Electrochemical cell bipolar plate for reducing short-circuit flows around headers

Abstract

The invention relates to a bipolar plate for an electrochemical cell comprising upper and lower metal sheets (10, 20) and first and second headers (2, 3), wherein, in a first intermediate zone extending between the upper (15) and lower (25) ribs of the first header on the one hand and the upper (12) and lower (22) external ribs on the other hand, the upper conductive sheet (10) comprises an upper recess (14), referred to as an intermediate upper recess, and the lower conductive sheet (20) comprises a lower rib (24), referred to as an intermediate lower rib, positioned over and in contact with the intermediate upper recess (14).

Description

BIPOLAR ELECTROCHEMICAL CELL PLATE ALLOWING A REDUCTION OF SHORT-CIRCUIT FLOWS AROUND THE COLLECTORS TECHNICAL FIELD

[0001] The field of the invention is that of electrochemical reactors comprising a stack of electrochemical cells, such as fuel cells and electrolysers, and relates more particularly to bipolar plates, of the conductive sheet type. STATE OF THE PRIOR ART

[0002] An electrochemical reactor, such as a fuel cell or an electrolyzer, usually consists of a stack of electrochemical cells, each of which has an anode and a cathode electrically separated from each other by an electrolyte. The cells are the site of an electrochemical reaction between two continuously introduced reactive fluids.

[0003] Generally speaking, in the case of a fuel cell, the combustible fluid (e.g. hydrogen) is supplied to the anode, while the oxidizing fluid (e.g. air) is supplied to the cathode. The electrochemical reaction is subdivided into two half-reactions, an oxidation reaction and a reduction reaction, which take place respectively at the anode / electrolyte interface and at the cathode / electrolyte interface. To take place, the electrochemical reaction requires the presence of an ionic conductor between the two electrodes, namely the electrolyte, for example, contained in a polymer membrane, and an electronic conductor formed by the external electrical circuit. The stack of cells is thus the site of the electrochemical reaction: the reactive fluids must be supplied there, the products and non-reactive species must be removed, as must the heat produced during the reaction.

[0004] Electrochemical cells are usually separated from each other by bipolar plates that provide electrical interconnection between them and the flow of reactive fluids. The bipolar plates have an anodic face at which a fuel fluid distribution circuit is formed, and an opposite cathodic face at which a combustion fluid distribution circuit is formed. Each distribution circuit takes the form of a network of channels arranged to bring the reactive fluid to the corresponding electrode. The bipolar plates may also have a cooling circuit formed by a network of internal conduits that ensure the flow of a heat transfer fluid to evacuate the heat produced locally during the electrochemical reaction by the cell.

[0005] Figure 1A is a schematic and partial view of an example of a bipolar plate 1, of the conductive sheet type, seen from above and from the cathode side. Figure 1B is a cross-sectional view of the bipolar plate 1 of Fig. 1A along the section line AA.

[0006] The bipolar plate 1 comprises two conductive sheets 10, 20 superimposed on each other. The conductive sheets 10, 20 each comprise a distribution circuit 7 adapted to convey a reactive fluid to the corresponding electrode. The distribution circuits 7 are superimposed on each other, and define, in the XY plane, the reaction zone of the electrochemical cell. An air collector 2 and a hydrogen collector 3 are adjacent to each other, and are here separated by a collector 4 of the heat transfer liquid. These collectors 2, 3, 4 are arranged, in the XY plane, opposite the inlet of the distribution circuit 7. The air collector 2 is adapted to supply the distribution circuit 7 with air, here via an injection zone 8 then a homogenization zone 9.

[0007] The conductive sheets 10, 20 comprise ribs superimposed two by two, and associated with sealing gaskets, which participate in ensuring the sealing of the bipolar plate. Thus, an upper rib 15 and a lower rib 25, called collector ribs, form a sealing line (called collector) which each surrounds a collector in the XY plane. They are in contact with sealing gaskets 6 called collector gaskets, and make it possible to prevent the reactive fluids from flowing on the same side of the bipolar plate 1 and from mixing. In addition, an upper rib 12 and a lower rib 22, called external, form a sealing line (called external) which extends over the periphery of the bipolar plate 1 and surrounds the collectors 2, 3, 4 and the distribution circuits 7 in the XY plane. They are in contact with so-called external seals 5, and prevent reactive fluids from flowing outside the bipolar plate 1.The upper ribs 12, 15 have the same profile as the respective lower ribs 22, 25, this profile being defined so as to provide the desired mechanical strength and elasticity.

