EP1368847A2

EP1368847A2 - Method for improving the water balance of fuel cells

Info

Publication number: EP1368847A2
Application number: EP02715437A
Authority: EP
Inventors: Erich Erdle; Alfred Koch; Gerhard Konrad; Ottmar Schmid
Original assignee: DaimlerChrysler AG
Current assignee: Daimler AG
Priority date: 2001-01-26
Filing date: 2002-01-16
Publication date: 2003-12-10
Also published as: US20040058206A1; DE10103568A1; WO2002059992A3; WO2002059992A2; AU2002225011A1; JP2004529458A

Abstract

The invention relates to a method for improving the water balance of an electrochemical cell stack, comprising: - a membrane electrode assembly (MEA) consisting of an anode, a cathode and an electrolyte arranged therebetween; - an anode gas chamber for distributing the anode gas to the membrane electrode assembly, said anode gas containing hydrogen and being de-humidified or partially humidified; - a cathode gas chamber for distributing the cathode gas to the membrane electrode assembly, said cathode gas containing oxygen; - a cooling chamber for distributing a coolant for cooling the cell stack, said cooling chamber being separated from the cathode gas chamber (3) by a porous separation plate (6). According to the invention, water is exchanged between the cathode gas and the coolant through the porous separation plate (6), located between the cathode gas chamber (3) and the cooling chamber (4), by means of an aqueous coolant, which has a lower temperature-dependent water-vapour partial pressure in relation to water.

Description

Process for improving the water balance of fuel cells

The invention relates to a method for improving the water balance of fuel cells, in particular low-pressure fuel cells.

Fuel cells are electrochemical units that generate electrical energy by converting chemical energy onto catalytic surfaces of electrodes.

Main types of electrochemical cells include:

- A cathode electrode on which the reduction reaction takes place by adding electrons. The cathode comprises at least one electrode support layer, which serves as a support for the catalyst.

- An anode electrode on which the oxidation reaction takes place through the release of electrons. Like the cathode, the anode consists of at least one support layer and catalyst layer.

- At least one matrix, which is arranged between the cathode and anode and serves as a support for the electrolyte. The electrolyte can be in a solid or liquid phase and as a gel. The solid-state electrolyte is advantageously converted into a

Matrix integrated, so that a so-called solid electrolyte is formed.

at least one separator plate which is arranged between the MEAs and is used for collecting reactants and oxidants in electrochemical cells, Anode gas chambers which are arranged between the anode-side separator plate and the MEA and through which the anode gas flows,

Cathode gas chambers which are arranged between the separator plate on the cathode side and the MEA and through which the cathode gas flows,

Cooling chambers through which a cooling medium flows and which serve to cool the cells,

- Sealing elements which both prevent mixing of the fluids in the electrochemical cells and also prevent the fluids from escaping from the cell to the environment

- Collection and distribution channels for the supply and discharge of the educts or products as well as the cooling media.

These three components listed first are also referred to as membrane electrode assemblies (MEA), the cathode electrode being applied to one side of the matrix and the anode electrode being applied to the other side.

If fuel cells are stacked on top of one another, a fuel cell stack is created, hereinafter also referred to as a stack. 1 shows an example of the structure of a fuel cell according to the prior art. The anode gas chamber is separated from the cathode gas chamber by the membrane electrode unit, comprising an anode, a cathode and an electrolyte. A cooling chamber is provided for cooling the cell, through which a cooling medium flows.

The electrical power supply runs in a series connection from cell to cell. The fluid management of the oxidants and reactants takes place via collection and distribution channels to the individual cells in a parallel connection.

In fuel cells based on hydrogen and oxygen, the sum reaction takes place according to the following equation

H2 + ¹ O ₂ -> H ₂ O + process heat + electrical current. Excess reactant and oxidant fluid is carried out of the cells. Furthermore, the product water must be removed from the cell to maintain the efficiency of the electrochemical reaction. The electrolyte state should not be negatively affected.

In the case of aqueous electrolytes, the electrolyte resistance strongly depends on the water content of the electrolyte. To keep the voltage drop across the cell as low as possible, the electrolyte resistance should be minimized during cell operation. In order to meet this requirement, homogeneous drainage of the product water is sought.

The product water is usually discharged by means of the steam loading of the reaction gases. If the reaction gases are supersaturated, the liquid water is also pressed out of the gas chambers of the cells by means of the dynamic pressure of the reaction gases. A smoothing of the product water discharge over the cell surface is usually achieved by pre-moistening the dry reaction gases before entering the cell. This pre-moistening prevents the electrolyte from drying out in the gas inlet area of the cell. If the electrolyte dries out, the cell will fail. The exemplary structure of a fuel cell system with separate pre-humidification of the reaction gases is shown in FIG. 2. The anode and cathode gases are pre-moistened in a humidifier before entering the fuel cell.

