Natural plant template derived cellular framework porous carbon as a

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Natural plant template derived cellular framework porous carbon as a high-rate and long-life electrode material for energy storage Xun Chen, Manzhou Chi, Linlin Xing, Xuan Xie, Simin Liu, Yeru Liang, Mingtao Zheng, Hang Hu, Hanwu Dong, Yingliang Liu, San Ping Jiang, and Yong Xiao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05777 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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ACS Sustainable Chemistry & Engineering

Natural plant template derived cellular framework porous carbon as a high-rate and long-life electrode material for energy storage Xun Chen†, Manzhou Chi†, Linlin Xing†, Xuan Xie†, Simin Liu†, Yeru Liang†, Mingtao Zheng†, Hang Hu†, Hanwu Dong†, Yingliang Liu*†, San Ping Jiang *‡, and Yong Xiao*† †College of Materials and Energy, South China Agricultural University, Guangzhou 510642, P. R. China ‡Fuels and Energy Technology Institute & Department of Chemical Engineering, Curtin University, Perth Western Australia 6102, Australia

*Corresponding authors E-mail address: [email protected] (Y.X), [email protected] (Y.L.), [email protected] (S.J.) Tel. and Fax: +86 020 85280319.

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Abstract A superb supercapacitor with high specific capacitance and high rate capacitance has been developed from taro epidermis biomass derived carbon materials through mild carbonizing and activation process. The taro epidermis derived porous carbon electrodes exhibit unique cellular frame porous structure with combined micro- and mesopores, and show excellent supercapacitance performance, achieving superior specific capacitance (466 F g-1 at 1 A g-1), high energy density (17.59 to 13.97 W h kg-1) and excellent rate capacitance (415 F g-1 at 5 A g-1 and 342 F g-1 at 50 A g-1) in aqueous 6 M KOH electrolyte. Meanwhile, the taro epidermis derived electrodes demonstrate excellent cycling stability: almost no fading after 40 000 cycles at 5 A g-1 in aqueous 6 M KOH electrolyte and 10 000 cycles at 2.5 A g-1 in organic 1 M TEABF4/AN electrolyte. The results demonstrate the effective use of the framework structure of plant cell walls as the highly porous carbon materials with superior supercapacitance performance.

Keywords: Supercapacitor; taro epidermis biomass; cellular framework porous structure; electrode materials; TEABF4/AN organic electrolyte.


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INTRODUCTION Thanks to their high power density, fast charging time and good cycle stability, supercapacitors (SCs) have been targeted as a promising technology to complement or replace batteries in portable electronics and hybrid vehicles.1-4 At present, carbon materials, especially those with high porosity and high specific surface area, are recognized as the most promising electrode materials for SCs owing to their good electrical conductivity, excellent physicochemical stability, large surface area, and low cost.5-8 However, the current carbon-based SCs exhibit good power density, long lifespan but possess relatively low specific capacitance (< 250 F g−1) and energy density (typically 3-8 W h kg−1), making them less attractive for the practical application.3, 9-11 A grand challenge in carbon-based SCs is to develop carbon materials not only with high porosity and high specific surface area but also with appropriate porous structure to further enhance the specific capacitance and energy densities. An effective way to tackle this challenge is to manipulate and design porous structures of carbon materials to increase the active area for charge storage, thereby improving SCs performance.5, 12-14 For example, increasing the ratios of the micropores (< 2 nm) and small mesopores (2-4 nm) in carbon materials can effectively increase the specific surface area (SSA) and the number of active sites.15-16 Chmiola et al shows that pores with diameter less than 1 nm can evidently enhance the specific capacitance of SCs.17 However, the use of microporous carbon (MPC) for SCs is hindered by the high diffusion resistance of electrolyte under high charge/discharge rates due to the fact that micropores with narrow but long channels/pores are difficult to be fully utilized.18 3 ACS Paragon Plus Environment


