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High performance supercapacitor from chromium oxide-nanotubes based electrodes
Chemical Physics Letters 434 (2007) 73–77 www.elsevier.com/locate/cplett
High performance supercapacitor from chromium oxide-nanotubes based electrodes Grzegorz Lota a, Elzbieta Frackowiak b
a,*
, Jagjiwan Mittal b, Marc Monthioux
b
a Poznan University of Technology, Institute of Chemistry and Technical Electrochemistry, 60-965 Poznan, Piotrowo 3, Poland Centre d’Elaboration des Mate´riaux et d’Etudes Structurales (CEMES), UPR #8011 CNRS, BP 94347, F-31055 Toulouse cedex 4, France
Received 3 October 2006; in final form 18 November 2006 Available online 26 November 2006
Abstract Single wall carbon nanotubes (SWNTs) filled and doped with chromium oxide have been used as attractive electrodes for supercapacitors. Pseudocapacitance effects related to the presence of nanosized chromium oxide finely dispersed at the nanoscale together with high conducting properties of SWNTs allow building efficient electrodes from this hybrid material. Even if capacitance values are not very high (ca. 60 F g1), however, extremely quick charge propagation was observed, doubtless due to the overall physical and textural properties of SWNT material. The positive effect – with respect to empty-SWNTs – brought by the presence of chromium oxide in and probably in-between the SWNTs indicates that chromium oxide is accessible to the electrolyte in spite of its encapsulated location, because of the numerous side entries created all along the SWNT walls during the filling step. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Single wall carbon nanotubes (SWNTs), among many other applications, are of great interest also for electrochemical use because of their suitable characteristics. Electrode materials should exhibit high conducting properties, high mesoporosity to enable easy electrolyte circulation and access of ions to interface for exchange of charges. Carbon nanotubes, and specifically SWNTs, perfectly fulfil these requirements. One of the very actual electrochemical application is a high power supercapacitor able to supply or collect a charge in a short time, e.g. for starting vehicle or recovery energy during deceleration. Supercapacitors (ultracapacitors) are attractive energy sources because of fast energy delivery and long-durability. It is well accepted now that the capacitance values of pure nanotubes (multiwalled as well as SWNTs) are very moderate most often from 10 to 40 F g1 (per electrode material) [1–3]. However, capacitance values can be greatly enhanced through pseudocapacitance effects by additional faradaic reactions, using *
Corresponding author. Fax: +48 61 665 2571. E-mail address: [email protected] (E. Frackowiak).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.11.089
nanotubes composites with conducting polymers [3–5], Ncontaining polymers [6] and transition metal oxides [7]. In this Letter, we propose a promising alternative route by using hybrid nanotubes, i.e. SWNTs whose inner cavity is partially filled with a foreign compound, chromium oxide in this case. The exceptional capability of the subsequent hybrid nanotubular material for quick charging/discharging process will be emphasized. 2. Experimental The preparation of chromium oxide-SWNT hybrid material was described elsewhere [8]. Briefly, the related procedure is based on soaking raw SWNTs material with a mixture of CrO3 and hydrochloric acid at room temperature for one day, then washing with water. Raw SWNT material was obtained from the electric arc process. The amount of SWNTs was 60 vol.% with associated impurities such as catalyst particles (Ni and Y), carbon shells, and amorphous carbon. X-photo-electron spectroscopy (XPS) showed that no Ni catalyst particle and only a little amount of Y particles (0.2 at%) were left after completion of the filling procedure due to their dissolution by HCl.
