U.S. patent application number 14/236220 was filed with the patent office on 2014-07-03 for method for producing alloy catalyst for fuel cells using silica coating.
This patent application is currently assigned to INDUSTRY-ACADEMIC COOPERATION FOUNDATION, YONSEI UNIVERSITY. The applicant listed for this patent is Hansung Kim, Jonggil Oh. Invention is credited to Hansung Kim, Jonggil Oh.
Application Number | 20140186748 14/236220 |
Document ID | / |
Family ID | 47177134 |
Filed Date | 2014-07-03 |
United States Patent
Application |
20140186748 |
Kind Code |
A1 |
Kim; Hansung ; et
al. |
July 3, 2014 |
METHOD FOR PRODUCING ALLOY CATALYST FOR FUEL CELLS USING SILICA
COATING
Abstract
Disclosed is a method for producing an alloy catalyst supported
on carbon, including the steps of: dispersing alloy particles into
a mixed solution of water with alcohol, introducing a silica
precursor thereto, and carrying out sol-gel reaction in the
presence of a basic catalyst to obtain silica-coated alloy
particles; supporting the silica-coated alloy particles onto a
carbon carrier to obtain silica-coated alloy particles supported on
carbon; heat treating the silica-coated alloy particles supported
on carbon to increase an alloying degree; and removing silica
coating by using inorganic base solution and a surfactant. The
method for producing an alloy catalyst provides a high-quality and
high-durability alloy catalyst by increasing the alloying degree of
a catalyst through a heat treatment step, while forming a silica
coating layer effectively on small alloy particles having a size of
several nanometers to inhibit growth of the size of alloy
particles. In addition, the catalyst may be used advantageously as
an electrode for fuel cells.
Inventors: |
Kim; Hansung; (Seoul,
KR) ; Oh; Jonggil; (Incheon, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kim; Hansung
Oh; Jonggil |
Seoul
Incheon |
|
KR
KR |
|
|
Assignee: |
INDUSTRY-ACADEMIC COOPERATION
FOUNDATION, YONSEI UNIVERSITY
Seoul
KR
|
Family ID: |
47177134 |
Appl. No.: |
14/236220 |
Filed: |
January 13, 2012 |
PCT Filed: |
January 13, 2012 |
PCT NO: |
PCT/KR2012/000326 |
371 Date: |
January 30, 2014 |
Current U.S.
Class: |
429/532 ;
502/182; 502/185 |
Current CPC
Class: |
H01M 2008/1095 20130101;
Y02E 60/50 20130101; H01M 4/926 20130101; H01M 4/8605 20130101;
H01M 4/921 20130101 |
Class at
Publication: |
429/532 ;
502/182; 502/185 |
International
Class: |
H01M 4/92 20060101
H01M004/92 |
Foreign Application Data
Date |
Code |
Application Number |
May 13, 2011 |
KR |
10-2011-0045164 |
Claims
1. A method for producing an alloy catalyst supported on carbon,
comprising the steps of: (1) dispersing alloy particles into a
mixed solution of water with alcohol, introducing a silica
precursor thereto, and carrying out sol-gel reaction in the
presence of a basic catalyst to obtain silica-coated alloy
particles; (2) supporting the silica-coated alloy particles onto a
carbon carrier to obtain silica-coated alloy particles supported on
carbon; (3) heat treating the silica-coated alloy particles
supported on carbon to increase an alloying degree; and (4)
removing silica coating by using aqueous hydrofluoric acid (HF)
solution or inorganic base solution and a surfactant.
2. The method for producing an alloy catalyst supported on carbon
according to claim 1, wherein the alloy particle comprises an alloy
of at least two metals selected from platinum, palladium, gold,
iridium, ruthenium, vanadium, chrome, manganese, iron, cobalt,
nickel, copper, zinc and titanium.
3. The method for producing an alloy catalyst supported on carbon
according to claim 1, wherein the silica precursor is selected from
TEOS (tetraethyl orthosilicate), TMOS (tetramethyl orthosilicate),
TBOS (tetrabutyl orthosilicate) and a mixture thereof.
4. The method for producing an alloy catalyst supported on carbon
according to claim 1, wherein the alloy particles have a size of
2-10 nm and the silica coating layer has a thickness of 3-50
nm.
5. The method for producing an alloy catalyst supported on carbon
according to claim 1, wherein the basic catalyst in the sol-gel
reaction is selected from aqueous ammonia, sodium hydroxide and
potassium hydroxide.
6. The method for producing an alloy catalyst supported on carbon
according to claim 1, the sol-gel reaction is carried out at
10-50.degree. C. for 3-48 hours under agitation.
7. The method for producing an alloy catalyst supported on carbon
according to claim 1, wherein the carbon carrier is at least one
selected from carbon black, carbon nanotubes, carbon nanofibers,
carbon nanocoils and carbon nanocages.
8. The method for producing an alloy catalyst supported on carbon
according to claim 1, wherein the heat treatment in step (3) is
carried out under inert gas atmosphere of argon or nitrogen, or
mixed gas atmosphere of argon or nitrogen with hydrogen at
400-1000.degree. C. for 2-4 hours to increase an alloying
degree.
