Research On Electrode Interface Of All-solid-state Supercapacitor Modified By Zwitterionic Polymer

Flexible solid supercapacitors have the characteristics of flexibility and simplicity,and show a huge application prospect in wearable electronic devices,smart skin and portable smart phones.In flexible solid supercapacitors,flexible electrode materials and polymer hydrogel based electrolytes are important components.Hydrogel electrolytes have the advantages of high ionic conductivity,safety and relatively low price,and have been widely used.However,due to their poor mobility,they cannot form effective contact with some of the pores of the electrode,as a result,it can not form effective contact with some pores of the electrode,so that the utilization rate of the specific surface area of the material is very limited,and the energy storage part is less.Therefore,the low energy density limits the development of flexible solid-state supercapacitors.Although a large number of excellent electrode and electrolyte materials have been developed,the interfacial issues between electrolytes and electrode materials have received less attention.The interface between the electrode layer and the electrolyte layer is an important medium for material and energy exchange in the working process of the supercapacitor.The interface compatibility directly determines the ion and electron transport of the supercapacitor under the charge and discharge state,and plays a key role in the rate performance and energy storage capacity of the device.Based on this,this thesis has carried out targeted research on the interface between electrode and electrolyte of supercapacitors,mainly including the following two aspects:(1)Conductive polymers containing zwitterionic groups(PEDOTSB)are introduced onto graphene electrodes through electrochemical polymerization.Due to the strong hydrophilic nature of the zwitterionic groups,the wettability of graphene electrodes can be improved.It was found that the surface capacitance of the graphene electrode modified by an amphoteric polymer increased from 99 m F/cm~2 to 224m F/cm~2.The contact angle of the graphene film decreased from 82.6°to 25.3°.From the AC impedance(EIS)curve,it can be seen that the charge transfer resistance of the graphene film has been effectively reduced after modification,which indicates that the zwitterionic group improves the interfacial compatibility between the electrode and the electrolyte,reduces the charge transfer resistance,and thus increases the specific capacitance.In order to confirm this,we have studied the effect of conductive polymers containing other different side groups on graphene films,namely,the introduction of poly(3,4-ethylenedioxythiophene)(PEDOT)and conductive polymers containing hydrophobic thiophene structures(PEDOTCH)through electropolymerization to modify graphene films.The results confirmed that the modification effect of PEDOTSB with zwitterionic groups was better than that of PEDOT,while the modification effect of PEDOTCH with hydrophobic side groups on graphene films was poor.When too much PEDOTCH was electropolymerized,its performance was even worse than that of unmodified graphene.(2)The electrode of a planar capacitor is limited by a limited porosity,resulting in a low energy density of the capacitor.We have designed a"two birds with one stone"method by adding the zwitterionic polymer PPDP to the graphene dispersion solution,dispersing it through ultrasound,and then conducting suction filtration to form a graphene film.Finally,after heat treatment,the graphene electrode is made.During this process,due to the fact that PPDP is insoluble in dimethylformamide,its particles can act as spacers,prevent graphene from accumulating again,and improve the specific surface area of the graphene film.Since zwitterionic polymers can be dissolved in salt solutions,we further immerse the graphene electrode containing PPDP in an aqueous Li Cl solution,during which part of the PPDP on the graphene membrane will be dissolved.Due to this method can make the zwitterionic polymer disperse on the surface and inside of the graphene film,PPDP can better improve the hydrophilicity of the inside and surface of the graphene electrode,so that the hydrogel can more easily enter into the inside of the electrode,forming a large and well compatible electrode electrolyte interface,and improving the utilization ratio of the specific surface area of the electrode.The study found that the specific capacitance of the graphene electrode modified with PPDP increased from 99 m F/cm~2 to 250 m F/cm~2,and the magnification performance of the capacitor was significantly improved.From electrochemical impedance spectroscopy,it can be found that the charge transfer resistance of the device has been greatly reduced.