Ultrathin Manganese Dioxide Nanosheets Grown on Mesoporous

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Ultrathin Manganese Dioxide Nanosheets Grown on Mesoporous Carbon Hollow Spheres for High Performance Asymmetrical Supercapacitors Leicong Zhang, Xuecheng Yu, Lulu Lv, Pengli Zhu, Fengrui Zhou, Gang Li, Rong Sun, and Chingping Wong ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01022 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 22, 2018

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ACS Applied Energy Materials

Ultrathin Manganese Dioxide Nanosheets Grown on Mesoporous Carbon Hollow Spheres for High Performance Asymmetrical Supercapacitors Leicong Zhang,†,‡ Xuecheng Yu,†,‡ Lulu Lv,† Pengli Zhu,*,† Fengrui Zhou,† Gang Li,† Rong Sun,† and Ching-ping Wong†,ǁ †

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences,

Shenzhen 518055, China ‡

Shenzhen College of Advanced Technology, University of Chinese Academy of

Sciences, Shenzhen 518055, China ǁ

School of Materials Science and Engineering, Georgia Institute of Technology,

Atlanta, Georgia 30332, United States

*Corresponding author, email: [email protected]

1

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ABSTRACT Designing and fabricating the electrochemical active materials with outstanding electrochemical performance is crucial for supercapacitors in applications of energy storage devices. In this study, a structure of ultrathin manganese dioxide (MnO2) nanosheets deposited on the surface of mesoporous carbon hollow spheres (MCHSs) is designed and fabricated through a simple method. MCHSs are served as both substrate materials for growth of MnO2 nanosheets and electrical conductive layer for improving the poor electrical conductivity of MnO2, thus obtaining MCHS/MnO2 with

good

electrochemical

performance.

What’s

more,

the

synergy

of

pseudocapacitance and electrical double-layer capacitance (EDLC) in asymmetrical configuration plays a crucial role in enhancing energy density of assembled supercapacitors. The obtained results exhibit that asymmetrical MCHS/MnO2//MCHS supercapacitor displays large specific capacitance (116.4 F/g at 0.1 A/g) and superior cycling stability (90.3% of capacitance retention after 6000 cycles), and delivers high energy density of 64.6 Wh/kg at power density of 100 W/kg. It has been demonstrated that assembled supercapacitor could be used as a small power supply to light a red LED. This study may provide a feasible method in energy storage applications. KEYWORDS:

mesoporous

carbon

hollow

spheres,

supercapacitor, energy density, operating voltage

2


MnO2,

asymmetrical

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Fossil fuels, the continued backbone energy sources of the world, are dried up day by day with national and global development. And there are also challenges of serious environmental pollution and rising global warming that humanity shares in common at present.1−3 Therefore, the unprecedented survival crisis human faced with forces us to develop and explore the renewable and sustainable energy sources,1 such as solar, water, wind, nuclear energy, and so on, which all can be converted to electricity to promote the long-term development of human.4,5 And the research of energy storage and conversion devices has drawn growing attentions from researchers and governments in the world. Among them, supercapacitor is regarded as one of the most promising candidates due to its fast charge/discharge rates, large power density, high security and stability, as well as the extremely outstanding cycling life for the future energy device which can alleviate the increasingly aggravated contradiction between economic development and environmental protection.6−8

Despite its great advantages and importance, challenges to fabricate supercapacitors with high-quality and high-performance restrict the practical applications of supercapacitors, are always there.9−11 Therefore, in the past few decades, much more effort has been put to develop the efficient electrochemical active materials and facile preparation methods for supercapacitors.12,13 At present, there are two efficient strategies to change the situation of low energy density of supercapacitor, such as improving specific capacitance of electrode material and broadening the operating potential window.14−16 The way can be introduced in detail as follows: (1) In order to increase the capacitance of the whole cell, pseudocapacitance materials with large 3



