Facile synthesis of cobalt and nitrogen co-doped graphene networks from polyaniline for oxygen reduction reaction in acidic solutions

Article abstract of DOI:10.1039/c5ta09615d

Direct pyrlysis of a mixture comprising polyaniline nanofibers and cobalt(ii) nitrate is found to be a simple and scalable approach to the synthesis of Co and N co-doped graphene networks for oxygen reduction reaction in acidic solutions with high catalytic activity and excellent durability.

Full text of DOI:10.1039/c5ta09615d

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Materials Chemistry A
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Facile synthesis of cobalt and nitrogen co-doped
graphene networks from polyaniline for oxygen
reduction reaction in acidic solutions†
Cite this: DOI: 10.1039/c5ta09615d
Received 25th November 2015
Accepted 9th February 2016
Shuqin Peng, Hongmei Jiang, Youming Zhang, Lu Yang, Shuqin Wang, Wenfang Deng
*
and Yueming Tan
DOI: 10.1039/c5ta09615d
www.rsc.org/MaterialsA
Direct pyrolysis of a mixture comprising polyaniline nanofibers and oxygen reduction reaction (ORR).5 Pt and its alloy nanoparticles
cobalt(II) nitrate is found to be a simple and scalable approach to the have been extensively used as effective catalysts for ORR, but
synthesis of Co and N co-doped graphene networks for oxygen their high cost, limited supply, and weak durability severely
reduction reaction in acidic solutions with high catalytic activity and hinder their applicability to broad commercialization. Great
excellent durability.
research efforts have therefore been focused on the develop-
ment of non-precious metal catalysts with ORR activity
comparable to that of Pt-based catalysts but much lower cost.6
Although many materials can show high catalytic activities in
alkaline media,7 few non-precious metal catalysts can show
high ORR activities in acidic solutions. M–N–C (M ¼ Co and/or
Fe) catalysts are only one of several successful examples in
acidic solutions.8 M–N–C catalysts are a class of carbonaceous
materials doped with nitrogen functionalities possibly coordi-
nated by metallic species, which can be simply prepared via
pyrolysis of precursors comprising nitrogen, carbon and metal
salts. Despite the great progress, the ORR activities of M–N–C
catalysts are still less competitive than those of Pt-based cata-
lysts. To further improve the ORR activity, synthesis of a novel
M–N–C catalyst with high specic surface area, abundant pore
structure, and high electrical conductivity, is still of great
interest and importance.
In our design concept, Co and N co-doped 3D graphene can
integrate the advantages of 3D graphene with M–N–C catalysts,
and thus are expected to serve as a highly active catalyst for
ORR. In this work, novel Co and N co-doped graphene networks
(Co–N-GNWs) were facilely prepared by simple pyrolysis of
a mixture comprising polyaniline nanobers and cobalt(II)
nitrate. The as-prepared Co–N-GNWs with an interconnected
porous structure showed high specic surface area, high crys-
tallization degree, and Co and N co-doping, all of which are
benecial to the improvement of ORR activity. As a result, the
Co–N-GNWs showed comparable catalytic activity for four-
electron ORR in acidic solutions but superior stability and
methanol tolerance to a commercial Pt/C catalyst.
Three-dimensional (3D) graphene with interconnected porous
structures can show high specic surface area, excellent
conductivity, low mass density, elegant exibility and good
mechanical strength, due to the combination of 3D porous
structures and intrinsic properties of graphene.1 Various 3D
graphene materials have been extensively prepared and inves-
tigated for various applications including supercapacitors,
microbial fuel cells, lithium-ion batteries, electrocatalysis, solar
cells, environmental protection, and chemical sensors, with
better functions than 2D graphene sheets.2 3D graphene
materials derived from graphene oxide and reduced graphene
oxide have been prepared via controlled self-assembly and
template-directed deposition.3 However, graphene oxide prod-
ucts are generally prepared by conventional and improved
Hummers methods that involve complicated procedures,
making the whole synthesis procedure for 3D graphene time-
consuming. Highly conductive 3D graphene can be prepared on
porous substrates (e.g., Ni foam) by chemical vapor deposition
from molecular carbon precursors (e.g., CH4).4 However, special
porous metal substrates are strongly needed, which may
increase the cost of 3D graphene products. Therefore, it is
interesting and important to develop a simple and scalable
approach for 3D graphene preparation.
