Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
2002-05-21
2004-11-02
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S010000, C429S006000, C429S006000, C429S006000, C429S006000
Reexamination Certificate
active
06811905
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to fuel cells and relates more particularly to direct organic fuel cells.
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Because of their comparatively high inherent efficiencies and comparatively low emissions, fuel cells are presently receiving considerable attention as a possible alternative to the combustion of nonrenewable fossil fuels in a variety of applications.
A typical fuel cell comprises a fuel electrode (i.e, anode) and an oxidant electrode (i.e., cathode), the two electrodes being separated by an ion-conducting electrolyte. The electrodes are connected electrically to a load, such as an electronic circuit, by an external circuit conductor. Oxidation of the fuel at the anode produces electrons that flow through the external circuit to the cathode producing an electric current. The electrons react with an oxidant at the cathode. In theory, any substance capable of chemical oxidation that can be supplied continuously to the anode can serve as the fuel for the fuel cell, and any material that can be reduced at a sufficient rate at the cathode can serve as the oxidant for the fuel cell.
In one well-known type of fuel cell, sometimes referred to as a hydrogen fuel cell, gaseous hydrogen serves as the fuel, and gaseous oxygen, which is typically supplied from the air, serves as the oxidant. The electrodes in a hydrogen fuel cell are typically porous to permit the gas-electrolyte junction to be as great as possible. At the anode, incoming hydrogen gas ionizes to produce hydrogen ions and electrons. Since the electrolyte is a non-electronic conductor, the electrons flow away from the anode via the external circuit, producing an electric current. At the cathode, oxygen gas reacts with the hydrogen ions migrating through the electrolyte and the incoming electrons from the external circuit to produce water as a byproduct. The overall reaction that takes place in the fuel cell is the sum of the anode and cathode reactions, with part of the free energy of reaction being released directly as electrical energy and with another part of the free energy being released as heat at the fuel cell.
It can be seen that as long as oxygen and hydrogen are fed to a hydrogen fuel cell, the flow of electric current will be sustained by electronic flow in the external circuit and ionic flow in the electrolyte. Oxygen, which is naturally abundant in air, can easily be continuously provided to the fuel cell. Hydrogen, however, is not so readily available and specific measures must be taken to ensure its provision to the fuel cell. One such measure for providing hydrogen to the fuel cell involves storing a supply of hydrogen gas and dispensing the hydrogen gas from the stored supply to the fuel cell as needed. Another such measure involves storing a supply of an organic fuel, such as methanol, and then reforming or processing the organic fuel into hydrogen gas, which is then made available to the fuel cell. However, as can readily be appreciated, the reforming or processing of the organic fuel into hydrogen gas requires special equipment (adding weight and size to the system) and itself requires the expenditure of energy.
Accordingly, in another well-known type of fuel cell, sometimes referred to as a direct organic fuel cell, an organic fuel is itself oxidized at the anode. Examples of such organic fuels include methanol, ethanol, propanol, isopropanol, trimethoxymethane, dimethoxymethane, dimethyl ether, trioxane, formaldehyde, and formic acid. Typically, the electrolyte in such a fuel cell is a solid polymer electrolyte or proton exchange membrane (PEM). (Because of the need for water in PEM fuel cells, the operating temperature for such fuel cells is limited to approximately 130° C.) In operation, the organic fuel is delivered to the anode in the form of a fuel/water mixture, and airborne oxygen is delivered to the cathode. (Oxidants other than oxygen, such as hydrogen peroxide, may also be used.) Protons are formed by oxidation of the organic fuel at the anode and pass through the proton exchange membrane to the cathode. Electrons produced at the anode in the oxidation reaction flow in the external circuit to the cathode, driven by the difference in electric potential between the anode and the cathode and, therefore, can do useful work. A summary of the electrochemical reactions occurring in a direct organic fuel cell (with methanol illustratively shown as the organic fuel) are as follows:
Anode: CH
3
OH+H
2
O→CO
2
+6H
+
+6e
−
(1)
Cathode: 1.5O
2
+6H
+
+6e
−
→3H
2
O (2)
Overall: CH
3
OH+1.5O
2
→CO
2
+2H
2
O (3)
At present, there are two different types of systems that incorporate direct organic fuel cells, namely, liquid feed systems and vapor feed systems. Examples of liquid feed systems are disclosed in the following U.S. patents, all of which are incorporated herein by reference: U.S. Pat. No. 5,992,008, inventor Kindler, issued Nov. 30, 1999; U.S. Pat. No. 5,945,231, inventor Narayanan et al., issued Aug. 31, 1999; U.S. Pat. No. 5,599,638, inventors Surampudi et al., issued Feb. 4, 1997; and U.S. Pat. No. 5,523,177, inventors Kosek et al., issued Jun. 4, 1996.
In a typical liquid feed system, a dilute aqueous solution of the organic fuel (i.e., approximately 3-5 wt % or 0.5-1.5 M organic fuel) is delivered to the fuel cell anode whereupon said aqueous solution diffuses to the active catalytic sites of the anode, and the fuel therein is oxidized. The liquid feed system is typically operated at 60° C.-90° C. although operation at higher temperatures is possible by pressurizing the anode and the fuel supply system. (For operation at temperatures greater than 100° C., cathode pressurization is additionally required.)
As can readily be appreciated, it would be desirable to increase fuel cell performance in a liquid feed system by using a more concentrated solution of the organic fuel than the approximately 3-5 wt % solution described above. Unfortunately, however, the proton exchange membrane typically used in a liquid feed system is rather permeable to the organic fuel. As a result, a substantial portion of the organic fuel delivered to the anode has a tendency to permeate through the proton exchange membrane, instead of being oxidized at the anode. Moreover, much of the fuel that transits the proton exchange membrane is chemically reacted at the cathode and, therefore, cannot be collected and recirculated to the anode. This type of fuel loss, which can total as much as 50% of the fuel, is referred to in the art as crossover. In addition, this problem of cross-over is exacerbated if the concentration of organic fuel in the aqueous solution is increased beyond the approximately 3-5 wt % described above since the permeability of the proton exchange membrane increases exponentially as the organic fuel concentration increases.
Consequently, because the concentration of organic fuel in the aqueous solution must remain relatively low to minimize cross-over, large quantities of water must be made available for diluting the organic fuel to appropriate levels. However, as can be appreciated, the required quantities of water can be heavy and space-consuming and can pose a problem to the portability of the system. Moreover, equipment for mixing the water and the organic fuel in the appropriate amounts, for re-circulating water generated at the cathode and for monitoring the concentration of the organic fuel in the aqueous solution is often needed as well.
Another complication resulting from the high concentration of water present in the aqueous solution is that a considerable amount of water delivered to the anode also permeates through the proton exchange membrane to the cathode. This excess water arriving at the cathode limits the accessibility of the cathode to gaseous oxygen, which must be reduced at the cathode to complement the
Cropley Cecelia C.
Stone Simon G.
Giner Electro Chemical Systems, LLC
Kriegsman & Kriegsman
Parsons Thomas H.
Ryan Patrick
LandOfFree
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