Fabrication of manganese dioxide nanosheet-based thin-film electrode and its electrochemical capacitance performance

Article abstract of DOI:10.1016/j.electacta.2012.05.154

A manganese dioxide (MnO₂) nanosheet-based thin film deposited on a conductive Ni substrate has been synthesized via a hydrothermal route. Field emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS) showed that the as-prepared thin film had a porous network structure, which consisted of interlaced MnO₂ nanosheets oriented perpendicular to the substrate. Electrochemical tests demonstrated that the MnO₂ nanosheet-based thin-film electrode exhibited excellent capacitance performance with high rate properties and good cycling stability. A specific capacitance of 385 F g^-1 was obtained at a current density of 0.5 A g^-1, with a capacitance retention of about 81% when the current density was increased from 0.5 to 5 A g^-1. When cycled at a higher current density of 1.25 A g^-1, 93% of the initial specific capacitance was retained over 5000 cycles. The excellent electrochemical properties of this MnO₂ thin-film electrode can be attributed to its thin-sheet morphology, porous structure and the good contact between the MnO₂ active material and the Ni substrate. Considering the excellent performance and facile preparation, this thin-film electrode should have great potential for application in energy storage and conversion devices.

Full text of DOI:10.1016/j.electacta.2012.05.154

Electrochimica Acta 78 (2012) 115–121
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Fabrication of manganese dioxide nanosheet-based thin-film electrode and its
electrochemical capacitance performance
∗
Xiaojun Wang, Haimei Liu, Xu Chen, David G. Evans, Wensheng Yang
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A manganese dioxide (MnO ) nanosheet-based thin film deposited on a conductive Ni substrate has been
synthesized via a hydrothermal route. Field emission scanning electron microscopy (FESEM) and X-ray
2
Received 18 March 2012
Received in revised form 28 April 2012
Accepted 10 May 2012
photoelectron spectroscopy (XPS) showed that the as-prepared thin film had a porous network structure,
which consisted of interlaced MnO2 nanosheets oriented perpendicular to the substrate. Electrochemical
tests demonstrated that the MnO2 nanosheet-based thin-film electrode exhibited excellent capacitance
Available online 18 June 2012
−
1
performance with high rate properties and good cycling stability. A specific capacitance of 385 F g was
Keywords:
Manganese dioxide
Nanosheets
Thin-film electrode
Supercapacitor
−1
obtained at a current density of 0.5 A g , with a capacitance retention of about 81% when the current
−1
−1
density was increased from 0.5 to 5 A g . When cycled at a higher current density of 1.25 A g , 93% of
the initial specific capacitance was retained over 5000 cycles. The excellent electrochemical properties
of this MnO2 thin-film electrode can be attributed to its thin-sheet morphology, porous structure and
the good contact between the MnO2 active material and the Ni substrate. Considering the excellent
performance and facile preparation, this thin-film electrode should have great potential for application
in energy storage and conversion devices.
©
2012 Elsevier Ltd. All rights reserved.
1
. Introduction
reported a dendritic MnO thin-film with a specific capacitance
2
−
1
of 700 F g prepared by anodic oxidation of sputtered Mn films.
However the high specific capacitance obtained at a slow charg-
ing current decreased rapidly with increasing current or sweep
rate. Zhang et al. [26] prepared a MnO2 nanocluster/carbon nan-
otube composite thin film by combining chemical vapor deposition
and electrochemical deposition and the composite thin-film elec-
An electrochemical capacitor (or supercapacitor) is a type of
economically competitive energy storage device, which has great
potential applications in hybrid electric vehicles, portable electron-
ics, and other devices owing to its high energy storage ability and
high power output [1]. Three main kinds of materials are used
in electrochemical capacitors, namely carbon materials, conduct-
ing polymers, and transition metal oxides [2–7]. Among various
transition metal oxides, ruthenium oxide presents a high specific
capacitance and excellent reversibility [8]. However, ruthenium
oxide cannot be used on a large scale owing to its high cost and
toxicity.
−
1
trode showed a specific capacitance of 199 F g . Wu [22] reported a
MnO nanowire thin film on a nickel substrate by electrodeposition
2
and the thin-film electrode showed a maximum specific capaci-
−
1
tance of 350 F g through optimizing experimental conditions.
Nanosheets are a novel class of two-dimensional nanomaterials
with a thickness on the order of nanometers and lateral dimen-
sions of submicron to several microns [28]. The most significant
features of such nanosheets are their exceptionally high specific
surface area [29] and large number of exposed surface active sites,
making them an ideal building block for the fabrication of thin
films [24,30,31]. In our previous work [24], we fabricated a multi-
layer thin film through layer-by-layer self-assembly of pre-formed
Manganese dioxide (MnO ) is considered to be a promising
2
candidate for the replacement of RuO2 as the next generation
of supercapacitor materials because of its low cost, environmen-
tal friendliness, and high theoretical specific capacitance [9–15].
To data, various approaches have been employed to fabricate
MnO2 thin films as supercapacitor electrodes, including sol–gel
synthesis [16], electrochemical deposition [17–22], layer-by-layer
assembly [23,24], and a combination of chemical vapor deposition
and electrochemical deposition [25,26]. Broughton and Brett [27]
MnO nanosheets and polyethylenimine, which showed good elec-
2
−
1
trochemical activity with a specific capacitance value of 288 F g
.
However, the film was insufficiently robust for long-term practical
application because of the relatively weak adhesion between the
active material and substrate.
∗ Corresponding author. Tel.: +86 10 64435271; fax: +86 10 64425385.
E-mail addresses: yangws@mail.buct.edu.cn, yangws buct@126.com (W. Yang).
This paper describes an attempt to overcome the problem of
poor adhesion between film and substrate by growing a thin film of
0
013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.05.154