[0008] Furthermore, the ribs are bordered by flat portions. The flat portions 11, 13, 16 of a conductive sheet are superimposed and in contact with the flat portions 21, 23, 26 of the opposite conductive sheet at the level of the same reference plane P re r (parallel to the XY plane). Thus, if we consider the upper sheet 10 (cathodic), an external flat portion 11 extends between the external upper rib 12 and the edge of the bipolar plate 1; an intermediate flat portion 13 extends between the external upper rib 12 and the upper collector rib 15; and an internal flat portion 16 extends between the upper collector rib 15 and the edge of the air collector 2.

[0009] Thus, in operation, the air is supplied by the inlet manifold 2, flows through the injection zone 8 then the homogenization zone 9, and comes into contact with the cathode by means of the distribution circuit 7. The unconsumed air then flows through the homogenization zone, the injection zone, to finally reach the outlet manifold 2 (see continuous arrows in fig.lA). The same is true for the hydrogen on the anode side.

[0010] However, it appears that air can bypass the reaction zone by flowing into lateral short-circuit passages located between the reaction zone and the external sealing line. Indeed, as shown by the dotted arrows in Fig. 1A, air can escape between the injection zone 8 and the homogenization zone 9, flow on one side to bypass the hydrogen collector 3 and join the lateral short-circuit passage (on the left in the figure), and flow on the other side to join the lateral passage on the right (on the right in the figure). These flows are short-circuit flows insofar as they do not participate in the electrochemical reaction, which degrades the performance of the fuel cell. This is the case for air on the cathode side, but also for hydrogen on the anode side.There is therefore a need to have bipolar plates whose structure makes it possible to reduce these short-circuit flows. STATEMENT OF THE INVENTION

[0011] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose a bipolar plate whose structuring of the conductive sheets around at least one reactive fluid collector makes it possible to reduce the short-circuit flow bypassing the reaction zone, thus improving the performance of the electrochemical cell.

[0012] For this purpose, the subject of the invention is a bipolar plate of an electrochemical cell, comprising: o an upper conductive sheet and a lower conductive sheet, made of an electrically conductive material, and superimposed on each other; o a first collector of a first reactive fluid and a second collector of a second reactive fluid different from the first reactive fluid, adjacent to each other; o the upper conductive sheet comprising: an upper distribution circuit, for conveying the first reactive fluid to an upper electrode; an upper rib called the first collector, surrounding the first collector; an upper rib called the external rib, surrounding at least the upper rib of the first collector and the upper distribution circuit; o the lower conductive sheet comprising: a lower distribution circuit, for conveying the second reactive fluid to a lower electrode different from the upper electrode, superimposed on the upper distribution circuit; a lower rib called the first collector, surrounding the first collector, superimposed on the upper rib of the first collector; a lower rib called the external rib, surrounding at least the lower rib of the first collector and the lower distribution circuit, superimposed on the external upper rib.

[0013] According to the invention, in a first intermediate zone extending between the upper and lower ribs of the first collector on the one hand and the external upper and lower ribs on the other hand, the upper conductive sheet comprises an upper recess called an intermediate recess, and the lower conductive sheet comprises a lower rib called an intermediate recess, superimposed and in contact with the upper intermediate recess. The upper intermediate recess and the lower intermediate rib extend longitudinally at least partly around the first collector.

[0014] Some preferred but not limiting aspects of this bipolar plate are as follows.

[0015] The intermediate lower rib may be coplanar with the first collector lower rib and with the external lower rib.

[0016] The intermediate lower rib can extend planarly by connecting the first collector lower rib with the outer lower rib.

[0017] The bipolar plate may comprise two first collectors, ensuring the supply and evacuation of the first reactive fluid, each first collector being at least partly bordered by a first intermediate zone comprising the upper intermediate recess and the lower intermediate rib.

[0018] In a second intermediate zone extending between the upper and lower ribs of the second collector on the one hand and the external upper and lower ribs on the other hand, the upper conductive sheet may comprise an upper rib called an intermediate rib, and the lower conductive sheet may comprise a lower recess called an intermediate, superimposed and in contact with the upper intermediate rib. The upper intermediate rib and the lower intermediate recess may extend longitudinally at least partly around the second collector.

[0019] The intermediate upper rib may be coplanar with the first collector upper rib and with the external upper rib.

[0020] The intermediate upper rib can extend in a planar manner by connecting the first collector upper rib and with the external upper rib.

