Modelling water dissociation, acid-base neutralization and ion transport in bipolar membranes for acid-base flow batteries

Research on flow batteries based on water dissociation and acid-base neutralization reactions at bipolar membranes is driven by the possibility of a low-cost and environmentally friendly technology. However, their application in energy storage requires a high round-trip efficiency, which has yet to be realized. In order to establish which critical factors determine their efficiency, this work examines the distribution of potential and concentration in a laboratory scale acid-base flow battery by using fundamental models. Transport mechanisms of diffusion, convection and migration were incorporated into the Nernst-Planck equation. Water dissociation during the charging step was modeled by the second Wien effect combined with the catalytic effect produced by functional groups or by catalysts present in the bipolar junction and compared to the water dissociation equilibrium model. The discharge was modeled by neutralization reaction kinetics and was also compared to the equilibrium model. All model parameters were firmly established and were determined or estimated from information available from the membrane supplier or the literature. The current-potential behavior predicted by the model for both charge and discharge closely matches experimental data and provides a lead for future work on full-scale modeling of acid-base flow batteries.