Hierarchical porous carbon derived from recycled waste filter paper as high-performance supercapacitor electrodes

Abstract

Hierarchical activated porous carbon (APC) was synthesized through convenient chemical activation with ZnCl₂ using recycled waste filter paper as the carbon precursor. The micro/mesopore ratio, pore volume and specific surface area of these APC materials could be controlled by adjusting the dosage of the ZnCl₂ chemical agent and the activation temperature. The optimal sample, APC-7-4, was activated at 700 °C with a ZnCl₂/carbon precursor mass ratio of 4 : 1, and exhibited a large surface area of 2169.8 m² g⁻¹, a suitable pore size and prominent porosity. Remarkably, when the resultant APC-7-4 material acted as the electrode material for a supercapacitor, APC-7-4 displayed an outstanding charge storage capacity with a satisfactory specific capacitance of 302.3 F g⁻¹ in 6 M KOH at a current density of 1 A g⁻¹. Moreover, the APC-7-4 electrode possessed good long-term cycling stability, and ca. 95.4% of its initial specific capacitance at 5 A g⁻¹ was retained even after 10 000 cycles. Based on these electrochemical test results, the approach proposed in this work provides a new view of converting waste products to design high-performance carbon-based electrode materials for great and promising applications in electric double-layer capacitors.

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1. Introduction

Currently, the growing global energy demand has initiated the exploration of new energy systems, as well as new energy conversion and storage systems. Among them, supercapacitors have attracted much attention and been considered to be promising candidates for next generation energy storage applications due to their high power density, excellent cycling stability, low maintenance cost, fast charge–discharge rate and environmental friendliness.^1–3 According to the energy storage mechanism, supercapacitors can be divided into pseudocapacitors and electrical double-layer capacitors (EDLCs). Pseudocapacitors store energy faradaically depending on the electrosorption, reduction–oxidation reactions, and intercalation taking place on the surface of electrode materials.⁴ In spite of possessing high specific capacitance and energy density, poor cycling stability and inferior electrical conductivity seriously restrict the practical application of pseudocapacitors.^5,6 However, EDLCs store energy in the double layers by charge accumulation between the surfaces of the electrodes and electrolyte, which feature in high power density, long cycle life and fast charge–discharge capability. Consequently, EDLCs have aroused considerable interest for applications in electric vehicles, uninterruptible power sources and other high-power apparatuses.^7,8

In general, the design of highly capacitive performance EDLCs is mainly dependent on the development of effective electrode materials. Carbon is one of the most abundant materials and has a variety of morphology and structural forms.⁹ Various carbon materials, such as activated carbon (AC), porous carbon, carbon nanotubes, carbon nanofibers, graphite and so on, have been investigated as EDLC electrode materials. Among them, activated carbons with high surface area and developed micro/mesoporosity are the most widely used electrode materials for EDLC applications, which exhibit a relatively satisfactory specific capacitance of around 150–250 F g⁻¹.^10,11 Thus, clear efforts aimed at obtaining high quality carbons for improving capacitive properties while concurrently reducing cost are a desirable research aspect. Usually, petroleum-derived coke, pitch, and coal are the common precursors for the production of commercial ACs.^12–14 The decreasing availability of fossil-based carbon compounds has driven the industry to search for sustainable resources to prepare ACs.

Recently, the recycling of waste products for the synthesis of activated carbon-based supercapacitor electrodes has attracted increasing attention. Waste products, such as agricultural waste, food, human hair, leaves, wood, tires and even insects etc., have already been utilized as carbon sources for synthesizing ACs by pyrolyzing these waste materials with some chemical (KOH, ZnCl₂, H₃PO₄ etc.) or physical (CO₂, H₂O) additives as agents for activation.^15–20 For instance, activated carbons derived from dead leaves exhibited a high specific capacitance of 400 F g⁻¹ and an energy density of 55 W h kg⁻¹ in 1 M H₂SO₄.²¹ Qian et al. reported activated porous carbon formed by activating human hair with KOH, and these ACs presented a satisfactory specific capacitance of 340 F g⁻¹ in 6 M KOH and good stability over 20 000 cycles.²² Zhi et al. prepared high-performance activated micro/mesoporous carbon from recycled waste tires.²³ Compared with those traditional fossil-based carbon sources, the advantages of low cost and environmental friendliness promote the utilization of waste-product-based high performance supercapacitors in practical applications.^24,25

