Supercapacitive properties of ultra-fine MnO2 prepared by a solid-state coordination reaction

Article abstract of DOI:10.1016/j.jallcom.2010.08.010

An ultra-fine MnO₂ powder was synthesized by a novel route of solid-state coordination reaction. The structure and morphology were characterized by X-ray diffraction, scanning electron microscope and infrared spectroscopy. The MnO₂ powder with a primary particle size of ～200 nm was composed of α-MnO₂ and γ-MnO₂. Electrochemical properties of the MnO₂ electrode were examined by cyclic voltammetry, and galvanostatic charge/discharge and electrochemical impedance spectroscopy measurements in 6 mol L^-1 KOH electrolyte. The MnO₂ electrode exhibited good supercapacitive performance. Specific capacitance values of 337 and 197 F g^-1 were obtained for the MnO₂ electrode at scan rates of 2 and 50 mV s^-1, respectively. The MnO₂ electrode subjected to 500 cycles at a current density of 800 mA g^-1 delivered a specific capacitance of 228 F g^-1, retaining 75% of its initial specific capacitance.

Full text of DOI:10.1016/j.jallcom.2010.08.010

Journal of Alloys and Compounds 507 (2010) 526–530
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Supercapacitive properties of ultra-fine MnO2 prepared by a solid-state
coordination reaction
Dao-Lai Fang∗, Bing-Cai Wu, Ai-Qin Mao, Yong Yan, Cui-Hong Zheng
School of Materials Science and Engineering, and Anhui Key Laboratory of Metal Materials and Processing, Anhui University of Technology, Ma’anshan, Anhui 243002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
An ultra-fine MnO2 powder was synthesized by a novel route of solid-state coordination reaction. The
structure and morphology were characterized by X-ray diffraction, scanning electron microscope and
infrared spectroscopy. The MnO2 powder with a primary particle size of ∼200 nm was composed of
␣-MnO2 and ␥-MnO2. Electrochemical properties of the MnO2 electrode were examined by cyclic voltam-
metry, and galvanostatic charge/discharge and electrochemical impedance spectroscopy measurements
in 6 mol L−1 KOH electrolyte. The MnO2 electrode exhibited good supercapacitive performance. Specific
capacitance values of 337 and 197 F g−1 were obtained for the MnO2 electrode at scan rates of 2 and
50 mV s−1, respectively. The MnO2 electrode subjected to 500 cycles at a current density of 800 mA g−1
delivered a specific capacitance of 228 F g−1, retaining 75% of its initial specific capacitance.
© 2010 Elsevier B.V. All rights reserved.
Received 23 May 2010
Accepted 4 August 2010
Available online 12 August 2010
Keywords:
Oxides
Chemical synthesis
Electrochemical measurements
Electrochemical properties
1. Introduction
fore, it is of great significance to develop an effective preparation
method for improving the supercapacitive properties, especially
Recently, electrochemical supercapacitors have aroused great
interest because they can be used as complementary charge
storage devices to conventional batteries in various applications
requiring peak power pulses, such as hybrid electric vehicles,
nano-electronics, burst power generation, and memory back-
up devices [1,2]. Generally the supercapacitors can be classified
into two categories: electrical-double-layer capacitors (EDLCs),
which build up electrical charge at the electrode/electrolyte
interface, and pseudocapacitors, which are based on reversible
faradic redox reactions at the interfaces at certain poten-
tials.
Due to its favorable pseudocapacitive characteristics, low cost
and environmental friendliness, tetravalent manganese oxide
(MnO2) is a most promising candidate for supercapacitor mate-
rials [3,4]. Regardless of its theoretical specific capacitance (SC) as
high as 1370 F g−1 [5], MnO2 material prepared so far still exhibits a
much lower SC. The powder MnO2 material prepared by the usual
methods such as sol–gel, co-precipitation, thermal decomposition
and hydrothermal synthesis possesses a lower SC ranging from
100 to 300 F g−1 [6–9]. The film MnO2 material usually delivers a
higher SC, e.g. 698 F g−1 for the sol–gel derived film (mass loading
∼4 ␮g cm−2) [10], and 460 F g−1 for the electrodeposition derived
film [11]. Nevertheless the SC of the film MnO2 decreases rapidly
with increasing film thickness due to its low conductivity. There-
the SC, of the MnO2 material.
