Nitrogen-doped graphene supported cobalt oxide nanocomposite as high performance electrocatalyst for oxygen reduction reaction

Article abstract of DOI:10.1166/jnn.2017.13103

Nitrogen doped reduced graphene oxide-supported cobalt oxide catalyst has been synthesized by a simple one step chemical reduction method (denoted as NrGO-Co₃O₄) for electrocatalytic oxygen reduction reaction (ORR). This material has been characterized by various instrumental methods. The morphological analysis shows the Co₃O₄ nanocomposites are well set on to the reduced graphene oxide with better dispersion. The X-ray photoelectron spectroscopy (XPS) shows electrochemical reduction has been done successfully with the increasing C/O ratio. Also, the Raman data reveals that the Co is presents with the oxidized form. The electrocatalytic activities have been verified using cyclic voltammetry (CV) and hydrodynamic voltammetry techniques in 0.1 M KOH electrolyte. The as prepared catalyst has shown more positively shifted onset and half wave potential (-0.091 V and -0.276 vs. Ag/AgCl) and high cathodic current density 2.57 mA cm^-2 and high methanol, ethanol crossover tolerance than Pt/C. It is the introduction of strongly bonded cobalt nanocomposite into the network of NrGO that modulate the electronic properties of the NrGO-Co₃O₄, resulting in the superb electrocatalytic performance. The reaction kinetics have confirmed that the ORR at NrGO-Co₃O₄ catalyst follows a four electron transfer reaction process.

Full text of DOI:10.1166/jnn.2017.13103

Article
Journal of
Nanoscience and Nanotechnology
Vol. 17, 3959–3966, 2017
Copyright © 2017 American Scientific Publishers
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Printed in the United States of America
Nitrogen-Doped Graphene Supported Cobalt Oxide
Nanocomposite as High Performance Electrocatalyst
for Oxygen Reduction Reaction
Sabina Yasmin, Mohammad Shamsuddin Ahmed, and Seungwon Jeon∗
Department of Chemistry and Institute of Basic Science, Chonnam National University, Gwangju 500-757, Republic of Korea
Nitrogen doped reduced graphene oxide-supported cobalt oxide catalyst has been synthesized by
a simple one step chemical reduction method (denoted as NrGO–Co3O4) for electrocatalytic oxy-
gen reduction reaction (ORR). This material has been characterized by various instrumental meth-
ods. The morphological analysis shows the Co3O4 nanocomposites are well set on to the reduced
graphene oxide with better dispersion. The X-ray photoelectron spectroscopy (XPS) shows elec-
trochemical reduction has been done successfully with the increasing C/O ratio. Also, the Raman
data reveals that the Co is presents with the oxidized form. The electrocatalytic activities have been
verified using cyclic voltammetry (CV) and hydrodynamic voltammetry techniques in 0.1 M KOH
electrolyte. The as prepared catalyst has shown more positively shifted onset and half wave poten-
tial (−0.091 V and −0.276 vs. Ag/AgCl) and high cathodic current density 2.57 mA cm−2 and high
Delivered by Ingenta to: State University of New York at Binghamton
methanol, ethanol crossover tolerance than Pt/C. It is the introduction of strongly bonded cobalt
IP: 5.189.201.34 On: Wed, 28 Jun 2017 07:07:47
nanocomposite into the network of NrGO that modulate the electronic properties of the NrGO–
Copyright: American Scientific Publishers
Co3O4, resulting in the superb electrocatalytic performance. The reaction kinetics have confirmed
that the ORR at NrGO–Co3O4 catalyst follows a four electron transfer reaction process.
Keywords: Cobalt Oxide, Fuel Cell, Nanocomposites, Nitrogen Doping, Oxygen Reduction
Reaction, Reduced Graphene Oxide.
