U.S. patent application number 11/796469 was filed with the patent office on 2007-10-04 for method to manufacture composite polymer electrolyte membranes coated with inorganic thin films for fuel cells.
This patent application is currently assigned to Korea Institute of Science & Technology. Invention is credited to Heung-Yong Ha, Seong-Ahn Hong, Daejin Kim, Soon Jong Kwak, Tae-Hoon Lim, Suk-Woo Nam, In-Hwan Oh, Juno Shim.
Application Number | 20070231655 11/796469 |
Document ID | / |
Family ID | 33448323 |
Filed Date | 2007-10-04 |
United States Patent
Application |
20070231655 |
Kind Code |
A1 |
Ha; Heung-Yong ; et
al. |
October 4, 2007 |
Method to manufacture composite polymer electrolyte membranes
coated with inorganic thin films for fuel cells
Abstract
The present invention relates to a method for manufacturing
composite polymer electrolyte membranes coated with inorganic thin
films for fuel cells using a plasma enhanced chemical vapor
deposition (PECVD) method or a reactive sputtering method, so as to
reduce the crossover of methanol through polymer electrolyte
membranes for fuel cells and enhance the performance of the fuel
cells. The manufacturing method of composite polymer electrolyte
membranes coated with inorganic thin films for fuel cells according
to the present invention is characterized to obtain composite
membranes by coating the surface of commercial composite polymer
electrolyte membranes for fuel cells with inorganic thin films
using a PECVD method or a reactive sputtering method. The inorganic
materials to form the inorganic thin films are chosen one or more
from the group comprising silicon oxide (SiO.sub.2), titanium oxide
(TiO.sub.2), zirconium oxide (ZrO.sub.2), zirconium phosphate
(Zr(HPO.sub.4).sub.2), zeolite, silicalite, and aluminum oxide
(Al.sub.2O.sub.3). The present invention, by coating the polymer
electrolyte membranes for fuel cells with inorganic thin films via
a PECVD method or a reactive sputtering method, reduces the
methanol crossover sizably without seriously reducing the ionic
conductivity of polymer electrolyte membranes, thereby, when
applied to fuel cells, realizes a high performance of fuel
cells.
Inventors: |
Ha; Heung-Yong; (Seoul,
KR) ; Kwak; Soon Jong; (Seoul, KR) ; Kim;
Daejin; (Gyeongjoo-si, KR) ; Shim; Juno;
(Seoul, KR) ; Oh; In-Hwan; (Seoul, KR) ;
Hong; Seong-Ahn; (Seoul, KR) ; Lim; Tae-Hoon;
(Seoul, KR) ; Nam; Suk-Woo; (Seoul, KR) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Assignee: |
Korea Institute of Science &
Technology
Seoul
KR
|
Family ID: |
33448323 |
Appl. No.: |
11/796469 |
Filed: |
April 27, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10751138 |
Dec 30, 2003 |
|
|
|
11796469 |
Apr 27, 2007 |
|
|
|
Current U.S.
Class: |
429/483 ;
429/325; 429/492; 429/496; 429/516; 429/535 |
Current CPC
Class: |
Y02E 60/523 20130101;
C08J 2327/18 20130101; C08J 5/2287 20130101; H01M 8/1025 20130101;
H01M 4/881 20130101; H01M 8/103 20130101; Y02P 70/56 20151101; H01M
8/1069 20130101; Y02E 60/50 20130101; H01M 8/04197 20160201; H01M
8/1023 20130101; H01M 8/1039 20130101; H01M 2300/0094 20130101;
Y02P 70/50 20151101; H01M 8/1055 20130101 |
Class at
Publication: |
429/033 ;
429/325 |
International
Class: |
H01M 8/10 20060101
H01M008/10 |
Foreign Application Data
Date |
Code |
Application Number |
May 31, 2003 |
KR |
10-2003-0035127 |
Claims
1. A composite polymer electrolyte membrane coated with inorganic
thin films for fuel cells manufactured by a method comprising
coating the surface of polymer electrode membranes with inorganic
thin films using a plasma enhanced chemical vapor deposition
(PECVD) method or a reactive sputtering method.
2. The composite polymer electrolyte membrane of claim 1, wherein
the inorganic thin films are chosen from one or more of the group
consisting of silicon oxide (SiO.sub.2), titanium oxide
(TiO.sub.2), zirconium oxide (ZrO.sub.2), zirconium phosphate
(Zr(HPO.sub.4).sub.2), zeolite, silicalite, and aluminum oxide
(Al.sub.2O.sub.3).
