WO2010033118A1

WO2010033118A1 - Bipolar plate for a fuel cell

Info

Publication number: WO2010033118A1
Application number: PCT/US2008/076771
Authority: WO
Inventors: Thomas H. Madden
Original assignee: Utc Fuel Cells, Llc
Priority date: 2008-09-18
Filing date: 2008-09-18
Publication date: 2010-03-25

Abstract

A device for use in a fuel cell includes a bipolar plate having a reactant side that includes a reactant flow field with a plurality of ribs that are spaced apart to define a plurality of reactant channels extending between the plurality of ribs. The plurality of reactant channels open to the reactant side of the bipolar plate. At least one of the plurality of ribs includes a water channel that also opens to the reactant side of the bipolar plate.

Description

BIPOLAR PLATE FOR A FUEL CELL

BACKGROUND OF THE DISCLOSURE

[0001] This disclosure relates to fuel cells. More particularly, this disclosure relates to a bipolar plate for facilitating water management within a fuel cell.

[0002] Fuel cells are widely known and used for generating electricity. Typically, a fuel cell unit includes an anode, a cathode, and an ion conducting polymer exchange membrane (PEM) between the anode and the cathode. The anode and cathode are located between bipolar plates (also referred to as transport plates) that include flow field channels for circulating reactant gases to the PEM to generate electricity in a known electrochemical reaction.

[0003] One issue associated with fuel cells relates to cooling the fuel cell unit to maintain a desirable operating temperature. This is often achieved by the circulation of a cooling fluid in some way throughout the fuel cell stack to sensibly maintain the stack temperature. Another means involves the removal of heat through evaporation of water. For instance, the anode- side bipolar plate of some fuel cell designs may be formed of a porous material that transports water to the PEM. Since water is required for proton transport through the polymer electrolyte membrane, a water pressure gradient across the PEM pulls the water from the anode to the cathode where the water evaporates to cool the fuel cell. However, a drawback of using the porous anode- side bipolar plate is that localized portions of the plate may dehydrate under certain conditions and thereby allow reactant gas to escape through the porous bipolar plate and perhaps to mix with the cathode reactant (e.g. air) on the other side.

SUMMARY OF THE DISCLOSURE

[0004] In an example embodiment, a device for use in a fuel cell includes a bipolar plate having a reactant side that includes a reactant flow field having a plurality of ribs that are spaced apart to define a plurality of reactant channels extending between the plurality of ribs. The plurality of reactant channels open to the reactant side of the bipolar plate. At least one of the plurality of ribs includes a water channel that opens to the reactant side. [0005] The example bipolar plate may be a solid anode bipolar plate that utilizes water channels to deliver cooling water to the electrode assembly. It does this through connections to the coolant and appropriate pressure control to facilitate water flow, for example. The plate thereby provides the benefit of a solid plate architecture in combination with delivering cooling water to the electrode assembly, without the drawback of pore dehydration as in porous plate architectures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The various features and advantages of the disclosed examples will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.

[0007] Figure 1 illustrates an example fuel cell.

[0008] Figure 2 illustrates a portion of the fuel cell of Figure 1.

[0009] Figure 3A illustrates a cross-section of a unitized cell of the fuel cell of Figure 1.

[0010] Figure 3B illustrates a cross-section of another unitized cell.

[0011] Figure 4 illustrates a cross-sectioned perspective view of a cathode bipolar plate of the unitized cell.

[0012] Figure 5 illustrates a cross-sectioned perspective view of an anode bipolar plate of the unitized cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0013] Figure 1 schematically illustrates selected portions of an example fuel cell 10 for generating electricity. In the illustrated example, the fuel cell 10 includes at least one unitized cell 12 (Figure 3A or 3B). For example, a plurality of the unitized cells 12 may be used to form a fuel cell stack, depending on the desired electric power to be generated.

[0014] In the illustrated example, the fuel cell 10 includes a first external manifold 14 and a second external manifold 16 for delivering reactant gases to the unitized cells 12. For example, the first external manifold 14 may deliver fuel, such as hydrogen gas, and the second external manifold 16 may deliver an oxidant, such as air. [0015] With reference to the first external manifold 14 and the second external manifold 16, the term "external" refers to the manifolds being separate and distinct structures from the unitized cells 12 rather than manifolds that are integrally formed within the unitized cells 12. Although the fuel cell 10 includes the first external manifold 14 and the second external manifold 16, it is to be understood that the disclosed examples are not limited to such a design and that in other examples the fuel cell 10 may include internal manifolds. These include combinations of internal and external manifolds for the fuel, oxidant, and coolant. Thus, given this description, one of ordinary skill in the art will recognize that the disclosed examples are applicable to a variety of different fuel cell configurations.

