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Redox flow batteries have demonstrated attractive attributes in large-scale stationary energy storage, but practical applications are impeded by high capital cost. Polysulfides are exceedingly cost-effective candidates of redox-active materials for achieving cost reduction, and a recent revival has been witnessed. But the slow conversion kinetics and irreversible crossover loss of polysulfides are daunting challenges that have caused severe technoeconomic stress and even system failure. Solutions to these issues capitalize on the innovations of powerful electrocatalysts and permselective membranes. To inspire viable development strategies and further advance polysulfide redox, this Review presents a critical overview of the state of the art of electrocatalysts and membranes, highlighting their working mechanisms, design protocols, and performance metrics. We briefly describe the complicated processes of the polysulfide reaction and the major spectroscopic methods for polysulfide speciation. Next, we point out the specific characteristics of polysulfide redox and summarize the metallic, metal sulfide, and molecular electrocatalysts to elucidate the fundamental requirements for imparting strong catalytic effects. We then discuss the possible origins of polysulfide crossover and outline the major families of membrane chemistries targeting polysulfide retention. Finally, the remaining challenges and the future perspectives for potential considerations are provided, aiming to realize efficient, durable polysulfide flow batteries. 

TEMPO has been widely explored as one of the most promising catholyte redox scaffolds in aqueous redox flow batteries, but the often-observed performance degradation raises concern with respect to its chemical instability. In this work, we demonstrate that the charged TEMPO species (i.e., TEMPO+) lack sufficient stability and also determine the major decomposition pathways. The decay products of TEMPO+ are experimentally analyzed using combined tools including nuclear magnetic resonance and mass spectroscopy. Reductive conversion to 2,2,6,6-tetramethylpiperidine (TEMPH) is commonly observed for a variety of 4-O-substituted TEMPO derivatives. The general detection of alkene and related carbonyl signals, in conjunction with the electrolyte acidification, reveals a deprotonation-initiated ring opening route that proceeds towards TEMPO decay. The protons on the β carbon are susceptible to chemical extraction by nucleophilic agents such as hydroxyl and the formed piperidine. This finding highlights the intrinsic structural factors for TEMPO degradation and will shed light on the potential stabilization strategies to afford long-cycling TEMPO-based flow batteries. 

The inexpensive sulfur raw material is promising to enable cost-effective redox flow batteries for long duration energy storage. But the catastrophic through-membrane crossover of polysulfides remains a severe challenge resulting in irreversible performance degradation and short cycle life. In this work, we demonstrate that use of a permselective cation exchange membrane yields a two orders of magnitude enhancement in polysulfide retention compared to the benchmark Nafion membrane. Combined physico-chemical, spectroscopic, and microscopic analyses suggest more disordered sidechain structures, which lead to the more hydrophobic nature and smaller hydrophilic domains in the membrane. The microstructural features contribute to the effective mitigation of polysulfide crossover. As a result, the cycle life of polysulfide/ferricyanide flow cells is boosted over a substantially extended test time. This finding sheds light on the fundamental membrane factors that cause polysulfide permeation and can provide feasible directions in the development of permselective membranes for polysulfide flow batteries.