Novel synthesis route for preparation of porous nitrogen-doped carbons from lignocellulosic wastes for high performance supercapacitors

Journal Pre-proof Novel synthesis route for preparation of porous nitrogen-doped carbons from lignocellulosic wastes for high performance supercapacitors Ali Pourjavadi, Hamed Abdolmaleki, Mohadeseh Doroudian, Seyed Hassan Hosseini PII:

S0925-8388(20)30479-5

DOI:

https://doi.org/10.1016/j.jallcom.2020.154116

Reference:

JALCOM 154116

To appear in:

Journal of Alloys and Compounds

Received Date: 3 October 2019 Revised Date:

28 January 2020

Accepted Date: 29 January 2020

Please cite this article as: A. Pourjavadi, H. Abdolmaleki, M. Doroudian, S.H. Hosseini, Novel synthesis route for preparation of porous nitrogen-doped carbons from lignocellulosic wastes for high performance supercapacitors, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2020.154116. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.

Ali Pourjavadi: Project administration; Supervision; Validation; Resources; Data curation Hamed Abdolmaleki: Experiments; manuscript writing Mohadeseh Doroudian: Supervision; Visualization; Software Seyed Hassan Hosseini: Supervision; Methodology

Novel synthesis route for preparation of porous nitrogen-doped carbons from lignocellulosic wastes for high performance supercapacitors Ali Pourjavadia*, Hamed Abdolmalekia, Mohadeseh Doroudiana, Seyed Hassan Hosseinib a

Polymer Research Laboratory, Department of Chemistry, Sharif University of Technology, Azadi Avenue,P.O.Box11365-9516, Tehran, Iran b

Department of Chemical Engineering, University of Science and Technology of Mazandaran, Behshahr 01134, Iran

ABSTRACT Porous nitrogen-doped carbons derived from biomass wastes are considered as promising materials for energy storage devices. Herein, a novel and scalable synthesis route for preparation of these materials from sugarcane bagasse waste is demonstrated. The synthesis process includes a hydrogel intermediate in which delignified bagasse and polyacrylamide networks are interlaced in molecular scale and calcium acetate serves as a hard template within the polymeric chains. After pre-carbonization and chemical activation of the hydrogel, porous nitrogen-doped carbon with high surface area of 1834.3 m2 g-1 and considerable pore volume of 1.03 cm3 g-1 was obtained with nitrogen and oxygen contents of 3.6 % and 14.5%, respectively. The fabricated supercapacitor electrode demonstrated high specific capacitance of 241 F g-1 at 0.5 A g-1 current density with capacitance retention of 96.5% after 5000 charge-discharge cycles at 2 A g-1. In a symmetrical two-electrode cell, the porous nitrogendoped carbon demonstrated high energy density of 14.8 W.h kg−1 at power density of 1001.6 W kg-1, which is comparable to many nitrogen-doped carbon materials found in the literature. Keywords: Nitrogen-doped carbon; Bagasse; Supercapacitor; Hydrogel

1. Introduction To face with challenges that arise from an accelerating consumption of fossil fuels, like global warming, air pollution, and climate changes, it is necessary to develop energy storage devices with high capacitance and energy density [1-4]. Among the existing energy storage technologies, supercapacitors have drawn remarkable attention due to their excellent long cycling life, low self-discharging, good reversibility and exceptional power density [5-7]. From the energy storage mechanism, supercapacitors can be classified into pseudocapacitors and electrical double-layer capacitors (EDLC). Pseudocapacitors store charge through a reversible faradic reaction on the surface of the active materials, while the EDLC´s energy storage mechanism is based on electrostatic ion adsorption along the interface of electrode and electrolyte [8-10]. Since electrode material is the key component determining the overall performance of supercapacitors, many efforts have been devoted to the fabrication of high performance electrode materials [11]. Among different capacitive electrode materials, biomass-derived carbons are among the most promising materials being studied extensively due to their low

