DE20016734U1 - Polymer electrolyte membrane - fuel cell system with cooling by non, weak or strong electrically conductive fluid media - Google Patents

Description

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K 51 787/8

POLYMER ELECTROLYTE MEMBRANE FUEL CELL SYSTEM WITH COOLING BY NON, WEAKLY OR STRONGLY ELECTRICALLY CONDUCTIVE FLUID MEDIA

The present invention relates to a fuel cell system with a plurality of polymer electrolyte membrane fuel cells which have a membrane electrode unit, an anode-side bipolar plate and a cathode-side bipolar plate and which are arranged one above the other in the form of a stack, wherein the fuel cell system is suitable for cooling with non-, weakly or strongly electrically conductive fluid media. The fuel cells are preferably operated with fuel gas and oxidizing agent under low pressure, wherein hydrogen or a methanol-water mixture in liquid or gaseous form is preferably used as the fuel gas and air or oxygen is used as the oxidizing agent.

Polymer electrolyte membrane fuel cells contain an anode, a cathode and an ion exchange membrane arranged between them. Several fuel cells form a fuel cell stack, with the individual fuel cells being separated from one another by bipolar plates acting as current collectors. To generate electricity, a fuel gas, e.g. hydrogen, is introduced into the anode area via gas distribution channels and an oxidizing agent, e.g. air or oxygen, is introduced into the cathode area via gas distribution channels. The reactants can be introduced under excess pressure (approximately 2 · 10 ⁵ to 4 · 10 ⁵ Pa) or at approximately atmospheric pressure (approximately 1.3 · 10 ⁵ to 1.5 · 10 ⁵ Pa abs). In the

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Each area in contact with the polymer electrolyte membrane has a catalyst layer. In the anode catalyst layer, the fuel is oxidized to form cations and free electrons, while in the cathode catalyst layer, the oxidizing agent is reduced by absorbing electrons. The cations migrate through the ion exchange membrane to the cathode and react with the reduced oxidizing agent, producing water when hydrogen is used as the fuel gas and oxygen as the oxidizing agent. When the fuel gas and oxidizing agent react, large amounts of heat are released that must be removed by cooling. Cooling can be achieved using gaseous media, e.g. air, or fluid media.

In conventional fuel cell stacks with liquid cooling, cooling is carried out by cooling channels in the bipolar plates, which are fed from central distribution and collection lines. Since typically between 20 and 50 up to several hundred individual fuel cells are connected in series, the cooling medium must be guided lengthways in or against the direction of current through the fuel cell stack in the central supply and discharge channels. In order to prevent the cooling medium from electrically connecting the different electrical potentials of the individual cells connected in series, thus causing short circuits between cells or material decomposition, deionized water is used as the cooling liquid. However, deionized water has a high absorption capacity for soluble ions of all kinds, so when used in fuel cell systems it must be continuously replaced or cleaned, e.g. by ion exchange systems. Such cleaning is often necessary, since the cooling media are usually

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Heat exchanger systems, where they become enriched with foreign ions. Such foreign ions not only increase the electrical conductivity of the cooling medium in an undesirable way, but many of the foreign ions (Cu ²⁺ , Ni ²⁺ ) also damage the solid polymer electrolyte membrane when the cooling medium comes into direct contact with the membrane.

The choice of suitable cooling media for fuel cell systems with conventional cooling is therefore very limited. Cooling media that could damage the membrane, catalyst layer, gas diffusion layer, sealing system and/or bipolar plates when in direct contact with them cannot be used, as is the case with oils, cooling water enriched with foreign ions from heat exchanger systems, cooling water with antifreeze or alcohols.

These restrictions regarding the cooling media that can be used result in limitations on the possible applications of the fuel cell systems. For example, if a fuel cell system that uses deionized water as a cooling medium is switched off in a very cold environment, the cooling medium can be frozen by the time it is switched on again and cause irreversible damage to the fuel cell system.

Especially in fuel cells that use hydrogen as fuel gas, which has excellent diffusion properties, there is a risk that fuel gas can escape directly into the environment due to a lack of suitable containment. In particular, when using the

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The risk of explosion in fuel cells in enclosed spaces cannot therefore be reliably ruled out.

