Generally, electroactive polymers often swell or shrink when they are used as electrode materials, and they are likely to degrade during cycling. Therefore, the long-term stability of conductive polymers during cycling is a problem to be solved. Compared with common conductive polymers such as polyaniline, polypyrrole, polycarbazole and their substituted derivatives, polyindole has relatively excellent thermal stability, high redox activity, good chemical stability and slower degradation rate. It is a good choice as electrode materials for energy storage devices such as supercapacitors.

        For a long time, the synthesis methods of polyindole mainly include chemical oxidation synthesis and electrochemical synthesis. The reagents used in these two synthetic methods (such as acetonitrile and chloroform) are expensive and polluting to the environment, which limits the synthesis and application of polyindole to a certain extent. Therefore, how to synthesize polyindole with green and low cost and high efficiency is a barrier to the practical application of polyindole.

       Based on the above problems, Professor Zhu Meifang, associate professor of Yang Shengyuan, Donghua University, used ethanol as the solvent of indole monomer and the mixture of ammonium persulfate (APS) and water as the oxidant to synthesize polyindole in the atmosphere of 0 ~C and inert gas, avoiding the use of traditional toxic solvents, and the synthesized polyindole has good thermal stability. At 500 ~C, the weight loss rate is only 0 ~C. It is 24%. The green synthesis method lays a foundation for the application of polyindole in energy, biomedical and other fields.

(Morphology of polyindole composite nanofibers synthesized by green synthesis)

       On this basis, we use stainless steel gauze as the base of the electrodes and fabricate polyindole/carbon nanotubes composite nanofibers electrodes by electrospinning in one step. The performance of the assembled symmetrical flexible supercapacitors has been greatly improved. With Ag/AgCl as reference electrode, the specific capacitance of polyindole/carbon nanotube composite nanofiber electrode can reach 476 F g-1 when the current density is 1.0 A g-1. In addition, supercapacitors have high power density, energy density and good cycle stability. After 2000 cycles of charge and discharge, the capacitance retention rate reaches 95%.

(One-step preparation of polyindole nanofibre-based electrodes by electrospinning and assembly of flexible supercapacitors)

        In order to further improve the performance of polyindole/carbon nanotube composite nanofibers as electrodes, the research group combines electrospinning and electrostatic spraying to control the micro-morphology of composite materials by adjusting the distance between electrospinning and electrospraying needles, thus realizing the synergistic effect of the two technologies and two materials, rather than simply superimposing the three-dimensional sponge. It is the first time that nano-sized flexible supercapacitors have been used. When the distance between the two needles is 200 px, the nano-sponge is used as the electrode material, which further improves the performance of the assembled supercapacitor, because the three-dimensional framework of the nano-sponge greatly enhances the specific surface area of the electrode material. The specific capacitance of polyindole/carbon nanotube composite nanosponge electrodes can reach 555 F g-1 when the current density is 1.0 A g-1 and Ag/AgCl is used as reference electrode in a three-electrode test system. This process not only provides a simple way to improve the performance of polyindole nanofibers, but also effectively combines zero-dimensional nanoparticles (such as transition metal oxides such as V2O5, Co3O4, carbon-based particles and conductive nanospheres such as copper) with one-dimensional nanofibers (such as polyindole, polypyrrole and other conductive polymer nanofibers) to form high performance nanofibers. The spongy three-dimensional structure of electrode materials provides a controllable way.

(Three-dimensional nano-skeleton structure based on synergistic effect of electrospinning and electrojet and its performance comparison)

       In summary, our research group has realized the systematic research from polymerization-spinning-device, which provides a reference for the application of polyindole nanofibers in energy storage devices such as supercapacitors. This series of research work has recently been published on Materials Letters, Electrohimica Acta, Journal of Materials Chemistry A (selected as the cover of the current period). Mike Tebyetekerwa, a foreign student from the School of Materials and the State Key Laboratory of Fiber Material Modification, Donghua University (from Uganda), is the first author of the paper. Professor Yang Shengyuan and Professor Zhu Meifang are co-authors. 。 This work is supported by Shanghai Morning Light Plan, Shanghai Science and Technology Innovation Action Plan, National Natural Science Foundation of China and Innovation Team of Ministry of Education.

Reference：

1. M. Tebyetekerwa, S. Yang*, M. Zhu* et al., Mater. Lett., 2017, 209, 400-403.

2. M. Tebyetekerwa, S. Yang*, M. Zhu* et al., Electrochim. Acta, 2017, 247, 400-409.

3. M. Tebyetekerwa, S. Yang*, M. Zhu* et al., J. Mater. Chem. A, 2017, DOI: 10.1039/C7TA06242G (Back Cover)