[0021] The bipolar plate may comprise two second collectors, ensuring the supply and evacuation of the second reactive fluid, each second collector being at least partly bordered by a second intermediate zone comprising the intermediate upper rib and the intermediate lower recess.

[0022] The invention also relates to an electrochemical cell, comprising at least one bipolar plate according to any one of the preceding characteristics, and a membrane / electrode assembly in contact with the bipolar plate.

[0023] The invention also relates to an electrochemical reactor, comprising at least one electrochemical cell according to the preceding characteristic. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which: Figure 1A, already described, is a schematic and partial view, in top view, of a bipolar plate according to an example of the prior art; Figure 1B, already described, is a schematic and partial view, in cross-section, of the bipolar plate of Fig. 1A along the section line AA; Figure 2A is a schematic and partial view, in top view, of a bipolar plate according to one embodiment; Figure 2B is a schematic and partial view, in cross-section, of the bipolar plate of Fig. 2A, along the section line Bl-Bl at the level of the air collector; Figure 2C is a schematic and partial view, in cross-section, of the bipolar plate of Fig.2A, along section line B2-B2 at the hydrogen collector; Figure 2D is a schematic and partial cross-sectional view of the bipolar plate of Fig. 2A, along section line B3-B3 between the collector sealing lines and the external sealing line; Figure 2E is a schematic and partial view, in cross-section, of the bipolar plate, according to an alternative embodiment of Fig. 2B; Figure 3A is a schematic and partial view, in top view, of a bipolar plate according to another embodiment, where the collectors of the heat transfer liquid are located along an axis orthogonal to the main axis of flow of the reactive fluids; Figure 3B is a schematic and partial view, in cross-section, of the bipolar plate of Fig. 3A, along the section line CC at the air collector; Figure 4A is a schematic and partial view, in top view, of a bipolar plate according to another embodiment; Figure 4B is a schematic and partial view, in cross-section, of the bipolar plate of Fig. 4A, along the section line Dl-Dl at the hydrogen collector; Figure 4C is a schematic and partial cross-sectional view of the bipolar plate of FIG.4A, following the section line D2-D2 at the air manifold. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0025] In the figures and in the remainder of the description, the same references represent identical or similar elements. In addition, the different elements are not shown to scale so as to enhance the clarity of the figures. Furthermore, the different embodiments and variants are not mutually exclusive and may be combined with each other. Unless otherwise indicated, the terms "substantially", "approximately", "of the order of" mean to within 10%, and preferably to within 5%. Furthermore, the terms "between ... and ..." and equivalent mean that the limits are included, unless otherwise indicated.

[0026] The invention relates to an electrochemical cell bipolar plate for electrochemical reactors such as fuel cells and electrolyzers. The bipolar plate is of the type with conductive sheets, which have ribs and recesses. The invention more specifically relates to a structural configuration of the bipolar plate, where the positioning of a superposition of a rib and a recess, at at least one reactant fluid collector, makes it possible to reduce the short-circuit flow of the reactant fluid.

[0027] Various embodiments and variants will be described with reference to a fuel cell, and in particular to a PEM (Proton Exchange) type fuel cell. Membrane, in English) whose cathode is supplied with oxygen and the anode with hydrogen. The invention applies, however, to any type of fuel cell, in particular to those operating at low temperature, i.e. at a temperature below 200°C, as well as to electrochemical electrolysers.

[0028] Figure 2A is a schematic and partial top view of a portion of the bipolar plate 1 according to one embodiment, at the level of the air 2, hydrogen 3 and heat transfer liquid 4 collectors. Figures 2B, 2C and 2D are schematic and partial cross-sectional views of the bipolar plate 1 of Fig. 2A, following, respectively, the section lines Bl-B1, B2-B2, and B3-B3.

[0029] Here and for the rest of the description, we define a direct orthogonal reference XYZ, where the Z axis is oriented along the thickness of the bipolar plate 1 (from the lower conductive sheet 20 to the upper conductive sheet 10), and where the X and Y axes define a plane parallel to the plane of the bipolar plates.

[0030] The electrochemical cells here belong to a stack of cells of a fuel cell. Each electrochemical cell comprises a membrane / electrode assembly (not shown) formed of an anode and a cathode separated from each other by an electrolyte here comprising a polymer membrane. The membrane / electrode assemblies of the electrochemical cells are arranged between bipolar plates adapted to bring reactive species to the electrodes and to evacuate the heat produced during the electrochemical reaction.