[0016] In addition to the first external manifold 14 and the second external manifold 16, the fuel cell 10 may also include a water source 18 for supplying coolant water to the unitized cells 12. For example, the water source 18 may include a positive displacement pump 20 for circulating coolant water through the unitized cells 12, as will be described below. The positive displacement is selected to deliver a relatively low flow rate of the coolant water against the relatively high head pressure of the coolant water within the unitized cells 12. Additionally, the positive displacement pump may utilize the power generated by the fuel cell 10 in some way.

[0017] Figure 2 illustrates a view of a section of the fuel cell 10 as illustrated in Figure 1. The view shown in Figure 2 represents a top view looking down onto the unitized fuel cells 12 and first external manifold 14. In this example, the first external manifold includes ports 22 for delivering the reactant fuel to the unitized cells 12. Connectors 24 (see also Figure 3) include an internal passage 26 for distributing the coolant water from the water source 18 to the unitized cell 12.

[0018] Figure 3A illustrates an example of one of the unitized cells 12. The unitized cell 12 includes an anode bipolar plate 30, a cathode bipolar plate 32, and an electrode assembly 34 located between the anode bipolar plate 30 and the cathode bipolar plate 32.

[0019] The unitized electrode assembly 34 includes an ion exchange membrane 36 for conducting ions in the electrochemical reaction, an anode 37a and a cathode 37b that generate an electric current, and gas diffusion layers 38 that distribute reactant gases and conduct the generated current. For example, the gas diffusion layers 38 may be porous to uniformly distribute the reactant gases and coolant water. In one example, at least the gas diffusion layer 38 between the anode 37a and the anode bipolar plate 30 may include a microporous layer of variable thickness that contains a nominal pore size of about 0.1 - 1 micrometers (3.9 - 39.4 microinches). The example nominal pore size provides the benefit of facilitating uniform coolant water distribution and limiting water pockets that may otherwise contribute to undesirable build-up of water in anode reactant channels. Such a condition may lead to reverse current decay and/or excessive anode pressure drop.

[0020] In a further example, one or both of the gas diffusion layers 38 may be hydrophobic to facilitate lateral distribution of the coolant water. For example, at least the gas diffusion layer 38 between the anode bipolar plate 30 and the anode 37a may be a porous carbon material, such as a woven cloth having laterally extending fibers relative to the ion exchange membrane 36 and electrodes 37 or non-woven carbon cloth, impregnated with a hydrophobic material. The hydrophobic material may include a fluoropolymer, or other type of hydrophobic material. In the illustrated example, the gas diffusion layer 38 between the anode bipolar plate 30 and the anode 37a includes a first sub-layer 38a adjacent the anode bipolar plate 30 that is hydrophobic and a second sub-layer 38b adjacent the anode 37a that includes a hydrophilic woven cloth having laterally extending fibers. As will be described, the first sub-layer 38a and the second sub-layer 38b facilitate coolant water distribution. In one example, a hydrophobic treatment may be applied only to the first sub-layer 38a (e.g., a microporous layer) of the gas diffusion layer 38 and the second sub-layer 38b may be hydrophilic.

[0021] Figure 4 illustrates a sectioned perspective view of one of the cathode bipolar plates 32. In this example, the cathode bipolar plate 32 includes a reactant side 50 having a first reactant flow field 52 for delivering the oxidant to the cathode 37b of the electrode assembly 34. The reactant flow field 52 includes reactant channels 54 that extend between ribs 56.

[0022] Figure 5 illustrates an example of one of the anode bipolar plates 30. In this example, the anode bipolar plate 30 includes a reactant side 60 having a second reactant flow field 62 for delivering the fuel to the anode 37a of the electrode assembly 34. For example, the term "side" used in reference to the reactant side 50 of the cathode bipolar plate 32 and the reactant side 60 of the anode bipolar plate 30 refers to the broad sides of the plates rather than the relatively thin, thickness direction sides.

[0023] The anode bipolar plate 30 includes a plurality of ribs 64 that are spaced apart to define a plurality of reactant channels 66 that extend between the plurality of ribs 64. The reactant channels 66 open to the reactant side 60 of the anode bipolar plate 30. In general, the reactant channels 66 extend parallel to each other to form the second reactant flow field 62, although in other examples the reactant channels 66 may be non-parallel or curvilinear.

