U.S. patent application number 10/034094 was filed with the patent office on 2002-05-16 for catalyst materials for fuel cells.
This patent application is currently assigned to California Institute of Technology, a California corporation. Invention is credited to Halpert, Gerald, Narayanan, Sekharipuram R., Surampudi, Subbarao.
Application Number | 20020058178 10/034094 |
Document ID | / |
Family ID | 26685748 |
Filed Date | 2002-05-16 |
United States Patent
Application |
20020058178 |
Kind Code |
A1 |
Narayanan, Sekharipuram R. ;
et al. |
May 16, 2002 |
Catalyst materials for fuel cells
Abstract
An improved direct liquid-feed fuel cell having a solid membrane
electrolyte for electrochemical reactions of an organic fuel.
Improvements in interfacing of the catalyst layer and the membrane
and activating catalyst materials are disclosed.
Inventors: |
Narayanan, Sekharipuram R.;
(Altadena, CA) ; Surampudi, Subbarao; (Glendora,
CA) ; Halpert, Gerald; (Pasadena, CA) |
Correspondence
Address: |
FISH & RICHARDSON, PC
4350 LA JOLLA VILLAGE DRIVE
SUITE 500
SAN DIEGO
CA
92122
US
|
Assignee: |
California Institute of Technology,
a California corporation
|
Family ID: |
26685748 |
Appl. No.: |
10/034094 |
Filed: |
December 27, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10034094 |
Dec 27, 2001 |
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09305249 |
May 4, 1999 |
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09305249 |
May 4, 1999 |
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08827319 |
Mar 26, 1997 |
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60014166 |
Mar 26, 1996 |
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Current U.S.
Class: |
429/494 ;
429/524; 429/528; 429/530; 429/532; 429/535; 502/101 |
Current CPC
Class: |
H01M 4/92 20130101; H01M
4/90 20130101; Y02E 60/50 20130101; H01M 8/1004 20130101; H01M
8/1018 20130101; H01M 8/1009 20130101; H01M 4/921 20130101; H01M
8/04156 20130101; H01M 2300/0082 20130101; Y02P 70/50 20151101;
H01M 4/923 20130101; H01M 4/8652 20130101; H01M 4/9016
20130101 |
Class at
Publication: |
429/42 ;
502/101 |
International
Class: |
H01M 004/90; H01M
004/88; H01M 004/90; H01M 004/92 |
Claims
What is claimed is:
1. A method for fabricating a catalyst layer for a fuel cell,
comprising: preparing a catalyst material for either the
electro-reduction or electro-oxidation reaction in the fuel cell;
introducing a substance in the catalyst material, wherein the
substance is insoluble in the catalyst material; and subsequently
removing the insoluble substance from the catalyst material to
increase a surface area of the catalyst material compared to the
catalyst material prior to introducing and removing the
substance.
2. The method as in claim 1, wherein the catalyst material
comprises a catalyst of about 7-10 wt. %, perfluorovinylether
sulfonic acid of about 60-70 wt. %, and polytetrafluoroethylene of
about 15-20 wt. %.
3. The method as in claim 2 wherein the catalyst material is
obtained by: mixing the catalyst and the polytetrafluoroethylene in
a diluted solution to form a mixture liquid; performing sonication
to the mixture liquid; subsequently adding the perfluorovinylether
sulfonic acid in a diluted solution to the mixture liquid to form a
new mixture liquid; and performing sonication to the new mixture
liquid.
4. The method as in claim 2, wherein the catalyst comprises
platinum and ruthenium.
5. The method as in claim 1, wherein the insoluble substance is a
surface active substance which prevents particle agglomeration and
is volatilized at a high temperature.
6. The method as in claim 5, wherein the surface active substance
is a non-ionic surfactant.
7. A method for fabricating a catalyst material for a fuel cell:
mixing a catalyst and a polytetrafluoroethylene in a diluted
solution to form a mixture liquid; performing sonication to the
mixture liquid; subsequently adding a perfluorovinylether sulfonic
acid in a diluted solution to the mixture liquid to form a new
mixture liquid solution; performing sonication to the new mixture
liquid solution; and placing dry ice into the new mixture liquid to
evaporate the liquid portion without agglomeration and growth of
particles to form a catalyst material.
8. The method as in claim 7, wherein the catalyst material
comprises the catalyst of about 7-10 wt. %, the perfluorovinylether
sulfonic acid of about 60-70 wt. %, and the polytetrafluoroethylene
of about 15-20 wt. %.
9. A method for fabricating a catalyst material for a fuel cell:
mixing a catalyst and a polytetrafluoroethylene in a diluted
solution to form a mixture liquid; performing sonication to the
mixture liquid; subsequently adding a perfluorovinylether sulfonic
acid in a diluted solution to the mixture liquid to form a new
mixture liquid solution; performing sonication to the new mixture
liquid solution; and adding a gas through the new mixture liquid
solution to cause bubbles to promote formation of a foam-type
catalyst material.
10. The method as in claim 9, wherein the catalyst material
comprises the catalyst of about 7-10 wt. %, the perfluorovinylether
sulfonic acid of about 60-70 wt. %, and the polytetrafluoroethylene
of about 15-20 wt. %.
11. The method as in claim 9, wherein the gas is an inert gas,
nitrogen, or air.
12. A method for fabricating a catalyst material for a fuel cell,
comprising: mixing a catalyst of about 7-10 wt. %, a
perfluorovinylether sulfonic acid of about 60-70 wt. %, and a
polytetrafluoroethylene of about 15-20 wt. % to form a catalyst
material; and thermally quenching the catalyst material from a high
temperature to a low temperature to activating the catalyst
material.
13. The method as in claim 12, wherein the thermal quenching is
performed in a liquid nitrogen to decrease the temperature from an
ambient temperature to about 77K.
14. The method as in claim 12, wherein the catalyst comprises
platinum and ruthenium with a relative percentage ratio from about
10 wt. % platinum and 90 wt. % ruthenium to about 90 wt. % platinum
and 10 wt. % ruthenium.
