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Electrochromic Fiber-Shaped Supercapacitors Xuli Chen, Huijuan Lin, Jue Deng, Ye Zhang, Xuemei Sun, Peining Chen, Xin Fang, Zhitao Zhang, Guozhen Guan, and Huisheng Peng*

Smart electronic products represent a new and promising direction in modern electronics.[1–8] The popularity of smart electronic devices such as smart phones, intelligent bracelets, and eye-wear has renewed interest in their energy-storage accessories. To this end, energy-storage devices such as supercapacitors,[9] which have generally been used to power these smart electronic products, are key to the intelligentization of products. Making an energy device such as a supercapacitor change its appearance according to its charged state, therefore, conforms to the concept of intelligence. If a supercapacitor can sense changes in the level of stored energy and communicate about the changes in a predictable and noticeable manner, we may quickly determine that the energy has been consumed before a device stops working, which introduces a high level of convenience and efficiency to a wide range of potential applications. However, although several attempts have recently been made to synthesize responsive electrode materials with chromatic transitions under different potentials,[10] they remain unavailable for smart supercapacitors. On the other hand, electrochromic materials can reversibly change their colors at various potentials based on redox reactions. Two types of electrochromic materials have been widely explored, that is, inorganic transition metal oxides such as WO3 and conducting organic polymers such as polyaniline (PANI), because of their fast redox reactions and obvious color changes.[10–13] The incorporation of electrochromic materials into supercapacitors may provide a general and effective strategy to realize intelligent supercapacitors that show different colors during use. Meanwhile, wearable and portable electronic devices are also becoming more and more desired and may further dominate life in the future.[6,14–18] A matching power system is badly needed to support them. Fiber-shaped supercapacitors have therefore been widely explored as a general and effective strategy to solve the above problem.[19–26] Compared with the conventional planar structure, the fiber shape enables many unique and promising advantages. For example, fiber-shaped supercapacitors can be woven into energy-storage textiles or integrated into clothes. A great deal of effort has been devoted X. L. Chen, H. J. Lin, J. Deng, Y. Zhang, X. M. Sun, P. N. Chen, X. Fang, Z. T. Zhang, G. Z. Guan, Prof. H. S. Peng State Key Laboratory of Molecular Engineering of Polymers Department of Macromolecular Science and Laboratory of Advanced Materials Fudan University Shanghai 200438, PR China E-mail: [email protected]

DOI: 10.1002/adma.201403243

Adv. Mater. 2014, DOI: 10.1002/adma.201403243

to developing new materials, optimizing structures, and improving the electrochemical properties of fiber-shaped supercapacitors. However, it is necessary to extend the applications of fiber-shaped supercapacitors by incorporating functional guests; this is critically important for miniaturization, a mainstream direction in wearable and portable electronics. In this Communication, we describe the development of an electrochromic fiber-shaped supercapacitor by electro-depositing polyaniline (PANI) onto sheets of aligned carbon nanotubes (CNT) as two electrodes. The resulting electrochromic fiber-shaped supercapacitors can be further woven into fabrics to display designed patterns (Figure 1a). In a typical fabrication process (Figure S1, Supporting Information), CNTs were wound next to each other onto a stretched elastic fiber (with a strain of 100%) to form sheet-like structures of aligned CNTs (typically 1 cm across) with a gap of ca. 1 mm between them after release. PANI was then electro-deposited on the two separated CNT sheets to prepare CNT/PANI composite electrodes (Figure 1a). The two CNT/PANI composite electrodes were further coated with poly(vinyl alcohol) (PVA)/H3PO4 gel electrolyte to produce the desired fiber-shaped supercapacitor. In the fiber-shaped supercapacitor, charges are stored and released mainly through the redox reaction of PANI, with a small contribution from the electrical double layers at the electrode–electrolyte interface. During the charge–discharge processes, the composite electrodes change color in response to the varying potentials. For instance, the positive electrode demonstrates rapid and reversible chromatic transitions between blue, green, and light yellow under different working states (Figure 1b). Of course, the negative electrode also displays similar color changes owing to the symmetric structure. These color changes can be directly observed by the naked eye, which suggests many promising applications in smart devices. Besides the chromatic transition properties, the supercapacitor also delivered a decent electrochemical performance. A specific capacitance of 255.5 F g–1 (0.1890 mF cm–1) has been achieved with energy density of 12.75 W h kg–1 and power density of 1494 W kg–1. Moreover, as the aligned CNT/PANI composite sheet electrodes are wound on an elastic fiber substrate, the electrochromic fiber-shaped supercapacitor is also stretchable and flexible with high stability. As a demonstration, 93.8% of the specific capacitance was retained after the supercapacitor had been bent for 1000 cycles and 97.9% after being 100% stretched for 100 cycles. The aligned CNT sheets were first dry-drawn from spinnable CNT arrays that had been synthesized by chemical vapor deposition. The CNT sheet was then wound onto an elastic rubber fiber. Figure 2a,b show that the CNTs remain highly aligned and closely attached on the elastic fiber substrate. PANI was

