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Sustainable Low-Cost Green Electrodes with High Volumetric Capacitance for Aqueous Symmetric Supercapacitors with High Energy Density Qinxing Xie, Rongrong Bao, Anran Zheng, Yufeng Zhang, Shihua Wu, Chao Xie, and Peng Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01417 • Publication Date (Web): 14 Jan 2016 Downloaded from http://pubs.acs.org on January 17, 2016

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ACS Sustainable Chemistry & Engineering

Sustainable Low-Cost Green Electrodes with High Volumetric Capacitance for Aqueous Symmetric Supercapacitors with High Energy Density Qinxing Xiea,*, Rongrong Baoa, Anran Zhenga, Yufeng Zhanga, Shihua Wub, Chao Xiec, Peng Zhaob a

Tianjin Key Laboratory of Fiber Modification and Functional Fibers, School of Materials

Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China b

Department of Chemistry, Nankai University, Tianjin 300017, China

c

School of Materials Science and Engineering, Shandong University of Technology, Zibo

255049, China

1 ACS Paragon Plus Environment


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ABSTRACT Low-cost and high performance electrode materials are critical for rapid development and wide applications of supercapacitors. Herein, we report a cost-effective approach to prepare nitrogen-doped active carbon/graphene (N-AC/Gr) composites using renewable corn straw and soy protein as precursors, respectively. The as-prepared composites have moderate specific surface areas in a range of 1233.6-1412.9 m2 g-1 and abundant porosity. Furthermore, the composites demonstrate high single electrode gravimetric and volumetric specific capacitances of 378.9 F g-1 and 257.7 F cm-3 at 0.05 A g-1, and of 321.1 F g-1 and 213.2 F cm-3 at 20 A g-1, accounting for an excellent rate capability with a retention of 66.4 % in 6 M KOH electrolyte. The assembled N-AC/Gr-based aqueous symmetric supercapacitor exhibits superior cycling durability with 93% retention after 10000 cycles at 2 A g-1, and delivers a high energy density of 13.1 Wh kg-1 (11.1 Wh L-1) with a power density of 12.5 W kg-1 (10.6 W L-1).

KEYWORDS: Supercapacitor, Graphene, Active carbon, Corn straw, Composite


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INTRODUCTION

Today, there is a growing demand for sustainable and renewable energy, which is stimulated by the global depletion of fossil fuel and simultaneous increase of environmental concerns. Hence, the related energy conversion and storage materials and devices have drawn a great deal of attention. Supercapacitors, which are also called electrochemical capacitors (ECs), are considered as a new type of energy storage systems with superior advantages including high power density, long cycle life, high rate capability and reversibility compared to conventional capacitors and Li-ion batteries (LiBs).1-2 Usually, they can be classified into two categories including electrical double-layer capacitors (EDLCs) and pseudocapacitors based on two distinct charge storage mechanisms.1-2 In the last decades, supercapacitors have been intensively investigated and widely applied in fields from small portable electronic devices to large-scale systems including electric vehicles

To fabricate high performance supercapacitors, one of the most critical factors must be considered is the energy storage capability of electrode materials. Carbons, such as activated carbon (AC), mesoporous carbon (MC), carbon aerogels (CA), carbon nanofibers (CNFs), carbon nanotubes (CNTs) and graphene nanosheets (GNs) are fascinating electrodes because of their advantageous characteristics including high specific surface area, excellent electric conductivity, abundant porosity and excellent physicochemical stability.3-10 Among of these carbons, GNs are most attractive due to the superior electronic and mechanical properties, as well as an exceptionally large theoretical specific surface area of ca. 2650 m2 g-1 and an extremely high specific capacitance of ca. 550 F g-

1

if all surface area is utilized.7,

11



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However, it is still a big challenge at present to realize a large-scale application because of the high-cost and complicated synthesis process. Additionally, a macroscopic manufacturing process generally result in the loss of advantages and intrinsic properties of individual graphene nanosheet due to the severe restacking effect.3 This gives an explanation of why the reported specific capacitances are still far below the theoretical value.12-15 It is very strategic to fabricate graphene composites with other electrochemically active components, including metal oxides, conductive polymers or other porous carbons that would be helpful to inhibit the agglomeration of GNs functionalizing as effective spacers.11-12, 15

In this work, we reported a new kind of active carbon/graphene (AC/Gr) composites prepared using crop straws such as corn straw as the starting material. To further enhance the capacitive performance, the carbon composites were doped with nitrogen atoms using soy protein as the nitrogen source for the first time. It has been demonstrated that nitrogen atoms doped in carbon structure is capable of providing extra peusodocapacitance, and improving the conductivity and wettability of carbon materials.16-19 This research is very meaningful as crop straws are abundant and renewable in nature. Actually, billions of tons of crop straws have to be abandoned or burned off every year because of the huge global yield, consequently resulting in serious air pollution. In this respect, we intended to provide a more efficient and economic utilization way of crop straws.

