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DOI: 10.1039/C4RA05881J (Paper) RSC Adv., 2014, 4, 39037-39044

Easy procedure to prepare nitrogen-containing activated carbons for supercapacitors

Tong-Xin Shang, Ming-Yang Zhang and Xiao-Juan Jin*
MOE Key Laboratory of Wooden Material Science and Application, 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. E-mail: jxj0322@163.com; Tel: +8613718160441

Received 18th June 2014 , Accepted 7th August 2014

First published on 7th August 2014

Abstract

Electrodes for electrochemical measurements have been fabricated using activated carbons (AC) prepared by waste medium density fiberboard containing 12% of urea-formaldehyde resin adhesive. The AC samples were activated by KOH at activation temperatures ranging from 700 to 900 °C (AC700–AC900) with an activation time of 60 minutes. As the elemental analysis show, the nitrogen content of all the AC samples decreased from 2.42% to 1.68% on increasing the activation temperature. The surface areas of the AC samples are in the range of 1308–1598 m2 g−1 with higher microspore volume between 0.5 and 2.0 nm. In addition, as shown by galvanostatic charge–discharge curves, cyclic voltammetry and alternating current impedance measurements, AC electrodes show superior capacitive performances. The electrochemical double layer capacitor was measured in a 7 M KOH electrolyte, which showed that the specific capacitances of AC700–AC900 varied from 176–232 F g−1 under a current density of 50 mA g−1. The AC800 exhibited the best electrochemical behavior with a specific gravimetric capacitance of 232 F g−1 and rectangular cyclic voltammetry curves at a scan rate of 2 mV s−1, which remained at 211 F g−1 even at a current density of 10 A g−1.

1. Introduction

The idea of developing supercapacitors has attracted the attention of researchers worldwide due to their numerous applications as power storage devices for electric vehicles, electronic appliances and memory back-up systems in computers. Supercapacitors have resulted in a new technical innovation that is unique and can combine the energy properties of batteries with the power discharge characteristics of capacitors.1 We can classify the supercapacitors into two types according to their energy storage mechanism, viz., redox supercapacitors and electrochemical double layer capacitors (EDLCs). EDLCs have emerged as a promising energy storage device for applications that require high power along with remarkable storage and cycle life.2 Because energy storage arises mainly from the accumulation of electronic and ionic charges at the interface between electrode materials and electrolyte solution in the EDLCs, the porosity and surface area of electrode active materials become the major factors that influence the specific capacitance of supercapacitors.3 Porous carbon materials have been widely applied in many fields such as catalyst supports,4 absorbents for bulky molecules,5 biomedical devices,6 and electrode materials.7 Specifically, they can be directly used for electrode materials of electric double-layer capacitors8 because of their high surface areas, large pore volumes, excellent conductivities and suitable pore sizes and textures, as well as chemical stability, which can significantly improve the specific capacity, rate capability, and life cycle of energy storage systems.

However, the specific capacitance of activated carbon is significantly lower than theoretically expected, resulting in EDLCs with lower energy densities than predicted, which considerably hinders their practical application. Previous research has shown that9 higher capacitance is not only attributed to higher specific surface area, but is also affected by the chemical surface composition of carbon materials. The energy storage of EDLCs can be enhanced by enriching the surface of carbon materials with heteroatoms such as oxygen or nitrogen.10–12 These heteroatoms modify the electron donor/acceptor properties of the carbon surface and are consequently expected to affect the charging of the electrical double-layer and yield pseudo-capacitance Faradaic reactions.13 Various surface modification methods have been investigated to introduce heteroatoms onto the surfaces of carbon materials, for example, chemical, plasma and flame treatments, coronal discharge, and direct fluorination.14–18 Recently, nitrogen containing activated carbons (ACs) are of particular interest for researchers because they have shown a high versatility and efficiency in the processing of waste products (in both gas and liquid phase, such as CO2, NOx, H2S, SO2, CH4 and in a wide range of organic compounds and heavy metals).19–26 Furthermore, they obtain excellent complexants and heterogeneous catalysts27,28 and prepare efficient capacitor materials.29,30 The methodologies most frequently used for the introduction of nitrogen functionalities to the structure of ACs are: (a) by carbonization and further activation of polymer containing nitrogen in their structure,31 and (b) by thermal treatment of carbon materials in the presence of nitrogen supplying agents such as ammonia, urea, melamine or nitrogen oxides.32–34 In both the cases, the incorporated nitrogen can be found as pyridines, pyrrolidines, lactams, imides, amines or nitriles depending on the procedure. Moreover, a third procedure, consisting the impregnation of the ACs with polyamines (e.g. polypropylenimine,35 polyamidoamine35 or polyethyleneimine (PEI)36,37), which are nitrogenated molecules with a well-defined structure, has also been used. Although some of these methodologies can fix high nitrogen contents, all of them have important drawbacks. In particular, it is worth mentioning the requirement of high temperature and/or high pressure and the difficulty in understanding and controlling the reaction mechanisms.

