BIPOLAR PLATE AND FUEL CELL STACK

Description

[0001] The invention relates to a bipolar plate formed from two interconnected individual plates. Each of the individual plates has a reactant flow field for a reactant of the fuel cell reaction, with one of the flow fields being present on each of the plate surfaces that face away from each other when connected. Each reactant flow field comprises a plurality of flow channels for a reaction medium delimited by walls of webs. In an active region, the webs and flow channels of one of the individual plates lie opposite the webs and flow channels of the other of the individual plates, meaning that in the active region they are aligned with each other. In this way, the mutually facing plate surfaces of the individual plates form a coolant flow field extending between the individual plates.The invention also relates to a fuel cell stack having a plurality of fuel cells to which such bipolar plates are assigned.

[0002] Fuel cell devices are used for the chemical conversion of a fuel with oxygen to water in order to generate electrical energy. For this purpose, fuel cells contain the so-called membrane electrode assembly (MEA) as a core component. This is a composite of a proton-conducting membrane and an electrode (anode and cathode) arranged on either side of the membrane. In addition, gas diffusion layers (GDL) can be arranged on either side of the membrane electrode assembly on the sides of the electrodes facing away from the membrane. During operation of the fuel cell device with a plurality of fuel cells combined to form a fuel cell stack, the fuel, in particular hydrogen H2 or a hydrogen-containing gas mixture, is fed to the anode where an electrochemical oxidation of H2 to H+< takes place with the release of electrons.The protons H+< are transported from the anode compartment to the cathode compartment via the electrolyte or membrane, which separates the reaction chambers from each other in a gas-tight manner and electrically insulates them. The electrons provided at the anode are conducted to the cathode via an electrical conductor. Oxygen or an oxygen-containing gas mixture is supplied to the cathode, resulting in a reduction of O2 to O2-< with the absorption of electrons. At the same time, these oxygen anions react with the protons transported across the membrane in the cathode compartment to form water.

[0003] The reactant gases are fed to the fuel cell electrodes via bipolar plates. In addition to the reactant gases, a cooling medium is also passed through the bipolar plates due to the heat generated during the fuel cell reaction, so that three different media are passed through the bipolar plates in a very small space.

[0004] When supplying the fuel cells with reactants, they are fed into the bipolar plates via main channels (ports). These ports are designed to distribute the reactants into an active area, thereby ensuring that the entire surface of the electrodes is supplied as evenly as possible using a flow field. Since the fuel cell stack consists of multiple bipolar plates stacked with membrane electrode assemblies, seals are used to seal the main channels along the length of the fuel cell stack. In addition, a good seal must be provided against the cooling medium flowing in the coolant channels.

[0005] Bipolar plates with distribution areas, which are intended to equalize the reactant flows in a distribution area, can be found in the documents US 2017 / 0 214 063 A1, DE 10 2015 225 228 A1 and DE 10 2014 206 333 A1.

[0006] In particular, the routing of the cooling medium over or within the bipolar plates has so far been given less importance during the development phase than the routing of the reactant flows. As a result, conventional bipolar plates do not yet provide optimal uniform distribution of the cooling medium, so that local overheating (a so-called "hotspot") can occur due to a locally low flow rate.

[0007] DE 10 2015 214 517 A1 describes a bipolar plate which has a crossing guidance of the two reaction media in its distribution area.

[0008] DE 10 2015 200 573 A1 describes a bipolar plate which has flattened webs in the distribution area for reactant guidance so that the coolant can be distributed.

[0009] DE 10 2014 206 335 A1 describes a bipolar plate whose distribution area is realized with intersecting media flows. The anode channels are arranged offset from the cathode channels.

[0010] DE 10 2008 056 900 A1 also describes a bipolar plate formed with intersecting flow channels in the distribution area. It is described that the inflow channels of the two plates of the bipolar plate intersect in the distribution area, thus guiding the coolant through the gaps between the intersecting inflow channels.

[0011] It is therefore the object of the present invention to provide a bipolar plate and a fuel cell stack that avoid such local overheating.

[0012] This object is achieved with a bipolar plate having the features of claim 1 and by a fuel cell stack having the features of claim 7. Advantageous embodiments with expedient further developments of the invention are specified in the dependent claims.

