Patent Publication Number: US-2010129732-A1

Title: Electrochemical Cell Stack Assembly

Description:
This application claims the benefit under 35 U.S.C.§119(e) to U.S. Provisional Patent Application Ser. No. 61/126,138, entitled, “ELECTROCHEMICAL CELL STACK ASSEMBLY,” which was filed on May 1, 2008, and is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The invention generally relates to an electrochemical cell stack assembly. 
     A fuel cell is a type of electrochemical cell, which converts chemical energy directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM) that permits only protons to pass between an anode and a cathode of the fuel cell. Typically PEM fuel cells employ sulfonic-acid-based ionomers, such as Nafion, and operate in the room temperature to 90° Celsius (C) temperature range. Another type employs a phosphoric-acid-based polybenziamidazole, PBI, membrane that operates in the 150° to 200° temperature range. At the anode, diatomic hydrogen (a fuel) is reacted to produce protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water. The anodic and cathodic reactions are described by the following equations: 
       Anode: H 2 →2H + +2 e   −   Equation 1 
       Cathode: O 2 +4H + +4 e   − 2H 2 O  Equation 2 
     A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power. 
     The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Electrically conductive gas diffusion layers (GDLs) may be located on each side of a catalyzed PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from both the anode and cathode flow-fields may diffuse through the GDLs to reach the catalyst layers. 
     The PEM fuel cell is only one type of fuel cell. Other types of fuel cells include direct methanol, alkaline, phosphoric acid, molten carbonate, and solid oxide fuel cells. 
     In contrast to a fuel cell, an electrochemical cell may alternatively be configured to function as an electrolyzer, which produces hydrogen and oxygen from electricity and water. More specifically, the following reactions occurring at the anode and cathode of the cell: 
       Anode: 2H 2 O→O 2 +4H − +4 e   −   Equation 3 
       Cathode: 4H − +4 e   − 2H 2   Equation 4 
     The electrochemical cell may also be configured to function as gas purifier, or pump. In this configuration, electrical energy is provided to the electrochemical cell to cause a gas species (such as hydrogen) at the anode side of the cell to be selectively transported to the cathode side of the cell. 
     Within the next decade, the demand for purified, compressed reactant gas is expected to increase dramatically. One factor that is driving this demand is the expected shift from oil-based fuels and internal combustion engines to hydrogen fuel and fuel cells. 
     Hydrogen production will likely be conducted by a variety of means. Examples include water electrolysis, methane reformation, propane reformation, alcohol reformation, sugar reformation, and/or oil and gasoline reformation. In the case of each of these examples, the product of the generation process is most frequently an impure, low pressure stream, which contains hydrogen gas as one of many constituents. The ideal product, however, would be a pure, dry pressurized hydrogen stream, which can be used directly by either the end application or which can be easily stored in pressurized gas containers. 
     Another factor driving an increased demand for purified, compressed reactant gases is the continued rapid increase in the industrialization of processes, which utilize reactant gases for materials microstructure processing. The semiconductor industry, as an example, uses large quantities of extremely pure, compressed reactant gases, such as hydrogen and oxygen. 
     SUMMARY 
     In an embodiment of the invention, a technique includes providing a loading force member to extend between end plates to compress flow plates of an electrochemical cell stack assembly together. The technique includes positioning the loading force member in a region of the stack near where a maximum expansion force is exerted on the flow plates due to the operation of the stack. 
     In another embodiment of the invention, a technique includes compressing flow plates of an electrochemical cell stack between end plates and extending a loading force member through the stack to compress the flow plates. The technique includes positioning the loading force member to extend substantially along a center line of the stack. 
     In an another embodiment of the invention, an apparatus includes a stack of flow plates to form electrochemical cells, which generate a pressure force on the flow plates due to operation of the cells. The apparatus includes end plates and a loading member that extends between the end plates to maintain compression of the flow plates. The loading force member is positioned to maximize a compression force on the flow plates in a region of the stack in which the pressure force is maximized. 
     In yet another embodiment of the invention, an apparatus includes a stack of flow plates to form electrochemical cells and a loading force member. The loading force member extends substantially along a center line of the stack to maintain compression of the flow plates. 
