Preparation And Properties Of Polyacrylic Acid Based Flexible Supercapacitors

Flexible supercapacitors can not only retain some excellent characteristics of supercapacitors like high power density,safety,and stability,but also maintain stable electrochemical performance and complete structure in the shape changes caused by external forces such as bending,folding,and extrusion.In this paper,polyacrylic acid（PAA）and graphene oxide（GO）composite gel electrolytes（PAA/GO）were prepared.Then the porous carbon with PAA as the precursor and the flexible free-standing electrode with the porous carbon as the active material were prepared.The flexible electrode and gel electrolyte were then assembled into flexible supercapacitors,and their performance and potential applications were explored.（1）PAA/GO composite hydrogel electrolyte was prepared by free radical polymerization.The oxygen-containing functional groups of GO can form hydrogen bond with the carboxyl group on PAA,thus significantly improving the mechanical properties of PAA gel electrolyte.After compounding with GO,the tensile strength of PAA gel increased from 4.0 MPa to 6.1 MPa,and the maximum tensile strain increased from 1556%to 1950%.At the same time,GO can also promote the transportation of electrolyte ions,which is conducive to improving the ionic conductivity of PAA hydrogels.The experimental results show that when the concentration of GO is 0.5%,the highest ionic conductivity of PAA/GO composite hydrogel electrolyte is16.81×10^-2 S cm^-1.The supercapacitor based on PAA/GO composite hydrogel electrolyte has a capacitance retention rate of 64.3%at a high current density of 20 A g^-1,and shows excellent cycle stability(after 10000 cycles at 5 A g^-1,the capacitance remains unchanged).In addition,the flexible supercapacitors have excellent flexibility and can maintain stable electrochemical performance at different bending angles of 0-90°.（2）Potassium polyacrylate gel was used as the precursor to prepare self-activation porous carbon by one-step carbonization-activation method without additional activator.The principle is that the-COOK group in the gel decomposes into K₂CO₃ when heated,and the decomposition product of K₂CO₃ reacts with carbon at high temperature to form pores.During the preparation of gel,KCl could be added as a template.After carbonation and activation,impurities can be completely removed only by washing with water.Under the action of the template,the porous carbon has a cheese like morphology,resulting in micron-scale large pores and thinner pore walls,which makes the activation of K₂CO₃ on the carbon matrix more fully,with a specific surface area of 1410 m²g^-1(much higher than 921 m² g^-1 of template free porous carbon),a pore volume of 0.60 cm³ g^-1,a capacitance of 265.9 F g^-1 at 0.1 A g^-1,and a capacitance of 117.8 F g^-1 at 100 A g^-1.The porous carbon has excellent specific capacitance and rate performance.（3）Flexible free-standing electrodes with PAAK/KCl porous carbon as active material and reduced graphene oxide（r GO）as conductive binder were prepared by tape casting and thermal reduction.Compared with the traditional electrode,the flexible electrode with PAA gel based porous carbon as the active material and r GO as the three-dimensional framework has greater specific capacitance and better rate performance.The flexible film with PC-r GO-7:3 and PAA/GO composite gel electrolyte were assembled into a flexible solid supercapacitor,which showed a maximum specific capacitance of 281.6 F g^-1 and good cycle stability（after 10000 cycles,the capacitance remains 91.3%）.Besides,electrochemical performance of the flexible device hardly changes within a bending angle of 0-90°,and it has good fatigue resistance during repeated bending.At the same time,connecting three bent devices in series can light up small bulbs,demonstrating the potential application value of PAA-based flexible supercapacitors.