Single cell and fuel cell stack with elastic structure for uniform distribution of operating medium

Abstract

The invention relates to a single cell for a fuel cell stack (1) having a membrane electrode assembly (7) arranged in an active area (13), which membrane electrode assembly is assigned at least one single plate (8) which is formed with a flow field for supplying an operating medium into the active area (13) of the membrane electrode assembly (7), which flow field is connected in terms of flow dynamics to a medium port (4, 5) present next to the active area (13). With respect to the membrane electrode assembly (7) transversely, there is a plate piece (12) which at least partially covers the flow cross section of the medium port (4, 5), which plate piece is elastically bendable by the force of the reaction medium flowing axially through the medium port (4, 5) in order to change the available flow cross section of the medium port (4, 5). The invention also relates to a fuel cell stack (1) having a plurality of such single cells.

Description

Technical Field

[0001] The present invention relates to a single cell for a fuel cell stack of a fuel cell system. A membrane electrode assembly (MEA) is arranged in the active area of ​​the cell. The MEA is equipped with at least one single plate, preferably a bipolar plate made of two interconnected single plates. The single plates form a flow field for supplying an operating medium to the active area of ​​the MEA. This flow field is fluidically connected to a media port located adjacent to the active area. Multiple media ports, stacked one above the other and aligned with one another, form a main channel for the operating medium, particularly cathode gas, fuel, or coolant. The present invention also relates to a fuel cell stack having a plurality of single cells stacked one above the other. Background Art

[0002] Fuel cell devices are used to chemically convert fuel and oxygen into water to generate electrical energy. To this end, fuel cells contain a so-called membrane electrode unit (MEA, i.e., membrane electrode assembly) as a core component, which is a composite consisting of a proton-conducting membrane and electrodes (anode and cathode) arranged on both sides of the membrane. In addition, gas diffusion layers (GDL) can be arranged on both sides of the membrane electrode unit on the side of the electrode facing away from the membrane. During the operation of a fuel cell device having a plurality of fuel cells combined into a fuel cell stack, fuel, in particular hydrogen H2 or a hydrogen-containing gas mixture, is supplied to the anode, where the conversion of H2 to H2 occurs with the release of electrons. + Electrochemical oxidation of H by protons is carried out via an electrolyte or membrane that hermetically separates the reaction chambers from each other and electrically insulates them. + The electrons provided at the anode are input to the cathode via an electrical circuit. Oxygen or an oxygen-containing gas mixture is supplied to the cathode, thereby converting O2 to O2 while receiving electrons. 2- At the same time, in the cathode compartment, these oxygen anions react with protons transported via the membrane to form water.

[0003] The reaction gases are supplied to the electrodes of the fuel cell via the bipolar plates. In addition to the reaction gases, a cooling medium is also passed through the bipolar plates due to the heat generated in the fuel cell reaction, so that three different media are passed through the bipolar plates in a very small space.

[0004] When reactants are supplied to a fuel cell, they are directed via main channels (ports) into the bipolar plates. These plates distribute the reactants to the active areas so that the entire electrode surface is supplied as evenly as possible thanks to the flow field. Because multiple bipolar plates and membrane electrode units are stacked in a fuel cell stack, seals are used to seal the main channels longitudinally through the fuel cell stack. Furthermore, a good seal must be achieved against the coolant flowing through the coolant channels.

[0005] To achieve uniform distribution of the operating medium introduced into the stack, DE 10 2017 202705 A1, DE 10 2014 220 682 A1, and DE 10 2017 211 755 A1 propose using inserts or plug-in elements in the main channels that alter the available flow cross section along the longitudinal extent of the respective main channel. This allows for uniform pressure distribution and, therefore, flow distribution of the operating medium. Using inserts or plug-in elements allows bipolar plates to be manufactured with uniform dimensions, thereby increasing the number of standard parts and reducing costs.

