Fabrication of Nitrogen-Doped Mesoporous-Carbon-Coated Palladium Nanoparticles: An Intriguing Electrocatalyst for Methanol and Formic Acid Oxidation

Article abstract of DOI:10.1002/asia.201600173

Inspired by the attractive catalytic properties of palladium and the inert nature of carbon supports in catalysis, a concise and simple methodology for in situ nitrogen-doped mesoporous-carbon-supported palladium nanoparticles (Pd/N-C) has been developed by carbonizing a palladium dimethylglyoximate complex. The as-synthesized Pd/N-C has been exfoliated as a fuel cell catalyst by studying the electro-oxidation of methanol and formic acid. The material synthesized at 400 °C,namely, Pd/N-C-400,exhibitssuperior mass activity and stability among catalysts synthesized under different carbonization temperaturesbetween300 and 500 °C. The unique 1D porous structure in Pd/N-C-400 helps better electron transport at the electrode surface, which eventually leads to about five times better catalytic activity and about two times higher stability than that of commercial Pd/C. Thus, our designed sacrificial metal-organic templatedirected pathway becomes a promising technique for Pd/N-C synthesis with superior catalytic performances. Wrap it up: A new sacrificial template-directed synthesis for the preparation of in situ nitrogen-doped carbon-supported palladium nanostructures (Pd/N-C) as electrocatalysts is reported. The electrocatalysts were tested in the methanol oxidation reaction (MOR) and formic acid oxidation reaction (FAOR; see figure) as model reactions for applications in fuel cells.

Full text of DOI:10.1002/asia.201600173

DOI: 10.1002/asia.201600173
Full Paper
Supported Catalysts
Fabrication of Nitrogen-Doped Mesoporous-Carbon-Coated
Palladium Nanoparticles: An Intriguing Electrocatalyst for
Methanol and Formic Acid Oxidation
Chaiti Ray,[a] Soumen Dutta,[a] Ramkrishna Sahoo,[a] Anindita Roy,[a] Yuichi Negishi,[b] and
Tarasankar Pal*[a]
Abstract: Inspired by the attractive catalytic properties of
palladium and the inert nature of carbon supports in cataly-
sis, a concise and simple methodology for in situ nitrogen-
doped mesoporous-carbon-supported palladium nanoparti-
cles (Pd/N-C) has been developed by carbonizing a palladium
dimethylglyoximate complex. The as-synthesized Pd/N-C has
been exfoliated as a fuel cell catalyst by studying the elec-
tro-oxidation of methanol and formic acid. The material syn-
thesized at 4008C,namely, Pd/N-C-400,exhibitssuperior mass
activity and stability among catalysts synthesized under dif-
ferent carbonization temperaturesbetween300 and 5008C.
The unique 1D porous structure in Pd/N-C-400 helps better
electron transport at the electrode surface, which eventually
leads to about five times better catalytic activity and about
two times higher stability than that of commercial Pd/C.
Thus, our designed sacrificial metal–organic templatedirect-
ed pathway becomes a promising technique for Pd/N-C syn-
thesis with superior catalytic performances.
Introduction
of this strong affinity-based interaction, the doped material ap-
pears to be a better catalyst.[17–19] Recently, research into
carbon materials as a support for palladium nanocatalysts has
been in huge demand and applied in various fields, even in
sensing activities,[20]surface-enhanced Raman scattering spec-
troscopy (SERS),[21]hydrogen storage, and batteries,[22–30] in addi-
tion to catalysis[31–34] and electrocatalysis.[35–44] Therefore, the se-
lection of nitrogen-doped carbon support for palladium nano-
particles as a catalyst is clear.
Supported materials influence the catalytic efficiency of metal
nanoparticles by affecting their valence state, size, available
surface area, and stability.[1–3] To capitalize on the advantages
of easy recovery and reusability of noble-metal nanocatalysts
in heterogeneous catalysis, various organic or inorganic sup-
ports, such as zeolites,[4,5] polymers,[6]mesoporous silica,[7]inor-
ganicoxides,[8]graphene,[9,10] carbon nanotubes,[11,12] activated
carbon,[13]and porous carbon,[14] are now in great demand.
Among them, carbon materials have become a popular sup-
port candidate for of features including large surface area, tun-
able functionalities, chemical inertness, easy availability, effec-
tive channels, porosity, and cost effectiveness. Furthermore, to
harness electron density in the support, heteroatoms such as
nitrogen or boron are often utilized as dopants to enhance the
catalytic activity in carbon-supported materials. For instance,
doping with electron-rich nitrogen atoms results in the in-
crease in p-electron density in carbon supports.[15,16] This nitro-
gen doping helps the nanoparticles to adhere strongly onto
the carbon support owing to enhanced electrostatic interac-
tions between the support and metal nanoparticles. As a result
The general approach for synthesis of palladium nanoparti-
cles fabricated on carbon supports involves multistep proce-
dures in which separately synthesized carbon materials are
mixed with metal nanoparticles.[26,45]For example, Ramaprabhu
et al. synthesized palladium-decorated nitrogen-doped gra-
phene by solar exfoliation of graphene oxide (GO)with a mix-
ture of melamine and PdCl2.[30] Wang et al. synthesized Pd@N-
doped carbon by means of a two-step procedure:the first step
was the preparation of carbon support andthe second step
was deposition of the palladium nanoparticles on the sup-
port.[46] Wang et al. also prepared Pd@N-doped carbon in
a three-step procedure.[15] First, a mixture of glucose, poly(ionic
liquid)s, and borax was treated at 2008C for 8 h. The obtained
product was then carbonized at 5508C for 4 h and 10008C for
1 h to give a nitrogen-doped carbon material. In the third step,
palladium was deposited on the support by reducing
PdCl2with NaBH4. Feng et al. synthesized porous worm-like pal-
ladium nanotubes (Pd WNTs) in a two-step process.[37] In the
first step, the palladium dimethylglyoximate complex was cal-
cined at 4508C for 2 h in a muffle furnace to giveworm-like
PdO nanotubes. Then, Pd WNTs were synthesized by the re-
duction of PdO nanotubes in the presence of NaBH4.
[a] C. Ray, S. Dutta, R. Sahoo, A. Roy, Prof. Dr. T. Pal
Department of Chemistry, Indian Institute of Technology
Kharagpur (India)
E-mail: tpal@chem.iitkgp.ernet.in
[b] Prof. Dr. Y. Negishi
Department of Applied Chemistry, Tokyo University of Science
Tokyo1628601 (Japan)
Supporting information for this article can be found under http://
dx.doi.org/10.1002/asia.201600173.
