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Biomass-swelling Assisted Synthesis of Hierarchical Porous Carbon Fibers for Supercapacitor Electrodes Yang Liu, Zijun Shi, Yanfang Gao, Weidan An, Zhenzhu Cao, and Jinrong Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11558 • Publication Date (Web): 04 Feb 2016 Downloaded from http://pubs.acs.org on February 8, 2016

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ACS Applied Materials & Interfaces

Biomass-swelling Assisted Synthesis of Hierarchical Porous Carbon Fibers for Supercapacitor Electrodes Yang Liu, Zijun Shi, Yanfang Gao,* Weidan An, Zhenzhu Cao, Jinrong Liu.

College of Chemical Engineering, Inner Mongolia University of Technology, Hohhot, 010051, P.R. China

* E-mail: [email protected]

KEYWORDS. biomass-swelling, hierarchical porous carbon, binderless, carbon fibers, cellulose, supercapacitor.

ABSTRACT. The preparation of porous materials from renewable energy sources is attracting intensive attention due to in terms of the application / economic advantage, and pore structural design is core in the development of efficient supercapacitors or available porous media. In this work, we focused on the transformation of natural biomass, such as cotton, into more stable porous carbonaceous forms for energy storage in practical applications. Biomorphic cotton fibers are pretreated under the effect of NaOH/urea swelling on cellulose and are subsequently used as a biomass carbon source to mold the porous microtubule structure through a certain degree of

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calcining. As a merit of its favorable structural features, the hierarchical porous carbon fibers exhibit an enhanced electric double layer capacitance (221.7 F g-1 at 0.3 A g-1) and excellent cycling stability (only 4.6% loss was observed after 6000 cycles at 2 A g-1). A detailed investigation displays that biomass-swelling behavior plays a significant role, not only in improving the surface chemical characteristics of biomorphic cotton fibers but also in facilitating the formation of a hierarchical porous carbon fiber structure. In contrast to traditional methods, nickel foams have been used as the collector for supercapacitor that requiring no additional polymeric binders or carbon black as support or conductive materials. Because of the absence of additive materials, we can further enhance capacitance. This remarkable capacitive performance can be due to sufficient void space within the porous microstructure. By effectively increasing the contact area between the carbon surface and the electrolyte, which can reduce the ion diffusion pathway or buffer the volume change during cycling. This approach opens a novel route to produce the abundant differently morphology of porous biomass-based carbon materials and proposes a green alternative methods to meet sustainable development needs.

1. INTRODUCTION

Supercapacitors (SCs) are the emerging energy storage technology,1,2 that has attracted much more attention because of the rapid charging/discharging rate, high power density, and superlong cycle life,3,4 especially electrical double layer capacitors (EDLCs) on the basis of carbon materials, where electrical energy is stored by the electrostatic accumulation of charges,5 such as


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graphene,6,7 graphene oxide,8 and carbon aerogels.9,10 The ease of production and low-cost of supercapacitors can meet the ever-increase energy demands and the stability cyclic of feature renewable energy sources.11

Among various available candidates, carbon-based materials ranging from activated carbons12 to carbon nanotubes13 have attracted extensive attention in many energy-related applications, 14

which are the most widely available electrodes by reason of their high surface area, good

electrical conductivity, and low cost.15,16,17 These well-designed, porous, carbon-based materials with relatively extensive surface areas, controllable porosity and sufficient electroactive active sites are popular as ideal EDLC electrode materials.15,18 Compared with conventional porous materials that can’t be easily adjusted under a relatively narrow range of length scales, hierarchical porous materials consist of some macropores (＞50 nm), mesopores (2-50 nm), and micropores (＜2 nm), whose structures are based on various different length scales and/or all kinds of morphologies.19,20 Generally, the hollow macro-chamber can ensure ion transportation at high rates and plays the role of an ion reservoir. The low-resistance pathways benefit from interconnected mesopores for the ions diffuse at the inner-pore surface, and the large space of micropores can enhance the possible electrostatic adsorption area.21,22 Therefore, by coupling the advantages of different blocks, we can construct novel porous architectures. Hierarchically porous materials because of the unique nature of its structure, that has aroused extremely great interest within the field of energy storage.23 To develop the pore network based on carbon


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materials, most studies are based on templating techniques, such as the hard-template and softtemplate methods. The hard-template synthesis method pre-synthesizes hard templates through impregnation, followed by carbonization and template removal.24 The soft-template synthesis method uses soft templates as substrate for self-assembly, followed by high-temperature carbonization and etching.25 Both methods offer a useful way to produce hierarchical porous materials. Compared with other techniques, these methods involve complicated steps, which are time-consuming and expensive, reducing their commercial value to a large extent.26,27 As a consequence, the development of a simple, eco-friendly and no template synthesis method, especially from a bio-renewable source based on readily available carbon sources, is of great significance and is a difficult challenge.

