Electrochemical energy storage performance of heterostructured SnO2@MnO2 nanoflakes

Author’s Accepted Manuscript Electrochemicalenergy storage performance of heterostructured SnO2@MnO2 nanoflakes Huanhao Xiao, Shunyu Yao, Fengyu Qu, Xu Zhang, Xiang Wu www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(16)31370-0 http://dx.doi.org/10.1016/j.ceramint.2016.08.064 CERI13516

To appear in: Ceramics International Received date: 6 June 2016 Revised date: 2 August 2016 Accepted date: 10 August 2016 Cite this article as: Huanhao Xiao, Shunyu Yao, Fengyu Qu, Xu Zhang and Xiang Wu, Electrochemicalenergy storage performance of heterostructured SnO2@MnO2 nanoflakes, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.08.064 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Electrochemical energy storage performance of heterostructured SnO2@MnO2 nanoflakes Huanhao Xiao, Shunyu Yao, Fengyu Qu, Xu Zhang*, Xiang Wu* College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, P. R. China

[email protected]

[email protected],

*

Corresponding authors.

1

Abstract In this work, we report synthesis of SnO2@MnO2 nanoflakes grown on nickel foam through a facile two-step hydrothermal route. The as-obtained products are characterized by series of techniques such as scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The as-obtained SnO2@MnO2 nanoflakes are directly used as supercapacitor electrode materials. The results show that the electrode possesses a high discharge areal capacitance of 1231.6 mF cm-2 at 1 mA cm-2 and benign cycling stability with 67.2 % of initial areal capacitance retention when the current density is 10 mA cm-2 after 6000 cycles. Moreover, the heterostructured electrode shows 41.1 % retention of the initial capacitance when the current densities change from 1 to 10 mA cm-2, which reveals good rate capability. SnO2@MnO2 nanoflakes products which possess excellent electrochemical properties might be used as potential electrode materials for supercapacitor applications. Keywords:

SnO2@MnO2

nanoflakes;

supercapacitor

2

hydrothermal;

electrode

materials;

1. Introduction In recent years, there is an increasing requirement for efficient, clean and sustainable energy sources and new technologies associated with energy storage because of rapid development of global economy, the depletion of fossil fuels and increasing environmental pollution [1-5]. The supercapacitor, also known as electrochemical capacitor, has attracted extensive attention due to its unique properties such as high power density, fast charge-discharge rate, long cycle life and safe operation [6-12]. As we know, the electrode materials are the heart of supercapacitors, which immediately determine the capability, delivery rates and efficiency [13-14]. Metal oxides have been widely investigated as electrode materials for supercapacitors because of their environmental benignity, low cost, and high capacitance [15-21]. Among them, SnO2 with a wide energy gap (Eg=3.6 eV), is a kind of typical n-type semiconductor, which possesses the traits including low cost, low toxicity, widespread availability and great flexibility in structure and morphology [22-24]. Moreover, SnO2 as the electrode material could be used not only in neutral aqueous electrolytes but also alkaline electrolytes with a wide potential window [25]. MnO2 is an alternative pseudocapacitive material and possess high theoretical capacitance (1370 F g-1), low toxicity and low cost [15]. However, MnO2 suffers from the intrinsically low electrical conductivity, which restricts fast electron transport [26]. Hence, to satisfy the requirements of high capacitance, high power and energy densities and structural stability for electrode materials, designing novel composited electrode materials is very important. Composited metal oxides as prospective