[0013] The bipolar plate according to the invention is characterized in particular in that outside and / or in an edge region of the active region there is a lateral offset between the webs of the individual plates, such that adjacent coolant channels of the coolant flow field are fluidically connected to one another by means of passage openings for distributing a coolant flow.

[0014] In this way, the coolant is evened out or homogenized just before the active area or at the very edge of the active area, effectively preventing local overheating or hot spots. The area where the coolant channels intersect due to the lateral offset of the webs can, for example, also be a distribution area directly adjacent to the media ports. The intersection area with the coolant flow openings can also be located between the active area and a distribution area.

[0015] It has proven advantageous if the webs of one of the individual plates outside and / or in an edge area of ​​the active region are angled by a first angle relative to the webs in the active region. The first angle is selected depending on the specific design of the bipolar plate. The length of the intersections or through-openings present there also depends, in particular, on the desired design of the bipolar plate.

[0016] Additionally, it is possible for the webs of the other individual plates outside and / or in an edge area of ​​the active region to be angled at a second angle relative to the webs in the active region. This also creates a passage opening that leads to an even distribution of the coolant flow across the bipolar plate.

[0017] The first angle differs from the second angle, and both angles are different from zero. The first angle is a positive angle and the second angle is a negative angle, but both angles are equal in magnitude. Therefore, all webs are angled by the same amount to achieve the desired even distribution of the cooling medium.

[0018] The length of the angled webs and the flow channels running between them is preferably between 5 and 50 mm. This creates a uniform distribution of the coolant flow over a very narrow area, allowing the bipolar plate itself to be designed very compactly. Only then do the webs and flow channels transition into the opposing configuration in the active area.

[0019] To achieve a slight inclination of the webs and the flow channels running between them, it has proven advantageous for the angle of the webs of the first and / or second individual plate to be between 10 degrees and 45 degrees. Such an angle is sufficient to avoid hot spots across the entire length of the bipolar plate.

[0020] In order to produce an even more uniform, particularly turbulent, flow of the coolant, it has proven advantageous if the angle by which the webs of the first and / or second individual plate are angled or positioned is between 45 degrees and 80 degrees.

[0021] A compact bipolar plate design can also be achieved by fluidically connecting the flow channels defined by the angled webs to a main channel located in a corner of the individual plates. The angled webs are preferably positioned as close as possible to the active region, since the active region preferably contains the same number of cathode and anode channels due to the opposing configuration.

[0022] In addition, it has proven advantageous if the coolant channels are fluidically connected to a main coolant channel, which is arranged at the plate edge between two main channels of the reactants.

[0023] The advantages, advantageous configurations, and effects explained in connection with the bipolar plate according to the invention apply equally to the fuel cell stack according to the invention, which is equipped with a plurality of fuel cells with the bipolar plates according to the invention. This stack is also characterized by improved thermal management due to the avoidance of local overheating points.

[0024] The features and feature combinations mentioned above in the description, as well as the features and feature combinations mentioned below in the description of the figures and / or shown alone in the figures, can be used not only in the respective combinations specified, but also in other combinations or on their own, without departing from the scope of the invention. Thus, embodiments are also to be considered encompassed and disclosed by the invention that are not explicitly shown or explained in the figures, but which emerge and can be produced from the explained embodiments through separate feature combinations.

[0025] Further advantages, features, and details of the invention will become apparent from the claims, the following description of preferred embodiments, and the drawings. These show: Fig. 1 a schematic representation of a fuel cell stack comprising a plurality of fuel cells with the bipolar plates showing the main channels, Fig. 2 a schematic detailed representation of the section II-II of the bipolar plate from Figure 1 , Fig. 3 a schematic detailed view of section III-III of the bipolar plate from Figure 1 , Fig. 4 a schematic detailed plan view of the webs and flow channels of the reactant flow fields, with the channels of the first reactant shown as solid lines and the channels of the second reactant shown as dashed lines, and Fig. 5 one of the Figure 4 corresponding representation with the coolant crossing / coolant distribution resulting from the positioning of the webs / flow channels.

