Porous carbon derived from sorghum stalk for symmetric supercapacitors

Abstract

Sorghum stalk based porous carbons (SSCs) have been synthesized through a simple carbonization method at 800 °C used sorghum stalk as carbon precursor and ZnCl₂ as activating agent. The morphology and structure of the SSCs are investigated by scanning electron microscopy, transmission electron microscopy, nitrogen adsorption–desorption, Raman spectra and X-ray diffraction. Undergoes activation at optimal amount of zinc chloride (sorghum stalks to ZnCl₂ is 1 : 1), the resulting samples, labeled as SSC1.0 has a porous texture with high specific surface area and efficient ion diffusion channels (1354.7 m² g⁻¹ specific surface areas and 0.765 cm³ g⁻¹ pore volumes), and the sample also has superhydrophilicity characterized by water contact angles. As supercapacitor electrode, it can deliver 216.5 F g⁻¹ specific capacitance at 0.5 A g⁻¹, 75% capacitance retention even at 8 A g⁻¹ in 2 mol L⁻¹ KOH aqueous electrolyte and excellent cyclic stability with 92% capacitance retention after 5000 cycles at 5 A g⁻¹. Moreover, the assembled SSC1.0//SSC1.0 symmetric cell has wide voltage range of 1.8 V, and high energy density in 0.5 mol L⁻¹ Na₂SO₄ aqueous electrolyte.

---

1. Introduction

Supercapacitors have received great attention in recent years owing to their reliable cycle life, rapid charge–discharge capability, safe operation, high energy and power density.^1–4 In addition, the applications of supercapacitors are very extensive, and include hybrid electric vehicles, portable electronics and mobile electrical systems. In principle, supercapacitors mainly operate via electrostatic sorption of electrolyte ions onto porous electrodes, namely electric double-layer capacitors (EDLCs), and the fast and reversible surface redox reactions of active electrodes or the redox intercalation of metal ions into layer electrode (pseudocapacitors).^5–7 It is well known that electrode active materials play a vital role for their electrochemical performance, and the structural and surface feature of electrode material directly determines the capacitance and energy density of supercapacitor. Recently, the metal compounds and conductive polymers or porous carbon materials have been used as electrodes materials for supercapacitor.^8,9 However, the practical applications of the metal compounds and polymers materials have been limited because of their low electrical conductivity, poor cycle stability and high price.^10–12 While carbon materials are recommended as the most promising candidates for supercapacitor electrodes because their rich porous structure, large surface areas, high conductivity, physical and chemical stability.

Among various carbon materials, activated carbons prepared by biomass waste materials are becoming a trend due to their renewable, low cost, unique structure and eco-friendly properties.^13–16 There are numerous biomass materials have been reported for using supercapacitor electrodes materials. For example, eggplant peel,¹⁷ fungus,¹⁸ coconut shell,¹⁹ potato,²⁰ corn cob,²¹ paulownia flower,²² prawn shells,²³ bamboo,²⁴ loofah sponges²⁵ and cow dung.²⁶ Li et al. used waste eggplant as raw material to produce a sheet-like porous carbon by direct carbonization at different temperatures. The carbon SPC-1000 shows high rate capability and capacitance retention. The good capacitor performance of the carbon is attributable to interconnected porous structure and higher nitrogen-doping content.¹⁷ Long et al. prepared a three dimensional (3D), densely porous graphene-like carbon (PGC) through the hydrothermal treatment of fungus in 0.1 M KOH solution and subsequent carbonization process, which shows high specific capacitances and excellent cycling stability.¹⁸ Sun et al. used coconut shell as raw materials by a combination of chemical and physical activation to preparation activated carbon for supercapacitor electrodes. The graphitic catalyst precursor (FeCl₃) and activating agent (ZnCl₂) were introduced simultaneously into the skeleton of the coconut shell through coordination of the metal precursor with the functional groups in the coconut shell, which have improved the performance of the capacitor performance.¹⁹

Sorghum is known as one of the most important agricultural commodities in the world, and sorghum stalks as biological resource are cheap, abundant, environmentally safe, commercially available and sustainable. In general, some of sorghum stalks were used as poultry feed, and others are directly abandoned in the country sides, which result in great waste. The sorghum stalks are mainly contained of crystalline cellulose, hemicellulose and lignin.²⁷ The crystalline cellulose polymers are comprised in densely packed nanosheet stacks which are surrounded by the amorphous components.²⁸ So it is an efficient way to utilize sorghum stalks as the carbon source to prepare carbon materials.

