Bipolar plate, fuel cell stack and fuel cell system

Abstract

Bipolar plate (15) for a fuel cell stack, comprising: - at least four reactant supply ports (154, 155, 156, 157) for supplying and removing reactants, wherein a transition device (1540, 1550, 1560, 1570) is connected to each of the reactant supply ports (154, 155, 156, 1570) and is configured to supply a reactant from a reactant supply port (154, 156) into an active area (AA) or to remove a reactant from the active area (AA) to a reactant supply port (155, 157); - at least two main supply ports for coolant (158, 159), each of which has a transition device for supplying or removing coolant for cooling the active area (AA);characterized in that the bipolar plate (15) comprises a coolant opening (150) for the passage of coolant, wherein the coolant opening (150) is associated with one of the reactant supply openings (154, 155, 156, 157) and is positioned on a side of this associated reactant supply opening (154, 155, 156, 157) facing away from the transition device (1540, 1550, 1560, 1570).

Description

The invention relates to a bipolar plate for a fuel cell stack, a fuel cell stack, a fuel cell system and a motor vehicle with a fuel cell system. Fuel cells utilize the chemical reaction of a fuel with oxygen to produce water, thereby generating electrical energy. As a core component, fuel cells contain the membrane electrode assembly (MEA), which consists of an ion-conducting, usually proton-conducting, membrane and a catalytic electrode, an anode and a cathode, positioned on either side of the membrane. These cathodes typically comprise supported precious metals, especially platinum. Additionally, gas diffusion layers (GDLs) can be arranged on both sides of the MEA, on the electrodes facing away from the membrane. A fuel cell is generally formed by a stack of multiple MEAs, whose electrical voltages are additive.Bipolar plates, also called flow field or separator plates, are typically arranged between the individual membrane electrode assemblies. These plates ensure the supply of operating media (i.e., reactants) and coolant to the individual cells. Furthermore, the bipolar plates provide an electrically conductive connection to the membrane electrode assemblies. In the operation of the fuel cell, the fuel (anode operating medium), in particular hydrogen (H₂) or a hydrogen-containing gas mixture, is supplied to the anode via an open flux field on the anode side of the bipolar plate. There, electrochemical oxidation of H₂ to protons (H⁺) takes place, releasing electrons (H₂ → 2H⁺ + 2e⁻). The electrolyte or membrane, which separates the reaction compartments gas-tight and provides electrical insulation, transports the protons (either water-based or anhydrous) from the anode compartment to the cathode compartment. The electrons supplied at the anode are then transferred to the cathode via an electrical conductor. An oxidizing agent, for example, oxygen or an oxygen-containing gas mixture (e.g., air), is supplied to the cathode via an open flux field on the cathode side of the bipolar plate as the cathode operating medium. This causes the reduction of O₂ to O₂⁻, with the uptake of electrons (½ O₂ + 2e⁻ → O₂⁻).At the same time, in the cathode compartment, the oxygen anions react with the protons transported across the membrane to form water (O2-+ 2H+→ H2O). The fuel cell stack is supplied with the reactants—that is, the fuel or anode operating gas, for example, hydrogen, and the cathode operating gas (for example, air), and the coolant—via main supply channels that run the entire length of the stack and from which the operating media are fed to the individual cells via the bipolar plates. At least two such main supply channels are provided for each operating medium: one for supplying and one for discharging the respective operating medium. Figures 1 and 2 show a typical fuel cell stack 10 and a typical bipolar plate 15 of the prior art. The fuel cell stack 10 shown comprises a plurality of membrane electrode assemblies 14 and bipolar plates 15 arranged alternately at their flat sides. In total, several stacked individual cells 11 thus form the fuel cell stack 10, whereby both an individual cell 11 and the fuel cell stack 10 can generally be referred to as a fuel cell. Unipolar plates are located between the end plates 18 and an adjacent membrane electrode assembly 14. Between the bipolar plates 15 and the respective membrane electrode assemblies 14, anode and cathode compartments (not shown) are arranged, which are delimited by circumferential seals 17. Among other things, to establish the sealing function of the seals 17, the fuel cell stack 10 is compressed or pressed together in the stacking direction S by means of clamping devices 19. Figure 2 shows an exemplary prior art bipolar plate 15 in a top view. The