Chemistry Reference
In-Depth Information
sulfonated polymers are cheaper than fluorinated ones and recent successful
studies have shown similar or even improved membrane properties, such as
mechanical strength, conductivity, and water retention.
71
The first breakthrough was achieved with high-temperature PEM fuel cell
membranes made of phosphoric acid-doped polybenzimidazole (PBI), a
highly stable aromatic polymer.
72
PBI membranes are usually produced by
dissolving the material in a mixture of an organic solvent (e.g. dimethyla-
cetamide) and inorganic salts (e.g. lithium chloride), applying the solution as
a thin film, drying and washing the film (to remove solvent, salts, and water),
and placing the film in phosphoric acid to generate a phosphoric acid-
doped, robust, and highly conductive membrane.
72-77
These membranes
exhibit perfect properties for PEM fuel cell membranes: high ionic con-
ductivity at high temperature and low humidity (in the order of 0.1 S cm
1
at
160 1C), low electro-osmotic drag, high CO tolerance, and low gas permea-
bility. This leads to good fuel cell performance, for example 0.35 W cm
2
for
H
2
/air and 0.55 W cm
2
for H
2
/O
2
, both at 160 1C and 0.55 V.
71
An extensive
review of polybenzimidazole derivatives for high-temperature PEMFCs has
been published recently.
78
Challenges related to PBI are the low phosphoric
acid loading and its retention during operation, membrane durability, and
low molecular weight.
71
A novel method to overcome these issues related to PBI is the creation of a
highly robust and stable, high molecular weight PBI film by using poly-
phosphoric acid as the polycondensation agent, polymerization solvent, and
casting solvent.
79-82
However, an effective alternative replacing Nafion as the
standard electrolyte for PEMFCs is still missing.
An interesting approach besides sulfonated polymers is the synthesis of
organic-inorganic membranes consisting of Nafion and hygroscopic oxides,
such as TiO
2
,
83,84
SiO
2
,
85,86
and ZrO
2
87
for PEMFCs operating at 100-150 1C.
88
The hygroscopic nanoparticles enhance water retention in the membrane and
improve transport properties, thus increasing the fuel cell performance at
higher temperatures.
86
In principle, there are two methods to incorporate
inorganic nanoparticles in organic electrolytes: on the one hand, a mixture of
the oxide nanoparticles and Nafion solution in a solvent is applied by casting
as a so-called composite membrane. On the other hand, oxides can be formed
in situ inside the polymer membrane by sol-gelation, creating a hybrid
membrane.
89
The synthesis of casting composites allows for the production of mem-
branes with well-defined composition, volume fraction, and oxide distri-
bution, which strongly affect the water retention and electrochemical
performance of the membranes.
90
In hybrid membranes, the oxides are mostly created by hydrolysis or
condensation reactions on hydrophilic sites of the Nafion during the sol-
gelation process, due to the anity of the inorganic precursor with water.
61
It is important that the synthesis takes place at a low temperature (
o
100 1C)
and that the oxide particle size is compatible with the ionic clusters, to
ensure that the intrinsic properties of the polymer remain unaltered.
d
n
3
r
4
n
g
|
4
.
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