Oxygen Reduction Activity of PtxNi1-x Alloy Nanoparticles
on Multiwall Carbon Nanotubes
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Citation
Kim, Junhyung, Seung Woo Lee, Christopher Carlton, and Yang
Shao-Horn. Oxygen Reduction Activity of PtxNi1-x Alloy
Nanoparticles on Multiwall Carbon Nanotubes. Electrochemical
and Solid-State Letters 14, no. 10 (2011): B110. © 2011 ECS The Electrochemical Society.
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http://dx.doi.org/10.1149/1.3613677
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Final published version
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Wed May 25 23:24:20 EDT 2016
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http://hdl.handle.net/1721.1/81295
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Detailed Terms
Electrochemical and Solid-State Letters, 14 (10) B110-B113 (2011)
B110
C The Electrochemical Society
1099-0062/2011/14(10)/B110/4/$28.00 V
Oxygen Reduction Activity of PtxNi1-x Alloy Nanoparticles on
Multiwall Carbon Nanotubes
Junhyung Kim, Seung Woo Lee, Christopher Carlton, and Yang Shao-Horn*,z
Electrochemical Energy Laboratory and Department of Mechanical Engineering, Massachusetts Institute of
Technology, Cambridge, Massachusetts 02139, USA
PtxNi1-x nanoparticles (Pt:Ni; 1:0, 4:1, 3:1 and 0.7:1) of 5 nm, were synthesized on carboxylic acid-functionalized multiwall carbon nanotubes (PtxNi1-x NPs/MWNT). The oxygen reduction reaction (ORR) activity measurements in 0.1 M HClO4 showed an
activity order of Pt NPs/MWNTs < Pt4Ni NPs/MWNTs Pt0.7Ni NPs/MWNTs < Pt3Ni NPs/MWNTs, where Pt3Ni NPs/MWNTs
had mass activity 2.5 times higher than a commercial 46 wt % Pt/C catalyst. The enhanced ORR activity of Pt3Ni NPs relative to
Pt NPs could be attributed to reduced coverage of oxygenated species on surface Pt sites as evidenced by the positive shift in the
onset voltage for OH adsorption.
C 2011 The Electrochemical Society.
V
[DOI: 10.1149/1.3613677] All rights reserved.
Manuscript submitted May 23, 2011; revised manuscript received June 28, 2011. Published July 20, 2011.
Among electrochemical energy conversion devices, proton
exchange membrane fuel cells (PEMFCs) have been at the forefront
as promising energy sources for automotive applications.1,2 However, the major challenge of PEMFCs is the kinetically sluggish oxygen reduction reaction (ORR) on Pt-based catalysts at the cathode,
which limits the efficiency and contributes to the high cost of this
technology.3 Fundamental studies of Pt alloy bulk surfaces show
that Pt alloy surfaces can exhibit ORR activity higher than Pt.4–8
For example, Pt can segregate in the outermost layer on bulk Pt
alloy surfaces to form a “sandwich-segregation”9,10 or “Pt-skin
structure,”8 which is shown to weaken the bond strength to oxygenated species relative to pure Pt (Refs. 5 and 7–11) and increase specific ORR activity. Following this concept, researchers have shown
that the Pt3Ni (111) surface with Pt segregation to the top surface
exhibits ORR specific activity 10 times greater than Pt (111).7
Remarkable progress toward fundamental understanding of ORR activity descriptor12–14 and very high activity established on single
crystal surfaces has motivated recent research in developing highly
active Pt3M nanoparticles (NPs) with sizes of practical relevance.15–17
For example, Wu et al.16 and Zhang et al.17 have synthesized truncated-octahedral Pt3Ni catalysts of 6 nm, and Pt3Ni nanoctahedra of
10 nm terminated with {111} facets by organic routes, which
show mass ORR activity of 0.53 A/mgPt and 0.11 A/mgPt at 0.9 V
vs. RHE without iR correction at room temperature, respectively.
