WO2021228525A1

WO2021228525A1 - Fuel cell unit

Info

Publication number: WO2021228525A1
Application number: PCT/EP2021/060480
Authority: WO
Inventors: Ernst Hauk; Harald Bauer; Lars Olems; Eberhard Maier; Andreas Rettich; Jochen Wessner
Original assignee: Robert Bosch Gmbh
Priority date: 2020-05-11
Filing date: 2021-04-22
Publication date: 2021-11-18
Also published as: DE102020205877A1

Abstract

The invention relates to a fuel cell unit in the form of a fuel cell stack for electrochemically producing electrical energy, comprising fuel cells arranged in a stacked manner, the fuel cells each comprising, as layered components (5, 6, 7, 8, 9, 10), a proton exchange membrane (5), an anode (6), a cathode (7), a bipolar plate (10) and a gas diffusion layer (9), wherein at least one electrical resistance heating element (37) is integrated in at least one layered component (5, 6, 7, 8, 9, 10) of at least one fuel cell (2) in order to heat the fuel cell unit (1) with electrical energy.

Description

description

title

Fuel cell unit

The present invention relates to a fuel cell unit according to the preamble of claim 1 and a fuel cell system according to the preamble of claim 9.

State of the art

Fuel cell units as galvanic cells convert continuously supplied fuel and oxidizing agent as process fluids into electrical energy by means of redox reactions at an anode and cathode. Fuel cells are used in a wide variety of stationary and mobile applications, for example in houses without a connection to a power grid or in motor vehicles, in rail transport, in aviation, in space travel and in shipping. In fuel cell units, a large number of fuel cells are arranged in a stack in an aligned manner. As a further process fluid, a cooling fluid, in particular a cooling liquid, is used to dissipate the waste heat generated in the exothermic redox reaction during operation in order to control the temperature of the fuel cell unit.

When the fuel cell unit is put into operation at low temperatures, in particular below 0 ° C., there is a risk that the process fluids can no longer be properly glided through the channels and gas spaces. The gas diffusion layers form gas spaces for the passage of fuel and oxidizing agent. The gas diffusion layer on the cathode distributes the oxidizing agent evenly from channels for oxidizing agents of a bipolar plate onto the catalyst layer on the cathode. Water is created at the cathode and this water accumulates in the gas diffusion layer. the The gas diffusion layer is made up, for example, of a hydrophobized carbon paper and a bonded layer of carbon powder, so that the water produced at the cathode is partially stored or absorbed by the gas diffusion layer. At low temperatures of below 0 ° C before the fuel cell unit is started up, this water freezes and blocks the passage of the oxidizing agent air. This makes it difficult to start up the fuel cell unit and, at the start of start-up, the fuel cell unit disadvantageously produces a significantly reduced electrical output. This problem also occurs to a lesser extent on the gas diffusion layer of the anodes, because water can also accumulate here.

In fuel cell units as fuel cell stacks, there is a current collecting plate on the top and bottom fuel cells. It is already known to arrange a heating plate or a heating foil as an electrical resistance heating element on the outside of the current collecting plate. This electrical resistance heating element is electrically isolated from the current collecting plate and heats the current collecting plate as well as fuel cells in the vicinity of the current collecting plate. In a disadvantageous manner, a substantial part of the heat generated by the electrical resistance heating element is not dissipated into the fuel cells, but into the environment, because the electrical resistance heating element is arranged on the outside of the fuel cell unit. Furthermore, only a small part of the fuel cells in the vicinity of the electrical resistance heating element is heated.

Disclosure of the invention

Advantages of the invention

Fuel cell unit according to the invention as a fuel cell stack for the electrochemical generation of electrical energy, comprising stacked fuel cells, the fuel cells comprising as layered components in each case a proton exchange membrane, an anode, a cathode, a bipolar plate and a gas diffusion layer, with at least one layered component at least one fuel cell an electrical resistance heating element is integrated to heat the fuel cell unit with electrical energy. The fuel cell unit can thus be effectively heated by the electrical resistance heating element before start-up and / or during operation, so that start-up and / or operation in the initial phase of operation is possible without any problems even at temperatures below 0 ° C. because ice has thawed and / or is thawed in a very short time by means of the heat made available by the at least one electrical resistance heating element. Due to the integration of the electrical resistance heating element in a layered component of the fuel cell, essentially all of the heat generated by the electrical resistance heating element is used to heat the fuel cells.

