EP0985239A1

EP0985239A1 - Cooling and wetting polymer-electrolyte fuel cells

Info

Publication number: EP0985239A1
Application number: EP97920648A
Authority: EP
Inventors: Arthur Koschany; Christian Lucas; Thomas Schwesinger
Original assignee: Magnet Motor Gesellschaft fuer Magnetmotorische Technik GmbH
Current assignee: L3 Magnet Motor GmbH
Priority date: 1997-04-10
Filing date: 1997-04-10
Publication date: 2000-03-15
Also published as: CA2286701A1; WO1998045889A1; JP2001519080A

Abstract

A polymer-electrolyte fuel cell (1) using air as oxidizing agent, under low overpressure, and different combustion gases, in particular hydrogen, is cooled by feeding liquid water directly into the gas passages (5, 9) of the combustion air and possibly of the combustible gas. The water introduced into the gas passages also serves to wet the solid polymer electrolyte (4).

Description

Cooling and humidification of polymer fuel cells

The invention relates to fuel cells which contain solid polymer membranes as the electrolyte, preferably hydrogen as the fuel gas and air or oxygen under low pressure as the oxidizing agent. The invention further relates to a method for simultaneously cooling the fuel cells and moistening the polymer electro membranes.

Polymer electrolyte fuel cells, as are commonly used to generate electrical current, contain one anode, one

Cathode and an ion exchange membrane arranged between them. A plurality of fuel cells form a fuel cell stack, the individual fuel cells being separated from one another by bipolar plates acting as current collectors. To generate electricity, a fuel gas, for example hydrogen, is introduced into the anode area and an oxidizing agent, for example air or oxygen, into the cathode area. Anode and cathode each contain a catalyst layer in the areas in contact with the polymer electrolyte membrane. In the anode catalyst layer, the fuel is oxidized to form cations and free electrons, in the cathode catalyst layer the oxidizing agent is reduced by taking up electrons. The cations migrate through the ion exchange membrane to the cathode and react with the reduced oxidizing agent, water being produced when hydrogen is used as the fuel gas and oxygen as the oxidizing agent. The reaction of fuel gas and oxidizing agent releases considerable amounts of heat, which have to be removed by cooling. Cooling was previously achieved by cooling channels in the bipolar plates, through which deionized water flowed. With this type of cooling there are enormous material problems, because typically about 50 to 300 bipolar plates are connected in series, so the cooling water electrically connects different potentials to each other. The result is material decomposition. Accordingly, only graphite or gold-plated metal can be used as the material for the bipolar plates.

It is also necessary to keep the polymer membrane moist, because the conductivity of the membrane depends heavily on its water content. In order to prevent the membrane from drying out, a complex system for moistening the reaction gases was therefore necessary.

The object of the invention is to provide a polymer electrolyte fuel cell or a polymer electrolyte fuel cell stack, the polymer electrolyte membrane of a fuel cell always having the optimum moisture content during operation and at the same time ensuring adequate cooling.

It is also an object of the invention to provide a method which enables the polymer electrolyte membrane to be

To keep the polymer electrolyte fuel cell at an optimal moisture content while the fuel cell is in operation and to cool the fuel cell sufficiently at the same time.

These tasks are solved by the polymer electrolyte

Fuel cell according to claim 1, the polymer electrolyte fuel cell stack according to claim 11, the method for cooling and humidifying a polymer electrolyte fuel cell according to claim 12 and the method according to claim 22.

Polymer electrolyte membranes require a high water content in order to ensure optimal conductivity for H ⁺ ions. The water content must generally be maintained by supplying water, since otherwise the fuel and oxidant gas streams flowing through the cell dry out the membrane. One possible

Counteract dehydration by adding an excess of water, However, it does not make sense, because water leads to flooding of the electrodes in too large quantities, ie the pores of the electrodes are blocked. A simple determination and regulation of the amount of water required was previously not possible.

Preferred embodiments are specified in the respective subclaims.

The drawings show:

Fig. 1 shows a preferred embodiment of a fuel cell according to the invention.

Fig. 2 shows a circuit for measuring the impedance of a fuel cell.