Electrochemical energy conversion devices, ranging from
fuel cells to metal–air batteries, require effective catalysts for
Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research
(Ministry of Education of China), College of Chemistry and Chemical Engineering,
Hunan Normal University, Changsha 410081, China. E-mail: tanyueming0813@
126.com
The synthesis procedure of Co–N-GNWs is shown in Scheme
S1 in the ESI.† In brief, the pre-synthesized polyaniline nano-
bers (Fig. S1 in the ESI†) were mixed with a cobalt(II) nitrate
powder in a weight ratio of 1 : 1 by a dry method using a mortar
† Electronic supplementary information (ESI) available: Experimental details and
additional gures. See DOI: 10.1039/c5ta09615d
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and pestle. The mixture was subjected to heat treatment at
200 ꢀC for 2 h and then at 900 ꢀC for 5 h under a N2 atmosphere,
at a heating rate of 10 ꢀC minꢁ1. Aer cooling to room
temperature, the resulting sample was etched in excessive 3 M
HCl aqueous solution. Finally, the pre-leached catalyst under-
went a second heat treatment at 900 ꢀC for 2 h under a N2
atmosphere.
The morphology of the as-prepared Co–N-GNWs was char-
acterized by scanning electron microscopy (SEM). As shown in
Fig. 1a–c, the Co–N-GNWs show an interconnected macro-
porous structure with pore sizes ranging from 200 to 400 nm.
The network structure was further conrmed by transmission
electron microscopy (TEM). As shown in Fig. 1d–f, the Co–N-
GNWs are obviously composed of 2D graphene nanosheets. The
thickness of the few-layer graphene wall is ꢂ3 nm, which was
conrmed by the high-resolution TEM (HR-TEM). Some small
carbon-coated metal nanoparticles also formed during the
synthesis process; so some mesopores with pore sizes smaller
than 30 nm can also be observed aer acid leaching (Fig. S2 in
the ESI†). The lattice fringes clearly show an interplanar spacing
of 0.33 nm, which is consistent with the (002) lattice spacing of
graphite.
Fig. 2 XRD patterns (a and d) and TEM images (b, c, e, and f) of
a mixture of polyaniline nanofibers and cobalt(II) nitrate after being
heated to 200 (a–c) and 900 ꢀC (d–f). Note here that these samples
were prepared only by heat treatment without acid leaching.
destroyed aer cobalt salt penetration; so no well-dened
nanowire structure is observed. The decomposition of cobalt(II)
nitrate can be explained as follows:
To understand the formation mechanism of graphene
networks, the mixture of polyaniline nanobers and cobalt(II)
nitrate was heated to different temperatures. Fig. 2a shows an
X-ray diffraction (XRD) pattern of the mixture aer being heated
to 200 ꢀC, which clearly indicates that some crystallized CoO
(JCPDS 89-7099) nanoparticles exist. Some diffraction peaks at
36.5ꢀ, 42.4ꢀ, 61.5ꢀ, 73.7ꢀ and 77.5ꢀ are observed, in good
agreement with (111), (200), (220), (311) and (222) planes of the
cubic CoO. Although some diffraction peaks of Co3O4 are
overlapped with those of CoO, small diffraction peaks at 18.8ꢀ,
31.3ꢀ, 59.4ꢀ and 65.2ꢀ are still observed, indicating that some
Co3O4 (JCPDS 64-0437) also exists at 200 ꢀC. The TEM image
(Fig. 2b) clearly indicates that small CoO nanoparticles are
embedded in a polyaniline matrix. The HRTEM image (Fig. 2c)
indicates that the lattice spacing is 0.205 nm, consistent with
the (200) lattice spacing of the cubic CoO. As is well known,
cobalt(II) nitrate (Co(NO3)2$6H2O) has a lower melting point
2Co(NO3)2 / 2CoO + 4NO2[ + O2[
(1)
(2)
6Co(NO3)2 / 2Co3O4 + 12NO2[ + 2O2[
i.e. Co(NO3)2 is converted to CoO and Co3O4. Aer pyrolysis
under a N2 atmosphere, polyaniline is converted to amorphous
carbon, which can react with CoO according to the following
reaction:
CoO + C / Co + CO[
(3)
The thermodynamic feasibility of formation of Co via the
reduction of CoO by carbon can be examined using Ellingham
diagrams. Kannan has reported that the thermodynamic
reduction temperature for CoO is ca. 505 ꢀC by using Ellingham
diagrꢀams.9 The heat treatment temperature in our experiment
(900 C) is higher than the thermodynamic reduction temper-
ature for CoO; so the reduction of CoO is feasible. The forma-
tion of Co is also conrmed by XRD. Fig. 2d shows an XRD
pattern of the mixture of polyaniline nanobers and cobalt(II)
nitrate aer being heated to 900 ꢀC, which indicates that some
well crystallized Co (JCPDS 15-0806) nanoparticles also exist
besides CoO and Co3O4. Three sharp diffraction peaks at 44.2ꢀ,
51.5ꢀ, and 75.9ꢀ are observed, in good agreement with (111),
(200), and (220) planes of the cubic Co. The average size of Co
nanoparticles calculated through Scherrer's equation by
analyzing the XRD diffraction peaks is ca. 30 nm. The diffrac-
tion peaks in Fig. 2d for various Co species are much sharper
than those in Fig. 2a, indicating that cobalt oxides undergo
aggregation at high temperatures. As shown in Fig. 2e, carbon
coated cobalt-based aggregates are observed. As shown in
Fig. 2f, the lattice spacing of 0.205 nm is consistent with the
(220) lattice spacing of Co3O4, and the lattice spacing of
0.177 nm is consistent with the (200) lattice spacing of cubic Co.
