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Carbon aerogels as electrode material for electrical double layer supercapacitors—Synthesis and properties
Electrochimica Acta 55 (2010) 7501–7505
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Carbon aerogels as electrode material for electrical double layer supercapacitors—Synthesis and properties Agnieszka Halama, Bronislaw Szubzda ∗ , Grzegorz Pasciak Electrotechnical Institute, Division of Electrotechnology and Materials Science, Wroclaw, Poland
a r t i c l e
i n f o
Article history: Received 15 September 2009 Received in revised form 10 March 2010 Accepted 13 March 2010 Available online 20 March 2010 Keywords: Quality of electricity Carbon aerogels Electric double layer supercapacitor Electrochemical properties Surfactant
a b s t r a c t This paper constitutes a description of technological research the aim of which was to design a symmetric supercapacitor dedicated for the system of quality of electrical energy improvement (supply interruption, voltage dip). The main task was to use the carbon aerogel technology as the efficient method for production of electrode material with desirable properties. Carbon aerogels were prepared by carbonization of resorcinol–formaldehyde (RF) polymer gels. RF-gels were synthesized by curing polycondensation and by the inverse emulsion polymerization of resorcinol with formaldehyde, followed by microwave drying. The morphostructural characteristics of the carbon aerogels were investigated by atomic force microscopy (AFM) and the N2 adsorption (BET method). The electrochemical properties were characterized by means of cycle voltammetry, galvanostatic charging/discharging, and self-discharge. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, the quality of electrical energy has become one of the most important problems in energetic industry. The presence of high advanced electronic and electro-technical machines, whose number is constantly increasing, necessitates a high level of the quality of electricity. However, the increasing number of active receivers of energy has a negative influence on the quality of electricity. It is especially noticeable in the case of: electric machines, devices modifying current frequency, high-power receivers, etc. Additionally, more and more frequently occurring connections from various energy sources to the common electrical circuit also requires a homogeneous quality. Under all these conditions highpower systems, which are capable of active current characteristic shaping, become essential. In these systems capacitors are very important ingredients which due to large amount of controlled energy have to be replaced by supercapacitors. In this sort of applications special properties of supercapacitors are utilized, other than the ones being currently under development worldwide. These devices should be symmetric and characterized by very short response time, should operate at high current intensity, even at the expense of self-discharge. These properties can be obtained by the presence of active groups red-ox on the surface of electrode material. Presented descriptions constitute part of the technological research leading to the valuation of possibilities to produce such
∗ Corresponding author. Tel.: +48 0713283061; fax: +48 0713282551. E-mail address: [email protected] (B. Szubzda). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.03.040
materials using the carbon aerogels technique. The main aim of our investigation was to determine whether it was possible to control and design properties of the final carbon material of electrodes in the phase of production of its polymer precursor (Tables 1 and 2). Carbon aerogels belong to the special class of nanostructure carbons prepared via the pyrolysis of organic polymer precursors at elevated temperatures under inert atmospheres [1]. More importantly, the texture of aerogels (surface area and pore size distribution) can be modified as a function of different synthesis parameters e.g.: selection of the synthesis route; type of the precursor (reactants, solvent, catalyst); the curing and drying methods and the pyrolysis conditions [2]. Even though the properties of carbon aerogels can be enhanced by a choice of activation conditions, the structure of organic precursors significantly influences the final carbon materials [3,4]. Theoretically, the storage capacity (and charging speed) in EDLCs is proportional to the surface area of the porous carbon electrodes. Therefore, the correlation between a capacitance and pore structure has been widely investigated by many researchers. These results, reported in the literature, show that the electrochemical capacitance is generally proportional to the specific surface area for the same kind of carbon materials, which have been prepared with the same preparation method just by changing the preparation parameters. However, according to the literature data, the capacitance depends not only on the surface area but also on the carbon structure, pore volume, pore size distribution, particle size, electrical conductivity, the surface functional groups of the electrode materials as well as the electrolyte composition [5–11].
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A. Halama et al. / Electrochimica Acta 55 (2010) 7501–7505
Table 1 Preparation conditions of RF-gels via polycondensation. Sample
Gelation temperature (◦ C)
pH
RF 0
6
Gelation time (h)
20
24
Heat treated Temperature (◦ C)
Time (h)
30 50
15 13
Table 2 Preparation conditions of the RF-gels via inverse emulsion polymerization. Sample
Surfactant
Concentrations of RF in cyclohexane (%)
pH
Temperature (◦ C)
Time (h)
Stirring speed (rpm)
RFSL
Ludwik
10
6
60
6
300
Carbon aerogels are mostly obtained through reactions of sol–gel with resorcinol (1,3–dihydroksybenzene) and formaldehyde. This reaction was described by Pekala in 1987 and it is so far the best known and examined reaction of achieving carbon aerogels of desired structure. The synthesis of these materials is usually based on the polycondensation of resorcinol with formaldehyde using Na2 CO3 as catalyst (C). Supercritical drying of RF-gels followed by a pyrolysis treatment usually gives rise to mesoporous carbon aerogels [12,13]. In this method, the control of meso- and micro-porosity takes place through the proper choice of molar ratios of resorcinol to catalyst (R/C) and resorcinol to water (R/W). However, this process of producing carbon aerogels as sug˛ gested by Pekala is time-consuming and expensive because it requires the employment of supercritical conditions in the process of solvent removal. Therefore, many researchers seek other methods of synthesis of carbon materials which are characterized by controlled texture. These methods include solvent removal in freeze-drying conditions [14,15], with the employment of nitrogen [16], drying in ambient conditions [17,18], or with the use of microwaves [19,20]. Some other methods include modifications of the synthesis conditions, namely, the usage of a silicon matrix [21,22] or synthesis in emulsion in the presence of surfactant [15,23,24]. The aim of this study was the comparison of the influence of the synthesis method of resorcinol-formaldehyde (RF) polymer gel precursors on electrochemical characteristics of carbon aerogel electrodes for the application of EDL supercapacitor. In this study only the synthesis method was changed while the molar ratios (R/C) and (R/W) were maintained constant.
