Effect of Manganese Valence on Specific Capacitance in Supercapacitors of Manganese Oxide Microspheres

Article abstract of DOI:10.1002/chem.202100113

Manganese oxides have attracted great interest in electrochemical energy storage due to high theoretical specific capacitance and abundant valence states. The multiple valence states in the redox reactions are beneficial for enhancing the electrochemical properties. Herein, three manganese microspheres were prepared by a one-pot hydrothermal method and subsequent calcination at different temperatures using carbon spheres as templates. The trivalent manganese of Mn₂O₃ exhibited multiple redox transitions of Mn³⁺/Mn²⁺ and Mn⁴⁺/Mn³⁺ during the intercalation/deintercalation of electrolyte ions. The possible redox reactions of Mn₂O₃ were proposed based on the cyclic voltammetry and differential pulse voltammogram results. Mn₂O₃ microsphere integrated the advantages of multiple redox couples and unique structure, demonstrating a high specific capacitance and long cycling stability. The symmetric Mn₂O₃//Mn₂O₃ device yielded a maximum energy density of 29.3 Wh kg^?1 at 250 W kg^?1.

Full text of DOI:10.1002/chem.202100113

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Chemistry—A European Journal
Effect of Manganese Valence on Specific Capacitance in
Supercapacitors of Manganese Oxide Microspheres
+
[a, b]
+ [a]
[c]
[b]
[a]
Xing Chen ,
Lei Li , Xiaoli Wang, Kun Xie, and Yuqiao Wang*
3
+
2+
4+
3+
Abstract: Manganese oxides have attracted great interest in
electrochemical energy storage due to high theoretical
specific capacitance and abundant valence states. The multi-
ple valence states in the redox reactions are beneficial for
enhancing the electrochemical properties. Herein, three
manganese microspheres were prepared by a one-pot hydro-
thermal method and subsequent calcination at different
temperatures using carbon spheres as templates. The
trivalent manganese of Mn O exhibited multiple redox
transitions of Mn /Mn and Mn /Mn during the inter-
calation/deintercalation of electrolyte ions. The possible redox
reactions of Mn O were proposed based on the cyclic
2
3
voltammetry and differential pulse voltammogram results.
Mn O microsphere integrated the advantages of multiple
redox couples and unique structure, demonstrating a high
specific capacitance and long cycling stability. The symmetric
Mn O //Mn O device yielded a maximum energy density of
2
3
2
3
2
À 1
3
À 1
29.3 Whkg at 250 Wkg .
2
3
Introduction
improving the electrochemical properties of manganese oxides
in neutral electrolyte.
Manganese oxides have attracted considerable interest as
electrode materials for supercapacitors due to their high
theoretical capacity, abundant raw materials and nontoxicity.
The manganese valence plays an important role in the
intercalation/deintercalation redox reactions. As is well known,
[1]
n+
(nÀ 1)+
the redox couple of Mn /Mn
can contribute most to the
In general, the manganese oxides store charges through the
valence state transition between Mn(IV) and Mn(III) by inter-
calation/deintercalation electrolyte ions into the electrode
total capacitance. For instance, Mn O /MnO -NG with mixed-
3 4 2
valence state cations was favorable for the formation of ion and
electron defects, leading to excellent electrochemical
[2]
[5]
materials. However, manganese oxides commonly exhibit
electrochemically inactive in neutral electrolyte, leading to low
properties. Besides, the core/shell a-Mn O @a-MnO exhibited
2
3
2+
2
3
+
4+
3+
more redox couples of Mn (Mn /Mn
instead of single manganese oxide. As an intermediate
valence, the trivalent manganese possesses two pair redox
and Mn /Mn )
[3]
[6]
specific capacitance. The main reason can be ascribed to the
intercalation/deintercalation of electrolyte ions only on the
surface without effective utilization of the Mn(IV)/Mn(III) redox
couple. This means that the intercalation/deintercalation reac-
tions only occur on the surface and the bulk of the electroactive
materials contributes little to the total capacitance. It is well
known that the neutral aqueous electrolyte is promising in
portable and wearable electronic devices due to their safeties
3
+
2+
4+
3+ [7]
couples of Mn /Mn and Mn /Mn
.