Carbon nanotubes having high electronic conductivities and high
chemical stability18 are a promising support for these highly active
alloy nanoparticles for PEMFC cathode.19,20 Although previous
studies21–23 have shown that Pt-alloy NPs can be synthesized on
CNTs for ORR, detailed ORR activity analysis of Pt-alloy NPs on
CNTs has not been reported. In this letter, we report a simple aqueous synthetic method for carboxylic acid-functionalized MWNTs
(MWNT-COOH) supported PtxNi1-x (Pt:Ni; 1:0, 4:1, 3:1 and 0.7:1)
NPs (PtxNi1-x NPs/MWNTs) via an electrostatic attraction as well
as ORR activity dependence in term of composite ratios in the
PtxNi1-x alloy NPs (Fig. 1). The intrinsic and mass ORR activity of
Pt3Ni NPs/MWNTs (2.67 mA/cm2Pt, 0.85 A/mgPt at 0.9 V with iR
correction vs. RHE at room temperature) is 7 and 2.5 times higher
than that of a commercial 46 wt % Pt/C catalyst (Tanaka Kikinzoku
Kogyo, TKK), respectively.
Experimental
For the synthesis of PtxNi1-x alloy NPs on MWNTs, we utilized
carboxylic group-functionalized MWNTs.24 Commercial pristine
MWNTs (NANOLAB) were refluxed in concentrated H2SO4/HNO3
(3/1 v/v, 96 and 70%, respectively) at 70 C to prepare carboxylic
acid-functionalized MWNTs, and then washed by Milli-Q water.
* Electrochemical Society Active Member.
z
E-mail: shaohorn@mit.edu
The prepared 20 mg carboxylic-functionalized MWNTs were dispersed in Milli-Q water (15 ml) and PtCl4 (15.5 mg in 20 ml H2O)
and NiCl2 (12.2 mg in 20 ml H2O) were introduced into the dispersed solution under vigorous stirring for 1 h, anchoring metal salts
onto the carboxylic-functionalized MWNTs support via an electrostatic attraction. The mixed solution was reduced in an ice bath
(4 C) for 30 min. by NaBH4 (0.037 g in 10 ml H2O) and PtxNi1-x
NPs/MWNTs catalyst was separated by filtration and further washed
thoroughly by Milli-Q water. The atomic ratios (Pt:Ni; 1:0, 4:1, 3:1
and 0.7:1) of the PtxNi1-x NPs/MWNTs were controlled by PtCl4
and NiCl2 feed ratios. Direct current plasma-atomic emission spectroscopy (DCP-AES) was used to analyze the weight loading of Pt
and Ni on the MWNTs using a Beckman spectra Span VI DCP-AES
instrument at Luvac INC. Transmission electron microscopy (TEM)
imaging was performed to determine particle size distributions at
200 KeV on a JEOL 2010F microscope equipped with a field emission gun. Energy dispersive spectroscopy (EDS) was used to analyze the atomic ratio of Pt/Ni.
Freshly prepared PtxNi1-x NPs/MWNT samples were dispersed in
Milli-Q (18 MX) water via ultrasonicator (Sonics & Materials, Inc)
for ORR activity measurement. Uniform, thin film electrodes were
prepared on the glassy carbon electrode (5 mm in diameter) by dropping 20 l of suspension of PtxNi1-x NPs/MWNT. After evaporation
of water, 10 l of a dilute NafionV solution (0.025 wt %), which was
prepared from 5 wt % NafionV solution (Ion Power, Inc.) was
dropped on the electrodes. The saturated calomel electrode (SCE)
potential scale was calibrated with the reversible hydrogen electrode
(RHE) scale via H2 electro-oxidation reaction. Cyclic voltammetry
(CV) of PtxNi1-x NPs/MWNT samples was performed in 0.1 M
HClO4 electrolyte at room temperature under an oxygen-free condition obtained by bubbling ultra high-purity Argon gas for at least 30
min. CV cycling was continued until steady state was reached with
the potential range between 0.05 and 1.1 V vs. RHE at a scan rate of
50 mV/s. Pt electrochemically active surface area (ESA) was calculated from the charge associated with hydrogen under-potential deposition on Pt (210 C/cm2Pt) after double layer current subtraction in
the potential range between 0.05 and 0.4 V vs. RHE.25 The ORR
activity of PtxNi1-x NPs/MWNTs samples was measured by sweep
voltammetry in O2-saturated HClO4 using a rotating disk electrode at
room temperature. After the electrolyte was purged with pure O2 for
at least 30 min, polarization curves were recorded between 0.1 and
1.1 V (RHE) at a scan rate of 10 mV/s. The Pt specific kinetic
activity (jk, mA/cm2Pt) of PtxNi1-x NPs/MWNTs was calculated
from Koutecky-Levich analysis with background current subtraction
and Pt ESA. The Koutecky–Levich plots based on
1 1 1
1
1
¼ þ ¼ þ
at 0.35, 0.65, and 0.75 V revealed an
j jk jD jk BC0 x1=2
excellent linear behavior between 1/j and 1=x1=2 . The slope, 1/BC0,
reflects the number of electrons involved in the reaction, which were
found to be 0.177, 0.160 and 0.175 mAcm2x1/2 for TKK Pt/C, and
R
R
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Electrochemical and Solid-State Letters, 14 (10) B110-B113 (2011)
Figure 1. (Color online) Schematic of PtxNi1-x NPs synthesis on carboxylic
acid-functionalized MWNTs.
