UNIT FUEL CELL, FUEL CELL STACK AND BIPOLAR PLATE ASSEMBLY

Abstract

A fuel cell stack includes a plurality of bipolar plates wherein each bipolar plate has at least an anode plate and a cathode plate, and a plurality of membrane electrode assemblies being sandwiched by the bipolar plates, wherein each membrane electrode assembly has at least an anode and a cathode which are separated by a membrane, wherein the bipolar plates sandwich the membrane electrode assembly in such a way that the anode of the membrane electrode assembly faces the anode plate of a first bipolar plate and the cathode of the same membrane electrode assembly faces the cathode plate of a second bipolar plate; and wherein a cell pitch of the fuel cell stack is defined by a distance of two adjacent membrane electrode assemblies, wherein at borders of the bipolar plates of the fuel cell stack, an overall distance between the anode plate of the first bipolar plate and the cathode plate of the second bipolar plate, which is measured over the sandwiched membrane electrode assembly, is equal to the cell pitch of the fuel cell stack.

Claims

1. Fuel cell stack comprising a plurality of bipolar plates wherein each bipolar plate has at least an anode plate and a cathode plate, and a plurality of membrane electrode assemblies being sandwiched by the bipolar plates, wherein each membrane electrode assembly has at least an anode and a cathode which are separated by a membrane, wherein the bipolar plates sandwich the membrane electrode assembly in such a way that the anode of the membrane electrode assembly faces the anode plate of a first bipolar plate and the cathode of the same membrane electrode assembly faces the cathode plate of a second bipolar plate; and wherein a cell pitch of the fuel cell stack is defined by a distance of two adjacent membrane electrode assemblies wherein at borders of the bipolar plates of the fuel cell stack, an overall distance (d) between the anode plate of the first bipolar plate and the cathode plate of the second bipolar plate, which is measured over the sandwiched membrane electrode assembly, is equal to the cell pitch of the fuel cell stack.

2. Fuel cell stack according to claim 1, wherein at the borders of the bipolar plates of the fuel cell stack, the anode plate of the first bipolar plate has a first distance to the membrane electrode assembly and the cathode plate of the second bipolar plate has a second distance to the membrane electrode assembly, wherein the first distance is different from the second distance.

3. Fuel cell stack according to claim 1, wherein the membrane electrode assembly further has a subgasket, which is at least partly arranged in an encompassing way around the anode and the cathode and the first and second distance are determined between the anode plate and the subgasket and the cathode and the subgasket, wherein preferably the subgasket encompasses the anode and cathode in a frame-like manner.

4. Fuel cell stack according to claim 1, wherein the anode plate and/or the cathode plate of at least one bipolar plate has a first area with a first structure and a second area with a second structure, wherein in the first area, the first structures of the anode and the cathode plate are identical channel-like structures comprising recesses and elevations, and in the second area, the second structures of the anode and cathode plate are also channel-like structures, wherein the second structure of the anode plate differs from the second structure of the cathode plate.

5. Fuel cell stack according to claim 4, wherein the first area is formed in an active region and the second area is formed in a border region, wherein, on the anode side, the active region is defined by the extent of the anode, and, on the cathode side, the active region is defined by the extent of the cathode, and the border region is defined by the extent of the subgasket which extends over the anode and/or cathode.

6. Fuel cell stack according to claim 4, wherein in at least one bipolar plate the second structure of either anode plate or cathode plate is provided with a first set of elevations and a second set of elevations, and the second structure of the respective other plate, namely cathode plate or anode plate, is provided with recesses and elevations, wherein the elevations of the first set of elevations of anode/cathode plate are arranged to face and/or contact the elevations of cathode/anode plate and the elevations of the second set of elevations of the anode/cathode plate are arranged to face the recesses of the cathode/anode plate, so that the elevations of the second set of elevations of anode/cathode plate are accommodated in the recesses of the cathode/anode plate.

7. Fuel cell stack according to claim 4, wherein the anode and cathode plate of the bipolar plate have a front side and a backside, wherein the first and second structures are arranged at the backside, and wherein, in the first area, the recesses of the backsides of the anode and cathode plate are arranged opposite of each other, thereby forming cooling fluid flow field channels of the bipolar plate.

