U.S. patent application number 17/459479 was filed with the patent office on 2022-06-30 for catalyst for fuel cell and method for preparing the same.
This patent application is currently assigned to HYUNDAI MOTOR COMPANY. The applicant listed for this patent is HYUNDAI MOTOR COMPANY, Kia Corporation. Invention is credited to Jee Youn HWANG, Ji-Hoon JANG, Dahee KWAK, Eunjik LEE, Songi OH.
Application Number | 20220209248 17/459479 |
Document ID | / |
Family ID | |
Filed Date | 2022-06-30 |
United States Patent
Application |
20220209248 |
Kind Code |
A1 |
HWANG; Jee Youn ; et
al. |
June 30, 2022 |
CATALYST FOR FUEL CELL AND METHOD FOR PREPARING THE SAME
Abstract
A catalyst for a fuel cell includes: a crystalline carbon
support having a specific surface area of about 200 m.sup.2/g to
about 500 m.sup.2/g; and intermetallic active particles of a
transition metal and a noble metal, wherein the intermetallic
active particles are supported on the crystalline carbon support
and have a particle diameter of greater than or equal to about 3
nm.
Inventors: |
HWANG; Jee Youn; (Seoul,
KR) ; LEE; Eunjik; (Uiwang-si, KR) ; KWAK;
Dahee; (Gunpo-si, KR) ; JANG; Ji-Hoon;
(Suwon-si, KR) ; OH; Songi; (Suwon-si,
KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HYUNDAI MOTOR COMPANY
Kia Corporation |
Seoul
Seoul |
|
KR
KR |
|
|
Assignee: |
HYUNDAI MOTOR COMPANY
Seoul
KR
Kia Corporation
Seoul
KR
|
Appl. No.: |
17/459479 |
Filed: |
August 27, 2021 |
International
Class: |
H01M 4/86 20060101
H01M004/86; H01M 4/92 20060101 H01M004/92 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 24, 2020 |
KR |
10-2020-0182778 |
Claims
1. A catalyst for a fuel cell, the catalyst comprising: a
crystalline carbon support having a specific surface area of about
200 m.sup.2/g to about 500 m.sup.2/g; and intermetallic active
particles of a transition metal and a noble metal, wherein the
intermetallic active particles are supported on the crystalline
carbon support and have a particle diameter of greater than or
equal to about 3 nm.
2. The catalyst of claim 1, wherein the crystalline carbon support
has a Raman spectrum intensity ratio of (1360) plane and (1590)
plane, I.sub.G/I.sub.D ((I(1580 cm.sup.-1)/I(1360 cm.sup.-1)) of
greater than or equal to about 0.9.
3. The catalyst of claim 1, wherein the crystalline carbon support
has an interplanar spacing (d.sub.002) of the (002) plane of less
than or equal to about 0.355 nm.
4. The catalyst of claim 1, wherein the crystalline carbon support
has a carbon shell thickness of about 3 nm to about 6 nm.
5. The catalyst of claim 1, wherein the crystalline carbon support
comprises carbon black, graphite, or a combination thereof.
6. The catalyst of claim 1, wherein the catalyst comprises more
than about 60% by number of the intermetallic active particles
having a particle diameter of greater than or equal to about 3 nm
with respect to a total number of the intermetallic active
particles.
7. The catalyst of claim 1, wherein the catalyst comprises about
40% or less by number of the intermetallic active particles present
in pores of the carbon support with respect to a total number of
the intermetallic active particles.
8. The catalyst of claim 1, wherein the catalyst comprises more
than 60% by number of the intermetallic active particles
participating in catalytic activity with respect to a total number
of the intermetallic active particles.
9. The catalyst of claim 1, wherein the intermetallic active
particles comprise an intermetallic core of a transition metal and
a noble metal, and a noble metal skin layer surrounding the
intermetallic core.
10. The catalyst of claim 1, wherein an atomic ratio of the noble
metal and the transition metal in the intermetallic active
particles is about 1:0.2 to about 1:0.6.
11. The catalyst of claim 1, wherein the noble metal comprises
platinum (Pt), ruthenium (Ru), osmium (Os), iridium (Ir), palladium
(Pd), an alloy thereof, or a mixture thereof.
12. The catalyst of claim 1, wherein the transition metal comprises
cobalt (Co), iron (Fe), nickel (Ni), zinc (Zn), tin (Sn), manganese
(Mn), copper (Cu), scandium (Sc), titanium (Ti), vanadium (V),
chromium (Cr), zirconium (Zr), yttrium (Y), niobium (Nb),
molybdenum (Mo), ruthenium (Ru), rhodium (Rh), osmium (Os),
palladium (Pd), cadmium (Cd), iridium (Ir), gold (Au), silver (Ag),
an alloy thereof, or a mixture thereof.
13. A method of preparing a catalyst for a fuel cell, the method
comprising: supporting a noble metal and a transition metal on a
crystalline carbon support having a specific surface area of about
200 m.sup.2/g to about 500 m.sup.2/g; and annealing the crystalline
carbon support on which the noble metal and the transition metal
are supported.
14. The method of claim 13, wherein the method further comprises:
coating a protective layer on the surface of the crystalline carbon
support on which the noble metal and the transition metal are
supported before annealing.
15. The method of claim 14, wherein the protective layer is an
organic protective layer including polydopamine, polyaniline,
polypyrrole, or a combination thereof, or an inorganic protective
layer including carbon, metal oxide, ceramic, or a combination
thereof.
16. A method of preparing a catalyst for a fuel cell, the method
comprising: irradiating ultrasonic waves to a precursor mixed
solution including a noble metal precursor, a transition metal
precursor, and a crystalline carbon support having a specific
surface area of about 200 m.sup.2/g to about 500 m.sup.2/gm, and
forming core-shell particles including a transition metal oxide
coating layer; annealing the core-shell particles and forming
intermetallic particles including a transition metal oxide coating
layer; and removing the transition metal oxide coating layer from
the intermetallic particles.
17. The method of claim 16, wherein the core-shell particles
comprise: a transition metal core; a noble metal shell surrounding
the transition metal core; and a transition metal oxide coating
layer surrounding the noble metal shell.
18. The method of claim 16, wherein irradiating of the ultrasonic
waves is performed for about 20 minutes to about 2 hours at an
output of about 125 W to about 200 W based on 100 mL of the
precursor mixed solution.
