HYBRID MEMBRANE-ELECTRODE ASSEMBLY WITH
MINIMAL INTERFACIAL RESISTANCE AND PREPRATION
METHOD THEREOF
Techniccal Field
The present invention relates to a membrane-electrode
assembly (MEA) , a key component of a fuel cell, as well as a
production method thereof. More particularly, the present
invention relates to a membrane-electrode assembly which is
suitable for mass production and has a low inter facial
resistance between the membrane and the electrode, as well as
a production method thereof.
Background Art
Fuel cells have recently received much attention as new
electric generators. In the near future, the fuel cells will
substitute for the existing electric generators as either
automobile batteries, power sources for electric generation
or portable electric sources.
A polymer electrolyte fuel cell is a kind of a direct
current generator converting the chemical energy of fuel
directly to electric energy by electrochemical reaction. It
conprises a continuous stack complex equipped with membrane-
electrode assemblies which are the heart of the fuel cell and
bipolar plates which serves to collect generated electricity
and to supply fuel. The membrane-electrode assembly refers to
an assembly comprising: an electrode where electrochemical
catalytic reaction occurs between fuel (aqueous methanol
solution or hydrogen) and air; and a polymer membrane where
the transfer of hydrogen ions occurs.
Meanwhile, all electrochemical reactions consist of two
individual reactions^ i.e., oxidation reaction occurring at
an anode(fuel electrode), and reduction reaction occurring at
a cathode(air electrode), in which the two electrodes are
separated from each other by a polymer electrolyte memk)rane.
In a direct methanol fuel cell, methanol and water in place
of hydrogen are supplied to the anode, and hydrogen ions
produced in an oxidation process of methanol are transferred
to the cathode through the polymer electrolyte membrane and
generates electricity by reduction reaction with oxygen
supplied to the cathode. Such reactions are as follows:
anode (fuel electrode) : CH3OH + H2O -> CO2+ SH+ + 6e"
cathode (air electrode): 3/2O2 + SH"" + 6e"-y 3H2O
Overall reaction: CH3OH + 3/2O2-> CO2 + 3H2O
The electrode of the direct methanol fuel cell is
typically a diffusion electrode. The electrode consists of
two layers, a gas diffusion layer (electrode support layer)
and 6in active layer. The gas diffusion layer serves as a
support and to diffuse fuel, and is made of carbon paper or
carbon cloth. The active layer is adjacent to the polymer
electrolyte membrane to cause substantial electrochemical
reaction and is made of either a platinum catalyst particle
dispersed in a carbon particle or platinum or alloy black.
The electrochemical reaction occurs at a three-phase
inter facial zone in which fuel diffused from the gas
diffusion layer is exposed to the interface between the
electrolyte membrane and the platinum catalyst particle of
the active layer. Thus, it is important for the improvement
of performance to enlarge the area of the three-phase
reaction zone, which is available in the electrochemical
reaction, and to place the platinum catalyst in the three-
phase reaction zone to the maximum possible extent. However,
unlike a liquid electrolyte, a depth to which the solid
polymer electrolyte membrane can be impregnated into the
electrode is limited to 10 |am, so that the area of the three-
phase reaction zone, which can be enlarged, is limited, and
only a portion of the platinum catalyst, which is exposed to
the three-phase reaction zone, can participate in the
electrochemical reaction in the fuel cell. Accordingly, in
order to increase the power density of the fuel cell, an
electrode structure is required in which the area of the
three-phase reaction zone is maximized and the maximum
possible amount of platinum is placed in the active layer
which is in contact with the electrolyte.
In the initial development stage of the direct methanol
fuel cells, an electrode was used which had been prepared by
adding Pt-black particles onto carbon paper or carbon cloth
used as a diffusion layer by a spray, etc., so as to form an
active layer, and adhering the active layer to an electrolyte
membrane by a hot-pressing process. However, the prior
structure had problems in that the interfacial resistance
between the active layer amd the electrolyte membrane was
high to make the structure inefficient, and a significant
amount of the catalyst particles penetrated into the
diffusion layer and thus did not participate in the reaction,
indicating that the expensive noble catalyst was useless.
In attempts to solve such problems, methods of forming
a catalytic layer directly on an electrolyte membrane as in a
decal process (US Patent No. 6,391,486) and a sputter
deposition process {US Patent No. 6,171,721) were proposed.
However, the decal process is one comprising forming an
active layer separately and then laminating the active layer
with an electrolyte membrane, but requires a higher
temperature than the glass transition temperature of the
electrolyte membrane upon the laminating step, thus requiring
separate pretreatment which makes the process complex.
Another problem is that the transfer of the separately formed
active layer is not proferly performed.
In the sputter deposition process, the efficiency of a
catalyst can be increased, but a thin film is formed at a
thickness of more than 1 µm due to the crystalline nature of
the catalyst, thus preventing the transport of cations.
Accordingly, only a very small amount of the catalyst will
inevitably be used, resulting in a reduction in power
density. Also, the sputter deposition process has problems in
that a higher power density than a given level can not
obtained, and a high-vacuum region is used due to the
characteristic of a semiconductor process, resulting in
increases in production cost and time, which renders the
process unsuitable for mass production.
While the above-described coating methods have their
own advantages, they have a serious disadvantage in that it
is difficult to form a stable interface between a solid
polymer electrolyte meitibrcine and nanosized catalyst
particles.
Brief Description o£ the Drawings
FIG. 1 is a schematic diagram showing a hybrid
membrane-electrode assembly according to the present
invention.
