Preparation of Y-doped ZrO2 coatings on MnO2 electrodes and their effect on electrochemical performance for MnO2 electrochemical supercapacitors

Article abstract of DOI:10.1039/c5ra20543c

To enhance the cycling stability and conductivity of manganese dioxide (MnO₂) electrodes for supercapacitors, yttrium (Y) doped zirconia (ZrO₂) (denoted as Y/ZrO₂) is coated on MnO₂ supercapacitor electrodes (Y/

Full text of DOI:10.1039/c5ra20543c

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Preparation of Y-doped ZrO coatings on MnO electrodes and their effect on
2
2
electrochemical performance for MnO
2
electrochemical supercapacitor
ab1,
Yuqing Zhang
Yufeng Zhai
a
School of Chemical Engineering and Technology, Tianjin University, 300072, Tianjin, P. R. China.
b
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P R China
Abstract: To enhance the cycling stability and conductivity of manganese binoxide (MnO2)
electrodes for supercapacitors, yttrium(Y) doped zirconia (ZrO ) (denoted as Y/ZrO ) are coated
2
2
2 2 2
on MnO electrodes as supercapacitors electrodes (Y/ZrO @MnO eletrodes), so protecting the
2 2
MnO electrodes in electrolytes and enhancing the electrochemical performance of MnO
2 2
electrodes in sodium sulfate electrolytes. Y/ZrO @MnO eletrodes are characterized by scanning
electron microscope (SEM), transmission electron microscope (TEM), energy dispersive xray
spectrom (EDX) and XRD analysis. The electrochemical properties of electrodes are tested and
analyzed by galvanostatic charge/discharge tests, cyclic voltammetry (CV) and electrochemical
impedance spectroscopy (EIS). The Y/ZrO
2
@MnO
2
electrodes get a specific capacity of 282.1 F
ꢀ1
g with a specific capacity loss of only 6.3% after 100 cycles at the current density of 50 mA
ꢀ1
g .The results show that MnO particles are successfully deposited by Yꢀdoped ZrO while the the
2
2
2 2
Y/ZrO @MnO electrodes display better cycling stability and capacity performance. Therefore,
2
Yꢀdoped ZrO coating is the potential choice to improve the cycling stability and conductivity of
2
MnO electrode.
2
Keywords: Yꢀdoped ZrO coating; MnO2; Supercapacitors; Specific capacity; Cycling stability.
*
Corresponding author. Phone: +86ꢀ22ꢀ27890470; fax: +86ꢀ22ꢀ27403389;
Eꢀmail address: zhangyuqing@tju.edu.cn
1

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1
. Introduction
After entering 21 century, with increasing consumption of fossil fuels for the development
st
of the economy, the environment pollution and energy crisis have become the most important
problems that we must deal with. Therefore, there is a great need of looking for renewable and
sustainable energy sources and developing technologically advanced devices for energy
1
ꢀ4
conversion and storage . In recent years, electrochemical supercapacitors (ESs), as a new sort of
efficient device for energy conversion and storage, have caused the extensive concern because of
their high power density, fast charge and discharge rate, better cycling stability, and longer
1
,2
lifeꢀcycle . These properties and characteristics make ESs to be applied in many domains, such
as electric vehicles, computer, electrochemical power source, military, power distribution system,
5
ꢀ7
etc . Therefore, supercapacitors are expected to become a novel energy resource to solve the
problem about energy storage and environmental pollution.
The electrochemical properties of supercapacitors, such as specific capacity and cycling
2
stability, intimately depend on the electrode materials . Thus, improving the electrochemical
performance of electrode materials is an effective strategy to enhance the electrochemical
properties of supercapacitors. Generally, various materials for electrodes of supercapacitors such
as carbonaceous materials, conducting polymers and transitionꢀmetal oxides have been researched.
2
Among these materials, MnO , as a sort of transitionꢀmetal oxides, is considered as a promising
alternative class of materials for electrochemical supercapacitors, for its superior characteristics
ꢀ1
such as large theoretical specific capacity (1370 F g ), environmental safety, low toxicity and
8
ꢀ10
natural abundance, as well as low cost . In spite of all of the superiorities, commercial
applications of supercapacitors based on MnO
2
electrode have not been implemented mostly due
2

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to some existing drawbacks about MnO as follows. Firstly, partial dissolution of MnO electrode
2
2
2
,11
in the electrolyte happens spontaneously during cycling, leading to the fading capacity
.
Secondly, poor electronic conductivity and ionic conductivity result in large resistence of MnO
2
12
electrode in supercapacitors . Hence, it is a key task for researchers to solve these disadvantages
of MnO material mentioned above.
