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Rational Synthesis of Highly Porous Carbon from Waste Bagasse for Advanced Supercapacitor Application Peifeng Yu, Yeru Liang, Hanwu Dong, Hang Hu, Simin Liu, Lin Peng, Mingtao Zheng, Yong Xiao, and Yingliang Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03763 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 4, 2018

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ACS Sustainable Chemistry & Engineering

Rational Synthesis of Highly Porous Carbon from Waste Bagasse for Advanced Supercapacitor Application Peifeng Yu, Yeru Liang*, Hanwu Dong, Hang Hu, Simin Liu, Lin Peng, Mingtao Zheng, Yong Xiao, Yingliang Liu* College of Materials and Energy, South China Agricultural University, 483 Wushan Road, Tianhe District, Guangzhou 510642, P. R. China *Corresponding Authors E-mail: [email protected] (Y.R. Liang); [email protected] (Y.L. Liu). KEYWORDS: Bagasse, biomass, highly porous carbon, hydrothermal carbonization, supercapacitor

ABSTRACT: Development of ultrahigh-surface-area biomass-based carbonaceous electrode materials is a major science and engineering challenge for high-performance supercapacitors. Here we present a type of highly porous carbon materials derived from waste bagasse by purposeful combination hydrothermal carbonization with chemical activation. The obtained waste bagasse-based carbon materials not only exhibit a valuable hierarchically porous structure with honeycomb-like texture, but also have very high specific surface area. The highest specific surface area reaches 3151 m2 g-1, which is superior to those of other bagasse-based porous carbons reported so far. Benefitting from the combination of hierarchical pore structure and well-developed porosity, such type of carbon materials serve as very well when used as electrodes in both 1.0 and 1.8 V aqueous supercapacitors. For example, the as-prepared carbon electrode gives a high capacitance of 413 F g-1 at 1 A g-1 and a satisfied cycling

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stability of 93.4% capacitance retention after 10000 cycles in 1.0-V aqueous supercapacitors. A remarkably high energy density of 22.3 Wh kg-1 at a power density of 220.9 W kg-1 can be achieved in 1.8-V aqueous symmetrical supercapacitors. These very attractive electrochemical performances enable this highly porous carbon to go far beyond many previously reported carbonaceous electrodes, which presents a great potential for bridging the electrochemical performance gap between conventional nonaqueous and aqueous supercapacitors, and opens up new avenues to high-value materials from waste bagasse.

As a kind of green energy storage devices, supercapacitor has attracted great interests during the past decades, due to its unique characteristics such as rapid charge/discharge rates, high power density, and excellent cycling stability.1-6 These intriguing features permit their use in many practical applications including smart grids, hybrid vehicles and portable electronics.7 Despite of rapid advancements, there are still some tough challenges, which limit the further commercialization of supercapacitors. For example, with the fast-growing demands of future systems, the electrochemical properties of supercapacitors have to be substantially improved. Additionally, the productive strategies need to be simplified as well as high cost-effectiveness. Development of high-performance but low-cost electrode materials has been considered to be an essential and efficient way to transcend these limitations.8-10 Carbon materials including porous carbons,11-15 graphene,16 carbon nanofibers,17-18 carbon nanotubes19 are fundamental candidates used as supercapacitor electrode materials. Taking into consideration the decisive role of specific surface area in determining charge storage capacity, porous carbons with large specific surface area (SSA) turn out to be the most promising candidates.20 Development of exceptionally high SSAs has already been a long-term pursuit for exploring advanced carbonaceous electrodes, which could offer an opportunity to further boost their electrochemical capacitive properties. However, many carbon materials with high SSAs are prepared from high-cost and unsustainable chemical petroleum resources, leading to side effects for environment.21 To address these


