Electrospinner: Flexible polymer-based composite thermoelectric devices: regulating fiber morphology and improving thermoelectric performance!

Background

Flexible thermoelectric devices can convert heat released by the human body into electrical energy and have broad application prospects in the field of wearable electronics. Covalent organic frameworks (COFs) have the advantages of diversified structural design, precisely adjustable micro/nanoporous structure, rich physical and chemical activity, and low thermal conductivity; carbon nanotubes (SWCNTs) show excellent mechanical and electrical properties, but poor dispersion during the preparation process. In recent years, COFs, as a type of crystalline porous polymer materials, have potential application prospects in the field of thermoelectrics.

Recently, the research group of Professor Du Yong of Shanghai University of Technology and the research group of Professor Zhang Fan of Shanghai Jiaotong University published a research result entitled "Nano-/Micro-fiber Engineering of Vinylene-Linked Polymeric Frameworks for Flexible Free-Standing Thermoelectric Films" in Advanced Fiber Materials. This work adopts the Knoevenagel condensation strategy developed by the team to obtain a series of vinylene bridged triazine core polymer framework nano/micro fibers, and effectively regulates the diameter and length of the fiber by changing the copolymer monomer, and then prepares a self-supporting flexible film with SWCNT composite, and studies its thermoelectric behavior. For the first time, the influence of COFs fiber morphology on the performance of flexible composite thermoelectric materials and devices was revealed, providing new ideas for the development of flexible polymer-based composite thermal Nanofiber Membranes.

The main point of this paper

COFs materials are a class of crystalline porous polymer materials with rich chemical structures and high specific surface areas, and have broad development prospects in the fields of gas separation/adsorption, catalysis, energy storage and optoelectronics. Among them, two-dimensional dynamic ethylene bond-linked COFs show excellent stability and π electron delocalization behavior. Combined with its low thermal conductivity and precise functional modification, a series of ethylene bond-linked triazine core polymer framework nano/micro fibers with different lengths and diameters were prepared by Knoevenagel condensation reaction with 2,4,6-trimethyl-1,3,5-triazine as the core monomer. Through vacuum filtration, a flexible self-supporting COF/SWCNTs composite film was constructed with SWCNTs. As a thermoelectric single arm, a silver electrode was connected in series to assemble a flexible thermoelectric device.

Based on the previous work, the morphology of the dynamic ethylene bond-bridged COFs fiber was regulated by comonomers (or functional building blocks), as shown in Figure 2. Among them, the high rigidity, small steric hindrance and poor charge properties of the triazine unit can provide strong π-π interactions, which can act as the main driving force to induce and promote the assembly of the two-dimensional conjugated skeleton COFs in a face-to-face stacking mode to form long-range ordered fibers. The proportion of the triazine unit in the two-dimensional conjugated skeleton is flexibly adjusted by the aromatic ring unit, thereby affecting the assembly behavior of the two-dimensional sheet and changing the morphology of the fiber (Figure 2c). On this basis, the content and dosage of COFs and SWCNTs are optimized, and alcohol is used as the solvent. The uniform dispersion system is obtained by ultrasound assistance, and vacuum filtration is used to obtain COFs/SWCNTs flexible self-supporting composite Nanofiber Membrane of different thicknesses and sizes.

The thermoelectric properties of the COF/SWCNTs composite Nanofiber Membrane are further analyzed, as shown in Figure 3. At 300 K, the conductivity of g-C18N3-COF/SWCNTs composite Nanofiber Membrane is as high as 643.94 S/cm; when the temperature rises from 300 K to 420 K, the Seebeck coefficient of the composite Nanofiber Membrane increases, while the conductivity gradually decreases, and the power factor increases with the increase of temperature; at 420 K, the power factor reaches 68.93 μW/(m·K2), and after being bent 1000 times with a radius of 4 mm, the conductivity still maintains more than 90% of the initial value, showing excellent flexibility

In summary, this work uses the Knoevenagel dynamic condensation polymerization developed by the team to prepare vinylidene-linked triazine core polymer framework fibers. Through the design and configuration of copolymer monomers (or building blocks), the diameter and length of nano/micron fibers were effectively regulated. Combined with the vacuum filtration process, a flexible self-supporting COF/SWCNTs composite film was prepared, and a flexible thermoelectric device was assembled as a thermoelectric arm, achieving a maximum power factor of 68.93 μW/(m·K2). This work reveals the relationship between the morphology of COF fibers and their thermoelectric properties, and provides new ideas for the development of high-performance flexible polymer-based composite thermoelectric Nanofiber Membranes

Electrospinning Nanofibers Article Source:

https://link.springer.com/article/10.1007/s42765-024-00477-7