specific capacitance can be chosen, i.e., transition metal oxides, hydroxides and sulfides; and electrical conductivity and specific surface area of electrode should be improved as much as possible.14,17−20 Pseudocapacitance materials, including MnO2, Co3O4, NiO, Ni(OH)2, NiS and MoS2, etc., have more excellent specific capacitance than most of carbon/graphite materials.8,15,16,21−26 Among the various candidates of the pseudocapacitance materials in most studies, MnO2 has always been regarded as a promising material because of its environmental friendliness, natural availability, low cost, but above all, it has excellent theoretical specific capacitance which is as high as 1370 F/g.16 Nevertheless, the MnO2 shows low specific capacitance and terrible rate capability in practical, suffered from its poor electrical conductivity, which restricts its practical application in energy storage.16,27,28 The important and effective consideration for solving these problems is to deposit MnO2 on highly conductive substrates, i.e., carbon clothes, Ni foam and electric conductive paper or conductive nanostructures, i.e., well-ordered metal nanocore arrays and conductive carbon nanomaterials, which can effectively and drastically promote the conductivity and specific surface area of electrode materials.16,29−33 (2) Cell configuration and electrolyte are two major factors that have specific effects on cell’s potential window. Asymmetrical system can introduce additional electrochemical potential difference as using different electrolytes.34,35 Therefore, effective approaches to broaden the potential window are mainly focused on choosing the asymmetrical assemble style and using solid-state gel/ion electrolyte. For example, in our previous work, highly electric conductive porous Ni(II)−filter paper was employed to electro-deposition

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MnO2 nanostructure and obtain positive electrode with high performance, and then asymmetrical assemble configuration in which active carbon (AC) was used as negative electrode material was applied to broaden the potential window to 2.5 V;33 Zijin Su et al. reported electrodeposition of MnO2 onto the highly ordered ultrathin 3D Ni nanocore arrays used as positive electrode, AC used as negative electrode and ionic liquid gel electrolyte are selected to prepare asymmetric supercapacitor, which behaved the excellent energy density of 52.2 W·h/kg and allowed the wide potential window of 2.5 V.16 These methods introduce the large specific capacitance and operating voltage to enhance the final energy density of cell. However, the whole fabrication process is still complicated and the productivity is not high enough. Therefore, there is still an urgent to explore low cost, facile and large-scale techniques for producing the electrical conductive materials-based MnO2 composites for supercapacitors.

Herein, we fabricated the mesoporous carbon hollow spheres (MCHS) with good electrical conductivity and high specific surface area, then MCHS-based MnO2 nanostructure composites were successfully prepared by convenient hydrothermal method. MCHS is not only used as the conductive substrate material, but also the reactant to react with KMnO4 to generate MnO2 improving adhesion between electrochemical active material layer MnO2 and conductive layer MCHS and decreasing the ion diffusion path. In view of the above-mentioned analysis, the beneficial asymmetrical configuration strategy was also adopted to achieve the broad operating potential window to obtain the asymmetrical supercapacitors with 5



distinguished energy density. Specifically, porous MCHS/MnO2, MCHS and natural Na2SO4 solution were used as positive electrode material, negative electrode material and electrolyte, respectively. The assembled asymmetrical MCHS/MnO2//MCHS supercapacitor (AMS) shows excellent energy density of 64.6 W·h/kg and distinguished cycling stability, exhibiting strong advantages in high electrochemical performance and facile preparation process.

RESULTS AND DISCUSSION The preparation schematic diagram of MCHS and MCHS/MnO2 is depicted in Figure 1. The method to prepare mesoporous carbon hollow spheres includes the following steps: (1) TEOS, which possesses faster hydrolysis and condensation than TPOS, generates abundant silica particles to form the larger silica cores during the early stage; (2) A large amount of silica particles derived from both TPOS and residual TEOS and the resorcinol−formaldehyde (RF) oligomers co-deposit on the surface of the silica cores in the second stage, which the core-shell SiO2@SiO2/RF nanospheres form; (3) After high temperature carbonization, the formaldehyde (RF) oligomers would be transformed to carbon；The silica component inside and outside can be easily removed by HF solution, and mesoporous carbon hollow spheres (MCHSs) are prepared.35 To fabricate the MCHS/MnO2 hollow nanospheres, MCHSs are performed as the primary reactants slowly reacted with KMnO4, forming a small quantity of MnO2 nanocrystallines tightly coated on the surface of MCHSs, followed by a large number of MnO2 nanoflakes generated by the fast decomposition of

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Figure 1. Schematic diagram of fabrication process for MCHS and MCHS/MnO2.

