Ultrafine N-doped carbon nanoparticles with controllable size to enhance electrocatalytic activity for oxygen reduction reaction

Article abstract of DOI:10.1039/C6RA22145A

There are several drawbacks to the current oxygen reduction reaction (ORR) involving the kinetically sluggish process and expensive catalysts which impede the mass production of noble-metal materials for application in clean and efficient energy conversion devices. Herein, a type of ultrafine carbon nanoparticle with high nitrogen doping concentration can be simply prepared from a conveniently available precursor through a green and cost-effective hydrothermal process. With the aid of water, not only is the product's specific surface area enlarged, but the pore structure is also enriched, and there is improvement in the degree of graphitization and nitrogen-doped content. Meanwhile, the size of the carbon particle can be readily tuned to a nanoscale by changing the duration of the hydrothermal reaction. Interestingly, electrocatalytic activity is dramatically enhanced with the transformation of the carbon size and nitrogen structure. The characterization reveals that we have achieved a highly satisfactory electrocatalytic activity for ORR, a relatively positiv-e onset potential of -0.02 V (vs. Hg/HgO) in alkaline media and 0.55 V (vs. Ag/AgCl) in acidic media, a higher diffusion-limiting current density than that of Pt/C, as well as outstanding stability and superior tolerance durability to methanol. Both the simplicity of the operation and the employed biomass precursor exactly meet the criteria for significant cost-saving, easy scale-up and eco-friendly demands for energy storage.

Full text of DOI:10.1039/C6RA22145A

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Ultrafine N-doped carbon nanoparticles with
controllable size to enhance electrocatalytic
Cite this: RSC Adv., 2016, 6, 110758
activity for oxygen reduction reaction†
Wenxi Wang,‡ Yang Shi,‡ Minchan Li, Zhenyu Wang, Shaofei Wu, Fucong Lyu,
*
Chaoqun Shang and Zhouguang Lu
There are several drawbacks to the current oxygen reduction reaction (ORR) involving the kinetically
sluggish process and expensive catalysts which impede the mass production of noble-metal materials
for application in clean and efficient energy conversion devices. Herein, a type of ultrafine carbon
nanoparticle with high nitrogen doping concentration can be simply prepared from a conveniently
available precursor through a green and cost-effective hydrothermal process. With the aid of water, not
only is the product's specific surface area enlarged, but the pore structure is also enriched, and there is
improvement in the degree of graphitization and nitrogen-doped content. Meanwhile, the size of the
carbon particle can be readily tuned to a nanoscale by changing the duration of the hydrothermal
reaction. Interestingly, electrocatalytic activity is dramatically enhanced with the transformation of the
carbon size and nitrogen structure. The characterization reveals that we have achieved a highly
satisfactory electrocatalytic activity for ORR, a relatively positive onset potential of ꢀ0.02 V (vs. Hg/HgO)
in alkaline media and 0.55 V (vs. Ag/AgCl) in acidic media, a higher diffusion-limiting current density than
that of Pt/C, as well as outstanding stability and superior tolerance durability to methanol. Both the
simplicity of the operation and the employed biomass precursor exactly meet the criteria for significant
cost-saving, easy scale-up and eco-friendly demands for energy storage.
Received 4th September 2016
Accepted 13th November 2016
DOI: 10.1039/c6ra22145a
www.rsc.org/advances
Recent progress has resulted in the incorporation of
heteroatoms (N, P and B) into a sp2-hybridized carbon frame-
1 Introduction
work, giving rise to enhanced electrocatalytic activities and
reaction kinetics as a result of the changes in the charge and
spin densities of the carbon lattice, which originates from the
difference in electronegativity between carbon atoms and
heteroatoms.19,22–25 In the past, the most frequently employed
approaches to multiple-doping carbon have been high
temperature post-treatment, the electrochemical method,
liquid phase polymerization and plasma treatment or chemical
vapour deposition.19,26–36 These methods are generally decient
and involve tedious and complex experimental procedures or
harsh reaction conditions due to the inclusion of various
synthetic chemicals. In the current case, it is expected that
heteroatom doping will be realized by simple and eco-friendly
methods for fabrication of affordable carbonaceous material
and ultimately be employed as a substitute for Pt-based ORR
electrocatalysis. Moreover, it is widely accepted that low cata-
lytics offer the nonuniform and insufficient exposure of the
active catalytic sites of pyrolytic products,37–39 which is inu-
enced, to a large extent, by the size, shape and nanoscale
porosity of the electrocatalyst. As catalytic character alters
different with carbonaceous precursors, a large quantity of
precursors have been investigated over past years. To date, the
precursor development of doped carbon material has moved
The increasing global energy crisis and environmental
contamination have stimulated worldwide efforts to rationally
explore alternative clean and sustainable energy generation.1–3
In particular, it is highly anticipated that fuel cells and metal–
air batteries will be adopted in the automotive eld in the
future.4,5 Nevertheless, outside the connement of the sluggish
kinetics of ORR at the cathode, sufficient high-efficiency elec-
trocatalysts are generally desired for practical applications.
