Constructing Cellulosic Ionogel And Study On Strengthened Performances

Ionic conductors,intrinsically conductive materials,which are made of ionic liquids（ILs）and polymers,show the characteristics of conductivity,softness,and tunable mechanical properties.Ionogels are applied in various fields,including health monitoring,skin sensing,soft robots,touch screens,and others.Green ionogel,based on natural cellulose and ILs,is a major topic of research.Currently,the deconstruction of cellulosic fiber via ILs is time-consuming and easy to induce the degradation of cellulose molecules.Furthermore,the developed cellulose ionogel exhibits poor mechanical and electrical properties,as well as insufficient stability.In this study,natural cellulose is the raw material and 1-allyl-3-methylimidazolium chloride salt（[AMIM]Cl）is the deconstruction solvent.Microwave-ILs technology was designed for superfast yet completed deconstruction of the cellulosic fibers to cellulose molecules,further,constructing cellulose ionogel.By constructing multiple forces in the ionogel system,the mechanical properties and ionic conductivity of cellulose ionogel were considerably improved.The surface densification strategy was designed to further strengthen the mechanical properties and long-term stability of cellulose ionogel,and realize its applications of mechanical-electrical signals and thermal-electric conversion.（1）The microwave（MW）-assisted ILs strategy was designed to trigger the high-frequency rotation of ILs ions in the microwave radiation field,achieving the controllable and rapid increase of system temperature;meanwhile,the vibrate ions effectively collide the cellulose fibers to realize them rapid deconstruction in the system.The results show that MW radiation enables the high-frequency swing of ILs,which gives rise to a rapid and homogeneous improvement of the system temperature,for example,MW heats the ILs system from～27℃ to～110℃ in just 4 s,while conventional technology（CT）takes 900 s for improving the temperature and leads to a non-uniform distribution of the system temperature,such as～112℃ for the outer sample and～107℃ for the inner sample.In such an MW irradiation field,the ions of ILs with violent rotation and vibration are capable of strongly attacking the cellulose fibers,inducing the characteristic behavior of the deconstruction of"fragmentation".In addition,MW-ILs technology makes sure the integrity of cellulose molecular structure,and the intrinsic viscosity of cellulose after deconstruction via MW-ILs strategy is still much higher than that of CT,laying the foundation for the construction of high-performance cellulose ionogel.（2）Via introducing carboxyl functional groups in cellulose molecules and incorporating ZnCl₂ into the system,we prepared a high strength cellulose ionogel,where the multiple forces,including coordination interaction between cellulose carboxyl groups and Zn²⁺,hydrogen bonding and ion-dipole dynamic bonding,were constructed in our cellulose ionogel system.As a result,the developed cellulose ionogel demonstrates high tensile strength（4.46 MPa）,and its elastic modulus and toughness increased by 15 and 10 times,respectively.In addition,the interaction between ZnCl₂ and cellulose molecules can construct the confinement structure between cellulose molecules,allowing the transport of small-size Cl^-,while repelling the large[AMIM]⁺and Zn²⁺,resulting that excellent ionic conductivity(up to 67.43 mS cm^-1)of cellulose ionogel.Besides,the cellulose ionogel exhibits the advantages of high transparent（94%）,recyclable,self-healing and freeze-resistant（-103℃）.The designed cellulose ionogel functions as a rare N-type ionic thermoelectric（iTE）material to convert low-grade thermal energy into high-quality electrical energy（～110 mV atΔT=36 K）.（3）Surface densification strategy of cellulose ionogel is designed via removing ILs on the surface of cellulose ionogel following by vacuum annealing,in which the cellulose ionogel has a multi-layer structure,including a 3D network inner component including ILs and cellulose molecules,and a dense surface just composed of cellulose molecules.Such special structure endows cellulose ionogel tunable yet superhigh mechanical properties（6.27～182.04 MPa）,excellent ionic conductivity(13.57～69.19 mS cm^-1)and improved thermoelectric properties（Seebeck coefficient and ZT_i increased by 2.7 times and 4 times,respectively）.Furthermore,the cellulose ionogel demonstrated outstanding stability（the tensile strength and ionic conductivity are basically unchanged after 60days in the air）due to the densified surface,which eliminates the impact of airborne water molecules on the ionogel.The ionic thermoelectric capacitor constructed based on the cellulose ionogel can realize the all-day thermoelectric energy conversion in a real environment.