Preparation And Electrochemical Performance Of Nanocarbon Materials From Petroleum Asphalt

Carbon nanomaterials and their composite with iron are promising materials forvarious applications in the fields of energy, information technology, biology, andenvironment due to their unique structure and physiochemical properties. Controlledsynthesis of these nanomaterials is of vital importance because many of the potentialapplications rely on the morphology and structure of the carbon nanomaterials. Thecarbon source used for the synthesis is an important factor influencing the productioncost of carbon nanomaterials. In this study, petrol asphalt, a carbon-rich and low-costby-product from the petrol refining, was employed as carbon source for the synthesisof carbon nanoballs, carbon nanofibers, sea urchin-like carbon structure andspiral-like carbon nanomaterials. The obtained materials were test as anode in lithiumion batteries and their electrochemical performance was evaluated. The followingtopics were covered in this work:1. Carbon nanobeads were synthesized by the pyrolysis of petrol asphalt at450°C. The carbonization of the carbon beads were subsequently performed at1500°C innitrogen. The obtained carbon nanobeads with the diameter in the range of50–100nmshowed relatively low degree of graphitization, and different functional groups weredetected on their surface. The nanobeads showed an excellent coulomb efficiency anda high stability with a capacity of500mAh/g after100cycles at1C. The capacity ishigher than200mAh/g even at10C. The treatment at high temperature led to thevolumetric contraction of the nanobeads and the decrease of the amount of surfacefunction groups. Simultaneously, the crystallinity was enhanced by the thermaltreatment. However, the electrochemical performance decreased clearly with a smallercapacity of200mAh/g at1C.2. Carbon nanobeads with controllable diameters were synthesized by chemicalvapor deposition using petrol asphalt as carbon source. The influence of reactiontemperature, time and temperature zone on the structure and size of carbon beads wasinvestigated. In the low temperature zone, nanobeads with small diameters wereobtained. The size of the carbon beads increases with increasing reaction temperature and time. Carbon beads of20–100nm with low graphitization degree were obtainedat different reaction temperatures accompanied by a small amount of polymers. In thehigh temperature zone, carbon beads with diameters of20nm–2μm and a clearlyhigher degree of graphitization were obtained in connected form. The capacity of thecarbon beads from the low temperature zone was determined to be300mAh/g after100cycles at1C, similar with the samples obtained in the high temperature zone atthe same rate. The capacity of the samples of the high temperature zone stabilized at150mAh/g at5C and further decrease was not observed at10C, which outperformedthe sample from the low temperature zone in terms of cyclability and coulombefficiency apparently due to its high graphitization degree.3. The synthesis of carbon nanomaterials and carbon-iron composites bycatalytic pyrolysis of petrol asphalt in inert gas with iron as catalysts. When ferrocenewas used as catalyst, the amount of ferrocene and reaction temperature were studied.In the low temperature zone nanobeads （50-100nm）, carbon nanotubes （20nm） andhierarchical carbon nanotube-iron composites were obtained when ferrocene of0wt%,10wt%and20wt%were applied, respectively. The crystallinity of the carbonmaterials increased first with the amount of ferrocene, and decreased with furtherincrease of ferrocene amount. In the high temperature zone, branched carbonmaterials were obtained with the diameter of about400nm. Additionally, inorganiciron source was studied as catalysts including Fe powder, FeS, FeCl₃, and Fe₃O₄,where Fe presents in different oxidation state from0to+3. When Fe or Fe₃O₄wasused as catalysts, chain-like fibers were obtained as disclosed by FESEM studies. Incase of FeS and FeCl₃, branched fiber-bundles were obtained. All the four samplesshow spiral type structures under TEM, and the degree of graphitization appears to benot high. Catalysts were not detected in the samples. The sample synthesized in thehigh temperature zone with ferrocene as catalyst was tested as anode for lithiumintercalation. Surprisingly, the capacity increased to380mAh/g after100cycles at1C, and the coulomb efficiency was stabilized at100%after the11cycles. At high rateof5C and10C, similar capacities of no less than200mAh/g were obtained. Hence, the carbon material showed a better electrochemical performance than those obtainedwithout catalyst due to a higher degree of graphitization and the presence of ironspecies.