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Effects of surface chemical properties of activated carbon modified by amino-fluorination for electric double-layer capacitor
Journal of Colloid and Interface Science 381 (2012) 152–157
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Effects of surface chemical properties of activated carbon modified by amino-fluorination for electric double-layer capacitor Min-Jung Jung a, Euigyung Jeong a, Seho Cho a, Sang Young Yeo b, Young-Seak Lee a,⇑ a b
Department of Applied Chemistry and Biological Engineering, BK21-E2M, Chungnam National University, Daejeon 305-764, Republic of Korea Heracron Research Institute, Kolon Industries, Inc., 212 Gongdan-dong, Gumi-Si, Gyungsangbuk-do 730-030, Republic of Korea
a r t i c l e
i n f o
Article history: Received 28 March 2012 Accepted 16 May 2012 Available online 24 May 2012 Keywords: Activated carbon Surface modification Amino-fluorination Electric double-layer capacitor
a b s t r a c t The surface of phenol-based activated carbon (AC) was seriatim amino-fluorinated with solution of ammonium hydroxide and hydrofluoric acid in varying ratio to fabricate electrode materials for use in an electric double-layer capacitor (EDLC). The specific capacitance of the amino-fluorinated AC-based EDLC was measured in a 1 M H2SO4 electrolyte, in which it was observed that the specific capacitances increased from 215 to 389 F g1 and 119 and 250 F g1 with the current densities of 0.1 and 1.0 A g1, respectively, in comparison with those of an untreated AC-based EDLC when the amino-fluorination was optimized via seriatim mixed solution of 7.43 mol L1 ammonium hydroxide and 2.06 mol L1 hydrofluoric acid. This enhancement of capacitance was attributed to the synergistic effects of an increased electrochemical activity due to the formation of surface N- and F-functional groups and increased, specific surface area, and mesopore volumes, all of which resulted from the amino-fluorination of the electrode material. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction The electric double-layer capacitor (EDLC) is a unique electrical storage device that can store much more energy than conventional capacitors and offers a much higher power density than batteries. There is currently much interest in EDLCs related to their quick response and the high mobility of ions in the electrolyte, resulting in high energy storage capacities due to the double-layer capacitor effect [1–4]. Porous carbon materials, especially activated carbon (AC), is a promising material for EDLC electrodes due to its high specific surface area, large pore volume, good mechanical stability, chemical inertness, and relatively low cost [5–7]. However, the specific capacitance of activated carbon is much lower than theoretically expected, resulting in EDLCs with lower energy densities than predicted, which greatly hinders their practical application. The energy storage of EDLCs can be enhanced by enriching the surface of the carbon materials with heteroatoms such as oxygen or nitrogen [8–10]. 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 [11]. Various surface modification methods have been investigated to introduce heteroatoms onto the surfaces of carbon materials, for example, chemical,
⇑ Corresponding author. Fax: +82 42 822 6637. E-mail address: [email protected] (Y.-S. Lee). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.05.031
plasma, and flame treatments, coronal discharge, and direct fluorination [12–16]. In our previous work, amino-fluorination was demonstrated as a novel chemical surface modification method for AC [17]. Using a simple impregnation with ammonium hydroxide and hydrofluoric acid solutions, amino-fluorination can introduce both nitrogen and fluorine functional groups and can be used to control the specific surface area and pore volume of a porous material [17]. Therefore, optimal conditions for amino-fluorination are required to enhance the specific capacitance of ACs for EDLC applications. In this study, the surface of phenol-based AC was modified by amino-fluorination with seriatim mixtures of ammonium hydroxide and hydrofluoric acid in various ratios for use as an EDLC electrode material. The effects of the treatments on the surface morphology and the electrochemical properties of the AC were also investigated.
2. Experimental 2.1. Amino-fluorination of activated carbon Phenol-based activated carbon (MSP-20) was purchased from Kansai Coke and Chemicals Company, Ltd., Japan. Ammonium hydroxide (NH3; 28.0–30.0%) was provided by Junsei Chemical Co., Ltd., Japan. Hydrofluoric acid (48.0–51.0%) was purchased from J.T. Baker, USA. 300 ml of ammonium hydroxide (7.43 mol L1) was mixed with 3 g of the raw AC (R-AC) and then 50 ml of various concentrations
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M.-J. Jung et al. / Journal of Colloid and Interface Science 381 (2012) 152–157 Table 1 The manufacturing conditions for the synthesis of amino-fluorinated activated carbons. Sample
NH4OH (mol L1)
HF (mol L1)
R-AC NF10-AC NF25-AC NF50-AC
– 7.43
– 0.83 2.06 4.11
2.2. Physicochemical characterization The pore structure of the as-prepared samples was assessed by N2 adsorption at 77 K using an ASAP2020 (Micromeritics, USA). The specific surface areas of the samples were evaluated using the Brunauer–Emmett–Teller (BET) equations. The pore-size distributions of the samples were determined by the methods of Horvath–Kawazoe (H–K) and Barrett–Joyner–Halenda (BJH). To investigate the changes in the functional groups on the surfaces of the untreated and amino-fluorinated ACs, X-ray photoelectron spectroscopy (XPS, VG Multilab 2000) analysis was performed using Mg Ka radiation. XPS elemental analysis was shown the elemental average value of the sample. The C1s and N1s peaks were deconvoluted to several pseudo-Vogit functions (sums of a Gaussian–Lorentzian function) using a peak analysis program obtained from Unipress Co., USA. The pseudo-Vogit function is given by the following equation [18]:
E E0 6 FðEÞ ¼ H4ð1 SÞ exp lnð2Þ RFWHM
3
2 ! þ 1þ
S EE0 RFWHM
7 2 5
3. Results and discussion 3.1. Surface chemical properties of prepared ACs
of hydrofluoric acid (0.83, 2.06, or 4.11 mol L1) were each added seriatim, respectively. These were mixed with a mechanical shaker for 48 h at room temperature. The treatment conditions of the amino-fluorinated ACs and the names of the samples are listed in Table 1. The treated AC was washed with distilled water until neutral pH and then dried for 24 h at 393 K. The untreated AC was named R-AC. And the amino-fluorinated AC samples are hereinafter referred to as NF10-AC, NF25-AC, and NF50-AC, depending on the concentrations of hydrofluoric acid used, which were 0.83, 2.06, and 4.11 mol L1, respectively.
