Fuel Cells and Voltage Tapping Devices

Abstract

A fuel cell having a plurality of contact surfaces on a plurality of flow field plates (10) includes at least one conductive rubber element (50) having alternating conductive regions (51) and electrically insulating regions (52) arranged at an angle (α) relative to a direction perpendicular (O) to the contact surfaces of the flow field plates (10) such that the condition B≧3.5·p·cos(α)-b·sin(α) is satisfied, where B is the width of the contact surfaces of the flow field plates (10), b is the width of the 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

[Technical field]

[0001] The present invention relates to an electrical contact between a fuel cell and at least one conductive rubber element. Furthermore, the present invention relates to a voltage tapping device for such a fuel cell. [Background technology]

[0002] Fuel cells convert chemical energy into electrical energy with high efficiency. They often consist of a pack of individual cells, a so-called stack. An individual cell usually contains a bipolar flow field plate on either side, between which an electrolyte and a membrane are placed. A voltage generated by the chemical reaction is applied across the membrane. This voltage can be measured at the bipolar plates and is an important parameter for operating a fuel cell.

[0003] For example, cell voltages can be measured using spring contacts mounted on a holder that is positioned at each individual cell and fixed by a holder or housing to a circuit board having conductor tracks that carry the individual cell voltages to a measurement device.

[0004] DE102007038153A1 describes a fuel cell including a conductive rubber element that is used to tap the voltage between two bipolar plates. This conductive rubber element includes alternating conductive and electrically insulating areas. The conductive rubber element is arranged at an angle of 90° to the surfaces of the bipolar plates. The conductive areas of the conductive rubber element make electrical contact with the bipolar plates. The electrically insulating areas prevent short circuits between the bipolar plates. In contrast to the use of spring contacts, this type of contact is geometrically easier to adjust. In addition, the contact pairing of the rubber with the bipolar plates prevents many degradation mechanisms that can occur with contact pairings of individual metals and / or precious metals. Furthermore, the conductive rubber element as an electrical contact element is relatively robust against external vibration and shock loads.

[0005] However, there are applications in which the area over which the conductive rubber element can contact the bipolar plate is much smaller than that described in the above patent specification, for example, it is conceivable to contact the bipolar plate only via a narrow end face in the edge region of the bipolar plate. Summary of the Invention [Problem to be solved by the invention]

[0006] It is an object of the present invention to provide a fuel cell equipped with a voltage tapping device including at least one conductive rubber element having very good vibration and impact resistance. [Means for solving the problem]

[0007] In a first aspect of the invention, this object is solved by a fuel cell having a plurality of bipolar plates electrically contacted by at least one conductive rubber element. Such a conductive rubber element is also called zebra rubber. The conductive rubber element comprises successive alternating conductive and electrically insulating areas. The conductive areas are in particular 10 5 This is understood to mean the area with a maximum electrical resistance of 10 Ω / cm. 9 It is understood to mean an area having an electrical resistance of more than Ω / cm. The electrically conductive and electrically insulating areas have in particular the same dimensions along the longitudinal axis of the conductive rubber element. The geometrical sum of the conductive and non-conductive layers (also called grid) is smaller than the width of the bipolar plate. The grid of the conductive rubber element is preferably in the range of 0.05 mm to 1.00 mm. If the grid of conductive rubber elements and its core width are selected to match the grid or spacing of the bipolar plates of the stack, short circuits between two adjacent bipolar plates can be prevented even as the stack shortens or lengthens over its life cycle.

[0008] The conductive rubber elements are arranged at an angle α to the perpendicular to the bipolar plate, satisfying the condition according to Eq. B≧3.5·p·cos(α)-b·sin(α) (Equation 1)

[0009] where p is the grid of conductive rubber elements, B is the width of the contact surface of the bipolar plate and b is the width of the conductive rubber element. This defines the minimum value of the angle α, so that a better overlap is achieved between the conductive area of ​​the conductive rubber element and the bipolar plate than if the conductive rubber element were placed orthogonally to the bipolar plate (α=0). This increases the robustness of the electrical contact against vibration and shock loads, even as the fuel cell's bipolar plate undergoes geometric changes over its life cycle.

[0010] Furthermore, it is preferred that the condition according to equation 2 is met.

