U.S. patent application number 15/471206 was filed with the patent office on 2018-09-27 for membrane electrode assembly for electrochemical cell.
The applicant listed for this patent is Elchemtech Co., Ltd.. Invention is credited to Yun Ki CHOI, Chang Hyun HAN, Su Hyun IM, Chang Hwan MOON, Sang Bong MOON, Dae Jin YOON.
Application Number | 20180274112 15/471206 |
Document ID | / |
Family ID | 63581636 |
Filed Date | 2018-09-27 |
United States Patent
Application |
20180274112 |
Kind Code |
A1 |
MOON; Sang Bong ; et
al. |
September 27, 2018 |
MEMBRANE ELECTRODE ASSEMBLY FOR ELECTROCHEMICAL CELL
Abstract
Disclosed is a membrane electrode assembly that includes a
polymer electrolyte membrane, a first electrochemical reaction
layer formed on one side of the polymer electrolyte membrane to
allow an oxidation reaction to occur thereon, a first
electron-conductive layer formed between the polymer electrolyte
membrane and the first electrochemical reaction layer, a second
electrochemical reaction layer formed on a remaining side of the
polymer electrolyte membrane to allow a reduction reaction to occur
thereon, and a second electron-conductive layer formed between the
polymer electrolyte membrane and the second electrochemical
reaction layer. The first electron-conductive layer and the second
electron-conductive layer include a porous metal.
Inventors: |
MOON; Sang Bong; (Seoul,
KR) ; CHOI; Yun Ki; (Seoul, KR) ; HAN; Chang
Hyun; (Seoul, KR) ; IM; Su Hyun; (Gyeonggi-do,
KR) ; YOON; Dae Jin; (Gyeonggi-do, KR) ; MOON;
Chang Hwan; (Seoul, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Elchemtech Co., Ltd. |
Seoul |
|
KR |
|
|
Family ID: |
63581636 |
Appl. No.: |
15/471206 |
Filed: |
March 28, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 8/1004 20130101;
Y02E 60/36 20130101; H01M 2008/1095 20130101; C25B 1/10 20130101;
H01M 4/8605 20130101; H01M 8/00 20130101; C25B 13/08 20130101; Y02E
60/50 20130101; H01M 4/9075 20130101; H01M 4/925 20130101; C25B
9/08 20130101; C25B 11/04 20130101 |
International
Class: |
C25B 9/08 20060101
C25B009/08; C25B 1/10 20060101 C25B001/10; C25B 13/08 20060101
C25B013/08; C25B 11/04 20060101 C25B011/04 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 23, 2017 |
KR |
10-2017-0036654 |
Claims
1. A membrane electrode assembly for an electrochemical cell
comprising: a polymer electrolyte membrane; a first electrochemical
reaction layer formed on one side of the polymer electrolyte
membrane to allow an oxidation reaction to occur thereon; a first
electron-conductive layer formed between the polymer electrolyte
membrane and the first electrochemical reaction layer; a second
electrochemical reaction layer formed on a remaining side of the
polymer electrolyte membrane to allow a reduction reaction to occur
thereon; and a second electron-conductive layer formed between the
polymer electrolyte membrane and the second electrochemical
reaction layer, wherein the first electron-conductive layer and the
second electron-conductive layer include a porous metal.
2. The membrane electrode assembly of claim 1, wherein the porous
metal further includes a coating layer including a platinum group
applied on a surface thereof.
3. The membrane electrode assembly of claim 1, wherein the first
electron-conductive layer and the second electron-conductive layer
each have a thickness of 0.1 to 1 mm.
4. The membrane electrode assembly of claim 1, wherein the first
electron-conductive layer and the second electron-conductive layer
include any one among platinum, palladium, rhodium, iridium,
ruthenium, osmium, carbon, gold, tantalum, tin, indium, nickel,
tungsten, manganese, and titanium, or a complex including two or
more thereof.
5. The membrane electrode assembly of claim 1, wherein the first
electrochemical reaction layer and the second electrochemical
reaction layer each includes an electrochemical catalyst (or the
electrochemical catalyst on a carrier), an ionic conductor, and a
binder.
6. The membrane electrode assembly of claim 5, wherein the
electrochemical catalyst includes any one among platinum group
elements including platinum, palladium, ruthenium, iridium,
rhodium, and osmium, any one metal among iron, lead, copper,
chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium,
and aluminum, or alloys, oxides, or double oxides thereof.
7. The membrane electrode assembly of claim 1, wherein elements
constituting the membrane electrode assembly include the polymer
electrolyte membrane, the first electron-conductive layer, the
second electron-conductive layer, the first electrochemical
reaction layer, and the second electrochemical reaction layer in
descending order of size.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to a membrane electrode
assembly for an electrochemical cell, and more particularly to a
membrane electrode assembly for an electrochemical cell including a
separate electron conduction layer for an electron movement path
between electrode catalysts therein.
2. Description of the Related Art
[0002] Generally, an electrochemical cell is an energy converter
that uses electric energy or generates electric energy, and is
classified into an electrolytic cell and a fuel cell. In order to
put the electrochemical cell to practical use, the fuel cell needs
to have improved output density (reduced electric energy
consumption in the case of water electrolysis), improved
durability, and a low price.
[0003] FIGS. 1 to 4 show a unit structure of a typical
electrochemical cell, an electrochemical stack structure, and a
system structure.
[0004] FIG. 1 is a view showing the concept of a membrane electrode
assembly 100 which constitutes a portion of a typical electrolytic
cell, which electrochemically decomposes water to generate hydrogen
and oxygen gases. FIG. 1 shows the thickness of the layer of each
of the elements at a lower part thereof.
[0005] The electrochemical cell for electrolysis, which
electrolyzes water (H.sub.2O) to generate oxygen gas (O.sub.2) and
hydrogen gas (H.sub.2), includes a first electrochemical reaction
layer 104, a second electrochemical reaction layer 108, a membrane
106, a first diffusion layer 102, and a second diffusion layer 110.
