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High-performance polypyrrole electrode materials for redox supercapacitors
Electrochemistry Communications 8 (2006) 937–940 www.elsevier.com/locate/elecom
High-performance polypyrrole electrode materials for redox supercapacitors Li-Zhen Fan, Joachim Maier
*
Max-Planck-Institut fu¨r Festko¨rperforschung 70569 Stuttgart, Germany Received 1 March 2006; received in revised form 17 March 2006; accepted 20 March 2006 Available online 27 April 2006
Abstract Highly active polypyrrole electrodes for redox supercapacitors were prepared by electrodeposition on Ti foil via cyclic voltammetry at a scan rate of 200 mV/s in oxalic acid solution and subsequently characterized in 1 M KCl. Scanning electron microscopy showed that the polypyrrole has a highly porous nanostructure leading to a very high specific capacitance of about 480 ± 50 F/g. The electrode exhibted a high stability during cycle life test. The effects of the mass of polypyrrole electrode materials on the specific capacitance were also investigated. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Polypyrrole; Supercapacitor; Electrodeposition; Specific capacitance
1. Introduction Supercapacitors are currently widely investigated due to their interesting characteristics in terms of power and energy densities. These devices offer an interesting and technologically relevant compromise with respect to power density (greater than for batteries) and energy density (greater than for conventional capacitors) [1]. Conducting polymers are an attractive class of electrode materials to be used in supercapacitors due to their advantageous properties including low cost compared to noble metal oxides, high doping–dedoping rate during charge–discharge process, high charge densities compared to high surface carbon, easy synthesis through chemical and electrochemical processing [2–4]. The conducting polymers, however, exhibit the disadvantage of a lower specific capacitance and a lower cycle life compared with carbon based electrodes because the redox sites in the polymer backbone are not sufficiently stable for many repeated redox process. A key point to obtain a high and stable specific capacitance lies in the design of the electroactive regions, e.g. by a better *
Corresponding author. Tel.: +49 711 689 1721; fax: +49 711 689 1722. E-mail address: [email protected] (J. Maier).
1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.03.035
control of the film microstructure, i.e. grain size, thickness, specific surface area and pore characters. Variation of the microstructure affects the penetration of electrolyte into pores, as well as the ion mobility within the conducting polymer. In the case of polypyrrole, in recent years more and more researchers focused on obtaining a high specific capacitance by preparing porous or polypyrrole/carbon composite electrodes via chemical polymerization or electrochemical polymerization [5–16]. Carbon (e.g. graphite [8], single-walled nanotubes (SWNT) [9,10], multiwalled nanotubes (MWNT) [11–16], or) was used to control the structure and conductivity of polypyrrole. Up to now the highest capacitance that we are aware of for polypyrrole/ MWNT composite was 506 F/g at a scan rate of 5 mV/s prepared by chemical polymerization method [16]. Here we report a high-performance polypyrrole prepared by a simple electrochemical deposition method and by choosing a suitable electrolyte solution. This electrode material displays a very high specific capacitance and a high cycle life. 2. Experimental Pyrrole (98%) from Aldrich, oxalic acid dihydrate (GR for analysis) and potassium chloride (GR for analysis)
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L.-Z. Fan, J. Maier / Electrochemistry Communications 8 (2006) 937–940
from Merck were used as received. Deionized water was used to prepare solution. Electrochemical deposition and characterization of polypyrrole were carried out in a onecompartment cell connected to a solartron 1255 impedance/gain-phase analyzer coupled with a solartron 1287 electrochemical interface instrument. The cell was equipped with a Ti foil (0.2 mm thickness, Goodfellow) as working electrode, a Platinum foil as counter electrode and a saturated calomel electrode (SCE) as reference electrode. All potentials reported here are measured vs. SCE. The area for deposition was 2 cm2, the remaining part of the Ti foil was covered by teflon tape. Polypyrrole was deposited onto the Ti foil by cyclic voltammetry between potential limits of 0.2 and 0.8 V at scan rates of 200 mV/s. Electrolyte solution consisting of 0.2 M oxalic acid and 0.1 M pyrrole was used for the deposition of polypyrrole. In order to avoid interference due to the change in the pyrrole concentration by the deposition rate, every electrode was plated in a freshly prepared solution. Subsequent to deposition, the electrodes were washed with distilled water and then dried at 40 °C. The mass of polypyrrole was determined by the differences of Ti foil before and after electrodeposition. The microstructure of the surface and cross section of the polypyrrole formed on Ti foil were measured by JEOL scanning electron microscope (SEM) JSM6340F.
