Fuel cell cathode utilizing multiple redox couples

Details Fuel cell cathode utilizing multiple redox couples Fuel cell cathode utilizing multiple redox couples

2002-10-04

2004-03-09

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having earth feature

C429S006000, C429S047000, C429S047000, C429S010000

Reexamination Certificate

active

06703156

ABSTRACT:

FIELD OF THE INVENTION
The instant invention relates generally to useful cathode active materials for fuel cells, more specifically to their use as the cathode material for Ovonic instant startup alkaline fuel cells. These inventive oxygen electrodes open up a tremendous number of degrees of freedom in fuel cell design by utilizing reduction/oxidation (redox) couples, such as metal/oxide couples, or simply couples which provide electrochemical oxidizer, preferably oxygen, to the fuel cell electrolyte for electrochemical “combustion”. These redox couples, due to their electrochemical potential, provide the fuel cells employing them with an increased operating voltage that is adjustable by varying the redox couple used. Additionally the redox couples provide the fuel cell with the ability to store oxidizer within the electrode which not only provides for instant startup, but also provides the capability to provide short surge bursts of energy during demand surges and also allows for recapture of regenerative energy.
BACKGROUND OF THE INVENTION
The instant application for the first time provides oxygen electrodes, and fuel cells using such electrodes, which use oxide couples to yield a wide selection of operating voltages. Specifically, the instant inventors have determined materials, which used in combination with hydrogen-side electrodes, particularly with those constructed of Ovonic (Trademark of Energy Conversion Devices, Inc.) hydrogen storage material, both of which, in combination, yield high performance fuel cells having hydrogen storage capacity within the hydrogen electrode and oxygen electrodes which take advantage of low-cost, in comparison with the traditional platinum electrodes, oxide couples which allow selection of specific ranges of operating voltage of the electrochemical cells with a broad operating temperature range and the opportunity to provide instant-start by use of the hydrogen storage capability of the short-range order available in the material of the Ovonic hydrogen electrode.
As the world's human population expands, greater amounts of energy are necessary to provide the economic growth all nations desire. The traditional sources of energy are the fossil fuels which, when consumed, create significant amounts of carbon dioxide as well as other more immediately toxic materials including carbon monoxide, sulfur oxides, and nitrogen oxides. Increasing atmospheric concentrations of carbon dioxide are warming the earth; creating the ugly specter of global climatic changes. Energy-producing devices which do not contribute to such difficulties are needed now.
A fuel cell is an energy-conversion device that directly converts the energy of a supplied gas into an electric energy. Highly efficient fuel cells employing hydrogen, particularly with their simple combustion product of water, would seem an ideal alternative to current typical power generations means. Researchers have been actively studying such devices to utilize the fuel cell's potential high energy-generation efficiency.
The base unit of the fuel cell is a cell having an oxygen electrode, a hydrogen electrode, and an appropriate electrolyte. Fuel cells have many potential applications such as supplying power for transportation vehicles, replacing steam turbines, and power supply applications of all sorts. Despite their seeming simplicity, many problems have prevented the widespread usage of fuel cells.
Presently most of the fuel cell R & D is focused on P.E.M. (Proton Exchange Membrane) fuel cells. Unfortunately, the P.E.M. fuel cell suffers from relatively low conversion efficiency and has many other disadvantages. For instance, the membrane and the electrolyte for the system is acidic. Thus, noble metal catalysts are the only useful active materials for the electrodes of the system. Unfortunately, not only are the noble metals costly, they are also susceptible to poisoning by many gases, specifically carbon monoxide (CO). Also, because of the acidic nature of the P.E.M fuel cell electrolyte, the remainder of the materials of construction of the fuel cell need to be compatible with such an environment, which again adds to the cost thereof. The proton exchange membrane itself is quite expensive, and because of it's low proton conductivity at temperatures below 80° C., inherently limits the power performance and operational temperature range of the P.E.M. fuel cell as the PEM is nearly non-functional at low temperatures. Also, the membrane is sensitive to high temperatures, and begins to soften at 120° C. The membrane's conductivity depends on water and dries out at higher temperatures, thus causing cell failure. Therefore, there are many disadvantages to the P.E.M. fuel cell which make it somewhat undesirable for commercial/consumer use.
The conventional alkaline fuel cell has some advantages over P.E.M. fuels cells in that they have higher operating efficiencies, they use less costly materials of construction, and they have no need for expensive membranes. The alkaline fuel cell also has relatively higher ionic conductivity in the electrolyte, therefore it has a much higher power capability. While the conventional alkaline fuel cell is less sensitive to temperature than the PEM fuel cell, the platinum active materials of conventional alkaline fuel cell electrodes become very inefficient at low temperatures. Unfortunately, conventional alkaline fuel cells still suffer from their own disadvantages.
For example, conventional alkaline fuel cells still use expensive noble metal catalysts in both electrodes, which, as in the P.E.M. fuel cell, are susceptible to gaseous contaminant poisoning. The conventional alkaline fuel cell is also susceptible to the formation of carbonates from CO
2
produced by oxidation of the hydrogen electrode carbon substrates or introduced via impurities in the fuel and air used at the electrodes. This carbonate formation clogs the electrolyte/electrode surface and reduces/eliminates the activity thereof. The invention described herein eliminates this problem from the hydrogen electrode.
Fuel cells, like batteries, operate by utilizing electrochemical reactions. Unlike a battery, in which chemical energy is stored within the cell, fuel cells generally are supplied with reactants from outside the cell. Barring failure of the electrodes, as long as the fuel, preferably hydrogen, and oxidant, typically air or oxygen, are supplied and the reaction products are removed, the cell continues to operate.
Fuel cells offer a number of important advantages over internal combustion engine or generator systems. These include relatively high efficiency, environmentally clean operation especially when utilizing hydrogen as a fuel, high reliability, few moving parts, and quiet operation. Fuel cells potentially are more efficient than other conventional power sources based upon the Carnot cycle.
The major components of a typical fuel cell are the hydrogen electrode for hydrogen oxidation and the oxygen electrode for oxygen reduction, both being positioned in a cell containing an electrolyte (such as an alkaline electrolytic solution). Typically, the reactants, such as hydrogen and oxygen, are respectively fed through a porous hydrogen electrode and oxygen electrode and brought into surface contact with the electrolytic solution. The particular materials utilized for the oxygen electrode and hydrogen electrode are important since they must act as efficient catalysts for the reactions taking place.
In an alkaline fuel cell, the reaction at the hydrogen electrode occurs between the hydrogen fuel and hydroxyl ions (OH-) present in the electrolyte, which react to form water and release electrons:
H
2
+2OH
−
→2H
2
O+2e
−
(E
0
=−0.828 v).
At the oxygen electrode, the oxygen, water, and electrons react in the presence of the oxygen electrode catalyst to reduce the oxygen and form hydroxyl ions (OH
−
):
O
2
+2H
2
O+4e
−
→4OH
−
(E
0
=−0.401 v).
The total reaction, therefore, is:
2H
2
+O
2
&rarr

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