WO2021023441A1

WO2021023441A1 - Fuel cell stack and method for the production and use of a fuel cell stack

Info

Publication number: WO2021023441A1
Application number: PCT/EP2020/068470
Authority: WO
Inventors: Isabel ZANDER; Harald Bauer
Original assignee: Robert Bosch Gmbh
Priority date: 2019-08-07
Filing date: 2020-07-01
Publication date: 2021-02-11
Also published as: DE102019211823A1

Abstract

The invention relates to a fuel cell stack (4) comprising at least one fuel cell (3), at least one current collector (52) and at least one end plate (48), wherein an end layer (10) is arranged between the at least one current collector (52) and the at least one end plate (48), and the end layer (10) is constructed from an electrically conductive polymer material (12), and wherein the electrically conductive polymer material (12) is electrically connected to a power source (14) and is suitable for heating up when an electrical voltage is applied. The invention further relates to a vehicle comprising the fuel cell stack (4) and to a method for producing the fuel cell stack (4) and to a use of the fuel cell stack (4).

Description

Fuel cell stack and method for making and using one

Fuel cell stack

The invention relates to a fuel cell stack, a vehicle comprising the fuel cell stack and a method for producing and using the fuel cell stack.

State of the art

A fuel cell is a galvanic cell that converts the chemical reaction energy of a continuously supplied fuel and an oxidizing agent into electrical energy. A fuel cell is therefore an electrochemical energy converter. In known fuel cells, hydrogen (H ₂ ) and oxygen (O ₂ ) in particular are converted into water (H ₂ O), electrical energy and heat.

An electrolyzer is an electrochemical energy converter which splits water (H ₂ O) into hydrogen (H ₂ ) and oxygen (O ₂ ) using electrical energy.

Among other things, proton exchange membranes (Proton Exchange Membrane = PEM) fuel cells are known, which are also referred to as polymer electrolyte fuel cells. Anion exchange membranes are also known for both fuel cells and electrolysers. Proton exchange membrane fuel cells have a centrally arranged membrane which is conductive for protons, i.e. for hydrogen ions. The oxidizing agent, in particular atmospheric oxygen, is thereby spatially separated from the fuel, in particular hydrogen.

Proton exchange membrane fuel cells also have an anode and a cathode. The fuel is fed to the anode of the fuel cell and catalytically oxidized to protons with the release of electrons. The protons pass through the membrane to the cathode. The released electrons are diverted from the fuel cell and flow to the cathode via an external circuit.

The oxidizing agent is fed to the fuel cell's cathode and it reacts to water by absorbing electrons from the external circuit and protons that have passed through the membrane to the cathode. The resulting water is drained from the fuel cell.

The gross response is:

O ₂ + 4H ⁺ + Ae - 2H ₂ O

A voltage is applied between the anode and the cathode of the fuel cell. To increase the voltage, several fuel cells can be arranged mechanically one behind the other to form a fuel cell stack, which is also referred to as a fuel cell stack, and electrically connected in series.

A fuel cell stack, to which hydrogen and oxygen and optionally a cooling medium are fed, is usually closed by two end plates. Current collectors are arranged within the end plates.

When electricity is generated through the production of water in a fuel cell, dissolved ions can be present in the water produced as a result of impurities and / or side reactions. The ions lead to an undesirable electrical conductivity, which causes corrosion effects and thus degradation, i.e. a loss of performance, on the fuel cell or the fuel cell stack.

Fuel cells are often supplied with media such as hydrogen and oxygen via supply channels arranged perpendicular to the membrane of the fuel cell. Media are also discharged via these feed channels. The supply channels are connected to the fuel cell or the fuel cell stack by port structures, which can also be referred to as fluid connections. Between different port structures one Fuel cells or a fuel cell stack can generate undesirable leakage currents.

US 2013 / 0295481A1 and DE 10 2012 220 705 Al deal with fluid connections for polymer electrolyte fuel cells. Molded parts or injection molded parts made of polymers are described, which electrically isolate the gas connections for the supply and discharge of air and hydrogen and a cooling water connection in the area of the end plate from one another. A manifold block for a fuel cell has electrical insulation for a coolant flow channel in an internal flow channel.

