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Nitrogen-enriched waste medium density fiberboard-based activated carbons as materials for supercapacitors
Industrial Crops and Products 43 (2013) 617–622
Contents lists available at SciVerse ScienceDirect
Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Nitrogen-enriched waste medium density fiberboard-based activated carbons as materials for supercapacitors Xiao-Juan Jin ∗ , Ming-Yang Zhang, Yu Wu, Ji Zhang, Jun Mu School of Material Science and Technology, Beijing Forestry University, 35 Qinghua East Road, Haidian District, 100083, PR China
a r t i c l e
i n f o
Article history: Received 18 April 2012 Received in revised form 5 August 2012 Accepted 8 August 2012 Keywords: Supercapacitors Waste medium density fiberboard Activated carbon
a b s t r a c t Nitrogen-enriched activated carbons were prepared from waste medium density fiberboard base using K2 CO3 as an activating agent for electrochemical capacitors. The effect of different temperatures of carbonization (300, 400, 500, and 600 ◦ C) and the influence of the sequence of activation process (a mass K2 CO3 /coke ratio of 3, temperature of 800 ◦ C and the activation time 1 h) on the electrochemical performance have been investigated. All of the samples were characterized in terms of surface chemical composition by elemental analysis and X-ray photoelectron spectroscopy, porosity by N2 sorption at 77 K, surface area by Brunauer–Emmett–Teller measurement. Electrochemical behavior as electric double-layer capacitors was determined using galvanostatic, voltammetric charge/discharge, specific capacitance values versus current densities, and impedance spectroscopy techniques in a 7 mol L−1 KOH aqueous solution. A maximum value of specific capacitance of 230 F g−1 was achieved for the sample carbonized at 400 ◦ C. The good electrochemical performance of the activated carbon was ascribed to high surface area, wide pore size distribution, and the presence of nitrogen functional groups. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Supercapacitors have attracted much attention because of their higher power density, better efficiency, and longer durability in comparison to the rechargeable batteries. Among various electrode candidates for supercapacitors, porous carbons are mainly investigated due to their advantages, such as good electrical conductivity, electrochemical stability, high surface area, large capacitance, long cycling life, and relatively low cost. However, a further advancement in the application of porous carbons, and more specifically activated carbons (ACs), in adsorption, catalysis or electrochemistry requires taking advantage of surface chemistry (Machnikowski et al., 2005; Radovic et al., 2001; Radovic, 1997). In the case of ACs, the chemistry is mostly related to the nature and concentration of heteroatoms which can be both substituted for carbon in the graphene layer and/or attached at the layer edges as functional groups, Among a variety of heteroatoms (O, N, S, P and Cl), Nitrogen has received appreciable attention. On one hand, the functional species of nitrogen improve wettability so enlarge the working area of microporous structures and, in consequence, increase the contribution of the capacitance related to the electrostatic
∗ Corresponding author. E-mail addresses: [email protected] (X.-J. Jin), [email protected] (M.-Y. Zhang), [email protected] (Y. Wu), [email protected] (J. Zhang), [email protected] (J. Mu). 0926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2012.08.006
interactions; on the other hand, the nitrogen groups can contribute pseudocapacitance for the carbonaceous materials to improve its electrochemical performance. The methods used for the synthesis of nitrogen containing porous carbons at a laboratory scale can be categorized into two groups. First, it is the treatment of conventional activated carbon at an elevated temperature with ammonia, ammonia–air or ammonia–steam mixtures (Boudou, 2003; Jansen and Bekkum, 1994; Mangun et al., 2001) and the surface chemistry is controlled by the reaction temperature. The second approach comprises the condensed phase pyrolysis of nitrogen containing polymers (Lahaye et al., 1999; Laszlo et al., 2001; Stanczyk et al., 1995) or co-pyrolysis of a suitable nitrogen carrier with a common carbon precursor, followed by the activation of the resultant char, as a result, thermally stable nitrogen atoms with different chemical states are present in the porous carbons. We here describe the significance of a novel nitrogen rich carbon material, ‘waste medium density fiberboard’ (MDF) rejected by the furniture factory for supercapacitors. The common method to treat the waste MDF is burning which is a kind of wasting of the resources, and more important, the poisonous gas containing nitrogen will be discharged in the atmosphere. Therefore, the development of methods for re-using the waste MDF materials is highly desired, and the production of activated carbon for supercapacitors from these wastes is an interesting possibility. The nitrogen atoms of the waste MDF originates from urea–formaldehyde resin adhesive used in the MDF manufacturing process, and some of nitrogen
