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stretchable SC. (b) SEM image of the buckled CNT film. (c) Photos of buckled CNT film under various deformations. (d) Normalized electrical resistance of the stretchable SC under stretching−releasing cycles at a strain of 200%. (e) CV curves of the fabricated SC at various stretching states.

      Source: Reproduced with permission [66]. © 2016, American Chemical Society.

      2.2.2.3 Stretchable On‐Chip Micro Supercapacitors (MSCs)

In the past decades, great progress has been achieved in the development of on chip stretchable 2D devices due to their advantages of ultra‐thin thickness, low weight, easy handing in appearance, feasibility of integration into miniaturized different kinds of wearable electronics like sensors, detector, nanorobot, etc. on the same elastic substrate and excellent mechanical performances under various deformation [16]. To overcome the low operation voltage windows of single on‐chip MSC, MSC arrays with series or parallel connection often directly designed [38,67–71]. To date, several papers refer to stretchable on‐chip MSC have been published. For example, Ha's group fabricated a multi‐walled carbon nanotubes (MWCNTs) @ Mn₃O₄ electrode based stretchable and patchable MSC array by dry‐transformation method [2]. Figure 2.8a showed the fabrication process of the planar MSC array on a stretchable substrate. At first, typical photolithography technology was employed to fabricate single MSC devices, which were then embedded into Ecoflex substrate with microchannels to build interconnections between MSC arrays. The cross‐section scheme of the single MSC was displayed in Figure 2.8b. It can be observed that single MSC itself had no stretch ability, but all‐solid‐state organic solvent‐based gel electrolyte made it easy to be transformed into Ecoflex substrate. Figure 2.8c depicted the digital photography of the stretchable and patchable MSC arrays. No observable change in CV curves under repeated bending, twisting, and stretching state presented in Figure 2.8d, suggested the stable electrochemical performance of the MSC array. Importantly, even in water, a micro‐light‐emitting diode (μ‐LED) can be easily lit by the fabricated MSC arrays. Moreover, the long‐term stability with 85% of capacitance retention for two weeks in ambient air without encapsulation opens up the possibility of promising commercial potential.

Serpentine interconnects have been deemed to an effective method to prepare stretchable MSCs. A related work was done by Ha and co‐workers in 2013, as shown in Figure 2.9 [39]. To achieve the stable electrochemical performance over deformation, the stretchable MSC arrays were fabricated as follows (Figure 2.9a): 400 nm thick polyimide (PI) film was spread on the SiO₂/Si substrate via spin‐coating, then a Ti/Au (5/50 nm) film was sputtered on the PI film to form current collector with serpentine interconnections. The second PI film was then spread to form a neutral mechanical plane for Ti/Au electrode. After that, SWCNT as active materials was spread on the metal current collector. The MSC precursor was the transfer from rigid SiO₂/Si substrate to elastic PDMS substrate though tape. After drop‐casting of ion‐gel electrolyte and encapsulation, the stretchable MSC arrays were finally fabricated.

      The obtained SWCNT electrodes based MSC array exhibited a capacitance of 100 μ F at the scan rate of 0.5 V s⁻¹, power density of 70.5 kW kg⁻¹ at energy density of 11.5 W h kg⁻¹. It also showed a stable electrochemical performance at the tensile strain of 30% due to the smooth deformation of serpentine interconnections. The MSC array with voltage window of 3 V can easily light a μ‐LED, as shown in Figure 2.9b. Even under bending and stretching, no noticeable degradation can be observed, demonstrating its good mechanical stability and wide application in wearable and portable electronics.

Design of the wave shaped electrode is also an efficient way to obtain a stretchable MSC arrays. Our group proposed a 3D print assisted technology to assembled MWCNT/PANI electrode based stretchable MSC array, as illustrated in Figure 2.10 [72]. Figure 2.10a displayed the fabrication procedures. In details, we used 3D printer to prepare a photosensitive resin‐based mold with a convex wavy electrode. Next, this convex wavy channel was transferred to PDMS film. Then 50 nm of Au film PDMS was sputtered on the surface of PDMS as the current collector. MWCNT/PANI electrode and polyvinylidene fluoride (PVDF) was mixed together to form an adhesive slurry, which was then injected to the concave wavy electrode. After coating the PMMA‐PC‐LiClO₄ gel electrolytes, the MSC array was prepared. The initial specific area capacitance calculated from CV curves was 44.13 mF cm⁻² for the single MSC without applied strain, which showed negligible difference under various strain ranging from 0% to 40%. The maximum energy density was 0.004 mWh cm⁻² at power density of 0.07 mW cm⁻². Moreover, the fabricated MSC device showed an excellent cycling stability with capacitance retention of 87% even after 20 000 charge‐discharge cycles. Figure 2.10b showed the optical images of the stretchable MSC array and rolled up devices. The MSC arrays were connected as 4S (series) + 2P (parallel), so it can be charged to 3.2 V. During the test, we found after charging, the MSC arrays can maintain at 1.8 V for more than 20 000 seconds, which could provide a stable power to light a LED (Figure 2.10c). Even after 20 minutes, the LED could work normally. Significantly, the brightness of the LED didn't change after three days under air ambient conditions because of the air‐stable gel electrolyte and encapsulation of the PDMS. Figure 2.10d depicted the normalized capacitance (C/C₀) of the device under different deformation, respectively. The retention of the capacitance was kept nearly 100% under stretching, twisting, crimping as well as winding, confirming the electrochemical stability of the MSC arrays, which is a promising candidate for powering other stretchable electronics.

Figure 2.8 (a) Schematics of the fabrication procedures for a MWNT/Mn₃O₄ based planar stretchable MSC. (b) The encapsulation of the MSC. (c) The stretchable MSCs array with embedded liquid metal interconnections. (d) CV curves of the MSC array measured under different types of deformations.

      Source: Reproduced with permission [2]. © 2015, The Royal Society of Chemistry.

Figure 2.9 (a) Schematics of fabricating a stretchable MSC array on a PDMS substrate with serpentine interconnects. (b) Photographs of the μ‐LEDs lighting test under bent and 30% stretched state.

      Source: Reproduced with permission [39]. © 2013, American Chemical Society.

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