KMnO4. Using this method, both the positive electrode material of MCHS/MnO2 and the negative electrode material of MCHS are successfully synthesized, and the preparation process is low cost, facile and large-scale.

Further, SEM and TEM measurement were employed to identify the microstructure and surface morphology of prepared nanospheres. Figure 2a reveals that the uniform MCHSs with an average size of 370 nm were successfully obtained and can be easily arrayed in order. The magnified SEM images of MCHS are presented in Figure 2b and Figure S1a, which suggest that there are lots of pores with size less than 10 nm on surface of MCHS, allowing MnO4− ions permeated into inner cavity of MCHS that results in MnO2 nanosheets depositing on the inwall of MCHS, and providing more active sites to enhance the capacitance of MCHS and MCHS/MnO2. And TEM image of MCHS provided in Figure 2c indicates hollow structure of MCHS and MCHS wall’s thickness is measured to be about 32 nm. High resolution TEM image of MCHS obviously exhibits that there are many channels on MCHS wall from the inside out, allowing the easy ions diffusion process, as exhibited in Figure S2a and Figure S3a. Therefore, rough surface and direct redox reaction between MCHS and MnO4− ions strengthen the binding force between MCHS and MnO2 which is 7



Figure 2. SEM (a and b) and TEM (c) images of MCHS; SEM (d and e) and TEM (f) images of MCHS/MnO2 composite.

beneficial to electrons transfer and ions diffusion during the electrochemical reaction process. SEM image of MCHS/MnO2 at low magnification clearly shows that MnO2 is uniformly coated on MCHS and the average diameter of MCHS/MnO2 is larger than MCHS. What’s more, there are a few MnO2 nanowires intertwining and lying on some MCHS/MnO2 nanospheres. The broken sample of MCHS/MnO2 treated by ultrasound for SEM characterization is shown in Figure S1b, from which MnO2 nanosheets grown on both inside and outside surface of MCHS can be obviously observed, confirming the porous surface and hollow structure of MCHS. In Figure 2e, magnified SEM image exhibits that the nanosized pores existed on surface of MCHS are completely covered and dense MnO2 nanosheets grow vertically, producing the MCHS/MnO2 hollow structures that internal MCHS as electrical conductive layer and MnO2 nanosheets as active layer which can improve the rate capability. To further 8


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evaluate the structure of prepared MCHS/MnO2, TEM measurements were also carried out. In Figure 2f, vertically grown MnO2 nanosheets are homogeneously coated on MCHS and the thickness of MCHS wall barely changes, producing the MCHS/MnO2 hollow nanosphere structure with the average diameter of 460 nm. The magnified TEM images of MCHS/MnO2 exhibit the ultrathin MnO2 nanosheets is presented in Figure S2b and Figure S3b, indicating much more sufficient active sites for electrochemical reactions. Furthermore, in order to confirm the elements distribution on MCHS/MnO2, the HAADF-STEM image and element mapping images are provided, which show that C, Mn and O elements exist in prepared MCHS/MnO2 hybrid materials and Mn and O elements are uniformly distributed on MCHS, as shown in Figure S4.

To accurately identify the composition of MCHS and MCHS/MnO2, powder XRD test was performed and results are depicted in Figure S5. The broad XRD peak located at about 25o is corresponding to the carbon derived from the high temperature carbonization of RF. Furthermore, the peaks respectively located at 12o, 37o and 66o in XRD pattern (marked in red) definitely show that MnO2 phase really exists after the hydrothermal reaction. And Raman spectroscopy technology was used for the characterization of MCHS and MCHS/MnO2, and the spectra are shown in Figure S6. Two main vibrational features can be obviously recognised at wavenumber of 1346 cm-1 (D band) and 1593 cm-1 (G band) derived from MCHS prepared by high temperature carbonization, demonstrating that both disordered and graphited carbon exists in MCHS. After MnO2 nanosheets growing on surface of MCHS, the intensity 9