3. The composite polymer electrolyte membrane of claim 1, wherein
the polymer electrolyte membranes are perfluorosulfonic acid
membranes, electrolyte membranes made of proton conducting
hydrocarbon materials, or electrolyte membranes made of proton
conducting fluorine materials.
4. The composite polymer electrolyte membrane of claim 1, wherein
the PECVD method used to make the composite polymer electrolyte
membrane uses reactants comprising one or more monomers selected
from the group consisting of organic metal compounds containing
aluminum, titanium, silicon, and zirconium in conjunction with one
or more gases selected from the group consisting of oxygen,
nitrogen, hydrogen, steam, and argon.
5. The composite polymer electrolyte membrane of claim 1, wherein
the reactive sputtering method used to make the composite polymer
electrolyte membrane uses a 99% or higher purity metal target such
as Si, SiO2, SiNH, Al, Zr, or Ti, and maintains its initial
pressure at a high vacuum range of 1.0.times.10.sup.-3 torr to
1.0.times.10.sup.-6 torr.
6. The composite polymer electrolyte membrane of claim 1, wherein
the PECVD or reactive sputtering method used to make the composite
polymer electrolyte membrane has as microwave power range of about
10 watts to about 500 watts.
7. The composite polymer electrolyte membrane of claim 1, wherein
the reaction chamber pressure for the PECVD method used to make the
composite polymer electrolyte membrane is in the range of about 1.0
to about 1000 millitorr.
8. The composite polymer electrolyte membrane of claim 1, wherein
the PECVD method or reactive sputtering method used to make the
composite polymer electrolyte membrane has an argon pretreatment
electromagnetic wave power in the range of about 10 watts to about
500 watts.
9. The composite polymer electrolyte membrane of claim 1, wherein
the PECVD method used to make the composite polymer electrolyte
membrane has an argon pretreatment pressure of about 1.0 to about
500 millitorr.
10. The composite polymer electrolyte membrane of claim 1, wherein
the reaction chamber pressure for the PECVD method used to make the
composite polymer electrolyte membrane is in the range of about 10
to about 500 millitorr.
11. The composite polymer electrolyte membrane of claim 1, wherein
the thickness of the inorganic thin films is in the range of about
1.0 to about 500 nm.
12. The composite polymer electrolyte membrane of claim 1, wherein
the composite membrane is further manufactured by a method
comprising a step of coating the surface of an electrolyte membrane
with a proton-conducting ionomer solution after coating the
inorganic thin film on the electrolyte membrane surface, so as to
enhance contact with the electrodes during manufacturing.
13. A membrane-electrode assembly (MEA) employing composite polymer
electrolyte membranes coated with inorganic thin films manufactured
by a method comprising coating the surface of polymer electrode
membranes with inorganic thin films using a plasma enhanced
chemical vapor deposition method or a reactive sputtering
method.
14. A fuel cell employing composite polymer electrolyte membranes
coated with inorganic thin films, or employing an MEA containing
composite polymer electrolyte membranes with inorganic thin films,
wherein the composite polymer membranes are manufactured by a
method comprising coating the surface of polymer electrode
membranes with inorganic thin films using a plasma enhanced
chemical vapor deposition method or a reactive sputtering method.
Description
[0001] This application is a division of U.S. application Ser. No.
10/751,138 filed on Dec. 30, 2003, which claims priority to Korean
Application 10-2003-0035127 filed on May 31, 2003 which
applications are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method to manufacture
composite polymer electrolyte membranes coated with inorganic thin
films for fuel cells and applications of these membranes; more
particularly to a method of coating the surface of commercial
polymer electrolyte membranes with inorganic thin films using a
plasma enhanced chemical vapor deposition (PECVD) method or a
reactive sputtering method, thus reducing the methanol permeability
without a sizable decrease of ionic conductivity, thereby realizing
a lower methanol permeability than that of conventional Nafion.RTM.
membranes or other composite polymer electrolyte membranes and,
therefore, enhancing the performance of methanol fuel cells, and
also relates to composite polymer electrolyte membranes coated with
inorganic thin films for fuel cells, which are manufactured by said
method
[0004] Also, the present method relates to an membrane-electrode
assembly (MEA) employing composite polymer electrolyte membranes
coated with inorganic thin films for fuel cells, which are
manufactured by the aforementioned method, and a method to
manufacture the same.
