U.S. patent application number 11/272805 was filed with the patent office on 2006-05-18 for membrane-electrode assembly for fuel cell and fuel cell system comprising same.
Invention is credited to Hee-Tak Kim, Young-Mi Park, Hae-Kwon Yoon.
Application Number | 20060105225 11/272805 |
Document ID | / |
Family ID | 36386729 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060105225 |
Kind Code |
A1 |
Kim; Hee-Tak ; et
al. |
May 18, 2006 |
Membrane-electrode assembly for fuel cell and fuel cell system
comprising same
Abstract
A membrane-electrode assembly for a fuel cell of the present
invention includes an anode and a cathode facing each other, and a
polymer electrolyte membrane interposed therebetween. At least one
of the anode and the cathode includes a catalyst layer and an
electrode substrate. The catalyst layer includes a catalyst and a
porous ionomer. The polymer electrolyte membrane contacts one side
of the catalyst layer and the electrode substrate contacts the
other side of the catalyst layer.
Inventors: |
Kim; Hee-Tak; (Suwon-si,
KR) ; Yoon; Hae-Kwon; (Suwon-si, KR) ; Park;
Young-Mi; (Suwon-si, KR) |
Correspondence
Address: |
Robert E. Bushnell;Suite 300
1522 K Street, N.W.
Washington
DC
20005
US
|
Family ID: |
36386729 |
Appl. No.: |
11/272805 |
Filed: |
November 15, 2005 |
Current U.S.
Class: |
429/483 ;
429/129; 429/450; 429/529; 429/532; 429/535 |
Current CPC
Class: |
H01M 4/8882 20130101;
H01M 4/8828 20130101; H01M 8/0234 20130101; H01M 4/8605 20130101;
H01M 8/1004 20130101; H01M 4/926 20130101; Y02E 60/50 20130101;
Y02P 70/50 20151101; H01M 4/8878 20130101; H01M 4/921 20130101 |
Class at
Publication: |
429/040 ;
429/129 |
International
Class: |
H01M 4/86 20060101
H01M004/86; H01M 2/14 20060101 H01M002/14 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 16, 2004 |
KR |
10-2004-0093428 |
Claims
1. A membrane-electrode assembly for a fuel cell, comprising: an
anode and a cathode facing each other, at least one of the anode
and the cathode comprising: a catalyst layer comprising a catalyst
and a porous ionomer layer; and an electrode substrate enabling a
reactant to diffuse into the catalyst layer; and a polymer
electrolyte membrane interposed between the anode and the
cathode.
2. The membrane-electrode assembly of claim 1, wherein the porous
ionomer layer has a porosity ranging from about 40 volume % to
about 80 volume %.
3. The membrane-electrode assembly of claim 1, wherein the porous
ionomer layer has a pore size ranging from about 10 nm to about
1,000 nm.
4. The membrane-electrode assembly of claim 1, wherein the porous
ionomer layer is present on a surface of the catalyst.
5. The membrane-electrode assembly of claim 1, wherein the catalyst
comprises a metal catalyst supported on a carrier.
6. The membrane-electrode assembly of claim 1, wherein the
electrode substrate comprises a conductive substrate selected from
the group consisting of a water-repellent treated carbon paper and
carbon cloth.
7. A fuel cell system, comprising: at least one electricity
generating element for generating electricity through oxidation of
fuel and reduction of an oxidant, comprising: a membrane-electrode
assembly comprising: an anode and a cathode facing each other, at
least one of the anode and the cathode comprising: a catalyst layer
comprising a catalyst and a porous ionomer layer; and an electrode
substrate enabling the fuel or the oxidant to diffuse into the
catalyst layer; and a polymer electrolyte membrane interposed
between the anode and the cathode; and separators positioned at
both sides of the membrane-electrode assembly; a fuel supplier
providing the fuel to the electricity generating element; and an
oxidant supplier supplying the oxidant to the electricity
generating element.
