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Room temperature synthesis of
cobalt-manganese-nickel oxalates
micropolyhedrons for high-performance
flexible electrochemical energy storage
device
Yi-Zhou Zhang
1
, Junhong Zhao
2
, Jing Xia
2
, Lulu Wang
2
, Wen-Yong Lai
1
, Huan Pang
1,2,3
& Wei Huang
1
1
Key Laboratory for Organic Electronics & Information Displays (KLOEID), Institute of Advanced Materials (IAM), National Jiangsu
Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts and Telecommunications (NUPT),
Nanjing, 210023, China,
2
Key Laboratory for Clearer Energy and Functional Materials of Henan Province, College of Chemistry
and Chemical Engineering, Anyang Normal University, Anyang, 455000, China,
3
State Key Laboratory of Coordination
Chemistry, Nanjing University, Nanjing, 210093, China.
Cobalt-manganese-nickel oxalates micropolyhedrons were successfully fabricated by a room temperature
chemical co-precipitation method. Interestingly, the Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O micropolyhedrons and
graphene nanosheets have been successfully applied as the positive and negative electrode materials (a
battery type Faradaic electrode and a capacitive electrode, respectively) for flexible solid-state asymmetric
supercapacitors. More importantly, the as-assembled device achieved a maximum energy density of
0.46 mWh?cm
23
, a decent result among devices with similar structures. The as-assembled device showed
good flexibility, functioning well under both normal and bent conditions (06–1806). The resulting device
showed little performance decay even after 6000 cycles, which rendered the Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O//
Graphene device configuration a promising candidate for high-performance flexible solid-state asymmetric
supercapacitors in the field of high-energy-density energy storage devices.
High-performance energy conversion-storage devices have been receiving a lot of research attention
recently
1–5
. Electrochemical capacitors, also known as supercapacitors (SCs), are widely regarded as
suitable candidates for the next generation power source owing to their high power density, high stability
and low fabrication cost
6,7
. SCs are widely applied in many fields such as emergency power supplies, electronic
devices, and hybrid-electric machines. However, the energy density of SCs are generally not high enough to be
used as sustainable power sources for many applications. Much effort has been devoted to improving the energy
density of SCs. One promising strategy is to design nanostructured electrode materials with a large amount of
active sites and high structural stability. Various micro/nanomaterials including nanoparticles
8
, nanowires
9,10
,
nanotubes
11
and nanosheets
12,13
have been successfully applied as SC electrodes with high energy density. On the
other hand, solid-state asymmetric supercapacitors (SASCs) comprising a battery type Faradaic electrode as an
energy source and a capacitive electrode as a power source are promising alternatives to conventional SCs which
are symmetrical using liquid electrolytes
14–18
. SASCs using solid-state electrolytes can avoid the potential problem
of electrolyte leakage and are more environmentally friendly. What’s more, compared with conventional SCs,
SASCs have other advantages including light-weight, small-size, good reliability, ease of handing, and a wider of
operating temperature range
19,20
, making SASCs suitable for wearable and flexible electronics. As an example, Lu
et al have successfully reported a SASC based on MnO
2
//Fe
2
O
3
nanowires
21
. However, it still remains a great
challenge to fabricate high-performance SASCs.
Transition metal oxalate micro/nano materials have been recently explored as precursors for the synthesis of
porous transition metal oxides (NiO
22,23
,Mn
2
O
3
22
, CeO
2
24
and so on) and porous mixed transition metal oxides
(NiMn
2
O
4
25
,Ni
0.3
Co
2.7
O
4
26,27
, ZnO-NiO
28
, Co-Ni-Mn oxide nanowires
29
and so on). As a matter of fact, however,
metal oxalates themselves can be used as electrode materials. Compared with the synthesis of traditional metal
OPEN
SUBJECT AREAS:
ELECTRONIC MATERIALS
BATTERIES
Received
28 October 2014
Accepted
23 January 2015
Published
23 February 2015
Correspondence and
requests for materials
should be addressed to
W.-Y.L. (iamwylai@
njupt.edu.cn); H.P.
