Shape-controlled Pd nanocrystal-polyaniline heteronanostructures with modulated polyaniline thickness for efficient electrochemical ethanol oxidation

Article abstract of DOI:10.1039/c9ta08349a

Constructing metal nanocrystal (NC)-polymer heteronanostructures (HNSs) with excellent properties in terms of ethanol adsorption and activation will lead to catalysts with a significantly enhanced electrochemical ethanol oxidation reaction (EOR). Herein, we present a facile and effective synthesis method for shape-controlled Pd NC-polyaniline (PANI) HNSs with various PANI thicknesses, which are achieved by using cubic and octahedral Pd NCs as metal NC seeds. By investigating the electrocatalytic properties of various Pd NC-PANI HNSs and their counterparts, we found that the electrocatalytic properties of the Pd NC-PANI HNSs highly depend on both the exposed facets of Pd NCs and thickness of the PANI coating. The cubic Pd NC-PANI (Pd_cube-PANI) HNSs with cubic Pd NCs and a 0.7 nm PANI coating significantly improved the electrocatalytic performance for the EOR compared to the Pd_cube-PANI HNSs with different PANI coating thicknesses, octahedral Pd NC-PANI (Pd_octa-PANI) HNSs with octahedral Pd NCs, cubic Pd NCs, and commercial Pd/C because of synergistic advantages of the favorable exposed facet and optimal PANI coating thickness.

Full text of DOI:10.1039/c9ta08349a

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J Po lue ran sael od fo Mn aot te rai ad l js u Cs ht emm ai rs gt ri yn sA
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DOI: 10.1039/C9TA08349A
ARTICLE
Shape-Controlled Pd Nanocrystal-Polyaniline
Heteronanostructures with Modulated Polyaniline Thickness for
Efficient Electrochemical Ethanol Oxidation
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
b,c,d
Yusuke Yamauchi,*b,e and Jong Wook Hong *a
Heon Chul Kim,‡ Yena Kim,‡ Yoshio Bando,
DOI: 10.1039/x0xx00000x
Constructing metal nanocrystal (NC)-polymer heteronanostructures (HNSs) with excellent properties in terms of ethanol
adsorption and activation will lead to catalysts with significantly enhanced electrochemical ethanol oxidation reaction (EOR).
Herein, we present a facile and effective synthesis method for shape-controlled Pd NC-polyaniline (PANI) HNSs with various
PANI thicknesses, which are achieved by using cubic and octahedral Pd NCs as metal NC seeds. By investigating the
electrocatalytic properties of various Pd NC-PANI HNSs and their counterparts, we found that electrocatalytic property of
the Pd NC-PANI HNSs highly depends on both the exposed facets of Pd NCs and thickness of the PANI coating. The cubic Pd
NC-PANI (Pdcube-PANI) HNSs with cubic Pd NCs and 0.7 nm PANI coating significantly improved electrocatalytic performance
for the EOR than the Pdcube-PANI HCNs with different PANI coating thicknesses, octahedral Pd NC-PANI (Pdocta-PANI) HNSs
with octahedral Pd NCs, cubic Pd NCs, and commercial Pd/C because of synergistic advantages of the favorable exposed
facet and optimal PANI coating thickness.
abundance of Pt hampers its extensive employment in the PEMFC
system.
Introduction
Recently, shape-controlled metal nanocrystals (NCs) are being
8
-12
increasingly used as an efficient platform for PEMFCs.
The field of environment-friendly and sustainable energy has
become drastically important because of the ever-increasing
consumption of fossil fuels due to high energy demand. Fuel cells
are one of the promising future energy-supplying technologies; in
particular, the polymer electrolyte membrane fuel cell (PEMFC) is of
special interest because of its high energy density, low operation
temperature, and low environmental impact.
tremendous potential as an efficient system to convert chemical
energy into electric energy, its practical application is limited by the
unsatisfactory performance of the catalysts used in it. For example,
Pt is considered to be the most effective catalyst for PEMFCs.
However, poisonous intermediates such as CO are produced during
potential operation, and the Pt catalysts are deformed by
agglomeration and dissolution, diminishing their active surface
catalytic sites and leading to an unfavorable increase in overpotential
for catalytic reactions in the PEMFC. In addition, high price and low
Synthesizing NCs bound by particular facets is an effective strategy
for the preparation of PEMFC catalysts because the surface atomic
arrangements of NCs have a great influence on their catalytic
properties. Thus, shape-controlled NCs with effective facets enhance
the catalytic activity and stability of PEMFCs by mitigating their
1
-3
1
3-16
limitations.