74
G. Lota et al. / Chemical Physics Letters 434 (2007) 73–77
The supercapacitor electrochemical characterization was performed in a two-electrode cell. The electrodes contained 85 wt% of the chromium oxide-SWNT hybrid material, 10 wt% of polyvinylidene fluoride (PVDF Kynar Flex 2801) and 5 wt% of acetylene black. Electrodes were formed in the form of pellets after weighing, mixing and pressing. A similar mass of both electrodes (varied in the range of 9–11.5 mg) was selected for building the capacitor. The geometrical surface area of one electrode was 0.8 cm2. Either 1 mol l1 H2SO4 or 6 mol l1 KOH aqueous solutions served as electrolyte. The capacitance properties of the nanotubular materials were studied by galvanostatic (20 mA g1–50 A g1) and potentiodynamic cycling at voltage scan rates from 1 mV s1 to 1000 mV s1 and by impedance spectroscopy at open circuit voltage with ±10 mV amplitude. Capacitance values are expressed per mass of one electrode. For electrochemical measurements ARBIN Instruments BT2000 – USA, VMP2/Z BIOLOGIC (France) and AUTOLAB ECOCHEMIE BV-PGSTAT 30/FRA2 (The Netherlands) operating in the frequency range 100 kHz to 1 mHz were used. 3. Results and discussion An opening of SWNTs is the first step in the expected mechanism of their filling with CrO3. It is proposed that CrO3 during its reaction with hydrochloric acid forms CrO2Cl2, which attacks both the SWNT tips (because of the well-known presence of pentagons) and the SWNTs walls (because of the presumable presence of topological defects such as Stone–Wales defect [9]). The result is the creation of openings through which CrO3 enters the SWNTs. Since opening is ultimately destructive and concomitant with filling the SWNTs, the suitable materials are obtained for conditions corresponding to sufficient SWNT filling (ca. 25%) while still limited SWNT structure damaging. This optimum corresponds to 1–2 day soaking time. This mechanism is consistent with what is proposed in [8], although alternatives are possible, e.g. by considering the transient formation of chromic acid then chromium hydroxide according to the following reaction: CrO2 Cl2 + 2H2 O
!
H2 CrO4 + 2HCl
ð1Þ
2H2 CrO4 + 6HCl
!
2Cr(OH)3 + 3Cl2 + 2H2 O
ð2Þ
The presence of chromium compound inside the tube is convincingly imaged by transmission electron microscopy (TEM) (Fig. 1). The actual filling of the inner cavity of SWNTs is further supported by Raman spectroscopy results which showed a hindering of the Radial Breathing Mode (RBM) that lowered the related band heights with respect to empty-SWNTs [10]. RBM-related peaks can decrease in intensity when electrons are taken from the system (at the level of van Hove singularities), typically when nanotubes are doped. CrO3 is likely to be able to dope SWNTs as an electron acceptor, as stated in [11].
Fig. 1. Examples of TEM images of CrO3@SWNT material: (a) in a multi-SWNT bundle and (b) in a two-SWNT bundle. CrO3 appears as short, dark contrast segments dispatched along and within the SWNTs.
Although it is not possible to ascertain it by mean of TEM, it is however believed that chromium oxide should be present at two sites in the sample at least, i.e. between SWNTs within the bundles in addition to inside the SWNT inner cavity, because CrO3 easily intercalates in-between graphene layers in graphite [12]. Additionally, the chemical identification of the filling material as chromium oxide (presumably CrO3) was ascertained by local EELS analysis. Interestingly, chlorine atoms were found associated to bundles, not to single filled-SWNTs, suggesting that a material is intercalated between SWNTs within bundles whose composition is different from the material filling the SWNTs, e.g., typically CrO2Cl2 + CrO3 instead of CrO3 only [8]. Thus, our material is probably not filled only, but also doped with chromium oxide and/or oxychloride moieties (where ‘doped SWNTs’ here stands for ‘SWNTs intimately associated with electron donor or acceptor elements or compounds with an association mechanism that closely relates to that of intercalation compounds in graphite’ according to Ref. [13]). An important requirement for practical applications, specifically for energy storage devices, is stability with time. TEM investigation of a CrO3@SWNT material aged for three years at room temperature showed that filling is still present [14], suggesting a convenient time stability. Electrochemical characterization of chromium oxideSWNTs was performed using three techniques, each of them supplying different useful information. Fig. 2 shows voltammetry characteristics at 20 mV s1 scan rate for both chromium oxide-SWNT electrodes and raw nanotubemade electrodes in two types of aqueous electrolytes, i.e. 1 mol l1 H2SO4 and 6 mol l1 KOH. Comparison of both electrodes shows perfectly the role of chromium oxide. Faradaic redox reactions are responsible for irregular shape of voltammograms especially from 0 to 0.3 V for acidic medium and from 0 to 0.6 V in alkaline solution in the operating voltage range of capacitor. However, the measurements are performed in a two-electrode cell, so it is difficult to
G. Lota et al. / Chemical Physics Letters 434 (2007) 73–77
75
60 40
C (F/g)
20 0 -20 SWNT
-40
SWNT + CrO3
-60 0
0.2
0.4
0.6
0.8
1
U (V)
Fig. 2. Voltammetry experiments at 20 mV s1 scan rate for SWNT-based (dotted lines) and CrO3@SWNT-based (solid lines) electrodes, using acidic (dark lines) or alkaline (grey lines) electrolyte.