(huanpangchem@
hotmail.com) or W.H.
(iamwhuang@njupt.
edu.cn)
SCIENTIFIC REPORTS | 5 : 8536 | DOI: 10.1038/srep08536 1
oxides, the synthesis of the metal oxalates generally involves only
simple synthetic methods in aqueous solution (usually, a simple
and scalable co-precipitation method under room temperature) thus
the materials are obtained with low cost, environmental friendliness
and safety. Micro/nanostructural nickel oxalates are an interesting
class and have been successfully applied to make electrodes for elec-
trochemical capacitors
30
and asymmetric supercapacitors
31
.
In this work, we have successfully synthesized uniform cobalt-
manganese-nickel oxalates (Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O) micropo-
lyhedrons by a room temperature chemical co-precipitation method.
The synthesis method is green with low energy consumption. More
importantly, Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O micropolyhedrons can be
successfully used as the positive electrode materials for SASCs (gra-
phene nanosheets as the negative electrode material). A maximum
energy density of 0.46 mWh?cm
23
was obtained easily by the as-
assembled SASC, which was higher than most of previous results.
The good flexiblity of the as-assembled SASC device enabled it to
work under both the normal and the bent condition (0u–180u). An
efficiency cycle ability was found after 6000 cycles, which made
Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O//Graphene SASC a promising candid-
ate for high-performance flexible SASC in the field of high-energy-
density energy storage devices.
Results
Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O micropolyhedrons were obtained by
a room temperature chemical co-precipitation condition (See experi-
mental section). The XRD patterns of the as-prepared samples are
shown in Figure 1. In view of the single oxalate phases (JCPDS#01-
0296-cobalt oxalate hydrate, JCPDS#01-0283-manganese oxalate
hydrate and JCPDS#01-0299-nickel oxalate hydrate), all the peaks
of the as-prepared product are from the coupling result of three
phases, which could not be indexed to a single oxalate. It indirectly
indicates that the mixed oxalates have formed. Figure 1b,c show the
typical SEM images of the as-prepared sample, and the uniform
micropolyhedron with 10 mm was the main product. To identify
the correct element ratio of the as-prepared sample, EDS-Mapping
has been measured (Figure 1e–g). The contrast of light-shade
intensity is related with the element content. Clearly, the order of
the element content is Co .Mn .Ni from the results of
Figure 1e–g. And the detailed quantitative calculation is also shown
in Figure S1: C 12.0%, O 54.2%, Mn 12.1%, Co 16.2%, Ni 3.2%. The
three metal element atom ratio is Co5Mn5Ni 555451, which
is highly consistent with the raw material ratio. Additionally, Co,
Mn and Ni contents were analyzed by ICP-OES (PE-3300DV) after
the sample was dissolved. The ICP result confirmed the three
element atom ratio was Co5Mn5Ni 555451, consistent with the
EDS result.
Discussion
The electrochemical property of the as-prepared Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O electrode was first studied in the three electrode con-
figuration by Cyclic Voltammetry (CV) and Chronopotentiometry
(CP) measurements (Figure 2). From Figure 2a and Figure S2, unlike
the shape of electric double-layer capacitance, CV curves suggested
that the electrochemical capacity was mainly pseudocapacitive. What’s
more, the Faradaic pseudocapacitive characteristics may result from
the redox mechanism of the surface metal ion. A reversible redox
reaction was proposed to occur on the as-prepared electrode:
Figure 1
|
(a) XRD patterns of as-prepared samples, JCPDS#01-0296-cobalt oxalate hydrate, JCPDS#01-0283-manganese oxalate hydrate and
JCPDS#01-0299-nickel oxalate hydrate; (b–d) SEM images, and (e–g) EDS-mapping images for different elements.
