KR100420375B1 - Liquid feed fuel cell using fuel mixed with perfluorooctansulfonic acid - Google Patents

Abstract

A liquid organic fuel cell 10 using the solid electrolyte membrane 18 is provided. Organic fuel, such as a methanol / water mixture, circulates past the anode 18 of the cell and oxygen or water circulates past the cathode 16. The battery electrolyte membrane is preferably made of Nafion. Also provided is a method for improving the performance of a carbon electrode structure, wherein a high surface area carbon particle / teflon binder structure is immersed in a Nafion / methanol bath to impregnate the electrode with Nafion. An anode manufacturing method for this fuel cell is described, in which metal alloys are deposited on an electrode from a solution containing perfluorooctane sulfonic acid. Fuel additives containing this acid and new organic fuels are also described.

Description

Liquid feed fuel cell using fuel mixed with perfluorooctansulfonic acid}

The present invention has been made under a contract with NASA and is subject to the Public Law 96-517 (35 USC 202), which the contractor intends to retain.

FIELD OF THE INVENTION The present invention relates to organic fuel cells, and more particularly to liquid supply organic fuel cells.

A fuel cell is an electrochemical cell in which free energy change due to fuel oxidation is converted into electrical energy. In organic / air fuel cells, organic fuels such as methanol, formaldehyde or formic acid are oxidized to carbon dioxide at the anode and air or oxygen is reduced to water at the cathode. In part, fuel cells using organic fuels are extremely attractive, both on installation and portable, because of the high specific energy of organic fuels (eg, the specific energy of methanol is 6232 wh / kg).

Two types of organic / air fuel cells are generally known.

1. An "indirect" or "reformer" fuel cell in which organic fuel is catalytically reformed and treated with carbon monoxide-free hydrogen with hydrogen obtained by oxidation at the anode of the fuel cell.

2. A "direct oxidation" fuel cell in which an organic fuel is fed directly to the fuel cell without prior chemical modifications where the fuel is oxidized at the anode.

Direct oxidation fuel cells do not require a fuel processing step. Thus, direct oxidation fuel cells offer significant advantages in weight and volume over indirect fuel cells. Direct oxidation fuel cells use a vapor or liquid supply of organic fuel. Currently used direct oxidation fuel cells typically employ a liquid supply scheme that circulates through the anode of the fuel cell of a liquid mixture of organic fuel and sulfuric acid electrolyte.

The use of sulfuric acid electrolytes in the latest technology direct methanol fuel cells presents several problems. The use of highly corrosive sulfuric acid poses significant limitations to fuel cell structural materials. Typically expensive corrosion resistant materials are required. Sulfate anions, which are produced in fuel cells, have a strong tendency to absorb on the electrocatalyst, impeding the rate of reaction of the electrical oxidation of the fuel and degrading the performance of the fuel electrode. In addition, sulfuric acid is decomposed at temperatures above 80 ° C. and the products of decomposition generally contain sulfur which can harm the electrocatalyst. The use of sulfuric acid electrolytes in many cell stacks can result in parasitic decentralized currents.

Examples of fuel cells of both direct and indirect types are described in US Pat. No. 3,013,908; 3,113,049; No. 4,262,063; 4,407,905; 4,407,905; No. 4,390,603; 4,612, 261; No. 4,478,917; No. 4,537,840; No. 4,562,123; And 4,629,664.

US Pat. Nos. 3,013,908 and 3,113,049 describe, for example, liquid fed direct methanol fuel cells using sulfuric acid electrolytes. U.S. Patent Nos. 4,262,063, 4,390,603, 4,478,917 and 4,629,664 describe improvements in methanol fuel cells based on sulfuric acid, wherein a high molecular weight electrolyte or solid quantum conductive membrane is positioned as an ionic conductive layer between the cathode and the anode. To reduce the crossover of organic fuel from anode to cathode. Although the use of ionic conductive layers helps to reduce crossover, ionic conductive layers are only used in conjunction with sulfuric acid electrolytes. Therefore, fuel cells have several of the above disadvantages of using sulfuric acid as an electrolyte.

In view of the above various problems associated with using sulfuric acid as an electrolyte, it is desirable to provide a liquid feed fuel cell that does not require sulfuric acid as an electrolyte.

In addition to improvements in the operating characteristics of liquid feed fuel cells, existing methods of producing high surface area electrocatalytic electrodes used in such fuel cells also need to be improved. Existing methods for producing fuel cell electrodes are time consuming and expensive. In particular, electrode production requires that high surface area carbon-supported alloy powders be prepared initially by chemical methods, which takes 24 hours in total. Once ready, the carbon-supported alloy powder is bound to a Teflon ^™ binder and applied to a carbon fiber based support to produce a gas diffusion electrode. The electrode is heated to 200-300 ° C. to volatilize impurities from the Teflon ^™ binder and obtain a Teflon ^™ fiber matrix. During this heating process, oxidation and sintering of the electrocatalyst can occur, resulting in a decrease in the activity of the electrode surface. Therefore, the electrode may be required to be reactivated before use.

In addition, the electrode produced by the conventional method is usually a gas diffusion type, so that the electrode is not properly wetted by the liquid fuel, and thus cannot be effectively used in the liquid supply type fuel cell. In general, the structure and characteristics of a fuel oxide electrode (anode) used in a fuel supply type fuel cell are significantly different from gas / vapor supply fuel cells such as hydrogen / oxygen fuel cells. The structure of the electrode used in the fuel supply fuel cell is very porous and all liquid fuel solutions must be wetted with holes. Carbon dioxide generated at the fuel electrode must be effectively released from the reaction zone. Proper wetting of the electrodes is a major problem in liquid feed fuel cells, even using sulfuric acid electrolytes.

As can be appreciated, it is desirable to provide an improved method for producing electrodes, particularly for liquid feed fuel cells. It is also highly desirable to devise a method of improving an electrode originally adapted for a gas feed fuel cell for use in a liquid feed fuel cell.

In addition to improving the liquid supply fuel cell itself, it is also desirable to provide a new effective fuel to provide an improved method of producing the electrodes of the fuel cell. In general, it is desirable to provide a liquid fuel that performs clean and effective electrochemical oxidation in a fuel cell. Effective use of organic fuels in direct oxidation fuel cells generally depends on the ease with which the organic compounds are oxidized anodically in the fuel cell. Conventional organic fuels, such as methanol, present significant challenges for electro-oxidation. In particular, the electrooxidation of organic compounds such as methanol is a process involving the transfer of a large number of electrons and also interrupted by several intermediate steps. These steps are involved in the dissosiative absorption of fuel molecules to form active surface species that perform relatively easy oxidation. The ease of dissociation absorption and surface reaction usually determines the ease of electrooxidation. Other existing fuels, such as formaldehyde, are more easily oxidized but also have other disadvantages. For example, formaldehyde is very toxic. In addition, formaldehyde is extremely water soluble and thus crosses the cathode of the fuel cell, thus degrading the performance of the fuel cell. Other conventional organic fuels, such as formic acid, are corrosive. In addition, many existing organic fuels harm the electrodes of the fuel cell during electro-oxidation, preventing reliable operation. As can be appreciated, particularly for liquid feed fuel cells, it is desirable to provide improved fuels that overcome the disadvantages of existing organic fuels such as methanol, formaldehyde, formic acid.

It is a general object of the present invention to provide an improved direct liquid feed fuel cell. One particular object of the present invention is to provide a direct liquid feed fuel cell that does not require a sulfuric acid electrolyte. Another particular object is to achieve proper wetting of the electrodes in the liquid feed fuel cell. It is a further object of the present invention to provide an improved method for wetting electrodes used in fuel cells with sulfuric acid electrolytes. Another specific object of the present invention is to provide an improved fuel for use in liquid feed fuel cells.

1 is a schematic diagram of an improved liquid supply organic fuel cell having a solid polymer membrane formed in accordance with a preferred embodiment of the present invention;

FIG. 2 is a schematic diagram of a multiple fuel system using the improved liquid higher organic fuel cell of FIG. 1; FIG.

