Electrochemical Capacitors for Miniaturized Self-powered Systems

Licentiate thesis, 2018

Miniaturized self-powered systems with harvest-store-use architectures have been recognized as a key enabler to the internet-of-things (IoT), and further the internet-of-everything (IoE), 5G communication and tactile internet. Electrochemical capacitors (ECs), also known as supercapacitors, are promoted to be the energy storage component in such systems, because of their advantages such as an almost limitless cycle life that is ideal for the vision of “fit-and-forget” maintenance-free networks. Moreover, ECs are able to undertake tasks beyond energy storage. For example, high-frequency ECs can potentially replace the bulky electrolytic capacitors as AC line filters, with benefits in sizing down the circuitry boards and thus constructing compact systems which are pursued by the IoT technology.

Bringing the IoT high-level requirements down to the device-level specifications, challenges to ECs are identified in different aspects, including device electrochemical performance, and device encapsulation/integration. Regarding the performance, challenges exist in (1) improving the energy density, (2) maximizing the operating voltage limit, (3) widening the working temperature range, (4) minimizing the self-discharge and leakage current, and (5) enhancing the frequency response property. Regarding the encapsulation and integration aspect, challenges exist in device design and fabrication. Novel encapsulation and integration EC concepts are thus appreciated to be compatible with the surface mount technology, allow for convenient adaption in the form factor and arbitrary choice of the EC materials (electrodes, electrolytes and separators). Moreover, the EC materials should be durable under the ambient conditions that occur during the encapsulation and integration processes, such as high-temperature exposure for the reflow soldering technique.

The thesis research work addresses the device performance challenges. Specifically, the use of redox electrolytes is promoted for improving the energy density of ECs towards a battery-level, and at the same time keeping the capacitor-level power capability and cycling stability. With a redox-active electrolyte KBr, hybrid devices combining the features of both batteries and ECs are constructed, and a 1.9 V maximum operating voltage is achieved in the aqueous system. Furthermore, voltage- and history-dependent behaviors are revealed, reminding the complexity of hybrid systems.

To explore the extreme high-temperature performance, a special measurement setup is customized and an EMImAc (1-Ethyl-3-methylimidazolium acetate) ionic liquid (IL) electrolyte is employed to enable an operation at a maximum of 150 °C. It is observed that the energy and power densities at high temperatures may not be sacrificed when decreasing the operating voltage limit, therefore it is proposed that for neat IL-based ECs, a strategy of trading the voltage limit for gaining stability at extreme high-temperatures can be considered.

With a graphite and carbon nanotubes hybrid material, it is demonstrated that the self-discharge and leakage current can be suppressed by employing a gel polymer electrolyte. Using the same electrode material, high-frequency ECs that are suitable for AC line filtering tasks are fabricated. The working frequency range is up to kHz with a state-of-art level areal (1.38 mF cm-2) and volumetric capacitances (345 mF cm-3), benefiting from a possible covalent bonding between graphite substrate and the CVD grown CNTs.

Not limited to the above research findings, this thesis has critically reviewed and summarized the general strategies and methods to address all the identified challenges to ECs for their application in miniaturized self-powered systems.