Nano Res

1

Tailoring uniform γ-MnO2 nanosheets on highly

conductive three-dimensional current collectors for

high-performance supercapacitor electrodes

Sangbaek Park1†, Hyun-Woo Shim2†, Chan Woo Lee1, Hee Jo Song1, Ik Jae Park1, Jae-Chan Kim2, Kug

Sun Hong1, and Dong-Wan Kim2()

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0581-1

http://www.thenanoresearch.com on September 11, 2014

© Tsinghua University Press 2014

Just Accepted

This is a “Just Accepted” manuscript, which has been examined by the peer-review process and has been

accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance,

which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP)

provides “Just Accepted” as an optional and free service which allows authors to make their results available

to the research community as soon as possible after acceptance. After a manuscript has been technically

edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP

article. Please note that technical editing may introduce minor changes to the manuscript text and/or

graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event

shall TUP be held responsible for errors or consequences arising from the use of any information contained

in these “Just Accepted” manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®),

which is identical for all formats of publication.

Nano Research DOI 10.1007/s12274-014-0581-1

1

TABLE OF CONTENTS (TOC)

Tailoring uniform γ-MnO2 nanosheets on highly

conductive three-dimensional current collectors for

high-performance supercapacitor electrodes

Sangbaek Park1, Hyun-Woo Shim2, Chan Woo Lee1, Hee

Jo Song1, Ik Jae Park1, Jae-Chan Kim2, Kug Sun Hong1,

and Dong-Wan Kim2*

1 Department of Materials Science and Engineering, Seoul

National University, Seoul 151-744, Korea

2 School of Civil, Environmental and Architectural

Engineering, Korea University, Seoul 136-713, Korea

Ultrathin 2-D MnO2 nanosheets with highly conductive 3-D

current collectors was fabricated and assembled to obtain

excellent electrochemical performance.

Dong-Wan Kim, http://dwkim.ajou.ac.kr

2

Tailoring uniform γ-MnO2 nanosheets on highly conductive three-dimensional current collectors for high-performance supercapacitor electrodes

Sangbaek Park1†, Hyun-Woo Shim2†, Chan Woo Lee1, Hee Jo Song1, Ik Jae Park1, Jae-Chan Kim2, Kug Sun Hong1, and Dong-Wan Kim2()

1 Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea 2 School of Civil, Environmental and Architectural Engineering, Korea University, Seoul 136-713, Korea

† These authors contributed equally

Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher)

© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011

ABSTRACT Recent efforts have focused on the fabrication and application of 3-D nanoarchitectured electrodes, which can

exhibit excellent electrochemical performance. Herein, a novel strategy towards the design and synthesis of

size- and thickness-tunable two-dimensional (2-D) MnO2 nanosheets on highly conductive 1-D backbone arrays

were developed via a facile, one-step enhanced chemical bath deposition (ECBD) method at a low temperature

(~ 50 °C). Inclusion of an oxidizing agent, BrO3-, in solution was crucial in controlling the heterogeneous

nucleation and growth of the nanosheets, and in inducing the tailored and uniformly arranged nanosheet

arrays. We fabricated supercapacitor devices based on 3-D MnO2 nanosheets with conductive Sb-doped SnO2

nanobelts as the backbone. They achieved a specific capacitance of 162 F g-1 at an extremely high current

density of 20 A g-1, and good cycling stability that shows the capacitance retention of ~92 % of its initial value,

along with a coulombic efficiency of almost 100% after 5,000 cycles in an aqueous solution of 1 M Na2SO4. The

results were attributed to the unique hierarchical structures, which provided a short diffusion path of

electrolyte ions by 2-D sheets and direct electrical connections to the current collector by 1-D arrays as well as

aggregation prevention by the well-aligned 3-D structure.

KEYWORDS Nanosheets, Manganese oxide, Chemical bath deposition, SnO2 nanobelts, Supercapacitor

1. Introduction

Manganese dioxide (MnO2) is one of the most

attractive inorganic materials and is a stable

compound with excellent chemical and physical

properties [1-4]. Due to its low cost, environmental

friendliness, non-toxicity, as well as its structural

flexibility and rich polymorphism (α-, β-, γ-, δ-, λ-,

and ε-type), wide applications have been reported

including as a catalyst [5], ion-sieve [6], ion exchange

[7], biosensor [8], lithium ion battery [9], and

Nano Res DOI (automatically inserted by the publisher) Review Article/Research Article Please choose one

————————————

Address correspondence to D.-W. Kim, dwkim@ajou.ac.kr

3

supercapacitor [10, 11]. In particular, MnO2 is a

promising material as a replacement for RuO2 in

pseudo-capacitors owing to its high theoretical

capacitance of 1233 F g-1 for a one-electron transfer

and complete reduction of MnIV to MnIII [12].

Moreover, it can be utilized in mild neutral aqueous

electrolytes, which ensures environmental

compatibility, safety, non-flammability, and

convenient assembly in air [13]. Among various

phases, γ-MnO2 is most widely used in energy

storage devices because its electrical activity

decreases more slowly than other forms during the

electrochemical process [14]. However, only 100 ~

300 F g-1 have been reported for MnO2 powders [15],

owing to their poor electrical conductivity (10-5 to 10-6

S cm-1)[16] and low accessible surface areas [17].

Thus, a MnO2 electrode that provides a short

diffusion path for electrolyte ions and a high

electrical conductivity of the active materials is

needed.

Several approaches have utilized in an attempt to

meet the essential criteria in the design of

high-performance MnO2-based electrodes for

supercapacitors. Given that pseudo-capacitance is

primarily generated by surface Faradaic redox

reactions, it has been quite a challenge to develop

MnO2 nanostructures with an ultrathin morphology

for advanced supercapacitors [18-21]. Additionally,

hybrid composite structures with highly conductive

materials have been explored in order to improve the

rate capabilities of MnO2 electrodes for high power

performance [22-31]. Carbon-based materials such as

carbon nanotubes, graphene sheets, and activated

carbon have been investigated as hybrid composites

with MnO2 [22-27]. Nevertheless, the aggregation of

carbon caused by van der Waals interactions

prevents its use in practical applications [24].

Recently, three-dimensional (3-D) MnO2

nanostructures with conductive backbones, e.g.,

SnO2 or Zn2SnO4, showed improved electrochemical

capacitance under carbon and binder-free conditions

[28, 29]. Thus, the design and synthesis of 3-D MnO2

nanostructures composed of 1-D metallic backbones

and 2-D ultrathin MnO2 nanosheets are attractive for

the development of superior supercapacitors.

A key challenge in the fabrication of 3-D MnO2

nanostructures is a facile and controllable synthetic

route for well-defined MnO2 nanosheets on the 1-D

backbone. Some efforts have been devoted to

obtaining high-quality MnO2 nanosheets by using

various methods, including delamination (exfoliation)

[32, 33], reduction [34, 35], and templating [36, 37].

For example, Liu et al. successfully deposited MnO2

nanosheets on various metal oxide nanowires using

sacrificial carbon templates coated on nanowires, in

which the carbon layers functioned as reducing

agents [38]. In spite of its wide applicability [35], this

process is limited by the inevitable carbon layer

deposition step. Although the structural design of 2D

nanosheets on 1D conductive array with high

performance in supercapacitor also has been

extensively explored in other materials [38-40], most

of them are synthesized in high temperature or

pressure, which make it hard to modulate the

morphology. A decade ago, Unuma et al. reported

that MnO2 thin layers were heterogeneously grown

on a substrate by chemical bath deposition (CBD)

with NaBrO3 as the oxidizing agent [41]. Because

CBD is a low-temperature and surfactant-free

technique, it is very useful in tailoring 3-D

nanoarchitectures [42]. Inspired by these pioneering

studies, we designed a broad and general approach

for forming MnO2 nanosheets on 1-D backbones by

CBD.

Herein, we present an alternative route by

organizing a 3-D MnO2 nanoarchitectured electrode

with 2-D nanosheets and 1-D conductive backbones

for supercapacitor devices. The synthetic route

consists of two stages: (i) construction of 1-D

backbones by conventional methods such as

vapor-liquid-solid, anodizing, or AAO templating,

and (ii) decoration of 2-D MnO2 nanosheets on 1-D

arrays by CBD (Scheme 1). Moreover, we adopted

enhanced CBD (ECBD) composed of an oxidation

reaction, which provided a facile, mass-producible,

and controllable way for forming MnO2 nanosheets

on arbitrary 1-D backbones. As a proof of concept,

Sb-doped SnO2 (ATO) nanobelt arrays having a

metallic conductivity [43] were selected as the

conductive 1-D backbone. In neutral aqueous

solutions, this 3-D ATO@MnO2 nanostructure

showed superior capacitance and stability at high

currents without any binders or conductors owing to

the highly conductive pathway generated by the 1-D

current collector, fast ion transport by the 2-D sheets,

4

Scheme 1 Illustration of 3-D hierarchical γ-MnO2 nanosheets.

Fabrication of conductive backbone arrays directly grown on the

current collector, and controllable γ-MnO2 nanosheets on each

1-D backbone.

and aggregation prevention by the well-defined 3-D

structure.

2. Results and discussion

2.1. Morphology and structure

The 3-D nanostructure decorated with 2-D MnO2

nanosheets was verified by morphological analysis.

As shown in Figure 1, ultrathin (< 10 nm) MnO2

nanosheets with typical diameters of 500 nm were

successfully formed on the ATO nanobelt surfaces by

ECBD. Specifically, the reaction was carried out at 50 oC for 3.5 h in a solution containing 0.05 M Mn2+ and

0.3 M BrO3-. It can be clearly seen from Figure 1b that

all nanobelts were uniformly covered with MnO2

nanosheets. Interestingly, aligned nanobelt arrays

(Figure S1) were preserved (Figure 1a and b),

suggesting that little damage was caused by the mild

ECBD.

Typical XRD patterns and Raman spectra of ATO

and ATO@MnO2 are presented in Figure 2. With the

exception of the SnO2- and Ti-related peaks, the most

intensive peak of the MnO2 nanosheets at around 2θ

= 36.9o corresponded to the (131) peak of the nsutite

γ-MnO2 (JCPDS 14-0644, a = 6.36 Å, b = 10.15 Å, c =

4.09 Å), consistent with previous studies for MnO2

thin films fabricated in solution containing Mn2+ and

Figure 1 Typical SEM images of (a) ATO nanobelts and (b)

ATO@MnO2 3-D nanostructures grown on Ti substrates.

