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Preparation of a flexible polypyrrole nanoarray and its capacitive performance
Materials Letters 132 (2014) 417–420
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of a flexible polypyrrole nanoarray and its capacitive performance Hongxiu Du a,b, Yibing Xie a,b,n, Chi Xia a,b, Wei Wang a,b, Fang Tian a, Yingzhi Zhou a a b
School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China Suzhou Research Institute of Southeast University, Suzhou 215123, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 May 2014 Accepted 21 June 2014 Available online 28 June 2014
Polypyrrole nanotube embedded nanopore array (PNENA) was prepared using an independent TiO2 nanotube array template-assisted electrodeposition and template removal process. All polypyrrole nanotubes were fully embedded inside polypyrrole nanopores to form a coaxial nanoarray structure and flow-through characteristic. The PNENA thin film electrode exhibited a specific capacitance of 88.6 F g 1 in a potential range of 0.2–0.6 V and a capacity retention of 67% after the 1000 cycles in 1.0 M H2SO4 aqueous solution. A solid-state flexible PNENA supercapacitor was assembled using two symmetric PNENA electrodes and gel electrolyte, showing a specific capacitance of 13 F g 1 at a current density of 0.005 A g 1. This PNENA supercapacitaor also exhibited a stable capacitive performance at both planar state and bending state, showing its possibility for flexible energy storage. & 2014 Elsevier B.V. All rights reserved.
Keywords: Polymers Nanoarray Supercapacitor Flexible Thin films
1. Introduction Nowadays, the flexible and lightweight electrochemical supercapacitors have aroused broad interest due to its potential applications in roll-up displays, portable electronic devices and hybrid electric vehicles [1–3]. An ideal flexible supercapacitor should have high electrochemical capacitance and bendable characteristic. The electrode materials are critical factors that determine the capacitive performance of supercapacitors [4–6]. Polypyrrole (PPy) is regarded as a promising electrode material of supercapacitors [7]. The nanoporous structure is expected to improve the utilization of electroactive PPy [8]. PPy nanobelts, nanobricks and nanosheets have been synthesized by the controlled electrochemical polymerization processes [8]. PPy nanotubes have also been well fabricated using V2O5 nanofibers or TiO2 nanotubes as the sacrificial template [9,10]. In this study, a novel flexible PNENA was synthesized by electrodepositing PPy into independent TiO2 nanotube array and then removing TiO2 template. More importantly, the feasibility of PNENA acting as the electrode material of flexible supercapacitor was also verified.
sheet by an anodization process at 30 V for 2 h in water and ethylene glycol mixture solution (volume ratio, 50/50) containing 0.2 M ammonium fluoride and 0.5 M phosphoric acid. An annealing treatment at 450 1C for 2 h was conducted to form anatase TiO2 nanotube array. Then, PPy-TiO2 nanotube hybrid was formed in a three-electrode system using TiO2 as a working electrode, Pt as a counter electrode and Hg/Hg2Cl2 as a reference electrode in an organic acetonitrile solution containing 0.15 M pyrrole monomer and 0.18 M lithium perchlorate (LiClO4) supporting electrolyte by a normal pulse voltammetry deposition method. The pulse potential was increased from 0.7 to 1.1 V with a pulse potential increment of 0.001 V. The pulse width was 0.06 s and the pulse period was 4 s. Finally, PNENA was obtained by dissolving the TiO2 template from the PPy-TiO2 nanotube hybrid in 2.0 M hydrofluoric acid aqueous solution. As-formed PNENA was repeatedly washed with double-distilled water and finally dried at room temperature. The morphology was investigated using scanning electron microscopy (SEM, Hitachi S-3000, Japan). Electrochemical measurements were carried out using an electrochemical workstation (CHI760C, CH Instruments, USA).
3. Results and discussion 2. Experimental PNENA was prepared through an electrochemical synthesis route. Firstly, independent TiO2 nanotube array was directly formed on Ti n Corresponding author at: School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China. E-mail address: [email protected] (Y. Xie).
http://dx.doi.org/10.1016/j.matlet.2014.06.140 0167-577X/& 2014 Elsevier B.V. All rights reserved.
