strains. The initial specific capacitances calculated from CV curves were 261 F g−1. It can be seen that no obvious change appeared when the SC was stretched to 30%. Moreover, the stretching cycle tests revealed that the SC maintained high mechanical strength and stability over 100 cycles.
2.2.2.2 Omnidirectionally Stretchable Planar SCs
From the aforementioned stretchable actions, it can be concluded that stretchability can be realized without affecting on the electrochemical performances of the SCs. There are even some operations with enhanced electrochemical performance when the SCs devices was stretched due to the more contact between electrode and gel electrolyte under stretching. Unfortunately, these actions can only be stretched along one direction, hence, if it is possible to make SCs isotropic stretchable, the electrochemical performance, such as specific capacitance, cycle stability etc. also could be improved in a certain degree.
Figure 2.6 (a) Fabrication process of the stretchable SCs by buckling electrode materials on an elastomeric PDMS substrate. (b) SEM image of a buckled CNT macro film. (c) CV profiles of the stretchable SCs measured at 30% strain.
Source: Reproduced with permission [64]. © 2009, Wiley‐VCH.
(d) Schematics of the stretchable SCs fabrication. (e) CV curves of the stretchable SCs at different tensile strains.
Source: Reproduced with permission [65]. © 2014, The Royal Society of Chemistry.
In 2016, Yu et al. designed a novel isotropic wavy shaped CNT film electrode based omnidirectionally stretchable SCs, as shown in Figure 2.7 [66]. Figure 2.7a showed the fabrication process. In detail, the PDMS elastic film was uniformly pre‐strained in all direction, and then CNT film was transformed to the PDMS substrate, finally, after relaxation, the omnidirectionally stretchable SCs were assembled. The SEM images of side view of buckled CNT film on silicon rubber substrate were presented in Figure 2.7b. The buckled CNT film and tight connection will benefit the stretchability. Figure 2.7c demonstrated the excellent isotropic stretchability of the buckled CNT film. The resistance variation after 10 000 uniaxially stretching−releasing cycles was also measured (Figure 2.7d), which showed a little increase (< 3%) at tensile strains of 200%. Figure 2.7e displayed the CV curves of the fabricated SCs with various deformable states. No significant change was observed in the CVs with 200% applied omnidirectional strains. Moreover, the specific area capacitance was improved from 1160.43 to 1230.61 mF cm−2 during uniaxial, biaxial, and omnidirectional elongations, which showed wide potential application in stretchable electronics.
Figure 2.7 (a) Schematic illustration of steps for fabricating omnidirectionally 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,