ꢀ
(55 C) than other cobalt(II) salts. The melted cobalt(II) nitrate
can penetrate into polyaniline nanobers. As shown in Fig. 2b,
the one dimensional structure of polyaniline was completely
Fig. 1 SEM (a–c) and TEM (d–f) images of Co–N-GNWs.
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Co can serve as a catalyst for graphitization at low tempera- ratio of D-band to G-band (ID/IG) is generally used to evaluate
tures;10 so graphitic carbon shells are coated on cobalt-based the crystallization degree of carbon materials. The values of
aggregates (Fig. 2f). Aer removing cobalt-based aggregates by ID/IG for Co–N-GNWs and N-CNWs are 0.46 and 1.18, respec-
acid leaching, open and interconnected carbon shells, i.e. gra- tively. The low ID/IG ratio also indicates a higher graphitization
phene networks, were obtained. Note that the second heat degree of Co–N-GNWs than that of N-CNWs.
treatment is not strongly needed for the formation of graphene
Fig. 3b shows nitrogen adsorption–desorption isotherms
networks, but is favorable for improving the catalytic activity and pore size distribution of Co–N-GNWs and N-CNWs. The
toward ORR.8a As shown in Fig. S3 in the ESI,† graphene Co–N-GNWs shows a high Brunauer–Emmett–Teller (BET)
networks with an interconnected macroporous structure are surface area of 401 m2 gꢁ1. A distinct hysteresis loop is observed
still observed in the absence of the second pyrolysis.
at high relative pressures (P/P0 > 0.43) for Co–N-GNWs, indi-
From the above discussion, we can conclude that cobalt(II) cating the presence of a mesoporous structure in Co–N-GNWs.
nitrate underwent melting, decomposition, and partial reduc- The pore size distribution of Co–N-GNWs (inset of Fig. 3b)
tion in the formation process of graphene networks. Thus calculated from absorption data using the Barrett–Joyner–
cobalt(II) nitrate plays very important roles in the synthesis of Halenda (BJH) model shows that abundant mesopores exist in
graphene networks. As shown in Fig. S4 in the ESI,† only the range of 3–30 nm. The N-CNWs show a BET surface area of
nitrogen-doped carbon nanowires (N-CNWs) can be obtained in 696 m2 gꢁ1. An obvious N2 adsorption at low relative pressures
the absence of cobalt(II) nitrate. The weight ratio of polyaniline (P/P0 < 0.1) but no hysteresis loop at high relative pressures can
nanobers and cobalt(II) nitrate is also important for controlled be observed for N-CNWs, indicating the presence of abundant
synthesis of Co–N-GNWs. When the weight ratio of polyaniline micropores but few mesopores in N-CNWs. The pore size
nanobers to cobalt(II) nitrate is 2 : 1, only carbon networks distribution (inset of Fig. 3b) also shows that Co–N-GNWs have
without a well-dened nanosheet structure are observed more abundant mesopores than N-CNWs.
