Abstract
One of the approaches to improve the specific energy of supercapacitors is to mix soluble or semi-soluble redox materials or “mediators” with electrolyte in electrode materials. The redox materials can undergo reversible electrochemical reactions at the interfaces between electronic conductor (carbon materials) and electrolyte phases thereby storing charges in the form of reduced and oxidized mediators. Often this kind of SCs are referred to “redox electrolyte SCs” or “mediator SCs”. A three-dimension model is established to simulate the transport process and electrochemical process of mediator-enhanced solid-state supercapacitors. The roles of mediators are studied in wide ranges of supercapacitor parameters, including concentration, conductivity, and diffusivity. The simulation results are validated using experimental data previously published. Specifically, the present study is aimed to analyze the roles of mediators in the so-called battery-like versus capacitor-like behaviors, the effect of release of free ions by mediators during charging, and the effect of mediator shuttling. The simulation results show that with mediators concentration greater than 0.2 mol L
-1
and diffusivity greater than 3 × 10
-12
m
2
s
-1
in electrodes, the charge/discharge behaviours are generally battery-like. However, at adiffusivity below 3 × 10
-12
m
2
s
-1
, the charge/discharge behaviours are generally capacitor-like. The simulation results clearly reveal that if free ions are given off by mediators during the charging process, the specific capacity of the supercapacitors can be extended. The simulation results also indicate that if the diffusivity of mediators in the separator membrane is below 10
-12
m
2
s
-1
, the self-discharge of the supercapacitor due to shuttling of the redox species between the electrodes is negligible. Thus, an operational mediator supercapacitor relies on a selective separator membrane and a high diffusivity in the composite electrodes.