A hierarchical porous carbon material from a loofah sponge network for high performance supercapacitors

Abstract

Environmentally friendly, low-cost and renewable biomass is a promising raw material for a high-performance supercapacitor electrode material. Herein, high surface area, hierarchical porous carbon materials were obtained by a carbonization and activation process of a loofah sponge. The specific surface area, pore volume and the pore size distribution of the porous carbon were controlled by adjusting the activation temperature and suitable activation agents. The results show that the carbon materials possess a high proportion of micropores and a few mesoporous structures. Amazingly, among them, the carbon materials prepared at 800 °C with an optimal structure and high surface area (1733 m² g⁻¹) have an outstanding specific capacitance of 304 F g⁻¹ at a current density of 1 A g⁻¹ and excellent rate capability (60.2% capacitance retention at a current density of 50 A g⁻¹). The enhanced electrochemical performance is attributed to the large surface area, good electrical conductivity, and fast charge transfer. Hierarchical porous carbon materials demonstrate superior good cyclic stability such as high capacitance retention of 98% over 10 000 charge–discharge cycles in a 6 M KOH electrolyte. Notably, in the two-electrode symmetric supercapacitors, the energy density could be up to 10 and 64.4 W h kg⁻¹ at a power density of 500 W kg⁻¹ and 11.3 kW kg⁻¹ in aqueous and organic electrolytes, respectively. Our results indicated that the strategies developed here would provide a novel route for the synthesis of porous carbon materials from a low-cost loofah sponge and show the possibility for application in energy storage.

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1. Introduction

The development of human society depends on sustainable energy and advanced energy technologies. With a fast-growing market for the development of manufacturing industries such as hybrid electric vehicles, there has been an urgent demand for environmentally friendly high-power energy resources. Supercapacitors as ideal energy-storage devices have attracted significant attention in the electric vehicle field.^1–5 Among them, electrochemical double layer capacitors (EDLC) exhibit excellent performance due to their extraordinary cycle stability, high power capability, low cost, and easy fabrication.^6–10 EDLC store energy through rapid and reversible adsorption of electrolyte ions onto the surface of the electrodes which are generally made from various carbon-based materials.^11–14 Present porous activated carbons have been commercially used as supercapacitor electrode materials for several decades, which generally possess high specific surface areas (>2000 m² g⁻¹). Unfortunately, the poor electronic conductivity and the single micropore structure present within activated carbon strongly affects its capacitive performance because it is difficult for inner pores to be fully accessed by electrolyte, resulting in a relatively low specific capacitance (<200 F g⁻¹) with low energy density (5 ∼ 8 W h kg⁻¹).^15–17 The effective combination of micropores and mesopores would facilitate the ion diffusion and charge propagation through the inner structures of porous carbon.^18,19 Moreover, highly electronic conductivity of carbon material is benefit to rapid transport of the electrons. Therefore, there is a critical need to develop new and advanced carbon-based materials with high specific surface area, high electrical conductivity, and proper pore size distribution through a facile synthesis route for high performance supercapacitors.