(Fig. S5a in the ESI†). When the weight ratio of polyaniline
The involvement of both N and Co in Co–N-GNWs was
nanobers to cobalt(II) nitrate is 1 : 2, only graphene nano- veried by X-ray photoelectron spectroscopy (XPS). As shown in
sheets without an interconnected macroporous structure are Fig. 3c, XPS data reveal that the N content in Co–N-GNWs is as
observed (Fig. S5b in the ESI†).
high as 4.13 at%, which is a little lower than that in N-CNWs
The Co–N-GNWs were further studied by XRD and Raman (4.57 at%, Fig. S7a in the ESI†). Fig. 3d shows high resolution
spectroscopy. A strong and sharp diffraction peak observed at XPS spectra of N 1s for Co–N-GNWs. The N 1s peaks can be
26.1ꢀ (Fig. 3a) is attributed to the (002) plane of the hexagonal tted by four component peaks, namely, pyridinic N (398.6 eV)
graphite structure, indicating a high crystallization degree for bonded to two C atoms in six-membered rings at the edge of
Co–N-GNWs. In contrast, the N-CNWs synthesized in the carbon matrix, pyrrolic N bonded to two C atoms in ve-
absence of cobalt(II) nitrate shows no obvious diffraction peak membered rings at the edge of carbon matrix (399.8 eV),
in the XRD pattern. Three peaks at 1347 (D-band), 1572 (G graphitic N (401.2 eV) bonded to three C atoms in central or
band), and 2695 cmꢁ1 (2D band) can be observed in the Raman valley position of carbon matrix, and oxidative N species (403
spectrum for Co–N-GNWs (Fig. S6 in the ESI†). The intensity eV). The peak deconvolution analysis shows that the percent-
ages of pyridinic N, pyrrolic N, graphitic N, and oxidative N in
Co–N-GNWs are 22.7%, 15.8%, 52.4%, and 9.1%, respectively.
The percentages of pyridinic N, pyrrolic N, graphitic N, and
oxidative N in N-CNWs are 28.1%, 21.0%, 36.7%, and 14.2%,
respectively (Fig. S7b in the ESI†). Obviously, the percentage of
graphitic N in Co–N-GNWs (52.4%) is higher than that in
N-CNWs (36.7%). Because cobalt incorporation can result in
higher graphitization,10 other N components tend to be trans-
formed to graphitic N.11 Pyridinic and pyrrolic nitrogens might
coordinate with a metal to form M–Nx (M ¼ Fe or Co) moieties,
which serve as active sites of M–N–C catalysts for ORR in acidic
solutions.8d,8e XPS analysis (inset of Fig. 3c) also reveals the
presence of a trace amount of Co species (0.17 at%) in Co–N-
GNWs. Two sets of peaks are observed in the cobalt region for
Co–N-GNWs, with binding energies of ꢂ780 and ꢂ795 eV, cor-
responding to 2p3/2 and 2p1/2
.
As shown in Fig. S8 in the ESI,† tting of the C 1s spectrum
can be resolved into three individual component peaks at
binding energies of 284.7, 287.1 and 289.5 eV, representing
graphitic carbon, carbonyl group and carboxyl group, respec-
tively. XPS results reveal that the atom ratio between C and O in
Co–N-GNWs is ꢂ32 : 1, while the atom ratio between C and O in
N-CNWs ꢂ11 : 1 (Fig. S7a in the ESI†), indicating that heat
Fig. 3 (a) XRD patterns of Co–N-GNWs and N-CNWs prepared at
a carbonization temperature of 900 ꢀC. (b) Nitrogen adsorption/
desorption isotherms and pore distribution (inset) of Co–N-GNWs and
N-CNWs. (c) XPS survey spectrum and Co 2p peak (inset) of Co–N-
GNWs. (d) High resolution XPS spectrum of N 1s.
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treatment in the presence of cobalt(II) nitrate can result in graphene-like network structure can be prepared only when the
higher graphitization and lower O content. weight ratio of polyaniline nanobers to cobalt nitrate is 1 : 1
The Co–N-GNWs with unique structure and composition are (Fig. 1 and S5 in the ESI†). As shown in Fig. S10 in the ESI,† the
expected to have high ORR activity. The Co–N-GNWs were tested Co–N-GNWs show the highest catalytic activity, highlighting the
as a catalyst for ORR, and cobalt-free N-CWs and Pt/C were used advantage of the well-dened interconnected graphene-like
as reference catalysts. The ORR measurements of Co–N-GNWs network structure of Co–N-GNWs. The well-dened inter-
and N-CNWs were performed in an O2-saturated 0.5 M H2SO4 connected graphene-like network structure of Co–N-GNWs can
aqueous solution at room temperature by using a rotating disk not only increase electrochemically accessible active sites but
electrode (RDE). As is well known, the ORR activity of Pt/C also facilitate electron/mass transfer, resulting in enhanced
measured in a non-adsorbing electrolyte (HClO4) is much higher catalytic activity.
