Fuel cell and fuel cell system

Abstract

The invention relates to a fuel cell (10) for a fuel cell system (12), comprising a first bipolar plate (14) and a second bipolar plate (16) and comprising a polymer membrane (18) arranged between the first bipolar plate (14) and the second bipolar plate (16), wherein the polymer membrane (18) is arranged between the first separator plate (20) of the first bipolar plate (14) and the second separator plate (22) of the second bipolar plate (16), wherein the first separator plate (20) and the second separator plate (22) each have a profile (24) such that a cooling channel structure (26) for a coolant (28) is provided between the first separator plate (20) and the second separator plate (22) of the respective bipolar plate (14, 16),and that a first channel structure (30) for a first fluid (32) is provided between the first separator plate (20) of the first bipolar plate (14) and the polymer membrane (18), and that a second channel structure (34) for a second fluid (36) is provided between the second separator plate (22) of the second bipolar plate (16) and the polymer membrane (18). It is proposed that the cooling channel structure (26) has a channel section (38), wherein at least two ribs (42) are provided in the channel section (38) for fluid-tightly separating at least one channel (40) from the cooling channel structure (26), in particular for separating a plurality of channels (40), wherein the at least one channel (40) is fluidically connected to the first channel structure (30) of the first separator plate (20) or to the second channel structure (34) of the second separator plate (22) of the first bipolar plate (14) or the second bipolar plate (16).

Description

The invention relates to a fuel cell having the features of independent claim 1 and to a fuel cell system having the features of independent claim 10.Nowadays, low-temperature proton exchange membrane fuel cells (PEMFC) are mainly used in mobile fields. The membranes used in this case have a strong dependence of the conductivity of the protons on the moistening of the membrane or of the media to be reacted.Currently, the humidification in the fuel cell is typically effected via an external humidifier, which uses the product water from the (mostly air-side) exhaust gas of the fuel cell in order to moisten the compressed fresh air before the inlet of the PEMFC or via an internal recirculation of the product water within the PEMFC through the membrane. For this purpose, hydrogen and air flow are usually arranged in countercurrent.In both cases, the air near the entry of the flow field of the fuel cell has a low moisture content, which can lead to a locally low water content of the membrane. A consequence of this may be a locally low conductivity of the membrane or ionomer in the electrodes and thus an unequal distribution of the current density. An unequal distribution of the current density in a PEMFC leads to increased losses and ultimately to a lower efficiency of the fuel cell.A further consequence of a locally low water content of the membrane may be a locally increased aging rate of the membrane. This is relevant in particular for applications with a high lifetime requirement, such as for the use of the PEMFC in commercial vehicles.The invention relates to a fuel cell having the features of independent claim 1 and to a fuel cell system having the features of independent claim 10. Features and details which are described in connection with the fuel cell according to the invention naturally also apply in connection with the fuel cell system according to the invention and vice versa, so that with regard to the disclosure reference is or can always be made reciprocally to the individual aspects of the invention.According to the invention, a fuel cell for a fuel cell system is provided, having a first bipolar plate and a second bipolar plate and having a polymer membrane arranged between the first bipolar plate and the second bipolar plate. In this case, the first bipolar plate and the second bipolar plate each have a first separator plate and a second separator plate arranged above the first separator plate. The polymer membrane is disposed between the first separator plate of the first bipolar plate and the second separator plate of the second bipolar plate.The first separator plate and the second separator plate each have a profile, such that a cooling channel structure for a coolant is provided between the first separator plate and the second separator plate of the respective bipolar plate. A first channel structure for a first fluid is provided between the first separator plate of the first bipolar plate and the polymer membrane, and a second channel structure for a second fluid is provided between the second separator plate of the second bipolar plate and the polymer membrane.The cooling channel structure has a channel section, wherein at least two ribs are provided in the channel section for fluid-tightly separating at least one channel from the cooling channel structure, in particular for separating a plurality of channels. Separating is understood here to mean that the channel or the channels are separated in a fluid-tight manner with respect to the cooling channel structure. The two ribs can thus be designed as a circumferential separation or sealing of the channel or channels. However, it is also conceivable for the two ribs