Synergistic effect between redox additive electrolyte and PANI-rGO nanocomposite electrode for high energy and high power supercapacitor

Supercapacitors (SCs), as high power density energy storage resources, have recently attracted considerable attention. Increasing the energy of supercapacitors is a key parameter to success in achieving far more demanding energy storage applications. There are two approaches to achieve this goal: the first is to widen the operating potential window, and the second is to develop emerging capacitive or pseudocapacitive materials. Polyaniline (PANI) with different morphologies, and hence different supercapacitive performances, has been considered as one of the intriguing active materials due to its high conductivity, intrinsic flexibility, and high redox active specific capacitance. In this work, we report on a simple and convenient method to synthesize PANI nanotubes with rectangular morphology. Then we synthesized PANI/reduced graphene oxide (PANI-rGO) nanocomposite via a convenient electrochemical reduction process. We investigated electrochemical performance of the electrode in the presence and absence of catechol, as a redox-additive molecule incorporated into the electrolyte, by CV, EIS and GCD methods. With incorporation of the catechol in the electrolyte the operating potential window of the electrode increased from 1.0 to 1.2 V, and its capacitance boosted about 5-fold from 429 to 1967 F/g at a current density of 1 A/g. The astonishing supercapacitive performance of the electrode is attributed to; (i) the rectangular nanotube morphology of the PANI, which provides both a better electrical conductivity and a facilitated ion transport pathway, and (ii) the synergetic effect between PANI-rGO and catechol. The electrode showed a superior long cycle life with 81% retention of the initial specific capacitance after 5000 cycles, and demonstrated an excellent rate capability with specific capacitances of 267 and 546 F/g, at a current density of 20 A/g, in the absence and presence of catechol, respectively. Furthermore, we fabricated a symmetric device using the PANI-rGO film on a flexible stainless steel substrate in the catechol-containing electrolyte. The device showed specific capacitance of 409 F/g at 1 A/g, energy density of 81.8 Wh/kg and power density of 7.7 kW/kg. These findings could be beneficial for further development of diverse energy storage devices and open up new prospects for the fabrication of high energy and power supercapacitors.