BrO3- [41]. Relatively weak and broad MnO2 peaks

represent the amorphous nature of nanosheets. To

further identify the local Mn environment in the

γ-MnO2 nanosheets, Raman analysis was carried out

(Figure 2b). Three main bands were observed in the

ATO nanobelts; the band near 627 cm-1 was the A1g

mode of SnO2 and bands near 452 and 245 cm-1 were

related to doping of Sb in SnO2 lattice [42]. The

Raman spectra of ATO@MnO2 revealed two main

vibration bands (ν1 = 644 and ν2 = 575 cm-1) in the

wavenumber range of 500 – 700 cm-1, characteristic

features of γ-MnO2 [14, 44]. The crystal structure of

γ-MnO2 can be described as an arrangement of

octahedral MnO6 units with a random intergrowth of

pyrolusite (β-MnO2) layers within a ramsdellite

(R-MnO2) matrix. Thus, the fraction of single chain

slabs (pyrolusite) in the double chain (ramsdellite)

framework affects vibrational interactions in the

lattice of γ-MnO2, inducing the blue shift in the

vibration peak (ν1) of γ-MnO2 from that of R-MnO2

(ν1 = 630 cm-1) to that of β-MnO2 (ν1 = 665 cm-1) [44].

Therefore, the amount of pyrolusite intergrowth,

known as DeWolff defects, can be determined by

linear interpolation; γ-MnO2 nanosheets have about

32 % pyrolusite in the ramsdellite. All these features

confirm that the MnO2 nanosheets were synthesized

as nsutite γ-MnO2.

The morphology and crystal structure of the

ATO@MnO2 nanostructures were further

investigated by TEM (Figure 3). ATO nanobelts were

regularly enclosed by MnO2 nanosheets, which were

perpendicular to the growth axis of the nanobelt and

formed circular bands surrounding the nanobelt. To

characterize the nanobelt and sheet separately,

respective selected area electron diffraction (SAED)

patterns were obtained. The nanobelt was assigned

to the SnO2 rutile phase according to the measured

SAED pattern with [102̅] zone axis (Figure 3b). From

5

Figure 2 (a) XRD graphs of ATO nanobelts and ATO@MnO2

3-D nanostructures. The line pattern show reference JCPDS

#41-1445 (SnO2) and #14-0644 (MnO2). (b) Raman spectra of

ATO and ATO@MnO2.

the SAED ring patterns (Figure 3c), it is clear that the

nanosheet is γ-MnO2 with an orthorhombic lattice of

d131 = 0.242 nm. Moreover, EDS mapping (Figure 3d)

revealed that oxygen existed in the core and sheet

components, whereas tin and manganese were only

detected in the core and sheet, respectively.

Therefore, 2-D γ-MnO2 nanosheet arrays could be

formed normal to the 1-D backbone axis by one-step

ECBD.

2.2. The function of the oxidizing agent

The key step in the presented synthesis was the use

of an oxidizing agent (BrO3-), which played a

significant role in the formation of the MnO2

nanosheets on the 1-D backbone arrays. In the

presence of BrO3-, the oxidation of Mn2+ to MnO2

occurred via two reaction steps:

6Mn2+ + BrO3– + 6H+ → 6Mn3+ + 3H2O + Br– (1)

2Mn3+ +2H2O → Mn2+ + MnO2 + 4H+ (2)

Thus, the overall reaction was as follows:

6Mn2+ + BrO3– + 3H2O → 3MnO2 + Br– + 6H+ (3)

From the Pourbaix diagram for manganese [41],

reaction (3) has a negative Gibbs free energy (∆G) in

all pH ranges, indicating that it is

thermodynamically feasible. Moreover, given that

there was no Mn3+ in the starting solution, the rates

of reaction (1) & (2) were predominately determined

by the concentration of Mn2+ and BrO3-. According to

thermodynamics, the Gibbs free energy of

heterogeneous nucleation is smaller than that of

homogeneous nucleation, implying that

Figure 3 (a) A TEM image of an ATO@MnO2 3-D

nanostructure, (b) SAED pattern of ATO nanobelt, (c) SAED

pattern of γ-MnO2 nanosheets, and (d) EDS mapping of a

ATO@MnO2 3-D nanostructure.

heterogeneous nucleation can be predominant when

the rate of (1) is slow enough. Thus, heterogeneous

nucleation and growth of MnO2 nanosheets can be

controlled by the concentration of the oxidizing

agent. Figure 4 shows the various ATO@MnO2

nanostructures synthesized with different

concentrations of BrO3- and Mn2+. Nanosheets were

irregularly attached on the nanobelts at low

concentrations of BrO3- (0.1 M) (Figure 4a, d, and g).

As the concentration of BrO3- increased (0.2 M), the

deposition amount and uniformity of the nanosheets

increased (Figure 4b, e, and h). At high

concentrations of BrO3- (0.3 M), nanosheets randomly

wrapped the nanobelts (Figure 4c and f) and some

precipitates were observed (Figure 4i). These results

were also confirmed by XRD and EDS analysis

(Figure S2&Table S1). The loading amount was

obviously different with different concentration of

BrO3-. Based on nucleation and growth theory, the

dependency on the concentrations of the oxidizer can

be interpreted as follows: First, with low

concentrations of BrO3-, only a few monomers were

formed in solution due to the very slow reaction rate,

which induced the slow heterogeneous nucleation

rate. Accordingly, the formed monomers were

mostly consumed by the growth of already attached

MnO2, rather than by the formation of other

heterogeneous nucleation on the nanobelts, inducing

abnormal growth and irregular deposition of MnO2

nanosheets. Second, with moderate concentrations of

BrO3-, the solution was filled with enough monomers

for heterogeneous nucleation to dominate, inducing

the thorough covering of nanosheets on the

nanobelts. Moreover, the nucleated MnO2 on the

6

Figure 4 SEM images of γ-MnO2 nanosheets synthesized in the

presence of MnCl2 and KBrO3 at different concentrations: (a-c)

MnCl2 0.02 M, (d-f) MnCl2 0.05 M, (g-i) MnCl2 0.1 M. (a, d, g)

KBrO3 0.1 M, (b, e, h) KBrO3 0.2 M, (c, f, i) KBrO3 0.3 M. The

reaction temperature is fixed at 50 oC. Scale bar of images and

insets is 5 μm and 1 μm, respectively. M and K refers to MnCl2

and KBrO3, respectively.

nanobelt was uniformly grown by sufficient

monomers. Lastly, with high concentrations of BrO3-,

fast reaction rates produced large amounts of

monomers, higher than the supersaturation level,

and homogeneous nucleation as well as

heterogeneous nucleation become pronounced.

Therefore, a large amount of MnO2 attached and

some precipitates settled. These behaviors were also

confirmed by the transparency of the solutions

(Figure S3): with low concentrations of BrO3-, the

solution was clear, meaning that only heterogeneous

nucleation occurred, and with high concentrations of

BrO3-, the solution became cloudy after the reaction,

revealing that homogeneous nucleation also

occurred. Therefore, fast heterogeneous nucleation

and moderate growth rates by adjusting the oxidizer

concentration are essential in growing uniform and

regular nanosheet arrays on 1-D backbones.

2.3. Controllable synthesis of γ-MnO2 nanosheets

To control the size of the MnO2 nanosheets, the

formation as a function of reaction time was

observed by ex situ SEM (Figure 5). The

concentrations of Mn2+ and BrO3- were fixed at 0.05

M and 0.2 M, respectively. As the reaction time

increased, the MnO2 nanosheets grew on the

nanobelts with diameters ranging from 400 nm to 1.5

Figure 5 SEM images of γ-MnO2 nanosheets prepared in the

presence of 0.05 M MnCl2 and 0.2 M KBrO3 with different

reaction times: (a) 2 h 15 m, (b) 2 h 30 m, (c) 2 h 45 m, (d) 3 h,

(e) 3 h 30 m, and (f) 4 h. The reaction temperature is fixed at 50 oC. Scale bar of images and insets is 5 μm and 1 μm,

respectively.

μm. This was consistent with the XRD graphs, in

which the (131) peak intensity of γ-MnO2 increased

as the reaction time increased (Figure S4). This

fine-tuning technique was attributed to the slow

growth rate by the low temperature in ECBD, which

is beneficial for the fabrication of 3-D hierarchical

heterostructures. Interestingly, when the ECBD

reaction was long enough (63 h), skein-like

microspheres were obtained with an average

diameter of 2.5 μm (Figure S5). Because irregular

powders precipitated when the ATO nanobelt was

not inserted in the solution, the formation of the

mesoporous microspheres was due to the

aggregation of the nanosheets on the nanobelt. This

result also supports the fact that nanosheets were

formed by heterogeneous nucleation and growth on

the surface of the nanobelts. In addition, the

thickness of the nanosheets could also be controlled

by temperature variation in ECBD. Figure S6 shows

the SEM images of the nanosheets formed at

different reaction temperatures and concentrations of

MnCl2. At 50 oC, all nanosheets synthesized with

various concentrations of MnCl2 were thin layers

with a thickness less than 10 nm. However, as the

temperature increased, the thickness of the

nanosheets increased up to 100 nm due to the fast

growth rate. While only 2-D growth dominantly

occurs at low temperatures, 3-D growth could occur

at high temperatures, inducing an increase in

thickness. These results demonstrated that size- and

dimension-tunable 2-D MnO2 nanosheets can be

formed on 1-D backbones by ECBD.