H2O separation via a porous layer adjacent to the cathode chamber is also known. An exemplary embodiment is shown in FIG. 3. In this case, the cooling chamber and the cathode chamber are arranged adjacent to one another by the porous layer, for example a porous separator plate. Water usually flows through the cooling chamber. Because of the diffusion through the porous separator plate, this arrangement allows, on the one hand, the humidification of the cathode gas in the line entry area. On the other hand, the liquid water in the supersaturated area of the cell surface is transported out of the cathode gas chamber through the porous layer. This arrangement partially compensates for the water load across the cell surface and thus also homogenizes the water content in the electrolyte, although despite these measures there is a high water vapor partial pressure difference between the cathode gas inlet and outlet. The desired minimum of the electrolyte resistance is achieved in such cells only in parts of the cell area. Additional, separate anode gas humidification is also required here. The water for this is usually obtained from the cathode gas. One disadvantage is that this version cannot be used at temperatures below 0 ° C without frost protection measures for the cooling medium, water. In addition, at an operating temperature above 65 ° C, additional water, for example from separate water tanks, must be fed to the cooling circuit, which is particularly undesirable in mobile applications. At operating temperatures above 65 ° C, more water is then discharged via the fuel cell exhaust air than is generated by the fuel cell process.

The object of the invention is to find a method with which it is possible to achieve a homogeneous electrolyte state over the entire cell area of the fuel cell for a wide temperature range without any technical measures or effort.

The object is achieved with the subject matter of patent claim 1. Advantageous designs are the subject of dependent claims.

According to the invention, a water exchange between the cathode gas and the cooling medium is realized with an aqueous cooling medium which has a temperature-dependent water vapor partial pressure which is lower than that of water, via the porous separator plate arranged between the cathode gas chamber and the cooling chamber. In the inlet area of the fuel cell, due to the water vapor partial pressure difference between the cooling medium in the cooling chamber and the dry cathode gas in the cathode gas space, water is transported from the cooling medium in the direction of the cathode gas through the porous separator plate. The dry cathode gas is thus moistened. In the wider cell area, product water is created in the cathode gas chamber due to the fuel cell reaction, which causes the water vapor partial pressure in the cathode gas increases. If the water vapor partial pressure of the cathode gas exceeds the water vapor partial pressure of the cooling medium, the porous separator plate causes water to be transported from the cathode gas in the direction of the cooling medium. A largely homogeneous electrolyte state is thus established over the entire cell area.

In addition, the inventive method humidifies and dehumidifies the electrolyte in a cell area.

To further homogenize the water content of the electrolyte, a porous separator plate with the function described above can also be arranged on the anode side.

Another advantage is that a homogeneous electrolyte state can be achieved even at temperatures above 65 ° C without additional measures, e.g. a water tank will be needed.

With a suitable choice of the cooling medium, it is possible to achieve a balanced water balance within the cell. The mass flow of dehumidification predominates - product water is transported from the cathode gas chamber in the direction of the cooling chamber - the mass flow of humidification - water is transported from the cooling chamber to humidify the cathode gas in the direction of the cathode gas chamber. This leads to an increase in the water content in the cooling medium. A balance between the water vapor partial pressure in the cathode gas and the water vapor partial pressure in the cooling medium is thus established for a specific operating temperature of the fuel cell.

Inorganic solutions, for example buffer solutions or organic solutions, for example glycols, glycerol or salts of organic acids, can advantageously be used as the cooling medium. Furthermore, the cooling medium can be a non-corrosive aqueous solution, emulsion or suspension. These solutions have a lower temperature-dependent water vapor partial pressure than pure water. As a result, the method according to the invention can be used without additional frost protection measures at temperatures below 0 ° C. In an advantageous embodiment, the surface of the separator plate facing the cathode gas space can be hydrophilic and the surface facing the cooling chamber can be hydrophobic. This prevents the cooling medium from reaching the cathode gas space through the pores of the separator plate from the cooling chamber. It is also achieved that the product water formed in the cathode gas space can be transported through the pores into the cooling chamber.

The water transport between the cooling chamber and the cathode gas chamber can, however, also be influenced by the liquid level in the porous separator plate or by the mass transport path length of the water vapor in the porous separator plate, which can be adjusted by the pressure within the cooling chamber and / or cathode gas chamber.