9. The method for producing an alloy catalyst supported on carbon
according to claim 1, wherein the aqueous inorganic base solution
is aqueous sodium hydroxide (NaOH) solution or aqueous potassium
hydroxide (KOH) solution.
10. The method for producing an alloy catalyst supported on carbon
according to claim 1, wherein the surfactant is a non-ionic
surfactant selected from polyoxyethylene glycol sorbitan fatty acid
esters, sorbitan fatty acid esters, aliphatic alcohols and
polyoxyethylene alkyl ethers.
11. The method for producing an alloy catalyst supported on carbon
according to claim 1, which comprises, instead of step (2),
supporting the alloy particles from which the silica coating is
removed onto a carbon carrier after step (4).
12. An alloy catalyst supported on carbon, obtained by a method
comprising the steps of: (1) dispersing alloy particles into a
mixed solution of water with alcohol, introducing a silica
precursor thereto, and carrying out sol-gel reaction in the
presence of a basic catalyst to obtain silica-coated alloy
particles; (2) supporting the silica-coated alloy particles onto a
carbon carrier to obtain silica-coated alloy particles supported on
carbon; (3) heat treating the silica-coated alloy particles
supported on carbon to increase an alloying degree; and (4)
removing silica coating by using aqueous hydrofluoric acid (HF)
solution or inorganic base solution and a surfactant.
13. The alloy catalyst supported on carbon according to claim 12,
which has a particle size of 2-10 nm.
14. An electrode for fuel cells, comprising an alloy catalyst
supported on carbon, said alloy catalyst obtained by a method
comprising the steps of: (1) dispersing alloy particles into a
mixed solution of water with alcohol, introducing a silica
precursor thereto, and carrying out sol-gel reaction in the
presence of a basic catalyst to obtain silica-coated alloy
particles; (2) supporting the silica-coated alloy particles onto a
carbon carrier to obtain silica-coated alloy particles supported on
carbon; (3) heat treating the silica-coated alloy particles
supported on carbon to increase an alloying degree; and (4)
removing silica coating by using aqueous hydrofluoric acid (HF)
solution or inorganic base solution and a surfactant.
15. A fuel cell comprising an electrode for fuel cells comprising
an alloy catalyst supported on carbon, said alloy catalyst obtained
by a method comprising the steps of: (1) dispersing alloy particles
into a mixed solution of water with alcohol, introducing a silica
precursor thereto, and carrying out sol-gel reaction in the
presence of a basic catalyst to obtain silica-coated alloy
particles; (2) supporting the silica-coated alloy particles onto a
carbon carrier to obtain silica-coated alloy particles supported on
carbon; (3) heat treating the silica-coated alloy particles
supported on carbon to increase an alloying degree; and (4)
removing silica coating by using aqueous hydrofluoric acid (HF)
solution or inorganic base solution and a surfactant.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a method for producing an
alloy catalyst for fuel cells using silica coating. More
particularly, the present disclosure relates to a method for
producing an alloy catalyst for fuel cells, including forming a
silica coating layer on alloy particles, carrying out heat
treatment, and then removing silica to inhibit growth of particle
size while enhancing catalytic activity and alloying degree.
BACKGROUND ART
[0002] Fuel cells are energy conversion devices which convert
chemical energy of fuel directly into electric energy, have higher
efficiency as compared to conventional internal combustion engines,
and show high energy density and eco-friendly characteristics.
Thus, many attentions have been given to such fuel cells.
[0003] Polymer electrolyte membrane fuel cells (PEMFC) and direct
methanol fuel cells (DMFC) are operated at a low temperature of
80.degree. C. or less in general, and thus an electrode catalyst is
required in order to increase the rate of oxidation and reduction
in fuel cells. Particularly, platinum is the only catalyst that can
accelerate oxidation of fuel (hydrogen or alcohol) and reduction of
oxygen up to a temperature of approximately 100.degree. C., and
thus has been used frequently as an electrode catalyst for fuel
cells. However, since the deposit of platinum is limited and
platinum is expensive, it is important to reduce the use of
platinum or to maximize the catalytic activity per unit mass for
the purpose of commercialization of fuel cells.
[0004] To accomplish this, many studies have been conducted about
platinum alloy catalysts supported on carbon. Platinum alloy
catalysts have higher activity and stability by virtue of the
electrical and structural characteristics of the surface of
particles in principle, and thus have been spotlighted as a
reliable substitute for fuel cell electrode materials.
[0005] In general, platinum alloy catalysts supported on carbon are
obtained by depositing a transition metal precursor on a platinum
catalyst supported on carbon as a starting material and carrying
out heat treatment at 700-1200.degree. C. by using a gaseous
reducing agent such as hydrogen. However, such heat treatment
causes an increase in size of alloy particles, resulting in
degradation of catalytic activity.
[0006] Under these circumstances, many studies have been conducted
about methods for producing an alloy catalyst without heat
treatment at high temperature, the method including: a chemical
reduction process (J. Power Sources 141 (2005), 13), carbonyl
complex process (J. Phys. Chem. B 108 (2004), 1938), microemulsion
process (Electrochim. Acta 50 (2005), 2323), and a polyol process
(Electrochim. Acta 49 (2004), 1045). However, the alloy catalysts
obtained by the above processes are problematic in that a
significant amount of non-alloyed transition metal is present on
the particle surface and is leached out easily during the operation
of a fuel cell, resulting in degradation of catalytic activity and
durability.