[0031] Each bipolar plate 1 is formed of two conductive sheets, upper 10 and lower 20, superimposed and assembled to each other. These conductive sheets 10, 20 are deformed locally so as to form flow channels for the reactive fluids, and in particular an injection zone 8, a homogenization zone 9 and a distribution circuit 7, as well as a cooling circuit located between the conductive sheets 10, 20. Thus, the lower conductive sheet 20, for example anode, is intended to be in contact with the anode of the membrane / electrode assembly of an electrochemical cell, while the upper conductive sheet 10, here cathode, is intended to be in contact with the cathode of the membrane / electrode assembly of an adjacent electrochemical cell.

[0032] Each conductive sheet 10, 20 has an external face and an opposite internal face, the conductive sheets 10, 20 being superimposed on each other at the internal faces. An external face is said to be anodic when it is intended to be in contact with the anode, or is said to be cathodic when it is intended to be in contact with the cathode. The anodic face of a sheet conductive sheet comprises the distribution circuit of a combustible reactive fluid, for example here hydrogen, and the cathode face of the other conductive sheet comprises the distribution circuit of the oxidizing reactive fluid, for example here air.

[0033] The conductive sheets 10, 20 are in the form of sheets, or elementary plates of low thickness, made of an electrically conductive material, for example a metal or even a composite material for example loaded with graphite. The thickness can be of the order of a few tens of microns to a few hundred microns, for example from 50 pm to 200 pm approximately in the case of metal sheets.

[0034] Each conductive sheet 10, 20 comprises ribs and recesses, obtained for example by stamping, embossing, or any other shaping technique, the shape of which on one face is the complement of the shape on the opposite face. The ribs make it possible in particular to define the distribution circuit of a reactive fluid at the external face, as well as the cooling circuit of the heat transfer fluid at the internal face.

[0035] Collectors 2, 3, 4 are openings which pass through each of the bipolar plates 1. The collectors of the reactive fluids 2, 3 are located on either side of the reaction zone, along a main axis of flow of the reactive fluids, here along the Y axis. In this example, a collector of the heat transfer liquid 4 is located between the collectors of the reactive fluids 2, 3. Alternatively (see fig. 3A), they can be located along an axis orthogonal to the main Y axis.

[0036] The reactive fluid collectors 2, 3 are adjacent to each other (separated or not by the heat transfer liquid collector 4), and are arranged opposite the same opening (inlet or outlet) of the reaction zone. In this example, an air inlet collector 2 and a hydrogen collector 3 (inlet or outlet) are located opposite the inlet of the cathode distribution circuit 7.

[0037] The conductive sheets 10, 20 comprise injection zones 8 to allow the circulation of fluids from or to the collectors. Thus, an air injection zone 8 ensures the fluid connection between the air collector 2 and the homogenization zone 9. It is formed of conduits which cross the sealing line of the air collector and open onto the external face of the cathode conductive sheet 10, to allow the air to flow towards the homogenization zone 9. The same is true for the anodic conductive sheet 20, the injection zone of which communicates with the hydrogen collector 3 and the homogenization zone. The heat transfer liquid injection zone is formed of conduits which open between the two cathode 10 and anodic 20 conductive sheets. Similar injection zones are described in particular in document EP3136492A1.

[0038] Each conductive sheet 10, 20 here comprises a homogenization zone 9 which communicates with the injection zone 8 on the one hand, and with the distribution circuit 7 on the other hand. Such a homogenization zone 9 makes it possible to homogenize the air flow at the inlet of the distribution circuit 7. It can be formed by homogenization conduits made in the conductive sheet 10. The document EP3136492A1 also describes an example of such a homogenization zone.

[0039] Each conductive sheet 10, 20 comprises a distribution circuit 7, formed of channels which extend between an inlet and an outlet aligned along a main direction (here the Y axis). These channels are separated two by two by a separation rib which is in contact with the corresponding MEA. The cathode and anodic distribution circuits define in the XY plane the reaction zone of the electrochemical cell.

[0040] Furthermore, we define a reference plane P re f of the bipolar plate 1 as being the contact plane of the conductive sheets 10, 20 at their internal (at the collectors) and external edges. This is the reference plane P re f with respect to which the ribs and recesses are defined, when considered from the external face of each conductive sheet 10, 20. Thus, a rib is obtained by local deformation of the conductive sheet 10, 20, from its internal face towards its external face. It is therefore a relief or a boss of the conductive sheet 10, 20, seen from the external face, which moves away from the reference plane P ref along the Z axis (and therefore which moves away from the other conductive sheet). Conversely, a recess is obtained by local deformation of the conductive sheet 10, 20, from its external face towards its internal face. It is therefore a depression or a hollow of the conductive sheet 10, 20, seen from its external face, which extends below the reference plane P re f along the Z axis (and therefore which approaches the other conductive sheet).

