U.S. patent application number 11/253146 was filed with the patent office on 2006-05-18 for direct methanol feed fuel cell and system.
This patent application is currently assigned to California Institute of Technology. Invention is credited to William Chun, Harvey A. Frank, Gerald Halpert, Barbara Jeffries-Nakamura, Andrew Kindler, Sekharipuram R. Narayanan, Subbarao Surampudi.
Application Number | 20060105210 11/253146 |
Document ID | / |
Family ID | 27533193 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060105210 |
Kind Code |
A1 |
Surampudi; Subbarao ; et
al. |
May 18, 2006 |
Direct methanol feed fuel cell and system
Abstract
Improvements to non-acid methanol fuel cells include new
formulations for materials. The platinum and ruthenium are more
exactly mixed together. Different materials are substituted for
these materials. The backing material for the fuel cell electrode
is specially treated to improve its characteristics. A special
sputtered electrode is formed which is extremely porous.
Inventors: |
Surampudi; Subbarao;
(Glendora, CA) ; Frank; Harvey A.; (Encino,
CA) ; Narayanan; Sekharipuram R.; (Altadena, CA)
; Chun; William; (Los Angeles, CA) ;
Jeffries-Nakamura; Barbara; (San Marino, CA) ;
Kindler; Andrew; (San Marino, CA) ; Halpert;
Gerald; (Pasadena, CA) |
Correspondence
Address: |
FISH & RICHARDSON, PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Assignee: |
California Institute of
Technology
|
Family ID: |
27533193 |
Appl. No.: |
11/253146 |
Filed: |
October 17, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10932521 |
Sep 1, 2004 |
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11253146 |
Oct 17, 2005 |
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10930505 |
Aug 30, 2004 |
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10932521 |
Sep 1, 2004 |
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10797625 |
Mar 9, 2004 |
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10930505 |
Aug 30, 2004 |
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09894022 |
Jun 27, 2001 |
6703150 |
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10797625 |
Mar 9, 2004 |
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09437331 |
Nov 9, 1999 |
6254748 |
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09894022 |
Jun 27, 2001 |
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09006846 |
Jan 14, 1998 |
6146781 |
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09437331 |
Nov 9, 1999 |
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08569452 |
Dec 8, 1995 |
5773162 |
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09006846 |
Jan 14, 1998 |
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08478801 |
Jun 7, 1995 |
6248460 |
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08569452 |
Dec 8, 1995 |
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08135007 |
Oct 12, 1993 |
5599638 |
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08478801 |
Jun 7, 1995 |
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Current U.S.
Class: |
429/410 ;
429/414; 429/448; 429/450; 429/506; 429/532 |
Current CPC
Class: |
H01M 4/921 20130101;
Y02E 60/523 20130101; H01M 8/1009 20130101; H01M 8/0234 20130101;
H01M 4/8663 20130101; H01M 8/0687 20130101; H01M 8/04194 20130101;
H01M 8/0228 20130101; H01M 8/04186 20130101; H01M 8/22 20130101;
Y02E 60/50 20130101; H01M 8/04156 20130101; H01M 8/1018 20130101;
H01M 8/0202 20130101; H01M 8/1011 20130101; H01M 8/04201 20130101;
H01M 8/1004 20130101; H01M 4/8605 20130101; H01M 8/2455 20130101;
H01M 2300/0082 20130101; H01M 8/0247 20130101 |
Class at
Publication: |
429/022 ;
429/034; 429/026; 429/030; 429/013; 429/015 |
International
Class: |
H01M 8/04 20060101
H01M008/04; H01M 8/10 20060101 H01M008/10 |
Goverment Interests
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] The U.S. Government may have certain rights in this
invention pursuant to NASA Contract No. NAS7-1407.
Claims
1. A methanol consuming system feeding a methanol consumption
device, comprising: a methanol consumption device comprising a
direct feed methanol fuel cell that produces an electrical output;
a fuel storage tank, having a first container area for containing
methanol-containing fuel therein, a water containing area adapted
for holding water therein, and a mixer element, connected to both
said water containing area and said first containing area, an area
associated with said mixer element storing a mixture of methanol
and water, said mixer element also including an output orifice
which feeds said methanol and water mixture to the methanol
consumption device for consumption; and a concentration sensor,
coupled to said mixer element, sensing a concentration of methanol
in said area associated with said mixer element, and connected to
adjust the concentration of methanol.
2. A system as in claim 1, further comprising a water recycling
element, recovering water from an output of said methanol
consumption device and feeding said water back toward said mixer
element.
3. An apparatus as in claim 2, wherein said recovering uses a
condenser.
4. A system as in claim 1, further comprising a controller, said
controller powered by said electrical output from said methanol
fuel cell.
5. A system as in claim 1, further comprising a first valve between
said methanol fuel storage tank and said mixer element, and a
second valve between said water storage tank and said mixer
element, said first and second valves controlled according to an
output of said methanol concentration sensor.
6. An apparatus as in claim 1, further comprising a first valve
between said methanol fuel storage tank and said mixer element, and
a second valve between said water storage tank and said mixer
element, said first and second valves controlled according to an
output of said methanol concentration sensor sensed by said
controller.
7. An apparatus as in claim 1, wherein said direct feed methanol
fuel cell has an anode impregnated with a solid electrolyte, proton
conducting, material.
8. A method of operating a methanol consuming system, comprising:
operating a direct feed methanol fuel cell to produces an
electrical output; storing liquid materials in a fuel storage tank,
having a first container area for containing methanol-containing
fuel therein, a water containing area holding water therein, and a
mixer element, connected to both said water containing area and
said first containing area, an area associated with said mixer
element storing a mixture of methanol and water; feeding an output
of said mixer element to the methanol consumption device for
consumption; and sensing a concentration of methanol in said area
associated with said mixer element, and adjusting the concentration
of methanol based on said sensing.
9. A method as in claim 8, further comprising recovering water from
an output of said methanol consumption device and feeding said
water back toward said fuel storage tank.
10. A method as in claim 8, further comprising powering said
controller using an electrical output from said methanol fuel
cell.
11. A method of operating a fuel cell stack, comprising pressing
the material against an anode of the fuel cell stack at least at
portions of a surface area of the anode; and using a porous
material for said pressing; and supplying and alcohol containing
fuel to said anode while said material is pressed thereagainst.
12. A method as in claim 12, wherein said material is formed into a
shape which equalizes alcohol supplying to the different parts of
the surface area of the anode.
13. A method as in claim 12, wherein said porous material is porous
carbon.
14. A fuel cell assembly, comprising: a fuel cell stack, having an
anode, a cathode, and a proton-conducting electrolyte layer between
said anode and said cathode; a material pressed against the anode
of the fuel cell stack at least at portions of a surface area of
the anode, said material formed of a porous material, at least at
the area where pressing; and a connection to a alcohol containing
fuel supply, adjacent to said material.
15. An assembly as in claim 14, wherein said material is formed
into a shape which equalizes alcohol supplying to the different
parts of the surface area of the anode.
16. An assembly as in claim 14, wherein said porous material is
porous carbon.
17. A system, comprising: a direct fed methanol fuel cell (10)
stack, of a type which operates substantially without an acid
electrolyte (18), and which includes a first input portion for
methanol fuel, coupled to said fuel to an anode (14) portion of the
fuel cell (10) stack, and a second input portion for air to be
applied to a cathode (16) portion of the fuel stack, and which
includes first and second voltage output terminals; and an air
filter, coupled to said second input portion, and operating to
clean the air prior to its introduction into said fuel cell (10)
stack.
18. A system as in claim 17, further comprising an air
pressurization part, coupled to deliver pressurized air to said air
filter.
19. A system as in claim 18, wherein said air pressurization part
uses a pressure driven turbine which recycles pressure.
20. A system as in claim 18, further comprising means to recycle
pressure to drive said air pressurization part.
21. A system as in claim 17, further comprising a fuel filter,
coupled to said first input portion, and operative to filter
methanol fuel.
22. A system as in claim 21, wherein said fuel filter is optimized
to filter hydrocarbon impurities.
23. A system as in claim 21, wherein said fuel filter includes
zeolite crystals.
24. A system as in claim 21, wherein said fuel filter includes a
plurality of different layers of zeolite crystals, each having a
different filter characteristic.
25. A system as in claim 21, wherein said fuel filter includes a
plurality of different layers of filtering materials, each having a
different filtering characteristic.
26. A system as in claim 25, wherein said different filtering
characteristic is a different pore size.
27. A system as in claim 21, wherein said filter includes a zeolite
which acts as a molecular sieve.
28. A system as in claim 21, wherein said fuel filter removes a
specified impurity from the methanol fuel.
29. A system as in claim 28, wherein said fuel filter includes
materials acting as a molecular sieve.
30. A system comprising: a membrane electrode stack, formed of an
anode (14) material, a proton conducting solid electrolyte
membrane, and a cathode (16) material, arranged into a stack, and
operative to produce electricity based on applied methanol via an
electrochemical reaction; a cathode (16) input structure, coupled
to an area of said cathode (16) in said membrane electrode stack,
and including an air filtering part coupled to said cathode (16)
input structure, thereby delivering filtered air to an area of said
cathode (16); and an anode (14) input structure, coupled to an area
of said anode (14) in said membrane electrode stack, and including
a methanol filtering part coupled to said anode (14) input
structure delivering filter methanol to said area of said anode
(14).