The use of biomass resources for a wide range of electrode materials reveals that a important part of complex functionalities of living systems are on the basis of the complex hierarchical organization from the nanometer to the macroscopic scale, which strongly inspires scientists and engineers to develop these novel materials.28,29 Biomass sources are becoming increasingly important due to the large availability and additional value derived from their renewable and biodegradable character. Many types of biomass sources, such as cotton fibers, which are available with high quality and in large amounts, are composites of a variety of celluloses, which possess useful structures and properties. It is precisely because of organic polymer contained rich


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carbon, the porosity of proper biomass precursors is useful to obtain a porous structure in the carbonized transformation process.30,31

Herein, we have proposed a distinctive synthesis of hierarchical porous carbon from biomass materials by a facile activation route, which is inexpensive, available worldwide, and nontoxic. To synthesize the hierarchical porous carbon materials, we chose cotton as an environmentally friendly carbon precursor via the pre-treatment of fiber in NaOH/urea solution and a subsequent carbonization process. NaOH/urea solution plays crucial roles in the fabrication of the fiberbased hierarchical porous structure.32,33 It is key for biomass swelling to make good use of the NaOH and urea in the aqueous solution.34 After the swelling treatment, the crystal structure of the cellulose remains unchanged, but the degree of natural crystallinity and dimensions of unique crystallites are modified,35 which decreases the cellulose crystallinity and the degree of polymerization and increases internal surface area.36 The NaOH/urea solution is conducive to develop an inter-connected pore network structure, which can fabricate micropores and mesopores in the walls of macropores by using the chemical activating agent. The synthesized carbon fibers show hierarchically porous structures with a vast surface area and produce a considerable capacitor electrode with a long cycle life and excellent rate capabilities.

2. EXPERIMENTAL SECTION

Materials preparation: Two steps strategy were used to preparation of electrode material. In the first swelling process, clean raw cotton was immersed into NaOH/urea aqueous solutions


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(7% NaOH/12% urea, the ratio of raw cotton to NaOH/urea aqueous solutions by weight was 75:100) with the desired mold followed by shaking for several hours. Then, the sample was cleaned by absolute ethanol and deionized (DI) water for 6 min each, and finally dried in the vacuum at 80 °C for 12 h.

In the second calcination process, the pretreated samples were transferred into a tubular furnace for pyrolysis under the protection of argon, then the carbon precursor was heated to 800 °C at a heating rate of 1 °C min−1 and was maintained at this temperature for 2 h, before being left to cool to room temperature naturally to yield black and ultralight hierarchical porous carbon fibers. As a control experiment, the carbon fibers were treated at the same calcination process but were with the notable absence of the swelling process. In addition, nickel foams were cleaned using ultrasonic by adding acetone, 6 M HCl, deionized (DI) water and finally absolute ethanol for 10 min respectively, and then were used for clamping the samples. The electrodes were named respectively following: the carbon fibers without swelling process, acetylene black and polytetrafluoroethylene (PTFE) were mixed in a mass ratio of 8:1.5:0.5, and then the mixture was pressed to form the CF Ⅰ electrodes; the carbon fibers without swelling process were pressed directly between two layers of nickel foam to form the CF Ⅱ electrodes; the carbon fibers with swelling process were pressed directly between two layers of nickel foam to form the CF Ⅲ electrodes;


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Material characterizations: The Raman spectra were recorded (RAMAN; Renishaw, UK) at a laser excitation of 532 nm. The X-ray diffraction of samples was investigated from 10° to 80° at a scan rate of 3°min-1. (XRD; Bruker, D8-Advance) Elemental composition analyses of the samples were performed through X-ray photoelectron spectroscopy (XPS; Thermo, ESCALAB 250Xi) The scanning electron microscopy (SEM; Hitachi, SU8020) and optical microscopy (Axio Scope A1, ZEISS, Germany) of the samples were used for characterizing morphology. The porous characteristics consisted of the surface area and pore size distribution, the surface area could be analyzed using the Brunauer-Emmett-Teller (BET) model and the pore size distribution could be analyzed using the Barrett-Joyner-Halenda (BJH) method (Quantachrome, Quadrasorb SI).