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constraints, waste biomass with little or no economic value, is investigated as an inexpensive and sustainable alternative for fabrication of high added-value carbonaceous materials. Many kinds of waste biomass such as peanut shells,22 banana peels,23 catkin,24 almond shells25 have proved to be very attractive to obtain high-SSA carbon materials with excellent supercapacitive performances. Nevertheless, most of the current biomass sources are featured by lab scale and low yield, which restrict their really practical applications in production of high-SSA carbon materials. There are only limited examples with abundant yield (e.g., coconut shell and wood) that can be industrial production of porous carbon. Therefore, exploration of more suitable substitutes that can be sustainable for long-period use still remains a critical concern. As the main sources for manufacturing sugar, sugarcane is one of the most abundant crop species worldwide (billions of tons per year).26-27 During the extraction of sucrose in sugar industry, a waste product, i.e., bagasse is generated on a large scale. For every ten tons of sugarcane harvested, three tons of bagasse is produced, amounting to seven hundred million tons of bagasse per year all over the world.28 In this context, together with rice husk, bagasse is one of the two highest-volume agricultural process residues. Considering the massive scale of global production, the utilization of bagasse has been an extensive research topic for decades. However, practical applications of bagasse have been limited to a narrow range of low-value items, such as fodder additives, papermaking, and fuels. To explore highvalue materials from bagasse, bagasse is employed as precursor for preparing porous carbons applied in adsorption and energy storage.29-35 However, the SSAs of the reported bagasse-based carbon materials are normally below 2500 m2 g-1 (Table S1). Considering the SSA is the most important structural parameter in improving electrochemical performance, it is still a great challenge for synthesizing bagasse-based carbon materials with ultrahigh SSA, which can provide new opportunities to boost their electrochemical performance to a new stage. Herein, we present a type of ultrahigh-SSA carbon materials synthesized from waste bagasse by purposeful combination hydrothermal carbonization with chemical activation (Figure 1). The key to this


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synthesis strategy is careful utilization of hydrothermal carbonization, which results in a rigid semicarbonized framework for creating a highly porous structure. The obtained waste bagasse-based highly porous carbon (WB-HPC) not only exhibits a hierarchically porous structure with honeycomb-like texture, but also has high specific surface area. The highest Brunauer-Emmett-Teller (BET) SSA reaches 3151 m2 g-1, which is higher than those of all the reported carbon materials from bagasse. Such a combination of the valuable hierarchical pore structure and the well-developed porosity is found to offer a positive opportunity to boost their supercapacitive performances of bagasse-based carbons. For instance, the typical WB-HPC exhibits a high capacitance of 413 F g-1 at 1 A g-1 and a satisfied cycling stability of 93.4% capacitance retention after 10000 cycles in 1.0-V aqueous supercapacitors. A remarkably high energy density of 22.3 Wh kg-1 at a power density of 220.9 W kg-1 can be achieved in 1.8-V aqueous symmetrical supercapacitors. These very attractive electrochemical performances enable the WB-HPC to go far beyond many previously reported carbonaceous electrodes, which present a great potential for bridging the electrochemical performance gap between conventional non-aqueous and aqueous supercapacitors. Furthermore, considering that waste bagasse is a fast-growth, sustainable and low-cost biomass with abundant yield, the fabrication of WB-HPCs would be easily scalable, offering an interesting opportunity for the development of high-value materials from waste bagasse at the industrial scale.


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Figure 1. Schematic illustration of the formation of waste bagasse-based highly porous carbon. Results and discussion Waste bagasse has been proven to be a promising carbon precursor in other literatures. In our first exploration, we started the direct carbonization of waste bagasse under inert conditions at 700 oC for 2 h, leading to the formation a waste bagasse-based porous carbon (WB-PC) matter. However, the assynthesized WB-PC possesses a low SSA of 478 m2 g-1 (Figure S1). To further increase the porosity, a chemical activation procedure is usually utilized. This method mainly involves mixing of carbon sources and activating agent (e.g., KOH, NaOH and ZnCl2), high-temperature activation, and removal of inorganic impurities.36-41 Based upon this, we try to extend this approach here to the production of waste bagasse-based highly porous carbon (WB-HPC) by using KOH as activating agent. Unfortunately, we found that no carbon material but only K2CO3 is obtained under our experimental conditions (Figure S2). After that, we decided to introduce hydrothermal carbonization technology prior to activation. Fortunately, we found that after incorporating the hydrothermal carbonization process, substantial carbon materials can be generated, which gives us the possibility to prepare highly porous