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are further turned over to waste MDF-based ACs. The utilization of the waste MDF can significantly reduce the environmental impact and afford attractive products. To the best of our knowledge, synthesizing such an activated carbon material for supercapacitors in this way has not yet been reported. In present work, four kinds of waste MDF-based ACs were prepared using potassium carbonate (K2 CO3 ) activation. The aim of this work was to investigate the effect of carbonization temperature and activation process on the specific surface area, pore size distribution, and nitrogen functional groups and on the electrochemical performance of the ACs. 2. Materials and methods 2.1. Materials The waste MDF was kindly provided by Beijing Jiahekailai Furniture and Design Company, which was obtained in the furniture manufacturing process containing 12% urea–formaldehyde resin adhesive of the mass. Other chemicals were analytical grades and were purchased from Beijing Lanyi Chemical reagent. Double distilled water was used for preparation of all required solutions. 2.2. Preparation of activated materials Carbonization process was carried out in nitrogen atmosphere at the temperature increase rate of 10 ◦ C/min to the final temperature of 300 ◦ C, 400 ◦ C, 500 ◦ C, 600 ◦ C, separately, maintained for 1 h. The carbonization products were marked as C300, C400, C500, and C600. The samples were then ground and screened out with sieves. The fraction in the particle diameter ranged from 40 meshes to 60 meshes. The particle samples were dried in a 105 ◦ C oven for 8 h; in activation step, 3 g of the oven-dried samples were soaked in a 50% K2 CO3 solution for 16 h at the mass ratio impregnation (3:1). The samples were then activated at 800 ◦ C for 60 min in nitrogen atmosphere. The obtained activated carbons (AC300, AC400, AC500, and AC600) were boiled first with 1 M HCl solution and then with distilled water until the pH of solution reach to about 6–7. Finally, these activated carbons were dried at 105 ◦ C in an oven for 8 h. 3. Characterization of the activated carbon 3.1. Chemical surface composition Chemical surface composition and state of the samples were determined by X-ray photoelectron spectroscopy (XPS) and elemental analysis. (i) X-ray was performed on an ESCALAB250 (VGScientific, UK) using a monochromatic Al K␣ radiation. The acceleration tension and power of X-ray source were 15 kV and 100 W, respectively. The sample charging was corrected by 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. (ii) The elemental analysis (contents in carbon, hydrogen, and nitrogen) of the activated carbons was under taken in a CHNS Analyzer (Thermofinnigan Flash, EA, 1112 series).
Table 1 The elemental analysis of the raw material, carbonization and AC samples. Samples
N (%)
C (%)
H (%)
Raw material C300 C400 C500 C600 AC300 AC400 AC500 AC600
8.39 7.06 6.84 6.31 5.77 3.86 3.12 2.95 2.47
45.16 70.35 74.85 77.50 79.38 88.96 93.97 94.44 95.11
5.76 3.62 2.68 2.09 1.50 2.74 2.50 2.22 1.93
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. 4. Electrode preparation and electrochemical measurements 4.1. Electrode preparation The dried activated carbon samples including AC300, AC400, AC500, and AC600 were grinded in an agate mortar. Electrodes for the electrochemical measurements were fabricated by mixing the sample with acetylene black and 60% polytetrafluoroethylene in a mass ratio of 87:10:3. The mixtures were sandwiched by nickel foam (square, about 1 cm2 ) and pressed under a pressure of 20 MPa with a nickel tape for connection to one disk. 4.2. Electrochemical analysis Electrochemical characters were tested in 7.0 M KOH solution. Constant current density charge–discharge and rate performance were tested using the BT2000 battery testing system (Arbin Instruments, USA) at room temperature. 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 gravimetric capacitance (Cp) analyzed, which means the specific capacitance per mass weight activated carbon in the electrode, is expressed in F g−1 and calculated by the following formulas of (1) and (2): C=
Q t Q t I = × =I× = U t U U U/t
Cp =
C I/m I/m = = m v U/t
(1) (2)
where I is observed value (A), m is average weight of each activated carbon disk (g), v is the voltage scan rate (mV s−1 ). 5. Results and discussion
3.2. Porous texture
5.1. Elemental analysis
The N2 adsorption–desorption isotherms of activated carbon prepared under optimum conditions were measured with an accelerated surface area and porosimetry system (ASAP 2010, Micromeritics) for determining the surface areas. Prior to the measurements, the samples were outgassed at 573 K, under nitrogen
Table 1 shows the elemental analysis of the raw material, carbonized samples and AC samples, respectively. It is can be seen that as temperature increased, there is an increase in the element of C and a decrease in H and N contents. Nitrogen is present in all AC samples ranging typically from 2.47% to 3.86%, which suggest that
