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Ren, R., Shang, T., Jin, X., and Gao, J. (2016). "Preparation of N-, S-Co-doped activated carbons derived from waste medium density fiberboard for supercapacitors," BioRes. 11(2), 4055-4068.


Nitrogen-, sulfur-co-doped activated carbons were obtained from sulfur-modified and nitrogen-doped activated carbons that were prepared from waste medium density fiberboard. The electrochemical capacitive performance of the activated carbon samples obtained was investigated in 7.0 M KOH electrolyte. The morphology, structure, and surface properties of the samples were investigated by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), N2 adsorption, X-ray photoelectron spectroscopy (XPS), and elemental analysis. The S-doping could be tuned by controlling the dosage of sulfur sublimed. Because of the introduction of sulfur functional groups, the prepared carbons exhibited better conductivity and higher specific capacitance, reaching 264 F/g under a current density of 50 mA/g and a 7.0 mol/L KOH electrolytic solution. A mechanism for improving the conductivity and capacitive performance of sulfur functional groups on activated carbons was discussed.

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Preparation of N-, S-Co-Doped Activated Carbons Derived from Waste Medium Density Fiberboard for Supercapacitors

Ruquan Ren, Tongxin Shang, Xiaojuan Jin * and Jianmin Gao*

Nitrogen-, sulfur-co-doped activated carbons were obtained from sulfur-modified and nitrogen-doped activated carbons that were prepared from waste medium density fiberboard. The electrochemical capacitive performance of the activated carbon samples obtained was investigated in 7.0 M KOH electrolyte. The morphology, structure, and surface properties of the samples were investigated by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), N2 adsorption, X-ray photoelectron spectroscopy (XPS), and elemental analysis. The S-doping could be tuned by controlling the dosage of sulfur sublimed. Because of the introduction of sulfur functional groups, the prepared carbons exhibited better conductivity and higher specific capacitance, reaching 264 F/g under a current density of 50 mA/g and a 7.0 mol/L KOH electrolytic solution. A mechanism for improving the conductivity and capacitive performance of sulfur functional groups on activated carbons was discussed.

Keywords: N, S-co-doped activated carbons; Waste medium density fiberboard; Sulfur functional groups

Contact information: Beijing Key Laboratory of Lignocellulosic Chemistry, MOE Engineering Research Center of Forestry Biomass Materials and Bioenergy, Beijing Forestry University, 35 Qinghua East Road, Haidian, 100083, Beijing, China; *Corresponding author: (Xiaojuan Jin); (Jianmin Gao)


Electrochemical supercapacitors, which can be divided into electrochemical double layer capacitors (EDLCs) and pseudo-capacitors, have incurred universal research interest as energy storage devices because of their high power density, long lifecycle, and reversibility. Electrochemical double layer capacitors are based on the accumulation of charges at the electrode/electrolyte interface; therefore, capacitance is strongly dependent on the surface area of the electrode material. Pseudo-capacitors make use of a reversible redox reaction to store charge, and the charge storage mechanism is faradic because the charge passing across the double layer is integral to the workings of the devices (Djurfors et al. 2003; Huang et al. 2014). Electrical conductive polymers and metal oxides, common pseudo-capacitive materials, have shown high specific capacitance. However, high price, low conductivity, and poor cycle capacity have limited their practical application as pseudo-capacitive materials (Si et al. 2013).

Activated carbons (ACs) are starting materials for EDLCs because of their electrochemical inertness, high electrical conductivity, large specific surface area, and good adsorption property. The electrochemical performance of EDLCs is dependent upon the surface area, pore structure, and surface chemistry of the activated carbons. Therefore, ACs, with a high specific surface area, are expected to be applied as electrode materials to enhance EDLCs with high performance (Faraji and Ani 2015). However, the EDLCs cannot be enhanced illimitably because the excessive increase in surface area would result in the decrease of mesoporosity, resulting in a reduction in the mass transfer capacity. It is desirable to improve the EDLCs capacitance by introducing Faradaic redox reaction without sacrificing of cycling stability (Sari and Ting 2015). The introduction of heteroatoms to ACs, such as nitrogen (N), phosphorus (P), sulfur (S), and boron (B), is suitable for combining the advantages of pseudo-capacitance with high surface area carbon (Tsubota et al. 2011). Activated carbons, modified with nitrogen heteroatoms, have attracted much interest because nitrogen functional groups were found to improve the wettability of the carbon network and contribute to pseudo-capacitance through faradic reactions. Zhou et al. (2014) reported that N-doped porous carbon capsules with tunable porosity were high-performance supercapacitors, and Shang et al. (2015) also studied the mechanism of nitrogen functional groups of carbon materials.