The first electrochemical reaction layer 104 includes a first
electrochemical catalyst 112 and a first carrier 114, and the
second electrochemical reaction layer 108 includes a second
electrochemical catalyst 116 and a second carrier 118.
[0006] The first diffusion layer 102 and the second diffusion layer
110 help to move electrons, reactants, or products to or from the
first and the second electrochemical catalysts 112 and 116. The
first and the second electrochemical catalysts 112 and 116 are the
most important materials that are used to perform electrolysis or
generate electric energy, and the first and the second carriers 114
and 118 function to support the first and the second
electrochemical catalysts 112 and 116 and provide an electron
movement path.
[0007] The first and the second electrochemical catalysts 112 and
116 are mixed with the first and the second carriers 114 and 118, a
binder, and a solvent to form a slurry or paste, which is then
applied on the membrane 106 or on the first and the second
diffusion layers 102 and 110 to form the first and the second
electrochemical reaction layers 104 and 108. The manufactured
assembly of "electrochemical reaction layers 104 and 108-membrane
106" or "electrochemical reaction layers 104 and 108-membrane
106-diffusion layers 102 and 110" is called a membrane electrode
assembly (hereinafter, referred to as "MEA").
[0008] The interval between the first electrochemical reaction
layer 104 and the second electrochemical reaction layer 108, which
are formed in the MEA, has a physical thickness value of the
membrane. Bubbles are not present in the first electrochemical
reaction layer 104 or in the second electrochemical reaction layer
108, thereby making it possible to perform high-current operation
at low voltages. Further, since the conductivity of an electrolyte
solution is not used, unlike in an alkali electrolytic cell, water,
which is a raw material, may be used while ensuring high purity,
and accordingly, high-purity hydrogen and oxygen may be
obtained.
[0009] The process of electrolyzing water will be described below
using the constitution shown in FIG. 1. The place at which an
oxidation reaction occurs is considered the first electrochemical
reaction layer 104, and the place at which a reduction reaction
occurs is considered the second electrochemical reaction layer 108.
The oxidation and reduction reactions occur simultaneously.
[0010] First, when water (H.sub.2O) is supplied through the first
diffusion layer 102 to the first electrochemical reaction layer
104, the water is decomposed into oxygen gas (O.sub.2), electrons
(e.sup.-) , and hydrogen ions (H.sup.+) (protons) at the first
electrochemical catalyst 112 (also called an oxidation catalyst, an
anode active material, or an oxygen-gas generating electrode), as
shown in the following Reaction Scheme 1. The oxygen gas (O.sub.2)
is discharged to the outside of the electrolytic cell via
diffusion, and the hydrogen ions (H.sup.+) are moved through the
membrane 106 to the second electrochemical catalyst 116 (also
called a reduction catalyst, a cathode active material, or a
hydrogen-gas generating electrode) by an electric field. The
electrons (e.sup.-), which are generated due to the aforementioned
reaction, are moved from the first electrochemical catalyst 112
through the first diffusion layer 102 and an external circuit (not
shown) to the second diffusion layer 110 and the second
electrochemical catalyst 116.
[0011] Meanwhile, the hydrogen ions (H.sup.+) and the electrons
(e.sup.-), which are moved from the first electrochemical catalyst
112, react at the second electrochemical catalyst 116 to generate
hydrogen gas (H.sub.2), as shown in Reaction Scheme 2. In addition,
a portion of the water supplied to the first electrochemical
reaction layer 104 is moved to the second electrochemical reaction
layer 108 by an electric field to thus be discharged together with
the hydrogen gas (H.sub.2) to the outside of the electrolytic
cell.
[0012] The electrochemical reactions, which occur at the first
electrochemical catalyst 112 and the second electrochemical
catalyst 116, are shown in the following Reaction Schemes 1 and 2.
The overall reaction at the first electrochemical catalyst 112 and
the second electrochemical catalyst 116 is shown in Reaction Scheme
3.
2H.sub.2O.fwdarw.4H.sup.++4e.sup.-+O.sub.2 (Anode) [Reaction Scheme
1]
4H.sup.++4e.sup.-.fwdarw.2H.sub.2 (Cathode) [Reaction Scheme 2]
2H.sub.2O.fwdarw.O.sub.2+2H.sub.2 [Reaction Scheme 3]
[0013] Meanwhile, a reverse reaction of electrolysis of water
occurs in the fuel cell, and will be described below (See Reaction
Schemes 4 to 6).
[0014] First, hydrogen gas is introduced into a first
electrochemical reaction layer 104, and oxygen gas is supplied to a
second electrochemical reaction layer 108. The hydrogen gas is then
converted into hydrogen ions (proton) and electrons via an
electrochemical reaction at a first electrochemical catalyst 112,
and the electrons are moved through an external load, which is
electrically connected to the fuel cell, and the protons are moved
through a membrane to a second electrochemical catalyst 116. The
protons and the electrons, which are generated at and moved from
the first electrochemical catalyst 112, react with oxygen gas,
which is supplied from the outside, at the second electrochemical
catalyst 116 to generate water, energy, and heat.
2H.sub.2.fwdarw.4H.sup.++4e.sup.+(Anode) [Reaction Scheme 4]
4H.sup.++O.sub.2+4e.sup.-.fwdarw.2H.sub.2O (Cathode) [Reaction
Scheme 5]
O.sub.2+2H.sub.2.fwdarw.2H.sub.2O [Reaction Scheme 6]
[0015] The following description pertains mainly to water
electrolysis for the electrolysis of water, but is applicable not
only to water electrolysis but also to fuel cells.
[0016] FIG. 2 is a view showing the structure of a typical
electrochemical cell which includes the MEA of FIG. 1 to
electrolyze water. An electrochemical cell 200, like that shown in
FIG. 2, includes a first end plate 202, a first insulating plate
204, a first current application plate 206, a first electrochemical
reaction chamber frame 208, a first electrochemical reaction
chamber 210, the MEA 100 of FIG. 1, a second electrochemical
reaction chamber 212, a second electrochemical reaction chamber
frame 214, a second current application plate 216, a second
insulating plate 218, and a second end plate 220. A direct current
power supply is used as a power converter 224, which applies
current to the electrochemical cell.
