Patent Application
20170229717
The present invention relates generally to an apparatus and method for improved reactant and coolant flow sealing within joined or fluidly-cooperating fluid-delivery plates used in a fuel cell assembly, and more particularly to the use of a microseal disposed on top of a metal bead that is integrally formed on a cooperating surface of one or both of the plates to provide more effective fluid isolation for the reactant or coolant that is conveyed through channels defined within the plate surfaces.
Fuel cells convert a fuel into usable energy via electrochemical reaction. A significant benefit to such an approach is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines (ICEs) for propulsion and related motive applications. In a typical fuel cell —such as a proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell—a pair of catalyzed electrodes are separated by an ion-transmissive medium (such as Nafion™) in what is commonly referred to as a membrane electrode assembly (MEA). The electrochemical reaction occurs when a first reactant in the form of a gaseous reducing agent (such as hydrogen, H2) is introduced to and ionized at the anode and then made to pass through the ion-transmissive medium such that it combines with a second reactant in the form of a gaseous oxidizing agent (such as oxygen, O2) that has been introduced through the other electrode (the cathode); this combination of reactants form water as a byproduct. The electrons that were liberated in the ionization of the first reactant proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load (such as an electric motor, as well as various pumps, valves, compressors or other fluid delivery components) where useful work may be performed. The power generation produced by this flow of DC electricity can be increased by combining numerous such cells into a larger current-producing assembly. In one such construction, the fuel cells are connected along a common stacking dimension in the assembly—much like a deck of cards—to form a fuel cell stack.
In such a stack, adjacent MEAs are separated from one another by a series of reactant flow channels, typically in the form of a gas impermeable, electrically conductive bipolar plates (also referred to herein as flow field plates). In one common form, the channels are of a generally serpentine layout, although other forms—including those with generally straight or sinusoidal patterns—may also be used. Regardless of the channel shape, it covers the majority of the generally planar surfaces of each plate. The juxtaposition of the plate and MEA promotes the conveyance of one of the reactants to or from the fuel cell, while additional channels (that are fluidly decoupled from the reactant channels) may also be used for coolant delivery. In one configuration, the bipolar plate is itself an assembly formed by securing a pair of thin metal sheets (called half plates, or more simply, plates) that have the channels stamped or otherwise integrally formed on their surfaces, while in another, the assembly includes an additional interstitial sheet with channels to put coolant into thermal communication with the adjacent anode and cathode channels of the outer sheets. Regardless of whether the assembly is two-sheet or three-sheet variety, the various reactant and coolant flowpaths formed by the channels on each of these sheets typically convene at a manifold (also referred to herein as a manifold region or manifold area) defined on one or more opposing edges of the plate. Examples of all of these features—as well as a typical construction of such bipolar plate assemblies that may be used in PEM fuel cells—are shown and described in commonly-owned U.S. Pat. Nos. 5,776,624 and 8,679,697, the contents of which are hereby incorporated by reference.
It is important to avoid leakage and related fluid crosstalk within a PEM fuel cell stack. This is of particular concern in the manifold regions of the bipolar plates where by virtue of higher pressures relative to the plate active regions, there is a greater tendency of fluids therein to be forced out through holes, surface undulations and related sealing discontinuities. To mitigate against leakage of such high pressure fluids, the Assignee of the present invention has used separate thick elastomeric seals placed between at least these regions of adjacently-stacked bipolar plates. In one form, the seals were overlaid on the surface to define a generally picture-frame type of structure as a way to circumscribe the region of the plate as a way to form a cooperative interface with an adjacent plate or other component. In other configurations, the Assignee of the present invention has formed grooves into the plate surface such that generally cylindrical or string-shaped seals placed within the grooves can providing the sealing interface. Regardless of whether the seals are configured to cooperate with grooved or non-grooved surfaces, even slight misalignment between adjacent plates under compression (such as that attendant to stack assembly) leads to variations in pressure applied to the corresponding seals, which in turn leads to seal deformation and concomitant gap formation and reactant or coolant leakage. Moreover, the use of separately-formed thick seal assemblies—while generally helpful in achieving improved sealing—is incompatible with commercial automotive fuel cell assembly applications, where high volume manufacturing requirements may involve the production of large numbers of fuel cell stacks per year. Given that each cell requires a bipolar plate assembly on both opposing surfaces of the MEA, even low volume production would require that a significant number of plates be made. As such, these thick sealing approaches would be a cost-prohibitive way to achieve the sealing methods needed to reduce reactant or coolant channel flow losses.