Chem. Asian J. 2016, 11, 1588 – 1596
1588
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Herein, we report on the development of a novel, inexpen-
sive, cost-effective, easy synthetic strategy for the production
of porous nitrogen-doped carbon-supported palladium (Pd/N-
C) nanomaterials, involving palladium dimethylglyoximate
complex as a precursor, which is easily prepared from dime-
thylglyoxime (dmg) and PdCl2in an acidic solution. The Pd/N-C
nanomaterial was obtained after carbonization of palladium di-
methylglyoximate complex at 4008C for 1 h. This method is
easy, does not need skilled operation techniques, and is eco-
nomically viable with an impressive yield of Pd/N-C, even from
a waste precipitate from the undergraduate laboratory. No
extra precious precursor materials or post-synthetic purification
steps are required in this case.
The gradual exhaustion of fossil fuels has resulted in an in-
crease in the demand for renewable energy sources. In recent
years, fuel cells have attracted tremendous interest from both
energy and environmental considerations by converting chem-
ical energy from fuels such as methanol or formic acid into
electrical energy.[47,48]Platinum is known to be the best catalyst
for the alcohol oxidation reaction, but platinum-based catalysts
have several disadvantages, such as high cost, low abundance,
and easy CO poisoning.[49] Considering these facts, palladium-
based catalysts have benefits in fuel cell applications over plat-
inum-based catalysts in terms of cost and CO poisoning behav-
ior. Nowadays, nitrogen-doped mesoporous-carbon-supported
palladium nanostructures are becoming an attractive material
in the field of electrocatalysis. Numerous reports are found on
alcohol oxidation in basic medium with palladium-based nano-
material owing to better stability of the electrocatalyst than
that in acidic medium.[35,46,50] Again, for improved stability and
robustness in palladium-based catalysts, porous carbon is used
as a support for the catalyst.[30] Herein, the as-prepared nitro-
gen-doped 1D framework of nanocatalysts with a carbon sup-
port are applied as electrocatalysts for fuel cell application. The
oxidation of small organic molecules, such as methanol and
formic acid, is a notable feature.
Scheme 1. Schematic representation of the synthesis of Pd/N-C nanomateri-
al.
thylglyoximate complexes was supported by observations of
Sharpe and Wakefield,[51]who proved that weak PdÀPd bond-
swere present in the palladium dimethylglyoximate complex.
This kind of bonding is explained by the facile overlap of filled
dz2 orbitals of palladium and results in a 1D columnar struc-
ture, leading to the formation of [Pd(dmg)2]n nanorods. Car-
bonization of the complex converts the coordination complex
into Pd/N-C, with appreciable shrinkage of the primary rod-like
structure. Carbonization of the complex at 4008C for 1 h re-
sults in the formation of palladium(0) on the nitrogen-doped
carbon support. The FESEM images in Figure 1band c reveal
the porous nature of Pd/N-C at low and medium magnifica-
tion; this indicates the formation of interconnected chains
with retention of the 1D structure. Reduction of palladium(II)
and the affinity of palladium for nitrogen engenders the
doped structure of Pd/N-C.
Detailed morphology characterization was performed
through TEM measurements. Figure 2a–c displays TEM images
of the Pd/N-C nanostructure at low, medium, and high resolu-
tion, respectively. The porous nature of the Pd/N-C nanostruc-
ture is attained through the interconnected chains. The defi-
nite contrast discrepancy is accounted in the TEM images,
which confirm the porous architecture of the synthesized ma-
terials. Figure 2c clearly reveals that the Pd/N-C composite is
composed of palladium nanoparticles about 20 nm in diameter
uniformly distributed in a continuous carbon support. HRTEM
analysis indicates that the sample contains a thin layer of
carbon (ꢀ3 nm) coating on the surface of palladium nanopar-
ticles. The carbon matrix is mostly amorphous in nature,
Results and Discussion
The dmg precursor is a well-known complexing agent for vari-
ous metal ions, especially for nickel(II) and palladium(II). In our
designed synthetic procedure for porous Pd/N-C nanomateri-
als, this inexpensive and commonly available compound, dmg,
was judiciously introduced as a source of carbon support as
well as for nitrogen doping. An aqueous solution of PdCl2 was
used as a precursor for the noble metal. Carbonization of the
initially prepared palladium dimethylglyoximate complex from
the precursor was performed at 4008C for 1 h to produce the
desired product. The presence of hydrogen in the carboniza-
tion environment plays a key role in the reduction of palladiu-
m(II) to palladium(0). The effortless and scalable synthetic strat-
egy for producing Pd/N-C is outlined in Scheme 1.
Field-emission (FE) SEM images indicate a smooth surface,
with rectangular rod-like structures of palladium dimethyl-
glyoximate complex with diameters of 400–700 nm and
lengths of 7–10 mm. The strong interactions between central
palladium(II) ions of adjacent square-planar palladium dime-
Figure 1. FESEM images of a) [Pd(dmg)2]n nanorods and Pd/N-C-400 (Pd/N-C
calcined at 4008C) nanostructure in low (b) and medium (c) resolution.
Chem. Asian J. 2016, 11, 1588 – 1596
www.chemasianj.org
1589
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
rous interconnections for effective electron transport and an
admirable catalytic activity of Pd/N-C are achieved if the com-
plex is carbonized at 4008C.
To demonstrate the nature of doping in the as-synthesized
carbonaceous materials, elemental area mapping analysis of
carbon, nitrogen, and palladium was performed by means of
scanning transmission electron microscopy (STEM). Figure 3
displays a homogeneous distribution of all elements over a 1D
matrix. The linescan and energy-dispersive X-ray spectroscopy
(EDXS) analyses support the synthesis of Pd/N-C quantitatively
(Figure S3 in the Supporting Information).
Figure 2. TEM images of the Pd/N-C-400 nanostructure at a) low, b) medium,
and c) high magnification. d) High-resolution (HR) TEM image; the fringe
spacing of Pd0is given in the inset. e) Close-up view of the fringe spacing of
Pd0.
whereas palladium nanoparticles are crystalline (Figure 2d).
