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TiO2 nanotube composites electrode
Electrochimica Acta 51 (2006) 1289–1292
Preparation and electrochemical capacitance of Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composites electrode He KuanXin a , Zhang Xiaogang a,b,∗ , Li Juan a b
a Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, PR China College of Material Science & Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, PR China
Received 11 January 2005; received in revised form 20 June 2005; accepted 25 June 2005 Available online 18 August 2005
Abstract In this paper, Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composites were synthesized by a simple chemical co-precipitation method. Electrochemical properties of the composites were examined by cyclic voltammetry, galvanostatic and impedance measurements. The highest specific capacitance values of 1053 F/g could be achieved with Me double hydroxides loaded on the TiO2 nanotube, which was comparable to that of hydrated ruthenium oxide. © 2005 Elsevier Ltd. All rights reserved. Keywords: TiO2 nanotube; Me double hydroxides (Me = Co and Ni); Electrochemical capacitance
1. Introduction Due to the increasing demands for electrical energy storage in certain current applications like digital electronic devices, implantable medical devices and stop/start operation in vehicle traction which need very short high power pulses, supercapacitors have drawn increasing attention in the areas of energy storage systems because supercapacitors possess many advantages over the second battery and conventional capacitor. However, the lower energy density of supercapacitors compared to that of secondary battery is a drawback for practical devices such as electric vehicles [1]. From the application point of view, it is necessary to develop supercapacitors with high energy density. It is well established that the energy density is given by CV2 /2 m where V is the initial voltage, and C the specific capacitance [2]. Obviously, maximizing the capacitance and the initial voltage of electrode materials is the key to increase the energy density. In order to increase the performance of supercapacitors in terms of energy, power and voltage, metal oxides like RuO2 , MnO2 and TiO2 in combination with carbon ∗
Corresponding author. Tel.: +86 02584892902; fax: +86 02584892951. E-mail address: [email protected] (Z. Xiaogang).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.06.020
materials have been investigated. The metal oxide/carbon composite electrodes utilize both advantages of double layer capacitance and pseudo-capacitance [3–7]. Recently, Co(OH)2 /USY nanocomposites have been prepared and the capacitance was reported to be as large as 1492 F/g [8]. In our earlier work, Me/Al double hydroxides (Me = Co and Ni) have been synthesized, and the maximum specific capacitance was up to 960 F/g [9]. All above show that developing alternative electrode material with improved characteristics and performance is the next logical work. It is well known that nanotubular material is an excellent material with their microscopic and porous structures and electrochemical behavior [10–12]. Recently, TiO2 nanotubes were obtained by alkaline hydrothermal treatment of titania [13–15]. TiO2 is usually employed in the dielectric capacitors. However, the especial nanotubular stucture seems to increase the dispersion of active materials and result in the enhancement of capacitance. RuO2 /TiO2 nanotubes [16] and NiO/TiO2 nanotubes [17] composites showed the good electrochemical capacitive performance. In this paper, Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composites were synthesized by the chemical co-precipitation method and then used as the electrode material for electrochemical supercapacitors.
H. KuanXin et al. / Electrochimica Acta 51 (2006) 1289–1292
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Electrochemical studies showed that the composite electrode has superior capacitive performance, and the maximum specific capacitance is up to 1053 F/g (for single electrode system), which could be also comparable to that of hydrated ruthenium oxide [18,19].
2. Experimental 2.1. Composites synthesis All chemicals used in this work were A.R. reagents from the Beijing Chemical Factory, except where otherwise indicated. TiO2 nanotubes were synthesized by mild hydrothermal method [13–15]. Me/TiO2 nanotube double hydroxides (Me = Co and Ni) composites were prepared by using the chemical co-precipitation route. The (Co + Ni)/TiO2 mole ratio was 2:1 and the mole ratios of Co/Ni were 6:4 [9]. Stoichiometric metal nitrate was solved in distilled water at room temperature. The mixed solution was co-precipitated in 2 mol/L NaOH solution containing 2 mol/L sodium carbonate in which TiO2 nanotubes were dispersed. The suspension was stirred at 50 ◦ C for 48 h. The precipitating precursors were separated by centrifugation and then were washed carefully with distilled water to remove the nitrate. The products were dried at 70 ◦ C for 18 h in air atmosphere conditions. 2.2. Synthesis of Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composite electrodes Electrodes for electrochemical capacitors were prepared by mixing the prepared powders with 25 wt% carbon black and 5 wt% PTFE binder of the total electrode mass. A small amount of water was then added to those composites to make more homogeneous mixture, which was pressed on nickel grid (1.2 × 107 Pa) to get an approximate thickness of 1 mm. The surface area of the composite electrode was about 1 cm2 , and the loading density of the electroactive materials was 5 mg/cm2 .
