DE102021119655A1 - Membrane electrode assembly, fuel cell stack, fuel cell device and motor vehicle with such - Google Patents

Abstract

The invention relates to a membrane electrode assembly (20), which comprises a membrane which has an electrochemically active area (A) on its two opposite surfaces, in which the membrane is coated on both sides with catalyst particles to form an electrode and is set up to Reactants involved in the fuel cell reaction are to be coated, the membrane being catalyst-free on both of its surfaces away from the active region (A) in the flow direction before the active region (A) and/or away from the active region (A) in the flow direction after the active region (A). formed and arranged to be swept by moisture or liquid. The invention also relates to a fuel cell stack (1), a fuel cell device (1) and a motor vehicle.

Description

The invention relates to a membrane electrode arrangement, through which improved moisture or water management can be ensured. The invention also relates to a fuel cell stack with such a membrane electrode arrangement, a fuel cell device with such a fuel cell stack and a motor vehicle with such a fuel cell device.

Fuel cells include a membrane electrode assembly formed from a proton conducting membrane having the anode on one side and the cathode on the other side. Reactant gases are supplied to the electrodes by means of polar plates, namely hydrogen in particular on the anode side and oxygen or an oxygen-containing gas, in particular air, on the cathode side. When supplying the fuel cell with the reactants, these are conducted via a channel into the polar plate, which is intended to distribute the reactants using a plurality of channels in order to cover the entire surface of the electrode with the reactants as evenly as possible. Unused reactants are discharged again via a gas outlet channel. During the electrochemical reaction, product water is also formed from the educts, in particular on the cathode side, but product water also reaches the anode side as a result of diffusion or osmosis. A liquid water discharge is therefore also necessary so that the fuel cell can be operated reliably and permanently. However, for reliable proton transport it is necessary for the membrane to be moisture-saturated, for which purpose it is known to use humidifiers, in particular on the cathode side, in order to precondition the cathode gas supplied to the fuel cell. Such an additional humidifier requires additional installation space, which is typically only available to a limited extent when the fuel cell device is used in a motor vehicle. In addition, a wide variety of hoses are available, which also take up a lot of space. Due to the running times for the individual media resulting from the tubing to the fuel cell stack, there is also always a time delay, which makes it difficult to react quickly to changing circumstances in the fuel cell stack.

For this reason it is, for example, from the CN 103 915 638 A , from the KR 2011 0 014 455 A and U.S. 5,176,966 A known to integrate additional humidifier plates in the stack structure of the fuel cell stack. In this way, the reactants are moistened very close to the cell and a quick reaction to changing circumstances is possible.

Compared to these known configurations, the object of the present invention is to bring the actual humidification function even closer to the individual unit cell of the fuel cell stack.

This object is achieved by a membrane electrode assembly having the features of claim 1, by a fuel cell stack having the features of claim 3, by a fuel cell device having the features of claim 8 and by a motor vehicle having the features of claim 10. Advantageous configurations with expedient developments of Invention are set out in the dependent claims.

The membrane electrode assembly according to the invention comprises a membrane which has an electrochemically active region on its two opposite surfaces, in which the membrane is coated on both sides with catalyst particles to form an electrode and is set up to be coated by a reactant involved in the fuel cell reaction. According to the invention, the membrane is formed catalyst-free on both of its surfaces away from this active area in flow direction before the active area and/or away from the active area in flow direction after the active area and is set up to be coated by moisture or liquid.

In this way there is the advantage that the membrane itself provides improved water transfer within the membrane due to the catalyst-free area, which can be coated with liquid or moisture.

This configuration makes it possible to prevent the membrane from drying out, particularly in the distribution areas where the reactants flow at a very high rate, since the membrane itself acts like a wick and causes the entire membrane to be moistened in its active area.

In this context, there is the possibility that a catalyst-free area is present upstream of the active region and downstream of the active region in the flow direction, and that the catalyst-free regions provide a different area. This area of the two catalyst-free areas can be adapted to the corresponding conditions within the fuel cell stack, so that where there is typically increased drying out, the catalyst-free area tends to be larger, which is then used to moisten the membrane.

