WO2009092433A1

WO2009092433A1 - Fuel cell, in particular for arrangement in a fuel cell stack, and fuel cell stack

Info

Publication number: WO2009092433A1
Application number: PCT/EP2008/011116
Authority: WO
Inventors: Cosimo Mazzotta; Gerald Hornburg; Matthias Jesse; Hans-Jörg Heidrich
Original assignee: Daimler Ag; Ford Global Technologies, Llc
Priority date: 2008-01-22
Filing date: 2008-12-24
Publication date: 2009-07-30
Also published as: DE102008005502A1

Abstract

The invention relates to a fuel cell (1), in particular for arrangement in a fuel cell stack, having a first bipolar plate (2) and a second bipolar plate (3) and having a membrane electrode assembly (4), which is arranged between the two bipolar plates (2, 3) arranged plane-parallel to one another, an anode flow field (F1) being formed between the first bipolar plate (2) and the membrane electrode assembly (4) and a cathode flow field (F1) being formed between the second bipolar plate (3) and the membrane electrode assembly (4) by channel structures respectively with anode channels (K1) and cathode channels (K2) introduced respectively into the first bipolar plate (2) and the second bipolar plate (3), and the membrane electrode assembly (4) comprising at least one opening (8), which introduces some of the anode waste gas from the flow outlet side of the anode flow field (F1) into a cathode gas on the flow inlet side of the cathode flow field (F2). According to the invention at least one sequence element (9) is arranged in the region of the opening (8) and controls discharge of some of the anode waste gas into the cathode gas.

Description

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FUEL CELL, IN PARTICULAR FORARRANGEMENT IN A FUEL CELL STACK, AND

FUEL CELLSTACK

The invention relates to a fuel cell, in particular for arrangement in a fuel cell stack, according to the preamble of claim 1 and to a fuel cell stack according to the preamble of claim 13.

A fuel cell stack (also known for short just as stack) consists of a plurality of fuel cells arranged stacked in a plane-parallel manner one above the other and connected electrically in series. Each fuel cell comprises an anode, a cathode and an electrolyte arranged therebetween, for example in the form of a polymer electrolyte membrane (PEM for short), which together form a membrane electrode assembly (MEA for short). Between the membrane electrode assemblies adjoining one another in the fuel cell stack there is in each case arranged a bipolar plate (also known as a bipolar separator plate unit). The bipolar plate here serves to space adjacent membrane electrode assemblies, to distribute reactants for the fuel cell, such as fuel and oxidizer, over the adjacent membrane electrode assemblies and to discharge the reactants in channels provided for this purpose and in each case open towards the membrane electrode assemblies, to dissipate the heat of reaction via a coolant conveyed in separate coolant channels and to bring about an electrical connection between the anode and the cathode of adjacent membrane electrode assemblies.

A fuel and an oxidizing agent are used as reactants. In general, gaseous reactants (reaction gases for short) are used, for example hydrogen or a gas containing hydrogen (for example reformate gas) as the fuel and oxygen or a gas containing oxygen (for example air) as the oxidizing agent. Reactants are understood to mean any substances which participate in the electrochemical reaction, including the reaction products, such as for example water.

The respective bipolar plate consists for example of two shaped parts, in particular plates, connected together in a plane-parallel manner - a first plate for connection with the anode of the one membrane electrode assembly and a second plate for connection with the cathode of another membrane electrode assembly. At the surface of the first plate facing the one membrane electrode assembly there are in this case arranged anode channels for distributing a fuel along the membrane electrode assembly, cathode channels being arranged at the surface of the second plate facing the other membrane electrode assembly for distributing the oxidizer over the latter membrane electrode assembly. The cathode channels and the anode channels are not connected together.