31. A system as in claim 30, wherein said methanol filtering part
includes materials forming a molecular sieve.
32. A system as in claim 30, wherein said methanol filtering part
includes three different portions with three different filtering
characteristics.
33. A system as in claim 30, wherein said methanol filtering part
includes a zeolite.
34. A system as in claim 30, further comprising an air pressurizing
part, providing pressurized air to said air filtering part.
35. A system as in claim 34, wherein said air pressurizing part
operates based on recycled pressure within the system.
36. A system as in claim 34, wherein said air pressurizing part
includes a pressure driven turbine.
37. A method, comprising: using a direct fed methanol fuel cell
(10) to extract electricity from an electrochemical reaction of
methanol which electrochemical reaction occurs substantially
without an acid electrolyte.(18); providing fuel to said direct fed
methanol fuel cell (10); and filtering air and providing filtered
air to said direct fed methanol fuel cell (10).
38. A method as in claim 37, wherein said providing fuel comprises
filtering fuel prior to providing said fuel.
39. A method as in claim 37, wherein said filtering fuel comprises
using a molecular sieve to remove particles of specified sizes
prior to providing said fuel.
40. A method as in claim 37, wherein said filtering fuel comprises
using a zeolite to filter the fuel.
41. A method as in claim 37, wherein said providing filtered air
comprises pressurizing air which is provided to said direct fed
methanol fuel cell (10).
42. A method as in claim 37, further comprising recycling pressure
used elsewhere in the system to provide said filtered air.
Description
[0001] This application is a continuation of U.S. application Ser.
No. 10/932,521, filed Sep. 1, 2004; which is a continuation of U.S.
application Ser. No. 10/930,505, filed Aug. 30, 2004; which is a
continuation of U.S. application Ser. No. 10/797,625, filed Mar. 9,
2004; which is a continuation of U.S. application Ser. No.
09/894,022, filed Jun. 27, 2001 (now U.S. Pat. No. 6,703,150);
which is a continuation of U.S. application Ser. No. 09/437,331,
filed Nov. 9, 1999 (now U.S. Pat. No. 6,254,748); which is a
divisional of U.S. application Ser. No. 09/006,846, filed Jan. 14,
1998 (now U.S. Pat. No. 6,146,781); which is a continuation of U.S.
application Ser. No. 08/569,452, filed Dec. 8, 1995 (now U.S. Pat.
No. 5,773,162); which is a continuation-in-part of U.S. application
Ser. No. 08/478,801, filed Jun. 7, 1995 (now U.S. Pat. No.
5,645,573); which is a continuation of U.S. application Ser. No.
08/135,007, filed Oct. 12, 1993 (now U.S. Pat. No. 5,599,638).
FIELD OF THE INVENTION
[0003] The present invention relates to direct feed methanol fuel
cell improvements for a system that operates without an acid
electrolyte or a reformer.
BACKGROUND AND SUMMARY
[0004] Transportation vehicles which operate on gasoline-powered
internal combustion engines have been the source of many
environmental problems. The output products of internal combustion
engines cause, for example, smog and other exhaust gas-related
problems. Various pollution control measures minimize the amount of
certain undesired exhaust gas components. The process of burning,
however, inherently produces some exhaust gases.
[0005] Even if the exhaust gases could be made totally benign,
however, the gasoline based internal combustion engine still relies
on non-renewable fossil fuels.
[0006] Many groups have searched for an adequate solution to the
energy problems.
[0007] One possible solution has been fuel cells. Fuel cells
chemically react using energy from a renewable fuel material.
Methanol, for example, is a completely renewable resource.
[0008] Moreover, fuel cells use an oxidation/reduction reaction
instead of a burning reaction. The end products from the fuel cell
reaction are typically mostly carbon dioxide and water.
[0009] Some previous methanol fuel cells used a "reformer" to
convert the methanol to H.sub.2 gas for a fuel cell. Methanol fuel
cells used a strong acid electrolyte. The present inventors first
proposed techniques which would allow a fuel cell to operate
directly from methanol and without an acid electrolyte--a direct
feed fuel cell. The subject matter of this improvement is described
in our U.S. Pat. No. 5,599,638, the disclosure of which is herewith
incorporated by reference to the extent necessary for proper
understanding. Since this is the work of the present inventors, of
course, there is no admission made here that this patent
constitutes prior art against the present invention.
[0010] The subject matter of the present invention describes
further refinements of such a direct fed fuel cell. Various
improvements to the fuel cell structure itself are described
herein, based on the inventors' further work on this concept. These
improvements include improved formulations for the electrode which
improve its operation. The electrode operation includes an improved
catalyst, which improves the efficiency of methanol production.
Fuel cells use an expensive platinum catalyst. The electrode
formulations given herein define techniques which reduce or obviate
the need for the platinum catalyst.
[0011] Techniques for forming the cathode electrode are also
described herein. These techniques optimize the operation of the
cathode for use with non-pressurized air. This even further
improves the efficiency of the fuel cell by allowing ambient
temperature and atmospheric pressure air as the reduction
mechanism.
[0012] Formation techniques for the electrodes are also described,
including techniques to condition the membrane. A formation of a
particularly preferred membrane electrode assembly is also
defined.
[0013] The present invention also defines flow field designs which
facilitate supplying the liquid fuel to the catalyst.
[0014] The fuel cell system eventually needs to be used in a final
product. This final product could be an internal combustion engine
or could be much simpler electronic devices, such as a radio. Any
electrically-driven product could operate based on electrical power
produced from these fuel cells. The inventors of the present
invention have discovered certain techniques to improve the
operation and ameliorate these problems which might otherwise
exist.
[0015] The techniques of the present invention also enable a
"system operation" by describing techniques to operate the fuel
cell as part of an overall system.
[0016] These system techniques includes sensors for measuring
methanol concentration and other important parameters. The
inventors realized that various sensors for various parameters
would be necessary. The inventors could not find a commercial
sensor. The present invention describes a way of modifying the
techniques which they use in their fuel cell to form a sensor. This
sensor operates with high reliability using the techniques of this
fuel cell.
[0017] Another technique defines formation of monopolar cells.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] These and other aspects of the invention will now be
described in detail with reference to the accompanying drawings,
wherein:
[0019] FIGS. 1 and 2 show a basic fuel cell according to the
present invention;
[0020] FIG. 3 shows the drying dish used for drying teflon encoded
carbon paper sheets;
[0021] FIG. 4 shows the basic platinum sputtering device of the
present invention;
[0022] FIG. 5 shows a basic flow field apparatus according to the
first embodiment of the present invention;
[0023] FIG. 6 shows a cross-sectional view of the preferred flow
field along the line 66 in FIG. 5;
[0024] FIG. 7 shows a first embodiment of the structure of the
biplate of the present invention;
[0025] FIG. 8 shows a second embodiment of the biplate
structure;
[0026] FIG. 9 shows a system operation of the direct methanol field
fuel cell;
[0027] FIG. 10 shows how the fuel cell concepts described above
would be modified for use in a methanol sensor;
[0028] FIG. 11 shows the methanol concentration versus current
relationship of the present invention;
[0029] FIG. 12 shows a graded molecular sieve fuel cell for
methanol according to the present invention;
[0030] FIG. 13 shows a first, expanded, figure of a monopolar
approach to a fuel cell of the present invention;
[0031] FIG. 14 shows the packaging of this monopolar approach;
[0032] FIG. 15 shows a second embodiment of the monopolar approach
in expanded view;
[0033] FIG. 16 shows how this monopolar approach would be assembled
into an operating system; and
[0034] FIG. 17 shows the different expanded layouts of the
monopolar approach assembly.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0035] The liquid feed system described in our above noted patent
uses a platinum-ruthenium catalyst on the anode and a platinum
catalyst on the cathode. A perfluorsulfonic acid membrane,
preferably DuPont's Nafion.TM. 117
tetrafluoroethylene-perfluorovinyl ether sulfonic acid copolymer,
is used as the polymer electrolyte membrane.
[0036] Importantly, this system operated without the necessity for
any acid electrolyte, or reformer. Various characteristics of
various materials were changed to allow this improvement.
[0037] The anode electrode was made more hydrophilic using an
ionomeric additive to improve access of the methanol water
solutions to the anode surface.
[0038] An alternative way of making the anode more hydrophilic was
to use an electrolyte which included a super acid ("a C8
acid").
[0039] Alternative methanol derivative fuels, such as
trimethoxymethane "TMM", reduce fuel crossover due to their
increased molecule size, and other characteristics.
