Application of Polyelectrolyte Multilayer Coatings to Reduce Chloride Ion Crossover in an Asymmetric Seawater Electrolysis Cell

Relatively inexpensive polyamide thin-film composite (TFC) membranes are being explored as separators in asymmetric seawater electrolysis cells for hydrogen gas production. However, the Cl– crossover from the saltwater catholyte to the anode must be minimized to prevent chlorine gas formation. We therefore examined whether coating the TFC active layer with negatively charged polyelectrolyte multilayers could reduce Cl– transport to the anode, using NO3– as a stable surrogate for Cl–. The coating reduced the crossover of NO3– by 19%, unlike in the pressure-driven reverse osmosis process where polyelectrolyte multilayer coatings on such TFC membranes have minimal effect on salt permeation. Using higher salt concentrations for both the inner and the last layers during the polyelectrolyte multilayer fabrication increased the negative charge of the multilayer and reduced the level of NO3– transport. Modeling of the electrolysis cell indicated that OH– was the dominant water ion in the polyelectrolyte multilayer (facing the alkaline catholyte). The concentrations of NO3– and OH– in the polyelectrolyte layer decreased with an increasing charge of the polyelectrolyte layer due to Donnan exclusion, resulting in a reduction of their migration and total fluxes. Overall, our findings demonstrated the feasibility of using polyelectrolyte multilayer coatings to control NO3– and thus Cl– ion transport during water electrolysis using a seawater catholyte.