Method for assembling a fuel cell stack and appropriately designed fuel cell stacks

Abstract

Method for assembling a fuel cell stack (1), the method comprising: a plurality of planar fuel cells (15) are arranged along a stacking dimension within a stacking support (200), each of the fuel cells (15) comprising at least one membrane electrode arrangement positioned facing adjacent to a bipolar plate which defines at least one integrally formed edge extension in the form of an outwardly extending tab (17G); a potting material is poured into a section of the holder (200) that defines a shape corresponding to the extension in the form of the tab (17G), whereby the tab (17G) is overmolded with the potting material, so that after curing the potting material forms a reference point (18) that is attached to the arranged fuel cells (15) along the stacking dimension to provide increased resistance to intercellular movement of the arranged fuel cells (15) along a dimension orthogonal to the stacking dimension; and the arranged fuel cells (15) are mounted within a housing (20).

Description

[0001] The present invention relates generally to an improved design for assembling a fuel cell stack and more specifically to a way of distributing an acceleration load over a fuel cell stack in order to secure and maintain the relative position of the fuel cells within the stack after it has been subjected to collisions and other high acceleration loads.

[0002] A key advantage of using fuel cells to convert a fuel into usable electricity via an electrochemical reaction is that this is achieved without relying on combustion as an intermediate step. As such, fuel cells offer several environmental advantages over internal combustion engines (ICEs) for vehicle propulsion and similar drive applications. In a typical fuel cell—for example, a proton exchange membrane (PEM) or polymer electrolyte membrane (PEM) fuel cell—a pair of catalyzed electrodes is separated by an ion-permeable medium (e.g., Nafion™), commonly referred to as a membrane electrode assembly (MEA). The electrochemical reaction occurs when a gaseous reducing agent (such as hydrogen, hydrogen, or hydrogen) is introduced into the fuel cell.Hydrogen (H₂) is introduced and ionized at the anode and then forced to pass through the ion-permeable medium, where it combines with a gaseous oxidizing agent (such as oxygen, O₂) introduced through the other electrode (the cathode). This combination of reactants forms water as a byproduct. The electrons released during the ionization of the hydrogen travel onward as direct current (DC) through an external circuit, typically including a load (such as an electric motor), where useful work can be performed. The power output produced by this flow of DC electricity can be increased by combining many such cells into a larger power-generating array.In such a design, the fuel cells are connected along a common stacking dimension - much like a stack of cards - to form a fuel cell stack.

[0003] The supply of reactants to the MEA—as well as the removal of the byproduct water and the supply of the electrical current generated by the cells to the load—is facilitated by the stacked interaction of the MEA, a gas-permeable diffusion medium (also known as a gas diffusion medium (GDM)), and a multi-channel bipolar plate. In addition to establishing a planar, facing relationship with the MEA and the GDM, the bipolar plate defines a collector / distributor as part of a frame-like structure dimensioned to be placed around the perimeter of the MEA and the GDM to facilitate the movement of the reactant, coolant, and byproduct within the stack.

[0004] Fuel cells installed in vehicles must be able to withstand significant load changes caused by vehicle acceleration and deceleration, as well as collisions, accidents, and similar impact events. In particular, the relative positions of the fuel cells forming the stack must be maintained after being subjected to high acceleration loads (e.g., up to 160 g or more) during destructive events such as a vehicle collision, in order for them to continue functioning. During such events, high shear forces can cause sliding between adjacent cells in the stack (specifically within the XZ plane of the previously mentioned Cartesian coordinate system). Small displacements between individual cells are amplified over the height of a large stack arrangement (e.g.,A cell displacement of 100 micrometers can result in a cell block displacement of 30 mm for a stack of 300 cells. These problems can be exacerbated by cold start conditions, where thermally induced contraction can reduce the Y-axis compression restraint load imposed on the cells during stack assembly, as well as by reduced intercellular friction, which can be caused by the use of surface treatments or inserts that may exhibit properties with a low coefficient of friction.

