Fuel cell stack and fuel cell system

Abstract

A fuel cell stack (10) comprising a plurality of alternately stacked bipolar plates (16) and membrane-electrode assemblies (14), each comprising a membrane (152), two catalytic electrodes (143) adjoining it on both sides, and a supporting and / or sealing layer (150) surrounding the membrane (152) in a frame-like manner and forming edge regions, wherein supply openings (144-149) are provided for the supply and removal of operating media, and wherein an edge (151) of the membrane-electrode assembly (14), which runs along its circumference and / or along its supply openings (144-149), has a cross-sectional profile that is flattened on one side or on both sides, characterized in that the flattened edge (151) of the membrane-electrode assembly (14) has an outwardly tapered profile and extends at least partially between two bipolar plates (16),and wherein the membrane electrode assembly (14) protrudes from the fuel cell stack (10) relative to the bipolar plates (16) or the membrane electrode assembly (14) recedes relative to the bipolar plates (16) such that the bipolar plates (16) protrude from the fuel cell stack (10) relative to the membrane electrode assemblies (14).

Description

[0001] The invention relates to a fuel cell stack comprising a plurality of alternately stacked bipolar plates and a membrane-electrode assembly, each comprising a membrane, two catalytic electrodes adjoining it on both sides, and a supporting and / or sealing layer surrounding the membrane in a frame-like manner and forming edge regions, wherein supply openings are provided for the supply and removal of operating media, characterized in that an edge of the membrane-electrode assembly, which runs along its circumference and / or along its supply openings, has a profile in cross-section that is flattened on one side or both sides. The invention further relates to a fuel cell system comprising such a fuel cell stack.

[0002] Fuel cells use the chemical conversion of a fuel with oxygen to form water to generate electrical energy. For this purpose, fuel cells contain the membrane electrode assembly (MEA) as their core component. This assembly consists of an ion-conducting (usually proton-conducting) membrane and a catalytic electrode (anode and cathode) arranged on either side of the membrane. The latter usually comprise supported precious metals, particularly platinum. Gas diffusion layers (GDLs) can also be arranged on either side of the membrane electrode assembly, on the sides of the electrodes facing away from the membrane. Typically, the fuel cell is formed by a plurality of MEAs arranged in a stack, whose electrical outputs add up.Bipolar plates (also called flow field or separator plates) are usually arranged between the individual membrane-electrode assemblies. These plates ensure the supply of the operating media, i.e., the reactants, to the individual cells and usually also serve for cooling. The bipolar plates also ensure electrically conductive contact with the membrane-electrode assemblies.

[0003] During operation of the fuel cell, the fuel (anode operating medium), in particular hydrogen H2 or a hydrogen-containing gas mixture, is fed to the anode via an anode-side open flow field of the bipolar plate, where an electrochemical oxidation of H2 to protons H + with the release of electrons (H2 →2 H + + 2 e -). Via the electrolyte or the membrane, which separates the reaction chambers from each other in a gas-tight manner and electrically insulates them, a (water-bound or water-free) transport of the protons from the anode chamber to the cathode chamber takes place. The electrons provided at the anode are conducted to the cathode via an electrical line. Oxygen or an oxygen-containing gas mixture (e.g., air) is supplied to the cathode via a cathode-side open flow field of the bipolar plate, so that a reduction of O2 to 2 O2- takes place with absorption of the electrons (½ O2 + 2 e - → O 2- ). At the same time, the oxygen anions in the cathode compartment react with the protons transported across the membrane to form water (O 2- + 2 H + → H2O).

[0004] The fuel cell stack is supplied with its operating media—the anode operating gas (e.g., hydrogen), the cathode operating gas (e.g., air), and the coolant—via main supply channels formed by corresponding supply openings in the membrane electrode assemblies and bipolar plates. These channels extend throughout the stack. The operating media are fed from the main supply channels to the individual cells via the bipolar plates. At least two such main supply channels are provided for each operating medium: one for supplying the respective operating medium and one for discharging the respective operating medium.

[0005] US 2009 / 0004543 A1 and DE 197 13 250 A1 show membrane electrode assemblies in which a frame-like support and sealing layer has an edge which has a profile which is flattened on both sides in cross-section or is rounded.

