Note: Descriptions are shown in the official language in which they were submitted.
CA 02952192 2016-12-20
Docket No.: P83 H 23/CA/I
IMPROVED STRUCTURES AND PREPARATION METHODS FOR CATALYST COATED
MEMBRANES FOR FUEL CELLS
BACKGROUND
Field of the Invention
This invention relates to simplified methods for fabricating a catalyst coated
membrane (CCM) for solid
polymer electrolyte membrane fuel cells. The invention further relates to CCM
structures comprising
reinforcement layers of expanded polymer sheets and which exhibit improved
mechanical strength. The
improved CCM structures can be fabricated using the simplified methods of the
invention.
Description of the related art
Reinforcement is one of the important approaches for improving the durability
of a polymer electrolyte
membrane fuel cell (PEMFC). For instance, a state-of-art proton exchange
membrane (PEM) can be
mechanically reinforced by either a porous substrate or nanofibers to improve
its mechanical strength and
durability. Among the possible reinforcement materials, expanded
polytetrafluoroethylene sheet (ePTFE)
is probably the one mostly used due to its excellent chemical inertness and
stability. Much academic and
patent literature has been published in which ePTFE is utilized to improve the
mechanical strength and
subsequent durability of a PEM during fuel cell operation [e.g. W02003095552,
US8431285,
US20030211264, US20130022894, US20140080031, Journal of Power Sources
175(2008) 817-825,
Journal of Membrane Science 306 (2007) 298-306].
In contrast to the PEM, there is little effort being devoted to reinforcing
the catalyst layers (i.e. cathode and
anode layers) in such fuel cells. It is known that microcracks can form or
develop in catalyst layers during
catalyst layer fabrication or fuel cell operation (e.g. during freeze-thaw and
wet-dry cycles). The
microcracks can further generate stress (both chemical and mechanical) to the
underlying PEM, which will
likely lead to the premature failure of the PEM. Thus, there is a need to
develop reinforced catalyst layers
for PEMFCs in order to meet long-term durability requirements for automotive
applications.
Recently, several patent applications were filed in which porous ePTFE was
applied to reinforce the catalyst
layers to prevent the formation of cracks and improve the mechanical strength
of the catalyst layers (e.g.
US20110236788A1; US20120183877A1; US20120189942A1; US20140261981A1;
US20140261983A1).
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The mechanical stability of the catalyst layer was improved without impacting
the performance. However,
the hydrophobic nature of ePTFE required the usage of organic solvents in
preparing the catalyst ink in
order to properly rinse and impregnate the ink into the ePTFE. The presence of
organic solvents complicates
the CCM coating process, increases safety concerns, and adds extra cost to the
process. To avoid the usage
of organic solvents, pure aqueous ink would be preferred. Unfortunately, a
pure aqueous ink is not
compatible with traditional ePTFE and thus reinforcement in this manner has
not been reported to date.
The current invention involves a novel method to fabricate a catalyst layer
reinforced by an expanded
polymer sheet (e.g. ePTFE). In particular, the ePTFE used is hydrophilic which
can be achieved by surface
treatment. The hydrophilicity of the ePTFE allows for the use of aqueous
catalyst ink, which simplifies the
coating process and reduces the cost. The reinforced catalyst layers show
improved mechanical strength
compared to their non-reinforced counterparts.
SUMMARY
Here, a novel method is disclosed for preparing reinforced catalyst layers for
catalyst coated membranes in
fuel cells. A hydrophilic expanded polymer sheet (e.g. ePTFE) is used for
reinforcement in order to improve
the mechanical durability of the catalyst layers. Advantageously, a pure
aqueous based catalyst ink can be
used in the coating process for preparing the catalyst layers. The hydrophilic
expanded polymer sheet can
be prepared by surface treatment of a conventional hydrophobic equivalent. A
catalyst coated membrane
(CCM) of the invention, containing such reinforced catalyst layers, shows
improved mechanical strength
compared to conventional unreinforced CCMs (e.g. as indicated by the better
electrical resistance
characteristics in the following Examples). In addition, the hydrophilicity of
the hydrophilic expanded
polymer sheets used can help maintain the hydration of the catalyst layers
under hot and dry fuel cell
operating conditions, which ultimately will improve fuel cell performance.
