Facile synthesis of flower-like Bi2MoO6 hollow microspheres for high

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Facile synthesis of flower-like Bi2MoO6 hollow microspheres for high-performance supercapacitors Ting Yu, Zhao-Qian Li, Shuanghong Chen, Youcai Ding, Wangchao Chen, Xuepeng Liu, Yin Huang, and Fantai Kong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04673 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Sustainable Chemistry & Engineering

Facile synthesis of flower-like Bi2MoO6 hollow microspheres for high-performance supercapacitors Ting Yua,b, Zhaoqian Lia, Shuanghong Chena,*, Youcai Dinga,b, Wangchao Chenc,a, Xuepeng Liua,b, Yin Huanga,b, Fantai Konga,* a

1

Key Laboratory of Photovoltaic and Energy Conservation Materials, CAS, Institute of Applied

Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, No 2221 Changjiangxi Road, Shushan District, Hefei, Anhui, 230088, P.R. China b

University of Science and Technology of China, No 96 Jinzhai Road, Baohe District, Hefei,

230026, P.R. China c

School of Chemistry and Chemical Engineering, Anhui Province Key Laboratory of Advanced

Catalytic Materials and Reaction Engineering, Hefei University of Technology, No 193 Tunxi Road, Baohe District, Hefei 230009, P. R. China

ABSTRACT Here, we synthesize the flower-like bismuth molybdate (Bi2MoO6) hollow microspheres via a facile hydrothermal method. Morphology characterization suggests that this structure possess numerous mesopores as well as high specific surface area. When we use the as-synthesized Bi2MoO6 as supercapacitors’ electrode material, it shows a high specific capacitance (182 F g-1 at current densities of 1 A g-1) as well as excellent rate property (retaining 80% of the capacitance at a current

1

Address correspondence to [email protected] (S. Chen); [email protected] (F. Kong). 1

ACS Paragon Plus Environment

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density of 5 A g-1) and cyclic stability. Therefore, we can take into account the characteristics of hollow sphere and its outstanding performance when we design and synthesis electrode materials for future energy storage systems.

Keywords: flower-like, hollow microsphere, bismuth molybdate, supercapacitor.

Introduction Considerable work have been devoted to alleviate the environmental pollution by developing clean and renewable energy storage technologies.1-5 Supercapacitors are currently emerging as an attractive energy storage device because of their excellent properties for example, high power density, fast charge/discharge rates as well as long lifetimes.6-8 In general, the electrode materials with high conductivity and effective charge transport pathways has a great influence on the performance of supercapacitors.9-11 Owing to its lost cost, good cycling stability, and high power density, carbon has been used as a representative electrode material,12-17 however, it still suffers from low energy density. Supercapacitors with high specific energy density can be achieved by using metal oxides electrode material, such as RuO2,18-20 NiO,21,22 and MnO2.23-25 Unfortunately, the high cost of RuO2 and the poor electrochemical conductivity of almost all metal oxides limited its further development. Recently, considerable research has focused on ternary transition metal oxides,26-29 owing to their rich redox reactions involving different cations and improved electronic conductivity compared with single component oxides. Moreover, through utilize the synergistic effect of two different metal species, e.g., ternary metal 2


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molybdates (Bi2MoO6,30-34 CoMoO4,35 and NiMoO4,36 etc), their electrochemical performance could be enhanced. Bi2MoO6 have been used in many fields, for example, photocatalysis and gas sensors due to its special optical and electric properties.37,38 Cheeringly, in recent years, the application of Bi2MoO6 nanomaterials has been expanded to energy storage field, especially supercapacitors. Liu et al. prepared Bi2MoO6 nanowires for supercapacitor applications and it shows predominant electrochemical performances (the specific capacitance is 1075 F g-1 when the current density is 0.1 A g-1).31 Ma et al. synthesized Bi2MoO6 nanotubes using template strategy and a reflux reaction, which gave a specific capacitance of 171.3 F g-1 when the current density is 0.585 A g-1.34 Nevertheless, it still suffers from a low specific capacitance under a higher current density. Thus more efforts need to be devoted to improve the related performance. Hierarchical hollow spheres have drawn broad interests since they not only increase the spatial dispersion but also lighten the mass weight, effectively. Consequently, the as-synthesized materials have a high surface area as well as more ion diffusion channel.39-42 Compared with bare MnO2 nanowire, Lee et al. found that MnO2 nanowire with a hierarchical structure exhibited an enhanced supercapacitor performance in electrolytes with different PH values.43 Therefore, it is meaningful to synthesis ternary Bi2MoO6 materials with hierarchical hollow microspheres to further enhance the electrical performances of electrode material. Herein,

