Controllable fabrication of nitrogen and phosphorus co-doped lignin-based carbon nanospheres for high-performance supercapacitor

Lignin-based carbon nanospheres have been considered as ideal electrode materials for supercapacitors because of regular morphology and high specific surface area. Heteroatom-induced defects on the carbon chain can resolve the poor hydrophilicity and surface functional group deficiency of pure carbon nanospheres, improving electrochemical performance. Herein, nitrogen and phosphorus co-doped lignin-based carbon nanospheres (NPCS) with different doping content were constructed by solvent exchange, hydrothermal treatment and carbonization treatment. The results showed that the NPCS could maintained perfect spherical morphology and good dispersion. When the ratio of covalent-bond stabilized lignin nanospheres to ammonium dihydrogen phosphate was 1:1 and the carbonization temperature was 700 ℃, the optimal specific surface area (89.8 m 2 g −1 ) and doping ratio of nitrogen and phosphorus (14 at%) were demonstrated in the NPCS. The obtained NPCS showed excellent capacitance storage capacity (312 F/g at 0.2 A/g) and significant cycle stability (93.7% capacitance retention after 10,000 cycles). The symmetric capacitor assembled based on NPCS had an energy density of 4.9 Wh /kg at the power density of 129.8 W/kg. This is mainly attributed to the fact that the NPCS has a high specific surface area and an enhanced affinity toward K ions. The study presents a novel approach to heteroatom co-doping in lignin-based carbon nanospheres, highlighting their potential for use in electrode materials.