Facile fabrication of a fully biodegradable and stretchable serpentine-shaped wire supercapacitor

Chemical Engineering Journal 366 (2019) 62–71

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Facile fabrication of a fully biodegradable and stretchable serpentine-shaped wire supercapacitor

T

Hanchan Leea, Geumbee Leeb, Junyeong Yuna, Kayeon Keuma, Soo Yeong Honga, Changhoon Songa, Jung Wook Kima, Jin Ho Leeb, Seung Yun Oha, Dong Sik Kima, Min Su Kimb, ⁎ Jeong Sook Haa,b, a b

Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea KU-KIST Graduate School of Converging Science and Technology, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

wire supercapacitor is fabricated • The using only biodegradable materials. embedded wire supercapacitor in • The elastomer is fabricated as serpentine shape.

uniform oxide layer grown on a • Amolybdenum wire via simple anodization.

wire supercapacitor can • Encapsulated be stable for a certain period in deionized water.

A R T I C LE I N FO

A B S T R A C T

Keywords: Biodegradable supercapacitors Transient electronics Wire-shaped supercapacitors Stretchable supercapacitors Anodized molybdenum oxides Biodegradable polymers

With the rapidly growing interest and usage in wearable electronics, considerable attention has been paid to the environmental concern caused by electronic waste and the toxicity of constituent materials. In this study, we report the facile fabrication of a fully biodegradable and stretchable serpentine-shaped wire supercapacitor by using a water-soluble molybdenum (Mo) wire, polyvinyl-alcohol-based biodegradable gel polymer electrolyte, and biodegradable elastomer—poly(1,8-octanediol-co-citrate) (POC). A thin oxide layer grown on a Mo wire via simple anodization drastically improves the electrochemical capacitance by inducing pseudocapacitance. As a result, the fabricated supercapacitor exhibits areal capacitance of 4.15 mF cm−2 at 0.05 mA cm−2, energy density of 0.37 µWh cm−2, and power density of 0.8 mW cm−2. The design of the serpentine-shaped wire supercapacitor encapsulated with POC gives mechanical and electrochemical stability against deformations of repetitive stretching. The biodegradable property of the supercapacitor is confirmed by the measurements of the change in mass of its constituent materials with elapsed time in water. Furthermore, the transient electrochemical performance of the fabricated wire supercapacitor in water over a certain period of time is observed to depend on the encapsulation.

⁎

Corresponding author at: Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea. E-mail address: [email protected] (J.S. Ha).

https://doi.org/10.1016/j.cej.2019.02.076 Received 28 November 2018; Received in revised form 4 February 2019; Accepted 12 February 2019 Available online 13 February 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Chemical Engineering Journal 366 (2019) 62–71

H. Lee, et al.

1. Introduction

application in eco-friendly or transient energy storage devices. In this study, we report a facile fabrication of a fully biodegradable wire supercapacitor via the use of a biodegradable electrode and electrolyte materials: a water-soluble Mo wire as a current collector, anodized molybdenum oxide film as an electrode, biodegradable and biocompatible poly(1,8-octanediol-co-citrate) (POC) as an encapsulation polymer, and polyvinyl alcohol (PVA) containing ionic salts (Na+ and Cl−) as a biodegradable electrolyte. In order to utilize the pseudocapacitive behavior of metal oxides, the Mo wire is anodized, and the capacitance of the supercapacitor is increased by 46 times as compared to bare Mo, as a result. The fabricated wire supercapacitor exhibits a capacitance retention of 82% after 5000 repetitive cycles of charge/ discharge. It can be fabricated in various lengths and has a constant length capacitance regardless of the total length. Moreover, it exhibits mechanical stability under repetitive bending deformation. In addition, the serpentine-shaped wire supercapacitor with POC encapsulation is endowed with stable performance under repetitive stretching by 50%. The degradation of all materials in de-ionized (DI) water is confirmed by measuring the changes in mass and electrochemical properties with elapsed time. This study demonstrates that the fabricated wire supercapacitor can be used stably for a certain period in DI water depending on the degradation rate of the encapsulation material.

