Facile synthesis of hierarchical MnO2sub-microspheres composed of nanosheets and their application for supercapacitors

Article abstract of DOI:10.1039/c4ra07880b

Hierarchical MnO₂sub-microspheres were fabricated by using a simple, green and efficient low-temperature route. These sub-microspheres were formed via the aggregation of ultrathin nanosheets with a thickness of 2-4 nm. An electrode made of hierarchical MnO₂sub-microspheres exhibited a specific capacitance of 120 F g^-1at a current density of 0.167 A g^-1. For an asymmetric AC//MnO₂supercapacitor, it exhibited a superior electrochemical stability in 1 M Na₂SO₄aqueous solution with 88% retention of the initial specific capacitance after 1000 cycles, and the Coulombic efficiency was above 97%, indicating good charge-discharge reversibility and electrochemical stability. This journal is

Full text of DOI:10.1039/c4ra07880b

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Facile synthesis of hierarchical MnO2 sub-
microspheres composed of nanosheets and their
Cite this: RSC Adv., 2014, 4, 40753
application for supercapacitors†
ab
Lifang Lian,ab Jie Yang,ab Peixun Xiong,ab Weifeng Zhangab and Mingdeng Wei
*
Hierarchical MnO2 sub-microspheres were fabricated by using a simple, green and efficient low-
temperature route. These sub-microspheres were formed via the aggregation of ultrathin nanosheets
with a thickness of 2–4 nm. An electrode made of hierarchical MnO2 sub-microspheres exhibited a
specific capacitance of 120 F gꢀ1 at a current density of 0.167 A gꢀ1. For an asymmetric AC//MnO2
supercapacitor, it exhibited a superior electrochemical stability in 1 M Na2SO4 aqueous solution with 88%
retention of the initial specific capacitance after 1000 cycles, and the Coulombic efficiency was above
97%, indicating good charge–discharge reversibility and electrochemical stability.
Received 31st July 2014
Accepted 26th August 2014
DOI: 10.1039/c4ra07880b
www.rsc.org/advances
supercapacitor. Moreover, nanoscale dimension may tolerate
more distortion and strain caused by the intercalation/dein-
tercalation of the ions, leading to long-term stability.14 There-
Introduction
Supercapacitors, due to their high power density, fast charge–
discharge ability and long cycling life, have been considered as
complementary to batteries, or even could replace batteries in
applications where high power delivery is needed.1–4 Super-
capacitors could be classied into two categories. One is the
electrical double layer capacitors that store charge electrostati-
cally using reversible absorption of electrolyte ions onto active
materials; the other is the pseudocapacitors that depend on the
reversible redox reaction at the surface of active materials.3,5 To
date, the most widely used electrode materials in super-
capacitors are carbonaceous materials (such as carbon nano-
tube,6 carbon nanober7 and grapheme8 etc.), metal oxides
(such as RuO2,9 V2O5 (ref. 10) and MnO2 (ref. 15–19) etc.), and
conducting polymers.11 Among them, manganese oxides, char-
acterized by high theoretical specic capacitance, nontoxicity,
abundance, low-cost and environmental compatibility, have
fore, many efforts have been devoted into the synthesis of MnO2
with different morphologies, sizes, nanostructures and prop-
erties through various methods, such as hydrothermal reac-
tion,15 sol–gel method,16 template method,17 electro-deposition
method,18 precipitation method,19 etc. In general, these
methods usually need some rigorous reaction conditions
including high temperature, high pressure, long reaction time,
complex and costly procedures, which are not versatile or
environmental friendly. Therefore, the development of a facile
and green route for synthesizing MnO2 is still of great
signicance.
