www.advmat.de www.MaterialsViews.com
COMMUNICATION
Water Surface Assisted Synthesis of Large-Scale Carbon Nanotube Film for High-Performance and Stretchable Supercapacitors Minghao Yu, Yangfan Zhang, Yinxiang Zeng, Muhammad-Sadeeq Balogun, Kancheng Mai, Zishou Zhang,* Xihong Lu,* and Yexiang Tong As an emerging field, stretchable electronics that can sustain large mechanical strain without degradation in their electronic performance have large potential to be widely applied in bioimplantable system, wearable system, wireless sensors and so on.[1–7] Over the past few years, various kinds of stretchable devices such as organic light-emitting diode devices,[8,9] radio frequency devices,[10] field effect transistors,[11] pressure and strain sensors,[12,13] temperature sensors,[14] and artificial skin sensors[15] have been developed. To power the stretchable electronics and achieve a fully power-independent and stretchable system, it is urged to explore new power source devices with high stretchability. In recent years, extensive efforts have been devoted to studying stretchable conversion or storage devices like solar cells,[16] photovoltaic cells,[17,18] Li-ion batteries[19,20] and supercapacitors (SCs).[21–28] Among these power source devices, SCs are perceived as one of the most promising energy storage devices due to their high power density, modest energy density, fast charge-discharge capability and long cycle life.[29–31] The key point for fabricating stretchable SCs is the design of stretchable SCs electrodes. Currently, the general strategy to fabricate a stretchable SC electrode is coating a thin layer of electrochemically active material on the surface of some stretchable substrates such like poly(dimethylsiloxane) (PDMS),[22,24,26] elastic rubber fiber,[27] nylon lycra fabric[25] and cotton sheet.[23] For example, Jiang et al.[22] combined a layer of purified carbon nanotubes (CNTs) macro film with pre-strained PDMS. After the prestrain released, the CNTs became periodically sinusoidal. These buckled macro films exhibited a high specific capacitance
M. H. Yu, Y. F. Zhang, Y. X. Zeng, M.-S. Balogun, Dr. Z. S. Zhang, Dr. X. H. Lu, Prof. Y. X. Tong MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry School of Chemistry and Chemical Engineering Sun Yat-Sen University Guangzhou, 510275, People’s Republic of China Fax: (+86)20 84112245 E-mail: [email protected]; [email protected] Y. F. Zhang, Prof. K. C. Mai, Dr. Z. S. Zhang MOE of Key Laboratory for Polymeric Composite and Functional Materials Key Laboratory for High Performance Polymer Based Composites of Guangdong Province Sun Yat-Sen University Guangzhou, 510275, People’s Republic of China
DOI: 10.1002/adma.201401196
Adv. Mater. 2014, DOI: 10.1002/adma.201401196
of 52 F g−1 with 30% strain applied at a current density of 1 A g−1, while 54 F g−1 without stain. The elastic deformation of the fabricated supercapacitors always deeply relied on the stretchability of the substrates, and the non-conductive property of these substrates requires that the applied electrochemically active materials must have a high conductivity. As a result, the current electrode materials for stretchable supercapacitors are limited. Additionally, recent effort has shown that the use of Au interlayer between the poly(styrene-block-isobutylene-block-styrene) (SIBS) and PANI could significantly improve the conductivity and electrochemical performances. Wallace et al. reported a kind of stretchable SC electrodes by firstly deposited a layer of Au on SIBS substrate and then coated PANI onto Au/SIBS surface.[28] The realization of low-cost, stretchable SC device with high energy and power density is still highly desirable. In this work, we present a simple and large-scale water surface assisted synthesis of multiwall carbon nanotube (MWCNT) based stretchable film. Taking advantage of extremely flat surface of water, uniform films were formed which combined the excellent conductivity of MWCNTs with the high stretchability of PDMS. The MWCNT/PDMS film containing 10% MWCNTs exhibited a high conductivity of 4.19 S cm−1 and could be stretched to a high strain of 50% without damaging its conductivity and structure. In addition, the size and shape of the film can be easily tuned by changing the area and shape of water surface. More importantly, the prepared MWCNT/PDMS film is an excellent conductive and mechanical substrate to support electrochemically active materials. When a layer of polyaniline (PANI) nanofibers was deposited on the MWCNT/PDMS film, the PANI/MWCNT/PDMS film electrode exhibited a benchmark specific capacitance of 1023 F g−1 and areal capacitance of 481 mF cm−2 at a scan rate of 5 mV s−1. A solid-state symmetric supercapacitor (SSC) device with remarkable stretchability and outstanding electrochemical properties was also assembled for demo by the PANI/MWCNT/PDMS films as stretchable electrodes. The as-fabricated SSCs possessed good and stable capacitive behavior even under dynamic stretching conditions, with more than 95% capacitance retention after 500 cycles during the dynamic stretching and releasing process. Moreover, this device was able to deliver a maximum energy density of 0.15 mWh cm−3 (11 Wh kg−1), which is considerably higher than most of SSCs reported recently.[32–39] Figure 1a illustrates the schematic diagram of the manufacturing processes of the stretchable MWCNT films. Typically, xylol, MWCNTs (proportions: 3, 5, 8, and 10 wt%), silicone-elastomer base and curing agent were mixed well with ultrasonic
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 1. (a) Schematic diagram of fabricating stretchable MWCNT/PDMS film. (b) SEM images of the as-prepared MWCNT/PDMS film containing 10 wt% MWCNTs. (c) Stress-recovery curves of MWCNT/PDMS film containing 10% MWCNTs. (d) linear sweep voltammograms (LSV) curves for MWCNT/PDMS film containing 10 wt% MWCNTs with different strains collected at 1 V s−1. (e) Photographs of the MWCNT/PDMS film containing 10 wt% MWCNTs with different stains.
