Bipolar plate and fuel cell stack having channel divisions present in active area

Abstract

The invention relates to a bipolar plate (2) for a fuel cell, which forms a first reactant flow field (3) at its first plate side, which forms a second reactant flow field (3) at its second plate side, which is arranged opposite the first plate side, wherein the flow fields (3) each comprise, in an active area (10), a plurality of flow channels (9) for one of the two reactants participating in the fuel cell reaction, which are limited by bridges (7), wherein the active areas (10) are each connected on the inlet side in flow technology to an inlet-side media port (4, 5) for one of the two reactants, which extends from the first plate side to the second plate side, via a distribution area (11) which extends beyond the active area (10), and wherein the active areas (10) are each connected on the outlet side in flow technology to an outlet-side media port (4, 5) which extends from the first plate side to the second plate side, via a collection area (16) which extends beyond the active area (10). At least one of the flow fields (3, 54) forms, in its active area (10), on the inlet side, a bridge (7) which has an abutment face (12) for the adjacent layer of the fuel cell, which is larger than an abutment face (13) of the outlet-side bridge (7), which is produced by a channel division (14) of at least some of the flow channels (9) present in the active area (10). The invention also relates to a fuel cell stack (1).

Description

TECHNICAL FIELD

[0001] The invention relates to a bipolar plate for a fuel cell, which bipolar plate is formed with a first reactant flow field at a first plate side thereof and with a second reactant flow field at a second plate side thereof, which is opposite the first plate side. The flow fields each comprise a plurality of flow channels for one of the two reactants participating in the fuel cell reaction, which are limited by bridges in an active area, wherein the active area is each in flow-technical connection on the inlet side with an inlet-side media port for one of the two reactants extending from the first plate side to the second plate side via a distribution area extending beyond the active area. The active area is each in flow-technical connection on the outlet side with an outlet-side media port extending from the first plate side to the second plate side via a collection area extending beyond the active area. The invention furthermore relates to a fuel cell stack having such a bipolar plate. BACKGROUND

[0002] Fuel cell devices are used to chemically convert fuel and oxygen into water in order to generate electrical energy. For this purpose, a fuel cell comprises as a core component a so-called membrane electrode assembly (MEA), which is a composite of a proton-conducting membrane and a corresponding one of the electrodes (anode and cathode) arranged at the membrane on both sides. In addition, gas diffusion layers (GDL) can be arranged at the electrodes on the side facing away from the membrane on both sides of the membrane electrode assembly. In the operation of a fuel cell device having a plurality of fuel cells combined into a fuel cell stack, fuel, in particular hydrogen H2or a hydrogen-containing mixed gas, is supplied to the anode, where an electrochemical oxidation of H2to H+occurs with release of electrons. Via an electrolyte or membrane, which separates the reaction chambers from one another in a gas-tight manner and is electrically insulating, the transport of protons H+from the anode chamber into the cathode chamber takes place. The electrons provided at the anode are transmitted to the cathode via an electrical line. Oxygen or an oxygen-containing mixed gas is supplied to the cathode, whereby a reduction of O2to O2takes place with the reception of electrons. At the same time, these oxygen anions react in the cathode chamber with the protons transported via the membrane to form water. + + 2-

[0003] Reactant gases are supplied to the electrodes of a fuel cell by means of bipolar plates. In addition to the reactant gases, cooling media are also routed through the bipolar plates due to the heat generated in the fuel cell reaction process, so that three different media are guided through the bipolar plates in a minimum of space.

[0004] ​​​When supplying the fuel cell with reactants, these are guided via main channels (ports) into the bipolar plate, which should promote the distribution of the reactants into the active area in order to supply the entire face of the electrode as uniformly as possible by means of the flow field. Since a plurality of bipolar plates with membrane electrode units are stacked in a fuel cell stack, a seal is used which seals the main channels longitudinally through the fuel cell stack. Additionally, a good seal must be made for the cooling medium flowing in the coolant channels.

