U.S. patent application number 15/389703 was filed with the patent office on 2017-07-06 for synthesis of nanoparticles using ethanol.
The applicant listed for this patent is Brookhaven Science Associates, LLC. Invention is credited to Jia Xu Wang.
Application Number | 20170194654 15/389703 |
Document ID | / |
Family ID | 49291441 |
Filed Date | 2017-07-06 |
United States Patent
Application |
20170194654 |
Kind Code |
A1 |
Wang; Jia Xu |
July 6, 2017 |
Synthesis of Nanoparticles Using Ethanol
Abstract
The present disclosure relates to methods for producing
nanoparticles. The nanoparticles may be made using ethanol as the
solvent and the reductant to fabricate noble-metal nanoparticles
with a narrow particle size distributions, and to coat a thin metal
shell on other metal cores. With or without carbon supports,
particle size is controlled by fine-tuning the reduction power of
ethanol, by adjusting the temperature, and by adding an alkaline
solution during syntheses. The thickness of the added or coated
metal shell can be varied easily from sub-monolayer to multiple
layers in a seed-mediated growth process. The entire synthesis of
designed core-shell catalysts can be completed using metal salts as
the precursors with more than 98% yield; and, substantially no
cleaning processes are necessary apart from simple rinsing.
Accordingly, this method is considered to be a "green" chemistry
method.
Inventors: |
Wang; Jia Xu; (East
Setauket, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Brookhaven Science Associates, LLC |
Upton |
NY |
US |
|
|
Family ID: |
49291441 |
Appl. No.: |
15/389703 |
Filed: |
December 23, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13860316 |
Apr 10, 2013 |
9550170 |
|
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15389703 |
|
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|
61622374 |
Apr 10, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 11/04 20130101;
B22F 9/24 20130101; B22F 9/18 20130101; B01J 13/02 20130101; B82Y
40/00 20130101; B22F 2301/25 20130101; Y10S 977/948 20130101; B01J
23/462 20130101; B22F 2998/10 20130101; C25B 1/04 20130101; Y10S
977/896 20130101; B82Y 30/00 20130101; B22F 1/0018 20130101; Y02E
60/50 20130101; H01M 4/926 20130101; B01J 23/44 20130101; Y10S
977/892 20130101; Y02E 60/36 20130101; B22F 1/025 20130101; B32B
15/02 20130101; Y10S 977/777 20130101; B22F 2009/245 20130101; B22F
2998/10 20130101; B22F 9/24 20130101; B22F 1/0085 20130101; B22F
9/24 20130101 |
International
Class: |
H01M 4/92 20060101
H01M004/92; C25B 1/04 20060101 C25B001/04; B22F 9/24 20060101
B22F009/24; B22F 1/02 20060101 B22F001/02; B22F 1/00 20060101
B22F001/00; B01J 13/02 20060101 B01J013/02; C25B 11/04 20060101
C25B011/04 |
Goverment Interests
STATEMENT OF GOVERNMENT RIGHTS
[0002] This invention was made with Government support under
contract number DE-AC02-98CH10886, awarded by the U.S. Department
of Energy. The Government has certain rights in the invention.
Claims
1. A core-shell nanoparticle comprising: an atomically ordered
metal core, comprising a first metal; and a shell comprising a
second metal, wherein there is no migration metal of atoms between
the atomically ordered metal core and the shell.
2. The core-shell nanoparticle of claim 1, wherein the core-shell
nanoparticle has an average particle size diameter of between about
1 nm and about 10 nm.
3. The core-shell nanoparticle of claim 2, wherein the atomically
ordered core comprises ruthenium or palladium.
4. The core-shell nanoparticle of claim 3, wherein the shell
comprises a layer of platinum with a thickness of about one to two
platinum atoms.
5. The core-shell nanoparticle of claim 3, wherein the atomically
ordered core comprises ruthenium and the shell comprises a
palladium layer with a thickness of about one to two palladium
atoms.
6. A catalyst for the hydrogen evolution reaction comprising the
nanoparticle of claim 1.
7. A catalyst for the hydrogen oxidation reaction comprising the
nanoparticle of claim 1.
8. A catalyst for the oxygen reduction reaction comprising the
nanoparticle of claim 1.
Description
CROSS-REFERENCE TO A RELATED APPLICATION
[0001] This application is a continuation application of copending
U.S. patent application Ser. No. 13/860,316, filed on Apr. 10,
2013, which claims the benefit under 35 U.S.C. 119(e) of U.S.
Provisional Application No. 61/622,374 filed on Apr. 10, 2012, the
disclosure of which is incorporated herein in its entirety.
FIELD OF THE INVENTION
[0003] This disclosure relates to nanoparticle processing. In
particular, it relates to methods of synthesizing noble metal
nanoparticles and applications of the nanoparticles so
produced.
BACKGROUND
[0004] Active, durable metal nanocatalysts with low platinum (Pt)
content are desired for commercializing fuel cells, and for
lowering the cost of hydrogen generators through water
electrolysis. While several core-shell nanoparticles with a narrow
distribution of particle size have displayed high catalytic
performance, methods to produce them in large quantity, uniformly
and inexpensively are essential for marketing them.
[0005] Core-shell nanocatalysts have been made mainly by (1) Pt
galvanic replacement of an underpotentially deposited (UPD) Cu
monolayers, and (2) forming a Pt-rich shell through
high-temperature annealing or acid leaching of Pt-bimetallic alloy
particles. To narrow down the spread of particle size, surfactants
or capping agents were used during wet-chemical syntheses.
[0006] The synthesis of monodispersed noble metal nanoparticles
using ethylene glycol as the solvent and reducing agent was
reported in 1999. PVP was used as capping agent in synthesizing
nanoparticles of Pt, Pd, Au, Ru, and Ir at various temperatures
ranging from 100.degree. C. to 150.degree. C.
