Polyaniline as a charge storage material in an aqueous aluminum-based electrolyte: Can aluminum ions play the role of protons?

Abstract

The high redox activity of polyaniline emeraldine salt (PANI-ES) was revealed in the aqueous solution of Al-salt, which makes this polymer attractive as an electrode material for aqueous aluminum electrochemical storage devices. Its redox behavior in Al(NO3)3, Al(NO3)3+HCl, AlCl3 and HCl was investigated by Cyclic Voltammetry and Chronopotentiometry. While the proton exchange determines PANI's redox behavior in strong acidic solutions, anion doping/dedoping is a more dominant process in less acidic Al-salt solutions. The formation/dissolution of solid-state nitrate complexes is proposed to happen during PANI's redox switching in Al(NO3)3, which causes disappearance and reappearance of grain boundaries, as revealed by AFM. Combined experimental and DFT approaches identify Al-salt as a secondary dopant of protonated PANI-ES (by Lewis acid-base complexation), which causes polaron→bipolaron conversion. The change in the redox mechanism of PANI-ES, caused by the substitution of HCl with Al(NO3)3, did not attenuate its charge storage ability. Moreover, PANI-ES delivers a higher capacitance in Al(NO3)3, amounting to 269 F g−1 at 10 A g−1. Furthermore, the use of Al(NO3)3 results in attenuated electrochemical PANI overoxidation, when compared to HCl, thus providing better capacitance retention upon potentiodynamic cycling. The results open novel perspective of using PANI-based materials for more suitable energy storage devices.