Fig. 3 shows the dependence of the conductivity of a Nafion 'membrane on the water content of the membrane.

A polymer electrolyte fuel cell according to the invention is used

Air or oxygen at low overpressure as an oxidizing agent. An excess pressure of less than 2 bar is preferred, particularly preferably less than 0.5 bar. The required pressure difference can also be achieved by suction. Hydrogen is preferably used as the fuel gas, but the use of other fuel gases is also possible in principle. Nafion ^® is preferably used as the polymer electrolyte membrane. Hydrogen is supplied to the individual fuel cells in a stack and distributed via gas channels in the anode area. At the same time air is supplied and distributed via gas channels in the cathode area. The hydrogen migrates to

Anode catalyst layer and forms cations there, which migrate through the electrolyte, a proton exchange membrane, to the cathode. At the cathode, oxygen migrates to the cathode catalyst layer and is reduced there. When reacting with the cations, water is formed as the reaction product. The water formed evaporates due to the heat of reaction, which results in a certain cooling. The On the one hand, the cooling effect is not sufficient, on the other hand, the membrane becomes increasingly poor in moisture during the operation of the fuel cell.

As can be seen from Fig. 3 for Nafion ^* NE 105 (30 ° C), the

Conductivity of ion-conducting membranes with the H ₂ O content. N (H ₂ O) / N (SO ₃ H) denotes the number of water molecules per sulfonic acid residue in the membrane.

A reduction in the moisture content of the solid

Polymer electrolyte membrane of a fuel cell therefore has the consequence that its internal resistance increases, that is to say its conductivity decreases. The conductivity of the membrane is extremely dependent on its water content. It is therefore essential for an efficient operation of a polymer electrolyte fuel cell that the polymer electrolyte membrane always has the optimum moisture corresponding to the respective working conditions (temperature, load, air ratio).

In order to maintain the optimum moisture, according to the invention, during operation of the fuel cell, preferably regularly or continuously, it can be determined whether the membrane is optimally moistened or whether water addition is required or what amount of water addition is required.

Basically, the amount of water added can vary widely. It depends on the respective working conditions of the fuel cell, and in particular also depends on the type of cooling of the fuel cell. Fuel cells are often supplied for cooling, which, depending on the design of the fuel cells, also moistens the membrane to a certain extent. Then in the

As a rule, less additional water is added than for cells with, for example, only air cooling.

The conductivity of the membrane depends on its water content. However, the conductivity of the membrane cannot be measured directly while a fuel cell is in operation. According to the invention preferably the impedance of the fuel cell (amount of the impedance or particularly preferably real part of the impedance) is determined. Since the conductivity of the membrane is a constant, monotonous function of these variables, the amount of water required can also be regulated on the basis of the impedance.

A possible circuit for measuring the impedance of a fuel cell is shown in FIG. 2.

The direct measurement of the conductance and thus the moisture content of a

Polymer electrolyte membrane of a fuel cell by determining the impedance is done by modulating the cell voltage with an alternating signal with a frequency of 1 to 20 kHz. In a fuel cell stack, the average moisture content of several membranes is suitably measured. The quotient from

AC voltage and the resulting current response is a measure of the humidity. In FIG. 2, BZ represents the fuel cell and R _{L represents} the load resistance. An arrangement of capacitor C, resistor R and AC voltage source U, which is suitable and small AC voltages, is connected in parallel with the load resistor

(Order of about 10 mV) and large currents (order of about 10 A). The voltage of the fuel cell is modulated by the alternating signal (approximately 1-20 kHz) from the alternating voltage source. The alternating voltage component U causes the fuel cell current to be superimposed with an alternating current I. The quotient from

AC voltage and alternating current is a measure of the impedance of the fuel cell and thus a measure of the moisture of the polymer electrolyte membrane, or of the amount of water required to be supplied.