The reversible tran-
3
+
2+
4+
3+
sition of Mn /Mn and Mn /Mn redox couples originated
from the intercalation/deintercalation of electrolyte ions can
enhance the specific capacitances of manganese oxides.
Notably, the trivalent manganese in NiCo-LDH can facilitate the
2
+
4+
rapid and effective transition of Mn /Mn
redox couple,
[4]
[8]
and electrochemical stabilities. Therefore, it is critical to
further enhancing the redox reaction activity and reversibility.
Therefore, the trivalent manganese with two redox couples of
2
+
3+
3+
4+
Mn /Mn
and Mn /Mn
is expected to maximize the
degree of redox reactions, leading to superior specific capaci-
tance.
+
+
[
a] Dr. X. Chen, L. Li, Prof. Y. Wang
School of Chemistry and Chemical Engineering
Southeast University
The morphologies of the electroactive materials also play an
important role in the intercalation/deintercalation redox reac-
tions. A well-defined nano/microstructure with a large surface
area and a porous structure can expose more electroactive sites
and facilitate better electrolyte ions accessibility into the inner
surface of active materials. Recently, 3D hierarchical nano/
microspheres have gained ever-increasing interest as a special
type of nano/microstructures. The unique structures can
integrate the merits of nanostructures and microstructures,
providing short electron/ion diffusion channels, large specific
2
11189 Nanjing (P. R. China)
E-mail: yqwang@seu.edu.cn
+
[
b] Dr. X. Chen, Prof. K. Xie
Chongqing Key Laboratory of Water Environment Evolution and Pollution
Control in Three Gorges Reservoir
Chongqing Three Gorges University
4
04020 Wanzhou (P. R. China)
[
[
c] Dr. X. Wang
College of Optoelectronic Engineering
Chongqing University
4
00044 Chongqing (P. R. China)
[
9]
+
surface area and more exposed electrochemical sites. How-
ever, exploiting a facile method to prepare the hierarchical
manganese oxide nano/microsphered is still a challenge.
] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202100113
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In this work, we reported the preparation of MnO, Mn O
2
3
and Mn O microspheres by a one-pot hydrothermal method
3
4
and subsequent calcination at different temperatures
Scheme 1). The prepared hierarchical manganese oxide micro-
(
spheres exhibit a similar configuration and size. The unique
structures can provide facile diffusion of electrolyte ions, large
specific surface area and robust structural stability. For Mn O ,
2
3
3
+
2+
4+
3+
two redox couples of Mn /Mn
and Mn /Mn
were
involved in the reversible intercalation/deintercalation of so-
3
+
2+
dium cations. The multiple redox transitions of Mn /Mn and
4
+
3+
Mn /Mn can enhance the specific capacitance of Mn O . The
2
3
possible redox reaction equations of intercalation/deintercala-
tion of sodium cations into Mn O were proposed according to
2
3
the cyclic voltammetry and differential pulse voltammogram
results. Mn O with multiple redox couples, ordered mesopo-
2
3
rous structure and large specific surface area was expected to
demonstrate excellent electrochemical performance for super-
capacitors.
Results and Discussion
Morphology and structure characterization
The SEM and TEM images show that the carbon sphere
possesses a smooth surface with a diameter of about 1.74 μm
(
Figure 1a1 and b1). MnO microsphere consisted of tiny MnO
2
2
nanowires on the surface of the carbon sphere (Figure 1a2 and
b2). Subsequently, Mn O microsphere was obtained after
1
x
y
calcination of MnO at 400°C. Mn O microsphere maintains the
2
x
y
sphere-like structure, which is covered with thread-like nano-
wires (Figure 1a3 and 1b3). With increasing the calcination
temperature to 500°C, the morphology change was observed
Figure 1. SEM and TEM images of (a1, b1) carbon sphere, (a2, b2) MnO
2
, (a3,
b3) Mn O , (a4, b4) Mn O and (a5, b5) Mn O ; (c) Schematic diagram of the
x
y
2
3
3
4
2 x y 2 3 3 4
formation process of MnO , Mn O , Mn O and Mn O microspheres.
for Mn O sample (Figure 1a4 and Figure 1b4). Numerous
2
3
interlaced nanorods were stacked together to form a porous
microsphere. When the temperature was increased to 800°C,
numerous worm-like nanoparticles uniformly covered on the
surface to assemble Mn O microsphere (Figure a5 and b5).
