Thermoset Solutions for Fuel Cell Seals

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1 Thermoset Solutions for Fuel Cell Seals Daniel Ramrus, Sr. Research Scientist, PhD. Paul Kozak, R&D Manager, P. Eng., MBA. Date of presentation: September 15, 2008, 4:30-5:00 pm Ballard Power Systems 9000 Glenlyon Parkway Burnaby, British Columbia Canada V5J 5J Presented at a meeting of the Thermoset Resin Formulators Association at the Hilton Suites Chicago Magnificent Mile in Chicago, Illinois, September 15 through 16, This paper is presented by invitation of TRFA. It is publicly distributed upon request by the TRFA to assist in the communication of information and viewpoints relevant to the thermoset industry. The paper and its contents have not been reviewed or evaluated by the TRFA and should not be construed as having been adopted of endorsed by the TRFA. 1

2 Abstract Silicones, polymer films, thermoplastic elastomers and epoxy resins are all possible sealing components for fuel cells. Although the needs of current programs at Ballard have been met with these materials and designs, they will not survive future demands, which include longer lifetimes at higher temperatures and the shorter cycle times necessary for increased volumes. As part of the technology development road map, Ballard is driving towards a lean design process that utilizes fewer components. This effort will reduce labour costs and increase robustness to meet specific product requirements. There are materials engineering and processing challenges that must be overcome to reach this goal. The final design can be altered to suit the manufacturing and process requirements of the materials. The current processes behind these assemblies include liquid injection moulding, dispensing, and screen-printing. This paper will detail the next generation of thermoset resin requirements in the areas of membrane electrode assembly (MEA) encapsulation and plate sealing. Commercialization of fuel cell technology depends on high volume capability required to drive the unit cell cost down to a level where the assembly is competitive with incumbent technologies. Introduction PEMFCs (Proton exchange membrane fuel cells) generally employ a membrane electrode assembly ("MEA") consisting of an ion exchange membrane coated with thin catalyst layers and disposed between two gas diffusion layers (GDLs) comprised of porous, electrically conductive sheet materials. The membrane conducts protons, and acts as a barrier for isolating the reactant streams as seen in Figure 1. 2

Figure 1 Schematic of a fuel cell Bi-polar flow field plates provide gas flow channels, coolant flow and electrical connects to the outside circuit to complete the unit cell.

3 Figure 1 Schematic of a fuel cell Bi-polar flow field plates provide gas flow channels, coolant flow and electrical connects to the outside circuit to complete the unit cell. For a PEMFC stack, sealing materials are used in 1) sealing MEA edges to prevent reactant gas crossover (internal sealing); 2) sealing bi-polar plates and gas inlet/outlet ports to prevent gas leaking to environment; 3) sealing coolant; and 4) electrical insulation between the bi-polar plates. Reliable sealing is critical to ensure efficient and safe operation. In PEMFCs, the sealing materials must perform in a wet and slightly acidic environment at a temperature range of 30 to 90 C and potentially at even more aggressive conditions of 40 to 120 C in the future. Chemical stability of the sealing material is very important in PEMFC applications, given the possible exposure to contaminants and MEA components, like platinum, that can accelerate degradation. The sealing materials must also possess low gas permeability, thermal stability, and excellent mechanical properties to survive freeze start, and startup/shut-down operating conditions. In addition, they should be low cost and easily adaptable to various unit cell and stack designs. The use of elastomers, a typical example being silicone rubber, is common in PEMFC sealing applications. Silicones have many merits as sealing materials (e.g. they possess excellent thermal stability and good mechanical properties). However, gas permeability is high and chemical stability can be low. The leachable and volatile species in most silicones can lead to the formation of by- 3

