Person:

Wu, Min

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.

We demonstrate a carbon capture system based on pH swing cycles driven through proton-coupled electron transfer of sodium (3,3’-(phenazine-2,3-diylbis(oxy))bis(propane-1-sulfonate)) (DSPZ) molecules. Electrochemical reduction of DSPZ causes an increase of hydroxide concentration, which absorbs CO2; subsequent electrochemical oxidation of the reduced DSPZ consumes the hydroxide, causing CO2 outgassing. The measured electrical work of separating CO2 from a binary mixture with N2, at CO2 inlet partial pressures ranging from 0.1 to 0.5 bar, and releasing to a pure CO2 exit stream at 1.0 bar, was measured for electrical current densities of 20 to 150 mA/ cm2. The work for separating CO2 from a 0.1 bar inlet and concentrating into 1 bar exit is 61.3 kJ/molCO2 at a current density of 20 mA/cm2 and extrapolates to 57.1 kJ/molCO2 in the low-current-density limit. At this limit, the cycle work for capture from 0.4 mbar extrapolates to 108-212 kJ/ molCO2 depending on the initial composition of the electrolyte. We also introduce an electrochemical rebalancing method that extends cell lifetime by recovering the initial electrolyte composition after it is perturbed by side reactions. We discuss the implications of these results for future low-energy electrochemical carbon capture devices.

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.