15. A catalyst material for a fuel cell, comprising: a catalyst
comprising tungsten carbide; a perfluorovinylether sulfonic acid;
and a polytetrafluoroethylene.
16. The material as in claim 15, wherein said catalyst further
includes ruthenium or ruthenium oxide.
17. The material as in claim 16, wherein said catalyst further
comprises platinum.
18. A catalyst material for a fuel cell, comprising: a catalyst
comprising zirconium dioxide; a perfluorovinylether sulfonic acid;
and a polytetrafluoroethylene.
19. The material as in claim 18, wherein said catalyst further
comprises platinum.
20. The material as in claim 19, wherein said catalyst further
comprises ruthenium dioxide.
21. A catalyst material for a fuel cell, comprising: a catalyst
comprising zeolites incorporated with platinum and ruthenium; a
perfluorovinylether sulfonic acid; and a
polytetrafluoroethylene.
22. The material as in claim 21, wherein said catalyst further
includes iridium in said zeolites.
23. The material as in claim 21, wherein said catalyst further
includes osmium in said zeolites.
24. The material as in claim 21, wherein said catalyst further
includes tungsten in said zeolites.
25. The material as in claim 21, further comprising an
electrically-conductive carbon material.
Description
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/305,249 filed May 4, 1999, which is a
continuation of U.S. patent application Ser. No. 08/827,319 filed
Mar. 26, 1997 (now U.S. Pat. No. 5,945,231, issued Aug. 31, 1999).
U.S. patent application Ser. No. 08/827,319 claims the benefit of
U.S. Provisional Application No. 60/014,166 filed on Mar. 26, 1996.
The disclosures of the prior applications are incorporated herewith
in their entirety by reference.
ORIGIN OF THE INVENTION
[0002] The invention described herein was made in the performance
of work under a NASA contract and is subject to the provisions of
Public Law 96-517(35 U.S.C. 202) in which the Contractor has
elected to retain title.
FIELD OF THE INVENTION
[0003] The present invention relates to fuel cells for generating
energy by electrochemical reactions, and more specifically to a
direct-feed oxidation fuel cell and manufacturing thereof.
BACKGROUND AND SUMMARY OF THE INVENTION
[0004] Organic fuels can be used to generate electrical power by
converting energy released from electro-chemical reactions of the
fuels. Organic fuels, such as methanol, are renewable. Typical
products from the electrochemical reactions are mostly carbon
dioxide and water. These products are environmentally safe.
Therefore, organic fuel cells are considered as an alternative
energy source to non-renewable fossil fuels for many applications.
In addition, use of fuel cells can eliminate many adverse
environmental consequences associated with burning of fossil fuels,
for example, air pollution caused by exhaust from gasoline-powered
internal combustion engines.
[0005] Direct liquid-feed oxidation fuel cells are of particular
interest due to a number of advantages over other fuel cell
configurations. For example, the organic fuel is directly fed in to
the fuel cell. This eliminates the necessity of having a chemical
pre-processing stage. Also, bulky accessories for vaporization and
humidification in gas-feed fuel cells are eliminated. Thus, direct
liquid-feed cells generally have simple cell construction and are
suitable for many applications requiring portable power supply.
[0006] Conventional direct liquid-feed cells usually use a liquid
mixture of an organic fuel and an acid/alkali electrolyte liquid,
which is circulated past the anode of the fuel cell. Problems
associated with such a conventional direct liquid-feed cell are
well recognized in the art. For example, corrosion of cell
components caused by the acid/alkali electrolyte places significant
constraints on the materials that can be used for the cell; fuel
catalysts often exhibit poor activity due to adsorption of anions
created by the acid electrolyte; and the use of sulfuric acid
electrolyte in multi-cell stacks can result in parasitic shunt
currents. As a result, the performance of the conventional cells is
limited to about less than 0.3 volt in output voltage and less than
about 30 mA/cm.sup.2 in output current. In addition, a number of
safety issues arise with the use of acidic and alkaline
solutions.
[0007] NASA's Jet Propulsion Laboratory (JPL) developed an improved
direct liquid-feed cell using a solid-state membrane electrolyte.
One of the advantages of the JPL fuel cell is the elimination of
the liquid acidic and alkaline electrolyte by the membrane
electrolyte. This solves many problems in the conventional fuel
cells. A detailed description of JPL's fuel cell can be found, for
example, in U.S. Pat. No. 5,599,638 and in U.S. patent application
Ser. No. 08/569,452, filed on May 28, 1996, both of which are
incorporated herein by reference.
[0008] FIG. 1 shows a typical structure 100 of a JPL fuel cell with
a membrane electrolyte 110 enclosed in housing 102. The electrolyte
membrane 110 is operable to conduct protons and exchange cations.
An anode 120 is formed on a first surface of the membrane 110 with
a first catalyst for electro-oxidation and a cathode 130 is formed
on a second surface thereof opposing the first surface with a
second catalyst for electro-reduction. An electrical load 140 is
connected to the anode 120 and cathode 130 for electrical power
output.
[0009] The membrane 110 divides the fuel cell 100 into a first
section 122 on the side of the anode 120 and a second section 132
on the side of the cathode 130. A feeding port 124 in the first
section 122 is connected to a fuel feed system 126. A gas outlet
127 is deployed in the first section 122 to release gas therein and
a liquid outlet 128 leads to a fuel recirculation system 129 to
recycle the fuel back to the fuel feed system 126. In the second
section 132 of the cell 100, an air or oxygen supply 136 (e.g., an
air compressor) supplies oxygen to the cathode 130 through a gas
feed port 134. Water and used air/oxygen are expelled from the cell
through an output port 138.
[0010] In operation, a mixture of an organic fuel (e.g., methanol)
and water is fed into the first section 122 of the cell 100 while
oxygen gas is fed into the second section 132. Electrochemical
reactions happen simultaneously at both the anode 120 and the
cathode 130, thus generating electrical power. For example, when
methanol is used as the fuel, the electro-oxidation of methanol at
the anode 120 can be represented by
CH.sub.3OH+H.sub.2O-CO.sub.2+6H.sup.++6e.sup.-,
[0011] and the electro-reduction of oxygen at the cathode 130 can
be represented by
O.sub.2+4H.sup.++4e.sup.-2H.sub.2O.
