US20060016122A1

US20060016122A1 - Performance additive for fuel cells

Info

Publication number: US20060016122A1
Application number: US11/029,964
Authority: US
Inventors: Hongli Dai; Joe Druliner; Mark Scialdone; Gary Johansson
Original assignee: Individual
Current assignee: EIDP Inc
Priority date: 2004-01-08
Filing date: 2005-01-05
Publication date: 2006-01-26
Also published as: WO2005081349A3; TW200534529A; WO2005081349A2

Abstract

A fuel mixture comprising at least one performance additive, wherein the performance additive comprises a cyclic tertiary amine having a molecular weight of at least about 200. Alternately, the performance additive may be present in the electrode composition. The performance additive is capable of improving the electrooxidation reaction rate of the fuel mixture, measured as current density, by at least 2% compared to a fuel mixture not containing the performance additive.

Description

FIELD OF THE INVENTION

This invention relates to additives for fuel cells, and in particular performance additives that improve power density of polymer electrolyte membrane (PEM) fuel cells.

BACKGROUND OF THE INVENTION

Electrocatalysts are known for oxidation of a variety of fuels such as hydrogen, reformate, methanol, ethanol, propanol, ethylene glycol, formaldehyde and formic acid etc. High power density fuel cells require electrocatalysts with high electrocatalytic activity. Various electrocatalyst compositions for polymer electrolyte membrane (PEM) fuel cells are known in the art, most of them contain expensive noble metal Pt. Therefore, there is a strong economic driver towards improving activity of the electrocatalyst. As an example, for methanol electro-oxidation, bimetallic electrocatalysts Pt/Ru have been developed to replace pure Pt electrocatalyst because Pt/Ru is more active. As seen in H. Gasteiger, Journal of Physical Chemistry, 97, 12020-12029,1993 and A. Crown, Surface Science, 506, L268-L274, 2002 various optimizations of the atomic composition, structure and particle morphology of Pt/Ru are being developed to further improve its methanol electro-oxidation activity.
To reduce costs, electrocatalysts that do not contain noble metals are also known in the art and might be applied to PEM fuel cells. These include metal carbides, metal borides and organometallic complexes such as iron and colbalt phthalocyanines and porphyrins. However, these non-noble metal electrocatalysts suffer low activity, usually orders of magnitude lower than that of noble metal electrocatalysts.
The present invention deals with organic additives that can promote the activity of electrocatalysts for PEM fuel cells. Many organic compounds are known electrocatalyst inhibitors or even poisons, notably sulfur containing compounds and most aromatic compounds. The absorption of these organic compounds on metal surfaces tends to block electrocatalyst active sites. In contrast, organic additives that promote electrocatalytic reactions are rare. H. Saffarian et. al. (Proceedings of Power Sources Conference, vol 39, page 116-119 (2000)) reported that some alkyl derivatives of uracil may be able to promote the oxygen electrochemical reduction reaction rate in acidic solutions. J. S. Bett et. al. (Electrochimica Acta, Vol. 43, No. 24, pp 3645-3655, 1998) and R. Venkataraman (Journal of Electrochemical Society, 151(5),A703-A709, 2004) reported that Ru and tetraaza-macrocycle complexes were able to promote the activity of Pt towards methanol electrochemical oxidation. However, the activity of the promoted Pt catalysts is still lower than that of Pt/Ru bimetallic catalyst.
For fuel cell electrocatalysts and in particular for electrocatalytic oxidation of small hydrocarbon molecules, there is a need to make the electrocatalyst as active as possible.