State of the art electrocatalysts used in the ORR predominantly
rely on the best known platinum or its alloys,6–9 although the
high cost and declining activity of these noble metal-bases have
severely obstructed their widespread commercialization.10–14 In
this regard, researchers have their hearts set on extensively
developing highly active and cost-effective non-precious metal
catalysts15–18 or metal-free heteroatom-enriched (e.g. N, ꢀB, S and
P) carbonaceous materials19–21 for the four-electron (4e ) ORR.
Department of Material Science and Engineering, South University of Science and
Technology of China, Shenzhen, Guangdong 518055, China. E-mail: luzg@sustc.
edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra22145a
‡ These authors contribute equally to this job.
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from expensive, hazardous inorganic and organic chemicals to further kept for 2 h in Ar atmosphere. Thereaer, the temper-
sustainable biomass,40–42 such as edible biomass and organic- ature sequentially increased to 800, 900 and 1000 ꢁC at a heating
rich wastes.
rate of 5 C minꢀ1, and remained nal temperature for 1.5 h.
ꢁ
At this juncture, it is therefore meaningful to directly design For simplicity, the carbon specimens were denoted as MCN-800,
a type of well-structured porous and heteroatom doping carbon MCN-900, MCN-1000. Separately, the carbon material was ob-
material from conveniently available biomass without using any tained from direct carbonization of raw Sterculia scaphigera
ꢁ
synthetic chemicals, which also simplies the procedure and without water assistance and hydrothermal process at 900 C,
decreases expense. The spindle-shaped Sterculia scaphigera, which labeled as R-900. Finally, another sample only absorbed
commonly found as an affordable herbal remedy in Indonesian abundant water was treated by freeze drying and carbonized at
ꢁ
and Chinese medicine, primarily consists of bassorin, galactose 900 C, which is designated as A-900.
and pentaglucose. Based on the previous work,43 the natural
Sterculia scaphigera expands signicantly in volume when
immersed in hot water, and hence an abundant pore structure
develops as it is moved in and out of water. Moreover the
morphology has a signicant variation through hydrothermal
treatment, tending to produce nanospheres with a diameter of
several nanometers. To the best of our knowledge, this is the
rst recorded instance that microporous nanosphere-shaped
carbon has been prepared from natural biomass Sterculia sca-
phigera, which displays highly efficient and selective catalytic
activity in ORR.
2.2 Physical characterizations
High-resolution TEM (JEOL, JEM-2010F at 200 kV) and eld
emission SEM (FESEM, JEOL 7001F) were performed to char-
acterize the size and morphologies of the prepared materials.
The degree of graphitization these samples was also detected by
Raman spectroscopy using Jobin-Yavon T6400 confocal Raman
spectrometer with 532 nm diode laser excitation on 1800-line
grating. Nitrogen adsorption/desorption isotherms were
measured on a Micromeritics, ASAP2020 gas sorption analyzer
at 77 K. All samples were rstly degassed at 200 ꢁC on a vacuum
line following a standard protocol before measurement. The
specic surface area was determined from N2 adsorption
isotherm by applying Brunauer–Emmett–Teller (BET) theory
and the pore size distributions were obtained from the
adsorption branch by means of Barrette–Joynere–Halenda
(BJH). The total pore volumes were determined from the
amount adsorbed at high relative pressure of 0.99. X-ray
photoelectron spectroscopy (XPS) was investigated using a VG
scientic ESCALAB 220iXL spectrometer with an Al Ka (hn ¼
1486.69 eV) X-ray source. All binding energies were carefully
aligned by reference to the C 1s peak (284.6 eV) arising from
surface hydrocarbons or possible adventitious hydrocarbon.
2 Experimental section
2.1 Preparation of microporous carbon nanoparticles (MCN)
For the synthesis of MCN, 50 g of thoroughly dried Sterculia
scaphigera (Fig. 1a) was rst immersed into hot water of
approximately at 80 ꢁC for 2 h. Aer immersion, the volume of
dried Sterculia scaphigera will swell signicantly, as shown in
Fig. 1b. The transparent colloform analogs were selected as
a hydrothermal precursor, and subsequently treated in a Teon
lined autoclave at 180 ꢁC for several hours. To identify the effect
of hydrothermal time on the oxygen reduction reaction activity,
the precursor was treated for 6, 12 and 18 h, respectively.