2
stant currents of 0.1 and 1.0 A g1. All of the electrochemical measurements were performed in a 1 M H2SO4 electrolyte.
ð1Þ
where F(E) is the intensity at energy E, H is the peak height, E0 is the peak center, RFWHM is the full width at half-maximum (FWHM), and S is the shape function related to the symmetry and the Gaussian– Lorentzian mixing ratio. The FWHM values of C1s and N1s were 1.4 and 1.8 eV, respectively. 2.3. Electrochemical characterization EDLC electrodes were fabricated by mixing 80 wt.% untreated or amino-fluorinated AC, 10 wt.% carbon black (Super P, Timcal Ltd., Switzerland), and a 10 wt.% solution of polyvinylidene fluoride (PVDF, Aldrich, USA) in N-methyl pyrrolidone (NMP, Aldrich, USA) to form a slurry, which was then painted onto a titanium plate. The untreated and amino-fluorinated AC-based electrodes were electrochemically characterized using a computer-controlled potentiostat–galvanostat (Ivium Technologies, Netherlands) equipped with a three-electrode assembly. The AC electrode was used as the working electrode, and Ag|AgCl was used as the reference electrode. A platinum plate was used as the counterelectrode. Cyclic voltammetry (CV) of the electrode materials was performed over a potential range of 0–1 V at scan rates of 5 and 50 mV s1. The capacitance of the electrode was measured by a galvanostatic charge–discharge cycle in the potential range of 0–0.9 V at con-
XPS elemental analysis was performed to compare the chemical compositions at the surfaces of the untreated AC and amino-fluorinated ACs. Table 2 lists the atomic percentages of C, O, N, and F for the amino-fluorinated ACs synthesized in this study. Based on the XPS elemental analysis, the O, N, and F contents of the amino-fluorinated ACs were increased over those of the R-AC, rising from an O/C ratio of 8.1, an N/C ratio of 0.0, and an F/C ratio of 0.0 to ratios of 9.9, 2.3, and 0.5, respectively. During amino-fluorination, active sites were introduced on the AC surface by fluorine attack. Hence, the O, N, and F components on the AC increased with increasing hydrofluoric acid concentration because the active sites of the AC surface formed by hydrofluoric acid attack reacted with water to produce other functional groups [17]. However, the NF50-AC, the sample prepared with the highest concentration of hydrofluoric acid in this study, had fewer O, N, and F functional groups than the NF25-AC. In this case, it is likely that the neutralization reaction treated in seriatim with solution of ammonium hydroxide and hydrofluoric acid would occur as the dominant than introducing active sites on the AC surface. In addition, the changes in the chemical bonds of the AC after the treatments were investigated by C1s and N1s deconvolution; the results are depicted in Fig. 1 and 2, respectively. The C and N components are also listed in Table 3 along with the binding-energy peak positions, concentrations, and chemical-bond assignments. In the C1s deconvolution for R-AC, five main peaks for C(1), C(3), C(4), C(5), and C(6) were observed at 284.5, 285.2, 286.1, 287.2, and 288.8 eV, respectively, corresponding to sp2(CAC), sp3(CAC), CAO, C@O, and (C@O)OH bonds. After the amino-fluorination of AC, two additional main peaks for C(3) and C(7) were observed at 285.8 and 290.2 eV, respectively, which correspond to CAN and CAF bonds [19–21]. In the N1s deconvolution for amino-fluorinated ACs, four main peaks were observed at 398.2, 399.4, 400.6, and 405.2 eV, which correspond to pyridine, pyrrole/pyridone, quaternary nitrogen [22,23], and ANAF [17,24,18] groups, respectively. The different types of functional groups were introduced with similar concentrations irrespective of whether the samples had been treated with different concentrations of hydrofluoric acid during amino-fluorination. Both nitrogen functional groups have electron-donor tendencies [25]; therefore, these nitrogen functional groups were expected to impact the electrochemical properties of the AC-based EDLCs. 3.2. Textural properties of the prepared ACs Changes in the pore textures of the ACs after amino-fluorination were investigated by nitrogen adsorption analysis at 77 K. The
Table 2 XPS surface elemental analysis parameters of the untreated and amino-fluorinated activated carbons. Sample
R-AC NF10-AC NF25-AC NF50-AC
Elemental content (At%) C1s
O1s
N1s
F1s
92.5 90.6 88.8 90.2
7.5 7.9 8.8 8.2
– 1.4 2.0 1.3
– 0.2 0.4 0.4
O/C (%)
N/C (%)
F/C (%)
N/F (%)
8.1 8.7 9.9 9.1
– 1.5 2.3 1.4
– 0.2 0.5 0.5
– 10.7 20.5 32.8
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(a) 40000
(b)
35000
50000
C(1)
C(1)
40000
Intensity
Intensity
30000 25000 20000 15000 10000 5000
C(6)
C(2) C(5) C(4)
30000 20000 10000
0
C(3) C(4) C(6) C(5) C(7)
0 292
290
288
286
284
282
280
292
290
Binding energy (eV)
(d) C(1)
40000
Intensity
Intensity
20000
290
288
286
C(2)
282
280
282
C(1)
40000 30000 20000 10000
284
284
60000
C(3) C(4) C(6) C(5) C(7)
0 292
0 292
286
50000
30000
C(3) C(4) C(6) C(5) C(7)
288
Binding energy (eV)
(c)
10000
C(2)
280
Binding energy (eV)
290
288
286
C(2)
284
282
280
Binding energy (eV)
Fig. 1. Deconvolution of the core-level C1s spectra of (a) R-AC, (b) NF10-AC, (c) NF25-AC, and (d) NF50-AC.
