Electrochemical Preparation Of Metal Oxides Encapsulated Carbon Nanofibers And Their Supercapacitive Performance

As a new type of energy storage device, supercapacitors have a lot ofadvantages such as low costs, long cycle life, high power density, andenvironmental friendliness. Supercapacitor is a kind of energy storage device withthe performance intermediate between physical capacitors and chemical batteries.With the capacitance hundreds to thousands times higher than the conventionalcapacitors and energy density about one hundred times higher than chemicalbatteries supercapacitors possess excellent pulse charging/discharging and largeenergy storage capability. Electrode materials are the key constituting parts of thesupercapacitors and play key roles in the supercapacitor performance. Metal oxidesare of high promise as the supercapacitor electrode materials due to the highcapacitance, low cost, and simple preparation process, and thus receive greatattention. In this thesis, current research and development status of thesupercapacitors and their electrode materials were reviewed; oxides and hydroxidesof Ni, Co and Mn encapsulated carbon nanofibers （CNFs） were prepared based onelectrospinning and electrochemical deposition; and the supercapacitiveperformance of the obtained materials were investigated.In this thesis, The maincontents are as follows:The precursor polyacrylonitrile （PAN） nanofibers were prepared byelectrospinning. The CNFs were prepared from those fibers by preoxidation andcarbonization. SEM observation indicates that the diameter of the electrospun PANnanofibers is about100nm and that of the obtained CNFs after preoxidation andcarbonization is in the range of50-70nm.Ni（OH）₂, Co（OH）₂, and MnO₂encapsulating layer were coated successfully onthe surface of the CNFs by pulse electrochemical deposition. NiO and Co₃O₄encapsulating layers were obtained by heating the Ni（OH）₂and Co（OH）₂layers.The struture of the obtained materials was characterized by SEM, Raman, XRD, andFT-IR，which indicates that the deposits on the CNF surface by electrochemicaldeposition are nanofibrous MnO₂, nanosheet structured α-Ni（OH）₂and nanosheetstructured α-Co（OH）₂. The α-Ni（OH）₂and α-Co（OH）₂were converted into NiO andCo₃O₄after heat treatment. The electrochemical performance of all samples were investigated using cyclicvoltammetry（CV）, galvanostatic charge-discharge（CD）, and electrochemicalimpedance spectroscopy（EIS） in2mol/L KOH aqueous solution. The resultsindicate that all oxide and hydroxide coatings exhibit obvious pseudocapacitiveperformance. The specific capacitance of Ni（OH）₂and Co（OH）₂coated CNFs canreach819F/g and375F/g and that of MnO₂, NiO and Co₃O₄coated CNFs are391F/g,439F/g, and528F/g, respectively, at the current density of1A/g.The encapsulating layers with different thickness of the above deposits wereprepared by changing the depositing time, followed by the measurements on theirstructure and electrochemical performance. The dependence of the supercapacitiveperformance on the depositing time was investigated measured and the optimaldepositing time at the current density of3mA/cm²was determined.