Nitrogen and sulfur co-doping of partially exfoliated MWCNTs as 3-D structured electrocatalysts for the oxygen reduction reaction

Article abstract of DOI:10.1039/c6ta00490c

Preventing the stacking of graphene sheets is of vital importance for highly efficient and stable fuel cell electrocatalysts. In the present work, we report a 3-D structured carbon nanotube intercalated graphene nanoribbon with N/S co-doping. The nanocomposite is obtained by using high temperature heat-treated thiourea with partially unzipped multi-walled carbon nanotubes. The unique structure preserves both the properties of carbon nanotubes and graphene, exhibiting excellent catalytic performance for the ORR with similar onset and half-wave potentials to those of Pt/C electrocatalysts. Moreover, the stereo structured composite exhibits distinct advantages in long-term stability and methanol poisoning tolerance in comparison to Pt/C.

Full text of DOI:10.1039/c6ta00490c

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Nitrogen and Sulfur Co-doping of Partially Exfoliated MWCNTs as
Three Dimensional Structured Electrocatalyst for the Oxygen
Reduction Reaction
Received 00th January 20xx,
Accepted 00th January 20xx
Jie Wang,a Zexing Wu,a Lili Han,b,c Ruoqian Lin,b,d Weiping Xiao,a Cuijuan Xuan,a Huolin. L Xin,b,d
and Deli Wanga*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Preventing the stacking of graphene sheets is of vital importance for highly efficient and stable fuel cell electrocatalysts. In
the present work, we report a 3-D structured carbon nanotube intercalated graphene nanoribbon with N/S co-doping. The
nanocomposite is obtained by high temperature heat-treated thiourea with partially unzipped multi-walled carbon
nanotubes. The unique structure reserves both the properties of carbon nanotube and graphene, exhibiting excellent
catalytic performance for the ORR with similar onset and half-wave potential relative to that of Pt/C electrocatalysts.
Moreover, the stereo structured composite exhibits distinct advantages in long-term stability and methanol poisoning
tolerance in comparison to Pt/C.
graphene based two-dimensional catalysts suffer from an
irreversible re-stacking of the nanosheets due to the strong π-
Introduction
7, 8
9
and I ) doped carbon
interaction during thermal annealing process and
electrochemical measurements 18-20. The re-stacking would
result in a lower surface area, limiting the mass transfer rate
and even decreasing the catalytic activity as well as running
stability. Great efforts have been devoted to preventing the re-
stacking of graphene, such as inserting carbon nanospheres or
multi-walled carbon nanotubes (MWCNTs) into graphene
layers 21-23, growing graphene on hierarchical porous carbon
materials 5, 24, conversion of graphene into three dimensional
aerogel to form a stereo structure 25. Recently, a carbon
nanotube-graphene complex was obtained through unzipping
Heteroatom (e.g. B 1, 2, N 3, 4, P 5, 6, S
materials as efficient noble metal-free catalysts have, to a
certain extent, enhanced the kinetics of the oxygen reduction
reaction (ORR) and reduced the cost compared with
commercial Pt/C in alkaline solution. Currently, co-doping of
heteroatoms into carbon materials as ORR catalysts have been
further investigated to improve electrocatalytic activity.
Synergistic catalytic effect for the ORR could be acquired
among different dopants through creating new non-electron-
10, 11
neutral sites
in the materials. Among the heteroatom
elements, S atom has brought particular interest and was
found to easily replacing the C atom when co-doping with N 12,
13
of MWCNTs or few-walled carbon nanotubes 4, 26
In this paper, we report nitrogen and sulfur co-doped 3-D
structured carbon nanotubes/graphene nanoribbon
.
.
Graphene,
a
single-atom-thick hexagonally arrayed sp2
composites. The unique structure was obtained by partially
exfoliation of outer walls of MWCNTs, to forming graphene
nanoribbons attached to intact inner walls of nanotubes. The
ratio of graphene to nanotubes was controlled by tuning the
amount of MWCNT and oxidizing agents. The internal
nanotube skeletons effectively prevent the graphene
nanoribbon from agglomeration during high temperature and
carbon atoms bonded sheet, has attracted a great deal of
interest in renewable energy conversion and storage devices
due to its unique properties, especially ultrahigh specific
surface area and superior electron conductivity 14-17. However,
electrochemical process. As
graphene/nanotubes composites exhibited
electron pathway for the ORR, which is comparable to Pt/C.
