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Figure 2.11 (a) Optical images of the stretchable cellular CNT film under increasing strain. (b) CV curves under stretching. (c) Photographs of the “watch strap” powered by cellular MSC array.

      Source: Reproduced with permission [40]. © 2016, The Royal Society of Chemistry.

      (d) Optical images of honeycomb 4 × 4 MSC arrays under different stretching state. (e) Capacitance retention versus elongation (the inset figure is the CV curves for 0–150% elongation, respectively). (f) LED powered by a honeycomb MSC device under stretching.

      Source: Reproduced with permission [73]. © 2016, American Chemical Society.

      Another type of 3D stretchable SCs are fabricated by embedding several flexible MSCs devices into a cellular form thus make the MSCs stretchable. This assembled method provides a general integration way, not only in the area of energy storage, as well as energy harvester like solar cell, wireless charging units and wearable electronics such as sensors, detectors. For example, Pu et al. introduced a stretchable cellular PDMS support for flexible 4*4 MSC arrays, as shown in Figure 2.11d–f [73]. The mechanical performance of honeycomb MSC array was displayed by the optical images of the devices with strain ranging from 0% to 275%. The mechanical performance of the devices also simulated by finite element analysis (FEA). From the CV curves, the specific capacitance of single SWCNT based MSC was calculated to 1.86 F cm⁻³ at scan rate of 0.05 V S⁻¹, the corresponding volumetric capacitance of the 4*4 MSC arrays was 0.15 F cm⁻³. Figure 2.11e showed the capacitance retention versus strains, it is very clearly seen that the capacitance of the MSC arrays kept unchanged when applied strains varying from 0 to 150%, which was also suggested by the invariable CV curves (Inset). A commercial LED lighting test driven by a honeycomb MSC devices that attach to a Nike wrist band under stretching was provided in Figure 2.11f, demonstrating the mechanical stability of the cellular MSC arrays at different stretching state. Noticeably, the voltage and current can be easily controlled through the intrinsic configuration of the cellular structure (different series or parallel interconnection modes), this also make the cellular stretchable SCs competitive among various types of stretchable SCs.

      2.2.3.2 Editable SCs

Editable SCs can be directly transferred into arbitrary shapes with stretchability along arbitrary direction. Figure 2.12a–c provided a mature method to prepare arbitrary CNT based electrode materials [74]. Figure 2.12a showed the schematics of out‐of‐plane deformation of 3D stretchable SCs. To obtain the SWCNT with pyramid structure, the pyramid catalyst containing 5 nm thick of Al₂O₃ and 1.2 nm thick of Fe was sputtered on the silicon wafer by photolithography and electron beam evaporation deposition. Then the aligned CNT array was synthesized on the silicon by employing CVD method. Finally, the PANI was deposited on the CNT film via electrochemical deposition to improve the specific capacitance of the electrode materials. Figure 2.12b showed the optical images of the CNT film with increasing strain along the z axis. From the images, we can see the structure and integrity of the CNT film was not damaged during the stretching process. The stretchable SCs were assembled by two pieces of PANI@CNT film and a PVA/H₃PO₄ gel electrolyte in the middle. The specific capacitances of the fabricated stretchable SCs calculated from the charge–discharge curves reached an areal capacitance of 61.4 mF cm⁻², which was maintained at 93.3% after 3000 strain‐release cycles along the y and z axes. Figure 2.12c displayed the gavanostatic charge–discharge profiles of the fabricated stretchable SCs with increasing strains from 0% to 16% along the x and y axis. It can be seen that there is no degradation in both charge–discharge time and operation voltage window, showing an outstanding stability under stretching.