Recently, many new carbon materials have been developed based on above research target.^20–24 Designing novel carbon nanostructures with high specific surface area and more active sites can shorten the diffusion pathways and offer minimized diffusive resistance to mass transport on a large electrode/electrolyte interface.^25–30 The hierarchical porous including micro- and mesopores can offer rapid ion transport with improved rate capability. These multiple synergistic effects of the above-mentioned features will boost the related performance of carbon materials in energy storage applications.^31–35 In addition, it is necessary to balance the conflict between their specific surface area and electrical conductivity.^26,27 Therefore, there are still huge challenges in satisfying all the aforementioned characteristics of carbon materials simultaneously through existing technologies. Fortunately, the biomass-derived carbons exhibited outstanding energy storage performance due to its porous structures and good electronic conductivity.^36–46 For instance, high-performance carbonaceous material for supercapacitors was obtained by one-step carbonization of seaweed biopolymer.⁴¹ This material had higher density and higher electrical conductivity than any activated carbon without requiring any conductivity additive in the electrodes. The fibers derived from wood were also selected as the carbon source for electrode materials.⁴² Their specific capacitance was directly related to the micropore surface area and the micropores seem to contribute equally to the electric double layer capacitance. We recently demonstrated an in situ self-generating template strategy for the synthesis of porous graphitic carbon nanosheets for advanced electrode materials of supercapacitors.⁴³ The capacitors can deliver high energy and power densities in both aqueous and organic electrolytes. Meanwhile, we also developed a simple and cost-effective method for the production of large surface area porous carbon materials from coconut shell.⁴⁴ The resulting porous carbon materials had outstanding specific capacitance and good cycling stability. Loofah sponge as a kind of sustainable and renewable biomass material has been widely used as food and cleaning tools because of its low cost and abundance in south China. Furthermore, it has a natural porous network structure which is showed as Fig. 1a. If the loofah sponge can be effective utilized as novel and renewable energy material, which would be of great significance application.

Fig. 1 (a) The photograph of loofah sponge after natural drying. (b) SEM images of activated loofah sponges (LS-800). (c and d) High-resolution transmission electron microscopy (HRTEM) images of the LS-800 derived from activated loofah sponges.

Herein, the loofah sponge could be directly transformed into hierarchical porous carbon materials with both micropores and mesopores structures, accompanying a good electronic conductivity by a simple process including air-drying, carbonization, and KOH activation process. The porosity and pore size distribution of the carbon materials were successfully controlled by adjusting the activation temperature from 600 to 1000 °C. The plentiful hierarchical porous structures could decrease the ion transport resistance and diffusion distance, whereas the good electronic conductivity could increase electron transport at electrode with better capacitance performance, so the synthesized hierarchical porous carbon materials from low-cost loofah sponge could be used as a high-performance supercapacitor electrode material.

2. Experimental section

2.1 Preparation of the porous carbon from loofah sponge

The loofah sponges where were collected from the farm nearby were air-dried. We cut the loofah sponge into pieces and calcined them at 600 °C for 1 h under an argon atmosphere; the pre-carbonized material was obtained (denoted as LS). The LS materials 0.2 g were then mixed with 0.4 g KOH in 30 mL distilled water (weight _carbon/weight _KOH = 1 : 2). Following a stirring step at room temperature for 10 h and then drying at 80 °C in a conventional oven. Then LS was activated in a tube furnace at 600, 700, 800, 900 and 1000 °C for 1 h under an argon atmosphere, which were denoted as LS-T, where the T represents the activated temperature. The heating rate was 5 °C min⁻¹. The final products were cooled down to room temperature, washed with 1 M HCl solution and distilled water until its pH reached the value of 7. As a reference sample, the LS activated by ZnCl₂, 0.2 g LS was mixed with 0.4 g ZnCl₂ in 30 ml distilled water stirring for 10 h. Then the LS activated in a tube furnace at 800 °C for 1 h under an argon atmosphere. The products were washed until its pH reached the value of 7 by following the same steps mentioned above.

2.2 Characterization of the porous carbon

X-ray powder diffraction (XRD) was conducted by using a Bruker/D8 diffractometer with nickel-filtered Cu Kα radiation. Raman spectra were obtained with a Jobin Yvon HR 800 micro-Raman spectrometer at 457.9 nm. SEM was performed by using a Hitachi S-4800 instrument operating at 5 kV. Transmission electron microscopy (TEM, JEOL JEM-2100) with an acceleration voltage of 200 kV was used to characterize the morphology of samples. X-ray photoelectron spectroscopy (XPS) analysis was performed on a VG ESCALAB MK II with an Mg Kα (1253.6 eV) achromatic X-ray source. The Fourier transform infrared spectra (FT-IR) of the samples are collected with a PE Spectrum One B IR spectrometer. Nitrogen adsorption–desorption isotherms measurement was carried out at 77 K using a Micromeritics Tris-tar II. Using the Brunauer–Emmett–Teller (BET) theory measures the specific surface area of the materials. The nonlocal density functional theory (NLDFT) method used to calculate the pore size distribution.

