2D-dynamic phenomenological modelling of vanadium redox flow batteries - Analysis of the mass transport related overpotentials

Abstract

Flow batteries exhibit relatively low power density owing to ohmic and concentration overpotentials, which leads to higher system costs. In this work, a phenomenological model of a vanadium redox flow battery (VRFB) equipped with an anion exchange membrane (AEM) was developed and validated. The model is used to assess the concentration overpotential during charge-discharge cycling at different operating conditions and a method to determine the mass transfer coefficient is presented. Also, a strategy to reduce the concentration overpotential is proposed. The simulated charge-discharge curve displays the lowest relative error reported in the literature for VRFB equipped with an AEM; the results reveal that the mass transfer coefficient is overestimated in most models in the literature. It is demonstrated that the concentration overpotentials during charging and discharging steps are not equal owing to a mismatch between the state of charge and the state of discharge. Also, the current density has a greater impact on this overpotential than the flow rate. Higher overpotentials were found near the membrane since the electronic conductivity is higher than the ionic conductivity. The simulation results show that positioning the distribution channels close to the membrane allows a reduction of the concentration over potential up to 3.9%.