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electrode materials, the fabricated SCs exhibited a specific capacitance of 60.435 mF cm−2 at the scan rate of 10 mV s−1 and a high energy density of 18.88 μW h cm−2 with an extended potential window of 1.5 V. The specific areal capacitance curves in Figure 2.5e showed a slightly decrease when the SC was stretched to 20%. Despite the limited stretchability of the coaxial SCs, the electrochemical performance was much improved, paving a great support to the real application of the stretchable SCs.

      2.2.2 Planar Stretchable SCs

      Stretchable 2D planar SCs with excellent properties of small size, low weight, excellent lifespan, high security and easy integration have become a preferred choice as energy storage to power the wearable electronics [37, 61–63]. There are two main categories including layer by layer sandwich structure and micro supercapacitors (MSCs). The fabrication method of the 2D planar stretchable SCs is similar to 1D fiber shaped SCs. One is via embedding rigid independent devices to the stretchable substrate or establishing serpentine interconnects between rigid devices to realize stretchability. Another one is replacing the rigid unit by stretchable component. In this section, we will introduce the typical fabrication method reported during the last few years.

      2.2.2.1 Fabrication of the Stretchable Planar SCs with Sandwich Structure

      Most of the deformable substrate used in the field of stretchable SCs is PDMS. In 2014, Xie et al. reported a flat Ni foam based stretchable all‐solid‐state SC with wavy shaped polyaniline (PANI)/graphene electrode [65]. Figure 2.6d showed the schematics of the fabrication process for fabricating the PANI/graphene electrodes based stretchable SCs. First, a flat Ni foam with a thickness of 200 mm was manually made into a wavy shape, next, the porous graphene was synthesized on the buckled Ni foam via atmospheric pressure chemical vapor deposition (CVD). Then the graphene coated Ni foam was put in a solution of 3 M HCl to etch nickel foam to obtain wavy‐shaped graphene film. In order to improve the electrochemical performance of the SCs, the PANI was deposited on the wavy shaped graphene film. Finally, two PANI covered graphene films with PVA/H3PO4 wall were encapsulated into Elastic substrate (Ecoflex). Figure 2.6e depicted the CV curves of the stretchable SCs at different tensile 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

      Schematic illustrations of (a) the 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. Schematic illustrations of (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.

Schematic illustrations of (a) the 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.

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