Sulfur has a lone pair of electrons, which could introduce pseudo-capacitance to carbon by the incorporation of S functional groups in the carbon framework. It has also been reported that S-doped activated carbons exhibit higher specific capacitance than plain activated carbons. Hasegawa et al. (2011) first reported that S-doped carbon could be used as a supercapacitor electrode material because of the redox faradic reactions provided by the additional sulfur-containing functional groups. Consequently, the specific capacitance of the carbon material was increased by 38% in an aqueous system compared with the conventional-ordered mesoporous carbon in their research. Huang et al. (2014) produced sulfurized activated carbons by coating the pore surface with thiophenic sulfur functional groups from the pyrolysis of sulfur flakes. Sulfurized activated carbon electrodes exhibit better conductivity and increased specific capacitance that is almost 40% higher than plain activated carbon electrodes at a high current density of 1.4 A/g. The addition of thiophene and sulfone species enhance faradic reactions and improve the specific capacitance of carbon in aqueous electrolytes, according to previous research (Sevilla and Fuertes 2012; Gu et al. 2013; Huang et al. 2014; Zhou et al. 2014). Zhao et al. (2012) also explored the idea of sulfoxide-, aromatic sulfide-, and sulfone-mediated mesoporous carbon for supercapacitors (Zhao et al. 2012).

Activated carbons produced from natural materials such as different biomasses are an attractive and economic option for use as electrode materials (Inal et al. 2015). Waste medium density fiberboard (MDF), as reported by Shang et al. (2014), is a good material for supercapacitors. Medium density fiberboard contains many nitrogen atoms, which originate from urea-formaldehyde resin. The recycling of MDF represents an efficient and environmentally friendly way to utilize this waste material.

In this work, N-, S-co-doped activated carbon was made by coating the pore surface of nitrogen-doped activated carbon with sulfur functional groups from the pyrolysis of sublimed sulfur. In this manner, nitrogen and sulfur species could be introduced into the target carbon materials. The performance of N-, S-co-doped activated carbon was investigated as an electrochemical capacitor. Additionally, the effects of sulfur on MDF electrochemical performance are discussed in detail.



The waste MDF was provided by the Jiahekailai Furniture and Design Company, in Beijing, China. Other chemicals were purchased from the Beijing Yixiubogu Chemical Reagent Co. (Beijing, China). All chemicals were of analytical grade, and the solutions were prepared with distilled water.

Preparation of Activated Carbon

The carbonization process was conducted according to Shang et al. (2014). In the activation step, 3.0 g of the oven-dried samples was mixed with KOH at a mass ratio impregnation of (3:1). The samples were then activated at the temperature of 800 °C for 1 h in a nitrogen atmosphere. Then, the activated carbons were boiled with 1.0 M HCl solution and then with distilled water until the pH of the solution reached approximately 6 to 7. Thereafter, samples were dried at 115 °C in an oven for 4 h. The obtained activated carbon samples and sublimed with sulfur at a weight ratio of 6:1, 8:1, 10:1, and 12:1. Then, the samples were mixed with agate mortar and pestle ground into uniform powders. Then, the mixture was heat-treated at 400 °C for 2 h, with a heating rate of 5 °C/min under a nitrogen atmosphere with a flow rate of 10 mL/min. The heat-treated samples were referred to as AC6, AC8, AC10, and AC12, depending on the ratio of activated carbon to sublimed sulfur, which were 6:1, 8:1, 10:1, and 12:1, respectively. The sample that was not sulfur-modified was referred to as AC. Finally, the activated carbons (AC, AC6, AC8, AC10, and AC12) were dried at 115 °C in an oven for 4 h.

Characterization of the Activated Carbons

Through elemental analysis, the content of carbon (C), hydrogen (H), N, and S were determined using a CHN analyzer (Thermo Finnigan Flash, EA 1112 series, Thermo Fisher Scientific, Waltham, MA). X-ray photoelectron spectroscopy (XPS) was used to determine the surface functional groups of the samples. The acceleration tension and power of the X-ray source were 15 kV and 100 W, respectively.

