process for manufacturing fuel cells of different power outputs, and corresponding fuel cells.

Abstract

The invention relates to a method for manufacturing fuel cells comprising a stack of plates, each including first channels for circulating reactive gases, respectively dihydrogen and air, and second channels for circulating a heat transfer fluid, with a proton exchange membrane interposed between two adjacent plates. According to the invention, plates of a single format and at least two membrane formats, each having different dimensions, are used. These plates are assembled with one of the membrane formats to produce at least two types of fuel cells, delivering different power outputs, from identical plates. (Shorthand figure: Figure 9)

Description

Title of the invention: method for manufacturing fuel cells of different powers, and corresponding fuel cells. 1. Field of the invention

[0001] The field of the invention is that of hydrogen fuel cells. More specifically, the invention relates to the improvement of such fuel cells, and in particular of the bipolar and monopolar plates of which they are made.

[0002] Such plates, and therefore such batteries, find applications in many fields, whenever it is necessary to produce electrical energy, in particular autonomously, for example in vehicles (cars, utility vehicles, trucks, buses, trains, boats, aircraft, etc.), generator sets, etc.

[0003] In particular, the invention relates to the manufacture of batteries delivering power adapted to a given application or need, in an efficient and economical manner. 2. Prior art

[0004] The principle of the fuel cell has been known for many years. It has been implemented in particular in the space sector, and numerous projects have also been developed by various car manufacturers.

[0005] A hydrogen fuel cell is based on the principle illustrated in [Fig. 1], and produces electrical energy by a chemical reaction between dihydrogen (H2) and dioxygen (O2). This chemical reaction is described by the equations below:

[0006] [Chem 1] / / 2 -> 2H+ + 2e O2 + 4 ZT + 2e -> 2H2O H2 + l / 2O2 -> H2O ;ArH<0 The optional term A,77 < 0 only indicates that the reaction is exothermic.

[0007] This reaction occurs in what is called an active zone of an assembly of an electrolyte membrane and electrodes (MEA - or Membrane Electrode Assembly in English), that is to say a stack of membranes allowing the exchange of H+ ions, placed between an anode, receiving dihydrogen from a reservoir, and a cathode receiving dioxygen (O2) from the outside air.

[0008] As illustrated in [Fig.l], the fuel, dihydrogen (H2), is introduced (Fl) into the cell, to come into contact with the anode A. A part of the dihydrogen enters (F2) the anode A, in which the dihydrogen molecules are separated into electrons e and H+ ions which pass through the electrolyte E towards the cathode C. The latter is in contact with the air brought in from outside (F3) whose oxygen molecules O2 (F4) combine with H+ ions and e electrons to produce (F5) water (H2 O). This water (F6) and the unused air (F7) are evacuated.

[0009] Anode A is therefore the element in which oxidation occurs: H2 —>2H+ + 2e , and cathode C is the element in which reduction occurs: O2 + 4H+ + 4e —> 2H2O. Electrons e circulate (F8) between anode A and cathode C, producing an electric current E, which is used to drive an electric motor and / or charge a battery.

[0010] This reaction is exothermic, and the various constituent elements of the battery can heat up quickly. The entire system must therefore be cooled. The design of the mechanical parts must therefore be adapted to be supplied with coolant, or heat transfer fluid.

[0011] The structure of a fuel cell, implementing this chemical reaction, is illustrated in [Fig. 2]. The cell 21 consists of a stack of cells 22, placed between two end plates 231 and 232.

[0012] The constituent elements of a cell 22 are detailed, exploded, in [Fig. 3]. It comprises two plates, bipolar or monopolar, between which an electrolyte membrane is placed. The monopolar plates are the first and last plates of the stack of cells. They generally have the same design as a bipolar plate, but inlets are closed so that the plate does not receive one of the gases. The cell thus comprises a cathode monopolar plate not receiving dihydrogen and an anodic monopolar plate not receiving dioxygen.

[0013] The bipolar plates, stacked between the two monopolar plates, are formed by the assembly of two metal half-plates 31, 32 (an anodic half-plate 31 and a cathodic half-plate 32), which can be welded, brazed or glued. A space between the two metal half-plates is defined by the forming of these to define on the one hand zones 33, 34 receiving a cooling liquid and on the other hand channels allowing the circulation of gases, respectively dihydrogen and dioxygen (extracted from the air). MEA membranes 35 are interposed between the half-plates.

