Relative Insignificance of Polyamide Layer Selectivity for Seawater Electrolysis Applications

Abstract

Low-cost polyamide thin-film composite (TFC) membranes are being explored as alternatives to cation exchange membranes for seawater electrolysis. An optimal membrane should have a low electrical resistance to minimize applied potentials needed for water electrolysis and be able to block chloride ions present in a seawater catholyte from reaching the anode. The largest energy loss associated with a TFC membrane was the Nernstian overpotential of 0.74 V (equivalent to 37 Ω cm 2 at 20 mA cm −2 ), derived from the pH difference between the anolyte and catholyte and not the membrane ohmic overpotential. Based on analysis using electrochemical impedance spectroscopy, the pristine TFC membrane contributed only 5.00 Ω cm 2 to the ohmic resistance. Removing the polyester support layer reduced the resistance by 79% to only 1.04 Ω cm 2 , without altering the salt ion transport between the electrolytes. Enlarging the pore size (∼5 times) in the polyamide active layer minimally impacted counterion transport across the membrane during electrolysis, but it increased the total concentration of chloride transported by 60%. Overall, this study suggests that TFC membranes with thinner but mechanically strong supporting layers and size-selective active layers should reduce energy consumption and the potential for chlorine generation for seawater electrolyzers.

Thin-Film Composite Membranes for Hydrogen Evolution with a Saline Catholyte Water Feed

“…However, in the context of the electrotransport process across a membrane, the transport number is defined as the fraction of the current that is transported by the ion in the membrane with no restriction on its transport mechanisms (Figure e) . Therefore, transport number calculation here based on the other ion crossover results includes the contribution of electric current from all three components of ion diffusion, electromigration, and convection …”

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

“…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. 13 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 same amount of total salt ion transport after 2 h in the batch electrolyzer occurs after 6 h in the zero-gap electrolyzer. 12 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. 13 Model Validation by Changing Electrolyte Concentrations.…”

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

“…20,21 Using a membrane with a more selective PA active layer with negative charge was recently shown to moderately reduce the permeation of Cl − ions but have little effect on the transport of counterions (Na + ) during electrolysis. 22 The Nernstian overpotential remained nearly constant and thus did not further decrease the energy demands. Therefore, other methods are needed to further reduce Cl − ion transport in TFC membranes, ideally along with the deceleration of Na + ion permeation.…”

“…Although TFC membranes favor the transport of water ions over salt ions, some cross-membrane charge is still carried by salt ions, increasing the concentrations of H + in the anolyte and OH – in the catholyte. This pH difference can contribute to the voltage increment during electrolysis, referred to as the Nernstian overpotential. , Using a membrane with a more selective PA active layer with negative charge was recently shown to moderately reduce the permeation of Cl – ions but have little effect on the transport of counterions (Na + ) during electrolysis . The Nernstian overpotential remained nearly constant and thus did not further decrease the energy demands.…”