Moreover, the dual-doped stereo structure of
a
result, the N/S co-doped
a
direct four-
nanotubes/graphene nanoribbon composites exhibit distinct
advantages in long-term stability and methanol poisoning
tolerance relative to Pt/C.
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ink was drop onto the glassy carbon substrate, and dried
naturally. Pt/C electrode with Pt mass loading of 15 μL cm-2 on
GC electrode serves as working electrode for comparison. The
cyclic voltammetry (CV) and linear sweep voltammetry (LSV) of
the samples was operated in N2- or O2-saturated 0.1 M KOH
solution at a scanning rate of 50 mV s-1 for CV and 5 mV s-1 for
LSV measurements. Electrons transfer number (n) was
calculated from the following Koutecky-Levich (K-L) equation:
The main text of the article should appear here with headings
as appropriate.
Experimental Section
Preparation of graphene/nanotubes composites
The exfoliation of MWCNTs was prepared similar to Hummers
method 26. 2.0 g of MWCNT was added into a 500 mL three-
necked round-bottomed flask filled with 300 mL of
concentrated H2SO4. The mixture was continuously mechanical
stirred for 12 h at room temperature. 6.0 g of KMnO4 was then
slowly added into the flask and mechanical stirred for 1 h, and
then heated to 70 oC for 1 h. After cooling down to room
temperature, the mixture was poured into a 3000 mL baker
filled with 25 mL of H2O2 and 500 mL of ice. Resulting
precipitate was obtained suction-filtered in air and washed
with copious amounts of distilled water until the pH was 7 and
then underwent a freeze-drying. The obtained material was
denoted as CNT-3 (the mass ratio of MWCNT to KMnO4 was
1:3). Similarly, by changing the ratio to 1:0, 1:2, 1:4 and 1:6,
the materials were denoted as CNT-0, CNT-2, CNT-4 and CNT-
6, respectively.
1
1
1
(1)
(2)
=
+
I
IK ID
ID =0.62nFAD2/3ω1/2ν −1/6C
O2
Rotating ring-disc electrode (RRDE) measurements were
performed in O2-saturated 0.1 M KOH solution at a sweep rate
of 5 mV s-1. The ring potential was set as 1.2 V and the electron
Preparation of N/S co-doped exfoliation MWCNT
0.1 g of partially unzipped CNTs mixed with 1.0 g of thiourea
was grounded into fine powder in a mortar. The mixture was
then heated at 900 °C for 2 h under flowing N2 in a tube
furnace to obtain nitrogen and sulfur co-doped MWCNTs
(NSCNT-3). For comparison, NSCNT-0, NSCNT-2, NSCNT-4 and
NSCNT-6 were also prepared using the same procedure.
The main text of the article should appear here with headings
as appropriate.
-
transfer number (n) and HO2 yield were calculated by using
equation (3) and (4). The collection efficiency (N) was
calibrated with and without a thin active layer on the disk via
Fe2+/Fe3+ couple in a 10 mM K3[Fe(CN)6] solution. After
calibration, the value of N was calculated to be 0.37.
4ID
(3)
n =
IR
ID +
N
Physical characterization
2IR
Powder X-ray diffraction (XRD) was conducted by using an
X'Pert PRO diffractometer, and diffraction patterns were
collected using Cu Ka (λ = 1.5406 Å) radiation at a scanning
rate of 4 °/min. Thermal gravimetric analysis (TGA) was carried
out on TA-Q500 Instrument with a heating rate of 5 °C/min. X-
ray photoelectron spectroscopy (XPS) data were obtained
using an AXIS-ULTRA DLD-600W Instrument. Fourier transform
infrared (FT-IR) spectra of the samples were collected on
VERTEX 70, BRUKER Inc. S/TEM images were obtained using
aberration-corrected Hitachi HD2700C Scanning Transmission
Electron Microscope operated at 200 keV. Raman spectra were
taken by a LabRam HR800 spectrometer with a 532 nm laser
excitation.
(4)
N
H2O2 % =
×100%
IR
N
ID +
In the four equations, I, IK, ID represent the measured current,
kinetic current and limiting diffusion current, respectively, n is
the overall electrons transfer number, F is the Faraday
constant (96 485 C mol-1), A is the electrode area (A = 0.196
cm2), D is the O2 diffusion coefficient (D = 1.9×10-5 cm2 s-1), ν is
the kinematic viscosity of the electrolyte (ν = 1.13×10-2 cm2 s-1),
ω is the rotating speed of RDE, CO2 is the concentration of O2
(CO2 = 1.2×10-3 mol L-1). Before adding the data into the
equation, rotating speed was converted from rpm into rad s-1.
