Redox Flow-Based Energy Storage and Water Desalination
Creators: name: Yue, Diqing
Publisher: name: Purdue University Graduate School
Year: 2024
Subjects: classificationCode: 401703
subject: Energy generation, conversion and storage (excl. chemical and electrical)

description: Energy storage has become a promising solution to stabilize renewable energy outputs and to solve the peak/off-peak issues of the power grid. Redox flow battery (RFB) possesses separated energy and power, high capacity, long cycle life and safety, and therefore is regarded as a potential candidate of energy storage. In this thesis, we have researched the degradation pathway of TEMPO derivative redoxmers, obtained long-time stable cycling of a non-aqueous RFB with synthetic redoxmers and permselective ceramic membranes, and extended the redox flow approach to the field of water desalination.The properties of redoxmers are the main elements that affect RFB performance. Organic redoxmers come to sight due to their facile property improvement based on structural diversity and molecular tailorability. But the majority of reported redoxmers are anolytes; catholytes are less developed. Also, the mechanism of limited long-time cycling stability is still not well understood. In our experiment, we have progressively unraveled a series of degradation mechanisms of TEMPO-based redoxmers, including oxidation, crossover, ring-opening and possibly deoxygenation. The initial candidate, 4-hydro-TEMPO (TEMPOL), presents combined decomposition pathways. The charged oxoammonium species oxidizes the alcohol group (-OH) in its structure to a ketone (C=O) bond and also undergoes a protonation-induced ring-opening side reaction forming an alkene structure, evidenced by the characteristic 13C NMR chemical shifts of C=O and C=C groups. Due to its non-ionic structure, crossover through the anion exchange membrane used in flow cells is another issue that causes capacity loss. A hydroxyl-free TEMPO derivative bearing an anionic sulfonate group (‒SO3‒) also suffers from deprotonation-induced ring opening. By eliminating nucleophilic moiety, we have designed the third TEMPO derivative that has a cationic tetraalkylammonium end group. This molecule exhibits greatly improved cycling stability in flow cells, yet still with slow capacity fading that may hypothetically be a result of parasitic deoxygenation reaction. With the carefully designed analyses, the obtained mechanistic understanding of molecular decomposition has paved the way for rationale structural design toward stable TEMPO catholyte candidates.Nonaqueous RFBs hold promise for higher cell voltage and energy density given their wider electrochemically stable voltage windows, but their performance is often plagued by the crossover of redox compounds. In this study, we used permselective lithium superionic conducting (LiSICON) ceramic membranes to enable reliable long-term cycling of organic redox molecules in nonaqueous flow cells. With different solvents on each side, enhanced cell voltages were obtained for a flow battery using viologen-based negolyte and TEMPO-based posolyte molecules. The thermoplastic assembly of the LiSICON membrane realized leakless cell sealing, thus overcoming the mechanical brittleness challenge. As a result, stable cycling was achieved in the flow cells, which showed good capacity retention over an extended test time (e.g. two months).Desalination of saline water is becoming an increasingly critical strategy to overcome the global challenge of drinkable water shortage, but current desalination methods are often plagued with major drawbacks of high energy consumption, high capital cost, or low desalination capacity. To address these drawbacks, we have developed a unique continuous-mode redox flow desalination approach capitalizing on the characteristics of redox flow batteries. The operation is based on shuttled redox cycles of very dilute Fe2+/Fe3+ chelate redoxmers with ultralow cell overpotentials. The air instability of Fe2+ chelate is naturally compensated for by its in situ electrochemical generation, making the desalination system capable of operations with electrolytes at any specified state of charge. Under unoptimized conditions, fast desalination rates up to 404.4 mmol·m−2·h−1 and specific energy consumptions as low as 7.9 Wh·molNaCl−1 have been successfully achieved. Interestingly, this desalination method has offered an opportunity of sustainable, distributed drinkable water supplies through direct integration with renewable energy sources such as solar power. Therefore, our redox flow desalination design has demonstrated competitive desalination performance, promising to provide an energy-saving, high-capacity, robust, cost-effective desalination solution.