Chemistry: electrical current producing apparatus – product – and – Having earth feature
Reexamination Certificate
1999-11-18
2002-12-03
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having earth feature
C429S010000
Reexamination Certificate
active
06489052
ABSTRACT:
BACKGROUND
This invention relates to integrated power generation and air purification and conditioning systems for stationaryand mobile applications. Specifically, an air purification subsystem may be installed in a fuel cell system across a cathode gas diffusion layer or along a cathode flow path to enhance air purification by utilization of fuel cell operating conditions.
A fuel cell is a device which converts chemical energy of a fuel into electrical energy, typically by oxidizing the fuel. In general a fuel cell includes an anode and a cathode separated by an electrolyte. When fuel is supplied to the anode and oxidant is supplied to the cathode, the cell electrochemically generates a useable electric current which is passed through an external load. The fuel typically supplied is hydrogen and the oxidant typically supplied is oxygen. In such cells, oxygen and hydrogen are combined to form water and to release electrons. The chemical reaction for a fuel cell using hydrogen as the fuel and oxygen as the oxidant is shown in equation (1).
H
2
+½O
2
→H
2
O (1)
This process occurs through two half-reactions which occur at the electrodes: Anode Reaction
H
2
→2H
+
+2e
−
(2)
Cathode Reaction
½O
2
+2H
+
+2e
−
→H
2
O (3)
In the anode half-reaction, hydrogen is consumed at the fuel cell anode releasing protons and electrons as shown in equation (2). The protons are injected into the fuel cell electrolyte and migrate to the cathode. The electrons travel from the fuel cell anode to cathode through an external electrical load. In the cathode half-reaction, oxygen, electrons from the load, and protons from the electrolyte combine to form water as shown in equation (3).
The directional flow of protons, such as from anode to cathode, serves as a basis for labeling an “anode” side and a “cathode” side of the fuel cell.
Fuel cells are classified into several types according to the electrolyte used to accommodate ion transfer during operation. Examples of electrolytes include aqueous potassium hydroxide, concentrated phosphoric acid, fused alkali carbonate, stabilized zirconium oxide, and solid polymers, e.g., a solid polymer ion exchange membrane.
An example of a solid polymer ion exchange membrane is a Proton Exchange Membrane (hereinafter “PEM”) which is used in fuel cells to convert the chemical energy of hydrogen and oxygen directly into electrical energy. A PEM is a solid polymer electrolyte which when used in a PEM-type fuel cell permits the passage of protons (i.e.,H
+
ions) from the anode side of a fuel cell to the cathode side of the fuel cell while preventing passage of reactant fluids such as hydrogen and oxygen gases.
A PEM-type cell includes an electrode assembly disposed between an anode fluid flow plate and a cathode fluid flow plate. An electrode assembly usually includes five components: two gas diffusion layers; two catalysts; and an electrolyte. The electrolyte is located in the middle of the five-component electrode assembly. On one side of the electrolyte (the anode side) a gas diffusion layer (the anode gas diffusion layer) is disposed adjacent the anode layer, and a catalyst (the anode catalyst) is disposed between the anode gas diffusion layer and the electrolyte. On the other side of the electrolyte (the cathode side), a gas diffusion layer (the cathode gas diffusion layer) is disposed adjacent the cathode layer, and a catalyst (the cathode catalyst) is disposed between the cathode gas diffusion layer and the electrolyte.
Several PEM-type fuel cells may be arranged as a multi-cell assembly or “stack.” In a multi-cell stack, multiple single PEM-type cells are connected together in series. The number and arrangement of single cells within a multi-cell assembly are adjusted to increase the overall power output of the fuel cell. Typically, the cells are connected in series with one side of a fluid flow plate acting as the anode for one cell and the other side of the fluid flow plate acting as the cathode for an adjacent cell.
The anode and cathode fluid flow plates are typically made of an electrically conductive material, typically metal or compressed carbon, in various sizes and shapes. Fluid flow plates may act as current collectors, provide electrode support, provide paths for access of the fuels and oxidants to the electrolyte, and provide a path for removal of waste products formed during operation of the cell.
The cell also includes a catalyst, such as platinum on each side of the electrolyte for promoting the chemical reaction(s) that take place in the electrolyte in the fuel cells. The fluid flow plates typically include a fluid flow field of open-faced channels for distributing fluids over the surface of the electrolyte within the cell.
Fluid flow plates may be manufactured using any one of a variety of different processes. For example, one technique for plate construction, referred to as “monolithic” style, includes compressing carbon powder into a coherent mass which is subjected to high temperature processes to bind the carbon particles together, and to convert a portion of the mass into graphite for improved electrical conductivity. The mass is then cut into slices, which are formed into the fluid flow plates. Typically, each fluid flow plate is subjected to a sealing process (e.g., resin impregnation) in order to decrease gas permeation therethrough and reduce the risk of uncontrolled reactions.
Fluid flow plates may also have holes therethrough which when aligned in a stack form fluid manifolds through which fluids are supplied to and evacuated from the stack. Some of the fluid manifolds distribute fuel (such as hydrogen) and oxidant (such as air or oxygen) to, and remove unused fuel and oxidant as well as product water from, the fluid flow fields of the fluid flow plates. Additionally, other fluid manifolds circulate coolant to control the temperature of the stack. For example, a PEM fuel cell stack may be maintained in a temperature range of from 60° C. to 200° C. The temperature of the anode and cathode exhaust streams may also be within this range as they leave the fuel cell. Cooling mechanisms such as cooling plates are commonly installed within the stack between adjacent single cells to remove heat generated during fuel cell operation.
PEM fuel cell systems using hydrogen as a fuel may include a fuel processing system such as a reformer to produce hydrogen by reacting a hydrocarbon such as natural gas or methanol. Many such fuel processing systems are well known in the art. Where a reformer is used, the reformed fuel gas is referred to as reformate, and may typically contain predominantly hydrogen, carbon dioxide and water. In some cases, reformate may also a relatively small amount of carbon monoxide. Since carbon monoxide, even in trace amounts, acts as a poison to most fuel cell catalysts, for example platinum-based catalysts, methods have been developed to minimize or eliminate carbon monoxide in reformate streams. Such methods include, for example, using a preferential oxidizer system to convert carbon monoxide into non-poisoning carbon dioxide, or optimizing fuel processor operating conditions such as temperature and air flow to minimize the production of carbon monoxide in the reformer.
Typically only a portion of the reactants (e.g., reformate containing hydrogen on the anode side, and air containing oxygen on the cathode side) flowing through a fuel cell will react. For example, the amount of reactants in the anode and cathode streams that are reacted may depend on factors including temperature, pressure, residence time, and catalyst surface area. For this reason, it may be desirable to feed excess reactants to a fuel cell in order to increase the reaction level to a point corresponding to a desired power output of the fuel cell. For example, it may be that 100 standard liters per minute (slm) of hydrogen must be reacted in a fuel cell to achieve a desired power output, but it is determined that 140 slm of hydrogen must be fed to t
Chaney Carol
Dove Tracy
Plug Power Inc.
Trop Pruner & Hu P.C.
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