3

electrode materials, which could inherit advantages from individual electrode and generate better electrochemical properties owing to their synergistic effects between two individual components [27]. Yang et al. reported hierarchical NiCo2O4@NiO core-shell heterostructured nanowire arrays on carbon cloth and showed a high gravimetric capacitance of 1792 F g-1 at 5 mA cm-2 with high rate capability and cycle property [28]. Qu’s group synthesized ZnO@Ni(OH)2 core-shell heterostructure via an electrospinning method combined with a hydrothermal approach and demonstrated ultrahigh specific capacitance of 2218 F g-1 at 2 mV s-1 [29]. In previous work, we reported fabrication of large scale α-Fe2O3@NiO heterostructures grown on carbon cloth and used as active materials for supercapacitors, the results showed the electrochemical performance of α-Fe2O3@NiO heterostructures was obviously better than the single α-Fe2O3 and NiO materials [18]. Herein, we report synthesis of large scale SnO2@MnO2 nanoflakes via a cost-effective and facile two-step hydrothermal method. Firstly, we fabricated single SnO2 nanoflakes directly grown on nickel foam. Secondly, SnO2 nanoflakes are used as the backbone to grow MnO2 layers. The as-obtained heterostructures are used as electrode materials for supercapacitors, revealing high areal capacitance of 1231.6 mF cm-2 at a current density of 1 mA cm-2 and good cycle stability of 67.2 % after 6000 cycles at 10 mA cm-2. The as-synthesized SnO2@MnO2 nanostructures could be anticipated to be ideal electrode materials for supercapacitor applications.

2. Experimental section 2.1. Preparation of SnO2 nanoflakes grown on nickel foam

4

Prior to experiment procedure, a piece of nickel foam (3 × 4 cm2) was first ultrasonically cleared continuously in acetone, ethanol, and distilled water for 15 min, respectively. Then the nickel substrate was dried under atmospheric condition. All reagents in the experiment are of analytical grade and were used without further purification. In a typical synthesis, 20 mmol SnCl2·2H2O was added in a 50 mL deionized water under magnetic stirring for 10 min to form a homogeneous solution. Whereafter, 40 mmol NH4F was slowly added to the above SnCl2·2H2O solution under continuous stirring. Then, a clear transparent solution was obtained after being continuous stirred for another 15 min. The solution was transferred into a 100 mL Teflon-lined stainless steel autoclave, and kept at 180 oC for 10 h. When reaction finished, the autoclave was cooled down naturally and the nickel foam substrate with pale yellow precursor was taken out. Subsequently, the product was washed with deionized water and absolute ethanol for several times, and dried at 60 oC for 6 h. Finally, the sample was obtained through annealing in a muffle kiln at 350 oC for 2 h. 2.2. Preparation of SnO2@MnO2 heteroarchitectures In a typical hydrothermal synthesis process, 5 mmol KMnO4 was dissolved in a 50 mL deionized water under magnetic stirring at room temperature for 15 min. Afterwards, the above mentioned KMnO4 solution was transferred into a 100 mL Teflon-lined autoclave and the nickel foam substrate coated with SnO2 nanoflakes was immersed into it. Then the Teflon-lined autoclave was sealed and maintained in an electric oven at 160 oC for 3 h. After the hydrothermal reaction, the nickel foam was taken out and cleaned with distilled water and absolute ethanol, and dried in a vacuum

5

oven at 60 oC for 6 h. At last, the sample was annealed at 350 oC for 2 h in air. 2.3. Characterization The morphology and microstructure of the as-synthesized products were conducted and analyzed by scanning electron microscope (SEM, Hitachi S-4800), transmission electron microscope (TEM, JEOL-2010). The crystallographic structure of the as-obtained products was determined by using X-ray powder diffraction (XRD, Rigaku Dmax-rB, CuKα radiation, λ= 0.1542 nm, 40 kV，100 mA). Chemical state analysis was performed by X-ray photoelectron spectroscopy (Perkin-Elmer model PHI 5600 XPS system). The specific surface area of the as-synthesized sample was figured out by employing the Brunauer-Emmett-Teller (BET) equation according to the nitrogen adsorption isotherm obtained with a Belsorp-max. The pore size distribution was investigated by the Barrett-Joyner-Halenda (BJH) method which applied to the desorption branch of the adsorption-desorption isotherm. 2.4. Electrochemical measurements Electrochemical tests of the products were carried out with a three-electrode electrochemical configuration on a electrochemical workstation (CHI660E, Shanghai, Chenhua) at room temperature. SnO2@MnO2 composite grown on nickel foam was cut into the size of 1× 1cm, which was directly served as the working electrode. Meanwhile, Pt foil and saturated calomel reference electrode (SCE) were used as counter and reference electrodes, respectively. 3 M KOH aqueous solution was employed as the electrolyte. Cyclic voltammetry (CV) curves were tested between -0.8 and 0 V (vs. SCE) at different scan rates. Galvanostatic charge-discharge tests