[0011]

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[0012] Here, P is the distance between two adjacent contact surfaces of the bipolar plate.

[0013] This defines the maximum value of the angle α, which provides a practical compromise between good electrical contact between the conductive rubber element and the bipolar plate and a sufficient number of electrically non-contacting conductive layers in conjunction with the protection of clearances and creepage distances (avoiding short circuits).

[0014] In order to make it easier to mount the conductive rubber element in the electromechanical assembly and to eliminate the risk of short circuits with other electrical components of the fuel cell, it is preferred that the conductive and insulating areas of the conductive rubber element are arranged between two electrically insulating edge layers. It is particularly preferred that the two electrically insulating edge layers are formed in one piece with the conductive rubber element and are made of the same material as the electrically insulating areas of the conductive rubber element. However, it is still understood that in this embodiment of the fuel cell, the width b of the conductive rubber element according to formulas 1 and 2 is only the width of the conductive and insulating areas, and not the total width of the conductive rubber element, which is increased by the two edge layers.

[0015] In order to ensure a constant elasticity in all areas of the conductive rubber element, it is preferred that the conductive areas, the electrically insulating areas and any electrically insulating edge layer are formed from the same plastic material, which is electrically insulating in the unmodified state, whereby the conductive areas of the plastic material are provided with a conductive finish, for example by adding carbon, silver or gold.

[0016] A circuit board containing several electrical circuit board contact elements is preferably arranged on the surface of the conductive rubber element facing away from the bipolar plates. A circuit board contact element is arranged on the circuit board above each contact surface of the bipolar plate that is electrically contacted by a conductive rubber element. Thus, a precisely dimensioned conductive rubber element establishes an electrical contact between the contact surface of the bipolar plate and the contact surface of the printed circuit board. This allows a voltage tap between the different bipolar plates of the fuel cell via the circuit board, which represents the relevant input signal for the control of the fuel cell.

[0017] By using an angle α greater than 0°, a longer distance is covered by the conductive rubber elements perpendicular to the stacking direction of the bipolar plates than in the case of an orthogonal arrangement of the conductive rubber elements and the bipolar plates. In order to be able to provide the fuel cell with a compact voltage tapping device, it is therefore preferred that several individual conductive rubber elements are arranged in the fuel cell, which are arranged offset from one another in a direction orthogonal to their respective longitudinal axes. In particular, the fuel cell comprises two conductive rubber elements. For this purpose, a plastic housing can be provided which is positioned in the fuel cell and has a recess which accommodates the two conductive rubber elements. Each individual conductive rubber element preferably contacts 6 to 10 contact surfaces of the bipolar plate. In this embodiment of the fuel cell, the circuit board includes an array of electrical circuit board contact elements on its underside, each of the electrical circuit board contact elements being positioned at a specific point above one of the conductive rubber elements.

[0018] The circuit board and at least one conductive rubber element are preferably components of a voltage tapping device, i.e. an assembly that usually also includes a plastic housing that aligns and fixes the circuit board and the conductive rubber element to each other. A voltage tapping device is understood here to be a device that can tap a voltage at several bipolar plates and that can be mechanically connected to the bipolar plates via the plastic housing, but can also be separated from the bipolar plates, for example.

[0019] For this purpose, the housing can preferably contain components and / or fixing elements of suitable shape. An element for arranging the voltage tapping device in the stack can be arranged between and parallel to the bipolar plates. This element can thus engage between the bipolar plates in a rib-like manner, thereby allowing a reliable alignment of the voltage tapping device on the bipolar plates. To ensure that no short circuits occur between the bipolar plates due to the voltage tapping device, the components and / or fixing elements are designed in particular to be electrically insulating.

[0020] The printed circuit board is preferably mechanically and electrically connected to the plug connector. A mechanically detachable electrical connection with a measuring device can be established via this interface, thereby allowing the tapped cell voltage to be evaluated.

[0021] In a second aspect, the present invention relates to a voltage tapping device configured to be placed on a fuel cell according to the first aspect. Such a voltage tapping device can then be attached to a conventional fuel cell to electrically contact its bipolar plates, thereby obtaining a fuel cell according to the first aspect of the invention that can be appropriately monitored and controlled. For maintenance and repair purposes, the entire voltage tapping device can also be disconnected again from the fuel cell.

