Design Strategies to Enhance Energy Conversion and Storage Performance for Dye-Sensitized Photo-Rechargeable Batteries

Abstract

In recent decades, dye-sensitized energy conversion and storage systems have garnered a lot of attention because of their effectiveness and versatility in a range of lighting conditions. This popularity stems from their use of earth-abundant components, easily tunable energy levels, and exceptional performance under low light intensity. To enabwle their widespread application, researchers have developed molecular engineering strategies, such as intramolecular charge transfer, molecular dye sensitization, and redox-active charging and discharging processes, which have significantly enhanced the performance of dye-sensitized solar cells (DSSCs) and enabled the successful development of dye- sensitized photo-rechargeable batteries (DSPBs), particularly under ambient light conditions. Despite these advancements, the development of DSPBs as a novel system has revealed significant challenges for practical applications. Among these, the low energy density, which is a critical performance component, remains a major limitation. To address this limitation, the following studies were conducted. In chapter 1, to further understand how DSPBs work and maximize energy density, the impact of electrolyte concentration, specifically the concentration of redox mediators, on DSPB performance was thoroughly examined. This study reveals that the photo-charge and discharge processes of DSPBs are strongly influenced by the concentration of the redox mediator. Using a cobalt mediator to increase the concentration, it was observed that higher electrolyte concentration enhances conductivity, reducing the photo-charge voltage and consequently increasing the discharge capacity. However, a higher concentration also leads to increased diffusion resistance during discharging, resulting in a decrease in discharge voltage. Ultimately, the discharge energy density was improved by a factor of 1.8 compared to the previous study, demonstrating that increasing the electrolyte concentration effectively boosts capacity. In chapter 2, a solid redox-targeting material capable of spontaneous oxidation and reduction was introduced to increase discharge energy density using Copper mediator. Although the Copper mediator is a key component for maximizing discharge voltage–a critical factor in energy density–its low solubility limits the ability to increase electrolyte concentration as in Chapter 1. To address this limitation, the redox-targeting material was incorporated to replenish the Copper mediator consumed during photo-charging through spontaneous electron transfer, as governed by the energy level of the liquid electrolyte, which is characterized by the Nernst equation. The application of this solid redox- targeting material demonstrated its influence on the photo-electrode, enabling the development of a novel design. Furthermore, the system was evaluated under both 1 SUN and indoor light conditions, achieving discharge energy density improvements of 2.3 and 2.2 times, respectively. In chapter 3, an innovative DSPBs was developed to address the limited discharge energy density at 1 SUN condition. To overcome the challenges of conventional DSPBs including the gap distance and membrane resistance caused by the ceramic membrane, a cascade layer was introduced between the photo-electrode and the coin-type battery. This cascade layer enables physical separation of the electrolytes while selectively allowing only electron transfer. This led to a 3.5-fold increase in light energy to electrical conversion efficiency and a 4-fold increase in discharge energy density of DSPB when compared to conventional systems under 1 SUN conditions.