Porous waxberry-like MnO2/La2O3 microspheres for high performance asymmetric supercapacitor

Article abstract of DOI:10.1016/j.electacta.2017.04.175

In this paper, porous waxberry-like microspheres structures of MnO₂/La₂O₃ (Mn-La) composites were synthesized via a facile two-pot hydrothermal method. The microstructure and chemical composition of as-prepared Mn-La composites were characterized by a series of techniques such as XRD, SEM, TEM, HRTEM, XPS and BET. The results show that the pure MnO₂ microspheres were covered with its large amount of well-ordered nanorods, and La₂O₃ was distributed evenly on MnO₂ surface to constitute Mn-La composites. Meanwhile, the electrochemical tests reveal that sample Mn-La10 (molar ratio ⁿMnO₂:ⁿLa₂O₃?=?1:0.1) exhibited the highest specific capacitance (C_m) (245.4?F?g^?1 at a current density of 0.3?A?g^?1, 1.5 times of MnO₂), good rate performance and excellent cycling stability (97.5% capacitance retention after 3000 cycles at current density of 2.0?A?g^?1). In general, suitable amount of La₂O₃ doping, based on the synergistic effect between MnO₂ and La₂O₃, is helpful to improve the capacitance performance of composites. Furthermore, an asymmetric supercapacitor (Mn-La10//AC), with Mn-La10 as the positive electrode and activated carbons as the negative electrode, displays high energy density of 25.8 Wh kg^?1 with a maximum power density of 0.3?kW?kg^?1 under potential window of 2.0?V.

Full text of DOI:10.1016/j.electacta.2017.04.175

Accepted Manuscript
Title: Porous waxberry-like MnO /La O microspheres for
2
2
3
high performance asymmetric supercapacitor
Authors: Yu Li, Bing Guan, Aimee Maclennan, Yongfeng Hu,
Dandan Li, Jing Zhao, Yaqiong Wang, Huaihao Zhang
PII:
S0013-4686(17)30969-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2017.04.175
EA 29433
To appear in:
Electrochimica Acta
Received date:
Revised date:
Accepted date:
17-1-2017
29-3-2017
30-4-2017
Please cite this article as: Yu Li, Bing Guan, Aimee Maclennan, Yongfeng
Hu, Dandan Li, Jing Zhao, Yaqiong Wang, Huaihao Zhang, Porous waxberry-
like MnO2/La2O3 microspheres for high performance asymmetric supercapacitor,
Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.04.175
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Porous waxberry-like MnO
2
/La
2
O microspheres for high performance
3
asymmetric supercapacitor
a
a
b
b
a
a
Yu Li , Bing Guan , Aimee Maclennan , Yongfeng Hu , Dandan Li , Jing Zhao ,
a
a,
Yaqiong Wang , Huaihao Zhang *
School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou
25002, PR China
a
2
b
Canadian Light Source, 44 Innovation Boulevard, Saskatoon, SK, Canada S7N 2V3
Graphical abstract
The porous waxberry-like Mn-La10 microspheres were synthesized via a facile two-pot
hydrothermal method. Electrochemical measurements showed that Mn-La10 exhibits the
maximum C and good rate capability.
m
1

Highlights

The hierarchical porous waxberry-like MnO
2
/La
2
3
O (Mn-La) microspheres were
synthesized via a simple two-step hydrothermal process.
-
1
-1


The Mn-La10 exhibits the maximum C
m
of 245.4 F g at 0.3 A g .
-1
The Mn-La10//AC delivers high energy density of 25.8 Wh kg at a power
-1
density of 0.3 kW kg .

An Interaction mechanism between MnO
2
and La
2
3
O is investigated and
proposed.
Abstract: In this paper, porous waxberry-like microspheres structures of MnO
2
/La
2
O
3
(Mn-La) composites were synthesized via a facile two-pot hydrothermal method. The
microstructure and chemical composition of as-prepared Mn-La composites were
characterized by a series of techniques such as XRD, SEM, TEM, HRTEM, XPS and
BET. The results show that the pure MnO
amount of well-ordered nanorods, and La
2
3
microspheres were covered with its large
was distributed evenly on MnO surface to
2
O
2
constitute Mn-La composites. Meanwhile, the electrochemical tests reveal that sample
Mn-La10 (molar ratio nMnO2:nLa O =1:0.1) exhibited the highest specific capacitance
2 3
-1
-1
(C
m
) (245.4 F g at a current density of 0.3 A g , 1.5 times of MnO
2
), good rate
performance and excellent cycling stability (97.5% capacitance retention after 3000
-1
cycles at current density of 2.0 A g ). In general, suitable amount of La
2
O
3
doping,
based on the synergistic effect between MnO
2
2
and La
2
O
3
, is helpful to improve the