Fig. 1(a) shows SEM images of TiO2 template and PNENA. TiO2 nanotubes have highly ordered and independent structure with an average inner diameter of 120–150 nm, the wall thickness of 10–20 nm and the interspace between the neighboring nanotubes of 35–60 nm. Fig. 1(b) shows the top surface view of SEM images of the PNENA. All PPy nanotubes are fully embedded inside polypyrrole nanopores, exhibiting an ordered and coaxial nanoarray
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Fig. 1. SEM images of (a) top surface view of TiO2; (b) top surface view, (c) bottom surface view and (d) cross-section view of PNENA.
Fig. 2. Schematic illustration for formation of PNENA.
structure. The inner diameter and the wall thickness of nanopores are 125–225 nm and 15–45 nm, respectively. The inner diameter and the wall thickness of nanotubes are 65–115 nm and 10–25 nm, respectively. Fig. 1(c) shows the bottom surface view of SEM images of the PNENA. Obviously, the PNENA still keeps an open tube mouth and independent tube wall structure, exhibiting a flow-through characteristic. Fig. 1(d) shows the cross-section view of SEM images of the PNENA. The PNENA exhibits a high-density and uniform alignment characteristic. Total length of the PNENA is about 900– 1100 nm. So, this PNENA with oriented nanoarray structure and flowthrough characteristic offers a highly accessible surface area and ion diffusion channel in an electrochemical process. The proposed synthesis pathway to PNENA is illustrated in Fig. 2. In this work, independent TiO2 nantube array is synthesized and utilized as template. Then, the electropolymerization of PPy is conducted successively on the external surface and internal surface of TiO2 nantube walls to form PPy-TiO2 nanotube hybrid. Finally, PNENA is fabricated by dissolving TiO2 template from PPyTiO2 nanotube hybrid in 2.0 M hydrofluoric acid aqueous solution [11]. This method shows following advantages. Each TiO2 nantube has its own wall and is separated from other. Such an independent nanotube array structure is beneficial to deposit the PPy inside these nanotubes and in the interspace between these neighboring nanotubes [12,13]. The independent TiO2 nanotube array can act as a
favourable template to form highly ordered and vertically oriented PNENA. Fig. 3(a) shows the cyclic voltammogram (CV) curves of PNENA at different scan rates in a potential window from 0.2 to 0.6 V vs. SCE. The CV current response is directly proportional to the scan rate. It indicates that this electrode material has excellent electrochemical reversibility and an ideal capacitive behavior [14]. The specific capacitance decreases from 60.3 to 45.3 F g 1 (or 14.1 to 10.6 F cm 2), presenting a capacity retention of 75.1%, when the scan rate was increased from 2 to 100 mV s 1. Such a decrease in capacitance with increasing scan rate is attributed to the presence of inner active sites that cannot completely sustain the redox transition at higher scan rates. This is highly related to the ion diffusion effect within the electrode. The electrolyte ions mostly approach the external surface of PNENA at higher scan rates. So, the specific capacitance obtained at the slower scan rate is close to that for full utilization of the electroactive material. Fig. 3(b) displays the galvanostatic charge–discharge curves of PNENA at different current densities. It can be seen that all of the curves are not ideal straight lines, indicating the involvement of Faradaic reaction process. The corresponding specific capacitances of PNENA are 88.6, 46.9, 33.1, 26.2 and 22.1 F g 1 at different current densities of 0.5, 1.0, 1.5, 2.0 and 2.5 A g 1, respectively. The capacitance decreases when the discharge current is gradually increased. The
H. Du et al. / Materials Letters 132 (2014) 417–420
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Fig. 3. (a) Cyclic voltammograms at different scan rates, (b) galvanostatic charge–discharge curves at different current densities and (c) variation of the specific capacitance with cycle number at a current density of 2.5 A g 1 g in 1 M H2SO4 aqueous solution for PNENA electrode.
