Mesoporous Titania for High Rate Electrochemical Energy Storage
Abstract: Mesoporous titanium oxides are increasingly attracting interest as potential candidates for fast electrochemical energy storage. In this thesis, mesoporous titania of different polymorphs have been evaluated in relation to their ability to reversibly store Li+ ions inside their structure. Mesoporous anatase beads with high crystallinity and specific morphology show high electrochemical capacity and high rate performance. The materials store charge through Li+ ion insertion of both faradaic and extrinsic pseudocapacitive nature. Moreover, the discharge capacity retention after 150 cycles is >95%. On the other hand, ordered mesoporous titania, prepared via low-temperature spray deposition and mainly amorphous, shows a linear correlation between voltage and capacity, typical of an intrinsic pseudocapacitive material of insertion type. The material exhibits exceptionally high electrochemical capacity of 680 mAh g-1 during the first cycle, which, however, rapidly decreases over the following cycles. A combination of electrochemical and structural characterization techniques is used to study the charge/discharge behavior of the material and the origin of the irreversible capacity. X-ray absorption spectroscopy and energy-filtered TEM are carried out to analyse pristine and cycled samples in charged and discharged state. The results suggest that the irreversible loss in the capacity is due to the formation of electrochemically inactive phases mainly located at the surface of the material. Additionally, electrodes based on mesoporous anatase beads are paired with a commercial activated carbon electrode that presents a broad distribution of mesopores and very high surface area to design a hybrid asymmetric supercapacitor. The device shows extraordinary stable performance with energy densities of 27 Wh kg-1 at the relatively fast discharge current of 2.5 A g-1 for 10 000 cycles and high power densities during fast cycling.
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