[0005] One way to prevent displacement between plates or intercellular displacement of automotive fuel cells during these high-acceleration events is to leave in place the reference pins used in the stacking arrangement and coupled to the stack, even after the assembly process is complete. In this way, the pins provide additional resistance to shear motion between adjacent stacked plates or cells. In this context, these shear displacements or displacements in the same plane between adjacent cells or plates are based on the understanding that the cell or plate stack axis is orthogonal to the direction of travel of the vehicle powered by that stack. The stack axis can be defined as such along a substantially vertical Cartesian axis (i.e.,the Y-axis) such that the majority of any intercellular or plate-to-plate movement that should be minimized lies in the XZ-plane. Experts in the field will recognize that the specific orientation of the cell, plates, and stack is not critical; rather, the means employed to prevent or reduce intercellular or plate-to-plate displacement are preferentially arranged in an orientation that maximizes such prevention. While the use of conventional reference pins and similar structures is effective in maintaining the relative stacking alignment of the cells or plates when subjected to high acceleration loads in the same plane, it significantly increases the cost of stack assembly.Their continued presence within the stack also makes disassembly difficult in the event that one of the cells or other stack components needs to be replaced for maintenance.

[0006] Another way to prevent displacement between the plates or intercellular displacement of automotive fuel cells during such a destructive event is to use adhesives or a supplementary support structure that can be formed between a casing wall and the stack. An example of this is described in US patent application US 2014 / 0272662 A1. The approach described herein employs a deployable, adhesive-like potting compound between the lateral edges of the stacked plates and a rigid housing or similar enclosure. However, this approach is only applied after the cells and plates have been aligned and stacked, and thus does not assist in aligning the cells and plates during the stacking process. Moreover, the permanent nature of the compound used is not conducive to subsequent stack disassembly for repair or diagnostic analysis.

[0007] Yet another approach involves welding (or otherwise attaching) a tab projecting laterally from one or more of the edges of the generally rectangular bipolar plate. These tabs can be engaged with each other along the dimension through the thickness (i.e., the Y-axis), so that the tendency of each cell or plate within the stack to move in response to a shear force (i.e., in the same plane) is counteracted by the engaging contact of the tab and recess. While effective in preventing movement between the plates or intercellular movement, each plate must be individually attached to its corresponding plate. SUMMARY OF THE INVENTION

[0008] According to the invention, a method for assembling a fuel cell stack is presented, characterized by the features of claim 1. The method comprises arranging a plurality of bipolar plates along a stacking dimension and adding a potting material to at least one circumferential edge formed by the stacked plates; the resulting reference point (also referred to as a reference point structure) has an increased thickness dimension along the cell stacking dimension; the increased thickness corresponds to the number of stacked cells and plates of a specific group. Thus, a group of 8 plates would have a corresponding thickness of 8 reference points, while a group of 16 or 32 plates would have a corresponding reference point thickness. The plates preferably define at least one lateral waveform to accommodate the complementary shape of the potting material.In this way, the reference point is attached to the stacked cells within a multi-cell group, forming a tight, interlocking fit between them. In the present context, the attachment of the reference point to the stacked cells within a group is achieved via the cured potting compound, which bonds with or is otherwise coupled to the edge waveform on or around each of the cells. According to a preferred embodiment, the waveform is an outwardly projecting tab that may be overmolded with the potting compound. The potting compound, in turn, defines a shape such that the resulting projection can fit outwardly into a complementarily shaped recess or cavity formed within a stacking housing or similar enclosure.In a preferred embodiment, the potting compound is built up along the stack dimension to be as thick as the number of plates. In a more preferred embodiment, the interaction between the reference point formed by the potting compound and the various plates is achieved by overmolding the potting compound onto integrally formed tabs extending laterally from the plate circumference. In this latter embodiment, the multilayered, thick reference point formed by the potting compound can be shaped to interact with a complementarily shaped lateral waveform formed within the stack casing or housing, such that any shear motion transmitted to the various stacked plates is directed through the thick reference point and into the housing to provide the increased resistance.One or more supports can be used to facilitate the stacking process, as well as to form a mold cavity or shape on the lateral edge into which the potting material can be poured.