Herein, the morphology, microstructure and electrochemical properties of the as-prepared composites were investigated and discussed in detail. As expected, the N-doped active carbon/graphene (N-AC/Gr) composites as electrodes for aqueous symmetric supercapacitors


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exhibit a high specific capacitance of 379.0 F g-1 at a current density of 0.05 A g-1, and 251.6 F g-1 at 20 A g-1. In addition, the assembled capacitor demonstrates an excellent long-term cycling stability, high energy density and high power density.

EXPERIMENTAL SECTION

Materials. All chemicals are analytical grade and used as purchased. Graphite oxide (GO) was prepared by a modified Hummers method.20 Corn straw powders were obtained from Liaocheng city, China, and washed with distilled water and dried at 110 °C for 12 h. Soy protein was obtained from Qingdao city, China.

Materials preparation. The N-doped AC/Gr composites were prepared using KOH as activation agent. In a typical process, 5 g of corn straw powders were ball milled for 2 h at a speed of 600 r/min, then probe-sonicated for 2 h in 200 ml of distilled water at 1500 W. The mixture was centrifuged to get a pulp of corn straw fibers (CSFs). A certain amount of GO (0, 1, 3, 5 wt.%, respectively) was sonicated for 30 min in 50 ml of distilled water, and added into the pulp drop by drop under stirring.

After that, 5 wt.% of soy protein was added and

stirred further for 30 min. The mixture was then dried and pressed into a pellet of 25 mm in diameter at 10 Mpa. The pellet was carbonized at 800 °C for 2 h in a flow of N2, then activated at 700 °C for 2 h by KOH with a C/KOH mass ratio of 4:1. The product was pulverized and washed with 0.1 mol/L hydrochloric acid and distilled water for several times until the pH of filtrate reached 7. After dried, the N-AC/Gr composites were obtained as black powders and designated as N-AC@Gr1, N-AC/Gr3 and N-AC/Gr5, respectively.



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For comparison, the pure CSFs and the mixture of CSFs with 5 wt.% soy protein was carbonized and activated with the same procedure, and the products were designated as AC and N-AC,

respectively .

Materials characterizations. The microstructure, surface composition and morphology of the samples were characterized by X-ray powder diffractions (XRD, Rigaku D/Max 2500), field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800), high resolution transmission electron microscopy (HRTEM, FEI Tecnai G20), X-ray photoelectron spectroscopy (XPS, Thermo scientific K-Alpha). The Raman spectra were recorded on a Thermo Scientific DXR Raman microscope at a laser excitation of 532 nm. Nitrogen adsorption/desorption

isotherms

were

collected

using

a

Tristar-3000

sorptometer

(Micrometics, USA). The surface area was calculated using the Brunauere-Emmette-Teller (BET) method, and the total pore volume was determined from the amount of nitrogen adsorbed at a relative pressure (P/P0) of 0.97. The micropore volume was determined by t-plot method. The pore sizes and distribution were analyzed via the Barrett–Joyner–Halenda (BJH) method using nitrogen adsorption data and assuming a slit pore model.