The object of the present invention is to overcome the disadvantages of the prior art and to provide a fuel cell system which is not restricted to the use of electrically non-conductive cooling media.

The object of the invention is in particular to provide a fuel cell system which can be cooled by means of non-, weakly or strongly electrically conductive fluid media.

Another object of the invention is to provide a fuel cell system that can be cooled by means of cooling media that can damage any components of the fuel cells if they come into contact with them.

A further object of the invention is to provide a fuel cell system which, in the event of leaks in a reactant chamber, does not release the reactant directly into the environment.

The object is achieved by the fuel cell system with a plurality of polymer electrolyte membrane fuel cells which have a membrane electrode unit, an anode-side bipolar plate and a cathode-side bipolar plate, and which are arranged one above the other in the form of a stack, wherein current collectors are arranged on the top and bottom of the stack, and wherein the fuel cell system is characterized in that

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between adjacent fuel cells of the stack, a gap is provided for receiving a cooling medium,

a cooling medium jacket is provided which surrounds the stack at least on its side surfaces, and the

at least one cooling medium distribution chamber into which at least one cooling medium inlet opening opens, and
forms at least one cooling medium collecting chamber into which at least one cooling medium outlet opening opens,

- wherein the cooling medium jacket is connected to at least one end plate which is arranged on an outer surface of the current collector facing away from the stack and/or to at least one cover element for forming a space through which cooling medium can flow, and

- wherein the outer surfaces of the fuel cells are covered with protective material and/or electrically insulating material.

The method for cooling the fuel cell system with a plurality of polymer electrolyte membrane fuel cells, which have a membrane electrode unit, an anode-side bipolar plate and a cathode-side bipolar plate, and which are arranged one above the other in the form of a stack, is characterized in that a cooling medium jacket is provided which surrounds the stack at least on its side surfaces and is connected to at least one end plate of the stack and/or to at least one cover element to form a cooling medium space, and a fluid cooling medium is flowed through the cooling medium space, wherein the cooling medium flows through at least intermediate spaces between adjacent cells of the stack and thereby comes into contact with the surfaces of the

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Bipolar plates of the fuel cells and cools the fuel cells.

To cool the fuel cell stack, the cooling medium is introduced into the cooling medium chamber on one side and flows through the stack, i.e.

flows through the spaces between the fuel cells and exits the cooling medium space again on another side, preferably the opposite side.

A single fuel cell is made up of the following components: membrane electrode unit, consisting of membrane, cathode and anode-side catalyst and gas diffusion layer, an anode-side and a cathode-side bipolar plate and the sealing system. Gas distribution structures are often incorporated into or applied to the bipolar plates. Each individual fuel cell also requires inlets and outlets for fuel gas and oxidizing agents. The fuel cells are usually rectangular, but can also have any other desired shape.

Likewise, the cooling medium jacket can basically have any shape as long as it is ensured that the cooling medium can flow sufficiently evenly through the spaces between the fuel cells. With certain cell shapes, such as octagonal cells, it can be advantageous to provide several cooling medium distribution spaces and/or collection spaces to ensure a sufficiently even flow.

The spatial arrangement or orientation of the fuel cell system is arbitrary. If the system is arranged so that the fuel cells

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form a stack of cells arranged next to each other, so if the system is rotated by 90° compared to the "upright" stacking, the cooling medium can flow vertically through the spaces between the cells.

The fuel cell system according to the invention and the method for cooling the fuel cell system basically only require the presence of a single fuel cell (there are then no gaps between adjacent cells), but it will usually contain several fuel cells stacked to form a fuel cell stack, typically about 10 to 100 fuel cells.

The fuel cells are stacked on top of each other in such a way that a gap remains between adjacent cells through which the cooling medium can flow. Current collectors, typically in the form of current collecting and discharging plates, are provided on the underside and the top of the stack. The fuel cells and the current collectors are electrically connected in series, i.e. the gaps between the individual fuel cells are each bridged by an electrically conductive connector. The fuel cell stack is finally closed off by end plates that are attached to the side of the current collectors facing away from the stack.

