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Electrochimica Acta 130 (2014) 693–698

Contents lists available at ScienceDirect

Electrochimica Acta

j ourna l ho me page: www.elsev ier .com/ locate /e lec tac ta

anowire K0.19MnO2 from hydrothermal method as cathode materialor aqueous supercapacitors of high energy density

.H. Zhanga,b, Y. Liua,b, Z. Changb, Y.Q. Yangb, Z.B. Wena,∗, Y.P. Wub,∗

College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, ChinaNew Energy and Material Laboratory (NEML), Department of Chemistry & Shanghai Key Laboratory of Molecular Catalysis and Innovative Material,udan University, Shanghai 200433, China

r t i c l e i n f o

rticle history:eceived 16 January 2014eceived in revised form 27 February 2014ccepted 2 March 2014vailable online 15 March 2014

a b s t r a c t

Nanowire K0.19MnO2·0.2H2O is prepared by a hydrothermal method. It presents better electrochem-ical behavior than K0.45MnO2·0.3H2O from the solid-state method. The capacitance of nanowireK0.19MnO2·0.2H2O (148 F g−1) is much higher than that of K0.45MnO2·0.3H2O (132 F g−1). It presentsexcellent cycling performance even when the oxygen in the aqueous electrolyte is not removed, andthere is no evident capacitance fading after 2500 cycles. The supercapacitor based on activated carbon

−1

eywords:upercapacitor0.19MnO2·0.2H2Oanowireathodeydrothermal method

and nanowire K0.19MnO2·0.2H2O delivers an energy density 41.3 Wh kg (based on the total mass of theactive electrode materials) at a power density of 156.8 W kg−1, higher than that based on activated carbonand K0.45MnO2·0.3H2O, 28.4 W h kg−1 at a power density of 115.1 W kg−1. In combination with our formerwork, it suggests that both the nano structure and the surface area are important to the capacitance ofKxMnO2 especially the latter.

© 2014 Elsevier Ltd. All rights reserved.

. Introduction

In recent years, rechargeable lithium batteries have been widelynvestigated such as Li//S [1,2], aqueous rechargeable lithium bat-eries [2–7], and Li//air [8,9] to try to replace lithium ion batteriesn the future. Considering that the natural resources of lithium areimited, new cheap electrode materials with enough sources areeing searched for and a feasible strategy is to explore novel energytorage systems using Na+ or K+ as working ions [10–14]. More-ver, both Na and K elements are naturally more abundant than Lilement, and their inorganic/organic salts are usually cheaper andore readily available than those based on Li element [15]. Con-

equently, several sodium- and potassium-based energy storageevices including supercapacitors have been reported [16–21].

Supercapacitors can deliver much higher power density andossess a much longer cycling life than lithium ion batteries. Theyan be a good complement to lithium ion batteries, which do nothow very high power density. Traditional materials such as MnO222], vanadium oxides [23] and MoO3 [24–26] have attracted great

ttention as cathode materials for aqueous solution-based super-apacitors [27].

∗ Corresponding author.E-mail address: wuyp@fudan.edu.cn (Y.P. Wu).

ttp://dx.doi.org/10.1016/j.electacta.2014.03.026013-4686/© 2014 Elsevier Ltd. All rights reserved.

For the aqueous supercapacitors based on intercalation com-pounds, cations (M+: M = Li, Na or K) can be provided by theintercalation compounds during the charge process, which come tothe anode via the aqueous solution. During the discharge process,the cations come back to the cathode [13,18,22,27] via the aqueoussolution, again. As a result, the concentration and the ionic conduc-tivity of the aqueous electrolytes are very stable during the chargeand discharge process. The required amount of electrolytes will beless than those for symmetric supercapacitors, leading to higherutilization of the electrode materials and higher practical energydensity [26,27]. In addition, the electrolytes are neutral, which aremore environmentally friendly than those based on acid or alkalinesolutions.

