Supercritical Fluid Deposition of Vanadium Pentoxide within Carbon Nanotube Buckypaper for Electrochemical Capacitor

There is a pressing need from a broad range of industries for high-performance energy storage devices with high power, high energy capacity, light weight, long lifetime, high efficiency, and low cost. A typical energy storage device, current electrochemical capacitors do not possess sufficient energy density to meet the needs. Recently utilization of oxide materials as pseudocapacitance materials has attracted a great deal of interest. However obtaining a high pseudocapacitance using an affordable oxide, while maintaining the high rate performance, remains elusive. This dissertation work aims to develop high-performance carbon nanotube (CNT) vanadium oxide hybrid nanostructured electrode materials for electrochemical capacitors. The CNT was in a form of freestanding thin film buckypaper (BP), which served as the current collector whilst providing double-layer capacitance, and vanadium oxide, coated on the CNT, was the pseudocapacitance material. Using a novel supercritical fluid deposition process, ultrathin vanadium oxide were uniformly deposited throughout the buckypaper with exceptional conformity at relatively low temperature, enabled by the unique properties of the supercritical fluids such as high solvation power, high diffusivity and zero surface tension. This overcame many of the transport limitations associated with the vanadium oxide material and indeed excellent electrode performance, particularly high rate performance, was achieved. The deposition process, the morphology and structure, and the capacitance behaviors of the composites were studied in detail, and the processing-morphology-electrochemical properties of the composites were elucidated. A high-pressure deposition system was constructed first for this dissertation research. Thereafter several deposition processes were investigated: physical adsorption - annealing, and in-situ reactive deposition. In the physical adsorption approach, the V2O5-buckypaper composite electrodes were fabricated by firstly physical adsorption of vanadium precursor in supercritical carbon dioxide (scCO2), followed by oxidation in air under elevated temperature. This approach resulted in the conformal deposition of V2O5 of molecular thickness onto the CNT and uniformly distributed throughout the BP. The V2O5 specificpseudo-capacitance of more than 1000 F/g were realized, even at high working power. To improve the active materials loading in the composite electrodes, two strategies were explored. In the first strategy, based on the qualitative fundamental understanding of the adsorption process, important physical parameters were identified, and the adsorption process was optimized by synergic use of physical understanding and statistical experiment design methods. The study resulted in an estimated second order model, which facilitated the search for adsorption conditions for higher precursor loading and higher total capacitance. The second strategy aimed to increase the available surface area for adsorption by the use of high specific surface area substrate material. Thus binder-free single-walled carbon nanotube (SWCNT)-activated carbon (AC) composite substrate was studied in comparison with the traditional activated carbon electrode. Based on thermogravimetric investigation of the precursor oxidation behavior, a conversion process was designed to maximize oxide materials conversion whilst minimizing substrate perturbation and degradation. The loading in the SWCNT-AC was increased by several times, and the composite electrodes showed excellent capacitance. In-situ reactive deposition was explored to further increase the oxide materials loading whiling maintaining the conformity and uniformity. Oxygen was used as the oxidizer and oxidation took place within the reactor. Conformal thin film of V2O5 layer with thickness varying from a few atomic layers to a few nm, with weight loading ~60%, was achieved. Together with the high V2O5 loading and high specific pseudo-capacitance enabled by the ultrathin film structure, excellent high-rate total capacitance was achieved. For example, the composite electrode with a 40% V2O5 showed a total capacitance ~130 F/g at a scan rate of 100 mV/s.