agitation for 30 min. Then, the previous mixture was decanted onto the distilled water. Due to that the mixture is insoluble with water and possesses lower density than that of water, it could float on the surface of water allowing silicone-elastomer base and curing agent reacting to form continuous networks. After the xylol had volatilized completely, a flat and uniform black film of MWCNT/PDMS with a thickness of about 300 um was obtained (For more detailed information about the manufacturing process please see Experimental section.). The MWCNT/PDMS film could be easily scaled up by employing a water surface with a larger area when the ratio of mixture to the water surface area remains the same. For example, a size of 30 cm × 40 cm was readily obtained as shown in Figure S1. Scanning electron microscopy (SEM) images reveal that the MWCNT/PDMS film containing 10% MWCNTs have a relatively rough surface (Figure 1b), which is valuable for the growth of electrochemically active materials. For an ideal stretchable substrate acting as current collector and mechanical support, it should have high conductivity and good mechanical properties. The conductivity and mechanical properties of the MWCNT/PDMS film could be optimized by adjusting the MWCNTs content. Significantly, the conductivity of the MWCNT/PDMS film dramatically increases from 0.55 to 4.19 S cm−1 with the increased MWCNTs content from 3 to 10 wt% (Figure S2a). However, meanwhile, the Young modulus and tensile strength of MWCNT/PDMS film increase (Figure S2b), and it is found that the film is hard to be continuous when the content of MWCNTs is over than 10%. Therefore, we chose 10% MWCNT/PDMS film as stretchable conductive substrate in terms of its relatively high conductivity (4.19 S cm−1) and good mechanical properties such as 113% elongation at break that meet the need of the substrate for stretchable SCs electrode. To further evaluate the feasibility of the 10% MWCNT/PDMS film as stretchable substrate, we studied the stress-recovery ability of the 10% MWCNT/PDMS film by extending it with different strains (10, 20, 40, 60, 80 and 100%), and followed with the length recovery after each extension. Due
2
wileyonlinelibrary.com
to molecular chain of PDMS and curling naturally WMCNTs could disperse in xylol well, the cross-linking reaction of PDMS occurred without impressed pressure, leading to a loose and porous MWCNT/PDMS structure. Hence, along with the cycles of stretching-releasing-stretching process increasing, the tensile strength and permanent deformation are both obviously increased, which can be attributed to the strain-induced orientation of PDMS together with MWCNTs (Figure 1c). However, the accumulated losing in stress-recovery of the 10% MWCNT/ PDMS film was only about 8% after repeatedly stretching and releasing and stretching to 60%, which means a largest strain of 60% can be applied to 10% MWCNT/PDMS film with almost no structural destruction. Moreover, the 10% MWCNT/PDMS film holds great potentials as stretchable conductive substrate. In the interior of CNT/PDMS film, numerous carbon nanotubes are tangled and form 3D-like net confirming efficient electron transport through the film. When strain was applied to the film, this unique structure is hard to be breaked. Therefore, the change of linear sweep voltammograms (LSV) curves collected for 10% MWCNT/PDMS film under different reversible stretch is subtle, even at a large applied strain of 50% (Figure 1d and 1e), indicating its excellent mechanical and conductive stability. All these results fully validate that our 10% MWCNT/PDMS film is a promising stretchable and conductive substrate for SCs. The high conductivity and rough surface of the prepared MWCNT/PDMS film enable it to serve as an excellent conductive substrate for electrochemically active electrode materials to form stretchable electrodes. For example, PANI nanofibers with outstanding electrochemical properties were readily grown on the MWCNT/PDMS film by a simple electrodeposition method in a solution of 0.1 M aniline and 1 M H2SO4 (Details please see Experimental section, the optimization of PANI mass loading is detailedly described in Figure S3). SEM images show these PANI nanofibers have a diameter of around 100–200 nm and formed a network structure with interconnected macroand meso-pores (Figure 2a). Transmission electron microscopy
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201401196
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 2. (a) SEM and (b) TEM images of PANI/MWCNT/PDMS electrode. Inset in Figure 2b is the corresponding SAED pattern. (c) CV curves of the MWCNT/PDMS and PANI/MWCNT/PDMS electrodes collected at 20 mV s−1. (d) Areal and specific capacitance as a function of the scan rate of PANI/MWCNT/PDMS electrode.
(TEM) and selected-area electron diffraction (SAED) analyses indicate the prepared PANI nanofibers have rough surfaces and amorphous nature (Figure 2b). Raman analysis confirms the presence of PANI on the MWCNT/PDMS substrate (Figure S4). And there is almost no change for the conductivity of the PANI/MWCNT/PDMS electrode with different strain applied (Figure S5). As shown in Figure 2c, the MWCNT/ PDMS film has a very small contribution to PANI/MWCNT/ PDMS electrode, and the rectangular shape and large current density of the CV curve clearly indicates the good pseudocapacitive behavior of PANI nanofibers. The redox peaks are not clear, which can be attributed to the diffusion of species is insufficient to the electrode surface at such high scan rate. CV curves of PANI/MWCNT/PDMS electrode at various scan rates are collected in Figure S6a. All curves exhibit approximately rectangle-like shape and unchanged with the increased scan rate from 5 to 100 mV s−1, further revealing the ideal capacitive behavior and fast charge/discharge property of PANI/MWCNT/ PDMS electrode. Significantly, as shown in Figure 2d, the PANI/MWCNT/PDMS electrode achieved remarkable areal and specific capacitance of 481 mF cm−2 and 1024 F g−1 at 5 mVs−1, which are substantially higher than those of recently reported PANI based electrode, such as PANI nanotubes (860 F g−1),[40] RGO/PANI (553 F g−1),[41] SLS/MWCNTs/PANI (401 F g−1)[42] and RGO/CNT/PANI (747 F g−1).[43] The approximately linear and symmetrical charge/discharge curve again confirms that the PANI/MWCNT/PDMS electrode has good coulombic efficiency and excellent reversibility (Figure S6b). In addition, the PANI/MWCNT/PDMS electrode has a good cycling stability with only 6% of its capacitance decreased after 2000 cycles (Figure S6c), which is fairly good value for the conducting polymer electrodes.[40–45] The superior capacitive behavior of
Adv. Mater. 2014, DOI: 10.1002/adma.201401196
the PANI/CNT/PDMS electrodes can be attributed to (1) the high conductivity of the CNT/PDMS film enables efficient electron transport and accessible diffusion of the electrolyte; (2) the direct connection between electrochemically active PANI nanofibers with CNT/PDMS film can effectively facilitate the interfacial charge transfer; (3) the unique porous architecture composed of numerous 1D PANI nanofibers not only significantly increases the accessible surface area as well as active sites for redox reaction, but also promotes the fast intercalation and deintercalation of ions. These results convincingly show that the MWCNT/PDMS film is a good support for constructing highperformance SC electrodes. To examine the practical application of the MWCNT/PDMS film as conductive substrate for stretchable SCs, a solid-state stretchable symmetric SC device based on the prepared PANI/ MWCNT/PDMS electrodes (denoted as PANI-SSC) was fabricated. As schematically represented in Figure 3a, the stretchable SC device was assembled by sandwiching two identical PANI/ MWCNT/PDMS electrodes with an elastomeric polyurethane textile separator and PVA/H2SO4 gel electrolyte, and followed by encapsulation using PDMS (Details please see our experimental section). The whole PANI-SSC device can be stretched as an integrated unit, and an as-fabricated PANI-SSC device is inserted in Figure 3a. Electrochemical performance of the PANI-SSC device was firstly conducted without strain applied. Figure 3b shows the CV curves of the PANI-SSC device collected at different scan rates. All the curves present essentially similar and symmetric shape as the scan rate increases from 5 to 100 mV s−1, suggesting the good capacitive behavior of the device. Variations in the volumetric capacitance (based on the volume of the entire device) and specific capacitance (based on the total mass of PANI) of the PANI-SSC device as a function of the scan
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
3
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 3. (a) A schematic diagram of stretchable PANI-SSC. Inset: Digital photograph of a PANI-SSC device. (b) CV curves of the PANI-SSC device collected at various scan rate. (c) Volumetric capacitance and specific capacitance calculated for the PANI-SSC device based on (b) as a function of the scan rate. (d) Galvanostatic charge/discharge curves collected at different current densities for the PANI-SSC device. (e) CV curves and corresponding volumetric capacitances of the PANI-SSC device with different strain collected at 10 mV s−1. (f) Cycling performance of the PANI-SSC device collected at 10 mV s−1 under dynamic stretching and releasing condition.