[0024] Each of the ribs 64 includes a first side 68 and a second side 70. At least one of the ribs 64 includes a water channel 72 that opens to the reactant side 60 of the anode bipolar plate 30. In the illustrated example, each of the ribs 64 includes a water channel 72; however, fewer than all of the ribs 64 may include water channels 72 in alternate embodiments.

[0025] Each water channel 72 is directly between the first side 68 and the second side 70 of the respective rib 64 and extends between an open side 74 at the reactant side 60 of the anode bipolar plate 30, a bottom surface 76, a first side surface 78 of the rib 64, and a second side surface 80 of the rib 64. For example, the bottom 76 of the water channel 72 in the given example is curved. The curvature reduces the potential accumulation of gas bubbles in the water channels 72, which could disrupt the continuous flow of water.

[0026] In the illustrated example, the reactant channels 66 include a first cross- sectional area and the water channels 72 include a second cross-sectional area that is less than the first cross- sectional area. For instance, the difference in the cross-sectional areas facilitates establishing a desired balance between the pressure of the coolant water within the water channels 72 and the pressure of the reactant gas within the reactant channels 66.

[0027] Each of the water channels 72 is fluidly isolated from the reactant channels 66 within the anode bipolar plate 30. That is, there is no direct fluid connection between the water channels 72 and the reactant channels 66 through the solid portions of the anode bipolar plate 30 without traveling outside of the perimeter of the anode bipolar plate 30.

[0028] The anode bipolar plate 30 may also include a coolant flow field 90 having coolant channels 92 that open to a coolant side 94 of the anode bipolar plate 30. In this example, the coolant side 94 is located opposite from the reactant side 60 of the anode bipolar plate 30.

[0029] The coolant channels 92 are fluidly connected with the water source 18 to circulate coolant water through the coolant flow field 90. In the illustrated examples, the connectors 24 fluidly connect the coolant channels 92 with the water channels 72 on the reactant side 60 of the anode bipolar plate 30. For instance, the connectors 24 include a port 100 (Figure 3A) that opens to the coolant channels 92 for transporting coolant water from the coolant channels 92 through the internal passages 26 to the water channels 72.

[0030] The connectors 24 may include separate and distinct pieces from the anode bipolar plate 30 that are then attached in a suitable manner to the anode bipolar plate 30. For instance, if the anode bipolar plate 30 is formed of a metal material, the connectors 24 may also be formed of a metal material, and welded, brazed, or bonded with an adhesive to the anode bipolar plate 30. Alternatively, if the anode bipolar plate is formed from a carbon material or a composite, the connectors 24 may be bonded using an adhesive, such as an epoxy or other polymeric adhesive.

[0031] The connection from the coolant channels 92 to the water channels 72 may alternatively be facilitated by simply providing a through-hole 103, or a plurality of through- holes 103 as shown in Figure 3B. In this example, water may flow from a single or multiple ports from the coolant channels 92 to some or all of the water channels 72.

[0032] The anode bipolar plate 30 facilitates cooling the unitized cell 12 and managing the coolant water within the unitized cell 12. For example, the water channels 72 deliver coolant water to the gas diffusion layer 38 between the anode bipolar plate 30 and the anode 37a. The coolant water flows through the coolant channels 92 and through the internal passages 26 of the connectors 24, as represented by flow arrows 102. The gas diffusion layer 38 laterally distributes the coolant water from the water channels 72 as indicated by flow arrows 104.

[0033] In the illustrated example, the hydrophobicity and porosity of the gas diffusion layer 38 facilitates lateral distribution of the coolant water along the anode 37a. For example, the water channels 72 continually deliver coolant water to facilitate an appropriate level of hydration of the gas diffusion layer 38 between the anode bipolar plate 30 and anode 37a. Moreover, the first sub-layer 38a of the gas diffusion layer 38 is suitably hydrophobic to provide a desired degree of back pressure for the coolant water to be uniformly distributed and the second sub-layer 38b laterally distributes the coolant water to maintain the desired hydration level of the gas diffusion layer 38. Water is wicked along the hydrophilic fibers to the anode 37a, leaving much of the porosity of gas diffusion layer 38 free to transport H₂.

[0034] Electro-osmotic drag from proton flow and a water pressure differential across the electrode assembly 34 drives the coolant water from the gas diffusion layer 38 at the anode 37a to the cathode 37b. At the cathode 37b, the coolant water evaporates to cool the unitized cell 12.