2.3. Characterization of the materials and electrodes All electrochemical measurements were done in a threeelectrode system. The prepared electrode was used as working electrode, a platinum foil of the same area as counter electrode. And all potential values in the present study are reported against the Hg/HgO in the same electrolyte .All measurements were carried out in 6 mol/L KOH electrolyte. The galvanostatic charge/discharge was evaluated with multichannel battery test system (BT2042, USA) in a certain potential range. Cyclic voltammetry and the impedance of the electrodes were characterized by the electrochemical workstation system (CHI660A, USA). Data were collected in the frequency range of 105 to 10−2 Hz taking 10 points per decade. TEM (Hitachi 600, Japan) and SEM (Leo1430VP, Germany) were employed to examine the morphology of the composites.
3. Results and discussion Typical transmission electron microscopy (TEM) of pure TiO2 nanotubes and Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composites are shown in Fig. 1(a) and (b), respectively. It can be clearly observed that the pure nanotube is hollow with an outer diameter between 20 and 30 nm and a length between 1 and 5 m, with regular morphology and good dispersion. Due to the proper open mesoporous network of nanotubes, the easily accessible electrode/electrolyte interface allows quick charge propagation in the composite material and it is helpful for the electrolyte ions to access the active materials and result in Faraday reaction, and the H+ or OH− yielded to migrate in time, which may contribute to enhancement of capacitive performance. The present images illustrate that there is a fundamental morphology change in Fig. 1b. The long TiO2 nanotubes in Fig. 1a were cracked into short TiO2 nanotubes, and there are Co/Ni double hydroxides composites inside and outside the short nanotubes. As a result, the
Fig. 1. TEM images of TiO2 nanotubes (a) and Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composites (b).
H. KuanXin et al. / Electrochimica Acta 51 (2006) 1289–1292
Fig. 2. Cyclic voltammograms measured in 6 mol/L KOH at 5 mV/s for (a) pure TiO2 nanotubes; (b) Co/Ni double hydroxides composite; (c) Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composite.
Co/Ni double hydroxides composite is in a better dispersion state. Fig. 2 shows the cyclic voltammetric curves of pure TiO2 nanotubes (a), Co/Ni double hydroxides composite (b) and Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composite (c) in 6 M KOH electrolyte, respectively. The CV curves indicated that the charging and discharging took place at 5 mV/s in the potential range of 0.0–0.6 V versus Hg/HgO. Obviously, pure TiO2 nanotubes had very small specific capacitance. Moreover, curve b shows that the pseudocapacitance mainly comes from Co/Ni double hydroxides composite. Therefore, both the phase of Co(OH)2 and Ni(OH)2 in the composite are responsible for the capacitance measured in curve c. Furthermore, the shape of curves b and c revealed that the capacitance characteristic of Co/Ni double hydroxides phase was distinct from that of the electric doublelayer capacitor, which would produce a CV curve close to an ideal rectangular shape. One quasi-reversible electrontransfer processes is visible in curves b and c, indicating that the measured capacitance is mainly based on redox mechanism. It is interesting to be noted that the cathodic peak P2 and P4 appear similarly at about 0.2 V, but the anodic peak P3 appears at about 0.5 V, which is higher than that of P1 . The potential differences (Ep ) between the anodic peak P3 and cathodic peak P4 are >0.3 V, which is higher than that of our earlier work [9]. The reason may be due to the TiO2 nanotubes, which increase the dispersion of active materials. In addition, the cyclability of Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composite was measured at the 1st and 200th cycle shown in Fig. 3. There is very less change between the peak potential position and the current response even after 200 cycles. The above results imply that very excellent Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composite is the better electrode material for redox supercapacitors. Fig. 4 shows the charge-discharge behavior of the composite electrode. The shape of the discharge curve does not show the characteristic of pure double layer capacitor or pure supercapacitor, in agreement with the result of the CV curve.
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Fig. 3. Cyclic voltammograms of Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composite electrodes at the 1st and 200th cycle.
Fig. 4. Charging-discharging behavior of Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composite electrode in 6 mol/L KOH at 5 mA/cm2 .
