Describing the permeability of permanganate co-ions in a CEM with varying supporting electrolyte concentrations

Redox flow batteries (RFBs), a promising technology for large-scale power grid energy storage, utilize a membrane separator to facilitate an ionic current while preventing mixing of the anolyte and catholyte solutions. The anolyte and catholyte of an RFB each contain an electroactive species, and usually an additional supporting electrolyte to charge balance the anode and cathode redox reactions and reduce overall ohmic losses. This study investigates the impact of the supporting electrolyte concentration and composition on Nafion's transport properties using permanganate as the permeant. The permanganate concentration was kept constant at 0.1 m, while sodium chloride and sodium hydroxide were both studied as supporting electrolytes. The sodium chloride concentration was varied from 0.1 m to 4.0 m, while the sodium hydroxide concentration was varied from 0.1 m to 10 m. The permanganate permeability, water sorption coefficient, and salt sorption coefficients were measured for Nafion, with a peak permeability being observed at a supporting electrolyte concentration of 1 m for both electrolytes. A framework to describe the observed behavior was derived using first principles to determine the transport coefficients under these conditions. It was found that at low concentrations, the permanganate permeability is controlled by Donnan exclusion and salt sorption, while at high concentrations it is controlled by salt diffusion and osmotic deswelling.