[0012] Thus, the protons generated at the anode 120 traverse the
membrane 110 to the cathode 130 and are consumed by the reduction
reaction therein while the electrons generated at anode 120 migrate
to the cathode 130 through the electrical load 140. This generates
an electrical current from the cathode 130 to the anode 120. The
overall cell reaction is:
2CH.sub.3OH+3O.sub.2-2CO.sub.2+4H.sub.2O+Electrical Energy.
[0013] The inventors recognized the advantages and potential of the
JPL's membrane fuel cell. Importantly, the inventors have
discovered a number of new materials for various components and
processing methods that can be used to improve the performance of
this type of fuel cells.
[0014] One aspect of the present invention describes new material
compositions for catalysts with improved efficiency and methods for
forming catalyst layers on the membrane electrolyte including
transfer of catalyst decals and deposition of catalyst materials
onto a backing layer with minimized catalyst permeation.
[0015] Another aspect directs to improve catalyst efficiency and
reactivity by increasing the surface area thereof.
[0016] Yet another aspect is to increase the reactivity of a
catalyst by changing the electronic properties of a catalyst
layer.
[0017] Still another aspect of the invention is construction and
processing of the electrolyte membrane to improve coating, bonding,
and to reduce fuel crossover.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] These and other advantages of the present invention will
become more apparent in light of the following detailed description
of preferred embodiment thereof, as illustrated in the accompanying
drawings, in which:
[0019] FIG. 1 is a block diagram illustrating a typical direct
liquid-feed fuel cell having a solid-state membrane electrolyte
[0020] FIGS. 2A-2C are schematics to illustrate a processing
example for forming a catalyst decal on a TEFLON backing block or
sheet with a catalyst ink.
[0021] FIGS. 3A and 3B are schematics to illustrate a preferred
process for transferring decals from a backing block to a membrane
surface.
[0022] FIG. 4 is a schematic showing a zeolite cage having
decomposed carbon compound as conducting elements.
[0023] FIGS. 5A and 5B are schematics showing two preferred systems
for alloying Pt--RuO.sub.2 catalysts.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0024] Certain aspects of the preferred embodiments are disclosed
in the incorporated references, U.S. Pat. No. 5,599,638, and U.S.
patent application Ser. No. 08/569,452. Therefore, the brevity in
describing various parts of the present invention is supplemented
by the disclosure of the above references.
[0025] 1. Catalyst Decal: Decal Preparation and Transfer
[0026] Both the anode and cathode in the preferred fuel cell have
catalyst materials for the electrochemical reactions. The catalyst
for the electro-oxidation of the fuel at the anode can use a number
of materials including platinum/ruthenium alloy. The cathode
catalyst for the electro-reduction of oxygen can use materials such
as platinum. It is desirable to form a good mechanical and
electrical contact between a catalyst material and the respective
membrane surface in order to achieve a high operating efficiency.
An electrically conducting porous backing layer is preferably used
to collect the current from the catalyst layer and supply reactants
to the membrane catalyst interface. A catalyst layer, therefore,
can be formed on the backing layer. The backing layer can be made
of various materials including a carbon fiber sheet.
[0027] The inventors recognized that improvements can be made in
the well-known method of using transferable catalyst decals.
Information about catalyst decals can be found, for example, U.S.
Pat. No. 5,798,187 to M. Wilson et al. of Los Alamos National Lab,
which is incorporated herein by reference.
[0028] A decal layer is a layer of catalyst on a substrate (e.g.,
TEFLON). The purpose of this decal is to transfer the catalyst
layer onto a proton conducting membrane. This is described in
detail as follows.
[0029] I. Preparation of Transferable Catalyst "Decals" for
Application on NAFION and other Proton Exchange Membranes
[0030] FIGS. 2A-2C illustrate a preferred process for forming
catalyst decals. This is described as follows.
[0031] (1) A selected catalyst material, perfluorovinylether
sulfonic acid (e.g., NAFION by DuPont) and polytetrafluoroethylene
(e.g., TEFLON) in appropriate proportions are mixed. The mix
includes approximately 150 mg of the catalyst, 0.7-1.4 g of 5%
NAFION solution, and 0.2-0.4 g of a TEFLON emulsion such as TFE-30
(diluted to 11% in solids). The solvent can be a solution having
NAFION in higher alcohols commercially available from Aldvich
Chemical Co. Therefore, a preferred ratio of the constituents in
the mix is: 7-10% catalyst, 60-70% NAFION solution (5% solution),
15-20% in TFE-30 that has been diluted to 11% in solids.
[0032] (2) A preferred mixing order is first mixing the catalyst
and diluted TFE solution and performing a first sonication for an
appropriate amount of time (e.g., approximately from 5 to 20
minutes) and then adding the NAFION solution and performing a
second sonication (e.g., approximately from 5 to 20 minutes).
[0033] (3) A TEFLON sheet is prepared by degreasing the
surface.
[0034] (4) The TEFLON substrate on which a catalyst layer will be
cast is flattened out and laid on a substantially horizontal
surface (e.g. a bubble-leveled table). The area of coating can be
marked out using TEFLON adhesive tape.
[0035] (5) The mix of catalyst, NAFION, and TEFLON solution is
poured into the blank area on the TEFLON substrate previously
marked.
[0036] (6) The mix is then uniformly spread out in the coating area
to form a catalyst layer therein. This can be done, for example, by
using a glass rod with a rounded end in a rotational motion (e.g.,
executed over a hundred times).
[0037] (7) The catalyst layer on the TEFLON backing layer is
allowed to dry out slowly so that evaporation occurs at a
pre-determined slow rate to eventually form a uniformly-dried
coating layer This can be done, for example, by enclosing the
sample in a container such as petridish or glass cover for about
12-24 hours.
