WO1997024914A1

WO1997024914A1 - Process for operating pem fuel cells

Info

Publication number: WO1997024914A1
Application number: PCT/DE1996/002459
Authority: WO
Inventors: Horst GRÜNE; Peter Buchner; Rittmar Von Helmolt
Original assignee: Siemens Aktiengesellschaft
Priority date: 1996-01-04
Filing date: 1996-12-19
Publication date: 1997-07-17
Also published as: DE19600200C1

Abstract

A process for operating PEM fuel cells with gaseous reactants and a coolant comprises the steps: at least one of the gaseous reactants is fed to the cells either dry or slightly moistened; between the inlet and outlet of the dry or slightly moistened reactants, a temperature difference is established across the cells, where the inlet temperature is lower than the outlet temperature; the dry or slightly moistened reactants are taken through the cells parallel to the temperature gradient; the coolant is air, fed through between the cells.

Description

description

Process for operating PEM fuel cells

The invention relates to a method for operating PEM fuel cells with gaseous reactants and a cooling medium.

In fuel cells that are operated with gaseous reactants, water is produced as the reaction product, but the reactants flowing out continuously remove moisture from the cells. In the case of fuel cells with an ion-conducting polymer membrane as the electrolyte, so-called PEM fuel cells (PEM = polymer electrolyte membrane), sufficient moisture must therefore be provided in the membrane. If this is not the case, the membrane dries out, which leads to a deterioration in performance or even to a defect or failure of the fuel cell. However, too much water in the cells also results in a drop in performance, because then the supply of the gaseous reactants to the cell becomes poorer.

Until now, PEM fuel cells have been kept moist by moistening the gaseous reactants supplied, ie the reaction gases. For this purpose, these gases are brought to the temperature prevailing in the cells in humidifiers which are connected upstream of the cells, so that they are saturated with water vapor at the gas inlet, ie when they enter the fuel cell (see, for example, DE-OS 42 01 632). Although this procedure ensures adequate moistening of the membrane, in particular at the gas inlet, a not inconsiderable design effort is required for the humidifiers. When humidifying via a membrane (see DE-OS 42 01 632), a very large membrane area is required for air operation of the fuel cells in order to completely humidify the air at a typical operating temperature of approx. 80 ° C. In order to deliver the required heat of vaporization from the fuel cells to the humidifiers, a liquid is generally used as the transport medium because of the large amount of heat; the cooling water circuit of the fuel cell battery serves in particular for this. However, the periphery of a fuel line unit, which must also include, for example, the return of the product water to the humidifiers, requires a relatively large outlay.

If, on the other hand, the heat of vaporization of the water supplied to the humidifiers is not applied from the waste heat of the fuel cells, then the overall system becomes less efficient. For this reason too, only cooling with water has been considered in fuel cell batteries. Disadvantages of water cooling are the risk of frost during breaks in operation, which requires frost protection for the coolant or constant heating, as well as the electrical losses in the coolant and the risk of electrochemical corrosion, which necessitates time-consuming control and treatment of the cooling water.

It is an object of the invention to provide a method by which the problems with the humidification of the reaction gases which have previously occurred during the operation of PEM fuel cells are avoided as far as possible and liquid cooling can also be dispensed with.

This is achieved according to the invention in that at least one of the gaseous reactants is supplied to the cells dry or only partially moistened, that a temperature difference is set via the cells - between the inlet and the outlet of the dry or partially moistened reactants, the inlet temperature T (E) is lower than the outlet temperature T (A), that the dry or partially moistened reactants are passed through the cells parallel to the temperature gradient, that air is used as the cooling medium. is turning, and that the air between the cells is passed ^¬.

An essential feature of the method according to the invention is thus the so-called internal moistening. This means that external humidification of the reaction gases is completely or at least partially eliminated. Accordingly, this process is preferably carried out in such a way that both reactants, namely hydrogen on the one hand and oxygen or air on the other hand, dry the cells, i.e. are supplied without moisture. It is therefore possible to dispense with (external) humidifiers, as a result of which the associated disadvantages are eliminated.

In the method according to the invention, however, one of the two reaction gases can also be humidified, in particular the hydrogen; the other reaction gas is then generally fed to the cells dry. However, it is also possible to use partially humidified reaction gases.

In the context of the present patent application, the term “partially moistened” means that the dew point of the reaction gas is significantly lower than the average cell temperature, in particular by approximately 20 to 40 ° C. The reaction gas then generally contains only a third of that amount of water , which corresponds to the saturation at the mean cell temperature.

Dry or partially humidified reaction gases are passed through the cells parallel to the temperature gradient. If a reaction gas is completely humidified, such a parallel guidance can be omitted with this gas.

The temperature difference required in the method according to the invention is preferably set via the cooling medium, ie via the air, specifically via the flow rate. The air flows freely between the cells through, ie there are no cooling elements or cooling layers between the cells. In addition, the temperature difference can be generated by conduction, in particular by heating or cooling at the cell ends with water.