     Advantages and other features of the invention will become apparent from the following drawing, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic diagram illustrating an electrochemical cell that is configured to produce a purified gas according to an embodiment of the invention. 
         FIG. 2  is a schematic diagram of an electrochemical cell pump formed from cascaded electrochemical cell stacks according to an embodiment of the invention. 
         FIG. 3  is a schematic diagram of a system to purify, compress and store gas according to an embodiment of the invention. 
         FIG. 4  is a schematic diagram of a fuel cell system according to an embodiment of the invention. 
         FIGS. 5 and 6  illustrate different electrochemical cell stack assemblies. 
         FIG. 7  is a flow diagram depicting a technique to prevent bowing of flow plates of an electrochemical cell stack assembly according to an embodiment of the invention. 
         FIG. 8  is an illustration of pressure forces generated by operation of an electrochemical cell stack according to an embodiment of the invention. 
         FIG. 9  is a schematic diagram of an electrochemical cell stack assembly according to an embodiment of the invention. 
         FIG. 10  is a view of the cathode side of a flow plate of the stack assembly of  FIG. 9  according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     An electrochemical cell may be configured to produce a significantly pure and compressed gas flow. For example, referring to  FIG. 1 , an electrochemical cell  12  may include an anode inlet  14  that receives a mixture of gases, which are communicated to the anode side of the cell  12 . The electrochemical cell  12  extracts a species (hydrogen, for example) of the incoming gas mixture and transports the selected species to the cathode side of the cell  12  to produce a purified and compressed flow of the selected species at the cell&#39;s cathode exhaust outlet  18 . As a more specific example, the electrochemical cell  12  may produce a relatively pure flow of hydrogen (a flow of 99.9% hydrogen by volume, for example) at the cathode exhaust outlet  18  in response to a reformate flow (a flow of 50% of hydrogen by volume, for example) that is received at the anode inlet  14 . 
       FIG. 1  also depicts a voltage source  20  that provides energy to facilitate the transport of the selected gas species between the anode and cathode sides of the electrochemical cell  12 . The non-transported gas species exits the electrochemical cell  12  at its anode exhaust outlet  16 . 
     Turning now to more specific details, the electrochemical cell  12  routes the incoming gas mixture (received at the anode inlet  14 ) over an anode catalyst/electrode assembly of the cell  12 . At this assembly interface, the gas species to be selectively transported is broken down and ionized. Following this, the species is transported through an electrolyte layer of the electrochemical cell  12 . The voltage source  20  provides the electromotive force for this transport. At a cathode catalyst/electrode assembly of the cell  12 , the species is re-assembled and neutralized. The gas species, which has been selectively transported exits the electrochemical cell  12  at the cathode outlet  18 ; and the gas species which were not transported exit the electrochemical cell  12  at the anode exhaust outlet  16 . 
     Selective hydrogen gas transport is achieved using a proton conductor of the cell  12 . This proton conductor might be an alkaline electrolyte; liquid or solid acid; or a proton-exchange-membrane (PEM). In the case of the PEM, the membrane may be catalyst coated Nafion. 
     As a more specific example, the electrochemical cell  12  may include a PEM and may be configured to produce a substantially pure hydrogen flow. For this cell, a gas mixture that contains hydrogen enters the anode side of the cell. At the anode catalyst/electrode, the hydrogen is broken down and ionized. Electrons are moved by the voltage produced by the voltage source  20  to the cathode side of the device, and protons are transported across the PEM electrolyte. At the cathode catalyst/electrode layer the hydrogen molecules re-assemble. The external voltage source  20  provides the energy to drive both the transportation of electrons and protons. Purified hydrogen is then free to exit the cathode of the electrochemical cell  12  at the cathode exhaust outlet  18 . Species which are not transported exit the electrochemical cell  12  as an anode exhaust waste stream at the anode exhaust outlet  16 . 