cost, sustainability, renewability, and earth abundancy[12]. There are several recently published research papers reporting fabrication of efficient electrode materials from agricultural biomass wastes such as coconut shell[13], orange peel[14], pomelo peel[15], waste tea[16], corncobs[17] and etc. Sugarcane bagasse, a residue remaining after extraction of sucrose from sugarcane, is an agricultural waste that have been be used as precursor for preparation of porous carbons. Recently, Tang et al.[18] reported microwave assisted production of CO2-activated carbon from sugarcane bagasse wastes for electrochemical desalination. Their product possessed the surface area of 1019 m2 g-1 and electrosorption capacity of 28.9 mg g-1. Guo et al.[19] used porous sugarcane-derived carbons for CO2 adsorption. Their NaOH-activated carbon with the surface area of 1149 m2 g demonstrated CO2 uptake of 5.50 mmol CO2 g-1. Babu, D.B and Ramesha, K.[20] reported synthesis of nitrogen-doped graphene-like carbon nanosheets from bagasse wastes and melamine, and they also investigated the application of obtained product as cathode electrode material for Li-S batteries. Their device displayed high reversible capacity of 1169 mAh g−1 at 0.2 C with 77% capacity retention after 200 cycles. It is well established that doping of specific heteroatoms inside the carbon networks can remarkably enhance the capacitive performance of carbon materials. There are many reports in literature approving that the presence of dopants such as sulfur, phosphorous, boron, and nitrogen has a positive impact on the specific capacitance of the carbon materials [14, 21-25]. Among these heteroatoms, nitrogen is the most studied dopant since it exists in many biomass precursors and also demonstrate better electrical conductivity due to the presence of lone pair electrons and conjugated systems [26, 27]. Cordero-Lanzac et al. [27] carried out comprehensive research to investigate the role of different nitrogen functionalities on the electrochemical performance of activated carbons. They observed that nitro and aminecontaining activated carbons exhibit a typical pseudocapacitive behavior by undergoing a reversible reaction to form hydroxylamine groups. Quaternary nitrogen-containing activated carbons present a fast electrical double layer formation without undergoing any faradic reactions. Finally, pyrrolic and pyridinic nitrogens not only participate in pseudocapacitive redox reactions but can also further improve the ionic diffusion on carbonaceous structures[27]. So far, many synthesis routes have been proposed for the preparation of efficient carbon materials with controlled morphology and chemical composition. These routes often pursue two general strategies for doping of nitrogen heteroatoms into carbon networks. One is direct thermal treatment of nitrogen containing precursors[28, 29], and the other is pretreatment of carbon precursors with nitrogen containing molecules like melamine, urea, and ammonia[20, 30]. Pore creation can also be carried out by the use of hard templates[31], physical activation[32], chemical activation[19], and microwave-induced activation[18]. Herein, we demonstrate a novel and scalable synthesis method for preparation of porous nitrogen-doped carbons from lignocellulosic wastes such as sugarcane bagasse. The main novelty of this work is offering a systematic approach to benefit from both hard template and chemical activation methods for pore creation. This path opens up a plethora of new capabilities for preparation of size-controlled pores in a carbon matrix by benefiting from different hard templates.

2. Experimental section 2.1. Materials Sugarcane Bagasse wastes were prepared from Dehkhoda sugarcane Co. Ahwaz, Iran. Industrial grade acrylamide was purchased from Iran-tejarat Chemicals. Potassium hydroxide, hydrochloric acid (37%), ammonium persulphate (APS), and N, Nʹ-methylene bis (acrylamide) purchased from Merck, Poly vinylidene difluoride (PVDF) and monohydrated calcium acetate purchased from SigmaAldrich. All these chemicals were used without further purifications.

2.2. Synthesis of hydrogels Sugarcane Bagasse wastes were washed, milled and passed through a 60-mesh sieve for separation of large particles. Alkali delignification of sugarcane bagasse was carried out with the method described by Harmsen[33]. Briefly, 13 g of powdered bagasse added to 1000 mL of 1.5 M NaOH - 1 M Na2S aqueous solution and stirred at 70 oC for 2 hours. The resulting delignified bagasse was filtered, washed thoroughly, and dried at 80 oC for 24 hours. Afterwards, 4 g of delignified bagasse, 8.85 g of acrylamide and 0.19 g of MBA were added to 90 mL of distilled water with a temperature of 75 oC. Subsequently, 0.1 g of APS was introduced to the reaction vessel as an initiator for free radical polymerization, the mixture was stirred slowly until formation of a 3D bagasse-acrylamide hydrogel (BAH). Calcium acetate containing bagasse-acrylamide hydrogel (BAH-C) was further prepared using a similar procedure only with an addition of 6.2 g monohydrated calcium acetate to the reaction vessel along with bagasse, acrylamide, and MBA.

