Patent Publication Number: US-2022223882-A1

Title: Fuel cell stack

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 16/460,236 filed Jul. 2, 2019 (Attorney Docket No. 1404.286), which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This invention relates in general to electrochemical cells, and more particularly to fuel cells systems and methods. 
     BACKGROUND OF THE INVENTION 
     Fuel cells electrochemically convert fuels and oxidants to electricity and heat and can be categorized according to the type of electrolyte (e.g., solid oxide, molten carbonate, alkaline, phosphoric acid or solid polymer) used to accommodate ion transfer during operation. Moreover, fuel cell assemblies can be employed in many (e.g., automotive to aerospace to industrial to residential) environments, for multiple applications. 
     A Proton Exchange Membrane (hereinafter “PEM”) fuel cell converts the chemical energy of fuels such as hydrogen and oxidants such as air directly into electrical energy. The PEM is a sold polymer electrolyte that permits the passage of protons (i.e., H+ ions) from the “anode” side of the fuel cell to the “cathode” side of the fuel cell while preventing passage therethrough of reactant fluids (e.g., hydrogen and air gases). The Membrane Electrode Assembly (hereinafter “MEA”) is placed between two electrically conductive plates, each of which has a flow passage to direct the fuel to the anode side and oxidant to the cathode side of the PEM. 
     Two or more fuel cells can be connected together to increase the overall output of the assembly. Generally, the cells are connected in series, wherein one side of a plate serves as an anode plate for one cell and the other side of the plate is the cathode plate for the adjacent cell. These are commonly referred to as bipolar plates (hereinafter “BPP”). Alternately, the anode plate of one cell is electrically connected to the separate cathode plate of an adjacent cell. Commonly these two plates are connected back to back and are often bonded together (e.g., bonded by adhesive, weld, or polymer). This bonded pair becomes as one, also commonly called a bipolar plate, since anode and cathode plates represent the positive and negative poles, electrically. Such a series of connected multiple fuel cells is referred to as a fuel cell stack or fuel cell system. The stack typically includes means for directing the fuel and the oxidant to the anode and cathode flow field channels, respectively. The stack usually includes a means for directing a coolant fluid to interior channels within the stack to absorb heat generated by the exothermic reaction of hydrogen and oxygen within the fuel cells. The stack generally includes means for exhausting the excess fuel and oxidant gases, as well as product water. 
     The stack also includes an endplate, insulators, membrane electrode assemblies, gaskets, separator plates, electrical connectors and collector plates, among other components, that are integrated together to form the working stack designed to produce electricity. The different plates may be abutted against each other and connected to each other to facilitate the performance of particular functions. 
     Such fuel cell plates are typically formed of stainless steel and machine molded, or stamped. The only viable process for bonding such metal plates together is welding. Welding is a relatively slow and costly process compared to solvent bonding and friction welding, which are not possible. 
     If fuel cell plates were formed of thermoset molded composite material the bonding of two thermal set plates typically requires a cooler side of a plate to be coated with a conductive glue to bond adjacent plates together. Such glue is not as conductive as a base product and the glue typically creates defects by inaccurate placement thereof or incomplete coverage of the glue which may create bonding problems. The gluing process is also typically time intensive. 
     Thus, a need exists for improved fuel cell systems and improved methods for manufacturing fuel cells that allow features to be formed in fuel cell plates. 
     SUMMARY OF THE INVENTION 
     The present invention provides, a method for use in forming a fuel cell plate which includes providing a sheet of conductive thermoplastic material and heating the sheet to soften the sheet. The sheet is pressed between two molds having a fuel cell plate feature forming element to form a fuel cell plate having a fuel cell feature conforming to the element. The fuel cell plate is cooled in the molds. 