2.3 Electrochemical behavior of LS-T in a three-electrode system

Electrochemical capacitive performance of LS-T was evaluated and assessed in a three-electrode system. Firstly the activated LS-T capacitive performance was evaluated through the use of three-electrode system in 6 M KOH aqueous electrolyte solution under room temperature. In the three-electrode system, Pt slice and a saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. The working electrode was prepared by mixing slurry which containing 90 wt% active materials LS-T, 5 wt% poly(tetrafluoro-ethylene, PTFE) and 5 wt% acetylene black on a nickel foam. Then the above mixtures (approximate 4 mg) were coated onto nickel-foam current collector, and dried at 80 °C in a vacuum oven for 6 h. Firstly, the electrochemical performance was tested in 6 M KOH aqueous electrolyte at room temperature. The capacitive performance of samples was studied by using cyclic voltammetry (CV), galvanostatic charge–discharge and electrochemical impedance spectroscopy (EIS) techniques. EIS was performed with a computer-controlled IM6e Impedance Analyzer in a frequency range from 0.01 Hz to 100 kHz at the open circuit potential with 5 mV amplitude. The CV test was performed in a BAS100B electrochemical workstation. The constant current charge–discharge capacitance test was performed with a CHI 660D (Shanghai CH Instruments Co., China). The working voltage windows were between −1.0 to −0.1 V. The galvanostatic charge–discharge values through the use of the following equation:where C is the capacitance (F g⁻¹), I (A) is the constant discharge current, Δt (s) is the discharging time, and ΔV (V) is the discharge voltage excluding the IR drop, and m (g) refer to the weight of active materials in the working electrode.

2.4 Electrochemical performance of LS-T in two-electrode system

Two-electrode symmetrical supercapacitor cells (2025-type coin cell) were constructed using both aqueous (6 M KOH) and organic (1 M Et₄NBF₄-PC) electrolytes. Electrochemical measurements were carried out on a LAND CT2000 (Wuhan Jinnuo Electronics, Ltd, Wuhan, China). The specific capacitance, power and energy density were calculated based on the total mass of anode and cathode materials. The specific capacitance of the two-electrode symmetrical supercapacitor cell was calculated using the following equation:In the above formula, I is the discharge current (A), Δt is the discharge time (s), m is refer to the both mass of anode and cathode materials (g) and ΔV is the potential change in discharge (V). The energy density was calculated using the following formula:C_cell is the total cell specific capacitance (F g⁻¹) and ΔV is the cell-operation potential (V). The average power density was obtained by using the following formula:The P is the power density, E is energy density and Δt is the discharge time.

3. Results and discussion

To optimize the experimental conditions, we prepared a series of samples by changing the activation temperature. As a result, an optimized sample LS-800 with well developed hierarchical porosity and highly ion-accessible surface area was obtained. The morphology, microstructure and compositions of LS-800 were investigated in detail. As observed in the scanning electron microscopy (SEM) images (Fig. 1b), LS-800 shows a reticular loose porous structure. It is noteworthy that the activation progress of LS-800 not only attack on the surface, but also inner structure after a suitable activation temperature. It is indicative that effectively broken apart from the material which might be due to the weak interaction in the structure, which activated with KOH is necessary to create new pores, increase total pore volume and surface areas. The porous structure is suitable for the rapid diffusion of ions by providing the interconnected and low-resistance channels to the interior for electrolyte ions.⁴³ The structure was further characterized by high-resolution transmission electron microscopy (HRTEM) image (Fig. 1c and d). We can observe that the sample has a disordered texture without crystalline impurities (Fig. 1c); porosity structure can also be found. Fig. 1d shows HRTEM images of the selected regions in Fig. 1c. We can find that at the edge of the picture exists degree of graphitization ribbon. The crystal structure of the sample was further confirmed by the powder X-ray diffraction (XRD) and the Raman spectra.

