Preparation Of Manganese Oxide/Carbon Nanofiber Composites And Their Electrochemical Performance

As intermediate systems between conventional capacitors and batteries, supercapacitors possess high power and reasonably high energy density, excellent charge/discharge efficiency and long cycle life. Supercapacitors have been paid much attention in many fields, such as electric vehicles, mobile telecommunication, electronics technology, aviation vs. aerospace, military force, and so on. Electrode material is the key to deciding the performance of supercapacitor. Among the electrode materials, manganese oxide (MnOx) has many advantages, such as its good electrochemical behavior, low cost and environmental benignity. It has been reported that, carbon material with good conductivity, high surface-area-to-volume ratios and abundant pores can serve as a supportive matrix to improve the efficiency of MnOx. In this work, carbon nanofiber (CNF) containing MnOx was prepared as freestanding electrode for supercapacitor, with nanostructrued MnO2and Mn(CH3COO)2as manganese sources. Various characterization and electrochemical techniques were conducted to investigate influence of manganese precursors on the structure and capacitive behavior of different MnOx/CNF composites.Nanostructrued MnO2with different morphologies, including rod, needle, plate and petal, was prepared from KMnO4and MnS04by hydrothermal method, in which reaction conditions (temperature and pH) were adjusted to control the structures. The needle-like MnO2was chosen as the precursor since it displayed best dispersion in MnO2/polyacrylonitrile (PAN) nanofibers by electrospinning, then after pre-oxidization and carbonization, the MnOx/CNF composites were fabricated. The morphology and micro-structure were characterized by means of FESEM, TEM, EDS, XRD and XPS. As a result, the MnOx/CNF films present uniform, continuous and porous structure inside the nanofibers. Electrochemical property was investigated by cyclic voltammetry, galvonostatic charging/discharging and impedance measurement techniques. The resulting MnOx/CNF composites exhibit good electrochemical behavior. The sample containing30wt%MnO2displays a specific capacitance as high as104F g-1at the scan rate of2mV s-1, and the sample containing10wt%MnO2retains82.1%of the initial capacitance after2000cycles of charging/discharging at the current density of1A g-1. In the composites, CNF with large specific surface area and porous structure provides a conductive framework for MnOx, which can accelerate charge transfer, reduce the diffusion pathway of ions in electrolyte, and improve cycling stability.To further enhance the electrochemical performance, polyvinyl pyrrolidone (PVP), a surfactant, was introduced into hydrothermal reaction to control the nanosize of MnO2. The resulting MnO2displays the morphology of nanorod-agglomeration microsphere, with obvious decrease of unit size compared with needle-like MnO2. PVP was also added into PAN solution as a dispersant to avoid aggregation of MnO2. Via the same electrospinning and carbonization process, MnOx particles present better distribution in the nanofibers. Compared to previous composite system, this product exhibits better electrochemical performance. The sample containing25wt%MnO2displays a specific capacitance as high as129F g-1at2mV s-1, and retains82.9%of the initial capacitance after2000cycles of charging/discharging at1A g-1, as well as better rate capability and smaller resistance. The improvement of the electrochemical property is ascribed to the reduced dimention of MnOx nanoparticles and the increased porosity in CNF, which enlarges the MnOx surface area exposed to the electrolyte and provides more electrochemical activated sites. Also, the better dispersion and more stable structure result in more effective electron-transfer and longer cycle ability.Using Mn(CH3COO)2as manganese source has successfully avoided the uneven dispersion of MnOx nanoparticles, because Mn(CH3COO)2can dissolve in PAN solution. Then by electrospinning and carbonization treatment, Mn(CH3COO)2changed to MnOx. The two MnOx/CNF composite systems prepared from different manganese precursors have the similar MnOx crystal structure composed of MnO and Mn3O4, due to the same carbonization condition. Compared with MnOx/CNF products fabricated from MnO2, the MnOx/CNF composites from Mn(CH3COO)2display better distribution and less aggregation of small nanoparticles. Therefore, the latter structure promotes the charge transfer from MnOx to CNF and enhances the availability of MnOx. The sample containing50%Mn(CH3COO)2·4H2O exhibits a specific capacitance as high as175F g-1at2mV s-1, and retains78.3%of the initial capacitance after2000cycles of charging/discharging at1A g-1.