www.nature.com/scientificreports
SCIENTIFIC REPORTS | 5 : 8536 | DOI: 10.1038/srep08536 2
MIIðÞC2O4nH2OzOH{
soluM IIIðÞ{(OH{)C2O4nH2Oze{
ð1Þ
CP curves of the as-prepared electrode with different current
densities are shown in Figure 2b. Clearly, the discharge time
decreased with the increase of current density. However, it exhibited
rather high electrode polarization even at low current densities as
shown in Figure 2b, we thus supposed that irreversible reactions
happened. The specific capacitances calculated from the discharging
curves with different current densities are shown in Figure 2c. The
specific capacitances achieved 990 F?g
21
at 0.6 A?g
21
, and 600 F?g
21
at even 4.0 A?g
21
in 3.0 M KOH solution. Interestingly, the as-pre-
pared electrode exhibited stable cycling performance with a specific
capacitance of 968 F?g
21
even after 6000 cycles (Figure 2d). Clearly,
the as-prepared electrode showed no capacity decay after 1000 cycles
in the inset of Figure 2d, further confirming the pragmatic value of
the Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O electrode.
The graphene electrode in three-electrode system have been eval-
uated. As depicted in Figure S3, the graphene electrode shows a good
specific capacitance (277 F g
21
, 0.6 A g
21
) which also offers a good
cycle life (88.8%, Retention 10000 cycle).
We have successfully assembled a SASC device using Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O micropolyhedrons and graphene nanosheets. To
measure the electrochemical property of the as-prepared device,
CV and CP measurements were tested carefully. Unlike the three-
electrode electrochemical feature, the SASC device displayed a quasi-
rectangular CV geometry with feeble redox peaks, indicating a
combination of both pseudocapacitive and electric double-layer
capacitor properties at all scan rates in Figure 3a. Moreover, galva-
nostatic charge-discharge curves of the as-prepared device with dif-
ferent current densities (0.50–4.0 mA?cm
22
) are shown in Figure 3b.
The good symmetry of curves showed excellent reversibility of the
as-prepared device. The linear sloping of galvanostatic charge-dis-
charge curves is not only characteristic of EDLC, but also of the
combination the pseudocapacitive behavior with EDLC behavior.
In other word, the electrochemical property of the SASC device
results from the combination of a battery type Faradaic electrode
and a capacitive electrode. The largest specific capacitance of the as-
prepared device can reach up to 86.3 mF?cm
22
at a current density
of 0.50 mA?cm
22
,55mF?cm
22
at 4.0 mA?cm
22
in Figure 3c. In
order to test the flexibility of the as-prepared device, the SASC was
bended with different angles (0u,30u,90u, and 180u, Inset-corres-
ponding optical images), while corresponding CV tests were carried
out (Figure 3d). Remarkably, throughout the bending processes, the
shape of CV curve was nearly unchanged, suggesting good flexibility
of the device. In fact, we have measured the performance of as-
prepared device after 400 bending times as depicted in Figure S4.
The as-prepared device showed only 0.1% performance decay after
400 bendings. The SASC maintained 98.6% of the initial specific
capacitance after 2000 cycles and at least 98.6% after 6000 cycles
as shown in Figure 3e. The stable cycling performance is much better
than most previous results, such as PANI/CNT//PANI/CNT (88.6%
after 1000 cycles)
17
,MnO
2
NW/graphene//graphene (79% after 1000
cycles)
32
, and RuO
2
-graphene//graphene (95% after 2000 cycles)
33
.In
Figure 2
|
In a three-electrode system in 3.0 M KOH solution. (a) CV curves with different scan speeds; (b) CP curves with different current densities; (c)
Specific capacitance calculated based on the discharge curve from (b), and (d) Cycling life test at 0.6 A?g
21
.
www.nature.com/scientificreports
SCIENTIFIC REPORTS | 5 : 8536 | DOI: 10.1038/srep08536 3
addition, the electrochemical impedance spectroscopy (EIS) analysis
before and after the cycling are compared in Figure S5. From the EIS
result in Figure S5, the resistance only changed slightly, which fur-
ther comfirms the stability of the electrochemical performance.