[0038] II. Decal Processing
[0039] (1) After the decal is dry, it is sprayed with a layer of
water on the decal. Alternatively, the decal can be sprayed with
isopropanol or a mixture of isopropanol and water
[0040] (2) The sprayed decal is submerged in water (e.g. in a
sealed plastic bag containing water) for a soaking period ranging
approximately from 1 to 24 hours. Other liquids can also be used
for soaking. For example, soaking in pure isopropanol ("IPA")
rather than water can reduce the soaking time. The inventors
discovered that semi-dry coatings are not easy to work with and
spraying on semi-dry coatings may result in loss of materials and
disintegration of the catalyst coating. Therefore, a long drying
period (e.g., 24 hours) for the catalyst layer under slow
evaporation is preferred prior to decal processing.
[0041] (3) The decal is ready for transfer after the above soaking
process.
[0042] III. Transferring Decal onto Membrane
[0043] (1) The membrane is preferably conditioned by a three-step
process of treatment: boiling at a temperature approximately from
90.degree. C. to 100.degree. C. in 5% H.sub.2O.sub.2 solution, then
treating in H.sub.2SO.sub.4, and followed by boiling in water.
[0044] (2) Next, the membrane can be either soaked in water on in a
mixture of isopropanol and water for a predetermined soaking time
(e.g., 24 hours). A mixture of about 10% of isopropanol and 90% of
water by volume is preferred to achieve a desired degree of
swelling of the membrane for the decal transfer. The inventors
discovered that this makes the membrane accept the decal
better.
[0045] (3) The soaked membrane is then mopped dry and placed
between two rigid and compressible sheets with the coated decal
facing the membrane. The sheets are preferably made of
corrosion-resistant materials such as gold-plated materials,
carbon-coated materials, and titanium.
[0046] (4) Next, the sandwich of membrane and decal block is hot
pressed with the metal sheets under a pressure of about 500-1500
psi and at a temperature of about 140-155.degree. C. for about 3-5
minutes and furtner treated in the same way as the
membrane-electrode assembly (MEA) (e.g., application of backing
layer and soaking in water or a water solution). See, for example,
U.S. patent application Ser. No. 08/569,452, filed on May 28, 1996,
which is incorporated herein by reference.
[0047] (5) The pressed pieces are cooled down under pressure at a
low temperature (e.g., <55.degree. C.).
[0048] (6) The pressed parts can be removed. The TEFLON backing
layer can be peeled off, leaving the catalyst layer fixed on the
membrane.
[0049] The above decal transfer process is illustrated in FIGS. 3A
and 3B.
[0050] The inventors investigated a number of parameters that
affect the decal transfer process as described above. In
particular, a Taguchi style experiment was conducted to optimize
many processing parameters. A Taguchi experiment is a factorial
design experiment wherein factors governing process performance are
varied in a systematic manner for optimization. See, for example,
"Quality Engineering Using Robust Design", by M. S. Phakhe,
Prentice Hall, New Jersey, 1989. The inventors studied following
parameters:
[0051] 1. NAFION content;
[0052] 2. TEFLON content;
[0053] 3. Drying time;
[0054] 4. Soak process (yes/no);
[0055] 5. Temperature of pressing;
[0056] 6. Pressure of "pressing"; and
[0057] 7. Membrane soak process (IPA or water).
[0058] The catalyst was substituted with battery grade graphite in
the experiment
[0059] The above investigation indicated that increasing NAFION
content and decreasing TEFLON content can be beneficial to the
decal transfer. Performing the membrane soak process in IPA
resulted in a good transfer, i.e., a large extent of the decal
(e.g., over 85%) is transferred. The inventors also discovered that
the coating soak process in water was important to attaining good
transfer. Using pressure in an appropriate range was also found to
be important in attaining good transfer. However, the drying time
appeared to have no observable effect as long as the decal is
completely dry.
[0060] The inventors found that the effects of various parameters
can be summarized as follows: (1) increasing NAFION amount
increased the extent of transfer; (2) increasing the TEFLON content
decreased the amount of transfer; (3) drying time has no
significant impact except that the coating has to be dry; (4) the
soaking process improved the transfer;(5) increase of pressure
favored decal transfer; and (6) membrane presoak in a solution
having IPA and water significantly enhanced transfer.
[0061] The following are the values for all these parameters for
the experiment that resulted in an optimized transfer:
[0062] Catalyst: 0.144 g over 2".times.2"
[0063] NAFION: 1.430 g
[0064] TEFLON: 0.213 g
[0065] Temperature: 145.degree. C.
[0066] Pressure: 500-1500 psi
[0067] Drying time: 24 hours (or less)
[0068] Decal soak process: IPA or water for 24 hours
[0069] Membrane soak: IPA/water (10-20/90-80) for 24 hours.
[0070] 2. Methods for Improving Catalyst Utilization
[0071] Catalyst utilization refers to the extent to which the
catalyst used in the preparation techniques is actually operative
in the fuel cell reactions. The inventors discovered several
approaches to improve the catalyst utilization.
[0072] I. Methods for Depositing Catalyst Material onto Carbon
Backing Layer without Permeation
[0073] Catalyst layers are applied in a conventional process by
painting an ink on the carbon porous backing layer. This ink often
soaks into the backing layer. This results in unusable catalyst
that is locked in the pores of the backing layer. The inventors
recognized that this permeation of the catalyst into the porous
backing layer reduces accessability of the catalyst for reactions.
The inventors further found that many techniques can be used to
apply catalyst layers onto a back layer without permeation of the
catalyst material. Several examples are give as follows.
[0074] (1) Instant drying of the catalyst layer as soon as the ink
touches the paper. The ink is a mixture of catalyst, TEFLON, NAFION
solution and water. The instant drying can be achieved by first
heating the paper to a temperature at which the solvent used in the
ink evaporates. The surface of the paper is subsequently sprayed
with the catalyst ink to leave behind a good superficial coat of
the catalyst without significant preparation of the catalyst layer
into the porous backing layer.
[0075] The inventors found that the fast drying process could give
a very different coat morphology which may have an impact. This,
however, can be obviated by applying multiple coats as discovered
by the inventors.
