Regulation Of Graphene Surface Microstructure And Its Application In Supercapacitors

Polyvinyl alcohol（PVA）gel electrolyte,has been widely studied as an important component of supercapacitors.Although its preparation technology has been quite mature,the physical and chemical properties of the synthesized PVA gel electrolyte still suffer from numerous defects,such as poor mechanical properties,low ionic conductivity,high inherent internal resistance,etc.Therefore,in order to improve the ion transport efficiency,many researchers have paid attention to introduce various conductive agents into PVA gel electrolyte.Among them,graphene and its oxide（GO）are frequently used as conductive agents for gel electrolytes due to their excellent physical and chemical properties.In recent years,most researchers have optimized the structural characteristics of gel electrolyte by directly introducing GO or graphene nanosheets.Nevertheless,little attention has been paid to the controllable optimization of PVA electrolyte structure and the properties of the electrolyte-electrode interface via utilizing graphene and GO nanosheets,so that the conductive agent in electrolyte cannot give full play to"build a channel"effects.The above-mentioned energy storage devices also have some disadvantages such as high interfacial resistance and poor rate performance in electrochemical process,which severely restricts the further application of graphene and its oxide in PVA gel electrolyte system.As a result,carefully studying the influence of graphene and its oxide on the structure and properties of PVA electrolyte system is expected to controllably improve the surface microstructure of PVA electrolyte and the interfacial properties between electrolyte and electrode,which possesses important reference significance for further enhancing the electrochemical performance of energy storage devices.Moreover,Graphene nanosheets has also been widely studied as an electrode for supercapacitors.However,the graphene sheets prepared in most reports are usually difficult to combine with the following structural characteristics:effectively avoid the irreversible accumulation and more retention of beneficial oxygen-containing groups,resulting in a significant reduction in capacitance.Hence,it is extremely important to explore a new strategy for regulating the microstructure of graphene nanosheets.Herein,we report the mechanism improving the performance of the electrolyte system and electrode materials.Namely,from the optimization of PVA-based gel electrolyte structures and the modification of graphene electrode materials,the physical,chemical and electrochemical properties of PVA-based electrolytes and graphene nano-materials were studied in detail,and the central conclusions are as follows:（1）PVA-based gel electrolyte was optimized by introducing GO samples with different microstructures.Based on the hydrogen bonding between GO sheets and PVA matrix,the hydrogel system（PVA/GO-Ⅱ/H₂SO₄）with higher ionic conductivity was prepared,and then by analyzing its structural performance in depth,the structural optimization mechanism of PVA-based gel electrolyte was established.The results of electrochemical performance test show that the supercapacitor based on activated carbon electrode and PVA/GO-Ⅱ/H₂SO₄ gel electrolyte possesses good rate performance and cycle stability.The primary reason is that the nearly ideal ion transport channels in the gel electrolyte and many shallow active sites in the electrolyte-electrode interface,which provides more space for the rapid electrochemical reaction;the interconnected three-dimensional network structure can also enhance the mechanical properties of the gel electrolyte,so that the device can maintain stable capacitive performance after multi-angle bending.（2）In order to further broaden the application scope of GO,a hydrogel system（PVA/Nr GO-Ⅱ/KOH）with excellent microstructures was prepared by the integrated reduction of GO-Ⅱby hydrazine hydrate.The SEM results confirm that Nr GO-Ⅱchip possesses the best effect on PVA/KOH gel electrolyte structure in an alkaline environment.The reasons are mainly manifested in two aspects:on the one hand,the GO-Ⅱsheets and hydrazine hydrate will apply to the PVA matrix successively and make it fully crack finally,sequentially forming the loose skeleton;then Nr GO-Ⅱsheets obtained by reduction of hydrazine hydrate can connect the cracked PVA matrix to each other,resulting in a continuous three-dimensional network structure and numerous micron pores.The optimized structure of PVA gel is beneficial to expand the effective contact areas between the electrolyte and electrode,thereby improving the electron and/or ion transfer efficiency,resulting in lower interface resistance and longer cycle life.The capacitive retention of the device is as high as 62.5%at 0.5～10 A/g,and the capacity loss is 7.1%after 1000 cycles at 1 A/g.In addition,the electrochemical behavior after the loop is the same as that of the original state,showing relatively good capacitive reversibility.（3）The optimized r GO30-oxalic electrode with good surface microstructure were obtained by utilizing Mt sheets to intercalate to GO precursor.The retention of oxygen-containing groups,distribution control of pore structures and alleviation of lamellar accumulation were achieved,and the electrochemical active sites of r GO30-oxalic were significantly increased,which improves the electrochemical performance of supercapacitors.When r GO30-oxalic acts as the electrode material of supercapacitors based on 6 M KOH electrolyte,it displays excellent excellent rapid charge-discharge performance and cycling stability:the specific capacitance retention is 79.8%at 0.25～20 A/g;after 10000 cycles at 5 A/g,the capacitance retention rate is still 89.7%.Furthermore,this electrode was also assembled into flexible supercapacitors with PVA/GO-Ⅱ/H₂SO₄ and PVA/Nr GO-Ⅱ/KOH gel electrolyte respectively.The performance testing results indicated that the specific capacitance based on PVA/Nr GO-Ⅱ/KOH electrolyte was slightly lower at 0.5～5 A/g.However,when the current density is higher than 5 A/g,this device initially exhibits the advantages of high specific capacity and excellent rate performance（0.5～8 A/g:71.1%）.Besides,the device also exhibits excellent cycling stability,with a capacity loss of less than 5%after 1000 cycles at 1 A/g.the results above confirm that r GO30-oxalic sample with good surface microstructure can show better electrochemical stability in alkaline electrolytes when assembled into supercapacitors.