The cathode and anode channels are in this case formed by recesses, separated from one another by raised portions, in the surfaces of the first and second plates in each case facing the membrane electrode assemblies. The first and second plates are preferably shaped, in particular embossed. The raised portions and recesses are produced for example discontinuously by stretch forming, deep drawing, impact extrusion or the like, or continuously by rolling or drawing. Depending on the configuration of the two plates, they may in turn comprise cathode or anode channels of an adjacent further fuel cell on the side remote from the membrane electrode assembly. In this case the respective bipolar plate is made from a shaped part with channel structures introduced on both sides and thus as a plate. The plates may also comprise coolant channels for conveying coolant therethrough.

Certain fuel cells additionally require a specific water content, i n order to display sufficient ion conductivity. This is particularly true of polymer electrolyte membrane fuel cells, which consist of materials based on fluorinated sulfonic acids, such as Nafion for example.

In addition, it is necessary in particular under certain operating conditions, such as for example on cold starting or during operation at low temperatures or low power or in the case of partial humidification of the cathode gas, to heat the fuel cell. To this end it is known to feed surplus anode waste gas, which is present in the anode waste gas for example due to non-homogeneous distribution of the hydrogen on the membrane, to the cathode gas, in order to provide combustion either in the anode channels or in the cathode channels.

A fuel cell system is known from DE 10 2005 045 319 A1 which provides for the anode waste gas to flow into the cathode side of the fuel cell without allowing the anode waste gas flow and the cathode inlet flow to be able to mix in any large volume. To this end, perforations in the form of a group of small holes are provided in the membrane electrode assembly. In this respect, the fuel cell system disclosed therein makes provision for the anode stoichiometry to be controlled and the perforations to be appropriately delimited to control how much anode waste gas flows into the cathode gas.

The object of the invention is therefore to provide a fuel cell which allows simple anode waste gas discharge. Furthermore, an improved fuel cell stack is provided.

The object is achieved according to the invention by the features stated in claim 1 and in claim 13.

Advantageous further developments of the invention constitute the subject matter of the subclaims.

The fuel cell comprises a first bipolar plate and a second bipolar plate and a membrane electrode assembly, which is arranged between the two bipolar plates arranged plane- parallel to one another. An anode flow field is formed between the first bipolar plate and the membrane electrode assembly and a cathode flow field is formed between the second bipolar plate and the membrane electrode assembly as a result of channel structures respectively with anode channels or cathode channels introduced respectively into the first bipolar plate or the second bipolar plate. To utilize the unreacted hydrogen present in the anode waste gas, the membrane electrode assembly comprises at least one opening, which, on the flow outlet side of the anode flow field, introduces some of an anode waste gas therefrom into a cathode gas on the flow inlet side of the cathode flow field. According to the invention, at least one sequence element is arranged in the region of the opening, said element controlling discharge of the anode waste gas into the cathode gas.

Incorporating a sequence element in the region of the opening for discharging the anode waste gas into the cathode gas enables purposeful control of the feed of hydrogen into the cathode gas, in particular air, in order to restrict any resultant catalytic combustion. It is possible in particular purposefully to control the temperature of the fuel cell by means of such metered discharge of the anode waste gas and thus to establish the temperature necessary for a cold start or similar operating processes requiring heat. Furthermore, control of anode waste gas discharge ensures that uncontrolled combustion does not arise. Thus, damage to the membrane electrode assembly, in particular the gas diffusion layers thereof, is reliably prevented. Moreover, uncontrolled feed of anode waste gas into the cathode gas via the opening is reliably prevented by the sequence element. For temperature-dependent control of anode waste gas discharge, the monitoring of the cell or stack operating temperature which is already provided may in particular be utilized and incorporated into control of the anode waste gas discharge.

To prevent accumulation of the mixture of anode waste gas and cathode gas and reliably to prevent damage to gas diffusion layers of the membrane electrode assembly by purposefully initiated, controlled combustion, the opening is conveniently arranged in a region of the membrane electrode assembly which has no catalyst. Furthermore, the opening, in particular the size thereof, is appropriately adjusted. For example, the opening is formed at least of one or more through-holes or of one or more slits. In this respect, the openings have a particularly small diameter in the μm or mm range.

