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Edge-Selectively Sulfurized Graphene Nanoplatelets as
Efficient Metal-Free Electrocatalysts for Oxygen Reduction
Reaction: The Electron Spin Effect
In-Yup Jeon, Sheng Zhang, Lipeng Zhang, Hyun-Jung Choi, Jeong-Min Seo,
Zhenhai Xia,* Liming Dai,* and Jong-Beom Baek*
The commercialization of fuel cells has been largely hindered by the sluggish oxygen reduction reaction (ORR) at the
cathode.[1,2] It is important to develop efficient cathodic electrocatalysts for ORR. Hitherto, Pt and its alloys have generally
been considered as the most effective electrocatalysts for ORR
in fuel cells.[3–6] However, they suffer not only from the high
cost of Pt but also from poor long-term stability under the operating conditions.[7–9] Furthermore, Pt-based electrocatalysts
have poor tolerance against fuel crossover and carbon monoxide (CO) poisoning effects. Therefore, the development of
non-precious metal[10–13] and/or metal-free[14–17] electrocatalysts
with a high activity and, more importantly, practical durability
has attracted considerable interest.[18]
Recent progress has demonstrated that heteroatom-doping,
such as nitrogen (N),[15,19–26] boron (B),[27] sulfur (S),[17,28] and
their mixture,[29–31] of carbon-based materials showed promising metal-free electrocatalytic activities due to the charge
polarization stemming from the difference in electronegativity
between carbon (χ = 2.55) and heteroatoms. In this context, vertically aligned nitrogen-doped carbon nanotubes (VA-NCNTs)
generated by pyrolysis of iron(II) phthalocyanine were first
reported to exhibit higher electrocatalytic activity and better
long-term durability compared to commercially available Pt/C
electrocatalysts.[15] Subsequently, nitrogen-doped graphene
(N-graphene) was also demonstrated to actively catalyze ORR
via a four-electron process free from the methanol crossover and
Dr. I.-Y. Jeon,[+] H.-J. Choi, J.-M. Seo, Prof. J.-B. Baek,
Interdisciplinary School of Green
Energy/Low-Dimensional Carbon Materials Center
Ulsan National Institute of Science
and Technology (UNIST)
100 Banyeon, Ulsan 689–798, South Korea
E-mail: [email protected]
S. Zhang,[+] Prof. L. Dai
Department of Macromolecular Science and Engineering
Case Western Reserve University
10900 Euclid Avenue, Cleveland, OH 44106, USA
E-mail: [email protected]
L. Zhang,[+] Prof. Z. Xia
Department of Materials Science and Engineering
and Department of Chemistry
University of North Texas
Denton, TX 76203, USA
E-mail: [email protected]
[+]These
authors contributed equally to this work.
DOI: 10.1002/adma201302753
Adv. Mater. 2013,
DOI: 10.1002/adma201302753
carbon monoxide poisoning effects.[32] The origin of improved
ORR electrocatalytic activity arising from heteroatom-doping of
carbon-based materials has been explained by charge polarization according to quantum mechanics calculations.[15] Namely,
the high electronegativity of the nitrogen atom (χ = 3.04) creates a net positive charge on neighboring carbon atoms to
facilitate the ORR. Following the earlier studies, many reports
have further demonstrated the great potential of heteroatomdoped carbon-based materials as metal-free ORR electrocatalysts. However, methods used for materials preparation often
involve tedious chemical vapor deposition (CVD)[15] and/or
environmentally hazardous Hummers’ methods.[25] The development of a simple and eco-friendly method for the scalable
production of heteroatom-doped carbon-based materials with
a significantly improved ORR electrocatalystic activity is highly
desirable, but still remains as important challenges for fuel cell
and energy communities.