19. The method of claim 16, wherein the intermetallic particles
comprise: intermetallic particles of a transition metal and a noble
metal; and a transition metal oxide coating layer surrounding the
intermetallic particles.
20. The method of claim 13 or claim 16, wherein annealing is
performed at about 700.degree. C. to about 1200.degree. C. for
about 2 hours to about 4 hours.
21. The method of claim 13 or claim 16, wherein annealing is
performed under a mixed gas including hydrogen (H.sub.2) and argon
(Ar), and the mixed gas comprises hydrogen (H.sub.2) in an amount
of about 1 volume % to about 10 volume % based on a total volume of
the mixed gas.
22. The method of claim 16, wherein removing the transition metal
oxide coating layer from the intermetallic particles is performed
by an acid treatment at about 60.degree. C. to about 94.degree. C.
for about 2 hours to 4 hours.
23. The method of claim 22, wherein an acid used for the acid
treatment comprises HClO.sub.4, HNO.sub.3, H.sub.2SO.sub.4, HCl, or
a combination thereof.
24. The method of claim 22, wherein a concentration of an acid used
for the acid treatment is about 0.01 M to about 1.0 M.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of
Korean Patent Application No. 10-2020-0182778, filed on Dec. 24,
2020, the entire contents of which are incorporated herein by
reference.
FIELD
[0002] The present disclosure relates to a catalyst for a fuel cell
and a method for preparing the same.
BACKGROUND
[0003] The statements in this section merely provide background
information related to the present disclosure and may not
constitute prior art.
[0004] A fuel cell is an energy conversion device that directly
converts chemical energy of a fuel into electrical energy. The fuel
cell has superior efficiency compared with existing internal
combustion engines, and is spotlighted as a next-generation energy
source due to its high energy density and
environment-friendliness.
[0005] Polyelectrolyte fuel cells (PEMFC) and direct methanol fuel
cells (DMFC) mainly operate at a low temperature of less than or
equal to about 80.degree. C., and thus an electrode catalyst is
desired to increase rates of oxidation and reduction reactions of
the fuel cell. In particular, platinum is mainly used as an
electrode catalyst for a fuel cell because it is the only catalyst
capable of promoting oxidation of fuel (hydrogen or alcohol) and
reduction of oxygen from room temperature to around 100.degree. C.
However, since platinum reserves are limited and very expensive, it
is very important to reduce the amount of platinum used or increase
catalytic activity per unit mass for commercialization of fuel
cells.
[0006] Studies on platinum alloy catalysts are being conducted.
Platinum alloy catalysts theoretically have higher activity and
stability than pure platinum catalysts due to electrical and
structural characteristics of the particle surface, and thus are
attracting attention as a reliable alternative to fuel cell
electrode materials. Among them, the regularly arranged alloy
catalyst structure (intermetallic structure) is spotlighted,
because it shows high durability when applied to fuel cells because
heterogeneous alloy metals do not melt out.
[0007] In order for the platinum alloy catalyst to exhibit this
intermetallic catalyst structure, a site at which a catalyst having
a particle size of greater than or equal to about 3 nm may be
synthesized is desired. However, when a high specific surface area
carbon support is used, catalyst particles are not only mostly
adsorbed into pores of the carbon support but also synthesized to
have a size of less than or equal to about 3 nm. Therefore, the
catalyst particles present in the pores of the carbon support may
not be exhibit the intermetallic structure, eventually
deteriorating a yield rate of intermetallic catalysts.
SUMMARY
[0008] One form of the present disclosure provides a catalyst for
fuel cells having a high area ratio of active catalyst by reducing
a contact area between active particles and a carbon support,
easily forming a three-phase interface and providing appropriate
adhesion, and reducing the use of platinum by maximizing an
utilization rate of the active particles.
[0009] Another form of the present disclosure provides a method of
preparing a catalyst for a fuel cell capable of increasing a yield
rate and a utilization rate of intermetallic active particles.
[0010] Another form of the present disclosure provides an electrode
including a catalyst for a fuel cell.
[0011] Another form of the present disclosure provides a
membrane-electrode assembly including an electrode.
[0012] Another form of the present disclosure provides a fuel cell
including a membrane-electrode assembly.
[0013] According to one form of the present disclosure, a catalyst
for a fuel cell includes a crystalline carbon support having a
specific surface area of about 200 m.sup.2/g to about 500
m.sup.2/g, and intermetallic active particles of a transition metal
and a noble metal wherein the intermetallic active particles have a
particle diameter of greater than or equal to about 3 nm and are
supported on the crystalline carbon support.
[0014] The crystalline carbon support may have a Raman spectrum
intensity ratio of (1360) plane and (1590) plane, I.sub.G/I.sub.D
((I(1580 cm.sup.-1)/I(1360 cm.sup.-1)) of greater than or equal to
about 0.9.
[0015] The crystalline carbon support may have an interplanar
spacing (d.sub.002) of the (002) plane of less than or equal to
about 0.355 nm.
[0016] The crystalline carbon support may have a carbon shell
thickness of about 3 nm to about 6 nm.
[0017] The crystalline carbon support may include carbon black,
graphite, or combinations thereof.
[0018] The catalyst for a fuel cell may include more than about 60%
by number of intermetallic active particles having a particle
diameter of greater than or equal to about 3 nm with respect to the
total number of intermetallic active particles.
[0019] The catalyst for a fuel cell may include about 40% or less
by number of intermetallic active particles present in the pores of
the carbon support with respect to the total number of
intermetallic active particles.
[0020] The catalyst for a fuel cell may include more than 60% by
number of intermetallic active particles participating in catalytic
activity with respect to the total number of intermetallic active
particles.
[0021] The intermetallic active particles may include an
intermetallic core of a transition metal and a noble metal, and a
noble metal skin layer surrounding the intermetallic core.
[0022] An atomic ratio of the noble metal and the transition metal
in the intermetallic active particles may be about 1:0.2 to about
1:0.6.
[0023] The noble metal may include platinum (Pt), ruthenium (Ru),
osmium (Os), iridium (Ir), palladium (Pd), an alloy thereof, or a
mixture thereof.