The XRD patterns of as-synthesized samples were shown in Fig. 2a. One broad peak located at 23.3° and a weak peak located at 43.8° was the characteristic peak of amorphous graphitic carbon. Especially the weak peak centered at 43.8° were greatly improve the electrical conductivity.⁴⁷ A new peak located at 25.8° can be observed in LS-800, which indicates the graphitization degree. In addition, with temperature increasing with graphitization degree strengthened which can be attributed to the formation of a higher degree of intralayer condensation, and the results are confirmed to the Fig. 1d. The Raman spectroscopy can give a further elucidation about loofah sponge material (Fig. 2b). The peaks located around 1320 and 1580 cm⁻¹ represent the characteristic D (defects) and G (graphitic) bands of carbon materials, respectively. The D band (1320 cm⁻¹) is due to the breathing mode of k-point phonons of A_1g symmetry and the G band (1580 cm⁻¹) is assigned to the E_2g phonon of sp² carbon atoms.^43,44 The D band suggests the appearance of a high degree of structural disorder in the carbon materials associated with the porous structures, the ratio of G/D indicates the perfect graphitic structure in the carbon material, and the ratios of LS-600 to LS-1000 were determined to be 1.15, 1.27, 1.38, 1.78 and 2.35, respectively. The appearance of 2D band further confirm the higher carbonization temperature leads to a higher structural alignment.

Fig. 2 (a) XRD patterns of the LS-600 to LS-1000, and (b) Raman spectra of LS-600 to LS-1000.

The N₂ sorption data were also collected at 77 K to investigate the porosity properties of carbon materials. The nitrogen adsorption isotherms for LS-600 to LS-1000, the pore size distribution and electrochemical properties of LS-600 to LS-1000 are summarized in Fig. 3 and Table 1, respectively. It can be seen that the porosity of the resultant carbon materials was significantly influenced by the activation temperatures. With increasing the activated temperatures from 600 to 900, both surface area and pore volumes of the material has an apparent improvement from 943 to 1841 m² g⁻¹, while the pore volumes enlarged from 0.44 to 1.05 cm³ g⁻¹. More importantly, the isotherms of LS-800 shows a type-IV with an increasing slope at higher relative pressure, this effect is commonly related to capillary condensation in mesopores. A hysteresis loop extending from P/P₀ = 0.45 to 0.95 is observed for LS-800 samples, indicating the coexistence of both micropore (0.44–1.05 nm) and mesopore (14.5–15.3 nm) structures in these materials. It is well known that the energy storage occurs primarily in micropores. However, only the micropore structure will limit the increasement in electric double-layer capacitance because it is difficult for inner pores to be fully accessed by electrolyte, it also needs a certain amount of mesopores. As the activation temperature increases to 1000 °C, it can be found that both the surface area and pore volumes decreased instantly to 1554 m² g⁻¹ and 0.94 cm³ g⁻¹, respectively. This phenomenon may due to the fact that a high temperature can cause the degradation of the carbon skeleton. Additionally, compared with LS-800, the sample of LS activated by ZnCl₂, the surface area can only reach 903 m² g⁻¹, and the pore size is about 2.37 nm (Fig. S1†). Combine these factors can be able to determine a good capacitive performance correlation with suitable higher surface area, uniform pore size and appropriate ratio of G/D to make a great contribution to a typical result of LS-800. So the LS-800 is an advantageous in charge storage and to be a promising material.

Fig. 3 (a) N₂ sorption isotherms and porosity characteristics of LS-600 to LS-1000. (b) Pore size distribution of LS-600 to LS-1000 measured by mercury porosimetry.