DE 11 2007 002 945 B4 describes a shear-elastic element that ensures tolerance compensation in the fuel cell stack. The shear elastic member is inserted between a clamping member and an end surface of the fuel cell stack in the stacking direction and elastically deforms in a shearing direction that is perpendicular to the stacking direction. Furthermore, the fuel cell stack has a cushion element which is positioned between the stack and a plate covering at least one side section of the side face of the stack.

If fuel cell stacks are put into operation at a temperature below the freezing point of water, frozen water can appear on the port structures. This can also occur if the fuel cell stack can be heated electrically by means of an intermediate layer, since the port structures can be thermally insulated by a polymer intermediate plate and thus separated from the heating.

Disclosure of the invention

A fuel cell stack is proposed, comprising at least one fuel cell, at least one current collector and at least one end plate, wherein an end position is arranged between the at least one current collector and the at least one end plate and the end position is made up of an electrically conductive polymer material and the electrically conductive polymer material electrically connected to a power source and is suitable to heat up when an electrical voltage is applied.

Furthermore, a vehicle comprising the fuel cell stack is proposed as well as a method for producing the fuel cell stack, characterized in that at least one port structure is produced from the electrically conductive polymer material by injection molding and then a mechanical connection between the at least one port structure and a second port structure is Structure or between the at least one port structure and parts of the end position that are different from the at least one port structure, is produced by injection molding of an electrically insulating component.

The end position can also be inserted into the fuel cell stack as an injection molded part in the form of an insert.

The fuel cell stack, which is in particular a PEM fuel cell stack, can comprise more than one end position, in particular two end positions. The end position made of electrically conductive polymer material, which is also referred to as polymer trace heating, can compensate for both shape tolerances and position tolerances within the fuel cell stack structure.

In the context of the present invention, the current collector is understood to mean an electrically conductive surface element which is electrically connected directly to an outer fuel cell of the fuel cell stack, in particular to the outer electrode of the outer fuel cell. Over the length of the fuel cell stack, the current collector brings together the current transported over the entire fuel cell surface in the plane of the surface element on one or more electrical connection points so that the electrical power of the fuel cell stack can be dissipated without impermissible ohmic heating.

The outer fuel cell of the fuel cell stack usually ends with a flow field, which can also be referred to as a flow plate. The flow field can be formed by a graphitic or metallic structure. Since there is no further fuel cell connected to the outer fuel cell, it can also be referred to as a monopolar plate. The current that flows through the fuel cell stack in the stacking direction and that forms the electrical fuel cell stack voltage between the two fuel cell stack ends as the sum of the individual fuel cell voltages must be electrically transmitted from the inside of the fuel cell stack to the outside, for example via seals. With smaller powers, this can be done in that the outer flow plate or monopolar plate has an electrical current tap in the plane of the monopolar plate. The monopolar plate is therefore enlarged by an extension or flag for a cable connection.

Alternatively, the monopolar plate can be electrically connected to the end plate or pressed and the current can be tapped at an end collector, preferably comprising stainless steel or aluminum, which bundles the current to an electrical cable connection. However, if there is an elastic intermediate layer made of a non-conductive polymer for mechanical tolerance compensation, the end plate is electrically insulated. Therefore, an electrically highly conductive plate, for example comprising copper, with low contact resistance, which can be achieved by a coating, for example with gold, can be arranged between the monopolar plate and the end plate, and this can be electrically connected to a connection cable. The plate, which has good electrical conductivity, is preferably used when the fuel cell stack can deliver a large amount of electrical power.

The end plate can transmit forces from tensioning elements to the electrically active cell surfaces such as the membrane surfaces evenly, that is to say with a uniform local pressing force and the same local deformation, the tensioning elements being outside the electrically active cell surfaces.

The current collector is electrically conductive and ensures electrical contact between the outer electrode of the fuel cell stack and a connection cable, while the end plate ensures mechanical stability of the fuel cell stack.