[0006] US 7070 876 B2 discloses a membrane electrode assembly with an improved integrated seal, wherein the seal comprises an inner pad attached to the edge of the electrodes, a flexible connection adjacent to the pad, and a sealing element adjacent to the connection. The sealing element is significantly thicker than the pad, and the flexible connection insulates the pad from the stress occurring in the sealing element. In this way, greater compression can be exerted on the sealing element, resulting in an improved and more reliable seal without excessive compression and damage to the attached pad. Within a fuel cell stack, the individual cells must be electrically insulated from one another to prevent electrical short circuits or arcing. However, as fuel cell current densities continue to rise, the cell heights of the individual cells are becoming increasingly smaller.However, this reduces the gap between two individual cells, especially between two adjacent bipolar plates, which shortens the creepage distance and thus increases the risk of an electrical short circuit or arcing.

[0007] A known measure to counteract this problem is to slightly increase the plate sizes of the membrane electrode assemblies compared to those of the bipolar plates, so that the edge regions of the membrane electrode assemblies extend slightly beyond the edges of the bipolar plates. This increases the creepage distance. The disadvantage of this solution, however, is that increasing the size of the MEA results in an increase in the overall volume of the stack. Thus, there are limits to the extent of the MEA overhang relative to the bipolar plates.

[0008] The present invention is therefore based on the object of enabling improved internal electrical insulation of the individual cells of a fuel cell stack without having to accept an excessive increase in the installation space of the stack.

[0009] This object is achieved by a fuel cell stack and a fuel cell system having the features of the independent claims.

[0010] The membrane electrode assembly comprises an ion-conductive, in particular proton-conductive, membrane and two catalytic electrodes connected to it on both sides. The MEA further comprises a support and / or sealing layer surrounding the membrane in a frame-like manner and forming an edge region. Furthermore, supply openings for the supply and removal of operating media are formed in the MEA. The MEA is characterized in that an edge, which runs along the circumference of the membrane electrode assembly and / or along its supply openings, has a profile that is flattened on one or both sides in cross-section.

[0011] Because the edge profile does not have a rectangular profile, unlike in the prior art, it is possible to extend the creepage distance that electrical charge carriers have to travel between two adjacent bipolar plates. In this way, the electrical insulation of the individual cells within the fuel cell stack is improved. The fuel cell stack according to the invention is characterized by improved internal electrical insulation of the individual cells with a comparatively small installation volume. In the invention, the flattened edges of the membrane-electrode assemblies extend at least partially between two adjacent bipolar plates. In other words, the flattened edge is at least partially concealed by the bipolar plate when viewed from above.In this way, a gap or undercut is formed between the flattened edge and an adjacent bipolar plate, in which there is no electrical contact between the two components.

[0012] In a preferred embodiment of the invention, the flattened profile is formed by the support and / or sealing layer, which surrounds the membrane in a frame-like manner. In this way, the flattened profile obtains its necessary mechanical stability and flexural rigidity. Basically, the support and / or sealing layer serves to mechanically stabilize the fragile membrane and facilitates its handling during assembly of the fuel cell stack. Furthermore, it can have a sealing effect. For this purpose, it is preferably formed from an elastic material, for example silicone or the like. The support and / or sealing layer can be a layer laminated to the membrane on one or both sides, or a one-piece structure enclosing the membrane edge in a U-shaped cross-section, which can be injection-molded, for example.

[0013] According to the invention, the flattened edge of the membrane electrode assembly has an outwardly tapered profile, meaning that the thickness of the membrane layer decreases toward the outside. In particular, the flattened edge has a triangular cross-section, tapering to a point. The tapered profile ensures that the edge of the MEA in the fuel cell stack loses contact with the adjacent bipolar plates. This further extends the creepage distance and improves the electrical insulation.

[0014] With respect to a plan view of a flat side of the membrane-electrode assembly, the flattened edge preferably has a width in the range of 1 to 16 mm, in particular in the range of 1 to 5 mm. Within these ranges, on the one hand, a sufficiently long creepage distance and thus good insulation effect is achieved, and on the other hand, the space required to form the flattened profile is kept to a minimum. The width of the flattened edge can depend in particular on the stack voltage, with a larger width being selected as the stack voltage increases. For stack voltages of up to 2000 V (based on DIN-EN 60664-1), a width of 1 to 16 mm is preferably selected, while for stack voltages of up to 630 mm, widths in the range of 1 to 5 mm are sufficient. Current stack voltages in automotive applications are around 300 V.