Fig. 4. (a) Schematic showing assembly structure of PNENA supercapacitor and its solid-state flexible prototype device, (b) cyclic voltammogram curves of PNENA supercapacitor at a planar state and bending state, (c) galvanostatic charge–discharge curve of PNENA supercapacitor.
values of specific capacitance obtained by charge–discharge and CV technique are comparable. To evaluate the electrochemical stability of the PNENA, the continuous galvanostatic charge–discharge measurement was carried out at the current density of 2.5 A g 1. The corresponding capacitance decay curve is shown in Fig. 3(c). The specific capacitance of the PNENA electrode was decreased from 23 F g 1 to 15.4 F g 1 after 1000 cycles, presenting a capacity retention of 67%. To explore its potential application in flexible and lightweight energy storage devices, a solid-state flexible supercapacitor as an energy storage device has been constructed using two symmetric PNENA electrodes separated by a porous dialysis membrane saturated with PVA/H2SO4 gel electrolyte. The device is encapsulated by polyethylene terephthalate (PET) film, and electrodes are connected with copper wires. Fig. 4(a) shows the schematic structure and the photographs of the flexible PNENA supercapacitor. The as-fabricated PNENA supercapacitor, having a total thickness of 0.6 mm, can be bent without destroying its structural integrity. Fig. 4(b) shows CV curves of the PNENA supercapacitor at the same scan rate of 2 mV s 1 when this supercapacitor keeps at a planar state and bending state. The corresponding capacitance is estimated to be 1.2 and 1.17 F g 1 on the base of the enclosed areas of the CV curves. It means that there is a fluctuation of about 2.5% of the capacitive performance, showing a good mechanical stability and flexibility of this PNENA supercapacitor. Fig. 4(c) shows the galvanostatic charge–discharge curve of PNENA supercapacitor. The specific capacitance is determined to be 13 F g 1 at the current density of 0.005 A g 1. It demonstrates a high possibility for this PNENA supercapacitor to be used as flexible electronic devices in future.
4. Conclusions A flexible PPy nanoarray thin film with a complex nanostructure has been successfully prepared using independent TiO2 nanotube
array template-assisted electrodeposition route and template removal process. Microstructure characterization proves that the PPy nanoarray keeps a coaxial nanotube embedded nanopore array structure and flow-through characteristic. A specific capacitance of 88.6 F g 1 was obtained at a current density of 0.5 A g 1 in 1.0 M H2SO4 aqueous solution. A flexible PNENA supercapacitor was assembled using two symmetric PNENA electrodes and gel electrolyte. The stable capacitive performance of PNENA supercapacitor at both planar and bending state suggests its prospective application of flexible energy storage devices.
Acknowledgements The work was supported by National Natural Science Foundation of China (no. 21373047 and 20871029), Program for New Century Excellent Talents in University of the State Ministry of Education (no. NCET-08-0119), Science & Technology Program of Suzhou City (no. ZXG2012026, SYN201208, SYG201017).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.06. 140. References [1] Koo M, Park KI, Lee SH, Suh M, Jeon DY, Choi JW, et al. Bendable inorganic thinfilm battery for fully flexible electronic systems. Nano Lett 2012;12:4810–6. [2] Duay J, Gillette E, Liu R, Lee SB. Highly flexible pseudocapacitor based on freestanding heterogeneous MnO2/conductive polymer nanowire arrays. Phys Chem Chem Phys 2012;14:3329–37. [3] Xie Y, Fang X. Electrochemical flexible supercapacitor based on manganese dioxide-titanium nitride nanotube hybrid. Electrochim Acta 2014;120:273–83.