capacitance performance of composites. Furthermore, an asymmetric supercapacitor
Mn-La10//AC), with Mn-La10 as the positive electrode and activated carbons as the
(
-1
negative electrode, displays high energy density of 25.8 Wh kg with a maximum power
-1
density of 0.3 kW kg under potential window of 2.0 V.
Keywords: Microspheres; MnO
2
/La
2
O
3
composite; Supercapacitor; Capacitance
performance
1
. Introduction
Supercapacitors (SCs), also known as electrochemical capacitor, a new kind of
electrochemical energy storage device featured with high power density, fast charge and
discharge, long cycle life and safe operation, have been considered to be a promising
candidate for filling the gap between conventional capacitors and batteries. Additionally,
it is widely used in various fields, such as aviation, military, consumer electronics, for
sustainable and renewable energy society [1-3]. Based on the mechanisms of energy
storage, SCs can be divided into two categories: electric double-layer capacitors (EDLCs)
and pseudocapacitors. Compared with EDLCs, pseudocapacitors can provide higher
specific capacitance (C ) and energy densities, mainly due to the utilization of fast
m
reversible Faradic reactions at the electrode surface [4-6].
Therefore, numerous efforts have been devoted to developing high performance
electrode materials for pseudocapacitors, including transition metal oxides, hydroxides
and conductive polymers [7-8]. Among the transition metal oxides, MnO , an
2
environmentally friendly candidate, is considered as one of the most promising active
3

materials for SCs due to its wider operating potential window, abundant resources and
high theoretical capacitance [9]. Nevertheless, the practical applications of pure MnO as
active materials have been hindered because of its low C and poor cycle stability and
other defects. In this context, extensive research has been carried out to modify MnO in
order to enhance its capacitance performance. For example, MnO was doped with Ni
10], Co [11] and other transition metals, or combined with carbon materials, such as
graphene, to form composite with various nanostructures [12].
2
m
2
2
[
Rare earth (RE) metal oxides, with variety in the composition and structure and
presence of 4f electrons, have attracted much attention in recent years due to their
unique properties, such as good semiconductor properties, optical, magnetic properties
and heat resistance. These properties make them widely used in electronics,
petrochemical, metallurgy, machinery, energy, light industry, environmental protection,
pigments and agriculture and so on [13]. La
2
3
O , an important RE metal oxide, has been
intensively researched in applications for optical [14], alloy [15], gas sensor [16] and
gate dielectric [17] fields owing to its excellent redox reversibility, thermal stability and
2
+
3+
abundant resources. Furthermore, as active material for SCs, La and La can coexist
during the process of charge and discharge, so as to promote its redox reaction to
enhance the capacitance performance. For instance, Yadav et al. utilized chemical
-1
deposition method to synthesize La
2
O
3
thin films with a C
m
of 147 F g at a scan rate of
-1
5
mV s [18]. Han et al. used a facile hydrothermal method to fabricate composite
3
+
microspheres NiO doped with La . The obtained active materials delivered a high C of
m
4

-1
-2
2
53 F g at 5 mA cm (2 times higher than pure NiO) and a cycling efficiency of 134 %
over 500 cycles [19]. In another study, Xue et al. reported a La-MnO /MWCNT
2
composite with noticeable capacitive performance. To be specific, La addition into
composites can increase conductivity and capacitance of the material [20]. Similarly, Xu
et al. prepared the La
2
O
3
/Co
3
O
4
/MnO -CNT hybrid nanocomposites by hydrothermal
2
-1
method for zinc-air batteries with a maximum energy densities of 970 Wh Kg [21].
In general, these previous relevant studies showed that some transition metal oxides
doped with La
few works has been devoted to the investigation of MnO
SCs active material. In this work, based on the two-pot hydrothermal method, MnO
waxberry-like microspheres covered with its nanorods were synthesized; then, La
was prepared and distributed evenly on MnO
composites with different La mole ratios. The Mn-La composites as electrode
2
O
3
can improve the C
m
of SCs. However, to the best of our knowledge,
2
/La (named as Mn-La) as
2
O
3
2
3
2
O
2
surface to obtain a series of Mn-La
2
O
3
material demonstrated advantages as follows: First, Mn-La, with porous hierarchical
structure, high specific surface area (SBET) and porosity, can provide larger contact area,
greatly reduce the ion diffusion resistance and facilitate the ionic and electronic kinetics
during reversible insertion/deinsertion processes. Second, MnO lattice interface doped
2
into La atoms is more conducive to establish electronic conduction system, shorten ion
diffusion paths of electrolyte to electrode material, and hence increase redox reaction
rate and the material conductivity. Third, the La
2
O
3
, acted as an accelerator, can enhance
the degree of Faradaic reaction between MnO
2
and electrolyte. The electrochemical tests
5

