*3.8. Self-Healable and Flexible Performance of the Supercapacitor*

The self-healable and flexible solid-state supercapacitor was fabricated based on TOCNF-CNT@PANI/PVA-2 hydrogel electrode and TOCNF/PVA hydrogel electrolyte in a sandwich structure. The detailed fabrication process was described in the experimental part. Due to the inherent flexibility and self-healing ability of the PVA hydrogel, the interfaces between hydrogel electrode and electrolyte could be completely combined, thereby manufacturing an integrated solid supercapacitor device. The assembled supercapacitor could withstand cutting, bending, and another mechanical damage, but the electrochemical performance was not obviously affected. As demonstrated above, the dynamically reversible PVA-borate cross-linking network provided the inherent, repeatable, and effective self-healable ability for the composite hydrogel (Figure 8a).

Figure 8b,c shows the electrochemical performances of the self-healing supercapacitor after multiple cutting/healing cycles. The CV and G-CD curves of supercapacitor had no obvious deformation after multiple cutting/healing cycles, indicating that the capacitance had not been significantly reduced. As calculated by the G-CD curves at 0.6 A g−<sup>1</sup> current density, the initial specific capacitance of the supercapacitor was 138 F g−1, and the specific capacitance after 1, 5, 10 cutting/healing cycles was 137.3, 134.7, and 124.1 F g<sup>−</sup>1, respectively. The corresponding capacitance retention was 99.5%, 97.6%, and 90.0%, respectively. For the as-prepared supercapacitor device, the self-healing capability was more outstanding than those reports. For example, a CNT film was spread on a self-healable substrate to manufacture electrode, combining the self-healable electrodes and polyvinylpyrrolidone-sulfuric acid (PVP-H2SO4) gel electrolyte to fabricate a supercapacitor. Its capacitance retention reached 85.7% after the 5th cutting/healing cycles [53]. By coating PANI and CNT nanomaterials on the surface of polymer fibers with self-healing ability to develop a novel filamentous self-healable supercapacitor, its capacitance retention was 92% after one cutting/healing cycle [54]. In Figure 8c, the voltage drop

in the G-CD curves was due to the electron transfer resistance of the solid-state hydrogel-based electrolyte, which resulted in the specific capacitance of the supercapacitor being smaller than that of the electrode [3].

**Figure 8.** (**a**) Schematic and structural illustration of the self-healing process due to the dynamic borate bond of the supercapacitor. (**b**) CV curves at initial and after the 1st, 5th, and 10th self-healing at a scan rate of 40 mV s<sup>−</sup>1. (**c**) G-CD curves at initial and after the 1st, 5th, and 10th self-healing at a current density of 0.6 A g<sup>−</sup>1. (**d**) Self-healing efficiency derived from both G-CD and CV curves at self-healing cycles from 1st to 10th. (**e**) Capacitance retention over 1000 bending and twisting cycles at an angle of 180◦.

The in-situ measurement method was used to evaluate the electrochemical performance of the assembled supercapacitor device under the bending or twisting state. Figure 8e shows the variation of capacitance according to the number of cycles, which were calculated from their G-CD curves at a constant current density of1Ag−<sup>1</sup> after each bending cycle. A bending cycle started from flat-state, passed through a 180◦ bending-state, and then returned to flat-state. One twisting cycle was similar. In Figure 8e, the capacitance retention of the supercapacitor device was 85.0% and 82.3% after 1000 bending cycles and twisting cycles, respectively. The performance was comparable supercapacitor to be tested in a flat state. Such a flexible solid-state supercapacitor with PANI hydrogel electrode possessed capacitance retention of 86% after 1000 consecutive charge-discharge cycles [55]. It even was superior to the polyaniline-sodium alginate (PANI-SA) hydrogel supercapacitor reported previously (typically 71% retention for over 1000 cycles) [51]. The improved cycling stability could be due to the quasi-solid hydrogel, further protecting the active PANI and avoiding the delamination of the CNT fibers as conductive pathways. The delamination from continuous expansion and shrinkage of the PANI molecular chain during the charge-discharge cycles could cause performance degradation. The superior capacitance retention under bending/twisting cycles also suggested that the contact between different layers of the supercapacitor device was excellent, which could benefit from the inherent self-healing property of PVA-based hydrogels [43]. The superior flexibility and self-healing solid-state supercapacitor had promising potential applications in a flexible electronic device.