bipolar plate 15 is divided into an active region AA and inactive regions IA. The active region AA is characterized by the fact that the fuel cell reactions take place in this region. The inactive regions IA can each be subdivided into supply regions SA and distribution regions DA. Within the supply regions SA, supply openings 154 to 159 are arranged on the bipolar plate 15, which, in the stacked state, are essentially aligned with one another and form main supply channels within the fuel cell stack 10. The fuel inlet opening 154 serves to supply the fuel or the anode operating gas, for example, hydrogen. The fuel outlet opening 155 serves to discharge the fuel after it has flowed over the active region AA. The oxidizer inlet opening 156 serves to supply the cathode operating gas or oxidizer, which is in particular oxygen or an oxygen-containing mixture, preferably air.The oxidizer outlet opening 157 serves to discharge the oxidizer after it has flowed over the active area AA. The coolant inlet opening 158 serves to supply the coolant, and the coolant outlet opening 159 serves to discharge it. The bipolar plate 15 has a cathode side 152, visible in the illustration, and an anode side 151, which is not visible. In typical designs, the bipolar plate 15 is constructed from two joined plate halves, the anode plate and the cathode plate. On the illustrated cathode side 152, reactant channels 153 are formed as open, trough-like channel structures, which connect the oxidizer inlet opening 156 with the oxidizer outlet opening 157. Only five exemplary reactant channels 153, here oxidizer channels, are shown; usually, a significantly larger number are present. Similarly, the anode side 151, not visible here, has corresponding reactant channels for the fuel, which connect the fuel inlet opening 154 with the fuel outlet opening 155. These reactant channels for the anode operating medium are also designed as open, trough-like channel structures.Inside the bipolar plate 15, particularly between the two plate halves, there are also typically enclosed coolant channels which connect the coolant inlet opening 158 with the coolant outlet opening 159. However, it has been observed that at low temperatures below the freezing point of water vapor, water condenses and then freezes in the fuel and oxidizer channels. This occurs particularly in the area of ​​the transition devices 1540, 1550, 1560, and 1570, which are located within the distributor areas DA and which guide and direct the reactants into and away from the active area AA. The transition devices thus represent critical areas for water freezing and are typically positioned inside the reactant supply openings 154, 155, 156, and 157. The situation is particularly detrimental if the transition devices include backfeed slots, where freezing is especially problematic. Frozen water or ice can block the unit cells in critical areas or transition devices, such as the backfeed slots. A negative consequence is a lack of air or an insufficient supply of reactants to the active area, especially during restarts. Various approaches have been employed, including water removal, thermal insulation, special designs for the return slots with a wicking effect, ice-resistant MEA designs, or cooling fingers. The disadvantages of these methods include low effectiveness, limited suitability for automotive systems, difficulty in applying them to small individual cells, degradation of other MEA properties, or deterioration of individual cell properties due to cell flow division. Prior art, such as DE 10 2015 015 229 A1, describes fuel cell stacks in which each coolant channel forming the flow field is flow-connected to a corresponding coolant inlet opening. Each of these inlet openings can be controlled by a valve. In other words, there is no main supply channel for the coolant, but instead individual, separate channels. Furthermore, US patent 2007 / 0042258 A1 states that the retention of water in the reactant channels can be reduced by forming a bypass through one or more inactive single cells in the area of ​​the end plates. WO 2008 / 110906 A2 describes a fuel cell or fuel cell stack in which temperature differences between the active area and the areas around the gas outlet openings (especially those for the cathode gas) are to be reduced. For this purpose, a thermally conductive element with a higher thermal conductivity than that of the bipolar plate is arranged so that, in the stack direction, it overlaps areas outside or at the edge of the active area and / or around the reactant outlet openings, thereby selectively introducing heat into colder zones. Additionally or alternatively, coolant flow paths in the bipolar plate are designed to cool or temper not only the active area but also the edge zones and regions around the reactant outlets. The object