An official website of the United States government
Abstract Understanding multi‐component transport behavior through hydrated dense membranes is of interest for numerous applications. For the particular case of photoelectrochemical CO2reduction cells (PEC‐CRC), it is important to understand the multi‐component transport behavior of CO2electrochemical reduction products including mobile carboxylates (formate and acetate) and alcohols (methanol and ethanol) in the ion exchange membranes as one role of the membrane in these devices is to minimize the permeation of these CO2reduction products to the anolyte as they often oxidize back to CO2. Cation exchange membranes (CEM) are promising candidates for such devices as they act to minimize the permeation of mobile anions, such as carboxylates. However, the design of new CEMs is necessary as the permeation of carboxylates often increases in co‐permeation with alcohols. Here, we investigate the transport behavior of carboxylates and alcohols in two types of CEMs (1) a crosslinked CEM was prepared by free‐radical copolymerization of a sulfonated monomer (AMPS) with a crosslinker (PEGDA), and (2) Nafion® 117. We observe an increase in both PEGDA‐AMPS and Nafion® 117 diffusivities to carboxylates in co‐diffusion with alcohols. We attribute this behavior to charge screening by co‐diffusing alcohol that reduces the electrostatic repulsion between bound sulfonates and mobile carboxylates.
more »
« less
Forbidden ion transport through cation exchange membranes
Cation exchange membranes (CEMs) are widely used in many applications. The fixed anionic groups e.g., COO , –SO3 - , etc. in the polymer matrix ideally allows the passage only of oppositely charged cations, driven by a potential or a concentration gradient. Anions, charged negative, the same as the membrane matrix, cannot pass through the membrane due to electrostatic repulsion. Such “Donnan-forbidden” passage can, however, occur to some degree, if the electrical or concentration gradient is high enough to overcome the “Donnan barrier”. Except for salt uptake/transport in concentrated salt solutions, the factors that govern such Forbidden Ion Transport (FIT) have rarely been studied. In most applications of transmembrane ion transport, whether electrically driven as in electrodialysis, or concentration-driven, it is the transport of the counterion to the fixed charged groups, such as that of the proton through a CEM, that is usually of interest. Nevertheless, CEMs are also of interest in analytical chemistry, specifically in suppressed ion chromatography. As used in membrane suppressors, both transport of permitted ions and rejection of forbidden ions are important. If the latter is indeed governed by electrostatic factors, other things being equal, the primary governing factor should be the charge density of the membrane, tantamount to its ion exchange capacity (IEC). In fabricating microscale suppressors, we found useful to synthesize a new ion exchange polymer that can be easily molded to make tubular microconduits. Despite a high IEC of this material, FIT was also found to be surprisingly high. We measured several relevant properties for thirteen commercial and four custom-made membranes to discover that while FIT is indeed linearly related to 1/ IEC for a significant number of these membranes, for very high water-content membranes, FIT may be overwhelmingly governed by the water content of the membrane. In addition, FIT through all CEMs differ greatly among strong acids, they may still be transported as the molecular acids and the extent is in the same order as the expected activity of the molecular acid in the CEM. These results are discussed with the perspective that even for strong acids, the transport does take place as un-ionized molecular acids.
more »
« less
- Award ID(s):
- 2003324
- PAR ID:
- 10555436
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Talanta
- Volume:
- 279
- Issue:
- C
- ISSN:
- 0039-9140
- Page Range / eLocation ID:
- 126581
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Ion-exchange membranes (IEMs) are widely used in water, energy, and environmental applications, but transport models to accurately simulate ion permeation are currently lacking. This study presents a theoretical framework to predict ionic conductivity of IEMs by introducing an analytical model for condensed counterion mobility to the Donnan-Manning model. Modeling of condensed counterion mobility is enabled by the novel utilization of a scaling relationship to describe screening lengths in the densely charged IEM matrices, which overcame the obstacle of traditional electrolyte chemistry theories breaking down at very high ionic strength environments. Ionic conductivities of commercial IEMs were experimentally characterized in different electrolyte solutions containing a range of mono-, di-, and trivalent counterions. Because the current Donnan-Manning model neglects the mobility of condensed counterions, it is inadequate for modeling ion transport and significantly underestimated membrane conductivities (by up to ≈5× difference between observed and modeled values). Using the new model to account for condensed counterion mobilities substantially improved the accuracy of predicting IEM conductivities in monovalent counterions (to as small as within 7% of experimental values), without any adjustable parameters. Further adjusting the power law exponent of the screen length scaling relationship yielded reasonable precision for membrane conductivities in multivalent counterions. Analysis reveals that counterions are significantly more mobile in the condensed phase than in the uncondensed phase because electrostatic interactions accelerate condensed counterions but retard uncondensed counterions. Condensed counterions still have lower mobilities than ions in bulk solutions due to impedance from spatial effects. The transport framework presented here can model ion migration a priori with adequate accuracy. The findings provide insights into the underlying phenomena governing ion transport in IEMs to facilitate the rational development of more selective membranes.more » « less
-
Affordable thin-film composite (TFC) membranes are a potential alternative to more expensive ion exchange membranes in saltwater electrolyzers used for hydrogen gas production. We used a solution-friction transport model to study how the induced potential gradient controls ion transport across the polyamide (PA) active layer and support layers of TFC membranes during electrolysis. The set of parameters was simplified by assigning the same size-related partition and friction coefficients for all salt ions through the membrane active layer. The model was fit to experimental ion transport data from saltwater electrolysis with 600 mM electrolytes at 10 mA cm–2. When the electrolyte concentration and current density were increased, the transport of major charge carriers was successfully predicted by the model. Ion transport calculated using the model only minimally changed when the negative active layer charge density was varied from 0 to 600 mM, indicating active layer charge was not largely responsible for controlling ion crossover during electrolysis. Based on model simulations, a sharp pH gradient was predicted to occur within the supporting layer of the membrane. These results can help guide membrane design and operation conditions in water electrolyzers using TFC membranes.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 eventualin situspectral 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.more » « less
-
Abstract 2D layered nanomaterials have attracted considerable attention for their potential for highly efficient separations, among other applications. Here, a 2D lamellar membrane synthesized using hexagonal boron nitride nanoflakes (h‐BNF) for highly efficient ion separation is reported. The ion‐rejection performance and the water permeance of the membrane as a function of the ionic radius, ion valance, and solution pH are investigated. The nonfunctionalized h‐BNF membranes show excellent ion rejection for small sized salt ions as well as for anionic dyes (>97%) while maintaining a high water permeability, ≈1.0 × 10−3L m m−2h−1bar−1). Experiments show that the ion‐rejection performance of the membrane can be tuned by changing the solution pH. The results also suggest that the rejection is influenced by the ionic size and the electrostatic repulsion between fixed negative charges on the BN surface and the mobile ions, and is consistent with the Donnan equilibrium model. These simple‐to‐fabricate h‐BNF membranes show a unique combination of excellent ion selectivity and high permeability compared to other 2D membranes.more » « less