3 is a graph showing the performance of solid polymer membrane electrolytes and sulfuric acid electrolytes in liquid organic fuels;

4 is a graph showing the performance of the liquid feed fuel cell of FIG. 1 for methanol / air and methanol / oxygen bonds;

5 is a graph showing the effect of fuel concentration on the performance of the liquid supply fuel cell of FIG. 1;

6 is a graph showing the polarization form of a fuel electrode and a cathode in the fuel cell of FIG. 1;

FIG. 7 is a block diagram illustrating a method of making an electrode containing a hydrophilic quantum conduction ionomer additive for use in a liquid supply cell; FIG.

8 is a graph showing the polarization properties for methanol oxidation at an electrode containing the ionomer additive and prepared according to the procedure shown in FIG. 7;

9 is a block diagram showing a method for producing an electrode using perfluorooctane sulfonic acid in an electro-deposition bath;

10 is a schematic of an electrochemical electrode for use in performing the method of FIG. 9;

FIG. 11 shows a polarization curve for an electrode fabricated using the method of FIG. 9;

12 is a graph showing polarization curves of a fuel cell using a sulfuric acid electrolyte and using perfluorooctane sulfonic acid as a fuel additive;

FIG. 13 is a graph showing the polarization curve of a fuel cell using dimethoxymethane as fuel for various fuel concentration levels in a half cell with sulfuric acid electrolyte;

FIG. 14 is a graph showing the polarization curve of a fuel cell using dimethoxymethane as fuel for varying temperature and concentration in a half cell with sulfuric acid electrolyte; FIG.

15 is a graph showing cell voltage as a function of current density for the fuel cell of FIG. 1 using dimethoxymethane as fuel;

FIG. 16 is a graph showing the polarization curve of a fuel cell using trimethoxymethane as fuel for various fuel concentration levels in a half cell with sulfuric acid electrolyte;

FIG. 17 is a graph showing the polarization curve of a fuel cell using trimethoxymethane as a fuel at different temperatures and concentrations in a half cell with sulfuric acid electrolyte;

FIG. 18 is a graph showing cell voltage as a function of current density for the fuel cell of FIG. 1 using trimethoxymethane as fuel; FIG.

19 is a graph showing the polarization curve of a fuel cell using trioxane as fuel for various fuel concentration levels in a half cell with 2M sulfuric acid electrolyte;

FIG. 20 is a graph showing the polarization curve of a fuel cell using trioxane as a fuel against varying temperatures and concentrations of sulfuric acid electrolyte in the half cell;

21 shows the cell voltage as a function of the current density of the fuel cell of FIG. 1 using trioxane as the fuel.

The object of the present invention to provide a direct liquid feed fuel cell that does not require a sulfuric acid electrolyte is achieved in part by using a solid polymer electrolyte membrane combined with a battery type anode that is porous and can be wetted with fuel. In an improved liquid supply fuel cell, the battery type anode structure and the cathode are coupled to one side of the solid polymer quantum conductive film forming the membrane electrode assembly. A solution of methanol and water that is substantially free of sulfuric acid is circulated through the anode side of the assembly.

Solid polymer electrolyte membranes are used, in part because these membranes have high ionic conductivity with excellent electrochemical mechanical stability and can act as both electrolytes and separators. In addition, the electrooxidation process of methanol and the electrokinetic reaction of air or oxygen (kinetic) are easier than the electrode / sulfuric acid interface at the electrode / membrane-electrolyte interface. The use of the membrane allows the operation of the fuel cell at high temperatures of 120 degrees. Since the solution of fuel and water is essentially free of sulfuric acid, no expensive corrosion resistant elements are needed in the fuel cell and its accessories. Also the absence of conductive ions that may be present when sulfuric acid electrolyte is used in a solution of fuel and water essentially eliminates the possibility of any parasitic shunt current in the multi-cell stack.

The solid polymer electrolyte is preferably a proton conducting cation exchange membrane, such as perfluorinated sulfonic acid polymer membrane, Nafion ^™ . Nafion is a copolymer of tetrafluoroethylene and perfluorovinylethersulfonic acid. Composite membranes of modified perfluorinated sulfonic acid polymers, polyhydrosulfonic acid and two or more proton exchange membranes may also be used.

The anode is preferably formed from high surface area particles of a platinum-based alloy of noble or non-noble metals. Binary and ternary compositions can be used for the electrooxidation of organic fuels. Platinum-ruthenium alloys having compositions varying in the range of atomic percent from 10 to 90 platinum are the preferred anode electrocatalysts for the electrooxidation of methanol. The alloy particles are unsupported or supported by pure metal powders, ie high surface area carbon materials.

Existing fuel cell anode structures (gas diffusion type) are not suitable for the use of liquid supply type organic / air fuel cells. These conventional electrodes have low fuel wettability. These conventional electrodes can be modified for use in liquid supply type fuel cells by coating materials that improve their wettability. Nafion having an equivalent weight of at least 1000 is a preferred material. The additive reduces the interfacial tension at the liquid / catalyst interface and leads to uniform wetting of the electrode pores and particles by a solution of fuel and water, thereby creating an enhanced use of the electrocatalyst. In addition to the improvement in wettability, Nafion additives also provide ionic continuity with the solid electrolyte membrane and allow effective delivery of proton or hydronium ions produced by fuel oxidation reactions. Furthermore, the additive facilitates the release of carbon dioxide in the pores of the electrode. By using perfluorinated sulfonic acid as an additive, anionic groups are not strongly adsorbed on the electrode / electrolyte interface. As a result, the electrokinetic oxidation of methanol is easier than that of sulfuric acid electrolyte. Other hydrophilic proton conducting additives with desirable features include montmorrolinite clay, alkoxycellulose, cyclodextrins, zeolite mixtures and zirconium hydrogen phosphate.

By using perfluorooctanesulfonic acid as an additive in the electrodeposition tank used when producing the electrodes, the object of partially improving the electrodes operating in the liquid feed fuel cell is achieved. Electrodeposition methods using perfluorooctane sulfonic acid additives include placing a high surface area carbon electrode structure in a bath containing a metal salt, placing an anode in the bath, and applying a voltage between the anode and the cathode. Adding positive metal until it is deposited on the electrode. After depositing the metal on the electrode, the electrode is removed from the bath and washed with deionized water.

Preferably, the metal salt comprises hydrogen hexachloroplatinate and potassium pentachloroaquaruthenium. The anode consists of platinum. The carbon electrode structure comprises high surface area carbon particles bound by polytetrafluoroethylene sold under the trade name Teflon ^™ .

The object provided to properly wet the electrode in a liquid feed fuel cell with a sulfuric acid electrolyte is achieved by using perfluorooctane sulfonic acid as an additive to the fuel mixture of the fuel cell. Preferably perfluorooctane sulfonic acid is added to the organic fuel and water mixture at a concentration of 0.001 to 0.1 M.

The general purpose of providing new fuels for use in organic fuel cells is achieved by using either trimethoxymethane or dimethoxymethane or trioxane. All three new fuels can be oxidized to carbon dioxide and water at high rates in the fuel cell without damaging the electrodes. Furthermore, neither trimethoxymethane, dimethoxymethane or trioxane are corrosive. The oxidation rate of the three new fuels is comparable to or better than that of existing organic fuels. For example, the oxidation rate of dimethoxymethane is higher than that of methanol at the same temperature. Trioxane is comparable to formaldehyde in its oxidation rate. However, trioxane has a much higher molecular weight than formaldehyde, so that the molecules of trioxane do not cross the cathode of the fuel cell as easily as the molecules of formaldehyde.

Trimethoxymethane, dimethoxymethane and trioxane can be used in fuel cells with any of the above improvements. However, the improved fuel can be advantageously used in other organic fuel cells, including entirely existing fuel cells.

Various objects and advantages of the present invention will become more apparent with reference to the following detailed description and accompanying drawings.