1: a diffusion layer of a anode;
2: an active layer of the anode;
3; an active layer of an electrolyte membrane;
4: an electrolyte membrane (polymer membrane);
5: an active layer of the electrolyte membrane;
6: an active layer of the cathode;
7: a diffusion layer of the cathode; and
8: a catalyst coated with electrolyte.
FIG. 2 shows the comparison of the laminating state of
an electrolyte membrane to a catalytic active layer between a
case where the catalytic layer is formed on a diffusion layer
(the upper portion of the figure) and a case where the
catalytic layer is formed on an electrolyte membrane (the
lower portion of the figure).
FIG. 3 shows microscope photographs of carbon cloth
(left side) and carbon cloth (right side), each of which is
used as a diffusion layer.
FIG. 4 shows the participation of catalyst particles in
reaction according to the viscosity of catalyst ink used in
coating a diffusion layer.
FIG. 5 is a schematic diagram showing a process of
preparing catalyst ink.
FIG. 6 is a perspective view showing an electrolyte
membrane which had been coated with a catalytic active layer
using a mask.
FIG. 7 is a schanatic diagram showing a method of
coating an active layer on a gas diffusion layer by a screen
printing process.
9: a screen printer;
10: an automatic feed head;
11: an active layer (catalytic layer);
12: a compression roller; and
13: a gas diffusion layer (carbon paper or carbon
cloth).
FIG. 8 shows the current-voltage curve and povjer
density curve of membrane-electrode assemblies produced in
Comparative Examples 1 and 2 and Example 1.
FIG. 9 shows not only schematic diagrams illustrating
the coating state of an active layer according to the
viscosity of catalyst ink in electrodes which had been
produced by coating catalyst inks having viscosities of 75
cPs (Comparative Exanple 3), 1,000 cPs (Example 1), and
15,000 cPs (Comparative Example 4), respectively, on a
diffusion layer by using a screen printing process, but also
photographs of the electrodes.
FIG. 10 shows the current-voltage curve and power
density curve of membrane-electrode assemblies produced in
Example 1 and Comparative Exaitples 3 and 4.
FIG. 11 shows photographs of the front side (left
figure) and backside (right figure) of an electrode where an
active layer had been coated at a condition of 75 cPs by a
die coating process in Comparative Example 5.
FIG. 12 shows the current-voltage curve and power
density curve of membrane-electrode assemblies produced in
Example 1 and Comparative Example 6.
FIG. 13 shows photographs of an electrolyte membrane
which had been coated with an active layer by a screen
printing technique in Comparative Example 7.
FIG. 14 shows the structure of a catalytic layer coated
on a gas diffusion layer in membrane-electrode assemblies
produced in Examples 2 and 3.
FIG. 15 shows a current-voltage curve and power density
curve in hydrogen fuel cells (PEMFC) which include membrane-
electrode assemblies produced in Examples 2 and 3,
respectively.
Disclosure of the Invention
Therefore, it is an object of the present invention to
provide an epoch-making method which can solve the problem of
formation of the unstable interface occurring in the prior
art and allows catalyst availability to be increased.
The present inventors have extensive studies to produce
a membrane-electrode assembly (MEA.) overcoming the above-
described disadvantages occurring in the prior art, and
consequently found that if a hybrid coating technique in
which an active layer containing a catalyst material is
coated on each of an electrolyte membrane and a diffusion
layer forming interfacial resistance was used, interfacial
resistance could be reduced as compared to that in a case of
coating the active layer on either of the electrolyte
membrane and the diffusion layer, and also the control of
viscosity in forming the active layer on the diffusion layer
could result in an increase in catalyst availability. On the
basis of these findings, the present invention has been
perfected.
Furthermore, the coating of the active layer on the gas
diffusion layer by the hybrid coating technique was performed
by a curtain coating process, such as screen printing, die
coating or blade coating, which facilitates mass production.
In one aspect, the present invention provides a
membrane-electrode assembly comprising: electrodes consisting
of a anode coitprising a gas diffusion layer and a catalyst
material-containing active layer, and an cathode comprising a
diffusion layer and a catalyst material-containing active
layer; and an electrolyte membrane interposed between the
anode and the cathode and comprising a catalyst material-
containing active layer at one or both sides, the electrodes
being hot-pressed to the electrolyte membrane, wherein the
viscosity of the active layer in coating the active layer on
the gas diffusion layer is in a range of 100 to 10,000 cPs
(see FIG. 1).
In another aspect, the present invention provides a
method for producing a membrane-electrode assemi^ly,
comprising the steps of: (a) forming a catalyst material-
containing active layer on the surface of an electrolyte
membrane; (b) forming a catalyst material-containing active
layer on the surface of a gas diffusion layer; and (c) hot-
pressing the gas diffusion layer to the electrolyte membrane,
wherein the viscosity of the active layer, which is applied
on the gas diffusion layer at the step (b) , is controlled in
a range of 100 to 10,000 cPs. In this method, the steps (a)
and (b) may be performed simultaneously, seguentially or in
reverse order.
The present invention is characterized in that, in
producing the membrane-electrode assembly, the catalytic
active layer is coated on each of the electrolyte menibrane
and the diffusion layer, and in forming the active layer on
the diffusion layer, the viscosity of the active layer is
controlled, in order to reduce interfacial resistance and to
increase catalyst availability and production rate.
(1) Reduction in interfacial resistance
The reaction in fuel cells mainly occurs at the
interface between the catalytic active layer and the
electrolyte membrane. Namely, cations generated from a
catalyst are passed through the electrolyte membrane and
subjected to electrochemical reaction at an opposite-side
catalyst. For this reason, the laminating state between the
electrolyte membrane and the catalytic active layer
(formation of three-phase reaction zone) is very important.
