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Cite this: RSC Advances, 2011, 1, 1349–1357
PAPER
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Nitrogen-doped graphene nanosheet-supported non-precious iron nitride
nanoparticles as an efficient electrocatalyst for oxygen reduction{
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Published on 07 October 2011 on http://pubs.rsc.org | doi:10.1039/C1RA00373A
Chi-Wen Tsai,a Meng-Hsiu Tu,a Chih-Jung Chen,a Tai-Feng Hung,a Ru-Shi Liu,*a Wei-Ren Liu,b
Man-Yin Lo,b Yu-Min Peng,b Lei Zhang,c Jiujun Zhang,c Der-Shiuh Shyd and Xue-Kun Xingd
Received 27th June 2011, Accepted 10th August 2011
DOI: 10.1039/c1ra00373a
Nitrogen-doped graphene-supported carbon-containing iron nitride (FeCN/NG) was synthesized by
the chemical impregnation of iron and nitrogen-containing precursors in the presence of ammonia
under thermal treatment. The resultant graphene-based material acted as an electrode with a much
higher electrocatalytic activity in the catalysis via a 4-electron pathway in fuel cells. The results of
X-ray diffraction, scanning electron microscopy, Fourier transform infrared and X-ray photoelectron
spectroscopy indicated that graphite oxide was successfully reduced to nitrogen-doped graphene.
X-Ray absorption spectroscopy further confirmed that carbon was incorporated into iron nitride,
demonstrating that Fe–N–C catalytic active sites may be responsible for the oxygen reduction
reaction. To the best of our knowledge, this is the first report of the combination of N-doped
graphene with non-precious metal for oxygen reduction in fuel cells, and may open up a new
possibility for preparing graphene-based nanoassemblies for intensive applications.
Introduction
In recent decades, most scientific activity in the search for
outstanding catalysts has been driven primarily by the demands
of fuel cells, which are regarded as important devices in the
future, because of major improvements over gasoline combustion and reduction in CO2 emissions. Noble ultrafine pieces of
metal have long been regarded as important electrocatalysts in
fuel cells. Such metals include Pt, Pd and their alloys (Pt-based
and Pd-based).1–4 Although these materials perform well in
the electrochemical reaction of fuels to generate energy, their
drawbacks include their susceptibility to time-dependent drift,
CO deactivation and cost and lack of abundance in nature, all of
which limit any mass market for commercially developed fuel
cells.5–8 Recently, research has been conducting into reducing
their use by using substitutes for the noble metal. Non-precious
metal nitrides are possible candidates due to their low cost and
acceptable catalytic activity, comparable to that of Pt in the
oxygen reduction reaction (ORR).9–13 In 1964, cobalt phthalocyanine was observed to catalyze the ORR. Since then extensive
research has been carried out to develop practical non-precious
a
Department of Chemistry, National Taiwan University, Taipei, 106,
Taiwan, Republic of China. E-mail: [email protected];
Fax: +886 2 23636359; Tel: +886 2 33661169
b
Material and Chemical Research Laboratories, and Industrial Technology
Research Institute, Hsinchu, 300, Taiwan, Republic of China
c
Institute for Fuel Cell Innovation, National Research Council of Canada,
Vancouver, BC, Canada
d
Synergys ScienTech Corporation, Science Tech-based Industrial Park,
Hsinchu, 300, Republic of China
{ Electronic supplementary information (ESI) available. See DOI:
10.1039/c1ra00373a
This journal is ß The Royal Society of Chemistry 2011
metal catalysts (NPMCs).14 Most investigations have attributed
the catalysis of oxygen to Me–N4 or Me–N2 centers of metallomacrocycles and C-containing distribution of MNxCy sites.15,16
N4-coordination, being similar to that in N4-macrocycles, is
indicative of catalysts that are produced at lower temperatures (T ¡ 550 uC).17 On the other hand, catalysts that are
formed at higher temperature (700 uC ¡ T ¡ 900 uC) exhibit
N2-coordination (N2-macrocycles), in which two nitrogen atoms
are included in each graphene sheet.18 These metal active centers
maintained nitrogen-coordinated structures and bonded to
the carbon support, providing the enhanced stability in this
pyrolized system.
Another key issue that contributes to the high electrocatalytic
activity is related to the nature of the carbon support, which can
assist in both dispersing the metal catalysts and dominating
electron transfer, facilitating mass transport kinetics at the
electrode surface. The discovery of graphene began a new era
of 2-dimensional (2D) fundamental science and technology.