[0006] Alternatively, however, the media ports of the bipolar plates can be designed with different flow cross-sections, for example, by selecting the flow cross-section of the outlet main channel to be larger than the flow cross-section of the inlet main channel. The larger cross-section on the outlet side slows the flow velocity and increases the pressure in the outlet, which impairs uniform distribution. Due to the resulting asymmetry of the individual cells, a large amount of structural space is required for a fuel cell stack formed in this manner, and the tensile forces of the individual cells relative to the fuel cell stack are also subject to an asymmetric force distribution that must be balanced. Summary of the Invention

[0007] The object of the present invention is to provide a single cell and a fuel cell stack which take at least one of the above-mentioned disadvantages into account.

[0008] This object is achieved by the individual cells according to the invention and the fuel cell stack according to the invention. Advantageous embodiments with expedient developments of the invention are described in the dependent claims.

[0009] The bipolar plate according to the present invention is particularly characterized by the presence of a plate that at least partially covers the flow cross-section of the media ports transversely to the membrane electrode assembly. This plate is elastically bendable or yields to the force of the reaction medium flowing axially through the media ports, thereby changing the available flow cross-section of the media ports. In this manner, optimized uniform distribution is achieved by utilizing multiple bent or folded plates along the associated main channels generated by the aligned, stacked arrangement of the media ports of the individual cells.

[0010] Preferably, different (especially discrete) positions are provided for the plate, which defines the available flow cross section of the media port. Advantageously, the plate can therefore be adjusted, depending on the forces acting on it by the flowing operating medium, between a tilted configuration, in which the available flow cross section of the media port is increased, and a non-deflected configuration, in which the available flow cross section of the media port is reduced relative to the increased available flow cross section.

[0011] In order to keep the axial extension of the fuel cell stack small, it is advantageous if the membrane electrode assemblies of the individual cells are enclosed in an insulating layer and if the metal sheets form part of this insulating layer.

[0012] This offers the possibility of the insulating layer being a frame surrounding the membrane electrode assembly and / or a sealing layer surrounding the membrane electrode assembly (e.g., a so-called "subgasket" or RIM). These components are already present in the fuel cell, so that the available flow cross section of the main channels or media ports can be influenced by the additional use of plates without requiring the provision of additional components or components.

[0013] If at least one groove, open on one side, is introduced into the portion of the insulating layer that projects into the available flow cross section of the media port, the plate can be formed in a simple manner. For example, using only a single open groove, two plate halves that project into the media port can be produced.

[0014] However, it is preferred to introduce two grooves running parallel to one another and open on one side in the part of the insulating layer that protrudes into the available flow cross section of the media port, since this facilitates "folding" of the plate due to the force of the flowing operating medium.

[0015] In order to be able to adjust the available flow cross section in a targeted manner, it has proven advantageous if the plates include a concave section that projects into the available flow cross section of the media port. A design with a convex section that projects into the flow cross section is also possible. Other geometries that facilitate uniform distribution across the stack can also be used.

[0016] To increase the number of standardized components in a fuel cell stack, it has proven advantageous to connect the flow fields of the single or bipolar plates to the inlet-side media ports and to the outlet-side media ports, and to design the inlet and outlet media ports to have the same dimensions. This allows for the production of highly symmetrical single cells, preferably with multiple, and in particular all, existing plates having the same dimensions.

[0017] The advantages, advantageous embodiments, and effects explained in conjunction with the individual cells according to the present invention also apply to the fuel cell stack according to the present invention, which is formed with a plurality of such individual cells, in which the media ports are aligned with one another. This stack is further characterized by improved uniform distribution of the operating medium via the main channel formed by the media ports.

[0018] There is the possibility of dimensioning the plates of the cells in such a way that, during a force-induced deflection, the plates of a first cell are supported against the plates of a cell stacked adjacent to the first cell. In this way, the plates are thus supported against one another, with a gradually increasing accumulation of plates as the distance from the media connection in the flow direction of the operating medium increases. Due to this accumulation of plates, the flow cross section gradually narrows, which has a positive effect on uniform distribution.At the same time, such a configuration enables additional transverse sealing due to the overlapping plates.

[0019] The features and feature combinations mentioned above in the description and the features and feature combinations mentioned below in the description of the figures and / or shown individually in the figures can be used not only in the respectively specified combination but also in other combinations or individually without departing from the scope of the present invention. Therefore, the following embodiments are also to be considered as included and disclosed by the present invention, which are not explicitly shown or explained in the figures but are inferred and can be produced by the individual feature combinations in the explained embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Further advantages, features and details of the invention are apparent from the claims, the following description of preferred embodiments and with reference to the accompanying drawings.

