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- Haque, Mohammad Mazharul, 1984
(författare)
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Electrolyte evaluation and engineering for the performance enhancement of electrochemical capacitors
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 2. |
- Haque, Mohammad Mazharul, 1984, et al.
(författare)
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Identification of self-discharge mechanisms of ionic liquid electrolyte based supercapacitor under high-temperature operation
- 2021
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Ingår i: Journal of Power Sources. - : Elsevier BV. - 0378-7753. ; 485
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Tidskriftsartikel (refereegranskat)abstract
- Ionic liquids (ILs) are promising electrolytes for supercapacitors (SCs) aimed for high-temperature applications, where increased ionic conductivity results in superior capacitive performance compared to room temperature (RT) performance. However, an increased temperature also accelerates the self-discharge rate that adversely affects energy retention and restricts the usage of SCs in standalone applications. In this study, a detailed electrochemical investigation on the self-discharge behaviour of carbon-based SCs containing an IL, 1-Ethyl-3-methylimidazolium acetate (EMIM Ac), has been carried out in the temperature range RT - 60 °C, and the underlying self-discharge mechanisms are identified. The results reveal that at a high voltage of 1.5 V, the self-discharge is characterized by a combination of charge redistribution and diffusion at both RT and 60 °C. At 60 °C, the diffusion-controlled mechanism dominates at lower voltages over the charge redistribution effect, while at RT both mechanisms contribute to a similar extent. The observed difference in the self-discharge mechanism between RT and 60 °C is explained in terms of a decreased RC time constant (τRC) at elevated temperature, and the same conclusions are potentially applicable to other IL-containing SCs as well.
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| 3. |
- Li, Qi, 1990, et al.
(författare)
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Finger Number and Device Performance: A Case Study of Reduced Graphene Oxide Microsupercapacitors
- 2021
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Ingår i: Physica Status Solidi (B): Basic Research. - : Wiley. - 1521-3951 .- 0370-1972. ; 258:2
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Tidskriftsartikel (refereegranskat)abstract
- Microsupercapacitors (MSCs) are recognized as suitable energy storage devices for the internet of things (IoTs) applications. Herein is described the work conducted to assess the areal energy and power densities of MSCs with 2, 10, 20, and 40 interdigital finger electrodes on a fixed device footprint area (the finger interspacing is fixed at 40 μm, and the finger width and length are allowed to vary to fit the footprint area). The MSCs are based on reduced graphene oxide (rGO) materials and fabricated with a spin-coating and etch method. The performance evaluation indicates a strong dependency of areal capacitance and energy density on the number of fingers, and the maximum (impedance match) power density is also influenced to a relatively large extent, whereas the average power density is not sensitive to the configuration parameters in the present evaluation settings (scan rate 20–200 mV s−1 and current density of 100 μA cm−2). For the rGO-based devices, the equivalent distributed resistance may play an important role in determining the device resistance and power-related performance.
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| 4. |
- Vyas, Agin, 1992, et al.
(författare)
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Comparison of Thermally Grown Carbon Nanofiber-Based and Reduced Graphene Oxide-Based CMOS-Compatible Microsupercapacitors
- 2021
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Ingår i: Physica Status Solidi (B): Basic Research. - : Wiley. - 1521-3951 .- 0370-1972. ; 258:2
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Tidskriftsartikel (refereegranskat)abstract
- Microsupercapacitors as miniature energy storage devices require complementary metal-oxide-semiconductor (CMOS) compatible techniques for electrode deposition to be integrated in wireless sensor network sensor systems. Among several processing techniques, chemical vapor deposition (CVD) and spin coating, present in CMOS manufacturing facilities, are the two most viable processes for electrode growth and deposition, respectively. To make an argument for choosing either of these techniques to fabricate MSCs utilizable for an on-chip power supply, we need a comparative assessment of their electrochemical performance. Herein, the evaluation of MSCs with CVD-grown carbon nanofiber (CNF)-based and spin-coated reduced graphene oxide (rGO)-based electrodes is reported. The devices are compared for their capacitance, energy and power density, charge retention, characteristic frequencies, and ease of fabrication over a large sweep of scan rates, current densities, and frequencies. The rGO-based MSCs demonstrate 112 mu F cm(-2) at 100 mV s(-1) and a power density of 12.8 mW cm(-2). The CNF-based MSCs show 269.7 mu F cm(-2) and 30.8 mW cm(-2). CVD-grown CNF outperforms spin-coated rGO in capacitive storage at low frequencies, whereas the latter is better in terms of charge retention and high-frequency capacitance response.
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