A few days ago, Ma Yanwei's research group from the Institute of Electrical Engineering of the Chinese Academy of Sciences and Wei Zhixiang's research group from the National Nanoscience Center jointly prepared a new solid-state flexible supercapacitor with high areal specific capacity, excellent charge-discharge cycle performance and flexibility. Relevant research results were published in the international material science journal Advanced Materials (Adv. Mater, 2015, doi:10.1002/ adma.201503543), and a national invention patent has been applied for.  

Most of the current solid-state flexible supercapacitors are multilayer film stacks formed by stacking two self-supporting flexible electrode films and an intermediate gel-state electrolyte film. The high viscosity and diffusion kinetics of the gel limit the diffusibility of electrolyte ions inside the electrode, so it is difficult to obtain a high areal specific capacitance. In addition, the multi-layer stacked device is prone to mechanical peeling damage between layers when it is continuously bent, which increases the internal resistance of the device and even degrades the overall capacitance performance. Therefore, how to fabricate high-performance flexible supercapacitors remains challenging.

To solve these problems, the research team designed and integrated the key components of flexible supercapacitors, the electrode-electrolyte-electrode layer, on a single flexible hydrogel film to form an all-in-one new device structure, as shown in the figure. 1 shown. Compared with the traditional device structure of the current multilayer film stack, this structure is beneficial to the diffusion of gel electrolyte ions inside the thicker electrode layer and improves the mechanical bending resistance.

In the specific preparation process, the team used the chemical crosslinking-casting method to prepare a self-supporting chemical hydrogel film, which has excellent ionic conductivity (0.082S cm-1) and mechanical tensile properties (can be stretched to 300%), as shown in Fig. 2a–c. Then, the conductive polymer is deposited on the upper and lower surfaces and the interior of the near surface of the hydrogel through chemical in-situ polymerization to form a composite hydrogel film. As shown in Figure 2d-e, the film has an arrangement of conductive polymer layer-hydrogel layer-conductive polymer layer inside, so an all-in-one integrated solid-state flexible supercapacitor can be formed. The new structural solid-state flexible supercapacitor has a very outstanding area specific capacity (488 mF cm-2) and excellent charge-discharge cycle stability (capacity does not decay after tens of thousands of cycles), as shown in Figure 3. In addition, the capacitance performance is not degraded after continuous bending for thousands of times. The excellent performance is expected to enable this new type of flexible supercapacitor to be used in power storage devices for next-generation wearable electronic devices.

In the related previous research work of the research group, the researchers explored the application of gel-state electrode materials in solid-state flexible supercapacitors. The researchers prepared a conductive polyaniline-based hydrogel electrode material. Compared with solid-state electrode materials, the gel-state electrode materials exhibit excellent capacitance performance and charge-discharge rate performance due to more sufficient ion accessibility. Relevant research results were published in the Royal Society of Chemistry's "Materials Chemistry: Series A" (J. Mater. Chem. A, 2014, 2, 19726).

The above research has received strong support from the National Natural Science Foundation of China and the Innovative Talents Introduction Program of the Institute of Electrical Engineering.