SUMMARY OF THE INVENTION

In the first aspect, the invention provides a fuel mixture comprising at least one said performance additive, wherein the performance additive comprises a cyclic tertiary amine having a molecular weight of at least about 200.
In the first aspect, the invention further provides a fuel mixture comprising a performance additive capable of improving the electrooxidation reaction rate of the fuel mixture, measured as current density, by at least 2% compared to a fuel mixture not containing the performance additive.
In the first aspect, the invention further provides a cyclic tertiary amine comprising saturated monocyclic, bicyclic or polycyclic rings containing at least one tertiary N atom; wherein the cyclic tertiary amine contains one or more atoms selected from C, N, O and Si, and wherein the number of ring atoms ranges from 3 to 18.
In the first aspect, the invention further provides a cyclic tertiary amine comprising at least two saturated cyclic amine rings, wherein the rings are linked to one another by means of linking groups selected from the group of linear or cyclic alkylidene, oxaalkylidene, polyoxaalkylidene, alkoxylated alkylidene and alkylidenylaryl groups having 1 to 100 carbon atoms.
In the first aspect, the invention further provides a cyclic tertiary amine comprising at least two saturated cyclic amine rings attached to polymeric cores by means of side chains.
In the first aspect, the invention further provides a cyclic tertiary amine, wherein the cyclic tertiary amine is an end group of a dendrimer. U.S. Pat. No. 4,507,466 defines dendrimers as a novel class of branched polymers containing dendritic branches having functional groups uniformly distributed on the periphery of such branches. Some suitable dendrimers include those disclosed in U.S. Pat. No. 4,507,466, which is incorporated herein by reference.
In the second aspect, the invention provides a fuel supply for supplying fuel to the anode of a fuel cell, the fuel supply supplying fuel comprising at least one performance additive.
In the third aspect, the invention provides an electrode composition comprising an electrocatalyst and at least one performance additive, wherein the performance additive comprises a cyclic tertiary amine having a molecular weight of at least about 200.
In a fourth aspect, the invention provides a membrane electrode assembly comprising an electrode composition, wherein the electrode composition comprises an electrocatalyst and at least one performance additive, wherein the performance additive comprises a cyclic tertiary amine having a molecular weight of at least about 200.
The membrane electrode assembly further comprises a solid polymer electrolyte membrane and at least one gas diffusion backing.
In a fifth aspect, the invention provides an electrochemical cell, such as a direct methanol fuel cell, comprising an anode and a cathode, a membrane comprising ionomer having ion-exchange groups separating the anode and cathode, and a fuel supply for supplying fuel, such as liquid methanol fuel, to the anode, the fuel supply comprises at least one performance additive.
In a sixth aspect, the invention provides an electrochemical cell, such as a fuel cell, comprising a membrane electrode assembly, wherein the membrane electrode assembly comprises at least one electrode composition, wherein the electrode composition comprises an electrocatalyst and at least one performance additive. The membrane electrode assembly further comprises a solid polymer electrolyte membrane wherein the membrane comprises an ionomer having proton conductive ion-exchange groups separating the anode and cathode. The membrane electrode assembly further comprises at least one gas diffusion backing. The fuel cell further comprises a fuel supply for supplying fuel, such as liquid methanol fuel, to the anode.
In a seventh aspect, the invention provides a process for operating a direct methanol fuel cell comprising an anode and a cathode, a membrane comprising ion-exchange groups separating the anode and cathode, and a fuel supply for supplying liquid methanol fuel to the anode, the process comprising adding at least one performance additive to the methanol fuel during the operation of the fuel cell, wherein the performance additive comprises a cyclic tertiary amine having a molecular weight of at least about 200.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration of a single cell assembly.
FIG. 2 shows the chronoamperometry of MeOH oxidation at 0.4V vs. Normal Hydrogen Electrode (NHE) at 40° C. The starting electrolyte is 2M methanol with 0.05M H2SO4, wherein at 600 sec., 0.15M N-methylmorpholine was injected into the solution, as described in Example 1.
FIG. 3 shows the cyclic voltammogram of 2M MeOH/0.05M H₂SO₄before and after 0.15M N-methylmorpholine was added, as described in Example 1.
FIG. 4 shows the chronoamperometry of MeOH oxidation at 0.4V vs. NHE at 40° C. wherein at 600 sec., 0.15M 4,4′-(Oxydi-2,1-ethanediyl)bismorpholine was injected into the solution, as described in Example 2.
FIG. 5 shows the cyclic voltammogram of 2M MeOH/0.05M H₂SO₄before and after 0.15M 4,4′-(Oxydi-2,1-ethanediyl)bismorpholine was added, as described in Example 2
FIG. 6 shows the current vs. time for a direct methanol fuel cell at a constant voltage of 0.45 V, as described in Example 3, wherein at about the 1800th second, 4,4′-(Oxydi-2,1-ethanediyl)bismorpholine is injected into the fuel compartment of the cell to result in 0.175M 4,4′-(Oxydi-2,1-ethanediyl)bismorpholine in the 2M methanol fuel. Current is seen to increase upon the injection.
FIG. 7 shows the current vs. time for a direct methanol fuel cell at a constant voltage of 0.45 V, as described in Example 3, wherein at about the 1800th second, 4,4′-(Oxydi-2,1-ethanediyl)bismorpholine is injected into the fuel compartment of the cell to result in 0.125M 4,4′(Oxydi-2,1-ethanediyl)bismorpholine in the 2M methanol fuel. Current is seen to increase upon the injection.
FIG. 8 shows the current vs. time for a direct methanol fuel cell at a constant voltage of 0.45 V, as described in Example 3, wherein at about the 1800th second, N-methylmorpholine is injected into the fuel compartment of the cell to result in 0.15M N-methyl morpholine in the 2M methanol fuel. Current is seen to decrease upon the injection.

DETAILED DESCRIPTION OF THE INVENTION

Fuel cells electrochemically oxidize hydrogen or hydrocarbon fuels such as alcohols, aldehydes, formic acid, and methoxy-methane to generate electricity. Among these, H₂is the most reactive fuel and there is little kinetic overpotential when Pt is used as the electrocatalyst. On the other hand, electrooxidation of CO containing reformate and hydrocarbon fuels, such as methanol, suffers from low reaction rate and high reaction overpotential compared to that of H₂. Thus, fuel cells employ hydrocarbon or CO containing reformate have lower power density than H₂fuel cells.
The present invention provides a novel approach that has several surprising and important benefits over the present state of the art. In the present invention, a performance additive herein described when added to the fuel supply or to the electrode structure provides surprising enhancement of fuel cell power density while the performance additive is not substantially decomposed or consumed in the process. While there are many known catalytic and electrocatalyst poisons, there are few known organic additives that promote catalytic reaction. The fact that such an organic performance additive exists for fuel cell oxidation reactions is surprising. When used as a fuel or electrode additive, the performance additive of the present invention is capable of promoting the electrocatalytic reaction therefore providing higher fuel cell power density than that without the additive. It is also surprising that the performance additive of this invention may simultaneously provide lower fuel crossover from the anode side to the cathode side. Given the fact that these improvements come without any increase of noble metal loading and without any complex chemical or physical modification to the electrocatalyst, this is a novel and comparatively low cost way to improve fuel cell performance.
Performance Additive:
The performance additive of this invention comprises a cyclic tertiary amine having a molecular weight of at least about 200. These performance additives are capable of increased electrooxidation reaction rate, measured as current density, by at least 2%, and more typically by 5 to 300% than that achieved without the performance additive. Suitable cyclic tertiary amines comprise saturated monocyclic, bicyclic or polycyclic rings containing at least one tertiary N atom; wherein the cyclic tertiary amine contains one or more atoms selected from C, N, O and Si, and wherein the number of ring atoms ranges from 3 to 18.
In one embodiment, the cyclic tertiary amine may comprise at least two saturated cyclic amine rings, wherein the rings are linked to one another by means of linking groups selected from the group of linear or cyclic alkylidene, oxaalkylidene, polyoxaalkylidene, alkoxylated alkylidene and alkylidenylaryl groups having 1 to 100 carbon atoms.
A representative structure for these cyclic tertiary amines would be:

where X1 and X2 are independently selected from C, N, O, and Si.
Some specific examples include:
In another embodiment, a cyclic tertiary amine comprises at least two saturated cyclic amine rings attached to polymeric cores by means of side chains. A representative structure corresponding to this embodiment would be:

where n=1 to 50000.
In a further embodiment, the cyclic tertiary amine is a part of a dendrimer. U.S. Pat. No. 4,507,466 defines dendrimers as a novel class of branched polymers containing dendritic branches having functional groups uniformly distributed on the periphery of such branches. Some suitable dendrimers include those disclosed in U.S. Pat. No. 4,507,466, which is incorporated herein by reference.
A representative structure corresponding to this embodiment would be a dendrimeric polyether ending with saturated cyclic tertiary amines (Dendrimer I) and a dendrimer based on successive generations of polypropylenimines substituted onto a central diamine, such as ethylenediamine, in which the terminal amine end groups are part of saturated cyclic rings (Dendrimer II).
Typically, the cyclic tertiary amine should not diffuse into the membrane of the fuel cell. To achieve this, the amines may be made with large enough molecular size so as to eliminate or minimize their diffusion into the membrane. A molecular weight of at least 200 is typical. Said amines may be immobilized by means known in the art, including but not limited to grafting onto insoluble substrates such as polymers, metal catalysts, inorganic oxides, and amorphous carbon, carbon nanotubes and carbon nanohorns, etc. Other immobilization means such as chelating and coordination may also be used.
In one aspect of this invention, the performance additive is used as a fuel additive, in which case, the molecular weight of the additive is typically about 200 to about 10,000 and still more typically about 400 to about 5,000, and most typically about 400 to about 2000. The fuel cell utilizes a fuel source that may be in the gas or liquid phase, and may comprise hydrogen or a hydrocarbon. In the case of gaseous fuel, the performance additive in its gases form is mixed with the gaseous fuel. In the case of liquid fuel, the performance additive is dissolved in and mixed with the liquid fuel.
Typically, the performance additive is used as liquid methanol fuel additive. In the present application, “methanol fuel” refers to the fuel in contact with the anode and the membrane. “Fuel mixture” is methanol and the performance additive with or without water. “Fuel supply” is the apparatus for supplying methanol fuel to the anode.
It is desirable that the performance additive selected be soluble in the methanol fuel under the conditions of temperature and concentration at which the methanol fuel is used in the fuel cell. Similarly, for any fuel mixtures to be supplied to the fuel cell which incorporate the performance additive, the performance additive should be soluble in the fuel mixture at the desired concentration.
The concentration of the performance additive in the methanol fuel is at least about 0.001 molar, typically at least about 0.05 molar, and more typically at least about 0.1 molar; no more than about 2.0 molar, typically no more than about 1.0 molar, and more typically no more than about 0.3 molar. The performance additive may advantageously be premixed with the fuel mixture supplied to the fuel cell, in which case concentrations can be in the same preferred ranges as the methanol fuel, or may be added to the fuel mixture during operation of the fuel cell.
The methanol fuel for the fuel cell can be supplied by a fuel supply, which can be a single container, or it may be supplied by a plurality of containers from which feeds are mixed. For example, a three-container system could have separate containers of methanol, water, and performance additive. A two-container system could have separate containers, one for up to 100% aqueous methanol, and the other for pure performance additive. Alternatively, a two-container system could have one for fuel mixture, typically performance additive in pure methanol, the other for methanol fuel, typically aqueous mixture of methanol and performance additive. If water is present, the fuel mixture advantageously has the same percentages of methanol and water discussed above for the methanol fuel. Water generated by the fuel cell can be a source of water for the methanol fuel. Therefore, the fuel supply can be fed from a fuel concentrate, which may contain up to 100% methanol or only methanol and the performance additive. The concentrate is added to the operating fuel cell to keep the methanol fuel in desired concentration range for both methanol and the performance additive.
The performance additive is not consumed at all or is not consumed at the rate at which methanol is consumed in the fuel cells of this invention and therefore, depending upon operating conditions such as temperature and current density, the concentration of the performance additive in the methanol fuel during operation may increase in the anode compartment and/or the fuel reservoir. In this event, the performance additive concentration can be maintained by stop adding the performance additive or reducing the amount of performance additive being added to the methanol fuel. The performance additive may be added to the methanol fuel intermittently when operating conditions indicate a drop in performance. This process can be automated, for example, by monitoring the amount of methanol and water being consumed and using the monitor signal to control a metering system to add performance additive to the methanol fuel as necessary to maintain performance. Alternatively, the performance additive concentration may be adjusted in real time so as to meet the power demand on the fuel cell.
Because the amount of performance additive in the fuel mixture needed for “make up” or to maintain the desired steady-state concentration of performance additive in the methanol fuel may be less than needed at the start of operation, the concentration of performance additive in the fuel mixture of a fuel cell that is in operation may be lower than that at the start of operation.
For portable devices powered by fuel cells designed according to this invention, containers of fuel will be convenient for refueling the cell. The containers can be made from polymer or metal materials suitable for the fuel, i.e. having low permeability to the fuel components and being resistant to interaction with the fuel components. It is preferred that the container be substantially nonvitreous, that is, not be made of glass or other vitreous material, though such material may comprise no more than about 10% of the total mass of the container, typically no more than about 5%. Such containers will have at least one dispensing port, sealed by a cap or plug, or other sealing means, such as by a foil membrane, or typically a septum of elastomeric material. The contents of the container may be used to fill the anode compartment of the fuel cell when fuel replenishment is necessary. Alternatively, the fuel cell can be designed to accept such containers, so they may be joined to the cell, replacing empty containers that have been removed. In either case, the container may hold a concentrated fuel mixture to which water is added to achieve the desired methanol fuel composition. The water may be in a separate compartment and may be water that is generated during operation of the fuel cell. In this respect, the containers may be used as disposable batteries are now used in devices such as flash lights and portable radios and may be used to provide an instant “recharge” for devices such as cell phones, portable computers, and portable digital assistants which currently employ rechargeable batteries.
Electrode Compositions:
In another embodiment, the invention provides an electrochemical cell, such as a fuel cell, comprising a membrane electrode assembly comprising at least one electrode composition, wherein the electrode composition comprises an electrocatalyst, a polymer binder, typically a highly fluorinated ion-exchange polymers (as described below) and at least one performance additive. The weight percentage of the polymer binder in the electrode composition is at least about 5%, and typically at least about 10%; no more than about 30%, and typically no more than about 20%. The weight percentage of the performance additive in the electrode composition is at least about 0.1%, and typically at least about 1%; no more than 15% and typically no more than 5%.
The molecular weight of the performance additive in the electrode composition is typically about 200 to about 1,000,000 and still more typically about 2000 to about 500,000, and most typically about 4000 to about 100,000. The electrode compositions that form the anode and cathode in a membrane electrode assembly may be made from well-known electrically conductive, catalytically active particles or materials and may be made by methods well known in the art. The electrode compositions may be formed as a film of a polymer that serves as a binder for the catalyst particles.
Electrocatalyst:
Electrocatalysts in the composition are selected based on the particular intended application for the membrane electrode assembly. Electrocatalysts suitable for use in the present invention include one or more noble group metals such as platinum, ruthenium, rhodium, and iridium and electroconductive oxides thereof, and electroconductive reduced oxides thereof. The catalyst may be supported or unsupported. For direct methanol fuel cells, a (Pt—Ru)O_Xelectocatalyst has been found to be useful. Typically used electorcatalysts for hydrogen fuel cells are platinum on carbon, for example, 60 wt % carbon, 40 wt % platinum such as the material with this composition obtainable from E-Tek Corporation Natick, Mass., and 60% platinum, 40% carbon obtainable from John Matthey as FC-60.
