Controllable Fabrication of Flexible and Foldable Carbon Nanofiber Films

Abstract

Flexible carbon films are considered the priority choice for flexible sensor electrodes or battery current collectors because of their high electrical conductivity and excellent mechanical flexibility. [9][10][11][12] Among different nanoscopic forms of carbon, 1D CNFs have aroused great interest since it is easy to be processed into flexible films. In particular, when the fiber size is reduced from the traditional micron to nanoscale, the ultrahigh specific surface area and rich defect structure brought by diameter refinement can greatly improve the physicochemical properties of CNF films. [13][14][15] Therefore, CNFs have become a research hotspot in both academia and industry. [16,17] The commonly used strategies to prepare CNFs include the template method, [18][19][20] arc approach, [21,22] chemical vapor deposition (CVD), [23][24][25][26] blend spinning, and electrospinning. [27][28][29][30] For template and arc techniques, the production efficiency of them is very low and it is difficult for them to yield continuous 1D CNFs. For CVD, the expensive equipment limits its large-scale manufacturing of CNFs. Although blend spinning can scalably produce CNFs at high speed in a cost-effective manner, the diameter of the CNFs is generally uneven and large than 500 nm, causing difficulty in controlling the fiber structure. In contrast, electrospinning, as a new fiber forming technology to realize polymer fluid stretching and refinement through a high electrostatic voltage, exhibits many advantages for preparing CNFs. For example, the size effect and surface effect of CNFs prepared by electrospinning are prominent, such as high specific surface areas, surface energy, and surface porosity. Moreover, the structure and surface of CNF can be easily controlled and functionalized, respectively, which endow the electrospun CNFs with a broad application prospect.Despite all these benefits, the mechanical brittleness of electrospun CNFs is still an obstacle that remains to be tackled. [31][32][33] From a microscopic point of view, the mechanical brittleness of CNFs is caused by stress concentration at the defects in CNFs during bending. The main strategy to fabricate flexible CNFs is to ex situ or in situ construct heterogeneous phases into carbon matrix by using surface decoration or doping metal oxide nanoparticles (Nps), [34,35] non-metallic oxide Nps, [36][37][38] and carbide NPs. [39][40][41] The common point of these methods is to establish heterogeneous interfaces between NPs and carbon components Carbon nanofibers (CNFs) fabricated through traditional methods are generally composed of amorphous carbon and graphite sheets, which usually endow CNFs with high modulus and brittleness. Here, a strategy of controlling graphite sheet structures in CNFs for the scalable fabrication of silk-like flexible CNF films that can be arbitrarily folded in any shape and can undergo thousands of dynamic bending deformation cycles is reported. In detail, the carbon precursor of polyacrylonitrile (PAN) is first electrospun into NFs and the...