Electrochemistry Communications 30 (2013) 9–12
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Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Sulfur-containing carbon by flame synthesis as efficient metal-free electrocatalyst for
oxygen reduction reaction
Shaukatali Inamdar, Hyuck-Soo Choi, Peng Wang, Min Young Song, Jong-Sung Yu ⁎
Department of Advanced Materials Chemistry, Korea University, 2511 Sejong-ro, Sejong 339-700, Republic of Korea
a r t i c l e
i n f o
Article history:
Received 1 December 2012
Received in revised form 26 January 2013
Accepted 29 January 2013
Available online 6 February 2013
a b s t r a c t
Sulfur-containing carbon soot is prepared for the first time through simple flame synthesis by burning
flammable thiophene, and it exhibits excellent catalytic activity for oxygen reduction reaction in alkaline
media. The flame synthesis provides not only a rapid single step low-cost approach, but also very efficient
and scalable methodology for metal-free sulfur-containing carbons with good control on the sulfur content.
© 2013 Elsevier B.V. All rights reserved.
Keywords:
Sulfur-containing carbon
Flame pyrolysis
ORR electrocatalyst
1. Introduction
The energy-conversion efficiency of low temperature fuel cells (FCs)
is limited by slow kinetics of oxygen-reduction reaction (ORR) [1]. Pt and
its alloys are the most effective cathode electrocatalysts for ORR [1]. The
large-scale application of FCs is hindered mainly by high cost, limited
supply, and poor durability of Pt. Therefore, numerous efforts have
been made to reduce or replace the Pt-based catalysts in FCs, searching
for alternative catalysts based on non-precious metals and metal-free
doped carbons. In particular, new cost-effective catalysts with high activity and practical durability for ORR become a prime requirement in the
commercialization of FCs. Recently, several studies confirmed enhancement in ORR activity by doping hetero atoms to carbon materials, especially nitrogen [2,3], phosphorus [4,5], boron [6] and their mixtures [7].
Very recently, we have demonstrated that P-doped mesoporous carbon
shows very prominent catalytic activity for ORR [5]. Yang et al. reported
that doped elements with similar electronegativity to that of carbon
favor ORR activity [8]. Theoretical studies by simulation/calculations
have established that breaking the electroneutrality of graphitic materials by the presence of dopant atoms in the carbon framework creates
charged sites favorable for O2 adsorption, enhancing ORR activity [6,9].
Sulfur (electronegativity value of 2.58) has a similar electronegativity
to that (2.55) of carbon and can be an ideal dopant [10].
There are a few reports on the synthesis of S-doped carbons by
multistep processes, which requires high temperature treatment ranging from 600 to 1000 °C [10–14]. The S-doped carbon has been used as a
good electrocatalyst for ORR [8], and as an anode material in Li-ion battery [11]. Recently, Inamdar and Haram showed a new flame pyrolysis
⁎ Corresponding author. Tel.: +1 82 44 860 1494; fax: +1 82 44 860 1331.
E-mail address: [email protected] (J.-S. Yu).
1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.elecom.2013.01.023
method for preparing spherical iron oxide nanoparticles by burning
ferrocene solution using a spirit lamp [15]. In our earlier report,
maghemite polyhedrons and maghemite–carbon composite were also
prepared by similar flame pyrolysis approach [16]. Flame pyrolysis
method has many advantages as the method is very simple, fast and
cost effective, and also can effectively produce carbon soot as dry powders in gram scale [15,16]. To the best of our knowledge, the literatures
lack report on flame synthesis method for sulfur-containing carbon. In
this communication, we report flame synthesis of S-containing carbon
for the first time by directly burning thiophene without any solvent or
metal source, and its high electrocatalytic activity for ORR. The
S-containing carbon exhibits excellent catalytic activity, as well as outstanding selectivity and long-term stability for ORR. This rapid, cheap
and simple method holds great promise to scalable synthesis of
S-containing carbon with high electrocatalytic activity.