As can be seen in FIG, 2, if the catalytic active layer
is formed directly on the electrolyte membrane, a good
laminating state between the electrolyte membrane and the
catalytic active layer will be obtained. On the other hand,
coating a catalyst on the diffusion layer results in
resistance to the transport of cations generated from the
catalyst to the electrolyte membrane even if hot pressing is
performed, since a depth to which the electrolyte membrane
(nafion) penetrates into the catalytic active layer is
limited.
Interfacial resistance can be examined by impedance
measurement. For exaitple, the measurement of iirpedance of MEA
in a single cell by a two-electrode impedance method using a
Zahner IM6 inpedance analyzer, i.e., the measurement of
impedance in a IM-lkHz region at an alternating current
amplitude of 5 mV using the flow of 400 scon hydrogen gas
through a reference electrode and the flow of 2000 scan air
through a working electrode, showed an interfacial resistance
of 25-30 mO*6.25 for the direct coating (the case of forming
a catalytic layer on an electrolyte membrane), and an
interfacial resistance of 35-40 mO*6.25 for the indirect
coating (the case of forming a catalytic layer on a diffusion
layer).
Thus, the present invention aims to reduce interfacial
resistance by coating the catalytic active layer on both the
electrolyte membrane and the gas diffusion layer in the
membrane-electrode assembly.
(2) Increase in catalyst availability
A. first approach to increase catalyst availability is
to increase the electrochemical utilization efficiency of the
catalyst in MEIA, and a second approach is to increase the
amount of the catalyst coated during the production of Me:A.
(a) The electrochettiical reaction in a fuel cell occurs
in a three-phase reaction zone where a catalyst (e.g., Pt or
Pt-Ru), fuel (e.g., methanol solution), and ionomer (e.g.,
nafion ionomer) are present together. Thus, a catalyst which
is not present in the three-phase reaction zone does not
participate in the reaction and causes a reduction in
catalytic efficiency.
If dilute catalyst ink is used in forming the catalytic
layer on the gas diffusion layer (GDL), the catalyst
particles with a size of less than about 1 µm will penetrate
into the pores of the diffusion layer (see FIGS. 3 and 11),
thus reducing the amount of the catalyst used in the
reaction. In order to increase the catalytic efficiency
(i.e., the efficiency of MEA in the electrochemical
reaction), the catalyst particles must not be lost into the
diffusion layer (GDL) (see FIG. 4) .
For this purpose, the present invention is
characterized in that the ionomer membrane (Nafion membrane) ,
an electrolyte mennbrane where the main reaction occurs, is
coated with the catalytic active layer by an air spray
process, and catalyst ink with high viscosity is coated on
the gas diffusion layer (GDL) by screen coating or die
coating. This allows the fabrication of the catalytic active
layer without the loss of the catalyst into the diffusion
layer.
(b) In the air spray process used in coating the
catalytic layer on the electrolyte membrane, the sprayed
catalyst particles shows a very low adhesion rate of about 30
wt% and a loss rate of the remaining 70%, resulting in a
reduction in catalyst availability. Thus, the present
invention is characterized in that a small amount of the
catalytic layer is coated on the electrolyte membrane in
order to reduce the interfacial resistance of the electrode,
and most of the catalytic active layer is coated on the
diffusion layer (an adhesion rate of more than 90% for a
rotary screen printing process) in order to increase the
catalyst availability.
(3) Increase in production rate
In coating most of the catalytic active layer on the
diffusion layer, the present invention utilizes a curtain
coating process, such as screen printing, die coating or
blade coating, which makes mass production easy.
Hereinafter, the present invention will be described in
detail.
1. Preparation of ca-balyst ink for active layer
Both an anode (fuel electrode) and a cathode (air
electrode) in a fuel cell contain a catalytic material for
electrochemical reaction in an active layer.
Catalyst materials which can be used in the present
invention include Pt black, Pt-Ru black, platinum-supported
carbon (platinized carbon, Pt/C) , platinura-ruthenium-
supported carbon (Pt-Ru/C), platinum-molybdenum (Pt-Mo)
black, platinum-molybdenum (Pt-Mo)-supported carbon,
platinum-rhodium (Pt-Rh) black, platinum-rhodium-support ed
carbon, and other platinum based alloys.
The catalyst for the anode is preferably Pt-Ru or Pt-
Ru/C, and the catalyst for the cathode is preferably Pt or
Pt/C. The reaction in the anode generates CO which poisons
the catalyst, thus lowering the catalyst activity. In order
to prevent this phenomenon, Ru is preferably used as a
cocatalyst.
The catalyst particles may be dispersed in carbon
particles or made of platinum or alloy blacks.
Examples of catalyst supports which can be used in the
present invention include all general carbon black based
supports such as Vucan XC-72R, Vulcan XC-72, acetylene black,
Kejon black, and black pearl, as well as conducting complex
oxides such as platinum oxide and ruthenium oxide.
The catalyst material of active layer applied on the
surface of the electrolyte membrane, which is opposite to the
surface of the anode, is preferably the same as the active
layer material on the anode, and the catalyst material of
active layer applied on the surface of the electrolyte
m^nbrane, which is opposite to the surface of the cathode, is
preferably the same as the active layer material on the
cathode.
