Preparation Of New High-performance Thermoelectric Composites Based On Single-walled Carbon Nanotubes And Organometallic Framework Materials

With the vigorous development of the national economy,society’s demand for energy is getting higher and higher.Thermoelectric(TE)materials can realize direct energy conversion between electricity and heat,on the basis of inner carrier motion in solids.Being emerging green energy materials,organic/inorganic thermoelectric composites have attracted much current interest and witnessed a rapid development due to their diverse advantages,such as low density,low thermal conductivity,high flexibility,and highly tunable molecular structure.By adjusting the organic structure and fabrication of their composites,the TE performance can be significantly enhanced.Seebeck coefficient(S),electrical conductivity(σ)and thermal conductivity(κ)play the important role in materials’ TE performance.However,S andσ are always interdependent,i.e.the increase of one parameter usually leads to the reduction of the other.In this work,we mainly study the thermoelectric properties of metal-organic frameworks/CNT(MOF/CNT)composite thermoelectric materials,and then we propose a novel method to simultaneously enhance the S and the σ by construction of MOF structure.Our method benefits the significant enhancement of TE performance.The main contents are as follows:1.A high-performance self-supporting thermoelectric composite flexible film was prepared by in-situ synthesis using a high-Seebeck coefficient MOF material and a high-conductivity SWCNT.First,SWCNTs were acidified with a mixed acid(nitric acid: sulfuric acid = 3 m L: 1 m L)to further increase the conductivity of SWCNTs and graft carboxyl groups on the surface of SWCNTs;then MOF crystals were grown in situ centering on the carboxyl groups on the surface of SWCNTs.The prepared mixture is finally subjected to a series of steps such as suction filtration,washing,drying and annealing to finally obtain a thermoelectric composite film.The micro structure and properties of the samples were characterized by SEM,TEM,Hall effect and Seebeck coefficient apparatus.Finally,we successfully prepared a high-performance MOF/CNT thermoelectric composite flexible film.It is important to note that the conductivity(825.7 ± 12.0 S cm-1)and the power factor(255.6 ± 11.8 μW m-1K-2)of the materials we reported are among the highest in the MOF-based thermoelectric materials.2.We explored the effect of annealing on the thermoelectric properties of MOF/CNT materials and explained its mechanism.First,the optimal conditions for annealing are sought by TGA and control variable methods.Then,TEM and FESEM were used to observe the changes of microstructure and morphology of the materials before and after annealing.Combined with FIIR,XRD and Raman spectroscopy,the reason why annealing treatment can significantly improve the thermoelectric properties of MOF/CNT materials is explained.Finally,the annealing treatment is extended to other MOF/CNT materials,and the same results are obtained.We predict that the annealing treatment is an effective and simple strategy to improve the thermoelectric properties of MOF-based materials.