WITHDRAWN: Hybrid symmetric supercapacitor assembled by renewable corn silks based porous carbon and redox-active electrolytes

WITHDRAWN: Hybrid symmetric supercapacitor assembled by renewable corn silks based porous carbon and redox-active electrolytes

Accepted Manuscript Hybrid symmetric supercapacitor assembled by renewable corn silks based porous carbon and redox-active electrolytes Kanjun Sun, Zh...

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Accepted Manuscript Hybrid symmetric supercapacitor assembled by renewable corn silks based porous carbon and redox-active electrolytes Kanjun Sun, Zhiguo Zhang, Hui Peng, Guohu Zhao, Guofu Ma, Ziqiang Lei PII:

S0254-0584(18)30185-8

DOI:

10.1016/j.matchemphys.2018.03.022

Reference:

MAC 20425

To appear in:

Materials Chemistry and Physics

Received Date: 26 July 2017 Revised Date:

2 January 2018

Accepted Date: 7 March 2018

Please cite this article as: K. Sun, Z. Zhang, H. Peng, G. Zhao, G. Ma, Z. Lei, Hybrid symmetric supercapacitor assembled by renewable corn silks based porous carbon and redox-active electrolytes, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.03.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Hybrid Symmetric Supercapacitor Assembled by Renewable Corn Silks Based Porous Carbon and Redox-active Electrolytes Kanjun Sun*a,b, ZhiguoZhangb, Hui Pengb,GuohuZhaoa, Guofu Ma*b, ZiqiangLeib

College of Chemistry and Environmental Science, Lanzhou City University, Lanzhou730070, China

b

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a

Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Key Laboratory of

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Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China

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*Corresponding authors. Tel./Fax.: +86 931 7975121

E-mailaddresses: [email protected] (K. Sun), [email protected] (G. Ma). Abstract

We have prepared nitrogen doped biomass activated carbon (CSC-1) using the agricultural wastes corn silks as

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raw materials and ZnCl2 as activating agent (the same weight of corn silks and ZnCl2) carbonized at 800 °C. The activated carbon has high specific surface areas of 1764.8 m2 g-1, large specific capacitance of 358.0 F g-1 at 0.5 A

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g-1, 67% of the capacitance retention at 20 A g-1, and 99.2 % of initiatory specific capacitance after 5000 consecutive cycles. To improve the energy density of the symmetric supercapacitor based on CSC-1 electrodes, we

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employed the 1 M H2SO4 aqueous solution with alizarin red (AR) and bromoamine acid (ABA) as an advanced electrolyte to fabricate a novel hybrid symmetric cell. Surprisingly, the above device obtains a high specific capacitance of 260.8 F g-1 and a high energy density of 17.8 Wh kg-1, which are markedly higher than those in the conventional electrolyte. The improved energy storage is attributed to Faradaic pseudocapacitance related to the redox-active species of AR and ABA in the H2SO4 electrolyte. Based on the excellent characteristics, the device is expected as a promising candidateto fabricate high performance supercapacitors.

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ACCEPTED MANUSCRIPT Keywords: Supercapacitor, Alizarin red, Bromoamine acid, Biomass carbon 1. Introduction With the growing energy demand and the consumption of fossil fuels in recent years, there has been

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requirements to exploit cost-effective, environmentally-friendly and high performance electrochemical energy storage devices for sustainable mobile power source [1,2]. Supercapacitors (SCs) have attracted much attention because of the maintenance-free operation, high power density and significantly long cycling life than lithium ion

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battery. In addition, supercapacitors can be directly utilized or coupled with other batteries for energy output toward

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certain electrical facilities, such as electric braking assistance, starter and generators[3].

Based on the charge storage mechanisms, supercapacitors can be divided into electrical double layer capacitor (EDLC) and pseudocapacitor[4]. The energy storage in EDLCs results from the electrostatic attraction of the opposite charges and the charge accumulation occurred on double-layers at the electrode/electrolyte interface,

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whereas the capacity of pseudocapacitor depends on the Faradaic reactions of electroactive species on the surface of the electrode [5]. Specially, the electrode active material of SCs are three groups: metal oxide, electronically conducting polymer and carbon materials [6]. Among them, carbon materials especially derived from biomass

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precursors for EDLC application have been the most developed because of the relatively low cost, easy