Table 1 Physical and electrochemical properties of products

Sample	SBET [m^2 g^−1]	SLanguir [m^2 g^−1]	Vpore [cm^3 g^−1]	Daver [nm]	C [F g^−1]
LS-600	943	1176	0.44	1.9	232
LS-700	1177	1486	0.55	1.89	267
LS-800	1733	2250	0.86	1.97	304
LS-900	1841	2403	1.05	2.25	187
LS-1000	1554	2031	0.94	2.36	204

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LS materials were then characterized by X-ray photoelectron spectroscopy (XPS) which was then used to confirm the surface chemical performance of the interpenetrating macroporous carbon materials.^48–50 The fully scanned spectra of the LS (see Fig. S2†) and LS-800 (Fig. 4a) materials final product demonstrate three peaks, at 284.8, 399.6 and 531.9 eV, corresponding to C 1s, N 1s, and O 1s, respectively. With the carbonization temperature increase, the contents of N and O are gradually decreased. On the contrary, the content of C element is increased, and the results are listed as Table S1.† It was similar to the measurement in IR, when the temperature increase, the peaks of functional group came down (see Fig. S3†). To realize the electronic states of the elements, we analyze the higher resolution spectra (Fig. 4b). The C 1s peak of the LS-800 material was fitted with four peaks. The main peak at 284.6 eV could be assigned to sp² hybridized carbon, indicating that most of C atoms are aromatic carbon, at same time a narrow trend could be observed in the XPS C 1s spectra of LS-800 which refer to an enhanced degree of graphitic order. In Fig. S2c,† the N 1s spectrum of LS had one component: the binding energies centered at 398.4 eV belong to pyridinic nitrogen. After further activated, the LS-800 keeps diverse N content with LS (Fig. 4b). The peak located at 400.8 eV is assigned to pyrrolic nitrogen, which could improve the electrical transmission of carbon materials. Two distinct peaks around 531.5 and 533.5 eV in the O 1s spectra reveal the presence of oxygen atoms in LS (Fig. S2d†) and LS-800 (Fig. 4d).⁴⁹ The introduction of N and O could also enhance the electrochemistry performance of carbon material based on the pseudocapacitance.

Fig. 4 The total XPS spectra of (a) LS-800, and C 1s (b), N 1s (c) and O 1s (d) spectra of LS-800.

The electrochemical performance of the LS-T series as supercapacitor electrodes was first characterized by cyclic voltammograms (CV) measurement in 6 M KOH aqueous solution at room temperature (Fig. S4a–d†). All the samples exhibit quasi-rectangular shape under the typical capacitive behavior from −1.0 to −0.1 V at various scan rates, indicating an ideal double-layer capacitor nature of the charge–discharge process. Compare with other samples the ideal rectangular shape refers to the sample of LS-800 is the most promising material and has the best capacitance behaviour due to the high surface area, and proper hierarchical porous structure. However, for the sample of LS-600 and LS-700, they have lower specific surface, relatively limited capacitor property. Although LS-900 possesses the highest surface area, the capacitance is not highest, same to the fact that a high temperature can cause the degradation of the carbon skeleton, leading to the negative effects for mass transport process. It is noteworthy that all the specific capacitance of carbon materials not only relies on the effective specific surface area but also the uniform pore size distribution based on the electrolyte ion sizes.