Previously we reported a cheap cathode material,K0.27MnO2·0.6H2O by solid state reaction, for the aqueous superca-pacitor [28]. It shows good reversible intercalation/deinteralationof K+ ions in the K2SO4 solution. In addition, we also found thatactivated carbon (AC) [29] and MnO2 [18] presented the bestrate behavior in the K2SO4 aqueous solution among alkali sulfateaqueous electrolytes. The asymmetric supercapacitor based onAC//K0.27MnO2·0.6H2O shows excellent cycling behavior in thevoltage region of 0–1.8 V without the removal of oxygen in the

electrolyte and deliver an energy density of 25.3 Wh kg−1 atpower density of 140 W kg−1 based on the total mass of the activeelectrode materials. If the redox reaction of K0.27MnO2·0.6H2O canbe more utilized, evidently its energy density can be higher. It was

6 imica

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2

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94 B.H. Zhang et al. / Electroch

eported that nano architecture and high specific surface area areavorable for the improvement of capacitive behavior of electrode

aterials for supercapacitors [30,31].In this work, we prepared nanowire K0.19MnO2·0.2H2O by a

imple and low energy consumption hydrothermal method andtudied its electrochemical properties as a cathode material for anqueous supercapacitor. Its capacitance is much larger than thatf K0.45MnO2·0.3H2O from solid-state reaction though the amountf K is much lower. It presents excellent cycling performance evenhen the oxygen in the aqueous electrolyte is not removed, and

here is no evident capacitance fading after 2500 cycles. It showsromise for the application due to its low price and environmentalriendliness.

. Experimental

.1. Materials synthesis

All reagents were of analytical grade. At first, nanowireirnessite-MnO2 (�-MnO2) (See Fig. S1 for its morphology) wasydrothermally synthesized by reacting solutions of MnSO4,NH4)2S2O8 and (NH4)2SO4 in a molar ratio of 1:1:4 at 140 ◦C.hen, nanowire K0.19MnO2·0.2H2O was prepared by a hydro-hermal method [32]. Typically, 0.2 g of the above prepared �-MnO2owder was dispersed in 35 mL of 5 mol L −1 KOH aqueous solu-ion. This solution was placed in a Teflon-lined autoclave (50 mL).he autoclave was heated at 205 ◦C for 48 h. After that, the mix-

ure was filtered, washed with water three times, and then driedt 60 ◦C. For comparison, the K0.45MnO2·0.3H2O was synthesizedsing a solid-state reaction. In detail, K2CO3 and the �-MnO2 (molaratio of 1:2) were mixed by ball-milling for 12 h, followed by

Fig. 1. (a) TG curves, (b) XRD patterns, (c) and (d) SEM micrograp

Acta 130 (2014) 693–698

heating at 700 ◦C for 8 h under air, which is similar to our previouslyreported heat-treatment process [28].

2.2. Materials characterization

The obtained products were characterized using a BrukerAdvance 8 powder X-ray diffractometer (XRD) with monochro-matized Cu K�-radiation (� = 1.54056 A). SEM micrographs wereobtained with a Philip XL30 microscope operated at 25 kV.Elemental analysis was obtained by Thermo E. IRIS Duo induc-tively coupled plasma (ICP) and thermal gravimetric analysis(TGA, PerkinElmerTGA7) for the as-prepared powder indicates anapproximate composition of K0.27MnO2·0.6H2O.

2.3. Electrochemical measurements

The cathode was prepared by pressing a powdered mixtureof nanowires K0.19MnO2·0.2H2O or K0.45MnO2·0.3H2O, acetyleneblack and poly(tetrafluoroethylene) (PTFE) in a weight ratio of8:1:1 onto nickel grid. Activated carbon (AC) from Ningde Xinseng(Chemical Industrial Ltd., Co.) with a specific BET-surface area ofabout 2800 m2 g−1 was used as received. The AC electrode was pre-pared in the same way as the anode. The aqueous K2SO4 solution of0.5 mol L−1 was used as the electrolyte. Cyclic voltammetric (CVs)tests were done with a three-electrode cell, in which a saturatedcalomel electrode (SCE) and Ni-grid were used as the reference

and the counter electrodes, respectively. Galvanostatic charge-discharge performance was tested using a two-electrode cell. Thecapacitance was calculated based on the weight of the active cath-ode material, and the energy density was calculated based on the

hs of K0.45MnO2·0.3H2O and nanowire K0.19MnO2·0.2H2O.