Electrochemical measurements: The electrochemistry performance of samples were examined using electrochemical workstation (Chenhua, CHI 760E) for galvanostatic charge-discharge (CD) measurements and cyclic voltammetry (CV). The three-electrode cell consisted of an Ag/AgCl reference electrode and a platinum foil counter electrode. Electrochemical impedance spectroscopy (EIS) measurements within a frequency range of 0.01-100 kHz were performed by potentiostat/galvanostat (Ametek, PAR 2273). The nickel foams were clamped with electroactive materials as the working electrode and were pressed at 10 M Pa for 60 s. Before the test, the working electrode was wetted in 3 M KOH aqueous solution for 2 h under vacuum conditions. The mass loading of active materials was 3.5 mg/cm2.


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3. RESULTS AND DISCUSSIONS

Figure 1. (a) The carbon fibers of XRD pattern and (b) Raman spectra. Optical microscope images of the (c) native fiber and (d) the fiber in NaOH/urea aqueous solutions. Inset in (e) showed the swollen fibers into a series of rod-like structure under low resolution.

To analyze the composition of the material, the carbon fibers were characterized by XRD, as shown in Figure 1a. The diffraction peaks of the carbon fibers, centered at 2θ = 25.0° and 43.5°, were attributed to the (002) and (101) reflections of a disorder carbon structure, respectively.37 Besides, the two characteristic peaks centered at approximately 1358 and 1588 cm-1 exhibited in the Raman spectra corresponded to the D-band and G-band of polycrystalline carbon materials respectively, as shown in Fig. 1b. As we known, the intensity ratio between D and G bands (ID/IG) could exactly reflect the graphitic degree of the carbon materials. The CF Ⅲ have


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possessed a defective nature and lower graphitized degree.38 This could be attributed to the high porosity of carbon fibers by NaOH/urea activation and doping of oxygen, consistent with the analysis of XRD. The purity of the carbon fibers was investigated using X-ray photoelectron spectroscopy (XPS), and the XPS survey spectrum of the CF Ⅲ revealed the presence of carbon and oxygen with no evidence of impurities (Figure S1 in the ESI†).

Besides, we need to understand the role of NaOH/urea aqueous solutions, the surface morphology of the carbon fibers was observed by an optical polarizing microscope. Figure 1c shows the native fibers in a series of rod-like structures with a regular diameter in alcohol of approximately 20μm.39 As soon as contact is made with the NaOH and urea aqueous solution, the cellulose fibers are broken, producing large rod-like pieces of cellulose fibers, which are between 30 and 50μm wide, as shown in Figure 1d and e. Cellulose can be swollen by intracrystalline swelling agents, such as NaOH/urea aqueous solution.34,40 When the transport of NaOH/urea aqueous solution through a system of pores and channels, swelling is occurring and the intra- and inter-molecular hydrogen bonding of cellulose is destroyed.41 The reason is that Na+ ions possess relatively small ionic radius and high charge density, which can make them more easily penetrate into cellulose, even embed on the cellulose chains to achieve swollen and dissolution.32

From a micro perspective, cellulose is a semi-crystalline polymer, includes non–crystalline regions of different levels and crystalline regions of the cellulose I. Cellulose I arranged in


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parallel is relatively unstable structure for a cellulose crystal, but the cellulose chains could be in an anti-parallel arrangement when swollen and dissolved. Hence, cellulose adopts a new crystalline structure called Cellulose II. Significant work is performed in the transition from Cell I to Cell II.41,32,42 The Cellulose I and II structures coexist in cellulose during the swelling. From a macro perspective, when raw cotton fibers are immersed in the swelling agents, the primary wall bursts, leading to the radial expansion of the cellulose in the secondary wall. In the primary wall, the swollen cellulose expanding through the tears, limiting the uniform expansion of the fiber leads to the lotus root shape forms.43,44,45 Thus, the NaOH/urea aqueous solution as swelling agents is an excellent medium for the preparation of functional materials.46


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Figure 2. SEM images of carbon fibers at different magnifications. (a, c and e) CFⅠwas obtained just by calcinations process. (b, d and f) CFⅢ was obtained contain swelling process and calcinations process. Inset in (h) showed cross-section image of CF Ⅲ.

