Palladium nanoparticle-decorated porous carbon spheres as high-activity catalyst in the electrooxidation of alcohol

Article abstract of DOI:10.1166/jnn.2017.14797

Herein, we report a facile technique for preparing a multicomponent electrocatalyst that has demonstrated promising activity in the electrooxidation of alcohol. The synthesis protocol involved the preparation of hyperbranched polynaphthalene (HPN) solid microspheres that were mixed with PdCl₂, and subsequently reduced to Pd (0) using an aqueous NaBH₄ solution. Finally, the HPN microsphere-supported palladium nanoparticles (Pd/HPNC) were obtained after undergoing thermal treatment. The unique porous construction of the microspheres resulted in higher surface area and lower resistance. The evaluation of the Pd/HPNC electrocatalyst showed improved performance, with a high degree of reaction efficiency.

Full text of DOI:10.1166/jnn.2017.14797

Article
Journal of
Nanoscience and Nanotechnology
Vol. 17, 7668–7671, 2017
Copyright © 2017 American Scientific Publishers
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Palladium Nanoparticle-Decorated Porous Carbon
Spheres as High-Activity Catalyst in the
Electrooxidation of Alcohol
1
2
2
2ꢀ∗
2ꢀ∗
Xinhua Huang , Anuraj Varyambath , Mi Ri Kim , Mi-Ra Kim , and Il Kim
1
School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, P. R. China
2
BK21 PLUS Centre for Advanced Chemical Technology, Department of Polymer Science and Engineering,
Pusan National University, Pusan 609-735, Korea
Herein, we report a facile technique for preparing a multicomponent electrocatalyst that has demon-
strated promising activity in the electrooxidation of alcohol. The synthesis protocol involved the
preparation of hyperbranched polynaphthalene (HPN) solid microspheres that were mixed with
PdCl , and subsequently reduced to Pd (0) using an aqueous NaBH solution. Finally, the HPN
2
4
microsphere-supported palladium nanoparticles (Pd/HPNC) were obtained after undergoing thermal
treatment. The unique porous construction of the microspheres resulted in higher surface area and
lower resistance. The evaluation of the Pd/HPNC electrocatalyst showed improved performance,
with a high degree of reaction efficiency.
IP: 91.243.90.56 On: Mon, 15 Oct 2018 12:38:09
Keywords: Alcohol, Carbon, Electrooxidation, Nanoparticle, Palladium, Polynaphthalene.
Copyright: American Scientific Publishers
Delivered by Ingenta
1
. INTRODUCTION
to the development of catalysts, there has also been a
focus on developing new carbon materials as supports to
facilitate the optimization of catalysis performance. High
surface area and well-developed porosity are both essen-
tial traits for catalyst supports in order to achieve high
Noble metal nanoparticles (NP) such as Au or Pd that
are supported on carbon materials have received a great
deal of attention owing to their excellent properties, which
are suited for applications in catalysis, electrochemistry,
sensors, as well as fuel cells and batteries (especially
1
0
activity.
1
–5
methanol fuel cell). These nanocomposites are capable
of exhibiting high activities in oxygen reduction and/or
fuel oxidation because of their high surface-to-volume
ratio, a property responsible for improving the Fermi lev-
We thus prepared porous carbon microspheres with
hyperbranched polynaphthalene (HPN) as carbon source
1
1
in template free. The prepared carbon microspheres as a
support for nanoparticles possess high specific area owing
to their hierarchical pore structure. These porous carbon
microspheres are in tight contact with each other, which
efficiently reduces resistance and increases the catalytic
performance during the reaction. Herein, the activity of the
Pd/HPNC catalyst for alcohol oxidation was investigated
and discussed. Based on our results, it is believed that the
Pd/HPNC catalyst could potentially become an ideal elec-
trochemical catalyst.
6
–8
els in redox reactions. The properties and efficiency of
nanoparticles are strongly dependent not only on their size
distribution, but also on the structure of the support mate-
rials. Thus identifying an appropriate support is the most
efficient approach in improving the catalytic efficiency of
nanoparticles.
Direct methanol fuel cell (DMFC) has promising poten-
tials to be used as a clean and ideal power source. Exten-
sive studies have been devoted toward the development
of new metal catalysts in order to increase the catalytic
activity for methanol electrooxidation. These catalysts are
generally small particles dispersed on conductive supports,
such as carbon material with a high surface area, to achieve
2. EXPERIMENTAL DETAILS
2.1. Materials
Naphthalene (NT, 98%), chloromethyl methyl ether (CME;
95%), PdCl (98%), and sodium borohydride (NaBH , CP)
2
4
9
high dispersion of the catalysts. Recently, in addition
were purchased from Sigma-Aldrich and used without fur-
ther purification. Zinc dichloride (ZnCl , 98%) sourced
from Sigma-Aldrich was dried at 110 C before use.
2
∗
ꢀ
Authors to whom correspondence should be addressed.
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J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 10
1533-4880/2017/17/7668/004
doi:10.1166/jnn.2017.14797