In a further embodiment, the at least one layered component with the integrated electrical resistance heating element is the gas diffusion layer. The gas diffusion view is well suited for the installation of the at least one electrical resistance heating element due to the material used and, in addition, with the gas diffusion view, especially in the case of external, last gas diffusion layers, there is the greatest risk of blocking or reducing the conduction of oxidizing agent or fuel due to frozen water or Ice cream.

In a further variant, the at least one layered component with the integrated electrical resistance heating element is the last layered component in a direction perpendicular to the fictitious planes spanned by the layered components. The last layer-shaped component is most exposed to the risk of thermal cooling below 0 ° C, so that the electrical resistance heating element is particularly useful here.

In a further embodiment, the last layered component with the integrated electrical resistance heating element is the last gas diffusion layer in the direction perpendicular to the fictitious planes spanned by the layered components. In an additional embodiment, an electrical resistance heating element is integrated in each of the last two gas diffusion layers in the direction perpendicular to the fictitious planes spanned by the layered components. This means that an electrical resistance heating element is integrated in the top and bottom gas diffusion layers.

Electrical resistance heating elements are expediently integrated into several layered components, in particular gas diffusion layers.

In a supplementary variant, the fuel cell unit comprises at least one temperature sensor, preferably several temperature sensors, and depending on the temperature detected by the at least one temperature sensor, the electrical heating power of the at least one electrical resistance heating element can be controlled and / or regulated, in particular before the fuel cell unit is started up. For example, at temperatures below 0 ° C, the at least one electrical resistance heating element can be used to heat the fuel cell unit and / or at least one fuel cell to a specified temperature above 0 ° C, and when the specified temperature, for example 5 ° C, is reached, the at least one electrical resistance heating element can be switched off .

In a supplementary variant, the electrical resistance heating element is designed as a heating wire and / or conductor track and / or flat heating element, in particular heating foil.

Fuel cell system according to the invention, in particular for a motor vehicle, comprising a fuel cell unit as a fuel cell stack with fuel cells, a compressed gas storage device for storing gaseous fuel, a gas delivery device for delivering a gaseous oxidizing agent to the cathodes of the fuel cells, the fuel cell unit being designed as a fuel cell unit described in this patent application.

In a further embodiment, the fuel cell system comprises a battery for operating the at least one electric battery

Resistance heating element before commissioning the fuel cell system. The at least one electrical resistance heating element can thus also be operated without electrical energy from the fuel cell unit.

In a further embodiment, the fuel cell system comprises a control and / or regulating unit for controlling and / or regulating the heating power of the at least one electrical resistance heating element, in particular as a function of the temperature of the fuel cell unit detected by at least one temperature sensor.

The at least one temperature sensor is preferably integrated into at least one layered component, in particular into the at least one layered component with the integrated electrical resistance heating element.

Electrical resistance heating elements are expediently integrated in at least 3, 5, 10, 20, 30 or 50 layered components, in particular gas diffusion layers and / or bipolar plates.

In a further embodiment, electrical resistance heating elements are integrated into every second, third, fourth or fifth layered component, in particular gas diffusion layers and / or bipolar plates.

In a further embodiment, the at least one electrical resistance heating element is integrated into the layered component so that the electrical resistance heating element in the layered component is at least partially, in particular completely, encased by the layered component and / or on an outside and / or inside of the layered component rests.

In a supplementary embodiment, the at least one electrical resistance heating element is integrated in layered components as bipolar plates, in particular in that the at least one electrical resistance heating element is arranged on an outside of the bipolar plate and / or on an inside of a channel for fuel and / or oxidizing agent and / or coolant of the Bipolar plate is arranged. In an additional variant, the at least one electrical resistance heating element is designed as a heating plate.

