Fuel cell and voltage tapping device

Abstract

A fuel cell having a plurality of contacting regions on flow field plates (10) comprises at least one conductive rubber element (50), which has electrically conductive regions (51) and electrically insulating regions (52) following one another in alternation. The conductive rubber element (50) is disposed at an angle (α) to an orthogonal (O) to the contacting surfaces on the flow field plates (10), the condition B ≥ 3.5 ∙ p ∙ cos(α) - b ∙ sin(α) being satisfied, where B is the width of the contacting surface on the flow field plates (10), b is the width of the electrically conductive regions (51) and electrically insulating regions (52) of the conductive rubber element (50) and p is the grid pitch of the conductive rubber element (50).

Description

[0001] Fuel cell and voltage tapping device

[0002] The present invention relates to the electrical contacting of a fuel cell with at least one conductive rubber. Furthermore, the present invention relates to a voltage tapping device for this fuel cell.

[0003] State of the art

[0004] Fuel cells convert chemical energy into electrical energy with high efficiency. They often consist of stacks of individual cells. The individual cells typically have a bipolar plate on each side, between which an electrolyte and a membrane are arranged. The electrical voltage generated by the chemical reaction drops across the membrane. This voltage can be measured at the bipolar plates and is a key parameter for operating the fuel cell.

[0005] To measure cell voltages, spring contacts mounted on a holder can be used. These contacts are positioned on or at each individual cell and secured to a circuit board with a holder or housing. This circuit board has conductive paths that transmit the electrical voltage of the individual cells to a measuring device.

[0006] DE 10 2007 038 153 A1 describes a fuel cell in which a conductive rubber is used to tap the voltage between two bipolar plates. This conductive rubber has alternating electrically conductive and electrically insulating areas. It is arranged on the surface of the bipolar plates at a 90° angle to their surfaces. The electrically conductive areas of the conductive rubber establish electrical contact with the bipolar plates. The electrically insulating areas prevent a short circuit between the bipolar plates. In contrast to the use of spring contacts, this type of contact is easier to adjust geometrically. Furthermore, a contact pairing between rubber and bipolar plate prevents many degradation mechanisms that can occur with individual metal and / or precious metal contact pairs. A conductive rubber as an electrical contact element is also relatively robust against external vibration and shock loads.

[0007] However, there are applications in which the area over which the conductive rubber can contact the bipolar plate is significantly smaller than described in the above patent specification. For example, it is conceivable to contact a bipolar plate solely via a narrow end face at the edge of the plate.

[0008] It is an object of the present invention to provide a fuel cell having a voltage tapping device with at least one conductive rubber, which has very good vibration and shock resistance.

[0009] Disclosure of the Invention In a first aspect of the invention, this object is achieved by a fuel cell with several bipolar plates that are electrically contacted by at least one conductive rubber. Such a conductive rubber is also referred to as zebra rubber. The conductive rubber has alternating electrically conductive areas and electrically insulating areas. Electrically conductive refers in particular to areas with an electrical resistance of a maximum of 10 5 Ohm / cm. Electrically insulating refers in particular to areas with an electrical resistance of more than 10 9Ohm / cm. The electrically conductive areas and the electrically insulating areas each have the same dimension along the longitudinal axis of the conductive rubber. The geometric sum of the conductive and non-conductive layer (also referred to as the pitch) is less than the width of a bipolar plate. The pitch of the conductive rubber is preferably in the range of 0.05 mm to 1.00 mm. If the pitch of the conductive rubber and its core width are selected to suit the pitch or spacing of the bipolar plates in a stack, a short circuit between two adjacent bipolar plates can be prevented, even if the stack shortens or lengthens over its life cycle.

[0010] The conductive rubber is arranged at an angle α to an orthogonal to the bipolar plates, which satisfies the condition according to formula 1:

[0011] B ≥ 3.5 • p • cos(α) - b • sin(α ) (Formula 1)

[0012] Here, p denotes a grid of the conductive rubber, B the width of a contact surface on the bipolar plates, and b the width of the conductive rubber. This defines a minimum value for the angle α, so that a better overlap between the conductive areas of the conductive rubber and the bipolar plates is achieved than if the conductive rubber were arranged orthogonally (α = 0) to the bipolar plates. This increases the robustness of the electrical contact against vibration and shock loads, even if the bipolar plates change geometrically over their life cycle in the fuel cell.