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accessibility, extraordinary cycling stability, high rate performance and high power density. On the other hand, the carbon materials derived from agricultural wastes might be inherently rich in hetero-atom species (such as nitrogen, oxygen, boron and sulfur) [7], which can improve the surface wettability of the carbon materials, significantly facilitate rapid electrolyte ion transport within the micropores and mesopores and induce pseudocapacitive behavior of carbon materials [8]. Except to hetero-atom species, high specific surface area and appropriate pore size have also been considered important factors to improve the specific capacitance of carbon materials [9]. And it is believed to achieve larger specific surface area and more developed pores of carbon materials commonly through 2

ACCEPTED MANUSCRIPT chemical activation methods. Compared to the KOH and NaOH, the ZnCl2 activating agent shows practical advantages such as minor erosion and low cost [10]. Making use of the agricultural wastes can not only reduce the environmental pollution but also create the economic value. The carbons prepared by the biomass precursors would

carbon materials is limited by their relatively low energy density [11].

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have a profound and lasting impact on the investigation of SCs. However, the large-scale industrialization of the

Since the energy density (E) of SCs is proportional to the capacitance (C) and the square of the voltage (V),

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according to the equation: E = 1/2CV2, increasing the capacitance or the cell voltage can effectively increase the

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energy density. In order to improve the capacitance of supercapacitiors, there are two strategies can be used, one way is to prepare transition metal oxides/hydroxides, metal chalcogenides and conducting polymers composite carbon materials as electrode materials. Unfortunately, they still suffer from low power density and short cycle life because of their poor electrical conductivity and instability [12]. Another way is to introduce redox additives in

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electrolytes. Many inorganic cationic redox couples such as VO2+/VO2+[13], I2/I-[14], and organic redox mediators such as p-benzenedial[15], p-phenylenediamine [16], methylene blue [17] and indigo carmine [18] are as redox additives have increased the total specific capacitance and energy density of the EDLCs through reversible redox

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reactions. Frackowiak et al. [19] applied KI and VOSO4 aqueous solutions as electrolytes on the sides of the

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positive and negative electrodes of supercapacitor based on activated carbon electrodes, and used an expensive Nafion Membrane to separate them into two compartments. Except for two different electrolytes with the positive and negative electrodes with a membrane, it is a challenge to find different redox-active electrolytes coexisting in the solution without expensive Nafion Membrane which can produce two or more redox reactions as well. Generally, there are some necessary properties of the redox additives into the liquid electrolytes immensely: involve in redox reaction, better solubility, electrochemically stable, easy preparation and nontoxic [20]. Therefore, to build high-performance supercapacitors, the electrode materials and advanced electrolyte should be elaborately 3

ACCEPTED MANUSCRIPT designed. In this work, we have prepared novel SCs using AC derived from corn silk as the efficient electrode materials and 1 M H2SO4 aqueous solution with two redox additives of alizarin red and bromoamine acid as electrolyte.

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Benefiting from the high specific surface area and effective hetero-atom doping, the activated carbon exhibits high specific capacity, excellent rate capability and long cycling life. More importantly, the energy densities of symmetric supercapacitors by assemblingactivated carbon electrode and redox-active H2SO4 electrolytes are

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obviously higher than that of the symmetric supercapacitors based onactivated carbon electrode and pristine H2SO4

and supply the extra pseudocapacitance. 2. Experimental section 2.1 Materials

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electrolyte, because the reversible redox reactions of redox-active species effective enhance the potential window

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Corn silks are derived from the local environment (Lanzhou Gansu province, China), zinc chloride (ZnCl2, Yantai Shuangshuang Chemical Co., Ltd, China), Nafion solution (DuPont, USA), alizarin red (AR) (Shanghai Zhonqin Chemical Co., Ltd, China), bromoamine acid (ABA) (Tokyo Chemical Industry Co., Ltd, Japan) and

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N-methyl-2-pyrrolidone (Shanghai Zhonqin Chemical Co., Ltd, China) were in analytical grade and used as

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produced without any further treatment.

2.2 Synthesis of nitrogen-doped porous carbon based on corn silk The physical mixing as the contacting method in most cases can lead to better porosity development than impregnation. In a typical preparation of the activated carbon CSC-1: 1.0 g of dried corn silk was thoroughly mixed with the same quality powder of ZnCl2 in the crucible, and carbonized at 800 °C for 2 h with the heating rate of 5 °C min-1 in a tubular furnace in nitrogen atmosphere. For comparison purpose, the raw material was carried out under the same conditions without mixing activating agent, and the resulting sample was named as CSC. The two 4

ACCEPTED MANUSCRIPT samples were washed in 2 M HCl at room temperature for 6 h to remove the impurities, and then washed with distilled water until the pH reached neutral. In final, CSC and CSC-1 are dried at 60 °C overnight. 2.3. Materials characterization

