Ant-Nest-Like Hierarchical Porous Carbon

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Manganese Dioxide/Ant-Nest-Like Hierarchical Porous Carbon Composite with Robust Supercapacitive Performances Qun Lu, Xianyou Wang, Manfang Chen, Bing Lu, Meihong Liu, Ting Xing, and Xingyan Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04492 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Sustainable Chemistry & Engineering

Manganese Dioxide/Ant-Nest-Like Hierarchical Porous Carbon Composite with Robust Supercapacitive Performances Qun Lu, Xianyou Wang*, Manfang Chen, Bing Lu, Meihong Liu, Ting Xing, Xingyan wang* (National Base for International Science & Technology Cooperation, National Local Joint Engineering Laboratory for Key Materials of New Energy Storage Battery, Hunan Province Key Laboratory of Electrochemical Energy Storage and Conversion, School of Chemistry, Xiangtan University, Xiangtan 411105, China)

ABSTRACT: Fullerenes (C60) are made of sp2-hybridized carbon atoms like carbon nanotubes and graphene. Herein C60 is sheared by chemical methods, involving alkali solution reactions that start to break carbon-carbon bonds, and then the MnO2/three-dimensional ant-nest-like hierarchical porous carbon composite (MnO2/ANHPC) is successfully prepared. ANHPC has the ant-nest-like hierarchical porous structure, and MnO2 is deposited on the surface of ANHPC. The MnO2/ANHPC composite reveals a high specific capacitance of 662 F g-1 at 1 A g-1 since it is the combination of both double-layer capacitances of ANHPC and pseudocapacitance of MnO2. The symmetric supercapacitor using the MnO2/ANHPC composite as the active material shows excellent rate performance and outstanding cycle stability (with 93.4 % capacity retention rate after 5000 cycles at 1 A g-1) as

*

Correspondingauthor: Tel: +86 731 58293377; Fax: +86 732 58292052. E-mail address: [email protected] (X. Wang), [email protected](X. Wang).

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well as a remarkable energy density of 21.5 Wh kg-1. The improved supercapacitive performance is closely related with the integrated advantage of 3D ant-nest-like hierarchical porous structure of ANHPC, stacked graphene sheets of linked hexagonal rings after chemically sheared C60 and pseudocapacitance behavior of MnO2. Therefore, the design and preparation of the MnO2/ANHPC composite is a promising strategy for active material of high performance supercapacitor. KEYWORDS: Ant-nest-like hierarchical porous structure; Chemical scissors; Fullerenes; Faradic capacitance; Supercapacitor

INTRODUCTION The depletion of fossil fuel resources and serious environmental pollution bring a good opportunity for the new energy development. [1-2] Due to its high power density, good rate capability, fast charge–discharge and long cycle lifetime, supercapacitors have attracted significant attention in recent years in many fields, such as lead–acid battery replacement, large cranes, vehicles, photovoltaic storage. [3-5] Usually, a wide variety of carbon materials have been widely used as electrode materials for supercapacitors on account of its large specific surface area, high conductivity and advanced pore structure. However, the carbon electrodes still suffer from low specific capacitance as it is mainly determined by the electrical double layer surrounding the surface of electrode.

[6-8]

Compared to various carbon materials,

transitional metal oxides such as RuO2, [9] Co3O4, [10] MnO2, [11] have high theoretical specific capacitance that results from fast and reversible surface redox reaction. [12–13] 2


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In many metal oxides materials, MnO2 has been considered to be one of the most competitive materials for supercapacitors as a result of its high theoretical specific capacitance (~1370 F g-1), wide potential window, low cost and natural abundance. Unfortunately, the low surface area and poor electrical conductivity of MnO2 are not effectively improved so that leading to the low specific capacitance, inferior rate capability and poor cycling stability, which restrict its applications in supercapacitors. To overcome the aforementioned drawbacks, incorporation of pseudocapacitive materials into carbon materials as a conductive substrate and support skeleton is a very effective strategy. Therefore, it is imminent to develop MnO2/C composite materials to improve the capacitive performance through rational structural design, which can realize the synergistic effect between pseudocapacitive materials and high conductivity, large surface area carbon materials. Thereinto, the characteristics of carbon skeleton, such as its structure and specific surface area, become the key factors to improve the performance of MnO2 active material. Great efforts have been made to combine various types of carbon materials with MnO2 to improve the electrochemical performance. For instance, Zhou et al.

