Electrochemical capacitors for miniaturized self-powered systems: challenges and solutions

Abstract: Electrochemical capacitors (ECs), also known as supercapacitors, are recognized as a key technology that will enable miniaturized self-powered systems, which will constitute the hardware base nodes of the internet of things (IoT), the internet of everything (IoE) and the tactile internet. Systems employing ECs can be designed to be maintenance-free thanks to the ultra-long cycling stability of ECs. Besides the function as a main or backup energy storage unit, advanced ECs can be used to support batteries at peak power load and they can be a substitute for conventional electrolytic capacitors used in a.c. line filtering, with clear advantages for system down-sizing due to their superior capacitance density. However, a number of challenges remain to be solved to advance the development of ECs for miniature systems. Regarding the performance as a competitor to e.g. batteries, the ECs suffer from inferior energy density, low working voltage, severe self-discharge and leakage current. For IoT systems embedded in a harsh environment, the ability to enduring extreme temperature is inadequate for most general-purpose ECs. The response at high frequency needs to be enhanced to enable functions such as a.c. line filtering. As for encapsulation and integration, novel concepts are appreciated for compatibility with surface mount technology and reflow soldering, allowing convenient adaption in the form factor and making possible an arbitrary choice of EC materials (electrodes, electrolytes and separators). To address the challenges, the thesis (1) explores the utilization of the redox electrolyte KBr to enhance the energy density of EDLCs; (2) adopts an ionic liquid electrolyte EMImAc to achieve working temperature beyond 120 °C; (3) uses an advanced graphite/VACNTs material for high-frequency ECs as a.c. line filters and low loss storage units in microsystems; (4) develops a bipolar EC prototype that doubles the working voltage limit; (5) mitigates the self-discharge and leakage current through the liquid crystal additive in an electrolyte; and (6) presents a cellulose-derived carbon nanofiber-based electrode material with enhanced capacitive performance. Generic strategies and methods to address each identified challenge are provided in the thesis, highlighting a step-by-step optimization route starting from the material properties, moving on to the electrode structures, and further to the device design.