     The anode catalyst layer may be selected to prevent build-up of contaminant species. For example, when carbon monoxide (CO) is present as one of species in the incoming gas mixture, a Pt—Ru catalyst mixture or alloy may be used for the anode catalyst layer. Then, when oxygen is present in sufficient volume, CO is oxidized by the Ru catalyst layer, preventing contamination of the Pt catalyst. As an example, air flow rates equal to four to eight percent of the hydrogen flow rate are sufficient to oxidize CO levels of about 100 parts per million (ppm). Such methods may be very useful in the case of hydrogen-rich gas mixtures, which are produced via fossil fuel reforming processes, because these mixtures often contain trace amounts of CO. 
     The electrochemical cell  12 , when operated as a pump, may compress the purified product to the desired use or storage pressure. Therefore, need for additional mechanical compression may be eliminated. Compression is achieved by placing a back-pressure on the cathode side of the cell  12 . When this is done, the species being transported (hydrogen, for example) becomes compressed upon transport from the anode side to the cathode side of the cell  12 . 
     Hydrogen is not the only species that may be transported across the membrane of the electrochemical cell  12  and thus, purified. For example, when the electrolyte material is an oxygen ion conductor, such one of the solid oxide electrolytes that may be used in a solid oxide fuel cell, oxygen pumping may be performed to achieve the same functions listed above for hydrogen, except with oxygen as the process gas. 
     The electrochemical cell may be combined with additional cells to further purify and compress the end product. For example, an electrochemical cell pump  25 , which is depicted in  FIG. 2  may be used. The electrochemical cell pump  25  includes N cascaded electrochemical cell stacks  30  (electrochemical cell stacks  30   1 ,  30   2  . . .  30   N  being depicted as examples). Each electrochemical cell stack  30  includes an anode inlet  32  that receives an incoming gas mixture, and the cell stack  20  is configured to selectively transport a gas species of this mixture to its cathode exhaust outlet  36 . Thus, each electrochemical cell stack  30  further purifies the gas mixture that is received at its anode inlet  32 . More specifically, the electrochemical cell stack  30   1  receives the incoming gas mixture to the electrochemical cell pump  25 ; the anode inlet  32  of the electrochemical cell stack  30   1  receives the anode exhaust (provided at anode exhaust outlet  34 ) from the electrochemical cell stack  30   1  and so forth. A cathode outlet  36  of each electrochemical cell stack  30  is connected to a shared exhaust line  40 , which communicates a substantially pure flow of the selected gas species from the pump  25 . 
     The efficiency of electrochemically compressing a purified hydrogen stream is far higher than can be achieved with the use of a mechanical compressor. Therefore, it is desirable to compress the gas that is produced by an electrochemical cell pump, without passing the gas through a mechanical compressor. For example,  FIG. 3  depicts a system  50  in which an electrochemical cell pump  52  receives an incoming gas flow at its anode inlet  54  and produces a corresponding purified and compressed gas flow at its cathode exhaust outlet  56 . As shown in  FIG. 3 , the compressed and purified gas flow may be stored in a compressed gas tank  60  that is connected directly to the cathode exhaust outlet  56 , without an intervening mechanical compressor being located between the pump  52  and tank  60 , in accordance with some embodiments of the invention. 
     As another example of the application of an electrochemical pump that directly furnishes a compressed and purified gas flow,  FIG. 4  depicts a fuel cell system  70  indicates an electrochemical cell pump  74  to furnish a purified and compressed hydrogen gas flow, which is used as fuel for a fuel cell stack  90  of the system  70 . In this regard, the electrochemical cell pump  74  includes an anode inlet  72  that receives a reformate flow from a reformer  80 . In response to this flow, the electrochemical cell pump  74  produces a substantially pure hydrogen flow (as its cathode outlet  73 ) that is received (without further compression) at an anode inlet  91  of the fuel cell stack  90 . 
     In response to the incoming hydrogen flow and an oxidant flow that is received from an oxidant source  84 , the fuel cell stack  90  produces electrical power for a load  98 . The fuel cell stack  90  may also include an anode exhaust outlet  92 , as well as a cathode exhaust outlet  94 . It is noted that the fuel cell system  70  may include power conditioning circuitry  96  that conditions the DC stack voltage that is provided by the fuel cell stack  90  into the appropriate form for the load  98 . 
