Effect of membrane thickness on performance of all-vanadium redox flow batteries

Species crossover through the ion exchange membrane leads to a decrease in the cell performance and lifetime for all-vanadium redox flow batteries (VRFBs). As an important design parameter, membrane thickness is one of the major factors for ion transport through the membrane. In this paper, a transient electrochemical model incorporating the Donnan effect for VRFBs is utilized to study the effect of membrane thickness on the ion transport in the membrane and on the cell performance. The simulated results show that with an increase in the membrane thickness, the coulombic efficiency is promoted while the extent gradually slows down. When the thickness is small, the coulombic efficiency deteriorates seriously. The study found that the ionic potential through the membrane gradually reaches a steady state, which process is controlled by the H⁺ concentration difference across the membrane and the membrane thickness. Moreover, with a decrease in the thickness, the vanadium ion fluxes are enhanced and the diffusion terns into the domination; while the migration is always controlled by the concentration overpotential.