B.H. Zhang et al. / Electrochimica Acta 130 (2014) 693–698 695

-0.8 -0.4 0.0 0.4 0.8 1.2-0.2

-0.1

0.0

0.1

0.2

0.3C

urr

ent / m

A

Potential / V vs. SCE

K0.19

MnO2 0.2H 2

O

K0.45

MnO2 0.3H 2

O

AC

FKo

tt

t

C

wdo

3

ffa0rnidlt

apmtt

Kte5isoowcro0t0t

0 25 0 50 0 75 0 100 0 125 0 150 0 175 00.0

0.2

0.4

0.6

0.8

1.0

1.6 A g-1

0.8 A g-1

0.4 A g-1

Po

ten

tial / V

vs . S

CE

Time / Second

0.2 A g-1

(a)

0 30 0 60 0 90 0 120 00.0

0.2

0.4

0.6

0.8

1.0

Po

ten

tial / V

vs . S

CE

Time / Second

1.6 A g-1

0.8 A g-1

0.4A g-1

0.2 A g-1

(b)

Fig. 3. Charge and discharge curves of (a) the nanowire K0.19MnO2·0.2H2O and (b)

ig. 2. (a) Cyclic voltamogramms of the AC, nanowire K0.19MnO2·0.2H2O and0.45MnO2·0.3H2O electrodes in 0.5 mol L−1 K2SO4 aqueous solution at the scan ratef 5 mV s−1.

otal weight of two electrodes. Electrochemical performance wasested without removal of the oxygen from the solution.

The specific discharge capacity C (F g−1) is calculated accordingo the following equation (1) [33,34]:

= (I�t)/(m�V) (1)

here I (mA) is the applied working current, �t (s) represents theischarge time, �V (V) is the voltage range, and m (mg) is the massf active materials.

. Results and discussions

The ICP analysis results show that the molar ratios of K to Mnor the product from the hydrothermal method is 0.19 and thatrom the solid-state method is 0.45, and the TG analysis resultsre shown in Fig. 1(a), that the molar ratios of H2O are 0.2 and.3, respectively. The reasons are mainly due to the difference ofeaction time and temperature. The XRD patterns of the preparedanowires K0.19MnO2·0.2H2O and K0.45MnO2·0.3H2O are shown

n Fig. 1(b). The peaks at about 12.6◦ and 26◦ are assigned to theiffraction of (001) and (002) planes, respectively, indicating the

ayered structure [35,36]. Since the amount of K is not near to 1,here are some impurities such as Mn2O3.

SEM micrographs of K0.19MnO2·0.2H2O and K0.45MnO2·0.3H2Ore shown in Fig. 1(c) and 1(d). It can be observed that the pre-ared K0.45MnO2·0.3H2O does not present the original nanowireorphology. The main reason is perhaps due to the high heat-

reatment temperature. In contrast, the K0.19MnO2·0.2H2O keepshe nanowire morphology from the nanowire MnO2.

CVs of the AC, the prepared nanowire K0.19MnO2·0.2H2O and0.45MnO2·0.3H2O electrodes in 0.5 mol L−1 K2SO4 aqueous elec-

rolyte in the three-electrode cell with nickel mesh as the counterlectrode and SCE as the reference electrode at the scan rate of

mV s−1 are shown in Fig. 2. The current collector, Ni-mesh,s very stable between 1.0 < ESCE < 1.2 V in the aqueous K2SO4olution owing to the existence of over-potentials [37]. The CVf the AC electrode shows an ideal rectangular shape with-ut any noticeable redox peaks from 0.1 to −0.8 V (vs. SCE),hich is characteristic of charging/discharging of the double layer

apacitance [29]. The nanowire K0.19MnO2·0.2H2O shows twoedox peaks corresponding to the intercalation/deintercalationf K+ ions [38]. Two oxidation peaks located at 0.42 and

