Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Docket No 2016P02401CA
METHODS FOR FABRICATING MEMBRANE ELECTRODE ASSEMBLIES WITH
PROTECTIVE FILM FOR ENHANCED DURABILITY IN FUEL CELLS
BACKGROUND
Field of the Invention
This invention relates to membrane electrode assemblies and manufacturing
processes for solid polymer
electrolyte fuel cells. In particular, it relates to a process for fabricate
such assemblies with thin protective
film at the edges for enhanced fuel cell durability.
Description of the Related Art
Proton exchange membrane fuel cells convert reactants, namely fuel (such as
hydrogen) and oxidant (such
as oxygen or air), to generate electric power. A membrane electrode assembly
(MEA) is the core component
of a proton exchange membrane fuel cell stack. Its structure is sandwich-like,
comprising a proton exchange
membrane (PEM), electro-catalyst layers and gas diffusion layers (GDLs). MEA
durability and cost are
crucial issues for the development and commercialization of fuel cell systems
in either stationary or
transportation applications. In automotive applications for instance, a MEA
may be required to demonstrate
durability of about 6,000 hours.
The membrane serves as a separator to prevent mixing of reactant gases and as
an electrolyte for
transporting protons from anode to cathode. Perfluorosulfonic acid (PFSA)
ionomer, e.g., Nation , has
been the material of choice and the technology standard for membranes. Nafion
ionomer consists of a
perfluorinated backbone that bears pendent vinyl ether side chains,
terminating with SO3H. A hydration-
dehydration dimensional change of the membrane during operation affects the
durability of fuel cells,
because it causes mechanical weakness in the membrane. In particular, the
edges, such as the boundary
between membrane and electrode in the MEA, crack easily due to excessive
pressure and stress.
Protecting the MEA edges with protective film has been used to improve
durability. D.M. Yu et al reported
edge protection using polyacrylonitrile (PAN) thin film for hydrocarbon based
membrane electrode
assemblies (Journal of Industrial and Engineering Chemistry, 28, 190-196,
2015). In this design, a thin PAN
film with 6 + I um thickness was attached to the peripheral region of the
membrane (active area: 5 cm x 5
cm) to protect the edges using rubbery epoxy adhesives and cured by hot
pressing at 80 C for 2 hours
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(Figures 1 a and lb). The cyclic hydration-dehydration durability of
sulfonated poly(arylene ether sulfone)
hydrocarbon membrane was significantly improved, because PAN film alleviated
inordinate pressure and
mechanically reinforced the boundary area during hydration-dehydration cycles.
However the ohmic and
interfacial contact resistance between the membrane and electrodes was
increased by the non-electrically
conducting PAN film.
In US2005/0089746, a thin barrier film was interposed between microporous
layers (MPLs) of GDLs and
a catalyst coated membrane (CCM) or between membrane and catalyst layers (same
as that reported in
Journal of Industrial and Engineering Chemistry, 28, 190-196, 2015) to inhibit
contact of other components
(e.g. sealant) with the catalyst layers or the membrane to suppress or
eliminate acid catalyzed hydrolysis of
sealant, and to prevent membrane contamination from any degradation byproduct
from the sealant. MEA
lifetime was significantly increased.
Because the output voltage of a single cell is of order of IV, a plurality of
cells is usually stacked together
in series for commercial applications. In such a stack, the anode flow field
plate of one cell is thus adjacent
to the cathode flow field plate of the adjacent cell. For assembly purposes, a
set of anode flow field plates
is often bonded to a corresponding set of cathode flow field plates prior to
assembling the stack. The
thickness of any protective film used will of course also impact MEA thickness
and consequently the stack
thickness and size. In US2005/0089746 Al, protective film with 501.tm
thickness was used. For automotive
application, a fuel cell stack may have as many as 400 single cells or more.
The total thickness of 400
MEAs with a 50 i.tm protective film on both sides might thus be increased by
¨4 cm. This will significantly
impact automotive fuel cell system design and the energy density of the fuel
cell stack. It also can be
expected that the thickness of any protective film between electrode layers
and GDL might affect the contact
resistance between them. The thicker the protective film, the higher the
contact resistance between GDL
and electrode layer.
For these reasons, using the thinnest protective film possible is desirable.
