Printed Transition Metal Oxide Electrochemical Capacitors for Energy Harvesting Applications

With the rapid development of Internet-of-Things (IoT), it is estimated that a trillion sensors will be needed around the globe by 2023 to connect most things around us. This rapid growth of portable devices is stimulating the development of flexible, wearable, and conformal embedded electronics with the unprecedented need for next-generation energy storage systems fully adaptable to diverse form factors. With such an enormous production demand of electronic devices, the manufacturing technologies used for current silicon microelectronics are prohibitively expensive. In addition, conventional fabrication methods, such as photolithography for electronics and electrode winding/stacking for energy storage systems, struggle as fabrication strategies to produce devices with advanced form factors (\textit{i.e.} three-dimensional, flexible, stretchable, wearable, conformal etc.). Printed electronics have been accepted as one of the ways forward to meet this high demand at a lower cost, but also represent a paradigm shift in greener electronic manufacturing. Additionally, the solution process, and in many cases digital nature of printed electronics, not only enable the rapid transfer of a prototype straight to the manufacturing process but also enable the fabrication of highly versatile and creative multifunctional designs. In this thesis, the fabrication of high-energy-density and high-power-density, nickel-(II) oxide co-planar micro-supercapacitors fabricated through inkjet printing is demonstrated. The developed micro-supercapacitors showed remarkable areal and volumetric specific capacitances of up to 155 mF/cm2 and 705 F/cm3 respectively (at 5 mV/s scan rate), surpassing the state-of-the-art inkjet-printed supercapacitors but also a few of the best micro-supercapacitors known to date. Moreover, the fabrication of supercapacitors on 3D objects through inkjet and water-transfer printing is also demonstrated. Electrochemical studies were performed to investigate the effect of substrate and printing resolution, different types of current collector silver inks, sintering temperatures and carbon residues in electrodes, different types of electrolytes, binders and other organic compounds in electrolytes, and the effect of electrode gaps on the electrochemical response and capacitance of inkjet-printed, co-planar NiO supercapacitors. This work provides a compelling platform to simplify the fabrication process of next-generation supercapacitors, with focus on digital design, scalable manufacturing, and direct integration with printed electronics to enable a variety of design flexibility needed for countless new IoT applications, including wearable health systems.