3
4
MnO , Mn O , Mn O and Mn O microspheres exhibited a
2
x
y
2
3
3
4
similar size with an average diameter of 3.62 μm.
The XRD diffraction peaks of MnO at 12.3°, 17.7°, 28.4°,
2
3
7.3°, 41.8°, 49.5°, 56.1°, 59.7°, 65.1°, 68.8° and 72.6° are indexed
to the (110), (200), (310), (211), (301), (411), (600), (521), (002),
(
541) and (312) planes of the tetragonal MnO phase (JCPDS
2
No. 44–0141), respectively (Figure 2a). However, the diffraction
peaks of the carbon are absent, which is probably due to the
2 x y 2 3 3 4
Figure 2. XRD patterns of (a) MnO , (b) Mn O , (c) Mn O and (d) Mn O .
[10]
low crystallinity and mass loading of the carbon. For Mn Oy
x
(
4
Figure 2b), the diffraction peaks located at 23.1°, 33.0°, 38.4°,
Scheme 1. Schematic illustration of manganese oxide microspheres.
9.4°, 55.1° and 65.9° are assigned to the (211), (222), (400),
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2
+
(
431), (440) and (622) planes of the orthorhombic Mn O phase
described as follows: At the beginning of the reaction, Mn
2
3
(
3
JCPDS No. 41–1442), whereas the other peaks at 18.1°, 28.9°,
ions may be adsorbed on the surface of the carbon sphere.
2.4°, 36.3°, 44.5°, 60.0° and 64.6° can be assigned to the (101),
Then MnO crystal whiskers are formed and assembled on the
2
[12]
(
(
112), (103), (202), (220), (224) and (400) planes of Mn O phase
surface of the carbon sphere
. During the calcination
3
4
JCPDS No. 24-0734). These results confirm that Mn O is
processes, MnO2 microsphere will be reduced to different
x
y
[
13]
2À
composed of Mn O and Mn O phase. For Mn O (Figure 2c),
materials under different temperatures . Simultaneously, CO3
2
3
3
4
2
3
À
the diffraction peaks correspond to the standard orthorhombic
and HCO3 ions inside the carbon sphere start to decompose
and form loose structures. The thread-like Mn O microsphere is
Mn O3 (JCPDS No. 41-1442). The dominant diffraction peaks
2
x
y
detected in Figure 2d are ascribed to Mn O phase (JCPDS
obtained at a low calcination temperature of 400°C. As the
temperature is increased to 500°C, the thread-like nanowires
are transformed into interlaced nanorods, forming Mn O micro-
3
4
No. 24-0734). The crystallite sizes of manganese oxides were
estimated by the Scherrer equation from MnO (211), Mn O
2
2
3
2
3
[11]
(
222) and Mn O4 (211) reflections,
and the corresponding
sphere. When the temperature is increased to 800°C, the
interlaced nanorods are changed to worm-like nanoparticles.
Mn O microsphere is obtained.
3
values were approximately 14.9, 29.3 and 55.0 nm, respectively.
