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Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability
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RAPID COMMUNICATION
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Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability
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Yan Huanga, Jiayou Taob, Wenjun Menga, Minshen Zhua, Yang Huanga, Yuqiao Fua, Yihua Gaob, Chunyi Zhia,c,n a
Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong, China b Wuhan National Laboratory for Optoelectronics (WNLO), and School of Physics, Huazhong University of Science and Technology (HUST), 1037 Luoyu Road, Wuhan 430074, China c Shenzhen Research Institute, City University of Hong Kong, Shenzhen 518000, China
31 Received 2 August 2014; received in revised form 14 October 2014; accepted 15 October 2014
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KEYWORDS
Abstract
High rate; Stretchable supercapacitors; Polypyrrole; Cycling stability; Electrodeposition
The performance and cycling stability of stretchable energy storage devices, such as supercapacitors and batteries, are limited by the structural breakdown arising from the stretch imposed and large volumetric swelling/shrinking. This work demonstrates a very facile and low-cost approach to fabricate stretchable supercapacitors with high performance and excellent cycling stability by electrochemical deposition of polypyrrole (PPy) on smartly-tailored stretchable stainless steel meshes. The fabricated solid-state supercapacitors possess a capacitance up to 170 F/g at a specific current of 0.5 A/g and it can be effectively enhanced to 214 F/g with a 20% strain. Moreover, they can be operated at a very high scan rate up to 10 V/s, which are 1–2 orders of magnitude higher than most rates for the PPy electrodes measured even in aqueous electrolytes. Even significantly, the fabricated solid-state supercapacitors under 0% and 20% strains achieve remarkable capacitance retentions of 98% and 87% at a very high specific current of 10 A/g after 10,000 cycles, respectively, which are the best for PPy-based solid-state flexible supercapacitors, to the best of our knowledge. The key factors and mechanisms to achieve such high performance are discussed. This facile and lowcost approach developed for fabricating stable and stretchable supercapacitors with high performances could pave the way for next-generation stretchable electronics. & 2014 Elsevier Ltd. All rights reserved.
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Corresponding author at: Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong, China. Tel.: +852 3442 7891; fax: +852 3442 0538. E-mail address: [email protected] (C. Zhi).
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http://dx.doi.org/10.1016/j.nanoen.2014.10.031 2211-2855/& 2014 Elsevier Ltd. All rights reserved.
Please cite this article as: Y. Huang, et al., Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.10.031
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Introduction
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Cost-effective and performance-attractive devices with longterm stability are essential for stretchable energy storage and conversion technologies, which receive considerable attention recently [1,2], for allowing many unprecedented applications possible such as future smart biosensors and clothes [3–6]. The supercapacitor in particular is one important category of energy storage devices for its high power density, fast rate of charge–discharge, long cycling life, etc. [7–17]. These features make it superior to protect against unstable energy supply situations [11,18–23]. However, several problems need to be solved when introducing the stretchability into conventionally rigid supercapacitors. First, it is much challenging to fabricate stretchable devices. Second, the performance and cycling stability of stretchable supercapacitors are limited by the non-effective solid state electrolyte and the structural breakdown resulting from the stretch applied and large volumetric swelling/shrinking during the charge/discharge process. Substantial effort has been devoted to the fabrication of stretchable devices. For example, non-coplanar buckled structures [24–26], coplanar serpentine and wavy structures [27–30], percolating nanostructured films [31], elastomers and stretchable textile substrates [32–36] have been utilized to achieve the stretchability of devices. However, on one hand, multi-step complicated fabrications are involved in these designs, which make them cost-expensive and time-consuming; on the other hand, the high performances are very difficult to be achieved with the solid state devices even the electrodes exhibit very good electrochemical activities when they are tested in the liquid electrolytes. Regarding the structural breakdown of electrode substances under strains, it has been found that conducting polymers such as PPy can maintain or even enhance performance under stretch due to their intrinsic stretchability [36–38]. But most reported electrochemical performances of PPy-based supercapacitors such as the shape of cyclic voltammograms (CV) and galvanostatic charge–discharge curves (CD) as well as the capacitance are somewhat frustrating. As part of efforts to improve the poor cycling stability of conducting polymers [39– 46], Liu et al. [47] substantially increase the cycle number from a commonly-demonstrated 1000 to a much higher value of 10,000 and achieve a remarkable capacitance retention of 85% by deposition of a thin carbonaceous shell onto the surface of PPy. However, no cycling test under stretched states was performed in the reference. Yue et al. [36] reported that under a strain of 20%, PPy coated on elastic Nylon fabrics lose 50% of capacitance after merely 500 cycles. Thus, it is crucial to develop a facile and low-cost method that can fabricate high-performance stretchable PPy supercapacitors with an excellent long-term cycling stability for their practical applications. Here we demonstrate a facile and low-cost strategy to effectively improve the performance and cycling stability of stretchable PPy-based supercapacitors through electrochemical polymerization of purified pyrrole monomers on smartly tailored stretchable stainless steel meshes. By cutting along a certain direction, a knitted stainless steel mesh was found to become very stretchable. Then purified pyrrole monomers were electrochemically polymerized onto the mesh. The fabricated supercapacitors increase the capacitance from the initial 170 F/g at a relaxed state to
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214 F/g at a 20% strain at a specific current of 0.5 A/g. Surprisingly, the solid-state supercapacitors can be operated at super-high rates up to 10 V/s, being 1–2 orders of magnitude higher than most scan rates for PPy electrodes measured even in aqueous electrolytes. More importantly, they achieve a capacitance retention of 98% under 0 strain, and 87% under a strain of 20% applied after 10,000 cycles at a very high specific current of 10 A/g. To our best knowledge, these are the best capacitance retention values reported so far for PPy-based solid-state supercapacitors.
Material and methods PPy electrodeposition Stretchable stainless steel meshes with a width of 0.5 cm were washed in acetone, ethanol and deionized water, and then used as substrates. Anodic electrodeposition of PPy was conducted up to 10 min at a constant current density of 0.33 mA/cm2 in a solution of 0.1 M p-Toluenesulfonic acid, 0.3 M sodium toluenesulfate, and 0.5% pyrrole monomer (v:v) at 0 1C. Prior to electrodeposition, pyrrole was distilled in order to purify pyrrole monomers.
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Fabrication of solid-state supercapacitors
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Two identical as-synthesized samples were assembled together with a gel electrolyte of H3PO4 and PVA (electrolyte composition: 6 g H3PO4, 6 g PVA, and 60 ml deionized water). After gel solidification at room temperature, the solid-state supercapacitor was obtained with the electrolyte also serving as a separator.