[0007] Multiple processes have been used in preparing catalysts,
often involving costly or poisonous agents. For example, a recent
study prepared monodispersed Pt-bimetallic alloy nanocatalysts,
Pt.sub.3M (where M=Fe, Ni, or Co) by an organic solvothermal method
in several steps. First, metal salts were dissolved in organic
solvents with various surfactants and reduced at elevated
temperatures. Next, the metal nanoparticles were separated by
centrifuge, washing, and then mixed with carbon black. After
drying, the catalysts were heated in an oxygen-rich atmosphere to
remove the surfactants. Finally, they further were annealed in a
reducing atmosphere to eliminate surface oxides. Hence, simpler
procedures employing only inexpensive, environmentally benign
agents are highly desirable.
SUMMARY
[0008] The present disclosure relates to methods for producing
nanoparticles. The nanoparticles may be made using ethanol as the
solvent and the reductant to fabricate noble-metal nanoparticles
with a narrow particle size distributions, and to coat a thin metal
shell on other metal cores. With or without carbon supports,
particle size is controlled by fine-tuning the reduction power of
ethanol, by adjusting the temperature, and by adding an alkaline
solution during syntheses. The thickness of the added or coated
metal shell can be varied easily from sub-monolayer to multiple
layers in a seed-mediated growth process. The entire synthesis of
designed core-shell catalysts can be completed using metal salts as
the precursors with more than 98% yield; and, substantially no
cleaning processes are necessary apart from simple rinsing.
Accordingly, this method is considered to be a "green" chemistry
method.
[0009] In one aspect, the disclosure relates to a method for
producing nanoparticles. The method includes: dissolving a first
metal salt in ethanol, heating the combination of the first metal
salt and the ethanol to a first temperature sufficient to partially
reduce first metal ions of the first metal salt, adding an alkaline
solution, to fully reduce the first metal ions, thereby causing
precipitation of nanoparticles. The first temperature may be
between about 50.degree. C. and about 120.degree. C.
[0010] In another aspect, the disclosure relates to a method for
producing atomically ordered core-shell nanoparticles. The method
includes: combining nanoparticle cores comprising a first metal and
a second metal salt with ethanol, heating the combination of the
nanoparticle cores, second metal salt, and the ethanol to a second
temperature high enough to substantially fully reduce second metal
ions of the second metal salt onto the nanoparticle cores and form
a conformal shell of second metal around the nanoparticle cores. In
various aspects, the second temperature is also sufficiently low to
prevent formation of second metal nanoparticles.
[0011] In various aspects, the disclosure also includes a method
for producing atomically ordered core-shell nanoparticles. The
method includes: dissolving a first metal salt in ethanol, heating
the combination of the first metal salt and the ethanol to a first
temperature sufficient to partially reduce first metal ions of the
first metal salt, adding an alkaline solution, to fully reduce the
first metal ions, thereby causing precipitation of nanoparticles.
The first temperature may be between about 50.degree. C. and about
120.degree. C. The method further provides for combining the
nanoparticle cores and a second metal salt with ethanol, heating
the combination of the nanoparticle cores, second metal salt, and
the ethanol to a second temperature high enough to substantially
fully reduce second metal ions of the second metal salt onto the
nanoparticle cores and form a conformal shell of second metal
around the nanoparticle cores. In various aspects, the second
temperature is also sufficiently low to prevent formation of second
metal nanoparticles.
[0012] In additional aspects, the nanoparticle cores are annealed
in hydrogen or hydrogen mixed with inert gas at between about
.degree. C. and about 500.degree. C. or about 0.5 hours to about 3
hours before the metal shell is formed on the nanoparticle
core.
[0013] The described embodiments of organic photovoltaic devices of
the disclosure, which are to be read in conjunction with the
accompanying drawings, are illustrative only and not limiting,
having been presented by way of example only to describe the
invention. As described herein, all features disclosed in this
description may be replaced by alternative features serving the
same or similar purpose, unless expressly stated otherwise.
Therefore, numerous other embodiments of the modifications thereof
are contemplated as falling within the scope of the present
invention as defined herein and equivalents thereto.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 depicts the standard reduction potential for ethanol
and [PtCl.sub.6].sup.2- as a function of pH and metal ion
concentration.
[0015] FIG. 2 depicts XRD intensity profiles showing, respectively,
fcc-dominant and hcp-dominant structures for 1:1 Ru@Pt
nanoparticles made with as-synthesized Ru cores and annealed
cores.
[0016] FIG. 3 depicts XRD intensity profiles for Ru@Pt/C samples
made with annealed Ru cores and the Pt:Ru ratio increasing from 0.5
to 1.33.
[0017] FIG. 4 depicts TEM images for the 1:1 Ru@Pt nanopaerticles
on (A) as-synthesized Ru/C, (B) annealed Ru/CNT, and (C) annealed
Ru/C.
[0018] FIG. 5 depicts high-resolution STEM image and line-scan
profiles of STEM-HAADF and Ru-EELS for a well-defined Ru@Pt
nanoparticle made using annealed Ru/CNT, and depicts a schematic
model for the corresponding partial-alloyed and well-defined Ru@Pt
NPs, respectively, made using, as-synthesized and annealed Ru
cores.
