Preparation Of PVA Gel Electrolyte And Research On Properties Of Flexible Supercapacitor

With the improvement of technology,the rapid development of smart and wearable electronics has promoted the research of flexible energy storage devices with wide operating temperature range,stable properties and safety.Flexible quasi-solid-state supercapacitors have become one of the hotspots in the field of flexible energy storage devices due to their long cycle life,safety and other advantages.As a critical part of flexible quasi-solid-state supercapacitor,the gel polymer electrolyte has a significant impact on the performance of energy storage devices.Polyvinyl alcohol(PVA)is a kind of gel polymer matrix materials with low price,safety,stable structure,and good hydrophilicity.It can be mixed with aqueous electrolytes with a wide range of p H to prepare gel electrolytes suitable for different electrodes,and is widely used in flexible quasi-solid supercapacitors.However,the issues,such as poor flexibility,low ion conductivity,and narrow work temperature range restrict commercialization.In this thesis,conductive polymers are used as electrodes to study the electrochemical performance of capacitor effected by the assembly method of PVA gel electrolyte and electrodes,to explore the electrochemical properties and low-temperature performance improvement of PVA gel electrolyte,and to research the influence from the modified PVA gel for the electrochemical performance and low-temperature properties of the capacitor.The high-performance flexible electrode was prepared by cyclic voltammetry electrodepositing polypyrrole(PPy)as the electrode material with bacterial cellulose(BC)as the substrate,and reduced graphene oxide(RGO)and carbon nanotubes(CNT)as the conductive layer.The electrode obtains a high area specific capacitance of 715.0 m F cm^-2 at 1 mA cm^-2,and retains 86.9%of capacitance after 5,000 charge-discharge cycles.Assembling two electrodes into a symmetrical capacitor,it obtains an area capacitance of 368.8 m F cm^-2 in the aqueous electrolyte at the current density of 1 mA cm^-2.As the current density increases to 30 mA cm^-2,30.5%of initial capacitance is retained.Using the conventional PVA hydrogel as quasi-solid-state electrolyte,the flexible quasi-solid-state capacitor assembled via infiltrating the flexible electrode has an area capacitance of 215.0 m F cm^-2 at 1 mA cm^-2,and retains53.0%of capacitance at 6 mA cm^-2.Using the chemically cross-linked PVA hydrogel as the solid electrolyte,the flexible quasi-solid-state capacitor assembled by layer-by-layer stacking has an area capacitance of 147.5 m F cm^-2,and only 15.7%of capacitance remains at 6 mA cm^-2.The above results indicate that the immersion assembly between the PVA gel electrolyte and the electrode has a significant impact on the rate performance of the capacitor.The freeze-partial dehydration method is used to treat the chemically cross-linked PVA hydrogel.The mechanical strength of the as-prepared f-PVA gel increases from 0.038 MPa to 0.68 MPa.The ionic conductivity increases from 0.048 S cm^-1 to0.077 S cm^-1.By using f-PVA as gel electrolyte,the all-in-one configured PPy-f-PVA obtains a high area capacitance of 644.4 m F cm^-2 at 1 mA cm^-2,and retains 58.2%of capacitance at 50 mA cm^-2.After 10,000 charge-discharge cycles,the capacitor maintains 86.4%of initial capacitance.With PANi as electrolyte and f-PVA as gel electrolyte,the all-in-one configured flexible quasi-solid-state PANi-f-PVA has a high area capacitance of 280 m F cm^-2 at 0.5 mA cm^-2,and retains 80.4%at 50 mA cm^-2.After 10,000 charge-discharge cycles in constant current density,the capacitor maintains 80.1%of capacitance.Comparing the influence of PVA and f-PVA gel electrolyte on the electrochemical performance,it can be observed that the hydrogel electrolyte treated by the freeze-partial dehydration method can effectively improve the rate performance of quasi-solid capacitors.After introducing the high-concentration H₂SO₄ electrolyte into f-PVA,the as prepared f-PVA3 exhibits a good flexibility and high ionic conductivity(5.92 m S cm^-1)at-40 ^oC.Using f-PVA3 hydrogel as solid electrolyte,PPy-based and PANi-based supercapacitors were prepared by all-in-one configuration,and studied the electrochemical properties at low temperatures.At-40 ^oC,the results show that PPy-f-PVA3 retains 61.3%of the capacitance at room temperature,and delivers a high capacitance of 119.4 m F cm^-2 at 50 mA cm^-2.PANi-f-PVA3 capacitor retains 95.9%of capacitance at low temperatures,and maintains 59.8%of initial capacitance at 50mA cm^-2.By adjusting the concentration of the H₂SO₄ electrolyte in the PVA hydrogel,with the help of the property of the H₂SO₄ solution,the formation of ice crystals in the gel is suppressed and more electrolyte ions are provided,which is an improvement for low-temperature resistant quasi-solid-state capacitors.