X.-J. Jin et al. / Industrial Crops and Products 43 (2013) 617–622
thermally stable nitrogen in the waste MDF maintained in the AC samples in the process of carbonization and activation. 5.2. XPS study Fig. 1 shows N1s XPS spectra of the carbonization and AC samples. According to the literatures (Bagreev et al., 2004; Jurewicz et al., 2003, 2004; Kapteijn et al., 1999; Kim et al., 2007; Lahaye et al., 1999; Machnikowski et al., 2004; Raymundo-pinero et al., 2004), the chemical state of nitrogen atoms in graphene layers could be assigned five types; N-6 (pyridinic nitrogen, 398.7 ± 0.3 eV), N-5 (pyrrolic nitrogen and pyridinic nitrogen in association with oxygen functionality, 400.3 ± 0.3 eV), N-Q (quaternary nitrogen, nitrogen substituted with carbons in the aromatic grapheme structure, 401.4 ± 0.5 eV), N-X and Oxidized nitrogen (402–405 eV). Except for the N-Q, all nitrogen functionalities are located at the edge of the graphene structure. The relative contributions of each nitrogen species to the total peak area are summarized in Table 2. The results indicated that the chemical state of nitrogen could be sensitively varied by the carbonization temperature and the activation process. 5.3. Textural studies of ACs The N2 adsorption–desorption isotherms, as shown in Fig. 2, which is used to determine the surface area and pore-size distribution of the ACs. All of the samples exhibit type-I isotherms according to the IUPAC classification. This is the characteristic of microporous solids. However, the small hysteresis loops observed on the adsorption–desorption isotherms were due to the existence of mesopores (Guo and Rockstraw, 2007). With increasing the temperature from 300 ◦ C to 600 ◦ C, the adsorption increased. It is probably because that pores became more when the temperature increased. The curve of AC400 was located in the top, which indicated that it has the highest microspore and mesopore volume. The parameters of the porous texture of ACs calculated from the isotherms are presented in Table 3. Table 3 showed that as the carbonization temperature increased from 300 ◦ C to 400 ◦ C, the BET surface area increased. When the carbonization temperature was 400 ◦ C, the maximum BET surface area could get 1199 m2 /g and total pore volume was 0.582 cm3 /g. When carbonization temperature increased from 400◦ C to 600 ◦ C, the BET surface area decreased. It is proved that textural properties of ACs are strongly depending on the carbonization temperature. The microspore and mesopore volume of AC400 are the highest in all samples. Fig. 3 shows the pore size distribution of the ACs. As we know, micropores are less than 2 nm wide, mesopores are 2–50 nm wide, and macropores are more than 50 nm wide. As can be seen from Fig. 3, the pore size of all the prepared samples included micropores and mesopores (Gyu and Park, 2008). 5.4. Electrochemical characteristics Galvanostatic charge/discharge measurements are commonly used to test the performance of capacitors. Fig. 4 shows galvanostatic charge/discharge curves of all the AC samples at a current density of 50 mA g−1 between 0 and 1.0 V in a 7.0 mol L−1 KOH electrolyte. A sudden potential drop at the very beginning of the constant current discharge is usually observed for EDLCs and this drop has been designated as the IR drop, which can be attributed to the resistance of electrolyte solution and the inner resistance of ion diffusion in carbon micropores (Conway, 1999; Zhang et al., 2010). The charge/discharge curves of all the samples approach a triangular shape reflecting good charge/discharge capacitive performance. However, the IR drop of the sample AC400 is slightly smaller that the other samples due to its wide pore size distribution.
Fig. 1. N1s XPS spectra of the carbonization and AC samples.
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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, Pyrollic nitrogen, pyridone (400.3 ± 0.3 eV)
N-Q, Quaternary nitrogen (401.4 ± 0.4 eV)
N-X, Oxidized nitrogen (402–405 eV)
A300 A400 A500 A600
37.79 48.46 49.31 46.15
48.03 47.00 45.84 47.22
14.17 3.04 – –
– 1.50 4.85 6.63
Table 3 Textural parameters of the ACs obtained by K2 CO3 activation with variable carbonization temperature. SBET (m2 /g)
Sample
804 1199 932 849
Sme (m2 /g)
Vtot (cm3 /g)
Vmi (cm3 /g)
Vme (cm3 /g)
Smi (%)
Vmi (%)
Sme (%)
Vme (%)
547 949 750 670
257 250 182 180
0.388 0.582 0.456 0.414
0.250 0.439 0.348 0.309
0.138 0.143 0.108 0.105
68.01 79.16 80.44 78.86
64.44 75.52 76.31 74.65
31.99 20.84 19.56 21.14
35.56 24.48 23.69 25.35
1.1
365
3
Volume Adsorbed(cm /g)
AC300 AC400 AC500 AC600
Smi (m2 /g)
1
325
0.9
285
AC400
AC500
0.7
AC600
1 65 0. 2
0
0. 4 0. 6 Relative pressure(P/P 0 )
0. 8
1
Voltage/V
AC300
205
0.5
0.3 0.2 0.1
The specific gravimetric capacitances were calculated from the galvanostatic charge/discharge process on the basis of Eq. (3). i × t m × v
0.6
0.4
Fig. 2. Nitrogen adsorption–desorption isotherms for prepared activated carbons by K2 CO3 activation with variable carbonization temperature.