The volumetric energy and power densities of the as-prepared
flexible SASC calculated based on the data in Figure 3b are shown
in Figure 4. For comparasion, the volumetric power and energy
densities of other energy storage devices are also plotted. The as-
fabricated flexible SASC possessed a maximum volumetric energy
density of 0.46 mW?h?cm
23
at 0.5 mA?cm
22
, and 0.29 mW?h?cm
23
at 4.0 mA?cm
22
, also showing good rate performance of the flexible
SASC device. Moreover, the maximum volumetric energy density
of the as-prepared flexible SASC was considerably higher in com-
parison with those of recently reported devices
9a,18,21,34–37,39,40
,suchas
TiO
2
@MnO
2
//TiO
2
@C (0.5 mA?cm
22
–0.30 mW?h?cm
23
)
18
,MnO
2
//
Fe
2
O
3
(0.5 mA?cm
22
–0.41 mW?h?cm
23
)
21
and ZnO@MnO//Graphene
(0.5 mA?cm
22
–0.234 mW?h?cm
23
)
39
.However,theobtainedmaxi-
mum volumetric energy density was lower than those from ref. 9b,
15, 38, 41 and 42. Additionally, the SASC device can offer a maximum
power density of 46 mW?cm
23
at 4.0 mA?cm
22
, which is much higher
than that of recently reported ZnO@MnO
2
37
, polyaniline//WO
x
@
MoO
x
38
,andZnO@MnO
2
//Graphene
39
, and NiO//C
40
,butlowerthan
Figure 3
|
(a) Cyclic voltammetry of the as-prepared SASC device with different scan rate from 5 to 100 mV s
21
; (b) The galvanostatic charge-discharge
curves with different current densities, 0.50–4.0 mA?cm
22
; (c) Corresponding specific capacitance calculated by discharge curves in (b); (d) CV curves at
a scan rate 30 mV s
21
with different bended degrees (0u, 30, 90uand 180u, Inset-corresponding optical images), and (e) Cycle life testing at 0.5 mA?cm
22
for 6000 cycles, and 2000 cycles-Inset.
www.nature.com/scientificreports
SCIENTIFIC REPORTS | 5 : 8536 | DOI: 10.1038/srep08536 4
that of other devices
9b,15,18,21,34–36,41,42
. The results above confirmed that
the Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O micropolyhedron is a promising
anode material for SASCs. To demonstrate the potential application
of the as-prepared ASC device, an ASC device was employed to power
a red light-emitting-diode (LED) as shown in the inset of Figure 4. The
ASC device can power a red LED (1.5 V) for about 2 min after char-
ging at 0.5 mA cm
22
for 30 s.
We attribute the excellent electrochemical energy storage behavior
to the desirable synergy of composition and nanostructure of as-
prepared materials. Specifically, the primary nanopores (Figure S6)
provide high electrochemical activity and relatively high active sur-
face area (89 m
2
g
21
), while the secondary micropolyhedrons in
micrometer dimensions prevent the undesirable agglomeration
and ensure the stability of the porous structure. It is worth mention-
ing that the abundant mesopores are of great significance for the
electrochemical processes.
In summary, Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O micropolyhedron has
been successfully synthesized via a room temperature chemical co-
precipitation method. More importantly, a flexible SASC device has
been successfully constructed with using the resulting Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O micropolyhedrons and graphene nanosheets. The
assembled Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O//graphene SASC achieved a
maximum energy density of 0.46 mWh?cm
23
, which was higher than
most of the reported solid state SCs. The resulting SASC exhibited
excellent efficiency cycle stability for 6000 cycles, which rendered it
as one of the top high-performance flexible solid-state asymmetric
supercapacitors. Other applications are anticipated by fully exploiting
the advantages of flexibility and high energy density originated from
both the materials architecture and the novel design of the device.