There has been an increased usage of portable and wearable devices in everyday lives [1–5]. In particular, interest in the miniaturized electronic devices that can be attached to the skin or inserted inside the human body has been intensified recently [6–10]. Accordingly, active research has been also conducted to develop small-sized energy storage devices for stably powering the miniaturized wearable devices without wired connection to external power sources [11–13]. Among various energy storage devices, high-performance supercapacitors have attracted considerable attention because of their high power density, good cycle stability, fast charge/discharge rates, and simple structure compared to batteries [14–17]. Supercapacitors can directly operate wearable devices or provide immediate power as an auxiliary device for batteries [18–20]. For their application to wearable devices attached to skin or human organs, supercapacitors are required to be stretchable and biocompatible [7,21]. Stretchable supercapacitors have been manufactured in various forms such as stacked, planar, and wire-type forms, depending on the purposes [22–27]. In particular, yarn- or wireshaped supercapacitors have mechanical stability upon various deformations such as bending, knotting, and coiling [12,27–30], and their performance can be easily controlled by adjusting their length [18]. In addition, such supercapacitors can be facilely integrated with other devices by weaving into a fabric [31–33]. Structural variation, such as pre-strained wavy [26], coiled [12,34], and serpentine-shaped structures [35], imparts stretchability to these wire-shaped supercapacitors, thus extending their scope of application to skin-attachable or implanted devices. However, electronic wastes and the toxicity of materials constituting such electronic devices hinder the realization of environmentfriendly wearable devices because of environmental pollution and bioincompatibility [36,37]. A new field of technology called green electronics or transient electronics can be a solution to this concern [38]. All conductors, insulators, semiconductors, and substrate materials for green or transient electronics are required to have biodegradable and biocompatible properties. For example, water-soluble transition metals such as magnesium (Mg), zinc (Zn), molybdenum (Mo), tungsten (W), and iron (Fe) can be used as conductive electrodes or interconnects [39–41]. Biodegradable polymers such as silk, polycaprolactone (PCL), polylactic acid (PLA), polylactic-co-glycolic acid (PLGA), poly(1,8-octanediol-co-citrate) (POC) and rice paper are used as substrate materials [42,43]. In addition, bioresorbable SiO2 can be applied as an excellent insulator [43,44]. Therefore, the devices fabricated using these materials can be stably used for a predetermined period and can be physically dissolved in water or body fluids after use. Recently, various transient electronic devices such as biodegradable sensors, energy storage devices, and harvesters has been reported [43,45–47]. Since the energy storage devices are essential for the stable operation of biodegradable devices without external power supply, biodegradable batteries and supercapacitors, have been also fabricated to meet this requirement. Chen et al. reported an all-wood-structured asymmetric supercapacitor based on an activated wood carbon anode, wood membrane separator, and a MnO2/wood carbon cathode [48]. Although this supercapacitor has high capacitance, only some parts of the supercapacitor are biodegradable. In addition, Lee et al. reported a fully biodegradable planar supercapacitor fabricated by water-soluble metal electrodes (W, Fe, and Mo), an agarose/NaCl electrolyte, and a PLGA substrate [39]. This supercapacitor was proven to be flexible because of its small thickness of 160 µm; however, it involves a complex fabrication process. Huang et al. developed a fully biodegradable Mg–MoO3 battery consisting of all soluble materials, including Mg, MoO3, Mo, sodium alginate hydrogel, PLGA, and a polyanhydride encapsulation layer [49]. The battery provides a high output voltage (up to 1.6 V) and prolonged lifetime of approximately 13 days. However, it is neither deformable nor flexible. Such disadvantages of recently developed biodegradable supercapacitors and batteries limit their practical

