Developing Novel Charge-Carriers for High-Energy Density Aqueous and Non-Aqueous Redox Flow Batteries
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The main objective of this thesis has been to investigate new charge carriers for redox flow batteries (RFBs) to address stability, solubility, and energy density limitations in existing systems. Using cyclic voltammetry (CV), chronoamperometry, and charge-discharge cycling as primary electroanalytical techniques, a range of redox active molecules were evaluated. In addition, UV-vis spectroscopy, spectroelectrochemistry and mass-spectrometry were also employed to further analyze the redox active materials. A fluorescent boron‐dipyrromethene (BODIPY) dye (PM567) was explored as suitable active material for an all organic symmetric non-aqueous RFB. A theoretical cell potential of 2.32 V was predicted from CV experiments. It was found that the oxidized and reduced, PM567 does not remain intact; however, the products of bulk electrolysis evolved over time to show stable redox behavior, making the dye a precursor for the active species of an RFB. In a separate study, we hypothesized that the redox properties of an environmentally damaging organic dye, methylene blue, make it suitable for use in the electrolyte solutions of aqueous redox flow batteries. Electrochemical analyses of aqueous methylene blue solutions established the dye as a pH tunable, two‐electron redox transfer agent with fast transfer kinetics, motivating the recycling of dye- containing wastewater as the basis for flow battery electrolyte solutions. The active material concentration is critically linked to the energy capacity of a RFB. We identified a research gap in electrochemical behaviors at the solubility limit of charge carriers compared to dilute conditions. This thesis features a systematic study which revealed that the cycling behaviors are concentration dependent and membrane crossover together with fouling are major contributors to performance losses. We also demonstrated the suitability of the mixed-component catholyte and anolyte solutions for high-energy density non-aqueous RFBs, which has the effect of increasing the overall concentration and the number of electrons transferred.