Stretchable array of high-performance micro-supercapacitors charged with solar cells for wireless powering of an integrated strain sensor

Author’s Accepted Manuscript Stretchable array of high-performance microsupercapacitors charged with solar cells for wireless powering of an integrated strain sensor Junyeong Yun, Changhoon Song, Hanchan Lee, Heun Park, Yu Ra Jeong, Jung Wook Kim, Sang Woo Jin, Seung Yun Oh, Lianfang Sun, Goangseup Zi, Jeong Sook Ha

PII: DOI: Reference:

www.elsevier.com/locate/nanoenergy

S2211-2855(18)30332-X https://doi.org/10.1016/j.nanoen.2018.05.017 NANOEN2725

To appear in: Nano Energy Received date: 13 April 2018 Revised date: 4 May 2018 Accepted date: 6 May 2018 Cite this article as: Junyeong Yun, Changhoon Song, Hanchan Lee, Heun Park, Yu Ra Jeong, Jung Wook Kim, Sang Woo Jin, Seung Yun Oh, Lianfang Sun, Goangseup Zi and Jeong Sook Ha, Stretchable array of high-performance microsupercapacitors charged with solar cells for wireless powering of an integrated strain sensor, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.05.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Stretchable array of high-performance micro-supercapacitors charged with solar cells for wireless powering of an integrated strain sensor

Junyeong Yuna, Changhoon Songa, Hanchan Leea, Heun Parka, Yu Ra Jeonga, Jung Wook Kima, Sang Woo Jinb, Seung Yun Ohb, Lianfang Sunc, Goangseup Zic, Jeong Sook Haa,b,*

a

Department of Chemical and Biological Engineering, Korea University, 5-1 Anam-dong,

Seoul 13l-701, Korea b

KU-KIST Graduate School of Converging Science and Technology, 5-1 Anam-dong, Seoul

13l-701, Korea c

Department of Civil, Environmental and Architectural Engineering, Korea University, Seoul

136-701, Republic of Korea *

Corresponding author: Tel: +82-2-3290-3303; [email protected]

Abstract The aim of this paper is to report on the fabrication of a stretchable array of high-performance solid-state micro-supercapacitors (MSCs), which can be charged with integrated, commercial Sibased solar cells (SCs). This would facilitate the powering of an integrated strain sensor. The planar MSCs comprised electrodes of potentiostatically deposited polypyrrole, on spray-coated multi-walled carbon nanotube film, and a gel-type electrolyte of LiCl/polyvinyl alcohol with a redox additive of 1-methyl-3-propylimidazolium iodide. The fabricated MSC achieved an areal capacitance of 5.17 mF cm-2. After 5,000 charge/discharge cycles, the MSC retained 80% of their initial capacitance. A strain sensor was fabricated utilizing a composite film of fragmentized graphene foam and polydimethylsiloxane. Such fabricated twelve parallel connected MSCs, a 1

strain sensor, and SCs were integrated on a single deformable polymer substrate with embedded stiff platforms of negative epoxy series resist (SU-8) via long serpentine interconnections of polyimide encapsulated Ti/Pt metal film for mechanical stability under stretching. After 1,000 repetitive biaxial stretching/releasing cycles by 30%, no noticeable change was observed in the charge/discharge behavior of the MSC array. Furthermore, both the photo-charge/discharge characteristics and electrochemical performance remained stable. When the whole integrated system was attached to the wrist, the integrated strain sensor could detect both externally applied strain and the arterial pulse using the energy stored in the MSCs from the SCs.

Graphical Abstract:

Keywords Stretchable; micro-supercapacitor; strain sensor; solar cell; integration; self-charge system 2

1. Introduction With increasing demand for health monitoring and biomedical devices, there have been considerable developments in deformable electronic devices in recent years [1-6]. These devices interact with the human body to monitor signals created by external strain, demonstrating that they work in the same way as human skin. To be used as skin-attachable or wearable devices, they should be both stretchable and flexible [7, 8]. Therefore, extensive research has been conducted on stretchable devices, including electronic skin, radio frequency antennae, lightemitting diodes, and energy storage devices [9-12]. Wireless operation of these stretchable devices will require several technical advances [13]. First, it is necessary to obtain either wirelessly transmitted power froby condddddddm an external source or utilize internally generated power. Second, the whole integrated system should be mechanically stable under deformations caused by body movements. Therefore, for the successful realization of electronic devices that can be attached to the body or skin, it is essential to create a cohesive interconnected system of an active sensor, energy harvesting, and energy storage devices on a single, deformable substrate. A self-charged power pack, that integrates energy harvesting and energy storage functions into a single device, would constitute a very promising energy system [14, 15]. For decades, energy generation methods have relied heavily on nuclear and fossil fuels to satisfy ever increasing energy demands [16, 17]. Moreover, serious environmental problems, such as global warming, airborne particulate matter, and the exhaustion of fossil fuels, are serious and increasing concerns [18]. At the same time, there is a need for renewable energy systems with low weight, low cost, and mechanical deformability for use in portable/wearable electronics, electric vehicles, and the Internet of things [19]. Among the various energy systems, a combination of SCs and batteries (or supercapacitors) has been considered a promising next generation power source for integrated devices [20, 21]. Photovoltaic cells that convert sunlight into electricity are the most efficient energy conversion device [22, 23]. However, they have disadvantages: the inherent instability of light intensity (depending on the daytime cycle, weather, and position of the sun), and failure of the electronic devices that are connected to the solar cell [24]. Despite improvements in storage capacity, cycle life, and the operational safety of energy 3

storage systems there are still key challenges to be addressed when designing alternate power supplies, including limited power capacity and long charging times. Supercapacitors have attracted attention as a promising candidate for energy storage, due to their fast charge/discharge characteristics, long cycle life, and high-power density [25, 26]. Supercapacitors are classified as either electrical double layer capacitors (EDLCs) or pseudocapacitors, depending on the charge storage mechanism [27, 28]. An EDLC typically uses carbon-based electrodes (including carbon nanotubes, graphene, or activated carbon), to accumulate electrostatically adsorbed charges at the interface between the electrode and electrolyte [29, 30]. Pseudocapacitors store energy by redox reactions of electrode materials (such as conducting transition metal oxides and conductive polymers), and by the adsorption of charged ions onto electrodes [31, 32]. In addition, various techniques involving asymmetric and hybrid supercapacitors have been introduced, to further improve their performance [33, 34]. Recently, another method was proposed to improve the performance of supercapacitors, where additional redox reactions are induced at the electrode-electrolyte interface, by introducing redox additives (or mediators) to the electrolyte [35]. The use of redox additives is an efficient, low cost, and eco-friendly strategy for obtaining high-performance energy storage devices [36]. Wearable strain sensors (SSs) for sensing subtle human motion have been utilized in various areas, such as the medical, entertainment, and sports industries [37-39]. As a wearable device, the SS should be thin and lightweight, and have a wide sensing range from very small deformations (pulse < 1%), to large deformation regions (elbow bending > 50%) [40]. In addition, for efficiency reasons, the production of sensors must be simple and low cost. Research on advanced SSs has been conducted on a variety of composite sensing materials, including carbon nanotubes/polydimethylsiloxane (PDMS), carbon black/thermoplastic elastomers, graphene/epoxy, and silicon nanomembrane/Ecoflex. Graphene foam (GF), with a macroporous network structure, has also attracted great attention due to its large surface area, and excellent electrical and mechanical properties [41, 42]. Composites of GF and an elastomer (such as Ecoflex and PDMS), exhibit stable electrical and mechanical properties when subjected to various types of deformation, such as stretching and bending [43]. When tensile strain is applied to a GF/elastomer composite, cracking and fracture of the graphene occurs that increases the electrical resistance [44]. 4

In this paper, we report on the fabrication of a stretchable sensor system comprising highperformance solid-state micro-supercapacitors (MSCs), commercial Si solar cells (SCs), and a strain sensor. This unique integration is achieved via use of serpentine interconnections on a single deformable polymer substrate with embedded stiff platforms of SU-8 photoresist. To realize a high level of performance, we suggest the optimum selection of materials, and a design for the fabricated MSC. Furthermore, from the analysis of strain distribution under stretching deformation, we also propose a novel strain-relief design of the substrate and interconnections, for integrating energy generation, energy storage, and the sensing devices. In this way, highperformance MSCs can be wirelessly charged (using the integrated SCs) to continuously operate the strain sensor, regardless of body movements. By conducting this research, we have successfully demonstrated the potential application of our stretchable self-charging power/sensor system to skin-attachable health-monitoring devices.

2. Experimental section 2.1 Array of MSCs with electrodes of Polypyrrole (Ppy)/CNT and serpentine interconnections For the spin coating process of polyimide (PI) solution (DFCPI-101, Dongbaek Fine-Chem, Korea), silica substrate (SiO2, dry type 1000 Å, P type, INexus, Inc.) was rinsed by simple blowing of a N2 gas gun. After spin coating (500 rpm for 10 s, 4,000 rpm for 1 min) the PI solution onto a SiO2 substrate, it was cured on a hot plate at 250 °C for 2 h. The Ti/Pt current collector of the MSC array, and Ti/Pt serpentine interconnection, were obtained via photolithography and the e-beam evaporation process (KVE-T4065, Korea Vacuum Tech.). After an additional spin coating of PI (and subsequent curing process), the structure of PI/Pt/PI was realized. In order to deposit electrode materials, the undesired top PI layer on the MSC was removed using a reactive ion etching process (RIE, JVRIE-8M, J Vac., power: 100 W, pressure: 130 mTorr, and O2: 20 sccm). Carboxylic acid (-COOH) functionalized multi-walled carbon nanotubes (CNTs) were synthesized through an acid treatment process, as shown in our previous work [12]: 500 mg of CNTs was refluxed with a solution of 30 mL sulfuric acid (H2SO4, 99.999%, Sigma Aldrich) and nitric acid (HNO3, 70%, ACS reagent, Sigma Aldrich) at 70 °C for 5

4 h. After the refluxing process, the CNTs/acid was mixed with 1 L of DI water and cooled at room temperature. Then, this solution was vacuum filtered using a mixed cellulose filter (0.2 μm, Advantec MFS, Inc.). To remove any residual acid, the filtered CNT powder was dispersed in 500 mL of DI water and poured into dialysis tubing of cellulose membrane (MW: 14 K, Sigma Aldrich). The dialysis tubes of the CNT dispersion solution were dispersed in a DI water bath and stirred for 5 days. After the dialysis process, the CNT dispersion solution was vacuum filtered and dried at room temperature. Then, prepared CNT-COOH was dispersed in water (1 g L-1) and spray coated onto the Ti/Pt current collector. Next, Ppy was potentiostatically deposited (0.8 V was applied for 5 s) on the CNT/Ti/Pt electrode (counter electrode: Pt, reference electrode: Ag/AgCl, Pyrrole monomer (98%, reagent grade, Sigma Aldrich) in 0.3 M sodium perchlorate (NaClO4, Anhydrous, cica-reagent, Sigma Aldrich)). Thus, the array of MSCs connected with serpentine interconnections was fabricated. 2.2 Synthesis of LiCl/PVA/MPII gel redox additive electrolyte First, 10 g of lithium chloride (LiCl, anhydrous, ACS reagent, Sigma Aldrich), 100 mL of DI water, and 10 g of PVA (MW: 89,000–98,000, Sigma Aldrich) were mixed in a beaker at 130 °C, until the gel electrolyte became clear. Then, the gel electrolyte was cooled to room temperature, and 2 g of MPII (Alfa Aesar) was stirred into the gel electrolyte. Finally, the LiCl/PVA/MPII gel electrolyte was obtained. 2.3 Fragmentized graphene foam (FGF) strain sensor Multilayer GF was synthesized on Ni foam (0.45g cm-3 bulk density, 95% porosity, 1.6 mm thickness, 1 cm × 2 cm, Goodfellow Cambridge, Ltd.) via chemical vapor deposition (CVD). After placing Ni foam in the middle of a quartz tube, it was annealed with Ar/H2 gas flow at 1,000 °C for 10 min. Then the multilayer graphene was synthesized with CH4 gas flow for 20 min. After the synthesis, the graphene/Ni foam was immersed in 3 M hydrochloric acid (HCl, Daejung) on a hot plate at 90 °C to remove the Ni foam. The GF was rinsed with DI water several times, then a 10 mg solution of GF/IPA was prepared in a vial. Next, it was located on vortex machine, and operated with an intensity level of 7 for 20 min. Subsequently, the weight of FGF/IPA solution was adjusted to 0.4 g by drying the IPA in an oven at 65 °C. The parallelogram pattern (7 mm × 1.5 mm) was formed on a glass slide by cutting the PI tape. The FGF/IPA 6

solution of 10 μL was drop casted, and IPA was vaporized in the oven at 65 °C for 20 min. Then the mixture (MPE, w/w, 1:1) of PDMS (Sylgard 184, K1 solution) and Ecoflex (Ecoflex 0030, SFX Korea) was poured on and cured in an oven at 65 °C for 2 h. After the curing process, the FGF/MPE film was peeled off the slide. Thus, the FGF strain sensor was fabricated. 2.4 Ecoflex substrate with an embedded stiff SU-8 photoresist platform array After curing spin-coated Ecoflex on a SiO2 substrate, it was exposed to UV light for 20 min. Then a stiff platform array of SU-8 photoresist (SU-8 3025, SU-8 negative epoxy series resists, MicroChem) was fabricated on UV-treated Ecoflex/SiO2 substrate via a photolithography process (spin coating: 2,000 rpm for 2 min, soft baking: 95 °C for 2 h, UV exposure: 150 mJ cm 2

for 30 s, development for 6 min). Next, the uncured Ecoflex was poured onto the stiff SU-8

photoresist platform array. After the curing process, this stiff SU-8 photoresist platform array on Ecoflex film was peeled off. Thus, the fabrication of the stiff SU-8 photoresist platform embedded Ecoflex substrate was completed. 2.4 Biaxially stretchable MSC array integrated with FGF SS and SCs The fabricated MSC array on PI substrate was transferred onto the Ecoflex substrate with the embedded stiff SU-8 photoresist platform array. Then the undesired PI area was removed using an RIE process (O2: 20 sccm, pressure: 130 mTorr, time: 5 min). The commercial Si-based solar cell (0.5 W/0.5 V, Dig Dog Bone) was hexagonally cut by a water based sawing process (AP Technologies). Commercial solder paste (Chip Quik Solder, Sn42/Bi57.6/Ag0.4) was placed on the positive and negative electrodes of the hexagonally cut SCs, and was also applied to the electrode of an integrated array. In order to melt and anneal the solder paste of the hexagonally cut solar cell onto the integrated array, the assembly was placed onto a 150 °C hot plate for 20 s. At the ends of the FGF/elastomer mixture films (PDMS:Ecoflex=1:1, w/w), holes were punched using a syringe needle (22G Kovax-needle, Korea Vaccine). Then, these holes were filled with liquid metal (Galinstan, 68.5% Ga, 21.5% In, and 10% Sn; Rotometals), to connect with the Ti/Pt electrode underneath. The synthesized gel electrolyte (2 μL) was drop casted onto the MSCs by micro pipette. Thus, the fabrication of the biaxially stretchable MSC array (integrated with the FGF SS and SCs), was complete.

7

3. Results and discussion

Figure 1. (a) Optical image of the biaxially stretchable MSC array with integrated SS and SCs. Inset shows the circuit diagram. Schematic illustrations of (b) the MSC, (c) the FGF SS, and (d) SCs on PI substrates. (e) Magnified image of serpentine interconnection. (f) Schematic of the stiff SU-8 platform array embedded Ecoflex substrate. Figure 1 (a) shows an optical image of the biaxially stretchable MSC array with integrated SCs and SS. For the fabrication of such a stretchable device, we adopted the following design concepts. Integrated system of energy harvesting, energy storage, and sensor devices: We believe that this is the first time integrated MSCs (Figure 1 (b)), SS (Figure 1 (c)), and Si-based commercial SCs (Figure 1 (d)) on a single, stretchable substrate. With this integrated system, the strain sensor can be driven without any external wires or connected power source. Instead, it can be operated by using the stored energy of the MSCs, which are charged by the integrated solar cells. This design demonstrates the potential application of the system as a wirelessly operated skin-attachable health-monitoring system. High-performance MSC with selectively deposited gel electrolyte: Hexagonal shaped allsolid-state MSCs, with interdigitated electrodes and a high fill factor, were fabricated for the close packed array. The detailed design specification is shown in Figure S1. The Ti/Pt current 8

collector, which was not oxidized by iodide (I-) during the charge/discharge process, was used with a LiCl/PVA/MPII gel electrolyte. To increase the specific area and electrical conductivity of the electrodes, CNT was spray coated on the Ti/Pt electrodes. Then the Ppy was potentiostatically deposited, while maintaining the porous network structure. Figure S2 shows that the LiCl/PVA/MPII exhibits selective deposition (only onto the MSC electrode), which enables full stretchability of the serpentine interconnections during the application of biaxial strain. FGF SS: CVD grown GF was fragmentized by a vortex mixer to form the percolation network, maintaining their 3D structure. Figure S3 shows that the resistance of the FGF SS could be controlled by adjusting the volume of the FGF/IPA solution during filling of the patterned PI film. In this way, it was possible to control the current flow to enable long-term operation of the SS using the stored energy in the MSCs/SCs system. The electrical connection, between the FGF SS and the whole integrated system, was made from liquid metal Galinstan. Serpentine interconnections and stretchable substrate: Narrow Ti/Pt thin film encapsulated with PI (thickness: PI/Ti/Pt/PI= (800/5/100/800) nm) is in a neutral mechanical plane (Figure 1 (e)), ensuring the accommodation of the applied strain. Figure S4 shows the detailed design specification of the serpentine interconnection. In addition, SU-8 (which is one of the negative photoresists having a high Young’s modulus), was patterned and inserted into Ecoflex substrate to localize the applied strain. The schematic of the Stiff SU-8 photoresist platform array embedded Ecoflex substrate is shown in Figure 1 (f). As a result, the MSCs and SCs located on the stiff SU-8 photoresist platform array can maintain their stable performance during repetitive stretching/releasing cycles. The insertion of the stiff SU-8 photoresist platform array enabled precise alignment control. Figure 2 investigates the chemical and structural properties of the component materials for the MSC electrodes and SS. Figures 2 (a)–(d) show scanning electron microscopy (SEM) images taken from the MSC electrode of CNT/Ppy, with variation of the deposition time of Ppy. Figure 2 (a) shows that the spray-coated high-density CNTs are entangled to form a porous structure, providing an ion path that allows ions to transfer faster at the interface between the electrode and electrolyte. In addition, the carboxylic acid functional group of CNT film has hydrophilicity 9

directly related to the electrical double layer capacitance in an aqueous electrolyte [45]. Figure 2

10

(b) shows the porous network structure of CNT/Ppy after deposition of Ppy for 5 s. Figures 2 (c)

11

and (d) show that as the deposition time for Ppy increased further to 15 s, CNT network pores

12

were filled with Ppy film, and then the growth of Ppy nanowires started after 20 s. Figure S5

13

shows the change of areal capacitance with the variation of Ppy deposition time. The selection of

14

optimized conditions for CNT/Ppy, in LiCl/PVA and LiCl/PVA/MPII gel electrolytes, was based

15

upon this data set. Figure 2 (e) shows the Fourier transform infrared (FT-IR) spectra for CNT

16

and CNT/Ppy. In the FT-IR spectrum of CNT, the C = O stretching by the carboxylic group of

17

CNT is observed at 1,741 cm-1 [46]. The absorption peak is observed at 1,647 cm-1,

18

Figure 2. SEM images of CNT/Ppy electrodes with various deposition times of Ppy: (a) 0, (b) 5, (c) 15, and (d) 20 s. (e) FT-IR spectra, and (f) Raman spectra of CNT (blue) and CNT/Ppy (red). SEM images of (g) GF, and (h) FGF. (i) Raman spectra of GF (blue) and FGF (red). corresponding to the C = C stretching mode of CNT [47]. The FT-IR spectrum of CNT/Ppy

19

shows peaks at 1,559 and 1,479 cm-1, corresponding to antisymmetric and symmetric pyrrolering fundamental vibrations, respectively [48]. The peaks at 1,275 and 1,229 cm-1 are attributed to C–N and C–C stretching vibrations, respectively [49]. Moreover, the peak at 956 cm-1 is attributed to C–H out-of-plane vibration [50]. Figure 2 (f) shows the Raman spectra of CNT and CNT/Ppy. The CNT spectrum shows a D-band, indicating disordered graphite at 1,354 cm-1, Gband attributed to the E2g mode of graphite wall at 1,597 cm-1, and 2D-band at 2,690 cm-1 [51]. In the spectrum of CNT/Ppy, the same D-, G-, and 2D-bands are observed [52]. Moreover, additional peaks at 940, 980, 1, 059, and 1,420 cm-1 are observed with the deposition of Ppy on the CNT. The peaks at 940 and 980 cm-1 are attributed to ring deformation with di-cation (dipolaron) and radical cation (polaron), respectively [53]. The peak at 1,059 cm-1 originates from C–H in plane deformation, and the 1,420 cm-1 peak is attributed to the C–N stretching mode [54, 55]. This information confirms the existence of CNT and CNT/Ppy on the electrodes of the MSC. Figure 2 (g) shows the SEM image, taken from the CVD grown GF, after etching the Ni. The figure shows a macroporous structure with an average pore size of ~300–600 μm. These 3D branch shape cylinders are connected in the form of a network, which is fragmentized during the vortex mixing process, as shown in Figure 2 (h). However, the graphene layer is not randomly torn or crushed into small particles. Most of the FGF preserves the 3D structure of the GF, and the vertices of the branch shape are maintained. Therefore, the strain is different at all parts of the FGF, but it is expected to have a concentrated strain at the vertex of GF [40]. Figure 2 (i) shows the Raman spectra of GF synthesized from CVD and FGF, obtained after the vortex mixing process. The peak intensity ratios of 2D-band to G-band (I2D/IG), and D-band to G-band (ID/IG) for GF are 0.59 and 2.76, respectively. This signifies the growth of multilayer graphene with a low defect density via CVD [56]. The similarity of the Raman spectra for GF and FGF indicates that the lattice structure is preserved, even after the vortex mixing process. Figure S6 shows the electrochemical characteristics and stable voltage windows of CNT and CNT/Ppy electrodes, investigated in the three-electrode systems, by taking the cyclic voltammetry (CV) curves, galvanostatic charge/discharge curves, electrochemical impedance, and areal capacitance, at various current densities. The following setup was used: 1 M LiCl as the electrolyte, Pt coil as the counter electrode, and Ag/AgCl (saturated 3 M NaCl) as the reference electrode. Figures S6 (a) and (b) show the CV curves of CNT and CNT/Ppy electrodes 20

Figure 3. Electrochemical performance of the CNT/Ppy MSCs with LiCl/PVA (black) and LiCl/PVA/MPII (red) gel electrolytes. (a) CV curves at a scan rate of 100 mV s-1. (b) Galvanostatic charge/discharge curves at a current density of 100 μA cm-2. (c) Nyquist impedance plots in the frequency range from 500 kHz to 0.1 Hz. (d) Areal capacitance at various current densities with error bars from 15 different MSCs. (e) Ragone plot. (f) Capacitance retention for the charge/discharge cycles. at various scan rates. CV curves of the CNT and CNT/Ppy electrodes at a scan rate of 200 mV s-1 were compared in Figure S6 (c). Both electrodes show the rectangular shape of the CV curves, indicating fast ion diffusion and current response to a voltage scan. In the same potential range of 0–0.8 V, the CNT/Ppy electrode showed more than three times the enhanced current density compared to the CNT electrode. This is attributed to the redox reaction of Ppy. Figures S6 (d) and (e) show the galvanostatic charge/discharge curves at various current densities. In Figure S6 (f), the galvanostatic charge/discharge curves of the CNT and CNT/Ppy electrodes (at a current density of 30 A g-1) show symmetric triangular shapes, and one charge/discharge cycle takes 15 s and 88 s, respectively. In Figure S6 (g), the specific capacitance of the CNT electrode is 135 F g1

at 20 A g-1, and 112 F g-1 at 100 A g-1. The specific capacitance of the CNT/Ppy electrode is 783

F g-1 at 20 A g-1, and 594 F g-1 at 100 A g-1. In Figure S6 (h), the Nyquist impedance plots were measured in a frequency range of 500 kHz to 0.1 Hz. The CNT and CNT/Ppy electrodes showed equivalent series resistance (ESR) values of 81 Ω and 58 Ω, respectively, which signified the 21

lower charge transport resistance of the CNT/Ppy electrode. These results confirmed the significant improvement in electrochemical performance of the CNT electrode via the addition of a Ppy film. Figure 3 compares the electrochemical properties of CNT/Ppy MSCs with the two different electrolytes of LiCl/PVA, and LiCl/PVA/MPII. Figure 3 (a) shows the CV curves at a scan rate of 100 mV s-1. CNT/Ppy MSC with LiCl/PVA shows the typical rectangular shape of EDLC, indicating the rapid response of current to voltage change, owing to the fast ion diffusivity. The CV curve of the CNT/Ppy MSC with LiCl/PVA/MPII exhibits typical double redox peaks, with a highly increased response current density value. This is attributed to the additional redox reaction of MPII (used as a redox additive) on the electrode during the charge/discharge processes. The redox pairs of 3I-/I3-, 2I-/I2, 2I3-/3I2, and I2/2IO3- show the following redox reactions [57]: 3𝐼 − ↔ 𝐼3− + 2𝑒 −

(1)

2𝐼 − ↔ 𝐼2 + 2𝑒 −

(2)

2𝐼3− ↔ 3𝐼2 + 2𝑒 −

(3)

𝐼2 + 6𝐻2 𝑂 ↔ 2𝐼𝑂3− + 12𝐻 + + 10𝑒 −

(4)

Moreover, Figure S7 shows that these additional redox reactions are observed at lower scan rates of 10, 20, 30, and 50 mV s-1. Figure 3 (b) shows the galvanostatic charge/discharge curves of CNT/Ppy MSCs, with LiCl/PVA and LiCl/PVA/MPII gel electrolytes, at a current density of 100 μA cm-2. The charge/discharge process for one cycle took 8 and 103 s, respectively. It can be explained that the addition of MPII to the LiCl/PVA electrolyte is able to enhance the electrochemical performance of the CNT/Ppy MSC. The galvanostatic charge/discharge curve of CNT/Ppy MSC with the LiCl/PVA/MPII gel electrolyte exhibits the peaks of redox reaction at approximately 0.55 and 0.35 V. Figure 3 (c) shows the measured electrochemical impedance spectra in the frequency range of 500 kHz to 0.1 Hz. The Nyquist plot shows the impedance behavior of the supercapacitor in the form of frequency change. The Y-axis is an imaginary region indicating the capacitive parameter, and the X-axis is a real component exhibiting the Ohmic parameter. The equivalent series resistance (ESR) value, that signifies the charge transport resistance, is measured at the X-axis [58]. The ESR values of MSCs with LiCl/PVA and LiCl/PVA/MPII are 450 and 150 Ω, respectively. This indicates that MSC with the LiCl/PVA/MPII gel electrolyte has a lower charge transport resistance than with LiCl/PVA. 22

Moreover, the impedance plot of MSC with LiCl/PVA shows a larger semicircle in the highfrequency region, which indicates the larger contact impedance of the LiCl/PVA electrolyte than LiCl/PVA/MPII, with CNT/Ppy electrodes [59]. In the low-frequency region, the impedance plot is close to being parallel with the Y-axis, signifying the low diffusion resistance in the electrode structure [60]. Figure 3 (d) shows the areal capacitance (Ca, F cm-2) calculated at various current densities, according to the following equation: 𝐶𝑎 (F 𝑐𝑚−2 ) =

𝐼 (𝐴) × ∆𝑡 (𝑠) ∆𝑉 (𝑉) × 𝐴 (𝑐𝑚2 )

(5)

where, I, Δt, ΔV, and A are the discharge current (A), discharge time (s), voltage drop (V), and total area of electrodes (cm-2), respectively. The total area of the electrodes was measured, including the interspace (μm) of the two electrodes. The CNT/Ppy with LiCl/PVA and LiCl/PVA/MPII gel electrolytes gave areal capacitances of 0.48 and 5.17 mF cm-2, respectively, at a current density of 100 μA cm-2. In addition, the low standard deviation (measured from 15 different devices) indicates a high reproducibility of device fabrication. Figure S8 shows the Coulombic efficiency, with various current densities, indicating the charging/discharging efficiency. At a current density of 50 μA cm-2, MSCs with LiCl/PVA and LiCl/PVA/MPII electrolytes exhibited a Coulombic efficiency of 92% and 79%, respectively. However, at the higher current density, the corresponding Coulombic efficiency increased to 96% and 97%, respectively. This is attributed to the poor charge transport efficiency of the redox material (MPII) at lower current densities. Figure 3 (e) shows the Ragone plot for energy density (E, Wh cm-2), and power density (P, W cm-2), of the fabricated device. The energy density and power density are calculated with the following equations: E (Wh 𝑐𝑚−2 ) = P (W 𝑐𝑚−2 ) =

𝐶𝑎 (𝐹 𝑐𝑚−2 ) × ∆ 𝑉 2 (𝑉) 7200

(6)

𝐸 (𝑊ℎ 𝑐𝑚−2 ) × 3600 ∆𝑡 (𝑠)

(7)

where, E, P, Ca, ΔV, and Δt are the energy density, power density, areal capacitance, voltage window, and discharge time, respectively. Here, the calculated values of areal capacitance, energy density, and power density are also dependent on the thickness of the electrodes 23

(nanoscale, micro scale, or thicker), the type of electrode (e.g., carbon-based materials, transition metal oxides, or conducing polymer), or the electrolyte (e.g., solution, gel, or solid) [61]. Since the thickness of our CNT (270 nm) and CNT/PPy (430 nm) is very thin at nanometer scale, as shown in Figure S9, the electrochemical performance was evaluated in areal metric. The fabricated MSCs with LiCl/PVA and LiCl/PVA/MPII displayed a maximum energy density of 0.045 and 0.44 μWh cm-2, and a maximum power density of 30.2 and 176.5 μW cm-2, respectively. Our CNT/Ppy MSC achieved a reasonable performance compared to the previous studies, shown by the electrochemical performance of the MSCs in areal metric. Table S1 (supporting information) compares the performance of MSCs, such as graphene quantum dots/polyaniline asymmetric MSC (thickness: 1.3 nm, Ca: 667.5 μF cm-2, E: 0.093 μWh cm-2, P: 7.52 μW cm-2) [62], K2CO3(P2O7)2·2H2O nanocrystal whiskers/graphene nanosheet asymmetric MSC (thickness: 10 μm, Ca: 30 mF cm-2) [63], reduced graphene film MSC (thickness: 40 nm, Ca: 80.7 μF cm-2, E: 2.5 mWh cm-3, P: 495 W cm-3) [64], MnO2 nanoflakes on Si nanowires MSC (thickness: 50 μm, Ca: 13 mF cm-2, E: 9.1 μWh cm-2) [65], porous carbon MSC (Ca: 800 μF cm-2) [66], nanoporous Au/MnO2/Ppy MSC (1.3 mF cm-2, 45.3 mWh cm-3, 440.4 W cm-3) [67] and Au/MnO2/Au MSC (thickness: 1.6 μm, Ca: 11.9 mF cm-2) [68]. Figure 3 (f) shows the measured cyclic stability of CNT/Ppy MSCs with LiCl/PVA and LiCl/PVA/MPII, which show a retention of 37% and 80% of the initial capacitance, respectively, after 5,000 repetitions of the charge/discharge process. The areal capacitance of CNT/Ppy MSC with LiCl/PVA/MPII continuously increased to 126% of the initial value, up to 600 cycles of the charge/discharge process, then it gradually decreased. This phenomenon is known as electrochemical activation, commonly observed in electrochemical processes, which (in this study) was due to the addition of MPII. [69]. It is clearly shown that the electrochemical performance of the fabricated MSC with LiCl/PVA/MPII was significantly enhanced, even with the same electrode of CNT/PPy. The electrochemical stability of the fabricated MSC with CNT/Ppy and LiCl/PVA/MPII, under air ambient condition, was measured as shown in Figure S10. The capacitance retention was calculated from the charge/discharge curves at a current density of 100 μA cm-2 after the drying process of LiCl/PVA/MPII gel electrolyte on CNT/Ppy electrodes. Every 6 h, the measurement was repeated with 20 different MSCs, and the gradual decrease was observed with an elapsed time of 18 h. However, 95% of the initial capacitance was retained after 72 h. 24

Figure 4. Electrochemical performance of the integrated system with the MSC array and solar cells. (a) Photo-charging curves under light from a solar simulator, and discharge curves, at current densities of 50, 70, 100, and 150 µA cm-2 in the dark states. (b) Areal capacitances at various current densities (red: charge/discharge process by external power supply, green: photo-charge/discharge process by solar simulator, and blue: photo-charge/discharge process by sunlight). (c) Capacitance retention for the photo-charge/discharge cycles. (d) Selfdischarge profile with an open circuit voltage of 0.8 V. (e) Optical images of the integrated systems with an externally connected LCD. Figure 4 shows the electrochemical performance of the integrated system of MSC arrays and Si-based commercial SCs. Two serially connected Si-based commercial SCs were connected to the MSC array, and the resulting photo-charge/discharge curves are shown in Figure 4 (a). The stop voltage of the photo-charge process was set to 0.8 V for the integrated photo-charge system, then the discharge process was performed at a current density of 50, 70, 100, and 150 μA cm-2. The photo-charge process to 0.8 V took approximately 2 s, and the discharge curves show the shape to be similar to that of the MSC charged by an external power source. This signifies that our fabricated MSCs can be reliably discharged, regardless of the charging method. Figure 4 (b) compares the areal capacitance of the fabricated MSC/SC array for three different charging methods: co-charging by (red) an external power supply, (green) a solar simulator, and (blue) sunlight. The three systems show areal capacitances of 4.23, 4.01, and 4.1 mF cm-2 at a current 25

Figure 5 (a) Schematic definition of the biaxial strain εbiaxial. (b) Optical images (left) of the results obtained from the FEM analysis (right) of the serpentine interconnection in various biaxial stretching states. The cross-sectional FEM analysis view of the serpentine interconnection at a strain of 30% is shown below right. (c) Optical image of the MSC array (left), and the strain distribution estimated by FEM analysis (right), under a biaxial strain of 30%. (d) CV curves with variation in the applied biaxial strain. Normalized capacitance with repetitive stretching by 30% in the case of (e) MSC array, and (f) MSC with the solar cells system. Insets: (e) charge/discharge, and (f) photo-charge/discharge curves, with variation in the applied biaxial strain. density of 50 μA cm-2; 5.17, 4.93, and 5.17 mF cm-2 at a current density of 100 μA cm-2; and 5.25, 4.96, and 5.16 mF cm-2 at a current density of 150 μA cm-2, respectively. This result indicates that our integrated system of MSCs and SCs is stable, regardless of the charging method. Figure 4 (c) shows the capacitance retention of the integrated system after 500 cycles of the photocharge/discharge process. The capacitance increased to 120% of the initial capacitance, which is similar to the behavior of capacitance retention after the charge/discharge process when implemented by an external power supply, as shown in Figure 3 (f)). Figure 4 (d) shows the self-discharge profile after being charged up to 0.8 V. It took approximately 1.2 h to reach an open circuit voltage of 0.4 V. Figure 4 (e) is the optical microscope image taken of the liquid crystal display (LCD), driven by our integrated system using an external wire connection. After charging with the integrated SCs, the stored energy of the MSC array was used to operate the 26

LCD for approximately 25 min. This signifies that our integrated system satisfies the highenergy density required for long-term operation of the LCD. Figure 5 shows the strain distribution, estimated by a finite element method (FEM) analysis, for biaxial stretching of the whole system. Commercial software (ABAQUS) was used for this analysis. The neo-Hookean constitutive model was used for the Ecoflex substrate. Due to the large strain deformation of the device, it was modeled by a second order solid element (C3D20). The applied biaxial strain was calculated using the following equation: 𝜺𝒃𝒊𝒂𝒙𝒊𝒂𝒍 =

(𝒍′ −𝒍) 𝒍

(8)

where l and l’ are the length of the initial state and stretched state of the unit module, respectively, as illustrated in Figure 5 (a). Figure 5 (b) shows the optical microscope image, and the FEM analysis of the serpentine interconnection, taken after biaxial stretching of up to 30%. The explicit solver in ABAQUS was adopted to simulate the behavior of the serpentine interconnection, due to its highly complicated deformation. In the model, the serpentine interconnection was not allowed to penetrate the substrate, instead it “popped-up” due to the stretching. As shown in the optical images taken from the experimental results and FEM analysis (Figure 5 (b)), the strain of the serpentine interconnection was measured at less than 0.001% during the stretching process (from 5% to 30%). This is attributed to the pop-up structure of serpentine interconnections that have a narrow and long design. Hence, a biaxially stretchable integrated system with mechanical stability was fabricated. Figure 5 (c) shows the optical microscope image, and the corresponding strain distribution by FEM analysis, under 30% biaxial stretching. Even under the applied biaxial strain of 30%, a very small strain of approximately 0.04%–0.08% was applied to the area of the stiff SU-8 photoresist platform array, while it reached approximately 102%–118% strain in other areas. The experimentally measured strain was similar to the theoretical strain (calculated by FEM analysis), which indicated the mechanical stability of the fabricated integrated system. Next, the electrochemical performance of the integrated system was measured under biaxial stretching up to 30%. Figures 5 (d), (e)-Inset, and (f)-Inset show the CV curves, galvanostatic charge/discharge curves, and photo-charge/discharge curves with a variation of applied strain of 0%, 10%, 20%, and 30%, respectively. There is no significant change, indicating the mechanical 27

stability of the fabricated integration system. Figures 5 (e) and (f) show the normalized capacitance (C/C0), with an interval of 50 cycles (during repetitive 30% biaxial stretching) from the charge/discharge and photo-charge/discharge curves, respectively. Here, C0 and C are the capacitance before and after the stretching cycles, respectively. Both graphs show the degradation of the initial capacitance by just 2%, even after repetitive stretching of 1,000 cycles. This result implies the mechanical stability of our fabricated integrated system.

28

The fabricated resistor type FGF SS was also integrated with the MSC and SC array, to detect

29

Figure 6 (a) Sensing mechanism of the FGF SS. (b) Resistance change of the FGF SS under applied strain with error bars from 15 different samples, and Inset: I–V curves of the FGF SS taken at stretching states of (0%, 10%, 20%, and 30%). (c) Photograph of integrated system attached to the skin of the wrist. (d) Resistance change of the FGF sensor driven by stored energy of the MSCs, charged with (red) external power supply, (green) solar simulator, and (blue) solar light, in response to the pulse signal of the radial artery. (e) Photograph of wrist bending. (f) Resistance change of the FGF sensor driven by stored energy of the MSC, charged with (red) external power supply, (green) solar simulator, and (blue) solar light, in response to the bending of the wrist. bio signals (such as arterial pulse and wrist motion) by using the stored energy of the integrated

30

system, without any wired connection to an external power supply. Figure 6 (a) shows the sensing mechanism of the fabricated FGF SS, when the elastomer substrate is stretched with the applied strain. The distance between the adjacent FGF pieces on the elastomer substrate was elongated upon stretching, which resulted in an increase of contact resistance over a large area [40]. Figure 6 (b) shows the change of resistance with various applied strains of 0%, 10%, 20%, and 30%. The resistance linearly increased from 134 to 193 kΩ, with increases of the strain. This signifies that the fabricated FGF SS shows stable electrical performance up to a 30% strain. Figure 6 (b)-inset shows the current–voltage (I–V) curves at stretching states of 0%, 10%, 20%, and 30%. The current value also changed with a variation of the strain. The linear I–V curves exhibit the Ohmic behavior of the fabricated FGF SS. Figure 6 (c) shows a photograph of the integrated system attached to the wrist of a 30-year-old male graduate student (using a commercial Scotch tape (3M)), for detecting the pulse signal of the radial artery under sunlight. Figure 6 (d) compares the pulse signals of the radial artery for three differently charged MSCs: MSCs charged with (red) an external power supply, (green) solar simulator, and (blue) sunlight, respectively. Important information about the arterial wall, such as the stiffness of the aortic system, can be obtained via analysis of the arterial pulse waveforms using pulse rate and pressure. Regardless of the charging method, all three graphs show similar repeated resistance variations. The pulse signal of the radial artery exhibits 15 beats per 10 s, and the interval between pulses is uniform. Figure 6 (e) is a photograph of the integrated system detecting wrist motion. Top left and bottom right of Figure 6 (e) show the flat and bending states. Tensile strain was applied to the integrated system toward the black arrows, in the case of its bending state. This wrist motion can be detected by the change of resistance, as shown in Figure 6 (f). When the wrist was bent, the resistance increased from 134 to 145 Ω, then gradually returned to 134 Ω as the wrist was straightened. At the points of tensile bending, and during the relaxation process of the wrist motion, sharp peaks are observed, which are attributed to the viscoelasticity of the PDMS/Ecoflex composite used for fabricating the FGF strain sensor [70, 71]. Similarly, the wrist motion is clearly detected in all three graphs, regardless of the charging method. In addition, the stability of our integrated system under sunlight was measured, as shown in Figure S11. The relative strain response was calculated by the following equation. 𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑠𝑡𝑟𝑎𝑖𝑛 𝑟𝑒𝑠𝑝𝑜𝑛𝑠𝑒 = 31