Spacer structures are provided in the gaps between the fuel cells. These spacer structures can be made of an electrically conductive material such as metal or carbon-containing materials, such as carbon fiber paper with porous structures, and at the same time act as electrical connectors between the individual fuel cells. However, they can also

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Electrically non-conductive spacer structures, e.g. made of plastic, can be used and separate electrical connectors can be provided. The spacer structures can be separate components, but they can also be formed integrally with a bipolar plate or integrally with two bipolar plates. Electrically conductive materials are preferred as materials for the spacer structures. When formed integrally with one or both of the adjacent bipolar plates, they consist of an identical material to the bipolar plates, typically metal or carbon-containing materials.

The dimensions of the gaps must be selected so that they allow unhindered flow of the cooling medium and uniform cooling. Depending on the size of the fuel cells, a distance between adjacent cells of approximately 0.1 to 10 mm, preferably 0.2 to 5 mm, particularly preferably 0.2 to 1 mm, can be considered suitable. A spacer structure provided in the gaps should impede the flow of the cooling medium as little as possible. Spacer structures that form channel- or pore-shaped structures for the cooling medium are preferred.

The stacking sequence - fuel cell, intermediate space (with spacer structure), fuel cell, etc. - results in a hydraulic parallel connection of the cooling medium across the fuel cell stack, which achieves very uniform cooling with very low pressure differences. Typically, the pressure loss is in the range of a few hundred Pa to a few thousand Pa, depending on the size of the fuel cells and the heat absorption of the cooling medium.

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In addition to the gaps between the individual fuel cells, a gap can also be provided between the lowest and highest fuel cells in a stack and the adjacent current collector. These additional gaps have the advantage that coolant also flows over the very lowest and very highest bipolar plates in a stack. In this case, an electrical connection is required between these final bipolar plates and the current collector.

The fuel cells can be any commercially available polymer electrolyte membrane fuel cells, e.g. with carbon fiber fleece electrodes, bipolar plates made of metal or graphite and a Nafion® membrane or Gore membrane. For the purposes of the present invention, it is only important that the individual fuel cells are sealed in such a way that no cooling medium gets into the interior of the fuel cells. It is irrelevant whether each cell is sealed individually or whether the sealing only takes place when the fuel cells are clamped together to form a stack. The requirements for sealing the cells against the cooling medium are relatively easy to meet, since the pressure in the fuel cells is higher than in the coolant system. Fuel gas pressures of approx. 0.3 · �IΩ ⁵ to 0.5 are typical.

•&Igr;&Ogr; ⁵ Pa above atmospheric pressure and oxidant pressures of approx. 0.1

• 10 ⁵ to 0.5 · &Igr;&Ogr; ⁵ Pa above atmospheric pressure. The system according to the invention is basically suitable for any pressure on the fuel gas side and the oxidizing agent side, e.g. also for pressures of 2 ■ 10 ⁵ to 4 · 10 ⁵ Pa or higher. If sealing problems occur, the reaction gas is more likely to leak into the cooling medium. The penetration of cooling medium into the cells is not encouraged.

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The fuel cell stack is encased on its periphery with a cooling medium jacket. The jacketing is done in such a way that a preferably chamber-like distribution space for the cooling medium is created on one side of the fuel cell stack and an identical or similar collection space for the cooling medium is created on another, preferably the opposite, side of the fuel cell stack. If the distribution space and collection space are not on opposite sides, it is preferable to form spacers between the individual cells so that the cooling medium is guided from the distribution space to the collection space. The collection space can also be shaped differently than the distribution space, as long as it is suitable for receiving and passing on the cooling medium emerging from the stack. The distribution space has a cooling medium inlet opening through which the cooling medium enters the distribution space, and the cooling medium collection space has a cooling medium outlet opening through which the cooling medium leaves the cooling jacket again. Particularly in the case of high fuel cell stacks or large-area fuel cells, it can be advantageous to introduce a distribution structure into the distribution chamber to improve the flow of the cooling medium. Alternatively or additionally, several or many cooling medium inlet openings can be provided, which also promote an even distribution of the introduced cooling medium. The collection chamber can also have several outlet openings. The inlet opening(s) and the outlet opening(s) can be integrated into the cooling medium jacket or into one or both end plates or into both the cooling medium jacket and the end plates.