2
Among these drawbacks of MnO electrode material mentioned above, the partial dissolution
2
in the electrolyte is the most prominent issue and draws great attention of researchers. Pang and
1
3,14
Anderson
utilized solꢀgel method to prepare thinꢀfilm MnO
2
electrode, the result
ꢀ1
2 4
of test indicating that more than 10% capacity was lost after 1500 cycles , in 0.1mol L Na SO
ꢀ1
15
electrolyte, at a scanning rate of 50 mV s . Reddy et al. prepared MnO
2
electrode, at a sweep
ꢀ1
13
rate of 5 mV s , suffering more than 50% capacity loss after 800 cycles. Recently, Hsieh et al
also reported the capacity fading of electrochemical capacitors based on the MnO ·nH
2
2
O
electrodes, ranging from 5% to 30% in 1000 cycles, which was close to current rate and binder
content. Moreover, it has been reported that the protective coatings on the electrodes surface are
an effective strategy to improve the cycling stability of the electrode and enhance its
1
6
17
electrochemical properties . Walz et al deposited nanoporous silica coating on solid barium
ferrate particles and obviously enhanced its stability, comparing to the uncoated materials. Licht et
2 2 4
al have used ceramic material of zirconia (ZrO ) as protective coatings on the K FeO electrodes,
and their investigation results indicate that the ZrO
2
coating effectively enhances the stability of
1
8,19
these electrodes, simultaneously improves the energy storage capacity of superꢀiron batteries
.
Nevertheless, as we all know, ZrO can form cubic, tetragonal, and monoclinic phases or
2
orthorhombic phases. Among these phases, the cubic form has caused the extensive concern
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attributed to its high conductivity, excellent thermal stability, mechanical properties, chemical
resistance, and oxygen conductivity. In spite of all of the superiorities, the stability of the cubic
form at room temperature is not satisfactory. Therefore, the stabilization of cubic form at room
2
0,21
2 2
temperature is very necessary . It is reported that doping Y to ZrO can stabilize cubic of ZrO
at room temperature; meanwhile the conductivity of cubic ZrO
2
at room temperature can be
successfully investigated and developed Yꢀdoped
2 4
zirconia coatings on K FeO particles, significantly enhancing the stability and conductivity of the
2
2
23,24
improved as well . In addition, Zhang et al
2 4
coated K FeO electrodes and provided a good strategy for our further research.
Based on the previous work and investigation above, it is necessary to research and develop a
novel sort of electrode materials for supercapacitors. Therefore, in order to enhance the cycling
stability and conductivity of manganese binoxide (MnO ) electrodes for supercapacitors in this
2
2 2 2
paper, yttrium (Y) doped zirconia (ZrO ) (denoted as Y/ZrO ) are coated on MnO electrodes as
2 2 2
supercapacitors electrodes (Y/ZrO @MnO eletrodes), so protecting the MnO electrodes and
2
enhancing the electrochemical performance of MnO electrodes in sodium sulfate electrolytes.
And then their structure and electrochemical performance were observed and researched through
SEM, TEM, EDX, galvanostatic charge/discharge test, CV and EIS.
2
. Experimental
2
.1 Materials
2 2
Zirconium oxychloride octahydrate (ZrOCl ·8H O) (AR grade, ≥99.0%), ammonia water
(NH
3 2
·H O) (AR grade, 25%ꢀ28%), and cetyltrimethyl ammonium bromide (CTAB) (AR grade,
99.0%) were purchased from Tianjin GuangFu fine chemical research institute. Manganous sulfate
(
MnSO
4
2 2 5
·H O) (AR grade, ≥99.0%), ethanol absolute (C H OH) (AR grade, ≥99.7%), and sodium
4

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sulfate (Na SO ) (AR grade, ≥99.0%) were supplied by Tianjin Guangfu Fine Chemicals Co., Ltd.
2
4
4 3 3 2
Potassium permanganate (KMnO ) (AR grade, ≥99.0%) and yttrium nitrate (Y(NO ) ·6H O) (AR
grade, ≥99.0%) were obtained from Tianjin Jiangtian chemical Co., Ltd. Acetylene black (AR
grade, ≥99.0%, Tianjin Jinqiushi Chemical Co., Ltd.), Poly vinylidenedi fluoride (PVdF) (AR
grade, ≥99.0%, Chengdu Chenguang Research Institute of Chemical Industry), nickel foam, and
Nꢀmethyl pyrrolidone (NMP) (AR grade, ≥99.0%, Tianjin Bo di Chemical Co., Ltd.) were
obtained and used asꢀreceived.
2
2 2
.2 Preparation of Y/ZrO @MnO electrodes
2
.2.1 Preparation of MnO
A certain amount of CTAB was dissolved in 67 ml deionized water under magnetic stirring.