In this report, edge-sulfurized graphene nanoplatelets
(SGnP) were prepared by simply ball-milling the pristine
graphite in the presence of sulfur (S8) and their electrocatalytic activity for ORR in alkaline medium was evaluated. Electrochemical measurements showed that the SGnP exhibited
an improved electrocatalytic activity with long-term operation
stability, and tolerance to methanol crossover/CO poisoning
effects compared to those of the pristine graphite and commercial Pt/C electrocatalysts. Since the charge polarization stemming from the difference in electronegativity between carbon
(χ = 2.55) and sulfur (χ = 2.58) is almost negligible,[33] another
new important factor should have contributed to the enhanced
ORR activity of SGnP. On the basis of theoretical calculations,
a new mechanism of improved ORR activity associated with the
contribution from “electron spin” is proposed, which is supported by experimental results. As also supported experimentally, the oxidized SGnP (SOGnP) was found to show further
improvement in ORR activity due to the increased spin and
charge densities.
Figure 1a shows the mechanochemical sulfurization driven
by ball-milling graphite in the presence of sulfur (S8) to produce edge-selectively sulfurized graphene nanoplatelets
(SGnP).[34,35] As shown in Figures 1b and c, a typical scanning
electron microscopy (SEM) image of the pristine graphite with
a large grain size (ca. 150 µm) and a plane flake morphology
(Figure 1b) was dramatically altered by C–C bond breaking
of graphitic frameworks and delamination of graphitic layers
upon ball-milling. As a result, the SEM image of the resultant
SGnP displays drastically reduced grain size (~1 µm) with a
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Figure 1. a) A schematic representation of the ball-milling process. SEM images: b) the graphite; and, c) SGnP. The scale bars are 1 μm. d) Energy
dispersive X-ray (EDX) spectra taken from the rectangle areas in (b) and (c). e) Sulfur mapping of SGnP taken from the rectangle area in (c); the scale
bar is 125 nm. TEM images of SGnP: f) at low magnification; and, g) at high magnification; the insets are selected area diffraction (SAED) patterns.
flower-like morphology (Figure 1c). The observed grain size
reduction indicates graphitic C–C bond cleavages to generate
active carbon species (carboradicals, carbocations, and carbanions); homolytic cleavages are favorable to produce mostly
carboradicals[36], which are reactive enough to pick up sulfur
to yield SGnP (Figure S2 in the Supporting Information).
The energy dispersive X-ray (EDX) spectrum of SGnP shows
a strong sulfur peak with minor oxygen peak in addition to a
dominant carbon peak, whereas the pristine graphite exhibits
only carbon element (Figure 1d). Along with the carbon and
oxygen element mappings (Figures S3 and S4), the SGnP further displays uniformly-distributed sulfur element (Figure 1e).
TEM images of SGnP exhibit flake structures at low magnification (Figure 1f) with clear selected area electron diffraction (SAED) pattern (Figure 1f, inset), indicating that SGnP has
a high crystallinity. High-resolution TEM (HR-TEM) images
(Figure 1g) show SGnP with a highly ordered structure, with
honeycomb lattice in the basal area and some distortion at the
edge region (arrow, Figure 1g); the associated SAED is a clear
six-fold pattern (Figure 1g, inset). Interestingly, electron densities along the darker edge lines appear to be higher than basal
area (arrows, Figures S5a and b in the Supporting Information), due probably to sulfur moieties at the edges that have
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higher electron density than carbon atoms in the basal plane.
In all cases, TEM images exhibit less than five graphitic layers
with a high crystallinity in basal areas (Figures S5c–f). The
results indicate that the edge-selective sulfurization of graphite
produces SGnP with minimal distortion on its basal plane.
