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Robust Electrodes Based on Coaxial TiC/C–MnO 2 Core/Shell Nanofi ber Arrays with Excellent Cycling Stability for High-Performance Supercapacitors Xuming Zhang , Xiang Peng , Wan Li , Limin Li , Biao Gao , Guosong Wu , Kaifu Huo , * and Paul K. Chu *
considerable attention due to the high power density and
excellent cycle life. [ 1–7 ] According to the energy storage mech-
anism, SCs can be classifi ed into electric double-layer capaci-
tors (EDLCs) and faradic pseudocapacitors. [ 8 ] Generally,
faradic pseudocapacitance-type SCs, including conducting
polymers [ 9–11 ] and transition-metal oxides/hydroxides, [ 12–16 ]
have been studied more extensively than EDLCs based on
carbon-based materials because the pseudocapacitance mate-
rials exhibit higher specifi c capacitance stemming from the
reversible redox reactions. [ 3 ]
Manganese oxide (MnO 2 ) is a one of promising candi-
date for SCs on account of its high theoretical capacitance
of about 1,370 F g −1 , low cost, low toxicity, and natural abun-
dance compared to other transition metal oxides. [ 17 ] Further-
more, the capacitance of MnO 2 could be achieved in neutral
electrolytes (usually in Na 2 SO 4 ) which bode well for safe
and environmentally friendly operation. However, in prac-
tice, the capacitive performance of MnO 2 is not satisfactory DOI: 10.1002/smll.201402519
A coaxial electrode structure composed of manganese oxide-decorated TiC/C core/shell nanofi ber arrays is produced hydrothermally in a KMnO 4 solution. The pristine TiC/C core/shell structure prepared on the Ti alloy substrate provides the self-sacrifi cing carbon shell and highly conductive TiC core, thus greatly simplifying the fabrication process without requiring an additional reduction source and conductive additive. The as-prepared electrode exhibits a high specifi c capacitance of 645 F g −1 at a discharging current density of 1 A g −1 attributable to the highly conductive TiC/C and amorphous MnO 2 shell with fast ion diffusion. In the charging/discharging cycling test, the as-prepared electrode shows high stability and 99% capacity retention after 5000 cycles. Although the thermal treatment conducted on the as-prepared electrode decreases the initial capacitance, the electrode undergoes capacitance recovery through structural transformation from the crystalline cluster to layered birnessite type MnO 2 nanosheets as a result of dissolution and further electrodeposition in the cycling. 96.5% of the initial capacitance is retained after 1000 cycles at high charging/discharging current density of 25 A g −1 . This study demonstrates a novel scaffold to construct MnO 2 based SCs with high specifi c capacitance as well as excellent mechanical and cycling stability boding well for future design of high-performance MnO 2 -based SCs.
Supercapacitors
X. Zhang, X. Peng, W. Li, L. Li, Prof. G. Wu, Prof. P. K. Chu Department of Physics and Materials Science City University of Hong Kong Tat Chee Avenue , Kowloon , Hong Kong , P. R. China E-mail: [email protected]
B. Gao, Prof. K. Huo Wuhan National Laboratory for Optoelectronics Huazhong University of Science and Technology Wuhan 430074 , China E-mail: [email protected]
1. Introduction
To satisfy the increasing energy and power demands for
modern communication tools, electrical vehicles, and
electronic devices, supercapacitors (SCs) have received
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because of the low conductivity (10 −5 –10 −6 S cm −1 ) and poor
proton diffusion distance (20 nm). [ 18–20 ] Nanostructured
MnO 2 such as amorphous hollow spheres, amorphous nano-
particles, and layered nanosheets, thus have been widely
investigated, [ 19,21–23 ] as capacitive electrodes, which could
deliver better capacitive performance than other crystalline
MnO 2 because of more electrochemically active sites, effec-
tive disordered tunnel structure in the amorphous struc-
ture, large surface area, and easy electrolyte diffusion in
the nanosheets. [ 2,24,25 ] However, the capacitive behavior and
cycling stability of MnO 2 with different crystallinity is still
not well understood. In addition, the capacitive performance
of dispersive nanostructured MnO 2 is usually limited and
lower than 300 F g −1 due to the large contact resistance and
instability between the active materials and conductive addi-
tive or organic binders, leading to permanent damage and
precipitous capacitance drop in cycling. [ 26 ]
The design and fabrication of SCs with excellent electron
transport, short ion diffusion path, direct contact between
the active materials and electrolyte without a binder are cru-
cial to commercial adoption. The core/shell nanofi ber (NF)
array prepared on a current collector is expected to present
a robust architecture by combining a highly conductive one-
dimensional nanostructure core and electroactive transition
metal oxide or hydroxide shell. Hence, much effort has been
made to prepare hybrid structures suitable for high-perfor-
mance electrodes. Lu et al. [ 27 ] have produced hydrogen-
treated TiO 2 NWs on carbon fi bers as the core (conducting
scaffold) to support thin amorphous MnO 2 (7 nm) and
carbon shells (18 nm) for high performance asymmetric SCs.
