| 1. |
- Hansson, Josef, 1991, et al.
(författare)
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Bipolar electrochemical capacitors using double-sided carbon nanotubes on graphite electrodes
- 2020
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Ingår i: Journal of Power Sources. - : Elsevier BV. - 0378-7753. ; 451
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Tidskriftsartikel (refereegranskat)abstract
- The electrochemical capacitor (EC) is a key enabler for the miniaturized self-powered systems expected to become ubiquitous with the advent of the internet-of-things (IoT). Vertically aligned carbon nanotubes (VACNTs) on graphite holds promise as electrodes for compact and low-loss ECs. However, as with all ECs, the operating voltage is low, and miniaturization of higher voltage devices necessitates a bipolar design. In this paper, we demonstrate a bipolar EC using graphite/VACNTs electrodes fabricated using a joule heating chemical vapor deposition (CVD) setup. The constructed EC contains one layer of double-sided VACNTs on graphite as bipolar electrode. Compared to a series connection of two individual devices, the bipolar EC has 22% boost in volumetric energy density. More significant boost is envisaged for stacking more bipolar electrode layers. The energy enhancement is achieved without aggravating self-discharge (71.2% retention after 1 h), and at no sacrifice of cycling stability (96.7% over 50000 cycles) owing to uniform growth of VACNTs and thus eliminating cell imbalance problems.
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| 2. |
- Haque, Mohammad Mazharul, 1984, et al.
(författare)
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Self-discharge and leakage current mitigation of neutral aqueous-based supercapacitor by means of liquid crystal additive
- 2020
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Ingår i: Journal of Power Sources. - : Elsevier BV. - 0378-7753. ; 453
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Tidskriftsartikel (refereegranskat)abstract
- Self-discharge is being recognized as one of the main obstacles to implementing the supercapacitor (SC) in standalone self-powered systems. Strategies for addressing this issue include the modification of electrodes, electrolytes, separators, and diverse device configurations. However, an improved self-discharge behavior is often achieved with a large compromise on other prominent figures of merit such as capacitance, energy density, or cycle life of the device. In this work, a thorough comparative electrochemical investigation of SCs containing a neutral aqueous electrolyte, 1 M Li2SO4, and with a liquid crystal (LC) additive, 2% 4-n-pentyl-4′-cyanobiphenyl (5CB) in 1 M Li2SO4, has been carried out at different states of charge. The results demonstrate that the device containing the LC additive 5CB exhibits a reduced self-discharge and leakage current without compromising the capacitive performance at different nominal voltages compared to the behavior of the device without 5CB. We suggest an explanation of the difference of the self-discharge behavior between the devices through tunability of the effective conductivity of the electrolyte composite upon applied voltages. As a result, in an open circuit condition, the device containing LC shows a slower diffusion of ions that facilitates a decreased self-discharge and leakage current.
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| 3. |
- Li, Qi, 1990
(författare)
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Electrochemical capacitors for miniaturized self-powered systems: challenges and solutions
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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.
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| 4. |
- Li, Qi, 1990, et al.
(författare)
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Explanation of anomalous rate capability enhancement by manganese oxide incorporation in carbon nanofiber electrodes for electrochemical capacitors
- 2020
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Ingår i: Electrochimica Acta. - : Elsevier BV. - 0013-4686. ; 340
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Tidskriftsartikel (refereegranskat)abstract
- Electrochemical capacitors (ECs) can provide ultra-long cycle life and ultra-fast energy delivery, characteristics which most battery technologies lack. Making composites out of carbon and pseudocapacitive materials is a popular strategy directed on narrowing the gap in energy density with regard to batteries. Usually, the incorporation of pseudocapacitive materials leads to a decrease in power performance compared to a pure carbon matrix, due to inferior electrical conductivity. This work, however, presents significant improvement in rate capability demonstrated by a composite electrode containing carbon nanofibers (NCNF) and manganese oxides (MnO2). The NCNF/MnO2 is prepared with a common method through the reaction with permanganate. The material has excellent performance metrics, especially a 78.2% rate capability (capacitance retention at 15 A g−1 relative to 0.5 A g−1), more than 10 times that for the NCNF carbon matrix. The exceptional enhancement can be explained by the development of micropores and surface area of NCNF, thus alleviating the “pore starvation” issue, and surface functional groups variation that enhances capacitive performance. This work highlights the importance of paying attention to the modification of carbon substrate when investigating carbon composite electrodes e.g. carbon/MnO2 networks.