In one possible embodiment the opening is arranged in a supporting frame for the membrane electrode assembly, which is conveniently adjoined by an anode outlet (also known as an anode port) of the anode flow field and a cathode inlet (also known as a cathode port) of the cathode flow field.

To achieve a maximally compact fuel cell construction, the sequence element is arranged wholly or partially in the region of the anode outlet or of the cathode inlet at or in the first bipolar plate or second bipolar plate.

Preferably, the sequence element is of very compact construction and acts automatically. This allows the fuel cell or fuel cell stack to be of a simple, compact construction.

In one possible embodiment, the sequence element takes the form of a pressure sequence valve. Like a pressure relief valve, the pressure sequence valve here closes off the opening until a pressure set at the pressure sequence valve is reached. Once the set pressure is reached, the pressure sequence valve opens and some of the anode waste gas is introduced into the cathode gas. Conveniently, the pressure sequence valve is arranged at one of the sides of the opening, either on the cathode side or the anode side. The pressure sequence valve takes the form, for example, of an automatic spring-loaded flap valve. In this case, the spring-loaded flap is opened in accordance with the set spring tension of the spring-loaded flap valve when a correspondingly high overpressure is reached in the anode flow field at the anode outlet, so allowing discharge of the anode waste gas into the cathode gas. In this respect, both the size of the opening and the spring tension set are conformed to the type, size and pressure conditions of the fuel cell.

Since the membrane electrode assembly consists largely of soft, very flexible materials, a spring tension of the spring-loaded flap valve may be difficult to set. For this reason, the invention alternatively provides for setting of the spring tension to be assisted by fittings in at least one of the bipolar plates. In one possible embodiment, a resilient, compressible material, in particular a plastic, for example an elastomer, such as silicone, or polypropylene, is provided for this purpose, which is arranged for example as a shaped part between the opening and the sequence element constructed as a spring-loaded flap valve.

As an alternative to use of a spring-loaded flap valve and/or of a resiliently compressible material, the pressure sequence valve may also take the form of an electrically, pneumatically, hydraulically or electropneumatically driven control valve. For example, the pressure sequence valve may take the form of a movably driven cylinder valve, which extends substantially at least over the length of the opening. By rotation or longitudinal displacement of the cylinder valve arranged in the opening, discharge of some of the anode waste gas into the cathode gas may be controlled.

Preferably, the fuel cell is part of a fuel cell stack. The fuel cell is in particular a "polyelectrolyte membrane fuel cell" (known as a PEMFC for short). This conventionally takes the form of a fuel cell stack made up of a plurality of fuel cells arranged in a plane- parallel manner one above the other with a plurality of bipolar plates, between which is arranged in each case a membrane electrode assembly with an anode flow field on one side and a cathode flow field on the other side, some of the anode waste gas from the flow outlet side of the anode flow field being introduced into a cathode gas on the flow inlet side of the cathode flow field. According to the invention, an anode collection channel arranged in end plates and a cathode collection channel are connected fluidically together via at least one opening, at least one sequence element being arranged in the region of the opening and controlling discharge of some of the anode waste gas into the cathode gas.

In one possible embodiment the opening is arranged in the form of a through-hole or of a channel in the end plate between the anode collection channel and cathode collection channel arranged parallel next to one another in a single end plate, the sequence element being arranged laterally on the anode collection channel or the cathode collection channel in or at the opening in the relevant end plate.

In an alternative embodiment the opening between the anode collection channel and cathode collection channel arranged parallel one above the other is introduced into two adjoining end plates in the form of recesses, the sequence element extending over at least two end plates at the top or end face of the fuel cell stack.

