Rapid Preparation Of Lignin-based Carbon Aerogels By Microwave-assisted Method

Supercapacitors,as new type of energy storage device,have high specific capacitance,good cycle stability,and fast charging,which make them widely used in backup energy,electric vehicles,and wind power etc.Carbon aerogels are a new type of nano-scale porous carbon material with excellent properties such as high porosity,controllable pore size distribution,low density,high specific surface area and high electrical conductivity.Therefore,carbon aerogels have a good application prospect in the electrode materials of supercapacitors and rechargeable batteries.At present,carbon aerogels are mainly prepared by sol-gel method using petroleum-based raw materials,which has high cost,long production cycle,certain harm to human body and environment.In this study,renewable biological resource-alkali lignin was used as raw material to partially replace resorcinol.With microwave-assisted technology,alkali lignin and resorcinol react with formaldehyde rapidly to gel.Then biomass-based carbon aerogels are prepared by drying and carbonization.In order to further improve the electrochemical performance,the lignin-based carbon aerogels were modified by metal oxide.The effects and mechanism of microwave reaction conditions,lignin addition and metal oxide modification process on the microstructure and electrochemical properties of carbon aerogels were investigated.Finally,the effects of CO₂ activation process on the microstructure and electrochemical properties of carbon aerogels were studied by step-by-step optimization method using optimal microwave reaction conditions,lignin addition and metal oxide modification.The results are as following.?1?Alkali lignin has phenolic properties and can be used in place of resorcinol to prepare biomass-based carbon aerogels.Lignin-based carbon aerogels can be prepared in 60 minutes using microwave assisted technology,while the traditional sol-gel method takes 7 days,which greatly shortens the production cycle,reduces production costs,and increases productivity.Scanning electron microscopy?SEM?characterization shows that the lignin-based carbon aerogels do not show obvious three-dimensional network structure.Alkali lignin has a low active site,and the gel network formed by condensation with resorcinol and formaldehyde under the action of microwave has low crosslink density and gel strength.The network structure is easily contracted and collapsed during drying and carbonization process.When the reaction time is 60 min and the microwave power is 10 W,the prepared lignin-based carbon aerogels have the most developed network structure with a specific surface area of 179 m²/g.The carbon aerogels exhibit excellent electrochemical performance with a specific capacitance of 76.97 F/g at a current density of 1 A/g.?2?The microstructure of the lignin-based carbon aerogels was adjusted by the addition of alkali lignin under a microwave reaction condition of 60 min-10 W.When the mass ratios of lignin to resorcinol are 0:100 and 100:0,lignin-based carbon aerogels cannot be prepared by microwave assisted technology;when the mass ratios of lignin to resorcin are 30:70,50:50,70:30,and 90:10,lignin-based carbon aerogels can be prepared using microwave assisted techniques.Lignin is a large-sized molecule with low reactivity.At the same time,as the amount of alkali lignin added increases,the pH of the precursor solution increases,the gel speed increases,and the shrinkage rate increases,and the three-dimensional network structure of the carbon aerogels cannot be retained.The lignin-based carbon aerogels prepared with a mass ratio of lignin to resorcinol of 30:70 exhibit a high degree of graphitization,and developed network structure,with a specific surface area of up to 256 m²/g.The specific capacitance reaches 140.26 F/g at a current density of 1 A/g.?3?The lignin-based carbon aerogel was modified by metal oxide.The SEM characterization shows that the metal oxide modified lignin-based carbon aerogels have an obvious three-dimensional network structure.The metal ions act as catalyst in the sol-gel process,promote the gelation reaction,retain the three-dimensional network structure of the carbon aerogels.At the same time,the metal ions can be uniformly dispersed in the carbon aerogels under the action of microwave.The structural characterization shows that Cu and Mn are present in the carbon aerogel in the form of Cu and MnO,respectively.The specific surface area of the composite carbon aerogels prepared by adding Mn ions to 5%is 675 m²/g,which is 31.3%higher than that of the carbon aerogels prepared by adding Cu ions to 5%,and is 163.6% higher than that of the carbon aerogels prepared without Mn ions.The composite carbon aerogels prepared by adding Mn ions to 10%has a well-developed microstructure,and optimal electrochemical performance.The specific capacitance is as high as 231.06 F/g,which is 64.7%larger than that of carbon aerogels prepared without Mn ions.?4?The MnO/lignin composite activated carbon aerogels were prepared by one-step CO₂activation process.SEM characterization shows that the CO₂ activation process does not destroy the skeleton structure of the composite carbon aerogels and retains its network structure.After activation process,the specific surface area of the composite carbon aerogels increases from 582 m²/g to 918 m²/g.The CO₂ activation process introduces a large number of micropores and some mesopores to the composite carbon aerogels,providing a pathway for ion migration.The composite carbon aerogels prepared after CO₂ activation process exhibit a relatively developed microstructure,and highest specific capacitance of 267.89 F/g.The internal resistance of the MnO/lignin composite activated carbon aerogels is the lowest.