Preparation And Thermoelectric Properties Of MOFs-derived Carbon Wrapped Selenide / PEDOT:PSS Composites

With the rapid development of industry and the increasing demand for energy,the excessive burning of fossil fuels has caused energy shortages and environmental pollution,which has become a major problem facing human beings.Thermoelectric（TE）materials can realize the mutual conversion of thermal energy and electrical energy,and have great application potential in the field of TE power generation and TE refrigeration.As a typical organic TE material,organic conductive polymer poly（3,4-ethylenedioxythiophene）:polystyrene sulfonic acid（PEDOT:PSS）has became a research hotspot due to its high conductivity,low thermal conductivity and good film forming property.At the same time,metal-organic frameworks（MOFs）have low thermal conductivity due to their porous structure,and selenides have high Seebeck coefficients Fabricating organic-inorganic composite materials through appropriate processes is an effective way to improve the TE properties by using the advantages of each component.Herein,flexible free-standing TE composite films were prepared by vacuum filtration combining cold pressing process,using organic conductive polymer PEDOT:PSS as matrix,and MOFs-derived carbon coated selenides（Bi_xSe_y@C,γ-In₂Se₃@C,Mo Se₂@C）as inorganic filler phase,respectively.The composition of inorganic powders was analyzed by X-ray diffractometer（XRD）,scanning electron microscope（SEM）and transmission electron microscope（TEM）.The composition,morphology and element valence of flexible free-standing composites films were characterized.The TE properties of composite films were measured by MRS-3 film TE tester.The effects of the type and content of carbon coated selenide powders derived from MOFs on the TE properties of composite films were investigated.The main research contents are as follows:（1）Bi-MOFs powders were firstly synthesized by a solvothermal method.After the Bi-MOFs powders were transformed into carbon coated bismuth selenide（Bi_xSe_y@C）nanosheets by selenidation strategy,the flexible free-standing Bi_xSe_y@C/PEDOT:PSS（BSCP）composite films were prepared by a vacuum filtration combining with cold pressing process,using conductive polymer PEDOT:PSS as the organic phase.Obtained Bi_xSe_y@C powders has a hexagonal sheet shape with average size of 7.32μm.When the MOFs-derived Bi_xSe_y@C content is 70%,BSCP-70composite film got the highest power factor of 11.23μW m^-1 K^-2at 340 K.This composite film exhibited good flexibility and low resistance change rate of 6.52%after500 bending cycles with a radius of 4 mm.（2）In-MOFs powders were synthesized by a solvothermal method.Carbon coated indium selenide（γ-In₂Se₃@C）powders were obtained from In-MOFs powders after undergoing a selenidation pyrolysis treatment.Flexible free-standingγ-In₂Se₃@C/PEDOT:PSS（ISCP）composite films were fabricated by vacuum filtration combining with cold pressing,using PEDOT:PSS as the organic phase.The influence ofγ-In₂Se₃@C powder content on TE properties of the composite films was studied.Asγ-In₂Se₃@C content increase,the thickness of ISCP composite TE films increased and the conductivity decreased.When the MOFs-derivedγ-In₂Se₃@C content is 60%,ISCP-60 composite film got the highest power factor of 17.06μW m^-1 K^-2 at 320K.（3）Using Molybdenum ion(Mo⁵⁺)as the metal body and trimesic acid as the organic ligand,Mo-MOFs powders were synthesized by a solvothermal method.Carbon coated molybdenum selenide（Mo Se₂@C）microspheres were prepared form Mo-MOFs powders after undergoing a selenidation pyrolysis treatment.Similarly,using PEDOT:PSS as the organic phase,flexible free-standing Mo Se₂@C/PEDOT:PSS（MSCP）composite TE films were fabricated by vacuum filtration combining with cold pressing.The influence of MOFs-derived Mo Se₂@C content on the distribution,micro-structure and TE properties of the MSCP composite films were explored in detail of.It is found that the Mo Se₂@C microspheres are uniformly distributed in the whole composite films.With the increase of Mo Se₂@C microspheres content,the thickness of MSCP composite TE films increased from 25.7μm to 28.9μm.When the Mo Se₂@C content is 70%,MSCP-70 composite film acquired the highest power factor of 27.06μW m^-1 K^-2at 360 K.The composite film displayed good flexibility and the resistance change rate was only 6.06%after 500 bending cycles with a radius of 4 mm for the MSCP-70 composite film.