2.3. Synthesis of porous nitrogen-doped carbons 4 g of BAH and BAH-C were separately carbonized at 650 oC for 2 hours under N2 atmosphere with heating rate of 5 oC min-1. The products were washed with 1 M HCl solution and deionized water to eliminate impurities. After drying, the obtained carbons were named CBAH and CBAH-C, respectively. Both products were separately impregnated with KOH with mass ratio of 1:1, then placed in a furnace and heated to 750 oC at a rate of 5 oC min-1 under N2 atmosphere for 1 hour. After cooling, the products were thoroughly washed with 1 M HCl solution and deionized water, whereafter they were dried at 100 oC overnight. The resulting activated carbons from CBAH and CBAH-C were named NDC-1 and NDC-2, respectively.

2.4. Characterization The morphology of samples was investigated using Field Emission Scanning Electron Microscopy (FE-SEM, MIRA3 TESCAN) and Transmission Electron Microscopy (TEM; FEI Tecnai G2 F20). Thermal gravimetric analyses (TGA) (Pyris1 Perkin-Elmer) were carried out under N2 atmosphere in order to investigate carbonization behavior of the hydrogels. Elemental analyses (EA) were performed using Vario EL CHNS Elemental Analyzer. Surface area, porosity, and gas adsorption-desorption properties of the carbon materials were measured using a Brunauer−Emmett−Teller (BET) surface analysis instrument (Belsorp mini II) under N2 atmosphere. Crystallinity of the synthesized carbons was investigated by X-ray Diffraction (XRD) with a powder diffractometer (PANalytical B.V.

C=

Empyrean) using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was performed using an AXIS ULTRA spectrometer (Kratos Analytical, England) for precise determination of nitrogen and oxygen functionalities.

2.5. Electrochemical measurements The electrochemical properties of activated carbons were investigated in a three-electrode system with a saturated calomel electrode (SCE) as a reference electrode, a platinum wire as a counter electrode and 6M KOH aqueous electrolyte. The working electrodes were prepared by pressing a paste comprising of synthesized activated carbons, acetylene black, and PVDF at a ratio of 8:1.5:0.5 onto nickel foam as a current collector under the pressure of 6 MPa. The mass of the active material in the working electrodes was around 1.5 mg/cm2. Cyclic Voltammetry (CV) curves of fabricated electrodes were recorded within the potential window of -1 and 0 V at scan rates of 25, 50, 100, 150 and 200 mV/s. Galvanostatic charge-discharge (GCD) curves of the electrodes were obtained within the same potential window at current densities of 0.5, 1, 2, 5 and 10 A g-1. Electrochemical impedance spectroscopy (EIS) measurements were carried out at open-circuit voltage in frequency range of 10 mHz to 100 kHz with an AC perturbation of 5 mV. The specific capacitance (Cs) of the working electrodes in a 3 electrode system can be derived from GCD curves according to the following equation[34].

=

(1)

Where C (F g-1) is the specific capacitance, I (A) is the discharge current, (s) indicates discharge time, m (g) is the mass of the active material loaded on nickel foam and (V) refers to the voltage change within the time ( ). The gravimetric specific capacitance, energy density (E) and power density (P) of the symmetrical two electrode system were also calculated by the following equations[34]: = ED =

(2) (3)

. ⋅

= 3600



∆

(4)

Where I is the applied current density, V is voltage window, is the time of full discharge in seconds, and m is the weight of active material on both electrodes.

3. Results and discussion 3.1. Delignification of sugarcane bagasse Sugarcane bagasse is an agricultural waste remains after sucrose extraction from sugarcane and is mainly composed of cellulose, hemicellulose and lignin. Lignin acts like a glue to cellulose and hemicellulose fibers[35], and severely weakens the bagasse dispersibility in water. Delignification of sugarcane bagasse was carried out to overcome this problem. The mechanism of lignin degradation is depicted in Fig. 1. In this process moieties containing phenolic groups and alkoxide substitutes at alpha position (A) are converted into quinomethide (B), which is attacked by hydrogen sulfide ions at alpha-carbon position to form a benzylthiolate anion (C)[36]. Then,

(C) loses a B-phenolate (E) group by entering the sulfide anion through a neighboring replacement reaction. This process continues until all the lignin contents of bagasse degrade and dissolve in the aqueous phase. The FE-SEM images of untreated sugarcane bagasse and delignified sugarcane bagasse are shown in Fig. 2a and Fig. 2b, respectively. It is observable that tissue integrity of untreated bagasse is damaged and replaced by fibrous structure after the delignification process.