Exemplary embodiments of the invention will be explained in greater detail with reference to drawings, in which:

Fig. 1 is a schematic, sectional view of a fuel cell with a first bipolar plate and a second bipolar plate, between which is arranged a membrane electrode assembly, which comprises at least one opening for fluidic connection of an anode flow field with a cathode flow field, a sequence element being arranged in the region of the opening,

Fig. 2 is a schematic, sectional view of a fuel cell with an alternative embodiment of a sequence element,

Fig. 3 is a schematic, sectional view of a fuel cell with a further alternative embodiment of a sequence element,

Fig. 4 is a schematic, sectional view of a fuel cell with a further alternative embodiment of a sequence element, and

Fig. 5 is a schematic, sectional view of a fuel cell stack formed from a plurality of fuel cells and having an opening through an end plate for fluidic connection of anode and cathode flow field and a sequence element for controlling the opening cross-section,

Fig. 6 is a schematic, sectional view of a fuel cell stack formed from a plurality of fuel cells, with an alternative arrangement of an opening through an end plate for fluidic connection of anode and cathode flow field and a sequence element for controlling the opening cross-section.

Mutually corresponding parts are provided with the same reference numerals in all the figures.

Figure 1 is a schematic, sectional representation of a fuel cell 1 with a first bipolar plate 2 and a second bipolar plate 3 and a membrane electrode assembly 4 arranged therebetween.

In this case, an anode flow field F1 is arranged between the one side, in particular the bottom, of the first bipolar plate 2 and the membrane electrode assembly 4 and a cathode flow field F1 is arranged between the one side, in particular the top, of the second bipolar plate 3 and the membrane electrode assembly 4. The anode flow field F1 and the cathode flow field F2 are in each case formed by channel structures respectively comprising anode channels K1 and cathode channels K2 introduced respectively into the first bipolar plate 2 or the second bipolar plate 3. The other sides of the bipolar plates 2 and 3 likewise comprise channel structures, which form the cathode or anode side of adjacent fuel cells. Depending on type and construction, the respective bipolar plates 2 and 3 may be provided in a manner not described in any more detail with coolant channels for adjusting the temperature of the fuel cell 1.

The membrane electrode assembly 4 comprises in detail a membrane, on either side of which an appropriate catalyst and gas diffusion layers 5 or 6 respectively are in each case applied and positioned in a known manner.

In the vicinity of the frame, a seal 7 is arranged on either side of the membrane electrode assembly 4 to prevent fluidic connection of anode flow field F1 and cathode flow field F2.

To achieve controlled combustion inter alia for the purpose of adjusting the temperature of the fuel cell 1 , an opening 8 is arranged in the membrane electrode assembly 4 according to the invention in the region without a catalyst, which opening 8 connects the anode flow field F1 fluidically to the cathode flow field F2. For purposeful control of combustion, at least one sequence element 9 is arranged in the region of the opening 8, which sequence element controls discharge of some of the anode waste gas into the cathode gas. The opening 8 is preferably arranged in the supporting frame of the membrane electrode assembly in that region adjoined by an anode outlet Ka of the anode flow field F1 and a cathode inlet Ke of the cathode flow field.

In this case, the opening 8 is formed in particular of a plurality of through-holes not described in any greater detail or of one or more slits, whose size is conformed to the amount of anode waste gas to be discharged.

Preferably, the sequence element 9 is arranged on or in the second bipolar plate 3 wholly or partially in the region of the cathode inlet Ke and completely covering the opening 8. Alternatively, the sequence element 9 may also be arranged wholly or partially in the region of the anode outlet Ka of the first bipolar plate 2.

Figure 1 shows as a possible embodiment of the sequence element 9 a pressure sequence valve, in particular an automatic spring-loaded flap valve 9.1 , which opens and closes automatically as a function of the set spring tension under appropriate pressure conditions. For instance, when in particular an elevated overpressure prevails in the anode flow field F1, the spring-loaded flap valve 9.1 is opened automatically, such that anode waste gas may be introduced into the cathode gas.

To set the spring tension, it is additionally possible to arrange a flexible, compressible material M, for example an elastomer, such as silicone, or another suitable material in the form of a shaped part between spring-loaded flap valve 9.1 and opening 8. The spring tension may also be set by a mechanical spring element arranged in the relevant bipolar plate 2 or 3 (in the exemplary embodiment in the second bipolar plate 3).