Sun et al. [ 28 ] have prepared a 3D ZnO/MnO 2 core/shell nano-
structure on a metal substrate with advanced rate capability
and enhanced charge-discharge stability. Liu et al. [ 15 ] have
fabricated Co 3 O 4 nanowire/MnO 2 ultrathin nanosheet core/
shell arrays on stainless steel by using sacrifi cial reactive 3D
carbon template layers to produce SCs with high capacitance
and good cycling stability. However, most of MnO 2 coating
methods involve an additional step to fabricate the sacrifi cial
carbon layer and load the manganese oxide via reduction,
thereby making the fabrication procedure relatively compli-
cated. Furthermore, the bonding force between the substrate
and nanostructure may be weakened by mechanical stress in
cycling. [ 29 ] Therefore, fabrication of high capacitive materials
directly on a current collector with the appropriate structural
and physiochemical performance is still challenging.
In our previous study, we have demonstrated the produc-
tion of TiC/C core/shell NF arrays on a Ti alloy substrate by
a one-step thermochemical reaction. [ 30 ] The unique struc-
ture not only exhibits robust construction, but also provides
good bonding and low resistance between the substrate
and nanostructure, thus serving as a good scaffold for high-
performance silicon anode for lithium-ion batteries and
highly sensitive biosensing devices. [ 31–34 ] Furthermore, the
TiC/C core/shell structure with a carbon layer is a natural
micro-current collector for MnO 2 introduction combined
with the in situ chemical redox reaction in KMnO 4 . [ 35 ]
Herein, we fabricated the coaxial core/shell architecture com-
prising the amorphous MnO 2 decorated TiC/C core/shell NFs
electrode (labeled as MnO 2 @TiC/C) and described remark-
able specifi c capacitance as well as excellent mechanical and
cycling stability. The well-designed nanocomposite electrode
possesses strong bonding between the formed MnO 2 shell
and pristine NF while exhibiting a high specifi c discharge
capacity of 645 F g −1 at a current density of 1 A g −1 , and 99%
capacity retention after 5,000 charging/discharging cycles
in a three-electrode system as well as in the symmetrical
two-electrode device. However, after a thermal treatment
in nitrogen up to 500 °C, almost 70% of the capacitance is
lost. Interestingly, the lost capacitance at high charging/dis-
charging cycling current can restore to primitive capacitance
due to the phase transformation from bulk crystallites to
layered nanosheets during cycling. The process and phase
transformation are demonstrated in Scheme 1 . The results
demonstrate a remarkable electrode architecture and pro-
vide fundamental information about the mechanism and sta-
bility of MnO 2 with different crystallinity in cycling which are
important to the design and fabrication of high-performance
energy storage devices.
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Scheme 1. Schematic illustration of the fabrication and structural transformation of the MnO 2 @TiC/C electrode.
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2. Results and Discussion
The MnO 2 decorated TiC/C NFs electrode were prepared by
a simple hydrothermal reaction between the carbon shell of
the TiC/C NFs and KMnO 4 at 160 °C for various durations,
and the mass of the loaded MnO 2 is determined by the hydro-
thermal reaction time. Figures 1 a–d display the evolution of
the morphology and microstructure of the electrode with
time. As shown in Figure 1 a, quasi-aligned NFs with diame-
ters of about 150–200 nm and lengths up to 5 micrometers are
obtained after 15 min. When the reaction time is increased to
30 min, rod-shape NFs with larger diameters of about 300 nm
are formed due to the larger mass loading of MnO 2 as shown
in Figure 1 b. However, if the reaction time is increased to 1 h,
the fi bers cannot be distinguished and condensed rods appear
(Figure 1 c), which could block the effective electrolyte dif-
fusion pathway and reduce the specifi c area. If the reaction
time is further lengthened to 3 h, the carbon layer fully reacts
with KMnO 4 to produce a thick MnO 2 layer with a rough sur-
face and thickness of about 7–8 micrometers (Figure 1 d). On
the bottom of the layer (inset image in Figure 1 d), there are
many thin bare TiC nanowires sticking into the MnO 2 layer.
The electrochemical resistance of the as-prepared MnO 2 @
TiC/C electrode increases dramatically if the hydrothermal
time is prolonged (Figure S1 and Table S1).