However, the composition and coating method of the
catalyst ink applied on each of the diffusion layer cind the
electrolyte membrane are different between the diffusion
layer and the electrolyte membrane. In other words, the
electrolyte membrane is coated using a catalyst ink having a
low viscosity of less than 10 cPs as it is preferably coated
by an air spray process. Also, the gas diffusion layer (GDL)
is preferably coated using a catalyst ink having a high
viscosity of 100-10,000 cPs and more preferably 1,000-10,000
cPs, so that it is preferably coated by a screen printing,
blade coating or die coating process.
A solvent/dispersion medium for the active layer of the
anode is preferably the same as a solvent/dispersion medium
for the active layer of the cathode. Non-limited examples of
the solvent/dispersion medium, which can be used in the
present invention, include water, butanol, isopropanol (EPA),
methanol, ethanol, normal propanol, normal butyl acetate, and
ethylene glycol.
The content of the solvent/dispersion medium in the
catalyst ink is preferably 1-30% times the weight of the
catalyst used. The viscosity of the catalyst ink may vary
depending on the amount of the catalyst, and a coating
process may be determined depending on the viscosity.
In order to increase the utilization efficiency of the
catalyst, it is important to make catalyst ink where the
catalyst is well dispersed without aggregation. For this
purpose, it is preferable in the present invention that
isopropanol (IPA), NAFION solution and water are mixed with
each other at suitable amounts to prepare a well-dispersed
solvent mixture, which is mixed and stirred with the catalyst
so as to disperse the catalyst well and then subjected to a
ultrasonic milling process for 5 minutes so as to be
uniformly mixed with the catalyst. On a preparation method of
the catalyst ink, see FIG. 5.
The composition of the catalyst ink for the active
layer -of the anode contains;, but not limited to, Pt-Ru or Pt-
Ru/C, Nafion ionomer (30 wt% based on the weight of the
catalyst), and solvent (1-30 times the weight of the
catalyst) , and the coirposition of the catalyst ink for the
active layer of the cathode contains, but not limited to, Pt
or Pt/C, Nafion ionomer (30 wt% based on the weight of the
catalyst), and solvent (1-30 times the weight of the
catalyst)
2. Coating of catalyst ink on electrolyte membrane
(first step)
The electrolyte membrane acts as a hydrogen ion (H*)
conductor.
Non-limited examples of the electrolyte membrane, which
can be used in the present invention, include Nafion™
membrane (manufactured by DuPont Corp, perfluoro sulfonic
acid), Flemion (Asahi glass Co.), aciplex (Asahi Chemical
Co.), and Gore-select (Gore Co.), as well as all cation
electrolyte membranes.
The electrolyte membrane may be either a complex
electrolyte membrane or a rtiorabrane, the surface of which had
been hydrophilically treated.
Coating the electrolyte membrane with catalyst ink will
be preferably performed by supplying the catalyst ink by a
gas pressure method and coating the catalyst ink on a
completely dried electrolyte moanbrane by a spray process. In
this case, the viscosity of the active layer which is coated
on the electrolyte membrane is preferably in a range of 1-10
cPs.
In the first step, the coating of the catalyst ink on
the surface of the electrolyte raeitibrane is preferably
performed by a spray coating process. This is because the
spray coating process has an advantage in that it allows a
thin catalyst layer to be formed directly on the surface of
the polymer electrolyte membrane, thus securing a continuous
interface and increasing the availability of the catalyst.
Another reason is that coating the catalytic active layer on
the electrolyte membrane by a screen method results in the
deformation of the electrolyte membrane (see FIG. 13) .
At this time, in order to prevent the electrolyte
membrane from being swolJen by the solvent of the catalyst
ink during the production of the membrane-electrode assemloly,
it is iitportant to maintain the electrolyte membrane in a
dried state. It is thus preferable to continuously evaporate
the catalyst ink solvent by a thermal dryer during the spray
coating, so as to maintain the electrolyte membrane in a
dried state. For this purpose, an air spray process is
preferably used.
An exanple of the spray coating process uses a spray
gun.
Examples of carrier gas which can be used in the spray
coating using the spray gun include inert gases, such as
nitrogen and air. The pressure of the carrier gas in the
spray coating using the spray gun may be in a range of 0.01
to 2 atm.
An operation terrperature at which the active layer is
formed on the electrolyte membrane is preferably in a range
of 20 to 100 "C. An operation terrperature of more than 120 °C
may cause the problem of Nafion degradation as it approaches
140 °C, the glass transition tenperature (Tg) of Nafion.
Furthermore, the operation temperature may vary depending on
the Tg of an electrolyte membrane used.
In addition to the spray process, processes of coating
the catalyst ink on the electrolyte membrane include a
physical vapor deposition process of coating a very small
amount of a catalyst on a membrane using a RF magnerron
sputter or a thermal evaporator. Particularly, in order to
prevent the crystal growth of the catalyst upon sputtering,
co-sputtering with carbon or conductive material may be
adopted.
The electrolyte membrane is preferably coated with the
active layer using a mask. Namely, it is preferable to form
patterns. This is because the electrolyte membrane needs to
have a larger area than an area required for reaction since
it must not only act as a passage for the transport of an
electrolyte but also act to inhibit the flow of reactants
(methanol or hydrogen, oxygen) (see FIG. 6) .
The amount of the active layer formed on the
electrolyte membrane is preferably 1-100% by weight based on
the weight of the active layer formed on the diffusion layer.
The active layer on the electrolyte membrane is preferably
formed to a thickness of less than 1 |Jm. This is because it
is advantageous to coat most of the catalyst on the diffusion
layer since a coating process such as screen printing is more
advantageous for mass production than that of the spray
process.