Table 3 C1s and N1s peak parameters of the untreated and amino-fluorinated activated carbons.
(a) Intensity
1600
N(2) Peak position Concentration (%) (eV) R-AC NF10-AC NF25-AC NF50-AC
1200
N(3)
N(1)
800
N(4) 400 0 408 406 404 402 400 398 396 394
Binding energy (eV)
(b)
1600
N(2)
Intensity
1200
76.8 11.3 – 7.2 4.6 1.0 – – – –
405.2
–
62.5 13.2 6.3 7.4 5.3 3.6 1.7 15.7 56.6 23.5
59.6 13.5 8.4 7.4 5.5 3.6 2.1 13.8 53.5 28.1
62.3 13.0 6.1 7.1 5.6 3.7 2.1 16.0 54.3 25.8
4.2
4.6
4.0
N(1)
N(4)
0 408 406 404 402 400 398 396 394
Binding energy (eV) 1200
N(2)
Intensity
284.5 285.2 285.8 286.4 287.3 288.8 290.3 398.2 399.4 400.6
N(3)
800
400
(c)
CAC(sp2) CAC(sp3) CAN CAO CAO CAOOH CAF Pyridine Pyrrole /pyridone Quaternary nitrogen N(4) NAF C(1) C(2) C(3) C(4) C(5) C(6) C(7) N(1) N(2) N(3)
800
N(4)
N(3)
N(1)
400
0 408 406 404 402 400 398 396 394
Binding energy (eV) Fig. 2. Deconvolution of the core-level N1s spectra of (a) NF10-AC, (b) NF25-AC, and (c) NF50-AC.
nitrogen adsorption isotherms of the untreated and amino-fluorinated AC samples are presented in Fig. 3. The R-AC sample showed a dramatic increase in the quantity of nitrogen adsorbed below a P/ P0 of 0.01, followed by no significant increase, indicating that a microporous structure was developed [26]. The isotherm curves of the amino-fluorinated ACs increased markedly below a P/P0 of 0.1 and then increased gradually, resulting from the conversion of some micropores into mesopores [27]. Fig. 4 presents the size distributions of the micropores and mesopores calculated using both the HK model and the BJH model. Fig. 4a shows that the number of small micropores (approximately 0.4–0.7 nm in size) was perceptibly decreased by amino-fluorination, whereas the number of large micropores (greater than 1 nm in size) was increased. Furthermore, as illustrated in Fig. 4b and C, the numbers of large micropores and mesopores were increased in the amino-fluorinated ACs compared with the untreated AC. Detailed information regarding the textural properties of the ACs is presented in Table 4. The ACs had specific surface areas of 1924–2319 m2 g1, total pore volumes of 0.87–1.33 cm3 g1, and mesopore volumes of 0.11–0.65 cm3 g1. The specific surface areas,
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M.-J. Jung et al. / Journal of Colloid and Interface Science 381 (2012) 152–157 Table 4 Textural properties of the untreated and amino-fluorinated activated carbons.
3
Quantity Adsorbed (cm /g)
800 700
Sample
Specific surface area (m2 g1)a
Total pore volume (cm3 g1)b
Micropore volume (cm3 g1)c
Mesopore volume (cm3 g1)d
Mesopore volume ratio (%)
R-AC NF10-AC NF25-AC NF50-AC
1924 2319 2254 2237
0.87 1.22 1.14 1.33
0.76 0.74 0.73 0.69
0.11 0.48 0.41 0.65
12.6 39.2 35.8 48.4
600 500 R-AC NF10-AC NF25-AC NF50-AC
400 300
a
0
Specific surface area calculated using the BET method. The total pore volumes were obtained from the volume of nitrogen adsorbed at a relative pressure of 0.98. c Micropore volume calculated using the H-K method. d Calculated as the total pore volume – the micropore volume. b
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0) Fig. 3. Nitrogen adsorption isotherms at 77 K for the untreated and aminofluorinated activated carbons.
(a)
0.007 R-AC NF10-AC NF25-AC NF50-AC
0.005
3
Volume (cm /g)
0.006
3.3. Electrochemical characteristics of the prepared ACs
0.004 0.003 0.002 0.001 0.000 0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Pore width (nm)
(b)
0.12 R-AC NF10-AC NF25-AC NF50-AC
3
Volume (cm /g)
0.10 0.08 0.06 0.04 0.02 0.00 1
2
3
4
5
6
7
8
9
10
Pore width (nm)
(c)
0.12 R-AC NF10-AC NF25-AC NF50-AC
Volume (cm /g)
0.10 3
mesopores than the R-AC, as reported in Table 4. Furthermore, the amino-fluorinated AC samples also had relatively higher specific surface areas. Therefore, we hypothesized that amino-fluorinated ACs should provide better electrochemical performance than the R-AC.
