fuel cells

Abstract

The present invention discloses a fuel cell comprising a pair of flow field plates, each flow field comprising a plurality of reactant flow channels separated by platforms, wherein at least one platform in at least one flow field plate has a top cross-section with a curve. In particular, the cathode plate can have a platform with a partially or continuously curved cross-section. In some embodiments, the cross-section of the top of the platform is a circular arc. The fuel cell further comprises a membrane electrode assembly having a cathode gas diffusion layer, the cathode gas diffusion layer having a compressible surface, which is compressed when in contact with the cathode plate platform when the fuel cell is assembled, so that the compressed area of the cathode GDL surface is in close contact with the curved top of the platform.

Description

Technical Field

[0001] The present invention relates to a fuel cell, in particular to a flow field plate of a hydrogen fuel cell. Background Art

[0002] Fuel cells electrochemically convert hydrogen fuel and an oxidant (such as oxygen from the air) into electrical energy and reaction products. One type of fuel cell, known as a polymer electrolyte membrane ("PEM") fuel cell, typically employs a membrane electrode assembly ("MEA") comprising a solid polymer ion exchange membrane with catalyst layers applied on both sides to form a catalyst coated membrane ("CCM") disposed between two gas diffusion layers ("GDL"). The catalyst layers contain a catalyst, such as finely divided platinum, to initiate the desired electrochemical reaction. The GDL effectively acts as a facilitator, facilitating diffusion of reactants across the CCM and typically comprises a porous conductive sheet, such as carbon fiber paper or carbon cloth. In operation, the electrodes are electrically coupled to provide an electrical circuit that conducts electrons between the electrodes through an external circuit.

[0003] In a typical fuel cell, the MEA is positioned between two electrically conductive fluid flow field plates, or separators. The fluid flow field plates have at least one fluid flow channel formed in at least one of their major planar surfaces. The fluid flow field plates act as current collectors, providing support for the electrodes, providing access for the fuel and oxidant to the respective anode and cathode surfaces, and providing channels for removing reaction products (such as water) formed during fuel cell operation. One fluid plate, called the anode plate, has open fuel flow channels that direct hydrogen fuel to the anode side of the MEA, while the other fluid plate, called the cathode plate, has open oxidant flow channels that direct oxidant (such as air) to the cathode side of the MEA.

[0004] Both the anode and cathode plates typically have multiple parallel flow channels separated by elongated platforms. These flow channels typically include a flat top surface in contact with the adjacent GDL surface. The portion of the flow field plate containing the flow channels and platforms is typically referred to as the flow field area. The portion of the CCM adjacent to the GDL that is in fluid communication with the flow field area is typically referred to as the active area of the CCM, i.e., the portion of the CCM where electrochemical reactions occur. Optimal fuel cell performance depends on the reactants reaching the active area of the CCM where the electrochemical reactions occur. Taking the cathode as an example, the oxidant supply must migrate from the oxidant channels of the cathode plate through the adjacent GDL to the active area of the CCM. However, conventional platform designs tend to hinder the flow of reactants from the flow channels to those portions of the CCM adjacent to the platforms, thereby reducing fuel cell performance. Fuel cell performance is particularly sensitive to the oxygen concentration in the cathode catalyst area, especially when air is used as the oxidant, as the oxygen concentration in air is only about 21%.

[0005] It is therefore an object of the present invention to provide an improved fuel cell flow field plate which addresses some of the shortcomings of prior art flow field plate designs. Summary of the Invention

[0006] According to one aspect, a fuel cell includes an anode plate, a cathode plate, and a membrane electrode assembly sandwiched between the anode plate and the cathode plate. The anode plate includes a separator having an active side with a fuel flow field, wherein the fuel flow field has an open-face fuel flow channel with at least one platform. The cathode plate includes a separator having an active side with an oxidant flow field, wherein the oxidant flow field has open oxidant flow channels and at least one platform. The membrane electrode assembly includes a catalyst-coated ion exchange membrane sandwiched between anode and cathode gas diffusion layers. At least one of the platforms of the anode plate and the cathode plate has a top with a curved cross-section. In some aspects, both the anode plate and the cathode plate have a top with a curved cross-section.

[0007] At least one of the anode and cathode gas diffusion layers is compressible and is compressed around at least a portion of the platform having a top portion with a curved cross-section.

[0008] The oxidant flow field may include a plurality of parallel platforms that are straight along their length and have a top portion with a curved cross section. The curved cross section may be continuously curved and, for example, may have a fixed radius (full circle) to define a circular portion, or may have a variable radius to define a portion with varying curvature. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a perspective view of the active side of a cathode plate of a PEM fuel cell according to one embodiment of the present invention.

[0010] Figure 2 yes Figure 1 A perspective view of the inactive side of a cathode plate is shown.

[0011] Figure 3 yes Figure 1 A top view of the end of the cathode plate is shown.

[0012] Figure 4 is a cross-sectional view along the section line BB, showing a portion of the flow field of the cathode plate.

[0013] Figure 5 Detailed view of region C of one channel and two adjacent platforms of the cathode plate flow field.