[0041] As indicated with reference to Figs. 1A and 1B, the bipolar plate 1 comprises ribs 15, 25, called collector ribs, superimposed on one another, which entirely surround each collector 2, 3, 4 in the XY plane. Thus, the air collector 2 is surrounded by upper 15 and lower 25 air collector ribs, just like the hydrogen collector 3 and the heat transfer liquid collector. Seals 6, called collector seals, are arranged respectively in contact with the upper 15 and lower 25 collector ribs. The seals 6 may also be in contact with a sealing film (not shown), such as the electrolytic membrane or a lateral reinforcement, which extends between the bipolar plates 1. This configuration, which forms a collector sealing line, makes it possible to prevent the fluid from flowing onto the conductive sheet without passing through the corresponding injection zone 8.

[0042] Furthermore, the bipolar plate 1 comprises ribs 12, 22 called external, superimposed on each other, which entirely surround in the XY plane the collectors as well as the reaction zone. Thus, upper ribs 12 and lower ribs 22 external are superimposed on each other, and extend at the edge of the bipolar plate 1. Seals 5, called external, are arranged respectively in contact with the upper ribs 12 and lower ribs 22 external. They can also be in contact with the waterproof film. This configuration, which forms an external sealing line, makes it possible to prevent the fluids from flowing outside the bipolar plate 1.

[0043] The conductive sheets 10, 20 comprise flat portions, upper and lower, superimposed on each other, and in contact with each other at the level of the reference plane P ref. Thus, flat portions called internal, upper 16 and lower 26, extend in the XY plane between the edge of each collector and the corresponding collector ribs 15, 25, over the entire periphery of the collector. Conversely, flat portions called external, upper 11 and lower 21, extend in the XY plane between the external ribs 12, 22 and the edge of the bipolar plate 1, over the entire periphery thereof.

[0044] According to the invention, in a first intermediate zone extending between the sealing line of a first collector (which is connected to the distribution circuit of the upper conductive sheet), and on the other hand, the external sealing line, the upper conductive sheet 10 comprises an upper recess 14 called intermediate, and the lower conductive sheet 20 comprises a lower rib 24 called intermediate, superimposed and in contact with the upper intermediate recess 14. The intermediate zone is the zone where a collector rib and an external rib are located opposite each other. In the example of fig. 2A where the air collector 2 has four sides, the intermediate zone extends along the top side and along the right side.

[0045] Consequently, thanks to the intermediate lower rib 24 of the lower conductive sheet 20, the possibility for the reactive fluid associated with the second collector to flow around the first collector in the first intermediate zone, on the side of the lower conductive sheet 20, and thus to reach the lower lateral short-circuit passage located on the side of the first collector is reduced. This reduces short-circuit flows, and improves the performance of the electrochemical cell.

[0046] This structural configuration results in the fact that the two conductive sheets 10, 20 have a contact plane P int i, in this first intermediate zone, different from the reference plane P re f. This contact plan P inti is defined by the contact between the upper recess intermediate 14 and the lower intermediate rib 24. The cross-section of flow of the reactive fluid, associated with the second collector, is then reduced on the side of the lower intermediate rib 24, which increases the fluidic flow resistance, which therefore limits this unwanted flow of the reactive fluid.

[0047] Note that it is advantageous for the intermediate lower rib 24 to be coplanar with the collector lower rib 25 and with the external lower rib 22. This reduces the possible flow cross-section as much as possible. However, it is possible for the intermediate lower rib 24 not to be coplanar with the collector lower rib 25 and / or with the external lower rib 22.

[0048] In the example of Fig. 2A, the first intermediate zone extends in the XY plane between the sealing line of the air collector (see the seal 6 of the air collector) and the external sealing line (see the external seal 5). As shown in Fig. 2B, the upper conductive sheet 10 (cathodic) has an intermediate upper recess 14 and the lower conductive sheet 20 (anodic) has an intermediate lower rib 24, superimposed and in contact with the intermediate upper recess 14. Also, the first contact plane P in ti is different from the reference plane P re f, and is located below the latter in the direction -Z. The upper intermediate recess 14 and the lower intermediate rib 24 each comprise a flat portion which is connected laterally to the adjacent collector rib 15, 25 and to the adjacent external rib 12, 22 by curved portions.

