K2CO3-KCl acts as a molten salt flame retardant to prepare N and O doped honeycomb-like carbon in air for supercapacitors

This paper uses coal tar pitch (CTP) as carbon precursors, K2CO3-KCl as activator and molten salt flame retardant to prepare N, O doped honeycomb-like carbon via a simple carbonization process in the muffle furnace in the air. The molten salt not only plays the role of flame retardant to prevent the combustion of CTP but also as the liquid medium to contact with the surface of inert CTP, which improves the activation efficiency of K2CO3. Profiting from the honeycomb morphology, heteroatom doped, high specific surface area and the protection of flame retardant, the as-prepared carbon electrode shows excellent specific capacitance (309.5 F g−1) and high carbon yield (32.5 wt%). Meanwhile, we also use K2CO3 to prepare a new type of K2CO3-ethylene glycol deep eutectic solvent (DES) electrolyte. The composed symmetrical supercapacitor displays a superior energy density of 29.1 Wh kg−1 in this DES electrolyte. Importantly, it can operate at −50 to 105 °C and exhibits satisfactory energy density. This work uncovers a universal and facile strategy to prepare porous carbon materials in the air and provides new possibilities for the exploitation of supercapacitors with superior energy density using K2CO3 based DES as electrolyte and can work under extreme temperature conditions.