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Characterization and electrochemical measurement Microstructures of electrodes were characterized by SEM (Philips XL30). FTIR (AVATAR 380) was used to get the spectra of the as-synthesized PPy. Cyclic voltammetry and galvanostatic charging/discharging measurements were performed on an electrochemical station (CHI 760E). Electrochemical impedance spectroscopy was measured at frequencies ranging from 0.01 Hz to 100,000 Hz with potential amplitude of 5 mV. Supercapacitors were mounted into two clips, and stretched horizontally by a test bench (AMH-500, Yiding Co.).
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Results and discussion 111 It is noticed that the stainless steel mesh possess the same weaving structure as conventional textiles but electrically conductive. Although the mesh woven by a plain weave technique is not stretchable along directions of the weft and the warp, it can be stretched up to a strain of 40% along a direction of 451 to the weft or the warp, as schematically shown in Fig. 1a and b. By cutting along the dashed black lines in Fig. 1, the as-cut mesh is stretchable biaxially. This provides a facile and low-cost method to fabricate stretchable devices without resorting complicated treatments to obtain conductive and stretchable substrates. As conducting polymers such as PPy possess the best deformability among various functional
Please cite this article as: Y. Huang, et al., Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.10.031
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Fig. 1 Schematic illustration: (a) preparation of a stretchable steel mesh by cutting along the dashed black lines. (b) Stretch the mesh biaxially. (c) PPy electrodeposition on the stretchable steel mesh. (d) Supercapacitor device assembly by coating H3PO4/PVA gel electrolyte.
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materials for supercapacitors, it was chosen as active electrode materials and electrochemically deposited onto the conductive mesh, followed by a device assembly by coating gel electrolytes, as schematically shown in Fig. 1. Scanning electron microscopy (SEM) images (Fig. 2a and b) of a PPy-stainless steel mesh show clearly the two-dimensional (2D) hierarchical structure with a continuous PPy film uniformly electrodeposited onto the conductive mesh, including the cross intersections. Fig. 2b demonstrates that, unlike films formed by inorganic particles, the thin layer of PPy is soft and can be squeezed without the appearance of any cracks. This suggests the high potential of the PPy for stretchable devices which have to sustain severe deformations. The surface of stainless steel mesh underneath is not smooth, explaining the rough PPy layer observed in Fig. 2b and c. A Fourier transform infrared (FTIR) spectrum in Fig. 2d confirms the species of as-synthesized PPy. The broad band from 1320 to 1260 cm 1 is attributed to C–H or C–N in-plane vibrations. The region at around 1140 cm 1 corresponds to the breathing vibration of the pyrrole ring. The peak at around 1090 cm 1 corresponds to the mode of in-plane deformation vibration of NH2+ which forms on the PPy chains via protonation. The bands at 1010 cm 1 and 940 cm 1 are attributed to C–H and N–H in-plane ring deformation vibration, and C–C out-of-plane ring deformation vibration [36,48]. The electrochemical measurements of stretchable PPybased solid-state supercapacitors were conducted at room temperature. As demonstrated in Fig. 3a and b, the device can be operated at various scan rates up to 10 V/s and CVs keep the rectangular shape even at a high scan rate of 2 V/s. It should be noted that both numbers are 1–2 orders of magnitude higher than most scan rates for PPy electrodes measured even in aqueous electrolytes [36,40,41,47–52], and are even equivalent to the highest values reported in solidstate the carbon nanotubes-MnO2 devices, which also keep good rectangular shape at most scan rates [53]. These indicate the PPy-based solid-state supercapacitors fabricated
can endure ultrafast voltage/current change rates, which is believed to be a result of effective electrochemical dynamic processes in the supercapacitors. This is confirmed by the well-defined rectangular shapes of most CVs and the wider working potential windows (Fig. S1) than those in most literatures [36,40,41,47–52]. Fig. 3c shows CD behaviors at various specific currents up to 20 A/g. The devices can charge–discharge ultrafast meanwhile retaining the shape of ideal isosceles triangles. All these features indicate excellent supercapacitor performances. Fig. 3d and e demonstrates CVs of devices with PPy deposition times of 5 min and 10 min at scan rates of 25 and 250 mV/s, respectively. Currents dramatically increase with the increased PPy deposition time for both scan rates, suggesting that more PPy are deposited onto the mesh and contribute to the charge storage. As the scan rate increases from 25 to 250 mV/s, the shape of CVs remains rectangular, with a little deviation for samples with higher mass loadings. This again indicates the good electrochemical performance of the supercapacitors. Fig. 3f shows specific capacitances of PPy with respect to scan rates. Capacitances of 5 min deposited PPy are higher than those of 10 min at the same scan rate. Moreover, the decay rate of capacitances with respect to the scan rate for the 5 min deposition is lower than that of 10 min. These are believed that a thinner PPy facilitates electron transportation and ion accessibility, which maximizes the utilization of the PPy. The specific capacitances obtained in the fabricated solid-state devices are comparable, or even higher than many reported results with PPy as the electrode material tested in liquid electrolytes [36,40,41,47–51]. It should be noted that we use the lower capacitances obtained from CVs for comparison with those in the literatures, instead of the higher numbers from CDs which are considered to be more accurate. Considering a much lower ionic conductivity of the solid electrolytes than liquid electrolytes, the specific capacitances achieved here in the solid-state supercapacitors are remarkable. Considering that the thickness of the PPy electrodeposted is in the magnitude
Please cite this article as: Y. Huang, et al., Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.10.031
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Fig. 2 Characterization of PPy electrodes: (a) an overall view of uniformly coated 2D hierarchical PPy-mesh electrode. (b) Squeezed PPy electrodeposited on the skeleton of the mesh exhibiting excellent ductility of the PPy film. (c) High magnification of a PPy film showing a rough surface. (d) FTIR spectrum of the as-synthesized material showing the species of PPy.
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Fig. 3 Electrochemical performances of PPy-based supercapacitors: (a, b) CVs of 5 min deposited PPy at low and high scan rates, respectively. (c) CDs of 5 min deposited PPy at various specific currents. (d, e) CVs with different electrodeposition times at scan rates of 25 and 250 mV/s, respectively. (f) Specific capacitances as functions of scan rates with various electrodeposition times: Q6 5 min (red and circle), and 10 min (blue and triangle). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
of micrometer (1 mm), these remarkable capacitances of PPy are believed not to arise from the ultrathin electrodes which can contribute to high specific capacitances [10].