The HRTEM image presents the nanowire-structure on the
3
4
surface of MnO microsphere (Figure 3a) and the corresponding
selected area electron diffraction (SAED) pattern exhibits several
clear diffraction rings, indexing to the (310), (211), (301) and
As is well known, the specific surface area and pore sizes of
the electrodes play a crucial role in enhancing the electro-
2
chemical performance. N adsorption/desorption isotherms of
2
(
411) planes of the tetragonal phase MnO (JCPDS No. 44-0141)
Mn O and Mn O show a type III isotherm and the adsorption-
2
x
y
3
4
(
Figure 3b). Figure 3c shows the HRTEM image of Mn O with
desorption isotherms are nearly overlapped, suggesting no
evidence of mesopores (Figure 4a). MnO and Mn O exhibit a
x
y
distinct lattice fringes. The high-magnification TEM image of the
violet dashed circle shows a lattice spacing of 0.67 nm,
corresponding to the (011) plane of the cubic Mn O phase
2
2
3
type IV isotherm with a distinct hysteresis loop at the P/P range
0
[14]
of 0.4–1.0, indicating the existence of mesopores. The specific
surface area and total pore volumes of MnO , Mn O , Mn O and
2
3
(
Figure 3d). The inverse fast Fourier transform (IFFT) image of
the yellow dashed circle displays a lattice fringe spacing of
.24 nm, assigning to the (004) plane of the tetragonal Mn O
2
x
y
2
3
Mn O are presented in Table S1. Mn O possesses a higher
3
4
2
3
2
À 1
0
specific surface area (141.2 m g ) and a larger pore volume
(0.38 cm g ). Additionally, the BJH pore size distribution curves
of MnO and Mn O exhibit a major peak in the range of 10–
3
4
3
À 1
(Figure 3e). Figure 3f presents the nanorod-like structure of
Mn O . The high-magnification TEM image of the white dashed
2
3
2
2
3
square exhibits a lattice fringe spacing of 0.38 nm, correspond-
ing to the (211) plane of the cubic Mn O . The diffraction rings
40 nm (Figure 4b), verifying the existence of mesoporous
structures. Mn O and Mn O samples show no evidence of
2
3
x
y
3
4
of Mn O are ascribed to the (100), (102) and (103) planes of the
mesoporous structures. The large surface area, large pore
volume and mesoporous structure can expose more electro-
chemical active sites for charge storage and supply abundant
3
4
tetragonal Mn O (Figure 3h–i).
3
4
The possible reaction mechanisms lead to different mor-
phology of Mn-based oxides at different temperatures can be
Figure 3. TEM images of (a) MnO
2
, (c) Mn
x
O
y
, (f) Mn
2
O
3
and (h) Mn
3
O
4
. The selected area electron diffractions of (b) MnO
2
and (i) Mn
3
O
4
. The high-magnification
TEM images of (d) Mn and (g) Mn
x
O
y
2
O
3
. The IFFT image of (e) Mn O .
x y
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ments were conducted. The DPV curves show two oxidation
peaks and two reduction peaks at the amplitude of 50 mV
(
Figure 5b). Due to the existence of manganese (III) in Mn O ,
2
3
2+
3
+
the two pairs of redox couples can be ascribed to Mn /Mn
and Mn /Mn redox couples. The broad peaks at about 0.31
and 0.83 V can be attributed to the oxidation of Mn to Mn
and Mn to Mn , respectively. The peaks at 0.78 and 0.30 V
4
+
3+
[8]
2
+
3+
3
+
4+
4
+
3+
3+
can be ascribed to the reduction of Mn to Mn and Mn to
2
+
Mn , respectively. The DPV curves at different amplitudes were
also recorded and the corresponding oxidation and reduction
peaks still can be observed (Figure 5c). The DPV results
provided a convinced confirmation that two pairs of redox
couples were involved in the redox reactions. The probable
2
Figure 4. (a) N adsorption/desorption isotherms. (b) The plots of BJH pore
size distributions of manganese oxides.
[
16]
channels for ion and electron transfer, resulting in high specific
capacitance.
redox reaction equations of Mn O were proposed as follows:
2 3
(
(
1)
2)
Electrochemical measurements
The CV curves of MnO , Mn O and Mn O electrodes exhibited
2
x
y
3
4
a shape of quasi-rectangular, indicating the capacitive behavior
Figure 5a). This was in accordance with the previously reported
Mn O electrode possessed a larger integrated area of CV
2 3
(
curves, demonstrating a higher specific capacitance. Addition-
ally, the GCD curves of MnO , Mn O , Mn O and Mn O4
work that the intercalation/deintercalation redox reactions
commonly occurred on the surface of manganese oxides
2
x
y
2
3
3
electrodes exhibited a nearly symmetric triangular shape (Fig-
ure 5d), illustrating the excellent rate performance and reversi-
bility. Among the four samples, Mn O electrode delivered a
[15]
without no obvious redox peaks in Na SO electrolyte.
2
4
Particularly, a pair of redox peaks obviously appeared in the CV
curve of Mn O , while no redox peaks were observed in the CV
2
3
longer discharge time, manifesting a higher specific capaci-
tance, which was consistent with the CV results. The IR drops of
the four electrodes in Figure 5d are mainly due to the high
2
3
curves of MnO , Mn O and Mn O . The appearance of the redox
2
x
y
3
4
peaks is mainly due to the redox reactions. The redox peaks
originated from the reversible intercalation/deintercalation of
sodium cations into the interior of Mn O . To explore the
[17]
solution resistances
.