7

2.4. Supercapacitive performance of 3-D

hierarchical ATO@MnO2 nanosheets

To highlight the advantages of the 3-D ATO@MnO2

hetero-nanostructures as electrode materials for

supercapacitors, their electrochemical performance

was investigated using cyclic voltammetry (CV) and

galvanostatic charge-discharge (CD) test in a

three-electrode system in 1 M Na2SO4. Figure 6a

shows the typical CV curves of the ATO@MnO2

electrode fabricated for 3.5 h (Figure 1) in the

potential range of 0 to 1.0 V (vs. Ag/AgCl) at scan

rates from 10 up to 500 mV s-1. This CV is an effective

experimental technique to evaluate the characteristic

capacitive behaviors of electrode materials in

supercapacitors [45]. The CV responses presented

quasi-rectangular shapes, indicating fast

charging-discharging processes characteristic such as

ideal electrical double-layer capacitance (EDLC)

behavior. Besides, it should be also noted that a

broad oxidation/reduction couple was observed at

around 0.6 and 0.3 V. This means that the

ATO@MnO2 electrode exhibited a pseudo-capacitive

behavior and a reversible Faradaic transition of Mn

oxide; that is, the ATO@MnO2 electrode undergoes

an electrochemical charge transfer reaction in basic

electrolytes, making it a potential candidate for

pseudo-capacitor applications. According to the

literature, such charge-discharge processes or the

capacitive reaction of MnO2-based electrodes near

the surface during CV or CD measurements in

aqueous solutions can be expressed by the following

faradaic reactions [46-48]:

(Mn4+O2)surface + xC+ + xe- ↔ C+x(Mn1-x4+Mnx3+O2)surface (4)

(Mn4+O2)bulk + xC+ + xe- ↔C+x(Mn1-x4+Mnx3+O2)bulk (5)

(C+ = K+, Li+, Na+, or H+)

This first mechanism (reaction (4)) illustrates the

adsorption/desorption of electrolyte cations (C+) on

the MnO2 surface and reaction (5) involves the

insertion of cations dissolved in the electrolyte. The

literature reveals that a pair of redox peaks,

MnO2/MnOOC, are generally involved in this system

and demonstrate that the overall redox reaction

given by reactions (4) and (5) are involved in the

charge storage mechanism. In the present study, the

CV characteristics of the ATO@MnO2 electrode are

mainly consistent with those previously reported for

MnO2 in Na2SO4 electrolytes [47, 49-52], even though

such a CV shape may vary from sample to sample

and may strongly depend on the morphology and

surface properties of the electrodes. Furthermore, the

linear relationship of the plot of the anodic peak

current density versus the scan rate up to 500 mV s-1

(inset in Figure 6a and Figure S7) also indicated the

occurrence of surface redox reactions. Therefore,

these CV pattern characteristics substantiate

pseudo-capacitive behaviors of the ATO@MnO2

electrode; that is, the charge storage of the

ATO@MnO2 electrode is characteristic of the

pseudo-capacitive process originating from the

reversible redox reaction.

More importantly, the CV curves showed no

obvious distortion from 10 to 200 mV s-1 and

exhibited a nearly rectangular shape, indicating that

the redox reaction is fast enough for the ATO@MnO2

electrode at the measured scan rate. Even at the scan

rate of 500 mV s-1, the CV retained a similar shape

without the significant change, indicating not only

excellent ion diffusion in the electrode but also a

better high-rate response of the ATO@MnO2 material.

In other words, this result suggests that the 3-D

hierarchical formations as well as the presence of the

self-supported ATO nanowires (NWs) on the current

collector and thin 2-D MnO2 nanosheets combined

loosely with ATO NWs can actually facilitate

electronic and ion transport and thus improve the

kinetics of the capacitive reaction. The mass-specific

capacitance (SC, F g-1) of ATO@MnO2 electrode was

estimated from the CV curves, and was denoted as

SCCV (see Supporting Information; Calculation of

specific capacitance). As a function of scan rate, the

calculated SCCV values of the ATO@MnO2 electrode

were not only the maximum specific capacitance of

278 F g-1 at a scan rate of 10 mV s-1, but also 183 F g-1

at a scan rate as high as 200 mV s-1. Furthermore,

even at a high scan rate of 500 mV s-1, the

ATO@MnO2 electrode retained a specific capacitance

of 178 F g-1.

Figure 6b shows the CD study of the ATO@MnO2

electrode carried out at current densities of 0.5, 1, 2, 5,

10, and 20 A g-1 within the potential range of 0 to 1.0

V (vs. Ag/AgCl). The sloped variation with some

broad and short plateaus in the discharge profiles,

which illustrated the near linearity, demonstrates the

representative pseudo-capacitive behavior of the

8

Figure 6 Electrochemical performance of ATO@MnO2 electrode. (a) Cyclic voltammetry (CV) behaviors measured at six different scan

rates of 10, 20, 50, 100, 200, and 500 mV s-1. The inset shows the linearity of the anodic current density with scan rates, (b) Evolution of

the galvanostatic discharge profiles obtained at current densities of 0.5, 1, 2, 5, 10, and 20 A g-1, (c) Corresponding mass-specific

capacitance (SCCD) as a function of discharge current densities, (d) Long-term cycling performance (closed-circle) measured at a

constant current density of 20 A g-1. The closed-squares indicate a superior Coulombic efficiency over average 99%, and each inset of (d)

shows highly symmetric charge-discharge profiles and excellent SCCD retention until 5,000 consecutive cycling tests. All measurements

were recorded on the potential window of 0 to 1.0 V (vs. Ag/AgCl).

ATO@MnO2 electrode. Although the time

dependence discharge curves in this work exhibited

overall linear variation with broad and short

plateaus, they clearly indicated suitable and fast

pseudo-capacitive characteristics caused by the

surface redox reaction of ATO@MnO2, which

corresponded to the results of the broad redox pair

and quasi-rectangular shapes observed in the CV

profiles. We also calculated the mass-specific

capacitance values of the ATO@MnO2 electrode from

the CD curves, and were denoted as SCCD (see

Supporting Information; Calculation of specific

capacitance), and plotted as a function of discharge

current densities as shown in Figure 6c. The SCCD

values were 216, 203, 192, 180, 171, and 162 F g-1 at

current densities of 0.5, 1, 2, 5, 10, and 20 A g-1,

respectively. In addition, the calculated area-specific

capacitances (SCCD,area) of ATO@MnO2 electrode were

also shown in Figure S8. Importantly, the

ATO@MnO2 electrode exhibited a high SCCD of up to

216 F g-1, and 75% of the specific capacitance was

retained even at a high current density of 20 A g-1. In

particular, an increase in the discharge current

generally leads to a large potential drop (i.e., iR drop)

that results in a decrease in the capacitive

performance, but the SCCD value of the ATO@MnO2

electrode showed nevertheless over 90% retention

even at 20 A g-1 as compared to the value at 5 A g-1. Among ATO@MnO2 heterostructures prepared with

various synthetic condition, the sample prepared from

0.05 M Mn2+ and 0.3 M BrO3- at 50 oC for 3.5 h

showed the best capacitance performance (Figure S9),

which might be originated from relatively higher

surface area of 2D nanosheets (Figure 4), shorter

diffusion path to 1D ATO (Figure 5), and larger loading

amount of MnO2 active material (Table S1). Here, the

highest specific capacitances of the ATO@MnO2

electrode in terms of high-rate capability were 178 F

g-1 at a scan rate of 500 mV s-1 (for CV) and 162 F g-1

at a current density of 20 A g-1 (for CD); these values

exceeded other MnO2-based electrodes in previous

reports (Table S2) [46, 47, 49, 50, 52-72, 76]. In

addition, we showed the Ragone plot (power density

9

vs. energy density) for the ATO@MnO2 electrode

(Figure S10). It is also impressive that the

ATO@MnO2 electrode delivered a specific power

density from 250.6 to 10,625 W kg-1 and a specific

energy density from 108 to 85 Wh kg-1 as the

galvanostatic charge-discharge current density was

increased from 0.5 to 20 A g-1.

Long-term cycling performance of electrode

materials at higher rate operation conditions is

essential for real supercapacitor operations. Figure

6d presents the cycle number dependence of the

specific capacitance of the ATO@MnO2 electrode. As

can be seen, the SC value of the ATO@MnO2

electrode was over 165 F g-1 and maintained

approximately 91.5% of its initial value after 5,000

charge-discharge cycles (the inset on the far right in

Figure 6d), demonstrating the excellent cycling

stability of the ATO@MnO2 hetero-nanostructures as

a supercapacitor electrode. Furthermore, the ~100%

Coulombic efficiency (η) for the CD tests of 5,000

cycles, a measure of the feasibility of the redox

process calculated from the discharge and charging

time (denoted as tD and tC, respectively) in CD

measurements [73, 74]: 𝜂(%) =𝑡𝐷

𝑡𝐶× 100 , clearly

demonstrates the electrochemical suitability of the

ATO@MnO2 hetero-nanostructures and the higher

feasibility of the redox process. It can be also seen,

the charge curves are symmetric to their

corresponding discharge counterparts for 5,000

cycles (the inset on the far left in Figure 6d), further

indicating its excellent reversibility. The similar

symmetric triangular charge-discharge curves (as

compared to the initial profile) and high Coulombic

efficiency demonstrate that no significant structural

or phase change of the ATO@MnO2

hetero-nanostructures were induced during the CD

processes, contributing to its long-term

electrochemical stability. The microstructural

stability of the ATO@MnO2 hetero-nanostructures

after the long-term cycling tests was further

confirmed using TEM and EDS elemental mapping

analyses (Figure S11) as well as SEM observation

(Figure S12), and it illustrated that the 3-D

hierarchical ATO@MnO2 hetero-nanostructures were

retained rather well.

In addition, we also investigated the

electrochemical performance of bare ATO nanobelt

electrode as a reference sample of comparison in

order to emphasize the supercapacitive performance

of ATO@MnO2 nanostructure electrode (Figure S13).

Importantly, it is noted that the bare ATO nanobelt

electrode did not showed the notable characteristics

in the CV curves, and its specific capacitance values

calculated from the CD measurement were

approximately ~1 F g-1 and below, which is negligible.

Therefore, it is believed that the ATO nanobelts

(backbone) here can only offer the effective electron

pathway, but their capacitive performance is not

affected on the supercapacitive performance of

ATO@MnO2 nanostructure electrode.