In order to meet the futuristic needs of next-generation implantable and portable electronics, steady performances of devices under stretched states are indispensable. Fig. 4 shows
Please cite this article as: Y. Huang, et al., Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.10.031
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Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability 1 3 5 7 9 11 13 15 17 19 21 23 25
electrochemical performances of the fabricated supercapacitor under different strains. Insets of Fig. 4a show the photographs of the device under relaxed and stretched states. The device gets much longer and narrower under the strain, while no cracks in the device are observed upon stretching. The stretchability of the device arises from the stretchable components: the smartly tailored mesh, PPy, and the gel electrolyte. As far as electrochemical performance is concerned, the shape of the CVs is slightly distorted for the stretched supercapacitor (Fig. 4a). Moreover, the discharge time increases and the internal resistance decreases remarkably (Fig. 4b). It should be noted that the strain induces enhanced capacitances, as revealed in Figs. S3a, b and S4a, b and confirmed by more CDs under strains shown in Fig. S3c and d [36]. To clarify this point in details, specific capacitances under different strains as functions of specific currents and scan rates are plotted in Fig. 4c and Fig. S3e, respectively. Specific capacitances over 210 F/g and 204 F/g were achieved for 10 min deposited PPy under a 20% strain at 0.5 A/g and 5 mV/s, respectively. These are remarkable and much better than PPy based solid-state supercapacitors reported. For example, at a scan rate of 100 mV/s and 20% strain, 138 F/g was achieved for our supercapacitors vs. less than 60 F/g achieved for the nylon lycra fabric supercapacitors [36]. The capacitance decreases with the specific current, being consistent with the decreasing tendency of rate-dependent performance shown in Fig. 3f.
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Under 0% and 20% strain, the electrochemical impedance spectra (EIS) of 10 min deposited PPy are very similar as shown in Fig. 4d, revealing the good conductivity is well preserved under stretch. The well preserved conductivity is attributed to the stretchable PPy. There is no crack at all from the SEM image of the PPy electrode after repeated stretch (Fig. S3g). In addition, it is noteworthy that both the physical shape and performance of the device return to the initial relaxed state after the strain is removed (Fig. S3h). This indicates an excellent stretch recoverability and a good candidacy for next-generation stretchable electronics applications. The cycling performance at a very fast charging–discharging rate of 10 A/g under both relaxed and stretched states are studied as shown in Fig. 4e. The cycling stability of most previously reported PPy is poor because of sturctural breakdown induced by repeatedly large volume increase (charging) and decrease (discharging) [47]. Most PPy-based electrodes lose over 50% of the initial capacitance after 1000 cycles. However, in our case, the capacitance retention under 0 strain is remarkably kept to be 98% in the solidstate supercapacitor (5% fluctuation). This is even much better than the state-of-the-art best result of 85% achieved with the liquid electrolyte [47]. Even with a 20% strain applied all over the duration of 10,000 cycles, there is still 87% of the original capacitance retained. To the best of our knowledge, such long-term cycling stability with an existence
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115 Fig. 4 Electrochemical performances of PPy-based supercapacitors: (a) CVs of 10 min deposited PPy under various strains at a scan rate of 25 mV/s. The insets show photographs of the device under relaxed and stretched states. (b) CDs of 10 min deposited PPy under various strains at a specific current of 2 A/g. (c) Specific capacitances of devices with 10 min deposited PPy as functions of specific current under various strains. (d) Nyquist plot of the device with 10 min deposited PPy under 0 (black, solid) and 20% strain (blue, hollow). The inset shows a zoom out of the Nyquist plot showing semi-circles. (e) Capacitance retentions as functions of cycle number at a specific current of 10 A/g under 0 (black) and 20% strain (blue). (f) CVs of the fabricated stretchable supercapacitors experiencing various kinds of deformation: knot, fold and bend, at the scan rate of 10 mV/s. The insets show photographs of the deformed supercapacitor. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article as: Y. Huang, et al., Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.10.031
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Y. Huang et al. These features may result from three effects as follows. First, before electrochemical polymerization, pyrrole monomers are purified by distilling [64–66]. In view of pyrrole monomers which are easily oxidized and self-polymerized during storage, the purification of pyrrole may effectively improve electronic and ionic transportation of the polymerized PPy. Second, electrochemical polymerization helps forming a much more uniform PPy film, in comparison to those obtained by other methods such as chemical polymerization [36,49,50]. Moreover, polymerization induced by electrochemical reactions may induce ordered molecule chain structure, which may dramatically enhance structural stability of the PPy electrodes. This is believed to be the key factor to achieve the outstanding cycling stability under both relaxed and stretched status. The detailed investigations on the mechanism behind are underway. Third, the highly electric-conductive and surface-unsmooth mesh provides a tight contact with PPy during electrodeposition. The close contact can maximize the PPy effectively utilized for charge storage, guarantee the effective dynamic processes, and maintain the performance when cycled and deformed eventually. The stretch-induced capacitance enhancements observed in Fig. 4c can be explained as follows: (a) resistances are effectively reduced upon stretch by the improvement of electrical contacts [33,36]. This is confirmed by the decreasd IR drop shown in Fig. 4b and Fig. S3c, as well as EIS results shown in Fig. 4d and Fig. S3f. In contrast to cases of 0 strain, the system resistance of 10 min deposited PPy decrease 9.5% under the 20% strain. (b) The strain may induce a molecular ordering of PPy along the stretch direction, which can increase ionic and electronic conductivities of PPy [65]. (c) The wrinkles of deposited PPy may be expanded and provide a larger electrochemically effective areas upon
of strain on a stretchable supercapacitor all along has not been performed and achieved even with the most stable carbon materials as electrodes. Fig. 4f shows the excellent flexibility of the supercapacitors fabricated. The CVs at a quite low scan rate of 10 mV/s completely overlap when the device is knotted, folded and bent. It should be noted that, usually at such a low scan rate, CV measurements become very sensitive and can be easily distorted or affected by any small interruptions. This is very impressive in comparison with what has been studied, in which usually the device flexibilities were only demonstrated by bending or slight twisting. In addition, after conforming to complex nonplanar surfaces in these deformation tests, the supercapacitors can recover to their original shapes without permanent deformation preserved, revealing an excellent structure-performance reliability. Table 1 compares performances of various PPy-based solidstate supercapacitors with our devices [54–63]. It is clear that our stretchable PPy supercapacitors possess outstanding performances, including excellent cycling stability, high rate capability, decent capacitance and great flexibility. For example, at a very high specific current of 10 A/g, capacitance retentions of 98% and 87% were achieved after 10,000 cycles under 0% and 20% strain, respectively, which are excellent in comparison with most previously reported retentions of less than 80% after 1000–2000 cycles without strain at low current densities. As for the maximum scan rate recorded, there has been no value higher than 1 V/s so far in comparison with a super-high scan rate of 10 V/s achieved here. It is even more valuable that our solid-state supercapacitors possess all these merits and they are stretchable, knottable and foldable with great structure and performance recoverability and preservability.