Specifically, the inventive method is for preparing a catalyst coated membrane
assembly for a solid polymer
electrolyte fuel cell in which the catalyst coated membrane assembly comprises
a solid polymer electrolyte
membrane comprising a proton-conducting ionomer and first and second catalyst
layers comprising first
and second catalysts respectively. The first and second catalyst layers are
bonded to first and second sides
respectively of the solid polymer electrolyte membrane. And at least the first
catalyst layer comprises a
hydrophilic expanded polymer sheet. The method comprises the steps of
obtaining the hydrophilic
expanded polymer sheet, applying the hydrophilic expanded polymer sheet to a
polymer backing sheet,
preparing a first catalyst ink slurry comprising the first catalyst and an
aqueous solvent, coating the first
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catalyst ink slurry onto the hydrophilic expanded polymer sheet, drying the
first catalyst ink coating thereby
creating a first catalyst layer comprising the hydrophilic expanded polymer
sheet mounted on the backing
sheet, and decal transferring the first catalyst layer to the first side of
the solid polymer electrolyte.
In one embodiment, the first catalyst layer can be the cathode catalyst layer
and the first catalyst can be the
cathode catalyst. Alternatively, the first catalyst layer can be the anode
catalyst layer and the first catalyst
can be the anode catalyst. In a preferred embodiment, both the first and
second catalyst layers comprise a
hydrophilic expanded polymer sheet.
In the CCMs of the invention, the proton-conducting ionomer can be
perfluorosulfonic acid ionomer or
hydrocarbon ionomer. The expanded polymer sheet can be an expanded PTFE
(ePTFE) sheet. The polymer
backing sheet can be polyethylene terephthalate. The first catalyst ink slurry
can additionally comprise other
common constituents used in the art, e.g. Pt/carbon or Pt alloy/carbon and
perfluorosulfonic acid ionomer.
The decal transferring step can comprise the steps of applying the first
catalyst layer comprising the
hydrophilic expanded polymer sheet mounted on the backing sheet against the
first side of the solid polymer
electrolyte membrane, hot pressing the applied first catalyst layer and the
solid polymer electrolyte
membrane together at 150 C at an applied force of 5000 lb for 3 minutes, then
removing the backing sheet.
The invention further comprises a catalyst coated membrane assembly for a
solid polymer electrolyte fuel
cell made according to the aforementioned methods and a solid polymer
electrolyte fuel cell comprising a
catalyst coated membrane assembly made according to the aforementioned
methods.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 schematically illustrates processes of the invention for catalyst
coating and for fabrication of a half
CCM.
Figure 2 shows various SEM images of hydrophilic ePTFE, catalyst layers, and a
half CCM which have
been reinforced in accordance with the invention. Figure 2A shows the
morphology of hydrophilic
porous ePTFE; Figure 2B shows a cross-sectional image of a catalyst layer
reinforced by hydrophilic
ePTFE; Figure 2C shows a cross-sectional image of a reinforced catalyst layer
with catalyst missing;
and Figure 2D shows a cross-sectional image of a half CCM comprising a
reinforced catalyst layer on
Nafion membrane (EW= 875).
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Figure 3A shows a photograph of a reinforced catalyst layer being peeled from
a PET backing substrate.
Figure 3B shows a photograph of the handling of a free-standing catalyst layer
reinforced by hydrophilic
ePTFE.
Figure 4 shows a schematic illustration of the apparatus used in the breakdown
voltage testing performed
in the Examples.
DETAILED DESCRIPTION
Herein, a material (and particularly an expanded polymer sheet) is considered
to be "hydrophilic" if it
exhibits a static water contact angle of less than 300.
In the present invention, a hydrophilic expanded polymer sheet (such as ePTFE)
is employed as a coating
substrate onto which is cast an aqueous ink. The aqueous ink coats, wets, and
fills the porous expanded
polymer sheet. Moreover, this ePTFE will further serve as a reinforcement
layer to improve the mechanical
strength of the catalyst layer, consequently improving the durability of a CCM
and a MEA made therewith.
In addition, the hydrophilicity of the ePTFE used can help maintain the
hydration of the catalyst layer under
hot and dry fuel cell operation conditions, which will ultimately improve the
fuel cell performance.
