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Visualization of the electrocatalytic activity of three- dimensional MoSe2@reduced graphene oxide hybrid nanostructures for oxygen reduction reaction
Shuli Xin1,§, Zhengqing Liu2,§, Li Ma1, Yao Sun1, Chunhui Xiao1, Fei Li1,3 (), and Yaping Du2 ()
1 Department of Chemistry, School of Science, Xi’an Jiaotong University, Xi’an 710049, China 2 Frontier Institute of Science and Technology jointly with College of Science, State Key Laboratory for Mechanical Behavior of
Materials, Xi’an Jiaotong University, Xi’an 710049, China 3 Bioinspired Engineering and Biomechanics Center (BEBC), Xi’an Jiaotong University, Xi’an 710049, China § These authors contributed equally to this work.
with a concentration of 4 mg·mL−1 were prepared by
dispersing 4 mg of catalyst samples in 1 mL of a 2:1
(v/v) mixture of DMF and deionized water, followed
by addition of 20 μL Nafion solution and sonication
for 10 min. A 5 μL aliquot of the catalyst suspension
was uniformly loaded onto the GCE surface using a
micropipette, and then dried in air to provide a
catalyst loading of ca. 0.285 mg·cm−2. The EIS Nyquist
plots of the samples were recorded in a 0.1 M KOH
aqueous solution at an overpotential of 250 mV in the
range of 100 kHz–0.1 Hz with amplitudes of 5 mV.
2.4.2 Cyclic voltammetry measurements
CV measurements were performed using the same
system as that described in the EIS measurements. A
9 μL aliquot of catalyst suspension was uniformly
loaded on the GCE surface using a micropipette and
then dried in air to provide a catalyst loading of ca.
0.51 mg·cm−2. For comparison, a commercial Pt/C
catalyst suspension was prepared using the same
quantity and method as those described above. All the
potentials reported in electrochemical measurements
were calibrated with respect to a reversible hydrogen
electrode (RHE) (details in the Electronic Supplemen-
tary Material (ESM)). All the CV measurements were
taken in an O2- or N2-saturated 0.1 M KOH aqueous
solution and scanned from 0.17 to 0.97 V at a scan
rate of 10 mV·s−1 at room temperature, except for the
comparison of the MoSe2@rGO and Pt/C samples,
which employed potentials from 0.17 to 1.07 V.
2.4.3 Linear sweep voltammetry (LSV) measurements
LSV measurements were carried out on a glassy carbon
rotating-disk electrode modified with 0.6 mg·cm−2 of
as-prepared sample suspensions or 50 μg·cm−2 Pt/C
suspension using a Pine biopotentiostat combined
with a rotation speed controller (Pine Instrument Co.
Ltd., USA). The LSV experiments were performed in
O2- or N2-saturated 0.1 M KOH aqueous solution and
scanned from 0.19 to 1.09 V at a scan rate of 10 mV·s−1
and various rotation speeds (400, 625, 900, 1,225, 1,600,
and 2,025 rpm, respectively).
The overall transfer electron numbers per oxygen
molecule in the ORR process were calculated based
on the slopes of Koutecky–Levich plots (J−1 vs. ω−1/2,
Eq. (1))
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5 Nano Res.
1 2K L K
1 1 1 1 1
J J J J B (1)
where J is the measured current density, JK and JL are
the kinetic current density and the limiting diffusion
current density, respectively, and ω is the electrode
rotation speed. B was determined from the slope of the
K–L plot according to the Levich equation (Eq. (2)) [11]
2 2
2 31 6
O O0.2B nF D C (2)
where n is the overall number of electrons transferred
per oxygen molecule, F is the Faraday constant, DO2 is
the diffusion coefficient of O2 in 0.1 M KOH (1.73 ×
10−5 cm2·s−1), is the kinetic viscosity of the electrolyte
(0.01 cm2·s−1), and CO2 is the bulk concentration of O2
(1.21 × 10−6 mol·cm−3) [57].
2.4.4 SECM measurements
SECM measurements were performed on a CHI920C
SECM workstation (Chenhua Co., Ltd., China) using a
four-electrode system. A homemade 25 μm-diameter
Pt-disk microelectrode was used as the SECM probe
and also as working electrode 1 (WE1), and the GCE
loaded with the catalyst dispersions was used as
the substrate electrode and also as working electrode
2 (WE2). The RE and CE used in the SECM mea-
surements were the same as those used in the CV and
LSV experiments. The reported potential values applied
to both WE1 and WE2 in the SECM experiments were
also calculated with respect to the RHE. The catalyst
suspensions were prepared using the same method
as that described above. A 0.1 μL aliquot of catalyst
dispersion with a concentration of 4 mg·mL−1 was
dropped onto the surface of the GCE with a micro-
dispenser, and dried in air. All the SECM scanning
experiments were conducted using the redox com-
petition mode of SECM (RC-SECM) [51–54] (or the
equivalent shielding mode [55]) in an O2-saturated
0.1 M KOH aqueous solution with probe-to-sample
distances of ca. 50 μm. For area scan measurements,
the SECM probe applied with a potential of 0.17 V was
scanned in the XY plane over the substrate electrode
modified with catalyst spot array at the sample
potentials (ES) of 0.57, 0.62, 0.67, 0.72, and 0.77 V,
respectively, with an increment distance of 5 μm and
an increment time of 0.02 s. The x- and y-line SECM
scans were taken over the most active areas of the
catalyst spot arrays at a scan rate of 25 μm·s−1.
2.4.5 Stability and methanol tolerance measurements
Both the stability and methanol tolerance experiments
of MoSe2@rGO with respect to the commercial Pt/C
were carried out using chronoamperometry at 0.77 V
in an O2-saturated 0.1 M KOH solution employing
the same three-electrode system as that used in the CV
experiments. The stability experiments were performed
with prolonging chronoamperometry measurements
to 15,000 s and the methanol tolerance experiment was
tested after injection of 10% methanol at ca. 300 s.
3 Results and discussion
3.1 Structure and composition of MoSe2@rGO
hybrid nanostructures
The structures of the prepared MoSe2 and MoSe2@rGO
hybrid nanostructures can be observed by the obtained
SEM, TEM, and HAADF-STEM images (Figs. 1(a)–1(d)).
As shown in Fig. 1(a), the TEM image of pure MoSe2
shows the formation of aggregated MoSe2 consists of
layers. In the MoSe2@rGO hybrid, MoSe2 nanostructures
with an average size of ca. 150 ± 5 nm are shown
to be uniformly distributed on the rGO nanosheets
(Figs. 1(b)–1(d)), confirming that the rGO nanosheets
provide an ideal platform for growing MoSe2 layers.
Furthermore, the MoSe2@rGO hybrid exhibits a unique
rosebud-like 3D architecture. The possible mechanism
for the growth of MoSe2 layers on the rGO nanosheets
involves the precursors being anchored to the surface
of the rGO by carboxyl, hydroxyl, and epoxy functional
groups [58–60], and the defects of graphene nanosheets
acting as the nucleation sites [61]. Corrugations in the
MoSe2@rGO hybrid can be more clearly observed in
the inset of Fig. 1(b) and the HAADF-STEM image in
Fig. 1(d), which show that the MoSe2 in the MoSe2@rGO
hybrid is composed of thin layers. The energy disperse
spectroscopy data presented in Fig. S1(a) in the
Electronic Supplementary Material (ESM) further verify
the Mo:Se stoichiometric atomic ratio of 1:2.
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6 Nano Res.
Figure 1 (a) TEM image of the as-prepared layered MoSe2 nano-structures. (b) SEM (inset: the enlarged SEM image of (b)), (c) TEM, and (d) STEM images of the as-prepared 3D MoSe2@rGO hybrid nanostructure.
The detailed crystal structure of the as-prepared
MoSe2@rGO hybrid was further characterized using
HRTEM (Figs. 2(a) and 2(b)). A single rosebud-like
MoSe2@rGO structure can be more clearly observed
Figure 2 (a) TEM image of MoSe2 grown on rGO to form the MoSe2@rGO hybrid nanostructure. (b) HRTEM image of MoSe2 taken from the rim of the MoSe2@rGO hybrid. Structural models of MoSe2 viewed from the (c) <010> and (d) <001> direction.
in Fig. 2(a). Figure 2(b) displays the HRTEM image
taken of the rim of the MoSe2 layers in the MoSe2@rGO
hybrid, in which MoSe2 exhibits defined crystal lattice
fringes with an interplanar spacing of 0.28 nm and an
interlayer spacing of 0.64 nm, corresponding to the
(100) and (002) planes of MoSe2, respectively. To provide
a deeper understanding of the MoSe2 nanostructure,
the atomic structure models with a normal interlayer
spacing of 0.64 nm were built (Figs. 2(c) and 2(d)). In
Fig. 2(c), the viewing from the <010> direction, the (100)
and (002) planes are marked, which are as shown
in the vertically stacked MoSe2 layers in the HRTEM
image in Fig. 2(b). The view from the <001> direction
exhibited in Fig. 2(d) indicates that the MoSe2 possesses
graphene-like structure. The (100) plane perfectly
matches the HRTEM image of the planar orientation
in Fig. 2(b).
The composition of the MoSe2@rGO hybrid nano-
structure was initially confirmed by XRD. In the XRD
patterns of pure MoSe2 and the MoSe2@rGO hybrid
nanostructure displayed in Fig. 3(a), all the diffraction
peaks of hexagonal MoSe2 (2H-type, space group:
P63/mmc, a = b = 0.329 nm, c = 1.293 nm, JCPDS Card
No. 29-0914) are observed in the MoSe2@rGO hybrid,
demonstrating that the rGO serves as the platform
for growing MoSe2. The chemical states of Mo, Se,
and C in the MoSe2@rGO hybrid were then analyzed
using XPS (Figs. 3(b) and 3(c), and Fig. S1(b) in the
ESM). In Fig. 3(b), the double peaks arising from the
core levels of Mo 3d3/2 and Mo 3d5/2 orbitals are
located at 232.2 and 228.9 eV, respectively, indicating
a characteristic of Mo4+ in the obtained MoSe2@rGO
hybrid [62, 63]. The Se 3d XPS spectra shown in Fig. 3(c)
show peaks at 55.0 and 54.2 eV, which are assigned to
the core levels of Se 3d3/2 and Se 3d5/2, respectively,
indicating the presence of Se2− [64, 65]. The C 1s
peaks at 284.6 eV in Fig. S1(b) (in the ESM) originate
from the graphene nanosheets with low oxygen content,
verifying the reduction of GO to rGO [66]. Raman
spectroscopy was further applied to characterize
the structure of the MoSe2@rGO hybrid. As displayed
in Fig. 3(d), the Raman peaks at 236 and 284 cm−1
correspond to the A1g and E12g of MoSe2 [32], respec-
tively, and the observed D, G, and 2D bands of graphene
in the hybrid [60] indicate that the MoSe2@rGO hybrid
is successfully synthesized.
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7 Nano Res.
The loading percentage of the MoSe2@rGO hybrid
was determined by TGA. As demonstrated by our
previous report [32], MoSe2 is gradually oxidized
with increasing temperature. At the low temperature
of 200 °C, MoSe2 starts to pyrolyze and produces MoO3
and SeO2 according to the following reaction
2MoSe2 + 7O2 = 2MoO3 + 4SeO2 (3)
When the temperature reaches the sublimation tem-
perature of SeO2 (315 °C), SeO2 begins to gasify along
with the total weight residue oxide of MoO3. As
shown in Fig. 4(a), for the as-reduction to 80% of
the initial weight, and leads to the final prepared
MoSe2@rGO hybrid, the TGA curve exhibits a process
of two-stage weight loss, in which SeO2 volatilizes
at the first stage and rGO converts into CO2 at the
second stage. Through analysis of the TGA data, the
loading percentage of MoSe2 in the MoSe2@rGO hybrid
was calculated to be ca. 60.4%.
3.2 Electric resistance of MoSe2@rGO hybrid
nanostructures
The charge transfer resistance is an important
characteristic that affects the electrocatalytic perfor-
mance of ORR catalysts. Therefore, electrochemical
impedance spectroscopy measurements for the pure
MoSe2 and the MoSe2@rGO hybrid were conducted
in 0.1 M KOH solution in the range of 100 kHz–
0.1 Hz. The obtained EIS Nyquist plots of these two
samples are shown in Fig. 4(b). Both pure MoSe2 and
MoSe2@rGO exhibit arc-like profiles at high frequencies,
and long straight lines at low frequencies, which
correspond to the charge transfer resistance and mass
transfer resistance, respectively [67, 68]. The diameter
of the semi-circle at high frequencies is significantly
reduced for the plot of the MoSe2@rGO hybrid com-
pared with that of pure MoSe2. This indicates that the
charge-transfer resistance at the electrode/electrolyte
interface is significantly decreased upon combining
Figure 3 (a) XRD patterns, (b) and (c) XPS spectra, and (d) Raman spectrum of the as-obtained MoSe2@rGO hybrid nanostructures. (b) Mo 3d and (c) Se 3d signals of the MoSe2@rGO hybrid nanostructure.
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8 Nano Res.
MoSe2 with the conductive rGO platform, which leads
to improvement of the charge transfer kinetics of
the MoSe2 grown on the rGO support and further
contributes to the enhancement of the ORR electro-
catalytic activity of MoSe2.
3.3 ORR electrocatalytic activity of MoSe2@rGO
hybrid nanostructures
To evaluate the electrocatalytic activity of the prepared
MoSe2@rGO hybrid for ORR, and to compare its
electrocatalytic performance with those of pure MoSe2,
rGO, and their physical mixture (MoSe2+rGO), CV
measurements were performed in 0.1 M KOH solution
using a GCE modified with one of these four materials.
CV curves of a bare GCE and a Pt/C-modified GCE
were also recorded as control experiments. From the
obtained CVs of these samples shown in Figs. 5(a)
and 5(b), compared with the CV curves (dashed curves)
recorded in N2-saturated KOH solution, all the GCEs
exhibit obvious reduction currents in the potential
range of 0.17 to 1.07 V in the O2-saturated KOH solution
(solid curves), proving that the observed cathodic
currents are from the ORR at these GCEs. In Fig. 5(a),
the ORR peak potentials of these sample-modified
GCEs are in the order of MoSe2@rGO (0.77 V) > MoSe2
GCE (0.58 V), from which we can obtain the following
information about these samples: Firstly, the rGO-
modified GCE has a more positive ORR peak potential
(i.e., better electrocatalytic activity for the ORR) than
the bare GCE, which could be due to that the large
surface area and the relatively good conductivity of
rGO contribute to increasing the oxygen adsorption
area of GCE and improving the electron transfer
kinetics from GCE to oxygen in solution [40, 41].
Secondly, the MoSe2-modified GCE has better ORR
electrocatalytic activity than those of the GCEs modified
with rGO and the physical mixture of MoSe2 and rGO.
This indicates that MoSe2 is the main contributor to
the electrocatalysis of the ORR, which could be due
Figure 4 (a) TGA analysis curve of the MoSe2@rGO hybrid. (b) EIS Nyquist plots of the pure MoSe2 and the MoSe2@rGO hybrid.
Figure 5 CV curves of different sample-modified GCEs in O2-saturated (solid curves) or N2-saturated (dashed curves) 0.1 M KOH aqueous solution at scan rates of 10 mV·s−1. (a) From top to bottom, the bare GCE, the rGO-modified GCE, the MoSe2+rGO-modified GCE, the MoSe2-modified GCE, and the MoSe2@rGO-modified GCE. (b) The GCEs modified with MoSe2@rGO and Pt/C.
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9 Nano Res.
to the ultrathin MoSe2 layers with large surface area
providing numerous active sites for ORR, and the
abundant exposed Mo edges for adsorbing oxygen
from solution and thus achieving oxygen replacement
[27]. This result also indicates that the decreased
loading of MoSe2 in the mixture of MoSe2 and rGO
also results in a lower ORR electrocatalytic activity
compared to that of the same mass of pure MoSe2.
Thirdly, compared with the pure MoSe2, the ORR
peak potential of the MoSe2@rGO hybrid moves to
more positive potential by ca. 50 mV, demonstrating
the enhanced ORR electrocatalytic reactivity of MoSe2
grown on rGO. The substantially improved ORR
electrocatalytic activity of the MoSe2@rGO hybrid can
be ascribed to the rGO support efficiently ameliorating
the aggregation of layered MoSe2, and thus maintaining
its electrocatalytic activity during the ORR [38, 39]. As
proved by the above EIS results, the conductive rGO
support accelerates the electron transfer from the GCE
to oxygen through the MoSe2@rGO hybrid during the
ORR process. In addition, the 3D rosebud-like nano-
structure of the MoSe2@rGO hybrid also provides
excellent structural rigidity [38, 39], and the interlayer
MoSe2 spacing of 0.64 nm allows the MoSe2@rGO to
react with the electrolyte more efficiently and provides
abundant active edge sites accessible to oxygen
molecules [31, 32], both of which further enhance ORR
benefits from the synergetic effect of MoSe2 and rGO
hybridization, rather than their simply physical mixing.
In Fig. 5(b), compared to the commercial Pt/C catalyst
with an ORR peak potential of 0.81 V, the ORR peak
potential of the MoSe2@rGO hybrid is only ca. 40 mV
smaller under the same mass loading, demonstrating
that the MoSe2@rGO hybrid presents as a potential
non-Pt ORR catalyst with favorable ORR electrocatalytic
activity.
LSV measurements using a RDE modified with
MoSe2@rGO in O2-saturated 0.1 M KOH solution were
performed to further understand the ORR process
occurring on the MoSe2@rGO hybrid (Fig. (6)). For
comparison, the LSV curves of RDEs modified with
Figure 6 (a) LSV curves of the RDE modified with different catalysts in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV·s−1
and a rotation speed of 1,600 rpm. From top to bottom: rGO, MoSe2+rGO, MoSe2, MoSe2@rGO, and Pt/C. (b) LSV curves of the RDE modified with the MoSe2@rGO hybrid at a scan rate of 10 mV·s−1 at different rotation speeds. (c) K–L plots of the MoSe2@rGO hybrid under different applied potentials and at different rotation speeds, and (d) Tafel plots for MoSe2@rGO and Pt/C.
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10 Nano Res.
rGO, MoSe2, MoSe2+rGO, and commercial Pt/C were
also recorded under the same experimental conditions,
with a rotation rate of 1,600 rpm and a constant scan
rate of 10 mV·s−1. As shown in Fig. 6(a), the onset and
half-wave potentials of MoSe2@rGO are located at
0.90 and 0.73 V, respectively, which are more positive
than those of rGO (0.74 V, 0.58 V), MoSe2+rGO (0.83 V,
0.61 V), and MoSe2 (0.85 V, 0.63 V). The oxygen reduction
current densities of these catalysts at the potential of
0.5 V are in the order of MoSe2@rGO (3.18 mA·cm−2) >
MoSe2 (1.82 mA·cm−2) > MoSe2+rGO (0.75 mA·cm−2) >
rGO (0.24 mA·cm−2) under the same catalyst loading.
The more positive onset and half-wave potentials, and
the higher current density of the MoSe2@rGO hybrid,
further demonstrate its superior ORR electrocatalytic
performance to those of pure MoSe2, rGO, and their
physical mixture, which could also confirm the
synergetic effect between the ultrathin MoSe2 layers
with abundant ORR catalytically active edges and the
conductive rGO support to facilitate the electron
transfer from the less conductive MoSe2 to the RDE
[31, 32, 38, 39]. Compared to the commercial Pt/C,
a difference of 40 mV in both onset and half-wave
potentials is obtained for the MoSe2@rGO hybrid,
indicating the comparable ORR electrocatalytic activity
of MoSe2@rGO and Pt/C. In addition, the current pla-
teaus observed in the LSV curves also demonstrate that
the oxygen reduction processes is diffusion-controlled
on the catalysts modified GCE at the rotation rate.
The LSV results show that the ORR activities of these
catalysts increase in the order MoSe2@rGO > MoSe2 >
MoSe2+rGO > rGO, which is in good accordance with
the CV results and also provides a quantitative com-
parison of the ORR current densities of these samples.
To obtain the kinetic and mechanistic information
of the MoSe2@rGO-catalyzed ORR, LSV measurements
using a MoSe2@rGO-modified RDE were performed
at various rotation speeds with a constant scan rate.
As has been demonstrated [69], the shortened diffusion
distance at higher speeds enhances the diffusion of
oxygen to the RDE surface. Therefore, the limiting
current density increases with an increase in rotation
rates from 400 to 2,025 rpm, whereas the onset potentials
of the ORR catalyzed by MoSe2@rGO remain constant
(Fig. 6(b)). From the obtained LSVs of MoSe2@rGO
hybrid, the corresponding K–L plots at electrode
potentials from 0.45 to 0.65 V were calculated and
are presented in Fig. 6(c). All the linear K–L plots at
different potentials show inverse current density (j−1)
as a function of the inverse of the square root of the
rotation speed (ω−1/2) at these potentials, indicating
first-order reaction kinetics with respect to the con-
centration of dissolved oxygen, and revealing a similar
oxygen reduction-involved electron transfer number
per oxygen molecule (n) at different potentials [9, 70].
Through analyzing the slopes of the K–L plots according
to the K–L equation, the calculated average electron
transfer number per oxygen molecule for ORR at the
potential ranging from 0.45 to 0.65 V (vs. RHE) are
3.9~4.1, revealing a nearly four-electron dominated
ORR process for the MoSe2@rGO hybrid in alkaline
aqueous solution [6, 10, 11], similar to that of the
commercial Pt/C-catalyzed ORR. The good ORR elec-
trocatalytic activity of MoSe2@rGO is also confirmed
by the Tafel plot shown in Fig. 6(d). The kinetic
currents derived by mass-transport correction of the
RDE data give a Tafel slope of 62 mV per decade for
MoSe2@rGO at low overpotentials, which is close to
the 57 mV per decade of the commercial Pt/C catalyst.
The adjacent slopes observed for MoSe2@rGO and
Pt/C at low overpotentials in alkaline medium suggest
that the rate-determining step of both catalysts is
the first electron reduction of oxygen [71]. Moreover,
for ORR, with closed onset and peak potentials for
oxygen reduction. Compared to other well-investigated
ORR catalysts, such as N-, B-, or Fe-doped graphene
[12, 13] and Co3O4 [6], the MoSe2@rGO hybrid has a
more positive peak potential for ORR and a larger
value of n close to 4.0. Even the ORR electrocatalytic
activity of MoSe2@rGO is still inferior to that of the
golden standard Pt/C catalyst [7]. However, considering
its advantages of low cost and the relatively efficient
ORR catalytic activity, the MoSe2@rGO composite
shows promise as a potential nonprecious metal ORR
catalyst.
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11 Nano Res.
3.4 Quantitative comparison of local ORR electro-
catalytic activities of catalyst samples using SECM
The above CV and RDE results elucidated and com-
pared the average electrocatalytic activities of these
samples for ORR. To investigate the local catalytic
activity of MoSe2@rGO with high spatial resolution,
and to simultaneously compare its reactivity with the
other catalyst samples in a single experiment, SECM
in RC-SECM mode [51–54] (or the equivalent shielding
mode [55]) with a four-electrode system was employed.
Samples of MoSe2@rGO, MoSe2, MoSe2+rGO, and rGO
with the same mass loading were simultaneously
modified on the same GCE, and the substrate GCE
and the SECM probe were both applied with oxygen
reduction potentials in an O2-saturated 0.1 M KOH
solution. Catalyst samples with higher ORR electro-
catalytic activity consume more local oxygen, resulting
in a noticeable current decrease at the SECM probe
across the active catalyst surface. The ORR electro-
catalytic activities of different samples can then
be distinguished by comparing the recorded ORR
currents at the SECM probe (i.e., the color contrast in
the SECM image) across the corresponding sample
surfaces. And the difference in the ORR electrocatalytic
activities of samples can be more clearly observed by
applying different polarized potentials to the samples.
Figure 7(a) is the optical photograph of the GCE
surface loaded with spots of MoSe2@rGO, MoSe2,
MoSe2+rGO, and rGO. Figures 7(b)–7(f) are the corres-
ponding SECM images of these four catalyst samples
under different sample potentials (i.e., the polarized
potentials at the GCE) ranging from 0.57 to 0.77 V with
a gradient of 0.05 V, which were selected according to
the oxygen reduction peak potentials of these four
samples shown in Fig. 5(a). The currents presented in
the SECM images are the decreases of the ORR currents
recorded at the SECM probe (∆i) after subtraction of
the background currents at corresponding sample
potentials, which was defined as the SECM probe
currents over the bare GCE at the corresponding
applied potentials. As shown in Fig. 7(b), at a sample
potential (ES) of 0.57 V, all four sample spots exhibit
brighter images (yellow color) than that of the
background GCE (green color), which is due to the
large decrease in the ORR currents at the SECM probe
across the four active catalyst spots owing to the
consumption of oxygen around the SECM probe
by the electrocatalytic reaction. The images indicate
that all four samples show good ORR electrocatalytic
activities at this sample potential. The MoSe2@rGO
spot is more visible than the other three spots,
confirming that the MoSe2@rGO hybrid is more active
than MoSe2, MoSe2+rGO, and rGO towards ORR at
an ES of 0.57 V, which is consistent with the CV and
LSV results. Moreover, the local ORR electrocatalytic
activities of the MoSe2@rGO hybrid and the other
three samples can be more clearly distinguished by
comparing the values of ∆i at more positive sample
potentials. As shown in Figs. 7(c)–7(f), the color contrast
of the MoSe2@rGO hybrid with those of the MoSe2,
MoSe2+rGO, and rGO samples becomes more pro-
nounced when adjusting the sample potentials to
more positive values (i.e., from 0.62 to 0.77 V), further
proving that the MoSe2@rGO hybrid exhibits higher
ORR electrocatalytic activity than MoSe2, MoSe2+rGO,
and rGO at more positive sample potentials. For
example, at sample potentials of 0.72 and 0.77 V
(Figs. 7(e) and 7(f)), the MoSe2, MoSe2+rGO, and rGO
samples are hardly visualized compared to the
background GCE, which could be due to the ORR
peak potentials of the MoSe2, MoSe2+rGO, and rGO
samples all being lower than 0.72 V, thus not much
amount of O2 could be consumed by these three samples
at this potential. Whereas, the obtained maximum
values of ∆i for the MoSe2@rGO hybrid sample at ES =
0.72 and 0.77 V are 7.29 and 6.31 nA, respectively,
which is due to the ORR peak potential of MoSe2@rGO
hybrid being 0.77 V (Figs. 5(a) and 6(a)). This indicates
that the MoSe2@rGO hybrid still exhibits ORR elec-
trocatalytic activity at these two potentials.
To quantitatively compare the local ORR electro-
catalytic activities of the MoSe2@rGO hybrid with
those of MoSe2 and MoSe2+rGO, x-line scans along
the most active areas of MoSe2@rGO and MoSe2, and
y-line scans along the most active areas of MoSe2@rGO
and MoSe2+rGO were performed by recording the
oxygen reduction currents at the SECM probe under
different sample potentials. The differences in the local
electrocatalytic activities of MoSe2@rGO vs. MoSe2
and MoSe2+rGO at all five potentials can be more
clearly observed from the differences in ∆i in the
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12 Nano Res.
RC-SECM x- and y-line scans and the extracted bar
graphs shown in Figs. 7(g)–7(j). In Figs. 7(g) and 7(i),
the values of ∆i for the MoSe2@rGO hybrid (from
10.70 to 3.73 nA) are higher than those of MoSe2
(from 10.07 to 2.35 nA) at all five sample potentials.
In Figs. 7(h) and 7(j), the values of ∆i for MoSe2@rGO
(from 14.00 to 3.73 nA) are also higher than those
of MoSe2+rGO (from 13.29 to 2.15 nA) at the same
Figure 7 (a) Optical photograph of the catalyst spots of MoSe2@rGO (upper-left), MoSe2 (upper-right), MoSe2+rGO (bottom-left) and rGO (bottom-right) loaded on a GCE. (b)–(f) Corresponding RC-SECM area scan images of the catalyst spots in O2-saturated 0.1 M KOH aqueous solution with different potentials applied to the GCE (ES): (b) 0.57 V, (c) 0.62 V, (d) 0.67 V, (e) 0.72 V, and (f) 0.77 V (vs.RHE). SECM probe diameter: 25 μm, probe-to-catalyst distances: ca. 50 μm, probe potential: 0.17 V (vs. RHE). (g) Background current-subtracted RC-SECM x-line scans (at y = 420 μm) along the most active sites of the MoSe2@rGO and MoSe2 spots at different catalyst potentials, and (i) the corresponding bar graph. (h) Background current-subtracted RC-SECM y-line scans (at x = 500 μm) along the most active areas of the MoSe2@rGO and MoSe2+rGO spots at different catalyst potentials, and (j) the corresponding bar diagram.
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
13 Nano Res.
potentials. And with increasing ES from 0.57 to 0.77 V,
the ∆i values of these three samples all display
decreasing trends, which is due to the three samples
becoming less active for electrocatalyzing ORR at
more anodic sample potentials. The differences in
∆i between MoSe2@rGO and MoSe2, and MoSe2@rGO
and MoSe2+rGO, become more apparent, further
confirming that the MoSe2@rGO hybrid exhibits
superior ORR electrocatalytic performance than those
of MoSe2+rGO and pure MoSe2. The SECM area and
line scan results above provide a comparison of the
local ORR electrocatalytic activities of these catalyst
samples from both qualitative and quantitative points
of view. Furthermore, the SECM results confirm that
the MoSe2@rGO hybrid exhibits improved ORR elec-
trocatalytic activity compared with those of the other
three, supporting the CV and LSV results, and com-
plementing the CV and LSV measurements by providing
local ORR electrocatalytic information for the catalyst
samples. In addition, compared to the CV and LSV
techniques, SECM also has the advantages of only
needing a small amount of sample and the capability
of distinguishing the catalytic activities of samples
with close ORR peak potentials in a single SECM
measurement by applying different sample potentials.
3.5 Stability and methanol tolerance of MoSe2@rGO
hybrid nanostructures
For practical application in fuel cells, the stability and
tolerance to methanol of an ORR catalyst are the
other two crucial parameters. The stability of the
MoSe2@rGO hybrid compared with that of the
commercial Pt/C in O2-saturated 0.1 M KOH was
investigated with chronoamperometry. As shown in
Fig. 8(a), at a constant ORR potential of 0.77 V, both
the current densities of MoSe2@rGO and Pt/C exhibit
decaying trends with time. However, the current–time
(i–t) response of MoSe2@rGO exhibits a relatively
slow attenuation with high current retention (91.3%)
over 15,000 s of continuous ORR processing, whereas
the Pt/C catalyst exhibits relatively low stability with
a low current retention (86.0%) under the same
experimental conditions. These results indicate the
better electrochemical stability of MoSe2@rGO than
that of Pt/C, which could also be due to the com-
bination of MoSe2 with the rGO substrate ameliorating
the possible aggregation of layered MoSe2 nanostruc-
tures and therefore maintaining the ORR catalytically
active sites of the MoSe2 [27]. The methanol tolerance
of the MoSe2@rGO hybrid and Pt/C were comparatively
tested using chronoamperometry in O2-saturated
0.1 M KOH at a constant potential of 0.77 V (Fig. 8(b)).
Initially, before injecting methanol, both MoSe2@rGO
and Pt/C generate negative currents at 0.77 V due to
only the ORR occurring. After injection of 10 vol.%
methanol into the solution at ca. 300 s, a sharp current
density change to positive can be observed for the
commercial Pt/C catalyst, which could be due to
methanol adsorption and oxidation on the Pt/C surface
[9, 10]. However, the MoSe2@rGO hybrid exhibits only
a slight current change, and the current response
recovers rapidly, indicating the resistance of MoSe2@rGO
to methanol [27]. The above results of the stability
and methanol tolerance of MoSe2@rGO proves that it
is a promising ORR electrocatalyst in alkaline solution
for practical fuel cell applications.
Figure 8 Chronoamperometry curves of the MoSe2@rGO hybrid and commercial Pt/C on GCEs in O2-saturated 0.1 M KOH at 0.77 V (vs. RHE) (a) for 15,000 s and (b) before and after adding 10 vol.% methanol at ca. 300 s.
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14 Nano Res.
4 Conclusions
A 3D rosebud-like MoSe2@rGO hybrid nanostructure
was successfully synthesized through a simple and
facile hydrothermal approach. Its structure, composition,
and electric resistance were thoroughly characterized
using TEM, SEM, XRD, XPS, Raman spectroscopy,
TGA, and EIS. The superior ORR catalytic activity
of the MoSe2@rGO hybrid than those of the pure
MoSe2, rGO, and physically mixed MoSe2 and rGO
was confirmed through CV and LSV measurements,
which could be attributed to the synergetic effect of
the abundantly active edge sites of the layered MoSe2
nanostructures, the highly conductive rGO nanosheet
support, and the unique 3D nanostructure of the hybrid.
The nearly four-electron dominated ORR process, as
confirmed through K–L plots, and the close slopes
of the Tafel plots drawn from the RDE data further
confirmed the comparable ORR catalytic performances
of the MoSe2@rGO hybrid and the commercial Pt/C
catalyst in alkaline medium. Furthermore, the results
of methanol tolerance and stability testing illustrated
the superior robustness of the MoSe2@rGO hybrid to
that of the commercial Pt/C. The SECM technique
was also applied to provide both qualitative and
quantitative comparison of the local ORR catalytic
activities of the MoSe2@rGO hybrid, pure MoSe2, rGO,
and physically mixed MoSe2 and rGO under different
polarized potentials. This work indicates that MoSe2
is a promising new TMD ORR catalyst for energy
conversion, and its ORR electrocatalytic performance
can be improved through the use of highly conductive
carbon nanomaterials as supports.
Acknowledgements
This work was financially supported by the National
Natural Science Foundation of China (Nos. 21105079
and 21405119), the Fundamental Research Funds for
the Central Universities of China (Nos. 0109-1191320016
and cxtd2015003), the Scientific Research Foundation
for the Returned Overseas Chinese Scholars by the
State Education Ministry of China, and the Interna-
tional Science and Technology Cooperation and
Exchange Program of Shaanxi Province of China (No.
2016KW-064). Yaping Du gratefully acknowledges
the financial support from the start-up funding from
Xi'an Jiaotong University, the Fundamental Research
Funds for the Central Universities of China (No.
2015qngz12), and the the National Natural Science
Foundation of China (Nos. 21522106 and 21371140).
Electronic Supplementary Material: Supplementary
material (calibration of reversible hydrogen electrode;
EDX of MoSe2, and XPS of MoSe2@rGO hybrid nano-
structures; Table S1) is available in the online version
of this article at http://dx.doi.org/10.1007/016-1249-9.
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