However, the amount of the impedance depends, apart from the conductivity of the membrane, on other parameters, namely the size of the catalyst surface that is in contact with the membrane, the ohmic resistance of the electrodes and the poisoning of the membrane by foreign ions. These sizes are subject to in the course of

Service life of a fuel cell of some change, where the deviations due to changes in the ohmic resistance of the electrodes and poisoning of the membrane by foreign ions are generally negligible. In the course of the service life of a fuel cell, the amount of impedance that corresponds to the optimum membrane moisture under given operating conditions (setpoint value of the amount of impedance) can vary. Therefore, the setpoint of the amount of the impedance to be maintained should be reset in the course of maintenance work. The new setpoint is determined by maximizing the performance of the fuel cell. During operation of the fuel cell, the optimum setpoint can alternatively be readjusted by fuzzy logic or similar methods familiar to the person skilled in the art, in accordance with the changed conditions.

A largely independent measure of the conductivity of the membrane is obtained from the catalyst surface (whose change is essentially responsible for the change in the setpoint value of the impedance). if, in addition to the magnitude of the impedance, its phase angle is also taken into account. If the real part of the impedance determined electronically from this is considered as a controlled variable, a single setpoint value can even be used over the entire service life of the fuel cell.

During the operation of the fuel cells, the impedance (amount or real part) can be measured continuously or at regular intervals. If the conductance of the membrane or the membranes is calculated to be too low, water is supplied to the system, for example by opening water inlet valves electronically controlled in the usual way until the target value of the impedance is reached again.

In the case of fuel cell stacks with a plurality of fuel cells, it is advantageous not to use the amount or the real part of the impedance for each

Membrane individually to determine, but average values for a plurality of cells of the stack or even for all cells of the stack to determine together and to adjust the required water addition accordingly.

Regardless of the type of determination of the optimal water content of the membrane and the regulation of the water feed, it is possible according to the invention to use membrane dampening water at the same time for cooling the fuel cell and thus to ensure adequate cooling. This is achieved according to the invention in that, in the case of a fuel cell which is designed as stated above, ion-free into the gas channels for the combustion air

Water in liquid form is introduced immediately. Alternatively, the water can also be introduced directly into the gas channels for the fuel gas.

A proven solution is the introduction of water in both

Cathode as well as in the anode area, especially under operating conditions that cause the membrane to dry out heavily.

The liquid water evaporates in the hot fuel cell and, due to the phase change taking place, causes efficient cooling of the cell. It also penetrates the polymer electrolyte membrane and keeps it moist.

The easiest way to get the amount of water required

To add air flow or the air and / or hydrogen flow consists in the water using a metering pump in numerous thin lines, e.g. Capillaries to insert into the gas channels. There is no significant mixing of the water with the air or the fuel gas, so the free water surface available for evaporation is relatively small.

A significantly larger free water surface and thus a faster one

Moistening of the membrane and more efficient cooling can be achieved if the required amount of water is added to the reaction gas streams in mixed form, i.e. as an aerosol. The water in the air Aerosol and possibly the water in fuel gas Aerosol contain water in the form of droplets of 2 to 20 μm in size, which ensure rapid evaporation or evaporation. The aerosol can be produced, for example, with the aid of ultrasonic atomizers or nozzles. The simplest and at the same time the least energy-intensive generation of the aerosol takes place by means of ultrasonic atomizers at frequencies of at least 100 kHz.

A particularly advantageous embodiment of the invention is the design of the channels for receiving water in air aerosol or water-in-fuel gas aerosol, as shown in FIG. 1. In a fuel cell stack, each fuel cell is delimited by a bipolar plate 10, 6 on the anode and cathode sides. The anode-side bipolar plate is simultaneously the cathode-side bipolar plate of a neighboring cell and the cathode-side bipolar plate is simultaneously the anode-side bipolar plate of the other neighboring cell.

The bipolar plate has corrugated sheet structure at least in a partial area, so it alternately has elevations and depressions. A surface of the bipolar plate 6 touches with its elevations 7 the cathode region 2 of the fuel cell, as a result of which the depressions 8 between the two adjacent elevations form channels 5 with the cathode region for receiving water in air aerosol. In the same way, the surface of the bipolar plate 10 touches the anode region 3 of the cell, so that the depressions 12 located between two adjacent anode-side elevations 11 also form channels 9 with the anode region 3. These can be used to absorb water in fuel aerosol.