Figure 1 schematically illustrates processes of the invention for catalyst
coating and for fabrication of a half
CCM. A layer of hydrophilic ePTFE is initially obtained, for instance via wet
chemical treatment or surface
plasma polymerization of conventional hydrophobic ePTFE (e.g. CA2864541). As
shown in Figure 1, the
hydrophilic ePTFE layer is first laid onto a polyethylene terephthalate (PET)
substrate under controlled
tension. Subsequently, a desired aqueous catalyst ink (e.g. containing
Pt/carbon catalyst, Aquivion
(EW=790) ionomer, and water as solvent) can be cast onto the ePTFE via Mayer
bar coating. The ink is
readily impregnated into the ePTFE and will form a thin wet layer between the
ePTFE and the underlying
PET substrate. A thin layer of partially impregnated ePTFE layer thus forms
adjacent to the PET. The
viscosity of the ink, thickness, porosity, and pore size of ePTFE can have
significant impact on the
properties and morphology of the formed catalyst layer. A lower ink viscosity
combined with thinner
ePTFE with larger pore size and higher porosity can result in more thorough
impregnation and uniform
reinforcement. After evaporation of the water solvent, the catalyst layer can
be transferred onto a PEM (e.g.
Nafion type membrane) via hot bonding (decal transfer). Due to the very low
adhesion force between the
PET and the ePTFE, such decal transfer can be smooth and complete.
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The following Examples have been included to illustrate certain aspects of the
invention but should not be
construed as limiting in any way.
EXAMPLES
Hydrophilic ePTFE samples, reinforced catalyst layers, and half CCMs were
prepared as described below.
Surface plasma polymerization to prepare hydrophilic ePTFE
Hydrophilic ePTFE with a static water contact angle of 100 was prepared from
conventional sheets of
hydrophobic ePTFE that was pre-treated via surface polymerization of
allylamine (as disclosed in the
aforementioned CA2864541). Plasma treatment was performed on a M4L plasma
system provided by PVA
Tepla Co. Ltd. The ePTFE was pre-cut into 15 cm x 15 cm size and placed on the
plate of the plasma
chamber. The thickness of the ePTFE was about 4 um, while the porosity and
pore size was 90% and 1.2
um respectively. The chamber was evacuated to 50 mTorr at a speed of 40 L/min.
Then, nitrogen gas was
injected at a rate of 100 standard cubic centimeters (SCCM) with a glow
discharge at 75 W for 3 minutes.
The system was evacuated to 100 mTorr. Then, allylamine was injected at a
speed of 18 SCCM until] the
pressure reached 300 mTorr. Radio frequency (RF) glow discharge power was set
at 150 W and the
treatment time was 10 min. Thereafter, the system was evacuated again to a
pressure below 50 mTorr and
air was introduced into the chamber to atmospheric pressure for 15 min.
Treated ePTFE film was removed
from the chamber and stored between two pieces of examining paper before
further usage.
Catalyst ink slurry preparation
0.5 g of Pt catalyst deposited on carbon particles plus 50 g of grinding media
(5 mm Yttria-stablized zirconia
beads) were mixed with 5g of distilled and deionized water and 2g of
perfluorosulfonic acid polymer
dispersion (Aquivion 790). The ink slurry was jar milled for several days
before usage.
Preparation of reinforced catalyst layers
The ePTFE (10 cm x10 cm) treated by surface polymerization of allylamine was
laid carefully on a
polyethylene terephthalate (PET) substrate. Wrinkles and air bubbles were
minimized to avoid coating
defects. The catalyst ink slurry prepared as above was coated onto the ePTFE
with a loading of 0.25 mg/cm2.
The coating substrate was then dried on a hot plate at 55 C for 30 minutes
before usage.
Preparation of half CCMs
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Membrane electrolyte was fabricated by coating a layer of Nafion 875 ionomer
solution (20% solids
content, n-propanol/water = 60/40) onto a casted PTFE (polyethylene
tetrafluoroethylene polymer)
substrate (from Saint-Gobain) via knife coating. After quick drying, the
coating substrate was annealed at
150 C for 30 minutes to produce a membrane electrolyte layer on the PTFE
substrate. The thickness of the
membrane electrolyte layer was about 15 um. The reinforced catalyst layer
prepared as above was then
decal transferred to the Nafion 875 membrane via hot bonding at 150 C with
applied force of 5000 lb for
3 minutes.
The SEM image in Figure 2A shows the morphology of the prepared hydrophilic
porous ePTFE. In Figure
2A, a porous structure was observed for the hydrophilic ePTFE. Figure 2B shows
a cross-sectional SEM
image of a catalyst layer reinforced by hydrophilic ePTFE on a PET substrate.
