Molten salt synthesis of nitrogen and oxygen enriched hierarchically porous carbons derived from biomass via rapid microwave carbonization for high voltage supercapacitors

Highlights: • Nitrogen and oxygen enriched hierarchically porous carbons derived from carrot. • Rapid microwave carbonization of carrot powder is realized in only 4 min. • The as-prepared carbon material possesses a high mesopore ratio of 70%. • The symmetrical supercapacitor achieves high voltage window of 0–1.6 V in Na₂SO₄. • Excellent cycle stability is obtained for the symmetrical supercapacitor. Nitrogen and oxygen enriched hierarchically porous carbons (NOHPCs) derived from biomass have been successfully prepared by rapid microwave carbonization coupled with molten salt synthesis method in only 4 min. ZnCl₂ plays important roles as microwave absorber, chemical activation agent and porogen in this process. NOHPC-1:10 sample possesses the maximum specific surface area of 1899 m² g⁻¹ with a pore volume of 1.16 cm³ g⁻¹ and mesopore ratio of 70%, as well as nitrogen content of 5.30 wt% and oxygen content of 14.12 wt%. When evaluated as an electrode in a three-electrode system with 6 M KOH electrolyte, the material exhibits a high specific capacitance of 276 F g⁻¹ at 0.2 A g⁻¹, with a good rate capability of 90.9% retention at 10 A g⁻¹. More importantly, the symmetric supercapacitor based on NOHPC-1:10 in 1 M Na₂SO₄ electrolyte exhibits a high energy density of 13.9 Wh kg⁻¹ at a power density of 120 W kg⁻¹ in a wide voltage window of 0–1.6 V, an excellent cycling stability with 95% of capacitance retention after 10,000 cycles. Our strategy provides a facile and rapid way for the preparation of advanced carbon materials derived from biomass towards energy storage applications.