The layered components of the fuel cells are useful: proton exchange membranes, anodes, cathodes, gas diffusion layers and bipolar plates.

The components of the fuel cells are preferably stacked in an aligned manner.

In a further embodiment, the fuel cells of the fuel cell unit are stacked in an aligned manner.

In a further embodiment, the fuel cells each comprise a proton exchange membrane, an anode, a cathode, at least one gas diffusion layer, preferably two gas diffusion layers, and at least one bipolar plate.

In a further embodiment, the connecting device is designed as a bolt and / or is rod-shaped.

The clamping elements are expediently designed as clamping plates.

In a further variant, the gas delivery device is designed as a fan or a compressor.

In particular, the fuel cell unit comprises at least 3, 4, 5 or 6 connection devices.

In a further embodiment, the tensioning elements are plate-shaped and / or disk-shaped and / or flat and / or are designed as a grid.

Preferably the fuel is hydrogen, hydrogen-rich gas, reformate gas or natural gas. The fuel cells are expediently designed to be essentially flat and / or disk-shaped.

In a supplementary variant, the oxidizing agent is air with oxygen or pure oxygen.

The fuel cell unit is preferably a PEM fuel cell unit with PEM fuel cells.

Brief description of the drawings

In the following, embodiments of the invention are under

Described in more detail with reference to the accompanying drawings. It shows:

Fig. 1 is a greatly simplified exploded view of a

Fuel cell system with components of a fuel cell,

2 shows a perspective view of part of a fuel cell,

3 shows a longitudinal section through a fuel cell,

4 shows a perspective view of a fuel cell unit as a fuel cell stack, i.e. H. a fuel cell stack,

5 shows a section through the fuel cell unit according to FIGS. 4 and

6 shows a view of an electrical resistance heating element in a first embodiment as a heating wire,

7 shows a view of the electrical resistance heating element in a second exemplary embodiment as a conductor track on a circuit board and FIG

8 shows a view of the electrical resistance heating element in a third exemplary embodiment as a heating film. 1 to 3 show the basic structure of a fuel cell 2 as a PEM fuel cell 3 (polymer electrolyte fuel cell 3). The principle of fuel cells 2 is that electrical energy or electrical current is generated by means of an electrochemical reaction. Hydrogen H2 is fed as a gaseous fuel to an anode 7 and the anode 7 forms the negative pole. A gaseous oxidizing agent, namely air with oxygen, is passed to a cathode 8, ie the oxygen in the air provides the necessary gaseous oxidizing agent. A reduction (electron uptake) takes place at the cathode 8. The oxidation as the release of electrons is carried out at the anode 7.

The redox equations for the electrochemical processes are:

Cathode:

0 ₂ + 4 H ⁺ + 4 e- ~ »2 H ₂ 0

Anode:

2 H _{2 ~} »4 H ⁺ + 4 e-

Sum reaction equation of cathode and anode:

2 H ₂ + 0 _{2 ~} »2 H ₂ 0

The difference between the normal potentials of the electrode pairs under standard conditions as reversible fuel cell voltage or open circuit voltage of the unloaded fuel cell 2 is 1.23 V. This theoretical voltage of 1.23 V is not achieved in practice. In the idle state and with small currents, voltages over 1.0 V can be reached and in operation with larger currents, voltages between 0.5 V and 1.0 V are reached. The series connection of several fuel cells 2, in particular a fuel cell unit 1 as a fuel cell stack 1 of several stacked fuel cells 2, has a higher voltage, which corresponds to the number of fuel cells 2 multiplied by the individual voltage of each fuel cell 2.