2. Experimental
The synthesis of carbon soot with varied sulfur contents was carried out by flame pyrolysis method. Three samples were prepared
with different sulfur content as follows. Sample S-3 (sulfur 3.4 at.%):
5 mL thiophene was burned and soot collected over cold glass surface
with yield of 150 mg. Sample S-2 (sulfur 2.1 at.%): 2 mL thiophene in
1 mL benzyl alcohol was burned and soot collected with yield of
102 mg. Sample S-1 (sulfur 1.0 at.%): 1 mL thiophene in 1 mL benzyl
alcohol was burned and soot collected with yield of 75 mg.
X-ray powder diffraction (XRD) was measured using Bruker D-8
diffractometer with Cu-Ka radiation (1.5406 A°) operated at 40 kV
and 40 mA. HR-TEM images were recorded using JEOL FE 2010 microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS)
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S. Inamdar et al. / Electrochemistry Communications 30 (2013) 9–12
analyses were made by AXIS-NOVA (Kratos) X-ray photoelectron
spectrometer.
Electrochemical experiments were carried out at room temperature
with three electrode setup by electro-analyzer (Autolab-PGSTAT30).
The working electrode was prepared by drop-casting S-containing
carbon over glassy carbon (GC) electrode. An Ag/AgCl (saturated KCl)
and Pt wire were used as reference and counter electrodes, respectively.
For ORR study, cyclic voltammetry (CV) and linear sweep voltammetry
(LSV) were recorded for 0.2 mg/cm2 active catalyst loaded by dropcasting 5 μL of the suspension, which was prepared by mixing 3 mg
catalyst and 30 μL 5% nafion in 560 μL deionized water, on GC rotating
disk electrode. The CVs and LSVs were recorded in O2-saturated 0.1 M
KOH with 100 mV/s and 10 mV/s scan rates, respectively.
3. Results and discussion
S-containing carbons were prepared by burning thiophene as a
sulfur as well as carbon source. Fig. 1a is an actual photograph of
the synthesis method, while XRD pattern of the sample S-3 is given
in Fig. 1b. Two characteristic peaks observed at 24.6° and 42.3° correspond to 002 and 100 planes of turbostratic carbon with low crystallinity between graphite and amorphous carbon. The first (002) peak
at 24.6° indicates the formation of turbostratic phase, while weaker
broad peak observed at 42.3° is related to (100) plane of the phase
[17,18]. The interlayer distance for (002) plane has increased a little
after the addition of sulfur from 3.354 Å expected for graphite to
3.616 Å, as the diameter of sulfur atom (204 pm) is larger than that
of carbon atom (154 pm) [10,11,13].
Fig. 1c shows HR-TEM image for S-containing carbon S-3. Burning
of thiophene in air typically yields disordered, nonporous carbon with
size around 30–40 nm. To investigate the actual distribution of the
sulfur inside the carbon matrix, EDX elemental mapping was done
on a relatively large area. The EDX mapping also confirmed that sulfur
was homogeneously dispersed over the carbon matrix throughout
the sample (figure is not shown).
It is reported that sulfur acts as n-type dopant in graphite and diamond semiconductors, and sulfur doping increases the conductivity
of amorphous carbons at optimum concentration, but decreases conductivity if present in excess [13,19]. The incorporation of dopants results in a disordered graphitic lattice, and additional charge carriers
are introduced into the system [13]. The nature of heteroatom bonding with carbon framework is a crucial factor for getting good electrocatalytic activity [7,8,20]. For the flame-synthesized sample S-3,
bonding nature of sulfur in carbon soot was investigated through
XPS analysis. Fig. 1d shows the XPS survey scan of the sample S-3
along with high-resolution S2p spectrum shown as inset. Sulfur is
found to be incorporated into the carbon in two distinct forms,
~ 35% as \C\S\C\ at 163.4 eV and ~ 65% in the form of sulfate
(\C\SO4\C\) or sulfonate (\C\SO3\C\) at 168.5 eV [11–14].