As shown in Figure 2a, the X-ray photoelectron spectroscopy
(XPS) spectrum of the pristine graphite shows a typical C1s peak
at 284.3 eV associated with the graphitic C–C and a minor O1s
peak at 532.1 eV, arising most probably from physical adsorption of oxygen/moisture in air onto the surface of the pristine
graphite.[37] In addition to the O1s and C1s peaks observed from
the pristine graphite, SGnP shows obvious S2s and S2p peaks at
227.2 and 163.4 eV, respectively (Figure 2a), indicating that a significant amount of sulfur (4.94 at%) has been covalently introduced into SGnP. However, this value of 4.94 at% after being
converted into 9.64 wt% is still much lower than those obtained
from elemental analysis (EA, 17.84 wt%, Table S1 in the Supporting Information) and EDX (11.92 at%, Table S2), respectively. This is because XPS analysis, unlike EA and EDX, is more
sensitive to the surface chemical composition.[38] Nevertheless,
XPS measurements have been widely used for qualitative analyses of carbon-based materials. High-resolution XPS spectra,
together with the curve fittings and deconvolutions, shows that
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Figure 2. XPS surveys: a) full spectra of the pristine graphite (its high-resolution spectra are presented in Figure S6) and SGnP. High-resolution XPS
spectra: b) C1s of SGnP; c) O1s of SGnP; and, d) S2p of SGnP (Inset is S2s of SGnP). e) X-ray diffraction (XRD) patterns of the pristine graphite and
SGnP (Inset is the magnified XRD curve of SGnP). f) Thermogravimetric analysis (TGA) thermograms of the pristine graphite and SGnP obtained with
a ramping rate of 10 °C min−1 in nitrogen.
the C1s spectrum of SGnP consists of a graphitic C–C peak
centered at 284.2 eV, along with C–O and C–S (284.8 eV) and
O=C–OH (288.7 eV) peaks with much higher relative peak
intensities to the C–C peak (Figure 2b) than those for the pristine graphite (Figure S6a).[39] The O1s spectrum of SGnP also
shows O=C–OH and C–OH peaks at 531.7 and 533.1 eV, respectively (Figure 2c). The S2p spectrum exhibits two major C–S and
a minor –SO3H peaks at 163.4, 164.6, and 169.0 eV, respectively
(Figure 2d), while the S2s spectrum shows a single C–S peak at
227.2 eV (Figure 2d, inset), indicating a covalent nature for the
C–S bond at the edges of SGnP.
Adv. Mater. 2013,
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X-ray diffraction (XRD) powder patterns of the pristine
graphite demonstrates a prominent [002] peak at 26.5°, which
corresponds to a layer-to-layer d-spacing of 0.34 nm with the
highly ordered graphitic structure (Figure 2e). All other minor
peaks are attributable to three dimensional diffraction lines
associated to graphite powder.[40] The SGnP also shows the [002]
peak at 26.1°, which is very close to that of the pristine graphite
(Figure 2e). However, its peak intensity has catastrophically
decreased to 0.5% of that of the pristine graphite (Figure 2e,
inset), indicating that SGnP has a high degree of exfoliation
even in the solid state. The characteristic difference between
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SGnP and the most studied graphene oxide (GO) is that the
GO has a large shift of the [002] peak from 26.5° (d-spacing of
0.34 nm) to as low as 10.5° (d-spacing of 0.83 nm),[41] while the
d-spacing of SGnP remains almost identical to that of pristine
graphite. These results implicate that the pristine graphite has
been delaminated into graphene nanoplatelets (GnP) to a great
extent without significant basal plane distortion and thus minimal lattice expansion. The significant increase in BET surface
area (Table S3 in the Supporting Information) further supports
the delamination of the pristine graphite into SGnP. Therefore,
it is noteworthy that the ball-milling process involves not only
mechanochemical C–C bond cleavage driving edge-selective
functionalization, but also delamination of graphite into graphene nanoplatelets (GnP). As observed from TEM images
(Figures 1f, 1g, and S5), the SGnP could be further exfoliated
into few-layer GnP upon dispersion in polar solvents (vide
infra), for example in ethanol used for the preparation of SGnP
dispersions onto the TEM grids.
Thermogravimetric analysis (TGA) has also been used for
quantitative estimation of the sulfurization degree (Figure 2f).