Graphene, a single-atom-thick sheet of an sp2-hybridized carbon
network, has attracted intense fascination since its discovery in
2004.19 Regardless of types for 0D buckyballs, 1D nanotubes
and 3D graphite, they all consist of similar carbon atom
networks, whose parent structure is graphene. Owing to
disclosure of their enormous surface area (theoretical surface
area of 2630 m2 g21), excellent electric conductivity (theoretical
conductivity of 1250 S m21) and mechanical strength, graphene
can be adopted in many fields such as electronic devices,20,21
energy storage,22,23 hydrogen storage24,25 and biosensors26,27,
having established many of their electronic and optical properties during their discovery by scientists. Early studies of
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graphene-based electrocatalysts demonstrated their promising
potential applications in fuel cells.28,29 One method of fabricating efficient electrocatalysts is nitrogen doping,30–32 which not
only enhances the conductivity of graphene, but also creates
carbon atoms around nitrogen atoms as active sites.33,34 The
incorporation of nitrogen atoms into the matrix of carbon results
in electron withdrawing from the carbon because of the higher
electronegativity of nitrogen.35 The N-induced charge-transfer
from neighboring carbon atoms can reduce the barrier of the
ORR potential, weakening the O–O bonding on the surface
of the catalysts and releasing the ORR obstruction at the
electrodes.31 Furthermore, nitrogen plays a role as an n-type
carbon dopant that induces the formation of disordered carbon
nanostructures and donates electrons to the carbon, dominating
the ORR procedure.36 An investigation of the CVD-deposited
N–graphene sheets for ORR at the cathode revealed a much
greater electrocatalytic activity, indicating that metal-free
N-containing graphene is an efficient catalyst.37 However,
ORR yields two possible products, H2O and H2O2 via 4-electron
and 2-electron pathways, respectively, and only one of those
carbon atoms that are directly bonded to nitrogen (CxNy
catalytically active sites) may enable a 2-electron reduction to
hydrogen peroxide,32,38 originating from the weak associative
adsorption of O2 to form OOH2 intermediate.39 This undesired
product may cause serious etching damage to the proton
exchange membrane (PEM) in a fuel cell, resulting in a decreased
duration. Thus, numerous challenges must be overcome.
Drawing inspiration from the above studies, this work uses a
combination of iron nitride and N-doped graphene. Herein
N-doped graphene and FeCN (carbon-containing iron nitride,
described in the latter analysis) nanoparticles that are supported
on N-doped graphene are synthesized by using graphite oxides
(GO) that are mixed with Fe and N precursors in an atmosphere
of ammonia (NH3). 1,10-Phenanthroline, an N-containing ligand
that has aromatic rings, can be adsorbed to the surface of graphite
oxide through a p–p stacking interaction and can harvest Fe by
using its lone-pair electrons on nitrogen atoms, revealing a
bifunctional ability. Hence, a cooperative p–p superimposition
between graphite oxide and a planar aromatic molecule utilized
here is provided with explosion to grow graphene-based nanoassemblies. X-Ray photoelectron spectroscopy reveals important
features of the chemical environment of nitrogen that is
incorporated into the graphene layers as pyridinic, pyrrolic and
graphitic type species.40–42 X-Ray absorption spectroscopy (XAS)
yields information about the graphitization of carbon networks
and the coordination of incorporated nitrogen with Fe under
different modification. Cyclic voltammetry (CV) exhibits the
electrochemical performances, revealing that conductivity can be
increased and a close 4-electron reaction can proceed via suitable
decoration. The whole procedure for the preparation of graphenebased electrocatalysts and the XPS characteristics of N species
are shown in Scheme 1.
Experimental
Synthesis of graphite oxide
The preparation procedure for graphite oxide (GO) followed
that reported in the literature.43 2.0 g graphite (G) powder
(20 mm, Timrex1) and 1.0 g sodium nitrate (NaNO3, 99%,
Acros) were added into 140 mL concentrated sulfuric acid
(H2SO4, 95–97%, Sigma-Aldrich) and stirred for 2 h under icecooling in a hood. Then, 6.0 g potassium permanganate
(KMnO4, 99%, Acros) was slowly added under the same
treatment again. Subsequently, 500 mL deionized (DI) water
was gradually poured into the reacted slurry for 2 h with
agitation. Then, 30 mL hydrogen peroxide (H2O2, 34.5–36.5%,
Scheme 1 Growth of FeCN nanoparticles on N-doped graphene. Magnified picture shows nitrogen species doping in the graphene layers.