Referring to the drawings, preferred embodiments of the present invention are described. First, an improved liquid feed organic fuel cell using a solid polymer electrolyte membrane and ionomer anode additive is mainly described with reference to FIGS. Then, a method for producing an anode with ionomer additive is described with reference to FIGS. 7 and 8. A method of achieving improved wetting by making an electrode in a bath containing perfluorooctane sulfonic acid is described with reference to FIGS. 9-11. A fuel cell using perfluorooctanesulfonic acid as a fuel additive is described with reference to FIG. 12. Fuel cells using dimethoxymethane, trimethoxymethane and trioxane as fuels are described with reference to FIGS. 13 to 21.

Fuel Cell Using Solid Quantum Conductive Electrolyte Membrane

1 shows a liquid supply organic fuel cell 10 with a housing 12, an anode 14, a cathode 16 and a solid polymer quantum-conducting cation exchange electrolyte membrane 18. As described in more detail below, the anode 14, cathode 16 and solid polymer electrolyte membrane 18 are preferably a single multilayer composite structure referred to as a membrane-electrode assembly. The pump 20 is provided for pumping a solution of organic fuel and water into the anode chamber 22 of the housing 12. The mixture of organic fuel and water is recovered through discharge port 20 and recycled through the re-circulation system described in FIG. 2 below, which includes a methanol tank 19. The carbon dioxide formed in the anode compartment leaks out through the port 24 in the tank 19. An oxygen or air compressor 26 is provided to supply oxygen or air to the cathode chamber 28 in the housing 12. 2, described below, illustrates a fuel cell system that combines a stack of each fuel cell that includes a recirculation system. The following detailed description of the fuel cell of FIG. 1 focuses primarily on the structure and function of the anode 14, the cathode 16 and the membrane 18.

Prior to use, the anode chamber 22 is filled with a mixture of organic fuel and water and the cathode chamber 28 is filled with air and oxygen. During operation, organic fuel is circulated past the anode 14, while oxygen or air is pumped into the chamber 28 and circulated past the cathode 16. When an electrical load (not shown) is connected between the anode 14 and the cathode 16, the electro-oxidation of the organic fuel occurs at the anode 14, and the electro-reduction of oxygen occurs at the cathode 16. The occurrence of different reactions at the anode and the cathode results in a voltage difference between the two electrodes. Electrons generated by electro-oxidation at the anode 14 are conducted through an external load (not shown) and ultimately trapped at the cathode 16. Hydrogen ions or both produced at the anode 14 pass directly through the membrane electrolyte 18 to the cathode 16. Therefore, the flow of current is maintained by the flow of ions passing through the cell and electrons passing through an external load.

As noted above, anode 14, cathode 16 and film 18 form a single composite layered structure. In a preferred embodiment, the membrane 18 is formed from Nafion, which is a perfluorinated proton-exchange membrane material. Nafion is a copolymer of tetrafluoroethylene and perfluorovinylethersulfonic acid. Other membrane materials can also be used. For example, a composite membrane of modified perfluorinated sulfonic acid polymer, polyhydrosulfonic acid and two kinds of proton exchange membranes can be used.

The anode 14 is formed from platinum-ruthenium alloy particles as fine metal powder, ie unsupported or dispersed on high surface area carbon, ie in supported form. The high surface area carbon is a material such as Vulcan XC-72A provided by Carbot Inc., USA. Carbon fiber sheet backing (not shown) is used to make electrical contact with the electrocatalyst particles. Commercially available Toray ^™ paper is used as the electrode backing sheet. Supported alloy electrocatalysts on Toray Paper Backings can be obtained from E-tek, Inc. of Framingham, Mass., USA. In addition, unsupported and supported electrocatalysts can be prepared by a chemical method that is combined with a Teflon ^™ binder and sprayed on toray paper backing to produce an anode. An efficient and time-saving method for producing an electrode-catalytic electrode is described in detail below.

A platinum based alloy whose second metal is any of tin, iridium, osmium or rhenium can be used in place of the platinum-ruthenium alloy. In general, the choice of alloy depends on the fuel used in the fuel cell. Platinum-ruthenium is suitable for the electrooxidation of methanol. For platinum-ruthenium, the loading of alloy particles in the electrocatalyst is preferably in the range of 0.5 to 4.0 mg / cm ² . More efficient electro-oxidation is obtained with higher loading levels than with low loading levels.

The cathode 16 is a gas diffusion electrode in which platinum particles are bonded to one side of the membrane 18. The cathode 16 is preferably formed from unsupported or supported platinum bonded to one side of the membrane 18 opposite the anode 14. The cathode is either unsupported platinum black (fuel cell grade) obtained from Johnson Matthey Inc., USA, or supported platinum material obtained from E-tek, Inc., USA. Suitable for As with the anode, the cathode metal particles are preferably mounted on a carbon backing material. The loading of the electrocatalyst particles into the carbon backing is preferably in the range of 0.5 to 4.0 mg / cm ² . Electrocatalyst alloys and carbon fiber backings comprise 10 to 50 weight percent of Teflon to create the three-phase region and provide the hydrophobicity necessary to achieve efficient removal of water generated by electrical reduction of oxygen.

During operation, a fuel and water mixture (containing no acidic or alkaline electrolyte) with a concentration of 0.5 to 3.0 mol / l is circulated past the anode 14 in the anode chamber 22. Preferably a flow rate in the range of 10 to 500 ml / min is used. Since the mixture of fuel and water circulates past the anode 14, for example in a methanol cell, the following electrochemical reaction causes electron emission.

_{Anode: CH 3 OH + H 2 O} → CO 2 + 6H + + 6e -

The carbon dioxide produced by the reaction is recovered together with the solution of fuel and water through the discharge port 23 and separated from the solution in the gas-liquid separator (described below with respect to FIG. 2). The solution of fuel and water is then recycled in the cell by the pump 20.

Simultaneously with the electrochemical reaction described in Scheme 1 above, another electrochemical reaction involving electro-reduction of oxygen takes place at the cathode 16, as follows.

^{_{Cathode: O 2 + 4H + + 4e}} - → H 2 O

The individual electrode reactions described by Schemes 1 and 2 result in all of the reactions of the exemplary methanol fuel cell given below.

Battery: CH ₃ OH + 1.50 ₂ → CO ₂ + 2H ₂ O

In sufficiently large concentrations of fuel, a current density of at least 500 mA / cm 2 can be maintained. However, at such concentrations, the crossover rate of fuel across the membrane 18 to the cathode 16 increases such that the efficiency and electrical performance of the fuel cell is significantly reduced. Concentrations of 0.5 mol / l or less limit cell operation to current densities of 100 mA / cm 2 or less. It is known that low flow rates can be applied at low current densities. High flow rates are required when operating under high current densities to increase the mass transfer ratio of organic fuel to the anode as well as to remove carbon dioxide produced by the electrochemical reaction. Low flow rates also reduce the crossover of fuel from the anode to the cathode through the membrane.

Preferably, it circulates past the cathode 16 at a pressure in the range of 10 to 30 psig of oxygen or air. In particular, under high current densities, pressures above ambient improve the mass transport of oxygen to the site where electrochemical reactions occur. Water generated by the electrochemical reaction at the cathode is delivered to the outside of the cathode chamber 28 by the flow of oxygen through the port 30.

In addition to the electro-oxidation at the anode described above, the liquid fuel dissolved in water is permeated through the solid polymer electrolyte membrane 18 to combine with oxygen on the cathode electrocatalyst surface. This process is described in Scheme 3 for an example of methanol. This phenomenon is called "fuel crossing". Fuel crossover lowers the operating potential of the oxygen electrode and consumes fuel without generating useful electrical energy. Fuel crossover is generally a parasitic reaction that reduces efficacy and generates heat in the fuel cell. Therefore, it is desirable to minimize the rate of fuel crossover. The crossover rate increases with increasing concentration and temperature in proportion to the permeability of the fuel through the solid electrolyte membrane. By selecting a solid electrolyte membrane with a low water content, the permeability of the membrane into the liquid fuel can be reduced. Reduced penetration into the fuel results in a low cross rate. In addition, fuels with large molecular sizes have smaller diffusion coefficients than fuels with small molecular sizes. Therefore, the permeation force can be reduced by selecting fuel having a large molecular size. Water soluble fuels are preferred, and moderately water soluble fuels exhibit reduced permeability. Fuels with high boiling points do not evaporate, and delivery through the membrane takes place in the liquid. Since the permeability for evaporation is higher than that of liquids, fuels with high boiling points generally have low crossovers. The concentration of liquid fuel can also be lowered to reduce the crossover rate. The anode structure is sufficiently wetted by the liquid fuel to maintain the electrochemical reaction with optimal distribution of hydrophobic and hydrophilic sites and prevents excessive amounts of fuel from accessing the membrane electrolyte. Thus, proper selection of the anode structure can lead to good performance and desirable low crossover rates.