In the first method, resorcinol (C6 H4 (OH)2 ) was initially dissolved in deionized water, followed by adding sodium carbonate (Na2 CO3 ) and formaldehyde (HCOH) into the solution, which was stirred by a magnetic stirrer. After gelation was performed, the sample was heat treated. In the other method, spherical RF-gel particles were synthesized by emulsion polymerization; viscous RF-sols obtained by the first method were dispersed into cyclohexane solution containing
2. Experimental 2.1. Materials Resorcinol (98%) was purchased from Aldrich Chemical Co. Formaldehyde (36–38% in water, stabilized with methanol), sodium carbonate (anhydrous), potassium hydroxide and cyclohexane was purchased from POCH. Ludwik was purchased from Inco-Veritas S.A. All reagents were used without further purification. 2.2. Preparation of carbon aerogels RF-gels were synthesized by the polymerization of resorcinol (R) and formaldehyde (F), by using sodium carbonate as a basic catalyst (C) and deionized water (W) as a solvent. The molar ratios of resorcinol to formaldehyde (R/F), resorcinol to catalyst (R/C) and the molar ratio of resorcinol to water (R/W) were fixed at 0.5 mol/mol, 100 mol/mol and 0.08 mol/mol, respectively. Two methods of synthesis of RF-gels were used: a sol–gel polycondensation and an inverse emulsion polymerization.
Fig. 1. (A) AFM planar image of RFO polymer precursor after drying (projected area 49 m2 ). (B) AFM planar image of RFSL polymer precursor after drying (projected area 16 m2 ).
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Fig. 2. (A) AFM image of fragment of RFO carbon aerogel. (B) AFM image of fragment RFSL carbon aerogel.
a surfactant. The concentrations of RF solutions in the organic continuous phase (v/v; volume of RF solution/volume of cyclohexane) were 10%. The ratio of surfactant to volume of RF solution was fixed at 1 (g/ml). The emulsions were agitated at the speed of 300 rpm and the constant temperature (60 ◦ C) for 6 h to obtain RF microspheres. The wet gels were immersed in acetone to exchange the water inside the pores and then dried by microwave drying, using the power of 450 W for 10 min. The pyrolysis of the dried aerogels was conducted in a tubular furnace under a constant argon gas flow of 60 cm3 /min. The RC aerogels were first heated to 1100 ◦ C at the constant heating rate of 6 ◦ C/min, and were kept at this temperature for 120 min. After pyrolysis, the furnace was left to cool down to room temperature under argon flow. 3. Results and discussion In order to evaluate the influence of the method and conditions of obtaining polymer–precursor–on properties of final carbon material, two samples of material with different properties were prepared. Our task was to determine the correlation between specific technological changes at the synthesis stage and the structure and electrochemical properties of carbon electrodes of a supercapacitor, which were formed from final material after pyrolysis. The results of the research are presented by comparison of two materials, obtained by two methods: RFO–polycondensation method and RFSL–polymerization in emulsion. The aim of replacement of polymer gellation in polycondensation process by polymerization in emulsion was to answer the question whether high granularity of substrates in uniformly dispersed emulsion in the phase of precursor production causes increased micro- and meso-porosity of the output material.
3.1. Structure and physical properties In order to research the relationship between structures and electrochemical properties of the samples, the morphology of their surface was studied with the use of atomic force microscopy (AFM) (Innova Veeco microscope) in the tapping mode. The particle size distributions of the carbon aerogels were measured by a laser diffraction particle size analyzer (FRITSCH, analysett22 MicroTec). The pore size distribution and specific surface area of carbon samples were measured by the N2 adsorption–desorption method with an ASAP 2020 (Micromeritics) instrument. The AFM images of the surface topography of RFO and RFSL polimer precursors were shown in Fig. 1A and B. Both materials show a granular morphology, being the result of the cluster aggregation process and roughness of the surface. Therefore, the structure of carbon material obtained after pyrolysis remains in correlation with the structure of precursors. In Fig. 2A and B the surface topography of carbon single grain RFO and RFSL is presented. We can observe here the roughness of surface as well as the development of surface – a particle and pore size. Differences in the scale are due to various agglomeration of carbon particles and their size, therefore, in order to illustrate the roughness of surface we have employed various scales. It is easy to notice that the surface of RFSL grain is highly smooth (Rms = 21.6 nm), while the surface of RFO is characterized by high roughness which would suggest that carbon particles create clusters which are more packed. The numerical analysis of AFM graphs shows the average particle size of 200 nm for RFO and 21 nm for RFSL. Estimated average pore diameters are 16 nm for RFO and 1.5 nm for RFSL materials.