[0007] Therefore, it is essential to carry out a heat treatment
process at high temperature in order to obtain an alloy catalyst
satisfying high catalytic activity and durability applicable to
fuel cell catalysts. In addition to this, a novel process by which
the growth of particle size is inhibited is required.
DISCLOSURE
Technical Problem
[0008] A technical problem to be solved by the present disclosure
is to provide a method for producing an alloy catalyst, including
carrying out heat treatment at high temperature to increase an
alloying degree and catalytic activity while controlling the size
of alloy particles to several nanometers.
[0009] Another technical problem to be solved by the present
disclosure is to provide an electrode for fuel cells including the
alloy catalyst obtained by the above method and fuel cells using
the same.
Technical Solution
[0010] In one general aspect, there is provided a method for
producing an alloy catalyst supported on carbon, including the
steps of: [0011] (1) dispersing alloy particles into a mixed
solution of water with alcohol, introducing a silica precursor
thereto, and carrying out sol-gel reaction in the presence of a
basic catalyst to obtain silica-coated alloy particles; [0012] (2)
supporting the silica-coated alloy particles onto a carbon carrier
to obtain silica-coated alloy particles supported on carbon; [0013]
(3) heat treating the silica-coated alloy particles supported on
carbon to increase an alloying degree; and [0014] (4) removing
silica coating by using aqueous hydrofluoric acid (HF) solution or
inorganic base solution and a surfactant.
[0015] According to an embodiment, the alloy particle includes an
alloy of at least two metals selected from platinum, palladium,
gold, iridium, ruthenium, vanadium, chrome, manganese, iron,
cobalt, nickel, copper, zinc and titanium, preferably
platinum-cobalt alloy particle.
[0016] According to another embodiment, the silica precursor may be
selected from TEOS (tetraethyl orthosilicate), TMOS (tetramethyl
orthosilicate), TBOS (tetrabutyl orthosilicate) and a mixture
thereof, and preferably TEOS (tetraethyl orthosilicate).
[0017] According to still another embodiment, the alloy particles
may have a size of 2-10 nm and the silica coating layer may have a
thickness of 3-50 nm.
[0018] According to still another embodiment, the basic catalyst in
the sol-gel reaction may be selected from aqueous ammonia, sodium
hydroxide and potassium hydroxide, preferably ammonia.
[0019] According to still another embodiment, the silica coating
layer may be formed by carrying out the sol-gel reaction at
10-50.degree. C. for 3-48 hours under agitation.
[0020] According to still another embodiment, the carbon carrier
may be at least one selected from carbon black, carbon nanotubes,
carbon nanofibers, carbon nanocoils and carbon nanocages.
[0021] According to still another embodiment, the heat treatment in
step (3) may be carried out under inert gas atmosphere such as
argon or nitrogen, or mixed gas atmosphere of argon or nitrogen
with hydrogen, preferably 90 vol % of argon with 10 vol % of
hydrogen.
[0022] According to still another embodiment, the heat treatment in
step (3) may be carried out at 400-1000.degree. C. for 2-4 hours,
preferably at 750-850.degree. C. for 3 hours to increase an
alloying degree.
[0023] According to still another embodiment, the aqueous inorganic
base solution may be aqueous sodium hydroxide (NaOH) solution or
aqueous potassium hydroxide (KOH) solution, and aqueous
hydrofluoric acid solution, sodium hydroxide solution or potassium
hydroxide solution (preferably, aqueous hydrofluoric acid solution)
may be used in step (4) to remove silica coating.
[0024] According to yet another embodiment, the surfactant may be a
non-ionic surfactant selected from polyoxyethylene glycol sorbitan
fatty acid esters, sorbitan fatty acid esters, aliphatic alcohols
and polyoxyethylene alkyl ethers.
[0025] In a variant, there is provided a method for producing an
alloy catalyst supported on carbon, including, instead of step (2),
a step of supporting the alloy particles from which the silica
coating is removed onto a carbon carrier after step (4).
[0026] In other words, there is provided a method for producing an
alloy catalyst supported on carbon, including the steps of: [0027]
(1') dispersing alloy particles into a mixed solution of water with
alcohol, introducing a silica precursor thereto, and carrying out
sol-gel reaction in the presence of a basic catalyst to obtain
silica-coated alloy particles; [0028] (2') heat treating the
silica-coated alloy particles supported on carbon to increase an
alloying degree; [0029] (3') removing silica coating by using
aqueous hydrofluoric acid (HF) solution or inorganic base solution
and a surfactant; and [0030] (4') supporting the silica-coated
alloy particles onto a carbon carrier to obtain silica-coated alloy
particles supported on carbon.
[0031] In another general aspect, there are provided an electrode
for fuel cells including the alloy catalyst supported on carbon
obtained by the above-mentioned method, and a fuel cell using the
same.
