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Enhanced rate performance of flexible and stretchable linear supercapacitors based on polyaniline@Au@carbon nanotube with ultrafast axial electron transport
Journal of Power Sources 340 (2017) 302e308
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Enhanced rate performance of flexible and stretchable linear supercapacitors based on polyaniline@Au@carbon nanotube with ultrafast axial electron transport Jiang Xu a, 1, Jianning Ding a, b, *, 1, Xiaoshuang Zhou a, Yang Zhang a, Wenjun Zhu a, Zunfen Liu e, f, Shanhai Ge d, **, Ningyi Yuan a, ***, Shaoli Fang c, e, Ray H. Baughman c, e a
Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Jiangsu Key Laboratory for Solar Cell Materials and Technology, Changzhou University, Changzhou, 213164, China Micro/Nano Science and Technology Center, Jiangsu University, Zhenjiang, 212013, China c Alan G. MacDiarmid NanoTech Institute, University of Texas at Dallas, Richardson, TX, 75080, USA d Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, PA, 16802, USA e Jiangnan Graphene Research Institute, Changzhou, 213149, China f State Key Laboratory of Medicinal Chemical Biology, Nankai University, Tianjin, 300071, China b
h i g h l i g h t s Synthesis of Au nanograin decorated aligned multiwall carbon nanotube (CNT) sheets. Au nanograins results in fast radial ion diffusion and enhance axial electron transport. The flexible linear solid supercapacitor exhibits an ultrahigh rate performance. A remarkable capacitance retention of about 95% over 1000 stretch/release cycles.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2016 Received in revised form 18 October 2016 Accepted 22 November 2016
Linear supercapacitors suffer a severe loss of capacity at high rates due to the trade-off of radial ion diffusion and axial electron transport. Optimizing axial conductivity of electrodes is a key to circumvent this trade-off. We report here the synthesis of Au nanograin decorated aligned multiwall carbon nanotube (CNT) sheets, followed by the incorporation of polyaniline (PANI). The embedded Au nanograins results in fast radial ion diffusion and enhance axial electron transport in the linear electrodes. The flexible linear solid supercapacitor fabricated by twisting two PANI@Au@CNT yarns exhibits an outstanding electrochemical performance with a total volumetric capacitance of ~6 F cm3 at scan rate up to 10 V s1. Diameter of the electrode has little effect on volumetric capacitance even at high scan rates because of its high electrical conductivity. Highly stretchable supercapacitors with high rate performance and excellent cycling and stretching stability have been also fabricated using buckled linear electrodes made by wrapping PANI@Au@CNT sheet on elastic rubber fibers. The stretchable linear supercapacitor possesses a stable total volumetric capacitance of up to ~0.2 F cm3 at scan rate of 1 V s1 and at 400% strain, and remarkable capacitance retention of about 95% over 1000 stretch/release cycles. © 2016 Elsevier B.V. All rights reserved.
Keywords: Supercapacitor Flexible Stretchable Au nanograin Aligned CNT
* Corresponding author. Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Jiangsu Key Laboratory for Solar Cell Materials and Technology, Changzhou University, Changzhou, 213164, China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (J. Ding), [email protected] (S. Ge), nyyuan@ cczu.edu.cn (N. Yuan). 1 These authors contribute equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2016.11.085 0378-7753/© 2016 Elsevier B.V. All rights reserved.
1. Introduction Flexible and stretchable linear (fiber, wire or yarn) supercapacitors and batteries are receiving more and more attention with the increasing demand for miniaturized electronic devices [1e4], such as micro-robots, wearable electronic textiles and implantable medical devices. Aligned carbon nanotube (CNT) is regarded as one of the most suitable candidates for constructing
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linear supercapacitors [5e11]. To improve the energy density, one common approach is to coat redox-active materials on carbonaceous materials to increase pseudo-capacitance [12,13], such as conducting polymers [6] and transition metal oxide [14e16]. However, while pseudo-capacitors provide high energy storage capability, energy delivery at high power is a big challenge unless the active material layer is very thin [17e19]. For linear supercapacitors, due to its large length to diameter ratio, even if the active material layer is very thin, e.g. coated on the well-aligned CNT, having much higher electrical conductivity (~3 104 S m1) [20] than widely used pseudo-capacitance materials [21e24], the rate performance is still limited [7,25]. To solve this problem, metal fibers (the conductivity of ~108 S m1) were introduced as current collectors in core(metal)/sheath(CNT) structured yarn supercapacitors [25] or biscrolled (metal and CNT) yarn supercapacitors [7]. In the core/sheath structured yarn, CNT forms a thin surface layer around the metal filament core, resulting in a significant improvement in electrochemical performance. For biscrolled CNT/ PEDOT yarn (~20 mm diameter)/Pt wire, the volumetric capacitances are 13 and 10 F cm3 in liquid and solid electrolytes at a scan rate of 10 V s1, respectively, which are the best values reported. However, metal wire cannot improve the electron transport in the body of CNT/PEDOT yarn, thus the capacitance decreased sharply with increase of yarn diameters at high scan rates [7]. In addition, due to metal fatigue and relatively heavy weight, these materials may hardly meet the demand of multiple bending, stretching and light-weight as an energy storage component. Compared with the flexible linear supercapacitors, the stretchable linear supercapacitors have much poorer rate performances and they are usually tested at low scan rate (0.01e0.1 V s1) [26e28]. Based on the above consideration, we proposed a novel electrode using Au nanograin decorated aligned CNT (denoted as Au@CNT) sheet followed by the deposition of polyaniline (PANI) (denoted as PANI@Au@CNT) to fabricate flexible and stretchable linear supercapacitors, which taking the form of a two-ply structure consisting of two identical single fibers. The single fiber was made by scrolling PANI@Au@CNT sheet or wrapping the sheet on an elastic core fiber. The design of the novel electrodes provide following aims: (i) Au nanograins embedded in the PANI/CNT layers increases the electrical conductivity of the electrode. (ii) Au nanograins increase the roughness of linear electrode. This will increase the utilization rate of PANI. (iii) Au nanograins do not affect mechanic strength of aligned CNT. In a word, high electrical conductivity, high flexibility and lightweight of the PANI@Au@CNT linear electrodes offer outstanding performance of supercapacitors in electrochemical, bending and stretching tests. 2. Experimental 2.1. Materials All chemicals were analytical grade and were used without further purification. Drawable CNT forests (height of ~350 mm, an outer diameter of ~9 nm and contained ~6 walls) were grown on a Si wafer by chemical vapor deposition, as previously described [29,30]. The freestanding CNT sheet was drawn from CNT forests, and then supported by a rectangular frame. The rectangular frame was made of two plastic rods and two metal (stainless steel) rods, as shown in Fig. S1. The sheet resistance was measured from two metal rods directly using a multimeter. The measured value for a single CNT layer ranges from ~700 to 1000 U/sq. in the draw direction, depending upon the areal density of the CNT sheet (which is a function of the forest height). Polyvinyl alcohol (PVA) powder and aniline were bought from Sigma-Aldrich Corporation.