rate are displayed in Figure 3c. Significantly, the device was able to deliver a highest volumetric capacitance of 2.1 F cm−3 and a highest specific capacitance of 159 F g−1 at 5 mV s−1. To the best of our knowledge, these values are the highest among those reported stretchable SCs[21–28] and substantially higher than most of symmetric SSCs.[32–39] Additionally the close to triangle shape of charge/discharge curves further confirmed its high Coulombic effciency, excellent reversibility, and good charge propagation across the two electrodes (Figure 3d). In order to highlight the stable electrochemical behavior of our as-prepared PANI-SSC device under stretch condition, CV studies were performed under both statically fixed stretching state and dynamic stretching/releasing state. Particularly worth mentioning is that the largest strain applied reached 50% due to that the strong bonding between PANI nanofibers and CNT/PDMS film enables that the electrode could reserve the nature of its stretchability. And this is much larger than other stretchable energy storage devices reported recently (always < 35%).[20–22,24,28] The inset of Figure 3e compares the CV curves of the PANI-SSC device upon application of strain between 0 and 50% at 10 mV s−1. Significantly, almost no difference was observed in these CV curves. The calculated volumetric capacitance based on these curves ranged between 1.37 and 1.39 F cm−3, which means less than 2% variation of volumetric capacitance under strain. In addition, the cyclic stability of the device was further tested at a constant stretching and releasing speed of 5% strain per second between 0 and 50% strain. As shown in Figure 3f, the volumetric capacitance of the device slightly altered with only 4.4% decrease of capacitance after 500 cycles, which accords with the long-term cycling test conducted without strain applied (Figure S7). The loss of the capacitance may be ascribed to the structural broken and the deterioration of chemical properties for PANI, which could be further improved by optimizing the structure, coating a thin layer of carbon on the PANI surface and so on. Overall, the PANI-SSC device exhibited stable electrochemical performance under both static and dynamic stretching conditions. The stable
4
wileyonlinelibrary.com
electrochemical performance under strech condition of our asfabricated PANI-SSC device is mainly due to (1) these crooked PANI nanofibers formed a network structure with interconnected macro- and meso-pores, which has a good strain accommodation can endure a certain degree of strain; (2) the strong bonding between electrochemically active PANI nanofibers and CNT/PDMS film through their intermolecular forces enables the PANI/CNT/PDMS electrode to reserve the natural stretchability of CNT/PDMS film without damage of electrochemical performance;[25,28] and (3) the PVA/H2SO4 polymer gel electrolyte functions as an elastic shell to retain the PANI nanofiber structure under stretching.[37] Figure 4 shows Ragone plots of the PANI-SSC device reported in this paper and some other recently reported SSCs for comparison. The PANI-SSC device achieved a remarkable volumetric energy density of 0.15 mWh cm−3 (11 Wh kg−1) at current density of 2 mA cm−2. This is a very high energy density
Figure 4. Ragone plots of the PANI-SSC device. The values for other SC devices are added for comparison.[32–39]
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201401196
www.advmat.de www.MaterialsViews.com
COMMUNICATION
among the reported stretchable SCs.[21,22,25,26,28] Moreover, this present value is substantially higher than values reported for most of reported SSCs, such as TiO2@C-based SSCs (0.011 mWh cm−3),[32] TiO2@PPY-based SSCs (0.013 mWh cm−3),[33] SWCNTs-based SSCs (0.02 mWh cm−3),[39] ZnO@ MnO2-based SSCs (0.04 mWh cm−3),[34] TiN-based SSCs (0.05 mWh cm−3),[37] graphene-based SSCs (0.06 mWh cm−3),[38] VN-based SSCs (0.079 mWh cm−3),[35] and MnO2/carbon particle (CNP)-based SSCs (0.09 mWh cm−3).[36] Additionally, assembling asymmetric SCs or using ionic liquid can further enlarge the energy density of our device, which will be our next work. In summary, a kind of MWCNT/PDMS film with excellent conductivity and mechanical properties was developed using a facile and large-scale water surface assisted synthesis method. The as-prepared MWCNT/PDMS film containing 10% MWCNTs yields a remarkable conductivity of 4.19 S cm−1 and could endure up to 50% strain without damage of its conductivity and overall structure. When using MWCNT/PDMS film as a conductive support for electrochemically active PANI nanofibers, the PANI/MWCNT/PDMS electrode achieved a maximum areal capacitance of 481 mF cm−2 and specific capacitance of PANI/MWCNT/PDMS1023 F g−1 at a scan rate of 5 mV s−1. Such high performance of PANI/MWCNT/PDMS electrode makes them very promising in design and fabrication of stretchable SCs. A high-performance SSCs with high energy density and good mechanical property based on the PANI/ MWCNT/PDMS electrodes and PVA/H2SO4 gel electrolyte is also demonstrated. The as-fabricated PANI-SSC device not only delivered an outstanding energy density of 0.15 mWh cm−3 (11 Wh kg−1), but also showed good and stable capacitive behavior even under static and dynamic stretching conditions. All these findings are expected to enlighten a broad area of stretchable energy-storage devices.
Fabrication of PANI-SSCs: The PANI-SSCs were assembled by two pieces of identical PANI/MWCNT/PDMS electrodes (work area: 2 cm × 0.5 cm) with a separator (elastomeric polyurethane textile) sandwiched between. H2SO4/PVA gel was used as a solid electrolyte. H2SO4/PVA gel electrolyte was simply prepared as follows: 2 mL H2SO4 (98 wt%), 20 mL distilled water and 2g PVA were mixed together and heated at 80 °C for 30 min under vigorous stirring. All the electrodes and separator were soaked with the H2SO4/PVA solution and make them solidified at room temperature for about 3 h. Then they were assembled together and kept at 40 °C for another 3 h to remove excess water in the electrolyte. Finally, the whole device was encapsulated by PDMS, while care was taken to ensure both two end of the device was not included in PDMS in order to function as the electrical contact of the device with external circuit. The area and thickness of the fabricated SSCs are about 1 cm2 and 0.07 cm, respectively. Material Characterization and Electrochemical Measurement: For mechanical testing, all specimens were conditioned at 25 °C and 50% relative humidity for 5 days. Tensile properties were characterized using a Hounsfield THE 10K-S testing machine. The stretch of the MWCNT/ PDMS film and PANI-SSCs was conducted on a programmable custommade stretchable stage (AH-SS3). The conductivity of MWCNT/PDMS film was measured by a standard four-probe method using a physical property measurement system (ST2253). The morphology, structure and composition of electrode materials were characterized by field-emission SEM (FE-SEM, JSM-6630F), TEM (TEM, JEM2010-HR, 200 kV), and Raman spectroscopy (Renishaw inVia). All the electrochemical measurements were conducted with an electrochemical workstation (CHI 760E). The electrochemical studies of the individual electrode were performed in a conventional three-electrode cell with a Pt counter electrode and a saturated calomel reference electrode in 1 M H2SO4 solution.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements Experimental Section Preparation of MWCNT/PDMS Film: All reagents used were of analytical grade and used directly without any purification. MWCNTs (Nanocyl NC 7000) were used as electrical conductor. The two component silicone elastomer (Bluesil RTV 3040) was used as matrix, and the ratio of A : B equals to 10:1. Firstly, a mixture of xylol (500 mL), MWCNTs (3 mg, 5 mg, 8 mg, 10 mg corresponding to proportions: 3%MWCNTs/PDMS, 5%MWCNTs/PDMS, 8%MWCNTs/PDMS, and 10%MWCNTs/PDMS), and two component of the silicone-elastomer was mixed well with ultrasonic agitation for 30 min. and then poured onto the distilled water with a surface area of 1200 cm2. Due to that the mixture is insoluble with water and possesses lower density than that of water, the mixture floated on the surface of water allowing siliconeelastomer base and curing agent reacting to form continuous networks. After two days, polyaddition reaction of silicone-elastomer at room temperature had finished and the xylol had volatilized completely, a flat and uniform black film of MWCNT/PDMS with a thickness of about 300 um was directly peeled off from the water surface. Preparation of PANI/MWCNT/PDMS Electrode: PANI was electrodeposited on the MWCNT/PDMS film through cyclic voltammetry technique in a conventional three-electrode cell with a Pt counter electrode and a saturated calomel reference electrode using an electrochemical workstation (CHI 760E). The reaction was performed in a solution of 0.1 M aniline and 1 M H2SO4. And the detailed parameter of cyclic voltammetry method is: potential range: 0 – 1 V, scan rate 100 mV s−1, cycles: 50.
Adv. Mater. 2014, DOI: 10.1002/adma.201401196
We acknowledge the financial support by the Natural Science Foundations of China (21273290, 91323101, 51303215 and J1103305), the Natural Science Foundations of Guangdong Province (S2013030013474), and the Young Teacher Starting-up Research of Sun Yat-Sen University. Received: March 17, 2014 Revised: April 9, 2014 Published online:
[1] D. Y. Khang, H. Jiang, Y. Huang, J. A. Rogers, Science 2006, 311, 208. [2] H. L. Filiatrault, G. C. Porteous, R. S. Carmichael, G. J. Davidson, T. B. Carmichael, Adv. Mater. 2012, 24, 2673. [3] D. H. Kim, J. H. Ahn, W. M. Choi, H. S. Kim, T. H. Kim, J. Song, Y. Y. Huang, Z. Liu, C. Lu, J. A. Rogers, Science 2008, 320, 507. [4] P. Lee, J. Lee, H. Lee, J. Yeo, S. Hong, K. H. Nam, D. Lee, S. S. Lee, S. H. Ko, Adv. Mater. 2012, 24, 3326. [5] T. Sekitani, Y. Noguchi, K. Hata, T. Fukushima, T. Aida, T. Someya, Science 2008, 321, 1468. [6] K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J. H. Ahn, P. Kim, J. Y. Choi, B. H. Hong, Nature 2009, 457, 706. [7] Y. Zhu, F. Xu, Adv. Mater. 2012, 24, 1073. [8] Z. Yu, X. Niu, Z. Liu, Q. Pei, Adv. Mater. 2011, 23, 3989.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
5
www.advmat.de
COMMUNICATION
www.MaterialsViews.com [9] G. Shin, M. Y. Bae, H. J. Lee, S. K. Hong, C. H. Yoon, G. Zi, J. A. Rogers, J. S. Ha, ACS Nano 2011, 5, 10009. [10] S. Cheng, Z. Wu, Lab on a Chip 2010, 10, 3227. [11] G. Shin, C. H. Yoon, M. Y. Bae, Y. C. Kim, S. K. Hong, J. A. Rogers, J. S. Ha, Small 2011, 7, 1181. [12] S. C. Mannsfeld, B. C. Tee, R. M. Stoltenberg, C. V. Chen, S. Barman, B. V. Muir, A. N. Sokolov, C. Reese, Z. Bao, Nat. Mater. 2010, 9, 859. [13] T. Yamada, Y. Hayamizu, Y. Yamamoto, Y. Yomogida, A. IzadiNajafabadi, D. N. Futaba, K. Hata, Nat. Nanotech. 2011, 6, 296. [14] C. Yu, Z. Wang, H. Yu, H. Jiang, Appl. Phys. Lett. 2009, 95, 141912. [15] D. J. Lipomi, M. Vosgueritchian, B. C. Tee, S. L. Hellstrom, J. A. Lee, C. H. Fox, Z. Bao, Nat. Nanotech. 2011, 6, 788. [16] D. J. Lipomi, B. C. K. Tee, M. Vosgueritchian, Z. Bao, Adv. Mater. 2011, 23, 1771. [17] Y. Sun, W. M. Choi, H. Jiang, Y. Y. Huang, J. A. Rogers, Nat. Nanotech. 2006, 1, 201. [18] J. Yoon, A. J. Baca, S. I. Park, P. Elvikis, J. B. Geddes, L. Li 3rd, R. H. Kim, J. Xiao, S. Wang, T. H. Kim, M. J. Motala, B. Y. Ahn, E. B. Duoss, J. A. Lewis, R. G. Nuzzo, P. M. Ferreira, Y. Huang, A. Rockett, J. A. Rogers, Nat. Mater. 2008, 7, 907. [19] A. M. Gaikwad, A. M. Zamarayeva, J. Rousseau, H. Chu, I. Derin, D. A. Steingart, Adv. Mater. 2012, 24, 5071. [20] C. Wang, W. Zheng, Z. Yue, C. O. Too, G. G. Wallace, Adv. Mater. 2011, 23, 3580. [21] D. Kim, G. Shin, Y. J. Kang, W. Kim, J. S. Ha, ACS Nano 2013. [22] C. Yu, C. Masarapu, J. Rong, B. Wei, H. Jiang, Adv. Mater. 2009, 21, 4793. [23] L. Hu, M. Pasta, F. L. Mantia, L. Cui, S. Jeong, H. D. Deshazer, J. W. Choi, S. M. Han, Y. Cui, Nano Lett. 2010, 10, 708. [24] X. Li, T. Gu, B. Wei, Nano Lett. 2012, 12, 6366. [25] B. Yue, C. Wang, X. Ding, G. G. Wallace, Electrochim. Acta 2012, 68, 18. [26] Z. Niu, H. Dong, B. Zhu, J. Li, H. H. Hng, W. Zhou, X. Chen, S. Xie, Adv. Mater. 2013, 25, 1058.
6
wileyonlinelibrary.com
[27] Z. Yang, J. Deng, X. Chen, J. Ren, H. Peng, Angew. Chem. Int. Edit. 2013, 52, 13453. [28] C. Zhao, C. Wang, Z. Yue, K. Shu, G. G. Wallace, ACS Appl. Mater. Interfaces 2013, 5, 9008. [29] X. Xiao, X. Peng, H. Jin, T. Li, C. Zhang, B. Gao, B. Hu, K. Huo, J. Zhou, Adv. Mater. 2013, 25, 5091. [30] C. Wu, X. Lu, L. Peng, K. Xu, X. Peng, J. Huang, G. Yu, Y. Xie, Nat. Commun. 2013, 4, 2431. [31] J. Feng, X. Sun, C. Wu, L. Peng, C. Lin, S. Hu, J. Yang, Y. Xie, J. Am. Chem. Soc. 2011, 133, 17832. [32] W. C. Chen, T. C. Wen, H. S. Teng, Electrochim. Acta 2003, 48, 641. [33] V. Gupta, N. Miura, Electrochim. Acta 2006, 52, 1721. [34] W.-C. Chen, T.-C. Wen, J. Power Sources 2003, 117, 273. [35] H. Zhou, H. Chen, S. Luo, G. Lu, W. Wei, Y. Kuang, J. Solid State Electr. 2005, 9, 574. [36] L. Yuan, X. H. Lu, X. Xiao, T. Zhai, J. Dai, F. Zhang, B. Hu, X. Wang, L. Gong, J. Chen, C. Hu, Y. Tong, J. Zhou, Z. L. Wang, ACS Nano 2012, 6, 656. [37] X. Lu, G. Wang, T. Zhai, M. Yu, S. Xie, Y. Ling, C. Liang, Y. Tong, Y. Li, Nano Lett. 2012, 12, 5376. [38] F. Fusalba, P. Gouerec, D. Villers, D. Belanger, J. Electrochem. Soc. 2001, 148, A1. [39] V. Gupta, N. Miura, Mater. Lett. 2006, 60, 1466. [40] Z.-L. Wang, R. Guo, G.-R. Li, H.-L. Lu, Z.-Q. Liu, F.-M. Xiao, M. Zhang, Y.-X. Tong, J. Mater. Chem. 2012, 22, 2401. [41] W. Chen, R. B. Rakhi, H. N. Alshareef, Nanoscale 2013, 5, 4134. [42] M. Ammam, J. Fransaer, Chem. Commun. 2012, 48, 2036. [43] J. Yan, T. Wei, Z. Fan, W. Qian, M. Zhang, X. Shen, F. Wei, J. Power Sources 2010, 195, 3041. [44] K. Wang, Q. Meng, Y. Zhang, Z. Wei, M. Miao, Adv. Mater. 2013, 25, 1494. [45] L. Hu, J. Tu, S. Jiao, J. Hou, H. Zhu, D. J. Fray, Phys. Chem. Chem. Phys. 2012, 14, 15652.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201401196
COMMUNICATION
Water Surface Assisted Synthesis of Large-Scale Carbon Nanotube Film for High-Performance and Stretchable Supercapacitors Minghao Yu, Yangfan Zhang, Yinxiang Zeng, Muhammad-Sadeeq Balogun, Kancheng Mai, Zishou Zhang,* Xihong Lu,* and Yexiang Tong As an emerging field, stretchable electronics that can sustain large mechanical strain without degradation in their electronic performance have large potential to be widely applied in bioimplantable system, wearable system, wireless sensors and so on.[1–7] Over the past few years, various kinds of stretchable devices such as organic light-emitting diode devices,[8,9] radio frequency devices,[10] field effect transistors,[11] pressure and strain sensors,[12,13] temperature sensors,[14] and artificial skin sensors[15] have been developed. To power the stretchable electronics and achieve a fully power-independent and stretchable system, it is urged to explore new power source devices with high stretchability. In recent years, extensive efforts have been devoted to studying stretchable conversion or storage devices like solar cells,[16] photovoltaic cells,[17,18] Li-ion batteries[19,20] and supercapacitors (SCs).[21–28] Among these power source devices, SCs are perceived as one of the most promising energy storage devices due to their high power density, modest energy density, fast charge-discharge capability and long cycle life.[29–31] The key point for fabricating stretchable SCs is the design of stretchable SCs electrodes. Currently, the general strategy to fabricate a stretchable SC electrode is coating a thin layer of electrochemically active material on the surface of some stretchable substrates such like poly(dimethylsiloxane) (PDMS),[22,24,26] elastic rubber fiber,[27] nylon lycra fabric[25] and cotton sheet.[23] For example, Jiang et al.[22] combined a layer of purified carbon nanotubes (CNTs) macro film with pre-strained PDMS. After the prestrain released, the CNTs became periodically sinusoidal. These buckled macro films exhibited a high specific capacitance
M. H. Yu, Y. F. Zhang, Y. X. Zeng, M.-S. Balogun, Dr. Z. S. Zhang, Dr. X. H. Lu, Prof. Y. X. Tong MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry School of Chemistry and Chemical Engineering Sun Yat-Sen University Guangzhou, 510275, People’s Republic of China Fax: (+86)20 84112245 E-mail: [email protected]; [email protected] Y. F. Zhang, Prof. K. C. Mai, Dr. Z. S. Zhang MOE of Key Laboratory for Polymeric Composite and Functional Materials Key Laboratory for High Performance Polymer Based Composites of Guangdong Province Sun Yat-Sen University Guangzhou, 510275, People’s Republic of China
DOI: 10.1002/adma.201401196
Adv. Mater. 2014, DOI: 10.1002/adma.201401196
of 52 F g−1 with 30% strain applied at a current density of 1 A g−1, while 54 F g−1 without stain. The elastic deformation of the fabricated supercapacitors always deeply relied on the stretchability of the substrates, and the non-conductive property of these substrates requires that the applied electrochemically active materials must have a high conductivity. As a result, the current electrode materials for stretchable supercapacitors are limited. Additionally, recent effort has shown that the use of Au interlayer between the poly(styrene-block-isobutylene-block-styrene) (SIBS) and PANI could significantly improve the conductivity and electrochemical performances. Wallace et al. reported a kind of stretchable SC electrodes by firstly deposited a layer of Au on SIBS substrate and then coated PANI onto Au/SIBS surface.[28] The realization of low-cost, stretchable SC device with high energy and power density is still highly desirable. In this work, we present a simple and large-scale water surface assisted synthesis of multiwall carbon nanotube (MWCNT) based stretchable film. Taking advantage of extremely flat surface of water, uniform films were formed which combined the excellent conductivity of MWCNTs with the high stretchability of PDMS. The MWCNT/PDMS film containing 10% MWCNTs exhibited a high conductivity of 4.19 S cm−1 and could be stretched to a high strain of 50% without damaging its conductivity and structure. In addition, the size and shape of the film can be easily tuned by changing the area and shape of water surface. More importantly, the prepared MWCNT/PDMS film is an excellent conductive and mechanical substrate to support electrochemically active materials. When a layer of polyaniline (PANI) nanofibers was deposited on the MWCNT/PDMS film, the PANI/MWCNT/PDMS film electrode exhibited a benchmark specific capacitance of 1023 F g−1 and areal capacitance of 481 mF cm−2 at a scan rate of 5 mV s−1. A solid-state symmetric supercapacitor (SSC) device with remarkable stretchability and outstanding electrochemical properties was also assembled for demo by the PANI/MWCNT/PDMS films as stretchable electrodes. The as-fabricated SSCs possessed good and stable capacitive behavior even under dynamic stretching conditions, with more than 95% capacitance retention after 500 cycles during the dynamic stretching and releasing process. Moreover, this device was able to deliver a maximum energy density of 0.15 mWh cm−3 (11 Wh kg−1), which is considerably higher than most of SSCs reported recently.[32–39] Figure 1a illustrates the schematic diagram of the manufacturing processes of the stretchable MWCNT films. Typically, xylol, MWCNTs (proportions: 3, 5, 8, and 10 wt%), silicone-elastomer base and curing agent were mixed well with ultrasonic
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 1. (a) Schematic diagram of fabricating stretchable MWCNT/PDMS film. (b) SEM images of the as-prepared MWCNT/PDMS film containing 10 wt% MWCNTs. (c) Stress-recovery curves of MWCNT/PDMS film containing 10% MWCNTs. (d) linear sweep voltammograms (LSV) curves for MWCNT/PDMS film containing 10 wt% MWCNTs with different strains collected at 1 V s−1. (e) Photographs of the MWCNT/PDMS film containing 10 wt% MWCNTs with different stains.
agitation for 30 min. Then, the previous mixture was decanted onto the distilled water. Due to that the mixture is insoluble with water and possesses lower density than that of water, it could float on the surface of water allowing silicone-elastomer base and curing agent reacting to form continuous networks. After the xylol had volatilized completely, a flat and uniform black film of MWCNT/PDMS with a thickness of about 300 um was obtained (For more detailed information about the manufacturing process please see Experimental section.). The MWCNT/PDMS film could be easily scaled up by employing a water surface with a larger area when the ratio of mixture to the water surface area remains the same. For example, a size of 30 cm × 40 cm was readily obtained as shown in Figure S1. Scanning electron microscopy (SEM) images reveal that the MWCNT/PDMS film containing 10% MWCNTs have a relatively rough surface (Figure 1b), which is valuable for the growth of electrochemically active materials. For an ideal stretchable substrate acting as current collector and mechanical support, it should have high conductivity and good mechanical properties. The conductivity and mechanical properties of the MWCNT/PDMS film could be optimized by adjusting the MWCNTs content. Significantly, the conductivity of the MWCNT/PDMS film dramatically increases from 0.55 to 4.19 S cm−1 with the increased MWCNTs content from 3 to 10 wt% (Figure S2a). However, meanwhile, the Young modulus and tensile strength of MWCNT/PDMS film increase (Figure S2b), and it is found that the film is hard to be continuous when the content of MWCNTs is over than 10%. Therefore, we chose 10% MWCNT/PDMS film as stretchable conductive substrate in terms of its relatively high conductivity (4.19 S cm−1) and good mechanical properties such as 113% elongation at break that meet the need of the substrate for stretchable SCs electrode. To further evaluate the feasibility of the 10% MWCNT/PDMS film as stretchable substrate, we studied the stress-recovery ability of the 10% MWCNT/PDMS film by extending it with different strains (10, 20, 40, 60, 80 and 100%), and followed with the length recovery after each extension. Due
2
wileyonlinelibrary.com
to molecular chain of PDMS and curling naturally WMCNTs could disperse in xylol well, the cross-linking reaction of PDMS occurred without impressed pressure, leading to a loose and porous MWCNT/PDMS structure. Hence, along with the cycles of stretching-releasing-stretching process increasing, the tensile strength and permanent deformation are both obviously increased, which can be attributed to the strain-induced orientation of PDMS together with MWCNTs (Figure 1c). However, the accumulated losing in stress-recovery of the 10% MWCNT/ PDMS film was only about 8% after repeatedly stretching and releasing and stretching to 60%, which means a largest strain of 60% can be applied to 10% MWCNT/PDMS film with almost no structural destruction. Moreover, the 10% MWCNT/PDMS film holds great potentials as stretchable conductive substrate. In the interior of CNT/PDMS film, numerous carbon nanotubes are tangled and form 3D-like net confirming efficient electron transport through the film. When strain was applied to the film, this unique structure is hard to be breaked. Therefore, the change of linear sweep voltammograms (LSV) curves collected for 10% MWCNT/PDMS film under different reversible stretch is subtle, even at a large applied strain of 50% (Figure 1d and 1e), indicating its excellent mechanical and conductive stability. All these results fully validate that our 10% MWCNT/PDMS film is a promising stretchable and conductive substrate for SCs. The high conductivity and rough surface of the prepared MWCNT/PDMS film enable it to serve as an excellent conductive substrate for electrochemically active electrode materials to form stretchable electrodes. For example, PANI nanofibers with outstanding electrochemical properties were readily grown on the MWCNT/PDMS film by a simple electrodeposition method in a solution of 0.1 M aniline and 1 M H2SO4 (Details please see Experimental section, the optimization of PANI mass loading is detailedly described in Figure S3). SEM images show these PANI nanofibers have a diameter of around 100–200 nm and formed a network structure with interconnected macroand meso-pores (Figure 2a). Transmission electron microscopy
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201401196
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 2. (a) SEM and (b) TEM images of PANI/MWCNT/PDMS electrode. Inset in Figure 2b is the corresponding SAED pattern. (c) CV curves of the MWCNT/PDMS and PANI/MWCNT/PDMS electrodes collected at 20 mV s−1. (d) Areal and specific capacitance as a function of the scan rate of PANI/MWCNT/PDMS electrode.
(TEM) and selected-area electron diffraction (SAED) analyses indicate the prepared PANI nanofibers have rough surfaces and amorphous nature (Figure 2b). Raman analysis confirms the presence of PANI on the MWCNT/PDMS substrate (Figure S4). And there is almost no change for the conductivity of the PANI/MWCNT/PDMS electrode with different strain applied (Figure S5). As shown in Figure 2c, the MWCNT/ PDMS film has a very small contribution to PANI/MWCNT/ PDMS electrode, and the rectangular shape and large current density of the CV curve clearly indicates the good pseudocapacitive behavior of PANI nanofibers. The redox peaks are not clear, which can be attributed to the diffusion of species is insufficient to the electrode surface at such high scan rate. CV curves of PANI/MWCNT/PDMS electrode at various scan rates are collected in Figure S6a. All curves exhibit approximately rectangle-like shape and unchanged with the increased scan rate from 5 to 100 mV s−1, further revealing the ideal capacitive behavior and fast charge/discharge property of PANI/MWCNT/ PDMS electrode. Significantly, as shown in Figure 2d, the PANI/MWCNT/PDMS electrode achieved remarkable areal and specific capacitance of 481 mF cm−2 and 1024 F g−1 at 5 mVs−1, which are substantially higher than those of recently reported PANI based electrode, such as PANI nanotubes (860 F g−1),[40] RGO/PANI (553 F g−1),[41] SLS/MWCNTs/PANI (401 F g−1)[42] and RGO/CNT/PANI (747 F g−1).[43] The approximately linear and symmetrical charge/discharge curve again confirms that the PANI/MWCNT/PDMS electrode has good coulombic efficiency and excellent reversibility (Figure S6b). In addition, the PANI/MWCNT/PDMS electrode has a good cycling stability with only 6% of its capacitance decreased after 2000 cycles (Figure S6c), which is fairly good value for the conducting polymer electrodes.[40–45] The superior capacitive behavior of
Adv. Mater. 2014, DOI: 10.1002/adma.201401196
the PANI/CNT/PDMS electrodes can be attributed to (1) the high conductivity of the CNT/PDMS film enables efficient electron transport and accessible diffusion of the electrolyte; (2) the direct connection between electrochemically active PANI nanofibers with CNT/PDMS film can effectively facilitate the interfacial charge transfer; (3) the unique porous architecture composed of numerous 1D PANI nanofibers not only significantly increases the accessible surface area as well as active sites for redox reaction, but also promotes the fast intercalation and deintercalation of ions. These results convincingly show that the MWCNT/PDMS film is a good support for constructing highperformance SC electrodes. To examine the practical application of the MWCNT/PDMS film as conductive substrate for stretchable SCs, a solid-state stretchable symmetric SC device based on the prepared PANI/ MWCNT/PDMS electrodes (denoted as PANI-SSC) was fabricated. As schematically represented in Figure 3a, the stretchable SC device was assembled by sandwiching two identical PANI/ MWCNT/PDMS electrodes with an elastomeric polyurethane textile separator and PVA/H2SO4 gel electrolyte, and followed by encapsulation using PDMS (Details please see our experimental section). The whole PANI-SSC device can be stretched as an integrated unit, and an as-fabricated PANI-SSC device is inserted in Figure 3a. Electrochemical performance of the PANI-SSC device was firstly conducted without strain applied. Figure 3b shows the CV curves of the PANI-SSC device collected at different scan rates. All the curves present essentially similar and symmetric shape as the scan rate increases from 5 to 100 mV s−1, suggesting the good capacitive behavior of the device. Variations in the volumetric capacitance (based on the volume of the entire device) and specific capacitance (based on the total mass of PANI) of the PANI-SSC device as a function of the scan
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
3
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 3. (a) A schematic diagram of stretchable PANI-SSC. Inset: Digital photograph of a PANI-SSC device. (b) CV curves of the PANI-SSC device collected at various scan rate. (c) Volumetric capacitance and specific capacitance calculated for the PANI-SSC device based on (b) as a function of the scan rate. (d) Galvanostatic charge/discharge curves collected at different current densities for the PANI-SSC device. (e) CV curves and corresponding volumetric capacitances of the PANI-SSC device with different strain collected at 10 mV s−1. (f) Cycling performance of the PANI-SSC device collected at 10 mV s−1 under dynamic stretching and releasing condition.
rate are displayed in Figure 3c. Significantly, the device was able to deliver a highest volumetric capacitance of 2.1 F cm−3 and a highest specific capacitance of 159 F g−1 at 5 mV s−1. To the best of our knowledge, these values are the highest among those reported stretchable SCs[21–28] and substantially higher than most of symmetric SSCs.[32–39] Additionally the close to triangle shape of charge/discharge curves further confirmed its high Coulombic effciency, excellent reversibility, and good charge propagation across the two electrodes (Figure 3d). In order to highlight the stable electrochemical behavior of our as-prepared PANI-SSC device under stretch condition, CV studies were performed under both statically fixed stretching state and dynamic stretching/releasing state. Particularly worth mentioning is that the largest strain applied reached 50% due to that the strong bonding between PANI nanofibers and CNT/PDMS film enables that the electrode could reserve the nature of its stretchability. And this is much larger than other stretchable energy storage devices reported recently (always < 35%).[20–22,24,28] The inset of Figure 3e compares the CV curves of the PANI-SSC device upon application of strain between 0 and 50% at 10 mV s−1. Significantly, almost no difference was observed in these CV curves. The calculated volumetric capacitance based on these curves ranged between 1.37 and 1.39 F cm−3, which means less than 2% variation of volumetric capacitance under strain. In addition, the cyclic stability of the device was further tested at a constant stretching and releasing speed of 5% strain per second between 0 and 50% strain. As shown in Figure 3f, the volumetric capacitance of the device slightly altered with only 4.4% decrease of capacitance after 500 cycles, which accords with the long-term cycling test conducted without strain applied (Figure S7). The loss of the capacitance may be ascribed to the structural broken and the deterioration of chemical properties for PANI, which could be further improved by optimizing the structure, coating a thin layer of carbon on the PANI surface and so on. Overall, the PANI-SSC device exhibited stable electrochemical performance under both static and dynamic stretching conditions. The stable
4
wileyonlinelibrary.com
electrochemical performance under strech condition of our asfabricated PANI-SSC device is mainly due to (1) these crooked PANI nanofibers formed a network structure with interconnected macro- and meso-pores, which has a good strain accommodation can endure a certain degree of strain; (2) the strong bonding between electrochemically active PANI nanofibers and CNT/PDMS film through their intermolecular forces enables the PANI/CNT/PDMS electrode to reserve the natural stretchability of CNT/PDMS film without damage of electrochemical performance;[25,28] and (3) the PVA/H2SO4 polymer gel electrolyte functions as an elastic shell to retain the PANI nanofiber structure under stretching.[37] Figure 4 shows Ragone plots of the PANI-SSC device reported in this paper and some other recently reported SSCs for comparison. The PANI-SSC device achieved a remarkable volumetric energy density of 0.15 mWh cm−3 (11 Wh kg−1) at current density of 2 mA cm−2. This is a very high energy density
Figure 4. Ragone plots of the PANI-SSC device. The values for other SC devices are added for comparison.[32–39]
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201401196
www.advmat.de www.MaterialsViews.com
COMMUNICATION
among the reported stretchable SCs.[21,22,25,26,28] Moreover, this present value is substantially higher than values reported for most of reported SSCs, such as TiO2@C-based SSCs (0.011 mWh cm−3),[32] TiO2@PPY-based SSCs (0.013 mWh cm−3),[33] SWCNTs-based SSCs (0.02 mWh cm−3),[39] ZnO@ MnO2-based SSCs (0.04 mWh cm−3),[34] TiN-based SSCs (0.05 mWh cm−3),[37] graphene-based SSCs (0.06 mWh cm−3),[38] VN-based SSCs (0.079 mWh cm−3),[35] and MnO2/carbon particle (CNP)-based SSCs (0.09 mWh cm−3).[36] Additionally, assembling asymmetric SCs or using ionic liquid can further enlarge the energy density of our device, which will be our next work. In summary, a kind of MWCNT/PDMS film with excellent conductivity and mechanical properties was developed using a facile and large-scale water surface assisted synthesis method. The as-prepared MWCNT/PDMS film containing 10% MWCNTs yields a remarkable conductivity of 4.19 S cm−1 and could endure up to 50% strain without damage of its conductivity and overall structure. When using MWCNT/PDMS film as a conductive support for electrochemically active PANI nanofibers, the PANI/MWCNT/PDMS electrode achieved a maximum areal capacitance of 481 mF cm−2 and specific capacitance of PANI/MWCNT/PDMS1023 F g−1 at a scan rate of 5 mV s−1. Such high performance of PANI/MWCNT/PDMS electrode makes them very promising in design and fabrication of stretchable SCs. A high-performance SSCs with high energy density and good mechanical property based on the PANI/ MWCNT/PDMS electrodes and PVA/H2SO4 gel electrolyte is also demonstrated. The as-fabricated PANI-SSC device not only delivered an outstanding energy density of 0.15 mWh cm−3 (11 Wh kg−1), but also showed good and stable capacitive behavior even under static and dynamic stretching conditions. All these findings are expected to enlighten a broad area of stretchable energy-storage devices.
Fabrication of PANI-SSCs: The PANI-SSCs were assembled by two pieces of identical PANI/MWCNT/PDMS electrodes (work area: 2 cm × 0.5 cm) with a separator (elastomeric polyurethane textile) sandwiched between. H2SO4/PVA gel was used as a solid electrolyte. H2SO4/PVA gel electrolyte was simply prepared as follows: 2 mL H2SO4 (98 wt%), 20 mL distilled water and 2g PVA were mixed together and heated at 80 °C for 30 min under vigorous stirring. All the electrodes and separator were soaked with the H2SO4/PVA solution and make them solidified at room temperature for about 3 h. Then they were assembled together and kept at 40 °C for another 3 h to remove excess water in the electrolyte. Finally, the whole device was encapsulated by PDMS, while care was taken to ensure both two end of the device was not included in PDMS in order to function as the electrical contact of the device with external circuit. The area and thickness of the fabricated SSCs are about 1 cm2 and 0.07 cm, respectively. Material Characterization and Electrochemical Measurement: For mechanical testing, all specimens were conditioned at 25 °C and 50% relative humidity for 5 days. Tensile properties were characterized using a Hounsfield THE 10K-S testing machine. The stretch of the MWCNT/ PDMS film and PANI-SSCs was conducted on a programmable custommade stretchable stage (AH-SS3). The conductivity of MWCNT/PDMS film was measured by a standard four-probe method using a physical property measurement system (ST2253). The morphology, structure and composition of electrode materials were characterized by field-emission SEM (FE-SEM, JSM-6630F), TEM (TEM, JEM2010-HR, 200 kV), and Raman spectroscopy (Renishaw inVia). All the electrochemical measurements were conducted with an electrochemical workstation (CHI 760E). The electrochemical studies of the individual electrode were performed in a conventional three-electrode cell with a Pt counter electrode and a saturated calomel reference electrode in 1 M H2SO4 solution.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements Experimental Section Preparation of MWCNT/PDMS Film: All reagents used were of analytical grade and used directly without any purification. MWCNTs (Nanocyl NC 7000) were used as electrical conductor. The two component silicone elastomer (Bluesil RTV 3040) was used as matrix, and the ratio of A : B equals to 10:1. Firstly, a mixture of xylol (500 mL), MWCNTs (3 mg, 5 mg, 8 mg, 10 mg corresponding to proportions: 3%MWCNTs/PDMS, 5%MWCNTs/PDMS, 8%MWCNTs/PDMS, and 10%MWCNTs/PDMS), and two component of the silicone-elastomer was mixed well with ultrasonic agitation for 30 min. and then poured onto the distilled water with a surface area of 1200 cm2. Due to that the mixture is insoluble with water and possesses lower density than that of water, the mixture floated on the surface of water allowing siliconeelastomer base and curing agent reacting to form continuous networks. After two days, polyaddition reaction of silicone-elastomer at room temperature had finished and the xylol had volatilized completely, a flat and uniform black film of MWCNT/PDMS with a thickness of about 300 um was directly peeled off from the water surface. Preparation of PANI/MWCNT/PDMS Electrode: PANI was electrodeposited on the MWCNT/PDMS film through cyclic voltammetry technique in a conventional three-electrode cell with a Pt counter electrode and a saturated calomel reference electrode using an electrochemical workstation (CHI 760E). The reaction was performed in a solution of 0.1 M aniline and 1 M H2SO4. And the detailed parameter of cyclic voltammetry method is: potential range: 0 – 1 V, scan rate 100 mV s−1, cycles: 50.
