- Email: [email protected]
Enhanced capacitance of carbon nanotubes through chemical activation
24 July 2002
Chemical Physics Letters 361 (2002) 35–41 www.elsevier.com/locate/cplett
Enhanced capacitance of carbon nanotubes through chemical activation E. Frackowiak a, S. Delpeux b, K. Jurewicz a, K. Szostak D. Cazorla-Amoros c, F. Beguin b,* a
a,b
,
Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, ul. Piotrowo 3, 60-965 Poznan, Poland b Centre de Recherche sur la Mati ere Divis ee, CNRS-Universit e, 1B rue de la F erollerie, 45071 Orl eans Cedex 02, France c Departamento de Quimica Inorganica, Universidad de Alicante, Apartado 99, 03080 Alicante, Spain Received 7 February 2002; in final form 22 April 2002
Abstract Microporosity of pure multi-walled carbon nanotubes (MWNTs) has been highly developed using chemical KOH activation. Depending on the nanotubular material, the burn-off ranged from 20% to 45% after the activation process. At least twofold increase of surface area has been obtained with maximum values of ca. 1050 m2 =g for KOH/C ratio of 4:1. The activated material still possesses a nanotubular morphology with many defects on the outer walls that give a significant increase of micropore volume, while keeping a noticeable mesoporosity. Such activated MWNTs have been used as electrode material for supercapacitors in alkaline, acidic and aprotic medium. Enhanced values of capacitance were always observed after activation: in some cases it increased almost seven times from 15 F/g (for non-activated nanotubes) to 90 F/g (after chemical activation). Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Supercapacitors are energy devices with a high power due to fast charge propagation between the negatively or positively charged plates and ions from the electrolytic solution. Very unique properties of carbon nanotubes, especially a marked mesoporous character allowed them to be proposed as attractive electrode materials for supercapacitors [1–5]. The interconnected network of nanotubes forms open mesopores which perfectly
*
Corresponding author. Fax: +33-2-3863-3796. E-mail address: [email protected] (F. Beguin).
facilitate the transportation of ions while playing also an adsorption role. Hence, due to the accessible electrode/electrolyte interface and a low electrical resistance of nanotubes, a high power is expected for devices built with electrodes from these materials. Capacitance values are strongly depending on the type of nanotubes and on the presence of side products. For example pure single wall nanotubes (SWNTs) from Rice University in the form of bucky paper used as capacitor electrodes in 6 M KOH give a specific capacitance of 40 F/g [4], whereas values up to 180 F/g were claimed for asreceived SWNTs with an estimated purity of 30% and contaminated by catalyst and other carbon
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 6 8 4 - X
36
E. Frackowiak et al. / Chemical Physics Letters 361 (2002) 35–41
forms [5]. The highly purified multi-walled nanotubes (MWNTs) obtained by decomposition of acetylene supply values from 10 to 80 F/g depending on catalyst, support and production temperature. Due to the presence of pyrolytic carbon coating on the MWNTs surface capacitance increases up to 130 F/g after functionalisation by hot nitric acid [2,3]. However, the contribution of pseudocapacitance connected with the presence of surface groups diminishes while cycling due to the irreversible character of some redox reactions. Additional capacitance increase has been obtained by doping of MWNTs with conducting polymers, e.g., polypyrrole [6,7]. By such a coating a stable faradaic pseudocapacitance was reached and cycleability could exceed 2000 cycles at 350 mA/g current load [6]. The mesoporous character of as-received nanotubes essentially determines their electrochemical properties. However, even if accessibility to the electrode/electrolyte interface is excellent, their specific surface area is low, from 200 to ca. 400 m2 =g, with almost negligible microporosity. For efficiently charging the electric double layer, i.e., for getting high values of capacitance, a developed surface area is demanded and the presence of micropores is crucial. In the present work, a significant increase of nanotubes microporosity has been obtained by chemical KOH activation that gives a novel attractive material with various potential applications as capacitor electrode, catalyst support and gas storage.