[0017] The first end plate 202 and the second end plate 220 have
bolt/nut fastening holes (not shown) for assembling the unit
electrochemical cells, and provide paths (not shown) through which
reactants and products are moved. The first insulating plate 204
and the second insulating plate 218 provide an electric insulation
function between the first end plate 202 and the first current
application plate 206 and between the second end plate 220 and the
second current application plate 216. The first current application
plate 206 and the second current application plate 216 are
connected to the power converter 224 to apply required current to
the electrochemical cell 200.
[0018] Meanwhile, when the first electrochemical catalyst 112 is
positioned in the first electrochemical reaction chamber 210 to
allow an oxidation reaction to occur, the first electrochemical
reaction chamber 210 becomes a space through which water, as the
reactant, and oxygen, as the product, are moved. The second
electrochemical reaction chamber 212, which is positioned at the
opposite side of the first electrochemical reaction chamber 210
while the membrane 106 is interposed between the first and the
second electrochemical reaction chambers, provides a space through
which hydrogen, which is generated in the reduction reaction, and
water, which is moved from the first electrochemical reaction
chamber 210, are moved.
[0019] The first electrochemical reaction chamber 210 is isolated
from the outside by the first electrochemical reaction chamber
frame 208, and the second electrochemical reaction chamber 212 is
isolated from the outside by the second electrochemical reaction
chamber frame 214. In addition, a gasket (or packing) 222 is
provided between the MEA 100 and the first electrochemical reaction
chamber frame 208 and between the MEA 100 and the second
electrochemical reaction chamber frame 214 in order to prevent the
reactants and the products from leaking to the outside.
[0020] Among the elements constituting the electrochemical cell
200, the first electrochemical reaction chamber frame 208, the
second electrochemical reaction chamber frame 214, and the gasket
222 have predetermined holes, through which the reactants or the
products are easily supplied to and discharged from the
electrochemical cell. The first electrochemical reaction chamber
frame 208 and the second electrochemical reaction chamber frame 214
have fluid paths (represented by the dotted line in (A) of FIG.
2).
[0021] Meanwhile, another electrochemical cell 200 may have a
pressure pad (not shown, refer to 304 of FIG. 3) between the second
electrochemical reaction chamber frame 214 and the second current
application plate 216 so as to maintain the balance of the
electrochemical cell 200.
[0022] FIG. 3 is a view showing the concept of a known
electrochemical stack. A plurality of unit electrochemical cells is
required in order to obtain a desired amount of products during an
electrolysis reaction, and an assembly of the two or more layered
electrochemical cells is called an electrochemical stack.
[0023] When the electrochemical cells are layered in order to
constitute the electrochemical stack 300 shown in FIG. 3, unit
electrochemical cells are repeatedly disposed in a desired number
between the basic electrochemical cells 200. A pressure pad 304 is
interposed between the unit electrochemical cells in order to press
the elements to each other. Bolts 306 are fastened with nuts 310
through holes, which are formed through edges of the first and the
second end plates 202 and 220, in order to assemble the unit
electrochemical cells in the electrochemical stack.
[0024] FIG. 4 is a view showing a system for electrolyzing water
using the electrolysis stack that is the same as the
electrochemical stack of FIG. 3 in order to produce hydrogen. A
hydrogen-generating system 400, as shown in FIG. 4, includes an
electrolysis stack 420, a water-treating unit for treating water,
which is supplied to the electrolysis stack 420, and a gas-treating
unit for purifying hydrogen gas, which is generated from the
electrolysis stack 420, and controlling pressure.
[0025] Pure water of 1 Mega ohm cm or more is used as water, which
is a raw material used in the electrolysis stack 420. An automatic
valve 402, which is provided in a pure water-supplying line s1, is
adjusted to supply pure water, and the automatic valve 402 is
controlled using a level sensor 405, which is used to sense the
level, in an oxygen-water separation bath 404 (dotted line e2).
Water is supplied from the oxygen-water separation bath 404 to the
electrolysis stack 420 using a circulation pump 406, which is
provided in a circulation pipe s2, joins water circulating through
a circulation line s9 from a hydrogen-water separation bath 424,
and then passes through a pipe in which a heat exchanger 408, a
water-quality sensor 410, and an ion-exchanging filter 412 are
provided. The water is then supplied to a first electrochemical
reaction chamber 414 (the place in which an oxidation reaction
occurs) of the electrolysis stack 420. Meanwhile, when direct
current is supplied from a power converter 440 through a wire e1 to
the electrolysis stack 420, the water undergoes a decomposition
reaction.
[0026] Oxygen, which is generated from the first electrochemical
reaction chamber 414, and unreacted water are moved through a
discharge pipe s4 to the oxygen-water separation bath 404, and a
temperature sensor 416 is provided in the discharge pipe s4 to
sense the temperature. Oxygen, which is separated in the
oxygen-water separation bath 404, is discharged through an
oxygen-discharge pipe s5 to the outside, and the water is subjected
to a re-circulation process.
[0027] The hydrogen gas, which is generated from the second
electrochemical reaction chamber 422, entails water, and is moved
through a discharge pipe s6 to the hydrogen-water separation bath
424 so as to be separated from water. A level sensor 426, which is
used to sense the level, is provided in the hydrogen-water
separation bath 424 so as to adjust the level. When the level of
the hydrogen-water separation bath 424 is a predetermined value or
more, an automatic valve 428 is opened (electric signal of e3) to
supply the water through the circulation line s9 to the circulation
pipe s2.