we

synthesized

the

hierarchical

flower-like

Bi2MoO6

hollow

microspheres by a simple method. The unique architecture is constructed by 3



nanosheets in all directions which provide high surface area. Numerous nanosheets could promote charge transport and ions diffusion greatly. Besides, the as-synthesized Bi2MoO6 hollow microspheres exhibit remarkable electrochemical performance (146 F g-1 at a current density of 5 A g-1).

Experimental section Synthesis of hierarchical Bi2MoO6 nanostructures

All solvents and chemicals were not purified and used directly. Using a improved hydrothermal method that is built on the reported method previous, the hierarchical Bi2MoO6 materials were synthesized.44 First, Bi(NO3)3·5H2O (3.5 mmol) and Na2MoO4 (1.75 mmol) were dissolved in ethylene glycol (10 mL) under constant magnetic stirring for 1 h. Then, ethanol (20 mL) was transferred into the above solution slowly, and in order to achieve a homogeneous solution, the solution was stirring for a certain period of time. The clear solution was added to the teflon-lined stainless steel autoclave (100 mL). The autoclave was cooled down to room temperature after heated and maintained at 160 °C for 16 hours. Through washed with ethanol, followed by DI water for three times, we removed the attached products on the surface of synthesized yellow samples. Finally, the products were dried at 80 °C overnight in air.

Materials characterizations

The Micro Diffractometer (model D5005, λ = 1.5406 Å, Cu Kα radiation) is used for 4


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the X-ray diffraction (XRD) patterns test. The morphology and structure of products was recorded through transmission electron microscopy (JEOL; JEM-200CX) and scanning electron microscopy (SEM, FEI XL-30 SFEG which coupled with a TLD). Using

nitrogen

adsorption/desorption

apparatus

(Micromeritics

Instrument

Corporation, TriStar II 3020 V1.03), the pore volume, surface area and pore size were presented. In vacuum and an excitation source of monochromatic Al Ka radiation at 15KV and 150W, using the Kratos Axis Ultra X-ray photoelectron spectrometer to illustrate the X-ray photoelectron spectroscopy (XPS). In N2 atmosphere and with a heating rate of 10 ℃ per minute, the Q5000 IR thermal gravimetric analyzer is used to obtain the thermal gravimetric (TG) analysis of samples. The conductivity of the as-synthesized sample was established by a four point probe instrument (RTS-9, Guangzhou four-point probe Co. Ltd., China) with volume resistivity within 106 Ω cm at room temperature. We press the powder into sheet under the pressure (10 MPa) for 5 minutes.

Electrodes preparation and electrochemical measurements

To prepare a working electrodes, Bi2MoO6 (80 wt %) was well mixed with carbon black (10 wt %) and polytetrafluoroethylene (PTFE, 10 wt %), and grind till it forms slurry in ethanol solution. Then, we coated the electroactive material (~2.0 mg) on nickel grid (1.0 cm × 1.0 cm) that served as a current collector. Finally, the electroactive material was sintered at 80 °C in air for 8 hours. We investigated the electrochemical properties of samples on electrochemical 5



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workstation (IM6ex, Germany, Zahner Company) in a three-electrode system that composed by working electrode, reference electrode (saturated calomel electrode (SCE)) and the counter electrode (platinum plate). All of the electrodes were immersed into KOH (3 M) electrolyte. Within a suitable potential range (0 - 0.6 V), the cyclic voltammetry (CV) curve was performed through employing the scan rates from 5 to 50 mV s-1. Within a suitable potential range (0 - 0.5 V) and at different current densities which varying from 1 to 5 A g-1, the galvanostatic charge–discharge (GCD) curves was measured. The follows equation shows the relationship between current densities and specific capacitance: ‫=ܥ‬

୍×Δ୲ ୫×Δ୴

(1)

m(g): the mass of active materials, ∆V(V): the working potential excluding the IR drop, I(A): the discharge current, ∆t(s): the discharge time. The Coulomb efficiency (િ) is measured by the following formula: ୲

η = ×100% ୲ᇱ

(2)

t: the diacharge time, t': the charge time. At a frequency range varying from 10 mHz to 100 kHz amplitude of 10 mV, we obtained the electrochemical impedance spectroscopy (EIS) curves.