The advantages, advantageous configurations and effects mentioned in connection with the membrane electrode arrangement according to the invention apply in the same way to a fuel cell stack according to the invention which is equipped with such a membrane electrode arrangement. In addition to this membrane electrode arrangement, it also includes a polar plate applied in a stacking direction for supplying one of the electrodes of the membrane electrode arrangement with the reactant, the polar plate having a media port as an inlet for the reactant and a media port as an outlet for the reactant, as well as a flow field that fluidically connects the two media ports includes. It is preferred that the polar plate provides media ports for three media, namely the two reactants and a coolant. In addition, the polar plate is preferably formed free of further media ports.

This fuel cell stack also offers improved humidification and thus more efficient operation. Any external humidifier can be made smaller by such a configuration, with the possibility also being present of completely dispensing with such an external humidifier. This offers significant space savings.

A plurality of membrane electrode assemblies and a plurality of polar plates are preferably present in the fuel cell stack, the membrane electrode assemblies and the polar plates being stacked alternately between two end plates. In this way, each unit cell in the fuel cell stack can be formed with a membrane electrode assembly having a membrane with a catalyst-free region. These catalyst-free regions of the membranes can vary depending on the number of unit cells stacked, so that the unit cells further from the ports provide a different area for the catalyst-free regions than the membranes close to the ports.

The fuel cell stack is therefore preferably formed with a connection for the first reactant that is fluidically connected to the media ports on the inlet side, with a connection for the first reactant that is fluidly mechanically connected to the media ports on the outlet side. Furthermore, the fuel cell stack is formed with a connection for the second reactant that is fluidically connected to the media ports on the inlet side, with a connection for the second reactant that is fluidly mechanically connected to the media ports on the outlet side. The arrangement of the connections for the first reactant and the second reactant are selected in such a way that a countercurrent principle prevails for the two reactants. In this way, it is possible for there to be reliable transfer of water and high humidity, leading to saturation of the membranes for efficient ion transport within the fuel cell stack.

The position of the connections is preferably selected in such a way that the liquid or moisture is guided in a ring shape. In particular, there is a flow from a cathode outlet to an anode inlet, with the flow running along an anode flow field to an anode outlet and flowing towards a cathode inlet, where it runs along a cathode flow field and arriving back at the cathode outlet. There is no direct flow of liquid relative to the membrane. Rather, it is brushed by moisture on one side when the gas flow is guided through the anode flow field. It is swept by moisture on its other side when the gas flow is passed through the cathode flow field.

In this way, it is possible for the moisture to be distributed in an annular manner over the anode spaces and cathode spaces of the fuel cell stack. Locally very dry areas are thereby avoided. At the same time, an external humidifier can be dispensed with, or one can be designed to be much smaller. The selected annular flow allows the relative humidity to be adjusted over the stack as a whole, which runs in a circle. In this way, there is improved homogeneity and greater dynamics, since the humidification paths are shorter, which increases the stack performance and the service life of the fuel cell stack. Because the humidification can be regulated internally in the stack, there is a simplification for the frost start.

In addition, there is a cost reduction due to the omission of the external humidifier and the omission of the media-connecting components from the external humidifier to the stack inlet. However, in order to additionally reduce the space requirement, or to be able to make better use of it, it has proven to be advantageous if the connections are all present on the same end plate.

The advantages, advantageous configurations and effects mentioned in connection with the membrane electrode arrangement according to the invention and the fuel cell stack according to the invention apply in the same way to a fuel cell device according to the invention which is equipped with such a fuel cell stack.

This is also characterized by the omission of an external humidifier, although this can also be made smaller. Such a fuel cell device makes reliable use of the available installation space.

In this context, it is advantageous if the fuel cell device is formed without a humidifier on the outside of the stack, since the available installation space is then optimally utilized.

The advantages, advantageous configurations and effects mentioned in connection with the membrane electrode arrangement according to the invention, the fuel cell stack according to the invention and the fuel cell device according to the invention apply in the same way to the motor vehicle according to the invention which is equipped with such a fuel cell device.

In this motor vehicle, too, reduced fuel consumption is to be noted, since the fuel cell stack is lighter due to the more compressed design. Due to the ideal, even humidification, the fuel cell stack can be operated even more efficiently in addition to the reduced weight, which also reduces fuel consumption. In this way, the range of the motor vehicle is increased.