[0024] The transition metal may include cobalt (Co), iron (Fe),
nickel (Ni), zinc (Zn), tin (Sn), manganese (Mn), copper (Cu),
scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr),
zirconium (Zr), yttrium (Y), niobium (Nb), molybdenum (Mo),
ruthenium (Ru), rhodium (Rh), osmium (Os), palladium (Pd), cadmium
(Cd), iridium (Ir), gold (Au), silver (Ag), an alloy thereof, or a
mixture thereof.
[0025] According to another form of the present disclosure, a
method of preparing a catalyst for a fuel cell includes supporting
a noble metal and a transition metal on a crystalline carbon
support having a specific surface area of about 200 m.sup.2/g to
about 500 m.sup.2/g; and annealing the crystalline carbon support
on which the noble metal and the transition metal are
supported.
[0026] The method of preparing a catalyst for a fuel cell may
further include coating a protective layer on the surface of the
crystalline carbon support on which the noble metal and the
transition metal are supported before the annealing process.
[0027] The protective layer may be an organic protective layer
including polydopamine, polyaniline, polypyrrole, or a combination
thereof, or an inorganic protective layer including carbon, metal
oxide, ceramic, or a combination thereof.
[0028] According to another form of the present disclosure, a
method of preparing a catalyst for a fuel cell includes irradiating
ultrasonic waves to a precursor mixed solution including a noble
metal precursor, a transition metal precursor, and a crystalline
carbon support having a specific surface area of about 200
m.sup.2/g to about 500 m.sup.2/g to form core-shell particles
including a transition metal oxide coating layer; annealing the
core-shell particles to form intermetallic particles including a
transition metal oxide coating layer; and removing the transition
metal oxide coating layer from the intermetallic particles.
[0029] The core-shell particles may include a transition metal
core, a noble metal shell surrounding the transition metal core,
and a transition metal oxide coating layer surrounding the noble
metal shell.
[0030] The irradiating of the ultrasonic waves may be performed for
about 20 minutes to about 2 hours at an output of about 125 W to
about 200 W based on 100 mL of the precursor mixed solution.
[0031] The intermetallic particles may include intermetallic
particles of a transition metal and a noble metal, and a transition
metal oxide coating layer surrounding the intermetallic
particles.
[0032] The annealing may be performed at about 700.degree. C. to
about 1200.degree. C. for about 2 hours to about 4 hours.
[0033] The annealing may be performed under a mixed gas including
hydrogen (H.sub.2) and argon (Ar), and the mixed gas may include
hydrogen (H.sub.2) in an amount of about 1 volume % to about 10
volume % based on a total volume of the mixed gas.
[0034] The removing of the transition metal oxide coating layer
from the intermetallic particles may be performed by acid treatment
at about 60.degree. C. to about 94.degree. C. for about 2 hours to
4 hours.
[0035] The acid used for the acid treatment may include HClO.sub.4,
HNO.sub.3, H.sub.2SO.sub.4, HCl, or a combination thereof.
[0036] A concentration of the acid may be about 0.01 M to about 1.0
M.
[0037] The catalyst for a fuel cell according to one form of the
present disclosure may have a high area ratio of an active catalyst
by reducing a contact area between active particles and a carbon
support, easily form a three-phase interface and provide an
appropriate adhesion, and reduce the use of platinum by increasing
an utilization rate of the active particles.
[0038] The method of preparing a catalyst for a fuel cell according
to another form of the present disclosure may increase a yield rate
and a utilization rate of intermetallic active particles.
[0039] Further areas of applicability will become apparent from the
description provided herein. It should be understood that the
description and specific examples are intended for purposes of
illustration only and are not intended to limit the scope of the
present disclosure.
DRAWINGS
[0040] In order that the disclosure may be well understood, there
will now be described various forms thereof, given by way of
example, reference being made to the accompanying drawings, in
which:
[0041] FIG. 1 is a schematic view showing a catalyst for a fuel
cell according to one form of the present disclosure;
[0042] FIG. 2 is a schematic view showing a catalyst for a fuel
cell according to the prior art;
[0043] FIG. 3 is a schematic view showing a method of preparing a
catalyst for a fuel cell according to another form of the present
disclosure;
[0044] FIG. 4 is a schematic view showing a method of preparing a
catalyst for a fuel cell according to another form of the present
disclosure;
[0045] FIG. 5 is a scanning electron micrograph (SEM) image and a
transmission electron micrograph (TEM) image of the catalyst
prepared in Example 1;
[0046] FIG. 6 is a scanning electron micrograph (SEM) image and a
transmission electron micrograph (TEM) image of the catalyst
prepared in Comparative Example 1;
[0047] FIG. 7 is a graph showing the results of XRD analysis of the
catalysts prepared in Example 1 and Comparative Example 1;
[0048] FIGS. 8 and 9 are graphs showing the performance and
durability evaluation results of the catalyst prepared in Example
1; and
[0049] FIGS. 10 and 11 are graphs showing the performance and
durability evaluation results of the catalyst prepared in
Comparative Example 1.
[0050] The drawings described herein are for illustration purposes
only and are not intended to limit the scope of the present
disclosure in any way.
DETAILED DESCRIPTION
[0051] The following description is merely exemplary in nature and
is not intended to limit the present disclosure, application, or
uses. It should be understood that throughout the drawings,
corresponding reference numerals indicate like or corresponding
parts and features.
[0052] The advantages and features of the present disclosure and
the methods for accomplishing the same will be apparent from the
forms described hereinafter with reference to the accompanying
drawings. However, the forms should not be construed as being
limited to the forms set forth herein. Unless otherwise defined,
all terms (including technical and scientific terms) used herein
have the same meaning as commonly understood by one of ordinary
skill in the art. In addition, terms defined in a commonly used
dictionary are not to be ideally or excessively interpreted unless
explicitly defined. In addition, unless explicitly described to the
contrary, the word "comprise", and variations such as "comprises"
or "comprising," will be understood to imply the inclusion of
stated elements but not the exclusion of any other elements.
[0053] Further, the singular includes the plural unless mentioned
otherwise.
[0054] The catalyst for a fuel cell according to one form of the
present disclosure includes a crystalline carbon support and
intermetallic active particles supported on the crystalline carbon
support.
[0055] FIG. 1 is a schematic view showing a catalyst for a fuel
cell according to one form of the present disclosure, and FIG. 2 is
a schematic view showing a catalyst for a fuel cell according to
the prior art.