     Thus, because the electrochemical cell pump  74  directly compresses the purified fuel for the fuel cell stack  90 , the overall efficiency of the fuel cell system  70  is increased, as compared to an arrangement in which a mechanical compressor is used to further pressurize the incoming fuel flow to the fuel cell stack  90  to the appropriate level. 
     Referring to  FIG. 5 , for such purposes of energizing seals between flow plates and minimizing contact resistances, the flow plates of an electrochemical cell stack assembly are compressed together. Therefore, as depicted in  FIG. 5 , flow plates  122  of an electrochemical cell stack assembly  120  may be situated between upper  124  and lower  126  end plates. The end plates  124  and  126  in combination with tie rods  128  that extend between the upper  124  and lower  126  end plates maintain a compression force on the flow plates  122 . During the assembly of the electrochemical cell stack assembly  120 , the end plates  124  and  126 ; flow plates  122 ; and the gasket seals of the stack assembly  120  are compressed together by a press. The tie rods  128  are inserted through the end plates  124  and  126  and are secured to the stack assembly  120  via threaded connections (for example). Thus, the tie rods  128  are loading force members that maintain the compression on the components of the stack assembly  120  after the assembly  120  is removed from the press. 
     A difficulty with the stack assembly  120  of  FIG. 5  is that when the stack is operated as an electrochemical pump, expansion forces are produced, which tend to bow, or warp, the flow plates  122 . For example, when part of an electrochemical pump, the stack may produce a highly pressurized gas (a gas having a pressure on the order of 1000 to 5000 pounds per square inch (p.s.i.), for example). The internal expansion forces in the stack, which result from this highly pressurized gas may cause warping, or bowing, of the flow plates  122 , if appropriate measures are not taken. Referring also to  FIG. 8 , the expansion force may be conceptualized as upward  176  and lower  178  expansion forces that extend generally along a center line  174  of a stack  170  of flow plates. In other words, the expansion forces are maximized along the center line  174  of the stack  170 . Locating the tie rods  128  outside of the stack, as depicted in  FIG. 5 , may fail to properly compensate for the expansion forces, thereby leading to warping, or bowing, of the flow plates  122 . 
     Alternatively, a fuel cell stack assembly  150  (see  FIG. 6 ) may include tie rods  128  that extend through the stack of flow plates  122  near the outer perimeter of the stack. However, this positioning of the tie rods  128  may still fail to offset the expansion forces that are generated by the operation of the stack; and as a result, the flow plates may still experience bowing, or warpage. 
     It has been discovered that by locating a compression loading force mechanism, such as one or more tie rods, near the center line of the electrochemical cell stack, bowing of the flow plates may be minimized, if not eliminated. More specifically, referring to  FIG. 7 , in accordance with some embodiments of the invention, a technique  160  includes using a force loading mechanism to maintain the compression of flow plates of an electrochemical cell stack, pursuant to block  162 . The loading force mechanism is positioned (block  164 ) near the center line of the expansion pressure that is produced by operation of the stack. In some embodiments of the invention, this means that the loading force mechanism is positioned near the center line of the stack of flow plates. Due to this arrangement, the compression forces are maximized in a region of the stack in which the expansion forces are at their maximums. 
     As a more specific example,  FIG. 9  depicts an exemplary embodiment  200  of an electrochemical cell stack assembly in accordance with the invention. It is noted that the electrochemical cell stack assembly  200  is depicted as being formed from circular, or disk-shaped, flow plates  204 . However, other shapes, such as rectangularly-shaped flow plates may be used, in accordance with other embodiments of the invention. 
