Study On The Preparation Of Lignin-based Heteroatom-doped Carbon Nanospheres And Their Electrochemical Properties

Lignin-based carbon nanospheres have the high specific surface area,large porosity and excellent stability to be used in catalyst carriers,adsorbents and energy storage materials.However,the low hydrophilicity and poor chemical activity of pure carbon nanospheres make it difficult to meet these functional requirements.In this paper,the covalently stabilised lignin nanospheres were used as carbon precursors.The precursors were doped and carbonised to obtain lignin-based heteroatom-doped carbon nanospheres.The mechanism of the effect of different heteroatoms and their doping forms on the electrochemical properties was investigated.In addition,graphene was further compounded with the prepared lignin-based heteroatom-doped carbon nanospheres to enhance the electrical conductivity and improve the electrochemical properties while retaining the pseudocapacitance.The main studies and findings are as follows:(1)Lignin nanospheres with high solid content and high yield were obtained using a yvalerolactone/water binary solvent system.On this basis,nitrogen-doped lignin-based carbon nanospheres(NCS)were successfully constructed using urea as the nitrogen source.By varying the urea doping ratio and carbonisation temperature,the nitrogen atom content of the NCS varied between 5.0 and 10.9 at.%.When assembled into electrochemical capacitors,the NCS exhibited an excellent capacitance of 232 F g-1 at 0.5 A g-1 and a long cycle stability of 10 000 cycles(retention rate of 96.6%).Compared to lignin-based carbon nanospheres(LCS)undoped with nitrogen atoms,the specific capacitance was increased by approximately 40%.(2)Based on the study of nitrogen-doped lignin-based carbon nanospheres,nitrogenphosphorus co-doped lignin-based carbon nanospheres(NPCS)with different nitrogen and phosphorus atom contents were prepared by introducing phosphorus atoms along with nitrogen atoms using ammonium dihydrogen phosphate.Due to the dispersed spherical structure and the co-doping of N/P atoms,NPCS can provide excellent pore structure,abundant defects and active sites.the NPCS-derived electrode materials exhibit excellent charge storage capacity(312 F g-1 at 0.2 A g-1)and remarkable cycling stability(93.7%capacitance retention after 10 000 cycles).The symmetrical capacitors assembled on the basis of NPCS have an energy density of 4.9 Wh kg-1 at a power density of 129.8 W kg-1.(3)Based on the study of nitrogen and phosphorus co-doped lignin-based carbon nanospheres,composites of nitrogen and phosphorus doped lignin-based carbon nanospheres and graphene(NPCS@rGO)were prepared by hydrothermal treatment.By tuning the ratio of carbon spheres and graphene,the NPCS@rGO prepared under optimized conditions possessed a specif c surface area of 869.5 m2 g-1 and a moderate amount of heteroatom content(N:5.88 at.%;P:0.70 at.%).Due to the abundant micro/mesopores of the carbon spheres,the high electrical conductivity of graphene and the synergistic effect of the "layer-sphere-layer"structure,the prepared composites exhibit good ion penetration and migration,significantly enhancing the electrochemical properties.The NPCS@rGO-based electrode materials can provide a specific capacitance of 330.7 F g-1,and the assembled supercapacitor achieves an energy density of 5.8 Wh kg-1.