After CTAB powders were completely dissolved, 0.6320g KMnO granules were added into the
2
particles
4
4
CTAB solution, which was kept stirring for 30 minutes. Then the MnSO aqueous solution
4 2 4
(1.014g MnSO ·H O powders dissolved in 100 ml deionized water) was dropped into KMnO
solution by a dropping funnel at a speed of 2 drops per second under powerful magnetic stirring.
The whole process was kept at constant temperature of 30℃. The products were collected by
centrifugation, washed by ethyl alcohol and deionized water for several times and dried at 60 ℃
in the air for 6 hours.
2
2 2
.2.2 Preparation of Y/ZrO @MnO electrodes
2 2 3 3 2
Firstly, a certain amount of ZrOCl ·8H O and Y(NO ) ·6H O was added into 100 ml
deionized water in a 250 ml conical flask, kept stirring for 10 minutes after the powders were
completely dissolved. Then a certain amount of CTAB was added into the solution. After complete
2
dissolution, the solution was kept stirring for 30 minutes. Finally, 1 g prepared MnO particles
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were added and then handled with ultrasonic treatment for 30 minutes. After that, the dilute
ammonia solution was dropwise added into the prepared solution above slowly by a dropping
funnel. The solution pH value was adjusted from 8 to 9 and then the solution was kept stirring for
several hours. The whole process is under constant 30 ℃. Finally, the products were collected by
centrifugation, washed by ethyl alcohol and deionized water for several times and dried for 6
hours at 60 ℃ in the air. After above reactions, the dried particles were calcined in the air in pipe
furnace at 450 ℃ for 3 h and the Y/ZrO
.3 Fabrication of electrodes
2 2
@MnO powders were obtained.
2
The electrodes for evaluating the electrochemical properties were fabricated by mixing
Y/ZrO @MnO (80 wt%) sample with acetylene black (10 wt%) and PVdF (10 wt%) in a beaker.
2
2
Then moderate amount of NMP was added into the electrode material to form slurry. After that,
2
the slurry was coated (coating area: 1 × 1 cm ) on a nickel foam (1.8 mm, washed by ethyl alcohol
under sonication for 1h). Next, the electrodes were dried at 60 ℃ for 3h under vacuum (vacuum
degree: ≥ 0.08 MPa), and then pressed at 10 MPa for 10 seconds. Finally, the electrodes were
dried at 60 ℃ for 12 h in the air.
2
.4 Characterization
2
.4.1 SEM analysis
The surface morphologies and structures of the asꢀprepared samples were observed using a
scanning electron microscope (SEM, Hitachi Sꢀ4800, Japan).
2
.4.2 TEM analysis
The surface morphologies and structures of the asꢀprepared samples were observed using a
transmission electron microscopy (TEM, JEOL JEMꢀ2100F, Japan).
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2
.4.2 EDX analysis
The content of elements in the surface of the asꢀprepared samples were observed using a
Energy dispersive Xꢀray spectrometer (EDX, Tecnai G2F20 FEI, Netherlands).
.4.3 XRD analysis
2
The XRD patterns were recorded using a RINT2000 diffractometer (Rigaku Corporation, Japan)
equipped with a CuKα Xꢀray source. Data were collected in the 2θ range 10ꢀ80°.
2
.5 Electrochemical tests
Electrochemical tests of the asꢀprepared electrodes, including the galvanostatic
charge/discharge test, the cyclic voltammetry (CV) test, and the electrochemical impedance
ꢀ1
spectroscopes (EIS) test. All the electrochemical tests were performed using 0.5 mol L aqueous
Na SO solution as the electrolyte at 25 ℃ in this study.
2
4
2
.5.1 Galvanostatic charge/discharge tests
The galvanostatic charge/discharge tests of electrodes were studied via Land Battery Test
System (CT2001A, Wuhan Jinnuo Electronic Ltd., China) in the potential range between 0 and 0.9
A at constant current and in the potential range between 0 and 0.8 V. In this paper, the specific
capacity is calculated on the basis of the galvanostatic charge/discharge test curve for testing a
single electrode. There are the same two electrodes having identical quality in the process, with a
ꢀ
1
current density of 100 mA g .
2
.5.2 CV tests
CV tests were performed using a CHI650C electrochemical workstation in a threeꢀelectrode
2
configuration, with Y/ZrO @MnO particles coated Ni foam, Pt foil (1 × 1 cm ) and saturated
2
2
calomel electrode (SCE) as working, counter and reference electrodes, respectively. The potential
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voltage range of CV was from ꢀ0.5V to 0.5V.
.5.3 EIS tests
EIS tests were also performed using a CHI650C electrochemical workstation in a
threeꢀeletrode configuration as the same as that in the CV tests. And the electrochemical
2
ꢀ2
impedance spectra were measured by imposing a sinusoidal alternating voltage frequency of 10
5
to 10 Hz.