     As depicted in  FIG. 9 , in accordance with some embodiments of the invention, the disk-shaped flow plates  204  are located between upper  206  and lower  208  disk-shaped end plates, although other shapes for the end plates are possible in accordance with other embodiments of the invention. The end plates  206  and  208 , in combination with a loading force mechanism  210 , maintain a compression force on the flow plates  204 . In this regard, in accordance with some embodiments of the invention, the loading force member  210  extends through a center line of the stack of the flow plates  204 . The center line may be a line that extends through the center of each disk-shaped flow plate of the stack  204 . This center line, in turn, also coincides with the region of the flow plate stack  204 , which is associated with the highest expansion forces that are exerted by the electrochemical cells of the stack  204  during their operation. As a result, the effect of the compression force that is applied through the end plates  206  and the loading member  210  countering the pressure forces is therefore maximized to prevent bowing of the flow plates  204 . 
     In accordance with some embodiments of the invention, the stack may use a one flow plate cell design, in which each electrochemical cell is formed from the upper surface of one flow plate and the lower surface of the adjacent flow plate. More particularly, one surface, or side, of the flow plate may be a cathode side of the flow plate and thus may be used to form the cathode chamber of the cell; and the opposite side of the flow plate may be the anode side and thus, be used to form the anode chamber of the adjacent cell. Membrane electrode assemblies (MEAs) are situated between each adjacent pair of flow plates of the stack. 
     As a more specific example,  FIG. 10  depicts an exemplary embodiment  250  of a disk-shaped flow plate in accordance with some embodiments of the invention. In particular,  FIG. 10  depicts the cathode side of the flow plate  250 . 
     The flow plate  250  is formed from a disk-shaped flow plate body  252  that is generally symmetrical with respect to a center  251  of the flow plate  250 . Thus, the center line of the flow plate stack extends through the center  251  of the flow plate  250  and through the centers of the other flow plates of the stack. In accordance with some embodiments of the invention, an opening  270 , which is concentric with respect to the center line of the stack, is formed in the center of the flow plate  250 . The opening  270  receives the loading force member that extends through the stack along the stack&#39;s center line. In accordance with some embodiments of the invention, the opening  270  also serves to form part of the cathode exhaust plenum passageway for the stack. In this regard, when the flow plate  250  is assembled in the stack, the openings  270  of the flow plates align to form the cathode exhaust plenum passageway. 
     The flow plate  250  includes other openings  294  and  296 , which form portions of other plenum passageways of the stack. In general, the openings  294  and  296  each are eccentric with respect to the center line  251 , are located near the outer perimeter of the flow plate  250  and partially circumscribe the center line  251 . In accordance with some embodiments of the invention, the openings  294  and  296  may be associated with anode flows. In this regard, the opening  294  may form a portion of the plenum passageway to communicate an incoming anode flow to the electrochemical cell, and the opening  296  may form part of the plenum passageway to communicate an anode exhaust from the electrochemical cell. Therefore, because the openings  294  and  296  are associated with anode flows, seals or gaskets  297  and  298  surround the openings  294  and  296 , respectively, to isolate the anode flows from the cathode side (depicted in  FIG. 10 ) of the flow plate  250 . 
     The cathode side of the flow plate  250  includes an active region to communicate a cathode flow. For the case in which the electrochemical cell stack is an electrochemical pump, or purifier, this active region communicates the purified gas from the MEA of the cell. As depicted in  FIG. 10 , in accordance with some embodiments of the invention, the active region on the cathode side of the flow plate  250  may include flow channels  260  that are, in general, radially directed to flow cathode chamber gas into the cathode exhaust plenum. More specifically, the flow channels  260  are configured to flow the cathode chamber gas into small openings, or “dive-throughs,” which generally surround the periphery of the opening  270 . The dive-throughs  280  communicate the cathode flow to a sealed region on the opposite side of the flow plate  250 . This sealed region, in turn, is in fluid communication with the cathode exhaust plenum passageway. As depicted in  FIG. 10 , a seal  271 , which is concentric to the center line  251 , may be located radially inside the openings  280  and generally circumscribe the opening  270 . The seal  271  in combination with another concentric seal  290  seal off the active region in between. 
     In accordance with some embodiments of the invention, a small number (four, for example) of openings  292  may be formed in the active region near the outer cell  290 . The openings  292  effectively establish a bleed flow between the cathode and anode sides of the cell. 
     Other stack and plate designs are envisioned, all of which fall within the scope of the appended claims. 
     While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.