However, it is difficult to place a
very thin film (such as a film with thickness 2-5 p.m) into a MEA using
conventional continuous
manufacturing methods due to the insufficient mechanical properties of very
thin plastic films. A
conventional method for example involves peeling off a very thin film from a
backer, then placing it
between a GDL and an electrode layer, followed by bonding together of a MEA.
Although protective film technology can improve MEA durability, it comes with
additional cost in material
and in the MEA manufacturing process. The trade-off between improved
durability and additional cost
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should be considered for commercialization of fuel cells. There remains a
continuing need to improve
processes for manufacturing MEAs with protective film to reduce the cost of
solid polymer electrolyte fuel
cells. This invention fulfills these needs and provides further related
advantages.
SUMMARY
In this invention, a thin polymer film is used to protect the edges of one or
more of a proton exchange
membrane (PEM), a catalyst coated membrane (CCM), and a gas diffusion layer
(GDL) in a solid polymer
electrolyte fuel cell. The polymer film may be decal transferred or
alternatively a roll hot pressing technique
may be used to transfer the film to the desired component or components. The
latter process allows for the
continuous manufacture of MEA components with protective film. Compared to
those methods reported in
the prior art (Journal of Industrial and Engineering Chemistry, 28, 190-196,
2015, Journal of the
Electrochemical Society, 155(4), B411-B422, 2008), no adhesives or glues are
needed to attach the
protective film into the MEA. Such curable adhesives or glue would add extra
steps to MEA fabrication.
Also degradation byproducts from such adhesives or glues during fuel cell
operation could introduce new
contaminants to the MEA, and consequently impact MEA performance and
durability.
With decal transfer and/or roll hot pressing manufacturing processes, a very
thin protective film with
thickness of order of 2-5 um can easily and quickly be interposed into a MEA.
This technique is well
suited for continuous manufacturing of a MEA. MEAs with very thin protective
film (2-5 um) will allow
for more flexibility in fuel cell system design. In particular, roll hot
pressing offers the potential for cost
reduction in a continuous manufacturing process.
These and other aspects of the invention are evident upon reference to the
attached Figures and following
detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
Figures 1 a and lb are schematic surface and cross-section diagrams
respectively of a prior art MEA with
protective film between the membrane and the electrode layers and are
reproduced from Journal of
Industrial and Engineering Chemistry, 28, 190-196, 2015.
Figure 2 is a schematic diagram of a prior art MEA with protective film
between the fluid diffusion layers
(i.e. GDLs) and the catalyst layers and is reproduced from US2005/0089746.
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Figures 3a, b, and c illustrate the decal transfer preparation of several
embodiments of a MEA with
protective film and cross-sectional views of those MEAs. Figure 3a shows a MEA
with film on the MPL
of the GDL at both the reactant inlet and outlet. Figure 3b shows a MEA with
film on the MPL of the GDL
periphery. Figure 3c shows a MEA with film on the CCM periphery on both the
cathode and anode sides.
Figures 4a, b, and c show the continuous preparation of GDL, CCM, and membrane
respectively in which
protective film has been applied using a roll hot pressing technique.
Figure 5 shows the OCV lifetime of a baseline MEA and MEAs employing
protective film in the
configurations depicted in Figure 3a and Figure 3b.
Figure 6 shows plots of OCV versus time for a baseline MEA and for MEAs
employing protective film in
the configurations depicted in Figure 3a and Figure 3h.
Figure 7 shows plots of conductivity versus time of the effluent water from
the OCV tests of the MEAs in
Figure 6.
DETAILED DESCRIPTION
MEA durability can be improved by incorporating a thin polymer film between
the membrane and the
electrode layers or between the GDLs or MPLs (in relevant MEAs) and the
electrode layers. For instance,
Figures la and lb are reproduced from the Journal of Industrial and
Engineering Chemistry, 28, 190-196,
2015 and show schematic surface and cross-section diagrams respectively of a
prior art MEA with
protective film between the membrane and the electrode layers. Further, Figure
2 is reproduced from
US2005/0089746 and shows a schematic diagram of a prior art MEA with
protective film between the fluid
diffusion layers (i.e. GDLs) and the catalyst layers. However, a new process
has been needed for MEA
fabrication to eliminate potential contamination from using adhesives/glues in
the MEA fabrication, and
also to reduce cost in MEA manufacturing. Preferably, the process should be
feasible for very thin film so
that the fuel cell stack size will not be significantly impacted. Attaching a
thin protective film (e.g. 1 to 15
lam, and more preferably 2 to 5 [tm) appropriately into a MEA with a decal
transfer or roll hot press method
provides benefits over the prior art.