0.08 0.06 0.04 0.02 0.00 1.0
1.5
2.0
2.5
3.0
Pore width (nm) Fig. 4. Pore-size distributions of the untreated and amino-fluorinated activated carbons (a) H–K, (b) BJH, and (c) an expansion of (b).
total pore volumes, and mesopore volumes of the ACs were markedly increased by amino-fluorination, whereas the micropore volumes decreased compared with those of the R-AC. This decrease in the micropore volume occurred because mesopores were formed by the collapse of the smaller micropores due to the etching effect of amino-fluorination [17]. These mesopores are beneficial to the access of the electrolyte into the pore volume and consequently improve ionic transport in EDLCs [28]. The amino-fluorinated ACs had relatively more
The cyclic voltammograms (CVs) of the untreated and aminofluorinated ACs are presented in Fig. 5. The cyclic voltammetry tests on the three-electrode system were conducted within a potential range of 0–1.0 V at several different potential sweep rates. At a scan rate of 5 mV s1 (Fig. 5a), the CV profiles of the as-prepared samples were close to being rectangular shape, and small redox peaks were observed at 0.3–0.4 V, indicating the occurrence of a pseudo-Faradaic reaction caused by O or N functional groups [29–32]. However, we observed no electrochemical reaction peak caused by the F functional group in these CVs [33]. In the CV profiles of the as-prepared samples obtained at 50 mV s1 (Fig. 5b), the CVs displayed a leaf-like shape with no electrochemical reaction peaks [30,34]. The leaf-like shape is attributed to the influence of the ohmic resistance due to electrolyte motion in the carbon pores upon double-layer formation [35,36]. All the samples of aminofluorinated ACs had higher inner-integral areas on the CVs than that of the R-AC; the NF25-AC had the highest specific capacitance. The galvanostatic charge–discharge curves of the untreated and amino-fluorinated ACs at current densities of 0.1 and 1.0 A g1 are presented in Fig. 6. All the charge–discharge curves exhibit triangular shapes, indicating typical capacitive behavior. However, we observed that the amino-fluorinated AC samples had higher specific capacitances than that of the R-AC, and the NF25-AC had the highest specific capacitance. These results are in agreement with the CV results described above. Fig. 6a presents a galvanostatic charge–discharge curve at the relatively small loading current of 0.1 A g1, showing no obvious ohmic drop, indicating that all the AC-based electrodes displayed good capacitive behavior at this loading current. Additionally, at a current density of 1.0 A g1 (Fig. 6b), a sharp change in voltage (an IR drop) was observed for all the samples. However, the IR drop of the amino-fluorinated ACs decreased when compared with that of the R-AC, going from 0.09 to 0.03 V. The introduction of polar nitrogen atoms can improve the affinity of the carbon surface for aqueous electrolytes, thereby increasing the surface area accessible by the electrolytes [37]. The specific capacitances of the untreated and amino-fluorinated AC-based electrodes were estimated from their galvanostatic charge–discharge curves at various current–density loads (Table 5). The specific capacitance was calculated from the following equation [38]:
C ¼ iDt=mDV
ð2Þ
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M.-J. Jung et al. / Journal of Colloid and Interface Science 381 (2012) 152–157
(a)
Table 5 Specific capacitances at various current densities of untreated and amino-fluorinated activated carbons.
3
R-AC 10F-AC 25F-AC 50F-AC
Current (mA/mg)
2
Sample
1
R-AC NF10-AC NF25-AC NF50-AC
0
Specific capacitance at various current density (F g1) 0.1 (A g1)
0.2 (A g1)
0.25 (A g1)
0.5 (A g1)
1.0 (A g1)
2.0 (A g1)
215 244 389 271
188 214 337 234
183 208 325 225
159 192 299 205
138 171 273 187
119 151 250 171
-1
-2 0.0
0.2
0.4
0.6
0.8
1.0
0.8
1.0
Voltage (V)
(b) R-AC 10F-AC 25F-AC 50F-AC
16
Current (mA/mg)
12 8 4 0 -4 -8 -12 0.0
0.2
0.4
0.6
Voltage (V)
4. Conclusions
Fig. 5. Cyclic voltammograms of the untreated and amino-fluorinated activated carbon-based electrodes obtained at (a) 5 mV s1 and (b) 50 mV s1.
(a)
1.0
R-AC NF10-AC NF25-AC NF50-AC
Potential (V)
0.8 0.6 0.4 0.2 0.0 0
1000
2000
3000
4000
5000
where i is the current density, Dt is the discharge time, m is the mass of AC, and DV is the potential range of the charge–discharge cycle. As a result, the specific capacitance of the amino-fluorinated ACs was markedly increased. The NF25-AC exhibited the highest specific capacitance of 389 F g1 at a current density of 0.1 A g1 and 250 F g1 at a current density of 1.0 A g1. The enhancement of the specific capacitance of the amino-fluorinated AC-based EDLC can be attributed to the observed increases in the specific surface area and the changes in pore structure of the AC in combination with an increased electrochemical activity due to the N and F functional groups after amino-fluorination. In a previous study, we found that the effect of simultaneous N and F introduction on AC was better than the individual introduction of N or F with respect to EDLC performance [17]. The results of this study demonstrate that the NF25-AC is suitably optimized for use as an EDLC electrode material.
6000
Amino-fluorination is a novel surface modification method for porous materials such as AC during which simultaneous reactions occurs between AC, ammonium hydroxide, and hydrofluoric acid. In this work, amino-fluorination was found to induce changes in the surface chemical and textural properties and the electrochemical properties of AC-based EDLCs. The amino-fluorinated ACs exhibited remarkably increased specific surface areas, total pore volumes, mesopore volumes, and numbers of N-, O-, and F-containing surface functional groups. These changes resulted in the highest specific capacitance of 389 F g1 when the amino-fluorination solution was optimized by treated seriatim with solution of 7.43 mol L1 ammonium hydroxide and 2.06 mol L1 hydrofluoric acid. Therefore, the amino-fluorination of AC is a simple and efficient way to enhance the performance of AC-based EDLCs. Acknowledgments
7000
Time (s)
(b)
R-AC NF10-AC NF25-AC NF50-AC
0.8
Potential (V)
This work was supported by the IT R&D program of MKE/KEIT [KI002177-2012-04, Pouch/Radial type Lithium Ion Capacitor for Ubiquitous Electronics].