Fig. 1. Degradation mechanism of lignin

3.2 Preparation of porous nitrogen-doped carbons 3.2.1 Synthesis of hydrogels Both hydrogels were prepared by polymerization of acrylamide in the presence of delignified bagasse with MBA crosslinker and APS as a free radical initiator. TGA curves of both hydrogels are shown in Fig. 3a. The curve attribute to BAH shows only one distinct degradation stage at about 350-400 oC which means all components of the hydrogel carbonize at approximately the same temperature range. For a better comprehension of the carbonization process, DTG curve of the BAH is also depicted in Fig. 3b. A shoulder at 319 oC corresponds to carbonization of hemicellulose and two adjacent peaks at 366 oC and 379 oC are attributed to the degradation of cellulose and polyacrylamide, respectively[37]. Despite BAH, the TGA curve of BAH-C shows three distinct weight losses. First stage at 200-230 oC stems from the loss of water of hydration in monohydrated calcium acetate, the second step at 400-450 o C corresponds to carbonization of bagasse-acrylamide gel, and a weight loss at 500-550 oC is attributed to the decomposition of calcium acetate to CaCO3 and acetone[38]. The carbonization temperature of BAH-C is around 50 oC higher than BAH which indicates that the presence of calcium acetate has increased the thermal resistance of the hydrogel. Furthermore, formation of calcium carbonate molecules acting as hard template, and liberation of acetone gas during pyrolysis process, can significantly increase the porosity of the carbons obtained from BAH-C in comparison with those obtained from BAH. FE-SEM images of BAH and BAH-C are shown in Fig. 2c and Fig. 2d, respectively. Unlike BAH, which shows a smooth surface, BAH-C has a spotty surface, which is due to the presence of calcium acetate inside the hydrogel.

Fig. 2. FE-SEM images of a) sugarcane bagasse; b) delignified bagasse; c) BAH; d) BAH-C; e) CBAH; f) CBAH-C; g) NDC-1; h) NDC-2 . TEM images of i) NDC-1 j) NDC-2

Fig. 3. a) TGA curves of BAH and BAH-C b) DTG curve of BAH

3.2.2. Synthesis of NDC-1 and NDC-2 FE-SEM images of CBAH and CBAH-C obtained from direct carbonization of hydrogels are shown in Fig. 2e and Fig. 2f, respectively. Unlike CBAH, that has planar laminar structure, CBAH-C demonstrates high porosity and high surface area, which is due to the impact of calcium acetate as a hard template. BET data (Table 1) further support this fact and quantitatively show that BAH-C has a much higher surface area (324.4 m2 g-1) and porosity (0.19 cm3 g-1) in comparison with CBAH, which has a surface area of 22.1 m2 g-1, and pore volume of 0.03 cm3 g-1. The elemental analysis data (Table 1) also indicate that both products have considerable nitrogen content (7.6% for CBAH and 8.1% for CBAH-C), which means nitrogen stabilization was successfully carried out in both products during the carbonization step. In addition, the high oxygen-content of both carbons is also helpful for the creation of further pores during the activation process[29]. Fig. 2g and Fig. 2h demonstrate FE-SEM images of NDC-1 and NDC-2, respectively. The well-defined irregular porous structure of both products indicates the crucial role of KOH activation in developing and expanding new pores within the carbon products. TEM image of NDC-1 and NDC-2 are also provided in Fig. 2i and Fig. 2j. For a precise investigation of pore structures, N2 adsorption-desorption isotherms are demonstrated in Fig. 4a. In both NDC-1 and NDC-2, the isotherms were similar to type IV with a hysteresis loop demonstrating the presence of mesoporous structure in both products[39]. Also, a significant raise at low relative pressures (P/P0<0.1) reflects the existence of large number of micropores in both nitrogen-doped carbons. In general, NDC-2 shows higher nitrogen adsorption and wider hysteresis loop in comparison with NDC-1, supporting that it has higher microporous and mesoporous structures. Additionally, table-1 provide the elemental analysis and BET data of both products, in which, NDC-1 has the surface area of 826 m2 g-1 and nitrogen content of 3.1%, and NDC-2 possesses the surface area of 1834.3 m2 g-1 and nitrogen content of 3.6%. BJH curves that are displayed in Fig. 4 demonstrate pore size distribution of samples. The BJH curve of NDC-1 shows only one extreme peak at the region of >2 nm (micropore region), however for NDC-2 two distinct peaks exist at >2 nm and 4 to 6 nm (mesopore region). The peak at mesopore region for NDC-2 stem from calcium acetate and calcium carbonate templates. Raman spectra of both activated carbons (Fig. 4d) possess two peaks at