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Figure 1. Schematic illustration of the structure and display function of the electrochromic, wearable fiber-shaped supercapacitor.

electrochemically deposited onto the CNT sheet, and the CNTs retained their aligned structure after incorporation of the PANI (Figure 2c,d). Figure 2d further shows that PANI was uniformly deposited on the surfaces of the aligned CNTs. Two aligned CNT/PANI composite electrodes were mainly studied to form a fiber-shaped supercapacitor. The impact of the PANI content in the composite on the electrochemical properties was first carefully investigated. The weight of PANI was calculated by Faraday’s law and is detailed

in the Supporting Information. Figure 3a shows cyclic voltammograms of the fiber-shaped supercapacitors with different PANI weight percentages in the composite electrodes. The redox peaks between 0 and –0.5 V or +0.5 V correspond to the redox reactions in PANI. The enclosed cyclic voltammetry areas first increased as the PANI weight percentages increased to 70% and then decreased with a further increase to 90%. Accordingly, the specific capacitance also increased with increasing PANI weight percentage from 20% to 70% and then decreased with

Figure 2. SEM images. a,b) Aligned-CNT sheet on an elastic fiber at low and high magnifications, respectively. c,d) Aligned-CNT/PANI composite on an elastic fiber at low and high magnifications, respectively. PANI weight percentage: 70%. The insets in (b) and (d) show higher magnifications.

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COMMUNICATION Figure 3. Electrochemical characterization of the electrochromic fiber-shaped supercapacitor. a) Cyclic voltammograms at 10 mV s–1. b) Galvanostatic charge–discharge profiles at 1 A g–1. c) Specific capacitance at different current densities. d) Cyclic voltammograms at various scan rates. e) Galvanostatic charge–discharge profiles at different current densities. f) Dependence of specific capacitance on cycle number at 1 A g–1. The PANI weight percentages are 20%, 50%, 70%, and 90% in (a–c) and 70% in (d–f).

a further increase to 90% at 1 A g–1 (Figure 3b). This conclusion was also verified by galvanostatic charge–discharge measurements in the current density range 1–10 A g–1 (Figure 3c). For instance, the specific capacitance based on the total weight of PANI and CNT in the composite electrode increased from 82.20 to 255.5 F g–1 as the PANI weight percentage increased from 20% to 70% and then decreased to 188.9 F g–1 with a further increase to 90% at the current density of 1 A g–1. The energy and power densities of the composite electrode (PANI weight percentage of 70%) can therefore be calculated as 12.75 and 1494 W kg–1 at 1 and 10 A g–1, respectively. The increased specific capacitance with increasing PANI weight percentage below 70% should be ascribed to the pseudo-capacitance provided by PANI, which was uniformly deposited on the aligned CNTs. However, at a higher PANI weight percentage, such as 90%, the excess PANI was coated on the outer surface of the CNT/PANI composite fiber electrode and could not effectively

