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.