Cspec =
AC300 AC400 AC500 AC600
0.8
245
0 0
500
1000
(3)
where Cspec is single-electrode specific gravimetric capacitance (farad per gram), i is the discharge current (milliamperes), t is the total discharge time (seconds), m is the total mass of active material in single-electrode (milligrams), and v is the potential difference during the discharging (volts). The specific capacitance of the AC300–AC600 varied from 208–230 F g−1 under this current density. The highest capacitance of the AC400 is 230 F g−1 , which attribute to its higher specific surface area and wide pore size distribution. The presence of mesopores can enhance utilization of the exposed surface for charge separation and provide resistance pathways for the ions through the porous particles (Zhang et al., 2010).
1500 2000 2500 3000 Charge-discharge time/s
3500
4000
Fig. 4. Galvanostatic charge/discharge curves of the ACs at the current density of 50 mA g−1 .
Fig. 5 shows the dependence of the specific capacitance values on the current density for activated samples. It can be seen that the capacitance decreased with current density to a similar extent for all of the samples. In addition, for the sample AC300 and AC600, the sample with the higher capacitance was not the sample with the higher BET surface area. Therefore, other factors, apart from the 250
Capacitance/F.g-1
225 200
AC300 AC400
175
AC500 AC600
150 125 100 75 50 25
Fig. 3. Pore volume of prepared activated carbons.
0
10
20
30 40 -1 Current density/A.g
50
60
Fig. 5. Specific capacitance values versus current densities for ACs.
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Capacitance/F.g-1
150 2mV 10mV
50
20mV 50mV
- 50
100mV
- 150 - 250 - 0. 05
0. 15
0. 35
0. 55
0. 75
0. 95
Voltage/V
Fig. 6. Cyclic-voltammograms (CVs) of the waste MDF carbon-based AC400 with scan rates from 2 to 100 mV s−1 .
surface area, must be contributing to the enhancement of capacitance (Roldan et al., 2010). It is revealed that the nitrogen groups in the AC samples can improve the performance of the capacitance. Fig. 6 shows the CV results of the selected sample of AC400 which has the best electrochemical performance at various scan rates. At 2, 10, 20, 50, and 100 mV s−1 , the curves present the rectangle shapes for the charge/discharge processes. At the highest scan rate 100 mV s−1 , a rectangular shape with only slight deviation from the ideal rectangular shape can be observed .Cyclic voltammetry in a three-electrode configuration is an excellent technique for studying the presence of pseudocapacitive phenomena. Materials with pseudo-capacitance show redox peaks related to electron-transfer reactions. The voltammograms in Fig. 6 show the redox processes which are the contribution of the nitrogen groups in AC samples. In addition, a small hump during the sweep at 0.8–1.0 V was clearly observed for the AC400 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 electron-donating effect of nitrogen heteroatoms. It also suggests that AC400 can be an excellent candidate as electrode materials for supercapacitor. Electrochemical impedance spectroscopy (EIS) was utilized to obtain information on the supercapacitors performance, such as
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their frequency dependence and ESR (Biniaket al, 1997; Li et al., 2010). Fig. 7 illustrates the Nyquist plots of the CA400 supercapacitors. A semicircle of very small radius was obtained at 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 CA400 was about 0.75. 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. Obviously, the straight line part of carbon AC400 is more close to vertical line along the imaginary axis, suggesting AC400 has good capacitive behavior. The sample of AC400 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 the nitrogen functionalities. 6. Conclusion N-enriched electrode materials for supercapacitors were prepared from medium density fiberboard base by K2 CO3 activation. The effects of physical properties and chemical composition of waste MDF-based ACs on the electrochemical performance were explored. The porosity characterization results showed that all of the ACs produced are essentially microporous. Activated carbon with a highest surface area and mesopore volume was produced when the activation temperature was 400 ◦ C. The ACs show a similar electrochemical behavior at all of the carbonization temperatures. The activated carbon AC400 exhibited the best electrochemical behavior with a specific gravimetric capacitance of 230 F g−1 , with rectangular cyclic voltammetry curves at a scan rate of 2 mV s−1 , which remained at 156 F g−1 even at a current density of 20 A/g. Correlating the capacitive behavior with textural characteristics, the good electrochemical properties were ascribed to the high surface area, wide pore size distribution, and nitrogen functionalities. Acknowledgments This study was funded by the State Forestry Administration under the Project 201204807: the study on the technology and mechanism of the activated carbon electrode preparation from waste hard board. References
Fig. 7. Nyquist plot of the CC and CA (inset: enlarged high-frequency region of Nyquist plot).