than that in an adsorbing electrolyte (H2SO4); so the ORR
To investigate the active sites of Co–N-GNWs, X-ray absorp-
measurement of Pt/C was performed in an O2-saturated 0.1 M tion ne structure (XAFS) spectroscopy measurement for Co–N-
HClO4 aqueous solution. A characteristic set of polarization GNWs was carried out. Fig. S11 in the ESI† shows the XAFS
curves for ORR on Co–N-GNWs, N-CNWs and Pt/C catalyst spectrum of Co–N-GNWs. The bonding environment of the Co
modied RDEs are displayed in Fig. 4a. The RDE polarization atoms is investigated by Fourier transforms of k2-weighted Co
curves show that the Co–N-GNW catalyst has an onset potential K-edge extended XAFS (EXAFS). The quantied tting result
of 0.95 V, which is much more positive than that of the cobalt- shows that Co–N bonding is found and its coordination number
free N-CNW catalyst (0.77 V). In addition, the diffusion-limiting was estimated to be ꢂ4.6. These results conrm the formation
current of the Co–N-GNW catalyst is much higher than that of of Co complexes. In fact, the formation of metal–nitrogen
the N-CNW catalyst. These results indicate that the Co–N-GNW complexes during heating treatments has also been reported
catalyst exhibits ORR activity superior to that of the N-CNW elsewhere.8d,8e Therefore, the high ORR activity of Co-NGNWs is
catalyst. The half-wave potential (E1/2) of the Co–N-GNW catalyst mainly attributed to the involvement of ORR-active Co–N
is 0.81 V, which is only a 30 mV negative shi compared with complex moieties in Co–N-GNWs.
a commercial Pt/C catalyst (0.84 V). Fig. 4b shows the corre-
To investigate the ORR catalytic pathway, rotating ring-disk
sponding Tafel plots of Co–N-GNW and Pt/C catalysts. A similar electrode (RRDE) measurements were carried out to monitor
slope was observed for Co–N-GNWs (63 mV decꢁ1) and Pt/C the formation of H2O2 in the ORR. As shown in Fig. S12 in the
(67 mV decꢁ1), suggesting a similar reaction mechanism of ORR ESI,† the H2O2 yield measured for the Co–N-GNW catalyst is
on the catalyst surface where the rate-determining step at both below 2%, and the corresponding electron transfer number (n)
catalysts was likely the rst electron reduction of oxygen. The is above 3.96 over the potential range from 0.1 to 0.9 V, indi-
kinetic mass activity (jm) of the Co–N-GNW catalyst at 0.80 V cating virtually complete reduction of O2 to H2O in a four-
calculated through the Koutecky–Levich equation is 13.2 A gꢁ1
,
electron process. In contrast, the N-CNW catalyst produced
which is higher than the values of polyaniline-based Fe–N–C 13.1–21.8% H2O2 and the value of electron transfer number is
catalyst (ꢂ6.7 A gꢁ1),8a VB12-based mesoporous Fe–N–C (5.0 A 3.56–3.74 over the potential range from 0.10 to 0.60 V. Fig. S13
gꢁ1),8b and phenylenediamine-based Fe–N–C (11.5 A gꢁ1),8c in the ESI† shows polarization curves for a Co–N-GNW catalyst
highlighting the high ORR activity of our Co–N-GNW catalyst. at different rotation rates. The limiting currents of the Co–N-
Note that a second heat treatment is strongly needed for the GNW catalyst increased with increasing rotation speed (from
improvement of ORR activity. As shown in Fig. S9 in the ESI,† the 400 to 2025 rpm). The corresponding Koutecky–Levich plots
half-wave potential of the catalyst prepared without a second within the potential range from 0.40 to 0.70 V exhibit good
pyrolysis is ꢂ120 mV negative shi compared with that of the linearity with a rather consistent slope, suggesting rst-order
catalyst prepared with a second pyrolysis, consistent with reaction kinetics for ORR with respect to oxygen concentration
previous report.7a The catalytic activities of the samples with in the solution. The slopes of their linear t lines are used to
different weight ratios of polyaniline nanobers to cobalt nitrate calculate the electron transfer number on the basis of the
were tested. Co–N-GNWs with a well-dened interconnected Koutecky–Levich equation. The Co–N-GNW catalyst exhibits the
electron transfer number of 3.95–3.96 depending on the
potentials, well consistent with the result obtained from RRDE
measurements.