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Fig. 4. Current–voltage curves obtained in the cyclic voltammetry test of RFO and RFSL carbon materials.
Fig. 3. Particle size distribution of RFO and RFSL carbon aerogels.
In Fig. 3 the distribution of sizes of particles granulated for 2 h in the planetary mill (Fritsch) is presented. Another property of carbon material, which is controllable by means of carbon aerogels technique, is easiness and efficiency of granulation of final carbon material. In Fig. 3 one can notice that the average particle size of material RFSL is two times smaller than RFO. The analysis results mentioned above are fully confirmed by the examination of porosity made by the BET nitrogen adsorption method (adsorption–desorption isotherms were measured using ASAP 2020 Micromeritics). The results refer to the porosity examination which shows the specific surface of final carbon material RFSL (1670 m2 /g) – whose precursor was obtained by polymerization in the emulsion method – about ten times larger than the specific surface of RFO (189 m2 /g). It proves that a carbon aerogels method allows to design the structure of materials used in production of supercapacitor electrodes in the phase of preparation of its polymer precursor. RFSL material has a very high value of the specific surface which results from over 90% contribution of micropores of size smaller than 2 nm (Table 3).
• galvanostatic charging and discharging at the voltage 0–600 mV, value of the current intensity 300 mA, mass of each investigated electrodes was 300 mg, • outflow current measurement during self-discharge of supercapacitor, voltage 600 mV. In Fig. 4 the comparison of charts resulting from the cyclic voltammetry test is presented. The analysis of curves in the figure gives additional information about the influence of parameters of precursor preparation on properties of the supercapacitors electrodes obtained from the final carbon material. Additionally, one can notice two times higher value of intensity of the current, which resulted from electrodes potential on the whole scanning segment. This indicates higher electrical capacitance of the capacitor, which should be expected given previous results concerning porosity of both considered materials. An important feature of both graphs is that the current–voltage characteristics are not parallel to the horizontal voltage axis. It means that the process of charging of the double layer of capacitor is accompanied by electrode processes occurring at electrodes. This phenomena is undesirable in energystoring supercapacitors, since it results in rapid self-discharges and
3.2. Electrochemical properties of electrode material in supercapacitor test model Electrochemical tests were carried out in a model supercapacitor cell in which electrodes were formed by the described carbon aerogels, comminuted to the fraction <100 m. The supercapacitor model tests were carried out with ATLAS 0531 Electrochemical Unit & Impedance Analiser. There was a twoelectrode measurement system and 6 M KOH solution was used as an electrolyte. Final carbons aerogels were formed as round electrodes, diameter 40 mm, thickness 0.6 mm, each mass 300 mg. Single experiment consisted of: • cyclic voltammetry test, carried out at a rate of potential increase 1 mV/s to the value of 0.9 V, • impedance spectroscopy, test voltage 0,4 V and sinusoidal impulses 100 mV with frequency from 1 mHz to 10 kHz,
Fig. 5. Nyquist plot–impedance spectroscopy of RFO and RFSL carbon materials.
Table 3 Specific surface of RFSL I material measured with BET method. SBET [m2 /g]
1670
<2 nm
2–3 nm
3–5 nm
5–10 nm
10–50 nm
>50 nm
[m3 /g]
[%]
[m3 /g]
[%]
[m3 /g]
[%]
[m3 /g]
[%]
[m3 /g]
[%]
[m3 /g]
[%]
0.677
90.3
0.041
5.5
0.015
2.0
0.008
1.1
0.008
1.1
0.001
0.1
A. Halama et al. / Electrochimica Acta 55 (2010) 7501–7505
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Table 4 Values of capacities and outflow current of carbon materials, obtained in galvanostatic discharge and self-discharge tests. Investigated material
Capacitance (F g−1 )
RFO RFSL
68 150
low energetic efficiency. However, as pointed out in the introduction, it is a desirable effect in the considered application and it is caused on purpose by utilization of additives which produce chemical active groups on the surface of materials. Active groups were projected for hydrophilic properties of final carbons for water based electrolytes. After preparation also hydrophobic groups we noticed. During the further research, samples with both water and organic electrolytes will be tested. Both materials differ in the kind and method of precursor production. One can notice that it yielded different effects – the slope of characteristic curve for RFO material is higher than the one for RSFL. Moreover, for potential values of 0.4–0.5 V a clear effect of electrochemical process occurring during discharges can be observed. This effect is likely to be related to the red-ox process occurring at the surface or inside the carbon material with the potential value close to the solvents (i.e. water) decomposition potential. This process may affect the value of supercapacitors capacitance. Nyquist plot in Fig. 5 shows the electrochemical impedance results of tested RFO and RFSL materials. In the figure we can see two various regions depending on frequencies. At high frequencies semicircles and linear part at low frequencies are similar for both investigated materials. Semicircles describe resistance of the solution and the linear part of the graph-diffusion to the electrodes. The linear part of RFO plot is parallel to 45◦ line and it is closer to the vertical axis. It seems a better capacitive behavior then RFSL. The charge transfer resistance is similar for both materials, i.e. about 1 cm−2 . The results of the following electrochemical tests, which are presented in Table 4, constitute really valuable information that confirm advantages of the carbon aerogels method applied to obtain any configurations of electrochemical properties of electrode carbon materials. Both special properties of investigated materials and the applied method of preparation of their precursors create a great tool for designing carbon materials that possess the expected features. The comparison of materials RFO and RFSL analyzed above shows over two times higher value of specific capacitance of RFSL material. It is correlated with the results of cyclic voltammetry tests; however, it does not result from the surface area results, where the value for RFSL was ten times higher (Table 3). The value of capacitance higher than a specific porosity ratio may result from the electrode phenomenon, which introduces pseudocapacitance. Perhaps, the image of this phenomenon can be observed on the abovementioned voltammetry graph (Fig. 4). RFSL carbon material exhibited the highest value of specific capacitance. Therefore, further research, the aim of which is to determine the relationship between technological methods of precursor production and electrochemical properties of electrodes obtained from final material, will be focused on RFSL material. As it was mentioned before, the comparison of polycondensation and polymerization in emulsion processes showed that the granularity of reagents during polymerization of precursor influenced the structure of the final material.
