Preparation Of Spring-like Mesoporous Carbon Nanofibres For Intercalated Supercapacitors

Supercapacitors are a new kind of green energy-storage devices with high power density and long cycle life, which can be operated under wide range of temperature. Extensive research efforts are directed towards boosting the energy density of supercapacitors.In this research, graphitic carbon nanofibers (CNFs) prepared via chemical vapor decomposition were used as starting materials to introduce nanogate-carbon characteristics and spring-like mesoporous structure for intercalated supercapacitors with high energy density. The major conculsions were summaried as follows.(1) A simple method was developed to introduce mesoporous channels into catalytically grown CNFs via an oxidation-heat expansion method. The pristine CNFs were oxidated by the Hummers method to introcude the large amounts of oxygen functionalities into the graphene layers of CNFs. After a slow heat treatment, the oxygen functionalities vaporized gradually and, thus, forming spring-like mesoporous CNFs. The BET surface area and microcrystalline layer distance increased remarkably as a result of the oxidation-heat expansion.(2) Voltage-driven electrochemical activation took place in mesoporous CNFs electrodes, during which anions and cations intercalated into the graphitic layers. The result of the intercalation process was that, the interlayer distance was further expanded by8.1%to match the size of the ions. Besides, the surface area aviablable for ions was enlarged, so that there was much more room for ions to adsorb. Finally, the specific capacitance for the cell increased from2.8F/g to31.5F/g, and the capacitances for positive and negative electrodes were inproved to115.2F/g and138.1F/g, respectively.(3) Other influential factors were also dicussed in this work. As oxidation degree went deeper, the interlayer distance became wider, the Intercalation Starting Point became lower indicating an easier process, and the resulting capacitances also increased. Then, as the reason of structures, the platelet CNFs behaved better than the herring-bone CNFs during the process. Also, high working voltage of intercalation capacitors required the stability of the electrolyte. So SBPBF4electrolyte was more suitable for intercalation process than Ef4NB4.(4) The expanded carbon nano-fibres were also used as the positive electrode for the Li-ion capacitors. The negative electrode was pre-lithiated CNFs, so the cell voltage window was higher, which could contribute to a larger energy density. And the result showed that the energy density could be as high as18.8Wh/kg, while the power density was11.7kW/kg, which was much better than average supercapacitors.