[0014] Figure 6 is a cross-sectional view of a portion of a fuel cell including a cathode plate according to an embodiment of the present invention and portions of two adjacent fuel cells.

[0015] Figure 7is a detailed view of the image of area D showing a portion of the MEA in contact with the platform of the cathode plate. DETAILED DESCRIPTION

[0016] Embodiments disclosed herein generally relate to a fuel cell comprising a pair of flow field plates, each flow field plate having a fluid flow field comprising a plurality of reactant flow channels separated by platforms, wherein at least one platform in at least one of the flow field plates has a top with a continuously curved cross-section. In some embodiments, the cathode flow field comprises a plurality of parallel straight flow channels, the corresponding platforms having a continuously curved cross-section. The cross-section of the top of the platform can have a fixed radius, thereby defining a circular portion, or a variable radius, thereby defining a portion with a varying curvature. The fuel cell further comprises an MEA having a cathode GDL, the cathode GDL having a compressible surface that is compressed when in contact with the cathode plate platform when assembling the fuel cell, so that the compressed area of the cathode GDL surface conforms to the curved top of the platform.

[0017] refer to Figure 1-7 According to one embodiment, the fuel cell 10 includes a MEA 12 sandwiched between a cathode plate 14 and an anode plate 16 (see Figure 6 A plurality of fuel cells 10 may be stacked together to form a fuel cell stack ( Figure 6 1 and 2. The anode plate 16A of one adjacent fuel cell and the cathode plate 14A of another adjacent fuel cell are shown in FIG.

[0018] The cathode plate 14 has a generally planar separator with an oxidant flow field 18 on one surface, referred to as the "active side" (see FIG. Figure 1 ). The opposite surface is called the "inactive side" (see Figure 2 ), and facing the coolant channel 20 on the coolant side of the adjacent fuel cell anode plate 16A. The oxidant flow field 18 includes a plurality of open-face oxidant channels 22, which are surrounded by platforms 24 (see FIG. Figure 4 ) are separated. In this embodiment, the oxidant flow field 18 includes a plurality of longitudinally parallel and straight oxidant channels and corresponding platforms; however, other embodiments may feature flow fields having different channel geometries, such as a serpentine shape.

[0019] At one end of the cathode plate 14, fluid inlets 26, 28, and 30 introduce fuel (hydrogen), oxidant (air), and coolant flows, respectively, into the fuel cell 10. At the other end of the cathode plate 14, fluid outlets 32, 34, 36 exhaust the fuel, oxidant, and coolant flows, respectively, from the fuel cell 10. The fluid outlet ports 32, 34, 36 are fluidically coupled to the corresponding fluid inlet ports 26, 28, 30 through flow passages within the fuel cell 10. In particular, fuel flows from the fuel inlet 26 via spaced-apart fuel return channels 40 on the inactive side of the cathode plate 14 to the anode flow field channels 38 in the adjacent anode plate 16A. The return channels 40 extend from the fuel inlet 26 to a fuel return slot (not shown) in the anode plate 16A; the fuel return slot extends through the thickness of the anode plate 16A and is fluidically coupled to the anode flow field channels 38. Oxidant flows from the oxidant inlet 28 to the oxidant flowfield channels 22 via spaced-apart oxidant return channels 42 on the inactive side of the cathode plate 14, which extend from the oxidant inlet 28 to return slots 44 in the cathode plate 14; the oxidant return slots 44 extend through the thickness of the cathode plate 14 and connect to the oxidant flowfield channels 22 via transition regions 45. Coolant flows from the coolant inlet 30 to the coolant channels 20 of the adjacent anode plate 16A via coolant backfield channels 46 on the inactive side of the cathode plate 14. Similarly, the fuel, oxidant, and coolant outlets 32, 34, 36 are fluidly coupled to their respective anode flowfield channels 38, oxidant flowfield channels 22, and coolant channels 20 via return channels. Peripheral seals (not shown) surround the ports and return channels to prevent leakage.

[0020] Special References Figures 4 to 5 As shown, oxidant flow field 18 features a platform 24 with a curved cross-sectional platform top 52. Without being bound by theory, an ideal cathode flow field would theoretically have a flow field platform that is as narrow as possible to maximize oxygen diffusion beneath the platform while providing sufficient electrical connectivity to ensure adequate current distribution and low resistive losses. It is desirable to provide a platform with a curved cross-sectional top, minimizing the width of platform 24 and, conversely, maximizing the width of oxidant channel 22. In this embodiment, platform top 52 is fully circular, i.e., continuously curved with a constant radius R, thereby defining a circular portion. A suitable range for the platform radius is between 0.05 and 0.5 mm. Utilizing a fully circular design minimizes platform width and is also expected to simplify manufacturability, as the channels and platform can be specified with fully circular dimensions. Alternatively, other embodiments (not shown) may be provided in which the platform top has other continuously curved geometries, such as an oval or elliptical shape. In still other embodiments, platform top 52 may have a continuously curved cross-section with a variable radius to define a portion with varying curvature.