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2.2. Preparation of PANI@Au@CNT sheet Au was coated on highly oriented CNT sheets using thermal evaporation system (MINI-SPECTROS, Kurt J. Lesker, USA). The loading amount of Au was controlled by deposition time. After evaporation, two ends of Au covered CNT (denoted as Au@CNT, Fig. S2) sheet were fixed on glass using thin tape. PANI was deposited on the surface of Au@CNT as active material by oxidation of aniline at a potential of 0.75 V in an aqueous solution of aniline (0.1 M) and H2SO4 (1 M). Potassium chloride-saturated Ag/AgCl and platinum wire were used as reference and counter electrodes, respectively. Then the as-prepared PANI@Au@CNT sheet was dried at room temperature and schematically shown in Fig. 1a. For comparison, PANI was also coated on CNT sheet (denoted as PANI@CNT). 2.3. Fabrication of flexible and stretchable linear supercapacitors PVA-H3PO4 gel electrolyte was prepared by mixing PVA (Mw 75,000e80,000) powder (5 g), H3PO4 (5 g) and deionized water (45 ml) together. The mixture was heated to 90 C under vigorous stirring until it becomes clear gel. Typical fabrication process of the flexible and stretchable supercapacitors is schematically shown in Fig. 1a and b. For flexible yarn supercapacitors, firstly, the above prepared PANI@Au@CNT sheets were scrolled into yarns (Fig. S3). Depending upon the thickness and width of the hybrid sheet, the diameters of the PANI@Au@CNT yarn are in the range of 20e40 mm. Two identically prepared PANI@Au@CNT yarns were individually coated with 1 M PVA-H3PO4 gel electrolyte. The yarn was immersed in the gel electrolyte (one end of yarn was kept out of gel) for 5 min, then fished out and hung up to dry. After 10 h of drying in air to vaporize the excess water, the two-electrodes were then plied together (the two ends of yarns without gel were placed in the opposite direction) and biscrolled to form a yarn supercapacitor, which could be directly woven into a textile structure without the need of further packaging (Fig. 1b). The thickness of the gel coating was measured by optical microscope and the value was about 5 mm. Stretchable linear supercapacitor was fabricated by twisting two core-sheath stretchable fibers (PANI@Au@CNT sheets as the active material sheaths wrapped on the pre-stretched rubber core fibers). The rubber core wires having uniform diameters were fabricated by dipping a wood stick into the melted styrene-(ethylene-butylene)styrene (SEBS) copolymer containing a plasticizer (ExxonMobil, Marcol 82) which was subsequently pulled out as the reported method [31]. The diameter of the original rubber wire was controlled at 280 mm, which reduced to 150 mm at 400% strain (called the fabrication strain). The elastic rubber wire was stretched to a predetermined strain, ranging from 0 to 400%. Then, the PANI@Au@CNT sheet was wrapped around the pre-stretched wire, with the direction of CNT parallel to the rubber fiber. Upon releasing the pre-stretched wire to its natural state, the PANI@Au@CNT-wrapped elastic wire showed a buckling structure of highly overlapped PANI@Au@CNT sheets, which facilitated the elasticity of the entire wire-shaped supercapacitor up to an extremely high strain (~400%) without detrimental effects to its electrochemical properties. 2.4. Material characterization and electrochemical measurement The morphology of the samples was probed by high-resolution field emission scanning electron microscopy (FE-SEM, Hitachi S4800). PANI was examined by Fourier transform infrared spectroscopy (FTIR, E55 þ FRA106, Bruker Optiks, Germany). The electrochemical measurements were carried out in both two- and three-electrode configurations at room temperature using CHI
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660E electrochemical workstation. For three-electrode configuration, PANI@CNT sheet or PANI@Au@CNT sheet was used as working electrode, while Ag/AgCl and Pt were used as reference and counter electrodes, respectively. All three-electrode measurements were performed in 1 M H2SO4 aqueous electrolyte and the areal capacitance (CA) was calculated according
Za CA ¼ b
IðUÞdU vAða bÞ
(1)
where I is the discharging current, v and A refer to the scan rate and area of the activated sheet, respectively. a and b are the ending and initial voltage (U) of discharge, respectively. For two-electrode configuration, the volumetric capacitance (CV) of the solid-state capacitance was calculated according to the equation
Za CV ¼ b
IðUÞdU vVða bÞ
(2)
where V refers to the volume of the full-cell, including two fibers and gel electrolyte.