[0005] 2. Description of the Related Art
[0006] A direct methanol fuel cell (hereinafter referred to as
"DMFC") has the same structure and operates on the same principle
as a polymer electrolyte membrane fuel cell (hereinafter referred
to as "PEMFC") using hydrogen, but in case of the DMFC, methanol is
directly fed to the anode as a fuel instead of hydrogen. Therefore,
its fuel feeding system and overall device is simple compared with
the PEMFC, which makes it available in a compact-size. Also, the
DMFC has other advantages that the liquid fuel composed of methanol
and water functions as a coolant as well as a fuel, the whole
device is compact and light-weighted, the operating temperature is
much lower than that of the conventional fuel cells, and it can
operate for a longer duration at a time due to its convenient
refueling.
[0007] However, the DMFC has drawbacks that its electrode
performance is low due to the methanol oxidation at the cathode
side, the platinum catalyst is poisoned by carbon monoxide which is
one of reaction products, and the power density is lower than that
of PEMFCs. Also, the DMFC has other drawbacks of excessive
consumption of expensive platinum catalyst and gradual performance
degradation. Yet, the most serious problem of the DMFC is the
degradation of its cell performance due to methanol crossover from
the anode to the cathode.
[0008] The DMFC can overcome limitations on small-sized batteries
and inconveniences caused by recharging needs and, therefore, has
high prospects of being used as portable power sources for mobile
phones, PDAs, and notebook computers. Further, with more
improvement in performance, the DMFC could be made available as an
automobile power source.
[0009] In these DMFCs, an electrolyte membrane carries out not only
the role as a proton conductor from the anode to the cathode but
also the role as a barrier to methanol and oxygen. Therefore,
polymer electrolyte membranes for fuel cells should have a high
ionic conductivity and yet a low electronic conductivity. Also,
polymer electrolyte membranes for fuel cells should transfer less
methanol or water, and be highly stable mechanically, thermally,
and chemically.
[0010] However, although Nafion.RTM. membranes of Du Pont in
general use or other commercially available membranes have a
superior ionic conductivity, they have the problem that methanol is
permeated from the anode to the cathode. This permeated methanol is
oxidized on the cathode, poisoning the platinum catalyst thereby
causing mixed potentials and, therefore, degrading the whole
performance of the cell.
[0011] Lots of researches have been performed to resolve this
crossover problem in DMFCs. The researches are carried out in two
different directions. One is to develop new polymer electrolyte
membranes; the other is to improve conventional commercial polymer
electrolyte membranes.
[0012] As a former example, U.S. Pat. No. 6,503,378 describes a
method of manufacturing a composite polymer electrolyte membrane
superior in thermal, chemical, and mechanical characters, in which
the polymer electrolyte membrane comprised of a hydrophobic
hydrocarbon region and a hydrophilic region that are covalently
bound to form a single polymer molecule. However, this method is
short of reducing the methanol crossover.
[0013] Korean Unexamined Patent Publication No. 2003-0004065
describes a method of manufacturing partly fluorinated copolymers
based on vinyl compounds substituted with trifluorostyrene, and
ionic conductive polymer electrolyte membranes made of the same. It
is described that electrolytes can be manufactured with a superior
mechanical property at low cost and the swelling can be reduced
compared with conventional cases. Yet, it does not report that the
methanol permeability can be reduced.
[0014] Korean Unexamined Patent Publication No. 2002-0074582
describes a method, in which mixed polymer solutions are made by
adding perfluorinate ionomers (eg. Nafion.RTM. solution) in polymer
matrix, and then polymer membrane is manufactured by casting
method, and composite membrane is obtained by coating the
perfluorinate ionomers on both sides of the membrane. This method
is described to manufacture composite membranes with a superior
performance characteristics at a lower cost compared with
commercially available Nafion.RTM. membranes. Yet, it has drawbacks
that the mechanical property of the composite membrane is inferior
and the manufacturing process is complicated.
[0015] As a second example of modifying Nafion.RTM. membranes, some
researchers proposed a method producing Nafion.RTM./silicon oxide
composite membranes via sol-gel reaction using Nafion.RTM. 115 and
tetraethylorthosilicate (TEOS) [D. H. Jung, S. Y. Cho, D. H. Peck,
D. R. Shin and J. S. Kim, Journal of Power Sources, 4683 1-5
(2002)]. This method showed that the methanol permeability
decreases with increasing silicon oxide content in the membrane. In
cells using this composite membranes according to said method, the
current density was 650 mA/cm.sup.2 at a cell voltage of 0.5 V and
temperature of 120, which is a superior result when compared with
other commercial membranes. However, this method has drawbacks that
the ionic conductivity is decreased compared with Nafion.RTM.
membranes and the performance is decreased with increasing silicon
oxide content more than 12%.