When operating fuel cells in accordance with the method according to the invention, a temperature difference is set in the cells, specifically starting from the cell entrance. The incoming dry reaction gas takes water with it at the cell entrance, but the cell or battery temperature in the inlet area of the reaction gases is kept so low that the membrane cannot dry out appreciably. In the further course, an increase in the temperature is then permissible or possible because the controlled absorption and entrainment of product water from the membrane prevents subsequent areas from drying out. With such a temperature profile above the cell - despite the relatively low temperature at the gas inlet - effective cooling is possible due to the higher temperature towards the gas outlet.

An essential feature of the invention is thus a high temperature difference ΔT = T (A) -T (E) over each cell. This temperature difference is preferably approximately 10 to 80 ° C., in particular approximately 20 to 40 ° C. Typical temperatures in stationary operation are - depending on the load, operating pressure and ambient temperature - for example: T (E) = 20 to 40 ° C and T (A) = 50 to 100 ° C. This temperature distribution is preferably achieved by a parallel flow of cooling medium, ie air, and reaction gas. The cell or battery power reduced in the entrance area of the cells - due to the relatively low temperature - may require a somewhat larger battery (up to approx. 30%). However, this disadvantage is more than compensated for by an overall smaller construction volume. In the method according to the invention it is important that water is not used as the cooling medium. In this method, air is used as the cooling medium. Air cooling is particularly advantageous when using fuel cells for electric traction.

The method according to the invention can be used particularly advantageously for operating fuel cells or batteries, as described in DE-PS 44 42 285; air cooling takes place here. This procedure offers the following additional advantages:

- there is a very simple operating system;

- No own coolant is required;

- There is no electrochemical corrosion by the cooling medium;

- high battery voltages are possible (some 100 V);

- the membrane is not destroyed (T (E) is low);

- The regulation is very simple.

In principle, these advantages also exist if the reaction gases are partially humidified, which could be necessary under certain operating conditions.

The invention will be explained in more detail with the aid of examples and a figure.

The water absorption or release of electrolyte membranes in PEM fuel cells, which are ion exchange membranes, depends, among other things, on the difference in the water vapor partial pressure in equilibrium and the water vapor partial pressure of the gas flowing through the cell. So that's it

Water delivery to a dry inflowing gas is strongly temperature-dependent. If a temperature profile corresponding to the formation of product water is set over the membrane surface, as is the case with the method according to the invention, then the water can be removed in gaseous form without the membrane drying out. For example, when using air as the reaction gas, which is available dry at the gas inlet with an air ratio of λ = 2, saturation with water vapor from the resulting product water is possible at a temperature T (A) of approx. 62 ° C. The cooling capacity saved by the evaporation of the product water is P _t h / I = 0.23 W / A. This corresponds to about a third of the total thermal power at the operating point (P _e / I <0.75 V). For air cooling of a cell stack of 40 cells (area: 100 cm ² ), a cooling air flow of approx. 144 m ³ / h is necessary at a pressure difference of 0.8 mbar.

The figure shows an air-cooled cell stack of a PEM fuel cell battery according to the invention, which has a lower temperature at the cell or gas inlet than at the outlet. The associated temperature profile is shown schematically.

A cell (active area: 100 cm ² ), which is operated with oxygen and hydrogen, is cooled at the oxygen inlet and heated at the outlet. In this way, a temperature of 35 to 40 ° C is set at the input and a temperature of 65 to 75 ° C at the output. While the oxygen is used dry, ie not humidified, the hydrogen is humidified. Measurements with an excess of oxygen of λ = 5 and a load current of 40 A, corresponding to 400 mA / cm ² , show that the voltage does not change within a period of approx. 70 h; it is in the range between about 720 and 740 mV. Despite the high oxygen throughput, no drying out of the membrane can be observed, which would be associated with a drop in the voltage. The water of reaction formed in the cell is therefore sufficient for gas humidification. This also applies to an excess of oxygen up to λ = 10, which corresponds to air operation with λ = 2. If the cell is operated with dry hydrogen, only a slight drop in the voltage can be observed over a period of several days.

Claims

claims

1. A method for operating PEM fuel cells with gaseous reactants and a cooling medium, characterized in that at least one of the gaseous reactants is supplied to the cells dry or only partially moistened, that via the cells - between the entry and the exit of the dry or partially moistened reactants - a temperature difference is set, the inlet temperature being lower than the outlet temperature, that the dry or partially moistened reactants are passed through the cells parallel to the temperature gradient, that air is used as the cooling medium, and that the air is passed between the cells.

2. The method of claim 1, d a d u r c h g e - k e n n z e i c h n et that the dry or partially moistened reactants are passed through the cells parallel to the cooling medium.

3. The method of claim 1 or 2, d a d u r c h g e ¬ k e n n z e i c h n e t that both reactants are supplied to the cells dry.

4. The method according to any one of claims 1 to 3, so that a temperature difference between 10 and 80 ° C, in particular between 20 and 40 ° C, is set.

5. The method according to one or more of claims 1 to 4, that the temperature difference is set by means of the flow rate of the cooling medium.