[0009] Furthermore, a method for assembling many fuel cells is described. Each cell comprises a MEA (Modulation Energy Array) positioned adjacent to a bipolar plate, which defines at least one integrally formed edge extension within it. The method includes defining one or more mold shapes within a stacking holder, which are designed to receive a potting material. The fuel cells are arranged along a stacking dimension within the stacking holder, such that a liquid form of the potting material is poured into the mold. Upon cooling, the potting material forms a reference point that is attached to the various arranged fuel cells along their stacking dimension, thereby providing increased resistance to intercellular movement of the arranged fuel cells along their dimension, which is substantially orthogonal to the stacking dimension.As discussed elsewhere, the number of cells that can be stacked to form a module, with a reference point acting as the attachment or connection point, may be determined by other stacking requirements of the fuel cell system; according to one form, the number of cells arranged in each module may be a multiple of 8, such as 8, 16, 24, or 32, with limits at the upper boundary of cells imposed by the mechanical properties (such as shear strength) of the potting material.

[0010] Furthermore, a fuel cell stack is presented according to the invention, characterized by the features of claim 6. The stack comprises many fuel cells arranged in an adjacent relationship along a stacking dimension (e.g., the aforementioned Y-axis in a conventional Cartesian coordinate system), and a respective bipolar plate for each of the cells. A potting material is attached to the stacked fuel cells along their stacking dimension to provide increased resistance to intercellular movement along a dimension that has at least one component which is substantially orthogonal to the stacking dimension. BRIEF DESCRIPTION OF THE DIFFERENT VIEWS OF THE DRAWINGS

[0011] The following detailed description of the preferred embodiments of the present invention is best understood when read in conjunction with the following drawings, in which identical structures are designated by the same reference numerals and in which: Fig. 1 is a simplified exploded view of a fuel cell stack; Fig. 2 a perspective view of a vehicle with the fuel cell stack of Fig. 1 is; Fig. 3 is a simplified exploded view of a bipolar plate, which is used in the fuel cell stack of Fig. 1 is used; Fig. 4 is a perspective view of a stacked fuel cell block before placing a housing support structure around it, with the protruding tabs and overmolded reference point placed along a lateral edge corresponding to the bipolar plates; Fig. 5A shows an alternative embodiment of the placement of the protruding tabs and the overmolded reference point along the corners of a bipolar plate placed within a stacking holder according to one aspect of the present invention; Fig. 5B a detailed section of the reference point placed at the corner of Fig. 5A shows; and Fig. 6 shows the use of a housing with a feature shaped to align with the reference point of Fig. 4 to work together to provide another form of complementary intercellular sliding resistance. DETAILED DESCRIPTION OF PREFERRED EXECUTION FORMS

[0012] First, referring to the Fig. 1 and Fig. Figure 3 shows a fuel cell stack 1 comprising a plate 5 of the dry end unit, a plate 10 of the wet end unit, and a block of fuel cells 15 placed in a stacking orientation between the plates 5 and 10 of the end units. Although not shown in detail, each fuel cell 15 within the block generally comprises the anode, the cathode, and a membrane, arranged together to form the generally planar sandwich structure of the MEA, which is pressed between a pair of gas-permeable, electrically conductive diffusion media. These media serve both to supply reactants (i.e., H2 on the anode side of the MEA and O2 (usually in the form of air) on the cathode side of the MEA) and to receive electrical current catalytically generated at the anode and cathode.The fuel cell 15 also includes bipolar plates (hereinafter also referred to as bipolar plate assemblies) 17, which provide a support structure to the MEA and the diffusion media. In the present context, the stacking dimension, which corresponds to the height of the assembled stack 1, is defined along the Y-axis, as shown, although those skilled in the art will recognize that this is a matter of expediency and that any linear dimension, depending on the orientation of the stack 1 during the assembly process, is equally suitable.

[0013] The many individual cells 15 within the stack 1 are held in place by the casing or housing 20, which consists of a pair of opposing end covers 25, 30, side walls 35, 40, and rigid bracket elements 45 arranged vertically along each of the side walls 35, 40 to attach the plate 10 of the wet end unit to the plate 5 of the dry end unit. According to one design, the plate 10 of the wet end unit is fixedly attached to the opposing end covers 25, 30, while the plate 5 of the dry end unit is adjustably attached. This latter connection is particularly advantageous in stacking applications, since the attached endpoint position is unknown until final assembly; this, in turn, allows the attachment to be adjusted to accommodate a range of possible endpoint attachment positions.In contrast, the fixed attachment of the wet end plate 10 means that the positional relationship of the attached components to each other is essentially unchanging. Additional details, in connection with the molded features that can be formed into one or more end covers 25, 30 and side walls 35, 40, are described in relation to [reference]. Fig. 6 discussed below.

