Ion Transport Mechanisms in Bipolar Membranes for Electrochemical Applications

Electrochemistry can be a useful technology to enable the transition toward renewable energy sources and to prevent further climate change. Some electrochemical applications are already industrially implemented, like water electrolysers, while others, like CO2 reduction, need further development to be industrially competitive. Here, a bipolar membrane can provide the conditions for a step forward. It will not only separate (gaseous) products from the two electrodes in an electrochemical cell, but it will also allow the use of different pH and electrolyte at either side. This enables optimization of electrode compartments. As it is composed of a cation and anion exchange layer, no ion transport can theoretically occur across the entire membrane. To still provide the required charge transport, the BPM can dissociate water at the interface layer of the BPM, where often a catalyst is deposited to enhance this reaction. An ideal bipolar membrane has highly conductive membrane layers, fast kinetics at the interface layer and therefore high water permeability to the interface, a long lifetime, and a low ion crossover. As the BPM can be implemented in various electrochemical energy applications, likewater and CO2 electrolysis, batteries, fuel cells and resource recovery, different specific requirements exist per application. For batteries and resource recovery a low ion crossover is crucial, while for fuel cells, water and CO2 electrolysis fast kinetics are essential. Hence, for each application a specifically developed BPM is required for future applications. The development should be based on knowledge gained by studying the performance of BPMs in these conditions with techniques like electrochemical impedance spectroscopy.