[0021] Figure 1 A schematic diagram of a fuel cell stack having a plurality of fuel cells is shown, wherein a single cell shows the main channel,

[0022] Figure 2 Shown by Figure 1 Schematic detailed illustration of the cross section II-II of the active area of ​​the fuel cell stack,

[0023] Figure 3 shows a schematic detailed top view of a media port on the inlet side of one of the individual cells, wherein arrows indicate the media flow or media guidance,

[0024] Figure 4 A schematic detailed top view of the other inlet-side media port of one of the individual cells is shown, wherein arrows indicate the media flow or media guidance,

[0025] Figure 5 A schematic detailed top view of the other inlet-side media port of one of the individual cells is shown, wherein arrows indicate the media flow or media guidance,

[0026] Figure 6 A schematic detailed top view of the other inlet-side media port of one of the individual cells is shown, wherein arrows indicate the media flow or media guidance,

[0027] Figure 7 shows a schematic detailed cross-sectional view of a fuel cell stack in the region of a main channel, and

[0028] Figure 8 Another schematic detailed cross-sectional view of a fuel cell stack in the region of a main channel is shown. DETAILED DESCRIPTION

[0029] Figure 1 The fuel cell stack 1 shown in FIG. 1 is composed of a plurality of fuel cells 2 connected in series. Each fuel cell 2 includes an anode and a cathode, and a proton-conducting membrane separating the anode and cathode. The two electrodes and the membrane together form a membrane electrode assembly 7 (abbreviated: MEA). The membrane is formed from an ionomer, preferably a sulfonated tetrafluoroethylene polymer (PTFE) or a perfluorosulfonic acid polymer (PFSA). Alternatively, the membrane can be formed as a sulfonated hydrocarbon membrane.

[0030] Fuel (e.g. hydrogen) is supplied to the anode via the anode chamber within the fuel cell stack 1. In a polymer electrolyte membrane fuel cell (PEM fuel cell), the fuel or fuel molecules are split into protons and electrons at the anode. The membrane allows protons (e.g. H + ) passes, but the electron (e - ) is impermeable. At the anode, the following reaction occurs: 2H2 → 4H + +4e - (Oxidation / electron release). During the passage of protons through the membrane to the cathode, electrons are conducted to the cathode or energy storage via an external circuit. Cathode gas (e.g., oxygen or oxygen-containing air) can be supplied to the cathode via the cathode chamber within the fuel cell stack 1, thereby performing the following reaction on the cathode side: O2 + 4H + +4e - →2H2O (reduction / electron acceptance).

[0031] Air compressed by a compressor is supplied to the fuel cell stack 1 via a cathode fresh gas line. Furthermore, the fuel cell stack 1 is connected to a cathode exhaust gas line. On the anode side, hydrogen stored in a hydrogen tank is supplied to the fuel cell stack 1 via an anode fresh gas line to provide the reactants required for the electrochemical reactions in the fuel cells 2. These gases are transferred to the bipolar plates 3, which have main channels 4 extending from the media ports for distributing the gases to the membranes and outlets. Furthermore, the bipolar plates have main coolant channels 5 extending from the media ports for guiding the coolant in coolant channels 6, allowing three different media to be channeled into a very small space. Figure 1 Also shown are the main channels 4, 5 of the respective pairs of fuel cells 2 forming the fuel cell stack 1 with the bipolar plates 3. The membrane electrode assemblies 7 together with their associated bipolar plates 3 form the individual cells of the fuel cell stack 1.

[0032] exist Figure 2 The fuel cell stack 1 is shown along Figure 1Detailed section II-II in FIG. This section extends through the active area 13 of the fuel cell stack 1. The active area 13 of the bipolar plate 3 is not itself electrochemically active, but is arranged adjacent to components of the fuel cell 2 in which the electrochemical fuel cell reaction takes place, namely at or on the membrane electrode assembly 7. It can be seen that the bipolar plate 3 has reactant flow fields in this active area 13 on its surfaces facing away from each other, which are arranged opposite each other and thus have an oppositely disposed structure. The reactant flow field distribution has a plurality of flow channels 9 for the respective reaction media, which are delimited by the walls 11 of the bridges 10. The bridges 10 and flow channels 9 of one of the individual plates 8 are therefore designed in this active area 13 to extend oppositely to the bridges 10 and flow channels 9 of another of the individual plates 8. In this way, coolant channels 6 of the coolant flow field extending between the individual plates 8 are formed.