[0019] FIG. 6 depicts favorable stacking sequences for two layers
of Pt on four layers Ruslab models with four atoms in a (2.times.2)
hexagonal cell in each layer. The differences in energy, .DELTA.E,
are given relative to the bilayer model with the all-hcp sequence,
Ru(ABAB)-Pt(AB), shown at the bottom. The additional .DELTA.E value
in square bracket for the partially-alloyed model at the top is
with respect to the partially-alloyed model with all-hcp sequence,
i.e., Ru(AB)-RuPt(ABAB). The numbers of layers involved in fcc-ABC
and hcp-AB sequences are highlighted, respectively, by the
triangles and rectangles, and depicts (Right panel) XRD spectra
generated using the corresponding structural models to illustrate
the effect of the stacking sequence on the relative intensities at
the fcc-(111) and hcp-(101) positions. Atomic factors and the
effects of lattice stain and particle size are not included.
[0020] FIG. 7 depicts high-resolution STEM images of Ru@Pt NPs with
2 to 3 Pt atomic layers on hcp Ru cores. The numbers of layers
involved in fcc-ABC and hcp-AB sequences are highlighted,
respectively, by the triangles and rectangles. The stacking
sequence at the interface is ABAB-AC(B) in (A), and ABAB-CA in
(B).
[0021] FIG. 8 depicts polarization curves for water electrolysis
using Ru@Pt/C nanocatalysts at the cathode, showing a similar
performance with a 99% reduction of Pt loading.
[0022] FIG. 9 depicts fuel cell tests showing the better CO
tolerance of the Ru@Pt.sub.1.33/C core-shell nanocatalysts than
that of Pt/C.
[0023] FIG. 10 depicts fuel cell tests showing diminishing CO
stripping peak on Ru@Pt core-shell NPs, sweep rate 20 mV
s.sup.-1.
[0024] FIG. 11 depicts fuel cell tests showing performance
stability against anode potential cycling.
[0025] FIG. 12 depicts EDX-determined Pt/(Pt+Pd) atomic ratio as a
function of particle size for two distinct samples, showed by dots
and squares, compared to the calculated curves for monolayer
(solid) and bilayer (dash) of Pt on Pd nanoparticles.
[0026] FIG. 13 depicts oxygen reduction reaction polarization (10
mV s.sup.-1) and cyclic voltammetry (inset) curves (50 mV s.sup.-1)
obtained on a Pd@Pt.sub.ML sample before and after 5000 pulse
potential cycles, 0.6 V (10 s)-1.0 V (10 s).
DETAILED DESCRIPTION
[0027] The present disclosure relates to methods for producing
nanoparticles. The nanoparticles may be made by using ethanol as a
solvent and reducing agent. Thus, it is possible to form
noble-metal nanoparticles with narrow particle size distributions.
Furthermore, the nanoparticles may be formed that have a core of a
first metal and a thin shell of a second metal.
[0028] It has been found that ethanol may serve as both a solvent
and a reducing agent of metals in metal salts. FIG. 1 depicts the
standard reduction potential of ethanol, as well as the reduction
potential of [PtCl.sub.6].sup.2-. As a reductant, ethanol itself is
oxidized in two steps (Eq. 1):
##STR00001##
[0029] In the first step two protons are removed and in the second
step one oxygen molecule is added. The top pair of lines in FIG. 1
shows the 2e.sup.- pH-dependent standard reduction potential for
both steps of the ethanol oxidation. The more negative the redox
potential is, the higher is the reducing power. The standard
reduction potentials are low enough for reducing noble metals
typically used for electrocatalysis, such as, Ru, Rh, Pd, Os, Ir,
Pt, and Au.
[0030] It has also been found that ethanol's reducing power can be
enhanced and fine-tuned by adding alkaline solution to neutralize
the protons from ethanol oxidation. Adding H.sub.2O provides oxygen
and enables the second step. Furthermore, the OH.sup.- of the
alkaline solution reacts with the protons given off from both of
the oxidation steps, thus further driving both the redox reactions.
Thus, the addition of alkaline solutions may be used to control the
reducing power of ethanol during synthesis for various purposes.
The larger a potential gap with a metal precursor is, the easier
the metal reduction occurs. The addition of the alkaline solution
can make the residual of metal precursor negligible at the end of
synthesis. It is also noted that the intermediate, acetaldehyde, is
less stable, and is a stronger reductant than ethanol.
[0031] Thus, the first two-electron ethanol oxidation has a high
activation barrier, which allows for a uniform ethanol solution
containing partially reduced metal precursors stable at room
temperature. For example, Ru.sup.3+ may be reduced to Ru.sup.2+ by
the first two-electron oxidation of ethanol. This solution is
stable for weeks at room temperature; there is no nucleation in the
absence of water. However, upon the addition of an aqueous alkaline
solution, the acetaldehyde's oxidation is enabled, which triggers
Ru nucleation in a very uniform manner, and thus, results in a
narrow particle size distribution.
[0032] Metal salts used may be any suitable salts based on Ru, Rh,
Pd, Os, Ir, Pt, and Au metals. Combinations of salts may also be
used. In certain embodiments the metal salts are based on Ru, Pd,
Pt, or combinations thereof. For example, the metal salts may be
selected from ruthenium (III) chloride trihydrate, palladium (II)
chloride, and chloroplatinic acid hexahydrate. Other suitable salts
may include chloroauric acid, chloroiridic acid, iridium(III)
chloride, osmium chloride, and rhodium (III) chloride.
[0033] The metal salts may be added to the ethanol in
concentrations ranging from about 1 mM to50 mM, such as for example
from about 1 mM to 10 mM. In certain embodiments anhydrous (200
proof) ethanol is used.
[0034] The ethanol may be heated to provide energy to overcome the
activation barrier of the first two-electron ethanol oxidation. The
ethanol and salt combination may be heated to temperatures between
about 40.degree. C. and about 150.degree. C., such as between about
50.degree. C. and about 120.degree. C. In certain embodiments the
temperature is about 80.degree. C. and in other embodiments about
110.degree. C.
