Ionic Resistance and Permselectivity Tradeoffs in Anion Exchange Membranes

Abstract

Salinity gradient energy technologies, such as reverse electrodialysis (RED) and capacitive mixing based on Donnan potential (Capmix CDP), could help address the global need for noncarbon-based energy. Anion exchange membranes (AEMs) are a key component in these systems, and improved AEMs are needed in order to optimize and extend salinity gradient energy technologies. We measured ionic resistance and permselectivity properties of quaternary ammonium-functionalized AEMs based on poly(sulfone) and poly(phenylene oxide) polymer backbones and developed structure−property relationships between the transport properties and the water content and fixed charge concentration of the membranes. Ion transport and ion exclusion properties depend on the volume fraction of water in the polymer membrane, and the chemical nature of the polymer itself can influence fine-tuning of the transport properties to obtain membranes with other useful properties, such as chemical and dimensional stability. The ionic resistance of the AEMs considered in this study decreased by more than 3 orders of magnitude (i.e., from 3900 to 1.6 Ω m) and the permselectivity decreased by 6% (i.e., from 0.91 to 0.85) as the volume fraction of water in the polymer was varied by a factor of 3.8 (i.e., from 0.1 to 0.38). Water content was used to rationalize a tradeoff relationship between the permselectivity and ionic resistance of these AEMs whereby polymers with higher water content tend to have lower ionic resistance and lower permselectivity. The correlation of ion transport properties with water volume fraction and fixed charge concentration is discussed with emphasis on the importance of considering water volume fraction when interpreting ion transport data.

Theoretical evaluation of electrochemical cell architectures using cation intercalation electrodes for desalination

“…In our treatment of IEMs we also neglect water transport through them, as well as their resistance. In reality it is known that IEM permselectivity and resistance are coupled and depend on thickness, swelling, 59 and solution concentration. 60,61 Though incorporating detailed IEM properties is clearly important to obtain agreement between theoretical predications and experimental results when using particular IEMs, we consider ideally permselective IEMs to focus on the "best case" scenario of IEM performance among all cell architectures that we consider.…”

Influence of solution concentration and salt types on the performance of reverse electrodialysis cells

“…It has been demonstrated that high solution concentrations suppress the Donnan exclusion capacity of the ion exchange membranes, severely reducing the permselectivity and diminishing energy conversion efficiency in a RED stack [25,35]. An increase in the concentration of charged groups in the membranes may improve the ion exchange capacity, but other factors (e.g, electrical resistance, swelling degree, permselectivity, and mechanical strength) should also be taken into account [13,36,37].…”

Fouling resistant nanocomposite cation exchange membrane with enhanced power generation for reverse electrodialysis

“…However, membrane ionic resistance is a parameter that is highly related to membrane thickness, i.e., reduction of the IEM thickness results in the decrease of ionic (area) resistance [32,65]. Intrinsic ionic resistance was introduced in order to exclude the membrane thickness effect and directly compare the intrinsic membrane properties with different thicknesses.…”

“…Intrinsic ionic resistance was introduced in order to exclude the membrane thickness effect and directly compare the intrinsic membrane properties with different thicknesses. Intrinsic ionic resistance is a membrane intrinsic property that is obtained from dividing the ionic resistance by the membrane thickness [65]. The change of membrane intrinsic resistance with O-MWCNT loading is shown in Fig.…”