8. The fuel cell system of claim 7, wherein the porous ionomer
layer has a porosity ranging from about 40 volume % to about 80
volume %.
9. The fuel cell system of claim 7, wherein the porous ionomer
layer has a pore size ranging from about 10 nm to about 1,000
nm.
10. The fuel cell system of claim 7, wherein the porous ionomer
layer is present on a surface of the catalyst.
11. A method for manufacturing a membrane-electrode assembly,
comprising: preparing an anode and a cathode, at least one of the
anode and the cathode comprising: a catalyst layer comprising a
catalyst and a porous ionomer layer; and an electrode substrate
enabling a reactant to diffuse into the catalyst layer; and
preparing a polymer electrolyte membrane interposed between the
anode and the cathode.
12. The method of claim 11, wherein the catalyst layer is formed by
a process comprising: coating a composition including the catalyst,
an ionomer and a plasticizer onto the electrode substrate; and
extracting the plasticizer.
13. The method of claim 12, wherein the plasticizer is at least one
polymer selected from the group consisting of a C1 to C10
polyalkyleneglycol; a C1 to C10 polyalkyleneoxide; a C1 to C10
poly(alkyl)acrylic acid; an aromatic or fluorine polymer having a
sulfonic acid group; and a cellulose-based polymer.
14. The method of claim 12, wherein the plasticizer is extracted by
dipping the coated composition onto the electrode substrate in an
extraction solvent.
15. The method of claim 14, wherein the extraction solvent is
selected from the group consisting of an alcohol-based solvent, an
ether-based solvent, tetrahydrofuran, and a mixture thereof.
16. The method of claim 12, wherein the weight ratio of the
plasticizer to the ionomer ranges from about 20:80 to 70:30.
17. The method of claim 11, wherein the catalyst layer is formed by
a process comprising: coating a composition including the catalyst,
an ionomer and fumed silica onto the electrode substrate; and
firing the coated composition.
18. The method of claim 17, wherein the weight ratio of the fumed
silica to the ionomer ranges from about 10:90 to 50:50.
19. An membrane-electrode assembly manufactured according to claim
12.
20. The fuel cell system comprising the membrane-electrode assembly
of claim 19.
21. A membrane-electrode assembly for a fuel cell, comprising: an
anode and a cathode facing each other, at least one of the anode
and the cathode formed by coating a composition including a
catalyst, an ionomer and one of a plasticizer or fumed silica onto
an electrode substrate, and extracting the plasticizer when the
plasticizer is used or firing the coated composition when the fumed
silica is used; and a polymer electrolyte membrane interposed
between the anode and the cathode.
22. The membrane-electrode assembly of claim 21, wherein the
plasticizer is at least one polymer selected from the group
consisting of a C1 to C10 polyalkyleneglycol; a C1 to C10
polyalkyleneoxide; a C1 to C10 poly(alkyl)acrylic acid; an aromatic
or fluorine polymer having a sulfonic acid group; and a
cellulose-based polymer.
23. The membrane-electrode assembly of claim 21, wherein the
plasticizer is extracted by dipping the coated composition onto the
electrode substrate in an extraction solvent.
24. The membrane-electrode assembly of claim 21, wherein the weight
ratio of the plasticizer to the ionomer ranges from about 20:80 to
70:30.
25. The membrane-electrode assembly of claim 21, wherein the
plasticizer has a number average molecular weight ranging from 200
to 50,000.
26. The membrane-electrode assembly of claim 21, wherein the weight
ratio of the fumed silica to the ionomer ranges from about 10:90 to
50:50.
27. The membrane-electrode assembly of claim 21, wherein the fumed
silica has a specific surface area ranging from 100 to 1,200
m.sup.2/g and a particle size of 10 nm to 1,000 nm.