In the present work, a simple and efficient reduction route
was successfully developed for fabricating hierarchical sub-
microspheres of MnO2 on a large scale. Furthermore, hierar-
chical MnO2 sub-microspheres were used as an electrode for
supercapacitor and exhibited
performance.
a
good electrochemical
attracted much attention as
supercapacitor.2,12
a promising candidate for
Nanostructurization is one of major strategies in the devel-
opment of high-performance manganese oxide-based elec-
trodes for supercapacitor.13 It is well known that the
nanomaterials synthesized with controlled sizes, shapes, crystal
phases, dimensions and compositions make it possible to tune
material parameters for optimizing performance in
Experimental
Hierarchical MnO2 sub-microspheres were synthesized via a
facile reduction route. In a typical synthesis, 1.0 g of SDBS and
0.632 g of KMnO4 were added to 50 mL of 0.2 M sulfuric acid
ꢁ
solution with stirring, and then was kept at 50 C for 4 h in a
water bath. Finally, the precipitate was ltered and washed
several times with deionized water and then dried at 70 ꢁC
for 12 h.
aState Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou
University, Fuzhou, Fujian 350002, China. E-mail: wei-mingdeng@fzu.edu.cn; Fax:
+86-591-83753180; Tel: +86-591-83753180
XRD patterns were recorded on PANalytical X'Pert spec-
trometer using the Co-Ka radiation and the data were changed
to Cu-Ka data. X-ray photoelectron spectroscopy (XPS)
measurements (ESCALAB 250) were performed to analyze the
bInstitute of Advanced Energy Materials, Fuzhou University, Fuzhou, Fujian 350002,
China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra07880b
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 40753–40757 | 40753

View Article Online
RSC Advances
Paper
surface species and their chemical states. Scanning electron
microscopy (SEM) and transmission electron microscopy (TEM)
were taken on a Hitachi S4800 instrument and a FEI F20 S-
TWIN instrument, respectively.
Electrochemical properties of the samples were evaluated in
a standard three electrode test pool (Hangzhou Saiao Electro-
chemistry Technology Co., Ltd). For the working electrode, a
mixture containing of 70 wt% active material, 20 wt% acetylene
black, and 10 wt% PTFE binder was well mixed. Then the
mixture was pressed with several drops of isopropyl alcohol
solvent to form a thin sheet. The sheet was rolled to get
approximate thickness of 100 mm and then pressed onto the
nickel foam. The mass loading of the activated material was
about 5 mg cmꢀ2. Pt electrode and saturated calomel electrode
(SCE) were used as the counter and reference electrodes,
respectively. All electrochemical measurements were carried out
in 1 M Na2SO4 aqueous electrolyte using an IM6 Electro-
chemical Workstation (Zahner). For the fabrication of the
asymmetric supercapacitor, active carbon (AC) electrodes were
rst prepared by the way same with MnO2 electrodes. Then the
MnO2 electrode was used as positive electrode and AC electrode
was used as negative electrode, respectively. The mass ratio of
positive electrode/negative electrode was 1 : 2 (based on charge
balance theory and their respectively specic capacitance). The
charge–discharge tests for the asymmetric supercapacitors were
performed on the Land automatic batteries tester (Land CT
2001A, Wuhan, China). The discharge specic capacitance was
calculated according to the following equation:
Fig. 1 (a) XRD pattern of MnO2, (b and c) SEM and (d) TEM images of
the MnO2 sample. The inset in (d) is the SAED pattern of the MnO2
sample.
I ꢂ Dt
Csp
¼
m ꢂ DU
where I (A) is the applied current, Dt (s) is the discharge time,
DU (V) is the tested potential range, and m (g) is the mass of
tested active material within the electrodes.
Results and discussion
Fig. 1a shows the XRD pattern of MnO2 and clearly shows two
characteristic peaks at 37.1ꢁ and 66.3ꢁ, indicating the amor-
phous nature of MnO2.20 Fig. 1b reveals that the synthesized
MnO2 exhibited a spherical structure with a diameter of 300–
500 nm. A SEM image in Fig. 1c demonstrates that the spherical
structure was formed by the aggregation of nanosheets. A TEM
image focused on a single sphere is shown in Fig. 1d and further
conrmed that the spherical structure was formed via the
aggregation of nanosheets with a typical thickness of 2–4 nm.
The selective area electronic diffraction (SAED) (Fig. 1d inset)
shows that the synthesized MnO2 was amorphous,21 which was
consistent with XRD.
Fig. 2 XPS spectra of MnO2: (a) Mn 2p and (b) O 1s.