[0076] (2) Dual-Layer structure of carbon paper. A layer of porous
carbon is applied on the carbon paper to create a less porous super
structure. This substantially prohibits the catalyst from
penetrating into the carbon paper. Graphite, a highly conducting
material, can also be used in place of the porous carbon, specially
sub-micron size graphite. Other conducting materials that are
resistant to acids, such as gold, can be used according to the
invention.
[0077] The porosity of this layer will be determined by the size of
the carbon or graphite particles and the binder used therein. A
good binder, e.g., NAFION, will allow the next coat of catalyst to
have a good ionic and electronic contact. The carbon layer can be
coated by any technique, including but not limited to, spray
painting, screen printing, brushing or electropainting.
[0078] The catalyst layer can be coated on the carbon paper by any
coating technique. The carbon layer should not inhibit mass
transfer of the methanol solution and the removal of carbon dioxide
bubbles if it is hydrophilic and is also formed from porous carbon.
The formation of the dual layer structure has been verified under
the microscope by the inventors.
[0079] The inventors recognized that the instant drying technique
described in (1) can be integrated with the above dual layer
structure of the carbon paper.
[0080] (3) It is also possible to teflonize a sheet of carbon paper
and coat it with a water suspension of the catalyst. When this is
dry, then the whole electrode can be dipped in NAFION solution and
subsequently dried. This also prevents the particles from migrating
into the paper.
[0081] (4) A thin super coat of the metal can be used to substitute
carbon in smoothing the surface and forming a relatively porous
second layer. This will allow painting or spraying of the surface
with the catalyst ink without letting the material soak into the
carbon paper or any other backing structure. Such a deposition of
metal to form the second layer on the gas diffusion structure can
also be carried out electrochemically, such as by electroplating of
the metal.
[0082] II. Improving Properties of Anode and Cathode Catalysts
[0083] The inventors recognized that increasing the surface area of
the catalyst can enhance the reactivity of the catalyst towards the
carrying out fuel oxidation (e.g., methanol) and increase the
reaction rate. Several exemplary methods in accordance with the
present invention are as follows.
[0084] (1) Temporary introduction of a substance insoluble in the
catalyst and removable by subsequent leaching may be used to
increase the surface area of the catalyst. Such a substance can be
a surface active substance which will prevent particle
agglomeration and can be volatilized at a higher temperature.
Non-ionic surfactants may be preferable. Metals such as zinc,
aluminum, or tin incorporated in the catalyst during preparation
can also serve this function.
[0085] (2) Freeze drying the solution during catalyst preparation
using dry ice and subsequently evaporating the solution can yield
highly active catalysts, because agglomeration of particles and
growth of particles will be prevented by a freeze-drying
process.
[0086] (3) Using a high surface area carbon (activated type) in the
preparation will work as a particle isolation method in
agglomeration processes and also enhance the conductivity of the
catalyst itself. Examples of such carbons are acetylene black,
Shawanigan Black, vulcan-XC-72, Black Pearls 2000, and alike.
[0087] (4) Adding large amount of air, an inert gas or nitrogen
will promote the formation of foam-type structures. This can be
performed during catalyst particle formation in order to prepare a
high surface area catalyst. Air or preferably an inert gas and
nitrogen should be bubbled through the solution during the
precipitation of the Platinum and ruthenium hydroxides. This can be
achieved by adding a volatile surfactant to the precipitating
solution or to the starting solution. This can provide the
condition for production of foam-like structures.
[0088] (5) The Pechini process (citrate gel) for forming oxides can
be used as a precursor for the formation of high surface area
structures. Pechini process is disclosed in U.S. Pat. No.
3,330,697, which is incorporated herein by reference.
[0089] (6) Formation of the catalyst in the presence of large
amounts of conducting substances such as lead dioxide can be used
for effective dispersion (i.e., even distribution of the catalyst
material). Lead dioxide is an inert conductive material resistant
to acids.
[0090] III. Changing Electronic Properties of Interfacing Surface
for Activating Catalyst
[0091] (1) Varying the ratio of Pt:Ru and varying the temperature
of preparation can be used to alter the electronic properties of
the catalyst for a desired effect. The Pt:Ru ratio can be varied
from about 10%Pt to 90%Pt, corresponding to 90%Ru to 10%Ru.
[0092] (2) Titanium can be used to activate the catalyst. Titanium
and ruthenium form a solid solution of oxides. Use of isoproxide of
titanium prior to the precipitation in the above methods of
preparation of the catalyst can be beneficial.
[0093] (3) Quenching the catalyst after reduction, i.e., rapid
cooling the catalyst from a high temperature (e.g., about
300.degree. C.) to a low temperature (e.g., liquid nitrogen
temperature of 77K), would create stresses in the particles. This
generates new activation forces. Segregation of phases and stresses
in the catalyst is produced by quenching. Such a quenching process
can be done in liquid nitrogen.
[0094] (4) Use of a chemical field caused by zeolites may be
important in orienting the molecules at the surface. Zeolites are
capable of interacting with metals, such as Pt and Ru, and changing
the electronic properties of the metals. This modification of the
chemical nature of the catalysts can be exploited to enhance the
activity of the catalysts.
[0095] (5) Addition of silica or titanium dioxide during
preparation would alter the local electronic fields.
[0096] (6) An additive material can be used to modify the nature of
the surface in such a way that the harmful effects of the
adsorption are not observed. Such an additive could clean off the
poisoning species or increase the ease of removal of the poisoning
species. Suitable additive materials include Ir, Rh, Os, and
alike.
[0097] (7) Reducing the size of particles to a nanoscale
environment for the decomposition of methanol is another way of
enhancing activity of methanol oxidation catalysts.
[0098] (8) Concentrating the additives on the surface instead of
the bulk can be achieved by "adatom" approaches (i.e., forming thin
atom layers from solutions). This method can be used to provide the
additive in a selected region. Suitable additives include metals
such as Bi, Sn, Sb, Ir, Rh, Os, deposited from solutions in thin
layers on the catalyst.
[0099] For example, electroless deposition of other metals on the
existing catalyst can be used for surface modification with adatom.