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Sigma-Aldrich) was added, and the dispersed slurry turned from
dark brown to bright yellow which was accompanied by
bubbling. The top of the solution was decanted after the
bubbling stopped and then 0.1 M hydrochloric acid (HCl, 37%,
Sigma-Aldrich) was added to a total volume of 500 mL. This
process was repeated several times until barium sulfate (BaSO4)
was no longer formed upon the addition of barium chloride
(BaCl2, 99.995%, Merck) in order to precipitate out the slightly
sulfonated functional groups. The slurry was then further rinsed
with DI water until the pH of the washing solution was almost
neutral (6.5). The remaining solid was dried at 80 uC and ground
to a fine powder.
Chemicals and materials
Iron acetate (Fe(CH3COO)2, Fe content: 29.5%, Alfa Aesar),
1,10-phenanthroline monohydrate (C12H8N2?H2O, 99.8%,
Riedel-deHaen), graphite oxide (GO) and sulfuric acid (H2SO4,
95–97%, Fluka) were purchased from commercial chemicals.
GO was prepared in our laboratory. All chemicals were used
without further purification. The water used in the experiment
was of reagent grade and was produced by using a Milli-Q SP
ultrapure-water purification system from Nihon Millipore Ltd.,
Tokyo, Japan.
Fabrication method of graphene-based electrocatalysts
GO powder was treated directly in an NH3 atmosphere at 800 uC
for 2 h to obtain N-doped graphene (NG). The metal-containing
catalyst was impregnated by mixing iron acetate and 1,10phenanthroline with the desired Fe proportion of 4.1 wt%. They
were mixed in a solution of 10 mL of ethanol to form an
N-coordinated Fe complex. The complex turned red during
sonication for 0.5 h and was then immediately dried in an
atmosphere of air at approximately 70 uC for 12 h. The dried
powder was ball-milled for 3 h at 400 rpm, yielding a welldispersed powder. Eventually, the obtained sample was sintered
under the same condition with NG to obtain FeCN nanoparticles that were supported on N-doped graphene (FeCN/NG).
Characterization of electrocatalysts
The crystallographic structures of graphene-based electrocatalysts were obtained by using an X-ray powder diffractor (XRD,
X’Pert PRO diffractometer) under Cu Ka radiation (l =
1.5418 Å). The morphologies of the samples were determined
by using a field emission scanning electron microscope (FESEM, JEOL JSM-6700F). The chemical environments of the
target samples were measured using a Fourier transform infrared
spectrometer (FTIR, Varian 640-IR). The chemical states of the
samples were characterized by using an X-ray photoelectron
spectrometer (XPS, PHI Quantera) under Al Ka radiation
(l = 8.3406 Å). The vibrational properties of these samples were
analyzed by using appropriate equipment (Raman, Ibin-Yvon
LabRam) with an exciting source (632.8 nm He-Ne laser).
as the working electrode and a platinum plate was used as the
counter electrode. The surface of the glassy carbon (GC) electrode
(5 mm in diameter) was polished by using alumina powder
(0.05 mm). Then, powdered catalyst was uniformly dispersed in
deionized water via an ultrasonic shock to obtain the catalyst ink
for subsequent preparation. Then, 20 mL of the catalyst ink
(catalyst loadings of 200 mg cm22) was applied onto the prepolished GC electrode. After it had been dried in air at 30 uC, the
surface of the electrode was covered with 20 mL of a 0.05% Nafion
solution (Du Pont DE2020) that was diluted by using isopropyl
alcohol and dried in air at room temperature, forming a thin
proton exchange film. The cyclic voltammograms (CVs) were
obtained under N2 atmosphere in 0.5 M sulfuric acid electrolyte at
a scanning rate of 20 mV s21 over 20 cycles (activation for steady
CV) to evaluate the electrochemical properties between 0 and 1 V.