1
16
X. Wang et al. / Electrochimica Acta 78 (2012) 115–121
Fig. 1. (a) FESEM image, (b) magnified FESEM image, (c) cross-sectional FESEM image and (d) CA measurement of the as-prepared MnO2 thin film; the inset of (a) is a typical
photograph of the as-prepared MnO2 thin-film electrode under bending stress.
MnO2 nanosheets directly in situ on a conductive nickel substrate,
by means of a hydrothermal process. In addition to improved adhe-
sion, in situ growth of such a film should also facilitate the charge
transport between active material and substrate [31–34]. The as-
prepared thin film was characterized by field emission scanning
electron microscopy (FESEM), X-ray photoelectron spectroscopy
The reaction was repeated at different temperatures for varying
times.
2.3. Characterization
FESEM images were obtained with a Zeiss Supra 55 scanning
(
XPS) and contact angle (CA) measurements. The capacitive behav-
electron microscope at an accelerating voltage of 15 kV. XPS mea-
surements were performed using a Thermo Scientific ESCALAB 250
instrument equipped with a Mg K␣ X-ray source (1253.6 eV) at a
pass energy of 30.0 eV. Curve fitting was performed using a com-
bination of Gaussian and Lorentzian functions, while background
subtraction was carried out according to the Shirley method.
CA measurements were performed on a commercial drop-shape
analysis system (DSA100, KRÜSS GmbH, Germany) at ambient
temperature. The mass of deposited MnO2 on the nickel sub-
strate was determined by dissolving the film in an H2O2/HNO3
aqueous solution and using inductively coupled plasma-atomic
emission spectroscopy (ICP-AES) (Shimadzu ICPS-7500) to analyze
the resulting solution. Based on the measured concentration of Mn
per unit volume, the total mass of deposited MnO2 was calculated
ior of the as-prepared MnO2 thin-film electrode was evaluated by
cyclic voltammetry (CV) and galvanostatic charge–discharge tests.
2
. Experimental
2.1. Reagents
All chemicals including manganese chloride (MnCl ), tetram-
2
ethylammonium hydroxide (TMAOH), and hydrogen peroxide
H O ) were of analytical grade, and were used as received without
(
2
2
further purification. The nickel foil was 99.9% purity. All aqueous
solutions were prepared in distilled water.
−
1
based on the formula weight of stoichiometric MnO2 (87 g mol ).
Electrochemical measurements were performed using a three-
2.2. Synthesis of the MnO2 thin film
electrode system with the MnO thin-film electrode as the working
Nickel foil was rinsed well with ethanol and deionized water.
2
electrode, and a Pt sheet and a saturated calomel electrode (SCE)
as the counter and reference electrodes, respectively. CV scans
were recorded from −0.1 to 0.8 V (vs. SCE) in 1 M Na SO aque-
5
mL of a mixed aqueous solution of 3 wt% H O2 and 0.6 M TMAOH
2
was added to 25 mL of 0.03 M MnCl ·4H O aqueous solution
2
2
within 10 s. After stirring for 2 h at room temperature, the nickel
foil was immersed in the precursor solution, which was then
transferred into a 100 mL Teflon-lined stainless steel autoclave,
2
4
ous solution on a CHI 660C electrochemical workstation (Shanghai
Chenhua Instrument Co., China). Galvanostatic charge–discharge
tests were investigated using an Arbin MSTAT4+ multichannel gal-
vanostat/potentiostat in the potential range of −0.1 to 0.8 V (vs.
SCE).
◦
sealed and maintained at 140 C for 12 h. After cooling the auto-
clave, the nickel foil was removed, washed with distilled water
and ethanol several times, and then dried at room temperature.

