Flexible all-solid-state supercapacitors for self-powered nanosystems

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Flexible all-solid-state supercapacitors for self-powered nanosystems


Author: Jin, Huanyu
Title: Flexible all-solid-state supercapacitors for self-powered nanosystems
Degree: M.Phil.
Year: 2016
Subject: Supercapacitors.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Physics
Pages: xi, 134 pages : color illustrations
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2856406
URI: http://theses.lib.polyu.edu.hk/handle/200/8378
Abstract: A supercapacitor (SC) is an energy storage component which has many advantages such as high power density, very fast charge and discharge rate and long cycle life. Supercapacitors using solid-state electrolyte are flexible, environmentally friendly, and light weight. However, their performance is still limited by several issues such as low active material loading per unit area, low operating voltage, low energy density, complicated fabrication process and poor integrated ability. To overcome these limitations, several methods were conducted in this research to improve the capacitance, fabrication process, operating voltage, and integrated ability. First, a simple oxidation and annealing method was introduced to fabricate functionalized carbon fabric (FCF) electrodes. The as-prepared FCF electrode could be operated at both negative potential and positive potential. Furthermore, a flexible symmetric solid-state supercapacitor with large operating voltage (1.6 V) and high volumetric capacitance was successfully developed. The volumetric energy density and volumetric power density are higher than most previously reported CF-based SCs. Next, the focus of research is turned to fiber-based supercapacitors as they have higher integrated ability than fabric-based supercapacitors. A high-performance and flexible fiber-shaped supercapacitor (FSC) with organic-inorganic hybrid structure was fabricated. After adding a thin layer of polyaniline (PANI) on the manganese oxide (MnO₂)-coated carbon fiber thread (CFT) electrode, the capacitance of the electrode was increased by 2530 %. These CFT-MnO₂-PANI hybrid electrodes were further assembled into solid-state FSCs which exhibited high volumetric capacitance, good rate capability and excellent mechanical stability.
Energy density has a squared dependency on operating voltage. Herein, a solid-state asymmetric fiber-shaped supercapacitor made of carbon fiber thread@polyaniline and functionalized carbon fiber thread electrodes was developed. The as-prepared device showed a high operating voltage (1.6 V), high volumetric energy density and good flexibility. To optimize the capacitance of fiber-based asymmetric supercapacitor, carbon nanoparticles coated on carbon fiber (CF@CNPs) was chosen as the substrate owing to its notable features. The manganese oxide nanosheet grown on CF@CNPs and functionalized CF@CNPs were employed as the positive and negative electrode of the micro-asymmetric SC respectively. The as-prepared asymmetric SC can operate at 1.8 V and exhibit a high volumetric energy density of 2.1 mWh cm⁻³, which is comparable to that of a thin-film Li-battery. The final work of this research project was to develop a promising method for making supercapacitors to overcome the complicated fabrication process and polluted by-products caused by the electrochemical deposition method. An efficient and convenient method was developed to deposit a highly uniform and semi-transparent MnO₂ film by suppressing the coffee-ring effect (CRE) for transparent capacitive energy storage devices. By carefully controlling the amount of ethanol added in the MnO₂ droplet, we could optimize the film uniformity. Comparison measurements for the devices with or without CRE were carefully conducted, showing the efficiency and necessity of the method. The SC device without CRE shows a superior capacitance, high rate capability and lower contact resistance. This device could be used for both flexible and rigid applications, demonstrating its potential for flexible self-powered systems and capacitive energy storage window.

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