Preparation Of Ultra-high Specific Surface Area Activated Carbon By Lignin Thermal Pretreatment And Its Storage Mechanism

Natural biomass material is an important precursor to prepare activated carbon material instead of coal.Super activated carbon is the most mature commercial energy storage material at present,but there are still some problems such as high cost,low yield and unclear aperture control mechanism.Lignin has natural spatial network structure and high carbon content,which is a high quality material for the preparation of activated carbon.In this paper,on the basis of studying the influence of different thermal treatment methods on the structure of lignin,the synthesis of super activated carbon and the study of the effect of pore size on the electrochemical characteristics.This study has important theoretical significance for the application and development of biomass carbonaceous materials.The research contents were as follows:（1）The hydrocarbon ratio of lignin was close to 11,and the aromatization degree was high.With the increase of torrefaction pretreatment temperature,the carbon element in lignin increased,and the aromatic ring began to decompose at higher temperature.The dehydration reaction of alkali thermal and hydrothermal process resulted in the removal of hydroxyl group in lignin,and condensation reaction also took place.The torrefaction treated lignin has lower oxygen-containing group concentration and higher aromatization degree than the raw material.Alkaline conditions promote the decarboxylation of lignin,and hydrothermal processes promote the hydrolysis and cleavage of C-C bonds.Meanwhile,hydroxyl groups in lignin increased gradually under the action of OH-in hydrothermal process.The number and weight average molecular weights of lignin in alkali thermal pretreatment were up to 1564 and 1595,respectively,while those in hydrothermal pretreatment were 1447 and 1464,respectively.When the temperature was higher than 200℃,the molecular weight of lignin increased and a new irreversible polymerization reaction occurred between lignin units.（2）TG/DTG curves of lignin after roasting and pretreatment were similar to those of raw materials.The main weight loss temperature range was from 200℃to 700℃,and there were significant weight loss peaks near 360℃and 735℃.The first weight loss peak temperature of alkali thermal lignin increased to 369℃and the weight loss rate increased to 6.86%,while the second weight loss peak was not obvious.The first weight loss peak of hydrothermal lignin was227℃,which was significantly lower than that of other treatments.The second weight loss peak was near 400℃,and the weight loss rate was 7.16%,which was the highest in all DTG curves.The final carbon residue rate was 49.25%,which was the lowest among the three thermal pretreatment methods.TG-FTIR showed typical characteristic peaks of H₂O,CH₄,CO₂ and CO,as well as aldehydes,ketones,organic acids,ethers and phenols in the gas products.After pretreatment,the pyrolysis gas products of lignin were different.The release trend of CO₂ was closest to the peak of DTG curve,and the absorption peak intensity was the highest among the products.The output of CO increased gradually with the increase of temperature,while the peak time of H₂O was earlier,and the intensity increased gradually with the occurrence of pyrolysis.The absorption peak of CH₄appeared near 380℃,which was generated by the fracture of methoxide group.With the rise of temperature,the secondary pyrolysis of volatile compounds increases the yield of CH₄.（3）FWO method fitted the pyrolysis activation energy of lignin in the range of36.69～334.21k J/mol,and the pyrolysis activation energy increased after roasting.The activation energy of alkali thermal pretreatment lignin was from 34.64 k J/mol to 302.13 k J/mol,and that of hydrothermal pretreatment lignin was the lowest from 32 k J/mol to 282 k J/mol.The activation energies of the gaussian multi-peak fitting model were 112.01,185.36,166.59 and 163.40 k J/mol,respectively,which were lower than those calculated by FWO method.The pyrolysis products of lignin before and after thermal pretreatment were mainly phenols,among which the most relative content was guaiacol,followed by 3-methoxy-4-hydroxybenzaldehyde.Alkali thermal and hydrothermal pretreatment increased the content of monomer phenols,such as catechol,4-methylguaiacol,4-methylcatechol,3-methylcatechol,etc.,indicating that alkali-thermal and hydrothermal pretreatment promoted the depolymerization of lignin.（4）The micropore specific surface area and pore volume ratio of activated carbon prepared by torrefaction at 150℃reached 2087 m²/g and 75%.When the alkali thermal pretreatment temperature was at 150℃,the activated carbon had the highest total specific surface area and total pore volume of 2564 m²/g and 1.44 cm³/g,respectively.Under hydrothermal pretreatment at 200℃,the total specific surface area of activated carbon was up to 3559 m²/g,and the total pore volume was up to 2.13cm³/g.The total specific surface area and total pore volume decreased at 250℃.Electrochemical experiments show that the maximum specific capacitance was 288.75F/g when the current density was 0.5 A/g.In constant current charge-discharge,the curve of activated carbon was isosceles triangle.When the scanning rate was from 5 m V/V to 20 m V/V,the shape of the electrode material was approximately rectangular,and the charge and discharge performance of the electrode material was better and the electrochemical performance was stable.（5）In order to further improve the rate performance of ultra-high specific surface area activated carbon,CO₂ secondary activation was used to control the pore size.The activation kinetics of CO₂isothermal and non-isothermal activation was calculated by tg curve.The fitting degree R² of the first stage water release was lower than that of endothermic reaction,and the activation energy was33.21k J/mol.The fitting degree of the second and third stage was above 0.96,indicating a high fitting degree.The third stage was the main activation stage,and the activation energy was178.32k J/mol.After the secondary activation,the specific surface area of the activated carbon did not decrease significantly,but the total pore volume decreased significantly,from 2.13 cm³/g to1.81cm³/g,the micropore volume decreased from 0.90cm³/g to 0.80cm³/g,and the mesopore ratio increased from 0.27 to 0.33.After secondary activation,the mass capacitance of activated carbon decreases to 284F/g,but it still remained high with the increase of current density.After 10000charge-discharge cycles,the capacitance retention of the activated carbon increased from 81.02%to 90.12%after the second activation,indicating that it had a certain effect on improving the electrochemical performance of the activated carbon.