To overcome some of the cost and manufacturing issues related to the use of thick elastomeric sealing approaches, the Assignee of the present invention has developed integrally-formed bipolar plate sealing where the plate surfaces are stamped in a manner similar to that which is used to form the reactant or coolant channels. This stamping produces outward-projecting metal beads to establish generally planar plateaus that define discrete contact points between adjacent plate surfaces. While such a configuration is more compatible with the high-volume production needs mentioned above, proper sealing remains difficult to achieve, especially in view of the inherent vagaries of fuel cell stack manufacturing where both dimensional tolerances of the components as well as the cell-to-cell alignment of one hundred or more individual cells within the stack are likely to be present.
It would be desirable to provide enhanced sealing between adjacently-stacked plates (whether within a single bipolar plate assembly or across numerous plates within a fuel cell stack), including ensuring that such seals are impervious to the effects of component tolerances, interplate misalignment and other hard-to-control manufacturing factors. It would likewise be desirable to achieve such sealing in a repeatable, cost-effective manner.
It is an object of the disclosure to provide a microseal that will help make the process of joining bipolar plates and their metal beads relatively impervious to the plate-to-plate misalignment and component tolerances. According to a first aspect of the present invention, a bipolar plate assembly for a fuel cell system includes a pair of plates (often referred to as half-plates) that are joined together such that a microseal disposed on the metal bead surfaces of at least one of the half-plates increases the fluid-tight seal between them. A subgasket is placed between the pair of plates and can cooperate with the microseal and an MEA that is disposed between the pair of plates. In addition, the subgasket is sized and shaped from a non-conductive and gas impermeable material such that it forms a frame-like periphery around the MEA as a way to separate and prevent contact between the electronically conductive layers (electrodes and gas diffusion layer) on the cell's anode and cathode sides. By the cooperation of the microseal and the engaging surface of the respective metal bead and subgasket, fluid isolation between the pair of plates is maintained, while the cooperation of the subgasket and the MEA ensures the desired electrical isolation.
In the present context, one or both of the half-plates will be understood to be made from a thin underlying metal structure that includes planar opposing surfaces at least one of which defines one or more of a reactant channel, reactant manifold, coolant channel and coolant manifold. Likewise, the metal bead is defined by a rectangular, trapezoidal, hemispherical or related shape cross section that is integrally-formed with and projects out of the surface of the half-plates; this metal bead provides the necessary seal force and related fluid isolation between cooperatively engaged plates via suitable balance of out-of-plane elasticity and stiffness. Furthermore, the microseal is a layer of polymeric material (such as that discussed in more detail below) that may be deposited via various methods (also discussed in more detail below) onto the metal bead or subgasket. Together, the microseal and the underlying metal bead make up a metal bead seal (MBS), where the functions of the microseal are to (a) fill in the imperfection of the metal bead and subgasket surfaces, (b) induce more uniform seal force per length along the MBS length by providing a compliant cushion to make up the non-uniform compressed height of the metal bead, (c) prevent the gas/fluid permeation through the bulk of the microseal and (d) to prevent leakage at the subgasket/microseal or metal bead/microseal interfaces. Moreover in the present context, the peripheral formation of the subgasket does not require complete edgewise coverage around the MEA, but rather complete through-the-thickness electrical isolation between the MEA's anode and cathode.
According to another aspect of the present invention, a method of assembling a fuel cell system includes placing numerous fuel cells on top of one another in a stacked configuration and placing a microseal on top of a metal bead that is integrally formed as part of at least one plate of a bipolar plate assembly. The plate, metal bead and microseal configuration are similar to those discussed in the previous aspect.
These and other objects, features, embodiments, and advantages will become apparent to those of ordinary skill in the art from a reading of the following detailed description and the appended claims.
The following detailed description of the preferred embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which the various components of the drawings are not necessarily illustrated to scale:
Referring initially to
The fuel cell 30 includes a substantially planar proton exchange membrane 35, anode catalyst layer 40 in facing contact with one face of the proton exchange membrane 35, and cathode catalyst layer 45 in facing contact with the other face. Collectively, the proton exchange membrane 35 and catalyst layers 40 and 45 are referred to as the MEA 50. An anode diffusion layer 55 is arranged in facing contact with the anode catalyst layer 40, while a cathode diffusion layer 60 is arranged in facing contact with the cathode catalyst layer 45. Each of diffusion layers 55 and 60 are made with a generally porous construction to facilitate the passage of gaseous reactants to the catalyst layers 40 and 45. Collectively, anode catalyst layer 40 and cathode catalyst layer 45 are referred to as electrodes, and can be formed as separate distinct layers as shown, or in the alternate (as mentioned above), as embedded at least partially in diffusion layers 55 or 60 respectively, as well as embedded partially in opposite faces of the proton exchange membrane 35. In fact, as will be appreciated by those skilled in the art, the precise placement of the catalyst layers 40, 45 on the membrane 35 or diffusion layers 55, 60 is not critical to the operation of the embodiments of the invention discussed herein; as such, MEAs 50 may be in one of two conventional forms the first of which is a catalyst coated membrane (CCM), and the second of which is catalyst coated diffusion media (CCDM) that is subsequently attached to the PEM. Either variant is deemed to be compatible with and therefore within the scope of the present invention.