Fig. 3. Capacitance calculated from voltammetry experiments carried out in acidic electrolyte (1 mol l1 H2SO4) at 1 V s1 scan rate for a capacitor built with chromium oxide-SWNTs and empty-SWNT-based electrodes.
predict which redox transformations of chromium compounds are responsible for this electrochemical response, however, such an irregular shape surely proves faradaic reactions. Considering pseudocapacitance effects, Cr is present in the various oxidation state, e.g. (CrVI) as CrO3. It is well known that CrO3 forms H2Cr2O7 in 1 mol l1 sulphuric acidic solution. During the subsequent electrochemical reactions, Cr2 O2 7 converts into a lower oxidation state species (CrIII) as sulfate. Cr3+ remains in the solution and can be oxidized during the reverse reaction. On the other hand, in VI basic medium, CrO3 converts into CrO2 4 (Cr ), then into Cr(OH)3 during the subsequent reduction reactions. It is worth noting that Cr(OH)3 exhibits a low solubility in water and a lower electrical conductivity, hence allowing to predict that basic medium will definitely not be preferable for high current load experiments, nor long, multi-cycle, durability testing. It is clear that considering profitable pseudocapacitance effects a simple redox transfer from Cr6+ to Cr3+ should be rather excluded and the general redox reactions with a mixed valence of chromium oxide should be taken into account according to the following equations, where hydrous state of oxide is obviously present:
A good capacitor performance of hybrid electrodes was also confirmed during galvanostatic charge/discharge experiments in 1 mol l1 H2SO4 where even at current load of 2 A g1 the ohmic drop (which would appear as a sharp voltage jump during changing of polarization) was almost not observed, while it was pronounced for empty-SWNTbased electrodes. Profit of using hybrid material as capacitor electrode was especially observed during galvanostatic charging/discharging tests at the extremely high loads (Fig. 4). The capacitance values decrease from 70 F g1 at low regime to ca. 50 F g1 at 5 A g1 current loads but preserve still 30 F g1 at 50 A g1 regimes whereas typical capacitor materials hardly tolerate the loads of 5 A g1. To complete the characterization of the capacitor performance, an impedance spectroscopy method was also used to estimate specifically all the resistive components. A Nyquist plot taken using chromium oxide-SWNTs-based electrodes at frequency from 100 kHz to 10 mHz in acidic electrolyte shows a perpendicular dependence of the imaginary part versus real one (Fig. 5). This demonstrates a perfect capacitive behaviour with very moderate diffusion
CrOa ðOHÞb þ dHþ þ de
$
CrOad ðOHÞbþd
ð3Þ
Capacitance values were calculated from voltammetry experiments performed for chromium oxide-SWNTs-based electrodes in 1 mol l1 H2SO4 solution at different scan rates (from 10 to 1000 mV s1). The higher the scan rate, the moderately lower the response from faradaic reactions, hence, revealing that some diffusion and resistance limitations take place. However, a quite good shape of the voltammetry curves performed in acidic medium at extremely high scan rate of 1 V s1 (Fig. 3) proves that a capacitor built with such hybrid nanotubes-based electrodes can easily operate and still supply capacitance of 40 F g1. On the contrary, the aggravated shape of the curve (revealing a more resistive character) was obtained when using empty-SWNT-based electrodes, thereby demonstrating the benefit of CrO3 filling.
Fig. 4. Capacitance values versus current load for SWNT-based electrodes and chromium oxide-SWNT-based electrodes in acidic and alkaline electrolyte.
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G. Lota et al. / Chemical Physics Letters 434 (2007) 73–77
4. Conclusions
Fig. 5. Nyquist plot at frequency from 100 kHz to 10 mHz (inset from 100 kHz to 1 Hz) for a capacitor built with chromium oxide-SWNT-based electrodes. Acidic electrolyte.
limitations. The capacitance versus frequency dependence for capacitors built with hybrid based electrodes in two different electrolytes (acidic and alkaline) shows that acidic medium is preferable, giving at 1 Hz the capacitance values of 50 F g1 in 1 mol l1 H2SO4. On the other hand, a capacitance value of 25 F g1 is even observed at 10 Hz frequency, which is exceptional. It is noteworthy that most of carbon materials supply negligible capacitance values already at 1 Hz. The values of time constant, which is a very important characteristic for capacitor performance, were extremely low, reaching 0.07 s for chromium oxide doped electrodes and 0.04 s for empty-SWNT-based electrodes. Higher time constant for hybrid material is obviously connected with its higher capacitance values. Finally, it is well known that pseudocapacitance effects are not always stable during cycling. Hence, galvanostatic charging/discharging tests at 200 mA g1 were performed. The capacitance values of 60 F g1 were easily maintained over 5000 cycles, which proves that chromium oxideSWNT material is very stable during cycling. However, this is true only in acidic medium, since a gradual aggravation of stability with cycling is observed in alkaline solution. The latter is consistent which was predicted above. It is noteworthy to stress that even if presented capacitance values are moderate, the target of this Letter is to show a possibility of using nanotubes as a perfect conducting and mechanical component in composite where different type of transition metals oxide could play a role of active material. The concomitant presence of nanotubes enables to fully benefit of the pseudocapacitance effects brought by oxides.
