Designing Redox-Active Oligomers for Crossover-Free, Nonaqueous Redox-Flow Batteries with High Volumetric Energy Density

Abstract

Here we show how to design organic redox-active solutions for use in redox-flow batteries, with an emphasis on attaining high volumetric capacity electrodes that minimize active-material crossover through the flow cell’s membrane. Specifically, we advance oligoethylene oxides as versatile core motifs that grant access to liquid redox-active oligomers having infinite miscibility with organic electrolytes. The resulting solutions exhibit order-of-magnitude increases in volumetric capacity and obviate deleterious effects on redox stability. The design is broadly applicable, allowing both low potential and high potential redox centers to be appended to these core motifs, as demonstrated by benzofurazan, nitrobenzene, 2,2,6,6-tetramethylpiperidin-1-yl)­oxyl, and 2,5-di-tert-butyl-1-methoxy-4-(2′-methoxy)­benzene pendants, whose reduction potentials range from −1.87 to 0.76 V vs Ag/Ag+ in acetonitrile. Notably, the oligoethylene oxide scaffold minimizes membrane crossover relative to redox-active small molecules, while also providing mass- and electron-transfer kinetic advantages over other macromolecular architectures. These characteristics collectively point toward new opportunities in grid-scale energy storage using all-organic redox-flow batteries.

Design Rules for Membranes from Polymers of Intrinsic Microporosity for Crossover-free Aqueous Electrochemical Devices

“…These experimentally validated guiding principles should accelerate the identification of ion-selective polymer membranes for the broad palette of emerging aqueous cell chemistries, including those based on inorganics, 15,19,20 metal coordination complexes, 14,[21][22][23] organometallics, 24,25 polyoxometalates, [26][27][28] redox-active organic molecules, 13,25,[29][30][31][32][33][34][35][36] and related macromolecular redoxmers. [37][38][39][40][41][42][43][44]…”

A Comparative Review of Metal‐Based Charge Carriers in Nonaqueous Flow Batteries

“…Macromolecular systems fall within the growing trend of adopting large redox carriers to discourage crossover and incorporate less expensive membranes. [23,53,54] In 2013, Anderson and co-workers established the utility of macromolecular systems for RFBs, demonstrating the ability of several Keggin-type ([XM 12 O 40 ] nÀ ) clusters to function as active species in aqueous flow cell configurations ( Figure 4). [55][56][57] Recent examples are benchmarks of the progress made towards effective aqueous RFBs.…”

“…Whereas molecular charge carriers discussed above focus on mononuclear complexes functionalized with ligands to improve its performance and solubility, macromolecular carriers are polynuclear with a dynamic ligand scaffold that does not suffer significant reorganization energies upon redox. Macromolecular systems fall within the growing trend of adopting large redox carriers to discourage crossover and incorporate less expensive membranes [23,53,54] . In 2013, Anderson and co‐workers established the utility of macromolecular systems for RFBs, demonstrating the ability of several Keggin‐type ([XM 12 O 40 ] n − ) clusters to function as active species in aqueous flow cell configurations (Figure 4).…”

Comparison of Separators vs Membranes in Nonaqueous Redox Flow Battery Electrolytes Containing Small Molecule Active Materials

“…3,[12][13] The lack of well-studied membranes presents a challenge for those developing new active materials due to the difficulty in deconvoluting causes of capacity fade within a cycling cell. 5,[14][15] For example, in a separated cell-meaning one in which the positive and negative electrolytes only contain the positive and negative active species-capacity fade due to crossover can dominate performance evaluations during cell cycling. In this situation, one must conduct further analysis (e.g.…”