Truncated Octahedral Pt3Ni ORR Electrocatalysts
Jianbo Wu,† Junliang Zhang,‡ Zhenmeng Peng,† Shengchun Yang,† Frederick T. Wagner‡ and
Hong Yang*,†
†
Department of Chemical Engineering, University of Rochester, Gavett Hall 206, Rochester,
New York, 14627
‡
Electochemical Energy Research Lab, General Motors Research and Development,
Honeoye Falls, NY 14472
* CORRESPONDING AUTHOR
E-mail: [email protected]
Telephone: (585) 275-2110; Fax: (585) 273-1348
S1
Experimental Details
Synthesis of Pt3Ni Nanoparticles. A mixture of borane-tert-butylamine complex
(TBAB, Aldrich, 97%, 1.14 mmol), adamantanecarboxylic acid or adamantaneacetic acid
(ACA or AAA, Aldrich, 99%, 1.2 mmol), hexadecanediol (Aldrich, 96%, 6.2 mmol), one of
the following long alkane chain amines-hexadecylamine (HDA, TCI, 90% 8.28 mmol),
dodecylamine (DDA, Aldrich, 98%, 8.28 mmol), or octadecylamine (ODA, Aldrich, 97%,
8.28% mmol)-and diphenyl ether (DPE, Aldrich, 90%, 2 ml) was added into a 25-mL
three-neck round-bottle flask under argon protection. The reaction mixture was maintained at
190 °C using an oil bath. Platinum acetylacetonate (Pt(acac)2, Strem, 98%, 0.127 mmol) and
nickel acetylacetonate (Ni(acac)2, Aldrich, 95%, 0.0424 mmol) were dissolved in 2-mL DPE
at 60 °C followed by rapid injection into flask. The reaction was maintained at 190 °C for 1 h.
After the reaction, 200 μL of the product was mixed with 800 μL of chloroform in a plastic
vial (1 mL), followed by the addition of 1 mL of ethanol. The precipitate was separated from
the mixture by centrifugation at 5000 rpm for 5 min. The supernatant was decanted and the
black product was dispersed in 1 mL of chloroform. This process was repeated three times.
Preparation of Carbon-Supported Catalysts. Carbon black (Vulcan XC-72) was used
as support for making platinum nickel catalysts (Pt3Ni/C). In a standard preparation, carbon
black particles were dispersed in hexane and sonicated for 1 h. A designed amount of
platinum
nickel
nanoparticles
were
added
to
this
dispersion
at
the
nanoparticle-to-carbon-black mass ratio of 20:80. This mixture was further sonicated for 30
min and stirred overnight. The resultant solids were precipitated out by centrifugation and
dried under an argon stream.
The solid product was then re-dispersed in n-butylamine at a concentration of 0.5
mg-catalyst/mL. The mixture was kept under stirring for 3 days and then collected using a
centrifuge at a rate of 5000 rpm for 5 min. The precipitate was re-dispersed in 10 mL
methanol by sonicating for 15 min and then separated by centrifugation. This procedure was
repeated three times. The final samples were dispersed in ethanol for further characterization.
Characterization. Transmission electron microscopy (TEM) and high-resolution
transmission electron microscopy (HR-TEM) images were taken on a FEI TECNAI F-20
field emission microscope at an accelerating voltage of 200 kV. Scanning transmission
electron microscopy (STEM) and elemental maps were carried out using the high-angle
annular dark field (HAADF) mode on the same microscope. The optimal resolution of this
microscopy is 1 Å under TEM mode and 1.4 Å under STEM mode. Energy dispersive X-ray
(EDX) analysis of particles was also carried out on a field emission scanning electron
microscope (FE-SEM, Zeiss-Leo DSM982) equipped with an EDAX detector. Powder X-ray
diffraction (PXRD) patterns were recorded using a Philips MPD diffractometer with a Cu Kα
X-ray source (λ=1.5405 Å).
Electrochemical Measurement. The alloy mass of each Pt3Ni/C catalyst was
determined by thermogravimetric analysis (TGA) using an SDT-Q600 TGA/DSC system
S2
from TA Instruments, Inc at a ramp rate of 10 °C/min to 600 °C in air followed by annealing
at 600 °C for 30 min under a forming gas of 5 % hydrogen in argon at a flow rate of 50
ml/min. A three-electrode cell was used to do the electrochemical measurements. The
working electrode was a glassy-carbon Rotating Disk Electrode (RDE) (diameter: 5mm and
area: 0.196 cm2) from Pine Instruments. The procedure for the RDE measurements followed
those established in literature.1-3 A platinum foil with the size of 1cm×1cm was used as the
counter electrode and an Ag/AgCl (3-M Cl-) was used as the reference electrode. To
effectively prevent chloride ions from cross-contaminating the electrolyte in the working cell,
the reference electrode was placed in a separate compartment via a stopcock that was wetted
with the electrolyte. All potentials in this paper are referenced to the Reversible Hydrogen
Electrode (RHE), which was calibrated with H2 oxidation/evolution on a Pt polycrystalline
RDE electrode. All the liquid junction potentials were cancelled out in adjusting for the
RHE scale. The electrolyte used for all the measurements was 0.1-M HClO4, diluted from
70% double-distilled perchloric acid (GFS Chemicals, USA) with Millipore®ultra pure water.
