Study On Preparation Of Electrode Material And Electrolyte For High-Performance Supercapacitor Using Deep Eutectic Solvent(DES)

In recent years,the paradox between human’s dependence on energy and the burning of fossil fuels has promoted the development and utilization of various clean energy sources.However,in view of the intermittent nature of these clean energy sources,there is an urgent need to develop efficient and environmentally friendly energy conversion and storage systems.Among them,supercapacitors have become the subject of extensive research by scientists due to their advantages such as high power density,outstanding rate capability,fast charge/discharge and long cycle life.However,it still faces an urgent problem that needs to be solved,that is the problem of low energy density.In this paper,the method of using eutectic solvent to assist phenolic resin condensation was used to prepare hierarchical porous carbon materials with micropores as the mainstay.At the same time,the effects of different doses of deep eutectic solvent on the amount of heteroatom doping and pore structure of carbon materials were investigated.In addition,a binary and ternary deep eutectic solvent are also prepared for use as the electrolyte of supercapacitors,which not only realize normal operation in a wide temperature range,but also significantly improve its energy density.The specific research results are as follows:1.By using deep eutectic solvent synthesized from urea and Zn Cl₂as well as phenol-formaldehyde resin as starting materials,the hierarchically nanoporous carbon material was synthesized via direct carbonization process.Notably,the largest surface area can reach up to 1414 m²g^–1,coexistent with high nitrogen（8.09 at.%）and oxygen（14.77 at.%）contents.Moreover,we use theoretical calculation to determine the optimum molar ratio between urea and Zn Cl₂（1:1）.As a consequence,it delivers large specific capacitance(204 F g^–1),superior cycling stability（96.97%）and excellent energy density(5.28 Wh kg^?1)in 6M KOH electrolyte.For further increasing energy density,we employ an electrolyte consisting of 1-ethyl-3-methylimidazolium tetrafluoroborate and acetonitrile in an equivalent mass into a symmetric supercapacitor configuration.Resultantly,the energy density is as high as 40.13 Wh kg^?1due to large voltage window（3 V）.The present deep eutectic solvent carbonization method opens an avenue for producing carbon materials especially possessing with porosities and functionalities for favorable of achieving superior supercapacitor.2.Through simple heat treatment of choline chloride and ethylene glycol,a binary deep eutectic solvent was obtained and used as the electrolyte of supercapacitors.The relationship between the viscosity,electrical conductivity and electrochemical performance of binary deep eutectic solvents under extreme conditions（-40～115°C）is revealed.With the increase of temperature,the viscosity of electrolyte decreases along with the enhancement of electrical conductivity;similar phenomenon toward capacitive behaviors under extreme conditions also occurs.The electrolyte with molar ratio of 1:2（choline chloride:ethylene glycol）under 115°C shows a specific capacitance of 362 F g^–1(with energy density of 50.33 Wh kg^-1),which can remain as 102 F g^–1(with energy density of 14.13 Wh kg^-1)even at-40°C.In addition,density functional theory also confirms that the electrolyte with molar ratio of 1:2 has larger absolute value of adsorption energy as 4.42 e V.The present deep eutectic solvent by choline chloride and ethylene glycol can extend to other electrolytes especially for wide temperature range supercapacitors.3.Add the third effective component urea on the basis of the above binary deep eutectic solvent,we successfully synthesize a novel ternary deep eutectic solvent electrolyte for wide-temperature region supercapacitor（-40～115°C）through adjusting the molar ratio between choline chloride,ethylene glycol and urea.Notably,when the molar ratio is 1:2:1（choline chloride:ethylene glycol:urea）,the electrolyte under115°C not only exhibits prominent transport properties(viscosity:3.39 c P;conductivity:13.41 m S cm^–1),but also delivers the excellent electrochemical performances with specific capacitance up to 317 F g^–1(specific energy:74.37 Wh kg^-1),and the stability after 10,000 cycles close to 100%.Besides,the theoretical calculation by density functional theory reveals that the total energy of electrolyte with a molar ratio of 1:2:1is the largest（24.66 e V）,confirming its outstanding performances.More importantly,hydrogen bonding indeed occurs in such electrolytes which helps the electrolyte to achieve normal operation over a wide-temperature range.This electrolyte possesses the capacity for lighting bulb and maintaining brightness for 50s.The present ternary deep eutectic solvent offers a bran-new research idea for wide-temperature region electrolyte employed to supercapacitors.