Coated Bipolar Membranes with Improved Forward Bias Performance for Energy Harvesting from Salt-Contaminated Acid and Base

Background sodium chloride in hydrochloric acid and sodium hydroxide solutions leads to large overpotentials when a bipolar membrane (BPM) is operated under forward bias (FB). Under FB polarization, the accumulation of salt ions at the junction hinders the transport of H+ and OH– ions, thus increasing the mass transport resistance and lowering the water recombination rate. The “ionic blockade” phenomenon is mainly observed if the base is contaminated with Cl– ions due to the poor OH–/Cl– selectivity of the BPM’s anion exchange layer (AEL). This shortcoming is successfully reduced by modifying the AEL with a sub-micrometer thick poly(benzimidazole) (PBI) coating. Ionic crosslinking between the AEL and PBI leads to a denser interface that enhances the size exclusion of Cl– ions. Furthermore, the negative charges of deprotonated benzimidazole units at the basic operating conditions contribute to the Donnan exclusion of Cl– ions, while the OH– ions can still hop between the alkaline-doped free volumes of the PBI film. The enhanced OH–/Cl– selectivity prevents the accumulation of Cl– ions at the junction and leads to lower overpotentials during the forward bias operation of BPMs in salt-contaminated acid and base. As a result, the PBI-coated BPM has a peak power density 1.6 times higher than that of an uncoated BPM when harvesting electrical energy from a pH gradient. The BPM modification also benefits flow battery applications, as the calculated BPM voltaic efficiency at 100 A/m2 (dis)charge current density is increased from −3.7% to 57% with the addition of the PBI coating.