The overall sulfur content was found to be 3.4, 2.1 and 1.0 at.% for
S-3, S-2 and S-1, respectively.
For investigations of the prepared S-containing carbon as ORR catalyst, CV and LSV measurements were carried out in alkaline medium.
CVs at a scan rate of 100 mV/s and LSVs using rotating disk electrode
(RDE) at scan rate 10 mV/s were recorded in 0.1 M KOH and shown
in Fig. 2a and b, respectively. A featureless voltammetric curve is observed for the S-containing carbon in the N2-saturated solution, while
a well-defined cathodic peak appears near at − 0.27 V vs Ag/AgCl in
O2-saturated 0.1 M KOH solution (Fig. 2a), clearly demonstrating
high electrocatalytic activity of the S-containing carbon towards electrochemical reduction of O2. The ORR peak potential demonstrated
here is similar to or little less than N-doped graphitic arrays or
N-containing CNTs [3,21], and is superior than earlier reports on
S-doped carbons [10,14] or even better than P-doped graphite [2].
Methanol tolerance of the prepared catalyst was also tested, and the
result confirms that the presence of methanol has null effect on the
ORR performance of the flame-synthesized S-containing carbon
b
Intensity, (a.u.)
a
002
100
10
20
30
40
50
60
70
80
90
2θ
–C–SO4 –C– &
–C–SO3 –C–
C1s
d
c
Intensity, (a.u.)
–C–S–C–
O1s
155
160
165
170
175
Binding energy, (eV)
S2p
0
200
400
600
800
1000
1200
Binding energy, (eV)
Fig. 1. a) Actual photograph of flame pyrolysis setup used to prepare sulfur-containing carbon, b) XRD pattern, c) HR-TEM image recorded for as-prepared sample S-3, and d) XPS
survey spectrum of S-3 with an inset showing high-resolution S2p spectrum with two distinct peaks.
S. Inamdar et al. / Electrochemistry Communications 30 (2013) 9–12
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Fig. 2. (a) CVs recorded for S-3 in N2-saturated and O2-saturated 0.1 M KOH solution with and without presence of methanol, (b) LSVs of S-3 catalyst on glassy carbon RDE in O2-saturated
0.1 M KOH solution at various rotation speeds, (c) Comparative LSVs at 1600 rpm for varied sulfur content along with commercial 20%Pt@C (E-TEK), and (d) Koutecky–Levich plots of
oxygen reduction at different electrode potentials.
catalyst, indicating superb selectivity towards oxygen reduction even
in the presence of methanol as shown in Fig. 2a. The area of the
voltammograms arises from capacitive currents of the carbon material. Onset potential value of − 0.16 V is derived from LSV measurements using a RDE in O2-saturated 0.1 M KOH solution.
The effect of the sulfur content on the ORR performance was
checked, and the comparative LSVs at 1600 rpm are shown in
Fig. 2c along with that for state of the art commercial 20% Pt@C
from E-TEK. The S-containing carbons show similar onset potentials,
but S-3 illustrates much better activity than S-1 and S-2 with lower
S content, suggesting that activity increases with S content, although
the flame synthesized S-containing carbons as ORR catalyst is not
very competitive to commercial Pt@C in terms of onset potential.
Number (n) of electrons transferred in oxygen reduction process is
an important factor for ORR catalysts which determines the catalytic
efficiency towards formation of H2O surpassing H2O2 formation. Platinum usually catalyzes oxygen reduction by a 4 electron process,
which is desired for efficient fuel cell operation [3–7]. Fig. 2d shows
that Koutecky–Levich plots of the S-containing carbon in 0.1 M
KOH, reveal good linearity although the slopes are varied slightly
with applied potentials, which indicates the mechanism is not very
selective for either 2 or 4 electron process, which is more or less
similar to earlier reports [11–14]. Fig. 3a depicts the number of electrons transferred during the catalytic reaction calculated from the
Koutecky–Levich plots at various potentials [4]. The Koutecky–Levich
equation is given below.
