Specific ion effects on the permselectivity of sulfonated poly(ether sulfone) cation exchange membranes

Effect of Divalent Cations on RED Performance and Cation Exchange Membrane Selection to Enhance Power Densities

Reverse electrodialysis (RED) is a membrane-based renewable energy technology that can harvest energy from salinity gradients. The anticipated feed streams are natural river and seawater, both of which contain not only monovalent ions but also divalent ions. However, RED using feed streams containing divalent ions experiences lower power densities because of both uphill transport and increased membrane resistance. In this study, we investigate the effects of divalent cations (Mg²⁺ and Ca²⁺) on RED and demonstrate the mitigation of those effects using both novel and existing commercial cation exchange membranes (CEMs). Monovalent-selective Neosepta CMS is known to block divalent cations transport and can therefore mitigate reductions in stack voltage. The new multivalent-permeable Fuji T1 is able to transport divalent cations without a major increase in resistance. Both strategies significantly improve power densities compared to standard-grade CEMs when performing RED using streams containing divalent cations.

When Salt-Rejecting Polymers Meet Protons: An Electrochemical Impedance Spectroscopy Investigation

Polymeric membranes are widely used for salt removal, but mechanism of ion permeation is still insufficiently understood. Here we analyze ion transport in polymers relevant to desalination, dense aromatic polyamide Nomex and cellulose acetate (CA), using impedance spectroscopy, focusing on the effects of the salt type, concentration and pH. The results highlight the role of proton uptake in ion permeation. For Nomex the exceptionally high affinity to proton results in a power-low scaling of conductivity with salt concentrations with an unusual exponent 1/2. The results for CA suggest dominance of pore transport, with pore charge increasing with decreasing pH, which contradicts previous view of CA as a weakly acidic polymer and points to proton uptake as possible pore-charging mechanism. The observed effects may have far-reaching consequences in desalination, as even at neutral pH they may both enhance and suppress salt permeation and affect pH changes.

Osmotic Ballasts Enhance Faradaic Efficiency in Closed-Loop, Membrane-Based Energy Systems

Aqueous processes for energy storage and conversion based on reverse electrodialysis (RED) require a significant concentration difference across ion exchange membranes, creating both an electrochemical potential and an osmotic pressure difference. In closed-loop RED, which we recently demonstrated as a new means of energy storage, the transport of water by osmosis has a very significant negative impact on the faradaic efficiency of the system. In this work, we use neutral, nonpermeating solutes as "osmotic ballasts" in a closed-loop concentration battery based on RED. We present experimental results comparing two proof-of-concept ballast molecules, and show that the ballasts reduce, eliminate, or reverse the net transport of water through the membranes when cycling the battery. By mitigating osmosis, faradaic and round-trip energy efficiency are more than doubled, from 18% to 50%, and 7% to 15%, respectively in this nonoptimized system. However, the presence of the ballasts has a slightly negative impact on the open circuit voltage. Our results suggest that balancing osmotic pressure using noncharged solutes is a promising approach for significantly reducing faradaic energy losses in closed-loop RED systems.

Effect of cationic groups in poly(arylene ether sulfone) membranes on reverse electrodialysis performance

Swelling of poly(arylene ether sulfone) anion exchange membranes increases the permselectivity in reverse electrodialysis performance while maintaining the high ion conductivity.