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Contents lists available at SciVerse ScienceDirect
Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Nitrogen-enriched waste medium density fiberboard-based activated carbons as materials for supercapacitors Xiao-Juan Jin ∗ , Ming-Yang Zhang, Yu Wu, Ji Zhang, Jun Mu School of Material Science and Technology, Beijing Forestry University, 35 Qinghua East Road, Haidian District, 100083, PR China
a r t i c l e
i n f o
Article history: Received 18 April 2012 Received in revised form 5 August 2012 Accepted 8 August 2012 Keywords: Supercapacitors Waste medium density fiberboard Activated carbon
a b s t r a c t Nitrogen-enriched activated carbons were prepared from waste medium density fiberboard base using K2 CO3 as an activating agent for electrochemical capacitors. The effect of different temperatures of carbonization (300, 400, 500, and 600 ◦ C) and the influence of the sequence of activation process (a mass K2 CO3 /coke ratio of 3, temperature of 800 ◦ C and the activation time 1 h) on the electrochemical performance have been investigated. All of the samples were characterized in terms of surface chemical composition by elemental analysis and X-ray photoelectron spectroscopy, porosity by N2 sorption at 77 K, surface area by Brunauer–Emmett–Teller measurement. Electrochemical behavior as electric double-layer capacitors was determined using galvanostatic, voltammetric charge/discharge, specific capacitance values versus current densities, and impedance spectroscopy techniques in a 7 mol L−1 KOH aqueous solution. A maximum value of specific capacitance of 230 F g−1 was achieved for the sample carbonized at 400 ◦ C. The good electrochemical performance of the activated carbon was ascribed to high surface area, wide pore size distribution, and the presence of nitrogen functional groups. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Supercapacitors have attracted much attention because of their higher power density, better efficiency, and longer durability in comparison to the rechargeable batteries. Among various electrode candidates for supercapacitors, porous carbons are mainly investigated due to their advantages, such as good electrical conductivity, electrochemical stability, high surface area, large capacitance, long cycling life, and relatively low cost. However, a further advancement in the application of porous carbons, and more specifically activated carbons (ACs), in adsorption, catalysis or electrochemistry requires taking advantage of surface chemistry (Machnikowski et al., 2005; Radovic et al., 2001; Radovic, 1997). In the case of ACs, the chemistry is mostly related to the nature and concentration of heteroatoms which can be both substituted for carbon in the graphene layer and/or attached at the layer edges as functional groups, Among a variety of heteroatoms (O, N, S, P and Cl), Nitrogen has received appreciable attention. On one hand, the functional species of nitrogen improve wettability so enlarge the working area of microporous structures and, in consequence, increase the contribution of the capacitance related to the electrostatic
∗ Corresponding author. E-mail addresses: [email protected] (X.-J. Jin), [email protected] (M.-Y. Zhang), [email protected] (Y. Wu), [email protected] (J. Zhang), [email protected] (J. Mu). 0926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2012.08.006
interactions; on the other hand, the nitrogen groups can contribute pseudocapacitance for the carbonaceous materials to improve its electrochemical performance. The methods used for the synthesis of nitrogen containing porous carbons at a laboratory scale can be categorized into two groups. First, it is the treatment of conventional activated carbon at an elevated temperature with ammonia, ammonia–air or ammonia–steam mixtures (Boudou, 2003; Jansen and Bekkum, 1994; Mangun et al., 2001) and the surface chemistry is controlled by the reaction temperature. The second approach comprises the condensed phase pyrolysis of nitrogen containing polymers (Lahaye et al., 1999; Laszlo et al., 2001; Stanczyk et al., 1995) or co-pyrolysis of a suitable nitrogen carrier with a common carbon precursor, followed by the activation of the resultant char, as a result, thermally stable nitrogen atoms with different chemical states are present in the porous carbons. We here describe the significance of a novel nitrogen rich carbon material, ‘waste medium density fiberboard’ (MDF) rejected by the furniture factory for supercapacitors. The common method to treat the waste MDF is burning which is a kind of wasting of the resources, and more important, the poisonous gas containing nitrogen will be discharged in the atmosphere. Therefore, the development of methods for re-using the waste MDF materials is highly desired, and the production of activated carbon for supercapacitors from these wastes is an interesting possibility. The nitrogen atoms of the waste MDF originates from urea–formaldehyde resin adhesive used in the MDF manufacturing process, and some of nitrogen