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Therefore, it is of great technical significance to substantially reduce the resistance of ion diffusion in MPC. In this respect, hierarchically porous carbon (HPC) with the interconnected porous structure (macro-, meso-, and micropores) is considered to be promising alternative to the conventional MPC, taking advantages of the integrated macroporous and mesoporous structure in the electrolyte transportation.19-20 Unfortunately, the current HPCs suffer low adsorption sites and high electrolyte diffusion resistance due to the difficulty in the optimization and control of pore distribution and structure during the synthesis process.18 Ionic adsorption/desorption within the micropores depends strongly on the ion migration and diffusion through the meso- and macro-pores. Without properly distributed and well balanced porous structure, the phenomenon of microporous shielding occurs at high current densities, leading to the decrease in active sites and specific capacitance. Thus, new and unique multidimensional structured carbon, such as honeycomb,10, 2122

egg-box,15,

23

flower-like,24 and sheet structure16,

25

have attracted a significant

attention because their exquisite multidimensional structure could shorten ion-diffusion length and realize fast electron transfer in an electrochemical reaction process. For example, the sheet carbon with open structure has inherent advantages of high exposed surface atoms and fast ion adsorption and desorption behavior.25 The space and channels between the sheet layers improve the ion accessibility and shorten the diffusion length to and from the electrode/electrolyte interface. Graphene is a typical 2dimensional carbon nanosheet and has been studied extensively because of their unique properties especially the high theoretical surface area.19,

26-27

However, the SSA is 4

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decreased deeply by the irreversible self-stacking due to the strong π–π interactions between graphene nanosheets. Graphene with a spatial structure could avoid selfstacking with enhanced performance.28-29 Nature probably provides the best integrated porous structures. The properly distributed and well-balanced nano-, micro- and macroscale structures existed in natural biomaterials have been precisely evolved and controlled with the aid of biological energy metabolism over billions years.30-31 Thus, in recent years, the use of biomaterials in particular the waste biomass for the fabrication of diverse functional carbon materials with naturally formed morphologies and microstructures is attracting considerable attention due to its sustainability and renewability.32-41 It has been demonstrated that the SC performance of biomass-based carbon materials is compatible or better than that derived from synthetic carbon.38-39, 42-43 By utilizing their inherently uniform and biologically optimized nanostructures, further processing and carbonization can yield carbon electrode materials with specific and unique porous structures.32, 44-46 Peng et al. took advantage of the unique hollow and multi-layered structure of willow catkin fiber and adopted a facile one-step pyrolysis and activation synthesis method to convert willow catkin biomass into interconnected porous carbon nanosheets.32

Mitlin’s

group

reported

the

development

of

a

hierarchical

macro/microporous carbon material originated from the eggshell membrane, in which the typical structure of the natural microporous and an interwoven fiber network was successfully preserved by simple carbonization and activation procedure.46 Utilization of biologically evolved and optimized microstructure of biomass materials is one of 5 ACS Paragon Plus Environment


most effective strategies in the development of highly effective porous carbon materials of SCs. In this paper, taro epidermis (TE), as the by-product of the taro, was chosen as the raw biomass to synthesize highly porous and functional carbon materials for SCs. A mild carbonization and activation procedure was adopted to preserve cellular framework porous structure of taro epidermis, forming cellular framework porous carbon (CFPC). CFPC electrodes exhibit excellent electrochemical performance in both aqueous and organic electrolytes and the as-assembled CFPCs/CFPCs symmetrical cells show superior capacitance, good conductivity, excellent rate capability, long cycling stability, and excellent energy/power densities, achieving the superior energy density of 17.59 and 40.8 W h kg-1 in aqueous electrolyte (6 M KOH) and organic (1 M TEABF4, AN) electrolytes, respectively. EXPERIMENTAL SECTION Synthesis of materials. Fresh taro epidermis were obtained from an agriculture market in Guangzhou, China. Potassium hydroxide (KOH), Hydrochloric acid (36% HCl), tetraethylammonium tetrafluoroborate acetonitrile solution (TEABF4/AN) and ethanol (99.8% CH3CH2OH) were purchased from Shanghai Chemical Reagents Co., Ltd. Commercial activated carbon (YP-50) was purchased from Kuraray Chemicals. All chemical reagents were analytical grade and used as received without further purification. As obtained taro epidermis was washed with deionized water and dried at 105 oC for 12 h before use. Subsequently, the taro epidermis was transferred to a rectangular 6 ACS Paragon Plus Environment