The cooling jacket does not necessarily have to be made in one piece, but can also be composed of several parts,

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which are welded, glued or otherwise tightly connected to one another. For example, a separate distribution chamber and collection chamber can be provided, and the two remaining, not yet covered side surfaces of the fuel cell stack can each be provided with a plate which, when connected to the distribution chamber on the one hand and the collection chamber on the other, forms a continuous cooling medium jacket.

Non-conductive plastic materials are preferably used as materials for the cooling medium jacket and the distributor structure. However, this is not mandatory. When cooling with a non-conductive cooling medium, electrically conductive materials can also be used, for example stainless steel. It is only necessary that the bipolar plates as well as the current collectors and the connectors that provide electrical contact between the fuel cells and between the fuel cells and current collectors do not have electrical contact with the cooling medium jacket. For electrical insulation, for example, an electrically insulating layer can be applied to the inside of the cooling medium jacket side surface that is adjacent to the fuel cell stack, or a gap can be left between the cooling medium jacket side surfaces and the opposite side surfaces of the fuel cell stack. Such a gap also has the advantage that cooling medium also flows past the outer surfaces of the fuel cell stack. Such a gap would typically have a width of up to 10 mm, but preferably only up to about 1 mm, in order to ensure that the cooling medium flows preferentially through the gaps between the cells.

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For electrical insulation of the current collectors, these can also be designed in such a way that physical contact with the cooling medium jacket is avoided, or electrical insulation can be provided between the current collector and the cooling medium jacket.

When using electrically conductive cooling media, it is recommended to use non-conductive materials for all components. The bipolar plates, the current collectors and the electrical connectors, e.g. the spacer structure between the fuel cells, must of course always be made of electrically conductive material. When using a conductive or aggressive cooling medium, these components are coated with an appropriate protective layer, e.g. with insulating varnish when using conductive cooling media, or another material that is resistant to the cooling medium used. It is possible to treat all parts that are to be coated with a protective or insulating layer separately. However, this has the disadvantage that the layer must be removed again at the electrical contact points. It is therefore preferable to fully assemble the fuel cell stack including spacers and only then coat it with insulating varnish, for example by dipping. In this way, the entire fuel cell stack is completely encapsulated, insulated and protected.

The cooling medium jacket encloses the side surfaces of the fuel cell stack. End plates, e.g. made of glass fiber reinforced plastic, are attached to the top and bottom of the fuel cell stack, whereby the contact points between the cooling jacket and the end plates must be sealed coolant-tight. This can

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For example, this can be done by machining a circumferential groove in the end plates that corresponds to the circumference of the cooling medium jacket, whereby the width of the groove must be slightly larger than the wall thickness of the cooling medium jacket. A sealant, such as silicone, is introduced into the groove and the cooling medium jacket is then sunk or pressed into the sealant. Alternatively, the end plate can simply be placed on the cooling medium jacket and the resulting edges sealed with sealant. In the case of end plates whose surface is the same shape as the cooling medium jacket cross-section but slightly smaller than the inner cross-section of the cooling medium jacket, sealant can also be applied to the front sides of the end plate and this can then be inserted into the cooling medium jacket.

It is also possible to design the cooling medium jacket in the form of a trough into which the fuel cell stack is inserted. In this case, only one of the end plates of the fuel cell stack is used to form the space through which the cooling medium can flow. Alternatively, the fuel cell stack, including both end plates, can be completely accommodated in the space through which the cooling medium can flow. The cooling medium jacket is then closed with separate cover elements, whereby passages for gases and for power collection must be provided.

To supply the fuel cells with fuel gas and oxidant, the fuel cells each have a fuel gas supply and a fuel gas outlet as well as an oxidant supply and an oxidant outlet. These reactant supplies and/or outlets must be arranged and sealed in such a way that no contact is possible between the reactants and the cooling medium.