catalyst were dissolved by 72% hydrofluoric and nitric acids, respectively. The sample A=Cox Mgð1xÞ O was easily purified only by concentrated hydrochloric acid. The KOH activation of MWNTs was performed at 800 °C under argon flow (500 ml/min) with a strict control of KOH:C weight ratio (4:1) and time [9]. Activated samples were carefully washed by demineralised water and the weight loss of carbon was estimated after drying. Specific surface area was measured by nitrogen adsorption at 77 K using a Micromeritics apparatus ASAP 2010. Before adsorption the samples were outgassed at 350 °C for 24 h until pressure reached 106 mbar. Transmission electron microscopy (TEM, Philips CM20) has been used for the observation of the activated material. The capacitor electrodes were pellets formed by pressing a mixture of nanotubular material (85 wt%) + acetylene black (5 wt%) + polyvinylidene fluoride (PVDF-Kynarflex, Atochem, 10 wt%). Two electrode capacitors were built with a glassy fibrous separator and gold current collectors, using a Swagelokâ type system. The mass of electrodes ranged from 4 to 8 mg. Three types of electrolytic solution have been used: aqueous 6 M KOH or 1 M H2 SO4 and organic 1.4 M TEABF4 in acetonitrile (Merck). The values of capacitance were estimated by voltammetry (scan rate of potential from 1 to 10 mV/s), galvanostatic charge/ discharge cycling (VMP-Biologic-France and BT2000-ARBIN-USA) and impedance spectroscopy from 10 kHz to 1 mHz (AUTOLAB FRA2).
2. Experimental
3. Results and discussion
Two types of MWNTs have been selected for chemical activation by KOH: (a) nanotubes obtained by decomposition of acetylene at 700 °C on cobalt (12.5%) supported on silica; (b) prepared by decomposition of acetylene at 600 °C on Co particles from a solid solution of cobalt oxide and magnesium oxide, Cox Mgð1xÞ O [8]. They will be referenced A/CoSi700 and A=Cox Mgð1xÞ O, respectively. After nanotubes preparation, purification has been performed in both cases. In the case of the A/CoSi700 material, the support and the
Microtexture of as-prepared and activated nanotubes has been characterised by TEM and nitrogen adsorption at 77 K. In the case of nanotubes A/CoSi700, the specific surface area increased from 430 to 1035 m2 =g after KOH activation (4KOH:C) with a 45% weight loss. For the second type of nanotubes, A=Cox Mgð1xÞ O, the specific surface area changed from 220 to 885 m2 =g whereas the weight loss was definitively smaller (20%). The nanotubular materials present a type IV nitrogen adsorption isotherm with a hysteresis loop
E. Frackowiak et al. / Chemical Physics Letters 361 (2002) 35–41
typical of a mesoporous material where the desorption requires definitively higher energy than adsorption. Fig. 1 shows an example of isotherm for pristine and activated A=Cox Mgð1xÞ O nanotubes. The hysteresis is definitively more pronounced in the case of the activated nanotubes, that confirms a wide distribution of pores. However, the most important information, comparing the isotherms of activated and pristine nanotubes is a marked uptake at low relative pressure, which demonstrates the development of micropores in the activated sample. Before the activation process, the micropore volume of both types of nanotubes was almost negligible. After the activation it increases to 0.47 and 0:40 cm3 =g for A/CoSi700 and A=Cox Mgð1xÞ O nanotubes, respectively. As-received A=Cox Mgð1xÞ O nanotubes are quite different from A/CoSi700 due to the in situ formation of catalyst during growth of nanotubes [8]. The average outer diameter of A=Cox Mgð1xÞ O is about 12 nm whereas the diameter of the central canal ranges from 3 to 6 nm. The tube walls are thinner than in the case of A/CoSi700 where the
37
central canal ranges from 5 to 10 nm and the outer diameter from 15 to 25 nm. The TEM image of a general population of activated nanotubular material (Fig. 2) matches very well with the nitrogen adsorption/desorption isotherm (Fig. 1). Both activated materials still possess a mesoporous character due to the entanglement and presence of a central canal. However, very defected walls, cracks and surface irregularities introduced by KOH activation well visible in Figs. 3a, b supply additionally micropores. Whereas the pristine A=Cox Mgð1xÞ O tubes are closed, all the tips are open after the KOH activation treatment. Development of surface area of carbon by KOH activation is well known, being discovered over 20 years ago [9], and successfully applied for the industrial production of the microporous carbon PX21, however, the mechanism of this process is still discussed. During KOH activation, considerable amounts of potassium carbonate and hydrogen are formed. Crucial role is played by metallic potassium formed above 700 °C according to the reactions:
Fig. 1. Nitrogen adsorption/desorption at 77 K for A=Cox Mgð1xÞ O nanotubes: (a) as-prepared; (b) activated by KOH.