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Table 1 Comparison of our stretchable PPy-based supercapacitors with the reported PPy-based solidstate supercapacitor devices in terms of cycling stability, scan rate, capacitance and flexibility.
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Ref.
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Electrode materials
Capacitance retention (%)
43 45
This study PPy
47
[54]
49
[55]
51
[56] [57]
53
[58] [59]
55 57
Cycling stability
[60] [61] [62]
PPy PPy-nanoporous gold PPy PPy-PANI PPy-reduced graphene oxide PPy PPy-carbon fiber paper PPy-MnO2 PPy PPy-carbon PPy
98 87 83.6 74.4
Maximum scan rate recorded (V/s) Cycle number
Specific current (A/g)
10,000
10
Capacitance (F/g)
Strain (%)
105 10
170 214 51 49
0 20 0 30
2000
0.5
0.1
78
900
4.4
1
250
0
80 60
10,000 4000
0.1 0.05
187 25
0 0
75
2000
0.1
37
0
78
1000
0.1
93
2000
86.7 14.3 45.4 67
1000 2000
1
0.1
0.01
Knottable, foldable, bendable, and stretchable Stretchable
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Bendable
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82.4
0 0
320a 433a 205.5
0
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[63]
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a The high capacitances were achieved at very slow scan rate of 1 mV/s. At 10 mV/s, the capacitances of PPy and PPy-carbon are 180 F/g and 320 F/g, respectively.
10,000
0.1
107
113 115
0
0.2 1
Flexibility demonstration
0
Please cite this article as: Y. Huang, et al., Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.10.031
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stretch. This may also contribute to the observed capacitance enhancements.
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Conclusions
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In summary, we designed a very facile and low-cost approach to fabricate solid-state PPy supercapacitors with great over-all performances, including outstanding cycling stability (98% capacitance retention after 10,000 cycles), super-high rate capacity (10 V/s, 1–2 orders of magnitude higher than values of PPy reported ever), high capacitances, and excellent stretchability and flexibility etc. The supercapacitors are fabricated by utilizing an electrochemically polymerized PPy on smartly-tailored stainless steel mesh as electrodes. The fabricated solid state supercapacitors possess a capacitance up to 170 F/g at a current density of 0.5 A/g and it can be effectively enhanced to 214 F/g with a 20% strain. Remarkably, they can be operated at a very high scan rate up to 10 V/s, which is 1–2 orders of magnitude higher than reported scan rates for the PPy electrodes measured even in aqueous electrolytes. It is also equivalent to the highest values reported for solid-state the carbon nanotube-MnO2 devices. Moreover, the fabricated solid-state supercapacitors achieve remarkable capacitance retentions of 98% and 87% after 10,000 cycles under 0% and 20% strains, respectively. To the best of our knowledge, this is the best capacitance retentions for PPy-based solid-state supercapacitors (Table 1). The great over-all performances achieved in the fabricated stretchable supercapacitors are attributed to the utilization of well purified pyrrole monomers, electrochemical polymerization and the highly electric conductive mesh. Our approach opens up new opportunities for the development of low-cost, high performance, stable and stretchable energy storage devices.
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Acknowledgments 39 This research was supported by the Early Career Scheme of the Research Grants Council of Hong Kong SAR, China, under Project number CityU 9041977, the Science Technology and Innovation Committee of Shenzhen Municipality (Grant Q3 number JCYJ20130401145617276), and Grant from the City University of Hong Kong. The authors thank Dr. J. P. Wang, W. Chen, T. C. Lau and F. Ai for their experimental support. Q2
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Appendix A.
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Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2014.10.031.
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Jiayou Tao received his master's degree in School of Optical and Electronic Information from Huazhong University of Science and Technology. He is a Ph.D. candidate in School of Physics at HUST. His research interests include supercapacitors, various nanocomposite materials and their applications.
Yan Huang received her Ph.D. degree (2013) in Materials Science, University of Rochester. Now she is a Postdoctoral Fellow at the City University of Hong Kong. Her research interests include fuel cells and supercapacitors.
Yihua Gao is a professor of Wuhan National Laboratory for Optoelectronics and School of Physics, Huazhong University of Science and Technology from 2006. He received his Ph.D. degree (1998) from Institute of Physics, Chinese Academy of Sciences (CAS). After that, he worked as a postdoctoral fellow in National Institute for Materials Science (NIMS) in Japan. His research interests include semiconductions, solar cells and supercapacitors.
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Wenjun Meng received his Ph.D. degree in 2012 from Hokkaido University. Now he is a senior research associate in City University of Hong Kong and mainly focuses on the research of energy-related materials and thermal conductive polymer composites.
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Minshen Zhu received his Bachelor degree in Materials Science and Engineering from Beijing University of Chemical Technology, and received the Master degree from City University of Hong Kong. He is now a Ph.D. candidate under direction of Dr. Chunyi Zhi. His research focuses on the materials for electrochemical capacitors.
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Yang Huang received his B.S. (2009) and M.S. degree (2012) in Materials Science and Engineering from Southwest Jiaotong University. Now he is a Ph.D. candidate in City University of Hong Kong and his current research focuses on the nanostructured materials for energy and environmental applications.
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Yuqiao Fu received her Master degree in City University of Hong Kong in 2014. She is currently a research assistant in Department of Physics and Materials Science at City University of Hong Kong. Her research interests focus on BN materials in the field of ductility and UV-shielding polymer composites.
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Please cite this article as: Y. Huang, et al., Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.10.031
Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability 1 3 5 7 9 11
Chunyi Zhi received his Ph.D. degree in physics from Institute of Physics, CAS in 2004. Then he moved to National Institute for Materials Science (NIMS) in Japan as a post-doctoral fellow, followed by an ICYS Research Fellow, Researcher (faculty) and Senior Researcher (permanent). He began his academic appointment in the Department of Physics and Materials Science at the City University of Hong Kong in 2012. His research interests include mass production of BN/BCN nanostructures, polymer nano-composites, and energyrelated electrochemical and photoelectro-chemical devices.
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Please cite this article as: Y. Huang, et al., Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.10.031
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RAPID COMMUNICATION
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Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability
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Yan Huanga, Jiayou Taob, Wenjun Menga, Minshen Zhua, Yang Huanga, Yuqiao Fua, Yihua Gaob, Chunyi Zhia,c,n a
Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong, China b Wuhan National Laboratory for Optoelectronics (WNLO), and School of Physics, Huazhong University of Science and Technology (HUST), 1037 Luoyu Road, Wuhan 430074, China c Shenzhen Research Institute, City University of Hong Kong, Shenzhen 518000, China
31 Received 2 August 2014; received in revised form 14 October 2014; accepted 15 October 2014
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KEYWORDS
Abstract
High rate; Stretchable supercapacitors; Polypyrrole; Cycling stability; Electrodeposition
The performance and cycling stability of stretchable energy storage devices, such as supercapacitors and batteries, are limited by the structural breakdown arising from the stretch imposed and large volumetric swelling/shrinking. This work demonstrates a very facile and low-cost approach to fabricate stretchable supercapacitors with high performance and excellent cycling stability by electrochemical deposition of polypyrrole (PPy) on smartly-tailored stretchable stainless steel meshes. The fabricated solid-state supercapacitors possess a capacitance up to 170 F/g at a specific current of 0.5 A/g and it can be effectively enhanced to 214 F/g with a 20% strain. Moreover, they can be operated at a very high scan rate up to 10 V/s, which are 1–2 orders of magnitude higher than most rates for the PPy electrodes measured even in aqueous electrolytes. Even significantly, the fabricated solid-state supercapacitors under 0% and 20% strains achieve remarkable capacitance retentions of 98% and 87% at a very high specific current of 10 A/g after 10,000 cycles, respectively, which are the best for PPy-based solid-state flexible supercapacitors, to the best of our knowledge. The key factors and mechanisms to achieve such high performance are discussed. This facile and lowcost approach developed for fabricating stable and stretchable supercapacitors with high performances could pave the way for next-generation stretchable electronics. & 2014 Elsevier Ltd. All rights reserved.
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Corresponding author at: Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong, China. Tel.: +852 3442 7891; fax: +852 3442 0538. E-mail address: [email protected] (C. Zhi).
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http://dx.doi.org/10.1016/j.nanoen.2014.10.031 2211-2855/& 2014 Elsevier Ltd. All rights reserved.
Please cite this article as: Y. Huang, et al., Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.10.031
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Introduction
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Cost-effective and performance-attractive devices with longterm stability are essential for stretchable energy storage and conversion technologies, which receive considerable attention recently [1,2], for allowing many unprecedented applications possible such as future smart biosensors and clothes [3–6]. The supercapacitor in particular is one important category of energy storage devices for its high power density, fast rate of charge–discharge, long cycling life, etc. [7–17]. These features make it superior to protect against unstable energy supply situations [11,18–23]. However, several problems need to be solved when introducing the stretchability into conventionally rigid supercapacitors. First, it is much challenging to fabricate stretchable devices. Second, the performance and cycling stability of stretchable supercapacitors are limited by the non-effective solid state electrolyte and the structural breakdown resulting from the stretch applied and large volumetric swelling/shrinking during the charge/discharge process. Substantial effort has been devoted to the fabrication of stretchable devices. For example, non-coplanar buckled structures [24–26], coplanar serpentine and wavy structures [27–30], percolating nanostructured films [31], elastomers and stretchable textile substrates [32–36] have been utilized to achieve the stretchability of devices. However, on one hand, multi-step complicated fabrications are involved in these designs, which make them cost-expensive and time-consuming; on the other hand, the high performances are very difficult to be achieved with the solid state devices even the electrodes exhibit very good electrochemical activities when they are tested in the liquid electrolytes. Regarding the structural breakdown of electrode substances under strains, it has been found that conducting polymers such as PPy can maintain or even enhance performance under stretch due to their intrinsic stretchability [36–38]. But most reported electrochemical performances of PPy-based supercapacitors such as the shape of cyclic voltammograms (CV) and galvanostatic charge–discharge curves (CD) as well as the capacitance are somewhat frustrating. As part of efforts to improve the poor cycling stability of conducting polymers [39– 46], Liu et al. [47] substantially increase the cycle number from a commonly-demonstrated 1000 to a much higher value of 10,000 and achieve a remarkable capacitance retention of 85% by deposition of a thin carbonaceous shell onto the surface of PPy. However, no cycling test under stretched states was performed in the reference. Yue et al. [36] reported that under a strain of 20%, PPy coated on elastic Nylon fabrics lose 50% of capacitance after merely 500 cycles. Thus, it is crucial to develop a facile and low-cost method that can fabricate high-performance stretchable PPy supercapacitors with an excellent long-term cycling stability for their practical applications. Here we demonstrate a facile and low-cost strategy to effectively improve the performance and cycling stability of stretchable PPy-based supercapacitors through electrochemical polymerization of purified pyrrole monomers on smartly tailored stretchable stainless steel meshes. By cutting along a certain direction, a knitted stainless steel mesh was found to become very stretchable. Then purified pyrrole monomers were electrochemically polymerized onto the mesh. The fabricated supercapacitors increase the capacitance from the initial 170 F/g at a relaxed state to
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214 F/g at a 20% strain at a specific current of 0.5 A/g. Surprisingly, the solid-state supercapacitors can be operated at super-high rates up to 10 V/s, being 1–2 orders of magnitude higher than most scan rates for PPy electrodes measured even in aqueous electrolytes. More importantly, they achieve a capacitance retention of 98% under 0 strain, and 87% under a strain of 20% applied after 10,000 cycles at a very high specific current of 10 A/g. To our best knowledge, these are the best capacitance retention values reported so far for PPy-based solid-state supercapacitors.
Material and methods PPy electrodeposition Stretchable stainless steel meshes with a width of 0.5 cm were washed in acetone, ethanol and deionized water, and then used as substrates. Anodic electrodeposition of PPy was conducted up to 10 min at a constant current density of 0.33 mA/cm2 in a solution of 0.1 M p-Toluenesulfonic acid, 0.3 M sodium toluenesulfate, and 0.5% pyrrole monomer (v:v) at 0 1C. Prior to electrodeposition, pyrrole was distilled in order to purify pyrrole monomers.
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Fabrication of solid-state supercapacitors
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Two identical as-synthesized samples were assembled together with a gel electrolyte of H3PO4 and PVA (electrolyte composition: 6 g H3PO4, 6 g PVA, and 60 ml deionized water). After gel solidification at room temperature, the solid-state supercapacitor was obtained with the electrolyte also serving as a separator.
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Characterization and electrochemical measurement Microstructures of electrodes were characterized by SEM (Philips XL30). FTIR (AVATAR 380) was used to get the spectra of the as-synthesized PPy. Cyclic voltammetry and galvanostatic charging/discharging measurements were performed on an electrochemical station (CHI 760E). Electrochemical impedance spectroscopy was measured at frequencies ranging from 0.01 Hz to 100,000 Hz with potential amplitude of 5 mV. Supercapacitors were mounted into two clips, and stretched horizontally by a test bench (AMH-500, Yiding Co.).
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Results and discussion 111 It is noticed that the stainless steel mesh possess the same weaving structure as conventional textiles but electrically conductive. Although the mesh woven by a plain weave technique is not stretchable along directions of the weft and the warp, it can be stretched up to a strain of 40% along a direction of 451 to the weft or the warp, as schematically shown in Fig. 1a and b. By cutting along the dashed black lines in Fig. 1, the as-cut mesh is stretchable biaxially. This provides a facile and low-cost method to fabricate stretchable devices without resorting complicated treatments to obtain conductive and stretchable substrates. As conducting polymers such as PPy possess the best deformability among various functional
Please cite this article as: Y. Huang, et al., Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.10.031
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Fig. 1 Schematic illustration: (a) preparation of a stretchable steel mesh by cutting along the dashed black lines. (b) Stretch the mesh biaxially. (c) PPy electrodeposition on the stretchable steel mesh. (d) Supercapacitor device assembly by coating H3PO4/PVA gel electrolyte.
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materials for supercapacitors, it was chosen as active electrode materials and electrochemically deposited onto the conductive mesh, followed by a device assembly by coating gel electrolytes, as schematically shown in Fig. 1. Scanning electron microscopy (SEM) images (Fig. 2a and b) of a PPy-stainless steel mesh show clearly the two-dimensional (2D) hierarchical structure with a continuous PPy film uniformly electrodeposited onto the conductive mesh, including the cross intersections. Fig. 2b demonstrates that, unlike films formed by inorganic particles, the thin layer of PPy is soft and can be squeezed without the appearance of any cracks. This suggests the high potential of the PPy for stretchable devices which have to sustain severe deformations. The surface of stainless steel mesh underneath is not smooth, explaining the rough PPy layer observed in Fig. 2b and c. A Fourier transform infrared (FTIR) spectrum in Fig. 2d confirms the species of as-synthesized PPy. The broad band from 1320 to 1260 cm 1 is attributed to C–H or C–N in-plane vibrations. The region at around 1140 cm 1 corresponds to the breathing vibration of the pyrrole ring. The peak at around 1090 cm 1 corresponds to the mode of in-plane deformation vibration of NH2+ which forms on the PPy chains via protonation. The bands at 1010 cm 1 and 940 cm 1 are attributed to C–H and N–H in-plane ring deformation vibration, and C–C out-of-plane ring deformation vibration [36,48]. The electrochemical measurements of stretchable PPybased solid-state supercapacitors were conducted at room temperature. As demonstrated in Fig. 3a and b, the device can be operated at various scan rates up to 10 V/s and CVs keep the rectangular shape even at a high scan rate of 2 V/s. It should be noted that both numbers are 1–2 orders of magnitude higher than most scan rates for PPy electrodes measured even in aqueous electrolytes [36,40,41,47–52], and are even equivalent to the highest values reported in solidstate the carbon nanotubes-MnO2 devices, which also keep good rectangular shape at most scan rates [53]. These indicate the PPy-based solid-state supercapacitors fabricated
can endure ultrafast voltage/current change rates, which is believed to be a result of effective electrochemical dynamic processes in the supercapacitors. This is confirmed by the well-defined rectangular shapes of most CVs and the wider working potential windows (Fig. S1) than those in most literatures [36,40,41,47–52]. Fig. 3c shows CD behaviors at various specific currents up to 20 A/g. The devices can charge–discharge ultrafast meanwhile retaining the shape of ideal isosceles triangles. All these features indicate excellent supercapacitor performances. Fig. 3d and e demonstrates CVs of devices with PPy deposition times of 5 min and 10 min at scan rates of 25 and 250 mV/s, respectively. Currents dramatically increase with the increased PPy deposition time for both scan rates, suggesting that more PPy are deposited onto the mesh and contribute to the charge storage. As the scan rate increases from 25 to 250 mV/s, the shape of CVs remains rectangular, with a little deviation for samples with higher mass loadings. This again indicates the good electrochemical performance of the supercapacitors. Fig. 3f shows specific capacitances of PPy with respect to scan rates. Capacitances of 5 min deposited PPy are higher than those of 10 min at the same scan rate. Moreover, the decay rate of capacitances with respect to the scan rate for the 5 min deposition is lower than that of 10 min. These are believed that a thinner PPy facilitates electron transportation and ion accessibility, which maximizes the utilization of the PPy. The specific capacitances obtained in the fabricated solid-state devices are comparable, or even higher than many reported results with PPy as the electrode material tested in liquid electrolytes [36,40,41,47–51]. It should be noted that we use the lower capacitances obtained from CVs for comparison with those in the literatures, instead of the higher numbers from CDs which are considered to be more accurate. Considering a much lower ionic conductivity of the solid electrolytes than liquid electrolytes, the specific capacitances achieved here in the solid-state supercapacitors are remarkable. Considering that the thickness of the PPy electrodeposted is in the magnitude
Please cite this article as: Y. Huang, et al., Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.10.031
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Fig. 2 Characterization of PPy electrodes: (a) an overall view of uniformly coated 2D hierarchical PPy-mesh electrode. (b) Squeezed PPy electrodeposited on the skeleton of the mesh exhibiting excellent ductility of the PPy film. (c) High magnification of a PPy film showing a rough surface. (d) FTIR spectrum of the as-synthesized material showing the species of PPy.
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Fig. 3 Electrochemical performances of PPy-based supercapacitors: (a, b) CVs of 5 min deposited PPy at low and high scan rates, respectively. (c) CDs of 5 min deposited PPy at various specific currents. (d, e) CVs with different electrodeposition times at scan rates of 25 and 250 mV/s, respectively. (f) Specific capacitances as functions of scan rates with various electrodeposition times: Q6 5 min (red and circle), and 10 min (blue and triangle). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
of micrometer (1 mm), these remarkable capacitances of PPy are believed not to arise from the ultrathin electrodes which can contribute to high specific capacitances [10].
In order to meet the futuristic needs of next-generation implantable and portable electronics, steady performances of devices under stretched states are indispensable. Fig. 4 shows
Please cite this article as: Y. Huang, et al., Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.10.031
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Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability 1 3 5 7 9 11 13 15 17 19 21 23 25
electrochemical performances of the fabricated supercapacitor under different strains. Insets of Fig. 4a show the photographs of the device under relaxed and stretched states. The device gets much longer and narrower under the strain, while no cracks in the device are observed upon stretching. The stretchability of the device arises from the stretchable components: the smartly tailored mesh, PPy, and the gel electrolyte. As far as electrochemical performance is concerned, the shape of the CVs is slightly distorted for the stretched supercapacitor (Fig. 4a). Moreover, the discharge time increases and the internal resistance decreases remarkably (Fig. 4b). It should be noted that the strain induces enhanced capacitances, as revealed in Figs. S3a, b and S4a, b and confirmed by more CDs under strains shown in Fig. S3c and d [36]. To clarify this point in details, specific capacitances under different strains as functions of specific currents and scan rates are plotted in Fig. 4c and Fig. S3e, respectively. Specific capacitances over 210 F/g and 204 F/g were achieved for 10 min deposited PPy under a 20% strain at 0.5 A/g and 5 mV/s, respectively. These are remarkable and much better than PPy based solid-state supercapacitors reported. For example, at a scan rate of 100 mV/s and 20% strain, 138 F/g was achieved for our supercapacitors vs. less than 60 F/g achieved for the nylon lycra fabric supercapacitors [36]. The capacitance decreases with the specific current, being consistent with the decreasing tendency of rate-dependent performance shown in Fig. 3f.
5
Under 0% and 20% strain, the electrochemical impedance spectra (EIS) of 10 min deposited PPy are very similar as shown in Fig. 4d, revealing the good conductivity is well preserved under stretch. The well preserved conductivity is attributed to the stretchable PPy. There is no crack at all from the SEM image of the PPy electrode after repeated stretch (Fig. S3g). In addition, it is noteworthy that both the physical shape and performance of the device return to the initial relaxed state after the strain is removed (Fig. S3h). This indicates an excellent stretch recoverability and a good candidacy for next-generation stretchable electronics applications. The cycling performance at a very fast charging–discharging rate of 10 A/g under both relaxed and stretched states are studied as shown in Fig. 4e. The cycling stability of most previously reported PPy is poor because of sturctural breakdown induced by repeatedly large volume increase (charging) and decrease (discharging) [47]. Most PPy-based electrodes lose over 50% of the initial capacitance after 1000 cycles. However, in our case, the capacitance retention under 0 strain is remarkably kept to be 98% in the solidstate supercapacitor (5% fluctuation). This is even much better than the state-of-the-art best result of 85% achieved with the liquid electrolyte [47]. Even with a 20% strain applied all over the duration of 10,000 cycles, there is still 87% of the original capacitance retained. To the best of our knowledge, such long-term cycling stability with an existence
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115 Fig. 4 Electrochemical performances of PPy-based supercapacitors: (a) CVs of 10 min deposited PPy under various strains at a scan rate of 25 mV/s. The insets show photographs of the device under relaxed and stretched states. (b) CDs of 10 min deposited PPy under various strains at a specific current of 2 A/g. (c) Specific capacitances of devices with 10 min deposited PPy as functions of specific current under various strains. (d) Nyquist plot of the device with 10 min deposited PPy under 0 (black, solid) and 20% strain (blue, hollow). The inset shows a zoom out of the Nyquist plot showing semi-circles. (e) Capacitance retentions as functions of cycle number at a specific current of 10 A/g under 0 (black) and 20% strain (blue). (f) CVs of the fabricated stretchable supercapacitors experiencing various kinds of deformation: knot, fold and bend, at the scan rate of 10 mV/s. The insets show photographs of the deformed supercapacitor. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article as: Y. Huang, et al., Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.10.031
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Y. Huang et al. These features may result from three effects as follows. First, before electrochemical polymerization, pyrrole monomers are purified by distilling [64–66]. In view of pyrrole monomers which are easily oxidized and self-polymerized during storage, the purification of pyrrole may effectively improve electronic and ionic transportation of the polymerized PPy. Second, electrochemical polymerization helps forming a much more uniform PPy film, in comparison to those obtained by other methods such as chemical polymerization [36,49,50]. Moreover, polymerization induced by electrochemical reactions may induce ordered molecule chain structure, which may dramatically enhance structural stability of the PPy electrodes. This is believed to be the key factor to achieve the outstanding cycling stability under both relaxed and stretched status. The detailed investigations on the mechanism behind are underway. Third, the highly electric-conductive and surface-unsmooth mesh provides a tight contact with PPy during electrodeposition. The close contact can maximize the PPy effectively utilized for charge storage, guarantee the effective dynamic processes, and maintain the performance when cycled and deformed eventually. The stretch-induced capacitance enhancements observed in Fig. 4c can be explained as follows: (a) resistances are effectively reduced upon stretch by the improvement of electrical contacts [33,36]. This is confirmed by the decreasd IR drop shown in Fig. 4b and Fig. S3c, as well as EIS results shown in Fig. 4d and Fig. S3f. In contrast to cases of 0 strain, the system resistance of 10 min deposited PPy decrease 9.5% under the 20% strain. (b) The strain may induce a molecular ordering of PPy along the stretch direction, which can increase ionic and electronic conductivities of PPy [65]. (c) The wrinkles of deposited PPy may be expanded and provide a larger electrochemically effective areas upon
of strain on a stretchable supercapacitor all along has not been performed and achieved even with the most stable carbon materials as electrodes. Fig. 4f shows the excellent flexibility of the supercapacitors fabricated. The CVs at a quite low scan rate of 10 mV/s completely overlap when the device is knotted, folded and bent. It should be noted that, usually at such a low scan rate, CV measurements become very sensitive and can be easily distorted or affected by any small interruptions. This is very impressive in comparison with what has been studied, in which usually the device flexibilities were only demonstrated by bending or slight twisting. In addition, after conforming to complex nonplanar surfaces in these deformation tests, the supercapacitors can recover to their original shapes without permanent deformation preserved, revealing an excellent structure-performance reliability. Table 1 compares performances of various PPy-based solidstate supercapacitors with our devices [54–63]. It is clear that our stretchable PPy supercapacitors possess outstanding performances, including excellent cycling stability, high rate capability, decent capacitance and great flexibility. For example, at a very high specific current of 10 A/g, capacitance retentions of 98% and 87% were achieved after 10,000 cycles under 0% and 20% strain, respectively, which are excellent in comparison with most previously reported retentions of less than 80% after 1000–2000 cycles without strain at low current densities. As for the maximum scan rate recorded, there has been no value higher than 1 V/s so far in comparison with a super-high scan rate of 10 V/s achieved here. It is even more valuable that our solid-state supercapacitors possess all these merits and they are stretchable, knottable and foldable with great structure and performance recoverability and preservability.
