Effect Of Organic Additives On The Interface Properties And Energy Density Of Supercapacitors

Low energy density is one of the bottleneck problems hindering the development of double layer supercapacitors.The interface of materials and electrolytes serves as the place for ion adsorption and energy storage,which has a great influence on the specific capacitance of supercapacitors.In this paper,by adding different organic solvents into the ionic liquid and the aqueous electrolyte,the effects of organic additives on the interfacial wettability and the ion concentration distribution at interface of electrode were investigated respectively.The interface characteristics were associated with the specific capacitance and energy density of supercapacitors.The main conclusions are shown in the followings:(1)High viscosity and poor wettability are the main problems of ionic liquid as the electrolyte of supercapacitors.The acetonitrile was selected in this study and added into ionic liquid[EMIM]BF4 to improve the wettability.It was showed that when the volume ratio of[EMIM]BF4 and acetonitrile was 1:3,the contact angle of interface decreased from 43° to 15°,which resulted in a good wettability and a high utilization rate of specific surface area.The specific capacitance of materials increased from 12.1 F/cm3 to 29.4 F/cm3 as a result of the addition of acetonitrile(2)The effect of isopropanol on the specific capacitance and energy density of aqueous electrolyte was investigated in this study.It was found that the addition of isopropanol not only enhanced the wettability of the interface,but also improved the specific capacitance retention after long-term cycles effectively.In addition,20vol%addition of isopropanol inhibited the polarization at high potential significantly,which increased the voltage window from-0.8?0 V to-0.8?0.5 V,leading to the energy density improved from 7.0 Wh/kg in 6 M KOH to 19.4 Wh/kg.The specific capacitance attenuation at high current density(10 A/g)was also improved greatly by the addition of isopropanol,which dropped from 30.3%to 13.9%.(3)By the experiments and simulations,the influence of ion distribution in the surface of electrode on the specific capacitance and its kinetic regulation mechanism was revealed.Firstly,the effect of aqueous electrolyte concentration on the specific capacitance was investigated.The DFT simulation revealed that there was multi-layered orderly packing of ions at the interface of electrode at the high concentration of electrolyte,which resulted in a high specific capacitance.By regulating the transport rate of ions in electrolyte and the charging rate of ions,the influence of kinetics process on the specific capacitance was investigated.A model with multi-layered adsorption of ions was proposed for a high capacitance,and the kinetic in favor of multi-layered adsorption was pointed out.