Waste filter paper, a readily recycled waste product generated in daily chemical experiments, is usually thrown away as experimental garbage. In this work, we demonstrate the synthesis of hierarchical activated porous carbon through chemical activation with ZnCl₂ using recycled waste filter paper as the carbon precursor. The high surface area and prominent porosity were generated by the ZnCl₂ activation agent during the carbonization process. By tuning the dosage of ZnCl₂ and the activation temperature, the hierarchical micro/mesoporous structure could be controlled. Meanwhile, the optimal activated porous carbon material exhibited a superior capacitive property with a specific capacitance of 302.3 F g⁻¹ at a current density of 1 A g⁻¹ in 6 M KOH. More importantly, a high energy density and good long-term cycling stability were also presented.

2. Experimental

2.1 Synthesis of materials

The hierarchical activated porous carbon (APC) was prepared through a chemical activation route using ZnCl₂ as a chemical activating agent and recycled waste filter paper (FP) as the carbon precursor. In a typical synthesis, 1 g of filter paper was impregnated in 50 mL of ZnCl₂ solution and then stirred for 4 h. Subsequently, the solution was dried at 110 °C for 10 h to obtain a ZnCl₂-impregnated filter paper precursor, and then the precursor was activated and carbonized under a N₂ atmosphere at different temperatures (700, 800 and 900 °C). After cooling down, the activated samples were repeatedly washed with HCl solution (0.5 M) and distilled water. Finally, these materials were dried under vacuum at 80 °C for 8 h to obtain the final hierarchical activated porous carbon products, which were defined as APC-x-y (x = 7, 8 or 9, meaning the activation temperature was 700, 800 or 900 °C, respectively; y = 1, 2 or 4, referring to the mass ratio of ZnCl₂/FP which was 1 : 1, 2 : 1 or 4 : 1, respectively).

2.2 Characterization

X-ray diffraction (XRD) patterns were recorded using a Bruker D8 diffractometer using Cu Kα radiation (λ = 0.15418 nm) as an X-ray source. Nitrogen adsorption–desorption isotherms were carried out at −196 °C using a micromeritics ASAP 2020HD88 analyzer. Before adsorption, the samples were out-gassed at 200 °C for 10 h. The specific surface area (S_BET) was evaluated using the Brunauer–Emmett–Teller (BET) method, and the pore size distributions were calculated according to the Density Functional Theory (DFT) method. The morphology was observed using transmission electron microscopy (TEM, FEI Tecnai G2 20) with an accelerating voltage of 200 kV and scanning electron microscopy (SEM, Quanta 250 FEG). Raman spectra were recorded on a Raman spectrometer (WITEC Spectra Pro 2300I) operating with a 532 nm laser. X-ray photoelectron spectra (XPS) were obtained on a VG ESCALAB MK II X-ray photoelectron spectrometer with a Mg Kα exciting source (1253.6 eV).

2.3 Electrochemical measurements

The products were tested via a two-electrode testing device in a 6 M KOH electrolyte solution, which was performed on a CHI660D electrochemical workstation at room temperature. The working electrodes were prepared by mixing active material, carbon black and polytetrafluorene polytetrafluoreneethylene (PTFE) binder in a weight ratio of 8 : 1 : 1. After coating the above slurries on foamed Ni grids (1 cm × 1 cm), the electrode was dried overnight at 120 °C before being pressed under a pressure of 20 MPa. Cyclic voltammetry (CV) curves were obtained in the potential range of −1.0 to 0 V by varying the scan rate from 5 to 100 mV s⁻¹. Charge–discharge measurements were done galvanostatically at 1–50 A g⁻¹ over a voltage range of −1.0 to 0 V. Electrochemical impedance spectroscopy (EIS) was measured in a frequency range of 10 kHz to 0.01 Hz at open circuit voltage with an alternate current amplitude of 5 mV. Two nearly identical working electrodes were prepared using the previous method, using glass fiber as the separator. The mass loading of active material in a single electrode was about 1 mg.