ratio ID/IG increases, indicating that the defects in MCHS/MnO2 are more than MCHS. What’s more, there are three weak peaks respectively located at wavenumber of 500, 572 and 629 cm-1 in Raman spectrum of MCHS/MnO2 that are corresponding to MnO2. The pore size distributions and specific surface areas of MCHS and MCHS/MnO2 were identified by N2 adsorption/desorption analysis and BET calculation process, as shown in Figure S7. Hysteresis loop can be clearly observed both in N2 adsorption/desorption curves of MCHS and MCHS/MnO2, and the MCHS shows a higher absorbed volume than MCHS/MnO2, reflecting the larger specific surface area of MCHS, as shown in Figure S7a. And specific surface areas of MCHS and MCHS/MnO2 were respectively calculated to be 956.7 and 154 m2/g by BET method. Figure S7b shows pore size distributions of MCHS and MCHS/MnO2, and MCHS exhibits the much more pore volume with the pore diameter ranged from 1.7 to 10 nm, so the pore with size of less than 10 nm is dominated. And the pore size ranged from 1.7 to 100 nm exists in MCHS/MnO2 after MnO2 nanosheets growing on surface of MCHS, indicating that the micropores, mesopores and macropores exist.

XPS technology was implemented to further verify the information of MnO2 grown on MCHS. Spectra plotted in Figure 3 show a full spectrum of prepared sample and fine spectra of Mn 2p, Mn 3s and O 1s. In Figure 3a, three peaks located at 642.3, 529.8 and 84.5 eV in survey spectrum are respectively attributed to Mn 2p, O 1s and Mn 3s, indicating the manganese dioxide found.36 The fine Mn 2p spectrum is presented in Figure 3b, which has two peaks located at 653.9 and 642.1 eV to be well 10


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(b) O 1s

800

600

400

200

Mn 2p1/2

Intensity (a.u.)

Mn 3s

C 1s

1000

11.8 eV

Mn 2p

Mn 2p3/2

Intensity (a.u.)

Mn 2p

(a)

0

660

655

Binding energy (eV) Mn 3s

645

(d)

4.74 eV

O 1s

92

90

88

86

84

640

Raw line Base line H-O-H Mn-OH Mn-O-Mn Fitting line

Intensity (a.u.)

(c)

650

Binding energy (eV)

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


82

80

536

534

532

530

528

526

Binding energy (eV)

Binding energy (eV)

Figure 3. (a) XPS full spectrum of MnO2 grown on MCHS, and the multiplet splitting peaks for the core level XPS spectrum of Mn 2p (b), Mn 3s (c) and O 1s (d) regions.

corresponding to Mn 2p1/2 and Mn 2p3/2, the separation of which is 11.8 eV that is consistent with MnO2 previously reported.36,37 What’s more, the result reported from previous study shows that the average valence state of Mn element in manganese oxide depends linearly on the peak energy separation (∆E) in Mn 3s and trivalent and tetravalent manganese oxides correspond to ∆E values of about 5.4 and 4.7 eV, respectively.36,38,39 And ∆E value of prepared manganese oxide from Figure 3c is 4.74 eV, therefore, the mean valence state of Mn element is determined to be 3.94 on the basis of above studies. Also, the mean valence state of Mn element can be obtained 11



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from the signal of Mn−O−Mn and Mn−OH components in fine O 1s spectrum (Figure 3d) according to the following formula (1): X Mn =

IV × ( S Mn −O − Mn − S Mn −OH ) + Ⅲ × S Mn −OH S Mn −O − Mn

(1)

where S is the Mn−O−Mn or Mn−OH component signal in fine O 1s spectrum.36 XPS-peak-differentation imitating analysis results from O 1s show that Mn−O−Mn and Mn−OH component signals are respectively 65.73 and 26.12 area%, from which the mean valence state of Mn element is calculated to be 3.6 that is consistent with the result obtained from Mn 3s.