Figure 2 a shows the cyclic voltammetry (CV) curves of
the as-prepared MnO 2 @TiC/C NFs in the potential range
between 0 and 0.8 V (vs. SCE) in a 1 M aqueous Na 2 SO 4 at
a small scanning rate of 10 mV s −1 . After the hydrothermal
treatment for 15 min, the CV curve retains an approximately
rectangular shape indicating an ideal capacitive behavior. For
the sample of 30 min hydrothermal reaction, the CV loop
exhibits larger area. However, the capacitance decreases
with further extension of hydrothermal reaction time due
to the reduced contact area and poor ion diffusion. The
optimal area and gravimetric capacitance of the electrode
(HR 30 min) are 65 mF cm −2 and 325 F g −1 at a scanning rate
of 10 mV s −1 , respectively, as shown in Figure 2 b.
The supercapacitive performance of the optimal elec-
trode (HR 30 min) is studied by of CV and galvanostatic
charge-discharge (GCD) measurements, as shown in
Figure 3 . The as-prepared electrode has much larger capac-
itance than the pristine TiC/C NFs at a scanning rate of
20 mV s −1 (Figure 3 a). The CV curve shows anodic and
cathodic waves centered at 0.51 and 0.39 V vs SCE, respec-
tively, attributable to cation deintercalation during oxida-
tion and cation insertion during reduction. [ 36 ] Even at a large
scanning rate of 200 mV s −1 (Figure 3 b), the CV curve shows
a similar shape, indicating fast ion diffusion in the as-pre-
pared MnO 2 shell. The GCD curves of the electrode acquired
at different current densities from 1 to 25 A g −1 are shown
in Figure 3 c. The discharge curves are not straight lines and
composed of two slope regions, a large slope at high potential
region (0.5–0.8 V) and a small slope at low potential region
(0–0.5 V), suggesting that the faradic capacitance was mainly
contributed by the low oxidation state of manganese which
consists with the observed redox peaks at low potential in
CV curve (Figure 3 a) and further proved by the XPS anal-
ysis. [ 37,38 ] Figure 3 d shows the calculated discharge capaci-
tance of the electrode as a function of current density. The
capacitances are calculated to be 645, 476, 355, 335, 314, and
288 F g −1 at current densities of 1, 2.5, 5, 7.5, 10, and 25 A g −1 ,
respectively. The IR drops in all curves are similar and not
obvious indicating small resistance and good contact between
the MnO 2 shell and micro current collectors.
Thermogravimetric analysis (TGA) is performed on
the as-prepared MnO 2 @TiC/C NFs powder (HR 30 min)
scraped from the electrode in N 2 at a ramping rate of 10 °C/
min ( Figure 4 ). The profi le indicates 11.3% weight loss when
heated from room temperature to 260 °C, corresponding to
the loss of water from the surface and in the lattice of the
MnO 2 nanostructure. A considerable amount of water con-
tent in the tunnel or layered structure of MnO 2 is expect
to enhance the electrochemical perfor-
mance in a mild aqueous electrolyte. [ 2 ] If
the annealing temperature is higher than
480 °C, a weight loss of 7.7% is observed
because of lattice oxygen release accom-
panied by phase transformation from
MnO 2 to Mn 2 O 3 or Mn 3 O 4. [ 39,40 ]
Transmission electron microscopy
(TEM) images are used to further investi-
gate the microstructure of the as-prepared
and annealed electrode (HR 30 min, with
and without thermal treatment at 500 °C
in N 2 ). Figure 5 depicts the representative
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Figure 1. Typical SEM images of the as-prepared MnO 2 @TiC/C NFs electrode after the hydrothermal reaction (HR) at 160 °C for different time: (a) 15 min, (b) 30 min, (c) 1 h, and (d) 3 h.
Figure 2. (a) CV curves and (b) Corresponding gravimetric and areal capacitances plots.
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TEM images of the coaxial structure. A typical core/shell
structure is shown in Figure 5 a and composed of a thin core
with a diameter of about 30 nm and thick shell 230 nm in
diameter. The HR-TEM images in Figure 5 b and 5 d show
apparent contrast between the inner and outer layers and the
outer layer with poor crystallinity covers the NF uniformly.
The corresponding EDS spectra (Figure 5 c) obtained from
the following small regions: (1) outer dark zone, (2) interface,
and (3) inner light area, show different signal intensities for
potassium, carbon, and manganese indicating that the thick-
ness of the as-prepared MnO 2 shell is about 17 nm. The
HR-TEM images and elemental maps are employed to con-
fi rm the thickness and uniform distribution of the MnO 2 layer
(Figure S2). By further heating to 500 °C in N 2 , the amor-
phous MnO 2 layer is converted to crystalline nanoparticle
clusters and the thickness shrinks to 7–8 nm (Figure 5 d). The
lattice spacings between adjacent lattice planes of the crystal-
line cluster are approximately 0.344 and 0.267 nm (Figure 5 e),
corresponding to the distance between the two (022) and
(023) planes of Mn 3 O 4 , respectively. As shown in the XRD
pattern (Figure 5 f), before thermal treatment, the as-pre-
pared electrode shows no peaks related to MnO 2 indicating
the amorphous nature of the hydrothermal MnO 2 shell. After
annealing, diffraction peaks of Mn 3 O 4 (JCPDS No: 75–0765)
at 32.4° and 33.5° emerge. In addition, EDS confi rms the
existence of abundant K element and the K/Mn atomic ratio
is about 0.92 in the corresponding as-prepared electrode.