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The morphology of the carbons were performed using the field emission scanning electron microscopy (FE-SEM, Ultra Plus, Carl Zeiss) at an accelerating voltage of 5.0 kV. X-ray diffraction (XRD) of samples was examined on a Rigaku D/Max-2400 diffractometer (Tokyo,Japan).) with Cu Kα radiation (λ = 1.5418 Å) at 40 kV

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and 100 mA. Raman spectra was recorded through an Via Raman spectrometer (Renishaw) with an Argon ion laser

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(λ = 514.5 nm) at ambient temperature. The Brunauer-Emmett-Teller surface area (SBET) and pore structure of the carbon samples were analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus (U.S.A.), and all samples were degassed at 200 °C prior to nitrogen adsorption measurements. The structure of CSC-1 was further carried out by a transmission electron microscopy (TEM, JEM-2010 Japan). X-ray

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photoelectron spectroscopy (XPS) measurement was performed on an Escalab 210 system (Germany). 2.4. Electrochemical measurements 2.4.1 Three-electrode systems

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The electrochemical performance of the CSC and CSC-1 were investigated and compared using

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three-electrode systems. A high purity carbon rod and saturated calomel electrode (SCE) in 1 M H2SO4 solution were used as the counter electrode and reference electrode, respectively. Typically, 4 mg of CSC or CSC-1 were dispersed in 0.4 mL of 0.25 wt% Nafion ethanol solutions with assistance by ultrasonic vibration. Then, 8 µL of above suspension were dropped onto the glassy carbon electrode as working electrode and dried for 10 minutes at room temperature. 2.4.2 Two-electrode systems Electrochemical measurements were further taken using two-electrode systems consisting of two symmetric 5

ACCEPTED MANUSCRIPT CSC-1-based electrodes in different electrolytes. The fabrication of the SCs consists of the preparation of the electrodes and electrolyte. Firstly, the working electrodes were prepared as following: In general, 80 wt% CSC-1 (16.0 mg), 10 wt%

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PTFE (2.0 mg), and 10 wt% acetylene black (2.0 mg) were mixed with some N-methyl-2-pyrrolidone to form homogeneous slurry. The as-prepared slurry was spread uniformly onto the rounded stainless steel mesh current collector with an area of 2 cm2, and the coating was left in an oven for overnight at 60 °C. Afterward, the coating

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was pressed with a pressure of up to 10 MPa to ensure adherence between the active material and the current

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collector. The mass of CSC-1 activated carbon electrodes coated on one stainless steel mesh after pressed was calculated to be 4.0 mg (excluding the mass of carbon black and PVDF).

Secondly, the electrolyte is prepared as following: 0.1 g of alizarin red (AR) and 0.1 g bromoamine acid (ABA) were added into 20 mL of 1 M H2SO4 solution and constantly stirred for 2 h to form a homogeneous solution. For

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comparison, 0.1 g of AR and 0.1 g ABA were added into conventional 1 M H2SO4 electrolyte, respectively. Finally, the hybrid symmetric supercapacitors (SC) were assembled using the prepared CSC-1 activated carbon electrodes, separated by a filter paper. The redox-mediated electrolytes were prepared by doping 0.1 g AR,

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0.1g ABA and 0.1 g AR-0.1g ABA into 20 ml 1 M H2SO4 aqueous solution in the SCs, which are named Device 2,

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Device 3 and Device 4, respectively. As a comparison, the symmetric supercapacitor with conventional 1 M H2SO4 was named as Device 1.

The electrochemical performance of the all carbons in both three-electrode system and two-electrode system were characterized by cyclic voltammetry, galvanostatic charge/discharge measurements, and electrical impedance spectroscopy (EIS) measurements with an electrochemical workstation (CHI 660D). In addition, the cycle-life stability was obtained on the cycling testing equipment (CT2001A, Wuhan Land Electronic Co. Ltd., China). The specific capacitance of the single electrode (Csp), cell capacitance (CCell), energy density (E) and power 6

ACCEPTED MANUSCRIPT density (P) are calculated using the following equations [21]: Ccell =I∆t/(m∆V)

(1)

CSP = 4×Ccell (2) E = 1000×Ccell∆V2/ (2×3600)

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P = E/∆t

(3)

(4)

where I is the discharge current, m is the total mass of the active materials in both electrodes, ∆t is the discharge

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time and∆V is the potential range.