[0040] FIG. 1 illustrates a liquid feed organic fuel cell 10 having
a housing 12, an anode 14, a cathode 16 and a solid polymer
proton-conducting cation-exchange electrolyte membrane 18. As will
be described in more detail below, anode 14, cathode 16 and solid
polymer electrolyte membrane 18 are preferably a single multi-layer
composite structure, referred to herein as a membrane-electrode
assembly. A pump 20 is provided for pumping an organic fuel and
water solution into an anode chamber 22 of housing 12. The organic
fuel and water mixture is withdrawn through an outlet port 23 and
is re-circulated through a re-circulation system described below
with reference to FIG. 2 which includes a methanol tank 19. Carbon
dioxide formed in the anode compartment is vented through a port 24
within tank 19. An oxygen or air compressor 26 is provided to feed
oxygen or air into a cathode chamber 28 within housing 12. FIG. 2
illustrates a fuel cell system incorporating a stack 25 of
individual fuel cells including the re-circulation system, which
includes a heat exchanger 37 receiving the output from the anode
outlet port 23 of the stack 25, fuel/water circulation tank 35 and
a pump 20 to inject a fuel and water solution into the anode
chamber 22 of the stack 25. Methanol from a methanol storage tank
33 enters a fuel and water injection system 29, which provides an
input stream to the circulation tank 35. An oxidant supply system
26 supplies air or oxygen to the cathode chamber 28 of the stack
25. Carbon dioxide and water emitted from the cathode chamber 28 at
outlet 30 are provided to a water recovery unit 27, which in turn
supplies water to the fuel/water injection system 29. The following
detailed description of the fuel cell of FIG. 1 primarily focuses
on the structure and function of anode 14, cathode 16 and membrane
18.
[0041] Prior to use, anode chamber 22 is filled with the organic
fuel and water mixture and cathode chamber 28 is filled with air or
oxygen. During operation, the organic fuel is circulated past anode
14 while oxygen or air is pumped into chamber 28 and circulated
past cathode 16. When an electrical load (not shown) is connected
between anode 14 and cathode 16, electro-oxidation of the organic
fuel occurs at anode 14 and electro-reduction of oxygen occurs at
cathode 16. The occurrence of different reactions at the anode and
cathode gives rise to a voltage difference between the two
electrodes. Electrons generated by electro-oxidation at anode 14
are conducted through the external load (not shown) and are
ultimately captured at cathode 16. Hydrogen ions or protons
generated at anode 14 are transported directly across membrane
electrolyte 18 to cathode 16. Thus, a flow of current is sustained
by a flow of ions through the cell and electrons through the
external load. As noted above, anode 14, cathode 16 and membrane 18
form a single composite layered structure. In a preferred
implementation, membrane 18 is formed from Nafion.TM., a
perfluorinated proton-exchange membrane material. Nafion.TM. is a
co-polymer of tetrafluoroethylene and perfluorovinylether sulfonic
acid. Other membrane materials can also be used. For example,
membranes of modified perflourinated sulfonic acid polymer,
polyhydrocarbon sulfonic acid and composites of two or more kinds
of proton exchange membranes can be used.
[0042] Anode 14 is formed from platinum-ruthenium alloy particles
either as fine metal powders, i.e. "unsupported", or dispersed on
high surface area carbon, i.e. "supported". The high surface area
carbon may be material such as Vulcan XC-72A, provided by Cabot
Inc., USA. A carbon fiber sheet backing (not shown) is used to make
electrical contact with the particles of the electrocatalyst.
Commercially available Toray.TM. paper is used as the electrode
backing sheet. A supported alloy electrocatalyst on a Toray.TM.
paper backing is available from E-Tek, Inc., of Framingham, Mass.
Alternately, both unsupported and supported electrocatalysts may be
prepared by chemical methods, combined with Teflon.TM. binder and
spread on Toray.TM. paper backing to produce the anode. An
efficient and time-saving preferred method of fabrication of
electro-catalytic electrodes is described in detail
hereinbelow.
[0043] Platinum-based alloys in which a second metal is either tin,
iridium, osmium, or rhenium can be used instead of
platinum-ruthenium. In general, the choice of the alloy depends on
the fuel to be used in the fuel cell. Platinum-ruthenium is
preferable for electro-oxidation of methanol. For
platinum-ruthenium, the loading of the alloy particles in the
electrocatalyst layer is preferably in the range of 0.5-4.0
mg/cm.sup.2. More efficient electro-oxidation is realized at higher
loading levels, rather than lower loading levels.
[0044] Cathode 16 is a gas diffusion electrode in which platinum
particles are bonded to one side of membrane 18. Cathode 16 is
preferably formed from unsupported or supported platinum bonded to
a side of membrane 18 opposite to anode 14. Unsupported platinum
black (fuel cell grade) available from Johnson Matthey Inc., USA or
supported platinum materials available from E-Tek Inc., USA are
suitable for the cathode. As with the anode, the cathode metal
particles are preferably mounted on a carbon backing material. The
loading of the electrocatalyst particles onto the carbon backing is
preferably in the range of 0.5-4.0 mg/cm.sup.2. The electrocatalyst
alloy and the carbon fiber backing contain 10-50 weight percent
Teflon.TM. to provide hydrophobicity needed to create a three-phase
boundary and to achieve efficient removal of water produced by
electro-reduction of oxygen.
[0045] During operation, a fuel and water mixture (containing no
acidic or alkaline electrolyte) in the concentration range of
0.5-3.0 mole/liter is circulated past anode 14 within anode chamber
22. Preferably, flow rates in the range of 10-500 ml/min. are used.
As the fuel and water mixture circulates past anode 14, the
following electrochemical reaction, for an exemplary methanol cell,
occurs releasing electrons: Anode:
CH.sub.3OH+H.sub.2OCO.sub.2+6H.sup.++6e.sup.- (1)
[0046] Carbon dioxide produced by the above reaction is withdrawn
along with the fuel and water solution through outlet 23 and
separated from the solution in a gas-liquid separator (described
below with reference to FIG. 2). The fuel and water solution is
then re-circulated into the cell by pump 20.
[0047] Simultaneous with the electrochemical reaction described in
equation 1 above, another electrochemical reaction involving the
electro-reduction of oxygen, which captures electrons, occurs at
cathode 16 and is given by: Cathode:
O.sub.2+4H.sup.+4e.sup.-H.sub.2O (2)
[0048] The individual electrode reactions described by equations 1
and 2 result in an overall reaction for the exemplary methanol fuel
cell given by: Cell: CH.sub.3OH+1.5O.sub.2CO.sub.2+2H.sub.2O
(3)
[0049] At sufficiently high concentrations of fuel, current
densities greater than 500 mA/cm can be sustained. However, at
these concentrations, a crossover rate of fuel across membrane 18
to cathode 16 increases to the extent that the efficiency and
electrical performance of the fuel cell are reduced significantly.
Concentrations below 0.5 mole/liter restrict cell operation to
current densities less than 100 mA/cm.sup.2. Lower flow rates have
been found to be applicable at lower current densities. High flow
rates are required while operating at high current densities to
increase the rate of mass transport of organic fuel to the anode as
well as to remove the carbon dioxide produced by electrochemical
reaction. Low flow rates also reduce the crossover of the fuel from
the anode to the cathode through the membrane.
[0050] Preferably, oxygen or air is circulated past cathode 16 at
pressures in the range of 10 to 30 psig. Pressures greater than
ambient improve the mass transport of oxygen to the sites of
electrochemical reactions, especially at high current densities.
Water produced by electrochemical reaction at the cathode is
transported out of cathode chamber 28 by flow of oxygen through
port 30.
[0051] In addition to undergoing electro-oxidation at the anode,
the liquid fuel which is dissolved in water permeates through solid
polymer electrolyte membrane 18 and combines with oxygen on the
surface of the cathode electrocatalyst. This process is described
by equation 3 for the example of methanol. This phenomenon is
termed "fuel crossover". Fuel crossover lowers the operating
potential of the oxygen electrode and results in consumption of
fuel without producing useful electrical energy. In general, fuel
crossover is a parasitic reaction which lowers efficiency, reduces
performance and generates heat in the fuel cell. It is therefore
desirable to minimize the rate of fuel crossover. The rate of
crossover is proportional to the permeability of the fuel through
the solid electrolyte membrane and increases with increasing
concentration and temperature. By choosing a sold electrolyte
membrane with low water content, the permeability of the membrane
to the liquid fuel can be reduced. Reduced permeability for the
fuel results in a lower crossover rate. Also, fuels having a large
molecular size have a smaller diffusion coefficient than fuels
which have small molecular size. Hence, permeability can be reduced
by choosing a fuel having a large molecular size. While water
soluble fuels are desirable, fuels with moderate solubility exhibit
lowered permeability. Fuels with high boiling points do not
vaporize and their transport through the membrane is in the liquid
phase. Since the permeability for vapors is higher than liquids,
fuels with high boiling points generally have a low crossover rate.
The concentration of the liquid fuel can also be lowered to reduce
the crossover rate. With an optimum distribution of hydrophobic and
hydrophilic sites, the anode structure is adequately wetted by the
liquid fuel to sustain electrochemical reaction and excessive
amounts of fuel are prevented from having access to the membrane
electrolyte. Thus, an appropriate choice of anode structures can
result in the high performance and desired low crossover rates.
[0052] Because of the solid electrolyte membrane is permeable to
water at temperatures greater than 60.degree. C., considerable
quantities of water are transported across the membrane by
permeation and evaporation. The water transported through the
membrane is condensed in a water recovery system and fed into a
water tank (both described below with reference to FIG. 2) so that
the water can be re-introduced into anode chamber 22.