of the present invention is to provide a bipolar plate, a fuel cell stack and a fuel cell system in which the negative effect of water freezing is reduced. This problem is solved by a bipolar plate having the features of claim 1, by a fuel cell stack according to claim 5, by a fuel cell system according to claim 7 and a motor vehicle according to claim 10. The bipolar plate according to the invention for a fuel cell stack comprises at least four reactant supply ports for supplying and removing reactants, wherein a transition device is connected to each of the reactant supply ports and is configured to introduce a reactant from a reactant supply port into an active region or to introduce a reactant from the active region to a reactant supply port. Furthermore, the bipolar plate comprises at least two main supply ports for coolant, each of which has a transition device for supplying or removing coolant to cool the active region. The bipolar plate also comprises a coolant port for introducing coolant, wherein the coolant port is associated with one of the reactant supply ports and is positioned on a side of the associated reactant supply port facing away from the transition device.A transition device can include a backfeed slot and / or further narrow, channel-like structures for distribution. These are located within the distribution area of ​​the bipolar plate. The transition device is typically positioned on the inside of the reactant supply port, although the invention is not limited to this. A reactant here and in the following comprises an oxidizing agent, preferably oxygen or an oxygen-containing mixture, and a fuel, preferably hydrogen. The coolant port can be located at an oxidation inlet port, an oxidation outlet port, or a fuel outlet or fuel inlet port. In other words, "the side facing away from" also refers to the outside of the reactant supply port with respect to the transition device or the active area. The additional coolant opening advantageously allows for cooling of the side of the reactant supply opening facing away from the transition device. Rapid cooling, especially when the main coolant supply opening is shut off, can cause residual gaseous water to condense and then freeze. However, with the coolant opening, this occurs on the non-critical side of the reactant supply opening, i.e., the side facing away from the transition device. This results in preferential condensation and freezing on the outside of the reactant supply opening, preventing or at least reducing freezing in the critical internal areas. The critical freezing point can thus be spatially shifted by the coolant opening. In particular, such an additional coolant opening can be easily integrated into an existing design at low cost.The risk to the plate and the stack is minimized by shifting the water to a non-critical area. Furthermore, the invention allows operation in wet conditions, which is also advantageous for the performance of the MEA. Moreover, effective startup is still possible even in freezing conditions. In a preferred embodiment, the bipolar plate comprises at least two coolant openings, each of which is associated with a reactant supply opening, and each coolant opening is positioned on the side of the associated reactant supply opening facing away from the transition device. This advantageously protects two or more transition devices from ice formation in the critical area. Any combination of the four reactant supply openings is possible. Preferably, a first coolant opening is assigned to a reactant supply opening for introducing a reactant into the active area, and a second coolant opening is assigned to a reactant supply opening for introducing the same reactant out. This allows for the protection of a specific reactant supply opening or its transition device, which is particularly critical, for example, with regard to water and ice formation. In a preferred embodiment, at least one coolant opening is shaped in cross-section such that each of the at least one coolant opening for the passage of coolant is associated with two reactant supply openings and is positioned on a side of the reactant supply openings facing away from the transition devices of these two reactant supply openings. This allows transition devices of two reactants to be protected with only one coolant opening. The fuel cell stack according to the invention comprises a stack of membrane electrode assemblies and bipolar plates arranged between two end plates according to one of the above embodiments. At least four main supply channels for the supply and removal of reactants are formed through the stacked reactant supply openings, which penetrate the stack in one stacking direction. At least two main supply channels for the supply and removal of coolants are formed through the stacked main supply openings