Synthesis and Characterization of Asymmetric Supercapacitors

• Main Authors: Yen-Po Lin, 林彥伯
• Other Authors: Nae-Lih Wu
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/27208589236574033021

博士 === 國立臺灣大學 === 化學工程學研究所 === 99 === A new type of aqueous asymmetric supercapacitor that contains MnFe2O4 pseudocapacitive anode and LiMn2O4 battery cathode with 9M LiNO3(aq) as electrolyte has been synthesized and characterized. The anode and cathode electrodes were characterized separately in 1M and 9M LiNO3(aq). Both electrodes showed superior performance in high concentration electrolyte and high temperature. The nanocrystalline MnFe2O4 anode material have a specific capacitance of ca. 99 F/g and the LiMn2O4 cathode a specific capacity of ca. 128~100 mAh/g under 10~100 C-rate. The cell has a maximum operating voltage window of ca. 1.3 V, limited by irreversible reaction of MnFe2O4 toward reducing potential. The specific power and specific energy of the full cell were found to increase with increasing anode-to-cathode mass ratio (A/C) and saturate at A/C~4.0, which gives specific cell energies, based on total mass of two electrodes, of 10 and 5.5 Wh/kg at 0.3 and 1.8 kW/kg, respectively. The 4-1 cell shows good cycling stability that only within 5% capacitance loss after 5000 cycles, and exhibits significantly slower self-discharge rate than the MnFe2O4 symmetric cell and other asymmetric capacitors. In addition, the physical and electrochemical properties of two kinds of MnO2@C composite materials in 1M NaCl aqueous solution were determined and discussed. MnO2 was deposited onto muti-walled carbon nanotubes (CNT) and carbon black (CB) by chemical co-deposition to form composite materials. From X-ray powder diffraction characterizations, composites are spinel-type MnO2@CNT (abbreviated as S-MnO2@CNT) and birnessite-type MnO2@CB (denoted as B-MnO2@CB). SEM and TEM observations reveal that S-MnO2 was well-dispersed onto MWCNT with nano-flake structure, and the morphology of the B-MnO2 of the other composite was nano-particulate, and the BET surface area of B-MnO2@CB and S-MnO2@CNT are 138 and 156 m2/g. The S-MnO2@CNT-based electrode delivered 309 F/g-MnO2 and 247 F/g-MnO2 at 2 mV/s and 200 mV/s, respectively, which showed superior performance than that of B-MnO2@CB (229 F/g-MnO2 and 132 F/g-MnO2 at 2 mV/s and 200 mV/s), and therefore, the excellent performance is attribute to a larger contact area with the electrolyte, more homogeneous dispersion of oxide and the highly conductive substrate (CNT) also helped to enhance the performance of the electrode. A long-term stability test of the S-MnO2@CNT-based symmetric cell was carried out at 50 mV/s and 100 mV/s, and each sweep rate involved 5000 cycles. After 10000 cycles, the capacitance of the S-MnO2@CNT-based symmetric cell remained above 96%, but the B-MnO2@CB-based symmetric cell only retained 76.7% capacitance after 10000 cycles at 50 mV/s. Self-discharge tests show that S-MnO2@CNT could store charge longer than the B-MnO2@CB composite electrode or the amorphous MnO2@CB composite electrode, indicating that S-MnO2@CNT has superior performance for the application of supercapacitors. Besides, LiMn2O4@CB and LiMn2O4@CNT composite materials were synthesized successfully through a one-step hydrothermal process that employed B-MnO2@CB and S-MnO2@CNT as Mn precursors. The XRD characterizations show the features of spinel LiMn2O4 structure and graphitic structure of carbons, and the crystallite size of LiMn2O4 were calculated by using Debye-Sherrer equation individually. The carbon content among LiMn2O4@CB and LiMn2O4@CNT composite were analyzed using TGA, which are 37.6 wt% and 19.2 wt%, respectively. The electrochemical performance of LiMn2O4@CB composite was characterized in aqueous and organic electrolyte contain with Li ion, and the LiMn2O4@CB electrode exhibited 128 mAh/g reversible capacity in both electrolytes. Moreover, LiMn2O4@CB showed long-term stability while charging/discharging in a half cell at 25 oC and 55 oC, which remained 97% capacity after 660 cycles. The LiMn2O4@CNT electrode showed a reversible capacity of 130 mAh/g at 0.5 C-rate and presented an extremely high power density in 9M LiNO3 aqueous electrolyte, which delivered 111 mAh/g at 500 C charging/discharging rate.