The triangular-shape galvanostatic charge–discharge experiments of the LS-800 were done at different current densities in a three-electrode configuration (Fig. 5a). According to the discharging time, the capacitance are estimated to be 304, 293, 282, 261, 233, 211, 202 and 191 F g⁻¹ at the current density of 1, 2, 3, 5, 7, 10, 20 and 30 A g⁻¹, respectively. Notably, the promising material has good rate capability even at a fast charge–discharge rate of 50 A g⁻¹, a high specific capacitance value of 183 F g⁻¹ still could be obtained which is much higher than that of other samples (Fig. 5a) and be superior to other biomass-derived carbons from biomass resource (Table S2†).^{43,44,50–56} It is suggested that the material has an advantageous in charge storage because of the good conductivity promotes the electron transport during the charge–discharge process. However, the LS samples activated by merely ZnCl₂ only show lower capacitance 223 F g⁻¹ at the current density of 1 A g⁻¹ in the 6 M KOH electrolyte (Fig. S1†). The results show that the selection of activating agent is the very important for controlling the surface area and pore size distribution. In order to have a further investigate about the performance of LS-800, the cyclic voltammetry and electrochemical impedance spectra are also tested in three-electrode system in the 6 M KOH solution. As shown in Fig. 5b, a nearly rectangular shaped loop is obtained at a sweep rate of 10, 20, 50 and 100 mV s⁻¹. Even at a sweep rate of 200 mV s⁻¹, the rectangular shape can be kept as usual, the performance was represented a quick dynamics and a good charge propagation. Fig. 5c exhibits the specific capacitance of LS-600 to LS-1000 under different current densities. A Nyquist plot of LS-T carbon electrode materials in 6 M KOH in a frequency ranges from 10 kHz to 10 MHz (Fig. 5d). The LS-800 electrodes show an arc in the high frequency region and a straight line in low frequency region. The equivalent series resistance (ESR) of LS-600 to LS-1000 was obtained from the curve fitted to the Nyquist plot and was determined to be 0.1213, 0.1265, 0.1135, 0.1374 and 0.141 Ω, respectively, which imply high electrode conductivity. Moreover, the long-term cyclic life of the LS-800 was monitored by using galvanostatic charge–discharge measurement at a current density of 2 A g⁻¹ with potential window from −1.0 to −0.1 V (Fig. 6). The specific capacitance decreased slowly even after 10 000 cycles at the current density of 2 A g⁻¹. The long term performance maintains at about 98% of the initial specific capacitance and could obtain the value about 297 F g⁻¹ over 10 000 cycles, which indicates the porosity LS-800 excellent structural stability.

Fig. 5 Electrochemical performance measured in a three-electrode system with 6 M KOH electrolyte. (a) Charge–discharge curves of LS-800 with the current densities from 1 to 50 A g⁻¹. (b) Cyclic voltammograms of LS-800 with scan rates from 10 to 200 mv. (c) Specific capacitance of LS-600 to LS-1000 under different current densities. (d) Electrochemical impedance spectra under the influence of an ac voltage of 5 mV, inset are the electrical equivalent circuit used for fitting the impedance spectra for LS-800.

Fig. 6 Capacitance retention of LS-800 at a constant current density of 2 A g⁻¹ for 10 000 cycles in 6 M KOH solution.

The performance of two-electrode cell is an important evaluation in practice application. The synthetic LS-800 was further evaluated in a fully assembled two-electrode cell with the 6 M KOH electrolyte. As shown in Fig. 7a, when at a relatively low scan rate, the cyclic voltammogram curves are almost like a perfect rectangular shape as well as the results from the above three-electrode test, implying quick dynamics and good charge propagation. When the sweep rate with an increase from 20 to 100 mV s⁻¹, it shows the perfect rectangular shape. In this supercapacitor even under the sweep of 200 mV s⁻¹ still could achieved such a rectangular shape, it is represents an ideal electrical double layer formation across the surface of carbon, it also means that the reversible adsorption and desorption of the ions, which is an evidence to explain the microporous character of the material has high specific surface area. The specific capacitances at different current densities are shown in Fig. 7b. According to the discharging time, specific capacitances are estimated to be 72, 44.3 and 42 F g⁻¹ at the current density of 1, 10 and 20 A g⁻¹, respectively. A Nyquist plot for this supercapacitor is shown in Fig. 7c. The smaller semicircle indicates a higher ionic conductivity of the aqueous electrolyte, and the relatively longer length of phase shift segment implies higher Warburg impedance. Fig. 7d shows a Ragone plot of the corresponding power and energy densities. The LS-800 based supercapacitor with a cell voltage of 1.0 V exhibits an energy density of 10 W h kg⁻¹ at a power density of 500 W kg⁻¹. Remarkable, the material of LS-800 power performance in aqueous electrolytes is much better than the commercial carbon-based supercapacitors. In addition, the long-term cyclic performance of the LS-800 is monitored in a two-electrode system at a current density of 1 A g⁻¹ (Fig. S5†). The specific capacitance decreased slowly even after 5000 cycles. The long term performance maintains at about 93% (66.9 F g⁻¹) of the initial specific capacitance over 5000 cycles, which shows the LS-800 excellent stability in a two-electrode system.

