Effect of Membrane Properties on Ion Crossover in Vanadium Redox Flow Batteries

Ion crossover through the membrane is a critical issue associated with the performance and reliability of vanadium redox flow batteries (VRFBs). In this work, we develop a numerical model to investigate the effects of thickness, fixed charge concentration and electro-kinetic permeability of the cation-exchange membrane (CEM) on the ion crossover from diffusion, migration and electro-osmotic convection (EOC) in VRFBs. The cycling performances of VRFBs in different conditions are compared. The results show that thicker membranes reduce the diffusion flux but have little influence on the migration and EOC fluxes. The net vanadium ion crossover from the negative side to the positive side increases with the state-of-charge (SOC), and the thicker membranes lower the asymmetric degree of the vanadium ion crossover between charge and discharge. Both higher fixed charge concentration and enhanced electro-kinetic membrane permeability increase the migration and EOC effects in membranes, and thus increase the crossover of V2+ and V3+; these effects decrease the crossover of VO2+ and VO2+ at discharge, and vice versa for charge. The coulombic efficiency decreases with the increase of the electro-kinetic permeability, and the reduction of coulombic efficiency is more obvious when the membrane fixed charge concentration is higher. (C) 2019 The Electrochemical Society.