[0014] Next, referring to Fig. 2 comprises a vehicle 100 (e.g. a car, a van, a bus, a truck or a motorcycle), a fuel cell-based drive system 110, which consists of an electric motor 130, which obtains its electrical power from the fuel cell stack 1 of Fig. 1, which comprises many individual fuel cells 15. The drive system 110 can include one or more fuel gas storage tanks 130, 140, as well as power converters or similar electronics, electrical storage devices (e.g., batteries, ultracapacitors, or the like) 160, and controllers that provide operational management, and any number of valves, compressors, piping systems, temperature controllers, and other auxiliary equipment.

[0015] Any number of different types of fuel cells 15 can be used to form the stack of the drive system 110; the cells 15 can be made of metal hydride, alkaline, electrogalvanic, or other variants. According to a preferred (though not necessary) embodiment, the cells 15 are PEM fuel cells, as discussed above, and the remainder of the present invention is based on this embodiment. According to a preferred embodiment, the cells 15 within the stack 1 are combined in series, parallel, or a combination of both to produce a higher voltage or current output, depending on the requirements of the vehicle 100. It will be understood that the stack 1 can be used for purposes other than motor vehicles.

[0016] Referring again to Fig. 3 defines each bipolar plate arrangement 17, as in the MEAs and diffusion media (not shown) to which it is attached, as a generally rectangular, planar surface section 17A with four edge sections 17B forming a perimeter around the surface section 17A. Between the surface section 17A and the two edge sections 17B along the longitudinal dimension is a pair of collector (or distributor) sections 17C, 17D, each arranged on opposite edges, so that the introduced reactant passes through the many winding flow field channels formed on the surface section 17A. According to one form, one of the collector sections 17C, 17D defines a supply (or inlet) collector, while the other defines an exhaust collector.According to an alternative embodiment (not shown), the supply and exhaust collector sections 17C, 17D can be located adjacent to each other along the same edge of the plate assembly 17; each variant is compatible with the present invention. According to one embodiment, the bipolar plate assembly 17 can be made of separate sheets 17E, 17F (generally between about 75 and 100 micrometers along the thickness (i.e., stacking) dimension) of a suitable corrosion-resistant material (such as 304 stainless steel), which can be joined together by brazing, laser welding, or some other process; in such an embodiment, the flow channels formed by each surface section 17A can generally define mirror images of each other, so that when two adjacent cells 15 are stacked, the surface sections 17A of the two within a single assembly 17 are in a back-to-back arrangement.In addition to these plates, which comprise the flow channels for the reactant gas flow field, similarly shaped plates (not shown) can be stacked to provide a separate coolant flow circuit; such coolant flow circuits can define any suitable flow channel shape (such as the coiled shape shown in the flow channels). As with plates 17E, 17F, each such additional coolant flow circuit can be joined by brazing or another suitable joining technique. In the present context, the bipolar plate arrangement may or may not include the coiled flow channels formed on the two opposing planar surfaces; if, for example, such a plate forms the last plate in a stack 1, it would not be necessary for channels to be present that are formed in the side facing the unit end plates (such as unit end plates 5, 10 of ). Fig. 1) are facing each other.

[0017] A lateral waveform (also referred to as a lateral extension, lateral projection, or the like) in the form of a tab 17G is formed integrally within the plane of each arrangement 17. In this context, the integral connection between the plate and the tab 17G differs from those requiring separate fastening, such as by bolting, welding, or the like. This avoids additional manufacturing steps. A reference point 18 is overmolded with a suitable potting compound over the tab 17G, defining a thickness along the stacking dimension (i.e., the Y-axis, as shown). As discussed in more detail below, a conical recess 18A helps to provide resistance to sliding between the plates or similar relative movement.Other shapes besides the conical shape 18A, including a column or pin (not shown), may also be used, which mimics the projection of an arrangement (e.g., reference point) pin along the stacking dimension; these and other shapes are considered to be within the scope of protection of the present invention.

