Person:

Fell, Eric

A highly stable phosphonate‐functionalized viologen is introduced as the redox‐active material in a negative potential electrolyte for aqueous redox flow batteries (ARFBs) operating at nearly neutral pH. The solubility is 1.23 m and the reduction potential is the lowest of any substituted viologen utilized in a flow battery, reaching −0.462 V versus SHE at pH = 9. The negative charges in both the oxidized and the reduced states of 1,1′‐bis(3‐phosphonopropyl)‐[4,4′‐bipyridine]‐1,1′‐diium dibromide (BPP−Vi) effect low permeability in cation exchange membranes and suppress a bimolecular mechanism of viologen decomposition. A flow battery pairing BPP−Vi with a ferrocyanide‐based positive potential electrolyte across an inexpensive, non‐fluorinated cation exchange membrane at pH = 9 exhibits an open‐circuit voltage of 0.9 V and a capacity fade rate of 0.016% per day or 0.00069% per cycle. Overcharging leads to viologen decomposition, causing irreversible capacity fade. This work introduces extremely stable, extremely low‐permeating and low reduction potential redox active materials into near neutral ARFBs.

We demonstrate the electrochemical oxidation of an anthracene derivative to a redox-active anthraquinone at room temperature in a flow cell without the use of hazardous oxidants or noble metal catalysts. The anthraquinone, generated in situ, was used as the active species in a flow battery electrolyte without further modification or purification. This potentially scalable, safe, green, and economical electrosynthetic method is also applied to another anthracene-based derivative and may be extended to other redox-active aromatics.

Aqueous organic redox flow batteries are promising candidates for large-scale energy storage. However, the design of stable and inexpensive electrolytes is challenging. Here, we report a highly stable, low redox potential, and potentially inexpensive negolyte species, sodium 3,3′,3′′,3′′′-((9,10-anthraquinone-2,6-diyl)bis(azanetriyl))tetrakis(propane-1-sulfonate) (2,6-N-TSAQ), which is synthesized in a single step from inexpensive precursors. Pairing 2,6-N-TSAQ with potassium ferrocyanide at pH=14 yielded a battery with the highest open-circuit voltage, 1.14 V, of any anthraquinone-based cell with a capacity fade rate <10 %/yr. When 2,6-N-TSAQ was cycled at neutral pH, it exhibited two orders of magnitude higher capacity fade rate. The great difference in anthraquinone cycling stability at different pH is interpreted in terms of the thermodynamics of the anthrone formation reaction. This work shows the great potential of organic synthetic chemistry for the development of viable flow battery electrolytes and demonstrates the remarkable performance improvements achievable with an understanding of decomposition mechanisms.

Synthetic cost and long-term stability remain two of the most challenging barriers for the utilization of redox-active organic molecules in redox flow batteries for grid scale energy storage. Starting from potentially inexpensive 9,10-dihydroanthracene, we developed a new synthetic approach for two extremely stable anthraquinone negolytes, i.e., 3,3'-(9,10- anthraquinone-diyl)bis(3-methylbutanoic acid) (DPivOHAQ) and 4,4'-(9,10- anthraquinone-diyl)dibutanoic acid (DBAQ). Pairing with a ferrocyanide posolyte at pH 12, DPivOHAQ and DBAQ can transfer up to 1.4 M and 2 M electrons with capacity fade rates of 0.014%/day and 0.0084%/day, respectively, and exhibit 1.0 V of open circuit voltage. By adjusting the supporting electrolytes to pH 14, DPivOHAQ exhibited a record low capacity fade rate of <1%/year. We attribute the capacity loss of these flow batteries to be caused primarily by the formation of anthrone, which can be suppressed by increasing the pH of the electrolyte and reversed by exposure to air.

Water‐soluble redox‐active organic molecules have attracted extensive attention as electrical energy storage alternatives to redox‐active metals that are low in abundance and high in cost. Here an aqueous zinc–organic hybrid redox flow battery (RFB) is reported with a positive electrolyte comprising a functionalized 1,4‐hydroquinone bearing four (dimethylamino)methyl groups dissolved in sulfuric acid. By utilizing a three‐electrolyte, two‐membrane configuration this acidic positive electrolyte is effectively paired with an alkaline negative electrolyte comprising a Zn/[Zn(OH)4]2− redox couple and a hybrid RFB is operated at a high operating voltage of 2.0 V. It is shown that the electrochemical reversibility and kinetics of the organic redox species can be enhanced by an electrocatalyst, leading to a cyclic voltammetry peak separation as low as 35 mV and enabling an enhanced rate capability.