To visualize the microstructure of the carbon fibers, scanning electron microscopy images of two samples were performed, as shown in Figure 2. The CF Ⅰ was obtained immediately after the calcinations process, and the CF Ⅲ was obtained with the help of an extra swelling process. Under low resolution of SEM observations, the preparation of carbon materials had a hollow fibrous structure, and a straight orientation, as shown in figure 2a, 2b, 2h. In Figure 2c, the


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surface of the CF Ⅰ is smooth and without clear pores, indicating the poor porosity of this carbon fibers. There are no changes in the morphology and shape of the carbonized fiber after the carbonization process. Comparatively, the SEM images of the CF Ⅲ had an obviously rough surface and consisted of a well-developed, disordered and interconnected pore system, as shown in Figure 2d and f. Under high resolution SEM observations, CF Ⅲ showed irregular structures with the sizes ranging within a few micrometers, as shown in Figure 2f. The CF Ⅲ had a very large area of micrometric pores on the surface, and had a wide coverage rate of interconnected pores within the texture. Void pores formed with a diameter of approximately ~1 nm, which was further confirmed by the N2 sorption isotherms. Contrary to CF Ⅰ, the interconnected pores was absent in the case of the non-swollen fibers, as shown in Figure 2e. The irregular morphology of the particles was directly correlated with the degree of swelling (Figure .S2, in the ESI†).The SEM images of the fibers at the different swelling times, such as 1 h, 2 h, 3 h, showed similar morphology. When the swelling time is increased, the porosity of resulting fibers also increases. Moreover, the carbon fibers with a swelling time of two hours have a more optimal structure compared with other fibers. During the occurrence of morphological transformation in swelling process, which demonstrates that a sufficient amount of NaOH/urea aqueous solutions is necessary to form the special pore structure. The excellent configuration may reduce the diffusion distances of charges to the interior, which plays the role of reservoirs for electrolyte. These extraordinary textural features are of great significance for fast charge transport and are superior to a solid structure without space inside to ensure.


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Figure 3. Nitrogen adsorption-desorption isotherms at 77.3 K and inset showed pore size distribution by BJH method using the adsorption branch of isotherm. (a) CF Ⅰ, (b) CF Ⅲ. Table 1. Pore parameters of CF Ⅰ and CF Ⅲ. BET sample

surface area (m2 g-1)

t-method micropore surface area 2

-1

(m g )

total pore

average pore

volume

diameter

(cm3 g-1)

(nm)

CFⅠ

136.66

9.561

0.1607

4.70

CFⅢ

584.49

436.8

0.3846

2.63

The porosity properties of the carbon materials were investigated, as shown in Figure 3. The nitrogen adsorption/desorption isotherms for the carbon fibers, as shown in Figure 3b, which exhibited the combined characteristics of multimodal porosity, micropores, and mesopores together with macropores. At the low relative pressure, the sharp increase in the adsorption curve is characteristic of microporous materials, indicating the presence of micropores in the carbon fibers.47 The hysteresis loop between the adsorption and desorption branches suggests the presence of mesopores. Additionally, the slightly steep adsorption at the relative pressure of 0.8– 1.0 demonstrated the presence of macropores. The porous carbon fibers formed after


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carbonization, has a larger surface area suitable for use as supercapacitors. The pore size distributions of mesopores are inserted in Figure 3. The mesopores with a pore size of approximately 2.63 nm are consistent with the mesoporous carbon materials. Moreover, much larger pore volumes of mesopores with pore sizes exist in the porous carbon fibers. The surface area and pore volume parameters of the carbon fibers were measured by the Brunauer–Emmett– Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively, and were summarized in Table 1. The swelling serves as the activation process and greatly influences the specific surface area and pore volume. The wider size distribution demonstrates the hierarchical porous characteristic and also accounts for the higher specific surface area.48 The large range of the pore size distribution is significant to improve electrolyte diffusion when used EDLCs.


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Figure 4. Electrochemical performances of the CF Ⅲ. (a) Cyclic voltammograms at different scan rates. (b) Charge-discharge curves at different current densities. (c) The specific capacitance (●) and capacity retention ( ) with different current densities. (d) Ragone plot performed in 3M KOH aqueous solution. (e) Cycling test of CF Ⅲ. The potential range is -1 to 0 V after 6000 cycles with a current density of 2 A g−1. The CF Ⅲ could retain 95.4% of its initial specific capacitance. The bottom insets were the initial profiles for five charge-discharge cycles each.

