DE102013021627A1 - Method for determining humidity of polymer electrolyte membrane of fuel cell for vehicle e.g. motor car, involves determining humidity of membrane based on the change of pressure in fuel cell with respect to time - Google Patents

Abstract

The method involves detecting the change of prevailing pressure in a fuel cell (12) with respect to time. The humidity of a polymer electrolyte membrane (18) which separates an anode chamber (14) from a cathode chamber (16) of the fuel cell is determined based on the change of pressure in the fuel cell over time. The change in prevailing pressure in the anode chamber of the fuel cell with respect to time is detected. Independent claims are included for the following: (1) a method for operating a fuel cell system; (2) a device for determining humidity of polymer electrolyte membrane; and (3) a vehicle.

Description

The invention relates to a method for determining a moisture content of a membrane which separates an anode space from a cathode space of a fuel cell. Furthermore, the invention relates to a method for operating a fuel cell system, a device for determining a humidity of a membrane and a vehicle with such a device.

In the case of fuel cells with a polymer electrolyte membrane, a certain moistening of the membrane is to be provided in order to ensure the best possible function of the fuel cell.

The basic structure of a polymer electrolyte membrane fuel cell - PEMFC for short - is as follows. The PEMFC contains a membrane-electrode assembly - MEA short, which is composed of an anode, a cathode and a polymer electrolyte membrane arranged between them (also ionomer membrane) - PEM - constructed. In turn, the MEA is interposed between two separator plates, with one separator plate having fuel distribution channels and the other separator plate having channels for the distribution of oxidant and the channels facing the MEA. The channels form a channel structure, a so-called flow field or flow field. The electrodes, anode and cathode, are generally designed as gas diffusion electrodes - in short GDE. These have the function of dissipating the electricity generated in the electrochemical reaction (for example 2H ₂ + O ₂ → 2H ₂ O) and allowing the reactants, starting materials and products to diffuse through. A GDE consists of at least one gas diffusion layer or gas diffusion layer (GDL for short) and a catalyst layer which faces the PEM and at which the electrochemical reaction takes place. The GDE may also have a gas distribution layer, which adjoins the gas diffusion layer and which faces in the PEMFC a Separatorplatte.

Gas diffusion layer and gas distribution layer differ mainly in their pore sizes and thus in the nature of the transport mechanism for a reactant (diffusion or distribution).

Such a fuel cell can produce high power electrical power at relatively low operating temperatures. Real fuel cells are usually stacked into so-called fuel cell stacks - stacks - to achieve a high power output, instead of the monopolar separator plates bipolar separator plates, so-called bipolar plates are used and form monopolar separator plates only the two terminal terminations of the stack. They are sometimes called end plates and can be structurally different from the bipolar plates.

The bipolar plates are generally composed of two partial plates. These sub-plates have substantially complementary and with respect to a mirror plane mirror-image forms. However, the partial plates do not necessarily have to be mirror images. It is only important that they have at least one common contact surface to which they can be connected. The partial plates have an uneven topography. As a result, the channel structures already mentioned above arise at the surfaces of the partial plates that point away from each other. For example, in the case of embossed metallic partial plates, the channel structure complementary to the above-mentioned channel structure exists on the surfaces of the partial plates facing one another. When the two sub-panels are placed on top of each other, a cavity, which consists of a system of several interconnected tunnels, thus arises between the sub-panels, on their mutually facing surfaces. The cavity or the system of the tunnel is liquid-tight surrounded by a substantially peripheral part of the plates in the edge region joining, with openings for coolant supply and removal are provided so that the cavity for the distribution of a coolant can be used.

Thus, one of the tasks of a bipolar plate is: the distribution of oxidizing agent and reducing agent; the distribution of coolant and thus the cooling (better tempering) of the fuel cell; The fluidic separation of the individual cells of a stack from each other; Furthermore, the electrical contacting of the series-connected individual cells of a stack and thus the passage of the electric current generated by the individual cells.