2. Experimental section 2.1. Electrochemical oxidation of Mo wires Anodization, one of the electrochemical oxidation processes, was adopted to grow molybdenum oxide on the Mo wire surface. To remove the native oxide formed on the Mo wire surface, a Mo wire (diameter: 250 µm, Alfa Aesar) was sonicated in a mixed solution composed of ethanol, acetone, and 40% hydrochloric acid. A constant voltage of 0.8 V was applied to the Mo wire for 6 min with a Pt coil as a counterelectrode, Ag/AgCl as a reference electrode, and a Mo wire as a working electrode in a 0.05 M NaCl (MERCK) solution. The distance between the Pt coil and the Mo wire was kept constant during the anodization. The oxidized Mo wires were carefully washed with DI water and dried in vacuum for 30 min. 2.2. Preparation of NaCl/PVA gel electrolyte PVA (0.1 g, Mw 89,000–98,000, Sigma-Aldrich) was added slowly to DI water (9 mL) and the mixture was stirred at 110 °C until the solution became clear. After 30 min, the transparent PVA gel was cooled down to 25 °C. Then, a solution of NaCl (0.03 g) in DI (1 mL) water was added slowly to the PVA gel and stirred for 30 min to obtain the 0.05 M NaCl/PVA electrolyte. 2.3. Synthesis of poly(1,8-ocatanediol-co-citrate) Octanediol (Alfa Aesar) and citric acid (Alfa Aesar) were added into a 250-ml three-neck round-bottom flask at the same molar ratio and stirred at 200 rpm in a nitrogen gas atmosphere to be dissolved at 160–165 °C. When all the contents were dissolved and became transparent, the temperature of the system was lowered to 140 °C for prepolymerization, and stirring was continued for 60 min. The pre-polymer was dissolved in ethanol to produce a 30% v/w solution and the solution was poured into a flat glass chalet. The chalet was stored in a 55 °C oven for one week to complete the post-polymerization [50]. 2.4. Fabrication of stretchable serpentine-shaped wire supercapacitor After coating of two electrochemically oxidized Mo wires with NaCl/PVA electrolyte, they were dried in air to solidify the electrolyte. The two wires were then twisted, and the electrolyte was coated once more to form a wire supercapacitor. The fabricated wire supercapacitor 63

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Fig. 1. (a) (i) Schematic illustration of a fabricated stretchable serpentine-shaped wire supercapacitor (left). Optical images of constituent materials according to elapsed time in DI water (right). (ii) Fabrication process of a wire supercapacitor with anodization of the Mo wire in three-electrode system. (b) SEM images of bare Mo (left) and anodized Mo (right) wire surfaces, respectively. The inset is the cross-sectional SEM image of anodized Mo wire.

3. Results and discussion

was bent in a serpentine form, and it was inserted between two POC films (3.5 cm × 6 cm) and encapsulated to complete a stretchable serpentine-shaped wire supercapacitor.

Fig. 1a shows a schematic diagram of the entire structure for a stretchable serpentine-shaped wire supercapacitor, the dissolution images of all constituent materials and the fabrication process of wire supercapacitor. The wire supercapacitor was fabricated using only biodegradable materials and transformed into a serpentine-shape, encapsulated with biodegradable elastomer, POC, to have stretchable properties. Optical microscope images taken from the individual components of our wire supercapacitor inserted in DI water at temperatures of 65 and 90 °C show their dissolution. Here the measurements were done at elevated temperatures for accelerating the dissolution: Mo wire with a diameter of 13 µm became thinner after 21 days in DI water at 90 °C. POC film in DI water at 65 °C was decomposed into small pieces after 48 h. Other images show the changes of POC and NaCl/PVA film in DI water at 65 °C. After 1 h in DI water at 65 °C, NaCl/PVA film was also decomposed into several fractions. After anodic oxidation of a Mo

2.5. Dissolution tests of biodegradable materials The Mo wires (diameter: 13 µm, Alfa Aesar), used to identify the biodegradable images, were immersed in DI water at 90 °C. The NaCl/ PVA film used as the electrolyte and the POC film used as the encapsulation elastomer were immersed in DI water at 65 °C. The change in the mass of each component was measured periodically to evaluate the dissolution kinetics. After measuring the mass of each material, the DI water was changed with fresh water, periodically, to eliminate the effect of aged DI water on the dissolution kinetics.

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under a constant voltage is considered to be MoO3. As shown in Fig. S3, the O 1s spectrum also demonstrates the growth of surface oxide: the intense peak from the anodized Mo wire at 531.0 eV corresponds to the 1s peak of O2− in MoO3 [56]. Both EDS and XPS analysis confirm the growth of the MoO3 layer via anodic oxidation of the Mo wire. Fig. 2c shows the galvanostatic charge-discharge (GCD) curves obtained with variation of anodization time in three-electrode measurements. With these GCD curves, the areal capacitance (CA) of each electrode was calculated following Eq. (2) [57]

wire, dip-coating and drying of the gel-type NaCl/PVA electrolyte was performed several times to cover the entire surface of the electrodes. Then, two electrolyte-coated wires are twisted and dip-coated with electrolyte once more to obtain the wire supercapacitor. Here, the NaCl/PVA gel serves as a separator between the two electrodes as well as electrolyte. Before anodization of the Mo wire using the three-electrode system, it was cleaned with ethanol, acetone, and hydrochloric acid, and then cut to the appropriate length. In the anodization of the Mo wire in 0.05 M NaCl, a Pt coil was used as a counter-electrode to control the duration of the applied voltage of 0.8 V, and Ag/AgCl was used as a reference electrode. Scanning electron microscope (SEM) images taken from the Mo wire before and after anodization show the clear difference in the morphology in Fig. 1b. Upon anodic oxidation of Mo, there irregularly appeared a cracked membrane with a thickness of ∼1 µm, estimated from the cross-sectional SEM image of the inset. In the atomic force microscope line profiles taken from both Mo and anodized Mo wire electrodes by scanning 40 µm in x-axis over the wire surface at 5 µm intervals along y-axis (Fig. S1), the rms roughness values are estimated to be 133 ± 4.0 nm and 57 ± 13 nm, respectively. Since the skeleton of the wire supercapacitor is a stiff metal wire, it is not stretchable. In this study, however, the wire supercapacitor was made stretchable by embedding it inside a biodegradable elastomeric polymer, POC, after deforming it into a serpentine shape. Here, in addition to the elasticity, POC films played two important roles as an encapsulation layer: It prevented the performance degradation of the wire supercapacitor exposed to air. Recently, the problem of instability in ambient air of PVA-based electrolytes, which can affect the electrochemical stability of devices without encapsulation, has been reported [51,52]. The solvent evaporation in electrolyte or deterioration of polymers has been argued as the main reasons. The other role was maintaining stable performance in DI water by preventing the dissolution of component materials. An encapsulation strategy is required in transient electronics to maintain the performance in water or biofluid for the desired period [39]. In this context, the encapsulation of POC films could extend the lifetime of a wire supercapacitor in water. The feasibility of the Mo metal as a water-soluble electrode of a planar supercapacitor was demonstrated in our previous study [39]. In this study, we suggest a simple but efficient way to enhance the electrochemical performance of the biodegradable wire supercapacitor based on a Mo wire, inducing the pseudocapacitance via anodization of the Mo wire. The anodizing process is an oxidation reaction taking place on the anode. When a constant voltage of 0.8 V is applied to the Mo anode in the 0.05 M NaCl electrolyte, the anode loses electrons, and an oxide film is formed on the Mo surface according to the following mechanism (1) [53]

Mo + 3H2 O ↔ MoO3 + 6H+ + 6e−

CA =

I × ∆t ∆V × A

(2)

where I is the discharge current and Δt is the discharge time. The operating voltage window is ΔV which is 0.8 V in this experiment, and A is the area of one electrode. The area is calculated to be 0.24 cm2, using the length of 3 cm for the Mo wire and a diameter of 0.25 mm for the base plane due to its cylindrical shape. The anodically oxidized Mo wire electrode showed an improved capacitance compared to that of the bare Mo wire regardless of the anodization time (Fig. 2d). This indicates that the oxide film formed on the electrode surface contributed to the increase in capacitance probably due to the pseudocapacitive behavior. For up to 6 min, the discharge time increased with anodization time but decreased afterwards. It was observed that a part of the thick oxide layer peels away from the anodized electrode surface during the anodization for longer times of 9 min and 12 min. The decrease in the capacitance after 6 min is attributed to loss of oxide-layer. Therefore, a 6min-anodized electrode with a capacitance of about 46 times larger than that of bare Mo was used as an optimized electrode of our wire supercapacitor. The 6-min-anodized Mo wire has an average areal capacitance of 18.2 mF cm−2, at a current density of 1.0 mA cm−2. For a detailed comparison of the electrochemical performance between bare Mo and 6-min-anodized Mo electrode, we conducted threeelectrode measurements. In 0.05 M aqueous NaCl, a Pt coil and Ag/ AgCl were used as the counter-electrode and the reference electrode, respectively. The CV curve area and discharge time of an anodized Mo electrode are larger and longer than those of bare Mo electrode, as shown in Fig. 3a and b, indicating the dramatically enhanced performance via the use of anodized Mo electrodes and it is tentatively attributed to the pseudocapacitance of the MoO3 layer. In Fig. 3c, the difference in the capacitance of the bare Mo electrode and the anodized Mo electrode was confirmed. Recently, various transition metal oxides (e.g., MoO3, RuO2·nH2O, WO3, NiO and MnO2) have been found to contribute to the improvement of energy storage capability of supercapacitors via their pseudocapacitive behavior [57–62]. Materials with pseudocapacitive behavior are expected to have some typical characteristic electrochemical signals in three-electrode measurements [63,64]. First, the shape of the CV curve is almost rectangular, and the GCD curve is observed to exhibit an isosceles triangular shape, indicating the ideal capacitive behavior. Also, in the AC impedance measurement, the Nyquist plot exhibits a vertical line with a phase angle of 90° or less. Fig. 3d shows CV curves measured at a scan rate from 10 to 5000 mV s−1. At various scan rates, a rectangular shape was maintained. Fig. 3e shows triangular GCD curves without a potential plateau, independent of the current density. In the Nyquist plot, covering frequencies from 500 kHz to 0.1 Hz, a vertical line with a phase angle of almost 90° is clearly exhibited in the low frequencies, and a semicircle, associated with charge-transfer resistance, is present in the high frequency region (Fig. 3f). These are the general features of the pseudocapacitive materials in the Nyquist plot [64]. Also, a steep slope from the imaginary part to the real part of the resistance reflects the capacitive behavior of the electrode. The closer the slope is to 90°, the more pseudocapacitive behavior is represented. These electrochemical behaviors are very similar to those observed with carbon-based electrodes and are consistent with the electrochemical characteristics of pseudocapacitive materials. Therefore, we suggest that the MoO3 layer formed by the anodic oxidation of the Mo wire

(1)

The optical images of bare Mo and oxidized Mo wires are shown in Fig. S2. The surface of the Mo wire before anodization exhibits light reflection, while the surface after anodization does not reflect light and appears black. To confirm the uniform growth of molybdenum oxide layer over the Mo wire surface, energy dispersive X-ray spectroscopy (EDS) mapping of O and Mo was performed on three sites as shown in Fig. 2a. From the distribution of O and Mo elements in the EDS analysis, it was confirmed that the surface of the Mo wire was uniformly oxidized. The EDS analysis shows that the relative atomic composition of oxygen to Mo is 2.6:1, which is close to the stoichiometric composition of MoO3 and the same composition was observed in three different locations. XPS spectra were also taken to analyze the detailed oxidation state of the oxide layer as shown in Fig. 2b. The two 3d peaks of bare Mo were measured at 228.7 eV (3d5/2) and 231.8 eV (3d3/2), respectively, with an interval of 3.1 eV. Two peaks from Mo after an anodization for 6 min were observed at 232.5 eV (3d5/2) and 235.7 eV (3d3/2), which correspond to the binding energies of the Mo6+ state [54,55]. Therefore, the oxide layer formed on the surface of the Mo wire via anodic oxidation 65

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Fig. 2. (a) SEM image and EDS mapping images taken on the anodized Mo wire at a position ‘2’. Atomic ratios of oxygen and molybdenum obtained from EDS mapping measured at three regularly separated sites. All scale bars correspond to 300 nm. (b) XPS spectra of Mo 3d peak collected from bare (bottom) and anodized Mo wires (top). (c) Charge/discharge curves taken from the Mo wire electrodes with various anodization time in the three-electrode measurement system. (d) Areal capacitance of the electrode in a three-electrode configuration with variation of anodization time.

supercapacitor, and the capacitance per length was maintained at about 0.37 mF cm−1, independent of the total length. This highly reproducible performance indirectly verifies that the MoO3 formed by anodic oxidation is uniformly grown regardless of the length, which is consistent with the EDS analysis of Fig. 2a. Thus, the cell capacitance can be determined by controlling the length of the supercapacitor. Generally, water-containing electrolytes cause degradation of supercapacitor performance due to the evaporation of water upon exposure to air. Therefore, the wire supercapacitor was encapsulated with biodegradable polymer, POC, to guarantee the stability in air-ambient environments while evaluating the electrochemical performance. As shown in Fig. S5, POC encapsulation ensured the fabricated wire supercapacitor with 97% retention of the initial capacitance after 10 h. Fig. 4e shows the capacitance retention and coulombic efficiency of the wire supercapacitor, encapsulated by POC film, during 5000 charge/ discharge cycles at a current density of 0.5 mA cm−2. At the beginning of the charge/discharge cycles, the capacitance continuously increases due to the additional electrochemical oxidation of Mo by aqueous NaCl/PVA electrolyte and the continuous charge/discharge currents applied to the electrode surface [39]. However, after 1500 cycles, it gradually decreases. This is explained in terms of the rather thick oxide layer hindering the transport of charges from the electrode surface to the collector of the bare Mo wire. Nonetheless, the fabricated wire supercapacitor retains about 82% of its initial capacitance after 5000

should be a pseudocapacitive material. As a result, the electrochemical performance of the electrode was improved due to pseudocapacitance through the redox reaction of MoO3 and ions in the NaCl electrolyte following the Eq. (3) [58]

MoO3 + xNa+ + xe− ⇄ Na x MoO3

(3)

Fig. 4 shows the electrochemical properties of a 3-cm long wire supercapacitor fabricated with a 6-min-anodized Mo electrode in a twoelectrode system. CV curves taken at various scan rates over a voltage range of 0–0.8 V in Fig. 4a show a very stable rectangular profile in which the current density increases with the scan rate. The GCD curves at various current densities maintained the symmetric triangular shapes with coulombic efficiency close to 100% in Fig. 4b. This verifies that the manufactured wire supercapacitor is an efficient supercapacitor. The areal capacitance (CA) at various current densities in Fig. 4c was calculated by the Eq. (2). In this calculation, A is the total area of the two electrodes, which is 0.48 cm2. It is the area of the two Mo wires with a length of 3 cm and a base plane diameter of 0.25 mm. The maximum areal capacitance of the wire supercapacitor was calculated to be 4.15 mF cm−2 at a current density of 0.05 mA cm−2, which correspond to the maximum volumetric capacitance of 166 mF cm−3 at a current density of 2 mA cm−3 (Fig. S4a). Fig. 4d shows cell capacitance and length capacitance with variation of the length of wire supercapacitor. The total cell capacitance increased with the length of the wire 66

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Fig. 3. (a) Cyclic voltammetry (CV) and (b) GCD curves of bare Mo wire and anodized Mo wire electrodes in 0.05 M NaCl solution with Pt coil as a counter-electrode, and Ag/AgCl as a reference electrode. (c) Areal capacitances of electrodes with various current densities from 0.3 to 5 mA cm−2 (d) CV curve of anodized Mo electrode at scan rates between 10 and 500 mV s−1, (e) GCD curves obtained at current densities from 0.3 to 5.0 mA cm−2, (f) Nyquist plot of anodized Mo wire electrode.

medical devices. The mechanical stability of the fabricated supercapacitor under various deformations is evaluated in Fig. 5. The fabricated wire supercapacitor retained its stable electrochemical performance without any significant change in capacitance and CV curves through various deformations such as bending, knotting, and coiling (Fig. 5a). The optical images of the wire supercapacitor under various deformations are shown in Fig. S7. In Fig. 5b, it is observed that the capacitance remained stable even after 600 bending cycles with a bending radius of 5 mm. Fig. S8 also shows stable operation under bending deformation with various bending radii. Beyond the flexibility, there have been various attempts to fabricate stretchable wire supercapacitors. Pre-strained and coiled elastic polymers with carbon nanotube were used to fabricate a stretchable wire supercapacitor [26,27,34]. In this study, stretchability was endowed to our wire supercapacitor via embedding the serpentine-shaped wire supercapacitor inside the elastomeric polymer. Serpentine-shaped design is widely used for making stretchable interconnects with a very thin and metallic film encapsulated with thin polymer film [22,57]. In Fig. S9, the serpentine-shaped wire supercapacitor without encapsulation was found to have fatigue at 50% strain although it returned to its original length at 30% strain. However, the POC encapsulated supercapacitor was confirmed to return to its original length without any change under 50% strain. The applied strain (ε), ε = (l − l0)/l0, is defined with optical images, as shown in Fig. S10, where l0 and l are the total lengths of the serpentine-shaped wire supercapacitor embedded inside POC before and after stretching, respectively. The serpentine design used to fabricate this supercapacitor is detailed in Fig. S11. Depending on the size or design of the serpentine structure, the overall stretchability of the supercapacitor can be controlled [68]. Stretchability of the biodegradable POC film depends on the conditions of synthesis such as temperature and time for polymerization [69]. In this study, we synthesized POC in 55 °C oven for one week, which was

cycles and exhibits excellent cycle stability, with a coulombic efficiency close to 100%. In Fig. 4f, the areal energy density (EA) and power density (PA) are calculated by Eqs. (4) and (5) [65]

EA =

CA × ∆V 2 7200

(4)

PA =

EA × 3600 tdischarge

(5)

where ΔV and tdischarge correspond to the operating cell voltage and the discharge time, respectively. The EA is 0.37 µWh cm−2 at a PA of 20 µW cm−2 and PA is 0.8 mW cm−2 at an EA of 0.14 µWh cm−2, which correspond to volumetric energy density (EV) of 14.8 µWh cm−3 at volumetric power density (PV) of 0.8 mW cm−3 and PV of 32 mW cm−3 at an EV of 5.6 µWh cm−3 (Fig. S4b). Compared with previously reported planar supercapacitor based on Mo thin films [39], wire supercapacitors with multi-walled carbon nanotube/VOx electrodes [18], wet-spun yarn [66] and composite yarn of single-walled carbon nanotube/Polyaniline [67], our wire supercapacitor provides comparable performance. In particular, it is notable that such performance was obtained simply by a facile anodic oxidation of the Mo wire. In Fig. S6, it is confirmed that we can control the operation voltage window and the total capacitance via connecting multiple wire supercapacitors in series and parallel so that we can meet the required voltage and capacitance; three serial connections provide three-fold higher voltage but 1/3 of the capacitance. Likewise, three parallel connections provide three-fold higher capacitance. So, we could successfully operate a red LED (Digi-key, 1.6 V, USA) using three serially connected wire supercapacitors with an operation voltage of 2.4 V. The purpose of this study is to develop fully biodegradable supercapacitors made of biodegradable and biocompatible materials to increase the possibility of application to clothing, skin, or implantable 67

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Fig. 4. Electrochemical performance of a wire supercapacitor. (a) CV curves at scan rates from 0.01 to 1 V s−1. (b) GCD curves obtained at current densities from 0.05 to 1.0 mA cm−2. (c) Areal capacitance calculated from GCD curves at various current densities from 0.05 to 2 mA cm−2. (d) Cell and length capacitance with cell length of 3, 6, 9, 12, and 15 cm. (e) Normalized capacitance retention and coulombic efficiency during repetitive charge-discharge cycles at a current density of 0.5 mA cm−2. The inset shows GCD curves at initial, after 1500 cycles, and after 5000 cycles. (f) Ragone plots.

shaped wire supercapacitor demonstrates potential application in textiles and wearable electronics. Fig. 6 shows the dissolution kinetics of individual components of our wire supercapacitor. The change in mass was measured with the elapsed time in DI water. According to previous studies, dissolution rates might be different for the same material with varying parameters, such as thickness, surface morphology, deposition method for thin film, pH level, and ionic concentration of solution [44]. In case of molybdenum, 10 µm thick Mo foil and thin film of 40 nm have a dissolution rate of 0.02 µm d−1 at 37 °C in PBS (pH 7.4) and 0.001 µm h−1 at 37 °C in DI water, respectively [41,70]. We performed dissolution tests at elevated temperatures of 90 °C and 65 °C, to accelerate the dissolution of each material. A Mo wire was immersed in DI water at 90 °C (Fig. 6a). The initial average mass of the Mo wire with a diameter of 250 µm is 5.2 mg, and the mass change rate extracted through linear fitting is about 0.035 mg d−1. The surface of the Mo wire turns into

observed to be stretchable by 170% and the strain–stress curve is given in Fig. S12. The serpentine-shaped wire supercapacitor encapsulated with POC film maintained the capacitance for strains in the 10% to 50% range (Fig. 5c). Fig. 5d exhibits 91% capacitance retention after 1000 repetitive stretching/releasing cycles with 30% strain. The slight decrease of capacitance after 1000 cycles of stretching seems to be due to water evaporation in electrolyte through the gap between two POC films, resulting from repetitive mechanical friction with repetitive stretching/releasing cycles. As shown in Fig. S13, the electrolyte volume of a supercapacitor completely embedded in PDMS was maintained regardless of stretching cycles, whereas that of the device without any encapsulation prominently decreased in ambient air condition. As expected, the capacitance of the wire supercapacitor without encapsulation decreased dramatically during 500 stretching cycles in air ambient condition within 40 min (Fig. S14). The excellent mechanical flexibility and stretchability of the fabricated serpentine68