The XPS spectrum was used to further determine the
oxidation state of the obtained MnO2. As depicted in Fig. 2a, it
Fig. 3a shows the CV curves of MnO2 sub-microspheres at
clearly reveals that the binding energy peaks of Mn 2p3/2 and Mn different scan rates ranging from 5 to 100 mV sꢀ1 with a
2p1/2 centered at 642.25 and 654.05 eV, respectively, which were potential window of 0 to 0.9 V. It clearly reveals an approxi-
in good agreement with the reported date for MnO2.12 The mately rectangular shape which can keep a nearly rectangular at
average oxidation state of Mn was around 3.55 based on the scan rate even as high as 100 mV sꢀ1, indicating a typical
analysis of O 1s spectrum (Fig. 2b).12,20,22
pseudocapacitive behavior of MnO2.22,23 In addition, a pair of
40754 | RSC Adv., 2014, 4, 40753–40757
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Fig. 4 Cycling performance of MnO2 electrode.
estimate the stable potential windows of MnO2 and AC. The
MnO2 electrode was measured within a potential window of 0 to
0.9 V, while AC electrode was measured within a potential
window of ꢀ0.9 to 0 V at a scan rate of 10 mV sꢀ1, as shown in
Fig. 5a. Both MnO2 and AC electrodes exhibited a nearly ideal
rectangular shape at their corresponding potential windows,
demonstrating their stable electrochemical performance,
therefore, it is expected that the operation voltage can be
extended to 1.8 V in 1 M Na2SO4 aqueous solution. Fig. 5b shows
Fig. 3 (a) CV curves at different scan rates and (b) charge–discharge
profiles at different current densities of MnO2 electrodes.
light redox peaks could be observed at low scan rate of 10 mV
sꢀ1 (Fig. S1†), revealing a fast and reversible Faradaic reaction
between alkali cations (Na+) and ultrathin MnO2 nano-
sheets.22–24 Fig. 3b shows the galvanostatic charge–discharge
curves of MnO2 sub-microspheres at different current densities
and the specic capacitances were 120 and 107 F gꢀ1 at 0.167
and 3.333 A gꢀ1, respectively. When the current density was
increased from 0.167 to 3.333 A gꢀ1, 89.16% of the initial
capacitance could be retained, demonstrating a good rate
capability.
Fig. 4 shows the cycling performance of MnO2 sub-micro-
spheres at a current density of 1 A gꢀ1. The specic capacitance
increased slightly over the whole 3000 cycles which was similar
to some reports on manganese oxide electrode materials.25,26
The increase of the specic capacitance may be attributed to the
activation effect of electrochemical cycling.25 This demon-
strated that the electrode of MnO2 sub-microspheres had a good
electrochemical stability.
To further investigate the electrochemical performance of
MnO2 sub-microspheres, an asymmetric supercapacitor based
on MnO2 and AC was fabricated. Before the test of a full cell, CV
Fig. 5 (a) CV curves of the MnO2 and AC electrodes, (b) charge–
discharge profiles of the individual electrode (MnO2 and AC), along
measurements were performed on the two electrodes to with the voltage profile of AC//MnO2 asymmetric supercapacitor.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 40753–40757 | 40755

View Article Online
RSC Advances
Paper
the charge–discharge proles of the individual electrodes vs.
SEC reference electrode and the voltage prole of the asym-
metric AC//MnO2 supercapacitor in 1 M Na2SO4 aqueous solu-
tion. It can also be observed from Fig. 5b that the AC//MnO2
asymmetric supercapacitor displayed a slope voltage prole
from 0 to 1.8 V, which was resulted from the potential difference
between the positive electrode of MnO2 and the negative elec-
trode of AC.
Fig. 6a shows the CV curves of an asymmetric AC//MnO2
supercapacitor at different scan rates. It exhibited a good
capacitive behavior with a nearly rectangular CV curves, indi-
cating that the stable electrochemical window of the asym-
metric supercapacitor can be extended to 1.8 V. Fig. 6b shows
the galvanostatic charge–discharge proles for an asymmetric
AC//MnO2 supercapacitor at different current densities. The
specic capacitance at a current density of 1 A gꢀ1 (based on the
total mass of active materials on the two electrodes) was 34.4 F
Fig. 7 Cycling performance of AC//MnO2 asymmetric supercapacitor.