The specifi c capacitance of manganese oxide corre-
lates with the ionic conductivity which is clearly related to
the crystallographic microstructure. [ 41 ] Figure 6 shows the
capacitance behavior of the as-prepared electrode thermally
treated at different temperature. The CV results acquired at a
scanning rate of 10 mV s −1 are presented in Figure 6 a. When
the annealing temperature is over 300 °C, the calculated
capacitance from the CV curves begin to decline dramati-
cally and 70% of the initial capacitance is lost upon further
heating to 500 °C (Figures 6 a,b). It has been reported that
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Figure 3. (a) CV curves of the as-prepared MnO 2 @TiC/C (HR 30 min) and pristine TiC/C NFs electrodes; (b) Corresponding CVs at different scanning rates; (c) GCD curves at different current densities; (d) Current density dependence of the gravimetric capacitance.
Figure 4. TGA curve of the as-prepared MnO 2 @TiC/C NF powder scraped from the electrode (HR 30 min).
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the lost capacitance arises from the decreased BET and loss
of water from MnO 2. [ 18 ] Water molecules increase the volume
of the MnO 2 structure to benefi ts ion diffusion, which is also
illustrated by the TEM image and TGA analysis reported
here. It is interesting to note that redox peaks disappear
when water is totally lost from the MnO 2 shell (at 300 °C,
TGA curve in Figure 4 ), suggesting that the remaining capac-
itance after the thermal treatment stems from the surface
faradic reaction or electric double-layer capacity. Conversely,
the amorphous MnO 2 with a loose structure and large water
content improves ion diffusion and reversible redox reac-
tions. Electrochemical impedance spectroscopy (EIS) is con-
ducted on the annealed electrodes and the corresponding
Nyquist plots in the frequency range between 100 kHz and
100 mHz are displayed in Figure 6 c which can be fi tted well
by the equivalent circuit in the inset fi gure. In the equivalent
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Figure 5. (a) TEM and (b) HR-TEM images of the as-prepared MnO 2 @TiC/C NF (HR 30 min); (c) EDS spectra of the region in (b); (d) TEM and (e) HR-TEM of the annealed MnO 2 @TiC/C NF (500 °C); (f) XRD patterns of the MnO 2 @TiC/C electrode before and after annealing at 500°C in N 2 ambient; (1) pristine TiC/C electrode, (2) as-prepared and (3) annealed MnO 2 @TiC/C electrodes.
Figure 6. (a) CVs of the as-prepared MnO 2 @TiC/C electrodes (HR 30 min) and annealed at 200, 300, 400 and 500 °C; (b) Corresponding capacitance retention compared with the as-prepared electrode; (c) Impedance Nyquist plots of corresponding annealed electrodes; (d) XPS spectra of the Mn 2p signals from the as-prepared and annealed electrodes (500 °C).