In addition to providing a substantially porous flowpath for reactant gases to reach the appropriate side of the proton exchange membrane 35, the diffusion layers 55 and 60 provide electrical contact between the electrode catalyst layers 40, 45 and an assembly of bipolar plates 65 that in turn acts as a current collector. Although shown notionally as having a thick-walled structure in
Subgaskets 75 may be used to promote seal attachment or related cooperation between the half-plates 65A and 65B and the MEA 50; in one form, subgasket 75 may be made from a plastic material to define an electrically non-conductive picture frame-like profile that may be placed peripherally to protect the edge of the MEA 50. This subgasket 75—which is preferably between about 50 μm and 250 μm in thickness—is often used to extend the separation of gases and electrons between the catalyst layers 40 and 45 to the edge of MEA 50 as a way to increase the membrane 35 active surface area.
In operation, a first gaseous reactant, such as H2, is delivered to the anode side of the MEA 50 through the channels 70 from half-plate 65A, while a second gaseous reactant, such as O2 (typically in the form of air) is delivered to the cathode side of the MEA 50 through the channels 70 from half-plate 65B. Catalytic reactions occur at the anode 40 and the cathode 45 respectively, producing protons that migrate through the proton exchange membrane 35 and electrons that result in an electric current that may be transmitted through the diffusion layers 55 and 60 and bipolar plate 65 by virtue of contact between it and the layers 55 and 60. Related channels (not shown) may be used to convey coolant to help control temperatures produced by the fuel cell 1. In situations where the half-plates 65A, 65B are configured for the flow of coolant, their comparable features to their reactant-conveying plate counterparts are of similar construction and will not be discussed in further detail herein.
Referring with particularity to
Referring next to
Although
As mentioned above, the cooperation of the metal bead 105 and microseal 110 on each of joined half-plates 65A, 65B defines the MBS 115; this structure promotes a more robust, leakage-free sealing, regardless of whether such sealing is formed in the active area 80 or manifold area 85. In another version (not shown), the microseal 110 can be attached or directly formed onto the subgasket 75 as part or extension of the MEA 50; either variant is deemed to be within the scope of the present invention. The microseal 110 shown in this embodiment is noteworthy for its relative large aspect (i.e., thickness-to-width) ratio.
Referring with particularity to
In a configuration where the microseal 110 is formed on the subgasket 75, the microseal 110 would need to be wider than that of the engaging surface 107 of metal bead 105. Furthermore, in a configuration where the microseal 110 is formed directly on the engaging surface 107 of metal bead 105 through screen printing, the microseal 110 defines a thickness of no greater than about 300 μm at its thickest, and an overall width of no more than about 3000 μm (i.e., 3 mm) on the engaging surface 107 of metal bead 105. More particularly, the thickness is preferably between about 30 and 300 μm, with widths of between about 1.0 and 3.0 mm.
The present inventors have observed that in traditional elastomeric seals, the sealing pressure is simply proportional to the contact pressure and width, where the proportionality constant is the material's modulus of elasticity (or tensile modulus) E. However, the present inventors have discovered that the MBS 115 of the present invention does not mimic these idealized pressure conditions, and instead needs to take spatial constraints into consideration; these constraints are due to (1) the comparatively rigid facing metal bead 105 and subgasket 75 substrates that the microseal 110 is placed on, (2) the thinness of the microseal 110 layer that is made possible by improved manufacturing process capabilities (such as the screen printing discussed herein) and (3) the assumption of substantially complete adhesion between the microseal 110 and its respective substrate as a way to enable ease of part handling during both initial manufacturing as well as during reworking or rebuilding.
Importantly, the present inventors have discovered that these constraints cause the material of the microseal 110 to exhibit a much higher stiffness (referred to herein as the effective modulus, Eeff) that relates the designed geometry (h′ for engaged microseal 110 height, a′ for engaged microseal 110 width and η for the aspect ratio of the microseal 110) and the material properties (including tensile modulus E and Poisson's ratio υ) to the applied loads and resulting deflections. In other words, the effective modulus or stiffness modifies the values inherent in the material makeup of the microseal 110 by taking into consideration the spatial constraints placed on the microseal 110. Even more important is that the present inventors have discovered that due to the thin geometry of the microseal 110 relative to conventional thick seals, the effective modulus Eeff (which better explains the leakage phenomena than recourse to a conventional nominal sealing pressure) is very sensitive to plate-to-plate and cell-to-cell misalignment. An analytical solution representing the mechanical behavior of a constrained system is presented in The Effect of Compressibility on the Stress Distributions in Thin Elastomeric Blocks and Annular Bushings by Yeh-Hung Lai, D. A. Dillard and J. S. Thornton in The Journal of Applied Mechanics (1992) the contents of which are incorporated by reference in their entirety. In equation form, this analytical representation is shown as:
where Δ represents the deflection, F represents the force (for example, in Newtons) such as that associated with compressing the cells 30 within stack 1 along their axial stacking dimension, S represents the stiffness and:
As such, Δ/h′ equals the strain (or change in thickness along the thickness dimension in response to the applied force F). From the above, the present inventors believe that the effective modulus Eeff can be related to the local sealing pressure in a manner similar to how the modulus E of the material can be related to the sealing pressure in a traditional elastomer seal design.