.68 V (vs. SCE), respectively, can be seen clearly, whereas,here are two corresponding reduction peak located at 0.40 and.61 V (vs. SCE). The behavior of the K0.45MnO2·0.3H2O elec-rode slightly deviates from the ideal rectangular shape with

the K0.45MnO2·0.3H2O at different current densities from 0.2 to 1.6 A g−1 at thepotential range of 0 - 1.0 V vs. SCE.

two redox couples, indicative of both the capacitive and thepseudocapacitive property of the K0.45MnO2·0.3H2O cathode,whose two oxidation peaks are located at 0.61 and 0.73 V (vs. SCE)and two reduction peaks at 0.42 and 0.61 V (vs. SCE), respectively.The area in the CV curve for the nanowire K0.19MnO2·0.2H2O islarger than that of K0.45MnO2·0.3H2O, indicating that the capaci-tance of the former will be larger though its content of potassiumis lower.

The charge-discharge curves at different current density from0.2 to 1.6 A g−1 for the prepared nanowire K0.19MnO2·0.2H2Oand K0.45MnO2·0.3H2O cathodes are shown in Fig. 3. Both thenanowire K0.19MnO2·0.2H2O and K0.45MnO2·0.3H2O electrodespresent good rate capability since the ionic conductivity of theaqueous K2SO4 solution is higher than those of the Li2SO4 andNa2SO4 aqueous electrolytes. However, at the same current density,the charge and discharge time for the nanowire K0.19MnO2·0.2H2Ois longer than that for the K0.45MnO2·0.3H2O, suggesting that thenanowire K0.19MnO2·0.2H2O has a higher capacitance than theK0.45MnO2·0.3H2O.

Charge and discharge curves for the AC, the preparednanowire K0.19MnO2·0.2H2O and K0.45MnO2·0.3H2O electrodes at200 mA g−1 and the cycling behavior of the prepared nanowireK0.19MnO2·0.2H2O and K0.45MnO2·0.3H2O electrodes are shownin Fig. 4. The AC as the anode presents a capacitance of 113F g−1 with a linear charge and discharge curves. Both thenanowire K0.19MnO2·0.2H2O and K0.45MnO2·0.3H2O do not showsharp or linear charge and discharge curves due to the exist-

ence of the pseudo-capacitance from the redox reactions anddouble-layer electric capacitance. The initial capacitance of thenanowire K0.19MnO2·0.2H2O (148 F g−1) is higher than that of

696 B.H. Zhang et al. / Electrochimica Acta 130 (2014) 693–698

0 30 60 90 12 0 15 00.0

0.2

0.4

0.6

0.8

1.0

Po

ten

tia

l /

V v

s.S

CE

Capacity / F g-1

K MnO 0.3H O K MnO 0.2H O

(b)

0 20 40 60 80 10 00.0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

-0.8

Po

ten

tia

l / V

vs. S

CE

Capacity / F g-1

AC(a)

0 500 1000 1500 2000 25000

50

100

150

200

250

300

Cycle Number

Dis

ch

arg

e C

ap

acity/F

g-1

0

20

40

60

80

100

120

K0.19 MnO2 0.2H 2O

Co

ulo

mb

ic E

fficie

ncy/%

(c)

0 500 1000 1500 2000 25000

60

120

180

240

300

Cycle N umbe r

Dis

ch

arg

e C

ap

acity/ F

g-1

0

20

40

60

80

100

120

K0.45 MnO2 0.3H 2O

Co

ulo

mb

ic E

fficie

ncy/%

(d)

F 200 mK ) at theK

KibKfmo(btccwoI1c

ig. 4. Charge and discharge curve of (a) the AC electrode at a current rate of

0.45MnO2·0.3H2O electrodes at a current rate of 200 mA g−1 between 0-1 V (vs. SCE0.19MnO2·0.2H2O and (d) the K0.45MnO2·0.3H2O electrodes.