[0035] In one example, the anode bipolar plate 30 is formed of a solid, nonporous material. The anode bipolar plate 30 thereby provides the benefit of a solid plate architecture in combination with delivering the coolant water to the electrode assembly 34, without the drawback of dehydration as in porous plate architectures.

[0036] For instance, the anode bipolar plate 30 may be formed of a solid carbon material, metal or metal alloy material, or other suitable type of material. Further, the anode bipolar plate 30 may be molded or machined, depending on the selected material and/or other parameters. For instance, if the anode bipolar plate 30 is formed of a metal alloy, the coolant channels 92, water channels 72, and reactant channels 66 may be machined. In one example, machining may be selected if the water channels 72 are relatively small (e.g., less than 1-2 millimeters or 0.04-0.08 inches in width). If the anode bipolar plate is carbon or a composite, the coolant channels 92, water channels 72, and reactant channels 66 may be formed in a molding process, such as an injection or compression molding process.

[0037] Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments.

[0038] The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.

Claims

CLAIMSWhat is claimed is:

1. A device for use in a fuel cell, comprising: a bipolar plate including a reactant side having a reactant flow field that includes a plurality of ribs that are spaced apart to define a plurality of reactant channels extending between the plurality of ribs, the plurality of reactant channels opening to the reactant side of the bipolar plate, and at least one of the plurality ribs including a water channel that opens to the reactant side.

2. The device as recited in claim 1, wherein the bipolar plate further includes a coolant flow field having coolant channels that open to a coolant side of the bipolar plate.

3. The device as recited in claim 2, wherein at least one of the coolant channels is fluidly connected with the water channel.

4. The device as recited in claim 2, further comprising at least one connector having a passage fluidly connecting one of the coolant channels with the water channel.

5. The device as recited in claim 2, further comprising at least one through -hole extending through the bipolar plate that fluidly connects one of the coolant channels with the water channel.

6. The device as recited in claim 1, wherein each reactant channel includes a first cross- sectional area and the water channel includes a second cross-sectional area that is less than the first cross-sectional area.

7. The device as recited in claim 1, wherein the reactant channels comprise rectangular cross-sections and the water channel comprises a cross-section having a curved surface.

8. The device as recited in claim 1, wherein at least one of the plurality of ribs includes a first side that is a boundary of one of the plurality of reactant channels and a second side that is a boundary of another of the plurality of reactant channels, and the water channel is directly between the first side and the second side.

9. The device as recited in claim 1, further comprising a reactant gas source fluidly connected with the reactant flow field and a water source fluidly connected with the water channel.

10. The device as recited in claim 9, wherein the water source comprises a positive displacement pump.

11. The device as recited in claim 9, wherein the water source is maintained at a desired purity through the use of ion exchange resin.

12. The device as recited in claim 1, wherein the plurality of reactant channels are fluidly isolated from the water channel within the bi-polar plate.

13. The device as recited in claim 1, wherein the water channel extends between an open side at the reactant side of the bipolar plate, a bottom side opposite from the open side, a first side of one of the plurality of ribs that extends between the open side and the bottom side, and a second side of another of the plurality of ribs that extends between the open side and the bottom side opposite of the first side.

14. The device as recited in claim 1, wherein the plurality of ribs each include one of the water channels.

15. The device as recited in claim 1, further comprising an electrode assembly located adjacent to the bipolar plate.

16. The device as recited in claim 14, further comprising a cathode bipolar plate located adjacent to the electrode assembly, where the electrode assembly is between the cathode bipolar plate and the bipolar plate.

17. The device as recited in claim 14, wherein the electrode assembly includes at least one porous gas diffusion layer having a nominal pore size of about 0.1 - 1 micrometers (3.9 - 39.4 microinches).

18. The device as recited in claim 16, wherein the at least one porous gas diffusion layer includes a portion that is hydrophobic and another portion that is hydrophilic.

19. The device as recited in claim 1, wherein the bipolar plate is formed of a solid, nonporous material.

20. A method of processing a device for a fuel cell, comprising:

forming a bipolar plate including a reactant side having a reactant flow field that includes a plurality of ribs that are spaced apart to define a plurality of reactant channels extending between the plurality of ribs, the plurality of reactant channels opening to the reactant side of the bipolar plate, and at least one of the plurality ribs including a water channel that opens to the reactant side.

21. The method as recited in claim 19, further comprising machining at least the water channel into the bipolar plate.

22. The method as recited in claim 19, further comprising molding at least the water channel into the bipolar plate.