Fuel cells are well-known and are commonly used to produce electrical energy from reducing and oxidizing reactant fluids to power electrical apparatus, such as apparatus on-board space vehicles, transportation vehicles, or as on-site generators for buildings. Each individual fuel cell generally includes an anode catalyst and a cathode catalyst separated by an electrolyte, such as a proton exchange membrane (“PEM”) as known in the art. Frequently, a diffusion layer is secured between the catalyst and a substrate layer. The substrate layer is usually secured between the diffusion layer and a flow field. If there is no diffusion layer, the substrate is secured between the catalyst and the flow field. Flow fields define flow channels for directing reactant streams through the fuel cell in fluid communication through the diffusion and substrate layers with the catalysts. As is known, the flow fields may be porous water transport plates or solid separator plates.
The diffusion layer is typically a highly porous, electrical conductor made from carbon black and a hydrophobic polymer, such as polytetrafluoroethylene. Depending on the fuel cell design, the diffusion layer is usually hydrophobic, however, the diffusion layer may be partially hydrophobic and partially hydrophilic to facilitate simultaneous liquid and gaseous transport through the layer. The diffusion layer is usually about 25-100 microns thick. The diffusion layer facilitates transfer of the reactant streams through the fuel cell by minimizing the thickness of water films on the surface of the catalysts. The diffusion layer also facilitates the removal of product water from the fuel cell.
The substrate layer adjacent the diffusion layer is highly porous and made from expensive carbon fibers and a well known manufacturing process that requires high temperature graphitizing. Depending on the cell design, the substrate layer may be either hydrophobic or hydrophilic. The substrate is usually about 150-300 microns thick. The substrate facilitates the transport of reactant streams, water vapor, liquid water and electrons. The substrate conducts electrons both through the plane of the substrate and in the plane of the substrate from a centerline of an adjacent flow channel to ribs of the flow field, wherein the flow channel is defined between ribs of the flow field. The substrate also facilitates the diffusion and flow of reactant streams and product water both through the plane of the substrate and in the plane of the substrate from the flow channel to the centerline of the flow channel beneath the rib of the flow field, or vice versa. Also, the substrate must have a flexural strength adequate to distribute an axial pressure load relatively uniformly over total surface areas of adjacent layers. For example, the pressure load must be distributed evenly across the flow channels of the flow field to prevent the substrate from deforming into the flow channels. An exemplary flexural strength is about 200 kilogram force per square centimeter (“kgf/cm2”). U.S. Pat. No. 4,851,304 to Miwa et al. describes properties of typical fuel cell substrates. Planar types of fuel cells are secured in compression in a fuel cell stack by a combination of pressure plates and tie-rods, as is well known. This axial compressive force minimizes the resistance of the cells and is required to obtain suitable fluid seals. The substrate must have a compressive strength that is typically at least two times the axial force on the cell stack. Typical compressive strength of a prior art substrate is greater than 10 kgf/cm2.
In fuel cells of the prior art, it is known that carbon or graphite layers or papers are traditionally secured between the catalysts and flow fields as diffusion and/or substrate layers. However, the use of known carbon or graphite layers or papers presents significant problems, including high manufacturing costs, impeding the diffusion of hydrogen and oxygen through pores defined by the layers, and impeding the outflow of fuel cell product water from a cathode catalyst.
A partial solution to the use of carbon or graphite paper is disclosed in U.S. Pat. No. 5,707,755, entitled “PEM/SPE Fuel Cell” that issued on Jan. 13, 1998 to Grot. The patent discloses, instead of the carbon or graphite layers or papers, the use of a plurality of electrically conductive filaments secured with a specific orientation with respect to flow channels, or grooves of a flow field. The specific orientation is longitudinal so that the filaments extend across, and do not fall into, the flow channels, or grooves, that direct reactant flow through the fuel cell.
Nonetheless, the prior art has limitations. Manufacturing a fuel cell with conductive filaments having such a specific orientation imposes substantial cost and manufacturing burdens. Moreover, the prior art fails to rectify the significant substrate manufacturing costs associated with the mass production of fuel cells. The substrates are costly because they include expensive carbon fibers and are manufactured through a costly high temperature graphitizing process well known in the art. Accordingly, there is a need for a fuel cell that minimizes substrate costs by replacing the prior art substrates with a cost effective material.