Self-discharge Outflow current (mA)
Voltage after 1 h/V
3.8 22.1
0.177 0.033
4. Conclusions The investigation results fully confirm the basic assumption of this work, namely that it is possible to control to a wide extent these properties of carbon aerogels which are important for the application in supercapacitors. The obtained materials were characterized by a large surface which, during measurements, showed high specific capacitance. A possibility of achieving hydrophilic and hydrophobic properties was evaluated. After carrying out voltammetry tests it is possible to select materials for which no unfavorable electrochemical processes take place (such as water decomposition) in the range of working potentials. The influence of the method of material synthesis on the value of outflow current of supercapacitor was observed under experimental conditions. Summarizing the electrochemical investigations, it should be stated that the materials which were successfully produce may be very well suited for the production of electrodes for symmetric supercapacitors working on the principle of double electric layer; however, these materials will be subject to further investigations. References [1] G.A.M. Reynolds, A.W.P. Fung, Z.H. Wang, M.S. Dresselhaus, P.W. Pekala, J. NonCryst. Solids 188 (1995) 27. [2] S.A. Al-Muhtaseb, J.A. Ritter, Adv. Mater. 15 (2003) 101. [3] Y. Hanzawa, K. Kaneko, R.W. Pekala, M.S. Dresselhaus, Langmuir 12 (1996) 6167. [4] D. Wu, Z. Sun, R. Fu, J. Appl. Polym. Sci. 99 (5) (2006) 2263. ˜ [5] E. Raymundo-Pinero, K. Kierzek, J. Machnikowski, F. Béguin, Carbon 44 (2006) 2498. [6] H. Liu, G. Zhu, J. Power Sources 171 (2007) 1054. [7] A.G. Pandolfo, A.F. Hollenkamp, J. Power Sources 157 (2006) 11. [8] T.A. Centeno, F. Stoeckli, J. Power Sources 154 (2006) 314. [9] Q.H. Meng, L. Liu, H.H. Song, R. Zhang, L.C. Ling, J. Inorg. Mater. 19 (2004) 593. [10] P. Azais, L. Duclaux, P. Florian, D. Massiot, M.A. Lillo-Rodenas, A. Linares-Solano, J.P. Peres, C. Jehoulet, F. Beguin, J. Power Sources 171 (2007) 1046. [11] X.G. Zhuang, Y.S. Yang, Y.J. Ji, D.P. Yang, Z.Y. Tang, ACTA Phys. -Chem. Sin. 19 (2003) 689. [12] R.W. Pekala, US Pat. 4873218 (1988). [13] R.W. Pekala, US Pat. 4997804 (1991). [14] H. Tamon, H. Ishizaka, T. Yamamoto, T. Suzuki, Carbon 38 (2000) 1099. [15] S.I. Kim, T. Yamamoto, A. Endo, T. Ohmori, M. Nakaiwa, J. Ind. Eng. Chem. 12 (2006) 484. [16] C. Lin, J.A. Ritter, Carbon 35 (1997) 1271. [17] S.T. Mayer, J.L. Kaschmitter, R.W. Pekala, US Pat. 5420168 (1995). [18] J. Li, X.Y. Wang, Q.H. Huang, S. Gamboa, P.J. Sebastian, J. Power Sources 158 (2006) 784. [19] T. Yamamoto, T. Nishimura, T. Suzuki, H. Tamon, Dry Technol. 19 (2004) 1319. [20] L. Zubizarreta, A. Arenillas, A. Domínguez, J.A. Menéndez, J.J. Pis, J. Non-Cryst. Solids 354 (2008) 817. [21] S. Han, K. Sohn, T. Hyeon, Chem. Mater. 12 (11) (2000) 3337. [22] J.B. Joo, P. Kim, W. Kim, J. Kim, J. Yi, Catal. Today 111 (2006) 171. [23] C.T. Alviso, R.W. Pekala, J. Gross, X. Lu, R. Caps, J. Fricke, J. Mater. Res. Soc. Symp. Proc., Micropor. Macropor. Mater. 431 (1996) 521. [24] N. Tonanon, W. Tanthapanichakoon, T. Yamamoto, H. Nishihara, S.R. Mukai, H. Tamon, Carbon 41 (2003) 2981.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Carbon aerogels as electrode material for electrical double layer supercapacitors—Synthesis and properties Agnieszka Halama, Bronislaw Szubzda ∗ , Grzegorz Pasciak Electrotechnical Institute, Division of Electrotechnology and Materials Science, Wroclaw, Poland
a r t i c l e
i n f o
Article history: Received 15 September 2009 Received in revised form 10 March 2010 Accepted 13 March 2010 Available online 20 March 2010 Keywords: Quality of electricity Carbon aerogels Electric double layer supercapacitor Electrochemical properties Surfactant
a b s t r a c t This paper constitutes a description of technological research the aim of which was to design a symmetric supercapacitor dedicated for the system of quality of electrical energy improvement (supply interruption, voltage dip). The main task was to use the carbon aerogel technology as the efficient method for production of electrode material with desirable properties. Carbon aerogels were prepared by carbonization of resorcinol–formaldehyde (RF) polymer gels. RF-gels were synthesized by curing polycondensation and by the inverse emulsion polymerization of resorcinol with formaldehyde, followed by microwave drying. The morphostructural characteristics of the carbon aerogels were investigated by atomic force microscopy (AFM) and the N2 adsorption (BET method). The electrochemical properties were characterized by means of cycle voltammetry, galvanostatic charging/discharging, and self-discharge. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, the quality of electrical energy has become one of the most important problems in energetic industry. The presence of high advanced electronic and electro-technical machines, whose number is constantly increasing, necessitates a high level of the quality of electricity. However, the increasing number of active receivers of energy has a negative influence on the quality of electricity. It is especially noticeable in the case of: electric machines, devices modifying current frequency, high-power receivers, etc. Additionally, more and more frequently occurring connections from various energy sources to the common electrical circuit also requires a homogeneous quality. Under all these conditions highpower systems, which are capable of active current characteristic shaping, become essential. In these systems capacitors are very important ingredients which due to large amount of controlled energy have to be replaced by supercapacitors. In this sort of applications special properties of supercapacitors are utilized, other than the ones being currently under development worldwide. These devices should be symmetric and characterized by very short response time, should operate at high current intensity, even at the expense of self-discharge. These properties can be obtained by the presence of active groups red-ox on the surface of electrode material. Presented descriptions constitute part of the technological research leading to the valuation of possibilities to produce such
∗ Corresponding author. Tel.: +48 0713283061; fax: +48 0713282551. E-mail address: [email protected] (B. Szubzda). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.03.040
materials using the carbon aerogels technique. The main aim of our investigation was to determine whether it was possible to control and design properties of the final carbon material of electrodes in the phase of production of its polymer precursor (Tables 1 and 2). Carbon aerogels belong to the special class of nanostructure carbons prepared via the pyrolysis of organic polymer precursors at elevated temperatures under inert atmospheres [1]. More importantly, the texture of aerogels (surface area and pore size distribution) can be modified as a function of different synthesis parameters e.g.: selection of the synthesis route; type of the precursor (reactants, solvent, catalyst); the curing and drying methods and the pyrolysis conditions [2]. Even though the properties of carbon aerogels can be enhanced by a choice of activation conditions, the structure of organic precursors significantly influences the final carbon materials [3,4]. Theoretically, the storage capacity (and charging speed) in EDLCs is proportional to the surface area of the porous carbon electrodes. Therefore, the correlation between a capacitance and pore structure has been widely investigated by many researchers. These results, reported in the literature, show that the electrochemical capacitance is generally proportional to the specific surface area for the same kind of carbon materials, which have been prepared with the same preparation method just by changing the preparation parameters. However, according to the literature data, the capacitance depends not only on the surface area but also on the carbon structure, pore volume, pore size distribution, particle size, electrical conductivity, the surface functional groups of the electrode materials as well as the electrolyte composition [5–11].
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A. Halama et al. / Electrochimica Acta 55 (2010) 7501–7505
Table 1 Preparation conditions of RF-gels via polycondensation. Sample
Gelation temperature (◦ C)
pH
RF 0
6
Gelation time (h)
20
24
Heat treated Temperature (◦ C)
Time (h)
30 50
15 13
Table 2 Preparation conditions of the RF-gels via inverse emulsion polymerization. Sample
Surfactant
Concentrations of RF in cyclohexane (%)
pH
Temperature (◦ C)
Time (h)
Stirring speed (rpm)
RFSL
Ludwik
10
6
60
6
300
Carbon aerogels are mostly obtained through reactions of sol–gel with resorcinol (1,3–dihydroksybenzene) and formaldehyde. This reaction was described by Pekala in 1987 and it is so far the best known and examined reaction of achieving carbon aerogels of desired structure. The synthesis of these materials is usually based on the polycondensation of resorcinol with formaldehyde using Na2 CO3 as catalyst (C). Supercritical drying of RF-gels followed by a pyrolysis treatment usually gives rise to mesoporous carbon aerogels [12,13]. In this method, the control of meso- and micro-porosity takes place through the proper choice of molar ratios of resorcinol to catalyst (R/C) and resorcinol to water (R/W). However, this process of producing carbon aerogels as sug˛ gested by Pekala is time-consuming and expensive because it requires the employment of supercritical conditions in the process of solvent removal. Therefore, many researchers seek other methods of synthesis of carbon materials which are characterized by controlled texture. These methods include solvent removal in freeze-drying conditions [14,15], with the employment of nitrogen [16], drying in ambient conditions [17,18], or with the use of microwaves [19,20]. Some other methods include modifications of the synthesis conditions, namely, the usage of a silicon matrix [21,22] or synthesis in emulsion in the presence of surfactant [15,23,24]. The aim of this study was the comparison of the influence of the synthesis method of resorcinol-formaldehyde (RF) polymer gel precursors on electrochemical characteristics of carbon aerogel electrodes for the application of EDL supercapacitor. In this study only the synthesis method was changed while the molar ratios (R/C) and (R/W) were maintained constant.