Electrochemical measurement. The electrochemical measurements were performed in a two-electrode cell at room temperature on a CHI 660E electrochemical workstation (CH Instrument, China), using 6 M KOH aqueous solution as electrolyte. The working electrodes were fabricated by casting a mixture of the as-prepared carbon materials with 60% PTFE emulsion in a mass ratio of 90:5 onto a nickel foam of 1 × 1 cm2. Each carbon electrode contains ca. 3 mg cm-2 of active material. The electrochemical impedance spectroscopy (EIS) 6 ACS Paragon Plus Environment

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measurements were carried out at an open circuit potential with an amplitude of 5 mV in a frequency range of 100 kHz to 0.01 Hz. The cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements were performed with a potential window of 0 to 1.0 V. The cycling performance of the assembled supercapacitors was evaluated by GCD measurements at a current density of 2 A g-1 for 10000 cycles. The gravimetric specific capacitance of the materials (Cm, F g-1)) was calculated according to the CV measurements by using the equation (1): 1, 21-22

= ∆

(1)

where I represents the response current (A), S is the potential scan rate (V s-1), ∆V corresponds to the applied voltage range (V), and m is the mass (g) of active material on each electrode. For comparison, the specific capacitance (Cm, F g-1) was also calculated based on GCD measurements by using the equation (2):1, 21, 23

=

2 I∆ t m∆ V

(2)

where I is the constant discharge current (A), Δt is the discharge time (s), m is the mass of active material (g) on each electrode, and ΔV is the voltage difference (V). The volumetric specific capacitance (Cv, F cm-3) was calculated by using the equation (3):24-25

Cv = ρ × Cm

(3)

where ρ is the particle density (g cm-3) of carbon that can be calculated according to the equation (4):24-25 7 ACS Paragon Plus Environment


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ρ=

( ⁄ )

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(4)

(cm3 g-1), ρT is the true density of carbon (2 g cm-3 )

where Vt is the total pore volume

The gravimetric specific energy density (Eg, Wh kg-1) and power density (Pg, W kg-1) of the cells were calculated using the equation (3) and (4), respectively1, 21, 23:

= =

∆ .

×!"## ∆$

(5)

(6)

where Cs represents the gravimetric specific capacitance of the cell (Cm/4, F g-1),ΔV corresponds to the voltage change during the discharge process after IR drop (V), Δt is the discharge time (s). The volumetric energy density Ev (Wh L-1) and power density Pv (W L-1) were calculated according to equation Ev = ρEg and Pv = ρPg, respectively.25

RESULTS AND DISCUSSION

The SEM and TEM measurements were performed to examine the morphology and porous structure of the as-prepared samples. As shown in Figure 1, the samples all partially reserve the fibrous structure of corn straw. The pure corn straw derived actived carbon (AC) exhibits slightly wrinkled and rough surface, while N-AC exhibits relatively smooth surface. It can be seen as well that both N-AC and N-AC/Gr composites demonstrate surfaces with distinct stereoscopic lamellar morphology, because of the carbonization of soy protein and/or surface coating of graphene. Moreover, the surface of N-AC/Gr1 clearly reveals the presence of crumpled graphene nanosheets. The elemental mapping for C and N in N-AC/Gr1 8 ACS Paragon Plus Environment

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composite reveals a homogeneous distribution of nitrogen atoms in the material, as shown in Figure S1. Figure 2 shows the TEM images of N-AC/Gr1 composite, it reveals that the quasi-two-dimensional wrinkled graphene sheets as well and abundant nanosized pores. This kind of porous structure can significantly accelerate the penetration of electrolyte ions by reducing the transportation path, which can realize better electrochemical performance.26-27

Figure 1, Figure 2 Figure 3a shows the XRD patterns of the samples. It can be observed that all XRD patterns exhibit two broad peaks of (002) at ca. 23o and (100) at ca. 43o, indicating the amorphous carbon structure. The microstructure was also investigated by Raman spectroscopy. Figure 3(b) shows the resulted Raman spectra for the as-prepared samples, and hydrothermally reduced graphene for comparison. All spectra exhibit D band of carbon at ca. 1350 and G band at ca. 1595 cm-1, respectively. The D band is related to the disorder or defects in carbon structure, while the G band is ascribed to the ordered graphitic structure. The intensity ratio of D and G band (ID/IG) represents the relative amount of ordered graphite crystallites in the carbon materials, which is 0.88 for AC and N-AC, 0.86 for N-AC/Gr1 and N-AC/Gr5, 0.80 for N-AC/Gr3, respectively. Apparently, AC/Gr composites exhibit lower ID/IG, which indicates less defects or disorder in structure. As disorder in graphene is generally abundant, the hydrothermally reduced graphene exhibit a higher ID/IG ratio of 0.92. Hence, the decreased defects of AC/Gr composites shall be attributed to a synergistic effect of graphene and active carbon.