Surface morphologies from cross sections of the samples were observed using field emission scanning electron microscopy (FESEM; JSM-7000, JEOL Ltd., Tokyo, Japan). Transmission electron microscopy (TEM) measurements were conducted on a JEM-1010 transmission electron microscope (JEOL Ltd, Japan).

The pore volume, pore diameter, and Brunauer Emmett-Teller (BET) surface area of the samples were determined from the corresponding N2 adsorption-desorption isotherms obtained at -196 °C with an accelerated surface area and porosimetry system (ASAP 2010, Micromeritics Instrument Corp., Atlanta, GA). The BET method was used to determine the specific surface area (SBET). The total pore volume was calculated from the N2 adsorption isotherm at a relative pressure of 0.995 (P/P0). For the micropore volume and mesoporous surface area, the V-t and Barrett-Joyner-Halenda (BJH) methods were used, respectively. The pore size distribution was calculated using the density functional theory (DFT).

Electrode Preparation and Electrochemical Measurements

Electrodes for electrochemical measurements were prepared from the samples by first grinding the particles into fine powders. The measurements were fabricated by mixing the samples with acetylene black and 60% polytetrafluoroethylene at the mass ratio of 87:10:3. Then, the mixture was coated onto a nickel foam substrate square (1.0 cm2) and dried in an oven at 115 °C for over 4 h. All electrochemical tests were carried out using two-electrode cells in 7.0 mol/L KOH electrolyte on the BT 2000 battery testing system (Arbin Instruments, College Station, TX, USA) and the 1260 electrochemical workstation (Solartron Metrology, West Sussex, UK).


Elemental Analysis and XPS Study

Table 1 shows the elemental analysis of the samples. After modification, there was an increase in the presence of sulfur contents with increasing sulfur dosage. The sulfur content of N-, S-co-doped activated carbons (SAC) ranged from 1.45% to 1.93%, indicating the presence of sulfur in the SAC samples after the heat-treatment, and a partial loss of sulfur, which could have been attributed to evaporative losses during the heating process. The nitrogen content of the samples was similar, ranging from 1.13% to 1.19%. This suggested that a portion of the nitrogen atoms were turned over (some of the nitrogen atoms of the waste MDF originate from urea-formaldehyde resin adhesive used in the MDF manufacturing process, and they are further turned over to waste
MDF-based ACs) to the obtained samples, and that the nitrogen-containing functional groups weren’t damaged during the modification process.

To obtain further information about the surface chemistry of the samples, the surface elemental composition of the samples was investigated by XPS (Table 2, Fig. 1). Many researchers have reported the action mechanism of nitrogen-containing functional groups for improving electrochemical performance (Nolan et al. 2013; Shang et al. 2014; Ferrero et al. 2015; Shang et al. 2015; Wang et al. 2016; Gao et al. 2016). The nitrogen species of the carbon matrix include pyridinic (N-6), pyrrolic/pyridone (N-5), quaternary nitrogen (N-Q), and pyridine-N-oxide (N-X). The relative contributions of each nitrogen species to the total peak area are summarized in Table 2. According to a previous work by the authors (Shang et al. 2014), there are three nitrogen species (N-6, N-5, N-X) in the activated carbon derived from waste medium density fiberboard. Thus, the chemical state of nitrogen could be sensitively varied by the carbonization temperature and activation temperature. According to the analysis the optimum experimental conditions are 500 °C for carbonization temperature and 800 °C for activation temperature. After modification, the photoemission spectrum of AC10 did not show the obvious intensity change of C1s and O1s compared with the AC sample, because the tunable SAC samples were obtained by controlling the dosage of sublimed sulfur. Table 2 reveals the existence of three types of sulfur functionalities at the binding energies of S2- (sulfide) (161.4 to 161.6 eV), thiophene (163.9 to 164.5 eV), and sulfone (168.0 to 168.9 eV). Eventually, a small amount of sulfur in the AC sample formed the main sulfur species: –C–S–C– thiophene and –C–S(O)2–C– sulfone bridges (Fig. 2). According to the XPS analysis, sulfur atoms in the SAC samples were incorporated into the carbon framework through the formation of C-S and C-SO2-C bonds to produce similar thiophene and sulfone bridge structures. The increased capacitance of the SAC samples was primarily attributed to the thiophene functional groups (Huang et al. 2014; Zhou et al. 2014). This type of structure is characterized by a lone electron pair among the sulfur atom p-orbital overlapping with π-orbitals of the graphite sp2 hybridization, forming an extended π-system with a filled valence band that is conjugated with the backbone carbon structure. Surface electron density of SAC samples was higher because of the synergistic activation of conjugated carbon in combination with the electron-rich sulfur groups, thus increasing of dipole moment. According to the “like-dissolves-like” theory, the total polarization of the medium would enlarge as a result of the enhanced dipole moment, which could facilitate the penetration of polar substrate into the pores (Zhao et al. 2012). Additionally, sulfone bridges, resulting from the oxidative conditions, could also participate in faradic reactions and improve the specific capacitance of carbon.

