Preparation And Thermoelectric Properties Of Tellurium Based Inorganic Nanostructures/PEDOT:PSSs Composite Film

Thermoelectric?TE?material is a promising candidate for energy conversion which can directly convert heat into electricity or verse vice through the transport of carriers.With the rapid development of wearable devices and flexible electronic devices,there is an increasing demand for energy that can continuously provide electricity.Besides,with the development of miniaturization of devices,there is an increasing demand for miniaturized energy devices,thus promoting the development of flexible TE materials and devices.Tellurium?Te?nanowires have attracted much attention owing to their high TE responses in TE research field.Compared to other materials,Te owns relatively high Seebeck coefficient near room temperature.While its low electrical conductivity limits the enhancement of power factor as well as the ZT value.Silver telluride?Ag₂Te?is one of the most important n type TE materials near room temperature,and the thermal conductivity of Ag₂Te near room temperature is only 0.1-0.4 W/mK.Using Te as the template,Ag₂Te can be synthesized at room temperature,the preparation of Ag₂Te is simple and easy to be scaled up.In recent years,the rapid development of conducting polymer materials and carbon nanostructured materials has provided novel ideas for the development of traditional TE materials.Conducting polymers possess variesties of advantages,such as low thermal conductivity,easy to synthesis,abundant,low cost and so on.In addition,the electrical conductivity of conducting polymers can be adjusted by doping/de-doping.However,since its Seebeck coefficient is relatively low,power factors as well as ZT values of conducting polymers are relatively low.The maximum ZT value of pure conductive polymers currently reported is only 0.42,which is much lower than that of the conventional inorganic TE materials.In this paper,in order to prepare flexible TE thin films with good TE performance as well as superior flexibility,composite films are prepared by combining Te/Te-based nanostructured materials with polymers.The obtained composites films own good TE performance and superior flexibility through the synergistic effects.In addition,the TE performance can be improved by different post-treatments.The main research contents are shown as follows:?1?PEDOT:PSS coated Te?PC-Te?nanorods are prepared by in-situ polymerization method,both the dispersibility in solution and the stability in the environment of the obtained PC-Te nanorods are improved.PC-Te/PEDOT:PSS composite films with different PC-Te content are prepared by drop-coating method,The maximum power factor of the untreated PC-Te/PEDOT:PSS composite film containing 87 wt%PC-Te is 51.6?W/mK².After H₂SO₄ treatment,electrical conductivity of the composite films increases while the Seebeck coefficient decreases.When the concentration of H₂SO₄ is 12 M,the maximum power factor of the composite film containing 90 wt%PC-Te is 141.9?W/mK² at room temperature,which is 2.75 times as high as that of the maximum power factor of the untreated samples.XPS and GIWAXS analysis reveal that H₂SO₄ treatment selectively removes the insulating PSS units,PEDOT chains are rearranged and the?-?coupling effects between PEDOT molecular chains are enhanced,thereby increasing the carrier concentration.A TE generator prototype consisting of 9 legs is fabricated using the optimized sample.The maximum output power and power density produced from the prototype are 47.7 nW and 57.2?W/cm²,respectively,at the temperature difference of 40 K.?2?Single-walled carbon nanotubes?SWCNTs?