Abstract
The effect of two atmospheric post-treatment conditions directly after the KOH activation of polyacrylonitrile-based nanofibres is studied in this work. As post-treatment different N(2) : O(2) flow conditions, namely high O(2)-flow and low O(2)-flow, are applied and their impact on occurring reactions and carbon nanofibres' properties is studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), Raman spectroscopy, elemental analysis and CO(2) and Ar gas adsorption. At high O(2)-flow conditions a pyrophoric effect was observed on the KOH-activated carbon nanofibers. Based on the obtained results from the TGA and DSC the pyrophoric effect is attributed to the oxidation reactions of metallic potassium formed during the KOH activation process and a consequent carbon combustion reaction. Suppression of this pyrophoric effect is achieved using the low O(2)-flow conditions due to a lower heat formation of the potassium oxidation and the absence of carbon combustion. Compared to the high O(2)-flow samples no partial destruction of the carbon nanofibers is observed in the SEM images. The determination of the adsorption isotherms, the surface area, the pore size distribution and the isosteric enthalpies of adsorption show the superior properties under low O(2)-flow conditions. The present micropore volume is increased from 0.424 cm(3) g(-1) at high O(2)-flow to 0.806 cm(3) g(-1) for low O(2)-flow samples, resulting in an increase of CO(2) adsorption capacity of 38% up to 6.6 mmol g(-1) at 1 bar. This significant improvement clearly points out the importance of considering highly exothermic potassium oxidation reactions and possible post-treatment strategies when applying KOH activation to electrospun carbon nanofiber materials.