Notably, shape-controlled Pd NCs such as cubic NCs
4, 5
In spite of its
with only {100} facets exhibit remarkable electrocatalytic activities
toward electrochemical oxidation reactions in both acidic and
8
, 17-19
alkaline electrolytes,
indicating their suitability as a substitute
6
, 7
for Pt catalyst. However, despite recent advances in the catalytic
performance of PEMFCs by using metal NCs with well-controlled
shapes, the fuel cells still have insufficient catalytic activity and
stability during catalytic reactions. This is a major limitation for the
commercialization of PEMFCs. One of the promising strategies to
overcome this limitation is to incorporate conductive polymers into
2
0, 21
metal NCs for preparing metal NC-polymer nanostructures.
Of
various conductive polymers, polyaniline (PANI) has been considered
well suited for preparing electrocatalysts because its high electric
a. Department of Chemistry and Energy Harvest-Storage Research Center (EHSRC)
University of Ulsan, Ulsan 44610, South Korea. E-mail: jwhong@ulsan.ac.kr
International Center for Materials Nanoarchitectonics (WPI-MANA) National
Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044,
Japan
Institute of Molecular Plus, Tianjin University, No. 11 Building, No. 92 Weijin
Road, Nankai District, Tianjin, 300072, PR China
Australian Institute for Innovative Materials, University of Wollongong, Squires
Way, North Wollongong, NSW, 2500, Australia
School of Chemical Engineering and Australian Institute for Bioengineering and
Nanotechnology (AIBN), The University of Queensland Brisbane, QLD 4072,
Australia. E-mail: y.yamauchi@uq.edu.au
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
These authors contributed equally to this work.
conductivity originates from π-π conjugation and because it has
b.
22-25
remarkable stability against deformation during operation.
In
addition, PANI can not only modify the electronic structure of metal
NCs to promote the electrooxidation of alcohols, but also trigger the
formation of the surface-bound hydroxide ion (OHads) and make
possible electrooxidation without the formation of surface poisoning
c.
d.
2
6, 27
e.
molecules such as CO.
Therefore, metal NC-PANI nanostructures
are particularly desirable for various electrooxidation reactions of
alcohol. For example, Wang et al. demonstrated that Pd-PANI
nanotube array catalysts showed improved electrocatalytic activity
for ethanol oxidation reaction (EOR) because of the benefits of
†
‡
26
coupling Pd and PANI. In addition, PANI-decorated Pt/C@PANI
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catalysts exhibited better electrochemical performance than pristine Synthesis of octahedral Pd NCs: In a typical synthesis of
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Pt catalysts in an oxygen reduction reaction.28 Nevertheless, in the octahedral Pd NCs, citric acid (60 mg) a
D
O
I: 1
0
.10
3
9
/C
9T
A
08
349A
n
d
A
A
(
6
0
m
g
)
w
e
re
aforementioned studies, the nanostructures were primarily
produced by using metal NCs with irregular shape and size.
Furthermore, the previously reported metal NC−polymer
nanostructures lack fine−control of PANI thickness, which is
significant to the preparation of active metal NC−PANI
electrocatalysts. Against this background, it is desirable to prepare
electrocatalysts using metal NC with effective exposed facets and
PANI and to study their electrocatalytic performance for developing
advanced catalysts for various electrocatalysis.
dissolved in 8.0 mL of deionized (DI) water and mixed with an
aqueous CTAC solution (200 mM) of 4 mL. After, the reaction
mixture was heated until 100 °C under stirring for 10 min.
2 4
Subsequently, Na PdCl (60 mg) was dissolved in deionized
water (3 mL) and then injected into the pre-heated reaction
mixture. The solution maintained at 100 °C for 3 h. The product
was washed three times with DI water by centrifugation at
1
0000 rpm for 20 min.