1.0
References
0.6 0.4 0.2 0.0 0
100
200
300
400
500
Time (s) Fig. 6. Galvanostatic charge–discharge curves of the untreated and amino-fluorinated ACs at current densities of (a) 0.1 A g1 and (b) 1.0 A g1.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Effects of surface chemical properties of activated carbon modified by amino-fluorination for electric double-layer capacitor Min-Jung Jung a, Euigyung Jeong a, Seho Cho a, Sang Young Yeo b, Young-Seak Lee a,⇑ a b
Department of Applied Chemistry and Biological Engineering, BK21-E2M, Chungnam National University, Daejeon 305-764, Republic of Korea Heracron Research Institute, Kolon Industries, Inc., 212 Gongdan-dong, Gumi-Si, Gyungsangbuk-do 730-030, Republic of Korea
a r t i c l e
i n f o
Article history: Received 28 March 2012 Accepted 16 May 2012 Available online 24 May 2012 Keywords: Activated carbon Surface modification Amino-fluorination Electric double-layer capacitor
a b s t r a c t The surface of phenol-based activated carbon (AC) was seriatim amino-fluorinated with solution of ammonium hydroxide and hydrofluoric acid in varying ratio to fabricate electrode materials for use in an electric double-layer capacitor (EDLC). The specific capacitance of the amino-fluorinated AC-based EDLC was measured in a 1 M H2SO4 electrolyte, in which it was observed that the specific capacitances increased from 215 to 389 F g1 and 119 and 250 F g1 with the current densities of 0.1 and 1.0 A g1, respectively, in comparison with those of an untreated AC-based EDLC when the amino-fluorination was optimized via seriatim mixed solution of 7.43 mol L1 ammonium hydroxide and 2.06 mol L1 hydrofluoric acid. This enhancement of capacitance was attributed to the synergistic effects of an increased electrochemical activity due to the formation of surface N- and F-functional groups and increased, specific surface area, and mesopore volumes, all of which resulted from the amino-fluorination of the electrode material. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction The electric double-layer capacitor (EDLC) is a unique electrical storage device that can store much more energy than conventional capacitors and offers a much higher power density than batteries. There is currently much interest in EDLCs related to their quick response and the high mobility of ions in the electrolyte, resulting in high energy storage capacities due to the double-layer capacitor effect [1–4]. Porous carbon materials, especially activated carbon (AC), is a promising material for EDLC electrodes due to its high specific surface area, large pore volume, good mechanical stability, chemical inertness, and relatively low cost [5–7]. However, the specific capacitance of activated carbon is much lower than theoretically expected, resulting in EDLCs with lower energy densities than predicted, which greatly hinders their practical application. The energy storage of EDLCs can be enhanced by enriching the surface of the carbon materials with heteroatoms such as oxygen or nitrogen [8–10]. 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 [11]. Various surface modification methods have been investigated to introduce heteroatoms onto the surfaces of carbon materials, for example, chemical,
⇑ Corresponding author. Fax: +82 42 822 6637. E-mail address: [email protected] (Y.-S. Lee). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.05.031
plasma, and flame treatments, coronal discharge, and direct fluorination [12–16]. In our previous work, amino-fluorination was demonstrated as a novel chemical surface modification method for AC [17]. Using a simple impregnation with ammonium hydroxide and hydrofluoric acid solutions, amino-fluorination can introduce both nitrogen and fluorine functional groups and can be used to control the specific surface area and pore volume of a porous material [17]. Therefore, optimal conditions for amino-fluorination are required to enhance the specific capacitance of ACs for EDLC applications. In this study, the surface of phenol-based AC was modified by amino-fluorination with seriatim mixtures of ammonium hydroxide and hydrofluoric acid in various ratios for use as an EDLC electrode material. The effects of the treatments on the surface morphology and the electrochemical properties of the AC were also investigated.
2. Experimental 2.1. Amino-fluorination of activated carbon Phenol-based activated carbon (MSP-20) was purchased from Kansai Coke and Chemicals Company, Ltd., Japan. Ammonium hydroxide (NH3; 28.0–30.0%) was provided by Junsei Chemical Co., Ltd., Japan. Hydrofluoric acid (48.0–51.0%) was purchased from J.T. Baker, USA. 300 ml of ammonium hydroxide (7.43 mol L1) was mixed with 3 g of the raw AC (R-AC) and then 50 ml of various concentrations
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M.-J. Jung et al. / Journal of Colloid and Interface Science 381 (2012) 152–157 Table 1 The manufacturing conditions for the synthesis of amino-fluorinated activated carbons. Sample
NH4OH (mol L1)
HF (mol L1)
R-AC NF10-AC NF25-AC NF50-AC
– 7.43
– 0.83 2.06 4.11
2.2. Physicochemical characterization The pore structure of the as-prepared samples was assessed by N2 adsorption at 77 K using an ASAP2020 (Micromeritics, USA). The specific surface areas of the samples were evaluated using the Brunauer–Emmett–Teller (BET) equations. The pore-size distributions of the samples were determined by the methods of Horvath–Kawazoe (H–K) and Barrett–Joyner–Halenda (BJH). To investigate the changes in the functional groups on the surfaces of the untreated and amino-fluorinated ACs, X-ray photoelectron spectroscopy (XPS, VG Multilab 2000) analysis was performed using Mg Ka radiation. XPS elemental analysis was shown the elemental average value of the sample. The C1s and N1s peaks were deconvoluted to several pseudo-Vogit functions (sums of a Gaussian–Lorentzian function) using a peak analysis program obtained from Unipress Co., USA. The pseudo-Vogit function is given by the following equation [18]:
E E0 6 FðEÞ ¼ H4ð1 SÞ exp lnð2Þ RFWHM
3
2 ! þ 1þ
S EE0 RFWHM
7 2 5
3. Results and discussion 3.1. Surface chemical properties of prepared ACs
of hydrofluoric acid (0.83, 2.06, or 4.11 mol L1) were each added seriatim, respectively. These were mixed with a mechanical shaker for 48 h at room temperature. The treatment conditions of the amino-fluorinated ACs and the names of the samples are listed in Table 1. The treated AC was washed with distilled water until neutral pH and then dried for 24 h at 393 K. The untreated AC was named R-AC. And the amino-fluorinated AC samples are hereinafter referred to as NF10-AC, NF25-AC, and NF50-AC, depending on the concentrations of hydrofluoric acid used, which were 0.83, 2.06, and 4.11 mol L1, respectively.
