CN1311583C - Suspensions for use as fuel for electrochemical fuel cells - Google Patents

Abstract

A fuel composition for fuel cells includes a polar solvent such as water, a first portion of a first fuel dissolved in the solvent at a saturated concentration, and a second portion of the first fuel suspended in the solvent to serve as a reservoir of fuel as the dissolved portion is consumed. Preferably, the first fuel is a hydride such as NaBH4. Optionally, the fuel composition also includes a second fuel such as an alcohol that also controls the solubility of the first fuel in the solvent, inhibits decomposition of the first fuel and stabilizes the suspension. Preferably, the fuel composition also includes an additive such as an alkali for stabilizing the first fuel.

Description

Suspension for use as fuel for electrochemical fuel cells

Background of the invention

The present invention relates to a suspension fuel composition for use in an electrochemical fuel cell, a method of generating electrical energy using the suspension fuel composition, and a fuel cell for generating electrical energy using the suspension fuel composition.

A fuel cell is a device that converts the energy of a chemical reaction into electrical energy. Fuel cells offer the advantages of high efficiency and environmental friendliness compared to other sources of electrical energy. Although fuel cells are increasingly accepted as a source of electrical energy, there are technical difficulties that prevent their widespread use in many applications, particularly in automotive and portable appliances.

Fuel cells produce electrical energy by contacting a fuel with a catalytically active anode, while contacting an oxidant with a catalytically active cathode. When in contact with the anode, the fuel is oxidized at the catalytic center to produce electrons: the electrons travel from the positive electrode to the negative electrode via a circuit connecting the electrodes. At the same time, the oxidant is catalytically reduced at the cathode, consuming the electrons generated at the anode. Mass balance and charge balance can be maintained by the corresponding generation of ions at either the negative or positive electrode and diffusion of these ions through the electrolyte with which the electrode is in contact to the other electrode.

A common type of fuel cell uses hydrogen as the fuel and oxygen as the oxidant. Specifically, hydrogen is oxidized at the positive electrode, releasing protons and electrons, as shown in formula 1 below:

(1)

the protons move through the electrolyte to the negative electrode. Electrons travel from the positive electrode, through the electrical load and to the negative electrode. At the cathode, the oxygen is reduced, combining with electrons and protons generated from the hydrogen to form water, as shown in formula 2 below:

(2)

although fuel cells using hydrogen as a fuel are simple, clean and efficient, the extreme flammability of hydrogen and the large gas cylinders required for storage and admission of hydrogen mean that hydrogen-powered fuel cells are unsuitable for many applications.

In general, storage,handling and transportation of liquids are simpler than for gases. Therefore, liquid fuels have been proposed for use in fuel cells. A number of processes have been developed to convert liquid fuels such as methanol to hydrogen in situ. These methods are not simple and require a fuel pre-treatment stage and a complex fuel conditioning system.

Fuel cells that directly oxidize liquid fuels are a solution to this problem. Because the fuel is fed directly into the fuel cell, a direct liquid-feed fuel cell is relatively simple. Most commonly, methanol has been used as a fuel in these types of cells because it is inexpensive, available from different sources and has a high specific energy (5020 ampere-hours per liter).

In a direct-feed methanol fuel cell, methanol is catalytically oxidized at the positive electrode to produce electrons, protons, and carbon monoxide, as shown in formula 3 below:

(3)

carbon monoxide is tightly bound to the catalytic sites on the positive electrode. The number of vacancies available for further oxidation will be reduced, reducing the power output. One solution to this problem is to use positive catalysts, such as platinum/ruthenium alloys, but they are less sensitive to CO adsorption. Another solution is to introduce the fuel into the cell as a "anolyte", which is a mixture of methanol and an aqueous liquid electrolyte. The methanol reacts with water at the positive electrode to generate carbon dioxide and hydrogen ions as shown in the following formula 4:

(4)

in fuel cells using a positive electrolyte, the composition of the positive electrolyte is an important design consideration. The anolyte must have both high conductivity and high ion mobility at theoptimum fuel concentration. Acidic solutions are most commonly used. Unfortunately, acidic positive electrolytes are most effective at higher temperatures, but the temperature causes the acidity to passivate or destroy the positive electrode. A positive electrolyte with a pH close to 7 is positive friendly, but has too low conductivity to be used for efficient electricity generation. Thus, most of the prior art relates to methanol fuel cells using Solid Polymer Electrolyte (SPE) membranes.