around 1345 cm-1 (D-band) and 1590 cm-1 (G-band). The former attributes to disordered structures of carbon, which results from A1g symmetry phonons near the K-zone boundary, while the latter is associated with the phonon mode with E2g symmetry of graphitic carbon [52]. The higher the intensity ratio of D peak to G peak, the lower graphitization degree of carbon, which indicates greater structural defects and pores. Comparing of ID/IG ratio in NDC-1 (0.92) and NDC-2 (0.98) demonstrates that NDC-2 has lower crystallinity and accordingly higher pores and structural defects.

Table 1- Elemental analysis and BET data of synthesized carbons Sample

C content (%)

H content (%)

N content (%)

O content (%)

SBET (m2 g-1)

VTotal(cm3 g-1)

CBAH

59.2

7.1

7.6

26.1

22.1

0.03

56

7.3

8.1

28.6

324.4

0.19

NDC-1

79.7

3.2

3.1

14

826

0.62

NDC-2

78.1

3.8

3.6

14.5

1834.3

1.03

CBAH-C

Fig. 4. a) N2 adsorption−desorption isotherm of NDC-1 and NDC-2 b) Pore-size distribution curve of NDC-1. c) Pore-size distribution curve of NDC-2 d) Raman spectra of NDC-1 and NDC-2

XPS analysis was also carried out in order to investigate different surface functionalities of nitrogen and oxygen in NDC-2. The survey spectrum (Fig. 5a) shows three main peaks at 285, 400 and 532 eV, which are assigned to C1s, N1s and O1s spin-orbit couplings. In the

evaluation of the nitrogen bonding conFig.urations, the high resolution spectrum of N1s is deconvoluted into three subpeaks with binding energies of 398.5, 400.4, and 401 eV, each of which corresponds to pyridinic nitrogens, pyrrolic nitrogens, and graphitic nitrogens, respectively[40] (Fig. 5b). The O1s core level peak can also be resolved into three components (Fig. 5c). The peak at 531.4 eV is assignable to isolated C=O/ O-H/ O−C=O, the peak at 532.6 eV can be ascribed to C=O/ O-C=O, and the peak in 533.6 originates from CO-C/ C-O-OH/C-OH functionalities[41]. The crystallinity of NDC-1 and NDC-2 products were investigated by X-ray powder diffraction (XRD) analysis, as shown in Fig. 5d. Both samples do not possess sharp and distinct characterization peaks, which means both products demonstrate the structure of disordered carbon materials. However, two broad diffraction peaks at 2θ=23o and 43o are indexed to the (002) and (100) crystallographic plane reflections of carbon materials, respectively. The peaks at 2θ = 23° in both NDC-1 and NDC-2 shifts to a lower diffraction angle compared with standard graphite at 26.6°, indicating broadening of interlayer d-spacing between (002) planes in the Bragg formula. The broad diffractions at 2θ=43o also indicate the presence of honeycomb sp2 hybridized carbon bonds in both carbon products.