[0021] The platforms 24 also each have a platform bottom 54 that extends downwardly to the oxidant channel bottom 56 of the adjacent oxidant channel 22. In this embodiment, the platform bottom 54 provides a sloped wall for the adjacent oxidant channel 22. The slope angle of the sloped wall is defined as the ratio of the top radius R to the platform bottom width W, and can be between 0.1 and 0.5.

[0022] The flow channel walls are connected to the oxidant channel bottom 56 by fillets 58 for ease of manufacturing; however, in other embodiments, the oxidant channel 22 may have other cross-sectional geometries, for example, the oxidant channel 22 may have a circular cross-section defining a circular segment (not shown).

[0023] The cathode plate 14 can be made of expanded graphite and manufactured by stamping a blank plate to form the desired structure and then performing appropriate post-processing. Alternatively, the cathode plate 14 can be made of other suitable materials known in the art and manufactured by other suitable techniques known in the art (e.g., molded graphite particle / resin composites, processed into shaped conductive materials such as molded graphite particle / resin composites, stamped into metal plates).

[0024] As previously mentioned, the platform design should provide adequate conductive connections to ensure adequate current distribution and low resistive losses. Platforms with a continuously curved top may reduce the available surface area for electrical contact with the MEA. To ensure adequate electrical contact, the MEA 12 is equipped with a compressible GDL. Figure 7 The portion of the GDL (not shown) in contact with the platform 24 is compressed to conform to the curved platform top 52 and maintain good electrical contact. An example of a suitable compressible MEA includes a catalyst-coated perfluorosulfonic acid (PFSA) membrane sandwiched between a pair of carbon fiber paper GDLs.

[0025] According to yet another embodiment, the anode plate 16 includes a fuel flow field having a platform with a top portion having a curved cross-section. The cross-section of the platform top portion may be partially or continuously curved, and may be completely circular, for example, to define a circular portion.

[0026] The terms used herein are for the purpose of describing specific embodiments only and are not intended to be limiting. Accordingly, as used herein, the singular forms "a", "an" and "the" are intended to also include the plural forms, unless the context clearly indicates otherwise. It will be further understood that, when used in this specification, the terms "include" and "comprise" specify the presence of one or more of the described features, integers, steps, operations, elements and components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and groups. Directional terms such as "top", "bottom", "upward", "downward", "vertical" and "lateral" used in the following description are only used to provide relative references and are not intended to imply any restrictions on how any item is positioned during use, or how it is installed in an assembly or in relation to the environment. In addition, unless otherwise stated, the term "coupling" and its variations used in this specification are intended to include indirect and direct connections. For example, if a first device is coupled to a second device, the coupling can be through a direct connection or through an indirect connection via other devices and connections. Similarly, if a first device is communicatively coupled to a second device, the communication can be carried out through a direct connection or through an indirect connection via other devices and connections.

[0027] As used herein, reference to "about" or "approximately" a number or "substantially" equal to a number means within plus or minus 10% of the number.

[0028] It is contemplated that any portion of any aspect or embodiment discussed in this specification can be implemented or combined with any portion of any other aspect or embodiment discussed in this specification.

[0029] The scope of the claims should not be limited by the preferred embodiments set forth in the Examples, but should be given the broadest interpretation consistent with the overall description.

Claims

1. Fuel cells include: an anode plate comprising a separator having an active side with a fuel flow field including an open-face fuel flow channel having at least one land; a cathode plate comprising a separator having an active side with an oxidant flow field, the oxidant flow field comprising open oxidant flow channels and at least one platform; and a membrane electrode assembly sandwiched between an anode plate and a cathode plate, the membrane electrode assembly comprising a catalyst-coated ion exchange membrane sandwiched between anode and cathode gas diffusion layers; characterized in that at least one of the platforms of the anode plate and the cathode plate has a top portion with a curved cross-section; At least one of the anode gas diffusion layer and the cathode gas diffusion layer is compressible and is compressed around at least a portion of the platform having a top portion with a curved cross-section; The fuel cell further includes an MEA having a cathode GDL having a compressible surface that compresses when in contact with the cathode plate platform when the fuel cell is assembled such that the compressed area of the cathode GDL surface conforms to the curved top of the platform.

2. The fuel cell according to claim 1, wherein The oxidant flow field includes a plurality of straight platforms parallel in a length direction, and the platforms have tops with curved cross sections.

3. The fuel cell according to claim 1, wherein The top of the at least one platform has a continuously curved cross-section.

4. The fuel cell according to claim 3, wherein The continuously curved cross section is a circular arc segment with a given radius.

5. The fuel cell according to claim 4, wherein The radius is between 0.05 and 0.5 mm.

6. The fuel cell according to claim 3, wherein The continuously curved cross section is composed of multiple curved segments, and the curvature of each curved segment corresponds to a different radius.

7. The fuel cell according to claim 6, characterized in that The top cross-section of the at least one platform is an ellipse or a curved segment of an ellipse-like shape.

8. The fuel cell according to claim 1, wherein The cathode plate platform and the anode plate platform both have tops with curved cross-sections.

Images