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3. Results and discussion 3.1 Morphologies and chemical features of activated carbons

As shown in the Fig. 1a, the carbon without activation (CSC) is composed of the densely packed carbon layers mixed up with some irregular tubulose structure, and there are some slit-shaped patterns at the surface of CSC in

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the high magnification (Fig. 1b). This is probably due to the carbonization at high temperature started from the inside of corn silks, break up the large molecules (cellulose, hemicellulose, lignin etc.), break down the original structures and produce some porosity. Compared with the CSC, CSC-1 not only retains the basic structures of CSC

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(Fig. 1c), but also obviously displays unconnected mesopores and macropores on the surface of the carbon

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skeletons (Fig. 1d). During the chemical activation, the ZnCl2 penetrated the raw material to etch and created mesopores in the CSC-1.

The nitrogen adsorption/desorption isotherm of porous carbon is a significant impact of the activation atmosphere on their porosity. As shown in Fig. 2a, the adsorption isotherms of CSC and CSC-1 show the obvious limiting uptakes in the low pressure region, indicating the existence of micropores; there exists the distinct plateau even at high relative pressure, meaning the presence of mesopores as well. In addition, both CSC and CSC-1 have the distinct type H-4 hysteresis loop associated with narrow slit-like pores. While there are also clearly defined step 7

ACCEPTED MANUSCRIPT loops at the relative pressures from 0.4 to 0.6, suggesting a characteristic of uniform mesopores structures. More importantly, the increase of the adsorbed volume for CSC-1 suggests that fresh pore creation and widening happened at the activated process. According to the calculations of the Barrett-Joyner-Halenda (BJH) model, the

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main pore sizesof the two samples mainly range from 1.7 to 10 nm (Fig. 2b), indicating that the existence of micropores and small mesopores. Obviously, the pore sizes of the CSC-1are distributed narrowly and centered in the range of 2-5 nm, suggesting the existence of abundant mesopores (inset plot in Fig. 2b). The appropriate pore

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size is favorable to the diffusion and transportation of electrolyte ion while improving the energy density and the

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power capability of supercapacitor [22]. Notably, the BET specific surface area and total pore volume of CSC-1 are up to 1764.8 m2 g-1 and 2.0 cm3 g-1 respectively, which are higher than those of CSC (736.9 m2 g-1 and 0.4 cm3 g-1). Therefore, it is reasonable to conclude that the porous carbon CSC-1 prepared using ZnCl2 as activating agent presents more well-developed porosity and higher surface area than those of CSC [23]. The CSC-1 with high

supercapacitors.

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specific surface area and appropriate pore size is a promising electrode material for high performance

Fig. 3a shows powder X-ray diffraction (XRD) in the wide angle region of CSC and CSC-1. It is generally

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shown that the two samples exhibit low crystallinity. In the patterns of CSC, there are two obvious broad diffraction

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peaks centered at 24.1° and 43.7°, which can be indexed to the (002) and (100) diffraction from graphitic phase. It also can be seen that the same peaks of CSC-1 are not as visible as that of CSC, indicating the more amorphous structure of CSC-1 after the ZnCl2 activation. It also can be seen that the (002) diffraction peak of CSC-1 becomes weaker and broader after activation[24]. Raman spectroscopy analysis was further employed to investigate the structure of the two samples. The Raman spectroscopy (Fig. 3b) for the two samples clearly exhibit two peaks centered at 1350 cm-1 (D band) and 1662 cm-1 (G band), in which the D-band is related to disordered graphite carbon while the G-band corresponds to the graphitic carbon phase with an sp2 electronic configuration. The D 8

ACCEPTED MANUSCRIPT band and G band are characteristic Raman peaks for carbon materials and the intensity ratio of D band and G band (ID/IG) suggests the degree of the structural order with respect to a perfect graphitic structure [25], and the higher the ratio, the lower the degree of graphitization [26]. Here, the ID/IG values are determined to be 0.86 for CSC and 1

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for CSC-1, respectively, indicating their low graphitic order degree. In addition, CSC-1 after chemical activation possesses the higher ID/IG values and the lower graphitization than CSC, which might be attributed to the higher surface area and larger pore volume. Based on the above analysis, it is worth mentioning the chemical activation

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can make well-developed porous structure, high specific surface area and reduce the orderly graphitic structure of

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the carbon materials, which is conducive for electrolyte ion to penetrate the interior of carbon electrodes. CSC-1 is further characterized by transmission electron microscopy (TEM). The high resolution TEM micrographs (Fig. 4a and Fig. 4b) show the primarily defective microporous morphology with highly structural disorder and it can be clearly seen the CSC-1 is composed of multiple carbon layers and abundant disordered pores.

