Abstract
This article provides an overview on manufacturing composite carbon nanofiber-based aerogels through freeze casting technology. As known, freeze casting is a relatively new manufacturing technique for generating highly porous structures. During the process, deep cooling is used first to rapidly solidify a well-dispersed slurry. Then, vacuum drying is conducted to sublimate the solvent. This allows the creation of highly porous materials. Although the freeze casting technique was initially developed for porous ceramics processing, it has found various applications, especially for making aerogels. Aerogels are highly porous materials with extremely high volume of free spaces, which contributes to the characteristics of high porosity, ultralight, large specific surface area, huge interface area, and in addition, super low thermal conductivity. Recently, carbon nanofiber aerogels have been studied to achieve exceptional properties of high stiffness, flame-retardant and thermal-insulating. The freeze casting technology has been reported for preparing carbon nanofiber composite aerogels for energy storage, energy conversion, water purification, catalysis, fire prevention etc. This review deals with freeze casting carbon nanofiber composite materials consisting of functional nanoparticles with exceptional properties. The content of this review article is organized as follows. The first part will introduce the general freeze casting manufacturing technology of aerogels with the emphasis on how to use the technology to make nanoparticle-containing composite carbon nanofiber aerogels. Then, modeling and characterization of the freeze cast particle-containing carbon nanofibers will be presented with an emphasis on modeling the thermal conductivity and electrical conductivity of the carbon nanofiber network aerogels. After that, the applications of the carbon nanofiber aerogels will be described. Examples of energy converters, supercapacitors, secondary battery electrodes, dye absorbents, sensors, and catalysts made from composite carbon nanofiber aerogels will be shown. Finally, the perspectives to future work will be presented.
Highlights
As synthetic ultralight porous materials, aerogels were first reported by Kistler [1,2] in the early 1930s
Composite carbon nanofiber aerogels are new materials characterized by having a high specific area, high porosity, and being lightweight
Freeze-casting and drying followed by high temperature pyrolysis are the major procedures for the preparation of carbon nanofiber aerogels
Summary
As synthetic ultralight porous materials, aerogels were first reported by Kistler [1,2] in the early 1930s. The unique structure of aerogels is derived from gels where the major component as a liquid phase is replaced by a gas through evaporation or sublimation. Such a phase substitution without causing obvious shrinkage results in solid materials containing one or more constituents. Nanoparticles, for example Fe3O4, were embedded into carbon nanofibers to form composite aerogels as high capacity energy storage materials to make anodes for lithium-ion batteries [4]. The pores in aerogels become numerous light scatters in the materials. This review article deals with recent development of composite carbon nanofiber aerogels. Various materials for making carbon nanofiber aerogels will be discussed. Typical applications including energy conversion and storage, absorption, sensing, and catalysis will be discussed
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