[0035] The two-electron ethanol oxidation may take between about 10
minutes and several hours, such as between about 0.5 hours and
about 2 hours. Typically, a color change may be observed. For
example, the Ru.sup.3- to Ru.sup.2+ reduction can be followed by
monitoring the color going from brown (Ru.sup.3+) to green
(Ru.sup.2+) over a period of about 1 hour.
[0036] The final oxidation step may be triggered by the addition of
an aqueous alkaline solution. Suitable aqueous alkaline solutions
include sodium hydroxide (NaOH) solutions, potassium hydroxide
(KOH) solutions. The aqueous alkaline solutions may have Molar
concentrations between about 0.05 M and about 1.0 M, such as
between about 0.1 M and about 0.2 M. Upon the addition of the
aqueous alkaline solution the metal ions of the metal salt are
fully reduced and form a nanoparticle core of the first metal. The
nanoparticle cores may be rinsed with water, ethanol or other
suitable solvent.
[0037] After formation, the nanoparticle core may then be annealed
at temperatures ranging between about 350.degree. C. and about
500.degree. C. The annealing may be performed for between about 0.5
hours and about 2 hours, such as for example 1 hour. The annealing
may be performed under hydrogen gas or a mixture of hydrogen and
argon or nitrogen.
[0038] In certain embodiments the nanoparticle cores may be
produced in the presence of a support. The support can be any
suitable support. For example, the support can be carbon, carbon
nanotubes, fullerenes, alumina, silica, silica-alumina, titania,
zirconia, calcium carbonate, barium sulphate, a zeolite,
interstitial clay, and the like. For the embodiments where a
support is used, the support may be added to the ethanol solution
before or after the partial reduction of the first metal salt.
[0039] The nanoparticle core (either annealed or not and/or on
support or not) may then be added to a second solution of a second
metal salt in ethanol. The second metal salt may be any of the
suitable salts mentioned above. However, to form a core-shell
nanoparticle the second metal salt may have a different metal than
the metal of the first metal salt. In other words, to form or add a
thin shell coating on the nanoparticle core, a second metal salt
different than the first metal salt is used.
[0040] The ethanol may then be heated to a temperature meeting two
parameters. The desired temperature is sufficiently high to fully
reduce the second metal ions of the second metal salt onto the
nanoparticle cores, yet sufficiently low to prevent formation of
second metal nanoparticles, such as between about 50.degree. C. and
about 150.degree. C., or about 50.degree. C. and about 80.degree.
C. In certain embodiments the temperature is about 80.degree.
C.
[0041] At the desired temperature a conformal shell will form
around the nanoparticle core. By varying the amount and
concentration of the second metal salts, it is possible to form
conformal shells that are atomic monolayers, bilayers, or
trilayers. Shell formation may occur without the addition of an
aqueous alkaline solution. However, aqueous alkaline solution may
be added towards the end of the shell formation in order to
accelerate the metal reduction towards completion.
[0042] The resulting core-shell nanoparticles may be atomically
ordered, in that they form particles that have distinct cores and
distinct shells, with minimal or no migration between metal atoms
between core and shell.
[0043] The core-shell nanoparticles may have average particle size
diameters of between about 1 nm and about 10 nm, such as between
about 2 nm and about 6 nm.
[0044] The core-shell nanoparticles made according to the method
described herein may be well suited as catalysts for hydrogen
evolution reactions. For example, RuPt and PdPt core-shell
nanoparticles have been found to exhibit excellent catalytic
performances at ultra-low metal loading for hydrogen evolution in
water electrolyzers. Additionally, RuPt and PdPt core-shell
nanoparticles have been found to be stable during oxygen reduction
reactions, practically remaining unchanged, and with cyclic
voltammetry curves showing negligible loss of electrochemical
surface area.
EXAMPLES
[0045] The following materials were used: anhydrous ethanol (200
proof, ACS/USP Grade, Pharmco Aaper), ruthenium (III) chloride
trihydrate (technical grade, Aldrich), chloroplatinic acid
hexahydrate (ACS reagent, >37.50% Pt basis, Sigma-Aldrich),
palladium (II) chloride (>99.9%, Aldrich), commercial
carbon-supported Pd nanoparticles (30 wt % Pd/C, NEC), sodium
hydroxide (reagent grade, 97%, powder, Sigma-Aldrich) and potassium
hydroxide (semiconductor grade, pellets, 99.99% trace metals basis,
Sigma-Aldrich). MilliQ ultrapure deionized water (18.2 M.OMEGA.,
Millipore UV Plus) was used to prepare all aqueous solutions and to
rinse off anions after filtering synthesized nanocatalysts. Ketj
enblack EC-600JD (AkzoNobel) and OH-functionalized carbon nanotubes
(15 nm in diameter, CheapTubes) were used as the carbon supports.
MilliQ ultrapure deionized water (18.2 M.OMEGA., Millipore UV Plus)
was used to prepare all aqueous solutions in syntheses and
electrochemical measurements. The electrolytes used in
electrochemical measurements were prepared with optima grade
perchloric acid (Fisher Scientific). Oxygen gas (research purity,
Matheson Tri-Gas), argon gas (4.8 grade, BNL) and hydrogen gas
(extra dry grade, 99.95 PCT, GTS Welco) were used to saturate the
electrolytes.