The catalytic activity, determined by ORR, was explored in the
solution of an oxygen-rich 0.5 M sulfuric acid electrolyte that was
equipped by using a rotating disk electrode (RDE) that revolved at
1600 rpm. All current–voltage (I–V) curve measurements were
made using a potentiostat (Eco Chemie AUTOLAB) and GPES
(General Purpose Electrochemical System) software. The rotating
ring disk electrode (RRDE) was employed to detect the number
of transferred electrons and the H2O2 yield by using the bipotentialstatic electrochemical analyzer (CHI 760D, USA). These
measurements were made in 0.5 M sulfuric acid electrolyte at a
scan rate of 10 mV s21, purged with O2 gas. All of the tests were
conducted at room temperature and all potentials were recorded
versus the potential of the reversible hydrogen electrode (RHE).
XAS data analysis
The Fe K-edge (7112 eV) and C K-edge (284.2 eV) XAS spectra
were obtained in transmission mode at the BL17C1 and in
fluorescence mode at the BL20A stations at the National
Synchrotron Radiation Research Center (NSRRC), Taiwan.
The BL17C1 beamline moved 2 mrad of radiation from the tip to
the left wing of the horizontal radiation fan of the superconducting-wavelength-shifter (SWLS) X-ray source. The energy
resolution (DE/E), estimated from the rocking curves of the
double crystal Si (111) monochromator, was (1.7–3.0) 6 1024
and the energy ranged from 4 to 15 keV. The ion chamber was
filled with a mixture of gaseous nitrogen and helium and was
used to measure the incident beam intensity (I0). Meanwhile, a
mixture of argon and nitrogen gases was introduced to measure
the transmitted beam intensity (It). The BL20A beamline carried
out at a 6 m high-energy spherical grating monochromator
(HSGM). In front of a microchannel plate (MCP) detector with
a dual set of MCPs that was mounted an electrically isolated grid
and the energy ranged from 0.11 to 1.5 keV. The photon flux of
the incident beam (I0) was monitored by using a nickel mesh
(80% transmission) that was situated at the rear of the exit of the
monochromator. All of the XAS experiments were carried out at
room temperature.
Extended X-ray absorption fine structure (EXAFS) data analysis
Electrochemical measurements
A module of a three-electrode system was constructed to make
electrochemical measurements. A glassy carbon electrode (GC
electrode, 5 mm in diameter, PINE: AFE3T050GC) was prepared
This journal is ß The Royal Society of Chemistry 2011
The backscattering amplitude and phase shift functions for
particular atom pairs were calculated ab initio by using the
FEFF7 code. EXAFS analyses were performed by using an
analytical package called ‘‘REX2000’’ coded by Rigaku. The raw
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X-ray absorption data modified using standard procedures,
including pre-edge and post-edge background subtraction,
normalization with respect to edge height, Fourier transformation and nonlinear least-squares curve fitting.44 The Fourier
transformed (FT) k3-weighted EXAFS spectra ranged from 3 to
13.2, and was used to estimate the contribution of each bonding
pair to the FT peak. The experimental Fourier-filtered spectra
were obtained by performing an inverse FT with a Hanning
window function with r between 0.83 and 2.85 Å. The value of
the amplitude reduction factor (S02) value for iron was fixed at
0.88 to evaluate the structural parameters for each bond pairs.
Structure-related parameters, including coordination number,
interatomic distance, absorption energy shift and Debye–Waller
factor were given to elucidate information about FeCN/NG
electrocatalyst.
Results and discussion
Structural and morphological analyses via XRD and SEM
Further calculation of their average particle sizes are the 7.29 Å,
6.27 Å and 3.98 Å, for GO, NG and FeCN/NG respectively.
Accordingly the theoretical thickness of graphene monolayer is
0.34 Å, their numbers of layers can be obtained as follows: GO
10, NG 18 and FeCN/NG 12, demonstrating an exfoliation from
the graphite. Their different morphologies recorded by SEM are
shown in Fig. 2. The image of GO describes rippled and fluffy
sheets possibly owing to the introduction of O-containing
functional groups with dimensions of several hundreds of
nanometers to several micrometers (Fig. 2a). Single sheets are
usually difficult to photograph with good contrast, but stacked
ones can readily be imaged by using electron microscopy. The
sheets of NG are slightly thicker than those of GO, demonstrating the removal of functional groups and a regenerative p–p
stacking interaction by thermal annealing (Fig. 2b). After further
deposition of iron nitride nanoparticles, their dispersion on the
N-doped graphene sheets can be observed clearly (Fig. 2c).
The morphologies of electrocatalysts influence their activity as
determined by probing CV performance in subsequent analyses.
