HyperCoal-derived porous carbons with alkaline hydroxides and carbonate activation for electric double-layer capacitors

• ACs for EDLC electrodes were prepared by activating HyperCoal with NaOH and KOH.
• AC from NaOH activation at 700 °C reaches a capacity of 43.9 F/g and a SSA of 3110 m2/g.
• CaCO3 significantly inhibits the pore development during KOH activation.
• Electrochemical performance of the ACs depended on SSA and pore size distribution.
• HyperCoal is an excellent and low-cost candidate for EDLC electrode preparation.

Electrical double layer capacitors (EDLCs) with HyperCoal-derived activated carbons (ACs) as electrodes are reported in this work. The ACs were prepared by the carbonization of HyperCoal at 600 °C followed by the activation with KOH and NaOH for 2 h between 500 and 800 °C to achieve high total pore volume (TPV) and specific surface area (SSA). The porosity of the ACs was characterized with N2 adsorption at 77 K. The results show that the ACs prepared with KOH and NaOH activations at 700 °C have a SSA of 2594 and 3010 m2 g− 1 and a TPV of 1.36 and 1.77 cm3 g− 1, respectively, leading to the highest specific capacitance of 43.1 and 43.9 F g− 1, respectively, in a two-electrode EDLC cell with 0.5 M tetraethylammonium tetrafluoroborate/propylene carbonate (TEABF4/PC) electrolyte. Activations with CaCO3 and KOH/CaCO3 mixture were also investigated to understand the effect of CaCO3 on the properties of the ACs. CaCO3 significantly inhibited the porosity development during KOH activation and gave ACs with quite low SSA and specific capacitance. The results demonstrated that the alkaline hydroxide activation is an efficient approach to the preparation of ACs from HyperCoal with high-performance for EDLCs.