Facile and scalable construction of nitrogen-doped lignin-based carbon nanospheres for high-performance supercapacitors

Lignin-based carbon nanospheres have a high specific surface area, large porosity, and excellent stability, while being used in potential applications such as catalyst carriers, adsorbents, and energy storage materials. However, the low hydrophilicity and poor chemical activity of pure carbon nanospheres make it challenging for those materials to meet the needs of various functions. The introduction of nitrogen atoms can increase the active site of the materials, thereby improving the performance. In this study, lignin nanospheres (LNS) with a solid content of 5.8 mg mL-1 in the suspension and an actual yield of 85% were prepared using a gamma-valerolactone/water binary system. Based on that, nitrogen-doped lignin-based carbon nanospheres (NCS) were successfully constructed. The doping ratio of urea and carbonization temperature significantly affected the physicochemical properties of the carbon materials. By varying the urea doping ratio and temperature, the nitrogen atom content of the NCS varied from 5.0 to 10.9 at.%. After assembling them into electrochemical capacitors, NCS-15-700 exhibited excellent capacitance of 232 F g-1 at 0.5 A g-1 and 10,000 cycles long cycling stability (retention of 97.2%). The specific capacitance was enhanced by approximately 40% compared to undoped lignin-based carbon nanospheres (LCS). It is evident that nitrogen-doped lignin-based carbon nanospheres have promising applicability in supercapacitors.