Preparation And Performance Study Of PEDOT:PSS Thermoelectric Fibers

The demand for flexible wearable devices is exploding due to the rapid growth of smart wearable electronics in areas such as soft robotics,the Internet of Things,and medical diagnostic devices.The continuous use of flexible wearable electronics requires a flexible power system that can provide a continuous and stable power supply.Thermoelectric（TE）devices can directly convert thermal energy into electrical energy and use the temperature difference between the human body and the environment to achieve sustainable and stable power generation,providing an effective solution for the development of wearable power supply systems.Fiber-based thermoelectric materials are receiving more and more attention because of their advantages of being soft,lightweight,comfortable to wear,diverse structures,and high-density integration.Compared with conductive fibers prepared from metal or inorganic carbon materials,poly（3,4-ethylenedioxythiophene）:poly（styrene sulfonic acid）（PEDOT:PSS）has received much attention because of its high electrical conductivity,water solubility,easy processing,non-toxicity,and good environmental stability.At present,there are still many problems with PEDOT:PSS fibers,such as poor molding of pure PEDOT:PSS fibers,difficulty in forming continuous uniform filaments,insufficient conductivity of untreated fibers,and the flexibility,strength,and elasticity of fibers cannot be adapted to practical applications.PEDOT:PSS/EMIM DCA fibers were prepared in this study using the wet spinning method to meet practical applications and develop high-performance thermoelectric fibers.PEDOT:PSS was modified using an ionic liquid,and the effect of ionic liquid（EMIM DCA）on the spinning solution spinnability and viscosity was investigated.The post-treatment of PEDOT:PSS/EMIM DCA thermoelectric fibers by the impregnation-stretching method was used to explore the preparation process,fiber morphology,thermoelectric and mechanical properties of the thermoelectric fibers,and the chemical composition and structural changes of PEDOT:PSS/EMIM DCA fibers were analyzed.Based on this,fiber-based thermoelectric generators were prepared and validated for temperature-sensing applications by stitching the fibers to fabrics,and their electrical and sensing properties were tested.The results show that EMIM DCA can increase the viscosity of the spinning solution and accelerate the molding speed of the fibers in the solidification bath.The electrical conductivity of the fiber after sulfuric acid immersion-stretching treatment was 4288 S·cm^-1,the power factor was 85.5μW·m^-1·K^-2,and the tensile strength reached 956 MPa due to the ion-exchange reaction between sulfuric acid and PEDOT:PSS,which removed the excess PSS.Meanwhile,the stretching caused the macromolecules in the fiber to stretch from a convoluted conformation to a linear one,which increased the carrier migration.In addition,the P/ED fibers exhibit excellent flexibility,ultra-low bending radius（0.1 mm）,and Young’s modulus（3.8 GPa）.The fibers remained stable in terms of Seebeck coefficient and（S）resistance（R）and electrical conductivity after 1000repeated bends and in the range of 50%to 90%humidity.Based on this,fiber-based thermoelectric generators were prepared by stitching the fibers to the fabric,and their power reached 421.7 p W.To further improve the fiber elasticity and structure,P-N thermoelectric fibers were prepared in this study by compounding PEDOT:PSS/SWCNT with elastomeric polyurethane based on microfluidic spinning technology.Firstly,P-type PEDOT:PSS/SWCNT/PU fibers were prepared using isopropyl alcohol/deionized water as a coagulation bath,and the optimal ratio of spinning slurry and preparation process were explored to analyze and characterize the thermoelectric fiber morphology,conductive mechanism,thermoelectric and mechanical properties.Then,P-type fibers were modified with PEI to prepare n-type PEDOT:PSS/SWCNT/PU fibers.Respiratory monitoring masks were prepared and tested using P-N fibers.The strain sensing behavior of PEDOT:PSS/SWCNT/PU fibers were investigated,and the working mechanism of the strain sensor was studied by tunneling theory.The results showed that the incorporation of SWCNT improved the fiber orientation,and the conductivity and Seebeck coefficient of the fiber increased with the increase of SWCNT content,and the best performance was obtained with 26 wt%SWCNT content.At this time,the fiber conductivity was 67.16 S·cm^-1,the Seebeck coefficient was 32.08μV·K^-1,and the power factor was 6.91μW·m^-1·K^-2.This basis was explored for the N-type spinning solution,which performed best with a conductivity of 69.26 S·cm^-1 and a Seebeck coefficient of-22.42μV·K^-1 when the relative PEI content was 30%.The PEDOT:PSS/SWCNT/PU fibres were converted from P-type to N-type thermoelectric fibres due to the self-assembly effect of the cationic PEI and anionic PSS in the spinning solution.Finally,the fibers with P-N composite thermoelectric fibers were successfully prepared using microfluidic wet spinning with a stress of 15.3 MPa and a strain of 69.3%.The output voltage of the integrated flexible thermoelectric generator was 0.452 m V and the output power was 2.11 p W at a temperature difference of 20 K.The fibers had appropriate sensing van and sensitivity（3.8,38.0,and 87.7）,a fast response time（416 ms、500 ms and 583 ms）,and showed good repeatability and excellent durability in multi-cycle tests.Finally,the sensors were prepared by fiber integration in respiratory monitoring,energy harvesting,and strain sensors,demonstrating their potential applications in human health monitoring.