6

were measured from the different current densities with the potential window of -0.8 and 0 V (vs. SCE) and the electrochemical impedance spectroscopy (EIS) tests were performed by using an AC voltage with 1 mV amplitude in the frequency range from 0.1 Hz to 100 kHz. C (mF cm-2) called the areal capacitance of the working electrode material was calculated from the galvanostatic discharge curves based on the following equation: C = IΔt/(ΔVS). Where I is the discharge current density, t is the discharge time, S is the geometrical area of the working electrode, and ΔV is the voltage change (V) excluding IR drop in the discharge procedure.

3. Results and discussion X-ray diffraction technique is used to analyze the crystallinity and crystal phases of the as-synthesized samples. Fig. 1a shows XRD patterns of the as-obtained product. Three diffraction peaks with high intensity belong to nickel substrate (JCPDS card no.04-0850). Other visible diffraction peaks could be indexed to tetragonal rutile SnO2 phase and the crystalline birnessite-type MnO2 with JCPDS Card (no.41-1445) and JCPDS Card (no.80-1098), respectively. Energy dispersive spectrometry (EDS) mapping analyses are further conducted to confirm the composition of composite materials and mapping images indicate the elements distribute across the structure. As shown in Fig. 1d-f, the composite nanoflakes consist of Sn, O, Mn elements and the elements are well-distributed. SEM is used to examine the morphology of the as-obtained samples grown on nickel foam. Different magnification SEM images of the as-prepared products are shown in Fig. 2a-d, respectively. From Fig. 2a, it can be seen that large scale

7

nanoflakes grow on nickel foam, forming a three dimensional network structure. Fig. 2b shows high magnification SEM image of the sample, which reveals nanoflakes interconnect with each other. Meanwhile, the nanoflakes are very thin. Fig. 2c-2d demonstrates SEM images of SnO2@MnO2 composite. Compared with single SnO2 nanoflakes, The morphology of the composite samples don’t change. But the surface of the nanoflakes becomes very rough and contains numerous wrinkles. Further detailed microstructure information of SnO2@MnO2 composite is elucidated by TEM. Fig. 2e shows low magnification TEM image of the as-obtained SnO2@MnO2 nanoflake, which is consistent with the observations from SEM images. A high-resolution TEM image (HRTEM) of the composite indicates that the well lattice fringes with the lattice spacings of 0.337 nm, 0.265 nm and 0.67 nm, which correspond to the (110), (101) and (001) lattice plane of rutile SnO2 structure and birnessite-type MnO2, respectively. The inset in Fig. 2f is the selected-area electron diffraction (SAED) pattern, which indicates the polycrystalline nature of SnO2@MnO2 nanoflake. To further confirm the structure and component information of SnO2@MnO2 products, X-ray photoelectron spectroscopy (XPS) is used to investigate the binding energy of atoms, from which one can study various chemical states and gain the detailed elemental composition of the hybrid samples. Fig. 3a shows the overall range XPS spectrum of SnO2@MnO2 nanoflakes, from which one can discover the diffraction peaks belong to Sn, Mn, O elements, respectively, revealing the products include Sn, Mn and O composition, which is well consistent with XRD analysis result.