In the first method, resorcinol (C6 H4 (OH)2 ) was initially dissolved in deionized water, followed by adding sodium carbonate (Na2 CO3 ) and formaldehyde (HCOH) into the solution, which was stirred by a magnetic stirrer. After gelation was performed, the sample was heat treated. In the other method, spherical RF-gel particles were synthesized by emulsion polymerization; viscous RF-sols obtained by the first method were dispersed into cyclohexane solution containing
2. Experimental 2.1. Materials Resorcinol (98%) was purchased from Aldrich Chemical Co. Formaldehyde (36–38% in water, stabilized with methanol), sodium carbonate (anhydrous), potassium hydroxide and cyclohexane was purchased from POCH. Ludwik was purchased from Inco-Veritas S.A. All reagents were used without further purification. 2.2. Preparation of carbon aerogels RF-gels were synthesized by the polymerization of resorcinol (R) and formaldehyde (F), by using sodium carbonate as a basic catalyst (C) and deionized water (W) as a solvent. The molar ratios of resorcinol to formaldehyde (R/F), resorcinol to catalyst (R/C) and the molar ratio of resorcinol to water (R/W) were fixed at 0.5 mol/mol, 100 mol/mol and 0.08 mol/mol, respectively. Two methods of synthesis of RF-gels were used: a sol–gel polycondensation and an inverse emulsion polymerization.
Fig. 1. (A) AFM planar image of RFO polymer precursor after drying (projected area 49 m2 ). (B) AFM planar image of RFSL polymer precursor after drying (projected area 16 m2 ).
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Fig. 2. (A) AFM image of fragment of RFO carbon aerogel. (B) AFM image of fragment RFSL carbon aerogel.
a surfactant. The concentrations of RF solutions in the organic continuous phase (v/v; volume of RF solution/volume of cyclohexane) were 10%. The ratio of surfactant to volume of RF solution was fixed at 1 (g/ml). The emulsions were agitated at the speed of 300 rpm and the constant temperature (60 ◦ C) for 6 h to obtain RF microspheres. The wet gels were immersed in acetone to exchange the water inside the pores and then dried by microwave drying, using the power of 450 W for 10 min. The pyrolysis of the dried aerogels was conducted in a tubular furnace under a constant argon gas flow of 60 cm3 /min. The RC aerogels were first heated to 1100 ◦ C at the constant heating rate of 6 ◦ C/min, and were kept at this temperature for 120 min. After pyrolysis, the furnace was left to cool down to room temperature under argon flow. 3. Results and discussion In order to evaluate the influence of the method and conditions of obtaining polymer–precursor–on properties of final carbon material, two samples of material with different properties were prepared. Our task was to determine the correlation between specific technological changes at the synthesis stage and the structure and electrochemical properties of carbon electrodes of a supercapacitor, which were formed from final material after pyrolysis. The results of the research are presented by comparison of two materials, obtained by two methods: RFO–polycondensation method and RFSL–polymerization in emulsion. The aim of replacement of polymer gellation in polycondensation process by polymerization in emulsion was to answer the question whether high granularity of substrates in uniformly dispersed emulsion in the phase of precursor production causes increased micro- and meso-porosity of the output material.
3.1. Structure and physical properties In order to research the relationship between structures and electrochemical properties of the samples, the morphology of their surface was studied with the use of atomic force microscopy (AFM) (Innova Veeco microscope) in the tapping mode. The particle size distributions of the carbon aerogels were measured by a laser diffraction particle size analyzer (FRITSCH, analysett22 MicroTec). The pore size distribution and specific surface area of carbon samples were measured by the N2 adsorption–desorption method with an ASAP 2020 (Micromeritics) instrument. The AFM images of the surface topography of RFO and RFSL polimer precursors were shown in Fig. 1A and B. Both materials show a granular morphology, being the result of the cluster aggregation process and roughness of the surface. Therefore, the structure of carbon material obtained after pyrolysis remains in correlation with the structure of precursors. In Fig. 2A and B the surface topography of carbon single grain RFO and RFSL is presented. We can observe here the roughness of surface as well as the development of surface – a particle and pore size. Differences in the scale are due to various agglomeration of carbon particles and their size, therefore, in order to illustrate the roughness of surface we have employed various scales. It is easy to notice that the surface of RFSL grain is highly smooth (Rms = 21.6 nm), while the surface of RFO is characterized by high roughness which would suggest that carbon particles create clusters which are more packed. The numerical analysis of AFM graphs shows the average particle size of 200 nm for RFO and 21 nm for RFSL. Estimated average pore diameters are 16 nm for RFO and 1.5 nm for RFSL materials.
