Aqueous Solubility of Organic Compounds for Flow Battery Applications: Symmetry and Counter Ion Design to Avoid Low-Solubility Polymorphs

Abstract

Flow batteries can play an important role as energy storage media in future electricity grids. Organic compounds, based on abundant elements, are appealing alternatives as redox couples for redox flow batteries. The straightforward scalability, the independence of material sources, and the potentially attractive price motivate researchers to investigate this technological area. Four different benzyl-morpholino hydroquinone derivatives were synthesized as potential redox active species. Compounds bearing central symmetry were shown to be about an order of magnitude less soluble in water than isomers without central symmetry. Counter ions also affected solubility. Perchlorate, chlorate, sulfate and phosphate anions were investigated as counter ions. The formations of different polymorphs was observed, showing that their solubility is not a function of their structure. The kinetics of the transformation can give misleading solubility values according to Ostwald’s rule. The unpredictability of both the kinetics and the thermodynamics of the formation of polymorphs is a danger for new organic compounds designed for flow battery applications.

Improvement in aqueous solubility of achiral symmetric cyclofenil by modification to a chiral asymmetric analog

“…Medicinal chemists knows that methylation rarely leads to improved aqueous solubility 34 . And the mechanism of improved solubility by methylation is not fully understood.…”

Novel, Stable Catholyte for Aqueous Organic Redox Flow Batteries: Symmetric Cell Study of Hydroquinones with High Accessible Capacity

“…Therefore, a series of four HQ derivatives incorporating the morpholinomethylene and/or methyl groups were designed to understand the relationship between the structure of this particular compound class and the important parameters of a redox flow battery. The influence of molecular structure on the solubility of the four molecules were thoroughly discussed in our previous report [31]. In this work, the stability of the four molecules was investigated in symmetric flow cells in different supporting electrolytes using different cycling methods.…”

“…The stability of the four hydroquinone derivatives was primarily investigated in a symmetric cell setup at a concentration of 0. [31]. An anion-exchange membrane was employed to retain the protonated, cationic hydroquinones and conduct the counter ions at the same time.…”

Cerium/Ascorbic Acid/Iodine Active Species for Redox Flow Energy Storage Battery

“…The RFB has been developed for large-scale energy storage systems (KWh–MWh) during the last decade to address the power instability of renewable energy resources (e.g., wind power and solar power) because of climate change. RFBs have been regarded as suitable for large-scale energy storage because of their modular design, good scalability, flexible operation, high storage capacity and efficiency, long operating life, safety, and environmentally friendly properties [ 1 , 2 , 3 , 4 , 5 ]. E. Sánchez-Díez et al provided a review of the status of and perspective towards sustainable stationary energy storage of the RFB, which can potentially fulfill cost requirements and enable large-scale storage [ 1 ].…”

“…F. Pan et al reviewed different categories of storage active species for RFB applications, such as inorganic ions (metal complexes) and metal-free organic compounds (e.g., polysulfide/sulfur and lithium) storage and suggested the future development of redox species towards higher energy density [ 4 ]. S. N. Garcia et al synthesized four different benzyl-morpholino hydroquinone derivatives as potential redox active species [ 5 ]. The all-vanadium redox flow battery (all-VRFB) is one of the most promising technologies for mid- to large-scale application (KW–MW) and has the advantages of fast response time, flexible design, high cycle life, and non-flammable and non-explosive properties.…”