Understanding the Membrane-Electrolyte System for Designing Aqueous Organic and Metalorganic Flow Batteries

Redox flow batteries with aqueous electrolytes hold promise for large scale energy storage, offering independent scaling of power from the electrochemical stack and energy stored in tanks of dissolved reactants. The all-vanadium redox flow battery (VRFB) has been the most developed chemistry to date, although more recently organic and metalorganic redox reactants have been proven viable for batteries with long calendar life. Ion exchange membranes for flow batteries must suppress crossover of redox reactants while enabling conductivity of a charge-carrying ion for efficient battery cycling. Studies on VRFBs have shown that the concentration and composition of the solutions contacting the membrane have crucial influence on transport phenomena through the membrane pores. Building on this understanding, the research presented here contributes an evaluation of the effects of iron(II/III) hexacyanide electrolyte concentration and pH on cation exchange membrane performance, which will inform the design of aqueous organic and metalorganic redox flow batteries (AORFB).<br/><br/>In AORFBs, the size and charge numbers of organic molecules and metalorganic complexes afford them more crossover resistance than metal ions like iron or vanadyl, but many of the most stable chemistries use neutral or alkaline electrolytes where sodium or potassium ions carry charge through the membrane. The widely-used cation exchange membrane Nafion has a factor-of-ten reduction in conductivity in potassium form compared to the proton form available in acidic environments. Therefore in AORFBs, ohmic resistance of the membrane is a substantial limiting factor on practical current densities and power densities for battery operation.<br/><br/>The composition and concentration of the electrolyte contacting the membrane influences the water content of the membrane, which determines the pathways available for transport, and hence the conductivity. Here 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 of both iron hexacyanide concentration and supporting electrolyte (sodium or potassium hydroxide) on water content and conductivity of both Nafion cation exchange membranes and a non-fluorinated, hydrocarbon-based cation exchange membrane from Fumatech (E-620K). These results emphasize that total ion concentration of the electrolyte affects membrane water content, and therefore supporting electrolyte should be minimized when concentrations of redox active species are high enough to provide ionic 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. Considering membrane and electrolyte properties as an interrelated system supports the design of battery electrolytes for compatibility with a given membrane, and may also accelerate the creation and selection of new membranes tailored for flow batteries.