Xiaojun He

Synthesis of hierarchical porous carbons for supercapacitors from coal tar pitch with nano-Fe2O3 as template and activation agent coupled with KOH activation

Hierarchical porous carbons (HPCs) for supercapacitors were synthesized from coal tar pitch using nano-sized γ-Fe2O3 as a template and activation agent coupled with KOH activation by conventional and microwave heating. The HPCs were characterized by scanning electron microscopy, transmission electron microscopy, N2 adsorption and X-ray diffraction techniques. The results show that the specific surface area (SBET) of HPCs is tunable, and increases from 761 m2 g−1 to 1330 m2 g−1 as the mass ratio of γ-Fe2O3 to the pitch increases in the mixture. The nano-sized γ-Fe2O3 is reduced to Fe3O4, FeO, Fe in the activation reaction step. The carbon dioxide generated from the oxidation reactions of carbon monoxide via γ-Fe2O3 reacts with carbon that is a kind of in situ physical activation, which results in the development of the porosity in HPCs. The large SBET in HPCs are due to the synergistic effects including γ-Fe2O3 template, KOH chemical activation, and physical activation resulting from the reactions of γ-Fe2O3 and KOH activation. Under optimum conditions with the mass of coal tar pitch, γ-Fe2O3, KOH at 4.2 g, 16.8 g and 6 g, the HPC made by conventional heating shows a high capacitance of 194 F g−1 in 6 M KOH aqueous electrolyte and an energy density of 20.3 Wh kg−1 in 1 M tetraethylammonia tetrafluoroborate in propylene carbonate electrolyte at a current density of 0.1 A g−1. This work may pave a new way to produce high performance HPCs for energy storage devices.

Effect of different gas medium on carbon nanotubes synthesis by arc discharge

Carbon nanotubes (CNTs) were synthesized in different gas medium by arc discharge. Optical emission spectroscopy (OES) was used in situ to investigate the formation mechanism of CNTs. Field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) were used to characterize CNTs. FE-SEM observation reveals that a few CNTs synthesized in N₂/C₂H₂/CO₂ medium (CNTs_N2/C2H2/CO2) are open-ended or porous ones and the impurity amount adhered on CNTs_N2/C2H2/CO2 or CNTs_He/C2H2/CO2 is smaller than that adhered on CNTs_N2/C2H2 or CNTs_He. TEM characterization results show that the wall of CNTs_N2/C2H2/CO2 is partly damaged because of being oxidized, which led to its diameter is smaller than that of CNTs_N2/C2H2. OES diagnosis results show that active radicals including C₂ and CH are the precursors of CNTs formation while CO₂ acts as the etching species to eliminate amorphous carbon in the presence of CO₂. A possible formation process of CNTs is proposed on the basis of the results of OES diagnosis and FE-SEM as well as TEM observation.