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carbon from waste bagasse (Figure S3). Such a significant comparison also indicates that the hydrothermal carbonization procedure is vital for the conversion of waste bagasse to carbonaceous matter. To investigate the porosity of the as-prepared sample WB-HPC-700, N2 adsorption-desorption measurement was performed. It is shown in Figure 2a that the N2 adsorption-desorption isotherm of WB-HPC-700 displays a steep enhancement of nitrogen uptake at low relative pressure (P/P0), indicating that a great deal of micropores exist in its carbon frameworks.42 According to the pore size distribution (PSD) curve calculated by density functional theory, the sizes of these micropores are mainly centered at 1.3 and 1.7 nm. Additionally, when further raising the P/P0, the nitrogen adsorption progressively increases until the P/P0 near 0.4, illustrating the existence of mesopores.43 Furthermore, the mesopores have a maximum PSD peak at 2.4 nm (Figure 2b). The calculated BET SSA of WBHPC-700 is as high as 3151 m2 g-1, which is not only nearly 6.6 times higher than that of the WB-PC prepared without KOH activation, but also the highest one for all the reported carbon materials derived from bagasse (Table S1). Moreover, such an ultrahigh SSA is also is comparable to or even superior to those of many porous carbons obtained from other biomass precursors (Table S2).


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Figure 2. (a) N2 adsorption-desorption isotherms and (b) pore size distribution curves of the as-obtained WB-HPCs. Figure 3 presents the morphological observation of the WB-HPCs. The representative TEM images in Figure 3a reveal that WB-HPC-700 have a typical three-dimensional (3D) hierarchical honeycomblike texture. The carbonaceous skeletons encircle and bridge to generate a 3D macropore structure, in which the open macropores with size between tens of nanometers up to hundreds of nanometers are well interconnected to each other. As illustrated in magnified image of Figure 3b, many small-sized nanopores are found in the carbon skeleton, indicative of the well-developed porosity of the sample.


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These results are well agreed with N2 adsorption results. The characteristic of these carbon skeletons is further determined by X-ray diffraction (XRD) pattern, Raman spectrum and X-ray photoelectron spectroscopy (XPS). It is found in the XRD pattern that WB-HPC-700 exhibits two broad diffraction peaks centre at around 25o and 43o, which reveals that the amorphous nature and low graphitization degree of carbon framework (Figure S4).44 In the Raman spectrum of Figure S5, the peaks can be found at around 1346 cm-1 (D-bond) and 1588 cm-1 (G-bond), which correspond to the disorder structure or defects of carbon and graphite in-plane vibrations, respectively.45 The integral area ratio of G-bond and D-bond (IG/ID) is calculated to be 0.39, implying a low degree of graphitization in WB-HPC-700. Moreover, it can be seen that the as-prepared WB-HPC-700 is comprised of carbon, oxygen and nitrogen elements based on the XPS result (Figure S6a). The contents of carbon, oxygen and nitrogen in WB-HPC-700 are measured to be 87.5, 12.2, 0.3 at.%, respectively (Figure S6b).

Figure 3. TEM images of (a) WB-HPC-700, (c) WB-HPC-600 and (d) WB-HPC-800. (b) Highresolution TEM image of WB-HPC-700.