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is possible. In the fuel cell system according to the invention, there is a complete decoupling of the media - oxidizing agent, fuel gas, cooling medium - and the respective sealing systems in the individual fuel cells and in the fuel cell stack. By decoupling the sealing systems, the fuel cell stack can be constructed from individual cells that have already been checked for leaks before assembly. Only the individual gas supply channels and gas discharge channels for fuel gas and oxidizing agent between the individual cells need to be sealed.

If the fuel cell stack is to be provided with a protective or insulating coating, it is possible to fully assemble the stack with all gas supplies, coat it and then insert it into the cooling medium jacket and close the cooling medium space by means of end plates or cover elements.

The sealing for the cooling medium is carried out exclusively via the end plate or the end plates of the fuel cell stack or other cover plates and is thus removed from the immediate interior of the fuel cell. Problems such as those that occur in fuel cell systems in which the sealing of the individual media-carrying layers in the individual cells as well as the media supply and
-discharge channels are only created when the stack is completely assembled, thus avoiding problems due to the diversity of the individual media sealing systems and sealing requirements.

A preferred way to decouple all media is to install a separator between the membrane electrode assembly and the

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to provide seals on both sides of the bipolar plates adjacent to it in such a way that partial areas of the bipolar plates and the membrane electrode unit lie outside this seal, whereby these are non-overlapping areas for the anode-side bipolar plate and the cathode-side bipolar plate. These can be corner areas, but also other areas, e.g. central areas, so that a cross-flow of the reaction gases results, for example. The area within the seal between the membrane electrode unit and the anode-side or cathode-side bipolar plate is the active reaction area with supply and discharge for the reaction gas required in each case. In the areas outside the seal are the supply and discharge lines for the reaction gas not required in the respective active reaction area, which are each sealed separately. A particularly advantageous feature of this arrangement is that the fuel gas supply and discharge lines and the oxidizing agent supply and discharge lines are in the cooling medium space and are washed around by the cooling medium. This has the advantage that the cooling medium encloses the entire fuel cell stack, preventing the escape of reaction gases from the cells into the atmosphere, i.e. the system is intrinsically safe against leaks on the fuel gas side and the air/oxygen side. Conversely, the entry of the cooling medium into the reaction gas circuits can be prevented if the pressure in the reaction gas circuits is higher than the pressure in the cooling medium circuit.

The supply and discharge of the reaction gases into or out of the space through which the cooling medium flows is preferably carried out through passages in the end plates.

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A gas separator can be installed at the outlet of the cooling medium, which allows simple and rapid detection of even small amounts of escaped reaction gases.

Due to the design of the fuel cell system according to the invention, if tap water is used as the cooling medium, the heated cooling medium can be coupled directly into a service water circuit. However, a heat exchanger can also be used. In any case, only a small pumping power is required for pumping due to the low pressure loss when flowing through the fuel cell system.

In the following, preferred embodiments of the invention are explained in more detail with reference to the figures.
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The drawings show:

Fig. 1 a vertical section through two fuel cells with

intermediate spacer structure,
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Fig. 2a is a vertical section through a fuel cell system according to the invention,

Fig. 2b shows a horizontal section through a fuel cell system according to the invention,

Fig. 3a a seal according to the invention between the cooling medium jacket and the end plate,

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Fig. 3b shows another seal according to the invention between the cooling medium jacket and the end plate,

Fig. 4a a plan view of a seal according to the invention between the membrane electrode unit and the anode-side bipolar plate

as well as the supply and removal of oxidizing agents,

Fig. 4b is a plan view of a seal according to the invention between the membrane electrode unit and the cathode-side bipolar plate as well as the fuel gas supply and fuel gas discharge.

As can be seen from Fig. 1, a single fuel cell 8 is basically made up of the components membrane electrode unit 1 - consisting of membrane, cathode and anode-side catalyst and gas diffusion layers - as well as an anode-side 2 and cathode-side 3 bipolar plate and the sealing system. Gas distribution structures are incorporated in the bipolar plates, which are indicated in Fig. 1 by the dashed lines. The two fuel cells are arranged at a distance from one another, with a conductive spacer structure 4' ensuring electrical contact between the anode-side bipolar plate of one cell and the cathode-side bipolar plate of the other cell. The spacer structure 4' has passages for a cooling medium 13, which flows along the anode-side bipolar plate 2 of one cell and the cathode-side bipolar plate 3 of the neighboring cell, thereby cooling the cells. The passages for the cooling medium are preferably channel- or pore-shaped structures. The spacer structure can either be part of a bipolar plate of a

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fuel cell, part of adjacent bipolar plates from neighboring fuel cells, or a standalone component.