The gravimetric specific capacitance, C_sp (F g⁻¹), for a single electrode was calculated from each galvanostatic charge–discharge curve according to the following equation:

where I (A) is the constant current, Δt is the discharge time (s), m (g) is the total mass for both carbon electrodes, and ΔV (V) is the voltage change excluding the IR drop during the discharge process.

The energy density (E) was estimated by using the following formula:

The power density (P) was calculated using the following equation:

where R_ESR is the equivalent series resistance, m is the total mass for both carbon electrodes, and V is the maximum discharging voltage.

3. Results and discussion

3.1 Structure characterization

Scheme 1 illustrates the schematic diagram for the fabrication of filter paper derived activated hierarchical porous carbon through a simple ZnCl₂ activation route. The recycled and dried waster filter paper was cut into small pieces, and then impregnated with ZnCl₂ solution and stirred for 4 h. Filter paper not only contains abundant inorganic and organic components, but also possesses an excellent adsorptive capacity to accommodate a large amount of ZnCl₂ activating agent, which favor obtaining well-developed porous structures. ZnCl₂-impregnated filter paper was dried, and then further carbonized and activated at different temperatures. After washing with HCl and H₂O, activated hierarchical porous carbon materials were obtained.

Scheme 1 Schematic diagram of the preparation of APC-x-y materials from recycled waste filter paper and application in a supercapacitor.

ZnCl₂ is one of the most common chemical activating agents for activation of carbonaceous precursors, resulting in superior porosity.^26,27 To reveal the activation process of filter paper by ZnCl₂, XRD patterns of activated porous carbon unwashed with HCl (APC-r) and the APC-x-y samples are shown in Fig. 1. In the XRD pattern of the APC-r sample, the strong diffraction peaks at 2θ = 30–70° should be indexed to the hexagonal phase of zinc oxide,^28,29 demonstrating the formation of zinc oxide during the process of activation. One weak and broad diffraction peak at a 2θ angle of 20–30° can be observed in all APC-x-y materials, suggesting a turbostratic carbon structure with low crystallinity between graphitic and amorphous carbon, which endows the enhanced in-plane conductivity.³⁰ Meanwhile, a weak diffraction peak at 2θ = 43.3° arises in all of these activated samples, which should be ascribed to the formation of a high degree of interlayer condensation, further improving the electrical conductivity.³¹

Fig. 1 The XRD patterns of the APC-x-y samples before (a) and after (b) acid treatment.

The Raman spectra of all the APC-x-y materials are displayed in Fig. 2a. It can be clearly observed that there are two peaks appearing in these samples. The peak located at about 1334 cm⁻¹ is assigned to the D-band, which should be related to the vibration of carbon atoms with dangling bonds in planar terminations of the disordered graphite-like framework.³² The other peak centered at about 1585 cm⁻¹ refers to the G-band, corresponding to ideal graphite in-plane vibrations with E_2g symmetry.³³ Besides, the degree of graphitization can be further characterized by the ratio of the relative intensities of the D- and G-band peaks (I_D/I_G). The values of I_D/I_G for APC-7-1, APC-7-2, APC-7-4, APC-8-4 and APC-9-4 are 0.97, 0.95, 0.94, 0.99 and 1.08, respectively. The relatively lower I_D/I_G intensity ratio for APC-7-4 can be indicative of a reduced amount of heteroatom doping.²² Moreover, the peak intensities of the D- and G-bands were used to evaluate L_a that reflects the in-plane crystallite size, according to the following equation: L_a = (4.35 nm)*(I_G/I_D).^34,35 Thus, the parameter L_a in these samples is estimated and listed in Table 1.

