The carbonization of polyacrylonitrile-derived electrospun carbon nanofibers studied by in situ transmission electron microscopy.

Roland Schierholz,Hermann Tempel,Henning Weinrich,Rüdiger-A Eichel,Hans Kungl,Daniel Kröger,Joachim Mayer,Markus Gehring

doi:10.1039/c8ra10491c

Abstract

Cathode structures derived from carbonized electrospun polyacrylonitrile (PAN) nanofibers are a current line of development for improvement of gas diffusion electrodes for metal–air batteries and fuel cells. Diameter, surface morphology, carbon structure and chemical composition of the carbon based fibers play a crucial role for the functionality of the resulting cathodes, especially with respect to oxygen adsorption properties, electrolyte wetting and electronic conductivity. These functionalities of the carbon fibers are strongly influenced by the carbonization process. Hitherto, fibers were mostly characterized by ex situ methods, which require great effort for statistical analysis in the case of microscopy. Here, we show the morphological and structural evolution of nanofibers during their carbonization at up to 1000 °C by in situ transmission electron microscopy (TEM). Changes in fiber diameter and surface morphology of individual nanofibers were observed at 250 °C, 600 °C, 800 °C and 1000 °C in imaging mode. The structural evolution was studied by concomitant high resolution TEM and electron diffraction. The results show with comparatively little effort shrinkage of the nanofiber diameter, roughening of the surface morphology and formation of turbostratic carbon with increasing carbonization temperature at identical locations.

Highlights

Electrospinning is an efficient technique to provide 1-D nanostructured polymeric or polymer-based materials and composites.[1]
Shrinkage in diameter as observed on individual nano bers over the whole temperature range mounts up to 20% reduction of the initial size
No marked in uence of the initial nano ber diameter on its relative reduction was detected

Summary

Introduction

Electrospinning is an efficient technique to provide 1-D nanostructured polymeric or polymer-based materials and composites.[1]. A er electrospinning the standard procedure to convert polymeric PAN- bers into carbon bers is oxidative stabilization, carbonization and graphitization.[10] The rst of these three steps is performed under air between 200 C and 300 C and the reactions involved are the cyclisation of nitrile and incorporation of oxygen as described by Goodhew et al.[11] This step is important to avoid votalilization, maximize the carbon yield and avoid the formation of hollow core bers in the subsequent carbonization step.[10,12] The subsequent carbonization step is conducted under inert gas atmosphere up to temperatures of about 1500 C. The corresponding evaporation processes start at temperatures just above the stabilization temperature of e.g. 400 C.13 most changes were reported for

Results

Discussion

Conclusion