X. Wang et al. / Electrochimica Acta 78 (2012) 115–121
117
3
. Results and discussion
3.1. Characterization of the as-prepared MnO2 thin film
Fig. 1 shows FESEM images and a CA measurement for the as-
prepared MnO thin film. As can be seen from Fig. 1a, a uniform thin
2
film with an open framework was obtained, which is formed by
randomly interlaced nanosheets without any evidence of cracking
or aggregation. The arrangement of nanosheets generates a porous
network structure. Importantly for practical applications as a flex-
ible supercapacitor, no loss of the active material was observed on
repeatedly folding the film (inset in Fig. 1a), implying there is good
adhesion between the active material and the substrate. A magni-
fied image of the thin film (Fig. 1b) shows that the individual MnO2
nanosheets are approximately 10 nm in thickness. A high magnifi-
cation FESEM cross-section image of the film (Fig. 1c) reveals that
the thickness of the thin film is about 400 nm. Furthermore, the as-
prepared MnO2 thin film exhibited highly hydrophilic properties.
As shown in Fig. 1d, the as-preparedMnO thin film has a water con-
2
◦
tact angle as low as 17 . This is also important for supercapacitor
applications, because thin films with such high wettability should
give an intimate contact with an electrolyte, facilitating penetra-
tion of the electrolyte into the film electrode and charge transport
during charge–discharge cycling.
The composition of the as-prepared thin film was investigated
by X-ray photoelectron spectroscopy. Fig. 2a displays the Mn 2p
core level XPS spectra. The peaks located at 642.2 eV and 654 eV are
characteristic of the Mn 2p3/2 and Mn 2p1/2 peaks of Mn(IV) in MnO2
[
35]. The peak separation (ꢀE) of the multiplet splitting of the Mn 3s
peak, arising from exchange interactions of the 3d electrons with
the ejected 3s electron, is also sensitive to the average oxidation
state of manganese [36]; the value of ꢀE increases linearly from
4
+
3+
∼
4.8 for Mn to ∼5.3 eV for Mn [37]. As shown in Fig. 2b, the ꢀE
value of 4.84 eV for Mn 3s core level in XPS of the thin film indicates
that the oxidation state of the manganese is ca. +4, confirming the
formation of a MnO2 thin layer on the substrate.
Fig. 2. XPS spectra of the as-prepared thin film: (a) Mn 2p spectra and (b) Mn 3s
spectra.
3.2. Influence of varying reaction temperature and time on the
formation of the thin film and the mechanism of its formation
octahedra, and TMAOH serves to facilitate the two-dimensional
growth of the resulting MnO2 crystal seeds [29]. Ni foils are
invariably coated with a thin layer of nickel oxide once a fresh
nickel substrate is exposed to air. This oxide layer will become
covered with a large number of hydroxyl groups when immersed
in the above reaction solution (pH ca. 13) since its isoelectric point
has a pH value of 8.5–10. These hydroxyl groups on the substrate
may act as “hunters” through hydrogen bonding as reported in the
A better understanding of the mechanism of formation of the
MnO2 nanosheet-based thin film would be useful in terms of
extending the method to the fabrication of other metal oxide thin
films. A schematic illustration of the formation process of the
thin film is shown in Scheme 1. In solution, H O oxidizes MnCl2
2
2
to form MnO2 crystal seeds consisting of edge-shared {MnO6}
Scheme 1. Schematic illustration of the formation process of the MnO2 nanosheet-based thin film.

1
18
X. Wang et al. / Electrochimica Acta 78 (2012) 115–121
◦
◦
◦
◦
Fig. 3. FESEM images of MnO2 thin films prepared at different reaction temperatures for 12 h: (a) 80 C, (b) 120 C, (c) 140 C, and (d) 160 C.
literature [33,34] to capture the originally formed MnO2 crystal
seeds. At elevated temperatures, these crystal seeds initiate the
subsequent growth of the nanosheet-based thin film on the Ni
substrate.
We investigated the influence of varying the reaction tem-
perature and time on the formation of the thin film. As shown
in Fig. 3, after reaction for 12 h at a relatively low temperature
◦
of 80 C, it was found that only few thin sheets were adsorbed
◦
Fig. 4. FESEM images of MnO2 thin films reacted at 140 C for different hydrothermal reaction times: (a) 30 min, (b) 1 h, (c) 2 h, and (d) 4 h.