A high specific surface area and synergistic effect between different types of pore structure are


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beneficial for improve electrochemical performance for supercapacitors, such as capacitance, cycling stability and rate capability. In order to accurately evaluate the electrochemical performance, we chose and compared the results of samples of CF Ⅰ, CF Ⅱ and CF Ⅲ. The results are presented in Figures 4 and 5 and demonstrate noticeable double layer capacitive properties. The performance of carbon materials as supercapacitor electrodes was first characterized by cyclic voltammetry (CV) measurements. Figure 4a shows the typical CV curves of CF Ⅲ at various scan rates from 10 to 200 mV s-1 in the potential range between -1 and 0 V. Generally, an ideal electrochemically active electrode material has a nearly rectangular CV shape.38 At different scan rates, the curves exhibit a typical rectangular shape, indicating representative electric double layer capacitive behavior. Even at 200 mV s-1, the unique rectangular symmetry of the CV curves is maintained, and no clear distortion of the curve was observed, which demonstrated a highly reversible adsorption/desorption of electrolyte ions among the surface of the electrode materials within CF Ⅲ and indicates an excellent rate capability of CF Ⅲ. The good rate capability depends on the well-defined hierarchical porosity, which reflects that the ions of the electrolyte can transfer quickly and glidingly in the channels of this carbon fiber. Typical galvanostatic charge/discharge curves of CF Ⅲ at different current densities from 0.3 to 10 A g -1 are displayed in Figure 4b. The symmetrical, highly linear and triangular characteristics of the charging–discharging curves indicate good reversibility and major capacitive contribution from EDLC.49 The galvanostatic charge/discharge measurement is more accurate as a evaluate technique for EDLC.50 According to the discharge curves of the


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galvanostatic charge/discharge measurement, the specific capacitance (SC), which was calculated based on the total mass of the electrode, was 221.72 F g-1 at 0.3 A g-1. The more detailed mass SCs of CF Ⅲ were calculated as 209.8, 198.3, 189.7 and 172.8 F g-1 at specific current densities of 0.5, 1, 2 and 5 A g-1, respectively, as presented in Figure 4c. An increase in the discharge current leads to a large voltage drop, resulting in a decrease in the SC values. Nevertheless, the SC value of CF Ⅲ has ~77.9% SC retention, even at the large current density of 5 A g-1 compared with 0.3 A g-1. The Ragone plot of CF Ⅲ is shown in Figure 4d. The energy density in 3 KOH electrolyte decreases from 30.79 W h kg-1 to 24 W h kg-1 with the increase of current density from 0.3 A g-1 to 5 A g-1. The energy densities decrease slowly with gradually increasing power densities, also suggesting that CF Ⅲ is a hopeful electrode material for efficient supercapacitors. Another crucial parameter of supercapacitors for practical application is the long cycle life. To understand the electrochemical stability of CF Ⅲ , galvanostatic charge/discharge measurements were performed, at a current density of 2 A g-1, as shown in Figure 4e. The capacitance of CF Ⅲ remained at nearly 95.4% of the initial capacitance after 6000 cycles, and the morphology of CF Ⅲ showed no obvious change, indicating the excellent electrochemical cycling stability of CF Ⅲ. Generally, high cycling stability is attributed to the double layer charge–discharge process in the electrode materials during potential cycling.51


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Figure 5. Electrochemical performances of the as-prepared samples: (a) Cyclic voltammograms curves of the three samples at a scan rate of 5 mV s-1. (b) Charge-discharge curves of the three samples at a current density of 1 A g-1, (c) The calculation of the specific capacitance through discharge curves at different current densities. (d) Nyquist plots with the imaginary part vs the real part of (○) CF Ⅰ, (○) CF Ⅱ, and (○) CF Ⅲ, and the insets showed the high frequency region.