[0049] Thus, in this first intermediate zone which extends between the sealing line of the air collector and the external sealing line, the flow of air is allowed on the cathode side by the upper intermediate recess 14, while that of hydrogen on the anodic side is limited or even made impossible by the lower intermediate rib 24. This greatly reduces the possibility of hydrogen flowing from the anodic side around the air collector 2, and reaching the adjacent lower (anodic) short-circuit lateral passage (located on the side of the air collector 2).

[0050] Preferably, in order to limit short-circuit flows on both the cathode side and the anodic side, this first structural configuration is made at least partly around the first collector 2 (with an intermediate lower rib 24 - and the corresponding recess - which extends in the first intermediate zone at least partly around the first collector 2), and an opposite structural configuration is made at least partly around the adjacent second collector 3 (with an intermediate upper rib 17 - and the corresponding recess - which extends in a second intermediate zone at least partly around the second collector 3). By extending at least partly around a collector, it is meant that the intermediate rib considered (and the corresponding recess) extends over at least 25%, or even 50%, or even 75% or even 100% of the length of the intermediate zone.

[0051] Thus, the bipolar plate 1 advantageously comprises a second intermediate zone, which extends in the XY plane between the sealing line of the hydrogen collector and the external sealing line. As shown in fig. 2C, the upper conductive sheet 10 comprises an intermediate upper rib 17, and the lower conductive sheet 20 comprises an intermediate lower recess 27, superimposed and in contact with the intermediate upper rib 17. Also, the contact plane Pint? is also different from the reference plane P re f, and is located above the latter in the +Z direction. The upper intermediate rib 17 and the lower intermediate recess 27 each comprise a flat portion which is connected laterally to the collector rib 15, 25 and to the external rib 12, 22 by curved portions.

[0052] Also, in this second intermediate zone which extends between the sealing line of the hydrogen collector and the external sealing line, the flow of hydrogen is allowed on the anode side by the lower intermediate recess 27, while that of air on the cathode side is limited or even made impossible by the upper intermediate rib 17. This greatly reduces the possibility of air flowing from the cathode side around the hydrogen collector 3, and reaching the adjacent upper (cathodic) short-circuit lateral passage (located on the side of the hydrogen collector 3).

[0053] As shown in Fig.2D, the transition between the contact plane P inti(in the first intermediate zone) and the contact plane Pint? (in the second intermediate zone) can be located between the heat transfer liquid collector and the external sealing line. Also, this contact transition occupies a particularly limited area, since it does not extend longitudinally parallel to a collector sealing line.A non-zero volume, delimited by the internal faces of the conductive sheets 10, 20, comes from the fact that it is preferable for the upper curved portion 18 (ensuring the junction between the upper intermediate rib 17 and the upper intermediate recess 14) not to be in contact with the lower curved portion 28 (ensuring the junction between the lower intermediate recess 27 and the lower intermediate rib 24), this in order not to weaken the conductive sheets 10, 20 by a local transmission of mechanical stresses, while respecting the manufacturing and positioning tolerances.

[0054] Thus, this local configuration of the conductive sheets 10, 20 between the collector sealing lines and the external sealing line makes it possible to locally reduce the cross-section of flow of each of the reactive fluids, each in a given intermediate zone, which consequently limits the flows in the lateral short-circuit passages, and therefore improves the performance of the electrochemical cell.

[0055] Note that this configuration can advantageously be found on the other side of the reaction zone, at the outlet of the distribution circuits. Thus, in the example of fig.1A where the lateral short-circuit passage on the right side of the reaction zone connects an air collector 2 (upstream) to a hydrogen collector 3 (downstream), the air collector 2 is bordered at least in part by a first intermediate zone formed by an intermediate upper recess 14 and an intermediate lower rib 24 (see fig.2B), and the opposite hydrogen collector 3 is bordered in part by a second intermediate zone formed by an intermediate upper rib 17 and an intermediate lower recess 27 (as in fig.2C).

[0056] This is also the case in a configuration (not shown) where the same lateral short-circuit passage would extend in the XY plane along the reaction zone between an air (or hydrogen) inlet manifold to an air (or hydrogen) outlet manifold. In this case, the air manifolds 2 may be bordered at least in part by a first intermediate zone (intermediate upper recess 14 and intermediate lower rib 24), and the hydrogen manifolds 3 may be bordered at least in part by a second intermediate zone (intermediate upper rib 17 and intermediate lower recess 27).