Another method is addition of metal salts to the catalyst wash
bath, which may also provide adequate number of adatoms for this
purpose.
[0100] (9) Changing the phase of the substance could include the
formation of non-equilibrium phases. These phases are usually quite
active compared to the equilibrium phase. High-energy ball milling
might be an approach to produce such non-equilibrium phases. These
can be used for Pt--Ru, Pt--Ru--X wherein X includes Ir, Rh, Os, or
Ti.
[0101] IV. Modifications to Catalyst Layer with respect to Altering
Hydrophobic and Hydrophilic Constituents to Improve Performance
[0102] (1) The inventors discovered that TEFLON added in the
catalyst layer such as by using TFE-30 usually does not have any
hydrophobic characteristics because it is with a surfactant. Also,
the hydrophilic surfactant will compete for the catalyst active
surface Elimination of TEFLON emulsion from the catalyst mix will
eliminate the surfactant. However, TEFLON itself is beneficial. The
layer can then use TEFLON micron powder (e.g., MP1100). This will
provide the TEFLON properties without adding the surfactant. The
Inventors have used such an ink and have found it performs well.
Therefore, this new method of incorporating TEFLON would be
beneficial to the anode and the cathode.
[0103] Replacing with TEFLON powder (such as the MP series 1100
typically) or with PVDF can be beneficial. The use of large amounts
of macro particular TEFLON will allow access of air to the various
parts of the catalyst layer more efficiently than small microscopic
particles. Thus the TFE-30 addition can be replaced by using MP1100
or PVDF or Kynar material in the mix for the catalyst layer.
[0104] (2) The role of TEFLON in the catalyst layer could be
significant in reducing the open area for methanol transport. In
order to attain these properties, an alternative ink is to be made
out of the crosslinked polystyrene sulfonic acid and PVDF or Kynar
or a methanol rejecting zeolite such as mordenite. These layers can
be applied in one or more layers with or without the catalysts.
These additives can be applied in one or more layers. The inventors
contemplate that the primary catalyst may be used with mere
polyvinylidene fluoride (e.g., PVDF) for the first layer, whereas a
combination with the zeolite/crosslinked sulfonic acid PVDF can be
used for the second layer. PVDF can be obtained in very fine
powders and can be dissolved in some organic solvents and dispersed
easily with the catalyst ink.
[0105] 3. Other Catalysts and Preparation Methods
[0106] I. Laser Ablation
[0107] A new method for applying the catalyst on the gas diffusion
substrate is laser co-ablation of Pt and Ru. This process involves
the continuous or pulsed laser evaporation of a target followed by
deposition on a substrate.
[0108] II. Ion Implantation
[0109] Yet another method for preparation of catalyst is
ion-implantation. This can be carried out on existing catalysts or
already prepared electrodes where ions of various materials such as
Bi, Pb, Sn, P can be implanted to enhance the activity of the
catalyst.
[0110] III. Tungsten Carbide
[0111] Tungsten carbide behaves like platinum under many
circumstances of organic transformation and may be used as a
catalytic material. Tungsten carbide combined with platinum is a
good catalyst for Hydrogen evolution. Therefore, the inventors
contemplate that tungsten carbide be combined with ruthenium oxide
or ruthenium to prepare a new catalyst. Small amounts of platinum
can also be added to the tungsten carbide material so that the
activity of both tungsten carbide and platinum are enhanced. This
can be incorporated into the present procedure to lower the costs
of the catalysts.
[0112] High surface area tungsten carbide can be produced by the
following method: (1) tungsten oxide can be deposited on a high
surface area carbon sheet by a solution process which involves
precipitation and drying; (2) then the tungsten oxide dispersed on
carbon can be reduced with hydrogen at 500.degree. C. or higher in
order to substantially eliminate the oxygen. This will leave the
active tungsten on the surface; (3) the temperature is then raised
to about 900.degree. C. will provide the carbide. By choosing the
correct carbon to tungsten ratio, stoichiometric carbide with no
excess carbon can be produced.
[0113] IV. Zirconium Dioxide
[0114] Zirconium dioxide is another material for catalysts of
hydrogen and water based reactions. Combining Pt with zirconium
oxide would yield a good catalyst. This can be prepared by an
impregnation technique. The zirconium oxide is produced by a
hydrolysis process from zirconium chloride or zirconyl nitrate
solution. The platinum salt such as the chloride or nitrate is
added to it in desired quantities and sonicated until complete
dissolution of the platinum salt occurs and the platinum is
uniformly distributed. A reducing agent such as formaldehyde and
sodium formate is then added and the solution thereof is heated. Pt
will deposit on the zirconium oxide. Several methods can be used to
prepare ZrO.sub.2-based catalysts. Two preferred examples are as
follows.
[0115] Method 1:
[0116] The objective is to prepare a fine dispersed
Pt--RuO.sub.2--ZrO.sub.2 catalyst. Fine particle ZrO.sub.2 can be
produced by hydrolysis of zirconyl nitrate with a pH value at about
5-6. This can occur during the neutralization of the regular
catalyst preparation method. The zirconium hydroxide formed will be
the nucleus for precipitation of the Ru(OH).sub.3 and Pt(OH).sub.3.
This ensures a fine grainy precipitate evaporate as usual. The
resultant dry powder can be recovered by vacuum drying. This is
followed by hydrogen reduction for about 16 hours. The catalyst
Pt--RuO.sub.2 is then washed and recovered. The catalyst mixture
can include about 30-50% of ZrO.sub.2 as a support agent.
[0117] Method 2:
[0118] This method avoids the effect of chlorides and other amions
completely.
[0119] ZrO.sub.2 in a hydrous state is prepared by neutralization
of ZrO(NO.sub.3).times.H.sub.2O solutions Pt (Acetyl acetonate) and
Ru (Ac Hc) are added to ZrO.sub.2 and allowed to absorb by an
impregnation process. This solution is evaporated to dry out in
vacuum. The solid is then reduced in hydrogen at 200-400.degree. C.
to produce bimetallic Zirconia supported Pt--RuO.sub.2 catalysts.