The fuel cell 2 also includes a proton exchange membrane 5 (Proton Exchange Membrane, PEM), which between the anode 7 and the Cathode 8 is arranged. The anode 7 and cathode 8 are layer-shaped or disk-shaped. The PEM 5 acts as an electrolyte, catalyst carrier and separator for the reaction gases. The PEM 5 also functions as an electrical insulator and prevents an electrical short circuit between the anode 7 and cathode 8. In general, proton-conducting foils made from perfluorinated and sulfonated polymers are 12 μm to 150 μm thick. The PEM 5 conducts the protons H ⁺ and essentially blocks ions other than protons H ⁺ , so that the charge transport can take place due to the permeability of the PEM 5 for the protons H ^+. The PEM 5 is essentially impermeable to the reaction gases oxygen O2 and hydrogen H2, ie blocks the flow of oxygen O2 and hydrogen H2 between a gas space 31 on the anode 7 with hydrogen H2 fuel and the gas space 32 on the cathode 8 with air or Oxygen O2 as an oxidizing agent. The proton conductivity of the PEM 5 increases with increasing temperature and increasing water content.

The electrodes 7, 8 as the anode 7 and cathode 8 rest on the two sides of the PEM 5, each facing the gas spaces 31, 32. A unit composed of the PEM 5 and the electrodes 6, 7 is referred to as a membrane electrode assembly 6 (membrane electrode assembly, MEA). The electrodes 7, 8 are pressed with the PEM 5. The electrodes 6, 7 are platinum-containing carbon particles that are bound to PTFE (polytetrafluoroethylene), FEP (fluorinated ethylene-propylene copolymer), PFA (perfluoroalkoxy), PVDF (polyvinylidene fluoride) and / or PVA (polyvinyl alcohol) and are embedded in microporous carbon fiber, Glass fiber or plastic mats are hot-pressed. A catalyst layer 30 is normally applied to each of the electrodes 6, 7 on the side facing the gas spaces 31, 32. The catalyst layer 30 on the gas space 31 with fuel on the anode 7 comprises nanodisperse platinum ruthenium on graphitized soot particles that are bound to a binder. The catalyst layer 30 on the gas space 32 with oxidizing agent on the cathode 8 analogously comprises nanodisperse platinum. Nafion®, a PTFE emulsion or polyvinyl alcohol, for example, are used as binders.

A gas diffusion layer 9 (gas diffusion layer, GDL) rests on the anode 7 and the cathode 8. The gas diffusion layer 9 on the anode 7 distributes the fuel from channels 12 for fuel evenly on the Catalyst layer 30 on the anode 7. The gas diffusion layer 9 on the cathode 8 distributes the oxidizing agent from channels 13 for oxidizing agent evenly on the catalyst layer 30 on the cathode 8. The GDL 9 also draws water of reaction in the opposite direction to the direction of flow of the reaction gases, ie in one Direction from the catalyst layer 30 to the channels 12, 13. Furthermore, the GDL 9 keeps the PEM 5 moist and conducts the current.

The GDL 9 is composed, for example, of a hydrophobized carbon paper and a bonded layer of carbon powder.

A bipolar plate 10 rests on the GDL 9. The electrically conductive bipolar plate 10 serves as a current collector, to drain water and to conduct the reaction gases through a channel structure 29 and / or a flow field 29 and to dissipate the waste heat that occurs in particular during the exothermic electrochemical reaction at the cathode 8. In order to dissipate the waste heat, channels 14 for the passage of a liquid or gaseous coolant are incorporated into the bipolar plate 10. The channel structure 29 in the gas space 31 for fuel is formed by channels 12. The channel structure 29 in the gas space 32 for oxidizing agent is formed by channels 13. For example, metal, conductive plastics and composite materials or graphite are used as the material for the bipolar plates 10.

The layered components of a fuel cell 2 are thus the proton exchange membrane 6, the anode 7, the cathode 8, the two gas diffusion layers 9 and the bipolar plate 10. The layered components span fictitious planes 41.

