Preparation Of Multifunctional Sodium Alginate Based Hydrogel Electrolytes And Their Application In Flexible Supercapacitor

With the rapid development of wearable electronic devices,the research of novel flexible energy storage devices is urgently needed.Flexible supercapacitors have attracted widespread attention because of their high power density,fast charging and discharging speed and long cycle life.The main difficulty of traditional supercapacitors to achieve flexibility is that electrolytes cannot meet the requirements of flexible device structure.Hydrogels with unique three-dimensional network porous structure and great flexibility have become ideal candidates for flexible supercapacitor electrolytes.In addition,the three-dimensional network structure of hydrogels also provides abundant transport paths for ion migration.Conductive hydrogels have excellent ionic conductivity,showing their great potential for application in flexible supercapacitor electrolytes.However,the hydrogels also have some problems to be solved,including poor mechanical properties,evaporation of water,lack of self-healing and adhesion,and narrow operating temperature range.In this paper,a series of sodium alginate-based ionic conductive hydrogels with multifunctional properties including flame retardancy,thermal responsiveness,self-healing,adhesion and anti-freezing were designed and prepared for application in flexible supercapacitors.By integrating flexible supercapacitors and flexible sensors,a self-powered seamless integrated system based on hydrogel is successfully designed.The main contents are as follows:1.In order to solve the safety problem caused by thermal runaway,sodium alginate used as the basic framework in this part,added acrylamide and acrylic acid for free radical polymerization,and successfully prepared a sodium alginate-based ion-conducting hydrogel that can achieve double-safety protection.The hydrogel has high elongation at break（1578%）and tensile strength（0.24 MPa）,thermal responsiveness,good thermal insulation and flame retardancy（LOI=46%）.The thermal responsiveness and flame retardancy of the hydrogel can be used to achieve dual safety guarantee.When the temperature was above the transition temperature,the temperature alarm device based on the hydrogel would raise an alarm.The hydrogel electrolyte was also applied in flexible symmetric supercapacitors.When thermal runaway or even fire disaster occured,the hydrogel electrolyte had flame retardancy,which would reduce the harm of thermal runaway and economic losses.The flexible symmetric supercapacitor based on the hydrogel electrolyte exhibited a specific capacity of 510m F cm^-2 at a current density of 0.5 m A cm^-2,energy density of 62 m Wh cm^-2 at a power density of 234 m W cm^-2,and the capacitance retention rate of the device remained 92.3%after repeated bending for 800 cycles,which proved that the device had good flexibility and electrochemical stability.2.In order to solve the disadvantages of electrolyte breakage and poor electrode/electrolyte interface contact leading to poor electrochemical performance,in this part,using sodium alginate as basic framework,acrylic acid and tannic acid were introduced to prepare ionic conductive hydrogels through free radical polymerization.The self-healing and adhesive properties of hydrogels were benefitted from the coordination interaction between trivalent metal aluminum ions(Al³⁺)and carboxyl groups and the synergy of various hydrogen bonds.The ionic conductive hydrogel exhibited moderate mechanical properties（elongation at break of 1304%and tensile strength of 71.8 k Pa,and compressive strength of 53 k Pa under 70%compressive strain）.The hydrogel had good adhesion to different substrates and showed great repeated adhesion.Among them,the adhesion to hogskin was the highest,up to 38 k Pa.The flexible symmetrical supercapacitor with the ionic conductive hydrogel as electrolyte had excellent energy storage performance:a specific capacitance of 547 m F cm^-2 at a current density of 0.5 m A cm^-2,a power density of 250 m W cm^-2 at the energy density of 65.5 m Wh cm^-2,good cycle stability（a capacitance retention of 90%after6000 charge/discharge cycles）,and high coulombic efficiency（stabilized at 100%）.After five cutting/healing cycles,the capacitance of the supercapacitor remained98.5%.3.In order to solve the problem that the electrolyte was easy to lose water,resulting in poor electrochemical performance and could not be used at low temperature,multifunctional hydrogels were prepared by radical polymerization method in this part of work.Based on the second part,the solvent was changed to a water/ethylene glycol mixed binary solvent system.Sodium alginate and acrylic acid also were taken as basic framework,and introduced tannic acid and Al³⁺.The conductive hydrogels with multifunctional properties including self-healing,adhesive,antifreezing and moist were prepared through multiple hydrogen bonds and coordination interactions among the components.The hydrogel showed great self-healing and mechanical property（elongation at break of 973%,and tensile strength of 55.74 k Pa）,excellent freeze resistance（at-50°C）,moisture retention and good adhesion to various substrates.The flexible symmetric supercapacitor based on this hydrogel electrolyte exhibited high electrochemical performance with a specific capacitance of 302 m F cm^-2 at 0.6 m A cm^-2,a power density of 252.87 m W cm^-2 at the energy density of 29.84 m Wh cm^-2.After 5000 cycles of charging/discharging,it still maintained a specific capacitance of93.2%.In addition,supercapacitors exhibit excellent stability after repeatly cutting/healing of the electrolyte,and antifreezing resistance which could still work at a temperature of-50℃.Therefore,this work greatly expanded the application prospects of flexible supercapacitors.4.In order to solve the problem that the integrated devices of flexible supercapacitors and sensors were not lightweight and portable enough,a self-powered all-in-one integrated sensing system based on hydrogel was designed.Also using sodium alginate as basic framework,acrylamide and acrylic acid as the monomer,divalent zinc ion(Zn²⁺)as the conductive ions prepared a double-network ionic conductive hydrogel by free radical polymerization.The hydrogel had excellent mechanical properties（elongation at break of 1425.07%,and tensile strength of 0.345MPa）due to the synergistic effect of ionic coordination interaction and hydrogen bonding between Zn²⁺and carboxyl groups.Flexible asymmetric supercapacitors based on the hydrogel electrolyte showed excellent energy storage properties.The voltage window broadened to 2.2 V,the capacitance was 691.99 F g^-1 at 0.6 A g^-1,and the maximum energy density was 310.75 Wh kg^-1 at the power density of 530.46 W kg^-1.At the same time,the supercapacitor showed great cycle stability and high coulombic efficiency.Meanwhile,the supercapacitor exhibited excellent flexibility and stable electrochemical performance.Thus,the flexible supercapacitor can well supply energy for the self-powered integrated integrated sensing system.The hydrogel strain sensor had high strain sensitivity（GF=3.32）,fast response performance（165 ms）and excellent durability（1000 cycles）,low detection limit（0.5%）,which could sensitively detect the movements of human fingers,wrists,elbows and knees.The flexible supercapacitor with high capacitance and long self-discharge time,as internal energy storage units,can achieve long-term continuous energy supply to the sensor part（>2.9h）.They were designed and integrated into a self-powered sensing system without external wires,and the device achieved the goal of lightweight and portable.The system could detect different strains,human movements and pronunciations.