[0026] One in Figure 1The fuel cell stack 1 shown consists of a plurality of fuel cells 2 connected in series. Each of the fuel cells 2 comprises an anode and a cathode, as well as a proton-conductive membrane separating the anode from the cathode. The two electrodes together with the membrane form a membrane-electrode assembly 7 (MEA for short). The membrane is made of an ionomer, preferably a sulfonated tetrafluoroethylene polymer (PTFE) or a perfluorinated sulfonic acid polymer (PFSA). Alternatively, the membrane can be formed as a sulfonated hydrocarbon membrane.

[0027] Fuel (e.g., hydrogen) is supplied to the anodes via anode compartments within the fuel cell stack 1. In a polymer electrolyte membrane fuel cell (PEM fuel cell), fuel or fuel molecules are split into protons and electrons at the anode. The membrane allows protons (e.g., H +< ) to pass through, but is impermeable to electrons (e -< ). The following reaction occurs at the anode: 2H 2 → 4H +< + 4e -< (oxidation / electron release). While the protons pass through the membrane to the cathode, the electrons are conducted to the cathode or an energy storage device via an external circuit. Cathode gas (for example oxygen or oxygen-containing air) can be supplied to the cathodes via cathode chambers within the fuel cell stack 1, so that the following reaction takes place on the cathode side: O 2 + 4H +< + 4e -< → 2H 2 O (reduction / electron uptake).

[0028] Compressed air is supplied to the fuel cell stack 1 via a cathode fresh gas line by a compressor. The fuel cell stack 1 is also connected to a cathode exhaust gas line. On the anode side, hydrogen stored in a hydrogen tank is supplied to the fuel cell stack 1 via an anode fresh gas line to provide the reactants required for the electrochemical reaction in a fuel cell 2. These gases are transferred to bipolar plates 3, which have main channels 4 (ports) for distributing the gases to the membrane and for discharge. In addition, the bipolar plates have main coolant channels 5 (ports) for the passage of a cooling medium in a coolant channel 6, so that three different media are conveyed in a very small space.

[0029] In Figure 1Also shown are the main channels 4, 5 of a plurality of fuel cells 2 with bipolar plates 3 forming the fuel cell stack 1, each grouped in pairs.

[0030] A detailed section of the fuel cell stack 1 along section II-II from Figure 1 is in Figure 2shown. This section runs through the active region 13 of the fuel cell stack 1. The active region 13 of the bipolar plate 3 is of course not electrochemically active itself, but is arranged adjacent to those constituents of the fuel cell 2 in which the electrochemical fuel cell reaction takes place. It can be seen that the bipolar plate 3 in this active region 13 has the reactant flow fields on its surfaces facing away from one another, which are arranged opposite one another and thus form an opposite configuration. The reactant flow fields each have a plurality of flow channels 9 for the respective reaction medium, which flow channels are delimited by walls 11 of webs 10. The webs 10 and the flow channels 9 of one of the individual plates 8 are thus designed to run opposite the webs 10 and the flow channels 9 of the other of the individual plates 8 in this active region 13.In this way, coolant channels 6 of a coolant flow field extending between the individual plates 8 are formed.

[0031] If the coolant is not present at the same pressure or volume flow rate in the coolant channels 6, local overheating spots (so-called "hot spots") may occur. Therefore, it is necessary to distribute the coolant flow more evenly to avoid such local overheating spots.

[0032] This is where the bipolar plate 3 according to the invention comes into play, which Figure 3 in a detailed view along section III-III from Figure 1can be seen in more detail. The bipolar plate 3 provides a lateral offset between the webs 10 of the individual plates 8 outside and / or in an edge region of the active region 13. This offset is selected such that adjacent coolant channels 6 of the coolant flow field are fluidically connected to one another via through-openings 12 for distributing a coolant flow. In this way, the coolant can be evenly distributed shortly before or immediately at the edge of the active region 13 by a suitable splitting, so that a homogeneous coolant flow is present across the coolant flow field in the active region 13. Figure 3 only the offset of one of the two individual plates 8 can be seen, although the other of the two individual plates 8 can also be offset.

[0033] According to the Figure 4The webs 10 of one of the individual plates 8 outside and / or in an edge region of the active region 13 are angled relative to the webs 10 in the active region 13 by a first angle. It can also be seen that the webs 10 of the other of the individual plates 8 outside and / or in an edge region of the active region 13 are also angled relative to the webs 10 in the active region 13 by a second angle. Figure 4 According to the invention, the same angle is selected for the adjustment of the webs 10, whereby this angle is selected to be positive for one of the individual plates 8 and negative for the other of the individual plates 8. In this way, the passage openings 12 shown in Figure 3 are created in the coolant flow field located between the two individual plates 8, which lead to an equalization of the coolant across the bipolar plate 3.