[0033] Because it is necessary to distribute the operating medium as evenly as possible into the active area 13 of the bipolar plate 3, a distribution region 14 with a distributor field containing channels is typically introduced between the media ports 4, 5 forming the main channels and the active area 13. To this end, the channels have suitable branches for guiding the operating medium. Downstream of the active area 13, i.e., downstream of the corresponding flow field, the partial media flows are collected again in a collection region 18 and discharged via the outlet-side media ports 4, 5.

[0034] To achieve a uniform flow of the operating medium across the main channel, the media ports 4, 5 (at least on the inlet side) are provided with at least one plate 12. This plate is arranged transversely or laterally relative to the membrane electrode assembly 7 and at least partially covers the available flow cross section of the media ports 4, 5. The plate 12 is elastically bendable or yieldable due to the force of the reaction medium flowing axially through the media ports 4, 5, thereby changing the available flow cross section of the media ports 4, 5.

[0035] exist Figures 3 to 6 It can be learned from Figure 1 Detailed view of the media port 4 shown at the top left of the fuel cell stack 1 in FIG. Here, the membrane electrode assembly 7 is in an insulated position, in this case enclosed in a frame 16, wherein the plate 12 forms part of the frame 16. The plate can also be part of the sealing structure ("subgasket" / RIM).

[0036] In accordance with Figure 3In the embodiment of the present invention, two grooves 15 extending parallel to one another and open on one side are introduced into the portion of the frame that protrudes into the available flow cross section of the media ports 4, 5. These two grooves 15 form a plate 12 that can be elastically bent, in particular, folded into or out of the plane of the paper. As can be seen, the plate 12 is provided with a concave section 17 that protrudes into the available flow cross section of the media ports 4, 5 and serves to create the desired flow resistance. Other shapes for creating other desired flow resistances are also possible.

[0037] In accordance with Figure 4 In the embodiment of , there is exactly one groove 15, which is oriented in the present case centrally with respect to the media port, so that two smaller (sub) plates 12 are formed on the left and right. Here, the possibility is also shown of providing both plates 12 with a concave section 17 that projects into the available flow cross section of the media ports 4, 5. Figure 5 The design of the plate 12 corresponds to the Figure 4 The embodiment of the plate 12 of FIG. 1 is different in that the upper plate 12 in the figure does not have a concave section 17. Instead, it is provided with a more precisely convex shape.

[0038] Figure 6 A design of the plate 12 is shown in FIG. Figure 3 ) by two lateral grooves 15 and (as in the embodiment according to Figure 4 ) is formed by a central groove 15. Here, there are also concave sections 17 at the two plates 12.

[0039] Depending on the prevailing boundary conditions, the proportion of the plates 12 covering the plate-free, cut-off media ports 4, 5 can be varied and adjusted in a targeted manner to optimize the uniform distribution for the respective fuel cell stack 1. The size and shape of the plates 12 are specifically adapted to the material thickness of the frame (MEA frame) for insulation and mechanical properties.

[0040] In accordance with Figure 7A detailed cross-sectional view of a fuel cell stack 1 (formed by a plurality of such single cells stacked one above the other) shows that the media ports 4, 5 are aligned to form a main channel. In the non-flowing state shown, the plates 12 protrude horizontally into the main channel formed by the media ports 4, 5. If operating medium flows through the plates 12, they elastically flex, as illustrated by the dashed lines. Preferably, the plates 12 can be adjusted, depending on the force of the flowing operating medium, between a folded configuration (in which the available flow cross section of the media ports 4, 5 is increased) and a non-deflected configuration (in which the available flow cross section of the media ports 4, 5 is reduced relative to the increased available flow cross section).