−1
J
¼ Jk
−1
2=3 −1=6 1=2 −1
þ 0:62 n F C D ν
ω
where, J is the measured current density, Jk is the kinetic current density
of the ORR, and n is the overall number of electrons transferred during
the oxygen reduction. F is Faraday constant (F=96,485 C mol−1), C is
Fig. 3. (a) Number of electrons transferred (n) calculated at various potentials for S-3, and (b) the change of peak potentials and current densities with varied sulfur content (at.%).
Here, the peak potentials (−■-) are taken from CVs and current densities (−●-) from Fig. 2c at −1.2 V.
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S. Inamdar et al. / Electrochemistry Communications 30 (2013) 9–12
the bulk concentration of O2 (C= 1.2 × 10 −3 mol L −1), D is the diffusion coefficient of O2 in the KOH electrolyte (D= 1.9 × 10 −5 cm2 s−1),
ν is the kinetic viscosity of the electrolyte (ν = 0.01 cm2 s−1) for the
0.1 M KOH, and ω is the angular velocity of the disk (ω = 2πN, N is
the linear rotation speed). Typical n value of pristine carbon materials
such as Vulcan carbon and carbon nanotubes is 2 for ORR, which indicates peroxide formation [4,21,22]. The n is found tending towards 4
with decreasing potentials in this case. This suggests that sulfurdoping not only improves catalytic performance, but also the selectivity
towards a 4 electron process for oxygen reduction [10].
The effect of sulfur content on ORR peak potential and ORR current
density demonstrates linear trend as shown in Fig. 3b, illustrating
ORR activity increases with sulfur content. The ORR peak potentials
shift to less negative values with sulfur content. This also clearly indicates that sulfur is the active species for ORR. The sulfur in the
thiophenic (\C\S\C\) configuration in carbon is suggested as the
active site for promoting ORR in accordance with ORR study in
S-doped graphene [8,23]. The effect of the sulfur on enhancing the
ORR efficiency can be summarized in two facts; 1) the lone pairs of
sulfur possibly can contribute to favorable interaction with oxygen
molecules in the surroundings [10] and 2) the sulfur-content will induce greater strain and defects in the carbon material compared to nitrogen, facilitating charge localization for favorable chemisorption of
oxygen because of the large atomic radii of sulfur (102 pm) compared
to nitrogen (65 pm) in view of carbon (77 pm) [10,24,25].
The amount of the S-containing samples is high, indicating the
reactions can easily be scaled up for gram scale synthesis. Our motivation was to synthesize S-containing carbon using flame pyrolysis of
sulfur-rich precursors in air. The choice of precursor can be made
according to the sulfur content and its flammability. Even a wide range
of sulfur-rich precursors can be applied to produce sulfur-containing
carbon soot, and even non-flammable precursors can be utilized by
dissolving in appropriate fuels such as acetone and alcohols to get the
desired product.
4. Conclusions
Herein, we demonstrated an economic, scalable single-step synthesis for sulfur-containing carbon using flame pyrolysis method.
Our approach represents the first ever synthesis by directly burning
flammable thiophene as a sulfur and carbon source. The product
exhibited not only excellent catalytic activity compared to available
reports, but also remarkable tolerance for methanol oxidation with
superior selectivity as catalyst for ORR in alkaline media, holding
promise for future replacement of Pt-based catalysts. The effect of
sulfur content on the ORR performance shows linear trend. The
sustainable and simple nature of the flame pyrolysis can provides
economic, environmental feasible and scalable approach for the
heteroatom-containing carbon for various applications viz. ORR
electrocatalyst and the Li-ion battery anode materials.
Acknowledgments
This work was supported by NRF grant (NRF-2010-0029245) and
Global Frontier R&D Program on Center for Multiscale Energy System
(NRF-2011-0031571) funded by the Ministry of Education, Science
and Technology. We thank KBSI at Jeonju for SEM, XPS and HR-TEM.
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