Since the solid electrolyte membrane is permeable to water at temperatures above 60 ° C., a significant amount of water is passed through the membrane by permeation and evaporation. The water delivered through the membrane condenses in the water recovery system and is fed to a water tank (described below through FIG. 2) so that water can be re-introduced into the anode chamber 22.

Proton produced at the anode 14 and water produced at the cathode 16 are transferred between the two electrodes by the quantum-conducting solid electrolyte membrane 18. Maintaining high quantum-conductivity of membrane 18 is important for the effective operation of organic / air fuel cells. The moisture content of the membrane is maintained by providing direct contact of the liquid fuel and water mixture. The thickness of the quantum-conducting solid electrolyte membrane should be in the range of 0.05 to 0.5 mm to be size stable. Membranes thinner than 0.05 mm result in membrane electrode assemblies with weak mechanical strength, while membranes thicker than 0.5 mm experience extreme and destructive size changes induced by the expansion of the polymer by liquid fuel and water solutions and also exhibit transient resistance. Can be. The ionic conductivity of the membrane must be greater than 1 ohm ^-1 cm ^-1 so that the fuel cell has an acceptable internal resistance. As noted above, the membrane should have a low permeability for liquid fuel. Although Nafion membranes are known to be effective as quantum-conducting solid polymer electrolyte membranes, their properties are similar to those of perfluorinated sulfonic acid polymer membranes such as Aciplex ^™ (manufactured by Asahi Glass Co., Japan) and Nafion. Similar to XUS13204. Polymeric membranes manufactured by Dow Chemical Co., such as 10, are also applicable. Membranes of polyethylenesulfonic acid, polypropylenesulfonic acid, polystyrenesulfonic acid and other polyhydrocarbon based-sulfonic acids (such as membranes made by RSA Corporation, USA) may also be used depending on the temperature and duration of fuel cell operation. Acid equivalent weight, or chemical component (e.g., modified acid group or polymer backbone), or moisture content, or the shape and extent of crosslinking (e.g., cross-linking by polyvalent cations, ie Al ³⁺ , Mg ²⁺ ) Composite membranes of two or more types of quantum-conducting cation exchange polymers can be used to achieve low fuel permeability. Such composite membranes can be prepared to achieve high ionic conductivity, low permeability to liquid fuels and good electrochemical stability.

As can be seen from the above description, liquid feed direct oxidation organic fuel cells can be achieved using quantum-conducting solid polymer membranes as electrolytes that do not require free soluble acid or base electrolytes. The only electrolyte is a quantum conducting solid polymer membrane. No acid is present in the liquid fuel and water mixture in free form. Since no free acid is present, corrosion due to the acid of the cell components which can occur in the acid based organic / air fuel cells of the present technology is avoided. This provides considerable flexibility in the selection of materials for fuel cell related subsystems. Moreover, unlike a fuel cell containing potassium hydroxide as a liquid electrolyte, the cell performance does not deteriorate because soluble carbonate is not formed. In addition, parasitic decentralized current is avoided by the use of the solid electrolyte membrane.

2, a fuel cell system using a stack of fuel cells similar to the fuel cell shown in FIG. 1 is described. The fuel cell system includes a stack 25 of fuel cells, each having a membrane / electrode assembly described above with respect to FIG. 1. Oxygen or air is supplied by, for example, an oxidant supply device 26 such as a containerized oxygen supply device, a feed blower fan or an air compressor. Air and water or oxygen and water mixtures exit the stack 25 through the discharge port 30 and are transferred to the water recovery unit 27. The water recovery unit 27 operates to separate air or oxygen from the water. Some of the air or oxygen separated by the unit 27 is returned to the oxidant supply device 26 for reentry into the stack 25. Fresh air or oxygen is added to the supply device 27. The water separated by the unit 27 is supplied to the fuel and water injection unit 29 which receives an organic fuel such as methanol from the storage tank 33. The injection unit 29 combines the water from the recovery unit 27 with the organic fuel from the tank 33 to produce a solution of fuel and water with fuel dissolved in water.

The solution of fuel and water provided by the injection unit 29 is supplied to the circulation tank 35. The mixture of fuel and water containing carbon dioxide exits the stack 25 through the port 23 and is supplied to the storage tank 33 through the heat exchanger 37. Therefore, the circulation tank 35 is supplied with both a solution of fuel and water from the injection unit 29 and a solution of fuel and water containing carbon dioxide from the heat exchanger 37. The circulation tank 35 extracts carbon dioxide from the mixture of fuel and water and releases carbon dioxide through the vent 39. The resulting solution of fuel and water is supplied to the stack 25 via a pump 20. The circulation tank 35 may also be located between the stack 25 and the heat exchanger 34 to remove carbon dioxide in front of the heat exchanger 34 to improve the performance of the heat exchanger.

The operation of the various configurations shown in FIG. 2 is described in more detail. The circulation tank 35 is a tower having a large head space. The mixture of liquid fuel and water supplied from the injection unit 29 is added to the top of the tower. A mixture of fuel and water with carbon dioxide is fed to the bottom of the tower. Carbon dioxide released from the mixture of fuel and water accumulates in the upper space and eventually leaks out. In addition, a mixture of fuel and water containing carbon dioxide may be passed through a tube bundle of microporous materials such as Celgard ^™ or GoreTex ^™ , allowing gas to be released through the walls of the tube of microporous material. And liquid fuel flows along the axis of the tube. Celgard and Gore-Tex are registered trademarks of Celanese Corp. and Gore Association.

Static recirculation systems (not shown) can be used in the anode chamber of stack 25 to separate carbon dioxide from the mixture of fuel and water so that no external circulation tanks need to be provided. With such a system, bubbles of carbon dioxide due to intrinsic buoyancy tend to occur vertically in the anode chamber. The viscous interaction with the liquid fuel mixture surrounding the gas bubbles causes the liquid fuel to be pulled upwards in the direction of the discharge port 23. When the liquid releases gas outside of the anode chamber, heat is exchanged with the surrounding refrigerant, thereby making it denser than the liquid in the cell. The densified solution is fed to the bottom of the anode chamber through an injection port. Instead of expanding the electrical energy in the pump, the static recycle system uses heat and gas generated in the cell. The above-described process is based on a static recycle system and will no longer be described in detail. The use of a static recirculation system can limit the basis on which the fuel cell can be operated and can only exist for fixed applications.

Test Results for Fuel Cells with Nafion Electrolyte Membranes

The electro-oxidation kinetics of methanol for sulfuric acid electrolytes and for Nafion electrolytes were studied by constant current polarization measurements of electrochemical cells (similar to the electro-deposition cells that were not described but described in FIG. 10 below). The battery consists of a working electrode, a platinum counter electrode and a reference electrode. The working electrode is polarized in a solution containing the selected electrolyte and liquid fuel. The potential of the working electrode to the reference electrode is monitored.

