Pt Cu nanocrystals with hexa-pod morphology and their ...We utilized a facile solvothermal approach...
Transcript of Pt Cu nanocrystals with hexa-pod morphology and their ...We utilized a facile solvothermal approach...
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Nano Res
1
PtxCuy nanocrystals with hexa-pod morphology and
their electrocatalytic performances towards oxygen
reduction reaction
Yujing Li1,2 (), Fanxin Quan2, Enbo Zhu3, Lin Chen2, Yu Huang3,4, and Changfeng Chen1,2
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0834-7
http://www.thenanoresearch.com on June 9, 2015
© Tsinghua University Press 2015
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Nano Research DOI 10.1007/s12274-015-0834-7
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1 Nano Res.
TABLE OF CONTENTS (TOC)
PtxCuy nanocrystals with hexa-pod morphology and their electrocatalytic performances towards oxygen reduction reaction
Yujing Li1,2*, Fanxin Quan2, Enbo Zhu3, Lin Chen2, Yu Huang3,4, Changfeng Chen1,2
1. State Key Laboratory of Heavy Oil, China
University of Petroleum, Beijing, Changping,
102249, China.
2. Department of Materials Science and
Engineering, College of Science, China University
of Petroleum, Beijing, Changping, 102249, China.
3. Department of Materials Science and
Engineering, University of California-Los Angeles,
Los Angeles, California, 90095, United States.
4. California NanoSystems Institute, University of
California-Los Angeles, Los Angeles, California,
90095, United States.
Hexa-pod PtxCuy synthesized with wet chemistry display
superior electro-catalytic activity towards oxygen reduction
reaction due to the existence of high-index facets.
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PtxCuy nanocrystals with hexa-pod morphology and
their electrocatalytic performances towards oxygen
reduction reaction
Yujing Li1,2 (), Fanxin Quan2, Enbo Zhu3, Lin Chen2, Yu Huang3,4, Changfeng Chen1,2
Received: day month year
Revised: day month year
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Oxygen reduction
reaction; Bimetallic alloy;
Morphology control; Fuel
cell Electro-catalysis;
High-Index.
ABSTRACT
Synthesis of bimetallic PtxCuy nanocrystals (NCs) with well-shaped hexa-pod
morphology through wet chemistry approach is reported. As-synthesized
convex NCs around 20nm in size expose by low-index (111) facets on seeds and
various high-index facets on pods. The growth mechanism involves preferred
growth along crystallographic direction on cuboctahedral seeds. The
synthetic protocol can be applied to synthesis of PtxCuy NCs with various Cu/Pt
ratios. The electro-catalytic activities of hexa-pod PtxCuy NCs supported on
carbon black towards oxygen reduction reaction (ORR) are studied. Hexa-pod
PtCu2/C catalysts exhibit the highest specific activity (3.7 mA/cm2Pt) and mass
activity (2.4 A/mgPt), the highest compared with data reported from literatures
on PtxCuy. Comparison with PtxCuy in other morphologies indicates that the
enhanced activity originates from morphology factor. Existence of high-index
facets, as well as abundant edges and steps on pods, could reasonably explain
the enhanced catalytic activity. The hexa-pod PtxCuy/C catalysts also show high
stability in morphology and activities after accelerated durability tests. As-
synthesized hexa-pod PtxCuy NCs are highly potential cathode electro-catalysts
for proton exchange membrane fuel cells.