Fig. 2a shows a section through a fuel cell system according to the invention perpendicular to the plane of the fuel cells of the fuel cell stack. Between the individual fuel cells 8 there are gaps 4 with spacers 4'. The upper and lower ends of the fuel cell stack 10 are formed by current collectors 7, 7'. The arrangement of fuel cell stack 10 and current collectors 7, 7 ' is surrounded on its entire circumference by a cooling medium jacket 9, which is shaped in such a way that a chamber-like space is formed on two opposite sides of the fuel cell stack. The space shown on the left side in Fig. 2a is the cooling medium distribution space 11, into which a cooling medium inlet opening 14 opens and in which a cooling medium distribution structure 12 is located. The space shown on the right side in Fig. 2a is the cooling medium collection space 11', into which a cooling medium outlet opening 14' opens. The upper and lower ends of the cooling medium jacket 9 are formed by end plates 6, 6', which are connected to the cooling medium jacket in such a way that no cooling medium can escape. Preferably, a sealant is used to seal between the cooling medium jacket 9 and the end plates 6, 6', which can be removed again without damaging the cooling medium jacket 9 and the end plates 6, 6' in order to allow maintenance work on the fuel cell stack without any problems. In the end plates 6, 6' there are passages for supplying fuel gas, e.g. hydrogen, and oxygen or air to the fuel cell stack and for removing fuel gas and oxygen or air from the fuel cell stack. When the fuel cell system is in operation, coolant escapes at the

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The coolant enters the coolant distribution chamber 11 through the cooling medium inlet opening 14, flows through the gaps 4 between the cells 8 and reaches the coolant collection chamber 11', which it leaves again through the cooling medium outlet opening 14'. The arrangement shown ensures a hydraulic parallel connection of the coolant across the fuel cell stack, whereby particularly uniform cooling with very low pressure differences is achieved.

Fig. 2b shows a section through an intermediate space 4 between two fuel cells 8 of the same arrangement as shown in Fig. 2a, cut parallel to the fuel cell plane. In the intermediate space 4 there are spacers 4', shown in the form of parallel lines, which simultaneously act as electrical connectors between the bipolar plates of adjacent cells. The cooling medium 13 flows through the intermediate spaces between the spacers 4'. On the two sides of the fuel cell stack where there is no cooling medium distribution space 11 and no cooling medium collection space 11', there are the cooling medium jacket side surfaces 15, 15', on each of which an electrically insulating layer 5, 5' is provided in order to prevent direct contact between the cooling medium jacket 9 and current-carrying components of the fuel cell stack 10.

Fig. 3a and 3b show seals according to the invention between the cooling medium jacket 9 and the end plates 6, 6'. In Fig. 3a, the end plate 6 has a recess into which the cooling medium jacket 9 is sunk. The recess is wider than the wall thickness of the cooling medium jacket 9, and the remaining space is filled with sealant 16.

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filled, wherein the cooling medium jacket 9 is completely surrounded by sealing agent 16 in the area of the recess.

In the embodiment shown in Fig. 3b, the sealing means 16 is applied to the inside of the upper and/or lower edge of the cooling medium jacket. The cooling medium jacket 9 encloses the end plate 6, i.e. the sealing means 16 seals the end plate 6 at its outer circumference.

Fig. 4a and 4b show a preferred embodiment of the decoupling of the cooling medium circuit from the fuel gas and air/oxygen circuits according to the invention. Seals are located between the membrane electrode unit of a fuel cell and the anode-side and cathode-side bipolar plates.