Adv. Mater. 2014, DOI: 10.1002/adma.201403243

contact with the aligned CNTs, which eventually led to slower and less effective ion diffusion (Figure S2, Supporting Information). In addition, at higher PANI weight percentages, with increasing thickness of the PANI layer the electrical resistance increased, thus decreasing the specific capacitance. This fact was also verified by the voltage drop (product of current and resistance, IR) from the galvanostatic charge–discharge profiles (Figure 3b). The IR drop in the supercapacitor remains low at PANI weight percentages below 70% and obviously increases with a further increase in weight percentage, for example, 0.30 V at 90%, at 1 A g–1. Therefore, a weight percentage of 70% was mainly investigated in the following discussion. The impact of the thickness of CNT sheet on the electrochemical properties was also studied. The specific capacitance increased with an increase in thickness from 20 to 100 nm (Figure S3, Supporting Information). For the convenience of fabrication, a thickness of 20 nm was mainly used in the studies described below.

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The redox peaks of cyclic voltammograms with scan rate increasing from 10 to 50 mV s–1 indicated a pseudo-capacitance of the fiber-shaped supercapacitor (Figure 3d). Specific capacitances of 215.6, 178.9, and 86.60 F g–1 were produced with increasing scan rates of 10, 20, and 50 mV s–1, respectively, lower than those calculated from galvanostatic charge– discharge profiles. This phenomenon may be explained by the different charge–discharge rates of the two methods. For instance, the discharge time of the galvanostatic charge–discharge process with a specific capacitance of 255.5 F g–1 was 127.7 s, while the discharge time of the cyclic voltammogram process with a specific capacitance of 215.6 F g–1 was 100 s. At the same time, the nearly symmetric shapes of the galvanostatic charge–discharge profiles at increasing current densities also suggested high Coulomb efficiencies (92% and 98% at 1 A g–1 and 10 A g–1, respectively; Figure 3e). The stability of the fiber-shaped supercapacitor was further studied by long-life galvanostatic charge–discharge at 1 A g–1 (Figure 3f). About 80% of the specific capacitance was retained after 500 cycles and 69% after 10000 cycles. In addition, the shape of the Nyquist plot was also well kept after 3000 cycles with similar electron transfer resistances (Figure S4, Supporting Information),[27] so the structure of the composite electrode was stable during the long-life cycling process. The horizontal intercept increased, indicating an increasing resistance of electrolyte as the gel electrolyte became drier with a higher proton transfer resistance. The aligned CNT/PANI composite electrode was flexible and stretchable. No obvious damage of the structure was found for the electrode under bending (Figure S5, Supporting Information). In addition, the resistance fluctuated by less than 0.20% under bending with increasing bending angles and by less than 2.3% after bending for 1000 cycles with a bending angle of 180° (Figure S6, Supporting Information). The structure of the composite electrode was investigated under stretching. The helical angle of the CNT sheet decreased and the distance between aligned CNTs increased in the stretched state (Figure S7, Supporting Information); after the electrode was released to the original state, the structure recovered (Figure S8, Supporting Information). Accordingly, the diameter of the fiber substrate decreased from 525 to 499 µm with a strain of ca. 10% and then returned to 521 µm during the above reversible deformation. Importantly, the aligned organization of the CNTs was well maintained during the reversible deformation. Figure S9 (Supporting Information) compares the resistance of the composite electrode as the strain is increased to 100%. The resistance varied by less than 0.50%, and it increased by less than 0.80% after 100 stretching cycles at a strain of 100%. As a result, the fiber-shaped supercapacitor can also be flexible and stretchable with high electrochemical performance. The specific capacitance varied by 0.60% when the bending angle was increased to 180° (Figure 4a and Figure S10 in the Supporting Information), and the supercapacitor retained ca. 94% of its specific capacitance after being bent for 1000 cycles with a bending angle of 180° (Figure 4b and Figure S11 in the Supporting Information). Similarly, the specific capacitance decreased slightly, by less than 3.0%, after the supercapacitor was stretched to 100% (Figure 4c and Figure S12 in the Supporting Information), and it was also well maintained after the supercapacitor was stretched for