Subsequently, the products were collected through centrifuga-
tion at 9000 rpm for 5 min several times, and then dried in
a freezer dryer for overnight. All precursors were then carbon-
ized as the following process. The temperature was increasꢀe1d
2.3 Electrochemical measurements
2.3.1 Rotating disk electrode (RDE) measurement. The
electrochemical experiments were carried out in a conventional
ꢁ
ꢁ
three-electrode cell using
a Bio-Logic VMP3 workstation
from ambient temperature to 250 C at a speed of 1 C min
,
controlled at ambient temperature. Platinum, Hg/HgO and Ag/
AgCl were used as the counter and reference electrodes,
respectively. A glass carbon (GC) disk with a diameter of 5.0 mm
served as the substrate for the working electrode. To coat the
working electrode with that catalyst, typically, 2 mg of each
sample was ultrasonically dispersed in the 1 ml water/
isopropanol (v/v ¼ 1 : 3) mix solvent for 1 h. Then a GC elec-
trode was covered with 5 ml of the catalyst ink, which was further
adhered using 5 ml 0.05 wt% Naon solutions. The electrolyte
was saturated with pure oxygen by bubbling O2 in 0.1 M KOH
and 0.1 M HClO4 solution before each electrochemical experi-
ment. The cyclic voltammetry (CV) and oxygen reduction tests
were completed by rotating the electrode in the range of
400 rpm to 2025 rpm at a scanning rate of 10 mV sꢀ1. The CV
measurements were also performed in Ar ow for control
experiments. The stability of the catalyst possessing
outstanding ORR was evaluated by 2000 cycles at a sweep rate of
10 mV sꢀ1. Koutecky–Levich plots were analyzed at different
Fig. 1 Schematic process for preparation of the carbon nanoparticles.
Images of the biomass (a) Sterculia scaphigera; and (b) biomass having
absorbed water; (c) sample for hydrothermal treatment and (d) freeze
drying; (e) carbonaceous material.
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electrode potentials. The slopes of their best linear t lines were
used to calculate the number of electrons transferred (n) on the
basis of the Koutecky–Levich equation:44,45
1
1
1
1
1
¼
þ
¼ þ
Bu1=2 Jk
J
JL Jk
B ¼ 0.62nFCo(Do)2/3 ꢀ1/6
v
where J is the measured current Jk is the kinetic-limiting current
and u is the electrode rotation angular velocity. n is the trans-
ferred electron number, F is the Faraday constant, Co is the bulk
concentration of O2, Do is the diffusivity of O2 molecule in the
electrolyte, and n is the kinematic viscosity of the electrolyte. For
the Tafel plot, the kinetic current was obtained from the mass-
transport correction of RDE by:
J ꢂ JL
Jk ¼
JL ꢀ J
Fig. 2 (a) SEM images of hydrothermal product; (b) MCN-900 TEM
image and (c) the corresponding high magnification TEM, the inset
shows d-spacing of 5 stacking layers of grapheme sheets; (d) STEM
image of MCN-900 and EDS mapping of C, N, O.
2.3.2 Rotating ring-disk electrode (RRDE) measurement.
Catalyst and electrodes were prepared by the same method as
above. The ring current (IR) was measured with a Pt ring elec-
trode in the alkaline and acidic solution. The four-electron
selectivity of the catalyst was evaluated based on the H2O2
yield. The H2O2 yield and the electron transfer number (n) were
determined by the following equations:
morphology in Fig. 2b is kept as well as the appearance pre-
sented in Fig. 2a, even though the sample is subjected to high
temperature treatment. Additionally, the HRTEM image of
MCN-900 in Fig. 2c possesses a clear microporous and graphi-
tized structure (marked by the red arrow), demonstrating the
simple and efficient approach to producing porous carbon
material through the hydrothermal method. In an attempt to
further conrm the function of water, TEM and HRTEM images
of the sample derived from carbonizing the original and
absorbed water Sterculia scaphigera are also presented in
Fig. S2.† Although both samples exhibit the irregular micro-
morphology in Fig. S1b and e,† the evident discrepancy in the
porous and graphitized structure can be effortlessly distin-
guished. The consistent discrepancy is directly reected from
N2 adsorption/desorption and Raman characterization. These
porous structures are believed to facilitate mass exchange
during the ORR process and thereby improve the catalyst's
performance.47 The element mapping illustrated in Fig. 2d
infers that carbon, nitrogen and oxygen are contained in the
product and are dispersed homogeneously, indicating the
successful doping of as-obtained carbon material.