[0016] As another example, some researches proposed a fabrication
method in which a polybenzimidazole layer is formed at the surface
of Nafion.RTM. membrane by screen printing method [L. J. Hobson, Y.
Nakano, H. Ozu and S. Hayase, Journal of Power Sources, 104,
1(2002)]. The composite polymer electrolyte membrane via this
method was shown to reduce the methanol permeability by 40 to 60%
and the cell performance was improved by 46%. However, the ionic
conductivity has been decreased by about 50% compared with
Nafion.RTM. membranes.
[0017] Also, another method to manufacture membranes has been
proposed, which improved the cell performance by 51%. This method
performs a surface treatment by exposing the surface of Nafion.RTM.
membrane in electron beam of 9.2 .mu.C/cm.sup.2 at 35 kV of
accelerated voltage [L. J. Hobson, H. Ozu, M. Yamaguchi, and
Hayase, Journal of The Electrochemical Society, 148, 10 (2001)].
However, this modified membrane does not reduce the methanol
crossover as compared with Nafion.RTM. membrane, and shows a
drawback that sulfonic groups on the surface are eliminated to a
sizable degree.
[0018] Therefore, a novel method to manufacture polymer electrolyte
membranes for fuel cells is required to improve the fuel cell
performance by resolving the drawbacks of conventional polymer
electrolyte membranes for fuel cells and even more reducing the
methanol crossover.
SUMMARY OF THE INVENTION
[0019] An object of the invention is to provide a method to
manufacture composite polymer electrolyte membranes coated with
inorganic thin films for fuel cells and the membranes made by the
same method, in which the surface of composite polymer electrolyte
membranes are coated with inorganic thin films using a PECVD method
or a reactive sputtering method, thus reducing the methanol
permeability without a sizable decrease of ionic conductivity,
thereby realizing a more reduced methanol permeability than that of
conventional Nafion.RTM. membranes or other composite polymer
electrolyte membranes and, therefore, enhancing the performance of
methanol fuel cells.
[0020] Another object of the invention is to provide a
membrane-electrode assembly (hereinafter referred to as `MEA`) and
a fuel cell which employs the composite polymer electrolyte
membranes coated with inorganic thin films, and the manufacturing
method of the same.
[0021] To accomplish the above objects, the method to manufacture
composite polymer electrolyte membranes coated with inorganic thin
films for fuel cells according to the present invention is
characterized to obtain composite membranes by coating the surface
of the composite polymer electrolyte membrane for fuel cells with
inorganic thin films using a PECVD method or a reactive sputtering
method.
[0022] In the method to manufacture composite polymer electrolyte
membranes coated with inorganic thin films for fuel cells according
to the present invention, inorganic materials of said inorganic
thin films are chosen one or more from the group comprising silicon
oxide (SiO.sub.2), titanium oxide (TiO.sub.2), zirconium oxide
(ZrO.sub.2), zirconium phosphate (Zr(HPO.sub.4).sub.2), zeolite,
silicalite, and aluminum oxide (Al.sub.2O.sub.3).
[0023] In the method to manufacture composite polymer electrolyte
membranes coated with inorganic thin films for fuel cells according
to the present invention, said polymer electrolyte membranes are
perfluorosulfonic acid membranes such as Nafion.RTM. membrane (Du
Pont), Dow membrane (Dow Chemical), Flemion membrane (Asahi Glass
Co.), Aciplex membrane (Asahi Chem.), BAM (Ballarde), or
Gore-select membrane (W.L. Gore, Inc.); polymer electrolyte
membranes made of proton conducting hydrocarbon polymers such as
sulfonic polysulfone, sulfonic polyethylene, sulfonic
polypropylene, sulfonic polystyrene, sulfonic polyphenol
formaldehyde, polystyrene divinylbenzene sulfonic acid, sulfonic
polybenzyimidasol, sulfonic polyamide, or sulfonic polyether-ether
ketone; or polymer electrolyte membranes made of proton conducting
polymers containing florine such as sulfonic polyvinylidene
fluoride, sulfonic polytetrafluorethylene, or fluoric ethylene
propylene.