Fig. 2 The Raman spectra of the APC-x-y samples (a); XPS spectra of APC-x-y: (b) survey; (c) C1s; (d) O1s.

Table 1 Pore structure properties of all of the APC-x-y samples

Sample	SBET^a (m^2 g^−1)	Smicro^b (m^2 g^−1)	Smeso^c (m^2 g^−1)	Vtotal^d (cm^3 g^−1)	Vmicro^e (cm^3 g^−1)	Dp^f (nm)	La (nm)	ID/IG
a BET surface area.b Micropore surface area calculated using the V–t plot method.c Mesopore surface area calculated using the V–t plot method.d The total pore volume calculated using single point adsorption at P/Po = 0.976.e The micropore volume calculated using the V–t plot method.f Pore diameter of the peak value in Fig. 5b and d.
APC-7-1	1665.1	919.1	746.0	0.82	0.45	0.64/1.27/2.00	4.48	0.97
APC-7-2	2231.6	177.6	2054.1	1.15	0.07	0.64/1.27/2.35	4.63	0.95
APC-7-4	2169.8	—	2169.8	1.49	—	0.64/1.27/2.73	4.48	0.94
APC-8-4	2096.9	—	2096.9	1.36	—	0.64/1.27/3.43	4.39	0.99
APC-9-4	2050.9	—	2050.9	1.32	—	0.64/1.27/2.74	4.03	1.08

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To obtain more information regarding the chemical elements on the outer surface of the materials, XPS was performed on a representative sample (APC-7-4). In the survey spectrum with a binding energy ranging from 0 to 1200 eV (Fig. 2b), peaks corresponding to C and O can be obviously observed, and the relative content of each was determined to be 96.2% and 3.8%, respectively, suggesting the high carbonization level of APC-7-4. To further reveal the chemical state of the carbon and oxygen in detail, deconvolution of the XPS peaks was performed using XPS PEAK Software. As shown in Fig. 2c, the C1s spectrum ranging from 277.5 to 300.0 eV can be approximately divided into three peaks. The peak located at ca. 284.8 ± 0.2 eV is related to the sp² C=C band of graphitic carbon,³⁶ while the peak at about 286.4 ± 0.2 eV should be attributed to the band of sp³ C–C or defective carbon atoms which are no longer in the regular graphitic structure.³⁷ The peak at ∼288.6 ± 0.2 eV derives from the contribution of the –C=O or –COOH bands.³⁸ The O1s spectrum displayed in Fig. 2d can also be fitted into three deconvoluted peaks located at ca. 531.5 ± 0.1, 532.3 ± 0.1 and 533.8 ± 0.1 eV. The peak at about 531.5 ± 0.1 eV is ascribed to the contribution of oxygen in the carboxyl groups; and the peak at ca. 532.3 ± 0.1 eV corresponds to the –C=O ester band; the one centered at ca. 533.8 ± 0.1 eV should be assigned to the band of –C–O–C.^6,39

It is well known that the main component of filter paper is lignocellulose with a long fiber structure. Fig. 3a shows a scanning electron microscopy (SEM) micrograph of filter paper directly carbonized at 900 °C (defined as FP-900), the tubular morphology of the resultant material is obvious, which demonstrates that the original structure can be retained during the high temperature carbonization treatment. After activation by ZnCl₂, these APC-x-y materials present an irregular morphology, however, discernible pores with different pore sizes can be clearly observed in every APC-x-y sample (Fig. 3b–d). In addition, it can be found that the pore sizes of APC-x-y gradually increase with an increase in the activation temperature from 700 to 900 °C. This phenomenon can be also testified by the results of the pore size distribution. The morphology of these APC-x-y samples can be further characterized using TEM images. From the TEM images of the representative sample APC-7-4 (Fig. 4), the developed meso/micropore structure can be clearly seen. The interconnected micro- and meso-porous structure of APC-7-4 provides a favorable route for transportation and penetration of electrolyte ions, which plays an important role in fast ion transfer. Consequently, these activated porous carbon materials derived from recycled waste filter paper could be promising electrode materials for supercapacitors.

