkaline fuel cells (AFCs), relative to well-established acidic fuel cells, is their enhanced reaction kinetics permitting the use of less costly, non-noble-metal catalysts. Developing anion exchange membranes (AEMs) for AFCs that are stable under high pH operating conditions is a significant challenge. Insufficient stability of the widely used quaternary ammonium (QA) cation is the main reason attenuate membranes lifetime. Ion conduction in AEMs is intrinsically slower than in proton exchange membranes is another factor impedes AEMs wide-scale application. Therefore, developing bulky cations with enhanced electronegativity and steric hindrance, and understanding the water/ion transport of the bulky cation functionalized AEMs were the focus of this thesis. In this work, three bulky cations attached to random copolymers were investigated in regard to OH- stability, water absorption, morphology, ion conduction, and water transport. A study of 1,4,5-trimethyl-2-(2,4,6-trimethoxyphenyl) imidazolium functionalized polyphenylene oxide (PPO) indicated the attachment of 2,4,6-trimethoxyphenyl groups to unmodified imidazolium improved OH- stability due to electron donating and bulk steric effect. Membranes with two ion exchange capacities (IECs) were studied showing high IEC sample (IEC=1.8 mmol/g) had a higher conductivity (23 mS/cm at 95% RH and 90°C) via increasing anion concentration. Different IECs also gave rise to membranes with different morphology and transport behavior. Study on phosphonium cation attached by three hydrophobic 2,4,6-trimethoxyphenyl groups suggested a method to enhance chemical stability as well as reduce water uptake. In this section, different solvents selected by Hansen solubility parameter were used to maximize membrane phase separation. As expected, the optimal mixed solvent (DMSO: ethyl lactate by 41:59 vol%) cast membrane showed highest conductivity, and transports among membranes cast from different solvents are varied. Cobaltocenium, with an 18 electron inert structure had highest chemical stability. By studying random and crosslinked membranes, we have obtained in-depth understanding of polymer configuration influence on membrane property. Crosslinked network impeded ion transport resulting in a smaller conductivity than random polymer. Also crosslinked membrane followed a water mediate transport instead of polymer chain movement dominated transport due to the crosslinked network hindered polymer chains movement.
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