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Fig. 4. Current–voltage curves obtained in the cyclic voltammetry test of RFO and RFSL carbon materials.
Fig. 3. Particle size distribution of RFO and RFSL carbon aerogels.
In Fig. 3 the distribution of sizes of particles granulated for 2 h in the planetary mill (Fritsch) is presented. Another property of carbon material, which is controllable by means of carbon aerogels technique, is easiness and efficiency of granulation of final carbon material. In Fig. 3 one can notice that the average particle size of material RFSL is two times smaller than RFO. The analysis results mentioned above are fully confirmed by the examination of porosity made by the BET nitrogen adsorption method (adsorption–desorption isotherms were measured using ASAP 2020 Micromeritics). The results refer to the porosity examination which shows the specific surface of final carbon material RFSL (1670 m2 /g) – whose precursor was obtained by polymerization in the emulsion method – about ten times larger than the specific surface of RFO (189 m2 /g). It proves that a carbon aerogels method allows to design the structure of materials used in production of supercapacitor electrodes in the phase of preparation of its polymer precursor. RFSL material has a very high value of the specific surface which results from over 90% contribution of micropores of size smaller than 2 nm (Table 3).
• galvanostatic charging and discharging at the voltage 0–600 mV, value of the current intensity 300 mA, mass of each investigated electrodes was 300 mg, • outflow current measurement during self-discharge of supercapacitor, voltage 600 mV. In Fig. 4 the comparison of charts resulting from the cyclic voltammetry test is presented. The analysis of curves in the figure gives additional information about the influence of parameters of precursor preparation on properties of the supercapacitors electrodes obtained from the final carbon material. Additionally, one can notice two times higher value of intensity of the current, which resulted from electrodes potential on the whole scanning segment. This indicates higher electrical capacitance of the capacitor, which should be expected given previous results concerning porosity of both considered materials. An important feature of both graphs is that the current–voltage characteristics are not parallel to the horizontal voltage axis. It means that the process of charging of the double layer of capacitor is accompanied by electrode processes occurring at electrodes. This phenomena is undesirable in energystoring supercapacitors, since it results in rapid self-discharges and
3.2. Electrochemical properties of electrode material in supercapacitor test model Electrochemical tests were carried out in a model supercapacitor cell in which electrodes were formed by the described carbon aerogels, comminuted to the fraction <100 m. The supercapacitor model tests were carried out with ATLAS 0531 Electrochemical Unit & Impedance Analiser. There was a twoelectrode measurement system and 6 M KOH solution was used as an electrolyte. Final carbons aerogels were formed as round electrodes, diameter 40 mm, thickness 0.6 mm, each mass 300 mg. Single experiment consisted of: • cyclic voltammetry test, carried out at a rate of potential increase 1 mV/s to the value of 0.9 V, • impedance spectroscopy, test voltage 0,4 V and sinusoidal impulses 100 mV with frequency from 1 mHz to 10 kHz,
Fig. 5. Nyquist plot–impedance spectroscopy of RFO and RFSL carbon materials.
Table 3 Specific surface of RFSL I material measured with BET method. SBET [m2 /g]
1670
<2 nm
2–3 nm
3–5 nm
5–10 nm
10–50 nm
>50 nm
[m3 /g]
[%]
[m3 /g]
[%]
[m3 /g]
[%]
[m3 /g]
[%]
[m3 /g]
[%]
[m3 /g]
[%]
0.677
90.3
0.041
5.5
0.015
2.0
0.008
1.1
0.008
1.1
0.001
0.1
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Table 4 Values of capacities and outflow current of carbon materials, obtained in galvanostatic discharge and self-discharge tests. Investigated material
Capacitance (F g−1 )
RFO RFSL
68 150
low energetic efficiency. However, as pointed out in the introduction, it is a desirable effect in the considered application and it is caused on purpose by utilization of additives which produce chemical active groups on the surface of materials. Active groups were projected for hydrophilic properties of final carbons for water based electrolytes. After preparation also hydrophobic groups we noticed. During the further research, samples with both water and organic electrolytes will be tested. Both materials differ in the kind and method of precursor production. One can notice that it yielded different effects – the slope of characteristic curve for RFO material is higher than the one for RSFL. Moreover, for potential values of 0.4–0.5 V a clear effect of electrochemical process occurring during discharges can be observed. This effect is likely to be related to the red-ox process occurring at the surface or inside the carbon material with the potential value close to the solvents (i.e. water) decomposition potential. This process may affect the value of supercapacitors capacitance. Nyquist plot in Fig. 5 shows the electrochemical impedance results of tested RFO and RFSL materials. In the figure we can see two various regions depending on frequencies. At high frequencies semicircles and linear part at low frequencies are similar for both investigated materials. Semicircles describe resistance of the solution and the linear part of the graph-diffusion to the electrodes. The linear part of RFO plot is parallel to 45◦ line and it is closer to the vertical axis. It seems a better capacitive behavior then RFSL. The charge transfer resistance is similar for both materials, i.e. about 1 cm−2 . The results of the following electrochemical tests, which are presented in Table 4, constitute really valuable information that confirm advantages of the carbon aerogels method applied to obtain any configurations of electrochemical properties of electrode carbon materials. Both special properties of investigated materials and the applied method of preparation of their precursors create a great tool for designing carbon materials that possess the expected features. The comparison of materials RFO and RFSL analyzed above shows over two times higher value of specific capacitance of RFSL material. It is correlated with the results of cyclic voltammetry tests; however, it does not result from the surface area results, where the value for RFSL was ten times higher (Table 3). The value of capacitance higher than a specific porosity ratio may result from the electrode phenomenon, which introduces pseudocapacitance. Perhaps, the image of this phenomenon can be observed on the abovementioned voltammetry graph (Fig. 4). RFSL carbon material exhibited the highest value of specific capacitance. Therefore, further research, the aim of which is to determine the relationship between technological methods of precursor production and electrochemical properties of electrodes obtained from final material, will be focused on RFSL material. As it was mentioned before, the comparison of polycondensation and polymerization in emulsion processes showed that the granularity of reagents during polymerization of precursor influenced the structure of the final material.