Here, we describe the significance of a novel nitrogen rich carbon material, ‘waste medium density fiberboard’ (MDF) rejected by the furniture industry for supercapacitors. The use of medium density fiberboard has rapidly increased in the past decade, replacing more and more solid wood lumber and plywood products. Since its first appearance in the 1960s, MDF has been steadily gaining market share worldwide. Meanwhile, in the international market, the MDF output of China is on the increase, reaching a record high of 50.23 million m3 in 2012, according to the available data.38 In 2001, the MDF output was 5.27 million m3, which implies that the MDF output increased 8.53 times during those eleven years. MDF is a wood-based composite material used extensively in furniture production. With the rapid development in the production and usage of MDF, recycling of the waste fiber needs to be addressed. The sources of MDF byproducts include trimmings, dust generated by the sanding of the panels and off-specification boards produced at a production facility, residues produced during remanufacturing and material removed from service at the end of its life-cycle. The traditional methods to manage byproducts from wood-based composites are by burning and landfilling. In China, waste MDF is usually burned to obtain heat energy, which could be considered a wastage of resources, and more importantly, a poisonous gas containing nitrogen is discharged in the atmosphere. Therefore, the development of methods for reusing the waste MDF materials is highly desired. The nitrogen atoms of the waste MDF originates from urea-formaldehyde resin adhesive used in the MDF manufacturing process, and some of the nitrogen is further converted to waste MDF-based ACs. The utilization of waste MDF can significantly reduce the environmental impact and afford attractive products. To the best of our knowledge, there is little literature on the synthesis of such an activated carbon material for supercapacitors.

In this paper, we report an efficient and straightforward methodology directed to use waste MDF to prepare nitrogen containing activated carbons with high specific surface area. This material is considered to be a novel nitrogen rich carbon material for high-performance electrode preparation. As it is known, MDF is a wood-based material prepared from lignocellulosic fibers bonded together by synthetic resin (usually urea-formaldehyde) under heat and pressure. MDF usually contains 10–12% of urea-formaldehyde resin adhesive, which is enriched by nitrogen. After the carbonization and activation processes, nitrogen is heat stabilized and reserved. The study has been focused on the procedure to improve the capacitive performance of all the carbon samples, remarking the effect of activation temperature on the electronic performance of MDF-based activated carbons. In addition, several measurements were introduced to describe the characterization of MDF-based activated carbons and carbon electrodes.

2. Material and methods

2.1 Materials

The waste MDF was kindly provided by Beijing Jiahekailai Furniture and Design Company, which was obtained during the furniture manufacturing process containing 12% of urea-formaldehyde resin adhesive. Other chemicals were of analytical grade and were purchased from Beijing Lanyi Chemical reagent. Double distilled water was used for the preparation of all the required solutions.

2.2 Preparation of activated materials

The carbonization process was carried out in a high-purity nitrogen atmosphere at a temperature increase rate of 10 °C min−1 up to a final temperature of 500 °C, which was maintained for 60 min. The samples were then ground and screened out with sieves. The fraction with a particle diameter ranging from 40 to 60 meshes was selected. The samples were dried at 105 °C in an oven for 8 h. For the activation step, 3 grams of the oven-dried samples were soaked in a 50% KOH solution for 16 h at a mass ratio impregnation of (3[thin space (1/6-em)]:[thin space (1/6-em)]1). The samples were then activated at the temperatures of 700, 750, 800, 850, 900 °C for 60 min in a nitrogen atmosphere. The obtained activated carbons (AC700, AC750, AC800, AC850 and AC900) were boiled first with 1 M HCl solution and then with distilled water, until the pH of the solution reached about 6–7. Finally, these activated carbons were dried at 105 °C in an oven for 8 h.