Description
CLAIM OF PRIORITY
[0001] This application makes reference to, incorporates the same
herein, and claims all benefits accruing under 35 U.S.C. .sctn.119
from an application for MEMBRANE-ELECTRODE ASSEMBLY FOR FUEL CELL
AND FUEL CELL SYSTEM COMPRISING SAME earlier filed in the Korean
Intellectual Property Office on 16 Nov. 2004 and there duly
assigned Ser. No. 10-2004-0093428.
FIELD OF THE INVENTION
[0002] The present invention relates to a membrane-electrode
assembly for a fuel cell and a fuel cell system comprising the
same. More particularly, the present invention relates to a
membrane-electrode assembly with high power and to a fuel cell
system comprising the same.
BACKGROUND OF THE INVENTION
[0003] A fuel cell is a power generation system for producing
electrical energy through an electrochemical redox reaction of an
oxidant and a fuel such as hydrogen or a hydrocarbon-based material
such as methanol, ethanol, or natural gas.
[0004] A fuel cell can be classified into a phosphoric acid type, a
molten carbonate type, a solid oxide type, a polymer electrolyte
type, or an alkaline type depending upon the kind of electrolyte
used. Although each of these different types of fuel cells operates
in accordance with the same basic principles, they may differ from
one another in the kind of fuel, the operating temperature, the
catalyst, and the electrolyte used.
[0005] Recently, a polymer electrolyte membrane fuel cell (PEMFC)
has been developed. The PEMFC has power characteristics that are
superior to conventional fuel cells, as well as a lower operating
temperature and faster start and response characteristics. Because
of this, the PEMFC can be applied to a wide array of fields such as
for transportable electrical sources for automobiles, distributed
power sources such as for houses and public buildings, and small
electrical sources for electronic devices.
[0006] A PEMFC is essentially composed of a stack, a reformer, a
fuel tank, and a fuel pump. The stack forms a body of the PEMFC,
and the fuel pump provides fuel stored in the fuel tank to the
reformer. The reformer reforms the fuel to generate hydrogen gas
and supplies the hydrogen gas to the stack. Fuel stored in the fuel
tank is pumped to the reformer using power which can be provided by
the PEMFC. Then, the reformer reforms the fuel to generate the
hydrogen gas, and the hydrogen gas is electrochemically oxidized
and the oxidant is electrochemically reduced in the stack to
generate the electrical energy.
[0007] Alternatively, a fuel cell may include a direct oxidation
fuel cell (DOFC) in which a liquid fuel is directly introduced to
the stack. Unlike a PEMFC, a DOFC does not require a reformer.
[0008] In the above-mentioned fuel cell system, the stack for
generating the electricity has a structure in which several unit
cells, each having a membrane electrode assembly (MEA) and a
separator (also referred to as a "bipolar plate"), are stacked
adjacent to one another. The MEA is composed of an anode (also
referred to as a "fuel electrode" or "oxidation electrode") and a
cathode (also referred to as an "air electrode" or "reduction
electrode") that are separated by a polymer electrolyte
membrane.
[0009] The polymer electrolyte membrane can be fabricated using a
perfluorosulfonic acid ionomer membrane such as Nafion.RTM. (by
DuPont), Flemion.RTM. (by Asahi Glass), Asiplex.RTM. (by Asahi
Chemical), and Dow XUS.RTM. (by Dow Chemical). The electrodes
including the catalysts supported on the carbon can be fabricated
by binding electrode substrates such as porous carbon paper or
carbon cloth with a carbon powder carrying pulverized catalyst
particles such as platinum (Pt) or ruthenium (Ru) using a
water-repellent binder.
[0010] What is needed is a high power membrane electrode assembly
and a fuel cell system where the transferring rate of the reactant
is fast and a high concentration of reactants can be present on the
surface of a catalyst.
SUMMARY OF THE INVENTION
[0011] An exemplary embodiment of the present invention provides a
membrane-electrode assembly for a fuel cell, wherein a catalyst
layer has pores and can maintain a concentration of hydrogen and an
oxidant on a surface of a catalyst and realize a high-power fuel
cell.
