Dual redox electrolytes for improving the performance of asymmetric supercapacitors constructed from heteroatom-doped green carbon spheres

Abstract

Although supercapacitors benefit from both long-cycle durability and extraordinary power density, unmet energy demands still remain the key challenge on the path of their practical applications. Boosting energy densities without sacrificing the remarkable power densities and long cycle life is of special significance in developing high-performance energy storage systems. Herein, the synergy between reversible redox mediator addition and heteroatom-doping of carbon-based electrode active materials represents an enhanced electrochemical performance of the fabricated energy storage devices. Green carbon spheres (CSs) were synthesized through the eco-benign hydrothermal carbonization (HTC) approach. CSs were then chemically decorated by metal nanoparticles. A carbothermic reduction, in which metallic nano particles nanoparticles played a role as a sacrificial agent, was conducted to prepare highly porous CSs (PCSs). The carbothermic reduction process was followed by heteroatom doping of the PCSs. Nitrogen- and boron-doped PCSs were then utilized as positive and negative electrode active materials, respectively. A combination of potassium iodide (KI) and sodium anthraquinone-2-sulfonic acid (AQSA) was employed as dual-redox mediators to boost the electrochemical performance of the fabricated N-PCS//B-PCS ASC devices. The assembled ASCs coupled with AQSA/KI dual redox additives rendered not only remarkable energy density (86.3 W h kg−1) at a reasonable power density of 340 W kg−1 but also exhibited outstanding cyclic stability (only 22 % decay of initial capacitance observed after 5000 charge/discharge cycle). The encouraging features parallel to the remarkable capacitive performance of the fabricated ASCs make these supercapacitors immensely promising for practical applications.