Fig. 3 The SEM photographs of all materials: (a) FP-900; (b) APC-7-4; (c) APC-8-4; (d) APC-8-4.

Fig. 4 The TEM images of the APC-7-4 sample.

The porosity of carbonaceous materials, including the specific surface area, pore volume and pore size, plays an essential role in determining the capacitive properties, which were analyzed using N₂ adsorption–desorption tests. The influence of the ZnCl₂ activation agent dosage on the pore structures of the APC-x-y materials was explored, and the results are displayed in Fig. 5a and b. With the increase of the ZnCl₂ dosage, the isotherms of these materials gradually transform from type I to type IV, and even a marked hysteresis loop at the relative pressure region from 0.45 to 0.80 can be clearly observed in the APC-7-4 sample. This result indicates that the porous structure of the resultant activated carbons gradually converts from microporous to mesoporous. Thus, the porous properties of these activated carbons greatly depend on the activating agent dosage, including the specific surface area, pore volume, pore size and the ratio of micro/mesopores (Table 1). From Fig. 5b, it can be seen that the majority of the pores in APC-7-1 are micropores, and APC-7-2 and APC-7-4 exhibit a hierarchical pore structure as well as the mesopore proportion and size gradually increasing with the enhancement of the ZnCl₂ dosage. This result should be related to a large fraction of micropore coalescence, bringing about the increase of the mesopore proportion. More importantly, the size of the micropores in all samples is around 0.64 nm, which is close to the size of hydrolyzed K⁺ ions (0.331 nm),⁴⁰ suggesting that the micropores exhibited in the APC-x-y materials are conducive to their capacitive performance. Activation temperature is another important parameter for the development of porosity. With the increase of activation temperature from 700 to 900 °C, all of the APC-x-4 materials exhibited a marked H4 type hysteresis loop at relative pressures of 0.40 to 0.85, indicative of the formation of mesoporous structures. Interestingly, when the activation temperature was enhanced from 700 to 900 °C, the surface areas and pore volumes of the APC-x-4 materials gradually decreased (Table 1), which should be related to the sintering effect at high temperature, followed by shrinkage of the char, and realignment of the structure.⁴¹ Fig. 5d reveals the evolution of the pore structure, where all APC-x-4 materials possess a hierarchical pore distribution comprising micropores and mesopores, and more importantly, widened mesopores of 5–8 nm in diameter exist in the APC-7-4 sample. This high surface area, compounded with a hierarchical pore structure, is dominant for supercapacitor applications. The specific surface area, micro/mesopore ratio and pore structure parameter data of all materials are listed in Table 1. It can be clearly found that the specific surface area, micro/mesopore area, total pore volume, micro/mesopore volume ratio and pore size distribution can be controlled by changing the activation parameter, which greatly determine the capacitive performance.

Fig. 5 The influence of the ZnCl₂ activating agent dosage on the pore structure: N₂ adsorption–desorption isotherms (a) and pore size distributions (b) of the APC-7-y samples; the influence of activation temperature on the pore structure: N₂ adsorption–desorption isotherms (c) and pore size distributions (d) of the APC-x-4 samples.

3.2 Electrochemical properties

It is known that the carbonization temperature of materials has an important influence on their electrochemical behavior.⁴² Prior to using these resultant materials as the electrode materials for supercapacitors, we first optimized the carbonization temperature. The electrochemical performance of the APC-x-y materials as supercapacitor electrodes was evaluated using cyclic voltammetry and galvanostatic charge–discharge measurements using a two-electrode system in 6 M KOH. Fig. 6a and b compare the cyclic voltammogram curves of all the samples at a scan rate of 30 mV s⁻¹ and the galvanostatic charge–discharge curves at a current density of 1 A g⁻¹, respectively. Obviously, the cyclic voltammogram and galvanostatic charge–discharge tests reveal the same result, which is that APC-7-4 has the largest encircled area and longest discharge time, implying that it has the best capacitance behavior. The specific capacitance of APC-7-4 is calculated to be 302.3 F g⁻¹ at a current density of 1 A g⁻¹, which is much higher than those of APC-7-1 (186.3 F g⁻¹), APC-7-2 (178.8 F g⁻¹), APC-8-4 (181.8 F g⁻¹), APC-9-4 (211.5 F g⁻¹) and other activated carbon materials.^43–46