[0015] In one embodiment of the invention, the membrane electrode assemblies protrude from the fuel cell stack relative to the bipolar plates. In other words, the membrane electrode assemblies have a larger cut and thus a certain overhang relative to the bipolar plates. This overhang can also be provided along the peripheral edge region of the MEA as well as the edge region of the supply openings. This measure further improves the electrical insulation of the cells.

[0016] According to an alternative embodiment, the membrane electrode assemblies do not protrude from the fuel cell stack relative to the bipolar plates. In this case, the membrane electrode assemblies end flush with the bipolar plates (this alternative is not part of the invention) or, according to the invention, even recede relative to them, so that the bipolar plates protrude from the stack relative to the membrane electrode assemblies. This embodiment results in a particularly small installation volume for the fuel cell stack.

[0017] Preferably, the depth to which the flattened edge of the membrane electrode assembly extends between two bipolar plates is in the range of 0 to 10 mm, in particular in the range of 0 to 3 mm. Adhering to these limits ensures good electrical insulation while simultaneously preventing unnecessary space requirements due to the flattened edge.

[0018] A further aspect of the invention relates to a fuel cell system comprising a fuel cell stack according to the invention. In particular, the fuel cell system comprises, in addition to the fuel cell stack, an anode supply and a cathode supply with the corresponding peripheral components.

[0019] A further aspect of the invention relates to a vehicle having a fuel cell system with a fuel cell stack according to the invention. The vehicle is preferably an electric vehicle, in which electrical energy generated by the fuel cell system serves to supply an electric traction motor and / or a traction battery.

[0020] Further preferred embodiments of the invention result from the remaining features mentioned in the subclaims.

[0021] The various embodiments of the invention mentioned in this application can be advantageously combined with one another, unless stated otherwise in the individual case.

[0022] The invention is explained below in exemplary embodiments with reference to the accompanying drawings. They show: Fig. 1 is a block diagram of a fuel cell system according to a preferred embodiment; Fig. 2 a plan view of a membrane electrode assembly; Fig. 3 a top view of a bipolar plate; Fig. 4 is a sectional view of a single cell of a fuel cell stack according to a known embodiment; Fig. 5 a sectional view of a single cell of a fuel cell stack according to a first embodiment of the invention Fig. 6 is a sectional view of a single cell of a fuel cell stack according to another embodiment of the invention; and Fig. 7 is a sectional view of a single cell of a fuel cell stack according to another embodiment not forming part of the invention.

[0023] Fig. 1 shows a fuel cell system, designated overall by 100, according to a preferred embodiment of the present invention. The fuel cell system 100 is part of a vehicle (not shown in detail), in particular an electric vehicle, which has an electric traction motor supplied with electrical energy by the fuel cell system 100.

[0024] The fuel cell system 100 comprises, as its core component, a fuel cell stack 10, which has a plurality of individual cells 11 arranged in a stacked form and formed by alternately stacked membrane electrode assemblies (MEAs) 14 and bipolar plates 16 (see detailed section). Each individual cell 11 thus comprises an MEA 14, which has an ion-conductive polymer electrolyte membrane (not shown in detail here), as well as catalytic electrodes arranged on both sides thereof, namely an anode and a cathode, which catalyze the respective partial reaction of the fuel cell conversion and can in particular be formed as coatings on the membrane. The anode and cathode electrodes comprise a catalytic material, for example platinum, which is supported on an electrically conductive carrier material with a large specific surface area, for example a carbon-based material.An anode chamber 12 is thus formed between a bipolar plate 16 and the anode, and the cathode chamber 13 is formed between the cathode and the next bipolar plate 16. The bipolar plates 16 serve to supply the operating media to the anode and cathode chambers 12, 13 and also establish the electrical connection between the individual fuel cells 11. Optionally, gas diffusion layers can be arranged between the membrane-electrode assemblies 14 and the bipolar plates 16.

[0025] In order to supply the fuel cell stack 10 with the operating media, the fuel cell system 100 has, on the one hand, an anode supply 20 and, on the other hand, a cathode supply 30.