In cells using SPE membranes, the negative electrode is exposed to oxygen in the air and separated from the positive electrode by a proton exchange membrane that acts both as an electrolyte and a physical barrier that prevents leakage from the positive electrode compartment containing the liquid positive electrolyte. One type of membrane commonly used as a solid electrolyte for fuel cells is the perfluorocarbon material sold under the trademark "Nafion" by e.i. dupont DE Nemours (Wilmington, DE). Fuel cells using SPE membranes have higher power densities and longer operating lifetimes than other anolyte type fuel cells.

A practical disadvantage of SPE membrane fuel cells arises from the tendency of high concentrations of methanol to dissolve the membrane and diffuse through it. As a result, a large proportion of the methanol supplied to the cell is not used to generate electrical energy, but is lost by evaporation or oxidized directly at the cathode, generating heat rather than electrical energy.

The problem of membrane breakthrough by the fuel can be overcome by using a catholyte with a low (up to 3%) methanol content. Low methanol content limits the efficiency of the fuel cell (when measured in terms of electrical energy output as a function of volume of fuel consumed) and increases problems with fuel transport, deadweight and waste disposal. Further limiting the application of low methanol content anolyte type liquid feed fuel cells, particularly for automotive and portable applications, is the expense and complexity of the required peripheral equipment for fuel recycling, replenishment, heating and degassing.

Finally, despite having a high specific energy, methanol is relatively unreactive at room temperature, which limits the specific power output of methanol fuel cells to about 15 milliwatts per square centimeter.

Other organic compounds, mainly higher alcohols, hydrocarbons and acetates, have been proposed as fuels for fuel cells. See, for example, O.Savadouo and X.Yang, "the oxidation of the solvent acids for Direct hydrocarbons across cells applications", IIIrd International Symposium on electrochemical analysis, Slovenia (Schroewinia), 1999, page 57, and C.Lamy et al, "Direct oxidation of the methanol, ethanol and higher alcohols and hydrocarbons in PEM cells", IIIrd International Symposium on electrochemical analysis, Slovenia (Schroewinia), 1999, page 95. Most of these options show little promise due to low electrochemical activity, high cost, and sometimes toxicity.

Inorganic water-soluble reducing agents, such as metal hydrides, hydrazine and hydrazine derivatives, have also been proposed as fuels for fuel cells. See, e.g., s.lel, "The conversion of an alkane aldehyde cell lines with metals stores alloys", journal of The Electrochemical Society, volume 149, No.5, pages a603-a606 (2002), J.O' m.bocgris and s.srinivasan, fucels: their electrochemistry, McGraw-Hill, New York, 1969, pp 589-593, and N.V.Korvin, Hydrazine, Khimiya, Moscow, 1980 (Russian), pp 205-224. Such compounds have high specific energies and are highly reactive.

One such compound is NaBH₄. In water, NaBH₄Dissociate to Na⁺And BH₄ ^-. In neutral solution, BH₄ ^-Oxidation on the positiveelectrode according to the following equation 5:

(5)

the greatest disadvantage of hydrogen-containing inorganic compounds as fuels is their decomposition in acids and neutral solutions. For example, BH₄ ^-Decomposition according to equation 6:

(6) 。

in alkaline solution, BH₄ ^-Oxidation on the positive electrode according to the following equation 7:

(7)

the corresponding reduction of gaseous oxygen on the negative electrode proceeds according to equation 8:

(8)

mass balance and charge balance are maintained by diffusion of hydroxyl ions from the negative electrode to the positive electrode through the electrolyte.

Albeit BH₄ ^-Is stable in alkaline solutions, but it decomposes when in contact with a catalyst, such as when present on the anode of a fuel cell, according to equation 6, even when the fuel cell is not electrically loaded. Although the hydrogen gas generated by this reaction can also be oxidized on the positive electrode according to equation 1, the half-reaction represented by equation 7 is more energy efficient than the half-reaction represented by the combination of equations 1 and 6. In addition, BH on the positive electrode₄ ^-Tends to shorten the service life of the positive electrode.

