# Healable, Flexible Supercapacitors Based on Shape Memory Polymers
## 2 Materials and Methods
### Investigations on the Heal-Ability of the Electrodes  
The PCL, as the healing reagent, not only played a critical role in the conduction of the composite, but also enhanced the self-healing of the composite. In order to disclose the thermal-triggered self-healing of the composite, CV measurements and galvanostatic charge-discharge measurements were performed. The CV measurements of MCP40S60, MCP20S80 and MCP60S40 at three different states were obtained at a sweep rate of 5 mV s\({}^{-1}\) as shown in Figure 4a, Figures S3a and S4a. The galvanostatic charge-discharge measurements of MCP40S60, MCP20S80, and MCP60S40 at three different states were executed at a current density of 0.2 mA cm\({}^{-2}\) as shown in Figure 4b, Figures S3b and S4b. When a scratch was made by a scalpel on the sample, most of the CNT/MnO\({}_{2}\) were cut and broken along the crack, which led to a damaged conductive layer and decreased capacitance. However, the capacitance could be reversibly restored as the sample was heated because of the self-healing of the composite. The areal capacitances of the MCP40S60 electrode at a current density of 0.2 mA cm\({}^{-2}\) were 5.8 mF cm\({}^{-2}\), 64.575 mF cm\({}^{-2}\), and 69 mF cm\({}^{-2}\), corresponding to the cracked, healed, and pristine state. The same results could be found from MCP20S80 and MCP60S40. This indicates that the composite with PCL possesses the excellent self-healing ability and the capacitance could be recovered even if it was scratched. According to the self-healing efficiency formula, the healing index of the MCPS electrode film is 85%, 93%, and 87%, respectively, as shown in Figure 4c. This implies that there exists an appropriate value of PCL content to obtain the optimal healing effect. The self-healing of the composite is resulted from the healing reagent PCL and the shape memory effect. It is well known  
Figure 3: (a) CV curves of the supercapacitors with different PCL contents at 5 mV s\({}^{-1}\); (b) galvanostatic charge/discharge curves of MCP40S60 supercapacitors collected at different current densities; (c) areal capacitances of MCP40S60 supercapacitors at different current densities; (d) cycle stability of MCP40S60 electrode film at 0.6 mA cm\({}^{-2}\). that PCL serves as a healing reagent in shape memory polymer systems, which experiences melting and recrystallization transitions during the healing process. The PCL can flow freely in the polymer as it heats up and fills in the defects of morphologies, and they are tightly bonded together as they cool. So as the PCL content increases, the more the melted PCL in the polymer matrix moves when it is heated, and the better the self-healing effect. To further verify the above discussion, a series of SEM images of the samples were taken. Figure 5 shows that the evolution of the surface morphologies under different contents of the PCL was monitored by SEM. A scratch with a width of dozens of micrometers was observed in the samples at the cracked states. However, heating at 60 \({}^{\circ}\)C for several minutes enabled the scratch to heal. The healing effect was improved with an increase in the PCL content. For instance, the samples with 40% and 60% of the PCL had better recovery under the thermal stimulations. What is more, heating not only made the PCL melt, but also triggered shape recovery enabling the cracked surface to be closer. The shape memory polymers are sensitive to temperature and high temperature endows them with good elasticity and shape recovery. So the shape memory effect contributed to the shrinking of the scratch and the melted PCL filled up the gap when the composite was healing. However, too much plastic PCL caused a lack of elastomeric force in the polymer matrix to drive the shape recovery as well as the crack closure. As a result, the sample containing 40 wt % of the PCL had optimized healable performance as the healable electrode.