3. Results and discussion 3.1. Morphologies The morphologies of CNT, Au@CNT, PANI@Au@CNT sheets are shown in Fig. 2. Fig. 2a and b are the high resolution SEM images of CNT and Au@CNT sheets, which clearly show that Au nanograins with diameters of a few nanometers randomly attached on the surface of each aligned nanotube (Fig. S4). Further, it is notable that the evaporated Au layer did not form a connected film, which means it did not change the aligned bundle morphology and the mechanical strength of CNT. The PANI uniformly covered on the nanotube was shown in Fig. 2c and d, which also keeps the porous structure in the PANI@Au@CNT sheet. The composition of PANI@Au@CNT was characterized by Fourier transform infrared spectra
(FTIR) and the result was shown in Fig. S5. The FTIR spectrum reveals four typical peaks at 878, 1026, 1122, and 1298 cm1 demonstrating the existence of PANI [32,33].
3.2. Electrochemical properties of the PANI@Au@CNT sheet The resistance of 0.5 cm-width Au@CNT sheet is 2.0 U cm1, while the resistance of CNT sheet with the same width is 1500 U cm1. After deposition of PANI, the produced PANI@Au@CNT sheet owns a high electrical conductivity of ~6 107 S m1. To explore the performance of PANI@Au@CNT for supercapacitors, cyclic voltammetry (CV) measurements were performed in 1 M H2SO4 aqueous electrolyte by the half-cell test (a three-electrode configuration) in a potential range of 0.2e0.6 V. The length of the electrode sheet immersed in the solution is 3 cm. CV curves in Fig. 3a indicate that at a scan rate of 100 mV s1, CNT sheet shows a rectangular shape due to its double-layer capacitance, while both PANI@CNT and PANI@Au@CNT electrodes demonstrate two strong peaks, which are attributed to the redox reaction of PANI. Considering the different deposition rates of PANI on CNT and Au@CNT, the deposition time for Au@CNT sheet (e.g. 10 s) was much shorter than that for CNT sheet (e.g. 120 s) to get an equal loading amount of PANI. It can be clearly seen that the CV curve for PANI@CNT begins to deviate at 0.1 V s1, however, the CV curve of PANI@Au@CNT is perfect. To further investigate the rate performance of PANI@Au@CNT sheet electrodes, CV measurements (Fig. 3b) were conducted at different scan rates of 1e100 V s1, the well-maintained curves at high scan rates (e.g. 100 V s1) suggest an excellent rate capability for this hybrid composite electrode. Fig. 3d shows the dependences of capacitance of PANI@Au@CNT sheet electrodes on scan rates. With the scan rate increasing from 1 to 100 V s1, the PANI@Au@CNT (PANI deposition time is10 s) sheet shows a very small capacitance decay of about 18%. Even at 100 V s1 scan rate, a CA of about 0.2 mF cm2 is achieved. When the PANI deposition duration increases to 20 or 30 s, the shapes of the CV curves obtained at 1.0 V s1 (Fig. 3c) were kept well and their CA increase to 0.58 and 1.72 mF cm2, respectively. However, the decay of the capacitance with the increase of scan rate is much fast. This may be caused by the increased thickness and connection of PANI (Fig. 2c and d).
Fig. 1. (a) PANI@Au@CNT sheet; Fabrication process of (b) flexible and (c) stretchable linear supercapacitors.
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Fig. 2. High-resolution SEM images of (a) CNT; (b) Au@CNT; (c) PANI@Au@CNT sheet (10s); (d) PANI@Au@CNT sheet (30s).
3.3. Electrochemical properties of flexible linear supercapacitors based on PANI@Au@CNT yarn electrodes The excellent property of PANI@Au@CNT was further explored by its application in solid-state linear supercapacitors. Unless
specifically mentioned, the deposition duration of PANI was set at 10 s. 0.5 cm-wide PANI@Au@CNT sheet was scrolled into a 20 mmdiameter yarn as shown in Fig. 4a. The inset in Fig. 4a shows that the yarn has pores. Electrolyte will penetrate into these pores and herein increase the utilization of PANI. The solid-state two-ply
Fig. 3. (a) CV curves of CNT, PANI@CNT, PANI@Au@CNT sheets obtained at scan rate of 0.1 V s1; (b) CV curves of PANI@Au@CNT sheet obtained at different scan rates; (c) CV curves of PANI@Au@CNT sheets with different deposition times of PANI; (d) Dependence of capacitance on scan rate for different PANI@Au@CNT sheets.
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Fig. 4. (a) SEM image of PANI@Au@CNT yarn; (b) Optical images; (c) CV curves and (d) Cycle life of flexible linear supercapacitor; (e) CV curves of flexible linear supercapacitor at different states; (f) Capacitance retention vs. yarn diameter. The inset in (a) is the amplified SEM image; the inset in (d) is galvanostatic charge-discharge curves of flexible linear supercapacitor.