1. INTRODUCTION
It is, however, among all kinds of FCs, alkaline fuel cells
(AFCs) are the most attractive due to their superior kinetic
activity for the ORR than that of FCs which utilizing acidic
media. In addition, AFCs do not require precious metals
for stable activity.11 The ORR takes place in the cathode
electrode and could be followed by two main pathways as
below:
Catalysts for the oxygen reduction reaction (ORR) are the
key components of fuel cells (FCs).1 Platinum (Pt)-based
catalysts are commonly utilized as the best electrocatalysts
for both anode and cathode in FCs due to the excellent cat-
alytic activity. Although, the large-scale commercialization
of FCs is impeded by the high cost and limited reserves
of Pt.2ꢀ3 In order to reduce the usage of precious Pt metal
and subsequently the cost of (FCs), the development of
nonprecious metal or metal-free ORR electrocatalysts has
generated a great deal of interest.4–9 Metal or metal oxide
catalysts frequently suffer from dissolution, sintering, and
agglomeration during catalysis, which can result in cata-
lyst degradation.10 To overcome this obstacle, nano-based
support materials such as carbon (active carbon, porous
carbon, carbon nanotubes, and/or graphene) have been
introduced to nanocatalysts for maximizing electroactive
surface area and improve their catalytic activity including
durability.10
O2 +4e− +4H+ −→ 4H2O
(1)
O2 +2e− +2H+ −→ H2O2 +2e− +2H+ −→ 2H2O (2)
In Eq. (1), O2 undergoes a single-step four-electron trans-
ferred reduction, directly forming H2O; meanwhile, in
Eq. (2), O2 first undergoes a two-electron reduction to
H2O2, followed by an additional two-electron reduction to
H2O.12 Therefore, because H2O2 production inhibits the
efficiency of FC, the development of a system that favors
the four-electron reduction pathway is crucially needed.
Graphene sheets are highly conductive and 2D flat lat-
tices consisting of a monolayer of sp2-hybridized carbon
atoms which have been attracted a great deal of research
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 6
1533-4880/2017/17/3959/008
doi:10.1166/jnn.2017.13103
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Nitrogen-Doped Graphene Supported Cobalt Oxide Nanocomposite as High Performance Electrocatalyst for ORR
Yasmin et al.
interest due to high electrical conductivity and electron
mobility, large surface area, long-term stability and unique
mechanical flexibility.13–15 Graphene has already been used
in various fields, especially in sensors,16ꢀ17 capacitor18ꢀ19
and ORR catalysts.4–6
N-doped reduced graphene oxide (NrGO) has been
reported to be good catalyst as well as good support
for ORR because of its large surface area, high elec-
tric conductivity, and N-related active sites.20ꢀ21 Therefore,
cobalt nanocomposite directly grown on the surface of
N-doped graphene support would be superior catalyst with
improved catalytic performance for ORR.
Lab 2000 (Thermo Electron Corporation, England) with a
14.9 keV Al Ka X-ray source. A three-electrode potentio-
stat (CHI 700C Electrochemical Workstation, USA) in a
grounded Faraday cage was used for voltammetric mea-
surements. A Pt wire was used as an auxiliary electrode.
A calibrated Ag/AgCl electrode from Bioanalytical Sys-
tems Inc. (BAS) in 3 M NaCl solution was used as a refer-
ence electrode. The current densities were normalized with
respect to the geometrical surface area of electrodes. An
RRDE was employed as a working electrode. An EG&G
Model 636 RRDE system and a CHI 700C electrochemical
workstation were used for hydrodynamic voltammetry.
In this article, we demonstrate N-doped reduced
graphene oxide supported cobalt oxide (NrGO–Co3O4)
which synthesized by simple chemical reaction. Co3O4
has been extensively studied due to high capacities
and multi-electron reaction ability in the electrochemi-
cal reaction.22 Meanwhile, Graphene-based metal oxide
or their alloy composites have shown superior electrocat-
alytic properties.16ꢀ23–25 Reduced-graphene is recognized
as a potential conductive support material to improve the
electrochemical properties of Co-based materials.22 The
as prepared NrGO–Co3O4 was characterized via X-ray
photoelectron spectroscopic (XPS), Transmission electron
microscopy (TEM) and Raman spectroscopy. The elec-
trochemical behavior of NrGO–Co3O4 for ORR has been
investigated using cyclic voltammetry (CV), rotating disk
2.2. Materials and Chemicals
Graphite powder (∼325 mesh, 99.999%), hydrazine mono-
hydrate (65%) were from Aldrich, Korea. Cobalt(II)
nitrate hexahydrate [Co(NO3ꢁ2 · 6H2O] was purchased
from Daejung Co., Korea. All other reagents were of ana-
lytical grade and used without further purification.
