WO2020128322A1

WO2020128322A1 - Bipolar plate for a fuel cell

Info

Publication number: WO2020128322A1
Application number: PCT/FR2019/053146
Authority: WO
Inventors: David Olsommer
Original assignee: Compagnie Generale Des Etablissements Michelin
Priority date: 2018-12-19
Filing date: 2019-12-18
Publication date: 2020-06-25

Abstract

The invention relates to a bipolar plate constituting the first polar plate of a first base element of a fuel cell and the second polar plate of a second base element which is adjacent to the first base element of the same fuel cell, the bipolar plate comprising two parallel thin plates joined by brazing, each thin plate comprising at least one fuel or oxidant distribution channel formed in the thickness of the polar plate, and a sealing channel which is intended to receive a seal and also formed in the thickness of the plate, the bipolar plate being characterised in that the sealing channel has, on the inner surface thereof, protrusions the height of which is less than 50% of the thickness of the sealing channel, and the surface of which is less than 50% of the total surface of the sealing channel.

Description

BIPOLAR PLATE FOR FUEL CELL

Description

The present invention relates to the field of fuel cells, and in particular that of bipolar plates used to form the stack of a fuel cell. Indeed, a fuel cell is composed of a stack of electrochemical cells comprising an anode and a cathode separated by an ion-exchange membrane, the assembly being called “membrane-electron assembly”, or AME, and each AME being disposed between two bipolar plates.

Fuel cells are currently the subject of numerous studies as part of efforts to limit environmental pollution, particularly in transport. Among the most studied at the moment is undoubtedly the electrolytic generators with hydrogen fuel, using as oxidant air or pure oxygen.

During the operation of a fuel cell, two simultaneous electrochemical reactions occur at the level of the membrane-electron assembly: an oxidation of the fuel at the anode, and a reduction of oxidant at the cathode. These two reactions produce ions, positive and negative, which combine at the membrane level and produce electricity in the form of a potential difference. In the case of an oxygen-hydrogen fuel cell, the H + and O- ions combine.

These electrochemical reactions take place at the level of each basic cell, each of these cells being separated from the adjacent cells by bipolar plates, which have several functions, in particular:

a first function for supplying fuel and oxidant, and

- a second heat exchange function, allowing refrigeration, or cooling, of the battery.

It should be noted that a bipolar plate generally consists of two thin plates, joined together by a means such as welding or bonding. In addition, the bipolar plates, by their different functions, must be electrically conductive, while remaining insensitive, in terms of corrosion, to oxidizer and fuel.

In order to fulfill the first function, a channel is provided on the entire face of the bipolar plates in contact with the membrane. Each channel has an inlet through which the fuel or the fuel enters, and an outlet through which the neutral gases, the water generated by the electrochemical reaction, and the residual moisture of the hydrogen from its side. To fulfill the second function, a coolant is generally sent between the two thin plates forming the bipolar plate.

In order to ensure correct operation of a fuel cell, while guaranteeing correct performance, it is necessary that the seal be perfectly ensured between the bipolar plates and the membrane-electron assemblies, in order to avoid any gas leakage. To guarantee this seal, seals made of a material such as silicone are used. It has been found that, for a given crushing value, the smaller the thickness of the joint, the higher its compression ratio, and the greater the sealing failures, due to excessive stress on the elastomer. .

In addition, it has been found that a thin joint requires more precise manufacturing and assembly tolerances, for example in terms of the thickness of the joint or the geometry of the bearing surface of the bipolar plate, which generates additional manufacturing costs. Thus, it seems appropriate to choose a thick joint, in a configuration having a compression ratio acceptable by the material.

However, some problems have been found in the fuel cell assembly with thick joints. For bipolar plates assembled by a strong vacuum brazing process, it turns out that the thickness of the joint forces the assembly tools to compensate for the difference in height due to this thickness. However, these soldering tools are often made of ceramic materials, and the creation of a “stepped” tool making it possible to accept height differences on the parts to be brazed can become extremely expensive and complex. In addition, the stepped shape of the tool requires significant positioning accuracy between the bipolar plate and the tool, which may prove to be incompatible with the high expansions encountered during heavy brazing.