[14]

prepared freestanding MnO2

nanoflakes/porous carbon nanofibers (MnO2/PCNFs) through combining an electro-spinning method and in-situ deposition and its specific capacitance is as high as 520 F g-1 at 0.5 A g-1. The CNT/MnO2 nanoflaky structure displayed a high specific capacitance of 108.5 F g-1 at 0.7 A g-1.

[15]

The MnO2 and polyvinyl alcohol derived

carbon composite showed a specific capacitance as high as 132 F g-1 at 0.5 A g-1. 3


[16]


Nevertheless, it can be found that the capacitance performance is still far from the theoretical capacity, which may be attributed to the low activity specific surface leading to the fact that MnO2 cannot fully contact with the electrolyte to carry out the rapid redox reaction. In our group, a lot of works on porous carbon materials such as carbon aerogel (CA), [19]

[17]

carbide-derived carbon (CDC),

[18]

hollow porous carbon spheres (HPCSs)

as well as the MnO2/ordered mesoporous carbon composites (MnO2/OMCs) [20] as

the electrode materials of the supercapacitors have been extensively studied. In this work, three-dimensional (3D) ant-nest-like hierarchical porous carbon (ANHPC) has been simply prepared via KOH chemical shearing C60 as carbon source. Fullerenes (C60) are made of sp2-hybridized carbon atoms like carbon nanotubes and graphene. [21-22]

After KOH chemical shearing C60, some C60 fragments with graphene-like

structure can be formed, and then the obtained C60 fragments can self-assemble into 3D ant-nest-like hierarchical porous structure that can effectively prevent agglomeration of nanostructures to preserve the basic structural features and contain the macro or mesopores for fast ion migration. In addition, the 3D hierarchical porous structure assembled by graphene-like structure fragments is propitious to the high electrical conductivity. MnO2 is then embedded into the 3D ant-nest-like hierarchical porous carbon skeleton to improve the conductivity of MnO2. Meanwhile, the large specific surface and interconnected pore structure of carbon skeleton can make the embedded MnO2 fully contact with the electrolyte to increase the effective active 4


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surface area of MnO2 so that it can react with electrolyte rapidly, thus greatly improving the specific capacitance and rate performance. The physical properties and electrochemical performance of the obtained composite are studied in detail.

EXPERIMENTAL SECTION Synthesis Preparation of ANHPC: In a typical synthesis, 0.4 g C60 powder was added to 200 mL toluene under stirring constantly at room temperature until dissolution. At the same time, 5 g potassium hydroxide (KOH) was added to the 100 mL absolute alcohol and subjected to ultrasonic treatment for 30 minutes to prepare a KOH solution. Afterwards, the KOH solution was gradually added into the C60 suspension under stirring constantly for 2 h, followed by rotary evaporation at 110°C until the toluene and ethanol were removed. The dried mixture was annealed in a tube furnace at 800°C under Ar atmosphere for 1 h with a heating rate of 5 °C min-1. The activated product was washed with deionized water, filtered, and dried in an oven at 60°C in an ambient atmosphere. The obtained dried product was annealed again at 900°C for 1 h in Ar gas flow with a heating rate of 5 °C min-1 to obtain the final product of ANHPC. Synthesis of the MnO2/ANHPC composite: 0.3 g ANHPC was dispersed into 30 mL water via 30 minutes sonication, producing stable dispersion. Then 0.2 mol L-1 KMnO4 was added into the dispersion with vigorous stirring. The reaction between C and KMnO4 was carried out at room temperature for 2 h. The final product of the 5



MnO2/ANHPC composite was collected by filtration and vacuum-dried at 100°C overnight.

Fig. 1 Schematic illustration of synthesis procedure of the MnO2/ANHPC composite.

Characterization of physicochemical properties The morphologies and structures of ANHPC, and MnO2/ANHPC composite were observed with scanning electron microscopy (SEM, Quanta FEG 250, FEI) rigged with energy dispersive X-ray spectrometer (EDS) and transmission electron microscopy (TEM, JEM-2100F, JEOL). Fourier transform infrared (FTIR) spectrometer (Perkin-Elmer Spectrum One) was utilized to characterize the functional group of the sample. X-ray diffraction (XRD) patterns were utilized to characterize the crystal structures of the samples. The four probe tester is used to measure the conductivity of the sample. The N2 adsorption/desorption measurements at 77.3 K nitrogen (JW-BK112) were performed to analyze the textural properties of ANHPC and MnO2/ANHPC composite. The specific surface area, pore volumes (Vtotal) and pore size distribution (PSD) of ANHPC and MnO2/ANHPC composite were estimated 6