Raman spectroscopy is a useful tool for revealing the elec-
tronic properties of carbon-based materials. The Raman spectra
of Fig. 3 reveals that the D and G band were located around 1360
cmꢀ1 and 1580 cmꢀ1, respectively. It can been seen that the G
band arises from the bond stretching of all sp2-bonder pairs,
including C–C, C–N and C–O, while the D band is associated
with the sp3-hybridized defect site.48 In the Raman spectra of
carbon-based materials, the parameter ID/IG is an interesting
indicator of the level of defects within the material.9 From Fig. 3,
the ID/IG value is 1.23, 0.97 and 0.95 in the sequence of R-900 to
MCN-900. Combined with the other results, it is conrmed that
Ir=N
ꢀ
% HO2 ¼ 200 ꢂ
Id þ Ir=N
Id
n ¼ 4 ꢂ
Id þ Ir=N
where Id is the disk current, Ir is the ring current, and N is
current collection efficiency of the Pt ring. N is 0.32 from the
reduction of K3Fe[CN]6.46
3 Results and discussion
3.1 Micromorphology and composition
Fig. 2a and S1† present the SEM morphology of the three
specimens as prepared before carbonization. Obviously, the
SEM image of Fig. S1d† shows distinct micro-appearance
differences with raw Sterculia scaphigera (Fig. S1a†), power-
fully indicating that water gives rise to the development of
porous structure. When further treated with a hydrothermal
process, a chain of nanoparticles with single diameter of 50 nm
is obtained, as shown in Fig. 2a. Meanwhile, the effect of
hydrothermal time on the sample morphology is also investi-
gated to determine the optimum nanostructure. The SEM
images have been supplied in Fig. S2.† The morphologies of the
sample obtained from 6 and 18 h hydrothermal process are
indeed inhomogeneous, which suggests that hydrothermal
time has a profound inuence on the nanostructure. Never-
theless, the sample treated with 12 h hydrothermal treatment
displays the nanosphere-shaped morphology. Furthermore, the
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Fig. 5 (a) Full-scan XPS survey spectra of R-900, A-900 and MCN-
900, the inset is atomic ratio of each N species for the corresponding
sample; (b) high resolution N 1s XPS of R-900, A-900 and MCN-900.
Fig. 3 Raman spectra for R-900, A-900 and MCN-900.
resolution N 1s deconvolution analyses is shown in Fig. 5b. It
can be seen in Fig. 5b that all of the samples contain three types
of nitrogen congurations: pyridinic-N (398.9 eV), pyrrolic-N
(399.8 eV), and quaternary N (401.2 eV). However an eye-
catching difference is the amount of N species. Considering
the inset of Fig. 5a and Table S2,† although a relatively high
amount of pyridinic-N belongs to R-900, the total N content is
merely 1.3 at%, far lower than in MCN-900. The N content will
undoubtedly affect the electrochemical performance. It is
supposed that this results from the consumption of impurities
in raw material when annealing at high temperatures.49 In
Fig. 5a, there was still an observed impurity peak of R-900,
shown by the red arrows. Based on these results, it can be
reasonably speculated that the hydrothermal process contrib-
utes to retaining the N content and forms the efficient N species
that promote electrochemical activity.50
there is not only a defect caused by N doping, but there are also
graphite-like planes with high temperature treatment (see
Fig. S3†).
Surface area and pore distribution are important features of
doped carbon materials in the context of their electrochemical
activity. Fig. 4a and b show the nitrogen adsorption/desorption
isotherms and pore size distribution of the three samples. A
conspicuous feature of Fig. 4 is the different type of isotherms
for these samples, as well as pore size distribution, which
demonstrates the role of water in the preparation of porous
carbon material. As shown in Fig. 4a, MCN-900 exhibits the
typical type-IV isotherms, in sharp contrast to the type I
isotherms of sample A-900 and R-900. In addition, the pore size
distribution calculated using the BJH method is presented in
Fig. 4b, indicating the hierarchical porous texture of MCN-900
and monotonous pore size of samples R-900 and A-900. The
impact of water and temperature on the surface and pore size is
listed in Table S1.† It can be seen from Table S1† that the BET
specic surface area has been increased to 353.87 m2 gꢀ1 for
MCN-900, far more than that of R-900 at 2 m2 gꢀ1. Meanwhile,
the pore size and volume of A-900 and MCN-900 have expanded
enough to be identied from the data. These results are in
accordance with the differences observed in the TEM image.