The present invention therefore aims to propose a solution making it possible to remedy these drawbacks.

Statement of the invention

Thus, the present invention relates to a bipolar plate constituting the first pole plate of a first base element of a fuel cell and the second pole plate of a second base element adjacent to the first base element of the same cell fuel, the bipolar plate comprising two parallel thin plates assembled by brazing, each thin plate comprising at least one fuel or oxidant distribution channel formed in the thickness of the pole plate, and a sealing channel intended to receive a seal , also arranged in the thickness of the plate.

The bipolar plate is characterized in that the sealing channel has, on its internal surface, protuberances whose height represents less than 50% of the thickness of the sealing channel, and whose surface represents less than 50% of the total area of the sealing channel. It is specified here that the terms “bearing surface” and “surface of the channel sealing "are used interchangeably in the present description, and must be understood with a similar meaning.

This invention advantageously makes it possible to remedy the drawbacks of the prior art, since it makes it possible to reconcile the benefits and the robustness of a thick joint with an ease of manufacture, in particular during the brazing operation. Indeed, the protrusions allow the thin plates to be correctly positioned during the brazing operation, without the need to use an expensive stepped tool. In addition, the use of point protrusions, the surface of which represents less than half of the surface of the sealing channel, allows sufficient space to be installed for the installation of a thick joint. In addition, the dimensions of the protuberances, in particular their height, can be chosen so as to offer a compression ratio which does not degrade the elastomer constituting the seal.

The invention presents various advantageous embodiments, which will be detailed later on with the aid of figures.

Thus, in a first particular embodiment, the protuberances are spikes located at regular intervals over the entire sealing channel.

In one example, these pins can be positioned on each of the thin plates so that the pins of one of the thin plates (that constituting the anode) are positioned face to face with the pins of the other thin plate (that constituting the cathode) when the bipolar plate is assembled.

In another example, however, the pins are not positioned face to face in the assembled bipolar plate. Preferably, the pins are positioned in staggered rows, that is to say that they are positioned at regular intervals on each of the thin plates, with the same interval step, but with an offset of half a step from one thin plate to another.

In another embodiment, the protuberances are in the form of wavelets over the entire sealing channel.

In another embodiment, the distribution channels are arranged so that when the first and second base elements of the fuel cell are stacked, a circulation channel is formed between the two pole plates, and in what this distribution channel communicates with a cooling fluid supply orifice.

Brief description of the drawings

Other objectives and advantages of the invention will appear clearly in the description which follows of a preferred but nonlimiting embodiment, illustrated by the following figures in which: [Fig 1]: Figure 1 shows a state of the art bipolar plate.

[Fig 2]: Figure 2 shows three sectional views of a step of brazing a bipolar plate of the prior art.

[Fig 3]: Figure 3 shows a partial view of an embodiment of a bipolar plate according to the invention, in front view and sectional view.

[Fig 4]: Figure 4 shows a partial view of a second embodiment of a bipolar plate according to the invention, in front view and sectional view.

detailed description

As shown in FIG. 1, a bipolar plate comprises a central skeleton 1 made up of two thin, parallel plates, joined together by a process such as gluing, soldering or welding. One side of this plate is intended to be backed by an anode, in a fuel cell, and the other side is intended to be backed by a cathode.

The thin plates are pierced with several holes at their periphery, in order to form collectors of fuel 2, oxidizer 3, and coolant 4. The plates also include a set of channels 5, arranged in their thickness, in order to allow the circulation, on the surface, of fuel or oxidizer. Furthermore, the thin plates have orifices allowing a collector to communicate with a gas circulation channel.

The bipolar plate also includes seals, for example of polymer material, located in at the level of the bearing surface shown by the arrow 15. A first seal makes it possible to produce a seal so that the gas circulating in the channels of the first plate does not come into contact with the gas circulating on the next bipolar plate in the stack, these two gases being by nature different, in order to allow correct operation of the fuel cell. A second seal makes it possible to seal between the two thin plates forming the bipolar plate, so that the gas following the path indicated by the arrow 14 does not come into contact with the coolant circulating in the bipolar plate.