As can be seen, SGnP shows weight loss of 17.3 wt% with temperature (Figure 2f and Table S2 in the Supporting Information) while the pristine graphite displayed a negligible amount
of weight loss (0.3 wt%) up to 800 °C. The weight loss value
of 17.3 wt% agrees well with that (17.84 wt%) of EA measurement as summarized in Table S1, suggesting that the amount
of weight loss is mainly caused by thermally stripping off the
edge-sulfur moieties.[42]
Hitherto, various structural characterization techniques have
been applied to indentify sulfur moieties at the edges of SGnP.
The edge-sulfurization and size reduction should contribute to
the enthalpic and entropic gains, respectively, for an efficient
dispersion of SGnP in various solvents. As expected, SGnP
is dispersible well in most protic and polar aprotic solvents,
including neutral water (Figures S7a and S7b). Among all fifteen solvents tested in this study, polar aprotic solvents, such
as N,N-dimethtylacetamide (DMAc), N,N-dimethylformamide
(DMF), and N-methyl-2-pyrrolidone (NMP), were found to be
good solvents for dispersing SGnP into stable dispersions with
concentrations as high as 0.1 mg mL-1. To further investigate
the dispersion stability, Zeta-potential measurements were carried out at different concentrations in DMF as a basic solvent
(Figure S7c). SGnP was found to show Zeta-potentials in the
range of –33.7 to –49.9 mV with respect to concentrations of
0.10 to 0.01 mg mL−1 (Table S4). These results imply that the
thermodynamic driving forces for the stable dispersions should
be due to the size reduction (entropic contribution) and the
strong interactions (enthalpic contribution) between polar functional groups at the edges and polar DMF.[43,44]
The electrocatalytic activities of SGnP were evaluated in
N2- or O2-saturated 0.1 M aq. KOH solutions. For comparison,
the pristine graphite and commercial Pt/C (20% Pt, E-TEK,
Vulcan XC-72R) were also tested under the same condition.
Cyclic voltammograms in Figures 3a and b show obvious
oxygen reduction peaks for carbon-based electrodes in the
O2-saturated 0.1 M aq. KOH solution. In Figure 3a, a single
Figure 3. CV curves obtained from the sample electrodes in N2- and O2-saturated 0.1 M aq. KOH solutions at a scan rate of 50 mV s-1: a) the pristine
graphite; and,b) SGnP. c) Linear sweep voltammograms of the sample electrodes in an O2-saturated 0.1 M aq. KOH solution at a scan rate of 10 mV s−1
with a rotation rate of 1600 rpm. d) Koutecky-Levich plots for the sample electrodes at –0.6 V.
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cathodic reduction peak at –0.47 V can be observed in an
O2-saturated solution for the pristine graphite, while the corresponding cathodic reduction peak for the SGnP shifted positively to –0.40 V. Meanwhile, the oxygen reduction current of
SGnP was calculated to be 0.62 mA cm−2 after correcting background current, which is over double that of pristine graphite
(0.30 mA cm−2). These results clearly indicate that SGnP has
a much higher ORR catalytic activity than pristine graphite.
Linear sweep voltammetry (LSV) was carried out on a rotating
disk electrode (RDE) in O2-saturated 0.1 M aq. KOH solution
at a scan rate of 10 mV s−1 with a rotation rate of 1600 rpm.
As shown in Figure 3c, the onset potential of oxygen reduction for the pristine graphite was approximately –0.40 V,
while a significant up-shift to –0.22 V was observed for SGnP.
Because the diffusion current of Pt/C was limited to –0.8 V in
alkaline electrolyte, the limiting currents of the sample electrodes were also measured up to and compared at this potential. In case of SGnP, its limiting current was –4.47 mA cm−2,
which is 2.7 times higher than that (–1.67 mA cm−2) of pristine graphite and approaches 93.4% (–4.79 mA cm−2) of commercial Pt/C. LSV at various rotating speeds were also performed for the pristine graphite (Figure S8a in the Supporting
Information) and SGnP (Figure S8b) to gain further insight
into the significant enhancement of ORR activity in SGnP.
The diffusion current densities increase with increasing the
rotating rates, and the limiting current densities for SGnP are
generally much higher than those for the pristine graphite
at any constant rotation rate. These results confirm that the
sulfur element on SGnP plays the key role in the improved
ORR activity.