As described in the previous section, graphite oxide was prepared
by the chemical exfoliation of commercial graphite powder.
Fig. 1 shows the XRD patterns of graphene-based samples.
Chemical oxidation breaks the ordering of graphite layers by
introducing a variety of functional groups (epoxide, carboxyl,
etc.).45 The exfoliation of individual sheets is assisted by these
functional groups, disrupting the interaction between layers and
resulting in the loss of the ordered lattice structure in the sheets.
This structural conversion increases the interplanar distance
between the sheets, shifting the XRD peak to smaller angles (12u)
in GO (ESI, Fig. S1{).46 After the doping of nitrogen using NH3,
the XRD peak of the graphene phase in NG shows a shift to
bigger angles (26u).47 Graphite oxide is reduced to graphene
because NH3 splits into N2 and H2 at high temperature.48 H2
serves as a reductant, causing a reduction of O-containing
groups on the GO. Upon comparing with NG, FeCN/NG
displays a Fe2N phase (JCPDS, file no. 89-3939) and the diffraction peaks centered at 37.58u, 40.82uand 42.94ucorrespond to
(110), (002) and (1̄1̄1) reflections, respectively. The presence of
an Fe2N phase demonstrates the formation of iron nitride that is
dispersed on the N-doped graphene. The measured d-spacing at
about 26u, ranging from the GO (7.59 Å), NG (3.36 Å) to the
FeCN/NG (3.38 Å), may enhance the number of active sites.
Fig. 1 XRD characterization of graphene-based electrocatalysts.
1352 | RSC Adv., 2011, 1, 1349–1357
Fig. 2 SEM images of graphene-based electrocatalysts: (a) GO, (b) NG
and (c) FeCN/NG.
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Chemical environment detection via IR, XPS and Raman
To corroborate the loss of O-containing groups on the GO
through N-doping, FTIR was performed (Fig. 3). The spectrum
of GO illustrates features that correspond to C–O (nC–O at
1060 cm21), C–O–C (nC–O–C at 1250 cm21), C–OH (nC–OH at
1400 cm21) and CLO in carboxylic acid and carbonyl moieties
(nCLO at 1720 cm21).49,50 The peak close to 1600 cm21 (CLC) can
be attributed to skeletal vibrations of unoxidized sp2 domains.51
However, the characteristic curve changes dramatically after
transition to NG. The heat eliminates almost all O-containing
groups and increases the intensity of the peaks associated with
unoxidized graphitic domains, demonstrating an increase in
graphitization as the stacked layers attract each other. A similar
phenomenon in the presence of FeCN/NG demonstrates that
thermal treatment can eliminate the barriers of repulsion of
O-containing groups that are introduced to the graphite,
indicating that GO is reduced to graphene nanosheets.47 To
probe the effect of nitrogen doping, XPS measurements were
made to investigate graphene-based samples. As can be seen in
Fig. 4 (a), the XPS survey spectrum before and after nitrogen
doping includes a predominant narrow graphitic C 1s peak at
284.2 eV, an N 1s peak at 400 eV, an O 1s peak at 540 eV,
together with an Fe 2p peak at 706 eV over a wide range of
energy (0 to 900 eV).37,52 It is obviously observed that the signal
of N 1s appears after nitrogen doping. The stronger intensity of
the FeCN/NG electrocatalyst indicates that addition of N from
1,10-phenanthroline also facilitates N content. Nevertheless, the
oxygen content in exfoliated GO decreases remarkably after
nitrogen doping, that is, this heat treatment eliminates the
oxygen functionalities on GO, which corresponds to the
observation in FTIR. Besides, the red arrow in Fig. 4 (a) points
out the presence of Fe, demonstrating the encapsulation of
Fe in FeCN/NG electrocatalyst. Fig. 4 (b) shows the fitted C 1s
peak of GO, which consists of five components that arise
from C–C (284.8 eV), C–OH/C–O/C–O–C (285.7 eV) and CLO
(287.2 eV).42,53 The additional component at 286.2 eV from NG
and FeCN/NG is assigned to the CLN bond.50 As revealed by
the fitted curves, the constituents of oxygen functionalities
decrease after nitrogen doping and the contribution of CLN is
Fig. 4 XPS surveys of graphene-based samples. (a) Full range, (b) fitted
C 1s and (c) N 1s.
Fig. 3 FTIR spectra of graphene-based electrocatalysts.