of polypyrrole is determined by the oxidation and reduction process of the polymer during its growth. The polymerization of pyrrole in the supporting oxalic acid electrolyte occurred between 0.5 and 0.8 V. In the experiments, the voltage was scanned between 0.2 and 0.8 V, eventually leading to the formation of porous nanosized polypyrrole (to what degree the ‘‘dead range’’ between 0.2 and 0.5 V was significant for the morphology behavior was not been investigated further). Miura et al. reported on the preparation of porous nanosized transition metal oxides by using this method [17,18]. Our polypyrrole film also displays high surface roughness which may be due to the solvent we used. Noh et al. reported that polypyrrole polymerized in H2O exhibited higher surface roughness and higher specific capacitance (maximum capacitance: 355 F/g) than if other solvents such as acetonitrile and diethyl ether have been used [7]. Cyclic voltammetry is an effective tool to reveal the capacitive behavior of a given material. Large current and rectangular forms of the voltammogram, symmetric in anodic and cathodic directions, are indications of an ideal capacitive nature [2]. Cyclic voltammetry characterization was carried out using a three-electrode arranged in one compartment cell and 1 M KCl as electrolyte. The
3. Results and discussion Electrochemical deposition techniques are known to allow for a better control over the mass of the deposited material than chemical synthesis. The relationship of cycle number and the specific mass of polypyrrole grown on Ti foil is presented in Fig. 1. At low cycle numbers the specific mass of polypyrrole polymerized on the Ti foil was a linear function of cycle number, while at high cycle numbers, slope decreases. It is seen from Fig. 2 that porous polypyrrole particles with average diameters of 50 nm were deposited onto the surface of Ti foil. The porous nanostructure
2
Specific mass (mg/cm )
0.8
0.6
0.4
0.2
0.0 0
50
100 150 Cycle numbers
200
Fig. 1. Specific mass of polypyrrole deposited on Ti foil as a function of cycle number.
Fig. 2. SEM images of polypyrrole polymerized on Ti foil (a) surface, and (b) cross section; specific mass of polypyrrole is 0.6 mg/cm2.
L.-Z. Fan, J. Maier / Electrochemistry Communications 8 (2006) 937–940
2
Current (mA/cm )
100
50
10 mV/s 50 mV/s 100 mV/s 200 mV/s 300 mV/s 500 mV/s
0
-50 -0.8
-0.4
0.0 E (V)
0.4
0.8
Fig. 3. Typical cyclic voltammograms of polypyrrole electrode in 1 M KCl electrolyte at different scan rates. Specific mass of polypyrrole is 0.6 mg/cm2.
Specific Capacitance (F/g)
(a)
500
2
0.1 mg/cm 2 0.2 mg/cm 2 0.4 mg/cm 2 0.6 mg/cm 2 0.7 mg/cm 2 0.8 mg/cm
400
300
200
100 0
100
200
300
400
500
Scan rate (mV/s) (b)
Specific Capacitance (F/g)
potential was scanned from 0.8 to 0.8 V and scan rate was changed from 10 to 500 mV/s. Specific capacitance values (C) of the material were calculated from cyclic voltammograms by means of C = i/m(dV/dt), with i being the average current in the capacitive potential region, dV/dt the scan rate, and m the mass of polypyrrole. All the specific capacitance data reported in this paper are calculated according to this equation. It is clearly seen from Fig. 3 that there are no redox peaks in the range between 0.4 and 0.6 V. All the curves are almost rectangular in shape and exhibit mirror image characteristics to the E-axis in the range of 0.4 to 0.6 V, indicating capacitive behavior. The cyclic voltammograms are rectangular and symmetrical even at the high scan rate of 500 mV/s, indicating high reversibility and high power density of polypyrrole deposited in the present study. Fig. 4(a) shows the specific capacitance of polypyrrole as a function of scan rate at different specific masses. A maximum specific capacitance value of 480 ± 50 F/g was obtained at a scan rate of 10 mV/s for a specific mass of 0.6 mg/cm2 (±10%). This value is distinctly higher than most of the data reported in the literature for polypyrrole and polypyrrole composite materials [5–16], and to the best of our knowledge, the largest specific capacitance observed so far for pure polypyrrole prepared by electrodeposition. (It is comparable to the maximum value of 500 F/g reported in the literature [16], which was obtained by a non-electrochemical method.) These very high specific capacitances can be attributed to the high roughness and porous microstructure of nanosized polypyrrole as presented in Fig. 2. Both particle size and porosity of polypyrrole play a crucial role in determining the ability of the electrolyte to enter and to enable local ion transfer processes. The nanosized polypyrrole results in a short diffusion distance for ions penetrating the entire particles. High surface roughness and porous structure permit rapid insertion/extraction of cations and anions from aqueous
939
500
10 mV/s 100 mV/s 200 mV/s 300 mV/s 500 mV/s
400
300
200
100
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
2
Specific mass (mg/cm ) Fig. 4. (a) Plot of scan rate vs. specific capacitance for polypyrrole electrodes at different specific masses, and (b) plot of specific mass vs. specific capacitance for polypyrrole electrodes at different scan rates.
