1D fiber/yarn supercapacitors for wearable energy storage

• Main Author: Zhai, Shengli
• Other Authors: Wang Xin
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Chemical engineering
• Online Access: https://hdl.handle.net/10356/87465; http://hdl.handle.net/10220/46757
• Institution: Nanyang Technological University

With the fast development of wearable electronics, there is an urgent demand for advanced wearable energy storage devices, which are stable, long-lasting, flexible, wearable, and safe to fulfil the requirements of emerging wearable electronics. As a new family of supercapacitors, fiber/yarn supercapacitors (F/YSCs) with one-dimensional (1D) cylindrically shaped fibers or yarns as electrodes have attracted significant interests as energy storage devices for wearable electronics since 2011. They show good potentials as either micro-devices to complement or even replace micro-batteries in miniaturized wearable electronics or being scaled up to macro-devices using textile technology to store large amounts of energy for larger wearable electronics or smart textiles. Thus, in this thesis, the research on 1D F/YSCs for wearable energy storage applications is divided into two directions: long yarn supercapacitors (YSCs) which is capable of being knitted or weaved into energy storage textiles and short fiber micro-supercapacitors (FMSCs). Presently, state-of-art YSCs have achieved high electrochemical performance via elaborated yarn electrode materials selection and engineering. However, the length of most reported YSCs is limited to several centimeters. In contrast, practical textile yarns are orders of magnitude longer. The reported length dependent performance often doesn’t increase linearly with the increase of length. Further, the cost of electrode materials has seldom been considered, while it can be expected that a large amounts of electrode materials are needed for fabricating long YSCs. Lastly, few works have demonstrated realistic knitted or woven energy storage textiles from YSCs. In these regards, the research focus for YSCs in this thesis is to fabricate long YSCs with high energy storage performance at reasonable cost, and develop the capability to be knitted or woven into practical energy storage textiles. To this end, all-carbon solid-state YSCs using commercially available activated carbon and carbon fiber yarns were demonstrated. Activated carbon with large surface area serves as electrode materials, while carbon fibers with excellent electrical conductivity and mechanical strength act as current collectors and substrate for depositing activated carbon. Long YSCs up to 50 cm with a total capacitance of 1164 mF were successfully fabricated. Utilized three long YSCs, a wearable wristband was knitted which is capable of lighting up a light-emitting diode (LED) indicator, demonstrating good potentials for wearable energy storage. On the other hand, previously reported FMSCs often suffer from inferior energy storage capacity to micro-batteries, and their production methods are also not scalable for commercialization. Moreover, few studies have investigated the integration strategy of FMSCs to meet a wide range of energy and power requirements by wearable devices. Therefore, the research focus for FMSCs in this thesis is to first improve their energy storage performance, followed by exploring scalable and time-energy-efficient fabrication methods of fiber electrodes, and subsequently new device integration strategies. To these perspectives, this study began with the demonstration of micro-nano-integrated core-sheath hybrid carbon fibers comprised of a microscale core made of commercial graphite fibers and a nanoscale hybrid sheath comprised of nitrogen-doped graphene oxide (GO) sheets and multi-walled carbon nanotubes (MWCNTs). The highly conductive graphite fiber core provides efficient pathways for fast electron transfer, while the high surface area nano-hybrid sheath with large surface area and enables large capacitance storage. The use of graphite fiber core dramatically improves the electrical conductive of the electrode which achieved a 6-times increase in capacitance retention compared to hybrid carbon fibers without graphite fiber core. To further improve the energy storage capacity, pseudocapacitive materials, RuO2 nanoparticles with ultrahigh high mass loading (up to 42.5%) was uniformly incorporated into hybrid carbon fibers derived from holey GO (HGO) sheets as the main components and single-walled carbon nanotubes (SWCNTs) as nanospacers. The composite fibers produced exhibited one of the highest volumetric capacitance of 1054 F cm−3 among reported fibers. Next, a new hydrothermal method was developed which allows the independent control of temperature and pressure in a wide range of subcritical and near-critical conditions. The near-critical hydrothermal conditions can dramatically shorten the hydrothermal assembly time of hybrid carbon fibers from several hours to 20 mins and achieve an 80% energy saving with enhanced physicochemical and capacitive performance compared to standard hydrothermal methods. Furthermore, a new concept of integration strategy of multiple FMSCs both in the X-Y plane and Z-axis direction to build a three-dimensional (3D) supercapacitors (SCs) was demonstrated, which could save 75% footprint compared to the typical strategy of integrating fiber devices on a flat substrate. In conclusion, this thesis shows that rational electrode material design, synthesis, and architecture engineering can address several key challenges in developing high-performance F/YSCs. The scalable and economical fabrication methods demonstrated can facilitate the practical applications of F/YSCs for wearable energy storage.