Results and discussion Characterization of hierarchical Bi2MoO6 hollow spheres

6


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Figure 1. (a-c) FESEM graphics, (d, e) TEM image, (f) high-resolution TEM graphic of as-synthesized samples. Figure 1a shows the morphology of the as-prepared Bi2MoO6 sample, from which numerous flower-like hierarchical microstructures with a diameter of approximately 1-2 µm were clearly observed. From Figure 1b we can confirm that individual flower-like microsphere consists of loosely arranged nanosheets and the thickness is around 20 nm. These nanosheets interleaved together and engendered a hollow structure. It can be foreseen that numerous nanosheets can improve the utilization ratio of the electroactive materials as well as supply numerous transport paths for small molecules.45-48 To obtain further information about the structure and morphology of as-obtained Bi2MoO6, we performed TEM and

HRTEM

measurements. Figure 1c further confirmed the hollow structure. HRTEM image (Figure 1d) presents the fringe spacing of 0.32 nm, which correspond well with the (140) planes of orthorhombic Bi2MoO6 well. The reaction mechanism was proved by corresponding SEM graphic (Figure S1) and illustrated in Figure 2. The homogeneous solution (step 1) might contains production of complex of Bi3+ or MoO42- with OH 7



through a unique coordination between Bi3+ (MoO42-) with ethylene glycol.49 In the primary stage of solvothermal reaction, the Bi2MoO6 nanoparticle is produced and gradually assemble into spherical agglomerates (step 2) in order to reduce the surface energy of nanoparticles. During this period, the smaller crystallites are gradually dissolved and generate abundant ions (Bi2O22+ and MoO42-) (step 3) because the solution is weakly acidic, which is caused by the special orthorhombic structure of Bi2MoO6.38,50 At the same time, these ions were transferred onto the surface of microspheres which provide lots of growing point for further nanocrystalline growth. The re-formed nanoparticles further grow into 2D sheet-like structure (step 4).38,51 After a appropriate solvothermal time, the internal solid microsphere would dissolved (step 5). Eventually, the hierarchical Bi2MoO6 hollow microspheres are formed.

Figure 2. Schematic formation mechanism of the Bi2MoO6 microspheres. Via the X-ray diffraction (XRD) test, we obtained the phase of as-synthesized Bi2MoO6 sample. The XRD pattern (Figure 3a) shows sharp and intense peaks, and 8


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the characteristic diffraction peaks at 10.93°, 23.52°, 28.31°, 32.64°, 33.14°, 36.06°, 39.56°, 47.18°, 55.44°, 55.59°, 56.25°, 58.48°, 68.20° coincide with (111), (220), (311), (400), (511), and (440) crystal planes of the orthorhombic phase of Bi2MoO6 (JCPDS card no. 84-0787), respectively. Moreover, the product is high purity because

100

(400)

(331)/(133) (191) (262)

(062) (240)

(060) (151)

(002)

as-prepared sample

(111)

(020)

Volume adsorbed (cm-3g-1)

b 120

(131)

a

JCPDS NO.84-0787 10

20

30

40

50

60

70

2 Theta (degree)

80 60 40

Pore volume (cm3g-1nm-1)

no impurity peaks were detected.

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.020

C

0.016 0.012 0.008 0.004 0.000 0

5

10

15

20

25

Pore diameter (nm)

20 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

Figure 3. (a) XRD and standard patterns, (b, c) the curves of Nitrogen adsorption/desorption and pore size distribution of as-prepared sample. To characterize the porous nature of hierarchical Bi2MoO6 hollow microspheres, the nitrogen adsorption/desorption isotherm was conducted (Figure 3b). The as-prepared Bi2MoO6 has a specific surface area of 44.6 m2 g-1, which is higher than the previously reported samples.38,52 In the isotherms the hysteresis loops implies the presence of mesopores, from Figure 3b we could also calculated the average pore diameter is 17.4 nm. These meso-pores and high surface area of flower-like Bi2MoO6 consist with the results of SEM and TEM. It is generally believed that mesopores play significant roles in electrochemical process, since it promote the penetration of electrolyte that promotes the near surface and surface redox reactions. Therefore, we can conclude that the flower-like Bi2MoO6 hollow microspheres could exhibit good 9



electrochemical performance for its abundant mesopores and high surface area.

a

b Mo 3d Intensity (a.u.)