Nano Research DOI (automatically inserted by the publisher)
Research Article
Address correspondence to Yujing Li, Email: [email protected]
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Carbon supported Pt nanoparticles are currently the
most widely accepted cathode catalysts for proton
exchange membrane fuel cells (PEMFCs), but Pt-
based alloy nanocrystals (NCs), mainly Pt alloyed
with transition metal M (denoted as PtM, M=Ni, Co,
Fe, Mn, etc.), have gained enormous attention in the
past decade due to their improved catalytic activities
compared with pure Pt in oxygen reduction reaction
(ORR).[1-10] The drastically improved electro-
catalytic performance of PtM alloys can mainly be
attributed to surface electronic effect, and geometric
effect induced by the surface strain or atomic
arrangement.[11-13] It has also been reported by
literatures that, for PtM alloys, the formation of Pt-
skin structure is crucial in achieving enhanced ORR
activities.[14, 15] It is generally accepted that the
formation of Pt-skin structure leads to the modified d-
band electronic structure and shortened Pt-Pt distance
on surface, both of which could benefit the adsorption
of oxygen molecules and breakage of O-O bond.[2, 16-
20]
Among various PtM alloys for PEMFC cathode
application, PtxCuy is a special alloy family that has
not received much attention.[21-24] Strasser et al
reported that electro-catalytic activities of PtxCuy
towards ORR can also be drastically enhanced
through voltammetric de-alloying approach, and
hypothesized that lattice-contracted alloy core
stabilizes the Pt-rich skin with surface strain and
hence improves the surface catalytic activity.[25, 26]
The same group also found that when increasing
Cu/Pt ratio for PtxCuy alloy, the catalysts show
elevated ORR activities in acidic media but declined
activity in alkaline electrolyte.[27] Cu atoms on the
surface remain stable in alkaline media, but tend to
deplete in acid, leading to the formation of ORR-
preferred Pt-rich skin in only acidic media, which
explains their observed distinctive behaviors in
different electrolytes. Stephens and Chorkendorff et al
engineered a Cu/Pt(111) near-surface alloy with Cu
atoms in subsurface up to 1 monolayer, and
demonstrated 8-fold enhancement of ORR activity
over Pt(111) with appropriate amount of deposited
Cu.[28] They proposed that the presence of subsurface
Cu can weaken the binding of surface Pt atoms to OH*
intermediates and expedite the ORR. From literatures,
PtxCuy alloys have clearly demonstrated enhanced
ORR activities via surface de-alloying.[29-31]
Nonetheless, to achieve ORR activities from PtxCuy
catalysts as high as those reported from PtxNiy-based
alloys remains difficult.
Both theoretical and experimental studies have
pointed out that, for Pt-based alloy catalysts, their
catalytic activities strongly depend on exposed
facets.[14, 32] For instance, Stamenkovic and
Markovic et al found that for single crystalline Pt3Ni
film, Pt3Ni(111) facet shows one-order-of-magnitude
enhancement in ORR activity compared with Pt(111)
surface, which is attributed to modified surface d-
band electronic structure induced by atomic
arrangement on (111) facet and Pt-rich surface
compositional profiles as mentioned above.[14] For
nano-sized catalysts, morphology of catalyst particle
determines exposed facets, and therefore affects their
activities towards ORR.[33-35] In addition, it is also
known that atoms at corners and edges of catalyst
particle, determined by morphology as well, situated
in special strain and bonding states and thus could
show distinctively high catalytic activities.[32, 36]
Therefore, controlling the morphology of alloy NCs is
crucial in optimizing catalytic performance and
investigating catalytic mechanism.[37-39] Large
amount of studies have been placed on morphology-
controllable syntheses of PtNi, PtCo, and PtFe alloy
families, while relative literatures are limited for PtCu
alloy. Earlier literatures on PtCu nanocatalysts
generally show irregular morphologies.[21, 23] Wang
and Li et al reported a unique synthetic protocol of
synthesizing PtxCuy with various Cu/Pt ratios in
different shapes, including mainly spheres, truncated
octahedron, flowers etc, demonstrating the possibility
to control the morphology of PtCu NCs.[40] Fang et al
reported the synthesis of octahedral and cubic PtCu
NCs with the typical oleylamine/oleic acid protocol
with the presence of tungsten hexacarbonyl.[41, 42]
Saleem and Wang et al employed a two-step synthetic
approach, and obtained ultrathin PtCu nanosheets,
based on which PtCu nanocones could further be
obtained by controllable rolling of nanosheets.[43]
However, none of the above work correlates
morphology of PtCu NCs with their catalytic activities
towards ORR, leading to the lack of data regarding
morphology-activity relation.
Herein, we demonstrate a facile and robust approach
for harvesting uniform monodisperse PtxCuy NCs
with hexa-pod morphology in various Cu/Pt ratios.
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3 Nano Res.
As-synthesized hexa-pod PtxCuy NCs are enclosed
mainly by high-index crystallographic facets,
hypothetically in a seeding scheme. The electro-
chemical activities towards ORR are determined for
the PtxCuy NCs with Cu/Pt ratio of 2/3, 1/1, and 2/1
supported on commercial carbon black. The highest
specific activity, 3.7 mA/cm2Pt, and mass activity, 2.4
A/mg Pt, are achieved with PtCu2/C, both of which are
higher than the best ever reported activities obtained
from PtCu alloy nano-catalysts.