to run parallel to one another and to be connected to one another by two further ribs in order to separate the channel from the cooling channel structure in a fluid-tight manner.In this case, the at least one channel is fluidically connected to the first channel structure of the first separator plate of the first bipolar plate. Additionally or alternatively, the at least one channel is fluidly connected to the first channel structure of the first separator plate of the second bipolar plate. Additionally or alternatively, the at least one channel is fluidly connected to the second channel structure of the second separator plate of the first bipolar plate. Additionally or alternatively, the at least one channel is fluidly connected to the second channel structure of the second separator plate of the second bipolar plate.During operation, the channel or the channels of the channel section are flowed through by the medium of the respective channel structure to which they are fluidically connected. This means that at least one channel of the channel section is fluidically connected to the first channel structure of the first separator plate and thus during operation a part of the total mass flow of the first fluid flows through. Additionally or alternatively, at least one channel of the channel section is fluidically connected to the second channel structure of the second separator plate and is thus flowed through during operation by a part of the total mass flow of the second fluid. As a result, the total mass flow of the first fluid and / or of the second fluid is divided in the region of the channel section and the divided part of the total mass flow now flows parallel to the first and / or second channel structure. In the region of the channel section of the cooling channel structure in the first and / or second channel structure, this leads to a reduction in the mass flow limited to the region of the channel section. At the same time, the cross-sectional area for the coolant is reduced parallel to the channels in the cooling channel structure and thus its speed is locally increased. The respective fluid or the separated mass flow portion of the respective fluid flows parallel to the non-separated mass flow portion of the respective fluid in a channel (or channels) running parallel to the first or second channel structure.By reducing the mass flow of the first fluid in the first channel structure or of the second fluid in the second channel structure in sections by the channel section, the moistening of the polymer membrane is optimized. Since less mass flow with a lower flow velocity is available for the reduction of polymer membrane in the region of the channel section, the consumption of the first and the second fluid can proceed in an optimum manner and at the same time the reaction products formed can wet the polymer membrane. It is advantageous here if the separator plate, via which the fluid with the lower relative humidity is introduced, has the channel section. This reduces the mass flow of the fluid which is to be enriched with a relative humidity, so that the reactants in this fluid are also reduced. In this case, the relative humidity desired for moistening the polymer membrane is achieved artificially on account of the lower amount of the fluid and the polymer membrane is moistened more quickly and efficiently.The polymer membrane is to be considered as a constituent of a membrane electrode unit (MEA).At the same time, the flow of the coolant is influenced by the separation of the channel or channels in the cooling channel structure, wherein the heat transfer is also influenced. As a result of the changed velocity, less heat is transferred at the surface of the polymer membrane on which flow takes place, so that a higher temperature gradient is available in the region after or before the channel section for removing the heat by the coolant. This leads to homogenization of the temperature distribution in the flow direction of the first fluid and / or of the second fluid over the length of the respective bipolar plate. This is advantageous in particular if the bipolar plates are bipolar plates through which flow takes place in direct current and the channel section is arranged in an inflow region of the media into a reaction region of the bipolar plates. Just at the beginning of the reaction region, the polymer membrane is dry and has to be moistened, wherein at the same time the reaction produces comparatively less heat according to the area fraction of the region. Following the channel section, significantly more heat is generated in cumulative fashion by the reactions, so that the wider distribution of the coolant is advantageous in order to remove it. The flow velocity of the first fluid and / or of the second fluid is reduced by the channel or channels, so that the polymer membrane can be prevented from drying out by an improved water absorption of the reaction gases at a higher velocity. Within the scope of the invention, it may be advantageous that the channel section has a separating region with at least one separating opening for separating a first mass flow fraction of the first fluid from the first channel structure of the first separator plate into the at least one channel of the first bipolar plate or of the second bipolar plate. Additionally or alternatively, the separating region has a separating opening for