A study of ion transport in anion exchange membranes

To achieve net-zero energy-related and industrial CO¬2 emissions by 2050, the International Energy Agency has declared that the share of renewables in the energy supply must increase from 20% in 2020 to 80% in 2050. A hydrogen-based economy is part of a solution to reduce CO2 emissions through water electrolysis and fuel cells. The use of an anion exchange membrane enables the utilization of a non-platinum group catalyst, reducing the commercialization cost of the electrochemical cell. However, a significant concern of using anion exchange membranes is the carbonation reaction of OH- with atmospheric CO2, converting OH- to larger and less mobile anions like HCO3- and CO32-. This thesis analyzes the conductivity of the anion exchange membrane to explore the physicochemical properties of the membrane in various ion forms (e.g., Cl-, HCO3-, CO32-, and OH-) with or without reinforcement composite. The conductivity was measured using linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) techniques in both the in-plane and through-plane directions. The water sorption properties of the membrane were characterized by dynamic vapour sorption (DVS). Additionally, the ion transport in the membrane is visualized using a pH indicator-doped membrane to investigate the effects of the carbonation reaction. The change of pH in the membrane is determined using UV-vis spectroscopy. As a result of water electrolysis, OH- is generated by electrolytically splitting water. The electrolytically generated OH- flushes out the HCO3- initially present in the membrane. A blueshift was observed in the UV-vis absorption spectrum of the indicator-doped membrane. Additionally, the pH range at which the indicator-doped membrane changes colour is lower than that of the indicator in an aqueous solution. This difference could be explained as a pKa change of the indicator in the membrane or an increased ion concentration in the ion-conducting domains compared to the bulk solution. Collectively, the work presented in this thesis suggests that the reinforcement substrate increased the through-plane conductivity but not the in-plane conductivity, and the conductivity in its pure OH form achieved by electrolytically generating OH- via water electrolysis is 400% times higher than its HCO3- form.