Advantageous Effects
[0032] According to the method for producing an alloy catalyst of
the present disclosure, it is possible to obtain a high-quality and
high-durability alloy catalyst by increasing the alloying degree of
a catalyst through a heat treatment step, while forming a silica
coating layer effectively on small alloy particles having a size of
several nanometers to inhibit growth of the size of alloy
particles. In addition, the catalyst obtained by the method may be
used advantageously as an electrode for fuel cells.
DESCRIPTION OF DRAWINGS
[0033] FIG. 1 is a schematic view illustrating each step of the
method for producing an alloy catalyst by using a silica coating
process according to an embodiment.
[0034] FIG. 2 is an HR-TEM (high-resolution transmission electron
microscopy) image illustrating each step of the method for
producing an alloy catalyst by using a silica coating process
according to an embodiment.
[0035] FIG. 3 is an HR-TEM image illustrating a platinum-cobalt
alloy catalyst supported on carbon, obtained by using a
room-temperature NaBH.sub.4 reduction process, and the same
catalyst heat treated at 800.degree. C.
[0036] FIG. 4 shows X-ray diffraction patterns of the
platinum-cobalt alloy catalysts obtained by using the silica
coating process according to the present disclosure, a
room-temperature NaBH.sub.4 reduction process and a
room-temperature NaBH.sub.4 reduction process followed by heat
treatment at 800.degree. C., as well as a commercially available
Pt/C catalyst.
[0037] FIG. 5 is a CV graph of the platinum-cobalt alloy catalysts
obtained by using the silica coating process according to the
present disclosure, a room-temperature NaBH.sub.4 reduction process
and a room-temperature NaBH.sub.4 reduction process followed by
heat treatment at 800.degree. C.
[0038] FIG. 6 is a graph illustrating the results of evaluation of
unit cell quality when using the platinum-cobalt alloy catalysts
obtained by using the silica coating process according to the
present disclosure, a room-temperature NaBH.sub.4 reduction process
and a room-temperature NaBH.sub.4 reduction process followed by
heat treatment at 800.degree. C.
[0039] FIG. 7 is a graph illustrating the results of ADT
(Accelerated Durability Test) when using the platinum-cobalt alloy
catalysts obtained by using the silica coating process according to
the present disclosure, a room-temperature NaBH.sub.4 reduction
process and a room-temperature NaBH.sub.4 reduction process
followed by heat treatment at 800.degree. C.
BEST MODE
[0040] Exemplary embodiments now will be described more fully
hereinafter with reference to the accompanying drawings, in which
exemplary embodiments are shown.
[0041] The method for producing an alloy catalyst according to the
present disclosure is characterized in that a silica coating layer
is formed on alloy particles and heat treatment is carried out to
increase the alloying degree of alloy particles, while silica is
removed after the silica coating and heat treatment to inhibit the
growth of alloy particle size, thereby providing an alloy catalyst
having a high alloying degree and catalytic activity.
[0042] In general, alloy particles are obtained by a NaBH.sub.4
reduction process using a strong reducing agent. However, in this
case, a difference in reduction rate between different metals
causes a drop in alloying degree, transition metals used as general
heterogeneous metals are present on the alloy surface at a high
ratio, and most of transition metals present on the surface are
leached out under the operation environment of a fuel cell due to a
low equilibrium potential, resulting in degradation of catalytic
activity and durability. In addition, when increasing the alloying
degree of an alloy catalyst, the ratio of transition metals present
on the catalyst surface is reduced, and thus the catalytic activity
is increased and durability is also improved. However, in order to
increase the alloying degree of an alloy catalyst, a heat treatment
process is essentially required but heat treatment at high
temperature is problematic in that it causes an increase in
particle size, leading to a decrease in catalytically active
area.
[0043] Therefore, the method for producing an alloy catalyst
according to the present disclosure includes the steps of
introducing silica during the heat treatment as a capping agent to
increase the alloying degree of a catalyst, while inhibiting the
growth of particle size, so that the alloy particles are coated
with silica, carrying out heat treatment, and removing the silica
coating.
[0044] According to the present disclosure, the method includes the
steps of: preparing alloy particles, coating the alloy particles
with silica, supporting the silica-coated alloy particles on a
carrier, heat treating the catalyst supported on the carrier, and
removing the silica coating layer. In brief, a silica layer is
coated on alloy particles, followed by heat treatment, and then
silica is removed, so that an alloy catalyst supported on a carrier
can be obtained efficiently.
[0045] An embodiment of the method according to the present
disclosure will now be explained in more detail referring to FIG.
1. The method includes the steps of: (1) dispersing alloy particles
into a mixed solution of water with alcohol, introducing a silica
precursor thereto, and carrying out sol-gel reaction in the
presence of a basic catalyst to obtain silica-coated alloy
particles; (2) supporting the silica-coated alloy particles onto a
carbon carrier to obtain silica-coated alloy particles supported on
carbon; (3) heat treating the silica-coated alloy particles
supported on carbon to increase an alloying degree; and (4)
removing silica coating by using aqueous hydrofluoric acid solution
or inorganic base solution and a surfactant.
[0046] The metals for use in alloying may be selected from the
group consisting of any combinations of platinum (Pt), palladium
(Pd), gold (Au), iridium (Ir), ruthenium (Ru), vanadium (V), chrome
(Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper
(Cu), zinc (Zn) or titanium (Ti). In addition, there is no
particular limitation in the method for preparing alloy particles,
and examples of the method include known processes such as chemical
reduction process using a reducing agent, alcohol reduction
process, polyol process, or the like.