[0013] It is preferred that the condition according to formula 2 is also fulfilled:

[0014] Here, P denotes a distance between two adjacent contact surfaces on bipolar plates.

[0015] This defines a maximum value for the angle a. This creates a practical compromise between good electrical contact between the conductive rubber and the bipolar plates and a sufficient number of non-electrically contacted conductive layers, which is relevant for clearance and creepage distance protection (preventing short circuits).

[0016] To enable easier installation of the conductive rubber in an electromechanical assembly and to rule out the risk of short circuits with other electrical components of the fuel cell, it is preferred that the electrically conductive regions and electrically insulating regions of the conductive rubber are arranged between two electrically insulating edge layers. Particularly preferably, the two electrically insulating edge layers are designed as one piece with the conductive rubber and consist of the same material as the electrically insulating regions of the conductive rubber. In this embodiment of the fuel cell, however, the width b of the conductive rubber according to formulas 1 and 2 continues to be understood as only the width of the electrically conductive and electrically insulating regions and not the total width of the conductive rubber increased by the two edge layers.To ensure consistent elasticity of the conductive rubber in all its areas, it is preferable that the electrically conductive areas, the electrically insulating areas, and any electrically insulating edge layers be made of the same plastic. This plastic is electrically insulating in its unmodified state, while the electrically conductive areas are made electrically conductive, for example, by adding carbon, silver, or gold.

[0017] On a side of the conductive rubber facing away from the bipolar plates, a circuit board is preferably arranged, which has several electrical circuit board contact elements. A circuit board contact element is arranged on the circuit board above each contact surface on a bipolar plate that is electrically contacted by the conductive rubber. Thus, the correctly dimensioned conductive rubber establishes electrical contact between the contact surface on the bipolar plate and the contact surface on the circuit board. This enables electrical voltage tapping between different bipolar plates of the fuel cell via the circuit board, which represents a relevant input signal for the control of a fuel cell.

[0018] By setting an angle a of more than 0°, a longer distance perpendicular to the stacking direction of the bipolar plates is covered with conductive rubber than would be the case with an orthogonal arrangement between the conductive rubber and the bipolar plates. In order to provide a compact voltage tapping device on the fuel cell, it is therefore preferred that several individual conductive rubbers are arranged on the fuel cell, which are offset from one another orthogonally to their respective longitudinal axes. In particular, the fuel cell has two conductive rubbers. For this purpose, a plastic housing can be provided which is positioned on the fuel cell and has recesses for receiving two conductive rubbers. Each individual conductive rubber preferably contacts 6 to 10 contact surfaces on bipolar plates.In this embodiment of the fuel cell, the circuit board has several rows of electrical circuit board contact elements on its underside, each of which is arranged at specific points above one of the conductive rubbers.

[0019] The circuit board and the at least one conductive rubber band are preferably components of a voltage tapping device, i.e., an assembly that typically also includes a plastic housing that aligns and secures the circuit board and the conductive rubber band. A voltage tapping device is understood here to be a device with which electrical voltages can be tapped at multiple bipolar plates and which can be mechanically connected to the bipolar plates, for example, via the plastic housing, but can also be separated from them.

[0020] For this purpose, the housing can preferably have suitably shaped arrangement and / or fastening elements. Elements for arranging the voltage tapping device on the stack can be arranged parallel to the bipolar plates between them. They can thus engage between the bipolar plates in the manner of ribs, thus enabling secure alignment of the voltage tapping device to the bipolar plates. To prevent the voltage tapping device from causing a short circuit between the bipolar plates, the arrangement and / or fastening elements are designed to be particularly electrically insulating.

[0021] The circuit board is preferably mechanically and electrically connected with a connector. This interface can be used to establish a mechanically detachable electrical connection to a measuring device, which can be used to evaluate the tapped cell voltages.

[0022] In a second aspect, the invention relates to a voltage tapping device configured to be arranged on a fuel cell according to the first aspect. Such a voltage tapping device can be subsequently attached to a conventional fuel cell to electrically contact its bipolar plates and thus obtain a fuel cell according to the first aspect of the invention that can be suitably monitored and controlled. For maintenance and repair purposes, the entire voltage tapping device can also be separated from the fuel cell again.