[0056] FIG. 1 shows intermetallic active particles 201 having a
particle diameter of greater than or equal to about 3 nm supported
on the surface of a crystalline carbon support 101, and FIG. 2
shows intermetallic active particles 201 having a particle diameter
of greater than or equal to about 3 nm supported on the surface of
a high specific surface area carbon support 102 and alloy particles
202 having a particle diameter of less than about 3 nm supported
inside pores of the high specific surface area carbon support
102.
[0057] As shown in FIG. 2, when the high specific surface area
carbon support 102 is used to synthesize the intermetallic active
particles 201, the alloy particles 202 located inside the pores of
the high specific surface area carbon support 102 are formed to
have a small particle diameter of less than about 3 nm. However, in
order to exhibit an intermetallic structure, the alloy particles
202 should be formed to have a particle diameter of greater than or
equal to about 3 nm, but when the particle diameter is less than
about 3 nm, the alloy particles 202 may be synthesized as an alloy,
but not the intermetallic structure. In addition, the active
particles 202 synthesized inside the pores of the high specific
surface area carbon support 102 hardly contact with an ionomer and
thus become electrochemically inert.
[0058] On the other hand, as shown in FIG. 1, when the crystalline
carbon support 101 is used to synthesize the intermetallic active
particles 201, most of the intermetallic active particles 201 are
positioned on the surface of the crystalline carbon support 101 and
synthesized to have a size of greater than or equal to about 3 nm
through an annealing process.
[0059] In other words, when the intermetallic active particles 201
are synthesized by using the crystalline carbon support 101, a
contact area between the intermetallic active particles 201 and the
crystalline carbon support 101 is reduced, increasing an area ratio
of active catalyst, the three-phase interface is easily formed,
while appropriate adhesion is provided, and the use amount of
platinum may be reduced by increasing an utilization rate of the
active particles.
[0060] The crystalline carbon support 101 may have a specific
surface area of about 200 m.sup.2/g to about 500 m.sup.2/g. When
the crystalline carbon support 101 has a specific surface area of
less than about 200 m.sup.2/g, dispersibility of the catalyst may
be deteriorated, when the crystalline carbon support 101 has a
specific surface area of greater than about 500 m.sup.2/g,
efficiency of the catalyst may be deteriorated.
[0061] The crystalline carbon support 101 may have a Raman spectrum
intensity ratio of (1360) plane and (1590) plane, I.sub.G/I.sub.D
((I(1580 cm.sup.-1)/I(1360 cm.sup.-1)) of greater than or equal to
about 0.9, for example, about 0.9 to about 1.25. When the Raman
spectral intensity ratio, I.sub.G/I.sub.D of the crystalline carbon
support 101 is less than about 0.9, the corrosion resistance of the
catalyst may be deteriorated.
[0062] The crystalline carbon support 101 may have an interplanar
spacing (d.sub.002) of the (002) plane of less than or equal to
about 0.355 nm, for example about 0.34 nm to about 0.355 nm. When
the interplanar spacing (d.sub.002) of the (002) plane of the
crystalline carbon support 101 exceeds about 0.355 nm, a yield rate
of catalyst particles may be lowered.
[0063] The crystalline carbon support may have a carbon shell
thickness of about 3 nm to about 6 nm. When the carbon shell
thickness of the crystalline carbon support 101 is less than about
3 nm, corrosion resistance may be reduced, and when it exceeds
about 6 nm, the catalyst particle distribution may be reduced.
[0064] The crystalline carbon support 101 may be a spherical carbon
support including carbon black, graphite, or a combination thereof.
The carbon black may include denka black, ketjen black, acetylene
black, channel black, furnace black, lamp black, thermal black, or
a combination thereof.
[0065] The intermetallic active particles 201 have an intermetallic
structure in which transition metals and noble metals are regularly
arranged.
[0066] The noble metal may include platinum (Pt), ruthenium (Ru),
osmium (Os), iridium (Ir), palladium (Pd), an alloy thereof, or a
mixture thereof.
[0067] The transition metal may include cobalt (Co), iron (Fe),
nickel (Ni), zinc (Zn), tin (Sn), manganese (Mn), copper (Cu),
scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr),
zirconium (Zr), yttrium (Y), niobium (Nb), molybdenum (Mo),
ruthenium (Ru), rhodium (Rh), osmium (Os), palladium (Pd), cadmium
(Cd), iridium (Ir), gold (Au), silver (Ag), an alloy thereof, or a
mixture thereof.
[0068] The intermetallic active particles 201 may have a particle
diameter of greater than or equal to about 3 nm, for example, about
3 nm to about 6 nm. When the intermetallic active particles 201
have a particle diameter of less than about 3 nm, a disordered
alloy structure, but not an intermetallic structure, may be
formed.
[0069] The catalyst for a fuel cell includes the crystalline carbon
support 101 and thus may include about 60% by number or more of the
intermetallic active particles 201 having a particle diameter of
greater than or equal to about 3 nm based on the total number of
the intermetallic active particles 201, for example, greater than
60% by number and less than or equal to about 100% by number and
also may include about 40% by number of the intermetallic active
particles 201 inside the pore of the crystalline carbon support
101, for example, about 0% by number to about 40% by number.
[0070] In addition, since intermetallic active particles 201 of the
crystalline carbon support 101 on the surface but not inside the
pores thereof is approximately proportional with electrochemical
performance of the catalyst for a fuel cell, when the catalyst for
a fuel cell includes more than about 60% by number of the
intermetallic active particles 201 having a particle diameter of
greater than or equal to about 3 nm or less than or equal to about
40% by number of the intermetallic active particles 201 inside the
pores of the crystalline carbon support 101 based on the total
number of the intermetallic active particles 201, the catalyst for
a fuel cell may include more than about 60% by number of the
intermetallic active particles 201 participating in catalytic
activity, for example, greater than about 60% by number and less
than or equal to about 100% by number based on the total number of
the intermetallic active particles 201.
[0071] For example, when the intermetallic active particles 201 are
located not inside the pores of the crystalline carbon support 101
but only on the surface of the crystalline carbon support 101, that
is, about 0% by number of the intermetallic active particles 201
inside the pores of the crystalline carbon support 101, the
catalyst for a fuel cell may exhibit both excellent electrochemical
performance and durability.
[0072] The intermetallic active particles 201 may include a noble
metal skin layer (not shown) surrounding these particles on the
surfaces.