In this study, we aim to use sorghum stalks as low-cost biomass precursor to reveal the inherent nanostructure, and ZnCl₂ as pore-forming agents to manufacture porous carbon material. Their relationships of morphology, structures, specific surface area and electrochemical characteristics of the carbon samples are also discussed in detail.

2. Experimental

2.1. Materials

Sorghum stalks collected from the local environment (Lanzhou Gansu province, China), zinc chloride (ZnCl₂, Aladdin Ltd, China). All the reagents used in experiments were in analytical grade and without further purification.

2.2. Preparation of sorghum stalks based porous activated carbon

Before the carbonization, the sorghum stalks were washed with deionized water, dried in an air-circulating oven at 60 °C for 24 h, and then smashed into powder by a disintegrator. The carbonization, graphitization and activation process were simultaneously carried out in a tubular furnace. Typically, 1.0 g of the dried sorghum stalks powder was mixed with ZnCl₂ and grinded to form evenly dispersed mixture, then the mixture was placed in porcelain boat and carbonized in tubular furnace at 800 °C for 2 hour with a heating rate of 5 °C min⁻¹ in a nitrogen atmosphere. After being cooled to room temperature naturally, the products were thoroughly washed with 2 mol L⁻¹ HCl to remove inorganic salts and other impurities, then dried at 60 °C for 24 h. The obtained samples with different content ZnCl₂ were designated as SSC0, SSC0.5, SSC1.0 and SSC1.5, respectively.

The preparation strategy was depicted in Scheme 1.

Scheme 1 The preparation process of SSCs.

2.3. Characterization of carbon materials

The morphologies and structures of the samples were characterized by scanning electron microscopy (FE-SEM, Ultra Plus, Carl Zeiss, Germany). Transmission electron microscopy (TEM, JEM-1200EX microscope operating at 200 kV). The Brunauer–Emmett–Teller surface area (BET) of the samples were analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus (U. S. A.). The pore size distribution plots were recorded from the desorption branch of the isotherms based on the Barrett–Joyner–Halenda (BJH) model and non-local density functional theory (NLDFT) model. Raman spectra was recorded with an in via Raman spectrometer (Renishaw) and X-ray diffraction (XRD) of samples was performed using a Rigaku D/Max-2400 diffractometer with Cu Kα radiation (k = 1.5418 Å) at 40 kV, 100 mA. The 2θ measure range was from 5° to 80°. X-ray photoelectron spectroscopy (XPS) measurement was performed on an Escalab 210 system (Germany).

2.4. Electrochemical measurement

The electrochemical properties of electrodes were analyzed by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) measurements in three-electrode system and two-electrode configuration was performed with an electrochemical workstation system (CHI 660D, Shanghai Chen Hua Co. Ltd, China). The cycle-life stability was recorded was performed with cycling testing equipment (CT2001A, Wuhan Land Electronic Co. Ltd, China).

2.4.1. Three-electrode system. In a three-electrode system, Hg/HgO electrode serves as the reference electrode, nickel foam as collector and platinum (1 cm × 1 cm) as the counter electrode, respectively. The working electrode was prepared by mixing the SSCs, binder polyvinylidene fluoride (PVDF) and commercial conductive carbon black in a mass ratio of 8 : 1 : 1 in N-methyl-2-pyrrolidone (NMP) solution until it formed homogeneous slurry. Further, the obtained slurry was coated on the nickel foam with an area of 1.0 cm² and dried at 60 °C for 24 h, then weighted and pressed into sheets under 15 MPa. The total mass of each electrode was limited to vary from 3.0 to 5.0 mg.²⁹ Then, the three-electrode system was tested in 2 mol L⁻¹ KOH aqueous electrolyte.

2.4.2. Two-electrode system. For a two-electrode system, the working electrodes were fabricated similar to the three-electrode system. Further, the two electrodes with identical or very close weight were measured. The SSC1.0 electrode fitted with the separator (thin polypropylene film) and electrolyte solution were symmetrically assembled into sandwich-type cells construction (electrode/separator/electrode). In order to make aqueous electrolyte solutions homogeneously diffuse into the SSC1.0 electrodes, the separator and SSC1.0 electrodes were immersed in 0.5 mol L⁻¹ Na₂SO₄ electrolytes for 12 h, after being assembled into the supercapacitor configuration.