Fig. 1. XPS curves (XPS full spectrum scanning) (a) AC and (b) AC10

Fig. 2. Chemical structure of sulfur-containing functional groups

Table 1. Elemental Analysis of the Samples

Table 2. Detailed XPS Analysis of the Samples

Textural Studies of Samples

Figure 3 shows FESEM and TEM images of AC and AC10 samples. The surface morphologies of AC and AC10 were similar, and the AC10 sample retained its porous structure, even after the modification with sulfur. Honeycomb structures resulting from the chemical activation process are visible in the FESEM images. Continuous pores structure can be seen from TEM images; the darker regions represent the carbon frame and the pores are shown as white areas. The results of FESEM and TEM analysis reveal that both mesopores and micropores were present in the activated carbon.

Porous structures in the samples were determined by BET analysis (Fig. 4). The surface area and pore volume, calculated according to the DFT (Vdif) method, are listed in Table 3. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, all isotherms presented a combination of type-I and type-II isotherms, which indicate that all of the samples possessed a combination of microporous and mesoporous structure. In addition, Fig. 4 shows a slight hysteresis loop (type IV) in the desorption branch, at a relative pressures of approximately 0.9 units (P/P0). This may indicate the presence of mesopores. A similar character could be concluded from the pore size distribution curves in Fig. 5. Apparently, the AC sample had a total surface area of 2081 m2/g, and after coating with sulfur, the total surface area showed an obvious decrease for all four SAC samples. Meanwhile, both the mesopore and micropore surface areas were decreased by differing amounts. These reductions could have resulted from the effect that the sulfur coating had on the surface of the pores. The sulfur coating could have penetrated the mesopores and micropores, causing blockages in the pore structure, which resulted in the notable decrease in the SBET and adsorption property.

Fig. 3. FESEM of the (a) AC and (b) AC10; TEM images of (c) AC and (d) SAC

Fig. 4. Nitrogen adsorption–desorption isotherms of the samples

Fig. 5. Pore size distribution of the prepared activated carbons

Table 3. Physical Properties of the Samples

Electrochemical Characteristics

The samples were investigated further as the electrodes of supercapacitors. A test of electrochemical performance was performed in a 7.0 mol/L KOH electrolyte. The galvanostatic charge–discharge, specific capacitance at different current densities, cyclic voltammetries (CV), and alternating current impedances were studied in order to determine the effect of sulfur loading on electrodes, as shown in Figs. 6 through 9. The galvanostatic charge-discharge curves of the AC and SAC electrodes, at a current density of 50 mA/g, are presented in Fig. 6. The shape of the curves for all of the samples was triangular, which confirmed that the ideal capacitive behavior was nearly symmetric for the charge/discharge curves. The corresponding specific capacitance value, at a current density of 50 mA/g, was calculated according to the following equation,


where Cm is the specific capacitance per mass weight of activated carbon in the electrode (F/g), Iis the discharge current (A), Δt is the time elapsed for the discharge branch from 0 to 1 V (s), Δvis the voltage difference within the time (V), and m is the active mass of carbon on the electrode (g).

The specific capacitance of the samples varied from 245 to 264 F/g (AC: 245 F/g, AC12: 259 F/g, AC10: 264 F/g, AC8: 255 F/g, and AC6: 252 F/g) under a current density of 50 mA/g. Heteroatom doping could have induced pseudo-capacitance, thus enhancing the entire capacitance of the carbon materials. The contribution of S-containing groups to pseudo-capacitance could be concluded from Tables 1 and 2. Generally, as the specific surface area of the substrate increased, the capacitance also increased. However, compared with AC and AC12, which possess larger specific surface areas, sample AC10 exhibited a greater capacitance because of the larger degree of pseudo-capacitance. Although samples AC8 and AC6 possessed higher sulfur content, the specific capacitance of these samples was lower than AC10 because of the smaller specific surface area. Compared with the AC sample, SAC samples exhibited higher specific capacitance after sulfur modification, and sample AC10 exhibited the highest specific capacitance of 264 F/g. According to the specific surface area and pore size distribution of the samples shown in Table 3, when the sulfur content reached 1.64% (Table 2), the contribution to pseudo-capacitance of sulfur is optimal and the sulfur function dominates the specific capacitance over the effect of the pore structure. Consequently, the synergistic effect of pseudo-capacitance and pore structure reached an optimum level when the carbon-sulfur ratio was 10:1, which was also the highest specific capacitance. Because of this result, sulfur-doping exhibited a positive effect on the enhancement of capacitance.

