Structure-property relationships in anion exchange membranes for electrochemical energy conversion and storage.

摘要

Polymer electrolyte membrane (PEM) fuel cells are promising candidates for powering automotive vehicles, but their advancement has been hindered by the costs associated with their platinum-based electrocatalysts. One strategy to resolve this problem is to replace the conventional acidic PEM with an alkaline anion exchange membrane (AEM) because fuel cells operated in alkaline media do not require platinum group metal catalysts. A significant challenge to realizing this concept is to design and implement an AEM that is chemically robust under alkaline conditions and that facilitates high ionic conductivity.;This dissertation presents a scientific approach to address the aforementioned problems through investigation of alternative cations, beyond quaternary trimethylammonium, to understand what chemical features influence ion conductivity and alkaline stability. It was postulated that selecting cations with larger free base conjugate pKA values (i.e., greater basicity) would yield improved AEM alkaline stability and ionic conductivity. The pKA value accounts for the steric hindrance, inductive, and resonance features of an organic cation and these features influence a cation's interaction with hydroxide. UdelRTM polysulfone (PSF) and poly(2,6-dimethyl 1,4-phenylene) oxide (PPO) were selected as the model polymer backbones because they can be tailored with different cation groups. The types of cations assessed were of the quaternary ammonium and phosphonium types and 1-methylimidazolium. The prepared AEMs demonstrated a direct correlation between the cation's free base conjugate pKA and anion conductivity for most cations assessed. Alkaline stability was assessed through multi-dimensional NMR to determine the degradation products in AEMs. NMR confirmed that the cation groups degraded through fundamentally different degradation mechanisms dependent upon their chemical make-up. Because the degradation mechanisms were different, the rate of degradation of the cation groups did not demonstrate a correlation to the cation's free base conjugate pKA. If the cations did proceed through the same degradation mechanism, then a correlation was observed. Additionally, it was discovered that the cation groups in PSF and PPO triggered polymer backbone degradation in alkaline despite the resiliency of both these pristine polymers in alkaline solutions. The AEMs prepared were successfully demonstrated in several electrochemical energy storage and conversion technologies (including alkaline fuel cell, alkaline water electrolyzer, and the all-vanadium redox flow battery).