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porcelain boat and heated in a tubular furnace at 400 oC for 2 h under nitrogen with a heating rate of 2 oC min-1, and the obtained powders donated as CFC. Then the powders were mixed with KOH at a weight ratio of powder:KOH=1:3, and activated in a nickel crucible at 600, 700, or 800 oC for 2 h under nitrogen with heating rate of 5 oC min-1. The products were then thoroughly washed with 2 M HCl solution to remove any inorganic salts, followed by washing with deionized water until neutral pH. Finally, the powders were dried at 80 oC for 12 h and were denoted as CFPC-600, CFPC-700, and CFPC-800, respectively. For the purpose of comparison, the powders after carbonization at 700 oC for 2 h under nitrogen with heating rate of 5 oC min-1 but without activation in KOH treatment were also prepared and were donated as CFC-700. Material characterization. X-ray diffraction (XRD) patterns were obtained using powder X-ray diffractometer (XD-2X/M4600, Cu Kα, λ=0.15405 nm) from 5 to 80°. Raman data were recorded on Jobin-Yvon HR 800 micro Raman spectrophotometer (λ= 457.9 nm). Porosity and pore distribution of samples were determined from N2 adsorption-desorption isotherms (ASAP 2020 Micromeritics sorption analyzer), and the samples were degassed at 350 °C for 8 h before the measurement. Specific surface area (SSA) was obtained from Brunauer-Emmett-Teller (BET) fitting and the non-local density functional theory (NLDFT) was employed to calculate the pore size distribution (PSD). XPS characterization was conducted by VG ESCALAB Mark II (VG Scientific, UK). The element analysis was performed by Vario Micro cube (Elementar, Gemany). Optical microscope (WSM500D, Micro optical instrument Co. Ltd.), field emission electron microscope (FESEM, ZEISS Ultra 55) and high-resolution transmission 7 ACS Paragon Plus Environment


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electron microscope (HRTEM, JEOL 2100 HR) were used to observe the morphology of the samples in this study. A standard four-probe (RTS-9) was employed to measure the electrical conductivity of the samples. Thermogravimetric analysis was performed by using a TGA instrument (NETZSCH TG 209F1 Libra, Gemany). Electrochemical measurement. Electrochemical performance of the electrodes was tested in 6 M KOH solution, using three-electrode system. The working electrodes were prepared

by

mixing

as-prepared

active

materials,

acetylene

black,

and

polytetrafluoroethylene (PTFE) binder with a mass ratio of 8:1:1 to form a slurry. The slurry was then dispersed on a nickel foam current collector (1 cm ×1 cm) sheet and pressed together, followed by drying under vacuum at 100 oC overnight. The loading of active materials was 4.0 mg cm-2. Pt foil and Hg/HgO reference was used as the counter and reference electrodes, respectively. The galvanostatic charge/discharge (GCD) and cyclic voltammetry (CV) measurements were carried out over a voltage range of -1.0-0 V vs. Hg/HgO reference electrode using the CHI 660D electrochemical workstation (Shanghai Chenhua, China). For comparison, commercial activated carbon materials (YP-50, Kuraray Chemicals) were tested. The electrochemical impedance spectroscopy (EIS) curves of samples were obtained on Im6ex (Zahnex Co., Germany) electrochemical workstation at an open circuit voltage with signal amplitude of 5 mV in the frequency range of 0.01 Hz to 100 kHz. The specific capacitance of electrode and Resr based on GCD were calculated by Eqs. (1) and (2): C

I  t m  V

Resr 

(1)

Vdrop

(2)