0.45MnO2·0.3H2O (132 F g−1) though the potassium contentn the former is much less than that in the latter. This cane mainly ascribed to the reason for the nanowire structure of0.19MnO2·0.2H2O and the larger surface area. The surface area

rom BET method for the nanowire K0.19MnO2·0.2H2O is 25.652 g−1 (Fig. 5a), which provides more K+ ions or sites at the surface

f the layer structured KxMnO2 than that of the K0.45MnO2·0.3H2O8.26 m2 g−1, Fig. 5b). This is similar to the former reports on car-onaceous materials as electrode materials for supercapacitors, i.e.he surface area is more important [39]. Both present excellentycling and their capacitances do not fade evidently after 2500ycles even the oxygen in the aqueous solution is not removed,hich suggest superior electrochemical performance of these cath-

de materials for the supercapacitors in the aqueous solutions [40].n addition, both of them show a Coulombic efficiency of nearly00% except in the first several cycles, indicative of a good electro-hemical stability.

0.0 0.2 0.4 0.6 0.8 1.0

0

20

40

60

80

100

Vo

lum

e a

bsorb

ed

(cm

3/g

)

Relative pressure (P/P0)

K0.45MnO 2 0.3H 2O

Surface area: 8.26 m2 g

-1

(a)

Fig. 5. Nitrogen aborption and desorption curves of the nanowire K0.19Mn

A g−1 between -0.8 - 0 V (vs. SCE) and (b) the nanowire K0.19MnO2·0.2H2O and 500th cycle, and cycling performance and Coulombic efficiency of (c) the nanowire

The Nyquist plots of the nanowire K0.19MnO2·0.2H2O andK0.45MnO2·0.3H2O electrodes are shown in Fig. 6. It can beseen clearly that the charge-transfer resistance of the nanowireK0.19MnO2·0.2H2O is much lower than that of K0.45MnO2·0.3H2O,suggesting that the larger surface area is more favorable for thecharge transfer process since the available sites at the surfaceof the nanowire K0.19MnO2·0.2H2O is larger than those of theK0.45MnO2·0.3H2O.

The weight ratio of the electrodes largely affects the elec-trochemical performance of the asymmetric supercapacitors. Theweight ratio of the asymmetric supercapacitors are optimized asfollows: AC: K0.19MnO2·0.2H2O and AC: K0.45MnO2·0.3H2O are 1.3:1 and 1: 1.3, respectively. The charge and discharge curves and

the Ragone plots of the assembled asymmetric supercapacitorsAC/0.5 mol L−1 K2SO4/K0.19MnO2·0.2H2O (K0.45MnO2·0.3H2O) areshown in Fig. 7a. For comparison the Ragone plot of the supercapa-citors AC/0.5 mol L−1 K2SO4/AC is also shown in Fig. 7b. The charge

0.0 0.2 0.4 0.6 0.8 1.0

0

20

40

60

80

100

Vo

lum

e a

bsorb

ed

(cm

3/g

)

Relative pressure (P/P0)

K0.19MnO 2 0.2H2O

Surface area: 25.65 m2g

-1

(b)

O2·0.2H2O and K0.45MnO2·0.3H2O to measure their BET surface area.

