Liquid fuel compositions for electrochemical fuel cells

Details Liquid fuel compositions for electrochemical fuel cells Liquid fuel compositions for electrochemical fuel cells

2001-01-03

2003-04-29

Toomer, Cephia D. (Department: 1714)

Fuel and related compositions

Liquid fuels

Organic oxygen compound containing

C044S445000, C044S457000, C429S010000, C429S010000, C429S006000, C429S047000

Reexamination Certificate

active

06554877

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to liquid fuel compositions for use in electrochemical fuel cells, a method of producing electricity with the fuel compositions, and a fuel cell using the fuel compositions to generate electricity.
A fuel cell is a device that converts the energy of a chemical reaction into electricity. Amongst 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.
A fuel cell produces electricity by bringing a fuel and an oxidant in contact with a catalytic anode and a catalytic cathode, respectively. When in contact with the anode, the fuel is catalytically oxidized on the catalyst, producing electrons and protons. The electrons travel from the anode to the cathode through an electrical circuit connected between the electrodes. The protons pass through an electrolyte with which both the anode and the cathode are in contact. Simultaneously, the oxidant is catalytically reduced at the cathode, consuming the electrons and the protons generated at the anode.
A common type of fuel cells 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 an 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 with electrons and protons produced from the hydrogen to form water as shown in equation 2:
½O
2
+2H
+
+2e
−
→H
2
O   (2)
Although fuel cells using hydrogen as a fuel are simple, clean and efficient the extreme flammability 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, the storage, handling and transport of liquids is simpler than of gases. Thus liquid fuels have been proposed for use in fuel cells. Method 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 for this problem. Since the fuel is directly fed into the fuel cell, direct liquid-feed fuel cells are generally 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 (5025 Wh/kg).
In direct-feed methanol fuel cells, the methanol is catalytically oxidized at the anode producing electrons, protons and carbon monoxide, 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 is to use anode catalysts which are less susceptible to CO adsorption, such as platinum/ruthenium alloys.
Another solution has been to introduce the fuel into the cell as an “anolyte”, a mixture of methanol with an aqueous liquid electrolyte. The methanol reacts with water at the anode to produce carbon dioxide and hydrogen ions, 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 anolyte 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 to 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 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 of Wilmington, Del. under the trademark “Nafion.” Fuel cells using SPE membranes have a higher power density and longer operating lifetimes compared to other anolyte based cells. One disadvantage SPE membrane fuel cells have arises from the tendency of methanol to diffuse through the membrane. As a result, much methanol is not utilized for generation of electricity but is lost through evaporation. In addition, if the methanol comes in contact with the cathode, a “short-circuit” occurs as the methanol is oxidized directly on the cathode, generating heat instead of electricity. Further, depending upon the nature of the cathode catalyst and of the oxidant, catalyst poisoning or cathode sintering often occurs.
The diffusion problem is overcome by using anolytes with a low (up to 5%) methanol content. The low methanol content limits the efficiency of the fuel cell as the methanol diffusion rate limits electrical output. Efficiency is also limited 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.
Lastly, despite a high specific energy, methanol is rather unreactive. As a result, the performance of direct-feed liquid methanol fuel cells is limited to about 5 mWcm
−2
.
An alternative fuel to consider is one composed of hydrogen-containing inorganic compounds with a high reduction potential such as methyl hydrides and hydrazine and its derivatives. Such compounds have a high specific energy and are highly reactive.
One such compound is NaBH
4
. In water, NaBH
4
dissociates to give BH
4
−
. In a neutral solution BH
4
−
is oxidized at the anode according to equation 5:
BH
4
−
+2 H
2
O→BO
2
−
+8 H
+
+8 e
−
  (5)
The greatest drawbacks of hydrogen-containing inorganic compounds as fuel is the spontaneous decomposition of these compounds in acidic and neutral solutions, equation 6:
BH
4
−
+2 H
2
O→BO
2
−
+4 H
2
  (6)
In a basic solution BH
4
−
is oxidized at the anode according to equation 7:
BH
4
−
+8 OH
−
→BO
2
−
+6 H
2
O+8e
−
  (7)
Although stable in basic solutions, BH
4
−
decomposes on contact with a catalyst, such as found on the anode of a fuel cell, even when the circuit is broken.
There is a need for a liquid fuel composition for fuel cells that can produce high power and is stable in contact with the catalytic anode when the electrochemical circuit is broken.
SUMMARY OF THE INVENTION
The above and other objectives are achieved by the innovative fuel composition provided by the invention. The fuel composition is made up of a combination of a primary fuel and an auxiliary fuel. The primary fuel is a mixture of one or more compounds, of which at least one is a surface active compound, most preferably an alcohol such as methanol. The auxiliary fuel is a mixture of one or more hydrogen-containing inorganic compounds with a high reduction potential such as metal hydrides, hydrazine and hydrazine derivatives.
The invention further provides the fuel composition as an “anolyte” where the electrolyte component of the fuel composition has a pH above 7, most preferably an aqueous solution of an alkali metal hydroxide such as KOH.
The invention further provides a fuel cell for the generation

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