The specific capacitance can be obtained from Eq. (1) C=
I [(dV/dt) × ω]
(1)
where I (mA) and dV/dt (mV/s) denote the applied galvanostatic current and the slope of these chronopentiometric, respectively; ω (g) represents the mass of electroactive material. The specific capacitance of the composite was 1053 F/g estimated from Fig. 4. Fig. 5 showed that the capacitive value of Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes
Fig. 5. Charging-discharging behavior of (a) Co/Ni double hydroxides composite; (b) Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composite measured in 6 mol/L KOH at 10 mA/cm2 .
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H. KuanXin et al. / Electrochimica Acta 51 (2006) 1289–1292
1053 F/g was obtained for this composite electrode, which could be comparable to that of hydrated ruthenium oxide. The impedance studies showed that the enhanced high frequency response was attributed to the TiO2 nanotubes, which promoted the dispersion and the utilization of Co/Ni double hydroxides composite considerably.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 20403014) and Key project of Chinese MOE. Fig. 6. Impedance plots of (a) Co/Ni double hydroxides composite; (b) Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composite.
composites was about 1.5 times higher than that of pure (Co and Ni) composites. Fig. 6 presented the Nyquist plots of Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composite electrode and Co/Ni double hydroxides composite electrode. At the higher frequencies, the measured resistance is composed of the following terms: the ionic resistance of electrolyte, the intrinsic resistance of the active material, and the contact resistance at the active material/current collector interface. Although the high frequency resistance of Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composites was smaller than that of pure (Co and Ni) composites, the intrinsic resistance is the same, about 0.5 . The results revealed that the electrical properties of the electrode materials are enhanced after TiO2 nanotubes were loaded with Co(OH)2 and Ni(OH)2 . As the consequence, Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composites have a better frequency response than that of Co/Ni double hydroxides composites, and their full capacitance reached at a higher frequency.
4. Conclusions Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composites were prepared by simple chemical chemical coprecipitation method and a maximum specific capacitance of
References [1] K.H. An, W.S. Kim, Adv. Funct. Mater. 11 (2001) 387. [2] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic Publishers/Plenum Press, New York, 1999 (Chapter 20). [3] J.K. Chang, C.T. Lin, W.T. Tsai, Electrochem. Commun. 6 (2004) 666. [4] K. Kierzek, E. Frackowiak, G. Lota, G. Gryglewicz, J. Machnikowski, Electrochim. Acta 49 (2004) 515. [5] H.C. Liang, F. Chen, R.G. Li, L. Wang, Z.H. Deng, Electrochim. Acta 49 (2004) 3463. [6] R.Z. Ma, B.Q. Wei, C.L. Xu, J. Liang, D.H. Wo, Bull. Chem. Soc. Jpn. 73 (2000) 813. [7] Y.T. Wu, C.C. Hu, J. Electrochem. Soc. 151 (2004) A2060. [8] L. Cao, F. Xu, Y.Y. Liang, H.L. Li, Adv. Mater. 16 (2004) 1853. [9] X.M. Liu, Y.H. Zhang, X.G. Zhang, S.Y. Fu, Electrochim. Acta 49 (2004) 3137. [10] S.C. Pang, M.A. Anderson, T.W. Chapman, J. Electrochem. Soc. 147 (2) (2000) 444. [11] C. Niu, E.K. Sichel, R. Hoch, D. Moy, H. Tennent, Appl. Phys. Lett. 70 (1997) 1480. [12] R.Z. Ma, J. Liang, B.Q. Wei, B. Zhang, C.L. Xu, D.H. Wu, J. Power Sources 84 (1999) 126. [13] T.K. Kasuga, M. Hiramastsu, A. Hoson, T. Sekino, K. Niihara, Langmuir 14 (1998) 3160. [14] Y.C. Zhu, H.L. Li, Y.R. Koltypin, Y.R. Hacohen, A. Gedanken, Chem. Commun. (2001) 2616. [15] D.S. Seo, J.K. Lee, H. Kim, J. Crystal Growth 229 (2001) 428. [16] W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami, Y. Takasu, Angew. Chem. Int. Ed. 42 (2003) 4092. [17] M. Hughes, G.Z. Cheng, M.S.P. Shaffer, D.J. Fray, A.H. Windle, Chem. Mater. 14 (2002) 1610. [18] Y.G. Wang, X.G. Zhang, Electrochim. Acta 49 (2004) 1957. [19] Y.G. Wang, X.G. Zhang, J. Electrochem. Soc. 152 (2004) A671.