Figure 2e supports the formation of palladium(0) nanoparticles
embedded in the carbon material, for which the fringe spacing
of 0.225 nm corresponds to the (111) interplanar spacing of
palladium nanoparticles. Generally, the type of structure of
metal nanoparticles embedded on a carbon support can only
be achieved by pyrolysis of metal–organic complexes in an
inert atmosphere.[52] Figure S1 in the Supporting Information
presents HRTEM images of Pd/N-C-400 from four different po-
sitions. In each image, the red dotted circle represents palladi-
um nanoparticles with diameters of about 17–20 nm and the
black arrow indicates the uniform carbon layer of about 3 nm
thickness on the palladium nanoparticle surface. Thus, HRTEM
analysis proves the formation of a uniform carbon coating on
the surface of palladium nanoparticles.
Figure 3. a) STEM image of the Pd/N-C-400 nanostructure and the corre-
sponding mapping images of the elements b) carbon, c) palladium, and
d) nitrogen.
To determine the palladium content (in wt%) in the Pd/N-C
nanocomposites in the bulk phase, inductively coupled plasma
mass spectrometry (ICP-MS) analysis was performed. The re-
sults show that the palladium contents in Pd/N-C-300, -400,
and -500 are 41.87, 35.55, and 34.82 wt%, respectively. The pal-
ladium content (wt%)in Pd/N-C determined by ICP-MS is simi-
lar to that determined by EDXS analysis.
The morphology and composition of the as-synthesized Pd/
N-C nanostructures are highly dependent on the carbonization
temperature of the palladium dimethylglyoximate complex. To
evaluate the effect of temperature on morphology alterations
of the interconnected chain, TEM analysis of Pd/N-C-300 and
Pd/N-C-500 was carried out and the corresponding TEM
images are shown in Figure S2 in the Supporting Information.
A question may be asked about the choice of carbonization
temperature of the palladium dimethylglyoximate complex.
Destruction of the complex skeleton, reduction of palladium(II)
to palladium(0) and retention of the porous carbon skeleton
are the desired features of the prescribed thermal treatment.
An investigation of the retention of parental morphology, that
is, a 1D structure with porous architecture, is of prime impor-
tant in the present case. To arrive at the desired morphology,
we conducted thermolysis of the complex at different temper-
atures. As expected, the lower temperature (3008C) could not
carbonize the rod-like morphology of the palladium dimethyl-
glyoximate complex quantitatively (Figure S2a in the Support-
ing Information). However, the higher temperature (ꢀ5008C)
destroys the 1D network of Pd/N-C (Figure S2b in the Support-
ing Information). Thus, retention of 1D network with mesopo-
BET analysis was performed to investigate the porous nature
and specific surface area of the Pd/N-C-400 nanostructure. The
nitrogen gas adsorption–desorption isotherm of this porous
material is shown in Figure 4a,from which the IV-type isotherm
confirms the mesoporous nature of the material with a clear
H1-type hysteresis loop in the 0.5–1.0 P/P0 range.[53] This materi-
al shows a nitrogen gas uptake capacity of about 136 ccgÀ1
and a calculated specific surface area of 124.095 m2 gÀ1 with
a pore volume 0.158 ccgÀ1. According to the Barret–Joyner–
Halenda (BJH) plot, the calculated half-pore diameter of the
material is 1.802 nm. The pore size distribution graph in Fig-
ure 4b confirms the mesoporous nature of the material. Fig-
ure S4a and c in the Supporting Information shows the nitro-
gen adsorption–desorption isotherms of the Pd/N-C-300 and
-500 nanostructures with surface areas of 33.38 and
60.19 m2 gÀ1, respectively. The calculated half-pore widths are
1.97 and 1.95 nm for the Pd/N-C-300 and -500 nanostructures,
respectively (Figure S4b and d in the Supporting Information).
Chem. Asian J. 2016, 11, 1588 – 1596
www.chemasianj.org
1590
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
0:9 lKa1
B2qcosqmax
ð1Þ
L ¼
in whichL is the size of palladium nanoparticles, lKa1 is the X-
ray wavelength (l=0.154 nm), B2q is the half-peak width, and
qmax is the Bragg angle. The calculated palladium nanoparticle
size is 19.2 nm for the Pd/N-C-400 nanostructure, which is in
good agreement with TEM results.
X-ray photoelectron spectroscopy (XPS) analysis was execut-
ed to scrutinize the electronic state and bonding configura-
tions of the elements (Pd, N, C) present in the as-synthesized
materials (Figure 6). The binding energy peaks of C 1s in the
XPS spectrum were observed at 284.6, 285.8, and 288.8 eV;
these could be attributed to the formation of the sp2-graphitic
structure, CÀN/CÀO/CÀC, and C=N/C=O, respectively. The in
situ sp2-graphitic structure formation facilitates electron trans-
fer throughout the nanostructure by improving the electron
conductivity properties. Analysis of the Pd 3d XPS spectra con-
firms the formation of pure palladium(0) in the Pd/N-C materi-
Figure 4. a) Nitrogen adsorption-desorption isotherm and b) pore size distri-
bution plot of the porous Pd/N-C-400 nanostructure.
Thus, Pd/N-C-400 has a higher specific surface area than those
of the other two as-prepared nanostructures.
The XRD pattern in Figure 5 confirms the crystalline nature
of the as-prepared Pd/N-C samples; sharp diffraction peaks at
40.1 (111), 46.5 (200), 68.2 (220), 82.3 (311), and 86.68 (222) are
in good agreement with characteristic peaks of palladium
(JCPDS, card no. 05-0681) and a broad peak at 23.48 (002) indi-
cates the presence of graphitic carbon. For comparison, the
XRD pattern of [Pd(dmg)2]n is also provided, in which the
strong peak at about 108 confirms the formation of a metal–
organic complex; this peak was found to disappear after car-
bonization, which confirmed the complete conversion of
metal–organic complex into our desired Pd/C material. The
palladium nanoparticle size in the Pd/N-C-400 nanostructure
can be evaluated by using the Scherrer equation [Eq. (1)]:[54,55]
Figure 6. XPS spectra of a) Pd 3d, b) N 1s, and c) C 1s elements of Pd/N-C-
Figure 5. XRD pattern of [Pd(dmg)2]n (black) and Pd/N-C-400 (red).
400.