for coolants, which penetrate the stack in the stacking direction. Furthermore, one or more coolant channels are formed through the one or more stacked coolant openings, which penetrate the stack in the stacking direction. The fuel cell stack has the advantage that the additional coolant channel allows condensation and icing to occur on the non-critical side facing away from the transition devices.Rapid cooling, especially when the main coolant supply channels are switched off, can lead to ice formation in these non-critical areas, so that the critical, opposite areas of the transition facilities do not ice up, or at least ice up less. In a preferred embodiment, where at least two coolant channels are formed, one of the at least two coolant channels can be fluidically connected to another of the at least two coolant channels, the connection being formed via a polar plate, in particular a bipolar plate or a unipolar plate, of the fuel cell stack. Unipolar plates typically terminate a stack and are therefore particularly suitable as a return connection. The advantage here is that the coolant does not have to be routed out of the fuel cell stack, but rather bypasses the stack. The fuel cell system according to the invention further comprises a fuel cell with a fuel cell stack according to one of the above embodiments, wherein the fuel cell stack comprises a main coolant inlet, to which the main supply channel for supplying coolant is connected, and a further coolant inlet, to which the coolant channel is connected. The fuel cell system further comprises a coolant supply, which is fluidically connected to the main coolant inlet via a main coolant supply path and to the coolant inlet via a coolant supply path. The coolant supply can also be a cooler, which cools heated coolant and functions as a chiller. The two inlets can thus be controlled separately. In a preferred embodiment, the two coolant supply paths are fluidically connected by a blocking device controllable via a control unit. The blocking device is fluidly connected to the coolant supply via a supply path, and the control unit is configured to close the main coolant supply path and open the coolant supply path by means of the blocking device when the fuel cell is switched off. This allows any water vapor present in the non-critical area, i.e., away from the transition device, to condense through the coolant channel and freeze in this non-critical area. The opening can occur during the shutdown process or after the shutdown is complete. In a preferred embodiment, the control unit is signal-connected to a temperature sensor, and the control unit is configured to close the main coolant supply path and open the coolant supply path by means of the blocking device when the fuel cell is switched off and a temperature determined by the temperature sensor is equal to or less than the freezing point of water. This more precisely specifies the conditions under which coolant should be circulated through the coolant channel to avoid unnecessary coolant consumption. The temperature is preferably an ambient temperature outside the fuel cell. Furthermore, a motor vehicle with a fuel cell system will be provided according to one of the above specifications. Further advantages, features, and details of the invention will become apparent from the claims, the following description of preferred embodiments, and the drawings. Figure 1 schematically shows a fuel cell stack according to the prior art; Figure 2 shows a bipolar plate according to the prior art; Figure 3 shows a bipolar plate according to a first embodiment of the invention; Figure 4 shows a bipolar plate according to a second embodiment of the invention; Figure 5 shows a bipolar plate according to a third embodiment of the invention; and Figure 6 shows a fuel cell stack and a fuel cell system according to an exemplary embodiment of the invention. For an explanation of the fuel cell stack 10 and the bipolar plate 15 shown in Fig. 1 and Fig. 2 according to the prior art, reference is made to the introductory description. Figure 3 shows a bipolar plate 15 according to a first embodiment of the invention. In addition to the following explanation of the bipolar plate 15 according to the invention, reference is made to Figure 2 of the prior art described in the introduction to the description for those features which are also present in the prior art and are not described separately again to avoid repetition. In this exemplary embodiment, the bipolar plate 15 comprises four reactant supply openings 154, 155, 156, 157 for supplying and removing reactants. A corresponding transition device 1540, 1550, 1560, 1570 is connected to each of the reactant supply openings 154, 155, 156, 1570. This transition device is preferably located on the inside of the reactant supply openings 154, 155, 