Various catalysts were subjected to testing the possible
crossover in the presence of 1 M methanol. Fig. S14 in the ESI†
compares the effect of methanol on the ORR over various
catalysts. The ORR on a Pt/C catalyst is seriously restrained in
the presence of methanol due to the occurrence of methanol
oxidation reaction on Pt. In contrast, the ORR onset potential
and current density for Co–N-GNW and N-CNW catalysts
remain almost unchanged in the presence of methanol. These
Fig. 4 (a) ORR polarization curves for Co–N-GNWs, N-CNWs and Pt/
results suggest that the Co–N-GNWs and N-CNWs catalysts are
methanol tolerant and can selectively perform the ORR in the
presence of methanol. The durability of various catalysts toward
C catalysts recorded at room temperature at a sweep rate of 10 mV sꢁ1
and a rotation rate of 900 rpm. (b) Corresponding Tafel plots of Co–N-
GNWs and Pt/C catalysts.
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ORR was evaluated by the accelerated durability tests, which
were carried out at room temperature in O2-saturated 0.5 M
H2SO4 solutions by applying cyclic potential sweeps between 0.6
and 1.1 V at 50 mV sꢁ1 for 3000 cycles. As shown in Fig. S15 in
the ESI,† the Co–N-GNW and N-CNW catalysts showed a nega-
tive shi of the half-wave potential by 2 and 5 mV, respectively.
In contrast, the Pt/C catalyst showed a negative shi of the half-
wave potential by 25 mV. The durability of Co–N-GNW and Pt/C
catalysts toward ORR was also evaluated through chro-
noamperometric measurements at 0.70 V in an O2-saturated
0.5 M H2SO4 aqueous solution. As shown in Fig. S16 in the ESI,†
the Co–N-GNW catalyst lost only ꢂ13% of its initial activity aer
10 000 s continuous operation, whereas Pt/C lost nearly ꢂ32%
of its initial activity. These results indicate a much better
stability of the Co–N-GNW catalyst than the Pt/C catalyst in
acidic solutions.
In conclusion, Co–N-GNWs were facilely synthesized from
a mixture comprising polyaniline nanobers and cobalt(II)
nitrate via simple pyrolysis and acid leaching. The Co–N-GNWs
showed many attractive features including interconnected
network structure, high specic surface area, high crystalliza-
tion degree, and Co and N co-doping. More importantly, the
Co–N-GNWs showed comparable catalytic activity but superior
stability and methanol tolerance to a commercial Pt/C catalyst
for four-electron ORR in 0.5 M H2SO4 aqueous solutions. This
work not only provides a simple and scalable approach to direct
synthesis of 3D graphene from polymers, but also demonstrates
Lett., 2013, 13, 6136; (d) Y. Zhang, M. Chu, L. Yang,
W. Deng, Y. Tan, M. Ma and Q. Xie, Chem. Commun., 2014,
50, 6382; (e) B. Tang, G. Hu, H. Gao and Z. Shi, J. Power
Sources, 2013, 234, 60; (f) F. Liu, S. Chung, G. Oh and
T. S. Seo, ACS Appl. Mater. Interfaces, 2012, 4, 922; (g)
F. Yavari, Z. Chen, A. V. Thomas, W. Ren, H.-M. Cheng and
N. Koratkar, Sci. Rep., 2011, 1, 166.
3 (a) S. H. Lee, H. W. Kim, J. O. Hwang, W. J. Lee, J. Kwon,
C. W. Bielawski, R. S. Ruoff and S. O. Kim, Angew. Chem.,
Int. Ed., 2010, 49, 10084; (b) J. L. Vickery, A. J. Patil and
S. Mann, Adv. Mater., 2009, 21, 2180; (c) Z. Tang, S. Shen,
J. Zhuang and X. Wang, Angew. Chem., Int. Ed., 2010, 49,
4603.