FIG. 3 shows the polarization curve, ie the current density expressed in polarization to milliampere / cm ² (mA / cm ² ) for the reaction rate of methanol oxidation in Nafion and sulfuric acid electrolytes, curve 41 is a 0.5M sulfuric acid electrolyte. Polarization for, curve 43 represents polarization for the Nafion electrode. Polarization is represented by potential versus NHE, where NHE represents a normalized hydrogen electrode. The curves show the measured data for a fuel consisting of a 1M mixture of methanol in water at 60 ° C. As can be seen from FIG. 3, the polarization loss is lower when the electrode contacts Nafion than sulfuric acid. Therefore, it can be concluded that the reaction rate of the electrooxidation of methanol is more useful when the electrolyte is Nafion. These observations can be explained by the fact that strong absorption of sulfate ions occurs at the electrode / sulfuric acid interface at both potentials which impedes the rate of reaction of electro-oxidation. This absorption does not occur because such ions are not produced when Nafion is used as an electrolyte. It is also believed that the reaction rate of the electrical reduction of oxygen or air is increased at the electrode / nafion interface as compared to the electrode / sulfuric acid interface. This lateral effect may be due to the high solubility of oxygen in Nafion and the absence of strongly absorbed anions. Thus, the use of quantum-conducting solid polymer membranes as electrolytes is advantageous for the reaction rate of electrode reactions and overcomes the disadvantages of sulfuric acid electrolytes.

In addition, the sulfuric acid electrolyte decomposes at a temperature of 80 ° C or higher. The degraded product can reduce the performance of the individual electrodes. The electrochemical and thermal stability of solid polymer electrolytes such as Nafion are significantly higher than in sulfuric acid, and solid polymer electrolytes can be used at temperatures as high as 120 ° C. Thus, the use of quantum-conducting solid polymer membranes allows long-term operation of the fuel cell at temperatures as high as 120 ° C., and the reaction rates of the electro-oxidation of the fuel and the electro-reduction of oxygen occur more conveniently as the concentration increases. It provides additional advantages.

4 shows the performance of the fuel cell shown in FIG. 2 when the methanol oxygen bond and the methanol / air bond are operated at 65 ° C. In FIG. 4, the voltage of the fuel cell is shown along axis 32 and the current density of mA / cm ² is along axis 34. Curve 36 represents the performance of the methanol / oxygen bond while curve 38 represents the methanol / air bond. As can be seen, the use of pure oxygen provides slightly better performance than air.

5 shows the effect of fuel concentration on cell performance. The fuel cell potential is shown along axis 40 while the current density of mA / cm ² is shown along axis 42. Curve 44 shows the performance for a 2.0 molar methanol solution at 150 degrees Fahrenheit. Curve 46 shows the performance for a 0.5 mole methanol mixture at 140 degrees Fahrenheit. Curve 48 shows the performance for 4.0 moles of methanol mixture at 160 degrees Fahrenheit. As can be seen, 2.0 moles of methanol mixture provide the best overall performance. 5 also shows that the fuel cell can maintain a current density as high as 300 mA / cm ² while at the same time maintaining an ideally high voltage. In particular, a 2.0 mole methanol mixture provides a high voltage of 0.4 volts or more at nearly 300 mA / cm ² . The performance shown in FIG. 5 represents an important improvement over the performance of previous organic cells.

The polarization form of the anode and cathode of the fuel cell is shown as a function of current density in mA / cm ² , with the voltage shown along axis 50 and the current density shown along axis 52. Curve 54 shows the polarization pattern for a 2.0 mole mixture at 150 degrees Fahrenheit. Curve 56 represents the form of polarization for fuel and curve 58 represents the form of polarization for oxygen.

Anode Structure for Liquid-Supply Fuel Cells

The anode structure of the liquid supply fuel cell should be completely different from that of the conventional fuel cell. Conventional fuel cells use gas diffusion electrode structures that can provide gas, liquid and solid equilibrium. However, liquid supply fuel cells require an anode structure similar to a battery. The anode structure must be porous and able to wet the liquid fuel. In addition, the structure must have both electrical and ionic conductivity to effectively transfer electrons to the anode current collector (carbon paper) and hydrogen / hydronium ions to the Nafion electrolyte membrane. Moreover, the anode structure should help the useful gas to characterize the anode. The electrodes required for a liquid supply fuel cell can be specially manufactured and also conventionally useful gas diffusion electrodes can be modified with suitable additives.

Electrode Impregnation with Ionomer Additives

The electrocatalyst layer and carbon fiber support of anode 14 (FIG. 1) are preferably impregnated with a hydrophilic quantum conductive polymer additive such as Nafion. This additive is provided in part in the anode to effectively deliver protons and hydronium produced by the electrooxidation reaction. The ionomer additive also promotes uniform wetting of the electrode pores with liquid fuel / aqueous solution and provides for improved availability of the electrocatalyst. By reducing the absorption of anions, the reaction rate of methanol electrooxidation is also improved. Moreover, the use of ionomer additives makes it possible to achieve exerting useful gas generating properties for the anode.

In order for the anode additive to be effective, it must be hydrophilic, proton-conductive, electrochemically stable, and should not interfere with the reaction rate of liquid fuel oxidation. Nafion is a preferred anode additive that satisfies these conditions. Hydrophilic proton conductive additives which are expected to have the same effect as Nafion are montmorolinite clays, zeolites, alkoxycelluloses, cyclodextrins and zirconium hydrogen phosphates.

7 is a block diagram showing the steps involved in impregnation of an anode with an ionomer additive such as Nafion. Initially, a carbon electrode structure is obtained or prepared. Commercially available high surface area carbon electrode structures using mixtures of high surface area electrocatalysts and Teflon binders applied on toray carbon fiber paper can be used. Electrocatalytic electrodes can be made from high surface area catalyst particles and toray paper using TFE-30 ^™ , an emulsion of polytetrafluoroethylene, both of which are sold by E-Tek Incorporated. Although these structures can be made from the constituent materials described above, prefabricated structures can be purchased directly from E-Tech Inc. in any size.

In step 302, the electrode is suitably in a solution containing 0.5-5% of an ionomer additive (Aldrich Chemical Co. or Methanol or isopropanol solution supplied by Solution Technologies Inc.). Dilution) to immerse the electrocatalyst particles for 5 to 10 minutes with ionomer additives such as Nafion. In step 304, the electrode is removed from the solution and then dried under air and vacuum at a temperature range of 20-60 ° C. to volatilize any higher alcohol residues associated with the Nafion solution. These impregnation steps 302-304 are repeated until the desired composition (weight of the electrode catalyst ranges from 2 to 10%) is achieved. A loading of 0.1 to 0.5 mg / cm ² is suitable. Electrode compositions with additives greater than 10% result in an increase in the internal resistance of the fuel cell and result in poor bonding with the solid polymer electrolyte membrane. Compositions with less than 2% additives typically do not improve electrode performance.

To form the electrode impregnated in the electrode catalyst particles, the electrode catalyst particles are mixed with a Nafion solution diluted to 1% with isopropanol. The solvent is then evaporated until it is a concentrated mixture. The concentrated mixture is then applied to the toray paper to form a thin layer of electrode catalyst. A mixture of about 200 m ² / g high surface area particles applied to a toray paper is typical. It should be noted here that the electrocatalyst layer thus formed has only Nafion, not Teflon. The electrode thus prepared is then dried in vacuo at 60 ° C. for 1 hour to remove higher alcohol residues and then ready for use in the liquid feed cell.

Commercially available high surface area platinum-tin electrodes are impregnated with Nafion according to the procedure described above. FIG. 8 compares the performance of Nafion-impregnated and non-impregnated electrodes in a half cell similar to the cell of FIG. 10 except that it contains sulfuric acid electrolyte. In particular, FIG. 8 shows the polarization measurement in a liquid formaldehyde fuel (1 mol) with sulfuric acid electrolyte (0.5 M). The current density (mA / cm ² ) is shown side by side on axis 306 and the potential of the voltage is shown side by side on axis 308. Curve 310 is a galvanostatic polarization curve of a platinum-tin electrode that does not contain Nafion. Curve 312 is a constant current polarization curve of the platinum-tin electrode that is not impregnated with Nafion.

As can be seen in FIG. 8, a larger current density can be obtained with an electrode impregnated with Nafion than an electrode without impregnation. In fact, the oxidation of formaldehyde hardly occurs with an electrode that is not impregnated. Thus, the addition of Nafion provides a dramatic improvement. In addition, the absence of any hystersis in the constant current polarization curve indicates that these coatings are stable.

