Active material for fuel cell anodes incorporating an...

Details Active material for fuel cell anodes incorporating an... Active material for fuel cell anodes incorporating an...

2001-10-31

2003-09-02

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having earth feature

C429S047000, C429S006000, C429S218200, C429S105000

Reexamination Certificate

active

06613471

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to active materials for fuel cell anodes, and more specifically, hydrogen storage alloy active materials for the anode of Ovonic instant startup/regenerative fuel cells. The active material includes a hydrogen storage alloy material with an additive, which, upon utilization of the active material in an anode of an alkaline electrolyte fuel cell, gives the anode added benefits, not attainable by using hydrogen storage alloy material alone. These added benefits include 1) precharge of the hydrogen storage material with hydrogen; 2) higher porosity/increased surface area/reduced electrode polarization at high currents; 3) simplified, faster activation of the hydrogen storage alloy; and optionally 4) enhanced corrosion protection for the hydrogen storage alloy. These benefits are achieved by adding a water reactive chemical hydride to the hydrogen storage alloy used as the active material of the negative electrode of the alkaline fuel cell.
BACKGROUND OF THE INVENTION
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 a cathode, an anode, 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. Regrettably, the P.E.M. fuel cell suffers from relatively low conversion efficiency and has many other disadvantages. For instance, 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 and of limited availability, but 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. While the conventional alkaline fuel cell is less sensitive to temperature than the PEM fuel cell, platinum active materials are used in conventional alkaline fuel cell electrodes. 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 anode 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 anode.
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 anode for hydrogen oxidation and the cathode 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 anode and cathode and brought into surface contact with the electrolytic solution. The particular materials utilized for the cathode and anode are important since they must act as efficient catalysts for the reactions taking place.
In an alkaline fuel cell, the reaction at the anode 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+2
e
−
E
0
=−0.828
v.
At the cathode, the oxygen, water, and electrons react in the presence of the cathode catalyst to reduce the oxygen and form hydroxyl ions (OH
−
):
O
2
+2H
2
O+4
e
−
→4OH
−
E
0
=−0.401
v.
The total reaction, therefore, is:
2H
2
+O
2
→2H
2
O
E
0
=−1.229
v
The flow of electrons is utilized to provide electrical energy for a load externally connected to the anode and cathode.
It should be noted that the anode catalyst of the alkaline fuel cell is required to do more than catalyze the reaction of H
+
ions with OH
−
ions to produce water. Initially the anode must catalyze and accelerate the formation of H
+
ions and e
− from H
2
. This occurs via formation of atomic hydrogen from molecular hydrogen. The overall reaction may be simplified and presented (where M is the catalyst) as:
M+H
2
→2M . . . H→M+2H
+
+2
e
−
.
Where M . . . H denotes atomic hydrogen adsorbed on the catalyst. Thus the anode catalyst must not only efficiently catalyze the electrochemical reaction for formation of water at the electrolyte interface, but must also efficiently dissociate molecular hydrogen into atomic hydrogen. Using conventional anode material, the dissociated hydrogen is transitional and the hydrogen atoms can easily recombine to form hydrogen if they are not used very efficiently in the oxidation reaction. With the hydrogen storage anode materials of the inventive instant startup fuel cells, hydrogen is stored in hydride form as soon as they are created, and

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