Polymer electrolyte membrane or proton exchange membrane (PEM) fuel cell systems have intrinsic benefits and a wide range of applications due to their relatively low operating temperatures and good balance of specific power (Watts/kg), power density (Watts/liter), specific energy (Watt-hours/kg) and energy density (Watt-hours/liter). The active portion of a PEM cell is a membrane sandwiched between an anode and a cathode layer. Fuel containing hydrogen is passed over the anode and oxygen (air) is passed over the cathode. The reactants, through the electrolyte membrane, react indirectly with each other generating an electrical voltage between the cathode and anode. Typical electrical potentials of PEM cells can range from 0.5 to 0.9 volts where the higher the cell voltage, the greater the electrochemical efficiency. At lower cell voltages, the current density is higher but there is eventually a peak value in power density for a given set of operating conditions. The electrochemical reaction also generates heat and water as byproducts that must be extracted from the fuel cell, although the extracted heat can be used in a cogeneration mode, and the product water can be used for humidification of the membrane, cell cooling or dispersed to the environment.
Multiple cells are combined by stacking, interconnecting individual cells in an electrical series configuration. The voltage generated by the fuel cell stack is effectively the sum of the individual cell voltages. There are designs that use multiple cells in parallel or in a combination series-parallel connection. Fluid flow field plates are inserted between the cells to separate the anode reactant of one cell from the cathode reactant of the next cell. These plates are typically graphite based or metallic in nature. To provide hydrogen to the anode and oxygen to the cathode without mixing, a system of fluid distribution and seals is required.
The dominant design at present in the fuel cell industry is to use fluid flow field plates with the flow fields machined, molded or otherwise impressed. An optimized flow field plate has to fulfill a series of requirements: very good electrical and heat conductivity; gas tightness; corrosion resistance; low weight; and low cost. The fluid flow field plate design ensures good fluid distribution as well as the removal of product water and heat generated. Manifold design is also critical to uniformly distribute fluids between each separator/flow field plate.
There is an ongoing effort to innovate in order to increase the specific power and power density (reduce weight and volume) of fuel cell stacks, and to reduce material and assembly costs.
In a fuel cell system (stack & balance of plant), the stack is the dominant component of the fuel cell system's weight and cost and the fluid flow field plates are the major component (both weight and volume) of the stack.
Fluid flow field plates are a significant factor in determining the specific power and power density of a fuel cell, typically accounting for 40 to 70% of the weight of a stack and almost all of the volume. For component developers, the challenge is therefore to reduce the weight, size and cost of the fluid flow field plate while maintaining the desired properties for high-performance operation.
The material for the fluid flow field plate must be selected carefully due to the challenging environment in which it operates. In general, it must possess a particular set of properties and combine the following characteristics:                High electrical conductivity, especially in through-plane direction        Low contact resistance with the gas diffusion layer (GDL)        High thermal conductivity, both in-plane and through-plane        Good thermal stability, limiting expansion and contraction due to temperature variations        Good mechanical strength and resistance to cracking        Able to maintain good feature tolerance for flow fields, etc.        Fluid impermeability to prevent reactant and coolant leakage, especially for the case of gaseous hydrogen        Corrosion resistance        Resistance to ion-leaching, so as not to contaminate the membrane electrode assembly (MEA)        Thin and lightweight        Low cost and ease of manufacturing        Recyclable        Environmentally benign        
A number of different methods have been used to manufacture fluid flow field plates including for example, U.S. Pat. No. 5,300,370 to Washington et al for “Laminated Fluid Flow Field Assembly for Electrochemical Fuel Cells” on Apr. 5, 1994. This patent describes a laminated fluid flow field assembly comprising a separator layer and a stencil layer, where in operation, the separator layer and stencil layer cooperate to form an open faced channel for conducting pressurized fluids. Although this patent is namely for discontinuous flow field configurations, it also addresses continuous flow field designs. This method, however, has a number of significant drawbacks which focus mainly on the fabrication of the stencil layer. When the flow channels in the stencil layer are formed, material is removed from the flow field plate, and therefore the remaining channel landings are left unsupported. Effectively, the landings of the stencil layer plate would move indiscriminately, therefore leaving the stencil layer to be very difficult to handle and position. Further, the tolerance required for the correct flow channel width to ensure accurate fluid flow distribution per channel would not be maintained, especially for the continuous flow field design.
Another example is provided in U.S. Pat. No. 5,521,018 to Wilkinson et al for “Embossed Fluid Flow Field Plate for Electrochemical Fuel Cells” on May 28, 1996. This patent namely describes an embossed fluid flow field plate comprising two sheets of compressible, electrically conductive material, where each sheet has two oppositely facing major surfaces, where at least one of the major surfaces has an embossed surface which has a fluid inlet and at least one open-faced channel embossed therein. A metal sheet is interposed between each of the compressible sheets. Although this patent focuses mainly on embossed fluid flow field plates, it provides an example of a coolant flow field plate where a single coolant flow channel is die-cut and the sealant channel is embossed. It is indeed an advantage to have a single channel joining the fluid inlet and fluid outlet when removing material to form the flow channel, as in this case, since the perimeter of the channel is effectively supported. With that said, the channel is of a complex, serpentine geometry and even though it is supported around the perimeter, the landings are not supported within the plate, therefore making it impractical to handle and position after it is fabricated.
U.S. Pat. No. 5,683,828 to Spear et al for “Metal Platelet Fuel Cells Production and Operation Methods” on Nov. 4, 1997 describes fuel cell stacks comprising stacked separator/membrane electrode assembly cells in which the separators comprise a series of stacked thin sheet platelets having individually configured serpentine micro-channel reactant gas humidification, active area and cooling fields within. Although this patent outlines a method to fabricate a metal platelet comprising a complex serpentine flow geometry which is supported throughout by a means to maintain the correct flow channel spacing, thereby allowing the platelet to be easily handled after fabrication without the flow channel landings shifting, the method described for manufacturing these flow channel supports is depth etching, which is a relatively costly manufacturing method and does not lend itself to higher volume production.
Thus, there is a need for an improved method for fabricating fluid flow field plates with complex fluid flow field geometries.