3. Coatong of catalyst: ink on diffusion layer (second
step)
Although catalyst ink used on the diffusion layer is
made of the same material as that of the catalyst ink used on
the electrolyte membrane, it preferably contains a solvent at
a different ratio from that of the catalyst ink used on the
electrolyte membrane. Specifically, it is preferable that the
catalyst ink on the diffusion layer has thick viscosity, and
the catalyst ink on the electrolyte membrane has dilute
viscosity.
The diffusion layer may be formed of carbon paper or
carbon cloth (manufactured by SGL, toray, etc.).
If methanol is used as fuel, it will be preferable that
the diffusion layer used in the anode is made of a carbon
paper or carbon cloth which had not been water-repellent-
coated with Teflon in order to effectively supply methanol.
In this case, the diffusion layer of the cathode is
preferably made of a carbon paper which had been water-
repellent-coated with 5-20 wt% of Teflon in order to remove
water generated after reaction.
The catalyst ink used in forming the active layer on
the gas diffusion layer preferably has a viscosity of 100-
10,000 cPs, and more preferably 1,000-10,000 cPs.
In the process of forming the active layer on the
diffusion layer, it is most important to control the
viscosity of the catalyst ink. This is because the current-
voltage curve cind power density of the membrane-electrode
assembly vary depending on the control of viscosity in
forming the active layer on the diffusion layer. Namely, if
the catalyst ink has low viscosity, catalyst particles will
infiltrate into the porous diffusion layer, so that catalyst
particles which do not participate in reaction will
significantly increase so as to make catalyst perforr[iance
insufficient and to require the use of a large amount of
catalysts. Particularly, the infiltrated catalyst particles
act as serious resistance to the diffusion of the fuel
methanol solution at the diffusion layer side of the anode.
For this reason, catalyst ink with high viscosity is
required.
Processes of coating the catalyst ink for the active
layer on the diffusion layer include a curtain coating
process (e.g., screen printing, blade coating or die coating)
and a spray process.
In order to prevent catalyst particles (less than 1 [nm]
from infiltrating into the internal pores of the diffusion
layer to lower catalyst availability, catalyst ink with the
highest possible viscosity needs to be used. Thus, the
curtain coating process is more advantageous than a spray
process such as air spray. Further, the curtain coating
process is advantageous for mass production
In forming a catalytic layer on the gas diffusion
layer, a catalyst coated with electrolyte powder is
preferably used in order to form a three-phase reaction zone
while facilitating the control of the catalyst ink viscosity.
Then, the electrolyte (Nafion)-coated catalyst powder is
preferably mixed with a solvent or dispersion medium.
Examples of the solvent or dispersion medium used include
water and alcohols, such as butanol, isopropanol (IPA), and
normal propanol (NPA) . If the Naf ion solution is added to the
catalyst ink so as to form the three-phase reaction zone, the
viscosity of the catalyst ink will be reduced to make the
catalyst ink unsuitable for screen printing. Thus,- it is
preferable to control the catalyst viscosity by performing a
pretreatment process where a Pt or Pt-Ru black catalyst is
added to Nafion solution, and the solvent of the Nation
solution is dried in a drying oven, thus coating only Nafion
electrolyte on the surface of the catalyst particles. By the
catalyst pretreatment process as described above, the
catalyst viscosity may be easily determined depending on the
mixing ratio of the catalyst to the solvent.
A process of coating the catalyst ink prepared as
described above on the gas diffusion layer using a screen
printer is shown in FIG. 7.
As a coating process such as screen printing is more
advantageous for mass production than the spray process, it
is advantageous to coat most of the catalyst on the diffusion
layer. Thus, it is preferable to form the active layer on the
electrolyte membrane to a thickness of less than 1 |jm while
forming most of the active layer on the diffusion layer.
In the step of forming the active layer on th€5 gas
diffusion layer, it is preferable to dry the solvent of the
catalyst ink remaining after coating the catalytic layer. The
active layer formed on the gas diffusion layer is preferably
dried by a hot rolling process, thus making an electrode.
4. Asseanbly of electrode and. elec-brolyte membrane
The active layer coated on the electrolyte membrane
adheres to the active layer (catalytic layer) coated on the
gas diffusion layer by hot pressing, thus producing a
membrane-electrode membrame (MEA) .
The process of hot-pressing the electrode to the
electrolyte membrane is preferably performed at a temperature
of 50-200 °C under a pressure of 5-100 kg/cm^.
The Advantageous Effect
The inventive membrane-electrode assembly has a low
interfacial resistance between the membrane and the
electrodes, as well as high catalyst availability and
excellent power density, and can be mass-produced.
Best Mode for Carrying Out the Invention
Hereinafter, the present invention will be described in
detail by exaitples. It is to be understood, however, that
these exaitples are given to illustrate the present invention
and not intended to limit the scope of the present invention.
Example 1
Step 1: Coating on electrolyte membrane side
Pt-Ru black was used as a catalyst on a anode side of
an electrolyte membrane, and Pt black was used as a catalyst
on an cathode side of the electrolyte membrane. Nation
solution, IPA and water were mixed with each other at
suitable amounts so as to prepare a well-dispersed solvent
mixture. The solvent mixture was mixed with each of the
catalysts at a ratio of catalyst : Nafion dry weight :
dispersion medium of 1: 0.3: 50, and stirred to disperse the
catalyst well, and uniformly mixed by sonification for 5
minutes, thus preparing catalyst ink. The catalyst ink had a
viscosity of 1 cPs.
A Dupont Nafion membrane used as an electrolyte
membrane was pretreated with hydrogen peroxide and sulfuric
acid and then inserted into a stainless grid used as a mask.
