This research aims to analyse the non-ohmic resistance of multiple ion-exchange membranes (IEM) in series, for a better understanding of membrane resistances. These membranes can be found in series in energy storage systems such as acid-base flow batteries (ABFB).
The effect of different current densities on the ohmic and non-ohmic resistance of bipolar membranes (BPM), under forward and reverse bias were studied using electrochemical impedance spectroscopy (EIS). Hereafter, an anion exchange membrane (AEM), cation exchange membrane (CEM) and BPM were examined individually and in series using EIS, to compose a simplified equivalent circuit for a whole ABFB triplet.
For the effect of different current densities on the BPM, an exponential decline of the non-ohmic resistance of the diffusion boundary layer (DBL) was found as a function of the current density. Under forward bias, the DBL resistance becomes significant. The resistance of the water dissociation reaction (WDR) was minimal under all current densities. Flow experiments validate a depletion and enrichment of the DBL during water association and dissociation, respectively.
To compose a simplified equivalent circuit for an AEM, BPM and CEM in series, it was found that the specific resistances of all membranes can be summed at their respective frequencies. This made it possible to construct a simplified equivalent circuit for a whole triplet containing an AEM, BPM and CEM.