[0057] Figure 2E is a variant of Fig. 2B. Here, the lower conductive sheet 20 has an intermediate lower rib 24 which connects the ribs 25 and 22 by projecting from a plane passing through these ribs, thus coming into contact with the lower sealing film 30. The upper sheet 10 then has an intermediate recess 14 which comes into contact with the intermediate rib 24. This configuration further reduces the short-circuit flow of hydrogen at the level of the lower sheet 20 around the air collector 2. Obviously, a similar configuration (which would then be a variant of Fig. 4B) can be present around the hydrogen collector 3.

[0058] Figure 3A is a schematic and partial top view of a bipolar plate according to another embodiment, where the collectors of the heat transfer liquid are not located between the collectors of the adjacent reactive fluids, but are arranged on either side of the zone reaction along an X axis orthogonal to the main flow axis Y. Figure 3B is a cross-sectional view of the bipolar plate of Fig. 3A along the section line CC.

[0059] The air collectors 2 are here at least partly bordered by a first intermediate zone formed by an intermediate upper recess 14 and an intermediate lower rib 24. The hydrogen collectors 3 are here at least partly bordered by a second intermediate zone formed by an intermediate upper rib 17 and an intermediate lower recess 27.

[0060] Thus, as in Fig. 2A, this configuration reduces the short-circuit flow of hydrogen going from one hydrogen collector 3 to the other, at the level of the lower conductive sheet 20 (anodic), due to the presence of the intermediate lower rib 24. Similarly, the short-circuit flow of air going from one air collector 2 to the other, at the level of the upper conductive sheet 10 (cathodic), is reduced due to the presence of the intermediate upper rib 17.

[0061] Note that a contact plane transition zone may extend between the air collector 2 and the hydrogen collector 3. The conductive sheet 10 may comprise an upper flat portion 16 which delimits the edge of the hydrogen collector 3, then an upper hydrogen collector rib 15, an intermediate upper rib 17, a curved contact plane transition portion 18, an intermediate upper recess 14, an air collector upper rib 15, and finally an upper flat portion 16 which delimits the edge of the air collector 2. The conductive sheet 20 comprises an intermediate lower recess 27 superimposed on the rib 17, a curved contact plane transition portion 28, and an intermediate lower rib 24 superimposed and in contact with the intermediate upper recess 14. The curved portions 18 and 28 are offset along the X axis to avoid weakening bipolar plate 1.

[0062] Figure 4A is a schematic and partial top view of a portion of the bipolar plate according to another embodiment, at the level of the air 2, hydrogen 3 and heat transfer liquid 4 collectors. Here, as in fig.2A, a heat transfer liquid collector 4 is located between the adjacent air 2 and hydrogen 3 collectors. Figures 4B and 4C are schematic and partial views of the bipolar plate of fig.4A, in cross section along the section lines D1-D1 and D2-D2.

[0063] Here we find, as in Figs. 2A and 2C, the second structural configuration which at least partly surrounds the hydrogen collector 3. In other words, the hydrogen collector 3 is bordered by the second intermediate zone, which extends in the XY plane between the sealing line of the hydrogen collector 3 and the external sealing line. The upper conductive sheet 10 (cathodic) has therein an intermediate upper rib 17 and the lower conductive sheet 20 (anodic) has therein an intermediate lower recess 27, superimposed and in contact with the intermediate upper rib 17. Thus, this structural configuration makes it possible to limit the flow of air on the cathodic side around the hydrogen collector 3, thereby reducing short-circuit flows, and improving the performance of the electrochemical cell.

[0064] In this example, the air collector 2 is bordered by a third intermediate zone similar to that described with reference to FIGS. 2A and 2B, which extends in the XY plane between the sealing line of the air collector 2 and the external sealing line. The upper conductive sheet 10 comprises an intermediate upper flat portion 13 followed by an intermediate upper rib 17. The lower conductive sheet 20 comprises an intermediate lower flat portion 23, superimposed and in contact with the intermediate upper flat portion 13 at the reference plane P re f, then an intermediate lower recess 27, superimposed and in contact with the intermediate upper rib 17.

[0065] An upper curved portion 19.1 of plane transition ensures the connection between the upper intermediate flat portion 13 and the upper intermediate rib 17. And a curved portion 29.1 ensures the connection between the lower intermediate flat portion 23 and the lower intermediate recess 27. These curved portions 19.1 and 29.1 are offset in the XY plane so as not to be in contact and thus avoid weakening the bipolar plate 1.

