Investigating electrochemical impedance and performance variation in nanostructured Mn₃O₄/activated carbon/reduced graphene oxide asymmetric supercapacitors with different electrolytes

The synthesis of Mn₃O₄ nanoparticles and activated carbon/reduced graphene oxide (AC/rGO) nanocomposite involved surfactant-assisted chemical precipitation and sonochemical methods, respectively, to produce high-quality electrode materials. The morphology of the spherical Mn₃O₄ nanoparticles and the wrinkled sheet-like structure of rGO were found to enhance the electrochemical performance and stability of the electrodes significantly. Electrochemical investigations were conducted using two electrolytes: 2 M KOH and LiNO₃. In half-cell analyses, Mn₃O₄ and AC/rGO exhibited specific capacitances of 138 F g⁻¹ and 609 F g⁻¹, respectively, with 2 M KOH, and 104 F g⁻¹ and 49.8 F g⁻¹ with 2 M LiNO₃ electrolyte, at 1 A g⁻¹. The observed differences in performance were discussed regarding ionic radius, ionic conductivity, and diffusional coefficient of ions. Furthermore, asymmetric supercapacitor pouch cell devices (Mn₃O₄//AC/rGO, MAGASC) were fabricated employing both electrolytes, demonstrating enhanced electrochemical performance. The MAGASC pouch cells exhibited specific capacitances of 273 F g⁻¹ and 130 F g⁻¹ at. 100 mV s⁻¹ with KOH and LiNO₃ electrolytes, respectively. Energy and power density were measured to be 35.2 Wh kg⁻¹ and 1.4 kW kg⁻¹ for KOH electrolyte, and 10.9 Wh kg⁻¹ and 1.6 kW kg⁻¹ for LiNO₃ electrolyte at 0.6 A g⁻¹. Electrochemical impedance spectroscopy (EIS) analysis revealed a lower equivalent series and charge transfer resistance for MAGASC with KOH electrolyte than for ASC pouch cells with LiNO₃ electrolyte. Complex capacitance and relaxation time constant of the MAGASC were determined using EIS data to analyze frequency behavior. Moreover, the ASC pouch cell demonstrated excellent cyclic stability, retaining 90% of its initial capacitance over 5000 cycles in both electrolytes. These findings underscore the superior energy storage capacity of MAGASC with KOH electrolyte and its broader operating potential with LiNO₃ electrolyte.