[0024] In the method to manufacture composite polymer electrolyte
membranes coated with inorganic thin films for fuel cells according
to the present invention, the PECVD method uses reactants being one
or more monomers chosen from the group of organic metal compounds
containing aluminum, titanium, silicon, and zirconium in
conjunction with one or more gases from the group of oxygen,
nitrogen, hydrogen, steam, and argon.
[0025] In the method to manufacture composite polymer electrolyte
membranes coated with inorganic thin films for fuel cells according
to the present invention, said organic metal compounds are one or
more chosen from the group comprising trimethyl disiloxanes
(TMDSO), hexamethyl disiloxane (HMDSO), hexamethyl disilane,
tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate,
tetrabuthyl orthosilicate, tetra-isopropyl orthosilicate, aluminium
methoxide, aluminium ethoxide, aluminium butoxide, aluminium
isopropoxide, titanium ethoxide, titanium methoxide, titanium
butoxide, titanium isopropoxide, zirconium ethoxide, and zirconium
butoxide.
[0026] In the method to manufacture composite polymer electrolyte
membranes coated with inorganic thin films for fuel cells according
to the present invention, said reactive sputtering method is
characterized to use a 99% or higher pure metal target such as Si,
SiO.sub.2, SiNH, Al, Zr, or Ti, and to maintain its initial
pressure at a high vacuum range of 1.0 10.sup.-3 torr to 1.0
10.sup.-6 torr.
[0027] In the method to manufacture composite polymer electrolyte
membranes coated with inorganic thin films for fuel cells according
to the present invention, said reactive sputtering method is
characterized to vapor-depositing an in organic film on the target
surface after cleaning by sputtering the surface in a 99.9% or
higher argon gas atmosphere so as to prevent oxidation of the
target surface during the sputtering.
[0028] In the method to manufacture composite polymer electrolyte
membranes coated with inorganic thin films for fuel cells according
to the present invention, said PECVD device or reactive sputtering
method is characterized to have a microwave power at the range of
10 watts to 500 watts.
[0029] In the method to manufacture composite polymer electrolyte
membranes coated with inorganic thin films for fuel cells according
to the present invention, the reaction chamber pressure of said
PECVD method or reactive sputtering method is in the range of 1.0
to 1000 millitorr.
[0030] In the method to manufacture composite polymer electrolyte
membranes coated with inorganic thin films for fuel cells according
to the present invention, the argon pre-treatment electromagnetic
wave power of said PECVD method or reactive sputtering method is in
the range of 10 watts to 500 watts.
[0031] In the method to manufacture composite polymer electrolyte
membranes coated with inorganic thin films for fuel cells according
to the present invention, the argon pre-treatment pressure of said
PECVD method is in the range of 1.0 to 500 millitorr.
[0032] In the method to manufacture composite polymer electrolyte
membranes coated with inorganic thin films for fuel cells according
to the present invention, the reaction gas pressure in the chamber
of said PECVD method is in the range of 10 to 500 millitorr.
[0033] In the method to manufacture composite polymer electrolyte
membranes coated with inorganic thin films for fuel cells according
to the present invention, the distance between electrodes of said
PECVD method is in the range of 1 to 30 cm.
[0034] In the method to manufacture composite polymer electrolyte
membranes coated with inorganic thin films for fuel cells according
to the present invention, the thickness of said inorganic films is
in the range of 1.0 to 500 nm.
[0035] The method to manufacture composite polymer electrolyte
membranes coated with inorganic thin films for fuel cells according
to the present invention further comprises a step of coating the
surface of composite polymer electrolyte membrane with an ionomer
solution of commercial polymer electrolyte membranes mixed with a
solution of water and isoprophyl alcohol, after coating said
inorganic film on the surface, so as to enhance contact with the
electrodes during manufacturing the fuel cells.
[0036] A composite polymer electrolyte membrane coated with
inorganic thin films for fuel cells according to the present
invention is characterized to be manufactured via the
aforementioned method.
[0037] A MEA according to the present invention employs the
composite polymer electrolyte membranes coated with inorganic thin
films for fuel cells manufactured via the aforementioned
method.
[0038] A method of manufacturing an MEA according to the present
invention includes a process of coating catalyst compounds for
electrodes directly on the composite polymer electrolyte membranes
coated with inorganic thin films for fuel cells manufactured via
the aforementioned method. Said direct coating of the electrode
catalyst, in which the electrode comprises catalysts and ionic
conductive materials, is to reduce the contact resistance between
the electrolyte membrane and the electrodes.