This problem is addressed in PCT application No. WO/02/054506 (which is incorporated herein by reference as if fully set forth herein) by adding an alcohol such as methanol to alkaline NaBH₄To solve the problem in solution. In addition to its use as a fuel, these alcohols inhibit hydride species such as BH₄ ^-Decomposition on the positive electrode. It is believed that the alcohol utilizes at least one of two mechanisms to inhibit decomposition of the hydride species at the positive electrode. The first mechanism is that adsorption of alcohol molecules onto the positive catalytic site sterically hinders access of hydride species to the catalytic site. The second mechanism is that the alcohol molecules solvate the hydride species.

Intuitively, it can be expected that the capacity of a fuel cell operating on a hydride fuel (measured in amp-hours) is a linear function of the hydride concentration. For example, NaBH₄Solubility in 3M KOH is 1.25 moles per liter, and NaBH₄The solubility in 3M NaOH is 4 mol/l, and therefore depends on the use of NaBH₄The capacity of a fuel cell operated with saturated 3M NaOH is expected to depend on the applicationNaBH₄Four times the capacity of a saturated 3M KOH operated fuel cell. Experimentally, this is not the case.

Fig. 1 shows diagrammatically a fuel cell 10 which consists of an electrolyte chamber 16 which is bounded on both sides by a negative electrode 14 and a positive electrode 20, respectively, and contains an electrolyte. The negative electrode 14 and the positive electrode 20 are shown connected by an electrical load 28. On the other side of the anode 20 of the electrolyte chamber 12 is a fuel chamber 22 containing a fuel solution. The oxidant is atmospheric oxygen that reaches the negative electrode 14 on the other side of the negative electrode 14 of the electrolyte chamber 12. In the particular fuel cell 10 used in the experiments reported herein, the volume of the electrolyte chamber 16 was 2cm³The volume of the fuel chamber 22is 15cm³And the area of each electrode 14 and 20 is 4cm². The negative electrode 14 was prepared by printing 20% platinum on activated carbon on waterproof paper by a screen printing method. The positive electrode 20 was prepared by screen printing 20% platinum and 10% ruthenium on activated carbon on hydrophilic carbon paper.

The capacity of the fuel cell 10 is determined by using different concentrations of NaBH in a 3.3M aqueous NaOH fuel solution in the fuel chamber 22₄And measured in electrolyte chamber 12 using 6M aqueous KOH electrolyte. NaBH used₄The effective mass mF of (A) is determined using Faraday's law as the initial NaBH₄Measured as a function of concentration:

(9)m_F＝CM/Fn

where C is the capacity measured in ampere hours, F is 26.8 ampere hours per mole is the Faraday constant, and M is 38 g/mole is NaBH₄And n ═ 8 is per BH in equation 7₄ ^-The number of electrons released by the anion. The results are depicted in fig. 2. mF with increasing initial NaBH₄The concentration increases, but not linearly. Initial NaBH₄The higher the concentration, the NaBH₄The less efficient the use. In addition, when NaBH of fuel solution₄At levels above about 50 grams per liter, there is enhanced fuel decomposition at the positive electrode 20. This in turn leads to active gas release and foam formation, positive process pulsing and gradual destruction of the positive electrode 20. Increasing NaBH₄Also promotes NaBH at the initial concentration of₄Transitioning from the positive electrode 20 to the negative electrode 14 via the electrolyte.

There is therefore a widely recognized need for, and it would be highly desirable to have, a fuel composition for a fuel cell that would allow the use of hydride fuels at their full capacity.

Summary of the invention

According to the present invention there is provided a fuel composition comprising: (a) a solvent; (b) a first portion of a first fuel dissolved in a solvent; and (c) a second portion of the first fuel suspended in the solvent.

According to the present invention, there is provided a method of generating electrical energy comprising the steps of: (a) providing a fuel cell comprising an anode and a cathode; (b) contacting an oxidant with the negative electrode; and (c) contacting a fuel composition with the anode, the fuel composition comprising: (i) a solvent, (ii) a first portion of the fuel dissolved in the solvent, and (iii) a second portion of the fuel suspended in the solvent.