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Fig. 5. (a) Capacitance retention of the wire supercapacitor measured under various deformations; bending, knotting, and coiling. The inset shows the corresponding CV curves at a scan rate of 100 mV s−1. (b) Capacitance retention as a function of bending cycles with a bending radius of 5 mm. (c) Capacitance retention of the serpentine-shaped wire supercapacitor obtained under stretching. The inset shows GCD curves under different strain at a current density of 1.0 mA cm−2. (d) Capacitance retention under stretching by 30% as a function of stretching cycles.

capacitance of the wire supercapacitor. These results suggest that the change in POC thickness and synthesis conditions, or the use of other biodegradable polymers that have less swelling and higher water resistance can further improve the lifetime of the entire device.

black after 1 h, with a mass retention of 88.5% after 15 days. It seems that the oxide layer grown on the Mo surface in water was fallen off after the Mo wire was taken out of the water and dried, as mentioned before in Fig. 2c. POC and NaCl/PVA were immersed in DI water at 65 °C (Fig. 6b and c). POC was confirmed to be dissolved at a constant rate over time. The initial average mass of the POC is about 73 mg, and the dissolution rate is about 5.3 mg d−1. NaCl/PVA showed a rapid mass change within 2 h. The initial average mass of NaCl/PVA is about 27 mg, and the dissolution rate is about 5.4 mg h−1 up to 2 h and 0.27 mg h−1 after 2 h. Here, the mass of POC and NaCl/PVA was weighed after vacuum filtration for accurate mass measurement. All the error bars were obtained with four different samples at each measurement time, and DI water was regularly replaced with fresh water to exclude the effect of the aged solution on the dissolution rate. Finally, the performance of POC-encapsulated fully biodegradable wire supercapacitor was evaluated in DI water at 37 °C (Fig. 6d). Encapsulation is an important strategy that enables practical application by adjusting the lifetime of the device as desired. Without encapsulation, the ions in the electrolyte (Na+ and Cl−) might escape into the surrounding water due to the difference in ionic concentration, and the performance deteriorated quickly in 5 min (Fig. S15). However, the encapsulated wire supercapacitor operated stably for 11 days since its water resistance was enhanced by POC film. In the initial days, the capacitance increased because of the additional oxidation of Mo in the presence of aqueous electrolyte, while it decreased afterwards probably because of the poor conductivity due to the thickness of the oxide film. Furthermore, the degradation of the POC film might have caused the infiltration of the surrounding water, which would have decreased the

4. Conclusion This paper reports the fabrication strategies for a biodegradable and stretchable serpentine-shaped wire supercapacitor that was built with biodegradable materials including water-soluble metal wire electrodes, PVA-based electrolytes, and biodegradable elastomer POC. In particular, this wire supercapacitor is fabricated through the facile anodization of the Mo wire for several minutes and dip-coating of the electrolyte without a separator. The wire supercapacitor exhibits stable performance under repetitive bending and the serpentine-shaped wire supercapacitor embedded inside the POC elastomer shows stable electrochemical performance under 50% strain. With the measurement of changes in mass of the component materials and the change in capacitance of the wire supercapacitor in DI water, transient behavior as well as the dissolution kinetics are confirmed. This systematic study demonstrates the potential of this facilely fabricated wire supercapacitor as a transient energy storage device applicable to environment-friendly electronics. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (Grant No. 69

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Fig. 6. Mass retention and optical images of component materials measured with elapsed time in water. (a) Mo wire with a diameter of 0.14 µm. (b) POC film with a thickness of 0.3 mm. (c) PVA film with a thickness of 0.06 mm. (d) Capacitance retention of the POC encapsulated wire supercapacitor measured with elapsed times. Inset is the optical image showing the gradual disappearance of POC encapsulation film.

NRF-2016R1A2A1A05004935). The authors also thank the KU-KIST graduate school program of the Korea University.

[12]

Conflict of interest [13]

The authors declare no conflict of interest. [14]

Appendix A. Supplementary data

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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.02.076.

[16]

[17]

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