It is obvious that the ATO@MnO2 electrode

exhibits high-capacitive performance, which may be

due to the unique and novel formation of the 3-D

hierarchical hetero-nanostructures (Figure 7a). First,

the loosely packed, thin 2-D layered architectures of

MnO2 and macro-/mesoporous characteristic caused

by the open space between each 2-D MnO2

nanosheet as well as the neighboring individual

ATO@MnO2 hetero-nanostructure can offer a large

electroactive surface area to electrolyte ions for fast

Figure 7 (a) Schematic illustration of facile accession of ions in

electrolyte and electron “superhighways” in the 3-D hierarchical

self-supported ATO@MnO2 nanosheet electrode, (b) The

complex-plane impedance plots (Nyquist plots; imaginary part,

Z" versus real part, Z') of the ATO@MnO2 electrode before and

after 3,000 cycles. The inset shows the equivalent circuit for the

EIS spectra, (c) Frequency-dependent specific capacitance values

of the ATO@MnO2 electrode calculated from the impedance

measurement. The EIS measurement was carried out by

imposing a sinusoidal alternating voltage frequency of 100 kHz

to 0.01 Hz at open circuit voltage (OCV), an alternating current

(AC) amplitude of 5 mV, and a constant direct current (DC) bias

potential of 1.0 V (vs. Ag/AgCl) under the same conditions: a

three-electrode system with an aqueous electrolyte solution (1 M

Na2SO4) at room temperature.

surface sorption reactions and easy accessibility of

10

ions for highly feasible redox reactions, which may

ensure sufficient electrochemical utilization of the

MnO2 during the capacitive reaction. Furthermore,

this morphology can also serve as an ‘ion-buffering

reservoir’ of electrolyte ions [73, 75], resulting in

better penetration and occurrence of efficient

Faradaic reactions even at very high-rates, thereby

leading to not only reduced internal and charge

transfer resistance, but also enhanced power

characteristic by shortening the diffusion length for

both electrolyte ions and charges during the

charge-discharge process. Second, the self-supported

conductive ATO NBs array combined with the thin

MnO2 nanosheets can provide excellent interfacial

contacts and highly conductive paths throughout the

MnO2 nanosheets for rapid electronic transport as a

“superhighway”. In our previous study, the

conductivity of an individual ATO NB was about

1.03×104 S m-1, which corresponds to the metallic

property [43]. Thus, ATO/Ti electrode can provide

1-D conductive path, which is also confirmed by low

sheet resistance of the electrode (6.74 mΩ/□). Even

after MnO2 nanosheets coating, the electrode has

quite low sheet resistance (42.7 mΩ/□). Third, the

3-D hierarchical framework and voids give the

microstructural stability and fast ion diffusion into

the entire electrode matrix, thus enhancing the

electrochemical kinetics, and also accommodating

the strain arising due to high-rate insertion and

extraction of electrolyte ions, which are

synergistically beneficial for a strong cycling life

along with high-rate capability.

To gain further insight into the origin of superior

electrochemical performance of the ATO@MnO2

electrode, electrochemical impedance spectroscopy

(EIS) measurements were carried out before

(denoted as after 1st cycle in Figure 7b) and after the

CD tests of 3,000 cycles in the frequency range of

0.01–105 Hz at room temperature, and the obtained

impedance spectra were analyzed by the CNLS

fitting method based on a Randles equivalent circuit

depicted in the inset of Figure 7b [51, 55].

Importantly, the measured specific internal

resistance (Rs) as well as the charge-transfer

resistance (Rct) fitting values of the ATO@MnO2

electrode were similar until 3,000 cycles (Rs: 13.19

and 13.22 Ω mg-1 and Rct: 34.17 and 36.12 Ω mg-1,

respectively), indicating not only a good electrolyte

conductivity and electrochemical stability of the

ATO@MnO2 electrode, but also the significant

feasibility of the redox reactions even after long-term

CD processes at high current densities, because of

the enhancement of the diffusivity of the electrolyte

ions in the electroactive sites and highly conductive

paths for rapid electronic transport, which in turn,

reduced the Rct without any appreciable structural

change during high-rate cycling, thereby leading to

an excellent pseudo-capacitive performance.

Additionally, the profile exhibited higher angles than

45 °, indicating the suitability as electrode materials

for supercapacitors. The detailed EIS analyses are

described in the Supporting Information:

Electrochemical impedance spectroscopy (EIS)

analysis. For a more informative representation, we

converted the measured EIS data of the ATO@MnO2

electrode before and after 3,000 cycles into specific

capacitance (denoted as SCEIS, F g-1) using the

following equation [68, 73]:

𝑆𝐶𝐸𝐼𝑆 =1

2𝜋𝑓𝑍" (6)

and plotted the data as a function of frequency, “f”,

as shown in Figure 7c. The calculated SCEIS values

before and after 3,000 cycles were relatively similar

at all frequency regions, even though the Rct is

slightly increased after 3,000 cycles. Moreover, for

the frequency range of 0.01–0.1 Hz, the ATO@MnO2

electrode showed a maximum SCEIS value. This is

because electrolyte ions could easily penetrate into

the ATO@MnO2 hetero-nanostructures with a 3-D

hierarchical form and are electrochemically more

accessible over a reasonable frequency range

measured for the electrode. However, a minimum of

~5% decrease in the SCEIS value after 3,000 cycles was

also observed at the lower frequency region, which

well corroborates with the result from the CD study.

Interestingly, no decrease in the SCEIS value at higher

operating frequency regions after long-term cycling

tests at elevated current density condition is very

crucial for the ATO@MnO2 as a possible electrode

material for high-performance supercapacitor

applications.

3. Conclusion

In conclusion, we successfully developed 3-D MnO2

11

nanostructures composed of 2-D ultrathin MnO2

nanosheets and a 1-D highly conductive backbone by

ECBD. Two-dimensional MnO2 nanosheets were

uniformly grown perpendicular to the z-axis of the

backbone and had a circular band shape

surrounding the backbone. In this system, the

addition of an oxidizer, particularly BrO3-, acted as a

novel morphology controller and compensated for

the demerit of the CBD method, namely the

non-uniform formation and slow reaction time,

enabling the one-step synthesis of the 2-D MnO2

nanosheets on an arbitrary 1-D backbone. Moreover,

the size and thickness of the nanosheets were easily

tunable over a wide range (diameter: 0.4 – 1.5 μm

and thickness: 10 – 100 nm) via the reaction duration

and temperature. Furthermore, the ATO@MnO2

electrode showed not only a high-rate capability of

178 F g-1 at a scan rate of 500 mV s-1 (for CV) and 162

F g-1 at a current density of 20 A g-1 (for CD), but also

excellent long-term cycle stability as it retained

approximately 91.5% of its initial value after 5,000

charge-discharge cycles. This can be attributed to the

unique 3-D hetero-nanostructure formation, namely

loosely packed, thin MnO2 nanosheets combined

with self-supported ATO NWs, and thereby, this

structure leaded to enhanced electrochemical

kinetics of the capacitive reaction. This work

suggests not only the development of tailored

nanosheets in 3-D hierarchical heterostructures, but

also the possibility of oxidizers, which can act as

controlling agents and as growth accelerators in

low-temperature CBD. We envisage that the ECBD

method can be expanded to achieve 2-D nanosheets

engineering in 3-D nanostructures in other fields,

such as photo-conversion devices, energy storage

systems, etc.

4. Methods

4.1. Synthesis

1-D Conductive ATO backbones were prepared by

the vapor-liquid-solid (VLS) mechanism using a

conventional procedure [42, 43]. In this study, 10 µm

ATO nanobelt arrays were deposited on Ti foil

(99.7%, thickness 0.127 mm, Sigma-Aldrich, USA) by

thermal evaporation. For VLS growth, a 2 nm-thick

Au catalyst was deposited on the Ti substrate by an

evaporator. A mixed powder of Sn (99.5%, Samchun,

Korea) and Sb (99.9%, high purity chemicals, Japan)

in a 3:1 ratio was used as the source and was loaded

on a quartz boat and placed into the center of a fused

silica tube. The as-prepared Au/Ti substrates were

inserted into the tube furnace 20 cm from the source.

The source was heated to 1073 K under vacuum

conditions (< 2×10-3 Torr) and the ATO nanobelts

were grown on the Ti substrate at 873 K with an

oxygen flow of 12 sccm.

MnO2 were synthesized in a manner similar to that

previously reported after a slight modification [41].

2-D MnO2 nanosheets were formed on the surface of

the ATO nanobelts via the oxidation reaction of Mn2+

by one-step CBD with BrO3- as the oxidizer. In a

typical synthesis, MnCl2·4H2O (99%, Sigma-Aldrich,

USA) was dissolved in deionized water and KBrO3

(99.8%, Sigma-Aldrich, USA) was added with

various molar concentrations of Mn2+ (0.005 M to 0.1

M) and BrO3- (0.1 to 0.3 M). The prepared suspension

was stirred for 5 min and sonicated for 10 min at

room temperature to dissolve all reagents. The ATO

nanobelt electrodes at the bottom of the vial

containing 10 mL of the solution were heated to

different reaction temperatures (40 oC to 100 oC) for

various amounts of time (2 h to 63 h). After the ECBD

reaction, the samples were rinsed in deionized water

and ethanol, and finally dried at room temperature.

4.2. Characterization

The crystallographic characteristics of the 3-D MnO2

electrodes were investigated by XRD using a Bruker

D8-Advance (Cu Kα1 radiation) instrument operated

at 40 kV and 40 mA. The crystal structures were

double checked by a Renishaw Raman spectrometer

(inVia Raman microscope). The morphologies of the

samples were investigated by a JEOL scanning

electron microscope (JSM-6330F). TEM studies were

performed using a JEM-2100F equipped with an

energy dispersive X-ray analysis (EDS) system.