According to the Koutecky-Levich equation,[45,46] the number
of electrons transferred (n)[47] can be calculated to be 3.3 and 2.0
for the SGnP and the pristine graphite, respectively (detailed
calculations are described in the Supporting Information). As
a control experiment, the n value for the commercial Pt/C was
also experimentally determined to be a four-electron transfer (n
= 4) process, which agrees well with the reported values.[48,49]
The electron transfer number (n = 2.0) of the pristine graphite
is close to the classical two-electron transfer process, similar to
carbon nanotubes,[16] while the corresponding n = 3.3 for the
SGnP electrode indicates a coexisting pathway of a two-electron
and a four-electron transfer processes, but closer to the latter.
Once again, therefore, sulfur doping plays an important role for
the improved ORR activity in SGnP.
The origin of the ORR activity enhancement with the
nitrogen-doped carbon nanomaterials has been previously
attributed to the higher electronegativity of the nitrogen
(χ = 3.04) than that of carbon (χ = 2.55),[33] which creates a
net positive charge on adjacent carbon atoms in the graphitic
structures to readily attract electrons and to induce a parallel
diatomic adsorption mode for oxygen molecules.[15] The parallel
diatomic adsorption could effectively weaken the O–O bond
and facilitate the O2 dissociation in the ORR process. Given
that the electronegativities of sulfur (χ = 2.58) and carbon are
nearly the same,[33] the change of atomic charge distribution for
the SGnP is relatively much smaller, compared with nitrogendoped carbon materials. Therefore, other factors, such as asymmetric electron spin, could have contributed to the significantly
improved ORR catalytic activity of SGnP.
To understand the fundamental role of sulfur for the ORR
activity of SGnP, a first-principles study on the electronic structure of SGnP was performed using density functional theory
(DFT) methods. The heteroatom sulfur may be either physically/chemically adsorbed on the graphene surface (in the case
of physical adsorption, free-standing sulfur will be washed
off by Soxhlet extraction and thus its contribution can largely
be ruled out) or covalently bonded at the edges in the form
of single sulfur atom or sulfur oxide (O=S=O) groups.[17,50]
Therefore, several possible types of S on graphitic structures,
including the sulfur on graphene basal plane (C100H26S-1
and C100H26S-2, Figures S9a and S9b, respectively), and covalently bonded sulfur at zigzag and armchair edges of graphene
(C99H25S, Figures S9c and S9d, respectively), and a sulfur
cluster ring connecting graphene (C144H40S4, Figure S9g),
were built based on our experimental results (see Figure 2) and
those reported in the literature.[29,51,52] The sulfur at the edge
of graphene could be easily oxidized into sulfur oxide (O=S=O)
(C99H25O2S, Figures S9e and S9f).[17] In Figures S9a and S9b,
the distance between the sulfur atom and the nearest carbon
atoms is about 1.9 Å, indicating that chemical bonds between
sulfur and carbon atoms on graphene basal plane. When
each sulfur atom combines with two carbon atoms, the bonds
become stable. The difference between Figures S9a and S9b is
that the sulfur atom adsorbed on two different carbon atoms in
the same hexagonal carbon ring. In Figure S9g, two graphenes
were connected by the sulfur cluster. There was an angle
between two different graphene planes. This type of structure
could be formed by two pieces of SGnP.
As mentioned above, the catalytic activity of the doped graphene is closely related to spin and charge distributions on
the graphitic framework.[53,54] The statistical distributions of
the spin and charge densities, as well as magnetic moments,
on the pristine, sulfur-doped and sulfur ring cluster-connected
graphene are calculated wilth Mulliken methods and are summarized in Table 1. Overall, the maximum charge and its percentage do not vary too much, but the spin density is quite
different on these types of graphene. More specifically, the graphene with sulfur atoms adsorbed on its surface (Figures S9a
and S9b) have almost the same charge distribution as the pristine graphite. The increase in the maximum charge is below
0.01 compared to the pristine graphene. Also, this type of sulfur
adsorption does not create any additional spin at all (Table 1).