Preparation and electrochemical capacitance of Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composites electrode He KuanXin a , Zhang Xiaogang a,b,∗ , Li Juan a b
a Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, PR China College of Material Science & Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, PR China
Received 11 January 2005; received in revised form 20 June 2005; accepted 25 June 2005 Available online 18 August 2005
Abstract In this paper, Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composites were synthesized by a simple chemical co-precipitation method. Electrochemical properties of the composites were examined by cyclic voltammetry, galvanostatic and impedance measurements. The highest specific capacitance values of 1053 F/g could be achieved with Me double hydroxides loaded on the TiO2 nanotube, which was comparable to that of hydrated ruthenium oxide. © 2005 Elsevier Ltd. All rights reserved. Keywords: TiO2 nanotube; Me double hydroxides (Me = Co and Ni); Electrochemical capacitance
1. Introduction Due to the increasing demands for electrical energy storage in certain current applications like digital electronic devices, implantable medical devices and stop/start operation in vehicle traction which need very short high power pulses, supercapacitors have drawn increasing attention in the areas of energy storage systems because supercapacitors possess many advantages over the second battery and conventional capacitor. However, the lower energy density of supercapacitors compared to that of secondary battery is a drawback for practical devices such as electric vehicles [1]. From the application point of view, it is necessary to develop supercapacitors with high energy density. It is well established that the energy density is given by CV2 /2 m where V is the initial voltage, and C the specific capacitance [2]. Obviously, maximizing the capacitance and the initial voltage of electrode materials is the key to increase the energy density. In order to increase the performance of supercapacitors in terms of energy, power and voltage, metal oxides like RuO2 , MnO2 and TiO2 in combination with carbon ∗
Corresponding author. Tel.: +86 02584892902; fax: +86 02584892951. E-mail address: [email protected] (Z. Xiaogang).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.06.020
materials have been investigated. The metal oxide/carbon composite electrodes utilize both advantages of double layer capacitance and pseudo-capacitance [3–7]. Recently, Co(OH)2 /USY nanocomposites have been prepared and the capacitance was reported to be as large as 1492 F/g [8]. In our earlier work, Me/Al double hydroxides (Me = Co and Ni) have been synthesized, and the maximum specific capacitance was up to 960 F/g [9]. All above show that developing alternative electrode material with improved characteristics and performance is the next logical work. It is well known that nanotubular material is an excellent material with their microscopic and porous structures and electrochemical behavior [10–12]. Recently, TiO2 nanotubes were obtained by alkaline hydrothermal treatment of titania [13–15]. TiO2 is usually employed in the dielectric capacitors. However, the especial nanotubular stucture seems to increase the dispersion of active materials and result in the enhancement of capacitance. RuO2 /TiO2 nanotubes [16] and NiO/TiO2 nanotubes [17] composites showed the good electrochemical capacitive performance. In this paper, Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composites were synthesized by the chemical co-precipitation method and then used as the electrode material for electrochemical supercapacitors.
H. KuanXin et al. / Electrochimica Acta 51 (2006) 1289–1292
1290
Electrochemical studies showed that the composite electrode has superior capacitive performance, and the maximum specific capacitance is up to 1053 F/g (for single electrode system), which could be also comparable to that of hydrated ruthenium oxide [18,19].
2. Experimental 2.1. Composites synthesis All chemicals used in this work were A.R. reagents from the Beijing Chemical Factory, except where otherwise indicated. TiO2 nanotubes were synthesized by mild hydrothermal method [13–15]. Me/TiO2 nanotube double hydroxides (Me = Co and Ni) composites were prepared by using the chemical co-precipitation route. The (Co + Ni)/TiO2 mole ratio was 2:1 and the mole ratios of Co/Ni were 6:4 [9]. Stoichiometric metal nitrate was solved in distilled water at room temperature. The mixed solution was co-precipitated in 2 mol/L NaOH solution containing 2 mol/L sodium carbonate in which TiO2 nanotubes were dispersed. The suspension was stirred at 50 ◦ C for 48 h. The precipitating precursors were separated by centrifugation and then were washed carefully with distilled water to remove the nitrate. The products were dried at 70 ◦ C for 18 h in air atmosphere conditions. 2.2. Synthesis of Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composite electrodes Electrodes for electrochemical capacitors were prepared by mixing the prepared powders with 25 wt% carbon black and 5 wt% PTFE binder of the total electrode mass. A small amount of water was then added to those composites to make more homogeneous mixture, which was pressed on nickel grid (1.2 × 107 Pa) to get an approximate thickness of 1 mm. The surface area of the composite electrode was about 1 cm2 , and the loading density of the electroactive materials was 5 mg/cm2 .