The moistening of the membrane is generally carried out via the fuel cell supplied reaction gases. However, in a fuel cell system with polymer electrolyte membrane fuel cells, it is difficult to determine the humidity of the membrane. The moisture directly measuring sensors are complex and still do not always allow reliable information about moisture in a fuel cell stack of the fuel cell system. For example, dew point sensors located on gas-carrying lines upstream and downstream of the fuel cell stack do not allow to make reliable statements about the humidity in the interior of the fuel cell stack.

Object of the present invention is therefore to provide a particularly simple and reliable method and apparatus of the type mentioned, a vehicle with such a device and a method for operating a fuel cell system.

This object is achieved by a method having the features of patent claim 1, a method having the features of patent claim 7, a device having the features of patent claim 9 and a vehicle having the features of patent claim 10. Advantageous embodiments with expedient developments of the invention are specified in the dependent claims.

In the method according to the invention, a change in a pressure prevailing in the fuel cell is detected over time. Depending on the change in pressure over time, the humidity of the membrane is determined. This is based on the finding that a membrane, such as is used as a polymer electrolyte membrane in a fuel cell, can absorb water and thereby swell. Consequently, as the water absorption increases, a thickness of the membrane increases. The thickness, which depends on the water content of the membrane, has a decisive influence on how fast the pressure prevailing in the anode space or the cathode space of the fuel cell can change. Namely, the medium present in the anode space or cathode space can diffuse through the membrane. However, the higher the water content or the moisture of the membrane, the thicker it is and the worse the gases will diffuse through the membrane.

Thus, if the membrane is in a swollen state in which it has a particularly large thickness, then another change in the pressure over time can be detected than with a dry and thus thinner membrane. With a moist membrane there is thus a lower pressure gradient than with a dry membrane. By taking into account the relationship between the water content, the thickness and the barrier effect of the membrane, the moisture content of the membrane can be determined particularly easily and reliably. Since the change in the pressure prevailing in the fuel cell pressure can be detected with a simple and conventional pressure sensor, a particularly simple and inexpensive method is realized, which allows a reliable estimate of the moisture balance of the membrane and thus the fuel cell.

On a complicated measurement technique for the direct detection of moisture can be dispensed with. The knowledge of the humidity of the membrane can be used in the operation of a fuel cell system having the fuel cell. For example, the degree of dryness of the membrane can be determined. If it is found that the membrane is too moist, the membrane can be dried. In this way it can be ensured that when starting up the fuel cell system non-condensing or frozen water causes problems. In particular, for optimizing a freeze-start of the fuel cell system, therefore, the method is advantageously used. However, even if the membrane is too dry, it can be moistened specifically before the fuel cell system is put into operation.

The reliable determination of the humidity of the membrane has an advantageous effect on the service life of a fuel cell stack having the fuel cell. Even with knowledge of the humidity of the membrane, the time to be provided for a cold start of the fuel cell system can be kept particularly low.

The change in the pressure prevailing in the anode compartment of the fuel cell is preferably detected over time. Namely, the anode compartment is usually part of a closed circuit in which the fuel is recirculated. Thus, in a conventional fuel cell system, no separate shut-off devices need to be provided in order to close off the anode space in such a way that the pressure drop can be detected well. In addition, the fuel present in the anode compartment diffuses particularly easily through the membrane. A pressure drop in the anode compartment of the fuel cell is therefore particularly easy to detect.

Particularly noticeable and therefore particularly easily detectable is a change in the pressure in the anode compartment of the fuel cell when the fuel is initially present in the anode compartment at overpressure, that is, at a pressure above the atmospheric pressure.

Accordingly, it has been shown to be advantageous if a pump device which conveys the fuel into the anode compartment is switched off and the change in the pressure in the anode compartment is detected after the pump device has been switched off.

However, an overpressure in the anode chamber can also be adjusted by first opening a shut-off element which closes a pressure container in which the fuel is stored. Accordingly, an admission of the anode chamber with fuel originating from the pressure vessel can be prevented by closing the shut-off element, wherein the change in the pressure in the anode chamber is detected after closing the shut-off element. With such a timed or pulsed actuation of the shutoff, the pressure drop can be normal Operation of the fuel cell system and thus repeatedly capture before a shutdown.

