A fuel cell converts chemical energy into electrical energy and some thermal energy by means of a chemical reaction between a fuel (e.g., hydrogen gas or a hydrogen-containing fluid) and an oxidant (e.g., oxygen or air). A proton exchange membrane (PEM) fuel cell uses hydrogen or hydrogen-rich reformed gases as the fuel, a direct-methanol fuel cell (DMFC) uses methanol-water solution as the fuel, and a direct ethanol fuel cell (DEFC) uses ethanol-water solution as the fuel, etc. These types of fuel cells that require utilization of a PEM layer as a proton transport electrolyte are collectively referred to as PEM-type fuel cells.
A PEM-type fuel cell is typically composed of a seven-layered structure, including (a) a central PEM electrolyte layer for proton transport; (b) two electro-catalyst layers on the two opposite primary surfaces of the electrolyte membrane; (c) two fuel or gas diffusion electrodes (GDEs, hereinafter also referred to as diffusers) or backing layers stacked on the corresponding electro-catalyst layers (each GDE comprising porous carbon paper or cloth through which reactants and reaction products diffuse in and out of the cell); and (d) two flow field plates (or a bi-polar plate) stacked on the GDEs. The flow field plates are typically made of graphite, metal, or conducting composite materials, which also serve as current collectors. Gas-guiding channels are defined on a GDE facing a flow field plate or, more typically, on a flow field plate surface facing a GDE. Reactants (e.g., H2 or methanol solution) and reaction products (e.g., CO2 at the anode of a DMFC, and water at the cathode side) are guided to flow into or out of the cell through the flow field plates. The configuration mentioned above forms a basic fuel cell unit. Conventionally, a fuel cell stack comprises a number of basic fuel cell units that are electrically connected in series to provide a desired output voltage. If desired, cooling channels and humidifying plates may be added to assist in the operation of a fuel cell stack.
In one common practice, a fuel flow field plate and an oxidant gas flow field plate are separately made and then assembled together to form a bipolar plate (one side of a bipolar plate serving as a negative terminal and the other side as a positive terminal, hence the name). In some cases, an additional separator is sandwiched between the two flow field plates to form a bipolar plate. It would be highly advantageous if the flow filed plates and the separator can be mass-produced into an integrated bipolar plate assembly. This could significantly reduce the overall fuel cell production costs and reduce contact ohmic losses across constituent plate interfaces. The bipolar plate is known to significantly impact the performance, durability, and cost of a fuel cell system. The bipolar plate, which is typically machined from graphite, is one of the most costly components in a PEM fuel cell.
Fluid flow field plates have open-faced channels formed in one or both opposing major surfaces for distributing reactants to the gas diffuser plates (the anode and cathode backing layers, typically made of carbon paper or fabric). The open-faced channels also provide passages for the removal of reaction products and depleted reactant streams. Optionally, a bipolar plate may have coolant channels to manage the fuel cell temperature. A bipolar plate should have the following desirable characteristics: high electrical conductivity (e.g., preferably having a conductivity no less than 100 S/cm), low permeability to fuel or oxidant fluids, good corrosion resistance, and good structural integrity.
Conventional methods of fabricating fluid flow field plates require the engraving or milling of flow channels into the surface of rigid plates formed of a metal, graphite, or carbon-resin composite. These methods of fabrication place significant restrictions on the minimum achievable fuel cell thickness due to the machining process, plate permeability, and required mechanical properties. Further, such plates are expensive due to high machining costs. The machining of channels into the graphite plate surfaces causes significant tool wear and requires significant processing times.
Alternatively, fluid flow field plates can be made by a lamination process (e.g., U.S. Pat. No. 5,300,370, issued Apr. 5, 1994), wherein an electrically conductive, fluid impermeable separator layer and an electrically conductive stencil layer are consolidated to form one open-faced channel. Presumably, two conductive stencil layers and one separator layer may be laminated to form a bipolar plate. It is often difficult and time-consuming to properly position and align the separator and stencil layers. Die-cutting of stencil layers require a minimum layer thickness, which limits the extent to which fuel cell stack thickness can be reduced. Such laminated fluid flow field assemblies tend to have higher manufacturing costs than integrated plates, due to the number of manufacturing steps associated with forming and consolidating the separate layers. They are also prone to delamination due to poor interfacial adhesion and vastly different coefficients of thermal expansion between a stencil layer (typically a metal) and a separator layer.