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are further turned over to waste MDF-based ACs. The utilization of the waste MDF can significantly reduce the environmental impact and afford attractive products. To the best of our knowledge, synthesizing such an activated carbon material for supercapacitors in this way has not yet been reported. In present work, four kinds of waste MDF-based ACs were prepared using potassium carbonate (K2 CO3 ) activation. The aim of this work was to investigate the effect of carbonization temperature and activation process on the specific surface area, pore size distribution, and nitrogen functional groups and on the electrochemical performance of the ACs. 2. Materials and methods 2.1. Materials The waste MDF was kindly provided by Beijing Jiahekailai Furniture and Design Company, which was obtained in the furniture manufacturing process containing 12% urea–formaldehyde resin adhesive of the mass. Other chemicals were analytical grades and were purchased from Beijing Lanyi Chemical reagent. Double distilled water was used for preparation of all required solutions. 2.2. Preparation of activated materials Carbonization process was carried out in nitrogen atmosphere at the temperature increase rate of 10 ◦ C/min to the final temperature of 300 ◦ C, 400 ◦ C, 500 ◦ C, 600 ◦ C, separately, maintained for 1 h. The carbonization products were marked as C300, C400, C500, and C600. The samples were then ground and screened out with sieves. The fraction in the particle diameter ranged from 40 meshes to 60 meshes. The particle samples were dried in a 105 ◦ C oven for 8 h; in activation step, 3 g of the oven-dried samples were soaked in a 50% K2 CO3 solution for 16 h at the mass ratio impregnation (3:1). The samples were then activated at 800 ◦ C for 60 min in nitrogen atmosphere. The obtained activated carbons (AC300, AC400, AC500, and AC600) were boiled first with 1 M HCl solution and then with distilled water until the pH of solution reach to about 6–7. Finally, these activated carbons were dried at 105 ◦ C in an oven for 8 h. 3. Characterization of the activated carbon 3.1. Chemical surface composition Chemical surface composition and state of the samples were determined by X-ray photoelectron spectroscopy (XPS) and elemental analysis. (i) X-ray was performed on an ESCALAB250 (VGScientific, UK) using a monochromatic Al K␣ radiation. The acceleration tension and power of X-ray source were 15 kV and 100 W, respectively. The sample charging was corrected by 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. (ii) The elemental analysis (contents in carbon, hydrogen, and nitrogen) of the activated carbons was under taken in a CHNS Analyzer (Thermofinnigan Flash, EA, 1112 series).
Table 1 The elemental analysis of the raw material, carbonization and AC samples. Samples
N (%)
C (%)
H (%)
Raw material C300 C400 C500 C600 AC300 AC400 AC500 AC600
8.39 7.06 6.84 6.31 5.77 3.86 3.12 2.95 2.47
45.16 70.35 74.85 77.50 79.38 88.96 93.97 94.44 95.11
5.76 3.62 2.68 2.09 1.50 2.74 2.50 2.22 1.93
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. 4. Electrode preparation and electrochemical measurements 4.1. Electrode preparation The dried activated carbon samples including AC300, AC400, AC500, and AC600 were grinded in an agate mortar. Electrodes for the electrochemical measurements were fabricated by mixing the sample with acetylene black and 60% polytetrafluoroethylene in a mass ratio of 87:10:3. The mixtures were sandwiched by nickel foam (square, about 1 cm2 ) and pressed under a pressure of 20 MPa with a nickel tape for connection to one disk. 4.2. Electrochemical analysis Electrochemical characters were tested in 7.0 M KOH solution. Constant current density charge–discharge and rate performance were tested using the BT2000 battery testing system (Arbin Instruments, USA) at room temperature. 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 gravimetric capacitance (Cp) analyzed, which means the specific capacitance per mass weight activated carbon in the electrode, is expressed in F g−1 and calculated by the following formulas of (1) and (2): C=
Q t Q t I = × =I× = U t U U U/t
Cp =
C I/m I/m = = m v U/t
(1) (2)
where I is observed value (A), m is average weight of each activated carbon disk (g), v is the voltage scan rate (mV s−1 ). 5. Results and discussion
3.2. Porous texture
5.1. Elemental analysis
The N2 adsorption–desorption isotherms of activated carbon prepared under optimum conditions were measured with an accelerated surface area and porosimetry system (ASAP 2010, Micromeritics) for determining the surface areas. Prior to the measurements, the samples were outgassed at 573 K, under nitrogen
Table 1 shows the elemental analysis of the raw material, carbonized samples and AC samples, respectively. It is can be seen that as temperature increased, there is an increase in the element of C and a decrease in H and N contents. Nitrogen is present in all AC samples ranging typically from 2.47% to 3.86%, which suggest that