Beneath a uniform catalyst
coating, a very thin interface containing ePTFE was observed. By cracking the
catalyst layer, the internal
structure of the reinforcement layer was better observed (as shown in the SEM
image of Figure 2C which
shows a cross-sectional image of the reinforced catalyst layer with catalyst
missing). A continuous porous
ePTFE layer was present, which supported the catalyst layer. It is noted that
the adhesion force between the
ePTFE and the PET backer was sufficient to keep the ePTFE from contracting
during the drying process.
After decal transfer of the catalyst layer to a Nation 875 membrane, a half
CCM was obtained. (Figure 2D
shows a cross-sectional image of the half CCM which comprises the reinforced
catalyst layer on Nafion
membrane, EW= 875.) It is noted in particular that, on the top of the catalyst
layer, a very thin layer of
ePTFE was observed, which can function as a protection layer to prevent
catalyst cracking during fuel cell
operation, as well as to prevent penetration of carbon fibres from the
adjacent gas diffusion layer. Due to
the high porosity and large pore size, this thin layer of ePTFE did not cause
a significant increase of
electrical resistance in the catalyst layer under compression. Moreover, the
hydrophilicity of the ePTFE is
helpful for maintaining the hydration level of the catalyst layer during fuel
cell operation, which should
therefore improve the fuel cell performance, particularly under hot and dry
operating conditions.
Figure 3A shows a photograph of a reinforced catalyst layer being peeled off
from the PET backing
substrate. As is evident from the photograph, the reinforced catalyst layer
can be readily peeled off. Figure
3B shows a photograph of the handling of the peeled off, free-standing
catalyst layer of Figure 3A. As is
evident from the photograph, the peeled off catalyst layer can be easily
handled without compromising the
original shape. These examples demonstrate the improved mechanical strength of
the reinforced catalyst
layer. To the best of our knowledge, this is the first report of free-standing
catalyst layer being fabricated.
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To study the effect of reinforcement layer on the potential for electrical
shorting through the catalyst layer,
a series of breakdown voltage (BDV) tests were carried out. Figure 4 shows a
schematic illustration of the
apparatus used in this BDV testing. As illustrated in Figure 4, a BDV testing
setup including a pair of
carbon plates, copper based compression heads, and power supply was used to
test the breakdown voltage
of half CCMs.
Sample preparation for breakdown voltage testing
First, a pair of carbon plates was pre-cut into 4 cm >< 4 cm, followed by
surface polishing using extra fine
sandpaper (e.g. Buehler P2500). A piece of reinforced half CCM prepared as
above was cut into 5 cm x 5
cm size and assembled between two Toray GDL samples (3 cm x 3 cm) so as to
create a GDL/half
CCM/GDL sandwich assembly. The sandwich assembly was then hot bonded at 150 C
with applied force
of 5000 lb for 1.5 minutes. After bonding, the test sample was placed on the
top of bottom carbon plate,
followed by aligning the top carbon plate. Then the whole assembly was moved
to the bottom plate of the
BDV head. On the top of the assembly, one piece of Grafoil sheet was added to
uniformly distribute the
compression. The Grafoil sheet was hot compressed prior to usage at 150 C,
7000 lbs for 5 minutes.
Breakdown voltage testing of prepared samples involved applying an increasing
voltage across the sample
until voltage breakdown occurred.
Four different samples with reinforced catalyst layers and four different
samples without reinforced catalyst
layers were prepared, tested, and compared. The breakdown voltages observed
for the catalyst layers
without reinforcement were 1.6, 1.8, 1.6, and 1.6 volts respectively. The
breakdown voltages observed for
the catalyst layers with reinforcement were much higher, namely 35, 37, 50,
and 32 volts respectively. The
electrolyte membrane used in the sample fabrication was 15 micron thick,
Nafion membrane (D2029,
EW-875) which was cast in-house. Due to the rough edges present on the GDLs,
the samples without
reinforcement showed very low BDVs, i.e. about 1.6 volts on average. In
contrast, the samples with the
reinforced catalyst layers showed much higher BDV, i.e. >30 volts, verifying
the protective effect of the
hydrophilic ePTFE reinforcement.
All of the above U.S. patents, U.S. patent application publications, U.S.
patent applications, foreign patents,
foreign patent applications and non-patent publications referred to in this
specification, are incorporated
herein by reference in their entirety.
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While particular elements, embodiments and applications of the present
invention have been shown and
described, it will be understood, of course, that the invention is not limited
thereto since modifications may
be made by those skilled in the art without departing from the spirit and
scope of the present disclosure,
particularly in light of the foregoing teachings. Such modifications are to be
considered within the purview
and scope of the claims appended hereto.
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