Preparations and Characterizations of Anion Exchange Membranes and Ionomers for Anion Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) that utilize anion exchangemembranes (AEMs) as solid polymer electrolytes have been attracting increasingattention in the past few decades. AEMFCs not only exhibit enhanced reactionkinetics for both fuel oxidation and oxygen reduction, but also possess improvedstability and durability of catalysts in alkaline media. As a key component ofAEMFCs, anion conductive components, including AEMs and anion exchangeionomers (AEIs), are particularly indispensable to anion conduction in the membraneand catalyst layer, respectively. Although the anion conductive components haveseen significant development and promising achievements in the past few decades,the critical issues facing anion conductive components still exist, that is, low ionicconductivity, poor chemical stability and carbonation. The primary objective of thisthesis is to prepare and characterize the anion conductive components that possess animproved conductivity and enhanced chemical stability. Based on the category ofanion conductive components, this thesis focuses on heterogeneous polymermembranes, homogeneous polymer membranes and inorganic membranes. The critical issue in heterogeneous polymer membranes is the progressiverelease of the doped alkali. Two approaches are adopted to address this issue in thisthesis: the addition of inorganic additives and the fabrication of a novel membranestructure. For the former approach, layered double hydroxides (LDHs) are added intoa cross-linked poly (vinyl alcohol) (PVA) structure to form uniform compositepolymer membranes, which exhibit an improved ionic conductivity and alkalinestability, even at elevated operating temperatures. For the latter approach, a novelsandwich-porous polybenzimidazole (sp-PBI) is designed based on an understanding of the influence of the alkaline doping process on physicochemical properties of PBImembranes. The properties of sp-PBI membranes are demonstrated with theenhancement of retention of the doped alkali by means of both the ionic conductivitymeasurement and the real H2/O2 AMEFC test. The membrane electrode assembly(MEA) fabricated with the sp-PBI shows a peak power density of 544 mW cm-2 at90°C, which is among the highest performance for this type of fuel cell. Two approaches are applied in this thesis for the homogeneous polymermembranes, to address the issue of low ionic conductivity and poor chemicalstability: the cross-linking method and the self-aggregating method. For the formerapproach, a diamine, featured with long aliphatic chains of alkyl groups and inherentdiamine structures is chosen to cross-link part of the functional groups on quaternaryammonia polysulfone (QAPSF). This cross-linking reaction stabilizes the cross-linkedQAPSF with an ionic exchange capacity (IEC) as high as 1.62 mmol g-1.Hence, high hydroxide conductivity is gained with increased IEC, while the swellingdegree is largely inhibited by the cross-linked networks. For the latter approach, ahydrophobic side chain is introduced to create the microphase separation inquaternary ammonia poly (2, 6-dimethyl-1, 4-phenylene oxide) (QAPPO). Theseadditional hydrophobic groups effectively drive the microscopic phase separation ofthe hydrophilic/hydrophobic domains and create nano-phase separated, well-connectedionic channels. Aggregation of the hydrophilic domains increase the localhydroxide concentration and enhance the hydroxide hopping conduction, whichboosts the hydroxide ion conductivity to 65 mS cm-1 at 80°C. A two-step approach is introduced in the preparation of Mg-Al LDHs forinorganic membranes. Superfine LDH nanoparticles enable the fabrication of anintegrated structure. Hydroxide ions can be transported smoothly due to LDH'suniform and thin hexagonal-platelet morphologies, a high surface area and well-crystalizedstructure. The as-synthesized Mg-Al LDHs exhibit high hydroxide ionconductivity and superior stability toward anion exchange membrane waterelectrolysis, which is the reverse process of AEMFCs. Keywords: Anion exchange membrane; Anion exchange ionomer; Anion exchangemembrane fuel cells; Heterogeneous polymer membranes; Homogeneous polymermembranes; Layered double hydroxides.