In summary, the pseudocapacitance effects related to the presence of chromium oxide in hybrid material combined with the high conducting properties of nanotubes make such a kind of hybrid material suitable for building efficient electrodes for supercapacitor. Nanosized chromium oxide particles finely dispersed at nanoscale in the SWNT material make possible the enhanced charging rate of the electrical double layer and allow fast faradaic reactions. Chromium-containing species present as CrO3 inside SWNTs as well as present in the form of CrO2Cl2 (possibly along with CrO3 too) between SWNTs within bundles supply redox reactions due to access by the electrolyte in spite of its encapsulated (and intercalated) location because of the numerous side openings created all along the SWNT defective walls during the filling step. Even if capacitance values of this hybrid material are not extremely high (60 F g1), however, exceptionally quick charge propagation was observed, doubtless due to the overall physical and textural properties of SWNT material. Especially, the unique conductivity of SWNTs due to their one dimensional electronic structure, such as ballistic electronic transport which is maintained even after openings have been created in their walls for filling purpose [15] enable them to carry high currents (50 A g1) mostly with no heating, which is very important for capacitor application. Acknowledgements The authors acknowledge the financial support from the Ministry of Science and Education (Poland) grant 3 T10A 00128, and Centre National de la Recherche Scientifique (France) for providing the Research Associate grant for J.M. The authors also thank C. Guimon (University of Pau, France) for the XPS analysis. References [1] S. Shiraishi, H. Kurihara, K. Okabe, D. Hulicova, A. Oya, Electrochem. Commun. 4 (2002) 593. [2] M. Endo et al., Appl. Phys. A 82 (2006) 559. [3] E. Frackowiak, in: J. Schwarz et al. (Eds.), Encyclopedia of Nanoscience and Nanotechnology, Marcel Dekker, Inc., New York, 2004, p. 537. [4] K.H. An, K.K. Jeon, J.K. Heo, S.C. Lim, D.J. Bae, Y.H. Lee, J. Electrochem. Soc. 149 (2002) A1058. [5] E. Frackowiak, V. Khomenko, K. Jurewicz, K. Lota, F. Be´guin, J. Power Sources 153 (2006) 413. [6] F. Be´guin, K. Szostak, G. Lota, E. Frackowiak, Adv. Mater. 17 (2005) 238. [7] E. Raymundo-Pin˜ero, V. Khomenko, E. Frackowiak, F. Be´guin, J. Electrochem. Soc. 152 (2005) A229. [8] J. Mittal, M. Monthioux, H. Allouche, O. Stephan, Chem. Phys. Lett. 339 (2001) 311. [9] M. Monthioux, Carbon 40 (2002) 1809. [10] J. Mittal, M. Monthioux, H. Allouche, O. Stephan, W. Bacsa, in: H. Kuzmany et al. (Eds.), Electronic Properties of Molecular Nanostructures, American Institute of Physics Conference Proceedings, vol. 591, 2001, p. 273.
G. Lota et al. / Chemical Physics Letters 434 (2007) 73–77 [11] P. Corio, A.P. Santos, P.S. Santos, M.L.A. Temperini, V.W. Brar, M.A. Pismenta, M.S. Dresselhaus, Chem. Phys. Lett. 383 (2004) 475. [12] J. Mittal, H. Konno, M. Inagaki, Synth. Met. 96 (1998) 103. [13] M. Monthioux, E. Flahaut, J.-P. Cleuziou, J. Mater. Res. 21 (2006) 2774.
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[14] J. Mittal, G. Lota, E. Frackowiak, M. Monthioux, in: The Proceedings of International Conference on Carbon Carbon’06, The Robert Gordon University, Aberdeen, Scotland (UK), Extend. Abstr. (CDRom), Oral/2B3, July 16–20, 2006. [15] P. Utko, J. Nygard, M. Monthioux, L. Noe´, Appl. Phys. Lett., in press.
