Interfacial Modification Of Gel Electrolyte All-Solid-State Supercapacitor Electrode Materials

With the wide application of portable and wearable devices,flexible electrochemical energy storage devices have attracted a lot of attention.Among them,all-solid-state supercapacitors have gradually become a research hotspot in recent years due to their advantages of good mechanical strength and flexibility,good safety,and high power density.Carbon material is one of the most widely used electrode materials for supercapacitors,among which graphene is the most representative material.Since its discovery,graphene has replaced activated carbon as a more concerned carbon-based electrode material due to its ultra-high specific surface area.However,the energy density of all-solid-state supercapacitors is generally low,and the poor interfacial compatibility between the electrolyte and the electrode is also one of the key factors.In order to improve its energy density,it is necessary to improve the wettability of the electrolyte to the electrode interface.In this paper,the wettability of graphene electrodes with gel electrolytes was investigated by interfacial modification of graphene electrodes,we attempted to clarify how the wettability of graphene electrodes with gel electrolytes intrinsically affects the performance of supercapacitors.In this paper,expanded graphite sheets were obtained from commercial graphite paper by electrochemical exfoliation,and then graphene films were obtained by ultrasound,suction filtration,and vacuum drying;and then the graphene films were then modified using a solution of the synthesized zwitterionic polymer poly（propylsulfonate dimethylammonium propylmethacrylamide）（PPDP）as an interfacial modifier.Finally,the unmodified and modified graphene films and PVA/H₂SO₄hydrogels were assembled into supercapacitors to test the electrochemical performance.Specifically,from the three perspectives of graphene film thickness,PVA gel electrolyte with different molecular weights and PPDP with different molecular weights as interfacial modifiers,the effects of the wettability of the gel electrolyte on the graphene film electrodes on the performance of the assembled supercapacitor were investigated respectively.First,we investigated the effect of the interfacial wettability of graphene thin-film electrodes with different thicknesses with the gel electrolyte before/after modification on the performance of supercapacitors.The area capacitances of the supercapacitors assembled by three graphene films with different thicknesses（FHG-10,FHG-20,and FHG-50,which gradually increase in thickness）without modification on the electrodes surface were given the area capacitances from the initial 31 m F/cm²,37 m F/cm²,51m F/cm²at the beginning respectively,increased to 72 m F/cm²,95 m F/cm²,130 m F/cm² after 10h infiltration;and the area capacitances of the three supercapacitors after the modification treatment on the electrodes surface increased from 58 m F/cm²,86m F/cm²,75 m F/cm² at first to 84 m F/cm²,158 m F/cm²,173 m F/cm² respectively after10h infiltration,which has proved that the hydrophilic PPDP modified electrodes interface can effectively promote the improvement of capacitor energy density.At the same time,as the thickness of the graphene film increases,the specific surface area of the interior of the graphene film electrodes of per unit area increases.The modification of PPDP can make the electrolyte fully infiltrate the inside of the electrodes,thereby improving the electrochemical performance of supercapacitors assembled with thicker electrodes and effectively increasing the energy density of the capacitor.Therefore,the effective infiltration of the electrolyte to the electrode interface to improve the electrochemical performance is essentially achieved by increasing the effective specific surface area of the electrode.Then,we selected FHG-20 as the research object,and prepared PVA/H₂SO₄ gel electrolyte with the same mass concentration of PVA with different molecular weights,studied the effect of electrolyte-electrode interface wettability on the performance of the assembled supercapacitors before/after electrodes modification.We found that for supercapacitors assembled with unmodified electrodes,as the molecular weight of PVA increased（M_w were 19,000,47,000,and 61,000 respectively）,the area capacitances increased from the initial 39 m F/cm²,38 m F/cm²,37 m F/cm² to 111 m F/cm²,103m F/cm²,95 m F/cm² after 10h infiltration respectively;and after the electrodes surface were modified,the area capacitance of the assembled supercapacitor was increased from the initial 90 m F/cm²,90 m F/cm²,86 m F/cm² to 162 m F/cm²,159 m F/cm²,158m F/cm² after 10h infiltration respectively,after the surface of the graphene membrane electrodes were modified,the area capacitance obtained by electrolyte of different viscosities were not much different.Since the concentration and number of conductive ions in the electrolyte are the same,it can be considered that after the modification,the electrolyte can effectively infiltrate the inside of the electrodes,thereby realizing the effective use of the internal surface area,and improving the electrochemical performance of the device.Therefore,using PPDP to modify the electrodes interface can increase the wettability of the gel electrolyte,and can weaken or even eliminate the effect of viscosity changes caused by the use of PVA with different molecular weights.Finally,we synthesized PPDP with smaller molecular weight and used it for the interfacial modification study of graphene membrane electrode FHG-20.The initial area capacitance of the supercapacitor assembled on this basis can reach 163 m F/cm²,and after 3h infiltration,the area capacitance reached 193 m F/cm².The graphene film electrode was modified after using a large molecular weight PPDP,the initial area capacitance was 86 m F/cm²,and the maximum area capacitance was 158 m F/cm².The AC impedance resulted show that the interfacial charge transfer resistance of the supercapacitor assembled from the electrodes treated with small molecular weight PPDP was also small.When the electrodes were interfacially modified with small molecular weight PPDP,its molecular chain can enter the deeper interior of the graphene film,which can further increase the effective specific surface area of the electrodes and significantly improve the electrochemical performance of the assembled supercapacitor.