Organic Molecule-based Electrolyte Materials for Aqueous Redox Flow Battery
AbstractSolar and wind energies have been growing so fast that in many regions around the world they have become the cheapest source for electricity production. Nevertheless, as we rely increasingly on solar and wind energies, one of their biggest technical barriers needs to be addressed, which is the intermittency: the sun does not always shine, and the wind does not always blow. Grid-level electrical energy storage (EES) can store these energies when they are generated in excess and reuse them whenever and wherever needed.
Batteries provide a means to store electrical energy in the form of chemical energy. Although traditional solid-electrode batteries such as lithium-ion batteries have been extensively developed over the past few years, they are deemed too costly for long discharge duration EES applications such as regulating solar and wind energy. Aqueous redox flow batteries (ARFB), however, provide a more cost-effective route for EES by separating its electro-active storage materials from the electrochemical conversion components. This allows independent adjustment of the battery power (by changing the size of its electrode) and its energy (by changing the volume or concentration of its electrolyte) so that both components can operate at their optimum performance for arbitrary discharge durations.
Traditional ARFBs utilize redox-active inorganic compounds dissolved in highly corrosive supporting electrolyte. These species also exhibit serious membrane crossover, sluggish redox kinetics, low solubility, or high chemical cost, which severely limit their battery performance and commercialization. Contrarily, redox-active organic molecules, such as quinones, have demonstrated high chemical diversity. Through appropriate chemical functionalization to improve their reduction potential, solubility, and stability, many of these shortcomings from inorganic systems can be effectively addressed. In Chapter 2 of the thesis, an overview of the design principles of quinone-based electrolyte materials will be first discussed. In Chapters 3 and 5, two alkaline ARFB systems based on alkali-soluble anthraquinone and alloxazine derivatives, respectively, will be demonstrated. Rational design of these electrolyte materials is provided, along with an electrochemical analysis of their full-cell performance by pairing them with alkali-compatible ferri/ferrocyanide redox pair. In Chapter 4, semiquinone (a singly reduced quinone intermediate during cell cycling) formation in alkaline condition, its characterization using ESR, and its impact on cell cycling performance will be presented. Based on preliminary results, by tuning the concentration of semiquinone, the battery capacity retention can be significantly improved.
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