[0028] Meanwhile, the hydrogen gas, which is separated in the
hydrogen-water separation bath 424, is supplied through a gas pipe
s7 to a hydrogen-gas purifier 430 to thus remove moisture from
hydrogen. Typically, a bed, which is filled with a moisture
absorbent, is applied to the hydrogen gas purifier 430. The
hydrogen that passes through the hydrogen gas purifier 430 is
supplied through a high-purity hydrogen gas pipe s8 to a field
requiring hydrogen. A pressure control valve 434 is provided in the
high-purity hydrogen gas pipe s8 to control the pressure of the
hydrogen gas generated from the electrolysis stack 420. Pressure
sensors 432 and 438 are provided in the front and the rear of the
pressure control valve 434 to measure pressure, and a check valve
436 is provided to maintain the flow of gas in a predetermined
direction.
[0029] In this water electrolysis system, a reduction in electric
energy consumption (in the case of a fuel cell, an increase in
output density), improvement in durability, and cost reduction are
required in order to realize practical use of an electrochemical
cell. In this regard, in the case of the structure of the
conventional MEA 100, since the structure for delivering electrons
between the electrode catalysts in the MEA is not developed, the
electrochemical activity is low and it is difficult to realize
performance at high current density.
[0030] FIG. 5 shows photographs of the catalyst distribution on the
surface of a typical MEA (Comparative Example 1, to be described
later) manufactured using a conventional method. As seen from FIG.
5, the electrochemical reaction layer is locally formed, causing a
lack of electron delivery layers (black portions in the
photographs). Accordingly, there is a problem in that a large
amount of catalyst is consumed in order to maintain the
electrochemical performance.
CITATION LIST
Patent Document
[0031] Korean Patent No. 10-1357146
[0032] Korean Patent Application Publication No.
10-2008-0032962
SUMMARY OF THE INVENTION
[0033] Accordingly, the present invention has been made keeping in
mind the above problems occurring in the related art, and an object
of the present invention is to provide a membrane electrode
assembly (MEA) for an electrochemical cell including a separate
electron conduction layer for an electron movement path between
electrode catalysts therein. In the MEA, it is possible to enable
high current density operation, reduce electric energy consumption
even using typical current density, improve durability, and reduce
manufacturing costs.
[0034] In order to accomplish the above object, the present
invention provides a membrane electrode assembly for an
electrochemical cell that includes a polymer electrolyte membrane,
a first electrochemical reaction layer formed on one side of the
polymer electrolyte membrane to allow an oxidation reaction to
occur thereon, a first electron-conductive layer formed between the
polymer electrolyte membrane and the first electrochemical reaction
layer, a second electrochemical reaction layer formed on a
remaining side of the polymer electrolyte membrane to allow a
reduction reaction to occur thereon, and a second
electron-conductive layer formed between the polymer electrolyte
membrane and the second electrochemical reaction layer. The first
electron-conductive layer and the second electron-conductive layer
include a porous metal.
[0035] Further, according to the present invention, the porous
metal may further include a coating layer including a platinum
group applied on a surface thereof.
[0036] Further, according to the present invention, the first
electron-conductive layer and the second electron-conductive layer
may each have a thickness of 0.1 to 1 mm.
[0037] Further, according to the present invention, the first
electron-conductive layer and the second electron-conductive layer
may include any one among platinum, palladium, rhodium, iridium,
ruthenium, osmium, carbon, gold, tantalum, tin, indium, nickel,
tungsten, manganese, and titanium, or a complex including two or
more thereof.
[0038] Further, according to the present invention, the first
electrochemical reaction layer and the second electrochemical
reaction layer may each include an electrochemical catalyst (or the
electrochemical catalyst on a carrier), an ionic conductor, and a
binder.
[0039] Further, according to the present invention, the
electrochemical catalyst may include any one among platinum group
elements including platinum, palladium, ruthenium, iridium,
rhodium, and osmium, any one metal among iron, lead, copper,
chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium,
and aluminum, or alloys, oxides, or double oxides thereof.
[0040] Further, according to the present invention, elements
constituting the membrane electrode assembly may include the
polymer electrolyte membrane, the first electron-conductive layer,
the second electron-conductive layer, the first electrochemical
reaction layer, and the second electrochemical reaction layer in
descending order of size.
[0041] According to the present invention, for an electron movement
path, electrons are sequentially moved through an anode catalyst,
an electron-conductive layer on a membrane, an external circuit,
the electron-conductive layer on the membrane, and a cathode
catalyst. Therefore, the electron movement path is short compared
to a known electrochemical cell having an electron movement path,
which is formed so that the electrons are sequentially moved
through an anode catalyst, an anode-chamber diffusion layer, an
anode-chamber current application plate, an external circuit, a
pressure pad, a cathode-chamber current application plate, a
cathode-chamber diffusion layer, and a cathode catalyst.
Accordingly, the current density-voltage characteristics of the
electrochemical cell are excellent, thereby reducing energy
consumption during electrolysis.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] The above and other objects, features and advantages of the
present invention will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0043] FIG. 1 is a view showing the concept of an MEA, which is a
portion of a typical electrolytic cell that electrochemically
decomposes water to produce hydrogen and oxygen gases;
[0044] FIG. 2 is a view showing the structure of a typical
electrochemical cell which includes the MEA of FIG. 1 to
electrolyze water;
[0045] FIG. 3 is a view showing the concept of a known
electrochemical stack;
[0046] FIG. 4 is a view showing a system for electrolyzing water
using the electrochemical stack of FIG. 3 to produce hydrogen;
[0047] FIG. 5 shows photographs of the catalyst distribution on the
surface of a typical MEA manufactured using a conventional
method;
[0048] FIG. 6 is a view showing an MEA according to an embodiment
of the present invention; and
[0049] FIG. 7 is a comparative graph showing the performance of
Example 1 of the present invention and Comparative Example 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0050] Hereinafter, preferred embodiments of the present invention
will be described in detail with reference to the appended drawings
so as to easily perform the present invention by the person having
ordinary skill in the related art. However, descriptions of known
techniques, even if they are pertinent to the present invention,
are considered unnecessary and may be omitted insofar as they would
make the characteristics of the invention unclear. Furthermore, the
same or similar portions are represented using the same reference
numeral in the drawings.