The catalysts should be washed to remove any absorbed ions.
[0120] The inventors have found that a catalyst can be formed with
or without ruthenium oxide by using the present process for making
Pt--Ru catalysts.
[0121] V. Zeolite-based Catalysts
[0122] Pt and Ru can be incorporated in zeolites and clays in order
to exploits the acid catalytic properties of these materials.
[0123] Pt cation complexes (e.g., ammine chlorides) and similar
ruthenium complexes can be exchanged with zeolite material (ZSM,
mordenites, etc.) and they can be treated with hydrogen at elevated
temperatures (200-300.degree. C.) to produce activated Pt--Ru
catalysts with enormous surface area. These will reduce cost of
catalysts for oxidation of methanol in fuel cells, and also provide
carbon monoxide tolerant materials for hydrogen oxidation in fuel
cells.
[0124] A preferred procedure for preparation of Zeolite based
Pt--Ru or Pt--Ru--Ir catalysts is as follows.
[0125] (1) Zeolites such as the ZSM and natural zeolites are all in
the sodium or potassium form. These need to be converted to the
ammonium form by a standard linde procedure which involves boiling
the zeolite repeatedly with ammonium chloride solutions and
subsequent washing.
[0126] (2) These Ammonium exchanged Zeolites are then exchanged
with Pt.sup.(lV) and Ru.sup.(III) or Pt.sup.(II) and Ru.sup.(II) by
stirring with heat, zeolite plus the cationic complex of the
appropriate metals. For example, Pt(NH.sub.3).sub.2.sup.4+Cl.sub.4
or Pt(NH.sub.3).sub.2Cl.su- b.2 or H.sub.2Pt(NO.sub.3).sub.6 or the
nitrite sulfate salt of Pt. These materials form a Pt exchanged
zeolite. Similar complexes such as Ru(NH.sub.3).sub.4 Cl.sub.3 can
be used to form the Ru-zeolite.
[0127] The inventors recognized that the following is preferably
observed in preparing Pt--RuO.sub.2 Zeolite catalysts.
[0128] (A) The Zeolite to be used should have fairly large pore
diameters to accommodate metals such as Pt, Ru, Ir etc. Mol siv 13x
is a zeolite with pores of about 10.degree. and would be suitable
for this process/purpose.
[0129] (B) At first the zeolite is exchanged with NH.sub.4+ by the
standard Linde Process--(NH.sub.4Cl reflux for 2-4 hours, repeated
2-3 times). Then the exchange with Pt.sup.ll and Ru.sup.lll is
carried out from tetrammine cationic complexes of Pt and Ru. Other
cationic complexes of the metal are also acceptable. Pt.sup.lV and
Ru.sup.lV cationic complexes can also be used. This exchange can be
carried out at 100.degree. C. for efficient exchange. The metal
loading in the zeolite can be controlled by choosing appropriate
amounts of the metal in solution.
[0130] (C ) The exchanged zeolite is now calcined at 550.degree. C.
by using flowing air. This results in the zeolite in the proton
form.
[0131] (D) Next, reduction of the Pt and Ru in the zeolite is
carried out in H.sub.2/Ar mixture (e.g., 3-10% H.sub.2) at a
temperature of about 100-400.degree. C. This method will result in
a zeolite catalyst with high catalytic activity
[0132] (E) After reduction, these catalysts can be thoroughly
washed in water to remove chlorides and then be vacuum dried.
[0133] Similar complexes of iridium, osmium or tungsten can be used
to form the appropriate catalyst. These catalysts are usually more
active than conventionally dispersed catalysts.
[0134] (3) In order to improve conductivity when used as an
electrode, the zeolites should preferably be mixed with a
conducting form of carbon, such as graphite, Acetylene black,
Shawanigan black, or Vulcan XC-72, to produce more conducting
forms. Such an electrode is preferably formed by addition of a
binder such as TEFLON/PVDF or EPDM.
[0135] Alternatively, it may be possible to introduce conducting
back bones in zeolites cages using high concentration of metals in
a selected zeolite with high exchange capacity. Also, carbon
compounds of the type that degrade to elemental carbon can also be
caused to decompose in the cage of the zeolite to form electrically
conducting structures as shown in FIG. 4.
[0136] After such treatment, the zeolite can be exchanged to give
the Pt, Ru or Pt, Pt--Ru--Iv(Os) catalysts.
[0137] VI. A Method of Alloying Pt--RuO.sub.2 Catalysts
[0138] An alloy of Pt--Ru may be used in place Pt--RuO.sub.2, with
some advantages. The method of alloying the Pt--RuO.sub.2 catalysts
to be described is electrochemical. The basis of this method is
that electrochemical hydrogen evolution and electrochemical oxide
reductions are more effective ways of reducing RuO.sub.2 to Ru. The
set up is illustrated in FIG. 5A. A single compartment having
catalyst-suspended solution is used. The potential of the working
electrode has a negative potential while the counter electrode (CE)
has a positive potential.
[0139] Preferably, separated cathode and anode compartments should
be used as shown in FIG. 5B. The catalyst particles are confined in
the center compartment with the working electrode (WE). In one
configuration, WE is held at -0.1 Volt with respect to the
potential at the normal hydrogen electrode("NHE").
[0140] When the particles strike the cathode, electrochemical
reduction will occur. The current and charge can be used to control
the extent of reduction. Such a process is likely to produce an
alloyed Pt--Ru which is more active than that produced by merely
H.sub.2/Ar reduction.
[0141] VII. Fine Particle Amorphous/Intermetallic Alloys by
Hydrolysis of Transition Metal-mixed Grignard Reagents
[0142] A method of preparation of amorphous intermetallic alloys
can be used to prepare catalysts for methanol and methane
oxidation. Aleandri et al. disclose one preparation method in Chem.
Mater., vol.7, pp. 1153-1170 (1995), which is incorporated herein
by reference. A preferred preparation procedure is described
below.