Further work is undergoing in our lab to improve the device perform-
ance and to extend the philosophy in this work into other systems
within the framework of flexible/stretchable energy devices.
Methods
Synthesis of the Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O micropolyhedron.All of the chemical
reagents were of analytical grade and used without further purification. In a typical
preparation, 10.0 mL 0.10 M Co(CH
3
COO)
2
-ethylene glycol solution, 8.0 mL
0.10 M Mn(CH
3
COO)
2
-ethylene glycol solution and 2.0 mL 0.10 M Ni(CH
3
COO)
2
-
ethylene glycol solution were mixed and stirred for 10 min. Then 40.0 mL 0.10 M
(NH
4
)
2
C
2
O
4
-H
2
O solution was added into the above solution, and then mixed
vigorously for 2 h, and the resulting mixture was incubated at room temperature for
1 h. The resulting pink precipitate was collected by centrifugation, washed with water,
ethanol several times and finally dried in air.
Preparation of graphite oxide.GO was produced from natural graphite powders
(universal grade, 99.985%) according to Hummers method. Firstly, natural graphite
powders were treated by 5% HCl twice, then filtered, washed with distilled water
thoroughly, and dried at 110uC for 24 h. Secondly, graphite powders (10 g) were
placed in cold (0uC) concentrated H
2
SO
4
(230 mL). KMnO
4
(30 g) was added
gradually with stirring and cooling. The temperature of the solution was not allowed
to go up to 20uC. The mixture was stirred for 40 min, and distilled water (460 mL)
was added slowly to an increase in temperature to 98uC. The temperature was held at
35 63uC for 30 min. Finally, distilled water (1.4 L) and 30% H
2
O
2
solution (100 mL)
were added after the reaction. The solution was kept at room temperature for 24 h
and then the mixture was filtered, washed with 5% HCl aqueous solution until sulfate
could not be detected with BaCl
2
. The reaction product was dried under vacuum at
50uC for 24 h.
Preparation of functionalized graphene sheets.The dried GO was thermally
exfoliated at 300uC for 5 min under air atmosphere. The obtained samples were
subsequently treated at 700uC in Ar for 3 h with a heating rate of 2uC/min.
Characterizations.The morphology of as-prepared samples was observed by a JEOL
JSM-6701F field-emission scanning electron microscope (FE-SEM) at an acceleration
voltage of 5.0 kV. The phase analyses of the samples were performed by X-ray
diffraction (XRD) on a Rigaku-Ultima III with Cu K
a
radiation (l51.5418 A
˚). Mn,
Co and Ni contents were analyzed by ICP-OES (PE-3300DV) after the sample was
dissolved.
Fabrication and electrochemical study on the Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O
micropolyhedron electrode in a conventional three-electrode system.All
electrochemical performances were carried out on Arbin-BT6000 electrochemical
instrument in a conventional three-electrode system equipped with platinum
electrode, a Hg-HgO as counter and reference electrode, respectively. Before
electrochemical measurement, we have purged out O
2
from the solution by the inert
gas-Ar. The working electrode was made from mixing of active materials-the
Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O micropolyhedron electrode, acetylene black, and PTFE
(polytetrafluoroethylene) with a weight ratio of 8051555, coating on a piece of nickel
foam of about 1 cm
2
, and pressing it to a thin foil at a pressure of 5.0 MPa. The typical
mass load of electrode material was 5.0 mg. The electrolyte was 3.0 M KOH solution.
Galvanostatic charge–discharge methods were used to investigate capacitive
properties of the Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O micropolyhedron electrode, which
were all carried out with an Arbin-BT6000 electrochemical instrument. Cyclic
voltammetry measurements of the Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O micropolyhedron
electrode was conducted by using PARSTAT2273.