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by means of the Brunauer-Emmett-Teller (BET) method. Electrochemical characterization In typically fabrication of working electrodes: the active material (80 wt%), acetylene black (10 wt%), and polyvinylidene fluoride (PVDF, 10 wt%) were well stirred for 3 h in N-methyl-2-pyrrolidone (NMP) and it subsequently was coated onto nickel foam substrate (1 cm2). Afterwards, the working electrodes was dried and then pressed under a pressure of 16 MPa for 60 seconds. The loading mass of active material in the electrode is about 4 mg cm-2. The electrochemical performance of all prepared working electrodes was measured via cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) on the electrochemical workstation (VersaSTAT3, Princeton Applied Research, USA). The above electrochemical measurements employed a conventional three-electrode system assembled with MnO2/ANHPC as the working electrode, Hg/HgO electrode as the reference electrode and nickel foam as the counter electrode. The cycle life was investigated by a supercapacitor test station (Arbin, BT2000, USA) on button cell supercapacitors. Besides, all the tests were measured in 6 mol L-1 KOH electrolyte at ambient temperature.

RESULTS AND DISCUSSION The synthetic route of the MnO2/ANHPC composite is illustrated in Fig.1. In the circumstance of KOH activation, the C60 cage can be destroyed by chemical scissors 7



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to produce some C60 fragments and leads to formation of carbon quantum dots, and thus the obtained C60 fragments can self-assemble into a 3D ant-nest-like hierarchical porous structure. [23，25] The MnO2 is deposited on the surface and channels of ANHPC based on the oxidation reduction reaction of MnO4- ions with carbon atoms as follows: [24]

4MnO-4 + 3C + H2O = 4MnO2 + CO32 + 2HCO3

(1)

In order to characterize the morphology and microstructure of ANHPC and MnO2/ANHPC composite, the representative SEM and TEM images are presented in Fig. 2. As expected in Fig. 2a and b, it can be observed that ANHPC has a 3D ant-nest-like structure (ant nest as shown in the illustration in Fig. 2a) with hierarchical pores with sizes from a few to several hundred nanometers. The ant-nest-like hierarchical porous structure is an excellent carbon substrate for 3D functional electrode material since its 3D interconnected porous structure can provide integral path for diffusion of electrolyte ions and is also beneficial to large amount of active materials to load. The SEM images in Fig. 2c and d display the microstructure of the MnO2/ANHPC composite. The composite still maintains an ant-nest-like hierarchical porous structure similar to ANHPC and the carbon skeleton structure is not destroyed or collapsed, which can effectively improve the space utilization of methodic porous structure, leading to a large active surface area for faradaic reactions. Comparing Fig. 2b with 2d, the visible holes of the MnO2/ANHPC composite are apparently reduced, suggesting that MnO2 is embedded on the outer surface and 8


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interior channels of ANHPC. Further details of the ant-nest-like microstructure can be seen in the TEM images. As presented in Fig. 2e and f, ANHPC is dominated by an amorphous structure, which is in contrast to the well-crystallized structure of C60 precursor, indicating that C60 cage is destroyed under KOH chemical shearing.

[20]

From the high resolution transmission electron microscopy (HRTEM) image in Fig. 2f, it can be manifested that the walls of ANHPC are composed of curved graphene-like layers with a certain interlayer spacing in the framework, and the corresponding holes and curved passages in the bright part can effectively promote the fast penetration of electrolyte into the pores, so as to achieve good energy storage. [26]

As for the MnO2/ANHPC composite, the TEM images in Fig. 2g and h show a lot

of MnO2 loading on the surface and internal holes of the porous carbon. The HRTEM image shows clear lattice fringes with an interplanar distance of ~ 0.7 nm, corresponding to the spacing of the (001) planes of the brinessite-type MnO2. [27]

9



Fig. 2 SEM images of (a) and (b) ANHPC, (c) and (d) MnO2/ANHPC composite; TEM images of (e) ANHPC and (g) MnO2/ANHPC composite, HRTEM images of (f) ANHPC and (h) MnO2/ANHPC composite, the inset of Fig. 2a shows the photograph of an ant nest.

10


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XRD diffractograms of C60, ANHPC, and MnO2/ANHPC composite are shown in Fig. 3a. The characteristic peak corresponding to diffraction from C60 crystal (fcc) has been labeled.

[28]

As a result of KOH chemical shearing , all these peaks totally

vanished in the patterns of ANHPC and MnO2/ANHPC composite, indicating that C60 crystal has been completely restructured by KOH chemical shearing, and it also indirectly illustrate that the cage-like fused-ring structure of C60 was chemically sheared by KOH. There are two broad weak diffraction peaks (002) and (100) of the graphite-like structure from the XRD pattern of ANHPC. There are three characteristic peaks at 12°, 37.3° and 65.6° in MnO2/ANHPC composite, which are attributed to birnessite-type MnO2 (JCPDS 42-1317) from the index.