Herein, we present a facile and effective synthesis method for
shape-controlled Pd NC-PANI core-shell heteronanostructures
Synthesis of Pdcube-PANI and Pdocta-PANI HNSs: In a typical
synthesis of Pdcube-PANI HNSs, 10 mg of cubic Pd NCs anchored
on carbon supports (20 wt%), 1 mL of aniline (13.4 mM), 1 mL
of HCl (1.1 mM) were subsequently injected into 9 mL of DI
water with stirring. Then, the reaction mixture was sonicated
for 10 min, followed by kept at 1 °C for 1 h. After that, 1 mL of
APS (13.4 mM) was added into the reaction mixture with
vigorously stirring. The resultant mixture was kept at 1 °C with
stirring for 2 h. The product was washed 2 times by
centrifugation at 5000 rpm for 20 min. For Pdocta-PANI HNSs, 10
mg of octahedral Pd NCs were used instead of cubic Pd NCs. The
other synthesis conditions are identical with those for the
Pdcube-PANI HNSs. For investigating the polymerization behavior
dependent on acidity of reaction mixture, 0, 11 μM, and 1.1 M
of HCl (1 mL) were added instead of 1.1 mM of HCl (1 mL) for
(HNSs) with controllable PANI thickness, which is achieved by
using cubic and octahedral Pd NCs as metal NC seeds.
Furthermore, since cubic and octahedral Pd NCs are composed
of exclusively {100} and {111} facets, respectively, the successful
syntheses of cubic and octahedral Pd NC-PANI HNSs with
tunable thickness of the PANI coating can provide an effective
platform for understanding the influences of metal surfaces and
PANI thickness on catalytic performance. Thus, the proposed
method will allow developing the efficient electrocatalysts for
alcohol electrooxidation. The present study demonstrates that
the electrocatalytic properties of Pd NC-PANI HNSs toward EOR
depend greatly on both, the exposed facets of Pd NCs and the
thickness of the PANI coating. It was found that {100}-facet-
terminated cubic Pd NC-PANI (Pdcube−PANI) HNSs composed of
an ultrathin PANI coating (thickness: 0.7 nm) exhibited the
largest electrocatalytic activity and stability for EOR because of
the synergistic advantages of the favorable exposed facet and
optimal PANI coating thickness.
employing pH 7, 6, and 1, respectively. For preparing Pdcube
PANI , Pdcube-PANI , Pdcube-PANI , and Pdcube-PANI HNSs,
.7, 26.8, 53.6, and 107.2 mM of aniline (1 mL) were added into
-
Ⅰ
Ⅱ
Ⅲ
Ⅳ
6
reaction mixtures instead of 1 mL of aniline (13.4 mM),
respectively.
Experimental
Electrochemical
measurements:
Electrochemical
measurements were conducted in a three-electrode cell using
Bio-logic EC-Lab SP-300. Pt wire and Ag/AgCl (3 M NaCl) were
used as counter and the reference electrodes, respectively. All
electrochemical data was obtained at room temperature. To
prepare the working electrode, a 10 μL of the catalyst ink
containing 1 μg of Pd based on Inductively coupled plasma-
optical emission spectrometry (ICP-OES) was dropped onto a
glassy carbon electrode (GCE, diameter: 5 mm), and then dried
at room temperature. The dried GCE was cleaned
electrochemically by 50 potential cycles between -0.85 and 0.35
V vs Ag/AgCl at a scan rate of 50 mV s- in 0.1 M KOH. The
Chemical and Materials: Sodium tetrachloropalladate
(
Na
polyvinylpyrrolodone (PVP, Mw 55,000), aniline (≥99.5 %), and
cetyltrimethylammonium chloride (CTAC, 25 wt% in H O) and
2
PdCl
4
,
98%), potassium bromide (KBr, ≥99%),
2
Nafion® perfluorinated resin solution (5 wt%) were obtained
from Sigma-Aldrich. Palladium on activated carbon (Pd/C, 20
wt%) and ammonium peroxydisulfate (APS, 98.0%) was
obtained from Alfa aesar. Hydrochloric acid (35.0 ~ 37.0%) and
potassium hydroxide (KOH, 95.0%) were obtained from
Samchun. L-ascorbic acid (AA, 99.5%) was obtained from
Daejung. Citric acid (99 %) was obtained from Acros Organic.
Other chemicals were reagent grade and deionized water with
a resistivity of greater than 18.3 MΩ·cm was used in the
preparation of reaction solutions.