Figure 3



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To evaluate the influence of nitrogen and oxygen functionalities on capacitive performance of the materials, N-ACGr1 was further investigated by XPS (Figure 4). It was found that the sample surface is mainly composed of carbon, nitrogen and oxygen elements with an atomic ratio of 88.7/9.3/2.1. The C1s spectrum was deconvoluted into five type peaks, corresponding to C-C at 284.6 eV (peak 1), C-N at 285.1 eV (peak 2), C-O at 286.4 eV (peak 3), C=O at 287.5 eV (peak 4), and O-C=O at 289.0 eV (peak 5), respectively.23 The emergence of C-N species indicates the successful doping of nitrogen atoms into the carbon network structure. The deconvolution of N1s spectrum results in four peaks that corresponds to four types of nitrogen functional species integrated into the carbon skeleton including pyridinic type nitrogen (N-6) at 398.8 eV (peak 1), pyridonic/pyrrolic nitrogen (N-5) associated with oxygen functionality at 400.3 eV (peak 2), quaternary type of N atoms (N-Q) substituted with carbons in the aromatic graphene structure at 401.8 eV (peak 3), and the oxidized nitrogen (N-X) at 403.6 eV (peak 4), respectively.28-29 Unlike N-Q, N-6, N-5 and N-X are all located at the edge of the graphene structure. The relative contributions of each nitrogen species to the total peak area is 29.1% for N-6, 26.1% for N-5, 25.4% for N-Q, 19.3% for N-X. The existence of N-Q is able to enhance the electrical conductivity of carbon materials, while the accessible nitrogen species of N-6 and N-5 provide the active sites of chemical reactions, accounting for improved capacitive performance.18, 30-32

Figure 4

The surface area and porous structure were analyzed by using the nitrogen sorption measurements. The isotherms (Figure 5a) reveal the type-I characteristics. Compared to AC, 10 ACS Paragon Plus Environment

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N-AC and N-AC/Gr composites exhibit remarkably enhanced nitrogen adsorption volume, indicating the increased surface area. The steep adsorption at a low relative pressure for all samples indicates the presence of micropores. Meanwhile, there is no significant hysteresis loop, which is ascribed to a small fraction of mesopores. Figure 5b shows the pore size distributions (PSDs) for all samples, demonstrating the pore sizes for all samples are mainly smaller than 5 nm. The BET specific surface area (SBET) and the detailed pore structure parameters of the samples are summarized in Table 1. The SBET is 1412.9 m2 g-1 for N-AC/Gr1, 1269.5 m2 g-1 for N-AC/Gr3, and 1233.6 m2 g-1 for N-AC/Gr5, revealing a slight decrease with increased doping amount of graphene. The doped graphene nanosheets are able to prevent the deep diffusion of KOH, and inhibit the over-etching of carbon during activation.33 The nitrogen-free AC exhibits the lowest SBET of 893.5 m2 g-1, while N-AC demonstrates a SBET of 1251.7 m2 g-1 that is comparable to those of N-AC/Gr composites. It has to be noticed that the reported specific surface areas for graphene were still far below the theoretical value (2675 m2 g-1) because of the face-to-face restacking of graphene sheets, for example, 517.9, 274. 9 and 106 m2 g-1 reported by Yan, Sun and Wang, respectively, which are greatly dependent upon the synthesizing methods and graphene morphology.34-36 In this case, the intermixed biomass-carbon particles of the composites are capable of inhibiting the restacking effect, which enables the increased surface utilization of graphene. According to Table 1, it can be concluded that the micropores in N-AC/Gr composites account for 75-83% of total pore volume, while in AC accounts for 67.4%, and in N-AC accounts for 37.5%. This indicates that the graphene in the composites is favorable for the development of micropores. It has been demonstrated that the micropores provide adsorbing sites for electrolyte ions that 11 ACS Paragon Plus Environment


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are responsible for the enhanced energy storage capability of carbon materials, while the mesopores provide effective paths for electron and ion transportation to micropores.37-40 Hence, N-AC/Gr composites, especially N-AC/Gr1, are expected to have much enhanced electrochemical performance.

