Nitrogen-doped Porous Carbon Materials Derived From The Potassium Hydroxide Activation Of Bagasse:Preparation And Electrochemical Performance

Nitrogen-doped porous carbons?NDPCs?,possessing good electrical conductivity,excellent plasticity,great chemical stability,and a wide stable operating temperature range,are one of the most desirable host substrates and electrode materials for hosting elemental sulfur in lithium-sulfur batteries?LIBs?and supercapacitors?SCs?,respectively.However,the above-mentioned nitrogen-doped porous carbon materials with high specific surface area,rich nitrogen content,and suitable pore structure are also faced with a series of problems,such as complicated preparation process,expensive raw materials,and etc.In view of the above issues,this paper presents a facile synthesis method for the fabrication of nitrogen-doped porous carbon materials by activation of the mixed precursor of urea and bagasse,using KOH as an activating agent.The influencing factors of the impregnation time,the ratio of the mixed precursor and KOH,as well as the activation temperature on the physical properties of the as-prepared materials were investstigated.Furthermore,the relationships between the physical properties and the electrochemical performances were also studied,to attempt to establish an environmental-friendly,cheap and highly effective synthesis route to prepare nitrogen-doped porous carbon materials for high performance LIBs and SCs.The main research contents of this thesis are the following sections:?1?The effects of impregnation time,pyrolysis temperature and ratio of KOH and the mixed precursor on the pore structure and nitrogen contents of the as-prepared porous carbon materials were investigated.?2?The electrochemical properties of nitrogen-doped porous carbon materials,prepareing from different experimental conditions,were used as the active electrode materials for SCs were studied.Also,the relationships between the physical properties and electrochemical performances of the tested electrode materials were analyzed and deduced.?3?The sulfur/nitrogen-doped porous carbon?S/NDPC?nanocomposites with different sulfur loadings were prepared and their electrochemical performances were mesuresed.Also,the possible factors resulted in the fifferent electrochemical performance of the S/NDPC were analyzed.Based on the above-mentioned works,some results and conclusions are as follows:?1?The change of the impregnation time can affect the dispersion degree of the activator and urea in the bagasse,therefore,the specific surface area,the pore volume,the nitrogen content and the pore size distribution of the as-prepared NDPCs can be optimized by choosing the best impregnation time.?2?The increase of temperature can improve the etching effect,however,very low yield of the final products carbon were achieved at the temperatures were above 800?.Therefore,All NDPCs-based materials were prepared at 800?.?3?The change of the ratio of KOH to urea greatly changes the nitrogen content,specific surface area,and pore structure of the resulting NDPCs.Therefore,a proper proportion of KOH and urea is very important for the preparation of ideal NDPCs if the carbon source?bagasse?is a certain amount.?4?The electrochemical performances of NDPCs prepared under different experimental conditions suggest that the sample?KNCb-111?obtained from pyrolysis of the mixed precursor with the same mass of bagasse,KOH and urea at 800?exhibited the best electrochemical performances.In a three-electrode system,the specific capacitances are up to 350.8,301.9 and259.5 F g^-1 at a current density of 1.0 A g^-1 in 1 M H₂SO₄,6 M KOH and 2 M Li₂SO₄electrolytes,respectively.Also,the symmetric capacitor assembled by KNCb-111-based electrodes shows a energy density of 39.1 Wh kg^-1 at a specific power of 20 kW kg^-1,with 95.5%capacitance retention after 10 000 cycles.?5?Two sulfur/nitrogen-doped porous carbon nanocomposites?KNCb-111-S50%and KNCb-111-S70%with sulfur loadings of 50%and 70%respectively?were prepared successfully by using KNCb-111 as the substrate material.The electrochemical results show that the KNCb-111-S50%sample exhibits much higher specific capacity and better cycle performance.Under the current densities of 0.5,1,2 and 4 C(1 C=1630 mA g^-1),the specific discharge capacities of this nanocomposite are up to 1367.5,975.2,810.7 and 705.8 m Ah g^-1,respectively.After 300 cycles at 0.5 C,it still displays a high capacity of 819.2 mAh g^-1,showing a good cycling stability.In summary,this paper investigated the effects of preparation conditions on the composition,pore structure and electrochemical performance of nitrogen-doped porous carbon materials.And it also provided a theoretical basis and technical support for the preparation of high-quality heteroatom-doped carbon-based electrode materials from bagasse.