[27]

Apparently,

the disappeared peaks of ANHPC at 2θ around 42.8° and 21.6° after deposition of MnO2 is attributed to too much MnO2 deposited on the carbon and the part carbon erosion on the crystal plane of ANHPC due to the redox reaction of KMnO4 with carbon. The functional groups on the surface of carbon and composite were characterized by FTIR measurements. Fig. 3b shows FTIR spectra of C60, ANHPC and MnO2/ANHPC. The valleys at 527, 574, 1180, and 1427 cm-1 in the C60 spectrum result from four infrared active modes with F1u symmetry, associated with the primarily radial motion of carbon atoms (527 and 574 cm-1) and the tangential motion of carbon atoms (1180 and 1427 cm-1) in C60.

[29]

All these vibrations disappear in

ANHPC and MnO2/ANHPC composite, except for the modes at 1577 and 3450 cm-1, which result in the π–π interaction and O-H stretching vibration, respectively. Besides, 11



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a characteristic Mn-O vibrational peak emerges at 532 cm-1 for the MnO2/ANHPC composite, indicating the presence of MnO2 in composites.

[30]

Above results clearly

show that the cages of C60 are completely ruptured and 3D ant-nest-like hierarchical porous carbon with no sign of any C60 molecules is obtained by KOH chemical shearing. Besides, it indicates that MnO2 is also successfully loaded on the carbon skeleton. The loading mass of MnO2 is about 56% via EDS analysis. (see supporting information)

Fig. 3 (a) XRD diffractograms and (b) FTIR spectra of C60, ANHPC, MnO2/ANHPC composite.

To confirm the specific surface area and texture and size of the pore, the N2 adsorption/desorption isotherms and pore size distribution curves for C60, ANHPC MnO2/ANHPC composite are presented in Fig. 4. The pore characteristic parameters of C60, ANHPC and MnO2/ANHPC composite are listed in Table 1. For the MnO2/ANHPC composite and ANHPC in Fig. 4a, the nitrogen adsorption/desorption isotherms are a combination of type I and type IV according to IUPAC classification, thus indicating that both are made up of micropore and mesopore, which are beneficial for electrolyte ion transfer.

[31]

The higher isotherm of ANHPC in the

low-pressure region suggests occurrence of micropore filling, so ANHPC has more 12


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micropores than the MnO2/ANHPC composite. The specific surface area of ANHPC and MnO2/ANHPC composite are 2372 and 543 m2 g-1, respectively, which is much higher than that of C60 (4.17 m2 g-1). Apparently, the 3D hierarchical porous structure assembled by C60 fragments possesses larger surface area, wider pore size distribution and relatively higher pore volume after destruction of C60 cage, which are in favor of rapid transfer and diffusion of ions, accumulation of charge and the loading of active substances. It is clearly found in Table 1 that the specific surface area of the MnO2/ANHPC composite decreases remarkably after incorporation of MnO2 into carbon network, which can be ascribed to MnO2 formation inside the pore channels of ANHPC and is consistent with the SEM and TEM results in Fig. 2. Besides, MnO2 deposited the channel and pore of ANHPC can be further proved via EDS elemental mapping of SEM and its cross-section (see supporting information)

Fig. 4 (a) Nitrogen adsorption/desorption isotherms at 77 K and (b) their corresponding BJH pore size distribution curves of C60, ANHPC, MnO2/ANHPC composite.

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Table 1 Pore structure parameters and the specific capacitance of C60, ANHPC and MnO2/ANHPC composite. SBET

Average pore size

Vmicro

Vtotal

Capacitance

(m2 g-1)

(nm)

(m3 g-1)

(m3 g-1)

(F g-1)

C60

4.17

3.5

0.001

0.018

-

ANHPC

2372

3.17

0.948

1.936

254

MnO2/ANHPC

543

7.21

0.182

0.859

662

Samples

The capacitances calculated from GCD at 1 A g-1 in three-electrode system.