B.H. Zhang et al. / Electrochimica

0 5 10 15 20 25 30 35 400

5

10

15

20

-Z''

/ O

hm

Z' / Ohm

K0.45 MnO

2 0.3H 2

O

K0.19 MnO

2 0.2H 2

O

Fe

atstetKptoh

FortA

ig. 6. Nyquist plots for the nanowire K0.19MnO2·0.2H2O and K0.45MnO2·0.3H2Olectrodes which were measured by using Ni mesh as the counter electrode.

nd discharge curves of the supercapacitors are similar to those ofheir cathode materials, which do not present an evident linearhape. This is due to the pseudo-capacitance of the redox reac-ions. The asymmetric supercapacitors do not show clear plateaus,ither, since the capacitive behavior is different from redox reac-ions. The asymmetric supercapacitor based on the nanowire0.19MnO2·0.2H2O presents an energy density of 41.3 Wh kg−1 at a

ower density of 156.8 Wkg−1, higher than that of the supercapaci-or based on K0.45MnO2·0.3H2O that only delivers an energy densityf 28.4 Wh kg−1 at a power density of 115.1 Wkg−1, which is alsoigher than that of our former work on K0.27MnO2·0.6H2O, 25.3 Wh

0 30 60 90 12 0 15 00.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Vo

lta

ge

/ V

Discharge capac ity / F g-1

AC//K0.19MnO2 0.2H2O

AC//K0.45MnO2 0.3H2O

(a)

0 50 0 100 0 150 0 200 0 250 00

10

20

30

40

50

AC//K0.19

MnO 2 0.3H

2O

AC//K0.45

MnO 2 0.2H

2O

AC//AC

En

erg

y d

en

sity / W

h k

g-1

Power density / W kg-1

(b)

ig. 7. (a) Charge-discharge profiles for the supercapacitors between 0 - 1.8 V basedn the nanowire K0.19MnO2·0.2H2O and K0.45MnO2·0.3H2O cathodes at a currentate of 200 mA g−1 and AC as the anode in the 500th cycle, and (b) Ragone plots ofhecapacitor of AC//AC, and supercapacitors of AC//nanowire K0.19MnO2·0.2H2O andC//K0.45MnO2·0.3H2O.

[

[[

[

Acta 130 (2014) 693–698 697

kg−1 at a power density of 140 W kg−1 [25]. The symmetric super-capacitors AC/0.5 mol L−1 K2SO4/AC shows a much lower energydensity of 5.6 Wh kg−1 at a power density of 51.5 Wkg−1 [41]. Whenthe power density increases, their energy density decreases. How-ever, the supercapacitor based on the nanowire K0.19MnO2·0.2H2Opresents higher energy density than that for the K0.45MnO2·0.3H2Oat the same power density. The main reasons are ascribed to thenano structure and the larger surface area of the K0.19MnO2·0.2H2O.In the case of the supercapacitor based AC//K0.45MnO2·0.3H2O, itsenergy density is about 20 Wh kg−1, higher than that based onAC//K0.27MnO2·0.6H2O [25]. This results show that the content ofpotassium amount is also favorable for the increase of capacitancethough its action is not as large as that of surface area.

4. Conclusions

Nanowire K0.19MnO2·0.2H2O is prepared by a hydrothermalmethod. It presents better electrochemical behavior thanK0.45MnO2·0.3H2O from the solid-state method as the cathodefor an asymmetric supercapacitor in 0.5 mol L−1 K2SO4 aqueoussolution using activated carbon as the anode. The capacitanceof the nanowire K0.19MnO2·0.2H2O (148 F g−1) is much higherthan that of K0.45MnO2·0.3H2O (132 F g−1) though its potassiumamount is less. Both of them present excellent cycling performanceeven when the oxygen in the aqueous electrolyte is not removed,and there is no evident capacitance fading after 2500 cycles.The K0.19MnO2·0.2H2O delivers an energy density 41.3 Wh kg−1

(based on the total mass of the active electrode materials) ata power density of 156.8 W kg−1 with activated carbon as theanode, higher than that of K0.45MnO2·0.3H2O, 28.4 Wh kg−1 at apower density of 115.1 W kg−1. This work shows great promisefor the application of these supercapacitors due to their low price,environmental friendliness, high power and energy densities, andexcellent cycling performance.

Acknowledgments

Financial supports from MOST (2010DFA61770), NSFC(21073046) and STCSM (12JC1401200) are gratefully appreciated.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.electacta.2014.03.026.

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