3. Results and discussion

3.1. The morphologies and structures of SSCs

The morphology and high porosity of the SSCs were investigated by FE-SEM techniques (Fig. 1). From Fig. 1a–e, they show that the morphology of SSCs is seriously affected by different mass ratio of sorghum stalks to ZnCl₂. The SSC0 presents a number of highly unordered knobs and has slightly crumpled surface but without apparent pores (Fig. 1a). Obviously, as the activating agent content increases, the morphologies of the sample gradually are changed. The surface of SSC0.5 appears little small pores (Fig. 1b). More interestingly, the SSC1.0 surface become more rough (Fig. 1c), in high resolution image (Fig. 1d), it clearly exhibit that the surface appears loose network structure and exist a number of irregular micropores, which might make ionic diffusion from bulk electrolyte into the inner space of carbon materials easier. However, the sorghum stalks are treated with excessive ZnCl₂, which might be lead to the collapse of the pores and formed macropores and carbon layers accumulated randomly in the process for SSC1.5 (Fig. 1e). Distinctively, TEM image (Fig. 1f) reveals the SSC1.0 has porous structure, soft wrinkles edges. The interconnected porous carbon would provide unique open-pore system and short diffusion path for electrolyte ions, which is helpful to improve the electrochemical performance.

Fig. 1 FE-SEM for the samples: (a) SSC0; (b) SSC0.5; (c) SSC1.0; (d) the high magnification SSC1.0; (e) SSC1.5 and (f) TEM for the samples SSC1.0.

In order to investigate the development of porosity by ZnCl₂ activation, the specific surface area and pore structures of the activated carbon samples were measured by the N₂ adsorption–desorption technique. As given in Fig. 2a, it indicates the presence of different pore sizes from micropores to mesoporous. The increasing adsorbed volume at low pressure (0–0.2) is related to micropores, the desorption hysteresis at medium relative pressure (0.2–0.9) reveals the presence of mesopores and the small tails at a relative pressure near to 1.0 (>0.9) reveals the presence of macropores.^30,31 The results show that SSC0 attributed to a type I absorption isotherm indicating the non-porous characteristics. But with the increased the mass ratio of sorghum stalks to ZnCl₂, the dramatically increased adsorption volumes show a much more developed porous structure. Specifically, the isotherms of the activated carbon samples (SSC0.5, SSC1.0, and SSC1.5) at relative pressures from 0 to 0.9 imply the coexistence of micropores and mesopores. Therefore, it is reasonable to conclude that the BET specific surface area increases with increasing activating agent content. The pore structure is also confirmed by the pore size distribution plots (Fig. 2b), which are calculated using the Barrett–Joyner–Halenda (BJH) model. And associated with non-local density functional theory (NLDFT) model is given (the inset of the Fig. 2b). As shown in Fig. 2b and the inset, the activated carbons have well-developed porous structure and exhibit much more pores with diameters centered in the microporous and mesoporous regions than SSC0. The specific surface area, pore volumes, and average pores size of SSCs were obtained by BET measurement that summarized in Table 1. As is shown inset Fig. 2b, the NLDFT pore size distribution indicate the SSC1.0 exhibits porous structure that mainly made up of micropores (1.6 nm) and mesopores (2.2 nm).

Fig. 2 (a) Nitrogen adsorption–desorption isotherms, (b) and inset (b) pore size distributions of the SSCs.

Table 1 BET surface area and pore structure characterization parameters, surface N, O contents and forms of SSCs

Samples	SBET^a (m^2 g^−1)	Vtotal^b (cm^3 g^−1)	D^c (nm)	O states^d (%)			N states^d (%)
				C–O–R	C=O	COOH	Pyridinic	Pyrrolic	Graphitic
a Specific surface area determined according to BET (Brunauer–Emmett–Teller) method.b Total pore volume at P/P0 = 0.99.c Adsorption average pore diameter.d Total amount of redox species represent the amounts C–O–R, C=O, pyridinic- and pyrrolic N functionalities per gram of samples (μmol g^−1), which are calculated from the N, O atomic contents and the states from fitted XPS data.
SSC0	535.8	0.273	2.04	42.3	13.3	2.4	27.8	34.8	37.4
SSC0.5	1041.6	0.561	2.15	38.4	16.0	2.8	22.4	32.8	44.8
SSC1.0	1354.7	0.765	2.26	32.7	19.4	8.5	16.2	22.0	61.8
SSC1.5	1443.9	0.838	2.32	34.1	17.7	7.2	26.8	24.0	49.2