[0023] Short description of the drawings

[0024] Embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description.

[0025] Fig. 1a shows a plan view of a fuel cell with a voltage tapping device according to an embodiment of the invention.

[0026] Fig. 1b shows an isometric view of the fuel cell with the voltage tapping device according to Fig. 1a.

[0027] Fig. 1c shows a view of the fuel cell with voltage tapping device according to Fig. 1a from below.

[0028] Fig. 2 shows a section of Fig. 1c, from which a conductive rubber was removed as an example.

[0029] Fig. 3 shows the underside of a printed circuit board with an electrical connector on its top side, which is used in an embodiment of the invention.

[0030] Fig. 4a shows an isometric view of the arrangement of a conductive rubber relative to two contact surfaces on bipolar plates in one embodiment of the invention. Fig. 4b shows a top view of the conductive rubber and the two bipolar plates of Fig. 4a.

[0031] Fig. 5 shows distances and angles on the conductive rubber and the contact surfaces on the bipolar plates according to Fig. 4b.

[0032] Embodiments of the invention

[0033] Relevant sections of a fuel cell according to an embodiment of the invention are shown in Figs. 1a to 1c. These have contact surfaces on bipolar plates 10. A voltage tapping device 20 is arranged on the contact surfaces of the parallel bipolar plates 10. It has a housing with a locking lever 21, which serves to fix the circuit board 40 in a suitable position in the housing. Comb-like arrangement and fastening elements 22 on the underside of the voltage tapping device 20 as positioning aids, which are arranged in two parallel rows, engage between the bipolar plates 10. An electrical connector 30, whose plug face points upwards, is provided for connecting an electrical circuit board 40 to a measuring device (not shown). On the underside of the circuit board 40, which faces the bipolar plates 10, two conductive rubbers 50a, 50b are arranged.These are each positioned at an angle of 45° to the bipolar plates.

[0034] Fig. 2 shows a representation of the fuel cell with the voltage tapping device 20 positioned on the bipolar plates 10, in which the second conductive rubber 50b has been removed for clarity. It can be seen that the removed conductive rubber 50b contacts the circuit board 40 on its side facing away from the bipolar plates 10. The circuit board 40 has an electrical circuit board contact element 41 above each bipolar plate 10 or each electrical tapping point contacted by the conductive rubber 50b. In this exemplary embodiment, eight circuit board contact elements 41 are arranged in a row on the circuit board 40.

[0035] Fig. 3 shows the entire underside of the printed circuit board 40. In addition to the row of eight printed circuit board contact elements 41, which are provided for contacting the second conductive rubber 50b, there is a further row of printed circuit board contact elements 41, which is provided for contacting the first conductive rubber 50a. This row also has eight printed circuit board contact elements 41. Each of the two conductive rubbers 50a, 50b thus establishes an electrical connection between eight contact surfaces on bipolar plates 10 and the associated contact surfaces of the printed circuit board 40, so that the electrical voltage at sixteen bipolar plates 10 can be tapped via the routing (not shown) on the printed circuit board and the connector 30.

[0036] 4a and 4b show, in one embodiment of the invention, the contact between a conductive rubber 50 and two bipolar plates 10 arranged at the edge of a fuel cell. The conductive rubber 50 has a central region consisting of alternating electrically conductive regions 51 and electrically insulating regions 52. These regions 51, 52 are sandwiched between electrically insulating edge layers 53a, 53b. All parts 51, 52, 53a, 53b of the conductive rubber 50 consist of a silicone rubber. This is electrically insulating in its untreated state. In the electrically conductive regions 51, it is made electrically conductive by the addition of carbon in the form of graphite.

[0037] Fig. 5 shows the geometric dimensions and angles of the contact surfaces on the bipolar plates 10 and the conductive rubber 50 in one embodiment of the invention. The bipolar plates 10 each have a width B of 0.2 mm. The distance P between two consecutive contact areas on bipolar plates is 1.0 mm. The distance P is the distance between the left edge of the contact surface of a bipolar plate 10 and the left edge of the next contact surface.