Synthesis of Unsupported Ru Nanoparticles:
[0046] In a typical synthesis of unsupported Ru nanoparticles, a 50
mL ethanol solution containing 150 .mu.mol RuCl.sub.3 was refluxed
at 110.degree. C. for 1 hour under rigorous stirring. The
solution's color turned from brown to greenish reflecting the
partial reduction of Ru.sup.3+ to Ru.sup.2+. Thereafter, 4.5 mL 0.1
M aqueous NaOH solution (450 .mu.mol=3 times of 150 .mu.mol of
RuCl.sub.3) was added, enabling a further reduction to metallic Ru
nanoparticles. After 2 hours, the complete reduction of Ru ions was
ensured by raising the pH to neutral with a little extra alkaline
solution (<200 .mu.mol). The mixture was cooled down to room
temperature, filtered out, rinsed and dried.
Synthesis of Carbon-Supported Ru Nanoparticles:
[0047] In a typical synthesis of carbon-supported Ru nanoparticles,
100 mL ethanol solution containing 400 .mu.mol RuCl.sub.3 was
refluxed at 110.degree. C. with rigorous stirring for 1 h in a
three-necked flask; meanwhile, 200 mg carbon powder or carbon
nanotubes were disbursed in 60 mL ethanol by sonication for 20 min.
The slurry was transferred into the reaction flask with additional
10 mL ethanol to assure a complete transfer. After the temperature
stabilized at 110.degree. C., 12 mL of a 0.2 M aqueous alkaline
solution of 1200 (3.times.400) .mu.mol NaOH was injected while
stirred vigorously. After 2 h, the color of the solution was
checked. If it was not completely colourless, additional
0.5.times.400=200 .mu.mol NaOH was added. The heater was turned
off, allowing the mixture to cool slowly in the oil bath to room
temperature before filtering.
Coating Shell Metal Onto Ru Core Nanoparticles:
[0048] In a typical synthesis of Ru@Pt core-shell nanoparticles, an
as-synthesized Ru/C (400 .mu.mol Ru) sample was annealed in H.sub.2
at 450 C for 1 hour, then dispersed in 100 mL ethanol and refluxed
at 110.degree. C. for 1 h. After cooling down to room temperature,
8 mL of 50 mM H.sub.2PtCl.sub.6 (400 .mu.mol) ethanolic solution
was added with vigorous stirring or sonication to ensure a uniform
dispersion. The mixture was heated to 80.degree. C. and maintained
there for 2 hours. The solution usually became colorless; if not,
up to 3.times.400 .mu.mol NaOH was added to ensure the complete
reduction of Pt. The mixture was cooled down to room temperature,
filtered, and rinsed with copious amount of water to eliminate the
Cl.sup.- ions. The synthesized sample had a 1:1 Ru:Pt atomic ratio,
along with 25 wt % Pt and 37 wt % Pt+Ru on carbon supports.
Coating Shell Metal Onto Core Pd Nanoparticles:
[0049] In a typical synthesis of Pd@Pt core-shell nanoparticles, 30
mL ethanol containing 60 mg Pd/C (30 wt %, 169 .mu.mol Pd) was
mixed with 50 mM ethanolic H.sub.2PtCl.sub.6 solution (62.6 .mu.mol
Pt, pH.about.2, yellow) with rigorous magnetic stirring at room
temperature. After the mixture was confirmed to be uniform, it was
heated to and refluxed at 80.degree. C. for 1-2 hours. The reaction
progress was checked by the color of supernatant. After the
solution color faded away, 125.2 (2.times.62.6) .mu.mol of aqueous
0.2 M NaOH solution was added with additional refluxing at
80.degree. C. for 0.5 hour, to ensure the complete reduction of Pt.
Then the mixture was cooled down, filtered out, washed with copious
water to eliminate Cl.sup.- ions, and dried at room temperature
under vacuum. The Pt and Pd weight percentages were determined by
ICP-MS and EDX to be 17% and 25%, respectively, consistent with the
calculated 1:2.7 Pt:Pd atomic ratio based on the amount of metal
precursors.
[0050] In the control experiment of Pd shell coating on Pd/C
nanoparticles, ethanolic PdCl.sub.2 solution was used instead of
H.sub.2PtCl.sub.6 solution. The procedure and reaction conditions
were similar to those for Pd@Pt nanoparticles. The Pd atomic ratio
was 1:4 for shell: core.
Characterization
[0051] Scanning transmission electron microscopy (STEM)
measurements were performed using a Hitachi HD2700C operated at 200
kV, equipped with a cold field emission electron source and a probe
aberration corrector. In a vibration-isolated and
temperature-stabilized room, the spatial resolution for imaging is
about 1.0 A. The probe current was in 50-100 pA range. The sample
for STEM was prepared by drop casting the ethanolic suspension of
carbon-supported nanoparticles on a carbon-coated copper grid
(Lacey carbon support film, 300 mesh, Ted Pella Inc.).
[0052] The Z.sup.n-contrast (Z is the atomic number and n is
approximately 1.7) STEM images were taken using a high angle
annular dark-field (HAADF) detector, and elementary line scans were
made with a high resolution EELS detector (Gatan Enfina-ER) . We
employed a convergence semi-angle of 28 mrad. With an energy
dispersion of 0.3 eV per channel and a collection semi-angle of 20
mrad, the energy resolution is around 0.45 eV. The exposure time
for each spot was about 0.07 s with a step size of 0.9 .ANG. for
EELS line scans. We extracted the Ru EELS signal from the EELS
spectrum using a power-law background model and an integration
window at the Ru M.sub.4,5 edge (3, 4). TEM images were also taken
with a JEOL 3000F TEM operating at 300 kV equipped with Gatan image
filter system.
[0053] X-ray diffraction (XRD) experiments were carried out on
beamline X7B(.lamda.=0.3196 .ANG.) of the National Synchrotron
Light Source at Brookhaven National Laboratory. Two dimensional
powder patterns were collected with a PerkinElmer image plate
detector, and the diffraction rings were integrated using the FIT2D
code. Lanthanum hexaboride (LaB6) was used as the instrumental
reference. We refined the fits to the XRD peaks based Pseudo-Voigt
function (the combination function of Gaussian and Lorentz
functions) using Trust-Region method.
