N–O–S Co-doped Hierarchical Porous Carbons Derived from Calcium Lignosulfonate for High-Performance Supercapacitors

We propose a simple and low-cost synthesis of N–O–S co-doped hierarchical porous carbons (MGC-x) by direct carbonization of calcium lignosulfonate. MGC-x possesses a large surface area (∼1500 m² g^–1) and high mesopore ratio (>40%) and heteroatom content. In the three-electrode system, MGC-700 and MGC-800 exhibit high specific capacitances of 417 and 312 F g^–1 at 0.5 A g^–1 and 184 and 170 F g^–1 even at 100 A g^–1 using a 1 M H₂SO₄ electrolyte. Notably, there exists a critical current density when evaluating the rate capability of MGC-x electrodes, which is closely correlated to their ion diffusion and electron transport properties. In two-electrode configuration with a 3 M KOH electrolyte, MGC-700 with ultrahigh mass loading (∼13 mg cm^–2, ∼210 μm) can deliver large specific capacitance of 249 F g^–1 and excellent rate capability with capacitance retention of 81.9% from 0.05 to 20 A g^–1. MGC-700 and MGC-800 both show superb cycling stabilities with 94 and 98% of original capacitances being retained after 10 000 cycles, respectively. Furthermore, the MGC-700-based supercapacitor with commercial-level electrode mass loading yields an impressive energy density of 7.2 Wh kg^–1 at 62.5 W kg^–1 and maintains 5.1 Wh kg^–1 even at 4585.4 W kg^–1. The MGC-800-based supercapacitor exhibits considerable energy densities of 26.7 and 9.8 Wh kg^–1 at 309.8 and 7000 W kg^–1 in a 1 M (C₂H₅)₄NBF₄/PC electrolyte, respectively, guaranteeing its utilization in high-performance supercapacitors.