gꢀ1. When the current density was increased from 1 to 2 A gꢀ1
,
96% of the initial capacitance can be retained, indicating a good
rate capability. The supercapacitor delivered an energy density
of 15.84 W h kgꢀ1 at a power density of 885 W kgꢀ1, and can be
maintained a value of 14.85 W h kgꢀ1 at a power density of
1725 W kgꢀ1, which were much higher than symmetrical
supercapacitors reported in literatures such as MnO2//MnO2,27
AC//AC,28 and CNTs//CNTs.29 This is also higher or comparable
to those of other asymmetric supercapacitors such as PANI//
MnO2,30 PPY//MnO2,30 PEDOT//MnO2,30 Fe3O4//MnO231 and AC//
MnO2.32,33
Fig. 7 shows the cycling performance for an asymmetric AC//
MnO2 supercapacitor at a current density of 1.5 A gꢀ1 in 1 M
Na2SO4 aqueous solution; this device exhibited a superior
electrochemical stability with a 88% retention of the initial
specic capacitance (34 F gꢀ1) aer 1000 cycles. Such cycling
performance was superior or at least comparable to those of
other asymmetric supercapacitors, such as graphene//MnO2/
grapheme,34 AC//a-MnO2$nH2O35 and AC//MnO2.36 The capaci-
tance fading was attributed to the corrosion of the current
collector arising from the presence of dissolved oxygen in the
aqueous electrolyte33 or matching problem.37 The Coulombic
efficiency of the asymmetric supercapacitor was above 97%,
indicating good charge–discharge reversibility and electro-
chemical stability.38
As mentioned above, MnO2 sub-microspheres used as the
electrode in a three-electrode system or an asymmetric super-
capacitor all exhibited a good supercapacitive performance,
which may be attributed to the intrinsic characteristics of MnO2
sub-microspheres. MnO2 sub-microspheres with a hierarchi-
cally porous structure can permit facile diffusion of the elec-
trolyte, and also enhance the contact between the electrode
surface and the electrolyte; while ultrathin nanosheets can
shorten the transport distance of electrolytes and electrons
during electrochemical cycling, leading to large capacitance,
high rate capability and cycling stability.
Conclusions
In summary, a simple and effective route was developed for
fabricating hieratical MnO2 sub-microspheres. These sub-
microspheres were formed via the aggregation of ultrathin
nanosheets with a thickness of 2–4 nm. The electrochemical
measurements indicated that hierarchical MnO2 sub-
Fig. 6 (a) CV curves at different scan rates and (b) charge–discharge
profiles at different current densities of AC//MnO2 asymmetric
supercapacitor.
40756 | RSC Adv., 2014, 4, 40753–40757
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
microspheres exhibited large capacitance, high rate capability 18 S. L. Chou, J. Z. Wang, S. Y. Chew, H. K. Liu and S. X. Dou,
and long-term cycling stability. Thus, such a hierarchically Electrochem. Commun., 2008, 10, 1724.
porous nanostructure can serve as promising electrode mate- 19 M. Toupin, T. Brousse and D. Belanger, Chem. Mater., 2002,
rials for supercapacitors.
14, 3946.
20 Y. Hou, Y. W. Cheng, T. Hobson and J. Liu, Nano Lett., 2010,
10, 2727.
Acknowledgements
21 R. Liu and S. B. Lee, J. Am. Chem. Soc., 2008, 130, 2942.
22 M. Toupin, T. Brousse and D. Belanger, Chem. Mater., 2004,
16, 3184.
This work was nancially supported by National Natural
Science Foundation of China (NSFC 21173049; J1103303).
23 Q. T. Qu, P. Zhang, B. Wang, Y. H. Chen, S. Tian and
Y. P. Wu, J. Phys. Chem. C, 2009, 113, 14020.
24 Z. B. Lei, J. T. Zhang and X. S. Zhao, J. Mater. Chem., 2012, 22,
153.
25 H. Jiang, T. Sun, C. Z. Li and J. Ma, J. Mater. Chem., 2012, 22,
2751.
Notes and references
1 G. P. Wang, L. Zhang and J. J. Zhang, Chem. Soc. Rev., 2012,
41, 797.
2 P. Y. Tang, Y. Q. Zhao, Y. M. Wang and C. L. Xu, Nanoscale,
2013, 5, 8156.