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| 5. |
- Vyas, Agin, 1992, et al.
(författare)
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Enhanced Electrode Deposition for On-Chip Integrated Micro-Supercapacitors by Controlled Surface Roughening
- 2020
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Ingår i: ACS Omega. - : American Chemical Society (ACS). - 2470-1343. ; 5:10, s. 5219-5228
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Tidskriftsartikel (refereegranskat)abstract
- On-chip micro-supercapacitors (MSCs), integrated with energy harvesters, hold substantial promise for developing self-powered wireless sensor systems. However, MSCs have conventionally been manufactured through techniques incompatible with semiconductor fabrication technology, the most significant bottleneck being the electrode deposition technique. Utilization of spin-coating for electrode deposition has shown potential to deliver several complementary metal-oxide-semiconductor (CMOS)-compatible MSCs on a silicon substrate. Yet, their limited electrochemical performance and yield over the substrate have remained challenges obstructing their subsequent integration. We report a facile surface roughening technique for improving the wafer yield and the electrochemical performance of CMOS-compatible MSCs, specifically for reduced graphene oxide as an electrode material. A 4 nm iron layer is deposited and annealed on the wafer substrate to increase the roughness of the surface. In comparison to standard nonroughened MSCs, the increase in surface roughness leads to a 78% increased electrode thickness, 21% improvement in mass retention, 57% improvement in the uniformity of the spin-coated electrodes, and a high yield of 87% working devices on a 2″ silicon substrate. Furthermore, these improvements directly translate to higher capacitive performance with enhanced rate capability, energy, and power density. This technique brings us one step closer to fully integrable CMOS-compatible MSCs in self-powered systems for on-chip wireless sensor electronics. ©
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| 6. |
- Vyas, Agin, 1992, et al.
(författare)
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Impact of electrode geometry and thickness on planar on-chip microsupercapacitors
- 2020
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Ingår i: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 10:52, s. 31435-31441
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Tidskriftsartikel (refereegranskat)abstract
- We report an assessment of the influence of both finger geometry and vertically-oriented carbon nanofiber lengths in planar micro-supercapacitors. Increasing the finger number leads to an up-scaling in areal power densities, which increases with scan rate. Growing the nanofibers longer, however, does not lead to a proportional growth in capacitance, proposedly related to limited ion penetration of the electrode.
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| 7. |
- Vyas, Agin, 1992, et al.
(författare)
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Towards Integrated Flexible Energy Harvester and Supercapacitor for Self-powered Wearable Sensors
- 2020
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Ingår i: 2019 19TH INTERNATIONAL CONFERENCE ON MICRO AND NANOTECHNOLOGY FOR POWER GENERATION AND ENERGY CONVERSION APPLICATIONS (POWERMEMS).
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Konferensbidrag (refereegranskat)abstract
- We present the first results of a flexible energy harvester and a foldable supercapacitor to power wearable and flexible sensors. The flexible energy harvester is fabricated by using 38 mu m piezoelectric polyvinylidene difluoride (PVDF) sandwiched between carbon electrodes. Both the design and process excel in simplicity and cost-effectiveness. The flexible harvester demonstrates a power output of 2.6 mu W cm(-3) at a resonant frequency of 50 Hz with a 3dB bandwidth of about 11 Hz, which is higher than devices previously reported and similar to a commercial PZT harvester film of same size. A flexible energy storage supercapacitor (GP-SC) was fabricated using a graphite/VACNTs (vertically aligned carbon nanotubes) material as electrodes. A prototype GP-SCs has an areal capacitance of about 1.2 mF cm(-2). Finally, an integrated scheme is proposed for future work.
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