Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2001-02-27
2003-07-15
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000
Reexamination Certificate
active
06593022
ABSTRACT:
RELATED APPLICATION
This application includes subject-matter incorporated from applicant's British Patent Application Ser. No. 9814121.1 filed on Jul. 1, 1998.
FIELD OF THE INVENTION
The invention relates to a stratum such as a separator plate for use as a subcomponent of a fuel cell stack, and especially a PEM-type fuel cell stack (“PEM” is an acronym for “proton exchange membrane”), and to preferred methods of use of such strata in a fuel cell stack.
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. Solid polymer fuel cells generally include a membrane electrode assembly, (“MEA”) comprising a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers. The electrode layers typically comprise porous, electrically conductive sheet material and an electrocatalyst at each membrane-electrode interface to promote the desired electrochemical reactions.
At the fuel cell anode, the fuel (typically hydrogen) moves through the porous electrode material and is oxidized at the anode electrocatalyst to form cations, which migrate through the membrane to the cathode. At the cathode, the oxidizing gas (typically air containing oxygen) moves through the porous electrode material and is reduced at the cathode electrocatalyst to form a reaction product, usually water. The anode and cathode also respectively donate and accept the electrons required for the electric current flow from and to the fuel cell and ultimately through the load across which the fuel cell alone or in electrical combination with other fuel cells (usually a series connection in a stack) is connected. More specifically, in the vicinity of the anode of each fuel cell in the stack, the hydrogen breaks down into (i) the positively charged protons that move through the polymeric membrane, and (ii) electrons that flow to the next fuel cell connected in series in the stack (or, if the fuel cell is a terminating fuel cell in a stack, through the load across which the fuel cell stack end terminals are connected). In the vicinity of the cathode, not only do the fuel and oxidant gases complete the exothermic chemical reaction that provides the water or other reaction product, but the electric circuit is also completed; the electrons that flow through the load combine with the cations and the oxidant to form an electrically neutral reaction product, usually water. The hydrogen may be supplied directly from a supply of same or may be a conversion product of, for example, a hydrocarbon such as methane.
In conventional fuel cells, the MEA is interposed between two substantially fluid-impermeable, electrically conductive plates, commonly referred to as separator plates. The plates serve as current collectors, provide structural support for the electrode layers, typically provide means for directing the fuel and oxidant to the anode and cathode layers, respectively, and typically provide means for removing products, such as water, formed during operation of the fuel cell. When reactant channels are formed in the separator plates, the plates are sometimes referred to as fluid flow field plates.
Fuel cell stacks typically comprise an aligned assembly of fuel cells connected together mechanically and electrically in series to obtain desired voltage and power output. An early example of a fuel cell stack is illustrated in Maru U.S. Pat. No. 4,444,851 granted Apr. 24, 1984; a later example is illustrated in Washington U.S. Pat. No. 5,514,487 granted May 7, 1996. Typically, in any such stack, one side of a given fluid flow field plate (separator plate) is the anode plate for one cell, and the other side of the plate is the cathode plate for the adjacent cell, and so on seriatim. For this reason, the plates are sometimes referred to as bipolar plates.
Fluid reactant streams are typically supplied to the fuel cell electrodes via channels in the flow field plates communicating with external plenum chambers or manifolds connected to the sides of the stack, or communicating with internal plenum chambers or manifolds formed by aligning openings formed within the plates and MEAs in the stack. Internal manifolds have been almost universally used in preference to external manifolds for proton exchange membrane (PEM) fuel cell stacks; external manifolds are more commonly found in high-temperature fuel cell stacks such as phosphoric acid fuel cell stacks. Similarly, fluid stream exhaust conduits or manifolds may be external or internal. Typically the stack also has coolant passageways extending within it for circulating a coolant fluid to absorb heat generated by the exothermic fuel cell reaction.
The requisite flow-field channels in a fuel cell separator plate may be formed as a pattern of parallel open-faced fluid-flow channels formed in a major surface of a rigid, electrically conductive plate. The parallel channels extend between an inlet manifold opening and an outlet manifold opening formed in the plate. 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. Fluid-flow-field plates for electrochemical fuel cells in which the inlet and outlet flow channels are mutually disconnected, so that in operation the reactant stream must pass through the porous electrode layer to get from the inlet to the outlet, have also been described.
In a companion British application Ser. No. 9814123.7 (McLean et al., assigned to the applicant herein) filed on Jul. 1, 1998 there is disclosed in one embodiment of the invention of that application an undulate MEA layer sandwiched between successive planar separator plates for use in a PEM-type fuel cell stack. (“MEA” is an acronym for “membrane electrode assembly”.) In conjunction with the separator plates, the undulations of the MEA layer serve to separate fuel flow channels from oxidant flow channels. Those flow channels bounded on one side by one separator plate of the sandwich are the fuel flow channels, and those channels bounded by the other of the two separator plates in the sandwich are the oxidant flow channels. In such undulate MEA layer/separator plate sandwiches, typically a given separator plate serves as the lower layer of a sandwich for one fuel cell and as the lower layer of a sandwich for one fuel cell immediately underneath it (assuming for the sake of this discussion, that the fuel cell stack is vertically oriented with the fuel cells forming generally horizontal strata in such stack—of course, the orientation is arbitrary). Consequently, for any given separator plate, fuel gas flows in a flow field on one side of the plate, and oxidant gas flows in a flow field on the other side of the plate. In each case, the flowpaths of the flow field are defined by the spaces formed by the undulations of the MEA layer. These flow channels extend in an axial sense from one side of the fuel cell stack to the opposite side.
In many conventional configurations, the flow channels form a serpentine flowpath. The term “serpentine” is understood to apply to a flowpath in which, in sequential flow channel components of the flowpath, a reversal of direction of gas flow occurs. Such flowpaths serve two principal objectives, viz to provide reactant gas efficiently to as much of the MEA layer surface as possible, and to provide flow channels that are long relative to their cross-sectional area.
If a serpentine flowpath design is chosen, it is often conveniently formed for the most part of straight channel segments that extend over most of one dimension of the working fuel cell surface and connect at their extremities to the next adjacent straight segment forming part of that parti
Ballard Power Systems Inc.
Dove Tracy
McAndrews Held & Malloy Ltd.
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