show that the sample Mn-La10 (molar ratio nMnO2:nLa2O3=1:0.1) displays the highest C
m
-1
-1
of 245.4 F g at a current density of 0.3 A g , good rate capability and outstanding
cyclic stability. In addition, the asymmetric supercapacitor (named as Mn-La10//AC)
was fabricated using Mn-La10 and active carbon (AC) as positive and negative
electrodes, respectively. The Mn-La10//AC has extended the operating potential to 2.0 V
-1
in 1.0 M Na
2
SO electrolyte, and achieved a high energy density of 25.8 Wh kg with a
4
-1
power density of 0.3 kW kg .
2
. Experimental
2
.1. Synthesis of MnO microspheres
2
All reagents in this work are analytical grade and used without any further
purification. The formation process of sample is depicted in Fig. 1. As for a typical
synthesis, 2 mmol MnSO ·H O, an equal amount of K SO and K were dissolved
in 50 mL distilled water, and stirred until completely dissolved. Then, 2.0 mL H SO
and 0.05 mmol solid AgNO were added to the solution, stirred for about 20 min,
transferred into a 100 mL Teflon-line stainless steel autoclave, and heated at 60 ºC for
2 h. Afterwards, the precipitate was cooled to room temperature, collected by filtration
4
2
2
4
2
S
2
O
8
2
4
3
1
and washed several times with deionized water and absolute ethanol, respectively.
Lastly, the products were dried at 60 ºC for 8 h.
2
.2. Synthesis of MnO
2
/La
2
O
3
composites
For the formation of Mn-La composites, La(NO
3
)
3
·6H O, with different molar
2
ratios of 5%, 10% and 15% (molar ratio nLa O3:nMnO2), were dissolved in 15 mL 0.35 M
2
6

aqueous ammonia solution, respectively. Then, 0.1 g as-prepared MnO was dispersed in
2
the above solution, sonicated for 1 h to form a uniform mixture solution. The resultant
solution was then transferred into a 100 mL Teflon-line stainless steel autoclave, sealed
and maintained at 150 ºC for 6 h. After the reaction, the precipitates were collected by
filtration and washed several times with deionized water and absolute ethanol,
respectively. After that, the products were dried at 60 ºC for 8 h. Finally, the resulting
product was further calcinated in air at 300 ºC for 1 h to obtain the target samples. The
obtained products were denoted as Mn-La5, Mn-La10 and Mn-La15, respectively.
2
.3. Material characterization
X-ray absorption near edge structure spectra (XANES) of Mn K-edge and La
3
L -edge were measured using soft X-ray as micro characteristics beam line at Canadian
Light Source. Elemental composition and chemical valence states of the samples were
studied by X-ray photoelectron spectroscopy (XPS, Thermo Science, ESCALAB 250Xi,
USA). The crystal structure of the samples was characterized by X-ray powder
diffraction (XRD, Bruker D8 super speed, Cu Kα radiation, λ = 1.5406 Å, tube voltage:
4
0 kV, tube current: 40 mA). The lattice parameters of the samples were determined by
high resolution transmission electron microscopy (HRTEM, Tecnai G2F30 S-TWIN
field emission transmission electron microscopy). The morphology and microstructure
of the product was analyzed by scanning electron microscopy (FESEM, Hitachi S-4800
II field emission scanning electron microscopy, 20 kV) and transmission electron
microscopy (TEM, Philips TECNAI 12 transmission electron microscopy). Qualitative
7

and quantitative analysis of sample elements was determined by energy dispersive X-ray
EDS, Norcrometer, USA). The specific surface area and pore size distribution of the
(
materials were measured by N
2
adsorption analyzer (BET, Micromeritics, TriStar 3000,
USA).
2
.4. Electrochemical measurements
Electrochemical performances of the individual electrodes were measured in 1.0 M
SO aqueous electrolyte with a standard three electrode system, with platinum foil
Na
2
4
and saturated calomel electrode (SCE) as counter and reference electrodes, respectively.
Electrochemical tests were performed under the potential window of -0.2 to 0.8 V (vs.
SCE). The working electrodes were prepared by mixing active material, conductive
graphite and polytetrafluoroethylene (PTFE) in a mass ratio of 80:15:5. Afterwards, the
2
above mixture was coated on the nickel foam (1×1 cm ), dried at 60 °C in vacuum for 8
h to remove solvent, and then pressed at 10 MPa for 30 s in order to improve the contact
between active material and foam. The loading mass of active material of each electrode
is about 2.5 mg.
The Mn-La10//AC device, Mn-La10 as the positive electrode and AC as the
negative electrode, was tested in a two-electrode system with a polyester fiber paper as
the separator and 1.0 M Na
2
SO as electrolyte. Also, the preparation method of negative
4
electrode was the same as the positive one. As for the Mn-La10//AC, the theoretically
optimal loading mass ratio of active materials was estimated to balance the charge with
Q  Q , whereQ and Q represent the charges stored at the positive and negative