PtNPs/MWNTs and Pt3Ni NPs/MWNTs, respectively. They are in
good agreement with the predicted value (0.1505 mAcm2x1/2) from
the relationship: jD ¼ 0:2nFCO2 ðDO2 Þ2=3 m1=6 x1=2 ,26 where n, the
apparent number of electrons transferred in the reaction, is equal to 4,
F is the Faraday constant, CO2 is the O2 concentration in 0.1 M
HClO4 (1.26 103 mol l1), DO2 is the diffusivity of O2
B111
(1.93 105 cm2 s1) in dilute electrolyte solutions and m is the kinematic viscosity of the electrolyte (1.009 102 cm2 s1). Knowing
ORR on both Pt and PtxNi1-x nanoparticles occurs via a 4-electron
pathway, the specific activity was calculated from jk at 0.9 V vs.
RHE. The ORR activity with iR correction corrected the uncompensated ohmic electrolyte resistance (35 X) measured via high frequency ac impedance in O2-saturated 0.1 M HClO4 by the following
equation: EiR-corrected ¼ Eapplied iR, where i is ORR current and R
is the uncompensated ohmic electrolyte resistance.
Results and Discussion
Transmission electron microscopy (TEM) imaging showed that
all the PtxNi1-x NPs/MWNT samples had an average PtxNi1-x particle size of 5 nm (60.70.8 nm, standard deviation) (Fig. 2).
Direct current plasma-atomic emission spectroscopy (DCP-AES)
analysis showed that atomic compositions of PtxNi1-x NPs/MWNTs
Figure 2. (Color online) TEM images of
PtxNi1-x NPs/MWNTs with histograms:
(a) Pt NPs/MWNTs, (b) Pt4Ni NPs/
MWNTs, (c) Pt3Ni NPs/MWNTs and (d)
Pt0.7Ni NPs/MWNTs. (e) Energy dispersive spectroscopy (EDS) and scanning
transmission electron microscopy (STEM)
high annular dark field (HAADF) image
of Pt3Ni NPs/MWNTs.
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B112
Electrochemical and Solid-State Letters, 14 (10) B110-B113 (2011)
Figure 3. (Color online) Intrinsic ORR
activity comparison of TKK (46 wt % Pt/C)
and PtxNi1-x NPs/MWNTs at 0.9 ViR
corrected vs. RHE. (a) cyclic voltammograms
(46 wt % Pt/C, TKK; 9.9 gPt/cm2geo, 55
wt % Pt NPs/MWNTs, 27.9 gPt/cm2geo
and 41 wt % Pt3Ni NPs/MWNTs, 17.8
gPt/cm2geo) in Ar-saturated 0.1 M HClO4,
(b) oxygen reduction polarization curves
collected from a positive-going potential
sweep at a 10 mV/s scan rate at room temperature, (c) ORR intrinsic activity as a
function of iR-corrected potential obtained
at 1600 rpm after background and iR correction (ohmic electrolyte resistance of 35
X) and (d) ORR specific (black) and mass
(blue) activity of TKK and PtxNi1-x NPs/
MWNTs from 1600 rpm in O2-saturated
0.1 M HClO4, where the error bars represent standard deviations obtained from three
independent measurements.
