CARBON NANOTUBES IN ELECTROCHEMICAL ENERGY STORAGE DEVICES
Date of Award
Doctor of Philosophy
This dissertation reports on the results of investigations performed on the electrochemical energy storage properties of carbon nanotubes. Specifically, the effect of heat treatment on the electrochemical energy storage properties of carbon nanotubes is reported. We will focus on single-walled carbon nanotubes (SWCNTs) produced by using the direct decomposition of metallocene (for example ferrocene). For understanding the basic properties related to electrochemical storage, electrochemical double-layer capacitors (EDLCs) were fabricated using CNTs as active electrode materials. Electrochemical characterization such as Cyclic Voltammetry (CV), Gavanostic Charge-Discharge (GCD) measurements as well as Electrochemical Impedance Microscopy (EIS) were performed. Other measurements to understand the physical nature of the nanotubes used were also performed. These include, Scanning Electron Microscopy (SEM), and Volumetric Adsorption measurements. Our investigations indicate that heat treatment of as-produced SWCNTs at 350oC for 1 hour can improve the specific capacitance of these materials several times > 300% when used with potassium hydroxide (KOH) as the electrolyte. We have also performed EDLC measurements using room temperature ionic liquid-based polymer electrolyte. This was performed to increase the operational voltages of the devices. Enhanced capacitances for heat-treated samples as compared to as-produced samples were also obtained in the case of this electrolyte. We have also shown that it is possible to fabricate flexible supercapacitors using aligned multi-walled carbon nanotubes (MWCNTs) grown directly on a metal (Inconel 600) substrate. The flexibility and robust electrode materials are the crucial characteristics to maintain through the solid-state electrolytes (gel polymer: PVA/H2SO4). Applying this type of gel on MWCNTs growth which is in the right alignment on the flexible metal sheet (Inconel) gives robust CNTs supercapacitors. To go further in understanding the charging mechanism in these supercapacitors, circuit modeling was applied to determine the role of pseudocapacitance in these devices. Even though the findings reported in this dissertation are performed on specific materials, fundamental understandings can be easily extrapolated to other energy storage devices. Understanding the role of electrochemical charge mechanisms to store the charges in various assembling of energy storage devices is a valuable insight for future investigations.
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