High performance supercapacitor from chromium oxide-nanotubes based electrodes Grzegorz Lota a, Elzbieta Frackowiak b
a,*
, Jagjiwan Mittal b, Marc Monthioux
b
a Poznan University of Technology, Institute of Chemistry and Technical Electrochemistry, 60-965 Poznan, Piotrowo 3, Poland Centre d’Elaboration des Mate´riaux et d’Etudes Structurales (CEMES), UPR #8011 CNRS, BP 94347, F-31055 Toulouse cedex 4, France
Received 3 October 2006; in final form 18 November 2006 Available online 26 November 2006
Abstract Single wall carbon nanotubes (SWNTs) filled and doped with chromium oxide have been used as attractive electrodes for supercapacitors. Pseudocapacitance effects related to the presence of nanosized chromium oxide finely dispersed at the nanoscale together with high conducting properties of SWNTs allow building efficient electrodes from this hybrid material. Even if capacitance values are not very high (ca. 60 F g1), however, extremely quick charge propagation was observed, doubtless due to the overall physical and textural properties of SWNT material. The positive effect – with respect to empty-SWNTs – brought by the presence of chromium oxide in and probably in-between the SWNTs indicates that chromium oxide is accessible to the electrolyte in spite of its encapsulated location, because of the numerous side entries created all along the SWNT walls during the filling step. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Single wall carbon nanotubes (SWNTs), among many other applications, are of great interest also for electrochemical use because of their suitable characteristics. Electrode materials should exhibit high conducting properties, high mesoporosity to enable easy electrolyte circulation and access of ions to interface for exchange of charges. Carbon nanotubes, and specifically SWNTs, perfectly fulfil these requirements. One of the very actual electrochemical application is a high power supercapacitor able to supply or collect a charge in a short time, e.g. for starting vehicle or recovery energy during deceleration. Supercapacitors (ultracapacitors) are attractive energy sources because of fast energy delivery and long-durability. It is well accepted now that the capacitance values of pure nanotubes (multiwalled as well as SWNTs) are very moderate most often from 10 to 40 F g1 (per electrode material) [1–3]. However, capacitance values can be greatly enhanced through pseudocapacitance effects by additional faradaic reactions, using *
Corresponding author. Fax: +48 61 665 2571. E-mail address: [email protected] (E. Frackowiak).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.11.089
nanotubes composites with conducting polymers [3–5], Ncontaining polymers [6] and transition metal oxides [7]. In this Letter, we propose a promising alternative route by using hybrid nanotubes, i.e. SWNTs whose inner cavity is partially filled with a foreign compound, chromium oxide in this case. The exceptional capability of the subsequent hybrid nanotubular material for quick charging/discharging process will be emphasized. 2. Experimental The preparation of chromium oxide-SWNT hybrid material was described elsewhere [8]. Briefly, the related procedure is based on soaking raw SWNTs material with a mixture of CrO3 and hydrochloric acid at room temperature for one day, then washing with water. Raw SWNT material was obtained from the electric arc process. The amount of SWNTs was 60 vol.% with associated impurities such as catalyst particles (Ni and Y), carbon shells, and amorphous carbon. X-photo-electron spectroscopy (XPS) showed that no Ni catalyst particle and only a little amount of Y particles (0.2 at%) were left after completion of the filling procedure due to their dissolution by HCl.
74
G. Lota et al. / Chemical Physics Letters 434 (2007) 73–77
The supercapacitor electrochemical characterization was performed in a two-electrode cell. The electrodes contained 85 wt% of the chromium oxide-SWNT hybrid material, 10 wt% of polyvinylidene fluoride (PVDF Kynar Flex 2801) and 5 wt% of acetylene black. Electrodes were formed in the form of pellets after weighing, mixing and pressing. A similar mass of both electrodes (varied in the range of 9–11.5 mg) was selected for building the capacitor. The geometrical surface area of one electrode was 0.8 cm2. Either 1 mol l1 H2SO4 or 6 mol l1 KOH aqueous solutions served as electrolyte. The capacitance properties of the nanotubular materials were studied by galvanostatic (20 mA g1–50 A g1) and potentiodynamic cycling at voltage scan rates from 1 mV s1 to 1000 mV s1 and by impedance spectroscopy at open circuit voltage with ±10 mV amplitude. Capacitance values are expressed per mass of one electrode. For electrochemical measurements ARBIN Instruments BT2000 – USA, VMP2/Z BIOLOGIC (France) and AUTOLAB ECOCHEMIE BV-PGSTAT 30/FRA2 (The Netherlands) operating in the frequency range 100 kHz to 1 mHz were used. 3. Results and discussion An opening of SWNTs is the first step in the expected mechanism of their filling with CrO3. It is proposed that CrO3 during its reaction with hydrochloric acid forms CrO2Cl2, which attacks both the SWNT tips (because of the well-known presence of pentagons) and the SWNTs walls (because of the presumable presence of topological defects such as Stone–Wales defect [9]). The result is the creation of openings through which CrO3 enters the SWNTs. Since opening is ultimately destructive and concomitant with filling the SWNTs, the suitable materials are obtained for conditions corresponding to sufficient SWNT filling (ca. 25%) while still limited SWNT structure damaging. This optimum corresponds to 1–2 day soaking time. This mechanism is consistent with what is proposed in [8], although alternatives are possible, e.g. by considering the transient formation of chromic acid then chromium hydroxide according to the following reaction: CrO2 Cl2 + 2H2 O
!