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H. Du et al. / Materials Letters 132 (2014) 417–420
[4] Dubal DP, Patil SV, Kim WB, Lokhande CD. Supercapacitors based on electrochemically deposited polypyrrole nanobricks. Mater Lett 2011;65:2628–31. [5] Andres B, Forsberg S, Paola Vilches A, Zhang R, Andersson H, Hummelgård M, et al. Supercapacitors with graphene coated paper electrodes. Nord Pulp Pap Res J 2012;27:481–5. [6] Wu X, Hong X, Luo Z, Hui KS, Chen H, Wu J, et al. The effects of surface modification on the supercapacitive behaviors of novel mesoporous carbon derived from rod-like hydroxyapatite template. Electrochim Acta 2013;89: 400–6. [7] Li X, Zhitomirsky I. Electrodeposition of polypyrrole-carbon nanotube composites for electrochemical supercapacitors. J Power Sources 2013;221:49–56. [8] Dubal DP, Lee SH, Kim JG, Kim WB, Lokhande CD. Porous polypyrrole clusters prepared by electropolymerization for a high performance supercapacitor. J Mater Chem 2012;22:3044–52. [9] Zhang XY, Manohar SK. Narrow pore-diameter polypyrrole nanotubes. J Am Chem Soc 2005;127:14156–7.
[10] Kowalski D, Schmuki P. Polypyrrole self-organized nanopore arrays formed by controlled electropolymerization in TiO2 nanotube template. Chem Commun 2010;46:8585–7. [11] Wang ZL, Guo R, Ding LX, Tong YX, Li GR. Controllable template-assisted electrodeposition of single- and multi-walled nanotube arrays for electrochemical energy storage. Sci Rep 2013:3. [12] Xie YB, Du HX. Electrochemical capacitance performance of polypyrroletitania nanotube hybrid. J Solid State Electrochem 2012;16:2683–9. [13] Du HX, Xie YB, Xia C, Wang W, Tian F. Electrochemical capacitance of polypyrrole-titanium nitride and polypyrrole-titania nanotube hybrids. New J Chem 2014;38:1284–93. [14] Shi K, Su Y, Zhitomirsky I. Characterization of Ni plaque based polypyrrole electrodes prepared by pulse electropolymerization. Mater Lett 2013;96: 135–8.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of a flexible polypyrrole nanoarray and its capacitive performance Hongxiu Du a,b, Yibing Xie a,b,n, Chi Xia a,b, Wei Wang a,b, Fang Tian a, Yingzhi Zhou a a b
School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China Suzhou Research Institute of Southeast University, Suzhou 215123, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 May 2014 Accepted 21 June 2014 Available online 28 June 2014
Polypyrrole nanotube embedded nanopore array (PNENA) was prepared using an independent TiO2 nanotube array template-assisted electrodeposition and template removal process. All polypyrrole nanotubes were fully embedded inside polypyrrole nanopores to form a coaxial nanoarray structure and flow-through characteristic. The PNENA thin film electrode exhibited a specific capacitance of 88.6 F g 1 in a potential range of 0.2–0.6 V and a capacity retention of 67% after the 1000 cycles in 1.0 M H2SO4 aqueous solution. A solid-state flexible PNENA supercapacitor was assembled using two symmetric PNENA electrodes and gel electrolyte, showing a specific capacitance of 13 F g 1 at a current density of 0.005 A g 1. This PNENA supercapacitaor also exhibited a stable capacitive performance at both planar state and bending state, showing its possibility for flexible energy storage. & 2014 Elsevier B.V. All rights reserved.
Keywords: Polymers Nanoarray Supercapacitor Flexible Thin films
1. Introduction Nowadays, the flexible and lightweight electrochemical supercapacitors have aroused broad interest due to its potential applications in roll-up displays, portable electronic devices and hybrid electric vehicles [1–3]. An ideal flexible supercapacitor should have high electrochemical capacitance and bendable characteristic. The electrode materials are critical factors that determine the capacitive performance of supercapacitors [4–6]. Polypyrrole (PPy) is regarded as a promising electrode material of supercapacitors [7]. The nanoporous structure is expected to improve the utilization of electroactive PPy [8]. PPy nanobelts, nanobricks and nanosheets have been synthesized by the controlled electrochemical polymerization processes [8]. PPy nanotubes have also been well fabricated using V2O5 nanofibers or TiO2 nanotubes as the sacrificial template [9,10]. In this study, a novel flexible PNENA was synthesized by electrodepositing PPy into independent TiO2 nanotube array and then removing TiO2 template. More importantly, the feasibility of PNENA acting as the electrode material of flexible supercapacitor was also verified.