[0047] In step (1), the surface of alloy particles is coated with
silica, wherein the alloy particles are dispersed into a mixed
solution of water with alcohol and a silica precursor is introduced
thereto to obtain silica-coated alloy particles.
[0048] Preferably, a lower alcohol is used as the alcohol in step
(1) since it has high miscibility with water and facilitates the
formation of a silica coating layer through a sol-gel reaction of
silica precursor. When the silica precursor is added to the
solution in which the alloy is dispersed and the resultant mixture
is agitated, a silica coating layer is formed on the alloy
particles through a sol-gel reaction in the presence of a basic
catalyst.
[0049] As the silica precursor, TEOS (tetraethyl orthosilicate),
TMOS (tetramethyl orthosilicate), TBOS (tetrabutyl orthosilicate)
or a combination thereof is used preferably. As the catalyst for
the silica sol-gel reaction, a basic compound such as aqueous
ammonia (NH.sub.4OH), sodium hydroxide (NaOH) or potassium
hydroxide (KOH) is used preferably.
[0050] The silica layer formed in step (1) preferably has a
thickness of 3-50 nm. When the thickness is less than 3 nm, it is
difficult to prevent the growth of alloy particles completely
during the heat treatment. When the thickness is larger than 50 nm,
it is not easy to remove the silica layer subsequently.
[0051] The agitation is carried out preferably for 3-48 hours at a
temperature of 10-50.degree. C., but is not limited thereto.
[0052] In step (2), silica-coated alloy particles are supported on
a carrier. The carrier that may be used herein is not particularly
limited, but at least one selected from carbon black, carbon
nanotubes, carbon nanocoils and carbon nanocages is used preferably
in view of the supportability and dispersibility of a catalyst.
There is no particular limitation in the method for supporting the
silica-coated alloy particles on the carrier. For example, the
silica-coated alloy particles are mixed with the carrier in an
adequate solvent, followed by agitation. Also, there is no
particular limitation in the solvent that may be used herein.
[0053] In step (3), the catalyst supported on a carrier is heat
treated. The alloy particles have an increased alloying degree and
the ratio of transition metals present on the particle surface is
decreased through the heat treatment step. The solvent contained in
the catalyst supported on a carrier is removed by using a freeze
dryer or rotary evaporator under vacuum, before the heat treatment
is carried out.
[0054] In addition, the heat treatment step is preferably carried
out under inert gas atmosphere such as argon or nitrogen, or mixed
gas atmosphere of inert gas with hydrogen. More preferably, the
heat treatment step is carried out under mixed gas atmosphere of 90
vol % of argon with 10 vol % hydrogen.
[0055] Further, the heat treatment step is preferably carried out
at a temperature of 400-1000.degree. C. When the heat treatment
temperature is less than 400.degree. C., it is not possible to
improve the alloying degree sufficiently, resulting in a limited
increase in catalytic activity. When the heat treatment is
excessively high (>1000.degree. C.), it is not possible to
inhibit the growth of particle size sufficiently, resulting in
degradation of catalytic activity.
[0056] In step (4), the silica coating layer is removed to obtain
an alloy catalyst supported on carbon. Aqueous hydrofluoric acid
(HF) solution or aqueous solution of inorganic base such as sodium
hydroxide (NaOH) or potassium hydroxide (KOH) is used to remove
silica.
[0057] While silica is removed, the alloy particles contained in
the silica layer in step (4) are dispersed in the surrounding
solution. Herein, a surfactant is used preferably to improve the
dispersibility. There is no particular limitation in the surfactant
that may be used herein. In view of dispersibility and easy
removability, non-ionic surfactants including polyoxyethylene
glycol sorbitan fatty acid esters, sorbitan fatty acid esters,
aliphatic alcohols, or polyoxyethylene alkyl ethers are used
preferably. More preferably, a non-ionic surfactant such as
polyoxyethylene (20) sorbitan monolaurate (Tween 20) is used.
[0058] When producing an alloy catalyst supported on carbon
according to the present disclosure, step (2) of supporting the
catalyst on a carbon carrier may be carried out after the alloy
particles from which the silica coating is removed are obtained in
step (4), if desired.
[0059] In another aspect, the present disclosure provides an
electrode for fuel cells including the alloy catalyst supported on
carbon obtained by the above-described method, and a fuel cell
using the same.
[0060] The alloy catalyst obtained according to the present
disclosure has a high alloying degree and the growth of particle
size thereof is inhibited to provide high quality and high
durability. Thus, the alloy catalyst according to the present
disclosure may be used advantageously for an electrode for fuel
cells and for a fuel cell.
MODE FOR INVENTION
[0061] The examples and comparative examples will now be described
in detail. The following examples are for illustrative purposes
only and it will be understood by those skilled in the art that the
scope of the present disclosure is not limited thereto.