Computational Method
[0054] The calculations were performed by using periodic DFT as
implemented in the Vienna ab-initio simulation package (VASP) (5,
6). Ion-core electron interactions were described using the
projected augmented wave method (PAW) (7, 8), and Perdew-Wang
functional (GGA-PW91) within the generalized gradient approximation
(GGA) (9, 10) was used to describe exchange-correlation effects.
The cutoff energy of plane-wave basis set was 400 eV. The five- or
six-layer slab models have four atoms in a (2.times.2) hexagonal
array within each layer and a vacuum of 12 .ANG. between the slabs.
The 9.times.9.times.1 k-points using the Monkhorst-Pack scheme (11)
and first-order Methfessel-Paxton smearing (12) of 0.2 eV was
employed in the integration to speed up the convergence. The
conjugate gradient algorithm was used in optimization, allowing the
convergence of 10.sup.-4 eV in total energy and 10.sup.-3 eV
.ANG..sup.-1 in Hellmann-Feynman force on each atom. All atoms were
allowed to relax except those of the bottom two layers that were
fixed at the hcp Ru bulk position with the optimized lattice
constant of a=b=2.731 and c=4.307 .ANG.. Simulated XRD spectra were
obtained using Reflex module embedded in Materials Studio 5.5 by
Accelrys [http://accelrys.com/products/materials-studio/] with
X-ray source being synchrotron beam (X=0.3196 .ANG.) as in the
experimental measurements, step size of 0.01.degree. , and
Pseudo-Voigt broadening of 0.1.degree..
Water Electrolysis Tests
[0055] In-house deionized water was used for all water electrolysis
testing. All testing was conducted using a custom test station
fabricated at Proton OnSite (Proton) for characterization of cell
materials. The test station used an integrated water purification
module, which maintained on-board conductivity near 18 M.OMEGA. cm.
Temperature control was regulated by a Teflon coated submersible
heater and all operational tests were conducted at 50.degree. C.
Commercially available fuel cell stack test hardware was modified
for electrolysis testing by replacing carbon flow fields on the
anode side of the cell with titanium flow fields designed and
fabricated at Proton. This test cell hardware has been validated
versus Proton's commercially available stack designs, in order to
predict full-scale operational performance. A current control
Sorensen power supply was used to power the cell stack, with over
current protection set at 2.0 A cm.sup.-2. Current was adjusted
through the scan region and allowed to stabilize for 5 minutes
before collecting cell potential measurements.
Fuel Cell Tests
[0056] Fuel cell acceleration stress tests (ASTs) for the anodes
were carried out on membrane electrode assemblies (MEAs) with an
active electrode area of 45 cm.sup.2. The cathode catalyst was Pt/C
(Pt loading: 0.4 mg cm.sup.-2), the membrane was Nafion.RTM.211,
and the GDLs were obtained from Ballard Material Products (BMP).
During the AST, the fuel cell was alternated between operating (1 A
cm.sup.-2) and shutdown modes, with the anode potential cycled
between .about.0.02 V and .about.0.95 V, and the cathode potential
cycled between .about.0.55 V and .about.0.93 V (13). In-situ cyclic
voltammetry measurements for CO stripping were performed using
CorrWare software with a PAR Model 263A potentiostat connected to a
20-A Kepco power booster, by flowing hydrogen on the cathode
(acting as a pseudo hydrogen reference electrode) and nitrogen on
the anode.
Structure Characterization of Ru@Pt Nanoparticles
[0057] In FIG. 1 XRD intensity profiles are provided which show,
respectively, the fcc-dominant and hcp-dominant structures for the
1:1 Ru@Pt nanoparticles made with as-synthesized Ru cores and
annealed cores. The curves for pure Ru nanoparticles on carbon
nanotube support, Ru/CNT, and Pt nanpoarticles on carbon powder
support, Pt/C, are plotted as references. FIG. 2 XRD provides
intensity profiles for the Ru@Pt/C samples made with annealed Ru
cores and the Pt:Ru ratio increasing from 0.5 to 1.33. The curve
for annealed Ru cores is also plotted for reference, and the
intensities of the Ru@Pt/C curves are normalized to the annealed Ru
core curve at the Ru(101) position. The average particle sizes were
estimated by TEM measurements (FIG. 3) and the average Pt shell
thicknesses in number of monolayer (ML) were calculated from
particle size and Ru:Pt atomic ratio assuming a well-defined
core-shell structure.
[0058] Ru(hcp) and Pt(fcc), both consist of closely packed planes
of atoms, differing in their stacking sequence: The hcp layers
cycle between two shifted positions, expressed as ABAB, whereas the
fcc layers cycle between all three equivalent shifted positions,
i.e., ABCABC. Their distinctive features in the XRD spectra (Seen
in FIG. 2) have been used in previous studies to distinguish the
Ru@Pt core-shell nanoparticles, synthesized by using a sequential
polyol process at 200.degree. C., from the RuPt alloy
nanoparticles. A similar XRD profile was obtained for the Ru@Pt
sample prepared with the as-synthesized Ru cores. It differs from
that for pure Pt nanoparticles by the notable intensity between the
fcc Pt (111) and (200) peaks, originating from the hcp Ru (002) and
(101) diffractions. Because of the relative weakness of the hcp
diffractions, the Ru cores were considered to be highly
disordered.
