A Membrane-Electrolyte System Approach to Understanding Ionic Conductivity and Crossover in Aqueous Organic and Metalorganic Flow Batteries

Abstract

The ion exchange membrane is a critical component of most aqueous flow batteries, where it provides a transport medium for charge carrying ions while suppressing undesired crossover of redox active species. The all-vanadium redox flow battery (VRFB), 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 therefore influence the transport phenomena through the membrane pores. In the present work, we build on this understanding by evaluating the effects of iron(II/III) hexacyanide electrolyte concentration, pH, and cations on cation exchange membrane performance, to inform design of aqueous organic and metalorganic flow batteries (AORFBs).The size and charge numbers of established redox active materials for AORFBs afford them lower crossover than metal ions like vanadyl or iron. However, many emerging AORFB chemistries with extreme stability use neutral or alkaline electrolytes where sodium or potassium, not protons, must carry ionic current through the membrane. In these flow batteries, the membrane resistance regularly contributes the most to polarization losses. The widely-used cation exchange membrane Nafion has 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 order to investigate the membrane-electrolyte system, we first 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 of pre-treatment, cation (sodium or potassium), supporting electrolyte, and iron hexacyanide concentration on Nafion conductivity and electrolyte uptake, and compare these results with a non-fluorinated Fumatech membrane (E-620). We also assess pre-treatment, concentration, and cation effects on ferricyanide permeability. Considering the membrane and contacting flow battery electrolyte as one system, our results emphasize that the total ion concentration of the electrolyte affects membrane water content, and there are conditions where additional supporting electrolyte can have a harmful effect on membrane conductivity. We also show that maximizing the concentration of iron hexacyanide by using mixed cation electrolytes results in increased membrane resistance, signifying that conductivity and volumetric capacity become a tradeoff.