A variety of composite bipolar plates have been developed, which are mostly made by compression molding of polymer matrices (thermoplastic or thermoset resins) filled with conductive particles such as graphite powders or fibers. Because most polymers have extremely low electronic conductivity, excessive conductive fillers have to be incorporated, resulting in an extremely high viscosity of the filled polymer melt or liquid resin and, hence, making it very difficult to process. Bi-polar plates for use in PEM fuel cells constructed of graphite powder/fiber filled resin composite materials and having gas flow channels are reviewed by Wilson, et al (U.S. Pat. No. 6,248,467, Jun. 19, 2001). Injection-molded composite-based bipolar plates are disclosed by Saito, et al. (U.S. Pat. No. 6,881,512, Apr. 19, 2005 and U.S. Pat. No. 6,939,638, Sep. 6, 2005). These thermoplastic or thermoset composites exhibit a bulk conductivity significantly lower than 100 S/cm (the US Department of Energy target value), typically not much higher than 10 S/cm. The plates produced tend to be relatively thick.
Besmann, et al. disclosed a carbon/carbon composite-based bipolar plate (U.S. Pat. No. 6,171,720 (Jan. 9, 2001) and U.S. Pat. No. 6,037,073 (Mar. 14, 2000)). The manufacture process consists of multiple steps, including production of a carbon fiber/phenolic resin preform via slurry molding, followed by a compression-molding step. The molded part is then pyrolyzed at a high temperature (1,500° C.-2,500° C.) to obtain a highly porous carbon/carbon composite. This is followed by chemical vapor infiltration (CVI) of a carbon matrix into this porous structure. It is well-known that CVI is a very time-consuming and energy-intensive process and the resulting carbon/carbon composite, although exhibiting a high electrical conductivity, is very expensive.
Instead of using pyrolyzation and CVI to produce carbon/carbon composites, Huang, et al. (US Patent Application Pub. No. 2004/0229993, Nov. 18, 2004) discloses a process to produce a thermoplastic composite with a high graphite loading. First, polymer fibers, such as thermotropic liquid crystalline polymers or polyester, reinforcing fibers such as glass fibers, and graphite particles are combined with water to form a slurry. The slurry is pumped and deposited onto a sieve screen. The sieve screen serves the function of separating the water from the mixture of polymer fibers, glass fibers and graphite. The mixture forms a wet-lay sheet which is placed in an oven. Upon heating to a temperature sufficient to melt the polymer fibers, the wet-lay sheet is allowed to cool and have the polymer material solidify. Upon solidification, the wet-lay sheet takes the form of a sheet material with reinforcement glass fibers held together by globules of thermoplastic material, and graphite particles adhered to the sheet material by the thermoplastic material. Several of these sheets are then stacked, preferably with additional graphite powder interspersed between sheets, and compression-molded in a hot press. After application of heat and pressure in the press, one or more formed bipolar plates are obtained, where the bipolar plates are a composite of glass fibers, thermoplastic matrix and graphite particles. Clearly, this is also a tedious process which is not amenable to mass production.
Alternatively, fluid flow field plates can be made from an electrically conductive, substantially fluid impermeable material that is sufficiently compressible or moldable so as to permit embossing. Flexible graphite sheet is generally suitable for this purpose because it is relatively impervious to typical fuel cell reactants and coolants and thus is capable of isolating the fuel, oxidant, and coolant fluid streams from each other. It is also compressible and embossing processes may be used to form channels in one or both major surfaces. The “flexible graphite” is the exfoliated reaction product of rapidly heated natural graphite particles which have been treated with an agent that intercalates into the crystal structure of the graphite to expand the intercalated particles at least 80 or more times (up to 1000 times) in the direction perpendicular to the carbon layers in the crystal structure. The exfoliated graphite may be compressed together into flexible sheets which, unlike the original graphite flakes, can be formed and cut into various shapes. These thin sheets (foils or films) are hereinafter referred to as flexible graphite. Flexible graphite can be wound up on a drum to form a roll of thin film, just like a roll of thin plastic film or paper.
Although highly conductive, flexible graphite sheets by themselves do not have sufficient stiffness and must be supported by a core layer or impregnated with a resin. For example, U.S. Pat. No. 5,527,363 (Jun. 18, 1996) discloses a fluid flow field plate comprising a metal sheet interposed between two flexible graphite (FG) sheets having flow channels embossed on a major surface thereof. These FG-metal-FG laminates are also subject to the delamination or blistering problem, which could weaken the plate and may make it more fluid permeable. Delamination or blistering can also cause surface defects that may affect the flow channels on the plate. These problems may be difficult to detect during fabrication and may only emerge at a later date. In particular, thermal cycling between frozen and thawed conditions as are likely to be encountered in an automobile application of the fuel cell, often results in delamination between a flexible graphite layer and the metal layer. Alternatively, Mercuri, et al. (U.S. Pat. No. 5,885,728, Mar. 23, 1999) discloses a flexible graphite sheet having embedded fibers extending from its surface into the sheet to increase the resin permeability of the sheet for the preparation of a resin-impregnated flexible graphite bipolar plate. The step of adding ceramic fibers significantly increases the process complexity and cost.