Fig. 5. a) XPS survey spectrum of NDC-2 b) N1s spectrum of NDC-1 c) O1s spectrum of NDC-1 d) XRD spectra of NDC-1 and NDC-2

3.3. Electrochemical properties

Capacitive performance of as-obtained carbon materials was evaluated by CV, GCD, and EIS measurements in a three-electrode system. The galvanostatic charge-discharge analysis were carried out at potential range of -1.0 – 0.0 and current density of 2 A g-1 to estimate and evaluate the capacitive behavior of CBAH, CBAH-C, NDC-1, and NDC-2 (Fig. 6a). According to equation 1, the specific capacitance of the products qualitatively follows the order of NDC-2 > NDC-1 > CBAH-C > CBAH based on the discharge duration in GCD curves. This trend is further supported by CV plots (Fig. 6b), indicating the fact that capacitive performance is directly proportional to the surface area and pore volume of the electrode materials demonstrated in Table 1. The presence of redox peaks in CV curve of CBAH-C demonstrates its pseudocapacitive behavior arising from a high degree of nitrogen and oxygen functionalities (totally 36.7 %), which undergo faradic reactions[42]. The galvanostatic charge-discharge curves of the NDC-2 electrode at different current densities are shown in Fig. 6c. The curves exhibit nearly triangular shape in potential range of -1.0 – 0.0 V even at high current density of 10 A g-1 representing excellent and closely ideal capacitive behavior of the electrode. Fig. 6d shows the relationship between the charge-discharge current density and the specific capacitance of CBAH, CBAH-C, NDC-1, and NDC-2 electrodes. The highest specific capacitance was observed in NDC-2 electrode at current density of 0.5 A g-1 equal to 241 F g-1 which is better than or comparable to many N-doped carbon materials found in the literature. The specific capacitance value of CBAH, CBAH-C and NDC-1 are also 13, 149 and 198 F g-1, respectively, at current density of 0.5 A g-1. By increasing current density, a slight decrease of specific capacitance was observed in all four electrodes. For instance, at current densities of 5 and 10 A g-1 the specific capacitance of NDC-2 is 175 and 161, and for NDC-1 is 152 and 146, respectively.

-1

Fig. 6. a) GCD curves of CBAH, CBAH-C, NDC-1 and NDC-2 at 2 A g . b) CV curves of CBAH, CBAH-C, NDC-1, and NDC-2 at scan rate of 25 mV/sec. c) GCD curves of NDC-2 at different current densities. d) Specific capacitance of CBAH, CBAH-C, NDC1 and NDC-2

The shape of CV curves can also be informative about ion transfer rate through the porous carbon structure. The more CV curves exhibit rectangle-like shape, the faster the ion diffusion process through the electrode material[43]. Rectangular shape of CV curves also indicates that capacitive behavior of electrode is mainly stems from formation of electrical double-layer along the interface of

electrode and electrolyte. The CV curves of NDC-2 electrode (Fig. 7a) show rectangular shape with small distortion even at high scan rate of 200 mV/sec, which represents fast ion transport during charge-discharge process. The small deviation from ideal rectangular shape can also be attributed to different nitrogen and oxygen functionalities on the surface of carbon structure which result in further increase of overall specific capacitance by undergoing faradic reactions[44]. The Nyquist plots of NDC-1 and NDC-2 electrodes are shown in Fig. 7b. The presence of semi-vertical line in low frequency regions for both samples indicates low diffusion resistance and Warburg element [57]. In middle and high frequency region, the intercept of the curves, which are shown in enlarged format, represent the equivalent series resistances (ESR) which are 1.08 Ω and 1.14 Ω for NDC-1 and NDC-2, respectively. Furthermore, the small semicircle in both curves can be attributed to charge transfer resistance (Rct) of electrodes, which is 0.4 Ω for NDC-1 and 0.3 for NDC-2. The other important factor that determines the practical application of a supercapacitor is cycling durability. To investigate that, the specific capacitance of the NDC-2 electrode was measured in a three-electrode setup within 5000 cycles at current density of 2 A g-1 and the results are shown in Fig. 7c. Notably, high capacitance retention of 96.5% was recorded after 5000 times of charge-discharge cycles, confirming excellent cycling stability of the carbon product resulting from its unique structural and surface properties. However, the observed drop in the specific capacitance by increasing the cycle number might be attributed to irreversible faradic reactions between surface nitrogen functionalities of the carbon material with cations in the electrolyte, which leads to decrease in pseudocapacitive performance of the electrode and accordingly decrease in overall capacitance.