This journal is ß The Royal Society of Chemistry 2011
distributed over the N-doped samples. These observations
indicate considerable deoxygenation through the process of
nitrogen doping. Moreover, the N 1s spectrum in Fig. 4 (c) is
deconvoluted to elucidate three components originating from
pyridine-like (399.04 eV), pyrrole-like (400.25 eV) and graphitelike (401.09 eV) nitrogen atoms within the graphene structure, all
of which play certain roles in the ORR process (Scheme 1).39 It is
preferentially believed that an O2 molecule adsorbs associatively
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at C sites on graphene-like zigzag edges, if a graphite-like N is
situated nearby. This substitutional N makes nearby C atoms
possess higher electronegativity owing to polarization and thereby
facilitate the strong binding of O2 to the C atoms in the outermost
layer.35 Therefore, the higher catalytic activity in the ORR is likely
to be related to the presence of graphite-like N. Based on the
fitting of the three components herein, the chemical state of
graphite-like N is characteristic of higher activity toward ORR for
the above statements. The intensity of graphite-like N for FeCN/
NG is stronger than that for NG, directly suggesting a better
validity of ORR catalysis. Furthermore, O2 molecule can also
adsorb onto FeCN nanoparticles, enhancing the ORR activity
further. Raman spectroscopy is a widely used approach for the
recognition of carbon materials and in particular the detail of
carbon–carbon bonds. Fig. S2 (ESI{) reveals that the G and the D
bands both undergo significant variations upon amorphization of
graphite from G to FeCN/NG transformation. The G line is
attributed to the E2g mode of the in-plane sp2 domains, while the
D line is characteristic of a breathing mode for k-point A1g.54
Specifically the G band broadens obviously and shows a shift to a
higher frequency as the intensity of the D band increases. Here,
the frequencies of all samples for the D bands are almost the same
(1328 cm21), while those for the G bands are 1585 cm21 in
addition to graphite at 1572 cm21, demonstrating a transition
between graphite layers. On the other hand, the D/G intensity
ratios from graphite to FeCN/NG are 0.2, 1.15, 1.28 and 1.47,
respectively. This result indicates that an increased number of edge
planes and the degree of disorder in the prepared graphene
sheets.55 The increase in the D/G ratio from NG to FeCN/NG is
caused by the addition of nitrogen, resulting in the presence of
disorder located in carbon–carbon rings.
Structural analysis via XAS
To confirm the electronic states and the bonds in a series of
graphene-based samples, XAS analyses were carried out. Since
XAS is delicate to the atomic bonding environment and can probe
even tiny changes in it, it is a highly effective approach for
analyzing the electronic states and bonding atoms.56 The C 1s
X-ray absorption near edge spectroscopy (XANES) spectrum
displayed in Fig. 5 (a) demonstrates the transformations on the
graphene layers by graphitization upon doping with N. The p*
states (285 to 289 eV) comprise graphitic rings (p*ring) and CO
functional groups (p*CO), resulting in the transitions of 1s to p*.57
On the other hand, the s* states (289 to 294 eV) are associated
with the same species, but they belong to s* transitions (s*ring
and s*CO).57 Peaks C1 and C2 at approximately 286 and 292 eV
are caused by C–C p* (ring) and C–C s* (ring) excitations
respectively.57,58 However, there is a trend that an increase of C1
and a decrease of C2, revealing the dominance of stronger p–p
network. Further changes can be significantly observed within the
C3 peak at about 290 eV, corresponding to carbon atoms that are
attached to oxygen atoms.59 The detected decrease in intensity
of this characteristic confirms the weakening of the oxygen
functionalities. To elucidate the Fe-coordinated environment, an
XAS of the Fe K-edge was conducted to obtain relative information. Fig. 5 (b) compares the normalized XANES spectrum of the
FeCN/NG electrocatalyst with those of standard materials. The
curve shown for the electrocatalyst is located between those of
1354 | RSC Adv., 2011, 1, 1349–1357
Fig. 5 XAS analyses of graphene-based electrocatalysts. (a) XANES of
C K-edge. (b) XANES of Fe K-edge for FeCN/NG electrocatalyst. Inset
shows its corresponding EXAFS curve.
iron foil and iron acetate (7110 to 7125 eV) consistent with the
oxidative valence of Fe in the Fe2N phase. The white line that is
centered at approximately 7130 eV reveals the local electronic
state and its intensity between signals from iron foil and iron
acetate, indicating a similar trend with the above observations.
