Skin-Like, Dynamically Stretchable, Planar Supercapacitors with

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Skin-Like, Dynamically Stretchable, Planar Supercapacitors with Buckled Carbon Nanotube/Mn-Mo Mixed Oxide Electrodes and Air-Stable Organic Electrolyte Geumbee Lee, Jung Wook Kim, Heun Park, Jae Yoon Lee, Hanchan Lee, Changhoon Song, Sang Woo Jin, Kayeon Keum, Chul-Ho Lee, and Jeong Sook Ha ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b08645 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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Skin-Like, Dynamically Stretchable, Planar Supercapacitors with Buckled Carbon Nanotube/Mn-Mo Mixed Oxide Electrodes and Air-Stable Organic Electrolyte Geumbee Lee,a Jung Wook Kim,b Heun Park,b Jae Yoon Lee,a Hanchan Lee,b Changhoon Song,a Sang Woo Jin,b Kayeon Keum,a Chul-Ho Lee,b and Jeong Sook Haa,b*

aKU-KIST

Graduate School of Converging Science and Technology, Korea University,

145 Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea

bDepartment

of Chemical and Biological Engineering, Korea University, 145 Anam-ro,

Seongbuk-gu, Seoul, 02841, Republic of Korea

Corresponding author *E-mail: [email protected] (Jeong Sook Ha)

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ABSTRACT For practical applications of high-performance supercapacitors as wearable energy storage devices attached to skin or clothes, the supercapacitors are recommended to have stable mechanical and electrochemical performances during dynamic deformations, including stretching, due to real-time movements of the human body. In this work, we demonstrate a skin-like, dynamically stretchable, planar supercapacitor (SPS). The SPS consists of buckled manganese/molybdenum (Mn/Mo) mixed oxide@multi-walled carbon nanotube (MWCNT) electrodes; organic gel polymer electrolyte of adiponitrile, succinonitrile, lithium bis(trifluoromethanesulfonyl)imide, and poly(methylmethacrylate); and a porous, elastomeric substrate. The addition of a Mn/Mo mixed oxide to the MWCNT film produces an 8-fold increase in the areal capacitance. The use of an organic solvent-based electrolyte enhances the operation cell voltage to 2 V and air stability to one month under ambient air conditions. The fabricated planar supercapacitors are biaxially stretchable up to 50% strain and maintain ~90% of their initial capacitance after 1000 repetitive stretching/releasing cycles. Furthermore, the SPS exhibits stable electrochemical performance under dynamic stretching in real time regardless of the strain rate and performs reliably during repetitive bending/spreading motions of an index finger while attached to skin.

KEYWORDS: planar supercapacitors, stretchable supercapacitors, pseudocapacitors, organic electrolyte, skin-attachable electronics


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Skin-attachable or wearable devices are required to be flexible/stretchable to maintain their performance under deformation when attached to curved surfaces. With tremendous advancements in flexible/stretchable devices, interests in soft power sources as a sustainable energy supply have dramatically increased.1-4 Among energy storage/conversion devices, including batteries, supercapacitors, photovoltaics, and generators, supercapacitors have many advantages, such as a simple fabrication process, fast charging/discharging, and excellent cycle stability, over batteries. Accordingly, various efforts have been made recently to demonstrate the high potential of supercapacitors as soft power supply devices. Strategies to obtain stretchable supercapacitors are classified according to their strain-relieving mechanisms into three conventional designs, wavy/buckled, bridge-island, and porous/textile. The buckled design is most widely used and leads to wavy electrodes via releasing a prestrained elastomeric substrate or wire.5,6 Planar rigid devices (islands) can be stretched when they are connected with stretchable interconnections (bridges) of serpentine, polymer-encapsulated thin metal films or patterned liquid metals on a stretchable, soft substrate.7,8 Porous/textile designs provide a continuous electrical pathway under stretching via contact between active materials coated on porous/textile elastomers.9,10 For practical applications of supercapacitors as power sources for skin-attachable electronics, several conditions should be satisfied. First, the supercapacitors should have excellent electrochemical performance, such as high operation voltage, capacitance, energy and power densities, and air stability. Second, they should have a small size and small thickness for facile integration with other devices while attached to skin. Third, they should have omnidirectional stretchability to accommodate possible complex deformations due to skin movement on elbows or knees. Furthermore, the supercapacitors must be dynamically as well as statically stretchable without any performance degradation under real-time deformations. 4 ACS Paragon Plus Environment

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In reality, however, difficulties in practical applications of stretchable supercapacitors have been frequently reported. Above all, the lack of intrinsically stretchable materials makes the fabrication of high-performance supercapacitors difficult. The necessity of using a large amount of electrode material for improved performance usually leads to supercapacitors with large dimensions.11,12 Even though a stretchable array of supercapacitors can be fabricated using stretchable interconnections, the fabrication process is complicated, and the resultant device is large.8,13,14 Compared to sandwich-type supercapacitors with two stacked electrodes and electrolyte, a planar supercapacitor has the advantage of a smaller thickness to facilitate compact packing with other electronic devices and application to portable/wearable devices.15,16 Furthermore, a few attempts for electrode arrangements in parallel are reported, including developments of thin supercapacitors via slicing method17 and supercapacitors with high operation voltage by mimicking the electric eel.18 For most stretchable supercapacitors, electrochemical measurements are conducted in static mode.19-21 However, confirming their stable performance under dynamically applied strain is recommended for practical application as skin-attachable energy storage devices. In this work, we introduce a strategy for fabricating a dynamically stretchable planar supercapacitor (SPS) with high electrochemical performance via the selection of materials and design of optimized fabrication processes. The electrodes consist of multi-walled carbon nanotubes (MWCNTs) and manganese/molybdenum (Mn/Mo) mixed oxide, and the electrolyte

is

made

of

adiponitrile/succinonitrile/lithium

bis(trifluoromethanesulfonyl)imide/poly(methyl methacrylate) (ADN/SN/LiTFSI/PMMA) in a gel state. These components are placed on a prestrained porous, elastomeric substrate, resulting in an SPS with mechanical stability during stretching and high electrochemical performance. In particular, a mixture of potentiodynamically electrodeposited Mn/Mo oxide 5 ACS Paragon Plus Environment

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results in a much higher capacitance than pure Mn oxide, and the deposition mass and specific capacitance linearly increase with the number of electrodeposition cycles. In addition, the use of an organic solvent-based electrolyte leads to a high operation cell voltage of 2 V and high air stability even without encapsulation of the planar supercapacitor. Recently, studies on the development of intrinsically stretchable planar supercapacitors have been reported. Xiao et al. reports the stretchable microsupercapacitors arranged in three tandem with output voltage of 2.4 V (0.8 V for a single device). Their mechanical stability was evaluated uniaxially under static strain mode.22 Park et al. describes fully laser-patterned stretchable microsupercapacitors with a low operation voltage of 0.8 V. Although, stretchability of the supercapacitor was investigated in both X- and Y-directions, respectively, the performance degradation occurred with stretching.23 Unlike previous works, we fabricated the high-performance, SPS with improved operation voltage and excellent air stability and confirmed their biaxial stretchability in both static and dynamic modes. The fabricated thin SPS with a thickness of ~270 µm can be conformally attached to skin on a finger, and its performance was stable during dynamic finger joint movements. This work demonstrates a skin-like, high-performance, dynamically stretchable planar supercapacitor with high potential as a soft power source in skin-attachable electronics.


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RESULTS AND DISCUSSION The whole structure and fabrication process of an SPS with buckled electrodes and organic solvent-based electrolyte are schematically illustrated in Figure 1a and described in more detail in the experimental section. A porous, deformable silicon rubber film with micropores on the surface is fabricated by using sandpaper with a roughness as a mold. The porous film is softer and more hydrophobic but less transparent (semitransparent) than the nonporous film (Figure S1). A chromium/gold (Cr/Au, 10/100 nm) film, a current collector, is deposited onto a 50% prestrained porous film using a shadow mask to create an interdigitated electrode pattern with 300-µm interspaces. Then, MWCNTs functionalized according to our previous report21 are spray coated onto the prestrained Cr/Au film, and then, Mn/Mo mixed oxide is electrodeposited to enhance the electrochemical performance. Although Mn oxide-based thin films have been widely used as supercapacitor electrodes in recent years, their charge-storage capabilities have been investigated via the incorporation of carbon or other transition metals, such as Ni or Pb, due to their low electrical conductivity.21,24 According to previous work, Mn/Mo mixed oxide films have a much higher conductivity than pure Mn oxide films.25 Therefore, we tried to apply a facile and simple method for fabricating a high capacity supercapacitor with a Mn/Mo mixed oxide film in this work. The scanning electron microscopy (SEM) images of the Mn/Mo@MWCNT electrode in Figure 1b show both the released and stretched states. Isotropic and uniform buckled structures are visible in the released state, while the buckled structures become smooth under biaxial stretching. The stretchability of the buckled film is strongly dependent on the extent of the applied prestrain. For example, films with smaller and more dense buckles can endure larger strains.26 Here, we apply a biaxial prestrain of 50%, which is sufficient to consider the possible strain induced by human joint movements while a device is attached to human skin.27,28 Figure 1c summarizes the mechanical stability of the buckled 7 ACS Paragon Plus Environment

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Mn/Mo@MWCNT electrode. In order to measure the resistance of buckled electrodes, we used two-point probe method with the square buckled electrode of 0.64 cm2 area, as shown in Figure S2a. Applied strain onto the buckled electrode is defined in Figure S2b. The normalized resistance is R/R0, where R and R0 (4.4 Ω) are the resistance of the electrode after and before application of biaxial stretching, respectively. As the applied strain increased up to 50%, the distance between two probes for the measurement also increased from ~11 to 17 mm. Accordingly, the resistance increased slightly by ~9% at 50% strain. On the other hand, the interval between probes was kept constant at ~11 mm in the resistance measurement for 1000 repetitive stretching/releasing cycles under 50% strain. In this way, the resistance was measured in released state after stretching cycle. Clearly, 1000 repetitive stretching/releasing cycles with up to 50% strain do not degrade the electrical conductivity of the buckled Mn/Mo@MWCNT electrode. After drop casting the gel polymer electrolyte (ADN/SN/LiTFSI/PMMA) on the relaxed Mn/Mo@MWCNT electrodes, the SPS is successfully prepared. Detailed dimensions of the relaxed and fully stretched SPS are shown in Figure S3. The fabricated thin (~270 µm) and small (8 mm×8 mm) SPS could be conformally attached onto a glass rod with a diameter of 5 mm and freely stretched. The optical images are shown in Figure S4. To observe the surface morphology of the electrodes fabricated using different thin-film preparation methods, SEM images of the spray-coated MWCNT film, electrodeposited Mn/Mo mixed oxide, and Mn/Mo@MWCNT composite film were collected, as shown in Figure 2a. The MWCNT film densely covers the whole electrode surface, and the Mn/Mo mixed oxide particles form a bumpy surface. In the composite film, MWCNT fibrils are wrapped with Mn/Mo mixed oxide particles smaller than those electrodeposited without the MWCNT film. The energy dispersive spectroscopy (EDS) mapping results, as shown in Figure 2b, confirm 8 ACS Paragon Plus Environment

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that the component elements Mn, Mo, O and C are homogeneously distributed over the entire Mn/Mo@MWCNT electrode surface. To investigate the oxidation state of the Mn/Mo mixed oxide, X-ray photoelectron spectroscopy (XPS) measurements were performed. In the Mn 2p spectrum (Figure 2c), doublets appear at 642.4 eV (Mn 2p3/2) and 654.2 eV (Mn 2p1/2) with a difference of 11.8 eV. These peaks perfectly coincide with the binding energies of Mn4+, suggesting the formation of manganese dioxide (MnO2).29,30 Figure 2d shows the Mo 3d XPS spectrum, and two peaks at 232.2 eV (Mo 3d5/2) and 235.3 eV (Mo 3d3/2) are observed with an interval of 3.1 eV. These binding energies confirm the oxidation state of Mo6+.31,32 The three deconvoluted O 1s peaks at 530.0, 531.4 and 532.4 eV (Figure 2e) correspond to the lattice oxygen in metal oxides (O2-), hydroxide (OH-) and residual water (H2O), respectively, indicating the growth of metal oxides.33 The Mn/Mo mixed oxide is grown via potentiodynamic electrodeposition, as described in the experimental section. To investigate the effect of the concentration of molybdate ions (MoO42-) on the capacitive performance, five different electrodeposition baths were prepared with 2 mM MnSO4 and 0, 10, 20, 40 and 60 mM Na2MoO4. As shown in Figure 2f, the Mn/Mo mixed oxide electrodeposited from 20 mM MoO42- exhibits the best capacitive performance. Thus, we used the electrodes fabricated at this concentration in this study. The Mn/Mo mixed oxide film exhibits a more than 3.5 times higher capacitance than the pure Mn oxide film, which is attributed to the absorption of free carriers originating from the enhanced film conductivity, as mentioned earlier.25 The cyclic voltammetry (CV) curves of the pure Mn oxide film and Mn/Mo mixed oxide film during potentiodynamic oxidation are shown in Figure S5. The anodic current appearing at approximately +0.6 V indicates that Mn2+ oxidation occurs via the following chemical reaction.25 𝑀𝑛2 + + 2𝐻2𝑂→𝑀𝑛𝑂2 + 4𝐻 + + 2𝑒 9 ACS Paragon Plus Environment

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As seen in Figure S6, the CV curve recorded in a pure MoO42- solution without Mn2+ precursors does not show an anodic current, suggesting that MoO42- is electrochemically inactive in this potential region. A possible mechanism to explain the growth of the Mn/Mo mixed oxide film has been suggested. Initially, the protons accumulated via electrooxidation of Mn2+ react with MoO42- ions around the electrode surface. Then, polymolybdate(VI) species, typically Mo7O246- with a +6 oxidation state, as expected from XPS spectra, are the dominant species formed under such acidic conditions.25,34 Based on this mechanism, we could reproducibly fabricate this electrode with high performance via an easy and simple one-step deposition process. Figure 2g represents the CV curves taken in 0.5 M Na2SO4 electrolyte at a scan rate of 50 mVs-1 with the Mn/Mo mixed oxide electrodes subjected to different numbers of deposition cycles. The SEM images corresponding to each electrode are in Figure S7. With an increase in the number of deposition cycles, the area of the CV curves also increases. In particular, the areal capacitances of the Mn/Mo mixed oxide films calculated from the CV curves in Figure 2g linearly increase with the number of deposition cycles (Figure 2h), leading to an increase proportional to the deposited mass of the Mn/Mo mixed oxide (Figure S8). The areal capacitances of each electrode is calculated at various scan rates including 50 mVs-1 as shown in Figure S9. Additionally, four Mn/Mo mixed oxide electrodes fabricated under the same deposition conditions have almost the same capacitive performance (Figure S10). These results demonstrate that this one-step electrodeposition of the Mn/Mo mixed oxide film is a highly reproducible fabrication technique in which the performance of the electrode can be easily controlled via adjusting the number of deposition cycles. Considering the fabrication time, we used the 36-cycle deposited Mn/Mo film with an electrodeposition time of 1 h in this work. To evaluate the pseudocapacitive behavior of the fabricated Mn/Mo mixed oxide film and its 10 ACS Paragon Plus Environment

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feasibility as a supercapacitor electrode, three-electrode measurements were conducted. Pseudocapacitors based on MnO2, Nb2O5 and RuO2, commonly known pseudocapacitive materials, exhibit improved capacitance due to redox reactions between electrode materials and ions in the electrolyte, while electrical double layer capacitors (EDLCs) store charges through the physical adsorption/desorption of ions in the electrolyte.35,36 The pseudocapacitive materials show the electrochemical signatures representative of carbon-based materials, i.e., rectangular-shaped CV curves, symmetric triangular profiles for the galvanostatic chargedischarge (GCD) curves and vertical lines parallel to the imaginary region in the Nyquist plots.37 The electrochemical properties of the Mn/Mo@MWCNT composite film satisfy the standards required for pseudocapacitive materials (Figure S11), i.e., rectangular CV curves, symmetric triangular GCD curves, and ideal Nyquist plots. These behaviors are consistent with the dramatically enhanced electrochemical performance of the Mn/Mo@MWCNT film compared to that of the MWCNT film (Figure S12). The areal capacitance increases by at least 8 fold with the addition of the Mn/Mo mixed oxide to the MWCNT film. Planar supercapacitors with both electrodes in the same plane guarantee a reduced thickness compared to stacked supercapacitors due to the absence of a separator, and the reduced thickness facilitates integration with other devices and conformal attachment onto skin. However, the critical issue of electrolyte degradation during exposure to ambient air conditions remains in planar supercapacitors without encapsulation. Accordingly, developing electrolytes with long-term air stability is essential.21 Aqueous electrolytes, which have been commonly used in recent planar-type supercapacitors, are reported to hinder high performance.38,39 Due to the electrochemical decomposition voltage of water, the operation voltage in supercapacitors is limited to below 1 V. In addition, the evaporation of water under ambient air conditions results in capacitance degradation due to the reduction in ion mobilities. Thus, we fabricate an 11 ACS Paragon Plus Environment

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organic solvent-based gel polymer electrolyte (GPE) consisting of adiponitrile, succinonitrile, lithium

bis(trifluoromethanesulfonyl)imide,

and

poly(methylmethacrylate)

(ADN/SN/LiTFSI/PMMA) and utilize it in our planar supercapacitor. The chemical structures of the major components in the electrolyte and a linear sweep voltammetry curve that confirms the electrochemical stability window (ESW) of the electrolyte are shown in Figure 3a. The ESW of the ADN/SN/LiTFSI/PMMA electrolyte is from -3.5 V to 3.5 V in a cell consisting of symmetric gold electrodes. In this measurement, inert metals such as gold, platinum or stainless steel are generally preferred. This is to understand only the decomposition of constituent materials of the electrolyte within a wide potential range. For this reason, we tried to check the electrochemical stability of the gel polymer electrolyte with symmetric 100 nm-thick gold electrodes, used as a current collector in our stretchable planar supercapacitors later. In recent studies, LSVs of GPEs (e.g., [BMIM][BF4]/ethoxylated trimethylolpropane triacrylate (ETPTA), [BMIM][BF4]/succinonitrile etc.) were also recorded with two symmetric platinum electrodes and identical stainless steel electrodes.16,40 The current density remained nearly 0 during scanning at a scan rate of 5 mVs-1, which means that no decomposition of the components occurs within this potential range. Figure 3b shows the impedance spectra (Nyquist plots) in the frequency region from 1 MHz to 10 Hz collected for one month. All the slopes for the plots and equivalent series resistance (ESR) values are nearly constant, indicating that the conductivity of the electrolyte is maintained regardless of the exposure time to ambient air for one month. Additionally, we evaluated the Nyquist plots of the electrolyte exposed to ambient air conditions for more than one month as shown in Figure S13. The plot after two months did not show any noticeable change compared to that of initial state. After four months, slope of the plot slightly decreased and the ESR value increased by ~16% compared to the initial value. To understand the high air stability of our GPE, we took the Raman spectra from the GPE exposed to air with elapsed time (Figure 3c). The Raman spectra of each component 12 ACS Paragon Plus Environment

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constituting the electrolyte and the fabricated GPE are also shown in Figure S14. As shown in the spectrum obtained in initial state of the GPE, Raman bands of all components are clearly observed. And no significant change in the Raman spectra during exposure to ambient air for one month also suggests the excellent air stability of the ADN/SN/LiTFSI/PMMA electrolyte. This stability is attributed to the low vapor pressure of the organic solvent.21 The vapor pressures of ADN and SN are ~0.1 Pa41 and ~1 Pa,42 respectively, while that of water is ~3,000 Pa at room temperature. Therefore, in the supercapacitors with aqueous electrolytes, ion mobility decreases due to the evaporation of water, leading to the degradation of device performance with elapsed time in air ambient condition.38,39 The fabricated ADN/SN/LiTFSI/PMMA can be a promising GPE with enhanced operation voltage and better air stability than water-based electrolytes. Although, ADN, SN and LiTFSI are known to be slightly toxic, these organic solvents and conducting salts are commonly used as an electrolyte material for high-performance batteries and supercapacitors. Mostly, organic solvents-based electrolytes are used in supercapacitors for high operational cell voltage or air stability.21,43,44 Concern on the toxicity of organic solvents can be alleviated via encapsulation. Complete encapsulation of the devices exposed to air can provide chemical safety to human skin while those devices are applied to skin-attachable/wearable electronics.21 Therefore, the issue of developing high-performance (e.g., wide operation voltage and air stability etc.) electrolyte with functionalities (e.g., environment-friendly, biocompatibility, non-flammability and non-toxic etc.) will be the subject of future investigations. The electrochemical performance of the stretchable planar supercapacitor (SPS) with buckled Mn/Mo@MWCNT electrodes and ADN/SN/LiTFSI/PMMA GPE is evaluated (Figure 4). CV curves measured at a scan rate of 200 mVs-1 (Figure 4a) clearly show that the fabricated SPS could stably operate in different voltage windows ranging from 1.0 to 2.0 V. As shown in 13 ACS Paragon Plus Environment

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Figure 4b, the CV curves show symmetric charge/discharge current levels and quasirectangular shapes at various scan rates. The GCD curves taken at various current densities indicate the almost linear profile during charging/discharging in Figure 4c. The areal capacitance (Ccell,A) of SPS is calculated from the GCD curves with respect to the current densities, as shown in Figure 4d. The total projection area of the SPS with interdigitated electrodes is ~0.64 cm2 including interspaces. The maximum Ccell,A of 7.5 mFcm-2 is obtained at a current density of 0.3 mAcm-2. Figure 4e presents the changes in capacitance over 5000 repetitive charge/discharge cycles at a current density of 1.0 mAcm-2. The Coulombic efficiency (η) is maintained during cycling, and the capacitance retention is approximately 88% of the initial capacitance after 5000 charge/discharge cycles. The slight increase in the capacitance appearing in the initial cycles is attributed to the activation process that occurs when the electrolyte wets the electrode surface.45 In addition, excellent cyclic stability is demonstrated with no significant change in the GCD curves after 5000 cycles (inset of Figure 4e). The self-discharge property is another important concern for the practical application of supercapacitors. Therefore, we obtained self-discharge curves after charging at different current densities (Figure S15). The self-discharge process occurred more slowly as the current density was smaller, which is consistent with the previous reports.46,47 When the SPS is fully charged to 2 V at current densities of 0.05, 0.25, and 0.5 mAcm-2, voltage drop to 1 V took 5.3, 2.4, and 0.6 h, respectively. As expected and shown in Figure 3, the performance of the SPS remains stable under ambient air for one month due to the good air stability of ADN/SN/LiTFSI/PMMA (Figure S16). In the Ragone plot in Figure 4f, the energy and power densities of our SPS calculated from the GCD curves are compared to those of other supercapacitors based on Mn oxide or Mo oxide electrodes. Our fabricated SPS has an energy density of 4.2 µWhcm-2 at a power density of 0.3 14 ACS Paragon Plus Environment

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mWcm-2 and a power density of 1.0 mWcm-2 at an energy density of 1.4 µWhcm-2, which are comparable or superior to those of recently reported supercapacitors.15,48-50 To apply a supercapacitor as a soft power source for skin-attachable electronics, the supercapacitor should be flexible and stretchable so as to retain its performance regardless of body movements. Thus, we check the mechanical stability of our SPS upon application of strain in static mode. Especially, skin-attachable/wearable electronics require the encapsulation strategies, which contributes to the protection of devices from external impacts and guarantees human safety from harmful chemicals. In our previous work on planar microsupercapacitors with organic solvent-based electrolyte, the devices were encapsulated using thin elastomer films.21 As a result, the skin-attached supercapacitors showed stable electrochemical properties under bending of the wrist or knee as well as in water, demonstrating the complete encapsulation of the device. However, since long-term air stability of SPS was guaranteed as shown in Figure S16, we conducted experiments in ambient air condition without any encapsulation of planar devices. The GCD curves in Figure 5a show no noticeable change with increasing biaxial strain up to 50%, indicating that the normalized capacitance remains unchanged. Furthermore, the capacitance retains ~90% of its initial value even after 1000 stretching/releasing cycles via application of steadily increasing strain from 10 to 50% (Figure 5b). At the end of every 50 cycles, charge/discharge measurements were performed three times at a current density of 1.0 mAcm-2 to calculate the capacitance. As a result, our stretchable planar supercapacitor experienced a total of 63 times charge/discharge tests, including the measurement for initial capacitance. As shown in Figure 4e, the capacitance retention is approximately 100% of the initial capacitance after 1,000 charge/discharge cycles. Therefore, the decrease in performance after 1,000 times fatigue test is originated from repeatedly applied strain, not from the electrochemical irreversibility of the electrodes during 63 times 15 ACS Paragon Plus Environment

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charge/discharge measurements. Although mechanical stability of a single buckled electrode was demonstrated in Figure 1c, whole device containing gel electrolyte might experience mild fractures and delamination of electrode materials under repeatedly applied strain.20,51 The operating voltage and capacitance can be adjusted by connecting multiple supercapacitors in series and parallel, respectively, as schematically demonstrated in the equivalent circuit diagrams in Figure 5c. The corresponding GCD curves are shown in Figure 5d. Three serially connected SPSs exhibit an output voltage of 6 V, which is three times larger than that of a single SPS, and three SPSs connected in parallel have a three times longer discharge time than a single SPS. After attaching an SPS array, consisting of two serially connected SPSs and a light emitting diode with a turn-on voltage of 1.6 V, onto a balloon using printed GaInSn liquid metal interconnections, the LED could operate for 2 min (Figure 5e). Detailed information on the LED is given in our previous work.15 Even after inflation of the balloon to induce a sheer stress to the attached SPS array with an applied strain of 50%, the brightness and operation time of the LED remain constant. The charging and discharging behavior of the SPS also remains constant. As shown in Figure S17, although our GPE is not completely in a solid state, it stays for a while on the substrate due to its viscosity. In contrast, slight falling is observed in liquid electrolyte fabricated without PMMA. Generally, the viscosity of GPE can be controlled by the content or molecular weight of the polymer. A little fluidity of the GPE not only enabled the stable wetting on top of the electrodes during repeated stretching/releasing experiments, but also allowed sufficient time for evaluating the device performance while attached on to a curved surface such as a balloon or finger joint. Furthermore, all stretching tests were conducted in plane under both static and dynamic modes (insets of Figure 5b). Therefore, the gel electrolyte did not flow out and we could perform stretching experiments without any 16 ACS Paragon Plus Environment

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encapsulation for SPSs. After estimating the static stretchability of our SPS, we further studied its electrochemical performance under the application of dynamic strain to evaluate the possible application of our SPS to skin-attachable electronics where unexpected deformation can always be induced by body movement. Most mechanical stability tests on stretchable supercapacitors have been performed via the application of static strain. Thus far, only a few groups have reported the dynamic stability of stretchable energy storage devices.52-54 Figure 6a displays the dynamically recorded CV curves of our SPS at various strain rates of 3, 6, and 10% s-1. Since it takes 10 s to reach a biaxially stretched state of 30% at a strain rate of 3% s-1, the CV curve measured at a scan rate of 200 mVs-1 includes a single stretching/releasing cycle. Similarly, two and three stretching/releasing cycles are represented in the CV curves obtained at a strain rate of 6 and 10% s-1, respectively. All the dynamically recorded CV curves under applied strain overlap with the CV curve taken prior to the dynamic stretching. Figure 6b shows the GCD curves and capacitance retention dynamically obtained at sequential strain rates of 3, 6, and 10% s-1. A single charge/discharge cycle was synchronized with a stretching/releasing cycle. The dynamic capacitance is maintained at the initial value regardless of the strain rate. Next, our SPS is attached to an index finger, and its performance was evaluated simultaneously with finger motion, as shown in Figure 6c. Here, the stretching/releasing cycle can be represented by the bending/spreading motion of the finger. A single bending/spreading process that takes 1.2 s is converted into a frequency of 0.8 Hz, which is reasonable considering the frequency range of human motions.55 The GCD curve dynamically recorded during four repetitive cycles of bending/spreading demonstrates very stable performance without any signal variations. It is also clearly shown that the GCD curve taken under finger motion does not deviate from that without applied strain. These results clearly indicate the excellent mechanical stability of our skin-like stretchable SPS and suggest its high potential for 17 ACS Paragon Plus Environment

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application in skin-attachable electronics as an integrated energy storage device.

CONCLUSION A high-performance, thin, planar supercapacitor with dynamic stretchability is fabricated via deliberate selection of both electrode and electrolyte materials and specialized fabrication of a soft substrate film. Electrodeposition of Mn/Mo mixed oxide on MWCNTs induces pseudocapacitance, dramatically increasing the capacitance. A porous elastomer film is used as a prestrained substrate, leading to deposition of a buckled Mn/Mo@MWCNT electrode. Via use of an organic gel polymer electrolyte (ADN/SN/LiTFSI/PMMA), the operation cell voltage is increased to 2 V, and the stability under air-ambient conditions is enhanced to up to one month without encapsulation. As a result, our fabricated planar supercapacitor exhibits an areal capacitance of 7.5 mFcm-2 at a current density of 0.3 mAcm-2, an areal energy density of 4.2 µWhcm-2 at a power density of 0.3 mWcm-2 and a power density of 1.0 mWcm-2 at an energy density of 1.4 µWhcm-2. The supercapacitor exhibits stability under both static and dynamic stretching deformation with strains up to 50 and 30%, respectively. These results demonstrate high-performance, biaxially stretchable, planar supercapacitors with high durability under realtime dynamic biaxial stretching, suggesting their application in various wearable, skinattachable electronics as soft energy storage devices.


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METHODS Electrodeposition of the Mn/Mo mixed oxide Electrodeposition was conducted to obtain the Mn/Mo mixed oxide via a three-electrode cell. The electrodeposition solution bath consisted of 2 mM MnSO4 (Sigma-Aldrich), 20 mM Na2MoO4 (Sigma-Aldrich) and a moderate amount of Na2SO4 (Sigma-Aldrich), where the total concentration of sodium was fixed to 120 mM. The spray-coated MWCNT film on Au current collector, platinum (Pt) coil and Ag/AgCl electrode served as the working electrode, counter electrode and reference electrode, respectively. Mn/Mo mixed oxide was deposited on the working electrode by potentiodynamic deposition in the potential window from 0 to +1 V at a scan rate of 20 mVs-1. Preparation of the ADN/SN/LiTFSI/PMMA gel polymer electrolyte For the preparation of the ADN/SN/LiTFSI/PMMA gel polymer electrolyte, 2.87 g of LiTFSI (Sigma-Aldrich) was added to 10 mL of an ADN/SN (Sigma-Aldrich) mixture with a weight ratio of 1:1. After completely dissolving the LiTFSI, 0.7 g of PMMA (Mw ~996,000, Aldrich) was added into the solution. By stirring and heating at 100℃ for 1 h, the solution became clear and changed into a gel state. Fabrication of a stretchable planar supercapacitor The porous elastomeric substrate was prepared by pouring an uncured mixture of 7 g polydimethylsiloxane (Dow Corning) and Ecoflex (Smooth-On) with a mass ratio of 8:2 into a 10 cm × 10 cm piece of sandpaper. After curing at 65℃ for 40 min, the elastomeric substrate was peeled off from the sandpaper. The porous substrate was biaxially stretched by 50%, and then, each edge of the substrate was fixed with metal clips. Subsequently, patterned layers of a Cr/Au thin film (10/100 nm), MWCNTs and Mn/Mo mixed oxide were deposited using a 19 ACS Paragon Plus Environment

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shadow mask via e-beam evaporation, spray coating and electrodeposition, respectively, and served as electrodes. After removing the prestrain from the porous elastomer, the prepared gel polymer electrolyte was cast on the patterned electrode to fabricate an intrinsically stretchable, planar supercapacitor with buckled electrodes. Characterizations The porous elastomer images were obtained using an optical microscope, and the morphologies of buckled and relaxed Mn/Mo@MWCNT electrodes were observed using scanning electron microscopy (SEM, Hitachi S4800 and Quanta 250 FEG). X-ray photoelectron spectroscopy (XPS, X-TOOL, ULVAC-PHI) measurement was performed to investigate the oxidation state of electrodeposited Mn/Mo mixed oxide. Raman spectra were taken using a home-built spectrometer equipped with a monochromator (Andor, SOLIS303i) and an excitation laser with wavelength of 532 nm and power of 3 mW to observe the stability of the gel polymer electrolyte during exposure to air. Electrochemical measurements and calculations To evaluate the electrochemical performance of individual electrode and as-prepared stretchable supercapacitor, CV and GCD curves were obtained in a three-electrode cell and two-electrode cell using an electrochemical analyzer (Ivium Technologies, Compact Stat). In the three-electrode cell with 0.5 M Na2SO4 electrolyte, Ag/AgCl, Pt wire, and the individual electrode are used as reference, counter, and working electrodes, respectively. Both the capacitance of individual electrode (Celectrode) and that of cell (Ccell) are calculated from the CV curves using the following equation:56 𝑉

𝐶𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒 = 𝐶𝑐𝑒𝑙𝑙 =

∫𝑉 + 𝑖(𝑉)𝑑𝑉 ―

2𝑉𝑣


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𝑉

where ∫𝑉 + 𝑖(𝑉)𝑑𝑉 is obtained by integrating the CV curve, V is the potential window, and v ―

is the scan rate. Both Celectrode and Ccell also can be calculated from GCD curves according to the following equation:

𝐶𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒 = 𝐶𝑐𝑒𝑙𝑙 =

𝑖 𝑑𝑉 ( ) 𝑑𝑡

where i and dV/dt correspond to the discharge current and the slope of the discharge curve, respectively. The areal capacitance of single electrode and that of cell were calculated using the relationships of Celectrode,A=Celectrode/Aelectrode and Ccell,A=Ccell/Acell, respectively. Aelectrode is the whole projection area of single electrode and Acell is the projection area of the planar supercapacitor including interspaces between the two electrodes. Here, the total area of the stretchable supercapacitor is approximately 0.64 cm2, as shown in Figure S3. The areal energy density (Ecell,A) and power density (Pcell,A) were calculated by the following equations.

𝐸𝑐𝑒𝑙𝑙,𝐴 =

𝑃𝑐𝑒𝑙𝑙,𝐴 =

𝐶𝑐𝑒𝑙𝑙,𝐴 × ∆𝑉2 2 × 3,600

𝐸𝑐𝑒𝑙𝑙,𝐴 × 3,600 𝑡𝑑𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑒

where ∆𝑉 is the voltage window and 𝑡𝑑𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑒 is the discharge time.


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Figure 1. (a) Schematic illustration of the structure and fabrication process of a biaxially stretchable planar supercapacitor. (b) SEM images of a buckled electrode before (left) and after (right) application of a 50% biaxial strain. (c) Normalized electrical resistance of the buckled electrode as a function of applied biaxial strain (top) and during 1000 biaxial stretching/releasing cycles under a biaxial strain of 50% (bottom).


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Figure 2. (a) Surface SEM images of MWCNT, Mn/Mo mixed oxide, and Mn/Mo@MWCNT electrodes, respectively. (b) EDS mapping of Mn, Mo, O, and C corresponding to the SEM image of the Mn/Mo@MWCNT electrode (all scale bars are 500 nm). XPS spectra of Mn/Mo mixed oxide in the energy regions of (c) Mn 2p, (d) Mo 3d, and (e) O 1s. (f) Comparison of the specific capacitance with the concentration of MoO42- ions. (g) CV curves and (h) areal capacitances with cycle number for the electrodeposition of Mn/Mo mixed oxide.


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Figure 3. (a) (Left) Scheme of the GPE consisting of 1 M LiTFSI in ADN/SN and PMMA, indicating Li+ and TFSI- ions. (Middle) Molecular structures of the component materials. (Right) Linear sweep voltammetry of the GPE recorded by two gold electrodes at a scan rate of 5 mVs-1. (b) Nyquist plots of a symmetric planar cell (Au/GPE/Au) with elapsed time in ambient air. The inset shows the change in the equivalent series resistance with elapsed time in air. (c) Raman spectra in the frequency region of 600-1600 cm-1 taken from GPE exposed to ambient air for a month.


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Figure 4. Electrochemical characterization of the fabricated supercapacitor with buckled Mn/Mo@MWCNT electrodes and ADN/SN/LiTFSI/PMMA GPE. CV curves measured at (a) different voltage ranges of 1.2-2.0 V and (b) various scan rates of 10-300 mVs-1. (c) GCD curves at current densities from 0.3 to 0.8 mAcm-2. (d) Dependence of areal capacitance on current density from 0.3 to 1.0 mAcm-2. (e) Capacitance retention during repetitive charge/discharge cycles at a current density of 1.0 mAcm-2. (f) Ragone plots in comparison to those of other supercapacitors with Mn- or Mo-based electrodes.


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Figure 5. Electrochemical performance of a biaxially stretchable planar supercapacitor (SPS) measured under static stretching. (a) GCD curves measured under a biaxial strain of 10-50% and the corresponding normalized capacitance values at a current density of 0.8 mAcm-2. (b) Capacitance retention as a function of stretching/releasing cycle number under different biaxial strains. The inset shows the optical images of SPS before (left) and after (right) stretching (scale bar, 1 cm). (c) Schematic diagram of three SPSs connected in (i) series and (ii) parallel. (d) GCD profiles of the SPS array connected in series and parallel with liquid metal interconnections. (e) Photographs (left) and charge/discharge behaviors (right) of two serially connected SPSs and an LED lit by them on a balloon without and with shear deformation. The inset shows a circuit diagram.


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Figure 6. Electrochemical performance of a SPS measured under real-time dynamic stretching. (a) CV curves measured during stretching/releasing cycles to 30% biaxial strain at different strain rates of 3 (green line), 6 (blue line), and 10% s-1 (red line). Here, the CV curves without applied strain (=0%) are in gray color. (b) Capacitance retention of a dynamically stretchable planar supercapacitor at strain rates of 3, 6, and 10% s-1 and corresponding strain-time curves obtained at a current density of 0.8 mAcm-2. (c) GCD curve dynamically recorded during finger bending/spreading motion (green) while the SPS is attached to an index finger. GCD curve taken prior to the finger motion (w/o motion) is presented in yellow color.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Jeong Sook Ha) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (Grant No. NRF-2016R1A2A1A05004935). The authors also thank the KU-KIST graduate school program of the Korea University.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Comparison of porous and nonporous elastomers; a schematic illustration for measuring the resistance of the electrode; definition of applied strain; dimensions of a SPS before and after biaxial stretching; photographs for a deformable SPS; CV curves of Mn and Mn/Mo electrodes during 36 electrodeposition cycles; SEM images, loading masses and capacitances on the scan rates of the Mn/Mo mixed oxides as a function of the number of deposition cycles; capacitive performance for four Mn/Mo electrodes; pseudocapacitive

behavior

of

a

Mn/Mo@MWCNT 28

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composite

electrode;

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electrochemical performance for MWCNT and Mn/Mo@MWCNT electrodes; Nyquist plots of the organic electrolyte for 4 months in ambient air; Raman spectra taken from ADN, SN, LiTFSI,PMMA and GPE; self-discharge curves of a SPS; capacitance retention of a SPS with elapsed time in ambient air; and photographs of the organic electrolyte


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Skin-Like, Dynamically Stretchable, Planar Supercapacitors with

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