Trade-off between capacitance and cycling at elevated temperatures in redox additive aqueous electrolyte based high performance asymmetric supercapacitors

High surface area mesoporous MWCNTs/ZrO₂ and MWCNTs/WO₃ composites are used to fabricate asymmetric supercapacitors (ASCs) in 1 M Li₂SO₄ aqueous electrolyte with and without addition of KI. The addition of redox electrolyte additive leads to simultaneous enhancement in specific energy and power. These ASCs, when operated upto temperatures ≤60 °C, show reasonably stable specific capacitance and cycling stability. It is observed that the temperature predominantly affects the capacitance fade during galvanostatic charging-discharging whereas specific capacitance actually increases slightly at elevated temperatures. The Gouy-Chapman theory fails to explain such improvements in the specific capacitance. The improvement is strongly governed by the activation energy associated with the diffusion of ionic species and their chemical potential. Further, the capacitance fade at higher temperatures can be attributed to: (a) the total "time spent" at a given temperature during cycling and (b) reduced kinetic barrier for iodine/iodide redox pairs at the positive electrode/electrolyte interface, which facilitates H₂ generation at the negative electrode and induce charge-imbalanced state.