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thermally stable nitrogen in the waste MDF maintained in the AC samples in the process of carbonization and activation. 5.2. XPS study Fig. 1 shows N1s XPS spectra of the carbonization and AC samples. According to the literatures (Bagreev et al., 2004; Jurewicz et al., 2003, 2004; Kapteijn et al., 1999; Kim et al., 2007; Lahaye et al., 1999; Machnikowski et al., 2004; Raymundo-pinero et al., 2004), the chemical state of nitrogen atoms in graphene layers could be assigned five types; N-6 (pyridinic nitrogen, 398.7 ± 0.3 eV), N-5 (pyrrolic nitrogen and pyridinic nitrogen in association with oxygen functionality, 400.3 ± 0.3 eV), N-Q (quaternary nitrogen, nitrogen substituted with carbons in the aromatic grapheme structure, 401.4 ± 0.5 eV), N-X and Oxidized nitrogen (402–405 eV). Except for the N-Q, all nitrogen functionalities are located at the edge of the graphene structure. The relative contributions of each nitrogen species to the total peak area are summarized in Table 2. The results indicated that the chemical state of nitrogen could be sensitively varied by the carbonization temperature and the activation process. 5.3. Textural studies of ACs The N2 adsorption–desorption isotherms, as shown in Fig. 2, which is used to determine the surface area and pore-size distribution of the ACs. All of the samples exhibit type-I isotherms according to the IUPAC classification. This is the characteristic of microporous solids. However, the small hysteresis loops observed on the adsorption–desorption isotherms were due to the existence of mesopores (Guo and Rockstraw, 2007). With increasing the temperature from 300 ◦ C to 600 ◦ C, the adsorption increased. It is probably because that pores became more when the temperature increased. The curve of AC400 was located in the top, which indicated that it has the highest microspore and mesopore volume. The parameters of the porous texture of ACs calculated from the isotherms are presented in Table 3. Table 3 showed that as the carbonization temperature increased from 300 ◦ C to 400 ◦ C, the BET surface area increased. When the carbonization temperature was 400 ◦ C, the maximum BET surface area could get 1199 m2 /g and total pore volume was 0.582 cm3 /g. When carbonization temperature increased from 400◦ C to 600 ◦ C, the BET surface area decreased. It is proved that textural properties of ACs are strongly depending on the carbonization temperature. The microspore and mesopore volume of AC400 are the highest in all samples. Fig. 3 shows the pore size distribution of the ACs. As we know, micropores are less than 2 nm wide, mesopores are 2–50 nm wide, and macropores are more than 50 nm wide. As can be seen from Fig. 3, the pore size of all the prepared samples included micropores and mesopores (Gyu and Park, 2008). 5.4. Electrochemical characteristics Galvanostatic charge/discharge measurements are commonly used to test the performance of capacitors. Fig. 4 shows galvanostatic charge/discharge curves of all the AC samples at a current density of 50 mA g−1 between 0 and 1.0 V in a 7.0 mol L−1 KOH electrolyte. A sudden potential drop at the very beginning of the constant current discharge is usually observed for EDLCs and this drop has been designated as the IR drop, which can be attributed to the resistance of electrolyte solution and the inner resistance of ion diffusion in carbon micropores (Conway, 1999; Zhang et al., 2010). The charge/discharge curves of all the samples approach a triangular shape reflecting good charge/discharge capacitive performance. However, the IR drop of the sample AC400 is slightly smaller that the other samples due to its wide pore size distribution.
Fig. 1. N1s XPS spectra of the carbonization and AC samples.
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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, Pyrollic nitrogen, pyridone (400.3 ± 0.3 eV)
N-Q, Quaternary nitrogen (401.4 ± 0.4 eV)
N-X, Oxidized nitrogen (402–405 eV)
A300 A400 A500 A600
37.79 48.46 49.31 46.15
48.03 47.00 45.84 47.22
14.17 3.04 – –
– 1.50 4.85 6.63
Table 3 Textural parameters of the ACs obtained by K2 CO3 activation with variable carbonization temperature. SBET (m2 /g)
Sample
804 1199 932 849
Sme (m2 /g)
Vtot (cm3 /g)
Vmi (cm3 /g)
Vme (cm3 /g)
Smi (%)
Vmi (%)
Sme (%)
Vme (%)
547 949 750 670
257 250 182 180
0.388 0.582 0.456 0.414
0.250 0.439 0.348 0.309
0.138 0.143 0.108 0.105
68.01 79.16 80.44 78.86
64.44 75.52 76.31 74.65
31.99 20.84 19.56 21.14
35.56 24.48 23.69 25.35
1.1
365
3
Volume Adsorbed(cm /g)
AC300 AC400 AC500 AC600
Smi (m2 /g)
1
325
0.9
285
AC400
AC500
0.7
AC600
1 65 0. 2
0
0. 4 0. 6 Relative pressure(P/P 0 )
0. 8
1
Voltage/V
AC300
205
0.5
0.3 0.2 0.1
The specific gravimetric capacitances were calculated from the galvanostatic charge/discharge process on the basis of Eq. (3). i × t m × v
0.6
0.4
Fig. 2. Nitrogen adsorption–desorption isotherms for prepared activated carbons by K2 CO3 activation with variable carbonization temperature.