1
2
electrolyte solutions were purged with N gas for 30 min before
performing electrochemical experiments. The CVs of all
catalysts were obtained between –0.85 and 0.35 V vs Ag/AgCl
-1
at a scan rate of 50 mV s in 0.1 M KOH or 0.1 M KOH + 0.5 M
ethanol. For CO-stripping experiments, the surface of the
catalysts loaded on GCE was saturated with CO by purging CO
gas in 0.1 M KOH while holding the working electrode at –0.3 V
versus Ag/AgCl for 15 min, and then CO dissolved in electrolyte
Synthesis of cubic Pd NCs: In a typical synthesis of cubic Pd NCs,
PVP (105 mg), AA (60 mg), and KBr (600 mg) were dissolved in
8
.0 mL of deionized (DI) water, and then the reaction mixture
was heated until 80 °C under stirring for 10 min. Subsequently,
Na PdCl (57 mg) was dissolved in deionized water (3 mL) and
2
was removed by purging with N gas for 30 min. The CO
2
4
stripping experiments were performed between –0.85 and 0.35
then injected into the pre-heated reaction mixture. It
maintained at 80 °C for 3 h. The product was washed three
times with DI water by centrifugation at 10000 rpm for 20 min.
-1
V vs Ag/AgCl at a scan rate of 20 mV s .
2
| J. Name., 2012, 00, 1-3
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ARTICLE
Characterizations: TEM and SEM images of the prepared indicate the formation of a PANI coating layer onVitehweArstiuclrefOanclienes
catalysts were obtained by using Jeol JEM-2100F and Jeol JEM- of cubic Pd NCs attached on the carbonDsOuIp: p10o.1r0ts39(/FCi9gT. A10d8)3.4T9oA
7
210F, respectively. FTIR measurement was performed by using further verify the formation of the PANI coating on the cubic Pd
an IRAffinity-1S (SHIMADZU). ICP-OES measurement was NCs, we obtained the Fourier transform IR (FTIR) spectra of the
carried out using a Spectroblue-ICP-OES (Ametek). X-ray Pdcube-PANI HNSs and pristine polyaniline (Fig. S2). The FTIR
diffraction (XRD) measurement were conducted on a Rigaku spectrum of the Pdcube-PANI HNSs reveals distinctive
−1
D/MAX2500V/PC. X-ray photoelectron spectroscopy (XPS) characteristic peaks such as those of amide (710 cm ), aromatic
−
1
−1
measurements were conducted on a ThermoFisher K-alpha.
amine (1135 cm ), carbonyl groups (1650 cm ), which
2
2, 23, 25, 30
correspond to PANI.
This observation implies that the
Results and discussion
surface of Pd NCs was covered with PANI.
To synthesize Pdcube-PANI HNSs, cubic Pd NCs with an average
edge length of 17.9 ± 0.6 nm (Fig. S1a) were first produced by
reduction of disodium tetrachloropalladate (Na PdCl ) in an
2 4
aqueous solution containing polyvinylpyrrolidone (PVP),
ascorbic acid (AA), and potassium bromide (KBr) (see the
Experimental details), which were used as seed NCs for the
preparation of Pdcube−PANI HNSs. Subsequently, the cubic Pd
NCs were anchored on carbon supports (Fig. S1b). Then, Pdcube
-
PANI HNSs were obtained by surface-initiated polymerization of
aniline on the cubic Pd NCs attached on the carbon supports, as
shown in Scheme 1. The typical transmission electron
microscopy (TEM) image of the Pdcube-PANI HNSs shown in Fig.
1
a shows that cubic nanostructures are highly monodispersed
on carbon supports. The single crystalline nature of the
products is observed in a high-resolution TEM (HRTEM) image
of an individual nanostructure (inset of Fig. 1b), and the
measured average thickness of the PANI coating is 0.7 ± 0.1 nm.