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circuit, a solution resistance (R s ) is in series with a constant
phase element (CPE dl ) for the double-layer capacitance
which is in parallel with the charge transfer resistance (R ct )
and pseudocapacitance (CPE p ). The semicircle intersecting
with the abscissa depends on the internal resistance and the
slope reveals the ion diffusion behavior in the supercapaci-
tors. [ 42 ] The as-prepared electrode without thermal treatment
has the smallest electrochemical resistance as well as largest
ion diffusion rate and capacitance. Although R s is slightly
enhanced with increasing temperature, R ct obtained from the
MnO 2 shell increases signifi cantly with annealing tempera-
ture from 1.07 Ω for the as-prepared electrode to 35.02 Ω for
the electrode annealed at 500 °C. The resistance evolution
illustrates the declining conductivity with increasing tem-
perature in conjunction with the deteriorated capacitive
performance of the SCs. The X-ray photoelectron spectros-
copy (XPS) results in Figure 6 d confi rm the chemical com-
position of the amorphous and crystalline MnO 2 shells. It is
known that the as-prepared MnO 2 shell contains a signifi cant
amount of suboxide state for the Mn 2p peak corresponding
to the spin-orbit doublet of Mn 2p 3/2 and Mn 2p 1/2 . The Mn
2p 1/2 peak is centered at 653.7 eV whereas the Mn 2p 3/2 peak
can be fi tted by three peaks including Mn 2+ (640.7 eV), Mn 3+
(641.8 eV), and Mn 4+ (643.15 eV), respectively. [ 43 ] However,
the area of the Mn 2+ peak decreases signifi cantly together
with the appearance of a sharp Mn 3+ peak and slight shift in
Mn 4+ (643.0 eV) with increasing annealing temperature. This
is probably due to the formation of Mn 3+ from the reaction
between Mn 2+ and Mn 4+ . [ 40 ]
The cycling performance is one of the important aspects
of faradic pseudocapacitance SCs because most of them
suffer from continuous capacity fading due to the mechanical
and chemical instability. The long-term cycling stability of the
electrodes annealed under different conditions is assessed
at GCD current densities of 5 A g −1 and 25 A g −1 in 1 a M
Na 2 SO 4 solution, as shown in Figure 7 a. The as-prepared
electrode (HR 30 min) shows almost constant capaci-
tance and over 99% of the initial capacitance (330 F g −1 ) is
retained after 5,000 cycles indicating the high cyclic stability,
which is better than those of hybrid capacitor, MnO 2 /NiO
(87.5% retained after 1,500 cycles), [ 44 ] MnO 2 /CNT sponge
(90.2% retained after 4000 cycles), [ 45 ] and MnO 2 /graphene
hydrogel (83.4% retained after 5,000 cycles). [ 46 ] However, the
annealed MnO 2 @TiC/C electrodes have small initial capaci-
tance which decreases to 310 F g −1 (94.2% retained) and
108 F g −1 (32.6% retained) after annealing at 300 °C and
500 °C, respectively. Interestingly, these annealed MnO 2 @
TiC/C electrodes exhibit strong self-recovery after cycling.
The capacitance of the electrode annealed at 300 °C could
restore to 323 F g −1 at a GCD current density of 5 A g −1 after
500 cycles. In contrast, the crystalline electrode annealed at
500 °C shows visibly self-recovered capacitance that remains
steady after 2,000 cycles. It is restored to 230 F g −1 , that is, 70%
of the capacitance of the as-prepared electrode. By increasing
the GCD current density to 25 A g −1 , stable capacitance is
obtained after 1,000 cycles and it is restored to 278 F g −1
which is 96.5% of the capacitance of the as-prepared elec-
trode (288 F g −1 at 25 A g −1 ). Figures 7 b-c present the typical
GCD cycles of the annealed electrode (500 °C) at current
densities of 5 A g −1 and 25 A g −1 , respectively. The longer dis-
charging time of the last 4 cycles compared to the fi rst ones
indicates increased capacitance. The CV tests before and
after cycling are used to identify the charge storage mecha-
nism. As shown in Figure 7 d, two pairs of redox peaks are
observed from the electrode (500 °C) after 5,000 cycles at
25 A g −1 and one pair of obscure redox peaks is found after
cycling at 5 A g −1 , while no redox peaks are observed before
cycling. It is demonstrated in details by the CV tests at dif-
ferent scanning rates as shown in Figure S3. Emergence of
the redox peaks suggests that the charge storage mechanism
is mainly based on the redox reaction of the interfacial oxy-
cation species with various oxidation states. The redox peaks
in the CV curves at 0.4–0.6 V (vs. SCE) are identical to the
theoretical transformation potential between Mn 4+ and Mn 3+
according to the Pourbaix diagram of manganese, [ 47 ] which
can be lowered by the large amount of small oxidation
state Mn. Our results indicated that a pair of redox peaks
is located at a high potential after the annealed electrode
(500 °C) undergo 5,000 cycles at 5 A g −1 . After cycling at
25 A g −1 , the intensity of the faradic redox peaks is appar-
ently enhanced and shifts to a smaller potential region with
about a 0.1 V potential difference, which may be attrib-
uted to that the structure change results in fast ion diffu-
sion and strong reversible redox reactions. [ 24,25 ] Figure 7 e
plots the specifi c capacitance at different scanning rates
for the as-prepared and annealed electrodes (500 °C)
before and after cycling at current densities of 5 A g −1 and
25 A g −1 . The initial specifi c capacitance of the annealed
electrode decreases gradually from about 100 F g −1 to
50 F g −1 when the scanning rate increases from 10 mV s −1 to
200 mV s −1 . After 5,000 cycles at 5 A g −1 , the specifi c capaci-
tance recovers to 250 F g −1 at a scanning rate of 10 mV s −1
and further increases to 300 F g −1 with good rate capability
at a large GCD current density of 25 A g −1 , which is close
to the capacitance of the as-prepared MnO 2 @TiC/C elec-
trode (HR 30 min). EIS is performed and the corresponding
Nyquist plots are shown in Figure 7 f to further illustrate
the fundamental electrochemical behavior of these elec-
trodes. The electrode after cycling at 25 A g −1 has smaller
R e (12.5 Ω) and R ct (1.8 Ω) and sharper EIS slope in the
low frequency range than that obtained at a smaller GCD
current density of 5 A g −1 (R e = 13.7 Ω and R ct = 3.3 Ω), indi-
cating improved conductivity and low ion diffusion resist-
ance resulting from the large GCD current.