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Fig. 4c and 4d indicate that the comparatively clear edge of a thin carbon flakes is interconnected with disordered wormhole-like porous structure. The developed porosity of the carbon materials is important for electrolyte ions to rapid transportation and penetration [27]. These results are in good agreement with the data of SEM and nitrogen

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adsorption/desorption isotherm analysis.

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To confirm the chemical composition of CSC and CSC-1, X-ray photoelectron spectroscopy (XPS) was conducted. As shown in the Fig. 5a and S1a, three peaks at 285.1, 400.1, and 533.1 eV are corresponding to C 1s, N 1s, and O 1s, respectively. The content analysis of C, N and O in CSC-1 are calculated from the XPS survey spectrum to be around 87%, 3.2% and 9.8%. Further, we performed a detailed analysis on the XPS pattern of CSC-1, and the magnified peaks for C1s, N1s and O1s are addressed together with the fitting curves. Specifically, the spectrum of C1s (Fig. 5b) can be fitted into four peaks with binding energies. The dominant peak at 284.5 ± 0.1 eV corresponds to C=C configurations. And the small peaks 285.8 ± 0.2 eV, 286.5 ± 0.1 eV and 288.5 ± 0.1 eV are 9

ACCEPTED MANUSCRIPT attributed to C-N, C-OR, and C=N/C=O functional groups, respectively, which clearly reveal the nitrogen atoms and oxygen groups have been doped [28]. It is worth mentioning that the hetero-atoms and functional groups in carbon materials can significantly change the electron/donor characteristics. The high-resolution of N 1s spectrum

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(Fig. 5c) demonstrates that there existmainly four kinds of nitrogen. The peak at 398.2 ± 0.2 eV corresponds to pyridinic N species (N-6), and the peak at 400.1 ± 0.2 eV can be ascribed to pyrrolic structure (N-5). The peak at 403.2 ± 0.2 eV can be assigned to the oxidized pyridine-N-oxide (N-X), and the peak at 401.1 ± 0.2 eV is attributed

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to the graphitic N (N-Q) [29,30]. Previously reported studies have demonstrated that N-5 and N-6 could create

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numerous extrinsic defects and active sites, which are beneficial for the fast diffusion of transporting ions and might give rise to pseudocapacitance, while N-Q can significantly increase the electronic conductivity [31]. The spectrum of O1s of the CSC-1 (Fig. 5d) can be deconvoluted into three single peaks with binding energies centered at 530.9 ± 0.3 eV, 532.7 ± 0.2 eV, and 533.8 ± 0.1 eV which correspond to quinone, C=O, and C-OH, respectively

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[32]. And the quinone and C=O functional groups are the most possible active sites for the C-H activation which has been confirmed [33]. Meanwhile, the presence of C-O species is beneficial to the wetting of porous carbon in electrolyte, resulting in efficient surface areas exposed for charge storage [34].For comparison purpose, we have

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alsoanalysised the spectrum of C1s, N1s and O1s (see Supplementary information,Fig. S1).

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3.2 Electrochemical propertiesof the carbon electrodes Based on its unique irregular tubulose structure with lots of mesopores which can provide high special surface area and high pore volume as well as short ion diffusion paths, CSC-1 would be expected to possess low diffusion resistance for protons/cations, easy electrolyte penetration and high electroactive areas. As shown in the Fig. 6a, the cyclic voltammetry curvesof CSC and CSC-1 are composed of double-layer behaviors and a pair of broad and reversible faradaic redox peaks. A couple of sensitive and reversible redox peaks around 0.4 V both in the cyclic voltammetry curve (CV) and galvanostatic charge/discharge curve (GCD) mainly are attributed to the well-known 10

ACCEPTED MANUSCRIPT quinone-hydroquinone redox reactions: Q + 2H+ + 2e- ↔ QH2 (Q: quinone), which are provided by oxygen-containing functional groups [35]. However, in the CVs and GCDs of the nitrogen-doped porous carbons CSC-1, the faradaic pseudocapacitance provided by nitrogen-containing functional groups is not obvious, which

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might be due to the low content or the pseudocapacitance contributed by the nitrogen-containing functional groups is minor and secondary. It is well known that the capacitance of electrode is higher if the area of CV is larger [36]. Comparatively, the CV of CSC-1 exhibits larger current response and covers the larger area, suggesting that CSC-1

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has better electrochemical performance and higher specific capacitance than CSC. As shown in Fig. 6b, the GCDs

