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

Abstract

Halogen (iodide, I À ) 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 ZnCl 2 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 À /I 3 À , 2I À / I 2 , 2I 3 À /3I 2 and I 2 /IO 3 À pairs produce redox peaks in CV and a large Faradaic plateau in charge-discharge curves. Similarly, I À 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 À1 and 9.5 W h kg À1 in aqueous solution of 1 M H 2 SO 4 . Interestingly, nearly two-fold improved specific capacitance and energy density of 912 F g À1 and 19.04 W h kg À1 were achieved when 0.08 M KI was added in 1 M H 2 SO 4 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 H 2 SO 4(572 F g À1 , 11.6 W h kg À1 ) and 0.08 M KI added to 1 M Na 2 SO 4 (604 F g À1 , 12.3 W h kg À1 ) as electrolytes.

Synergistic interaction between redox-active electrolyte and binder-free functionalized carbon for ultrahigh supercapacitor performance

“…S11b) can be attributed to reversible ion adsorption in porous carbon 46 . This phenomenon in the high voltage range and its explanation have been reported for other redox electrolytes 27,44,47 . The two-electrode cell capacitance, calculated from the galvanostatic discharge curve in total potential window of 1.35 V, was 294 F g À 1 , which is very close to the capacitance calculated by equation for two capacitors in series (1/C ¼ 1/C þ þ 1/C À ) 48 .…”

“…3a). A quasirectangular capacitive curve with pseudocapacitive broad peaks 27 at B0.35 V was observed because of the presence of additional functional groups. This capacitance is B2.5 times greater than the previously reported value of 185 F g À 1 for the same material in which a binder was used to attach the PCMSs to a glassy carbon substrate 24 .…”

“…Furthermore, the EIS curve at 0.2 V (Fig. 3b) displays a short tail indicating fast diffusion of the electrolyte ions into the pores 16 . Moreover, at 0.2 V, the greater deviation from 90°than the other two curves at 0.01 and 0.32 V is attributed to the high diffusion rate 41,42 and the strong scan-rate dependence 43 .…”

“…The polarized carbon-oxygen surface groups attract water molecules, increase the wettability of the PCMSs and fully utilize the surface of the PCMS 32 . These groups also provide active sites for reactions 24 and contribute to pseudocapacitance [33][34][35] , which leads to the observed quasi-rectangular cyclic voltammetric curves 27 (top inset of Fig. 3a).…”

“…Roldan et al 14 reported an increase in capacitance from 320 to 901 F g À 1 by adding hydroquinone to H 2 SO 4 electrolytes. Similarly, Senthilkumar et al 15 used hydroquinone addition into polyvinyl-alcohol H 2 SO 4 gel electrolyte to increase the capacitance from 425 to 941 F g À 1 , and they reported potassium iodide addition into aqueous H 2 SO 4 electrolyte 16 to increase the capacitance from 472 to 912 F g À 1 . These increases in capacitances were attributed to rapid faradaic reactions at the electrode/electrolyte interface that occurred by introducing mediators (hydroquinone/quinine, iodide/iodine pairs) into electrolytes, which augmented the pseudocapacitive contribution to the system.…”