Huang et al.
Pd NP-Decorated Porous Carbon Spheres as High-Activity Catalyst in the Electrooxidation of Alcohol
Dichloroethane (DCE) was purchased from Daejung
Chem. Co. and distilled before use.
were collected (95% yield) and dried overnight in a vac-
uum oven at 110 C.
ꢀ
2
.2. Characterization
2.4. Synthesis of Pd/HPNC Electrocatalyst
The morphologies and size of the Pd/HPNC catalyst were
characterized by X-ray powder diffraction (XRD) analysis
HPN (50 mg) and PdCl (10 mg) were added to a pre-
scribed amount of water and the mixture was stirred at
room temperature for 1 h prior to addition of aqueous
2
(
automatic Philips powder diffractometer with nickel-
NaBH solution (0.4 M, 1 mL). The resulting powdery
filtered Cu Kꢀ radiation). The microstructures of the sam-
ples and energy dispersive X-ray spectroscopy (EDX)
were recorded on an S-4800 scanning electron microscope
4
solid was then separated via centrifugation and dried at
ꢀ
5
0 C for 10 h. Subsequently, the Pd-loaded HPNC catalyst
ꢀ
was thermally treated at 600 C for 5 h under a nitrogen
atmosphere to yield the Pd/HPNC composite. The resulting
spheres were thoroughly washed with deionized water to
ensure complete removal of chloride ions. The palladium
content in the composite catalyst was about 20 wt%.
(
Hitachi, Japan) and a JEM-2100F transmission electron
microscope (JEOL Ltd., Japan). Cyclic voltammetry (CV)
experiments were performed in a three-electrode cell on
a CHI760D electrochemical workstation (Shanghai CH
Instruments, China), consisting of a glassy carbon working
electrode, platinum wire, and Hg/HgO, which were used
as the counter and reference electrodes, respectively.
2
.5. Preparation of the Electrode
Pd/HPNC (5 mg) was dispersed in a mixture of ethanol
®
(
1 mL) and Nafion 117 solution (1 mL, 0.5 wt%, DuPont,
USA) under ultrasonic stirring to form an ink. The ink
5 ꢁL) was then deposited onto the glassy carbon rod’s
2
.3. Synthesis of Hyperbranched Polynaphthalene
HPN) Microspheres
The HPN microspheres were prepared according to a pre-
(
(
surface (4 mm in diameter) and dried at room tempera-
ture overnight. The Pd/HPNC loading was established at
1
1
viously reported method. Briefly, a slurry of ZnCl in
2
DCE (5 mL) and CME was added to a solution of NT (1 g)
in DCE (25 mL) under nitrogen atmosphere, and the mix-
−
2
0
.025 mg cm .
ꢀ
ture was stirred for 18 h at 40 C. The resulting precipitate
was washed with water and methanol until the filtrate was
3. RESULTS AND DISCUSSION
Based on the aforementioned procedures, a hierarchical
clear. The polymer was then dried under vacuum for 8 h at
IP: 91.243.90.56 On: Mon, 15 Oct 2018 12:38:09
ꢀ
8
0 C. After undergoing extraction inCao Sp oy xr ihg l he tt :e Ax mt r aec rt ioc ra n S pc oi er on ut i sf i cn ePt wu bo rl iks ho ef r Hs PN polymer was successfully synthe-
using methanol for 24 h, the desired HPN micr Do sep l hi ve er er es d bys iI zn egd e un st ai ng a low concentration system (i.e., 1 g of NT in
Figure 1. SEM (A) and TEM (B, C) images of the HPN microspheres.
J. Nanosci. Nanotechnol. 17, 7668–7671, 2017
7669