solution even at a high scan rate of 500 mV/s. A decrease in specific capacitance of only 60% (for specific mass 0.6 mg/cm2) was observed when the scan rate was increased from 10 to 500 mV/s, indicating the high power density of the material. In particularly, the very high specific capacitance at high scan rates shows that our polypyrrole material is a promising material for redox supercapacitors applications. To examine the relationship between specific mass and specific capacitance, different polypyrrole masses were deposited at varied cycle numbers. It is interesting to note from Fig. 4(b) that the specific capacitance increases first with an increasing specific mass of polypyrrole, then decreases after a maximum value of 0.6 mg/cm2 has been reached. The reason for the increase of specific capacitance at the lower specific mass can be attributed to increased porosity which allows the electrolyte to penetrate into the mesopores of electrode material [19]. The mesopores between the nanosized polypyrrole can form channels that enable the access of the electrolyte. The observed decrease for specific masses larger than 0.6 mg/cm2 may be due to blockage of channels by over-growth of polypyrrole [7].
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L.-Z. Fan, J. Maier / Electrochemistry Communications 8 (2006) 937–940
and nanosized particles. It seems possible to further enhance the specific capacitance by optimizing experimental conditions such as potential range, scan rate and solution concentration.
400 50
300
Curr e n t (mA)
Specific capacitance (F/g)
500
200
40
Acknowledgements
1st 1000th
30 20
L.Z. Fan thank Alexander von Humboldt Foundation in Germany for financial support. The authors thank A. Fuchs, U. Klock et al. for their technical support; Dr. Y.S. Hu for helpful discussions.
10 0 -10
100
-20 -0. 8
-0. 4
0.0
0.4
0.8
E ( V)
0 0
200
400 600 Cycle number
800
1000
Fig. 5. Cycle-life data of polypyrrole electrode in 1 M KCl electrolyte. The inset shows the cyclic voltammograms of the 1st and the 1000th cycle of polypyrrole electrode in 1 M KCl electrolyte at 50 mV/s. Specific mass of polypyrrole is 0.6 mg/cm2.