Chem. Asian J. 2016, 11, 1588 – 1596
www.chemasianj.org
1591
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
als with increasing annealing temperature. The binding ener-
gies of Pd0 3d5/2 and Pd0 3d3/2 for Pd/N-C-400 and -500 are ob-
served to be comparable (335.3 and 340.4 eV for Pd/N-C-400;
335.4 and 340.6 eV for Pd/N-C-500), whereas there is a positive
shift in the case of Pd/N-C-300 (336.6 and 341.7 eV). Again, the
prominent peaks for the Pd 3d5/2 and Pd 3d3/2 electronic states
of palladium(II) appear at around 338.7 and 341.7 eV in the
case of Pd/N-C-300. Deconvolution of the Pd 3d XPS spectrum
shows almost the same content of Pd0 (52.7%) and Pd2+
(47.3%) in the Pd/N-C-300 material. XPS measurements of Pd/
N-C-400 show that most of the palladium species are present
as palladium(0) nanoparticles (85.1%), with a small amount of
Pd2+ (14.9%) in the nitrogen-doped carbon support. Upon in-
creasing the carbonization temperature to 5008C, characteristic
palladium peaks indicates a majority of palladium(0) (89%)
species in the Pd/N-C-500 material. The binding energies of N
1s for pyridinic, pyrrolic, and quaternary-N groups experience
a positive shift with increasing annealing temperature owing
to electron density transfer from nitrogen to palladium. Lower-
ing of the binding energy of Pd 3d in Pd/N-C-400 and -500 is
in good agreement with previous reports by the groups of
Wang[46] and Zhou.[56] The pyridinic and pyrrolic nitrogen
atoms can donate their p-electron density to support the
carbon network. As a result, the nitrogen-doped carbon sup-
port stabilizes palladium(0) by preventing its reoxidation and
facilitates the reduction of palladium(II) to palladium(0).[57,58]
The XPS spectra of Pd 3d, C 1s, and N 1s of various as-syn-
thesized materials are shown in Figure S5 in the Supporting In-
formation. The contents of palladium, nitrogen, and carbon in
as-synthesized Pd/N-C-300, -400, and -500 are summarized in
Table S1 in the Supporting Information. These results are in
good agreement with those obtained by ICP-MS analysis. As
a result, temperature-dependent, as-synthesized Pd/N-C nano-
structures vary in catalytic activity, as discussed below.
Figure 7. Cyclic voltammograms of GCEs modified with four catalysts in 1m
KOH at a scan rate 50 mVsÀ1. The current density was calculated by consid-
ering the surface area of GCE (0.07 cm2).
in whichQ0 is the surface charge obtained from the area under
the CV curves of palladium oxide reduction and q0 represents
the charge required for desorption of a monolayer of oxygen
from a clean palladium surface,that is, 424 mCcm .
À2 [35]In all
cases, 7 mg catalyst was used. For ECSA calculations, ICP-MS
analysis data forthe palladium content (wt%) in Pd/N-C was
used. Cyclic voltammograms of all catalysts in KOH medium
are shown in Figure 7. The CV profile of commercial Pd/C in
KOH medium demonstrates a clear peak for PdO reduction at
À0.46 V versus Ag/AgCl (Figure S6 in the Supporting Informa-
tion). The hydrogen desorption region is not prominent, but
a small hump is found. For as-synthesized Pd/N-C catalysts,
peaks of both PdO reduction and the hydrogen region are
prominent.
This type of anomaly in the cyclic voltammograms is mainly
found owing to differences in electrocatalytic activities. During
the cathodic sweep, the peak for palladium oxide reduction
appeared at around À0.39 V versus Ag/AgCl for Pd/N-C-400;
this is nearly the same as that for Pd/N-C-300 and identical to
that for Pd/N-C-500. Moreover, Pd/N-C-400 has a palladium
oxide reduction peak at a potential about 70 mV higher than
that of commercial Pd/C (À0.46 V), which indicates less interac-
tion between metal and oxygen in the case of the as-synthe-
sized Pd/N-C-400 nanostructure. The ECSA values of Pd/N-C-
400 (160.71 m2 gÀ1) are 1.9 and 1.5 times higher than those of
Pd/N-C-300 (84.95 m2 gÀ1) and -500 (103.91 m2 gÀ1), respective-
ly. It is important to note that all Pd/N-C catalysts have higher
ECSA values than that of Pd/C (59.4 m2 gÀ1). This information
leads us to draw the conclusion that Pd/N-C-400 has more
electrocatalytically active sites than other samples as a result
of its highly porous structure from the self-assembly of the
unique chain-like morphology.[59]
Electrocatalysis
The electrocatalytic activity of as-prepared Pd/N-C was tested
by considering the methanol oxidation reaction(MOR) and
formic acid oxidation reaction (FAOR) as model reactions for
fuel cell applications. These small organic molecules were elec-
trochemically oxidized to convert their chemical energy into
clean electrical energy, and the whole process was studied by
cyclic voltammetry (CV). The influence of annealing tempera-
ture on the catalytic behavior of our catalysts was also investi-
gated. Commercial 20 wt% Pd/C (Pd/C) was used as a reference
catalyst to estimate the effectiveness of our synthesized mate-
rial.
The electrocatalytic efficiency of a catalyst is highly depen-
dent on the available electrochemically active surface area
(ECSA). The ECSAs of as-prepared nanocatalysts and Pd/C were
evaluated from cyclic voltammograms by using a modified
glassy carbon electrode (GCE) in 1m KOH at a scan rate
50 mVsÀ1 (Figure 7). The ECSA values were calculated by using
Equation (2):
In Figure 8a, the oxidation of methanol by Pd/N-C shows
two oxidation peaks. The peak of the forward scan arises be-
cause of methanol oxidation to form CO2 and the peak for the
reverse scan indicates the oxidation of carbonaceous species
generated during methanol oxidation in the forward scan.
