Cycling Stability of Manganese Dioxide Polymorph Supercapacitors at Elevated Temperature
Date Issued
2014
Date
2014
Author(s)
Pan, Hsiao-An
Abstract
Cycling stability at elevated temperature (50°C) of supercapacitors made of two different MnO2 polymorphs, including cryptomelane (α), birnessite were investigated.
In the first part of the thesis, the cycle life of the α-MnO2 was analyzed by the cyclic voltammetry, which was found to be worse at 50°C. The cycling performances obtained from different potential regions reveals that the capacitance fading of the α-MnO2 while cycling at full range is caused by the intercalation/deintercalation process mainly occurs at 0.7 V. In addition, from the cycling tests in alternative scan tare, the capacitance fading of the sample became more dramatic for higher scan rate at 50°C. This means that the structure could not supply well enough electronic or ionic transfer rate after cycled at 50°C for 5000 times. From the synchrotron X-ray diffraction analysis, the peak intensity that related to the layer structure had decreased after 5000 cycles at room temperature, and decreased to an even lower value after cycled at elevated temperature. This result is believed to be caused by the Jahn-Teller effect which mainly presents at the potential (around 0.7 volts) for cations to intercalate into the bulk material. In-situ XAS was further done to investigate the valence change of the Mn atoms which confirmed the intercalation/deintercalation behavior at 0.7 Volts. The repeated volume change of the unit cell attributed from the Jahn-Teller effect during the charge/discharge upon cycling at elevated temperature leads to loss of capacitance, especially at higher scan rate.
High-temperature (50°C) cycling stability of birnessite MnO2 was also examined, while different finding comparing to the α-MnO2 was obtained. For the birnessite MnO2, only 30 and 27 percent of initial specific capacitance were remained after 5000 cycles under the same composition of electrode used for α-MnO2 in room temperature and 50°C respectively, which can be improved by adding more binder. Same procedure of changing the scan rate during the cycling test as we did for α-MnO2 samples was carried out for birnessite samples as well. Combining with the resistivity measurements of the electrode, results show that the decay of the capacitance for the birnessite sample at elevated temperature is due to the overall structure breakdown which increases the electronic resistivity of the electrode. The reason of the structure breakdown is the severe swelling and shrinking of the birnessite MnO2 that stretch and loose the PVdF binder, which causes the fatigue of the binder after 5000 cycles. The amount of cations intercalating or deintercalating through the layer structures was limited for smaller potential regions, so the difference between the samples that cycled at room temperature and 50°C was therefore unobvious in these cases.
Subjects
超高電容器
二氧化錳
結晶型
循環穩定性
高溫
Type
thesis
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