Fig. 7 Electrochemical performance of LS-800 in a two-electrode system. (a) Curves recorded at different scan rates. (b) Specific capacitances at different current densities. (c) Nyquist plots in the frequency range from 0.01 Hz to 100 kHz. (d) Energy and power densities for LS-800 in 6 M KOH solution under different current densities.

In addition, the material is evaluated in a fully assembled two-electrode cell with the 1 M Et₄NBF₄-PC electrolytes. The specific capacitances at different current densities are shown as Fig. 8. According to the discharging time, specific capacitances are estimated to be 51.5 and 33.8 F g⁻¹ at the current density of 1 and 10 A g⁻¹, respectively. Two fully assembled two-electrode series cells could light a LED bulb, and the long-term cyclic performance maintains at about 84% of the initial specific capacitance (43.2 F g⁻¹) over 2000 cycles (Fig. S6†). Moreover, a high specific energy density of 64.4 W h kg⁻¹ and higher power density of 11.3 kW kg⁻¹ could be retained. The superior performance could be attributed to the special nano-structure, large surface area, plenty of uniform micropore pore size and proper pore volumes of LS-800, confirming that the synthesized LS-800 sample would be a superior material as an electrode material for high-power and low-price supercapacitors. Moreover, because of their high surface area, electrical conductivity, and well-defined channel structures, the biomass carbons from loofah sponge could be also used materials as proper supports for metal oxides to further improve the range of operating voltage, energy density, and power density.

Fig. 8 Electrochemical performance of LS-800 in a two-electrode system in organic 1 M Et₄NBF₄-PC electrolyte (a) curves recorded at different scan rates. (b) Specific capacitances at different current densities. (c) Nyquist plots in the frequency range from 0.01 Hz to 100 kHz. (d) Energy and power densities for LS-800 under different current densities.

4. Conclusions

We have developed a novel and simple method to synthesize a high-performance carbon material from loofah sponge for super-capacitors. We found that product LS-800 can reach a high charge storage capacity with 304 F g⁻¹ in 6 M KOH electrolyte and high capacitance retention of 98% over 10 000 charge–discharge cycles. The specific capacitance of 72 F g⁻¹ of LS-800 is also verified in the two-electrode cell in aqueous electrolyte with a current density of 1 A g⁻¹. Notably, the energy density could be up to 10 and 64.4 W h kg⁻¹ at a power density of 500 W kg⁻¹ and 11.3 kW kg⁻¹ in aqueous and organic electrolytes, respectively. The excellent performance is attributed to the hierarchical porous structure of LS which is favorable for the rapid diffusion of electrolyte ions by providing low-resistant pathways. Moreover, a suitable micro/mesoporosity proportion also contributes to high specific capacitance. The synthesis strategy developed in this work could open up a novel strategy to prepare advanced capacitance materials from the abundant biomass loofah sponge in nature.

Acknowledgements

This work was supported by the Key Program Projects of National Natural Science Foundation of China (21031001), the National Natural Science Foundation of China (91122018, 21371053, 51372071, 21401048), the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (708029), the Natural Science Foundation of Heilongjiang Province of China (B201412, QC2014C007).

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Footnote

† Electronic supplementary information (ESI) available: Electrochemical performance of LS-800; XPS and FT-IR of serials of samples. See DOI: 10.1039/c5ra05688h

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