[0073] As described later, since the intermetallic active particles
201 in the presence of a protective layer are annealed, noble metal
particles may be exposed to the outer surface of the intermetallic
active particles 201 and then, formed into the noble metal skin
layer in which noble metal particles are dispersed at high density
on the surfaces of the intermetallic active particles 201.
[0074] In general, since a slurry preparation process for electrode
formation proceeds at a pH of less than or equal to about 1, and
the fuel cell is operated in an acidic atmosphere, the transition
metals in the alloy catalyst may be easily eluted, and the eluted
transition metals enter the ion exchange membrane to increase the
membrane resistance. As a result, deterioration of the fuel cell
performance may be caused.
[0075] However, since a catalyst for a fuel cell manufactured in
the method of preparing the catalyst for a fuel cell includes the
noble metal skin layer on the surface, elution of transition metals
is suppressed, thereby inhibiting the deterioration of performance
of the fuel cell.
[0076] The noble metal skin layer may have a thickness of less than
or equal to about 0.5 nm or about 0.2 nm to about 0.5 nm. When the
noble metal skin layer has a thickness of greater than about 0.5
nm, the catalyst has a similar surface structure to that of a
conventional platinum catalyst, having an insignificant performance
improvement effect due to alloying.
[0077] In the catalyst for a fuel cell, the noble metals and the
transition metals may have an atom ratio of about 1:0.2 to about
1:0.6. When the transition metal has an atom ratio of less than
about 0.2, the intermetallic structure may be difficult to form,
but when the atom ratio is greater than about 0.6, the noble metal
skin layer may have an insignificant thickness.
[0078] The method of preparing the catalyst for a fuel cell
according to another form of the present disclosure includes
supporting a noble metal and a transition metal on a crystalline
carbon support having a specific surface area of about 200
m.sup.2/g to about 500 m.sup.2/g; and annealing the crystalline
carbon support supported by the noble metal and the transition
metal.
[0079] FIG. 3 is a schematic view showing a method of preparing a
catalyst for a fuel cell according to another form of the present
disclosure. Referring to FIG. 3, a method of preparing a catalyst
for a fuel cell is described.
[0080] First, the noble metal 210 and the transition metal 220 are
supported on a crystalline carbon support (S1-1).
[0081] Specifically, a noble metal precursor and a transition metal
precursor are dispersed on the crystalline carbon support and then
reduced by using a reducing agent. The reduction of the dispersed
noble metal precursor and transition metal precursor may be
performed in various reduction methods (e.g., a polyol method).
[0082] For example, the crystalline carbon support is added to a
solvent (e.g., ethylene glycol) and then, dispersed by using
ultrasonic wave dispersion and/or magnetic stirring. After adding
the noble metal precursor and the transition metal precursor to the
crystalline carbon support dispersion, pH of the solution is
adjusted. Subsequently, the resultant is reacted at a temperature
higher than room temperature for a predetermined time to reduce the
noble metal precursor and the transition metal precursor, obtaining
the crystalline carbon on which the noble metal 210 and the
transition metal 220 are supported.
[0083] The noble metal precursor may be in a form of a noble metal
salt, and may include a nitrate, a sulfate, an acetate, a chloride,
an oxide, or a combination thereof, and the transition metal
precursor may be in a form of salts of the transition metal, and
may include, for example, a nitrate, a sulfate, an acetate, a
chloride, an oxide, or a combination thereof. Since the content of
the crystalline carbon support is the same as described above, a
repetitive description will be omitted.
[0084] Optionally, the method of preparing the catalyst for a fuel
cell may further include coating a protective layer 230 on the
surface of the crystalline carbon support on which the noble metal
210 and the transition metal 220 are supported before the annealing
process (S1-2). The protective layer 230 is annealed after the
coating to expose noble metal particles on the outer surfaces of
the active particles and thus form the noble metal skin layer in
which the noble metal particles are dispersed at high density on
the surface of the catalyst for a fuel cell.
[0085] The protective layer 230 may be an organic protective layer
including polydopamine, polyaniline, polypyrrole, or a combination
thereof, or an inorganic protective layer including carbon, metal
oxide, ceramic, or a combination thereof.
[0086] For example, the protective layer 230 may be formed by first
coating a polymer containing carbon atoms as a main component,
specifically, an organic polymer that is carbonizable through an
annealing process at a high temperature under an inert gas
atmosphere and then, annealing the organic polymer under the
hydrogen-deficient inert gas atmosphere at the high temperature to
convert the organic polymer into a carbon coating layer. The
organic polymer may include polypyrrole (PPy), polyaniline (PANI),
polydopamine (PDA), or a combination thereof.
[0087] For another example, the protective layer 230 may be formed
as a silica coating layer by dispersing the crystalline carbon
support on which the noble metal 210 and the transition metal 220
are supported in a mixed solution of water and alcohol and then,
adding a silica precursor thereto. The alcohol has good miscibility
with the water, and lower alcohol may be used to facilitate the
formation of the silica coating layer through a sol-gel reaction of
the silica precursor. When the silica precursor is added to the
solution in which the crystalline carbon support supported by the
noble metal 210 and the transition metal 220 is dispersed and then,
stirred, the silica coating layer is formed through the sol-gel
reaction under a base catalyst. As the silica precursor, TEOS
(tetraethyl orthosilicate), TMOS (tetramethyl orthosilicate), TBOS
(tetrabutyl orthosilicate), or a combination thereof may be used.
As a catalyst for the silica sol-gel reaction, a base compound of
aqueous ammonia (NH.sub.4OH), sodium hydroxide (NaOH), or potassium
hydroxide (KOH) may be used.
[0088] Next, the crystalline carbon support on which the noble
metal 210 and the transition metal 220 are supported is annealed
(S1-3).
[0089] Through the annealing process, an alloying degree of the
noble metal 210 and the transition metal 220 is increased, forming
the intermetallic active particles 201.
[0090] Herein, since the protective layer 230 controls sizes of the
intermetallic active particles 201 into several nanometers by
suppressing their growths during the annealing process, the
alloying degree may be increased by sufficiently performing the
annealing process at a high temperature annealing, thereby
enhancing composition uniformity and catalytic activity.
[0091] The annealing process may be performed at about 700.degree.