sheet by an anodization process at 30 V for 2 h in water and ethylene glycol mixture solution (volume ratio, 50/50) containing 0.2 M ammonium fluoride and 0.5 M phosphoric acid. An annealing treatment at 450 1C for 2 h was conducted to form anatase TiO2 nanotube array. Then, PPy-TiO2 nanotube hybrid was formed in a three-electrode system using TiO2 as a working electrode, Pt as a counter electrode and Hg/Hg2Cl2 as a reference electrode in an organic acetonitrile solution containing 0.15 M pyrrole monomer and 0.18 M lithium perchlorate (LiClO4) supporting electrolyte by a normal pulse voltammetry deposition method. The pulse potential was increased from 0.7 to 1.1 V with a pulse potential increment of 0.001 V. The pulse width was 0.06 s and the pulse period was 4 s. Finally, PNENA was obtained by dissolving the TiO2 template from the PPy-TiO2 nanotube hybrid in 2.0 M hydrofluoric acid aqueous solution. As-formed PNENA was repeatedly washed with double-distilled water and finally dried at room temperature. The morphology was investigated using scanning electron microscopy (SEM, Hitachi S-3000, Japan). Electrochemical measurements were carried out using an electrochemical workstation (CHI760C, CH Instruments, USA).
3. Results and discussion 2. Experimental PNENA was prepared through an electrochemical synthesis route. Firstly, independent TiO2 nanotube array was directly formed on Ti n Corresponding author at: School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China. E-mail address: [email protected] (Y. Xie).
http://dx.doi.org/10.1016/j.matlet.2014.06.140 0167-577X/& 2014 Elsevier B.V. All rights reserved.
Fig. 1(a) shows SEM images of TiO2 template and PNENA. TiO2 nanotubes have highly ordered and independent structure with an average inner diameter of 120–150 nm, the wall thickness of 10–20 nm and the interspace between the neighboring nanotubes of 35–60 nm. Fig. 1(b) shows the top surface view of SEM images of the PNENA. All PPy nanotubes are fully embedded inside polypyrrole nanopores, exhibiting an ordered and coaxial nanoarray
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Fig. 1. SEM images of (a) top surface view of TiO2; (b) top surface view, (c) bottom surface view and (d) cross-section view of PNENA.
Fig. 2. Schematic illustration for formation of PNENA.
structure. The inner diameter and the wall thickness of nanopores are 125–225 nm and 15–45 nm, respectively. The inner diameter and the wall thickness of nanotubes are 65–115 nm and 10–25 nm, respectively. Fig. 1(c) shows the bottom surface view of SEM images of the PNENA. Obviously, the PNENA still keeps an open tube mouth and independent tube wall structure, exhibiting a flow-through characteristic. Fig. 1(d) shows the cross-section view of SEM images of the PNENA. The PNENA exhibits a high-density and uniform alignment characteristic. Total length of the PNENA is about 900– 1100 nm. So, this PNENA with oriented nanoarray structure and flowthrough characteristic offers a highly accessible surface area and ion diffusion channel in an electrochemical process. The proposed synthesis pathway to PNENA is illustrated in Fig. 2. In this work, independent TiO2 nantube array is synthesized and utilized as template. Then, the electropolymerization of PPy is conducted successively on the external surface and internal surface of TiO2 nantube walls to form PPy-TiO2 nanotube hybrid. Finally, PNENA is fabricated by dissolving TiO2 template from PPyTiO2 nanotube hybrid in 2.0 M hydrofluoric acid aqueous solution [11]. This method shows following advantages. Each TiO2 nantube has its own wall and is separated from other. Such an independent nanotube array structure is beneficial to deposit the PPy inside these nanotubes and in the interspace between these neighboring nanotubes [12,13]. The independent TiO2 nanotube array can act as a
