Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2001-08-14
2004-11-02
Bell, Bruce F. (Department: 1746)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000, C429S006000, C252S511000, C252S502000, C252S503000, C252S506000, C252S508000, C252S512000, C252S513000, C252S514000, C252S515000
Reexamination Certificate
active
06811917
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to conductive materials, and in particular to conductive thermosetting compositions which find particular utility in the manufacture of electrochemical cell components, and methods of making thereof.
Conductive, moldable composite materials are actively being sought for use in electrochemical cells, which includes both electrolysis cells and fuel cells. A preferred type of electrochemical cell is the “proton exchange membrane” cell, wherein the cathode of the cell is separated from the anode by a proton exchange membrane that facilitates the diffusion of ions and/or water between the cathode and anode.
The typical electrochemical cell includes a number of individual cells arranged in a stack, with the working fluid directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a cathode, a proton exchange membrane, and an anode.
Membrane electrode assemblies (“MEAs”) for use in fuel cells are well known, being described for example in U.S. Pat. Nos. 5,272,017 and 3,134,697, which are incorporated by reference herein. The MEA for each cell is placed between a pair of electrically conductive elements which serve as current collectors for the anode/cathode, and which generally contain an array of grooves in the faces thereof for distributing the gaseous reactants (H
2
and O
2
/air) over the surfaces of the anode and cathode. The gaseous O
2
/air reactant is usually saturated, typically with water.
Electrochemical cell systems generally comprise a plurality of such cells, which are stacked together in electrical series separated from each other by an impermeable, electrically conductive plate referred to as a bipolar plate. The bipolar plate thus serves as an electrically conductive separator element between two adjacent cells, and generally also has reactant gas distributing grooves on both external faces thereof. In most cases the bipolar plate also has internal passages through which coolant flows to remove heat from the stack. In the electrochemical cell environment, the exterior faces of the bipolar plates are in constant contact with often highly corrosive, acidic solutions at elevated temperatures. Moreover, at least one of the electrode faces may be polarized in the presence of pressurized, saturated air or hydrogen. To survive in such an environment, the bipolar plates must be able to withstand these pressures and be highly resistant to corrosion and degradation.
Bipolar plates are often fabricated from graphite, which is lightweight, corrosion resistant, and electrically conductive. However, graphite is quite brittle and thus prone to cracking, and mechanically difficult to handle, thus increasing production costs. Additionally, graphite is porous, making it virtually impossible to make the very thin, gas-impervious plates that are desirable for low-weight, low-volume electrochemical cell stacks. The graphite plates must also be operably connected to the other components by seal rings. Typically the seal ring material contains plasticizers and additives that leach out over time and contaminate the catalyst, which generally halts energy production.
PCT Application 99/19389 to Choate et al., U.S. Pat. No. 6,248,467 to Wilson et al., and U.S. Pat. No. 6,251,308 to Butler disclose molding compositions with conductive fillers that can be formed into structures exhibiting electrical and thermal conductivity such as bipolar plates. The molding compositions typically employ a low viscosity resin system, specifically epoxy, epoxy vinyl ester, and phenolic resin systems. While these compositions may offer some improvements to electrochemical cell technology, they may not have sufficient long-term chemical resistance. Materials employed in the electrochemical cell must be resistant to degradation in a particularly punishing environment over periods of time measured in years.
Accordingly, there is a perceived need in the art for a low cost, conductive molding composition with a high chemical resistance for use in electrochemical cells and methods of making thereof.
BRIEF SUMMARY OF THE INVENTION
The above described drawbacks and deficiencies are overcome by a conductive, moldable composite material for the manufacture of electrochemical cell components, comprising a thermosetting resin system and conductive filler, wherein the thermosetting resin composition comprises: (1) a polybutadiene or polyisoprene resin; (2) an optional, functionalized liquid polybutadiene or polyisoprene resin; (3) an optional butadiene- or isoprene-containing copolymer; and (4) an optional low molecular weight polymer. In a preferred embodiment, the conductive moldable composite material is used to form a bipolar plate, current collector or other electrochemical cell component. In another embodiment, articles made of the conductive, moldable composite material are resistant to chemical attack and hydrolysis, have excellent mechanical strength and toughness, have a volume resistivity of about 0.116 ohm-cm or less, and have a thermal conductivity of at least about 5 watts/meter ° K. (W/m ° K.). Articles have a linear shrinkage per unit length of the molded composite in the X-Y plane less than or equal to about 0.005, measured as described in ASTM D-955. In addition, the conductive, moldable composite material as well as articles made from it are economical to produce due to the inexpensive starting materials as well as the use of conventional processing equipment.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A conductive, moldable composite material for the manufacture of electrochemical cell components comprises a thermosetting resin system and conductive filler wherein the thermosetting resin composition comprises: (1) a polybutadiene or polyisoprene resin; (2) an optional, functionalized liquid polybutadiene or polyisoprene resin; (3) an optional butadiene- or isoprene-containing polymer; and (4) an optional, low molecular weight polymer. In a preferred embodiment, the conductive moldable composite material is used to form a bipolar plate, current collector or other electrochemical cell component.
The resin system used to form the conductive, moldable composite material is a thermosetting composition generally comprising a polybutadiene resin, polyisoprene resin or mixture thereof. The polybutadiene or polyisoprene resins may be liquid or solid at room temperature. Liquid resins may have a molecular weight greater than 5,000, but preferably have a molecular weight of less than 5,000 (most preferably between 1,000 and 3,000). The preferably liquid (at room temperature) resin portion maintains the viscosity of the composition at a manageable level during processing to facilitate handling, and it also crosslinks during cure. Polybutadiene and polyisoprene resins having at least 90% 1,2-addition by weight are preferred because they exhibit the greatest crosslink density upon cure owing to the large number of pendant vinyl groups available for crosslinking. High crosslink densities are desirable because the products exhibit superior performance in an electrochemical cell environment at elevated temperatures. A preferred resin is B3000 resin, a low molecular weight polybutadiene liquid resin having greater than 90 weight percent (wt. %) 1,2-addition. B3000 resin is commercially available from Nippon Soda Co., Ltd.
The resin system used to form the conductive, moldable composite optionally comprises functionalized liquid polybutadiene or polyisoprene resins. Examples of appropriate functionalities for butadiene liquid resins include but are not limited to epoxy, maleate, hydroxy, carboxyl and methacrylate. Examples of useful liquid butadiene copolymers are butadiene-co-styrene and butadiene-co-acrylonitrile. Possible functionalized liquid polybutadiene resins include Nisso G-1000,
Fitts Bruce B.
Landi Vincent R.
Roy Saroj Kumar
Bell Bruce F.
Cantor & Colburn LLP
World Properties, Inc.
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