A plurality of fuel cells 2 are stacked in a fuel cell unit 1 and / or a fuel cell stack 1 and / or a fuel cell stack 1 (FIG. 4). 1 shows an exploded view of two fuel cells 2 arranged one above the other. A seal 11 seals the gas spaces 31, 32 in a fluid-tight manner. In a compressed gas reservoir 21 (FIG. 1), hydrogen H2 is stored as fuel at a pressure of, for example, 350 bar to 700 bar. From the pressurized gas reservoir 21, the fuel is passed through a high pressure line 18 to a pressure reducer 20 to reduce the pressure of the fuel in a medium pressure line 17 from approximately 10 bar to 20 bar. The fuel is transferred from the medium pressure line 17 to an injector 19 directed. At the injector 19, the pressure of the fuel is reduced to an injection pressure between 1 bar and 3 bar. From the injector 19, the fuel is fed to a feed line 16 for fuel (FIG. 1) and from the feed line 16 to the channels 12 for fuel, which form the channel structure 29 for fuel. The fuel thereby flows through the gas space 31 for the fuel. The gas space 31 for the fuel is formed by the channels 12 and the GDL 9 on the anode 7. After flowing through the channels 12, the fuel not consumed in the redox reaction at the anode 7 and possibly water from a controlled humidification of the anode 7 is diverted from the fuel cells 2 through a discharge line 15.

A gas delivery device 22, for example in the form of a fan 23 or a compressor 24, delivers air from the environment as an oxidizing agent into a supply line 25 for oxidizing agent. From the supply line 25, the air is fed to the channels 13 for oxidizing agents, which form a channel structure 29 on the bipolar plates 10 for oxidizing agents, so that the oxidizing agent flows through the gas space 32 for the oxidizing agent. The gas space 32 for the oxidizing agent is formed by the channels 13 and the GDL 9 on the cathode 8. After flowing through the channels 13 or the gas space 32 for the oxidizing agent 32, the oxidizing agent not consumed at the cathode 8 and the water of reaction arising at the cathode 8 due to the electrochemical redox reaction are discharged from the fuel cells 2 through a discharge line 26. A feed line 27 is used to feed coolant into the channels 14 for coolant and a discharge line 28 is used to discharge the coolant conducted through the channels 14. The supply and discharge lines 15, 16, 25, 26, 27, 28 are shown in FIG End region (not shown) of bipolar plates 10 lying on top of one another. The fuel cell stack 1 together with the compressed gas storage device 21 and the gas delivery device 22 form a fuel cell system 4.

In the fuel cell unit 1, the fuel cells 2 are arranged between two clamping elements 33 as clamping plates 34. An upper clamping plate 35 rests on the uppermost fuel cell 2 and a lower clamping plate 36 lies on the lowest fuel cell 2. The fuel cell unit 1 comprises approximately 300 to 400 fuel cells 2, which are not all shown in FIG. 4 for reasons of drawing. The clamping elements 33 apply a compressive force to the fuel cells 2, ie the upper clamping plate 35 rests on the top fuel cell 2 with a compressive force and the lower clamping plate 36 rests on the lowermost fuel cell 2 with a compressive force. The fuel cell stack 2 is thus braced in order to ensure the tightness for the fuel, the oxidizing agent and the coolant, in particular due to the elastic seal 11, and also to keep the electrical contact resistance within the fuel cell stack 1 as small as possible. To brace the fuel cells 2 with the tensioning elements 33, four connecting devices 39 are designed as bolts 40 on the fuel cell unit 1, which are subject to tensile stress. The four bolts 40 are firmly connected to the chipboard 34.