     The present invention provides in a second aspect, a fuel cell system including a first fluid flow plate including a first plurality of first channels for flow of an oxidant or a fuel. The plurality of first channels has first channel cross-sectional flow areas. A second fluid flow plate includes a second plurality of second channels for flow of an oxidant or a fuel. The plurality of second channels has second channel cross-sectional flow areas. A membrane electrode assembly is located between the first plate and the second plate. The first flow plate includes a passage for a flow of a fluid entirely on a same side of the first flow plate as the first plurality of first channels. The passage has a cross-sectional area for flow of the fluid smaller than the first channel cross-sectional flow area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention will be readily understood from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a block diagram of a fuel cell system in accordance with the invention; and 
         FIG. 2  is a perspective view of a portion of a fuel cell of the fuel cell system of  FIG. 1 ; 
         FIG. 3  is a perspective view of a channel and flow restrictor of the fuel cell system of  FIG. 1 ; 
         FIG. 4  is a perspective view of a fuel cell plate of the fuel cell system of  FIG. 1 ; 
         FIG. 5  is a perspective view of a portion of the fuel cell plate of  FIG. 4 ; 
         FIG. 6  depicts a process for forming the fuel cell plate of  FIG. 4 ; 
         FIG. 7  is a perspective view of a land of a fuel cell plate including a groove for a flow of fluid; 
         FIG. 8  is a side cross-sectional view of two fuel cell plates including the land and groove of  FIG. 7  and a channel including a groove; and 
         FIG. 9  depicts the channel and groove of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with the principles of the present invention, fuel cell systems and methods are provided. 
     In an example depicted in  FIG. 1 , a fuel cell system  10  is referred to as the assembled, or complete, system which functionally together with all parts thereof produces electricity and typically includes a fuel cell stack  20  and an energy storage device ( 30 ). The fuel cell is supplied with a fuel  13 , for example, hydrogen, through a fuel inlet  17 . Excess fuel  18  is exhausted from the fuel cell through a purge valve  90  and may be diluted by a fan  40 . In one example, fuel cell stack  20  may have an open cathode architecture of a PEM fuel cell, and combined oxidant and coolant, for example, air, may enter through an inlet air filter  10  coupled to an inlet  5  of fuel cell  20 . Excess coolant/oxidant and heat may be exhausted from a fuel cell cathode of fuel cell stack  20  through an outlet  11  to fan  40  which may exhaust the coolant/oxidant and/or excess fuel to a waste exhaust  41 , such as the ambient atmosphere. The fuel and coolant/oxidant may be supplied by a fuel supply  7  and an oxidant source  9  (e.g., air), respectively, and other components of a balance of plant, which may include compressors, pumps, valves, fans, electrical connections and sensors. 
       FIG. 2  depicts an internal subassembly  100  of fuel cell stack  20  of  FIG. 1  including a cathodic end fluid flow plate  110  at an outer end  115  and a flow plate seal  120  on an inner side thereof. A membrane electrode assembly (MEA)  130  is located between seal  120  and a second flow plate seal  150 . An anode flow plate  160  is on a second end  165  of subassembly  100 . 
     MEA  130  includes a membrane  140  between a cathode side catalyst layer  125  and an anode side catalyst layer  135 . A cathode side gas diffusion layer (GDL)  122  is located between cathode side catalyst layer  125  and flow plate  110 . An anode side gas diffusion layer  145  is located between anode side catalyst layer  135  and flow plate  160 . Seal  120  and seal  150  may be received in a channel of on an inner side of flow plate  110  and flow plate  160 , respectively. 
       FIG. 3  depicts a channel  200  of a plate  205  of a fuel cell (e.g., fuel cell  20 ) for receiving a flow of a fluid (e.g., a fuel or oxidant). Plate  205  may be a cathode plate or an anode plate, as in plate  110  and plate  160 , respectively, described above. A flow restrictor  210  may be upstream of (i.e., provides a flow of fluid to) channel  200  and may have a smaller cross-sectional area than channel  200 . An intersection  220  where channel  200  and flow restrictor  210  meet may provide a reduction of pressure of fluid flowing through flow restrictor  210  into channel  200  due to the difference in cross-section areas therebetween. 
     Flow restrictor  210  may include an entrance end  230  which receives fluid from a channel  240  (or other flow providing structure) having a larger cross-sectional area than flow restrictor  210  and larger than channel  200 . Thus, a flow volume per unit time may be less in flow restrictor  210  relative to channel  240  flowing into flow restrictor  210 . As the fluid flows from flow restrictor  210  into channel  200  an available cross-sectional area may increase compared to the cross sectional area of flow restrictor  210  such that a pressure drop may occur in channel  200 . 