To prepare the working electrode, 10 mg of the Pt3Ni/C catalyst was dispersed in 20 mL of a
mixed solvent and sonicated for 5 mins. The solvent contained a mixture of DI water,
isopropanol, 5% Nafion in the volume ratio of 4:1:0.025. 20 μL of the suspension was added
onto the RDE by a pipette and dried in flowing argon. The Pt loading on the RDE was
calculated as 9.3 μgPt/cm2. The electrochemical active surface area (ECSA) measurements
were determined by integrating the hydrogen adsorption charge on the cyclic voltammetry
(CV) at room temperature in nitrogen saturated 0.1-M HClO4 solution. The potential scan
rate was 20 mV/s for the CV measurement. Oxygen reduction reaction (ORR) measurements
were conducted in a 0.1-M HClO4 solution which was purged with oxygen for 30 min prior
to, and during, the measurement. The scan rate for ORR measurement was 10 mV/s in the
positive direction. The ORR polarization curves were collected at 1600 rpm. Due to the small
currents measured at 0.9 V on RDE, the effect of iR compensation is expected to be
negligible, thus the data presented in this paper were used without iR-drop correction. For
comparison, Pt/C (TKK, 50wt%Pt on Vulcan carbon) was used as the baseline catalyst, and
the same procedure as described above was used to conduct the electrochemical measurement,
except that the Pt loading was controlled at 11 μgPt/cm2.
Calculation for (111) Surface Ratio. A simple cube has only {100} facets and no {111}
facets, i.e. the fraction of (111) surface over the entire surface area, Scube(111)%, is equal to 0.
A truncated octahedron can be generated by cutting off six vertices from the octahedron,
generating {100} facets (Scheme S1). Thus we can calculate the (111) surface area by
subtracting the corresponding (111) surfaces of six square pyramids from the original
octahedron, Soctahedron(111), which can be derived from the following equation:
2
1
S octahedron (111) = 8 × • 2a • sin 600 = 4 3a 2
2
where a is the distance from the corner to the body center of the (untruncated) octahedron.
Assuming the height of each square pyramid is b, then the sum of {111} surface area of six
square pyramids being removed should be:
( )
∑S
square − pyramid
1
3
(111) = 6 × 4 × • ( 2b) 2 • sin 600 = 12 3b 2 , b ≤
a
2
3
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and the total area of the (100) surfaces of a truncated octahedron being created should be:
S t ,o {100} = 6 × ( 2b) 2 = 12b 2 , b ≤
3
a
3
Thus, the total (111) surface area of a truncated octahedron can be calculated by the
following equations:
St ,o (111) = S octahedron (111) − ∑ S square− pyramid (111) = 4 3a 2 − 12 3b 2
Thus, the ratio of (111) surface over the entire surface area of a truncated octahedron can be
calculated by the following formula:
2
⎛b⎞
1 − 3⎜ ⎟
2
2
St ,o (111)
4 3a − 12 3b
3
⎝a⎠
,b ≤
St ,o (111)% =
a
=
=
2
2
2
2
St ,o (111) + St ,o (100) 4 3a − 12 3b + 12b
3
⎛b⎞
1− 3 − 3 ⎜ ⎟
⎝a⎠
(
)
The overall fraction of (111) surface areas over the entire surface areas of a catalyst that is
composed of given percentages of truncated octahedral (αt,o) and cubic (αcube) shapes, is the
weighted average given by the following equation:
Scatalyst (111)% = St ,o (111)% × α t ,o + Scube (111)% × α cube
The average value of (b a ) is 0.11 for those truncated octahedral Pt3Ni nanocrystals in all
2
three samples based on the TEM measurement of about 200 nanocrystals, while the
population of truncated octahedra, αt,o, is 70%, 90% and 100%, respectively. The population
of cubes, αcube, in the samples was based on the TEM images and obtained by counting about
200 nanocrystals. The rest of nanocrystals were truncated octahedra and the population of
truncated octahedra, αt,o, could be derived using the following formula:
α t ,o = 1 − α cube
Thus, the fraction of (111) surfaces is 0.545 for the catalyst with 70% truncated octahedral
shape, 0.701 for that with 90%, and 0.779 for the one without cubes.
Reference:
(1) Schmidt, T. J.; Gasteiger, H. A.; Stab, G. D.; Urban, P. M.; Kolb, D. M.; Behm, R. J. J.
Electrochem. Soc. 1998, 145, 2354-2358.
(2) Paulus, U. A.; Schmidt, T. J.; Gasteiger, H. A.; Behm, R. J. J. Electroanal. Chem. 2001,
495, 134-145.
(3) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal. B-Environ. 2005,
56, 9-35.
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Scheme S1. Illustration of generation of truncated octahedron.
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Table S1. ECSA, Mass- and Area- Specific ORR Activities of Pt3Ni and Pt Catalysts*
sample name
100% t,o-Pt3Ni
90% t,o-Pt3Ni
70% t,o-Pt3Ni
Pt/C (TKK)
Pt loading
[μgPt /cm2disk]
ECSA
[m2/gPt]
mass activity
[A/mgPt]
specific activity
[mA/cm2Pt]
9.3
62.4
0.53
0.85
9.3
53.7
0.44
0.82
9.3
33.8
0.22
0.65
11
65
0.14
0.215
*: The activity was measured at 0.9 V (vs. RHE).
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Figure S1. TEM image of Pt3Ni nanoparticles produced with octadecylamine.
Figure S2. HR-TEM image of a Pt3Ni alloy nanocubes.
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Figure S3. TEM image of Pt3Ni nanoparticles made using TBAB as the reducing agent.
Figure S4. a) STEM image and its corresponding b) Pt (M line) and c) Ni (K line) elemental
maps, and d) EDX spectrum of t,o-Pt3Ni nanoparticles.
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Figure S5. EDX spectra of (a) 70% and (b) 90% truncated octahedral nanoparticles showing
the metal composition of Pt3Ni.
Figure S6. TEM images of carbon-supported Pt3Ni particles with various shapes: (a) 70%, (b)
90%, (c) 100% truncated octahedral Pt3Ni nanoparticles, respectively.
Figure S7. TEM image of the carbon-supported Pt catalyst (TKK, 50% loading).
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