H2 CrO4 + 2HCl
ð1Þ
2H2 CrO4 + 6HCl
!
2Cr(OH)3 + 3Cl2 + 2H2 O
ð2Þ
The presence of chromium compound inside the tube is convincingly imaged by transmission electron microscopy (TEM) (Fig. 1). The actual filling of the inner cavity of SWNTs is further supported by Raman spectroscopy results which showed a hindering of the Radial Breathing Mode (RBM) that lowered the related band heights with respect to empty-SWNTs [10]. RBM-related peaks can decrease in intensity when electrons are taken from the system (at the level of van Hove singularities), typically when nanotubes are doped. CrO3 is likely to be able to dope SWNTs as an electron acceptor, as stated in [11].
Fig. 1. Examples of TEM images of CrO3@SWNT material: (a) in a multi-SWNT bundle and (b) in a two-SWNT bundle. CrO3 appears as short, dark contrast segments dispatched along and within the SWNTs.
Although it is not possible to ascertain it by mean of TEM, it is however believed that chromium oxide should be present at two sites in the sample at least, i.e. between SWNTs within the bundles in addition to inside the SWNT inner cavity, because CrO3 easily intercalates in-between graphene layers in graphite [12]. Additionally, the chemical identification of the filling material as chromium oxide (presumably CrO3) was ascertained by local EELS analysis. Interestingly, chlorine atoms were found associated to bundles, not to single filled-SWNTs, suggesting that a material is intercalated between SWNTs within bundles whose composition is different from the material filling the SWNTs, e.g., typically CrO2Cl2 + CrO3 instead of CrO3 only [8]. Thus, our material is probably not filled only, but also doped with chromium oxide and/or oxychloride moieties (where ‘doped SWNTs’ here stands for ‘SWNTs intimately associated with electron donor or acceptor elements or compounds with an association mechanism that closely relates to that of intercalation compounds in graphite’ according to Ref. [13]). An important requirement for practical applications, specifically for energy storage devices, is stability with time. TEM investigation of a CrO3@SWNT material aged for three years at room temperature showed that filling is still present [14], suggesting a convenient time stability. Electrochemical characterization of chromium oxideSWNTs was performed using three techniques, each of them supplying different useful information. Fig. 2 shows voltammetry characteristics at 20 mV s1 scan rate for both chromium oxide-SWNT electrodes and raw nanotubemade electrodes in two types of aqueous electrolytes, i.e. 1 mol l1 H2SO4 and 6 mol l1 KOH. Comparison of both electrodes shows perfectly the role of chromium oxide. Faradaic redox reactions are responsible for irregular shape of voltammograms especially from 0 to 0.3 V for acidic medium and from 0 to 0.6 V in alkaline solution in the operating voltage range of capacitor. However, the measurements are performed in a two-electrode cell, so it is difficult to
G. Lota et al. / Chemical Physics Letters 434 (2007) 73–77
75
60 40
C (F/g)
20 0 -20 SWNT
-40
SWNT + CrO3
-60 0
0.2
0.4
0.6
0.8
1
U (V)
Fig. 2. Voltammetry experiments at 20 mV s1 scan rate for SWNT-based (dotted lines) and CrO3@SWNT-based (solid lines) electrodes, using acidic (dark lines) or alkaline (grey lines) electrolyte.