2.3 Characterization of the activated carbon

2.3.1 Chemical surface composition. Chemical surface composition and the state of the samples were determined by elemental analysis and X-ray photoelectron spectroscopy (XPS). (i) The elemental analysis (contents of carbon, hydrogen, and nitrogen) of the activated carbons was determined by a CHNS Analyzer (Thermofinnigan Flash, EA, 1112 series). (ii) XPS was performed on an ESCALAB250 (VG Scientific, UK) using monochromatic Al Kα radiation. The acceleration tension and power of X-ray source were 15 kV and 100 W, respectively. Sample charging was corrected using the C1s peak (284.6 eV) as an internal standard. The surface atomic ratios were calculated from the ratio of the corresponding peak areas after correction with the theoretical sensitivity factors based on the Scofield's photoionization cross-sections.
2.3.2 Porous texture. The N2 adsorption–desorption isotherms of activated carbon prepared under optimum conditions were measured with an accelerated surface area in a porosimetry system (ASAP 2010, Micromeritics) for determining the surface areas. Prior to the measurements, the samples were outgassed at 573 K under nitrogen flow for at least 2 h. The nitrogen adsorption–desorption data were recorded at a liquid nitrogen temperature of 77 K. The nitrogen adsorption isotherm was measured over a relative pressure (p/p0) range from approximately 10−6 to 1. The Brunauer–Emmett–Teller (BET) surface area was calculated using the BET equation from the selected N2 adsorption data within a range of relative pressure, p/p0, from 0.1 to 0.3. Pore size distribution in the micropore range was obtained by the Barrett–Joyner–Halenda (BJH) method.

2.4 Electrode preparation and electrochemical measurements

2.4.1 Electrode preparation. The dried activated carbon samples, including AC700, AC750, AC800, AC850 and AC900, were grinded in an agate mortar. Electrodes for electrochemical measurements were fabricated by mixing the sample with acetylene black and 60% polytetrafluoroethylene in a mass ratio of 87[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]3, and dispersed in C2H5OH aqueous solution forming a homogeneous slurry. The slurry was then pressed into the nickel foam (square, about 1 cm2) under a pressure of 20 MPa with a nickel tape for connection to one disk.
2.4.2 Electrochemical measurements. The capacitive performance of all the carbon samples was investigated in 7 M KOH using two-electrode cells. The electrodes were dried at 105 °C in an oven for 8 h and then weighted. Two electrodes with identical weight were selected for the measurements. Cyclic voltammetry, galvanostatic charge–discharge and alternating current impedance were used in the evaluation of the capacitive performance. Constant current density charge–discharge and rate performance were tested using the BT2000 battery testing system (Arbin Instruments, USA) at room temperature. Moreover, for the charge–discharge, the specific capacitance of the electrode can also be calculated according to the following equation:
 
image file: c4ra05881j-t1.tif(1)
where Cm is the specific capacitance per mass weight of activated carbon in the electrode (F g−1), I is the discharge current (A), Δt is the time elapsed for the discharge branch from 0 to 1 V (s), ΔV is the voltage difference within the time (V) and m is the active mass of carbon on the electrode (g).

Cyclic voltammetry (CV) and alternating current impedance were employed for the electrochemical measurements of each sample using the 1260 electrochemical workstation (Solartron Metrology, UK) at room temperature. The specific capacitance per weight of activated carbon in the electrode, that is the gravimetric capacitance (Cm), was analyzed. Cm is expressed in F g−1 and calculated by the following two formulas:

 
image file: c4ra05881j-t2.tif(2)
 
image file: c4ra05881j-t3.tif(3)
where I is the observed value (A), m is average weight of each activated carbon disk (g) and v is the voltage scan rate (mV s−1).

3. Results and discussion

3.1 Elemental analysis

Because the surface features are considered to be very important for the performance of carbonaceous materials as supercapacitors, the changes in the texture and chemistry of the composites are first analyzed to link them to electrochemical performance.

Extensive studies on the chemistry of nitrogen modified carbons revealed the dependence of surface functionality on the pyrolysis temperature.39–42 The pyrolyzed carbon is expected to have nitrogen mostly in the form of amine, amide, and ammonium.43 During pyrolysis, these groups decompose, and consequently some nitrogen is released while some is incorporated into the carbon matrix. Table 1 shows the elemental analysis of the raw material C0, carbonized sample C500 and AC samples. It shows that the initial material C0 contains some nitrogen functionalities resulting from the decomposition of the urea-formaldehyde resin adhesive used for its preparation. After activation, it can be seen that as the temperature increases, there is a sharp increase in the presence of C and a decrease in H and N contents. Nitrogen is present in all the AC samples, ranging typically from 1.68% to 2.42%, which suggest that 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.