[0012] Another embodiment of the present invention provides a fuel
cell system including the membrane-electrode assembly.
[0013] According to one embodiment, a membrane-electrode assembly
for a fuel cell includes an anode and a cathode facing each other,
and a polymer electrolyte membrane interposed therebetween. At
least one of the anode and the cathode includes a catalyst layer
and an electrode substrate (a reactant diffusion layer). The
catalyst layer includes a catalyst and a porous ionomer. The
polymer electrolyte membrane contacts one side of the catalyst
layer and the electrode substrate contacts the other side of the
catalyst layer.
[0014] According to another embodiment, a fuel cell system includes
at least one electricity generating element for generating
electricity through oxidation of fuel and reduction of an oxidant,
a fuel supplier for providing fuel to the electricity generating
element, and an oxidant supplier for supplying the oxidant to the
electricity generating element. The electricity generating element
includes the above membrane-electrode assembly and separators
positioned at both sides of the membrane-electrode assembly.
[0015] The porous ionomer layer has a porosity ranging from about
40 volume % to about 80 volume %.
[0016] The porous ionomer layer has a pore size ranging from about
10 nm to about 1,000 nm.
[0017] The porous ionomer layer is present on a surface of the
catalyst.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] A more complete appreciation of the present invention, and
many of the above and other features and advantages of the present
invention, will be readily apparent as the same becomes better
understood by reference to the following detailed description when
considered in conjunction with the accompanying drawing in which
like reference symbols indicate the same or similar components,
wherein
[0019] FIG. 1 is a schematic diagram showing processes of forming a
porous ionomer layer included in the catalyst layer of the present
invention;
[0020] FIG. 2 is a schematic diagram showing a fuel cell system
according to the present invention; and
[0021] FIG. 3 is a graph showing measurement results of voltage to
current of Examples 1 and 2 and Comparative Example 1.
DETAILED DESCRIPTION OF THE INVENTION
[0022] An embodiment of the present invention will hereinafter be
described in detail with reference to the accompanying
drawings.
[0023] A membrane-electrode assembly for a fuel cell includes a
cathode and an anode facing each other, and a polymer electrolyte
membrane interposed therebetween. The cathode and the anode include
each a catalyst layer which includes a catalyst (preferably, a
metal catalyst).
[0024] Fuel is supplied to the anode and an oxidant is supplied to
the cathode. The fuel is oxidized at the anode to generate protons
and electrons, and then the protons are transferred to the cathode
through a polymer electrolyte membrane and the electrons are
transferred to the cathode through an out-circuit. The transferred
protons and electrons are reacted with the oxidant to generate
water and electrical energy.
[0025] In order to realize good performance characteristics of a
fuel cell, the surface area of the catalyst to participate in the
electrochemical reaction should be large, and the concentration of
reactants on the surface of the catalyst should be maintained to be
high. For this purpose, it is preferable that an ionomer is present
to increase transfer of protons.
[0026] A large amount of ionomers in the catalyst layer surround
the metal catalyst act as a resistance against mass transfer to the
metal catalyst. According to one embodiment, the ionomer layer that
acts as an ion conductor in the catalyst layer has small pores and
thereby reactants such as fuel and the oxidant can easily pass
through 11 the porous ionomer layer. The fuel and oxidant can be
present at a high concentration on a surface of the metal catalyst
to realize a high power membrane-electrode assembly.
[0027] The porous ion conductive ionomer allows the reactants to
pass through the pores and be quickly transferred to the surface of
the metal catalyst. The path between an electrode substrate and the
surface of the catalyst is shorter compared to a catalyst layer
without pores, and thus the reactant can be quickly transferred,
and a limit for fuel cell performance due to a mass transfer limit
can be overcome.
