The Influence of Electric Field on Crossover in Redox-Flow Batteries

Transport of active species through the ion-exchange membrane separating the electrodes in a redox-flow battery is an important source of inefficiency. Migration and electro-osmosis have significant impacts on the crossover of reactive anions, cations, and neutral species. In this paper, these phenomena are theoretically and experimentally explored for commercial cation-exchange membranes. The theoretical analysis indicates that plotting the cumulative Coulombic mismatch between charge and discharge as a function of time can be used to assess crossover rates. The relative importance of migration and electro-osmosis over diffusion is quantified and shown to increase with increasing current density and membrane thickness because the contributions of migration and electro-osmosis to ionic flux are independent of membrane thickness and proportional to current density, while diffusion is inversely proportional to membrane thickness and independent of current density.