We utilized a facile solvothermal approach to
synthesize the hexa-pod PtxCuy NCs. [Pt(NH3)4](NO3)2
and Cu(NO3)2 were used as metal precursors, and 1,2-
propanediol (PDO) was used as solvent and reducing
agent for the synthesis. After numerous trials, we
found that a combination of polyvinylpyrrolidone
(PVP) and sodium bromide (NaBr) with appropriate
ratio as capping agent can be used to obtain NCs with
hexa-pod and truncated octahedral (TO)
morphologies. Previously reported shape-controllable
syntheses of PtxCuy NCs were mostly conducted in
non-polar organic solvents such as oleylamine, hence
the NCs commonly show hydrophobic surface and
need further treatment by either ligand exchange or
acid treatment for the purpose of obtaining best
electrochemical activity.[33, 39] In this work, the
reaction is conducted in hydrophilic solvent, and
PVP/NaBr is used as capping agent combination,
endowing the as-synthesized PtxCuy NCs with
hydrophilic surface. Besides, capping species PVP and
NaBr can be easily washed off to minimize their
possible hindering effects on catalytic activities.
Therefore, the synthetic protocol is purely green and
favored by the electrochemical application as well.
The morphology of the as-synthesized PtCu2 NCs is
characterized by transmission electron microscope
(TEM). Fig. 1(a) represents the typical view of the
sample at low magnification, indicating that NCs are
distributed within a narrow size window around 20
nm and all NCs show pod-like projections. At higher
magnification, NCs show well-defined morphology.
The NC at lower right clearly shows the hexa-pod
morphology, and other NCs in the same image also
show 2D projections of hexa-pod along different zone
axes. By examining 50 NCs, it is found that over 92%
of the NCs show projections of hexa-pod, indicating
fairly high yield of hexa-pod NCs with this synthetic
protocol. A schematic of hexa-pod NC with smoothed
facets is shown in the inset of Fig. 1(b). Elemental
mapping images of the NCs under scanning
transmission electron microscope (STEM) are shown
in Fig. 1(c), indicative of uniform distribution of Pt and
Cu in NCs. Atomic resolution TEM image of a NC
lying along zone axis is shown in Fig. 1(d-f). A
close view of the pod highlighted by red rectangle can
be found in Fig. 1(e). According to fast Fourier
transformation (FFT) in Fig. 1(f), surface on the pod
can mostly be assigned to low index facets such as (111)
(yellow), (100) (orange), and (220) (light green) with
abundant atomic steps and kinks. The NCs grow into
well-defined morphology but most facets are
smoothed out.
Figure 1. Typical view of as-synthesized PtCu2 NCs at low
magnification (a), high magnification (b) under TEM, as well as
elemental mapping (c) under STEM . High resolution TEM image
of one PtCu2 NC aligned in zone axis is shown in (d) with
close view of highlighted red square region in (e) and its fast
Fourier transformation (FFT) in (f). Color code in (e): Yellow for
(111) facet, Orange for (100) facets, and Light green for (220)
facets.
To further confirm the of hexa-pod morphology, TEM
sample was tilted in α direction to capture the
projections of NCs along different zone axes, as shown
in Fig. 2. Images were recorded at every 10-degree tilt
from 10˚ to -30˚ in the direction illustrated in Fig. 2(f),
by denoting horizontal position as 0˚. A polygon
model with similar morphology to a real NC, e.g. with
simulated lengths of the six pods, was constructed as
shown in Fig. 2(f). The selected model NC can be
found on the left in Figs. 2(a-e). The 3D model was
tilted in the same way as the real NC on the sample
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4 Nano Res.
stage, and the 3D schematics were displayed as insets
at corresponding angles. It can be implied through
comparison that the 3D models show similar
projections to the real NC viewed from different zone
axes, confirming the hexa-pod morphology of the
PtCu2 NCs.
Figure 2. TEM images taken when sample holder is tilted at -30o
(a), -20o (b), -10o (c), 0o (d) and 10o (e) along α direction. (f)
Schematic of hexa-pod 3D model tilted in the highlighted α
direction.
Understanding how the NCs grow into hexa-pod
morphology is of great importance to explore the
growth mechanism. To better understand the growth
mechanism, a time evolution experiment was carried
out to capture the morphology of NCs growing at
different stages, e.g. 30min, 2h and 6h, as shown in Fig.
3. When taken at 30min, most NCs show TO and
octahedral morphologies with the sizes around 9 nm.
A small amount of NCs start to show pod-like
structure with one or two pods as in the inset of Fig.
3(a).
Figure 3. TEM images of PtCu2 NCs captured from reactions
stopped at 30min (a), 2h (b), and 6h (c).
