Organic Active Species for Nonaqueous Redox Flow Batteries

Despite decades of research on redox flow batteries (RFBs), their widespread implementation for grid electricity storage has not manifested. In the U.S., the grid penetration of intermittent renewable power is currently too low to necessitate widespread storage, and alternatives less expensive than RFBs are available to utilities pursuing efficiency improvements. RFB development should focus both on improved performance and cost reduction. Most RFBs depend on high concentrations of expensive, environmentally impactful transition-metal-based active species to achieve viable energy densities [1]. One notable exception is the aqueous anthraquinone chemistry, based on an organic active species [2].

Polycyclic aromatic hydrocarbons (pAHs) have highly conjugated molecular structures, which offer redox activity that could be promising for all-organic RFBs. Byproducts of coal extraction and petroleum refining, pAHs are abundant and offer an inexpensive alternative to transition-metal complexes. Since they tend to offer widely separated, stable redox states, pAHs necessitate the wide electrochemical stability windows provided by polar, aprotic, nonaqueous solvents. Researchers previously attempted using the pAH rubrene in nonaqueous RFBs, but were discouraged by its extremely low solubility in conventional nonaqueous solvents.[3] Others have functionalized pAHs to form water-soluble quinone structures at the expense of large RFB operating potentials.[2]

We will focus on 9,10-diphenylanthracene(DPA)[4], a promising pAH for symmetric, nonaqueous RFBs. Figure 1 demonstrates the redox activity of DPA in acetonitrile, exhibiting couples at -2.2 V and 0.9 V vs. Ag/Ag+. A symmetric RFB based on DPA could, in principle, achieve a cell potential above 3V. As with rubrene, however, DPA suffers from solubility concerns: concentrations necessary for persuasive cell demonstration are unattainable. We modified DPA by appending various organic functional groups, examining the effects on solubility and redox activity. We functionalized the phenyl groups of DPA with both electron-withdrawing and electron-donating groups: methyl, methoxy, methoxycarbonyl, and more complicated ethylene-glycol based functionalities were investigated. Various nonaqueous solvents and supporting electrolytes are considered to optimize solubility.

[1] S. Eckroad. EPRI Report 1014836. 2007.

[2] Q. Liu et al. Electrochem. Comm. 12(2010): 1634.

[3] M. Chakrabarti, et al. Electrochimica Acta. 52(2007): 2189.

[4] K. Santhanam, A. Bard. JACS. 88(1966): 2669.

Figure 1: Cyclic voltammogram of 0.001 M DPA/0.05 M TEABF4 in acetonitrile, glassy carbon WE (1mm diameter), Pt CE, 500 mVs^-1, Argon atmosphere with <1ppm O₂,H₂O