Example 1
Preparation of PtCo/C Using Silica Coating Process
[0062] First, 50 mg of PtCl.sub.4 and 17.6 mg of
CoCl.sub.2.6H.sub.2O are dissolved into 100 ml of ultrapure water
and the mixture is agitated for 20 minutes. Next, 2.2 g of
polyvinyl pyrrolidone (PVP, molecular weight: .about.10,000) is
dissolved into 100 ml of ultrapure water, treated with ultrasonic
waves for 15 minutes, and then is mixed with the above metal
precursor solution. Then, 37.35 mg of NaBH.sub.4 is mixed with 10
ml of ultrapure water and added to the solution obtained as
mentioned above, followed by agitation for about 12 hours.
[0063] After the completion of reduction, platinum-cobalt alloy
particles (PtCo) are recovered by using a centrifugal separator and
redispersed into 160 ml of ethanol. Then, 1.6 ml of TEOS is mixed
with 14.4 ml of ethanol and the mixture is introduced to the alloy
solution, followed by agitation for 10 minutes. After that, 7.8 ml
of aqueous ammonia is added thereto and agitated for about 24 hours
to perform coating of PtCo with silica.
[0064] PtCo (PtCo@SiO.sub.2) on which a silica layer is formed is
recovered by using a centrifugal separator and redispersed into 200
ml of ethanol. Then, 31.8 mg of Ketjen black EC300j is added
thereto as a carrier and agitated for 12 hours. Then, the resultant
product is recovered by using a freeze dryer.
[0065] The silica-coated alloy particles supported on the carrier
(PtCo@SiO.sub.2/C) are introduced to a furnace and heat treated
under the atmosphere of air at 400.degree. C. for 30 minutes to
remove impurities such as PVP. To increase the alloying degree,
heat treatment is carried out at 800.degree. C. under the
atmosphere of 90 vol % of argon and 10 vol % of hydrogen for 3
hours.
[0066] After the completion of heat treatment, the sample is
introduced to 300 ml of 1% aqueous HF solution in which 300 mg of
Tween 20 is dissolved and agitated for 3 hours to dissolve out
silica. The resultant alloy catalyst is washed sufficiently with
ultrapure water and ethanol alternately, and dried at 80.degree. C.
for about 12 hours (PtCo/C). The method according to Example 1 is
shown schematically in FIG. 1.
Comparative Example 1
Preparation of PtCo/C Using Silica Coating Process
[0067] Example 1 is repeated, except that Tween 20 is not used
during the step of removing silica.
Comparative Example 2
Preparation of PtCo/C Using NaBH.sub.4 Reduction Process
[0068] First, 127 mg of Ketjen black EC300j is introduced to 400 ml
of ultrapure water and treated with ultrasonic waves for 10
minutes. Next, 200 mg of PtCl.sub.4 and 47 mg of
CoCl.sub.2.6H.sub.2O are added thereto, followed by agitation for
30 minutes. Then, 150 mg of NaBH.sub.4 is mixed with 220 ml of
ultrapure water and the mixture is added to the solution obtained
as described above, followed by agitation for about 12 hours. After
the completion of reduction, the platinum-cobalt catalyst supported
on carbon is recovered by using a filtration system under reduced
pressure, washed sufficiently with ultrapure water and ethanol
alternately, and dried at 80.degree. C. for about 12 hours. The
resultant PtCo/C catalyst is heat treated at 800.degree. C. under
the atmosphere of 90 vol % of argon and 10 vol % of hydrogen for 3
hours.
Test Example 1
Image Analysis Using High-Resolution Transmission Electron
Microscopy (HR-TEM)
[0069] FIG. 2 is an HR-TEM (high-resolution transmission electron
microscopy) image illustrating each step of the method for
producing the PtCo/C catalyst by using a silica coating process
(Example 1) according to the present disclosure.
[0070] First, (a) shows PtCo alloy particles on which a silica
layer is formed and corresponds to step (1) of FIG. 1. Referring to
(a) of FIG. 2, only one PtCo alloy particle is surrounded with the
silica layer. It can be seen that PtCo alloy particles have a size
of 1-4 nm and the silica layer has an average thickness of 7
nm.
[0071] Next, (b) shows the particles of (a) after they are
supported on carbon and subjected to heat treatment, and
corresponds to step (3) of FIG. 1. Referring to (b) of FIG. 2, it
can be seen that there is no change in particle size even after
carrying out heat treatment at 800.degree. C.
[0072] Then, (c) shows the final step of removing silica and
corresponds to step (4) of FIG. 1. It can be seen that PtCo
particles maintain a particle size of 4 nm or less and high
dispersibility even after carrying out heat treatment at
800.degree. C.
[0073] Finally, (d) shows particles obtained by using no surfactant
during the step of removing silica (Comparative Example 1). In this
case, while the alloy particles contained in the silica layer are
dispersed into the surrounding solution, they form agglomerates due
to the attraction force between particles. This can be seen from
(d) of FIG. 2.
[0074] Therefore, it can be seen that a surfactant is required to
improve the dispersibility of alloy particles upon the removal of
silica. In Example 1 according to the present disclosure, Tween 20
is used.
[0075] Meanwhile, FIG. 3 shows the HR-TEM image of the catalyst
obtained by a NaBH.sub.4 reduction process using no silica coating
(Comparative Example 2).