[0005] On the inlet side of the active area, in particular on the cathode side of the fuel cell, there is a relatively dry reactant flow, which can lead to drying of the inlet side region of the membrane of the fuel cell arranged adjacent to the bipolar plate. In order to compensate for the influence of the dry air at the inlet, a humidifier is therefore used in the system, which increases the humidity of the air at the cathode inlet. These humidifiers require a large volume, can reduce the power density of the fuel cell system and increase the overall costs. The further the reactant flow penetrates through the flow channels of the cathode-side reactant flow field, the more strongly the reactant flow is humidified, since the fuel cell reaction leads to the production of product water.

[0006] In order to make the flow of the reactants on the cathode side uniform, DE 10 2018 202 561 A1 proposes a bipolar plate for a fuel cell, in which the active area of the flow field is designed to divide its flow channels into two respective smaller sub-channels. A similar configuration can be known from the bipolar plate of US 2007 / 0178 359 A1, in which, however, there are two sections separated from one another in the active area with different numbers of channels in order to also make the flow of the reactants uniform there. In DE 10 2008 033 211 A1, a bipolar plate is shown, in which the flow channels are designed with a fan-shaped expansion, in which the available flow cross section of the channels continuously increases from the inlet to the outlet in order to distribute the reactants more uniformly.

[0007] Despite the solutions known to help make the flow of the respective reactants uniform, there is still a very large inflow of fresh cathode gas on the inlet side, in particular on the cathode side, so that a strong and active humidification of the cathode gas is required before it is supplied to the fuel cell stack. SUMMARY

[0008] It is therefore an object of the present application to provide a bipolar plate and a fuel cell stack with less need for pre-humidification of the reactants to make the fuel cell efficient.

[0009] This object is achieved by a bipolar plate having the features of claim 1 and a fuel cell stack having the features of claim 10. Advantageous design embodiments of the application with suitable refinements are specified in the dependent claims.

[0010] The bipolar plate according to the application is characterized in particular in that at least one of the flow fields forms a bridge on the inlet side in its active area, which bridge has an abutment surface for the adjacent layer of the fuel cell, which abutment surface is larger than the abutment surface of the outlet-side bridge, which is produced by the channel division of at least some of the flow channels present in the active area.

[0011] Since the abutment surface of the bridge limiting the flow channel occupies a larger area upstream of the channel division, a kind of "water reservoir" is created in this area due to the larger width of the bridge present there. In other words, therefore, not so much dry reactant is supplied to the adjacent layer, for example the gas diffusion layer or the membrane electrode assembly, which can lead to drying out at these sites. It is thereby possible to achieve a reduction in the humidifier volume up to the cancellation. Furthermore, with a selective design of the reactant flow field, the number of flow channels present in the active area on the inlet side is reduced, so that the distribution area still upstream of the active area can also be realized significantly more simply, i.e. with lower complexity.

[0012] It goes without saying that the term "active area" is to be understood in this connection in that the bipolar plate itself is not chemically active, but that there the electrochemical fuel cell reaction takes place adjacent to the reactant flow field.

[0013] In order to achieve a reliable channel division, it proves to be advantageous for the flow channels located downstream of the channel division to extend separated by an intermediate bridge. This intermediate bridge can also have a significantly reduced area of its bridge back, so that the abutment surface present downstream of the channel division is correspondingly designed smaller and thus ensures a reliable discharge of the product water created in the fuel cell reaction.

[0014] The reduced abutment surface on the outlet side is also achieved in that the available flow cross section of the flow channels located upstream of the channel division is the same as the available flow cross section of the flow channels located downstream of the channel division.

[0015] Preferably, each of the inlet-side flow channels has a channel division, so that after the channel division there is a doubling of the number of flow channels compared to the flow channels upstream of the channel division.

[0016] Depending on the operating conditions present within the fuel cell, it can be advantageous for only every second flow channel or only every third flow channel of a certain number of inlet-side flow channels to have a channel division.

[0017] It is possible for the channel division to not only result in a doubling of the flow channels, but also in a tripling or quadrupling. In this connection, it is therefore possible for the following advantageous design to be possible, according to which the part of the flow channels located upstream of the channel division is divided by the channel division into at least three flow channels located downstream of the channel division.