[0053] Protons generated at anode 14 and water produced at cathode
16 are transported between the two electrodes by proton-conducting
solid electrolyte membrane 18. The maintenance of high proton
conductivity of membrane 18 is important to the effective operation
of an organic/air fuel cell. The water content of the membrane is
maintained by providing contact directly with the liquid fuel and
water mixture. The thickness of the proton-conducting solid polymer
electrolyte membranes should preferably be in the range from
0.05-0.5 mm. Membranes thinner than 0.05 mm may result in membrane
electrode assemblies which are poor in mechanical strength, while
membranes thicker than 0.5 mm may suffer extreme and damaging
dimensional, changes induced by swelling of the polymer by the
liquid fuel and water solutions and also exhibit excessive
resistance. The ionic conductivity of the membranes should be
greater than 1 ohm.sup.-1 cm.sup.-1 for the fuel cell to have a
tolerable internal resistance.
[0054] As noted above, the membrane should have a low permeability
to the liquid fuel. Although a Nafion.TM. membrane has been found
to be effective as a proton-conducting solid polymer electrolyte
membrane, perfluorinated sulfonic acid polymer membranes such as
Aciplex.TM. (manufactured by Asahi Glass Co., Japan) and polymer
membranes made by Dow Chemical Co., Japan) and polymer membranes
made by Dow Chemical Co., USA, such as XUS13204.10 which are
similar to properties to Nafion.TM. are also applicable. Membranes
of polyethylene and polypropylene sulfonic acid, polystyrene
sulfonic acid and other polyhydrocarbon-based sulfonic acids (such
as membranes made by RAI Corporation, USA) can also be used
depending on the temperature and duration of fuel cell operation.
Composite membranes consisting of two or more types of
proton-conducting cation-exchange polymers with differing acid
equivalent weights, or varied chemical composition (such as
modified acid group or polymer backbone), or varying water
contents, or differing types and extent of cross-linking (such as
cross linked by multivalent cations e.g., Al 3+, Mg 2+ etc.,) can
be used to achieve low fuel permeability. Such composite membranes
can be fabricated to achieve high ionic conductivity, low
permeability for the liquid fuel and good electrochemical
stability.
[0055] As can be appreciated from the foregoing description, a
liquid feed direct oxidation organic fuel cell is achieved using a
proton-conducting solid polymer membrane as electrolyte without the
need for a free soluble acid or base electrolyte. The only
electrolyte is the proton-conducting solid polymer membrane. No
acid is present in free form in the liquid fuel and water mixture.
Since no free acid is present, acid-induced corrosion of cell
components, which can occur in current-art acid based organic/air
fuel cells, is avoided. This offers considerable flexibility in the
choice of materials for the fuel cell and the associated
sub-systems. Furthermore, unlike fuel cells which contain potassium
hydroxide as liquid electrolyte, cell performance does not degrade
because soluble carbonates are not formed. A solid electrolyte
membrane also minimizes parasitic shunt currents.
[0056] Further Improvements. The reactions of the direct
methanol/liquid-fed fuel cell are as follows: TABLE-US-00001 Anode
CH.sub.3OH + H.sub.2O = 6H.sup.+ + CO.sub.2 + 6e.sup.- Cathode
1.5O.sub.2 + 6H.sup.+ + 6e.sup.- = 4H.sub.2O Net CH.sub.3OH +
1.5O.sub.2 = CO.sub.2 + 2H.sub.2O
[0057] The present specification describes various improvements in
manufacturing and forming the preferred structure and materials
used according the present invention.
[0058] Various experiments carried out by the inventors have
ascertained that one particular preferred catalyst material is
platinum-ruthenium ("Pt--Ru"). Various formulations allowing
combination of those two metals are possible. The inventors found
that a bimetallic powder, having separate platinum particles and
separate ruthenium particles produced a better result than a
platinum-ruthenium alloy. The preferred Pt--Ru material used
according to the present invention has a high surface area to
facilitate contact between the material and the fuels. Both
platinum and ruthenium are used in the catalytic reaction, and the
inventors found that it was important that the platinum and
ruthenium compounds be uniformly mixed and randomly spaced
throughout the material, i.e., the material must be
homogeneous.
[0059] A first aspect of the present invention combines different
metals to form a platinum-ruthenium bimetallic powder which has
distinct sites of different materials. While there is some
combination between the particles, the techniques of the present
invention ensure that the extent of combination is minimal.
[0060] The process of forming the preferred materials is described
herein. First, a slurry of platinum salts and ruthenium salts in
hydrochloric acid is formed.
[0061] A chloroplatinic acid hexahydrate salt H2
PtCl.sub.6.6H.sub.2O is formed by dissolving chloroplatinic acid
crystals in hydrochloric acid.
[0062] A ruthenium salt K2RuCl.sub.5.H.sub.2O is formed from
potassium pentachloroaquoruthenium (III).
[0063] 12.672 grams of chloroplatinic acid crystals are mixed with
13.921 grams of potassium pentachloroaquoruthenium crystals and 600
ml of 1 molar hydrochloric acid. The mixture of acid and salt is
stirred for 15 to 30 minutes to obtain a homogeneous mixture.
[0064] The acid slurry is then neutralized and precipitated by
addition of 140 grams of sodium carbonate (Na.sub.2CO.sub.3) per ml
per minute at between 20-30.degree. C. During this time, carbon
dioxide will vigorously evolve from the solution. The sodium
carbonate is continuously added until the gas evolution ceases. At
this time, the solution turns brown-black. The inventors found that
this took about 15 minutes.
[0065] Maintaining proper pH during this operation is
important--the pH should be maintained at around 9.25 by the slow
addition of sodium carbonate.
[0066] The "grey powdery mass" is then processed to evaporate water
from the slurry. The evaporation takes between 1 and 2 hours and
eventually forms a black gluey solid with dry lumps of the
material. The black gluey solid is then dried in a vacuum or in
flowing nitrogen at 80 to 100.degree. C. A lumpy grey solid is
obtained. This solid includes materials which are still in solution
with the sodium chloride.
[0067] The chemical content of the grey powdery mass Ruthenium
hydroxide--Ru(OH).sub.3, Platinum hydroxide--Pt(OH).sub.4 and
"gunk" or chlorides, plus excess Na.sub.2CO.sub.3.
[0068] The inventors postulate that these extra materials maintain
the separation between the platinum and the ruthenium. If the
materials were maintained alone, they would sinter, causing them to
join and increase particle size. The carbonate buffer between the
particles prevents coalescing.
[0069] This lumpy solid material is then reduced in a hydrogen and
argon atmosphere to reduce the salt to a metal. The material is
transferred into a glass boat. The boat is placed in the center of
a glass tube of a tubular furnace. In a gaseous mixture of 7%
hydrogen, 93% argon or alternatively in a mixture of
hydrogen/nitrogen, the material is reduced at around 225.degree. C.
The gas should be flowing over the boat at a rate of 50 to 200 ml
per minute.
[0070] The gas flow is maintained in the heated atmosphere for 14
hours. Then, with hydrogen still flowing over the powder, the
catalyst powder is allowed to cool to around 40.degree. C. This
forms a mixture of particles of platinum, ruthenium, plus other
chlorides and carbonates.
[0071] The resulting material must then be washed. The material
takes several washes, e.g. six washes at 60.degree. C. Each wash
transfers the sample in the glass boat to a beaker having 1 liter
of de-ionized water at 60.degree. C.
[0072] Platinum-ruthenium is water insoluble. Hence, the washings
do not effect the platinum ruthenium materials, and only removes
the other materials. Each washing includes stirring the water
solution for 15 minutes, to dissolve the soluble chlorides and
carbonates. Since the metal particles are of submicron size, they
do not settle to the bottom, but instead form a colloidal
mixture.
[0073] The solution is allowed to cool to 40.degree. C. The
solution is later centrifuged at 3000 rpm for one hour. The
centrifuging process leaves a clear supernatant liquid. The
supernatant liquid is transferred off, and the black sediment is
transferred to a flask having 1 liter of 60.degree. de-ionized
water. This further washing removes any dissolved chlorides.
[0074] This washing process is repeated a total of six times. It
has been found that stirring the water and centrifuging is
important for total removal of the chlorides. These chlorides are
harmful to catalyst performance. However, the inventors found that
these chlorides are a necessary binder to minimize the material
coalescing but should be removed later.
[0075] After the final centrifuging operation, the powder is
transferred to a beaker and dried in a vacuum oven at 60.degree. C.
for three hours. Alternatively, the material can be freeze-dried.
This results in a free-flowing submicron size active
platinum-ruthenium catalyst. It is important to note that the dried
materials have submicron sizes and hence they can easily become
airborne. A submicron mask must be worn to ensure safety.
[0076] The active catalyst powder has been found to include a
homogeneous mixture of submicron size platinum particles and
ruthenium particles. There are also some trace residuals of
RuO.sub.2, ruthenium oxide, and ruthenium alloy.
[0077] This powder is used as a catalyst on the anode as described
herein.
[0078] The platinum salt and ruthenium salt which are the initial
products of this conversion can also be modified by adding titanium
dioxide (TiO.sub.2), iridium (Ir) and/or osmium (Os). These
materials can be used to improve the fuel cell performance at
relatively nominal cost.