Adv. Mater. 2014, DOI: 10.1002/adma.201401196
We acknowledge the financial support by the Natural Science Foundations of China (21273290, 91323101, 51303215 and J1103305), the Natural Science Foundations of Guangdong Province (S2013030013474), and the Young Teacher Starting-up Research of Sun Yat-Sen University. Received: March 17, 2014 Revised: April 9, 2014 Published online:
[1] D. Y. Khang, H. Jiang, Y. Huang, J. A. Rogers, Science 2006, 311, 208. [2] H. L. Filiatrault, G. C. Porteous, R. S. Carmichael, G. J. Davidson, T. B. Carmichael, Adv. Mater. 2012, 24, 2673. [3] D. H. Kim, J. H. Ahn, W. M. Choi, H. S. Kim, T. H. Kim, J. Song, Y. Y. Huang, Z. Liu, C. Lu, J. A. Rogers, Science 2008, 320, 507. [4] P. Lee, J. Lee, H. Lee, J. Yeo, S. Hong, K. H. Nam, D. Lee, S. S. Lee, S. H. Ko, Adv. Mater. 2012, 24, 3326. [5] T. Sekitani, Y. Noguchi, K. Hata, T. Fukushima, T. Aida, T. Someya, Science 2008, 321, 1468. [6] K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J. H. Ahn, P. Kim, J. Y. Choi, B. H. Hong, Nature 2009, 457, 706. [7] Y. Zhu, F. Xu, Adv. Mater. 2012, 24, 1073. [8] Z. Yu, X. Niu, Z. Liu, Q. Pei, Adv. Mater. 2011, 23, 3989.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
5
www.advmat.de
COMMUNICATION
www.MaterialsViews.com [9] G. Shin, M. Y. Bae, H. J. Lee, S. K. Hong, C. H. Yoon, G. Zi, J. A. Rogers, J. S. Ha, ACS Nano 2011, 5, 10009. [10] S. Cheng, Z. Wu, Lab on a Chip 2010, 10, 3227. [11] G. Shin, C. H. Yoon, M. Y. Bae, Y. C. Kim, S. K. Hong, J. A. Rogers, J. S. Ha, Small 2011, 7, 1181. [12] S. C. Mannsfeld, B. C. Tee, R. M. Stoltenberg, C. V. Chen, S. Barman, B. V. Muir, A. N. Sokolov, C. Reese, Z. Bao, Nat. Mater. 2010, 9, 859. [13] T. Yamada, Y. Hayamizu, Y. Yamamoto, Y. Yomogida, A. IzadiNajafabadi, D. N. Futaba, K. Hata, Nat. Nanotech. 2011, 6, 296. [14] C. Yu, Z. Wang, H. Yu, H. Jiang, Appl. Phys. Lett. 2009, 95, 141912. [15] D. J. Lipomi, M. Vosgueritchian, B. C. Tee, S. L. Hellstrom, J. A. Lee, C. H. Fox, Z. Bao, Nat. Nanotech. 2011, 6, 788. [16] D. J. Lipomi, B. C. K. Tee, M. Vosgueritchian, Z. Bao, Adv. Mater. 2011, 23, 1771. [17] Y. Sun, W. M. Choi, H. Jiang, Y. Y. Huang, J. A. Rogers, Nat. Nanotech. 2006, 1, 201. [18] J. Yoon, A. J. Baca, S. I. Park, P. Elvikis, J. B. Geddes, L. Li 3rd, R. H. Kim, J. Xiao, S. Wang, T. H. Kim, M. J. Motala, B. Y. Ahn, E. B. Duoss, J. A. Lewis, R. G. Nuzzo, P. M. Ferreira, Y. Huang, A. Rockett, J. A. Rogers, Nat. Mater. 2008, 7, 907. [19] A. M. Gaikwad, A. M. Zamarayeva, J. Rousseau, H. Chu, I. Derin, D. A. Steingart, Adv. Mater. 2012, 24, 5071. [20] C. Wang, W. Zheng, Z. Yue, C. O. Too, G. G. Wallace, Adv. Mater. 2011, 23, 3580. [21] D. Kim, G. Shin, Y. J. Kang, W. Kim, J. S. Ha, ACS Nano 2013. [22] C. Yu, C. Masarapu, J. Rong, B. Wei, H. Jiang, Adv. Mater. 2009, 21, 4793. [23] L. Hu, M. Pasta, F. L. Mantia, L. Cui, S. Jeong, H. D. Deshazer, J. W. Choi, S. M. Han, Y. Cui, Nano Lett. 2010, 10, 708. [24] X. Li, T. Gu, B. Wei, Nano Lett. 2012, 12, 6366. [25] B. Yue, C. Wang, X. Ding, G. G. Wallace, Electrochim. Acta 2012, 68, 18. [26] Z. Niu, H. Dong, B. Zhu, J. Li, H. H. Hng, W. Zhou, X. Chen, S. Xie, Adv. Mater. 2013, 25, 1058.
6
wileyonlinelibrary.com
[27] Z. Yang, J. Deng, X. Chen, J. Ren, H. Peng, Angew. Chem. Int. Edit. 2013, 52, 13453. [28] C. Zhao, C. Wang, Z. Yue, K. Shu, G. G. Wallace, ACS Appl. Mater. Interfaces 2013, 5, 9008. [29] X. Xiao, X. Peng, H. Jin, T. Li, C. Zhang, B. Gao, B. Hu, K. Huo, J. Zhou, Adv. Mater. 2013, 25, 5091. [30] C. Wu, X. Lu, L. Peng, K. Xu, X. Peng, J. Huang, G. Yu, Y. Xie, Nat. Commun. 2013, 4, 2431. [31] J. Feng, X. Sun, C. Wu, L. Peng, C. Lin, S. Hu, J. Yang, Y. Xie, J. Am. Chem. Soc. 2011, 133, 17832. [32] W. C. Chen, T. C. Wen, H. S. Teng, Electrochim. Acta 2003, 48, 641. [33] V. Gupta, N. Miura, Electrochim. Acta 2006, 52, 1721. [34] W.-C. Chen, T.-C. Wen, J. Power Sources 2003, 117, 273. [35] H. Zhou, H. Chen, S. Luo, G. Lu, W. Wei, Y. Kuang, J. Solid State Electr. 2005, 9, 574. [36] L. Yuan, X. H. Lu, X. Xiao, T. Zhai, J. Dai, F. Zhang, B. Hu, X. Wang, L. Gong, J. Chen, C. Hu, Y. Tong, J. Zhou, Z. L. Wang, ACS Nano 2012, 6, 656. [37] X. Lu, G. Wang, T. Zhai, M. Yu, S. Xie, Y. Ling, C. Liang, Y. Tong, Y. Li, Nano Lett. 2012, 12, 5376. [38] F. Fusalba, P. Gouerec, D. Villers, D. Belanger, J. Electrochem. Soc. 2001, 148, A1. [39] V. Gupta, N. Miura, Mater. Lett. 2006, 60, 1466. [40] Z.-L. Wang, R. Guo, G.-R. Li, H.-L. Lu, Z.-Q. Liu, F.-M. Xiao, M. Zhang, Y.-X. Tong, J. Mater. Chem. 2012, 22, 2401. [41] W. Chen, R. B. Rakhi, H. N. Alshareef, Nanoscale 2013, 5, 4134. [42] M. Ammam, J. Fransaer, Chem. Commun. 2012, 48, 2036. [43] J. Yan, T. Wei, Z. Fan, W. Qian, M. Zhang, X. Shen, F. Wei, J. Power Sources 2010, 195, 3041. [44] K. Wang, Q. Meng, Y. Zhang, Z. Wei, M. Miao, Adv. Mater. 2013, 25, 1494. [45] L. Hu, J. Tu, S. Jiao, J. Hou, H. Zhu, D. J. Fray, Phys. Chem. Chem. Phys. 2012, 14, 15652.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201401196