wire-shape supercapacitor consists of two symmetric PVA-H3PO4 gel coated electrodes, as shown in Fig. 4b. The thickness of PVAH3PO4 gel electrolyte layer was ~5 mm. Fig. 4c presents the CV curves of the two-ply supercapacitor with a potential range from 0 to 0.75 V at different scan rates of 1e100 V s1. As can be seen, the supercapacitor shows nearly rectangular CV curve even when the scan rate increases to 20 V s1, indicating an excellent rate performance. The calculated volumetric capacitance (Cv) of the capacitor is about 6 F cm3 at 10 V s1, which is comparable with the best values reported in the literature [7]. The inset of Fig. 4d displays galvanostatic charge-discharge curves of the supercapacitor measured at different current densities. It shows that these curves are almost symmetrical isosceles lines, demonstrating PANI@Au@CNT yarn electrode has typical supercapacitive behavior even at current density up to 1000 mA cm1. Fig. S6 shows the dependence of capacitance of the entire cell based on the PANI@Au@CNT electrodes on current density. Although the capacitance
decreased gradually with increasing current density, a CV of ~6 F cm3 can be got at a current density of ~70 A cm3. The durability of the supercapacitor was investigated using a cyclic galvanostatic charge-discharge test. After 5000 consecutive chargedischarge cycles, the capacitance maintained at 94.5% of its original value, indicating that the supercapacitor based on PANI@Au@CNT exhibits an excellent cycle life (Fig. 4d). The superior cyclic stability is attributed to thin PANI layer which can accommodate the stress changes in the doping/dedoping process and thus less susceptible to fatigue caused by cyclic mechanical stress. The electrochemical properties of the supercapacitor under different bending states were evaluated using an in-situ test and the corresponding CV curves obtained at 2 V s1 are shown in Fig. 4e. It can be seen that the capacitance of the supercapacitor changed little even when the bending angle approached to 180 . In addition, the capacitance is almost fully maintained after bending for 300 cycles. These indicate that the linear supercapacitor based
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based capacitor decreases very little as the electrode diameter increase. Because of the wire-shaped structure, multiple supercapacitors can be easily connected in series-parallel to meet specific energy and power needs for practical applications. Fig. 5 and Movie S1 shows that three prepared linear supercapacitors connected in series (Voltage, 2.0 V) successfully powered a digital watch. After the supercapacitor stack was charged at lower current, it can power the digital watch for ~60 s. When the charge current increases to 5 mA, the charge process lasts only 0.01 s, but it can still power the digital watch for ~20 s. The fast charging test validates that the prepared linear supercapacitor owns an excellent rate performance visually. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2016.11.085 3.4. Electrochemical properties of stretchable linear supercapacitors based on buckled PANI@Au@CNT@rubber fibers Fig. 5. Running time of digital watch vs. charge current of flexible linear supercapacitors. The background is the optical images of operating circuit.
on PANI@Au@CNT sheet exhibits a good flexibility. To increase the specific capacitance, the width (w) or thickness of PANI@Au@CNT sheets can be enlarged to increase the diameter (d) of fiber produced. It can be calculated that fiber diameter is proportional to the square root of the sheet width using the same twist mode, d1/d2zsquar(w1/w2). For example, three pieces of 0.5 cm-wide PANI@Au@CNT sheet are scrolled as fiber electrode, whose diameter increases to ~40 mm (Fig. S7). The dependence of capacitance on the electrode diameter was depicted in Fig. 4f. It is noteworthy that different from the reported biscrolled (Pt and CNT) yarn supercapacitors [7], the capacitance of the PANI@Au@CNT
The released core (rubber)-sheath (PANI@Au@CNT) fiber is shown in Fig. 6a. The diameter of the released fiber is ~280 mm, around which a layer of 1 mm thick aligned PANI@Au@CNT sheet was wrapped (Fig. S8). The observed highly overlapped and buckled PANI@Au@CNT coating imparted a high elasticity. The buckling periodicity for the PANI@Au@CNT coating was ~10 mm. The dependence of the resistance on strain is shown in Fig. 6c, the resistance of the elastic fiber electrode increases 15% when the fiber is stretched to 400% strain. This change is explained by the overlap of Au nanograins coated on the aligned CNTs after the strain release. Low resistance and high stability of these highly stretchable conductive fibers prompted us to fabricate highly stretchable linear supercapacitors by twisting two core-sheath fibers with PVAH3PO4 gel electrolyte (Fig. S9). Before measurements, a few cycles
Fig. 6. (a) SEM image of stretchable electrode at release state; (b) CV curves measured at different states; Resistance change and capacitance retention vs. (c) strain and (d) cycle number. Linear supercapacitor or electrode stretches to 400% and releases to 0% is one cycle.
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of stretch and release are performed to insure reversibility of structure and properties of the solid-state supercapacitor. Fig. 6b shows CV curves for the supercapacitor at different tensile strains. The electrochemical performance of the stretchable supercapacitor was similar to those of flexible supercapacitor. Volumetric capacitance of the entire cell was calculated to be about 0.2 F cm3 at a scan rate of 1 V s1 (including rubber, electrodes and gel electrolyte). Fig. 6c shows the normalized capacitance (i.e., a ratio of the capacitance at a specific strain to that at 0 strain) as a function of the tensile strain. As can be seen in Fig. 6c, the capacitance decreased by 4% as the tensile strain increased from 0 to 400%. The long-term elasticity of these stretchable supercapacitors has also been investigated by repeatedly stretching the device up to 400% strain (Fig. 6d). During the first few strain cycles, the capacitance decreased by about 5%. Thereafter, the capacitance remained stable even after 1000 cycles. Clearly, this is benefit from the stable resistance of the core-sheath Au@CNT@rubber fiber (Fig. 6d). So, the twisted wire-shaped supercapacitors are extremely elastic with a good tolerance to repeated stretching cycles even at high tensile strains, and outperformed not only the state-of-the-art fiber-like stretchable supercapacitor but also many film-type stretchable supercapacitors reported to date [26e28,31,34]. 4. Conclusions Flexible and stretchable linear supercapacitors having superhigh rate performance was fabricated. PANI@Au@CNT composite was used as electrodes, whose conductivity improves 3 orders of magnitude relative to the PANI@CNT. The supercapacitor shows nearly rectangular CV curve, and its CV is about 6 F cm3 at a scan rate of 10 V s1. The diameter of electrode yarn has very little effect on CV due to high electrical conductivity of electrodes. The charge/ discharge cycle and bending stability tests show that the produced supercapacitor is very durable. We also developed extremely stretchable wire-shaped supercapacitors, which exhibits a high CV of about 0.2 F cm3 at a scan rate of 1 V s1 and at a stain of 400%. The stretchable supercapacitors also show good long-term performance stability. The stretchable wire-shaped supercapacitors developed in this study outperformed all the state-of-the-art stretchable electronics reported to date in terms of the elasticity and physical and electrochemical performance. Acknowledgments This work was supported by the National Natural Science Foundation of China, China (51272033, 51572037, and 51335002), the Priority Academic Program Development of Jiangsu Higher Education Institutions. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.11.085.