[0014] The plates are designed to be assembled and form a battery of a defined power. The design of a battery is complex and specific. The designer of each plate must design it, specifically, to deliver the required power.

[0015] Depending on the applications, different powers may be required. Thus, it is necessary to design a plurality of suitable plates. This design requires precise characterization of the shapes and dimensions of the plate and its various elements, in particular the channels, to obtain the required intensity at the terminals of the stack, in the most efficient way possible. It is then necessary to develop specific tooling to manufacture each plate. This is expensive and complex.

[0016] 3. Main characteristics of the invention

[0017] The invention proposes a solution making it possible to simplify the manufacture of batteries of different powers, or intensities, at constant voltage, and to reduce manufacturing costs, in the form of a method for manufacturing fuel cells having a stack of plates each comprising first channels for circulating reactive gases, respectively dihydrogen and air, and second channels for circulating a heat transfer fluid, a proton exchange membrane being interposed between two neighboring plates.

[0018] According to the invention, plates of a single format and at least two membrane formats, each having different dimensions, are used, and said plates are assembled with one of said membrane formats, so as to produce at least two types of fuel cell, delivering different powers, from identical plates.

[0019] In this way, it is possible to have several types of batteries, and therefore to effectively adapt the battery used according to the needs, using a single type of plates, only by varying the format of the membrane. It is thus quick and simple to produce several types of batteries, without having to design and produce different types of plates, which is complex and expensive. In addition, economies of scale are obtained in the manufacture of these plates intended for several battery powers.

[0020] According to a particular implementation, the method of the invention uses at least three membrane formats:

[0021] - a first format covering a maximum active surface of said plates;

[0022] - a second format covering substantially 75% of said active surface;

[0023] - a third format covering substantially 50% of said active surface.

[0024] Of course, other formats and other dimensions are possible, depending on the needs.

[0025] To implement the method efficiently, it is desirable that said plates are designed so as to ensure substantially constant diffusion of said reactive gases and said heat transfer fluid in an area coinciding with the membrane used, whatever the surface area thereof.

[0026] According to a particular implementation, said first and / or said second channels follow a path defining undulations in a corrugation plane substantially perpendicular to the main plane of said plate.

[0027] In other words, the plate, and in particular its active surface, that is to say the surface ensuring the exchange of protons, located opposite the membrane, is defined in three dimensions, and no longer according to a plane. It presents undulations, or "hollows" and "bumps", determined so as to optimize the pressure of said gases and / or the flow rate of said heat transfer fluid.

[0028] According to a particular embodiment, said first channels extend in a direction orthogonal to the direction of said second channels.

[0029] In particular, said channels may extend parallel to the length and width of the plate respectively, crossing each other and following the undulations.

[0030] According to another aspect, which may where appropriate be implemented independently of the first, said channels have a variable section.

[0031] This makes it possible in particular to optimize the pressure of said gases and / or the current density delivered by said plate.

[0032] The invention also relates to fuel cells manufactured according to the method described above.

[0033] Some of these batteries have a first predetermined power, corresponding to a first membrane format covering a maximum active surface of said plates.

[0034] Other batteries may have a second predetermined power, corresponding to a second membrane format with a surface area less than said maximum active surface area of ​​said plates.

[0035] In particular, these batteries may belong to the group comprising membranes having:

[0036] - a second format covering substantially 75% of said active surface; or

[0037] - a third format covering substantially 50% of said active surface. 4. list of figures

[0038] Other characteristics and advantages of the invention will appear more clearly on reading the following description of an example of implementation, given as a simple illustrative and non-limiting example, and the appended figures among which:

[0039] [Fig.l]: [Fig.l], already described in the preamble, illustrates the general principle of a fuel cell;

[0040] [Fig.2]: [Fig.2], already described in the preamble, presents the structure of a fuel cell, comprising a stack of cells;

[0041] [Fig.3]: [Fig.3], already described in the preamble, presents the constituent elements of a cell of [Fig.2], in exploded view;

[0042] [Fig.4]: [Fig.4] schematically illustrates the main elements of a bipolar plate of the cell of [Fig.3];

[0043] [Fig.5A], [Fig.5B]: Figures 5A and 5B respectively illustrate the path of the flow of gas (dihydrogen or air) and the path of the heat transfer fluid, following undulations (only a portion of the half-plate is shown);

[0044] [Fig.6]: [Fig.6] is another view of a portion of the half-plate, making appear the waves according to the two orthogonal directions defined by the channels;

[0045] [Fig.7]: [Fig.7] schematically illustrates an example of undulation;

[0046] [Fig.8]: [Fig.8] illustrates the variation of the section of a gas channel;

[0047] [Fig.9]: [Fig.9] illustrates the principle of the invention;

[0048] [Fig. 10]: [Fig. 10] illustrates the method of the invention, in the form of a block diagram.