Cspec =
AC300 AC400 AC500 AC600
0.8
245
0 0
500
1000
(3)
where Cspec is single-electrode specific gravimetric capacitance (farad per gram), i is the discharge current (milliamperes), t is the total discharge time (seconds), m is the total mass of active material in single-electrode (milligrams), and v is the potential difference during the discharging (volts). The specific capacitance of the AC300–AC600 varied from 208–230 F g−1 under this current density. The highest capacitance of the AC400 is 230 F g−1 , which attribute to its higher specific surface area and wide pore size distribution. The presence of mesopores can enhance utilization of the exposed surface for charge separation and provide resistance pathways for the ions through the porous particles (Zhang et al., 2010).
1500 2000 2500 3000 Charge-discharge time/s
3500
4000
Fig. 4. Galvanostatic charge/discharge curves of the ACs at the current density of 50 mA g−1 .
Fig. 5 shows the dependence of the specific capacitance values on the current density for activated samples. It can be seen that the capacitance decreased with current density to a similar extent for all of the samples. In addition, for the sample AC300 and AC600, the sample with the higher capacitance was not the sample with the higher BET surface area. Therefore, other factors, apart from the 250
Capacitance/F.g-1
225 200
AC300 AC400
175
AC500 AC600
150 125 100 75 50 25
Fig. 3. Pore volume of prepared activated carbons.
0
10
20
30 40 -1 Current density/A.g
50
60
Fig. 5. Specific capacitance values versus current densities for ACs.
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Capacitance/F.g-1
150 2mV 10mV
50
20mV 50mV
- 50
100mV
- 150 - 250 - 0. 05
0. 15
0. 35
0. 55
0. 75
0. 95
Voltage/V
Fig. 6. Cyclic-voltammograms (CVs) of the waste MDF carbon-based AC400 with scan rates from 2 to 100 mV s−1 .
surface area, must be contributing to the enhancement of capacitance (Roldan et al., 2010). It is revealed that the nitrogen groups in the AC samples can improve the performance of the capacitance. Fig. 6 shows the CV results of the selected sample of AC400 which has the best electrochemical performance at various scan rates. At 2, 10, 20, 50, and 100 mV s−1 , the curves present the rectangle shapes for the charge/discharge processes. At the highest scan rate 100 mV s−1 , a rectangular shape with only slight deviation from the ideal rectangular shape can be observed .Cyclic voltammetry in a three-electrode configuration is an excellent technique for studying the presence of pseudocapacitive phenomena. Materials with pseudo-capacitance show redox peaks related to electron-transfer reactions. The voltammograms in Fig. 6 show the redox processes which are the contribution of the nitrogen groups in AC samples. In addition, a small hump during the sweep at 0.8–1.0 V was clearly observed for the AC400 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 electron-donating effect of nitrogen heteroatoms. It also suggests that AC400 can be an excellent candidate as electrode materials for supercapacitor. Electrochemical impedance spectroscopy (EIS) was utilized to obtain information on the supercapacitors performance, such as
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their frequency dependence and ESR (Biniaket al, 1997; Li et al., 2010). Fig. 7 illustrates the Nyquist plots of the CA400 supercapacitors. A semicircle of very small radius was obtained at 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 CA400 was about 0.75. 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. Obviously, the straight line part of carbon AC400 is more close to vertical line along the imaginary axis, suggesting AC400 has good capacitive behavior. The sample of AC400 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 the nitrogen functionalities. 6. Conclusion N-enriched electrode materials for supercapacitors were prepared from medium density fiberboard base by K2 CO3 activation. The effects of physical properties and chemical composition of waste MDF-based ACs on the electrochemical performance were explored. The porosity characterization results showed that all of the ACs produced are essentially microporous. Activated carbon with a highest surface area and mesopore volume was produced when the activation temperature was 400 ◦ C. The ACs show a similar electrochemical behavior at all of the carbonization temperatures. The activated carbon AC400 exhibited the best electrochemical behavior with a specific gravimetric capacitance of 230 F g−1 , with rectangular cyclic voltammetry curves at a scan rate of 2 mV s−1 , which remained at 156 F g−1 even at a current density of 20 A/g. Correlating the capacitive behavior with textural characteristics, the good electrochemical properties were ascribed to the high surface area, wide pore size distribution, and nitrogen functionalities. Acknowledgments This study was funded by the State Forestry Administration under the Project 201204807: the study on the technology and mechanism of the activated carbon electrode preparation from waste hard board. References
Fig. 7. Nyquist plot of the CC and CA (inset: enlarged high-frequency region of Nyquist plot).
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