2 I


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where C (F g-1) is the specific capacitance of the active materials calculated from the GCD test, I (A) presents the constant current, m (g) is the mass of active materials within the working electrode, Δt (s) is the discharge time, ΔV (V) is the voltage window during the discharge with IR drop corrected, Vdrop (V) is the IR drop between the first two points in the discharge curve, Resr (Ω) is the equivalent series resistance. For a symmetric supercapacitor in a two-electrode cell configuration was fabricated and tested in 6 M KOH and 1 M TEABF4/AN electrolytes, respectively. To prepare the electrodes, slurry of active materials, acetylene black and PTFE binder with a mass ratio of 8:1:1 was pressed on the nickel foam and aluminum foil in aqueous and organic electrolyte cells, respectively. The electrodes were dried under vacuum at 100 oC overnight. The electrode area was 1 cm2, and the active material loading was 4.0 mg. The cellulose membrane and polypropylene were adopted as separator in aqueous and organic electrolyte cell test systems, respectively. Two electrodes with the same size were assembled as a symmetric supercapacitor (CR2032-type coin cell), and in the case of the organic electrolyte system, the assembly was carried out in a glovebox with nitrogen atmosphere. The electrochemical measurements were performed on NEWARE (BTS 7.5.x) battery program-control system. The galvanostatic charge/discharge and cyclic voltammetry test for the coin cells were tested in the potential range of 0.01-1 V and 0.01-2.7 V in aqueous and organic electrolyte system, respectively. And the electrochemical impedance spectroscopy (EIS) curves were also measured at an open circuit voltage with signal amplitude of 5 mV in the frequency



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range of 0.01 Hz to 100 kHz. The capacitance of the total cell (Ccell) was calculated by Eq. (3): Ccell 

I  t m  V

(3)

where Ccell (F g-1) is the capacitance of the total cell, I (A) is the constant chargedischarge current, ΔV (V) is the voltage window during the discharge with IR drop corrected, Δt (s) is the discharge time, m (g) is the total mass of active material on the two carbon electrodes for supercapacitor. “IR drop corrected” stand for the voltage window between the first and second discharge point which has been removed from ΔV. The energy density (E, W h kg-1) and power density (P, W kg-1) of symmetric supercapacitors were also calculated by Eqs. (4) and (5):

E

Ccell  V 2 2  3.6

(4)

P

E  3600 t

(5)

where ΔV (V) is the voltage window during the discharge with IR drop corrected and Δt (s) is the discharge time.

RESULTS AND DISCUSSION Synthesis and microstructure. As the outer epidermis of the bulbs, taro epidermis has a large amount of stomata distributed on the surface with intensive inner cells filled with large vacuoles. These cells are separated by interconnected cell walls into small 10 ACS Paragon Plus Environment

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compartments. Moreover, the cell wall as plant protection organization with certain mechanical strength to maintain the survival and metabolism of plant cells, which provides the basis for the intact of the structure during the carbonization and activation process. Figure 1a shows the facile and simple procedure developed in this work to fabricate taro epidermis based carbon materials. After cleaning and low temperature carbonization treatment of fresh taro epidermis, the as-obtained carbonized precursor was chemically activated with KOH. Then, the highly porous and activated carbon materials were obtained after washing with acid and distilled water.

Figure 1. (a) Scheme of the process to synthesize taro epidermis derived cellular structured framework carbon (CFPC) materials. (b) Optical microscope image of taro epidermis, the inset in (b) shows the structure of the epidermis tissue. (c) FE-SEM 11 ACS Paragon Plus Environment


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images of CFC before the KOH treatment, (d-e) FE-SEM images and (f-g) HR-TEM images of CFPC-700 at different magnifications. The inset in (e) shows the cambered sheet structure and in (g) shows short ordered structure. Figure 1b-1g shows the microstructure of taro epidermis derived porous carbon, taking CFPC-700 as an example. The epidermis tissue consisted of many small rooms by the cell wall (see the inset, Figure 1b). The unique architecture and composition of epidermis tissue have been evolved to support the physiological functions in their living organisms. After a mild carbonization of taro epidermis at 400 oC, the cellula of taro epidermis shrunk but still maintained their natural cellular frame structure (Figure 1c), in accordance with the optical microscopy image. Interestingly, there are a lot of holes or cavities with the range of several hundred nanometers on the surface (Figure 1c), which is highly favorable for the activation process, as KOH can be absorbed into the channels of taro epidermis. In the activation process, KOH firstly reacts with carbon to form potassium, hydrogen, and potassium carbonate, at temperature range of 400 oC to 600 oC (6KOH + 2C→2K + 3H2 + 2K2CO3). Then the temperature being increased to 700 oC, KOH has been completely consumed and K2CO3 has started to decompose into K2O and CO2 (K2CO3 →K2O + CO2), and is completely consumed at 800 oC. Further, the produced CO2 and K compounds will react with carbon at 700 oC (CO2+C→2CO, K2CO3+2C →2K+3CO, K2O+C →2K+CO). The above process enables the material surface to generate a rich porous structure. The formed potassium carbonate is helpful to generate mesoporous, and the further activation will not only expand the width of existed pores but also generate more micropores.25, 47 KOH activation process removes 12 ACS Paragon Plus Environment

