Electrochemical Performance Of Activated Graphene Powder Supercapacitors Using A Room Temperature Ionic Liquid

Room temperature ionic liquids?RTILs?have been considered as a promising electrolyte for further improving supercapacitor's performances owing to their large potential window,high thermal stability,and working temperature range.However,RTILs suffer from the poor conductivity and high viscosity stemming from their strong cation-anion interactions.In this work,we investigated the influences of solvent on the capacitive performance within RTILs-based supercapacitors.Activated graphene powders were employed as electrode active materials,and 1-butyl-3-methyl-imidazolium tetrafluoroborate?[BMIM][BF₄]?was chosen as the electrolytes because of the widely applications in electrochemical energy storage.The mole fraction of[BMIM][BF₄](?_IL)in electrolytes could be regulated with adjusting the ratio of acetonitrile?ACN?.Electrochemical measurements suggested that the solvent effects on the charge storage capability of supercapacitors depended strongly on the applied potential scan rates?or current densities?.Specifically,at a lower scan rate of 10 mV s^-1(or lower current density of 1 A g^-1),solvent exhibited a negligible influence on the electrochemical performance.However,at an elevated scan rate of 200 mV s^-1(or current density of 5A g^-1),solvent addition could prominently enhance the capacitance by² folds.These results could resolve the controversial solvent effects reported in previous simulation and experimental studies.To interpret the as-obtained results,we further explored the solvent effects on the dynamic properties of electrolytes.It was found that solvent could effectively reduce the strong ion-ion interactions within pristine RTILs,thus decreasing the viscosity by²9 times.Further electrical impedance spectroscopy tests suggested that the addition of solvent was able to significantly suppress the series resistance?by⁵.5 times?and dielectric relaxation time?by⁶.3 times?,which thereby improved the rate capability of supercapacitors.We demonstrated that the maximum specific energy density of supercapacitor(?_IL=0.25)was calculated to be 65.2 Wh kg^-1 at 1 A g^-1,among the best performances in the state-of-art literatures.More importantly,under an elevated working temperature of 50°C,its energy density could reach up to 85.5 Wh kg^-1,which was much higher than that of aqueous or organic solution based supercapacitors(<10 Wh kg^-1)and lead-acid battery(20³5 Wh kg^-1),comparable to that of Ni metal hydride battery(40¹00 Wh kg^-1)and lithium-ion battery(80¹50 Wh kg^-1).Cyclic voltammetry tests were performed on the supercapacitors at different operating potential windows?3.0,2.5,2.0,1.5,1.0,and 0.5 V?.The tests found that the effect of the operating potential window on the solvent effects were related to the applied scan rates.Specifically,at a lower potential scan rate,under higher potential window,solvent exhibited an insignificant influence on the capacitive of supercapacitors;under the lower potential window,the capacitance exhibited firstly increased and then decreased with solvent addition.At an elevated potential scan rates,the operating potential window revealed a minor impact on the capacitance.The capacitance was enhanced first and then remained constant.When the potential scan rate was further increased,the capacitance was linear with solvent addition.