| The excessive consumption of fossil energy and the deteriorating environment promote the rapid development of clean energy and new energy storage systems.Supercapacitors have the advantages of high power density,fast charge and discharge capacity,high safety,good reversibility and long cycle life,which are widely used in electronic equipment,hybrid vehicles,medical equipment,etc.Porous carbon materials have attracted wide attention as electrode materials for supercapacitors due to their high specific surface area,good chemical stability,excellent electrical conductivity and low cost.Humic acid is a kind of macromolecular organic material with rich functional groups（such as carboxyl,hydroxyl,carbonyl,quinone and methoxy）and a certain pore structure.It can be divided into black humic acid（HA）,fulvic acid（FA）and brown humic acid（BA）.In this paper,firstly,HA,FA and BA were extracted from lignite in Guizhou by the method of oxygenolysis-alkali dissolution-acidification,and the optimal extraction conditions were explored.Secondly,porous carbon materials were prepared by oxidation-activation and carbonization using HA with the highest yield and FA with the largest content of oxygen-containing functional groups as carbon sources,respectively.The preparation conditions were optimized by orthogonal experiment.The effects of direct carbonization,oxidation-carbonization,activation-carbonization and oxidation-activation-carbonization on the structure and properties of carbon materials were compared,and they were used as electrode materials for supercapacitors to study their electrochemical properties.The specific research contents and results are as follows:（1）The optimum extraction process of FA was as follows:the liquid-solid ratio of H2O2 to lignite was 4,the reaction temperature was 60 ℃,and the reaction time was 6 h.Under these conditions,FA had the highest O content（49.72 wt%）,the total acid group content was 8.64mmol·g-1,the carboxyl content was 5.59 mmol·g-1,and the phenolic hydroxyl content was 3.05mmol·g1.At 800°C,the weight loss rate of FA was 56.06%,which is the lowest carbon formation rate.The optimum extraction process of HA was as follows:the liquid-solid ratio of H2O2 to lignite was 3,the reaction temperature was 50°C,and the reaction time was 2 h.Under these conditions,The HA had the highest C content（67.04 wt%）and the highest extraction rate（34.53%）.The total acid group content was 5.03 mmol·g-1,the carboxyl content was 3.24mmol·g-1,and the phenolic hydroxyl content was 1.79 mmol·g-1.At 800°C,the weight loss rate of HA was 38.27%,which is the highest carbon formation rate.The optimum extraction process of BA was as follows:the liquid-solid ratio of H2O2 to lignite was 2,the reaction temperature was 70 ℃,the reaction time was 8 h,Under these conditions,the total acid group content was 6.72 mmol·g-1,the carboxyl content was 3.39 mmol·g-1,the phenolic hydroxyl content was 3.33 mmol·g-1,and the weight loss rate of BA was 49.91%at 800°C,that is,the carbon formation rate is in the middle.（2）The optimal preparation conditions of humic acid-based porous carbons（X-HPCHs）were as follows:the liquid-solid ratio of oxidation（H2O2/HA）was 3,the alkali-carbon ratio（KOH/HA）was 1.5,and the activation temperature was 750 ℃.The specific surface area of OAHPCH was 1727 m2·g-1,the total pore volume was 1.00 cm3·g-1,and the mesoporous ratio was 87.00%.In the three-electrode system,the specific capacitance of OAHPCH electrode was226 F·g-1 when the electrolyte was 6 M KOH and the current density was 0.2 A·g-1.When the current density increased from 0.2 A·g-1 to 5 A·g-1,the specific capacitance retention rate of OAHPCH was 51.33%.At the scanning rate of 20 m V·s-1,the contribution of pseudo capacitance accounts for 7%of the total capacitance;After 5000 charge-discharge cycles,the specific capacitance attenuation is only 75.46%of the initial value.（3）The optimal preparation conditions of fulvic acid-based porous carbons（X-HPCFs）were as follows:the liquid-solid ratio of oxidation（H2O2/FA）was 3,the alkali-carbon ratio（KOH/FA）was 1.4,and the activation temperature was 800 ℃.The specific surface area of OAHPCF was 3362 m2·g-1,the total pore volume was 2.23 cm3·g-1,and the mesoporous ratio was 67.77%.In the three-electrode system,the specific capacitance of OAHPCF electrode was453 F·g-1 when the electrolyte was 6 M KOH and the current density was 0.2 A·g-1.When the current density was 5 A·g-1,the specific capacitance retention rate of OAHPCF was 68.76%.At the scanning rate of 20 m V·s-1,the contribution of pseudo capacitance accounts for 32%of the total capacitance;After 5000 charge-discharge cycles,the specific capacitance attenuation is 86.80%of the initial value.In summary,the porous carbon materials prepared by FA have better structural and electrochemical properties than those prepared by HA.FA is more suitable than HA as carbon source to prepare porous carbon materials for supercapacitors,and oxidization-activation-carbonization is an effective method to adjust pore structure. |