4 products in the membrane that accelerate failure of this component, as has been observed experimentally by Ballard. Commercial stacks can comprise hundreds of cells and consequently many yards of seals. It is important therefore to employ highly reliable seal materials and designs. However, achieving this objective within the design constraints of the unit cell is a continuing challenge. To obtain high volumetric power density, the trend is to employ the thinnest unit cells possible. This requirement applies equally to the sealing component of the unit cell, adding even greater challenges both to the materials and to the design philosophy by which the seal is incorporated into the unit cell. That is, the thinner the seal becomes, the wider the range of compression experienced for any given stack-up tolerances. Thus, either the seals must be capable of tolerating greater ranges of compression (e.g., by using multiple seals designed to accommodate different ranges of compression) or even tighter tolerances are required on the thickness of cell components, to avoid uneven compression and ultimate failure of the sealing function. Performance and cost requirements will drive the development of ancillary membrane electrode components, including seal materials, to be durable at a low cost to the manufacturer. Due to the aggressive conditions that exist at 90 C under compression, for 40,000 hours, these suitable seals will need to exhibit increased resistance to chemicallyand mechanically-induced failure modes in comparison with the currently available, commercial materials. To this end, the objective is to develop appropriately robust seal materials, in conjunction with compatible design and processing solutions, and provide a specification and commercially representative testing protocol to evaluate the combinations against Department of Energy (DOE), industry and internal product durability goals. These lifetime goals for robust seal materials are at least 40,000 hours for stationary applications. The developed specifications will include material properties before and after processing, in addition to other ex-situ, in-situ and accelerated ex-situ/in-situ testing parameters. There are many types of PEMFC sealing designs. Conventional methods include sealing around plate manifold openings and the active area of the MEA. This can involve framing the MEA with a resilient fluid impermeable gasket, placing preformed gaskets in channels in the electrode layers and/or bi-polar plates, or molding seals within grooves in the electrode layer or bipolar plate, thus separating the electrochemically active area and any fluid manifold openings. Examples of conventional methods are disclosed in U.S. Pats. 5,176,966 and 5,284,718. A second strategy is to injection mold the seal onto the MEA. It would be preferable to manufacture a sheet or roll of MEA material that already comprises the electrode and membrane layers, wherein this multi-layer material could then be cut to the desired size and shape for individual MEAs, such as that disclosed in Ballard patents (U.S. Pat. No. 6,057,054, US ). The sealing materials readily penetrate the porous GDLs on either side of the MEA, prior to the actual curing process. The seal extends laterally beyond the edge of the MEA, framing the entire periphery (See Figure 2A below). Such a seal can prevent fluid transfer around the edge of the MEA and 4

5 can also be used to effect fluid tight seals to both adjacent bi-polar plates (See Figure 2B below). Additional seals for internal ports or manifolds may also be incorporated at the same time as the edge seal for the MEA using an appropriate molding operation. However, as mentioned above, highly reliable seals are required to meet the durability targets for commercial PEMFCs, as such further improvements in materials and designs are required. GDL μm Flush-cut MEA Elastomer GDL impregnated with sealing materials Figure 2A. An MEA using an integrated seal technology with an elastomeric overmold (schematic diagram) Figure 2B. Depicts Figure 1A sealed MEA incorporated into a unit cell assembly ((1) Bi-polar plate (2) Elastomer off-board external seal (3) elastomer impregnated MEA forming internal sealing (anode to cathode sealing) (4) gas and/or water flow channels (5) GDL Ballard is looking for sealing solutions (consisting of combined material, process and design elements) for backup power, materials handling, and cogeneration applications. A leading approach is a thermoset solution where each unit cell can be injected with a material that can penetrate the gas diffusion layer of the MEA and prevent internal and external leaks. Ballard is not in the business of thermoset formulations, so we are looking for supplier support in this endeavor. 5

6 In a preferred unit cell design, the sealing of the MEA is achieved through a low pressure insert-molding application whereby a two component elastomeric or thermoset material is combined in a static mixer, pushed through a chilled barrel into a hot mold where it cures to form a seal profile over-molded onto the MEA insert. US Patent 6,057,054 describes this process and clearly states the requirement for flow processable materials which will easily saturate the GDL layers, and completely fill the thin wall seal features in the cavities of the mold. Results There are a number of technical challenges that need to be overcome to advance the incumbent sealing technology to realize cost effective, durable and reliable products for the fuel cell industry. The schematic diagrams in Figure 3 illustrate sealing technologies where uncured liquid sealing materials are introduced into the peripheral regions of a MEA. The seal material is then cured to prevent fuel-oxidant mixing around the edge of the MEA as well as efficient sealing with other components of the unit cell. While an excellent design concept, this technology does impose material constraints that result in a number of technical challenges that require resolution. Firstly, the viscosity of the uncured material must allow for saturation of the edge of the MEA. The MEA is made from a thin membrane and carbonized paper so it is fragile in nature. In addition, the seal materials must be durable enough to withstand the oxidizing and acidic environment of the fuel cell while providing sufficient properties to seal the MEA, for the lifetime of the product at temperatures that can reach 100 C Figure 3. Epoxy-encapsulated MEA (glued half cell) with off-board external seal ((1) Bi-polar plate (2) MEA glued to bipolar plate with epoxy (3) Epoxy encapsulated MEA (4) Membrane as part of 5-layer MEA) Thermoset materials, such as polyimides, polyesters and epoxies, are widely used as adhesives and bonding materials in packaging, coating and construction. Epoxy adhesives were introduced commercially in 1946 and have a wide application of use in the automotive, industrial and aerospace markets. Epoxies are probably the most versatile family of adhesives because they bond well to many substrates and can be modified to achieve widely varying properties. Cured epoxies exhibit excellent tensile-shear strength, resistance to moisture, acids and many solvents. Low shrinkage on curing and high resistance to creep under prolonged stress are all characteristics of many high quality epoxies. Epoxy resins have no evolution of volatiles during cure and are useful in gapfilling applications. The low viscosity of many epoxies at high temperature makes it easy to impregnate into porous GDLs to form an excellent seal. However, drawbacks include 6