[0041] according to Figure 8 The cross-sectional view shows that the plates 12 of the individual cells are dimensioned so that, during a force-induced folding, the plates 12 of a first individual cell are supported against the plates 12 of the adjacently stacked cells. In this way, the plates 12 are supported against one another, with a gradually increasing accumulation of plates 12 as the distance from the media connection in the flow direction of the operating medium increases. This accumulation of plates 12 gradually narrows the available flow cross-section of the main channel, which facilitates even distribution of the flowing operating medium. At the same time, this configuration enables additional transverse sealing due to the overlapping plates 12. Currently, the plates 12 have identical dimensions. Targeted, variable dimensions of the plates 12 across the stack are also conceivable.

[0042] Thus, the present invention already provides a uniform distribution of the medium flow in the main channels, so that there is an improved medium distribution over the entire bipolar plate 3 , over the entire individual cell and thus over the entire fuel cell stack 1 .

[0043] Reference Signs List

[0044] 1. Fuel cell stack

[0045] 2 Fuel Cells

[0046] 3 Bipolar plates

[0047] 4 Main channels / media ports for reactants

[0048] 5 Main coolant channels / media ports for cooling medium

[0049] 6 coolant channels

[0050] 7. Membrane Electrode Assembly (MEA)

[0051] 8 boards

[0052] 9 Flow channels

[0053] 10 Bridge

[0054] 11 walls

[0055] 12 plates

[0056] 13 Active area

[0057] 14 Allocation Area

[0058] 15 slots

[0059] 16 Frame

[0060] 17 concave section (plate)

[0061] 18 Collection Areas

[0062] 19 End Plate

Claims

1. A single cell for a fuel cell stack (1), comprising a membrane electrode assembly (7) arranged in an active area (13), the membrane electrode assembly being assigned at least one single plate (8), the single plate forming a flow field for supplying an operating medium into the active area (13) of the membrane electrode assembly (7), the flow field being fluidically connected to a medium port (4, 5) located adjacent to the active area (13), characterized in that: Transversely to the membrane electrode assembly (7), there is a plate (12) that at least partially covers the flow cross section of the media port (4, 5), and the plate is elastically bendable by the force of the reaction medium flowing axially through the media port (4, 5) to change the available flow cross section of the media port (4, 5).

2. The single cell according to claim 1, characterized in that The plate (12) can be adjusted between a folded configuration, in which an enlarged available flow cross section of the media port (4, 5) is present, and a non-deflected configuration, in which an available flow cross section of the media port (4, 5) is reduced relative to the enlarged available flow cross section, depending on the force of the flowing operating medium acting on it.

3. The single cell according to claim 1 or 2, characterized in that: The membrane electrode assembly (7) is enclosed in an insulating layer, and the plate (12) forms a part of the insulating layer.

4. The single cell according to claim 3, characterized in that The insulating layer is a frame (16) surrounding the membrane electrode assembly (7) and / or a sealing layer surrounding the membrane electrode assembly (7).

5. The single cell according to claim 3, characterized in that To form the plate (12), at least one groove (15) open on one side is introduced into the portion of the insulating layer that projects into the available flow cross section of the media ports (4, 5).

6. The single cell according to claim 5, characterized in that Two grooves (15) extending parallel to one another and open on one side are introduced into the portion of the insulating layer that projects into the available flow cross section of the media ports (4, 5).

7. The single cell according to claim 1 or 2, characterized in that: The plate (12) comprises a concave section (17) which projects into the available flow cross section of the media ports (4, 5).

8. The single cell according to claim 1 or 2, characterized in that: The flow field is fluidically connected to the inlet-side media ports (4, 5) and the outlet-side media ports (4, 5), wherein the inlet-side and outlet-side media ports (4, 5) are formed with the same size.

9. A fuel cell stack (1) comprising a plurality of individual cells according to any one of claims 1 to 8, wherein the media ports (4, 5) are oriented aligned with one another.

10. The fuel cell stack (1) according to claim 9, characterized in that The plates (12) of the cells are dimensioned such that, during a force-induced deflection, the plates (12) of a first cell are supported on the plates (12) of a cell stacked adjacent to the first cell, increasing the resistance on the stack.

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