A particularly accurate statement about the humidity of the membrane can be made when a gas composition present at the beginning of detecting the change in pressure in the fuel cell is taken into account in determining the humidity of the membrane. In particular, the differences in concentration of existing in the anode compartment and cathode compartment gases such as hydrogen, oxygen, nitrogen and water namely have an impact on the change in pressure over time. Preferably, therefore, at the beginning of detecting the change in pressure, the gas composition is known both in the anode compartment and in the cathode compartment. In that case, it is possible to use a particularly simple correlation of the membrane thickness as a determining factor for the pressure gradient and the change in pressure over time for the determination of the moisture.

In the method according to the invention for operating a fuel cell system, the humidity of the membrane of at least one fuel cell of the fuel cell system is determined by means of the method according to the invention. In this case, the membrane is moistened before starting up the fuel cell system, if a humidity of the membrane is less than a predetermined threshold. If the membrane is too dry, it can thus be specifically ensured that the humidity of the membrane permits operation of the fuel cell system with the highest possible degree of efficiency in the event of a subsequent restart or restart of the fuel cell system.

Furthermore, the membrane can be acted upon by a shutdown or during a shutdown of the fuel cell system with a dry medium, if a humidity of the membrane is greater than a predetermined threshold. This is particularly advantageous when temperatures are below freezing. In that case, too humid conditions in the fuel cell are unfavorable, because through frozen water the admission of the anode space and the cathode space as well as the catalysts with the fuel or the oxidizing agent can be made more difficult. Therefore, to prepare for a subsequent cold start or freeze start, it is advantageous to specifically dry a membrane that is too damp when it is shut down.

In particular ambient air can be used as the medium for drying the membrane, which is introduced into the cathode space of the fuel cell while bypassing a humidifier. Additionally or alternatively, a drying on the anode side of the fuel cell take place by fresh, dry hydrogen introduced into the anode compartment of the fuel cell and at the same time in a water separator of an anode circuit collected water is discharged.

The device according to the invention for determining a moisture content of a membrane comprises at least one fuel cell, in which the membrane separates an anode space from a cathode space of the fuel cell. The device comprises means for detecting a change in the pressure prevailing in the fuel cell over time and an evaluation device. The evaluation device is designed to determine the humidity of the membrane over time, depending on the change in pressure.

For this purpose, the evaluation device can be coupled, for example, via a communication path with a pressure sensor, which detects the pressure in the fuel cell, in particular the pressure in the anode chamber.

The invention also relates to a vehicle having such a device, wherein the vehicle may in particular be a motor vehicle.

The advantages and preferred embodiments described for the method according to the invention for determining the moisture and for the method according to the invention for operating a fuel cell system also apply to the device according to the invention and to the vehicle according to the invention.

The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of the figures and / or in the figures alone can be used not only in the respectively specified combination but also in other combinations or in isolation, without the scope of To leave invention. Thus, embodiments are also included and disclosed by the invention, which are not explicitly shown or explained in the figures, however, emerge and can be produced by separated combinations of features from the embodiments explained.

Further advantages, features and details of the invention will become apparent from the claims, the following description of preferred embodiments and from the drawings. Showing:

1 schematizes a fuel cell system of a vehicle, in which the humidity of a Membrane is determined by the pressure drop in the anode compartment; and

2 a graph illustrating the pressure drop in the anode compartment.

From a fuel cell system 10 for a vehicle, a fuel cell stack is shown which includes a plurality of fuel cells 12 includes. In each of the fuel cells 12 is an anode room 14 from a cathode compartment 16 through a membrane 18 separated. The membrane 18 is designed as a polymer electrolyte membrane. The anode compartment 14 is in a closed anode circuit 20 integrated, over which the anode space 14 leaving hydrogen gas, so the fuel is recirculated. This is in the anode circuit 20 a pump 22 intended. The in operation of the fuel cell system 10 through the anode circuit 20 flowing hydrogen comes from a pressure vessel 24 , The remaining, usual components of the fuel cell system 10 For example, components for moistening the reactants and devices for applying the cathode space 16 the fuel cell 12 with air as the oxidant are not shown for clarity.

In the anode circuit 20 is a pressure sensor 26 arranged. This pressure sensor 26 recorded in the anode circuit 20 and thus also in the anode compartment 14 prevailing pressure. About another pressure sensor 28 can be in the cathode compartment 16 capture prevailing pressure.