Results and discussion
Electrochemical measurements
Due to the harsh oxidizing of KMnO4 and concentrated H2SO4,
the process of MWCNT unzipping would introduce plenty of
oxygen functional groups and defects. Thus, the exfoliation
degree of MWCNTs could be represented by the water
dispersible ability, where, the more oxygen functional groups
and defects, the better hydrophilic. The water dispersible
ability for carbon nanotubes with different exfoliation degrees
are shown in the photograph illustrations (Figure S1). By
adding 5.0 mg of samples into the vials separately, the five
samples display gradient dispersible capabilities from non-
The electrochemical measurements were performed on a
Autolab PGSTAT302N electrochemical workstation equipped
with a high speed rotator from Pine Instruments in which a
three-electrode system consisting of a glassy carbon working
electrode (5 mm in diameter), a piece of carbon paper as
counter electrode to eliminate the contamination from Pt, and
reverse hydrogen electrode as reference electrode at room
temperature (298 K) in 0.1 M KOH solution. 5 mg of samples
was dispersed in 1 mL of isopropanol/Nafion hybrid solutions
and ultrasonic dispersed to form homogeneous ink. 16 μL of
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dispersible to easily dispersible property. After ultrasound for
The morphology of the NSCNT materials were first characterized
60 s, the five samples were successively standing for 15 min by scanning electron microscopy (SEM) measurement. The
and 30 min. It can be seen that the CNT-0 barely dispersed in integrated one dimensional nanotube structure can be observed in
water and the dispersible ability was successively enhanced Figure 2a. The defect-free structure hindered an adequate
from CNT-2 to CNT-6. The XRD pattern (Figure 1a) exhibits the interaction of the inner tube-wall of NSCNT-0. A transition state
structural transformation of CNTs with different exfoliation morphology was obtained by partial exfoliation of MWCNTs. As
degrees. The diffraction peak related to the (002) plane at shown in Figure 2b, the NSCNT-3 materials are in stereo structure
around 26 ° gradually disappeared, while the peak intensity with CNTs as skeleton supported exfoliated graphene nanoribbons.
related to (001) plane of graphene oxide at around 10 ° was In an electrochemical condition, the interconnected structure could
increasing, suggesting the increasing of the exfoliation degree store electrolyte initially, providing a quick supply and short
of CNTs. It is worth noting that the (001) plane was evidently diffusion distance. On the contrary, due to the full unzipping of
moved to low-angle region, indicating the enlargement of MWCNTs into graphene nanoribbons, the result NSCNT-6
lamellar spacing. FT-IR spectrum (Figure S2) shows the experienced severe agglomeration because of the π-π re-stacking
gradually enhanced absorption peaks of C=O bond (1735 cm-1), under high temperature calcination (Figure 2c). Raman spectra
C=C bond (1622 cm-1) and epoxy C-O bond (1228 cm-1), (Figure 1b) of all the three samples show characteristic D and G
respectively. The increased intensity of the three characteristic band located at 1350±20 cm-1 and 1575±20 cm-1, respectively. The
G band is attributed to the first order scattering of the E2g mode of
sp2 carbon atoms, while the D band is induced by defects or
27,
disorder
28. Intensity ratio (ID/IG) of NSCNT-0, NSCNT-3 and
NSCNT-6 were calculated to be 0.80, 1.46 and 1.30, respectively.
The increasing of ID/IG was due to the partial exfoliation of MWCNTs,
leaving many defects and disordered transition state. The following
decrease of ID/IG from NSCNT-3 to NSCNT-6 indicated the strong π-π
stacking and Van der Waals interactions between graphene
nanoribbons layers, leading to stacking of nanoribbons and
lowering the defects as well as disordered structures. Moreover,
the G band of NSCNT-3 and NSCNT-6 slightly shift toward higher
frequency with respect to that of NSCNT-0, indicating the
successfully p-type doping of heteroatoms in the composite
Figure 1. (a) XRD patterns of exfoliated CNT-0, CNT-2, CNT-3, CNT-4
and CNT-6; (b) Raman spectra of NSCNT-0, NSCNT-3 and NSCNT-6.
structure29,30
.
peaks represents the deepening of oxidation degree which is in
consistent with the XRD patterns in Figure 1a.