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impurities such as SiO2 but it appears that the KOH activation did not destroy the carbonized cellular frame structure (Figure 1d). It is worth noting that the evaporation of water and pyrolysis of organic matter make the pore size in Figure 1c largely contract during the heating and carbonization process, though the pore size in Figure 1c seems to much smaller than it before in Figure 1b. The original cellular structure and extensive porous structure are still intact after the activation temperature treatment from 600 to 800 oC (Figure 1d-1e and Figure S1). For comparing the structure effected by KOH activation, the SEM images of CFC-700 obtained without KOH treatment is shown in Figure S2. We can clearly observe the various diameter of pores exist in the surface of CFC-700. The result state the structure is came from the template of celluar framework, and still kept intact during activation process. HR-TEM analysis also shows that the structure of sheet of CFPC-700 sample remain (Figure 1f). There exist substantial number of nano-sized pores in the range of 1-2 nm and channels on the surface of cellular frame structured carbon, which is characterized by short-range ordered porous structure (Figure 1g, S3, and S4). The lattice fringes have been measured and shown in Figure S3, the selected area electron diffraction (SAED) image of CFPC-700 is shown in Figure S4, which indicates the partial graphitization structure of CFPC-700. The results indicate the existence of the cambered sheet structure in 3D connecting cellular frame network (Figure 1e) with integrated and uniformly distributed micro- and mesopores in taro epidermis derived carbon. Similar microstructure was also observed on taro epidermis derived porous carbon heat-treated at 600 and 800 oC, CFPC-600 and CFPC-800 (Figure S1). 13 ACS Paragon Plus Environment


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The chemical compositions of as-prepared CFPCs were analyzed by XPS and the results for typical CFPC powder, CFPC-700 are shown in Figure 2. The XPS curves of CFPC-600 and CFPC-800 are shown in Figure S5. There are three obvious peaks centered at 285, 532, and 401 eV in the XPS spectra (Figure 2a), which can be assigned to C1s, O1s, and N1s, respectively.48-49 This is also indicated by the energy-dispersive X-ray (EDX) results (Figure S6). The XPS C1s spectrum exhibits mainly single peak consisting of sp2-bonded carbon (C=C, 284.8 eV) with a small tail containing sp3bonded carbon (C−C/C−N, 285.7 eV), C-O (286.8 eV), and COOH (288.9 eV) at the higher-binding energy region (Figure 2b), which indicates the formation of the abundant conjugated systems to improve its conductivity.50 The O1s spectrum ranging from 520 to 543 eV shown in Figure 2c can be divided into three peaks centering at 531.5, 532.7 and 533.5 eV. Peaks at 531.5 eV and 532.7 eV can be fitted to C=O and C−O bonds, whereas peak at 533.5 eV can be assigned to C−O−C bonds.23, 48 In the N1s spectra, CFPC-700 displays three peaks centered at 398.5, 400.3, and 403.0 eV (Figure 2d), corresponding to N-6 (pyridinic-type or amine N), N-5 (pyrrolic and pyridonic type N), and N-X (oxidized N), respectively.48, 51 One pyridinic N atom is bonded with two sp2 C atoms and provides one pair of electrons to the π system, thus inducing electron donor properties to the carbon layers.10 Herein, pyridinic and pyrrolic are assumed to be the main contributing to the pseudocapacitance.49, 51 Therefore, the oxygen-containing groups and a small amount of N atoms doped into carbon networks can enhance the wettability of carbon electrode interface to electrolyte, and the electronrich nitrogen can also enhance the conductivity of the carbon materials.32, 48, 52-53 The 14 ACS Paragon Plus Environment

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ash content has been measured by TGA test shown in Figure S7. The original ash content of taro epidermis is 6.94 %, the ash content value decreased to 2.80 %, 0.93 %, and 0.56 % of CFPC-600, CFPC-700, and CFPC-800 after KOH activation, respectively.

