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Published online 13 October 2020 | https://doi.org/10.1007/s40843-020-1472-3

Improving the volumetric specific capacitance of flexible polyaniline electrode: solution casting method and effect of reduced graphene oxide sheets 1

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Jingjing Wang , Weihua Su , Jinmeng Zhang , An’an Zhou , Hua Bai and Tongyi Zhang ABSTRACT High volumetric specific capacitance is essential for flexible supercapacitors. In this paper, a flexible composite electrode with ultrahigh volumetric specific capacitance and good rate performance is designed and prepared. The film electrode is composed of non-porous compact polyaniline as the matrix and a small amount of reduced graphene oxide sheets as the conductive filler. The film is prepared by a newly developed solution casting method, where the casting solution is obtained by self-assembly of polyaniline and graphene oxide in a blended solution. A systematical investigation on the effect of reduced graphene oxide sheets reveals that they serve as both the conductive filler and the diffusion barrier. With the optimized reduced graphene oxide content, a large volumetric −3 −3 specific capacitance of 1354 F cm at 2.4 A cm is achieved, and good rate performance is also obtained because of the good ionic conductivity of polyaniline. This work provides a high-performance electrode for flexible supercapacitors, and the solution casting method is also valuable in fabricating other organic electrode materials. Keywords: conducting polymer, graphene, compact film, conductivity, diffusion barrier

INTRODUCTION Flexible energy storage devices are highly demanded with the rapid development of wearable electronic devices [1– 4]. Flexible supercapacitors are an important group of flexible energy storage devices due to their high rate performance and long cycling life [5–7]. Considering the practical application, volumetric specific capacitance is essential for flexible supercapacitors [8,9]. Generally, the volumetric specific capacitance of an electrode material can be calculated from its gravimetric specific capacitance

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and apparent density. For an electrode material with a certain composition, the main method of promoting its volumetric specific capacitance is to increase the apparent density, for example, by reducing the porosity. However, the interconnected pores inside the electrode serve as a “high way” for electrolyte diffusion and reducing the porosity of the electrode usually leads to a decrease in the rate performance. Therefore, it requires accurate control of the porosity and microstructure of the electrode materials to achieve a high volumetric specific capacitance at large current density. At present, carbon materials, conducting polymers and transition metal oxides are common electrode materials in supercapacitors [10–12]. Among these materials, polyaniline/reduced graphene oxide (PANI/RGO) composites have attracted considerable attention because PANI provides large pseudocapacitance via redox reactions and RGO sheets form conducting path for electron transport [13