[0049] 5. Description of a particular embodiment

[0050] 5.1 Method of the invention

[0051] The invention is therefore based on a completely new approach to the design and manufacture of fuel cells. According to the prior art, the person skilled in the art develops a specific plate design for the required power, and in particular for the active part of the plate, opposite which the membrane allowing the proton exchange will be placed.

[0052] According to the invention, on the other hand, a single plate and several (at least two) membrane dimensions are provided which can be placed between two plates. Thus, it is possible to easily manufacture several batteries, of different powers, from a single type of plate.

[0053] As illustrated in the example of [Fig.9], plates 91 are available, with which, in this example, three distinct plate dimensions 92A (100% of the active surface), 92B (75% of the active surface) and 92C (50% of the active surface) can be associated.

[0054] Three battery powers are thus available, from a single plate, that is to say without it being necessary to design specific plates or specific tools.

[0055] According to the illustrated embodiment, the membranes 92A, 92B or 92C are placed centered on the active part of the plate 91.

[0056] The manufacturing method of the invention, according to one implementation, is illustrated in [Fig.10],

[0057] Plates (El) are manufactured according to a single dimension and a single format. Membranes of different dimensions are also manufactured (E2A, E2B and E2C). Depending on the required power. The selected plates and membranes (E3A, E3B and E3C) are then assembled to manufacture batteries of different powers PA, PB or PC.

[0058] This approach goes against the habits of the person skilled in the art, who has always developed plates and batteries for maximum power, that is to say by exploiting 100% of the active surface. It is not obvious, for the person skilled in the art, to decide to exploit only a portion of the active surface.

[0059] Of course, it is appropriate that the plates used are as efficient as possible. One of the difficulties in the design of fuel cells, and of these plates in particular, is the optimization and homogenization of the path of this coolant, which is generally imposed by what has been designed for the circulation of the gases of the anode and cathode parts.

[0060] A particular embodiment of plates suitable for implementing the invention is described below.

[0061] 5.2 Main elements of a bipolar plate

[0062] Different types of channels are known for the circulation of gases, in particular straight channels, parallel to each other, in serpentines or in zigzag, extending over an active part of the plate.

[0063] The efficiency of these plates is not perfect. Despite the designers' efforts, there are significant variations in the pressure of the gases, particularly oxygen, in the active surface, and a non-homogeneous distribution of the heat transfer fluid, particularly partial in the middle of the plate. This introduces thermal inhomogeneity, particularly the presence of hot spots.

[0064] As a result, this introduces an inhomogeneity in the creation of the electric current in the plate.

[0065] Such plates are also difficult to manufacture and assemble. In particular, they have poor rigidity along their length, which leads to the use of relatively thick, therefore heavy, strips and requires the use of manufacturing techniques that are not well-suited to mass production, such as brazing or multiple welds.

[0066] There is therefore a need for a new approach to the production of such plates, to enable production more suited to series requirements and / or to improve their efficiency.

[0067] For information purposes, the main aspects of a bipolar plate are described below. As illustrated schematically in the example of [Fig.4], there are several essential areas in a bipolar plate:

[0068] - active zone AZ: this is the location where the reaction takes place, consisting of many channels. One of the design difficulties of these plates is the need to provide a homogeneous distribution of the gas and not to overfeed or underfeed the channels. The surface area of ​​the active area of ​​the plate is the same as that of the active area of ​​the AME;

[0069] - Homogenization zone HZ: location in which the flows mix to feed the channels of the active zone with the same quantity of gas / coolant. It is similar to a funnel, going from a flow inlet towards the start of the active zone;

[0070] - ABP anti-bypass zone: an anti-bypass zone is necessary for reasons mechanical such as good retention of the AME and / or good compression of the seal. It is then necessary to avoid a bypass effect for the gases, which could induce drops in gas pressure, and therefore the efficiency of the entire system;