Fabrication and electrochemical study of the flexible
Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O//graphene SASC.The PET substrates were first
deposited with a layer of Pt film (,335 nm thick) and then coated with the slurry
containing the active materials (Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O micropolyhedrons or
Graphene nanosheets) via a similar process to that in the three electrode system and
were used as the working electrode after drying. Caution: Graphene nanosheets
electrode was recoated for 6 times with the above graphene slurry. The ratio of the
mass of the positive electrode to that of the negative electrode is 1518. In the
meantime, the PVA/KOH gel electrolyte was prepared as follows: the gel electrolyte
(1.52 g PVA, 2.13 g KOH, and 15 mL DI water) was prepared at 75uC for 30 min
under vigorous stirring. Subsequently, two pieces of such electrodes were immersed in
the PVA/KOH gel solution for 5 ,10 min to adsorb a layer of solid electrolyte. After
the excess water was vaporized, two pieces of such electrodes containing electrolyte
were pressed together on a sheet out roller. Thus, the stacked SASC was fabricated.
CV measurements were carried out at 5, 10, 20, 30, 50 and 100 mV?s
21
on an
electrochemical work station (PARSTAT-2273). The flexible
Co
0.5
Mn
0.4
Ni
0.1
C
2
O
4
*nH
2
O//Graphene SASC was galvano statically charged and
discharged at the current density of 0.5–4.0 mA?cm
22
on the Arbin-BT6000
electrochemical instrument. All the electrochemical measurements were conducted
at room temperature.
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Figure 4
|
Ragone plots of the as-prepared SASC device. The values
reported for other previous devices are added for comparison
9,15,18,21,34–41
.
Inset shows a red LED (1.5 V) powered by the as-prepared SASC.
www.nature.com/scientificreports
SCIENTIFIC REPORTS | 5 : 8536 | DOI: 10.1038/srep08536 5
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Acknowledgments
We acknowledge financial support from the National Key Basic Research Program of China
(973 Program, 2014CB648300), the Program for New Century Excellent Talents in
University (grant no. NCET-13-0645, NCET-13-0872), the National Natural Science
Foundation of China (21201010, 21422402, 20904024, 51173081, 61136003, 61106036, and
U1304504), the Program for Innovative Research Team (in Science and Technology) in
University of Henan Province (14IRTSTHN004), the Science & Technology Foundatio n of
Henan Province (122102210253, 13A150019, 14B150001 and 14A430038), the Natural
Science Foundation of Jiangsu Province (BM2012010, BK20140060, BK20130037),
Specialized Research Fund for the Doctoral Program of Higher Education
(20133223110008), the Program for Graduate Students Research and Innovation of Jiangsu
Province (CXZZ12-0454), the China Postdoctoral Science Foundation (2012M521115), the
Ministry of Education of China (IRT1148), the Priority Academic Program Development of
Jiangsu Higher Education Institutions (PAPD), the Six Talent Plan (2012XCL035) and
Qing Lan Project of Jiangsu Province and the Opening Research Foundations of the State
Key Laboratory of Coordination Chemistry (Nanjing University).
Author contributions
Y.Z.Z., J.H.Z., J.X., L.L.W., H.G.F. and H.P. conceived and designed the experiments. Y.Z.Z.,
W.Y.L., H.P. and W.H. analyzed the measurements. H.P. wrote the manuscript in
collaboration with all the authors.
Additional information
Supplementary information accompanies this paper at http://www.nature.com/
scientificreports
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Zhang, Y.-Z. et al. Room temperature synthesis of
cobalt-manganese-nickel oxalates micropolyhedrons for high-performance flexible
electrochemical energy storage device. Sci. Rep. 5, 8536; DOI:10.1038/srep08536 (2015).
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SCIENTIFIC REPORTS | 5 : 8536 | DOI: 10.1038/srep08536 6