38
E. Frackowiak et al. / Chemical Physics Letters 361 (2002) 35–41
Fig. 2. Transmission electron microscopy (TEM) image of a general population of activated A=Cox Mgð1xÞ O nanotubes.
2KOH þ H2 ! 2K þ 2H2 O K2 O þ C ! 2K þ CO The mobility of free potassium vapour and its intercalation and/or insertion into the nanotubes walls through all the outer and inner defects are responsible for the separation of the graphitic layers and the development of the internal microporosity with a quite often cage-like structure. Careful TEM observation reveals that in some cases the creation of porosity takes place also inside the nanotubes, that proves the assumption of K insertion through the central canal. KOH activation seems to be an elegant and simple method for the enhancement of electrochemically active surface area of nanotubes. Indeed, ability for the charge accumulation is extremely enhanced by the creation of micropores. For instance, A=Cox Mgð1xÞ O nanotubes supplied a specific capacitance of only 10–15 F/g before activation, whereas after this process the values increase up to 90 F/g, especially in alkaline medium (6 M KOH). Comparison of ability for charge accumulation of this material at 2 and 10 mV/s scan rate of potential is presented in Fig. 4. It
is clearly seen that this capacitor, with a noticeable amount of mesopores in the active electrode material, can be easily charged/discharged at high current density keeping a box-like shape characteristic. Capacitance has also been measured in acidic medium 1 M H2 SO4 , and the values reached 85 F/g for A=Cox Mgð1xÞ O and 95 F/g for A/CoSi700 activated materials. In organic electrolytic solution (1.4 M TEABF4 in acetonitrile) the values of capacitance reached 65 F/g for both types of activated nanotubes (Fig. 5) with a mirror-like shape of characteristics. In all cases capacitance values were also estimated by galvanostatic charge/discharge and impedance spectroscopy, which gave the same results. The values of time constant RC estimated from impedance measurements varied from 0.1 to 1 s where higher values were found for activated samples. It confirms that charge propagation in the micropores takes longer time. Two kinds of pores are essential for capacitors performance using a carbon material. The micropores wetted by the electrolytic solution contribute extensively to the adsorption of ions on the electrochemically active surface of the electrodes. To some extent there is also a small contribution
E. Frackowiak et al. / Chemical Physics Letters 361 (2002) 35–41
39
Fig. 3. Examples of (0 0 2) lattice fringes images of activated nanotubes: (a) A/CoSi700, (b) A=Cox Mgð1xÞ O.
Fig. 4. Voltammetry characteristics of a capacitor built from activated A=Cox Mgð1xÞ O nanotubes at scan rates of 2 and 10 mV/s. Electrolyte: 6 M KOH.
Fig. 5. Voltammetry characteristics of a capacitor built from activated A/CoSi700 nanotubes at scan rates of 2 and 10 mV/s. Electrolyte: 1.4 M TEABF4 in acetonitrile.