It is commonly known that TO NCs can easily
transform into octahedron via preferable growth
along crystallographic direction, which makes
the transition between TO and octahedron difficult to
be discernible. It is a reasonable hypothesis that the
NCs start from TO seeds, and evolve into octahedron
through the faster growth in direction. From
30min to 2h, all NCs develop into multi-pod
morphology. For a TO NC, it has six (100) facets which
tend to shrink and disappear by the continuing fast
growth of direction. Eventually, the six (100)
facets can extrude and grow into pods, leading to the
convex morphology shown in Fig. 3(b). At 6h, the NCs
show obvious pod structure with longer pods. A close
look into Fig. 3(c) indicates that NCs show uniform
hexa-pod morphology, with pods varying in length on
each NC.
Figure 4. Schematic of growth mechanism illustrating the
morphology evolution of the hexa-pod PtxCuy NCs.
Through the time evolution experiment, we propose
that due to faster growing rate in
crystallographic directions, TO seeds quickly
transform into octahedral NCs that sit in a meta-stable
state due to the continuing fast growth in
directions, and eventually evolve into hexa-pod
morphology. The growth trajectory can be illustrated
more clearly as the schematic in Fig. 4. It can be easily
deduced from the hypothesis that the surface of the
hexa-pod NCs strongly depend on their growth stage,
i.e. pod length. Therefore, they can hardly be assigned
to a fixed crystallographic facet. Assuming that the
hexa-pod NCs were perfect polygons with sharp
facets, the NC should be enclosed by 32 facets,
including 8 facets on the seed with and other 24 facets
on the pods. It could be calculated that interfacial
angle is 70.6˚ between two (111) facets, and 54.7˚
between (111) and (100) facet, both of which are higher
than the maximum interfacial angle that could
possibly be between the facets on the pod (highlighted
with green dashed lines in Fig. 4) and (111) facets
(highlighted with yellow dashed lines). It means that
the surface on the pod can be neither (111) nor (100),
indicating that the pod should be enclosed by facets
with higher crystallographic indexes. Through tilt of
TEM sample holder, two high resolution TEM images
of selected PtCu2 NCs viewed from zone axis
shown in Fig. S1 (Supporting Information). Surface
highlighted by red dashed line can be designated to
high index crystallographic facets such as (420), (460),
(710), and (750).[44] It is noteworthy that the convex
NCs in this work commonly show smoothed pods,
adding that pods differ in length, meaning that pods
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5 Nano Res.
of NCs are enclosed by high-index facets with various
indexes.
Since the growth mechanism has been proposed, it is
of our interest to see if the composition of the alloy
NCs can be tuned. In this work, PtxCuy hexa-pod NCs
with three Cu/Pt ratios, determined by energy
dispersive X-Ray spectroscopy (EDX) equipped in
TEM and inductively coupled plasma optical emission
spectrometer (ICP-OES), are shown in Fig. 5. For the
alloy products with Cu/Pt ratio 2/1, 1/1 and 2/3, the
starting precursors, Cu(NO3)2 and [Pt(NH3)4](NO3)2,
were added with the Cu/Pt ratio at 3/1, 1/1, and 1/3.
The ratio difference between products and precursors
comes from the reduction potential difference of Cu2+
and Pt(NH3)42+ in PDO. In earlier work reported by
Wang and Li et al, synthesis was carried out at
temperatures generally above 240˚C when all Cu2+
ions can be fully reduced.[40] In this work, all
syntheses were conducted at 180˚C, leading to
incomplete reduction of precursors. Well-shaped
hexa-pod PtCu NCs can only be formed for PtCu2
alloy (Fig. 5(c)), while mixed multi-pod morphologies
can be formed for Pt3Cu2 and PtCu alloys as in Fig. 5(a-
b). However, it can be clearly seen for mult-pod Pt3Cu2
and PtCu alloy NCs that, they show longer pods. EDX
spectra of Pt3Cu2 and PtCu2 NCs can be found in Fig.
S2-S3.
Figure 5. TEM images of Pt3Cu2 (a, d), PtCu (b, e), and PtCu2 (c,
f).
It is an obvious trend that when increasing Cu/Pt ratio,
NCs tend to show more regular and uniform
morphology. It is possible due to the fact that
overgrowth could occur when more Pt sources exist,
while slower reaction kinetics could benefit
morphology evolution at the scenario with more Cu
sources.[45] To avoid misunderstanding, all PtxCuy
alloy NCs are denoted as hexa-pod NCs because most
NCs still show six pods for Pt3Cu2 and PtCu alloys.