[0051] FIG. 6 is a view showing an MEA according to an embodiment
of the present invention. As shown in FIG. 6, an MEA 500 according
to the embodiment of the present invention includes a first
electron-conductive layer 506, a first electrochemical reaction
layer 504, a membrane 502, a second electron-conductive layer 518,
and a second electrochemical reaction layer 508. The first
electron-conductive layer 506 and the first electrochemical
reaction layer 504 are sequentially formed on one side of the
membrane 502, and the second electron-conductive layer 518 and the
second electrochemical reaction layer 508 are sequentially formed
on the remaining side of the membrane 502. Meanwhile, the MEA 500
may or may not include a diffusion layer.
[0052] An electrolysis reaction of water in the MEA 500 of the
present embodiment will be described below. The description will be
given on the assumption that an oxidation reaction (oxygen
generation reaction) occurs at a first electrochemical catalyst and
a reduction reaction (hydrogen generation reaction) occurs at a
second electrochemical catalyst.
[0053] First, when water (H.sub.2O) is supplied to a first
electrochemical catalyst 510 (oxidation catalyst, oxygen catalyst),
water is decomposed into oxygen gas (O.sub.2), electrons (e.sup.-),
and hydrogen ions (H.sup.+) (protons). A portion of the water
(H.sub.2O) is discharged to the outside together with the oxygen
gas (O.sub.2), and the hydrogen ions (H.sup.+), which are obtained
due to decomposition, are moved through the membrane 502 to a
second electrochemical catalyst 516 (reduction electrode, hydrogen
electrode).
[0054] In addition, the decomposed electrons are moved to an
external circuit (not shown) via the first electron-conductive
layer 506 formed on the membrane 502 and the first electrochemical
catalyst 510 (oxidation catalyst, oxygen catalyst). Meanwhile, the
electrons (e.sup.-) moved along the external circuit (not shown)
for connecting the first electron-conductive layers 506 are moved
through the second electron-conductive layer 518. The electrons
moved to the second electron-conductive layer 518 reach the first
electrochemical catalyst 510. The electrons that are moved react
with hydrogen ions (H.sup.+) in the second electrochemical reaction
layer 508 to generate hydrogen gas. In addition, water (H.sub.2O)
which has passed through the membrane 502 in conjunction with the
hydrogen ions (H.sup.+) is discharged to the outside of the
electrolytic cell together with the hydrogen gas. The
electrochemical reaction that occurs under the first and the second
electrochemical catalysts 510 and 516 is shown in the
aforementioned Reaction Schemes 1 and 2.
[0055] Any membrane may be used as the membrane 502 of the present
embodiment as long as the membrane has hydrogen ion (proton)
conductivity, and a fluorine-based polymer electrolyte and a
hydrocarbon-based polymer electrolyte may be used. Examples of the
fluorine-based polymer membrane may include Nafion (Registered
trademark), manufactured by the DuPont Company, Flemion (Registered
trademark), manufactured by Asahi Glass Co., Ltd., Aciplex
(Registered trademark) manufactured by Asahi Kasei Corporation, and
Gore Select (Registered trademark) manufactured by Gore &
Associates, Inc. Examples of the hydrocarbon-based polymer membrane
may include an electrolyte membrane such as sulfonated polyether
ketone, sulfonated polyether sulfone, sulfonated polyether ether
sulfone, sulfonated polysulfide, and sulfonated polyphenylene.
Among the aforementioned examples, it is preferable to use a Nafion
(Registered trademark)-based material, which is manufactured by the
DuPont Company, as the polymer membrane.
[0056] The first and the second electron-conductive layers 506 and
518 of the present embodiment are formed on either side of the
membrane 502, and function to conduct electrons. The first and the
second electron-conductive layers 506 and 518 formed on the
membrane 502 are 0.1 to 1 mm and preferably 0.1 to 0.5 mm in
thickness. This is because when the thickness of the first and
second electron-conductive layers 506 and 518 is 0.1 mm or less, it
is difficult to form the electrochemical reaction layer on the
electron-conductive layer, and when the thickness of the
electron-conductive layer is 1 mm or more, the excessive formation
of the electrochemical reaction layer interferes with the movement
of the proton, thereby lowering the ionic conductivity. The
material of the first and second electron-conductive layers 506 and
518 may be a metal having excellent conductivity such as platinum,
palladium, rhodium, iridium, ruthenium, osmium, carbon, gold,
tantalum, tin, indium, nickel, tungsten, manganese, and titanium.
It is preferable that a platinum group coating be applied on
titanium from the viewpoint of chemical resistance.
[0057] Meanwhile, the first and second electron-conductive layers
506 and 518 may include a metal or may include a coating layer
including a platinum group applied on the metal. Meanwhile, a
material having excellent conductivity may be formed on the metal
by the principle of electroless plating of a process of
precipitating a metal precursor with a reducing agent, or by
thermal decomposition of the metal precursor.
[0058] The first and the second electrochemical reaction layers 504
and 508 of the present embodiment are formed on either side of the
membrane 502 having the first and the second electron-conductive
layers 506 and 518. The first and second electrochemical reaction
layers 504 and 508 may include an electrochemical catalyst (or the
electrochemical catalyst on a carrier), an ionic conductor, and a
binder. The catalyst ink for the first electrochemical reaction
layer 504 includes at least a first electrochemical catalyst 510, a
carrier 512 (may not be included), a polymer electrolyte, and a
solvent, and the catalyst ink for the second electrochemical
reaction layer 508 includes at least a second electrochemical
catalyst 516, a carrier 514 (may not be included), a polymer
electrolyte, and a solvent.
[0059] Examples of the polymer electrolyte included in the catalyst
ink of the present embodiment may include a fluorine-based polymer
electrolyte and a hydrocarbon-based polymer electrolyte exhibiting
proton conductivity. In addition, examples of the fluorine-based
polymer electrolyte may include a Nafion (Registered
trademark)-based material manufactured by the DuPont Company.