     The described change in cross-sectional area from channel  240  to flow restrictor  210  relative to channel  200  may provide a desired reduction in pressure to the fluid flowing in channel  200  relative to channel  240 . Multiple such pressure drops for multiple channels arranged in parallel prior to water generation may balance flows from channel to channel due to the reduction in flow at the entrances of the multiple inlet restrictors. Such a reduction in flow prior to entering channel  200  may inhibit a production of water in the channel of the fuel cell due to a reduction in the amount of flow present per unit time and any resultant dwell time. 
     For example, multiple instances of the channel (e.g., channel  200 ) and flow restrictor (e.g., flow restrictor  210 ) may form an anode inlet  207  of an anode plate (e.g., plate  205 ) as depicted in  FIGS. 4-5 . In another example, such channels and flow restrictors may form a cathode inlet of a cathode plate (not shown). 
     Plate  205  may include multiple iterations of channel  200  and flow restrictor  210  and may be formed from a thin conductive plastic sheet, such as Kynar, LCP and PPS which could have a dimension of 0.15 mm. Such thermoplastics may be formed into sheet stock by hot rolling.  FIG. 6  depicts a method for producing a fuel cell plate, such as plate  205 . A sheet  300  may be unwound off a roll  305 , and may be warmed and softened, by a heater  310  or oven (e.g., to a temperature such that sheet  305  holds together and soft enough such that sheet  305  would not crack during a stamping process) on the way to a press  320  (e.g., a sheet metal type speed stamping press) where the sheet is struck with a male and female cavity defining a part (e.g., a fuel cell plate having flow restrictor  210 ) desired. Once warm, the plastic sheet will form to the contours of inner surfaces  322  of the press and may be elongated (e.g., elongated 30%) to achieve a desired plate geometry, such as to replicate current stainless steel plates. Further, additional features not possible in current plates, such as flow restrictor  210 , may be formed using the described method. More specifically, inner surfaces  322  may include feature details  330  to form such features (e.g., flow restrictor  210 ) not possible using the method of the prior art. The press or tool remains cool, i.e., having a temperature such that the hot soft plastic may cool and solidify into the shape desired (e.g., fuel cell plate  205  with flow restrictor  210 ) as described. Laser cutting and welding may be performed using a trimming mechanism  340  (e.g., a laser cutting tool) to trim and weld the plate (e.g., plate  205 ) formed using the indicated method. 
     The plastic sheet (e.g., Kynar, LCP or PPS having a dimension of 0.15 mm) may be much less expensive, corrosion resistant and lighter than the  316 L coated stainless steel used in the prior art to form fuel cell plates. Further, the thickness of the completed fuel cell plate may be about that of a stainless steel fuel cell plate (e.g., about 0.2 mm) but would be lighter than such a stainless steel plate. 
     Further, the indicated method may also include the plastic sheet being pierced to provide a pressure drop or dive through hole in a fuel cell plate (e.g., fuel cell plate  205 ). Such piercing may be performed using a pin, such as a 0.010 inch diameter pin, and the piercing may be performed prior to the introduction of the sheet into the press or after such pressing. A pin could also be part of the press such that the piercing is done at the same time as the pressing. If the piercing were done before or after the pressing, the pin may be heated to facilitate the pin melting through the material. Such piercing of the plastic sheet would not be viable using a harder raw material such as stainless steel since the indicated pin would not be able to penetrate the steel or would not be durable enough for multiple uses. The use of a pin for such a process as is possible with the plastic sheet of the indicated method, relative to drilling or other processes as would be necessary for harder materials, is that the use of the pin is less expensive due to its durability for multiple piercings with the material. 