8

electrodes correspondingly. The charge stored in positive or negative electrode can be
calculated by equation:
Q  C Em
(1)
m
where C
m
is the specific capacitance, m is the mass and ΔE is the voltage difference
between the positive (+) and negative (-) electrodes. In order to achieve the charge
storage balance, the mass ratio between the positive and negative electrode materials
was estimated according to the following equation:
m / m  C E / C E
(2)


m

m



Base on potential windows and C
m
of Mn-La10 and AC electrode, the actual mass
ratio of two electrode was m (Mn-La10)/m (AC) = 0.8 (Fig. S1).
All the cycling voltammetry (CV) and galvanostatic charge-discharge (GCD) were
tested on an electrochemical workstation (CHI660E, Shanghai, China). Electrochemical
impedance tests (EIS) were performed on AutoLab (AutoLab-PGSTAT30, AutoLab,
-2
5
Netherlands) with a frequency range of 10 to 10 Hz.
3
. Results and Discussion
3
.1. Morphology and structure
The structure and crystallographic phase of as-prepared samples were determined
by XRD. From Fig. 2A, the diffraction peaks of all samples are observed at 12.8°, 18.1°,
8.8°, 37.5°, 41.9° and 49.8°, assigned to the (110), (200), (310), (211), (301) and (411)
planes of the α-MnO (JCPDS PDF:44-0141) correspondingly. Besides, no peaks
characteristic of La
2
2
2
O
3
can be observed, suggesting that the amount of doped La
2
O
3
9

could not change MnO
Mn-La samples were broader and weaker than those of pure MnO
crystallinity in Mn-La samples, as expected. In addition, with the increase of La
content, these peaks shifted slightly to smaller angles, ascribing to lattice expansion or
2
crystal phase. As shown in Fig. 2B, all characteristic peaks of
2
, implying the low
2
O
3
3
+
distortion of MnO
2
phase [19]. Specifically, the ionic radius of La (0.106 nm) is bigger
4
+
3+
than Mn (0.054 nm). When a small amount of La ions are introduced into the lattice
interface of MnO
would occur, leading to the shift of diffraction peaks [22]. These results demonstrate that
La can be doped into MnO successfully.
Furthermore, XANES measurements were performed to investigate the valence
state and structure compositions of the element. From Fig. 2C, the position of Mn
2
during the synthetic process, the lattice imperfection and expansion
2
O
3
2
4
+
K-edge spectrum of Mn-La10 is consistent with MnO -Ref (Mn ), suggesting that the
2
4+
oxidation state of Mn in Mn-La10 is tetravalent (Mn ) [23]. As for the La L
spectrum shown in Fig. 2D, the strong peak at 5491.2 eV with one broad shape
resonance at about 5520.3 eV is in accordance with the values reported for La -Ref
3
-edge
2
O
3
3
+
[
24], indicating the presence of trivalent lanthanum (La ).
The elemental composition and chemical state of Mn-La10 was also investigated
by XPS as shown in Fig. 3(A~E). As depicted in Fig. 3A, the XPS survey spectrum of
Mn-La10 verifies the presence of C, O, Mn and La elements. The Mn 2p spectrum (Fig.
3
2
B) shows two peaks at the binging energy 642.2 and 653.9 eV, corresponding to Mn
3/2 and Mn 2p1/2, respectively, with an energy separation 11.7 eV, which is in
p
1
0

accordance with the previous reports on MnO
2
[25]. Meanwhile, the Mn 3s spectrum in
Fig. 3C manifests that the binging energy difference between two peaks is 4.7 eV, also
4
+
ascribing to the Mn [26]. As for the O 1s spectrum in Fig. 3D, the peak at 529.8 eV
can be assigned to the Mn-O-Mn bond [26], whereas peak at 531.6 eV is attributed to
La-O bond [27]. The La 3d spectrum (Fig. 3E) is split into La 3d3/2 and La 3d5/2, located
at 852.0 eV and 835.2 eV, respectively, with two satellite peaks, indicating the existence
3
+
of La . Furthermore, the spin-orbit splitting between La 3d3/2 and La 3d5/2 peaks of 16.8
eV is characteristic of La
results.
2
3
O phase [28, 29], further proving the aforementioned XANES
BET specific surface area (SBET) and pore size distribution results in Fig. 3F are
obtained from N adsorption/desorption analysis. Obviously, all samples display a type
2
IV isotherm with H3 type hysteresis loop, revealing the mesoporous structure [30].
Mn-La has a wide relative pressure range of hysteresis loop with relatively more small
dimensional mesoporous, corresponding to the obvious hysteresis loop. The intensities
of pore size distribution of Mn-La are slightly higher than that of pure MnO , suggesting
2
higher pore volume of Mn-La. According to the BJH method, the pore-size distribution
curves can be calculated from the adsorption branch of isotherm (Fig. 3F inset), showing
the mesoporous structure with a broad pore-size distribution concentrated in the range of
2
~15 nm for all samples. The SBET of MnO
2
, Mn-La5, Mn-La10 and Mn-La15 samples
are 46.2, 65.8, 70.4 and 66.9 m g , respectively. As expected, SBET of Mn-La
composites are higher than pure MnO , illustrating the improved SBET of composites due
2
-1
2
1
1

