Reducing Chloride Ion Permeation during Seawater Electrolysis Using Double-Polyamide Thin-Film Composite Membranes

Abstract

Low-cost polyamide thin-film composite membranes are being explored as alternatives to expensive cation exchange membranes for seawater electrolysis. However, transport of chloride from seawater to the anode chamber must be reduced to minimize the production of chlorine gas. A double-polyamide composite structure was created that reduced the level of chloride transport. Adding five polyamide layers on the back of a conventional polyamide composite membrane reduced the chloride ion transport by 53% and did not increase the applied voltage. Decreased chloride permeation was attributed to enhanced electrostatic and steric repulsion created by the new polyamide layers. Charge was balanced through increased sodium ion transport (52%) from the anolyte to the catholyte rather than through a change in the transport of protons and hydroxides. As a result, the Nernstian loss arising from the pH difference between the anolyte and catholyte remained relatively constant during electrolysis despite membrane modifications. This lack of a change in pH showed that transport of protons and hydroxides during electrolysis was independent of salt ion transport. Therefore, only sodium ion transport could compensate for the reduction of chloride flux to maintain the set current. Overall, these results prove the feasibility of using a double-polyamide structure to control chloride permeation during seawater electrolysis without sacrificing energy consumption.

Modeling Ion Transport across Thin-Film Composite Membranes During Saltwater Electrolysis

“…These simulation results corroborate with previous experimental ion transport results in a zero-gap electrolyzer which demonstrated that membrane charge could be used to tune the ratio of salt ions transported but could not be used to increase the total fraction of charge carried by water ions . Recent studies have debated the impact of polyamide membrane charge on water purification systems using typical reverse osmosis and nanofiltration membranes, suggesting the RBS and EIS studies might not be capturing the effective membrane charge that exists during operation in high salinity and pH solutions .…”

“…Additional membrane and electrolyzer modifications are being studied to decrease Cl – permeation to the anode. For example, adding polyamide active layers to both sides of the TFC membrane successfully reduced Cl – permeation from the catholyte to the anolyte by 53% during electrolysis with a zero-gap electrolyzer . Additionally, the zero-gap electrolyzer configuration enables lower salt ion permeation in comparison with the batch electrolyzer used here with a membrane-electrode distance of 3.7 cm.…”

“…The same amount of total salt ion transport after 2 h in the batch electrolyzer occurs after 6 h in the zero-gap electrolyzer . The smaller electrode-membrane distance in the zero-gap configuration enables a higher percentage of charge to be carried by water ions generated at the electrodes, while water ions are forced to first migrate across the electrolyte chambers in the batch configuration …”

“…Initial tests showed that using different TFC membranes impacted electrolyzer performance, but some TFC membranes could produce current densities with applied potentials comparable to those needed with more expensive cation exchange membranes (CEMs) . Modifications to the TFC membrane have been proposed to decrease the overall resistance of the membrane while reducing chloride permeation, but these studies demonstrating the mechanisms governing the TFC membrane-based electrolyzer performance are not straightforward. − The multiple composite layers of the membrane have different transport properties, and the size-exclusive nature of the active layer causes the resistance of the membrane to change depending on the abundance of varying ions in solution.…”