The CV and GCD curves of Mn O electrode were conducted
2
3
2
3
specific redox couples in the redox peaks, the DPV measure-
to further investigate the electrochemical properties. When the
À 1
Figure 5. (a) CV curves of manganese oxides at 50 mVs . (b) DPV curves of Mn
2
O
3
at an amplitude of 50 mV. (c) DPV curves of Mn
2
O
3
at different amplitudes.
À 1
(
d) GCD curves of manganese oxides at 1 A g ; (e) CV curves of Mn
2
O
3
electrode at various scan rates. (f) GCD curves of Mn
2
O
3
electrode at various current
densities.
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scan rate was increased, the oxidation peaks moved to a more
positive potential while the reduction peaks shifted to a more
negative potential due to the polarization of Mn O electrode
charge transfer resistance (R ). In the low-frequency region, the
ct
straight line represents the Warburg impedance, which is
related to the ion diffusion resistance from the electrolyte to
2
3
[20]
(Figure 5e). However, the quasi-rectangular shapes of the CV
the electrode interface. An equivalent circuit was adopted to
simulate the EIS data. The C and C represent the double-layer
curves were still maintained, demonstrating good electrochem-
ical reversibility and rapid redox reactions. The GCD curves of
Mn O electrode exhibited nearly symmetric triangular shapes
dl
F
capacitance and a Faradaic pseudocapacitor, respectively. The
fitted data were shown in Table S4. In comparison with the Rs
2
3
2
with the increase of current densities, illustrating the high
coulombic efficiency during the charge/discharge process (Fig-
ure 5f). With the increasing of the current density, the IR drop
values increase, which is mainly attributed to the inefficient
transmission between electrolyte ions and electrons during the
(2.77 Ωcm ) of MnO electrode, Mn O , Mn O and Mn O4
2
x
y
2
3
3
electrodes exhibited relatively smaller R values (1.84, 2.09 and
s
2
2.01 Ω·cm , respectively), illustrating the faster charge transfer
kinetics. Mn O electrode demonstrated a lower R value of
2
3
ct
2
2
0.22 Ωcm than those of MnO (0.32 Ω·cm ), Mn Oy
(0.66 Ωcm ) and Mn O (0.37 Ωcm ), indicating the efficient
3 4
2
x
[18]
2
2
charge/discharge process . The CV and GCD curves of MnO2,
Mn Oy and Mn O4 electrodes display similar electrochemical
charge transport. In the low-frequency region, Mn O electrode
x
3
2
3
characteristics to Mn O electrode (Figure S1), exhibiting superi-
possessed the lowest Warburg impedance of 0.0068
2
3
0
.5
À 2
or electrochemical performance. According to the calculated
specific capacitances, Mn O electrode presents higher specific
S·sec ·cm , implying the faster diffusion of the electrolyte
ions. The EIS results demonstrate that Mn O electrode has
2
3
2
3
capacitances than those of MnO , Mn O and Mn O electrodes
better electrical conductivity and faster ion diffusion kinetics.
The long-term cycling stability of the electrode is a critical
2
x
y
3
4
at the same current densities (Figure S2 and Table S2). The
specific capacitances of Mn O electrode are 209.1, 132.2, 126.4,
parameter for practical application. For MnO2 and Mn O4
2
3
3
À 1
1
2
23.6, 120.8, 113.0 and 108.0 Fg at 1, 2, 4, 6, 8, 10 and
electrodes, the initial increase of the specific capacitance in the
first 1800 and 1000 cycles may be due to the self-activation
process caused by the electrolyte ions diffusion (Figure 6b). The
specific capacitances of MnO and Mn O electrodes decreased
À 1
0 Ag , respectively. Mn O electrode obtained a high capaci-
2
3
tance retention of 51.6% when the current density was
À 1
increased from 1 to 20 A g , demonstrating the superior rate
2
3
4
capability. Mn O electrode also exhibited a comparable specific
capacitance to previously reported works (Table S3).
slightly in the subsequent 3200 and 4000 cycles. After
5000 charge/discharge cycles, the capacitance retentions of
MnO , Mn O and Mn O electrodes were 100%, 106.2% and
2
3
Basing on the above analyses, the enhanced specific
capacitance of Mn O electrode can be ascribed to the
2
x
y
3
4
102.2%, respectively. In contrast, Mn O electrode maintained a
2
3
2
3
following reasons: (1) Mn O microsphere provided abundant
electroactive sizes and efficient ion diffusion pathways due to
its high specific surface area and unique mesoporous structure.
capacitance retention of around 97.6%. It is noteworthy that
the specific capacitance of Mn O always maintains the
maximum value. The excellent cycling stabilities of the four
electrodes can be attributed to the unique three-dimensional
sphere-like structure, alleviating the volume expansion during
the charge-discharge process.
