Anion exchange membrane fuel cells (AEMFCs) that utilize anion exchange membranes (AEMs) as solid polymer electrolytes have been attracting increasing attention in the past few decades. AEMFCs not only exhibit enhanced reaction kinetics for both fuel oxidation and oxygen reduction, but also possess improved stability and durability of catalysts in alkaline media. As a key component of AEMFCs, anion conductive components, including AEMs and anion exchange ionomers (AEIs), are particularly indispensable to anion conduction in the membrane and catalyst layer, respectively. Although the anion conductive components have seen significant development and promising achievements in the past few decades, the critical issues facing anion conductive components still exist, that is, low ion...[ Read more ]

Anion exchange membrane fuel cells (AEMFCs) that utilize anion exchange membranes (AEMs) as solid polymer electrolytes have been attracting increasing attention in the past few decades. AEMFCs not only exhibit enhanced reaction kinetics for both fuel oxidation and oxygen reduction, but also possess improved stability and durability of catalysts in alkaline media. As a key component of AEMFCs, anion conductive components, including AEMs and anion exchange ionomers (AEIs), are particularly indispensable to anion conduction in the membrane and catalyst layer, respectively. Although the anion conductive components have seen significant development and promising achievements in the past few decades, the critical issues facing anion conductive components still exist, that is, low ionic conductivity, poor chemical stability and carbonation. The primary objective of this thesis is to prepare and characterize the anion conductive components that possess an improved conductivity and enhanced chemical stability. Based on the category of anion conductive components, this thesis focuses on heterogeneous polymer membranes, homogeneous polymer membranes and inorganic membranes.

The critical issue in heterogeneous polymer membranes is the progressive release of the doped alkali. Two approaches are adopted to address this issue in this thesis: the addition of inorganic additives and the fabrication of a novel membrane structure. For the former approach, layered double hydroxides (LDHs) are added into a cross-linked poly (vinyl alcohol) (PVA) structure to form uniform composite polymer membranes, which exhibit an improved ionic conductivity and alkaline stability, even at elevated operating temperatures. For the latter approach, a novel sandwich-porous polybenzimidazole (sp-PBI) is designed based on an understanding of the influence of the alkaline doping process on physicochemical properties of PBI membranes. The properties of sp-PBI membranes are demonstrated with the enhancement of retention of the doped alkali by means of both the ionic conductivity measurement 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 at 90°C, which is among the highest performance for this type of fuel cell.

Two approaches are applied in this thesis for the homogeneous polymer membranes, to address the issue of low ionic conductivity and poor chemical stability: the cross-linking method and the self-aggregating method. For the former approach, a diamine, featured with long aliphatic chains of alkyl groups and inherent diamine structures is chosen to cross-link part of the functional groups on quaternary ammonia polysulfone (QAPSF). This cross-linking reaction stabilizes the cross-linked QAPSF 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 swelling degree is largely inhibited by the cross-linked networks. For the latter approach, a hydrophobic side chain is introduced to create the microphase separation in quaternary ammonia poly (2, 6-dimethyl-1, 4-phenylene oxide) (QAPPO). These additional hydrophobic groups effectively drive the microscopic phase separation of the hydrophilic/hydrophobic domains and create nano-phase separated, well-connected ionic channels. Aggregation of the hydrophilic domains increase the local hydroxide concentration and enhance the hydroxide hopping conduction, which boosts 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 for inorganic membranes. Superfine LDH nanoparticles enable the fabrication of an integrated structure. Hydroxide ions can be transported smoothly due to LDH’s uniform and thin hexagonal-platelet morphologies, a high surface area and well-crystalized structure. The as-synthesized Mg-Al LDHs exhibit high hydroxide ion conductivity and superior stability toward anion exchange membrane water electrolysis, which is the reverse process of AEMFCs.

Keywords: Anion exchange membrane; Anion exchange ionomer; Anion exchange membrane fuel cells; Heterogeneous polymer membranes; Homogeneous polymer membranes; Layered double hydroxides