Pd NP-Decorated Porous Carbon Spheres as High-Activity Catalyst in the Electrooxidation of Alcohol
Huang et al.
a high specific surface area.11 This type of hierarchi-
cal porous network improves the ion kinetics and facil-
1
2ꢂ13
itates charge accumulation on the pore surface area.
Such uniform carbon microspheres can thus be utilized
in catalytic and electrochemical reactions, and for build-
ing complex structures. In this study, we demonstrated an
application of the newly developed catalyst system via an
electrochemical catalysis reaction.
The morphologies and sizes of the prepared composite
catalyst are similar before and after carbonization, as con-
firmed via high-resolution SEM images (Figs. 2(A and B)).
Most of the Pd NPs (∼15 nm in diameter) are highly dis-
persed on the surface of the support. The XRD pattern of
Pd/HPNC (Fig. 3(A)) shows the characteristic Pd/HPNC
ꢀ
ꢀ
diffraction peaks at 2ꢃ = 39ꢄ8 and 46.7 , which corre-
spond to the (111) and (200) orientations of the face-
centered cubic structure of palladium crystals, respectively.
The Pd (220) peak was used to calculate particle size
based on Scherrer’s equation: d = Kꢅ/ꢆB cosꢃꢇ, where
d is the average diameter in nm, K denotes the Scher-
rer constant (i.e., 0.89), ꢅ denotes the wavelength of the
X-ray (ꢅ = 0ꢄ154 nm), B denotes the corresponding full
width at half maximum of the (220) diffraction peak, and
ꢃ denotes the Bragg diffraction angle. The size of Pd NPs
in the Pd/HPNC composite was established at around
20 nm, which is highly consistent with the estimated result
obtained from the TEM observation (Figs. 2(C and D)).
Figure 2. SEM and TEM images of the Pd NP-loaded HPNC catalyst.
Before (A, C) and after (B, D) carbonization at 600 C. (D) In particular,
shows the well-defined structure of Pd NPs dispersed on the surface of
HPNC spheres.
ꢀ
3
0 mL of DCE). The scanning electron microscopy (SEM)
and transmission electron microscopy (TEM) images in
Figure 1 confirmed that the HPN network was synthesized
as particles with a well-defined spherical structure, mea-
IP: 91.243.90.56 On: Mon, 15 Oct 2018 12:38:09
The TEM image (Fig. 2(C)) shows that the Pd NPs are
Copyright: American Scientific Publishers
suring 1 ꢁm in diameter.
densely dispersed on the surface of the porous spheres
without apparent overlaps. Even though a minor degree
of overlaps was observed, this is probably a result of the
TEM technique’s limitation as it is only capable of making
two-dimensional (2D) projections of 3D objects, and thus
not indicative of real particle overlaps.
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These particles aggregated into grape-like bunches that
interconnect in different directions, giving rise to a 3D
network that results in the formation of interconnected
meso- and macropores. Figure 1(C) shows the microporous
structure of the microspheres at the edge, which eases the
loading of nanoparticles into the spherical structure.
We evaluated the direct electrocatalytic activity of the
prepared composite via CV measurements. Figure 4 dis-
The obtained carbon support showed good nanostructure
inheritability, and the hierarchical pore structure exhibited
−1
plays the CV curves measured in 1 mol L of KOH
ꢀ
Figure 3. The XRD pattern (A) and EDX spectrum (B) of the Pd/HPNC composite nanocatalyst after calcination at 600 C.
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J. Nanosci. Nanotechnol. 17, 7668–7671, 2017