The growth mechanism of PtCu NCs is also explored
by time evolution experiment as shown in Fig. S4. The
hexa-pod PtCu NCs also start from octahedral seeds
and evolve gradually into hexa-pod alloy NCs,
indicating that PtCu and PtCu2 share similar growth
mechanism towards hexa-pod NCs. It can be
concluded that this protocol can be applied to hexa-
pod PtxCuy alloy NCs with various Cu/Pt ratios. X-ray
diffraction (XRD) patterns of PtCu and PtCu2 are
shown in Fig. S5. For PtCu2, the shift of (111)
diffraction peak verifies that the inclusion of more Cu
atoms leads to the shrink of unit cell.
As comparisons, PtxCuy NCs with two other
morphologies were synthesized to explore the effect
of morphology on ORR activities. By switching the
solvent to 1, 4-butanediol (BDO), uniform and
monodisperse TO and near-spherical (NS) irregular
PtCu NCs, shown in Fig. S6, can be obtained simply
by manipulating the Cu/Pt feeding ratio of precursors.
It has been mentioned that, at 180˚C, PDO and PVP
are not stong enough to reduce all Pt and Cu
precursors. The reducibility is even weaker for BDO at
this temperature, so the overgrowth can hardly occur
when using BDO, leading to the formation of
truncated octahedral NCs and NS NCs. The Cu/Pt
ratio is 1/1 for both alloy samples, determined by EDX
and ICP.
To maximize the active surface area and enhance the
stability of hexa-pod NCs, all PtxCuy NCs were loaded
onto high-surface-area carbon black (Vulcan XC-72,
CB), as shown in Fig. S7. The catalyst loadings are
determined by total masses, with the weight
percentage at 20%. According to Fig. S7, loadings of
PtxCuy NCs on CB are highly uniform. No aggregation
can be found in TEM view.
To evaluate ORR activities of the CB-supported NCs,
the catalysts were loaded onto glassy carbon electrode
(GCE). Masses of loaded noble metals (referred to Pt
only) were the same (12 μg/cm2) for all catalysts.
Catalyst films on GCE were prepared by covering the
pre-formed catalyst film with thin Nafion film.
Electrochemical surface areas (ECSAs) were
determined by integrating hydrogen
adsorption/desorption peak in cyclic voltammetric
(CV) curves (between ~0.05 and 0.40 V vs. reversible
hydrogen electrode denoted as RHE) shown in Fig.
4(a), by assuming the monolayer hydrogen adsorption.
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It is worth noting that the CVs in Fig. 6(a) were
recorded after the curves were stabilized. The initial
de-alloying cycles, e. g. 1st, 2nd, and 10th cycles, are
shown in Fig. S8. Although the characteristic
desorption peak of under-potentially deposited (UPD)
hydrogen is not well-shaped as on pure Pt surface,
they can still be recognizable even during the 1st cycle,
indicating the presence of substantial Pt atoms on NC
surface for all catalysts. Surface Cu/Pt ratio
determined by X-ray photoelectron spectroscopy (XPS)
is 5/11 for PtCu2 sample, which deviates from bulk
ratio but agrees well with the indication from
hydrogen UPD in CV. Since the reaction temperature
is not high enough to reduce all metallic precursors,
and, Cu atoms are more reductive than Pt at such
temperature, it results in the displacement reaction
between Cu atoms at surface and Pt ions in solution,
leading to the Pt-rich structure. The low anodic peaks
between 0.5 and 0.75V on the 1st and 2nd cycle are
attributed to selective Cu dissolution from NC surface.
The cathodic peaks associated with the reduction of Pt
oxide mixed with re-deposition peak of Cu atoms,
split on 2nd cycle into distinguishable peaks. The peak
around 0.75V related to reduction of Pt oxide shifts to
the positive potential, while the shoulder peak at 0.6V
gradually disappear on 10th cycle, implying
undetectable Cu dissolution from NC surface at this
stage.[25] CV curves in Fig. 6(a) are recorded once the
electrochemical activation curves stabilize, which
resemble that of pure Pt and indicate the formation of
Pt-rich surface.
Figure 6. CV curves (a) and polarization curves (b) of Pt3Cu2,
PtCu, and PtCu2. Inset of (b): Tafel plot of the PtCu/C electro-
catalysts compared with commercial JM Pt/C.
ORR activities were determined by rotating disk
electrode (RDE) approach. The electrode was
polarized between 0.1 and 1V in oxygen-saturated
0.1M HClO4 electrolyte via linear scanning
voltammetry (LSV) at the rotating rate of 1600 rpm.
Commercial Johnson Matthey (JM) Pt/C (20wt% Pt)
catalyst was selected as comparison. It can be seen in
Fig. 6(b) that onset potentials of PtCu/C catalysts are
at least 50 mV more positive than JM Pt/C catalyst,
among which Pt3Cu2/C and PtCu2/C show further 15
mV more positive onset potential than that of PtCu/C.