to the synergistic effect between La
2
O
3
and MnO [31]. Such unique mesoporous
2
structure with high SBET could effectively alleviate agglomeration and generate more
effective electroactive sites to promote ions transfer, thus to ensure the high capacitive
performance of composites.
The morphology and structure of samples were characterized by SEM and TEM.
As exhibited in Fig. 4A, MnO shows a porous microspheres structure with the diameter
2
about 2~3 μm. The magnified SEM image (Fig. 4E) reveals that the microspheres
surface is composed of large amount of nanorods, arranged orderly from inside to
outside. From the TEM image (Fig. 4I), the sample displays microspheres covered with
nanorods on surface, consistent with the SEM results. The growth of microspheres
MnO might follow a “nucleation growth self-assembly” process. At the initial stage of
2
the reaction, the particles rapidly aggregate to form spheres owing to the high reactant
concentration. With the reaction time extended successively, the concentration of
reactants decreased, slowing the formation of MnO
MnO nanorods to form the waxberry-like MnO
comparison of the SEM images (Fig. 4B, C and D), the surface of samples becomes
much rougher with the increase of La content. Quite evidently, compared with MnO ,
2
nuclei and promoting the growth of
2
2
microspheres [32]. From the
2
O
3
2
the microspheres of Mn-La5 turn from dense-assembly to loose assembly (Fig. 4F),
matching well with the conclusion of crystallinity variation in XRD. For the Mn-La10,
the nanorods arrangement of surface layer of the composite changed from the orderly
epitaxial growth to the disordered grid structure (Fig. 4G). Meanwhile, abundant pores
1
2

were produced by the detachment and agglomeration of nanorods (Fig. 4G and Fig. 4K)
during the reaction process, effectively enhancing the SBET and ensuring the rapid
transport and diffusion of electrolyte ions. As clearly observed in Fig. 4H, the surface
nanorods of Mn-La15 were closely arranged in disorder, leading to the lower porosity
resulted from the excessive La
2
O
3
coating on MnO
2
microspheres surface. What is more,
content, the surface nanorods
based on Fig. 4J, K and L, with the increasing La
2
O
3
arrangement of composites becomes disordered progressively, matching well with the
SEM observation.
HRTEM images of Mn-La10 are shown in Fig. 4 (M, N and S). The structure at the
edge of MnO
Mn-La10 (Fig. 4S) exhibits clear lattice fringes with a lattice spaces of 0.341 and 0.309
nm, corresponding to the (100) plane of La (PDF:05-0602), (310) plane of MnO
PDF:44-0141), respectively. This result is in good accordance with the previous XRD
results and confirms the co-existence of La and MnO phase. EDS mappings of
2
microspheres (Fig. 4M) is consistent with the SEM image. The image of
2
O
3
2
(
2
O
3
2
Mn-La10 are shown in Fig. 4 (O, P, Q and R), the observed signals of O and Mn are
primarily distributed in the core region, whereas the signal of La is found in entire
region. Notably, the widened signal distribution of La element demonstrates that La
2
O
3
was successfully grown on MnO surface evenly, further proving the conclusion of XRD
2
and HRTEM. Additionally, EDS analysis (Fig. S2) reveals that the Mn-La samples were
composed of O, C, K, Mn and La without any other impurities. The C element is
possible from conductive carbon tape as substrate, air exposure of sample and gold
1
3

sputtering treatment [33]. The K (inset in Fig. S2) from the reagents was present during
the entire reaction process, and didn’t affect the formation of MnO structure [34].
2
+
Furthermore, a small amount of K is favorable for the redox occurrence, beneficial to
the charge storage properties [31]. The La/Mn atomic percentages of MnO , Mn-La5,
2
Mn-La10 and Mn-La15 are about 0%, 5.0%, 10.5% and 14.3%, respectively, basically
matching the experimental control ratio (0%, 5.0%, 10% and 15%), in good agreement
with the XRD and EDS results.
3
.2. Electrochemical performance
The electrochemical properties of samples were evaluated by CV and GCD
measurements in a three-electrode configuration with a SCE reference electrode in 1.0
M Na SO aqueous electrolyte. Fig. 5A shows the typical CV curves with
quasi-rectangular shape of all samples measured at a scan rate of 5 mV s , suggesting
that La doping did not change the charge storage mechanism of MnO . Interestingly,
as the La content increases, the area of CV curves displays a trend of first increase
2
4
-1
2
O
3
2
2
O
3
and then decrease. Obviously, the CV area of Mn-La10 is larger than that of other
samples, reflecting its best capacitance performance [31]. As the scan rate increasing
-1
from 5 to 100 mV s , the CV profiles of pure MnO are rectangular shaped and similar
2
for all scan rates (Fig. S3A), demonstrating the good capacitive properties. Such kind of
CV curves, like the double-layer capacitor type response, is an envelope of the locus of
reversible redox peaks, shifting a little by few mV each cycle [35] and indicating a
pseudo-constant rate over the complete CV tests [32, 36]. Meanwhile, Fig. S3 (C, E, G)
1
4