Figure 2. (a) XPS survey spectrum of CFPC-700 and high-resolution spectrum of (b) C 1s, (c) O 1s, and (d) N 1s photoelectron spectra. Elemental composition in the surface of CFPC-700 is 92.04 at% C, 5.72 at% O, 2.14 at% N and 0.09 at% S, based on the XPS analysis. The S content is very low and is not expected to significantly contribute to the charge storage capacity. Both N and O contents are affected by the heat treatment temperature. The N content was 2.89 at% for CFPC-600 and decreased to 0.63 at% when carbonization temperature increased to 800°C (CFPC-800). At the same time, the content of oxygen decreased from 10.59 at% 15 ACS Paragon Plus Environment


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to 4.60 at%. This indicates that most organic have evolved and the oxygen content decreases because of removal of CO/CO2 with the increase in the carbonization temperature. The content of major elements is summarized in Table 1. The difference in the content data between the element analysis and XPS may be due to the fact that XPS analyze mainly the surface composition. The carbon content increases as the activated temperature raised and the C/O ratio of CFPC-600, CFPC-700, and CFPC800 calculated by XPS analyze is 8.16, 16.09, and 20.73, respectively, the increased C/O ratio of CFPCs is benefit to enhance the conductivity of the CFPCs.15 The H element contents in element analysis may come from the water vapor adsorption from the air. Table 1. Elemental composition information for as-prepared CFPCs Elemental analysis (wt%)

　

XPS (atom%)

　

　Sample 　C

O

N

S

H

　C

O

N

S

CFPC-600

78.24

15.17

2.85

0.32

0.84

86.43

10.59

2.89

0.08

CFPC-700

86.53

6.43

2.27

0.43

1.05

92.04

5.72

2.14

0.09

CFPC-800

　90.98

4.72

0.92

0.52

0.89

　 95.40

4.60

0.63

0.04

Figure 3a displays the N2 adsorption/desorption isotherms of CFPCs. Type-I sorption isotherm was observed with a relatively low saturation pressure at (P/P0) of ~0.1, indicating a high density of micropores. The pore-size distribution obtained from the isotherm shows that the activated temperature affects the porosity distribution of the synthesized carbon materials (Figure 3b). The specific surface area of CFPCs activated 16 ACS Paragon Plus Environment

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at 600, 700 and 800 oC were determined to be 1996, 3218, and 3105 m2 g-1 with corresponding pore volumes of 0.82, 1.46, and 1.50 cm3 g-1 and average pore diameter of 1.64, 1.81, and 1.93 nm, respectively. Both the pore volume and average pore diameter increased when the activated temperature. This indicates high temperature could intensify the activation process and increase the pore diameter. The decrease of SSA of CFPC activated at 800 oC is most likely due to the conversion of some of the micropores to mesoporous in the high temperature activation process.54 The porosity properties of the CFPCs are summarized in Table 2. The mesoporous can evidently enhance the circulation of electrolyte to improve efficiency of ion transferring for fast or large current density charge-discharge process. The micropores will provide more active sites to promote the ion adsorption and oxidation-reduction reaction happened on materials surface. The high specific surface area and optimized pore structure are beneficial for the enhanced electrochemical performances for supercapacitors, such as capacitance, cycling stability and rate capability.



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Figure 3. (a) N2 adsorption/desorption isotherms, (b) PSD curves, (c) XRD patterns, and (d) Raman spectra of the CFPC-600, CFPC-700 and CFPC-800, respectively. Table 2. Surface area and porous structure parameters of CFPC samples heat-treated at different temperatures Sample SBET a)

Smicrob)

Smesoc)

S(d

Natural plant template derived cellular framework porous carbon as a

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