Figures 3a, b, and c illustrate the decal transfer preparation of several
embodiments of a MEA with
protective film along with cross-sectional views of those MEA embodiments. In
Figure 3a, a protective
film is decal transferred (from a backer) to the MPLs of both cathode and
anode GDLs at both the reactant
inlets and outlets of the MEA active area window. Then the GDLs with the
transferred protective films are
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combined with a CCM to make a MEA. Thus, in Figure 3a, the thin protective
films are only interposed at
the inlets and outlets of the MEA. In Figure 3b, a protective film is decal
transferred to the MPLs of both
cathode and anode GDLs over the entire periphery of the MEA active area
window. Then the GDLs with
the transferred protective films are combined with a CCM to make a MEA. Thus,
in Figure 3b, the thin
protective films are interposed over the entire MEA active area periphery. In
the embodiment shown in
Figure 3c, a protective film is decal transferred to both sides of a CCM at
the peripheral MEA active area
window. Then the CCM with the transferred protective films are combined
appropriately with cathode and
anode GDLs to make a MEA. In this way, the thin protective films are also
interposed at the MEA active
area periphery. In all cases, the position of the protective films can be
adjusted and controlled during the
decal transfer steps.
For continuous manufacturing of MEA, protective film with a thickness between
1 and 15 p,m may be cast
onto a backer web in an appropriate pattern with a controlled width. Then the
web of protective film can
be transferred to a GDL roll or to a CCM roll using roll hot pressing
techniques as shown in Figures 4a and
b respectively. GDL rolls with transferred protective film (prepared as
depicted in Figure 4a) can thereafter
be applied to a CCM web to prepare a suitable continuous web of MEA.
Alternatively, a CCM roll with
transferred protective film (prepared as depicted in Figure 4b) can thereafter
be applied to appropriate GDL
rolls to prepare a suitable continuous web of MEA. In both these methods, the
thin protective film appears
interposed between the GDLs and the electrode layers. In a further embodiment,
webs of protective film
can be transferred to a membrane roll using a roll hot pressing technique as
shown in Figure 4c. A
membrane roll with transferred protective films (prepared as depicted in
Figure 4c) can thereafter be
combined with electrode webs to prepare a web of CCM, and then combined with
webs of GDL to prepare
a suitable continuous web of MEA. In this method, the thin protective films
appear interposed between
the membrane and the electrodes. In all these configurations, the thin
protective film or films can be either
on one side or on both sides of the relevant components.
The polymer materials which can be used as protective film include
poly(vinylidene fluoride) (Kynar),
polypropylene, polyethylene, polyolefins, PTFE (polytetrafluoroethylene),
polyaryl ethers, poly(ether ether
ketone), poly(ether sulfone), polyimide, FEP (fluorinated ethylene propylene),
ETFE (ethylene
tetrafluoroethylene), PFA (perfluoroalkoxy alkanes), PET (poly(ethylene
terephthalate), PEN
(polyethylene naphthalate), and poly(phenylene sulfide). The thickness of the
thin protective films used
can be between I and 15 rn, and preferably between 2 and 5 pm.