3
. Results and discussion
3
2 2
.1 Determination of preparation conditions of Y/ZrO @MnO particles
2 2
The electrochemical performances of Y/ZrO @MnO electrodes are affected by many factors
during depositing Y/ZrO
2 2
coatings on the surface of MnO electrodes. Among those factors, the
adding amount of ZrOCl ·8H O, the doping amount of Y(NO ) ·6H O (the mole ratio of
2
2
3 3
2
3 3 2 2 2
Y(NO ) ·6H O and ZrOCl ·8H O) (mol : mol), the concentration of CTAB, the reaction time, and
the calcination temperature were observed and researched. To determine the fitting preparation
conditions of Y/ZrO @MnO particles, a single factor experiment was used for research and five
2
2
different levels of every factor were chosen in experiments. Finally, electrodes were prepared
using Y/ZrO @MnO particles to further investigate.
2
2
In all the single factor experiments, the specific capacity of produced electrodes mentioned
above were evaluated by using the method of galvanostatic charge/discharge test.
3
2 2
.1.1 Determination of the adding amount of ZrOCl ·8H O
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Fig. 1 The effect of adding amount of zirconium oxychloride on the specific capacity of electrodes
Fig. 1 shows that the effect of the adding amount of ZrOCl ·8H O on the specific capacity of
2
2
the electrodes. At the initial stage, the specific capacity of electrodes increases with adding amount
of ZrOCl ·8H O being increased. When the adding amount of ZrOCl ·8H O is 0.6171 g, the
2
2
2
2
ꢀ1
specific capacity of electrodes reaches the highest value of 207.3 F g . When the adding amount
of ZrOCl ·8H O is increased continually, the specific capacity of the electrodes is decreased
2
2
2 2
sharply. The reason for the result suggests that: when the adding amount of ZrOCl ·8H O is less
than 0.6171 g, the electrodes can not be deposited completely, so the coatings are not enough
inhibit the fading of specific capacity. And the other hand, the adding amount of ZrOCl ·8H O is
2
2
more than 0.6171 g, the coatings are so thick that they make electrolyte transfer difficultly and
even prevent active materials from contacting with electrolyte, leading to the sharp decrease of the
2 2
specific capacity. In conclusion, 0.6171 g is chosen as the fitting adding amount of ZrOCl ·8H O.
3
3 3 2
.1.2 Determination of the doping amount of Y(NO ) ·6H O
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Fig. 2 The effect of yttrium doping amount on the specific capacity of electrodes
The curve in Fig. 2 shows the effect of yttrium (Y) doping amount on the specific capacity of
electrodes. At the initial stage, the specific capacity of the electrodes increases with the doping
3 3 2
amount of Y(NO ) ·6H O being added. The specific capacity of the electrodes reaches its
ꢀ1
maximum value of 233.6 F g when the mole ratio between Y(NO ) ·6H O and ZrOCl ·8H O
3
3
2
2
2
(
mol : mol) is 3:100. Then the specific capacity of the electrodes starts to decrease with the doping
amount of yttrium being increased unceasingly. Especially, the mole ratio between Y(NO ·6H
3
)
3
2
O
2 2
and ZrOCl ·8H O(mol : mol) is more than 5:100, the specific capacity of the electrodes decreases
sharply. The reason suggests that：the oxygen vacancies are few in the coating when the doping
amount of yttrium (Y) is less, so the resistance of charge transfer is strong, which makes the
specific capacity low. With increasing the doping amount of Y(NO
3 3 2
) ·6H O, the coatings of the
electrodes contain more yttrium (Y), which increases the oxygen vacancies and decreases the
3 3 2
resistance of charge transfer. But when the doping amount of Y(NO ) ·6H O is too much, the
coatings turn into mixtures of zirconium (Zr) and yttrium (Y), which makes the conductivity in the
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coatings of the electrodes decrease seriously. As the result, the resistance of charge transfer grows
highly and the specific capacity decreases much. Thereby, the fitting mole ratio between
3 3 2 2 2
Y(NO ) ·6H O and ZrOCl ·8H O(mol : mol) is 3:100 .