The inset presents the EXAFS spectrum of the FeCN/NG
electrocatalyst, listing the information in Table 1 by fitting its
curve. The number for the degrees of freedom (Nf) is formulated
by using the following equation: Nf = 2DkDr/p + 2 # 15.1, where
Dk expresses the effective k-range of the data and Dr is the R-range
of the data to be modified (as described in the part of the
experimental section for the Hanning window). The fitting
parameter (Np) is 12, which is lower than Nf, indicating a reliable
result to satisfy the minimal requirements of the IXS standard.60
Table 1 Fe K-edge EXAFS structural parameters of FeCN/NG
electrocatalyst
Sample
Path
R (Å)
CN
s2 (6 1023 Å2)
DE (eV)
FeCN/NG
Fe–Fe
Fe–N
Fe–C
2.70(1)
1.94(2)
2.07(1)
4.7(1)
2.1(1)
1.1(3)
10.4(1)
1.2(2)
11.4(4)
29.9(4)
26.7(6)
8.4(1)
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The Fe–N interatomic bond distance (1.94 Å) of the FeCN/NG
electrocatalyst is consistent with that of the standard Fe2N, and
representation of a Fe–N2 active center (coordination number:
2.1) is in agreement with the reported studies.15,16,18 However, the
Fe–Fe interatomic bond distance (2.70 Å) is slightly lower than
that in Fe2N, indirectly confirming the presence of the other Fe
species. It was reported that transition metals such as Fe, Co and
Ni could bind to carbon supports via an anchoring effect, and heat
treatment facilitated the formation of carbide species (MexC).61
Furthermore, the presence of C incorporation (Fe–C interatomic
bond) has been detected in the PANI–Fe–C electrocatalyst.62
Therefore, FexC species may be formed in this process by sintering
with carbon. The EXAFS fitting parameter is consistent with the
Fe–C bond length (2.07 Å) of the FeCN/NG electrocatalyst,
which is close to that of reported Fe3C (2.08 Å) and 2.66 Å.
In addition, the Fe–Fe interatomic bond length (2.70 Å) of
FeCN/NG is located between that of Fe2N (2.72 Å) and Fe3C
(2.66 Å), demonstrating a mixed phase. Accordingly, FeCN
nanoparticles (C-containing iron nitride) are indeed attached to
the sheets of N-doped graphene and the distribution of FeNxCy
sites may also be responsible for ORR catalysis.16
Electrochemical performance via CV
To evaluate the electrocatalytic activities of graphene-based
samples toward ORR, the CV approach was used to study the
performance. As displayed in Fig. 6 (a), there is an increase of
the capacitance from GO to FeCN/NG, indicating the increasing
dominance of the electrical conductivity and the faster transfer of
charges.63 The high catalyst surface area and the presence of
graphitic edges makes the carbon materials highly capacitive,64
contributing to the high capacitance and the possible high ORR
activity. However, the GO material has a lower conductivity
owing to the lack of an extended p-conjugated orbital system,
and thus the electric conductivity can be viewed as an indicator
to examine the extent for reduction of graphene.49 Consequently,
the majority of O-containing functional groups are removed in
our products after nitrogen doping. Furthermore, modification
of FeCN nanoparticles on the surface of NG leads to a bigger
enhancement of electric conductivity as compared with that of
NG alone. Furthermore, the well-developed redox peaks at
around 0.66 V indicate a correlation between the change in the
oxidative state of the Fe species in the electrocatalyst and the
relative features of reversible CV potential for the FeCN/NG
catalyst.13 Fig. 6 (b) plots ORR polarization curves obtained
with and without nitrogen doping, which reveal the modification
of FeCN nanoparticles. Comparing these three catalysts
indicates that NG outperforms GO in ORR, but FeCN/NG
has the highest catalytic activity (steady-state catalytic current
density at 0.05 V, GO: 20.18, NG: 20.60 and FeCN/NG:
23.53 mA cm22). The better performance can be attributed to
the formation of active centers for graphite-like N positions and
FeCN nanoparticles. Nitrogen doping reportedly introduces
Fig. 6 CV measured curves of graphene-based electrocatalysts. (a) CV measurements under an N2-saturated condition in 0.5 M sulfuric acid
electrolyte at a scan rate of 20 mV s21. (b) ORR measurements under an O2-saturated condition in 0.5 M sulfuric acid electrolyte at a scan rate of
10 mV s21. (c) Current–potential curves of the ORR measurement in an O2-saturated condition in 0.5 M sulfuric acid electrolyte at a scan rate of
10 mV s21 for FeCN/NG electrocatalyst at various rates of rotation. (d) Koutecky–Levich plots at various rotation rates.