The features and combinations of features mentioned above in the description and the features and combinations of features mentioned below in the description of the figures and/or shown alone in the figures can be used not only in the combination specified in each case, but also in other combinations or on their own, without going beyond the scope of the leave invention. Embodiments are therefore also to be regarded as included and disclosed by the invention which are not explicitly shown or explained in the figures, but which result from the explained embodiments and can be generated by means of separate combinations of features.

• 1 eine schematische Darstellung einer Brennstoffzellenvorrichtung,
• 2 eine schematische Darstellung einer Draufsicht auf die Oberfläche einer Membranelektrodenanordnung, wobei die Strömung eines Reaktanten illustriert ist,
• 3 eine schematische Darstellung einer Draufsicht auf die Oberfläche einer (Bi-)Polarplatte, wobei die Strömung eines Reaktanten illustriert ist, und
• 4 eine schematische Schnitt- oder Seitenansicht auf einen Brennstoffzellenstapel, wobei für die einzelnen Bereiche (A, B, C) die relative Feuchte im Betrieb in den nachstehenden Diagrammen zur jeweiligen Position illustriert ist; einerseits für die relative Feuchte der kathodenseitigen Luft (mittleres Diagramm) und andererseits für die relative Feuchte des anodenseitigen Wasserstoffs H₂ (unteres Diagramm).

Further advantages, features and details of the invention result from the claims, the following description of preferred embodiments and with reference to the drawings. show:

• 1 a schematic representation of a fuel cell device,
• 2 a schematic representation of a plan view of the surface of a membrane electrode assembly, wherein the flow of a reactant is illustrated,
• 3 a schematic representation of a plan view of the surface of a (bi)polar plate, wherein the flow of a reactant is illustrated, and
• 4 a schematic sectional or side view of a fuel cell stack, wherein for the individual areas (A, B, C) the relative humidity during operation is illustrated in the diagrams below for the respective position; on the one hand for the relative humidity of the cathode-side air (middle diagram) and on the other hand for the relative humidity of the anode-side hydrogen H ₂ (bottom diagram).

In the 1 a fuel cell device 1 is shown schematically, which has a fuel cell or a plurality of fuel cells combined to form a fuel cell stack 2 .

Each of the fuel cells includes a membrane electrode assembly 20 made up of an anode and a cathode and a proton conductive membrane separating the anode from the cathode. The membrane is formed from an ionomer, preferably a sulfonated tetrafluoroethylene polymer (PTFE) or a polymer of perfluorinated sulfonic acid (PFSA). Alternatively, the membrane can be formed as a sulfonated hydrocarbon membrane.

A catalyst is additionally admixed to the anodes and/or the cathodes, the membranes preferably being coated on their first side and/or on their second side with a catalyst layer made of a noble metal or mixtures comprising noble metals such as platinum, palladium, ruthenium or the like. which serve as a reaction accelerator in the reaction of the respective fuel cell.

Fuel (for example hydrogen) is supplied to the anodes via anode chambers within the fuel cell stack 2 . In a polymer electrolyte membrane fuel cell (PEM fuel cell), fuel or fuel molecules are split into protons and electrons at the anode. The membrane lets the protons (e.g. H ⁺ ) through, but is impermeable to the electrons (e ^- ). The following reaction takes place at the anode: 2H ₂ → 4H ⁺ + 4e ^- (oxidation/donation of electrons). While the protons pass through the membrane to the cathode, the electrons are conducted to the cathode or an energy storage device via an external circuit. Cathode gas (e.g. oxygen or oxygen-containing air) can be supplied to the cathodes via cathode spaces within the fuel cell stack 2, which are also provided by the bipolar plate 10, so that the following reaction takes place on the cathode side: O ₂ + 4H ⁺ + 4e ^- → 2H ₂ O (reduction /electron uptake).

Compressed air is supplied to the fuel cell stack 2 via a cathode fresh gas line 3 by a compressor 4 . In addition, the fuel cell is connected to a cathode exhaust gas line 6 . On the anode side, hydrogen is supplied to the fuel cell stack 2 through an anode fresh gas line 8 from a hydrogen tank 5 in order to provide the reactants required for the electrochemical reaction in a fuel cell. These gases are conducted by means of a media port 15 in polar plates or bipolar plates 10 to their active surface A or discharged from them, with flow channels 11 running separately from webs 13 being formed in the polar plate or bipolar plate 10 for the further distribution of the gases to the Membrane electrode assembly 20 and for their discharge from the fuel cell stack 2 and the passage of a cooling medium.

