Fuel and related compositions – Liquid fuels – Inorganic component
Reexamination Certificate
2002-08-29
2004-08-10
Toomer, Cephia D. (Department: 1714)
Fuel and related compositions
Liquid fuels
Inorganic component
C044S436000, C044S445000, C044S628000, C516S020000, C516S078000, C429S047000, C429S105000
Reexamination Certificate
active
06773470
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to suspension fuel compositions for use in electrochemical fuel cells, a method of producing electricity with the suspension fuel compositions, and a fuel cell using the suspension fuel compositions to generate electricity.
A fuel cell is a device that converts the energy of a chemical reaction into electricity. Among the advantages that fuel cells have over other sources of electrical energy are high efficiency and environmental friendliness. Although fuel cells are increasingly gaining acceptance as electrical power sources, there are technical difficulties that prevent the widespread use of fuel cells in many applications, especially mobile and portable applications.
A fuel cell produces electricity by bringing a fuel into contact with a catalytic anode while bringing an oxidant into contact with a catalytic cathode. When in contact with the anode, the fuel is oxidized at catalytic centers to produce electrons. The electrons travel from the anode to the cathode through an electrical circuit connecting the electrodes. Simultaneously, the oxidant is catalytically reduced at the cathode, consuming the electrons generated at the anode. Mass balance and charge balance are preserved by the corresponding production of ions at either the cathode or the anode and the diffusion of these ions to the other electrode through an electrolyte with which the electrodes are in contact.
A common type of fuel cell uses hydrogen as a fuel and oxygen as an oxidant. Specifically, hydrogen is oxidized at the anode, releasing protons and electrons as shown in equation 1:
H
2
→2H
+
+2e
−
(1)
The protons pass through the electrolyte towards the cathode. The electrons travel from the anode through an electrical load and to the cathode. At the cathode, the oxygen is reduced, combining smith electrons and protons produced from the hydrogen to form water as shown in equation 2:
O
2
+4H
+
+4e
−
→2H
2
O (2)
Although fuel cells using hydrogen as a fuel are simple, clean and efficient, the extreme flammability of hydrogen and the bulky high-pressure tanks necessary for storage and transport of hydrogen mean that hydrogen powered fuel cells are inappropriate for many applications.
In general, storage, handling and transport of liquids are simpler than for gases. Thus, liquid fuels have been proposed for use in fuel cells. Methods have been developed for converting liquid fuels such as methanol into hydrogen, in situ. These methods are not simple, requiring a fuel pre-processing stage and a complex fuel regulation system.
Fuel cells that directly oxidize liquid fuels are the solution to this problem. Because the fuel is directly fed into the fuel cell, direct liquid-feed fuel cells are comparatively simple. Most commonly, methanol has been used as the fuel in these types of cells, as it is cheap, available from diverse sources and has a high specific energy (5020 Ampere hours per liter).
In direct-feed methanol fuel cells, the methanol is catalytically oxidized at the anode, producing electrons, protons and carbon monoxide, as shown in equation 3:
CH
3
OH→CO+4H
+
+4e
−
(3)
Carbon monoxide tightly binds to the catalytic sites on the anode. The number of available sites for further oxidation is reduced, reducing power output. One solution to this problem is to use anode catalysts, such as platinum/ruthenium alloys, which are less susceptible to CO adsorption. Another solution is to introduce the fuel into the cell as an “anolyte”, a mixture of methanol Keith an aqueous liquid electrolyte. The methanol reacts with water at the anode to produce carbon dioxide and hydrogen ions, as shown in equation 4:
CH
3
OH+H
2
O→6H
+
+CO
2
+6e
−
(4)
In fuel cells that use anolytes, the composition of the anolyte is an important design consideration. The anolte must have both a high electrical conductivity and high ionic mobility at the optimal fuel concentration. Acidic solutions are most commonly used. Unfortunately, acidic anolytes are most efficient at relatively high temperatures, temperatures at which the acidity can passivate or destroy the anode. Anolytes with a pH close to 7 are anode-friendly, but have an electrical conductivity that is too low for efficient electricity generation. Consequently, most prior art direct methanol fuel cells use solid polymer electrolyte (SPE) membranes.
In a cell using a SPE membrane, the cathode is exposed to oxygen in the air and is separated from the anode by a proton exchange membrane that acts both as an electrolyte and as a physical barrier preventing leakage from the anode compartment wherein the liquid anolyte is contained. One membrane commonly used as a fuel cell solid electrolyte is a perfluorocarbon material sold by E. I. DuPont de Nemours (Wilmington, Del.) under the trademark “Nafion”. Fuel cells using SPE membranes have a higher power density and longer operating lifetimes than other anolyte-based fuel cells.
A practical disadvantage of SPE membrane fuel cells arises from the tendency of high concentrations of methanol to dissolve the membrane and to diffuse through it. As a result, a significant proportion of methanol supplied to the cell is not utilized for generation of electricity, but either is lost through evaporation or is oxidized directly at the cathode, generating heat instead of electricity.
The problem of membrane penetration by the fuel is overcome by using anolytes with a low (at most 3%) methanol content. The low methanol content limits the efficiency of the fuel cell when measured in terms of electrical output as a function of volume of fuel consumed and raises issues of fuel transportation, dead weight and waste disposal. Further limiting the use of low methanol content anolyte-based liquid feed fuel cells, especially for mobile and portable applications, is the expense and complexity of necessary peripheral equipment for fuel circulation, replenishment, heating and degassing.
Finally, despite having a high specific energy, methanol is rather unreactive at room temperature, which limits the specific power output of a methanol fuel cell to about 15 milliwatts per square centimeter.
Other organic compounds, notably higher alcohols, hydrocarbons and acetates, have been proposed as fuels for fuel cells. See, for example, O. Savadogo and X. Yang, “The electrooxidation of some acetals for direct hydrocarbons fuel cell applications”,
IIIrd International Symposium on Electrocatalysis
, Slovenia, 1999, p. 57, and C. Lamy et al., “Direct anodic oxidation of methanol, ethanol and higher alcohols and hydrocarbons in PEM fuel cells”,
IIIrd International Symposium on Electrocatalysis
, Slovenia, 1999, p. 95. Most of these candidates have shown very little promise, because of low electrochemical activity, high cost, and, in some cases, toxicity.
Inorganic water-soluble reducing agents, such as metal hydrides, hydrazine and hydrazine derivatives also have been proposed as fuels for fuel cells. See, for example, S. Lel, “The characterization of an alkaline fuel cell that uses hydrogen storage alloys”,
Journal of the Electrochemical Society
vol. 149 no. 5 pp. A603-A606 (2002), J. O'M. Bockris and S. Srinivasan,
Fuel Cells: Their Electrochemistry
, McGraw-Hill. New York, 1969, pp. 589-593, and N. V. Korvin,
Hydrazine
, Khimiya, Moscow, 1980 (in Russian), pp. 205-224. Such compounds have high specific energies and are highly reactive.
One such compound is NaBH
4
. In water NaBH
4
dissociates to Na
+
and BH
4
−
. In a neutral solution, BH
4
−
is oxidized at the anode according to equation 5:
BH
4
−
+2H
2
O→BO
2
−
+8H
+
+8e
−
(5)
The greatest disadvantage of hydrogen-containing inorganic compounds as fuel is their decomposition in acid and neutral solutions. For example. BH
4
−
decomposes according to equation 6:
BH
4
−
+2H
2
O→BO
2
−
+4H
2&
Finkelshtain Gennadi
Fishelson Nikolai
Katzman Yuri
Lurie Zina
Friedman Mark M.
More Energy Ltd.
Toomer Cephia D.
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