3
.1.3 Determination of the concentration of CTAB
Fig. 3 The effect of the concentration of CTAB on the specific capacity of electrodes
Fig. 3 shows that the specific capacity of the electrodes increases with the increasing of the
concentration of CTAB at the initial stage. The electrodes get the highest specific capacity of
ꢀ1
ꢀ1
2
25.3 F g while the concentration of CTAB is 7 mmol L . However, when the concentration of
ꢀ1
CTAB is more than 7 mmol L , the specific capacity begins to decline. This result suggests that:
CTAB is added to insure that channels are generated in the inorganic coating shells of the
electrodes, which makes active materials easily contact with electrolyte. Channels for electrolyte
transfer can not be produced enough if the amount of CTAB is little. So, with adding the
concentration of CTAB, the quantity of channels increases continuously, as a result the specific
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capacity of the electrodes augments. However, the formation of inorganic coating shells might be
prohibited by excess adding amount of CTAB. Therefore, the fitting adding amount of CTAB is 7
ꢀ1
mmol L .
3
.1.4 Determination of the reaction time
Fig. 4 The effect of reaction time on the specific capacity of electrodes
It is shown in Fig. 4 that the specific capacity increases with the coating reaction time extends.
The specific capacity of the electrodes changes slightly when coating reaction time exceeds 2 h.
ꢀ1
The specific capacity of the electrodes reaches its maximum value of 217.3 F g when coating
reaction time is 3 h. Then reaction time reaches 4 h, it has a weak influence on the specific
ꢀ1
capacity, the value of the specific capacity is 203.5 F g . The reason suggests that：short reaction
time can not enough impel the growth of the inorganic coating shell, in contrast, overlong reaction
time will add unnecessary costs. Therefore, the fitting reaction time should be 3 h.
3
.1.5 Determination of the calcination temperature
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Fig. 5 The effect of different calcination temperatures on the specific capacity of electrodes
As is shown in Fig. 5, the specific capacity of the electrodes increases with rising calcination
ꢀ1
temperature at the initial stage and reaches the maximum value (227.5 F g ) at 450 ℃, whereas
the specific capacity decreases sharply while calcination temperature exceeds 450 ℃.
2
Actually, different crystal structures of MnO are formed under different calcination
2
temperatures. When the calcinations temperature is below 300 ℃, MnO is still in amorphous
state. However, αꢀMnO appears when MnO is heated between 400 and 500 ℃ and transforms
2
2
3
2 3 2
to αꢀMn O above 500 ℃ . It is reported that if MnO was in amorphous state, it is not stable
during cycling as the electrode material, which is adverse to the diffusion of electrolyte. The
specific capacity of electrodes decreases sharply when calcination temperature exceeds 450 ℃,
2 3
because the specific capacity of αꢀMn O is very little. Therefore, 450 ℃ is adopted as the fitting
calcination temperature.
2 2 2 2
The fitting preparation conditions of Y/ZrO @MnO particles are: the mass of ZrOCl ·8H O
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is 0.6171 g, the mole ratio between Y(NO ) ·6H O and ZrOCl ·8H O (mol : mol) 3:100, the
3
3
2
2
2
ꢀ1
adding amount of CTAB 7 mmol L , the reaction time 3 h, the calcination temperature 450 ℃.
Characterizations and electrochemical tests of electrodes prepared under fitting preparation
conditions were analyzed and researched as below.
3
3
.2 Characterizations
.2.1 SEM analysis of materials
2 2
The Y/ZrO @MnO materials for SEM are prepared under the fitting preparation conditions.
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Fig. 6 SEM images: (A) pure MnO2 particles (B) Y/ZrO2@MnO2 particles
(
C) Y/ZrO2@MnO2 particles after 5000 cycles
The morphology of pure MnO particles and Y/ZrO @MnO particles are shown in Fig. 6.
2
2
2
SEM images in Fig. 6 (A) reveals that the pure MnO particles perform sphere morphology with
2
smooth surface; meanwhile they have a particle size about 450 nm. Compared to the SEM image
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of the pure MnO particles, the surface of Y/ZrO @MnO particles becomes relatively rough in
2
2
2
2 2
Fig. 6 (B). These results indicate that MnO materials have been indeed coated by Y/ZrO coatings.
Besides, by comparing the images in Fig. 6 (B) and Fig. 6 (C), it is proved that the morphology of
Y/ZrO2@MnO2 has no obvious changes after 5000 cycles and the inorganic coating shell
increases the cycle stability of the electrode.
3
.2.2 TEM and EDX analysis of materials
The Y/ZrO @MnO materials for TEM and EDX are prepared under the fitting preparation
conditions.
2
2
Fig. 7 TEM images and EDX spectrum of Y/ZrO @MnO particles
2
2
2
TEM images in Fig. 7(A), Fig. 7(B) and Fig. 7(C) show that MnO particles are successfully
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by Y/ZrO coatings. In these images, deep color regions stand for MnO particles and gray
2
2
sections represent Y/ZrO
2
coatings. The thickness of the coating shells is between 20 nm and 50
nm in Fig. 7(B), Fig. 7(A) and Fig. 7 (C). This result is congruent with the result of SEM analysis.