In the fuel cell unit 1, an electrical resistance heating element 37 is integrated and built into the top and bottom gas diffusion layers 9 as a heating wire 42 (FIG. 3). The electrical resistance heating element 37 is integrated centrally in a direction perpendicular to the fictitious planes 41 in the gas diffusion layer 9, so that the electrical resistance heating element 37 is completely surrounded by the gas diffusion layer 9 within the gas diffusion layer 9. The electrical resistance heating element 37 can thus heat the gas diffusion layers 9 essentially uniformly. The heated gas diffusion layers 9 conduct the heat by means of thermal conduction to the layered components 5, 6, 7, 8, 10 of the fuel cell 2 lying directly and indirectly on the gas diffusion layer 9, so that these components 5, 6, 7, 8, 10 are also heated will. A temperature sensor 46 is also built into the gas diffusion layers 9 with integrated electrical resistance heating element 37. The fuel cell system 4 also includes a control and / or regulating unit (not shown) and a battery 38. The temperature sensor 46 detects the temperature in the fuel cells 2 before and / or during operation of the fuel cell system 4. B. 0 °, before commissioning and / or during operation of the fuel cell system 4, electrical current is passed through the electrical Wderstandsheizelement 37, so that thereby the Fuel cells 2 are heated. If the fuel cells 2 are heated with the electrical resistance heating elements 37 before being put into operation, this is done with electrical energy from the battery 38 Fuel cell unit 4 generated electrical energy removed.

In a further exemplary embodiment of the fuel cell unit 1, not shown, the electrical resistance heating elements 37 are not only arranged in the upper and lower gas diffusion layers 9, but also in a part of the gas diffusion layers 9 in between, for example in every third gas diffusion layer 9. The fuel cell unit 1 comprises 300 fuel cells 2 and thus 600 gas diffusion layers 9, so that the electrical resistance heating elements 37 are integrated in approximately 200 gas diffusion layers 9. This enables the fuel cell unit 1 to be heated essentially uniformly in a direction perpendicular to the fictitious planes 41 spanned by the components 5, 6, 7, 8, 10 of the fuel cells 2.

In a further exemplary embodiment of the fuel cell unit 1, not shown, the electrical resistance heating elements 37 are integrated into the bipolar plates 10 by the electrical resistance heating element 37 being arranged on an outside of the bipolar plates 10, which rest on the gas diffusion layer 9 for the cathode 8. The metal bipolar plates 10 conduct the heat well, so that the gas diffusion layer 9 for the anode 7 lying on the opposite outside of the bipolar plate 10 is also well heated. For example, electrical resistance heating elements 37 are integrated on every third or fifth bipolar plate 10 or only on the top and bottom bipolar plates 10.

In Fig. 6 a view or top view of only the electrical resistance heating element 37 is shown as the heating wire 42 in a view looking perpendicular to the fictitious planes 41 spanned by the components 5, 6, 7, 8, 10 of the fuel cells 2. The heating wire 42 is arranged in a meander shape and thus also enables parallel in one direction to the fictitious planes 41 spanned by the components 5, 6, 7, 8, 10 of the fuel cells 2, a uniform heating of the gas diffusion layer 9.

7 shows a view or top view of only the electrical resistance heating element 37 as a conductor track 43 in a view with a viewing direction perpendicular to the fictitious planes 41 spanned by the components 5, 6, 7, 8, 10 of the fuel cells 2. The conductor track 43 is arranged on a circuit board 44.

8 shows a view or top view of only the electrical resistance heating element 37 as a heating film 45 in a view with a viewing direction perpendicular to the fictitious planes 41 spanned by the components 5, 6, 7, 8, 10 of the fuel cells 2. The electrical Conductive heating foil 45 has an electrical conductor (not shown) on the left and right edge, so that the current is conducted through the heating foil 45 from the left edge to the right edge or vice versa. The electrically conductive heating film 45 is preferably surrounded by electrical insulation (not shown).