Mass-transport-corrected specific activities between
0.88 and 0.96 V can be found through Tafel plot in the
inset of Fig. 6(b). The log j-V curve of JM Pt/C catalyst
shows two linear components, implying that oxygen
molecules probably experience a transition between
two distinct mechanism regimes when the potential is
elevated from 0.88 to 0.96 V. For PtxCuy/C catalysts,
their Tafel plots show high linearity and indicate
single-reaction-mechanism and high oxygen
reduction rates at the potential range.[46] It can be
clearly seen that the specific activities of hexa-pod
PtCu/C catalysts are higher than that of JM Pt/C by at
least one order of magnitude. The calculated number
of transferred electrons is 3.86 per O2 molecule, as
shown in Fig. S9. For PtxCuy/C catalysts with TO and
NS PtCu NCs, their ORR activities were also
determined which can be found in Fig. S10. With
almost the same composition, hexa-pod PtCu/C
shows over 3-time enhancement in specific activity
than those of TO and NS PtCu/C catalysts, and 4-time
enhancement in mass activity. Eliminating the
possible effect induced by composition, the
enhancement in activity mainly originates from the
morphology of NCs. It has been reported in abundant
literatures that atoms located at corners and edges of
catalysts are more active catalytic sites. In this work,
the hexa-pod NCs obviously show higher ratio of
atoms at corners and edges according to Fig. 1(d),
which possibly make the surface highly active.[32] In
addition, as is mentioned above, hexa-pod PtCu NCs
expose partially by high-index facets, which could
also endow the surface with higher activity towards
ORR. Specific and mass activity values of various
catalysts at 0.9V are listed in Tab. 1. The highest
activities were achieved by PtCu2/C catalyst, with
specific activity up to 3.7 mA/cm2Pt and mass activity
2.4 A/mgPt, both of which are the highest-ever
reported for PtxCuy alloys, and also higher than
commercial Pt and other Pt-based alloy catalysts.[10,
35] Table 1. ECSAs, specific and mass activities of hexa-pod Pt3Cu2,
PtCu, PtCu2.and TO PtCu.
Sample ECSA
(m2/g)
Specific
Activity
Mass
Activity
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7 Nano Res.
(mA/cm2Pt) (A/mgPt)
Pt3Cu2 55.3 3.4 1.6
PtCu 51.7 3.0 1.2
PtCu2 63.0 3.7 2.4
TO PtCu 52.3 0.8 0.3
Durability is also a crucial factor when evaluating the
practical usability of catalysts. Accelerated durability
tests (ADTs) were carried out for all materials. The
catalysts were polarized with CV curves between 0.6
and 1.1 V at the scanning rate of 50 mV/s, in 0.1 M
HClO4 with continuous oxygen bubbling to keep
electrolyte saturated. ORR activities were determined
every 4000 cycles until it reaches 20000 cycles. The
50th and 20000th cycles are shown in CV and LSV in
Fig. S11-S13 for Pt3Cu2/C, PtCu/C, and PtCu2/C,
respectively. Activities of PtCu2/C catalyst shows most
drop in activity, 32% in specific activity and 50% in
mass activity. With the decrease of Cu/Pt ratio, the
activity declines less during ADT, confirming that
PtxCuy/C with high Cu content tend to have Cu atoms
dissolved easily, thus activity tends to drop faster. For
Pt3Cu2/C, it loses 11.8% in specific activity to 3.0
mA/cm2Pt, and 25% in mass activity to 1.2 A/mgPt. The
amount of degradation, as well as the final activities
after ADT, still meets the DOE targets on durability for
fuel cell cathode catalyst. As a fair comparison, ADT
was also measured for TO PtCu/C catalyst. It is
interesting that PtCu/C with TO NC actually shows
enhanced specific activity during ADT. We can be
inferred from LSV in Fig. S14 that kinetic current at 0.9
V remains almost unchanged. As a result, the
declining ECSA leads to higher specific activity, while
the mass activity remains almost the same. The
microstructures of catalysts after ADT were
characterized with TEM, as shown in Fig. S15. The TO
NCs seem to be smoothed after ADT and show near-
spherical shape without distinguishable increase in
size. For hexa-pod NCs, they tend to show smoothed
pods after ADT without change in size, indicating the
high stability of hexa-pod NCs at ADT operation
condition.