Solid-state coordination reaction route, which lies in between
the solid-state and the wet-chemical method in a certain sense,
is a very facile yet highly effective method for synthesis of ultra-
fine or nanosized particles [12–14]. However, few investigations
were reported concerning preparation of MnO2 material for super-
capacitors by this route. In this work, the route was attempted to
synthesize manganese oxalate precursor, and a calcination tem-
perature as low as 250 ◦C was applied to decompose the oxalate
precursor for obtaining an ultra-fine MnO2 powder, which exhib-
ited good supercapacitive performance in terms of the SC and cycle
stability.
2. Experimental
2.1. Synthesis of the manganese oxalate precursor and the MnO2 powder
Analytical-grade manganese acetate Mn(CH3COO)2·4H2O and oxalic acid
H2C2O4·2H2O were used as starting materials. A powder mixture with a molar ratio
of Mn2+:oxalic acid of 1:1 was ball milled at room temperature for 5 h in a polyethy-
lene container using zirconia balls as milling medium. The milled mixture was then
dried at 70 ◦C and calcined for 10 h at 250 ◦C in air.
2.2. Structural characterization
˚
A Philips X’pert Pro X-ray diffractometer with Cu K␣ radiation (ꢀ = 1.5406 A)
was used to analyze the phase compositions of the dried milled mixture and the
calcined manganese oxide powder. Diffraction data were collected in the 2Â range
from 15◦ to 70◦, using the step-scan mode with a scanning speed of 0.02◦ step-
size and 1 s per step. Fourier transform infrared (FT-IR) spectroscopy of the calcined
manganese oxide powder was recorded on a Perkin-Elmer 577 spectrometer with
a resolution of 4 cm−1, using KBr as dispersing medium. The morphology of the
∗
Corresponding author. Tel.: +86 555 2315318; fax: +86 555 2311570.
E-mail address: fangdl@ustc.edu (D.-L. Fang).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.08.010

D.-L. Fang et al. / Journal of Alloys and Compounds 507 (2010) 526–530
527
Fig. 1. XRD patterns of the synthesized manganese oxalate (a) and the obtained
Fig. 2. FT-IR spectra of the obtained MnO2 powder.
MnO2 (b).
tion peaks at 2Â = 18.2◦, 28.7◦, 37.2◦ and 49.1◦ well match those
of ␣-MnO2 (JCPDS: 44-0141), while the peaks at 2Â = 21.9◦, 33.1◦,
38.6◦, 57.5◦, and 66.0◦ are in good agreement with those of ␥-MnO2
(JCPDS: 14-0644). This confirms that the obtained manganese oxide
is mainly composed of a mixture of ␣- and ␥-MnO2. Low inten-
sity and broadening of the peaks in Fig. 1(b) indicate that the
obtained manganese oxide powder is not well crystallized, and/or
that the crystallites in the powder are very fine. The characteris-
tic diffraction peaks of ␣-form manganese oxalate in Fig. 1(a) are
not observed in the XRD patttern in Fig. 1(b), which implies that
the isothermal decomposition of the manganese oxalate was com-
pleted at a calcination temperature of 250 ◦C in a duration of 10 h.
This conforms to the results given by Mohamed et al. [16], who
reported that at heating rate of 5 ◦C min−1 in air, decomposition of
anhydrated manganese oxalate was initiated at a temperature as
low as 275 ◦C, and attributed the significant diminishing of the ini-
tial decomposition temperature to the role of oxygen in promoting
decompositions of transition metal oxalates containing oxidizable
cations.