The Nafion membrane was heated at the backside of the
stainless grid by a thermal dryer for 20 minutes so as to
completely remove moisture. The temperature in the thermal
dryer was 80 "C. Next, each of the catalyst inks (Pt-Ru black
for the anode side, and Pt black for the cathode side)
prepared as described above was taken and sprayed on the
front side of the grid at the amount of 0.1-20 cc/cm^ by means
of a spray gun, thus forming an active layer with a thickness
of less than 1 µm. At this time, the pressure of carrier gas
was in a range of 0.01-2 atm, and the solvent of the catalyst
ink was continuously evaporated during the spray coating by
heating the Nation membrane at the backside of the grid by a
thermal dryer.
Step 2: Coating on diffusion layer
As a diffusion layer for use on the anode, carbon paper
and carbon cloth which had not been water-repellent-coated
with Teflon in order to efficiently supply methanol was used.
As a diffusion layer for use in the cathode, carbon p^iper
which had been water-repellent-coated with 5-20 wt% of Teflon
in order to remove water generated after reaction was used.
A pretreatment process was performed in which each of
Pt black and Pt-Ru black catalysts was added to Nation
solution, and the solvent of the Nafion solution was dried in
a drying oven, thus coating only Nafion electrolyte on the
surface of the catalyst particles.
As the anode catalyst, the Pt-Ru black pretreated as
described above was used, and as the cathode catalyst, the Pt
black pretreated as described above was used. IPA and water
were mixed with each other at suitable amounts so as to
prepare a well-dispersed solvent mixture, and then mixed with
each of the catalysts at a ratio of catalyst : nafion dry
weight : dispersion medium of 1 : 0.3 : 3. The resulting
mixture was stirred to disperse the catalyst well, and
uniformly mixed by sonication for 5 minutes, thus preparing
catalyst inks. The catalyst inks had a viscosity of 1000 cPs.
Each of the catalyst inks prepared as described above
was coated on a diffusion layer by means of a screen printer
shown in FIG. 1, thus making an electrode with a catalyst
content of about 4 mg/on^. The active layer formed or, the
diffusion layer was dried by a hot rolling process, thus
producing a diffusion electrode.
Step 3: Assembling of electrode and electrolyte
membrane
A Nafion electrolyte membrane containing an active
layer was put between the two electrodes prepared as
described above, and hot-pressed at 140 °C under a pressure of
5-100 kg/cm^ for 3-10 minutes, thus producing a membrane-
electrode assembly (MEA) . In this ME^A, the Nafion electrolyte
membrane was slightly larger than the electrodes, and the
sizes of the electrodes and the Nafion electrolyte memt.rane
were 5cm^ and 16cm^, respectively.
Comparative Example 1 (Direct Coating)
Exaitple 1 was repeated except that, in the preparation
of MEA, the catalytic layer (active layer) was fonned
directly only on the electrolyte membrane using catalyst ink
having a viscosity of 1 cPs by an air spray process.
Coioparative Exanple 2 (Indirect Coating)
Example 1 was repeated except that, in the preparation
of MEA, the catalytic layer (active layer) was coated only on
the diffusion layer using catalyst ink having a viscosity of
1 cPs by an air spray process and then assembled with the
electrolyte membrane by a hot pressing process.
Comparative Example 3 (Coating of diffusion layer with
catalyst ink having viscosity of less than 100 cPs
Example 1 was repeated except that catalyst ink with a
viscosity of 75 cPs was used in coating the catalytic layer
(active layer) on the diffusion layer.
Comparative Example 4 (Coating of diffusion layer with
catalyst ink having viscosity of more than 10,000cPs)
Example 1 was repeated except that catalyst ink with a
viscosity of 15,000 cPs was used in coating the catalytic
layer (active layer) on the diffusion layer.
Conparative Exaicple 5 (Die coating of diffusion layer
with catalyst ink having viscosity of less than lOOcPs)
Example 1 was repeated except that the catalytic layer
(active layer) was coated on the diffusion layer using
catalyst ink having a viscosity of 75 cPs by a die coating
process.
FIG. 11 shows photographs of the front side (left
figure) and backside (right figure) of the electrode prepared
according to Comparative Example 5.
As shown in FIG. 11, it can be found that if catalyst
ink with a viscosity of less than 100 cPs is used, catalyst
particles will penetrate the electrode support (diffusion
layer) so as to impregnate the diffusion layer to the
backside of the diffusion layer. The impregnation of the
diffusion layer with the catalyst particles causes an
increase in the mass transfer resistance of fuel, thus
showing a negative effect on performance.
Comparative Exanple 6 (Coating of catalyst ink__on
diffusion layer by spray coating process)
Example 1 was repeated except that the catalytic layer
(active layer) was coated on the diffusion layer using
catalyst ink having a viscosity of 1 cPs by a spray coating
process. Namely, the active layer was coated on both the
diffusion layer and the electrolyte membrane by a spray
coating process.
Comparative Example 7 (Coating of catalyst ink on
electrolyte membrane by screen printing
Example 1 was repeated except that the catalytic layer
(active layer) was coated on the electrolyte membrane using
catalyst ink having a viscosity of 1000 cPs by a screen
printing process.
As shown in FIG. 13, when the active layer was coated
on the electrolyte membrane by the screen printing processr
one of mass production processes, the Nafion membrane used as
the electrolyte membrane would be severely deformed. Due to
the severe deformation of the electrolyte membrane, the
catalyst particles were not uniformly coated to make the
production of MEA impossible.