Conclusion
To summarize, we synthesized PtxCuy NCs with hexa-
pod morphology. The synthetic protocol can be
employed to synthesize PtxCuy NCs with various
Cu/Pt ratios. The six pods on NCs grow from (100)
facets of TO seeds, along crystallographic
direction. The hexa-pod NCs expose by (111) and
other facets with high indexes. Due to the high ratio of
atoms at corners and edges, as well as the existence of
high index facets, hexa-pod NCs supported on carbon
black show drastically enhanced catalytic activities
towards ORR, with the highest specific activity 3.7
mA/cm2Pt and mass activity 2.4 A/mgPt obtained at 0.9
V by hexa-pod PtCu2/C catalysts. ADTs show high
stability for alloys with lower Cu/Pt ratio. The highest
durability was obtained from hexa-pod Pt3Cu2/C, with
11.8% lost in specific activity and 25% lost in mass
activity after ADT. The high catalytic performance of
hexa-pod PtxCuy NCs endows them with potential
application as cathode catalysts in PEMFC.
For more instructions, please see our web page at
http://www.thenanoresearch.com/.
Acknowledgements
We acknowledge the Microstructure Laboratory for
Energy Materials (MLEM) at CUP for the technical
support with TEM. We also acknowledge the funding
support from Natural Science Foundation of China
(No. 21303265), Ph.D. Programs Foundation of
Ministry of Education of China (No. 20130007120012)
and Young Talent Award of CUP (No. YJRC-2013-46).
Electronic Supplementary Material: Supplementary
material, such as Characterization, additional graphs
and electrochemistry data, is available in the online
version of this article at
http://dx.doi.org/10.1007/s12274-***-****-*
(automatically inserted by the publisher). References
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Electronic Supplementary Material
PtxCuy nanocrystals with hexa-pod morphology and
their electrocatalytic performances towards oxygen
reduction reaction
Yujing Li1,2 (), Fanxin Quan2, Enbo Zhu3, Lin Chen2, Yu Huang3,4, Changfeng Chen1,2
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
Synthesis of hexa-pod PtCu nanocrystals (NCs): For the synthesis of hexa-pod PtCu NCs, Pt[Pt(NH3)4](NO3)2
and Cu(NO3)2 were used as metallic precursors. Appropriate amount of precursors were dissolved in 14 mL 1, 2-
propanediol (PDO) which also serves as reducing agent. Polyvinylpyrrolidone (PVP) and sodium bromide (NaBr)
were also dissolved in PDO as stabilizing and capping agent. The starting solution was stirred for 10 minutes
and transferred into a Teflon-lined autoclave. The solution was heated from room temperature up to 190 oC at
the rate of 6 oC/min and kept at this temperature for 6 hours, and then cooled down in the air. The products were
separated from the reaction solution by centrifuge. The NCs were dispersed in 2 mL of ethanol, precipitated by
15 mL of acetone, sonicated for 10 min, and then centrifuged at 8000 RPM for 10 min. The washing procedure
was repeated three times. The NCs were stored in ethanol at the concentration of 1mg/mL. The ratios of added
precursors Pt[Pt(NH3)4](NO3)2/Cu(NO3)2 are 3/1, 1/1, and 1/3 for the synthesis of Pt3Cu2, PtCu, and PtCu2,
respectively.
For the synthesis of truncated octahedral and near-spherical PtCu NCs, all conditions were the same as above
except that 1, 4-butanediol (BDO) was used as solvent and reducing agent. The ratios of added precursors
Pt[Pt(NH3)4](NO3)2/Cu(NO3)2 are 1/3 and 1/1 for the synthesis of truncated octahedral and near-spherical PtCu
NCs, respectively.
Preparation of carbon-supported catalysts: Carbon black (Vulcan XC-72, CB) is used as the catalyst support.
For a typical catalyst preparation, CB was dissolved in ethanol at a concentration of 1 mg/mL, and sonicated for
30 min. Appropriate amount of NCs dissolved in ethanol was sonicated and mixed with the CB solution, with
the NC/CB mass ratio fixed at 1/4, further sonicated for another 1 hour, and then stirred overnight. The catalysts
were then collected by centrifuge at the rate of 8000 rpm, washed with ethanol, and then dried under a N2 stream.
As-prepared catalysts were stored in ethanol or isopropanol (IPA).
Characterization: Transmission electron microscopic (TEM) images with α angle tilt was taken on JEOL JEM
2100 at an accelerating voltage of 200 kV. High resolution transmission electron microscopy (HRTEM) and
energy dispersive X-Ray spectroscopy (EDX) were taken on FEI TECNAI F-20 field emission microscope at an
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accelerating voltage of 200 kV. The fast Fourier Transformations (FFTs) were obtained by high resolution image
under TEM. The simulated FFT was also obtained by constructing the same crystal structure with WinXMorph,
along the same zone axis with the real image. Both FFTs were aligned in the same angle (overlapped) to determine
the index of the exposed facets on the pod. All TEM samples were made by casting corresponding NC solution
on carbon film supported on molybdenum grid.
