Fabrication of ultrahigh-performance asymmetrical supercapacitor with pristine Zeolitic Imidazolate Framework-8 and a redox additive electrolyte

Abstract

In the present work, Zeolitic Imidazolate Framework-8 (ZIF-8) was prepared at room temperature using a simple solvothermal method. X-ray diffraction revealed the formation of a cubic phase with I −43 m space group. Scanning Electron Microscopy and High Resolution Transmission Electron Microscopy analysis showed the hexagonal-type morphology of as prepared pure ZIF-8. Brunauer–Emmett–Teller analysis, confirmed the formation of a narrow mesopores having surface area (877 m2/g) and average pore diameter (1.16 nm). The prepared ZIF-8 as electrode material investigated in aqueous (i.e. 2 M LiOH, 6 M KOH, and 6 M NaOH) and redox additive electrolytes RAE1 and RAE2 (i.e. 0.35 M [K4(Fe (CN)6)].3H2O in 6 M NaOH and 0.17 M [K4(Fe (CN)6)].3H2O in 1 M Na2SO4) for electrochemical analysis. The fabricated pristine ZIF-8 shows battery-type redox behavior in the three-electrode system in presence of redox additive electrolyte (RAE) and ultrahigh specific capacity (637.5 C/g) is obtained in RAE1 at a current density of 10 A/g. Furthermore, this electrode could maintain about 75% of its initial capacitance at 20 A/g after 1500 repeated charge-discharge cycles. In addition, an asymmetric supercapacitor (ASSC) device was constructed with graphene nanoplatelets (GNPs) as a negative electrode and pure ZIF-8 as a positive electrode. In the presence of RAE1, the fabricated device offers a significant energy density E of 36.91 Wh/kg at a power density P of 4429.08 W/kg in a wide voltage window range (0.0–1.9 V). The ASSC device retains 87% of its capacitance after 3200 galvanostatic charge-discharge cycles. Moreover, two asymmetric devices are joined in series to light up red and green LEDs for a practical utility.