FUEL CELL, FUEL CELL STACK AND METHOD OF OPERATING A FUEL CELL STACK

Abstract

The invention relates to a fuel cell (1) for a fuel cell stack (11), comprising a polymer membrane (2) which serves as an electrolyte and has respectively on both sides a catalyst layer (3, 4) for forming an anode (3) on the one side and a cathode (4) on the other side, a gas diffusion layer (5) and a bipolar plate (6) being applied to each of the two analyst layers (3, 4). According to the invention, a short-circuit element (7) is applied, preferably printed, to at least one bipolar plate (6). namely on the side facing away from the gas diffusion layer (5). The invention also relates to a fuel cell stack (11) and to a inetliod for operating a fuel cell stack (11).

Claims

1. A fuel cell (1) for a fuel cell stack (11), comprising a polymer membrane (2) which serves as an electrolyte and which comprises on either side a catalyst layer (3, 4) for forming an anode (3) on one side and a cathode (4) on the other, the two catalyst layers (3, 4) each bearing an applied gas diffusion layer (5) and also an applied bipolar plate (6), wherein at least one bipolar plate (6) bears an applied, short-circuit element (7) on a side facing away from the gas diffusion layer (5).

2. The fuel cell (1) as claimed in claim 1, wherein the short-circuit element (7) is an elastically deformable element.

3. The fuel cell (1) as claimed in claim 1, wherein the short-circuit element (7) bounds a pressurizable pressure compartment (8).

4. The fuel cell (1) as claimed in claim 3, wherein the pressure compartment (8) is pressurizable via a side channel (9).

5. The fuel cell (1) as claimed in claim 1, wherein the bipolar plate (6) and the short-circuit element (7) bear at least regionally an applied seal (10).

6. The fuel cell (1) as claimed in claim 1, wherein the short-circuit element (7) comprises different zones (A, B, C) which differ in terms of their electrical resistance.

7. A fuel cell stack (11) comprising at least two fuel cells (1) as claimed in claim 1, the fuel cells (1) being stacked such that the respective short-circuit elements (7) are disposed one above another in a mirrored disposition and in the event of a short circuit make contact with one another in a region of a common short-circuit face (12).

8. A method for operating the fuel cell stack (11) as claimed in claim 7, wherein the short-circuit elements (7) are selectively engaged and disengaged via a central pressure supply.

9. The method as claimed in claim 8, wherein the short-circuit elements (7) are pressurized via the central pressure supply and elastically deformed.

10. The method as claimed in claim 9, wherein the pressurization is carried out using a gas or a liquid.

11. The fuel cell (1) as claimed in claim 1, wherein the short-circuit element (7) is a printed short-circuit element.

12. The fuel cell (1) as claimed in claim 2, wherein the elastically deformable short-circuit element (7) is a plate or a membrane.

13. The fuel cell (1) as claimed in claim 3, wherein the short-circuit element (7), together with the bipolar plate (6), bounds the pressurizable pressure compartment (8).

14. The fuel cell (1) as claimed in claim 4, wherein the side channel (9) is formed in the bipolar plate (6) and/or traverses the bipolar plate (6).

15. The fuel cell (1) as claimed in claim 5, wherein the seal (10) is printed.

16. The fuel cell (1) as claimed in claim 5, wherein the seal (10) circumferentially surrounds and/or completely covers the short-circuit element (7).

17. The method as claimed in claim 10, wherein the gas is hydrogen or air.

18. The method as claimed in claim 10, wherein the liquid is a coolant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The invention is elucidated in more detail below by means of the appended drawings. In the drawings:

[0030] FIG. 1 shows a schematic representation of a fuel cell,

[0031] FIG. 2 shows a schematic plan view of a fuel cell of the invention,

[0032] FIG. 3 shows a schematic representation of the short-circuit element of the fuel cell of FIG. 2,

[0033] FIG. 4 shows a schematic longitudinal section through the fuel cell of FIG. 2,

[0034] FIG. 5 shows a schematic longitudinal section through a fuel cell stack comprising the fuel cell of FIG. 4, without short-circuiting,

[0035] FIG. 6 shows a schematic longitudinal section through a fuel cell stack comprising the fuel cell of FIG. 4, with short-circuiting,

[0036] FIG. 7 shows a schematic representation of an alternative short-circuit element,

[0037] FIG. 8 shows a schematic longitudinal section through a fuel cell with the short-circuit element of FIG. 7,

[0038] FIG. 9 shows a schematic diagram of the mode of operation of a fuel cell, and

[0039] FIG. 10 shows a distribution of potential in a fuel cell.

