Studies On The Composition, Structure And Performance Of Ultrafine Carbon Fibers

Ultrafine carbon fibers have aroused co nsiderable attention in the fields of energy and environment such as catalysis,hydrogen reserving,adsorption and so on due to its excellent chemical stability,high surface area,porous structure,superior conductivity and so on.However,the few active sites and large contact resistance restrict the efficient electron transfer and transport and the further enhancement of electrochemical performance.Doping carbon fibers with heteroatoms and fabricating interconnected carbon fibers with enhanced conductivity by introduction of graphene/CNT are effective ways to introduce more active sites and lower contact resistance,consequently enhancing its catalytic activity.Therefore,the following three parts are the studies the paper mainly focused on.Firstly,to gain carbon fibers based catalysts with large surface area and many active sites,the techniques of electrospinning and post-treatment doping are used to massively fabricate B,N-codoped carbon nanofibers.The impact of the two post-treatment methods,i.e.,heat-treatment of electrospun carbon nanofibers with the mixture of boric acid/urea in N2?BNCf-N?and subsequently activated in NH₃?BNCf-NA?,on the electrocatalytic activity and adsorption behavior is investigated.After B,N-codoping,B-N-C active sites was introduced into BNCf-N.B,N-codoping have synergetic effect in enhancing the electrocatalytic activity.NH₃ activation gives BNCf-NA a high surface area of 306.3 m2 g-1 with more microporous structure and optimized surface functionalities.When applied to the adsorption of MB,BNCf-NA display 99 % adsorption quantity in 4 minutes.These intriguing features render BNCf-NA higher electrochemical active surface area and excellent catalytic behavior with onset potential of-0.068 V?vs Ag/AgC l?,only ca.60 mV more negative than that on the Pt/C catalyst.Secondly,to obtain carbon fibers based catalysts with more active sites and higher conductivity,based on electrospinning and in-situ growth methods,NG/Co-CNT/CNF were prepared by the heat-treatment of air-cured Co-PANf,g-C3N4 and melamine.The in-situ growth mechanism of the graphene and CNT and their effect on the electrocatalytic and adsorption performance were studied.The hybrid show optimized ORR performance when the weight ratios of Co?NO₃?₂·6H₂O and PAN in electronspun solution are 1:10 and 1:15.Furthermore,when the weight ratio is 1:15,CNT were introduced.The simultaneous introduction of N G and CNT into electrospun CNF webs can ensure more active sites and facilitated electron transport,thus providing enhanced electrocatalytic performance for ORR.With a loading of 0.48 mg cm-2,the half-wave potential difference between NG/Co-CNT/CNF and Pt/C is only 33 mV,confirming that the interconnected 3D network architecture possess better ORR activity.Thirdly,to be environmental friendly,simplify the preparation process and lower the cost,based on the carbonization of silk cocoon and in-situ growth method,NG-SCC_f were prepared by the heat-treatment of silk cocoon,g-C3N4 and melamine.The corresponding composition,structure,electrocatalytic and photocatalytic performance were researched.Silk cocoon-derived carbon fibers?SCC_f?has thermally induced 3D interconnected web structure and a high surface area of 1032.1 m2 g-1.After in-situ growth of N G,the N content was increased from 2.08 to 7.19 at.%,more active sites were introduced,electrocatalytic activity was improved,H2 evolution rate was increased from 14.9 to 65.5 ?mol h-1 g-1.The interconnected SCC_f provide multipath to enhance the electron transport,and in-situ formed NG allows the further rapid transport of charge carriers and effective separation of electron-hole pairs,thus accelerating the reduction process to generate H2.NG-SCC_f are of broad application prospect as bifunctional catalysts.