Preparation And Capacitance Performance Study Of Carbon/MnO_x 3-dimensional Composite Nanofibers

As a new class of environmental friendly energy storage device, supercapacitor has greater energy density than conventional capacitor and higher power density than secondary battery. It can be widely applied in the fields such as mobile communication, portable mobile devices, electric vehicle and aviation-aerospace. Supercapacitor has attracted increasing attention in the field of current energy storage field. The property of electrode material is one of the key factors for the performance of supercapacitor. In other words, the performance of supercapacitor can be largely improved while proper electrode material is used.Carbon material with high specific surface area and excellent electrical conductivity shows a good capacitance performance, while transition metal oxide has a much higher capacity for electron storage than carbon material due to Faraday reaction with electrolyte ion. It has become a new branch for supercapacitor electrode material research. Among the transition metal oxides, MnOx is recognized as a kind of ideal supercapacitor electrode material due to the advantages of abundance, low cost, wide electrochemical window, environmentally friendly and high theoretical specific capacitance. But the poor electric conductivity limits its application as electrode material in supercapacitors without combination with other materials. So far, there were many reports about carbon/MnOx composite electrode to overcome the poor electric conductivity of MnOx, and some progress has been achieved in this area. But only combining nanomaterials with 2-dimensional interface composite is reported in these works. This kind of composite electrode is difficult to improve the loading capacity of MnOx while remaining its good electrical conductivity. Furthermore theirs synthesis methods are complex, and they are not suitable for practical production. In this work, a simple method has been proposed by adding manganese acetate into polyacrylonitrile (PAN) solution as electrospinning solution and a typical process that used for carbon nanofibers fabrication was applied to prepare carbon/MnOx 3-dimentional. The phase structure and micro-morphology were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), laser-raman spectrum, transmission electron microscopy (TEM), scanning electron microscope (SEM), etc. Electrochemical performance of the composite nanofiber electrodes was investigated by cyclic voltammetry and galvanostatic charge-discharge technique.In the first part, we studied the relationship of the pore structure and capacitance performance of porous carbon nanofibers to the addition amount of pore-forming agent polyethylene glycol (PEG). When the mass of PEG was 50% to that of PAN, the porous nanofibers electrode had the best capacitance performance. The specific capacitance achieved 207.2 F/g at the current density of 0.1 A/g. The specific capacitance retained 96.3% after 1000 charge-discharge cycles at the current density of 1 A/g, which demonstrated that the porous nanofibers electrode has an excellent long-term cycling stability.In the second part, carbon/MnOx 3-dimensional composite nanofibers were fabricated. The MnOx was consisted of MnO, Mn3O4 and Mn2O3. And it was dispersed in the composite nanofibers in the form of nanoparticle. As increasing the ratio of manganese acetate added into PAN solution, the main component of MnOx changed from Mn3O4 to MnO, the nanoparticles became larger and the reunion phenomenon aggravated at the same time. The composite nanofibers electrode has an excellent cycling stability but its rate performance is relative bad. When the addition mass of manganese acetate was 75% to that of PAN, the specific capacitance was 199.8 F/g under a low scan rate (1 mV/s), but it only retained 57.9% after the scan rate increased to 50 mV/s, as the composite nanofibers electrode was characterized via cyclic voltammetry technique. And after 1500 cycles at the scan rate of 100 mV/s, the specific capacitance retained 98.2% for the maximum value.In the third part, polyvinylpyrrolidone (PVP) was added into the electrospinning solution to fabricate porous carbon/MnOx 3-dimensional composite nanofiber, where PVP acted as roles of dispersing agent for MnOx and pore-forming agent. When the mass of PVP was 10% to PAN and the mass of manganese acetate was equal to PAN, the achieved porous composite nanofibers electrode had the best capacitance performance. The specific capacitance was 209 F/g under a low scan rate (1 mV/s), and retained 70% after the scan rate increased to 50 mV/s, it displayed a better rate performance than carbon/MnOx 3-dimensional composite nanofibers electrode fabricated in the former part, as the porous composite nanofibers electrode was characterized via cyclic voltammetry technique. After 1500 cycles at the scan rate of 100 mV/s, the specific capacitance retained 96.7% for the maximum value.As a conclusion, we demonstrated that porous carbon nanofibers electrode with good capacitance performance was fabricated when moderate PEG was added into PAN electrospinning solution. The composite electrode material with three-dimensional composite structure could increase the load capacity of the Faraday capacitance active material and showed a good capacitance performance at the same time, which shows its potential to be applied in practical supercapacitor electrode material.