Viewed overall, the fuel cell unit 1 according to the invention and the fuel cell system 4 according to the invention are associated with significant advantages. At temperatures below 0 ° C., starting up the fuel cell unit 1 is associated with difficulties because process fluids, namely the fuel, the oxidizing agent and the coolant, can only be passed through the fuel cell unit 1 with difficulty. In particular, frozen water as ice in the gas diffusion layers 9 on the cathodes 8 prevents or reduces effective conduction of air or oxygen to the cathodes 8, so that the fuel cell unit 1 generates no or only a reduced electrical current. The electrical resistance heating elements 37 are integrated into the fuel cells 2, so that effective and uniform heating of the fuel cells 2 before and / or during the initial phase of the operation of the fuel cells 2 is possible. The fuel cell unit 1 thus has no problems during commissioning and after commissioning or after the fuel cell unit 1 has been started, the fuel cell unit 1 can deliver a high level of electrical power. The electrical resistance heating element 37 of Fuel cell unit 1 can heat fuel cells 2 to a temperature above 0 ° C. in a very short time, so that a preheating phase of fuel cell unit 1 is very short before start-up and / or after start-up, fuel cells 2 to a temperature over a very short time 0 ° C. The fuel cell system 4 comprises a battery 38 which provides the electrical energy for the electrical resistance heating elements 37 before the fuel cell unit 1 is put into operation.

Claims

Expectations

1. Fuel cell unit (1) as a fuel cell stack (1) for the electrochemical generation of electrical energy, comprising stacked fuel cells (2), the fuel cells (2) comprising as layered components (5, 6, 7, 8, 9, 10) each one Proton exchange membrane (5), an anode (6), a cathode (7), a bipolar plate (10) and a gas diffusion layer (9), characterized in that at least one layered component (5, 6, 7, 8, 9, 10 ) at least one fuel cell (2) at least one electrical resistance heating element (37) is integrated for heating the fuel cell unit (1) with electrical energy.

2. Fuel cell unit according to claim 1, characterized in that the at least one layered component (5, 6, 7, 8, 9, 10) with the integrated electrical resistance heating element (37) is the gas diffusion layer (9).

3. Fuel cell unit according to claim 1 or 2, characterized in that the at least one layer-shaped component (5, 6, 7, 8, 9, 10) with the integrated electrical resistance heating element (37) in a direction perpendicular to the layer-shaped components (5 , 6, 7, 8, 9, 10) spanned fictional planes (41) the last layer-shaped Component (5, 6, 7, 8, 9, 10) is.

4. Fuel cell unit according to claim 3, characterized in that the last layer-shaped component (5, 6, 7, 8, 9, 10) with the integrated electrical resistance heating element (37), the last gas diffusion layer (9) in the direction perpendicular to the layer-shaped Components (5, 6, 7, 8, 9, 10) spanned fictional planes (41) is.

5. Fuel cell unit according to one or more of the preceding claims, characterized in that in the two last gas diffusion layers (9) in the direction perpendicular to the layered components (5, 6, 7, 8, 9, 10) spanned fictitious planes (41) an electrical resistance heating element (37) is integrated in each case.

6. Fuel cell unit according to one or more of the preceding claims, characterized in that electrical resistance heating elements (37) are integrated into several layered components (5, 6, 7, 8, 9, 10), in particular gas diffusion layers (9).

7. Fuel cell unit according to one or more of the preceding claims, characterized in that the fuel cell unit (1) comprises at least one temperature sensor (46), preferably a plurality of temperature sensors (46), and the electrical heating power of the at least one electrical resistance heating element (37) can be controlled and / or depending on the temperature detected by the at least one temperature sensor (46) can be regulated, in particular before the fuel cell unit (1) is started up.

8. Fuel cell unit according to one or more of the preceding claims, characterized in that the electrical resistance heating element (37) is designed as a heating wire (42) and / or a conductor track (43) and / or a flat heating element, in particular a heating foil (45) is.

9. Fuel cell system (4), in particular for a motor vehicle, comprising

- A fuel cell unit (1) as a fuel cell stack (1) with fuel cells (2),

- A pressurized gas reservoir (21) for storing gaseous fuel,

- A gas delivery device (22) for delivering a gaseous oxidizing agent to the cathodes (8) of the fuel cells (2), characterized in that the fuel cell unit (1) is designed as a fuel cell unit (1) according to one or more of the preceding claims.

10. Fuel cell system according to claim 9, characterized in that the fuel cell system (4) comprises a battery (38) for operation of the at least one electrical resistance heating element (37) before the start-up of the fuel cell system (4).