(337e) A Membrane-Electrolyte System Approach to Understanding Ionic Conductivity and Crossover in Alkaline Flow Batteries

The ion exchange membrane is a critical component of most aqueous redox flow batteries (RFBs), where it provides a transport medium for charge carrying ions while suppressing the undesired crossover of redox active species. The all-vanadium RFB, the most developed flow battery chemistry to date, is also the system for which most studies of flow battery membranes have been done. These studies have shown that the concentration and composition of the acidic vanadium electrolytes influence water content in the membrane phase, and thereby influence the transport phenomena through the membrane pores. However, many emerging RFB chemistries, such as those involving organic molecules with extreme chemical stability, use alkaline or neutral electrolytes in which sodium or potassium, rather than protons, carry ionic current through the membrane. In these flow batteries, the membrane resistance is often the largest contributor to polarization losses. The widely-used Nafion® cation exchange membrane family exhibits a factor-of-ten reduction of conductivity in potassium form compared to when it is protonated in acidic environments. Ohmic resistance of the membrane is therefore a substantial limiting factor on practical current densities and power densities for battery operation.

In this contribution, we report our investigation of the alkaline membrane-electrolyte system.¹ We describe simple methods for conductivity and electrolyte uptake measurements that can be used to screen a variety of membrane materials and electrolytes. We show the influence on Nafion® conductivity and electrolyte uptake of pre-treatment and the concentrations of cation species (sodium or potassium), supporting electrolyte, and iron hexacyanide (a common redox-active RFB material), and we compare these results with those for a non-fluorinated Fumatech membrane (E-620). We also assess pre-treatment, concentration, and cation effects on the permeability of ferricyanide. Considering the membrane and contacting flow battery electrolyte as one system, our results demonstrate that the total ion concentration of the electrolyte affects membrane water content and that there are conditions where additional supporting electrolyte can suppress membrane conductivity. We also show that increasing the concentration of iron hexacyanide by using mixed cation electrolytes results in increased membrane resistance, implying a tradeoff between conductivity and volumetric capacity.

[1] George, T. Y.; Thomas, I. C.; Haya, N. O.; Deneen, J. P.; Wang, C.; Aziz, M. J. Membrane-Electrolyte System Approach to Understanding Ionic Conductivity and Crossover in Alkaline Flow Cells. ACS Applied Materials and Interfaces 2023