Fig. 4a shows a plan view of the anode side of a membrane electrode unit with a seal 26 to the anode-side bipolar plate. The seal 26 encloses an active reaction area 21, but leaves two oppositely arranged corner areas of the membrane electrode unit free. The fuel gas supply 17 and the fuel gas discharge 18 are located within the active reaction area 21, the oxidant supply 19 and the oxidant discharge 20 are located outside the active reaction area 21. The oxidant supply and discharge are sealed against the cooling medium 13 by means of seals 24 and 25.

Fig. 4b shows a corresponding arrangement on the cathode side. The seal 27 between the membrane electrode unit and the bipolar plate defines an active reaction area 21'. The oxidizing agent

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Feed 19 and the oxidant discharge 20 are located within the active reaction area 2Γ. Oppositely arranged corner areas of the membrane electrode unit for a fuel gas feed 17 with seal 22 and a fuel gas discharge 18 with seal 23 are located outside the seal 27.

The corner areas of the membrane electrode unit located outside the seal 26 and the corner areas of the membrane electrode unit located outside the seal 27 do not overlap. This arrangement ensures that the reaction gases are supplied and discharged to the active reaction area of the cell without any overlap and without contact with the cooling medium, although the supply and discharge lines are flushed with the cooling medium.

In the fuel cell system according to the invention, the cooling medium surrounds the entire fuel cell stack and each individual fuel cell. The individual fuel cells are evenly flowed around with a very low pressure difference by the hydraulic parallel connection of the cooling medium, and low circulation rates are required for the cooling medium. The cooling medium circuit is completely decoupled from the supply and discharge of the reaction gases, and the sealing systems for fuel gas, oxidizing agent and cooling medium are also completely decoupled, both in the individual fuel cell and in the fuel cell stack. Cooling takes place exclusively on the surfaces of the anode and cathode-side bipolar plates of the individual fuel cells and on the surfaces of the assembled stack. This ensures that the cooling medium cannot cause any damage to the membrane electrode unit. The fuel cell stack can be made from already sealed

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Individual fuel cells can be constructed, i.e. during assembly only the corresponding reaction gas supply channels and reaction gas discharge channels to the individual cells need to be sealed. The cooling medium is sealed to the outside via the end plate system of the fuel cell stack.

The construction according to the invention enables the use of non-, weakly or strongly electrically conductive fluid media for cooling, since there is a complete separation and encapsulation of the cooling medium from the active membrane zone and, if necessary - especially in the case of highly conductive or aggressive fluid media - both the individual fuel cells and the fuel cell stack can be provided with an electrically non-conductive insulating layer or a protective layer resistant to the aggressive medium.

This results in a number of advantages:

The fuel cell stack can even be operated with cooling media which, if in direct contact with the membrane, catalyst layer, gas diffusion layer, sealing system and/or bipolar plate, could cause damage to them (e.g. oils, cooling water enriched with foreign ions (Cu ²⁺ , Ni ²⁺ ) from heat exchanger systems, cooling water with antifreeze, alcohols).

The fuel cell system can be made "winter-proof" by choosing a suitable cooling medium, i.e., for example, by adding antifreeze additives to the cooling medium, freezing of the fuel cell can be prevented in a wide subzero temperature range.

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Irrespective of this, the design according to the invention is also important from a safety perspective when using conventional coolants, because the fuel cell system is intrinsically safe against the leakage of fuel gas, since in the event of a leak it is completely absorbed by the cooling medium and cannot escape into the environment.

What is also remarkable is the technically very uncomplicated structure of the system.

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Claims (14)