Thus, what is described so far is an improved liquid feed fuel cell anode impregnated with an ionomer additive. A method of making an anode comprising an ionomer additive has also been described. Hereinafter, the use of perfluorooctanesulfonic acid as an additive in the electrodeposition tank used to fabricate the electrode and as a direct additive in the fuel is described. It also explains the new fuel.

Electrodeposition of Electrodes Using Perfluorooctane Sulfonic Acid Additives

9 to 11, an electrode fabrication method for use in an organic fuel cell will be described in detail below. This method is advantageously used to fabricate the cathode used in the liquid organic fuel cell described above. However, the electrodes produced by the method of FIGS. 9-11 can optionally be used in various organic fuel cells.

With reference to the first of FIG. 9, the steps of the method of manufacturing the anode will be described. In step 200, the carbon electrode structure is prepared by applying a mixture of high surface area carbon particles and Teflon binder to fibrous carbon paper. Preferably the carbon particles have a surface area of 200 m ² / g. Suitable carbon particle materials, named Vulcan XC-72, can be obtained from E-Tek Incorporated. Teflon binder is preferably added in an amount of 15% by weight. The fibrous carbon paper is preferably a toray paper which can also be obtained from E-Tec Incorporated. The carbon structure can be made from the constituent material. Optionally commercially available prefabricated structures are obtained directly from E-Tec Incorporated in the form of squares of 2 inches by 2 inches.

In step 202, an evaporation bath is prepared by dissolving hydrogen hexachloropaltinate (IV) and pentachloroaquaruthenium potassium (III) in sulfuric acid. Preferably, the resulting metal ion concentration is in the range of 0.01 to 0.05M. Also preferably, sulfuric acid has a concentration of 1M. The compound is used to obtain platinum-ruthenium deposition on carbon electrode structures. Alternatively a solution may be used. For example, to obtain platinum-tin deposition, tin chloride compounds are dissolved in sulfuric acid instead.

Metal ion salts are primarily dissolved in sulfuric acid to prevent hydrolysis of the solution. In ruthenium deposition, the resulting solution is preferably de-aerated to prevent the formation of higher oxidation states.

High purity perfluorooctanesulfonic acid (C-8 acid) is added to the bath in step 204. C-8 acid is added at a concentration ranging from 0.1 to 1.0 g / l. C-8 acid is provided to promote complete wetting of the carbon particles. C-8 acid is provided to promote complete wetting of the carbon particles. C-8 acids are electro-inert and do not adsorb specifically at metal sites in the structure. Thus, C-8 acid is not harmful to the subsequent electrodeposition process. The addition of C-8 acid has been found to be very beneficial and is probably necessary for successful electrodeposition on the electrode.

In step 206, the carbon electrode structure obtained from step 200 is located in the electrodeposition tank obtained from step 204. A platinum anode is also located within the bath. Another anode material may be used for the deposition of other metal ions.

The voltage is applied in step 208 between the carbon electrode structure and the platinum anode. The voltage is applied for about 5 to 10 minutes to obtain electrodeposition of platinum-ruthenium with a loading of about 5 mg / cm ^{2 to} the carbon electrode. Preferably, a voltage of approximately −0.8 V is applied to the mercury sulfate reference electrode.

After a certain amount of metal is deposited on the carbon electrode, the electrode is removed in step 210 and washed with deionized water. Preferably, the electrode is rinsed at least three times in deionized water for 15 minutes each time. The washing step is primarily provided to remove adsorbed chlorine and sulfate ions from the surface of the carbon electrode. The cleaning step has been found to be very beneficial and is probably necessary to obtain an effective electrode for use in organic fuel cells.

The electrode obtained from the manufacturing method of step 206 was found to have very uniform "cotton-ball" shaped particles with a significant amount of microstructure. It has been found that the average particle size can be similar to 0.1 micron.

9 is a deposition apparatus for use in performing the method is shown in FIG. 10. In particular, FIG. 10 shows three electrode cells 212 including a single carbon-structure electrode 214, a pair of platinum counter-electrodes (or anodes) 216, and a reference electrode 218. All electrodes are located in bath 220 formed of the metal / C-8 acid solution. Electrical contacts 222 and 224 are located on the inner side of the cell 212 above the bath 220. A magnetic stirrer 226 is positioned in the bath 220 to facilitate stirring and circulation of the bath. The circulating jacket 228 is provided around the cell 212 which is used to regulate the temperature in the cell. The platinum anode is located within the fine glass frits 230, where the glass frit is provided to separate the anode from the cathode to prevent diffusion of the oxidation product of the anode into the cathode.

The reference electrode 214 is a mercury / sulfuric acid first mercury reference electrode. The reference electrode is provided for monitoring and adjusting the electrical potential of the carbon electrode structure 214. Preferably both potential and constant current regulation methods are used. The composition of the alloy deposition is controlled by selecting a bath composition as outlined above and by performing electrodeposition at a current density greater than the limited current density for metal deposition. When choosing an appropriate bath composition, it is important to normalize the electrochemical equivalence of the metals in the composition.

During operation, the amount of charge passed from the anode to the cathode is used to monitor the amount of detected and deposited material. In this regard, the amount of charge used in any hydrogen evolution reaction is subtracted from each measurement of the total charge.

Proper electrodeposition typically occurs for 5 to 10 minutes depending on the operating conditions and the required catalyst loading.

The monitoring facility used to monitor and adjust the electrode potential is not shown in FIG. 10, but those skilled in the art will be familiar with the function and operation of such a device.

FIG. 11 shows the performance of electrodes deposited using the method of FIG. 9 in the electrodeposition cell of FIG. 7. In FIG. 11, the potential, expressed as voltage versus NHE, is provided alongside axis 240, while current density (mA / cm ² ) is provided alongside axis 242. Curve 246 shows a constant current polarization curve for a carbon-supported platinum-ruthenium alloy electrode made with a loading of 5 mg / cm ² . Curve 244 shows a constant current polarization curve for the electrode with 1 mg / cm 2 loading. In each case, the electrode is used in sulfuric acid electrolyte in the half cell. The fuel cell comprises an organic fuel consisting of 1 mole methanol and 0.5 mole sulfuric acid and is operated at 60 ° C. At a loading of 5 mg / cm ² , the electrode maintains a continuous current density of 100 mA / cm ² at 0.45 volts versus NHE.

The results shown in FIG. 11 show examples of performance that can be obtained using electrodes fabricated according to the method of FIG. 9. In addition, performance enhancement can be obtained by optimizing the electrodeposition conditions and alloy composition. Thus, the specific conditions and concentrations described above are not required for optimization, but represent the best way presently known for fabricating electrodes.

Perfluorooctane sulfonic acid (C-8 acid) as fuel additive

The use of C-8 acid as an additive in an electrolytic bath has been described above. It has been determined that C-8 acid can be preferably applied as an additive in the fuel of a liquid feed fuel cell using sulfuric acid electrolyte. In particular, it has been found that linear C-8 acids with a concentration of 0.001 to 0.1 M, having a molecular formula of C ₈ F ₁₇ SO ₃ H, are good wetting agents in liquid feed fuel cells.

12 shows the results of experiments comparing the use of C-8 acid as an additive with a fuel cell lacking an additive. In particular, FIG. 12 shows the results of a half cell experiment using Teflon coated high surface area carbon-supported platinum and platinum alloy electrodes installed in sulfuric acid electrolyte. The results are obtained using a half cell similar to the cell shown in FIG. FIG. 12 shows the potential parallel to the vertical axis 400 versus the current density (mA / cm ² ) parallel to the NHE and the horizontal axis 402. Four curves show fuels without additives (curve 404), fuels with 0.0001M additive (curve 406), fuels with 0.001M additive (curve 408), and fuels with 0.01M additive (curve 412). To provide polarized polarization.