A valve or a suction jet pump illustrated in the figure can be suitable in this case in order to realize the desired partial pressure of fresh fuel within an anode circuit, which comes about through the anode recirculation line 7 . With such an anode recirculation line 7 , the fuel not consumed in the fuel cell stack 2 can be fed back to the anode chambers upstream of the fuel cell, so that the anode recirculation line 7 opens back into the anode fresh gas line 8 . Spent fuel is removed from the fuel cell via the anode exhaust line 9 . Alternatively or additionally, a recirculation fan, also shown in the figure, can also be used to bring about the recirculation.

In 2 a membrane electrode assembly 20 is shown comprising a membrane having on both of its opposing surfaces an electrochemically active region A in which the membrane is coated on both sides with catalyst particles to form the electrode and arranged to be swept by a reactant involved in the fuel cell reaction as symbolized by the arrows. However, the membrane has on both of its surfaces away from the active area A in the direction of flow in front of the active area A and in the direction of flow after the active area A regions that are formed without a catalyst and set up to be coated by moisture or liquid. These two regions B and C only serve to moisten the membrane, the consequence being reduced drying out of the membrane electrode arrangement 20, particularly in the areas in which there is a particularly strong flow (on the inlet side). In this way, there is improved water management.

There is the possibility that the two catalyst-free areas B and C provide a different area, in particular depending on their position within a fuel cell stack 2 and also in particular depending on the degree of drying out by the respective reactant flow.

A bipolar plate 10 is used to distribute the reactant flow over the catalyst-free area B, C and over the active area A. In 3 Such a bipolar plate 10 is illustrated, in which three media ports 15 shown on the left for the inflow of the two reactants and the coolant and three media ports 15 on the right for their outflow are present. The flow of fuel from the media port 15 shown at the top left to the media port 15 shown at the bottom right, which extends distributed over the bipolar plate 10 or over its flow field 12, is shown purely by way of illustration. The flow field 12 has flow channels 11 , shown only schematically, which are separated by webs 13 . The electrochemical reaction takes place in the active area A, since in this area the membrane electrode arrangement 20 is coated on both sides by the two reaction media and it is also coated there with catalyst particles. Due to the present flow of reactants, there is increased drying of the membrane of the membrane electrode arrangement 20 on the inlet side, i.e. near the media port 15 shown at the top left. Therefore, the illustratively dotted highlighted region B is present, which is formed free of catalyst particles. In order to be able to reverse the direction of flow, there is the further area C, which is highlighted in a dotted manner and also prevents excessive drying out. Both areas B and C can act like a wick for liquid or moisture, so that the membrane is moistened overall and an external moistener can be omitted.

In 4 a side view or sectional view through the fuel cell stack 2 can be seen, it being made clear here that in each unit cell there is an area B, C on the inlet side as well as on the outlet side, which is formed without a catalyst. In this way, corresponding moistening can therefore take place on each side due to the product water formed during the fuel cell reaction.

It has turned out to be advantageous if the reactants flow through the fuel cell stack 2 in the countercurrent principle, since in this way there is improved moisture distribution. In such a fuel cell stack 2, a plurality of membrane electrode assemblies 20 and a plurality of polar plates 10 are present, the membrane electrode assemblies 20 and the polar plates 10 are alternately stacked between two end plates 16. The fuel cell stack has on one and the same end plate 16 a connection 17 for the first reactant which is fluidically connected to the media ports 15 on the inlet side. It also has a connection 17 for the first reactant, which is fluidically connected to the media ports 15 on the outlet side, and also includes a connection 17 for the second reactant, which is fluidically connected to the media ports 15 on the inlet side. Furthermore, there is a connection 17 for the second reactant, which is fluidically connected to the media ports 15 on the outlet side, the arrangement of the connections 17 for the first reactant and the second reactant being chosen such that a countercurrent principle prevails for the two reactants.

In the present case, the position of the connections 17 is selected in such a way that the liquid or moisture is guided in an annular manner, with the flow being guided from a cathode outlet 18 to an anode inlet 19 . Starting from the anode inlet 19 , it flows through an anode flow field to an anode outlet 21 .