C. to about 1200.degree. C. for about 2 hours to about 4 hours.
When the annealing process is performed at less than about
700.degree. C. or for less than about 1 hour, the alloying
degree-improving effect is deteriorated, limiting in the increase
of the catalytic activity, but when at greater than about
1200.degree. C. or for greater than about 10 hours, the particle
growth-suppressing effect is deteriorated, resulting in a decrease
of the catalytic activity.
[0092] The annealing process may be performed under an inert gas
atmosphere such as argon, nitrogen, and the like or under a mixed
gas atmosphere of inert gas and hydrogen (H.sub.2), wherein the
hydrogen may be included within the range of about 1 volume % to
about 10 volume % based on a total volume of the mixed gas.
[0093] Finally, the surface of the annealed intermetallic active
particles 201 is acid-treated to remove impurities and residual
acid (S1-4).
[0094] Through the acid-treatment, the protective layer 230
remaining on the surface of the prepared intermetallic active
particles 201, impurities, and transition metal remaining on the
surface may be removed (eluted).
[0095] For example, the intermetallic active particles 201 are
added to an acid aqueous solution and then, refluxed at a
predetermined temperature (e.g., about 80.degree. C.) for
predetermined times (e.g., about 3 hours). On the other hand, the
acid aqueous solution may include, for example, sulfuric acid
(H.sub.2SO.sub.4), nitric acid (HNO.sub.3), hydrochloric acid
(HCl), acetic acid (CH.sub.3COOH), or a combination thereof.
[0096] Residual acid may be removed by performing filtering and
drying processes together. In other words, the intermetallic active
particles 201 are filtered and then, several time treated with
distilled water to remove the residual acid solution. In addition,
in order to keep the surfaces of the intermetallic active particles
201 clean, the intermetallic active particles 201 may be dried in a
dry oven or a vacuum oven filled with an inert gas.
[0097] A method of preparing a catalyst for a fuel cell according
to one form of the present disclosure includes irradiating
ultrasonic waves to a precursor mixed solution to form core-shell
particles including a transition metal oxide coating layer,
annealing the core-shell particles to form intermetallic particles
including a transition metal oxide coating layer, and removing the
transition metal oxide coating layer from the intermetallic
particles.
[0098] FIG. 4 is a schematic view showing a method of preparing a
catalyst for a fuel cell according to another form of the present
disclosure. Referring to FIG. 4, a method of preparing a catalyst
for a fuel cell is described.
[0099] The core-shell particles 300 including the transition metal
oxide coating layer 350 are formed by irradiating ultrasonic waves
to the precursor mixed solution including the noble metal
precursor, the transition metal precursor and the support
(S2-1).
[0100] High frequency oscillation of the ultrasonic waves generates
bubbles in a cavity, resulting in oscillatory growth, and when the
oscillation finally reaches a certain scale, the cavity explodes.
This series of processes caused by the ultrasonic irradiation is
called "an acoustics cavitation mechanism."
[0101] The cavity explosion occurring in the final stage of the
acoustics cavitation mechanism may cause a huge amount of thermal
energy up to about 5000 K, which is dissipated in a very short time
of about 10.sup.-6 seconds.
[0102] When reactants in the chemical reaction combined with
ultrasonic irradiation are at least two materials having different
vapor pressures, the at least two reactants have different
evaporation rates to bubbles by a high frequency oscillation of
ultrasonic waves, so that structural and electrochemical
characteristics of the reaction resultants may be controlled using
the same. For example, when nanoparticles including at least two
metals are prepared by using a noble metal precursor and a
transition metal precursor as reactants and irradiating the same
with ultrasonic waves, distributions of the noble metal and the
transition metal elements in nanoparticles may be controlled
according to a vapor pressure difference of the noble metal
precursor and the transition metal precursor.
[0103] For example, in the nanoparticles, the noble metal having a
low vapor pressure may be disposed in shell portions, and the
transition metal having a high vapor pressure may be disposed in
core portions, forming core-shell particles 300.
[0104] The irradiating of the ultrasonic waves may be performed for
about 20 minutes to about 2 hours at an output of about 125 W to
about 200 W based on 100 mL of the precursor mixed solution. When
the irradiating of the ultrasonic waves is performed at an output
of less than about 125 W or for a time of less than about 20
minutes, metal ions may be insufficiently reduced; while when at
greater than about 200 W or for greater than about 2 hours, a
particle size thereof may be unnecessarily grown.
[0105] The noble metal precursor may include those having a lower
vapor pressure than the vapor pressure of the transition metal
precursor and contributing to a galvanic substitution reaction
after forming transition metal seed particles and enlarging the
sizes thereof. For example, the noble metal precursor may be in a
form of a noble metal salt, and may include a nitrate, a sulfate,
an acetate, a chloride, an oxide, or a combination thereof.
Specifically, the noble metal precursor may be an acetyl acetonate
of noble metal, a hexafluoroacetyl acetonate of the noble metal, or
a pentafluoroacetyl acetonate of the noble metal.
[0106] The transition metal precursor may be in a form of salts of
the transition metal, and may include, for example, a nitrate, a
sulfate, an acetate, a chloride, an oxide, or a combination
thereof. Specifically, the transition metal precursor may be an
acetyl acetonate of the transition metal, a hexafluoroacetyl
acetonate of the transition metal, or a pentafluoroacetyl acetonate
of the transition metal.
[0107] The transition metal precursor is rapidly volatilized by a
high vapor pressure and rapidly captured in a cavity by the
ultrasonic waves, so the transition metal may be disposed in a core
portion in the core-shell particles 300.
[0108] Since the content of the crystalline carbon support is the
same as described above, a repetitive description will be
omitted.
[0109] The precursor mixed solution may further include a reducing
solvent.
[0110] The reducing solvent may include an organic material having
no moisture and oxygen source, for example, a solvent having a
reducing power at a temperature of greater than or equal to about
70.degree. C. or a solvent having a reducing power at a temperature
of about 70.degree. C. to about 400.degree. C. Specifically, the
reducing solvent includes ethylene glycol, di-ethylene glycol,
tri-ethylene glycol, poly-ethylene glycol, or a combination
thereof.
[0111] The reducing solvent reduce reactants of a noble metal
precursor and a transition metal precursor in a cavity formed by
the ultrasonic treatment, and also, maintain a high boiling point
to create an external liquid environment for generating and
extinguishing a cavity.