Examples of the hydrocarbon-based polymer electrolyte may include
an electrolyte such as sulfonated polyether ketone, sulfonated
polyether sulfone, sulfonated polyether ether sulfone, sulfonated
polysulfide, and sulfonated polyphenylene. Considering the adhesion
between the first and second electrochemical reaction layers 504
and 508 and the first and second electron conductive layers 506 and
518, it is preferable to use the same material as the membrane 502,
that is, the Nafion ionomer.
[0060] Examples of the first and second electrochemical catalysts
510 and 516 used in the first electrochemical reaction layer 504
and the second electrochemical reaction layer 508 in the present
embodiment may include platinum group elements including platinum,
palladium, ruthenium, iridium, rhodium, and osmium, metal such as
iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium,
molybdenum, gallium, and aluminum, or alloys, oxides, or double
oxides thereof. It is preferable to use one or more metals, which
are selected from platinum, palladium, rhodium, ruthenium, and
iridium, or oxides thereof in order to ensure excellent electrode
reactivity and effectively perform a stable electrode reaction over
a long period of time.
[0061] In the present embodiment, when the particle diameter of the
first and the second electrochemical catalysts 510 and 516 is very
large, the activity of the catalyst is reduced, and when the
particle diameter is very small, the stability of the catalyst is
reduced. Accordingly, the particle diameter is preferably 0.5 to 20
nm, and more preferably 1 to 5 nm.
[0062] Meanwhile, carriers 512 and 514 for carrying the catalyst
include powder that exhibits electron conductivity, and titanium
oxide or carbon particles may be used. The carrier is provided in a
fine particle form and exhibits conductivity, and any carrier may
be used as long as the carrier does not intrude the catalyst.
However, it is preferable to use titanium oxides, carbon black,
graphites, black lead, activated carbon, carbon fibers, carbon
nanotubes, or fullerene.
[0063] In addition, when the particle diameter of the carriers 512
and 514 is very small, it is difficult to form an
electron-conductive path, and when the particle diameter is very
large, the diffusion of gas into the electrode catalyst layer
formed on the carrier is reduced, or the availability of the
catalyst is reduced. Accordingly, the particle diameter is
preferably 10 to 1,000 nm, and more preferably 10 to 100 nm.
[0064] In the present embodiment, the thickness of the first and
second electrochemical reaction layers 504 and 508 is the same as
or larger than that of the first and second electron-conductive
layers 506 and 518, and is preferably 0.1 mm or less.
[0065] Meanwhile, the size of each component constituting the MEA
500 of the present embodiment is as follows. The size Dc of the
membrane 502 is larger than that of first and second
electrochemical reaction chamber frames 208 and 214 in FIG. 2, and
the size Db of the first and second electron-conductive layers 506
and 518 is smaller than that of the first and second
electrochemical reaction chamber frames 208 and 214 in FIG. 2. The
size Da of the first and second electrochemical reaction layers 504
and 508 is preferably the same as the internal area of first and
second electrochemical reaction chambers 210 and 212 in FIG. 2.
Further, the components constituting the MEA 500 preferably include
the membrane 502, the first electron-conductive layer 506, the
second electron-conductive layer 518, the first electrochemical
reaction layer 504, and the second electrochemical reaction layer
508 in descending order of size.
[0066] A method of manufacturing an MEA according to the embodiment
of the present invention will be described hereinafter.
[0067] Process 1: Pre-Treatment Process of the Membrane 502
[0068] Pre-treatment of the membrane 502 is a process which
includes roughening the surface of the membrane using a mechanical
process, and physically and chemically treating organic and
inorganic impurities present in the membrane 502. The process will
be described in detail below.
[0069] Process 2: Process of Forming the First and the Second
Electron-Conductive Layers 506 and 518
[0070] The first and second electron-conductive layers 506 and 518
are integrated with the membrane 502 obtained during process 1. A
metal precursor solution having electron conductivity (for example,
copper, platinum, silver, gold, etc.) and a reducing agent are
added to a porous metal to form a metal thin film layer having
electron conductivity on the porous metal, thereby forming the
first and second electron-conductive layers 506 and 518. The
thickness of the metal thin film layer is obtained by repeating a
deposition reduction process. The detailed procedure thereof will
be described later. The procedure is constituted based on the
principle of electroless plating, but the constitution is not
limited thereto. The metal precursor may be thermally decomposed,
thus forming the metal thin film layer on the porous metal.
Meanwhile, the first and second electron-conductive layers 506 and
518 that are manufactured in the above-described manner may be
simply integrated with the membrane 502 by thermal pressing using a
hot press.
[0071] Process 3: Process of Forming the First and the Second
Electrochemical Reaction Layers 504 and 508
[0072] Process 3 is a process of forming the first and the second
electrochemical reaction layers 504 and 508 on the membrane 502
which has the first and the second electron-conductive layers 506
and 518 obtained during process 2. Process 3 includes a catalyst
synthesis process, a catalyst ink manufacturing process, a catalyst
ink transfer process, and a thermal-pressing process. The catalyst
synthesis process includes forming mixture oxides, using a reaction
of a desired catalyst precursor and an oxidant, and drying the
mixture oxides to obtain an electrochemical catalyst having a
powder structure. The catalyst ink manufacturing process includes
mixing the electrochemical catalyst, which is synthesized during
the catalyst synthesis process, a particulate material, a
dispersant, and a binder made of the same material as the membrane
502 to manufacture the catalyst ink. Further, the catalyst ink
transfer process includes transferring the catalyst ink, which is
manufactured during the catalyst ink manufacturing process, onto a
Teflon sheet using a spray and then drying the catalyst ink. The
thermal-pressing process includes attaching the Teflon sheet, which
is obtained during the catalyst ink transfer process, to both sides
of the membrane 502 having the first and the second
electron-conductive layers 506 and 518, which are formed during
process 2, and then thermal-pressing the Teflon sheet using a hot
press. The processes will be described in detail below.