8

High-resolution XPS spectrum of Mn 2p are shown in Fig. 3b and there are two major peaks (Mn 2p3/2 and 2p1/2) center at about 642.9 and 654.6 eV, which has a spin energy separation of approximately 11.7 eV, determining the oxidation state of Mn and Mn4+ ions are existed in the composite samples [30]. To ascertain the specific surface area and pore size of the SnO2@MnO2 composite, BET

nitrogen

adsorption-desorption

tests

are

conducted

and

nitrogen

adsorption-desorption isotherm and the homologous BJH pore diameter distribution of SnO2@MnO2 nanoflakes scraped from the Ni substrate are shown in Fig. 4. Obviously, the isotherm of the as-obtained SnO2@MnO2 nanoflakes is a type IV isotherm with H3-type hysteresis loops when the range was 0.2-1.0 P/P0 on the basis of the IUPAC classification. The Brunauer-Emmett-Teller (BET) surface area of SnO2@MnO2 nanoflakes is around 36.9 m2 g−1 by the calculation, as shown in the inset in Fig. 4. The pore size distribution of SnO2@MnO2 nanoflakes is centered at about 4.752 nm, which reveals the composite nanoflakes possess mesoporous characteristic. In order to investigate potential application of the as-synthesized composite as supercapacitor electrode materials, we assess their electrochemical capacitive performance by directly employing SnO2@MnO2 nanoflakes grown on nickel foam as the working electrode. The cyclic voltammetry (CV) tests of pure nickel foam substrate and SnO2@MnO2 hybrid electrodes are firstly measured in the potential window ranging from 0 to -0.8 V at 40 mV s-1 to make sure whether pure nickel substrate affect CV results. As shown in Fig. 5a, SnO2@MnO2 electrode material

9

presents much higher capacitive current density than pure nickel foam. The substrate shows no capacitive current density, demonstrating nickel foam effect can be neglected due to its little contribution to the total capacitance of the integral electrode material. Fig. 5b depicts CV curves of SnO2@MnO2 electrode at various scan rates varying from 3 to 40 mV s-1. The shape of the CVs is deviated from an ideal rectangular shape, which is pertained to the electric double-layer capacitance, and redox peaks over the whole range of scan rates can be distinctly discovered, demonstrating that the capacitance of SnO2@MnO2 composites mainly possesses Faradic pseudocapacitive characteristics. Meanwhile, with the increase of scan rate, the area enclosed by the CV curves enhances. The shape of the CV curves maintains unchanged even the scan rate is 40 mV s-1, implying SnO2@MnO2 electrode possesses good

rate

capability

and

electrochemical

reversibility.

By

applying

the

chronopotentiometry technique, the galvanostatic charge-discharge and cyclic stability performance of SnO2@MnO2 electrode are tested. The charge-discharge curves are measured at different current densities ranging from 1 to 10 mA cm-2 and the results are presented in Fig. 5c. From the charge-discharge curves, the form of curves are different from straight and flat line, and the curves present plateaus, indicating the electrode possesses typical pseudocapacitive characteristics, which is in line with the CV curves (Fig. 5b). Based on the discharge curves and the equation ( C = IΔt/(ΔVS)), we calculate the areal capacitances of SnO2@MnO2 composites at different current densities. When the current densities are 1, 2, 3, 5, 8 and 10 mA cm-2, the corresponding areal capacitances are calculated to be 1231.6, 810.6, 712.5, 612.5,

10

546.5, 506.25 mF cm-2, respectively, indicating that the areal capacitances reduce with the increasing of current densities. Table 1 shows the comparison of the capacitance between the as-prepared product and other pure oxide materials, revealing SnO2@MnO2 hybrid structure possesses higher areal capacitance than those of some reported electrode materials. The areal capacitance retention rate of SnO2@MnO2 nanoflakes electrode is calculated to about 41.1% when the current densities are increased from 1 to 10 mA cm-2 ( Fig. 6a), demonstrating the composite electrode possesses high rate capability and good rate performance. Fig. 6b shows the comparison of the cyclic voltammetry (CV) curves of SnO2@MnO2 hybrid composites, MnO2, SnO2 and Ni foam at a scan rate of 40 mV s-1 with the potential window ranging from 0 to -0.8 V. Apparently, the enclosed area of SnO2@MnO2 composit electrode is much larger than those of the other materials (MnO2, SnO2), revealing the composite have better electrochemical properties. As depicted in Fig. 6c, SnO2@MnO2 heterostructured electrode exhibits much longer discharging time and possesses higher areal capacitance than MnO2 and bare SnO2, which is consistent with the results in Fig. 6b. Such enhanced electrochemical performances of SnO2@MnO2 nanoflakes may be attributed to the structures of the composite, which could amplify the total electrochemical active sites and have the electrode materials adequately enter into KOH electrolyte and permit the electrolyte easily diffuse into the inner region of the electrodes. Moreover, MnO2 electrode materials can offer additional pseudocapacitance in the electrolyte and generate multifunctional and synergistic effects [35]. To further investigate the