DETAILED DESCRIPTION

[0040] The construction of a fuel cell 1 of the invention is described illustratively using FIG. 1. A central element is a polymer membrane 2 which serves as an electrolyte. The polymer membrane 2 has on either side a catalyst layer 3, 4, with the catalyst layer 3 forming an anode and the catalyst layer 4 a cathode. The catalyst layers 3, 4 are each followed on either side by a gas diffusion layer 5 and by a bipolar plate 6.

[0041] In the operation of the fuel cell 1, the anode 3 is supplied with hydrogen (H.sub.2) and the cathode 4 with oxygen (O.sub.2). In a chemical reaction, the hydrogen and oxygen reactants are transformed into electrical energy, and water (H.sub.2O) is also formed, and is taken off as product water.

[0042] In FIG. 1 it can be seen that on the outside of each of the bipolar plates 6 there is an element 7 disposed. This element is a short-circuit element 7, which is elucidated in more detail below by FIGS. 2 to 8.

[0043] From the plan view of FIG. 2 it is apparent that the short-circuit element 7 is applied on the outside to the bipolar plate 6 of the fuel cell, specifically in a corner region. The short-circuit element 7 may be, for example, a small plate or a membrane, and so has a comparatively flat construction. The short-circuit element 7 is elastically deformable.

[0044] The short-circuit element 7 bounds a pressure compartment 8, which is formed preferably between the short-circuit element 7 and the bipolar plate 6 (see FIG. 4). Via a side channel 9 formed in the bipolar plate 6, the pressure compartment 8 can be subjected to a pressure medium, causing elastic deformation of the short-circuit element 7. For the sealing of the pressure compartment 8, the short-circuit element 7 is surrounded by a seal 10, which extends to the bipolar plate 6 and also seals the side channel 9 to the outside (see FIGS. 3 and 4).

[0045] Where two fuel cells 1, 1′ each with at least one short-circuit element 7, 7′ are arranged one above the other to form a fuel cell stack 11, the arrangement is such that two bipolar plates 6, 6′ each lie opposite one another with a short-circuit element 7, 7′, so that the two short-circuit elements 7, 7′ are brought into overlap and are already slightly elastically deformed (see FIG. 5). When the pressure compartments 8, 8′ (not shown) are subjected to a pressure medium via a central pressure supply, the two short-circuit elements 7, 7′ undergo further deformation. In this way a short-circuit is produced via a common short-circuit face 12 (see FIG. 6). In order to regain the original state shown in FIG. 5, the pressure in the pressure compartments 8, 8′ is stepped down.

[0046] An alternative embodiment of a short-circuit element 7 for a fuel cell 1 of the invention is apparent from FIGS. 7 and 8. In this case the short-circuit element 7 has zones A, B, C, which differ in terms of their electrical resistance. The electrical resistance of zone A is the highest and is in the KS/range, while the electrical resistance of zone B is in the S2 range and the electrical resistance of zone C is in the mΩ range or less. In this way it is ensured that on each contacting of the short-circuit element 7, the electrical resistance is continuously reduced, and, respectively, on each elimination of the short-circuit, it is continuously increased.

[0047] FIG. 9 represents the state of a fuel cell 1 during start-up of the fuel cell stack 11. In the initial state, an anode region 13 and a cathode region 14 are full of air and oxygen (O.sub.2) respectively. For start-up, hydrogen (H.sub.2) is passed into the air-filled anode region 13, and so the hydrogen gradually displaces the air. In the state illustrated in FIG. 9, the left-hand side of the anode region 13 has already been supplied with hydrogen, whereas air is still present on the right-hand side of the anode region 13. In this case there is a distribution of potential of the kind represented illustratively in FIG. 10. On account of the gas distribution described, there are high differences in potential between the cathode 4 and the electrolyte, which is formed by the polymer membrane 2, and so the following equation is valid:

Δφ.sub.c<1 V

[0048] These differences in potential lead to carbon corrosion in the catalyst layer 4 of the cathode and hence to degradation of the fuel cell 1. The degradation continues for as long as the H.sub.2/O.sub.2 gas front is moving through the anode region 13.

[0049] With the aid of the present invention it is possible to counteract the degradation caused by high differences in potential during start-up and run-down of the fuel cell 1, specifically by means of cell-specific short-circuiting.