2
3
2
3
(
2) The intermediate valence state of manganese(III) exhibited
3
+
2+
4+
3+
two redox couples of Mn /Mn
and Mn /Mn
for the
reversible intercalation/deintercalation of sodium cations, re-
sulting in high specific capacitance.
The electrochemical kinetics of the obtained electrodes
were investigated by the electrochemical impedance spectro-
scopy (EIS) measurements. The Nyquist plots were presented in
Figure 6a. In the high-frequency region, the intercept on the
The relationships between the scan rate and current density
were used to investigate the reaction kinetics based on
[21]
equation (3):
(
3)
real axis is related to the solution resistance (R ), which is
s
composed of the intrinsic resistance of the active materials, the
ionic resistance of the electrolyte and the contact resistance
between the active materials and the current collector
Where a and b are constants. The value of b is the slope of
log(i) against log(v). Generally, when the b=1, the charge
storage process is capacitive-controlled. While the b=0.5, it
means that the charge storage process is dominated by
diffusion-controlled. The calculated b-values of the anodic and
cathodic peaks were 0.88 and 0.99, respectively, demonstrating
the capacitive-controlled behaviour during the charge-storage
process (Figure 7a).
[19]
interface.
The diameter of the semicircle represents the
The ratios of the capacitive-controlled and diffusion-con-
trolled processes to the total capacitance for Mn O , MnO and
2
3
2
9a]
[
Mn O electrodes were calculated based on equation (4):
3
4
(
4)
Where i(V) is the current at a certain potential of V, v is the
scan rate. k and k correspond to the capacitive-controlled and
diffusion-controlled processes, respectively. Figure 7b and Fig-
Figure 6. (a) Nyquist plots (insets: the high frequency area enlargement and
corresponding equivalent electrical circuit). (b) Cycling performance of
1
2
À 1
manganese oxides at 1 A g over 5000 cycles.
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capacitive-controlled process(Figure S5 and Figure S6b). Addi-
tionally, the capacitive contribution of Mn O , MnO and Mn O
4
2
3
2
3
electrodes were compared (Figure 7d). Mn O electrode demon-
2
3
strated a higher capacitive contribution than MnO and Mn O
4
2
3
electrodes. Consequently, these results further confirm that the
charge storage process of Mn O and Mn O is mainly
2
3
3
4
dominated by a capacitive-controlled process.
A symmetric supercapacitor was assembled using two slices
of Mn O electrodes. The CV curves were performed at
2
3
À 1
5
0 mVs under different potential windows to estimate the
optimum operating voltage of the fabricated symmetric super-
capacitor. The device exhibited a stable potential window up to
2
.0 V with a slight polarization phenomenon (Figure 8a). Herein,
we explored the electrochemical performance of the device at
the maximum operating voltage of 2.0 V. The CV curves at
various scan rates all exhibited a quasi-rectangular shape,
demonstrating the capacitive characteristic (Figure 8b).
2 3
Figure 7. (a) The plots of log(i) versus log(v) for Mn O electrode. (b) CV
curves of the capacitive-controlled and diffusion-controlled processes of
À 1
The GCD curves of the device at various current densities
Mn
2
O
3
electrode at 5 mVs . (c) The ratios of the capacitive-controlled and
À 2
ranging from 1 to 20 mAcm displayed a symmetric triangle
diffusion-controlled charge storage processes at different scan rates.
d) Capacitive contribution of MnO , Mn and Mn electrodes at different
scan rates.