Cyclic voltammetry measurement is conducted to evaluate the electrochemical performance of ANHPC and MnO2/ANHPC composite with a three-electrode system at 5 mV s-1 between -1.0 V and 0 V vs. Hg/HgO. It can be clearly observed from Fig. 5a that the CV curve of ANHPC shows a similar rectangular shape, indicating nearly ideal EDLC capacitive behavior and excellent reversibility, while the CV curve of the MnO2/ANHPC composite is distorted from mirror image symmetry. There is a pair of obvious redox peaks observed from the CV curves of the MnO2/ANHPC composite, which are ascribed to the redox reaction between Mn4+ and Mn3+, the

MnO2/ANHPC

composite

electrode

produces

an

[32]

suggesting that

oxidation/reduction

pseudocapacitance in the scanning potential range. The redox peak of CV curve was attributed to the following reaction Equation: MnO2 + H2O + e-  MnOOH + OH-

(2)

It indicates that supercapacitive behavior of the MnO2/ANHPC composite electrode is combination of EDLC of ANHPC and redox pseudocapacitance of MnO2. Generally, the specific capacitance of the carbon materials can be established by the following Eq. (3). 14


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Cs 

1 2mvV

 I (V )dV

(3)

Where m is the mass of the active substance (g), v is the constant scan rate (V s-1), V is the sweep potential window (V) and I (V) is the voltammetric current (A).

[33]

As

emerged in Fig. 5a, the area surrounded by the CV curve of the MnO2/ANHPC composite is obviously larger than that of ANHPC. The specific capacitances are 233 F g-1 and 536 F g-1 at 5 mV s-1 determined by Eq. (3) for ANHPC and MnO2/ANHPC composite, respectively. Obviously, the MnO2/ANHPC composite shows an optimal electrochemical performance and clearly high specific capacitance.

Fig. 5 (a) CV curves at 5 mV s-1 and (b) GCD curves at 1 A g-1 of ANHPC, MnO2 and MnO2/ANHPC composite electrodes; (c) GCD curves for the MnO2/ANHPC composite electrode at different current densities. (d) Specific capacitances of ANHPC and MnO2/ANHPC composite electrodes at different current densities.

15



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Fig. 5b demonstrates the galvanostatic charge/discharge curves at 1 A g-1 in 6 mol L-1 KOH electrolyte. ANHPC electrode shows a typical symmetrical triangle shape, demonstrating that ANHPC has good double-layer capacitive behavior, while the curve of the MnO2/ANHPC composite deviates the former triangle shape as a result of the faradaic reactions. Moreover, the obvious potential plateaus in the charge/discharge curves for the MnO2/ANHPC electrode is in accordance with the redox peaks observed in the CV curves. The specific capacitances can be calculated based on Eq. (4):

Cm 

itd mV

(4)

where Cm, is the specific capacitance (F g-1), i the discharge current (A), td the discharge time (s), V the potential window (V) and m the weight of active material (g). [34] According to Eq. (4), the specific capacitance of the MnO2/ANHPC is as high as 662 F g-1 at 1 A g-1. In order to analyze further the electrochemical behavior of the MnO2/ANHPC composite, Fig. 5c displays the GCD curves of the MnO2/ANHPC composite electrode at various current densities from 1 to 10 A g-1. Generally speaking, the specific capacitance of the MnO2/ANHPC composite decreases with the increase of the current density, that is the same as the electrochemical behavior of usual supercapacitor because at large current density the electrolyte ion can’t well penetrate into the inner of active materials due to slow diffusion. [35] Fig. 5d displays the curve of variation of the specific capacitance with the current density. It is noted that the specific capacitance of the MnO2/ANHPC composite is obviously higher than 16


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that of ANHPC at the same current density. The specific capacitance of the MnO2/ANHPC composite electrode is 662 F g-1 at 1 A g-1, and the value still retains 333 F g-1 even at 10 A g-1, showing an excellent stability and a good rate capability, while the ANHPC is just 254 F g-1 at 1 A g-1 compared with MnO2/ANHPC composite. The enhanced specific capacitance of the MnO2/ANHPC composite is probably attributed to the synergistic effect of ANHPC and MnO2. Besides, the ANHPC can also admit certain mechanical deformation in the redox process of the MnO2/ANHPC composite, which can avoid the damage of the electrode material and benefit to outstanding cycle stability. Moreover, the pseudocapacitance of MnO2 in the composite is enhanced by its highly conductive ANHPC substrate which is in favor of redox reaction of MnO2 component. [36] Simultaneously, MnO2 embedded on the surface and internal channels of ANHPC can also exhibit the enhanced active areas that provide high electro-active regions and shorten electrolyte diffusion paths. Fig. 6 displays the electrochemical impedance spectroscopy (EIS) of ANHPC, MnO2 and the MnO2/ANHPC composite electrodes in the frequency range from 10-2 to 105 Hz with 5 mV amplitude. It can be clearly observed from the Fig. 6a that the Nyquist diagrams shows a straight line owing to the Warburg impedance in the low frequency region, which is attributed to the frequency dependence of ion diffusion to the electrode interface in the electrolyte. The representative impedance response shows characteristic feature of ideal capacitive behavior of porous carbon electrodes, demonstrating that the porous structure of materials allow the rapid infiltration of 17



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electrolyte ions. Moreover, as presented in the high frequency region in the illustration of Fig. 6a, the inconspicuous arc-shaped curve in the high frequency region is related to equivalent series resistance (ESR), which is consisted of electronic contributions and ionic contributions.

