CA2970019A1 - Methods for fabricating membrane electrode assemblies with protective film for enhanced durability in fuel cells - Google Patents

Abstract

A method for fabricating a membrane electrode assembly (MEA) with thin polymer film for edge protection for a solid polymer electrolyte fuel cell is provided. The protective thin polymer film can be transferred to an appropriate cell component by decal transfer or roll hot pressing techniques. MEA
durability can be significantly improved by incorporating such protective thin polymer film.

Description

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 Docket No 2016P02401 CA
(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.

5 Docket No 2016P02401CA
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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Claims (5)

What is claimed is:

1. A method for fabricating a membrane electrode assembly for a solid polymer electrolyte fuel cell comprising:
casting a thin polymer film onto a backer; and transferring the thin polymer film from the backer to the surface of a component selected from the group consisting of: a gas diffusion layer, a catalyst coated membrane; and a proton exchange membrane.

2. The method of claim 1 wherein the transferring step comprises decal transferring.

3. The method of claim 1 wherein the transferring step comprises roll hot pressing.

4. A membrane electrode assembly for a solid polymer electrolyte fuel cell fabricated according to the method of claim 1.

5. A solid polymer electrolyte fuel cell stack comprising a series stack of solid polymer electrolyte fuel cells wherein the fuel cells in the stack comprise the membrane electrode assembly of claim 4.