4.3. Electrochemical characterization

A home-made, three-electrode cell system was used

to measure the response of the 3-D hierarchical,

self-supported ATO@MnO2 nanosheets on the Ti foil,

which were used directly as the working electrodes

(~1 cm2) without cohesive binders and conductive

12

additives. An aqueous solution of 1 M Na2SO4 was

utilized as the electrolyte with Pt gauze (2 × 3 cm2) as

the counter electrode and an Ag/AgCl (saturated KCl)

electrode as the reference electrode. Using this

three-electrode configuration, the electrochemical

characteristics were determined on an

electrochemical workstation (model Ivium-n-Stat

electrochemical analyzer, Ivium Technologies B. V.,

Netherlands). Cyclic voltammetry (CV) and

galvanostatic charge-discharge (CD) techniques were

employed to evaluate the electrochemical

performance of the supercapacitor electrodes. The

CVs were measured in the voltage window between

0 and 1.0 V (vs. Ag/AgCl) at scan rates from 10 to 500

mV s-1, and the CDs were conducted in the same

potential window range with current densities of 0.5,

1, 2, 5, 10, and 20 A g-1. The cycle stability was also

evaluated using CD measurement at a constant

current density of 20 A g-1 for over 5,000 cycles. All

operating current densities were calculated based on

the mass of the active material (mass of MnO2

nanosheets). Electrochemical impedance

spectroscopy (EIS) was carried out in the frequency

range of 0.01 Hz–100 kHz with a perturbation

amplitude of 5 mV versus the open circuit potential

(OCV), and the obtained impedance spectra were

analyzed quantitatively by curve fitting with Z-view

software (Version. 2.90, Scribner Associates Inc.). All

electrochemical measurements were performed at

room temperature.

Acknowledgements

This research was supported by Basic Science

Research Program through the National Research

Foundation of Korea(NRF) funded by the Ministry of

Science, ICT and future Planning

(2012R1A2A2A01045382 and 2010-0029027).

Electronic Supplementary Material: Supplementary

material (further TEM, SEM, XRD and EIS analysis)

is available in the online version of this article at

http://dx.doi.org/10.1007/s12274-***-****-*

References [1] Zordan, T.; Hepler, L. G. Thermochemistry and oxidation

potentials of manganese and its compounds. Chem. Rev.

1968, 68, 737-745.

[2] Cheng, F.; Zhao, J.; Song, W.; Li, C.; Ma, H.; Chen, J.;

Shen, P. Facile controlled synthesis of MnO2

nanostructures of novel shapes and their application in

batteries. Inorg. Chem. 2006, 45, 2038-2044.

[3] Subramanian, V.; Zhu, H.; Wei, B. Alcohol-assisted room

temperature synthesis of different nanostructured

manganese oxides and their pseudocapacitance properties

in neutral electrolyte. Chem. Phys. Lett. 2008, 453,

242-249.

[4] Zhang, H.; Cao, G.; Wang, Z.; Yang, Y.; Shi, Z.; Gu, Z.

Growth of manganese oxide nanoflowers on

vertically-aligned carbon nanotube arrays for high-rate

electrochemical capacitive energy storage. Nano Lett. 2008,

8, 2664-2668.

[5] Débart, A.; Paterson, A. J.; Bao, J.; Bruce, P. G. α‐MnO2 Nanowires: A Catalyst for the O2 Electrode in

Rechargeable Lithium Batteries. Angew. Chem. Int. Edit.

2008, 120, 4597-4600.

[6] Li, W. N.; Yuan, J.; Shen, X. F.; Gomez‐Mower, S.; Xu, L. P.; Sithambaram, S.; Aindow, M.; Suib, S. L.

Hydrothermal Synthesis of Structure‐and Shape‐Controlled Manganese Oxide Octahedral Molecular Sieve

Nanomaterials. Adv. Func. Mater. 2006, 16, 1247-1253.

[7] Long, J. W.; Rhodes, C. P.; Young, A. L.; Rolison, D. R.

Ultrathin, protective coatings of poly (o-phenylenediamine)

as electrochemical proton gates: making mesoporous MnO2

nanoarchitectures stable in acid electrolytes. Nano Lett.

2003, 3, 1155-1161.

[8] Luo, X.; Morrin, A.; Killard, A. J.; Smyth, M. R.

Application of nanoparticles in electrochemical sensors and

biosensors. Electroanal 2006, 18, 319-326.

[9] Sayle, T. X.; Maphanga, R. R.; Ngoepe, P. E.; Sayle, D. C.

Predicting the electrochemical properties of MnO2

nanomaterials used in rechargeable Li batteries: simulating

nanostructure at the atomistic level. J. Am. Chem. Soc.

2009, 131, 6161-6173.

[10] Ghodbane, O.; Pascal, J. L.; Favier, F. Microstructural

effects on charge-storage properties in MnO2-based

electrochemical supercapacitors. ACS Appl. Mater.

Interfaces 2009, 1, 1130-1139.

[11] Fan, Z.; Yan, J.; Wei, T.; Zhi, L.; Ning, G.; Li, T.; Wei, F.

Asymmetric Supercapacitors Based on Graphene/MnO2

and Activated Carbon Nanofiber Electrodes with High

Power and Energy Density. Adv. Func. Mater. 2011, 21,

2366-2375.

[12] Brousse, T.; Toupin, M.; Dugas, R.; Athouël, L.; Crosnier,

O.; Bélanger, D. Crystalline MnO2 as possible alternatives

to amorphous compounds in electrochemical

supercapacitors. J. Electrochem. Soc. 2006, 153,

A2171-A2180.

[13] Xu, C.; Du, H.; Li, B.; Kang, F.; Zeng, Y. Asymmetric

activated carbon-manganese dioxide capacitors in mild

aqueous electrolytes containing alkaline-earth cations. J.

Electrochem. Soc. 2009, 156, A435-A441.

[14] Xiong, Y.; Xie, Y.; Li, Z.; Wu, C. Growth of Well‐Aligned γ‐MnO2 Monocrystalline Nanowires through a Coordination‐Polymer‐Precursor Route. Chem.-Eur. J. 2003, 9, 1645-1651.

http://dx.doi.org/10.1007/s12274-***-****-*

13

[15] Jones, D. J.; Wortham, E.; Rozière, J.; Favier, F.; Pascal,

J.-L.; Monconduit, L. Manganese oxide nanocomposites:

preparation and some electrochemical properties. J. Phys.

Chem. Solids 2004, 65, 235-239.

[16] Lee, S. W.; Kim, J.; Chen, S.; Hammond, P. T.; Shao-Horn,

Y. Carbon nanotube/manganese oxide ultrathin film

electrodes for electrochemical capacitors. ACS Nano 2010,

4, 3889-3896.

[17] Dong, X.; Shen, W.; Gu, J.; Xiong, L.; Zhu, Y.; Li, H.; Shi,

J. MnO2-embedded-in-mesoporous-carbon-wall structure

for use as electrochemical capacitors. J. Phys. Chem. B

2006, 110, 6015-6019.

[18] He, Y.; Chen, W.; Li, X.; Zhang, Z.; Fu, J.; Zhao, C.; Xie,

E. Freestanding three-dimensional graphene/MnO2

composite networks as ultralight and flexible

supercapacitor electrodes. ACS Nano 2012, 7, 174-182.

[19] Jiang, H.; Li, C.; Sun, T.; Ma, J. A green and high energy

density asymmetric supercapacitor based on ultrathin

MnO2 nanostructures and functional mesoporous carbon

nanotube electrodes. Nanoscale 2012, 4, 807-12.

[20] Lei, Z.; Zhang, J.; Zhao, X. S. Ultrathin MnO2 nanofibers

grown on graphitic carbon spheres as high-performance

asymmetric supercapacitor electrodes. J. Mater. Chem.

2012, 22, 153.

[21] Zheng, H.; Wang, J.; Jia, Y.; Ma, C. a. In-situ synthetize

multi-walled carbon nanotubes@MnO2 nanoflake core–

shell structured materials for supercapacitors. J. Power

Sources 2012, 216, 508-514.

[22] Zhou, R.; Meng, C.; Zhu, F.; Li, Q.; Liu, C.; Fan, S.; Jiang,

K. High-performance supercapacitors using a nanoporous

current collector made from super-aligned carbon

nanotubes. Nanotechnology 2010, 21, 345701.

[23] Chen, S.; Zhu, J.; Wu, X.; Han, Q.; Wang, X. Graphene

oxide−MnO2 nanocomposites for supercapacitors. ACS

Nano 2010, 4, 2822-2830.

[24] Wu, Z.-S.; Ren, W.; Wang, D.-W.; Li, F.; Liu, B.; Cheng,

H.-M. High-energy MnO2 nanowire/graphene and

graphene asymmetric electrochemical capacitors. ACS

Nano 2010, 4, 5835-5842.

[25] Yu, G.; Hu, L.; Liu, N.; Wang, H.; Vosgueritchian, M.;

Yang, Y.; Cui, Y.; Bao, Z. Enhancing the supercapacitor

performance of graphene/MnO2 nanostructured electrodes

by conductive wrapping. Nano Lett.2011, 11, 4438-4442.

[26] Gao, H.; Xiao, F.; Ching, C. B.; Duan, H.

High-performance asymmetric supercapacitor based on

graphene hydrogel and nanostructured MnO2. ACS Appl.

Mater. Interfaces 2012, 4, 2801-2810.

[27] Mao, L.; Zhang, K.; On Chan, H. S.; Wu, J.

Nanostructured MnO2/graphene composites for

supercapacitor electrodes: the effect of morphology,

crystallinity and composition. J. Mater. Chem. 2012, 22,

1845-1851.

[28] Yan, J.; Khoo, E.; Sumboja, A.; Lee, P. S. Facile coating of

manganese oxide on tin oxide nanowires with

high-performance capacitive behavior. ACS Nano 2010, 4,

4247-4255.

[29] Bao, L.; Zang, J.; Li, X. Flexible Zn2SnO4/MnO2 core/shell

nanocable-carbon microfiber hybrid composites for

high-performance supercapacitor electrodes. Nano Lett.

2011, 11, 1215-1220.

[30] He, S.; Chen, W. High performance supercapacitors based

on three-dimensional ultralight flexible manganese oxide

nanosheets/carbon foam composites. J. Power Sources

2014, 262, 391-400.

[31] He, S.; Hu, C.; Hou, H.; Chen, W. Ultrathin MnO2

nanosheets supported on cellulose based carbon papers for

high-power supercapacitors. J. Power Sources 2014, 246,

754-761.

[32] Omomo, Y.; Sasaki, T.; Wang, L.; Watanabe, M.

Redoxable nanosheet crystallites of MnO2 derived via

delamination of a layered manganese oxide. J. Am. Chem.

Soc. 2003, 125, 3568-3575.

[33] Yang, X.; Makita, Y.; Liu, Z.-h.; Sakane, K.; Ooi, K.

Structural characterization of self-assembled MnO2

nanosheets from birnessite manganese oxide single crystals.