In the present case, depending on the time course of the pressure in the anode circuit 20 , So based on the change of the pressure sensor 26 detected pressure over time, the humidity of the membrane 18 certainly.

Here one uses the knowledge that the membrane 18 depending on the water content is different permeable to hydrogen and other gases. At high water content of the membrane 18 this is namely in a swollen state, which is associated with an increased thickness. The moister and thicker the membrane 18 For example, the poorer the hydrogen diffuses through it.

From respective values of the anode compartment 14 present and by means of the pressure sensor 26 detected pressure can be determined according to a pressure gradient, which conclusions about the humidity of the membrane 18 allows. To accomplish this, the pressure sensor is 26 with an evaluation device 30 coupled. This detects the change in the anode space 14 and cathode compartment 16 prevailing pressures as a function of time.

Corresponding pressure gradients are in an in 2 shown graphs 32 shown. First, the fuel cell system 10 so operated that in normal operation in the anode compartment 14 sets a substantially constant pressure. Accordingly, a curve 34 which in the graph 32 the time course of the pressure in the anode compartment 14 represents a first area 36 in which the pressure is substantially constant. The pressure is on an ordinate 38 of the graph 32 plotted and the time on an abscissa 40 ,

At a time 42 will shut down the fuel cell system 10 initiated. Accordingly, the pump promotes 22 no more hydrogen through the anode circuit 20 , The hydrogen begins, at first very rapidly and then progressively slower through the membrane 18 to diffuse. A corresponding pressure drop is in another area 44 the curve 34 seen.

The drop in pressure in the anode compartment 14 can be described by means of a pressure gradient. This in the field 44 the curve 34 Graphically illustrated pressure gradient is dependent on the concentration difference of the gases, which in the anode compartment 14 and in the cathode compartment 16 and the thickness of the membrane 18 ,

veranschaulichen. Hierbei beschreibt der Term auf der linken Seite der Gleichung den Druckgradienten, welcher eine Funktion des Konzentrationsunterschieds ΔcH₂ des Wasserstoffs, des Konzentrationsunterschieds ΔcN₂ des Stickstoffs, des Konzentrationsunterschieds ΔcO₂ des Sauerstoffs, des Konzentrationsunterschieds ΔCH₂O_g des gasförmig vorliegenden Wassers sowie der Dicke δ_M der Membran 18 ist.This can be explained by a formula:

illustrate. Here, the term on the left side of the equation describes the pressure gradient which is a function of the concentration difference ΔcH _{2 of} the hydrogen, the concentration difference ΔcN _{2 of} the nitrogen, the concentration difference ΔcO _{2 of} the oxygen, the concentration difference ΔCH ₂ O _{g of} the gaseous water and the thickness δ _{M of} the membrane 18 is.

With known gas composition in the anode compartment 14 and in the cathode compartment 16 at the time 42 is the thickness of the membrane 18 the determining factor for the pressure gradient, which is in the anode compartment 14 established. By detecting the itself after switching off the fuel cell system 10 and stopping the recirculation in the anode circuit 20 changing pressure in the anode compartment 14 So easy and precise statements about the humidity of the membrane 18 to be hit. The pressure change is dependent on the membrane thickness, and this in turn on the water content of the membrane 18 ,

Another in 2 shown curve 46 illustrates the in the cathode compartment 16 present pressure. This is essentially constant since in the cathode compartment 16 set immediately after switching off a compressor commonly used for compressing the air compressor, the ambient pressure. Due to the rapid diffusion of hydrogen through the membrane 18 , which is faster than the diffusion of oxygen into the anode compartment 14 , turns in the anode compartment 14 briefly a pressure below that in the cathode compartment 16 present pressure (see. 2 ).

