Highly Stretchable and Transparent Supercapacitor by AgâAu Core

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Highly Stretchable and Transparent Supercapacitor by Ag−Au Core− Shell Nanowire Network with High Electrochemical Stability Habeom Lee,†,∥ Sukjoon Hong,‡,∥ Jinhwan Lee,† Young Duk Suh,† Jinhyeong Kwon,† Hyunjin Moon,† Hyeonseok Kim,† Junyeob Yeo,*,§ and Seung Hwan Ko*,† †

Applied Nano and Thermal Science Lab, Department of Mechanical Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Korea ‡ Laser Thermal Lab, Department of Mechanical Engineering, University of California, Berkeley 94720, United States § Novel Applied Nano Optics Lab, Department of Physics, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 41566, Korea S Supporting Information *

ABSTRACT: Stretchable and transparent electronics have steadily attracted huge attention in wearable devices. Although Ag nanowire is the one of the most promising candidates for transparent and stretchable electronics, its electrochemical instability has forbidden its application to the development of electrochemical energy devices such as supercapacitors. Here, we introduce a highly stretchable and transparent supercapacitor based on electrochemically stable Ag−Au core−shell nanowire percolation network electrode. We developed a simple solution process to synthesize the Ag−Au core−shell nanowire with excellent electrical conductivity as well as greatly enhanced chemical and electrochemical stabilities compared to pristine Ag nanowire. The proposed core−shell nanowirebased supercapacitor still possesses fine optical transmittance and outstanding mechanical stability up to 60% strain. The Ag−Au core−shell nanowire can be a strong candidate for future wearable electrochemical energy devices. KEYWORDS: Ag−Au core−shell nanowire, stretchable and transparent supercapacitor, nanowire percolation network, stretchable electronics, wearable electronics, transparent energy device

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INTRODUCTION Supercapacitors, owing to long cycle life with rapid charging and discharging capabilities at high power density,1−3 have long become a most viable candidate to substitute conventional batteries.4 Recently, the interest toward supercapacitors is evergrowing, especially for those on nonrigid substrates,5,6 due to the rapid increase in demand for portable and wearable electronics.7−9 Since these devices are intended to be attached to the human body and operated under continuous mechanical disturbances such as bending, twisting, and stretching,10−12 supercapacitor has huge benefits over conventional batteries for their associated energy device in terms of portability and safety, having lighter weight as well as lower risk of explosion.13,14 However, the latest progress on the development of nextgeneration future electronic devices in both industry and academia15−18 further suggests that the ultimate form of supercapacitor should not only be stretchable, but going another step forward to be optically transparent19 to achieve additional versatility and support easy integration with other wearable devices. To these objectives, there have been few attempts to fabricate a stretchable and transparent supercapacitor based on © XXXX American Chemical Society

carbon nanomaterials including carbon nanotube (CNT) and graphene as its electrode,20−28 with limited performance in transparency, electrical conductivity, and stretchability. The use of these carbon-based materials typically requires additional schemes on the target substrates such as employing wrinkles or applying prestrain29−31 to attain meaningful stretchability over the device. These supercapacitors, which depend on the pretreated substrates, are difficult to be integrated into a stretchable electronic circuit.32 Moreover, these CNT or graphene-based transparent electrodes generally suffer from high electrical resistance due to the inherent defects in the pristine materials or derived from the fabrication process.33−35 For the high-performance stretchable and transparent supercapacitor, a novel conductive platform that possesses superior electrical conductivity together with good optical transparency and mechanical stretchability at the same time is therefore in need. Received: April 13, 2016 Accepted: June 2, 2016

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DOI: 10.1021/acsami.6b04364 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Ag−Au core−shell NW synthesis. (a) Schematic illustration of the Au coating process on the Ag NW surface. (b, d) TEM images of (b) pristine Ag NW and (d) Ag−Au core−shell NW with crystalline analysis. (c) Schematic illustration of the Au atom supply and its deposition procedure.

deformation up to 60% tensile strain, which corresponds to the maximum tensile strain commonly associated with human motion.42 These supercapacitors are further connected in series to turn on a red LED at the stretched state, confirming that the proposed supercapacitor exhibits significantly enhanced energy density compared to the previously reported stretchable and transparent supercapacitors23,24,31 even without a separate active material.

In this regard, silver nanowire (Ag NW) percolation network can be an excellent candidate as a platform for the stretchable and transparent supercapacitor. Ag NW percolation network has been intensively studied and become a markedly mature technology as a novel transparent conductor to replace indium−tin oxide (ITO),36−38 and its application has been successfully extended to stretchable and transparent electrodes to withstand repeated external strain.11,39 However, despite these advantages, its poor electrochemical stability has forbidden the use of Ag NW percolation network as a main material for wearable electrochemical energy devices including supercapacitor. Upon the application, the electrolyte in the supercapacitor dissolves the Ag NW through ionization, and the performance of the resultant supercapacitor with Ag NW network electrode deteriorates rapidly at typical operating voltage range.40 To overcome this problem and develop the next-generation platform for a stretchable and transparent supercapacitor, the electrochemical stability of the Ag NW percolation network must be enhanced in an essential respect. In this study, we introduce electrochemically stable Ag−Au core−shell (AACS) NW network with high stretchability and transparency for wearable energy device application. AACS NW, synthesized through a simple Au coating process in solution environment, is composed of the core Ag NW with a thin gold outer layer at nanoscale. Although the consumption of high-priced Au41 in preparing AACS NW is minimized compared to the synthesis of an ordinary Au NW, it is confirmed that the outer Au layer on AACS NW strikingly improves the chemical and electrochemical stability of the overall AACS NW. At the same time, the theories, processes, and practices developed for conventional Ag NW are directly applicable to the AACS NW to readily create AACS NW percolation network for highly stretchable and transparent electrode. On the basis of this novel platform, stretchable and transparent supercapacitor is successfully achieved. Compared with pristine Ag NW network-based supercapacitor, the resultant supercapacitor exhibits no degradation upon repeated charging/ discharging cycles, and it operates stably even at a large

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RESULTS AND DISCUSSIONS We developed the electrochemically stable sheath forming method around the original Ag NW through the facile solution process. The surface of the Ag NW is coated with Au atoms through reduction of Au ions as schematically illustrated in Figure 1a. The pristine Ag NWs (N&B Co., Korea) are introduced into aqueous solution that contains capping agent, reducing agent, and pH increasing agent, followed by the slow injection of Au precursor through motorized syringe pump. More detailed experimental conditions including injection speed, solution concentration, and chemicals are included in the Experimental Section. During the process, the key factors for conformal coating of Au atoms on the Ag NW surface are adequate pH of the reaction solution and the injection speed of the Au precursor. Failure in satisfying either condition results in porous Au nanotubes as shown in Figure S1, since those factors are crucial for blocking the galvanic reaction that causes the loss of Ag atoms from Ag NW.43,44 Au outer shell is successfully created on the surface of Ag NW at high conformity once these conditions are satisfied simultaneously. In adequately high pH solution, the reduction power of reducing agent is optimized to prevent the Au precursor involving in galvanic reaction with Ag NW.44 At the same time, as the Au precursor is injected very slowly, the Au atoms derived from the precursor do not nucleate themselves but epitaxially deposit on the surface of Ag NW in the reaction solution under vigorous stirring. Figure 1b shows transmission electron microscopy (TEM) image of the pristine Ag NW. The lattice distance of 2.20 Å measured from the image processing reveals that the periodic structure observed in the image corresponds to [220] direction of single B


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Figure 2. Fabrication and characterization of Ag−Au core−shell NW-based electrodes. (a) Schematic illustration of the electrode fabrication through vacuum filtration and transfer method. (b) Digital image of as-prepared transparent Ag−Au core−shell NW network electrodes at various sheet resistances. (c) Optical transmittance of the transparent electrodes at various areal NW densities. (d) Strain-dependent electrical resistance of Ag− Au core−shell NW electrode on an elastic PDMS substrate.

crystalline Ag.45 In the Au coating process illustrated in Figure 1c, Au atoms are deposited on the surface of Ag NW and eventually cover the whole surface to form a shell structure. After a complete coating process, AACS NW is obtained as shown in the TEM image in Figure 1d. The clear contrast change from light to dark in both ends signifies that there are heavy atoms as compared with the core region. More close examination on the outer region clarifies that the periodic structure matches to [111] direction of single crystalline Au.46 TEM results confirm that the Ag NW is conformally covered by thin Au layer and that the core Ag NW is not damaged or degraded during the Au coating process. Having the core Ag NW remains unchanged, and superior electrical conductivity as well as the mechanical robustness of the primary NW are expected to be preserved even after 5 h of Au coating process. TEM−energy-dispersive X-ray (EDX) line profile in Figure S2 further proves that the final product is indeed AACS NW that is composed of core Ag NW of ∼30 nm diameter with Au shell at ∼5 nm thickness. The thickness of Au layer also can be easily controlled by changing the amount of the injected precursor as verified in Figure S3. In the case shown in Figure S3a, the thickness of the deposited Au layer is ∼3 nm, and this value is well-matched with the half of the average NW diameter increase after the Au deposition process as shown in Figure S4. Therefore, it is worth mentioning that the proposed Au coating process does not involve any replacement of Ag atoms upon the reduction process implying that the reaction is largely galvanic replacement free. With the developed AACS NWs, the theories and practices developed for other metal NWs can be also applied to create NW network-based large-area transparent electrodes. The vacuum filtration and transfer method is used throughout this study to prepare the highly stretchable and transparent AACS

NW network electrodes, and the procedure is schematically illustrated in Figure 2a. For a typical preparation, measured amount of AACS NW solution is poured into the filtration cup, which is connected tightly with polytetrafluoroethylene (PTFE) filter, and after complete filtration, the resultant AACS NW network on the filter is transferred to the target substrate. Depending on the type of the target substrate, transparent electrodes or stretchable electrodes can be easily prepared. Figure 2b shows the AACS network electrodes on slide glass with various sheet resistances from 42.3 to 167.7 Ω/sq depending on the density of NW network. Figure 2c confirms that the AACS NW networks possess superior optical transparency, exceeding 85% in the entire range of visible wavelength, which is measured by a UV−vis spectrometer. The areal density corresponding to each case was calculated to be in the range from 52 to 268 mg/m2. It is noticeable that the prepared AACS NW electrodes are comparative to the Ag NWbased electrode in terms of both the transparency and the sheet resistance. When metallic NWs are deposited on the stretchable polydimethylsiloxane (PDMS) substrate instead of a rigid substrate, the electrode can sustain its electrical conductivity with its optical transmittance under a large strain due to the unique structural features of NW percolation network.11 Herein synthesized AACS NW can be likewise applied as a constituent of transparent and stretchable electrode on PDMS substrate. Subsequently, the AACS NW-based stretchable and transparent electrode is prepared, and its stretchability is analyzed through measuring the electrical resistance change in stepwise strain variation. The result is summarized in Figure 2d with an inset image that shows the experimental setup consisting of the AACS NW electrode with both ends connected to copper tape on the horizontally moving stage. Except for the first cycle, C


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Figure 3. Aging test of pristine Ag NW and Ag−Au core−shell NW electrodes under ambient condition. (a) Transient electrical resistance measurement for four weeks. (inset) Digital and SEM images of Ag NW electrodes at the 1st day. SEM images of (b) the pristine Ag NW and (c) Ag−Au core−shell NW after four weeks in the controlled ambient condition. (d) SEM image and (e) the corresponding EDS analysis of pristine Ag NW electrode after four weeks. Notice the disconnected lines for pristine Ag NW during the four weeks aging test, while Ag−Au core−shell NW still maintains good connection.

every resistance variation cycles follows the similar path.47 Although the resistance increases slightly during the stretching, it recovers after each releasing cycle, verifying that the AACS NW percolation network can be employed as a stretchable and transparent electrode. The resistance change under mechanical stretching can be further decreased with higher NW density, while the transparency will be degraded, because there is a trade-off between resistance change and transparency in metal NW percolation network. It is well-known that metal NWs such as Ag NW, those with smaller diameter in particular, have degrading stability problem to lose their electrical conductivity even in ambient condition.48 Being an inert noble metal with high reduction potential compared to Ag, Au sheath created around the surface of Ag is expected to increase the overall stability of the electrode to the environment. For comparison, three types of electrodes (pristine Ag NW, AACS NW with thin and thick Au sheath layers) are prepared at 2 × 2 cm2 size to investigate their electrical resistance change over time. The thickness of the Au shell of the thin and thick AACS NW is 3 and 5 nm, respectively, as can be found in the Figure S3. These electrodes are placed in controlled environment maintaining temperature of 22 °C and relative humidity of 45% to mimic the ambient condition. As can be seen in the second inset of Figure 3a, the surface of NW of each electrode is smooth without any residue except NW, yet the resistances of the electrodes increase gradually over time, and it is noticeable that pristine Ag NW

electrode (black line) degrades much faster than AACS NW electrodes (blue and triangle lines). It is apparent from Figure 3b that the surface of the Ag NW is covered with tiny nanoparticles after four weeks, and most of the NWs are disconnected to show electrical failure. In contrast, although similar small nanoparticles are observable on the surface, AACS NW network almost maintain their overall structure as close to its original state for both cases of thin and thick Au layer coating (Figure 3c). The electrical measurements in Figure 3a are also consistent with the SEM images, confirming the enhancement of electrode stability in ambient condition by Au coating on the Ag NW surface. To examine the mechanism behind the deterioration of the pristine Ag NWs in the air, EDX analysis is conducted with the resultant pristine Ag NW electrode after kept in the aforementioned environment for four weeks. Figure 3d shows the SEM image of the analyzed region, and two types of symbols are marked on the figure. The dot-lined circles indicate the newly emerged particles, and the red stars specify the disconnected spots of the Ag NWs. Comparing Figure 3d with the EDS analysis in Figure 3e, the particles are largely composed of Ag atoms. Seeing that there exist no other structures except Ag NW in the as-prepared network, it is reasonable to assume that the Ag atoms migrate from the original Ag NWs and assemble each other to create larger particles by Ostwald ripening.49 An atom having high surface energy tends to move to other location to decrease its surface D


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Figure 4. Chemical stability test of pristine Ag NW and Ag−Au core−shell NW electrodes. (a) Digital and microscope images of pristine Ag NW (left) and Ag−Au core−shell NW (right) based electrodes after H2O2 corrosion test. (b) Electrical resistance change under H2O2 exposure. (inset) Enlarged graph and schematic experimental setup. (c) SEM images of pristine Ag NW (top row) and Ag−Au core−shell NW (bottom row) before and after the corrosion test.

optical image in Figure 4a. This result implies that the Au shell layer successfully protects the core Ag NW against the attack of the corrosive chemicals and therefore maintains its performance even at such harsh condition in which the Ag NWs are dissolved instantly. The electrical resistance change upon the exposure to H2O2 solution also confirms the advantage of Au coating as shown in Figure 4b. The inset shows the same graph with enlarged y-axis, and from the graph it is noticeable that there is a minute change of the electrical resistance in both thin (red line) and thick (blue line) AACS NW electrodes. Figure 4c shows the SEM images of the electrodes after the end of the chemical corrosion test. On the one hand, the pristine Ag NWs (top row) are totally dissolved, and only negligible residue is found. On the other hand, the AACS NW electrode (bottom row) shows insignificant change in its feature, but with some tiny particles sparsely attached on the surface. It can be estimated that there are some fine pores or defects on the Au layer, and therefore a small fraction of the core Ag NW is exposed to the H2O2 solution and generates tiny particles similar to the ones shown in top right of Figure 4c. These partial damages are also consistent with the resistance transient change, which indicates an initial slight increase in the electrical resistance of AACS NW. Yet, increase in resistance is infinitesimally small, especially for the one with thicker Au shell (blue line), compared to the dramatic resistance upsurge in pristine Ag NW network (black line). These results clearly validate that the core Ag NW is well-protected by the Au shell to achieve highly enhanced chemical stability.

energy by increasing the number of neighboring atoms. As a result, the pristine Ag NW percolation network without any protection cannot sustain its initial shape permanently and lose electrical contact in due course as shown in Figure 3b,d. This problem becomes more severe when the NW diameter gets smaller. When the inert Au layer covers over the surface of pristine Ag NWs, the outer Ag atoms become more stable, and the migration of Ag atoms is delayed until the Au layer is removed from the Ag NW surface by similar mechanism. Therefore, AACS NW electrode can maintain the initial electrical resistance significantly longer than pristine Ag NW electrode, because the core Ag NWs, which are presumed to be primarily responsible for high electrical conductivity, remain far more stable. Enhancement in chemical stability by Au outer shell is valid even in the presence of environmental chemicals. As an extreme example, the effect of H2O2 electrolyte aqueous solution, which is known to dissolve Ag efficiently, is tested on the pristine Ag NW and AACS NW network. Ag NWs and AACS NWs electrodes are prepared at high densities, and half of the each electrode is immersed in 12.5% H2O2 aqueous solution for 1 min, followed by deionized (DI) water cleaning. The photographs and microscopic images of the electrodes are displayed in Figure 4a. After the short contact with H2O2 solution, the pristine Ag NWs (left pictures) are immediately degraded with a large amount of bubbles generation, and the immersed Ag NWs are mostly removed from the glass substrate. In contrast, for the AACS NW electrode (right pictures), there is no perceptible change as presented in the E


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Figure 5. Electrochemical stability test of Ag NW and Ag−Au core−shell NW electrodes. (a) CV curves of pristine Ag NW electrode to 2nd cycle. (b) CV curves of Ag−Au core−shell NW electrode to 50 cycles. (c) SEM images of the NWs after CV test for both interface (red box) and inner (blue box) region and schematic illustration of three-electrode measurement method. Inset scales are 300 nm. (d) Relative mass variation of Ag versus Au during the 50 cycles of CV test on Ag−Au core−shell NW electrode.

Even though the AACS NW electrode shows improved stability in ambient condition and harsh chemical solution, the electrochemical stability is essential for energy device to sustain its performance under the applied voltage. To examine the electrochemical stability of the resultant AACS NW electrode, cyclic voltammetry (CV) test is conducted. A typical threeelectrode measurement method is used with Ag/AgCl reference electrode and Pt counter electrode in the Na2SO4 aqueous electrolyte. The CV test result for the pristine Ag NW electrode shows an oxidation peak at the first cycle followed by rapid current drop and flattened curve without corresponding reduction peak at the reverse voltage scan region as displayed in Figure 5a. The potential value at the peak corresponds to the characteristic oxidation peak of typical Ag.40 This signifies that an irreversible reaction is occurred in the electrode material during the first oxidation and that the Ag ions are dissolved directly into the electrolyte solution. In contrast, CV result of AACS NW electrode shows typical CV curves for good electrodes as displayed in Figure 5b. Oxidation peaks are also found at ∼0.45 V potential value, which is the same as the oxidation peak of pure Ag NW electrode; however, there are corresponding reduction peaks at the reverse voltage scan state in every CV cycle without rapid drop in the current. As the AACS NW possesses tiny defects or pores in the Au surface, the core Ag NWs are partially exposed to electrolyte, and the Ag ions are generated only at the exposed spots. Since the Ag ions are largely protected by the Au shells, they mostly do not dissolve into the electrolyte nor diffuse away from the

electrode. As a result, these Ag ions in the vicinity of Au shell are likely to involve in the reduction procedure and generate reduction peak in the CV curve during the reverse voltage scan. It is also observable that those peaks diminish as the cycles continues, and the CV curves eventually converge to rectangular shape after 50 cycles. The stabilized CV curve of AACS NW electrode is included in Figure S5, drawn simultaneously with the second CV cycle of Ag NW electrode. There is a pair of peaks that are produced at different potential values with Ag. These peaks indicate the properties of Au,50 and it signifies that there are no more Ag atoms actively involved in the CV test. Figure 5c shows SEM images of the electrodes after the end of the CV test with the illustration that explains the setup of the three-electrode measurement method. The enlarged drawing of working electrode indicates the airelectrolyte interface region (red box and arrows) and the inner region (blue box and arrows) totally immersed in electrolyte. From the SEM image, it is found that the pristine Ag NWs of the air−electrolyte interface region are damaged and disconnected as marked with yellow triangle symbols in Figure 5c (top left). Since the NW electrochemical damage first starts at the air electrolyte interface and totally disconnects the electrode, the pristine Ag NWs of the inner region are not involved in the CV test any longer and thus maintain their initial shape. However, the AACS NW electrode maintains its initial shape at both regions as can be observed in Figure 5c (right column). This signifies that the Au shell is practically not damaged by the CV electrochemical test even though the F


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Figure 6. Demonstration of the stretchable and transparent supercapacitor. (a) Comparison of CV curves between pristine Ag NW-based supercapacitor and Ag−Au core−shell NW-based supercapacitor. (inset) A digital picture of the stretchable and transparent supercapacitor. (b) CV curves of Ag−Au core−shell NW-based supercapacitor at various strain condition (0%, 30%, 60%) and voltage scan rate (50, 200, 500 mV/s). (c) The relative capacitance change of the Ag−Au core−shell NW-based supercapacitor during the repeated stretching cycles. (d) The galvanostatic charge−discharge curve of the Ag−Au core−shell NW-based supercapacitor under various strain condition (0%, 30%, 60%). (e) Series connected Ag−Au core−shell NW-based supercapacitor at 30% strain. Note that a commercial LED can be turned on at the stretched conditions.

three-electrode CV test in aqueous solution (half-cell test), the CV test of full-cell that is assembled by the gel electrolyte with two identical AACS NW network electrodes shows neither oxidation nor reduction peaks as shown in Figure 6a. It is confirmed from Figure 5b,d that oxidation and reduction peaks of AACS NW are originated from the dissolution of the inner Ag NW into liquid electrolyte through the tiny defects or pores of the Au shell. Different from liquid electrolyte, the polymer gel electrolyte cannot penetrate into the tiny pores of Au shell due to its high viscosity, solid-like properties. As a result, the Au shell layer keeps the Ag core NW from contacting with the polymer electrolyte, and therefore the effect of the polymer electrolyte, which is to dissolve Ag atoms in contact, is almost insignificant. The benefit of employing AACS NW as the electrode of the supercapacitor is clearly observable from the CV curve of a Ag NW-based supercapacitor whose result is drawn simultaneously as black line in Figure 6a. Moreover, unlike with the Ag NW-based supercapacitor failed in second

electrolyte containing Ag ions continuously pass through the defects. X-ray photoelectron spectroscopy (XPS) analysis is conducted on AACS NW electrodes after they undergo different number of CV cycles to support our investigation, and the result is displayed in Figure 5d. The mass concentration of Ag atoms, which is calculated with fixed mass of Au, decreases steadily as the number of CV cycles increases to ensure that the loss of Au is negligible, or at least much smaller than Ag at the repeated CV measurements. Through successful realization of novel class of metal NW that possesses highly improved electrochemical stability, stretchable, and transparent supercapacitor composed of AACS NW percolation network is demonstrated for wearable electronics applications. The detailed process can be referred to in the Experimental Section with Figure S6. In the proposed supercapacitor, AACS NW electrode acts as both active material and current collector, while the H3PO4 polymer gel acts as both separator and electrolyte. Unlike the result of G


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introduce AACS NW in this study by introducing Au thin coating at nanoscale on the surface of Ag NWs by a simple solution process. On the one hand, the microscopic observations and the electrical resistance measurements confirm that the chemical and electrochemical stabilities of the AACS NW are markedly improved compared to the pristine Ag NW. On the other hand, the advantages of the metal NW percolation network, such as good optical transparency and high stretchability with superior macroscopic electrical conductivity, are well-preserved with the AACS NW percolation network. As a consequence, a novel class of transparent and stretchable supercapacitor is successfully demonstrated. The resultant supercapacitor not only shows stable performance up to 60% tensile strain but also succeeds in operating a commercial LED without separate active material to brighten the prospect of the percolation network as transparent and stretchable electrode for versatile wearable energy devices.

CV cycle, the AACS NW-based supercapacitor continues operating without any performance degradation or failure during repeated 500 times charge−discharge process as in Figure S7. In Figure S8a, the rectangular shape of the CV curves at various voltage scan rate explains the ideal electrochemical properties of the AACS NW-based supercapacitor. The AACS NW-based supercapacitor maintains its capacitance over 76% during the voltage scan rate increase by 10 times from 50 to 500 mV/s. The galvanostatic charge− discharge measurement result shown in Figure S9a also explains the good electrochemical properties of the AACS NW-based supercapacitor. All the curves preserve typical triangular shape with negligible IR drop at a wide range of input current condition. Also, as can be seen in Figure S9b, the capacitance calculated from the charge−discharge measurement is conserved over 65% (from 209.9 to 136.5 μF/cm2) even after the 20 times input current increase. Furthermore, to observe the mechanical stability of the AACS NW-based supercapacitor, CV test is performed at various voltage scan rate under increasing strain condition. As a result, the AACS NW-based supercapacitor not only exhibits optical transparency, but also sustains its performance in a very large strain condition as presented in Figure 6b. The CV curve slightly increases along with the strain up to 30%, and this trend could be explained by decrease in the gap between upper and lower electrode together with the enlarged working area.23 It is noticeable that the shape of CV curve slightly changes at 60% strain, especially in the fast voltage scan condition. The deformation of the CV curve under high strain and fast voltage scan condition might be derived from the temporary electrode resistance increase, which is recovered after strain releasing (cf. Figure 2d). The supercapacitor sustains its electrochemical performance even after 200 stretching cycles as shown in Figure 6c. First 100 cycles are conducted under 30% strain condition, and the other 100 cycles are conducted under 60% strain condition. The insets are digital images of AACS NW-based supercapacitor at 0% and 60% strain condition. The charge−discharge curves under different strain condition indicate, once again, the good capacitive properties with high mechanical stability of the resultant supercapacitor. The curves sustain their triangular shape with unremarkable IR drop under increasing strain condition up to 60%. Considering that the required strain range of wearable devices is ∼55%,42 these results are very encouraging for applying the prepared stretchable and transparent supercapacitors as an energy source for various wearable electronics to realize a fully stretchable human-integratable device. Additionally, the supercapacitors are connected in series to turn on a red light-emitting diode (LED) as shown in Figure 6e, and the CV curve for the series connected supercapacitors is attached in Figure S10. The series-connected supercapactiors are successfully charged and discharged lighting on the redLED even at 30% strain condition. To the best of authors’ knowledge, this is the very first example of operating a commercial electronic device with a stretchable and transparent supercapacitor at its stretched state.

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METHODS

Preparation of Ag−Au Core−Shell Nanowires. Aqueous solution (1 mM, 20 mL) of poly(vinylpyrrolidone) (PVP; MW 55 000, Sigma-Aldrich) is poured into a 50 mL Erlenmeyer flask with an elliptical stirrer. Aqueous solution (0.1 M, 5 mL) of L-ascorbic acid (FW 176.12, Sigma-Aldrich) and 0.2 M aqueous solution (5 mL) of NaOH (Sigma-Aldrich) are injected into the PVP solution sequentially, keeping the stirrer vigorously rotating. After the mixture becomes a clear solution, 100 μL of 0.5 wt % Ag NW aqueous solution (NW diameter: 30 to ∼35 nm, N&B Co.) is added into the reaction solution. As the final additives, 0.15 mM aqueous solution (6 or 12 mL for thin or thick Au layer) of HAuCl4 (FW 339.79, Sigma-Aldrich) is injected into the reaction solution through motorized syringe pump at 45 μL/min injection speed. Through this mild reaction over several hours, the Au atoms are deposited on the whole surface of the Ag NWs. After all the Au precursor is injected, the Ag−Au core−shell NWs are separated and washed by DI water three times through centrifugation. The separated Ag−Au core−shell NWs are kept in mixture of DI water and ethanol (2 mL of DI water, 18 mL of ethanol) in conical tube. Preparation of Nanowire Percolation Network Electrode. Slide glass is cleaned with acetone and ethanol as a substrate for transparent electrode. Also, as a substrate for a stretchable electrode, PDMS films are prepared by mixing PDMS solution and curing agent (Sylgard 184, Dow Corning Co.) at 10:1 ratio. After curing at 60 °C oven for 2 h, the PDMS film is cut into square shape. After a vacuum pump (GHP-240, KODIVAC), which is connected with glass filter kit (300 mL, STERLITECH), is operated, the Nylon and the PTFE filters are sequentially placed on the glass filter, and a holder cup is assembled to the glass filter tightly with the clamp. The synthesized Ag−Au core−shell NW solution is poured into the holder cup to filter the NWs. For the three types of transparent electrodes, 3, 5, and 7 mL of solution were used in each case. In all the following experiments, a fixed amount (10 mL) of Ag−Au core−shell NWs solution is used. The resultant network of the Ag−Au core−shell NWs on the PTFE filter is transferred to the target substrate by simple contact with the corona gun-treated substrate. The Ag NW electrode as a reference electrode is prepared by the same method except for the type of NW solution. Characterization of Ag Nanowires and Ag−Au Core−Shell Nanowires. Analysis of the micromorphology of the NWs was conducted through transmission electron microscopy (TEM, JEM2100F, JEOL Ltd) and field emission scanning electron microscopy (FE-SEM, AURIGA, Carl Zeiss). Characterization of Electrodes. The transmittance of the electrode is measured by UV−vis spectrometer (Cary 100, Agilent Technologies) in visible range. The sheet resistance of the electrode is measured by four-point probe. In the aging test (resistance monitoring test at ambient condition), the electrodes are prepared by abovementioned method on the slide glass, and the electrodes are cut into

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CONCLUSIONS In conclusion, percolative network with metal NW, Ag NW in particular, has long been a leading figure in the transparent and stretchable electrode, yet failed to be applied as an electrode of the electrochemical energy devices mainly due to its poor electrochemical stability. To overcome this problem, we newly H



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square shape of which both ends are connected to copper tape. During the whole aging test, all the electrodes are kept in the constant environment of 22 °C with 45% relative humidity. The electrochemical stability test is conducted through electrochemical analyzer system (VersaSTAT3−450, AMETEK) by three electrodes method composed of Ag/AgCl reference electrode, Pt counter electrode, and working electrode (target electrode). The mass concentration variation in cyclic voltammetry measurement is examined by XPS (AXIS-His, KRATOS). Supercapacitor Fabrication. The polymer electrolyte is made through the method reported previously.51 A pair of Ag−Au core− shell NW electrodes formed on the PDMS substrates is immersed in the prepared polymer electrolyte for 1 min. After the residual water is dried, the two electrodes are attached to each other with moderate pressure to assemble a supercapacitor. Refer to the illustration of Figure S6. Characterization and Demonstration of Supercapacitor. The performance of the supercapacitor is examined by two-electrodes method through aforementioned electrochemical analyzer system. CV is conducted in the voltage range from 0 to 0.8 V using fixed voltage scan rate of 500 mV/s. For the performance measurement under strain, the supercapacitor is mounted on the custom designed equipment. In every 10% strain up to 60%, CV is conducted in above-mentioned condition. In demonstration step, four supercapacitors are connected in series and operated at 30% strain.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b04364. Microstructure of the Au nano tube after the uncontrolled Au deposition method, TEM line profile of the Ag−Au core−shell NW, thickness control of the Au layer, average diameter change before and after Au deposition process, comparison of CV curves between pristine Ag NW and Ag−Au core−shell NW, Ag−Au core−shell NW-based supercapacitor fabrication process, CV curves, and capacitance retention of the Ag−Au core−shell NW-based supercapacitor at various voltage scan rate, galvanostatic charge−discharge curves of the Ag−Au core−shell NW-based supercapacitor at various input current, CV curves of series connected supercapacitor. (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (J.Y.) *E-mail: [email protected]. (S.H.K.) Author Contributions ∥

REFERENCES

(1) Pay, S.; Baghzouz, Y. In Effectiveness of Battery-Supercapacitor Combination in Electric Vehicles, Power Tech Conference Proceedings, 2003 IEEE; IEEE: Bologna, Italy, 2003; Vol. 3, p 6. (2) Wang, Q.; Wen, Z.; Li, J. A Hybrid Supercapacitor Fabricated with a Carbon Nanotube Cathode and a TiO2−B Nanowire Anode. Adv. Funct. Mater. 2006, 16 (16), 2141−2146. (3) Wang, Y.; Shi, Z.; Huang, Y.; Ma, Y.; Wang, C.; Chen, M.; Chen, Y. Supercapacitor Devices Based on Graphene Materials. J. Phys. Chem. C 2009, 113 (30), 13103−13107. (4) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7 (11), 845−854. (5) Hu, L.; Pasta, M.; Mantia, F. L.; Cui, L.; Jeong, S.; Deshazer, H. D.; Choi, J. W.; Han, S. M.; Cui, Y. Stretchable, Porous, and Conductive Energy Textiles. Nano Lett. 2010, 10 (2), 708−714. (6) Horng, Y.-Y.; Lu, Y.-C.; Hsu, Y.-K.; Chen, C.-C.; Chen, L.-C.; Chen, K.-H. Flexible Supercapacitor Based on Polyaniline Nanowires/ Carbon Cloth with Both High Gravimetric and Area-Normalized Capacitance. J. Power Sources 2010, 195 (13), 4418−4422. (7) Lee, Y.-H.; Kim, Y.; Lee, T.-I.; Lee, I.; Shin, J.; Lee, H. S.; Kim, T.-S.; Choi, J. W. Anomalous Stretchable Conductivity Using an Engineered Tricot Weave. ACS Nano 2015, 9 (12), 12214−12223. (8) Lee, C. H.; Ma, Y.; Jang, K. I.; Banks, A.; Pan, T.; Feng, X.; Kim, J. S.; Kang, D.; Raj, M. S.; McGrane, B. L.; Morey, B.; Wang, X.; Ghaffari, R.; Huang, Y.; Rogers, J. A. Soft Core/Shell Packages for Stretchable Electronics. Adv. Funct. Mater. 2015, 25 (24), 3698−3704. (9) Lim, S.; Son, D.; Kim, J.; Lee, Y. B.; Song, J. K.; Choi, S.; Lee, D. J.; Kim, J. H.; Lee, M.; Hyeon, T. Transparent and Stretchable Interactive Human Machine Interface Based on Patterned Graphene Heterostructures. Adv. Funct. Mater. 2015, 25 (3), 375−383. (10) Kim, J.; Banks, A.; Cheng, H.; Xie, Z.; Xu, S.; Jang, K. I.; Lee, J. W.; Liu, Z.; Gutruf, P.; Huang, X.; et al. Epidermal Electronics with Advanced Capabilities in Near-Field Communication. Small 2015, 11 (8), 906−912. (11) Hong, S.; Lee, H.; Lee, J.; Kwon, J.; Han, S.; Suh, Y. D.; Cho, H.; Shin, J.; Yeo, J.; Ko, S. H. Highly Stretchable and Transparent Metal Nanowire Heater for Wearable Electronics Applications. Adv. Mater. 2015, 27 (32), 4744−4751. (12) Zhang, Z.; Deng, J.; Li, X.; Yang, Z.; He, S.; Chen, X.; Guan, G.; Ren, J.; Peng, H. Superelastic Supercapacitors with High Performances During Stretching. Adv. Mater. 2015, 27, 356−362. (13) Kim, S.-K.; Kim, H. J.; Lee, J.-C.; Braun, P. V.; Park, H. S. Extremely Durable, Flexible Supercapacitors with Greatly Improved Performance at High Temperatures. ACS Nano 2015, 9 (8), 8569− 8577. (14) Lisbona, D.; Snee, T. A Review of Hazards Associated with Primary Lithium and Lithium-Ion Batteries. Process Saf. Environ. Prot. 2011, 89 (6), 434−442. (15) Lee, M.-S.; Lee, K.; Kim, S.-Y.; Lee, H.; Park, J.; Choi, K.-H.; Kim, H.-K.; Kim, D.-G.; Lee, D.-Y.; Nam, S. High-Performance, Transparent, and Stretchable Electrodes Using Graphene−Metal Nanowire Hybrid Structures. Nano Lett. 2013, 13 (6), 2814−2821. (16) Kim, U.; Kang, J.; Lee, C.; Kwon, H. Y.; Hwang, S.; Moon, H.; Koo, J. C.; Nam, J.-D.; Hong, B. H.; Choi, J.-B. A Transparent and Stretchable Graphene-Based Actuator for Tactile Display. Nanotechnology 2013, 24 (14), 145501. (17) Xiao, L.; Chen, Z.; Feng, C.; Liu, L.; Bai, Z.-Q.; Wang, Y.; Qian, L.; Zhang, Y.; Li, Q.; Jiang, K. Flexible, Stretchable, Transparent Carbon Nanotube Thin Film Loudspeakers. Nano Lett. 2008, 8 (12), 4539−4545. (18) Park, K.; Lee, D. K.; Kim, B. S.; Jeon, H.; Lee, N. E.; Whang, D.; Lee, H. J.; Kim, Y. J.; Ahn, J. H. Stretchable, Transparent Zinc Oxide Thin Film Transistors. Adv. Funct. Mater. 2010, 20 (20), 3577−3582. (19) Yang, Y.; Jeong, S.; Hu, L.; Wu, H.; Lee, S. W.; Cui, Y. Transparent Lithium-Ion Batteries. Proc. Natl. Acad. Sci. U. S. A. 2011, 108 (32), 13013−13018. (20) Yu, J.; Lu, W.; Pei, S.; Gong, K.; Wang, L.; Meng, L.; Huang, Y.; Smith, J. P.; Booksh, K. S.; Li, Q.; et al. Omnidirectionally Stretchable

S Supporting Information *

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Research Article

H.L. and S.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Research Foundation of Korea (Grant No. 2012-0008779) Global Frontier R&D Program on Center for Multiscale Energy System (Grant No. 2012-054172), Nano-Material Technology Development Program (R2011-003-2009), Creative Materials Discovery Program (NRF-2016M3D1A1900035) funded by the Ministry of Science, ICT & Future, and the R&D Convergence Program, and by Seoul National University, Institute of Advanced Machinery and Design. I


Research Article


(42) Yamada, T.; Hayamizu, Y.; Yamamoto, Y.; Yomogida, Y.; IzadiNajafabadi, A.; Futaba, D. N.; Hata, K. A Stretchable Carbon Nanotube Strain Sensor for Human-Motion Detection. Nat. Nanotechnol. 2011, 6 (5), 296−301. (43) Murshid, N.; Gourevich, I.; Coombs, N.; Kitaev, V. Gold Plating of Silver Nanoparticles for Superior Stability and Preserved Plasmonic and Sensing Properties. Chem. Commun. 2013, 49 (97), 11355−11357. (44) Yang, Y.; Liu, J.; Fu, Z.-W.; Qin, D. Galvanic Replacement-Free Deposition of Au on Ag for Core-Shell Nanocubes with Enhanced Chemical Stability and SERS Activity. J. Am. Chem. Soc. 2014, 136 (23), 8153−8156. (45) Li, S.; Shen, Y.; Xie, A.; Yu, X.; Qiu, L.; Zhang, L.; Zhang, Q. Green Synthesis of Silver Nanoparticles Using Capsicum Annuum L. Extract. Green Chem. 2007, 9 (8), 852−858. (46) Jin, Y.; Jia, C.; Huang, S.-W.; O’Donnell, M.; Gao, X. Multifunctional Nanoparticles as Coupled Contrast Agents. Nat. Commun. 2010, 1 (4), 1−8. (47) Xu, F.; Zhu, Y. Highly Conductive and Stretchable Silver Nanowire Conductors. Adv. Mater. 2012, 24 (37), 5117−5122. (48) Moon, I. K.; Kim, J. I.; Lee, H.; Hur, K.; Kim, W. C.; Lee, H. 2D Graphene Oxide Nanosheets as an Adhesive Over-Coating Layer for Flexible Transparent Conductive Electrodes. Sci. Rep. 2013, 3, 1112− 1118. (49) Hong, S.; Yeo, J.; Manorotkul, W.; Kang, H. W.; Lee, J.; Han, S.; Rho, Y.; Suh, Y. D.; Sung, H. J.; Ko, S. H. Digital Selective Growth of a ZnO Nanowire Array by Large Scale Laser Decomposition of Zinc Acetate. Nanoscale 2013, 5 (9), 3698−3703. (50) Braiek, M.; Rokbani, K. B.; Chrouda, A.; Mrabet, B.; Bakhrouf, A.; Maaref, A.; Jaffrezic-Renault, N. An Electrochemical Immunosensor for Detection of Staphylococcus Aureus Bacteria Based on Immobilization of Antibodies on Self-Assembled MonolayersFunctionalized Gold Electrode. Biosensors 2012, 2 (4), 417−426. (51) Lee, H.; Hong, S.; Kwon, J.; Suh, Y. D.; Lee, J.; Moon, H.; Yeo, J.; Ko, S. H. All-Solid-State Flexible Supercapacitors by Fast Laser Annealing of Printed Metal Nanoparticle Layers. J. Mater. Chem. A 2015, 3 (16), 8339−8345.