8. Fuel cell stack according to claim 7, wherein at least in the first area the anode plate and/or the cathode plate has a reactant flow field on the frontside, wherein each reactant flow field has recesses and elevations, which are formed by the respective elevations and recesses of the backsides.

9. Unit fuel cell for a fuel cell stack according to claim 1.

10. Bipolar plate for a fuel cell stack according to claim 1 comprising at least an anode plate with a front side and a backside and a cathode plate with a frontside and a backside, wherein the backsides of anode plate and cathode plate are facing each other, and wherein both the anode and cathode plate have a first area with a first structure on the backside and a second area with a second structure on the backside, wherein in the first area, the first structure is a channel like structures comprising recesses and elevations, wherein the elevations of the anode and cathode plate are arranged to face and contact each other, and the recesses of the anode and cathode plate are arranged opposite of each other thereby forming cooling fluid flow field channels of the bipolar plate, and wherein in the second area, the second structure of either the anode plate or the cathode plate is provided with a first set of elevations and a second set of elevations, and the second structure of the respective other plate is provided with recesses and elevations, wherein the first set of elevations is arranged to face and contact the elevations of the respective other plate and the second set of elevations is arranged to face the recesses of the respective other plate, so that the second set of elevations is accommodated in the recesses of the respective other plate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In the following, preferred embodiments of the invention are described in relation to the drawings, wherein the drawings are exemplarily only, and are not intended to limit the scope of protection. The scope of protection is defined by the accompanied claims, only.

[0030] The figures show:

[0031] FIG. 1: A schematic cross-sectional view of a fuel cell stack according to the state-of-the-art:

[0032] FIG. 2: a schematic cross-sectional view of a fuel cell stack according to a preferred embodiment of the present invention; and

[0033] FIG. 3: a schematic cross-section of a fuel cell stack according to a further preferred embodiment of the present invention.

DETAILED DESCRIPTION

[0034] In the following same or similar functioning elements are indicated with the same reference numerals.

[0035] FIGS. 1 and 2 show each a schematic cross-section of a part of a fuel cell stack 1. The fuel cell stack 1 has a membrane electrode assembly 10 which is sandwiched between two bipolar plate assemblies 100-1 and 100-2. The membrane electrode assembly 10 usually comprises a cathode 11 and an anode 12 which are separated by a membrane 13, and form the active region of the membrane electrode assembly 10. The active region is encompassed a subgasket 14.

[0036] As can be further seen in FIG. 1 and FIG. 2, the membrane electrode assembly 10 is sandwiched between two adjacent bipolar plate assemblies 100-1 and 100-2. Each bipolar plate assembly has a first flow field plate 20 (e.g. an anode plate), and a second flow field plate 30 (e.g. a cathode plate), which are in contact with the respective electrode of the membrane electrode assembly 10. Hence, the first flow field plate 20 of the first bipolar plate assembly 100-1, the MEA 10 and the second flow field plate 30 of the second bipolar plate assembly 30 form a unit fuel cell 50. In the following the first flow field plate 20 is regarded as the anode plate 20 and the second flow field plate 30 is regarded as the cathode plate. However, it should be noted that this may be the other way round without departing from the scope of the invention.

[0037] Each bipolar plate assembly 100-1, 100-2 or better each flow field plate 20, 30 has on its back side 21, 31 a cooling fluid flow field structure with cooling fluid flow field structures in the form of recesses 22, 32, and elevations 23, 33. Since both backsides 21, 31 are arranged to face each other the cooling fluid flow field structures form cooling fluid flow field channels 40 through which a cooling fluid may be guided for cooling the bipolar plate assembly 100-1, 100-2 and thereby the fuel cell stack 1.

[0038] On the front side 24, 34, namely at the side facing the electrodes, a reactant flow field is provided which also has also recesses 25, 35 and elevations 26, 36. In the depicted embodiments, the recesses 22, 32 and elevations 23, 33 of the cooling fluid flow field form the elevations 26, 36 and the recesses 25, 35 of the reactant flow field, respectively. This allows for a simplified manufacturing of the flow field plates 20, 30, as the flow field plate 20, 30 may be manufactured by a single coining or stamping process.

[0039] As can be further seen in FIGS. 1 and 2, the respective reactant flow fields are separated by the membrane electrode assembly 10 and by the subgasket region 14. Additionally, they are sealed from the outside by sealing elements 42 which are arranged between the flow field plates 20, 30, and the subgasket 14.