Figure 2. SEM images of NSCNT-0 (a), NSCNT-3 (b) and NSCNT-6 (c).
The transmission electron microscopy (TEM) image in Figure 3a the part of un-zipped CNTS. The internal nanotubes serve as
shows the criss-cross of NSCNT-0 sample which maintained the support intercalated outward graphene nanoribbons, forming a
integrated structure of MWCNTs after acid treatment. Figure 3b three dimensional stereo structure and preventing the graphene
and 3d present the TEM images and HAADF-STEM images of layers from re-stacking when suffering from high temperature
NSCNT-3 sample. Most of the nanotubes are in looseness state with annealing. In contrast, NSCNT-6 shows severe re-stacked structure
outer walls damaged and exfoliated, forming layered ribbon-like (Figure 3c) owning to the strong π-π stacking and Van der Waals
structures (yellow arrows in Figure 3b), which was different from interactions between graphene nanoribbons which is inconsistent
NSCNT-0 in Figure 3a. Graphene nanoribbons were existed in two with the SEM image in Figure 2c. From the corresponding HRTEM
forms: i, distributed around the remained CNTs; ii, connected with image (inset of Figure 3a and Figure 3b), it can be concluded that
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the wall thickness (between the two red arrows) become thinner represent the remained CNTs, while NSCNT-3 exhibited another
from NSCNT-0 to NSCNT-3. The d-spacing of NSCNT-3 between the main pore size at 20 nm and NSCNT-6 exhibited two more main
layers in the tubes are no longer dense after harsh oxidation pore sizes distribution at 20 – 30 nm and 80 nm. Both the increasing
process. The nitrogen adsorption, desorption isotherms and pore of specific surface area and enlarging of pore size distribution would
size distribution were obtained at 77.3 K (Figure S3a and S3b) for result in the improvement of ORR performance. Moreover, energy
NSCNT-0, NSCNT-3 and NSCNT-6. As a result, the specific surface dispersive spectrometer (EDS) analysis of NSCNT-3 illustrates a
area are 140.0 m2 g-1, 472.5 m2 g-1 and 322.3 m2 g-1 for NSCNT-0, homogenous distribution of N and S (Figure 3d-i). Considering that
NSCNT-3 and NSCNT-6 catalysts. Obvious hysteresis loop indicated thiourea was completely decomposed above 600 oC, as shown in
mesoporous structure of the catalysts. Meanwhile, the three thermo gravimetric analysis (TGA) (Figure S4), N and S elements are
catalysts showed a similar pore size distribution at 6.4 nm which proved to be doped into the exfoliated structure.
Figure 3. (a) TEM images of NSCNT-0 and the corresponding HR-TEM image (inset); (b) TEM image of NSCNT-3 and the corresponding HR-
TEM image (inset); (c) TEM image of NSCNT-6; (d) High-angle annular dark field (HAADF)-STEM image of NSCNT-3 with the corresponding
carbon (e), nitrogen (f), oxygen (g), sulfur (h) and the composite (i) of EDS elemental mapping images.
peaks at 283.9 eV and 287.2 eV are corresponding to C-S-C and C-N-
C bond 21, 32, 33, further confirming that N and S atoms have been
The surface elemental composition of NSCNT-3 was determined
successfully doped into the stereo framework. Meanwhile, different
by XPS analysis. As shown in Figure 4a, the XPS spectrum of NSCNT-
oxygen containing groups, C-OH (285 eV), C=O (285.7 eV) and COOH
3 exhibits five characteristic peaks located at ca. 165, 228, 285, 399
and 533 eV, corresponding to S 2p, S 2s, C 1s, N 1s, and O 1s31
,
(289.0 eV) were also observed in NSCNT-3. The high-resolution N1s
spectrum (Figure 4c) shows three single N species peaks at 398.2 eV,
399.8 eV and 401.4 eV, corresponding to pyridinic N, pyrrolic N and
graphitic N 34, 35, respectively. Moreover, NSCNT-3 contains S atoms
that form three single peaks shown in Figure 4d at 163.8 eV, 165.0
respectively. The elemental content of N and S for NSCNT-3 is 1.94%
and 1.52%, respectively. The high-resolution spectrum of C 1s in
NSCNT-3 (Figure 4b) was deconvoluted into six peaks. Among them,
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-
eV and 168.8 eV, which are attributed to sulfur in C-S-C bonds, mechanism with Pt/C: the protonation of O2 on the active sites of
conjugated -C=S- and -SOn- bonds 36, 37, respectively.
catalyst is the main rate-determining step 38-40
.