The present invention is a fuel composition for a fuel cell in which a first fuel is stored in two forms. The first portion of the first fuel is stored as a solution in a solvent. The second portion of the first fuel is stored as a suspension in a solvent. The effective concentration of the first fuel is the concentration of the first fuel in solution, and this concentration is kept low enough to prevent undesirable side effects such as decomposition of the first fuel at the anode and destruction of the anode. As the dissolved first fuel is depleted, there is a subsequent dissolution of the suspended first fuel. The effective mass of the first fuel is close to the total mass of the two portions of the first fuel.

Preferably, the solvent is a polar solvent such as water. Preferably, the concentration of the dissolved first fuel is the saturation concentration of the first fuel in the solvent. During operation of the fuel cell, as the dissolved first fuel is consumed, the suspended first fuel replaces the dissolved first fuel in solution and thus maintainsthe dissolved portion of the first fuel at its saturation concentration.

Preferably, the first fuel is a salt in a solvent (the anion of which is the product of the reduction half-reaction) that has a standard reduction potential that is more negative than the standard reduction potential of the hydrogen electrode in the solvent. For example, BH₄ ^-，NaBH₄The anion of (2) is an anion (in water) generated in the reduction half reaction

(10)

It has a standard reduction potential of-1.24 volts.

The first fuel is preferably a hydride, such as LiAlH₄，NaBH₄，LiBH₄，(CH₃)₃NHBH₃，NaAlH₄，NaCNBH₃，CaH₂LiH, NaH or KH. Most preferably, the first fuel is NaBH₄. Other preferred primary fuels include Na₂S₂O₃、Na₂HPO₃、Na₂HPO₂、K₂S₂O₃、K₂HPO₃、K₂HPO₂NaCOOH and KCOOH, which, like hydrides, are salts whose anions have standard reduction potentials in water that are more negative than the standard reduction potential of the hydrogen electrode in water. Generally for a solvent, a preferred fuel for any particular solvent includes salts whose anions have a standard reduction potential in the solvent that is more negative than the standard reduction potential of the hydrogen electrode in the solvent. Superior foodOptionally, the first fuel comprises from about 0.1% to about 80% by weight of the fuel composition. Most preferably, the first fuel comprises from about 5% to about 25% by weight of the fuel composition.

Optionally, the fuel composition of the present invention further comprises an alcohol, such as methanol, ethanol, propanol, butanol, pentanol, hexanol, ethylene glycol or glycerol. Preferably, the alcohol constitutes from about 0.1% to about 50% by weight of the fuel composition. Most preferably, the alcohol constitutes from about 1% to about 25% by weight of the fuel composition. Alcohols can perform four functions:

1. the alcohol is a second fuel that is oxidized at the anode of the fuel cell along with the first fuel.

2. The alcohol controls the solubility of the first fuel in the solvent to ensure that the saturation concentration of the first fuel is not too high.

3. And in WO/02/054506 for NaBH₄As such, the alcohol suppresses decomposition of the first fuel on the positive electrode of the fuel cell.

4. The alcohol stabilizes the suspension by being present in the solution of the first fuel in the solvent in a proportion such that the density of the solution is substantially equal to the density of the suspended portion of the first fuel, such that the suspended portion of the first fuel neither precipitates nor floats, but remains in suspension.

It is also within the scope of the present invention that any suitable additive be used for any of these four purposes, but alcohols are preferred additives.

Preferably, the fuel composition of the present invention comprises an additive for stabilizing the dissolved portion of the first fuel in the solvent. Preferably, this additive is a base such as LiOH, NaOH or KOH, or a basic salt. Preferably, this additive is present in the solvent at a concentration of between about 0.1 moles/liter and about 12 moles/liter. Most preferably, this additive is present in the solvent at a concentration of between about 0.2 moles/liter and about 5 moles/liter.

Osborg, in US4,081,252, teaches a fuel composition for combustion rather than for fuel cells which, like the present invention, includes a "hydrogen carrier" such as hydrazine, hydrazine derivatives or inorganic borohydride compounds which, according to the abstract of the patent, may be dissolved or suspended in the base fuel. However, all of the examples given by Osborg pertain to hydrogen carriers that are soluble in the base fuel. There is no indication in Osborg of any use of simultaneous dissolution and suspension of the hydrogen carrier in the base fuel.

Also within the scope of the invention are fuel cells fueled by the fuel compositions of the invention, and methods of using the fuel cells to generate electrical energy.