[0071] - M collectors: the collectors ("manifolds" in English) are elements acting on the sizing, including inlets and outlets for each gas and the refrigerant. Generally, their position and dimensions define the overall dimensions of the plate;

[0072] - Inlets I: the inlets are what allow the gas and the refrigerant liquid to go from collectors to the gas or liquid refrigerant side of the plate;

[0073] - Guide interfaces G: elements helping to keep the stack in position during the stacking process;

[0074] - V measuring electrodes: allowing a pin or a special device to be connected between the anode and the cathode to take measurements;

[0075] - Data matrix: allowing precise identification of the plate. It can including bearing a serial number, supplier identification number, time, date of manufacture, etc. Its dimensions are generally 4x4 or 5x5 mm;

[0076] - Poka-Yoke: mechanical element which constrains the positioning of the half-plates;

[0077] - Welds: holding the half-plates together, they can be watertight or simply intended to strengthen the structure.

[0078] 5.3 three-dimensional (3D) topology

[0079] The invention therefore proposes, according to a particular embodiment, a new approach to bipolar or monopolar plates, according to which the active zone thereof, that is to say substantially the zone opposite the membrane, allows the flows of gas and heat transfer fluid to move in the three dimensions of space.

[0080] More precisely, as illustrated by Figures 5A and 5B, respectively representing the path of the gas flow (dihydrogen or air) and the path of the heat transfer fluid, the channels 51 intended for the gas and the channels 52 intended for the heat transfer fluid extend in orthogonal directions D1, D2, respectively according to the length and the width of the plate, and intersect. In addition, these channels have undulations in both directions, the surface of the active part therefore not being flat, but having "hollows" and "bumps", as can be seen in particular in [Fig.6].

[0081] This approach presents, compared to the prior art according to which the channels extend in a plane, the plane defined by the plate, can present numerous advantages, depending on the implementations: - improvement of thermal convection, allowing to reduce the flow rate and / or improve the efficiency of the system; - better rigidity of the plate and / or weight saving, making it possible to reduce the thickness of the strip; - improvement of electrical contact and reduction of electrical contact resistance; - facilitation of the assembly of the battery, and in particular of the alignment of the cells with each other, making it possible in particular to avoid sealing problems; - homogenization of the partial pressure of oxygen along the active surface; - homogenization of current creation and improvement of current density.

[0082] [Fig.7] schematically illustrates an example of a ripple, characterized by its length, or period, P and its radius R, or maximum height of the ripple. Of course, the shape of the ripple may be different from that illustrated.

[0083] The inventors determined, taking into account in particular the tangency constraints of the fluid channels, a difference between the radius R and the period P less than or equal to 20% was effective, so as not to make the couple (radius; half-period) too strong and to obtain a balance between undulations that are too pronounced and undulations that are too tenuous, for example: R = P ± 20%.

[0084] As an indication, the dimensions can be of the order of: - / zP: between 8 and 12 mm; - R: between 16 and 24 mm.

[0085] As illustrated in [Fig.7], we could for example implement a radius R of 20 mm and a half-period of 10 mm.

[0086] More generally, the dimensions of the half-plate can be of the order of: - length: between 250 and 350 mm; - width: between 150 and 200 mm.

[0087] The number of gas channels is for example between 50 and 100, and the number of heat transfer fluid channels between 50 and 100.

[0088] 5.4 variable section channels

[0089] According to another aspect of the invention, which can where appropriate be implemented independently of the 3D topology described above, the section of the channels is varied.

[0090] Indeed, the inventors have found that the active zones according to the prior art, which have channels for conveying fluids of constant section, introduce heating and generate inhomogeneous partial pressures. In addition, this harms the rigidity of the plate.

[0091] To overcome these problems, gas channels are provided in particular whose section varies periodically, as illustrated by [Fig.8], in which it can be seen that, along the path 81 of the fluid, the section of the channel varies between a maximum section Smax 82 and a minimum section Smin 83.

[0092] This makes it possible in particular to improve the rigidity of the plate, to obtain better thermal convection, to balance the partial pressures and / or to obtain better electrical contact.

[0093] According to one embodiment, the maximum section Smax corresponds to a location where the maximum height H of the gas channel 84 and the minimum section Smin to a location where the gas channel 84 crosses a heat transfer fluid channel 85 (crossing, or "cross-channel" in English), the height being reduced to the minimum height h.