The stability of the polypyrrole electrode was examined by means of cycle-life tests performed for a large number of voltammetric cycles. A polypyrrole electrode with 0.6 mg/ cm2 of polypyrrole was subjected to 1000 cycles at a scan rate of 50 mV/s. As indicated in Fig. 5, a decrease of only 9% of the specific capacitance was observed after 1000 cycles (6% in the first 100 cycles and thereafter only 3% in the subsequent 900 cycles); the inset (Fig. 5) shows the similarity between the 1st and the 1000th cycle of the voltammogram. 4. Conclusions It can be concluded that a polypyrrole with high specific capacitance and cycle life can be obtained by using a high scan rate cyclic voltammetry deposition. In addition to easy control of thickness and morphology, this technique also combines formation and deposition of the polymer on the substrate to a single process. The high-performance of these electrode materials is attributed to the microstructure observed, characterized by high roughness, porosity
References [1] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, A.V. Schalkwijk, Nat. Mater. 4 (2005) 366. [2] B.E. Conway, Electrochemical Supercapacitors, Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, New York, 1997. [3] A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, J.P. Ferraris, J. Power Sources 47 (1994) 89, and references therein. [4] A. Malinauskas, J. Malinauskiene, A. Ramanavicius, Nanotechnology 16 (2005) R51, and references therein. [5] M.D. Ingram, H. Staesche, K.S. Ryder, Solid State Ionics 169 (2004) 51. [6] M.D. Ingram, H. Staesche, K.S. Ryder, J. Power Sources 129 (2004) 107. [7] K.A. Noh, D.W. Kim, C.S. Jin, K.H. Shin, J.H. Kim, J.M. Ko, J. Power Sources 124 (2003) 593–595. [8] J.H. Park, J.M. Ko, O.O. Park, D.W. Kim, J. Power Sources 105 (2002) 20. [9] K.H. An, K.K. Jeon, J.K. Heo, S.C. Lim, D.J. Bae, Y.H. Lee, J. Electrochem. Soc. 149 (2002) A1058. [10] H.T. Ham, Y.S. Choi, N. Jeong, I.J. Chung, Polymer 17 (2005) 6308. [11] M. Hughes, M.S.P. Shaffer, N.C. Renouf, C. Singh, G.Z. Chen, D.J. Fray, A.H. Windle, Adv. Mater. 14 (2002) 382. [12] M. Hughes, G.Z. Chen, M.S.P. Shaffer, D.J. Fray, A.H. Windle, Chem. Mater. 14 (2002) 1610. [13] K. Jurewicz, S. Delpeux, V. Bertagna, F. Beguin, E. Frackowiak, Chem. Phys. Lett. 347 (2001) 36. [14] Q.F. Xiao, X. Zhou, Electrochim. Acta 48 (2003) 575. [15] C.S. Du, J. Yeh, N. Pan, Nanotechnology 16 (2005) 350. [16] V. Khomenko, E. Frackowiak, F. Beguin, Electrochim. Acta 50 (2005) 2499. [17] K.R. Prasad, N. Miura, J. Power Sources 135 (2004) 354. [18] K.R. Prasad, N. Miura, Appl. Phys. Lett. 85 (2004) 4199. [19] K.R. Prasad, N. Miura, Electrochem. Commun. 6 (2004) 849.
High-performance polypyrrole electrode materials for redox supercapacitors Li-Zhen Fan, Joachim Maier
*
Max-Planck-Institut fu¨r Festko¨rperforschung 70569 Stuttgart, Germany Received 1 March 2006; received in revised form 17 March 2006; accepted 20 March 2006 Available online 27 April 2006
Abstract Highly active polypyrrole electrodes for redox supercapacitors were prepared by electrodeposition on Ti foil via cyclic voltammetry at a scan rate of 200 mV/s in oxalic acid solution and subsequently characterized in 1 M KCl. Scanning electron microscopy showed that the polypyrrole has a highly porous nanostructure leading to a very high specific capacitance of about 480 ± 50 F/g. The electrode exhibted a high stability during cycle life test. The effects of the mass of polypyrrole electrode materials on the specific capacitance were also investigated. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Polypyrrole; Supercapacitor; Electrodeposition; Specific capacitance
1. Introduction Supercapacitors are currently widely investigated due to their interesting characteristics in terms of power and energy densities. These devices offer an interesting and technologically relevant compromise with respect to power density (greater than for batteries) and energy density (greater than for conventional capacitors) [1]. Conducting polymers are an attractive class of electrode materials to be used in supercapacitors due to their advantageous properties including low cost compared to noble metal oxides, high doping–dedoping rate during charge–discharge process, high charge densities compared to high surface carbon, easy synthesis through chemical and electrochemical processing [2–4]. The conducting polymers, however, exhibit the disadvantage of a lower specific capacitance and a lower cycle life compared with carbon based electrodes because the redox sites in the polymer backbone are not sufficiently stable for many repeated redox process. A key point to obtain a high and stable specific capacitance lies in the design of the electroactive regions, e.g. by a better *
Corresponding author. Tel.: +49 711 689 1721; fax: +49 711 689 1722. E-mail address: [email protected] (J. Maier).