Chem. Mater. 2004, 16, 5581-5588.

[34] Deng, R.; Xie, X.; Vendrell, M.; Chang, Y.-T.; Liu, X.

Intracellular glutathione detection using

MnO2-nanosheet-modified upconversion nanoparticles. J.

Am. Chem. Soc. 2011, 133, 20168-20171.

[35] Sun, X.; Li, Q.; Lu, Y.; Mao, Y. Three-dimensional

ZnO@MnO2 core@shell nanostructures for

electrochemical energy storage. Chem. Comm. 2013, 49,

4456-4458.

[36] Zhao, G.; Li, J.; Jiang, L.; Dong, H.; Wang, X.; Hu, W.

Synthesizing MnO2 nanosheets from graphene oxide

templates for high performance pseudosupercapacitors.

Chem. Sci. 2012, 3, 433-437.

[37] Peng, L.; Peng, X.; Liu, B.; Wu, C.; Xie, Y.; Yu, G.

Ultrathin two-dimensional MnO2/graphene hybrid

nanostructures for high-performance, flexible planar

supercapacitors. Nano Lett. 2013, 13, 2151-2157.

[38] Liu, J.; Jiang, J.; Cheng, C.; Li, H.; Zhang, J.; Gong, H.;

Fan, H. J. Co3O4 Nanowire@ MnO2 Ultrathin Nanosheet

Core/Shell Arrays: A New Class of High‐Performance Pseudocapacitive Materials. Adv. Mater. 2011, 23,

2076-2081.

[39] Tian, W.; Wang, X.; Zhi, C.; Zhai, T.; Liu, D.; Zhang, C.;

Golberg, D.; Bando, Y. Ni(OH)2 nanosheet@Fe2O3

nanowire hybrid composite arrays for high-performance

supercapacitor electrodes. Nano Energ. 2013, 2, 754-763.

[40] Yang, Q.; Lu, Z.; Chang, Z.; Zhu, W.; Sun, J.; Liu, J.; Sun,

X.; Duan, X. Hierarchical Co3O4 nanosheet@ nanowire

arrays with enhanced pseudocapacitive performance. RSC

Adv. 2012, 2, 1663-1668.

[41] Unuma, H.; Kanehama, T.; Yamamoto, K.; Watanabe, K.;

Ogata, T.; Sugawara, M. Preparation of thin films of MnO2

and CeO2 by a modified chemical bath

(oxidative-soak-coating) method. J. Mater. Sci. 2003, 38,

255-259.

[42] Park, S.; Lee, S.; Seo, S. W.; Seo, S.-D.; Lee, C. W.; Kim,

D.; Kim, D.-W.; Hong, K. S. Tailoring nanobranches in

three-dimensional hierarchical rutile heterostructures: a

case study of TiO2–SnO2. CrystEngComm 2013, 15, 2939.

[43] Park, S.; Seo, S.-D.; Lee, S.; Seo, S. W.; Park, K.-S.; Lee,

C. W.; Kim, D.-W.; Hong, K. S. Sb:SnO2@TiO2

Heteroepitaxial Branched Nanoarchitectures for Li Ion

14

Battery Electrodes. J. Phys. Chem. C 2012, 116,

21717-21726.

[44] Julien, C.; Massot, M.; Rangan, S.; Lemal, M.; Guyomard,

D. Study of structural defects in γ-MnO2 by Raman

spectroscopy. J. Raman Spectrosc. 2002, 33, 223-228.

[45] Conway, B. E. Electrochemical Supercapacitors: Scientific

Fundamentals and Technological Applications, Kluwer

Academic/Plenum Press, 1999.

[46] Devaraj, S.; Munichandraiah, N. Effect of crystallographic

structure of MnO2 on its electrochemical capacitance

properties. J. Phys. Chem. C 2008, 112, 4406-4417.

[47] Toupin, M.; Brousse, T.; Bélanger, D. Charge storage

mechanism of MnO2 electrode used in aqueous

electrochemical capacitor. Chem. Mater. 2004, 16,

3184-3190.

[48] Pang, S. C.; Anderson, M. A.; Chapman, T. W. Novel

electrode materials for thin‐film ultracapacitors: comparison of electrochemical properties of sol‐gel‐derived and electrodeposited manganese dioxide. J.

Electrochem. Soc. 2000, 147, 444-450.

[49] Qu, Q.; Zhang, P.; Wang, B.; Chen, Y.; Tian, S.; Wu, Y.;

Holze, R. Electrochemical performance of MnO2 nanorods

in neutral aqueous electrolytes as a cathode for asymmetric

supercapacitors. J. Phys. Chem. C 2009, 113,

14020-14027.

[50] Li, S.; Qi, L.; Lu, L.; Wang, H. Facile preparation and

performance of mesoporous manganese oxide for

supercapacitors utilizing neutral aqueous electrolytes. RSC

Adv.2012, 2, 3298-3308.

[51] Kim, J. S.; Shin, S. S.; Han, H. S.; Oh, L. S.; Kim, D. H.;

Kim, J. H.; Hong, K. S.; Kim, J. Y. 1-D structured flexible

supercapacitor electrodes with prominent electronic/ionic

transport capabilities. ACS Appl. Mater. Interfaces 2014, 6,

268-274.

[52] Xie, X.; Zhang, C.; Wu, M. B.; Tao, Y.; Lv, W.; Yang, Q.

H. Porous MnO2 for use in a high performance

supercapacitor: replication of a 3D graphene network as a

reactive template. Chem. Commun. 2013, 49,

11092-11094.

[53] Yeager, M.; Du, W.; Si, R.; Su, D.; Marinković, N.; Teng,

X. Highly Efficient K0.15MnO2 Birnessite Nanosheets for

Stable Pseudocapacitive Cathodes. J. Phys. Chem. C 2012,

116, 20173-20181.

[54] Li, W.; Liu, Q.; Sun, Y.; Sun, J.; Zou, R.; Li, G.; Hu, X.;

Song, G.; Ma, G.; Yang, J.; Chen, Z.; Hu, J. MnO2

ultralong nanowires with better electrical conductivity and

enhanced supercapacitor performances. J. Mater. Chem.

2012, 22, 14864.

[55] Huang, M.; Zhang, Y.; Li, F.; Zhang, L.; Ruoff, R. S.; Wen,

Z.; Liu, Q. Self-assembly of mesoporous nanotubes

assembled from interwoven ultrathin birnessite-type MnO2

nanosheets for asymmetric supercapacitors. Sci. Rep. 2014,

4, 3878.

[56] Xu, C.; Zhao, Y.; Yang, G.; Li, F.; Li, H. Mesoporous

nanowire array architecture of manganese dioxide for

electrochemical capacitor applications. Chem.

Commun.2009, 7575-7.

[57] Feng, Z.-P.; Li, G.-R.; Zhong, J.-H.; Wang, Z.-L.; Ou,

Y.-N.; Tong, Y.-X. MnO2 multilayer nanosheet clusters

evolved from monolayer nanosheets and their predominant

electrochemical properties. Electrochem. Commun. 2009,

11, 706-710.

[58] Subramanian, V.; Zhu, H.; Wei, B. Nanostructured MnO2:

Hydrothermal synthesis and electrochemical properties as a

supercapacitor electrode material. J. Power Sources 2006,

159, 361-364.

[59] Xiao, W.; Xia, H.; Fuh, J. Y. H.; Lu, L. Growth of

single-crystal α-MnO2 nanotubes prepared by a

hydrothermal route and their electrochemical properties. J.

Power Sources 2009, 193, 935-938.

[60] Xu, M.; Kong, L.; Zhou, W.; Li, H. Hydrothermal

synthesis and pseudocapacitance properties of α-MnO2

hollow spheres and hollow urchins. J. Phys. Chem. C 2007,

111, 19141-19147.

[61] Jiang, R.; Huang, T.; Liu, J.; Zhuang, J.; Yu, A. A novel

method to prepare nanostructured manganese dioxide and

its electrochemical properties as a supercapacitor electrode.

Electrochim. Acta 2009, 54, 3047-3052.

[62] Vargas, O. A.; Caballero, A.; Hernán, L.; Morales, J.

Improved capacitive properties of layered manganese

dioxide grown as nanowires. J. Power Sources 2011, 196,

3350-3354.

[63] Sung, D.-Y.; Kim, I. Y.; Kim, T. W.; Song, M.-S.; Hwang,

S.-J. Room Temperature Synthesis Routes to the 2D

Nanoplates and 1D Nanowires/Nanorods of Manganese

Oxides with Highly Stable Pseudocapacitance Behaviors. J.

Phys. Chem. C 2011, 115, 13171-13179.

[64] Subramanian, V.; Zhu, H.; Vajtai, R.; Ajayan, P.; Wei, B.

Hydrothermal synthesis and pseudocapacitance properties

of MnO2 nanostructures. J. Phys. Chem. B 2005, 109,

20207-20214.

[65] Zhu, G.; Li, H.; Deng, L.; Liu, Z.-H. Low-temperature

synthesis of δ-MnO2 with large surface area and its

capacitance. Mater. Lett. 2010, 64, 1763-1765.

[66] Ragupathy, P.; Park, D. H.; Campet, G.; Vasan, H.; Hwang,

S.-J.; Choy, J.-H.; Munichandraiah, N. Remarkable

capacity retention of nanostructured manganese oxide upon

cycling as an electrode material for supercapacitor. J. Phys.

Chem. C 2009, 113, 6303-6309.

[67] Yuan, J.; Liu, Z.-H.; Qiao, S.; Ma, X.; Xu, N. Fabrication

of MnO2-pillared layered manganese oxide through an

exfoliation/reassembling and oxidation process. J. Power

Sources 2009, 189, 1278-1283.

[68] Yuan, C.; Gao, B.; Su, L.; Zhang, X. Interface synthesis of

mesoporous MnO2 and its electrochemical capacitive

behaviors. J. Colloid Interface Sci. 2008, 322, 545-550.

[69] Xu, M.-W.; Jia, W.; Bao, S.-J.; Su, Z.; Dong, B. Novel

mesoporous MnO2 for high-rate electrochemical capacitive

energy storage. Electrochim. Acta 2010, 55, 5117-5122.