are 78:22% (Pt:Ni) for Pt4Ni NPs/MWNTs, 72:28% (Pt:Ni) for
Pt3Ni NPs/MWNTs, and 39:61% (Pt:Ni) for Pt0.7Ni NPs/MWNTs,
respectively. Energy dispersive spectroscopy (EDS) was used to further verify the atomic Pt/Ni ratios of Pt3Ni NPs/MWNTs, which
were collected and quantified using the INCA software (version
4.08, Oxford instruments). Quantification of atomic Pt/Ni ratios was
performed using the Pt-L and Ni-K peaks with the sensitivity factors
from the software. EDS of a large number of Pt3Ni NPs was performed by spreading the beam in TEM mode and revealed that an
average atomic composition of 70:30% (Pt:Ni), which is in good
agreement with (DCP-AES) analysis (72:28%). In addition, scanning transmission electron microscopy (STEM) EDS analysis with a
nominal beam size of 0.7 nm showed that five randomly selected
Pt3Ni alloy NPs consisted of 67–72% Pt.
Cyclic voltammetry (CV) was used to determine the ESA of Pt
NPs. Figure 3a shows representative cyclic voltammograms of
PtxNi1-x NPs/MWNTs, Pt NPs/MWNTs and commercial TKK Pt/C
electrodes (Fig. 3a). The specific surface area of Pt3Ni NPs/MWNTs
averaged from three independent CV measurements was 32 m2/gPt,
which was lower than 80 m2/gPt of commercial TKK Pt/C catalysts.2 The measured specific surface area (32 m2/gPt) of Pt3Ni NPs/
MWNTs is in agreement with the estimated value of 40 m2/gPt
based on a surface Pt atomic percentage of 70% and a dv/a of 5 nm
of Pt3Ni NPs/MWNTs obtained from the particle size histogram.3
We examined the ORR activity of PtxNi1-x NPs/MWNTs using a
rotating disk electrode in O2-saturated 0.1 M HClO4 at room temperature. ORR polarization curves of each sample were collected at
different rotation rates in the range from 100 to 2500 rpm, from
which ORR kinetic current in the positive-going sweep was
extracted from the Koutecky-Levich equation.27 Figure 3b shows
representative ORR polarization curves of Pt3Ni NPs/MWNTs, Pt
NPs/MWNTs, and TKK Pt/C electrodes at 1600 rpm. Intrinsic ORR
activity based on the Pt ESA of PtxNi1-x NPs/MWNTs was compared with that of TKK (Pt/C) as a function of potential in Fig. 3c.
The Pt3Ni NPs/MWNTs show the highest intrinsic activity among
the catalysts, increasing in the order from Pt/C (TKK, 0.36 mA/
cm2Pt) < Pt NPs/MWNTs (0.84 mA/cm2Pt) < Pt4Ni NPs/MWNTs
(1.32 mA/cm2Pt) Pt0.7Ni NPs/MWNTs (1.33 mA/cm2Pt) < Pt3Ni
NPs/MWNTs (2.67 mA/cm2Pt) at 0.9 ViR-corrected vs. RHE (Table I).
The Tafel slopes of TKK Pt/C (64 mV/dec), Pt NPs/MWNTs (66
mV/decade) and Pt3Ni NPs (60 mV/decade) as determined by
[oE=oðlog is Þ], agree well with those of extended Pt and Pt3Ni polycrystals (80 mV/decade).28 The intrinsic activity of Pt/C (TKK) was
found to be 0.23 6 0.02 mA/cm2Pt without iR-correction at 0.9 V vs.
RHE, which is comparable to previous studies (Fig. 3d and Table
I).2,29 The intrinsic activity of Pt NPs/MWNTs (5.0 6 0.7 nm) is 2.3
times greater than that of 46 wt % Pt/C (TKK, 2.0 6 0.6 nm), which
can be attributed to reduced binding energy of oxygenated species
(OH and/or O) on larger sizes of Pt NP terrace as well as increasing
coordination number of surface Pt for Pt NPs/MWNTs relative to
TKK NPs (i.e., geometric effect).29–31 Overall, the PtxNi1-x alloy
NPs/MWNTs showed considerably higher intrinsic activity than
that of Pt NPs/MWNTs and TKK Pt NPs (Fig. 3d). The intrinsic
activity of Pt3Ni NPs/MWNTs (2.67 mA/cm2Pt) is three times
greater than 0.84 mA/cm2Pt at 0.9 ViR corrected vs. RHE of Pt
NPs/MWNTs in Fig. 3d. This enhancement of ORR activity can
Table I. ORR specific and mass activity at 1600 rpm, 0.9 V (RHE), 0.1 M HClO4; without and with iR-correction.