Fig. 3. Capacitance calculated from voltammetry experiments carried out in acidic electrolyte (1 mol l1 H2SO4) at 1 V s1 scan rate for a capacitor built with chromium oxide-SWNTs and empty-SWNT-based electrodes.
predict which redox transformations of chromium compounds are responsible for this electrochemical response, however, such an irregular shape surely proves faradaic reactions. Considering pseudocapacitance effects, Cr is present in the various oxidation state, e.g. (CrVI) as CrO3. It is well known that CrO3 forms H2Cr2O7 in 1 mol l1 sulphuric acidic solution. During the subsequent electrochemical reactions, Cr2 O2 7 converts into a lower oxidation state species (CrIII) as sulfate. Cr3+ remains in the solution and can be oxidized during the reverse reaction. On the other hand, in VI basic medium, CrO3 converts into CrO2 4 (Cr ), then into Cr(OH)3 during the subsequent reduction reactions. It is worth noting that Cr(OH)3 exhibits a low solubility in water and a lower electrical conductivity, hence allowing to predict that basic medium will definitely not be preferable for high current load experiments, nor long, multi-cycle, durability testing. It is clear that considering profitable pseudocapacitance effects a simple redox transfer from Cr6+ to Cr3+ should be rather excluded and the general redox reactions with a mixed valence of chromium oxide should be taken into account according to the following equations, where hydrous state of oxide is obviously present:
A good capacitor performance of hybrid electrodes was also confirmed during galvanostatic charge/discharge experiments in 1 mol l1 H2SO4 where even at current load of 2 A g1 the ohmic drop (which would appear as a sharp voltage jump during changing of polarization) was almost not observed, while it was pronounced for empty-SWNTbased electrodes. Profit of using hybrid material as capacitor electrode was especially observed during galvanostatic charging/discharging tests at the extremely high loads (Fig. 4). The capacitance values decrease from 70 F g1 at low regime to ca. 50 F g1 at 5 A g1 current loads but preserve still 30 F g1 at 50 A g1 regimes whereas typical capacitor materials hardly tolerate the loads of 5 A g1. To complete the characterization of the capacitor performance, an impedance spectroscopy method was also used to estimate specifically all the resistive components. A Nyquist plot taken using chromium oxide-SWNTs-based electrodes at frequency from 100 kHz to 10 mHz in acidic electrolyte shows a perpendicular dependence of the imaginary part versus real one (Fig. 5). This demonstrates a perfect capacitive behaviour with very moderate diffusion
CrOa ðOHÞb þ dHþ þ de
$
CrOad ðOHÞbþd
ð3Þ
Capacitance values were calculated from voltammetry experiments performed for chromium oxide-SWNTs-based electrodes in 1 mol l1 H2SO4 solution at different scan rates (from 10 to 1000 mV s1). The higher the scan rate, the moderately lower the response from faradaic reactions, hence, revealing that some diffusion and resistance limitations take place. However, a quite good shape of the voltammetry curves performed in acidic medium at extremely high scan rate of 1 V s1 (Fig. 3) proves that a capacitor built with such hybrid nanotubes-based electrodes can easily operate and still supply capacitance of 40 F g1. On the contrary, the aggravated shape of the curve (revealing a more resistive character) was obtained when using empty-SWNT-based electrodes, thereby demonstrating the benefit of CrO3 filling.
Fig. 4. Capacitance values versus current load for SWNT-based electrodes and chromium oxide-SWNT-based electrodes in acidic and alkaline electrolyte.
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G. Lota et al. / Chemical Physics Letters 434 (2007) 73–77
4. Conclusions
Fig. 5. Nyquist plot at frequency from 100 kHz to 10 mHz (inset from 100 kHz to 1 Hz) for a capacitor built with chromium oxide-SWNT-based electrodes. Acidic electrolyte.
limitations. The capacitance versus frequency dependence for capacitors built with hybrid based electrodes in two different electrolytes (acidic and alkaline) shows that acidic medium is preferable, giving at 1 Hz the capacitance values of 50 F g1 in 1 mol l1 H2SO4. On the other hand, a capacitance value of 25 F g1 is even observed at 10 Hz frequency, which is exceptional. It is noteworthy that most of carbon materials supply negligible capacitance values already at 1 Hz. The values of time constant, which is a very important characteristic for capacitor performance, were extremely low, reaching 0.07 s for chromium oxide doped electrodes and 0.04 s for empty-SWNT-based electrodes. Higher time constant for hybrid material is obviously connected with its higher capacitance values. Finally, it is well known that pseudocapacitance effects are not always stable during cycling. Hence, galvanostatic charging/discharging tests at 200 mA g1 were performed. The capacitance values of 60 F g1 were easily maintained over 5000 cycles, which proves that chromium oxideSWNT material is very stable during cycling. However, this is true only in acidic medium, since a gradual aggravation of stability with cycling is observed in alkaline solution. The latter is consistent which was predicted above. It is noteworthy to stress that even if presented capacitance values are moderate, the target of this Letter is to show a possibility of using nanotubes as a perfect conducting and mechanical component in composite where different type of transition metals oxide could play a role of active material. The concomitant presence of nanotubes enables to fully benefit of the pseudocapacitance effects brought by oxides.