In the embodiment shown in FIG. 1, hydrogen is fed in as the fuel gas through bores perpendicular to the plate surface. The hydrogen first enters channel 9, which is connected to the feed opening, and diffuses or flows from there into the adjacent porous anode region. From here the diffuses

Hydrogen partly to the anode catalyst layer, partly in the Level of the anode area in further gas channels 9. Because of the excellent diffusion properties of hydrogen, the entire anode area is evenly supplied with hydrogen without any problems.

If cooling water is also to be fed in together with the fuel gas, it is generally more advantageous to choose the same type of supply as in the cathode region, that is to say to feed fuel and water into each individual channel 9. Because of the poor diffusion properties of water compared to hydrogen, little else would

Water penetrate the anode, so the cooling effect would be low.

The construction has no separate cooling channels. A particular advantage is that the path of the aerosol through the channels 5 of the cell is a straight line. The

Corrugated sheet structure of the bipolar plate with straight gas paths makes it possible to minimize precipitation of the aerosol and to conduct the necessary volume flows with a small pressure drop.

As is often the case with porous plates, no

Flooding and clogging of the water pipes by water droplets. In addition, the "corrugated metal plate" is very simple and inexpensive to manufacture.

Anode and cathode areas are each a suitable one

Diffusion layers carrying catalyst are formed, which are arranged on the opposite sides of the polymer electrolyte membrane 4.

Air seals 15, 15 'and hydrogen seals 16, 16' close the

Cell gas-tight.

In order to increase the residence time of the water in the cell and thereby enable complete evaporation, the walls of the gas channels 5 and / or the gas channels 9 can be covered with a hydrophilic absorbent layer, for example with felt. The The hydrophilic, absorbent layer distributes the amount of water introduced evenly and holds it until it evaporates.

The amount of water required to achieve optimal membrane moistening can, as stated above, be electronic

Paths are determined and regulated. The amount of water introduced into the fuel cell has two functions: cooling the cell and moistening the membrane. However, only the setting of the suitable membrane moisture is taken into account to regulate the necessary amount of water. Depending on the

Parameters temperature, load, air ratio etc. the optimal membrane moisture and thus the optimal conductivity of the membrane is determined experimentally. The water addition varies depending on the conductivity to be achieved. The cell temperature varies widely depending on the operating conditions. However, as long as sufficient water is introduced to ensure optimal membrane moisture, an adequate cooling effect is guaranteed.

To the in a fuel cell or a fuel cell stack

To keep the moisture content of the reaction gases and their temperature as constant as possible along the flow direction, the reaction gas, in particular the air, can be passed through the cell stack several times. This is done by recycling the air / water mixture leaving the fuel cells or the fuel gas / water mixture leaving the fuel cells into the corresponding intake flow.

According to the invention, in the case of a polymer electrolyte fuel cell, the introduction of ion-free water in liquid form directly into the gas channels of the combustion air and / or of the fuel gas can simultaneously ensure that an optimum membrane moisture and thus an optimal conductance of the membrane and sufficient cooling of the fuel cell are maintained.

Claims

Expectations

1. Polymer electrolyte fuel cell (1) with an anode area (3), a cathode area (2), a polymer electrolyte membrane (4) arranged between them, a device for supplying air as an oxidizing agent to the cathode area, gas channels (5) for distributing the air in the cathode area , a device for supplying fuel gas to the anode area, and gas channels (9) for distributing the fuel gas in the anode area, characterized by a device for introducing water in liquid form directly into the gas channels (5) of the air in the cathode area and / or the gas channels (9 ) of the fuel gas in the anode area, as well as bipolar plates (10, 6) delimiting the anode side and / or cathode side, which are corrugated at least in a partial area and have elevations (11, 7) and depressions (12, 8), the cathode side located depressions (8) the gas channels (5) for distributing the air in the cathode area and / or the anode-side depressions n (12) form the gas channels (9) for distributing the fuel gas in the anode area.

2. Polymer electrolyte fuel cell (1) according to claim 1, characterized in that 'the device for introducing water in liquid form is designed so that no significant mixing of water and air and / or water and fuel gas takes place during the introduction.