Fig. 5a displays a comparison of the XPS survey spectra ob-
tained from R-900, A-900 and MCN-900 species, conrming the
presence of C, N and O elements with N/C atomic ratios of 1.8%,
3.25% and 6.71%, respectively. The high resolution C 1s spec-
trum of sample MCN-900 (Fig. S4†) affirms the existence of
effective function groups to improve ORR catalysis activity, such
as C–N (285.2 eV) and C]O (288.3 eV). For insight into the
amount and type of N-bonding conguration, the high-
3.2 Electrochemical activities
Inspired by the super chemical and physical properties of
demonstrated by the MCN-900 sample, we expected that porous
nanosphere-shaped carbon has great potential for application
in ORR catalysis. The electrocatalytic performance for ORR was
analyzed by rotating disk electrode (RDE) and rotating ring-disk
electrode (RRDE) measurement in alkaline media (0.1 M KOH)
and acidic media (0.1 M HClO4). A signicant redox peak is
found at approximately ꢀ0.11 V for MCN-900 electrode in
Fig. 5a and around ꢀ0.07 V for Pt/C electrode in Fig. 6b, rep-
resenting the reduction process in O2-saturated KOH solu-
tion.51,52 In order to compare the electrocatalytic performance of
all carbon materials in the ORR, linear sweep voltammetry (LSV)
curves were performed on a rotating disk electrode in an O2-
saturated 0.1 M KOH solution at a scan rate of 10 mV sꢀ1
(Fig. 6c). The ORR performance is clearly enhanced as the
preparation method changes. In particular, the sample MCN-
900 obtained from the hydrothermal treatment shows more
positive onset potential (around ꢀ0.02 V) than that of R-900
(ꢀ0.15 V) and A-900 (ꢀ0.095 V), which is on the verge of the
onset potential 0.03 V of commercial Pt/C.7,53 Moreover, the
diffusion-limiting current density of the MCN-900 electrode
reaches to ꢀ4.8 mA cmꢀ2, superior to the Pt/C electrode at ꢀ4.6
mA cmꢀ2. Considering that the electrocatalytic performance
changes with different temperature treatments, the LSV curves
of MCN-800 and MCN-1000 were also recorded at 1600 rpm,
presented in Fig. S6.† Apparently the MCN-900 exhibits the
Fig. 4 (a) Nitrogen adsorption/desorption isotherms for R-900, A-
900 and MCN-900; (b) pore size distribution of the corresponding
species.
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Fig. 7 The ORR performance in 0.1 M HClO4 solution. (a) LSV curves
of on R-900, A-900, MCN-900 and Pt/C at a rotation rate of
1600 rpm, the inset shows the corresponding electron transfer
numbers; (b) LSV curves of the MCN-900 with various rotation rates at
a scan rate of 10 mV sꢀ1, the inset is the corresponding Koutecky–
Levich plots (jꢀ1 vs. uꢀ1/2) at different potentials; (c) RRDE voltam-
mograms of MCN-900 in O2 saturated solution at rotation speed of
1600 rpm and sweep rate of 10 mV sꢀ1; (d) the electron transfer and
peroxide yields calculated from the RRDE measurement results; (e)
electrochemical stability comparison of MCN-900 and commercial
20% Pt/C as determined by continuous CV scanning in O2-saturated
0.1 M HClO4 solution, scan rate: 10 mV sꢀ1; (f) current–time chro-
noamperometric responses of MCN-900 and commercial 20% Pt/C
with the introduction of 3 M methanol at ꢀ0.3 V.
Fig. 6 The ORR performance in 0.1 M KOH solution. (a) LSV curves of
on R-900, A-900, MCN-900 and Pt/C at a rotation rate of 1600 rpm,
the inset shows the corresponding electron transfer numbers; (b) LSV
curves of MCN-900 with various rotation rates at a scan rate of 10 mV
s
ꢀ1, the inset is the corresponding Koutecky–Levich plots (jꢀ1 vs. uꢀ1/2
)
at different potentials; (c) RRDE voltammograms of MCN-900 in O2
saturated solution at rotation speed of 1600 rpm and sweep rate of
10 mV sꢀ1; (d) the electron transfer and peroxide yields calculated from
the RRDE measurement results; (e) electrochemical stability
comparison of MCN-900 and commercial 20% Pt/C as determined by
continuous CV scanning in O2-saturated 0.1 M KOH solution, scan
rate: 10 mV sꢀ1; (f) current–time chronoamperometric responses of
MCN-900 and commercial 20% Pt/C upon the introduction of 3 M
methanol at ꢀ0.3 V.
optimal ORR performance compared with what is mentioned
above. The more positive onset potential, as well as the higher
current density of MCN-900, may be attributed to the perfect
synergistic effect of N species as well as the abundant porous
structure, and enlarged specic surface area. Thus oxygen could
be reacted onto the catalyst surface with the preferred cong-
uration in a large quantity, and the electrons required for ORR
could be readily transferred into the catalyst.