Moreover, EDX spectrum indicates that the coating shell includes Y and Zr element, which proves
2
that yttrium is successfully doped into the ZrO coating shells.
3
.2.3 XRD analysis of materials
The PꢀMnO2 and Y/ZrO2@MnO2 materials for XRD are prepared under the fitting preparation
conditions: the mass of ZrOCl2·8H2O is 0.6171g, the mole ratio between Y(NO3)3·6H2O and
ZrOCl2·8H2O (mol : mol) 3:100, the adding amount of CTAB 7 mmol Lꢀ1; the reaction time 3 h;
the calcination temperature 450 ℃.
Fig. 8 XꢀRay diffraction patterns of different electrode materials
The XRD patterns for PꢀMnO2 and Y/ZrO2@MnO2 samples are she own in Fig. 8. In the pattern
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of the two samples, all diffraction peaks can be indexed to the αꢀMnO2 according to the JCPDS
4ꢀ0141 standard file25. It can be seen from the Fig. 8 that there are some new diffraction peaks
4
appearing and slight deviation of peak position through comparing the patterns of PꢀMnO2 and
Y/ZrO2@MnO2 samples. The new peaks correspond to the ZrO2 according to the JCPDS
07ꢀ0343 standard file26. The result indicates the MnO2 succeed to be coated by ZrO2. Meanwhile,
no extra peaks pertaining to Y are observed, indicating that no significant changes of the structure
occurred during the doping process.
3
.3 Electrochemical tests
2 2
As indicated by SEM and TEM results, Y/ZrO @MnO materials is the potential choice
application in energy field. To explore the potential applications in electrochemical energy storage,
the Y/ZrO @MnO materials are used to make supercapacitor electrodes, and characterized by
2
2
galvanostatic charge/discharge test, cyclic voltammetry (CV) and electrochemical impedance
spectroscopy (EIS).
3
.3.1 Galvanostatic charge/discharge tests of electrodes
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Fig. 9 The effect of cycle number on the specific capacity of electrodes：
(
A) 100 cycles (B) 5000 cycles
Cycle life is a key factor to be considered for supercapacitor. Thus, in order to validate the
effect of Y/ZrO coatings on the electrodes, the cycle stability tests of pure MnO electrodes and
2
2
ꢀ1
2 2
Y/ZrO @MnO electrodes are carried out under the same current densities of 50 mA g . As seen
in Fig. 9 (A) and Fig. 9 (B), the specific capacity of pure MnO2 electrodes reaches
57.3 F gꢀ1 at first cycle, and it decreases to 167.9 F gꢀ1 after 100 cycles and 153.0 F
2
gꢀ1 after 5000 cycles, with an overall specific capacity loss of 35% after 100 cycles
and 40.5% after 5000 cycles. Under the same current density, the Y/ZrO2@MnO2
electrodes get a specific capacity of 282.1 F gꢀ1 at the initial stage, and it decreases to
2
64.3 F gꢀ1 after 100 cycles and 243.6 F gꢀ1 after 5000 cycles, with a specific
capacity loss of only 6.3% after 100 cycles and 13.6% after 5000 cycles. The two
figures indicate that Y/ZrO2 inorganic coating shells increases the specific capacity
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and cycle stability of the pure MnO2 electrodes, because Y/ZrO2@MnO2 electrodes
shows a larger initial specific capacity and a less attenuation of specific capacity.
2
The reason suggests that: the protection of Yꢀdoped ZrO coating, which prevents direct
2
contact between MnO electrode and electrolyte, reduces partial dissolution of electrode materials
in the electrolyte during cycling test. Simultaneously, the expansion and contraction of MnO
2
crystal lattice is limited by Yꢀdoped ZrO coating, thus the cycle stability of the MnO electrode is
2
2
improved. Alternatively, the conductivity of ZrO
2
coating is improved by doping yttrium (Y),
which decreases surface resistance of ZrO
chargeꢀtransfer resistance of the whole electrode.
.3.2 CV analysis of electrodes
2 2
coating on the surface of MnO electrode and
3
Fig. 10 The effect of scanning rate on the specific capacity of electrodes
The effect of scanning rate on the specific capcacity of pure MnO electrodes and
2
2 2
Y/ZrO @MnO electrodes is shown in Fig. 10. As shown in Fig. 10, when the scanning rate is
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ꢀ1
5
mV s , the specific capacity of pure MnO electrode and Y/ZrO @MnO electrode are 272.4 F
2 2 2
ꢀ1
ꢀ1
g and 297.1 F g , respectively. With increasing scanning rate, the specific capacity of electrodes
ꢀ1
decreases sharply. As a result, when the scanning rate is 200mV s , the specific capacity of pure
ꢀ1
ꢀ1
2 2 2
MnO electrode and Y/ZrO @MnO electrode decline to 77.5 F g and 145.3 F g , and the
specific capacity retention ratio are around 28% and 49%, respectively. The result indicates that
the Y/ZrO inorganic coating shell increases the specific capacity of pure MnO electrode, and
2
2
makes the decreasing rate of the specific capacity to be less. The reason suggests that：with the
scanning rate increasing, the reaction processes of electrode become faster. Consequently,
the immigration and emigration of the electrolyte ion from the electrode can not occur timely. And
then, the active materials in the electrode can not have fully reaction, leading to the declination of
the specific capacity. Therefore, the Y/ZrO2 inorganic coating shell increases the overall
conductivity of the electrode, indeed.