[0066] This configuration reduces the short-circuit flow of air from one air collector 2 to the other, at the level of the upper conductive sheet 10 (cathodic), due to the presence of the intermediate upper rib 17.

[0067] Particular embodiments have just been described. Different variants and modifications will appear to those skilled in the art.

Claims

CLAIMS 1. Bipolar plate (1) of an electrochemical cell, comprising: o an upper conductive sheet (10) and a lower conductive sheet (20), made of an electrically conductive material, and superimposed on each other; o a first collector (2) of a first reactive fluid and a second collector (3) of a second reactive fluid different from the first reactive fluid, adjacent to each other; o the upper conductive sheet (10) comprising: • an upper distribution circuit, for conveying the first reactive fluid to an upper electrode; • an upper rib (15) called the first collector, surrounding the first collector (2); • an upper rib (12) called external, surrounding at least the first and second collectors (2; 3) and the upper distribution circuit; o the lower conductive sheet (20) comprising: • a lower distribution circuit, for conveying the second reactive fluid to a lower electrode different from the upper electrode, superimposed on the upper distribution circuit; • a lower rib (25) called the first collector, surrounding the first collector (2), superimposed on the upper rib of the first collector (15); • a lower rib called external (22), surrounding at least the first and second collectors (2; 3) and the lower distribution circuit, superimposed on the upper external rib (12); o characterized in that, in a first intermediate zone extending between the upper (15) and lower (25) ribs of the first collector (2) on the one hand and the upper (12) and lower (22) external ribs on the other hand, the upper conductive sheet (10) comprises an upper recess (14) called intermediate, and the lower conductive sheet (20) comprises a lower rib (24) called intermediate, superimposed and in contact with the upper intermediate recess (14), the upper intermediate recess (14) and the lower intermediate rib (24) extending longitudinally at least partly around the first collector (2).

2. Bipolar plate (1) according to claim 1, wherein the intermediate lower rib (24) is coplanar with the lower rib (25) of the first collector (2) and with the external lower rib (22).

3. Bipolar plate (1) according to claim 1 or 2, wherein the intermediate lower rib (24) extends in a planar manner connecting the lower rib (25) of the first collector (2) with the external lower rib (22).

4. Bipolar plate (1) according to claim 1, in which the intermediate lower rib (24) projects, in a direction opposite to the upper conductive sheet (10), with respect to a plane passing through the lower rib (25) of the first collector (2) and through the external lower rib (22).

5. Bipolar plate (1) according to claim 1 to 3, comprising two first collectors (2), ensuring the supply and evacuation of the first reactive fluid, each first collector (2) being at least partly bordered by a first intermediate zone comprising the upper intermediate recess (14) and the lower intermediate rib (24).

6. Bipolar plate (1) according to claim 1 to 4, in which, in a second intermediate zone extending between the upper (15) and lower (25) ribs of the second collector (3) on the one hand and the external upper (12) and lower (22) ribs on the other hand, the upper conductive sheet (10) comprises an upper rib (17) called intermediate, and the lower conductive sheet (20) comprises a lower recess (27) called intermediate, superimposed and in contact with the upper intermediate rib (17); the upper intermediate rib (17) and the lower intermediate recess (27) extending longitudinally at least partly around the second collector (3).

7. Bipolar plate (1) according to claim 5, wherein the intermediate upper rib (17) is coplanar with the upper rib (15) of the second collector (3) and with the external upper rib (12).

8. Bipolar plate (1) according to claim 5 or 6, wherein the intermediate upper rib (17) extends in a planar manner connecting the upper rib (15) of the second collector (3) and with the external upper rib (12).

9. Bipolar plate (1) according to claim 6, in which the intermediate upper rib (17) projects, in a direction opposite to the lower conductive sheet (20), with respect to a plane passing through the upper rib (15) of the second collector (3) and through the external upper rib (12).

10. Bipolar plate (1) according to claim 5 to , comprising two second collectors (3), ensuring the supply and evacuation of the second reactive fluid, each second collector (3) being at least partly bordered by a second intermediate zone comprising the intermediate upper rib (17) and the intermediate lower recess (27).

11. Electrochemical cell, comprising at least one bipolar plate (1) according to any one of the preceding claims, and a membrane / electrode assembly in contact with the bipolar plate (1).

12. Electrochemical reactor, comprising at least one electrochemical cell according to the preceding claim.

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