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of CSC and CSC-1 at the current density of 0.5 A g-1 exhibit partial arc shaped line contributed by the pseudofaradic reactions, which is in correspond with the CVs. Fig. 6c shows the CVs of CSC-1 at various scan rates from 10 to 200 mV s-1 in the potential range between 0 and 1.0 V. It can be observed that all the CV shapes at different scan rates are similar and the shape can remain when the scan rate ranges of 200 mV s-1, indicating a

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highly reversible system behavior of CSC-1. Fig. 6d displays the GCDs of CSC-1 at different current densities ranging from 0.5 A g-1 to 30 A g-1. It is clear that the shapes of the GCDs at the different densities are closely arc linear, suggesting the excellent capacitive behavior and electrochemical reversibility. The symmetric charge and

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discharge curves demonstrate its ideal capacitive behavior due to rapid ion transportation at the surface and in the

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inner structure of carbon electrode. Compared with the CSC electrode, the CSC-1 electrode not only exhibits much larger specific capacitance of 358.0 F g-1 at the current density of 0.5 A g-1 (Fig. 6b), but also can remain an ideal specific capacitance of 240.0 F g-1 when charge/discharge at a high-rate of 20 A g-1 remaining 67% of the specific capacitances at 0.5 A g-1 (Fig. 6e). Fig. 6e shows the change of specific capacitance with the increase of current density for CSC-1and the error barsrepresent the range of values obtained from measurements performed ontwo separate electrodes.As a result,the unique disordered mesoporous structure of CSC-1 can makethe electrolyte ionsdiffuse fast and create large specific capacitance and good rate capability. 11

ACCEPTED MANUSCRIPT Further, the long-term cycling stability is another important factor for estimating the practical application of supercapacitor electrode materials. And the cycle stability test was carried out at a relatively high current density of 5 A g-1 in a potential window of 0 to 1.0 V for 5000 cycles. As can be seen from Fig. 6f, the electrochemical

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capacitance of CSC-1 electrode has a high retention of 99.2 % after 5000 consecutive cycles, demonstrating the good charge/discharge stability as an electrode material for supercapacitor. It should be assigned to the stable mesopore structure favorable to ion transport and charge transfer. The first 3 cycles and the last 3 cycles are

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presented in the inset of Fig. 6f. There is only a slight decrease in the specific capacitance for CSC-1, which

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indicates excellent electrochemical stability and highlights the potential application of the CSC-1 in energy-storage devices.

3.3 Electrochemical propertiesof the as-fabricated supercapacitors

The CVsfor the symmetric two-electrode cells in 1 M H2SO4 with alizarin red, bromoamine acid and the mixture

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of alizarin red and bromoamine acid additive are exhibited in Fig. 7a. The CV of symmetric cell based on porous carbon CSC-1 in pristine H2SO4 shows a typical rectangular shape and the working voltage is limited to 1.0 V. In contrast, the CVs of symmetric cells studied in H2SO4 mixed with different electrolyte (AR, ABA and the mixture

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of AR and ABA) possess corresponding prominent redox humps, reflecting pseudocapacitive behavior assigned to

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the reversible electron transfer at the interface between the electrolyte and electrodes (Fig. 7a). It is well known that the redox reactions of anthraquinone compounds involve elementary steps containing 2H+ and 2e- for the reversible quinone/hydroquinone conversion [37]. Similarly, during the charging, H2AR and H2ABA diffuse on the interface of the positive carbon electrode, and release 2 H+ and 2 e- to form AR and ABA, respectively. Then AR and ABA diffuse back into the bulk electrolyte. In addition,the reactions are reverse of the charging process during discharge [38]. The proposed electron transfer mechanism during the redox reaction is illustrated in Scheme 1. The well-defined pair of Faradaic peaks in the Device 2, Device 3 and Device 4 are consistent with the possible 12

ACCEPTED MANUSCRIPT corresponding quasi-reversible reactions given below [39]: AR + 2H++ 2e-↔H2AR (Epa: 1.09 V; Epc: 0.90 V) ABA + 2H++ 2e-↔H2ABA (Epa: 0.76 V; Epc: 0.56 V)

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AR + ABA+ 4H++ 4e-↔H2AR + H2ABA (Epa:1.18 V and 0.67 V; Epc: 0.83 V and 0.40 V) The current responses of the electrodes with the different redox additive electrolytes are significantly higher than that of the capacitor without redox active additive. The total capacitance of Device 2, Device 3 and Device 4 should

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be composed of the electric double-layer capacitance and the pseudocapacitance provided bythe redox reactions. It