[0094] The ratio between H and h is preferably between 2 and 3, and for example such that H = 2.5 * h.

[0095] For simplification, we consider that the width L of the channel, which varies little, is constant. Thus, the section Smax is approximately L * H, and the section Smin is approximately (H - h) * L.

[0096] We therefore preferably choose a section variation ratio Smax / Smin of between 1.5 and 2, for example of the order of 1.6.

[0097] It should be noted that this [Fig.8] illustrates a portion of a half-plate, and therefore a half-section of the channels which are obtained by assembling two half-plates, to form a complete plate. The ratios, however, obviously remain the same.

[0098] As an indicative example, the dimensions may be of the order of: - H: between 0.2 and 0.35 mm; - h: between 0.1 and 0.15 mm; - L: between 0.5 and 1.3 mm.

[0099] 5.5 manufacturing and assembly

[0100] The approach of the invention makes it possible to manufacture half-plates from thinner strips than those conventionally used, for example 316L stainless steel with a double carbon coating, with a thickness of 0.075 mm. This makes it possible to produce batteries requiring less material, and therefore less heavy and less expensive.

[0101] Of course, the three-dimensional shape, the half-plates having undulations in two orthogonal directions (corresponding to the directions of the channels) and / or the presence of channels of variable sections require care especially when producing half-plates, for example by stamping or molding. A preferred manufacturing method is hydroforming, which has many advantages, such as precision in process repeatability, elasticity after forming, uniformity of wall thickness, efficiency of contact zones, adaptability...

[0102] On the other hand, this shape and / or the reduced thickness of the strips can allow a simpler and more reliable assembly of the two half-plates forming each plate, in particular for the joining of the half-plates, for example by welding, and for the assembly of the plates to form a stack.

[0103] The invention thus makes it possible to obtain very efficient plates and batteries, suitable for numerous applications, for example in motor vehicles, and more generally in any type of vehicle or means carrying fuel cells.

Claims

Claims

1. Method for manufacturing at least two types of fuel cells, delivering different powers according to said types of cells, said cells having a stack of plates each comprising first channels for circulation of reactive gases, respectively dihydrogen and air, and second channels for circulation of a heat transfer fluid, a proton exchange membrane being interposed between two neighboring plates, characterized in that: - plates of a single format are obtained; - membranes having at least two membrane formats each having different dimensions; - said plates are assembled with a first of said membrane formats, so as to produce a first type of fuel cell, delivering a first power;- said plates are assembled with a second of said membrane formats, so as to produce a second type of fuel cell, delivering a second power, so as to have several types of cells, delivering different powers, from identical plates and membranes specific to each type of cell, each cell of a given type of cell using membranes of the same format, intended for said type of cell.;

2. Method for manufacturing fuel cells according to claim 1, characterized in that said types of fuel cells respectively implement one of the membrane formats belonging to the group comprising: - a first format covering a maximum active surface of said plates; - a second format covering substantially 75% of said active surface; - a third format covering substantially 50% of said active surface.

3. A method of manufacturing a fuel cell according to any one of Claims 1 and 2, characterized in that said plates are designed to ensure a substantially constant diffusion of said reactive gases and said heat transfer fluid in a zone coinciding with the membrane used, regardless of the surface thereof.

4. A method of manufacturing a fuel cell according to Claim 3, characterized in that said first and said second channels extend in directions (D1, D2) that are orthogonal, respectively along the length and the width of said plate and follow a pattern defining undulations in a plane of undulation that is substantially perpendicular to the main plane of said plate.

5. Method for manufacturing fuel cells according to any one of claims 3 and 4, characterized in that said channels have a variable section between a maximum section (Smax) and a minimum section (Smin), said minimum section (Smin) corresponding to the locations where one of said first channels crosses one of said second channels.

6. Fuel cells manufactured according to the method of any one of claims 1 to 5, comprising at least: - a cell belonging to a first type of cell, having a first predetermined power, implementing a first membrane format covering a maximum active surface of said plates; and - a cell belonging to a second type of cell, having a second predetermined power, implementing a second membrane format with a surface area smaller than said maximum active surface of said plates.

7. Fuel cells according to claim 6, characterized in that they belong to the group comprising membranes having: - a second format covering substantially 75% of said active surface; or - a third format covering substantially 50% of said active surface.

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