The sequence element 9 may also be formed solely of the flexible, compressible material M, as is shown by way of example in Figure 2. To set the compressibility, the material M may be provided with gas inclusions, for example air, as contained in foamed plastics or elastomer.

Figures 3 and 4 show a further alternative exemplary embodiment of the sequence element 9, with Figure 3 showing a sequence element 9 in the form of an electropneumatically driven control valve 9.2. Alternatively, the control valve 9.2 may also be operated electrically, pneumatically or hydraulically. The energy necessary for actuating the control valve 9.2 may in this case be taken directly from the fuel cell 1. Alternatively, the control valve 9.2 may also be supplied by means of an external voltage source. By means of the control valve 9.2, the portion of the anode waste gas to be discharged may be more purposefully and better controlled than in the case of the automatically operated spring-loaded flap valve 9.1 or another suitable pressure relief valve.

Figure 4 shows as an alternative sequence element 9 a cylinder valve 9.3 rotatable about an axis of rotation D and longitudinally displaceable along an axis of translation T. A cylinder valve 9.4 is understood to mean in particular a cylindrical, movably arranged control valve. In this way, the quantity of anode waste gas to be discharged is set by changing the position of the cylinder valve 9.3. To this end, the cylinder valve 9.3 extends at least over the entire length of the opening 8.

Figures 5 and 6 show a plurality of fuel cells 1 arranged one above the other as part of a fuel eel I stack 10.

For fluidic connection of the anode flow field F1 to the cathode flow field F2, an opening 14 is provided, which connects fluidically together an anode collection channel 12 arranged in end plates 11 and a cathode collection channel 13. In this respect, the opening 14 may be arranged in that region of one of the end plates 11 in which the cathode collection channel 13 and the anode collection channel 12 are arranged parallel next to one another in the relevant end plate 11 , as shown in Figure 5. In this case one of the above-described types of sequence element 9 is arranged laterally on the anode collection channel 12 or the cathode collection channel 13 in the relevant end plate 11.

In an alternative embodiment, the opening 14 is arranged in that region of the end plates 11 in which the cathode collection channel 13 and the anode collection channel 12 are in each case formed by recesses in two mutually adjacent end plates, the two channels being arranged parallel one above the other in the adjacent end plates 11 , as shown in Figure 6. In this case the sequence element 9 is arranged so as to extend over at least two end plates 11 at the top or end face. In the exemplary embodiments according to Figures 5 and 6 in particular, with openings 14 arranged in the end plates 11 in the form of slits or through-holes or channels in the end plates 11 , electrically, pneumatically, hydraulically and/or electropneumatically operated control valves 9.2, in particular electrically operated solenoid valves or electropneumatic piezo valves, are used as the sequence elements 9. These are fitted and incorporated partially or completely into the end plate(s) 11 , such that additional feed lines are avoided, so making possible a very compact structure of the fuel cell stack 10.