CV measurements at a scan rate of 50 mVsÀ1 exhibit specific
peak current densities for Pd/N-C-400 that are 2, 3, and 8 times
higher than those of Pd/N-C-500, -300, and commercial Pd/C,
ECSA ¼ Q0=q0
ð2Þ
1592
Chem. Asian J. 2016, 11, 1588 – 1596
www.chemasianj.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
commercial Pd/C (À0.1 V). Again, to determine the actual elec-
trocatalytically active site in the composite, the MOR has been
performed by using nitrogen-doped carbon material as a cata-
lyst. As shown in Figure S7 in the Supporting Information, ni-
trogen-doped carbon material has a negligible specific peak
current density compared with that of Pd/N-C-400. Thus, from
the above results, we can infer that palladium atoms act as the
catalytically active site in the Pd/N-C nanostructure. The mass
activities (normalized with respect to the amount of catalyst)
at À0.164 V (vs. Ag/AgCl) of the four catalysts are 760.8, 358.5,
À1
259.1, and 158.3 mAmgPd for Pd/N-C-400, -500, -300, and
commercial Pd/C, respectively; these values suggest better
mass utilization of the precious palladium element to gain
maximum efficiency. Additionally, to investigate the tolerance
level of all catalysts, including commercial Pd/C, towards accu-
mulated CO during catalysis, the ratio of forward and back-
ward currents (If/Ib) was calculated. A higher If/Ib ratio implies
greater antipoisoning activity of the catalyst. Herein, as-synthe-
sized Pd/N-C-400 exhibits a higher If/Ib ratio (2.52; Table 1) than
those of the other catalysts, that is, Pd/N-C-300 (2.26), Pd/N-C-
500 (2.32), and commercial Pd/C (2.18). Moreover, Pd/N-C-400
demonstrates a higher specific activity (normalized with re-
spect to ECSA) than those of the other three catalysts (Table 1).
The chain-like array of Pd/N-C-400increases the electro-oxi-
dation behavior towards CH3OH by lowering the onset poten-
tial and exhibiting significantly higher peak current density
than the other three tested catalysts. The electrocatalytic activ-
ity of Pd/N-C-400, in terms of mass activity, is higher than
those of various reports in the literature reports,[35,41,60–66]as
shown in Table 2,which suggests that our synthesized material
shows better catalytic activity for methanol oxidation.
Herein, the electrocatalytic activity of Pd/N-C and Pd WNTs
(synthesized according to the procedure of Feng et al.[37]) were
investigated towards methanol oxidation in alkaline medium
(Figure S8 in the Supporting Information). The Pd/N-C catalyst
shows a higher mass activity (760.8 mAmgPdÀ1) than that of Pd
WNTs (149.6 mAmgPdÀ1). Thus, the above-stated synthetic pro-
cedure to prepare Pd/N-C has delivered a new and excellent
electrocatalyst with enhanced mass activity.
Figure 8. a) Cyclic voltammograms of various catalysts with a mixture of
0.5m KOH and 0.5m methanol at a scan rate 50 mVsÀ1. b) Onset potentials
of four electrocatalysts for methanol oxidation in alkaline medium. c) Chro-
noamperometric (CA) curves with various catalysts during the MOR at an ap-
plied potential of À0.20 V. In each case, current density is plotted with re-
spect to the surface area of the GCE (0.07 cm2).
Furthermore, the electrochemical stability of the catalysts for
methanol oxidation was studied by means of the CA method
at a given potential À0.2 V in 0.5m KOH and 0.5m CH3OH. The
results in Figure 8c reveal an initial rapid decrease in current
density for all catalysts, owing to the chemisorption of carbo-
naceous materials on the surface of the palladium catalyst
during electro-oxidation, and then a slower current density
decay of Pd/N-C-400 with time (5000 s) in comparison with Pd/
N-C-500, -300, and Pd/C. This data supports the higher restraint
of Pd/N-C-400 towards as-produced carbonaceous species,
respectively. In addition, as seen in Figure 8b, the onset poten-
tial of methanol oxidation in alkaline medium for Pd/N-C-400 is
significantly lower than those of Pd/N-C-500, -300, and com-
mercial Pd/C; this supports the higher catalytic efficiency of
Pd/N-C-400 for methanol oxidation over other catalysts. The
oxidation peak potential in the forward scan for Pd/N-C-400 is
observed at À0.17 V, which is similar to those of other catalysts
synthesized under different annealing temperatures, except for
Table 1. A comparison of the electrocatalytic activity of four catalysts towards the MOR.
À1
Catalyst
ECSAPd [m2 gÀ1
]
Current density [mAcmÀ2
]
If/Ib
Mass activity[mAmgPd
]
Specific activity [mAcmÀ2
]
Pd/N-C-300
Pd/N-C-400
Pd/N-C-500
Pd/C
84.95
160.71
103.91
59.4
8.29
24.92
11.78
3.01
2.26
2.53
2.32
2.18
259.1
760.8
358.5
158.3
0.305
0.473
0.345
0.266
Chem. Asian J. 2016, 11, 1588 – 1596
www.chemasianj.org
1593
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
The cyclic voltammogram of 0.25m formic acid in the pres-
ence of 0.5m KOH at a scan rate 50 mVsÀ1 (Figure 9a) shows
a peak for formic acid oxidation at À0.345 V in the forward
scan. On the backward scan, the peak near À0.374 V arises
owing to the further oxidation of carbonaceous intermediates
to form CO2.
Table 2. Comparison of the mass activity of MOR with various electroca-
talysts.
Material[a]
Mass activity
Ref.
À1
[mAmgPd
]
Pd-chestnutbur
HPN-CNT
PdNP/low-defect graphene
Pd/XC-72
566.3
ꢀ300
128.8
280
500
198
[35]
[41]
[60]
[61]
[62]
[63]
[64]
[65]
The Pd/N-C-400 catalyst exhibits an onset potential at
À0.84 V, which is 0.19 V lowers than that of Pd/C. Furthermore,
the formic acid oxidation peak of Pd/N-C-400 has a susceptibili-
ty to be more negative than with the other three catalysts,
which supports the proposal of the best electrocatalytic oxida-
tion activity of Pd/N-C-400 towards formic acid.
The mass activity of Pd/N-C-400 (525.7 mAmgPdÀ1) is 1.6,
2.4, and 5 times higher than those of Pd/N-C-500, -300, and
Pd/C, respectively (Table 3). Also, the specific activity of Pd/N-
C-400 (0.327 mAcmÀ2) is 0.97, 1.3, and 1.85 times higher than
those of Pd/N-C-500, -300, and Pd/C, respectively (Table 3). Fig-
ure S11in the Supporting Information shows a comparison of
the CV profiles of as-synthesized Pd/N-C-300, -400, -500, and
commercial Pd/C catalysts with respect to mass activity and
specific activity for the electro-oxidation of methanol and
formic acid.