X. Wang et al. / Electrochimica Acta 78 (2012) 115–121
119
on the Ni substrate (Fig. 3a). After reaction for 12 h at a higher
◦
temperature of 120 C, a large number of thin sheets became
adsorbed on the Ni substrate (Fig. 3b) with most of them stacking
parallel to the substrate. This is probably a result of the high surface
energy of the individual nanosheets formed at this relatively low
temperature, which favors the formation of a densely stacked thin
film in order to decrease the overall surface energy. However,
◦
when the reaction was carried out for 12 h at 140 C (Fig. 3c), the
desired open framework nanosheet-based thin film was obtained
on the substrate. This can be attributed to the reduced surface
energy of the nanosheets formed at a relatively high temperature.
When the reaction was carried out for 12 h at an even higher
◦
temperature of 160 C, much thicker nanosheets were obtained
(
Fig. 3d).
To further investigate the formation process of the MnO2 thin
◦
films under hydrothermal conditions at 140 C, we used different
reaction times of 30 min, 1 h, 2 h and 4 h. Although the reaction
did not stop immediately after the autoclave was removed from
the oven [38], we believe that the thin films obtained represent
distinct stages in the formation process. The different products
were examined by FESEM as shown in Fig. 4. In the initial stage
of the hydrothermal reaction, a large number of small nanosheets
were deposited on the Ni substrate (Fig. 4a); as the reaction con-
tinued, the deposited nanosheets grew in size (Figs. 4b–d) with
some small nanosheets redissolving into the solution phase and
the larger ones growing into much larger ones in an Ostwald ripen-
ing process [39]. This repeated dissolution and crystal growth on
the Ni substrate resulted in the in situ fabrication of the MnO2 thin
film.
3.3. Supercapacitor properties of MnO2 nanosheet-based
thin-film electrodes
CV and galvanostatic charge–discharge tests were employed
to investigate the electrochemical properties of the as-prepared
MnO2 nanosheet-based thin-film electrodes. Fig. 5a shows the CV
curves of an as-prepared MnO2 nanosheet-based thin-film elec-
Fig. 5. (a) CV curves of the as-prepared MnO2 nanosheet-based thin-film electrode
and Ni substrate within the potential window–0.1 to 0.8 V (vs. SCE) at a scan rate
−1
of 100 mV s , (b) CV curves of the as-prepared MnO2 nanosheet-based thin-film
electrode within the potential window −0.1 to 0.8 V (vs. SCE) at different scan rates:
−
1
−1
trode and a pure nickel foil electrode at a scan rate of 100 mV s
in 1 M Na SO solution in the potential range between −0.1 and
50, 150, 400, 600, 800, 1000 mV s
.
2
4
0.8 V (vs. SCE). The as-prepared MnO2 nanosheet-based thin-film
performance [40]. The specific capacitance can be calculated from
the charge–discharge curves using Eq. (1):
electrode shows a near-ideal rectangular profile with respect to the
zero-current line, indicating a perfect capacitive behavior; further-
more the contribution of the nickel substrate to the capacitance
is negligible since the nickel foil electrode shows a negligible
area within its CV curve compared with that of the as-prepared
MnO2 thin-film electrode. Fig. 5b shows CV curves of the as-
prepared MnO2 nanosheet-based thin-film electrode measured at
I × ꢀt
C =
(1)
m × ꢀV
where I is the current (A), ꢀt is the discharge time (s), ꢀV is the
potential change during the discharge process (V), and m is the
mass of active material in the electrode (g). As shown in Fig. 6b,
−
1
−1
various scan rates from 50 to 1000 mV s
tion. All the curves reveal typical rectangle-like shapes even up
in 1 M Na SO4 solu-
the specific capacitances are 385, 347, 325 and 312 F
at current
2
−
1
densities of 0.5, 1.25, 2.5 and 5 A g , respectively. The capacitance
retention was about 81% when the current density was increased
from 0.5 to 5 A g , demonstrating the good rate capacitance of the
as-prepared thin-film electrode.
−
1
to a scan rate of 1000 mV s , indicating the good rate proper-
−
1
ties of the as-prepared MnO nanosheet-based thin-film electrode.
2
It should be stressed that in this work, the as-prepared thin-film
electrode was constructed by direct growth of the MnO2 active
material on the conductive Ni substrate, and no binder or any
other ancillary materials, such as carbon black or super P were
added.
The electrochemical stability of the as-prepared nanosheet-
based thin-film electrode was examined by cycling tests at a current
−
1
density of 1.25 A g . The cycle performance is shown in Fig. 7a.
After 5000 consecutive cycles, the discharge capacitance retained
around 93% of its initial value. The porous network structure of the
thin film and the nanosheet morphology were also retained after
5000 cycles, as shown in Fig. 7b. These results indicate that the
thin-film electrode has a good long-term electrochemical stability
at a relatively high current density. To further test the efficacy of
the thin-film electrode, the cycling performance of the as-prepared
MnO2 nanosheet-based thin-film electrode was recorded at pro-
gressively increasing current densities, as shown in Fig. 8. The
electrode exhibited stable capacitance at each current density, even
when subjected to a sudden change in current density. After 2000
Galvanostatic charge–discharge measurements are
a reli-
able way to characterize the electrochemical capacitance of
materials under controlled current densities. Fig. 6a shows the
charge–discharge profiles of as-prepared MnO2 nanosheet-
based thin-film electrode at various current densities from 0.5
−
1
to 5.0 A g . The discharge curves and the corresponding charge
curves are almost symmetrical over the whole potential region, and
the slopes of curves are independent of the potential without any
obvious internal resistance (IR) drop during the charge–discharge
tests, indicating high coulombic efficiency and ideal capacitor