As can be seen from FIG. 12, the addition of C-8 additive slightly reduces the polarization. Although not shown, oxidation of methanol was investigated using 0.1M pure C-8 solution without sulfuric acid. Polarization curves (not shown) indicate that the reaction rate is not affected by the presence of perfluorooctane sulfonic acid ions.

Thus, Figure 12 shows that the use of C-8 acid at a concentration of 0.001 M or more as an additive is beneficial to liquid fuel solutions when using commercially available Teflon coated fuel cell electrodes for fuel cells using sulfuric acid as at least an electrolyte. Shows that

Referring to the remaining figures, three new fuels used in the liquid supply fuel cell will be described. Fuels are dimethoxymethane, trimethoxymethane and trioxane.

Dimethoxymethane as Fuel for Liquid-Supply Fuel Cells

13-15 show the results of experiments conducted using dimethoxymethane (DMM) as fuel for organic direct liquid feed fuel cells. In use, the DMM is mixed with water at a concentration in the range of 0.1 to 2M and fed to the fuel cell. Other concentrations may also be effective. The fuel cell may be of conventional design or may include one or more of the improvements described above. In fuel cells, the DMM is electrooxidized at the anode of the cell. Electrooxidation of the DMM involves a series of decomposition steps followed by surface reactions to form carbon dioxide and water. The electrochemical reaction is as follows.

_{(CH 3 O) 2 CH 2} + 4H 2 O ⇒ CO 2 + 16H + + 16e -

An experiment for testing the electrooxidation of the DMM was carried out with a half cell similar to that shown in FIG. 10, with temperature control using a 0.5 M sulfuric acid electrolyte with a Pt-Sn or Pt-Ru electrocatalyst electrode. The constant current polarization curves shown in FIG. 13 show the electro-oxidation characteristics of the DMM for platinum-tin electrodes for several different fuel concentrations. The platinum-tin electrode is a gas-diffusion type consisting of 0.5 mg / cm ² of total metal supported on Vulcan XC-72 obtained from E-Tec Incorporated, Framingham, Massachusetts. In FIG. 13, current densities appear side by side on axis 500, and polarization (expressed in terms of potential vs. NHE) is provided side by side on axis 502. Curves 504, 506, 508 and 510 each represent polarization for DMM concentrations of 0.1M, 0.5M, 1M and 2M. 13 shows that the increased concentration improves the oxidation kinetics of the DMM. The curve in FIG. 13 was measured in a half cell using 0.5M sulfuric acid as electrolyte with 0.1M C-8 acid. The measurement was carried out at room temperature.

It has been found that DMM can be oxidized at potentials that are significantly more negative than methanol. It has also been found that temperature significantly affects the rate of oxidation. However, the DMM has a low boiling point of 41 ° C. Thus, there are difficulties in attempting to use DMMs in liquid supply fuel cells for temperatures higher than the boiling point.

14 shows polarization for two different concentrations at two different temperatures. Current density is parallel to axis 512, and polarization (expressed as potential versus NHE) is provided along axis 514. Curve 516 shows the polarization of the DMM 1M concentration at room temperature. Curve 518 shows the polarization for 2M concentration of DMM at 55 ° C. As can be seen, improved polarization is obtained using high concentrations at high temperatures. In addition, the comparison of the curve 510 of FIG. 13 with the curve 518 of FIG. 14 shows that the increase in temperature results in improved polarization for the same concentration level. Thus, it is believed that an increase in temperature results in an improved reaction rate of electro oxidation.

In addition to the half cell experiments shown in FIGS. 13 and 14, fuel cell experiments were conducted with varying the efficiency of the DMM in the fuel cell. Direct oxidation of the DMM in the fuel cell was performed in a liquid feed type fuel cell as shown in FIGS. 1 and 2. Therefore, the fuel cell used a quantum conductive solid polymer membrane (Nafion 117) as the electrolyte. The membrane electrode assembly consisting of a fuel oxide electrode is made of an unsupported platinum-ruthenium catalyst layer (4 mg / cm ² ) and a gas-diffusion type unsupported platinum electrode (4 mg / cm ² ) for the reduction of oxygen. The fuel cell uses 1 M DMM solution on the fuel oxide side and oxygen at 20 psi on the cathode.

Analysis of the oxidation product of the DMM shows only methanol. Methanol is considered a possible intermediate for the oxidation of DMM to carbon dioxide and water. However, because the fuel cell system is compatible with methanol, the presence of methanol as an intermediate is not considered because methanol is ultimately oxidized to carbon dioxide and water.

The current-voltage characteristics of a liquid feed direct oxidation fuel cell using DMM as fuel is shown in FIG. 15. The fuel cell was operated at 37 ° C. In FIG. 15, the current density (mA / cm ² ) is provided along the axis 520 and the cell voltage (volts) is provided along the axis 522. Curve 524 represents the cell voltage as a relationship of current density for the 1M DMM solution described above. As shown from FIG. 15, the cell voltage reached 0.25 V at 50 mA / cm ² using a DMM, which was as high as that obtained using methanol (not shown). Higher performance can be achieved when operating at high temperatures and using Pt-Sn catalysts. The low boiling point of the DMM can be used in gas-fed type operation.

Thus, from half cell and full cell measurements, it was found that the DMM can be oxidized at very high rates. Therefore, DMM is considered to be an excellent fuel used in direct oxidizing fuel cells. In addition, DMMs are non-toxic, low vapor pressure liquids, and are easy to handle. In addition, DMM can be synthesized from natural gas (methane) by conventional techniques.

Trimethoxymethane as Fuel for Liquid-Supply Fuel Cells

16-18 show the results of experiments conducted using trimethoxymethane (TMM) as fuel for organic direct liquid feed fuel cells. As in the DMM described above, in use, the TMM is mixed with water in a concentration range of 0.1 to 2 M and injected into the fuel cell. Other concentrations can also be effective. The fuel cell can be of conventional design or can include one or more of the improvements described above. Within a fuel cell, the TMM is electrooxidized at the cell's anode. The electrochemical oxidation of TMM can be represented by the following scheme.

_{(CH 3 O) 3 CH +} 5H 2 O ⇒ 4CO 2 + 2OH + + 2Oe -

Experiments varying the electrooxidation of TMM were performed in a half-cell similar to the cell shown in FIG. 10 using a Pt-Sn electrode with 0.5 M sulfuric acid electrolyte with temperature control and 0.01 M C-8 acid. The results of these half cell experiments are shown in FIGS. 16 and 17.

16 provides constant current polarization curves for different concentrations of the TMM for Pt-Sn electrodes. The Pt-Sn electrode is a gas-diffusion type and consists of 0.5 mg / cm ² of total metal supported on Vulcan XC-72 obtained from E-Tec Incorporated, Framingham, Massachusetts. In FIG. 16, the current density (mA / cm ² ) is along the axis 600 and the polarization (potential vs. NHE) is provided alongside the axis 602. Curves 604, 606, 608 and 610 show polarizations for TMMs at concentrations of 0.1M, 0.5M, 1M and 2M, respectively. 16 shows that improved polarization can be obtained at high concentration levels. All measurements shown in FIG. 16 were taken at room temperature.

It has been found that TMM can be oxidized at potentials more negative than methanol. It has also been found that temperature affects the oxidation rate of the TMM. 17 shows polarization at two different concentrations and at two different temperatures. In FIG. 17, current density (mA / cm ² ) is provided side by side on axis 612, and polarization (expressed as potential vs. NHE) is provided side by side on axis 614. Curve 616 shows polarization for 1M TMM at room temperature, while curve 618 shows polarization for 2M concentration of TMM at 55 ° C. The curve in FIG. 17 is obtained using a Pt-Sn electrode in a 0.5 M sulfuric acid electrolyte containing 0.01 M C-8 acid. As can be seen, improved polarization is obtained using high concentrations at high temperatures. Comparison of the curve 618 of FIG. 17 with the curve 610 of FIG. 16 shows that an increase in temperature improves performance at the same concentration level. Although not shown, it was found that at temperatures as high as 60 ° C., the oxidation rate of the TMM was twice that at 25 ° C.

