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. ‎