[0040] In the fuel cell stack according to the state-of-the-art as depicted in FIG. 1, the anode plate 20 and the cathode plate 30 are formed identical. Hence, when arranging the flow field plates 20, 30 with their backsides 21, 31 facing each other, all recesses 22, 32 of the cooling fluid flow field of anode plate 20 and cathode plate 30 are facing each other. This design has the disadvantage that a first distance d1 between the cathode plate 30 of the bipolar plate assembly 100-1 and the respective adjoining subgasket 14, and a second distance d2 between the anode plate 20 of the bipolar plate assembly 100-2 and the respective adjoining subgasket 14, are are quite small. Consequently, there is a high risk for a short circuit, in case one of the bipolar plates is bended or the subgasket 14 is damaged or missing in this area, as the bipolar plate assemblies 100-1, 100-2 may come into contact with each other.

[0041] Referring now to FIG. 2, in contrast to that, the first and second flow field plates 20, 30 of the depicted embodiment of the present invention, are only identical in a first area I. In a second area II, the anode plate 20 has a first set of elevations 27 and a second set of elevations 28, whereas the cathode plate 30 still has elevations 37 and recesses 38. Thereby, the second set of elevations 28 is accommodated in the recesses 38. This in turn, allows for an enlarged distance d1 between the anode plate 20 of the first bipolar plate assembly 100-1 and the neighboring subgasket 14, wherein the distance d2 between the cathode plate 30 of the second bipolar plate assembly 100-2 and the same subgasket 14 is quite small, e.g. in the same range as known from the state of the art. Additionally, the overall distance is one cell pitch, which ensures an improved short circuit avoidance.

[0042] This newly developed design has the advantage that the border region (second area) of the bipolar plate assembly is more stable since two plates provide a higher stiffness than a single plate. Usually, an anode/cathode plate has a width of roughly 0.075 mm and is therefore very sensitive to bending or other damages.

[0043] This increased strength has the further advantage that the bipolar plate assembly may be welded in the very outer/border region. Due to the increased strength a counter-force may be applied by the opposite side of the bipolar plate assembly without damaging the assembly (e.g. bending the plates).

[0044] Preferably, the distance d1 is about the same as for the bead seal, so that when combining (stacking) the bipolar plate assemblies and the MEA, the MEA remains flat. In case the distance d1 is not large enough it is necessary to weld at the bottom of the flow field —namely in the recesses—which creates a bending in the membrane electrode assembly.

[0045] The overall distance of two adjacent plates is one cell pitch which is the maximal possible distance between two plates and therefore ensures that a short circuit may be avoided.

[0046] FIG. 3 shows a further preferred embodiment of the fuel cell stack, where the distance of the adjacent bipolar plate assemblies 100-1, and 100-2 is also one cell pitch. In contrast to the embodiment depicted in FIG. 2, there is no different distance between the plates to the gasket, but both are equally spaced by one cell pitch so that also in this embodiment a short circuit may be avoided.

[0047] In summary, due to the new design, the electrical insulation between adjacent bipolar plate assemblies 100-1, 100-2 is ensured even in regions where the subgasket part 14 is not sufficiently large compared to the extension of the bipolar plate assemblies 100-1, 100-2, or otherwise damaged, or insufficiently aligned. Additionally, the overall strength of the bipolar plate assembly and the fuel cells is improved.

REFERENCE SIGNS

[0048] 1 Fuel cell stack [0049] 10 membrane electrode assembly [0050] 100 Bipolar plate assembly [0051] I first area [0052] II second area [0053] 11 anode [0054] 12 cathode [0055] 13 membrane [0056] 14 subgasket [0057] 20 first (anode) flow field plate [0058] 30 second (cathode) flow field plate [0059] 21, 31 backside of the flow field plate [0060] 22, 32 elevations on the backside (first area) [0061] 23, 33 recesses on the backside (first area) [0062] 24, 34 frontside [0063] 25, 35 elevation on the frontside (first area) [0064] 26, 36 recess on the frontside (first area) [0065] 27 first set of elevations on the front side (second area) [0066] 28 second set of elevations on the front side (second side) [0067] 37 elevations (second area) [0068] 38 recess (second area) [0069] 40 Cooling fluid flow channels [0070] 50 unit fuel cell