Performance of five commercial bipolar membranes under forward and reverse bias conditions for acid-base flow battery applications

The list of bipolar membrane (BPM) applications is constantly growing, thanks to the ability of BPMs to enhance water dissociation at low voltage losses and maintain a stable pH gradient across the membrane. Despite several BPMs are available in the market, a comprehensive characterization of membrane properties is still lacking, as BPMs are typically tested only via a steady-state polarization sweep under reverse bias. This work aims to present a systematic characterization of five commercial bipolar membranes using different techniques, investigating BPMs in terms of material and electrochemical properties, as well as overall performance in an acid-base flow battery system (i.e., both under reverse and forward bias). In particular, when testing BPMs in a single-cell acid-base flow battery, we reached round-trip efficiencies up to 65% using current densities of 60 A/m² during discharge and 240 A/m² during charge. Moreover, this work shows how further research is needed to lower the proton leakage through the anion exchange layer and electrical resistance, to improve the performance of BPMs for acid-base flow battery and other energy applications.