Brief description of the drawings

The invention is described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a fuel cell;

FIG. 2 is a series of NaBH fuel compositions in accordance with the prior art₄Effective mass of to the initial NaBH₄A curve of concentration;

fig. 3A and 3B show plots of current and capacity for the fuel cell of fig. 1 for comparing the fuel composition of the present invention with the fuel composition of the prior art.

Description of the preferred embodiments

The present invention pertains to fuel compositions that can be used to generate electrical energy in a fuel cell. In particular, the present invention allows hydride fuels to be efficiently utilized by fuel cells.

The principles and operation of a fuel composition for a fuel cell in accordance with the present invention may be better understood with reference to the drawings and the accompanying description.

Turning now to the drawings, FIG. 1, in addition to illustrating a prior art fuel cell, is also illustrative of a fuel cell of the present invention in which a fuel composition of the present invention replaces a prior art fuel solution in fuel chamber 18.

The fuel composition of the present invention is prepared by preparing NaBH₄Saturated solution in 3M aqueous KOH and addition of NaBH as a solid powder₄And stirring with a magnetic stirrer to produce NaBH₄In NaBH₄-a suspension in saturated KOH solution. NaBH₄The average particle size is about 10 microns, and 90% of the NaBH₄The particles are less than 100 microns. The suspension was stabilized by adding 10 vol% glycerol as dispersing agent. The 10% glycerol dispersant is NaBH₄The saturated KOH solution had 1.12g/cm²Also maintains the NaBH₄The particles are uniformly dispersed in the suspension. The glycerol dispersant also retains the NaBO₂The reaction products are in a suspended state, and therefore the reaction products are prevented from decreasing the catalyst activity on the positive electrode 16 and also from decreasing the fuel efficiency. Suspended NaBH₄With dissolved NaBH₄Is 1: 1. The current produced by the fuel cell 10, and the corresponding capacity (integrated current), is the fuel cell 10vs NaBH fueled with the fuel composition₄Measured as a solution in 3M aqueous NaOH. Dissolved NaBH in fuel compositions of the present invention and in prior art fuel solutions₄Is 1.25M, it is NaBH₄Saturated concentration in 3M aqueous KOH. The load 20 is fixed at 0.5 volts. FIG. 3 shows the measured current, milliamps (left ordinate), and capacity, milliamp hours (right ordinate) asTime (hours). The curve labeled "a" belongs to the prior art fuel solution. The curve labeled "b" belongs to the fuel composition of the present invention. The fuel composition of the present invention provides more stable current and higher capacity than prior art fuel solutions.

While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention are possible.

Claims (30)

1. A fuel composition comprising:

(a) a polar solvent;

(b) a first portion of a first fuel dissolved in the solvent, wherein the concentration of the first portion of the first fuel is its saturation concentration, the first fuel being selected from Na₂S₂O₃、Na₂HPO₃、Na₂HPO₂、K₂S₂O₃、K₂HPO₃、K₂HPO₂NaCOOH or KCOOH, the anion of the first fuel being the product of a reduction half-reaction, which has a standard reduction potential in the polar solvent that is more negative than the standard reduction potential of the hydrogen electrode in the polar solvent;

(c) a second portion of the first fuel suspended in the polar solvent, and

(d) a first additive for stabilizing the suspension, wherein the additive is present in a ratio sufficient for a solution of a first portion of a first fuel in the polar solvent to have a density equal to a density of the second portion of the first fuel.

2. The fuel composition of claim 1, wherein the polar solvent is water.

3. The fuel composition of claim 1, wherein the first fuel is between 0.1 wt% of the fuel composition and 80 wt% of the fuel composition.

4. The fuel composition of claim 3, wherein the first fuel is between 5 wt% of the fuel composition and 25 wt% of the fuel composition.

5. The fuel composition of claim 1, further comprising:

(e) a second fuel dissolved in the polar solvent.

6. The fuel composition of claim 5, wherein the second fuel is an alcohol.

7. The fuel composition of claim 6, wherein the alcohol is selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol, hexanol, ethylene glycol, or glycerol.

8. The fuel composition of claim 5, wherein the second fuel is between 0.1 wt% of the fuel composition and 50 wt% of the fuel composition.

9. The fuel composition of claim 8, wherein the second fuel is between 1 wt% of the fuel composition and 25 wt% of the fuel composition.