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disorder in the stacked graphene layers and the formed defects
may act as anchoring sites for attachments to metal species.65,66
The improved activity may also be caused by the bonding of
the carbon-catalyst binding and an increase of electrical conductivity.67 On the other hand, the measured dissolution value of
Fe ion for FeCN/NG is 2.75 6 1025 wt%, so the Fe ion is almost
not washed away in this catalyst, maintaining the catalytic
activity. To even obtain quantitatively the number of transferred
electrons (n) of the ORR catalysis, the so-called Koutecky–
Levich test was measured at various rates of rotation of the
electrode.68 In this approach, the reciprocal of the plateau
current (ID21) is plotted as a function versus the reciprocal
rotating rate (v21) and the n merit is evaluated from the slope.
The RDE current–potential curves at various rotating speeds for
the FeCN/NG electrocatalyst are shown in Fig. 6c, demonstrating the high steady-state catalytic current density at high rotation
due to a large increase in oxygen concentration. Comparing the
experimental line of the FeCN/NG electrocatalyst with the
theoretical lines for the ORR pathways with n = 2 and 4
electrons yielded an n value that is approximately 3.91 (Fig. 6d).
This calculated number is extremely close to the theoretical
4-electron transferred process of the H2O product, suggesting
that the 2-electron transferred process of the H2O2 product could
not prefer involving in the ORR of this sample. Besides, the
optimization of RDE ink formulations has been adjusted
because of efficient active sites.69,70 The loading amount of the
catalyst is lowered and the distribution of the active sites are
sparse, promoting the undesirable diffusion of H2O2 product and
2-electron transport. To confirm in detail the H2O2 yield and the
number of transferred electrons of the FeCN/NG electrocatalyst,
RRDE was performed. H2O2 that is produced at the disk
electrode is detected by the ring electrode. The n value is
obtained from the ratio of the ring current (IR) to the disk
current (ID). The number of transferred electrons and H2O2 yield
can be recognized by using the formulas: n = 4 2 2(IR/N 6 ID)
and H2O2% = 100(IR/(N 6 ID))71,72. The transferred electrons
(approximately 3.93) obtained from Fig. S3 (ESI{) approaches
that obtained from the aforementioned Koutecky–Levich plot.
Furthermore, the generation of H2O2 is also small, indicating
a highly efficient catalysis for ORR. These electrochemical
measurements elucidate the improved ORR activity and the
4-electron pathway of the formation of H2O by catalyzed ORR.
Conclusions
In summary, graphene-based electrocatalysts with incorporated
N were developed. They can be readily treated to form a material
in ORR. The key component is the introduction of FeCN
nanoparticle supported on N-doped graphene. The FeCN/NG
electrocatalyst shows a superb performance in ORR applied to
fuel cells as compared with that of FeN and that of FeN/C. The
steady-state catalytic current density at the FeCN/NG electrode
is about 6 times higher than that at the NG electrode over a
measured potential range. The number of transferred electrons
indicate that H2O is a major product of this catalyzed ORR
process, elucidating that the presence of harmful H2O2 is almost
not produced. Comparing the ORR catalytic activity with
commercial Pt electrocatalyst, there is even an enhancement,
but this cost-effective electrocatalyst still has potential for fuel
1356 | RSC Adv., 2011, 1, 1349–1357
cell applications. We therefore anticipate that our findings will
result in the advanced development in fabrication of low-cost
and active graphene-based nanocomposites, and even brand new
materials for use in fields beyond fuel cells.
Acknowledgements
The authors would like to thank Mr. Szu-Hsueh Lai and Prof.
Chung-Hsuan Chen for the valuable measurements of Raman,
Mr. Chin-Chang Shen for the precious measurements of XPS,
Dr Jin-Ming Chen and Dr Jyh-Fu Lee for the treasurable XAS
measurements and the National Science Council of the Republic
of China, Taiwan (Contract No. NSC 97-2113-M-002-012-MY3)
and the Industrial Technology Research Institute (ITRI),
Taiwan for financially supporting this research.
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