Electrochemical measurement: All electrochemical measurements were carried out in a home-made three-
electrode cell. Catalyst inks were prepared by dispersing NC/CB in IPA/H2O (volume ratio: 3/1) with the
concentration of 2 mg/mL. Appropriate amounts of inks, were pipetted onto a rotating disk electrode (Pine
Instrumentation) made with glassy carbon (GCE) with diameter of 5 mm. The total loading masses of Pt were
controlled at the level of 12 μg/cm2. In this work, the electrode was prepared by a two-step approach. The carbon-
supported electro-catalysts form the first layer, followed by a Nafion® film formed by drying 10 μL of 100-time-
diluted Nafion® solution on top. The catalysts were electrochemically de-alloyed by applying cyclic voltammetric
(CV) scans between 0 and 1.1 V (vs. reversible hydrogen electrode, RHE) in 0.1 M HClO4 electrolyte, at the scan
rate of 50 mV/s. Electrochemical surface area (ECSA) were determined from CV curves recorded. The charges
induced by the adsorption/desorption of hydrogen species were calculated by integrating the area approximately
between 0.05 and 0.4 V (depending on the shape) under the CV curve. ORR was studied by recording linear scan
voltammetic (LSV) curves between 0 and 1.1 V in oxygen-saturated HClO4 electrolyte at the rotating rate of 1600
rpm. The electrolyte was bubbled with oxygen prior to, and during the measurement. The rotation rate
dependent polarization curves were recorded at the rotating rates of 400, 900, 1600, and 2500 rpm. The durability
tests were conducted by applying CV scans up to 4000 cycles each time at the scan rate of 50 mV/s, and recording
CV curves as mentioned above, and then studying ORR afterwards. After each set of measurement, another
round of ADT was applied, and a total of 20000 CV scans was applied to each catalyst for the evaluation of
durability.
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Figure S1. High resolution TEM images of (a) and (b) PtCu2 NCs projected from zone axis. Inset
of (a) is the 3D schematic of a hexa-pod particle with the same projection as the NC. Inset of (b) is
lattice in selected region showing that the zone axis of the NC is .
Figure S2. EDX spectrum PtCu2 by focusing electron beam on one NC.
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Figure S3. EDX spectrum PtCu2 by focusing electron beam on one NC.
Figure S4. TEM images of PtCu NCs captured from reactions stopped at 30min (a), 2h (b), and 6h (c).
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Figure S5. XRD patterns of PtCu2 (red) and PtCu (black), compared with pure Pt and Cu as shown in
black and blue lines.
Figure S6. TEM images of truncated octahedral (TO) PtCu NCs (a, b) and near-spherical (NS) PtCu
NCs (c, d).
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Figure S7. TEM images of CB supported Pt3Cu2 (a, b), PtCu (c, d), and PtCu2 (e, f) NCs.
Figure S8. Comparison of the 1st, 2nd and 10th CV curve of Pt3Cu2 (a), PtCu (b) and PtCu2 (c).
Figure S9. Polarization curves of PtCu2/C catalyst at the rotating rates of 400, 900, 1600 and 2500
rpm (a), and Levich-Koutecký plot of PtCu2/C compared with JM Pt/C at 0.5 V (b).
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Figure S10. Polarization curves (a) and Tafel plots (b) of hexa-pod PtxCuy/C catalysts, truncated
octahedral and near-spherical PtCu NCs.
Figure S11. CV curves (a) and polarization curves (b) of 50th and 20000th cycles during ADT for hexa-
pod Pt3Cu2/C catalysts.
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Figure S12. CV curves (a) and polarization curves (b) of 50th and 20000th cycles during ADT for hexa-
pod PtCu/C catalysts.
Figure S13. CV curves (a) and polarization curves (b) of 50th and 20000th cycles during ADT for hexa-
pod PtCu2/C catalysts.
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Figure S14. CV curves (a) and polarization curves (b) of 50th and 20000th cycles during ADT for TO
PtCu/C catalysts.
Figure S15. TEM images of TO PtCu/C (a, b) and hexa-pod Pt3Cu2/C (c, d) catalysts.
Address correspondence to Yujing Li, Email: [email protected]
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0834_Manuscript_2015-6-8