---

The surface chemical compositions of SSCs were examined by XPS. The chemical bonding between carbon and nitrogen were also summarized in Table 1. The XPS survey spectra of the SSC0, SSC0.5, SSC1.0 and SSC1.5 samples show common peaks assigned in typical carbon materials appears at 543.2, 412.2, and 298.2 eV corresponding to the orbitals peaks O1s, N1s, and C1s (Fig. 3a), respectively, showing the same elemental composition in all samples. To more comprehensively understand the detailed structural information, C1s and N1s peaks were deconvoluted. The C1s XPS spectrums of SSC0 (Fig. 3b), SSC0.5 (Fig. 3c), SSC1.0 (Fig. 3d) and SSC1.5 (Fig. 3e) can be fitted by four different binding states: C=C (284.5 eV), C–N/C–O–R (285.5 eV), C=N/C=O (286.4 eV) and apparent carboxyl groups (289.1 eV) bonds,³² showing the presence of N and O functionalities in the carbon motif. The carbon materials contains N and O atoms not only considerably contribute to improve wettability but also increase the number of chemically reactive sites and improve the holistic specific capacitance. Peak deconvolution of N1s spectrum (inset) reveals the presence of pyridinic- (398.6 eV), pyrrolic- (400.1 eV) and graphitic N (400.9 eV),^32,33 From inset Fig. 3, it is clear that with the increase of the sorghum stalk to ZnCl₂ mass ratio, the relative content of the pyridinic- and pyrrolic-N decrease, while the relative contents of the graphitic N increase, shows the partial transformation of the former two N forms into graphitic N, which is beneficial for a higher conductivity.^34,35 Especially, the transformation as shown in Table 1 of SSC1.0 is higher than SSC0, SSC0.5 and SSC1.5, which is helpful for the electrochemical performance. Meanwhile, the progress of the activation leads to an increase in the amount of surface oxygen groups. The O functionalities predominate in form of C–OH and C=O (Table 1), the possibly enhanced surface area by activation treatment also. Through means of faradaic reactions, the highly redox active N and O species are responsible for enhanced pseudocapacitance, which is important for the increase of the capacitance of the carbon materials.³⁶ The porous texture of samples benefits the sufficient compatibility to aqueous electrolyte, hence enhanced capacitance can be achieved.

Fig. 3 (a) XPS survey spectra of SSC0, SSC0.5, SSC1.0 and SSC1.5. C1s and N1s of (b) SSC0, (c) SSC0.5, (d) SSC1.0 and (e) SSC1.5.

To evaluate the wettability of SSCs, water contact angles of SSCs were measured. The optical images of water droplets on the surface of SSCs were shown in Fig. 4. The contact angle of SSC0 is 53°, reveals the weak hydrophilicity. In sharp contrast, the water drop can spread out rapidly onto the SSC0.5, SSC1.0 and SSC1.5 electrode surface with a contact angle of 0°, distinctly demonstrates the superhydrophilicity after activation. The enhanced hydrophilicity of SSC0.5, SSC1.0 and SSC1.5 is mainly derived from the appropriate nitrogen and oxygen functional groups on SSCs surface reinforce the interaction forces toward aqueous electrolytes through polar attraction and hydrogen bonds.³⁷ Therefore, a significant improvement in wettability is accomplished by activation treatment, which is beneficial for higher surface utilization ratio of SSCs. Considering the partial graphitization, enhanced specific surface area and the improved wettability, hence, the hydrophilicity SSC1.0 is expected to be more promising candidates as supercapacitor electrode materials.

Fig. 4 Contact angles of SSCs electrodes for deionized water.

Raman spectroscopy is an excellent tool for analyzing carbon materials. As Raman spectra have been shown to be sensitive to changes in the microscopic structure of the material.³⁸ The Raman spectra of SSCs sample (Fig. 5a) demonstrated the characteristic D and G peaks that can be observed at 1343 cm⁻¹ and 1579 cm⁻¹, respectively. The G band corresponds to the allowed Raman transition for large graphite crystals and indicates graphitic layers, while the D band corresponds to disordered carbon or defective graphitic structures. The relative intensity of the G and D bands (I_G/I_D), as calculated by the peak area, can be used as an indication of the molecular order within the crystalline carbon structure and has been related to the size of the graphite crystallite.³⁸ The I_G/I_D the ratios of SSC0, SSC0.5, SSC1.0 and SSC1.5 were determined to be 1.03, 1.06, 1.08 and 1.01, respectively. These results demonstrated that the degree of graphitization increasing with the increased of the sorghum stalk to ZnCl₂ mass ratio. However, when the mass ratio of ZnCl₂ to sorghum stalk increased to 3, the degree of graphitization began to decrease, which may be due to excessive activation of reduced nitrogen and oxygen content. Therefore, moderate use of ZnCl₂ can conduce to improve the graphitization degree of carbon and thereby enhance electric conductivity.

Fig. 5 (a) Raman spectra of SSCs sample and (b) XRD pattern of SSC1.0 sample.