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100 cycles to a strain of 100% (Figure 4d and Figure S13 in the Supporting Information). The flexible and stretchable fiber-shaped supercapacitors were able to be deformed into various knots without degradation of the electrochemical performance (Figure 4e,f). In particular, they can be woven into energy fabrics to power various portable devices, such as the light-emitting diode (LED) in Figure 4g. Interestingly, the fiber-shaped supercapacitor exhibited significant chromatic transitions during the charge–discharge process. Figure 5a shows a typical as-prepared fiber-shaped supercapacitor that appears green. Because of the symmetrical structure of the supercapacitor (Figure S1, Supporting Information), mainly the positive electrode was studied as a demonstration during the charge–discharge process. The positive electrode switched to blue upon charging to 1 V (Figure 5b), indicating the end of the charge process. It became green after discharge to 0.5 V (Figure 5c) and then turned to light yellow after further discharge to –0.5 V and –1 V (Figure 5d,e). So did the negative electrode. A more detailed illustration of the chromatic process is provided in Figure S14 (Supporting Information). The changing colors can visually demonstrate the energy storage state during the self-discharge process, which lasts for 70 h (Figure S15, Supporting Information), based on the dependence of the color on the potential. Furthermore, the chromatic transitions were also quantitatively verified by UV–vis spectroscopy (Figure S16, Supporting Information). The characteristic absorbance peaks for blue at 1 V), green (at 0.5 V), and light yellow (at –1 V) occurred at 565.5, 684.5, and 850.0 nm, respectively. After a cycle of the charge–discharge process, these absorbance peaks were well repeated with a high reversibility. Moreover, these chromatic transitions were repeated for more than 3000 cycles without fatigue (Figure S17, Supporting Information). The color change is derived from the redox reaction of PANI during the charge–discharge process.[28–30] Generally, PANI shows three oxidation states (see Figure S18 in the Supporting Information), and the structures of PANI change at different oxidation states, that is, the fully reduced state of leucoemeraldine, with amine linkers; the fully oxidized state of pernigraniline, with imine linkers; and the partially oxidized emeraldine, with both amine and imine linkers. Here n corresponds to the number of repeats of the unit, and x and y stand for the percentages of reduced and oxidized formats, respectively. In the corresponding cyclic voltammogram (Figure 3a), an obvious anodic peak is observed at 0.38 V, corresponding to both oxidation of the positive electrode and reduction of the negative electrode, while the peak at –0.41 V demonstrates the reduction of the positive electrode and oxidation of the negative electrode. At the positive electrode, after the charging process, PANI was fully oxidized into pernigraniline, exhibiting a blue color at 1 V, indicating that the charging process was completed; the PANI was partially reduced to emeraldine (green color) after discharge to 0.5 V, showing that the positive electrode was 50% charged. Accordingly, 50% of the PANI in the positive electrode was reduced according to Faraday’s law. After further discharge to –1 V the PANI at the positive electrode was completely reduced to leucoemeraldine (light yellow), indicating the discharge process was completed. The opposite color changes and redox reactions occurred at the negative electrode.

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COMMUNICATION Figure 4. a) Dependence of specific capacitance on bending angle. b) Dependence of specific capacitance on bending cycle number with a bending angle of 180°. c) Dependence of specific capacitance on strain. d) Dependence of specific capacitance on stretching cycle number with a strain of 100%. C0 and C correspond to the specific capacitances before and after bending or stretching, respectively. e,f) Electrochromic fiber-shaped supercapacitors that have been made into different shapes. g) Two energy storage textiles that have been woven from electrochromic fiber-shaped supercapacitors in series to light up a LED.