156, 1570 as shown in the figure, although the invention is not limited thereto. The transition devices 1540, 1550, 1560, 1570 can include so-called backfeed slots or at least form narrow channel structures to enable suitable transfer of the reactants into the active region AA. Even if the transitional facilities are only shown schematically, it is clear to the person skilled in the art that narrow transitional structures are formed on the side to the active area AA, in the sketched area. The transition devices 1540, 1560, located in the distribution area DA, are configured to introduce a reactant from a reactant supply opening 154, 156 into the active area AA of the bipolar plate 15. By way of example, the transition devices 1550, 1570, located opposite the active area AA, are configured to introduce the reactant from the active area AA to a reactant supply opening 155, 157. These transition devices may also include corresponding backfeed slots. Fuels such as hydrogen and oxidizing agents such as oxygen, air, or other oxygen mixtures can be used as reactants, although the invention is not limited to these. Furthermore, two main supply openings for coolant 158, 159 are provided. These also each feature, by way of example, a transition device for supplying or removing coolant for cooling the active area AA. The bipolar plate 15 further comprises a coolant opening 150 for the passage of coolant. The coolant opening 150 is associated with one of the reactant supply openings 154, 155, 156, 157, for example, a fuel supply opening 154. Furthermore, the coolant opening 150 is positioned on the side of this associated reactant supply opening 154 facing away from the transition device 1540. In alternative configurations, the coolant inlet 150 can be positioned at the other reactant supply inlets 155, 156, 157. This can be determined, for example, based on which of the transition devices exhibits particularly critical iron formation. Here and in the following, the cross-sectional shape of the coolant opening 150 is by no means limited to a rectangle, but can, for example, also be circular, a rounded rectangle or another suitable cross-sectional shape known to those skilled in the art. The additional coolant opening 150 allows cooling of the side facing away from the transition device. This side is not critical with regard to icing. Rapid cooling, for example when the main coolant supply opening is switched off, can lead to ice formation in these non-critical areas. This prevents or at least reduces icing in the transition areas. The invention thus aims to shift icing to the non-critical areas facing away from the respective transition device. Fig. 4 shows a bipolar plate 15 according to a second embodiment of the invention. In this exemplary embodiment, two coolant openings 150-1, 150-2 are provided. Each of the two coolant openings 150-1, 150-2 is assigned to a reactant supply opening 154, 155, 156, 157, here, for illustrative purposes, the fuel supply opening 154 and the fuel discharge opening 155. Furthermore, in this exemplary embodiment, each coolant opening 150-1, 150-2 is positioned on the side of the assigned reactant supply opening 154, 155, 156, 157 facing away from the transition device 1540, 1550, 1560, 1570. This allows the two transition devices and their critical areas to be protected from ice formation. As shown by way of example in Fig. 4, a first coolant opening 150-1 can be assigned to a reactant supply opening for introducing a reactant 154, 156 into the active region AA, and simultaneously a second coolant opening 150-2 can be assigned to a reactant supply opening for introducing the same reactant 155, 157. Ice formation can depend on the respective reactant and the water formed there. For example, more water may be formed in fuel transition areas than in oxidant transition areas. Thus, reactant-specific transition areas can be protected from ice formation. This can, for example, apply only to the oxidant, or, as in the present case, only to the transition areas 1540, 1550 of the fuel inlet openings or outlet openings 154, 155. Figure 5 shows a bipolar plate 15 according to a third embodiment of the invention. In this embodiment, a coolant opening 150-1, 150-2 is shaped in cross-section such that each of the at least one coolant opening 150-1, 150-2 is associated with two reactant supply openings 154, 155, 156, 157 for the passage of coolant. Furthermore, each coolant opening 150-1, 150-2 is positioned on a side of the two reactant supply openings 154, 155, 156, 157 facing away from the transition devices 1540, 1550, 1560, 157 of these two reactant supply openings 154, 155, 156, 157.In other words, a part of such coolant opening 150-1 is always positioned on a side of a first reactant supply opening 156 facing away from a first transition device 1560, and