Intensity (a.u.)

Bi 4f

O 1s Bi 4d5 Mo 3d

0

200

400

600

800

228

230

Binding Energy (eV)

232

234

236

238

Binding Energy (eV)

c

d Bi 4f

O 1s Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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156

158

160

162

164

166

168

527

528

529

530

531

532

533

534

535

Binding Energy (ev)

Binding Energy (eV)

Figure 4. XPS of flower-like Bi2MoO6 hollow microspheres: (a) survey spectra, (b)Mo 3d spectra, (c) Bi 4f spectra, and (d) O 1s spectra. To characterize the oxidation state and element composition of Bi2MoO6, we measured the X-ray photoelectron spectroscopy (XPS). As is shown in Figure 4a, four peaks located at 530.0, 440.0, 232.3 and 159.1 eV correspond to O 1s, Bi 4d5, Mo 3d and Bi 4f levels, respectively. The Mo 3d shows two peaks at 235.5 and 232.1 eV, related to the Mo 3d3/2 and Mo 3d5/2 of Mo6+. The peaks at 164.2 and 158.6 eV are consisted with the Bi 4f5/2 and Bi 4f7/2 of Bi3+. With regard to O 1s, we could get three peaks via deconvolution. Among them, the peak at 529.3eV belong to the Bi–O bonds in Bi2MoO6.53 The oxygen coordination in the small-size particles bring about a few defect sites, which caused the binding energy at peak of 530.4 eV. There are 10


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physic-chemisorbed waters on the interior and surface of the material because of the energy peak at 532.9 eV.54 The existence of physic-chemisorbed water could be confirmed by thermal gravimetric analysis (TGA), as shown in Figure S2b. The result of XPS is consistent with the observation from XRD. Electrical conductivities of Bi2MoO6 were measured using the four-point probe method. It was found that the conductivity of Bi2MoO6 is 2.57×10−3 S m−1, and through mixed with carbon black (10 wt %), the conductivity of electrode materials will increase.

Evaluation on electrochemical properties

In view of the desirable electrochemical performance of bismuth molybdenum oxide in supercapacitors and the unique microstructure of as-synthesized Bi2MoO6 products, the electrochemical behavior of flower-like Bi2MoO6 hollow microspheres were further measured in KOH (3.0 M) electrolyte in a suitable potential window. As shown in Figure 5a, well-defined redox peaks were exhibited, indicating the presence of faradaic redox reactions.55 When increase the scanning rate, we observed that the shapes of these curves keep almost unchanged and only the reduction peak moves negatively and the oxidation peak moves positively, which not only reveals perfect capacitive behavior and superior charge collection, but also exhibit the reversible faradaic redox reaction. The redox reactions of bismuth molybdate is based on the following equations that is similar to bismuth oxide in the alkaline electrolyte:56,57,30 11



BiO2− + e−→ BiO22−

(3)

2H2O + 3BiO22− →2BiO2− + 4OH− + Bi0

(4)

Bi0→ Bi

(5)

To get deep insight about the capacitance properties, the GCD measurements of bismuth molybdate electrode were tested at different applied current densities (Figure 5b). At different current densities of 1, 2, 3, 4 and 5 A g-1, the obtained specific capacitance are 182, 152, 156, 149 and 146 F g-1 (Figure S2d), respectively. It should be pointed out that Bi2MoO6 nanosheets growth on the nickel foam only gave a specific capacitance of 37.3 F g-1 when the current density is 2 A g-1(Figure S2a).33 To confirm the electrode materials’ charge storage mechanism, the relationship between sweep rate (v) and peak current (i) can be used (i=avb). When b is equal to 1, it is a typical capacitive behavior or pseudo-capacitive behavior, and if b is equal to 0.5, it is a battery behavior.55,58,59 In this study, we plotted the peak current (i) versus sweep rate (v) and found the peak current (i) of Bi2MoO6 electrode linearly increased with the sweep rate (v), indicating a b-value that is equal to 1 (Figure S2c). So the as-synthesized Bi2MoO6 is based on storage energy of a typical capacitive process.