Suitable electrocatalysts for this invention also include electrocatalysts that do not contain noble metal but have electrocatalytic activity for fuel cell reactions. These include Raney nickel, metal carbides, metal borides and organometallic complexes such as iron, copper, or zinc metalloporphyrins.
Binder:
Since the ion exchange polymer employed in the electrode composition serves not only as binder for the electrocatalyst particles but also assists in securing the electrode to the substrate, e.g. membrane, it is typical for the ion exchange polymers in the composition to be compatible with the ion exchange polymer in the membrane. Most typically, exchange polymers in the composition are the same type as the ion exchange polymer in the membrane.
Ion exchange polymers for use in accordance with the present invention are typically highly fluorinated ion-exchange polymers. “Highly fluorinated” means that at least 90% of the total number of univalent atoms in the polymer are fluorine atoms. Most typically, the polymer is perfluorinated. It is also preferred for use in fuel cells for the polymers to have sulfonate ion exchange groups. The term “sulfonate ion exchange groups” is intended to refer to either sulfonic acid groups or salts of sulfonic acid groups, typically alkali metal or ammonium salts. For applications where the polymer is to be used for proton exchange as in fuel cells, the sulfonic acid form of the polymer is preferred. If the polymer in the electrode composition is not in sulfonic acid form when used, a post treatment acid exchange step will be required to convert the polymer to acid form prior to use. Suitable highly fluorinated ion-exchange polymers include Nafion® polymers, available from E. I. DuPont de Nemours, Wilmington, Del.
The electrode compositions formed on the membrane should be porous so that they are readily permeable to the gases/liquids that are consumed and produced in cell. The average pore diameter is typically in the range of 0.01 to 50 μm, most typically 0.1 to 30 μm. The porosity is generally in a range of 10 to 99%, typically 10 to 60%.
Membrane:
The membranes may typically be made by known extrusion or casting techniques and typically have a thickness of about 5 μm to about 250 μm , more typically about 10 μm to about 200 μm, most typically about 20 μm to about 125 μm. While the polymer may be in alkali metal or ammonium salt form, it is typical for the polymer in the membrane to be in acid form to avoid post treatment acid exchange steps.
Ionomers for the membranes used in accordance with this invention may be any number of ion exchange polymers including polymers with cation exchange groups in the acid or proton form, hereinafter referred to as acid groups. Such acid groups include sulfonic acid groups, carboxylic acid groups, phosphonic acid groups, and boronic acid groups. Typically, the ionomer has sulfonic acid and/or carboxylic acid groups.
Polymers for use in accordance with the present invention are typically fluorinated, more typically highly fluorinated ion-exchange polymers having sulfonic acid and/or carboxylic acid groups. “Fluorinated” means that at least 10% of the total number of univalent atoms in the polymer are fluorine atoms. “Highly fluorinated” means that at least 90% of the total number of univalent atoms in the polymer are fluorine atoms. Most typically, the polymer is perfluorinated.
Typically, the polymer comprises a polymer backbone with recurring side chains attached to the backbone with the side chains carrying the acid groups. Possible polymers include homopolymers or copolymers of two or more monomers. Copolymers are typically formed from at least one monomer which is a nonfunctional monomer and which provides carbon atoms for the polymer backbone. A second monomer provides both carbon atoms for the polymer backbone and also contributes the side chain carrying the acid group or its precursor, e.g., a sulfonyl halide group such as sulfonyl fluoride (—SO₂F), which can be subsequently hydrolyzed and converted to a sulfonic acid group; or a carbomethoxy group (—COOCH₃) which can be subsequently hydrolyzed to a carboxylic acid group. For example, copolymers of a first fluorinated vinyl monomer together with a second fluorinated vinyl monomer having a sulfonyl fluoride group (—SO₂F) can be used. Possible first monomers include tetrafluoroethylene (TFE), hexafluoropropylene, vinyl fluoride, vinylidine fluoride, trifluorethylene, chlorotrifluoroethylene, perfluoro(alkyl vinyl ether), hexafluoroisobutylene ((CH₂═C(CF₃)₂), ethylene, and mixtures thereof. Possible second monomers include a variety of fluorinated vinyl ethers with sulfonic acid groups or precursor groups which can provide the desired side chain in the polymer. Additional monomers can also be incorporated into these polymers if desired.
Other sulfonic acid ionomers are known and have been proposed for fuel cell applications. Polymers of trifluorostyrene bearing sulfonic acid groups on the aromatic rings are an example (U.S. Pat. No. 5,773,480). The trifluorostyrene monomer may be grafted to a base polymer to make the ion-exchange polymer (U.S. Pat. No. 6,359,019).
Suitable perfluorinated sulfonic acid polymer membranes in acid form are available under the trademark Nafion® by E. I. du Pont de Nemours and Company.
Reinforced perfluorinated ion exchange polymer membranes can also be utilized in CCM manufacture. Reinforced membranes may be made by impregnating porous, expanded PTFE (ePTFE) with ion exchange polymer. ePTFE is available under the tradename “Goretex” from W. L. Gore and Associates, Inc., Elkton Md., and under the tradename “Tetratex” from Tetratec, Feasterville Pa. Impregnation of ePTFE with perfluorinated sulfonic acid polymer is disclosed in U.S. Pat. Nos. 5,547,551 and 6,110,333.
Alternately, the ion exchange membrane may be a porous support for the purposes of improving mechanical properties, for decreasing cost and/or other reasons. The porous support may be made from a wide range of components, for e.g., hydrocarbons such as a polyolefin, e.g., polyethylene, polypropylene, polybutylene, copolymers of those materials, and the like. Perhalogenated polymers such as polychlorotrifluoroethylene may also be used. The membrane may also be made from a polybenzimadazole polymer. This membrane may be made by casting a solution of polybenzimadazole in phosphoric acid (H₃PO₄) doped with trifluoroacetic acid (TFA) as described in U.S. Pat. Nos. 5,525,436; 5,716,727, 6,025,085 and 6,099,988.
Electrochemical Cell:
As shown in FIG. 1, an electrochemical cell, such as a fuel cell comprises a catalyst coated membrane (CCM) (10) in combination with at least one gas diffusion backing (GDB) (13) to form an unconsolidated membrane electrode assembly (MEA). The catalyst coated membrane (10) comprises an polymer electrolyte membrane (11) discussed above and catalyst layers or electrodes (12) formed from an electrocatalyst coating composition. The fuel cell is further provided with an inlet (14) for fuel, such as hydrogen; liquid or gaseous alcohols, e.g. methanol and ethanol; or ethers, e.g. diethyl ether, etc., an anode outlet (15), a cathode gas inlet (16), a cathode gas outlet (17), aluminum end blocks (18) tied together with tie rods (not shown), a gasket for sealing (19), an electrically insulating layer (20), graphite current collector blocks with flow fields for gas distribution (21), and gold plated current collectors (22).
Alternately, gas diffusion electrodes comprising a gas diffusion backing having a layer of an electrocatalyst coating composition thereon may be brought into contact with a solid polymer electrolyte membrane to form the MEA.
Catalyst Coated Membrane (CCM) and Membrane Electrode Assembly (MEA):
A variety of techniques are known for CCM manufacture which apply an electrocatalyst coating composition similar to that described above onto the solid fluorinated polymer electrolyte membrane. Some known methods include spraying, painting, patch coating and screen, decal, pad or flexographic printing.
In one embodiment of the invention, the MEA (30) may be prepared by thermally consolidating the gas diffusion backing (GDB) with a CCM at a temperature of under 200° C., preferably 140-160° C. The CCM may be made of any type known in the art. In this embodiment, an MEA comprises a solid polymer electrolyte (SPE) membrane with a thin catalyst-binder-performance additive layer disposed thereon. The catalyst may be supported (typically on carbon) or unsupported. In one method of preparation, a catalyst film is prepared as a decal by spreading the catalyst ink on a flat release substrate such as Kapton® polyimide film (available from the DuPont Company). After the ink dries, the decal is transferred to the surface of the SPE membrane by the application of pressure and heat, followed by removal of the release substrate to form a catalyst coated membrane (CCM) with a catalyst layer having a controlled thickness and catalyst distribution. Alternatively, the catalyst layer is applied directly to the membrane, such as by printing, and then the catalyst film is dried at a temperature not greater than 200° C.
The CCM, thus formed, is then combined with a GDB to form the MEA of the present invention. The MEA is formed, by layering the CCM and the GDB, followed by consolidating the entire structure in a single step by heating to a temperature no greater than 200° C., preferably in the range of 140-160° C., and applying pressure. Both sides of the MEA can be formed in the same manner and simultaneously. Also, the composition of the catalyst layer and GDB could be different on opposite sides of the membrane.
The following examples illustrate but do not limit the invention.