The ESR value is estimated to be 1.714 Ω

[37]

for the MnO2/ANHPC composite, which is lower than MnO2 (2.401 Ω) and only slightly higher than that of ANHPC (1.558 Ω). Fig. 6b presents the changes of specific capacitance with frequency for ANHPC and the MnO2/ANHPC composite electrodes. The specific capacitance is calculated based on the following Eq. (5): [38]

C=

1 (5)

2 fZ '' m

Here, C is the capacitance (F g-1), f is the frequency (Hz), Z" is the imaginary part of impedance (Ω), m is the mass of active material (g). The capacitance of all the samples decreases as the frequency increases, and electrodes behave like a pure resistance at high frequency region, demonstrating that the electrolyte ions can’t penetrate into most of the micropores under high frequencies.

[39]

It is observed that

the MnO2/ANHPC composite has a higher capacitance than ANHPC electrodes at low frequency range, which may be attributed to the superposition effect of redox pseudocapacitance of MnO2 and EDLC of ANHPC, which is also in accordance with aforementioned CV and GCD data.

18


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Fig. 6 (a) The Nyquist plots of ANHPC, MnO2 and MnO2/ANHPC composite electrodes (the inset shows the expanded high-frequency region of the plots) and (b) the curves of the specific capacitance with the frequency for different electrodes.

To further estimate the electrochemical properties of symmetric supercapacitor assembled by MnO2/ANHPC composite, the CV and GCD measurements are conducted in Fig. 7a and b, respectively. The CV curves of MnO2/ANHPC supercapacitor at different scan rates from 2 to 100 mV s-1 are presented in Fig. 7a and represent the rectangular-like shape with a little distortion, which is ascribed to the inherent resistivity of composite electrode.

[40]

In Fig.7a the CV curves are

obtained based on the symmetric supercapacitor with two electrode system and is different from the shape of the CV curves in three-electrode system in Fig.5a.

[41, 42]

As presented in Fig. 7b, the GCD curves of the MnO2/ANHPC supercapacitor show similar symmetrical triangle shape with slight deformation. At the beginning of discharge, a little sudden drop of potential can be observed, which is related to the ohmic resistance of the supercapacitor. According to Eq. (2), the specific capacitance of MnO2/ANHPC supercapacitor at 0.5, 1, 2, 5 and 10 A g-1 are 155, 130, 116, 111 19



Page 20 of 45

and 104 F g-1, respectively. Clearly, it shows a high specific capacitance and excellent rate performance, which is attributed to the combination of EDLC for ANHPC and redox pseudocapacitance for MnO2. Ragone plots of the corresponding power and energy densities are used to evaluate the capacitive performance of ANHPC and MnO2/ANHPC symmetric supercapacitors, as shown in Fig. 7c. The calculation of energy and power densities based on galvanostatic charging/discharging of supercapacitor at current densities ranges from 0.5 ~10 A g-1 according to Eqs. (6) and (7), respectively: [43] E

1 CV  2

(6)

where E, C and V are the energy density (Wh kg-1), the specific capacitance of symmetric supercapacitors (F g-1) and the voltage (V), respectively. P

E t

(7)

where P, E and t are the power density (W kg-1), the energy density (Wh kg-1) and discharge time (s), respectively. A maximum energy density of 21.5 Wh kg-1 is calculated for MnO2/ANHPC supercapacitor with a cell voltage of 1.0 V at 0.5 A g-1 in 6 mol L-1 KOH electrolyte from Fig. 7c, compared to a maximum energy density of 7.5 Wh kg-1 for ANHPC in the same conditions. It can be estimated that the energy density of MnO2/ANHPC symmetric supercapacitor still remains at 14.5 Wh kg-1 as the power density is increased to 5000 W kg-1. This obvious enhancement can be ascribed to the MnO2 grown on the surface and holes of ANHPC where most of the active sites for pseudo-capacitive species are provided. The energy densities for two 20


Page 21 of 45 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


electrodes slightly decrease with the increase of power density, which is probably attributed to the internal resistance arising from high loading of active material and the increase of ion diffusion resistance of electrode materials.