5 mV/s 15 mV/s 50 mV/s

20

b0.5

10 mV/s 25 mV/s

Potential (V vs.SCE)

a 30 Current (A g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10 0

-10

0.4 0.3

1 A g-1 2 A g-1

0.2

3 A g-1

0.1

4 A g-1

-20

5 A g-1

0.0 -30 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0

50

100

150

Time (S)

Potential (V vs. SCE)

12


200

250

c 2100 1500

1.0

1200

0.8

900

0.6

140

90 85

100

0.4 0.2 0.0

0

95

120

600 300

Specific capacitance (F g-1)

d160 After 1000 cycles Before charge-diacharge

1800

-Z'' (Ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

0.2

0.4

0.6

0.8

1.0

80

80 75 60

Coulombic efficiency (%)

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70

0

300

600

900

1200 1500 1800 2100

0

500

Z' (Ohm)

1000

1500

2000

2500

3000

Cycle number

Figure 5. (a) The cyclic voltammetry measurements of as-synthesized sample within 0.0 – 0.60 V at a scanning rate of 5 - 50 mV s-1 in KOH (3.0 M) electrolyte, (b) The galvanostatic charge–discharge characteristics of as-prepared sample with current densities from 1 to 5 A g-1 in KOH (3.0 M) electrolyte, (c) EIS Nyquist plots of as-prepared sample, (d) Charge/discharge cycling measurement and the corresponding coulombic efficiency at a current density of 3 A g-1. We investigated the device resistance before and after 1000 cycles to verify the above-mentioned results about charge transfer and electrolyte diffusion (Figure 5c). As can be seen in the high frequency region, the less obvious semi-circles revealed that the equivalent series resistance (ESR) of sample is 0.36 Ω, indicating a low interfacial resistance between electroactive material and Ni foam as well as a small charge transfer resistance. The slope of the curves (before charge-discharge) at the low frequency region is observed to be around 50°, and it illustrates the low diffusive resistance about electrolyte ions in electroactive materials. Moreover, after 1000 cycles the straight lines become more vertical. It may be due to the improved contact between the electroactive materials and electrolyte. The EIS plot also revealed the excellent electrochemical stability of as-fabricated electroactive materials.60,61 13



Stability is an important parameter for capacitors. Therefore, we measured the stability of supercapacitors composed by as-prepared hierarchical Bi2MoO6 hollow microspheres. As shown in Figure 5d, at the current densities of 3 A g-1, the device keeps about 95% retention after 400 cycles. After 3000 cycles, the electroactive material exhibited a slight decrease in capacitance by ~15%. Impressively, with increasing the current density from 1 to 5 A g-1, it still retains 80% of the capacitance, which displayed a high rate capability. The coulombic efficiency remains over 88% during the charge/discharge cycling (Figure 5d), which exhibit excellent stability of supercapacitors.

Conclusions In summary, we successfully synthesized the hierarchical flower-like Bi2MoO6 hollow microspheres by a simple hydrothermal method and employed as electroactive material for supercapacitors. The as-synthesized material shows a specific capacitance of 182 F g-1 at 1 A g-1, ultrahigh rate capability (retaining 80% of the capacitance at a current density of 5 A g-1), and well cyclic stability in aqueous alkaline electrolyte. The higher BET surface area and numerous transport paths which were provided by the hierarchical architectures were contributed to the excellent electrochemical performance. The simplicity of hollow sphere and their outstanding performances indicates an effective avenue to next-generation supercapacitors negative electrode material.

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ASSOCIATED CONTENT Supporting Information Additional data including the supporting images for the schematic illustration of Bi2MoO6, specific capacitance of as-synthesized Bi2MoO6 and two other Bi2MoO6 nanosheet reported previously for comparison, TGA analysis, b-Value determination of as-synthesized Bi2MoO6, and the specific capacitance of as-synthesized Bi2MoO6 at various current densities.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S. Chen);[email protected] (F. Kong). ORCID

Notes The authors declare no competing financial interest.

Acknowledgements This work is supported by the National Basic Research Program of China (No. 2015CB932200), National Natural Science Foundation of China (No. 61404142), CAS-Iranian Vice Presidency for Science and Technology Joint Research Project (No. 116134KYSB20160130), and Open research fund of the state key laboratory of alternate electrical power system with renewable energy sources (NCEPU): (No. 15



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LAPS17009).

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TOC/Abstract Graphic：：

The simplicity of hollow sphere and their outstanding performance establish an effective avenue to design hybrid materials for future energy storage systems.

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Facile synthesis of flower-like Bi2MoO6 hollow microspheres for high

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