Fig. 6 (a) CV curves of all electrode materials at the scan rate of 30 mV s⁻¹; (b) galvanostatic charge/discharge curves of all samples at a current density of 1 A g⁻¹; (c) galvanostatic charge/discharge curves of the APC-7-4 sample at different current densities; (d) specific capacitances of the APC-7-4, APC-8-4 and APC-9-4 electrode materials at different current densities.

To further estimate the electrochemical properties of the APC-7-4 electrode material, the galvanostatic charge–discharge measurements within a potential window of −1 to 0 V were carried out at various current densities ranging from 1 to 50 A g⁻¹ (Fig. 6c). It can be clearly seen that all of the charge and discharge curves are nearly linear and present approximately symmetrical triangular shapes, suggesting that the APC-7-4 electrode material possesses excellent electrochemical reversibility and almost ideal EDLC behavior without any redox reaction involved. The specific capacitances of the APC-7-4 electrode material are calculated to be 302.3, 259.8, 220.2, 207.4, 186.6 and 168.8 F g⁻¹ at current densities of 1, 2, 5, 10, 20 and 50 A g⁻¹, respectively. The decrement of the specific capacitance at higher current densities is due to the increment of the voltage (IR) drop and insufficient mass transportation at high current densities.⁴⁷ However, the voltage drop at the initiation of the discharge with a current density of 10 A g⁻¹ is 0.076 V, suggesting a relatively low internal resistance, which endows the prominent conductivity. To study the capacity for fast energy delivery and storage at high current densities, the correlation between the specific capacitance and the various current densities for the different electrode materials are displayed in Fig. 6d. It is obvious that the APC-7-4 material exhibits the most preferable capacitive performance. More importantly, even at 50 A g⁻¹, a specific capacitance of 168.8 A g⁻¹ was maintained for APC-7-4, about 55.8% of the capacitance was retained as compared to that at 1 A g⁻¹, which is considerably higher than the 50.3 and 52.1% capacitance retention ratios of APC-8-4 and APC-9-4, respectively. This result implies that the APC-7-4 material possesses a very fast and efficient charge transfer and a good rate capability, and thus the APC-7-4 material has great potential in practical applications for supercapacitor electrodes. The superior capacitive behavior of APC-7-4 should mainly depend on the following several advantages: (i) the extremely high surface area provides more electroactive sites for ion accumulation and energy storage; (ii) the developed hierarchical porous structure enhances the electrolyte accessibility to the total pore surface; (iii) the relatively low inner resistance and superior conductivity.