The electrochemical performance of the N/S dual-doped samples
The polarization curves for the ORR on NSCNT-3 at different
towards ORR was studied by cyclic voltammetry (CV) in N2- and O2- rotation speeds are shown in Figure 5c. Due to the difference of O2
saturated 0.1 M KOH solution. In N2- saturated KOH solution, there diffusion rate on the electrode surface, the limiting diffusion
was no redox peaks, however, the CVs exhibit obvious oxygen current density increased when accelerating the rotation speeds.
reduction peaks in O2 saturated solution (Figure S5). In addition, the The inset of Figure 5c shows the Koutecky-Levich (K-L) plots (j vs.
NSCNT-3 shows a sharp oxygen reduction peak at 0.79 V, while the
peak potential of NSCNT-0, NSCNT-2, NSCNT-4 and NSCNT-6 are at
ω
1/2) derived from equation (1) and (2) (analogous curves and K-L
plots for NSCNT-4 are given in Figure S6), respectively. The three
fitting straight lines show good linearity and demonstrate first-order
ORR kinetics at these potentials 1, 41. The electron transfer number
(n) for NSCNT-3 was quantitatively calculated to be 3.86, 3.81 and
3.97 (at potentials of 0.7 V, 0.75 V and 0.80 V), while NSCNT-4 was
3.47, 3.51 and 3.65 (at potentials of 0.65 V, 0.70 V and 0.75 V) by
using the slopes of these plots, indicating a 4-electron pathway
dominated ORR process. To gain further insights into the ORR
mechanism, rotating ring-disc electrode (RRDE) for the LSV
measurement was conducted on NSCNT-3. The negligible ring
current density (the oxidation of peroxide generated on the ring
electrode, inset of Figure 5d) in comparison to the disc current
-
density demonstrated little HO2 yield during the ORR process. The
-
yield of HO2 and electrons transfer number (n) derived from the
-
ring and disc current are presented in Figure 5d. Obviously, the HO2
yield and the electrons transfer number at a potential range from
0.1 V - 0.85 V on the NSCNT-3 were below 6 % and closed to 4,
respectively, which are close to the RRDE results of Pt/C (Figure S7).
-
The optimal negligible HO2 yield and near 4-electron transfer
number suggested a 4-electron reaction dominant pathway and
was in consistent with the RDE results.
Figure 4. (a) XPS spectrum of NSCNT-3 and the corresponding
high-resolution spectrum of C 1s (b), N 1s (c), and S 2p (d).
0.66 V, 0.75 V, 0.76 V and 0.75 V, respectively. The higher peak
potential indicates better electrocatalytic activity of the NSCNT-3
towards ORR. To better understand the ORR catalytic activity of the
samples, linear sweep voltammetry (LSV) measurement was
conducted with Pt/C catalyst for comparison. Among the
investigated samples, NSCNT-3 exhibits the best ORR activity in
terms of onset and half wave potentials (0.97 V and 0.81 V vs. RHE)
which were close to commercial Pt/C (0.96 V and 0.80 V, Figure 5a).
However, NSCNT-4 showed well onset and half wave potential (0.97
V and 0.78 V) but lower limiting diffusion current density. The
optimal limiting diffusion current density of NSCNT-3 (equal to Pt/C)
was due to the three dimensional structure of carbon nanotube
skeleton supported exfoliated graphene nanoribbons. The poor
performance of NSCNT-6 was caused by the severe agglomeration
of graphene nanoribbons after high temperature calcination Figure 5. (a) ORR polarization curves of NSCNT samples and Pt/C at
procedure. Therefore, it is concluded that an appropriate a rotation speed of 1600 rpm in O2-saturated 0.1 M KOH solution;
exfoliation degree of MWCNT was necessary for superior ORR (b) Tafel plots of ORR polarization curves of (a); (c) ORR polarization
performance, while higher or inadequate exfoliation would result in curves of NSCNT-3 in O2-saturated 0.1 M KOH solution at different
poor catalytic activity. Figure 5b shows the Tafel plots of the five rotating speed and its corresponding Koutecky-Levich plots under
NSCNT samples and Pt/C by plotting the logarithm of the kinetic potentials of 0.65 V, 0.70 V and 0.75 V (inset); (d) The electron-
current density derived from the ORR curves. The kinetic current transfer number n, H2O2 yield and RRDE voltammograms (inset) for
density was derived from equation 1. At low over-potentials, the NSCNT-3 catalyst.