[0073] Hereinafter, the method of manufacturing the membrane
electrode assembly according to Examples of the aforementioned
embodiment and Comparative Examples will be specifically described,
and experimental results will be given. However, the present
invention is not limited to the following Examples.
EXAMPLE 1
[0074] 1. Manufacture of the MEA 500
[0075] (1) Process 1: Pre-Treatment Process of the Membrane 502
[0076] Both surfaces of the membrane 502 (Nafion 117) were
scratched in four directions using sandpaper (Emery sand paper
1100CW), and then subjected to the swelling process in pure water
at 90.degree. C. Impurities were removed from the membrane, which
was subjected to the swelling process, using ultrasonic wave
treatment in pure water, and the membrane was treated in 3%
hydrogen peroxide (H.sub.2O.sub.2) for 30 min and in 0.5 to 1M
sulfuric acid (H.sub.2SO.sub.4) at 90.degree. C. for 30 min, and
then subjected to the aforementioned pure water process again.
[0077] (2) Process 2: Process of Forming First and Second
Electron-Conductive Layers 506 and 518
[0078] A titanium porous metal (thickness 0.2 mm, porosity 60%) was
deposited in a chloroplatinate
((NH.sub.3).sub.4PtCl.sub.2.H.sub.2O) precursor solution for 5
hours. After the deposition process, a NaBH.sub.4 solution was
added dropwise every 20 minutes for 2 hours in order to reduce the
metal precursor. After the reduction was finished, platinum-plated
titanium porous metal was obtained. In order to obtain a plated
layer having a desired thickness, the impregnation reduction
process was repeated as many times as desired, thus forming the
first and second electron-conductive layers 506 and 518. The first
and second electron-conductive layers 506 and 518, which were
manufactured in the above-described manner, were then integrated
with both sides of the membrane 502. The membrane 502 and the first
and second electron-conductive layers 506 and 518 were integrated
by being thermally pressed using a hot press.
[0079] (3) Process 3: Process of Forming First and Second
Electrochemical Reaction Layers 504 and 508
[0080] During the process of forming the first and the second
electrochemical reaction layers 504 and 508, the first and the
second electrochemical reaction layers 504 and 508 were formed on
the membrane 502 having the first and the second
electron-conductive layers 506 and 518. The process of forming the
first and the second electrochemical reaction layers 504 and 508
included a process of synthesizing first and second electrochemical
catalysts, a process of manufacturing ink for the first and the
second electrochemical catalysts, a process of transferring ink for
the first and the second electrochemical catalysts, and a
thermal-pressing process.
[0081] (3-1) Process 3-1: Process of Synthesizing the First and the
Second Electrochemical Catalysts
[0082] (3-1-1) Synthesis of the First Electrochemical Catalyst
[0083] An oxidized iridium-ruthenium mixture catalyst was
manufactured using a reaction of iridium chlorides
(IrCl.sub.3.xH.sub.2O) and ruthenium chlorides
(RuCl.sub.3.xH.sub.2O) in a sodium nitrate solution. In addition,
iridium chlorides and ruthenium chlorides were agitated in the
solution having sodium nitrates dissolved therein for about 2 hours
to be uniformly dissolved. The manufactured mixture catalyst
solution was heated to 100.degree. C. to vaporize distilled water
for 1 hour to thus perform concentration, and the concentrate was
sintered in an electric furnace at 475.degree. C. for 1 hour and
then slowly cooled. Subsequently, the resulting material was washed
with 9 L of distilled water and filtered in order to remove
generated sodium chlorides. The obtained solid was dried at
80.degree. C. for 12 hours to manufacture a final iridium-ruthenium
electrochemical mixture catalyst.
[0084] (3-1-2) Synthesis of the Second Electrochemical Catalyst
[0085] Commercially available Pt/C (Premetek Inc., amount of
carried platinum of 30%) was used as the second electrochemical
catalyst 516.
[0086] (3-2) Process 3-2: Process of Manufacturing Ink for the
First and the Second Electrochemical Catalysts
[0087] (3-2-1) Manufacture of Ink for the First Electrochemical
Catalyst
[0088] The oxidized iridium-ruthenium catalyst, which was
manufactured during process 3-1, nano-sized titanium dioxides as
the carrier, and the Nafion solution as the binder were used, and
the used catalyst and Nafion ionomers were mixed in an isopropyl
alcohol solvent at a ratio of 1:3.5 based on the weight of the
solid. Agitation for 1 hour and ultrasonic wave treatment for 1
hour were alternately performed twice in order to disperse the
catalyst.
[0089] (3-2-2) Manufacture of Ink for the Second Electrochemical
Catalyst
[0090] Pt/C (Premetek Inc., amount of carried platinum of 30%) was
used as the second electrochemical catalyst, and the Nafion
solution (a registered product from DuPont) was used as the binder.
The used catalyst and Nafion solution were mixed in an isopropyl
alcohol solvent at a ratio of 1:7.5 based on the weight of the
solid. Agitation for 1 hour and ultrasonic wave treatment for 1
hour were alternately performed twice in order to disperse the
catalyst.
[0091] (3-3) Process 3-3: Process of Transferring the First and the
Second Electrochemical Catalysts
[0092] (3-3-1) Transferring of the First Electrochemical Reaction
Layer 504
[0093] The polytetrafluoroethylene (PTFE) sheet was used as the
transfer sheet. The ink for the first electrochemical catalyst,
which was obtained during process 3-2, was moved to a syringe for
electrospraying only. The catalyst ink was transferred onto the
base material and then dried in the atmosphere at 90.degree. C. for
30 min to manufacture an electrochemical catalyst layer. The amount
of carried oxide catalyst was adjusted to about 4 mg/cm.sup.2 to
set the thickness of the first electrochemical reaction layer
504.