depicts the CV curves of Mn-La samples in the potential range from -0.2~0.8 V at
various scan rates. Apparently, no obvious redox peaks are observed for Mn-La. More
-1
importantly, even at the high scan rate of 100 mV s , the rectangular shape of CV curve
of Mn-La could be well kept without any obvious distortion, meaning their good
capacitance behavior and fast charge/discharge capabilities. The redox reactions of
+
composites, owing to the Na ion adsorption from electrolyte to Mn-La electrode surface,
cause intercalation/de-intercalation during the reduction and oxidation process [37].
Generally, the most reasonable conversion mechanisms were proposed to explain the
charge storage in Mn-La material as follows [34, 38]:
+
-
MnO +Na +e  MnOONa
(3)
(4)
2
+
-
La O +2Na +2e  La O Na
2
3
2
3
2
Fig. 5B illustrates the GCD curves for all samples at current density of 0.3 A g-1
within the potential range between -0.2 and 0.8 V. It is obvious that the good
symmetrical characteristic of GCD curves reveals the superior electrochemical
reversibility for all samples. Clearly, Mn-La10 exhibits a prolonged discharge time and
significantly enhanced C
with the CV curves. From Fig. S3 (D, F, H), the GCD profiles of Mn-La have good
symmetrical and linear characteristic, similar to that of MnO (Fig. S3B), revealing its
m
as compared to those of other samples, in good agreement
2
superior electrochemical reversibility and ideal charge/discharge properties. The C of
m
the electrode was calculated by the following equation:
C  I t / (mV)
(5)
m
1
5

-1
where C
m
, I, △t, m and △V are the specific capacitance (F g ), discharge current (A),
discharge time (s), mass of active material (g), potential window (V), respectively. The
C
C
m
m
calculated according to the discharge curves, is shown in Table 1. For instance, the
of MnO
2
, Mn-La5, Mn-La10 and Mn-La15 composites were 158.7, 188.7, 245.4 and
-1
-1
2
04.6 F g at 0.3 A g , respectively. Compared to MnO
2
, Mn-La composites possess
and MnO . On the
one hand, the Mn-La produce more defects to accept additional electrons, useful to store
more charges than pure MnO [39]; on the other hand, during the oxidation process, the
MnO and La release their electron simultaneously due to their inherent potential
difference. Nevertheless, the high activity of La
restored during the oxidation-reduction. Then, La
continuously, enhancing the degree of MnO
higher C
m
, mainly ascribing to the synergistic effect between La
2
O
3
2
2
2
2
O
3
2
O
3
makes it easy to be oxidized and
removes electrons from MnO
2
O
3
2
2
oxidation, and leading to a great
improvement in electrochemical performance. Consequently, the redox peaks, caused by
the quickly plunder process in electrochemical test [40], cannot be found in CV curves.
In particular, the C
m
of Mn-La10 is the highest, being the best capacitance performance
. Furthermore, when the current density
increased from 0.3 to 5.0 A g , the capacitance retention of Mn-La10 was still 169.8 F
and 1.5 times larger than that of pure MnO
2
-1
-1
g , proving its good rate capabilities. The promotion of C for Mn-La10 is due to that
m
the well interconnected porous structure and slit-shaped pore channel can facilitate facile
+
intercalation/deintercalation of Na , which can ensure the sufficient Faradic reactions for
energy storage. Nevertheless, the C of Mn-La15 was lower than that of Mn-La10,
m
1
6

probably caused by excessive coating of La
2
O
3
on MnO spherical surface [41], leading
2
to low porosity, SBET and ion transportation rate.
Electrochemical impedance spectroscopy (EIS) measurements are carried out to
study the storage kinetics. Nyquist plots are shown in Fig. 5C. Obviously, all the
Nyquist plots consist of a partial semicircle in high-frequency region and a straight line
in low-frequency region. Fig. 5D shows the corresponding equivalent circuit model and
the enlargement of Nyquist plots, including the equivalent series resistance R (the
s
material resistance, electrolyte migration resistance and the contact resistance between
the electrolyte and the current collector), Warburg impedance W (the electrolyte
diffusion rate in mesoporous electrode material), charge-transfer resistance Rct
,
double-layer capacitance C and pseudocapacitive C . The slope of curve in the
d
l
low-frequency region represents the W. For example, the higher slope of the straight line,
the faster diffusion of electrolyte in the electrode material. As displayed in Fig. 5C, in
the Warburg region, the slope of Mn-La10 is steeper than other electrodes, mainly
ascribing to its good wettability caused by large SBET. Overall, all Mn-La composites
exhibit a higher slope than that of pure MnO
beneficial to improve the ion diffusion rate on the electrode surface due to the
synergistic effect aforementioned. The R can be obtained from the high frequency
intercept on real axis. As illustrated in Fig. 5D, the R of MnO , Mn-La5, Mn-La10 and
2
, demonstrating that the La doping is
s
s
2
Mn-La15 were 3.50, 2.73, 2.21 and 2.31 Ω, respectively, indicating that Mn-La10 has
the highest conductivity. The semicircle in the high-frequency range corresponds to the
1
7

