Ion Selective Membranes

Abstract

DOI: 10.1002/admt.202100930 filters, revealed that a boost in ion selectivity can be achieved by engineering the ionic topology into an ionic nanochannel by adding an asymmetric element into both “chemistry” and “morphology” of the membranes.[1,2] This can be achieved by building an asymmetrical factor in membrane building blocks or during assembly. There is a need for research on how to assemble at scale ion-selective nanochannels into defect-free membrane-like morphologies with high packing density and long-term stability. Achieving this technological development will allow conversion to viable materials manufacturing and novel ion sensing systems or extraction processes. Our current understanding of ion transport based on electric double-layer overlapping, the dehydration of ions, ionic affinity difference, one-surface-charge-governed ion transport and higher mobility of target ions within nanochannels and membranes are not sufficient to explain new findings. Recent reports[3] identified that other contributory factors must be considered during ISM design such as Zig-Zag transport (twosurface-charge-governed transport because of spontaneous symmetry breaking of charge), different ionic velocity gradient (acceleration and deceleration behaviour of ions as a function of nanochannel dimensions, functional groups and asymmetry in morphology and chemistry), the effect of ice-like arrangements of water molecules on ion selectivity within the asymmetric nanoconfined areas, hydrated ion trapping phenomena, internal concentration polarization and accumulation of ions, orbital involvements of atoms of the nanoconfined areas, and gradual dehydration of ions within the asymmetric nanoconfined areas. The special section of ion-selective membranes covers both fundamental and practical topics that reflect the growing importance of the field over the years. To begin, Amiri et al. (2001308) reviewed recent reports on the design and development of ISMs to control proton transport within Vanadium Redox Flow Batteries (VRFB). A variety of modification strategies were reviewed and an attempt was made to introduce a design platform for future work. Jovanović et al. (2001136) reviewed recent advances in the performance of separators in Li–S batteries and proposed guidelines for measurements with respect to key properties. Ion-exchange membranes (IEMs) are categorized as one of the traditional types of ISMs. Shehzad et al. (2001171) reviewed systematically four types IEMs: self-assembled nanochannels, solid-state nanostructures, artificial surface structures, and fillers-integrated nanostructures. Although mixed matrix membranes have been extensively used for gas separation and water purification, their application for ion separation is yet to be fully explored. The new family of 2D materials called MXenes have attracted significant attention within the membrane community. In a comprehensive review, Mozafari et al. (2001189) reviewed the current status and prospects of ion-selective MXene-Based Membranes. Zhikao et al. (2000862) reviewed the potential of 2D material-based thin-film nanocomposite membranes for ion A. Razmjou Centre for Technology in Water and Wastewater University of Technology Sydney Sydney, NSW, Australia A. Razmjou, V. Chen UNESCO Centre for Membrane Science and Technology School of Chemical Engineering University of New South Wales Sydney, NSW 2052, Australia E-mail: amirr@unsw.edu.au M. Wessling RWTH Aachen University Chemical Process Engineering Forckenbeckstrasse 51, 52074 Aachen, Germany M. Wessling DWI–Leibniz Institute for Interactive Materials Forckenbeckstrasse 50, 52074 Aachen, Germany V. Chen School of Chemical Engineering University of Queensland Queensland 4072, Australia The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admt.202100930. Ion-selective membranes (ISMs) have recently gained significant attention as their functions have become more vital in many environmental and biomedical applications. ISMs play a critical role in the clean energy future and the battery industry. Rapid growth in renewable energy demand resulted in a significant increase in the price of energy-critical elements such as Lithium and rare earth elements. ISMs can be used to directly extract the elements from readily available resources such as seawater and underground brines without compromising the environment. They can also be used for numerous biomedical applications such as nanobiosensors and point-of-care testing. Although ISMs are highly demanded, their commercial implementation has encountered several limitations such as the trade-off between selectivity and permeability, long-term stability, and low throughput for example. This is mainly due to the lack of ability to observe and manipulate ion movement at the atomic scale, limited understanding of ion transport mechanisms, and insufficient knowledge about transport-controlling effects and contributing factors. To make a high-performance ion-selective membrane, its inner ionic topology (ionic domain size, domain spacing, and domain properties), as well as its surface chemistry (functional groups and surface charge) must be carefully designed with regards to environmental conditions (ionic strength and pH) and ion transport driving force (applied pressure or potential). However, tailoring the key internal and external design para meters can only enhance the ion selectivity to a certain level which usually does not meet industry requirements. Recent findings in the literature, inspired by biological ion