The d-spacing of adjacent lattice fringes along the edge of the
cubic NC is 1.95 Å, which matches well with that of the {200}
2
9
planes of face-centered cubic (fcc) Pd. This corroborates that
Fig. 1 (a) Low- and (b) high-magnification TEM images of Pdcube
-
the exposed surfaces of cubic NCs of the Pdcube-PANI HNSs are PANI HNSs. The insets in Fig. 1b show the visible lattice fringes and
100} facets, which are further confirmed from the fast Fourier an FFT pattern obtained from a cubic Pd NC in the Pdcube-PANI
transform (FFT) pattern viewed along the [001] zone axis (inset
of Fig. 1b). Fig. 1c presents the X-ray diffraction (XRD) pattern
of the Pdcube-PANI HNSs indexed to the fcc Pd crystalline
structure. The cubic shape of the product is also clearly
{
HNSs, respectively. (c) XRD pattern of Pdcube-PANI HNSs. (d)
HAADF-STEM and corresponding HAADF-STEM-EDS elemental
mapping images of Pdcube-PANI HNSs.
The procedure for synthesizing octahedral Pd NC-PANI (Pdocta
-
confirmed from a high-angle annular dark-field scanning TEM PANI) HNSs is identical to that for Pdcube-PANI HNSs, except for
HAADF-STEM) image (Fig. 1d). For examining the composition the introduction of octahedral Pd NCs with an average edge
of the Pdcube-PANI HNSs, an elemental mapping image was length of 21.3 ± 0.7 nm (Fig. S1c,d) instead of cubic Pd NCs in
obtained by HAADF-STEM-energy-dispersive X-ray the former (Scheme. 1). As seen in the TEM image (Fig. 2a), the
(
spectroscopy (HAADF-STEM-EDS). The intense nitrogen (N) octahedral NCs are clearly shown in a majority of products. The
signals observed around a cubic Pd NC and carbon supports HRTEM image of an octahedral NC in Pdocta-PANI HNS reveals
their high crystallinity (inset of Fig. 2b). The thickness of the
PANI coating is 0.7 ± 0.1 nm. The d-spacing of 2.25 Å between
adjacent lattice fringes matches well with that of the {111}
planes of fcc Pd, indicating that the {111}-facet-bounded
octahedral Pd NCs are included in the Pdocta-PANI HNSs (Fig.
3
1
2
b). The XRD patterns of the Pdocta-PANI HNSs display the
characteristic diffraction peaks seen in the reflections of the fcc
Pd crystalline nature of the prepared Pd-PANI HNSs (Fig. 2c).
The HAADF-STEM and HAADF-STEM-EDS mapping images of the
Pdocta-PANI HNS clearly show the formation of PANI coating
layers on the octahedral Pd NCs (Fig. 2d). Moreover, similar to
Pdcube-PANI HNSs, distinct intrinsic peaks corresponding to PANI
were also observed in the FTIR spectrum of the products (Fig.
S2).
Scheme 1 Schematic illustration for the formation of Pd-PANI
HNSs.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Given that the acidity of the reaction mixture for producing PANI HNSs with PANI coatings of different VitehwiAcrktincleeOssnelinse.
metal NC-polymer nanostructures greatly influences the Compared to an aniline concentration ofD1O.2I:210m.1M039f/oCr9sTtAa0n8d34a9rdA
polymerization behavior and deformation of metal NCs during
polymerization,3
2, 33
employing the optimal acidity is critical to
synthesizing metal NC-polymer nanostructures with the desired
morphology. The control experiments, wherein the HCl
concentration gradually increased under otherwise identical
conditions for Pdcube-PANI and Pdocta-PANI HNSs, were
conducted to verify the significance of acidity of the reaction
mixture (see the Experimental details). When reaction mixtures
Fig. 3 TEM images of (a, a’) standard Pdcube-PANI, (b, b’) Pdcube-PANI
, (c, c’) Pdcube-PANI , (d, d’) Pdcube-PANI , and (e, e’) Pdcube-PANI
HNSs.
Pdcube-PANI HNSs with a PANI coating of 0.7 ± 0.1 nm (Fig. 3a,a’),
Pdcube-PANI HNSs with thinner PANI coatings of 0.5 ± 0.1 nm
were formed when less amount of aniline (0.67 mM) was used
Fig. 3b,b’). In contrast, Pdcube-PANI HNSs with thicker PANI
coatings (1.4 ± 0.2, 1.9 ± 0.2, and 2.7 ± 0.1 nm) were produced
for Pdcube-PANI , and Pdcube-PANI HNSs,
Ⅰ
Ⅱ
Ⅲ
Ⅳ
Ⅰ
(
Ⅱ
, Pdcube-PANI
Ⅲ
Ⅳ
respectively, by injecting 2.44, 4.87, and 9.75 mM of aniline,
respectively (Fig. 3c-e).