Fig. 2 shows the FT-IR spectra of the obtained MnO2. The broad
absorption band at 3437 cm−1 is attributed to the O−H stretch-
ing vibration, and the absorption bands observed at 1635, 1387
and 1117 cm−1 are usually ascribed to the O−H bending vibra-
tions combined with Mn ions [17]. This suggests that a certain
amount of bound water exists in the obtained MnO2, which is gen-
erally believed to be beneficial to the electrochemical activity of
MnO2 materials. The broad absorption bands at 671, 617, 581 and
535 cm−1, as indicated by the inset in Fig. 2, are attributed to the
Mn−O vibrations in MnO6 octahedra of ␣- and ␥-MnO2 [17–19].
The FT-IR result reveals that the obtained MnO2 is a mixture of ␣-
and ␥-MnO2 containing a small amount of bound water, which is
in line with the XRD result in Fig. 1.
obtained manganese oxide powder was analyzed by using a JSM 6490 scanning
electron microscope (SEM).
2.3. Electrode fabrication and electrochemical characterization
The electrodes for electrochemical measurements were composed of the
obtained MnO2, acetylene black (AB) and polytetrafluoroethylene (PTFE), whose
weight ratio was 70:25:5. First, the MnO2 active material and the acetylene black
were fully mixed and grinded, then a proper amount of PTFE binder was added
into the powder mixture to achieve a homogenous slurry. The prepared slurry was
painted onto a nickel foam current collector with an area of 1 cm × 1 cm. Finally,
the painted current collector was dried for 10 h at 80 ◦C, and pressed at a pressure
of 10 MPa to form a reliable electrode. Each of the fabricated electrodes contained
∼4 mg of the MnO2 active material.
All the electrochemical measurements concerned were carried out in 6 mol L−1
KOH aqueous electrolyte. Cyclic voltammetry measurements were performed on
a CHI604C electrochemical workstation in a three-electrode cell set-up, using a
MnO2 electrode fabricated above as the working electrode, a platinum foil as the
counter electrode, a saturated calomel electrode (SCE) as the reference electrode.
Cyclic voltammograms at different scan rates were recorded between −0.4 and 0.4 V
vs. SCE. For evaluation of galvanostatic charge/discharge performance of the MnO2
electrode, a symmetrical capacitor was assembled by clamping a couple of the iden-
tical electrodes separated by a porous polypropylene separator. The galvanostatic
charge/discharge performance at different current densities was determined by a
battery test system of Neware BTS-3008W in a potential range from 0 to 0.8 V. Elec-
trochemical impedance spectroscopy (EIS) of the MnO2 electrode was determined
with the CHI604C electrochemical workstation, applying an a.c. amplitude of 5 mV
in a frequency range of 0.01–105 Hz.
3. Results and discussion
3.1. X-ray diffraction patterns and FT-IR spectra of the obtained
MnO2
After the starting materials of manganese acetate and oxalic acid
had been ball milled for 5 h, a cream-like, viscous substance was
formed, which was then dried at 70 ◦C, yielding a fine powder of
light pink colour. Fig. 1(a) gives XRD pattern of the synthesized
manganese oxalate. All the diffraction peaks in Fig. 1(a) are in
accordance with those of ␣-form manganese oxalate (JCPDS: 25-
0544), without the characteristic diffraction peaks of the starting
materials observed. This suggests that the solid-state coordination
reaction between manganese acetate and oxalic acid is completed
after ball milling for 5 h, forming a coordination compound of ␣-
form manganese oxalate. For avoiding formation of Mn2O3, the
synthesized manganese oxalate was calcined in air for 10 h at
250 ◦C, much lower than its usual nonisothermal decomposition
temperature (∼300 ◦C) [15], resulting in a manganese oxide pow-
der of brownish-black colour. Fig. 1(b) represents XRD pattern of
the obtained manganese oxide. As shown in Fig. 1(b), the diffrac-
3.2. Morphology of the obtained MnO2 powder
The morphology of the prepared MnO2 powder was investigated
by scanning electron microscopy (SEM). The lower-magnification
SEM image in Fig. 3(a) shows that the prepared MnO2 powder
consists of uniform and spherical particles with a particle size of
∼1 ␮m. The higher-magnification SEM image in Fig. 3(b) discloses
that each of the spherical particles observed under lower magnifi-
cation is actually an agglomerate, which is composed of a number
of smaller particles with a particle size of ∼200 nm. The solid-
state coordination route allows the prepared MnO2 powder with
an ultra-fine particle size, which is mainly related to the unique

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D.-L. Fang et al. / Journal of Alloys and Compounds 507 (2010) 526–530
Fig. 3. SEM images of the obtained MnO2 powder under lower (a) and higher (b) magnification.
