Observation of Complex Conjugate Roots and Resonant Behavior in Quasi-Solid Supercapacitors as an Indication of Its Electrochemical instability

Quasi-solid supercapacitors are promising electrochemical devices for energy storage applications due to their high-power density, long life cycle, and environmental benefits. However, their electrochemical performance can change over time as a result of interactions between the electrodes and electrolyte, as well as the fabrication process. In this study, the electrochemical behavior of quasi-solid supercapacitors with activated carbon electrodes immersed in 4 M H₂SO₄ poly(vinyl alcohol) electrolyte for periods of 10 min and 24 h were investigated. Initial measurements show a lack of energy storing properties in newly fabricated devices, which improve with the aging time, as observed in cyclic voltammetry and charge–discharge cycles. Anticlockwise arcs and resonant peaks were observed in Nyquist and Bode plots, respectively, and were modeled by introducing complex conjugate roots and a damping factor ξ in the transfer function of the electronic equivalent circuit. This unfavorable behavior disappeared after 14 days in devices with shorter immersion times. On the other hand, the effects persisted in devices with longer immersion times even after 28 days. The stability of quasi-solid supercapacitors is thus demonstrated to be linked to complex conjugate roots and resonant behavior in impedance spectroscopy.