[0079] A comparison with the prior art particles shows the
significant advantages of this process. The prior art particles
form 5 micron size particles. These particles included ruthenium
dioxide. An analysis of the particles of the present invention
shows a homogeneous mixture down to the point of micron particle
size. Under a scanning electron microscope there are no bright and
dull spots--all materials appear to be totally grey. This shows
that the mixing process has formed a totally homogeneous
material.
[0080] The material prepared according to this process is called
anode catalyst material. Further processing of this anode catalyst
by combining with nafion solution, etc. results in an "ink". As
described herein, this includes a combination of platinum metal and
ruthenium metal. The inventors have found the preferred ratio of
platinum to ruthenium can be between 60/40 and 40/60. The best
performance is believed to occur at 60% platinum, 40% ruthenium.
Performance degrades slightly as the catalyst becomes 100%
platinum. It degrades more sharply as the catalyst becomes 100%
ruthenium.
[0081] Other additions are added to the salt to improve
characteristics and to replace the catalyst materials by other
less-expensive materials. The inventors realized that this fuel
cell must be formed from inexpensive material. Unfortunately,
platinum is an extremely expensive material. As of today's writing,
platinum-ruthenium is the best material for the catalyst. The
inventors have investigated using replacements for all or part of
the platinum in the catalyst. The substitution is based on the way
that the platinum-ruthenium catalyst works.
[0082] The reaction which occurs at the anode is
CH.sub.3OH+H.sub.2O.fwdarw.CO2+H.sup.++e.sup.-. The inventors
believe that platinum-ruthenium catalyzes this reaction by aiding
in disassociating the materials on the catalyst surface. The
material draws the electrons out, and allows them to disassociate.
The reaction can be explained as follows.
[0083] Methanol is a carbon compound. The carbon atom is bound to
four other atoms. Three of the bonds are to hydrogen atoms. The
other bond is to a hydroxyl, OH, group. The platinum disassociates
methanol from its hydrogen bonds, to form: M=C--OH (M is the Pt or
other metal site catalyst)+3H.sup.+. The ruthenium disassociates
the hydrogen from the water molecule (HOH) to form M-OH. These
surface species then reassemble as CO.sub.2+6H.sup.++6e.sup.-. The
H.sup.+ (protons) are produced at the anode, and cross the anode to
the,cathode where they are reduced. This is called a bifunctional
catalyst.
[0084] Any material which has a similar function of disassociating
the methanol and water as described can be used in place of the
platinum. The inventors have investigated several such materials.
They found alternatives to platinum including palladium, tungsten,
Rhodium, Iron, Cobalt, and Nickel which are capable of dissociating
C--H bonds. Molybdenum (MoO.sub.3), niobium (Nb.sub.2O.sub.5),
zirconium (ZbO.sub.2), and rhodium (Rh) may also be capable of
dissociating H--OH as M-OH. A combination of these are therefore
good catalysts. The catalyst for dissociating the H--O--H bonds
preferably includes Ru, Ti, Os, Ir, Cr, and/or Mn.
[0085] Ruthenium can be replaced either wholly or partly by a
ruthenium-like material. The inventors found that iridium has many
characteristics which are similar to ruthenium. A first embodiment
of this aspect, therefore, uses a combination of platinum,
ruthenium and iridium in the relative relationship 50-25-25. This
adds the salt H.sub.2IrCl.sub.6.H.sub.2O to the initial materials
described above, in appropriate amounts to make a 50-25-25
(Pt--Ru--Ir) combination.
[0086] It has been found that this catalyst also operates quite
well, using less ruthenium.
[0087] Another material which has been found to have some
advantages is material including titanium compounds. Any titanium
alkoxide or titanium butoxide, e.g. titanium isopropoxide or
TiCl.sub.4--can also be added to the original mixture. This forms
an eventual combination of platinum-ruthenium--TiO.sub.2, also
formed in a 50-25-25 (Pt--Ru--TiO.sub.2) combination.
[0088] Platinum-ruthenium-osmium is also used. Osmium is added to
the mixture as a salt H.sub.2OsCl.sub.6.6H.sub.2O, and has also
been found to produce advantageous properties.
[0089] However formed, these materials used to form the platinum
ink must be applied to the anode. Various techniques can be used to
apply this material. Formation of the anode, therefore, is
described next.
[0090] Carbon Paper Formation.
[0091] Fuel crossover is a source of inefficiency in this fuel
cell. Fuel crossover in this fuel cell occurs when methanol passes
through the anode instead of reacting at the anode. The methanol
passes through the anode, the membrane electrode assembly, through
the membrane and then through the cathode. The methanol may react
at the cathode: this lowers the efficiency of the fuel.
[0092] The electrodes of the present invention are preferably
formed using a base of carbon paper. The starting material point is
TGPH-090 carbon paper available from Toray, 500 Third Avenue, New
York, N.Y. This paper, however, is first pre-processed to improve
its characteristics. The pre-processing uses a DuPont "Teflon 30"
suspension of about 60% solids.
[0093] The paper can alternately be chopped carbon fibers mixed
with a binder. The fibers are rolled and then the binder is burned
off to form a final material which is approximately 75% porous.
Alternately, a carbon cloth paper could be used. This will be
processed according to the techniques described herein.
Alternately, a carbon paper cloth could be used. This will be
processed according to the techniques described herein to form a
gas diffusion/current collector backing.
[0094] The preferably processed carbon paper includes paper within
embedded teflon particles. The spaces between the teflon particles
should preferably be small enough to prevent methanol from passing
therethrough. Even better characteristics are used when other
methanol derivatives, such as TMM are used. The anode assembly is
formed on a carbon paper base. This carbon paper is teflonized,
meaning that teflon is added to improve its properties. The
inventors have found that there is an important tradeoff between
the amount of teflon which is added to the paper and its final
characteristics.
[0095] It is important to maintain a proper balance of the amount
of teflon used, as described herein.
[0096] The paper is teflonized to make it water repellent, and to
keep the platinum ink mix from seeping through the paper. The paper
needs to be wettable, but not porous. This delicate balance is
followed by dipping and heating the paper. The inventors found a
tradeoff between the degree of wettability of the paper and the
amount of impregnation into the paper, which is described
herein.
[0097] First, the Teflon 30 emulsion must be diluted. One gram of
Teflon 30 is added to each 17.1 grams of water. One gram of Teflon
30 of weight 60% corresponds to 60 grams of teflon per 100 ml. This
material is poured into a suitable container such as a glass dish.
The carbon paper is held in the material until soaked.
[0098] The soaking operation corresponds to weighing a piece of
carbon paper, then dipping it into the solution for about 10
seconds or until obviously wet. The carbon paper is removed from
the solution with tweezers, making as little contact with the paper
as possible. However, the characteristics of teflon are such that
the tweezers themselves will attract the teflon, and cause an
uneven distribution of fluid. Teflon-coated tweezers are used to
minimize this possibility. The carbon paper is held with a corner
pointing down, to allow excess solution to drain off.
[0099] Teflon emulsion's surface tension characteristics are such
that if the material were laid on a glass surface, a lot of the
teflon would be dragged out by surface tension. Instead, a
paper-drying assembly is formed as shown in FIG. 3. A plurality of
teflon-covered wires 202 are stretched over a catchbasin such as a
dish 200. The stretched wires form two sets of
orthogonally-extending supports 202 and 204. The carbon paper which
has just been treated with teflon solution is held across these
supports.
[0100] Ideally, the wires are teflon-coated wires having a diameter
of 0.43 inches. While these dimensions are not critical, a smaller
amount of contact with the paper makes the suspension distribution
on the wire more even. Kinks 206 are formed in the wires to prevent
the carbon paper from touching the wires all along its length and
hence further minimize the area of contact.
[0101] The paper-drying assembly shown in FIG. 3 is then placed
into an oven at 70.degree. C. for one hour. The treated carbon
papers are removed from the dish after drying, and placed into
glass containers. These are then sintered in a furnace oven at
360.degree. C. for one hour. A properly processed paper will have
its weight increased by 5% over the course of this process. More
generally, any weight increase between 3 and 20% is acceptable. The
paper is weighed to determine if enough absorption has occurred
and/or if further paper processing will be necessary.
[0102] This substrate plus a catalyst layer forms the eventual
electrode.
[0103] Two preferred techniques of application of the catalyst
including layer are described herein: a direct application and a
sputtering application. Both can use the special carbon paper
material whose formation was described above, or other carbon paper
including carbon paper which is used without any special
processing. The direct application technique of the present
invention mixes materials with the platinum-ruthenium material
described above or any other formulation, more generally, catalyst
materials. The catalyst materials are processed with additional
materials which improve the characteristics.