2.3. Preparation of NrGO–Co3O4 Nanocomposite
GO was synthesized by using the modified Hummer’s
method3ꢀ26 through oxidation of graphite powder. The
NrGO–Co3O4 synthesis procedure has shown in Scheme 1.
In this procedure, GO (in 10 mg mL−1 in water) was taken
into a 25 mL bottle and sonicated for 1 h for making
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homogeneous GO suspension. 8 mL of 10 mM Co(NO3ꢁ2 ·
electrode (RDE) and rotating ring disk electrode (RRDE)
techniques in 0.1 M KOH solutions. CInotphyerieglhect:trAomcheermicia- n Scientific Publishers
IP: 5.189.201.34 On: Wed, 28 Jun 2017 07:07:47
6H2O solution was then added drop wise into GO suspen-
sion with magnetic stirring and then sonication further for
30 minutes. Then 10 ꢂL of N2H4 (65%) added under con-
tinuous stirring then heating at 65 C for overnight night.
cal study, we found that the NrGO–Co3O4 exhibits a more
positive onset potential (Eonset), higher cathodic current
density, lower H2O2 yield, and higher electron transfer
kinetics for the ORR in alkaline media. The results prove
that the NrGO–Co3O4 fabricated glassy carbon electrode
(GCE) has superior electrocatalytic activity for ORR than
that of NrGO and Pt/C.
ꢀ
Then, the product was cooled, filtered and washed sev-
eral times with DI water. Finally the product was dried at
ꢀ
45 C under vacuum condition for 24 h. For comparison,
NrGO was prepared through same way without addition
of Co(NO3)2.
2. EXPERIMENTAL DETAILS
2.4. Fabrication of NrGO–Co3O4 Modified GCE
2.1. Instruments
The surfaces of GCE (diameter: 3 mm) or rotating ring
disk electrode (RRDE, GC disk area: 0.196 cm2) were
carefully polished with 0.05 ꢂm alumina pastes, washed
several times with distilled water and rinse with methanol
to remove the physically adsorbed particles. The sus-
pension of NrGO–Co3O4 in ethanol (1 mg mL−1) was
The field emission transmission electron microscopy
(FE-TEM) and energy-dispersive X-ray spectroscopy
(EDX) observations were carried out in a JEM-2100F
microscope at 200 kV. The X-ray photoelectron spec-
troscopy (XPS) measurements were performed on a Multi
OH
OH
OHO
O
O
H
O
O
N
Co(NO3)2 . 6H2O
H2SO4
H3PO4
O
O
O
O
HO
N
O
O
HO
HO
N2H4 . H2O
65 ºC
HO
KMnO4
@90 ºC
N
OH
O
O
OH
O
NrGO-Co3O4
Graphite
24 hr
GO
Cobalt oxide
nanoparticles
Scheme 1. Synthesis procedure of NrGO–Co3O4 preparation.
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Yasmin et al.