As is well known, electrocatalysts with rapid ORR kinetics
and rich electron transfer to the adsorbed oxygen molecules are
highly desired. Therefore, to further clarify the kinetics of
carbon materials during the ORR process, their RDE curves
were performed under various rotation rates and analyzed using
the K–L equation, which is shown in Fig. 6b and S7.† The
electrons transferred per O2 of R-900, A-900 and MCN-900
electrode were calculated to be 2.69, 3.58 and 3.98, respec-
tively, suggesting the efficient one-step, 4-electron dominated
ORR process of MCN-900 sample, as is the case for Pt/C cata-
lyst.54,55 Meanwhile, the RRDE measurements conrm the
electron transfer number of approximately 4, as shown in
Fig. 6d, and the amount of H2O2 is less than 11%. Therefore the
method of using a four-electron pathway with high current to
catalyze O2 reduction is feasible and efficient.
In view of practical applications in fuel cells, the MCN-900
electrode was therefore subjected to further testing with
regards to the long-term stability and crossover effect for ORR.
Fig. 6e shows the electrochemical stability plot for MNC-900
and 20% Pt/C catalysts. A noticeable reduction current with
a well-dened cathodic peak (around ꢀ0.11 V) was maintained
very well for MCN-900 electrode, whereas Pt/C shows an obvious
diminution over the same period. This is possibly ascribed to
the higher strength of the covalent C–N bond and the absence of
activity degradation derived from Pt nanoparticle agglomera-
tion and dissolution.56–58 Fig. 6f illustrates the current–time
chronoamperometric responses to 3 M methanol at the MCN-
900 and Pt/C electrodes. There is no clear inversion of current
density for MCN-900 aer adding 3 M methanol, while a more
distinct drop is observed for commercial Pt/C, implying the
excellent tolerance of the MCN-900 electrode for methanol.
The ORR activity measurements in acidic media were carried
out to further test this excellent catalytic performance. It is
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obvious that a redox peak of MCN-900 with higher current Science Foundation of Shenzhen (No. JCYJ20150630145302231,
density is observed from Fig. S8a† compared with Fig. S8b,† JCYJ20150331101823677), the Shenzhen Peacock Plan
demonstrating an oxygen reduction process in acidic media. (KQCX20140522150815065). S. Y. thanks the support from the
And even better catalytic activity is observed in Fig. 7a in Guangdong and Shenzhen Innovative Research Team Program
comparison to that in alkaline media for MCN-900. The half- (No. 2011D052, KYPT20121228160843692).
potential difference between MCN-900 and Pt/C is only at
31 mV. The ORR kinetics in acidic media is reected in Fig. 7b,
which shows good linearity in the inset. Through the K–L plot, it
is calculated from linear t lines that the number of electrons
Notes and references
transferred is estimated to be 3.75–3.8 at the potential ranging
from ꢀ0.05 to 0.2 V. A four-electron pathway for ORR accom-
panying high current is revealed in acidic media. RRDE
measurements further conrm the ORR pathway, which is
consistent with the result obtained from K–L analysis. Mean-
while, the low ring current of MCN-900 suggests that a low
amount of hydro peroxide yields between 5–16% over a poten-
tial range of ꢀ0.2 to 0.6 V.
1 M. W. Kanan and D. G. Nocera, Science, 2008, 321, 1072–
1075.
2 J. Suntivich, K. J. May, H. A. Gasteiger, J. B. Goodenough and
Y. Shao-Horn, Science, 2011, 334, 1383–1385.
3 S. Wang, D. Yu and L. Dai, J. Am. Chem. Soc., 2011, 133, 5182–
5185.
4 M. Winter and R. J. Brodd, Chem. Rev., 2004, 104, 4245–4270.
5 P. G. Bruce, L. J. Hardwick and K. Abraham, MRS Bull., 2011,
36, 506–512.
6 Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier and
H. Dai, Nat. Mater., 2011, 10, 780–786.
7 S. Gao, K. Geng, H. Liu, X. Wei, M. Zhang, P. Wang and
J. Wang, Energy Environ. Sci., 2015, 8, 221–229.
8 W. Niu, L. Li, X. Liu, N. Wang, J. Liu, W. Zhou, Z. Tang and
S. Chen, J. Am. Chem. Soc., 2015, 137, 5555–5562.
9 Z. Ma, S. Dou, A. Shen, L. Tao, L. Dai and S. Wang, Angew.
Chem., Int. Ed., 2015, 54, 1888–1892.
The long-term stability and tolerance to methanol poisoning
effects in acidic media are also measured to reect the potential
for nal applications, as shown in Fig. 6e and f. An almost
negligible reduction current in the cathodic peak for MCN-900
is displayed in Fig. 7e, while Pt/C shows a dramatic current
reduction aer 2000 cycles. A more noticeable drop is found in
Fig. 7f when 3 M methanol is introduced, whereas a small
change is observed for MCN-900, demonstrating high methanol
tolerance even in acidic media.
10 A. Morozan, B. Jousselme and S. Palacin, Energy Environ. Sci.,
2011, 4, 1238–1254.