[0034] Depending on the bipolar plate design, it is preferred if the angled webs 10 and the flow channels 9 extending therebetween are between 5 mm and 50 mm long before they transition into an opposing configuration in the active region 13. Preferably, all webs 10 are angled by the same amount. Depending on the design, the angle is between 10 degrees and 45 degrees, with another alternative design of the bipolar plate 3 being between 45 degrees and 80 degrees.

[0035] Naturally, the flow channels 9 defined by the angled webs 10 are each fluidically connected to one of the main channels 4, which in this case is arranged in a corner of the two rectangular individual plates 8. The coolant channels 6, which are interlaced in their intersection area 15 due to the flow openings 12, are in turn fluidically connected to the main coolant channel 5, which is located at the plate edge between two of the main channels 4.

[0036] In Figure 5Finally, it is shown again how the coolant is distributed due to the passage openings 12, which leads to a more uniform coolant distribution across the bipolar plate 3. The majority of passage openings 12 can be located in the edge region 15 of the active region 13, although the intersection region 15 can also be located outside the active region 13. Preferably, a suitable distribution region 14 is provided between the intersection region 15 and the media ports 4, 5 in order to distribute the reactants evenly across the reactant flow fields.

[0037] As a result, the present invention enables an even distribution of heat, since the coolant can be distributed evenly over the bipolar plate 3 and thus evenly over the entire fuel cell stack 1. LIST OF REFERENCE SYMBOLS:

[0038] 1Fuel cell stack 2Fuel cell 3Bipolar plate 4Main channel for reactants 5Main coolant channel for cooling medium 6Coolant channel 7Membrane electrode assembly (MEA) 8Single plate 9Flow channel 10Bridge 11Wall 12Perforation 13Active region 14Distribution region 15Edge region / intersection region (active region)

Claims

1. Bipolar plate (3) made of two individual plates (8) which are connected together and each of which is formed with a reactant flow field on the plate surfaces facing away from each other, said reactant flow field comprising multiple flow channels (9) delimited by walls (11) of webs (10) for a reaction medium, wherein the webs (10) and the flow channels (9) of one of the individual plates (8) run in an active region (13) opposite the webs (10) and the flow channels (9) of the other individual plate (8) in order to form coolant channels (6) of a coolant flow field extending between the individual plates (8), wherein outside and / or in an edge region of the active region (13) there is a lateral offset between the webs (10) of the individual plates (8) such that coolant channels (6) of the coolant flow field running adjacently there are fluid-mechanically connected by passage openings (12) for distributing a coolant flow, wherein the webs (10) of one of the individual plates (8) outside and / or in an edge region of the active region (13) run angled by a first angle relative to the webs (10) in the active region (13), and wherein the webs (10) of the other individual plate (8) outside and / or in an edge region of the active region (13) also run angled by a second angle relative to the webs (10) in the active region (13), characterised in that the first angle is a positive angle and the second angle is a negative angle, wherein the two angles are equal in magnitude.

2. Bipolar plate (3) according to claim 1, characterised in that the angled webs (10) and the flow channels (9) running between them are between 5 and 50 millimetres long before they transition into an opposite configuration in the active region (13).

3. Bipolar plate (3) according to claim 1 or 2, characterised in that the angle is from 10 degrees to 45 degrees.

4. Bipolar plate (3) according to claim 1 or 2, characterised in that the angle is between 45 degrees and 80 degrees.

5. Bipolar plate (3) according to any one of claims 1 to 4, characterised in that the flow channels (9) delimited by the angled webs (10) are each fluid-mechanically connected to a main channel (4) which is arranged in a corner of the individual plates (8).

6. Bipolar plate (3) according to claim 5, characterised in that the coolant channels (6) are fluid-mechanically connected to a main coolant channel (5) which is arranged at the plate edge between two main channels (4).

7. Fuel cell stack (1) comprising a plurality of fuel cells (2) with bipolar plates (3) according to any one of claims 1 to 6.