[0059] With the annealed Ru cores, the average particle size of
ethanol-synthesized Ru@Pt nanoparticles increased and the XRD
spectra yielded features corresponding to well-ordered hcp-Ru cores
(See FIG. 2). With the same 1:1 Ru:Pt atomic ratio, we obtained
hcp-dominant XRD spectra on Ru@Pt/CNT samples that have an average
particle size 4.5 nm. Furthermore, four Ru@Pt/C samples were
prepared with different Ru:Pt atomic ratios and using as cores the
annealed Ru nanoparticles. The XRD spectra are plotted in FIG. 2
with the intensity normalized at the Ru(101) peak position with
that for the Ru nanoparticles before coating them with Pt. The
intensities at the fcc-(111) and fcc-(200) positions increase with
a rise in the Pt:Ru atomic ratio. These results suggest that
well-defined Ru@Pt core-shell nanoparticles have been synthesized
via controlled Pt coating in ethanol.
[0060] The core-shell elemental distribution was verified at the
atomic scale using various (scanning) transmission electron
microscopy ((S)TEM) techniques including a high angle annular
dark-field (HAADF)-STEM, high-resolution TEM, and electron
energy-loss spectroscopy (EELS). FIG. 4 shows the intensity
profiles in a line scan across a near-sphere Ru@Pt nanoparticle,
wherein the EELS signal for Ru indicates a 4.2 nm Ru core inside a
5.7 nm NP, as measured by the HAADF. The Pt shell is about 3-ML
thick, as is expected for 1:1 Ru@Pt nanoparticles with an average
diameter of 5.5 nm.
[0061] For the well-defined Ru@Pt core-shell nanoparticles, the
atomic structure at the hcp-fcc was studied interface using density
functional theory (DFT) calculations, and high-resolution STEM.
Focusing on the effect of the stacking sequence, slab models were
used composed of four Ru layers and one or two Pt layers with a
(2.times.2) hexagonal array within each layer. The atoms in the Pt
layers and the top two Ru layers were relaxed. For a Pt monolayer,
it was found that the energy is slightly lower for the
Ru(ABAB)-Pt(C) sequence (-33 meV) than for that of Ru(ABAB)-Pt(A)
(Tables 1 and 2).
TABLE-US-00001 TABLE 1 DFT calculated energy differences for slab
models with various stacking sequences. .DELTA.E (meV) vs. ABAB
Stacking sequence models sequence Ru--Pt monolayer ABAB-A 0 ABAB-C
-33 Ru--Pt bilayer Normal Ru hcp layers ABAB-AB 0 ABAB-AC -160
ABAB-CA -134 ABAB-CB 25 Top Ru layer shifted ABAC-BA 466 ABAC-AB
612 Ru--RuPt partial alloy AB-ACBA alloy vs. ABAB-AB bilayer 1000
AB-ACBA alloy vs. AB-ABAB alloy -250
TABLE-US-00002 TABLE 2 Lattice strain in the Ru cores and Pt shells
deduced from the refinement of the XRD spectra in FIG. 2. Strain in
core Pt Strain in shell Ru(101) % (111) % vs. Ru@Pt NP samples d
(.ANG.) vs. d.sub.bulk = 2.056 d (.ANG.) d.sub.bulk = 2.263
RuPt.sub.0.5/C 3.0 nm 2.057 0.05 2.242 -1.02 RuPt.sub.0.75/C 3.2 nm
2.056 0.0 2.243 -0.97 RuPt.sub.1.0/C 3.5 nm 2.058 0.1 2.25 -0.66
RuPt.sub.1.33/C 3.7 nm 2.058 0.1 2.242 -1.02 RuPt.sub.1.0/CNT 4.5
nm 2.061 0.24 2.251 -0.62
[0062] Adding the second Pt layer with two choices for each type of
monolayer creates four possible stacking sequences. The DFT
calculations revealed that two of them have lower energies than the
all-hcp Ru(ABAB)-Pt(AB) sequence. One is the Ru(ABAB)-Pt(AC)
sequence (-160 meV); the other is the Ru(ABAB)-Pt(CA) sequence
(-134 meV). FIG. 5 depicts their structural models. Exploration of
both structures via high-resolution STEM denoted that the former is
characterized by having three (2 Pt and 1 Ru) top layers involved
in the fcc sequence (the atoms in these layers align along the
triangular lines), and with five (1 Pt and 4 Ru) layers in the hcp
sequence (the atoms in the rectangle at every other layer aligned
vertically). For the Ru(ABAB)-Pt(CA) sequence, the fcc triangle
includes four (2 Pt and 2 Ru) top layers, while the hcp rectangle
has four Ru layers.
[0063] DFT-optimized structures were compared with high-resolution
STEM images for the Ru@Pt nanoparticles in the specified
crystallographic orientations. In FIG. 6, one image for a Ru@Pt/CNT
sample matches well with the Ru(ABAB)-Pt(AC) sequence, while
another other agrees with the Ru(ABAB)-Pt(CA) sequence. Since the
boundary between Ru core and Pt shell is not always apparent, it
can be questioned whether the top Ru layer could be shifted from
the normal A sites to the C sites by interaction with Pt layers. It
has been found that this scenario is unlikely because the energy
cost for such a shift is high, 466 or 612 meV, as listed in Table
1. Therefore, the observed images are well described by the
DFT-optimized structural models, confirming the formation of
well-defined, highly ordered Ru@Pt core-shell nanoparticles.
Catalytic Performances of Ru@Pt Nanoparticles
[0064] For hydrogen evolution in water electrolyzers, the best
performance was obtained using a 1:1 ratio Ru@Pt/C nanocatalysts
with bilayer-thick Pt shells. FIG. 7 compares the polarization
curve of such a catalyst measured with a standard baseline. A lower
cell voltage signifies that electricity is used more efficiently on
a better catalyzed electrode. The nearly-overlapping polarization
curves denote the achievement of a same performance with a
reduction of Pt loading by 99% (0.022 versus 3.0 mg cm.sup.-2). In
addition to the high Pt surface area per mass (.about.1 cm.sup.2
.mu.g.sup.-1) of the core-shell nanoparticles, optimized carbon
support and well-fabricated gas diffusion electrodes also are vital
for assuring the high activity at ultralow metal loadings.