without iR correction
TKK
Pt NPs/MWNTs
Pt4Ni NPs/MWNTs
Pt3Ni NPs/MWNTs
Pt0.7Ni NPs/MWNTs
2
jk (mA/cm Pt)
0.23 6 0.02
0.50 6 0.08
0.92 6 0.05
1.24 6 0.19
0.90 6 0.20
with iR correction
jm (A/mgPt)
jk (mA/cm2Pt)
jm (A/mgPt)
0.22
0.10 6 0.01
0.19 6 0.01
0.40 6 0.05
0.23 6 0.07
0.36 6 0.02
0.84 6 0.19
1.32 6 0.19
2.67 6 0.60
1.33 6 0.22
0.34 6 0.02
0.17 6 0.04
0.28 6 0.03
0.85 6 0.15
0.34 6 0.09
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Electrochemical and Solid-State Letters, 14 (10) B110-B113 (2011)
be explained by modification of the electronic structure of Pt (5d orbital vacancies) by introducing Ni,5,7,32,33 which can reduce
binding energy of oxygen containing species (OH and/or O) on
surface Pt of Pt3Ni NPs/WMNTs.33,34 The onset potential (Fig.
3a) for the adsorption of oxygenated species on the surface of Pt3Ni
NPs/MWNTs was shifted positively by 30 mV as compared to
that of Pt NPs/MWNTs, indicating weak chemical adsorption of
OHads-species.
The intrinsic activity of Pt3Ni NPs/MWNTs without iR-correction (1.24 6 0.19 mA/cm2Pt, Table I) is comparable to other Pt3Ni
(Ref. 35) and Pt3Co (Ref. 36) alloy NP catalysts reported previously.
The mass activity of Pt3Ni NPs/MWNTs was 0.85 A/mgPt, which is
2.6 times higher than that of TKK (Pt/C, 0.33 A/mgPt) at 0.9 ViR
corrected vs. RHE (Fig. 3d and Table I). In addition, without iR correction, Pt3Ni NPs/MWNTs (0.40 A/mgPt and 0.28 A/mgmetal) was
comparable to other reported Pt alloy NP catalysts such as truncated
octahedral Pt3Ni NPs (0.53 A/mgPt) (Ref. 35) and 4.5 nm Pt3Co
NPs (0.52 A/mgPt at 24 C),36 and higher than single-wall carbon
nanotubes supported 2–5 nm Pt NPs (0.24 A/mgPt),37 10 nm cube
Pt3Ni nanoclusters (0.11 A/mgPt) (Ref. 16) and 4 nm Pt3Ni/Vulcan
(0.09 A/mgPt).34
Conclusions
In conclusion, we utilized carboxylic acid-functionalized
MWNTs as a support for PtxNi1-x NPs to investigate ORR activity
in 0.1 M HClO4 at room temperature. The ORR activity increased
in the order of Pt NPs/MWNTs (0.84 mA/cm2Pt) < Pt4Ni NPs/
MWNTs (1.32 mA/cm2Pt) Pt0.7Ni NPs/MWNTs (1.33 mA/
cm2Pt) < Pt3Ni NPs/MWNTs (2.67 mA/cm2Pt), all of which are
higher than Pt/C (TKK, 0.36 mA/cm2Pt). This trend can be attributed
to the inhibition of Pt-OHads formation on Pt active site for ORR
due to the geometric and/or electronic effects. The most active
Pt3Ni NPs/MWNTs were found to have specific (2.67 mA/cm2Pt
with IR correction) and mass activity (0.85 A/mgPt with IR correction) among the highest for Pt alloy NPs reported to date.
Acknowledgments
This work was supported by National Science Foundation
MRSEC under the award number DMR – 0819762 and the U.S
Department of Energy grant number DE-AC02-98CH10886 through
Brookhaven National Laboratory. S.W.L. acknowledges a Samsung
Scholarship from the Samsung Foundation of Culture.
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