Composite gas distribution structure for fuel cell

Details Composite gas distribution structure for fuel cell Composite gas distribution structure for fuel cell

1999-10-25

2002-02-26

Chaney, Carol (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S010000, C429S010000, C429S010000, C429S006000, C429S047000, C429S047000

Reexamination Certificate

active

06350539

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a fuel cell system and more particularly to a system having a plurality of cells which consume an H
2
-rich gas to produce power for vehicle propulsion.
BACKGROUND OF THE INVENTION
Fuel cells have been used as a power source in many applications and have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A group of cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells hydrogen (H
2
) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O
2
), or air (a mixture of O
2
and N
2
). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies which comprise the catalyzed electrodes are relatively expensive to manufacture and require certain controlled conditions in order to prevent degradation thereof.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished heterogeneously within a chemical fuel processor, known as a reformer, that provides thermal energy throughout a catalyst mass and yields a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. The reforming reaction is an endothermic reaction that requires external heat for the reaction to occur.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. patent application Ser. Nos. 08/975,442 and 08/980,087, filed in November, 1997, and U.S. Ser. No. 09/187,125, filed in November, 1998, and each assigned to General Motors Corporation, assignee of the present invention. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively December 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
Efficient operation of a fuel cell depends on the ability to effectively disperse reactant gases at catalytic sites of the electrode where reaction occurs. In addition, effective removal of reaction products is required so as to not inhibit flow of fresh reactants to the catalytic sites. Therefore, it is desirable to improve the mobility of reactant and product species to and from the MEA where reaction occurs.
SUMMARY OF THE INVENTION
The present invention contemplates a diffusion structure which enhances mass transport to and from an electrode in a membrane electrode assembly (MEA) of a fuel cell. The diffusion structure cooperates and interacts with an electrode at a major surface of the electrode opposite the membrane electrolyte of the cell. The diffusion structure is a composite diffusion medium which facilitates the supply of reactant gas to the electrode. The diffusion structure also facilitates movement of water. The diffusion structure includes a characteristic bulk layer having two or more portions, each with properties defined below, including hydrophobicity and surface energy. The bulk layer is useable alone to function as a diffusion structure. However, it is preferably combined with an absorption layer and a desorption layer on respective sides of the bulk layer to form a preferred diffusion structure.
The diffusion structure preferably comprises an absorption layer which has a first electrically conductive material. The absorption layer has a surface facing or engaging the major surface of the electrode structure; and the absorption layer accepts water from the electrode structure. Water is a product of the reaction in the cell between hydrogen and air at the cathode.
The diffusion structure also comprises the bulk layer which has a second electrically conductive material. The bulk layer has a surface facing or engaging a major surface of the absorption layer opposite the electrode structure. The bulk layer has at least two portions, the first portion is less hydrophobic than the second portion. The first portion is nearest the absorption layer.
The diffusion structure preferably further comprises a desorption layer which has a third electrically conducted material. The desorption layer has a surface facing or engaging the second portion of the bulk layer, and an opposite surface facing away from the electrode structure. Water is released at this opposite surface of the desorption layer.
Preferably, the bulk layer comprises at least one intermediate portion between the first and second portions, where the hydrophobicity of each of the intermediate portions is greater than the first portion and less than the second portion. Preferably, the hydrophobic character of each intermediate portion is selected so that hydrophobicity increases in a direction away from the membrane electrode assembly. Preferably, a plurality of intermediate layers is arranged between the first and second portions, with decreasing surface energy and increasing hydrophobicity in the direction from the first portion to the second portion. Preferably, the diffusion structure is further characterized by increasing hydrophobicity and by decreasing surface energy in a direction from the electrode toward the opposite surface of the desorption layer.
In another aspect of the invention, specific materials are selected for the absorption layer, the bulk layer, and the desorption layer to provide the properties of surface energy, hydrophobicity, and corresponding hydrophilicity to optimize movement of reactant gases in a direction toward the membrane electrode assembly and to move product gases and water in a direction away from the membrane electrode assembly. Accordingly, the absorption layer preferably comprises the first electrically conductive material dispersed in a fluorinated polymeric binder (PVDF). The bulk layer first portion preferably consists essentially of the second electrically conductive material. The second portion of the bulk layer comprises the second electrically conductive material intermingled with polytetrafluoroethylene (PTFE). The amount by weight of the PTFE is less than the amount of the electrically conductive material in the second portion. The desorption layer preferably comprises the third electrically conductive material

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