EXAMPLES

Sample Preparation and Test Methods Chronoamperometry and Cyclic Voltammetry:
Chronoamperometry and cyclic voltammetry were conducted in a modified three-electrode cell commercially available from Princeton Applied research (microcell kit K0264). The cell temperature was precisely controlled within ±0.25° C. by supplying temperature controlled water to a glass jacket of the cell. The counter electrode was Pt (K0266 Princeton Applied Research) and the reference electrode was Ag/AgCl electrode commercially available from Bioanalytical Systems, Inc. (BAS) (RE-5B). The working electrode was made as described below.
After assembly of the cell, the cell electrolyte solution was purged with ultrahigh purity Argon for several minutes to remove air. The solution was blanketed by Argon during the experiments.
The cell was electronically connected to a potentiostat (Solatron 1287) which was controlled by the computer software CorrWare® (Scribner Associates Inc.) via a standard GPIB (General Purpose Input Board) interface.
The chronoamperometry was typically performed at 0.4V vs. NHE. The methanol oxidation current was recorded as a function of time at least twice per second. The performance additive of interest was typically injected into the cell in the middle of the chronoamperometry run using a digital pipette through an injection port. Cyclic voltammetry in the range of −0.5 to 1.1 V vs. NHE was performed before and after chronoamperometry. The scan rate was typically 20 mV/sec.
Preparation of the Working Electrode:
A working electrode was prepared by coating the tip (1 cm wide 1.5 cm long) of a 1 cm wide 6 cm long carbon paper strip (Toray) with 0.07 to 0.09 gram of catalyst ink. The ink formulation is 0.1 g Pt/Ru black, 2 g of Nafion® solution (proton form, 1% solids in water) and 2 g of nanopure water. The ink was stirred and sonicated to a uniform dispersion. The coated carbon paper was dried in the chemical hood before use.