[70] He, X.; Yang, M.; Ni, P.; Li, Y.; Liu, Z.-H. Rapid synthesis

of hollow structured MnO2 microspheres and their

capacitance. Colloid Surf. A-Physicochem. Eng. Asp. 2010,

363, 64-70.

[71] Yu, P.; Zhang, X.; Chen, Y.; Ma, Y. Self-template route to

MnO2 hollow structures for supercapacitors. Mater. Lett.

2010, 64, 1480-1482.

15

[72] Tang, N.; Tian, X.; Yang, C.; Pi, Z. Facile synthesis of

α-MnO2 nanostructures for supercapacitors. Mater. Res.

Bull. 2009, 44, 2062-2067.

[73] Meher, S. K.; Rao, G. R. Ultralayered Co3O4 for

High-Performance Supercapacitor Applications. J. Phys.

Chem. C 2011, 115, 15646-15654.

[74] Meher, S. K.; Justin, P.; Rao, G. R. Nanoscale morphology

dependent pseudocapacitance of NiO: Influence of

intercalating anions during synthesis. Nanoscale 2011, 3,

683-692.

[75] Shim, H. W.; Lim, A. H.; Kim, J. C.; Jang, E.; Seo, S. D.;

Lee, G. H.; Kim, T. D.; Kim, D. W. Scalable one-pot

bacteria-templating synthesis route toward hierarchical,

porous-Co3O4 superstructures for supercapacitor electrodes.

Sci. Rep. 2013, 3, 2325.

[76] Peng, Y.; Chen, Z.; Wen, J.; Xiao, Q.; Weng, D.; He, S.;

Geng, H.; Lu, Y. Hierarchical manganese oxide/carbon

nanocomposites for supercapacitor electrodes. Nano Res.

2011, 4, 216-225.

16

Electronic Supplementary Material

Tailoring uniform γ-MnO2 nanosheets on highly conductive three-dimensional current collectors for high-performance supercapacitor electrodes

Sangbaek Park1†, Hyun-Woo Shim2†, Chan Woo Lee1, Hee Jo Song1, Ik Jae Park1, Jae-Chan Kim2, Kug Sun Hong1, and Dong-Wan Kim2()

1 Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea 2 School of Civil, Environmental and Architectural Engineering, Korea University, Seoul 136-713, Korea

† These authors contributed equally

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

Calculation of specific capacitance

The following equations were used to derive the mass-specific capacitance (SC, F g-1), measured in Faradays

per gram, from the CV and CD profiles, respectively. Each SC value is denoted as SCCV and SCCD. First, the SCCV

values were calculated by integrating the area under the I–V curves and then dividing by the sweep rate ν (mV

s-1), the mass of the loaded active material (MnO2) in the electrode, and the working potential window (Va to Vc),

using the following equation [48,51,53,73]:

𝑆𝐶𝐶𝑉 =1

𝜈𝑚(𝑉𝑐−𝑉𝑎)∫ 𝑖(𝑉)𝑑𝑉𝑉𝑐𝑉𝑎

(1)

The other mass-specific capacitance, SCCD values were also calculated from the galvanostatic discharge curves

using the following equation [51,53,55]:

𝑆𝐶𝐶𝐷 =𝑖

𝑚(∆𝑉/∆𝑡) (2)

where, i (mA) is the applied constant discharge current, ∆t (s) is the discharge time, ∆V (V) is the potential

sweep window (1.0 V), and m (mg) is the mass of the loaded active material (MnO2) in the electrode.

-----------------------------------------------------------------------------------------------------------------------------------------------------------------

References

48. S. -C. Pang, M. A. Anderson and T. W. Chapman, J. Electrochem. Soc., 2000, 147, 444.

51. J. S. Kim, S. S. Shin, H. S. Han, L. S. Oh, D. H. Kim, J. -H. Kim, K. S. Hong and J. Y. Kim, ACS Appl. Mater. Interfaces, 2014,

6, 268.

53. M. Yeager, W. Du, R. Si, D. Su, N. Marinković and X. Teng, J. Phys. Chem. C, 2012, 116, 20173.

55. M. Huang, Y. Zhang, F. Li, L. Zhang, R. S. Ruoff, Z. Wen and Q. Liu, Sci. Rep., 2014, 4, 3878.

73. S. K. Meher and G. R. Rao, J. Phys. Chem. C, 2011, 115, 15646.

————————————

Address correspondence to D.-W. Kim, dwkim@ajou.ac.kr

17

Electrochemical impedance spectroscopy (EIS) analysis

The EIS spectra are composed of two major characteristic features in the frequency regions: i) a depressed arc

(partial semicircle) in the high- and middle-frequency regions, and ii) an inclined line (straight, sloped line

along the imaginary axis (Z″)) in the low-frequency region with a transition between the two regions called

the “knee frequency”, which illustrates typical capacitor behavior. These EIS characteristics are attributed to

various resistance phenomena during different interfacial processes in Faradaic reactions. The obtained

impedance spectra were analyzed by the CNLS fitting method based on a Randles equivalent circuit

depicted in the inset of Figure 8b, which was the same as the circuit recently employed for the analysis of a

MnO2-based electrode by Huang et al.[55] and Kim et al.,[51] where Rs and Rct are the internal resistance and

Faradaic interfacial charge-transfer resistance, respectively, and constant phase element (CPEn) represents the

capacitive performance. The interfacial diffusive resistance (Warburg impedance) has been denoted as “W”.

In this equivalent series resistance (ESR), the intercept at the real impedance (Z) axis in the high-frequency

region first corresponds to Rs, which represents the total resistance related to a combination of the ionic

resistance of the electrolyte, intrinsic resistance of the active materials, and contact resistance at the active

material-current collector interface. The higher Rs value indicates a lower electrical conductivity of the

electrode materials and vice versa [S1]. Meanwhile, the semicircular arc in the high- and middle-frequency

range, corresponding to the impedance behavior, also indicates contributing resistance from the Faradaic

redox process, named the charge-transfer resistance (Rct) at the electrode-electrolyte interface. This results

from the diffusion of electrons and can be calculated from the diameter of each semicircle. Such

charge-transfer resistance is directly related to the surface phenomena of the electrode materials: i) the

charge-transfer barriers, including the “electron transfer” between the current collector and the active

materials, and ii) the “ion-transfer” between the active materials and the electrolyte.49 In addition, the linear

parts of the Nyquist plot in the lower frequency range correspond to the Warburg impedance, W, which is

described as the diffusive resistance of the electrolyte ions in the host materials (active sites). Each profile

exhibited higher angles than 45 °, indicating the suitability as an electrode material for supercapacitors. In

particular, the profile after 3,000 cycles almost tended to a vertical asymptote along the imaginary line axis,

indicating the good electrochemical capacitance of the 3-D hierarchical ATO@MnO2 nanosheets in the

Na2SO4 aqueous electrolyte. Therefore, the overall EIS characteristics suggest superior accessibility of the

electrolyte ions and highly conductive paths for rapid electronic transport through the 3-D hierarchical

self-supported hetero-nanostructures and explicit contribution of the pseudo-capacitance to the energy

storage performance of the ATO@MnO2 material.

-----------------------------------------------------------------------------------------------------------------------------------------------------------------

References

51. J. S. Kim, S. S. Shin, H. S. Han, L. S. Oh, D. H. Kim, J. -H. Kim, K. S. Hong and J. Y. Kim, ACS Appl. Mater. Interfaces, 2014,

6, 268.

55. M. Huang, Y. Zhang, F. Li, L. Zhang, R. S. Ruoff, Z. Wen and Q. Liu, Sci. Rep., 2014, 4, 3878.

S1. G. A. Snook, P. Kao and A. S. Best, J. Power Sources, 2011, 196, 1.

18

Table S1. Quantitative elemental composition for ATO@MnO2 heterostructures synthesized in the presence of various

MnCl2 and KBrO3 concentrations. Energy dispersive spectroscopy (EDS) analysis was conducted to evaluate the atomic

concentration of Mn and Sn, and the ratio of Mn to Sn was calculated by obtained data.

Synthetic condition Mn (at%) Sn (at%) Mn/Sn

MnCl2 0.02 M and KBrO3 0.1 M 0.02 7.18 0.00279

MnCl2 0.02 M and KBrO3 0.2 M 3.14 8.46 0.371

MnCl2 0.02 M and KBrO3 0.3 M 10.9 5.86 1.87

MnCl2 0.05 M and KBrO3 0.1 M 8.90 6.11 1.46

MnCl2 0.05 M and KBrO3 0.2 M 29.5 1.05 28.1

MnCl2 0.05 M and KBrO3 0.3 M 36.9 -0.09 ―

MnCl2 0.1 M and KBrO3 0.1 M 22.9 2.94 7.79

MnCl2 0.1 M and KBrO3 0.2 M 39.0 -0.30 ―

MnCl2 0.1 M and KBrO3 0.3 M 40.0 -0.38 ―

19

Table S2. Comparison of the electrochemical performance with various reported MnO2-based electrode materials for

supercapacitors utilizing Na2SO4 aqueous electrolytes in the three-electrode system.

