Effect of alkali and halide ion doping on the energy storage characteristics of ionic liquid based supercapacitors

Highlights: • Electrochemical characteristics of hybrid-supercapacitors based on carbon electrodes. • Small amount of halide and alkali ion doping for microporous carbon electrodes. • Increased capacitance due to redox activity and specific adsorption of halide ions. • Reproducible capacitance values obtained from three measurement techniques. -- Abstract: The electrochemical properties of supercapacitors based on carbide-derived carbon electrodes doped with up to 5 wt% mixture of halide or alkali ion salts in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (EMImTFSI) have been studied to exploit their specific energy storage mechanism. To separately highlight the effect of redox active mixture doped electrodes, neat EMImTFSI is applied as the electrolyte for all studied supercapacitor systems. A noticeable increase of capacitance for carbon electrodes doped with halide ions has been observed, as measured by three different electrochemical analysis techniques. Compared with neat EMImTFSI based system, carbon electrodes doped with alkali ion salt mixtures do not show a noticeable improvement of energy storage characteristics. Interestingly, both 5 wt% alkali and halide ion salt mixture doped system exhibit a stable high capacitance up to 124 F/g at 3 V. It is shown that this enhancement of capacitance is due to reversible faradaic processes and the specific adsorption of halide ions within the micro- and mesoporous electrode material and does not limit the bulk electrolyte properties. It is seen that the alkali and halide ion doped systems display greater electrochemical stability compared with neat EMImTFSI based system within constant voltage floating tests.