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Fig. 11 Effect of cycle number on the cyclic voltammetry property of electrodes:
(
A) pure MnO2 electrode (B) Y/ZrO2@MnO2 electrode
The electrochemical properties of the two electrodes are evaluated using CV tests in 0.5 mol
ꢀ1
ꢀ1
L Na
2
SO
4
solution at the scan rate of 50 mV s . As shown in Fig. 11, the test result of the first
2
cycle is compared with that of 100 cycles under same conditions. For pure MnO electrode
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evaluated in Fig. 11 (A), although the shape of the CV curves maintain rectangle, the response
current decreases obviously after 100 cycles, which implies that the specific capacity of electrode
2 2 2
decreases obviously. Then, to compare with the pure MnO electrode, the Y/ZrO @MnO
2 2
electrode is shown in Fig. 11 (B), and the CV curves of Y/ZrO @MnO electrode change little
after 100 cycles, which indicates that the response current almost stays the same. Therefore, by
comparing the curves of the two electrodes, it is proved that the inorganic coating shell increases
the cycle stability of the electrode. This result is congruent with the conclusion of former analysis.
3
.3.3 EIS analysis of electrodes
Fig. 12 Nyquist plots of pure MnO2 electrodes and Y/ZrO2@MnO2 electrodes
The Nyquist plots of pure MnO electrodes and Y/ZrO @MnO electrodes before cycle tests
2
2
2
are shown in Fig. 12. In the Nyquist plots, the curve is a depressed arc at high frequency range,
corresponding to the charge transfer process. Arc radius stands for the resistance of the charge
transfer process and greater radius stands for bigger resistance. The straight line appeared at low
frequencies range corresponds to the electrolyte diffusion process. Normally, a higher slope for the
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impedance line presents a lower resistance of the electrolyte diffusion process. The two curves
display that the charge transfer resistance of pure MnO electrodes is seemingly almost the same
2
2 2
to that of Y/ZrO @MnO electrodes before cycle tests. However, the difference of charge transfer
2 2 2
resistance between pure MnO and Y/ZrO @MnO electrodes materials will be more clearly found
2
when the depressed arc part is enlarged in Fig. 12. It can be concluded that the Y/ZrO coatings
indeed decreases the charge transfer resistance of pure MnO electrodes. Then during the
2
electrolyte diffusion process, the slopes of the two lines are approach still, which implies the
inorganic coating shell hinders the diffusion of the electrolyte inside the electrode slightly.
Therefore, the two curves in Fig. 12 show that the EIS property of the two electrodes is similar to
each other before cycle tests.
Fig. 13 EIS equivalent circuit of electrodes
Table1
The results of Rs, RCt and W for different electrodes
Types of electrode
PꢀMnO2
Rs / ꢁ cm2
0.874
RCt /ꢁ cm2
1.853
W / ꢁ cm2
0.241
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YꢀZrO2@MnO2
0.842
0.921
0.256
For further investigation, electrodes are described by an equivalent circuit. As seen in Fig. 13,
the equivalent circuit is consisted of the electrolyte resistance (Rs), the charge transfer resistance
(
RCt), the Warburg impedance related to the diffusion in electrolyte (W) and the electrochemical
capacitance (C). Based on this equivalent circuit, the values of Rs, RCt and W can be obtained
via the software of ZsimpWin, and the corresponding results are listed in Table 1. It can be seen
from Table 1, the Rs values of two electrodes are very similar because of the consistent electrolyte.
The RCt values of YꢀZrO2@MnO2 electrode are significantly less than the PꢀMnO2 electrode and
the W values of two electrodes are basically the same. These results indicate that the YꢀZrO2
coating reduces the chargeꢀtransfer resistance of MnO2 and hardly hinders the diffusion of the
electrolyte inside the electrode, which conforms to the Nyquist plots in Fig. 13. The results further
demonstrate that the conductivity and the electrochemical properties of the whole MnO2 electrode
can be enhanced by Yꢀdoped ZrO2 coating significantly.