35
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37
99
39
Table 1 Comparison of our stretchable PPy-based supercapacitors with the reported PPy-based solidstate supercapacitor devices in terms of cycling stability, scan rate, capacitance and flexibility.
101
41
Ref.
103
Electrode materials
Capacitance retention (%)
43 45
This study PPy
47
[54]
49
[55]
51
[56] [57]
53
[58] [59]
55 57
Cycling stability
[60] [61] [62]
PPy PPy-nanoporous gold PPy PPy-PANI PPy-reduced graphene oxide PPy PPy-carbon fiber paper PPy-MnO2 PPy PPy-carbon PPy
98 87 83.6 74.4
Maximum scan rate recorded (V/s) Cycle number
Specific current (A/g)
10,000
10
Capacitance (F/g)
Strain (%)
105 10
170 214 51 49
0 20 0 30
2000
0.5
0.1
78
900
4.4
1
250
0
80 60
10,000 4000
0.1 0.05
187 25
0 0
75
2000
0.1
37
0
78
1000
0.1
93
2000
86.7 14.3 45.4 67
1000 2000
1
0.1
0.01
Knottable, foldable, bendable, and stretchable Stretchable
109
Bendable
111
82.4
0 0
320a 433a 205.5
0
59
[63]
61
a The high capacitances were achieved at very slow scan rate of 1 mV/s. At 10 mV/s, the capacitances of PPy and PPy-carbon are 180 F/g and 320 F/g, respectively.
10,000
0.1
107
113 115
0
0.2 1
Flexibility demonstration
0
Please cite this article as: Y. Huang, et al., Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.10.031
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stretch. This may also contribute to the observed capacitance enhancements.
3
Conclusions
5
In summary, we designed a very facile and low-cost approach to fabricate solid-state PPy supercapacitors with great over-all performances, including outstanding cycling stability (98% capacitance retention after 10,000 cycles), super-high rate capacity (10 V/s, 1–2 orders of magnitude higher than values of PPy reported ever), high capacitances, and excellent stretchability and flexibility etc. The supercapacitors are fabricated by utilizing an electrochemically polymerized PPy on smartly-tailored stainless steel mesh as electrodes. The fabricated solid state supercapacitors possess a capacitance up to 170 F/g at a current density of 0.5 A/g and it can be effectively enhanced to 214 F/g with a 20% strain. Remarkably, they can be operated at a very high scan rate up to 10 V/s, which is 1–2 orders of magnitude higher than reported scan rates for the PPy electrodes measured even in aqueous electrolytes. It is also equivalent to the highest values reported for solid-state the carbon nanotube-MnO2 devices. Moreover, the fabricated solid-state supercapacitors achieve remarkable capacitance retentions of 98% and 87% after 10,000 cycles under 0% and 20% strains, respectively. To the best of our knowledge, this is the best capacitance retentions for PPy-based solid-state supercapacitors (Table 1). The great over-all performances achieved in the fabricated stretchable supercapacitors are attributed to the utilization of well purified pyrrole monomers, electrochemical polymerization and the highly electric conductive mesh. Our approach opens up new opportunities for the development of low-cost, high performance, stable and stretchable energy storage devices.
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Acknowledgments 39 This research was supported by the Early Career Scheme of the Research Grants Council of Hong Kong SAR, China, under Project number CityU 9041977, the Science Technology and Innovation Committee of Shenzhen Municipality (Grant Q3 number JCYJ20130401145617276), and Grant from the City University of Hong Kong. The authors thank Dr. J. P. Wang, W. Chen, T. C. Lau and F. Ai for their experimental support. Q2
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Appendix A.
49
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2014.10.031.
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Jiayou Tao received his master's degree in School of Optical and Electronic Information from Huazhong University of Science and Technology. He is a Ph.D. candidate in School of Physics at HUST. His research interests include supercapacitors, various nanocomposite materials and their applications.
Yan Huang received her Ph.D. degree (2013) in Materials Science, University of Rochester. Now she is a Postdoctoral Fellow at the City University of Hong Kong. Her research interests include fuel cells and supercapacitors.
Yihua Gao is a professor of Wuhan National Laboratory for Optoelectronics and School of Physics, Huazhong University of Science and Technology from 2006. He received his Ph.D. degree (1998) from Institute of Physics, Chinese Academy of Sciences (CAS). After that, he worked as a postdoctoral fellow in National Institute for Materials Science (NIMS) in Japan. His research interests include semiconductions, solar cells and supercapacitors.
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Wenjun Meng received his Ph.D. degree in 2012 from Hokkaido University. Now he is a senior research associate in City University of Hong Kong and mainly focuses on the research of energy-related materials and thermal conductive polymer composites.
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Minshen Zhu received his Bachelor degree in Materials Science and Engineering from Beijing University of Chemical Technology, and received the Master degree from City University of Hong Kong. He is now a Ph.D. candidate under direction of Dr. Chunyi Zhi. His research focuses on the materials for electrochemical capacitors.
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Yang Huang received his B.S. (2009) and M.S. degree (2012) in Materials Science and Engineering from Southwest Jiaotong University. Now he is a Ph.D. candidate in City University of Hong Kong and his current research focuses on the nanostructured materials for energy and environmental applications.
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Yuqiao Fu received her Master degree in City University of Hong Kong in 2014. She is currently a research assistant in Department of Physics and Materials Science at City University of Hong Kong. Her research interests focus on BN materials in the field of ductility and UV-shielding polymer composites.
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Please cite this article as: Y. Huang, et al., Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.10.031
Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability 1 3 5 7 9 11
Chunyi Zhi received his Ph.D. degree in physics from Institute of Physics, CAS in 2004. Then he moved to National Institute for Materials Science (NIMS) in Japan as a post-doctoral fellow, followed by an ICYS Research Fellow, Researcher (faculty) and Senior Researcher (permanent). He began his academic appointment in the Department of Physics and Materials Science at the City University of Hong Kong in 2012. His research interests include mass production of BN/BCN nanostructures, polymer nano-composites, and energyrelated electrochemical and photoelectro-chemical devices.
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