Letter pubs.acs.org/NanoLett
Dynamic and Galvanic Stability of Stretchable Supercapacitors Xin Li,† Taoli Gu, and Bingqing Wei* Department of Mechanical Engineering, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *
ABSTRACT: Stretchable electronics are emerging as a new technological advancement, since they can be reversibly stretched while maintaining functionality. To power stretchable electronics, rechargeable and stretchable energy storage devices become a necessity. Here, we demonstrate a facile and scalable fabrication of full stretchable supercapacitor, using buckled single-walled carbon nanotube macrofilms as the electrodes, an electrospun membrane of elastomeric polyurethane as the separator, and an organic electrolyte. We examine the electrochemical performance of the fully stretchable supercapacitors under dynamic stretching/releasing modes in different stretching strain rates, which reveal the true performance of the stretchable cells, compared to the conventional method of testing the cells under a statically stretched state. In addition, the self-discharge of the supercapacitor and the electrochemical behavior under bending mode are also examined. The stretchable supercapacitors show excellent cyclic stability under electrochemical charge/discharge during in situ dynamic stretching/ releasing. KEYWORDS: Energy storage, dynamically stretchable, supercapacitor, elastomeric separator, carbon nanotube, self-discharge
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chemical and mechanical testing is required to simultaneously evaluate dynamic and galvanic stability of the stretchable energy storage devices. Here, we demonstrated for the first time a dynamically stretchable supercapacitor (DSS), with instantaneously monitored electrochemical behavior under dynamic stretching/ releasing modes. To achieve the full stretchability of the cells, we fabricated the stretchable supercapacitors using the buckled SWNT electrodes, novel elastomeric electrospun polyurethane (PU) separator, and a 1 M organic electrolyte of tetraethylammonium tetrafluoroborate in propylene carbonate, as the schematic depicted in Figure 1a. To be specific, the SWNT macrofilm electrodes were prepared by chemical vapor deposition30 and attached on 1mm-thick rectangular prestrained polydimethylsiloxane (PDMS; Dow Corning 184) to form a buckled structure (Supporting Information, Figure S1a) as the stretchable electrodes. Elastomeric separator was prepared by elastomeric electrospun polyurethane31 to form a nonwoven porous membrane (Supporting Information, Figure S1b). The large interconnected voids between the fibers create ample porous structures for the mobile ions in the electrolyte to move freely through the elastomeric separator during dynamic stretching/ releasing. Accordingly, this structure provides low equivalent series resistance (ESR), thus allowing a high charge and discharge rate. The ESR value is obtained from the impedance spectra discussed later. The separator is assembled with two buckled SWNT macrofilm electrodes attached on a compliant
he emerging biomedical devices, wearable devices, and wireless sensor applications require compliant features for the comfort of people and/or animals. With the recent availability of soft and bendable/stretchable substrates and elastomeric materials, stretchable electronics devices1,2 are able to function under reversible stretching and releasing, thus meeting the compliant requirements. To date, many stretchable electronic devices have been investigated, such as organic lightemitting diode (OLED) devices,3,4 interconnects,5 loudspeakers,6 pressure and strain sensors,7−9 temperature sensors,10 radio frequency (RF) devices,11 field effect transistors,12 and epidermal electronics.13 Thus, to power the stretchable electronics, stretchable energy conversion/storage devices become key components to achieve a fully power-independent and stretchable system. Stretchable energy conversion devices have been reported, such as the organic solar cell,14 GaAs photovoltaic cell,15 and various piezoelectric devices.16,17 As for energy storage devices, electrochemical supercapacitors store energy in double layers on the electrode/electrolyte interface with opposite charges, providing a high power density and modest energy density in complementing the battery system.18−22 To date, the research has only focused on the electrode components, such as graphene,23 buckled single-walled carbon nanotubes (SWNT) macrofilms24,25 and polypyrrole (PPy).26,27 However, the fully stretchable energy storage devices are relatively underdeveloped, with the exception of the stretchable SWNT/textile composite,28 and ultracompliant dry gel cell.29 Nevertheless, all of these reports have conducted electrochemical studies under a mechanically static state, which cannot fully reflect the true performance of the fully rechargeable and stretchable energy storage devices in practical applications. An in situ electro© 2012 American Chemical Society
Received: September 28, 2012 Revised: November 4, 2012 Published: November 20, 2012 6366
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Figure 1. (a) Schematic of the main components of the dynamically stretchable supercapacitor. (b) Schematic of the dynamically stretchable supercapacitor under in situ reversible dynamic stretching and releasing between 0% applied strain and 31.5% applied strain.
Figure 2. (a) CV curves under static 0% strain and 31.5% strain at a scan rate of 100 mV s−1. The cell (under the static 31.5% strain) gives approximately 4% larger specific capacitance than that of the cell under the static 0% strain. (b) Schematic of the expected CV curves under the DSR modes. The CV curve under the DSR modes is expected to shift between the two CV curves under statically applied minimum and maximum strain. (c) CV curve under the DSR mode at strain rate of 2.22% strain s−1, demonstrating the green CV curves shift between the two reference CV curves. (d) CV curve under DSR mode at strain rate of 4.46% strain s−1 also shifts between the two reference CV curves, however with higher (∼200%) frequency than that of 2.22% strain s−1, being consistent with the higher dynamic stretching/releasing strain rate (∼200%) on the cell.
authors, has not been reported and discussed in literature yet. Cyclic voltammograms (CVs) of the supercapacitors at a scan rate of 100 mV s−1 at static 0% strain (original length of the buckled SWNT macrofilm electrode) and static 31.5% strain (the buckled SWNT macrofilm electrode is stretched to be almost flattened) show a perfectly symmetrical rectangular shape (Figure 2a). The calculated gravimetric specific capacitance of the cell is ∼50 F g−1 at the scan rate of 100 mV s−1, using the method discussed previously.32 Interestingly, it is observed that the specific capacitance of the cell is improved at the maximum strain (31.5% strain) applied than that of the original unstretched state at 0% strain. Similar
PDMS substrate to form the stretchable supercapacitor (Figure 1a). The electrochemical properties of the stretchable supercapacitor were tested in a two-electrode configuration under an in situ dynamic stretching/releasing (DSR) and a conventional statically fixed stretching state (Figure 1b). The movie clips (Supporting Information, Movies 1 and 2) show the animation and the real test of the DSS under in situ DSR mode. We believe the electrochemical behavior of the full stretchable cells under the in situ dynamic stretching/releasing modes are more realistic and practical, revealing the true performance during charge and discharge, which, to the best knowledge of the 6367
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Figure 3. (a) Galvanostatic charge−discharge curve at the current density of 10 A g−1, during in situ DSR at strain rate of 4.46% strain s−1. (b) Comparison of the normalized capacitance (the capacitance divided by the initial capacitance value of the DSS fixed at static 31.5% strain) with different strain rates. (c) Enlarged figure (time from ∼18 s to ∼90 s) to compare the specific capacitance of the cell under different strain rate applied. (d) With in situ applied DSR cycling at the high strain rate of 1.11% strain s−1, and 4.46% strain s−1, the DSS shows good cycling stability. The current density is 10 A g−1. (e) Nyquist plots of the stretchable supercapacitor at different static strains and under different strain rates applied (inset: high-frequency region). (f) Schematic of the stretchable supercapacitor at different strains applied showing changing electrode/electrolyte interface due to stretching/releasing.
rate of 4.46% strain s−1 (cell stretching from minimum to maximum length in ∼7 s), respectively. It shows that both dynamic CV curves exhibited the shifting behavior as expected. The different frequencies of CV curves’ shifting are consistent with the different dynamic strain rates applied. The electrochemical behavior of the DSS under galvanostatic charge−discharge cycling was examined at a current density of 10 A g−1, exhibiting the high power capability of the SWNT electrodes. At this high current density, the calculated specific capacitance is ∼44 F g−1, the total capacitance is 13.2 m F, while the areal capacitance per unit area is in the range between 2.64 m F cm−2 (0% strain applied) to 3.47 m F cm−2 (31.5% strain applied). As shown in Figure 3a, under the DSR modes
behavior was also observed under other scan rates from 50 mV s−1 to 2000 mV s−1 (Figure S2 in the Supporting Information). It shows that the specific capacitances of the cell at 31.5% strain outperform that at 0% strain by various ratios, which could be attributed to the different ion diffusion at different scan rates.22 The trend that the specific capacitance at 31.5% strain is higher than that of originally unstrained cell remains the same. To highlight the effect of the DSR mode, it is expected that the CV curve (blue) would shift between the static CV curves of the maximum (red) and the minimum strain (black) (Figure 2b). Figure 2c and d shows the experimentally recorded CV curves under the strain rate of 2.22% strain s−1 (cell stretching from minimum to maximum length in ∼14 s) and the strain 6368
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with a high strain rate of 4.46% strain s−1, a reproducible and stable capacitive behavior shows that there is no significant effect from the physically in situ applied stretching/releasing (black curve) on the charge−discharge of the cell (red curve), which proves that the cell can be reversibly charged and discharged under the dynamic stretching and releasing at an extremely high strain rate. The movie clip (Supporting Information, Movie 3) shows the charge−discharge curve during test when the supercapacitor is under DSR mode at the high strain rate of 4.46% strain s−1. To study the effect of strain rate on electrochemical properties, the DSR modes on the cell was conducted with a strain rate of 2.22% strain s−1 and 1.11% strain s−1, at a same charge−discharge current density, and compared with that of 4.46% strain s−1 in Figure 3b. To clearly illustrate, the gravimetric capacitance of each state is normalized by dividing the present capacitance with the initial capacitance of the cell at the static 31.5% strain (highest capacitance). It shows that the capacitive behavior under all DSR strain rates is stable and within 2% fluctuation. It is observed that, at a strain rate of 2.22% strain s−1, and 1.11% strain s−1, there is a little higher fluctuation on the capacitive performance compared with that of 4.46% strain s−1. However, this deviation is only within ∼2%, indicating an excellent and stable electrochemical behavior of the stretchable cell under various applied strain rates. For better illustration, the dynamic behavior of the cell under three different strain rates is enlarged and depicted in Figure 3c. It shows the relatively more stable performance of the cell at 4.46% strain s−1 (blue) than at 2.22% strain s−1 (green) and 1.11% strain s−1 (yellow). The corresponding in situ applied DSR mode is highlighted with the solid line (stretching) and dotted line (releasing) to show the coupling effect. Besides the amplitude and the frequency of the fluctuation under a different strain rate, it is interesting to note that the average gravimetric capacitive performance of the cell under 1.11% strain s−1 is slightly lower (∼ 1%) than that under 2.22 strain s−1 and 4.46% strain s−1. The cyclic stability of the stretchable supercapacitor during the in situ applied DSR is illustrated in Figure 3d. After 1000 galvanostatic charge−discharge cycles at a current density of 10 A g−1, the cell achieved excellent 98.5% capacitance retention, under a low strain rate of 1.11% strain s−1, after finishing ∼677 DSR cycles. At an extremely high strain rate of 4.46% s−1, the cell still achieved 94.6% capacitance retention, after finishing ∼2521 DSR cycles. The small drop of the capacitance retention under the higher strain rate of 4.46% strain s−1, compared to that under 1.11% strain s−1, could be attributed to the much longer mechanical DSR cycles, thus resulting in possible fatigue-induced degradation of the cell. However, this high capacitance retention value is still very promising for a full stretchable supercapacitor, compared to previous reported literature24 in which the cell was stretched for 100 cycles and then tested for cyclic stability. To study the slight difference of the capacitive behavior of the cell, both under statically fixed strain and under the DSR modes with different strain rates applied, the electrochemical impedance spectroscopy (EIS) was performed, as shown in Nyquist plots of Figure 3e. The impedance result was fitted using ZView software with Randle’s equivalent circuit to get the fitting parameters (Supporting Information, Table 1). It is observed that the series resistance (Rs) is not significantly altered with the different strains or strain rates applied on the cell. However, it is interesting to see that, at a static applied
31.5% strain, the charge transfer resistance (Rct) is significantly decreased to 3.38 ohms, compared to 11.38 ohms at 0% strain, indicating a much improved electrode/electrolyte interface. The Warburg diffusion resistance (WoR) is also decreased from 19.34 ohms at 0% strain to 7.74 ohms at 31.5% strain. As discussed previously, the different strain rate applied on the cell at the DSR mode also slightly affects the capacitance performance, although this difference (∼1%) is almost negligible. The difference found is mainly contributed by the much increased Warburg diffusion resistance under the DSR mode, compared to the static state of the stretchable cell. In Figure 3e, the AC frequency of the cell at different strain rates is also marked. At 1.11% strain s−1, the calculated frequency of the mechanical DSR cycling is equal to 0.0176 Hz (blue mark). At 2.22% strain s−1 and 4.46% strain s−1, the corresponding frequency of the mechanical DSR cycling is 0.0352 Hz (pink mark) and 0.0708 Hz (green mark), respectively. It is easy to see that the corresponding ohmic element under those frequencies is in the sequence of Z′ (840 ohms at 1.11% strain s−1) > Z′ (510 ohms at 2.22% strain s−1) > Z′ (252 ohms at 4.46% strain s−1). In addition, as Supporting Information Table 1 shows, the Warburg diffusion resistance under the DSR mode is also in the sequence of WoR at 1.11% strain > WoR at 2.22% strain s−1 > WoR at 4.46% strain s−1. To further confirm the possible ion diffusion difference under the in situ DSR mode, the self-discharge33 process of the stretchable supercapacitor at the strain rates of 2.22% s−1 and 4.46% s−1 was examined, and the results are illustrated in the Supporting Information, Figure S3. The self-discharge process of the supercapacitor under the strain rate of 2.22% s−1 is