Self-discharge Outflow current (mA)
Voltage after 1 h/V
3.8 22.1
0.177 0.033
4. Conclusions The investigation results fully confirm the basic assumption of this work, namely that it is possible to control to a wide extent these properties of carbon aerogels which are important for the application in supercapacitors. The obtained materials were characterized by a large surface which, during measurements, showed high specific capacitance. A possibility of achieving hydrophilic and hydrophobic properties was evaluated. After carrying out voltammetry tests it is possible to select materials for which no unfavorable electrochemical processes take place (such as water decomposition) in the range of working potentials. The influence of the method of material synthesis on the value of outflow current of supercapacitor was observed under experimental conditions. Summarizing the electrochemical investigations, it should be stated that the materials which were successfully produce may be very well suited for the production of electrodes for symmetric supercapacitors working on the principle of double electric layer; however, these materials will be subject to further investigations. References [1] G.A.M. Reynolds, A.W.P. Fung, Z.H. Wang, M.S. Dresselhaus, P.W. Pekala, J. NonCryst. Solids 188 (1995) 27. [2] S.A. Al-Muhtaseb, J.A. Ritter, Adv. Mater. 15 (2003) 101. [3] Y. Hanzawa, K. Kaneko, R.W. Pekala, M.S. Dresselhaus, Langmuir 12 (1996) 6167. [4] D. Wu, Z. Sun, R. Fu, J. Appl. Polym. Sci. 99 (5) (2006) 2263. ˜ [5] E. Raymundo-Pinero, K. Kierzek, J. Machnikowski, F. Béguin, Carbon 44 (2006) 2498. [6] H. Liu, G. Zhu, J. Power Sources 171 (2007) 1054. [7] A.G. Pandolfo, A.F. Hollenkamp, J. Power Sources 157 (2006) 11. [8] T.A. Centeno, F. Stoeckli, J. Power Sources 154 (2006) 314. [9] Q.H. Meng, L. Liu, H.H. Song, R. Zhang, L.C. Ling, J. Inorg. Mater. 19 (2004) 593. [10] P. Azais, L. Duclaux, P. Florian, D. Massiot, M.A. Lillo-Rodenas, A. Linares-Solano, J.P. Peres, C. Jehoulet, F. Beguin, J. Power Sources 171 (2007) 1046. [11] X.G. Zhuang, Y.S. Yang, Y.J. Ji, D.P. Yang, Z.Y. Tang, ACTA Phys. -Chem. Sin. 19 (2003) 689. [12] R.W. Pekala, US Pat. 4873218 (1988). [13] R.W. Pekala, US Pat. 4997804 (1991). [14] H. Tamon, H. Ishizaka, T. Yamamoto, T. Suzuki, Carbon 38 (2000) 1099. [15] S.I. Kim, T. Yamamoto, A. Endo, T. Ohmori, M. Nakaiwa, J. Ind. Eng. Chem. 12 (2006) 484. [16] C. Lin, J.A. Ritter, Carbon 35 (1997) 1271. [17] S.T. Mayer, J.L. Kaschmitter, R.W. Pekala, US Pat. 5420168 (1995). [18] J. Li, X.Y. Wang, Q.H. Huang, S. Gamboa, P.J. Sebastian, J. Power Sources 158 (2006) 784. [19] T. Yamamoto, T. Nishimura, T. Suzuki, H. Tamon, Dry Technol. 19 (2004) 1319. [20] L. Zubizarreta, A. Arenillas, A. Domínguez, J.A. Menéndez, J.J. Pis, J. Non-Cryst. Solids 354 (2008) 817. [21] S. Han, K. Sohn, T. Hyeon, Chem. Mater. 12 (11) (2000) 3337. [22] J.B. Joo, P. Kim, W. Kim, J. Kim, J. Yi, Catal. Today 111 (2006) 171. [23] C.T. Alviso, R.W. Pekala, J. Gross, X. Lu, R. Caps, J. Fricke, J. Mater. Res. Soc. Symp. Proc., Micropor. Macropor. Mater. 431 (1996) 521. [24] N. Tonanon, W. Tanthapanichakoon, T. Yamamoto, H. Nishihara, S.R. Mukai, H. Tamon, Carbon 41 (2003) 2981.