The electrochemical performance of MCHS/MnO2 was characterized through three-electrode system in 1 M Na2SO4 electrolyte, and potential window of MCHS/MnO2 in both CV and GCD test is 0−1 V, as exhibited in Figure 4. CV curves of MCHS/MnO2 at scan rate of 5−100 mV are plotted in Figure 4a, prepared MCHS/MnO2 hollow nanosphere structure exhibits the approximately rectangular and symmetrical CV curves in potential window of 0−1 V, revealing the well reversibility of MCHS/MnO2 electrode, and it benefits from the structure of ultrathin MnO2 nanosheets grown on both inner surface and outer surface of MCHS that improves the rate of charge transfer and ion diffusion. GCD tests at different constant current densities are shown in Figure 4b, all the curves show nearly symmetrical triangular characteristic even at high current density of 5 A/g, indicating fast and reversible Faradaic reaction at interface between electrolyte and electrode. The fast charge-discharge process occurred at whether low or high current densities is also

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(a)

15

(b)

0.2 A/g 0.5 A/g 1.0 A/g 2.0 A/g 5.0 A/g

1.0 0.8

5 0 -5

5 mV/s 10 mV/s 20 mV/s 50 mV/s 100 mV/s

-10

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-Z" (ohm)

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-Z" (ohm)

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Z' (ohm)

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75 0

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Z' (ohm)


Figure 4. (a) CV curves of MCHS/MnO2 at 3−50 mV/s; (b) GCD curves of MCHS/MnO2 at 0.1−1.0 A/g; (c) Specific capacitance of MCHS/MnO2 calculated from GCD curves; (d) Nyquist plot of EIS for MCHS/MnO2 and the high-frequency region is magnified in the inset.

ascribed to the hollow and porous nanostructure of MCHS/MnO2, with the aid of which the ultrafast rate of electron transfer and ion diffusion can be easily achieved. The relationship curve of specific capacitance calculated from Figure 4b and current density is plotted in Figure 4c, specific capacitance is 137.4 F/g at 0.2 A/g, and specific capacitance decreases with current density increases from 0.2 A/g to 5.0 A/g, the final capacitance retention of 81.1% is achieved, demonstrating a good rate capability. Nyquist plot of EIS for MCHS/MnO2 that a semicircle connects with an approximately straight line at region from high-frequency of 100 kHz to 13



low-frequency of 10 mHz is shown in Figure 4d, indicating good electrical conductivity of electrode and ultrafast diffusion rate of ions at MnO2-electrolyte interface. The intercept of 1.11 Ω on real axis represents the equivalent series resistance (Rs) which is the sum of electrolyte ionic resistance, electrode self-resistance and the contact resistance at electrode-electrolyte, is pretty low to ensure the ultrafast electrons transfer.

The above fabricated hollow MCHS/MnO2 nanospheres exhibit excellent electrochemical properties. Therefore, the asymmetrical MCHS/MnO2//MCHS supercapacitor (AMS) was assembled by respectively using the prepared hollow MCHS/MnO2 and MCHS (the electrochemical performance of MCHS is provided in Figure S8, and more results are discussed in detail in Supporting Information) electrochemical active materials as positive and negative electrode in 1 M Na2SO4 aqueous solution. The total mass of electrochemical active materials loaded on positive and negative electrode is 2.78 mg. In Figure 5b, CV curves of MCHS over voltage range of -1−0 V and MCHS/MnO2 over voltage range of 0−1 V at 100 mV/s exhibit shape of approximate rectangle and no abnormal and sharp spike appears, indicating the possibility of maximum operating voltage of 2.0 V. Figure 5c provides the CV curves of AMS at 100 mV/s in different potential windows varying from 0−0.5 V to 0−2.0 V. It can be clearly seen that all CV curves show approximately rectangular shape, and there is still no sharp spike at the boundary of CV curve even in maximum potential window of 0−2.0 V, further indicating that operating voltage of assembled AMS might be able to reach to 2.0 V. And integral area of CV curve 14


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(b)

15

(c)