Exanple 2
Example 1 was repeated except that a Pt/C (Pt on
Carbon) in place of each of Pt-Ru black and Pt black was used
as a catalyst (see FIG. 14)
Example 3 (Use of catalyst, particles uncoated with
Nafion)
Exanple 2 was repeated except that, in the preparation
of catalyst ink to be coated on the diffusion layer, catalyst
particles non-pretreated with the Nafion solution were mixed
with a solvent mixture of IPA, Nafion solution and water to
prepare the catalyst ink (see FIG. 14) .
Experiments
1. Measurement of interfacial resistance (Exanple 1 and
Comparative Example 2)
The conductivity of MEA in a single cell was measured
with a Zahner IM6 analyzer by a two-electrode impedance
method. 400 seem of hydrogen gas was diffused through a
reference electrode, 2000 seem of air was diffused through a
working electrode, and the inpedance in a IM-lkHz region was
measured at an alternating current amplitude of 5 mV.
The membrane-electrode assembly of Comparative Example
2 (indirect coating) had an interfacial resistance of 35-40
m]Q*6.25, whereas the membrane-electrode assembly produced by
separately coating the active layer according to Example 1
(direct coating) had a low interfacial resistance of 25-30
mQ*6.26.
2. Measurement of power density (Example 1 and
Comparative Exairples 1 and 2)
The power densities of the membrane-electrode
assemblies produced in Example 1 and Comparative Examples 1
and 2 were measured.
The power density measurement was carried out under the
following operation conditions of a single cell: anode
catalyst: Pt/Ru black; cathode catalyst: Pt black; operation
teitperature: 80 °C; amount of use of catalyst: 4 mg/cm^; fuel:
2M CH3OH; 1000 cc/min of oxygen; and ambient pressure.
FIG. 8 shows power density curves according to the
production methods of the manbrane-electrode assembly. As
shown in FIG. 8, it can be found that the use of the hybrid
coating method according to Exaitple 1 shows a more than 50%
increase in power density.
Namely, it can be found frcati FIG. 8 that even the use
of the catalysts with the same amount shows the deviation in
performance, and the indirect-direct hybrid coating method
has excellent perfonnance. This suggests that the membrane-
electrode assembly produced according to the present
invention has high catalyst availability.
3. Measurement of coating state and power density
according to viscosity in screen coating on diffusion layer
(Example 1 and Comparative Examples 3 and 4)
FIG. 9 shows not only schematic diagrams illustrating
the coating state of the active layer according to the
viscosity of catalyst ink in the electrodes which had been
produced by coating catalyst inks having viscosities of 7 5
cPs (Comparative Example 3), 1,000 cPs (Example 1), and
15,000 cPs (Comparative Exanple 4), respectively, on a
diffusion layer by using a screen printing process, but also
photographs of the electrodes.
As is evident from PIG. 9, the electrode of Comparative
Example 3 which had been screen-printed with catalyst ink
having a viscosity of less than 100 cPs (75 cPs) showed the
penetration of catalyst particles into the diffusion layer
due to the low viscosity of the catalyst ink. Thus, the
electrode of Coir?iarative Exaitple 3 had catalyst loss and will
prevent the diffusion of fuel. The electrode of Cortparative
Example 4 which had been screen-printed with catalyst ink
having a viscosity of more than 10,000 cPs (15,000 cPs)
showed non-uniform coating due to the poor flowability of the
catalyst ink.
Meanwhile, the power densities of the membrane-
electrode assemblies produced in Exanple 1 and Comparative
Examples 3 and 4 were measured under the following operation
conditions of a single cell:
Anode catalyst: Pt/Ru black; cathode catalyst: Pt
black; operation temperature: 80 °C; amount of use of
catalyst: 4 mg/cm^; fuel: 2M CH3OH; 1000 cc/min of oxygen;
and ambient pressure.
As shown in FIG. 10, the case of Comparative Example 3
(75 cPs) showed a sharp reduction in performance^ since
catalyst particles plugged the pores of the diffusion layer
upon screen printing so as to cause the mass transfer
resistance of fuel in a high-current region. Meanwhile, the
case of Comparative Example 4 (15,000 cPs) showed low
performance in a low-current region, since catalyst particles
vere coated at a lower amount than the desired loading amount
due to the poor coating of the catalyst ink.
4. Power densities according to coating methods
(Example 1 and Comparative Ebtample S)
The power densities of the membrane-electrode
asseirtfaiies produced in Example 1 and Comparative Example 6
were measured.
The power density measurements were carried out under
the following operation conditions of a single cell;
Anode catalyst: Pt/Ru black; cathode catalyst: Pt
black.; operation temperature: 80 °C; amount of use of
catalyst: 4 mg/cm^; fuel: 2M CH3OH; 1000 cc/min of oxygen;
and ambient pressure.
In the case of Comparative Exaicple. 6 where the active
layer was formed on carbon paper or carbon cloth used as the
diffusion layer by spray coating process and assembled with
the electrolyte membrane by a hot pressing process, a
significantly large amount of catalyst particles penetrated
into the diffusion layer by the spray coating process so as
to make it impossible to participate in reaction, and acted
as resistance to the diffusion of the fuel methanol solution.
Accordingly, as shown in FIG. 12, it can be found that, in
the case of Coinparative Example 6, the mass transport in a
high-current region where reaction actively occurs to require
the active supply of fuel reactants (methanol or hydrogen),
is not smooth, resulting in a severe deterioration in
performance.