2
stant currents of 0.1 and 1.0 A g1. All of the electrochemical measurements were performed in a 1 M H2SO4 electrolyte.
ð1Þ
where F(E) is the intensity at energy E, H is the peak height, E0 is the peak center, RFWHM is the full width at half-maximum (FWHM), and S is the shape function related to the symmetry and the Gaussian– Lorentzian mixing ratio. The FWHM values of C1s and N1s were 1.4 and 1.8 eV, respectively. 2.3. Electrochemical characterization EDLC electrodes were fabricated by mixing 80 wt.% untreated or amino-fluorinated AC, 10 wt.% carbon black (Super P, Timcal Ltd., Switzerland), and a 10 wt.% solution of polyvinylidene fluoride (PVDF, Aldrich, USA) in N-methyl pyrrolidone (NMP, Aldrich, USA) to form a slurry, which was then painted onto a titanium plate. The untreated and amino-fluorinated AC-based electrodes were electrochemically characterized using a computer-controlled potentiostat–galvanostat (Ivium Technologies, Netherlands) equipped with a three-electrode assembly. The AC electrode was used as the working electrode, and Ag|AgCl was used as the reference electrode. A platinum plate was used as the counterelectrode. Cyclic voltammetry (CV) of the electrode materials was performed over a potential range of 0–1 V at scan rates of 5 and 50 mV s1. The capacitance of the electrode was measured by a galvanostatic charge–discharge cycle in the potential range of 0–0.9 V at con-
XPS elemental analysis was performed to compare the chemical compositions at the surfaces of the untreated AC and amino-fluorinated ACs. Table 2 lists the atomic percentages of C, O, N, and F for the amino-fluorinated ACs synthesized in this study. Based on the XPS elemental analysis, the O, N, and F contents of the amino-fluorinated ACs were increased over those of the R-AC, rising from an O/C ratio of 8.1, an N/C ratio of 0.0, and an F/C ratio of 0.0 to ratios of 9.9, 2.3, and 0.5, respectively. During amino-fluorination, active sites were introduced on the AC surface by fluorine attack. Hence, the O, N, and F components on the AC increased with increasing hydrofluoric acid concentration because the active sites of the AC surface formed by hydrofluoric acid attack reacted with water to produce other functional groups [17]. However, the NF50-AC, the sample prepared with the highest concentration of hydrofluoric acid in this study, had fewer O, N, and F functional groups than the NF25-AC. In this case, it is likely that the neutralization reaction treated in seriatim with solution of ammonium hydroxide and hydrofluoric acid would occur as the dominant than introducing active sites on the AC surface. In addition, the changes in the chemical bonds of the AC after the treatments were investigated by C1s and N1s deconvolution; the results are depicted in Fig. 1 and 2, respectively. The C and N components are also listed in Table 3 along with the binding-energy peak positions, concentrations, and chemical-bond assignments. In the C1s deconvolution for R-AC, five main peaks for C(1), C(3), C(4), C(5), and C(6) were observed at 284.5, 285.2, 286.1, 287.2, and 288.8 eV, respectively, corresponding to sp2(CAC), sp3(CAC), CAO, C@O, and (C@O)OH bonds. After the amino-fluorination of AC, two additional main peaks for C(3) and C(7) were observed at 285.8 and 290.2 eV, respectively, which correspond to CAN and CAF bonds [19–21]. In the N1s deconvolution for amino-fluorinated ACs, four main peaks were observed at 398.2, 399.4, 400.6, and 405.2 eV, which correspond to pyridine, pyrrole/pyridone, quaternary nitrogen [22,23], and ANAF [17,24,18] groups, respectively. The different types of functional groups were introduced with similar concentrations irrespective of whether the samples had been treated with different concentrations of hydrofluoric acid during amino-fluorination. Both nitrogen functional groups have electron-donor tendencies [25]; therefore, these nitrogen functional groups were expected to impact the electrochemical properties of the AC-based EDLCs. 3.2. Textural properties of the prepared ACs Changes in the pore textures of the ACs after amino-fluorination were investigated by nitrogen adsorption analysis at 77 K. The
Table 2 XPS surface elemental analysis parameters of the untreated and amino-fluorinated activated carbons. Sample
R-AC NF10-AC NF25-AC NF50-AC
Elemental content (At%) C1s
O1s
N1s
F1s
92.5 90.6 88.8 90.2
7.5 7.9 8.8 8.2
– 1.4 2.0 1.3
– 0.2 0.4 0.4
O/C (%)
N/C (%)
F/C (%)
N/F (%)
8.1 8.7 9.9 9.1
– 1.5 2.3 1.4
– 0.2 0.5 0.5
– 10.7 20.5 32.8
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(a) 40000
(b)
35000
50000
C(1)
C(1)
40000
Intensity
Intensity
30000 25000 20000 15000 10000 5000
C(6)
C(2) C(5) C(4)
30000 20000 10000
0
C(3) C(4) C(6) C(5) C(7)
0 292
290
288
286
284
282
280
292
290
Binding energy (eV)
(d) C(1)
40000
Intensity
Intensity
20000
290
288
286
C(2)
282
280
282
C(1)
40000 30000 20000 10000
284
284
60000
C(3) C(4) C(6) C(5) C(7)
0 292
0 292
286
50000
30000
C(3) C(4) C(6) C(5) C(7)
288
Binding energy (eV)
(c)
10000
C(2)
280
Binding energy (eV)
290
288
286
C(2)
284
282
280
Binding energy (eV)
Fig. 1. Deconvolution of the core-level C1s spectra of (a) R-AC, (b) NF10-AC, (c) NF25-AC, and (d) NF50-AC.