Similarly, for the sulfur ring cluster-connected graphene, there
is no spin density on the graphene although the percent content
of carbon atoms with higher charge density slightly increases
compared with the pristine graphite. Hence, these S-doped
structures with sulfur-adsorbed or sulfur ring cluster-connected
graphene may have a limited ORR catalytic capability like the
pristine graphite (Figure 3a). In contrast, the covalently bonded
sulfur or oxidized sulfur at the zigzag and armchair edges of
graphene induces both charge and spin densities on the graphene (Table 1, Figures S9c and S9d). As a result, those carbon
atoms with high positive charge and spin densities could serve
as active sites, which promotes catalytic activity (Figures 4a
and b). Furthermore, the sulfur atom itself is also an active site
for the ORR, since the charge density of sulfur at the zigzag
edge is increased as high as 0.22 (sky blue circle in Figure 4a
and Table 1), which could be, along with spin density, one of
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Table 1. Mulliken spin densities, charge densities, and magnetic moments in the structures of pristine graphene, SGnP, and SOGnP
Sample
Formula
Maximum spin density
(spin density > 0.14)
Maximum charge density
(charge density > 0.14)
Magnetic moment [debye]
Pristine graphene
C100H26
0
0.1855 (8%)
0
2.24
S on surface
C100H26S-1
0
0.1851 (8%)
C100H26S-2
0
0.1927 (8%)
2.21
S at zigzag edge
C99H25S
0.22 (3%)
0.1866 (5%)
0.94
S at armchair edge
C99H25S
0.37 (7%)
0.1895 (7%)
1.09
SO2 at zigzag edge
C99H25O2S
0.39 (3%)
0.1858 (3%)
8.00
SO2 at armchair edge
C99H25O2S
0.37 (7%)
0.1863 (7%)
10.30
Sulfur ring cluster
C144H40S4
0
0.1870 (11%)
0.0008
the important factors that improve ORR. It is noteworthy that
SOGnP has the highest magnetic moments among all these
types of S-doped structures (5–10 times higher than other
types). This large magnetic moment may contribute to catalytic
activities of the graphene since the magnetic moments relate to
electron motion and transfer. Thus, SOGnP could have better
catalytic properties than other type of S-doped graphene.
To further study the effect of heteroatom sulfur on chemical
reactivity of the graphene derivatives, we calculated the highest
occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distributions on the graphene,
which represent potentials of electron donor and acceptor,
respectively. The state of the orbital distributions is related to
the electron transfer during the ORR. In the case of the pristine graphene, the HOMO and LUMO orbitals are uniformly
distributed (Figure S10). While the HOMO and LUMO are
slightly polarized by the sulfur adsorbed on the graphene basal
Figure 4. HOMO and LUMO distributions: a) HOMO of SGnP; b) LUMO
of SGnP; c) HOMO of SOGnP; and, d) LUMO of SOGnP.
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plane (Figure S10), they are strongly polarized by the covalently
bonded sulfur atoms at the edges of graphene (Figures 4 and
S10). These polarized zones should serve as the active sites for
ORR. The results further support that only the edge-sulfurized
graphenes possess the high catalytic activity. On the basis of
theoretical calculations, it could be thus suggested that the
covalently bonded sulfur, especially sulfur oxide, at zigzag or
armchair edges of graphene shows strong catalytic activity for
ORR, while sulfur atoms adsorbed on graphene basal plane
and the sulfur cluster ring connecting graphenes are weak in
catalyzing ORR.