40
E. Frackowiak et al. / Chemical Physics Letters 361 (2002) 35–41
of mesopores to this phenomenon, however their main role is the transportation of the solvated ions from the solution to the electroactive surface. The value of capacitance is mainly controlled by the micropores, while possibilities of high power operation are given by mesopores. The high increase of capacitance values, which is observed after activation of the two kinds of nanotubes, is therefore directly related with the creation of micropores. For the characterisation of supercapacitors and for a comparison of different electrode materials, the evaluation of such parameters as leakage current Lc and self-discharge Sd is important. Side redox reactions with resistance in parallel to capacitance in the equivalent circuit are responsible for Lc, hence, the lower the value the better performance. Values of Lc and Sd of capacitors built from activated nanotubes and measured after charging to 1 V for aqueous medium and to 2 V in organic solution are shown in Table 1. It looks that the leakage current is definitively lower for the A=Cox Mgð1xÞ O material due to the lack of catalyst impurities which can give some redox reactions especially in aqueous electrolytes. Selfdischarge of capacitors varies from 10% to 50% depending on time and looks to be smaller for A=Cox Mgð1xÞ O material. In comparison to other carbon materials, e.g., the highly microporous carbon PX21, the values of Lc and Sd estimated in the same experimental conditions are definitively
Table 1 Leakage current Lc (mA/g) and self-discharge Sd (%) of capacitors built from activated nanotubes in 6 M KOH, 1 M H2 SO4 and 1.4 M TEABF4 in acetonitrile Electrolyte
A=Cox Mgð1xÞ O
A/CoSi700
Lc
Sd
Lc
Sd
6 M KOH After 1 h After 2 h
4.3 2.6
12.9 28.0
26.5 20.5
32.8 53.0
1 M H2 SO4 After 1 h After 2 h
2.5 1.6
11.5 34.7
29.5 26.1
20.1 48.8
1.4 M TEABF4 After 1 h After 2 h
7.1 6.5
12.2 23.6
5.5 5.0
13.2 26.2
lower, especially for high A=Cox Mgð1xÞ O nanotubes.
purity
activated
4. Conclusion For the first time the activation of MWNTs has been successfully realised with a significant increase of specific surface area up to 1035 m2 =g. The high efficiency of KOH activation in the development of surface area is attributed to redox reactions between carbon nanotubes and KOH followed by potassium intercalation and separation of the graphitic layers. This process is very efficient for the formation of pores, especially for A=Cox Mgð1xÞ O nanotubes with well graphitised walls, while preserving nanotubular morphology. The creation of defects on the nanotubes walls supplied the development of microporosity responsible for excellent capacitance behaviour. The values of capacitance increased after activation process especially for A=Cox Mgð1xÞ O nanotubes from 15 to 90 F/g in alkaline medium (6 M KOH). In organic electrolyte the capacitance of both activated material reached 65 F/g with a box-like shape characteristics. Such a significant modification of surface microporosity for essentially mesoporous nanotubular materials is of great interest for many electrochemical applications (supercapacitor electrodes and/or supports).
Acknowledgements This research has been partly supported by the NATO SfP project 973849 and by the European Community Research Training Network contract no. HPRNT-CT-2000-00037. Thanks are due to T. Cacciaguerra for the realisation of TEM images.
References [1] C. Niu, E.K. Sichel, R. Hoch, D. Moy, H. Tennet, Appl. Phys. Lett. 70 (1997) 1480. [2] E. Frackowiak, K. Metenier, V. Bertagna, F. Beguin, Appl. Phys. Lett. 77 (2000) 2421. [3] E. Frackowiak, F. Beguin, Carbon 39 (2001) 937.
E. Frackowiak et al. / Chemical Physics Letters 361 (2002) 35–41 [4] E. Frackowiak, K. Jurewicz, S. Delpeux, F. Beguin, J. Power Sources 97–98 (2001) 822. [5] K.H. An, W.S. Kim, Y.S. Park, Y.C. Hoi, S.M. Lee, D.C. Chung, D.J. Bae, S.C. Lim, Y.H. Lee, Adv. Mater. 13 (2001) 497. [6] K. Jurewicz, S. Delpeux, V. Bertagna, F. Beguin, E. Frackowiak, Chem. Phys. Lett. 347 (2001) 36.
41
[7] G.Z. Chen, M.S.P. Shaffer, D. Coleby, G. Dixon, W. Zhou, D.J. Fray, A.H. Windle, Adv. Mater. 12 (2000) 522. [8] Y. Soneda, K. Szostak, S. Delpeux, S. Bonnamy, F. Beguin, in: Proc. Carbon’01, Lexington (USA), 14–19 July, 2001, ACS, 2001, p. 30-1. [9] A.N. Wennerberg, T.M. O’Grady, U.S. Patent 4,082,694 (1978).