5. Power densities according to whether catalyst
particles are coated with Nafion or not (Examples 2 and 3)
The power densities of the mernbrane-electrode
assemblies produced in Examples 2 and 3 were measured in a
hydrogen fuel cell (PEMFC) .
The power density measurements were carried out under
the following operation conditions of a single cell:
Anode catalyst: Pt/C; cathode catalyst: Pt/C; operation
tettperature: 70 °C; amount of use of catalyst: 0.4 mg/cm'';
fuel: H2; and ambient pressure of air.
FIG. 15 shows the comparison between Example 2 where
the catalyst particles were coated with electrolyte (Nafion)
and Exanple 3 where the catalyst particles were not coated
with the electrolyte (Nafion) . As shown in FIG. 15, the
electrode fabricated using the electrolyte (Nafion)-coated
catalyst particles had a relatively low mass transport
resistance, indicating superiority in performance.
In the case of Example 2, an electrode structure was
formed in which the electrolyte (Nafion)-coated catalyst
particles were coated on the diffusion layer so as to
maintain the pores among catalyst particles at the maximum as
shown in the left figure of FIG. 14. The maintained pores
provide an inprovement in the electrode performance by
forming a passage through which fuel can be smoothly supplied
to the active section of the catalysts.
We Claim:
1. A membrane-electrode assembly comprising:
electrodes consisting of a anode comprising a gas diffusion Ia)'er and a
catalyst material-containing active layer, and a cathode comprising a diffusion layer
and a catalyst material-containing active layer; and
an electrolyte membrane interposed between the anode and the cathode and
comprising a catalyst material-containing active layer at one or both sides, the
electrodes being hot-pressed to the electrolyte membrane,
wherein the active layer is formed on the gas diffusion layer in electrode, by
coating the catalyst with electrolyte powder, mixing the coated catalyst powder with
a solvent so as to prepare catalyst ink, and coating the catalyst ink on the gas
diffusion layer; and
wherein the viscosity of the active layer in coating the active layer on the gas
diffusion layer is in a range of 100 to 10,000 cPs and the viscosity of the active layer
in coating the active layer on the electrolyte membrane is less than 10 cPs.
2. The membrane-electrode assembly as claimed in claim 1, wherein the
viscosity of the active layer in coating the active layer on the gas diffusion layer is in
a range of 1,000 to 10,000 cPs.
3. The membrane-electrode assembly as claimed in claim 1, wherein the
catalyst coated on a anode side-surface of the electrolyte membrane is the same as
the catalyst of the active layer in the anode, and the catalyst coated on an cathode
side-surface of the electrolyte membrane is the same as the catalyst of the active
layer in the cathode.
4. The membrane-electrode assembly as claimed in claim 1, wherein the
active layer on the gas diffusion layer is coated on the gas difftision layer by a
curtain coating process.
5. The membrane-electrode assembly as claimed in claim 1, wherein the
active layer on the electrolyte membmne is coated on the electrolyte membrane by a
spray coating process.
6. The membrane-electrode assembly as claimed in claim 1, wherein the
amount of the active layer formed on the electrolyte membrane is 1-100% by weight
based on the weight of the active layer formed on the gas diffusion layer.
7. A method for producing a membrane-electrode assembly as claimed in
any of the preceding claims, the method comprising the steps of:
(a) forming a catalyst material-containing active layer on the surface of an
electrolyte membrane;
(b) forming a catalyst material-containing active layer on the surface of a gas
diffusion layer; and
(c) hot-pressing the gas diffusion layer to the electrolyte membrane,
wherein the step (b) is performed by coating the catalyst with electrolyte
powder, mixing the coated catalyst powder with a solvent so as to prepare catalyst
ink, and coating the catalyst ink on the gas diffusion layer so as to form the active
layer; and
wherein the viscosity of the active layer, which is applied on the electrolyte
membrane at the step (a), is controlled in a range of less than 10 cPs and the
viscosity of the active layer, which is applied on the gas diffusion layer at the step
(b), is controlled in a range of 100 to 10,000 cPs.
8. The method as claimed in claim 7, wherein, at the step (a), catalyst ink fed
by a gas pressure method is coated on the dried electrolyte membrane by a spray
process.
9. The method as claimed in claim 8, wherein, at the step (a), the electrolyte
membrane is maintained in a dried state by a thermal dryer.
10. The method as claimed in claim 7, wherein the step (a) is carried out at
an operation temperature of 20-100 °C.
11. The method as claimed in claim 7, wherein the step (c) is carried out at
an operation temperature of 50-200 °C under a pressure of 5-100 kg/cm .
12. The method as claimed in claim 7, wherein the step (b) optionally
comprises performing a dry coating process to the gas diffusion layer.

The invention discloses a membrane-electrode assembly comprising: electrodes consisting of a anode comprising a gas diffusion layer (1) and a catalyst material-containing active layer (2), and a cathode comprising a diffusion layer (7) and a catalyst material-containing active layer (6); and an electrolyte membrane (4) interposed between the anode and the cathode and comprising a catalyst material-containing active layer (3, 5) at one or both sides, the electrodes being hot-pressed to the electrolyte membrane, wherein the active layer is formed on the gas diffusion
layer in electrode, by coating the catalyst with electrolyte powder, mixing the coated catalyst powder with a solvent so as to prepare catalyst ink, and coating the catalyst ink on the gas diffusion layer; and wherein the viscosity of the active layer in coating the active layer on the gas diffusion layer is in a range of 100 to 10,000 cPs and the viscosity of the active layer in coating the active layer on the electrolyte
membrane is less than 10 cPs. The invention is also for a method of producing such membrane-electrode assembly