Example 1

N-methvlmorpholine

N-methylmorpholine (>99.5%) was obtained from Aldrich and used as received. A carbon strip with Pt/Ru black (1:1 atomic ratio) catalyst was used as the working electrode. The starting electrolyte solution is 0.05M H₂SO₄with 2M MeOH in water. Cell temperature was at 40° C. A constant potential of 0.4 V vs. normal hydrogen electrode (NHE) was applied. At about 600 sec, N-methylmorpholine was injected into the solution to result in 0.15M N-methylmorpholine. FIG. 2 shows the result of this experiment: the methanol oxidation current increased by about 50% after injection.
It is known in the art, and shown here in FIG. 3 curve 1, methanol oxidation on Pt/Ru catalyst commences at about 0.25V vs. NHE, below which there is no appreciable oxidation current. However, in the presence of N-methylmorpholine, a significant oxidation current was obtained at voltages below 0.25V. In fact, below 0.42V, curve 2 has a higher oxidation current than curve 1 indicating increased methanol electrooxidation activity as a result of the additive.

Example 2

4,4′-(Oxydi-2,1-ethanedivl)bismorpholine

Example 1 was repeated with the following exception: the injected chemical was 4,4′-(Oxydi-2,1-ethanediyl)bismorpholine (Aldrich, GC, >95%) and its concentration in the cell solution was 0.15 M. FIG. 4 shows that the methanol oxidation current increased after injection. FIG. 5 shows cyclic voltammetry before (curve 1) and after (curve 2) the 4,4′-(Oxydi-2,1-ethanediyl)bismorpholine injection. In the presence of 4,4′-(Oxydi-2,1-ethanediyl)bismorpholine, the methanol oxidation current is higher than without it at potentials below 0.5V vs. NHE. This shows that 4,4′-(Oxydi-2,1-ethanediyl)bismorpholine enhances methanol oxidation rate.

Example 3

A direct methanol fuel cell was operated with a Nafion® 117 membrane, 1050 equivalent weight (EW), and 7 mils (178 μm) thick. The cell active area was 5 cm². The anode catalyst layer was comprised of 4 mg/cm²Pt/Ru (1:1 atom ratio) black and 0.5 mg/cm²Nafion® perflurosulfonic acid. The cathode catalyst was comprised of 4 mg/cm²Pt black and 0.5 mg/cm²Nafion® perflurosulfonic acid. The catalyst layers were applied to the Nafion® membrane by a screen printing process. Porous carbon paper obtained from SGL Inc. (GDL31BC) was used in both anode and cathode as diffusion backing. Stainless steel mesh was used in both anode and cathode as current collector. The cell body was made of polytetrafluroethylene and was comprised of an anode compartment that measured 4 cm×3 cm×2.5 cm. Methanol fuel in this compartment diffused onto the surface of the catalyst layer through the stainless mesh and porous carbon paper. There was no forced methanol fuel circulation. The cathode compartment was supplied with house air at a constant flow rate of 200 sccm. Operating temperature was 40° C. A Solartron 1287 potentiostat (Solartron Analytical Hampshire, England) was used to control the fuel cell.
Initially, 27 g of 2 molar methanol fuel was loaded in the fuel compartment. The cell potential was set at 0.45V constant and the current output was monitored as a function of time. At the 30 minutes mark, 1.15 cc of the additive 4,4′-(Oxydi-2,1-ethanediyl)bismorpholine were injected via a syringe into the fuel compartment to result in about 0.175 molar of the additive. As seen in the FIG. 6, the cell current increased from 134 mA to 142 mA. Methanol crossover in the cell with and without additive was measured voltammetrically according to the method of X. Ren et al. (J. Electrochemical Society, 147 (1), 92-98 (2000)) When measuring the crossover current, 200 sccm dry N₂was supplied to the cathode side of the fuel cell and a voltage of 0.8V was applied to the cell (cathode side positive). The steady state current was taken as the methanol crossover current. When the fuel was 2 molar MeOH without any additive, the crossover current density was 110 mA/cm²; when the fuel was 2 molar MeOH with 0.175 molar additive, the crossover current density was 7 mA/cm^2.

Example 4

Example 3 was repeated with the following exception: (Oxydi-2,1-ethanediyl)bismorpholine was injected, resulting in 0.125 molar of the additive. As shown in FIG. 7, the cell current increased from 196 mA to 206 mA upon injection of the additive. The crossover current reduced from 130 mA/cm²to 126 mA/cm²after additive injection.

Comparative Example A

Example 3 was repeated with the following exception: a cyclic tertiary amine, N-methylmorpholine having a molecular weight of 101 was injected resulting in 0.15 molar of the additive. As shown in FIG. 8, the cell current quickly decreased. The results show that the cyclic tertiary amine having a molecular weight of less than 200 failed the fuel cell test.

Claims

1. A fuel mixture comprising at least one performance additive, wherein the performance additive comprises a cyclic tertiary amine having a molecular weight of at least 200.

2. The fuel mixture of claim 1 wherein the performance additive is capable of improving the electrooxidation reaction rate of the fuel mixture, measured as current density, by at least 2% compared to a fuel mixture not containing the performance additive.

3. The fuel mixture of claim 1 wherein the cyclic tertiary amine has a molecular weight of about 200 to about 10,000.

4. The fuel mixture of claim 3 wherein the cyclic tertiary amine has a molecular weight of about 400 to about 4,000.

5. The fuel mixture of claim 4 wherein the cyclic tertiary amine has a molecular weight of about 400 to about 2000.

6. The fuel mixture of claim 1 wherein the cyclic tertiary amine comprises at least one saturated monocyclic, bicyclic or polycyclic ring containing at least one tertiary N atom, and wherein the number of ring atoms ranges from 3 to 18.