SEM and TEM are employed to identify the structure
and crystal evolution after cycling and the results are shown
in Figure 8 . Although the as-prepared electrode (HR 30 min)
retains almost the initial capacitance after long-term cycling
at 5 A g −1 , only a few nanosheets could be observed from
the surface of the NFs (Figure 8 a). On the other hand, the
annealed electrode is almost covered by nanosheets after
cycling at 5 A g −1 (Figure 8 b). The morphology of the cycled
electrodes still shows the intact shape without destruction due
to the coaxial architecture with a sturdy skeleton. The XRD
pattern in Figure 8 d exhibits the apparent diffraction peaks
at 12.4° and 25.2° for the annealed electrode after cycling
which can be assigned to the layered birnessite type MnO 2
(JCPDS No: 38–0965) [ 47,48 ] besides the remained Mn 3 O 4
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diffraction peak at 32.4°. However, no related MnO 2 peak
can be found from the as-prepared electrode after cycling,
suggesting that the amorphous MnO 2 is more stable than the
crystalline structure in the cycling testing. The EDS spectra in
Figure 8 e also confi rm the existence of Na in the nanosheet
structure. The HR-TEM images (Figure 8 c and Figure S4)
show that the nanosheets fully cover the fi ber with representa-
tive lattice plane distances of 0.351 nm (110), 0.241 nm (101),
and 0.227 nm (111) indicative of the layered birnessite type
MnO 2 (Na 0.91 MnO 2 ). [ 47 ] The results are consistent with the
XRD patterns. The good stability, reversibility and restored
capacitance can thus be attributed to the excellent electro-
chemical performance of the formed layered birnessite type
MnO 2 . [ 47 ] With regard to the amorphous structure, the small
mass of MnO 2 was dissolved and then be electrodeposited
back onto the surface with rare nanosheets. This process is
enhanced greatly on the surface of crystalline MnO 2 resulting
in the visible structure transformation and mass stability on
the electrode surface. This phenomenon leads to high capaci-
tive stability on the as-prepared MnO 2 @TiC/C electrode and
good capacitance recovery on the crystalline MnO 2 @TiC/C
electrode.
In order to demonstrate the feasibility of MnO 2 @TiC/C
electrode in SCs, a symmetrical prototype composed of two
pieces of the as-prepared MnO 2 @TiC/C electrodes are con-
structed (Figure S5) and the electrochemical performance is
illustrated in Figure S6. The capacitance shows no obvious
decay after 5,000 charging and discharging cycles at a current
density of 5 mA cm −2 thus corroborating the excellent long-
term stability of the as-prepared MnO 2 @TiC/C electrode. The
red light-emitting diode (LED) with the threshold voltage of
1.8 V can be driven by three devices in series.
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Figure 7. Cycling performance of the as-prepared (HR 30 min) and annealed electrodes (300 °C and 500 °C); (b) and (c) Typical charging-discharging curves of the annealed electrodes (500 °C) before and after 5,000 cycles at 5 A g −1 and 25 A g −1 ; (d) CVs of the annealed electrode before and after 5,000 cycles at 5 A g −1 and 25 A g −1 ; (e) Corresponding specifi c capacitance at various scanning rate; (f) Impedance Nyquist plots of the annealed electrode after 5,000 cycles at 5 A g −1 and 25 A g −1 .
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The above-mentioned results reveal a self-recovery phe-
nomenon derived from the structural transformation of
MnO 2 on high conductive TiC/C NFs during cycling, which
gradually retains the large capacitance close to that of the
amorphous MnO 2 . In our experiments, the larger resistance
of the MnO 2 shell caused by the increased crystallinity and
structure water loss impedes inner ion diffusion, resulting in
the large dissolution of surface active materials due to strong
surface reaction. Conversely, the amorphous MnO 2 shell with
a smaller resistance and faster ion diffusion reduces surface
dissolution due to the electrochemical reaction occurring in
the whole layer at the same current density. That is to say, the
dissolution process is sensitive to the crystallization degrees
and resistance of the materials. On the other hand, in the
presence of a cathode current on the electrode, the dissolved
materials can be further electrodeposited to form highly
stable birnessite type MnO 2 nanosheets
with the layered structure and lots of
electrochemical active sites which lead
to fast electron transport and ion diffu-
sion and excellent electrochemical capaci-
tance behavior. This process is similar to
that observed from the electrodeposition
method to fabricate MnO 2 nanosheets
in an electrolyte containing manganese
salts. [ 49,50 ] The larger cathodic current
density, the faster deposition rate of man-
ganese ions are achieved, resulting in fast
structural recomposition. In the process,
the highly conductive TiC/C NFs scaffold
on the substrate plays an important role in
lowering the interface resistance between
the active materials and current collector.
