Nitrogen-doped Prous Carbons Derived From Biomass For CO₂ Adsorption And Suspercapacitor Electodes

Porous carbons have the advantages of developed and adjustable porous structure,large specific surface area,low thermal expansion coefficient,chemical inertness,and excellent electrical conductivity.They can be used as highly efficient solid adsorbents for CO₂capture as well as electrodes for energy storage devices.In addition,the doping of nitrogen-containing basic groups into porous carbons can not only enhance the interaction between the adsorbent and CO₂molecules,thereby increasing the CO₂uptake,but also trigger pseudocapacitance effects and thus improve the electrochemical performance of the material.Therefore,nitrogen-doped porous carbons can be both promising CO₂adsorbents and electrode materials for supercapacitors.In order to obtain porous carbons with dual functions,in this thesis,nitrogen-doped porous carbons were obtained using biomass materials as the carbon precursors.The resulting porous carbons were carefully characterized and their CO₂adsorption and electrochemical performances were investigated.The specific research conclusions are as follows:1.Nitrogen-doped porous carbons were synthesized by NaNH₂activation using carbonized leaf as the precursor.Since NaNH₂can act as both as activator and nitriding agent,nitrogen-doped porous carbons can be obtained by a single-step NaNH₂activation.The resultant carbonaceous materials were characterized by different techniques such as nitrogen adsorption/desorption,SEM,TEM,XRD and XPS,and their CO₂adsorption and electrochemical performances were carefully investigated.It was found that the optimal sample exhibits the maximum CO₂uptake of 3.50 and 5.18 mmol/g at 1 bar,25°C and 0°C,respectively.After comprehensive analysis of porous structures,surface chemical properties and Porous carbons have the advantages of developed and adjustable porous structure,large specific surface area,low thermal expansion coefficient,chemical inertness,and excellent electrical conductivity.They can be used as highly efficient solid adsorbents for CO₂capture as well as electrodes for energy storage devices.In addition,the doping of nitrogen-containing basic groups into porous carbons can not only enhance the interaction between the adsorbent and CO₂molecules,thereby increasing the CO₂uptake,but also trigger pseudocapacitance effects and thus improve the electrochemical performance of the material.Therefore,nitrogen-doped porous carbons can be both promising CO₂adsorbents and electrode materials for supercapacitors.In order to obtain porous carbons with dual functions,in this thesis,nitrogen-doped porous carbons were obtained using biomass materials as the carbon precursors.The resulting porous carbons were carefully characterized and their CO₂adsorption and electrochemical performances were investigated.The specific research conclusions are as follows:2.In order to further improve the CO₂adsorption capacity and specific capacitance capacity of the carbonaceous materials,a series of nitrogen-doped porous carbon materials were prepared using hazelnut shells as raw materials exploring the same synthesis strategy described as the above section.HSC-500-1（NaNH₂/carbon ratio of 1,activation temperature of 550°C）exhibits the maximum CO₂uptake among this series of samples,i.e.4.23 and 6.23 mmol/g at 1 bar,25°C and 0°C,respectively.In addition,the three-electrode test results show that this sample has the best electrochemical performance in the 6 M KOH electrolyte,with specific capacitance of 309 F/g at a current density of 0.5 A/g.Under the current density of 5A/g,the retention rate of the specific capacitance of this sample is still as high as95.2%after 5000 cycles,indicating its long cycle life.The combined effect of the sample’s narrow microporosity,nitrogen content,pore size and pore size distribution determine its excellent CO₂adsorption capacity and high specific capacitance.3.In the above two sections,the CO₂adsorption and electrochemical performance of nitrogen-doped porous carbons derived from biomass were prepared and investigated.The recent theoretical and experimental studies have proved that heteroatom co-doping can significantly improve the CO₂adsorption capacity and electrochemical performance of carbonaceous materials.Therefore,in this part,Nitrogen-and sulfur co-doped porous carbons were synthesized by a carbonization-nitrogen and sulfur co-doping-activation process using water chestnut shell as the carbon precursor,thiourea as the nitrogen and sulfur source,and KOH as the activator.The obtained materials were characterized,and the effects of the porous structure and surface chemistry of the samples on the CO₂adsorption capacity and electrochemical performance were investigated.The maximum CO₂adsorption capacity of 4.34 and 6.31 mmol/g were found at 1 bar,25°C and 0°C,respectively,for this series of samples.The combined effects of narrow microporous volume,together with nitrogen and sulfur content of the adsorbents determine their CO₂adsorption capacities.In addition,the nitrogen integrated into the carbon framework exists in the form of pyridine nitrogen and pyrrole nitrogen,and the doped sulfur is present in the form of thiophene sulfur and oxidized sulfur for this series of N,S co-doped porous carbons.The synergistic effect between N,S co-doping can significantly increase the electrochemical active points on the surface,therefore leading to excellent electrochemical performance for these carbonaceous materials.In the 6 M KOH electrolyte,the specific capacitance of the optimized sample reaches318 F/g at the current density of 0.5 A/g.In addition,this sample also showed a longer cycle life.Due to the inexpensive raw materials,simple preparation process and excellent performances,these nitrogen and sulfur co-doped porous carbons have great potential in the application of CO₂capture and supercapacitors.