Huang et al.
Pd NP-Decorated Porous Carbon Spheres as High-Activity Catalyst in the Electrooxidation of Alcohol
For direct methanol fuel cells, different concentrations
of alcohol are desired in order to enhance the power-
to-volume densities. The CV curves for the methanol
oxidation at different concentrations using the Pd/HPNC
electrode are shown in Figure 5. Increasing the concen-
−1
tration of methanol from 0.5 to 2 mol L obviously
enhanced the oxidation peak, proving that the well-defined
porous structure of HPNC is favorable for mass transfer
and able to elevate the energy densities of direct methanol
fuel cells by using higher concentration of methanol.
4
. CONCLUSION
A hybrid Pd/HPNC composite electrocatalyst that shows
promising performance for methanol oxidation was
developed. Owing to the porous structure of the carbon
microspheres used as the support, the Pd/HPNC electro-
catalyst exhibits increased performance in the oxidation of
methanol (especially at a higher concentration), demon-
strating improved stability at larger current densities. These
advantages are attributed to the distinct porous structure of
the HPNC microspheres, which have a larger surface area
with good contact between the carbon spheres, in addition
to the high dispersibility of Pd NPs, which improves mass
transfer and electronic conductivity.
Figure 4. CV curves for different alcohol oxidation reactions using
−1
Pd/HPNC electrode in 1.0 mol L KOH solution (a), Pd/HPNC elec-
−
1
−1
trode in 1.0 mol L KOH and 1.0 mol L methanol solution (b),
−1
−1
and Pd/HPNC electrode in 1.0 mol L KOH and 1.0 mol L ethanol
solution (c) at 25 C. The scan rate used was 50 mV S for all the
experiments.
ꢀ
−1
solution, containing either methanol or ethanol, using the
−1
Pd/HPNC electrode at a scan rate of 50 mV s . The
results clearly indicate that Pd/HPNC showed higher cat-
alytic performance in the oxidation of the two alcohols.
Acknowledgments: This work was supported by the
Basic Science Research Program through the National
In particular, the methanol oxidation exhibited the most
IP: 91.243.90.56 On: Mon, 15 Oct 2018 12:38:09
Research Foundation of Korea (2015R1D1A1A09057372).
negative onset potential and highest c Cu r or ep ny tri dg eh nt s: i At y m. Ae rs i ca an Scientific Publishers
The authors also thank to BK21 PLUS Program for partial
control, the CV curve of Pd/HPNC was measu Dr e ed li vi ne r ea d by Ingenta
financial support.
−1
background solution of 1 mol L KOH aqueous solution.
Note that HPNC without bearing Pd nanoparticles showed
no activity.
References and Notes
1
2
3
4
5
6
7
8
9
. M. Nojima, Y. Ohta, and T. Yokozawa, J. Am. Chem. Soc. 137, 5682
(
2015).
. X. Bian, K. Guo, L. Liao, J. Xiao, J. Kong, C. Ji, and B. Liu, Talanta
9, 256 (2012).
9
. D. Riman, Z. Bartosova, V. Halouzka, J. Vacek, D. Jirovsky, and
J. Hrbac, RSC Adv. 5, 31245 (2015).
. X. Lin, Y. Shang, L. Li, and A. Yu, ACS Sustainable Chem. Eng.
3
, 903 (2015).
. K. Meng, Q. Liu, Y. Huang, and Y. Wang, J. Mater. Chem. A 3, 6873
2015).
(
. B. H. Wu, D. Hu, Y. J. Kuang, B. Liu, X. H. Zhang, and J. H. Chen,
Angew. Chem. Int. Ed. 48, 4751 (2009).
. W. Z. Li, X. Wang, Z. W. Chen, M. Waje, and Y. S. Yan, J. Phys.
Chem. B 110, 15353 (2006).
. S. Y. Wang, S. P. Jiang, T. J. White, J. Guo, and X. Wang, J. Phys.
Chem. C 113, 18935 (2009).
. L. Kong, W. Wei, Q. Zhao, J.-Q. Wang, and Y. Wan, ACS Catal.
2
, 2577 (2012).
0. G. Wen, S. Wu, B. Li, C. Dai, and D. S. Su, Angew. Chem. Int. Ed.
4, 4105 (2015).
1. X. H. Huang, S. Kim, M. S. Heo, J. E. Kim, H. Suh, and I. Kim,
Langmuir 29, 12266 (2013).
1
1
1
5
2. F. Xu, R. J. Cai, Q. C. Zeng, C. Zou, D. Wu, F. Li, X. Lu, Y. Liang,
and R. W. Fu, J. Mater. Chem. 21, 1970 (2011).
Figure 5. CV curves for methanol oxidation using the Pd/HPNC cat-
−
1
alyst in 1.0 mol L KOH solution at different methanol concentrations
(
13. L. D. Zhang, S. D. Zheng, D. E. Kang, J. Y. Shin, H. S, and I. Kim,
RSC Adv. 3, 4692 (2013).
ꢀ
−1
.
25 C). The scan rate was fixed at 50 mV S
Received: 21 July 2016. Accepted: 14 December 2016.
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