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Fig. 14 Effects of cycles number on EIS of pure MnO2 electrodes: (A) Nyquist plot
B) admittance plot (C) phase angleꢀfrequency plot
(
The EIS of pure MnO electrodes during electrochemical cycling is shown in Fig. 14
2
(
A).Nyquist plot shows that the radius of depressed arc at high frequencies after 100 cycles is
much larger than that after 1 cycle, which implies the resistance of charge transfer process
increases with increasing cycle number. The increasing resistance of charge transfer process might
27
2
be induced by the swells and contracts of MnO lattice during chargeꢀdischarge , which makes
the microꢀstructure collapse and enlarges internal resistance. Finally, the collapse of the
microstructure makes the specific capacity decrease sharply. For the low frequencies in Nyquist
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plot, the slope of the line after 100 cycles is lower than that after 1 cycle, which implies that the
diffusion resistance of electrolyte inside electrodes greatly increases after 100 cycles. This is
because the collapse of microstructure hinders the diffusion of electrolyte. The curve in Fig. 14 (B)
2 2
shows the admittance plot of pure MnO electrodes. The knee frequency for pure MnO electrodes
is reduced from 8.1 Hz (1 cycle) to 3.7 Hz (100 cycles), implying degraded electrochemical
response. According to phase angleꢀfrequency plot in Fig. 14 (C), the capacitor response
frequency is reduced from 0.12 Hz (1 cycle) to 0.026 Hz (100 cycles). It decreases by 80%, which
makes the capacitor response time increases from 8 s to 40 s. As shown in the curves in Fig. 14,
2
the EIS of pure MnO electrodes varies greatly after 100 cycles. The resistance of both charge
transfer process and electrolyte diffusion process increases, which reduces the electrochemical
response ability and increases capacitor response time.
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Fig. 15 Effects of cycles number on EIS of Y/ZrO2@MnO2 electrodes: (A) Nyquist plot
B) admittance plot (C) phase angleꢀfrequency plot
The effect of cycle numbers on the EIS of Y/ZrO @MnO electrodes is shown in Fig. 15.
(
2
2
Nyquist plot in Fig. 15 (A) shows that the radius of depressed arc at high frequencies after 100
cycles is similar to that after 1 cycle, which implies the resistance of charge transfer process
during electrodes process mainly remains the same with increasing cycle numbers. Although
2
MnO lattice continually swells and contracts during chargeꢀdischarge process, inorganic coating
shells effectively protect the microstructure of electrodes, which enhances the cycle stability of the
electrodes. For the straight line at the low frequency range, its slope changes little after 100 cycles,
indicating the electrolyte diffusion resistance inside the electrodes hardly changes. Fig. 15 (B)
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shows the admittance plot of Y/ZrO @MnO electrodes. The knee frequency is 2.1 Hz (1 cycle)
2
2
and 1.9 Hz (100 cycles), which implies the capacitor response ability changes little. According to
Fig. 13 (C), the capacitor response frequency is 0.12 Hz (1 cycle) and 0.098 Hz (100 cycles),
which only decreases by 19 %. At the same time, capacitor response time increases from 8 s (1
2 2
cycle) to 10 s (100 cycles). The curves in Fig. 13 show that the EIS of Y/ZrO @MnO electrodes
changes little after 100 cycles. The resistance of charge transfer process and electrolyte diffusion
process remains largely unchanged, which makes the electrochemical response ability keep the
2
same and the capacitor response time increase little. Compared to the EIS of pure MnO electrodes,
the results prove that the inorganic coating shell increases the cycle stability of the electrode
indeed.
4
. Conclusions
In this paper, a costꢀeffective and simple strategy is presented to design and fabricate novel
Y/ZrO @MnO material, to improve the cycle stability and conductivity property of electrode for
2
2
2 2
supercapacitor. The fitting preparation conditions of Y/ZrO @MnO particles are: the mass of
2 2 3 3 2 2 2
ZrOCl ·8H O is 0.6171 g, the mole ratio between Y(NO ) ·6H O and ZrOCl ·8H O (mol : mol)
ꢀ
1
3
:100, the adding amount of CTAB 7 mmol L ; the reaction time 3 h; the calcination temperature
50 ℃.The results of SEM, TEM, EDX and XRD about electrodes prepared under the fitting
materials are indeed coated by Yꢀdoped ZrO coatings and
4
preparation conditions show that MnO
2
2
2 2
Y/ZrO @MnO particles possess a coreꢀshell structure. The results of electrochemical tests show
2 2
that the Y/ZrO @MnO electrodes display better cycling stability and capacity performance.
Therefore, Yꢀdoped ZrO coating is the potential choice to improve the cycling stability and
2
2
conductivity of MnO electrode.
29