Are for the different operating conditions of the fuel cell system 10 the respective gas compositions in the anode compartment 14 and cathode compartment 16 known, it can be simple, inexpensive and reliable on the basis of the pressure gradient on the moisture or the water content of the membrane 18 getting closed. Such a determination of the humidity of the membrane 18 is particularly suitable if the fuel cell system 10 shut down, so in the so-called shutdown of the fuel cell system 10 ,

However, the pressure gradient can also be at a pulsed loading of the anode compartment 14 be detected with hydrogen. In such a pulsed anode operation becomes a pressure vessel 24 closing valve briefly open. Consequently, in the anode space arises 14 an increased pressure, which then decreases again until the valve opens again. The stationary state in which the hydrogen in the anode circuit 20 is no longer recirculated, so usually turns off when switching off the fuel cell system 10 and in pulsed anode operation. The humidity of the membrane 18 However, it can also be in the idle state of the fuel cell system 10 , So be detected in a so-called stand-by mode, in which the pump 22 not operated and that the pressure vessel 24 closing valve is not reopened.

Is based on the pressure gradient, which of the partial pressures of the gases in the anode compartment 14 and cathode compartment 16 and the thickness of the membrane 18 depends, the humidity of the membrane 18 determines, so this information for the further operation of the fuel cell system 10 be used.

For example, in preparation for a restart, so a re-commissioning of the fuel cell system 10 , if the membrane is too dry 18 These are specifically moistened. If the membrane is too moist 18 can after switching off the fuel cell system 10 a targeted drying done, such as by flushing with fresh hydrogen and / or by applying the cathode compartment 16 with at a humidifier of the fuel cell system 10 bypassed, so not humidified air.

LIST OF REFERENCE NUMBERS

10

The fuel cell system

12

fuel cell

14

anode chamber

16

cathode space

18

membrane

20

anode circuit

22

pump

24

pressure vessel

26

pressure sensor

28

pressure sensor

30

evaluation

32

graph

34

Curve

36

Area

38

ordinate

40

abscissa

42

time

44

Area

46

Curve

Claims (10)

Method for determining a humidity of a membrane ( 18 ) containing an anode space ( 14 ) from a cathode compartment ( 16 ) a fuel cell ( 12 ), characterized in that a change in one in the fuel cell ( 12 ) is detected over time and, depending on the change in pressure over time, the humidity of the membrane ( 18 ) is determined. Method according to claim 1, characterized in that the change in the anode space ( 14 ) of the fuel cell ( 12 ) of prevailing pressure over time is detected. A method according to claim 2, characterized in that a fuel in the anode space ( 14 ) conveying pump device ( 22 ) and the change in pressure in the anode compartment ( 14 ) after switching off the pumping device ( 22 ) is detected. A method according to claim 2 or 3, characterized in that an admission of the anode space ( 14 ) with a pressure vessel ( 24 ) is prevented by closing a shut-off element, wherein the change in the pressure in the anode compartment ( 14 ) is detected after closing the shutoff. Method according to one of claims 1 to 4, characterized in that at the beginning of detecting the change in the pressure in the fuel cell ( 12 ) gas composition in determining the humidity of the membrane ( 18 ) is taken into account. Method according to one of claims 1 to 5, characterized in that the change in pressure during a shutdown of a fuel cell ( 12 ) fuel cell system ( 10 ) is detected. Method for operating a fuel cell system ( 10 ), in which the humidity of the membrane ( 18 ) at least one fuel cell ( 12 ) of the fuel cell system ( 10 ) is determined by a method according to any one of claims 1 to 6, wherein the membrane ( 18 ) before commissioning the fuel cell system ( 10 ) is humidified if a moisture of the membrane ( 18 ) is less than a predetermined threshold. Method according to claim 7, characterized in that the membrane ( 18 ) after a shutdown or during a shutdown of the fuel cell system ( 10 ) is acted upon with a dry medium, in particular with ambient air and / or hydrogen, if a moisture of the membrane ( 18 ) is greater than a predetermined threshold. Device for determining a humidity of a membrane ( 18 ), with at least one fuel cell ( 12 ), in which the membrane ( 18 ) an anode space ( 14 ) from a cathode compartment ( 16 ) of the fuel cell ( 12 ), characterized in that the device comprises means ( 26 . 28 ) for detecting a change in one in the fuel cell ( 12 ) prevailing pressure over time and an evaluation device ( 30 ), which is designed, depending on the change in pressure over time, the humidity of the membrane ( 18 ). Vehicle, in particular motor vehicle, with a device according to claim 9.

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