Fig. 7. a) CV curves of NDC-2 at different scan rates b) Nyquist plots of NDC-1 and NDC c) Capacitance retention of NDC-2 during 5000 charge-discharge cycles

To further explore the potential application of our material as device, a two-electrode symmetric supercapacitor was fabricated from NDC-2 in 6 M KOH electrolyte. As shown in Fig. 8a GCD curves obtained from two-electrode system show a nearly triangular shape

indicating excellent rate capability, acceptable electrochemical reversibility, and good carrier transport within the electrodes for electrical double-layer supercapacitors. The CV curves are also demonstrated in Fig. 8b. All CV curves show semi-rectangular shape at the scan rates of 25 – 150 mV/sec with no clear change in CV profile even at high scan rates, indicating excellent rate performance and electrical double layer formation. To investigate cycle durability of the symmetrical supercapacitor it was subjected to 5000 charge-discharge cycles at current density of 5 A g-1 and the capacitance was calculated after each 100 cycles according to equation 2 and plotted in Fig. 8c. It was observed that the symmetrical supercapacitor retained about 93.9% of its initial capacitance after 5000 charge-discharge cycles, which represent nearly stable structure of synthesized NDC-2. The slight reduction in the specific capacitance may be attributed to irreversible faradic reactions between nitrogen and oxygen functionalities with the electrolyte. Ragon plot (Fig. 8d) of NDC-2 symmetric supercapacitor was plotted based on GCD curves by using equation 3 and equation 4. Based on this plot the highest energy density of 14.8 W.h kg−1 was obtained at the power density of 1001.6 W kg-1, which is comparable or better than many previously reported carbon based supercapacitors in the literature. Table-2 compares the capacitive performance of NDC-2 with some other reported nitrogen-doped carbon materials.

Fig. 8. a) GCD curves of symmetric supercapacitor of NDC-2 b) CV curves of symmetric supercapacitor of NDC-2 c) Cycle stability curve d) Ragon plot of NDC-2

Table 2- Comparison of different N-doped carbons as supercapacitor electrode material

Flexible, cross-linked N-doped carbon nanofiber Elastic N-doped carbon nanofibrous microspheres

175 F g-1 at 50 A g-1

Energy density (W h/kg) 5.9

219 F g-1 at 5 mV/s

Hollow particle-based nitrogen-doped carbon Nitrogen and Fluorine dual-doped carbon sheets Nitrogen-doped carbons from used cigarette filters Biomass derived nitrogen and oxygen-doped carbon N-doped carbon prepared by solid-solid grinding This work

Material

Specific capacitance

Power density (W/kg)

Reference

1200

[45]

58.7

300

[46]

307 F g−1 at 1.0 A g−1

10.96

25000

[47]

110 F g-1 at 1A g-1

3.82

0.5

[48]

263 F g-1 at 1 A g-1

20.2

720

[49]

163 F g-1 at 0.5 A g-1

50

372

[50]

228 F g-1 at 0.2 A g-1

26.3

1020

[51]

241 F g-1 at 0.5 A g-1

14.8

1001.6

-

4. Conclusions In summary, hierarchical porous nitrogen-doped carbons were prepared by carbonization and activation of bagasse-acrylamide hydrogels. The highest specific capacitance was obtained from NDC-2, which was 241 F g-1 at 0.5 A g-1 current density. This product also had a high surface area of 1834.3 m2 g-1 and remarkable pore volume of 1.03cm3 g-1 with 3.6% and 14.5% nitrogen and oxygen contents. A two-electrode symmetric supercapacitor was also fabricated to investigate the electrochemical performance and the potential application of our material in supercapacitors. The as-obtained supercapacitor showed an energy density of 14.8 W h/kg at power density of1001.6 W/kg.

Acknowledgment This work was supported by Sharif University of Technology, Grant Program (G940607).

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1- Highly porous nitrogen-doped carbon materials were prepared from delignified bagasse wastes and acrylamide. 2- The synthesis route includes a hydrogel intermediate in which bagasse and polyacrylamide chains are interlaced and calcium acetate is used as hard template 3- The resulted product shows high specific capacitance of 241 F g-1 and cycle stability of 96.5% after 5000 charge-discharge cycles

The authors declare no conflict of interest