1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.03.035
control of the film microstructure, i.e. grain size, thickness, specific surface area and pore characters. Variation of the microstructure affects the penetration of electrolyte into pores, as well as the ion mobility within the conducting polymer. In the case of polypyrrole, in recent years more and more researchers focused on obtaining a high specific capacitance by preparing porous or polypyrrole/carbon composite electrodes via chemical polymerization or electrochemical polymerization [5–16]. Carbon (e.g. graphite [8], single-walled nanotubes (SWNT) [9,10], multiwalled nanotubes (MWNT) [11–16], or) was used to control the structure and conductivity of polypyrrole. Up to now the highest capacitance that we are aware of for polypyrrole/ MWNT composite was 506 F/g at a scan rate of 5 mV/s prepared by chemical polymerization method [16]. Here we report a high-performance polypyrrole prepared by a simple electrochemical deposition method and by choosing a suitable electrolyte solution. This electrode material displays a very high specific capacitance and a high cycle life. 2. Experimental Pyrrole (98%) from Aldrich, oxalic acid dihydrate (GR for analysis) and potassium chloride (GR for analysis)
938
L.-Z. Fan, J. Maier / Electrochemistry Communications 8 (2006) 937–940
from Merck were used as received. Deionized water was used to prepare solution. Electrochemical deposition and characterization of polypyrrole were carried out in a onecompartment cell connected to a solartron 1255 impedance/gain-phase analyzer coupled with a solartron 1287 electrochemical interface instrument. The cell was equipped with a Ti foil (0.2 mm thickness, Goodfellow) as working electrode, a Platinum foil as counter electrode and a saturated calomel electrode (SCE) as reference electrode. All potentials reported here are measured vs. SCE. The area for deposition was 2 cm2, the remaining part of the Ti foil was covered by teflon tape. Polypyrrole was deposited onto the Ti foil by cyclic voltammetry between potential limits of 0.2 and 0.8 V at scan rates of 200 mV/s. Electrolyte solution consisting of 0.2 M oxalic acid and 0.1 M pyrrole was used for the deposition of polypyrrole. In order to avoid interference due to the change in the pyrrole concentration by the deposition rate, every electrode was plated in a freshly prepared solution. Subsequent to deposition, the electrodes were washed with distilled water and then dried at 40 °C. The mass of polypyrrole was determined by the differences of Ti foil before and after electrodeposition. The microstructure of the surface and cross section of the polypyrrole formed on Ti foil were measured by JEOL scanning electron microscope (SEM) JSM6340F.
of polypyrrole is determined by the oxidation and reduction process of the polymer during its growth. The polymerization of pyrrole in the supporting oxalic acid electrolyte occurred between 0.5 and 0.8 V. In the experiments, the voltage was scanned between 0.2 and 0.8 V, eventually leading to the formation of porous nanosized polypyrrole (to what degree the ‘‘dead range’’ between 0.2 and 0.5 V was significant for the morphology behavior was not been investigated further). Miura et al. reported on the preparation of porous nanosized transition metal oxides by using this method [17,18]. Our polypyrrole film also displays high surface roughness which may be due to the solvent we used. Noh et al. reported that polypyrrole polymerized in H2O exhibited higher surface roughness and higher specific capacitance (maximum capacitance: 355 F/g) than if other solvents such as acetonitrile and diethyl ether have been used [7]. Cyclic voltammetry is an effective tool to reveal the capacitive behavior of a given material. Large current and rectangular forms of the voltammogram, symmetric in anodic and cathodic directions, are indications of an ideal capacitive nature [2]. Cyclic voltammetry characterization was carried out using a three-electrode arranged in one compartment cell and 1 M KCl as electrolyte. The
3. Results and discussion Electrochemical deposition techniques are known to allow for a better control over the mass of the deposited material than chemical synthesis. The relationship of cycle number and the specific mass of polypyrrole grown on Ti foil is presented in Fig. 1. At low cycle numbers the specific mass of polypyrrole polymerized on the Ti foil was a linear function of cycle number, while at high cycle numbers, slope decreases. It is seen from Fig. 2 that porous polypyrrole particles with average diameters of 50 nm were deposited onto the surface of Ti foil. The porous nanostructure
2
Specific mass (mg/cm )
0.8
0.6
0.4
0.2
0.0 0
50
100 150 Cycle numbers
200
Fig. 1. Specific mass of polypyrrole deposited on Ti foil as a function of cycle number.