The end position comprises the electrically conductive polymer material. The end position can optionally include the electrically insulating component. The end position is preferably more than 50% by weight, more preferably more than 80% % By weight, more preferably more than 95% by weight, of the electrically conductive polymer material. The end position preferably comprises from 5% by weight to 20% by weight of the electrically insulating component. The end position is preferably manufactured using injection molding.

The electrically conductive polymer material, which is also referred to as Heat Inducing Thermoplastics (HIT), comprises a plastic matrix and an electrically conductive additive, which can also be referred to as a filler. The plastic matrix preferably consists of at least one thermoplastic or at least one thermoset. The plastic matrix more preferably comprises polyvinylidene fluoride (PVDF), polypropylene (PP), polyphenylene sulfide (PPS), polyamide (PA), polyether ether ketone (PEEK) or mixtures thereof. In particular, the plastic matrix consists of polyvinylidene fluoride, polypropylene, polyphenylene sulfide, polyamide, polyetheretherketone or mixtures thereof. The electrically conductive additive preferably comprises or consists of metal powder, metal fibers, carbon black, graphite, carbon fibers, carbon nano tubes or mixtures thereof. The proportion by weight of the electrically conductive additive, based on the electrically conductive polymer material, is advantageously above the percolation limit of the electrically conductive polymer material.

The electrical conductivity s of a material is determined by the number N of charge carriers per volume V, the elementary charge e, and the mobility m of the charge carriers: s = (N / V) p e

Depending on the type and amount of the electrically conductive additive in the electrically conductive polymer material, the specific resistance (r = 1 / s) of the electrically conductive polymer material is not more than 5 Ωcm.

Joule heat is generated by the electrical resistance of the electrically conductive polymer material, so that the electrically conductive polymer material is heated when an electrical voltage is applied, as is also described in DE 102012 212 798 A1. Furthermore, electrically conductive plastic compounds are shown in Hopmann et al., Development of electrically conductive plastic compounds based on filier combinations, Journal of Plastics Technology 10 (2014) 2, pages 50 to 67.

The electrical conductivity of the electrically conductive polymer material preferably decreases with increasing temperature. Accordingly, the specific resistance of the electrically conductive polymer material preferably increases with increasing temperature and there is a positive temperature coefficient (PTC). The decreasing electrical conductivity with increasing temperature is given in particular with thermoplastics as a plastic matrix. Due to the thermal expansion of the plastic matrix, the distances between particles of the electrically conductive additive become larger and electrical conduction paths are interrupted. The electrical resistance preferably increases exponentially to the softening temperature of the thermoplastic.

The heating of the electrically conductive polymer material is preferably limited to a maximum temperature of 200 ° C. when the electrical voltage is applied. By decreasing the conductivity of the electrically conductive polymer material at a higher temperature, self-regulation of the heating of the fuel cell stack by the end position is achieved, so that overheating is avoided.

The electrically conductive polymer material preferably has shear-elastic properties. The modulus of elasticity of the electrically conductive polymer material is preferably more than 1.5 GPa, more preferably more than 2 GPa. In particular, the E modulus of the electrically conductive polymer material is higher than the E modulus of the plastic matrix. For example, the modulus of elasticity of polyamide is from 1.0 GPa to 1.1 GPa and of polyamide containing 3% by weight of carbon nano fiber (CNT) is 2.1 GPa. The modulus of elasticity of polyamide containing 15% by weight of carbon black (CB) is, for example, 2.8 GPa.

The end position preferably has at least one port structure for the passage of a fluid such as hydrogen, oxygen, water or a cooling medium. The at least one port structure is preferably constructed from the electrically conductive polymer material. The at least one port structure therefore preferably comprises the electrically conductive polymer material, the at least one port structure more preferably consists of more than 50% by weight, more preferably more than 80% by weight, more preferably more than 95% by weight .-%, from the electrical conductive polymer material. The at least one port structure is advantageously designed in one piece with the end position.