Chemical Physics Letters 361 (2002) 35–41 www.elsevier.com/locate/cplett
Enhanced capacitance of carbon nanotubes through chemical activation E. Frackowiak a, S. Delpeux b, K. Jurewicz a, K. Szostak D. Cazorla-Amoros c, F. Beguin b,* a
a,b
,
Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, ul. Piotrowo 3, 60-965 Poznan, Poland b Centre de Recherche sur la Mati ere Divis ee, CNRS-Universit e, 1B rue de la F erollerie, 45071 Orl eans Cedex 02, France c Departamento de Quimica Inorganica, Universidad de Alicante, Apartado 99, 03080 Alicante, Spain Received 7 February 2002; in final form 22 April 2002
Abstract Microporosity of pure multi-walled carbon nanotubes (MWNTs) has been highly developed using chemical KOH activation. Depending on the nanotubular material, the burn-off ranged from 20% to 45% after the activation process. At least twofold increase of surface area has been obtained with maximum values of ca. 1050 m2 =g for KOH/C ratio of 4:1. The activated material still possesses a nanotubular morphology with many defects on the outer walls that give a significant increase of micropore volume, while keeping a noticeable mesoporosity. Such activated MWNTs have been used as electrode material for supercapacitors in alkaline, acidic and aprotic medium. Enhanced values of capacitance were always observed after activation: in some cases it increased almost seven times from 15 F/g (for non-activated nanotubes) to 90 F/g (after chemical activation). Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Supercapacitors are energy devices with a high power due to fast charge propagation between the negatively or positively charged plates and ions from the electrolytic solution. Very unique properties of carbon nanotubes, especially a marked mesoporous character allowed them to be proposed as attractive electrode materials for supercapacitors [1–5]. The interconnected network of nanotubes forms open mesopores which perfectly
*
Corresponding author. Fax: +33-2-3863-3796. E-mail address: [email protected] (F. Beguin).
facilitate the transportation of ions while playing also an adsorption role. Hence, due to the accessible electrode/electrolyte interface and a low electrical resistance of nanotubes, a high power is expected for devices built with electrodes from these materials. Capacitance values are strongly depending on the type of nanotubes and on the presence of side products. For example pure single wall nanotubes (SWNTs) from Rice University in the form of bucky paper used as capacitor electrodes in 6 M KOH give a specific capacitance of 40 F/g [4], whereas values up to 180 F/g were claimed for asreceived SWNTs with an estimated purity of 30% and contaminated by catalyst and other carbon
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 6 8 4 - X
36
E. Frackowiak et al. / Chemical Physics Letters 361 (2002) 35–41
forms [5]. The highly purified multi-walled nanotubes (MWNTs) obtained by decomposition of acetylene supply values from 10 to 80 F/g depending on catalyst, support and production temperature. Due to the presence of pyrolytic carbon coating on the MWNTs surface capacitance increases up to 130 F/g after functionalisation by hot nitric acid [2,3]. However, the contribution of pseudocapacitance connected with the presence of surface groups diminishes while cycling due to the irreversible character of some redox reactions. Additional capacitance increase has been obtained by doping of MWNTs with conducting polymers, e.g., polypyrrole [6,7]. By such a coating a stable faradaic pseudocapacitance was reached and cycleability could exceed 2000 cycles at 350 mA/g current load [6]. The mesoporous character of as-received nanotubes essentially determines their electrochemical properties. However, even if accessibility to the electrode/electrolyte interface is excellent, their specific surface area is low, from 200 to ca. 400 m2 =g, with almost negligible microporosity. For efficiently charging the electric double layer, i.e., for getting high values of capacitance, a developed surface area is demanded and the presence of micropores is crucial. In the present work, a significant increase of nanotubes microporosity has been obtained by chemical KOH activation that gives a novel attractive material with various potential applications as capacitor electrode, catalyst support and gas storage.