11

resistivity of the electrode materials, the electrochemical impedance spectroscopy (EIS) is tested in the condition of a frequency range from 100 kHz to 0.01 Hz at an AC perturbation amplitude of 5 mV, and the corresponding Nyquist plots are depicted in Fig. 6d. SnO2@MnO2 electrode possesses the lowest charge-transfer resistance (0.76 Ω) compared with MnO2 (1.22 Ω) and SnO2 (1.95 Ω), indicating the integration of SnO2@MnO2 nanoflakes can improve the holistic electron conductivity of hybrid electrode materials, Meanwhile, the particular heterostructure can provide an ideal pathway for electron and ion transport without kinetic limitations [35]. In order to explore the cycling stability of SnO2@MnO2 composites, we take continuous galvanostatic charge-discharge measurements for 6000 cycles when the current density is 10 mA cm-2 and the results are shown in Fig. 7. The inset in Fig. 7 shows the areal capacitance retention of SnO2@MnO2 composite electrode is approximately 67.2 % of the initial capacitance after 6000 cycles, which reveals SnO2@MnO2 materials possess a good cycling performance and could be promising electrode materials for supercapacitors.

4. Conclusions In conclusion, hybrid heterostructured SnO2@MnO2 nanoflakes grown on nickel foam have been successfully fabricated without any surfactants via two-step hydrothermal method. SnO2 nanoflakes directly grow on nickel substrate and then MnO2 grow on the surface of SnO2 nanoflakes to form heterostructured nanoflakes structure. SnO2@MnO2 composites as the working electrode show a large areal capacitance of 1231.6 mF cm-2 at the current density of 1 mA cm-2 and good rate

12

capability through the synergistic effects and good cycling performance after 6000 cycles at 10 mA cm-2. The results reveal SnO2@MnO2 composites are anticipated to be applicable electrode materials in energy storage devices.

Acknowledgement: This work was supported by the Scientific Research Fund of Heilongjiang Provincial Education Department (12531179).

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Figures caption Fig. 1 (a) Typical XRD patterns of the as-prepared nanoflakes grown on nickel foam (c-f) EDS mapping from Fig. 1 (b). Fig. 2 (a-d) SEM images of SnO2 nanoflakes and SnO2@MnO2 nanoflakes grown on nickel foam at different magnification (e-f) TEM images of the as-synthesized SnO2@MnO2 nanoflakes, the inset image in Fig. 2 (f) is the SAED pattern. Fig. 3 (a) XPS spectra of SnO2@MnO2 composites, (b) Mn 2P. Fig. 4 Nitrogen adsorption-desorption isotherm, the inset is BJH pore size distribution plot of SnO2@MnO2 nanoflakes Fig. 5 Electrochemical tests of the SnO2@MnO2 product (a) CV curves of SnO2@MnO2 and pure nickel foam at a scan rate of 40 mV s-1 (b) CV curves of SnO2@MnO2 electrode at various scan rates (3-40 mV s-1) (c) Galvanostatic charge-discharge curves of the composite samples at different current densities (d) areal capacitance of SnO2@MnO2 hybrid electrode at different current densities. Fig. 6 (a) The capacitance retention as a function of discharge current densities (b) CV curves of SnO2@MnO2 , MnO2, SnO2, pure nickel foam materials at a scan rate of 40 mV s-1 (c) The galvanostatic charge-discharge curves at a current density of 1 mA cm-2 of SnO2@MnO2 hybrid composites, MnO2 and SnO2, respectively (d) EIS spectra of the samples Fig. 7 Cycle performance of SnO2@MnO2 hybrid electrode at the current density of 10 mA cm-2

19

Table 1 Comparison of the capacitance of SnO2@MnO2 nanoflakes with several reported electrodes Materials

Capacitance

Ref.

