Development Of Hexamethylenetetramine Cross-linked Phenolic Fiber And Its Derived Flexible Activated Carbon Electrod

Phenolic fibers（PFs）is a synthetic fiber with three-dimensional cross-linked macromolecular network.It has been widely used in heat insulation and special protection and due to its excellent thermal stability and flame retardancy as well as chemical stability.The derived activated carbon fibers（PACF）show high char yield,large specific surface area and good mechanical robustness.These properties endow them a potential application in pollution treatment and energy storage.Formaldehyde,which is used as crosslinking agent in the preparation of phenolic fibers,is harmful to human body.in this work,we use hexamethylenetetramine（HMTA）as a non-toxic cross-linking agent instead.Melt-spun phenolic raw fiber was cross-linked with HMTA in a solution bath.The effect of crosslinking parameters on the properties of PFs were discussed.Following the similar crosslinking strategy,PFs mats were prepared by melt centrifugal spinning and microfluid air-jet spinning respectively.Ccorresponding flexible activated carbon fiber felt（PACF）and film（PCFM）were obtained after a one-step KOH chemical activation.Their structure were characterized by SEM,XRD,Raman and BET respectively.The electrochemical performance is studied through cyclic voltammetry,constant current charge and discharge and AC impedance.PFs derived activated carbon fibers show a promising candidate as flexible supercapacitor electrode.The details are included as follows:（1）cross-linking groups mainly composed of methylene and ether bonds are formed between the molecules,and the mechanical properties of the phenolic raw fiber are initially improved fter the raw fiber is pre-crosslinked in the hydrochloric acid solution of HMTA,the hydroxymethyl groups further condense to form more methylene groups,and the phenolic hydroxyl groups are converted into more stable carbonyl groups,the cross-linked network structure is formed inside the molecule After thermal cross-linking in a vacuum environment The maximum breaking strength of the cross-linked phenolic fibers reaches to 3.63 c N/d Tex and breaking elongation is 7.7%respectively.The initial decomposition temperature reaches to 386℃and the mass retention rate at 900℃in a nitrogen atmosphere is 60.7%.（2）phenolic fiber spunlace felt as the precursor,PACF is obtained under the condition of alkali-carbon ratio of 3,900℃for 1 hour activation has good flexibility,which can store more electrolyte ions as its specific higher surface(1124.5 m² g^-1),and has a fast response ability to large current densities due to its larger distribution of mesoporosity（20%）,meanwhile,lower defect density makes it have smaller resistance In 6 M KOH solution,the specific capacitance of PACF is 411 F g^-1 at the current density of 0.2 A g^-1 and the can maintain at 203 F g^-1 when the current was increased to10 A g^-1,with a capacitance retention rate 49.3%.Capacitance retention is 100%after10000 cycles at a current density of 5 A g^-1,the resistance of PACF is 3.05Ω.（3）Compared with PACF electrodes,PCFM is lighter,thinner and more flexible,with a mass specific capacitance of 408 F g^-1 at 0.2 A g^-1 and 211 F g^-1 at 10 A g^-1 in a6 M KOH solution,capacitance retention is 100%after 10000 cycles at a current density of 5 A g^-1,the resistance of PACF is 1.05Ω.