Wearable energy storage electrode based on liquid crystalline carbon nanotube fiber

Abstract

With the development of wearable devices and the miniaturization of various home appliances, fiber-type solid-state supercapacitors(FSSCs) are attracting a lot of attention. FSSCs made of carbon-based nanomaterials such as graphene and carbon nanotube has excellent flexibility, lightweight and robust. In particular, carbon nanotube fiber (CNTF) based on liquid crystal (LC) spinning has shown remarkable properties with electrical conductivity (~ MS/m), excellent mechanical strength (~ GPa), lightweight, and good flexibility. These outstanding properties of LC-spun CNTF originates from 1) the structural characteristics of the fiber with high-packing density and -alignment and 2) the fiber composed of highly crystalline CNTs with low defects prerequisite for the development of LC phase. Ironically, the nature of LC-spun CNTF also acts as disadvantages for directly applying to FSSCs because of the low specific area and low functional groups for incorporating active materials. Thus, it is essential to modify CNTF for FSSCs while preserving its outstanding properties. This thesis is about effective strategies for the architecture structure of LC-spun CNTF for use as a flexible electrode for wearable FSSCs. Chapter 1 in this thesis overviews the type of CNTF manufacturing method and the latest studies and trends on CNTF as energy devices. Especially, among the various methods for manufacturing CNTF, the liquid crystalline spinning method of CNT and liquid crystalline of CNTs are closely introduced. Chapter 2~4 in this thesis describes the strategies to overcome the disadvantages of LC-spun CNTF electrodes when applied directly to FSSCs. In Chapter 2, we propose an effective strategy to increase the surface area of electrodes with 2D materials to fabricate high-performance and flexible FSSCs. We employ graphene oxide (GO) as 3D architectural material of surface of CNTF. Chapter 3 is a continuous work of Chapter 2, which overcomes the process drawback of chapter 2. The issue of chapter 2 is that due to the weight of GO sheet, the 3D structures of GO easily collapse without a solidification process. Therefore, as a follow-up study, we employ tunicate cellulose nanofibrils (TCNF) as a filler to support a stereoscopic 3D structure of GO and fabricate a stable hybrid 3D structure on the CNTF without additional solidification process. Chapter 4 depicts a study to develop LC-spun CNTF using surface-functionalized CNTs, followed by an investigation of its electrochemical activity without any additional materials for directly use as a freestanding electrode for energy storage devices. In chapter 2 and 3, other nanomaterials such as GO and TCNF are employed, but CNTFs is solely used for FSSCs in this chapter. Oxygen-containing functional groups (–COOH and –OH) are introduced onto the surface of CNTs by well-known acid treatment and the lyotropic LC phase thereof is developed for use as a dope for liquid crystalline spinning (i.e., wet-spinning). The study on the electrochemical activity of CNTF as a function of functionalization provides new insight into the development of FSSCs. In this study, we utilize the outstanding properties of LC-CNTF and present effective surface modification strategies to improve the electrochemical properties of LC-CNTF for use as an electrode for FSSCs.