[0018] Next, referring to Fig. 4 in conjunction with Fig. Figure 3 shows details relating to the formation of a reference point 18 along two of the lateral edges 17B of many stacked cells (currently shown in simplified form comprising only the bipolar plate assemblies 17). As shown, openings may be formed in tabs 17G to further assist in overmolding the cast potting compound that forms the reference point 18. Although the tabs 17G (as well as the overmolded reference point formed thereon) are shown as arranged along the longitudinal edges of the bipolar plate assemblies 17, those skilled in the art will recognize that other locations on the plate circumference may also be used. As shown in connection with the Fig. 5A and Fig. As discussed in Section 5B below, for example, placing the tabs 17G and their overmolded reference point on the corners also falls within the scope of protection of the present invention. It will also be apparent that, while the shape of the tabs 17G is shown as rectangular, other shapes (e.g., trapezoidal, semicircular, or the like) can also be used, particularly if they are helpful in forming the overmolded reference point 18. In the specific shape shown in Fig. As shown in Figure 3, the reference point 18 defines a conical or similar thickness feature 18A which, by being substantially hollow within the region defined by the cone, allows two or more reference points 18 to be nested when stacking adjacent groups of arrangements 17. The conical feature 18A of the reference point 18 is discussed in greater detail in related U.S. patent application US 2016 / 0072145A1. Significantly, the use of the reference point 18 ensures that no pins are necessary for assembling the stack 1.

[0019] Specifically, the reference point 18 can be used to alleviate concerns regarding recent design improvements in intercellular sliding resistance by providing out-of-plane support (e.g., by being able to withstand loads of up to approximately 160 g). According to a specific mold, the reference point 18 can be made of a rigid, load-bearing plastic (e.g., an epoxy or elastomeric material) that can be overmolded onto the thin tabs 17G forming the integral extensions from the edges 17B or corners of the individual plates that make up the arrangement 17. In this way, a mold (shown below in conjunction with Fig. 6 shown and discussed in more detail), which serves as a mold for pouring the potting material during the stack assembly process, is helpful in constructing the reference point 18 in a column-like design for a complementary resistance fit between the reference point 18 and the housing 20. Although Fig. Figure 3 currently shows two reference points 18 (one each on opposite edges 17B) per bipolar plate 17. It will be evident that, depending on the configuration of the stack 1, more or fewer reference points may be formed. According to a preferred embodiment, several reference points 18 per grouped arrangement 17 are preferred, since this will not only help to promote better alignment during the assembly of the stack 1, but can also provide additional shear resistance between adjacent layers of cells 15 within the stack 1 in situations where higher levels of sliding resistance may be required.

[0020] Severe load changes resulting from a destructive event, such as a strong acceleration or deceleration of the vehicle 100, an impact involving the vehicle 100, or a similar impact on the fuel cell stack 1 itself, such as a vertical fall, can damage or disintegrate the fuel cell stack 1 by causing the individual fuel cells 15 within it to move relative to one another. The mechanical properties of the reference point 18 must be sufficient to withstand the maximum acceleration associated with such load changes. According to an exemplary design, the reference point 18 would have a shear strength of approximately 150 N for each corresponding bipolar plate of the arrangement 17 within the stack as one way to withstand a load of 160 g.

[0021] Next, referring to the Fig. 5A and Fig. Figure 5B shows a variant of the placement and shape of the reference point 18. Specifically, a support 200, used to stack the individual fuel cells 15, comprises an upper and a lower plate 210, 220, as well as the reference point shapes 230 and guide pins 240 mounted at the corners. Flange-mounted bearings 250 are attached to the upper plate 210 to receive the ends of the linear guide pins 240, which are mounted at their opposite ends on the lower plate 220. Additional pegs or similar alignment pins (not shown) may also be used. According to this variant, the lower plate 220 of the holder 200 comprises a generally trapezoidal shape or similar preform 230 located on the four corners of the lower plate 220 to receive the liquid potting material precursor which (after hardening) becomes the reference point 18.The corner of the bipolar plate arrangement 17 is shaped such that a tab-like extension (generally referred to as tab 17G of . Fig. 3 is similar) can intervene in reference point 18. As mentioned above, the number, shape, and placement of reference point 18 can be varied depending on the needs of the fuel cell system using stack 1.