separating a first mass flow fraction of the second fluid from the second channel structure of the second separator plate into the at least one channel of the first bipolar plate or of the second bipolar plate.The separating region is to be understood here as providing a transition from a common guidance of the first fluid or of the second fluid to the respective separating opening of the respective separator plate. This can be designed correspondingly in terms of flow technology in order to prevent flow separation and turbulence at the separation openings. At the same time, the separation ports are configured to separate a predetermined mass flow compartment. In this case, the mass flow to be separated can be determined via the cross-sectional area of the separation opening.It is conceivable that the respective mass flow portion of the first and / or second fluid is fed back to the reacting mass flow portion or recirculated only after the reaction region.In the context of the invention, it is conceivable for the channel section to have a feed region with at least one feed opening for feeding the first fluid, in particular the branched-off first mass flow fraction of the first fluid, from the at least one channel into the first channel structure of the first separator plate of the first bipolar plate or of the second bipolar plate. Additionally or alternatively, the feed region has at least one feed opening for feeding the second fluid, in particular the branched-off first mass flow fraction of the second fluid, from the at least one channel into the second channel structure of the second separator plate of the first bipolar plate or of the second bipolar plate.Advantageously, the respective separated mass flow component of the first fluid is supplied to the first channel structure or of the second fluid is supplied to the second channel structure. As a result, the respective fluid is available for reaction and energy generation. At the same time, the coolant is again provided with a larger surface area for the heat transfer in the cooling duct structure. This is particularly advantageous because this larger heat transfer area is provided in the region of the bipolar plates where the reaction between the first fluid and the second fluid produces more heat at the polymer membrane.It can be provided within the scope of the invention that the first bipolar plate and / or the second bipolar plate have a pre-distributor structure, a post-distributor structure and a reaction region, wherein the cooling channel structure is provided in the reaction region, and wherein the channel section of the cooling channel structure adjoins the pre-distributor structure of the first bipolar plate and / or of the second bipolar plate.The division of the mass flow of the first fluid and / or of the second fluid directly at the inflow region of the reaction region leads to optimum moistening of the polymer membrane. In the pre-manifold structure, the first fluid and the second fluid are separately supplied to the reaction region, and are to react at the reaction region with the polymer membrane to generate energy. However, at least one of the fluids is supplied with a lower relative humidity, which is compensated or equalized only by the reaction between, for example, air and hydrogen and the water formed in the process. Since now less fluid is available for the actual reaction, at least one reactant for the reaction is locally reduced, so that the water absorption or the water removal by the reactants is slowed down and thus the moistening of the polymer membrane in the region of the channel section moistening of the polymer membrane is optimized.It is also conceivable here for the channel section of the cooling channel structure to be arranged upstream of the post-distributor structure.The positioning is dependent on whether the first and the second fluid and the coolant flow through the bipolar plates in cocurrent or countercurrent principle, wherein the first and the second fluid flow mostly in countercurrent to one another.Within the scope of the invention, it is conceivable that the feed region of the channel section has a transverse distribution structure for transversely distributing the coolant into the further cooling channel structure.The transverse distribution structure can be provided in order to distribute the coolant after the channel section to the now available wider cross-sectional area of the cooling channel structure. As a result, dead-current regions and thus regions without heat transfer are to be prevented or avoided.The transverse distribution structure can be a partial or complete lowering of the feed region, as a result of which the cross section through which flow takes place is changed locally in such a way that the direction of the coolant coming from the channel section must change in order to then be distributed transversely. The transverse distribution structure is oriented at an angle between 45° to 90°, preferably 60° to 90°, further preferably at least 80° to 90°, to the flow direction of the first and second fluids. This ensures transverse distribution of the coolant.It is also conceivable for the transverse distribution structure to consist of fins or winglets in order to deflect the flow.It is also conceivable that the cooling channel structure, following the channel section, provides winglets along its length in the