SCE in 6 mol L−1 KOH aqueous electrolyte. Though slightly distorted
at higher scan rates, all the cyclic voltammograms obtained are
roughly rectangular in shape, suggesting good capacitive behavior
of the MnO2 electrode. Furthermore, all the cyclic voltammograms
present a couple of approximately symmetrical current peaks, i.e.
an oxidation one around 0.16 V and a reduction one around 0.08 V,
which are due to intercalation/detercalation of protons or alkaline
metal cations in the MnO2 electrode in KOH aqueous electrolyte,
indicating a considerable faradic pseudocapacitance contribution
[21]. The small potential difference between the anodic peak and
the cathodic peak, increasing from 0.07 to 0.18 V with increas-
ing scan rates from 2 to 50 mV s−1, reveals the high reversibility
of the MnO2 electrode. The SC of the MnO2 electrode can be cal-
conditions of the solid-state coordination reaction, under which the
nucleation process is usually much more favorable than the growth
process [12,20]. The prepared MnO2 powder with ultra-fine parti-
cles undoubtedly provides a larger effective surface area for redox
reactions and cation adsorption/desorption, which is beneficial to
promoting its supercapacitive performance.
3.3. Electrochemical properties of the MnO2 electrode
Cyclic voltammetry (CV) is considered to be an ideal tool to
indicate the capacitive behavior of any electrode material. Fig. 4(a)
shows the cyclic voltammograms of the MnO2 electrode measured
in a scan rate range from 2 to 50 mV s−1 between −0.4 and 0.4 V vs.
Fig. 4. Cyclic voltammograms with different scan rates (a), scan rate dependence of the SC (b) and galvanostatic charge/discharge profiles at various current densities (c) of
the MnO2 electrode.

D.-L. Fang et al. / Journal of Alloys and Compounds 507 (2010) 526–530
529
Fig. 6. Cycling life of the MnO2 electrode at a current density of 800 mA g−1 between
0 and 0.8 V.
Fig. 5. Electrochemical impedance spectra (EIS) of the MnO2 electrode in the fre-
quency range of 0.01–105 Hz.
tor interface [24], is estimated to be about 0.9 ꢃ from the point
intercepting the real axis in the high frequency range. In the high-
medium frequency region, a minor semicircle observed confirms
the slight faradic charge-transfer resistance at electrode/electrolyte
interface [24,25]. A straight line, nearly vertical to the real axis,
appears in the low frequency region, suggesting good capacitive
behavior of the prepared MnO2 electrode. The EIS results are in
good agreement with the CV results.
culated from the cyclic voltammograms according to the formula
Cs = I/mꢁ, where Cs is the SC (F g−1), m the mass (g) of the elec-
troactive material, ꢁ the potential scan rate (V s−1), and I the even
ꢀ
current response (A) defined by I = 1/(2(Vc − Va)) idV (Va and Vc
represent the lowest and highest potentials (V), respectively). As
shown in Fig. 4(b), the SC of the MnO2 electrode decreases from
337 to 197 F g−1 with increasing scan rates from 2 to 50 mV s−1
,
The long-term cycling stability of the MnO2 electrode
was investigated in 6 mol L−1 KOH electrolyte by galvanostatic
charge/discharge cycling at a current density of 800 mA g−1 for con-
secutive 500 cycles, as shown in Fig. 6. The SC of the prepared MnO2
electrode fades remarkably during the first 280 cycles, decreasing
by ∼19% of its initial SC. During the next 220 cycles, the SC of the
prepared MnO2 electrode becomes stable, decreasing by ∼6% of its
initial SC. A considerable SC of 228 F g−1, about 75% of its initial SC,
is still retained for the prepared MnO2 electrode after 500 cycles,
suggestive of good electrochemical stability, which is of practical
importance.