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can be inferred that the electrons transfer between the electric double layers and the redox reactions occurring at electrode-electrolyte interface is simultaneous[12]. Due to the two redox humps in the Device 4 are almost the sum of the redox hump of Device 2 and Device 3, the CV area of Device 4 is larger than that of others, indicating Device 4 possesses higher capacitance. Moreover, as shown in the Fig. 7b, the current response of Device 4

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increases with the increase of scan rate, confirming the excellent electrochemical reversibility while redox peaks are shifted and redox reactions processes at the electrolyte/electrode interface and controlled by diffusion [15]. The GCD of the symmetric cells based on porous carbon CSC-1 in 1 M H2SO4 solution displays an ideal

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triangular shape (Fig. 7c), which is consistent with a typical electric double-layer capacitive character. As the redox

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additives (AR, ABA and AR-ABA) are added into the 1 M H2SO4, there appear the non-ideal triangle shapes of the GCDs proving the Faradaic participated in the electric charge accumulation process [40]. Furthermore, the specific capacitance of Device 4 at 0.5 A g-1 is 260.8 F g-1 calculated according to the discharging time, which is higher than those of Device 2 (224.0 F g-1), Device 3 (205.2 F g-1) and Device 1 (115.6 F g-1). In addition, the Device 4 capacitor delivers Csp of 260.8 and 169.2 F g-1 at the current density of 0.5 and 8.0 A g-1, indicating the capacitance decreases with the increasing of current density due to the slower kinetic of the redox reactions of AR and ABA (Fig. 7d). 13

ACCEPTED MANUSCRIPT Due to the larger voltage window (extended up to 1.4 V) and the higher specific capacitance of the symmetric cells based on porous carbon CSC-1 and H2SO4 mixed electrolyte (Device 4), according to the equation: E = 1/2 CV2, Device 4 possesses larger energy density than others. In detail, the maximum energy density of Device 4 (17.8

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Wh kg-1) is higher than those of Device 1 (7.9 Wh kg-1), Device 2 (15.2 Wh kg-1) and Device 3 (14.0 Wh kg-1) at almost the same power density (ca. 360 W kg-1), as shown in Fig. 7e. In addition, it is also higher than that of other supercapacitors reported in recent literatures, such as activated carbon with KBr in the H2SO4 (11.6 Wh kg-1) [41],

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activated carbon with VOSO4 in the H2SO4 (13.7 Wh kg-1) [13], MnO2 with p-phenylenediamine in the KOH (10.1

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Wh kg-1) [42] and so on. Recently, the energy density of Bi2O3||AC asymmetric supercapacitor is high up to 35.4 Wh kg-1 by using KI as redox additive in Li2SO4 electrolyte [43], the symmetric PANI SC shows 228 F g-1 at 1 mA cm-2 in K4[Fe(CN)6] added 1 M H2SO4[44], and the addition of the redox quinone molecule can also be dissolved in ionic liquid todeveloped a moderate enhancement reaching values of 156 Fg-1 and 30 WhKg-1[45]. The cyclic

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durability of Device 4 symmetric capacitor was evaluated by repeating the galvanostatic charge/discharge tests at a current density of 1.0 A g-1 over 1000 cycles (Fig. 7f). The specific capacitance decreased 13% of the original, which might be due to the increasing aggregation of AR and ABA in pore structure of theelectroactive material

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during the long-time galvanostatic charge/discharge runs in aqueous solution [46].

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Electrochemical impedance spectroscopy is conducted to investigate the kinetics of the electrode materials. And electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency between 0.1 Hz and 105 Hz with the impedance amplitude of ± 5 mV at an open circuit potential. The solution resistance (Rs) is calculated from intercept of the Nyquist plot at Z' axis in the high frequency region, which is related to the ionic resistance of the electrolyte, the intrinsic resistance of electrode materials, and the contact resistance at the electrode/electrolyte interface. The charge-transfer resistance (Rct) is counted from the span of the small semicircle along the Z' axis, which describes the charge transfer resistance at the electrode/electrolyte interface. In addition, 14

ACCEPTED MANUSCRIPT the Warburg impedance (W) is the 45° segment in the EIS curve in middle-frequency region, which corresponds to the diffusion and migration resistance of ions from electrolyte to the interface of electrode materials. Meanwhile, the long line with high slope in low-frequency region is a result of the electrolyte diffusion and transport in the

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porous electrode, which can display the ideal capacitive behavior. As shown in Fig. 8, the Nyquist plots for both Device 1 and Device 4 electrodes have ideal electrochemical capacitance behaviors, such as, a small semicircle, a typical capacitor Warburg impedance and a nearly vertical linear. The Rs and Rct values of Device 4 are 4.51 Ω and

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0.20 Ω, respectively; however, the Rs and Rct values of Device 1 are 2.85 Ω and 0.14 Ω (the inset of Fig. 8).In

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addition, the Epa and Epc in the two-electrode systems are all higher than those in the three-electrode systems, which indicates that the redox-shuttle was not active(see Supplementary information,Fig. S2). The introduction of the redox additives into the 1 M H2SO4 electrolyte increase Rs and Rc simultaneously, may be due to the production of redox reactions at the electrolyte/electrode interface lead to the slower kinetics than electric double-layer

4. Conclusions

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capacitance.

