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Photoinitiated “Bottom-Up” Click Synthesis of Ion-Containing Networks as
Hydroxide Exchange Membranes
Andrew C. Tibbits
Advisors: Dr. Christopher Kloxin and Dr. Yushan Yan
Committee: Dr. Norman Wagner, Dr. Bingjun Xu, and Dr. Darrin Pochan
Fuel cells are energy conversion devices which directly convert chemical energy into
electrical energy and environmentally friendly byproducts (i.e., water) with potential versatility
for transportation and portable applications.
Hydroxide exchange membrane fuel cells
(HEMFCs) have the potential to decrease the overall fuel cell cost through the utilization of nonprecious metal catalysts such as nickel and silver as opposed to platinum which is used by the
current standard technology, proton exchange membrane fuel cells (PEMFCs).
However,
substantial improvements in thermal and alkaline stability, hydroxide conductivity, mechanical
flexibility, and processing are needed to create a competitive membrane for HEMFC
applications. Regardless of the type of membrane, the high water uptake that is typically
associated with increased ionic conductivity is problematic and can result in the dissolution of
the membrane during fuel cell operation. Covalent crosslinking of the membrane is an approach
which has been effectively applied to reduce water uptake without a significant compromise of
the hydroxide conductivity.
The synthesis and processing of membrane materials is vastly simplified by using click
polymerization schemes. Click chemistry is a collection of organic chemical reactions that are
rapid, selective, and high yielding. One of the most versatile and facile click reactions is the
thiol-ene reaction, which is the radical-mediated addition reaction between a thiol (an –SH
group) and an ‘ene’ (an electron rich vinyl group, C=C) in the presence of a photoinitiator and
light. The click attributes of the thiol-ene reaction enable potential of “bottom-up” design of ion-
containing polymers via photoinitiated crosslinking reaction in a single step with precise control
over structure and physicochemical properties not only for fuel cell membranes but also for a
range of other applications including separation membranes, sensors, flexible electronics, and
coatings.
However, a fundamental understanding of the formation and properties of ion-
containing thiol-ene materials and their implementation as hydroxide exchange membranes is
largely absent from the current literature.
The work described herein will highlight the versatility of click reactions, primarily the
thiol-ene reaction, for fabrication of ion-containing networks with tunable properties based on
the rational design and synthesis of photopolymerizable ionic liquid comonomers with an
emphasis on applicability for HEMFC applications. The role of ionic liquid monomer structure
on the kinetics and mechanism of thiol-ene ionic network formation and the subsequent
properties (i.e., ion conductive, thermomechanical, and structural) will be elucidated to establish
a guided framework for click ionic material development. This framework will be directed onto
the development of alkaline stable hydroxide-conductive membranes for fuel cell applications as
well as the incorporation of catalytic nanoparticles into a photocrosslinkable formulation as a
self-standing catalyst layer.
Finally, novel approaches to membrane fabrication will be
implemented to build on the foundational studies that will simultaneously enhance the ionic
conductivity and mechanical properties of the ion-containing polymer materials: these
approaches include the synthesis and crosslinking of photopolymerizable cationic surfactants for
microphase separated membranes as well as the first “bottom-up” ion-containing polymer
synthesized from the photoinitiated copper-catalyzed azide-alkyne cycloaddition (photoCuAAC) reaction which exhibits enhanced processability and hydroxide conductivity (>50
mS/cm).