Method of discharging reaction water in PEM fuel cells and...

Details Method of discharging reaction water in PEM fuel cells and... Method of discharging reaction water in PEM fuel cells and...

2001-03-30

2003-06-10

Chaney, Carol (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having magnetic field feature

C429S006000

Reexamination Certificate

active

06576358

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention lies in the technological field of PEM fuel cells and pertains, more specifically, to a method for discharging reaction water in PEM fuel cells and to a PEM fuel cell for implementing the method.
The operation of fuel cells gives rise—in the course of the electrochemical reaction of hydrogen (H
2
) with oxygen (O
2
)—to water (H
2
O). In PEM fuel cells (PEM=polymer electrolyte membrane), in which a cation exchange membrane serves as the electrolyte, the protons (H
+
) formed at the anode—via oxidation of the hydrogen—diffuse through the membrane and at the cathode combine with the O
2−
ions produced there to form water. The reaction water must be removed from the fuel cell so as not to affect the water budget and to keep it constant.
Various options of discharging the reaction water from PEM fuel cells are known:
Discharge from the cathode side (in liquid phase):
The reaction gases are humidified to completion (saturation concentration) at the operating temperature, e.g. about 60 to 80° C. The reaction water is then produced as a liquid and is carried off in the transport gas, from the cathode gas space by means of a gas excess (see Proceedings of the 26th Intersoc. Energy Conversion Eng. Conf., Boston, Mass., Aug. 4 to 9, 1991, Vol. 3, pp. 630-35; the publication also discloses the design principles for a PEM fuel cell).
If the cell is operated with air, the transport gas can be the inert gas fraction nitrogen (N
2
). A drawback of that process is that the humidification of the reaction gases is relatively involved.
Discharge on the cathode side (partially or completely in vapor phase):
The reaction gases are not humidified or only partially humidified, so that the reaction water can be discharged, at least in part, as a vapor (see U.S. Pat. No. 5,260,143 and European patent EP 0 567 499 B1). That type of operational approach places certain requirements on the electrolyte membrane in terms of mechanical stability and conductivity. In systems employing elevated working pressures it is possible to convert the reaction water into the vapor phase via expansion stages and to remove it from the fuel cell. Such a procedure is very involved however.
Discharge on the anode side:
The operational approaches in which the reaction water is discharged on the anode side employ a pressure differential—of the reactants—between cathode and anode (see U.S. Pat. No. 5,366,818). There, elevated gas pressure on the cathode side, e.g. air at 4·10
5
Pa (4 bar) compared with hydrogen at 2·10
5
Pa (2 bar), is employed to force the reaction water to the anode side where it is removed from the fuel cell by means of excess hydrogen. Setting an elevated pressure (on the cathode side) has severe drawbacks, however, since compression requires energy.
Furthermore, U.S. Pat. No. 5,272,017 and published European patent application EP 0 569 062 describe an “MEA” (Membrane Electrode Assembly) for use in a PEM fuel cell, in which two catalytically active, cathode- and anode-side layers are applied as electrodes to a polymer electrolyte membrane.
These layers consist of finely disperse carbon powder in which catalyst particles are present. On the anode side, the particles should have a pore size of from 9 to 11 nm, while a pore size of 6 to 8 nm obtains on the cathode side.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a method of discharging reaction water from fuel cells and a fuel cell for carrying out the method, which overcome the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which implements the discharge of reaction water in PEM fuel cells—comprising one porous layer each disposed on the cathode and on the anode—in such a way that neither humidification of the reaction gases nor elevated gas pressures are required.
With the above and other objects in view there is provided, in accordance with the invention, a method of discharging reaction water in a PEM fuel cell comprising an electrolyte membrane disposed between an anode and a cathode, the method which comprises placing one porous layer each on the cathode and on the anode and thereby forming a hydrophobic layer on the cathode having a smaller pore size than the porous layer on the anode, and discharging reaction water through the anode.
In other words, the objects of the invention are satisfied by the hydrophobic layer on the cathode side which has a smaller pore size than the anode-side layer and by the reaction water being discharged through the anode.
The invention therefore consists in discharging the reaction water on the anode side, with the advantage that gas humidification can be dispensed with and no elevated pressure is required, the invention providing a “gas conduction layer” on the cathode side. This gas conduction layer is gas-permeable, but impermeable to liquid water. Since during operation of the fuel cell—depending on the load—water is being formed continuously as a liquid, the internal pressure in the cell on the cathode side increases and the water is forced through the electrolyte membrane to the anode—and through the anode—whence it is removed by means of an excess reactant gas stream, i.e. is transported from the fuel cell. Humidification of the reaction gas on the anode side need not be ruled out as a matter of principle however. This is the case, for example, if the reaction gas which carries off the water is recirculated.
The inventive approach offers the following advantages:
The cathode gas (oxidant), namely air or oxygen, need not be humidified, i.e. it can be supplied to the fuel cell in dry form without the electrolyte membrane becoming desiccated and damaged.
The anode gas, namely hydrogen, likewise need not be humidified, since all of the reaction water is transported to the anode and there ensures adequate humidity. Desiccation during operation cannot occur therefore.
The problems which arise in controlling the water budget in PEM fuel cells on the cathode side are overcome with the method according to the invention, in that the discharge of water takes place systematically on the anode side. This means that it is not possible for water droplets in the porous gas conduction layer on the cathode to give rise to inert gas blankets (N
2
) which inhibit the diffusion of the oxygen toward the catalyst layer.
The effective pressure increase is achieved by means of an internal barrier layer. This means that the system works independently of the reaction pressures. No differential pressure is required which has to be achieved externally, e.g. via an air compressor.
With the above and other objects in view there is also provided, in accordance with a further feature of the invention, a PEM fuel cell for performing the above-noted method. The PEM fuel cell comprises an anode, a cathode, an electrolyte membrane between the anode and the cathode, a first porous, electron-conducting layer disposed on the anode and a second porous, electron-conducting layer disposed on the cathode. The second layer on the cathode side is hydrophobic and has, at least on a surface thereof, a smaller pore size than the first layer on the anode side.
In other words, the apparatus for implementing the method according to the invention, i.e. a fuel cell, includes—in addition to an anode, a cathode, and an electrolyte membrane (between anode and cathode)—one porous, electron-conducting layer each disposed on the anode and on the cathode, the layer on the cathode side (gas conduction layer) being hydrophobic and having, at least on the surface, a smaller pore size than the layer on the anode side. Thus the gas conduction layer forms a barrier to liquid water.
The gas conduction layer preferably has a smaller pore size, in the surface adjoining the cathode, than the layer on the anode side. Such an embodiment can be implemented for example by means of a gas conduction layer having an asymmetric pore structure. This has the advantage that the delivery of the reaction gas to the catho

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