After recognizing the further improvement of electrocatalytic activity in the oxidized version of SGnP (Figures 4c and d,
and Table 1), SGnP was oxidized in hydrogen peroxide (H2O2)
at ambient temperature to produce oxidized sulfur-doped graphene nanoplatelets (SOGnP) (detailed experimental procedure
described below). The nature of oxidized sulfur on SOGnP
was identified by XPS with curve fitting and deconvolution
(Figure S11). Compared with the SGnP starting material (see
Figure 2a), the SOGnP shows relatively higher intensity of O1s
peak and lower intensities of S2s and S2p peaks, indicating
the oxidation of sulfur in the form of sulfur dioxide (O=S=O;
Figure S11d) and some loss of sulfur-containing groups (i.e.,
the relative intensities of S2s and S2p peaks decreased compared with those of the starting SGnP). From the EA results,
the ratio of carbon to oxygen (C/O) decreased from 39.5 to
14.0 for SGnP and SOGnP, respectively, while that of carbon
to sulfur (C/S) ratio has significantly been increased from
11.8 to 39.4 (Table S1). Due to the more polar nature of SOGnP
than SGnP, the average contact angle (10 measurements) was
decreased from 38.2° (SGnP) to 22.0° (SOGnP) (Figure S7d).
As predicted by DFT calculations, the SOGnP shows a more
pronounced single reduction peak with higher cathodic current density (Figure S12a) than those of SGnP (see Figure 3b),
suggesting better ORR performance for SOGnP; LSV on RDE
further reveals that the onset-potential of SOGnP has positively shifted with respect to SGnP (Figure S12b), implying a
more facile ORR process for SOGnP in comparison with other
carbon-based heteroatom-doped systems (Table S5). In addition,
SOGnP shows a more efficient four-electron (n = 3.6) dominant
ORR pathway (Figures S12c and S12d) than that (n = 3.3) of
SGnP, and the value is comparable to that (n = 4.0) of commercial Pt/C electrocatalyst (see Figure 3d).
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Figure 5. a)Current–time (I–t) chronoamperometric responses of the pristine graphite, SGnP, SOGnP, and commercial Pt/C electrodes at –0.30 V in
an O2-saturated 0.1 M aq. KOH solution at a rotation rate of 1200 rpm b)Current-time (j–t) chronoamperometric responses for the sample electrodes
at –0.40 V in an O2-saturated 0.1 M aq. KOH solution upon addition of 3.0 M methanol at 300 s.
To investigate the electrode stability, an accelerated degradation test (ADT) was performed using the chronoamperometric I–t method at –0.30 V in an O2-saturated 0.1 M aq. KOH
electrolyte at a rotation rate of 1200 rpm. The I–t curves in
Figure 5a show that current densities on the pristine graphite,
SGnP, SOGnP, and Pt/C electrocatalysts all initially decreased
with time. The SGnP and SOGnP demonstrate a much slower
attenuation rate than commercial Pt/C (Figure 5a), indicating
that the SGnP and SOGnP have much higher stability than the
commercial Pt/C electrocatalyst. Furthermore, the tolerance to
the methanol crossover/CO poisoning effects is also an important consideration for fuel cells, as in methanol fuel cells, fuel
crossover from anode to cathode could diminish cathodic performance through the depolarizing effect. As seen in Figure 5b,
only the Pt/C electrocatalyst shows the fuel crossover effect
upon the addition of methanol. The above results unambiguously suggest that the SGnP, SOGnP and even the pristine
graphite have much higher selectivity toward ORR than the
commercial Pt/C electrocatalyst.
In summary, we have demonstrated that the edge-selectively
sulfurized graphene nanoplatelets (SGnP) could be produced
by simple but efficient dry ball-milling graphite in the presence
of sulfur (S8). The resultant SGnP can be used as an efficient
metal-free ORR electrocatalyst. Furthermore, the oxidation of
the SGnP into SOGnP further improves ORR capability to surpass the commercially available Pt/C electrocatalyst. To investigate the origin of high ORR activity, theoretical calculations
were conducted and showed that the electronic spin density,
in addition to generally considered charge density, plays a key
role in the high ORR activity of SGnP and SOGnP. Thus, those
sulfur (S) atoms and sulfur oxides (O=S=O) doped at the edges
of the graphene nanoplatlets (GnP) strongly promote the electrocatalytic activity. Furthermore, both SGnP and SOGnP demonstrate not only a high ORR electrocatalytic activity but also
a better fuel selectivity with a longer-term stability than those
of the pristine graphite and commercial Pt/C electrocatalysts.
