High Perfomance Ulta-Thin Microsupercapacitors Based on Carbon Nanotube Sheets
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Driven by increasing demand for computing, communications, and transportations; portable electronic devices are becoming more advanced and miniaturized, therefore require power supply and energy storage units with high power and energy densities that can be integrated with the same scale size. Batteries fall short in long-term cycling, due to their slow rate of redox reaction happening in bulk. Another state-of-the-art unit which has drawn much attention in the research communities are electrochemical microsupercapacitors (MSCs) as microscale power sources. Their long cycle life, excellent power density and lately improved energy density compared to traditional supercapacitors make them excellent candidates to replace or to complement microbatteries. Such devices can provide enhanced properties required for a variety of microelectronics such as MEMS, bio-implantable devices, RFID tags and renewable energy harvesters. New class of thin film microsupercapacitors with in-plane design has emerged for on-chip integration applications where bulky sandwich configuration and 3D MSCs indicate many shortcomings. Future advancement of 2D MSCs is dependent on development of integratable and scalable fabrication processes as well as improvement in their energy density. In this research, we leverage unique structure of carbon nanotube sheet as our electrode active material which offers high packing density, long-term chemical, mechanical stability, and high specific surface area. High degree of alignment of CNT sheet provides faster charge transport which resultsin higher rate capabilities. To fabricate these devices, we developed a process without compromising the structural alignments, electrical and mechanical properties of the CNT sheets. MSC devices with interdigitated electrode configuration (IDE) and various resolutions of 140, 60 and 40 µm were fabricated on rigid and flexible substrates using single step, rapid and reproducible method of plasma etching. Selective etching was done with assist of a shadow mask; without using any photoresist or binder materials. Specific capacitance, energy and power density of the devices were improved and characterized through various approaches; surface treatment (exposure to plasma), nitrogen doping (8X higher), configuration of CNT sheet (cross-stack versus undirectional; 2.5X higher), deposition of manganese oxide nanoparticles (3X higher), utilizing higher resolution electrodes (10X higher), and increasing thickness of electrodes. All-solid-state MSC devices were also demonstrated on flexible substrates in this dissertation. Flexible MSCs exhibited excellent mechanical stability under various bending states (from 0º to 180º angle) and repeated bending cycles (10,000 cycles) at bending angle of 180º with less than 20% change in areal capacitance. This research demonstrates potential of highly aligned CNT (HACNT) sheet for high-performing, lightweight thin film MSCs. Further improvement of performance is attainable via designing high resolution electrodes, in-situ doping, employing ionic electrolytes and controlled incorporation (thickness, morphology, and phase) of pseudo capacitive materials.