7. The fuel mixture of claim 6 wherein the cyclic tertiary amine further comprises at least one atom selected from the group of C, N, O and Si.

8. The fuel mixture of claim 6 wherein the cyclic tertiary amine comprises at least two saturated cyclic amine rings, and wherein the rings are linked to one another by means of a linking group selected from the group consisting of linear alkylidene, cyclic alkylidene, oxyalkylidene, polyoxyalkylidene, alkoxylated alkylidene and alkylaryl groups having 1 to 100 carbon atoms.

9. The fuel mixture of claim 8 wherein the cyclic tertiary amine is selected from the group consisting of:

10. The fuel mixture of claim 6 wherein the cyclic tertiary amine comprises at least two saturated cyclic amine rings, and wherein the rings are attached to polymeric cores by means of side chains.

11. The fuel mixture of claim 10 wherein the cyclic tertiary amine has the following structure:

wherein n=about 1 to about 50,000.

12. The fuel mixture of claim 11 wherein n=about 100 to about 50,000.

13. The fuel mixture of claim 12 wherein n=about 1000 to about 10,000.

14. The fuel mixture of claim 11 wherein n=1 to about 100.

15. The fuel mixture of claim 14 wherein n=1 to about 20.

16. The fuel mixture of claim 6 wherein the cyclic tertiary amine is an end group of a dendrimer.

17. The fuel mixture of claim 16 wherein the dendrimer is a dendrimeric polyether.

18. The fuel mixture of claim 16 wherein the dendrimer is based on successive generations of polypropylenimines substituted onto a central diamine.

19. The fuel mixture of claim 18 wherein the central diamine is ethylenediamine, in which the terminal amine end groups are part of saturated cyclic rings.

20. The fuel mixture of claim 1 wherein the concentration of the performance additive in the methanol fuel is at least about 0.001 M.

21. The fuel mixture of claim 20 wherein the concentration of the performance additive in the methanol fuel is at least about 0.05 M.

22. The fuel mixture of claim 21 wherein the concentration of the performance additive in the methanol fuel is about 0.1 M to about 2.0 M

23. A fuel supply for supplying fuel to the anode of a fuel cell, the fuel supply supplying fuel comprising at least one performance additive, wherein the performance additive comprises a cyclic tertiary amine having a molecular weight of at least about 200.

24. The fuel supply of claim 23 wherein the performance additive is capable of improving the electrooxidation reaction rate of the fuel mixture, measured as current density, by at least 2% compared to a fuel mixture not containing the performance additive.

25. The fuel supply of claim 23 wherein the cyclic tertiary amine has a molecular weight of about 200 to about 10,000.

26. The fuel supply of claim 23 wherein the cyclic tertiary amine comprises at least one saturated monocyclic, bicyclic or polycyclic ring containing at least one tertiary N atom, and wherein the number of ring atoms ranges from 3 to 18.

27. The fuel supply of claim 26 wherein the cyclic tertiary amine further comprises at least one atom selected from the group of C, N, O and Si.

28. The fuel supply of claim 23 wherein the cyclic tertiary amine comprises at least two saturated cyclic amine rings, and wherein the rings are linked to one another by means of a linking group selected from the group consisting of linear alkylidene, cyclic alkylidene, oxyalkylidene, polyoxyalkylidene, alkoxylated alkylidene and alkylaryl groups having 1 to 100 carbon atoms.

29. The fuel supply of claim 23 wherein the cyclic tertiary amine comprises at least two saturated cyclic amine rings, and wherein the rings are attached to polymeric cores by means of side chains.

30. The fuel supply of claim 23 wherein the cyclic tertiary amine is an end group of a dendrimer.

31. The fuel supply of claim 30 wherein the dendrimer is a dendrimeric polyether

32. The fuel supply of claim 30 wherein the dendrimer is based on successive generations of polypropylenimines substituted onto a central diamine.

33. The fuel supply of claim 23 wherein the concentration of the performance additive in the methanol fuel is at least about 0.001 M.

34. The fuel supply of claim 33 wherein the concentration of the performance additive in the methanol fuel is at least about 0.05 M.

35. The fuel supply of claim 34 wherein the concentration of the performance additive in the methanol fuel is about 0.1 M to about 2.0 M

36. An electrode composition comprising an electrocatalyst, a polymer binder and at least one performance additive, wherein the performance additive comprises a cyclic tertiary amine having a molecular weight of at least about 200.

37. The electrode composition of claim 36 wherein the performance additive is capable of improving the electrooxidation reaction rate of the electrode composition, measured as current density, by at least 2% compared to an electrode composition not containing the performance additive.

38. The electrode composition of claim 36 wherein the cyclic tertiary amine has a molecular weight of about 200 to about 1,000,000.

39. The electrode composition of claim 38 wherein the cyclic tertiary amine has a molecular weight of about 2000 to about 500,000.

40. The electrode composition of claim 39 wherein the cyclic tertiary amine has a molecular weight of about 4000 to about 100,000.

41. The electrode composition of claim 36 wherein the cyclic tertiary amine comprises at least one saturated monocyclic, bicyclic or polycyclic ring containing at least one tertiary N atom, and wherein the number of ring atoms ranges from 3 to 18.

42. The electrode composition of claim 41 wherein the cyclic tertiary amine further comprises at least one atom selected from the group of C, N, O and Si.

43. The electrode composition of claim 41 wherein the cyclic tertiary amine comprises at least two saturated cyclic amine rings, and wherein the rings are linked to one another by means of a linking group selected from the group consisting of linear alkylidene, cyclic alkylidene, oxyalkylidene, polyoxyalkylidene, alkoxylated alkylidene and alkylaryl groups having 1 to 100 carbon atoms.

44. The electrode composition of claim 43 wherein the cyclic tertiary amine is selected from the group consisting of:

45. The electrode composition of claim 41 wherein the cyclic tertiary amine comprises at least two saturated cyclic amine rings, and wherein the rings are attached to polymeric cores by means of side chains.