which is mainly due to that the redox reactions are based on the
intercalation/deintercalation of protons or alkaline metal cations
from the electrolyte [9,22]. At lower scan rates, more ions from the
electrolyte have a chance to gain access to the electroactive MnO2
material, and participate in redox reactions, resulting in a higher
SC. However, at higher scan rates, the effective interaction between
the ions and the electroactive MnO2 material is decreased accord-
ingly, leading to a lower SC. More importantly, a considerable SC
of 197 F g−1 was retained even at a higher scan rate of 50 mV s−1
indicating higher power characteristics of the MnO2 material.
,
For further investigation of the power properties of the MnO2
electrode, the galvanostatic charge/discharge profiles at different
current densities were determined in an electrochemical window
of 0.8 V, as shown in Fig. 4(c). The galvanostatic charge/discharge
profiles, especially the ones at lower current densities, are
somewhat curved, implying the predominance of the faradic pseu-
docapacitive nature over the electrical-double-layer capacitive one
of the MnO2 electrode. This result is coincident with the CV result
in Fig. 4(a), where a Faradic reaction characteristic, i.e. a couple of
redox peaks, could be observed obviously for the MnO2 electrode.
The SC of a symmetrical capacitor, converted to the SC of a single
electrode, can be obtained by the formula Cs = 4Iꢂt/mꢂV, where Cs
is the discharge SC (F g−1), I the discharge current (A), ꢂt the dis-
charge time (s), m the total mass (g) of the electroactive material
contained in a symmetrical capacitor, ꢂV the discharge potential
range (V) [23]. The SC of the symmetrical capacitor at current den-
sities of 174, 349 and 698 mA g−1 is calculated to be 348, 303 and
293 F g−1, respectively. Significantly, the capacitance degradation
4. Conclusion
An ultra-fine MnO2 powder with a particle size of ∼200 nm
was prepared by a facile and low-cost method, i.e. calcining the
solid-state coordination derived manganese oxalate for 10 h at
250 ◦C in air. The XRD results showed that the prepared MnO2
powder was a mixture of poorly crystallized ␣- and ␥-MnO2. Its
electrochemical properties were characterized by CV, galvanostatic
charge/discharge cycling, and EIS measurements in 6 mol L−1 KOH
aqueous electrolyte. The MnO2 electrode exhibited good superca-
pacitive performance, delivering a maximum SC of 337 F g−1 at a
scan rate of 2 mV s−1, and retaining a considerable SC of 197 F g−1
even at a scan rate of 50 mV s−1. Furthermore, a SC of 228 F g−1, i.e.
75% of the initial SC, was remained after 500 cycles at a current
density of 800 mA g−1, suggesting good electrochemical stability
of the prepared MnO2 electrode. Thus the ultra-fine MnO2 pow-
der obtained from the solid-state coordination reaction route is a
promising material for supercapacitors.
is ∼16% with increasing current densities from 174 to 698 mA g−1
,
indicating good rate capability of the MnO2 electrode.
For further investigation of the actual electrochemical diffu-
sion process, electrochemical impedance spectroscopy (EIS) was
performed in the frequency range from 0.01 to 105 Hz with an exci-
tation signal of 5 mV, as shown in Fig. 5. The internal resistance Ri
of the MnO2 electrode, consisting of the ionic resistance of elec-
trolyte, the intrinsic resistance of the electroactive material, and
the contact resistance at the electroactive material/current collec-
Acknowledgements
This work is financially supported by the Natural Science Foun-
dation of Education Commission of Anhui Province under Grant No.
KJ2008A003 and Grant No. KJ2009B050. The financial support pro-
vided by the Innovation Project of Anhui University of Technology
is gratefully acknowledged.

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