Fig. 2. SEM images of polypyrrole polymerized on Ti foil (a) surface, and (b) cross section; specific mass of polypyrrole is 0.6 mg/cm2.
L.-Z. Fan, J. Maier / Electrochemistry Communications 8 (2006) 937–940
2
Current (mA/cm )
100
50
10 mV/s 50 mV/s 100 mV/s 200 mV/s 300 mV/s 500 mV/s
0
-50 -0.8
-0.4
0.0 E (V)
0.4
0.8
Fig. 3. Typical cyclic voltammograms of polypyrrole electrode in 1 M KCl electrolyte at different scan rates. Specific mass of polypyrrole is 0.6 mg/cm2.
Specific Capacitance (F/g)
(a)
500
2
0.1 mg/cm 2 0.2 mg/cm 2 0.4 mg/cm 2 0.6 mg/cm 2 0.7 mg/cm 2 0.8 mg/cm
400
300
200
100 0
100
200
300
400
500
Scan rate (mV/s) (b)
Specific Capacitance (F/g)
potential was scanned from 0.8 to 0.8 V and scan rate was changed from 10 to 500 mV/s. Specific capacitance values (C) of the material were calculated from cyclic voltammograms by means of C = i/m(dV/dt), with i being the average current in the capacitive potential region, dV/dt the scan rate, and m the mass of polypyrrole. All the specific capacitance data reported in this paper are calculated according to this equation. It is clearly seen from Fig. 3 that there are no redox peaks in the range between 0.4 and 0.6 V. All the curves are almost rectangular in shape and exhibit mirror image characteristics to the E-axis in the range of 0.4 to 0.6 V, indicating capacitive behavior. The cyclic voltammograms are rectangular and symmetrical even at the high scan rate of 500 mV/s, indicating high reversibility and high power density of polypyrrole deposited in the present study. Fig. 4(a) shows the specific capacitance of polypyrrole as a function of scan rate at different specific masses. A maximum specific capacitance value of 480 ± 50 F/g was obtained at a scan rate of 10 mV/s for a specific mass of 0.6 mg/cm2 (±10%). This value is distinctly higher than most of the data reported in the literature for polypyrrole and polypyrrole composite materials [5–16], and to the best of our knowledge, the largest specific capacitance observed so far for pure polypyrrole prepared by electrodeposition. (It is comparable to the maximum value of 500 F/g reported in the literature [16], which was obtained by a non-electrochemical method.) These very high specific capacitances can be attributed to the high roughness and porous microstructure of nanosized polypyrrole as presented in Fig. 2. Both particle size and porosity of polypyrrole play a crucial role in determining the ability of the electrolyte to enter and to enable local ion transfer processes. The nanosized polypyrrole results in a short diffusion distance for ions penetrating the entire particles. High surface roughness and porous structure permit rapid insertion/extraction of cations and anions from aqueous
939
500
10 mV/s 100 mV/s 200 mV/s 300 mV/s 500 mV/s
400
300
200
100
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
2
Specific mass (mg/cm ) Fig. 4. (a) Plot of scan rate vs. specific capacitance for polypyrrole electrodes at different specific masses, and (b) plot of specific mass vs. specific capacitance for polypyrrole electrodes at different scan rates.
solution even at a high scan rate of 500 mV/s. A decrease in specific capacitance of only 60% (for specific mass 0.6 mg/cm2) was observed when the scan rate was increased from 10 to 500 mV/s, indicating the high power density of the material. In particularly, the very high specific capacitance at high scan rates shows that our polypyrrole material is a promising material for redox supercapacitors applications. To examine the relationship between specific mass and specific capacitance, different polypyrrole masses were deposited at varied cycle numbers. It is interesting to note from Fig. 4(b) that the specific capacitance increases first with an increasing specific mass of polypyrrole, then decreases after a maximum value of 0.6 mg/cm2 has been reached. The reason for the increase of specific capacitance at the lower specific mass can be attributed to increased porosity which allows the electrolyte to penetrate into the mesopores of electrode material [19]. The mesopores between the nanosized polypyrrole can form channels that enable the access of the electrolyte. The observed decrease for specific masses larger than 0.6 mg/cm2 may be due to blockage of channels by over-growth of polypyrrole [7].