In summary, the pseudocapacitance effects related to the presence of chromium oxide in hybrid material combined with the high conducting properties of nanotubes make such a kind of hybrid material suitable for building efficient electrodes for supercapacitor. Nanosized chromium oxide particles finely dispersed at nanoscale in the SWNT material make possible the enhanced charging rate of the electrical double layer and allow fast faradaic reactions. Chromium-containing species present as CrO3 inside SWNTs as well as present in the form of CrO2Cl2 (possibly along with CrO3 too) between SWNTs within bundles supply redox reactions due to access by the electrolyte in spite of its encapsulated (and intercalated) location because of the numerous side openings created all along the SWNT defective walls during the filling step. Even if capacitance values of this hybrid material are not extremely high (60 F g1), however, exceptionally quick charge propagation was observed, doubtless due to the overall physical and textural properties of SWNT material. Especially, the unique conductivity of SWNTs due to their one dimensional electronic structure, such as ballistic electronic transport which is maintained even after openings have been created in their walls for filling purpose [15] enable them to carry high currents (50 A g1) mostly with no heating, which is very important for capacitor application. Acknowledgements The authors acknowledge the financial support from the Ministry of Science and Education (Poland) grant 3 T10A 00128, and Centre National de la Recherche Scientifique (France) for providing the Research Associate grant for J.M. The authors also thank C. Guimon (University of Pau, France) for the XPS analysis. References [1] S. Shiraishi, H. Kurihara, K. Okabe, D. Hulicova, A. Oya, Electrochem. Commun. 4 (2002) 593. [2] M. Endo et al., Appl. Phys. A 82 (2006) 559. [3] E. Frackowiak, in: J. Schwarz et al. (Eds.), Encyclopedia of Nanoscience and Nanotechnology, Marcel Dekker, Inc., New York, 2004, p. 537. [4] K.H. An, K.K. Jeon, J.K. Heo, S.C. Lim, D.J. Bae, Y.H. Lee, J. Electrochem. Soc. 149 (2002) A1058. [5] E. Frackowiak, V. Khomenko, K. Jurewicz, K. Lota, F. Be´guin, J. Power Sources 153 (2006) 413. [6] F. Be´guin, K. Szostak, G. Lota, E. Frackowiak, Adv. Mater. 17 (2005) 238. [7] E. Raymundo-Pin˜ero, V. Khomenko, E. Frackowiak, F. Be´guin, J. Electrochem. Soc. 152 (2005) A229. [8] J. Mittal, M. Monthioux, H. Allouche, O. Stephan, Chem. Phys. Lett. 339 (2001) 311. [9] M. Monthioux, Carbon 40 (2002) 1809. [10] J. Mittal, M. Monthioux, H. Allouche, O. Stephan, W. Bacsa, in: H. Kuzmany et al. (Eds.), Electronic Properties of Molecular Nanostructures, American Institute of Physics Conference Proceedings, vol. 591, 2001, p. 273.
G. Lota et al. / Chemical Physics Letters 434 (2007) 73–77 [11] P. Corio, A.P. Santos, P.S. Santos, M.L.A. Temperini, V.W. Brar, M.A. Pismenta, M.S. Dresselhaus, Chem. Phys. Lett. 383 (2004) 475. [12] J. Mittal, H. Konno, M. Inagaki, Synth. Met. 96 (1998) 103. [13] M. Monthioux, E. Flahaut, J.-P. Cleuziou, J. Mater. Res. 21 (2006) 2774.
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[14] J. Mittal, G. Lota, E. Frackowiak, M. Monthioux, in: The Proceedings of International Conference on Carbon Carbon’06, The Robert Gordon University, Aberdeen, Scotland (UK), Extend. Abstr. (CDRom), Oral/2B3, July 16–20, 2006. [15] P. Utko, J. Nygard, M. Monthioux, L. Noe´, Appl. Phys. Lett., in press.