[0018] In order to achieve a uniform water discharge, it has also proven to be advantageous for the available flow cross section of the inlet-side flow channels to increase continuously up to the channel division. In this way, the inflow area of the adjacent layers is also increased during the flow, so that the degree of humidification is positively correlated with the increase or decrease of this area.

[0019] The design concept of a reduced number of inlet-side flow channels in comparison with the number of outlet-side flow channels additionally creates the advantageous possibility that in the distribution region there is a number of supply channels corresponding to the number of inlet-side flow channels. Due to the reduced number of inlet-side flow channels, the distribution region can be realized significantly more simply and thus with lower structural space requirements.

[0020] The advantages, advantageous design concepts and effects described in connection with the bipolar plate according to the application also apply to the fuel cell stack according to the application, in which there is also less need for pre-humidification.

[0021] The features and feature combinations mentioned in the description above, and the features and feature combinations mentioned in the description of the figures below and / or shown alone in the figures, can be used not only in the respective explicitly stated combinations, but also in other combinations or alone, without departing from the scope of the present application. Thus, embodiments which are not explicitly shown or explained in the figures, but which are derivable and producible from the explained embodiments by means of the individual feature combinations, are to be regarded as included and disclosed by the present application as well. BRIEF DESCRIPTION OF DRAWINGS

[0022] Further advantages, features and details of the application result from the claims, the following description of preferred embodiments and the drawings. Therein:

[0023] Figure 1 a schematic perspective view of a fuel cell stack is shown,

[0024] Figure 2 a detailed top view of a bipolar plate for a fuel cell stack according to Figure 1 is shown,

[0025] Figure 3 a detailed top view of another bipolar plate for a fuel cell stack according to Figure 1 is shown,

[0026] Figure 4 a detailed top view of yet another bipolar plate for a fuel cell stack according to Figure 1 is shown,

[0027] Figure 5 a detailed top view of yet another bipolar plate for a fuel cell stack according to Figure 1 is shown,

[0028] Figure 6 a detailed top view of yet another bipolar plate for a fuel cell stack according to Figure 1 a detailed cross-sectional view of yet another bipolar plate for a fuel cell stack according to

[0029] Figure 7 a detailed cross-sectional view of yet another bipolar plate for a fuel cell stack according to Figure 1 a detailed cross-sectional view of yet another bipolar plate for a fuel cell stack according to DETAILED DESCRIPTION

[0030] A bipolar plate 1 is shown in Figures 2 to 7 which is used in a fuel cell stack 1 shown in Figure 1 which consists of a plurality of fuel cells in series. Each of the fuel cells comprises an anode and a cathode and a proton-conducting membrane separating the anode and the cathode. The two electrodes together with the membrane form a membrane electrode assembly (in short: MEA). The membrane is formed of an ionomer, preferably a sulfonated tetrafluoroethylene polymer (PTFE) or a polymer of perfluorosulfonic acid (PFSA). Alternatively, the membrane can be formed as a sulfonated hydrocarbon membrane.

[0031] Fuel (e.g. hydrogen) is supplied to the anode via an anode chamber within the fuel cell stack. In a polymer electrolyte membrane fuel cell (PEM fuel cell), the fuel or fuel molecules are split into protons and electrons at the anode. The membrane allows protons (e.g. H + ) to penetrate, but is impermeable to electrons (e - ). At the anode the following reaction takes place: 2H2→ 4H + + 4e - (oxidation / electron release). During the passage of the protons through the membrane to the cathode, the electrons are conducted to the cathode or an energy store via an external circuit. Cathode gas (e.g. oxygen or oxygen-containing air) can be supplied to the cathode via a cathode chamber within the fuel cell stack, whereby the following reaction takes place on the cathode side: O2+ 4H + + 4e - → 2H2O (reduction / electron acceptance).