another part of the same coolant openings 150-1 is positioned on a side of a second reactant supply opening 154 facing away from a second transition device 1540. Here, by way of example, two coolant openings 150-1 and 150-2 are deformed accordingly, so that both the transition device 1560 of the oxidizer inlet opening 156 and the transition device 1540 of the fuel inlet opening 154, on the one hand, and the transition device 1570 of the oxidizer outlet opening 157 and the transition device 1550 of the fuel outlet opening 155, on the other hand, are protected from ice formation in the critical areas. This is achieved by only two additional coolant openings 150-1 and 150-2. In other embodiments, only one such deformed coolant opening 150-1 may be provided. It is clear to those skilled in the art that a membrane electrode arrangement 14 can also have corresponding openings at the same positions as shown for the present bipolar plates 15. Fig. 6 shows a fuel cell stack 10 and a fuel cell system 100 according to the invention. First, the fuel cell stack 10 according to the invention will be described in more detail. The fuel cell stack 10 comprises a stack of membrane electrode assemblies 14 and bipolar plates 15 arranged between two end plates 18. The lower end plate is not explicitly shown here for the sake of clarity. The bipolar plates 15 are designed, for example, according to one of the preceding embodiments of the invention. The stacking of the bipolar plates 15 results in four main supply channels for the supply and removal of reactants through the stacked reactant supply openings, which penetrate the stack in a stacking direction S. These main supply channels are not shown in this cross-sectional view of the fuel cell stack 10 for clarity. Furthermore, the stacking creates two main supply channels 102, 103 for the supply and discharge of coolants through the stacked main supply openings for coolants 158, 159, which also extend through the stack in the stacking direction S. These main supply channels 102, 103 are indicated in the figure by corresponding arrows for illustrative purposes only. From these main supply channels 102, 103 extending in the stacking direction S, the individual cells, in particular the bipolar plates 15 with the reactants, are guided into and away from the active region via the transition devices; see the previous figures relating to the bipolar plates 15 according to the invention. In this embodiment, the fuel cell stack 10 further comprises two additional coolant channels 105, 106, which are formed by the stacked, additional coolant openings 150. The coolant channels 105, 106 also extend through the stack in the stacking direction S. Viewed in the stacking direction S, these channels are located away from the transition devices and on the outside of the main supply channels 102, 103. Thus, the coolant channels 105, 106 face the non-critical area of ​​the main supply channels 102, 103. In this preferred embodiment, the two coolant channels 105, 106 are interconnected. As shown in this embodiment, the connection can preferably be made via a polar plate 108, which is, for example, part of the fuel cell stack 10 itself. For example, the polar plate 108 can be, in particular, a bipolar plate or a unipolar plate. Typically, unipolar plates are positioned at the end plates of the fuel cell stack 10. Therefore, with such a unipolar plate, the coolant channel 105, 106 can advantageously extend through the entire stack in the stack direction S, so that a maximum number of transition devices are protected from freezing by preferential ice formation in the non-critical area of ​​the main supply channel 102, 103. The illustrated rerouting of the coolant in the connected coolant channels 105, 106 around the fuel cell stack 10 thus forms a bypass. The coolant channels 105, 106 can also be designed, for example, with a deformed cross-section as shown in Fig. 5. In other alternative embodiments, for example with only one coolant channel 106, but also with two or more coolant channels 105, 106, a coolant outlet can be formed at the lower end of the fuel cell stack 10 of the respective coolant channel 105, 106. From such a coolant outlet, the coolant can then, for example, be transferred into a coolant circuit. A fuel cell system 100 is described below with respect to its coolant supply in a preferred embodiment. The fuel cell system 100 comprises the fuel cell stack 10 described above in preferred embodiments, the invention being not limited thereto. The fuel cell stack 10 further comprises a main coolant inlet 182, to which the main supply channel 102 for supplying coolant is connected. This is preferably incorporated, as can be seen in Fig. 5, in an end plate 18 of the fuel cell stack 10. The fuel cell stack 10 also comprises a further coolant inlet 186, to