/PC-Te free standing composite films are prepared by a simple vacuum assisted filtration method,and the effects of SWCNT with high content??10 wt%?on TE properties of the composite films are investigated.Before the H₂SO₄ treatment,the addition of SWCNTs apparently improves the electrical conductivity while reduces the Seebeck coefficient of the composite films,resulting in a reduction of the power factor.After H₂SO₄ treatment,the electrical conductivity is further improved,in which the PEDOT:PSS coating layer plays an important role,and the Seebeck coefficient decreases slightly.As a consequence,an optimized power factor of 104?W/m K²?corresponding to the electrical conductivity of 332 S/cm and Seebeck coefficient of 56?V/K,respectively?is obtained at 300 K for the sample containing 70 wt%SWCNTs.The dependence of electrical conductivity of the optimal composite film on temperature conforms to the one-dimensional?1D?variable range hopping model.The maximum open circuit voltage and output power delivered from the prepared prototype are 5.6 mV and 53.6nW,respectively,at a temperature difference of 44 K,correasponding to the power density of 21.4?W/cm².?3?Using the PC-Te nanorods as templates,we further synthesize Ag₂Te/PEDOT:PSS nanorods by introducing Ag into the PC-Te nanorods.Ag₂Te/PEDOT:PSS composite film on flexible polyethersulfone?PES?substrate is prepared using vacuum filtration followed by PTFE-aided cold pressing.The electrical conductivity of the composite film increases with Ag/Te ratio increasing.When the molar ratio of Ag/Te increases from 1:1 to 2:1,the conductivity changes from p-type to n-type.When the molar ratio of Ag/Te increases from 2:1 to 4:1,the Seebeck coefficient is negative and the absolute value of the Seebeck coefficient decreases with Ag/Te molar ratio increasing.The cold pressing pressure plays a great effect on the electrical conductivity,i.e.electrical conductivity of the composite film increases with cold pressure increasing,while the Seebeck coefficient changes little.As a consequence,when the molar ratio of Ag/Te is 2:1,after cold pressing at 35 MPa,electrical conductivity and Seebeck coefficient of the composite film are 369.3 S/cm and-62.3?V/K,respectively,at 300 K,resulting in a power factor of 143.3?W/mK².With temperature increasing,the absolute value of the Seebeck coefficient increases,while the electrical conductivity increases first and then decreases.As a concenquence,the maximum power factor of 244.4?W/mK² is obtained at 393 K,corresponding to the electrical conductivity of 379 S/cm and Seebeck coefficient of-80.3?V/K,respectively.The improvement of TE performance of the composite film is mainly attributed to the interfacial effect between PEDOT:PSS and Ag₂Te nanorods as well as the dense structure of the mechanical pressed composite film.In addition,the composite outstanding flexibility and tensile property.A TE generator prototype consisting of eight legs is assembled.The open circuit voltage and maximum output power of 20.3 mV and 209.4 nW are obtained,respectively,at a temperature difference of 30.3 K,corresponding to the power density of 141.5?W/cm².?4?Using polyvinyl pyrrolidone?PVP?/Te as the template,PVP coated Ag₂Te nanowires are synthesized When the initial molar ratio Ag/Te is 6:1,flexible PVP/Ag/Ag₂Te ternary composite film is prepared on nylon membrane using facile vacuum filtration followed by heat treatment.Ag nanoparticles are formed by separation from Ag-rich Ag₂Te nanowires during the treatment,and the Ag₂Te nanowires and Ag nanoparticles are bonded with PVP layers.Power factor of the composite film is 216.5?W/mK² at 300 K,corresponding to electrical conductivity and Seebeck coefficient of 360.9 S/cm and-77.5?V/K,respectively.As the temperature increasing,the Seebeck coefficient increases while the electrical conductivity increases first and then decreases.As a consenquence,the maximum power factor of 619.3?W/mK² is obtained at 413 K.At low temperature,Ag₂Te is monoclinic phase,and the carrier transport is controlled by the interface effect.At high temperature,Ag₂Te is transition from monoclinic phase into cubic phase,and the carrier transport is changed to be controlled by the electron-phonon scattering mechanism.Bending tests demonstrates superior flexibility of the hybrid film and the electrical conductivity decreases by only 9.4%after 1000 bending times along a rod with a radius of 5 mm.Additionally,a TE prototype composed of five legs is assembled.The open circuit voltage and maximum output power of 16.5 mV and 469nW,respectively,are obtained at a temperature gradient of 39.6 K,corresponding to a maximum power density of 341?W/cm².