[44]

Even so, the

MnO2/ANHPC supercapacitor can still deliver much higher energy density, demonstrating that the MnO2/ANHPC composite possesses great potential in the application of supercapacitors with high energy and power density.

Fig. 7 (a) CV curves at different scan rates from 2 to 100 mV s-1 and (b) GCD curves at current densities ranging from 0.5 to 10 A g-1 of MnO2/ANHPC supercapacitor; (c) Ragone plots and (d) The cycle life of at 1 A g-1 for ANHPC//ANHPC, MnO2/ANHPC//MnO2/ANHPC symmetric supercapacitors.

It is well-known that the cycle life is another key factor for the supercapacitor application with redox pseudocapacitance. As shown in Fig. 7d, the cycle life curves 21



Page 22 of 45

of coin supercapacitors for different electrode materials are tested at 1 A g-1 for 5000 cycles. It can be observed from Fig. 7d that the initial specific capacitances for ANHPC and MnO2/ANHPC supercapacitors are as high as 47 F g-1, 143 F g-1, respectively. The high specific capacitance of the MnO2/ANHPC composite is attributed to the combination of EDLC for ANHPC and redox pseudocapacitance for MnO2. After 5000 cycles, the capacity retention for the MnO2/ANHPC is still up to 93.4 %. Obviously, the outstanding cycle stability of the MnO2/ANHPC composite is due to its 3D ant-nest-like hierarchical porous structure, which can provide the stable conductive skeleton for MnO2 loading and effectively improve conductivity of MnO2. Furthermore, the electrochemical performance of the MnO2/ANHPC composite electrode has much higher specific capacitance and better cycle performance than other reported MnO2 based composites, as shown in Table 2. Consequently, the as-prepared

MnO2/ANHPC

composite

possesses

excellent

supercapacitive

characteristics and will be a potential candidate as the electrode active material of high performance supercapacitors.

22


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Table 2 Comparative representation of the specific capacity and cycle life with recently reported MnO2/C composites supercapacitors. Electrode materials

The maximum specific

Cycling performance

Ref.

93.1 F g-1 at 0.1 A g-1

92% after 2000 cycles (50 mV s-1)

[45]

367 F g-1 at 10 mV s-1


[46]

517 F g-1 at 5 mV s-1

75% after 1000 cycles (30 A g-1)

[47]

421 F g-1 at 1 mA cm-2


[48]

402.5 F g-1 at 1 A g-1


[49]

211.1 F g-1 at 1 A g-1

93.6% after 1000 cycles (1 A g-1)

[50]

247 F g-1 at 1 A g-1

94.7% after 2000 cycles (1 A g-1)

[51]

351 F g-1 at 1 A g-1


[52]

α-MnO2 thin films

663 F g-1 at 1 mA cm-2


[53]

MnO2/ANHPC

662 F g-1 at 1 A g-1

93.4% after 5000 cycles (1 A g-1)

this work

MnO2//CNT-CNF 3D graphene/CNTs/MnO2 CNT/CNF/MnO2 MnO2 /peptide hybrid nanowires MnO2-CNT/Ni Nest-like MnO2 hollow nanospheres CNTs/ACT/MnO2 CNT/MC/MnO2 hybrid networks

capacitance

CONCLUSIONS The MnO2/ant-nest-like hierarchical porous carbon composite (MnO2/ANHPC) has been successfully synthesized by the redox reaction of KMnO4 with porous carbon. The ANHPC has been simply prepared via KOH chemical shearing C60 as high conductive carbon source. The ANHPC presents hierarchical pore structure, large surface area (2372 m2 g-1), relatively high pore volume (1.936 m3 g-1), which are favorable to rapid ion transfer/diffusion. MnO2 is finely deposited on the surface and

23



channel of ANHPC as conductive substrate, which can increase the effective active surface area and improve electrical conductivity of MnO2. The MnO2/ANHPC composite exhibits a high specific capacitance (662 F g-1 at 1 A g-1) and excellent cycling stability (93.4 % capacitance retention after 5000 cycles at 1 A g-1) as well as a remarkable energy density of 21.5 Wh kg-1. Hence, the as-prepared MnO2/ANHPC composite is expected to be an ideal active material for the application of high performance supercapacitors.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Fig. S1. (a) SEM image of MnO2/ANHPC composite, EDS elemental mapping with respect to (b) O, (C) Mn, (d) C. Fig. S2. (a) SEM cross-section image of MnO2/ANHPC composite, EDS elemental mapping with respect to (b) O, (C) Mn, (d) C. Fig. S3. (a) EDS analysis of the as-prepared MnO2/ANHPC composite. (The plugged table in the diagram is the content of each element).