46. The electrode composition of claim 45 wherein the cyclic tertiary amine has the following structure:

wherein n=about 1 to about 50,000.

47. The electrode composition of claim 46 wherein n=about 100 to about 50,000.

48. The electrode composition of claim 46 wherein n=about 1000 to about 10000.

49. The electrode composition of claim 46 wherein n=about 1 to about 100.

50. The electrode composition of claim 46 wherein n=about 1 to about 20.

51. The electrode composition of claim 41 wherein the cyclic tertiary amine is an end group of a dendrimer.

52. The electrode composition of claim 51 wherein the dendrimer is a dendrimeric polyether.

53. The electrode composition of claim 51 wherein the dendrimer is based on successive generations of polypropylenimines substituted onto a central diamine.

54. The electrode composition of claim 53 wherein the central diamine is ethylenediamine, in which the terminal amine end groups are part of saturated cyclic rings.

55. The electrode composition of claim 36 wherein the polymer binder is a highly fluorinated ion-exchange polymer, optionally containing sulfonate ion exchange groups

56. The electrode composition of claim 55 wherein the weight percentage of the polymer binder is at least about 5%.

57. The electrode composition of claim 56 wherein the weight percentage of the polymer binder is at least about 10% and less than about 20%.

58. The electrode composition of claim 36 wherein the weight percentage of the performance additive is at least about 0.1%.

59. The electrode composition of claim 36 wherein the weight percentage of the performance additive is at least about 1% and less than about 5%.

60. A membrane electrode assembly comprising an electrode composition, wherein the electrode composition comprises an electrocatalyst, a polymer binder and at least one performance additive, wherein the performance additive comprises a cyclic tertiary amine having a molecular weight of at least about 200.

61. The membrane electrode assembly of claim 60 further comprising a solid polymer electrolyte membrane.

62. The membrane electrode assembly of claim 60 further comprising at least one gas diffusion backing.

63. The membrane electrode assembly of claim 60 wherein the performance additive is capable of improving the electrooxidation reaction rate of the electrode composition, measured as current density, by at least 2% compared to an electrode composition not containing the performance additive.

64. The membrane electrode assembly of claim 60 wherein the cyclic tertiary amine has a molecular weight of about 200 to about 1,000,000.

65. The membrane electrode assembly of claim 64 wherein the cyclic tertiary amine has a molecular weight of about 4000 to about 100,000.

66. The membrane electrode assembly of claim 64 wherein the cyclic tertiary amine comprises at least one saturated monocyclic, bicyclic or polycyclic ring containing at least one tertiary N atom, and wherein the number of ring atoms ranges from 3 to 18.

67. The membrane electrode assembly of claim 66 wherein the cyclic tertiary amine further comprises at least one atom selected from the group of C, N, O and Si.

68. The membrane electrode assembly of claim 66 wherein the cyclic tertiary amine comprises at least two saturated cyclic amine rings, and wherein the rings are linked to one another by means of a linking group selected from the group consisting of linear alkylidene, cyclic alkylidene, oxyalkylidene, polyoxyalkylidene, alkoxylated alkylidene and alkylaryl groups having 1 to 100 carbon atoms.

69. The membrane electrode assembly of claim 68 wherein the cyclic tertiary amine is selected from the group consisting of:

70. The membrane electrode assembly of claim 66 wherein the cyclic tertiary amine comprises at least two saturated cyclic amine rings, and wherein the rings are attached to polymeric cores by means of side chains.

71. The membrane electrode assembly of claim 70 wherein the cyclic tertiary amine has the following structure:

wherein n=about 100 to about 50000.

72. The membrane electrode assembly of claim 64 wherein the cyclic tertiary amine is an end group of a dendrimer.

73. An electrochemical cell comprising an anode and a cathode, a membrane comprising ionomer having ion-exchange groups separating the anode and cathode prepared from an electrode composition, and a fuel supply for supplying fuel to the anode, the fuel supply comprises at least one performance additive.

74. The electrochemical cell of claim 73 wherein the electrochemical cell is a fuel cell.

75. The fuel cell of claim 74 wherein the fuel is methanol.

76. The electrochemical cell of claim 74 wherein the performance additive is capable of improving the electrooxidation reaction rate of the electrode composition, measured as current density, by at least 2% compared to an electrode composition not containing the performance additive.

77. The electrochemical cell of claim 74 wherein the performance additive is a cyclic tertiary amine, and wherein the cyclic tertiary amine has a molecular weight of about 200 to about 1,000,000.

78. The fuel cell of claim 74 wherein the performance additive is a cyclic tertiary amine, and the cyclic tertiary amine comprises at least one saturated monocyclic, bicyclic or polycyclic ring containing at least one tertiary N atom, and wherein the number of ring atoms ranges from 3 to 18.

79. An electrochemical cell comprising a membrane electrode assembly, wherein the membrane electrode assembly comprises at least one electrode composition, wherein the electrode composition comprises an electrocatalyst, and wherein the electrochemical cell comprises a fuel supply comprising a performance additive.

80. The electrochemical cell of claim 79 wherein the electrochemical cell is a fuel cell.

81. The fuel cell of claim 80 wherein the membrane electrode assembly further comprises a solid polymer electrolyte membrane wherein the membrane comprises an ionomer having proton conductive ion-exchange groups separating the anode and cathode.

82. The fuel cell of claim 80 wherein the membrane electrode assembly further comprises at least one gas diffusion backing.

83. The fuel cell of claim 80 wherein the electrode composition comprises a performance additive.

84. A process for operating a direct methanol fuel cell comprising an anode and a cathode, a membrane comprising ion-exchange groups separating the anode and cathode, and a fuel supply for supplying liquid methanol fuel to the anode, the process comprising contacting one or all of the membrane, anode or fuel with at least one performance additive, wherein the performance additive comprises a cyclic tertiary amine having a molecular weight of at least about 200.