flow direction in order to always remain in the region of a turbulent boundary layer, in which the heat transfer is particularly advantageous, as a result of the breaking up of the boundary layer which builds up.Within the scope of the invention, it is furthermore conceivable that a length of the channel section corresponds to 15% to 45%, preferably 20% to 40%, more preferably 25% to 30%, of a length of the cooling channel structure.The length of the channel section or the length of the cooling channel structure is understood to mean the extension of the channel section or the cooling channel structure in the flow direction of the first and second fluids. In this case, the length of the cooling duct structure also corresponds to the length of the reaction region.It is furthermore conceivable that the at least one channel or at least one channel of the plurality of channels of the channel section has a cross-sectional constriction for regulating the first mass flow fraction of the first fluid or the first mass flow fraction of the second fluid.This cross-sectional narrowing can be carried out by tapering the channel or channels or lowering the web. By means of the cross-sectional constriction, the first mass flow proportion of the first fluid or of the second fluid can be adjusted. In this case, in the case of a plurality of the channels, each or every second or every third or every fourth channel can have a cross-sectional constriction of this type, such that the flow of the first fluid in the first channel structure or of the second fluid in the second channel structure can be influenced, or that the cooling or the heat transfer can be adapted to regions with a higher temperature. This optimizes both the reactivity at the polymer membrane and the temperature distribution along the length of the entire bipolar plate.It is also conceivable that the at least one separation opening of the separation region of the channel section of the first separator plate or of the second separator plate is designed such that the branched first mass flow fraction of the first fluid or the branched first mass flow fraction of the second fluid is 10% to 70%, preferably 15% to 60%, further preferably 20% to 30%, of a total mass flow of the first fluid or of a total mass flow of the second fluid.The diverted mass flow corresponds to the part of the total mass flow which is diverted in order to flow through the channel or channels.This reduces the mass flow of the respective fluid which should be humidified and thereby optimizes the humidification of the polymer membrane. In this case, sufficient mass flow of the respective fluid is available in the channel section for the reaction of the first and second fluids at the polymer membrane in order to supply the electrochemical reaction. At the same time, the quantity of reactant is increased again by the later supply of the respective branched-off fluid, so that an optimum reaction can always take place between the fluids on the polymer membrane over the entire length of the reaction region.Within the scope of the invention, it is optionally possible for the ribs to be welded seams, in particular laser-machined welded seams.These can be produced quickly and easily by laser welding and are fluid-tight.In order to ensure the positioning of the individual separator plates to form the defined channel structures and the cooling channel structure, the separator plates are joined to form a bipolar plate and are often welded to one another. Advantageously, the welds can be used to join the first and second separator plates to simultaneously create the ribs for forming the channels.A second aspect of the invention is a fuel cell system including a plurality of fuel cells according to the first aspect of the invention. Advantages described in detail with respect to the fuel cell according to the first aspect of the invention apply equally to the fuel cell system according to the second aspect of the invention.Further advantages, features and details of the invention will become apparent from the following description, in which a plurality of exemplary embodiments of the invention are described in detail with reference to the drawings. The features mentioned in the claims and in the description can be essential to the invention individually or in any combination. The invention is shown in the following figures: FIG. 1 shows a schematic illustration of a fuel cell in a plan view of a first bipolar plate, FIG. 2 shows a schematic illustration of a fuel cell in a sectional view A-A along the longitudinal extent of the bipolar plate, FIG. 3 shows a schematic illustration of a fuel cell in a sectional view D-D along the longitudinal extent of the bipolar plate, FIG. 4 shows a schematic illustration of a fuel cell in a sectional view B-B along the width extension of the bipolar plate, FIG. 5 shows a schematic illustration of a fuel cell system.FIGS. 1, 2 and 3 show a fuel cell 10 for a fuel cell system 12. The fuel cell 10 includes a first bipolar plate 14 and a second bipolar plate 16, and a polymer membrane 18 disposed between the first bipolar plate 14 and the second bipolar plate 16. In this case, the first bipolar plate 14 and the second bipolar plate 16 each have a first separator plate 20 and a second separator plate 22 arranged above the first separator plate 20. The polymer membrane 18 is disposed between the first