To further depict the electrochemical performance differences in terms of capacitive and impedance behavior, CF Ⅰ and CF Ⅱ were tested as control samples, as shown in Figure 5. In the potential range from -1 to 0 V, the induced currents of the CF Ⅲ samples were larger than those of the CF Ⅰ and CF Ⅱ samples, which indicates that after the introduction of a porous structure with the assistance of swelling, the capacitor built using the CF Ⅲ electrodes had a


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much greater capacitance. The lower capacitance of CF Ⅱ is ascribed to the lack of diffusion channels and thus slower ion transport in high rate operations.52

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This result was also obtained

from the comparison of the charge/discharge curves at the same current density, as shown in Figure 5b. Based on the large scale of carbon fibers in only one dimension, the active materials could be well-restrained between two pieces of Ni foam without any additional support or conductive materials. Additionally, CF Ⅱ has an approximately doubled capacitance value compared with CF Ⅰ, as shown in Figure 5c. The electrochemical impedance spectrum (EIS) was used to compare the electrochemical properties of the samples and is shown in Figure 5d. As power devices, the electrode materials of supercapacitors with lower resistance are electrochemically preferred for better commercial applicability. The series resistance and charge transfer resistance of the electrodes can be derived from the Nyquist plot. The impedance of the three samples was measured in the frequency range of 100-0.01 kHz at open circuit potential with an ac perturbation of 5 mV. During different interfacial processes, two major characteristics reflect various resistance phenomena in the high and middle frequency regions, the insert in Figure 5d. The intercepts of the high-frequency semicircle with the real axis are the bulk solution resistance Rs, and the diameters of the semicircles refer to the charge-transfer resistance Rct. The measured impedance spectra were analyzed using the CNLS fitting method based on the equivalent circuit, which is given in Figure S3 in the ESI†. The lower Rs value of CF Ⅲ is because of the binderless electrode system. The electroactive surface area increased are attributed to incremental specific surface area and enhanced electrical conductivity, and the


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larger the electroactive surface area, the lower the charge-transfer resistance.54,55 The minimum Rct value of CF Ⅲ illustrates that both highly specific and electroactive surface areas could obtained with the assistance of swelling, resulting in high EDLC capacitance. At different swelling times, the porous structure had optimal variation in pore size and pore size distribution. CF Ⅲ, with a swelling time of two hours, had the most suitable pore size and size distribution, which further enhanced its behavior as a capacitor, as shown in Figure S4 in the ESI†. 4. CONCLUSION Hierarchically porous materials possess macroporous chambers, mesoporous windows and a microporous skeleton, which were synthesized from cotton as a sustainable biomass carbon source through a well-designed swelling process followed by a post-carbonization treatment. As the result of favorable multiple synergistic effects, the hierarchical porous carbon fibers exhibit excellent supercapacitor performance, for example, a high specific capacitance of 221.72 F g-1 and 189.76 F g-1 at a current density of 0.3 A g-1 and 1 A g-1, respectively, as well as exhibit a considerable capacitance as an electrode with a long cyclic life and excellent rate capabilities in a three-electrode cell. The improved performance was attributed principally to the following features: (1) the bio-swelling behavior that improves the surface chemical characteristics of the materials and facilitates the formation of the hierarchical porous carbon fiber structure. The combination of the hierarchical porous structure with a high specific surface area (584.49 m2 g−1) and a large pore size of 2.63 nm produces accessible micropores and can facilitate fast ion


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transport between electrode materials and electrolytes. The macroporous structures and the mesoporous channels not only provide a commodiously accessible environment, but also provide a large comfortable space for electrolyte ion transport. The synergistic effect between different the morphology of pore, which is key r to the excellent capacitance, and (2) new methods attributed to improving capacity without polymeric binders or carbon black that serves as support or conductive materials. This method is easily performed and provides a green route for the development of novel electrode materials. Biomass carbon sources with an inter-connected multirole pore through a green swelling route are a new generation of electrode for commercial supercapacitors and/or other energy-related applications.

ASSOCIATED CONTENT

Supporting Information Available: X-ray photoelectron spectroscopy survey spectrum of the CFⅢ. SEM images of swollen carbon fibers at different time. Comparison of electrochemical performance of carbon fibers. AUTHOR INFORMATION

Corresponding Author: * [email protected]

Notes: The authors declare no competing financial interest.

ACKNOWLEDGEMENTS


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This work was financially supported by the National Natural Science Foundation of China (No.21266018 ， 21566030), Science and technology projects of Science and Technology Department of Inner Mongolia Autonomous Region, P. R. China (No.20110401 and No.20130409), the Natural Science Foundation of Inner Mongolia, P. R. China (No.2010MS0218), Program for Young Talents of Science and Technology in Universities of Inner Mongolia Autonomous Region (No. NJYT-15-A04), Ministry of Science and Technology China-South Africa Joint Research Program (No.CS08-L15).

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GRAPHICAL TABLE OF CONTENT


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