In addition to the half cell experiments shown in FIGS. 16 and 17, complete fuel cell experiments were performed with varying the efficiency of the TMM in the fuel cell. Direct oxidation of the TMM in the fuel cell was performed with a liquid feed type fuel cell of the type shown in FIGS. 1 and 2 above. Therefore, the fuel cell used a quantum conductive solid polymer membrane (Nafion 117) as the electrolyte. The membrane electrode assembly of the fuel cell included an unsupported platinum-ruthenium catalyst layer (4 mg / cm ² ) and a gas-diffusion type unsupported platinum electrode (4 mg / cm ² ) for the reduction of oxygen. The fuel cell used oxygen at 2 psi TMM solution on the fuel oxide side and 20 psi on the cathode.

As in the DMM, analysis of the oxidation product of TMM shows only methanol, which is considered a possible intermediate for the TMM to be oxidized to carbon dioxide and water. In fuel cells that are compatible with methanol, the presence of methanol as an intermediate product is not considered because methanol is ultimately oxidized to carbon dioxide and water.

The current-voltage characteristics of the liquid feed direct oxidation fuel cell described above are shown in FIG. 18 for both TMM and methanol. Current density (mA / cm ² ) is provided alongside axis 620, and current voltage is provided alongside axis 622. Curve 624 represents cell voltage as a relationship of current density to TMM 1M concentration. Curve 626 represents the cell voltage as a relationship of current density to methanol 1M concentration. The measurement shown in FIG. 18 was obtained at 65 degreeC. Although not shown, the cell voltage using TMM at 90 ° C. can reach 0.52 V at 300 mA / cm ² , which is higher than that obtained with methanol.

Thus, from both half-cell and full-cell measurements, it has been found that TMMs similar to DMMs can be oxidized at very high rates. In addition, like DMMs, TMMs are nontoxic, low vapor pressure liquids, easy to handle, and can be synthesized from natural gas (methane) by conventional methods.

Trioxane as fuel for liquid supply fuel cells

19-21 show the results of experiments conducted using trioxane as fuel for organic direct liquid feed fuel cells. Like the DMMs and TMMs described above, in use, trioxane is mixed with water at a concentration ranging from about 0.1 to 2M and fed to the fuel cell. Other concentrations may also be effective. The fuel cell can be of conventional design or can include one or more of the improvements described above. Within the fuel cell, the trioxane is electrooxidized at the anode of the cell. The electrochemical oxidation of trioxane can be represented by the following scheme.

_{(CH 3 O) 3 + 6H} 2 O ⇒ 3CO 2 + 12H + + 12e -

Experiments varying the electrooxidation of trioxane were performed in a half cell similar to the cell shown in FIG. 10 using a Pt-Sn electrode with 0.5M to 2.0M sulfuric acid electrolyte with temperature control and 0.01M C-8 acid. . The results of these half cell experiments are shown in FIGS. 19 and 20.

19 provides constant current polarization curves for different concentrations of trioxane for the Pt-Sn electrode. The Pt-Sn electrode is a gas-diffusion type and consists of 0.5 mg / cm ² of total precious metal supported on Vulcan XC-72 obtained from E-Tek Incorporated of Framingham, Massachusetts. In FIG. 19, the current density (mA / cm ² ) is provided along the axis 700, and the polarization (potential band NHE) is provided alongside the axis 702. Curves 704, 706, 708 and 710 show polarizations for trioxane at concentrations of 0.1M, 0.5M, 1M and 2M, respectively. 19 shows that improved polarization can be obtained at high concentration levels. All measurements shown in FIG. 19 were taken at 55 ° C.

Thus, for trioxane, increasing the fuel concentration increases the rate of oxidation. Furthermore, as can be seen from FIG. 19, a current density as high as 100 mA / cm ² is obtained at a potential of 0.4 V vs. NHE. This performance can be compared to the performance achieved with formaldehyde. Although not shown, it was determined through cyclic voltammetry that the mechanism of trioxane oxidation did not include the cleavage of formaldehyde prior to electrooxidation.

Increasing the acid concentration of the electrolyte has also been found to increase the rate of electrooxidation. 20 shows polarization at four different electrolyte concentrations and two different temperatures. In FIG. 20, current densities (mA / cm ² ) are provided alongside axis 712 and polarization (expressed as potential versus NHE) is provided alongside axis 714. Curve 716 shows polarization for 0.5M electrolyte concentration at room temperature. Curve 718 shows polarization for 0.5M electrolyte concentration at 65 ° C. Curve 720 shows polarization for 1M electrolyte concentration at 65 ° C. Finally, curve 722 shows the polarization for 2M electrolyte concentration at 65 ° C. Trioxane concentrations in both curves 716-722 are 2M.

The curve of FIG. 20 was obtained using a Pt-Sn electrode in a sulfuric acid electrolyte containing 0.01 M C-8 acid. As can be seen, improved polarization is obtained using high concentrations at high temperatures. Therefore, it was suggested that Nafion can be expected as an electrolyte because the very high rate of electrooxidation shows Nafion to have an acidity equivalent to 10M sulfuric acid.

In addition to the half cell experiments shown in FIGS. 19 and 20, complete fuel cell experiments were also conducted with varying efficiency of trioxane in the fuel cell. Direct oxidation of trioxane in the fuel cell was performed with a liquid feed type fuel cell of the type shown in FIGS. 1 and 2. Therefore, the fuel cell uses a quantum conductive solid polymer membrane (Nafion 117) as the electrolyte. The membrane electrode assembly of the fuel cell comprises an unsupported platinum-ruthenium catalyst layer (4 mg / cm ² ) and a gas-diffusion type unsupported platinum electrode (4 mg / cm ² ) for the reduction of oxygen. The fuel cell used 1 M trioxane solution on the fuel oxide side and oxygen at 20 psi on the cathode.

As in DMM and TMM, only methanol is shown in the analysis of the oxidation product of trioxane and methanol is considered a possible intermediate for trioxane to be oxidized to carbon dioxide and water. In fuel cells compatible with methanol, the presence of methanol as an intermediate product is not considered because methanol is ultimately oxidized to carbon dioxide and water.

The current-voltage characteristics of the liquid feed direct oxidation fuel cell described above are shown in FIG. 21 for trioxane. Current density (mA / cm ² ) is provided alongside axis 724, and current voltage is provided alongside axis 726. Curve 728 represents the cell voltage as a relationship of current density to trioxane 1M concentration. The measurement shown in FIG. 21 was obtained at 60 ° C. The performance shown in FIG. 21 can be significantly improved by using platinum-tin electrodes rather than Pt-Ru electrodes.

Although not shown, cross measurements in trioxane / oxygen fuel cells suggested that the cross rate was at least five times lower than methanol fuel cells. The reduced rate of crossover is highly desirable because crossover affects the efficiency and performance of the fuel cell as described above.

Thus, from both half-cell and full-cell measurements, trioxanes similar to DMM and TMM can be oxidized at very high rates.

The methods, embodiments, and experimental results shown herein are merely examples of the present invention, and do not limit the scope of the present invention.

Described are several improvements to liquid feed fuel cells, including improved electrolyte and electrode structure, improved methods of fabricating electrodes, additives to improve fuel performance, and a set of three new fuels. These various improvements can be met individually or the best parts can be combined to obtain better performance. However, it can be expected that the use of C-8 acid in the fuels described above may be effective only for fuel cells using acid electrolytes such as sulfuric acid, and may not be effective when using fuel cells using proton exchange membranes. It should be noted.

Claims (4)

A liquid supply fuel cell comprising: adding perfluorooctane sulfonic acid having a concentration of at least 0.0001 molar concentration to a fuel of the fuel cell. delete The method of claim 1, Wherein said perfluorooctane sulfonic acid is in the range of 0.0001 mol to 0.01 mol concentration. Anode; Cathode; Liquid organic fuels, water, acid electrolytes and perfluorooctanesulfonic acid additive solutions; Means for flowing the solution past the anode; And Means for flowing oxygen past said cathode.