Understanding the effects of polymer chemistry on membrane ion transport properties is critical for enabling efforts to design advanced highly permselective ion exchange membranes for water purification and energy applications. Here, the effects of fixed charge group type on anion exchange membrane (AEM) apparent permselectivity and ion transport properties were investigated using two crosslinked AEMs. The two AEMs, containing a similar acrylonitrile, styrene and divinyl benzene-based polymer backbone, had either trimethyl ammonium or 1,4-dimethyl imidazolium fixed charge groups. Membrane deswelling, apparent permselectivity and ion transport properties of the two AEMs were characterized using aqueous solutions of lithium chloride, sodium chloride, ammonium chloride, sodium bromide and sodium nitrate. Apparent permselectivity measurements revealed a minor influence of the fixed charge group type on apparent permselectivity. Further analysis of membrane swelling and ion sorption, however, suggests that less hydrophilic fixed charge groups more effectively exclude co-ions compared to more hydrophilic fixed charge groups. Analysis of ion diffusion properties suggest that ion and fixed charge group enthalpy of hydration properties influence ion transport, likely through a counter-ion condensation, ion pairing or binding mechanism. Interactions between fixed charge groups and counter-ions may be stronger if the enthalpy of hydration properties of the ion and fixed charge group are similar, and suppressed counter-ion diffusion was observed in this situation. In general, the hydration properties of the fixed charge group may be important for understanding how fixed charge group chemistry influences ion transport properties in anion exchange membranes.
more »
« less
Unlocking the potential of polymeric desalination membranes by understanding molecular-level interactions and transport mechanisms
Polyamide reverse osmosis (PA-RO) membranes achieve remarkably high water permeability and salt rejection, making them a key technology for addressing water shortages through processes including seawater desalination and wastewater reuse. However, current state-of-the-art membranes suffer from challenges related to inadequate selectivity, fouling, and a poor ability of existing models to predict performance. In this Perspective, we assert that a molecular understanding of the mechanisms that govern selectivity and transport of PA-RO and other polymer membranes is crucial to both guide future membrane development efforts and improve the predictive capability of transport models. We summarize the current understanding of ion, water, and polymer interactions in PA-RO membranes, drawing insights from nanofiltration and ion exchange membranes. Building on this knowledge, we explore how these interactions impact the transport properties of membranes, highlighting assumptions of transport models that warrant further investigation to improve predictive capabilities and elucidate underlying transport mechanisms. We then underscore recent advances in in situ characterization techniques that allow for direct measurements of previously difficult-to-obtain information on hydrated polymer membrane properties, hydrated ion properties, and ion–water–membrane interactions as well as powerful computational and electrochemical methods that facilitate systematic studies of transport phenomena.
more »
« less
- Award ID(s):
- 2136835
- NSF-PAR ID:
- 10436508
- Date Published:
- Journal Name:
- Chemical Science
- Volume:
- 14
- Issue:
- 4
- ISSN:
- 2041-6520
- Page Range / eLocation ID:
- 751 to 770
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
In many applications of hydrated, dense polymer membranes—including fuel cells, desalination, molecular separations, electrolyzers, and solar fuels devices—the membrane is challenged with aqueous streams that contain multiple solutes. The presence of multiple solutes presents a complex process because each solute can have different interactions with the polymer membrane and with other solutes, which collectively determine the transport behavior and separation performance that is observed. It is critical to understand the theoretical framework behind and experimental considerations for understanding how the presence of multiple solutes impacts diffusion, and thereby, the design of membranes. Here, we review models for multicomponent diffusion in the context of the solution-diffusion framework and the associated experiments for characterizing multicomponent transport using diffusion cells. Notably, multicomponent effects are typically not considered when discussing or investigating transport in dense, hydrated polymer membranes, however recent research has shown that these effects can be large and important for understanding the transport behavior.more » « less
-
null (Ed.)The decreasing cost of electricity produced using solar and wind and the need to avoid CO 2 emissions from fossil fuels has heightened interest in hydrogen gas production by water electrolysis. Offshore and coastal hydrogen gas production using seawater and renewable electricity is of particular interest, but it is currently economically infeasible due to the high costs of ion exchange membranes and the need to desalinate seawater in existing electrolyzer designs. A new approach is described here that uses relatively inexpensive commercially available membranes developed for reverse osmosis (RO) to selectively transport favorable ions. In an applied electric field, RO membranes have a substantial capacity for proton and hydroxide transport through the active layer while excluding salt anions and cations. A perchlorate salt was used to provide an inert and contained anolyte, with charge balanced by proton and hydroxide ion flow across the RO membrane. Synthetic seawater (NaCl) was used as the catholyte, where it provided continuous hydrogen gas evolution. The RO membrane resistance was 21.7 ± 3.5 Ω cm 2 in 1 M NaCl and the voltages needed to split water in a model electrolysis cell at current densities of 10–40 mA cm −2 were comparable to those found when using two commonly used, more expensive ion exchange membranes.more » « less
-
more » « less
Abstract Membrane desalination is a leading technology for treating saline waters to augment fresh water supply. The need for high-performance desalination membranes, particularly with high water/salt selectivity, has stimulated research into the fundamental structure-property-performance relationship of state-of-the-art membranes. In this study, we utilize a facile method for tuning properties of a polymeric desalination membrane to shed light on water and salt transport mechanisms of such membranes. A desalination membrane made of cellulose triacetate is treated in a plasticizer solution, followed by water rinsing. The modified membranes exhibit reduced salt flux without compromising water flux, indicating enhanced water/salt selectivity. An inspection of material characteristics using a model film system reveals a plasticizing-extracting process in changing the polymeric structure, which leads to the reduction of crystallite size in the polymer matrix, consequently affecting the transport properties of the membranes. Our findings highlight the potential of the plasticizing-extracting process in fabricating membranes with desired desalination performance.
-
more » « less
Cyclic voltammetry was applied to investigate the permselective properties of electrode-supported ion-exchange polymer films intended for use in future molecular-scale spectroscopic studies of bipolar membranes. The ability of thin ionomer film assemblies to exclude mobile ions charged similarly to the polymer (co-ions) and accumulate ions charged opposite to the polymer (counterions) was scrutinized through use of the diffusible redox probe molecules [Ru(NH3)6]3+and [IrCl6]2−. With the anion exchange membrane (AEM) phase supported on a carbon disk electrode, bipolar junctions formed by addition of a cation exchange membrane (CEM) overlayer demonstrated high selectivity toward redox ion extraction and exclusion. For junctions formed using a Fumion®AEM phase and a Nafion®overlayer, [IrCl6]2−ions exchanged into Fumion®prior to Nafion®overcoating remained entrapped and the Fumion®excluded [Ru(NH3)6]3+ions for durability testing periods of more than 20 h under conditions of interest for eventual
in situ spectral measurements. Experiments with the Sustainion®anion exchange ionomer uncovered evidence for [IrCl6]2−ion coordination to pendant imidazolium groups on the polymer. A cyclic voltammetric method for estimation of the effective diffusion coefficient and equilibrium extraction constant for redox active probe ions within inert, uniform density electrode-supported thin films was applied to examine charge transport mechanisms.
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d2sc04920a