Tafel slope of NSCNT-3 (67.1 mV dec-1) is close to the Pt/C (69.1 mV
dec-1), indicating that NSCNT-3 undergoes the similar ORR
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Long-term catalytic stability and fuel crossover effect are potential cycles; (b) i-t chronoamperometric stability measurement
important issues challenging the cathode catalysts in current fuel (at a potential of 0.7 V) of NSCNT-3 and Pt/C in O2-saturated 0.1 M
cell techniques 42-44. The durability of NSCNT-3 was first evaluated KOH solution at a rotating speed of 1600 rpm; (c) Polarization
by potential cycling between 0.05 and 1.0 V in O2-saturated 0.1 M curves of NSCNT-3 and Pt/C in O2-saturated 0.1 M KOH solution and
KOH solution at a scan rate of 200 mV s-1. As shown in Figure 6a, the the presence of 1 M methanol; (d) Methanol crossover tests for
NSCNT-3 catalyst exhibited negligible oxygen reduction peak NSCNT-3 catalyst performed on RDE with a rotation rate of 1600
potential shift after 5000 and even 10,000 potential cycles, rpm by injecting methanol into the electrolyte to reach
indicating superior catalytic stability. In addition, concentration of 1 M.
a
chronoamperometric measurements at constant potential of 0.7 V
and rotation speed of 1600 rpm were performed in O2-saturated
solution. As shown in Figure 6a, the NSCNT-3 displayed slight
activity loss while Pt/C suffered from gradually activity fading and
only 64 % current remaining after 15000 s. These results indicate a
much better stability of NSCNT-3 than Pt/C catalyst in alkaline
media. The CV curves (Figure S8) of Pt/C electrode in the presence
of 1 M methanol showed two characteristic oxidation peaks at 0.94
V and 0.78 V, which represent the methanol oxidation peak and CO
oxidation peak45, respectively. The methanol oxidation reaction and
oxygen reduction reaction created a competitive relationship which
would, to a certain degree, decrease the oxygen reduction activity.
While the CV curves of before and after adding 1 M methanol for
NSCNT-3 exhibited no obvious change in O2-saturated solution,
suggesting better methanol tolerance. Figure 6c compared the ORR
polarization curves on NSCNT-3 and Pt/C electrode in O2-saturated
0.1 M KOH in the presence of 1 M methanol at a rotation rate of
1600 rpm. NSCNT-3 showed negligible activity with and without 1 M
methanol. However, Pt/C exhibited an obvious oxidation peak at
about 0.7 V and significantly negative shift of half-wave potential
about 250 mV, indicating severe ORR activity loss in presence of
methanol. Moreover, potentiostat curves at 0.7 V was conducted
for NSCNT-3 and Pt/C by injected 1 M methanol into the solution at
400 s (Figure 6d). NSCNT-3 showed slight fluctuation and remains
stable current response while Pt/C suffered a sharp loss of 23.5 %
activity. These results indicate the better catalytic stability and
methanol tolerance of NSCNT-3 than commercial Pt/C catalyst
which is suitable as cathode catalyst for alkaline fuel cells.
Conclusions
In summary, nitrogen and sulfur co-doped 3-D structured
carbon nanotubes/graphene nanoribbon composites have
been successfully prepared. The internal nanotube skeletons
effectively prevent the outward graphene nanoribbons from
accumulation under high temperature and electrochemical
process. The unique stereo structure and efficient N/S co-
doping favor the adequate penetration with electrolyte and
provide more active sites for the ORR. NSCNT-3 showed
excellent catalytic activity including a highly positive onset
potential and half-wave potential which is similar to Pt/C
catalyst. Moreover, the NSCNT-3 catalyst exhibited superior
stability and methanol tolerance capability which made great
improvement compared with Pt-based catalysts.
Acknowledgements
This work was supported by the National Natural Science
Foundation (21306060, 21573083), the Program for New
Century Excellent Talents in Universities of China (NCET-13-
0237), the Fundamental Research Funds for the Central
University (2013TS136, 2014YQ009). The authors thank the
Analytical and Testing Center of HUST for XRD, STEM
measurements.
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