Process for making fuel cell plates

Details Process for making fuel cell plates Process for making fuel cell plates

2000-06-16

2002-09-24

Kalafut, Stephen (Department: 1745)

Plastic and nonmetallic article shaping or treating: processes

Forming electrical articles by shaping electroconductive...

C264S119000, C264S293000, C429S006000

Reexamination Certificate

active

06454978

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the manufacture of fuel cell fluid flow plates with surface indentations, and more particularly, to the manufacture of such plates in a very efficient and cost effective manner.
2. Description of the Related Art
Fuel cells are electrochemical devices which directly combine hydrogen from a fuel and oxygen, usually from the air, to produce electricity and water. With prior processing, a wide range of fuels, including hydrogen, natural gas, methanol, gasoline and coal-derived synthetic fuels, can be converted to electric power. The basic process is highly efficient (80-90%), pollution-free, quiet, free from moving parts and may be constructed to leave only heat and water as by-products. Since single fuel cells can be assembled into stacks of varying sizes, systems can be designed to produce a wide range of energy output levels and thus satisfy numerous kinds of applications.
Fuel cell construction generally consists of a fuel electrode (anode) and an oxidant electrode (cathode) separated by an ion conducting layer. In operation, current is generated by a reaction on the electrode surfaces which are in contact with an electrolyte. Fuel and oxidant are supplied as required by the current load; and water is continuously removed. The electrode reactions are comprised of the oxidation of hydrogen on the anode to hydrated protons with the release of electrons. Stated in another way, the hydrogen gas molecules split into protons and electrons. On the cathode, the reaction is of oxygen with protons to form water vapor including a consumption of electrons. Electrons flow from the anode through the external load to the cathode and the circuit is closed by an ionic current transported through the electrolyte.
There are several different types of fuel cells under such labels as phosphoric acid, alkaline, molten carbonate, solid oxide and proton exchange membrane (PEM). The basic components of a PEM fuel cell are the two electrodes separated by a polymer membrane electrolyte. Each electrode is coated on one side with a thin platinum catalyst layer. The electrodes, catalyst and membrane together form a membrane electrode assembly. In a manner analogous to that described above, hydrogen fuel dissociates or splits into free electrons and protons in the presence of the platinum catalyst at the anode. The free electrons are conducted in the form of usable electric current through the external circuit. The protons migrate through the membrane electrolyte to the cathode. At the cathode, oxygen from air, electrons from the external circuit and protons combine to form pure water and heat. Individual fuel cells produce about 0.6 volts and are combined into a fuel cell stack to provide the amount of electrical power required.
Fuel cells may be used as stationary electric power plants in buildings and residences, as vehicle power sources in cars, buses and trucks and as portable power in video cameras, computers and the like.
A single fuel cell consists of a membrane electrode assembly and two fluid flow field plates. Hydrogen and air supplied to the electrodes on either side of the PEM through channels formed in the flow field plates. Hydrogen flows through the channels to the anode where the platinum catalyst promotes separation into protons and electrons. On the opposite side of the PEM, air flows trough the channels to the cathode where oxygen in the air attracts the hydrogen protons through the PEM. The electrons are captured as useful electricity through the external circuit and combine with the protons and oxygen to produce water vapor at the cathode side.
Reference is made to U.S. Pat. No. 5,300,370 ('370) issued in 1994 which describes a typical fuel cell fluid flow plate from 1984. The plate, in the form of a rigid electrically conductive panel, includes a plurality of parallel open-faced fluid flow channels formed in a major surface of the panel. The parallel channels extend between an inlet header and an outlet header formed in the panel. The parallel channels are typically rectangular in cross section and about 0.030 inches deep and about 0.030 inches wide. The inlet header is connected to an opening in the plate through which a pressurized reactant, either fuel or oxidant, is supplied. The outlet header is also connected to an opening in the plate through which the exhaust reactant and water are discharged from the cell. The reactant runs from the inlet to the inlet header and then to the parallel channels. The reactant then diffuses through a porous electrode material to the electro catalytically active region of the membrane electrode assembly. The reactant then flows to the outlet header and then to the outlet from which it is exhausted from the fuel cell. A plurality of continuous open-face fluid flow channels formed in the surface of the plate traverse the central area of the plate in a serpentine manner. This patent goes on to disclose that the fluid flow plates are made of graphite and the channels are milled, engraved or molded.
The '370 patent discloses a new fluid flow field plate construction consisting of a stencil layer and a separator layer. The separator and stencil layers are formed of flexible graphite foil sheets having a thickness between about 0.003 inches and about 0.030 inches. Another patent, U.S. Pat. No. 5,521,018 ('018), discloses the concept of embossing a fluid flow field plate such as electrically conductive graphite foil sheet material. Other materials being sufficiently soft so as to permit embossing include porous electrically conductive sheet materials, such as carbon fibre paper, corrosive resistant metals, such as niobium; somewhat corrosive resistant material, such as magnesium or copper particularly when plated with noble metals such as gold or platinum to render them unreactive; and composite materials composed of corrosive metal powder, a base metal powder plated with corrosive resistant metal, and/or other chemically inert electrically conductive powders such as graphite and boron carbide bonded together with a suitable binder to produce a compressible electrically conductive sheet material. The embossing step is accomplished using a die where the channels are generally U-shaped or V-shaped in cross section. The '018 patent discloses that “the graphite foil sheet is embossed at an embossing pressure sufficient to impart into the compressible sheet material, smooth-surface channels, of substantially uniform depth, and having a clean, reverse image of the embossing die. Different flow field patterns and plate sizes will require different embossing pressures. The bulk of the sheet material (that is, the portions of the sheet material located apart from the channels) can also be compressed during the embossing operation and the embossing pressure can be selected to provide the appropriate channel depth in cross sectional profile, and also to impart the appropriate electrical conductivity and porosity to the bulk material.”
Still another U.S. Pat. No. 5,773,160 discloses the use of a coolant flow field plate in addition to a fuel flow field plate and an oxidant flow field plate. Yet another U.S. Pat. No. 5,981,098 ('098) issued in 1999 discusses fluid flow plates formed from a conductive material such as graphite where the flow channels are typically formed by machining. The patent also refers to an earlier fluid flow field plate comprising two outer layers of compressible electrically conductive material with an interposed center metal sheet. The outward faces of each of the two outer layers is embossed with flow field channels which are called “indentations”. The '098 patent goes on to describe fluid flow plates made by forming foil or sheet material into a design similar to a corrugation. Forming is accomplished by passing the plates between two rollers having patterns to make the channel grooves of preselected pitch and depth. One foil material is described as stainless steel. In this case, the height of the corrugated layer is 0.065 in

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