X-ray diffraction (XRD) analysis was used to investigate the structure of SSC1.0. As seen from Fig. 5b, two typical peaks can been observed, the clear observation a carbon peak is observed between 20° and 30°, which corresponds to the (002) plane and indicates that the SSC1.0 consists of small domains of ordered graphene sheets. The other broad peak around 43° is attributed to the plane of (100) diffraction and indicates graphite.³⁹ Notably, both diffraction peaks are broad in width and low intensity in height, it suggested that the amorphous feature of the SSC1.0 with partial graphitization.

3.2. Electrochemical characteristics of SSCs electrodes

In order to explore the potential application of the SSCs material as electrodes for supercapacitors, three-electrode system was used to evaluate the electrochemical properties of the SSCs in 2 M KOH aqueous solution. Fig. 6a is the CV curves of SSCs electrodes at a scan rate of 50 mV s⁻¹ with the potential range of −1.0 to 0 V. Typical CV curve for SSC1.0 exhibits larger area than that of others, indicating SSC1.0 has the higher specific capacitance. And the CV curves for SSC1.0 exhibits a nearly rectangular shape for all scan rates even at a potential scan rate of 100 mV s⁻¹, showing a typical electric double-layer capacitors behavior with excellent rate capability (Fig. 6b). Furthermore, the galvanostatic charge/discharge curves of SSC1.0 electrode at various current densities are shown in Fig. 6c. At the current density of 0.5 A g⁻¹, the SSC1.0 sample exhibits the utmost charge/discharge time because the electrolyte ions have sufficient time to enter and diffuse into the porosity at lower current densities. The charge/discharge time remarkably decreases with the increase of the current density. Even at 10 A g⁻¹ the galvanostatic curve maintains the ideal linear voltage time relation and displays minimal IR drop. The specific capacitance of electrodes can be calculated using the following equation:⁴⁰

C_m = (It)/(ΔVm) (1)

where C_m is specific capacitance (F g⁻¹), I is charge/discharge current (A), t is the time of discharge (s), ΔV is the voltage difference between the upper and lower potential limits, and m is the mass of the active electrode material. The capacitance retention of the electrodes for rates up to 10 A g⁻¹ is summarized in Fig. 6d. It exhibits that the highest specific capacitance of SSC1.0 is 216.5 F g⁻¹ at 0.5 A g⁻¹, and 162 F g⁻¹ even at 10 A g⁻¹, which remains about 75% of the highest capacitance. The SSC1.0 demonstrates high rate performance, while the large specific capacitance of the material is comparable to recent reports of other high performance nanostructure material.^19,41

Fig. 6 (a) CV curves of SSCs electrodes at a scan rate of 50 mV s⁻¹ in 2 M KOH solution; (b) CV curves of SSC1.0 electrode at different scan rates; (c) galvanostatic charge/discharge curves of SSC1.0 electrode at various current densities; (d) specific capacitance as a function of the current densities of the SSC1.0 electrode; (e) Nyquist plots of the SSC1.0 electrodes, (f) cycling stability of SSC1.0 in three-electrode system.

To understand the facilitated ion and electron transport behavior of SSCs materials, and excellent performance of SSC1.0, the electrochemical impedance spectroscopy (EIS) were investigated under the open circuit potential. Fig. 6e shows the Nyquist plots obtained at a frequency range from 0.1 to 10⁵ Hz. It is composed of three distinct parts at a different frequency range, including an uncompleted semicircle part at high frequency, an inclined portion of the curve (about 45°) at middle frequency and a linear part at low frequency. The intercept at the real axis gives the internal resistance value of the cell capacitor (R_s), which is the sum of the ionic resistance of the electrolyte, intrinsic active material resistance and the contact resistance at the interface of the active material and the current collector.⁴² The diameter of the semicircle reflects the charge transfer resistance (R_ct), which is related to the charge transfer through the electrode/electrolyte interface. A smaller semicircle means a lower charge-transfer resistance. The 45° slope of the line in the middle frequencies is ascribed to the Warburg impedance (W). The Nyquist plots are also fitted and interpreted with the help of an appropriate electric equivalent circuit (see the inset of Fig. 6e). The capacitor circuit consists of R_s, R_ct, W, C_L, C_dl and Q, in which C_dl and Q are related to the capacitor layer formed during the charge–discharge process, and C_L reflects the limited capacitance.⁴³ As can be concluded, the SSC1.0 not only has a low R_s (1.25 Ω cm²), but also possesses a small R_ct (0.52 Ω cm²), as well as a small Warburg resistance (1.28 Ω cm²), which indicates that the SSC1.0 electrode has a low resistance with a good ion response at high frequency ranges.⁴⁴ The EIS results further demonstrate that the porous carbon material SSC1.0 has the good electrical conductivity and the ability of rapid electron and ion transport.