Table 3 C1s and N1s peak parameters of the untreated and amino-fluorinated activated carbons.
(a) Intensity
1600
N(2) Peak position Concentration (%) (eV) R-AC NF10-AC NF25-AC NF50-AC
1200
N(3)
N(1)
800
N(4) 400 0 408 406 404 402 400 398 396 394
Binding energy (eV)
(b)
1600
N(2)
Intensity
1200
76.8 11.3 – 7.2 4.6 1.0 – – – –
405.2
–
62.5 13.2 6.3 7.4 5.3 3.6 1.7 15.7 56.6 23.5
59.6 13.5 8.4 7.4 5.5 3.6 2.1 13.8 53.5 28.1
62.3 13.0 6.1 7.1 5.6 3.7 2.1 16.0 54.3 25.8
4.2
4.6
4.0
N(1)
N(4)
0 408 406 404 402 400 398 396 394
Binding energy (eV) 1200
N(2)
Intensity
284.5 285.2 285.8 286.4 287.3 288.8 290.3 398.2 399.4 400.6
N(3)
800
400
(c)
CAC(sp2) CAC(sp3) CAN CAO CAO CAOOH CAF Pyridine Pyrrole /pyridone Quaternary nitrogen N(4) NAF C(1) C(2) C(3) C(4) C(5) C(6) C(7) N(1) N(2) N(3)
800
N(4)
N(3)
N(1)
400
0 408 406 404 402 400 398 396 394
Binding energy (eV) Fig. 2. Deconvolution of the core-level N1s spectra of (a) NF10-AC, (b) NF25-AC, and (c) NF50-AC.
nitrogen adsorption isotherms of the untreated and amino-fluorinated AC samples are presented in Fig. 3. The R-AC sample showed a dramatic increase in the quantity of nitrogen adsorbed below a P/ P0 of 0.01, followed by no significant increase, indicating that a microporous structure was developed [26]. The isotherm curves of the amino-fluorinated ACs increased markedly below a P/P0 of 0.1 and then increased gradually, resulting from the conversion of some micropores into mesopores [27]. Fig. 4 presents the size distributions of the micropores and mesopores calculated using both the HK model and the BJH model. Fig. 4a shows that the number of small micropores (approximately 0.4–0.7 nm in size) was perceptibly decreased by amino-fluorination, whereas the number of large micropores (greater than 1 nm in size) was increased. Furthermore, as illustrated in Fig. 4b and C, the numbers of large micropores and mesopores were increased in the amino-fluorinated ACs compared with the untreated AC. Detailed information regarding the textural properties of the ACs is presented in Table 4. The ACs had specific surface areas of 1924–2319 m2 g1, total pore volumes of 0.87–1.33 cm3 g1, and mesopore volumes of 0.11–0.65 cm3 g1. The specific surface areas,
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M.-J. Jung et al. / Journal of Colloid and Interface Science 381 (2012) 152–157 Table 4 Textural properties of the untreated and amino-fluorinated activated carbons.
3
Quantity Adsorbed (cm /g)
800 700
Sample
Specific surface area (m2 g1)a
Total pore volume (cm3 g1)b
Micropore volume (cm3 g1)c
Mesopore volume (cm3 g1)d
Mesopore volume ratio (%)
R-AC NF10-AC NF25-AC NF50-AC
1924 2319 2254 2237
0.87 1.22 1.14 1.33
0.76 0.74 0.73 0.69
0.11 0.48 0.41 0.65
12.6 39.2 35.8 48.4
600 500 R-AC NF10-AC NF25-AC NF50-AC
400 300
a
0
Specific surface area calculated using the BET method. The total pore volumes were obtained from the volume of nitrogen adsorbed at a relative pressure of 0.98. c Micropore volume calculated using the H-K method. d Calculated as the total pore volume – the micropore volume. b
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0) Fig. 3. Nitrogen adsorption isotherms at 77 K for the untreated and aminofluorinated activated carbons.
(a)
0.007 R-AC NF10-AC NF25-AC NF50-AC
0.005
3
Volume (cm /g)
0.006
3.3. Electrochemical characteristics of the prepared ACs
0.004 0.003 0.002 0.001 0.000 0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Pore width (nm)
(b)
0.12 R-AC NF10-AC NF25-AC NF50-AC
3
Volume (cm /g)
0.10 0.08 0.06 0.04 0.02 0.00 1
2
3
4
5
6
7
8
9
10
Pore width (nm)
(c)
0.12 R-AC NF10-AC NF25-AC NF50-AC
Volume (cm /g)
0.10 3
mesopores than the R-AC, as reported in Table 4. Furthermore, the amino-fluorinated AC samples also had relatively higher specific surface areas. Therefore, we hypothesized that amino-fluorinated ACs should provide better electrochemical performance than the R-AC.
