Post-treatment strategies for pyrophoric KOH-activated carbon nanofibres

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 N₂ : O₂ flow conditions, namely high O₂-flow and low O₂-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 CO₂ and Ar gas adsorption. At high O₂-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 O₂-flow conditions due to a lower heat formation of the potassium oxidation and the absence of carbon combustion. Compared to the high O₂-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 O₂-flow conditions. The present micropore volume is increased from 0.424 cm³ g^-1 at high O₂-flow to 0.806 cm³ g^-1 for low O₂-flow samples, resulting in an increase of CO₂ 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.