Ion Selective Membranes

Abstract

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 parameters 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 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 (two-surface-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 separation. The application of ISMs for sensing and biosensing applications was systematically reviewed by Soozanipour et al. (2000765) from a fundamental perspective to sensing applications and commercial implementation. In a perspective, Claus Hélix-Nielsen (2001177), discussed how biological mechanisms have inspired the development of biomimetic ISMs with a particular standpoint on the ion transport of planar aperture-type carbon-based membranes and their biological counterparts. Covalent/Metal−Organic Frameworks (C/MOFs) has recently been utilized as the building blocks of Li-ISMs. The C/MOF based ISNMs possesses Ångstrom-sized pores and various functional sites. In a progress report, Li et al. (2000790) introduced the rational strategies for the fabrication of MOF-Based ISNMs and summarized their applications in ion separation and energy conversion. In an original research work, Zakertabrizi et al. (2001049) used the reactive molecular dynamics (MD) simulation to examine the proton flow through hydrated graphene channels with different interlayer distance values. They found out that the combination of a spatial hindrance with the proton-selective Grotthuss mechanism for an interlayer distance smaller than 8 Å could make a membrane proton-exclusive. Ahmadi et al. (2000665) revealed that tannic acid possesses lithiophilic elements that can be used as a natural ion trapper if being incorporated inside graphene oxide nanochannel acts. Their work opens up an avenue of research for designing a new class of inexpensive Li-ISMs, based on the addition of naturally available lithiophilic guest molecules into 2D membranes. To recover water from highly concentrated brine, Rommerskirchen et al. (2100202) address the issue of water crossover through IEMs in a flow-electrode capacitive deionization process by a crosslinked coating of the membranes. They reported a 54% reduction in the water crossover with just a single coated IEM. Last but not the least, Cseri et al. (2000955) used electrospun nanofibrous polyimides enhanced with ion exchange properties to remove dye-loaded textile wastewater. It was an absolute pleasure to serve as the guest editor of this special section. We would like to thank all of the invited authors for their high-quality contributions to this special section. We would like to express our appreciation for the great support of the Editor-in-Chief of Advanced Materials Technologies, Dr. Esther Levy, for coordinating and handling all papers. The authors declare no conflict of interest. Amir Razmjou received his PhD from the University of New South Wales (UNSW), Australia in 2012 and since then he has held academic/visiting positions at Monash University, University of Isfahan, UNSW and MQ University. He is currently a Chancellor Research Fellow at the University of Technology Sydney (UTS). Dr Razmjou, a co-founder of a tech start-up and has supervised more than 20 postgraduate students. His current research focuses on Direct Extraction of Lithium (DEL), resource recovery and Artificial intelligence for material discovery. Matthias Wessling heads the Chair of Chemical Process Engineering at RWTH Aachen University. He is Vice-Rector for Research and Structure at the RWTH Aachen University and a member of the DWI Scientific Board. He was Vice Scientific Director of the DWI from 2015 to 2018, and his research covers the understanding and application of synthetic membranes for biomedical and sustainable processes. His research envisions a sustainable next-generation process engineering: this will be fossil-independent and will rely on biobased carbon resources as well as on electrochemical membrane reactors. The processes are aqueous-based where ion transport with synthetic membranes will be a key technology. Vicki Chen is the Executive Dean of Engineering, Architecture and Information Technology at the University of Queensland. She graduated with a Bachelor of Science in Chemical Engineering from the Massachusetts Institute of Technology and a PhD in chemical engineering from the University of Minnesota. She has over twenty-five years of research experience in the areas of membrane separation, gas separation, biocatalytic systems, nanomaterials, and water treatment. She was professor of chemical engineering at the University of New South Wales from 2008–2018, the Director of the UNESCO Centre for Membrane Science and Technology from 2006–2014 and head of school of chemical engineering from 2014–2018.

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