[0022] Examples of embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. [Brief description of the drawings]

[0023] [Figure 1a] FIG. 2 is a top view of a fuel cell including a voltage tapping device according to an embodiment of the present invention. [Figure 1b] FIG. 1b is an isometric view of a fuel cell with a voltage tapping device according to FIG. 1a. [Figure 1c]FIG. 1b shows a bottom view of a fuel cell equipped with a voltage tapping device according to FIG. 1a. [Diagram 2] FIG. 1c is a cross-sectional view of FIG. 1 with one conductive rubber element removed, for example. [Diagram 3] FIG. 2 illustrates the underside of a printed circuit board including an electrical connector on its top surface for use in an embodiment of the present invention. [Figure 4a] FIG. 13 is an isometric view of the arrangement of conductive rubber elements on two contact surfaces of a bipolar plate in an embodiment of the present invention. [Figure 4b] FIG. 4b is a top view of the conductive rubber element and two bipolar plates of FIG. 4a. [Diagram 5] FIG. 4b shows the distance and angle of the contact surfaces of the conductive rubber element and the bipolar plate according to FIG. 4b. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Relevant parts of a fuel cell according to an embodiment of the invention are shown in Figures 1a-1c. These parts include the contact surfaces of the bipolar plates 10. A voltage tapping device 20 is arranged on the contact surfaces of the parallel bipolar plates 10. The voltage tapping device 20 comprises a housing with a latch lever 21, which serves to fix the printed circuit board 40 in the appropriate position in the housing. On the underside of the voltage tapping device 20, comb-shaped components as positioning aids and fixing elements 22 are arranged in two parallel rows and engage between the bipolar plates 10. An electrical connector 30 with its mating surface facing upwards is for connecting the electrical circuit board 40 to a measuring device (not shown). Two conductive rubber elements 50a, 50b are disposed on the underside of the printed circuit board 40 facing the bipolar plate 10. The conductive rubber elements 50a, 50b are each positioned at an angle of 45° to the bipolar plate.

[0025] 2 shows a fuel cell with a voltage tapping device 20 positioned on a bipolar plate 10, with the second conductive rubber element 50b removed for clarity. It can be seen that the removed conductive rubber element 50b contacts the printed circuit board 40 with the surface facing away from the bipolar plate 10. The printed circuit board 40 includes an electrical circuit board contact element 41 above each bipolar plate 10 or each electrical tapping point contacted by the conductive rubber element 50b. In this embodiment, eight circuit board contact elements 41 are arranged in a row on the printed circuit board 40.

[0026] 3 shows the entire underside of the printed circuit board 40. In addition to the row of eight circuit board contact elements 41 provided for contacting the second conductive rubber element 50b, there is a further row of circuit board contact elements 41 provided for contacting the first conductive rubber element 50a. This row also includes eight circuit board contact elements 41. Each of the two conductive rubber elements 50a, 50b thus establishes an electrical connection between the eight contact surfaces of the bipolar plate 10 and the associated contact surfaces of the printed circuit board 40, and thus allows the voltage tapping at the 16 bipolar plates 10 via paths (not shown) on the printed circuit board and the plug connector 30.

[0027] In one embodiment of the present invention, Figures 4a and 4b show the contact of a conductive rubber element 50 with two bipolar plates 10 arranged at the edge of a fuel cell. The conductive rubber element 50 comprises a central region made up of alternating 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 element 50 are made of silicone rubber. The conductive rubber element 50 is electrically insulating in the unfinished state. The conductive rubber element 50 is made conductive in the conductive regions 51 by adding carbon in the form of graphite.

[0028] In Fig. 5, the geometric dimensions and angles of the contact surfaces of the bipolar plates 10 and the conductive rubber elements 50 are shown in an embodiment of the present invention. The bipolar plates 10 each have a width B of 0.2 mm. The distance P between two consecutive contact surfaces of the bipolar plates is 1.0 mm. The distance P is the distance between the left edge of the contact surface of one bipolar plate 10 and the left edge of the next contact surface.