Nitrogen-Doped Graphene Supported Cobalt Oxide Nanocomposite as High Performance Electrocatalyst for ORR
prepared by 1 h ultrasonic agitation. Exactly 10 ꢂL of
NrGO–Co3O4 suspension was then drop coated onto the
polished GCE (16 ꢂL for RRDE) and the allowed to dry
in air. All electrochemical experiments were performed in
a high purity oxygen or Argon purged (for at least 30 min)
high resolution spectra N1s at binding energy of 400 eV of
NrGO and NrGO–Co3O4 indicate N doping was success-
ful. The relatively high intensity of the N peak corresponds
to a high content (5.22 at%) for NrGO and (5.29 at%) for
NrGO–Co3O4. The corresponding peak of N1s spectrum
of NrGO and NrGO–Co3O4 indicate the pyridinic N (N1,
399.7 eV) and graphitic N (N2, 401.5 eV) and oxygenated
N (N3, 405.6 eV).26ꢀ32
A Co2p spectrum of NrGO–Co3O4 is presenting in
Figure 2(d). As shown in Figure 2(d), the Co2p spec-
trum contains of two segments of peaks leveled as Co2p3/2
and Co2p1/2, respectively.33ꢀ34 The presence of the Co2p3/2
and Co2p1/2 peaks are fitted considering two spin–orbit
doublets characteristic of Co2+ and Co3+, respectively, in
almost equal ration.33
ꢀ
0.1 M KOH solution at room temperature (∼26 C).
3. RESULTS AND DISCUSSION
3.1. Instrumental Characterizations
The surface morphology of NrGO and NrGO–Co3O4 has
investigated via TEM analysis. The TEM images of simply
prepared NrGO and NrGO–Co3O4 are shown in Figure 1.
The TEM image of NrGO exhibits a silky waves-like
morphology (Fig. 1(a)). It shows a transparent sheet-like
structure and turned it blackish when it’s layered, indi-
cating a broadened surface area. The TEM images of
NrGO–Co3O4 reveal that the as-formed Co3O4 (NPs) were
homogeneously and uniformly disperse onto the surface of
the NrGO demonstrating Co3O4NPs were set successfully
onto the NrGO without excess aggregation (Fig. 1(b)). The
EDX spectrum of NrGO–Co3O4 (Fig. 1(c)) shows signals
from C, O, Co and Cu (Cu peak was attributable to the
TEM holder). Indicating the NrGO–Co3O4 contained only
cobalt oxide with good population and without any con-
siderable metal contamination.
Raman spectroscopy can be used to characterize the
structural and electronic properties of carbon materials.
Figure 3 displays the Raman spectra of NrGO and NrGO–
Co3O4 which consists of a set of distinct peaks. The peaks
at 1354 cm−1 and 1596 cm−1 can be ascribed to the D
and G bands, respectively. The characteristic G band is
associated with the sp2 carbon atom vibrations and the D
peak is due to the breathing modes of six-atom rings and
requires a defect for its activation.30ꢀ31ꢀ35 The ratios of the
intensities of D band to G band (ID/IG) was used to under-
stand the structural disorder which was slightly decreased
from 1.28 to 1.18 for NrGO to NrGO–Co3O4, respectively,
because of the decrease in the sp2 domain size of carbon
The XPS spectra were performed to investigate the com-
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position of the samples. Figure 2 shows the XPS survey
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spectra of NrGO and NrGO–Co3O4 samples. The survey
spectra exhibit that three different peaks at about 285 eV,
533 eV, and 400 eV which corresponding to the C1s, O1s
and, N1s respectively.27–31 Only Co indicated peak was
observed along with all above mentioned peaks in NrGO–
Co3O4 upon Co addition (Fig. 2(a)). Figure 2(b) shows the
high-resolution C1s spectra of NrGO and NrGO–Co3O4.
The binding energies of 285.0 eV, 286.6 eV, 287.7 eV
and 289.9 eV were attributed to the C–C/C C, C–O,
3
2
Copyright: American Scientific Publishers
atoms and the reduction of sp to sp carbon during the
chemical reaction.36 The G band of NrGO–Co3O4 is neg-
atively shifted than that of NrGO, due to the ꢃ–ꢃ inter-
action between Co3O4NPs and graphene (Fig. 3 inset)31
which signifying well attachment of NPs onto graphene
sheet and better structural features.36
3.2. ORR on Modified Electrodes
3.2.1. CV Study
C
O and O–C O bonds, respectively.28 The intensity of
The electrocatalytic activity of NrGO–Co3O4 for ORR was
initially examined by CV in the potential range from 0.2
to −1.0 V versus Ag/AgCl at a scan rate of 50 mV s−1. As
shown in Figures 4(a, b) in the Ar-saturated 0.1 M KOH
oxygenated groups, especially for C–O and C O, signif-
icantly decreased compared to C–C/C C bond, confirm-
ing that GO was successfully reduced by reducing agent
for both NrGO and NrGO–Co3O4 samples. In Figure 2(c),
Figure 1. TEM images of NrGO (a), NrGO–Co3O4 (b), and EDX spectrum of NrGO–Co3O4 (c); inset: the HRTEM image of NrGO–Co3O4.
J. Nanosci. Nanotechnol. 17, 3959–3966, 2017
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Nitrogen-Doped Graphene Supported Cobalt Oxide Nanocomposite as High Performance Electrocatalyst for ORR
Yasmin et al.
C-C/C=C
C-O/C=O
COOH
O1s
N1s
Co2p3/2
C1s
Co2p
Co2p1/2
C1s
N1s
N3
(a)
(b)
(c)
(d)
N2
N1
Co2p
NrGO-Co3O4
NrGO
282 285 288 291
Binding energy (eV)
50 250 450 650 850
Binding energy (eV)
775 785 795 805
Binding energy (eV)
395
400
405
Binding energy (eV)
Figure 2. XPS survey spectra (a), core level of C1s (b) and N1s (c) of NrGO and NrGO–Co3O4 and Co2p spetram of NrGO–Co3O4 (d).
solution, a featureless voltammogram without any evi-
dent peak is observed. In contrast, as the KOH solution
is saturated with O2, a well-defined and strong cathodic
peak occurs, indicating the high catalytic activities of
NrGO–Co3O4 for ORR. The Eonset shifted positively to
around −0.112 V with increasing peak current density
(2.57 mA cm−1) than NrGO (Eonset − 0ꢄ2, current den-
sity 0.81 mA cm−1). This result clearly indicates that the
ORR catalytic activity of NrGO–Co3O4 is greater than that
of NrGO due to incorporation of Co3O4NPs onto NrGO
which acting as more active site for ORR.
with which conventional graphene materials can be con-
verted into cost-effective ORR electrocatalysts simply by
the addition of nonprecious metal oxide and simple chem-
ical treatment suggest considerable room for cost-effective
preparation of various nonprecious metal oxide catalysts
for ORR. These experimental results suggest that the cat-
alytic activity of ORR at NrGO–Co3O4 is obviously better
than that of NrGO catalyst in respect to any parameters
(Eonset, E1/2, or JL) and more or less good than Pt/C in
respect to the JL. It is undoubted that the well-dispersion
Co3O4 NPs and better reduction degree of NrGO play the
key role for ORR activity enhancement of this catalyst.
We further evaluated the catalytic activity of NrGO,
NrGO–Co3O4 and commercial 20% Pt/C catalysts toward
3.2.2. Hydrodynamic Voltammetry Study
ORR by the rotating disk electrode (RDE) measurements
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IP: 5.189.201.34 On: Wed,T2h8eJuRnD2E01h7yd0r7o:d0y7n:a4m7ic voltammetry is widely used
which recorded at a constant rotation speed of 1600 rp−m1
Copyright: American Scientific Publishers
for the determination of the electrocatalytic ORR path-
in an O2-saturated 0.1 M KOH solution at 10 mV s
scan rate. As shown in Figure 5, the linear sweep voltam-
metry (LSV) curves on RDE confirm the superior cat-
alytic activity of NrGO–Co3O4 with the Eonset of −0.091 V,
whereas the Eonset of ORR NrGO is −0.187 V. In addi-
tion, the ORR current density at any potential and half-
wave potential (E1/2) for NrGO–Co3O4 both are higher
than that of NrGO. Although, the ORR Eonset and E1/2
for NrGO–Co3O4 are lower than that of a commercial
Pt/C electrode, but the limiting current density (JL, from
−0.8 V) is higher than that of Pt/C. It is, however, the ease
way. The electrocatalytic activity of ORR was then eval-
uated by hydrodynamic voltammetry measurements in an
O2-saturated 0.1 M KOH solution in Figure 6. Figure 6
shows the RDE voltammograms of the glassy carbon disk
electrode modified with NrGO (a) NrGO–Co3O4 (b) with
the electrode rotation rate varied from 100 to 3600 rpm
at scan rate of 10 mV s−1. The increase in current with
faster rotating rates is actually expected, as faster rotat-
ing rates correlate with faster O2 flux to the catalyst sur-
face. The ORR JL of the NrGO–Co3O4 modified electrode
was always larger than that of the NrGO electrode at any
constant rotation rate. The Koutecky–Levich (K–L) plots
which are plotted from the plateau current show linear
dependence at cited potentials (5aꢁ–bꢁ). The linearity and
the parallelism of K–L plots are usually represented first
30
order kinetics with respect to O2 and the similar trans-
ferred electron number for per O2.35
Therefore, the transferred electron number (n) per O2
involved in ORR was calculated from the K–L equation as
given below.37
J−1 = Jk−1 +JL−1
(3)
(4)
JL = 0ꢄ62nFACoꢅDoꢁ2/3
V
−1/6ꢆ1/2
where J and JK are the measured and kinetic current den-
sity, ꢆ is the rotating rate (rpm), F is the Faraday con-
stant (96,485 C mol−1ꢁ, Co is the concentration of O2
Figure 3. Raman spectra of NrGO and NrGO–Co3O4; inset: G and D
bands.
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Yasmin et al.
Nitrogen-Doped Graphene Supported Cobalt Oxide Nanocomposite as High Performance Electrocatalyst for ORR
Figure 4. The CVs for ORR on NrGO–Co3O4 (a), NrGO (b) at 50 mV s−1 scan rate in O2 (solid line) and Ar (dotted line) saturated 0.1 M KOH.
in the electrolyte (1.2 mmol L−1),3ꢀ37ꢀ38 Do is the diffu-
sion coefficient of O2 in the 0.1 M KOH solution (1 ×
10−5 cm2 s−1),3ꢀ38 and ꢇ is the kinetic viscosity of the
electrolyte (1 ×10−2 cm2 s−1).34ꢀ35ꢀ39
importantly, the corresponding ring electrode current den-
sity was much lower at NrGO–Co3O4 than NrGO suggest-
ing a minimal amount of H2O2 intermediate was produced
at NrGO–Co3O4 during ORR and hence high efficiency of
the electrocatalytic ORR process.
Another calculation method had been used to verify the
n value using ring and disk electrodes current density of
RRDE measurement. However, the n value and the per-
centage (%) of produced H2O2 at modifier electrodes dur-
ing O2 reduction were calculated from the equations as
below:40ꢀ41
3.2.3. Kinetic Analysis
To further investigate the electron transfer pathway, the
RRDE voltammograms of the NrGO, NGO–Co3O4 and
Pt/C have been carried out in the same electrolyte solu-
tion with the electrode rotation rate 1600 rpm at scan rate
of 10 mV s−1 in Figure 7. Figure 7(a) shows better JL
on NrGO–Co3O4 even than that of commercially avail-
able Pt/C at disk electrode. The ring current is actually
4id
n =
(5)
(6)
id +ꢅir /N ꢁ
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corresponded to the amount of hydrogen peroxide (H2O )
which generated at the disk electrode during ORR. Very
IP: 5.189.201.34 On:2Wed, 28 Jun 2017 07:07:47
200ir /N
% of H2O2 =
Copyright: American Scientific Publishers
id +ir /N
where, id is the current of disk electrode, ir is the cur-
rent of ring electrode and N = 0ꢄ18, which is the RRDE
collection efficiency. The n value and the % of H2O2 cal-
culated using Eqs. (5) and (6) using obtained current data
from Figure 7(a). According to Figure 7(b), the trans-
ferred electron at NrGO–Co3O4 and Pt/C was closer to the
four and was always higher than NrGO. Also, the corre-
sponding % of H2O2 at NrGO–Co3O4 and Pt/C was lower
than that of NrGO (in Fig. 7(c)) suggesting that a signif-
icant amount (80%) of O2 is reduced directly to H2O at
this NrGO–Co3O4 modified electrode compared with other
tested catalysts.
Finally, the mass transport corrected Tafel plots were
produced to further investigate the kinetic properties for
the NrGO, NrGO–Co3O4 and Pt/C electrodes (Fig. 7(d)).
Figure 5. LSV curves of the NrGO, NrGO–Co3O4 and Pt/C electrocat-
alyst in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s−1
and at rotation speed of 1600 rpm.
Table I. The summary of the ORR performance on NrGO, NrGO–
Table II. Comparison of different cobalt oxide-based catalysts
Co3O4 and 20% Pt/C electrodes.
for ORR.
Name
NrGO
NrGO–Co3O4
20% Pt/C
jk @ −0.8 V
Name
Eonset (V) E1/2 (V) (mA cm−2
)
n value
Refs.
Eonset from LSV (V)
E1/2 (V)
−0ꢄ187
−0ꢄ418
4ꢄ37
20ꢄ1
131
−0ꢄ091
−0ꢄ276
4ꢄ98
15ꢄ4
120
0ꢄ042
−0ꢄ125
4ꢄ98
15ꢄ7
119
NrGO–Co3O4
Co-NGA
−0ꢄ091 −0ꢄ276
−0ꢄ16
4ꢄ98
4ꢄ80
3ꢄ9
–
3ꢄ9
2
This work
[47]
[48]
[49]
[50]
jk (mA cm−2ꢁ @ −0.9 V
% of H2O2 at −1.0 V
MnOx–Co3O4/C −0ꢄ056 −0ꢄ116
∼2ꢄ3
Tafel slope (mV dec−1
ꢁ
Co3O4
−0ꢄ233
−0ꢄ10
2ꢄ1
n value @ −1.0 V
3ꢄ6
3ꢄ9
4
CMO/GA
−0ꢄ22
4ꢄ00
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Nitrogen-Doped Graphene Supported Cobalt Oxide Nanocomposite as High Performance Electrocatalyst for ORR
Yasmin et al.
Figure 6. RDE voltammograms for ORR on NrGO–Co3O4 (a), and NrGO (b) at various rpm in O2-saturated 0.1 M KOH at scan rate of 10 mV s−1
;
and their corresponding K–L plots (aꢁ) and (bꢁ).
Two Tafel slopes are commonly seen in a Tafel plot such
as, 120 mV dec−1 and 60 mV dec−1. The 120 mV dec−1
the reaction speed is dominated by the diffusion limitation
inside the material. The Tafel slope of the NrGO–Co3O4
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has yielded the lower value of 120.5 mV dec−1 than that of
indicates the first electron transfer is the rate determin-
IP: 5.189.201.34 On: Wed, 28 Jun 2017 07:07:47
−1
42ꢀ43
Copyright: American Scientific Publishers
NrGO (131 mV dec ), and closely followed by the Pt/C
ing step to the transport of O2 to the electrocatalyst.
(118 mV dec−1), revealing the apparently facilitated mass
The Tafel slopes were obtained at high overpotential where
Figure 7. RRDE voltammograms at NrGO, NrGO–Co3O4 and Pt/C modified electrodes at 1600 rpm and at 10 mV s−1 scan rate in O2-saturated
0.1 M KOH (a), the corresponding transferred electron numbers and H2O2 production (b and c), and the Tafel plots of each electrodes (d).
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Yasmin et al.
Nitrogen-Doped Graphene Supported Cobalt Oxide Nanocomposite as High Performance Electrocatalyst for ORR
Acknowledgments: This research has supported by the
National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology
(NRF-2015R1D1A1A09059344).
References and Notes
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Received: 22 March 2016. Accepted: 23 May 2016.
Delivered by Ingenta to: State University of New York at Binghamton
IP: 5.189.201.34 On: Wed, 28 Jun 2017 07:07:47
Copyright: American Scientific Publishers
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