A transient model for vanadium redox flow batteries with bipolar membranes

The vanadium redox flow battery (VRFB) with a bipolar membrane promises to offer higher performance, since the cation exchange layer exhibits high conductivity, while the anion exchange layer reduces vanadium crossover. However, due to the wide range of optional structures and layer properties, the optimal design of bipolar membranes calls for a robust mathematical model. In this work, we propose a transient model of VRFB with a bipolar membrane, which improves the Poisson equation to describe the Donnan effect between the two layers to achieve parameter continuity at the interface, thereby providing a full description of charge and ion transport through the membrane. Numerical results show that, compared with cation exchange membranes, the presence of a thin anion exchange layer inhibits vanadium ion convection in the bipolar membrane, thereby significantly reducing the vanadium crossover, leading to alleviated capacity decay and improved coulombic efficiency. At large current densities, the bipolar membranes with thin anion exchange layer show higher energy efficiency than conventional ion exchange membranes, and the capacity decay is better alleviated when anion exchange layer is next to the negative electrode.