3 P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845.
4 L. H. Bao, J. F. Zang and X. D. Li, Nano Lett., 2011, 11, 1215.
5 M. J. Zhi, C. C. Xiang, J. T. Li, M. Li and N. Q. Wu, Nanoscale,
2013, 5, 72.
6 P. X. Li, C. Y. Kong, Y. Y. Shang, E. Z. Shi, Y. T. Yu, W. Z. Qian,
F. Wei, J. Q. Wei, K. L. Wang, H. W. Zhu, A. Y. Cao and
D. H. Wu, Nanoscale, 2013, 5, 8472.
26 M. Toupin, T. Brousse and D. Belanger, Chem. Mater., 2002,
14, 3946.
˜
´
27 V. Khomenko, E. R. Pinero and F. Beguin, J. Power Sources,
2006, 153, 183.
28 D. W. Wang, F. Li, M. Liu, G. Q. Lu and H. M. Cheng, Angew.
Chem., 2008, 120, 379.
29 M. Kaempgen, C. K. Chan, J. Ma, Y. Cui and G. Gruner, Nano
Lett., 2009, 9, 1872.
7 L. F. Chen, Z. H. Huang, H. W. Liang, W. T. Yao, Z. Y. Yu and
S. H. Yu, Energy Environ. Sci., 2013, 6, 3331.
8 U. N. Maiti, J. Lim, K. E. Lee, W. J. Lee and S. O. Kim, Adv.
Mater., 2014, 26, 615.
9 C. C. Hu, K. H. Chang, M. C. Lin and Y. T. Wu, Nano Lett.,
2006, 6, 2690.
30 V. Khomenko, E. Raymundo-Pinero, E. Frackowiak and
F. Beguin, Appl. Phys. A: Mater. Sci. Process., 2006, 82, 567.
31 T. Cottineau, M. Toupin, T. Delahaye, T. Brousse and
D. Belanger, Appl. Phys. A: Mater. Sci. Process., 2006, 82, 599.
32 X. Zhang, P. Yu, H. T. Zhang, D. C. Zhang, X. Z. Sun and
Y. W. Ma, Electrochim. Acta, 2013, 89, 523.
10 J. Yang, T. B. Lan, J. D. Liu, Y. F. Song and M. D. Wei,
Electrochim. Acta, 2013, 105, 489.
11 L. Z. Fan and J. Maier, Electrochem. Commun., 2006, 8, 937.
12 Z. J. Su, C. Yang, C. J. Xu, H. Y. Wu, Z. X. Zhang, T. Liu,
C. Zhang, Q. H. Yang, B. H. Li and F. Y. Kang, J. Mater.
Chem. A, 2013, 1, 12432.
13 W. F. Wei, X. W. Cui, W. X. Chen and D. G. Ivey, Chem. Soc.
Rev., 2011, 40, 1697.
14 S. Liu, S. H. Sun and X. Z. You, Nanoscale, 2014, 6, 2037.
15 V. Subramanian, H. W. Zhu and B. Q. Wei, J. Power Sources,
2006, 159, 361.
33 T. Brousse, P. L. Taberna, O. Crosnier, R. Dugas,
P. Guillemet, Y. Scudeller, Y. Zhou, F. Favier, D. Belanger
and P. Simon, J. Power Sources, 2007, 173, 633.
34 Z. S. Wu, W. C. Ren, D. W. Wang, F. Li, B. L. Liu and
H. M. Cheng, ACS Nano, 2010, 4, 5835.
35 M. S. Hong, S. H. Lee and S. W. Kim, Electrochem. Solid-State
Lett., 2002, 5, A227.
36 X. Zhang, X. Z. Sun, H. T. Zhang, D. C. Zhang and Y. W. Ma,
Mater. Chem. Phys., 2012, 137, 290.
37 J. L. Li and F. Gao, J. Power Sources, 2009, 194, 1184.
38 J. Y. Tao, N. S. Liu, L. Y. Li, J. Su and Y. H. Gao, Nanoscale,
2014, 6, 2922.
16 X. L. Wang, A. B. Yuan and Y. Q. Wang, J. Power Sources,
2007, 172, 1007.
17 X. Y. Wang, X. Y. Wang, W. G. Huang, P. J. Sebastian and
S. Gamboa, J. Power Sources, 2005, 140, 211.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 40753–40757 | 40757