Another way of influencing the water transport between the cooling chamber and the cathode gas chamber is to design the pore diameter. As a result, the equilibrium between the water vapor partial pressure of the cathode gas and the water vapor partial pressure of the cooling medium can be set. The water vapor partial pressure in the pores is set approximately according to the equation:

With

Q: surface tension r: pore radius

PD: water vapor pressure in the pore

PD: saturation vapor pressure in the pore qF: density of the cooling medium

T: temperature of the cooling medium RD: special gas constant of water vapor In an advantageous embodiment of the invention, a membrane separator with a membrane for moistening the anode gas is present. The cooling medium is guided in the membrane separator along one side of the membrane and the dry anode gas is guided along the other side of the membrane. Due to the water vapor partial pressure difference between the two media, there is a water transport from the cooling medium to the anode gas, whereby this is moistened. By means of the membrane separator, the product water absorbed by the cooling medium within the fuel cell is thus used to moisten the anode gas. A separate humidification of the anode gas is therefore not required. In a further advantageous embodiment of the invention, the cooling chambers of the electrochemical cell stack, the membrane separator and a metering device for metering the cooling medium are connected in a circuit for the cooling medium. This ensures a continuous supply of cooling medium to the fuel cell stack. The metering device advantageously ensures that only a certain part of the cooling medium emerging from the fuel cell stack is fed to the membrane separator. This allows the humidification of the dry anode gas to be set in a defined manner.

Another circuit for the anode gas is advantageously present, in which the electrochemical fuel cell stack and the membrane separator are connected. Thus, anode gas that was not consumed in the fuel cell stack can be reused by leading it from the fuel cell stack to the membrane separator.

However, it is also possible for the anode gas to be metered into the fuel cell stack in sufficient quantity so that no excess anode gas is produced after the fuel cell stack. A return of the anode gas from the fuel cell stack to the membrane separator can thus be dispensed with.

Further advantageous embodiments of the inventions are explained in more detail below with reference to drawings. Show it:

1 shows a structure of a fuel cell according to the prior art, as explained in the introduction to the description, 2 shows a structure of a fuel cell with a porous separator plate according to the prior art, as explained in the introduction to the description,

3 shows a structure of a fuel cell system with separate pre-humidification of the anode and cathode gases according to the prior art, as explained in the introduction to the description,

4 shows an inventive construction of a fuel cell system with a membrane separator for moistening the anode gas,

5 shows a comparison of the vapor pressure curves of different mixing ratios of a glycerol-water solution, FIG. 6 shows the voltage curve of a fuel cell with a glycerol-water solution as cooling medium.

4 shows a structure of a fuel cell system according to the invention. The cooling chamber 4 of the fuel cell stack 1, a metering valve 10 and a water separator 9 (e.g. membrane separator) for water separation are connected in a circuit 7 for the cooling medium. The membrane separator 9 comprises two chambers 17, 18 separated by a membrane 16. The membrane 16 serves to allow the water contained in the cooling medium to pass from the chamber 17 into the chamber 18. Humidification of the anode gas flowing through the chamber 18 is thereby achieved.

The metering valve 10 is connected between the fuel cell stack 1 and the membrane separator 9. After flowing through the cooling chamber 4 of the fuel cell stack 1 through the metering valve 10, the cooling medium thus reaches the chamber 17 of the membrane separator 9, where the water contained in the cooling medium is separated off.

A compensating vessel 11 is advantageously connected downstream of the membrane separator 9. This expansion tank 11 is used to compensate for volume fluctuations in the cooling medium, caused by the water content, which varies depending on the temperature (operating temperature of the fuel cell). A connection 12 is made between the metering valve 10 and the compensating vessel 11, so that it is possible to pass only a partial flow of the cooling medium emerging from the fuel cell stack 1 into the membrane separator 9.

A heat exchanger 14 connected in front of the fuel cell stack 1 ensures that the cooling medium is brought to the corresponding operating temperature of the fuel cell stack 1. By means of a pump 13 connected to the circuit 7, the cooling medium is transported from the membrane separator 9 through the heat exchanger 14 into the cooling chamber 4 of the fuel cell stack 1. In the fuel cell stack 1, the cooling chamber 4 is separated from the cathode gas chamber 3 by means of a porous separator plate 6, whereby water exchange between the chambers is achieved.

The anode gas chamber 2 of the fuel cell stack 1 and the chamber 18 of the membrane separator 9 are connected in a further circuit 8. The chamber 18 of the membrane separator 9 is arranged in front of the anode gas chamber 2 of the fuel cell stack 1, as seen in the flow direction of the anode gas. It is thus possible that unused anode gas from the fuel cell stack 1 can be reused. A metering device 15 (for example a jet pump) connected in front of the membrane separator 9 enables fresh, unused anode gas to be added to the circuit 8. In advantageous embodiments in which the anode gas is not completely consumed in the fuel cell, the anode gas can be recycled by means of of the circuit 8 are eliminated.