[0022] Next, referring to Fig. 6 in conjunction with Fig. 4. Other complementary means can be used to provide improved resistance to scissors and similar intercellular sliding. For example, shows Fig. 6 in particular bulges 20, which may be formed or attached to the housing 20. According to the version shown, the housing 20 may be an essentially integral structure (e.g., based on extruded aluminum) that is robust enough to provide support for the cell block 1 during an impact and to hold it in compression along the stacking axis. These shapes define a complementary resistive fit between the formed reference point and the bulges 20A formed within the housing 20. In such a case, the formed bulges 20A are dimensioned to allow a tight-tolerance fit along at least one sliding direction S within the XZ plane of each arrangement 17 within the stack 1 and are spaced apart to accommodate the lateral placement of the stacked groups of reference points 18 (currently shown in an embodiment without the conical feature 18A of the embodiment of Fig. 3 are shown), to coincide. According to a preferred embodiment, the protrusions 20A are integrally formed into the extruded aluminum housing 20, although according to another embodiment, the protrusions 20A can be formed separately and subsequently attached to the housing 20. Moreover, the protrusions 20A can form a generally C-shaped profile (viewed from above), such that any tendency to intercellular sliding movement along the sliding direction S is additionally resisted by the column-like shape, which extends over essentially the entire height (which is related to the stacking dimension of the Y-axis of Fig. 1 (coincides) of the housing 20 extends. The shape of the formed bulges 20A helps to ensure that they act as a support wall against movement in the X-direction of Fig. 1 acts as well as providing such additional resistance along a movement elsewhere within the XZ plane. An insulation 20B can be placed in the space between the perimeter of the stack and the inner housing 20 wall to provide electrical insulation.

[0023] As mentioned above, reference point 18 can be determined according to the in Fig. The embodiment shown in Figure 6 does not include the conical feature 18A. Likewise, such a variant (as well as the variant of Fig. 3) avoid the presence of a pin receptacle opening, since the uniform, constructed structure formed by the shaping of the reference point 18 according to one aspect of the present invention on many stacked fuel cells 15 facilitates secure, accurate placement of each of the cells 15 in the stacking dimension without reliance on pins or any other complementary structure. Furthermore, since the tabs 17G formed in the circumferential structure of each bipolar plate or arrangement 17 can be designed to engage with complementarily shaped recesses or indentations of the housing 20, the tendency of each cell or plate within the stack to move in response to a shear force (i.e., a force in the same plane) is resisted by the uniform nature of the reference point 18 and the tabs 17G to form a composite-like interplate structure through the thickness.

[0024] Referring again to Fig. 4. According to another embodiment of a structural reinforcement, a vertically elongated retaining bracket 50 (preferably made of aluminum or steel) can be positioned between the unit end plates 5, 10 of Fig. 1 extend such that it substantially encloses or otherwise covers the foreground stacked reference point 18 in a manner that mimics the C-shaped protrusions 20A which are integral to the embodiment of the extruded aluminium housing 20 of Fig.6. Such a retaining bracket can be screwed or otherwise attached to the unit end plates 5, 10 as one way to achieve structural integrity without having to form a separate housing 20. According to yet another embodiment, instead of being attached directly to the unit end plates 5, 10, the retaining bracket can be attached to a box-like frame formed around the stack 1. Such a frame can also include spacers 55 to help accommodate adjustments for variations in the height of the individual cells 15 that form the stack 1. Additional devices are also shown, including current collector plates 60, 65, which extend laterally from the respective end plates 5, 10 to connect to an electrical circuit (not shown).Insulating plates 70 can be placed between the lower surface of the end plates 5, 10 and a facing adjacent upper surface of the stack 1 to facilitate electrical and thermal insulation between them.

[0025] It is noted that, in order to describe and define the present invention, the terms "fuel cell" or the like are used herein to represent one or more individual cells used to provide an electric current, preferably for automotive, propulsion, or similar purposes. Furthermore, variants of the terms "car," "automotive," "vehicle," or the like are to be understood as generic unless the context indicates otherwise. As such, reference to an automobile is to be understood as covering cars, trucks, buses, motorcycles, and other similar modes of transport, unless a more specific meaning is addressed in the context.

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