charge transfer resistance (Rct), caused by faradic reactions [42]. Moreover, the Rct
obtained by ZSimpWin software for above samples were 3.45, 2.76, 1.61 and 2.22 Ω,
respectively. As is known, the smaller Rct means the faster charge transfer rate, which
could accelerate the reversible reaction between electrolyte and electrode. Therefore,
Mn-La10, being of smallest Rct among all samples, has superior charge transfer ability,
ascribing to the following reasons: (1) the loose network with numerous voids makes the
electrolyte soak the electrode easily and shorten the ion diffusion length. (2) The high
S
BET can enhance charge transfer rate, increase contact area between the electrode and
electrolyte, so as to reduce the resistances [43-45]. (3) The synergistic effect between
MnO and La changed the surface states of electron energy, accelerating the electron
transfer on the electrode surface. The La particles act as an electron transfer bridge
between MnO nanorods, and reduce the Rct of the composite electrodes. Besides, La ,
2
2
O
3
2
O
3
3
+
2
as a catalyst doped into the Mn4+ grain boundary edge, can effectively establish the
percolating network of electrical conduction, increase the electron transfer rate,
consequently lead to good capacitance performance [21].
To evaluate the cycling stability of samples, the C
m
, as a function of the cycling
firstly increased, followed by a slight
drop and finally stabilized to a smaller value for all samples. In the initial stages, the C
increase is due to the gradual activation of active materials, with improved penetration
of the electrolyte ions into the pores. Then, C gradually reduced and finally stabilized,
attributing to the degradation of active material to some extent at electrolyte during the
-1
number was measured at 2.0 A g (Fig. 5E). C
m
m
m
1
8

GCD processes [46]. Impressively, after 3000 GCD cycles test, the Mn-La10 displays
good cycling stability, retaining 97.5% of the initial value. For comparison, the
capacitance retention of MnO is about 90.9% after 3000 cycles, lower than that of
2
Mn-La samples. This result indicates the incorporation of La can increase the cycling
stability of the electrode. Meanwhile, the first 20 GCD cycle curves of Mn-La10,
displayed in Fig. 5F, reveal a good symmetry and reproducibility, manifesting its good
redox reversibility.
The desirable electrochemical performance of Mn-La10 may be attributed to the
following factors. First, the lattice imperfections, resulted from La atoms introduced into
MnO lattice, could accelerate the ion diffusion rate and effectively enhance the
2
utilization ratio of electrode active material [41]. Second, the disordered reticular porous
structure of Mn-La10 (as shown in Fig. 4G) with large SBET and high porosity can act as
ion-buffering reservoirs, greatly shorten the diffusion and transport paths of electrolyte
ions from bulk electrolyte to material interior surface, provide rich electroactive sites,
thereby improve the capacitance performance. Third, the smaller R and Rct of Mn-La10
s
ensure the fast charge transfer rate, which is beneficial for enhancing the rate capability
of active materials.
As for Mn-La10, the CV curves, before and after 3000 GCD cycles at scan rate of 5
-1
mV s in Fig. 6A, are highly similar in shape, showing the prominent cycling stability.
From Fig. 6B, the Mn-La10 displays an R value (2.21 Ω) and Rct value (1.61 Ω) before
cycle, slightly lower than the ones (R =2.56 Ω, Rct=1.77 Ω) after cycles. Meanwhile, the
s
s
1
9

