A R T I C L E S
Zhou et al.
and processing costs,7 diminished proton conductivity under
conditions of low water availability,8-10 and high fuel crossover
when used in direct methanol fuel cells (DMFC).11 Significant
efforts have been devoted worldwide to develop high perfor-
mance and reliable membranes. Some of the polymer electro-
lytes investigated to date include sulfonated poly(arylene
ether)s,12 graft13 or block copolymers14 based on sulfonated
polystyrene (PS), acid-doped polybenzimidazole (PBI),15 sul-
fonated poly(imides),16 and polyphosphazenes.17 Each of these
materials has their own advantages and disadvantages, and
Nafion still stands as the state-of-the-art PEM material.
Similar to Nafion, most of the conventional PEM materials
are processed into a membrane form by melt extrusion or solvent
casting and employed as preformed membranes in membrane
electrode assembly (MEA) fabrication. The requirement of using
preformed membranes may restrict new fuel cell designs and
developments. Holdcroft and co-workers18 proposed the concept
of making PEMs from curable liquid precursors and pointed
out that such a liquid-to-PEM approach may enable the
formation of PEMs conformable by injection molding, formed
as microchannels and unique shapes, or strongly adhering to
the catalyst layer without hot pressing. They dissolved a
preformed linear proton conducting polymer, sulfonated poly-
(ether ether ketone) (SPEEK), in a mixture of vinyl monomer
and cross-linking agent and polymerized this composition to
form a semi-interpenetrating network in which SPEEK stayed
as a guest polymer in a statistically cross-linked host polymer
matrix. By doing so, they demonstrated the fabrication of 1-mm
features using photolithographic techniques. However, such large
features are essentially no use for many fuel cell developments,
and the guest SPEEK is not chemically attached to the network.
In this work, we present a new strategy for making highly
proton conductive PEMs from easily processable, 100% curable,
low molecular weight reactive liquid precursors to solid
membranes. Highly fluorinated liquid precursors based on
styrenically functionalized reactive perfluoropolyethers (sPFPE)
were used in conjunction with a fluorinated derivative of
sulfonated styrenic (SS) monomers. Chemically cross-linked
PEMs with desired shape and thickness can be easily prepared
from the liquid precursors, and no further processing steps are
needed. By using imprint lithography/micromolding tech-
niques,19,20 three-dimensional, patterned PEMs with micron-
sized features can be easily fabricated with high fidelity. Up to
now, Nafion and other conventional membranes are flat and
smooth at the surface and therefore lacking enhanced perfor-
mance in such surface area-dependent catalytic electrochemical
cells. Patterned membranes can provide larger interfacial area
between the membrane and catalyst layer. In addition to other
issues, such as transport phenomenon, an increase in active
surface area without an increase in the geometric volume of
the MEA should result in higher power densities, which can
lead to the miniaturization of fuel cells.
Moreover, to achieve good proton conductivity, especially
at low relative humidity (RH) conditions, PEMs with high acid-
loading are highly desirable. For linear polymers, however, high
acid loading does yield better conductivities, but also leads to
compromised mechanical strength and swelling effects. Indeed
at sufficiently high ion exchange capacity (IEC), acid-containing
linear polymers can even become water-soluble.21 To achieve
high proton conductivity without the challenges associated with
linear polymers, chemically cross-linkable ionomeric systems
were proposed, and several systems were studied. Important
cross-linking methods included electron beam22 or γ-irradiation23
of preformed membranes and cross-linking through inter/
intrachain bridging links24 through the sulfonic acid groups.
However, irradiation cross-linking can result in PEMs with
nonuniform properties, and the consumption of sulfonic acid
groups results in a decrease in ion content of the membrane,
which will result in lower proton conductivity. Our sPFPE-SS
system is chemically cross-linked through the styrenic end
groups of the liquid precursors, and the cross-link density can
be controlled by using sPFPE precursors of different molecular
weights. By employing such a chemically cross-linked system,
PEMs with very high conductivity and good mechanical
integrity can be achieved. The fluorinated, 100% curable liquid
precursor to PEM approach can potentially enable the minia-
turization of fuel cells and many other opportunities for fuel
cell developments.
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