Pd/CMS
PtÀPd NPs/RGO
PdÀNiÀP NPs/C
Pd NPs on ordered
mesoporous carbon
Pd34Pt66 NPs
ꢀ380
197
ꢀ100
[66]
this work
Pd/N-C-400
760.8
[a] HPN-CNT=hollow palladium nanosphere–carbon nanotube, Pd/CMS=
palladium carbon microspheres, RGO=reduced graphene oxide.
such as CO, during methanol oxidation in alkaline medium
compared with other catalysts. The stability of the electrocata-
lysts was investigated by comparing the CV profiles of metha-
nol electro-oxidation before and after 1500 cycles. Figure S9in
the Supporting Information reveals that the decay of the origi-
nal current density of Pd/N-C-400 is about 45.5%, whereas Pd/
N-C-500, -300, and commercial Pd/C exhibit decreases in cur-
rent density of about 68.4, 85.5, and 89.3%, respectively. The
methanol electro-oxidation reaction was also performed in al-
kaline medium with commercial Pt/C as catalyst. The CV mea-
The CA data reveals that the current density decreases
sharply with time and reaches a plateau after about 500 s for
all Pd/N-C and Pd/C catalysts (Figure 9b). A magnified view re-
veals the comparatively higher stability of Pd/N-C-400 than the
other tested materials (Figure 9b, inset). These results provide
direct evidence of the better electrocatalytic performance and
surement clearly reveals
a specific peak current density
2.1 times lower (11.9 mAcmÀ2) and a mass activity 1.3 times
lower (600 mAmgPtÀ1) for commercial Pt/C than those for Pd/
N-C-400. In Figure 7e, the CV profile of the MOR represents
about 87% decay in current density after 1500 cycles. Thus,
the above results support the excellent electrocatalytic activity
and stability of as-synthesized Pd/N-C-400 towards the MOR.
The annealing temperature tunes the catalytic activities of
the synthesized Pd/N-C nanostructures. Heating the palladium
dimethylglyoximate complex at 3008C fails to convert it com-
pletely into the corresponding Pd/C-N-300 sample, as con-
firmed by TEM and XPS analyses, whereas the synthesized ma-
terial lost the unique arrangement upon heating at 5008C. On
the other hand, Pd/N-C-400 possesses a majority of palladi-
um(0) content with an exclusive 1D porous architecture and
electro-active sites that result in superior electrocatalytic activi-
ties and outstanding durability. TEM images of Pd/N-C-400
after the 1st and 1500thcycles of MOR (Figure S10 in the Sup-
porting Information) demonstrate insignificant changes in the
morphology to ensure the robustness of the as-synthesized
catalyst.
Herein, the electro-oxidation properties of carbon-supported
palladium catalysts was again investigated with formic acid in
alkaline medium. The low toxicity, non inflammability, and low
penetration efficiency makes formic acid a perfect candidate as
a fuel in polymer electrolyte membrane fuel cells.[38,67] The low
cost and lower CO poisoning of palladium compared with plat-
inum supports the use of as-synthesized PdÀN/C as a catalyst
for the FAOR.
Figure 9. a) Cyclic voltammograms of four catalysts in 0.25m formic acid and
0.5m KOH at 50 mVsÀ1. b) CA curves with various catalysts during the FAOR
at an applied potential of À0.35 V (inset shows an enlarged view of iversust
curve). In each case,the current density is plotted with in respect to the sur-
face area of the GCE (0.07 cm2).
Chem. Asian J. 2016, 11, 1588 – 1596
www.chemasianj.org
1594
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Pd/N-C-400, and Pd/N-C-500, respectively, according to the carbon-
ization temperature.
Table 3. A comparison of the mass activity and specific activity values of
four catalysts towards the FAOR.
Catalyst
ECSAPd
[m2 gÀ1
Mass activity
Specific activity
[mAcmÀ2
]
Electrochemical Measurements
À1
]
[mAmgPd
]
The electrochemical behavior of Pd/N-C was investigated by using
a three-electrode cell in a CHI 660E system with a GCE as a working
electrode, a platinum counter electrode, and a saturated solution
of Ag/AgCl (saturated KCl) as a reference electrode. First, the GCE
electrode was cleaned by polishing with 1, 0.3, and 0.05 mm alumi-
na powder consecutively. Then, under ultrasonic conditions, the
electrode was cleaned with distilled water and absolute ethanol,
respectively, for 10 min and completely dried in air at room tem-
perature. For electro-oxidation experiments, Pd/N-C catalysts dis-
persed in water (7 mL, 1 mgmLÀ1) were drop-cast on the GCE and
dried in air for 3 h. In this case, as a binder of the catalyst on the
GCE, a 0.01% aqueous solution of Nafion (7 mL) was applied. Then,
the completely dried GCE was introduced into the three-electrode
setup and, with a fixed scan rate of 50 mVsÀ1, all CV studies were
performed in different electrolyte solutions.
Pd/N-C-300
Pd/N-C-400
Pd/N-C-500
Pd/C
84.95
160.71
103.91
59.4
219.1
525.7
328.6
105.2
0.257
0.327
0.316
0.177
stability of the product obtained at an annealing temperature
of 4008C, namely, Pd/N-C-400, than those obtained at other
temperatures (Pd/N-C-500, Pd/N-C-300).
The better electrocatalytic activity of the as-synthesized Pd/
N-C-400 catalyst than that of commercial Pd/C can be ex-
plained by considering electronic aspects. When electron-rich
nitrogen atoms are introduced into palladium nanoparticles
embedded in a carbon matrix, a shift in the d band of palladi-
um takes place and, as a result, the Pd03d5/2 peak (335.3 eV) of
Pd/N-C-400 is negatively shifted by about0.5 eV relative to that
of commercial Pd/C (335.8 eV; Figure S12 in the Supporting In-
formation). Therefore, nitrogen doping provides a bigger elec-
tron-donation effect towards palladium nanoparticles, leading
to improved electrocatalytic activity of Pd/N-C-400.
Acknowledgements
We are thankful to the DST and CSIR, New Delhi, India, for fi-
nancial assistance, and the IIT Kharagpur for instrumental sup-
port. We also acknowledge Prof. S. B. Majumder (IIT Kharagpur)
for his help in annealing the palladium dimethylglyoximate
complex, and Prof. G. Mohan Rao (IISC, Bangalore) for the XPS
analyses of the Pd/N-C nanostructures.