3. Polymer electrolyte fuel cell (1) according to claim 1 or 2, characterized in that the device for introducing water has a plurality of thin lines which open into the gas channels (5; 9) of air and / or fuel gas.

4. Polymer electrolyte fuel cell (1) according to claim 1, characterized in that the device for introducing water contains means for producing an aerosol of water in air and / or fuel gas.

5. Polymer electrolyte fuel cell (1) according to claim 4, characterized in that the means for generating the aerosol are nozzles.

6. polymer electrolyte fuel cell (1) according to claim 4, characterized in that the means for generating the aerosol are ultrasonic atomizers (17).

7. polymer electrolyte fuel cell (1) according to any one of claims 1 to 6, characterized in that the gas channels (5) and / or the gas channels (9) have walls which are coated with a hydrophilic, absorbent layer.

8. Polymer electrolyte fuel cell (1) according to one of claims 1 to 7, characterized in that a device is provided for the electronic determination of the amount of water required for setting the optimal conductance of the membrane (4).

9. polymer electrolyte fuel cell (1) according to claim 8, characterized in that a device is provided for measuring the moisture content of the membrane (4) by modulating the cell voltage with an alternating signal.

10. Polymer electrolyte fuel cell (1) according to one of claims 1 to 9, characterized by a device for recycling air / water mixture leaving the fuel cell and / or fuel gas / water.

Mixture in the device for supplying air and / or fuel gas.

11. Fuel cell stack with a plurality of fuel cells (1) according to any one of claims 1-10, characterized by a device for measuring the average moisture content of a plurality of membranes.

12. Process for cooling and moistening a polymer electrolyte

Fuel cell (1) with an anode area (3), a cathode area (2), a polymer electrolyte membrane (4) arranged between them, a device for supplying air as an oxidizing agent to the cathode area, gas channels (5) for distributing the air in the cathode area, a device for

Supply of fuel gas to the anode area and gas channels (9) for distributing the fuel gas in the anode area. characterized in that for the simultaneous cooling of the fuel cell and humidification of the polymer electrolyte membrane, a required amount of water in liquid form is introduced directly into the gas channels of the air and / or the gas channels of the fuel gas, and that the fuel cell delimits bipolar plates (10; 6) are provided, which are corrugated in at least one partial area and elevations (11; 7) and depressions

(12, 8), and that water in air aerosol into the channels (5) formed by the depressions (8) on the cathode side and / or water in fuel gas aerosol in the channels formed by the depressions (12) on the anode side ( 9) is introduced.

13. The method according to claim 12, characterized in that the amount of water required for cooling and humidification of the air and / or the fuel gas is added without substantial mixing.

14. The method according to claim 12 or 13, characterized in that the required amount of water through a plurality of thin lines leading into the gas channels (5, 9) of air and / or

Fuel gas flow, is introduced.

15. The method according to claim 12, characterized in that the required amount of water is added to the air and / or the fuel gas in a mixed form using an aerosol.

16. The method according to claim 15, characterized in that the aerosol is produced with the aid of nozzles.

17. The method according to claim 15, characterized in that the aerosol is produced with the aid of ultrasonic atomizers (17).

18. The method according to any one of claims 12 to 17, characterized in that the residence time of the cooling water in the cell is increased by

Coating the walls of the gas channels (5) and / or the gas channels (9) with a hydrophilic, absorbent layer.

19. The method according to any one of claims 12 to 18, characterized in that the required amount of water is determined electronically by experimentally determining the optimal membrane moisture and regulating the addition of water depending on the

Membrane moisture.

20. The method according to claim 19, characterized in that the measurement of the moisture content of the membrane (4) by

The cell voltage is modulated with an alternating signal.

21. The method according to any one of claims 12-20, characterized in that the oxidizing agent and / or the fuel gas are recirculated.

22. A method for cooling and moistening a fuel cell stack with a plurality of fuel cells (1) according to any one of claims 12 to 21, characterized in that the average moisture content of several membranes is measured to determine the amount of water required.