catalyst were dissolved by 72% hydrofluoric and nitric acids, respectively. The sample A=Cox Mgð1xÞ O was easily purified only by concentrated hydrochloric acid. The KOH activation of MWNTs was performed at 800 °C under argon flow (500 ml/min) with a strict control of KOH:C weight ratio (4:1) and time [9]. Activated samples were carefully washed by demineralised water and the weight loss of carbon was estimated after drying. Specific surface area was measured by nitrogen adsorption at 77 K using a Micromeritics apparatus ASAP 2010. Before adsorption the samples were outgassed at 350 °C for 24 h until pressure reached 106 mbar. Transmission electron microscopy (TEM, Philips CM20) has been used for the observation of the activated material. The capacitor electrodes were pellets formed by pressing a mixture of nanotubular material (85 wt%) + acetylene black (5 wt%) + polyvinylidene fluoride (PVDF-Kynarflex, Atochem, 10 wt%). Two electrode capacitors were built with a glassy fibrous separator and gold current collectors, using a Swagelokâ type system. The mass of electrodes ranged from 4 to 8 mg. Three types of electrolytic solution have been used: aqueous 6 M KOH or 1 M H2 SO4 and organic 1.4 M TEABF4 in acetonitrile (Merck). The values of capacitance were estimated by voltammetry (scan rate of potential from 1 to 10 mV/s), galvanostatic charge/ discharge cycling (VMP-Biologic-France and BT2000-ARBIN-USA) and impedance spectroscopy from 10 kHz to 1 mHz (AUTOLAB FRA2).
2. Experimental
3. Results and discussion
Two types of MWNTs have been selected for chemical activation by KOH: (a) nanotubes obtained by decomposition of acetylene at 700 °C on cobalt (12.5%) supported on silica; (b) prepared by decomposition of acetylene at 600 °C on Co particles from a solid solution of cobalt oxide and magnesium oxide, Cox Mgð1xÞ O [8]. They will be referenced A/CoSi700 and A=Cox Mgð1xÞ O, respectively. After nanotubes preparation, purification has been performed in both cases. In the case of the A/CoSi700 material, the support and the
Microtexture of as-prepared and activated nanotubes has been characterised by TEM and nitrogen adsorption at 77 K. In the case of nanotubes A/CoSi700, the specific surface area increased from 430 to 1035 m2 =g after KOH activation (4KOH:C) with a 45% weight loss. For the second type of nanotubes, A=Cox Mgð1xÞ O, the specific surface area changed from 220 to 885 m2 =g whereas the weight loss was definitively smaller (20%). The nanotubular materials present a type IV nitrogen adsorption isotherm with a hysteresis loop
E. Frackowiak et al. / Chemical Physics Letters 361 (2002) 35–41
typical of a mesoporous material where the desorption requires definitively higher energy than adsorption. Fig. 1 shows an example of isotherm for pristine and activated A=Cox Mgð1xÞ O nanotubes. The hysteresis is definitively more pronounced in the case of the activated nanotubes, that confirms a wide distribution of pores. However, the most important information, comparing the isotherms of activated and pristine nanotubes is a marked uptake at low relative pressure, which demonstrates the development of micropores in the activated sample. Before the activation process, the micropore volume of both types of nanotubes was almost negligible. After the activation it increases to 0.47 and 0:40 cm3 =g for A/CoSi700 and A=Cox Mgð1xÞ O nanotubes, respectively. As-received A=Cox Mgð1xÞ O nanotubes are quite different from A/CoSi700 due to the in situ formation of catalyst during growth of nanotubes [8]. The average outer diameter of A=Cox Mgð1xÞ O is about 12 nm whereas the diameter of the central canal ranges from 3 to 6 nm. The tube walls are thinner than in the case of A/CoSi700 where the
37
central canal ranges from 5 to 10 nm and the outer diameter from 15 to 25 nm. The TEM image of a general population of activated nanotubular material (Fig. 2) matches very well with the nitrogen adsorption/desorption isotherm (Fig. 1). Both activated materials still possess a mesoporous character due to the entanglement and presence of a central canal. However, very defected walls, cracks and surface irregularities introduced by KOH activation well visible in Figs. 3a, b supply additionally micropores. Whereas the pristine A=Cox Mgð1xÞ O tubes are closed, all the tips are open after the KOH activation treatment. Development of surface area of carbon by KOH activation is well known, being discovered over 20 years ago [9], and successfully applied for the industrial production of the microporous carbon PX21, however, the mechanism of this process is still discussed. During KOH activation, considerable amounts of potassium carbonate and hydrogen are formed. Crucial role is played by metallic potassium formed above 700 °C according to the reactions:
Fig. 1. Nitrogen adsorption/desorption at 77 K for A=Cox Mgð1xÞ O nanotubes: (a) as-prepared; (b) activated by KOH.