The end position can be arranged directly between the end plate and the current collector, which adjoins the monopolar plate of the outer fuel cell of the fuel cell stack. The current collector attacks the

Fuel cell stack voltage. Since the electrically conductive polymer material can also be contacted by the current collector, the current collector can at the same time be a first power connection for the electrically conductive polymer material. The end plate can represent a second power connection. Alternatively, a conductive contact can be arranged in each case on a first and a second surface of the electrically conductive polymer material, so that during the assembly the first surface is contacted by the current collector and the second surface is contacted by the end plate. In this way, the end position becomes evenly warm, since the paths in the electrically conductive polymer material, i.e. the resistance, are roughly the same length.

The end position can have a non-planar shape, wherein the electrically conductive polymer material can enclose the port structure on the inside or outside, so that the end position has nozzles formed from the electrically conductive polymer material, which can also be referred to as fingers. There is a larger area of electrically conductive polymer material on the port structure and more heating cable can be emitted there.

The at least one port structure is advantageously already produced during the production of the end position, for example by injection molding the electrically conductive polymer material. In particular, more than one port structures can initially be produced, which are then mechanically connected to one another by subsequent injection molding of the electrically insulating component between the more than one port structures. The electrically insulating component, which preferably contains a second polymer material, simultaneously serves to mechanically connect the more than one port structure and also to electrically isolate the more than one port structure from one another. The second polymer material can correspond to the plastic matrix of the electrically conductive polymer material. The electrically insulating component preferably comprises polyvinylidene fluoride (PVDF), polypropylene (PP), polyphenylene sulfide (PPS), polyamide (PA), polyetheretherketone (PEEK) or mixtures thereof. In particular, the electrically insulating component consists of polyvinylidene fluoride, polypropylene, polyphenylene sulfide, polyamide, polyether ether ketone or mixtures thereof. The electrically insulating component advantageously has a material bond with the electrically conductive polymer material. The material of the plastic matrix of the electrically conductive polymer material can be used for this. To adapt mechanical and thermal properties such as thermal conductivity, the electrically insulating component can comprise a second additive such as ceramic powder.

The end position preferably has at least two areas which are electrically isolated from one another and which can also be referred to as parts. In particular, no more than one port structure is arranged on at least one of the at least two regions that are electrically isolated from one another.

The end position can consequently also be electrically segmented, that is to say divided into regions that are electrically isolated from one another. The current collector can be a first common power connection for each electrically isolated area.

A second power connection for each electrically isolated area can be integrated into the port structure.

One surface of the end position preferably has an electrically insulating film, which can also be referred to as a layer or layer. The electrically insulating film can also be referred to as an electrically insulating surface and is preferably a polymer film, preferably comprising polyethylene (PE), polypropylene (PP), polyetheretherketone (PEK), polyimide (PI), polyvinylidene fluoride (PVDF), polyamide (PA) ), Polytetrafluoroethylene (PTFE) or mixtures thereof, or a ceramic film. More preferably, the electrically insulating film consists of a polymer that can form a material bond with the plastic matrix of the electrically conductive polymer material. In particular, the electrically insulating film consists of a polymer of the same class as the plastic matrix of the electrically conductive polymer material. The film can be applied to the surface of the end position by placing, gluing, interposing or painting, for example with a PVDF paint. The electrically insulating film can represent additional electrical insulation with respect to the current collector and / or the end plate. The surfaces of components that have been injection molded from conductive polymer can have less electrical conductivity due to the formation of a cast skin. The electrically conductive polymer material advantageously additionally has metallic conductor tracks, which are arranged in particular near the surface, more preferably on the surface of the electrically conductive polymer material. The metallic conductor tracks advantageously penetrate the cast skin and ensure permanent electrical contact with the electrically conductive additive. The metallic conductor tracks can be used to make electrical contact between the electrically conductive polymer material and the power source.

The metallic conductor tracks can be produced by cold gas spraying of metal, in which metal is introduced into the top layer of the electrically conductive polymer material at high speed. The metallic conductor tracks can be introduced three-dimensionally into the injection-molded component, that is to say the end position, which may have port structures, so that a power connection can be made at almost any point in the end position.

Compared to the introduced metallic conductor tracks, the remaining surface of the end position, which may have a cast skin, may have poorer electrical conductor properties. If this is desired, the electrically insulating film can be applied to the surface of the end position for additional insulation.