[0029] The conductive regions 51 and the electrically insulating regions 52 of the conductive rubber element 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. The grid pitch p of the conductive rubber element may be 0.1 mm. The grid pitch is understood to be the distance from the beginning of one conductive region 51 to the beginning of the next conductive region 51.

[0030] When the conductive rubber element 50 is placed perpendicular to the bipolar plate 10, the overlap of the conductive area 51 and the electrically insulating area 52 of the conductive rubber element 50 with the bipolar plate 10 corresponds to the width B of the bipolar plate 10. However, when the conductive rubber element 50 is placed with an angle α (45° in this embodiment) with respect to the perpendicular O, the overlap x is calculated according to Equation 3:

[0031]

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[0032] The distance y that the conductive rubber element 50 partially protrudes beyond the bipolar plate 10 is calculated according to Equation 4: y=b tan(α) (Eq. 4)

[0033] This provides additional contact surface between the bipolar plate 10 and the conductive rubber element 50, increasing the electrical contact area between the bipolar plate 10 and the conductive rubber element 50 compared to the orthogonal arrangement. The contact area increases with increasing angle α and with increasing width b of the conductive region 51 and the electrically insulating region 52 of the conductive rubber element 50.

[0034] The distance from the left edge of one bipolar plate 10 to the left edge of the next bipolar plate 10 covered by the conductive rubber elements 50 corresponds to the distance P of the bipolar plates 10 in the orthogonal arrangement of the conductive rubber elements 50. With an increase in the angle α, this distance z comprises an increasing value according to Equation 5.

[0035]

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[0036] It has been found that the distance x and distance y associated with the grid p of the conductive rubber element 50 should satisfy the condition of Equation 6. x+y≧3.5·p (Equation 6)

[0037] Inserting Equation 3 and Equation 4 into Equation 6 results in Equation 1.

[0038] To limit the maximum value of the angle α, the conditions of Equation 7 and Equation 8 should be satisfied. x+y≦f (Equation 7) f=zxy (formula 8)

[0039] Therefore, the distance z should be at most half the sum of the distances x and y. To account for this, a distance f is also introduced to indicate the difference between the distance z and the distances x and y. Inserting Equations 3 to 5 into Equations 7 and 8 results in Equation 2.

Claims

1. A fuel cell having a plurality of contact surfaces on a plurality of bipolar plates (10), the fuel cell having at least one electrically conductive rubber element (50, 50a, 50b) comprising alternating successive electrically conductive regions (51) and electrically insulating regions (52), The conductive rubber elements (50, 50a, 50b) are arranged at an angle (α) with respect to a direction perpendicular (O) to the contact surface of the bipolar plate (10), and the condition B≧3.5・p・cos(α)−b・sin(α) where B is the width of the contact surface of one bipolar plate (10), b is the width of the conductive areas (51) and the electrically insulating areas (52) of the conductive rubber elements (50, 50a, 50b), and p is the grid pitch of the conductive rubber elements (50, 50a, 50b).

2. Condition α≦sin －１ 2. The fuel cell according to claim 1, further characterized in that ((P-2·B) / (2·b)) is satisfied, where P is the distance between two adjacent contact surfaces of the bipolar plate (10).

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

4. 4. The fuel cell according to claim 1, characterized in that a printed circuit board (40) having a plurality of electrical circuit board contact elements (41) is arranged on the surface of the conductive rubber element (50, 50a, 50b) facing away from the contact surface of the bipolar plate (10), each circuit board contact element (41) being arranged above each contact surface of the bipolar plate (10) that is electrically contacted by the conductive rubber element (50, 50a, 50b).

5. 5. A fuel cell according to claim 4, characterized in that it comprises a plurality of conductive rubber elements (50a, 50b) offset from one another in a direction perpendicular to their respective longitudinal axes.

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

7. 7. The fuel cell according to claim 6, wherein the voltage tapping device (20) includes components and / or fixing elements (22) arranged parallel to the stacking direction of the bipolar plates (10) and between the stacked bipolar plates (10).

8. 8. The fuel cell according to claim 6 or 7, characterized in that the printed circuit board (40) is mechanically and electrically connected to an electrical plug connector (30).

9. A voltage tapping device (20) configured for removably placement on a fuel cell according to any one of claims 6 to 8.

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