38
E. Frackowiak et al. / Chemical Physics Letters 361 (2002) 35–41
Fig. 2. Transmission electron microscopy (TEM) image of a general population of activated A=Cox Mgð1xÞ O nanotubes.
2KOH þ H2 ! 2K þ 2H2 O K2 O þ C ! 2K þ CO The mobility of free potassium vapour and its intercalation and/or insertion into the nanotubes walls through all the outer and inner defects are responsible for the separation of the graphitic layers and the development of the internal microporosity with a quite often cage-like structure. Careful TEM observation reveals that in some cases the creation of porosity takes place also inside the nanotubes, that proves the assumption of K insertion through the central canal. KOH activation seems to be an elegant and simple method for the enhancement of electrochemically active surface area of nanotubes. Indeed, ability for the charge accumulation is extremely enhanced by the creation of micropores. For instance, A=Cox Mgð1xÞ O nanotubes supplied a specific capacitance of only 10–15 F/g before activation, whereas after this process the values increase up to 90 F/g, especially in alkaline medium (6 M KOH). Comparison of ability for charge accumulation of this material at 2 and 10 mV/s scan rate of potential is presented in Fig. 4. It
is clearly seen that this capacitor, with a noticeable amount of mesopores in the active electrode material, can be easily charged/discharged at high current density keeping a box-like shape characteristic. Capacitance has also been measured in acidic medium 1 M H2 SO4 , and the values reached 85 F/g for A=Cox Mgð1xÞ O and 95 F/g for A/CoSi700 activated materials. In organic electrolytic solution (1.4 M TEABF4 in acetonitrile) the values of capacitance reached 65 F/g for both types of activated nanotubes (Fig. 5) with a mirror-like shape of characteristics. In all cases capacitance values were also estimated by galvanostatic charge/discharge and impedance spectroscopy, which gave the same results. The values of time constant RC estimated from impedance measurements varied from 0.1 to 1 s where higher values were found for activated samples. It confirms that charge propagation in the micropores takes longer time. Two kinds of pores are essential for capacitors performance using a carbon material. The micropores wetted by the electrolytic solution contribute extensively to the adsorption of ions on the electrochemically active surface of the electrodes. To some extent there is also a small contribution
E. Frackowiak et al. / Chemical Physics Letters 361 (2002) 35–41
39
Fig. 3. Examples of (0 0 2) lattice fringes images of activated nanotubes: (a) A/CoSi700, (b) A=Cox Mgð1xÞ O.
Fig. 4. Voltammetry characteristics of a capacitor built from activated A=Cox Mgð1xÞ O nanotubes at scan rates of 2 and 10 mV/s. Electrolyte: 6 M KOH.
Fig. 5. Voltammetry characteristics of a capacitor built from activated A/CoSi700 nanotubes at scan rates of 2 and 10 mV/s. Electrolyte: 1.4 M TEABF4 in acetonitrile.
