Toward Symmetric Organic Aqueous Flow Batteries: Triarylamine-Based Bipolar Molecules and Their Characterization via an Extended Koutecký-Levich Analysis.

Symmetric organic flow batteries (SOFBs) can potentially address membrane crossover problems by employing bipolar redox-active organic molecules (BROMs). Herein, a triarylamine (TAA) skeleton was chosen as a posolyte moiety for a new class of bipolar molecules for pH-neutral aqueous flow batteries (FBs). Pyridinium and viologen derivatives were tethered to the posolyte moiety, and the new compounds were characterized. Cyclic voltammetry revealed that only viologen with a highly hydrophilic substituent, connected to the TAA moiety via a Zincke reaction, could be reversibly reduced. Varying the supporting electrolyte concentration on the selected derivative revealed water solubility as a challenge for further development. The selected derivative, MeO-TPA-Vi-DMAE, was subjected to hydrodynamic voltammetry, and a modified Koutecký-Levich analysis was developed to investigate the observed potential-dependent currents at the hydrodynamically dominated region, which are often seen with redox-active organic molecules. This model discarded a purely Ohmic effect, showing a useful Levich slope at a certain overpotential before the onset of a secondary reaction. TAA-based BROMs hold promise for pH-neutral aqueous SOFBs, and the results will guide the design of new derivatives. The three-term Koutecký-Levich relation here introduced will be useful not only to develop BROM-based FBs but will most likely appeal to a much broader audience.