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The nanostructure of WB-HPCs can be regulated by adjusting the activation temperature. For example, as shown in Figure 2, when increasing the activation temperature from 600 to 700 oC, substantial new small mesopores with a PSD centered at 2.4 nm are generated. As the activation temperature further increases to 800 oC, the number of small mesopores continuously increases, and meanwhile some mesopores widen and become larger mesopores (ca. 3~ 6 nm in diameter). As a result of the above pore structure change, with increasing the activation temperature from 600 to 800 oC, the corresponding mesopore SSA, mesopore volume, total volume and average pore diameter continuously increase from 325 m2 g-1, 0.34 cm3 g-1, 1.19 cm3 g-1 and 1.6 nm to 1615 m2 g-1, 1.65 cm3 g-1, 2.23 cm3 g1

and 3.3 nm, respectively. Meanwhile, the SSA and volume of micropores decrease from 2187 m2 g-1

and 0.85 cm3 g-1 to 1091 m2 g-1 and 0.58 cm3 g-1, respectively. Correspondingly, the BET SSAs increase first and then decrease (see Table 1). As for the carbon framework parameters, the IG/ID ratios gradually decrease from 0.42 to 0.38 when increasing the activation temperature from 600 to 800 oC (Figure S5). It should be noted despite of their distinct different porous structure, the hierarchical honeycomb-like morphologies are well maintained in various WB-HPCs (Figures 3c and 3d), indicating good stability of nanostructure during different activation treatments. Table 1. Porosity parameters of the as-prepared samples.

Sample WB-HPC-600 WB-HPC-700 WB-HPC-800 WB-PC YP-50

SBET (m2 g-1) 2512 3151 2706 478 1627

Smic (m2 g-1) 2187 1895 1091 85 1521

Smeso (m2 g-1) 325 1256 1615 393 106

Vt (cm3 g-1) 1.19 1.73 2.23 0.20 0.72

Vmic (cm3 g-1) 0.85 0.84 0.58 0.06 0.60

Vmeso (cm3 g-1) 0.34 0.89 1.65 0.14 0.12

Daver (nm) 1.58 2.20 3.30 1.65 0.88

Benefiting from the combination of large SSA and valuable hierarchical porous structure, WB-HPCs are anticipated to possess outstanding supercapacitive performance. Electrochemical tests such as cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) are performed in a three-electrode system


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by using 6.0 mol L-1 KOH electrolyte solution to evaluate their supercapacitive properties. As shown in Figure 4a, all the CV curves of WB-HPC-700 under various scan rates exhibit quasi-rectangular appearance. Meanwhile, WB-HPC-700 typical quasi-triangular shape in GCD curves under different current densities (Figure 4b). These results demonstrate excellent and reversible capacitive behaviors in WB-HPC-700 electrodes. To evaluate superiority of WB-HPC-700 in the application of supercapacitors, an activated carbon YP-50 specifically used in commercial supercapacitors is utilized as electrodes and studied under the same experimental conditions (Figures S7 and S8). It can be seen that WB-HPC-700 distinctly processes larger area and longer charging/discharging time when compared with YP-50, indicating higher electrical storage capacities (Figure S9). A calculation based on galvanostatic discharge curves demonstrates that WB-HPC-700 possesses much higher specific capacitances than the commercially benchmark YP-50 at all current densities (Figure 4c). For instance, WB-HPC-700 displays an ultrahigh specific capacitance of 455 F g-1 at a current density of 0.5 A g-1, which is more than 2.76 times larger than that of YP-50 (165 F g-1). When increasing the current density to a high value of 50 A g-1, a large capacitance of 321 F g-1 still can be obtained for WB-HPC-700, showing favorable rate capability. These high capacitance values are significantly higher than those of most of porous carbons such as carbon nanotubes, graphenes, mesoporous carbons, hollow carbonaceous spheres, activated carbons and other highly porous carbons (Table S3), illustrating the great prospect of WB-HPC-700 for advanced supercapacitor. Furthermore, considering the difference in SSA (Table 1), the specific capacitances per specific surface area for WB-HPC-700 (10.2~14.4 µF cm-2) are also obviously superior to those of YP50 (8.3~10.1 µF cm-2) under various current densities, suggesting a higher electrochemically pore surface utilization in WB-HPC-700 (Figure 4d). Similar superiority in supercapacitive performances can also found in other WB-HPCs samples, i.e., WB-HPC-600 and WB-HPC-800 (Figure S10). All the specific capacitances of WB-HPC-600 and WB-HPC-800 are higher than those of YP-50, demonstrating the effectiveness and advantage of our WB-HPCs in the application of supercapacitors.