Electrochemical impedance spectroscopy (EIS) is a powerful method for evaluating the electrochemical properties of electrode materials, such as their resistivity and accessibility for electrolyte ions. Besides, EIS measurements are also performed to illustrate the kinetics at the interface between the electrode and the electrolyte, as well as to characterize the contribution of the hierarchical porous structure to the EDLC.⁴⁸ Consequently, to further investigate the capacitive performances of these activated porous carbons, typical Nyquist impedance spectra recorded at a potential of 5 mV within a frequency range of 10 kHz to 0.01 Hz for the APC-7-4, APC-8-4 and APC-9-4 materials are shown in Fig. 7. In all plots, a semicircle and a straight line can be clearly seen in the high frequency region and low frequency region, respectively. In the high frequency region (inset, Fig. 7), the diameter of the semicircle corresponds to the equivalent series resistance (ESR), and a smaller semicircle means a smaller ESR.⁴⁹ The contributions to the ESR include the resistance of the electrolyte solution, the intrinsic resistance of the active material and the contact resistance of the electrode interface/current collector. The equivalent series resistances are 0.27, 0.66 and 0.58 Ω for the APC-7-4, APC-8-4 and APC-9-4 electrodes, respectively. Thus, it is evident that APC-7-4 exhibits the lowest ESR value of all of the samples. Generally, a low ESR will bring about a small electrode-potential drop,⁵⁰ which is consistent with the observations in the charge/discharge experiments. A short slope of about 45° in the intermediate frequency can be seen in the Nyquist plots of all samples, which is a typical feature of porous electrodes.⁵¹ The 45° segment in the Nyquist plot corresponds to the Warburg resistance (W), which is related to the diffusion of the ions into the electrode particle interface inside the small pores.⁵² The projected length of the Warburg-type line on the real axis was short for the APC-7-4 electrode, suggesting reduced resistance is encountered by the fast ion transport into the internal pore network of the electrode. The nearly vertical lines at low frequencies represent the dominance of electrical double-layer capacitors.⁵³ Consequently, the APC-7-4 sample exhibits lower ESR and W values than those of other activated porous carbons, and thus it presents the most satisfactory capacitive properties, which should be mainly ascribed to its outstanding and predominantly hierarchical porous framework.

Fig. 7 Electrochemical impedance spectra of the APC-7-4, APC-8-4 and APC-9-4 electrode materials under the influence of an ac voltage of 5 mV (the inset is the high-frequency range in detail).

Fig. 8a shows the variation of the energy density of APC-7-4 with power density in 6 M KOH aqueous electrolyte. Though the energy density of APC-7-4 decreases with the increase of power density, the extractable energy density of the supercapacitor still maintains up to 21.9 W h kg⁻¹ at the power density of 24.7 kW kg⁻¹, which is superior to the performances of reported carbon-based electrode materials.^54–56 Furthermore, to validate the practical application efficiency of the electrode material, the long-term cycling stability of the APC-7-4 electrode was investigated through galvanostatic charge–discharge cycling at a current density of 5 A g⁻¹. As shown in Fig. 8b, no obvious variation can be found in the specific capacitance of the APC-7-4 electrode during the first 2000 cycles, and even after 10 000 cycles. The long-term cycling performance is maintained at ca. 95.4% of the initial specific capacitance and still reaches a capacitance value of about 210.1 F g⁻¹ over 10 000 cycles, which shows that the APC-7-4 electrode possesses a stable energy-storage process and a high degree of electrochemical reversibility.

Fig. 8 (a) Ragone plot of the APC-7-4 electrode material in 6 M KOH; (b) cycling stability of the APC-7-4 sample at a current density of 5 A g⁻¹ for 10 000 cycles in 6 M KOH electrolyte. Inset of the figure is the comparison of the 1st and 10 000th CV curves.

4. Conclusion

In summary, the activated porous carbons were successfully prepared using a facile chemical activation route with ZnCl₂ using recycled waste filter paper as the precursor. The ZnCl₂ activation process effectively produced a developed and interconnected hierarchical porous structure in the carbon precursor. The specific surface area, pore volume and meso/micropore ratio can be tuned by adjusting the ZnCl₂ dosage and activation temperature. The optimal sample, APC-7-4, exhibits excellent electrochemical properties for supercapacitor applications with a satisfactory specific capacitance of 302.3 F g⁻¹ at a current density of 1 A g⁻¹, as well as prominent cycling stability even after 10 000 charge–discharge cycles. This approach provides a good example of making full use of abundant sustainable resources to design novel carbon materials with superior porosity for promising applications in high performance energy storage devices.

Acknowledgements

The authors gratefully acknowledge the financial support from the program for New Century Excellent Talents in University (NCET-12-0696), the Leading Talents for Zhengzhou Science and Technology Bureau (Grant No. 131PLJRC649), the program for University Innovative Talents of Science and Technology in Henan Province (Grant No. 2012HASTIT03), and the National Natural Science Foundation of China (51472102).

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