Highly Value-added Porous Carbon Materials Derived From Aquatic Plants:Preparation,Characterization,and Performance

The effective utilization of waste biomass is always a hot topic in the fields of environmental and chemical engineering.As a kind of sustainable resource in nature,biomass materials have obvious environmental-friendly advantages along with a wide variety of sources,endowing them to be suitable precursors for carbons to meet the industrial production requirements,especially for carbon-enriched biomass materials.As compared terrestrial biomass,living conditions for aquatic plants are less affected by the nature without any requirements for the land to support their growth.Especially,most aquatic plants have abundant nanopores in structure to support their respiration depending on the living conditions,which is an obvious advantage in the preparation of high-performance carbons.The thesis reported the preparation of porous carbons with varied microstructure and surface-doping from two kinds of aquatic plants,Enteromorpha seaweeds(EP)and lotus stem,and studied the potential applications on CO2 capture and electrochemical storage.The main contents for the thesis are as follows:First,N,O-co-doped hierarchical carbons were obtained from direct pyrolysis of EP,using freeze-drying method to consolidate inherent peculiarities of EP.The results indicated that hierarchical carbons have surface areas of about 450 m2/g with 70%pores from mesopores,and N,O-species contents in the framework are of about 4.4 and 1.5 at.%,respectively.The gas adsorption results indicated that the carbons show the CO2 uptake of 5.14 mmol/g at 20 bar and ambient temperature.Electrochemical results indicated that the capacitance of carbon is up to 234 F/g at 0.5 A/g in 6M KOH with capacitance retention of 95%after 2000 cycles.Moreover,series of hierarchical carbons were synthesized from EP using KOH as activator.Significantly,the activated carbons exhibit the surface areas of up to 3345 m2/g with the N,O-species contents of 3.48 and 10.83 at.%,respectively,exhibiting high-performance for the CO2 capture and supercapacitor application.The CO2 uptake on these carbons is highly up to 3.85mmol/g at ambient conditions,and the corresponding uptake is up to 25.26 mmol/g at 20 bar.The capacitance of the carbon is up to 220 F/g at 1 A/g in 6M KOH with the with capacitance retention of 87%after 5000 cycles.Second,N-doped microporous carbons with some microsphere morphologies in were obtained from EP using hydrothermal carbonization method in a sealed reactor via a simply controlling temperature in activation process.The results indicated that the carbons have surface areas of up to 1979 m2/g with N-species of 4.3 at.%,exhibiting high-performance for CO2 capture and supercapacitors.Adsorption results indicated that CO2 uptakes on the carbons are of up to 3.87 mmol/g at ambient conditions with isosteric adsorption heat of up to 28 KJ/mol.Electrochemical results indicated that the capacitance is up to 200 F/g at 1 A/g in 6M KOH,and the symmetric supercapacitors show excellent cycling stability at high current density of 10 A/g with the capacitance retention up to 96%of initial capacitance after 10000 cycles.Third,after hydrothermal carbonization,we added an additional nitrogen source to increase N-species contents in the carbons during KOH activation,with an aim to understand the N-doping effect on the performance of carbons.The resulting microporous carbon shows surface area of 1098 m/g with N-loadings up to 15.5 at.%.Adsorption results indicated that the carbon exhibits the CO2 uptakes of 3.06 and 1.86 mmol/g at ambient pressure and 0 and 25 ?,respectively.The CO2 isosteric adsorption heat is highly up to be about 40 kJ/mol because of the high concentration of N-loadings.The oxygen reduction reaction measurements indicated that the current density is of about 4.1 mA/cm2 at 1600 rmp in 0.1 M KOH,showing excellent stability to methanol.The current density retention is highly up to 91.1%after 20000s cycles,which is better than commercial Pt/C electrode with current density retention of only 63.7%under the same conditions.The supercapacitor measurements indicated that the capacitance is up to 210 F/g at 0.5 A/g in 6M KOH,and capacitance exhibits almost no attenuation with the retention close to 100%at 2A/g after 10000 cycles.Finally,lotus stem bio-wastes as a local rich resource in Xiangtan city were used as the raw materials for the synthesis of carbons via the combination of hydrothermal carbonization and KOH activation method.The resulting porous carbons exhibit the surface areas is highly up to 2893 m2/g.Adsorption results indicated that the carbons exhibit the CO2 uptakes of 6.17 and 3.85 mmol/g at ambient pressure and 0 and 25 ?,respectively.The caiculated CO2 isosteric adsorption heat on carbon is about to be 42.5 kJ/mol.Based on the adsorption measurements under different conditions,we confirmed that the pore size distribution of carbons plays a critical role in the CO2 capture process especially at ambient pressure,providing effective guidance for the future preparation of high-performance adsorbents for CO2 capture.