Electric double layer capacitor and its improved specific capacitance using redox additive electrolyte

Halogen (iodide, I−) added aqueous electrolyte facilitates the capacitive behaviour of biomass derived activated carbon based electric double layer capacitors. To produce economically viable electrodes in large scale for supercapacitors (SCs), the activated carbons (ACs) prepared from Eichhornia crassipes (common water hyacinth) by ZnCl2 activation. The prepared ACs were characterized by XRD, Raman, FT-IR and surface area, pore size and pore volume analysis. The electrochemical properties of the SCs were studied using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (EIS) and cycling stability. The 3I−/I3−, 2I−/ I2, 2I3−/3I2 and I2/IO3− pairs produce redox peaks in CV and a large Faradaic plateau in charge–discharge curves. Similarly, I− ions improves the good ionic conductivity (lower charge transfer resistance) at the electrode/electrolyte interface which was identified through EIS studies. The calculated specific capacitance and energy density was 472 F g−1 and 9.5 W h kg−1 in aqueous solution of 1 M H2SO4. Interestingly, nearly two-fold improved specific capacitance and energy density of 912 F g−1 and 19.04 W h kg−1 were achieved when 0.08 M KI was added in 1 M H2SO4 electrolyte with excellent cycle stability over 4000 cycles. Subsequently, this improved specific capacitance and energy density was compared with 0.08 M KBr added to 1 M H2SO4 (572 F g−1, 11.6 W h kg−1) and 0.08 M KI added to 1 M Na2SO4 (604 F g−1, 12.3 W h kg−1) as electrolytes.