[0104] Platinum-ruthenium powder is mixed with an ionomer and with
a water repelling material. The preferred materials include a
solution of perfluorsulfonic acid (Nafion) and teflon
micro-particles. 5 grams of platinum-ruthenium powder are added per
100 ml of Nafion in solvent. A DuPont T-30 mix of 60% teflon solid
by weight appropriately diluted is added. These teflon
micro-particles are then mixed. Preferably, a dilute Teflon 30
suspension of 12 weight percent solids including 1 gram of Teflon
30 concentrate to 4 grams of de-ionized water is made. 300 mg of
de-ionized water is added to 350 mg of the 12 weight % teflon
solution described above. 144 mg of platinum-ruthenium is mixed to:
this solution. The resultant mixture is then mixed using an
ultrasonic mixing technique--known in the art as "sonicate". The
ultrasonic mixing is preferably done in an ultrasonic bath filled
with water to a depth of about 1/4 inch. The mixture is
"ultrasonicated" for about 4 minutes.
[0105] It is important that the Teflon must first be mixed with the
platinum-ruthenium as described above to form about 15% by weight
teflon. Only after this mixture is made can the Nafion be added.
The inventors have found that if Nafion is added first, it may
surround the particles of platinum and ruthenium. Therefore, the
order of this operation is critically important. At this point,
0.72 grams of 5 weight % Nafion is added to the jar, which is
sonicated again for 4 minutes. More generally, approximately 1 mg
of Nafion needs to be added per square cm of electrode to be
covered. The amount of teflon described above may also be modified,
e.g. by adding only 652 ml of the solution.
[0106] This process forms a slurry of black material. This slurry
of black material is then applied to the carbon paper. The
application can take any one of a number of forms. The simplest
form is to paint the material on the carbon paper backing, using
alternating strokes in different directions. A small camel hair
brush is used to paint this on. The preferred material amounts
described above, to form enough catalyst for one side of a 2-inch
by 2-inch piece of 5 weight % teflonized carbon paper. Accordingly,
the painting is continued until all the catalyst is used.
[0107] A drying time of two to five minutes between coats should be
allowed, so that the material is semi-dryed between coats and each
coat should be applied in a different direction. The anode needs to
then,dry for about 30 minutes. After that 30 minutes, the anode
must be "pressed" immediately. The pressing operation is described
herein.
[0108] The resulting structure is a porous carbon substrate used
for diffusing gases and liquids, covered by 4 per square cm of
catalyst material.
[0109] An alternative technique of applying the materials sputters
the materials onto the backing.
[0110] We have now described how to form the anode. Next, the
techniques involved in forming the preferred proton conducting
membrane (the Nafion) and then the techniques in forming the
cathode will be described.
[0111] Proton Conducting Membrane--The preferred material described
herein is Nafion 117. However, other materials can also be used to
form proton conducting membranes. For example, other
perfluorsulfonic acid materials can be used. It is postulated that
different materials with carboxylic acid groups can also be used
for this purpose.
[0112] The preferred embodiment starts with Nafion 117, available
from DuPont. This material is first cut to the proper size. Proper
sizing is important, since the end materials will be conditioned.
First, the Nafion is boiled in a hydrogen peroxide solution. A 5%
solution of hydrogen peroxide is obtained, and the membrane is
boiled in this solution for 1 hour in 80-90.degree. C. This removes
any oxidizable organic impurities.
[0113] Following this peroxide boiling step, the membrane is boiled
in de-ionized water, at close to 100.degree. C., for 30 minutes.
Hydrogen peroxide which was previously adsorbed into the membrane
is removed along with other water-soluble organic materials.
[0114] The thus-processed membrane is next boiled in a sulfuric
acid solution. A one molar solution of sulfuric acid is prepared by
diluting commercially available 18-molar concentrated ACS-grade
sulfuric acid. The ACS-grade acid should have metal impurities in
an amount less than 50 parts per million. The membrane is boiled in
the 1-molar sulfuric acid at about 100.degree. C. to more
completely convert the material into a proton conducting form.
[0115] The processed material is next boiled in de-ionized water at
90-100.degree. C. for thirty minutes. The water is discarded, and
this boiling step is repeated three more times to purify the
membrane.
[0116] After these washings, the membrane is free of sulfuric acid
and in completely "protonic" form. The membrane is stored in
de-ionized water in a sealed container until it is ready for
further processing.
[0117] Cathode construction. The cathode is constructed by first
preparing a cathode catalyst ink. The cathode catalyst ink is
preferably pure platinum, although other inks can be used and other
materials can be mixed into the ink as described herein. 250 mg of
platinum catalyst is mixed with 0.5 gram of water including 371/2
mg of teflon. The mix is sonicated for five minutes and combined
with a 5% solution of Nafion. The mix is again sonicated for five
minutes to obtain a uniform dispersal. This forms enough material
to cover one piece of 2.times.2'' carbon paper. Unprocessed Toray
carbon paper can be used with no teflon therein. However,
preferably the material is teflonized as discussed above. The
procedures are followed to make a 5% teflon impregnated paper. The
paper is then heated at 300.degree. C. for one hour to sinter the
teflon particles. Catalyst ink is then applied to the paper as
described above to cover the material with 4 mg/cm.sup.2/g of PT.
Teflon content of the paper can vary from 3-20%, 5% being the
preferred.
Sputtering
[0118] An alternative technique of cathode forming forms a
sputtered platinum electrode. This sputtered platinum electrode has
been found to have significant advantages when used as a plain air
electrode. The process of forming the sputtered platinum electrode
is described herein.
[0119] The cathode electrode carries out a reaction of
O.sub.2+H.sup.++e.sup.-.fwdarw.water. The O.sub.2 is received from
the ambient gas around the platinum electrode, while the electron
and protons are received through the membrane. This alternative
technique for forming the cathode electrode starts with fuel cell
grade platinum. This can be bought from many sources including
Johnson-Matthey. 20 to 30 gms per square meter of surface area of
this platinum are applied to the electrode at a particle size of
0.1 to 1 micron.
[0120] A platinum source is a solid rod of material. According to
this embodiment, the material is sputtered onto the substrate
prepared as described above. The platinum powder is first mixed
with aluminum powder. This mixing can be carried out using
mechanical means for example, or it can be done using salt
combination techniques as described above for the formulation of
the anode ink. The platinum-aluminum mixture is sputtered onto the
carbon paper backing using any known sputtering technique from the
semiconductor arts.
[0121] The platinum is sputtered as follows using the system
diagrammed in FIG. 4. A standard 4-inch target 250 holds the carbon
paper electrode 252. The target is rotated by a motor 254 at one
rotation between per 10 seconds. The preferred technique used
herein sputters platinum from a first Pt source and aluminum from
an Al source 262. The platinum is sputtered to 0.23 amps and the
aluminum at 0.15 amps at around 200 volts. The two sources impinge
from different directions in opposite sides towards the targets at
45.degree. angles.
[0122] The inventors found that 20 Torr was the ideal pressure for
this sputtering, although any pressure between 1 to and 50 to could
be used. The Argon pressure is about 30 mtorr. However, different
argon pressures can be used to form different particle sizes. The
sputtering is done for about 8 minutes.
[0123] Preferably, once sputtered, the etching is carried out by
dipping the sputtered backing into an etching solution, followed by
a washing solution followed by dipping.
[0124] The sputtered electrode is a mixture of Al and Pt particles
on the backing. The electrode is washed with potassium hydroxide
(KOH) to remove the aluminum particles. This forms a carbon paper
backing with very porous platinum thereon. Each of the areas where
the aluminum was formed is removed--leaving a pore space at that
location. The inventors found that a thick coating of the Pt--Al
material would prevent washing out the Al from some lower areas of
the catalyst. The present invention uses a thin coating--preferably
a 0.1 micron coating or less with a material density between 0.2 mg
per cm.sup.2 and 0.5 mg per cm.sup.2.
[0125] At this point in the processing, we now have an anode, a
membrane, and a cathode. These materials are assembled into a
membrane electrode assembly ("MEA")
MEA Formation
[0126] The electrode and the membranes are first laid or stacked on
a CP-grade 5 Mil, 12-inch by 12-inch titanium foil. Titanium foil
is used by the present inventors to present any acid content from
the membrane from leaching into the foil.
[0127] First, the anode electrode is laid on the foil. The proton
conducting membrane has been stored wet to maintain its desired
membrane properties. The proton conducting membrane is first mopped
dry to remove the macro-sized particles. The membrane is then laid
directly on the anode. The cathode laid on top of the membrane.
Another titanium foil is placed over the cathode.
[0128] The edges of the two titanium foils are clipped together to
hold the layers of materials in position. The titanium foil and the
membrane between which the assembly is to be pressed includes two
stainless steel plates which are each approximately 0.25 inches
thick.
[0129] The membrane and the electrode in the clipped titanium foil
assembly is carefully placed between the two stainless steel
platens. The two platens are held between jaws of a press such as
an arbor press or the like. The press should be maintained cold,
e.g. at room temperature.
[0130] The press then actuated to develop a pressure between 1000
and 1500 psi, with 1250 psi being an optimal pressure. The pressure
is held for 10 minutes. After this 10 minutes of pressure, heating
is commenced. The heat is slowly ramped up to about 146.degree. C.;
although anywhere in the range of 140-150.degree. C. has been found
to be effective. The slow ramping up should take place over 25-30
minutes, with the last 5 minutes of heating being a time of
temperature stabilization. The temperature is allowed to stay at
146.degree. C. for approximately 1 minute. At that time, the heat
is switched off, but the pressure is maintained.
