Modular TEMPO Dimerization for Water-in-Catholyte Flow Batteries with Extreme Energy Density, Power, and Stability

Abstract

Aqueous organic redox flow batteries (AORFBs) hold great promise for safe, sustainable, and cost-effective grid energy storage. However, developing catholyte redox molecules with desired energy density, power, and stability simultaneously has long been a critical challenge for AORFBs. Here, we report a novel class of ionic liquid mimicking TEMPO dimers (i-TEMPODs) that can be produced by our newly developed building block assembly synthetic platform. By systematically investigating 21 derivatives, we reveal i-TEMPODs have optimized size and charge that is compatible with highly conductive membrane and can form a “water-in-catholyte” (WiC) state. The tight coordination dynamics with water molecules deliver extreme solubility with promoted electrochemical stability at highly positive potentials. Leveraging these advances, we identify a champion molecule and demonstrate record overall AORFB performance in energy density (47.3 Wh/L), power density (0.325 W/cm2), and stability (no apparent capacity decay after 96 days) with low-cost and scalable chemistry.

A Hydroxylated Tetracationic Viologen based on Dimethylaminoethanol as a Negolyte for Aqueous Flow Batteries

“…This is in agreement with recent work with Dex-Vi, which shows that the viscosity of tetracationic viologens is manageable at practical concentrations, [24] in contrast to organic molecules suffering from excessive viscosity due to undesired oligomerization. [43] The need for battery performance and capacity retention evaluations using concentrated solutions of [(DMAE-Pr) 2 -Vi]Cl 4 is recognized. Under these conditions, intermolecular interactions will result in differences in viscosity, diffusion coefficient, conductivity, electrode kinetics and degradation rates dependent on molecular aggregation mechanisms in respect to relatively diluted conditions.…”

Viologen Hydrothermal Synthesis and Structure-Property Relationships for Redox Flow Battery Optimization

“…9,16 Although 1.0 M is often considered "high" volumetric capacity for AORFB, 2 it is much lower than the recently demonstrated i-TEMPOD catholyte with 4 M electron concentration. 17 To avoid this full cell AORFB capacity mismatch and improve the anolyte volumetric energy density, we hypothesized that viologens with neutral functionalization (i.e., alcohol moieties) will exhibit enhanced solubility and lower viscosity. This is because electrolyte with less charge will possess lower intermolecular interaction, resulting in reduced solution viscosity, while the smaller alcohol functionality permits hydrogen bonding network for water miscibility.…”

“…With energy dense and stable one-electron viologen performance, pH neutral anolyte is now comparable to alkaline anolyte, 1-8 while permitting pairing with high voltage, waster miscible, and electrochemically stable TEMPO catholytes. 17,[27][28][29][30][31][32] This highlights the importance of developing high-throughput synthetic methods to empirically screen complex structure-property relationships of organic redox molecules. Such insights will permit molecular engineering of ideal AORFB electrolyte for safe, sustainable, and reliable long-duration grid energy storage.…”

Viologen Hydrothermal Synthesis and Structure-Property Relationships for Redox Flow Battery Optimization

“…With energy dense and stable one-electron viologen performance, pH neutral anolyte is now comparable to alkaline anolyte, 1-8 while permitting pairing with high voltage, waster miscible, and electrochemically stable TEMPO catholytes (Figure S53-S57). 17,[27][28][29][30][31][32] This highlights the importance of developing high-throughput synthetic methods to empirically screen complex structure-property relationships of organic redox molecules. Such insights will permit molecular engineering of ideal AORFB electrolyte…”