1. Fuel cell system with a plurality of polymer electrolyte membrane fuel cells ( 8 ) which have a membrane electrode unit ( 1 ), an anode-side bipolar plate ( 2 ) and a cathode-side bipolar plate ( 3 ), and which are arranged one above the other in the form of a stack ( 10 ), current collectors ( 7 , 7 ') being arranged on the top and bottom of the stack ( 10 ), characterized in that - between adjacent fuel cells ( 8 ) of the stack ( 10 ) there is provided an intermediate space ( 4 ) for receiving a cooling medium ( 13 ), - a cooling medium jacket ( 9 ) is provided which surrounds the stack ( 10 ) at least on its side surfaces, and - at least one cooling medium distribution chamber ( 11 ) into which at least one cooling medium inlet opening ( 14 ) opens, and - at least one cooling medium collecting chamber ( 11 ') into which at least one cooling medium outlet opening ( 14 ') opens, - wherein the cooling medium jacket ( 9 ) is connected to at least one end plate ( 6 , 6 ') which is arranged on an outer surface of the current collector ( 7 , 7 ') facing away from the stack, and/or to at least one cover element for forming a space through which cooling medium ( 13 ) can flow, and - wherein the outer surfaces of the fuel cells ( 8 ) are covered with electrically insulating material and/or protective material. 2. Fuel cell system according to claim 1, characterized in that the fuel cells ( 8 ) are each equipped with a fuel gas supply ( 17 ) and/or a fuel gas discharge ( 18 ) and/or an oxidizing agent supply ( 19 ) and/or an oxidizing agent discharge ( 20 ) which is (are) arranged within the cooling medium jacket ( 9 ). 3. Fuel cell system according to claim 2, characterized in that a seal ( 26 ) is provided between the anode-side active region ( 21 ) of a fuel cell ( 8 ) and the anode-side bipolar plate ( 2 ) and the oxidizing agent supply ( 19 ) and/or discharge ( 20 ) is arranged outside the active region ( 21 ), and/or a seal ( 27 ) is provided between the cathode-side active region ( 21 ') of a fuel cell ( 8 ) and the cathode-side bipolar plate ( 3 ) and the fuel gas supply ( 17 ) and/or discharge ( 18 ) is arranged outside the active region ( 21 '). 4. Fuel cell system according to one of claims 1 to 3, characterized in that an intermediate space ( 4 ) is provided between at least one current collector ( 7 , 7 ') and the fuel cell ( 8 ) adjacent to the current collector. 5. Fuel cell system according to one of claims 1 to 4, characterized in that spacers ( 4 ') are arranged in at least some of the intermediate spaces ( 4 ). 6. Fuel cell system according to claim 5, characterized in that the spacers ( 4 ') are electrically conductive. 7. Fuel cell system according to one of claims 1 to 6, characterized in that the at least one cooling medium distribution space ( 11 ) and the at least one cooling medium collection space ( 11 ') are arranged on two opposite sides of the stack ( 10 ). 8. Fuel cell system according to one of claims 1 to 7, characterized in that all outer surfaces of the fuel cells ( 8 ) and the spacers ( 4 ') are covered with electrically insulating material and/or protective material. 9. Fuel cell system according to one of claims 1 to 8, characterized in that the cooling medium jacket ( 9 ) has cooling medium jacket side surfaces ( 15 , 15 ') which are arranged on opposite side surfaces of the stack ( 10 ), wherein a free space for receiving cooling medium ( 13 ) is provided between at least one cooling medium jacket side surface ( 15 , 15 ') and the opposite side surface of the stack ( 10 ). 10. Fuel cell system according to claim 9, characterized in that the cooling medium distribution space ( 11 ) and/or the cooling medium jacket side surfaces ( 15 , 15 ') and/or the cooling medium collection space ( 11 ') are formed in one piece. 11. Fuel cell system according to claim 9, characterized in that the cooling medium distribution space ( 11 ) and/or the cooling medium collection space ( 11 ') and/or the coolant jacket side surfaces ( 15 , 15 ') are designed as separate components. 12. Fuel cell system according to one of claims 1 to 11, characterized in that a cooling medium distribution structure ( 12 ) is arranged in the cooling medium distribution space ( 11 ). 13. Fuel cell system according to one of claims 1 to 12, characterized in that a seal ( 16 ) is formed between the coolant jacket ( 9 ) and at least one end plate ( 6 , 6 ') and/or at least one cover element in the plane of the end plate ( 6 , 6 ') and/or in the plane of the cover element. 14. Fuel cell system according to one of claims 1 to 12, characterized in that a seal ( 16 ') is formed between the cooling medium jacket ( 9 ) and at least one end plate ( 6 , 6 ') and/or at least one cover element on the front side of the end plate ( 6 , 6 ') and/or on the front side of the cover element.