Capacitance Performance On MnO₂ And MnO₂-C Composite Materials

Electrochemical supercapacitors as new energy storage devices show higher energy density than conventional dielectric capacitors, and higher power density and longer cycle life than batteries as well, filling the gap between conventional dielectric capacitors and batteries. With large discharge current, long cycle life and broad using temperature range, researching in supercapacitors appears unprecedented boom in recent years. Electrode material is the key to decide electrochemical capacitor performance. Manganese oxide with good electrochemical performance, low cost and environmental friendly has been used as electrode material, with loading of 10 mg·cm-2 and thickness of 100μm. Various material characterization and electrochemical methods were conducted to investigate influence of morphology structure, electrolyte type and concentration on capacitance, electrode preparation and capacitance characteristics of different manganese-carbon composite composites. The main contents and innovative points are as follows:1. Micro-nano-MnO2 with different morphologies, core-shell, sea urchin, nano slice agglomeration microspheres and nanorods agglomeration microspheres, have been prepared by room-temperature solution-based catalytic, wet chemical method or low-temperature hydrothermal method. The morphology, microstructure and supercapactor properties of MnO2 were characterized by XRD, SEM, N2 adsorption and desorption studies, cyclic voltammograms, constant current charge-discharge and AC impedance tests. The results show that crystallinity and specific surface area of as-prepared MnO2 are not the main reason causing capacitance differences, while morphology and micro-structure play more important roles. Closely network-mesoporous structure delivered higher capacitance performance than that of loose-open-structure. The sample with pore diameter at about 8 nm possessing high ratio hole volume showed better capability in Na2SO4 electrolyte. Nanorods agglomeration microspheres showed excellent electrochemical and cycle performance. The discharge specific capacitance of first cycle was 88 F·g-1 at the scan rate of 50 mV·s-1, after 2000 cycles by cyclic voltammograms, the specific capacitance achieved 114.2 F·g-1, which was 130% of the initial discharge specific capacitance.2. The influence of synthesis condition and electrode preparation on morphology, micro-structure and electrochemical performance of nanorods agglomeration microspheres were studied. The electrode showed better capacitance when dried at 70℃and pressured at 6 Mpa using the samples synthesized at 70℃for 30 min with stirring speed of 100 r/min as active materials.3. MnO2-C composites with good capacitance were prepared by magnetic mixing method. The composition, morphology and electrochemical performance were characterized by XRD, SEM, TEM, cyclic voltammograms, constant current charge-discharge and AC impedance tests. Different amount of CNTs were used as additional additive for electrode preparation and the composite electrode with 8:2 weight ratio of MnO2 and CNTs delivered the highest specific capacitance. The electrode delivers specific capacitance of 127.5 F·g-1 and energy density of 11.3 Wh-kg-1,61.8% and 61.4% more than that of MnO2 electrode, with power density of 400 W-kg-1 at a high current density of 1 A·g-1.