The Structure And Property Of Phenolated Lignin And Its Activated Carbon Fiber

In this work,autohydrolysis enzymolysis wheat straw residue(AER)was used as raw material to fabricate lignin-based activated carbon fibers(LACF)by steam activation.The mechanism of AER phenolation,the reactions of glutaraldehyde with phenolated lignin-based precursor fibers under alkaline condition,preoxidation and carbonization as well as activation was investigated comprehensively.The spinnability of AER was improved by phenolation modification and then the acquired as-spun fibers were cured using glutaraldehyde.To get the optimal process of steam activation by means of debugging activation parameters.After that,compare the effects of steam activation and chemical activation on pore structure,crystal structure and chemical structure.At last,the electrochemistry performance of LACF was detected.The main conclusions were as follows:(1)The phenolated lignin obtained through AER reacted with phenol at a ratio of 1:3 at 120℃ for 2h exhibited good melting property.The comprehensive characterization indicated that all lignin substructures(β-O-4’,β-5’/α-O-4’,β-β’,ferulate,p-coumarate and p-benzoate)were reacted during phenolation,resulting in decrease in aliphatic hydroxyl groups and increase in the phenolic hydroxyl groups.Phenolated lignin resulted in significant decrease in molecular weight.Phenol was incorporated onto both the sidechains and aromatic nuclei.β-elimination of γ-hydroxymethyl group as formaldehyde and the formation of diphenylmethane structures.Both T6 and Ts positions of tricin are highly activated towards electrophilic substitution,the disappear of T6 and Ts signals indicated these structures may condensed with themselves,lignin and/or phenol/formaldehyde during phenolation.(2)The precursor fibers could be made infusible in air after treating with glutaraldehyde(GA)under alkaline condition.Hydroxymethylene groups were formed and C=O,C-O-C and C-O were reduced during curing reaction.(3)After preoxidation,hydroxymethylene groups condensed meanwhile S and H units formed more cross-linkages,aromatic sidechain oxidation and demethoxylation during the oxidation process.Finally forming a heat-resistant mesh structure.(4)Different chemical reactions occured during carbonization.The removal of oxygen-containing functional groups such as carboxyl groups,ester groups and elimination of methylene groups mainly happened in 195-645℃.700℃ is a key turning point in the structural change of preoxidized fibers.Dehydrogenation of lignin aromatic ring at this temperature,benzene rings are combined with each other to form a polystyrene fused ring,and then transformed into graphite-like structure at 900℃.(5)The graphitization degree of fibers carbonized at 700℃ is very low.The amount and order of graphitized carbon both improved when temperature rises to 900 ℃.Meanwhile,since part of carbon six-ring structure were destroyed under high temperature,sp3 carbon defects were detected.During the graphitization process,the growth rate of the(002)crystal plane is greater than that of the(100)crystal plane,and the d002 value does not change much.(6)The specific surface area and total pore volume of steam activated LACF are the largest of these four activators.LACFs activated with steam and H3PO4 are mainly microporous.The total mesopore volume of LACF activated by KOH was the largest and the ZnCl2 had the lowest pore-forming rate.Among the four activators,steam activated LACF has the lowest degree of graphitization,followed by H3PO4.KOH activated LACF had a minimum graphite layer stack thickness Lc002 and base width of graphite crystallite La100.(7)The specific capacitance(Cs)of LACF activated by steam at 850℃ for 100 min reached 143 Fg-1 in 6M KOH electrolyte,with 79%of Cs retention tatio at 10 Ag-1.The Cs retention ratio after 2000 charging-discharging cycles at 3 Ag-1 reach up to 97.6%.