Abstract
When mediators (redox molecules) are mixed into an ion conducting polymers, the material will have a high ionic conductivity and electronic conductivity potentially providing a high storage capacity and charge/discharge rate. On the basis of our previous work of developing I
2
/NaI and K
3
Fe(CN)
6
/K
4
Fe(CN)
6
based supercapacitors, we studied the ionic conductivity, electronic conductivity, and capacity of the electrode materials as a function of concentration of mediators and temperature. A modified percolation model was proposed and used to describe the observations. In another line, we fabricated prototypes using commercially available materials.
The ionic and electronic conductivities were evaluated using a series of AC/DC measurements. The results indicate that for the electronic conductivity, there is a critical mediator concentration at which the conductivity steps up two orders of magnitude. For the ionic conductivity, the increase with mediator concentration is smooth. When the temperature decreases from 20°C to -20°C, the ionic conductivity decreases ~8 folds but is still greater than 10
-3
S/cm. This demonstrates that the presence of mediators in the material helps maintain a high ionic conductivity at -20°C. Cyclic voltammetry and charge/discharge test results are correlating and consistent with the conductivity measurement results. A percolation model was created with consideration of motion of the mediators at a given temperature. The simulation results agree well with the experimental data. The theoretical analysis indicates that charge exchange between mediator molecules promotes ionic conductivity via causing instant imbalance of the local charge distributions. Finally, supercapacitor prototypes were fabricated using the electrode materials with PVDF/LTFS and a mediator. The specific energy was 25 Wh/kg at 0.2 kW/kg or 10 Wh/kg at 25 kW/kg after cycling 2000 times.