[0076] In FIG. 3, (a) shows PtCo/C before heat treatment, (b) shows
the catalyst heat treated at 800.degree. C. under the atmosphere of
90 vol % of argon and 10 vol % of hydrogen for 3 hours. It can be
seen that the particle size increases from about 3 nm (before heat
treatment) to 20 nm or more (after heat treatment).
Test Example 2
X-Ray Diffraction Pattern Analysis
[0077] FIG. 4 shows X-ray diffraction patterns of the
platinum-cobalt alloy catalysts supported on carbon (PtCo/C),
obtained by using a room-temperature NaBH.sub.4 reduction process,
a room-temperature NaBH.sub.4 reduction process followed by heat
treatment at 800.degree. C. (Comparative Example 2) and the silica
coating process according to the present disclosure (Example 1).
After carrying out the analysis, the results are shown in the
following Table 1.
TABLE-US-00001 PtCo/C Production Process (111) peak in XRD 2 shift
(.degree.) vs commercially Particle size PtCo/C available Pt/C
catalyst (nm) NaBH.sub.4 Reduction process(RT) +0.6 3.1 NaBH.sub.4
Reduction process +0.9 24.6 (800.degree. C. heat treatment, Comp.
Ex. 2) Silica coating process +1.1 3.1 (800.degree. C. heat
treatment, Ex. 1) Commercially available Pt/C 0 4.1 (40 wt %,
Johnson Matthey)
[0078] In the X-ray diffraction patterns, the particle size is
calculated by applying Pt(111) peak in a range of
2.theta.=39.7.degree. to the Scherrer Formula. In addition, the
alloying degree is determined through the shift degree of Pt(111)
peak. As the 2.theta. value increases from the value in the X-ray
diffraction pattern of the commercially available Pt/C catalyst,
the alloying degree also increases.
[0079] In the case of PtCo/C obtained by the NaBH.sub.4 reduction
process at room temperature, 2.theta. is increased by 0.6.degree.
from the value of the commercially available Pt/C. When the above
catalyst (Comparative Example 2) is heat treated at 800.degree. C.,
2.theta. is increased by 0.9.degree.. This suggests that the alloy
catalyst obtained at room temperature has a low alloying degree and
requires heat treatment. However, the particle size increases from
3.1 nm to 24.6 nm after heat treatment. When the particle size of a
catalyst increases, the catalytically active area decreases.
[0080] Meanwhile, in the case of PtCo/C obtained by using the
silica coating process (Example 1), 2.theta. is increased by
1.1.degree. from the value of the commercially available Pt/C. In
addition, the alloying degree is increased as compared to the alloy
catalyst obtained by a room-temperature NaBH.sub.4 reduction
process followed by heat treatment at 800.degree. C. (Comparative
Example 2). This suggests that the catalyst obtained by using the
silica coating process provides a highly increased alloying degree.
Further, the catalyst has the same particle size (3.1 nm) as the
alloy catalyst obtained at room temperature. It can be seen that
the silica coating inhibits the growth of catalyst particle size
during the heat treatment.
[0081] Therefore, it can be seen that the silica coating process
according to the present disclosure provides an increased alloying
degree while maintaining a small particle size.
Test Example 3
Test for Determining Pt/Co Ratio on Alloy Surface
[0082] The platinum-cobalt alloy catalysts supported on carbon
(PtCo/C), obtained by using a room-temperature NaBH.sub.4 reduction
process, a room-temperature NaBH.sub.4 reduction process followed
by heat treatment at 800.degree. C. (Comparative Example 2) and the
silica coating process according to the present disclosure (Example
1) are examined by ICP (Inductively Coupled Plasma) and XPS (X-ray
Photoelectron Spectroscopy) to determine the overall Pt:Co ratios
and the Pt:Co ratios on the surfaces. The results are shown in the
following Table 2.
TABLE-US-00002 Pt:Co Atomic Ratio Overall Ratio Surface Ratio
PtCo/C Production Process from ICP from XPS NaBH.sub.4 Reduction
process (RT) 3.0:1 2.9:1 NaBH.sub.4 Reduction process 3.0:1 4.0:1
(800.degree. C. heat treatment, Comp. Ex. 2) Silica coating process
2.9:1 4.1:1 (800.degree. C. heat treatment, Ex. 1)
[0083] In the case of the room-temperature NaBH.sub.4 reduction
process, the overall Pt:Co ratio is 3.0:1 but the Pt:Co ratio on
the surface is 2.9:1, suggesting that the ratio of Co is higher on
the surface. However, when heat treating the catalyst at
800.degree. C. (Comparative Example 2), the ratio of the overall
Pt:Co is the same as the ratio before heat treatment (3.0:1) but
the Pt:Co ratio on the surface is 4.0:1. It can be seen from the
above results that heat treatment at high temperature increases the
ratio of platinum on the surface. Particularly, in the case of
PtCo/C obtained by the silica coating process (Example 1), the
overall Pt:Co ratio is 2.9:1 but the Pt:Co ratio on the surface is
4.1:1. Thus, it can be seen that the PtCo/C catalyst has the
highest ratio of platinum on the surface.