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Figure 4. (a) CV curves at different sweep rates and (b) GCD curves under various current densities of WB-HPC-700 electrode in a three-electrode system by using 6 mol L-1 KOH aqueous electrolyte. Comparison of (c) specific capacitances and (d) specific capacitances per SSA for WB-HPCs and YP-50. (e) Long-term cycling performance of WB-HPC-700 at a current density of 5 A g-1 in coin-type symmetrical supercapacitors. Inset of (e) is the charge-discharge curves of the first and last five cycles. To further confirm the preponderance of WB-HPCs in electrochemical performances, the tests of coin-type symmetrical supercapacitors are evaluated (Figure S11). Similar to those in the threeelectrode system, WB-HPC electrodes in coin-type supercapacitors demonstrate larger capacitances


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than YP-50 under various current densities (Figure S12a). The high specific capacitances render WBHPCs to high energy density values. For example, WB-HPC-700-based supercapacitor delivers a high energy density of 11.8 Wh kg-1 at a power density of 50.7 W kg-1 (Figure S12b), which is significantly higher than that of YP-50 (4.36 Wh kg-1). Even giving a high power density of 2781 W kg-1, WB-HPC700-based supercapacitor still displays a satisfied energy density of 9.1 Wh kg-1. Theses supercapacitive performances are preferably stable during the fast charge-discharge process. After 10000 cycles, WBHPC-700-based supercapacitor can maintain more than 93.4% of its initial capacitance at a high current density of 5 A g-1, showing a remarkable long-term electrochemical stability (Figure 4e). In order to further enhance the energy density, high-voltage aqueous symmetrical coin-type supercapacitors are construed by utilizing 1.0 mol L-1 Na2SO4 solution electrolyte. CV curves of the asassembled WB-HPC-700-based supercapacitor with voltage window from 1.0 to 1.8 V are depicted in Figure 5a. It can be seen that an ideal quasi-rectangular shape can be retained even increasing the voltage window to 1.8 V, demonstrating that a high operation voltage of 1.8 V can be realized by employing Na2SO4 aqueous solution as electrolyte. Such high operation voltage is rather stable under various sweep rates, even increasing the sweep rate to a high level of 200 mV s-1 (Figure 5b). As for the GCD measurements, 1.8-V WB-HPC-700-based aqueous supercapacitor exhibits good triangular-like shapes and small IR drops in the current density ranging from 0.5 to 10 A g-1 (Figure 5c). The specific capacitance of WB-HPC-700 in this 1.8-V aqueous supercapacitor is as high as 206 F g-1 at 0.5 A g-1, which is significantly larger than YP-50 of 85 F g-1 (Table S4). With further increasing the current density to 10 A g-1, a high specific capacitance value of 181.6 F g-1 is still observed, indicative of excellent rate capability in 1.8-V aqueous supercapacitor. These excellent capacitances are signiﬁcantly higher than many representative supercapacitors in high-voltage aqueous supercapacitors.46-49


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a

b

Current density (A g-1)

1.6 V 1.8 V

Current density (A g-1)