[0032] Air compressed by the compressor is supplied to the fuel cell stack 1 via the cathode fresh gas line. Additionally, the fuel cell stack 1 is connected with the cathode exhaust gas line. On the anode side, hydrogen stored in a hydrogen tank is supplied to the fuel cell stack 1 via the anode fresh gas line to provide the reactants required for the electrochemical reactions in the fuel cell. The gases are transferred at the bipolar plate 2 which has media ports 4, 5 for distributing the gases as well as the educts to the membrane or the adjacent gas diffusion layers. The flow of one of the two reactants, in particular the cathode gas, is illustrated via the stripe arrows running on the bipolar plate 2. Typically, a cooling medium is additionally guided through the bipolar plate 2 in order to remove the heat generated in the fuel cell reaction; the latter is also introduced or removed via the media ports 6.

[0033] All bipolar plates 2 shown have in common that they are formed at their first plate side with a first reactant flow field 3 and at their second plate side opposite the first plate side with a second reactant flow field 3. The flow fields 3 comprise in the active area 10 respectively a plurality of flow channels 9 for one of the two reactants participating in the fuel cell reaction, which are limited by bridges 7. The inlet-side media ports 4, 5 are in flow-technically connected at the inlet side with the active area 10 for which a distribution area 11 is used which runs outside the active area 10 with a supply channel 15. The active area 10 is in flow-technically connected at the outlet side via a collection area 16 running outside the active area 10 with the outlet-side media ports 4, 5 extending from the first plate side to the second plate side.

[0034] In order to reduce the need for pre-humidification of the reactants before they are introduced into the active area 10, the bipolar plate 2 is provided that at least one of its flow fields 3 is formed at the inlet side in its active area 10 with a bridge 7 having an abutment face 12 for the adjacent layers of the fuel cell, which is larger than an abutment face 13 of the outlet-side bridge 7, which results from a channel division 14 of at least some of the flow channels 9 present in the active area 10.

[0035] In this way, a larger area of the adjacent layers of the bipolar plate 2 is covered at the inlet side of the active area 10 by the abutment face 12 of the bridge 7, so that this area functions as an additional "moisture reservoir", since there is no fresh reactant output there via the plate side to the adjacent layers. This has advantages in particular on the cathode side, since the fuel cell reaction has H2O as a product, which leads to saturation of the cathode incoming air with increasing flow of the reactants via the reactant flow field 3.

[0036] Since the number of inlet-side flow channels 9 is less in the active area 10 than the number of outlet-side flow channels 9, it is also possible to implement the distribution area 11 between the inlet-side medium ports 4, 5 and the active area 10 significantly more simply and with lower structural space requirements, especially if there is a number of supply channels 15 in the distribution area 11 corresponding to the number of inlet-side flow channels 9. The supply channels 15 are currently represented by dashed lines, since the supply channels can also extend within the plate body of the bipolar plate 2 before they then come to the plate surface in the active area 10.

[0037] It can be seen in the figures that the flow channels 9 located downstream of the channel division 14 extend separately by the intermediate bridge 8. Furthermore, the available flow cross section of the flow channels 9 located upstream of the channel division 14 is the same as the available flow cross section of the flow channels 9 located downstream of the channel division 14, which contributes to reducing the outlet-side abutment face 13 of the bridge 7.

[0038] It is shown in Figure 2 that each of the inlet-side flow channels 9 has a channel division 14.

[0039] Figure 3 It is shown that each second flow channel 9 of a certain number of inlet-side flow channels 9 has a channel division 14.

[0040] Figure 4 It is shown that each third flow channel 9 of a certain number of inlet-side flow channels 9 has a channel division 14.

[0041] Figure 5 It is shown that the part of the flow channel 9 located upstream of the channel division 14 is divided by the channel division 14 into at least three flow channels 9 located downstream of the channel division 14. There are currently exactly three flow channels 9 located downstream of the channel division 14. These flow channels 9 located downstream of the channel division 14 are themselves separated by the intermediate bridge 8.

[0042] Figure 6 It is shown that the available flow cross section of the inlet-side flow channels 9 increases continuously up to the flow channel division 14. Conversely, the abutment face 12 also decreases in the flow of the reactants up to the reduced area of the abutment face 13 downstream of the channel division 14.