A key component of the present invention is to reduce the sensitivity of the microseal 110 effective modulus Eeff to misalignment that is directly related to the sealing pressure that the microseal 110 exerts on the facing interfaces, be it the metal bead 105 or subgasket 75. The present inventors have accounted for misalignment by considering that the engaged microseal 110 width a′ be defined as:
a′=a−α
where a equals the nominal microseal 110 width and α is the misalignment. For fuel cells of interest to the present invention, the misalignment is likely to be less than about 0.4 mm. Furthermore, in a case where there is an asymmetric microseal 110 design (such as that depicted in
h′=h
1
+h
2
where h1 equals the thickness of the first microseal 110A and h2 equals the thickness of the second microseal 110B. From this, the engaged microseal 110 aspect ratio η is:
Manufacturing and assembly process capabilities will determine the relevant range of the aspect ratio η. For example, assembly alignment capabilities contribute to defining a range of the misalignment α and the consistency of applied microseal 110 thickness contributes to defining a range for the total engaged thickness h′. From this, it is preferable to design the MBS 115 such that the effective modulus Eeff is robust against the full range of aspect ratios η that result from the preferred manufacturing and assembly processes. In this way, the present inventors have determined that a preferred way to reduce the sensitivity of the effective modulus Eeff to misalignment is to reduce the aforementioned spatial constraints on the microseal 110. More particularly, the present inventors have determined that there are several ways (via so-called “design knobs”) to achieve this, as discussed in more detail as follows.
The first approach is to increase the aspect ratio η in the designed geometry, such as by (a) increasing the entire microseal 110 height, (b) utilizing a more dome-like microseal 110 profile (such as that depicted in
In the table, the relative effective modulus Eeff sensitivity to misalignment for different designed geometries is shown, where the column entitled Increased Aspect Ratio corresponds to the configuration depicted in
Referring next to
The third approach is to reduce the adhesion or friction of the microseal 110 to the facing substrates of the subgasket 75 or the metal bead 105. Under compression, reduced friction or adhesion would allow the microseal 110 to expand laterally and escape the previously assumed spatial constraints. Nevertheless, caution must be exercised, as the addition of a lubricant as one way to lower friction or adhesion may introduce contaminants to the bipolar plate 65. Likewise, reducing the roughness of either the metal bead 105 or subgasket 75 may add a prohibitively expensive step to the substrate manufacturing, while removing microseal 110 adhesion entirely would make part handling difficult during stack 1 alignment and assembly, as well as during any needed rebuilding or reworking. With these concerns in mind, the present inventors have found that by judicious use of subgasket 75 materials, subgasket 75 reductions in surface roughness and the use of a lubricant between any or all of the interfacial regions between subgasket 75, metal bead 105 and microseal 110, may permit the relaxation of other design parameters in order to lower the pressure differential (Δp) that occurs as a result of misalignment, even in configurations where the initial pressure may be lower.
Referring next to
In a preferred form, microseal 110 may be made from various resilient plastic or elastomeric materials, including polyacrylate, alhydrated chlorosulphonated polyethylene, ethylene acrylic, chloroprene, chlorosulphonated polyethylene, ethylene propylene, ethylene vinyl acetate, perfluoroelastomer, fluorocarbon, fluorosilicone, hydrogenated nitrile, polyisoprene, microcellular polyurethane, nitrile rubber, natural rubber, polyurethane, styrene-butadiene rubber, TFE/propylene, silicone, carboxylated nitrile or the like), and is preferably applied by a screen printing process known in the art, although other approaches, such as pad printing, injection molding or other deposition techniques may also be used.
It is noted that terms like “preferably”, “generally” and “typically” are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention.
For the purposes of describing and defining the present invention, it is noted that the terms “substantially” and “approximately” and their variants are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.
Having described the invention in detail and by reference to specific embodiments, it will nonetheless be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. In particular it is contemplated that the scope of the present invention is not necessarily limited to stated preferred aspects and exemplified embodiments, but should be governed by the appended claims.