Considering that previous studies the electrochemical
oxidation have shown that pure PANI exhibits negligible
3
8
Fig. 2 (a) Low- and (b) high-magnification TEM images of Pdocta
PANI HNSs. The insets in Fig. 1b show the visible lattice fringes and
an FFT pattern obtained from an octahedral Pd NC in the Pdocta
PANI HNSs, respectively. (c) XRD pattern of Pdocta-PANI HNSs. (d)
HAADF-STEM and corresponding HAADF-STEM-EDS elemental
mapping images of Pdocta-PANI HNSs.
-
electrocatalytic activity toward EOR, the surface Pd atoms in
Pd-PANI HNSs can be presumed as catalytic active sites for EOR.
This signifies that an appropriate thickness of PANI coating,
which can not only efficiently facilitate the migration of
chemicals to the Pd surface, but also make possible obtaining
the full benefits of the Pd NC-PANI combination, effectively
-
with pH 6 and 7 were prepared, which have lower acidities improves their catalytic performance. Accordingly, investigating
compared with the standard condition (pH 4), a negligible PANI the optimal thickness of PANI coating is significant for designing
coating was observed on the Pd surfaces (Fig. S3a-d). This can the efficient Pd-PANI HNSs for electrocatalysis. To find the
be attributed to the predominant formation of the oligomers by optimal thickness of PANI coating, the EOR activities of various
the mixed ortho-para coupling than the polymerization of Pdcube-PANI HNSs with different PANI coating thicknesses,
3
4
aniline for PANI under pH higher than 4 ; this can hamper the standard Pdcube-PANI, Pdcube-PANI
Ⅰ Ⅱ Ⅲ
, Pdcube-PANI , Pdcube-PANI
growth of PANI on the surface of Pd NCs. On the other hand, , and Pdcube-PANI HNSs, were evaluated in an alkaline
Ⅳ
well-defined PANI coatings with very low thickness of 0.7 ± 0.1 electrolyte. The current densities obtained in cyclic
nm were obtained under the standard condition of pH 4 (Fig. voltammograms (CVs) were normalized based on the mass of
S3e,f). However, severe deformation of cubic Pd NCs was the catalysts loaded on a glassy carbon electrode (GCE) used as
observed in the reaction mixture with pH 1 (Fig. S3g,h). This is a working electrode. Fig. 4 shows that standard Pdcube-PANI
−1
attributed to the promoted oxidative etching due to the HNSs exhibited the highest mass activity of 1472.6 mA mg ,
3
5-37
presence of excessive HCl in the reaction mixture.
These which is substantially higher than the mass activities of Pdcube
-
−1
−1
results clearly show that the acidity of the reaction mixture PANI
highly influences the formation behavior of metal NC-polymer PANI
Ⅰ
(790.6 mA mg ), Pdcube-PANI
Ⅱ
(480.9 mA mg ), Pdcube
-
−1
−1
Ⅲ
(153.6 mA mg ), and Pdcube-PANI
Ⅳ
(84.1 mA mg ). As
nanostructures. We found that an acidity of around pH 4 is expected, this indicates that the EOR activity of Pd-PANI HNSs is
appropriate to produce Pd-PANI HNSs without deformation of highly dependent on their PANI coating thickness. Plotting the
Pd NCs. Considering these results, we could control the obtained EOR activities as a function of the PANI coating
thickness of the PANI coating of the Pd-PANI HNSs by using thickness results in a volcano curve according to the mass
different amounts of aniline. Fig. 3 shows the various Pdcube
-
activities of the different Pd-PANI HNSs (Fig. 4b). As the PANI
4
| J. Name., 2012, 00, 1-3
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ARTICLE
thickness decreased from 2.7 to 0.7 nm, there was a gradual Pd-PANI HNCs outperform previously reported Pd-based
View Article Online
DOI: 10.1039/C9TA08349A
enhancement of the EOR activities, whereas much thinner PANI conductive polymer electrocatalysts (Table S1).