Conclusion
We reported a facile and simple reaction strategy for the for-
mation of in situ Pd/N-C through carbonizing the metal–organ-
ic complex [Pd(dmg)2]n at 4008C. The as-synthesized Pd/N-C
samples acted as excellent electrocatalysts with superior stabil-
ity towards methanol and formic acid oxidation in alkaline
medium than that of Pd/C, owing to higher electrochemical
active surfaces as a result of the 1D nanostructure. This superb
electrocatalytic activity makes the porous Pd/N-C nanostruc-
ture a promising electrocatalyst for the construction of an effi-
cient fuel cell. The porous Pd/N-C nanostructure can also be
used as a useful catalyst in Suzuki coupling reactions under
ambient conditions.
Keywords: electrochemistry
palladium · supported catalysts
·
doping
·
nanoparticles
·
[1] S. Song, S. Yin, Z. Li, P. K. Shen, R. Fu, D. Wu, J. Power Sources 2010, 195,
1946–1949.
[2] J. Yang, C. Tian, L. Wang, H. Fu, J. Mater. Chem. 2011, 21, 3384–3390.
[3] X. S. Zhao, W. Z. Li, L. H. Jiang, W. J. Zhou, Q. Xin, B. L. Yi, G. Q. Sun,
Carbon 2004, 42, 3263–3265.
[4] L. Djakovitch, K. Koehler, J. Am. Chem. Soc. 2001, 123, 5990–5999.
[5] X. Liu, W. Liu, J. Li, Y. Zhang, L. Lang, L. Ma, B. Zhang, Ind. Eng. Chem.
Res. 2010, 49, 8826–8831.
[6] Y. Yang, S. Ogasawara, G. Li, S. Kato, J. Mater. Chem. A 2013, 1, 3700–
3705.
[7] S. El Hankari, A. E. Kadib, A. Finiels, A. Bouhaouss, J. J. E. Moreau, C. M.
Crudden, D. Brunel, P. Hesemann, Chem.Eur. J. 2011, 17, 8984–8994.
[8] P. D. Stevens, J. Fan, H. M. R. Gardimalla, M. Yen, Y. Gao, Org. Lett. 2005,
7, 2085–2088.
[9] G. M. Scheuermann, L. Rumi, P. Steurer, W. Bannwarth, R. Mülhaupt, J.
Am. Chem. Soc. 2009, 131, 8262–8270.
Experimental Section
Materials and Instruments
[10] S. Moussa, A. R. Siamaki, B. F. Gupton, M. S. EI-Shall, ACS Catal. 2012, 2,
145–154.
[11] A. R. Siamaki, Y. Lin, K. Woodberry, J. W. Connell, B. F. Gupton, J. Mater.
Chem. A 2013, 1, 12909–12918.
Information regarding materials and instruments are briefly dis-
cussed in the Supporting Information.
[12] B. Cornelio, G. A. Rance, M. Laronze-Cochard, A. Fontana, J. Sapi, A. N.
Khlobystov, J. Mater. Chem. A 2013, 1, 8737–8744.
[13] X. Sun, Y. Li, Angew. Chem. Int. Ed.2004, 43, 597–601; Angew. Chem.
2004, 116, 607–611.
[14] P. Veerakumar, R. Madhu, S.-M. Chen, V. Veeramani, C.-T. Hung, P.-H.
Tang, C.-B. Wang, S.-B. Liu, J. Mater. Chem. A 2014, 2, 16015–16022.
[15] P. Zhang, Y. Gong, H. Li, Z. Chen, Y. Wang, Nat. Commun. 2013, 4, 1593.
[16] P. Chen, L. M. Chew, A. Kostka, M. Muhler, W. Xia, Catal. Sci. Technol.
2013, 3, 1964–1971.
Synthesis of Pd/N-C
An aliquot (20 mL)of a 10À2 msolution of dmg was prepared in eth-
anol. The solution was added dropwise to a 10À2 msolution of
PdCl2 (10 mL; in water) and immediately a golden yellow precipi-
tate formed. The precipitate was collected by filtration and washed
several times with distilled water and dried at 808C. Carbonization
of the sample was performed in a tube furnace under a mixture of
Ar/H2 (Ar/H2 =90:10vol%) at different temperatures (300, 400, and
5008C) for 1 h. The obtained materials were labeled Pd/N-C-300,
[17] R. R. Salunkhe, J. Tang, Y. Kamachi, T. Nakato, J. H. Kim, Y. Yamauchi, ACS
Nano 2015, 9, 6288–6296.
Chem. Asian J. 2016, 11, 1588 – 1596
www.chemasianj.org
1595
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[18] J. Tang, R. R. Salunkhe, J. Liu, N. L. Torad, M. Imura, S. Furukawa, Y. Ya-
mauchi, J. Am. Chem. Soc. 2015, 137, 1572–1580.
[44] H. Qian, H. Huang, X. Wang, J. Power Sources 2015, 275, 734–741.
[45] N. Karousis, G. E. Tsotsou, F. Evangelista, P. Rudolf, N. Ragoussis, N. Tag-
matarchis, J. Phys. Chem. C 2008, 112, 13463–13469.
[46] H. Jin, T. Xiong, Y. Li, X. Xu, M. Li, Y. Wang, Chem. Commun. 2014, 50,
12637–12640.
[47] M. Liu, R. Zhang, W. Chen, Chem. Rev. 2014, 114, 5117–5160.
[48] A. Chen, P. Holt-Hindle, Chem. Rev. 2010, 110, 3767–3804.
[49] M. Gong, G. Fu, Y. Chen, Y. Tang, T. Lu, ACS Appl. Mater. Interfaces 2014,
6, 7301–7308.
[19] N. L. Torad, R. R. Salunkhe, Y. Li, H. Hamoudi, M. Imura, Y. Sakka, C.-C.
Hu, Y. Yamauchi, Chem. Eur. J. 2014, 20, 7895–7900.
[20] M. Jin, H. Liu, H. Zhang, Z. Xie, J. Liu, Y. Xia, Nano Res. 2011, 4, 83–91.
[21] M. E. Abdelsalam, S. Mahajan, P. N. Bartlett, J. J. Baumberg, A. E. Russel,
J. Am. Chem. Soc. 2007, 129, 7399–7406.
[22] D. Astruc, F. Lu, J. R. Aranzaes, Angew. Chem. Int. Ed.2005, 44, 7852–
7872; Angew. Chem. 2005, 117, 8062–8083.