The relative humidity of the two reactants can be seen in the diagrams below, with the middle diagram showing the relative humidity of the cathode gas as it flows through the fuel cell stack 2 shown above “from left to right”. Below that, the relative humidity of the fuel is drawn, with a flow through the fuel cell stack 2 “from right to left”.

In this way, locally very dry areas can be avoided, whereby external humidifiers can be completely dispensed with or they can only be used on a smaller scale. The relative humidity of the stack is guided "in a circle", which leads to an increased homogenization of the humidity over the entire stack.

The present invention is therefore distinguished by an improved homogenization of the moisture distribution, with shorter paths prevailing for the distribution of the moisture and thus greater dynamics being provided for the fuel cell stack 2 . Since the humidification can now be controlled internally in the stack, there are also simplifications for the frost start. There is no need for an external humidifier, and the media-connecting components from the external humidifier to the stack inlet are no longer required. This results in the fuel cell device 1 being designed to be more compact overall; it makes even better use of the available space. The overall weight of the fuel cell device and thus of the motor vehicle is also reduced by the design selected.

Reference List

1

fuel cell device

2

fuel cell stack

3

Cathode fresh gas line

4

compressor

5

hydrogen tank

6

cathode exhaust line

7

anode recirculation line

8th

anode fresh gas line

9

anode exhaust line

10

Polar Plate / Bipolar Plate

11

flow channel

12

flow field

13

webs

14

distribution area

15

media port

16

endplate

17

Connection

18

cathode outlet

19

anode inlet

20

membrane electrode assembly

21

anode outlet

22

cathode inlet

A

active area

B

first region away from the active area

C

second region away from the active area

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• CN 103915638A [0003]
• KR 20110014455 A [0003]
• US5176966A [0003]

Claims (10)

Membrane electrode assembly (20) comprising a membrane having on its two opposite surfaces an electrochemically active region (A) in which the membrane is coated on both sides with catalyst particles to form an electrode and arranged to be coated by a reactant involved in the fuel cell reaction characterized in that the membrane is formed catalyst-free on its two surfaces away from the active area (A) in the direction of flow before the active area (A) and/or away from the active area (A) in the direction of flow after the active area (A). and adapted to be swept by moisture or liquid. Membrane electrode assembly (20) after claim 1 , characterized in that upstream of the active region (A) and downstream of the active region (A) there is a catalyst-free region (B, C), and that the catalyst-free regions (B, C) provide a different area. Fuel cell stack (2) with a membrane electrode assembly (20). claim 1 or 2 and with a polar plate (10) applied in a stacking direction for supplying one of the electrodes of the membrane electrode assembly (20) with the reactant, the polar plate (10) having a media port (15) as an inlet for the reactant and a media port (15) as an outlet for the reactant and a flow field (12) that fluidically connects the two media ports (15). Fuel cell stack (2) after claim 3 , characterized in that there are a plurality of membrane electrode assemblies (20) and a plurality of polar plates (10), and that the membrane electrode assemblies (20) and the polar plates (10) are alternately stacked between two end plates (16). Fuel cell stack (2) after claim 4 , characterized in that there is a connection (17) for the first reactant that is fluidically connected to the media ports (15) on the inlet side, that there is a connection (17) that is fluidically connected to the media ports (15) on the outlet side for the first reactant, that a connection (17) that is fluidly connected to the media ports (15) is present the inlet-side media ports (15) connected connection (17) for the second reactant, that there is a flow-mechanically connected to the outlet-side media ports (15) connection (17) for the second reactant, and that the arrangement of the connections (17) for the first Reactants and the second reactant is chosen so that a countercurrent principle prevails for the two reactants. Fuel cell stack (2) after claim 5 , characterized in that the position of the connections (17) is selected such that there is an annular guidance of the liquid or moisture, the flow being guided from a cathode outlet (18) towards an anode inlet (19), with an anode flow field to passes along an anode outlet (21) and flows towards a cathode inlet (22) where it passes along a cathode flow field and arrives back at the cathode outlet (18). Fuel cell stack (2) after claim 5 or 6 , characterized in that the connections (17) are all on the same end plate (16). Fuel cell device (1) with a fuel cell stack (2) according to one of claims 3 until 7 . Fuel cell device (1) after claim 8 , which is formed without a humidifier on the outside of the stack. Motor vehicle with a fuel cell device (1). claim 8 or 9 .

Images