11 W. Wang, R. Wang, S. Ji, H. Feng, H. Wang and Z. Lei, J.
Power Sources, 2010, 195, 3498–3503.
4 Conclusions
In this work, nitrogen doped porous carbon nanoparticles were
successfully fabricated from Sterculia scaphigera, which is abun- 12 H. A. Gasteiger, S. S. Kocha, B. Sompalli and F. T. Wagner,
dantly available and a constantly renewed natural source. With Appl. Catal., B, 2005, 56, 9–35.
the assistance of water and hydrothermal treatment, it has been 13 H. Wang, R. Wang, H. Li, Q. Wang, J. Kang and Z. Lei, Int. J.
proven that not only pore texture and graphitization degree, but Hydrogen Energy, 2011, 36, 839–848.
also the relative nitrogen content of three types of nitrogen 14 Y. Bing, H. Liu, L. Zhang, D. Ghosh and J. Zhang, Chem. Soc.
species is signicantly improved. The preparation approach is
Rev., 2010, 39, 2184–2202.
ordinary, but extremely successful and efficient, producing highly 15 G. Wu, K. L. More, C. M. Johnston and P. Zelenay, Science,
conductive porous carbon. In this case, these characteristics of 2011, 332, 443–447.
carbon material show a perfect synergistic effect to enhance the 16 Z.-S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng and K. Mullen, J.
electrocatalytic oxygen reduction activity for fuel-cell technolo- Am. Chem. Soc., 2012, 134, 9082–9085.
gies. Further considering the effect of temperature on the struc- 17 K. Parvez, S. Yang, Y. Hernandez, A. Winter, A. Turchanin,
¨
¨
X. Feng and K. Mullen, ACS Nano, 2012, 6, 9541–9550.
tural and functional properties, the carbon material treated at
900 ꢁC serves as an optimal ORR electrocatalyst with high selec- 18 C. Shang, M. Li, Z. Wang, S. Wu and Z. Lu, ChemElectroChem,
tivity, remarkable stability and excellent methanol tolerance in
2016, 3, 1437–1445.
comparison to commercial 20% Pt/C electrodes in alkaline and 19 K. Gong, F. Du, Z. Xia, M. Durstock and L. Dai, Science, 2009,
acidic media. This kind of carbon material may be a promising 323, 760–764.
alternative for costly Pt-based electrocatalysts in fuel cells. The 20 L. Yang, S. Jiang, Y. Zhao, L. Zhu, S. Chen, X. Wang, Q. Wu,
preparation method validated in this study responds to the
requirement to produce high quality and outstanding perfor-
J. Ma, Y. Ma and Z. Hu, Angew. Chem., Int. Ed. Engl., 2011,
123, 7270–7273.
mance carbon-based electrocatalysts with a simple, cost-effective 21 Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X.
and environmentally benign approach.
Chen and S. Huang, CrossRef, CAS, Web of Science®
Times Cited, 37.
22 L. F. Chen, X. D. Zhang, H. W. Liang, M. Kong, Q. F. Guan,
P. Chen, Z. Y. Wu and S. H. Yu, ACS Nano, 2012, 6, 7092–
7102.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21671096), Natural Science Founda- 23 I. Y. Jeon, S. Zhang, L. Zhang, H. J. Choi, J. M. Seo, Z. Xia,
tion of Guangdong Province (No. 2016A030310376), the Natural
L. Dai and J. B. Baek, Adv. Mater., 2013, 25, 6138–6145.
This journal is © The Royal Society of Chemistry 2016
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RSC Advances
Paper
˜
´
24 I. Y. Jeon, H. J. Choi, S. M. Jung, J. M. Seo, M. J. Kim, L. Dai 42 E. Raymundo-Pinero, M. Cadek and F. Beguin, Adv. Funct.
and J. B. Baek, J. Am. Chem. Soc., 2012, 135, 1386–1393. Mater., 2009, 19, 1032–1039.
25 Y. Zheng, Y. Jiao, J. Chen, J. Liu, J. Liang, A. Du, W. Zhang, 43 W. Wang, Y. Wan, S. Wu, M. C. Li, L. Cao, F. Lv, M. Yang,
Z. Zhu, S. C. Smith and M. Jaroniec, J. Am. Chem. Soc.,
2011, 133, 20116–20119.
26 P. Chen, L.-K. Wang, G. Wang, M.-R. Gao, J. Ge, W.-J. Yuan,
S. Z. Fang, R. Sun and Z. Lu, RSC Adv., 2015, 5, 46558–46563.
44 N. S. Lewis and D. G. Nocera, Proc. Natl. Acad. Sci. U. S. A.,
2006, 103, 15729–15735.
Y.-H. Shen, A.-J. Xie and S.-H. Yu, Energy Environ. Sci., 2014, 45 C. Wei, L. Yu, C. Cui, J. Lin, C. Wei, N. Mathews, F. Huo,
7, 4095–4103.