[0065] FIG. 8 shows the polarization curves measured with
H.sub.2+10 ppm CO+1% air bleed, wherein Ru@Pt1.33/C performed
better than Pt/C. This may be attributed to the enhanced CO
tolerance to the weakened CO adsorption on the Pt shell (2-3 atomic
layer thick), as evidenced by the lack of a sharp CO stripping peak
compared with that for Pt nanoparticles (FIG. 9). DFT calculations
illustrated the weakened adsorption induced by lattice contraction.
It was found that Pt shells contracted by -0.62% to -1.0% based on
the XRD analyses (Table 2), largely due to the lattice mismatch
between Ru and Pt.
[0066] In previously used RuPt/C catalysts Ru may dissolve at
occasional high potentials during the starts and stops of fuel
cells. Ru ions can migrate through membrane to the cathode side;
the re-deposited Ru depresses the cathode activity for oxygen
reduction, and thus, lowers the cell voltage. A well-ordered
core-shell structure enhances dissolution resistance by assuring
the Ru core is covered completely with a Pt shell that itself is
more resistant to dissolution than Pt nanoparticles due to
Ru-induced lattice contraction. FIG. 10 shows the results of
accelerated stress test for the Ru@Pt.sub.1.33/C catalyst. After
2500 start/stop cycles that alternated the anode potential between
0.02 and 0.95 V, there were only minor losses in cell voltages. The
sustained activities at ultralow PGM loadings signify that
structural perfection matters for high activity and stability in
practical applications.
Structure Characterization of Pd@Pt Nanoparticles
[0067] Referring now to FIG. 11, for two distinct samples (dots and
squares) with different starting amount of H.sub.2PtCl.sub.6
precursor in synthesis, the measured Pt/(Pt+Pd) atomic ratios
decreased with the increasing particle size. The trend largely
followed the curves calculated for monolayer- and bilayer-thick Pt
shells on Pd cores, respectively (solid and dash lines). The
calculation was based on a cuboctahedron model and the normal
distribution of particle size (SI, experimental section). These
results denoted that the conformal and smooth Pt shell could be
epitaxially fabricated with variable thickness on Pd cores, simply
by adjusting the amount of H.sub.2PtC1.sub.6 used in syntheses. The
exclusive deposition of Pt atoms on Pd cores was verified by the
two-dimensional (2D) intensity mapping using scanning transmission
electron microscopy (STEM) technique. In addition, the Pt/(Pt+Pd)
atomic ratios were determined by energy dispersive X-ray
spectroscopy (EDX) for individual particles. No particles were
found without the existence of Pd, indicating that the
self-nucleation of Pt has been effectively prevented. Furthermore,
a nearly 100% yield was also verified by the very good agreement of
Pt and Pd weight percentages in the final products measured by
inductively coupled plasma mass spectrometry (ICP-MS) and by EDX
equipped on a scanning electron microscope (SEM), with those
calculated from the precursor amounts used in syntheses.
Catalytic Performances of Pd@Pt Nanoparticles
[0068] The uniform and smooth Pt shell was further inferred from
the stable oxygen reduction reaction activity measured for a Pd@Pt
core shell nanoparticles sample after pulse-potential stability
test (FIG. 12). As shown in FIG. 12, after 5000 cycles of potential
pulses (10 s at 0.6 V and at 1.0 V each), the oxygen reduction
reaction polarization curve remained unchanged, and the cyclic
voltammetry curve showed negligible loss of electrochemical surface
area.
[0069] Table 3 summarises the oxygen reduction reaction activities
for Pd@Pt.sub.mL and Pd@Pt.sub.2ML fabricated by the ethanol-based
approach, compared with those for Pd@Pt fabricated by a scale-up Cu
underpotential deposition (UPD) method. The complete and smooth Pt
surface formed at 80.degree. C. by ethanol led to smaller
electrochemical surface area than that formed at room temperature
by scale-up Cu UPD method. Pd@PtML fabricated by two distinct
methods exhibited similar mass activity (normalized by Pt or PGM
mass), indicating the ethanol-based route was an effective
large-quantity synthetic method to produce Pt monolayer
catalysts.
TABLE-US-00003 TABLE 3 oxygen reduction reaction activities for
Pd@Pt.sub.ML and Pd@Pt.sub.2ML Cu UPD 1 ML EtOH EtOH catalysts Pt
Pd@Pt.sub.ML Pd@Pt.sub.2ML Pt/(Pt + Pd) 0.27 0.27 0.33 atom ratio
Pt (wt %) 16.5 17.1 18.1 Pd (wt %) 24.5 24.9 19.7 MA.sub.Pt (A
mg.sup.-1) 0.62 0.64 0.62 MA.sub.PGM (A mg.sup.-1) 0.25 0.26 0.30
SA (mA cm.sup.-2) 0.32 0.58 0.70 ECSA (m.sup.2 g.sup.-1) 191 110
89
[0070] It should be apparent to those skilled in the art that the
described embodiments of the present invention provided herein are
illustrative only and not limiting, having been presented by way of
example only. As described herein, all features disclosed in this
description may be replaced by alternative features serving the
same or similar purpose, unless expressly stated otherwise.
Therefore, numerous other embodiments of the modifications thereof
are contemplated as falling within the scope of the present
invention as defined herein and equivalents thereto.
* * * * *
References