[0112] Meanwhile, on the surface of the core-shell particles 300
formed by the ultrasonic treatment, a transition metal oxide
coating layer 350 surrounding a noble metal shell 320 may be
included.
[0113] The transition metal oxide coating layer 350 may be formed
by insufficient solubility of a transition metal into a platinum
lattice, a difference of the reduction rates, and a component ratio
of an excessive amount of a transition metal during the ultrasonic
treatment.
[0114] The transition metal oxide coating layer 350 may have a
thickness of about 0.2 nm to about 0.88 nm. When the thickness of
the transition metal oxide coating layer 350 is less than about 0.2
nm, a non-uniform coating layer may be formed, and the particle
size may not be well controlled due to its thin thickness, while
when it exceeds about 0.88 nm, the transition metal oxide may be
crystallized after the annealing process and a residue may remain
after the acid treatment.
[0115] The transition metal oxide coating layer 350 is derived from
the transition metal precursor as in the transition metal core 310,
so the transition metal included in the transition metal oxide
coating layer 350 may be same as the transition metal included in
the transition metal core 310.
[0116] The method of preparing a catalyst for a fuel cell according
to one form of the present disclosure provides core-shell particles
300 including a transition metal oxide coating layer 350 in one
process using ultrasonic treatment, so that the process may be
simplified to save the cost.
[0117] Then the core-shell particles 300 are annealed to provide
intermetallic active particles 201 including a transition metal
oxide coating layer 350 (S2-2).
[0118] Through the annealing process, an alloying degree of the
noble metal and the transition metal increases, a ratio of the
transition metal core 310 decreases, and thus the intermetallic
active particles 201 are formed.
[0119] At this time, as the particle growth is suppressed by the
transition metal oxide coating layer 350, the sizes of the
intermetallic active particles 201 may be controlled to be sizes of
several nanometers during the annealing process, so the alloying
degree increases by performing the annealing process at a
sufficiently high temperature to enhance uniformity of the
composition and a catalytic activity.
[0120] The annealing may be performed at about 700.degree. C. to
about 1200.degree. C. for about 0.5 hours to about 16 hours. When
the annealing temperature is less than about 700.degree. C. or the
annealing time is less than about 30 minutes, an increase in
catalytic activity may be limited due to the lack of improvement of
the alloying degree. When the annealing temperature exceeds about
1200.degree. C. or the annealing time exceeds about 16 hours, an
effect of inhibiting particle size growth may decrease, resulting
in decreased catalytic activity.
[0121] The annealing process may be performed in an inert gas
atmosphere such as argon, nitrogen, or a mixed gas atmosphere of an
inert gas and hydrogen (H.sub.2), and an atmosphere including about
1 volume % to about 10 volume % of hydrogen based on a total volume
of the mixed gas.
[0122] Finally, the transition metal oxide coating layer 350 is
removed from the intermetallic active particles 201 (S2-3).
[0123] The removing of the transition metal oxide coating layer 350
in the intermetallic active particles 201 may be performed by acid
treatment.
[0124] The acid used for the acid treatment may include HClO.sub.4,
HNO.sub.3, HCl, or a combination thereof.
[0125] A concentration of the acid may be about 0.01 M to about 1.0
M. When the concentration of the acid is less than about 0.01 M,
residues may remain, while when the concentration of the acid
exceeds about 1.0 M, noble metal may be dissolved together.
[0126] The acid treatment may be performed at about 60.degree. C.
to about 94.degree. C. for about 2 hours to about 4 hours. When the
acid treatment temperature is less than about 60.degree. C. or the
acid treatment time is less than 2 hours, residues may remain. When
the acid treatment temperature exceeds about 94.degree. C. or when
the acid treatment time exceeds about 4 hours, the noble metal
catalyst may be dissolved.
[0127] Meanwhile, the catalyst for a fuel cell may include a noble
metal skin layer 340 on the surface of the intermetallic active
particles 201. That is, according to the method of preparing a
catalyst for a fuel cell according to one form of the present
disclosure, since the core-shell particles 300 formed by being
irradiated with the ultrasonic waves include a transition metal in
the core, the intermetallic active particles 201 obtained by
performing the same with the annealing process includes noble metal
particles exposed on the outer surface of the catalyst to provide a
noble metal skin layer 340 in which the noble metal particles are
dispersed with a high density on the surface of the intermetallic
active particles 201.
[0128] Another form of the present disclosure provides an electrode
for a fuel cell, including the catalyst for a fuel cell and an
ionomer mixed with the catalyst for a fuel cell.
[0129] Another form of the present disclosure provides a
membrane-electrode assembly including an anode and a cathode facing
each other, and an ion exchange membrane between the anode and
cathode, wherein the anode, the cathode, or both are the
aforementioned electrodes.
[0130] Another form of the present disclosure provides a fuel cell
including the aforementioned membrane-electrode assembly.
[0131] The electrode, the membrane-electrode assembly, and the fuel
cell are the same as those of the general electrode, the
membrane-electrode assembly, and the fuel cell, except that the
aforementioned catalyst for a fuel cell is included, so detailed
descriptions thereof will be omitted.
[0132] Hereinafter, specific examples of the disclosure are
described. However, the examples described below are for
illustrative purposes only, and the scope of the disclosure is not
limited thereto.
PREPARATION EXAMPLE
Preparation of Catalyst for Fuel Cell
Example 1
[0133] A crystalline carbon support having a specific surface area
of 230 m.sup.2/g, a Raman spectrum intensity ratio
(I.sub.G/I.sub.D) of 0.94, an interplanar spacing (d.sub.002) of a
(002) plane of 0.352 nm, and a carbon shell thickness of 4 nm was
prepared.
[0134] Pt(acac).sub.2, Fe(acac).sub.3, the crystalline carbon
support were added into ethylene glycol to prepare a precursor
mixed solution, and 100 mL of the precursor mixed solution was
irradiated with ultrasonic waves by using tip type ultrasonic waves
(Model VC-500, Sonic & Materials Inc., amplitude 30%, 13 mm
solid probe, 20 kHz) under an argon atmosphere at output of 150 to
200 W for 2 hours to provide core-shell particles including a
transition metal oxide coating layer.
[0135] At this time, the addition amounts of the noble metal
precursor and the transition metal precursor were adjusted so that
an atomic ratio of the noble metal and the transition metal may be
1:0.5, respectively.
