Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2002-01-04
2004-02-03
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S010000
Reexamination Certificate
active
06686084
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention pertains generally to polymer electrolyte membrane fuel cells and more particularly to an apparatus and method for disposal of water.
In the normal operation of a polymer electrolyte membrane “PEM” fuel cell, liquid water forms at the cathode side of the fuel cell. There are two sources for this liquid water. One source is the oxidizing gas being fed to the cathode side of the fuel cell. This gas is generally moisturized before being supplied to the cathode and some of this moisture condenses. The other source is an electrochemical reaction occurring on the cathode side which produces water. Discharging oxidizing gas has a limited capacity to carry away to outside the cell the liquid water formed at the cathode side.
When the quantity of liquid water formed on the cathode side exceeds the liquid water carried away by the oxidizing gas, there is a condition called a “flooded state” or “flooding.” In a flooded state, the liquid water remains on the surface of the cathode electrode and obstructs the dispersion of the oxidizing gas onto the surface of the cathode. This in turn results in a drop in the cell's voltage and amperage output. Ultimately, flooding will stop the cell's operation.
Similarly, the water accumulation can also take place in the anode plates of fuel cells. The anode flow is normally humidified before it is introduced to the anode plates. During operation when anode gas is consumed by chemical reaction of the fuel cell, the water content of the anode flow condenses to liquid water. To avoid flow blockage the condensed water has to be removed from the anode plates.
One conventional water removal technique is wicking, or directing the accumulated water away from the cathode using capillaries incorporated in the cathode. Another related water removal technique employs screens or meshes within the cathode to conduct water away from the catalyst layer. Still another go conventional water removal technique is to incorporate hydrophobic substances, such as polytetrafluoroethylene (trade name Teflon RTM,) into the cathode sheet material to urge accumulated water away from the cathode. This type of apparatus has the disadvantages of being limited in quantity of liquid water that can be removed from the flowfield and being limited in mass transfer of cathode gas to the polyelectrolyte membrane of the fuel cell.
It is known in the art to remove water at the cathode side by utilizing an electrode layer comprised of a porous base area with water repellency and plurality of penetration areas higher in water permeability scattered over or formed through the base area. This facilitates the oozing of water generated on the catalytic layer of the PEM fuel cell into the gas channels through the areas of higher permeability. These types of apparatus have the disadvantage of focusing on removing water at the catalytic area adjacent to the polyelectrolyte membrane; being limited in the quantity of water that can be removed and being limited in mass transfer of reactant gas to the polyelectrolyte membrane.
It is known in the art that an interdigitated flowfield will provide means for the reactant gases to flow through the gas diffusion layer (GDL). In this flowfield, the flow channels have dead-ends. The pressure difference between the inlet flow and exit flow in a plate provides the pressure head to force reactant gases to flow through the GDL. This flowfield configuration allows flow of reactants through a larger area of the GDL and provides a convection transport of reactants to support the reaction, improve the efficiency, and eliminate or reduce the mass transport limited operation. Also, flow of reactants through the GDL helps sweep the water produced in the GDL, and consequently, enhance the fuel cell operation.
It is also known in the art to remove fluid at electrodes by positioning a porous support layer near and in fluid communication with each electrode to facilitate fluid transport to and away from each electrode. The porous support layer includes hydrophobic pores and hydrophilic pores integrated throughout the layer. The fuel and oxidizing gasses are supplied through the hydrophobic pores and water is removed through the hydrophilic pores. This apparatus has the disadvantages of focusing on removing water at the electrode area; being limited in quantity of water that can be removed and being limited in mass transfer of cathode gas to the polyelectrolyte membrane.
A conventional fuel cell is comprised of a stack of PEM fuel cells. Such a conventional fuel cell may require a cooling plate for every cell. Reference is made to FIG. 2 in U.S. Pat. No. 5,840,414 and FIG. 3 in U.S. Pat. No. 5,853,909. To support the capillary action in the porous layers these patents require a cooling plate for every cell. This plurality of cooling plates increases the weight and volume of the fuel cell.
Accordingly, there exists a need for an apparatus and method to efficiently remove water produced at the cathode plates of a PEM fuel cell which enhances the flow of cathode gas to the catalytic area and avoids the loss of cathode gas. Further, there is a need for a fuel cell of reduced weight and volume. The present invention satisfies these needs, as well as others, and generally overcomes the presently known deficiencies in the art.
SUMMARY OF THE INVENTION
The present invention is directed to apparatus and method for disposal of water in an electrochemical fuel cell. The present invention of water removal technique can equally be used for anode or cathode plates. Taught here is an interdigitated flowfield with a gas block mechanism for water removal in fuel cells.
One aspect of the present invention is a cathode plate assembly for use with a cathode gas in polyelectrolyte membrane fuel cell. The assembly is comprised of the following major components: There is a cathode plate having a first major surface and a second oppositely opposed major surface (see FIG.
1
A). Within the first major surface is a flow field comprised of feed side interdigitated channels and exhaust side interdigitated channels that are in an interdigitated configuration. During the operation of the fuel cell there is flow of cathode gas from feed side interdigitated channels to exhaust side interdigitated channels.
Positioned adjacent to the flow field, are one or more porous gas block mediums. These porous gas blocks have pores sized such that water is sipped off to the outside of the flow field by capillary flow and cathode gas is blocked from flowing through the medium. There is a gas diffusion layer in close contact and over the first surface of cathode plate with its flow field. On the second major surface of the cathode plate can be an anode flowfield or a coolant flowfield. In both cases, the gas block is either in fluid communication with the coolant flowfield or a coolant manifold. Alternatively, the second major surface of the cathode plate may include an anode flowfield for delivering hydrogen, or a hydrogen mixture, to the fuel cell. In this case, the gas blocks are generally in communication to a liquid water manifold generally positioned at the perimeter of the plate for the purpose of delivering water to those plates that do have liquid water channels or to the second surface of the cathode plates. Note that the water channels can further serve the purpose of cooling channels, but are not required to have this function.
Similarly, an anode plate of design analogous to the cathode plate may encounter water accumulation. A gas block mechanism can also be used in an anode plate to help sweep water-to-water channels similar to those described above.
Another aspect of the present invention is a cathode plate assembly for use in a fuel cell with a pressurized cathode gas, and a pressurized coolant or an anode flowfield, comprised of the following components: A cathode plate having a first major surface and a second oppositely opposed major surface. A flow field for pressurized cathode gas within the first major surface having a feed side having a feed side
Issacci Farrokh
Rehg Timothy J.
Alejandro Raymond
Hybrid Power Generation Systems LLC
Ryan Patrick
Sutherland & Asbill & Brennan LLP
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