1
20
X. Wang et al. / Electrochimica Acta 78 (2012) 115–121
Fig. 6. (a) Charge–discharge curves of the as-prepared MnO2 nanosheet-based thin-
film electrode at different current densities within the potential window −0.1 to
Fig. 7. (a) Charge–discharge cycles of the as-prepared MnO nanosheet-based thin-
2
0
.8 V (vs. SCE), (b) plot of the variation in the specific capacitance as a function of
film electrode at a current density of 1.25 A g−1 within the potential window −0.1
discharge current density.
to 0.8 V (vs. SCE) and (b) FESEM image of the MnO nanosheet-based thin film after
2
5000 charge–discharge cycles.
continuous cycles at varying current densities, the current was
−1
returned to 0.5 A g , after which about 95.5% of the initial capac-
−1
itance (385 F g ) could be recovered and maintained for another
000 cycles without any noticeable decrease. This result again high-
1
lights the excellent high rate capability and long cycle lifetime of
the as-prepared MnO2 nanosheet-based thin-film electrode.
The good performance of the as-prepared MnO2 nanosheet-
based thin-film electrode can, we believe, be attributed to its
thin-sheet morphology, porous structure and the good contact
between the MnO active material and the Ni substrate current col-
2
lector. The good rate properties result from the following features
of the thin film: (1) the MnO2 nanosheets have a small thickness
of around 10 nm, which may shorten the diffusion path length for
ion diffusion into and out of the MnO2 active material [24]; (2) the
porous space between the interlaced nanosheets facilitates the dif-
fusion of electrolytes; (3) the as-prepared MnO2 thin film exhibits
highly hydrophilic properties and the high wettability of this thin-
film facilitates intimate contact of the active material with the
electrolyte; (4) the MnO2 nanosheets are formed by in situ growth
on the Ni substrate and thus can adhere well to the substrate and
facilitate the electrical transport at the interface between the active
material and the current collector. The good cycling stability of the
as-prepared thin-film electrode may be ascribed to: (1) the good
contact between the MnO2 active material and the Ni substrate
resulting from the in situ growth process; (2) the stable interlaced
structure of the MnO2 thin film, which is resistant to collapse or
aggregation of nanomaterials on the substrate. Our electrode fabri-
cation process is simplified by avoiding the various steps involved
in the conventional fabrication of binder-enriched electrodes [41],
Fig. 8. Cycling stability of the as-prepared MnO2 nanosheet-based thin-film elec-
trode at progressively varied current densities.
and may have potential applications in the fabrication of other
nanosheet-based thin-film electrodes for use in energy storage and
conversion devices.
4. Conclusions
A MnO2 thin-film electrode with an open framework consist-
ing of nanosheets has been directly grown in situ on a Ni substrate
using a hydrothermal approach. The as-prepared MnO nanosheet-
2
based thin-film electrode displays good capacitive performance

X. Wang et al. / Electrochimica Acta 78 (2012) 115–121
121
with high rate capability and excellent cycling stability without
the need for any other ancillary materials. The excellent perfor-
mance can be attributed to the favorable microstructure of the
as-prepared thin-film electrode and the good contact between the
MnO2 active material and the Ni substrate. Considering the good
performance and facile preparation process, the nanosheet-based
thin-film electrode shows considerable potential for applications
in the field of energy storage and conversion, and the fabrication
approach should have wide applicability in the fabrication of other
metal oxide thin-film electrodes.
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