Research On Supercapacitor Based On Mixture Of Imidazole-based Ionic Liquid

As a new generation of energy storage device developed in recent years,supercapacitor is characterized with higher cycle life,higher safety,and higher power density compared with traditional battery.The ionic liquid as a new generation of supercapacitor electrolyte is characterized with higher electrochemical stability than traditional salt solution or organic solvent.But the disadvantage of ionic liquid is higher viscosity and lower electrical conductivity due to the interaction between the anion and cation.In this study,imidazolium-based ionic liquid was selected as the research objective.Both the effect of alkyl modification and mixing effect on the ionic liquid physicochemical property and the performance of supercapacitor device are studied.In this dissertation,two ionic liquids of[BMIm][BF₄]and[PEIm][BF₄]with the same relative molecular mass were synthesized using classical two-step method.The results of thermogravimetric analysis and Raman spectroscopy showed that the purity of products was at a high level.When one methylene group from the butyl group was transferred to the methyl group,an ethyl group was formed.The study results showed that the thermal stability of the ionic liquid was enhanced in respect to the final decomposition temperature and the hygroscopicity was reduced,but the conductivity decreased from 3.19 to 2.22 mS/cm.When the ionic liquid mixture was formed using these two ionic liquids,the conductivity of ionic liquid mixture was located between the conductivity of two pure ionic liquid.Moreover,the fitting results of Vogel–Tammann–Fulcher?VTF?equation for the conductivity of all pure ionic liquid and ionic liquid mixture were matched very well.The parameters?₀ and B?of VTF equation of[PEIm][BF₄]were smaller than the corresponding parameters of[BMIm][BF₄],and for the ionic liquid mixture,both parameters increased with the concentration of[BMIm][BF₄].These results demonstrated that alkyl modification of methylene redistribution was not suitable to improve conductivity of ionic liquid,and the conductivity of ionic liquid mixture was determined mainly by the two components of[BMIm][BF₄]and[PEIm][BF₄].We assembled two supercapacitors using[BMIm][BF₄]and[PEIm][BF₄],the performance of the supercapacitor with[BMIm][BF₄]was better than the performance of the supercapacitor with[PEIm][BF₄]by CV test because the electrolyte conductivity dominated the electrochemical behavior of the supercapacitor.The calculated specific capacitance by GCD test under 3.2 V work voltage was 117F/g at 0.2 A/g and 63 F/g at 5 A/g for supercapacitor with[BMIm][BF₄],and the results were 108 F/g and 38 F/g correspondingly for supercapacitor with[PEIm][BF₄].The performance of the two supercapacitors showed little difference after 300 cycles of GCD test.The characteristic frequency was 0.36 Hz and 0.16 Hz respectively for the two supercapacitors by EIS test.When we assembled supercapacitors using different ionic liquid mixture,the behavior of electric double layer can be more stabilized by CV test when[BMIm][BF₄]was mixed a little with[PEIm][BF₄]or the other round.Because the structure difference between[BMIm][BF₄]and[PEIm][BF₄]was not significant and the conductivity of ionic liquid mixture was low,the capacitance and characteristic frequency for all ionic liquid mixture-based supercapacitors were located between that of pure ionic liquid-based supercapacitors.All those results demonstrated that alkyl modification was detrimental to supercapacitor performance and the performance of ionic liquid mixture-based supercapacitors were moderate.