Porous Carbon-based Materials Derived From Sodium Alginate:Preparation And Performances On Supercapacitors And Organic Pollutants Adsorption

Carbon materials are widely used in electrode materials of supercapacitors（SCs）and adsorbent materials because of their high specific surface area,good electrical conductivity,low price and chemical stability.However,the pore structure of traditional carbon materials with high specific surface area is generally single（pore diameter<2 nm）,which limits the rate performance of supercapacitors and also affects their adsorption capacity and kinetics as adsorbents.In order to improve the performance of carbon-based materials,this paper aims to prepare novel hierarchical porous carbon-based materials using sodium alginate as the carbon source,calcium chloride as the activating agent and urea as the nitrogen dopant.The effects of the modulation of relevant experimental factors on the composition and structure of the as-prepared carbon materials was investigated in detail,and the relationship between the physical properties and electrochemical properties/adsorption properties of the final carbon-based materials was also studied.It is desired to obtain a simple,environmentally friendly and low-cost method for preparing hierarchical porous carbon-based materials with high-performances in SCs and adsorption of organic pollutants.The main research contents of this thesis are the following sections:（1）The effects of the addition of green activator（calcium chloride）and nitrogen dopant（urea）on the physical properties of the porous carbon-based materials derived from sodium alginate,especially the specific surface area(S_BET)and pore structure of porous carbon materials,were explored respectively.（2）The electrochemical performance of porous carbon materials as electrode materials for supercapacitors was investigated,and the relationship between the physical properties and electrochemical properties of the materials was analyzed and discussed.（3）The adsorption performance of porous carbon mater ials as adsorbent materials for methylene blue was investigated,and the relationship between the physical properties and adsorption performance of the materials was analyzed and discussed.（4）The electrochemical performance of nitrogen-doped hierarchical porous carbon（NDHPC）as electrode materials for supercapacitors was investigated,and the relationship between the physical properties and electrochemical properties of the materials was analyzed and discussed.（5）The adsorption properties of nitrogen-doped hierarchical porous carbon materials as adsorbent materials for methylene blue and p-nitrophenol were investigated,and the relationship between the physical properties and adsorption properties of the materials was analyzed and discussed.Based on the above-mentioned works,some results/conclusions are as follows:（1）The addition of calcium chloride has a great influence on the structure and performance of the carbon materials.With the increase of calcium chloride,the disorder degree of carbon materials gradually decreases,the specific surface area gradually increases,the average pore size gradually increases,and the total pore volume（V_t）and micropore volume(V_micro)show a trend of first increasing and then decreasing.In particular,there is a significant difference in the pore size distribution of the three carbon materials.Among them,the SA-3 sample,derived from the largest addition amount of calcium chloride during the activation,shows almost no micropores,mainly mesopores and macropores of 10-100 nm.The SA-2 sample derived from a moderate amount of calcium chloride addtion has a large number of micropores below 2 nm and mesopores of 10-30 nm.The SA-1 sample derived from the least amount of calcium chloride addition has rich micropores,mesopores of 2-10 nm,and macropores in the range of 50-100 nm.（2）Under the condition that the content of calcium chloride is stable,the addition of urea also has a significant impact on the structure of the final NDHPC materials.With the increase in the amount of urea add tion,the degree of disorder,nitrogen content,specific surface area and micropore volume of the material is increased,and the average pore diameter decreases.In addition,the use of calcium chloride as the activator in the system to prepare NDHPC materials can effectively avoid the generation of toxic substances.It is also an environmentally friendly route.（3）Through the study of the electrochemical performa nce of porous carbon materials,it is found that the SA-1 sample prepared the best comprehensive electrochemical performance.In particular,its rate performance has been significantly improved due to the more rational pore size distribution.In the 6M KOH electrolyte,the specific capacity of SA-1 sample at 100A·g^-1 is up to 79.5%of that at 1 A·g^-1,which is higher than those of 57.2%for the SA-2sample and 46.1%for the SA-3 sample,respectively.In addition,after 5000 cycles under the three-electrode system,the capacity retention rate of SA-1 sample is as high as 99.61%.（4）The results of using porous carbon-based materials as adsorbents to adsorb methylene blue show that the V_tand V_micro of carbon materials both exhibit significant impact on the adsorption effect of methylene blue.Firstly,due to the larger V_micro of the SA-1 and SA-2samples,their adsorption capacity for methylene blue is greater than that of SA-3 sample.Secondly,compared with NPHPC samples with similar S_BETbut smaller V_t,the SA-1 sample of V_t of 1.51 cm³·g^-1 and the SA-2 sample of V_t of 1.56 cm³·g^-1 display obviously higher adsorption capacities for methylene blue.The adsorption kinetics results show that the adsorption processes for the three porous carbon-based materials（SA-1,SA-2 and SA-3）abide by pseudo-second-order kinetic model and Langmuir adsorption model with a spontaneous endothermic process.Moreover,the SA-3 sample takes the shortest time to reach the saturated adsorption capacity because of its unique macroporous structure.（5）After study of the electrochemical performance of NDHPC materials,it is found that the sample displays higher the specific capacitan when it has higher the external specific surface area,under the condition of the three samples show relatively similar specific surface area and pore structure.Specifically,in the 6 M KOH electrolyte,the N-111 sample with the highest external specific surface area(S外=977.1 m²·g^-1)exhibits the highest specific capacitance of 375.6 F·g^-1 at 1 A·g^-1,in contrast,the N-121 sample with S外of 546.9 m²·g^-1shows specific capacity of 301.5 F·g^-1,and the N-141 sample with S外 of 903.3 m²·g^-1displays specific capacity of 341.6 F·g^-1.（6）The adsorption results of p-nitrophenol in water using NDHPC materials as adsorbents show that the adsorption ability of the N-111,N-121 and N-141 samples all exhibit high adsorption level,with 224.0,231.3 and 216.8 mg·g^-1,respectively,although the total pore volumes of these three materials are relatively small,which may be that N-doping is beneficial to enhance the adsorption capacity of porous carbon materials.The kinetic results showed that the adsorption process of p-nitrophenol on the three NDHPC materials conformed to pseudo-second-order kinetic model and Langmuir adsorption isotherm with a spontaneous endothermic process.