Carbon Nanotubes/Manganese Dioxide Nano-Structured Planar Macro, 3D Micro, and Solid-State Supercapacitors
Global warming, fuel crisis and massive energy consumption of modern industrial societies have resulted in a greater focus on research and development of advanced energy storage devices. However, it is not possible to meet the rising global demands of energy, merely by using green energy sources without supplementing them with advanced energy storage systems. Batteries and supercapacitors (ECs) (also known as electrochemical capacitors or ultracapacitors) are the important electrical energy storage devices that have growing impact in our lives by storing energy in applications ranging from portable electronics, hybrid vehicles, military devices, power tools, bio-medical devices, and large industrial equipment. Supercapacitors are superior to batteries in power density, have excellent reversibility, long cycle life, greater safety, and ease of integration to into electronics. In this dissertation, I have proposed improving the energy and power density of supercapacitors by exploring nano-composite structures where low cost transition metal oxides are combined with three dimensional, high specific surface area and conducting carbon nanotubes (CNTs) to achieve the high performance, advanced devices. This has been achieved using 3 different configurations: a) planar supercapacitors; b) micropatterned 3D supercapacitors; and c) solid-state supercapacitors. Manganese dioxide deposition was achieved by optimizing an electrochemical technique using potassium permanganate and cyclic voltammetry. This technique results in-situ reduction of KMnO4 on the CNT surface to form MnO2 film; thus providing excellent control on the thickness and reproducibility. Such a MnO2/CNT architecture is beneficial for energy storage performance by virtue of reduced ion diffusion lengths, facile electron transfer kinetics, lower electrical contact resistance, and high specific surface area In addition, this approach eliminates the use of binders and/or any other additives. CNTs were synthesized on flexible graphite substrates to develop single sided and double-sided electrodes using low-cost thermal chemical vapor deposition (T-CVD) technique. A supercapacitor prototype cell was designed and fabricated using a stack of multiple, double-sided electrodes yielding high capacitance value of 3.7 F at 2 mV/s. A low equivalent series resistance (ESR) value of 0.8 Ω yields a maximum specific power value of 68.5 kW.kg-1 and a maximum specific energy of 111.6 Wh.kg-1 at 2.5V. Conventional silicon microfabrication process, hot filament CVD technique for CNT synthesis and the electrochemical technique for MnO2 deposition were combined to fabricate a novel 3D micropatterned supercapacitor electrodes that can deliver very high volumetric capacitance of 240 F/cm3 or high capacitance of 1.85 F/cm2. A new, efficient, simple and low-cost process has been developed for the fabrication of solid-state supercapacitors utilizing H3PO4/PVA solid polymer electrolyte. The resulting supercapacitor prototype yielded a very high capacitance value of 1.4 F or 830 F.g-1 and exhibited excellent cycle life with nearly 3000 cycles with 89% capacitance retention. Maximum specific power value of 73.9 kW.kg-1 and a maximum specific energy of 115.2 Wh.kg-1 was achieved. Results from fabrication, materials analysis using SEM and Raman spectroscopy and electrochemical characterization have been presented and discussed in this dissertation.