     In an example depicted in  FIGS. 7-8 , lands  400  may separate channels  410  of a fluid flow field of an anode fuel cell plate  420  or a cathode fuel cell plate  430 . One or more of lands  400  may include micro channels or grooves  440  therein which may be formed as features using appropriate feature details (e.g., feature details  330 ) of inner surfaces  330  of press  320  in the method described above and depicted in  FIG. 6 . Such grooves may allow a gas access to a portion of a MEA (e.g., MEA  130 ) under a land (e.g., a land  401  of lands  400 ) of a fuel cell plate (e.g., cathode plate  420 ). Also, such grooves may allow water a pathway to be removed without having to move in a plane of a GDL (e.g., GDL  522 ). Such grooves may have a depth and/or width dimension of 0.001″-0.010″, for example, and may be extend along a length of a land (e.g., land  40 ). In another example, the grooves may have a depth and/or width dimension of 0.0005″ to 0.015″. As indicated, such grooves may be formed using the method described above of heating a thermoset plastic and pressing the plastic to a desired shape including such grooves. Prior art methods would not allow the grooves to be easily attained since the groove would require material to be removed from the lands in a metal stamping process and in thermal set molded plates, the material would also need to be removed after molding where such removal would present a risk of damage to the molded plates, such as by scuffing the surface of the plate by a required removal process. 
     In another example depicted in  FIGS. 8-9 , channels  400  of cathode plate  420  and/or anode plate  430  may include micro grooves  450  therein to allow water to move therethrough, for example. Such flow of water in the micro grooves may occur without interfering with bulk gas flow in a channel  451  of channels  400  in which such gas may flow. For example, the water may flow with gravity in micro groove  450  in a direction opposite to that of the gas flow in channel  451 . Also, micro grooves  450  may have a depth and/or width dimension of 0.001″-0.010″, for example. Such micro grooves could also have depth and/or width dimensions of 0.0005″ to 0.015″. 
     Microchannels  450  may formed in a process similar to that of flow restrictors  210  and grooves  440  as described above. For example, microchannels may be formed as features using appropriate feature details  330  of inner surfaces  322  in the method described above and depicted in  FIG. 6 . Similar to grooves  440 , microchannels  450  are very difficult to achieve in the prior art, such as metal stamped plate because the microchannels would require material to be removed from the channels in a metal stamping process and in thermal set molded plates, the material would also need to be removed after molding which such removal presents a risk of damage to the plates. Also, in contrast, if formed using a molded thermal set process, the formation of grooves  440  and microchannels  450  may create stress risers in a plate material making it prone to cracking which is undesirable. More specifically, the machining of grooves into a compression molded plate, as in the prior art, could crack a plate during machining or a stress riser could be created during machining that could cause a crack to occur in the plate later. The formation of plate features in the above described method of the present invention, according to feature details  330  of press  320 , minimizes the likelihood of such stress risers since the features are formed during the pressing process and does not require additional machining to create such features which could cause cracks and/or stress risers. 
     The load described above could be any type of stationary or moveable load device, such as an industrial electrical vehicle or forklift truck. The fuel cell (e.g., fuel cell system  20 ) could be any type of fuel cell such as a proton exchange membrane fuel cell, solid oxide fuel cell, or any other fuel cell as would be known by one of ordinary skill in the art. The energy storage device described above could be any type of battery or other way of storing energy such as a lithium ion battery, lead acid battery, air compression energy storage device, water storage device, capacitor, ultra-capacitor, or any other device for storing energy. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include” (and any form of include, such as “includes” and “including”), and “contain” (and any form contain, such as “contains” and “containing”) are open-ended linking verbs. As a result, a method or device that “comprises”, “has”, “includes” or “contains” one or more steps or elements possesses those one or more steps or elements but is not limited to possessing only those one or more steps or elements. Likewise, a step of a method or an element of a device that “comprises”, “has”, “includes” or “contains” one or more features possesses those one or more features but is not limited to possessing only those one or more features. Furthermore, a device or structure that is configured in a certain way is configured in at least that way but may also be configured in ways that are not listed. 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to embodiments of the invention and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, and any alterations and further modifications in the described embodiments, and any further applications of the principles of the invention as illustrated therein as would normally occur to one skilled in the art to which the invention relates are contemplated an protected. 
     Although preferred embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims.