WO2018040070A1

WO2018040070A1 - Micro-supercapacitor array for integrated wearable electronic system and integrated wearable electronic system comprising the same

Info

Publication number: WO2018040070A1
Application number: PCT/CN2016/097965
Authority: WO
Inventors: Xiaohong Wang; Juan PU
Original assignee: Tsinghua University
Priority date: 2016-08-30
Filing date: 2016-09-02
Publication date: 2018-03-08

Abstract

A micro-supercapacitor array for integrated wearable electronic system with honeycomb structure comprising a plurality of honeycomb cell, said honeycomb cell comprising a polydimethylsiloxane bottom substrate (1), a bottom PI layer (2) on the honeycomb polydimethylsiloxane bottom substrate (1), a patterned Cr/Au bilayer (3) on the bottom PI layer (2), a top PI layer (4) on the Cr/Au bilayer (3), interdigital electrodes (5) on the top PI layer (4), polymer electrolyte layers (6) coated onto the interdigital electrodes (5), and a polydimethylsiloxane top substrate (7) on the PVA-H₃PO₄ gel electrolyte layers.

Description

MICRO-SUPERCAPACITOR ARRAY FOR INTEGRATED WEARABLE ELECTRONIC SYSTEM AND INTEGRATED WEARABLE ELECTRONIC SYSTEM COMPRISING THE SAME

Technical field of the invention

The present invention relates to a micro-supercapacitor array for integrated wearable electronic system and an integrated wearable electronic system comprising the same.

Background

The rapid growth of wearable electronic devices has increased the demand for reliable stretchable and flexible electronics. Several recent studies have been focused on the fabrication of various types of stretchable devices, such as wearable photovoltaics, sensitive robotic skin, epidermal electronics, soft surgical tools, and organic or inorganic LEDs. A critical need for all these stretchable devices is the development of energy-storage system with similar physical properties in order to facilitate direct integration with other electronics.

Supercapacitors are promising energy storage devices because they demonstrate higher power density, longer operation life, and better safety than batteries. To power stretchable electronics, the supercapacitors must be flexible and stretchable. The stretchability of most supercapacitors depends on the strain applied to the substrate before electrode loading, which hinders the integration with other electronic units. Miscrosupercapacitor (MSC) with two-dimensional (2D) in-plane electrode structure allows facile integration with on-chip and wearable electronic. The unique features of 2D in-plane electrodes reduce ionic diffusion pathways compared with conventional supercapacitors with sandwiched electrodes, thereby further improving the rate capability and power performance of the devices. Furthermore, a 2D in-plane structure reduces the device thickness, which not only endows the device with greater flexibility, but also enables the integration of MSC in the thickness direction to fully make use of the device area. This could greatly extend the output voltage range and enhance the energy density of MSC over limited device area. More recently, the stretchability of MSC array was increased by using serpentine-like metallic interconnectors, introducing stiff islands in the soft elastomer substrate, or employing suspended wavy structures of graphene microribbons. Although these technologies possess attractive features, none of these offer the capability of stretching to large deformations (e.g., >100％elongation) , which limits their applicability to only a few wearable electronic systems.

Achieving both high stretchability and good integration capability based on current strategies is challenging.

Summary of the invention

The one object of the invention is to provide a micro-supercapacitor (MSC) array for integrated wearable electronic system.

The unique feature of the MSC array is the stretchability of the MSC array. The stretchable MSC array has good capacitive performance and excellent rate capability.

In the invention, there is provided a stretchable MSC array comprising a plurality of honeycomb cell, said honeycomb cell comprising

a polydimethylsiloxane (PDMS) bottom substrate,

a bottom PI layer on the honeycomb PDMS bottom substrate,

a patterned Cr/Au bilayer on the bottom PI layer,

a top PI layer on the Cr/Au bilayer,

interdigital electrodes on the top PI layer,

polymer electrolyte layers coated onto the interdigital electrodes, and

a PDMS top substrate on the PVA-H₃PO₄ gel electrolyte layers.

According to the invention, the bottom PI layer, the patterned Cr/Au bilayer, the top PI layer and the PDMS top substrate have a shape corresponding to the honeycomb PDMS bottom substrate.

According to the invention, the interdigital pattern of the interdigital electrodes is the same as that of the patterned Cr/Au bilayer.

According to the invention, the Cr/Au bilayer is consisted of a Cr underlayer and an Au top layer.

According to the invention, the polymer electrolyte layers are polyvinyl alcohol-H₃PO₄ gel electrolyte layers.

According to the invention, the PDMS bottom substrate is the same as the PDMS top substrate, and the bottom PI layer is same as the top PI layer.

According to the invention, interdigital electrodes are interdigital single-walled carbon nanotube (SWCNT) electrodes.

The another object of the invention is to provide a method for fabricating the stretchable MSC array comprising

providing a honeycomb PDMS bottom substrate,

fabricating one or more micro-supercapacitor,

transferring the one or more micro-supercapacitors onto the honeycomb bottom PDMS substrate and bonding them onto the honeycomb PDMS bottom substrate,

coating a polymer electrolyte onto interdigital electrodes to form polymer electrolyte layers,

assembling a honeycomb PDMS top substrate together with the honeycomb bottom PDMS to encapsulate the micro-supercapacitor array.

According to the invention, the step of fabricating one or more micro-supercapacitor comprising

forming a bottom PI layer on a wafer,

forming a Cr/Au bilayer and patterning the Cr/Au bilayer to form interdigital current collectors and interconnects,

forming a top PI layer on the Cr/Au bilayer,

selectively etching the top PI layer to expose a interdigital electrode area and form honeycomb cell shape,

depositing electrode materials onto the interdigital electrode area exposed and patterning to form interdigital electrodes,

selectively etching the bottom PI layer to match the honeycomb cell shape of the top PI layer.

According to the invention, the step of transferring the one or more micro-supercapacitors onto the honeycomb bottom PDMS substrate comprising

peeling off the micro-supercapacitor from the wafer by a water-dissolvable tape and transferring it onto the honeycomb PDMS bottom substrate.

According to the invention, the step of bonding the one or more micro-supercapacitors transferred onto the honeycomb PDMS bottom substrate and bonding them onto the honeycomb PDMS bottom substrate comprising

spreading a liquid PDMS solution onto the honeycomb PDMS bottom substrate surface and curing to bond the one or more micro-supercapacitors transferred onto the honeycomb PDMS bottom substrate and removing the water-dissolvable tape.

According to the invention, a interdigital pattern of the interdigital electrodes is the same as that of the interdigital current collectors and interconnects.

According to the invention, the Cr/Au bilayer is consisted of a Cr underlayer and a Au top layer.

According to the invention, the honeycomb PDMS bottom substrate is the same as the honeycomb PDMS top substrate, and the bottom PI layer is same as the top PI layer.

According to the invention, the interdigital electrodes are interdigital SWCNT electrodes.

The still another object of the invention is to provide an integrated wearable electronic system comprising the above stretchable MSC array.

In this invention, the honeycomb PDMS structure can accommodate large deformation without producing excessive strain in the MSC and interconnect. Such stretchable MSC arrays exhibit excellent rate capability and power performance. The stable electrochemical performance of the stretchable MSC arrays can be maintained up to 150％stretching (275％under compressive pre-straining) and under excessive bending or twisting. The stretchable MSC arrays have high potential for integration with other electronics, such as energy harvesters, power management circuits, wireless charging circuits, and various sensors, encompassing a wide range of wearable, bio-implantable electronic systems.

Brief description of the figures

Figure 1 is a view showing undeformed and deformed configurations, structural layers, and electrode microstructure of the 4 × 4 stretchable MSC array: (a) schematic illustration of a stretched and bended device, (b) exploded view of various structural layers of the device, digital photograph of (c) undeformed, (d) compressed, and (e) twisted and stretched device, (f) optical microscope image of interdigital SWCNT electrodes, (g) top-view SEM image of SWCNT electrodes (the inset figure shows a TEM image of the SWCNT) , and (h) cross-sectional SEM image of the device on a silicon substrate (each layer is distinguished by a different color) .

Figure 2 is a view showing electrochemical performance of the honeycomb MSCs: (a) and (b) typical CV curves of single stretchable MSC and 4 × 4 stretchable MSC array, respectively, for a scan rate of 10, 50, and 100 V s^-1, (c) volumetric capacitance C_V versus scan rate, (d) and (e) potential versus time (GCD plot) for 5 nA applied current of single stretchable MSC and 4 × 4 stretchable MSC array, respectively, (f) volumetric energy density E_V versus power density P_V (Ragone plot) , (g) imaginary impedance Z″versus real impedance Z′ (Nyquist plot) , (h) normalized imaginary capacitance C″versus frequency, and (i) capacity retention versus charge/discharge cycles.

Figure 3 is a view showing optical images of the 4 × 4 stretchable MSC array at different deformation stages.

Figure 4 is a view showing electrical and mechanical performance of 4 × 4 stretchable MSC array: (a) capacity retention versus elongation (the inset figure shows CV curves for –50％compression and 0–150％elongation) , (b) experimental stress-strain curves of a honeycomb MSC, solid PDMS membrane, and Nike wrist band (the FEA stress-strain curve of the honeycomb MSC is also included for analysis validation) , (c) – (e) a commercial LED (turn-on voltage ＝ 2 V) lit by a stretchable MSC array bonded to a Nike wrist band at different elongations, (f) digital photograph of honeycomb MSC bended by 180° and (g) corresponding maximum (first principal) strain εmax distribution in the Au layer, (i) digital photograph of honeycomb MSC twisted by 60° and (j) corresponding εmax distribution in the Au layer and overlapping CV curves for different (h) bending and (k) twisting angles.

Figure 5 is a view showing fabrication process of stretchable 4 × 4 MSC arrays: (a) spin coating of the bottom PI layer on a Si wafer, (b) evaporation and micropatterning of the Cr/Au bilayer, (c) spin coating and micropatterning of the top PI layer, (d) spray deposition and micropatterning of interdigital SWCNT electrodes, (e) selective etching of the bottom PI layer, (f) transfer of the device onto a honeycomb PDMS substrate, (g) coating of the interdigital SWCNT electrodes with gel electrolyte, and (h) application of the top honeycomb PDMS 36 superstrate to encapsulate the MSC array.

Figure 6 is a view showing (a) Raman and (b) FTIR spectra of SWCNTs. For the Raman spectrum shown in (a) , ID/IG ＝ 0.078. The peaks at 1112, 1635, and 3448 cm^–1 in the FTIR spectrum shown in (b) are assigned to –CH bending, CNT back bone, and carboxylate –OH stretching, respectively.

Figure 7 is a view showing (a) Schematic diagram showing the location of MSCs in a 4 × 4 MSC array and (b) corresponding circuit diagram.

Figure 8 is a view showing digital photograph of an undeformed 3 × 3 MSC array.

Figure 9 is a view showing a 3 × 3 MSC array integrated with a power management chip on a honeycomb substrate.

Figure 10 is a view showing CV curves of Au and Au+SWCNT single MSCs for a scan rate of (a) 10, (b) 50, and (c) 100 V s-1 and (d) volumetric capacitance C_V versus scan rate of SWCNT and Au+SWCNT single MSCs.

Figure 11 is a view showing CV curves of a 3 × 3 MSC array for a scan rate equal to 10, 50, and 100 V s-1.

Figure 12 is a view showing digital photographs showing tensile testing of (a) a single honeycomb cell of the device, (b) a flat PDMS membrane, and (c) a cloth strip cut from a Nike wrist band.

Figure 13 is a view showing schematic illustration of regions referred to as the islands and beams of a 4 × 4 MSC array.

Figure 14 is a view showing optical images of 3 × 3 MSC array device configuration at different deformation stages.

Figure 15 is a view showing Capacity retention versus elongation of a 3 × 3 MSC array (the inset figure shows CV curves for –50％compression and 0–100％elongation) .

Figure 16 is a view showing maximum (first principal) strain εmax in the PDMS layer of a 4 × 4 MSC array (a) bended by 180°and (b) twisted by 60°.

Figure 17 is a view showing Maximum (first principal) strain εmax in the Au layer as a function of (a) bending and (b) twisting angle. The inset figures in (a) show side-view schematics of the deformed device for 0°, 90°, and 180° bending, whereas the inset figures in (b) show schematics of the deformed device for 0°, 30°, and 60°twisting.

Detailed description of preferred embodiments

Hereinafter, embodiments of the present invention will be described in detail with reference to accompanying drawings.

In embodiments of the invention, highly stretchable MSC arrays with SWCNT electrodes and PVA-H₃PO₄ gel electrolyte on honeycomb PDMS substrates are designed and fabricated. A unique feature of the design is that the stretchability of the MSC array. The stretchable MSC arrays are controlled by the deformation of the honeycomb PDMS structure. the honeycomb PDMS substrates can greatly increase the stretchability of stretchable device loaded with MSC array. The intrinsic deformation mechanism of honeycomb structures subjected to stretching results in highly localized strain at the corners of the structure. In contrast, a conventional (solid) PDMS substrate stretched by 100％exhibits a uniform strain distribution of ＝ 1.0. Thus, the MSC array and interconnects are placed at the low-strain regions of the honeycomb structure, the strain in these functional materials will be remarkably low during stretching. Moreover, the stretchable MSC array can be easily integrated with other electronic units, such as wireless charging system and energy harvesters, by placing different electronic devices in the low-strain regions.

Spray deposition of SWCNT electrodes

Purified SWCNTs (P3-SWNT, Carbon Solutions) with 1–3 at％carboxylic acid surface functional groups were used as electrode materials. The SWCNTs were dispersed in deionized (DI) water with a tip sonicator for 1–2 h to form a 0.5–1 mg/mL stable suspension. The suspension was then sprayed onto the Cr/Au layer and placed on a plate heated to 40–60℃ to form about 280-nm-thick SWCNT films. The sprayed SWCNT films were used as MSC electrodes without further treatment.

The microstructure of the sprayed SWCNTs was studied with an SEM (S-5500, Hitachi, Tokyo, Japan) . For cross-sectional SEM imaging of the electrodes, the MSCs on the silicon substrate were sectioned perpendicular to the finger-length direction and coated with about 1-nm-thick Au-Pd layer to improve the surface conductance. To easily distinguish the boundaries between adjacent layers in the cross-sectional SEM images, different layers were stained with various artificial colors using Adobe Photoshop.

To prepare the samples for TEM imaging, the SWCNT powder was dispersed in DI water with a tip sonicator for about 2 h to form a 0.05 mg/mL stable suspension. Then, one drop of the prepared suspension was applied on a standard Cu grid used for imaging with the TEM (JEOL JEM2011, Peabody, MA, USA) .

Fig. 1f shows an optical microscope image of the interdigital SWCNT electrodes. The width of the interdigital fingers is 500 μm, whereas the gap between fingers is 200 μm. Raman and Fourier transform infrared (FTIR) spectroscopy were used to examine the molecular structure and chemical structure of the SWCNTs, respectively (Fig. 6) . The very low D-to-G band ratio (ID/IG ＝ 0.078) calculated from the Raman spectrum (Fig. 6a) indicates that the SWCNTs are of high purity and low defect density. The carboxylate –OH stretch peak at 3448 cm–1 in the FTIR spectrum (Fig. 6b) confirms surface functionalization of the SWCNTs by carboxylic acid groups. The top-view scanning electron microscope (SEM) image shown in Fig. 1g reveals that the spray-deposition process produced an entangled random network of SWCNTs, while the inset figure shows a transmission electron microscope (TEM) image of the SWCNTs. The diameter of the SWCNTs ranges from 10 to 30 nm. Fig. 1h shows a cross-sectional SEM image of MSC electrodes fabricated on a silicon substrate. The SWCNT layer deposited onto the Cr/Au bilayer has a thickness of ～280 nm.

Preparation of the gel electrolyte

The polymer electrolyte (PE) was prepared by mixing 10 mL of DI water with 10 mL of phosphoric acid (H₃PO₄) using magnetic stirring for 30 min and dissolving 10 g of polyvinyl alcohol (PVA) in 90 mL of DI water at 90℃, also assisted by magnetic stirring for 1 h. The two solutions were then intermixed by magnetic stirring for 1 h. The PVA: H₃PO₄ ratio in the PE was fixed at 1: 1 vol/wt.

Fabrication of stretchable MSC array

MSC with SWCNT electrodes were fabricated on a Si substrate using standard microfabrication and then transferred onto a honeycomb PDMS substrate to assemble stretchable MSC array. The first step of the fabrication process was to spin coat the bottom PI layer onto a Si wafer (Fig. 5a) . The PI solution (ZKPI-306II, POME Sci-tech, Beijing, China) was mixed with a PI thinner (POME Sci-tech, Beijing, China) to a weight ratio of 5: 1 to form a diluted PI solution, and left overnight to allow the bubbles in the diluted PI solution to escape. The diluted PI solution was spin-coated onto a Si wafer in two steps (step (1) : 800 rpm for 18 s and step (2) : 6000 rpm for 60 s) , soft baked in oven at 80℃ for about 3 h to evaporate the solvent, and cured in oven at 250℃ for about 2 h. The obtained PI film was about 1.3 μm thick. A bilayer consisting of a 5-nm-thick Cr underlayer and a 50-nm-thick Au top layer was evaporated onto the wafer and patterned using a lift-off process to form the interdigital current collectors and the interconnects between the MSCs (Fig. 5b) . The lift-off process was performed in acetone with very gentle sonication. Another about 1.3-μm-thick PI layer (top PI) was spin-coated, soft-baked, and cured using the same process as for the bottom PI layer.

Photolithography and selective etching of the top PI layer with O₂ plasma (70 sccm, 10 Pa, 150 W, 20 min) was carried out to expose the interdigital electrode area for subsequent SWCNT deposition and formation of a honeycomb cell shape (Fig. 5c) . The bottom and top PI layers encapsulated the Cr/Au interconnects to protect them from stress exerted during peeling off of the MSC array from the Si substrate by placing them close to the neutral bending plane. The SWCNTs were deposited onto the exposed interdigital electrode area by spray-deposition and patterned to interdigital electrodes by a lift-off process (Fig. 5d) . The interdigital pattern of the SWCNT layer is the same as that of the underlying Cr/Au bilayer. The bottom PI layer was then selectively etched with O₂ plasma to match the honeycomb shape of the top PI layer (Fig. 5e) .

The honeycomb PDMS substrate was fabricated by a molding process. PDMS resin and curing agent (10: 1 weight ratio) were mixed for 5 min, degassed in vacuum for about 30 min, carefully poured into a poly (methyl methacrylate) (PMMA) mold to fill the grooves in the mold, kept at room temperature on a flat surface for about 30 min to allow the PDMS solution to flow, and then cured at 60℃ in oven overnight. Subsequently, the cured PDMS was carefully peeled off from the PMMA mold to release the honeycomb PDMS substrate. The weight of the poured PDMS solution was kept the same in each PDMS substrate to ensure a similar substrate thickness.

The fabricated MSCs were carefully peeled off from the Si wafer by 3M water-dissolvable tape, and transferred onto the honeycomb PDMS bottom substrate (Fig. 5f) . A very thin layer of liquid PDMS solution was spread onto the PDMS substrate surface and partially cured at 70℃ for about 15 min to bond the transferred MSCs onto the PDMS substrate. After placing the MSCs onto the honeycomb PDMS substrate, the whole structure was kept at room temperature for about 24 h to allow full curing of the PDMS binding layer. Then, the 3M tape was removed by submersing in DI water for about 8 h, the PVA-H₃PO₄ gel electrolyte was coated onto the interdigital electrodes, and the MSCs were left at room temperature overnight for the excess water to evaporate (Fig. 5g) . To encapsulate the whole MSC array, another identical honeycomb PDMS substrate was assembled together with the bottom PDMS to produce the stretchable MSC array (Fig. 5h) .

The stretchable MSC array has planar interdigital electrodes with SWCNTs and polyvinyl alcohol-phosphoric acid (PVA-H₃PO₄) gel electrolyte. A unique feature of the stretchable MSC array presented here is the honeycomb PDMS substrate (Fig. 1a) , which is specifically designed to accommodate large deformation upon stretching, bending, and twisting without generating excessive deformation in the stretchable MSC array. The stretchability of the stretchable MSC array is controlled by the deformation of the honeycomb PDMS structure. An exploded view of the multilayer construction of a honeycomb cell is shown in Fig. 1b. The current collectors and interconnects consist of a photolithographically patterned metallic bilayer of a 5-nm-thick chromium (Cr) underlayer and a 50-nm-thick gold (Au) top layer. Two about 1.3-μm-thick polyimide (PI) layers are used to sandwich the Cr/Au interconnects, placing them close to the mechanical neutral plane. The SWCNTs with interdigital patterns serve as an active material for energy storage. The PVA-H₃PO₄ gel electrolyte deposited onto the SWCNT electrodes to form PVA-H₃PO₄ gel electrolyte layers providing a medium for ionic transport. All of the above layers are encapsulated by two identical PDMS honeycomb substrates that provide long-term stability to the stretchable MSC array under ambient conditions and minimize the strain in the stretchable MSC array during stretching. A 4 × 4 stretchable MSC array has 16 MSCs of which four are connected in series and sets of four MSCs in series are connected in parallel (Fig. 7) .

A digital photograph of an undeformed, compressed, and twisted 4 × 4 stretchable MSC array are shown in Fig. 1c-e, respectively. We have also demonstrated the capability to fabricate the stretchable MSC array (Fig. 8) .

Fabrication of wearable electronic system

The islands in the honeycomb structure experience nearly rigid-body motion and very low strain during stretching； thus, these regions are dedicated to functional materials and electronic devices (e.g., MSCs) that should not experience large strains. This unique feature of the honeycomb structure enables facile integration of various electronic devices on a substrate. For example, the stretchable MSC array can be integrated with an energy harvester and a power management circuit on a honeycomb substrate to form a self-powered system, in which the energy collected by the energy harvester goes through the power management circuit and is stored in the stretchable MSC array for powering other electronic units, such as sensors, actuators, and mini-displays. We have demonstrated the integration of the power management chip with a stretchable 3 × 3 stretchable MSC array (Fig. 9) .

An example of wearable electronic system is the smart wrist band system. The objective here is to self-power these wearable electronics by storing the energy harvested from sport activities in stretchable MSC array. For this purpose, a stretchable MSC array must possess adequate stretchability, excellent mechanical stability, and sufficient compliance (softness) to prevent discomfort to the user.

Fig. 4c shows a commercial LED (with a turn-on voltage of 2 V) lit by a 4 × 4 stretchable MSC array attached to a honeycomb PDMS substrate, which is bonded to a Nike wrist band.

Electrochemical testing

The electrochemical performance of single stretchable MSC and 3 × 3 or 4 × 4 stretchable MSC array was evaluated with a two-electrode system and PVA-H₃PO₄ gel electrolyte. CV and GCD experiments were performed with a CHI 860D electrochemical workstation. In the CV tests, the scan rate was varied in the range of 0.05–100 V s^–1 and the voltage in the range of 0–0.8 V (single stretchable MSC) , 0–2.4 V (3 × 3 stretchable MSC array) , or 0–3 V (4 × 4 stretchable MSC array) . In the GCD experiments, the devices were charged and discharged using a charging/discharging current of 5 nA and a voltage in the range of 0–1 V (single stretchable MSC) or 0–3 V (4 × 4 stretchable MSC array) , respectively.

The volumetric capacitance C_V was determined from the CV responses for a scan rate in the range of 0.05–100 V s^–1 using the relation

where ΔV is the potential range (0.8 and 3 V for single stretchable MSC and 4 × 4 stretchable MSC array, respectively) , v is the total electrode volume, which includes the volume of both the electrodes and the space between them (for 4 × 4 stretchable MSC array, the electrode volume is 16 times that of single MSC) , I(t) is the current measured during CV testing, and t is the time.

Both the Au layer and the SWCNT layer contribute to the total capacitance of the MSCs. To determine the capacitance of the SWCNT layer, we subtracted the capacitance of the Au current collector from the total capacitance of the Au + SWCNT layers. CV curves of Au and Au +SWCNT for a scan rate of 10, 50, and 100 V s^–1 are shown in Fig. 10a–Fig. 10c, respectively. The calculated volumetric capacitance of Au + SWCNT and SWCNT versus scan rate is shown in Fig. 10d. In this invention, the capacitance of the MSCs is that contributed only by the SWCNT electrodes.

The volumetric energy density E_V and power density P_V were calculated from the CV responses for a scan rate in the range of 0.05–100 V s^–1 using the relations

where t (in seconds) is the discharge time. The energy and power density of the MSCs are those contributed by the SWCNT electrodes.

EIS tests were carried out by applying a 10 mV ac signal in the frequency range of 10^–1–10⁶ Hz with a Solartron 1260 Impedance/Gain-Phase Analyzer (AMETEK Advanced Measurement Technology, Farnborough, Hampshire, UK) . The real and imaginary parts of the impedance Z′and Z″, respectively, were recorded over the whole frequency range and plotted as Nyquist plots. The imaginary part of the capacitance C″was estimated from the relation

where f is the frequency and

is the absolute value of the impedance.

To investigate the electrochemical stability of the devices, cyclic CV tests were performed in the potential range of 0–0.8 V (single stretchable MSC) and 0–3 V (4 × 4 stretchable MSC array) at a scan rate of 10 V s^–1. A total of 10, 000 charge/discharge cycles were applied to single MSC and 4 × 4 stretchable MSC array.

Figs. 2a and b show typical CV curves of a single stretchable MSC and a 4 × 4 stretchable MSC array, respectively, for a scan rate of 10, 50, and 100 V s^–1. Both devices yield nearly rectangular CV curves even at a high scan rate of 100 V s^–1, which is indicative of good capacitive performance and excellent rate capability. The SWCNT electrodes on top of the Au current collector have much higher current than bare Au electrodes (Fig. 10) . The output voltage of the 4 × 4 stretchable MSC array extends to 3 V (Fig. 2b) , while that of the 3 × 3 stretchable MSC array extends to 2.4 V (Fig. 11) . For a low scan rate of 0.05 V s^–1, the volumetric capacitance CV of the single stretchable MSC with SWCNT electrodes is equal to 1.86 F cm_– ³, and decreases to 1.6 F cm^–3 upon the increase of the scan rate to 1 V s^–1 (Fig. 2c) . Further increasing the scan rate to 100 V s^–1 causes the volumetric capacitance of the single stretchable MSC to decrease to 1.0 F cm^–3, representing ～54％and ～63％of the volumetric capacitance at a scan rate of 0.05 and 1.0 V s^–1, respectively. The electrochemical properties of the SWCNT MSCs can be further elucidated by comparing their performance with those of other reported MSCs. The volumetric capacitance of the SWCNT MSC is comparable to those of most reported electric double-layer capacitor (EDLC) MSC (0.2–3 F cm^–3) . Some other reported EDLC MSC, such as reduced graphene oxide MSCs and graphene microribbon MSC, show higher volumetric capacitance, but poorer rate capability and frequency response. Electrochemically reduced graphene oxide MSCs and vertically aligned multi-walled carbon nanotube MSCs show a similar high rate capability but lower volumetric capacitance compared to the SWCNT MSC of this work. (A more detailed comparison of different EDLC MSCs is given in Table 1. ) For a scan rate of 0.05 V s^–1, the volumetric capacitance of the 4 × 4 stretchable MSC array is equal to 0.15 F cm^–3, and decreases to 0.08 F cm^–3 with the increase of the scan rate to 100 V s^–1, representing 53％capacitance retention (Fig. 2c) .

The galvanostatic charge/discharge (GCD) curves of the single stretchable MSC (Fig. 2d) and the 4 × 4 stretchable MSC array (Fig. 2e) are fairly symmetric, approximately triangular, and do not demonstrate an obvious IR drop, i.e., a sudden voltage decrease at the onset of GCD discharging. We believe that this is indicative of the high coulombic efficiency, fast charge propagation across the electrodes, and low equivalent series resistance (ESR) of these devices. Furthermore, the excellent performance of the current devices is illustrated by Ragone plots (Fig. 2f) . The single stretchable MSC and the 4 × 4 MSC array exhibit a maximum volumetric energy density of 0.17 and 0.18 mWh cm^–3 and a maximum volumetric power density of 40 and 11 W cm^–3, respectively. This performance is comparable to that of previously reported high-performance EDLC MSCs.

The frequency response of the devices was also examined by electrochemical impedance spectroscopy (EIS) . We observed a straight line of ～90° slope in the low-frequency region of the Nyquist plot (Fig. 2g) of both single stretchable MSC and 4 × 4 stretchable MSC array (see inset of Fig. 2g) , which is indicative of a nearly ideal capacitive performance. The absence of a semicircle-like response in the high-frequency region of the Nyquist plots indicates ultrahigh ionic conductivity at the electrode/electrolyte interface of the devices, which is consistent with the observed ultrahigh rate capability and high-power performance. We attribute the slightly higher ESR (estimated from the x-intercept in the Nyquist plots) of the 4 × 4 stretchable MSC array (106 Ω) compared with the single stretchable MSC (91 Ω) to the higher electrical resistance due to the longer Au interconnects in the 4 × 4 stretchable MSC array.

For a more informative analysis of the EIS results, we examined the frequency dependence of the normalized imaginary part C″of the capacitance (Fig. 2h) . The fast frequency responses of both single stretchable MSC and 4 × 4 stretchable MSC array is illustrated by the short relaxation time constant τ₀, which represents the minimum time for fully discharging the device with efficiency >50％, determined from the peak frequency f₀ of C″by τ₀ ＝ 1/f₀. The extremely small relaxation time constant (τ₀ ＝ 6.3 ms) of the single stretchable MSC further confirms the ultrafast charge/discharge and excellent power performance of this device. The relaxation time constant of the SWCNT MSCs is significantly lower than that of conventional EDLC MSCs (τ₀ ≈ 10 s) and comparable to that of aluminum electrolytic capacitors (τ₀ ≈ 1 ms) and high-rate MSCs (τ₀ ＝ 1–30 ms) (see Table 1) . Nevertheless, the 4 × 4 stretchable MSC array exhibits tenfold higher relaxation time constant (τ₀ ＝ 63 ms) because of its intrinsically higher electrical resistance, which agrees well with the Nyquist plots (Fig. 2g) . The relaxation time constant of the 4 × 4 stretchable MSC array can be further decreased by increasing the thickness of the Au layer. Fig. 2i shows the electrochemical stability of the single stretchable MSC and the 4 × 4 stretchable MSC array. Both devices maintain more than 90％of their initial capacity even after 10, 000 charge/discharge cycles.

Table 1. Performance comparison of MSCs developed in this work and previous studies.

RGO ＝ reduced graphene oxide；

ERGO ＝ electrochemically reduced graphene oxide；

va-MWCNT ＝ vertically aligned multiwalled carbon nanotubes；

[a] Estim ated from volumetric and areal capacitance data from the literature；

[b] Estim ated from specific capacitance vs scan rate data from the literature；

[c] Estim ated from areal capacitance and electrode thickness data from the literature；

[d] Estim ated from the CV curves and electrode thickness.

Mechanical testing

The mechanical tests were performed with an Instron machine. To obtain the stress-strain response of the devices, a honeycomb cell was subjected to three stretching cycles of 0–100％elongation. The flat PDMS membrane was prepared by pouring mixed PDMS resin and curing agent (10: 1 weight ratio) into a Petri dish and then curing at 60℃ in oven overnight. The thickness of the produced PDMS membrane is about 2 mm. For uniaxial tensile testing, the fully cured PDMS membrane was cut into 40-mm-long and 12-mm-wide strips using an ultra-sharp platinum-coated blade. The gauge length of the PDMS strips was 30 mm.

The Nike wrist band was cut into 70-mm-long and 22-mm-wide strips along the hoop direction with scissors. The thickness of a single cloth layer of Nike wrist band is about 2 mm. The gauge length of the wrist band strips for tensile tests was 60 mm. The flat PDMS strips and Nike wrist band strips were subjected to tensile loading up to 100％maximum elongation. A strain rate of 0.5 min^–1 was used in all the tensile tests. Digital photographs of the honeycomb cell of the device, flat PDMS membrane, and cloth strip cut from a Nike wrist band subjected to uniaxial tensile tests are shown in Fig. 12.

The deformation of the honeycomb structure during stretching comprises two stages. For the 4 × 4 stretchable MSC array with a four-cell honeycomb structure discussed here, the first stage of deformation includes the increase of the distance between the islands and the bending of the beams (see Fig. 13) connecting the islands. This deformation stage represents stretching up to 100％–120％. As the stretching increases to ～150％, the beams are also gradually stretched, and the strain begins to rapidly increase. This is defined as the practical stretching limit of the device. If the compression is regarded as a pre-strain in the device and the reference (initial) state of the stretching process, a zero nominal strain can be assigned to –50％pre-compression. Thus, –50％pre-compression increases the stretching limit of the whole system to 275％ (the red color data in Fig. 3) . In the subsequent analysis and discussion, we will refer to stretching as the elongation applied to an undeformed device (i.e., zero pre-strain) .

To examine the effect of stretching on the device capacitance, we used the CV responses for a scan rate of 10 V s^–1 and observed practically identical CV curves of 4 × 4 stretchable MSC array compressed by –50％and stretched by 0–150％ (Fig. 4a) and 3 × 3 stretchable MSC array compressed by –50％and stretched by 0–100％ (Fig. 15) . The excellent stretchability of MSC array with a honeycomb PDMS structure is demonstrated by the stable performance of both 4 × 4 and 3 × 3 stretchable MSC array up to 150％and 100％maximum elongation, respectively.

As observed in our trial tests, the in-use maximum elongation of a wrist band is ～100％, which is smaller than the stretchability limit of the devices presented here. To verify that the stretchable devices are softer and more compliant compared with wrist bands and flat PDMS membranes, we performed tensile tests in the 0–100％stretching range (Fig. 12) . In these experiments, we used a Nike wrist band and a solid PDMS membrane as controls. In both tensile experiments and FEA simulations, a honeycomb cell of a 4 × 4 stretchable MSC array was subjected to three stretching cycles of 100％maximum elongation. The stress-strain curves of the stretchable device obtained from these experiments and the FEA simulations were nearly identical (Fig. 4b) and the elastic modulus of the stretchable device extracted from the tensile tests and FEA is equal to 11 and 12.6 kPa, respectively, showing a good agreement between experiments and FEA. Moreover, the slopes of the stress-strain curves of the stretchable device are much smaller than those of the PDMS membrane and the Nike wrist band (see inset of Fig. 4b) , a proof of the greater compliance of the present devices. The elastic modulus of the Nike wrist band (about 30 kPa) is almost three times that of the stretchable device. The PDMS membrane exhibits a two-stage stress-strain response. In the 0–65％elongation range, the elastic modulus of the PDMS membrane is equal to 1.07 MPa, and increases to 1.26 MPa as the elongation increases beyond 65％. Both of these elastic modulus values are two orders magnitude higher than the elastic modulus of the stretchable devices. The stiffening of the PDMS membrane observed for elongations >65％is due to the unraveling and stretching of the polymer chains. Therefore, the present stretchable devices demonstrate high potential for wrist band system.

The process of putting on the wrist band with the stretchable device causes ～100％stretching of the whole system (Fig. 4d) , whereas ～50％stretching and slight bending were introduced during wearing the wrist band (Fig. 4e) . However, neither of the two deformation states causes any noticeable dimming of the LED, as shown in Figs. 4d and e.

Figs. 4f and i show digital photographs of a 4 × 4 stretchable MSC array on a honeycomb PDMS substrate subjected to 180° bending and 60° twisting, respectively. The corresponding device configurations obtained from FEA (Figs. 16 a and b for bending and twisting, respectively) are in good agreement with experimental results (Figs. 4f and i, respectively) . For 180° bending, εmax in the Au layer is equal to 0.0003, i.e., an order of magnitude smaller than the yield strain of Au (0.003) . For all bending angles in the 0°–180° range, εmax in the Au layer is significantly below the yield strain (Fig. 17a) . The extremely small strain in the Au layer is intrinsic of the stretchable device. Because the Au layer is sandwiched by two identical PI layers and another two identical PDMS substrates, it is located very close to the neutral plane of bending of the whole structure. Twisting introduces larger strains in the stretchable device compared with bending. For 60° twisting, εmax in the Au layer is equal to 0.0025 (Fig. 4j) . The Au layer exhibits purely elastic deformation in the 0°–60° twisting angle range (Fig. 17b) . In comparison with stretching, bending and twisting generate relatively smaller strains in the Au layer. These results suggest that both 0°–180° bending and 0°–60° twisting are not expected to degrade the electrochemical performance of the honeycomb MSC array. This is confirmed by the CV curves of bended and twisted devices. As shown in Figs. 4h and 4k, the response of the deformed devices are identical to those of undeformed devices, confirming the excellent mechanical stability of the stretchable stretchable devices.

The materials and schemes reported in this invention provide guidance for the design of energy storage devices suitable for powering stretchable electronic devices, such as optoelectronics, displays, sensors, and actuators. The rigid body-like motion of the islands in the honeycomb structure offers much higher stretchability than previous reports and facile integration of various electronics on the same substrate. The deployment of ultra-thin EDLC electrode materials endows the MSCs great flexibility, excellent electrochemical performance, and potential of extending device integration in the thickness direction. Further improvements can be made by optimizing the honeycomb structure to achieve even higher stretchability and surface coverage of effective devices and increased energy density of stretchable devices through surface modification of the SWCNTs. We believe that our work provides stimulus for new efforts toward the integration of energy storage devices, power management circuits, and energy harvesters on honeycomb-like highly stretchable substrates, which can enhance the design of new self-powered wearable electronics system.

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Claims

A micro-supercapacitor array for integrated wearable electronic system with honeycomb structure comprising a plurality of honeycomb cell, said honeycomb cell comprising

a polydimethylsiloxane bottom substrate (1) ,

a bottom PI layer (2) on the honeycomb polydimethylsiloxane bottom substrate (1) ,

a patterned Cr/Au bilayer (3) on the bottom PI layer (2) ,

a top PI layer (4) on the Cr/Au bilayer (3) ,

interdigital electrodes (5) on the top PI layer (4) ,

polymer electrolyte layers (6) coated onto the interdigital electrodes (5) , and

a polydimethylsiloxane top substrate (7) on the PVA-H₃PO₄ gel electrolyte layers.
The micro-supercapacitor array according to claim 1, wherein the bottom PI layer (2) , the patterned Cr/Au bilayer (3) , the top PI layer (4) and the polydimethylsiloxane top substrate (7) have a shape corresponding to the honeycomb polydimethylsiloxane bottom substrate (1) .
The micro-supercapacitor array according to claim 1 or 2, wherein a interdigital pattern of the interdigital electrodes (5) is the same as that of the patterned Cr/Au bilayer (3) .
The micro-supercapacitor array according to any one of claims 1-3, wherein the Cr/Au bilayer is consisted of a Cr underlayer and an Au top layer.
The micro-supercapacitor array according to any one of claims 1-4, wherein the polymer electrolyte layers (6) are polyvinyl alcohol-H₃PO₄ gel electrolyte layers.
The micro-supercapacitor array according to any one of claims 1-5, wherein the polydimethylsiloxane bottom substrate (1) is the same as the polydimethylsiloxane top substrate (7) , and the bottom PI layer (2) is same as the top PI layer (4) .
The micro-supercapacitor array according to any one of claims 1-6, wherein interdigital electrodes (5) are interdigital single-walled carbon nanotube electrodes.
A method for fabricating a micro-supercapacitor array for integrated wearable electronic system with honeycomb structure comprising

providing a honeycomb polydimethylsiloxane bottom substrate,

fabricating one or more micro-supercapacitor,

transferring the one or more micro-supercapacitors onto the honeycomb bottom polydimethylsiloxane substrate and bonding them onto the honeycomb polydimethylsiloxane bottom substrate,

coating a polymer electrolyte onto interdigital electrodes to form polymer electrolyte layers,

assembling a honeycomb polydimethylsiloxane top substrate together with the honeycomb bottom polydimethylsiloxane to encapsulate the micro-supercapacitor array.
The method according to claim 8, wherein the step of fabricating one or more micro-supercapacitor comprising

forming a bottom PI layer on a wafer,

forming a Cr/Au bilayer and patterning the Cr/Au bilayer to form interdigital current collectors and interconnects,

forming a top PI layer on the Cr/Au bilayer,

selectively etching the top PI layer to expose a interdigital electrode area and form honeycomb cell shape,

depositing electrode materials onto the interdigital electrode area exposed and patterning to form interdigital electrodes,

selectively etching the bottom PI layer to match the honeycomb cell shape of the top PI layer.
The method according to claim 8 or 9, wherein the step of transferring the one or more micro-supercapacitors onto the honeycomb bottom polydimethylsiloxane substrate comprising

peeling off the micro-supercapacitor from the wafer by a water-dissolvable tape and transferring it onto the honeycomb polydimethylsiloxane bottom substrate.
The method according to any one of claims 8-10, wherein the step of bonding the one or more micro-supercapacitors transferred onto the honeycomb polydimethylsiloxane bottom substrate and bonding them onto the honeycomb polydimethylsiloxane bottom substrate comprising

spreading a liquid polydimethylsiloxane solution onto the honeycomb polydimethylsiloxane bottom substrate surface and curing to bond the one or more micro-supercapacitors transferred onto the honeycomb polydimethylsiloxane bottom substrate and removing the water-dissolvable tape.
The method according to any one of claims 8-11, wherein a interdigital pattern of the interdigital electrodes is the same as that of the interdigital current collectors and interconnects.
The method according to any one of claims 8-12, wherein the Cr/Au bilayer is consisted of a Cr underlayer and a Au top layer.
The method according to any one of claims 8-13, wherein the polymer electrolyte layers are polyvinyl alcohol-H₃PO₄ gel electrolyte layers.
The method according to any one of claims 8-14, wherein the honeycomb polydimethylsiloxane bottom substrate is the same as the honeycomb polydimethylsiloxane top substrate (7) , and the bottom PI layer (2) is same as the top PI layer (4) .
The method according to any one of claims 8-15, wherein the interdigital electrodes are interdigital single-walled carbon nanotube electrodes.
An integrated wearable electronic system comprising the micro-supercapacitor array according to any one of claims 1-7.