Preparation Of Chitosan-based Conductive Hydrogels For Strain Sensor And Supercapacitor Application

In the past few years,flexible sensors have attracted increasing attention and were used for electronic skin,human motion monitoring,biosensing,and human-computer interaction,because of their novelty and diversity.In addition,compared with flexible batteries,supercapacitors possess good energy density（E_d）,high power density（P_d）,fast charge and discharge rate,low cost,and long-term life.Hydrogels are hydrophilic polymers with a three-dimensional network structure.It has been widely applied in flexible sensors and becomes one of the ideal electrolyte materials for flexible supercapacitors due to its superior mechanical properties and excellent flexibility.However,traditional hydrogels exist the problem of poor mechanical properties and seldom show self-healing behavior.It is difficult to maintain the electrochemical performance while withstanding periodic loads.Therefore,the development of self-healing conductive hydrogels with high strength and sensitivity has great application value.In this paper,we designed and synthesized two kinds of self-healing hydrogels with double crosslinking by adding different carbon-based materials（Chinese traditional ink or Ti₃C₂ MXene）into the chitosan matrix,and introducing ion cross-linking.Their structure and properties were characterized,and their applications in hydrogel-based strain sensors and supercapacitors were investigated.Specific research contents and results are as follows:（1）A cost-effective PAA-Fe³⁺/CS-ink hydrogel with high conductivity,mechanical properties,and sensitivity is fabricated by replacing carbon materials such as carbon black,carbon nanotubes,and graphene with Chinese traditional ink made of tung oil soot,and introducing PAA-Fe³⁺system.The mechanical properties,electrical conductivity,and self-healing ability of the hydrogels were improved through physical cross-linking and simple soaking strategies.Specifically,chitosan（CS）,ink,acrylic acid（AA）,and Fe Cl₃·6H₂O were polymerized into PAA-Fe³⁺/CS-ink pre-gel,then was soaked in saturated sodium chloride solutions to obtain PAA-Fe³⁺/CS-ink hydrogel.The hydrogel exhibits good mechanical properties（tensile strength and toughness are0.92±0.10 MPa and 2.53±0.20 MJ/m³,respectively）and self-healing ability.Besides,the hydrogel has good ion conductivity（6.69±0.19 S/m）because of the presence of Fe³⁺,Na⁺,and Cl^-.The introduced ink provides the photo-thermal responsiveness to PAA-Fe³⁺/CS-ink hydrogel.Based on the excellent performance of PAA-Fe³⁺/CS-ink hydrogel,it is applied to a strain sensor,which manifests high sensitivity（GF=2.66）and stability of strain and monitors the motion in different parts of the human body.The supercapacitor assembled using the PAA-Fe³⁺/CS-ink hydrogel as electrolytes shows a high specific capacitance of 257.08 F/g,with a maximum energy density of34.57 Wh/kg,a power density of 3.87 k W/kg,and capacitance retention rate of 86.59%after 5000 cycles.Besides,the electrochemical performance of hydrogel-based supercapacitors has hardly changed at different bending angles.The assembled supercapacitor can withstand mechanical deformation without affecting its use,and shows excellent flexibility.（2）The Zn²⁺-CS/PAM-MXene hydrogel with high electrical conductivity,mechanical properties,and self-healing capacity was synthesized using combined with physical and chemical cross-linking.Zn²⁺-chitosan（CS）complex was introduced into the polyacrylamide system to improve the mechanical properties of Zn²⁺-CS/PAM-MXene hydrogel,and the new two-dimensional material Ti₃C₂ MXene was added to further enhance the mechanical performances and conductivity of Zn²⁺-CS/PAM-MXene hydrogel.The effects of MXene addition on the properties of composite hydrogels were compared.The MXene improved the properties of composite hydrogels,resulted in the Zn²⁺-CS/PAM-MXene hydrogels exhibit better performance.The prepared Zn²⁺-CS/PAM-MXene hydrogels show good mechanical properties（tensile strength 410 k Pa）,electrical conductivity（2.58±0.317 S/m）,and self-healing properties.The supercapacitor assembled by Zn²⁺-CS/PAM-MXene hydrogels show higher specific capacitance（262.11 F/g,current density of 0.5 m A/cm²）and capacitance retention（70.91%,after 5000 cycles）.The maximum energy density and power density are 33.51 Wh/kg and 1.99 k W/kg,respectively.The electrochemical performance of Zn²⁺-CS/PAM and Zn²⁺-CS/PAM-MXene hydrogel electrolyte-based supercapacitors was studied at different bending angles,and their CV and GCD curves are similar size with little change in electrochemical performance,indicating the device has good physical deformation stability.