[0026] The anode supply 20 comprises an anode supply path 21, which serves to supply an anode operating medium (the fuel), for example, hydrogen, to the anode chambers 12 of the fuel cell stack 10. For this purpose, the anode supply path 21 connects a fuel reservoir 23 to an anode inlet of the fuel cell stack 10. The anode supply 20 further comprises an anode exhaust gas path 22, which discharges the anode exhaust gas from the anode chambers 12 via an anode outlet of the fuel cell stack 10. The anode operating pressure on the anode sides 12 of the fuel cell stack 10 is adjustable via an adjusting means 24 in the anode supply path 21. In addition, the anode supply 20 may include a fuel recirculation line 25 as shown, which connects the anode exhaust path 22 to the anode supply path 21.Fuel recirculation is common practice to return the fuel, which is usually used at a superstoichiometric level, to the stack and utilize it. A further adjusting device 26 is arranged in the fuel recirculation line 25, with which the recirculation rate can be adjusted.

[0027] The cathode supply 30 comprises a cathode supply path 31, which supplies the cathode chambers 13 of the fuel cell stack 10 with an oxygen-containing cathode operating medium, in particular air drawn in from the environment. The cathode supply 30 further comprises a cathode exhaust gas path 32, which removes the cathode exhaust gas (in particular the exhaust air) from the cathode chambers 13 of the fuel cell stack 10 and, if appropriate, feeds it to an exhaust system (not shown). A compressor 33 is arranged in the cathode supply path 31 to convey and compress the cathode operating medium. In the illustrated embodiment, the compressor 33 is designed as a compressor driven primarily by an electric motor, which is driven by an electric motor 34 equipped with corresponding power electronics 35.The compressor 33 can also be driven via a common shaft (not shown) by a turbine 36 (optionally with variable turbine geometry) arranged in the cathode exhaust gas path 32. The turbine 36 represents an expander, which causes the cathode exhaust gas to expand and thus reduces its pressure.

[0028] According to the illustrated embodiment, the cathode supply 30 can further comprise a wastegate line 37, which connects the cathode supply line 31 to the cathode exhaust line 32, thus representing a bypass of the fuel cell stack 10. The wastegate line 37 allows the operating pressure of the cathode operating medium in the fuel cell stack 10 to be reduced briefly without shutting down the compressor 33. An adjusting means 38 arranged in the wastegate line 37 allows the amount of cathode operating medium bypassing the fuel cell stack 10 to be controlled. All adjusting means 24, 26, 38 of the fuel cell system 100 can be designed as controllable or non-controllable valves or flaps. Corresponding further adjusting means can be arranged in the lines 21, 22, 31, and 32 in order to be able to isolate the fuel cell stack 10 from the environment.

[0029] The fuel cell system 100 may further comprise a humidifier module 39. The humidifier module 39 is arranged, on the one hand, in the cathode supply path 31 such that the cathode operating gas can flow through it. On the other hand, it is arranged in the cathode exhaust gas path 32 such that the cathode exhaust gas can flow through it. The humidifier 39 typically comprises a plurality of water vapor-permeable membranes, which are either flat or in the form of hollow fibers. The comparatively dry cathode operating gas (air) flows over one side of the membranes, and the comparatively humid cathode exhaust gas (exhaust gas) flows over the other side. Driven by the higher partial pressure of water vapor in the cathode exhaust gas, water vapor passes through the membrane into the cathode operating gas, which is thus humidified.

[0030] Various further details of the anode and cathode supply 20, 30 are shown in the simplified Fig. 1 for reasons of clarity. A water separator can be installed in the anode and / or cathode exhaust gas path 22, 32 to condense and drain the product water resulting from the fuel cell reaction. Finally, the anode exhaust gas line 22 can flow into the cathode exhaust gas line 32, so that the anode exhaust gas and the cathode exhaust gas are discharged via a common exhaust system.

[0031] The Fig. 2 and Fig. 3 each show a top view of an exemplary membrane electrode assembly 14 and bipolar plate 16, respectively, which constitute the fuel cell stack 10.