This joint is, for example, produced by brazing, welding, gluing or any other sealed connection means.

As indicated above, the known bipolar plates have many disadvantages during brazing operations. Thus, FIG. 2 shows three examples of plates of the state of the art seen in detail 10 in section AA with respect to FIG. 1. These views in section are represented at the time of the brazing operation by a tool 20. Thus, the first example (a) shows a conventional bipolar plate having a bearing surface for a conventional seal, not allowing the use of a thick seal. The second example (b) shows an improvement compared to the first example, since the bearing surface allows the presence of a thick joint. However, it can be seen in the figure that during the application of the soldering tool, a space is created between the plates and the tool. This leads to leaks because the thin plates are not properly plated during the operation.

To remedy this, the third example © shows the use of a stepped tool, that is to say a tool having a surface with an offset corresponding to the offset of the thin plate at the place intended to receive the seal. . This tool makes it possible to obtain a correct seal, but the manufacturing operations prove to be extremely expensive, on the one hand because of the more complex tooling, and on the other hand because of the need for precise positioning of the tool. compared to the plates.

FIG. 3 represents a bipolar plate according to the invention, making it possible to remedy this. This bipolar plate is shown in different forms: a partial front view (d), a sectional view (e) and a view (f) during a brazing operation by a tool 20. The other elements of the plate bipolar are similar to those shown in Figure 1, with the exception of the protuberances which are present on the entire seal sealing channel.

In this partial view we see a part 30 of a sealing channel appear, provided with protuberances 21 here having the shape of pins. These pins can for example take the form of circular pins, with a base diameter of between 0.2 mm and 2 mm, preferably around 0.6 mm. The height of the pins can be chosen between 0.02 mm and 0.2 mm. In a particular embodiment shown in Figure 3, the height of the pins is 0.07mm, the thickness of the joint can thus be increased by 2 x 0.07 mm = 0.14 mm compared to a version without pins. This front view (d) shows only a thin plate, but the sectional view B-B makes it possible to show the presence of the pins on the two thin plates. As previously indicated, the pins can be positioned so as to be face to face during assembly, or else, as in the sectional view, be positioned in staggered rows, that is to say with the same interval of spacing, offset by half a step from one plate to another. The third element in this figure shows such a bipolar plate during the brazing operation. It is clearly seen here that the presence of the pins makes it possible to compensate for the offset due to the depth of the seal sealing channel; this ensures correct sealing during brazing while using conventional tools.

Figure 4 shows another embodiment with a front view (g) of a thin plate, and a view (h) of a soldering operation by a tool 20. In this embodiment, in which the protuberances take the form of a ripple 22 which extends along the sealing channel.

Claims

1. Bipolar plate constituting the first pole plate of a first base element of a fuel cell and the second pole plate of a second base element adjacent to the first base element of the same fuel cell, the bipolar plate comprising two parallel thin plates assembled by brazing, each thin plate comprising at least one fuel or oxidant distribution channel formed in the thickness of the pole plate, and a sealing channel intended to receive a seal, also formed in the thickness of the plate, the bipolar plate being characterized in that the sealing channel (30) has, on its internal surface, protuberances (21) the height of which represents less than 50% of the thickness of the channel sealing, and whose surface represents less than 50% of the total surface of the sealing channel.

2. Bipolar plate according to claim 1, in which the protuberances are spikes located at regular intervals over the entire sealing channel.

3. Bipolar plate according to claim 2, in which the pins positioned on each of the thin plates are face to face in the assembled bipolar plate.

4. Bipolar plate according to claim 2, in which the pins positioned on each of the thin plates are not face to face in the assembled bipolar plate.

5. Bipolar plate according to claim 1, wherein the protuberances are in the form of wavelets on the entire sealing channel.

6. Bipolar plate according to one of the preceding claims, in which the distribution channels are arranged so that when the first and second base elements of the fuel cell are stacked, a circulation channel is formed between the two pole plates, and in that this distribution channel communicates with a cooling fluid supply orifice.