The results suggest a smart repairing
function to regulate the inappropriate
microstructure of MnO 2 by charging and
discharging cycles. The formed layered
birnessite type MnO 2 possesses fast ionic
diffusion and high electrochemical sta-
bility as reported previously. [ 47,51 ] In this
case, dissolution and electrodeposition of
MnO 2 with large resistance are promoted
by the large charging/discharging currents
which decrease the time required for the
structural transformation and recovered
capacitance.
3. Conclusion
Amorphous MnO 2 decorated TiC/C NFs
electrodes fabricated by a simple hydro-
thermal reaction in a KMnO 4 solution
have a high capacitance of 645 F g −1 at
a discharge current density of 1 A g −1
and 99% capacity retention after
5,000 charging/discharging cycles in a
three-electrode system as well as in sym-
metrical two-electrode device. The excellent capacitive per-
formance can be attributed to the amorphous structure, fast
ion diffusion, as well as robust and highly conductive TiC/C
NFs scaffold. However, when the hydrothermal formation
MnO 2 @TiC/C crystallized via thermal treatment, water mol-
ecules are lost resulting in diffi cult ion diffusion causing the
capacitance to fade to 30% of the initial capacitance. Inter-
estingly, after cycling, the annealed electrode shows structural
transformation depending on the dissolution and electrodep-
osition mechanism, causing a strong capacitance self-recovery
with 96.5% of the initial capacitance retained after 1,000
cycles at a GCD current density of 25 A g −1 . A large GCD
current is benefi cial to high capacitance recovery as a result
of the fast structural transformation. Our results demon-
strate a remarkable electrode architecture with high specifi c
capacitance and excellent mechanical stability for promising
Figure 8. Typical SEM images of the (a) as-prepared and (b) annealed MnO 2 @TiC/C electrode (500 °C) after 5,000 cycles at 5 A g −1 (HR 30min); (c) Representative HR-TEM of nanosheet; (d) XRD patterns of the (1) as-prepared and (2) annealed MnO 2 @TiC/C electrode (500 °C) after 5,000 cycles at 5 A g −1 ; (e) Corresponding EDS spectra of the formed nanosheet.
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small 2015, 11, No. 15, 1847–1856
SCs applications and also provide insights the mechanism and
stability of MnO 2 based SCs benefi ting the design and fabrica-
tion of high-performance energy storage systems.
4. Experimental Procedures
All the chemicals and solvents were of analytical reagent grade and used as received without further purifi cation. The pristine TiC/C NFs arrays were fabricated on the Ti6Al4V alloy substrate (1 × 1 × 0.2 cm 3 ) directly thermochemically under acetone vapor in a horizontal tube furnace. Acetone was introduced into the chamber at a fl ow rate of 150 sccm together with Ar as the carrier gas. The temperature was 800 °C and reaction time was 2 h. More details about the fabrication of the TiC/C NFs can be found from our previous papers. [ 30,52 ]
To prepare the MnO 2 shell decorated TiC/C NF, typically, a reduction reaction was carried out in a potassium permanganate (KMnO 4 ) solution. The reaction proceeded as follows: [ 35 ]
− − −4MnO +3C+H O=4MnO +CO +2HCO4 2 2 3
23
Initially, the cleaned pristine TiC/C NFs electrode was placed face-up and immersed in 10 mL of 0.03 M KMnO 4 solution in a 25 mL Tefl on-lined autoclave on a holder. The autoclave was then sealed and heated in an oven to 160 °C for different time periods. After the hydrothermal reaction, the specimens were removed from the vessel and slightly ultrasonic washed in DI water for 5 min-utes and dried in air to obtain the MnO 2 shell decorated TiC/C NFs electrode. The electrode was annealed at different temperature for 3 h in N 2 to increase the crystallinity of the MnO 2 shell. The mass of the pristine TiC/C and as-prepared MnO 2 @TiC/C electrodes were measured on an electrical balance with a resolution of 0.01 mg.
The samples were characterized by glancing angle X-ray dif-fraction at 1 o incidence (GAXRD, Philips X'Pert Pro). Field-emission scanning electron microscopy (FE-SEM, JSM-820), transmission electron microscopy (TEM, Philips CM20), and high-resolution TEM (HR-TEM, JEM-2010F) were performed to determine the various surface properties and morphology. Energy-dispersive X-ray spec-troscopy (EDS, Oxford INCA 200, Oxford Instruments, Oxfordshire, U.K.) and X-ray photoelectron spectroscopy (XPS, ESCALB MK-II, VG Instruments, U.K.) were also employed to determine the chemical composition. Thermogravimetric analysis (TGA) was performed using Pt pans to estimate the mass loss from the formed MnO 2 . The samples were fi rst dried at 50 °C for 1 h and then heated at a rate of 10 °C/min to 800 °C in N 2 .
