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optical images of LED powered by MSC array. (d) normalized capacitance (C/C0) measured before and after deformation, respectively."/>
Figure 2.10 (a) Schematics of the fabrication procedures of the stretchable MSC arrays. (b) Optical images of the stretchable MSC array under different types of deformations. (c) Real‐time optical images of LED powered by MSC array. (d) normalized capacitance (C/C0) measured before and after deformation, respectively.
Source: Reproduced with permission [72]. © 2017, Wiley‐VCH.
2.2.3 3D Stretchable SCs
The stretchability of 3D stretchable SCs are typically achieved by the configuration design, such as 3D cellular and pyramid structure that omits the utilization of elastic substrate, which is quite different with the strategy toward 1D fiber SCs and 2D planar SCs. Kirigami or patterning‐based editable technique is always employed to assemble the 3D stretchable SCs. Recently, many efforts have been devoted to design the 3D stretchable SCs, which can be divided into two types: cellular structure and editable SCs. The first one is to realize stretchability through cellular electrodes or embedding MSC arrays into a cellular structured elastic substrate. The later one represents the SCs devices arbitrary shape, which can be adjusted according to the demand of wearable electronics.
2.2.3.1 Cellular Structure
In nature, many animals like the North American elk or bird have a cellular bone that provides large deformations under attacking or during flying. Inspired by the biological materials, cellular structure is introduced to stretchable SCs because it can resist a broad spectrum of deformations including bending and stretching. A typical work has been reported by Peng's group, as shown in Figure 2.11a–c [40]. Figure 2.11a displayed the optical images of the stretchable cellular CNT film under increasing strain. The cellular CNT film was synthesized as follows: first, a paper mask with exposed cellular pattern was employed to coat a cellular catalyst in the silica wafer, then the CNT film was prepared by CVD on patterned catalyst covered silica wafer, after pressing and peeling off, the cellular CNT film was finally obtained, which could be stretched by 150%. Afterwards, a sandwiched stretchable SCs with two cellular CNT films separated by PVA/H3PO4 gel electrolyte was assembled. The electrochemical performance was also carried out in Figure 2.11b. The fabricated stretchable SCs showed a specific capacitance of 42.4 F g−1 with the electrode thickness of 38.3 μm. The identical CV curves for 0–140% strain demonstrated the excellent stability of the cellular stretchable SCs. The high specific capacitances can be reserved 98.3% after stretching by 140% for 3000 cycles. Figure 2.11c depicted a watch strap powered by the stretchable SCs that accommodate the deformation of the arm size, suggesting a novel class of possible designs for 3D stretchable SCs.
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.