[0032] Both components are 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. For this purpose, the membrane electrode assembly 14 has a catalytic electrode 143 in the active region AA on both sides of the polymer electrolyte membrane. The inactive regions IA can each be divided into supply regions SA and distribution regions DA. Within the supply regions SA, supply openings 144 to 147 are arranged on the side of the membrane electrode assembly 14 and 164 to 169 on the side of the bipolar plate 16, which are essentially aligned with one another when stacked and form main supply channels in the fuel cell stack. The anode inlet openings 144 and 164 serve to supply the anode operating gas, i.e. the fuel, for example hydrogen.The anode outlet openings 145 and 165 serve to discharge the anode exhaust gas after it has flowed over the active region AA. The cathode inlet openings 146 and 166 serve to supply the cathode operating gas, which is in particular oxygen or an oxygen-containing mixture, preferably air. The cathode outlet openings 147 and 167 serve to discharge the cathode exhaust gas after it has flowed over the active region AA. The coolant inlet openings 148 and 168 serve to supply the coolant, and the coolant outlet openings 149 and 169 serve to discharge the coolant.

[0033] The MEA 14 has an anode side 141 which is Fig. 2. Thus, the catalytic electrode 143 shown is designed as an anode, for example as a coating on the polymer electrolyte membrane. Fig. The cathode side 142, not visible in Figure 2, has a corresponding catalytic electrode, here the cathode. The polymer electrolyte membrane can extend over the entire width of the membrane electrode assembly 14, but at least over the active region AA.

[0034] The MEA 14 further comprises a support and sealing layer 150 that encloses the membrane and surrounds it like a frame. In the illustrated example, this support and sealing layer extends over the entire inactive region 1A, effectively forming it, and circumferentially enclosing the electrodes 143. The support and sealing layer 150 also forms an edge region 151 that surrounds the MEA 14 along its entire circumference and has a flattened profile, which will be explained in more detail later. Additionally, the supply openings 144 to 149 can also be formed with such a flattened edge (not shown here).

[0035] The Fig. The bipolar plate 16 shown in Figure 3 also has a cathode side 162, which is visible in the illustration, and an anode side 161, which is not visible. In typical embodiments, the bipolar plate 16 is constructed from two joined plate halves, the anode plate and the cathode plate. On the cathode side 162 shown, operating medium channels 163 are formed as open, groove-like channel structures, which connect the cathode inlet opening 166 to the cathode outlet opening 167. Only five exemplary operating medium channels 163 are shown, although a much larger number is usually present. Likewise, the anode side 161, which is not visible here, has corresponding operating medium channels, which connect the anode inlet opening 164 to the anode outlet opening 165. These operating medium channels for the anode operating medium are also formed as open, groove-like channel structures.Enclosed coolant channels run inside the bipolar plate 16, particularly between the two plate halves, and connect the coolant inlet opening 168 with the coolant outlet opening 161. The dashed lines in . Fig. 3 seals indicated.

[0036] As already mentioned, the Fig. 2 and Fig. 3, the membrane electrode assemblies 14 and bipolar plates 16 are arranged alternately on top of one another and pressed fluid-tight between two end plates to form a fuel cell stack 10. The bipolar plates 16 are interconnected via an external circuit to collect the generated current. On the other hand, the individual cells, particularly the bipolar plates 16, must be electrically insulated from one another.

[0037] The Fig. 4, Fig. 5, Fig. 6 and Fig. 7 show a section through an edge region of a single cell 11 of a corresponding fuel cell stack 10 according to the Fig. 2 and Fig. 3 shown cutting plane. Fig. 4 shows a known design and the Fig. 5, Fig. 6 and Fig. 7 each represent an embodiment according to the present invention. Corresponding elements are designated by the same reference numerals.

[0038] Each of the membrane electrode assemblies 14 of the Fig. 4 to 7 comprises a polymer electrolyte membrane 52, which is contacted in the active region AA by the two catalytic electrodes 143. The electrodes 143 can be present as a coating of the membrane, as a self-supporting layer, or as a coating of gas diffusion layers (not shown here) that adjoin the MEA 14 on both sides. Fig. 4 to 7 also show that the support and / or sealing layer 150 is integrally formed onto the edge of the membrane 152. This can be achieved, for example, by an injection molding process or the like. For example, the layer 150 is made of a silicone material. It can also have a bead (not shown here) that forms a circumferential seal with the bipolar plates 16. In an alternative embodiment, the membrane 152 can also be laminated in its edge region on one side with one or on both sides with two support and / or sealing layers (not shown).

