Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-04-06
2002-09-17
Maples, John S. (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S010000, C429S010000
Reexamination Certificate
active
06451466
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to a functional integration of multiple components for a fuel cell power plant, and deals more particularly with the functional integration of multiple components of a fuel cell power plant whereby the strong characteristics of each component may be utilized to compensate for the weak characteristics of the other components.
BACKGROUND OF THE INVENTION
Electrochemical fuel cell assemblies are known for their ability to produce electricity and a subsequent reaction product through the interaction of a fuel being provided to an anode electrode and an oxidant being provided to a cathode electrode, generating an external current flow there-between. Such fuel cell assemblies are very useful due to their high efficiency, as compared to internal combustion fuel systems and the like, and may be applied in many fields. Fuel cell assemblies are additionally advantageous due to the environmentally friendly chemical reaction by-products, typically water, which are produced during their operation. Owing to these characteristics, amongst others, fuel cell assemblies are particularly applicable in those fields requiring highly reliable, stand-alone power generation, such as is required in space vehicles and mobile units including generators and motorized vehicles.
Electrochemical fuel cell assemblies typically employ a hydrogen rich gas stream as a fuel and an oxygen rich gas stream as an oxidant where the reaction by-product is water. Such fuel cell assemblies may employ a membrane consisting of a solid polymer electrolyte, or ion exchange membrane, disposed between the anode and cathode electrode substrates formed of porous, electrically conductive sheet material—typically, carbon fiber paper. One particular type of ion exchange membrane is known as a proton exchange membrane (hereinafter PEM), such as sold by DuPont under the trade name NAFION™ and well known in the art. Catalyst layers are formed between the PEM and each electrode substrate to promote the desired electrochemical reaction. The catalyst layer in a fuel cell assembly is typically a carbon supported platinum or platinum alloy, although other noble metals or noble metal alloys may be utilized. In order to control the temperature within the fuel cell assembly, a water coolant is typically provided to circulate about the fuel cell assembly.
Other commonly known electrolytes used in fuel cell assemblies include phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix. It has, however, been found that PEM fuel cell assemblies have substantial advantages over fuel cells with liquid or alkaline electrolytes due to the superior performance of the PEM in providing a barrier between the circulating fuel and oxidant, while also being more tolerant to pressure differentials than a liquid electrolyte that is held by capillary forces within a porous matrix. Moreover, a PEM electrolyte is fixed and will not leach from the fuel cell assembly and retains a relatively stable capacity for water retention.
In the typical operation of a PEM fuel cell assembly, a hydrogen rich fuel permeates the porous electrode material of the anode and reacts with the catalyst layer to form hydrogen ions and electrons. The hydrogen ions migrate through the PEM to the cathode electrode while the electrons flow through an external circuit to the cathode electrode. At the cathode electrode, the oxygen-containing gas supply also permeates through the porous substrate material and reacts with the hydrogen ions and the electrons from the anode electrode at the catalyst layer to form the by-product water. Not only does the PEM facilitate the migration of these hydrogen ions from the anode to the cathode, but the ion exchange membrane also acts to isolate the hydrogen rich fuel from the oxygen-containing gas oxidant. The reactions taking place at the anode and cathode catalyst layers are represented by the equations:
Anode reaction: H
2
→2H
+
+2e
Cathode reaction: ½O
2
+2H
+
+2e→H
2
O
Conventional PEM fuels cells have the ion exchange membrane positioned between two permeable, electrically conductive plates, referred to as the anode and cathode plates. The plates are typically formed from graphite, a graphite-polymer composite, or the like. The plates act as a structural support for the two porous, electrically conductive electrode substrates, as well as serving as current collectors and providing the means for carrying the fuel and oxidant to the anode electrode and cathode electrode, respectively. They are also utilized for carrying away the reactant by-product water during operation of the fuel cell.
Moreover, the plates may have formed therein reactant feed manifolds which are utilized for supplying the fuel to the anode flow channels or, alternatively, the oxidant to the cathode flow channels. They may also have corresponding exhaust manifolds to direct unreacted components of the fuel and oxidant streams, and any water generated as a by-product, from the fuel cell. The construction and operation of a typical PEM fuel cell are well known and are described in detail in commonly owned U.S. Pat. No. 5,853,909, issued to Reiser, and incorporated herein by reference in its entirety. Alternatively, the manifolds may be external to the fuel cell itself, as shown in commonly owned U.S. Pat. No. 3,994,748, issued to Kunz et al., and incorporated herein by reference in its entirety.
Recent efforts at producing the fuel for fuel cell assemblies have focused on utilizing a hydrogen rich gas produced from the chemical conversion of hydrocarbon fuels, such as methane, natural gas, gasoline or the like, into a hydrogen rich stream. This process requires that the hydrogen produced must be efficiently converted to be as pure as possible, thereby ensuring that a minimal amount of carbon monoxide and other undesirable chemical byproducts are produced. This conversion of hydrocarbons is generally accomplished through the use of a steam reformer or an autothermal reformer. Reformed hydrocarbon fuels frequently contain quantities of ammonia, NH
3
, as well as significant quantities of carbon dioxide, CO
2
. These gases tend to dissolve and dissociate into the water which is provided to, and created within, the fuel cell assembly. The resultant contaminated water supply may cause the conductivity of the water to increase to a point where shunt current corrosion occurs in the coolant channels and manifold leading to degradation of fuel cell materials, as well as reducing the electrical conductivity of the PEM and thereby reducing the efficiency of the fuel cell assembly as a whole.
As disclosed above, the anode and cathode plates may be part of a coolant loop which provides coolant channels for the circulation of a water coolant, as well as for the wicking and carrying away of excessive water produced as a by-product of the fuel cell assembly operation. The water which is collected and circulated through a fuel cell assembly is therefore susceptible to contamination and may damage and impair the operation of the fuel cell assembly.
It is therefore necessary to provide a system which protects the fuel cell assembly from water contamination. One such system is described in commonly owned U.S. Pat. No. 4,344,850, issued to Grasso, and incorporated herein by reference in its entirety. Grasso's system for treating the coolant supply of a fuel cell assembly, as illustrated in FIG. 1 of 4,344,850, utilizes a separate filter and demineralizer for purifying a portion of the coolant supplied to the fuel cell assembly. A separate degasifier is also utilized to process the condensed water obtained from a humidified cathode exit stream. As discussed in Grasso, the heat exchange occurring between the coolant stream and the body of the fuel cell assembly is accomplished according to commonly assigned U.S. Pat. No. 4,233,369, issued to Breault et al., incorporated herein by reference in its entirety.
It is important to note that Grasso's coolant system does not provide
Grasso Albert P.
Van Dine Leslie L.
Vartanian George
Maples John S.
McCormick Paulding & Huber LLP
UTC Fuel Cells LLC
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