Charge/Ion Transport Properties of Self-assembled Organic and Polymeric Materials

Abstract

There are rising demands for developing electronics and energy storage system for more widespread uses in a diverse range of applications. This inevitably requires the development of new key materials with high electrochemical properties and good stability. Organic and polymeric materials are therefore expected to be an important elements for next generation electronics and energy storage system, due to their unique advantages such as sustainability, cost-efficiency, environmental friendliness and flexibility. Although organic and polymeric materials have various advantages compared to other conventional materials for energy storage system, the development of organic materials is still in its infant stage. Moreover, some of the drawbacks such as poor electrical conductivity, and slow redox kinetics also exist for enhancement of battery properties. One of the solution to enhance charge properties of organic and polymeric materials is determining and controlling their nano-/micro- structure. Structures of organic and polymeric materials are a crucial parameter in determining the efficiency of charge transfer. For development of sustainable and versatile electronics and energy storage system beyond current status, more fundamental structural studies are needed. Herein, in this thesis, investigation of charge transport properties of self-assembled organic and polymeric materials are described in the perspective of fundamental and application researches such as biosensors, electronics, and lithium batteries. In chapter 1, it gives a brief overview of energy storage system based on organic and polymeric materials. For a long time, organic materials have received much less attention compared to inorganic materials, mainly due of their poor stability and electrochemical performance. However, for the past decades, a lot of different organic molecules have been studied and exhibited great progress. Nowadays, some special candidates of organic materials show comparable or even superior electrochemical performance to the conventional inorganic cathodes. I present some candidates of organic materials for next generation electronics and energy storage systems in this chapter. In chapter 2, I have investigated the enhanced charge transport properties through nanostructured organometallic block copolymers. Organometallic block copolymers, poly(ferrocenyldimethylsilane-b-isoprene) (PFS-b-PI), containing electroactive ferrocene moieties are employed as electron mediators where the chemical cross-linking of PI chains greatly increases the stability of electrodes in physiological environments. Notably, catalytic current densities of the fabricated electrodes have proven be a sensitive function of the morphologies of electron mediators. Different nanoscale morphologies, i.e., bicontinous structure, nanowires, and nanoparticles, have been derived and the use of bicontinous PFDMS-b-PI confirms 2~50 times improved catalytic current response than the values obtained from other morphologies; the maximum catalytic current of glucose oxidation was 0.55 mA/cm2 at 60 mM glucose concentration. The bio-sensing ability of the fabricated electrode with structural optimization was also exploited and good sensitivity is obtained at the physiological concentration of glucose in blood. In chapter 3, the improved conductivity was obtained by developing 2-dimensional nanostructure of conducting polymer. I presented a new methodology to synthesize polyaniline (PANI) nanosheets using ice template. The PANI nanosheet demonstrated exceptional electrical conductivity that was a few orders of magnitude higher than that of most HCl-doped PANIs reported to date. Key to success stemmed from the use of ice, offering unique surfaces of water molecules to polymerize aniline molecules. The freestanding PANI nanosheets in tens of nanometers thickness are also easily attained by removal of ice through a simple melting process. In chapter 4, I have investigated the facile synthesis of new naphthoquinone (NQ)-derivatives with high charge transport properties for use in improved lithium-organic batteries. The rational design of these NQ-derivatives is based on theoretical calculations. Our lithium-organic batteries demonstrate remarkable charge-discharge properties, for example, a high discharge capacity of 250 mAh·g−1 (363 mAh·cm−3), discharge potential plateaus in the range of 2.32.5 V, and 99% capacity retention after 500 cycles at 0.2 C. In particular, the batteries had excellent rate performance up to 50 C with reversible redox behavior, unlike most other organic cathode materials. This was attributed a simple molecular substitution, addition amino groups at the 2- and 3- positions of the NQ ring, yielding 2,3-diamino-1,4-naphthoquinone (DANQ). DANQ has an exceptionally low band-gap of 2.7 eV and greater than 20-fold enhancement in the lithium diffusion rate compared to unmodified NQ. This chapter suggests that NQ-derivatives with modulated charge/ion transport properties are a viable alternative to the more widely studied lithium metal oxides.