The flow field plate should be constructed from inexpensive starting materials, materials that are easily formed into any plate configuration, preferably using a continuous molding process, and materials that are corrosion resistant in low temperature fuel cells and that do not require further processing such as high temperature pyrolization treatments. Any laminated or multi-layer plate should have adequate bonding between layers to ensure structural integrity and reduced contact resistance (reduced power loss due to joule heating).
In our earlier applications, we provided a sheet molding compound (SMC) composition for use as a fuel cell flow field plate or bipolar plate [Bor Z. Jang, “Sheet Molding Compound Flow Field Plate, Bipolar Plate and Fuel Cell,” U.S. patent application Ser. No. 11/293,540 (Dec. 5, 2005) and Bor Z. Jang, A. Zhamu, Lulu Song, “Method for Producing Highly Conductive Sheet Molding Compound, Fuel cell Flow Field Plate, and Bipolar Plate,” U.S. patent application Ser. No. 11/293,541 (Dec. 5, 2005)]. This SMC composition comprises a top flexible graphite (FG) sheet, a bottom FG sheet, and a resin mixture sandwiched between the two FG sheets. The resin mixture comprises a thermoset resin and a conductive filler. The flexible graphite sheet has a planar outer surface having formed therein a fluid flow channel. This SMC structure is simple and the production process is fast and continuous. However, commercially available FG sheets can be expensive. Furthermore, a FG sheet tends to have their constituent graphene platelets (exfoliated graphite platelets) being parallel to the FG sheet plane. The graphite crystal is known to have a high electrical conductivity on the basal plane (graphene plane), but not perpendicular to it. As a consequence, the in-plane electrical conductivity of a FG sheet is much greater than its thickness-direction conductivity. Unfortunately, it is the thickness-direction conductivity of a FG-based SMC that is important for a bipolar plate, rather than the in-plane conductivity. It would be highly desirable if the top and bottom graphite sheets have a higher thickness-direction conductivity (e.g., with graphite platelets oriented perpendicular to a bipolar plate plane).
In another earlier invention [Bor Z. Jang, A. Zhamu, and Lulu Song, “Highly Conductive Composites for Fuel Cell Flow Field Plates and Bipolar Plates,” U.S. patent application Ser. No. 11/324,370 (Jan. 4, 2006)], we provided an electrically conductive polymer composite as a bipolar plate. The composite is composed of (A) at least 50% by weight of a conductive filler, comprising at least 5% by weight reinforcement fibers, expanded graphite platelets, graphitic nano-fibers, and/or carbon nano-tubes; (B) a thermoplastic matrix at 1 to 49.9% by weight; and (C) a thermoset binder at 0.1 to 10% by weight; wherein the bulk electrical conductivity of the flow field or bipolar plate is at least 100 S/cm. The thermoset binder resin is used to hold the reinforcement elements (platelets, fibers, nano-tubes, etc.) together to form a composite preform. This composite preform that has a thermoplastic matrix material is at a later time molded into a thermoplastic composite. In one preferred embodiment of this invention, we suggested a bipolar plate that comprises such a thermoplastic composite having a skin layer less than 100 μm in thickness wherein the skin layer has a polymer volume fraction less than 20% and a conductive filler greater than 80%. This composite composition provides a high conductivity, which is a highly desirable feature of a bipolar plate. However, such a composite composition (containing at least 50% conductive reinforcements) tends to result in a thick or bulky molded structure and, hence, is not suitable for the fabrication of thin bipolar plates. A high filler proportion also means a high mixture viscosity and can present processing difficulty. The composite is limited to thermoplastic matrix materials and it requires the use of a thermoset binder to hold the filler particles together first prior to a shape molding operation. A simpler chemical formulation that enables simpler, more convenient, and faster processing of bipolar plates (hence, lower costs) is highly desirable.
Accordingly, an object of the present invention is to provide a new and improved fuel cell flow field plate or a bipolar plate that is a well-integrated, non-SMC component and can be made by using a simple, fast and cost-effective process. The process can be automated and adaptable for mass production. In particular, the bipolar plate has a thin, conductive composite core layer cladded between two carbon/graphite coating (clad) layers. The composite core layer provides some excess resin to help hold the carbon/graphite particles in the clad layers together. The clad layers can be very thin; they can be as thin as 1 μm or thinner and typically can be as thick as 20 μm. The resulting fuel cell system is highly conductive and well suitable for use as a current collector. This instant invention represents a significant improvement over and above the prior art bipolar plates, including our own previously invented plates.