[0043] According to Figure 7The cross-sectional view of the bipolar plate 2 moreover points out the possibility that the flow channel 9 is formed on the inlet side with a channel depth which is smaller than the outlet side channel depth of the flow channel 9, because in this way a different height of the bridge 7 is produced, which leads to a different pressing force on the adjacent layer, in this case the gas diffusion layer. In this way, therefore, a stronger pressing force is exerted on the adjacent gas diffusion layer by the higher bridge 7 and the deeper flow channel 9 which goes with it, so that its porosity is lower there than where a pressing force is achieved by a lower channel depth, which thus leads to a higher porosity of the gas diffusion layer in this area.

[0044] The present bipolar plate 2 and thus the fuel cell stack 1 provided with such a bipolar plate is therefore characterized in that in the active area 10 there is a "water reservoir" on the inlet side, which leads to a lower drying of the adjacent membranes. Here, stronger drying only takes place on the outlet side of the active area 10, but this is compensated by the product water produced there.

[0045] List of reference signs

[0046] 1 fuel cell stack

[0047] 2 bipolar plate

[0048] 3 reactant flow field

[0049] 4 medium port (e.g. oxidant / cathode) / main channel

[0050] 5 medium port (e.g. fuel / anode) / main channel

[0051] 6 medium port (e.g. coolant) / main channel

[0052] 7 bridge

[0053] 8 intermediate bridge

[0054] 9 flow channel

[0055] 10 active area

[0056] 11 distribution area

[0057] 12 abutment face (inlet side)

[0058] 13 abutment face (outlet side)

[0059] 14 channel division

[0060] 15 supply channel (distribution area)

[0061] 16 collection area

[0062] 17 sealing channel

Claims

1. Bipolar plate (2) for a fuel cell, the bipolar plate being formed at its first plate side with a first reactant flow field (3), the bipolar plate being formed at its second plate side opposite the first plate side with a second reactant flow field (3), wherein the reactant flow fields (3) each comprising in an active area (10) a plurality of flow channels (9) for one of the two reactants participating in the fuel cell reaction, which are limited by bridges (7), wherein the active areas (10) are each on the inlet side fluidically connected via a distribution area (11) running outside the active area (10) with an inlet-side media port (4, 5) for one of the two reactants running from the first plate side to the second plate side, and wherein the active areas (10) are each on the outlet side fluidically connected via a collection area (16) running outside the active area (10) with an outlet-side media port (4, 5) running from the first plate side to the second plate side, characterized in that at least one of the reactant flow fields (3) is formed in its active area (10) on the inlet side with a bridge (7) having an abutment face (12) for the adjacent layer of the fuel cell, which is larger than an abutment face (13) of an outlet-side bridge (7), which is produced by a channel division (14) of at least some of the flow channels (9) present in the active area (10).

2. The bipolar plate (2) according to claim 1, characterized in that The flow channels (9) located downstream of the channel division (14) run separately by an intermediate bridge (8).

3. Bipolar plate (2) according to claim 1 or 2, characterized in that The available flow cross section of the flow channels (9) located upstream of the channel division (14) is the same as that of the flow channels (9) located downstream of the channel division (14).

4. The bipolar plate (2) according to any one of claims 1 to 2, characterized in that Each of the inlet-side flow channels (9) has a channel division (14).

5. The bipolar plate (2) according to any one of claims 1 to 2, characterized in that Each second flow channel or each third flow channel (9) of a certain number of the inlet-side flow channels (9) has a channel division (14).

6. The bipolar plate (2) according to any one of claims 1 to 2, characterized in that The part of the flow channels (9) located upstream of the channel division (14) is divided by the channel division (14) into at least three flow channels (9) located downstream of the channel division (14).

7. The bipolar plate (2) according to any one of claims 1 to 2, characterized in that The available flow cross section of the inlet-side flow channels (9) increases continuously up to the channel division (14).

8. The bipolar plate (2) according to any one of claims 1 to 2, characterized in that The flow channels (9) are formed on the inlet side with a channel depth which is smaller than the outlet-side channel depth of the flow channels (9).

9. The bipolar plate (2) according to any one of claims 1 to 2, characterized in that There is a number of supply channels (15) in the distribution area (11) corresponding to the number of inlet-side flow channels (9).

10. Fuel cell stack (1) comprising a plurality of fuel cells with a bipolar plate (2) according to any one of claims 1 to 9.

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