The cycling stability is also a crucial parameter for supercapacitors electrode materials. To investigate the cycling stability, the galvanostatic charge/discharge cycling of the SSC1.0 was performed at a current density of 5 A g⁻¹ (Fig. 6f). After 5000 cycles, the capacity decay is only 8% compared with the starting value. It exhibits the good cycling stability which may be due to the porosity with the carbon material and keeping stable during cycling, this result makes it a promising candidate for long-term energy storage devices.

The two-electrode symmetric supercapacitor was also fabricated to further assess the actual electrochemical performance of SSC1.0 electrode. Compared with the acid and alkali solutions, the neutral Na₂SO₄ aqueous electrolyte has a higher operation voltage.¹⁹ CV cures of the SSC1.0//SSC1.0 symmetric cell in comparison to Ni foam at the same potential window was shown in Fig. 7a. It can be seen that the CV curve of Ni foam nearly approaches linear shape, indicating that Ni foam has no effect on the electrochemical properties of SSC1.0. Therefore, the working electrode SSC1.0 was prepared by coating on the Ni foam in the two-electrode system, and the SSC1.0//SSC1.0 symmetric supercapacitor was assembled and characterized in 0.5 mol L⁻¹ Na₂SO₄ aqueous electrolyte. Fig. 7b shows the CV curves of the symmetric cell at different voltage windows. When the high voltage extends to 1.8 V, the CV curves of the supercapacitor still retain rectangular-like shape which indicates ideal capacitive behavior and good reversibility. However, when the voltage increases to 2.0 V, the current is dramatically increased since the electrolyte is being decomposed with hydrogen and/or oxygen evolution. Therefore, the potential window of 1.8 V is selected to measure the electrochemical performance of the symmetric cell. Fig. 7c is the CV curves of the symmetric cell obtained at different scan rates from 10 to 100 mV s⁻¹ at the voltage range of 0 to 1.8 V. It can be seen that the CV curve still keep a quasi-rectangular shape even at a scan rate of 100 mV s⁻¹. Fig. 7d displays galvanostatic charge/discharge curves of the symmetric cell at different current densities from 1 to 15 A g⁻¹. Using the following formulas,³⁰ the specific energy density (E, W h kg⁻¹) and power density (P, W kg⁻¹) for a supercapacitor cell were counted.

E = 1/2CV² (2)

P = E/t (3)

where C is the specific capacitance of the cell (F g⁻¹), V is the voltage change during the discharge process after the IR drop in galvanostatic discharge curve, t is the time of discharge (s). In order to further confirm the electrochemical performance of the SSC1.0 carbon materials, Ragone plot (specific energy vs. specific power) of SSC1.0//SSC1.0 symmetric cell was calculated as shown in Fig. 8a. The SSC1.0//SSC1.0 symmetric cell exhibits the energy density 9.77 W h kg⁻¹ at a power density of 225.35 W kg⁻¹ and even remained 4.86 W h kg⁻¹ at a power density of 7170.5 W kg⁻¹, as shown in Fig. 8a. It is still apparently superior to the recently reported porous carbons.^22,45–47 Hence, the porous carbons prepared by bio-waste sorghum stalks residue are hopeful electrodes materials for supercapacitors.

Fig. 7 (a) CV curves of the SSC1.0//SSC1.0 symmetric cell in comparison to Ni foam//Ni foam symmetric cell at 50 mV s⁻¹ with the potential range of 0 to 1.8 V. (b) CV curves of the SSC1.0 symmetric cell at different voltage windows at a scan rate of 50 mV s⁻¹; (c) CV curves of the SSC1.0 symmetric cell at various scan rates with the potential range of 0 to 1.8 V; (d) galvanostatic charge/discharge curves of SSC1.0//SSC1.0 symmetric cell at various current densities.

Fig. 8 (a) Ragone plot of the SSC1.0//SSC1.0 symmetric cell and other previously reported carbon-based symmetric supercapacitors and (b) photograph of a green LED lighted by the three tandem-capacitors group.

To further verify the feasibility for energy supply of SSC1.0 capacitor, the three-tandem cell group was used to light up blue light-emitting diodes (LED). Then, the tandem cell group can light up the LED (Fig. 8b), which vividly shows the feasibility of SSC1.0 capacitor in amplified energy storage and output.