[0094] (3-3-2) Transferring of the Second Electrochemical Reaction
Layer 518
[0095] The ink for the second electrochemical catalyst 516, which
was obtained during process 3-2, was moved to a syringe for
electrospraying only. The catalyst ink was transferred onto the
carbon sheet and then dried in the atmosphere at 90.degree. C. for
30 min to manufacture an electrochemical catalyst layer. The amount
of the carried oxide catalyst was adjusted to about 1 mg/cm.sup.2
to set the thickness of the second electrochemical reaction layer
508.
[0096] (3-4) Process 3-4: Thermal-Pressing Process
[0097] (3-4-1) Formation of the First Electrochemical Reaction
Layer 504
[0098] The first electrochemical catalyst, which was obtained
during process 3-3 and loaded on the Teflon sheet, was
thermal-pressed twice on the membrane 502, which was obtained
during process 2, under a condition of 120.degree. C. and pressure
of 10 MPa for 3 min. The Teflon sheet was removed to transfer the
catalyst.
[0099] (3-4-2) Formation of the Second Electrochemical Reaction
Layer 508
[0100] The carbon sheet, on which the manufactured second
electrochemical catalyst 516 was loaded, was thermal-pressed under
a condition of 120.degree. C. and pressure of 10 MPa for 2 min on
the opposite surface of the membrane 502, with which the
manufactured first electrochemical reaction layer 504 was combined,
to obtain the MEA 500 shown in FIG. 6.
[0101] 2. Electrochemical Cell for Evaluation and Evaluation
System
[0102] The MEA of Example 1 had an electrochemically active area of
314 cm.sup.2 (Da=20 cm), an electron-conductive layer thickness of
0.5 mm, Db of 21 cm, an area of 346 cm.sup.2, and a membrane size
Dc of 25 cm. Titanium fibers were layered on the first
electrochemical reaction layer 504, and carbon fibers having high
diffusibility were layered on the second electrochemical reaction
layer 508 to perform evaluation. A cell for evaluation, shown in
FIG. 2, and a water electrolysis system, shown in FIG. 4, were
actually manufactured to perform evaluation.
[0103] The temperature of the cell was maintained at 80.degree. C.
(a temperature sensor 416 in FIG. 4), and the current-voltage
measurement of the cell for evaluation was performed. Meanwhile,
the discharge pressure of hydrogen (controlled using s8 and 434 in
FIG. 4) was maintained at about 10 bar.
[0104] 3. Measurement Result
[0105] From FIG. 7, it can be seen that the MEA manufactured in
Example 1 has a small voltage change, that is, a small slope,
depending on current density even when the current density is
increased. FIG. 7 is a comparative graph showing the performance of
Example 1 of the present invention and Comparative Example 1, and
shows the performance of the current density depending on
voltage.
COMPARATIVE EXAMPLE 1
[0106] 1. Manufacture of the MEA (Manufacture of the MEA According
to a Known Method)
[0107] The pre-treatment process of the membrane and the processes
of forming the first and the second electrochemical reaction layers
were performed using the same procedure and conditions as Example
1, and the processes of forming the first and the second
electron-conductive layers were not performed in order to compare
Comparative Example 1 and Example 1.
[0108] FIG. 5 shows photographs of the catalyst distribution on the
surface of a typical MEA that is manufactured using a conventional
method and is used as Comparative Example 1, and also shows the
concentration distribution for each noble metal. It can be seen
that the catalyst on the surface is not uniformly distributed, as
indicated by the arrows in the photographs.
[0109] 2. Electrochemical Cell for Evaluation and Evaluation
System
[0110] The same evaluation was performed in the electrochemical
cell and evaluation system to which the MEA (electrochemically
active area of 314 cm.sup.2) of Comparative Example 1 was applied,
like in Example 1.
[0111] 3. Measurement Result
[0112] From FIG. 7, it can be seen that the voltage slope is
significantly increased as the current density is increased in
Comparative Example 1.
[0113] [Evaluation of Example 1 and Comparative Example 1]
[0114] FIG. 7 shows the current density-voltage characteristic of
the MEAs of Example 1 and Comparative Example 1, region (1) shows
the high and low performance, depending on the electrochemical
catalyst, and region (2) is a high current density region. The
constitutions of the electrochemical catalyst of Example 1 and the
electrochemical catalyst of Comparative Example 1 are the same.
Accordingly, from FIG. 7, it can be seen that the MEA of Example 1
and the MEA of Comparative Example 1 have similar performance in
region (1). However, it can be seen that since the membrane having
the electron-conductive layer of Example 1 has excellent electron
conductivity to the electrochemical reaction layer in the MEA, the
membrane exhibits the low-voltage characteristic in region (2),
which is the high-density current region, compared to the membrane
of Comparative Example 1.
[0115] As described above, the present invention has a better
electron movement path than the conventional electrochemical cell,
and accordingly the current density-voltage characteristic of the
electrochemical cell is excellent and the energy consumption
required for electrolysis is reduced. In other words, in the
related art, electrons are sequentially moved through an anode
catalyst, an anode-chamber diffusion layer, an anode-chamber
current application plate, an external circuit, a pressure pad, a
cathode-chamber current application plate, a cathode-chamber
diffusion layer, and a cathode catalyst to thus form the electron
movement path. However, according to the present invention, the
electrons are sequentially moved through an anode catalyst, an
electron-conductive layer on a membrane, an external circuit, the
electron-conductive layer on the membrane, and a cathode catalyst
to thus form the electron movement path. Therefore, the
electrochemical cell of the present invention has an electron
movement path that is shorter than that of the known
electrochemical cell, and accordingly, the current density-voltage
characteristics of the electrochemical cell may be excellent due to
the short electron movement path, thereby reducing energy
consumption during electrolysis.
[0116] Although the preferred embodiments of the present invention
have been disclosed for illustrative purposes, those skilled in the
art will appreciate that various modifications, additions and
substitutions are possible, without departing from the scope and
spirit of the invention as disclosed in the accompanying
claims.
* * * * *