[0039] A fuel cell according to the present invention employs the
composite polymer electrolyte membranes coated with inorganic thin
films for fuel cells manufactured via the aforementioned
method.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 shows a schematic representation of a plasma enhanced
chemical vapor deposition (PECVD) device used to coat polymer
electrolyte membranes with inorganic thin films for fuel cells
according to the present invention.
[0041] FIG. 2 shows a scanning electron microscope (SEM) image of a
surface of composite polymer electrolyte membranes coated with
inorganic thin films for fuel cells manufactured according to a
third embodiment of the present invention.
[0042] FIG. 3 shows a diagram comparing the characteristic factors
of composite polymer electrolyte membranes coated with inorganic
thin films for fuel cells, which are manufactured according to
first to fifth embodiments of the present invention, and
conventional polymer electrolyte membranes.
DETAILED DESCRIPTION OF THE INVENTION
[0043] The PECVD technique utilizes a phenomenon that, when gas and
inorganic vapor are transferred into plasma under low pressure,
polymer materials are produced and coated on the substrate surface.
If polymerization reactions take place under near vacuum condition,
polymerized polymers grow to a film on the surrounding solid
surfaces. Therefore, the PECVD method is suitable for manufacturing
of membranes and improving of solid surfaces. The PECVD has the
following advantages.
[0044] {circle around (1)} Coating is uniform without flaws.
[0045] {circle around (2)} There are a variety of choices on
coating materials, since polymerization is possible even when
monomers has no functional group.
[0046] {circle around (3)} Coating is possible on any material if
it is stable under the vacuum.
[0047] {circle around (4)} Adhesive strength of the coated film is
superior.
[0048] {circle around (5)} Environmental pollution due to solvents
is prevented, since the method is performed in a dry condition.
[0049] {circle around (6)} The method is economic, since it
consumes less materials and energy.
[0050] A film manufactured by the PECVD method is generally known
to have a highly crosslinked and tight structure, a good mechanical
property, superior insolubility and thermal stability. A prepared
membrane is known to be uniform without pinholes and to have a
superior barrier property against gases and liquids. Also, since
the film is superior in its adhesive strength, it can be used as
protective materials for substrates.
[0051] FIG. 1 shows a schematic representation of a PECVD device
used in the present invention.
[0052] In the PECVD device of FIG. 1, power is supplied to the
upper aluminum electrode in the reaction chamber using a RF wave
generator with frequency of 13.56 MHz for plasma generation
connected with an impedance matching device. Monomer reactants are
fed in a spray from the upper electrode side with feeding rate
regulated by a fine flow regulation valve. The polymer electrolyte
membrane for surface-improvement (for example, a Nafion.RTM.
membrane) is mounted at the center of lower electrode in the
reaction chamber. The initial internal pressure of the reaction
chamber is lowered below 1 to 2 mTorr. When the flow rate of
monomer becomes stabilized, plasma treatment is performed for a
predetermined duration at a desired discharge power using the RF
generator at frequency of 13.56 MHz.
[0053] Compound film manufactured by the reactive sputtering method
is more favorable with respect to manufacturing process, degree of
purity, and cost, as compared with the direct sputtering on oxide
or nitride targets. That is because the gaseous atoms recoiled from
the target are very unstable and tend to react with reactive gases
so that they form a film on a substrate rapidly. If the substrate
temperature is increased during reactive sputtering, the rate of
film formation increases as the compound formation rate
increases.
[0054] In the method to manufacture oxide films via the reactive
sputtering according to the present invention, metal oxide films
are formed by using metal targets such as silicon, zirconium, and
titanium in the sputtering reaction chamber filled with oxygen or
steam in combination with nitrogen or argon.
[0055] When a DMFC is fabricated using composite polymer
electrolyte membranes coated with inorganic thin films for fuel
cells according to the present invention, in order to reduce the
contact resistance between polymer membranes and electrodes, the
surface of the composite membrane is sprayed with ionomer solution
of commercial electrolytes mixed with distilled water and
iso-propylene alcohol (IPA) at a pre-determined ratio and stirred
well to make a uniform solution.
[0056] An MEA is manufactured by coating with platinum-ruthenium
catalysts for anode on one side of composite polymer electrolyte
membranes and coating with platinum catalysts for cathode on the
other side. The catalyst loading of the anode and cathode are made
to be 0.1 to 10 mg/cm.sup.2 on the metal basis, respectively.
