Robust Electrodes Based on Coaxial TiC/C–MnO 2 Core/Shell … · 2015-04-16 · ©2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 1847 Robust Electrodes Based

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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.



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).



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).



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

small 2015, 11, No. 15, 1847–1856



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 .



small 2015, 11, No. 15, 1847–1856

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.



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.

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