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Incorporating protective film in accordance with the invention can alleviate
inordinate pressure and
mechanically reinforce the boundary area of the M EA. Without being bound by
any theory, it is expected
that mechanically weaker PFSA membrane would be subject to greater chemical
degradation than stronger
PFSA membrane. And thus, chemical degradation of membrane in MEAs without
protective film would
presumably be aggravated when compared to membrane in MEAs with protective
film. For instance, the
rate of crossover of reactants (either hydrogen or air/oxygen) through a PFSA
membrane with weaker
mechanical properties is expected to increase faster than that through a PFSA
membrane with stronger
mechanical properties. Consequently more hydrogen peroxide and free radical
production would be
expected in fuel cells comprising weaker PFSA membranes. The chemical
degradation of PFSA membrane
during fuel cell operation is proposed to proceed via the attack of hydroxyl
(.0H) or peroxyl (.00H) radical
species on weak groups (such as a carboxylic acid group) on the ionomer
molecular chain. The free radicals
may be generated by the decomposition of hydrogen peroxide with impurities
(such as Fe-) in a Fenton
type reaction. In fuel cells, hydrogen peroxide can be formed either at Pt
supported on carbon black in the
catalyst layers or during the oxygen reduction reaction. The hydroxyl radical
attacks the polymer unstable
end groups to cause chain zipping and/or could also attack an S03- group under
dry conditions to cause
polymer chain scission. Both attacks degrade the membrane and eventually lead
to membrane cracking,
thinning or forming of pinholes. However such effects may be reduced by
incorporating protective film
thereby significantly improving membrane chemical durability.
The following examples are illustrative of the invention but should not be
construed as limiting in any way.
EXAMPLES
Kynar PVDF thin films were used in accordance with the invention as protective
films to improve MEA
durability. Thin Kynar films were prepared by film casting onto backers.
First, Kynar polymer was
dissolved in methyl ethyl ketone solution to make a 10% (weight %)
concentration. Then the solution was
cast on a TPX (polymethylpentene) backer and was allowed to dry at room
temperature for 1 hour. The
thickness of the thin films was controlled to 3-5 !Am by adjusting the gap
between blade and TPX backer
(note that polymer film with any other thickness between 1 and 15 p.m could
also be cast and used as
protective film).
In inventive M EA embodiments, the obtained thin Kynar film was decal
transferred to the MPL of a GDL
to prepare GDLs with applied protective film at 270 F for few seconds under
low force. CCMs for the
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MEAs were prepared by decal transfer. Cathode Pt catalyst with 0.25 mg/cm2
loading and anode Pt catalyst
with 0.10 mg/cm2 loading were decal transferred to a reinforced
perfluorosulfonic acid (PFSA) membrane
with thickness of 15 m at a temperature above the glass transition temperature
of PFSA membrane for a
few minutes under high force.
Three types of MEA were fabricated and tested to check the durability or
chemical stability of the
membrane in different MEA configurations: a conventional or "baseline" MEA
without any protective film;
a MEA similar to the baseline MEA but with Kynar protective film on the MPL of
the GDL at both the
reactant inlet and outlet (i.e. the configuration of Figure 3a); and a MEA
similar to the baseline MEA but
with Kynar protective film on the MPL of the entire GDL periphery (i.e. the
configuration of Figure 3b).
In all cases, MEAs were fabricated by bonding together CCMs and appropriate
GDLs at 302 "F for few
minutes under high pressure.
The durability or chemical stability of the MEA samples was evaluated under
open circuit voltage (OCV)
conditions at 30% relative humidity (RH) and 95 C. For each type of MEA, 3
cell series stacks were made
with 48.4 cm2 active area. The supplied reactant gas flow-rates were 3.5 and
II slpm for hydrogen and air
respectively. The OCV of each cell in each stack was monitored over time. In
addition, the amount of
fluoride released as a result of decomposition of the membrane during testing
was determined over time
(i.e. the fluoride release rate) by measuring the fluoride ion found in both
the cathode and anode outlet
water. Testing was stopped and the OCV lifetime of each type of MEA was
defined as time at which the
OCV in any one of the 3 cells in the series stack reached 0.8V.
Figure 5 shows the OCV lifetime for the three different types of MEA. The OCV
lifetime of the baseline
MEA is only 147 hours, while the OCV lifetimes of the MEA with the Figure 3a
configuration is 375 hours
(2.5 times longer than that of the baseline MEA) and of the MEA with the
Figure 3b configuration is 534
hours (3.6 times longer than that of the baseline MEA.) Thus, the MEA whose
peripheral region is
completely protected with thin film showed the longest OCV lifetime.
Figure 6 shows OCV curves of average cell voltages of the 3 MEAs for each
stack. Figure 7 shows plots
of conductivity of combined effluent water from the MEAs in each of the series
stacks. These Examples
demonstrate that even though the thickness of protective film is only between
3-5 pm, MEA lifetime can
be significantly improved by incorporating thin protective film therein.
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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.
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