which the coolant channel 106 is connected. In alternative embodiments, several such coolant inlets 186 can also be provided. Furthermore, the fuel cell system 100 comprises a coolant supply 20, which is fluid-connected to the main coolant inlet 182 via a main coolant supply path 25-1 and to the coolant inlet 186 via a coolant supply path 25-2. In embodiments with multiple coolant inlets 186, several separate coolant supply paths 25-2 or fluid-connected coolant supply paths 25-2 can be provided accordingly. In this preferred embodiment, the two coolant supply paths 25-1, 25-2 are fluidically connected to a blocking device 30, which can be controlled via a control unit 40. The blocking device 30 is fluidically connected to the coolant supply 20 via a supply path 25. The locking device 30 can be a tap or a suitably appropriate valve. In this exemplary embodiment, the control unit 40 is configured to close the main coolant supply path 25-1 and open the coolant supply path 25-2 by means of the locking device 30 when the fuel cell is switched off. "Switched off" here means, for example, that a fuel cell in use, such as in a motor vehicle, is switched off because the motor vehicle is parked, although the invention is not limited to such a shutdown scenario. In particular, the control unit 40 can be signal-connected to a temperature sensor 50. The control unit 40 can then be configured to close the main coolant supply path 25-1 and open the coolant supply path 25-2 by means of the locking device 30 when the fuel cell is switched off and, in addition, an outside temperature determined by the temperature sensor 50 is equal to or less than the freezing point of water. Furthermore, in this exemplary embodiment, a main coolant outlet 181 and a coolant outlet 185 are formed in the end plate 18 of the fuel cell stack 10. These are fluidically connected to the coolant supply 20, thus creating a coolant circuit. The coolant supply 20 can accordingly also be a cooler that cools incoming coolant and returns the cooled coolant. At temperatures below or near freezing, the described ice blockages can occur at the transition devices. This typically happens after the system is switched off and cools down. The present invention makes it possible to condense and freeze the water gas in the channels at non-critical points. These non-critical points are those sides of the main supply channels that face away from the transition devices. The coolant channel according to the invention is located precisely there. This can preferably be coupled to the switching off or shutdown of the fuel cell and also to corresponding temperatures. In particular, such an additional coolant opening and coolant channel in the stack can be easily integrated into an existing design. This can be done at low cost. The risk to the plate and the stack is minimized by the described displacement of the water into the non-critical area. Furthermore, the invention allows operation in wet conditions, which is also advantageous for the performance of the MEA. Effective startup is also possible even in freezing conditions. The fuel cell system 100 can also be part of a motor vehicle not shown, in particular an electric vehicle, which has an electric traction motor that is supplied with electrical energy by the fuel cell stack 10. Reference symbol list AA Active Area IA Inactive Area DA Distribution Area SA Supply Area 10 Fuel Cell Stack 102 Main Supply Channel Inlet 103 Main Supply Channel Outlet 105 Coolant Channel Inlet 106 Coolant Channel Outlet 108 Polar Plate 14 MEA / Membrane Electrode Assembly 15 Bipolar Plate 150 Coolant Port 150-1 First Coolant Port 150-2 Second Coolant Port 151 Anode Side 152 Cathode Side 153 Reactant Channel / Oxidant Channel 154 Reactant Supply Port / Fuel Inlet Port 155 Reactant Supply Port / Fuel Outlet Port 156 Reactant Supply Port / Oxidant Inlet Port 157 Reactant Supply Port / Oxidant Outlet Port 158 ​​Main Coolant Supply Port / Coolant Inlet Port 159 Main Coolant Supply Port / Coolant Outlet Port 1540 Transition Device 1550 Transition device 1560 Transition device 1570 Transition device 158 Main coolant inlet 159 OutletMain coolant 17 Seal 18 End plate 182 Main coolant inlet 186 Coolant inlet 19 Clamping device 100 Fuel cell system 20 Coolant supply 25 Supply path 25-1 Main coolant supply path 25-2 Coolant supply path 30 Locking device 40 Control unit 50 Temperature sensor S Stack direction

Claims