10. The fuel composition of claim 1, further comprising:

(e) a second additive for controlling the solubility of the first fuel in the polar solvent.

11. The fuel composition of claim 10, wherein the second additive is an alcohol.

12. The fuel composition of claim 11, wherein the alcohol is selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol, hexanol, ethylene glycol, or glycerol.

13. The fuel composition of claim 10, wherein the second additive is between 0.1 wt% of the fuel composition and 50 wt% of the fuel composition.

14. The fuel composition of claim 13, wherein the second additive is between 1 wt% of the fuel composition and 25 wt% of the fuel composition.

15. The fuel composition of claim 1, further comprising:

(e) and a second additive for suppressing decomposition of the first fuel at the positive electrode of the fuel cell.

16. The fuel composition of claim 15, wherein the second additive is an alcohol.

17. The fuel composition of claim 16, wherein the alcohol is selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol, hexanol, ethylene glycol, or glycerol.

18. The fuel composition of claim 15, wherein the second additive is between 0.1 wt% of the fuel composition and 50 wt% of the fuel composition.

19. The fuel composition of claim 18, wherein the second additive is between 1 wt% of the fuel composition and 25 wt% of the fuel composition.

20. The fuel composition of claim 1, further comprising:

(e) a second additive for stabilizing the first portion of the first fuel inthe polar solvent.

21. The fuel composition of claim 20, wherein the second additive is a base.

22. The fuel composition of claim 21, wherein the base is selected from LiOH, NaOH, or KOH.

23. The fuel composition of claim 20, wherein the second additive is a basic salt.

24. The fuel composition of claim 20, wherein the concentration of the second additive in the second polar solvent is between 0.1 and 12 moles/liter.

25. The fuel composition of claim 24, wherein the concentration of the second additive in the polar solvent is between 0.2 and 5 moles/liter.

26. The fuel composition of claim 1, wherein the first additive is an alcohol.

27. The fuel composition of claim 26, wherein the alcohol is selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol, hexanol, ethylene glycol, or glycerol.

28. A fuel cell comprising the fuel composition of claim 1.

29. A fuel composition comprising

(a) A polar solvent;

(b) a first portion of a fuel dissolved in the polar solvent, the first fuel selected from LiAlH₄、NaBH₄、LiBH₄、(CH₃)₃NHBH₃、NaAlH₄、NaCNBH₃、CaH₂、LiH、NaH、KH、Na₂S₂O₃、Na₂HPO₃、Na₂HPO₂、K₂S₂O₃、K₂HPO₃、K₂HPO₂NaCOOH or KCOOH, wherein the concentration of the first portion of the fuel is its saturation concentration, the anion of the first fuel is the product of a reduction half-reaction, in the polar solvent, which has a standard reduction potential that is more negative than the standard reduction potential of the hydrogen electrode in the polar solvent;

(c) a second portion of the fuel suspended in the polar solvent;

(d) a first additive for controlling the stability of the fuel in the solvent; and

(e) a second additive for stabilizing the suspension, wherein the additive is present in a ratio sufficient for a solution of the first portion of the fuel in the solvent to have a density equal to a density of the second portion of the fuel.

30. A fuel composition comprising:

(a) a polar solvent;

(b) dissolved in the polar solventA first part of the fuel, the fuel being selected from LiAlH₄、NaBH₄、LiBH₄、(CH₃)₃NHBH₃、NaAlH₄、NaCNBH₃、CaH₂、LiH、NaH、KH、Na₂S₂O₃、Na₂HPO₃、Na₂HPO₂、K₂S₂O₃、K₂HPO₃、K₂HPO₂NaCOOH or KCOOH, the concentration of the first portion of the fuel being its saturation concentration, the anion of the fuel being the product of a reduction half-reaction having a standard reduction potential that is more negative than the standard reduction potential of the hydrogen electrode in the solvent;

(c) a second portion of the fuel suspended in the polar solvent;

(d) a first additive for suppressing decomposition of the fuel at the positive electrode of the fuel cell; and

(e) a second additive for stabilizing the suspension, wherein the additive is present in a ratio sufficient for a solution of the first portion of the fuel in the solvent to have a density equal to a density of the second portion of the fuel.

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