Size and Charge Effects on Organic Flow Battery Crossover Evaluated By Quinone Permeabilities through Nafion
Authors: George Thomas Young , Kerr Emily F. , Haya Naphtal O. , Gordon Roy G. , Aziz Michael J. 
Published: July 2022

Organic and metalorganic reactants have become promising for long-lifetime flow batteries. Synthetic chemistry unlocks a wide design space to tailor reactant redox potential, solubility, chemical and electrochemical stability, redox kinetics, and transport properties. Minimizing the crossover of reactants through the membrane or separator is one crucial design goal. To that end, this work contributes a systematic evaluation of size- and charge-based effects on small molecule permeability through Nafion. These results inform the design of flow battery electrolytes that improve the transport selectivity of ion exchange membranes.

Some recent flow battery designs have included crossover suppression strategies based on size and charge of reactants. One option is to leverage size-exclusion, for example by tethering redox-active moieties to polymer backbones,1,2 or by oligomerizing redox-active monomers.3-5 A charge-based strategy has been employed to decrease viologen crossover: sulfonate6 or phosphonate7 solubilizing groups were attached to the redox active core and paired with a cation exchange membrane, reducing crossover compared to previous iterations of this chemistry. Crossover rates of some organic-based flow battery molecules have been estimated to be very low, but other considerations must be balanced for designing viable battery technology. For example, electrolyte cost and solubility may be in direct tension with a crossover suppression strategy based on increasing redox mediator size.8 Untangling the effects of different membrane-molecule selectivity mechanisms is a valuable step on the path to advancing redox active molecule design.

This work evaluates a set of quinones in which size is varied by the number of aromatic rings (e.g. hydroquinone, anthraquinone) and charge number is varied almost independently through sulfonation. Each sulfonate moiety contributes a -1 charge, increasing the magnitude of the molecule charge number with the same sign as the fixed charges in Nafion. Effective size of solvated species is accessed through rotating disk electrode voltammetry: Stokes radii are calculated from measured diffusion coefficients. We found over an order of magnitude permeability reduction per sulfonate, emphasizing the importance of charge-based exclusion for ion exchange membranes. In comparison, size-exclusion effects are less impactful. For example, the Stokes radius of anthraquinone 2,6-disulfonate (AQDS) is twice that of hydroquinone 2,5-disulfonate but their permeabilities fall within the same order of magnitude.

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