The electrochemical measurements were carried out on a CHI 6144E instrument with the sample (1 × 1 cm 2 ) as the working elec-trode, 1 × 1 cm 2 Pt foil as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode. Cyclic voltam-metry (CV) was performed between 0 and 0.8 V (vs. SCE) at dif-ferent scanning rates in a 1.0 M Na 2 SO 4 aqueous solution at room temperature. The galvanostatic charging/discharging (GCD) meas-urements were performed at different current densities between 0 and 0.8 V (vs. SCE) and electrochemical impedance spectros-copy (EIS) was conducted at the open circuit potential (OCP) with an AC perturbation voltage of 10 mV. The symmetrical device was
assembled with two as-prepared electrodes (HR 30 min) with a separator inserted between them. The measurements were per-formed at between 0 and 0.8 V.
Acknowledgements
The work was fi nancially supported by City University of Hong Kong Applied Research Grant (ARG) No. 9667085 and Guang-dong – Hong Kong Technology Cooperation Funding Scheme (TCFS) GHP/015/12SZ and Fundamental Research Funds for the Central Universities (HUST: 0118187030). The authors also thank the facility support of the Center for Nanoscale Characterization & Devices (CNCD), WNLO-HUST.
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Received: August 22, 2014 Revised: October 20, 2014 Published online: December 28, 2014
Supporting Informationfor Small., DOI: 10.1002/smll.201402519
Robust Electrodes Based on Coaxial TiC/C-MnO2 Core-Shell Nanofiber Arrays with Excellent Cycling Stability for High-Performance Supercapacitors
Xuming Zhang, Xiang Peng, Wan Li, Limin Li, Biao Gao, Guosong Wu, Kaifu Huo*, and Paul K. Chu*
1
Supporting Information
Robust Electrodes Based on Coaxial TiC/C-MnO2 Core-Shell Nanofiber Arrays with
Excellent Cycling Stability for High-Performance Supercapacitors
Xuming Zhang, Xiang Peng, Wan Li, Limin Li, Biao Gao, Guosong Wu, Kaifu Huo,* and
Paul K. Chu*
Table S1. The simulated data of EIS spectra of different electrodes
Where Rs presents the solution resistanceis, CPEdl for the double-layer capacitance, Rct for the
charge transfer resistance and CPEp for the pseudocapacitance.
Rs (Ω cm-2) CPEdl (Ω−2cm2 s−n) Rct (Ω cm-2) CPEp (Ω−2 cm2 s−n)
As-prepared (HR 30min) 12.76 0.001338 1.37 0.00695
200°C 13.51 0.000695 2.36 0.00625
300°C 13.07 0.001675 5.14 0.00416
400°C 15.01 0.001192 14.73 0.00174
500°C 15.25 0.001085 35.02 0.00071
HR15 min 12.53 0.0003053 0.83 0.01002
HR 1 h 12.73 0.0003902 4.51 0.00944
HR 3 h 13.33 0.0000836 128.73 0.00826
Symmetric two-electrode system (as-prepared MnO2@TiC/C electrode, HR 30min)
// 0.73 0.002122 4.82 0.01919
2
Fig.S1. The Nyquist plots of as-prepared electrodes suffering from different hydrothermal
time.
Fig. S2. The TEM element maps of amorphous MnO2@TiC/C NF (HR30min).
3
Fig. S3. CV loops of (a) initial annealed MnO2@TiC/C electrode (HR 30min, 500°C), (b)
annealed MnO2@TiC/C electrode after 5000 cycles at current density of 5 A g-1and (c)
annealed MnO2@TiC/C electrode after 5000 cycles at current density of 25 A g-1.
4
Fig. S4 (a) element maps of annealed MnO2@TiC/C electrode after 5000 cycles at current
density of 5 A g-1 and (b) the HRTEM of the formed layered birnessite MnO2.
5
Fig. S5. The digital figure of two-electrode testing system
6
Fig. S6. Electrochemical performance of the symmetric SCs protocol composed of two as-
prepared MnO2@TiC/C electrodes (HR 30min): (a) CV curves acquired at different scanning
rates; (b) GCD curves at different current densities; (c) Area and volume capacitance as a
function of current density; (d) Nyquist plots; (e) Long-term cycle stability at high current
density of 5 mA cm-2; (f) Red light-emitting diode (LED) driven by three devices in series.