Biocarbon with large specific surface area and tunable pore structure from binary molten salt templating for supercapacitor applications

Tailoring the pore structure while maintaining a large specific surface area (SSA) of biocarbon remains a challenge, especially for single activating/templating system. Herein, a new binary ZnCl2-CaCl2 (Zn-Ca) molten salt system is explored for one-step carbonization of chitin to manufacture biocarbon with hierarchically tunable micro/mesoporous structure. By adjusting the salt to chitin ratio, biocarbon with large SSA (>1000 m2/g), controllable mesopore fractions (19.3%–67.9%) and high yield (up to 35%) has been fabricated. The optimal SSA, total pore and mesopore volume are obtained at 1671 m2/g, 1.32 cm3/g and 0.64 cm3/g, respectively, showing the synergistic effect of Zn-Ca on achieving well-balanced pore characteristics. Besides, the total nitrogen contents could also be enhanced. Such balanced physicochemical properties lead to excellent supercapacitor performance, achieving 301.2 F/g specific capacitance at a current density of 0.5 A/g as measured from a three-electrode system. The assembled supercapacitor cell also shows a notable rate capability of 70.2% at an ultrahigh current density of 50 A/g with a low internal resistance, which can reach a remarkable power density of 27 kW/kg. The outstanding capacitive performance is attributed to the modulated micro-mesoporous structure for efficient ion diffusion and adsorption, as well as appropriate heteroatoms for pseudo-capacitance contribution.