15 10

5 0 -5 -10

MCHS MCHS/MnO2



10

5 0 -5

0.5 V 1.0 V 1.5 V 2.0 V

-10

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0.0

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(g)

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0 6000

Cycle number

Figure 5. (a) A red LED lighted by one AMS device; (b) CV curves of MCHS and MCHS/MnO2 electrode at 20 mV/S over the potential window of -1−0 V and 0−1 V, respectively; (c) CV curves of AMS at 100 mV/s in different potential windows; (d) CV curves of AMS at various scan rates; (e) GCD curves of AMS at 2 A/g in different potential windows; (f) Discharged curves of AMS at different current densities; (g) Specific capacitance of AMS calculated from discharged curves; (h) Relationship between energy density and power density of AMS; (i) Capacitance retention of AMS as a function of GCD cycle number.

increases in proportion to potential window, suggesting ideal capacitive behavior of assembled AMS. Additionally, near rectangular shape of CV curves at all scan rates varying from 5 mV/s to 100 mV/s in maximum potential window of 0−2.0 V stably present in Figure 5d, indicating the good rate capability of AMS. To further confirm 15



the maximum operating voltage of AMS can reach to 2.0 V, GCD test at 2 A/g in different potential windows varying from 0−0.5 V to 0−2.0 V is implemented, as plotted in Figure 5e, all GCD curves are highly symmetric and no disordered curve appears in both small potential window of 0−0.5 V and large potential window of 0−2.0 V, effectively suggesting that the 2 V of maximum operating voltage of AMS is reliably realized. Figure 5f exhibits the discharged curves of AMS at various current densities; discharge time gradually decreases with increasing current density and longer discharge time achieves at a lower current density, revealing the good capacity behavior of AMS. The specific capacitance of AMS based on total weight of active materials calculated from Figure 5f versus current density is presented in Figure 5g, which exhibits that specific capacitance of AMS is 116.4 F/g at 0.1 A/g and is still 53 F/g even when current density increases to 2.0 A/g, therefore a capacitance retention ratio of 45.5% is achieved. Both high specific capacitance and large operating voltage of AMS, effectively help promoting the energy density of cell, are realized. Figure 5h presents the relationship between energy density and power density, all of which are obtained based on the total weight of active materials. The energy density of AMS reaches its maximum value of 64.6 Wh/kg at power density of 100 W/kg, which is higher than or comparable to most of reported supercapacitors (more details in Table S1, Supporting Information)15,17−19,23,24,29,30,40, and maintains 29.4 Wh/kg at power density of 2000 W/kg. Afterwards, the stability and cycle life of AMS were investigated by GCD test at 2 A/g, the result is presented in Figure 5i, and approximately 90.3% of capacitance retention is achieved according to initial

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capacitance and AMS still works normally after 6000 cycles. Finally, a red LED indicator was successfully lighted by AMS cell, as shown in Figure 5a, indicating that our prepared electrode materials include MCHS and MCHS/MnO2 are promising for the energy storage applications. These excellent electrochemical performance of AMS cell primarily owing to the synergy derived from asymmetrical configuration of AMS and the fast rate of electron transfer and ion diffusion from prepared electrode and aqueous electrolyte.

CONCLUSIONS In summary, the MCHS/MnO2 hollow nanospheres composed of the electrical conductive layer of MCHS and porous electrochemical active material of MnO2 have been successfully fabricated and exhibit outstanding electrochemical performance. The assembled asymmetrical MCHS/MnO2//MCHS supercapacitors could be operated in the large operating voltage of 2 V and deliver the high energy density of 64.6 Wh/kg at power density of 100 W/kg, which benefit from the synergy of both pseudocapacitance and EDLC that can effectively broaden the operating voltage of cell. Therefore, more energy is stored in AMS. Furthermore, AMS could be used as a power supply to light a red LED. Owing to these outstanding electrochemical performance of electrode materials and assembled supercapacitor, our facile and productive method may provide a strategy to design and prepare electrode materials and supercapacitors for energy storage applications.