slope for the straight lines becomes more gently after 3000 GCD cycles, further
confirming the good redox reversibility and cycling stability. Furthermore, the
morphology evolution of Mn-La10 electrode before and after 3000 GCD cycles was
displayed (Fig. 6C and D). For comparison, the images reveal that the morphology of
Mn-La10 spiny waxberry-like microspheres is mostly well-preserved after 3000 cycles.
Thus, such good cyclic stability is possibly attributed to this special structure, being the
buffer for the volume shrinking and expanding during the consecutive GCD process.
To further evaluate the Mn-La10 electrode for practical applications, the
performance of two-electrode Mn-La10//AC asymmetric supercapacitor, with Mn-La10
as the positive electrode and AC as the negative electrode, was tested (Fig. 7A). A series
-1
of CV curves with different potential windows at 50 mV s are performed to estimate
the appropriate operating potential of device (Fig. 7B). As expected, all CV curves
showed an ideal capacitive behavior with quasi-rectangular shapes. In addition, the
stable potential window of Mn-La10//AC can be extended to 2.0 V without obvious
polarization curves. Accordingly, a safe working potential window 2.0 V was employed
in this work. The shape of CV curves, under the potential of 0~2.0 V at different scan
rates in Fig. 7C, show the characteristic of a hybrid asymmetric supercapacitor, without
obvious distortion with the raised scan rate, suggesting desirable fast charge/discharge
property and rate capability of this device [47]. The main characteristics of constant
current discharge test include capacitance calculation (integral of the area contained
during the discharge) and resistances, such as ESR (equivalent series resistance) and
2
0

EDR (equivalent distributed resistance), which represents ESR and the electrolyte
resistance in the pores [48], The nonlinearities of GCD curves of Mn-La10//AC devices
at different current densities in Fig. 7D imply that the Faradaic reaction occurred. The
ESR was calculated by the following equations:
ESR U I
(6)
Where U (V) is the vertical part of the discharge voltage, I (A) is the discharge
current. The IR drop of curves increased with the rising current density from 0.52 V (0.3
-1
-1
A g ) to 1.12 V (5A g ). According to the formula (6), the larger the IR drop, the
smaller the resistance of cell. Based on the discharge curves at current densities of 0.3,
.5, 1.0, 2.0 and 5.0 A g , the corresponding C
are 46.5, 42.1, 33.8, 25.0 and 17.3 F g-1,
-
1
0
m
respectively (inset in Fig. 7E). With the increasing of current density, the specific
capacitance reduces, owing to the greater IR drop and insufficient active material in the
redox process. Furthermore, the capacitance retention rate is as high as 37.2% at 5.0 A
-1
g , testifying its good rate capability. To further illustrate the cycling performance of
-1
Mn-La10//AC device, 3000 GCD cycles curves at a current density of 5 A g are
depicted in Fig. 7E. Herein, with the increase of the cycle number, the C increases first
and gradually becomes stable, exhibiting excellent cycling stability along with 16%
increase of C after 3000 cycles compared to the initial one [47]. Based on the discharge
m
m
curves of Ragone plot in Fig. 7F, the energy density and power density can be calculated
using the following equations:
2
E  C 

V

/ 7.2
(7)
(8)
m
P  3.6E / t
2
1

-1
-1
where E (Wh kg ) is the energy density, C
m
(F g ) is the specific capacitance, △V (V) is
-1
the discharge potential, P (kW kg ) is the power density and △t (s) is the discharge time.
-1
-1
A maximum energy density of 25.8 Wh kg at a power density of 0.3 kW kg was
obtained, higher than MnO -based asymmetric supercapacitor [49]. The above results
show that the Mn-La10 is a promising candidate for energy storage.
. Conclusions
In summary, an effective method has been developed to fabricate waxberry-like
Mn-La microspheres by doping La into MnO based on a simple two-step
hydrothermal process. Among the obtained samples, the capacitance performance of
Mn-La10 was enhanced dramatically compared with other samples. The C of Mn-La10
. Moreover, the
2
4
2
O
3
2
m
-1
-1
reached up to 245.4 F g at 0.3 A g , about 1.5 times of pure MnO
2
capacitance retention of Mn-La10 is still as high as 97.5% after 3000 cycles. Such good
capacitive performance of Mn-La composites could be attributed to the following
aspects. Firstly, the presence of La
large SBET, significantly enhance the effective utilization of active materials. Secondly,
the synergistic effect between La and MnO leads to a faster charge storage process
and a higher charge storage capability. Finally, La can serve as an electrocatalyst to
accelerate electron transfer rate and thus improve the cycle stability. Meanwhile, the
2
3
O can cause lattice imperfection, so as to provide
2
O
3
2
2
O
3
-1
as-fabricated Mn-La10//AC device displayed a high energy density of 25.8 Wh kg at a
-1
power density of 0.3 kW kg , as well as the outstanding cycling stability with 116% C
m
2
2

retention after 3000 cycles. This work showed that the Mn-La10 composite is a
promising electrode material for the energy storage.
Acknowledgements
This work was financially supported by the Natural Science Foundation of China
(
NO. 21375116) and Postdoctoral Science Foundation of China (2014M551668) and the
Priority Academic Program Development of Jiangsu Higher Education Institutions
PAPD). The related measure and analysis instrument for this work was supported by
(
the Testing Center of Yangzhou University and Canadian Light Source. The CLS is
financially supported by NSERC Canada, CIHR, NRC, and the University of
Saskatchewan.
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