Figure 5, Table 1 The influence of the doped nitrogen and graphene on the electrochemical behavior and performance of N-AC/Gr composites was evaluated by CV, GCD and EIS measurements in 6 M KOH aqueous electrolyte. The CV curves of N-AC/Gr1 at scan rates varied from 5 to 200 mV s-1 exhibit a nearly ideal rectangular shape (Figure 6a,b). At the same time, there is no remarkable distortion even at a high scan rate of up to 200 mV s-1. This observation indicates a rapid current response upon the reversed direction of potential scan, which implies an excellent reversibility during charge-discharge process and significant double layer capacitive behavior of carbon electrodes. Figure S1a shows a comparison of CV curves at 200 mV s-1 for all samples, the enclosed area is relative to the magnitude of specific capacitance of the materials. The calculated gravimetric specific capacitances at 5 and 200 mV s-1 for N-AC/Gr1 and other samples for comparison are plotted in Figure 6d. It can be observed that N-AC/Gr composites have higher capacitances than AC and N-AC at both low and high scan rates as expected, demonstrating an outstanding enhancing effect of the doped graphene. For example, the capacitances at a scan rate of 5 mV s-1 are 165.5, 180.5 F g-1 for AC and N-AC, and 299.3, 238.1 and 221.7 F g-1 for N-AC/Gr1, N-AC/Gr3 and N-AC/Gr5, respectively. Apparently, N-AC exhibits improved capacitances as well because of the pseudocapacitive contribution


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from doped nitrogen. However, it is not as significant as N-AC/Gr composites although N-AC, N-AC/Gr3 and N-AC/Gr5 have nearly identical surface areas. Partially, it can be traced back to the low nitrogen content. Most importantly, it is because of a much lower micropore fraction for N-AC than other samples (ref. to Table 1). As mentioned above, the doped graphene in N-AC/Gr composites facilitates the formation of micropores, which accounts for the improved capacitance. However, the specific capacitance decreases with increased graphene doping amount, this phenomenon can be attributed to the decreased specific surface area and micropore fraction. Figure 6d shows a correlation of the specific capacitance with the micropore volume of samples, indicating that the capacitance increases with increased micropore volume except for N-AC because of the extra pseudocapacitance of nitrogen. Figure 6 The GCD measurements were performed as well for further evaluation of supercapacitive performance of N-AC/Gr composites (Figure 7). The GCD curves of N-AC/Gr1 at current densities varied from 0.05 to 20 A g-1 exhibit a typical isosceles-triangular shape without obvious voltage drop (IR) due to low internal resistance, which is an indication of excellent capacitive behavior, as shown in Figure S3. Generally, the discharge time decreases with increased current densities, indicating the decreased specific capacitance of porous carbons.1, 18-19

It is because the electrolyte ions do not have sufficient time to diffuse into the

micropores of electrode at high current densities, resulting in a low utilization rate of active materials. A comparison of GCD curves at 2 A g-1 for all samples reveals the improved supercapacitance of N-AC and three N-AC/Gr composites because of the doped nitrogen and 13 ACS Paragon Plus Environment


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graphene (Figure 7a), which is in consistent with the CV measurements. The calculated gravimetric capacitance as a function of current density is plotted in Figure 7b, demonstrating the superior rate capability of N-AC/Gr composites. The specific capacitance for N-AC/Gr1 is 378.9 F g-1 at 0.05 A g-1, and 251.6 F g-1 even at a high current density of 20 A g-1, resulting in a high retention rate of 66.4 %. Hence, the capacitance for the assembled cell is 94.7 F g-1 at 0.05 A g-1, and 62.9 F g-1 at 20 A g-1. Actually, the supercapacitor was also tested at a very high current density of 40 A g-1, 52.8 F g-1 can still be remained for the whole cell, demonstrating excellent energy storage capability that is even much better than graphene-based aqueous supercapacitors reported by other researchers.34-36 Additionally, N-AC/Gr1 demonstrates high volumetric specific capacitance as well, for example, 321.1 and 213.2 F cm-3 at current densities of 0.05 and 20 A g-1, respectively. To evaluate the penetration accessibility of electrolyte ions, the capacitances at 0.05 and 20 A g-1 were normalized based on the specific surface areas of the samples, as illustrated in Fig. 7c. It can be seen that N-AC/Gr1 demonstrates the highest capacitances per unit surface area (Csa), which is ascribed to the lowest diffusion resistance. On the contrary, N-AC exhibits the higher diffusion resistance than other samples. To elucidate the superior capacitive performance of N-AC/Gr composites, a comparison was made with other biomass-derived active carbons reported recently in literatures, as summarized in Table 2. Figure 7, Table 2 EIS measurements were performed for the assembled cells at room temperature to further explore the frequency response characteristics of the composites, the resulted Nyquist plots are depicted in Figure 7d. The vertical line at low frequencies demonstrates the excellent 14 ACS Paragon Plus Environment