coatings (0.5 nm) of Pdcube-PANI HNSs had attenuated
The electrochemical stability of Pdcube-PANI HNSs was
electrocatalytic activity. These results reveal that the reduction evaluated via accelerated durability test (ADT) and compared
of PANI coating thickness can lead to enhanced electrocatalytic with those of Pdocta-PANI HNSs, cubic Pd NCs, and commercial
activity, which is attributed to the improved mass transfer of the Pd/C catalysts. During the measurements, the catalysts loaded
on a working electrode were applied between −0.85 and 0.3 V
2
in N -saturated 0.1 M KOH and 0.5 M ethanol. After 500 cycles,
the Pdcube-PANI HNSs, Pdocta-PANI HNSs, cubic Pd NCs, and
commercial Pd/C catalysts exhibited 1113.0, 286.5, 379.8, and
−1
3
49.7 mA mg , respectively, which signifies losses of 24.4, 28.9,
Fig. 4 (a) CVs obtained with various Pdcube-PANI HNSs with different
PANI coating thicknesses in 0.1 M KOH + 0.5 M ethanol. (b) Plot of
the obtained EOR activities as a function of the PANI coating
thickness. (c) CVs obtained with various catalysts in 0.1 M KOH + 0.5
M ethanol. (d) Catalytic activities of various catalysts.
reaction species; however, excessively thin PANI coating (<0.7
nm) worsens the electrocatalytic activity by attenuating the
intrinsic positive benefits of PANI. On the basis of these findings,
Pdcube-PANI and Pdocta-PANI HNSs with a PANI coating 0.7 nm
thick obtained from the standard syntheses were chosen for
further electrochemical investigation.
To explore the influence of the surface morphology of Pd NCs
and formation of the PANI coating on the catalytic activity of
catalysts, the electrocatalytic properties of the Pdcube-PANI and
Pdocta-PANI HNSs were estimated, and the results were
compared with those of cubic Pd NCs and commercial Pd/C
Fig. 5 CVs obtained before and after ADT for (a) Pdcube-PANI HNSs,
(
b) Pdocta-PANI HNSs, (c) cubic Pd NCs, and (d) Pd/C catalysts in 0.1
-1
M KOH + 0.5 M ethanol at scan rate of 50 mV s . (e) The
normalized mass activity of various catalysts after 500 cycles. (f)
Mass activities of various catalysts before and after ADT.
4
6.3, and 49.9% in mass activity, respectively (Fig. 5). The
2
catalysts. Fig. S4a shows the CVs of different catalysts in N -
superior catalytic stabilities of the Pdcube-PANI and Pdocta-PANI
HNSs arise from the PANI coating because the coating can
effectively reduce the shape-deformation of the Pd NCs in the
Pdcube-PANI and Pdocta-PANI HNSs. Indeed, the TEM images of
Pd-PANI HNSs such as Pdcube-PANI and Pdocta-PANI HNSs
obtained after 500 cycles show that their initial morphologies
were well preserved (Fig. S6a,b), while cubic Pd NCs and Pd/C
catalysts underwent shape deformation (Fig. S6c,d). In addition,
the electrochemical stabilities of prepared samples and
commercial Pd/C toward EOR were investigated by comparing
their chronoamperometric curves obtained at -0.15 V vs
Ag/AgCl. The Pdcube-PANI HNSs also exhibited the highest
electrocatalytic stability compared with Pdocta-PANI HNSs and
Pd/C with respect to the steady-state oxidative current during
saturated 0.1 M KOH. The electrochemically active surface
areas (ECSAs) of the nanostructures obtained from the charge
required for oxygen species desorption in the CVs of the
2
−1
nanostructures were 33.0, 29.3, 23.2, and 47.9 m g for the
Pdcube-PANI HNSs, Pdocta-PANI HNSs, cubic Pd NCs, and Pd/C
catalysts, respectively (Fig. S4b). Fig. 4c shows the CVs obtained
for the EOR with different catalysts in 0.1 M KOH and 0.5 M
ethanol. It is apparent that the Pdcube-PANI HNSs showed the
highest EOR activity among the different catalysts. The mass
−1
activity of the Pdcube-PANI HNSs (1472.6 mA mg ) is roughly 3.7,
.1, and 2.1 times higher than those of the Pdocta-PANI HNSs
2
−1
−1
(402.9 mA mg ), cubic Pd NCs (707.1 mA mg ), and commercial
−1
Pd/C (697.9 mA mg ) catalysts, respectively (Fig. 4d).