[0136] The prepared core-shell particles were annealed at
800.degree. C. for 2 hours under a H.sub.2/Ar mixed gas atmosphere
to provide intermetallic particles including a transition metal
oxide coating layer.
[0137] The intermetallic particles were treated with a mixed
solution of 0.1 M HClO.sub.4 and ethanol at 94.degree. C. for 4
hours to prepare the catalyst for a fuel cell.
Comparative Example 1
[0138] A catalyst for a fuel cell was prepared according to the
same method as Example 1 except that a high specific surface area
carbon support (Tradename KB300J, Manufacturer: Lion) having a high
specific surface area of 783 m.sup.2/g, a Raman spectrum intensity
ratio (I.sub.G/I.sub.D) of 0.86, and an interplanar spacing
(d.sub.002) of a (002) plane of 0.367 nm was used.
Experimental Example 1: XRD Analysis of Catalyst for Fuel Cell
[0139] FIG. 5 is a scanning electron micrograph (SEM) image and a
transmission electron micrograph (TEM) image of the catalyst
prepared in Example 1, and FIG. 6 is a scanning electron micrograph
(SEM) image and a transmission electron micrograph (TEM) image of
the catalyst prepared in Comparative Example 1.
[0140] Referring to FIGS. 5 and 6, intermetallic active particles
had a different particle diameter depending on a pore structure of
the carbon supports. In other words, in Example 1 to which the
crystalline carbon support was applied, all active particles were
distributed on the carbon surface, but in Comparative Example 1 to
which the high specific surface area carbon support was applied,
about 60% by the number of particles was distributed on the carbon
surface, while 40% by the number of the particles was distributed
inside the carbon (embedded in pores).
[0141] In addition, the active particles present inside the pores
of the high specific surface area carbon support were mostly formed
to be small (<3 nm). In the process of forming the active
particles, the active particles were formed inside the micropores
and then, agglomerated due to a confinement effect, thereby no
longer growing and deteriorating a yield of an active catalyst.
[0142] FIG. 7 is a graph showing the results of XRD analysis of the
catalysts prepared in Example 1 and Comparative Example 1.
[0143] Referring to FIG. 7, Comparative Example 1 to which the high
specific surface area carbon support was applied exhibited a lower
intermetallic ordering feature peak than Example 1 to which the
crystalline carbon support was applied.
Experimental Example 2: Performance and Durability Evaluation of
Catalyst for Fuel Cell
[0144] The catalysts (40%) according to Example 1 and Comparative
Example 1 were respectively dispersed in an n-propanol solvent at
an ionomer carbon ratio (I/C) of 0.6 to prepare slurry. Each slurry
was coated on a releasing paper (a cathode: Pt loading of 0.1
mg/cm.sup.2, an anode: Pt loading of 0.025 mg/cm.sup.2). Between
the cathode and the anode, a nafion membrane was interdisposed and
then, bonded to manufacture a membrane electrode assembly
(MEA).
[0145] After connecting the manufactured MEA to a fuel cell
evaluation equipment, performance thereof was evaluated at
65.degree. C., 1 bar under 2500 sccm of air and 350 sccm of
H.sub.2, and a durability acceleration evaluation (AST 5k) of the
carbon supports was performed under cyclic voltammetry of 1.0 V to
1.5 V at 5000 cycles.
[0146] FIGS. 8 and 9 and Table 1 show the performance and
durability evaluation results of the catalyst prepared in Example
1, and FIGS. 10 and 11 and Table 2 show the performance and
durability evaluation results of the catalyst prepared in
Comparative Example 1.
TABLE-US-00001 TABLE 1 H.sub.2 350 ECSA Current Current Current
Cell Air 2500 m.sup.2/g density @ density @ density @ HFR voltage @
(BP 1 bar) Pt 0.8 V 0.7 V 0.6 V (m.OMEGA. cm.sup.2) 1.5 mA/cm.sup.2
Before 25.9 0.579 1.267 1.572 57.4 0.634 After ast 25.1 0.231 0.979
1.412 50.6 0.571 5k Retention 96.9% 40% 77% 90% -- .DELTA. 63
mV
TABLE-US-00002 TABLE 2 Current Current Current Cell H.sub.2 350
density @ density @ density @ voltage @ Air 2500 ECSA 0.8 V 0.7 V
0.6 V HFR 1.5 A/cm.sup.2 (BP 1 bar) m.sup.2/g Pt (A/cm.sup.2)
(A/cm.sup.2) (A/cm.sup.2) (m.OMEGA. cm.sup.2) (V) Before 39.1 0.406
1.04 1.47 67.4 0.590 After ast 5k 22.8 0.016 0.045 0.080 124.6 0
Retention 58.3 % -- -- -- -- .DELTA. 590 mV
[0147] Referring to FIGS. 8 to 11 and Tables 1 to 2, the catalyst
according to Example 1 exhibited a 120% initial performance
improvement and about 220% performance improvement after the
durability acceleration experiment, compared with the catalyst
according to Comparative Example 1.
[0148] In other words, the crystalline carbon support used in
Example 1 lowered a specific surface area and simultaneously, had
high crystallinity but relatively fewer defects, resulting in
improved carbon durability and performance.
[0149] On the contrary, the catalyst according to Comparative
Example 1 exhibited lower initial performance than the catalyst
according to Example 1 and a larger difference in the durability
results. The reason is that the high specific surface area carbon
support used in Comparative Example 1 had low crystallinity and
high bonding (defects) and thus poor resistance under carbon
corrosion conditions.
[0150] While this disclosure has been described in connection with
what is presently considered to be practical example forms, it is
to be understood that the disclosure is not limited to the
disclosed forms. On the contrary, it is intended to cover various
modifications and equivalent arrangements included within the
spirit and scope of the present disclosure.
DESCRIPTION OF SYMBOLS
[0151] 101: crystalline carbon support
[0152] 102: high specific surface area carbon support
[0153] 201: intermetallic active particles
[0154] 202: alloy particle
[0155] 210: noble metal
[0156] 220: transition metal
[0157] 230: protective layer
[0158] 300: core-shell particle
[0159] 310: transition metal core
[0160] 320: noble metal shell
[0161] 340: noble metal skin layer
[0162] 350: transition metal oxide coating layer
* * * * *