4. Conclusions

In summary, porous activated carbons from sorghum stalks have been successfully prepared utilize simple chemical activation method. By using the ZnCl₂ activating agent, the resulting porous carbon (SSC1.0) shows super-hydrophilicity, and a high specific surface area is up to 1354.7 m² g⁻¹, as well as high capacitive performance with a specific capacitance of 216.5 F g⁻¹ and excellent cycling performance (only 8% capacitance was lost after 5000 cycles) in three-electrode systems. Furthermore, the as-assembled SSC1.0//SSC1.0 symmetric supercapacitor device with an wide operation voltage of 0 to 1.8 V in 0.5 mol L⁻¹ Na₂SO₄ aqueous electrolyte delivers a high energy density of 9.77 W h kg⁻¹. Considering that large amount of sorghum stalks are directly abandoned in the country sides, which result in great waste. These findings suggest that the novel SSC1.0 material has great potential applications in low-cost energy storage devices. Therefore, the method taken in this study could develop a novel and eco-friendly approach for preparation of high performance supercapacitor electrode materials from renewable biomass waste.

Acknowledgements

This research was financially supported by the National Science Foundation of China (21664012, 51462032), the program for Changjiang Scholars and Innovative Research Team in University (IRT15R56), Innovation Team Basic Scientific Research Project of Gansu Province (1606RJIA324), Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, and Key Laboratory of Polymer Materials of Gansu Province. H. Peng thanks the financial support provided by the Outstanding Doctoral Dissertation Cultivation Program of Northwest Normal University.