EXPERIMENTAL SECTION 17



Fabrication of Mesoporous Carbon Hollow Spheres. The MCHSs were fabricated according to Chengzhong Yu and his co-workers’ previously reported method.35 Specifically, absolute ethanol (600 mL), deionized water (200 mL) and NH3·H2O (30 mL) were homogeneously mixed under stirring of 500 rpm. Then TPOS (13.6 mL) and TEOS (14.3 mL) were added to the above mixture, followed by the addition of resorcinol (4.0 g) and formaldehyde (5.6 mL) after 15 min. The reaction under stirring of 500 rpm was kept for 24 h. After being centrifuged and thoroughly rinsed by water and ethanol, the separated precipitates were dried at 50 °C overnight. To obtain MCHS, the dried precipitates were milled and annealed at 700 °C in nitrogen atmosphere for 5 h, and the inner silica particles were removed by hydrofluoric acid (5 wt%).

Fabrication of MCHS/MnO2 Hollow Nanospheres. MCHS/MnO2 spheres were fabricated through a hydrothermal reaction, in which MCHSs were used as reactants and templates. MCHSs (100 mg) were well dispersed in deionized water (200 mL), followed by addition of KMnO4 (4 mmol) under vigorous stirring. Then pour into Teflon-lined autoclave and maintained at 140 °C for 2 h. After redox reaction process, MCHS/MnO2 hollow nanospheres were collected through centrifugation, washing and drying for further use.

Assemble of Asymmetrical MCHS/MnO2//MCHS Supercapacitors. Prepared electrochemical active material MCHS/MnO2 or MCHS, carbon black and PVDF were mixed together at a weight ratio of 8:1:1 in 1-methyl-2-pyrrolidinone (NMP)

18


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solution by a mixer (SpeedMixerTM DAC 400.1 VAC−P, FlackTek, Inc.) at the speed of 2000 rpm for 3 min. Then obtained slurry was coated on compressed nickel foam and dried at 60 °C to prepare electrode. Na2SO4 solution (1 M) as electrolyte, MCHS/MnO2 as positive electrode and MCHS as negative electrode, separated by a glassy fibrous membrane, were assembled in a 2032 coin cell to obtain the asymmetrical supercapacitor.

Characterization

and

Evaluation.

The

microstructures

of

MCHS

and

MCHS/MnO2 were characterized by field-emission scanning electron microscope (FE-SEM, FEI Nova Nano SEM 450) and transmission electron microscope (Tecnai G2 F30 FEI). The X-ray diffraction pattern used for phase and structure analysis was obtained by XRD measurement (Rigaku D/Max 2500). The Raman spectra were obtained

by

Raman

spectroscopy

instrument

(Renishaw

inVia).

N2

adsorption/desorption isotherms and Brunauer-Emmett-Teller (BET) method were performed to characterize specific surface area and pore size distribution of materials by Micromeritics ASAP 2020 analyzer. The X-ray photoelectron spectroscope patterns used for surface chemical element compositions and chemical states analysis were obtained by XPS measurement (Escalab 250Xi). The electrochemical properties of electrodes, including cyclic voltammetry (CV), galvanostatic charging/discharging (GCD) and electrochemical impedance spectroscopy (EIS), were tested by station with a three-electrode system in 1M Na2SO4 or KOH electrolyte. The counter and reference electrodes were Pt plate and SCE, respectively.

19



ASSOCIATED CONTENT Conflict of Interest: No competing financial interest among all authors in this study.

Supporting Information Available: This file is available free of charge on the website at http://pubs.acs.org.

ACKNOWLEDGMENTS This work was ﬁnancially supported by the National Natural Science Foundation of China (21571186), Chinese Academy of Sciences Key Research Projects of Frontier Science (QYZDY-SSW-JSC010), Youth Innovation Promotion Association (2017411), Guangdong TeZhi plan youth talent of science and technology (2014TQ01C102).

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Graphic abstract A structure of ultrathin manganese dioxide nanosheets grown on both inside and outside walls of mesoporous carbon hollow spheres (MCHSs) was designed.


Ultrathin Manganese Dioxide Nanosheets Grown on Mesoporous

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