Electrochemical capacitors for miniaturized self-powered systems: challenges and solutions
Doktorsavhandling, 2020

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.





miniaturized self-powered systems

electrochemical capacitors

redox electrolytes

energy storage

a.c. line filters

MC2 Kollectorn, Kemivägen 9, Göteborg, Sweden
Opponent: Prof. Thierry Brousse, Jean Rouxel Institute of Materials, University of Nantes, France


Qi Li

Chalmers, Mikroteknologi och nanovetenskap, Elektronikmaterial

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Over years, the concept of the internet of things (IoT) has been one of the most common buzzwords in technology circles. The paradigm of the IoT capitalizes on gaining the identity of objects and the environment belonging to our daily life. For example, a smart thermostat is a piece of IoT technology. The thermostat learns your family’s routines and automatically adjusts the temperature based on when you are at home or away, awake or asleep, feeling hot or cold, to make your house more efficient and help you save on heating and cooling bills. The mobile app allows you editing schedules, changing the temperature away from home, and even receive alerts when it looks like something has gone wrong with your heating or cooling system. The full picture of IoT is far more complex, with applications ranging from home appliances to industrial automation and more. In the broadest sense, the term IoT encompasses everything connected to the internet, but it is more relevant to define objects that “talk” to each other.

To enable objects to “talk”, energy supply is indispensable, just as a mobile phone needs a battery. What about the objects being self-powered so that we do not need to recharge the battery anymore? The energy harvesting and energy storage technologies are an exciting combination that makes it possible. An energy harvester device can convert the around vibrations, heat or sunlight into electricity, and an energy storage device stores the converted electricity and provides it to the electronics. We need the devices to be long-lived, and also slim in size – therefore, miniaturized self-powered systems - because billions of such systems will be installed everywhere to make a really smart IoT world.

My thesis focuses on improving the energy storage part to better fit for IoT. The electrochemical capacitors, more known as supercapacitors, are different from normal batteries in a way that the capacitors can be recharged in seconds and have much longer lifespans. But the capacitors still need to be improved to be super - to store more electricity without increasing the size, to work in extremely cold or hot environments, and so on. Through proper design of the ingredients and packages, a super supercapacitor is probably not so distant from reality. The enchanting prospects of the IoT world may soon no more be part of science-fictions – the future is powered!

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Chalmers materialanalyslaboratorium




Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4780



MC2 Kollectorn, Kemivägen 9, Göteborg, Sweden


Opponent: Prof. Thierry Brousse, Jean Rouxel Institute of Materials, University of Nantes, France

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