favourable template to form highly ordered and vertically oriented PNENA. Fig. 3(a) shows the cyclic voltammogram (CV) curves of PNENA at different scan rates in a potential window from 0.2 to 0.6 V vs. SCE. The CV current response is directly proportional to the scan rate. It indicates that this electrode material has excellent electrochemical reversibility and an ideal capacitive behavior [14]. The specific capacitance decreases from 60.3 to 45.3 F g 1 (or 14.1 to 10.6 F cm 2), presenting a capacity retention of 75.1%, when the scan rate was increased from 2 to 100 mV s 1. Such a decrease in capacitance with increasing scan rate is attributed to the presence of inner active sites that cannot completely sustain the redox transition at higher scan rates. This is highly related to the ion diffusion effect within the electrode. The electrolyte ions mostly approach the external surface of PNENA at higher scan rates. So, the specific capacitance obtained at the slower scan rate is close to that for full utilization of the electroactive material. Fig. 3(b) displays the galvanostatic charge–discharge curves of PNENA at different current densities. It can be seen that all of the curves are not ideal straight lines, indicating the involvement of Faradaic reaction process. The corresponding specific capacitances of PNENA are 88.6, 46.9, 33.1, 26.2 and 22.1 F g 1 at different current densities of 0.5, 1.0, 1.5, 2.0 and 2.5 A g 1, respectively. The capacitance decreases when the discharge current is gradually increased. The
H. Du et al. / Materials Letters 132 (2014) 417–420
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Fig. 3. (a) Cyclic voltammograms at different scan rates, (b) galvanostatic charge–discharge curves at different current densities and (c) variation of the specific capacitance with cycle number at a current density of 2.5 A g 1 g in 1 M H2SO4 aqueous solution for PNENA electrode.
Fig. 4. (a) Schematic showing assembly structure of PNENA supercapacitor and its solid-state flexible prototype device, (b) cyclic voltammogram curves of PNENA supercapacitor at a planar state and bending state, (c) galvanostatic charge–discharge curve of PNENA supercapacitor.
values of specific capacitance obtained by charge–discharge and CV technique are comparable. To evaluate the electrochemical stability of the PNENA, the continuous galvanostatic charge–discharge measurement was carried out at the current density of 2.5 A g 1. The corresponding capacitance decay curve is shown in Fig. 3(c). The specific capacitance of the PNENA electrode was decreased from 23 F g 1 to 15.4 F g 1 after 1000 cycles, presenting a capacity retention of 67%. To explore its potential application in flexible and lightweight energy storage devices, a solid-state flexible supercapacitor as an energy storage device has been constructed using two symmetric PNENA electrodes separated by a porous dialysis membrane saturated with PVA/H2SO4 gel electrolyte. The device is encapsulated by polyethylene terephthalate (PET) film, and electrodes are connected with copper wires. Fig. 4(a) shows the schematic structure and the photographs of the flexible PNENA supercapacitor. The as-fabricated PNENA supercapacitor, having a total thickness of 0.6 mm, can be bent without destroying its structural integrity. Fig. 4(b) shows CV curves of the PNENA supercapacitor at the same scan rate of 2 mV s 1 when this supercapacitor keeps at a planar state and bending state. The corresponding capacitance is estimated to be 1.2 and 1.17 F g 1 on the base of the enclosed areas of the CV curves. It means that there is a fluctuation of about 2.5% of the capacitive performance, showing a good mechanical stability and flexibility of this PNENA supercapacitor. Fig. 4(c) shows the galvanostatic charge–discharge curve of PNENA supercapacitor. The specific capacitance is determined to be 13 F g 1 at the current density of 0.005 A g 1. It demonstrates a high possibility for this PNENA supercapacitor to be used as flexible electronic devices in future.