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Enhanced rate performance of flexible and stretchable linear supercapacitors based on polyaniline@Au@carbon nanotube with ultrafast axial electron transport Jiang Xu a, 1, Jianning Ding a, b, *, 1, Xiaoshuang Zhou a, Yang Zhang a, Wenjun Zhu a, Zunfen Liu e, f, Shanhai Ge d, **, Ningyi Yuan a, ***, Shaoli Fang c, e, Ray H. Baughman c, e a
Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Jiangsu Key Laboratory for Solar Cell Materials and Technology, Changzhou University, Changzhou, 213164, China Micro/Nano Science and Technology Center, Jiangsu University, Zhenjiang, 212013, China c Alan G. MacDiarmid NanoTech Institute, University of Texas at Dallas, Richardson, TX, 75080, USA d Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, PA, 16802, USA e Jiangnan Graphene Research Institute, Changzhou, 213149, China f State Key Laboratory of Medicinal Chemical Biology, Nankai University, Tianjin, 300071, China b
h i g h l i g h t s Synthesis of Au nanograin decorated aligned multiwall carbon nanotube (CNT) sheets. Au nanograins results in fast radial ion diffusion and enhance axial electron transport. The flexible linear solid supercapacitor exhibits an ultrahigh rate performance. A remarkable capacitance retention of about 95% over 1000 stretch/release cycles.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2016 Received in revised form 18 October 2016 Accepted 22 November 2016
Linear supercapacitors suffer a severe loss of capacity at high rates due to the trade-off of radial ion diffusion and axial electron transport. Optimizing axial conductivity of electrodes is a key to circumvent this trade-off. We report here the synthesis of Au nanograin decorated aligned multiwall carbon nanotube (CNT) sheets, followed by the incorporation of polyaniline (PANI). The embedded Au nanograins results in fast radial ion diffusion and enhance axial electron transport in the linear electrodes. The flexible linear solid supercapacitor fabricated by twisting two PANI@Au@CNT yarns exhibits an outstanding electrochemical performance with a total volumetric capacitance of ~6 F cm3 at scan rate up to 10 V s1. Diameter of the electrode has little effect on volumetric capacitance even at high scan rates because of its high electrical conductivity. Highly stretchable supercapacitors with high rate performance and excellent cycling and stretching stability have been also fabricated using buckled linear electrodes made by wrapping PANI@Au@CNT sheet on elastic rubber fibers. The stretchable linear supercapacitor possesses a stable total volumetric capacitance of up to ~0.2 F cm3 at scan rate of 1 V s1 and at 400% strain, and remarkable capacitance retention of about 95% over 1000 stretch/release cycles. © 2016 Elsevier B.V. All rights reserved.
Keywords: Supercapacitor Flexible Stretchable Au nanograin Aligned CNT
* Corresponding author. Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Jiangsu Key Laboratory for Solar Cell Materials and Technology, Changzhou University, Changzhou, 213164, China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (J. Ding), [email protected] (S. Ge), nyyuan@ cczu.edu.cn (N. Yuan). 1 These authors contribute equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2016.11.085 0378-7753/© 2016 Elsevier B.V. All rights reserved.
1. Introduction Flexible and stretchable linear (fiber, wire or yarn) supercapacitors and batteries are receiving more and more attention with the increasing demand for miniaturized electronic devices [1e4], such as micro-robots, wearable electronic textiles and implantable medical devices. Aligned carbon nanotube (CNT) is regarded as one of the most suitable candidates for constructing
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linear supercapacitors [5e11]. To improve the energy density, one common approach is to coat redox-active materials on carbonaceous materials to increase pseudo-capacitance [12,13], such as conducting polymers [6] and transition metal oxide [14e16]. However, while pseudo-capacitors provide high energy storage capability, energy delivery at high power is a big challenge unless the active material layer is very thin [17e19]. For linear supercapacitors, due to its large length to diameter ratio, even if the active material layer is very thin, e.g. coated on the well-aligned CNT, having much higher electrical conductivity (~3 104 S m1) [20] than widely used pseudo-capacitance materials [21e24], the rate performance is still limited [7,25]. To solve this problem, metal fibers (the conductivity of ~108 S m1) were introduced as current collectors in core(metal)/sheath(CNT) structured yarn supercapacitors [25] or biscrolled (metal and CNT) yarn supercapacitors [7]. In the core/sheath structured yarn, CNT forms a thin surface layer around the metal filament core, resulting in a significant improvement in electrochemical performance. For biscrolled CNT/ PEDOT yarn (~20 mm diameter)/Pt wire, the volumetric capacitances are 13 and 10 F cm3 in liquid and solid electrolytes at a scan rate of 10 V s1, respectively, which are the best values reported. However, metal wire cannot improve the electron transport in the body of CNT/PEDOT yarn, thus the capacitance decreased sharply with increase of yarn diameters at high scan rates [7]. In addition, due to metal fatigue and relatively heavy weight, these materials may hardly meet the demand of multiple bending, stretching and light-weight as an energy storage component. Compared with the flexible linear supercapacitors, the stretchable linear supercapacitors have much poorer rate performances and they are usually tested at low scan rate (0.01e0.1 V s1) [26e28]. Based on the above consideration, we proposed a novel electrode using Au nanograin decorated aligned CNT (denoted as Au@CNT) sheet followed by the deposition of polyaniline (PANI) (denoted as PANI@Au@CNT) to fabricate flexible and stretchable linear supercapacitors, which taking the form of a two-ply structure consisting of two identical single fibers. The single fiber was made by scrolling PANI@Au@CNT sheet or wrapping the sheet on an elastic core fiber. The design of the novel electrodes provide following aims: (i) Au nanograins embedded in the PANI/CNT layers increases the electrical conductivity of the electrode. (ii) Au nanograins increase the roughness of linear electrode. This will increase the utilization rate of PANI. (iii) Au nanograins do not affect mechanic strength of aligned CNT. In a word, high electrical conductivity, high flexibility and lightweight of the PANI@Au@CNT linear electrodes offer outstanding performance of supercapacitors in electrochemical, bending and stretching tests. 2. Experimental 2.1. Materials All chemicals were analytical grade and were used without further purification. Drawable CNT forests (height of ~350 mm, an outer diameter of ~9 nm and contained ~6 walls) were grown on a Si wafer by chemical vapor deposition, as previously described [29,30]. The freestanding CNT sheet was drawn from CNT forests, and then supported by a rectangular frame. The rectangular frame was made of two plastic rods and two metal (stainless steel) rods, as shown in Fig. S1. The sheet resistance was measured from two metal rods directly using a multimeter. The measured value for a single CNT layer ranges from ~700 to 1000 U/sq. in the draw direction, depending upon the areal density of the CNT sheet (which is a function of the forest height). Polyvinyl alcohol (PVA) powder and aniline were bought from Sigma-Aldrich Corporation.
