A Study On Structure And Supercapacitive Property Of Polyaniline Coating Layers On Carbon Nanofibers

As a new energy storage device, supercapacitor compared to conventionalbattery is highly promising for many important applications, including portableelectronics, computer backup systems and automotive auxiliary power systems. Allthese attributed to the advantages of high energy density, long cycle life, safety, andfast charge and discharge rates. The electrode materials play key roles in the overallperformance of the supercapacitors, which encourages researchers to explore theirsynthesis, characterization, and device application for supercapacitors. The carbonnanofibers (CNF) produced by electrospinning have excellent one dimensionstructure with the advantages of small diameter and good separation from each other.Coating polyaniline (PANI) on the CNF can provide both the electrical double layer(EDL) capacitance and pseudocapacitance, improving the overall performance whenused as the supercapacitor electrode materials. In this thesis, polyaniline coated oncarbon nanofibers were prepared via in situ polymerization and interfacialpolymerization methods, and the microstructures as well as the supercapacitiveproperties have been studied systematically.In in-suit polymerization, Fe2(SO4)3and APS/FeSO4were selected as theoxidant respectively, where the structure and property of the products wereinvestigated under differente reaction parameters. When using Fe2(SO4)3as theoxidant, the structure of polyaniline coating layers and the performance of theobtained supercarpacitors are dependent on the reaction time, oxidant concentration,and aniline concentration. The results indicate that when the reaction time is7days,the molar ratio of Fe2(SO4)3/An is8/1, the aniline concentration is0.05M, thecarbon nanofibers are uniformly coated with granular polyaniline particles, whichincrease the contact surface area with electrolyte and thus significantly enhance thesupercapaciance during the charge-discharge process, the specific capacitance canreach188.8F g-1. When using the complex oxidant APS/FeSO4,the reaction time isshorten effectively due to the high redox potential ofSO4and the microstructureof materials and their supercapacitance can be changed by changing the reactiontime and aniline concentration. The results show that when the reaction time is6h,the aniline concentration is0.03M, the polyaniline layers coated on the carbonnanofibers are oriented tapered particles, and the specific capacitance can reach 150.8F g-1.It is possible to get good morphology of polyaniline coating layers even at highaniline concentrations via a facile interfacial polymerization method, because thispolymerization method can efficiently control the growth rate. For this method,changing the reaction time and aniline concentration are able to prepare differentcoating morphology and achieve different supercapacitor properties. The resultsshow that when the reaction time is3h, the aniline concentration is0.1M,needle-like aligned polyaniline layers are covered on carbon nanofibers, and thelength of polyaniline needle-like structure is about50nm with the diameter beween30and50nm. This unique structure enables to increase the specific surface area ofthe electrode materials. The gap between the aligned nanowires could enhance theaccessibility of the electrolyte and shorten the diffusion distance of ion to thesurface of active material. Electrochemical tests reveal that the specific capacitanceof the obtained materials can reach281.2F g-1at the current density of1A g-1.Even the current density increases to5A g-1, the capacitance can still retain at222F g-1. Long term charge/discharge test at current density of1A g-1shows thatthe specific capacitance retains almost100%after850cycles.