Electrolyte evaluation and engineering for the performance enhancement of electrochemical capacitors
Doktorsavhandling, 2021

As a consequence of a fast-paced technological evolution along with the acknowledgment of utilizing clean and renewable energy resources over fossil fuels, the importance of energy storage devices is widely recognized. The electrochemical capacitor (EC), commonly known as a supercapacitor or ultracapacitor, is an energy storage device that is already being used in portable consumer electronics, electrification of transportation, and grid-level applications. High power density and long cycle life are the two most prominent properties of ECs, thanks to the electrostatic nature of their charge storage mechanism. These properties are well utilized in a system where ECs are used as a backup power-boosting device to rechargeable batteries. By providing the peak power required, they eventually prolong the battery lifetime.  However, the relatively low energy density of ECs compared to rechargeable batteries limits their application as a standalone device. In addition, low operating voltage, adverse self-discharge rate, severe leakage current, elevated temperature incompatibility are some of the crucial issues that are preventing the widespread application of ECs.

Besides a general discussion about ECs, the main objective of this thesis is to identify and address the above-mentioned critical challenges, and to propose and demonstrate corresponding solutions. Firstly, it is revealed that utilizing a redox-active KBr electrolyte can enhance both operating voltage and capacitance, and hence increases energy density without sacrificing power density or cycle life. Secondly, an evaluation of elevated temperature influence on the capacitive performance of ECs containing ionic liquid (IL) electrolyte demonstrates a high working temperature beyond 120 °C. Thirdly, a systematic investigation of ECs containing IL at elevated temperatures shows a significant increase of the self-discharge rate with temperature and pinpoints the underlying mechanisms; at lower initial voltages the self-discharge rate is dominated by diffusion of electrolyte ions rather than charge redistribution. Fourthly, the addition of a small amount of liquid crystals (LC) in neutral electrolyte shows a reduction of self-discharge and leakage current due to slower diffusion of ions in the device, which is proposed to originate from the anisotropic properties of LC. Finally, by utilizing the thermocapacitive effect, a thermal charging of ECs containing IL is demonstrated, where a high voltage of more than 900 mV could be recovered when two devices in series are exposed to a 60 °C temperature environment.

thermal charging


Energy storage

ionic liquid


thermoionic system


activated carbon

leakage current

liquid crystal

Kollectorn, Kemivägen 9, Göteborg
Opponent: Professor Ncholu Manyala, University of Pretoria, South Africa


Mohammad Mazharul Haque

Chalmers, Mikroteknologi och nanovetenskap, Elektronikmaterial

Our society is heavily dependent upon electrical energy and its demand has been growing enormously over the past few decades. The majority of industries are still relying on oil, natural gas, and coal as a primary source of energy, but the limited availability and negative environmental impact of such resources are strong driving forces towards renewable alternatives. One major obstacle in the use of renewable resources is their intermittent and somewhat unpredictable nature. Therefore, electrical energy storage devices able to accumulate energy from renewable sources and deliver on-demand are a vital prerequisite for renewables to become a viable solution.

As for today, there are two main types of electrical energy storage devices: rechargeable batteries and electrochemical capacitors (ECs). Rechargeable batteries are by far the most popular and widely used alternative in all kinds of applications, from electronic devices to transportation. However, typically rechargeable batteries based on Faradaic reactions suffer from two main limitations: long charging time, and limited cycle life. On the contrary, due to the unique electrostatic charge storage mechanism, ECs can charge in minutes (if not seconds) and have a nearly unlimited cycle life. These features make them immensely attractive for a wide range of emerging applications like regenerative braking for the electric vehicle, uninterruptible power supply, wearable electronics just to name a few.

However, there are some key limitations of ECs that need to be addressed before they can enter the market with their full potentials. First and foremost is their low energy density, meaning that their operation time between charging cycles is limited. They also tend to self-discharge faster than batteries when not in use. In certain applications, ECs need to operate in a wide temperature range which makes it very critical to select durable and compatible structural components and maintain a good performance.

In this thesis, we address the aforementioned challenges and provide potential solutions/improvements/suggestions to achieve high-performing ECs and expanding their range of usability.

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Hållbar utveckling







Annan materialteknik


Chalmers materialanalyslaboratorium




Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4971



Kollectorn, Kemivägen 9, Göteborg


Opponent: Professor Ncholu Manyala, University of Pretoria, South Africa

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