In summary,a nitrogen doped porous carbon (CSC-1) was prepared from bio-waste of corn silks with surface

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area of 1764.8 m2 g-1 and pore volume of 2.0 cm3 g-1 by ZnCl2 activation. The CSC-1 electrode possesses high

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specific capacitance (358.0 F g-1 at 0.5 A g-1), high-rate capabilities and excellent cyclic stability in 1 M H2SO4 solution. Moreover, the symmetric cell based on CSC-1 electrodes displays a specific capacitance and energy density of 115.6 F g-1 and 7.9 Wh kg-1 in aqueous H2SO4 electrolyte, respectively. Furthermore, after adding the alizarin red and bromoamine acid additives in the H2SO4 electrolyte, the specific capacitance and energy density of the hybrid symmetric cells are increased to 260.8 F g-1 and 17.8 Wh kg-1, as well as possesses excellent cycle stability over 1000 cycles. This result indicates that the addition of alizarin red and bromoamine acid into H2SO4 solution can improve the capacitive behavior via pseudo-capacitive reactions. The present method is very simple 15

ACCEPTED MANUSCRIPT and cost effective compared to prepare composites, polymers or gels to increase the energy density of supercapacitors. Acknowledgements

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This research was financially supported by the National Science Foundation of China (21664012, 51462032), the program for Changjiang Scholars and Innovative Research Team in University (IRT15R56), Innovation Team Basic Scientific Research Project of Gansu Province (1606RJIA324), Key Laboratory of Eco-Environment-Related

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Polymer Materials of Ministry of Education, and Key Laboratory of Polymer Materials of Gansu Province. H. Peng

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thanks the financial support provided by the Outstanding Doctoral Dissertation Cultivation Program of Northwest Normal University. References

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ACCEPTED MANUSCRIPT Figure captions Fig. 1(a, c) Low and (b, d) high magnification high-resolution SEM images of CSC and CSC-1 samples. Fig. 2 (a) Nitrogen adsorption-desorption isotherms, (b) and inset (b) pore size distributions of the as-prepared

Fig. 3 (a) XRD patterns and (b) Raman spectrum of the as-prepared carbons.

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carbons.

Fig. 4 (a, b) Low and (c, d) high magnification high-resolution TEM images of CSC-1.

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Fig. 5(a) XPS survey spectra and (b, c) and d) High-resolution XPS spectra of the deconvoluted C 1s, N 1s and O 1s peak of CSC-1.

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Fig. 6(a) CVs at a scan rate of 50 mVs-1 and (b) GCDs at the current density of 1A g-1 for all as-prepared carbons; (c) CVs with various scan rates, (d) GCDs at different current densities and (e) the change of specific capacitance with the increase of current density for CSC-1; (f) Cycling performances of CSC-1 at the current density of 5 A g-1 and the inset is GC curves of the first three and the last three cycles, respectively.

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Fig. 7 (a) CVs at a scan rate of 50 mVs-1 different symmetric supercapacitors (Device 1, Device 2, Device 3 and Device 4) in a two electrode system; (b) CVs at different scanrates of the Device 4; (c) GCDsat the current density

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of 1.0 A g-1of all symmetric supercapacitors; (d)GCDs at different current densities and (e) the cyclic durability a current density of 1.0 A g-1 over 1000 cycles and of the Device 4; (f) Ragone plotsof different symmetric

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supercapacitors.

Fig. 8 EIS of Device 1 and Device 4 symmetric supercapacitors. Scheme 1. Schematic illustration of the processes occurring on the AC electrode surface during charging.

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Scheme 1.

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ACCEPTED MANUSCRIPT Highlights ● Nitrogen-doped porous carbon derived from a renewable corn silks ● The CSC-1 displays high specific surface area and large specific capacitance

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● The alizarin red (AR) and bromoamine acid (ABA) are employed as redox-active species

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● The hybrid symmetric supercapacitor exhibit high energy density and cyclic stability