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2.2. Preparation of PANI@Au@CNT sheet Au was coated on highly oriented CNT sheets using thermal evaporation system (MINI-SPECTROS, Kurt J. Lesker, USA). The loading amount of Au was controlled by deposition time. After evaporation, two ends of Au covered CNT (denoted as Au@CNT, Fig. S2) sheet were fixed on glass using thin tape. PANI was deposited on the surface of Au@CNT as active material by oxidation of aniline at a potential of 0.75 V in an aqueous solution of aniline (0.1 M) and H2SO4 (1 M). Potassium chloride-saturated Ag/AgCl and platinum wire were used as reference and counter electrodes, respectively. Then the as-prepared PANI@Au@CNT sheet was dried at room temperature and schematically shown in Fig. 1a. For comparison, PANI was also coated on CNT sheet (denoted as PANI@CNT). 2.3. Fabrication of flexible and stretchable linear supercapacitors PVA-H3PO4 gel electrolyte was prepared by mixing PVA (Mw 75,000e80,000) powder (5 g), H3PO4 (5 g) and deionized water (45 ml) together. The mixture was heated to 90 C under vigorous stirring until it becomes clear gel. Typical fabrication process of the flexible and stretchable supercapacitors is schematically shown in Fig. 1a and b. For flexible yarn supercapacitors, firstly, the above prepared PANI@Au@CNT sheets were scrolled into yarns (Fig. S3). Depending upon the thickness and width of the hybrid sheet, the diameters of the PANI@Au@CNT yarn are in the range of 20e40 mm. Two identically prepared PANI@Au@CNT yarns were individually coated with 1 M PVA-H3PO4 gel electrolyte. The yarn was immersed in the gel electrolyte (one end of yarn was kept out of gel) for 5 min, then fished out and hung up to dry. After 10 h of drying in air to vaporize the excess water, the two-electrodes were then plied together (the two ends of yarns without gel were placed in the opposite direction) and biscrolled to form a yarn supercapacitor, which could be directly woven into a textile structure without the need of further packaging (Fig. 1b). The thickness of the gel coating was measured by optical microscope and the value was about 5 mm. Stretchable linear supercapacitor was fabricated by twisting two core-sheath stretchable fibers (PANI@Au@CNT sheets as the active material sheaths wrapped on the pre-stretched rubber core fibers). The rubber core wires having uniform diameters were fabricated by dipping a wood stick into the melted styrene-(ethylene-butylene)styrene (SEBS) copolymer containing a plasticizer (ExxonMobil, Marcol 82) which was subsequently pulled out as the reported method [31]. The diameter of the original rubber wire was controlled at 280 mm, which reduced to 150 mm at 400% strain (called the fabrication strain). The elastic rubber wire was stretched to a predetermined strain, ranging from 0 to 400%. Then, the PANI@Au@CNT sheet was wrapped around the pre-stretched wire, with the direction of CNT parallel to the rubber fiber. Upon releasing the pre-stretched wire to its natural state, the PANI@Au@CNT-wrapped elastic wire showed a buckling structure of highly overlapped PANI@Au@CNT sheets, which facilitated the elasticity of the entire wire-shaped supercapacitor up to an extremely high strain (~400%) without detrimental effects to its electrochemical properties. 2.4. Material characterization and electrochemical measurement The morphology of the samples was probed by high-resolution field emission scanning electron microscopy (FE-SEM, Hitachi S4800). PANI was examined by Fourier transform infrared spectroscopy (FTIR, E55 þ FRA106, Bruker Optiks, Germany). The electrochemical measurements were carried out in both two- and three-electrode configurations at room temperature using CHI
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660E electrochemical workstation. For three-electrode configuration, PANI@CNT sheet or PANI@Au@CNT sheet was used as working electrode, while Ag/AgCl and Pt were used as reference and counter electrodes, respectively. All three-electrode measurements were performed in 1 M H2SO4 aqueous electrolyte and the areal capacitance (CA) was calculated according
Za CA ¼ b
IðUÞdU vAða bÞ
(1)
where I is the discharging current, v and A refer to the scan rate and area of the activated sheet, respectively. a and b are the ending and initial voltage (U) of discharge, respectively. For two-electrode configuration, the volumetric capacitance (CV) of the solid-state capacitance was calculated according to the equation
Za CV ¼ b
IðUÞdU vVða bÞ
(2)
where V refers to the volume of the full-cell, including two fibers and gel electrolyte.