Table 1 Elemental analysis of the raw material, carbonization and AC samples
Sample CHN (wt%)
N (%) C (%) H (%)
C0 8.39 45.16 5.76
C500 6.81 74.50 2.69
AC700 2.42 63.27 1.67
AC750 2.35 74.87 1.47
AC800 2.09 85.96 1.73
AC850 1.89 88.84 1.36
AC900 1.68 90.25 0.84

3.2 XPS study

To further understand the surface chemistry of the AC samples, the nature of N species at the surface of the AC samples was investigated by XPS measurements. The N1s XPS spectra of AC samples are shown in Fig. 1. According to the literature,31,32,44–49 the chemical state of nitrogen atoms in graphene layers could be assigned as four types: N-6 (pyridinic nitrogen that bonds with two C atoms with one p-electron localized in the π conjugated system; 398.7 ± 0.3 eV), N-5/N-P (pyrrolic nitrogen that bonds with two C atoms with two p-electrons/pyridinic nitrogen in association with oxygen functionality marked pyridine; 400.3 ± 0.3 eV), N-Q (quaternary nitrogen, nitrogen substituted with carbons in the aromatic grapheme structure, 401.4 ± 0.4 eV), N-X and oxidized nitrogen (402–405 eV). Except for the N-Q, all the nitrogen functionalities are located at the edge of the graphene structure, as shown in Fig. 1. The relative contributions of each nitrogen species to the total peak area are summarized in Table 2. The results indicate that the chemical state of nitrogen could be sensitively varied by the carbonization temperature; on increasing the temperature, there is an increase in pyridinic nitrogen and oxidized nitrogen and a decrease in pyrrolic nitrogen and pyridone. J R Pels et al.41 also drew the same conclusion and our conclusion corresponds to the analysis of Claudia Weidenthaler et al.50 According to the analysis, under mild pyrolysis conditions the cyclization and aromatization, as schematically shown in Fig. 2, is not completed, although it already occurred partially. Meanwhile, N-X species are also present but only in low amounts. As the temperature increases, the aromatic extends its presence and the total amount of nitrogen is slightly reduced but a large fraction of N-5 nitrogen is transformed into N-X. Under severe pyrolysis conditions, N-6 and N-X increased as N-5 decreased. For a small part, it is released in the volatiles but the majority is converted to N-6 and N-X with the latter being the major constituent.
image file: c4ra05881j-f1.tif
Fig. 1 Fitted high-resolution XPS N1s spectra of the AC samples.
Table 2 Distribution of N species obtained from the deconvolution of the N1s peaks
Sample N-6, pyridinic nitrogen (%) (398.7 ± 0.3 eV) N-5/N-P, pyrrolic nitrogen, pyridone (%) (400.3 ± 0.3 eV) N-Q, quaternary nitrogen (%) (401.4 ± 0.4 eV) N-X, oxidized nitrogen (%) (402–405 eV)
AC700 36.86 55.67 — 8.47
AC750 45.41 46.50 — 8.09
AC800 46.52 41.69 — 11.79
AC850 48.40 39.56 — 12.03
AC900 49.06 38.24 — 12.70


image file: c4ra05881j-f2.tif
Fig. 2 Scheme of the graphitization of carbon materials.