7 poor wetting ability on some substrates with low surface energy, such as untreated plastics and elastomers making adhesion difficult. Many un-modified epoxies are brittle while modified epoxies may have limited shelf life, low peel strength and volatiles, which are released during curing. Significant efforts over the last five years have been made at Ballard to develop and use epoxies as sealing materials in PEMFCs. The PEMFCs sealed with Ballard formulated epoxies and designs have demonstrated lifetimes greater than 6,000 hours in an in-situ test under steady-state conditions. However, optimization of the epoxy formulations is still required to increase their durability and compatibility to plate materials. An optimized process with low cost and a defined process window are ultimately required for manufacture of the unit cell products at high volume. Ballard is in the midst of determining the thermoset material specifications necessary to glue a MEA to carbon bipolar plates by injection molding. The primary issues are flash of the thermoset into the active area of the cell, and viscosity of the material to flow through the seal groove channels of the mold surrounding the MEA. To work towards this goal, materials with a viscosity of 30,000 to 120,000 cps have been tested. The viscosity range has been narrowed to 30,000-60,000 cps. To reach this viscosity range, fillers may be added to the base epoxy. Filler size is an important factor related to the penetration into the GDL. Fillers up to 20 microns allow some penetration into the GDL without allowing for excessive flash. So far, the injection pressure has been limited to 60 psi resulting in an injection time of about 5 seconds. These initial trials have been tested on a 150 mm x 80 mm cell but cell size will be scaled up to meet platform requirements. Table 1 contains a list of current requirements. Single cells have been assembled and they have passed BOL (beginning of life) leak tests. They have not been run in-situ because the cell was not designed to allow for a non-conductive film to be placed around the MEA to provide electrical isolation between the bipolar plates. Future embodiments of this design will have this spacer incorporated into the plates. Figure 4 shows the flash of the thermoset into the GDL. MEA Thermoset seal groove Flash of thermoset into the GDL Figure 4 - Thermoset flash into GDL- Process trials 7

8 Table 1. Preliminary Technical Requirements for Epoxy Materials Property Measure a Measure After Aging b Ultimate tensile strength (MPa) > 40 > 30 Elongation at break (%) > 10 > 6 Tensile shear strength (MPa, steel/steel) >10 >5 Tg ( C) >100 >100 Viscosity (cps at 25 C) 30,000-50,000 N/A Shrinkage rate (%) <3 N/A Pot life (hour) > 2 N/A Shelf life (month) > 1 N/A Weight Gain (% loss) N/A <10% Leachable components(wt%, by water at 80 C ) < 1 N/A VOC (wt%) < 2 N/A Curing temperature ( C) < 150 N/A Molding time at curing temperature (min) < 10 N/A a Baseline measurement b Measured in 1M H2SO4 at 80 C Conclusions Initial trials with thermoset materials appear very encouraging and the data produced supports continued development of the injection molded epoxy cell. Other methods to seal a fuel cell with a thermoset are still being considered. The cost reductions that will be possible with this design are aligned with Ballard s objectives of a continued reduction in product cost and increase in product lifetime. The thermoset down-selection process is ongoing and as the process trials become more advanced, a customized thermoset will be required. The largest expected obstacles to this design were expected to be penetration into the GDL and the pressure drop of the thermoset in the mold but progress has been made towards overcoming both of these challenges. Acknowledgements Thank you to Dr. Paul Beattie, Simon Fearnley and Leya Behra for their guidance and work on this project. 8

Thermoset Applications in Fuel Cells

Thermoset Applications in Fuel Cells TRFA 2008 Annual Meeting Sept. 15 th 2008 Paul Kozak & Daniel Ramrus B A L L A R D P O W E R S Y S T E M S A C C E L E R A T I N G F U E L C E L L M A R K E T A D O