Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2001-04-09
2003-04-01
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S010000, C429S006000, C429S006000, C429S006000
Reexamination Certificate
active
06541147
ABSTRACT:
RELATED APPLICATION
This application includes subject-matter incorporated from applicant's British Patent Application Serial No. 9814123.7 filed on Jul. 1, 1998.
FIELD OF THE INVENTION
The present invention relates to electrochemical cells and particularly to fuel cells incorporating a proton exchange membrane. More particularly, the present invention relates to the use of printed circuit boards to form internal separator layers for non-planar electrolyte layered fuel cells.
BACKGROUND
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the reaction product is water. Conventional proton exchange membrane (“PEM”) fuel cells generally employ a planar, layered structure known as a membrane electrode assembly (“MEA”), comprising a solid polymer electrolyte or ion exchange membrane, which is neither electrically conductive nor porous, disposed between an anode electrode layer and a cathode electrode layer. The electrode layers are typically comprised of porous, electrically conductive sheets with elecro-catalyst particles at each membrane-electrode interface to promote the desired electrochemical reaction.
During operation of the fuel cell, hydrogen from a fuel gas stream moves from fuel channels through the porous anode electrode material and is oxidized at the anode electro-catalyst to yield electrons to the anode plate and hydrogen ions which migrate through the electrolyte membrane. At the same time, oxygen from an oxygen-containing gas stream moves from oxidant channels through the porous electrode material to combine with the hydrogen ions that have migrated through the electrolyte membrane and electrons from the cathode plate to form water. A useful current of electrons travels from the anode plate through an external circuit to the cathode plate to provide electrons for the reaction occurring at the cathode electro-catalyst.
In conventional fuel cells, the MEA is interposed between two rigid, planar, substantially fluid-impermeable, electrically conductive plates, commonly referred to as separator plates. The plate in contact with the anode electrode layer is referred to as the anode plate and the plate in contact with the cathode electrode layer is referred to as the cathode plate. The separator plates (1) serve as current collectors, (2) provide structural support for the MEA, and (3) typically provide reactant channels for directing the fuel and oxidant to the anode and cathode electrode layers, respectively, and for removing products, such as water, formed during operation of the fuel cell. Fuel channels and oxidant channels are typically formed in the separator plates; the plates are then normally referred to as fluid flow field plates. Herein, “fluid” shall include both gases and liquids; although the reactants are typically gaseous, the products may be liquids or liquid droplets as well as gases.
Multiple unitary fuel cells can be stacked together to form a conventional fuel cell stack to increase the overall power output. Stacking is typically accomplished by the use of electrically conductive bipolar plates which act both as the anode separator plate of one fuel cell and as the cathode separator plate of the next fuel cell in the stack. One side of the bipolar plate acts as an anode separator plate for one fuel cell, while the other side of the bipolar plate acts as a cathode separator plate for the next fuel cell in the stack. The bipolar plates combine the functions of anode and cathode plates referred to above and are provided with the fuel channels and oxidant channels. The internal structure of fuel cell stacks based on planar MEA elements requires complex bi-polar separator plates in which the fluid flow channels have been formed by removing material from the plate, usually through some form of machining process.
Watkins, U.S. Pat. Nos. 4,988,583 and 5,108,849, issued Jan. 29, 1991 and Apr. 28, 1992, respectively, describe fluid flow field plates in which continuous open-faced fluid flow channels formed in the surface of the plate traverse the central area of the plate surface in a plurality of passes, that is, in a serpentine manner, between an inlet manifold opening and an outlet manifold opening formed in the plate. These patents are typical of conventional fuel cell designs.
Undulate electrolyte layer fuel cells have also been proposed in high temperature, molten carbonate type fuel cells. For example, BBC Brown Boveri (FR 2306540) proposes a non-planar electrolyte layered molten carbonate fuel cell, and German Patent DE 3812813 proposes the use of a non-planar glass electrolyte layer. Japanese patent 1-292759 takes the non-planar electrolyte molten carbonate fuel cell concept one step further, proposing a different means of obtaining the non-planar structure. These molten carbonate cells are based entirely upon the use of planar separator layers and rely exclusively upon the use of metals and high temperature bonding techniques for cell construction. Construction of a PEM cell is impossible using the concepts disclosed in these patents.
McIntyre, U.S. Pat. No. 4,826,554, issued May 2, 1989, discloses a sinuously-formed “electrically conductive, hydraulically permeable matrix 130, which is also embedded into the membrane sheet 120”. However, there is no disclosure of alternating layers in a stack that contact one another to form interior flow conduits or channels.
Japanese Patent Publication No. 50903/1996, Futoshi et al., Feb. 20, 1996, discloses a solid polymer fuel cell having generally planar separators with alternating protruding parts serving to clamp a power generation element (apparently an MEA) into a non-planar but piecewise linear shape. The area of the MEA exposed to reactants is increased relative to planar MEA designs, but the portions of the MEA clamped between the protruding parts and the planar portion of each separator do not appear to be exposed to reactants. Further, significant clamping force appears to be required to reduce contact resistance. Such force, together with the abrupt changes in direction at the corners of the protruding parts, may introduce kinks and very large stresses in the MEA.
Separators that have been disclosed in the prior art are typically composed of flat sheets of simply conductive material such as metal or in some cases graphite.
British application Serial No. 9814123.7 (McLean et al., assigned to the applicant herein) filed on Jul. 1, 1998 and derivatives and divisionals thereof provide details of different aspects of non-planar MEA layers in PEM fuel cells, and other aspects of PEM fuel cell design.
SUMMARY OF THE INVENTION
In accordance with the present invention, a fuel cell stack comprises a stacked series of MEA structures alternating with aligned separator plates, each MEA structure being non-planar and having sufficient rigidity to retain its shape when the stack is placed under sufficient pressure in the stacking direction to maintain physical and electrical contact between each MEA structure and the adjacent separator plate and forming ,thereby, the fuel and oxidant channels between the MEA structure and the separator plates, each separator plate comprising an electrically insulating substrate overlaid on each surface by a selected pattern of electrically conductive traces, each trace on one surface of the substrate electrically connected to at least one trace on the opposite surface of the substrate by a conductive path, and the pattern of the traces selected so that the traces on each surface of the substrate are in electrical contact with the adjacent MEA structure in the fuel cell stack when the separator plate is aligned with the adjacent MEA structures and stacked in the fuel cell stack.
By employing non-planar MEA separator layers it becomes possible to build up a complex flow-field fuel cell stack by forming sheet elements into three dimensional structures based on periodic undulating waveforms. The resulting fuel cell stack can be manufactured in a continuous pro
Lindstrom Jeremy
McLean Gerard Francis
Ballard Power Systems Inc.
Chaney Carol
McAndrews Held & Malloy Ltd.
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