Test Example 4
CV (Cyclic Voltammetry) Test and Evaluation of Unit Cell
Quality
[0084] CV test is carried out for each of the platinum-cobalt alloy
catalysts supported on carbon (PtCo/C), obtained by using a
room-temperature NaBH.sub.4 reduction process, a room-temperature
NaBH.sub.4 reduction process followed by heat treatment at
800.degree. C. (Comparative Example 2) and the silica coating
process according to the present disclosure (Example 1).
[0085] In addition, the unit cell quality is evaluated for each
catalyst. As the anode, 0.4 mg/cm.sup.2 of the commercially
available catalyst (40 wt % Pt/C, Johnson Matthey) is used on the
basis of platinum. As the cathode, 0.4 mg/cm.sup.2 of each catalyst
is used on the basis of metal. Then, 150 ccm of hydrogen is
supplied to the anode and 150 ccm of oxygen is supplied to the
cathode. The unit cell is operated under ambient pressure at a
temperature of 75.degree. C. The quality of unit cell is evaluated
as the current density at 0.6V.
[0086] The results are shown in FIG. 5, FIG. 6 and the following
Table 3.
TABLE-US-00003 TABLE 3 Active surface area in CV Current density
PtCo/C Production process (m.sup.2/g) @ 0.6 V (A/cm.sup.2)
NaBH.sub.4 Reduction process (RT) 42.9 1.42 NaBH.sub.4 Reduction
process 12.6 1.08 (800.degree. C. heat treatment, Comp. Ex. 2)
Silica coating process 51.9 1.74 (800.degree. C. heat treatment,
Ex. 1)
[0087] After carrying out the CV test, it can be seen that PtCo/C
obtained by the silica coating process (Example 1) has a
catalytically active surface area of 51.9 m.sup.2/g, which is
higher than 42.9 m.sup.2/g in the case of the room temperature
NaBH.sub.4 reduction process and 12.6 m.sup.2/g in the case of the
a room-temperature NaBH.sub.4 reduction process followed by heat
treatment at 800.degree. C. (Comparative Example 2). This is
because PtCo/C obtained by using the silica coating process has an
increased alloying degree through the high-temperature heat
treatment step while the growth of particle size is inhibited and
the dispersibility is maintained uniformly.
[0088] In the unit cell quality test, the alloy catalyst (Example
1) obtained by using the silica coating process provides the best
result (1.74 A/cm.sup.2 at 0.6V), while the catalyst obtained by
the room-temperature NaBH.sub.4 reduction process provides 1.42
A/cm.sup.2 at 0.6V. Most of the transition metals present on the
alloy surface are molten under the operation environment of a fuel
cell due to a low equilibrium potential, resulting in degradation
of the quality of a fuel cell. The alloy catalyst obtained by using
the silica coating process shows increased catalytic activity by
virtue of a high alloying degree and a low surface ratio of Co, and
thus provides the best unit cell quality. However, the catalyst
obtained by using the room-temperature NaBH.sub.4 reduction process
followed by heat treatment at 800.degree. C. (Comparative Example
2) provides the lowest quality (1.08 A/cm.sup.2 at 0.6V) due to its
low active area.
Test Example 5
ADT (Accelerated Durability Test)
[0089] ADT is carried out for each of the platinum-cobalt alloy
catalysts supported on carbon (PtCo/C), obtained by using a
room-temperature NaBH.sub.4 reduction process, a room-temperature
NaBH.sub.4 reduction process followed by heat treatment at
800.degree. C. (Comparative Example 2) and the silica coating
process according to the present disclosure (Example 1). The
results are shown in FIG. 7.
[0090] As the electrode, a gold plate coated with 1 mg/cm.sup.2 of
each catalyst on the basis of metal is used. As the electrolyte,
0.5M H.sub.2SO.sub.4 is used. Then, 0.4V is applied for 120 hours
versus the hydrogen electrode, and the concentration of Co molten
into the electrolyte is determined periodically by using ICP.
[0091] FIG. 7 shows the amount of Co molten during ADT. In addition
to this, oxygen reduction current measured at a constant voltage of
0.4V is shown. It can be seen that the oxygen reduction current
decreases with the lapse of time. It can be also seen that
variations in the oxygen reduction current value corresponds with
variations in the amount of Co, suggesting that there is a strong
causal relation between the amount of molten Co and oxygen
reduction current. When the oxygen reduction current increases at
the initial stage of ADT until the electrode is activated to the
highest degree and reaches to the maximum value, it starts to
decrease and becomes have a constant value within 120 hours. In
addition, melting of Co occurs rapidly during the initial stage of
ADT, and then is stabilized.
[0092] In the case of the alloy catalyst (Example 1) obtained by
using the silica coating process, 9.8% of Co is molten during ADT.
This is very small as compared to the catalyst obtained by the
room-temperature NaBH.sub.4 reduction process (29.1%).
[0093] Meanwhile, in the case of the catalyst (Comparative Example
2) obtained by the room-temperature NaBH.sub.4 reduction process
followed by heat treatment at 800.degree. C., 0.7% of Co is molten
due to a decreased active area caused by an increase in particle
size.
INDUSTRIAL APPLICABILITY
[0094] As can be seen from the foregoing, the silica coating
process according to the present disclosure is suitable for the
production of a high-quality and high-durability catalyst for fuel
cells.
* * * * *