[0131] The press is then rapidly cooled using circulating water,
while the pressure is maintained at 1250 psi. When the temperature
reaches 45.degree. C., approximately 15 minutes later, the pressure
is released. The bonded membrane and electrodes are then removed
and stored in de-ionized water.
[0132] Flow Field. A fuel cell works properly only if fuel has been
properly delivered to the membrane to be reacted an/or catalyzed.
The membrane electrode assembly of the present invention uses a
flow field assembly as shown in FIG. 5. Each membrane electrode
assembly ("MEA") 302 is sandwiched between a pair of flow-modifying
plates 304 and 312 which include biplates and end plates. A flow of
fuel is established in each space 303 between each biplate/endplate
and MEA. The collection of biplates/endplates and MEA forms a
"stack". The biplate includes provisions for fluid flow at both of
its oppositely-facing surfaces. The end flowplate of the stack is
an end plate 312 instead of a biplate. The endplate has chambers on
one side only. The biplate 304 includes a plurality of separators
306 and a plurality of chamber forming areas 308. The separators
306 have the function of pressing against the membrane electrode
assembly 302. The end surface of separators 306 are substantially
flat surfaces that contact the surface of the MEA 302.
[0133] The biplates are formed of an electrically conductive
material in order to couple all the membrane electrode assemblies
302, 310 in series with one another.
[0134] Membrane electrode assemblies 302, as described above
include an anode, a membrane, and a cathode. The anode side 312 of
each membrane electrode assembly is in contact with an aqueous
methanol source in space 314. The cathode side of each membrane
electrode assembly is in contact with an oxidant air source 316
which provides the gaseous material for the reactions discussed
above. The air can be plain air or can be oxygen.
[0135] Flows of these raw materials are necessary to maintain
proper supply of fuel to the electrode. It is also desirable to
maintain the evenness of the flow.
[0136] One stack design of the present invention uses the system
shown in FIG. 6. The fuel is supplied from fuel supply chamber 602,
which is typically a high volume element which includes fuel under
pressure. Narrow nozzle-like elements 604 cause a large pressure
drop therealong. The pressure drop in the thin line is much greater
than any pressure drop along the supply. This evens the flow within
the cells and among the cells.
[0137] A careful trade-off must be balanced between the amount of
surface acting as a pressing element and the amount of surface that
acts as a holding element.
[0138] It is desirable to apply even pressure against the membrane
electrode assembly 202 from both sides for many reasons. However,
in places where the pressing surface 306 presses against the
membrane, the membrane electrode assembly 302 cannot be directly in
contact with the methanol. Instead it is pressed by the surface
315. Therefore, that part of the surface of MEA 302 does not react.
The different designs according to the present invention carry out
various functions to improve the flow or improve some
characteristic of its reliability.
[0139] Each of the nozzles 606 has a narrow width. The outlet 605
of each nozzle 606 faces one island pressing area 608 which
corresponds to a pressing surface 306. The supply of fuel from
nozzle 606 is supplied directly against the interface surface 610
of island 608. The islands in FIG. 6 are rectangular in shape.
Interface surface 610 is a narrow side of the rectangular island.
The wider side of the island is parallel to the flow. All input
flows face directly against one of the surfaces of an island.
[0140] The inventor found that this preferred narrow layout creates
turbulence in the area of the islands 608. The turbulence stirs the
fuel in the chamber and forms a more even flow through the system.
This turbulence also facilitates flow between each of the islands.
The output flow is eventually received by output nozzles 612 and
routed into output conduit 614. The output nozzles are analogously
placed adjacent surfaces 620 of the islands, thus causing further
turbulence.
[0141] The islands according to this embodiment are 50 mil on the
interface side 610, and 150 mil on the wider side. The pressure
drop across the stack is approximately 0.06 psi for the stack.
[0142] Other biplate configurations can also be used.
[0143] It is important that the biplates themselves be lightweight
and thin, to allow increase of the stacking pitch as much as
possible.
[0144] Graphite is difficult to machine, and is relatively thick.
Graphite's advantages include its imperviousness to liquid and
gas.
[0145] A number of alternative solutions are used according to the
present invention. A first modification of the present invention
uses the system shown in FIG. 7.
[0146] Interface layer 702 forms a dense, conductive liquid gas
impervious layer. This reduces the amount of fuel, gas or liquid
which can cross the biplate assembly over the materials. However, a
dense porous material is used as the crossing areas 700. The
porosity allows a certain amount of the material to seep into the
MEA through this interface layer.
[0147] The dense porous material can be conductive carbon, for
example, which is much easier to machine than graphite. The seepage
is stopped by interface material, which prevents the liquid and gas
from crossing across the whole biplate.
[0148] The porosity of the pressing parts allows the liquid and gas
to reach some of the parts of the membrane electrode assembly which
are being pressed by the pressing element. The methanol hence may
penetrate to these areas which would otherwise be less efficiently
convecting.
[0149] The central binding layer 704 is low-density ("LD") carbon.
LD carbon is relatively easy to work with and inexpensive. Since
the LD carbon is covered at all locations by graphite, however, its
undesirable characteristics are mostly masked.
[0150] A second embodiment used to form a biplate is shown in FIG.
8. This second biplate embodiment uses a layered titanium-carbon
ultrathin biplate. Any biplate should be thin; because we want the
stack to be as thin as possible for any given voltage. Each
membrane electrode assembly and biplate will produce a voltage when
energized, we call that the inherent voltage. The inherent voltage,
and the thickness of the device, sets the maximum possible
volts-per-inch of thickness of the device of the present invention.
One important component of the volts-per-thickness is the thickness
of the biplate.
[0151] FIG. 8 shows the second biplate embodiment of the present
invention. This material uses a layered concept to form a biplate
combining the best characteristics of the materials. A titanium
carbide interface layer 800 is bonded to titanium bonding layer
802. The titanium bonding layer 802 is preferably 3 mils thick.
These two layers together prevent migration of any protons across
the biplates and also ensure adequate electrical bonding
characteristics. The titanium materials are covered with separating
materials 804 which include surfaces to hold the biplates in place.
A certain measure of porosity is hence enabled as in the FIG. 7
embodiment.
[0152] Of course, the titanium could be replaced by any metal with
similar conducting and chemically stable characteristics.
[0153] The inventors of the present invention recognize that the
graphite material usually used must represent a trade-off between
the competing necessities.
[0154] Efficiency of operation requires that fuel from one side of
one biplate, e.g. the anode side, not seep across to to the other
side of the same biplate, which interfaces to a cathode. If the
biplate were porous, the fuel materials could seep across. However,
since no fluids can pass through the biplates, this has meant that
no fluids can reach the portions of the electron membrane assembly
being pressed by the pressing surfaces biplates, e.g. 306.
Therefore these portions of the membrane electrode assembly which
are being pressed by those pressing surface are not efficiently
producing electrical activity. This lowers the overall efficiency
of the cell.
[0155] These embodiment of the present invention provide a new kind
of trade-off. The membrane electrode assembly is pressed by a
porous portion of the biplate. This porous portion allows at least
some of the fuel to reach that portion of the electrode. This can
improve the electrical operation of the MEA. This feature of the
present invention also provides other bonded pieces which prevent
the fluid from passing over into the other portions of the
electrode membrane assembly.
[0156] System. The basic system of the present invention is shown
in FIG. 9. The system is based on the inventor's recognition of
ways of recycling the output of the fuel cell. The fuel cell
consumes methanol or methanol derivatives, water and produces
output products including methanol or derivatives, water, and
gases. Methanol represents the fuel that is to be consumed. Any
fuel cell system would need to carry quantities of methanol fuel to
be consumed. However, this reaction would also require equal
amounts of water. The inventors recognized that the water used in
the reaction can be recycled from the cathode. This, avoids the
need to The amount of power that a vehicle can produce is limited
by its payload--i.e. the weight of the vehicle and its occupants.
All vehicles are limited in power by the amount of weight that they
must carry. More weight limits a vehicle's power and hence makes
the vehicle less efficient. For example, a passenger car usually
does not hold more than 20-30 gallons of gasoline. This has been
determined by many to represent an optimum trade-off between the
distance that the vehicle can run before re-filling the tank, and
the excess weight that would result from a larger fuel tank.
[0157] Vehicle engineers decide how much payload weight they are
willing to allow. The inventors describe techniques which ensure
that this weight is taken up by fuel, not water.
[0158] One of the features of the system of the present invention
is to maintain the water balance so that most of the water is
recycled and no substantial source of water needs to be
carried.
[0159] The overall system is shown in FIG. 9. Methanol tank 900
stores pure methanol (or other methanol-type derivative fuel). A
first liquid pump 902 pumps the methanol through a valve 904 to a
circulation tank 906. Water tank 908 provides a supply of water
where necessary. The water is pumped by pump 910 through valve 912
to recirculation tank 906. A central controller 914 controls the
overall operation of the entire system. Controller 914 controls the
relative positions of the valves 904 and 912.
[0160] Methanol concentration sensor 916 is preferably located
either in the methanol or very close to it. Methanol sensor 916
detects the concentration of methanol in the circulation tank, and
controller 914 uses this information to control further operation
of the system.
