Nature of improved double-layer capacitance by KOH activation on carbon nanotube-carbon nanofiber hierarchical hybrids for supercapacitors

Carbon nanotubes (CNTs) and their related hybrids grown by chemical vapor deposition (CVD) method have been frequently used as energy-storage electrode materials. In most cases, the tips of CNTs are almost well closed, which often limits the utilization efficiency of their tubular structure in storage of charges. In this work, the authors have designed a type of tip-open CNT/CNF hierarchical hybrids via KOH activation on the CVD-grown Al/Fe-catalyzed CNT/CNF precursors. Results showed that the CNT hierarchies’ tips can be fully opened via KOH activation at 700–750 °C for 90 min. As expected, the tip-open CNT-CNF hybrids show a highly improved specific capacitance, which increases by 3.3 times that of the pristine ones. Moreover, sweep analysis indicated that the diffusion-type capacitance increases by 3.7 times while the Helmholtz-type capacitance increases only by 1.5 times, indicating that the tip-open CNTs contribute to ∼30% of the increase in double-layer capacitance. Further optimization on activation time and temperature indicated that activation at 700 °C for 90 min is suitable for opening the tips of CNT hierarchies and protecting the CNTs’ tubular structure. This research would give some useful advice for the preparation of high-performance CNT-related electrode materials for energy storage.