As previously mentioned, these fiber-shaped supercapacitors can be easily woven into fabrics or integrated into clothes or other flexible structures. Figure 5f shows an energy fabric that was composed of five aligned CNT/PANI composite electrodes in both the parallel and perpendicular directions. Each fiber was wound round with a continuous CNT/PANI composite film and coated with the PVA/H3PO4 gel electrolyte. Depending on the requirements, one or several modified fibers can serve as a positive electrode, while the others function as the negative electrode. Therefore, different electrochromic patterns may be realized during the charge–discharge process. For example, when the parallel electrode served as a positive electrode while the perpendicular electrode functioned as the negative electrode,

Adv. Mater. 2014, DOI: 10.1002/adma.201403243

they appeared light yellow and blue, respectively, after the discharge had been completed (Figure S19a, Supporting Information). In contrast, the parallel and perpendicular composite electrodes appeared blue and light yellow when the charging process had been completed at 1 V (Figure S19b, Supporting Information). Based on this strategy, a wide variety of symbols or signs can be designed and realized. For instance, Figure 5g,h and Figure S19c (Supporting Information) exhibit the three signs “+”, “F”, and “H”, respectively. The electrochromic fibershaped supercapacitor can also be woven into a cotton cloth. As a demonstration, a human face was formed by the fibershaped supercapacitor such that it appeared to be crying at –1 V and smiling at 1 V (Figure S20, Supporting Information). In

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Figure 5. Chromatic transitions during the charge–discharge process. a) An electrochromic fiber-shaped supercapacitor at 0 V. b–e) The positive electrode at 1, 0.5, −0.5, and −1 V, respectively. f) An energy storage textile woven from electrochromic fiber-shaped supercapacitors during the charge– discharge process. g,h) Electrochromic fiber-shaped supercapacitors that have been designed and woven to display the signs “+” and “F”, respectively.

other words, the crying face signified the end of the discharge process while the smiling face corresponded to the completion of the charge process. As expected, owing to the high flexibility and stretchability of the fiber-shaped supercapacitor, the resulting fabrics can be deformed without sacrificing either energy storage capability or display performance. Therefore, these chromic fiber-shaped supercapacitors are promising for display applications in addition to powering various portable and wearable electronic devices. In summary, a new family of electrochromic, flexible, and stretchable fiber-shaped supercapacitors has been developed by winding aligned CNT/PANI composite film electrodes on an elastic fiber. These fiber-shaped supercapacitors exhibit rapid and reversible chromatic transitions between different working stages, therefore providing dynamic and efficiently communicated information on their working status. Although three typical colors, that is, light yellow, blue, and green, are mainly demonstrated here based on the used PANI, other colors may also be realized by choosing other polymers or inorganic materials such as metal oxides according to the same strategy. These electrochromic fiber-shaped supercapacitors also show high electrochemical performance under deformations such as bending and stretching, and can be widely used as promising devices for displays in addition to storing energy. This work may serve as a starting point to develop display products by exploring chromic or other smart electronic devices with high efficiencies.

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Experimental Section Continuous and aligned CNT sheets were first drawn from spinnable CNT arrays synthesized by chemical vapor deposition.[31] The resulting CNT sheet was then wound onto a rubber fiber that had been prestretched with a strain of ca. 110% during the winding process to improve the stability during stretching. To enhance the attachment of the CNT sheet to the rubber fiber, a post-treatment with alcohol was performed at room temperature. PANI was then deposited onto two separated CNT sheets through electropolymerization of aniline at 0.75 V vs KCl-saturated Ag/AgCl in an aqueous solution including 0.1 M aniline and 1 M H2SO4, and a platinum wire was used as the counter electrode.[32] The PVA/H3PO4 gel electrolyte (mass ratio of 1.18) was prepared by adding H3PO4 to a PVA aqueous solution according to a previous report.[33] The specific capacitances were calculated on the basis of the composite electrode.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by MOST (2011CB932503), NSFC (21225417), STCSM (12nm0503200), the Fok Ying Tong Education Foundation, the Program for Special Appointments of Professors at Shanghai Institutions of Higher Learning, the Innovative Student Program at Fudan University

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Received: July 18, 2014 Revised: September 16, 2014 Published online:

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(EZH2203302/002), and the Program for Outstanding Young Scholars from the Organization Department of the CPC Central Committee.

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Electrochromic fiber-shaped supercapacitors.

An electrochromic fiber-shaped super-capacitor is developed by winding aligned carbon nanotube/polyaniline composite sheets on an elastic fiber. The f...
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