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Overview of Ion Exchange Membranes

1.1 Definition and Classifications

Ion exchange membranes (IEMs) are typically made up of a hydrophobic polymer matrix and ionic groups and can be classified into anion exchange membranes (AEMs) and cation exchange membranes (CEMs) according to the type of ionic groups grafted onto the membrane matrix. CEMs that are fixed with negatively charged groups (-SO3-, -COO-, etc.) conduct cations but repel anions, while AEMs containing positively charged groups (-NH3+, -NRH2+, -NR2H+, -NR3+, PR3+, -SR2+, etc.), permit the permeation of anions but retard cations [1, 2]. The typical polymer architectures of IEMs are shown in Figure 1.1a, while the typical groups are shown in Figure 1.1b [3].

According to the connection of ionic groups to the polymer matrix, IEMs can also be categorized as homogeneous and heterogeneous membranes. In homogeneous membranes, the charged groups are chemically bonded to the membrane matrix, and in heterogeneous membranes, they are physically mixed with the membrane matrix [4]. There are many other classification methods, and as a summary, we provide Table 1.1, listing the main categories of IEMs [5].

1.2 Profile of IEMs

Research on IEMs has a long history, dating back to 1890 when Ostwald investigated the properties of semipermeable membranes [6]. It was found that none of the electrolytes could permeate through membranes if these membranes were impermeable to either anions or cations. To explain this phenomenon, the author postulated the "membrane potential" at the boundary between a membrane and its surrounding solution, which was derived from the difference in concentration. The existence of such a boundary was confirmed by Donnan in 1911 [7]. The concentration equilibrium that led to the so-called "Donnan exclusion potential" was described with a mathematical equation. In 1925, Michaelis and Fujita used homogeneous, weak acid collodion membranes to initiate practical basic studies relevant to IEMs [8]. In the 1930s, Sollner demonstrated the idea of a mosaic or amphoteric membrane that had both positively and negatively charged moieties that revealed distinctive ion transport phenomena [9]. The increasing interest in IEMs that were used in industrial processes stimulated the development of synthetic IEMs based on phenol-formaldehyde polycondensation around 1940 [10]. Meyer and Strauss proposed an electrodialysis process that was performed in a configured cell [11]. This cell consisted of two compartments formed by an AEM and a CEM placed between two electrodes. In most practical electrodialysis processes, a stack into which multiple electrodialysis cells were arranged was used.

Figure 1.1 Schematic showing the structure of ion exchange membranes (IEMs). (a) Typical polymer architectures, (b) representative positively charged groups of anion exchange membranes (AEMs), while anionic groups, including sulfonate groups, carboxylate groups, and amidoxime groups, are usually introduced for cation exchange membranes (CEMs).

However, at that time, the industrial implementation of electrodialysis was still impeded by the absence of IEMs with good properties, especially low membrane resistance. This situation improved in the 1950s. Juda et al. from Ionics Incorporated [12] and Winger et al. from Rohm and Haas Company [13] promoted the development of IEMs that had improved performance in terms of chemical stability, selectivity, and electric resistance. Electrodialysis on the basis of these membranes is recommended as an industrial process that demineralizes and concentrates electrolyte solutions. Since then, IEM-based electrodialysis has witnessed wide applications in many fields. Some important examples are listed as follows: (i) In the 1960s, Asahi Co. performed the first salt production from seawater with monovalent ion permselective membranes [14]; (ii) In 1969, electrodialysis reversal (EDR) was invented that could run stably without any precipitation and deposition on both electrodes and membranes [15]; (iii) In the 1970s, DuPont developed a series of CEMs (widely known as Nafion®) made up of sulfonated polytetrafluorethylene [16]. These CEMs are chemically stable and highly conductive, and thus have still been widely used, such as in energy storage (e.g. flow batteries) or conversion systems (e.g. fuel cells); (iv) In 1976, Chlanda et al. presented a new concept of a bipolar membrane that consisted of an AEM layer and a CEM [17], and this concept is still widely studied [18, 19].

Table 1.1 The main categories of ion exchange membranes.

The basis of classification Type Description According to the type of ionic groups Cation exchange membrane Selectively transport cations but repel anions Anion exchange membrane Selectively transport anions but repel cations According to the arrangement of an ionic group within the membrane Amphoteric ion exchange membrane Membranes contain both positively and negatively charged groups and these groups are distributed randomly in membranes Zwitterionic ion exchange membrane Membranes contain an equal number of positively and negatively charged groups. Mosaic ion exchange membrane Membranes are made up of a set of anion and cation exchange elements arranged in parallel Bipolar ion exchange membrane A kind of composition membrane that at least is composed of a cation-selective and an anion-elective layer Monovalent anion perm-selective membranes Membranes can separate monovalent ions from multivalent ions According to the connection way of ionic groups Homogeneous ion exchange membrane Ionic groups are covalently connected to polymer backbone Heterogeneous ion exchange membrane Ionic groups are physically mixed with polymer matrix Semi-homogeneous ion exchange membrane The connections combine above two ways According to the variety of membrane materials Inorganic ion exchange membranes Mainly made up of inorganic materials Organic ion exchange membranes Mainly made up of organic polymer materials Inorganic-organic (hybrid) ion exchange membranes Mixing inorganic materials with organic polymers at the molecular level According to the cross-sectional structure of membrane Symmetric ion exchange membranes Symmetric structure of cross-section Asymmetric ion exchange membranes Asymmetric structure of cross-section Composite ion exchange membranes Covering asymmetric membranes with one or several layers According to pore structure of membranes Dense ion exchange membranes Having almost no free volume within membranes Microporous ion exchange membranes Membranes contain pores with size below 2 nm which consist of charged microporous polymers or mixed materials (polymers blended with microporous fillers, such as COFs, MOFs, and graphene) Porous ion exchange membranes Membranes prepared by phase inversion process contain pores with size above 2 nm and are mainly used for diffusion dialysis or electro-nanofiltration

Nafion® series combining both excellent electrochemical and physical properties is widely used but hampered by high manufacturing costs. Therefore, alternatives to low-cost hydrocarbon-based polymers have received much attention since around 2000, such as poly(phenylene oxide) (PPO), polyimide (PI), poly(ether sulfone) (PES), poly(ether ketone) (PEK), and polystyrene (PS) [20]. During the membrane-formation process, the charged moieties can self-assemble into a hydrophilic phase within the hydrophobic matrix, and thus, these membranes are called microphase-separated IEMs. This microstructure is crucial to IEM performance in terms of ion conductivity, ion selectivity, mechanical stability, etc. [20-25]. Molecular engineering strategies to enhance polymer self-assembly into highly ordered microstructures are required for high-performance IEMs. Over the past two decades of development, optimal self-assembly has been realized mainly via the following strategies [3]: (i) Densely grafting charged groups to increase the local size of the hydrophilic region (i.e. densely charged or block-type IEMs), (ii) enhancing the mobility of charged segments by introducing flexible spacers between charged groups and the backbone (i.e....