Reversible redox chemistry in azobenzene-based organic molecules for high-capacity and long-life nonaqueous redox flow batteries

Abstract

Redox-active organic molecules have drawn extensive interests in redox flow batteries (RFBs) as promising active materials, but employing them in nonaqueous systems is far limited in terms of useable capacity and cycling stability. Here we introduce azobenzene-based organic compounds as new active materials to realize high-performance nonaqueous RFBs with long cycling life and high capacity. It is capable to achieve a stable long cycling with a low capacity decay of 0.014% per cycle and 0.16% per day over 1000 cycles. The stable cycling under a high concentration of 1 M is also realized, delivering a high reversible capacity of~46 Ah L −1. The unique lithium-coupled redox chemistry accompanied with a voltage increase is observed and revealed by experimental characterization and theoretical simulation. With the reversible redox activity of azo group in π-conjugated structures, azobenzene-based molecules represent a class of promising redox-active organics for potential grid-scale energy storage systems.

Insights into the Redox Chemistry of Organosulfides Towards Stable Molecule Design in Nonaqueous Energy Storage Systems

“…The possible mechanism is that the resonance structure in the reduced product can handle the negative charge efficiently, accompanying with the electron delocalized in the molecule, which significantly reduces side reactions with organic solvents or supporting salts, thus enabling the stable cycling in multiple nonaqueous electrolytes, which is rarely reported for other redox‐active organic molecules (Figure 4 a). [20] Besides, the advantage of sulfur atoms in the resonance structure is that it has a large atomic size to help decrease the charge density, resulting in the better‐stabilized electron in the molecular structure.…”

Dual function organic active materials for nonaqueous redox flow batteries

“…The flow cell accessed 91% of the theoretical capacity, 18.3 Ah L -1 , which is one of the highest volumetric capacities reported for a nonaqueous RFBs (Figures 8a and 8b). 57 The two-electron cycling cell showed improved capacity retention (84% of capacity retention over 35 cycles; losing average capacity of 0.48% per cycle, 0.90% per day) compared to the 0.75 M one-electron cycling cell (74% of capacity retention over 30 cycles; losing average capacity of 0.88% per cycle, 0.99% per day), perhaps due to the lowered charge-discharge time (13 h per cycle), reduced rate of charged species crossover, and improved mass transfer at lower concentration of MEEV-(TFSI) 2 . In addition, the two-electron flow cell demonstrates 97% coulombic efficiency, 75-65% of voltage efficiency, and 73-64% of voltage efficiency.…”

Design Principles and Applications of Next‐Generation High‐Energy‐Density Batteries Based on Liquid Metals

“…A typical redox flow battery contains at least one redox active electrolyte with an opposite electrolyte or metal anode, that could allow pumped fluid supply from scalable tank containers for huge storage capacity, promising the recycling of electrolytes in the outer circulations. [82][83][84] However, the flow batteries normally have relatively low battery voltages for aqueous systems, relatively high viscosity for organic systems, and cross-over of redox species to be concerned for both. [85][86][87][88][89] The liquid metals could also be adopted as the "anolyte" to supply low enough anode potential for high energy output, and it could further prevent cross-over issue due to its high surface tension by more efficiently eliminating migrations across the nano-or micro-porous membranes, comparing with the smallsized water or organic solvent molecules.…”