[0143] The transition metal grignard reagents prepared by heating
MCl.sub.x (wherein M stands for transition metal elements including
Ni, Fe, Pd, Pt, Ru, Rh, Ir) with excess amounts of Aactive
magnesium@ or Et.sub.2Mg in THF generating soluble bimetallic
species with generalized formula
M.sup.1(MgCl).sub.m(MgCl.sub.2).sub.p, wherein m=1,2,3 and
p=0{tilde over ()}1. These inorganic grignard reagents react
further with M.sup.2Cl.sub.x in a mole ratio n:m, thus giving
intermetallics as follows:
n(M.sup.1(MgCl).sub.m(MgCl.sub.2).sub.p+m
M.sup.2Cl.sub.n-M.sup.1.sub.nM.s- up.2.sub.m+(mn+p)MgCl.sub.2
[0144] These M.sup.1.sub.nM.sup.2.sub.m alloys are amorphous
nano-scale crystalline. This method produces very fine particles
which will serve as active catalysts.
[0145] 4. Improvement in Bonding between Membrane and Coatings
[0146] The use of isopropanol in the previous experiment
demonstrates that swelling the membrane facilitates chain
entanglement.
[0147] The process of chain entanglement is important in improving
adhesion between two layers of polymers, such as in lamination or
in the bonding of dissimilar membranes which do not bond easily.
Thus swelling the membrane can be applied, for example, to bond
membranes which are not sufficiently thermoplastic to cause
intermingling of polymer chains. The inventors found that solvent
molecules can be used to decrease inter-chain interaction. This
allows the possibility of intermingling of chains between both
layers.
[0148] The inventors developed a method to improve the bonding
between coated electrodes and the membrane. After the carbon-paper
based electrodes are prepared and allowed to dry, they may be
sprayed with a mixture of isopropanol, t-butanol or water-based
mixture of these alcohols, having at least 50% by volume of the
alcohols to swell the dry catalyst layer. This can be done just
before the electrodes are laid on the membrane for pressing.
Alternately, swelling of the dry catalyst layer can also be
achieved by spraying the membrane or soaking the membrane in
alcohols (IPA/t-butanol) or alcohol water mixtures.
[0149] The inventors observed that the temperature resistance of
isopropanol soaked membranes appears to be lower than hydrated
membranes. This manifests itself as browning of the membranes as
the pressing of the MEAs at 145-150.degree. C. Unhydrated membranes
have been found to behave similarly.
[0150] Therefore, it may be preferable to treat the coating with
IPA/butanol, rather than the membrane. It is not clear whether the
ionomer also suffers a change as a result of the low hydration
level. In this case, hydrating the membrane in water may be a
better treatment.
[0151] 5. Water Removal from Air Electrode
[0152] (1) Water draining structures on the carbon paper can be
incorporated on the outside of the gas diffusion backing. This can
either be painted on imprinted by screen painting process. Even a
pressed on structure will be acceptable. The possible materials
which can form these hydrophilic patterns are NAFION, Platinum,
platinized titanium, titanium nitride or gold. NAFION for example
can be painted on to the surface to form lines and patterns. Metal
screens can be pressed on to the surface.
[0153] (2) The whole outer surface of the gas diffusion backing
could be made hydrophilic by spraying a thin layer of NAFION on a
hot substrate, or giving a thin coat of t substrate, or giving a
thin coat by sputter deposition process or electrodeposition
process.
[0154] (3) These hydrophilic structures (in 1 and 2 above) can then
be integrated with wicking structures. One way of testing out the
efficiency of the wicking process is to see how much water comes
off the cathode of a dummy cell when air is blown at a low flow
rate across it. If the amount of water removed would have to be
different for various structures in order for them to make an
impact on efficient water removal.
[0155] 5. Methods for Reducing Fuel Crossover
[0156] Ideally, all fuel should undergo the electro-oxidation at
the anode and be used to generate electrical energy. However, a
portion of the fuel which is dissolved in water permeates through
the solid polymer electrolyte membrane and directly combines with
oxygen. This portion of fuel does not generate any electrical
energy and is essentially wasted. This is called "fuel crossover".
Fuel crossover is undesirable since it reduces the operating
potential of the oxygen electrode and results in consumption of
fuel without producing useful electrical energy. Further, fuel
crossover generates undesirable heat in the fuel cell.
[0157] The inventors discovered that the permeability of the
membrane can be changed by processing the surface with zeolites.
The zeolite structure with the appropriate pore size can be used to
reduce the crossover of the fuel (e.g., methanol). Typically,
zeolites such as Mol-siv 3A, 4A, 5A from Union Carbide in the
protonic form would be candidate materials. These protonic forms
can be produced by standard methods of Ammonium ion exchange
followed by calcining at about 550.degree. C. in air. Such a
calcined zeolite can be used in a number of ways, including:
[0158] a) mixed with the catalyst to fill the voids between the
catalyst particles;
[0159] b) applied along with NAFION as a second layer on the
electrode; and
[0160] c) combined with conductive carbon, such as Shawanigan black
or graphite, and mixed in with NAFION ionomer to form a layer
adjacent to the membrane electrolyte.
[0161] The Zeolite containing layer may be formed to increase the
concentration/content of zeolite in the subsequent layers. In this
way, the catalyst utilization can be maintained without restricting
the access to methanol to the catalyst. When the methanol attempts
to enter the membrane, the zeolite particles will suppress such a
transport.
[0162] Zeolite as a "crossover inhibitor" is preferable to inert
materials such as TEFLON because Zeolites in the protonic form
offer some ionic conductivity.
[0163] This approach can be integrated with zeolite supported metal
catalysts. A mixture of zeolite catalyst and zeolite crossover
inhibitor may be applied. A zeolite, which excludes methanol, may
not be a good choice for catalyst preparation as it prevents access
of the methanol to the catalyst. This reduces the efficacy of
oxidation processes.
[0164] Although the present invention has been described in detail
with reference to a few preferred embodiments, one ordinarily
skilled in the art to which this invention pertains will appreciate
that various modifications and enhancements may be made without
departing from the scope and spirit of the present invention, which
are further defined by the following claims.
* * * * *