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L.-Z. Fan, J. Maier / Electrochemistry Communications 8 (2006) 937–940
and nanosized particles. It seems possible to further enhance the specific capacitance by optimizing experimental conditions such as potential range, scan rate and solution concentration.
400 50
300
Curr e n t (mA)
Specific capacitance (F/g)
500
200
40
Acknowledgements
1st 1000th
30 20
L.Z. Fan thank Alexander von Humboldt Foundation in Germany for financial support. The authors thank A. Fuchs, U. Klock et al. for their technical support; Dr. Y.S. Hu for helpful discussions.
10 0 -10
100
-20 -0. 8
-0. 4
0.0
0.4
0.8
E ( V)
0 0
200
400 600 Cycle number
800
1000
Fig. 5. Cycle-life data of polypyrrole electrode in 1 M KCl electrolyte. The inset shows the cyclic voltammograms of the 1st and the 1000th cycle of polypyrrole electrode in 1 M KCl electrolyte at 50 mV/s. Specific mass of polypyrrole is 0.6 mg/cm2.
The stability of the polypyrrole electrode was examined by means of cycle-life tests performed for a large number of voltammetric cycles. A polypyrrole electrode with 0.6 mg/ cm2 of polypyrrole was subjected to 1000 cycles at a scan rate of 50 mV/s. As indicated in Fig. 5, a decrease of only 9% of the specific capacitance was observed after 1000 cycles (6% in the first 100 cycles and thereafter only 3% in the subsequent 900 cycles); the inset (Fig. 5) shows the similarity between the 1st and the 1000th cycle of the voltammogram. 4. Conclusions It can be concluded that a polypyrrole with high specific capacitance and cycle life can be obtained by using a high scan rate cyclic voltammetry deposition. In addition to easy control of thickness and morphology, this technique also combines formation and deposition of the polymer on the substrate to a single process. The high-performance of these electrode materials is attributed to the microstructure observed, characterized by high roughness, porosity
References [1] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, A.V. Schalkwijk, Nat. Mater. 4 (2005) 366. [2] B.E. Conway, Electrochemical Supercapacitors, Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, New York, 1997. [3] A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, J.P. Ferraris, J. Power Sources 47 (1994) 89, and references therein. [4] A. Malinauskas, J. Malinauskiene, A. Ramanavicius, Nanotechnology 16 (2005) R51, and references therein. [5] M.D. Ingram, H. Staesche, K.S. Ryder, Solid State Ionics 169 (2004) 51. [6] M.D. Ingram, H. Staesche, K.S. Ryder, J. Power Sources 129 (2004) 107. [7] K.A. Noh, D.W. Kim, C.S. Jin, K.H. Shin, J.H. Kim, J.M. Ko, J. Power Sources 124 (2003) 593–595. [8] J.H. Park, J.M. Ko, O.O. Park, D.W. Kim, J. Power Sources 105 (2002) 20. [9] K.H. An, K.K. Jeon, J.K. Heo, S.C. Lim, D.J. Bae, Y.H. Lee, J. Electrochem. Soc. 149 (2002) A1058. [10] H.T. Ham, Y.S. Choi, N. Jeong, I.J. Chung, Polymer 17 (2005) 6308. [11] M. Hughes, M.S.P. Shaffer, N.C. Renouf, C. Singh, G.Z. Chen, D.J. Fray, A.H. Windle, Adv. Mater. 14 (2002) 382. [12] M. Hughes, G.Z. Chen, M.S.P. Shaffer, D.J. Fray, A.H. Windle, Chem. Mater. 14 (2002) 1610. [13] K. Jurewicz, S. Delpeux, V. Bertagna, F. Beguin, E. Frackowiak, Chem. Phys. Lett. 347 (2001) 36. [14] Q.F. Xiao, X. Zhou, Electrochim. Acta 48 (2003) 575. [15] C.S. Du, J. Yeh, N. Pan, Nanotechnology 16 (2005) 350. [16] V. Khomenko, E. Frackowiak, F. Beguin, Electrochim. Acta 50 (2005) 2499. [17] K.R. Prasad, N. Miura, J. Power Sources 135 (2004) 354. [18] K.R. Prasad, N. Miura, Appl. Phys. Lett. 85 (2004) 4199. [19] K.R. Prasad, N. Miura, Electrochem. Commun. 6 (2004) 849.