Sustainable eutectic mixture strategy of molten salts for preparing biochar with interconnected pore structure from algal residue and its performance in aqueous supercapacitor

Abstract

A green ternary (NaCl/KCl/ZnCl2) mixed salt-assisted carbonization of spirulina residues was proposed to produce the nitrogen-doped biochar (NBC) with a hierarchical porous structure for the application of supercapacitors. Modifying the ratio of mixed salt to residues and the carbonization temperature allows it to control the specific surface area (SSA) of biochars, the type and content of functional groups, and the graphitization of the carbon skeleton. Here, the eco-friendly mixed salt functioned for a multifunctional purpose by acting as both a pore regulating agent and the reaction medium. The optimal 650–5 sample, produced by a reaction temperature of 650 °C and ratio of 5, exhibited remarkable specific capacitance of 215 F/g and 207 F/g at 0.2 A/g in 1 M H2SO4 and 6 M KOH electrolytes, respectively, and the composed symmetrical capacitors showed excellent cycling stability in the four aqueous electrolytes (plus 1 M Na2SO4 and 15 mol/kg LiTFSI), with capacitance retention rate all ≥90 % after 10,000 cycles. In the LiTFSI system, the energy density was 25.28 Wh/kg at 375.04 W/kg. Furthermore, experiments using recycled salt confirmed the sustainability of this approach.