List of reference signs

1 Fuel cell

2 First bipolar plate

3 Second bipolar plate

4 Membrane electrode unit

5 Gas diffusion layer, anode side

6 Gas diffusion layer, cathode side

7 Seal

8 Opening

9 Sequence element

10 Fuel cell stack

11 End plates

12 Cathode collection channel

13 Anode collection channel

14 Opening

D Axis of rotation

F1 Anode flow field

F2 Cathode flow field

K1 Anode channel

K2 Cathode channel

Ka Anode outlet

Ke Cathode outlet

T Axis of translation

Claims

Patent Claims

1. A fuel cell (1), in particular for arrangement in a fuel cell stack, having a first bipolar plate (2) and a second bipolar plate (3) and having a membrane electrode assembly (4), which is arranged between the two bipolar plates (2, 3) arranged plane- parallel to one another, an anode flow field (F 1) being formed between the first bipolar plate (2) and the membrane electrode assembly (4) and a cathode flow field (F1) being formed between the second bipolar plate (3) and the membrane electrode assembly (4) by channel structures respectively with anode channels (K1) and cathode channels (K2) introduced respectively into the first bipolar plate (2) and the second bipolar plate (3), and the membrane electrode assembly (4) comprising at least one opening (8), which introduces some of the anode waste gas from the flow outlet side of the anode flow field (F1) into a cathode gas on the flow inlet side of the cathode flow field (F2), characterized in that at least one sequence element (9) is arranged in the region of the opening (8) and controls discharge of the anode waste gas into the cathode gas.

2. The fuel cell as claimed in claim 1 , characterized that the opening is arranged in a region of the membrane electrode assembly (4) which has no catalyst.

3. The fuel cell as claimed in claim 1 or 2, characterized in that the opening (8) is formed at least of one or more through-holes or of one or more slits.

4. The fuel cell as claimed in one of claims 1 to 3, characterized in that the opening (8) is arranged in a supporting frame for the membrane electrode assembly (4), which is adjoined by an anode outlet (Ka) of the anode flow field (F1 ) and a cathode inlet (Ke) of the cathode flow field (F2).

5. The fuel cell as claimed in claim 4, characterized in that the sequence element (9) is arranged wholly or partially in the region of the anode outlet (Ka) or of the cathode inlet (Ke) at or in the first bipolar plate (2) or second bipolar plate (3).

6. The fuel cell as claimed in claim 5, characterized in that the sequence (9) is made of a resiliently compressible material, in particular an elastomer or plastic.

7. The fuel cell as claimed in one of claims 1 to 6, characterized in that the sequence element (9) takes the form of a pressure sequence valve.

8. The fuel cell as claimed in claim 7, characterized in that the pressure sequence valve is an automatic spring-loaded flap valve (9.1).

9. The fuel cell as claimed in claim 7, characterized in that the pressure sequence valve is an el ectrically, pneumatically, hydraulically or electropneumatically driven control valve (9.2).

10. The fuel cell as claimed in one of claims 1 to 9, characterized in that the sequence element (9) is a movably driven cylinder valve (9.3).

11. The fuel cell as claimed in claim 10, characterized in that the cylinder valve (9.3) is arranged rotatably and/or longitudinally displaceably in the region of the opening (8).

12. The fuel cell as claimed in one of claims 1 to 11, characterized in that the fuel cell is part of a fuel cell stack (10).

13. A fuel cell stack (10) having a plurality of fuel cells (1) arranged one above the other with a plurality of bipolar plates (2, 3) and between which is arranged in each case a membrane electrode assembly (4) with an anode flow field (F1) on one side and a cathode flow field (F2) on the other side, some of the anode waste gas from the flow outlet side of the anode flow field (F1) being introducible into a cathode gas on the flow inlet side of the cathode flow field (F2), characterized in that an anode collection channel (12) arranged in end plates (11) and a cathode collection channel (13) are connected fluidically together via an opening (14), at least one sequence element (9) being arranged in the region of the opening (14) and controlling discharge of some of the anode waste gas into the cathode gas.

14. The fuel cell stack as claimed in claim 13, characterized in that the opening (14) is arranged between the anode collection channel (12) and cathode collection channel (13) arranged parallel next to one another in one of the end plates (11), the sequence element (9) being arranged laterally on the anode collection channel (12) or the cathode collection channel (13) in the relevant end plate (11).

15. The fuel cell stack as claimed in claim 13 or 14, characterized in that the opening (14) is arranged between the anode collection channel (12) and cathode collection channel (13), arranged parallel one above the other, of two adjacent end plates (11), the sequence element (9) being arranged so as to extend over both end plates (11) at the top or end face of the fuel cell stack (10).

16. The fuel cell stack as claimed in claim 14, characterized in that the opening (14) is introduced as a through-hole and/or slit in walls of the anode collection channel (12) or of the cathode collection channel (13).