Bipolar plate (15) for a fuel cell stack, comprising: - at least four reactant supply ports (154, 155, 156, 157) for supplying and removing reactants, wherein a transition device (1540, 1550, 1560, 1570) is connected to each of the reactant supply ports (154, 155, 156, 1570) and is configured to supply a reactant from a reactant supply port (154, 156) into an active area (AA) or to remove a reactant from the active area (AA) to a reactant supply port (155, 157); - at least two main supply ports for coolant (158, 159), each of which has a transition device for supplying or removing coolant for cooling the active area (AA);characterized in that the bipolar plate (15) comprises a coolant opening (150) for the passage of coolant, wherein the coolant opening (150) is associated with one of the reactant supply openings (154, 155, 156, 157) and is positioned on a side of this associated reactant supply opening (154, 155, 156, 157) facing away from the transition device (1540, 1550, 1560, 1570). Bipolar plate (15) according to claim 1, characterized in that the bipolar plate (15) comprises at least two coolant openings (150-1, 150-2), wherein each of the at least two coolant openings (150-1, 150-2) is associated with a reactant supply opening (154, 155, 156, 157), and wherein each coolant opening (150-1, 150-2) is positioned on a side of the associated reactant supply opening (154, 155, 156, 157) facing away from the transition device (1540, 1550, 1560, 1570). Bipolar plate (15) according to claim 2, characterized in that a first coolant opening (150-1) is associated with a reactant supply opening for introducing a reactant (154, 156) into the active area (AA), and a second coolant opening (150-2) is associated with a reactant supply opening for introducing the same reactant (155, 157) out. Bipolar plate (15) according to one of claims 1 to 3, characterized in that at least one coolant opening (150, 150-1, 150-2) is shaped in cross-section such that each of the at least one coolant opening (150, 150-1, 150-2) for the passage of coolant is associated with two reactant supply openings (154, 155, 156, 157) and is positioned on a side of the two reactant supply openings (154, 155, 156, 157) facing away from the transition devices (1540, 1550, 1560, 1570) of these two reactant supply openings (154, 155, 156, 157). Fuel cell stack (10), characterized in that the fuel cell stack (10) comprises a stack of membrane electrode assemblies (14) and bipolar plates (15) arranged between two end plates (18) according to one of the preceding claims 1 to 4, wherein: - at least four main supply channels for supplying and removing reactants are formed through the stacked reactant supply openings (154, 155, 156, 157), which penetrate the stack in a stacking direction (S); - at least two main supply channels (102, 103) for supplying and removing coolants are formed through the stacked main supply openings for coolants (158, 159), which penetrate the stack in the stacking direction (S); and- wherein one or more coolant channels (105, 106) are formed by the one or more stacked coolant openings (150, 150-1, 150-2) which penetrate the stack in the stacking direction (S). Fuel cell stack (10) according to claim 5, characterized in that, if at least two coolant channels (105, 106) are formed, one coolant channel (105) of the at least two coolant channels (105, 106) is flow-connected to another of the at least two coolant channels (105, 106), wherein the connection is formed via a polar plate (108), in particular a bipolar plate or a unipolar plate, of the fuel cell stack (10). Fuel cell system (100), comprising: - a fuel cell with a fuel cell stack (10) according to any one of the preceding claims 5 to 6, wherein the fuel cell stack (10) comprises a main coolant inlet (182) to which the main supply channel (102) for supplying coolant is connected, and a further coolant inlet (186) to which the coolant channel (106) is connected; - a coolant supply (20) which is fluidically connected to the main coolant inlet (182) via a main coolant supply path (25-1) and to the coolant inlet (186) via a coolant supply path (25-2). Fuel cell system (100) according to claim 7, characterized in that the two coolant supply paths (25-1, 25-2) are fluidically connected to a blocking means (30) controllable via a control unit (40), wherein the blocking means (30) is fluidly connected to the coolant supply (20) via a supply path (25), and wherein the control unit (40) is configured to close the main coolant supply path (25-1) and open the coolant supply path (25-2) by means of the blocking means (30) when the fuel cell is switched off. Fuel cell system (100) according to claim 8, wherein the control unit (40) is signal-connected to a temperature sensor (50), and wherein the control unit (40) is configured to close the main coolant supply path (25-1) and open the coolant supply path (25-2) by means of the locking means (30) when the fuel cell is switched off and a temperature determined by the temperature sensor (50) is equal to or less than the freezing temperature of water. Motor vehicle comprising a fuel cell system (100) according to any one of the preceding claims 7 to 9.

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