(
2
2
O
3
3
O
4
shape (Figure 8c), illustrating the excellent reversibility of the
symmetric supercapacitor. The device exhibited a high specific
À 2
À 2
capacitance of 210.7 mFcm at 1 mAcm (Figure 8d). When
À 2
the current density inceased to 20 mAcm , the specific
capacitance can still reach to 14.3 mFcm . Meanwhile, the
À 2
ure S3 clearly depict the CV curves of capacitive-controlled and
diffusion-controlled processes. The contributions of the capaci-
tive-controlled process were 61%, 59%, 66%, 78%, 96% and
device showed good coulombic efficiencies at various current
densities of nearly 100%. The device exhibited a comparable
À 1
À 1
À 1
8
9% at scan rates of 5, 10, 20, 50, 100 and 200 mVs ,
energy density of 29.3 Whkg at 250 Wkg to previous
À 1
À 1 [22]
respectively (Figure 7c). Mn O electrode also exhibited a
reports, such as MnO -CNT//AC (25 Whkg at 500 Wkg ),
2
3
4
À 1
À 1 [23]
capacitive-controlled process with a large percentage range
MnO /CNTs//AC (13.3 Whkg at 600 Wkg ), MnO -CSs-600//
2 x
À 1
À 1 [24]
À 1
from 50% to 90% (Figure S4 and Figure S6a). MnO electrode
AC (27.5 Whkg at 225 Wkg ), MnO //AC (11.0 Whkg at
2
2
À 1 [25]
À 1
demonstrated a low capacitive contribution (less than 50%) at
50 Wkg ),
birnessite-type MnO //AC (17.1 Whkg
100 Wkg ). Furthermore, the device demonstrated an ex-
cellent cycling stability with a capacitance retention of nearly
at
2
À 1
À 1 [26]
the scan rates range from 5 to 50 mVs . When the scan rates
À 1
increased to 100 and 200 mVs , MnO electrode showed a
2
2 3 2 3
Figure 8. Electrochemical performance of the assembled Mn O //Mn O device: (a) CV curves of the device tested at different potential windows. (b) CV curves
of the device at different scan rates. (c) GCD curves of the device at various current densities. (d) Specific capacitances of the device at various current
À 2
densities. (e) Ragone plots of the device. (f) Cycling stability and coulombic efficiency at 6 mAcm for 5000 cycles (Inset: a red LED lightened by two devices
connected in series).
Chem. Eur. J. 2021, 27, 1–9
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6
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À 2
1
00.0% for 6000 cycles at 6 mAcm (Figure 8f). The coulombic
Electrochemical tests: The cyclic voltammograms (CV) and galvano-
static charge-discharge (GCD) tests were evaluated by a three-
electrode method in 1 M Na SO electrolyte. Electrochemical
efficiency of the device is nearly 100.0% upon 5000 cycles. A
red LED can be lightened up by two devices connected in
2
4
impedance spectroscopy (EIS) was conducted with an amplitude of
mV in a frequency range from 0.01 Hz to 100 kHz at open circuit
series. The EIS measurements after the first cycle and
5
th
5
000 cycle were conducted to analyze the electrochemical
potential. The differential pulse voltammogram (DPV) was tested
from 0 to 1 V at different pulse amplitudes. The working electrode
was fabricated by mixing manganese oxides, acetylene black and
polytetrafluoroethylene (PTFE, 60 wt% dispersion in water) with a
weight ratio of 8:1:1. The mixture was coated on a graphite paper
kinetics and stability of the device (Figure S7). The device
2
demonstrated a low R value of 4.6 Ωcm after the first cycle.
s
After 5000 cycles, the R value and the radius of the semicircle
s
were nearly unchanged. The slop of the straight line at the low
frequency region after 5000 cycles slightly decreased. These
results indicates the excellent stability of the device during the
charge/discharge tests.
(
1 cm×1 cm). The mass loadings of metal oxides on each electrode
À 2
was around 2.0 mgcm . A Platinum wire (=1 mm in diameter) and
a Hg/HgO electrode were used as the counter and reference
electrodes, respectively. The symmetric supercapacitor was as-
sembled by two Mn O electrodes using 1 M Na SO4 aqueous
2
3
2
solution as the electrolyte. The obtained device was measured in a
two-electrode system.
