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

## TL;DR

Researchers developed a new type of flow battery using triarylamine-based molecules and a modified analysis method to study their performance in pH-neutral water.

## Contribution

A modified Koutecký–Levich analysis is introduced to study potential-dependent currents in bipolar organic molecules for flow batteries.

## Key findings

- Only viologen with a highly hydrophilic substituent connected to the TAA moiety can be reversibly reduced.
- A modified Koutecký–Levich analysis revealed useful Levich slopes before secondary reactions begin.
- TAA-based molecules show promise for pH-neutral aqueous flow batteries.

## Abstract

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.

Bipolar redox‐active organic molecules based on a triarylamine skeleton and a viologen unit for pH‐neutral aqueous flow batteries are presented. Cyclic voltammetry shows that only viologen with a highly hydrophilic substituent, connected to the TAA moiety can be reversibly reduced. A modified Koutecký–Levich analysis is presented to investigate the observed potential‐dependent currents at the hydrodynamically dominated region.