Novel electrochemical capacitors based on free-standing single-walled carbon nanotube films
Date
2012
Authors
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Publisher
University of Delaware
Abstract
In order to effectively store the energy from the renewable sources and release them when needed, energy storage devices play an important role. Among all the energy storage devices, the electrochemical capacitor currently holds the highest power density and modest energy density. Nanostructured materials have shown promising electrochemical performance as electrode materials for electrochemical capacitor and different carbon nanostructured materials are under close scrutiny. Among those, carbon nanotube films draw great attention owing to their extraordinary properties, such as high surface area, excellent electrical conductivity, and mechanical robustness. In this thesis, the electrochemical capacitors based on freestanding single-walled carbon nanotube (SWNT) film are investigated from three perspectives.
First, the effect of compressive stress on the electrochemical behavior of flexible supercapacitors based on the freestanding SWNT film electrodes and 1 M aqueous electrolytes with different anions and cations were investigated. The results demonstrated that the specific capacitance increased firstly and saturated in corresponding to decreases of the series resistance, the charge transfer resistance, and the Warburg diffusion resistance under an increased pressure from 0 to 1723.96 kPa. The wettability and the ion-size effect play important roles to determine the pressure dependence behavior of the supercapacitors. An improved high frequency capacitive response with 1172 Hz "knee" frequency was observed under the compressive pressure of 1723.96 kPa. Second, a dynamically stretchable supercapacitor (DSS) with highly reversible stretchability was demonstrated based on the elastomeric electrospun polyurethane separator, the buckled SWNT macrofilms electrodes, and organic electrolyte. The electrochemical behavior of the stretchable cells was instantaneously characterized under both dynamic stretch/release (DSR) and fixed static strain modes. The interesting capacitance variation was observed under the DSR mode, which corresponds with the state of the strain and strain rate applied. The capacitance variation is mainly attributed to the changes of charge transfer resistance and Warburg diffusion resistance during the stretching/releasing. The cells have shown excellent cycling stabilities under cyclic stretching/releasing.
Third, the SWNT/MnO2 hybrid films as supercapacitor electrode materials were synthesized using a new and facile approach, which is binder-free, robust and with pre-formed electrical pathways. The one-step precipitation process is facile, cost-effective and scalable for mass production of the electrode materials with enhanced electrochemical performances. The supercapacitors demonstrate a much improved energy density and equally high power density, as well as an ultra-high frequency response and excellent long-cycle stability. The energy density of 70 Wh kg-1 with power density of 77.3 W kg-1 shows its high competitiveness compared with the current Ni-MH battery systems, thus makes the supercapacitors very promising to be implemented as the electric energy storage devices for hybrid vehicles.