[0038] The electrically conductive regions 51 and electrically insulating regions 52 of the conductive rubber 50 each have a width b of 0.4 mm. The electrically insulating edge layers 53a, 53b each have the same width of 0.4 mm. A pitch p of the conductive rubber can be 0.1 mm. The pitch is understood to be the distance from the beginning of one electrically conductive region 51 to the beginning of the next electrically conductive region 51.

[0039] An overlap between the electrically conductive areas 51 and electrically insulating areas 52 of the conductive rubber 50 and a bipolar plate 10 would correspond to the width B of the bipolar plates 10 if the conductive rubber 50 is arranged orthogonally to the bipolar plates 10. However, if the conductive rubber 50 is arranged at an angle a, which in the present embodiment is 45°, relative to the orthogonal O, the overlap x is calculated according to formula 3:

[0040] A distance y, by which the conductive rubber 50 partially protrudes beyond a bipolar plate 10, is calculated according to formula 4:

[0041] It provides an additional contact area between bipolar plate 10 and conductive rubber 50, which increases the electrical contact area between bipolar plate 10 and conductive rubber 50 compared to an orthogonal arrangement. It increases with increasing angle a and with increasing width b of the electrically conductive areas 51 and electrically insulating areas 52 of the conductive rubber 50.

[0042] The distance from a left edge of a bipolar plate 10 to the left edge of the next bipolar plate 10, which is covered by the conductive rubber 50, corresponds to the distance P of the bipolar plates 10 in an orthogonal arrangement of the conductive rubber 50. With an increasing angle a, this distance z has an increasing value according to formula 5:

[0043] It was found that the distances x and y with respect to the grid p of the conductive rubber 50 should satisfy the condition of formula 6:

[0044] By inserting formulas 3 and 4 into formula 6 we get formula 1.

[0045] To limit the maximum value of the angle a, the conditions of formulas 7 and 8 should be met:

[0046] The distance z should therefore be at most half the sum of the distances x and y. To describe this, the distance f is introduced, which denotes the difference between the distance z and the distances x and y. Inserting formulas 3 to 5 into formulas 7 and 8 results in formula 2.

Claims

Claims 1. Fuel cell with multiple contact surfaces on bipolar plates (10), comprising at least one conductive rubber (50, 50a-b) which has alternating electrically conductive areas (51) and electrically insulating areas (52), characterized in that the conductive rubber (50, 50a-b) is arranged at an angle (a) to an orthogonal angle (O) to the contact surfaces on the bipolar plates (10), wherein the condition B 3.5 • p . cos(α) — b .. sin(α) is satisfied, where B is a width of the contact area on a bipolar plate (10), b is a width of the electrically conductive areas (51) and electrically insulating areas (52) of the conductive rubber (50, 50a-b) and p is a grid dimension of the conductive rubber (50, 50a-b).

2. Fuel cell according to claim 1, characterized in that furthermore the condition a sim 1((P - 2 • B) / (2 • b)) is satisfied, where P denotes a distance between two adjacent contact surfaces on bipolar plates (10).

3. Fuel cell according to claim 1 or 2, characterized in that the electrically conductive areas (51) and electrically insulating areas (52) of the conductive rubber (50, 50a-b) are arranged between two electrically insulating outer layers (53a, 53b).

4. Fuel cell according to one of claims 1 to 3, characterized in that a circuit board (40) is arranged on a side of the conductive rubber (50, 50a-b) facing away from the contact surfaces of the bipolar plates (10), the circuit board having several electrical circuit board contact elements (41), wherein above each contact area of ​​the bipolar plate (10) that is electrically connected to the conductive rubber (50, 50a-b) 5. Fuel cell according to claim 4, characterized in that it has several conductive rubbers (50a, 50b) which are arranged orthogonally to their respective longitudinal axes and offset from one another.

6. Fuel cell according to claim 4 or 5, characterized in that the printed circuit board (40) and the at least one conductive rubber (50, 50a-b) are components of a voltage tap device (20).

7. Fuel cell according to claim 6, characterized in that the voltage tap device (20) has arrangement and / or fastening elements (22) which are arranged between the bipolar plates (10) parallel to their stacking direction.

8. Fuel cell according to claim 6 or 7, characterized in that the printed circuit board (40) is mechanically and electrically connected to an electrical connector (30). 9.Voltage tap device (20) which is configured to be detachably arranged on a fuel cell according to one of claims 6 to 8.

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