1.0 V 1.2 V 1.4 V

8 4 0 -4

30 15 0 -15 -1

5 mV s 10 mV s-1

-30

-8 0.5

1.0

1.5

2.0

Voltage (V)

c 1.2

0.4

0.6

0

200

400

600

1.2

1.6

2.0

20 15 10 5

This work YP-50 Ref. 42 Ref. 47 Ref. 50

0

0.0

0.8

20 mV s-1 -1 50 mV s -1 100 mV s 200 mV s-1

Voltage (V)

d 25 0.5 A g-1 1 A g-1 2 A g-1 5 A g-1 10 A g-1

1.8

0.0

Energy density (Wh Kg-1)

0.0

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


800

Time (s)

Ref. 51 Ref. 52 Ref. 53 Ref. 54

103 Power density (W Kg-1)

104

Figure 5. Electrochemical results in coin-type supercapacitors in 1 mol L-1 Na2SO4 electrolyte solution. (a) CV curves of the as-assembled WB-HPC-700-based supercapacitor in a voltage window between 1.0 and 1.8 V at a sweep rate of 50 mV s-1. (b) CV curves of WB-HPC-700 in different sweep rates with the 1.8-V voltage window. (c) GCD curves of WB-HPC-700 in different current densities with the 1.8-V voltage window. (d) Ragone curves of WB-HPC-700, YP-50 and other typical carbon electrodes. Utilization of Na2SO4 aqueous electrolyte (1.8 V) instead of KOH aqueous electrolyte (1.0 V) can significantly enhance the energy density, because the energy density is proportional to the square of operation voltage. It is shown in Ragone plots of Figure 5d that WB-HPC-700-based 1.8-V supercapacitor delivers an energy density value as high as 22.3 Wh kg-1 when delivering a power density of 220.9 W kg-1, and holds 19.5 Wh kg-1 at a remarkably high power density of 9000 W kg-1, confirming its excellent capacitive performance. These outstanding supercapacitive characteristics such as high capacitances, preeminent cycling stability and excellent rate capability would offer WB-HPCs with a significant potential use as advanced supercapacitor electrodes.


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Conclusions In summary, waste bagasse-based porous carbons with high porosity are fabricated by combining hydrothermal carbonization with chemical activation. The typical WB-HPC material not only exhibits high BET SSA up to 3151 m2 g-1, but also has a valuable hierarchical porous structure with honeycomblike texture. By controlling the activation time, the pore structure can be well adjusted while retaining the hierarchically honeycomb-like architecture. This type of WB-HPCs serves as very well when used as electrodes in both 1.0 and 1.8 V aqueous supercapacitors, presenting a great potential for bridging the performance gap between conventional non-aqueous and aqueous supercapacitors. It is anticipated that this work could open a new avenue to upgrade the additional value of waste bagasse. Meanwhile, the obtained WB-HPCs are expected to be useful as advanced materials in many other applications, including drug delivery, lithium-sulphur battery, catalysis and adsorption. Methods Materials preparation Typically, the raw waste bagasse was rinsed with the deionized water and dried. Then the obtained waste bagasse was crushed by high speed hammer mill and sieved with a 200 mesh sieve. Subsequently, 2.0 g of the waste bagasse powder was dispersed into 20 ml deionized water, subsequently, the mixture transferred into a Teflon-sealed autoclave (40 ml) to go through a hydrothermal carbonization at 180 oC for 12 h. The resulting solid product was collected by vacuum filtration, washed abundantly with distilled water, followed by drying at 105 oC overnight. The above hydrochars were thoroughly mixed with KOH at a weight ratio of 1:4 in an agate mortar and then the mixture was heated to different temperatures with a heating rate of 5 oC min-1, hold at this temperature for 2 h under nitrogen flowing. The system was cooled down naturally under the N2 atmosphere. The obtained product were washed with 2 mol L-1 hydrochloric acid and rinsed with deionized water until the pH of filtrates closed to 7, leading to formation of WB-HPC-x. The x was denoted as the activation temperatures. In order to evaluate the role of hydrothermal carbonization, a control experiment was


