Why Aqueous Organic Redox Flow Battery

Aqueous redox flow batteries are promising for fire-safe and scalable grid energy storage to support the expansion of renewables. Current commercial flow battery systems employ inorganic redox active species, such as vanadium, iron, zinc, bromine, chromium, etc. However, the performance of these metals is limited by their intrinsic electrochemical properties, and there are few avenues to enhance the performance through chemical means. In contrast, organic redox active species present nearly boundless opportunities to directly tune the electrochemical properties through structural modification.  Aqueous flow battery devices utilizing organic redox active materials are called Aqueous Organic Redox Flow Batteries (AORFBs). Yet, designing organics with water solubility, chemical stability, facile kinetics, membrane compatibility, and high potential, while also being low-cost and scalable for practical application, has been difficult. 

Challenges of Conventional Approaches

Many organic anolytes with promising performance have been reported for alkaline condition, including quinones, phenazines, and fluorenones. However, no high-performance organic catholyte has been developed for alkaline conditions due to fundamental orbital and thermodynamic challenges in basic media. Instead, to showcase cycling stability, alkaline AORFBs typically utilize iron cyanide in excess as the catholyte, which has voltage, energy density, and membrane capability limitations. It is unclear, to date, whether an alkaline catholyte to match anolyte performance can be developed.

An alternative approach is to develop AORFB for pH neutral condition, where both anolyte and catholyte have shown promising stability. For example, our previous work has demonstrated highly stable, soluble, and efficient viologens anolytes, which can be easily produced in kilogram amounts in a chemistry lab. The design of organic catholytes has been more limited for pH neutral systems as well, with most research focusing on ferrocene and TEMPO derivatives. TEMPO catholyte shows more promise due to its facile kinetics, high potential, and water miscibility, yet its small size results in membrane crossover. This can be reduced by appending redox-inert charged groups at the 4-position, but this has been shown to sacrifice energy density, chemical stability, and  material cost. Thus, new approaches are needed to design AORFB catholyte. 

Key Findings

To address these aforementioned challenges in organic catholytes, we designed over 100 ionic liquid mimicking TEMPO dimers (i-TEMPODs). Instead of inflating the mass of TEMPO with redox-inert functionalization to prevent membrane crossover, we increase the size and charge with redox-active TEMPO components to retain high energy density. TEMPO was an ideal candidate for this strategy as it is made up of solely sp³ carbons with weak intermolecular forces (no pi-pi stacking). Furthermore, when the TEMPO monomers are linked using a soft cationic organic group, the structure mimics that of ionic liquid salts to spur water miscibility. 

A total of 21 i-TEMPODs were synthesized and characterized in this work to comprehensively establish this new class of catholyte. To promote high-throughput production, a building block assembly platform was develop, including TEMPO monomers and organic linkers with labile substitution groups. Then, these building blocks were reacted in various combinations to yield i-TEMPODs of varying 4-position functionalization, linker identity, size, and charge for systematic structure-property study. Each of these reactions possessed high yields with scalable methods. 

After synthesis, simple yet informative methods were used to characterize the physiochemical and electrochemical properties of i-TEMPODs. Cycling Voltammetry (CV) showed that the formal reduction potential can be tuned with the 4-position functionalization, while Electrochemical Impedance Spectroscopy (EIS) demonstrated that each retained facile redox kinetics characteristic of the nitroxide radical. Solubility and viscosity tests confirmed that i-TEMPODs were able to retain high water solubility and energy density with significantly deterred membrane crossover, even through higher power (lower resistance) ion-exchange membranes. Thus, our design hypothesis of i-TEMPODs was confirmed and useful structure-property trends were unraveled. 

Four different i-TEMPODs were cycled  in AORFB and demonstrated capacity stability over extended cycling. An optimized structure, N+TEMPOD, exhibited stable cycling at high energy density (2M N+TEMPOD, 4M electron) over 90 days of continuous cycling, confirming electrochemical stability. This test was done using AMVN anion-exchange membrane, which has relatively high resistance. AMVN (or AMV) is typically used in testing TEMPO catholytes to showcase stable cycling performance, but it is not fit for practical application due to its high area-specific resistance. Accordingly, N+TEMPOD was also tested with DSVN anion-exchange membrane, a lower resistance membrane more fit for practical device. In contrast to previously reported TEMPO monomers that exhibited facile crossover and cycling capacity decay with DSVN, no apparent capacity decay or membrane crossover was measured for N+TEMPOD at 2M over 23 days of cycling. Thus, N+TEMPOD possess advantageous properties for energy dense, high-power, and capacity stable AROFBs.

Next Steps

i-TEMPOD catholytes have been established and experimentally characterized in this work, showing competitive performance with all other flow battery systems. These materials have been produced in kilogram amounts in lab through our building-block assembly method. Organic redox active materials are often touted as abundant, green, and low cost, yet this is highly dependent on their raw material availability, synthetic methods, reaction yields, and waste streams. Thus, for impactful research, the AORFB field, ourselves included, must critically and honestly explore whether organic redox materials will translate from lab-scale to industrial production with truly sustainable and secure supply chains. If meaningful connections between lab and industry can be realized, then we believe that organic redox materials will play a key role in the future green economy.