Conclusion
À 1
Calculations: The specific capacitance (C , F g ), energy density (E,
s
À 1
À 1
Wh kg ) and power density (P, W kg ) were calculated according
In summary, hierarchical MnO , Mn O , Mn O and Mn O
4
2
x
y
2
3
3
[16]
to equations (5–7), respectively.
microspheres were prepared by a one-pot hydrothermal
method and subsequent annealing processes. Two pairs of
3
+
2+
4+
3+
(
(
(
5)
6)
7)
Mn /Mn and Mn /Mn redox couples were involved in the
intercalation/deintercalation of electrolyte ions. The trivalent
3
+
2+
4+
3+
manganese with Mn /Mn and Mn /Mn redox couples
can enhance the specific capacitance of Mn O . Two possible
redox reaction equations were proposed to illustrate the charge
storage mechanism of sodium cations intercalation/deintercala-
tion processes. The Mn O electrode integrated the merits of
multiple redox couples, ordered mesoporous structure and
large specific surface area, demonstrating excellent electro-
chemical performance for supercapacitors. The Mn O electrode
2
3
2
3
2
3
where I (A), Δt (s) and ΔV (V) represent the constant discharge
current, discharge time and potential window, respectively.
À 1
exhibited the maximum specific capacitance of 209.1 Fg at
À 1
1
Ag . All the manganese oxide electrodes yielded extraordi-
nary capacitance retentions of over 99.7% after 5000 cycles due
to the unique sphere-like structures. The symmetric
Mn O //Mn O device yielded a maximum energy density of
Acknowledgements
2
3
2
À 1
3
À 1
2
9.3 Whkg at 250 Wkg . This study will provide an important
This work was financially supported by the National Natural
Science Foundation of China (61774033), Scientific Research
Foundation of Graduate School of Southeast University
reference for the future research of manganese oxides.
(
YBYP2153) and Key Laboratory of Water Environment Evolution
Experimental Section
Synthesis of hierarchical manganese oxide microspheres: Glucose
and Pollution Control in Three Gorges Reservior (WEPKL2016LL-
01, WEPKL2019ZD-03). We also thank the Big Data Center of
Southeast University for providing the facility support on the
numerical calculations.
(
0.2 g) and MnSO (2.41 g) were added to distilled water (40 mL)
4
and stirred for 6 h. Then (NH ) S O (3.64 g) was added to the above
4
2
2
8
mixture under stirring. The obtained mixture was transferred into a
5
0 mL autoclave and maintained at 180°C for 4 h. The resulting
suspension was centrifuged and washed with deionized water and
ethanol three times. The products were dried at 80°C overnight to Conflict of Interest
obtain MnO microspheres. Then, MnO microspheres were calcined
2
2
at 400°C, 500°C and 800°C for 2 h in N atmosphere to obtain
2
The authors declare no conflict of interest.
Mn O , Mn O and Mn O microspheres, respectively.
x
y
2
3
3
4
Materials characterization: The X-ray diffraction (XRD) measure-
ments were conducted on a Shimazu/ XD-3 A using Cu Ka radiation
Keywords: manganese oxide
supercapacitor · tervalent
·
Mn O
· redox couple ·
2
3
(
λ=0.15406 nm). The structure and morphology were analyzed by
scanning electron microscopy (SEM, JEOL JSM-6700F) and trans-
mission electron microscope (JEOL JEM 200EX). The nitrogen
adsorption/desorption measurements were measured on Micro-
meritics ASAP 2020. The corresponding results were calculated
using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda
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Manuscript received: January 11, 2021
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Chem. Eur. J. 2021, 27, 1–9
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© 2021 Wiley-VCH GmbH
��
These are not the final page numbers!

FULL PAPER
Effect of manganese valence on
specific capacitance: Hierarchical
manganese oxides microspheres were
prepared by hydrothermal and calci-
Dr. X. Chen, L. Li, Dr. X. Wang, Prof. K.
Xie, Prof. Y. Wang*
1
– 9
3
nation methods. Mn O exhibited Mn
Effect of Manganese Valence on
Specific Capacitance in Supercapaci-
tors of Manganese Oxide Micro-
spheres
2
3
+
2+
4+
3+
/
Mn and Mn /Mn transitions
during the charge/discharge process.
The redox reaction equations of
Mn O was proposed based on the
2
3
electrochemical measurements.
Mn O exhibited superior specific ca-
2
3
pacitance due to the two redox
couples and unique structure.