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capacitive performance of all samples due to the rapid ion diffusion in electrolyte and adsorption onto electrode surface.51, 54 An intercept at Z' axis of high frequency corresponds to the total ohmic resistance (Rs) consisting of electrolyte resistance, intrinsic resistance of active material and electrical contact resistance,51, 55 which are in a range of 0.57-0.60 Ω for AC and N-AC, and of 0.54-0.55 Ω for N-AC/Gr composites, respectively. The doped graphene practically contributes to the improved conductivity of the composites. The small Rs values are beneficial for high power applications of supercapacitors. The semicircle region reflects the charge transfer process at the interface between electrode and electrolyte, the smaller semicircle represents the lower charge transfer resistance (Rct).55,56 Additionally, there is a line of 45° slope at the intermediate frequency region that is called the Warburg resistance (W), representing the frequency dependent characteristic of electrolyte diffusion into the interior pores of electrodes.46,47 Apparently, N-AC/Gr1 exhibits the lowest Rct and fastest ion diffusion rate in 6 M KOH aqueous electrolyte, this matches well to the CV and GCD measurements. Figure 8 To evaluate the cycling performance, the assembled symmetric supercapacitor with N-AC/Gr1 as electrodes in 6 M KOH electrolyte was charged-discharged for 10000 cycles at a current density of 2 A g-1 (Figure 8a). The capacitor demonstrates superior cycling durability with a high retention rate of 93 % after 10000 cycles, and excellent reversibility with a coulombic efficiency of nearly 100% for each charge/discharge process. The Ragone plots (Figure 8b) represent a correlation of the specific energy density with the power density of the cells, revealing significantly enhanced energy density and power density for 15 ACS Paragon Plus Environment


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N-AC/Gr-based supercapacitors. Especially, N-AC/Gr1 capacitor demonstrates a high specific energy density of 13.1 Wh kg-1 (11.1 Wh L-1) with a power density of 12.5 W kg-1 (10.6 W L-1) at 0.05 A g-1. Moreover, the energy density remains 8.1 Wh kg-1 (6.9 Wh L-1) with a high power density of 4668.0 W kg-1 (3956.1 W L-1) at 20 A g-1, revealing a retention of 61.8 %.

CONCLUSIONS

In summary, porous N-AC/Gr composites as electrodes for high performance supercapacitors were successfully prepared using corn straw and soy protein as the primary precursors. The as-prepared composites exhibit high specific surface area and abundant porosity. The doped graphene nanosheets are favorable for the development of micropores and the improvement of conductivity that are beneficial for significantly enhanced supercapacitive performance of the materials. A high single electrode specific capacitance of 378.9 F g-1 and 321.1 F cm-3 at 0.05 A g-1 was achieved for N-AC/Gr1 in 6 M KOH aqueous electrolyte. The assembled N-AC/Gr1-based symmetric supercapacitor demonstrates excellent rate capability, as well as superior cycling performance with 93% retention after 10000 cycles at 2 A g-1. In addition, the supercapacitor delivers a maximum energy density of 13.1 Wh kg-1 at a power density of 12.5 W kg-1. The results demonstrate a promising prospect for the fabrication and application of low-cost and high performance aqueous supercapacitors with high energy density and power density, as corn straw is abundant and naturally reproducible. In this respect, the proposed methodology technically offers a more efficient


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and economic utilization way of crop straws, and it can be readily extended to other types of biomass wastes.