2.3. Characterization of the materials and electrodes All electrochemical measurements were done in a threeelectrode system. The prepared electrode was used as working electrode, a platinum foil of the same area as counter electrode. And all potential values in the present study are reported against the Hg/HgO in the same electrolyte .All measurements were carried out in 6 mol/L KOH electrolyte. The galvanostatic charge/discharge was evaluated with multichannel battery test system (BT2042, USA) in a certain potential range. Cyclic voltammetry and the impedance of the electrodes were characterized by the electrochemical workstation system (CHI660A, USA). Data were collected in the frequency range of 105 to 10−2 Hz taking 10 points per decade. TEM (Hitachi 600, Japan) and SEM (Leo1430VP, Germany) were employed to examine the morphology of the composites.
3. Results and discussion Typical transmission electron microscopy (TEM) of pure TiO2 nanotubes and Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composites are shown in Fig. 1(a) and (b), respectively. It can be clearly observed that the pure nanotube is hollow with an outer diameter between 20 and 30 nm and a length between 1 and 5 m, with regular morphology and good dispersion. Due to the proper open mesoporous network of nanotubes, the easily accessible electrode/electrolyte interface allows quick charge propagation in the composite material and it is helpful for the electrolyte ions to access the active materials and result in Faraday reaction, and the H+ or OH− yielded to migrate in time, which may contribute to enhancement of capacitive performance. The present images illustrate that there is a fundamental morphology change in Fig. 1b. The long TiO2 nanotubes in Fig. 1a were cracked into short TiO2 nanotubes, and there are Co/Ni double hydroxides composites inside and outside the short nanotubes. As a result, the
Fig. 1. TEM images of TiO2 nanotubes (a) and Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composites (b).
H. KuanXin et al. / Electrochimica Acta 51 (2006) 1289–1292
Fig. 2. Cyclic voltammograms measured in 6 mol/L KOH at 5 mV/s for (a) pure TiO2 nanotubes; (b) Co/Ni double hydroxides composite; (c) Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composite.
Co/Ni double hydroxides composite is in a better dispersion state. Fig. 2 shows the cyclic voltammetric curves of pure TiO2 nanotubes (a), Co/Ni double hydroxides composite (b) and Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composite (c) in 6 M KOH electrolyte, respectively. The CV curves indicated that the charging and discharging took place at 5 mV/s in the potential range of 0.0–0.6 V versus Hg/HgO. Obviously, pure TiO2 nanotubes had very small specific capacitance. Moreover, curve b shows that the pseudocapacitance mainly comes from Co/Ni double hydroxides composite. Therefore, both the phase of Co(OH)2 and Ni(OH)2 in the composite are responsible for the capacitance measured in curve c. Furthermore, the shape of curves b and c revealed that the capacitance characteristic of Co/Ni double hydroxides phase was distinct from that of the electric doublelayer capacitor, which would produce a CV curve close to an ideal rectangular shape. One quasi-reversible electrontransfer processes is visible in curves b and c, indicating that the measured capacitance is mainly based on redox mechanism. It is interesting to be noted that the cathodic peak P2 and P4 appear similarly at about 0.2 V, but the anodic peak P3 appears at about 0.5 V, which is higher than that of P1 . The potential differences (Ep ) between the anodic peak P3 and cathodic peak P4 are >0.3 V, which is higher than that of our earlier work [9]. The reason may be due to the TiO2 nanotubes, which increase the dispersion of active materials. In addition, the cyclability of Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composite was measured at the 1st and 200th cycle shown in Fig. 3. There is very less change between the peak potential position and the current response even after 200 cycles. The above results imply that very excellent Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composite is the better electrode material for redox supercapacitors. Fig. 4 shows the charge-discharge behavior of the composite electrode. The shape of the discharge curve does not show the characteristic of pure double layer capacitor or pure supercapacitor, in agreement with the result of the CV curve.
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Fig. 3. Cyclic voltammograms of Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composite electrodes at the 1st and 200th cycle.
Fig. 4. Charging-discharging behavior of Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composite electrode in 6 mol/L KOH at 5 mA/cm2 .