[23] K. K. R. Datta, M. Eswaramoorthy, C. N. R. Rao, J. Mater. Chem. 2007, 17,
613–615.
[24] X. G. Hu, S. J. Dong, J. Mater. Chem. 2008, 18, 1279–1295.
[25] X. M. Chen, G. H. Wu, J. M. Chen, X. Chen, Z. X. Xie, X. R. Wang, J. Am.
Chem. Soc. 2011, 133, 3693–3695.
[26] M. L. Toebes, J. A. van Dillen, Y. P. de Jong, J. Mol. Catal. A 2001, 173,
75–98.
[27] M. J. Gronnow, R. Luque, D. J. Macquarrie, J. H. Clark, Green Chem. 2005,
7, 552–557.
[50] A. Datta, S. Kapri, S. Bhattacharyya, Green Chem. 2015, 17, 1572–1580.
[51] A. G. Sharpe, D. B. Wakefield, J. Chem. Soc. 1957, 281–285.
[52] T. Wang, Q. Zhou, X. Wang, J. Zheng, X. Li, J. Mater. Chem. A 2015, 3,
16435–16439.
[53] C. Ray, S. Sarkar, S. Dutta, A. Roy, R. Sahoo, Y. Negishi, T. Pal, RSC Adv.
2015, 5, 12446–12453.
[54] H. B. Pan, C. M. Wai, New J. Chem. 2011, 35, 1649–1660.
[55] S. Fu, C. Zhu, D. Du, Y. Lin, ACS Appl. Mater. Interfaces 2015, 7, 13842–
13848.
[28] Z. Chen, Z. M. Cui, F. Niu, L. Jiang, W. G. Song, Chem. Commun. 2010, 46,
6524–6526.
[56] B. Xiong, Y. Zhou, Y. Zhao, J. Wang, X. Chen, R. O’Hayre, Z. Shao, Carbon
2013, 52, 181–192.
[29] S. J. Guo, S. J. Dong, E. W. Wang, ACS Nano 2010, 4, 547–555.
[30] B. P. Vinayan, R. Nagar, S. Ramaprabhu, J. Mater. Chem. A 2013, 1,
11192–11199.
[31] C. Liu, Q. Ni, F. Bao, J. Qiu, Green Chem. 2011, 13, 1260–1266.
[32] M. Maity, U. Maitra, J. Mater. Chem. A 2014, 2, 18952–18958.
[33] M. Chtchigrovsky, Y. Lin, K. Ouchaou, M. Chaumontet, M. Robitzer, F.
Quignard, F. Taran, Chem. Mater. 2012, 24, 1505–1510.
[34] S. Das, S. Bhunia, T. Maity, S. Koner, J. Mol. Catal. A 2014, 394, 188–197.
[35] S. J. Ye, D. Y. Kim, S. W. Kang, K. W. Choi, S. W. Han, O. O. Park, Nanoscale
2014, 6, 4182–4187.
[57] B. P. Vinayan, K. Sethupathi, S. Ramaprabhu, Int. J. Hydrogen Energy
2013, 38, 2240–2250.
[58] Z. L. Li, J. H. Liu, Z. W. Huang, Y. Yang, C. Xia, F. W. Li, ACS Catal. 2013, 3,
839–845.
[59] S. Dutta, C. Ray, A. Mondal, S. K. Mehetor, S. Sarkar, T. Pal, Electrochim.
Acta 2015, 159, 52–60.
[60] H. Huang, X. Wang, J. Mater. Chem. 2012, 22, 22533–22541.
[61] Z. Liu, B. Zhao, C. Guo, Y. Sun, Y. Shi, H. Yang, Z. Li, J. Colloid Interface
Sci. 2010, 351, 233–238.
[62] C. Xu, L. Cheng, P. Shen, Y. Liu, Electrochem. Commun. 2007, 9, 997–
1001.
[63] Y. Lu, Y. Jiang, H. Wu, W. Chen, J. Phys. Chem. C 2013, 117, 2926–2938.
[64] M. Zhao, K. Abe, S. Yamaura, Y. Yamamoto, Chem. Mater. 2014, 26,
1056–1061.
[36] M. S. Jin, H. Zhang, Z. X. Xie, Y. N. Xia, Angew. Chem. Int. Ed. 2011, 50,
7850–7858; Angew. Chem.2011, 123, 7996–8000.
[37] J.-J. Feng, D.-L. Zhou, H.-X. Xi, J.-R. Chen, A.-J. Wang, Nanoscale 2013, 5,
6754–6757.
[38] Q. Wang, Y. Wang, P. Guo, Q. Li, R. Ding, B. Wang, H. Li, J. Liu, X. S. Zhao,
Langmuir 2014, 30, 440–446.
[65] G. Z. Hu, F. Nitze, X. Jia, T. Sharifi, H. R. Barzegar, E. Gracia-Espino, T. Wag-
berg, RSC Adv. 2014, 4, 676–682.
[39] S. Dutta, S. Sarkar, C. Ray, A. Roy, R. Sahoo, T. Pal, ACS Appl. Mater. Inter-
faces 2014, 6, 9134–9143.
[66] Y. Liu, M. Chi, V. Mazumder, K. L. More, S. Soled, J. D. Henao, S. Sun,
Chem. Mater. 2011, 23, 4199–4203.
[40] R. D. Morgan, A. Salehi-Khojin, R. I. Masel, J. Phys. Chem. C 2011, 115,
19413–19418.
[67] E. Antolini, Energy Environ. Sci. 2009, 2, 915–931.
[41] G. Hu, F. Nitze, H. R. Barzegar, T. Sharifi, A. Mikołajczuk, C.-W. Tai, A. Bor-
odzinski, T. Wagberg, J. Power Sources 2012, 209, 236–242.
[42] Q. Gao, M.-R. Gao, J.-W. Liu, M.-Y. Chen, C.-H. Cui, H.-H. Li, S.-H. Yu,
Nanoscale 2013, 5, 3202–3207.
[43] J. Chang, X. Sun, L. Feng, W. Xing, X. Qin, G. Shao, J. Power Sources
2013, 239, 94–102.
Manuscript received: February 11, 2016
Revised: March 5, 2016
Accepted Article published: March 27, 2016
Final Article published: April 23, 2016
Chem. Asian J. 2016, 11, 1588 – 1596
www.chemasianj.org
1596
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim