DOI: 10.1002/celc.201402349
Communications
Porous Dual-Layered MoOx Nanotube Arrays with Highly
Conductive TiN Cores for Supercapacitors
Xiang Peng,[a, c] Kaifu Huo,*[a] Jijiang Fu,[b] Biao Gao,[b] Lei Wang,[a] Liangsheng Hu,[b, c]
Xuming Zhang,[c] and Paul K. Chu[c]
Porous dual-layered molybdenum oxide (MoOx) nanotube
arrays (NTAs) with highly conductive titanium nitride (TiN)
cores (MoOx/TiN/MoOx NTAs) are fabricated by depositing
MoOx on porous TiN NTAs produced by nitriding as-anodized
TiO2 NTAs on Ti foils. The highly conductive TiN facilitates electron transfer and the porous MoOx wall allows the electrolyte
to permeate the nanotubes readily, maximizing the active sites
of the MoOx electroactive material to improve the specific capacitance and rate capability. The coaxial MoOx/TiN/MoOx NTA
electrode shows a high specific capacitance of 97 mF cm2
(323 F g1) at a current density of 1 mA cm2, and 60 % capacitance is retained when the current density is increased 20
times. A symmetrical device based on the MoOx/TiN/MoOx NTA
electrode exhibits a capacitance as high as 24 F cm3 (based on
the volume of MoOx/TiN/MoOx NTAs) and no significant decay
is observed after charging/discharging for 10 000 cycles.
Electrochemical capacitors or supercapacitors (SCs) boasting
superior power densities and long cycle lifetimes have attracted increasing attention, owing to promising applications in
hybrid electrical vehicles, as back-up power in portable electronics, as well as in power supplies.[1] However, commercial
SCs based on carbon electrodes typically have a relatively low
energy density, limiting their application.[2] Many attempts
have been devoted to increase the energy density while not
sacrificing the power density and cycle lifetime in order to satisfy commercial needs. Generally, SCs can be classified into two
categories: electrical double-layer capacitors (EDLCs) and pseudocapacitors, according to the charge-storage mechanism.
EDLCs commonly use carbonaceous materials as electrodes,
whereas pseudocapacitors utilize redox-active electrode materials. Transition-metal oxides (TMOs) such as manganese
[a] X. Peng, Prof. K. Huo, L. Wang
Wuhan National Laboratory for Optoelectronics (WNLO)
Huazhong University of Science and Technology
Wuhan 430074 (China)
E-mail: kfhuo@hust.edu.cn
[b] Prof. J. Fu, B. Gao, L. Hu
The State Key Laboratory of Refractories and Metallurgy
Wuhan University of Science and Technology
Wuhan 430081 (China)
[c] X. Peng, L. Hu, X. Zhang, Prof. P. K. Chu
Department of Physics and Materials Science
City University of Hong Kong
Tat Chee Avenue, Kowloon, Hong Kong (China)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/celc.201402349.
ChemElectroChem 2015, 2, 512–517
oxide,[3] vanadium oxide,[4] iron oxide,[5] NiO,[6] CoO,[7] and molybdenum oxide[8] exhibit high pseudocapacitance and have
been widely investigated as electrode materials for pseudocapacitors, owing to their multiple oxidation states available for
charge storage.[9] Among these oxides, molybdenum oxides,
MoOx (2 x 3) have attracted considerable interest as a promising supercapacitive material because of its low cost, nontoxicity, multiple oxidation states, high specific capacitance, and
the fact that it is environmentally benign.[8a] However, MoOx
alone cannot provide superior capacitive capabilities, owing to
poor electrical conductivity, resulting in a low power density
and sluggish faradaic redox kinetics.[10] Conversely, recent
works have been focused on crystalline MoOx,[3a, 11] mainly because of their stability and excellent electrochemical properties. However, the amorphous phase may offer a rather unique
electrochemical behavior in energy-storage applications that
may be exploitable in certain device applications.[12]
To improve the capacitive properties of MoOx and other
TMO-based electrodes, considerable attempts have been made
to combine MoOx and other TMOs with conducting nanowires,
carbon nanotubes (CNTs), graphene, or reduced graphene
oxide on the nanoscale[12a, 13] to overcome the low conductivity
of TMOs. Usually, high-performance MoOx-based electrodes are
fabricated by mixing MoOx nanowires/nanobelts with conducting CNTs or by depositing MoOx on the conducting nanowire
to produce synergistic properties for enhancing capacitive
properties.[11a, 14] However, a thin layer or low mass loading of
MoOx on conducting nanowires usually leads to low areal and
volumetric capacitance.[15] On the other hand, a thick layer or
high mass loading generally results in a low rate capability.[12a]
In this respect, highly conductive nanotube arrays (NTAs) with
open mouths constitute a desirable substrate for loading of
MoOx or TMOs, which can be coated on both the inner and
outer surfaces of the NTAs. The dual-layered MoOx nanotube
coated on both the inner and outer surfaces of the highly conductive core could increase the mass loading while keeping
a thin and uniform thickness, thus assuring high areal capacitance and high rate performance for SCs. In our previous
study, we reported porous TiN NTAs on Ti foil for loading polyaniline (PANI) to form coaxial PANI/TiN/PANI NTAs as high-performance electrodes for SCs.[16] The porous TiN NTAs have
a high conductivity and provide a large and ion-accessible surface area as well as a direct pathway for charge transport, thus
they are a promising substrate for loading highly pseudocapacitive MoOx for high-performance SCs.
In this work, porous dual-layered MoOx NTAs with highly
conductive TiN cores were prepared through electrochemical
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deposition of MoOx on porous TiN NTAs. The TiN NTAs with
mesoporous tube wall were obtained through the nitridation
of as-anodized TiO2 NTAs,[16] and the amorphous MoOx was
electrochemically deposited into the TiN nanotube along inner
and outer surfaces as well as in the mesoporous holes of the
TiN nanotube wall, forming a 3D MoOx/TiN/MoOx interconnected network. Such a nanocomposite allows easy permeation
and diffusion of the electrolyte into the amorphous MoOx material to maximize utilization of MoOx, owing to the increased
number of charge-storage sites. In addition, the high conductivity of TiN provides a direct pathway to transport electrons
from MoOx to the underlying conductive Ti current collector,
thus coaxial MoOx/TiN/MoOx exhibits good capacitive properties, with an areal specific capacitance of about 97 mF cm2 at
a current density of 1 mA cm2 and a capacitance retention of
60 % when the current density is increased 20 times from 1 to
20 mA cm2. Moreover, a symmetrical device based on two
MoOx/TiN/MoOx NTA electrodes exhibits a capacitance as high
as 24 F cm3 (based on the volume of MoOx/TiN/MoOx NTAs)
with no significant decay after charging/discharging for 10 000
cycles. The MoOx/TiN/MoOx electrode has outstanding properties and promising potential in SC applications.
Figure 1 a is a schematic illustration of the synthesis of a coaxial MoOx/TiN/MoOx NTA electrode on a Ti foil for SCs. The
field-emission scanning electron microscopy (FE-SEM) image
(Figure 1 b) reveals that the TiN has ordered vertical NTAs with
an inner diameter of approximately 100 nm and a wall thickness of approximately 20 nm. The side-view FE-SEM image
(Figure 1 d) reveals the porous structure along the nanotube
wall. After electrochemical deposition of MoOx for 10 s (Figure 1 c and Figure 1 e), the open-ended nanotubular structures
are preserved. However, the inner diameter of the NTAs decreases to approximately 80 nm and the wall thickness of the
nanotube increases to approximately 40 nm, indicating that
MoOx was deposited on both the inner and outer walls of the
TiN with a thickness of approximately 20 nm. The side-view
image of the MoOx-deposited NTAs also shows the porous
tube wall that provides easy access for ions in the electrolyte
to permeate and circulate the nanotubes, resulting in enhanced capacitive properties. Figure S1 a and S1 b (in the Supporting Information) indicate that the surface of the TiN NTAs
is fully covered by MoOx and the tubular structure disappears
when the electrochemical depositing time of MoOx is more
than 30 s.
Figure 2 a and 2 b are the transmission electron microscopy
(TEM) images of MoOx/TiN after 10 s deposition. The TEM
image in Figure 2 a shows porous holes along the tube wall
Figure 2. a) TEM and b) HR-TEM images of MoOx/TiN-10 s, c) XRD patterns of
TiN and MoOx/TiN-10 s, and d) high-resolution Mo 3d XPS spectra of MoOx/
TiN-10 s.
Figure 1. a) Schematic illustration of the synthesis of MoOx/TiN/MoOx NTAs.
b) Top-view and d) side-view FE-SEM images of TiN NTAs and c) top-view
and e) side view FE-SEM images of MoOx/TiN-10 s (insets show the corresponding magnified top-view images of the samples).
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and the energy-dispersive X-ray spectroscopy (EDS) in Figure S2 a reveals that the nanostructure contains Ti, N, Mo, and
O. High-resolution TEM (HR-TEM) discloses the multilayered microstructure of MoOx/TiN-10 s (Figure 2 b) and the outer layer is
homogeneous with a thickness of approximately 10 nm. The
pristine TiN NTAs and MoOx/TiN-10 s have similar X-ray diffraction (XRD) patterns, as shown in Figure 2 c, and all other peaks
are attributed to crystalline TiN (JCPDS 87-0633) with the exception of the two peaks centered at 39 and 708, stemming
from the Ti substrate. The HR-TEM image further reveals that
the TiN is crystalline and the deposited layer is amorphous,
which is agreement with the XRD results. To further determine
the composition of the amorphous layer on TiN, XPS and
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Raman scattering were performed. Figure 2 d shows the highresolution XPS spectrum of Mo 3d of the as-prepared MoOx/
TiN-10 s. The two strong peaks at 232.8 and 235.9 eV correspond to Mo6 + , and two medium peaks at 231.0 and 234.3 eV
arise from Mo5 + (Refs. [9a, 17]), indicating that MoOx is deposited on the TiN NTAs. By calculating the area of the peaks associated with Mo5 + and Mo6 + in the XPS spectra, the atomic ratio
of Mo/O is calculated to be 1:2.8, suggesting the formation of
a MoO2.8 coating on both the inner and outer surfaces of the
TiN nanotube. Figure S2 b shows the Raman spectra of TiN and
MoOx/TiN-10 s. Compared to TiN, the Raman spectra of MoOx/
TiN-10 s show additional peaks at 285 (O = Mo = O), 818 (Mo
OMo), and 990 cm1 (Mo = O), which are attributed to
MoOx,[18] providing further evidence that MoOx is deposited
onto the TiN NTAs.
The electrochemical properties of the as-prepared MoOx/TiN/
MoOx NTA electrodes are investigated in a 1 m Na2SO4 electrolyte, using a three-electrode system (CHI 660C electrochemical
work station) in the potential range between 0.2 and 1.2 V
(vs. Ag/AgCl). Figure 3 a shows the cyclic voltammetry (CV)
curves of the TiN, MoOx/TiN-10 s, and MoOx/TiN-30 s electrodes
at a scanning rate of 10 mV s1. Both the MoOx/TiN-10 s and
MoOx/TiN-30 s electrodes exhibit nearly rectangular and symmetrical CV curves, which is indicative of excellent capacitive
performance. Moreover, the area enclosed by the CV curve of
Figure 3. a) CV and b) GCD curves acquired from TiN, MoOx/TiN-10 s, and
MoOx/TiN-30 s electrodes; c) EIS plots of the TiN, MoOx/TiN-10 s, and MoOx/
TiN-30 s electrodes acquired by applying 5 mV ac under open-circuit potential conditions; d) CV curves of the MoOx/TiN-10 s electrode at different scanning rates; e) GCD curves of the MoOx/TiN-10 s electrode at different current
densities; and f) rate capability (based on areal capacitance) of the TiN,
MoOx/TiN-10 s, and MoOx/TiN-30 s electrodes. The electrolyte was a 1 m
Na2SO4 solution.
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the MoOx/TiN electrodes is much larger than that of pristine
TiN NTAs, suggesting that the capacitance is improved significantly after deposition of MoOx. In addition, the CV area increases with deposition time of MoOx, owing to the larger
amount of deposited MoOx. The CV curve of MoOx deposited
on Ti foil (MoOx/Ti-10 s) is also provided in Figure S3. MoOx/Ti10 s shows much lower capacitance compared to the eletrode
of MoOx/TiN-10 s, owing to the large area of active MoOx species and high conductivity rendered by TiN NTAs. Figure 3 b
presents the galvanostatic charge/discharge (GCD) curves of
the TiN, MoOx/TiN-10 s and MoOx/TiN-30 s electrodes at a current density of 1 mA cm2 in the potential range between 0.2
and 1.2 V (vs. Ag/AgCl). The specific capacitance (Csp), an important parameter widely used to evaluate the performance of
electrochemical SCs, is calculated by Equation (1):
Csp ¼
I Dt
A DU
ð1Þ
where Csp (mF cm2) is the areal specific capacitance, I (A) is
the discharge current, ~t (s) is the discharge time, A (cm2) is
the area of the electrode, and ~U (V) is the voltage range. According to Equation (1), the specific capacitances of TiN, MoOx/
TiN-10 s, and MoOx/TiN-30 s are calculated to be 21, 97, and
165 mF cm2, at a current density of 1 mA cm2, respectively.
After deposition of MoOx for 10 and 30 s, the specific capacitance increases 4 and 7 times, respectively, as a result of the
formation of a MoOx layer on the TiN nanotube. Electrochemical impedance spectroscopy (EIS, Figure 3 c) indicates that the
charge-transfer resistance (Rct) of MoOx/TiN-30 s is estimated to
be 1 W, which is larger than that of MoOx/TiN-10 s (0.35 W) and
the pristine TiN (0.2 W). Figure 3 d shows the CV curves of the
MoOx/TiN-10 s electrode at scanning rates of 5, 10, 20, 50, 100,
200, and 500 mV s1. The CV curves have a similar rectangular
shape despite of large scanning rate of 200 mV s1, suggesting
good capacitive properties. GCD experiments were further carried out to assess the MoOx/TiN-10 s electrode. The charging/
discharging curves in the potential range between 0.2 and
1.2 V (vs. Ag/AgCl) acquired from the MoOx/TiN-10 s electrode at rates ranging from 1 to 20 mA cm2 are displayed in
Figure 3 e. According to the Equation (1), the specific capacitances of the MoOx/TiN-10 s electrode are calculated to be 97,
88, 75, 65, and 58 mF cm2 at current densities of 1, 2, 5, 10,
and 20 mA cm2, respectively (Figure 3 f). Although the areal
capacitance of MoOx/TiN-10 s is smaller than that of MoOx/TiN30 s (Figure 3 f), the gravimetric capacitances of MoOx/TiN-10 s
are larger than those of MoOx/TiN-30 s at all current densities,
as shown in Figure 4 a. The gravimetric capacitance of MoOx/
TiN-10 s is as high as 323 F g1 at the current density of
3.3 A g1, whereas that of MoOx/TiN-30 s is only 235 F g1
based on the MoOx mass loading. MoOx/TiN-10 s shows excellent rate performance with 60 % of the capacitance maintained
when the current density is increased from 3.3 to 66.7 A g1
compared to 46 % obtained from the MoOx/TiN-30 s. The difference in the rate performance stems from the amount of MoOx
layers. The TiN has high conductivity, but MoOx is a semiconductor with low conductivity. Hence, the conductivity of the
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Figure 4. a) Rate performance (based on gravimetric capacitance) of the
MoOx/TiN-10 s and MoOx/TiN-30 s electrodes. b) Cycling performance of the
MoOx/TiN-10 s electrode at a current density of 2 mA cm2 in 1 m Na2SO4.
MoOx/TiN nanocomposite electrode drops after more MoOx
deposition, as suggested by the EIS results in Figure 3 c. In fact,
the larger the amount of MoOx loaded, the lower the conductivity. Our results reveal a special feature of MoOx deposited on
highly conductive TiN NTAs, which can be charged/discharged
at a high current density of approximately 70 A g1 with an
areal capacitance of 58 mF cm2 (gravimetric capacitance
193 F g1), which is much bigger than the values observed
from many other TMO-based electrodes, such as b-MnO2 nanorod@nanoflake (Ni, Co, Mn) oxides,[19] g-FeOOH nanosheets,[20]
MoO3 nanobelts,[21] and Co3O4/Co3(VO4)2 hybrid nanorods.[22]
An important performance gauge for SCs is the long-term
cycle life. Figure 4 b depicts the cycle stability of the as-prepared MoOx/TiN-10 s electrode at a current density of
2 mA cm2 in a 1 m Na2SO4 electrolyte. The MoOx/TiN-10 s electrode delivers excellent long-term cycle life performance with
almost no reduction in the capacitance after charging/discharging for 10 000 cycles. Moreover, after the cycling test, the
morphology of the electrode retained a tubular structure, and
porous holes along the nanotube wall could still be clearly observed (Figure S4), thereby suggesting excellent stability and
durability.
To demonstrate the feasibility of MoOx/TiN-10 s in SCs, a symmetrical prototype was constructed by using two MoOx/TiN10 s electrodes and 1 m Na2SO4 as the electrolyte (designated
as MoOx/TiN j j MoOx/TiN); electrochemical performance is illustrated in Figure 5. CV curves of MoOx/TiN j j MoOx/TiN at different scanning rates between 0 and 1 V are depicted in Figure 5 a. The CV curves are rectangular and symmetrical, even at
a large scanning rate of 2000 mV s1, revealing superior capacitive performance of the SC constructed by using MoOx/TiN10 s electrodes. The GCD curves obtained at different current
densities are depicted in Figure 5 b, which show symmetrical
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Figure 5. Electrochemical performance of the SC prototype composed of
two MoOx/TiN-10 s electrodes: a) CV curves acquired at different scanning
rates, b) GCD curves at different current densities, c) volume capacitance as
a function of current density, d) Ragone plot showing the energy–power
density relationship, e) Nyquist plots, and f) long-term cycle life time.
charging and discharging curves and excellent reversibility,
and high coulombic efficiency can be inferred. The SC prototype has a high volume capacitance (Figure 5 c) of 24 F cm3
(based on the geometric volume of MoOx/TiN-10 s NTAs) at
a current density of 1 A cm3, which is considerably larger than
that of TiN j j TiN (5.6 F cm3) and values obtained from reported C/MnO2 double-walled nanotube arrays (0.177 F cm3),
high-surface-area activated carbon (9 F cm3), onion-like carbon
micro-SCs (1.3 F cm3), hydrated GO micro-SCs (3.1 F cm3), and
carbon-coated sulfur-doped V6O13x//graphene-coated MnO2
nanoparticle-based asymmetric SCs (1.36 F cm3).[9b, 23]
The energy density and power density of the MoOx/TiN j j
MoOx/TiN device were also evaluated, as these properties are
highly relevant to energy storage. Figure 5 d shows the relationship between the energy and power densities of the
MoOx/TiN j j MoOx/TiN and TiN j j TiN SCs. The MoOx/TiN j j MoOx/
TiN device has a high energy density of 3.4 mWh cm3, which
is larger than that of TiN j j TiN of 0.8 mWh cm3. Additionally,
MoOx/TiN j j MoOx/TiN delivers the largest power density of approximately 5 W cm3. The EIS results in Figure 5 e indicate that
the EIS of the SC prototype is about 2 W, corresponding to the
high power density. The capacitance shows no obvious decay
after 10 000 charging/discharging cycles, as shown in Figure 5 f,
corroborating the excellent long-term cycle life. Figure S5
shows that two devices in series could drive a red light-emitting diode (LED) with a threshold voltage of 1.9 V.
In summary, nanostructured electrodes of porous dual-layered MoOx NTAs with highly conductive TiN cores (MoOx/TiN/
MoOx NTAs) deliver superior electrochemical performance. The
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areal specific capacitance of theh MoOx/TiN/MoOx NTA electrode increases up to four times compared to the pristine TiN
electrode and 60 % is retained when the current density is increased 20 times (from 1 to 20 mA cm2). After cycling 10 000
times, no significant decay in the capacitance is observed. The
SC prototype constructed with MoOx/TiN-10 s exhibits a large
capacitance of 24 F cm3, which does not degrade after 10 000
charging/discharging cycles. The excellent performance delivered by the MoOx/TiN-10 s electrode can be attributed to the
3D interconnected network and highly conductive core, and
the materials have large potential in energy-storage devices.
Experimental Section
The TiN NTAs were prepared according previously described procedures.[16] The TiO2 NTAs were prepared through electrochemical
anodization of Ti foils (Goodfellow, 10 10 0.1 mm3), which was
cut by using a wire-electrode-cutting machine and measured again
with a Vernier caliper. Anodization was carried out in a conventional
two-electrode cell equipped with a direct current (DC) power
supply (IT6834, ITECH, China) at 60 V for 4 h at room temperature
(RT). The Ti foil served as the anode and a graphite foil was the
counter electrode (1 cm separation). The electrolyte was ethylene
glycol containing 0.5 wt % ammonium fluoride, 5 vol % doubly distilled water, and 5 vol % methanol. The as-anodized TiO2 NTAs were
grown in situ on Ti foil, which were annealed at 450 8C in air for
3 h (designated as TiO2) to convert into the anatase phase, followed by thermal treatment in NH3 at 800 8C for 3 h to form the
TiN NTAs. The MoOx/TiN nanocomposite was produced through
electrochemical deposition of MoOx on TiN NTAs in a solution containing 0.1 m Na2MoO4, 0.1 m Na2EDTA, and 0.1 m CH3COONH4 and
potentiostatic electrolysis was performed at 2.0 V and 70 8C with
a Ag/AgCl electrode as the reference electrode and Pt foil (2 2 cm2) as the counter electrode for 10 and 30 s (designated as
MoOx/TiN-10 s and MoOx/TiN-30 s, respectively). The mass loading
of MoO3 for MoOx/TiN-10 s and MoOx/TiN-30 s was measured to be
0.3 and 0.7 mg cm2, respectively, according to the mass change
before and after electrochemical deposition by using a high-precision electronic microbalance.
The samples were characterized by using FE-SEM (FEI Nova 450
Nano), HR-TEM (Tecnai G2 U-TWIN), EDS, XRD (Philips X’ Pert Pro)
with CuKa radiation (1.5418 ) in the range of 20–808 (2q), XPS
(ESCALB MK-II, VG Instruments, U. K.), and micro-Raman scattering
at RT (HR RamLab with the 514.5 nm Ar + laser as the excitation
source).
The electrochemical measurements were conducted on a CHI 660C
instrument. The three-electrode cell consisted of a Ag/AgCl electrode as the reference electrode, platinum plate as the counter
electrode, and MoOx/TiN/MoOx as the working electrode. 1 m
Na2SO4 aqueous solution was used as the electrolyte. CV was performed between 0.2 and 1.2 V (vs. Ag/AgCl) at different scanning rates and the GCD curves were acquired at different current
densities in the voltage range between 0.2 and 1.2 V (vs. Ag/
AgCl). EIS was performed at the open-circuit potential with an ac
perturbation voltage of 5 mV and the cycling stability was assessed
on a charge/discharge tester (Land CT2001 A, Wuhan LAND Electronics Co., Ltd., China). A symmetrical prototype based on two
pieces of MoOx/TiN-10 s was assembled in a 2032-type stainlesssteel coin cell. In detail, two of MoOx/TiN-10 s electrodes were
placed at the opposite sides of the separator (NKK TF40) with the
MoOx/TiN-10 s side towards the separator. Then, several drops of
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1 m Na2SO4 electrolyte were added and sealed by using a buttoncell sealing machine with a press of 10 Mpa. The cycling performance was tested on a two-electrode battery test system (Land
CT2001 A). The CV, GCD, and EIS measurements were performed
on an electrochemical workstation (CHI 660C).
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Grant No. 50902104 and 21105077),
Fundamental Research Funds for the Central Universities (HUST:
0118187030), Outstanding Young and Middle-aged Scientific Innovation Team of Colleges and Universities of Hubei Province
(T201402) and City University of Hong Kong Applied Research
Grant (ARG) No. 9667085. The authors also thank the facility support of the Center for Nanoscale Characterization & Devices
(CNCD), WNLO-HUST, and the Analysis and Testing Center of
Huazhong University of Science and Technology.
Keywords: conductive core · electrochemistry · energy
conversion · molybdenum · TiN nanotube arrays
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Supporting Information
Porous Dual-Layered MoOx Nanotube Arrays with Highly
Conductive TiN Cores for Supercapacitors
Xiang Peng,[a, c] Kaifu Huo,*[a] Jijiang Fu,[b] Biao Gao,[b] Lei Wang,[a] Liangsheng Hu,[b, c]
Xuming Zhang,[c] and Paul K. Chu[c]
celc_201402349_sm_miscellaneous_information.pdf
Supporting information
(a)
(b)
2 μm
2 μm
Figure S1. FE-SEM images of the MoOx/TiN-30s: (a) Top-view and (b) Side-view.
Figure S2. (a) EDS spectrum of the MoOx/TiN-10s and (b) Raman scattering spectra
of TiN and MoOx/TiN-10s.
1
Figure S3 Comparison of CV curves of MoOx/TiN-10s, MoOx/Ti-10s, and pristine
TiN
Figure S3 shows the CV curves of MoOx/TiN-10s, MoOx/Ti-10s, and pristine
TiN. MoOx/Ti-10s was obtained by depositing MoOx on pure Ti foil (1×1 cm2)
under the same condition of preparing MoOx/TiN-10s.
Herein, MoOx/Ti-10s shows
the very low capacitance because of the bulk structure of MoOx film deposited on
pure Ti substrate. The MoOx/TiN-10s showed much higher capacitance compared
with MoOx/Ti-10s due to large surface area of active MoOx and high conductivity
rendered by TiN nanotube arrays. Both MoOx/TiN-10s and MoOx/Ti-10s show no
obvious redox peaks in their CV curves due to complexity of redox reactions of
transition metal oxides in aqueous solution. The very near potential of each reaction
make peaks overlay, thus almost rectangular and symmetrical shape in CV curve was
observed, which is also reported elsewhere[1].
2
Figure S4 Top-view (a) and side-view (b) SEM images of MoOx/TiN-10s after
charging/discharging for 10,000 cycles.
Figure S5. Red light-emitting diode (LED) demonstration with the diode driven by
two devices in series.
3
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4