Furthermore, a use of the fuel cell stack is proposed, the end position being used as surface heating with a specific power of 1.5 W / cm ² to 4 W / cm ² , in particular 2 W / cm ² to 3 W / cm ² .

The applied electrical voltage can preferably be 12, 24, 48 or 220 volts, or a multiple of the voltage of an individual fuel cell in the fuel cell stack.

Advantages of the invention Due to the end position of the fuel cell stack according to the invention, electrical trace heating is combined with a polymer intermediate layer and, if necessary, targeted port heating in one component. The areas near the end plates or their edge areas in the fuel cell stack are heated in particular by a simple polymer component, in particular a polymer injection-molded part. The end position is used in particular to heat the gas connection areas in the form of the at least one port structure.

With appropriate electrical contacting and electrical insulation of different port structures from one another, which are possible within the scope of the present invention, each port structure can be heated separately and the temperature of the respective port structure can be regulated separately.

The heatable port structures prevent condensation in the area of the port structures during a cold start of the fuel cell stack, and rapid heating of the edge areas of the fuel cell stack is possible.

Furthermore, shape tolerances and bearing tolerances of the fuel cell stack can be compensated for by the end position.

In addition, the electrically conductive polymer material enables heat to be dissipated homogeneously over the surface of the end position. The polymer material means that the end position can be freely shaped, for example by an injection molding process. Accordingly, the heat can be directed to individual areas of the fuel cell stack, such as the port structure, where there is a need for heat.

The combination of electrical trace heating, polymer intermediate layer and port heating can reduce the overall weight of the fuel cell stack.

Brief description of the drawings

Embodiments of the invention are explained in more detail with reference to the drawings and the following description. Show it:

Figure 1 shows a fuel cell stack,

Figure 2 shows a section of an end position made of electrically conductive polymer material,

FIG. 3 shows a graph of the temperature dependence of the electrical conductivity of the electrically conductive polymer material,

FIG. 4 shows a plan view of a fuel cell stack with end position and port structures and

FIG. 5 shows a side view of the fuel cell stack according to FIG. 4. Embodiments of the invention

In the following description of the embodiments of the invention, the same or similar elements are denoted by the same reference numerals, a repeated description of these elements being dispensed with in individual cases. The figures represent the subject matter of the invention only schematically.

FIG. 1 shows a schematic illustration of a fuel cell stack 4 with a plurality of fuel cells 3. Each fuel cell 3 has a membrane 23, two gas diffusion layers 1, an anode 31 and a cathode 32. The individual fuel cells 3 are delimited from one another by bipolar plates 50, which can include a cooling plate 45.

The fuel cell stack 4, to which hydrogen 40 and oxygen 42 and a cooling medium 44 are supplied, is closed by two end plates 48 and has current collectors 52. The various inlets are separated from one another by seals 46. Between the end plate 48 and the current collector 52, an end position 10, which serves as a shear-elastic, heatable compensation position, is arranged. FIG. 2 shows a section of an end position 10 constructed from an electrically conductive polymer material 12. The electrically conductive polymer material 12 is connected to a power source 14. In the illustration shown here, an electrical voltage is applied to the electrically conductive material 12, as a result of which the electrically conductive polymer material 12 is heated and gives off heat 34 to the surroundings and in particular to the fuel cell stack 4. The end position 10 can be produced by means of injection molding. This means that the shape for the end position 10 is flexible. The heat 34 is given off evenly over a surface 18 of the end position 10.

FIG. 3 shows a graph of the dependence of a conductivity 47 on a temperature 43 of the electrically conductive polymer material 12. The electrical conductivity 47 of the electrically conductive polymer material 12 and the temperature 43 of the electrically conductive polymer material 12 are plotted over time 41.

As the temperature 43 rises, the electrical conductivity 47 of the electrically conductive polymer material 12 decreases, so that heating of the electrically conductive polymer material 12 is self-regulating. The lower the conductivity 47, the less heat 34 is generated by the electrically conductive polymer material 12, so that a maximum temperature 36 of the electrically conductive polymer material 12 is not exceeded.

FIG. 4 shows a plan view of a fuel cell stack 4 with an end position 10 and port structures 16 and a current collector 52.