Electrode materials

C

(mol

L-1

)

Operating

voltage

(∆V)

High rate capability

Long-term cycle stability

Ref. Current

density

or

Scan rate

SC

Cycles

Current

density

or

Scan rate

SC

Mesoporous MnO2 walnuts 1 1 50 mV s

-1 ~25 F g

-1 2,000 834 mA g

-1 50

Porous MnO2 1 1 15 A g

-1

200 mV s-1

154 F g-1

~130 F g-1

4,000 2 A g-1

~160 F g-1

52

K0.15

MnO2 birnessite

nanosheets/PEDOT

0.1 0.9 10 A g-1

200 mV s-1

~ 30 F g-1

~ 50 F g-1

53

MnO2 1 0.8 100 mV s

-1 135 F g

-1 1,000 2 mA cm

-2 176 F g

-1 61

MnO2 powder 0.1 0.9 5 mV s

-1 150 F g

-1 47

α-MnO2 ultralong nanowires 0.5 1 50 A g

-1

100 mV s-1

~60 F g-1

137 F g-1

2,000 50 mV s-1

~180 F g-1

54

α-MnO2 hollow spheres and

urchins

1 1 10 mA g-1

124 F g-1

350 5 mA g-1

~130 F g-1

60

MnO2 1 1 50 mV s

-1 72 F g

-1 100 200 mA g

-1 ~135 F g

-1 58

MnO2 nanosheets 1 0.9 100 mV s

-1 ~25 F g

-1 1,000 10 mV s

-1 ~395 F g

-1 57

Mesoporous MnO2 nanowire

array

0.5 1 12 A g-1

84 F g-1

800 4 A g-1

~475 F g-1

56

α-MnO2 nanotubes 1 0.9 200 mV s

-1 136 F g

-1 59

Mesoporous MnO2 nanotubes 1 1 10 A g

-1 ~203 F g

-1 3,000 5 A g

-1 ~202 F g

-1 55

Birnessite-type MnO2 nanowires 0.5 1 100 mV s

-1 ~85 F g

-1 62

α-MnO2 1D nanostructures 1 1 20 mV s

-1 127 F g

-1 1,000 20 mV s

-1

0.5 mA cm-2

63

δ-MnO2 2D nanoplates 180 F g

-1

γ-MnO2-structured 3D urchins 100 F g

-1

MnO2 1 1 5 mV s

-1

200 mA g-1

168 F g-1

64

MnO2 nanorods 0.5 1 100 mV s

-1 ~60 F g

-1 49

20

Mesoporous MnO2 1 1 100 mV s

-1

2.3 A g-1

220 F g-1

244 F g-1

1,000 2 A g-1

~195 F g-1

68

MnO2 hollow spheres 1 0.9 2 A g

-1 ~88 F g

-1 71

α, β, γ, and δ-MnO2 0.1 1 10 mA cm

-2 ~260 F g

-1 500 0.5 mA cm

-2 ~210 F g

-1 46

Mesoporous MnO2 1 1 10 mA cm

-2 202 F g

-1 500 5 mA cm

-2 ~205 F g

-1 69

MnO2 0.1 1 10 mV s

-1 2,000 0.5 mA cm

-2 ~230 F g

-1 66

α-MnO2 1 1 10 mA g

-1 114 F g

-1 100 500 mA g

-1 ~131 F g

-1 72

Hollow α-MnO2 spheres 1 1 500 10 mV s

-1 84 F g

-1 70

δ-MnO2 1 1 50 mV s

-1 1,000 5 mV s

-1 263 F g

-1 65

MnO2-pillared layered structures 1 1 50 mV s

-1 600 10 mV s

-1 ~215 F g

-1 67

MnO2/carbon nanocomposites 1 0.8 20 mV s-1

2 A g-1

~152 F g-1

100 1 A g-1 ~170 F g-1 76

ATO@MnO2 hetero-structures 1 1 500 mV s

-1

20 A g-1

178 F g-1

162 F g-1

5,000 20 A g-1

~152 F g-1

This

work

-----------------------------------------------------------------------------------------------------------------------------------------------------------------

References

46. S. Devaraj and N. Munichandraiah, J. Phys. Chem. C, 2008, 112, 4406.

47. M. Toupin, T. Brousse and D. Bélanger, Chem. Mater., 2004, 16, 3184.

49. Q. Qu, P. Zhang, B. Wang, Y. Chen, S. Tian, Y. Wu and R. Holze, J. Phys. Chem. C, 2009, 113, 14020.

50. S. Li, L. Qi, L. Lu and H. Wang, RSC Adv., 2012, 2, 3298.

52. X. Xie, C. Zhang, M. -B. Wu, Y. Tao, W. Lv and Q. -H. Yang, Chem. Commun., 2013, 49, 11092.

53. M. Yeager, W. Du, R. Si, D. Su, N. Marinković and X. Teng, J. Phys. Chem. C, 2012, 116, 20173.

54. W. Li, Q. Liu, Y. Sun, J. Sun, R. Zou, G. Li, X. Hu, G. Song, G. Ma, J. Yang, Z. Chen and J. Hu, J. Mater. Chem., 2012, 22,

14864.

55. M. Huang, Y. Zhang, F. Li, L. Zhang, R. S. Ruoff, Z. Wen and Q. Liu, Sci. Rep., 2014, 4, 3878.

56. C. Xu, Y. Zhao, G. Yang, F. Li and H. Li, Chem. Commun., 2009, 7575.

57. Z. -P. Feng, G. -R. Li, J. -H. Zhong, Z. -L. Wang, Y. -N. Ou and Y. -X. Tong, Electrochem. Commun., 2009, 11, 706.

58. V. Subramanian, H. Zhu and B. Wei, J. Power Sources, 2006, 159, 361.

59. W. Xiao, H. Xia, J. Y. H. Fuh and L. Lu, J. Power Sources, 2009, 193, 935.

60. M. Xu, L. Kong, W. Zhou and H. Li, J. Phys. Chem. C, 2007, 111, 19141.

61. R. Jiang, T. Huang, J. Liu, J. Zhuang and A. Yu, Electrochim. Acta, 2009, 54, 3047.

62. O. A. Vargas, A. Caballero, L. Hernán and J. Morales, J. Power Sources, 2011, 196, 3350.

63. D. -Y. Sung, I. Y. Kim, T. W. Kim, M. -S. Song and S. -J. Hwang, J. Phys. Chem. C, 2011, 115, 13171.

64. V. Subramanian, H. Zhu, R. Vajtai, P. M. Ajayan and B. Wei, J. Phys. Chem. B, 2005, 109, 20207.

65. G. Zhu, H. Li, L. Deng and Z. -H. Liu, Mater. Lett., 2010, 64, 1763.

66. P. Ragupathy, D. H. Park, G. Campet, H. N. Vasan, S. -J. Hwang, J. -H. Choy and N. Munichandraiah, J. Phys. Chem. C, 2009,

21

113, 6303.

67. J. Yuan, Z. -H. Liu, S. Qiao, X. Ma and N. Xu, J. Power Sources, 2009, 189, 1278.

68. C. Yuan, B. Gao, L. Su and X. Zhang, J. Colloid Interface Sci., 2008, 322, 545.

69. M. -W. Xu, W. Jia, S. -J. Bao, Z. Su and B. Dong, Electrochim. Acta, 2010, 55, 5117.

70. X. He, M. Yang, P. Ni, Y. Li and Z. -H. Liu, Colloid Surf. A-Physicochem. Eng. Asp., 2010, 363, 64.

71. P. Yu, X. Zhang, Y. Chen and Y. Ma, Mater. Lett., 2010, 64, 1480.

72. N. Tang, X. Tian, C. Yang and Z. Pi, Mater. Res. Bull., 2009, 44, 2062.

76. Y. Peng, Z. Chen, J. Wen, Q. Xiao, D. Weng, S. He, H. Geng and Y. Lu, Nano Res., 2011, 4, 216.

22

Figure S1. Cross-sectional SEM images of the ATO nanobelt arrays.

Figure S2. XRD graphs of γ-MnO2 nanosheets synthesized in the presence of MnCl2 and KBrO3 at different

concentrations: (a-c) MnCl2 0.02 M, (d-f) MnCl2 0.05 M, (g-i) MnCl2 0.1 M. (a, d, f) KBrO3 0.1 M, (b, e, h) KBrO3 0.2

M, (c, f, i) KBrO3 0.3 M.

23

Figure S3. Photographs of the MnCl2 solution and reacted Ti coin cell under various concentrations of MnCl2 and

KBrO3. Reaction temperature and time was fixed to 50 oC and 6 h, respectively.

Figure S4. XRD graphs of γ-MnO2 nanosheets prepared in the presence of 0.05 M MnCl2 and 0.2 M KBrO3 with

different reaction times: (a) 2 h 15 m, (b) 2 h 30 m, (c) 2 h 45 m, (d) 3 h, (e) 3 h 30 m, and (f) 4 h.

24

Figure S5. (a) XRD graphs and (b) SEM images of γ-MnO2 particles precipitated in the solution of 0.05 M MnCl2 and

0.2 M KBrO3 with and without insertion of ATO nanobelt/Ti substrate. Reaction temperature and time was fixed to 40

oC and 63 h, respectively. Scale bar in the inset of b is 500 nm.

Figure S6. SEM images of nanosheets synthesized in the presence of 0.2 M KBrO3 with different concentrations of

MnCl2 and reaction temperatures. Reaction time was fixed to 6 h. Scale bar: 1 μm.

25

Figure S7. Enlarged electrochemical behavior graph of the inset of Figure 6a showing the linearity of the anodic current

density with scan rates.

Figure S8. The area-specific capacitance (SCCD,area) of ATO@MnO2 nanostructure electrode plotted as a function of

discharge current densities.

26

Figure S9. Comparison of the mass-specific capacitance of ATO@MnO2 nanostructures prepared from the different

synthetic condition.

27

Figure S10. Ragone plot derived from the galvanostatic discharge curves for the ATO@MnO2 electrode measured at six

different current densities from 0.5 to 20 A g-1. Here, the energy density (E, Wh kg-1) and power density (P, kW kg-1) are

calculated using the following equations [75]: E = (C × ∆V2)/2 and P = E/∆t, where C (F g-1) represents the specific

capacitance of the active material, ∆V (V) represents the cutoff voltage across the electrode, E represents the energy

density, and ∆t (s) represents the discharge time, respectively.

-----------------------------------------------------------------------------------------------------------------------------------------------------------------

References

75. H. -W. Shim, A. -H. Lim, J. -C. Kim, E. Jang, S. -D. Seo, G. -H. Lee, T. D. Kim and D. -W. Kim, Sci. Rep., 2013, 3, 2325.

28

Figure S11. (a) TEM, (b) SAED, and (c) EDS mapping of an ATO@MnO2 nanostructure after 5,000 cycles. Scale bar of c

is 100 nm.

Figure S12. Enlarged SEM images of the ATO@MnO2 nanostructure before and after 5,000 cycles.

29

Figure S13. Electrochemical performance of bare ATO nanobelt electrode. (a) CV behaviors, (b) Corresponding

mass-specific capacitance as a function of discharge current densities. Inset of (b) shows the galvanostatic discharge

profile obtained at six different current densities. All measurements were carried out using the three-electrodes system, and

were recorded on the potential window of 0 to 1.0 V (vs. Ag/AgCl).

a0581Revised NR_MS & Supporting_2014 0816