3.3 Textural studies of ACs

The N2 adsorption–desorption isotherms, as shown in Fig. 3, are used to determine the surface area and pore-size distribution of the ACs. The isotherms of all the AC samples resemble a combination of type-I and type-II isotherms, which is in accordance with the International Union of Pure and Applied Chemistry (IUPAC) classification. This adsorption behaviour exhibits a combination of microporous-mesoporous structure. Meanwhile, the isotherm shows an apparent hysteresis loop (H4 type) in the desorption branch at relative pressures above 0.8, indicating the presence of mesopores.51 The hysteresis loop is usually associated with the filling and emptying of the mesopores by capillary condensation.52 When activation temperature is increased from 700 to 850 °C, adsorption increased, and when the activation temperature is increased from 850 to 900 °C, the adsorption decreased. To the best of our knowledge, higher activation temperature are often followed by faster reactions, leading to a larger BET specific surface area, and thus contributes to larger nitrogen adsorption capacities. However, when the activation temperature reached 850 °C, the nitrogen adsorption capacity decreased. This probably implies that when the activation temperature reaches a certain value, the pores can be widened and burnt off such that the BET specific surface area decreases. As shown in Fig. 4, the curve of AC850 is located on the top, which indicates that it has the highest microspore and mesopore volume. Table 3 shows the parameters of the porous texture of ACs calculated from the isotherms. As observed in Table 3, the AC850 has the biggest BET surface area (1598 cm3 g−1) and the biggest total volume (0.862 cm3 g−1), which is in agreement with the results of Fig. 4, where the pore size distribution of ACs is shown. It is known that micropores are less than 2 nm wide, mesopores are between 2 and 50 nm wide, and macropores are more than 50 nm wide. As can be seen in Fig. 4, the sharpest peaks occurred when the pore diameter was above 1.0 nm, showing that a majority of the pores fall into the range of micropores. Furthermore, Fig. 4 shows that the pore size of all the prepared samples included a small amount of mesopores, which corresponds to the analysis results of the N2 adsorption–desorption isotherms in Fig. 3.
image file: c4ra05881j-f3.tif
Fig. 3 Nitrogen adsorption–desorption isotherms of the prepared activated carbons by KOH activation.

image file: c4ra05881j-f4.tif
Fig. 4 Pore size distributions of the prepared activated carbons.
Table 3 Physical properties of the ACs
Sample SBET (m2 g−1) Smi (m2 g−1) Vtot (cm3 g−1) Vmi (cm3 g−1) Sme (%) Vme (%)
AC700 1308 1255 0.586 0.505 4.10 13.81
AC750 1505 1443 0.679 0.588 4.14 13.43
AC800 1586 1499 0.795 0.688 5.47 13.40
AC850 1598 1508 0.862 0.737 5.66 14.49
AC900 1424 1334 0.784 0.663 6.35 15.47

3.4 Electrochemical characteristics

The galvanostatic charge–discharge curves of the AC electrodes at a current density of 50 mA g−1 are presented in Fig. 5. All the charge–discharge curves exhibit triangular shapes, with low IR drops, indicating typical capacitive behavior.53 Moreover, it is observed that the AC samples display straight discharge slopes, indicating excellent discharge capabilities and highly stability and reversibility features. Among the AC samples with different activation temperatures, AC800 exhibited the best capacitive behavior due to its big BET surface area and the high microspore and mesopore volume, which is beneficial to the access of the electrolyte into the pore volume, and consequently improves ionic transport in EDLCs.54 In addition, the corresponding specific capacitance values at a current density of 50 mA g−1 were calculated by eqn (1). The specific capacitance of the AC700–AC900 varied from 176–232 F g−1 (AC700: 199 F g−1, AC750: 214 F g−1, AC800: 232 F g−1, AC850: 221 F g−1, AC900: 176 F g−1) under this current density. Comparing AC850 with AC800, AC850 has the highest specific surface area and the largest amount of mesopores but it presents the lowest capacitance, which is probably due to AC800 having higher nitrogen content (2.09%) than AC850 (1.89%). Moreover, the specific surface area of AC850 is considerably bigger than that of AC750; however, the two samples exhibited similar capacitive behavior. The results also indicate that a higher capacitance is not only related to a higher specific surface area and a larger amount of mesopores, but also to the existence of nitrogen. The presence of mesopores can enhance the utilization of the exposed surface for charge separation and provide resistance pathways for the ions through the porous particles,9 but it cannot enhance the capacitance infinitely. At this point, the existence of nitrogen provides a significant advantage.
image file: c4ra05881j-f5.tif
Fig. 5 Charge–discharge curves of AC electrodes in 7 M KOH at a constant current density of 50 mA g−1.

The galvanostatic charge–discharge measurements were carried out to calculate the specific capacitances of all the AC samples at various current densities ranging from 0 to 10 A g−1 within a potential window from 0 to 1 V. The results are shown in Fig. 6. An obvious and sudden potential drop at the beginning of the constant current discharge is usually observed for EDLCs. This drop has been designated to be the IR drop, which can be attributed to the resistance of electrolyte solution and the inner resistance of ion diffusion in carbon micropores.9,55 It can be seen from Fig. 6 that capacitance decreased with current density to a similar extent in all the samples. However, the IR drop of the sample AC750 is slightly smaller than that one of AC850 due to its high nitrogen content. An opposite effect is observed for the carbon content, for which about 10% decrease in the capacitance is found. This can be related to a reduced pseudo-capacitive contribution as a result of the removal of nitrogen containing groups. In addition, in the case of AC800 and AC850 samples, the one with the highest capacitance was not the sample with the highest BET surface area. Therefore, other factors, apart from surface area, must contribute to the enhancement of capacitance.56 The nitrogen groups in the AC samples can improve the performance of the capacitance.


image file: c4ra05881j-f6.tif
Fig. 6 Calculated specific capacitance as a function of current density of AC electrodes.