4. Conclusions A flexible PPy nanoarray thin film with a complex nanostructure has been successfully prepared using independent TiO2 nanotube
array template-assisted electrodeposition route and template removal process. Microstructure characterization proves that the PPy nanoarray keeps a coaxial nanotube embedded nanopore array structure and flow-through characteristic. A specific capacitance of 88.6 F g 1 was obtained at a current density of 0.5 A g 1 in 1.0 M H2SO4 aqueous solution. A flexible PNENA supercapacitor was assembled using two symmetric PNENA electrodes and gel electrolyte. The stable capacitive performance of PNENA supercapacitor at both planar and bending state suggests its prospective application of flexible energy storage devices.
Acknowledgements The work was supported by National Natural Science Foundation of China (no. 21373047 and 20871029), Program for New Century Excellent Talents in University of the State Ministry of Education (no. NCET-08-0119), Science & Technology Program of Suzhou City (no. ZXG2012026, SYN201208, SYG201017).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.06. 140. References [1] Koo M, Park KI, Lee SH, Suh M, Jeon DY, Choi JW, et al. Bendable inorganic thinfilm battery for fully flexible electronic systems. Nano Lett 2012;12:4810–6. [2] Duay J, Gillette E, Liu R, Lee SB. Highly flexible pseudocapacitor based on freestanding heterogeneous MnO2/conductive polymer nanowire arrays. Phys Chem Chem Phys 2012;14:3329–37. [3] Xie Y, Fang X. Electrochemical flexible supercapacitor based on manganese dioxide-titanium nitride nanotube hybrid. Electrochim Acta 2014;120:273–83.
420
H. Du et al. / Materials Letters 132 (2014) 417–420
[4] Dubal DP, Patil SV, Kim WB, Lokhande CD. Supercapacitors based on electrochemically deposited polypyrrole nanobricks. Mater Lett 2011;65:2628–31. [5] Andres B, Forsberg S, Paola Vilches A, Zhang R, Andersson H, Hummelgård M, et al. Supercapacitors with graphene coated paper electrodes. Nord Pulp Pap Res J 2012;27:481–5. [6] Wu X, Hong X, Luo Z, Hui KS, Chen H, Wu J, et al. The effects of surface modification on the supercapacitive behaviors of novel mesoporous carbon derived from rod-like hydroxyapatite template. Electrochim Acta 2013;89: 400–6. [7] Li X, Zhitomirsky I. Electrodeposition of polypyrrole-carbon nanotube composites for electrochemical supercapacitors. J Power Sources 2013;221:49–56. [8] Dubal DP, Lee SH, Kim JG, Kim WB, Lokhande CD. Porous polypyrrole clusters prepared by electropolymerization for a high performance supercapacitor. J Mater Chem 2012;22:3044–52. [9] Zhang XY, Manohar SK. Narrow pore-diameter polypyrrole nanotubes. J Am Chem Soc 2005;127:14156–7.
[10] Kowalski D, Schmuki P. Polypyrrole self-organized nanopore arrays formed by controlled electropolymerization in TiO2 nanotube template. Chem Commun 2010;46:8585–7. [11] Wang ZL, Guo R, Ding LX, Tong YX, Li GR. Controllable template-assisted electrodeposition of single- and multi-walled nanotube arrays for electrochemical energy storage. Sci Rep 2013:3. [12] Xie YB, Du HX. Electrochemical capacitance performance of polypyrroletitania nanotube hybrid. J Solid State Electrochem 2012;16:2683–9. [13] Du HX, Xie YB, Xia C, Wang W, Tian F. Electrochemical capacitance of polypyrrole-titanium nitride and polypyrrole-titania nanotube hybrids. New J Chem 2014;38:1284–93. [14] Shi K, Su Y, Zhitomirsky I. Characterization of Ni plaque based polypyrrole electrodes prepared by pulse electropolymerization. Mater Lett 2013;96: 135–8.