The specific capacitance can be obtained from Eq. (1) C=
I [(dV/dt) × ω]
(1)
where I (mA) and dV/dt (mV/s) denote the applied galvanostatic current and the slope of these chronopentiometric, respectively; ω (g) represents the mass of electroactive material. The specific capacitance of the composite was 1053 F/g estimated from Fig. 4. Fig. 5 showed that the capacitive value of Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes
Fig. 5. Charging-discharging behavior of (a) Co/Ni double hydroxides composite; (b) Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composite measured in 6 mol/L KOH at 10 mA/cm2 .
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1053 F/g was obtained for this composite electrode, which could be comparable to that of hydrated ruthenium oxide. The impedance studies showed that the enhanced high frequency response was attributed to the TiO2 nanotubes, which promoted the dispersion and the utilization of Co/Ni double hydroxides composite considerably.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 20403014) and Key project of Chinese MOE. Fig. 6. Impedance plots of (a) Co/Ni double hydroxides composite; (b) Me double hydroxides (Me = Co and Ni)/TiO2 nanotube composite.
composites was about 1.5 times higher than that of pure (Co and Ni) composites. Fig. 6 presented the Nyquist plots of Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composite electrode and Co/Ni double hydroxides composite electrode. At the higher frequencies, the measured resistance is composed of the following terms: the ionic resistance of electrolyte, the intrinsic resistance of the active material, and the contact resistance at the active material/current collector interface. Although the high frequency resistance of Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composites was smaller than that of pure (Co and Ni) composites, the intrinsic resistance is the same, about 0.5 . The results revealed that the electrical properties of the electrode materials are enhanced after TiO2 nanotubes were loaded with Co(OH)2 and Ni(OH)2 . As the consequence, Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composites have a better frequency response than that of Co/Ni double hydroxides composites, and their full capacitance reached at a higher frequency.
4. Conclusions Me double hydroxides (Me = Co and Ni)/TiO2 nanotubes composites were prepared by simple chemical chemical coprecipitation method and a maximum specific capacitance of
References [1] K.H. An, W.S. Kim, Adv. Funct. Mater. 11 (2001) 387. [2] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic Publishers/Plenum Press, New York, 1999 (Chapter 20). [3] J.K. Chang, C.T. Lin, W.T. Tsai, Electrochem. Commun. 6 (2004) 666. [4] K. Kierzek, E. Frackowiak, G. Lota, G. Gryglewicz, J. Machnikowski, Electrochim. Acta 49 (2004) 515. [5] H.C. Liang, F. Chen, R.G. Li, L. Wang, Z.H. Deng, Electrochim. Acta 49 (2004) 3463. [6] R.Z. Ma, B.Q. Wei, C.L. Xu, J. Liang, D.H. Wo, Bull. Chem. Soc. Jpn. 73 (2000) 813. [7] Y.T. Wu, C.C. Hu, J. Electrochem. Soc. 151 (2004) A2060. [8] L. Cao, F. Xu, Y.Y. Liang, H.L. Li, Adv. Mater. 16 (2004) 1853. [9] X.M. Liu, Y.H. Zhang, X.G. Zhang, S.Y. Fu, Electrochim. Acta 49 (2004) 3137. [10] S.C. Pang, M.A. Anderson, T.W. Chapman, J. Electrochem. Soc. 147 (2) (2000) 444. [11] C. Niu, E.K. Sichel, R. Hoch, D. Moy, H. Tennent, Appl. Phys. Lett. 70 (1997) 1480. [12] R.Z. Ma, J. Liang, B.Q. Wei, B. Zhang, C.L. Xu, D.H. Wu, J. Power Sources 84 (1999) 126. [13] T.K. Kasuga, M. Hiramastsu, A. Hoson, T. Sekino, K. Niihara, Langmuir 14 (1998) 3160. [14] Y.C. Zhu, H.L. Li, Y.R. Koltypin, Y.R. Hacohen, A. Gedanken, Chem. Commun. (2001) 2616. [15] D.S. Seo, J.K. Lee, H. Kim, J. Crystal Growth 229 (2001) 428. [16] W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami, Y. Takasu, Angew. Chem. Int. Ed. 42 (2003) 4092. [17] M. Hughes, G.Z. Cheng, M.S.P. Shaffer, D.J. Fray, A.H. Windle, Chem. Mater. 14 (2002) 1610. [18] Y.G. Wang, X.G. Zhang, Electrochim. Acta 49 (2004) 1957. [19] Y.G. Wang, X.G. Zhang, J. Electrochem. Soc. 152 (2004) A671.