Preparation and performance research of carbon nanotube fiber-based thermoelectric devices
With the continuous development of flexible wearable electronic devices,thermoelectric(TE)devices,which can harvest energy from human body heat,are highly demanded as a potential wearable energy device.Wearable TE devices based on Seebeck effect,which can convert human body heat into electrical energy,are considered as one of the effective solutions to solve the energy problem of wearable electronic devices.TE devices can directly convert waste heat into electrical energy,which greatly improves energy utilization.And wearable TE devices have significant advantages in human body applications,such as flexibility,environmental protection,and sustainability.In the past few decades,people have been working on developing flexible TE devices.However,traditional inorganic thermoelectric materials have limitations such as heavy weight,high rigidity,and toxicity,which limit their application in flexible devices.In addition,conventional two-dimensional(2D)architecture thermoelectric devices can only harvest thermal energy on a flat surface,making it difficult to utilize the temperature difference in the vertical gradient formed between the human body and the environment.In order to improve the flexibility and TE performance of TE devices,and to better utilize the vertical temperature difference between the human body and the environment,carbon nanotube fibers with high conductivity,flexibility,and light texture are used as the matrix.Oleylamine(OA)and FeCl3 were selected as N-type and P-type dopants,and a simple impregnation method was used to perform N-type and P-type doping on the carbon nanotubes.By reasonably regulating the concentrations of OA solution and FeCl3 solution,the optimal concentration was selected for doping carbon nanotube fibers,resulting in the preparation of P-N structured carbon nanotube fibers.Due to the lack of metal electrode connections,carbon nanotube fibers exhibit excellent TE performance.Then,the carbon nanotube fibers with P-N structure were combined with a three-dimensional(3D)mold to prepare flexible TE devices with 3D structure.The successful doping of OA and FeCl3 was confirmed by SEM,EDS,and Raman characterization,and the impact of OA and FeCl3 on the structure of carbon nanotube fibers was analyzed.In addition,the study tested the effects of different concentrations of OA and FeCl3 solutions on the electrical conductivity,Seebeck coefficient,and power factor of carbon nanotube fibers,as well as the TE performance of TE devices on the human body.The study also investigated the potential of thermoelectric devices as flexible temperature sensors.The results indicate that when a 0.2 g/mL OA solution was used as the N-type dopant,the Seebeck coefficient of N-type carbon nanotube fibers was-69.7 μV/K,and the power factor was approximately 552.9 μW/(m·K2).On the other hand,when a 2.5 mmol/L FeCl3 solution was used for P-type doping of the carbon nanotube fibers,the Seebeck coefficient was 62.2 μV/K,and the power factor was around 431.4 μW/(m·K2).Furthermore,the N-row carbon nanotube fibers exhibited excellent air stability for over108 hours.Additionally,the flexible TE device was capable of generating an open-circuit voltage of 13 mV and an output power of approximately17.9 nW at ΔT =7℃,while also demonstrating favorable cycle stability.By utilizing OA and FeCl3 as dopants for N-type and P-type,respectively,a one-step doping process was employed to successfully fabricate carbon nanotube fibers with a P-N structure.These fibers were then integrated with a three-dimensional mold to create 3D TE devices.This approach enhanced the electrical conductivity and Seebeck coefficient of the carbon nanotube fibers,consequently improving the TE performance.Moreover,the TE devices based on carbon nanotube fibers exhibited superior flexibility and the ability to conform well to human skin,thereby offering valuable insights for the development of TE fibers and TE devices suitable for wearable applications.