40
E. Frackowiak et al. / Chemical Physics Letters 361 (2002) 35–41
of mesopores to this phenomenon, however their main role is the transportation of the solvated ions from the solution to the electroactive surface. The value of capacitance is mainly controlled by the micropores, while possibilities of high power operation are given by mesopores. The high increase of capacitance values, which is observed after activation of the two kinds of nanotubes, is therefore directly related with the creation of micropores. For the characterisation of supercapacitors and for a comparison of different electrode materials, the evaluation of such parameters as leakage current Lc and self-discharge Sd is important. Side redox reactions with resistance in parallel to capacitance in the equivalent circuit are responsible for Lc, hence, the lower the value the better performance. Values of Lc and Sd of capacitors built from activated nanotubes and measured after charging to 1 V for aqueous medium and to 2 V in organic solution are shown in Table 1. It looks that the leakage current is definitively lower for the A=Cox Mgð1xÞ O material due to the lack of catalyst impurities which can give some redox reactions especially in aqueous electrolytes. Selfdischarge of capacitors varies from 10% to 50% depending on time and looks to be smaller for A=Cox Mgð1xÞ O material. In comparison to other carbon materials, e.g., the highly microporous carbon PX21, the values of Lc and Sd estimated in the same experimental conditions are definitively
Table 1 Leakage current Lc (mA/g) and self-discharge Sd (%) of capacitors built from activated nanotubes in 6 M KOH, 1 M H2 SO4 and 1.4 M TEABF4 in acetonitrile Electrolyte
A=Cox Mgð1xÞ O
A/CoSi700
Lc
Sd
Lc
Sd
6 M KOH After 1 h After 2 h
4.3 2.6
12.9 28.0
26.5 20.5
32.8 53.0
1 M H2 SO4 After 1 h After 2 h
2.5 1.6
11.5 34.7
29.5 26.1
20.1 48.8
1.4 M TEABF4 After 1 h After 2 h
7.1 6.5
12.2 23.6
5.5 5.0
13.2 26.2
lower, especially for high A=Cox Mgð1xÞ O nanotubes.
purity
activated
4. Conclusion For the first time the activation of MWNTs has been successfully realised with a significant increase of specific surface area up to 1035 m2 =g. The high efficiency of KOH activation in the development of surface area is attributed to redox reactions between carbon nanotubes and KOH followed by potassium intercalation and separation of the graphitic layers. This process is very efficient for the formation of pores, especially for A=Cox Mgð1xÞ O nanotubes with well graphitised walls, while preserving nanotubular morphology. The creation of defects on the nanotubes walls supplied the development of microporosity responsible for excellent capacitance behaviour. The values of capacitance increased after activation process especially for A=Cox Mgð1xÞ O nanotubes from 15 to 90 F/g in alkaline medium (6 M KOH). In organic electrolyte the capacitance of both activated material reached 65 F/g with a box-like shape characteristics. Such a significant modification of surface microporosity for essentially mesoporous nanotubular materials is of great interest for many electrochemical applications (supercapacitor electrodes and/or supports).
Acknowledgements This research has been partly supported by the NATO SfP project 973849 and by the European Community Research Training Network contract no. HPRNT-CT-2000-00037. Thanks are due to T. Cacciaguerra for the realisation of TEM images.
References [1] C. Niu, E.K. Sichel, R. Hoch, D. Moy, H. Tennet, Appl. Phys. Lett. 70 (1997) 1480. [2] E. Frackowiak, K. Metenier, V. Bertagna, F. Beguin, Appl. Phys. Lett. 77 (2000) 2421. [3] E. Frackowiak, F. Beguin, Carbon 39 (2001) 937.
E. Frackowiak et al. / Chemical Physics Letters 361 (2002) 35–41 [4] E. Frackowiak, K. Jurewicz, S. Delpeux, F. Beguin, J. Power Sources 97–98 (2001) 822. [5] K.H. An, W.S. Kim, Y.S. Park, Y.C. Hoi, S.M. Lee, D.C. Chung, D.J. Bae, S.C. Lim, Y.H. Lee, Adv. Mater. 13 (2001) 497. [6] K. Jurewicz, S. Delpeux, V. Bertagna, F. Beguin, E. Frackowiak, Chem. Phys. Lett. 347 (2001) 36.
41
[7] G.Z. Chen, M.S.P. Shaffer, D. Coleby, G. Dixon, W. Zhou, D.J. Fray, A.H. Windle, Adv. Mater. 12 (2000) 522. [8] Y. Soneda, K. Szostak, S. Delpeux, S. Bonnamy, F. Beguin, in: Proc. Carbon’01, Lexington (USA), 14–19 July, 2001, ACS, 2001, p. 30-1. [9] A.N. Wennerberg, T.M. O’Grady, U.S. Patent 4,082,694 (1978).