THESIS
2014
xxxiv, 246 pages : illustrations ; 30 cm
Abstract
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...[
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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 H
2/O
2 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
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