Development Of Solid State Supercapacitors With Polypyrrole/Cotton Electrodes

In recent years, solid-state supercapacitors have attracted more and more attentionof researchers from all over the word because of good flexibility and security that isnot easy to leak. This phenomenon is due to the intelligent fabrics which choosesupercapacitors as energy storage component have been widely used in biomedicalfields such as health informatization and medical monitoring. Moreover, comparedwith the traditional supercapacitors using liquid electrolyte, supercapacitors usingsolid electrolyte are safer and easier to satisfy the wearable properties. In order toachieve this goal, first of all, people try to use fabric electrodes such as which coatedwith polypyrrole. Compared with common electrodes coated with porous carbonmaterials, polypyrrole/cotton electrodes have not only incomparable flexibility andtensile properties, but also characteristics of light weight, low cost, non-toxic and highconductivity, what’s more satisfy the wearable properties. At the same time, fabricelectrodes must be matched with solid and flexible electrolyte and then fully reflectwearable properties, so solid electrolyte must be firstly developed to complete theassembly of supercapacitors. But current research on solid-state supercapacitorsmainly focuses on the non-flexible electrodes, such as the research that use Ni(OH)2/activated carbon as electrode develops the corresponding solid electrolyte system ofPVA(polyvinyl alcohol)-PAAS(sodium polyacrylate)-KOH-H2O. In this research,supercapacitors’ electrochemical performance is improved, but the non-flexibleelectrodes make the wearable properties reduced greatly. Besides, related researchalways focuses on how to make the solid-state electrolyte and assemble thesupercapacitators, but the physical and chemical characteristics of electrolyte and itsstability are not explored.Main job of this project is to develop a kind of solid-state electrolyte of goodperformance that matched with polypyrrole/cotton electrodes, which will be used forassembly of supercapacitor. Appropriate basic electrolyte and gel material are choosedto form a gel electrolyte through experiments, whose stability of viscosity,conductivity and electrochemical properties over time are ensured. At last, assemblyof solid-state supercapacitor is completed through a certain method, theelectrochemical properties of which are characterized. Around these issues, thefollowing subjects are researched: (1)Cotton is used as the basic cloth for chemical polymerization, and electricaltests are taken on the polypyrrole fabrics obtained to examine its electricalconductivity. Basic electrolyte and gel material are blended to prepare the gelelectrolyte, good electrochemical properties and flexibility are both ensured.Two-electrode system is set up on the electrochemical station to collect the signals ofalternating current impedance plots and cyclic voltammetry curves.(2)Basic electrolyte is chosen from acid, alkali and inorganic salts. Not only theelectrochemical properties in two-electrode system, but also the adaptability withpolypyrrole electrodes are compared. Besides, it is tested that whether all kinds ofbasic electrolyte and gel material can form uniform system, and it is concludedthrough experiment that how much dosage of inorganic salt solution is needed toprecipitate5%mass fraction of PVA. About how to choose gel material PVA, it isdetermined by comprehensive consideration of both viscosity properties andperformance of supercapacitor it assembled.(3)Changes of electrolyte’s viscosity, conductivity and electrochemical propertiesdirectly affect the characteristics of supercapacitor after assembly, so they should bemonitored respectively by the measurement technique of viscosity, conductivity andelectrochemical figures over time. Stability of these properties over time can beimproved by the viscosity of gel electrolyte, which can be improved by added ineither organic or inorganic additives. Effect of these additives depend on the addingrate, so suitable organic additive need to be selected through tests on the viscositystability improvement under different adding rate. Ultimately, the best adding rate isdetermined.(4) Solid-state supercapacitor is assembled based on the gel electrolyte under bestconditions and polypyrrole/cotton electrodes completed. Charging reversibility, cyclelife, and specific capacitance under different voltage sweep speed of supercapacitorsare tested through galvanostatic charging/discharging and cyclic voltammetry curves.It comes to a conclusion on basis of above study as follows: Alkaline material andpolypyrrole cannot coexist. Inorganic salts make PVA precipitated. Therefore, themediate strong acid H3PO4is a better choice. Compared with other types, PVA-1750provide aqueous solution with suitable viscosity and still keep capacitance propertiesunder high viscosity, which make it a better choice of gel material used forassembling solid-state supercapacitors based on polypyrrole/cotton electrodes. It wasfound that electrolyte’s viscosity increased over time, accompanied with the decreaseof conductivity and electrochemical properties. When12%pyridine was added, the viscosity and conductivity were steady at around1.1Pa·s and53mS/cm, thestabilization of electrochemical properties was obviously improved. Electrochemicaltests are carried on the assembling complete solid state supercapacitor, potentialmaintains98%after galvanostatic charging/discharging1,000cycles, cyclicvoltammetry curve stays steady among potential window-0.3~0.7V, specificcapacitance is up to13.8F/g, which exhibits good capacitance characteristic.