Furthermore, the corresponding current density of the Pdcube
PANI HNSs is 4.46 mA cm , which is roughly 3.3, 1.5, and 3.1
times higher than those of the Pdocta-PANI HNSs (1.35 mA cm ),
cubic Pd NCs (3.07 mA cm ), and commercial Pd/C (1.43 mA
-
1
000 s (Fig. S9).
−
2
The significantly enhanced catalytic activity and stability of
Pdcube-PANI HNSs can be attributed to the synergistic effects of
the formation of the PANI coating and the Pd{100} facets.
Notably, the Pdcube-PANI HNSs in close contact with the cubic Pd
−2
−
2
−2
cm ) catalysts, respectively (Fig. S5). Notably, the as-prepared
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J. Name., 2013, 00, 1-3 | 5
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J Po lue ran sael od fo Mn aot te rai ad l js u Cs ht emm ai rs gt ri yn sA
Page 6 of 7
ARTICLE
Journal Name
NC and PANI coating showed improved catalytic performance Pdcube-PANI HNSs with PANI coating of differentVitehwiAcrktincleeOssnelinse.
for EOR than cubic Pd NCs. This may be a result of the The Pdcube-PANI HNSs with a PANI coatinDgOoI:f10th.1i0c3k9n/Ces9sTA00.8734n9mA
modification of the electronic structure of Pd surface atoms showed remarkable catalytic activity and stability for the EOR
through the delocalization of electrons in the Pd-PANI HNSs. To than their Pd-PANI counterparts, cubic Pd NCs, and commercial
elucidate the change in the electronic structure of surface Pd Pd/C. The XPS measurement and CO-stripping experiments
atoms in the Pdcube-PANI HNSs, X-ray photoelectron exhibited that the superb performance can be attributed to
spectroscopy (XPS) measurement on the Pd 3d core-level was modified electronic structures and high CO tolerance of Pdcube
conducted with the Pdcube-PANI HNSs and cubic Pd NCs. Fig. S7 PANI HNSs by PANI coating and Pd{100} facets. Such
shows the negative shift in the binding energies of the Pdcube hybridization technique of metal catalysts with functional
-
-
PANI HNSs compared with those of pure cubic Pd NCs. For polymers will be highly useful to further improve the
instance, the Pd 3d3/2 and Pd 3d5/2 binding energies of the electrochemical performance of metal catalysts in future,
Pdcube-PANI HNSs are 341.15 and 335.90 eV, respectively, while although many efforts have been made to prepare various types
4
7-51
those of the cubic Pd NCs are 341.55 and 336.20 eV, of metal catalysts with various shapes before.
respectively. The modified electronic structure of Pd in Pdcube
PANI HNSs may provide the optimal bonding strength between
surface-bound COCH ((COCH ads) and surface Pd atoms of the
Pdcube-PANI HNSs. This results in enhanced electrocatalytic
activities toward EOR because the decomposition of (COCH
-
Conflicts of interest
There are no conflicts to declare.
3
3
)
3 ads
)
is a rate-determining step in the electrochemical ethanol
oxidation pathway.3
9, 40
In addition, it is widely accepted that Acknowledgements
surface-bound OH (OHads) on the surface of catalysts can
This work was supported by the National Research Foundation of
Korea (NRF) grant funded by the Korea government (MSIT) (NRF-
4
1-43
promote EOR by reacting with (COCH
sufficiently provide OHads to the surface Pd atoms bound to
COCH owing to its high OHads formation ability and thus
enhance the catalytic activity of the Pd-PANI HNSs.
)
3 ads
.
PANI can
2
019R1H1A1079341) and Priority Research Centers Program
3
through the National Research Foundation of Korea(NRF) funded by
the Ministry of Education, Science and Technology (NRF-
2
6, 27
On the
other hand, despite the similar electronic structures of the
Pdcube-PANI and Pdocta-PANI HNSs as seen from the XPS
measurement (Fig. S7), through the electrocatalytic
experiments, we found that the Pdcube-PANI HNSs had larger
EOR activities than Pdocta-PANI HNSs. For investigating the
2
019R1A6A1A11053838)
higher catalytic activity of the Pdcube-PANI HNSs compared to References
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the Pdcube-PANI HNSs for the oxidative removal of CO to CO is
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