[0039] According to the execution according to Fig. 4, the membrane electrode assembly 14 overlaps the bipolar plates 16 in the form of an overhang with a length L. This results in a creepage distance which is determined from the distances denoted by K in Fig. 4 and which must be overcome by electrical charge carriers in order to short-circuit the two bipolar plates 16. The longer the creepage distance K, the lower the probability that it will be overcome by electrical charge carriers and the better the individual cells 11 are electrically insulated from one another. Thus, the insulation effect can be improved by increasing the overhang length L. However, this leads to an increase in the volume of the fuel cell stack 10, which is undesirable, particularly in mobile applications where installation space is limited.

[0040] Fig. 5 shows a corresponding sectional view of a single cell 11 of a fuel cell stack 10 with a membrane electrode assembly 14 according to a first embodiment of the invention. In this embodiment, the membrane electrode assembly 14 is formed with an edge 151 which, in the cross section shown, has a profile that is flattened on one side. This flattened edge 151 is clearly formed by the support and sealing layer 150. Due to the flattening of the frame 151, the membrane electrode assembly 14 loses contact with the upper bipolar plate 16 early on towards its outermost edge. In this way, the length of the creepage distance K is reduced compared to the rectangular profile of Fig. 4 was significantly extended and improved insulation was achieved.

[0041] Fig. Figure 6 shows a further embodiment of an MEA 14 according to the invention in a single cell 11 of a fuel cell stack 10. In this embodiment, the MEA 14, in particular the support and sealing layer 150, has an edge 151 with a profile that is flattened on both sides. This creates cavities or undercuts on both sides of the membrane electrode assembly 14 to the adjacent bipolar plates 16. In this embodiment, the creepage distance K is also significantly increased.

[0042] The length of the creepage distance K depends in particular on the width B of the flattened edge 151. Preferably, the width B is in the range of 1 to 5 mm. In a preferred embodiment, the flattened edge region 151, as in Fig. 5 and Fig. 6, at least partially arranged between the bipolar plates 16 by extending into the stack 10 with an undercut depth H. Thus, with the same overhang length L as in Fig. 4 and thus the same construction volume of the fuel cell stack 10, improved electrical insulation can still be achieved. The greater the undercut depth H, the less overhang length L is necessary to create a desired creepage distance K. The extent to which the flattened edge 151 is arranged between the two bipolar plates is shown in the Fig. 5 and Fig. 6 marked H.

[0043] The Fig. 7 differs from the version shown in Fig. 6, in that the MEA 14 does not protrude from the bipolar plates 16, but is flush or aligned with them (L = 0). The width B of the edge 151 corresponds here to the undercut depth H (B = H). Alternatively, the cut of the MEA 14 can even be smaller than that of the bipolar plates 16, so that L becomes negative. With the same creepage distance as in Fig. 6 In these embodiments, the size of the fuel cell stack is reduced.

Claims

[1] A fuel cell stack (10) comprising a plurality of alternately stacked bipolar plates (16) and membrane-electrode assemblies (14), each comprising a membrane (152), two catalytic electrodes (143) adjoining it on both sides, and a supporting and / or sealing layer (150) surrounding the membrane (152) in a frame-like manner and forming edge regions, wherein supply openings (144-149) are provided for the supply and removal of operating media, and wherein an edge (151) of the membrane-electrode assembly (14), which runs along its circumference and / or along its supply openings (144-149), has a profile that is flattened on one side or both sides in cross section, characterized byin that the flattened edge (151) of the membrane electrode assembly (14) has an outwardly tapering profile and extends at least partially between two bipolar plates (16), and wherein the membrane electrode assembly (14) protrudes from the fuel cell stack (10) relative to the bipolar plates (16) or the membrane electrode assembly (14) recedes relative to the bipolar plates (16), so that the bipolar plates (16) protrude from the fuel cell stack (10) relative to the membrane electrode assemblies (14). [2] Fuel cell stack (10) according to claim 1, characterized by that an undercut depth (H) with which the flattened edge (151) of the membrane electrode arrangements (14) extends between two bipolar plates (16) is in the range from 0 to 10 mm, in particular in the range from 0 to 3 mm. [3] Fuel cell system (100) comprising a fuel cell stack (10) according to one of claims 1 to 2.

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