0.08 0.06 0.04 0.02 0.00 1.0
1.5
2.0
2.5
3.0
Pore width (nm) Fig. 4. Pore-size distributions of the untreated and amino-fluorinated activated carbons (a) H–K, (b) BJH, and (c) an expansion of (b).
total pore volumes, and mesopore volumes of the ACs were markedly increased by amino-fluorination, whereas the micropore volumes decreased compared with those of the R-AC. This decrease in the micropore volume occurred because mesopores were formed by the collapse of the smaller micropores due to the etching effect of amino-fluorination [17]. These mesopores are beneficial to the access of the electrolyte into the pore volume and consequently improve ionic transport in EDLCs [28]. The amino-fluorinated ACs had relatively more
The cyclic voltammograms (CVs) of the untreated and aminofluorinated ACs are presented in Fig. 5. The cyclic voltammetry tests on the three-electrode system were conducted within a potential range of 0–1.0 V at several different potential sweep rates. At a scan rate of 5 mV s1 (Fig. 5a), the CV profiles of the as-prepared samples were close to being rectangular shape, and small redox peaks were observed at 0.3–0.4 V, indicating the occurrence of a pseudo-Faradaic reaction caused by O or N functional groups [29–32]. However, we observed no electrochemical reaction peak caused by the F functional group in these CVs [33]. In the CV profiles of the as-prepared samples obtained at 50 mV s1 (Fig. 5b), the CVs displayed a leaf-like shape with no electrochemical reaction peaks [30,34]. The leaf-like shape is attributed to the influence of the ohmic resistance due to electrolyte motion in the carbon pores upon double-layer formation [35,36]. All the samples of aminofluorinated ACs had higher inner-integral areas on the CVs than that of the R-AC; the NF25-AC had the highest specific capacitance. The galvanostatic charge–discharge curves of the untreated and amino-fluorinated ACs at current densities of 0.1 and 1.0 A g1 are presented in Fig. 6. All the charge–discharge curves exhibit triangular shapes, indicating typical capacitive behavior. However, we observed that the amino-fluorinated AC samples had higher specific capacitances than that of the R-AC, and the NF25-AC had the highest specific capacitance. These results are in agreement with the CV results described above. Fig. 6a presents a galvanostatic charge–discharge curve at the relatively small loading current of 0.1 A g1, showing no obvious ohmic drop, indicating that all the AC-based electrodes displayed good capacitive behavior at this loading current. Additionally, at a current density of 1.0 A g1 (Fig. 6b), a sharp change in voltage (an IR drop) was observed for all the samples. However, the IR drop of the amino-fluorinated ACs decreased when compared with that of the R-AC, going from 0.09 to 0.03 V. The introduction of polar nitrogen atoms can improve the affinity of the carbon surface for aqueous electrolytes, thereby increasing the surface area accessible by the electrolytes [37]. The specific capacitances of the untreated and amino-fluorinated AC-based electrodes were estimated from their galvanostatic charge–discharge curves at various current–density loads (Table 5). The specific capacitance was calculated from the following equation [38]:
C ¼ iDt=mDV
ð2Þ
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M.-J. Jung et al. / Journal of Colloid and Interface Science 381 (2012) 152–157
(a)
Table 5 Specific capacitances at various current densities of untreated and amino-fluorinated activated carbons.
3
R-AC 10F-AC 25F-AC 50F-AC
Current (mA/mg)
2
Sample
1
R-AC NF10-AC NF25-AC NF50-AC
0
Specific capacitance at various current density (F g1) 0.1 (A g1)
0.2 (A g1)
0.25 (A g1)
0.5 (A g1)
1.0 (A g1)
2.0 (A g1)
215 244 389 271
188 214 337 234
183 208 325 225
159 192 299 205
138 171 273 187
119 151 250 171
-1
-2 0.0
0.2
0.4
0.6
0.8
1.0
0.8
1.0
Voltage (V)
(b) R-AC 10F-AC 25F-AC 50F-AC
16
Current (mA/mg)
12 8 4 0 -4 -8 -12 0.0
0.2
0.4
0.6
Voltage (V)
4. Conclusions
Fig. 5. Cyclic voltammograms of the untreated and amino-fluorinated activated carbon-based electrodes obtained at (a) 5 mV s1 and (b) 50 mV s1.
(a)
1.0
R-AC NF10-AC NF25-AC NF50-AC
Potential (V)
0.8 0.6 0.4 0.2 0.0 0
1000
2000
3000
4000
5000
where i is the current density, Dt is the discharge time, m is the mass of AC, and DV is the potential range of the charge–discharge cycle. As a result, the specific capacitance of the amino-fluorinated ACs was markedly increased. The NF25-AC exhibited the highest specific capacitance of 389 F g1 at a current density of 0.1 A g1 and 250 F g1 at a current density of 1.0 A g1. The enhancement of the specific capacitance of the amino-fluorinated AC-based EDLC can be attributed to the observed increases in the specific surface area and the changes in pore structure of the AC in combination with an increased electrochemical activity due to the N and F functional groups after amino-fluorination. In a previous study, we found that the effect of simultaneous N and F introduction on AC was better than the individual introduction of N or F with respect to EDLC performance [17]. The results of this study demonstrate that the NF25-AC is suitably optimized for use as an EDLC electrode material.
6000
Amino-fluorination is a novel surface modification method for porous materials such as AC during which simultaneous reactions occurs between AC, ammonium hydroxide, and hydrofluoric acid. In this work, amino-fluorination was found to induce changes in the surface chemical and textural properties and the electrochemical properties of AC-based EDLCs. The amino-fluorinated ACs exhibited remarkably increased specific surface areas, total pore volumes, mesopore volumes, and numbers of N-, O-, and F-containing surface functional groups. These changes resulted in the highest specific capacitance of 389 F g1 when the amino-fluorination solution was optimized by treated seriatim with solution of 7.43 mol L1 ammonium hydroxide and 2.06 mol L1 hydrofluoric acid. Therefore, the amino-fluorination of AC is a simple and efficient way to enhance the performance of AC-based EDLCs. Acknowledgments
7000
Time (s)
(b)
R-AC NF10-AC NF25-AC NF50-AC
0.8
Potential (V)
This work was supported by the IT R&D program of MKE/KEIT [KI002177-2012-04, Pouch/Radial type Lithium Ion Capacitor for Ubiquitous Electronics].
1.0
References
0.6 0.4 0.2 0.0 0
100
200
300
400
500
Time (s) Fig. 6. Galvanostatic charge–discharge curves of the untreated and amino-fluorinated ACs at current densities of (a) 0.1 A g1 and (b) 1.0 A g1.
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