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

Abstract

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 (S_BET) 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 S_BET 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.