Tailor-made Organic Materials Controlling Redox Reactions and Molecular Interactions for Energy Conversion and Storage Devices

Abstract

Energy materials based on organic molecules with carbon skeletons appear promising due to their structural diversity and absence of rare earth elements. These energy materials can be broadly classified into two groups: energy harvesting materials, such as organic solar cells and thermoelectric devices, and energy storage materials, such as batteries and supercapacitors. This dissertation presents developments in organic materials for thermoelectric devices and redox-enhanced supercapacitors, and demonstrates important breakthroughs in overcoming previous limitations of these materials in practical applications. The first chapter of this dissertation presents novel acetal-functionalized indacenodithiophene (IDT) moieties, as well as the reason why the conventional IDT unit results in inferior conductivity. Conductive polymers containing IDT units have the potential to become high-performance thermoelectric materials due to their high charge-carrier mobility in the amorphous state which can lower the thermal conductivity without affecting the electrical conductivity. However, the conductivity of IDT-containing polymers, such as PIDT-EDOT, is poor (0.43 Scm−1). To overcome this, a novel strategy is proposed in this study which involves controlling the dopant position using a Lewis acid-base complex between a new acetal-functionalized IDT (IDTa) unit and a dopant, nitrosyl hexafluorophosphate. The polymer, PIDTa-EDOT, showed an increase in doping efficiency and condensed π-π stacking, which improved its electrical conductivity. Additionally, PIDTa-EDOT had a significantly lower thermal conductivity (0.09 Wm−1K−1) compared to PEDOT:PSS (0.16–0.39 Wm−1K−1) due to the reduction in grain size post-doping. Consequently, PIDTa-EDOT exhibited a ~30-fold increase in electrical conductivity (12.56 Scm−1) and an ~6-fold increase in thermoelectric performance (ZT = 7.57 X 10−3) compared to PIDT-EDOT. This research proposes a new approach to increase electrical conductivity and decrease thermal conductivity simultaneously, ultimately leading to the development of high-performance thermoelectric materials. The second chapter of this dissertation introduces the concept of a hydrotropic-supporting electrolyte. Redox molecules' solubility is a crucial factor that affects the performance of redox-enhanced electrochemical capacitors (redox ECs). However, the solubility of commonly used organic redox molecules in aqueous systems is low, which limits further performance enhancements. To overcome this issue, a complex and costly synthesis process is typically required. This study introduces the concept of a hydrotropic-supporting electrolyte (HSE) that acts as both a solubility-enhancing hydrotrope and an ion-conducting supporting electrolyte. The HSE increased the solubility of hydroquinone (HQ) and anthraquinone-derivative (AQM-Br) up to 7 and 3 times, respectively, compared to the aqueous H2SO4 electrolyte. A high-performance redox EC was developed by implementing a bipolar AQM-Br with p-toluenesulfonic acid as a hydrotrope. The mechanistic analysis provided insights into correlating the solute-hydrotrope interactions and hydrotrope efficiency. This study serves as a guide for designing energy-dense redox-active electrolytes and selecting optimal HSE and redox-active electrolyte pairs