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TABLES

Table 1 Specific surface area and pore structures of samples SBET (m2 g-1)

Vt (cm3 g-1)

Vmi (cm3 g-1)

Vme/Vmi

Smi (m2 g-1)

Sme (m2 g-1)

Dave (nm)

AC

893.5

0.43

0.29

0.48

630.4

263.0

1.92

N-AC

1251.7

0.64

0.24

1.66

539.0

712.7

2.00

N-AC/Gr1

1412.9

0.68

0.55

0.23

1194.2

218.7

1.93

N-AC/Gr3

1269.5

0.62

0.51

0.21

1098.1

171.5

1.95

N-AC/Gr5

1233.6

0.60

0.45

0.33

969.1

264.5

1.96

Note: SBET, the BET specific surface area; Smi, the micropore specific surface area; Sme, the mesopore specific surface area; Dave, the average pore size; Vmi, the micropore volume; Vt , the total pore volume.


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Table 2 Performances comparison of various biomass-based AC reported in literatures.

Materials

Electrolyte

Cm (F g-1)

Cv (F cm-3)

Paulownia sawdust

6 M KOH

195 (0.2 A g-1)

136.4 (0.2 A g-1)

41

Tobacco stems

1 M Li2SO4

167 (0.2 A g-1)

NA

42

Celtuce leaves

2M KOH

421 (0.5 A g-1)

190 (0.5 A g-1)

43

Argan seed shells

1 M H2SO4

325 (0.125 A g-1)

121 (0.125 A g-1)

44

Cassava peel

0.5 M H2SO4

153 (0.1 mA cm-2)

142(0.1 mA cm-2)

45

Rice husk

6 M KOH

245 (0.05 A g-1)

129.6 (0.05 A g-1)

46

Sugar canebagasse

1 M H2SO4

300 (0.25 A g-1)

134 (0.25 A g-1)

47

Sunflower seed

30 wt.%KOH

171 (10 A g-1)

152.7 (10 A g-1)

48

Banana peel

6 M KOH

206 (1A g-1)

117 (1A g-1)

49

Seaweeds

1 M H2SO4

264 (2 mV s-1)

208 (2 mV s-1)

50

Paulownia flower

1 M H2SO4

297 (1 A g-1)

NA

51

Bamboo

6M KOH

144 (5 A g-1)

68.6 (5 A g-1)

52

Pomelo peel

6M KOH

342 (0.2 A g-1)

171(0.2 A g-1)

53

N-AC/Gr1

6 M KOH

378.9 (0.05 A g-1)

321.1(0.05 A g-1)

This

299.3 (5 mV s-1)

253.6 (5 mV s-1)

work

Ref.



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FIGURES

Figure 1. SEM images of (a) AC, (b) N-AC, and N-AC/Gr1 with (c) low magnification and (d) high magnification.


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Figure 2. TEM images of N-AC/Gr1: (a) small magnification and (b) large magnification.



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Figure 3. (a) XRD patterns and (b) Raman spectra for all samples.


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Figure 4. (a) C1s and (b) N1s XPS spectra of N-AC/Gr1 composite.



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Figure 5. (a) Nitrogen sorption isotherms at 77 K, and (b) pore size distributions of all samples.


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Figure 6. (a-b) CV curves of N-AC/Gr1 at various scan rates from 5 to 200 mV s-1; (c) specific capacitances at 5 and 200 mV s-1 for all samples; (d) a correlation of specific capacitances at 5 mV s-1 with the micropore volume of the samples.



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Page 26 of 37

Figure 7. (a) A comparison of GCD curves at 2 A g-1, (b) rate performance, (c) specific capacitances per unit surface area at 0.05 and 20 A g-1, and (d) Nyquist plots for all samples, respectively.


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Figure 8. (a) The cycling performance at 2 A g-1 of N-AC/Gr1-based supercapacitor; (b) Ragone plot of energy density versus power density of the supercapacitors.



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ASSOCIATE CONTENT

Supporting information. The further characterization and electrochemical data of samples are included. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding author

* Email: [email protected] (Q. Xie)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was supported by National Natural Science Foundation of China (21271107), Tianjin Natural Science Foundation (10JCYBJC26200), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry ([2011]1568), respectively.


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Table of Contents (TOC) Graphic

Sustainable Low-Cost Green Electrodes with High Volumetric Capacitance for Aqueous Symmetric Supercapacitors with High Energy Density Qinxing Xie, Rongrong Bao, Anran Zheng, Yufeng Zhang, Shihua Wu, Chao Xie, Peng Zhao

Synopsis N-doped active carbon/graphene composites for sustainable supercapacitors with high energy density were prepared using corn straw and soy protein.


Sustainable Low-Cost Green Electrodes with High ... - ACS Publications

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