slower than that of 4.46% s−1, in consistent with the Warburg diffusion resistance results acquired from the EIS analysis that WoR at 2.22% strain s−1 > WoR at 4.46% strain s−1. At the strain rate of 4.46% s−1 (black curve), the observed wavy fluctuation in the self-discharge curve is corresponding to the state where the supercapacitor is stretched to 31.5% strain (a fast selfdischarge) and released back to 0% strain (a slow selfdischarge). At strain rate of 2.22% s−1 (red curve), the fluctuation is less obvious, due to the slower strain change. It is noted that the fluctuation of the self-discharge curve in DSR mode is in consistence with its CV curve shifting under the DSR mode. Figure 3f shows a schematic of the microscale view of the electrode/electrolyte interface to explain the possible reason of the variations in the charge transfer resistance and the Warburg diffusion resistance. At the static 0% strain, the buckled SWNT electrode forms periodic wavy structures on the microscale. The resistance on the electrode/electrolyte interface is high. The ionic diffusion process is also hindered at this state, due to the relatively long ion migration and diffusion length to move inside the inner Helmholtz layer, compared to the state of cell at a static 31.5% strain, in which the prebuckled SWNT electrode is stretched to be almost flattened, favoring faster kinetics and more effective ion diffusion. Under the DSR mode, the diffusion element increased favorably, owing to the moving of the liquid electrolyte inside the cell during stretching and releasing. At 4.46% strain s−1, the charge transfer at the electrode/electrolyte interface and the diffusion process is slightly improved, compared to that at 2.22% strain s−1 and 1.11% strain s−1, which could possibly be attributed to the faster changing of the wavy surface structure, which might favor the charged ions to be kept within the effective outer diffuse layer.18 However, this difference caused by different DSR strain rates is 6369
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found only within ∼1% range; thus for practical applications, this difference would not significantly affect the electrochemical performance of the cell. In addition to examining the electrochemical behavior of the supercapacitor under its stretched state (between 0% strain and 31.5% strain), it is also very important to ensure the functionality and stability of the supercapacitor under a bending/curving state when a compressive force is applied to the cell. The CV curves of the stretchable supercapacitor under (i) flat state (original length of 19 mm); (ii) fully bent state (the end to end distance reduced to 7 mm), and (iii) dynamic bending state (with a shrinking strain rate of 2.1% s−1) are shown in Figure S4 of the Supporting Information. During the dynamic bending, the cell reduced its lateral end-to-end distance from the flat state (19 mm) to the fully bent state (7 mm) in 30 s. It is noticed that the CV curve under the dynamic bending state shown a regular rectangular shape, fully overlapped with the CV curves at the flat and the static bending states, further proving the excellent stability of the cell. In summary, we have successfully demonstrated the stable performance of the stretchable supercapacitors under dynamic stretching/releasing and bending states, with various strain rates applied. This finding addresses the importance of dynamically testing of a fully stretchable energy storage device to fully reveal its true performance for practical purposes. The successful combination of elastomeric electrospun separator and elastomeric electrodes provides a good example and can be possibly further optimized to improve the energy density by anchoring redox materials.34 The stretchable supercapacitor shows excellent electrochemical performance and cyclic stability under intense in situ dynamic stretching and releasing. The slight variance on the capacitive performance under dynamic stretching is found to be within ∼2%, which is affected mainly by the ion diffusion process. The dynamically stretchable supercapacitor will provide an important power source for future stretchable electronics applications. The real-time dynamic testing method will provide an important guideline for the design optimization of all stretchable devices.
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0824790 and CMMI-0926093. The authors also thank Jinwen Qin, Chuan Cai, Qing Zhang, and Zeyuan Cao for discussions and Stephen Mulligan for proofreading.
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ASSOCIATED CONTENT
S Supporting Information *
Materials and method, detail of electrochemical measurements, optical and SEM images, AC impedance fitting parameters and Randel’s equivalent circuit, self-discharge of the capacitor at DSR mode, CV curve of the capacitor under fixed bending state and dynamic bending state, and movie data. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Telephone: 1-302-831-6438. Fax: 1302-831-3619. Present Address †
Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge funding support from US National Science Foundation (NSF) under the contracts of CMMI6370
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