Approach to chemical equilibrium of water dissociation for modeling bipolar membranes in acid-base flow batteries

Abstract

Understanding the behavior of bipolar membranes under forward and reverse bias in multi-ion solutions is essential for their application in acid-base flow batteries (ABFB). To this end, a two-dimensional steady-state model of the bipolar membrane is proposed based on the Nernst-Plank and Poisson equations coupled with water dissociation and acid-base neutralization reactions. The second Wien effect and catalytic activity are included in the water dissociation rate model at the bipolar junction. Experimental current-voltage curves were obtained using linear sweep voltammetry to validate the model. A commercial bipolar membrane was used in NaCl solutions at 0.25 and 0.5 mol L−1 concentrations, a mixture of NaCl-HCl and NaCl-NaOH solutions at different concentrations, and solutions of HCl and NaOH at 0.5 mol L−1 concentration. The catalytic activity coefficient was the only model parameter fitted to the experimental data; all other properties and parameters were obtained from the literature or calculated from the membrane supplier data. The numerical solution of the bipolar membrane model complements the experimental results. It shows a full view of conductivity, ion concentration, and potentials across the bipolar membrane as a function of its polarization.