[0057] A unit cell is manufactured by assembling after attaching a
carbon-cloth or carbon-paper as a gas diffusion layer to both sides
of a MEA fabricated by the aforementioned method.
[0058] The purpose, characteristics, and advantages of the present
invention will become more apparent through the descriptions on
preferred embodiments of the present invention. The following
embodiments show a method to manufacture polymer electrolyte
membranes for fuel cells according to the present invention and
some examples of the performance measurement results of polymer
electrolyte membranes manufactured by the same method. It should be
understood, however, that the detailed description and specific
embodiments are given by way of illustration only, since various
changes and modifications within the spirit and scope of the
invention will become apparent to those skilled in the art.
Embodiment 1
[0059] A composite polymer electrolyte membrane coated with
inorganic thin films was manufactured by coating with silica to a
thickness of 10 nm on the surface of a Nafion.RTM. 115 membrane (Du
Pont) via a PECVD method which uses silicon ethoxide (Product of
Aldrich) as reactants. For composite polymer electrolyte membrane
thus manufactured, the ionic conductivity was 0.091 S/cm and the
methanol permeability was 1.68.times.10.sup.-6 cm.sup.2/sec (see
Tables 1 and 2 below).
Embodiment 2
[0060] A composite polymer electrolyte membrane coated with
inorganic thin films was manufactured by coating with silica to a
thickness of 30 nm on the surface of a Nafion.RTM. 115 membrane (Du
Pont) via a PECVD method which uses silicon ethoxide (Product of
Aldrich) as reactants. For composite polymer electrolyte membrane
for fuel cells thus fabricated, the ionic conductivity was 0.075
S/cm and the methanol permeability was 8.25.times.10.sup.-7
cm.sup.2/sec (see Tables 1 and 2 below).
Embodiment 3
[0061] A composite polymer electrolyte membrane coated with
inorganic thin films was manufactured by coating with silica to a
thickness of 50 nm on the surface of a Nafion.RTM. 115 membrane (Du
Pont) via a PECVD method which uses silicon methoxide (Product of
Aldrich) as reactants. For composite polymer electrolyte membrane
thus manufactured, the ionic conductivity was 0.076 S/cm and the
methanol permeability was 9.09.times.10.sup.-7 cm.sup.2/sec (see
Tables 1 and 2 below).
Embodiment 4
[0062] A composite polymer electrolyte membrane coated with
inorganic thin films was manufactured by coating with alumina to a
thickness of 70 nm on the surface of a Nafion.RTM. 115 membrane (Du
Pont) via a PECVD method which uses aluminum-secondary-butoxide
(Product of Aldrich) as reactants. For composite polymer
electrolyte membrane for fuel cells thus manufactured, the ionic
conductivity was 0.071 S/cm, and the methanol permeability was
7.37.times.10.sup.-7 cm.sup.2/sec (see Tables 1 and 2 below).
Embodiment 5
[0063] A composite polymer electrolyte membrane coated with
inorganic thin films was manufactured by coating with alumina to a
thickness of 70 nm on the surface of a Nafion.RTM. 115 membrane (Du
Pont) via a PECVD method which uses titanium isopropoxide (Product
of Aldrich) as reactants. For composite polymer electrolyte
membrane thus manufactured, the ionic conductivity was 0.072 S/cm,
and the methanol permeability was 8.13.times.10.sup.-7 cm.sup.2/sec
(see Tables 1 and 2 below).
[0064] FIG. 2 is a scanning electron microscope image of the
surface of a the Nafion.RTM. 115 membrane coated with silica to a
thickness of 50 nm. As can be seen in FIG. 2, silica is uniformly
coated over the surface of Nafion.RTM. 115 membrane.