The end position 10 has at least one port structure 16 and a second port structure 17. Oxygen and hydrogen are supplied to the fuel cell stack 4 through the port structures 16, 17 and water is removed.

The end position 10 also has a first insulated area 24 and a second insulated area 26, which are insulated from one another by an electrically insulating component 30. The electrically insulating component 30 also establishes a mechanical connection 28 between the first isolated area 24 and the second isolated area 26. The at least one port structure 16 and the second port structure 17 are also electrically insulated from one another by the electrically insulating component 30, so that leakage currents be avoided. The regions 24, 26 that are electrically isolated from one another are electrically separated from one another by the electrically insulating component 30. Each isolated area 24, 26 has no more than one port structure 16, 17.

The end position 10 with the first insulated area 24 and the second insulated area 26 is made up of the electrically conductive polymer material 12 and has a surface 18.

The electrically conductive polymer material 12 has metallic conductor tracks 22 which are arranged in the electrically conductive polymer material 12 near the surface 18.

Furthermore, each port structure 16 has a single contact 33. Since the port structures 16, 17 are formed from the electrically conductive polymer material 12, the individual port structures 16, 17 and the areas 24, 26 isolated from one another can be heated separately from one another by means of the individual contacts 33.

FIG. 5 shows a side view of the fuel cell stack 4 according to FIG. 4 with the end position 10, port structures 16. The port structures 16 protrude through an end plate 48, where they have the individual contacts 33. The end plate 48 is used to generate mechanical pressure and to compress the fuel cell stack 4.

The end position 10 is subdivided into the regions 24, 26 that are electrically isolated from one another and are constructed from the electrically insulating component 30 by the mechanical connection 28. An electrically insulating film 20 is also located on the surface 18 of the end position 10.

The invention is not restricted to the exemplary embodiments described here and the aspects emphasized therein. Rather, within the range specified by the claims, a large number of modifications are possible that are within the scope of expert knowledge.

Claims

Expectations

1. Fuel cell stack (4) comprising at least one fuel cell (3), at least one current collector (52) and at least one end plate (48), with an end position (10) between the at least one current collector (52) and the at least one end plate (48) is arranged and the end position (10) is made up of an electrically conductive polymer material (12) and wherein the electrically conductive polymer material (12) is electrically connected to a power source (14) and is suitable to heat up when an electrical voltage is applied.

2. Fuel cell stack (4) according to claim 1, characterized in that the end position (10) has at least one port structure (16) for the passage of a fluid and the at least one port structure (16) made of the electrically conductive polymer material (12) is constructed.

3. Fuel cell stack (4) according to one of the preceding claims, characterized in that one surface (18) of the end position (10) has an electrically insulating film (20).

4. Fuel cell stack (4) according to one of the preceding claims, characterized in that the electrically conductive polymer material (12) has shear-elastic properties.

5. Fuel cell stack (4) according to one of the preceding claims, characterized in that the electrically conductive polymer material (12) additionally has metallic conductor tracks (22) which are arranged in particular close to the surface.

6. Fuel cell stack (4) according to one of the preceding claims, characterized in that the end position (10) has at least two electrically isolated regions (24, 26).

7. Fuel cell stack (4) according to one of the preceding claims, characterized in that no more than one port structure (16) is arranged on at least one of the at least two electrically isolated regions (24, 26).

8. A vehicle comprising a fuel cell stack (4) according to any one of claims 1 to 7.

9. The method for producing a fuel cell stack (4) according to one of claims 1 to 7, characterized in that at least one port structure (16) is produced from the electrically conductive polymer material (12) by means of injection molding and then a mechanical connection (28) between the at least one port structure (16) and a second port structure (17) or between the at least one port structure (16) and parts of the end position (10) that differ from the at least one port structure (16) are made by injection molding an electrically insulating component (30).

10. Use of a fuel cell stack (4) according to one of claims 1 to 7, characterized in that the end position (10) as a surface heating system with a specific power of 1.5 watts / cm ² to 4 watts / cm ² , in particular 2 watts / cm ² to 3 watts / cm ^{2 is} used.