[0028] FIG. 1 shows a preparation process of the porous ion
conductive ionomer. Referring to FIG. 1, a plasticizer 2 is added
to a mixture including a metal catalyst 6 and an ion conductive
ionomer 4 to prepare a composition for forming a catalyst layer,
and the composition is coated on an electrode substrate to form a
catalyst layer having an ionomer/plasticizer mixed layer thereon
and to fabricate an electrode. Then, the electrode including the
catalyst layer is dipped in a solvent that can dissolve the
plasticizer to extract the plasticizer 2 and to form the pores 8 in
the ionomer layer.
[0029] The ionomer layer may have porosity ranging from about 40
volume % to about 80 volume %. When the porosity is less than 40
volume %, reactant fluids are not diffused smoothly. When it is
more than 80 volume %, resistance against ion transfer
increases.
[0030] The pores may have a pore size of about 10 nm to about 1000
nm. When the pose size is less than 10 nm, reactant fluids are not
diffused smoothly. When it is more than 1000 nm, the large pores
may prevent formation of an ionic transfer pathway.
[0031] The electrode for a fuel cell according to the present
invention includes an electrode substrate and a catalyst layer, and
the catalyst layer includes a porous ionomer polymer layer having
pores.
[0032] The electrode for a fuel cell is prepared by coating a
composition for forming a catalyst layer onto one side of an
electrode substrate and drying it to form a catalyst layer.
Subsequently, the electrode substrate with the catalyst layer is
dipped in a solvent that can dissolve the plasticizer to extract it
and to form pores in the ionomer polymer layer.
[0033] The catalyst composition includes an ionomer polymer for a
binder, a metal catalyst, a plasticizer, and a dispersion
solvent.
[0034] The ionomer polymer is an ion conductive polymer and
transfers protons. The ionomer polymer has an equivalent weight
(EW) ranging from about 500 to about 2,000.
[0035] The micropores are three-dimensionally connected within the
ionomer to impart an ion transfer path. The ionomer may be any
proton conductive polymer having a cation exchange group selected
from the group consisting of a sulfonic acid group, a carboxylic
acid group, a phosphoric acid group, a phosphonic acid group, and
derivatives thereof, at its side chain.
[0036] Non-limiting examples of the proton conductive polymer
include perfluoro-based polymers, benzimidazole-based polymers,
polyether-based polymer, polyimide-based polymers,
polyetherimide-based polymers, polyamide-based polymers,
polyphenylene sulfide-based polymers, polysulfone-based polymers,
polyethersulfone-based polymers, polyetherketone-based polymers,
polyether-etherketone-based polymers, and
polyphenylquinoxaline-based polymers. In a preferred embodiment, at
least one ionomer may include but is not limited to a polymer
selected from the group consisting of poly(perfluorosulfonic acid),
poly(perfluorocarboxylic acid), co-polymers of tetrafluoroethylene
and fluorovinylether containing sulfonic acid groups, defluorinated
polyetherketone sulfides, aryl ketones,
poly(2,2'-(m-phenylene)-5,5'-bibenzimidazole), and
poly(2,5-benzimidazole).
[0037] For the plasticizer, a compound having hydrophilicity and/or
hydrophobicity and a number average molecular weight ranging from
200 to 50,000 may be used. When the molecular weight of the
plasticizer is less than 200, the plasticizer may volatilize during
fabrication of an electrode and a small amount of the plasticizer
may remain. Therefore, it is difficult to make pores by extracting
the plasticizer. On the contrary, when the molecular weight is more
than 50,000, entanglement of the plasticizer occurs excessively and
thereby the plasticizer is not easily extracted.
[0038] The plasticizer includes at least one polymer selected from
the group consisting of a C1 to C10 polyalkylene glycol such as
polyethylene glycol and polypropylene glycol; a C1 to C10
polyalkylene oxide such as polyethylene oxide and polypropylene
oxide; a C1 to C10 poly(alkyl)acrylic acid such as polyacrylic acid
and polymethacrylic acid; an aromatic or fluoro-based polymer such
as polystyrene having a sulfonic acid group and polyfluoro sulfonic
acid; and a cellulose-based polymer.