separator plate 20 of the first bipolar plate 14 and the second separator plate 22 of the second bipolar plate 16.For the flow of the first and second fluids 36 and the coolant 28 through the fuel cell 10, the first separator plate 20 and the second separator plate 22 each have a profile 24, such that a cooling channel structure 26 for a coolant 28 is provided between the first separator plate 20 and the second separator plate 22 of the respective bipolar plate 14, 16, and that a first channel structure 30 for a first fluid 32 is provided between the first separator plate 20 of the first bipolar plate 14 and the polymer membrane 18, and that a second channel structure 34 for a second fluid 36 is provided between the second separator plate 22 of the second bipolar plate 16 and the polymer membrane 18. This can be seen above all in the sectional views of FIGS. 2 and 3.In this case, the cooling duct structure 26 has a duct section 38, wherein at least two ribs 42 are provided in the duct section 38 for fluid-tightly separating at least one duct 40 from the cooling duct structure 26, see FIG. 3. In the present exemplary embodiment, a plurality of channels 40 are separated from the cooling channel structure 26 by two ribs 42 each. In the present case, the channels 40 are fluidly connected to the first channel structure 30 of the first separator plate 20 of the first bipolar plate 14. The ribs 42 are laser welded seams 62 here.However, it is also conceivable for the channels to be fluidically connected to the first channel structure 30 of the first separator plate 20 of the second bipolar plate 16 and / or to the second channel structure 34 of the second separator plate 22 of the first bipolar plate 14 and / or to the second channel structure 34 of the second separator plate 22 of the second bipolar plate 16.This depends on the function of the respective separator plate 20, 22 in the fuel cell 10.As can be seen in the plan view in FIG. 1 and the longitudinal section of FIG. 2, the channel section 40 has a separating region 44 having at least one separating opening 46 for separating a first mass flow fraction mp_ 1 of the first fluid 32 from the first channel structure 30 of the first separator plate 20 into the at least one channel 40 of the first bipolar plate 14 or of the second bipolar plate 16.The at least one separation opening 46 of the separation region 44 of the duct section 38 of the first separator plate 20 or of the second separator plate 22 is designed in the present case such that the branched first mass flow fraction mp_ 1 of the first fluid 32 or the branched first mass flow fraction mp_ 2 of the second fluid 26 is 10% to 70%, a total mass flow mp_ges 1 of the first fluid 32 or a total mass flow mp_ges 2 of the second fluid 26.If the channel section is provided in the second separator plate 22, it has the separating region 44 for separating a first mass flow fraction mp_ 2 of the second fluid 36 from the second channel structure 34 of the second separator plate 22 into the at least one channel 40 of the first bipolar plate 14 or of the second bipolar plate 16.In the present case, the channel section 38 has a feed region 48 having at least one feed opening 50 for feeding the first fluid, in particular the branched-off first mass flow fraction mp_ 1 of the first fluid 32, from the channels 40 into the first channel structure 30 of the first separator plate 20 of the first bipolar plate 14 and of the second bipolar plate 16.If the channel section 38 is provided in the second separator plate 22, the channel section 38 of the second separator plate 22 has the feed region 48 for feeding the second fluid, in particular the branched-off first mass flow fraction mp_ 2 of the second fluid 36, from the at least one channel 40 into the second channel structure 34 of the second separator plate 22 of the first bipolar plate 14 or of the second bipolar plate 16. In this case, the feed region 48 of the channel section 38 provides a transverse distribution structure 58 for transversely distributing the coolant 28 into the further cooling channel structure 26 and serves for the transverse distribution of the fluids, and in this case primarily the part of the first fluid which is fed back through the openings 50. This is designed here as a depression of the respective separator plate 20, 22.Each of the first bipolar plate 14 and the second bipolar plate 16 includes a pre-manifold structure 52, a post-manifold structure 54, and a reaction region 56. In this case, the cooling duct structure 26 is provided in the reaction region 56. In the illustrated embodiment, the channel portion 38 of the cooling channel structure 26 joins the pre-manifold structure 52 of the first bipolar plate 14 and the second bipolar plate 16.The channel section 38 is not provided along the entire length of the cooling channel structure 26 or of the reaction region 56. Rather, the length L 1 of the channel section 38 corresponds to 15% to 45%, preferably 20% to 40%, more preferably 25% to 30%, of the length L 2 of the cooling channel structure 26.For regulating the first mass flow fraction mp_ 1 of the first fluid 32 or the first mass flow fraction mp_ 2 of the second fluid 26 in the channels of the channel section 38, these each have a cross-sectional constriction 60.In FIG. 4, a fuel cell system 12 with a plurality of fuel cells 10 according to one of FIGS. 1 to 3 is shown.

