Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Inorganic carbon containing
Reexamination Certificate
2001-03-19
2003-12-30
Nguyen, Cam N. (Department: 1745)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Inorganic carbon containing
C502S326000, C502S339000
Reexamination Certificate
active
06670301
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to electrocatalyst compositions and the procedure for their preparations. More specifically, the invention relates to electrocatalysts with low platinum loading that can be used in fuel cells and which have a high tolerance to carbon monoxide.
A “fuel cell” is a device which converts chemical energy directly into electrical energy wherein the over-all cell reaction is the oxidation of a fuel by oxygen or suitable oxidizing gas, such as air. The chemicals are usually very simple, often just hydrogen and oxygen. In which case, the hydrogen is the “fuel” that the fuel cell uses to make electricity. The essential components of such a fuel cell are two electrodes in contact with the oxygen-containing gas and the fuel, respectively, and an electrolyte, which is in contact with both electrodes and which can be acidic, alkaline, solid or liquid. In accordance with generally recognized convention, the oxygen electrode may be considered as the positive electrode and the fuel electrode as the negative electrode with reference to the external circuit. The electrolyte functions to permit transport of ions without direct electrical contact between the fuel and oxidizing gas whereby the oxidation of the fuel can take place only as a result of a directed flow of ions across the electrolyte and a corresponding flow of electrons in an external circuit. The basic process of energy conversion is highly efficient and essentially pollution-free. Also, since a single cell can be assembled in stacks of varying sizes, systems can be designed to produce a wide range of output levels.
The fuel cell was invented in 1839. However, fuel cells capable of producing significant power were not developed until 1959 when an alkaline fuel cell capable of producing 5,000 watts (5 kW) was introduced. This fuel cell served as a starting point for the fuel cells developed by NASA and used to provide electrical power on both the Gemini and Apollo spacecraft. As a result of NASA's work, fuel cells were shown to be capable of efficient and reliable electrical power generation. Unfortunately, the fuel cells of that era were also inherently expensive due to the large amount of platinum needed to manufacture the fuel cells.
In a typical fuel cell, gaseous fuels are fed continuously to the anode (negative electrode) compartment and an oxidant (i.e., oxygen from air) is fed continuously to the cathode (positive electrode) compartment. Current is generated by reaction on the electrode surfaces, which are in contact with an electrolyte. The fuel is oxidized at the anode and gives up electrons to an external load. The oxidant accepts electrons and is reduced at the cathode. Ionic current through the electrolyte completes the circuit.
A fuel cell, although having components and characteristics similar to those of a typical battery, differs in several respects. The battery is an energy storage device. The maximum energy available is determined by the amount of chemical reactant stored within the battery itself. The battery will cease to produce electrical energy when the chemical reactants are consumed (i.e., discharged). In a secondary battery, recharging regenerates the reactants, which involves putting energy into the battery from an external source. The fuel cell, on the other hand, is an energy conversion device that theoretically has the capability of producing electrical energy for as long as the fuel and oxidant are supplied to the electrodes. In reality, degradation, primarily corrosion, or malfunction of components limits the practical operating life of fuel cells. Also, the fuel cell is distinguished from a battery in that its electrodes are catalytically active.
In a proton-exchange membrane (PEM) fuel cell, hydrogen is supplied to the anode where it breaks apart into protons and electrons. The membrane conducts protons but not electrons. The protons flow through the membrane while the electrons travel through the external circuit and provide electrical power. The electrons reduce oxygen which reacts immediately with protons coming from anode through the membrane to produce water.
Group VIII noble metal electrocatalysts, such as platinum-ruthenium (Pt—Ru) electrocatalysts, are commercially offered for the use in proton-exchange membrane (PEM) fuel cells. These electrocatalysts are Pt—Ru alloys dispersed on high surface area carbons with noble metal concentrations between 5 to 40 wt % with 1:1 Pt:Ru atomic ratio. The tolerance of these Pt—Ru alloy electrocatalysts to carbon monoxide, even when they are provided with high noble metal loading, is unsatisfactory and the high noble metal loading makes them expensive.
In many fuel cell systems, a hydrogen fuel is produced by converting a hydrocarbon-based fuel such as methane, or an oxygenated hydrocarbon fuel such as methanol, to hydrogen in a process called reforming. This reformate fuel contains, in addition to hydrogen, high levels of carbon dioxide, usually around 25%. The reformate fuel also contains small amounts of impurities, such as carbon monoxide, typically at levels of around 1%.
Other fuel cells, called “direct” or “non-reformed” fuel cells oxidize fuel high in hydrogen content directly, without the hydrogen first being separated by a reforming process. It has been known since the 1950s that lower primary alcohols (the C
1
-C
5
alcohols), particularly methanol, can be oxidized directly (i.e., without reformation to H
2
+CO or H
2
+CO
2
) at the anode of a fuel cell. A substantial effort has gone into the development of the so-called “direct methanol oxidation” fuel cell because of the obvious advantage of direct oxidation of methanol, which bypasses the reformation step. However, the major drawback of methanol fuel is its sluggish oxidation rate, which results in poor cell current and voltage characteristics that make it unattractive for practical applications. Accordingly, the present effort in fuel cell development is focused on the use of reformate hydrogen as a fuel.
Considerable research and development has gone into adapting fuel cell technology for electrically powered vehicles. However, hydrogen is expensive and poses handling and storage problems when used in fuel cells that power electric vehicles. The solution to these problems has been the development of systems for the on-board generation of hydrogen by reforming methanol or gasoline. Methanol is stored in a fuel tank on board the vehicle and converted in a steam reformer to a mixture of gases containing predominantly hydrogen. These systems provide a comparatively inexpensive source of hydrogen fuel and significantly reduce the handling and storage of hydrogen since methanol is converted to hydrogen as needed to power the vehicle. However, these systems have encountered some problems because, in addition to containing hydrogen, the gas also contains small amounts of carbon monoxide which is a catalytic poison for platinum.
In a typical hydrogen/oxygen fuel cell, hydrogen is oxidized to produce electricity and water. The following reactions occur on the anode and cathode:
Anode: 2H
2
→4H
+
+4e
−
(1)
Cathode: O
2
+4H
+
+4e
−
→2H
2
O (2)
Net reaction: 2H
2
+O
2
→2H
2
O (3)
In order for the oxidation and reduction reactions in a fuel cell to occur at useful rates and at desired potentials, electrocatalysts are required. In the absence of an electrocatalyst, a typical electrode reaction would occur, if at all, only at very high overpotentials. Catalysts that promote the rates of electrochemical reactions are referred to as electrocatalysts. Electrocatalysts promote the reactions that occur in fuel cells and allow the cells to operate at lower overpotentials. This is important because the energy efficiency of any cell is determined, in part, by the overpotentials necessary at the cell's anode and cathode. Supported electrocatalysts based on platinum and platinum alloys are preferred for the anode and cathode fuel cell reactions.
A maj
Adzic Radoslav
Brankovic Stanko
Wang Jia
Bogosian Margaret C.
Brookhaven Science Associates LLC
Nguyen Cam N.
LandOfFree
Carbon monoxide tolerant electrocatalyst with low platinum... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Carbon monoxide tolerant electrocatalyst with low platinum..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Carbon monoxide tolerant electrocatalyst with low platinum... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3101351