[0039] The weight ratio of the plasticizer to the ionomer may be
from 20:80 to 70:30, and preferably from 40:60 to 60:40. When the
amount of the plasticizer is less than 20 weight %, sufficient
pores may not be made, and when it is more than 70 weight %, the
pores may prevent formation of an ionic transfer pathway resulting
in increase resistance against ion transfer.
[0040] The dispersion solvent includes at least one selected from
the group consisting of isopropanol, dimethylsulfoxide,
dimethylacetamide, N-methylpyrrolidone, and a mixture thereof. When
the dispersion solvent is an alcohol-based solvent, it may be used
with water.
[0041] The extraction solvent for the plasticizer includes at least
one selected from the group consisting of an alcohol-based solvent
such as methanol, ethanol, isopropanol and so on; an ether-based
solvent such as dimethyl ether, diethyl ether, and so on;
tetrahydrofuran; and a mixture thereof.
[0042] The ionomer polymer is dispersed in the composition for
forming a catalyst layer. The composition may be coated using
screen printing, spray coating, or a doctor blade method depending
on the viscosity thereof, but is not limited thereto.
[0043] Alternatively, the catalyst including a porous ionomer
polymer layer may be formed using a catalyst composition which
includes an ionomer polymer for a binder, a metal catalyst, fumed
silica, and a dispersion solvent.
[0044] In the above catalyst composition, the ionomer polymer,
metal catalyst, and dispersion solvent are the same as
above-described. The fumed silica may have a specific surface area
ranging from 100 to 1200 m.sup.2/g, and a particle size of 10 nm to
1000 nm.
[0045] The weight ratio of the fumed silica to the ionomer may be
from 10:90 to 50:50, and preferably from 30:70 to 40:60. When the
amount of the fumed silica is less than 10 weight %, sufficient
pores may not be formed, which prevents diffusion of reactant
fluids, and when it is more than 50 weight %, the pores may prevent
formation of an ionic transfer pathway resulting in increased
resistance against ion transfer.
[0046] The composition for forming a catalyst layer is coated onto
one side of an electrode substrate and fired to form a catalyst
layer including the porous ionomer polymer layer. The composition
may be coated using screen printing, spray coating, or a doctor
blade method depending on the viscosity thereof, but is not limited
thereto. The firing may be performed at a temperature ranging from
60 to 130.degree. C.
[0047] The catalyst layer of the electrode preferably includes a
metal catalyst which enables a related reaction (the oxidation of
fuel and the reduction of the oxidant). Suitable choices for the
metal catalyst include at least one catalyst selected from the
group consisting of platinum, ruthenium, osmium, a
platinum-ruthenium alloy, a platinum-osmium alloy, a
platinum-palladium alloy, and a platinum-M alloy where a suitable M
is at least one transition metal selected from the group consisting
of Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn. Among them, it is
preferable to use at least one selected from the group consisting
of platinum, ruthenium, osmium, a platinum-ruthenium alloy, a
platinum-osmium alloy, a platinum-palladium alloy, a
platinum-cobalt alloy, or a platinum-nickel alloy.
[0048] The metal catalyst is preferably supported on a carrier. The
carrier may include carbon such as acetylene black, graphite, and
so on, or an inorganic material particle such as alumina, silica,
zirconia, titania, and so on. In one embodiment, the catalyst is a
commercially available catalyst, or a produced product in which a
noble 11 metal material is supported on the carrier. Since the
process to support the noble metal on a carrier is known to this
art, even though it is omitted from this description, one skilled
in the art may easily understand the present invention.