High Performance Supercapacitor Based on Isotropic Buckled Carbon Nanotube Films. ACS Nano 2016, 10 (5), 5204−5211. (21) Lamberti, A.; Clerici, F.; Fontana, M.; Scaltrito, L. A Highly Stretchable Supercapacitor Using Laser-Induced Graphene Electrodes onto Elastomeric Substrate. Adv. Energy Mater. 2016, 6 (10), 1600050. (22) Li, N.; Yang, G.; Sun, Y.; Song, H.; Cui, H.; Yang, G.; Wang, C. Free-Standing and Transparent Graphene Membrane of Polyhedron Box-Shaped Basic Building Units Directly Grown Using a Nacl Template for Flexible Transparent and Stretchable Solid-State Supercapacitors. Nano Lett. 2015, 15 (5), 3195−3203. (23) Chen, T.; Peng, H.; Durstock, M.; Dai, L. High-Performance Transparent and Stretchable All-Solid Supercapacitors Based on Highly Aligned Carbon Nanotube Sheets. Sci. Rep. 2014, 4, 3612. (24) Yu, A.; Roes, I.; Davies, A.; Chen, Z. Ultrathin, Transparent, and Flexible Graphene Films for Supercapacitor Application. Appl. Phys. Lett. 2010, 96, 253105. (25) Meng, Y.; Zhao, Y.; Hu, C.; Cheng, H.; Hu, Y.; Zhang, Z.; Shi, G.; Qu, L. All-Graphene Core-Sheath Microfibers for All-Solid-State, Stretchable Fibriform Supercapacitors and Wearable Electronic Textiles. Adv. Mater. 2013, 25 (16), 2326−2331. (26) Yang, Z.; Deng, J.; Chen, X.; Ren, J.; Peng, H. A Highly Stretchable, Fiber-Shaped Supercapacitor. Angew. Chem., Int. Ed. 2013, 52 (50), 13453−13457. (27) Wu, Q.; Xu, Y.; Yao, Z.; Liu, A.; Shi, G. Supercapacitors Based on Flexible Graphene/Polyaniline Nanofiber Composite Films. ACS Nano 2010, 4 (4), 1963−1970. (28) Zhang, K.; Zhang, L. L.; Zhao, X.; Wu, J. Graphene/Polyaniline Nanofiber Composites as Supercapacitor Electrodes. Chem. Mater. 2010, 22 (4), 1392−1401. (29) Niu, Z.; Dong, H.; Zhu, B.; Li, J.; Hng, H. H.; Zhou, W.; Chen, X.; Xie, S. Highly Stretchable, Integrated Supercapacitors Based on Single-Walled Carbon Nanotube Films with Continuous Reticulate Architecture. Adv. Mater. 2013, 25 (7), 1058−1064. (30) Yu, C.; Masarapu, C.; Rong, J.; Wei, B.; Jiang, H. Stretchable Supercapacitors Based on Buckled Single-Walled Carbon-Nanotube Macrofilms. Adv. Mater. 2009, 21 (47), 4793−4797. (31) Chen, T.; Xue, Y.; Roy, A. K.; Dai, L. Transparent and Stretchable High-Performance Supercapacitors Based on Wrinkled Graphene Electrodes. ACS Nano 2014, 8 (1), 1039−1046. (32) Kim, D.; Shin, G.; Kang, Y. J.; Kim, W.; Ha, J. S. Fabrication of a Stretchable Solid-State Micro-Supercapacitor Array. ACS Nano 2013, 7 (9), 7975−7982. (33) Liu, C.-H.; Yu, X. Silver Nanowire-Based Transparent, Flexible and Conductive Thin Film. Nanoscale Res. Lett. 2011, 6 (1), 75. (34) Gruner, G. Carbon Nanotube Films for Transparent and Plastic Electronics. J. Mater. Chem. 2006, 16 (35), 3533−3539. (35) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.-Y.; Hong, B. H. Large-Scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes. Nature 2009, 457 (7230), 706−710. (36) Hu, L.; Kim, H. S.; Lee, J.-Y.; Peumans, P.; Cui, Y. Scalable Coating and Properties of Transparent, Flexible, Silver Nanowire Electrodes. ACS Nano 2010, 4 (5), 2955−2963. (37) Krantz, J.; Richter, M.; Spallek, S.; Spiecker, E.; Brabec, C. J. Solution-Processed Metallic Nanowire Electrodes as Indium Tin Oxide Replacement for Thin-Film Solar Cells. Adv. Funct. Mater. 2011, 21 (24), 4784−4787. (38) Lee, J.; Lee, P.; Lee, H.; Lee, D.; Lee, S. S.; Ko, S. H. Very long Ag Nanowire Synthesis and Its Application in a Highly Transparent, Conductive and Flexible Metal Electrode Touch Panel. Nanoscale 2012, 4 (20), 6408−6414. (39) Akter, T.; Kim, W. S. Reversibly Stretchable Transparent Conductive Coatings of Spray-Deposited Silver Nanowires. ACS Appl. Mater. Interfaces 2012, 4 (4), 1855−1859. (40) Giovanni, M.; Pumera, M. Size Dependant Electrochemical Behavior of Silver Nanoparticles with Sizes of 10, 20, 40, 80 and 107 nm. Electroanalysis 2012, 24 (3), 615−617. (41) Shafiee, S.; Topal, E. An Overview of Global Gold Market and Gold Price Forecasting. Resour. Policy 2010, 35 (3), 178−189. J


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