Therefore, the present study provides not only new insights
on oxygen reduction mechanism for heteroatom-doped carbon
materials, but also a general approach for the low-cost and
high-performance metal-free ORR electrocatalysts for fuel cells.
Adv. Mater. 2013,
DOI: 10.1002/adma201302753
Experimental Section
Preparation of SGnP: SGnP was prepared by ball-milling the pristine
graphite in a planetary ball-mill machine (Pulverisette 6, Fritsch) in the
presence of sulfur (S8). The pristine graphite (5.0 g, Alfa Aesar, natural
graphite, 100 mesh (<150 μm), 99.9995% metals basis, Lot#14735,
Figure S1a) and sulfur (20.0 g, Aldrich Chemical Inc., Figure S1b) were
placed into a stainless steel ball-mill capsule containing stainless steel
balls (500.0 g, diameter 5 mm, Figure S1c). The capsule was sealed
and charged with argon after five charging–discharging cycles and then
it was fixed in the planetary ball-mill machine and agitated at 500 rpm
for 48 h. The resultant product was Soxhlet extracted with carbon
disulfide (CS2) to remove the remnant sulfur and 1 M aq. HCl solution
to get rid of any metallic impurities, if present. The final product was
freeze-dried at –120 °C under reduced pressure (0.05 mmHg) for 48 h
to yield 5.75 g (at least 0.75 g sulfur uptake) of dark black powder.
Elemental analysis (EA) revealed that weight percentage (wt%) of total
element contents (99.62%) consists of carbon (C, 78.85%), hydrogen
(H, 0.27%), oxygen (O, 2.66%), and sulfur (S, 17.84%), implying that
covalent sulfurization at the edges of graphite has been efficiently
occurred.
Oxidation of SGnP into SOGnP: SGnP (0.30 g) was treated with
phosphonitrilic chloride trimer (0.30 g) in aq. hydrogen peroxide (30%
H2O2, 30 mL) at room temperature for 1 h. After treatment, the resultant
product was collected by suction filtration and Soxhlet extracted with
methanol to remove residual phosphonitrilic chloride trimer and freezedried at –120 °C under a reduced pressure (0.05 mmHg) for 48 h to yield
0.28 g of dark black powder.
Electrochemical Measurements: The electrochemical tests were
carried out using a computer-controlled potentiostat (CHI 760C, CH
Instrument, USA) with a typical three-electrode cell. A platinum wire
was used as counter-electrode and an an Ag/AgCl (3 M KCl filled)
electrode as reference electrode. All the experiments were conducted
at ambient condition. The working electrodes were prepared by
applying each of the catalyst inks onto a pre-polished glassy carbon
disk electrode. Briefly, the electrocatalyst was dispersed in ethanol and
ultrasonicated for 15 min to form a uniform catalyst ink (2 mg mL−1).
A total of 7.5 μL well-dispersed catalyst ink was applied onto the prepolished glassy carbon (GC) disk electrode (5 mm in diameter). After
drying at room temperature, Nafion (0.05 wt%) stock solution (5 μL)
in ethanol was applied onto the surface of the catalyst layer to form
a thin protective film.[55] The addition of a small amount of Nafion
could effectively improve the dispersion of catalyst suspension and
enhance the binding onto the glass carbon electrode. The carefully
prepared electrodes were dried at room temperature overnight before
the electrochemical tests.
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
7
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COMMUNICATION
www.MaterialsViews.com
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors acknowledge the support of Basic Research, World Class
University (WCU), U.S.-Korea NBIT, Converging Research Center (CRC),
Mid-Career, and Basic Research Laboratory (BRL) programs through the
National Research Foundation (NRF) of Korea, and the NSF (CMMI1000768; CMS-1047655) and AFOSR (FA9550–09–1–0331, FA2386–10–
1–4071, FA9550–10–1–0546).
Received: June 17, 2013
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