4 (a) Z. P. Chen, W. C. Ren, L. B. Gao, B. L. Liu, S. F. Pei and
H. M. Cheng, Nat. Mater., 2011, 10, 424; (b) C. Wang, J. Xu,
M.-F. Yuen, J. Zhang, Y. Li, X. Chen and W. Zhang, Adv.
Funct. Mater., 2014, 24, 6372.
5 B. C. H. Steele and A. Heinzel, Nature, 2001, 414, 345.
6 (a) R. Bashyam and P. Zelenay, Nature, 2006, 443, 63; (b)
K. Gong, F. Du, Z. Xia, M. Durstock and L. Dai, Science,
2009, 323, 760; (c) M. Chu, Y. Zhang, L. Yang, Y. Tan,
W. Deng, M. Ma, X. Su, Q. Xie and S. Yao, Energy Environ.
Sci., 2013, 6, 3600; (d) K. Kwon, Y. J. Sa, J. Y. Cheon and
S. H. Joo, Langmuir, 2012, 28, 991.
7 (a) G. Du, X. Liu, Y. Zong, T. S. A. Hor, A. Yu and Z. Liu,
Nanoscale, 2013, 5, 4657; (b) B. Li, X. Ge, F. W. T. Goh,
T. S. A. Hor, D. Geng, G. Du and Z. Liu, Nanoscale, 2015, 7,
1830; (c) X. Ge, A. Sumboja, D. Wuu, T. An, B. Li,
F. W. T. Goh, T. S. A. Hor, Y. Zong and Z. Liu, ACS Catal.,
2015, 5, 4643; (d) R. Ning, J. Tian, A. M. Asiri, A. H. Qusti,
A. O. Al-Youbi and X. Sun, Langmuir, 2013, 29, 13146; (e)
R. Ning, C. Ge, Q. Liu, J. Tian, A. M. Asiri, K. A. Alamry,
C. M. Li and X. Sun, Carbon, 2014, 78, 60.
a
promising non-precious metal catalyst for fuel cell
applications.
Acknowledgements
We thank Shanghai Synchrotron Radiation Facility of Shanghai
Institute of Applied Physics for EXAFS measurements. This
work is supported by the National Natural Science Foundation
of China (21305041), Scientic Research Fund of Hunan
Provincial Education Department (13B063), and Program for
Science and Technology Innovative Research Team in Higher
Educational Institutions of Hunan Province.
8 (a) G. Wu, K. L. More, C. M. Johnston and P. Zelenay, Science,
2011, 332, 443; (b) H. W. Liang, W. Wei, Z. S. Wu, X. l. Feng
¨
and K. Mullen, J. Am. Chem. Soc., 2013, 135, 16002; (c)
Q. Wang, Z.-Y. Zhou, Y.-J. Lai, Y. You, J.-G. Liu, X.-L. Wu,
E. Terefe, C. Chen, L. Song, M. Rauf, N. Tian and S.-G. Sun,
J. Am. Chem. Soc., 2014, 136, 10882; (d) N. Ramaswamy,
U. Tylus, Q. Jia and S. Mukerjee, J. Am. Chem. Soc., 2013,
135, 15443; (e) A. L. Bouwkamp-Wijnoltz, W. Visscher,
J. A. R. van Veen and S. C. Tang, Electrochim. Acta, 1999,
45, 379.
References
1 S. Nardecchia, D. Carriazo, M. L. Ferrer, M. C. Gutierrez and
F. del Monte, Chem. Soc. Rev., 2013, 42, 794.
9 T. G. Devi and M. P. Kannan, Energy Fuels, 2007, 21, 596.
10 W. Deng, Y. Zhang, L. Yang, Y. Tan, M. Ma and Q. Xie, RSC
Adv., 2015, 5, 13046.
2 (a) J. Fan, J. Yan, L. J. Zhi, Q. Zhang, T. Wei, J. Feng,
M. L. Zhang, W. Z. Qian and F. Wei, Adv. Mater., 2010, 22,
3723; (b) L. Yang, S. Wang, S. Peng, H. Jiang, Y. Zhang,
W. Deng, Y. Tan, M. Ma and Q. Xie, Chem.–Eur. J., 2015,
21, 10634; (c) J. S. Luo, J. L. Liu, Z. Y. Zeng, C. F. Ng,
L. J. Ma, H. Zhang, J. Y. Lin, Z. X. Shen and H. J. Fan, Nano
˚
11 T. Shari, G. Hu, X. Jia and T. Wagberg, ACS Nano, 2012, 6,
8904.
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