[0161] The aqueous methanol in the circulation tank is maintained
by this control system at 1-2 M. Therefore, the methanol in line
918 should also be of the proper concentration. Pump 920 pumps the
methanol through a fuel filter 922 to the membrane electrode stack
924. The stack used herein can be a similar stack to those
described previously. The electrical output 926 of the stack 924 is
sent to the motor to drive the payload and also drives controller
914 and other electrical systems such as the compressor 930.
[0162] The stack is also driven with inlet air 932 through the
compressor 930. Air filter 934 cleans the air prior to its
introduction into the stack.
[0163] The fuel out of the stack includes two components: water and
methanol. Both components are processed using respective condensers
940 and 942 to lower the water temperature sufficiently to allow
both the methanol and the water to condense. Fans 944 may be used
to facilitate this cooling. The recycled methanol and water are
both resupplied to the circulation tank. The recycled methanol 946
from the output of the methanol stack, and the recycled air and
water from the inlet air 952 recycle into circulation tank 906.
[0164] Fluid engineers have recognized that pumping gas is
extremely expensive in terms of energy resources, while pumping
liquid is extremely inexpensive. One aspect of the present
invention may require pressurizing the air to the cathode. For
example, the air may need to be pressurized to 20 psi. However, the
output air on line 944 (after reacting with the cathode) will be
almost as highly pressurized. This output air 944 will be
pressurized to 19 psi. Accordingly, the output air 946 is coupled
to a pressure-driven turbine 946. This expander is run by pressure,
and used to drive the air compressor 930. Without this recycling of
the pressurized power, the air compressor might use as much as
20-30% of the power produced by the cell.
[0165] Expander output 948 includes an air and water combination.
This water and air is separated to vent the exhaust air at 950, and
the recycled water being returned to the circulation tank 902. A
vent for excess water 954 may also be necessary. This vent is
controlled by controller 914, and necessary at some time if too
much water is being recirculated.
[0166] As an alternative to the sensor and controller, the amount
of fuel which is supplied can be metered. The fuel cell first
starts its operation at room temperature. However, the current fuel
cell is intended to operate at about 90.degree. C. The
electrochemical fuel cell reaction will eventually heat up the fuel
cell to the proper temperature.
[0167] The present invention operates using methanol sensors. A
particularly preferred methanol sensor uses MEA technology
described above. As described above, a fuel cell is formed of an
anode and a cathode. The anode receives methanol. The cathode
receives air or oxygen.
[0168] This sensor uses the modified fuel cell shown in FIG. 10. A
Pt--Ru anode 1002 is connected to a nafion electrolyte 1004, which
is connected to a Pt cathode. The cathode is preferably larger than
the anode, e.g., the cathode is three times the area of the
anode.
[0169] The cathode 1006 (and anode) are immersed in the methanol
solution. Therefore, since the cathode 1006 is under fluid, it
cannot react with air, and hence the H.sub.2 cannot react to
H.sub.2O as in the basic fuel cell reaction. Applying a voltage to
the fuel cell changes, e.g. reverses, the reaction which occurs.
Under current, the anode reacts directly with methanol to form
CO.sub.2, and the cathode will change protons to hydrogen. A small
cathode, and a large anode to reduce protons, further enhances the
sensitivity of this methanol electrode.
[0170] The reactions, therefore, include: (+)
H.sub.2O+CH.sub.3.fwdarw.CO.sub.2+6H.sup.++6e.sup.- (-)
2H.sup.++2e.fwdarw.H.sub.2
[0171] A constant voltage is applied by constant voltage circuit
1010. Ammeter 1012 measures the current. FIG. 11 shows the
relationship between the current and the methanol concentration in
the solution. Controller 1014, which can be a process controller or
microprocessor, looks up the closest methanol concentration
corresponding to the measured current, using the plotted FIG. 11
relationship.
[0172] Since the FIG. 11 plot may be highly temperature dependent,
thermocouple 1016 may provide correction information.
[0173] Another important feature of the present invention is
related to practical use of this system in an automotive
environment. Practical use would require delivery of methanol from
the methanol equivalent of a gas pump. Methanol would have
hydrocarbon impurities when taken from the gas pump. Such
impurities would be very dangerous to the system described by the
present invention which requires highly pure methanol. According to
the present invention, a fuel filter is used. The fuel filter is
shown in FIG. 12. A three stage filter including zeolite crystals
therein of the synthetic 25M (Mobil) types or the natural types.
Typically a zeolite acts as a molecular sieve. The zeolite crystals
are used to filter the methanol to remove any hydrocarbon
impurities therefrom. These zeolites can include a set of layers of
three or more with pore sizes varying from 3-10 .ANG. gradually
from layer 1-3
[0174] 1 2 3 CH.sub.3OH, H.sub.2O, O.sub.2, H2 Layer 1 is typically
the large pore diameter zeolite X, offerite, A to remove large
molecules. Mordenite, a natural zeolite, is used in layer 2 to
exclude n-paraffins, n-butanes and n-alkanes. Zeolite 3A or 4A can
be used to remove small molecules such as propane and ethane in
layer 3. This preferably forms a graded molecular sieve.
Mono-Polar Approaches.
[0175] Previous approaches to fuel cells used a number of fuel
cells in series. The series connection of fuel cells adds the
output voltages to form a higher overall allowed the output of the
stack to be increased to a higher and more usable voltage. The
inventor of the present invention realized, moreover, certain
advantages which can be obtained from using a non stacked approach,
which the present inventor has labelled mono-polar. This monopolar
approach maintains each membrane electrode assembly completely
separately from all the others. This completely different approach
allows each element of the assembly to be made much larger, and
with a better efficiency. However, we can only get a lower output
voltage. Each mono-polar element can be assembled into a stack. The
important thing according to this feature is that each membrane
electrode assembly is separately connected,and the seperately
connected elements are connected in series, rather than assembling
them into a stack.
[0176] A first embodiment of the monopolar invention is shown in
FIG. 13. This embodiment could be used to form a fuel cell that
does not require contact forces in order to make electrical
connections. Membrane 1300 is preferably a Nafion membrane. The
Nafion membrane includes a central area with a termination of metal
cloth strips 1302, eg a screen. The metal cloth or screen 1302 is
covered with appropriate catalysts of the types described above.
Current carrying tabs 1304 bring the voltage which is produced to
the outside.
[0177] A plastic or metal flow field insert 1306 channels the
appropriate fuel material to the respective side of the
catalyst-covered cloth. Flow field element 1308 can be located on
the other side.
[0178] The material with the catalyst thereon is therefore attached
to the Nafion backing and pressed thereagainst to form a fuel cell
in a similar electrical but different mechanical way.
[0179] FIG. 14 shows a cross-section of the device. The tabs 1304
conduct the electricity to an electrode area 1400. Methanol is
brought into a methanol chamber 1402, into a sealed area on a first
side of the membrane. The seal is maintained by a ring sealing area
1406. Air is conducted to a second side of the membrane through air
chamber 1408, which is similarly sealed on the other side. Each of
these elements operates as a stand-alone unit, independent of the
other units. The current from these elements can be connected in
series to provide a higher voltage.
[0180] A second alternative embodiment of the invention is shown in
FIG. 15. This embodiment uses a membrane 1500, along with a
titanium sheet 1502. Titanium cloth 1504 is spot-welded to the
titanium sheet. The titanium cloth 1504 acts as the cathode and may
be coated with platinum. Titanium cloth 1506 acts as the anode and
may be coated with appropriate platinum ruthenium.
[0181] A gasket and bonding ring 1508 forms a chamber 1510 between
the membrane and the anode. In a similar way, another gasket and
bonding ring 1510 forms a chamber between the membrane and the
cathode.
[0182] The titanium sheet has a bead seal 1512 thereon to maintain
the chamber. Voltage produced by the titanium sheet is coupled to
the current takeoff area 1514.
[0183] This embodiment also includes places for rivets or
fasteners, since the bead sealing would allow metal fasteners to be
used. This integrated system could be extremely thin, especially if
titanium foil were used.
[0184] The elements could be used in a device shown in FIG. 16.
Each of these dual cell modules shown in FIGS. 13 or 15 includes a
cathode and anode thereon. The elements shown in FIG. 16 are
assembled to form two adjacent anodes up to cells 1602 and 1604
which face one another. A flow field 1606 is established between
the anode 1602 and 1604. This flow field should include an air flow
there between. In a similar way, two adjacent cathode face one
another and a flow field 1608 is formed therebetween to include the
appropriate airflow therebetween.
[0185] FIG. 17 shows an expanded view of how these cells would be
used. Flow field 1700 is an airflow field which faces the cathode
side 1702 of first cell 1704. The anode side 1706 faces a second,
methanol flow field 1708. Methanol is input through methanol input
port 1710 and out through output port 1712. The methanol flow field
also faces the anode side 1714 of a second bipolar cell 1716. The
cathode side 1720 of the second bipolar cell 1716 faces another
flow field element 1722 for air.
[0186] Although only a few embodiments have been described in
detail above, those having ordinary skill in the art will certainly
understand that many modifications are possible in the preferred
embodiment without departing from the teachings thereof.
[0187] All such modifications are intended to be encompassed within
the following claims.
* * * * *