Influence of carbon fibres crystallinities on their chemical activation by KOH and NaOH

Abstract

It has been proved that chemical activation with KOH or NaOH is an effective method to prepare activated carbon fibres and that both hydroxides exhibit different characteristics depending on the crystallinity of the carbon fibres. The role of carbon fibre crystallinity has been studied using three carbon fibres: (i) a pitch based carbon fibre (Osaka gas Co, obtained from an isotropic coal tar pitch), (ii) a high performance carbon fibre (Hexcel, obtained from polyacrylonitrile (PAN)) and (iii) a carbon nanofibre (Antolín Ingeniería, obtained from vapour grown). In addition, a graphite powder (Aldrich) has been studied. XRD patterns showed that the four precursors used have different structural order which decreases as follows: Aldrich graphite > Antolín carbon nanofibres > Hexcel carbon fibres > Donac carbon fibres. As the structural order increased, the hydroxide carbon activation became much more difficult being the activation of the nanofibres very little and impossible by none of these two activating agents in the case of the graphite. The present work also confirms that the nature of the hydroxide is an important variable. During carbon fibre activation, NaOH was more effective than KOH at developing porosity in the pitch-based carbon fibre sample that has the lowest crystallinity. In contrast, KOH generated more pore volume than NaOH when activating high performance PAN-based carbon fibres and nanofibres both having high crystallinity. In addition, the micropore size distribution obtained with KOH was narrower than with NaOH, especially when employing high ratios of chemical activating agent to carbon sample. This study provides data that support the formation of intercalation compounds during the activation process of the carbon fibres, which is favoured in the case of KOH, especially for the sample with a high structural order (PAN-based carbon fibres).