[0049] The electrode substrate supports the catalyst layer and
enables a reaction fluid to diffuse into the catalyst layer. The
electrode substrate may include carbon paper or carbon cloth, but
is not limited thereto. It may be treated with a fluorine-based
polymer in order to provide a water repellant property so as to
prevent deterioration of reactant diffusion efficiency by water
generated during driving of the fuel cell. The fluorine-based
polymer includes polyvinylidenefluoride, polytetrafluoroethylene,
fluorinated ethylenepropylene, polychlorotrifluoroethylene, a
fluoroethylene polymer, and so on.
[0050] The electrode may further include porous layers in order to
increase the reactant diffusion effects between the electrode
substrate and the catalyst layers.
[0051] The porous layer may be formed by coating a composition
including a conductive powder, a binder, and an ionomer as needed.
In general, the conductive powder with small diameter particles can
include carbon powder, carbon black, acetylene black, activated
carbon, or a nano-carbon such as carbon nanotubes, carbon
nanofiber, carbon nanowire, carbon nanohoms, carbon nanorings, and
the like. Non-limiting examples of the binder can be
polytetrafluoroethylene (PTFE), polyvinylidene fluoride, copolymers
of polyvinylidenefluoride-hexafluoropropylene (PVDF-HFP),
polyvinylalcohol, cellulose acetate, and so on.
[0052] The present invention also provides a membrane-electrode
assembly including the above electrode. The membrane-electrode
assembly is fabricated by positioning a polymer electrolyte
membrane between the anode and cathode and firing. The cathode and
anode may be the above-described electrode.
[0053] The polymer electrolyte membrane includes a proton
conductive polymer. The proton conductive polymer for the
electrolyte membrane of the present invention may be any polymer
resin having a cation exchange group selected from the group
consisting of a sulfonic acid group, a carboxylic acid group, a
phosphoric acid group, a phosphonic acid group, and derivatives
thereof at its side chain. The proton-conducting polymer may be
selected from the group consisting of perfluoro-based polymers,
benzimidazole-based polymers, polyether-based polymers,
polyimide-based polymers, polyetherimide-based polymers,
polyamide-based polymer, polyphenylene sulfide-based polymers,
polysulfone-based polymers, polyethersulfone-based polymers,
polyetherketone-based polymers, polyether-etherketone-based
polymers, and polyphenylquinoxaline-based polymers. In a preferred
embodiment, at least one proton-conducting polymer may include but
is not limited to a polymer selected from the group consisting of
poly(perfluorosulfonic acid), poly(perfluorocarboxylic acid),
co-polymers of tetrafluoroethylene and fluorovinylether containing
sulfonic acid groups, defluorinated polyetherketone sulfides, aryl
ketones, poly(2,2'-(m-phenylene)-5,5'-bibenzimidazole), and
poly(2,5-benzimidazole). According to the present invention, a
proton-conducting polymer included in a polymer electrolyte
membrane for a fuel cell is not limited to these polymers.
[0054] A fuel cell system of the present invention includes at
least one electricity generating element, a fuel supplier, and an
oxidant supplier. The electricity generating element includes at
least at least one unit cell where the above membrane-electrode
assembly is positioned between separators having reactant flow
channels and cooling channels.
[0055] The fuel cell system generates electricity through an
oxidation of fuel and reduction of an oxidant. The fuel includes
hydrogen or a hydrogen-containing hydrocarbon. The oxidant includes
air or pure oxygen. The fuel supplier supplies fuel to the
electricity generating element, and the oxidant supplier supplies
the oxidant to the electricity generating element.
[0056] The schematic structure of the fuel cell system according to
the present invention is illustrated in FIG. 2 and will be
described below referring to the drawing.
[0057] The fuel cell system 100 includes a stack 7 which includes
at least one electricity generating element 19 for generating
electrical energy through oxidation of fuel and reduction of an
oxidant, a fuel supplier 1, and an oxidant supplier 5.