References

1. Y. Zhai, Y. Dou, D. Zhao, P. F. Fulvio, R. T. Mayes and S. Dai, Adv. Mater., 2011, 23, 4828–4850.
2. J. Bae, M. Song, Y. Park, J. Kim, M. Liu and Z. Wang, Angew. Chem., Int. Ed., 2011, 50, 1683–1687.
3. S. Wang, W. li, L. Xin, M. Wu and X. Lou, RSC Adv., 2016, 6, 42633–42642.
4. X. Zhang, Y. Jiao, L. Sun, L. Wang, A. Wu, H. Yan, M. Meng, C. Tian, B. Jiang and H. Fu, Nanoscale, 2016, 8, 2418–2427.
5. H. Jiang, P. S. Lee and C. Li, Energy Environ. Sci., 2013, 6, 41–53.
6. Y. Tan, K. W. Wong and K. M. Nq, RSC Adv., 2016, 6, 91250–91255.
7. V. Augustyn, P. Simon and B. Dunn, Energy Environ. Sci., 2014, 7, 1597–1614.
8. X. Lu, M. Yu, G. Wang, T. Zhai, S. Xie, Y. Ling, Y. Tong and Y. Li, Adv. Mater., 2013, 25, 267–272.
9. Z. Li, Z. Xu, H. Wang, J. Ding, B. Zahiri, C. M. B. Holt, X. H. Tan and D. Mitlin, Energy Environ. Sci., 2014, 7, 1708–1718.
10. M. Sevilla and R. Mokaya, Energy Environ. Sci., 2014, 7, 1250–1280.
11. Z. Li, L. Zhang, B. S. Amirkhiz, X. Tan, Z. Xu, H. Wang, B. C. Olsen, C. M. B. Holt and D. Mitlin, Adv. Energy Mater., 2012, 2, 431–437.
12. D. Zhou, Y. Du, Y. Song, Y. Wang, C. Wang and Y. Xia, J. Mater. Chem. A, 2013, 1, 1192–1200.
13. R. Farma, M. Deraman, A. Awitdrus, I. A. Talib, E. Taer, N. H. Basri, J. G. Manjunatha, M. M. Ishak, N. Basri, B. N. M. Dollah and S. A. Hashmi, Bioresour. Technol., 2013, 132, 254–261.
14. K. Kuratani, K. Okuno, T. Iwaki, M. Kato, N. Takeichi, T. Miyuki, M. Majima and T. Sakai, J. Power Sources, 2011, 196, 10788–10790.
15. M. Chen, X. Kang, T. Wumaier, J. Dou, B. Gao, Y. Han, G. Xu, Z. Liu and L. Zhang, J. Solid State Electrochem., 2013, 17, 1005–1012.
16. H. Jin, X. Wang, Z. Gu and J. Polin, J. Power Sources, 2013, 236, 285–292.
17. Z. Li, W. Lv, C. Zhang, B. Li, F. Kang and Q. Yang, Carbon, 2015, 92, 11–14.
18. C. Long, X. Chen, L. Jiang, L. Zhi and Z. Fan, Nano Energy, 2015, 12, 141–151.
19. L. Sun, C. Tian, M. Li, X. Meng, L. Wang, R. Wang, J. Yin and H. Fu, J. Mater. Chem. A, 2013, 1, 6462–6470.
20. G. Ma, Q. Yang, K. Sun, H. Peng, F. Ran, X. Zhao and Z. Lei, Bioresour. Technol., 2015, 197, 137–142.
21. M. Genovese, J. Jiang, K. Lian and N. Holm, J. Mater. Chem. A, 2015, 3, 2903–2913.
22. J. Chang, Z. Gao, X. Wang, D. Wu, F. Xu, X. Wang, Y. Guo and K. Jiang, Electrochim. Acta, 2015, 157, 290–298.
23. F. Gao, J. Qu, Z. Zhao, Z. Wang and J. Qiu, Electrochim. Acta, 2016, 190, 1134–1141.
24. Y. Yang, L. Liu, Y. Tang, Y. Zhang, D. Jia and L. Kong, Electrochim. Acta, 2016, 191, 846–853.
25. Y. Luan, L. Wang, S. Guo, B. Jiang, D. Zhao, H. Yan, C. Tian and H. Fu, RSC Adv., 2015, 5, 42430–42437.
26. D. Bhattacharjya and J. S. Yu, J. Power Sources, 2014, 262, 224–231.
27. R. Kumar, G. Mago, V. Balan and C. E. Wyman, Bioresour. Technol., 2009, 100, 3948–3962.
28. P. Kumar, D. M. Barrett, M. J. Delwiche and P. Stroeve, Ind. Eng. Chem. Res., 2009, 48, 3713–3729.
29. H. Peng, G. Ma, K. Sun, J. Mu, Z. Zhang and Z. Lei, ACS Appl. Mater. Interfaces, 2014, 6, 20795–20803.
30. W. Huang, H. Zhang, Y. Huang, W. Wang and S. Wei, Carbon, 2011, 49, 838–843.
31. W. Xing, C. Huang, S. Zhuo, X. Yuan, G. Wang, D. Hulicova-Jurcakova, Z. Yan and G. Lu, Carbon, 2009, 47, 1715–1722.
32. J. Chang, Z. Gao, W. Zhao, L. Guo, M. Chu, Y. Tang, D. Wu, F. Xu and K. Jiang, Electrochim. Acta, 2016, 190, 912–922.
33. N. P. Wickramaratne, J. Xu, M. Wang, L. Zhu, L. Dai and M. Jaroniec, Chem. Mater., 2014, 26, 2820–2828.
34. J. Wu, D. Zhang, Y. Wang and B. Hou, J. Power Sources, 2013, 227, 185–190.
35. L. Sun, C. Tian, Y. Fu, Y. Yang, J. Yin, L. Wang and H. Fu, Chem.–Eur. J., 2014, 20, 564–574.
36. V. Barranco, M. A. Lillo-Rodenas, A. Linares-Solano, A. Oya, F. Pico, J. Ibanez, F. Agullo-Rueda, J. M. Amarilla and J. M. Rojo, J. Phys. Chem. C, 2010, 114, 10302–10307.
37. Z. Burton and B. Bhushan, Nano Lett., 2005, 5, 1607–1613.
38. R. Escribano, J. J. Sloan, N. Siddique, N. Sze and T. Dudev, Vib. Spectrosc., 2001, 26, 179–186.
39. S. T. Senthilkumar, R. K. Selvan, N. Ponpandian, J. S. Melo and Y. S. Lee, J. Mater. Chem. A, 2013, 1, 7913–7919.
40. G. Ma, H. Peng, J. Mu, H. Huang, X. Zhou and Z. Lei, J. Power Sources, 2013, 229, 72–78.
41. B. Xu, S. Hou, H. Duan, G. Cao, M. Chu and Y. Yang, J. Power Sources, 2013, 228, 193–197.
42. Y. Fan, X. Yang, B. Zhu, P. Liu and H. Lu, J. Power Sources, 2014, 268, 584–590.
43. L. Sun, C. Tian, Y. Fu, Y. Yang, J. Yin, L. Wang and H. Fu, Chem.–Eur. J., 2014, 20, 564–574.
44. P. Yu, Y. Li, X. Yu, X. Zhao, L. Wu and Q. Zhang, Langmuir, 2013, 29, 12051–12058.
45. X. Fan, C. Yu, J. Yang, Z. Ling and J. Qiu, Carbon, 2014, 70, 130–141.
46. X. He, R. Li, J. Qiu, K. Xie, P. Ling, M. Yu, X. Zhang and M. Zheng, Carbon, 2012, 50, 4911–4921.
47. K. H. An, W. S. Kim, Y. S. Park, J. M. Moon, D. J. Bae, S. C. Lim, Y. S. Lee and Y. H. Lee, Adv. Funct. Mater., 2001, 11, 387–392.

---