Claims

Fuel cell (10) for a fuel cell system (12), having a first bipolar plate (14) and a second bipolar plate (16), and having a polymer membrane (18) arranged between the first bipolar plate (14) and the second bipolar plate (16), wherein the first bipolar plate (14) and the second bipolar plate (16) each have a first separator plate (20) and a second separator plate (22) arranged above the first separator plate (20), wherein the polymer membrane (18) is arranged between the first separator plate (20) of the first bipolar plate (14) and the second separator plate (22) of the second bipolar plate (16), wherein the first separator plate (20) and the second separator plate (22) each have a profile (24), such that a cooling channel structure (26) for a coolant (28) is provided between the first separator plate (20) and the second separator plate (22) of the respective bipolar plate (14, 16), and in that a first channel structure (30) for a first fluid (32) is provided between the first separator plate (20) of the first bipolar plate (14) and the polymer membrane (18), and in that a second channel structure (34) for a second fluid (36) is provided between the second separator plate (22) of the second bipolar plate (16) and the polymer membrane (18), characterized in that the cooling channel structure (26) has a channel section (38), wherein at least two ribs (42) are provided in the channel section (38) for the fluid-tight separation of at least one channel (40) from the cooling channel structure (26), in particular for the separation of a plurality of channels (40), wherein the at least one channel (40) is fluidly connected to the first channel structure (30) of the first separator plate (20) of the first bipolar plate (14) and / or wherein the at least one channel (40) is fluidly connected to the first channel structure (30) of the first separator plate (20) of the second bipolar plate (16) and / or wherein the at least one channel (40) is fluidly connected to the second channel structure (34) of the second separator plate (22) of the first bipolar plate (14) and / or wherein the at least one channel (40) is fluidly connected to the second channel structure (34) of the second separator plate (22) of the second bipolar plate (16).Fuel cell (10) according to Claim 1, characterized in that the channel section (40) has a separating region (44) having at least one separating opening (46) for separating a first mass flow fraction (mp_1) of the first fluid (32) from the first channel structure (30) of the first separator plate (20) into the at least one channel (40) of the first bipolar plate (14) or of the second bipolar plate (16) and / or for separating a first mass flow fraction (mp_2) of the second fluid (36) from the second channel structure (34) of the second separator plate (22) into the at least one channel (40) of the first bipolar plate (14) or of the second bipolar plate (16).Fuel cell (10) according to Claim 1 or 2, characterized in that the channel section (38) has a feed region (48) having at least one feed opening (50) for feeding the first fluid, in particular the branched first mass flow fraction (mp_1) of the first fluid (32), from the at least one channel (40) into the first channel structure (30) of the first separator plate (20) of the first bipolar plate (14) or of the second bipolar plate (16) and / or for feeding the second fluid, in particular the branched first mass flow fraction (mp_2) of the second fluid (36), from the at least one channel (40) into the second channel structure (34) of the second separator plate (22) of the first bipolar plate (14) or of the second bipolar plate (16).Fuel cell (10) according to one of the preceding claims, characterized in that the first bipolar plate (14) and / or the second bipolar plate (16) have a pre-distributor structure (52), a post-distributor structure (54) and a reaction region (56), wherein the cooling duct structure (26) is provided in the reaction region (56), and wherein the duct section (38) of the cooling duct structure (26) adjoins the pre-distributor structure (52) of the first bipolar plate (14) and / or second bipolar plate (16).Fuel cell (10) according to one of the preceding claims, characterized in that the feed region (48) of the duct section (38) has a transverse distribution structure (58) for transversely distributing the coolant (28) into the further cooling duct structure (26).Fuel cell (10) according to one of the preceding claims, characterized in that a length (L1) of the channel section (38) corresponds to 15% to 45%, preferably 20% to 40%, more preferably 25% to 30%, of a length (L2) of the cooling channel structure (26).Fuel cell (10) according to one of the preceding claims, characterized in that the at least one channel (40) or at least one channel (40) of the plurality of channels (40) of the channel section (38) has a cross-sectional constriction (60) for regulating the first mass flow fraction (mp_1) of the first fluid (32) or the first mass flow fraction (mp_2) of the second fluid (26).Fuel cell (10) according to one of the preceding claims, characterized in that the at least one separation opening (46) of the separation region (44) of the duct section (38) of the first separator plate (20) or of the second separator plate (22) is designed such that the branched first mass flow fraction (mp_1) of the first fluid (32) or the branched first mass flow fraction (mp_2) of the second fluid (26) is 10% to 70%, preferably 15% to 60%, further preferably 20% to 30%, of a total mass flow (mp_ges1) of the first fluid (32) or of a total mass flow (mp_ges2) of the second fluid (26).Fuel cell (10) according to one of the preceding claims, characterized in that the ribs (42) are welded seams (62), in particular laser-machined welded seams (62).A fuel cell system (12) comprising a plurality of fuel cells (10) according to any one of claims 1 to 9.

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