[0058] The fuel supplier 1 is equipped with a fuel storage tank 9,
and a fuel pump 11 connected to the fuel tank 9. The fuel pump 11
discharges fuel stored in the fuel tank 9 with a predetermined
pumping force.
[0059] The oxidant supplier 5 for supplying oxidant to the
electricity generating element 19 of the stack 7 is equipped with
at least one pump 13 to provide the oxidant with a predetermined
pumping force.
[0060] The electricity generating element 19 includes a
membrane-electrode assembly 21 which performs oxidation of fuel and
oxidant reduction, and separators 23 and 25 which are positioned at
both sides of the membrane-electrode assembly and provide fuel and
oxidant to the membrane-electrode assembly 21.
[0061] In the fuel cell system of the present invention, fuel is
supplied to the anode and an oxidant is supplied to the cathode to
generate electricity through an electrochemical reaction between
the anode and cathode. At the anode, hydrogen or an organic raw
material is oxidized, and at the cathode, the oxidant is reduced so
that a voltage difference between the electrodes occurs.
[0062] The following examples illustrate the present invention in
further detail. However, it is understood that the present
invention is not limited by these examples.
EXAMPLE 1
[0063] 3 g of Pt/C including 20 weight % of platinum, 1 g of an
ionomer (from Dupont Company) and 2 g of dibutylphthalate as a
plasticizer were added to 20 g of IPA (isopropyl alcohol) to
prepare a catalyst slurry. Then, the catalyst slurry was coated on
water-repellent treated carbon paper (the electrode substrate) to a
catalyst layer.
[0064] The carbon layer including the catalyst layer was dried and
was dipped in methanol, which is capable of dissolving the
plasticizer, at 40.degree. C. for 2 hours to extract the
plasticizer and form an electrode including the porous ionomer
layer.
[0065] Two electrodes fabricated as above were positioned as an
anode and a cathode at both sides of a poly(perfluorosulfonic acid)
membrane (Nafion.RTM. of the DuPont Company) and the whole was
fired at 130.degree. C. for 1 minute and hot-pressed to fabricate a
membrane-electrode assembly.
[0066] The membrane-electrode assembly was inserted between two
gasket sheets and was then positioned between two separators having
predetermined shaped reactant flow channels and cooling channels.
Thereafter, it was interposed between copper end plates and pressed
to fabricate a unit cell.
EXAMPLE 2
[0067] A unit cell was fabricated by the same method as in Example
1, except that polyethyleneglycol having a molecular weight of 300
was used as the plasticizer.
Comparative Example 1
[0068] A unit cell was fabricated by the same method as in Example
1, except that the plasticizer was not added to the catalyst
slurry. In the electrode according to Comparative Example 1, the
porous ionomer layer was not formed.
[0069] With respect to the fuel cells fabricated in accordance with
Examples 1 and 2 and Comparative Example 1, 50% humidified air and
hydrogen were respectively supplied to the cathode and anode
without back pressure, and they were operated at 60.degree. C.
After operating the fuel cell systems according to Examples 1 and 2
and Comparative Example 1, voltage-current density values were
measured. The results are shown in FIG. 3.
[0070] Referring to FIG. 3, the electrodes according to Examples 1
and 2 including the porous ionomer layer show better performance
characteristics than those of Comparative Example 1.
[0071] The electrode for a fuel cell of the present invention
includes a porous ionomer layer in which reactants are transferred
to the surface of the catalyst through pores. The porous ionomer
layer reduces a path between the electrode substrate and the
surface of the catalyst, and thereby the transferring rate of the
reactant become fast and a high concentration of reactants can be
present on the surface of the electrode to realize a high power
membrane-electrode assembly and fuel cell system.
[0072] While this invention has been described in connection with
what is presently considered to be practical exemplary embodiments,
it is to be understood that the invention is not limited to the
disclosed embodiments, but on the contrary, is intended to cover
various modifications and equivalent arrangements included within
the spirit and scope of the appended claims.
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