The flow of the fluids in the fuel cell can largely be chosen arbitrarily. An exemplary flow guide is shown in FIG. 4. Here, the cathode gas flows in countercurrent to the anode gas and in cocurrent to the cooling medium.

5 shows the vapor pressure curves for various mixing ratios of a glycerol-water solution. The vapor pressure is plotted against the temperature. The vapor pressure of the solution decreases with increasing glycerin concentration. At a temperature of 80 ° C the vapor pressure for pure water about 474 mbar. At a glycerol concentration of 60%, the vapor pressure drops to approx. 370 mbar. A glycerin-water solution with a glycerin concentration of 90% has a vapor pressure of approx. 130 mbar. A higher concentration of glycerin in the glycerin-water solution lowers the freezing point of the solution. The freezing point of a glycerol-water solution with a glycerol concentration of 63% is approx. -40 ° C. Such a solution can thus be used as a cooling medium in the method according to the invention without additional frost protection measures.

The voltage curve of a fuel cell is shown in FIG. 6. A long-term test is shown in which the cell voltage is plotted against the measurement time. The test was carried out at a cell temperature of 70 ° C. and a current density of 0.5 A / cm ² . The left curve in the diagram shows the course of the cell voltage, using pure water as the cooling medium. The cell voltage drops by 0.4 mV / h, furthermore a large fluctuation range of the cell voltage can be seen.

The middle curve in the diagram shows the voltage curve of a fuel cell in which a glycerol-water solution with a glycerol concentration of 60% was used as the cooling medium. The voltage fluctuations over time have been significantly reduced in this experiment. This is due to the fact that a homogeneous electrolyte state was created over the entire cell area. In addition, with a drop in the cell voltage of 0.2 mV / h, a smaller drop in the cell voltage is observed than in the experiment with pure water as the cooling medium. The right curve again shows the voltage curve of the fuel cell in which pure water was used as the cooling medium. Here again a large fluctuation range of the cell voltage and a stronger voltage drop of 0.4 mV / h can be observed.

Claims

claims

1. A method for improving the water balance of an electrochemical cell stack, which comprises: a membrane electrode assembly (MEA) comprising an anode, a cathode and an electrolyte arranged between them, an anode gas chamber for distributing an anode gas to the membrane electrode assembly, wherein the anode gas contains hydrogen and is not or partially humidified, - a cathode gas chamber (3) for distributing a cathode gas to the

Membrane-electrode unit, wherein the cathode gas contains oxygen, a cooling chamber for distributing a cooling medium for cooling the cell stack, the cooling chamber and the cathode gas chamber (3) being separated by a porous separator plate (6), characterized in that with an aqueous cooling medium which has a lower temperature-dependent water vapor partial pressure than water, via which a porous separator plate (6) arranged between the cathode gas chamber (3) and the cooling chamber (4) realizes a water exchange between the cathode gas and the cooling medium.

2. The method according to claim 1, characterized in that the porous separator plate (6) arranged between the cathode gas chamber (3) and the cooling chamber (4) has a hydrophilic surface on the cathode side and a hydrophobic surface facing the cooling chamber.

3. The method according to any one of the preceding claims, characterized in that inorganic solutions such as buffer solutions or organic solutions such as glycols or salts of organic acids are used as the cooling medium.

4. The method according to any one of the preceding claims, characterized in that by means of the diameter of the pores of the porous separator plate (6) the water exchange between the cathode gas chamber and the cooling chamber is set.

5. The method according to any one of the preceding claims, characterized in that the water exchange between the cathode gas chamber and the cooling chamber by the pressure within the cathode gas chamber and / or

Cooling chamber is set.

6. The method according to any one of the preceding claims, characterized in that a metering device (10) is provided for metering the cooling medium.

7. The method according to any one of the preceding claims, characterized in that a water separator (9) for moistening the anode gas is present.

8. The method according to claim 7, characterized in that the water separator (9) is designed as a membrane separator, wherein the cooling medium along one side of the membrane (16) and along the other side of the

Membrane the dry anode gas is led.

9. The method according to claim 7 or 8, characterized in that a circuit (8) for the anode gas is present, in which the electrochemical cell stack (1) and the membrane separator (9) for moistening the anode gas are connected.

10. The method according to claim 8 or 9, characterized in that a circuit (7) for the cooling medium is present, in which the cooling chamber (4) of the electrochemical cell stack (1), the membrane separator (9) and the metering device (10) Metering of the cooling medium to the membrane separator (9) are switched.