(𝑅𝐵𝑒𝑛𝑑𝑖𝑛𝑔 −𝑅𝐹𝑙𝑎𝑡 ) 𝑅𝐹𝑙𝑎𝑡

(9)

where RBending and RFlat are the resistance values at the bent and straight states of the wrist motion, respectively. The charge/discharge process progressed while the whole integrated system was exposed to the light of a solar simulator, and the wrist motion was repeatedly detected at intervals of 2 h by the sensor of the integrated system, with error bars from 7 different inegrated systems. The relative strain response values were measured as 0.071 at the beginning (0 h), and 0.066 after 24 h, respectively. These values are within the error bars and can be classified as reasonable, considering that the relative strain responses of P1, P2, ···, P8, P9 in Figure 6 (f) vary from 0.058 to 0.077. This clearly shows the stability of our sensor system when exposed to sunlight.

4. Conclusion In this paper, we demonstrated the fabrication of a stretchable array of high-performance solidstate MSCs, which can be charged by integrated Si-based commercial SCs, to wirelessly power an integrated strain sensor. The planar MSC comprised electrodes of potentiostatically deposited Ppy, on spray-coated CNT film, and a gel-type electrolyte of LiCl/ PVA with a redox additive of MPII. A strain sensor was fabricated using a composite film of FGF and PDMS. The twelve fabricated parallel-connected MSCs, a strain sensor, and the two serially connected SCs were integrated on a single Ecoflex substrate with embedded stiff platforms of SU-8 photoresist via long serpentine interconnections of Ti/Pt metal film. After 1,000 repetitive biaxial stretching/releasing cycles by 30%, no noticeable change was apparent in the charge/discharge behavior. After attaching the entire integrated system onto the wrist of the author, the integrated strain sensor could detect externally applied strains and the arterial pulse using the energy stored in MSCs, charged with SCs. This work successfully demonstrates the potential application of our stretchable self-charging power/sensor system to skin-attachable health-monitoring devices.

32

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

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.nanoen.xxxxx.

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Junyeong Yun is currently a Ph. D. candidate under Prof. Jeong Sook Ha at the Department of Chemical and Biological Engineering in Korea University, Seoul, Korea. His research interests include synthesis and characterization of materials and their applications in supercapacitors for energy conversion and storage.

Changhoon Song is a Ph. D. candidate under Prof. Jeong Sook Ha at the Department of Chemical and Biological Engineering in Korea University, Seoul, Korea. His research interests include high-performance supercapacitors.

Hanchan Lee is currently a Ph. D. candidate under Prof. Jeong Sook Ha at the Department of Chemical and Biological Engineering in Korea University, Seoul, Korea. His recent research interests are focused on the fabrication and application of biodegradable and stretchable 38

electronics.

Heun Park is currently a Ph. D. candidate under Prof. Jeong Sook Ha at the Department of Chemical and Biological Engineering in Korea University, Seoul, Korea. His research interests mainly focus on stretchable and flexible sensors and their application in wearable electronics.

Yu Ra Jeong is currently a Ph. D. candidate under Prof. Jeong Sook Ha at the Department of Chemical and Biological Engineering at Korea University, Seoul, Korea. Her research interests include stretchable electronics for biomedical applications.

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Jeong Wook Kim is currently a Ph. D. candidate under Prof. Jeong Sook Ha at the Department of Chemical and Biological Engineering in Korea University, Seoul, Korea. His research interests mainly focus on stretchable sensor systems based on conducting polymers.

Sangwoo Jin is a Ph. D .candidate under Prof. Jeong Sook Ha at KU-KIST Graduate school of Converging Science and Technology, Seoul, Korea. His recent research interests are focused on the fabrication of flexible perovskite SCs for wearable energy-harvesting applications.

Seung Yun Oh is a Ph. D. candidate under Prof. Jeong Sook Ha at the KU-KIST Graduate School of Converging Science and Technology in Korea University, Seoul, Korea. His research 40

interests include electrochemical sensors for stretchable and wearable electronics.

Lianfang Sun is currently a Ph. D. candidate under Prof. Goangseup Zi in the School of Civil, Environmental, and Architectural Engineering at Korea University, Seoul, Korea. His recent research interests mainly focus on durability of concrete structures and development of numerical simulation methods.

Goangseup Zi is a Professor in the School of Civil, Environmental, and Architectural Engineering at Korea University, Seoul, Republic of Korea. He received his Ph. D. from Northwestern University, Evanston, IL, in 2002. His research interests include fracture mechanics, damage, and nonlinear analysis of structures and solids.

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Jeong Sook Ha is a professor of the Department of Chemical and Biological Engineering at Korea University. She received her Ph. D. degree in Chemistry from Brown University, Providence, USA, in 1989. Her research interests include stretchable electronic/acoustic/energy storage devices, nano-fabrication, and growth of nanomaterials.

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Highlights ● We fabricate a stretchable array of high performance micro-supercapacitors charged by solar cells for wireless powering of the integrated strain sensor. ● Serpentine interconnections with the embedded stiff SU-8 platform enable stable performance under stretching deformation. ● Strain sensor of fragmentized graphene foam detects pulse signal of the radial artery and bending motion of the wrist. ● The sensor is operated for 100 s with energy supplied by the micro-supercapacitors and solar cells. ● Under repeated biaxial stretching, the stretchable array retains stable performance.

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