Figure 7 illustrates the specific capacitance of electrodes at various current densities ranging from 50 mA to 5 A/g within a potential window from 0 to 1 V. The sudden potential drop at the beginning of the constant current discharge (IR drop) could be attributed to the resistance of electrolyte solution and the inner resistance of ion diffusion (Shang et al. 2014). Cyclic voltammetry (CV) testing (Fig. 8) showed rectangular curves from 0 to 1.0 V, at a scanning rate of 1.0 mV/s to 200 mV/s, respectively. The CV curves presented nearly rectangular shapes at different scan rates, which suggests good capacitive devices. The obvious hump during the sweep at 0.8 to 1.0 V was observed for both the AC and AC10 samples; this is evidence that complex reversible redox reactions occurred and overlapped, which is usually attributed to the involvement of sulfur and nitrogen functional groups (Tsubota et al. 2011). It has been widely accepted that heteroatom-containing groups have a positive effect on the electrochemical performance. Compared with the AC sample, the pseudo-faradic reactions of AC10 were more obvious from the hump in the CV curves. Therefore, apart from surface area and nitrogen functional groups, sulfur functional groups contribute greatly to the enhancement of capacitance.

Electrochemical impedance spectroscopy (EIS), also known as the Nyquist plot, can provide insight into how sulfur modification affects the performance of a sample. The Nyquist plots of AC and AC10 samples are shown in Fig. 9. The straight line in the low-frequency region and the small arc in the high-frequency region depict typical behaviors of supercapacitors. The impedance plots of both samples show almost identical shapes, with one semicircular arc in the very-high-frequency region and one linear part in the low-frequency region. The intercept at the real axis represents the equivalent series resistance (ESR) value at high frequencies, and the slope at the low-frequency region reflects the diffusive resistivity of the electrolyte ions within the pores. The straight line portion of AC10 close to the imaginary axis (y-axis) suggests that AC10 has better capacitive behavior than AC. The semicircular arc of AC10 is slightly decreased in size compared with that of AC, and the ESR values of AC10 and AC are approximately 0.16 and 0.24, respectively, revealing a decrease in the charge-transfer resistance.

Fig. 6. Charge-discharge curves of electrodes in 7.0 M KOH at a constant current density of 50 mA/g

Fig. 7. Calculated specific capacitance as a function of current density of electrodes

Fig. 8. Cyclic voltammograms of (a) AC10 and (b) AC electrodes in 7.0 M KOH at different scanning rates

Fig. 9. Nyquist plot of AC and AC10 electrodes (inset: enlarged high-frequency region of the Nyquist plot)


  1. This work disclosed a simple and efficient method for the production of N-, S-co-doped mesoporous and microporous carbons. The specific surface area of SAC samples showed an obvious reduction compared with the AC sample; however, the modification of nitrogen-containing activated carbon with sulfur was shown to increase the capacitance of electrodes for supercapacitor applications, deducted from the electrochemical measurements.
  2. The XPS test demonstrated that the enhancement in the electrochemical property for the N-, S-co-doped carbon materials was mainly attributable to the thiophene and sulfone sulfur-containing groups. The SAC electrode exhibited the highest specific capacitance of 264 F/g at a current density of 50 mA/g, when the ratio of C:S was 10:1.
  3. Nitrogen-, S-co-doped activated carbon from waste MDF is a good choice as a super-capacitor because it is low-cost and eco-friendly. The present work contributed valuable information towards the development of N-, S-co-doped carbon with high specific surface areas and excellent electrochemical performance properties from bio-waste materials.


The authors are greatly indebted to the National Natural Science Funding under Project: 51572028


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Article submitted: January 9, 2016; Pee review completed: February 19, 2016; Revisions accepted: March 9, 2016; Published: March 17, 2016.

DOI: 10.15376/biores.11.2.4055-4068