α-Fe2O3@NiO

557 mF cm-2

18

SnO2@Co3O4

587.5 mF cm-2

31

SnO2@NiO

660 mF cm-2

31

Fe3O4@SnO2

2.7 mF cm-2

32

ZnO/ZnS

217 mF cm-2

33

α-Fe2O3

681 mF cm-2

34

SnO2@MnO2

1231.6 mF cm-2

This work

20

MnO2 crystalline birnessite-ty pe structure

SnO2 tetragonal rutile structure

Fig. 1 Huanhao Xiao et al.

Fig. 1

21

Fig. 2

22

(a

0

Sn 3d3/2 O 1s Mn 2p3/2 Mn 2p Sn 3p3/2 1/2 Sn 3p1/2

Intensity(a.u.)

Mn 2p3/2 (MnO2) 11.7

Mn 2p1/2 (MnO2)

C 1s

Sn 4d Mn 3p Mn 3s

Sn 3d5/2

Intensity(a.u.)

(b

200

400

600

800

1000

1200

630

640

650

660

Binding Energy(eV)

Binding Energy(eV)

Fig. 3

23

670

680

1.6

1.2

0.020

Volume Adsorbed(cm3g-1)

Volume Adsorbed (cm3 g –1)

2.0

0.015

0.010

0.005

0.000 0

2

4

6

8

10

12

14

Pore Size(nm)

0.8

0.4

0.0 0.0

0.2

0.4

0.6

0.8

Relative Pressure (P/P0)

Fig. 4

24

1.0

(a)

(b)

0.03

0.02

Current density (mA cm-2)

Current density (mA cm-2)

0.02 0.01 0.00 -0.01 -0.02

0.01 0.00 3 mV s-1 5 mV s-1 8 mV s-1 10 mV s-1 20 mV s-1 40 mV s-1

-0.01 -0.02 -0.03

-0.03 0.0

-0.2

-0.4

-0.6

0.0

-0.8

-0.2

-0.4

-0.6

-0.8

Potential (V)

Potential (V)

(c)

(d) 1 mA cm-2

0.0

1400

2 mA cm-2 1200

Areal Capacitance(mF cm-2)

3 mA cm-2 -2

5 mA cm

-0.2

Potential（ V）

0.03

Ni foam SnO2@MnO2

8 mA cm-2 -2

10 mA cm -0.4

-0.6

-0.8

1000 800 600 400 200 0

0

200

400

600

800

1000

1200

1400

1600

0

2

4

6

8 -2

Time (s)

Current Density(mA cm )

Fig. 5

25

10

(a)

(b)

120

0.02

Current density (mA cm-2)

100 80

Retention(%)

0.03

60 40 20

0.01 0.00 -0.01

SnO2@MnO2 MnO2

-0.02

SnO2 Ni foam

-0.03 0 0

2

4

(c)

6

8

0.0

(d)

-0.4

-0.6

-0.8

16

SnO2

14

MnO2

12

-Z"/ohm

-0.4

-0.2

Potential (V)

SnO2@MnO2

-0.2

Potential（ V）

0.0

10

Current Density(mA cm-2)

SnO2@MnO2

10

MnO2

8

SnO2

6 4

-0.6 2 0

-0.8 0

200

400

600

800

1000

1200

1400

1600

0

2

4

Time (s)

6

8

10 Z'/ohm

Fig. 6

26

12

14

16

18

20

500 400

506.25 mF cm-2 140

300

120

340 mF cm-2

100

Retention (%)

Areal Capacitance (mF cm-2)

600

200

80

100%

60 67.2%

40

100

20 0 0

1000

2000

3000

4000

5000

6000

Number of cycles

0 0

1000

2000

3000

4000

5000

6000

Number of cycles

Fig. 7

27