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carried out. The preparation process was almost the same to that of WB-HPC-700, except that the hydrothermal carbonization process was bypassed. Meanwhile, for the purpose of evaluating the effect of activation on the porosity of the carbon material, a control sample WB-PC was fabricated. The preparation process of WB-PC was almost the same to that of WB-HPC-700, except that the activation process was bypassed. Morphology and structure characterization The morphology of sample was characterized by transmission electron microscopy (TEM, TECNAI 10), field emission electron microscope (FESEM, ZEISS Ultra 55) and high-resolution transmission electron microscope (HRTEM, JEM2100 HR). Raman spectra were gained by Jobin-Yvon HR800 micro Roman spectrophotometer (λ=457.9 nm). Xray diffraction (XRD) date were obtained using powder X-ray diffractometer (Rigaku-Ultima IV, Cu Kα, λ=0.15405 nm), from 10 to 80º). Pore structure of all samples was obtained from N2 adsorptiondesorption isotherms in 77 K (ASAP 2020 Micromeritics sorption analyzer). Before analysis, all samples were outgassed at 350 oC for 8 h. Moreover, the total pore volume (Vt) was estimated from the amount adsorbed at a relative pressure P/P0 of 0.990. The specific surface area was calculated by Brunauer-Emmett-Teller (BET) method. The micropore volume (Vmic) and micropore surface area (Smic) was determined by t-plot theory. The pore size distribution was analyzed by non-local density functional theory (NLDFT). Electrochemical performance evaluation The electrodes were prepared by mixing the as-prepared sample, carbon blank and PTFE binder with the weight ratio of 8: 1: 1 in ethanol. The obtained slurry was dispersed on nickel foam (current collector) as a 1 cm×1 cm sheet followed by pressing together and drying at 105 oC for 4 h. The mass of active materials loaded on each electrode is about 3.5 mg. Three-electrode system was executed in aqueous solutions of 6 mol L-1 KOH. In the test process, the Pt foil (1 cm2) was employed as counter electrode and the Hg/HgO electrode was used as the reference electrode. Galvanostatic charge/discharge (GCD), cyclic voltammetry (CV), and cycle-life stability was carried out on Chenhua electrochemical workstation (CHI660E, Shanghai, China). Moreover, in the


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three-electrode system, the specific capacitance (Cs) was calculated from the GCD curves by the following equation (1): =

×∆ ×∆

(1)

where the I is constant discharge current, ∆t is the discharge time, m is the active materials, ∆V is the voltage widow that reject the ohmic drop to the end of the discharge process. Then, in the coin-type supercapacitors, the specific capacitance (Cg) of single electrode was calculated from the GCD curves by the following equation (2): =

××∆ ×∆

(2) where the I represents the constant discharge current, the ∆t is the discharge time, the m is the total mass of the activated materials and the ∆V is the voltage widow that reject the ohmic drop to the end of the discharge process. Furthermore, the energy density (E) and power density (P) were calculated based on the symmetric two-electrode system by using the following equations:

= =

×∆ × ×. × △

(3) (4)

where the Cg stands for the specific capacitance of coin-type supercapacitor, and the ∆V represents the potential charge within the discharge time ∆t.50-54 AUTHOR INFORMATION

Corresponding Authors [email protected] (Y.R. Liang); [email protected] (Y.L. Liu). Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT: We gratefully acknowledge financial support from the projects of the National Natural Science Foundation of China (U1501242, 51602107, 21571066 and 21671069), Tiptop Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program (2017TQ04C419) and Program for Pearl River New Star of Science and Technology in Guangzhou (201710010104). Supporting Information: N2 adsorption-desorption isotherm, XRD patterns, Raman spectrum, XPS data, CV and GCD curves and comparison of the specific capacitance and energy density.

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For Table of Contents Use Only

Highly porous carbons with very attractive supercapacitive properties are prepared from waste bagasse by purposeful union of hydrothermal carbonization and chemical activation.


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Rational Synthesis of Highly Porous Carbon from ... - ACS Publications

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