Miniaturized Stretchable and High-Rate Linear Supercapacitors
- Wenjun Zhu†1,
- Yang Zhang†1,
- Xiaoshuang Zhou1,
- Jiang Xu1,
- Zunfeng Liu3,
- Ningyi Yuan1Email author and
- Jianning Ding1, 2Email author
© The Author(s). 2017
Received: 2 April 2017
Accepted: 26 June 2017
Published: 6 July 2017
Linear stretchable supercapacitors have attracted much attention because they are well suited to applications in the rapidly expanding field of wearable electronics. However, poor conductivity of the electrode material, which limits the transfer of electrons in the axial direction of the linear supercapacitors, leads to a serious loss of capacity at high rates. To solve this problem, we use gold nanoparticles to decorate aligned multiwall carbon nanotube to fabricate stretchable linear electrodes. Furthermore, we have developed fine stretchable linear supercapacitors, which exhibited an extremely high elasticity up to 400% strain with a high capacitance of about 8.7 F g−1 at the discharge current of 1 A g−1.
With increasing development of miniaturized electronic devices, the research on integrated power supplies become more urgent to meet the demanding applications, including micro-robots, smart bracelets, and strain sensors [1–3]. Miniaturized supercapacitors with high-rate performance are a promising candidate for powering these future devices [4, 5]. Moreover, linear supercapacitors have attracted much attention because their flexibility is well suited for wearable electronics [6, 7]. However, these fiber-shaped energy devices have to experience a dramatically stretching process in practical wearable applications. Therefore, it is necessary to evaluate their properties when they are dynamically stretched. Carbon nanotubes are the more suitable for the electrode materials of linear supercapacitors [8–10]. However, energy density of supercapacitors is not high, which hindered the further development of the linear supercapacitors in the field of wearable devices. In order to improve the energy density of supercapacitors, it is common to use pseudocapacitive material to modify electrodes, such as conductive polymers (e.g., PANI, PPy) or metal oxides (e.g., MnO2) [9, 11–14]. However, linear supercapacitors suffer a severe loss of capacity at high rates due to the trade-off of axial electron transport. Optimizing axial conductivity of electrodes is a key to circumvent this trade-off. Compared with the flexible linear supercapacitors, the stretchable linear supercapacitors have much poorer rate performances and they are usually tested at low scan rates (0.01–0.1 V s−1) [10, 11, 13]. Therefore, it is a key to improve the rate performance of the stretchable supercapacitors.
In this study, we fabricate a kind of stretchable linear supercapacitor based on aligned carbon nanotube (CNT) electrodes. To improve the conductivity of linear electrodes, we employed gold nanoparticles (AuNPs) to modify CNTs. The developed stretchable linear supercapacitor exhibited an extremely high elasticity up to 400% strain with a high capacitance of about 8.7 F g−1 at the discharge current of 1 A g−1.
Fabrication of PANI@Au@CNT Sheet
An aligned CNT sheet was drawn from an aligned CNT array (with heights of 350 μm and outer diameters of 9 nm) and simultaneously placed on a rectangular rack. The sheet resistance of a single CNT layer was about ~700–1000 Ω/cm, depending upon the areal density of the CNT sheet (which is a function of the forest height) . A thermal evaporation system (MINI-SPECTROS, Kurt J. Lesker, U S A) was used to deposit AuNPs on CNTs to prepare Au x @CNT sheet (x represents the deposition time of Au). To fabricate PANI@Au x @CNT sheet, polyaniline (PANI) was electrodeposited onto the aligned Au x @CNT sheets by immersing the Au x @CNT sheet into an aqueous solution of aniline (0.1 M) and H2SO4 (1 M) at 0.75 V.
Preparation of Fine Stretchable Supercapacitors
Finally, H3PO4/PVA gel electrolyte was prepared and dripped on the surface of PANI@Au@CNT@fiber. After drying for 6 h, the supercapacitor was assembled by twisting two gel-coated electrodes together and then drying for 12 h.
The morphology of the samples was detected by high-resolution field-emission scanning electron microscopy (FE-SEM, Hitachi S4800). The mass content of Au and C in Au@CNT was detected by an energy dispersive spectrometer (EDS) equipped on Hitachi S4800. The electrochemical performance of the stretchable supercapacitors was investigated by electrochemical cyclic voltammetry (CV), and galvanostatic charge-discharge (GCD) using CHI 660E electrochemical workstation. For the three-electrode system, an Au@CNT sheet or a PANI@Au@CNT sheet was used as a working electrode, with a potassium chloride-saturated Ag/AgCl reference electrode and a platinum wire counter electrode. All three-electrode measurements were performed in 1 M H2SO4 aqueous electrolyte.
Results and Discussions
Mass content of Au and C in Au x @CNT sheet (x = 5, 10, 15, 20)
Au (wt. %)
C (wt. %)
Figure 4c shows CV curves of wire-like symmetrical supercapacitors of CNT@fiber and PANI@Au15@CNT@fiber, respectively. A distinct difference between these two supercapacitors indicates a great improvement of capacitive behavior of PANI@Au15@CNT@fiber. Figure 4d shows GCD curves of these two symmetric supercapacitors. The symmetric triangular shape indicates that both two supercpacitors own a good supercapacitive performance. The specific capacitance of the CNT-based supercapacitor was about 1.6 F g−1 at the current density of 1 A g−1, for PANI@Au15@CNT-wrapped electrode, this value was about 8.7 F g−1. In order to ensure the accuracy of capacitance of the electrode materials, we weigh the electrode before and after deposition of PANI. The mass content of PANI is about 46 mg g−1 and the capacitance of PANI is about 360.8 F g−1.
In this work, a fine stretchable linear supercapacitor based on PANI@Au@CNT@fiber electrodes was fabricated. The fabricated supercapacitor can undergo strain of up to 400%. The supercapacitor based on PANI@Au15@CNT@fiber electrodes was approximately 8.7 F g−1 at the discharge current of 1 A g−1. The stretchable supercapacitors also showed a long-term stretching stability after 1000 stretching cycles and long life after 10,000 charge-discharge cycles.
This work was supported by the National Natural Science Foundation of China (91648109), The National Key Research and Development Program of China (2017YFB0307000), the Priority Academic Program Development of Jiangsu Higher Education Institutions.
WZ and YZ carried out the experiment and drafted the manuscript. XZ and JX participated in the preparation and measurement of the samples. ZL gave advice and performed the characterization. NY and JD supervised the whole work. All the authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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