Because of AC800 presenting more attractive features, such as highly interconnected porosity and higher surface area, as well as higher specific capacitance, cyclic voltammogram tests were performed from 2 to 100 mV s−1 between 0 and 1 V, to gain a qualitative understanding of the influence of pore structure on the rate dependence of the charge–discharge behavior. At 2, 10, 20, 50 and 100 mV s−1, the curves present quasi rectangular shapes for the charge–discharge processes. At 100 mV s−1, which is the highest scan rate, a rectangular shape with only slight deviation from the ideal rectangular shape can be observed. Cyclic voltammetry in a two-electrode configuration is an excellent technique for studying the presence of pseudo-capacitive phenomena. Materials with pseudo-capacitance show redox peaks related to electron-transfer reactions. The voltammograms in Fig. 7 show the redox processes that reflect the contribution of nitrogen groups in AC samples. In addition, a small hump during the sweep at 0.8–1.0 V was clearly observed for the AC800 sample, which is usually attributed to pseudo-faradaic reactions involving the quinone functional groups. The nitrogen functional groups, especially the pyrrolic and pyridinic nitrogen have been reported to be electrochemically active in the pseudo-faradaic reactions. It is believed that this kind of behavior is additionally enforced by the electron-donating effect of nitrogen heteroatoms. It also suggests that AC800 can be an excellent candidate electrode material for supercapacitor.


image file: c4ra05881j-f7.tif
Fig. 7 Cyclic voltammograms of AC800 electrode in 7 M KOH at different scan rates.

Electrochemical impedance spectroscopy (EIS) was utilized to obtain information regarding the supercapacitor performance, as shown in Fig. 8. Their frequency dependence and equivalent series resistance (ESR)57 illustrate the Nyquist plots of the AC800 super capacitors. A semicircle of very small radius was obtained at the high-frequency region and a straight line in the low frequency region. At very high frequencies, the intercept at the real axis is the ESR value; the ESR value of AC800 was about 0.21. The imaginary part of the impedance spectra at low frequencies represents the capacitive behavior of the electrode and approaches a 90° vertical line in an ideal capacitor. The straight line part of carbon AC800 is obviously closer to the vertical line along the imaginary axis, suggesting that AC800 has a good capacitive behavior. The AC800 sample showed a semicircle in the mid-high frequency zone, which is related to the high intrinsic electrical resistance and faradaic reactions. This resistance results in a high kinetic dependence upon faradaic phenomena with current density. This may be attributed to pseudo-faradaic reactions involving the quinone functional groups and nitrogen functionalities.


image file: c4ra05881j-f8.tif
Fig. 8 Nyquist plot of AC800 electrodes (inset: enlarged high-frequency region of Nyquist plot).

4. Conclusion

N-enriched electrode materials for supercapacitors were prepared from medium density fiberboard base by KOH activation. The effects of physical properties and chemical composition of waste MDF-based ACs on electrochemical performance were explored. The porosity characterization results showed that all the ACs produced are essentially microporous. ACs with a higher surface area and mesopore volume were produced when the activation temperature was 850 °C, then the better activation temperature was 800 °C. Including the influence of nitrogen content, the activated carbon AC800 exhibited the best electrochemical behavior with rectangular cyclic voltammetry curves at a scan rate of 2 mV s−1, and a specific gravimetric capacitance of 232 F g−1 at constant current densities of 50 mA g−1, which remained at 211 F g−1 even at a current density of 10 A g−1. By correlating the capacitive behavior with textural characteristics, the good electrochemical properties were attributed to the high surface area, wide pore size distribution and nitrogen functionalities. Therefore, the N-containing AC based on medium density fiberboard is a simple and efficient way to enhance the performance of AC-based EDLCs.

Acknowledgements

This study was funded by State Forestry Administration under Project 201204807: the study on the technology and mechanism of the activated carbon electrode preparation from waste hard board.

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