High Capacitance Flexible Zinc Ion Hybrid Supercapacitor Based On Multifunctional Hydrogel

With the rapid development of flexible wearable devices,more and more researchers begin to pay attention to flexible energy storage devices.Among them,flexible supercapacitor has gradually become a research hotspot because of its high power density and rapid charge and discharge.It is generally composed of electrodes and electrolytes.Electrolyte is a vital part for transferring and storing charges,and it is also a key part for giving flexibility and other properties to supercapacitors.Hydrogel electrolyte has excellent flexibility and good safety performance,and is the first choice to construct flexible supercapacitors.In order to expand the application of flexible supercapacitors under the conditions of low temperature,mechanical damage and deformation,three kinds of hydrogel electrolytes with different functions are designed in this paper to meet the practical use.The specific contents are as follows:1.A hydrogel electrolyte PVA/CNF/ZnSO₄/DMSO（PCD）with good low temperature performance was prepared with polyvinyl alcohol（PVA）as the main body,a small amount of oxidized cellulose nanofibers（CNF）,zinc sulfate（ZnSO₄）as the electrolyte ion and dimethyl sulfoxide-water（DMSO-H₂O）as the binary solvent.Its tensile elongation at room temperature（25°C）and-20°C is 363%and 300%respectively.The flexible zinc-based hybrid supercapacitor（FZHSC）assembled from it has excellent specific capacitance at room temperature and low temperature,which are 208.51 F g^-1and 135.4 F g^-1respectively.In addition,the FZHSC also shows excellent long-cycle performance of charge and discharge and energy density.After5000 cycles,80%of the original capacity can still be retained.The highest energy density can reach 212.48 Wh kg^-1.More importantly,the FZHSC still has excellent electrochemical stability in bending state,and the capacity retention rate is 93.1%when bending 180°.2.The oxidized CNF in PCD hydrogel was replaced by sodium alginate（SA）with a large number of carboxyl groups,and PVA/SA/ZnSO₄/DMSO（PS）gel with both self-healing and frost resistance was prepared.By adjusting the dosage of SA,the tensile strain and stress of PS can be restored to more than 90%of the original state after 12 h of healing.FZHSC based on this hydrogel has a high specific capacitance of 180.11 F g^-1at room temperature,and its capacity can be restored to94%after 12 h of fracture and recovery.At the same time,the FZHSC maintains a specific capacitance of 55.23 F g^-1at-20°C.In addition,it still retains 77%specific capacitance after 7000 charge and discharge cycles,and the coulombic efficiency is close to 100%.3.A hydrogel PVA/SA/TA/ZnSO₄/DMSO（PST）with anti-freezing,self-healing and self-adhesive properties was prepared by adding filler tannic acid（TA）into the previous hydrogel system.Compared with PS hydrogel,the self-healing property of PST hydrogel was greatly improved,and the original tensile strain could be restored within 1 h after fracture,and the electrical conductivity could be restored to 99%.The specific capacitance of FZHSC assembled by PST can reach 179.68 F g^-1(current density is 0.5 A g^-1)in the voltage window of 0-1.8 V,and its capacity can be restored to 95.53%after 1 h of healing.The FZHSC has a specific capacitance of 102.6 F g^-1at-20°C.After 5300 times of charge and discharge,the capacity retention rate is70.19%,showing its good electrochemical performance.