Liquid hetero-interface fuel cell device

Details Liquid hetero-interface fuel cell device Liquid hetero-interface fuel cell device

2001-11-21

2004-02-03

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having magnetic field feature

C429S010000, C429S047000

Reexamination Certificate

active

06686077

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention generally relates to fuel cells for generation of electricity and, more particularly, to a liquid hetero-interface fuel cell device.
As the worldwide energy shortage worsens, fuel cells have become attractive because of their high efficiency, low emission characteristics and exceptional reliability. Conventional methods of converting chemical energy of hydrocarbon fuels into electricity involve combustion. Such methods may use various types of steam turbines or internal combustion engines whose thermodynamic efficiency is limited to about 40%, with 25% being an average efficiency. Galvanic cells provide an alternate approach to converting chemical energy into electricity.
A galvanic cell which can oxidize hydrogen or hydrocarbon fuel is known as a fuel cell. Not limited by the Carnot or Stirling cycle efficiencies, a fuel cell can achieve a thermodynamic efficiency over 50% and possibly even higher than 80%. Economically viable fuel cells would find applications ranging from propulsion of automobiles, trains, and aircraft to generation of electricity in utility power stations, industrial plants, or private homes. Wide deployment of fuel cells would permit doubling or tripling the extractable energy from existing fossil fuel resources, thereby alleviating the energy shortage. Unlike conventional combustion, fuel cells do not pollute the environment and reduce contributions to the “green house effect.” Of the numerous fuel cell concepts investigated in the last 90 years with varying degree of success, however, relatively few were advanced into commercial products.
Probably the most successful fuel cell developed in a variety of forms uses gaseous hydrogen fuel and oxygen oxidizer. Some of these devices were developed into commercial products for use in space, or in remote applications such as marine buoys. Beyond these niche, high-value applications, however, fuel cells have not won wide acceptance. Reasons for this situation include low availability and poor storability of certain easy-to-use fuels (as is the case with hydrogen), difficulty in achieving complete oxidation of more complex hydrocarbon molecules, poor electric conductivity of commonly available petroleum-based fuels, and high cost of electrodes due to the use of rare elements or noble metals. One of the problems is that the membrane separating the fuel and the oxidizer must allow the chemical species to be physically transported across the membrane while the electrons associated with the oxidation-reduction reaction are collected and separately flow through the external load. A voltage is maintained across the membrane by the chemical potential gradients of the reacting species, which serve to separate electrical charge. Carbon compounds and intermediates do not readily cross the conventional membranes and so reaction stops. CO
2
is also a gaseous compound and must be removed in some way. These problems are sufficiently serious that, in current hydrocarbon fuel cells, the hydrogen is simply stripped away in the reforming process and the energy associated with carbon oxidation rejected as waste heat. Besides being bulky, heavy, and hazardous to operate, reformers add complexity and reduce efficiency of the fuel cell system as the energy in the fuel attributable to its carbon content is largely wasted in most such designs.
Fuel cell designers must also overcome numerous challenges which restrict operating characteristics of the cell such as removal of reaction products (typically carbon dioxide and/or water), lifetime of electrodes, and poisoning of electrolyte by parasitic reactions. The ideal fuel cell would use widely available, easily storable, low cost fuels (e.g., kerosene, alcohol, natural gas) and atmospheric oxygen. Construction and operation of the cell should allow it to compete against established electric power generating technologies in specific market segments. Some of the considerations in designing a fuel cell are reactivity, invariance, oxidizers, catalysts, cell separators, and polar and non-polar fluids.
Reactivity relates to both the speed and completeness of the reaction. Reaction speed requires high electrode activity, which is controlled by the rates and mechanisms of electrode reactions, and results in high current densities. Reaction completeness requires proper stoichiometry. For example, carbon should always be oxidized to CO
2
rather than CO so that a maximal amount of electrical energy is released in the reaction. In prior art, the reactivity requirement has been met by using porous materials to enlarge the active area of electrodes, by increasing pressure, by raising temperature, or by using catalysts.
Invariance relates to the objective that a fuel cell, unlike a conventional battery, should maintain constant performance throughout its life. This implies that there should be no corrosion or side reactions, and no changes in the electrolyte or the electrodes. In particular, fuel should not diffuse over and mix with the oxidizer. Catalysts can become poisoned and the pores of gas electrodes can become clogged with liquid (“drowning”), gas (“blowing”), or extraneous material making the electrode inoperative. If “wrong” ions carry the current, the electrolyte may lose its invariance, and the cathode and anode reactions may be thrown out of balance.
Oxidizers relate to the fact that most fuel cells use oxygen for fuel oxidation. Oxygen is first cathodically reduced to OH
−
cations, which react in the electrolyte with anions originating from fuel. Unfortunately, reactivity of OH
−
with many fuels is very slow, which leads to impracticably low current densities. While catalysts can often remedy this situation, they typically require use of expensive materials such as platinum or palladium, hence driving up the capital cost of the fuel cell system. Reactivity can be also increased by choosing a more reactive oxidizer such as the O
2
H
−
cation.
Catalysts previously used with fuel cells are typically in the form of coatings on electrode surfaces. Recently, a new soluble catalyst has been introduced, which is suitable for increasing reactivity of H
2
O
2
in oxidizing a broad variety of organic substances. This soluble catalyst is methyltrioxorhenium (CH
3
ReO
3
), also known as methylrhenium trioxide or MTO. Synthesis of MTO was first reported in 1979 and its use as a catalyst for hydrogen peroxide oxidation of a number of alkenes, alkynes, and ketones was first published in 1991 by W. A. Hermann et al. in the journal Angew. Chem., Intl. Ed. Eng., vol. 30, pp. 1638-41. This catalyst has important attractive features including ease of synthesis, stability in the air, stability and solubility in aqueous (low pH) as well as organic solvents, low toxicity, and effectiveness as either a homogeneous or heterogeneous catalyst. Unlike other catalysts, MTO alone does not decompose H
2
O
2
. Research shows that addition of a cocatalyst (preferably bromine ions) can further accelerate processes catalyzed by MTO as published in 1999 in the article “Bromide ions and methyltrioxorhenium as co-catalysts for hydrogen peroxide oxidations and brominations,” by J. H. Espenson et al. in the JournalOrg. Chem., vol. 54, pp. 1191-96.
Cell separators relate to the fact that it is impractical to mix large volumes of fuel and oxidizer. In most fuel cells, fuel and oxidizer are maintained in different compartments of the cell sharing a common wall known as a separator. Such a separator is permeable so that the fuel or the oxidizer can be contacted and reacted in a controlled fashion. Oxidation of the fuel takes place on the surface or within the separator. To promote high reaction rates, fuel cell separators often contain catalysts. A variety of separator designs have been used with varying degrees of success, including porous beds and ion exchange membranes. Key issues in design of fuel cell separators include maintaining high transport rates for reacting species and reaction products, and low susceptibility to flooding.
Polar and non-polar fluid

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