[0065] In Table 1, the ionic conductivities of composite polymer
electrolyte membranes coated with inorganic thin films manufactured
in aforementioned first to fifth embodiments and a Nafion.RTM. 115
membrane membranes are compared. TABLE-US-00001 TABLE 1
Resistance(.OMEGA.) Ionic Conductivity(S/cm) Nafion .RTM. 115
692.70 0.098 Embodiment 1 704.09 0.091 Embodiment 2 995.66 0.075
Embodiment 3 975.56 0.076 Embodiment 4 942.10 0.071 Embodiment 5
943.16 0.072
[0066] In Table 2, the methanol permeabilities of composite polymer
electrolyte membranes coated with inorganic thin films for fuel
cells manufactured in aforementioned first to fifth embodiments and
a Nafion.RTM. 115 membrane are compared. TABLE-US-00002 TABLE 2
Methanol Permeability Slope (cm.sup.2/sec) Nafion .RTM. 115 0.19112
2.77 .times. 10.sup.-6 Embodiment 1 0.11898 1.68 .times. 10.sup.-6
Embodiment 2 0.05807 8.25 .times. 10.sup.-7 Embodiment 3 0.06399
9.09 .times. 10.sup.-7 Embodiment 4 0.04737 7.37 .times. 10.sup.-7
Embodiment 5 0.06309 8.13 .times. 10.sup.-7
[0067] As can be seen in Table 1, in cases of embodiments 2 and 3
where composite polymer electrolyte membranes are coated with
silica using the PECVD method, the ionic conductivity is found to
reduce by about 20% compared with a Nafion membrane.
[0068] However, as can be seen in table 2, the methanol
permeability in cases 2 and 3 is decreased by about 70% compared
with a Nafion.RTM. 115 membrane.
[0069] In the result summarized from Table 1 and 2, it was
ascertained that, if composite polymer electrolyte membranes coated
with inorganic thin films for fuel cells was manufactured using a
PECVD method according to the embodiments of the present invention,
the methanol permeability is reduced sizably by about 70% compared
with Nafion.RTM. membranes without seriously decreasing the ionic
conductivity.
Embodiment 6
[0070] To properly represent the characteristics of composite
polymer electrolyte membranes for fuel cells, Characteristic Factor
defined as the ratio of ionic conductivity and methanol
permeability as shown in Eqn. 1 can be used. Characteristic .times.
.times. Factor .times. .times. ( .PHI. ) = Ionic .times. .times.
Conductivity Methanol .times. .times. Permeability .times. 10 - 3 [
Eqn . .times. 1 ] ##EQU1##
[0071] Characteristic factors using Eqn. 1 are obtained for
embodiments 1 through 5 and an Nafion.RTM. membrane, and the
results are compared in FIG. 3.
[0072] As shown in FIG. 3, the characteristic factor of composite
polymer electrolyte membranes for fuel cells coated with silicon
films according to the present invention is found to be superior by
about 2 to 3 times to Nafion.RTM. electrolyte membranes.
[0073] In other words, it was ascertained that the characteristics
of composite polymer electrolyte membranes for fuel cells coated
with inorganic thin films according to the present invention is far
better improved as compared with Nafion.RTM. electrolyte
membranes.
Embodiment 7
Composite Coating of Ionomer Solution
[0074] To enhance contact of polymer electrolyte membranes coated
with silica thin films with electrodes, a 5 wt % Nafion.RTM.
solution (Du Pont) is sprayed on the surface of membrane and the
amount of the Nafion.RTM. material coated on the membrane is as
much as 2 mg/cm.sup.2 on the dry weight basis.
[0075] Experiment 1: Performance Measurement of a Fuel Cell
[0076] A DMFC has been manufactured using composite polymer
electrolyte membranes for fuel cells coated with inorganic thin
films according this embodiment of the present invention, and the
performance of the cell has been measured. The measurement of the
DMFC was performed under the condition of passive methanol feed and
air breathing, in which a methanol solution of 4.5 M is used for
the anode and the cathode is exposed to the air so that oxygen is
naturally diffused and supplied to the electrode.
[0077] The results of performance measurement of the DMFC show that
DMFCs manufactured using the polymer electrolyte membranes for fuel
cells coated with silicon oxide thin films according to the present
invention have a higher performance by 30 to 40% as compared with
fuel cells employing unmodified Nafion.RTM. membranes.
[0078] That is because polymer electrolyte membranes coated with
silicon oxide thin films according to the present invention
enhances the performance of fuel cells by lowering the methanol
permeability while maintaining the ionic conductivity at about the
level as compared with commercial Nafion.RTM. electrolyte
membranes.
[0079] The present invention, by coating the polymer electrolyte
membranes for fuel cells with inorganic thin films via a PECVD
method or a sputtering method, shows the effects that the
performance of DMFCs is enhanced and the life of the cells is
extended in such a manner that the methanol permeability is
decreased sizably while the ionic conductivity is decreased just by
a small amount as compared with the existing commercial polymer
electrolyte membranes. Also, the PECVD method or sputtering method,
being very advantageous in the improvement of electrolyte membranes
in bulk, allows to manufacture low-methanol-permeable composite
electrolyte membranes efficiently at a low cost.
* * * * *