24


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AUTHOR IMFORMATION Corresponding Author *E-mail: [email protected]. (W. X.Y.) *E-mail: [email protected]. (W. X.Y.) Present Address *(X. W.) School of Chemistry, Xiangtan University, Xiangtan 411105, China. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51072173, 51272221 and 51302239), Specialized Research Fund for the Doctoral Program of Higher Education (Grant Nos. 20134301130001), the Natural Science Foundation of Hunan Province, China (Grant Nos. 13JJ4051).

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For Table of Contents Use Only

MnO2/ant-nest-like hierarchical porous carbon composite is prepared by the redox reaction of KMnO4 with carbon using C60 as carbon source. .

35



Text: MnO2/ant-nest-like hierarchical porous carbon composite is prepared by the redox reaction of KMnO4 with carbon using C60 as carbon source.


Page 36 of 45

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Table 1 Pore structure parameters and the specific capacitance of C60, ANHPC and MnO2/ANHPC composite. Samples C60 ANHPC MnO2/ANHPC

SBET

Average pore size

Vmicro

Vtotal

Capacitance

(m2 g-1)

(nm)

(m3 g-1)

(m3 g-1)

(F g-1)

4.17 2372 543

3.5 3.17 7.21

0.001 0.948 0.182

0.018 1.936 0.859

254 662



Page 38 of 45

Table 2 Comparative representation of the specific capacity and cycle life with recently reported MnO2/C composites supercapacitors. Electrode materials

The maximum specific capacitance

Cycling performance

Ref.

MnO2//CNT-CNF

93.1 F g-1 at 0.1 A g-1


[43]

3D graphene/CNTs/MnO2

367 F g-1 at 10 mV s-1


[44]

CNT/CNF/MnO2

517 F g-1 at 5 mV s-1


[45]

MnO2 /peptide hybrid nanowires

421 F g-1 at 1 mA cm-2


[46]

MnO2-CNT/Ni

402.5 F g-1 at 1 A g-1


[47]

Nest-like MnO2 hollow nanospheres

211.1 F g-1 at 1 A g-1

93.6% after 1000 cycles (1 A g-1)

[48]

CNTs/ACT/MnO2

247 F g-1 at 1 A g-1

94.7% after 2000 cycles (1 A g-1)

[49]

CNT/MC/MnO2 hybrid networks

351 F g-1 at 1 A g-1


[50]

α-MnO2 thin films

663 F g-1 at 1 mA cm-2


[51]

MnO2/ANHPC

662 F g-1 at 1 A g-1

93.4% after 5000 cycles (1 A g-1)

this work


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Fig. 1 Schematic illustration of synthesis procedure of the MnO2/ANHPC composite.



Fig. 2 SEM images of (a) and (b) ANHPC, (c) and (d) MnO2/ANHPC composite; TEM images of (e) ANHPC and (g) MnO2/ANHPC composite, HRTEM images of (f) ANHPC and (h) MnO2/ANHPC composite, the inset of Fig. 2a shows the photograph of an ant nest.


Page 40 of 45

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Fig. 3 (a) XRD diffractograms and (b) FTIR spectra of C60, ANHPC, MnO2/ANHPC composite.



Fig. 4 (a) Nitrogen adsorption/desorption isotherms at 77 K and (b) their corresponding BJH pore size distribution curves of C60, ANHPC, MnO2/ANHPC composite.


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Fig. 5 (a) CV curves at scan rate of 5 mV s-1 and (b) GCD curves at 1 A g-1 of ANHPC, MnO2 and MnO2/ANHPC composite electrodes; (c) GCD curves for the MnO2/ANHPC composite electrode at different current densities. (d) Specific capacitances of ANHPC and MnO2/ANHPC composite electrodes at different current densities.



Fig. 6 (a) The Nyquist plots of ANHPC, MnO2 and MnO2/ANHPC composite electrodes (the inset shows the expanded high-frequency region of the plots) and (b) the curves of the specific capacitance with the frequency for different electrodes.


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Fig. 7 (a) CV curves at different scan rates from 2 to 100 mV s-1 and (b) GCD curves at current densities ranging from 0.5 to 10 A g-1 of MnO2/ANHPC supercapacitor; (c) Ragone plots and (d) The cycle life of at 1 A g-1 for ANHPC, MnO2/ANHPC symmetric supercapacitors.


Ant-Nest-Like Hierarchical Porous Carbon

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