3. Results and discussion 3.1. Morphologies The morphologies of CNT, Au@CNT, PANI@Au@CNT sheets are shown in Fig. 2. Fig. 2a and b are the high resolution SEM images of CNT and Au@CNT sheets, which clearly show that Au nanograins with diameters of a few nanometers randomly attached on the surface of each aligned nanotube (Fig. S4). Further, it is notable that the evaporated Au layer did not form a connected film, which means it did not change the aligned bundle morphology and the mechanical strength of CNT. The PANI uniformly covered on the nanotube was shown in Fig. 2c and d, which also keeps the porous structure in the PANI@Au@CNT sheet. The composition of PANI@Au@CNT was characterized by Fourier transform infrared spectra
(FTIR) and the result was shown in Fig. S5. The FTIR spectrum reveals four typical peaks at 878, 1026, 1122, and 1298 cm1 demonstrating the existence of PANI [32,33].
3.2. Electrochemical properties of the PANI@Au@CNT sheet The resistance of 0.5 cm-width Au@CNT sheet is 2.0 U cm1, while the resistance of CNT sheet with the same width is 1500 U cm1. After deposition of PANI, the produced PANI@Au@CNT sheet owns a high electrical conductivity of ~6 107 S m1. To explore the performance of PANI@Au@CNT for supercapacitors, cyclic voltammetry (CV) measurements were performed in 1 M H2SO4 aqueous electrolyte by the half-cell test (a three-electrode configuration) in a potential range of 0.2e0.6 V. The length of the electrode sheet immersed in the solution is 3 cm. CV curves in Fig. 3a indicate that at a scan rate of 100 mV s1, CNT sheet shows a rectangular shape due to its double-layer capacitance, while both PANI@CNT and PANI@Au@CNT electrodes demonstrate two strong peaks, which are attributed to the redox reaction of PANI. Considering the different deposition rates of PANI on CNT and Au@CNT, the deposition time for Au@CNT sheet (e.g. 10 s) was much shorter than that for CNT sheet (e.g. 120 s) to get an equal loading amount of PANI. It can be clearly seen that the CV curve for PANI@CNT begins to deviate at 0.1 V s1, however, the CV curve of PANI@Au@CNT is perfect. To further investigate the rate performance of PANI@Au@CNT sheet electrodes, CV measurements (Fig. 3b) were conducted at different scan rates of 1e100 V s1, the well-maintained curves at high scan rates (e.g. 100 V s1) suggest an excellent rate capability for this hybrid composite electrode. Fig. 3d shows the dependences of capacitance of PANI@Au@CNT sheet electrodes on scan rates. With the scan rate increasing from 1 to 100 V s1, the PANI@Au@CNT (PANI deposition time is10 s) sheet shows a very small capacitance decay of about 18%. Even at 100 V s1 scan rate, a CA of about 0.2 mF cm2 is achieved. When the PANI deposition duration increases to 20 or 30 s, the shapes of the CV curves obtained at 1.0 V s1 (Fig. 3c) were kept well and their CA increase to 0.58 and 1.72 mF cm2, respectively. However, the decay of the capacitance with the increase of scan rate is much fast. This may be caused by the increased thickness and connection of PANI (Fig. 2c and d).
Fig. 1. (a) PANI@Au@CNT sheet; Fabrication process of (b) flexible and (c) stretchable linear supercapacitors.
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Fig. 2. High-resolution SEM images of (a) CNT; (b) Au@CNT; (c) PANI@Au@CNT sheet (10s); (d) PANI@Au@CNT sheet (30s).
3.3. Electrochemical properties of flexible linear supercapacitors based on PANI@Au@CNT yarn electrodes The excellent property of PANI@Au@CNT was further explored by its application in solid-state linear supercapacitors. Unless
specifically mentioned, the deposition duration of PANI was set at 10 s. 0.5 cm-wide PANI@Au@CNT sheet was scrolled into a 20 mmdiameter yarn as shown in Fig. 4a. The inset in Fig. 4a shows that the yarn has pores. Electrolyte will penetrate into these pores and herein increase the utilization of PANI. The solid-state two-ply
Fig. 3. (a) CV curves of CNT, PANI@CNT, PANI@Au@CNT sheets obtained at scan rate of 0.1 V s1; (b) CV curves of PANI@Au@CNT sheet obtained at different scan rates; (c) CV curves of PANI@Au@CNT sheets with different deposition times of PANI; (d) Dependence of capacitance on scan rate for different PANI@Au@CNT sheets.
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Fig. 4. (a) SEM image of PANI@Au@CNT yarn; (b) Optical images; (c) CV curves and (d) Cycle life of flexible linear supercapacitor; (e) CV curves of flexible linear supercapacitor at different states; (f) Capacitance retention vs. yarn diameter. The inset in (a) is the amplified SEM image; the inset in (d) is galvanostatic charge-discharge curves of flexible linear supercapacitor.