T. Sritharan and Z. Xu, Chem. Commun., 2014, 50, 7885–
27 J. J. Zeng and Y. J. Lin, Appl. Phys. Lett., 2014, 104, 133506.
7888.
28 S. Wang, L. Zhang, Z. Xia, A. Roy, D. W. Chang, J. B. Baek 46 Z. Y. Wu, P. Chen, Q. S. Wu, L. F. Yang, Z. Pan and Q. Wang,
and L. Dai, Angew. Chem., Int. Ed., 2012, 51, 4209–4212. Nano Energy, 2014, 8, 118–125.
29 L. Qu, Y. Liu and J. B. Baek, ACS Nano, 2010, 4, 1321–1326. 47 C. You, X. Zen, X. Qiao, F. Liu, T. Shu, L. Du, J. Zeng and
30 S. Wang, E. Iyyamperumal, A. Roy, Y. Xue, D. Yu and L. Dai,
Angew. Chem., Int. Ed. Engl., 2011, 123, 11960–11964.
31 Q. Li, S. Zhang, L. Dai and L. S. Li, J. Am. Chem. Soc., 2012,
134, 18932–18935.
S. Liao, Nanoscale, 2015, 7, 3780–3785.
48 X. Wang, J. Wang, D. Wang, S. Dou, Z. Ma, J. Wu, L. Tao,
A. Shen, C. Ouyang and Q. Liu, Chem. Commun., 2014, 50,
4839–4842.
32 J. Liang, Y. Jiao, M. Jaroniec and S. Z. Qiao, Angew. Chem., Int. 49 X. Jin, G. Zhang, Y. Hao, Z. Chang and X. Sun, Chem.
Ed., 2012, 51, 11496–11500. Commun., 2015, 51, 15585–15587.
33 Z. W. Liu, F. Peng, H. J. Wang, H. Yu, W. X. Zheng and 50 D. H. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo and
J. Yang, Angew. Chem., Int. Ed. Engl., 2011, 123, 3315–3319. J. Nakamura, Science, 2016, 351, 361–365.
34 C. H. Choi, M. W. Chung, H. C. Kwon, S. H. Park and 51 C. Zhang, N. Mahmood, H. Yin, F. Liu and Y. Hou, Adv.
S. I. Woo, J. Mater. Chem. A, 2013, 1, 3694–3699. Mater., 2013, 25, 4932–4937.
35 Y. Zheng, Y. Jiao, L. Ge, M. Jaroniec and S. Z. Qiao, Angew. 52 L. Wang, A. Ambrosi and M. Pumera, Angew. Chem., Int. Ed.,
Chem., Int. Ed. Engl., 2013, 125, 3192–3198. 2013, 52, 13818–13821.
36 F. Braghiroli, V. Fierro, M. Izquierdo, J. Parmentier, A. Pizzi 53 L. Chen, R. Du, J. Zhu, Y. Mao, C. Xue, N. Zhang, Y. Hou,
and A. Celzard, Carbon, 2012, 50, 5411–5420. J. Zhang and T. Yi, Small, 2015, 11, 1423–1429.
37 J. S. Lee, G. S. Park, S. T. Kim, M. Liu and J. Cho, Angew. 54 S. Gao, H. Fan, X. Wei, L. Li, Y. Bando and D. Golberg, Part.
Chem., Int. Ed. Engl., 2013, 125, 1060–1064. Part. Syst. Charact., 2013, 30, 864–872.
38 D. Zhao, J. L. Shui, L. R. Grabstanowicz, C. Chen, 55 M. S. El-Deab, T. Sotomura and T. Ohsaka, Electrochem.
S. M. Commet, T. Xu, J. Lu and D. J. Liu, Adv. Mater.,
2014, 26, 1093–1097.
39 G. Nam, J. Park, S. T. Kim, D. B. Shin, N. Park, Y. Kim,
J. S. Lee and J. Cho, Nano Lett., 2014, 14, 1870–1876.
40 D. S. Su, ChemSusChem, 2009, 2, 1009–1020.
Commun., 2005, 7, 29–34.
56 H. Jin, H. Zhang, H. Zhong and J. Zhang, Energy Environ. Sci.,
2011, 4, 3389–3394.
57 H. Zhong, H. Zhang, Z. Xu, Y. Tang and J. Mao,
ChemSusChem, 2012, 5, 1698–1702.
41 C. Lavorato, A. Primo, R. Molinari and H. Garcia, Chem.–Eur. 58 H. Zhong, H. Zhang, S. Liu, C. Deng and M. Wang,
J., 2014, 20, 187–194.
ChemSusChem, 2013, 6, 807–812.
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