wire-shape supercapacitor consists of two symmetric PVA-H3PO4 gel coated electrodes, as shown in Fig. 4b. The thickness of PVAH3PO4 gel electrolyte layer was ~5 mm. Fig. 4c presents the CV curves of the two-ply supercapacitor with a potential range from 0 to 0.75 V at different scan rates of 1e100 V s1. As can be seen, the supercapacitor shows nearly rectangular CV curve even when the scan rate increases to 20 V s1, indicating an excellent rate performance. The calculated volumetric capacitance (Cv) of the capacitor is about 6 F cm3 at 10 V s1, which is comparable with the best values reported in the literature [7]. The inset of Fig. 4d displays galvanostatic charge-discharge curves of the supercapacitor measured at different current densities. It shows that these curves are almost symmetrical isosceles lines, demonstrating PANI@Au@CNT yarn electrode has typical supercapacitive behavior even at current density up to 1000 mA cm1. Fig. S6 shows the dependence of capacitance of the entire cell based on the PANI@Au@CNT electrodes on current density. Although the capacitance
decreased gradually with increasing current density, a CV of ~6 F cm3 can be got at a current density of ~70 A cm3. The durability of the supercapacitor was investigated using a cyclic galvanostatic charge-discharge test. After 5000 consecutive chargedischarge cycles, the capacitance maintained at 94.5% of its original value, indicating that the supercapacitor based on PANI@Au@CNT exhibits an excellent cycle life (Fig. 4d). The superior cyclic stability is attributed to thin PANI layer which can accommodate the stress changes in the doping/dedoping process and thus less susceptible to fatigue caused by cyclic mechanical stress. The electrochemical properties of the supercapacitor under different bending states were evaluated using an in-situ test and the corresponding CV curves obtained at 2 V s1 are shown in Fig. 4e. It can be seen that the capacitance of the supercapacitor changed little even when the bending angle approached to 180 . In addition, the capacitance is almost fully maintained after bending for 300 cycles. These indicate that the linear supercapacitor based
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based capacitor decreases very little as the electrode diameter increase. Because of the wire-shaped structure, multiple supercapacitors can be easily connected in series-parallel to meet specific energy and power needs for practical applications. Fig. 5 and Movie S1 shows that three prepared linear supercapacitors connected in series (Voltage, 2.0 V) successfully powered a digital watch. After the supercapacitor stack was charged at lower current, it can power the digital watch for ~60 s. When the charge current increases to 5 mA, the charge process lasts only 0.01 s, but it can still power the digital watch for ~20 s. The fast charging test validates that the prepared linear supercapacitor owns an excellent rate performance visually. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2016.11.085 3.4. Electrochemical properties of stretchable linear supercapacitors based on buckled PANI@Au@CNT@rubber fibers Fig. 5. Running time of digital watch vs. charge current of flexible linear supercapacitors. The background is the optical images of operating circuit.
on PANI@Au@CNT sheet exhibits a good flexibility. To increase the specific capacitance, the width (w) or thickness of PANI@Au@CNT sheets can be enlarged to increase the diameter (d) of fiber produced. It can be calculated that fiber diameter is proportional to the square root of the sheet width using the same twist mode, d1/d2zsquar(w1/w2). For example, three pieces of 0.5 cm-wide PANI@Au@CNT sheet are scrolled as fiber electrode, whose diameter increases to ~40 mm (Fig. S7). The dependence of capacitance on the electrode diameter was depicted in Fig. 4f. It is noteworthy that different from the reported biscrolled (Pt and CNT) yarn supercapacitors [7], the capacitance of the PANI@Au@CNT
The released core (rubber)-sheath (PANI@Au@CNT) fiber is shown in Fig. 6a. The diameter of the released fiber is ~280 mm, around which a layer of 1 mm thick aligned PANI@Au@CNT sheet was wrapped (Fig. S8). The observed highly overlapped and buckled PANI@Au@CNT coating imparted a high elasticity. The buckling periodicity for the PANI@Au@CNT coating was ~10 mm. The dependence of the resistance on strain is shown in Fig. 6c, the resistance of the elastic fiber electrode increases 15% when the fiber is stretched to 400% strain. This change is explained by the overlap of Au nanograins coated on the aligned CNTs after the strain release. Low resistance and high stability of these highly stretchable conductive fibers prompted us to fabricate highly stretchable linear supercapacitors by twisting two core-sheath fibers with PVAH3PO4 gel electrolyte (Fig. S9). Before measurements, a few cycles
Fig. 6. (a) SEM image of stretchable electrode at release state; (b) CV curves measured at different states; Resistance change and capacitance retention vs. (c) strain and (d) cycle number. Linear supercapacitor or electrode stretches to 400% and releases to 0% is one cycle.
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of stretch and release are performed to insure reversibility of structure and properties of the solid-state supercapacitor. Fig. 6b shows CV curves for the supercapacitor at different tensile strains. The electrochemical performance of the stretchable supercapacitor was similar to those of flexible supercapacitor. Volumetric capacitance of the entire cell was calculated to be about 0.2 F cm3 at a scan rate of 1 V s1 (including rubber, electrodes and gel electrolyte). Fig. 6c shows the normalized capacitance (i.e., a ratio of the capacitance at a specific strain to that at 0 strain) as a function of the tensile strain. As can be seen in Fig. 6c, the capacitance decreased by 4% as the tensile strain increased from 0 to 400%. The long-term elasticity of these stretchable supercapacitors has also been investigated by repeatedly stretching the device up to 400% strain (Fig. 6d). During the first few strain cycles, the capacitance decreased by about 5%. Thereafter, the capacitance remained stable even after 1000 cycles. Clearly, this is benefit from the stable resistance of the core-sheath Au@CNT@rubber fiber (Fig. 6d). So, the twisted wire-shaped supercapacitors are extremely elastic with a good tolerance to repeated stretching cycles even at high tensile strains, and outperformed not only the state-of-the-art fiber-like stretchable supercapacitor but also many film-type stretchable supercapacitors reported to date [26e28,31,34]. 4. Conclusions Flexible and stretchable linear supercapacitors having superhigh rate performance was fabricated. PANI@Au@CNT composite was used as electrodes, whose conductivity improves 3 orders of magnitude relative to the PANI@CNT. The supercapacitor shows nearly rectangular CV curve, and its CV is about 6 F cm3 at a scan rate of 10 V s1. The diameter of electrode yarn has very little effect on CV due to high electrical conductivity of electrodes. The charge/ discharge cycle and bending stability tests show that the produced supercapacitor is very durable. We also developed extremely stretchable wire-shaped supercapacitors, which exhibits a high CV of about 0.2 F cm3 at a scan rate of 1 V s1 and at a stain of 400%. The stretchable supercapacitors also show good long-term performance stability. The stretchable wire-shaped supercapacitors developed in this study outperformed all the state-of-the-art stretchable electronics reported to date in terms of the elasticity and physical and electrochemical performance. Acknowledgments This work was supported by the National Natural Science Foundation of China, China (51272033, 51572037, and 51335002), the Priority Academic Program Development of Jiangsu Higher Education Institutions. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.11.085.
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