Electromembrane Processes: Basic Aspects and Applications

Electromembrane Processes: Basic Aspects and Applications

Recent developments on ion-exchange membranes and electro-membrane processes

A comprehensive review on the synthesis and applications of ion exchange membranes

Investigations on electrochemical properties of membrane systems in ion-exchange membrane transport processes by electrochemical impedance spectroscopy and direct current measurements

Chapter 6 Ion-Exchange Membrane Processes in Water Treatment

Electromembranes or “charged membranes”, representing ion-exchange membranes (IEMs), have been used in numerous processes, which are rather different in their basic concept, their practical applications, and their technical relevance. The IEM-based technologies, such as common electrodialysis (ED), bipolar membrane electrodialysis (BMED), capacitive deionization (CDI), and continuous electrodeionization (EDI), have further extended the range of applications of electromembrane processes beyond their traditional use in water treatment. The term “electromembrane process” is used to describe an entire family of processes that can be quite different in their basic concept and their application. But they are all based on the coupling of mass transport with an electrical current through ion perm-selective membrane. Electromembranes are used to remove ionic components such as salts from electrolyte solutions or to produce certain compounds such as NaOH and Cl2 from NaCl solutions. This chapter is concentrated mainly on technically relevant electromembranes for common electrodialysis, BMED, CDI, MCDI, and EDI, related to water treatment driven by electricity. To better illustrate the electromembrane process, some investigations on electrode, spacer, and setup used for electrodialysis have been included. In addition, two typical water treatment cases are also taken as examples.

Preparation of ion-exchange materials and membranes

The ion exchange membrane is a key component in many electrochemical membrane processes such as fuel cells, flow batteries and electrolysers. Typically, these ion exchange membranes are assembled in stacks and allow the transport of the charge carrying component, i.e. a cation (e.g. proton) or an anion, while retaining the other species and electrolytes preventing their crossover to the other side of the cell. Simultaneously electrons travel through an external circuit powering a device or to store energy.Conventional ion exchange membranes have two major problems: They are based on expensive materials (e.g. Nafion®e. PFSA; perfluorosulfonic acid) or on environmentally harmful chemicals and chemical reactions. Although due to its molecular structure and composition, PFSA membranes show good performances, the major limitation of PFSA membranes is the very high material costs often contributing for more than 35% to the total stack costs [1, 2].One of the major challenges of ion exchange membrane development is the tradeoff between high ion transport rates through the membrane while simultaneously preventing electrolyte crossover [3]. This talk will first present a comprehensive overview of required membrane characteristics and an extensive benchmark study of state-of-the-art performances of ion exchange membranes in different electro-membrane processes. Following on this, the challenges in ion exchange membrane development will be addressed and most importantly two new routes for the development of next generation ion exchange membranes will be presented and their characteristics will be compared to those of a series of extensively benchmarked commercially available ion exchange membranes.The first approach, electrospinning is an effective, versatile method to produce cheap ion exchange membranes [3-6]: Multiple polymers can be employed simultaneously during spinning and this is combined with high degrees of interchain entanglement. This results in an interconnected network of ionic pathways that promote high ionic conductivities confined in a matrix of an inert polymer that guarantees high rejections towards electrolytes to prevent crossover (Figure 1a). Moreover, it is a simple technique that can be easily adapted to large scale production.The second approach uses liquid crystalline (LC) polymers to make ion exchange membranes [7]. This approach has the potential to offer true molecular selectivity and a high degree of flexibility to actually tune this selectivity. LC polymer materials self-organize into structures with well defined isoporosity (Figure 1b). Subsequent template removal or chemical bond cleavage with an acid or base results in the formation of molecular pores. The pores of these materials can be functionalized and depending on the functionality, selectivity can be introduced. Depending on the bulkiness of the functional group also pore sizes can be smaller or bigger. Crosslinking of the formed structures allows control over the swelling of the material and with that reduces crossover. In this way one can rely on both charge-charge interactions as well on size sieving to separate species. The major challenge is the formation of organized structures over larger length scales and the identification of structure-property relationships and with that control over the membrane separation performance.Design principles of both newly developed membrane types are discussed, the membranes are extensively characterized and their performance in electrochemical processes is compared to that of conventional ion exchange membranes. The talk is concluded with a future outlook on the perspectives of ion exchanhe membrane development. T. Cho, et al., Energy Technol. 1 (2013) 596–608. https://doi.org/10.1002/ente.201300108.Lin, et al., J. Electrochem. Soc. 163 (2016) A5049–A5056. https://doi.org/10.1149/2.0071601jes.A. Hugo, et al., Journal of Membrane Science 566 (2018) 406. 10.1016/j.memsci.2018.09.006.Woo Park, et al., J. Membr. Sci. 541 (2017) 85–92. https://doi.org/10.1016/j.memsci.2017.06.086.Choi, et al., Macromolecules. 41 (2008) 4569–4572. https://doi.org/10.1021/ma800551w.J.B. Ballengee, P.N. Pintauro, Macromolecules. 44 (2011) 7307–7314. https://doi.org/10.1021/ma201684j.Kloos, et al., Journal of Membrane Science 620 (2021) 118849. https://doi.org/10.1016/j.memsci.2020.118849 Figure 1

Fabrication of hydrophilic cation exchange membrane with improved stability for electrodialysis: An excellent anti-scaling performance

The contamination of water sources with toxic metals and semi-metallic elements, including arsenic and selenium, is a matter of great concern worldwide, because of their potential negative impact on the ecosystems. While in very small amounts, many of these metals are necessary to support life, in larger amounts, they can become extremely toxic. Since, usually there are no detectable organoleptic changes in drinking water in the presence of toxic metal ions in trace levels, it is rather possible that some of them may easily remain undetected, thus additionally increasing possible health risks. Since most metal-containing species in water are either positively or negatively charged, use of electro-membrane processing for their removal appears as natural choice. Membrane processes that use ion-exchange membranes and electric potential difference as the driving force for ionic species transport are referred to as electromembrane processes. Various electro-membrane separation processes include electrodialysis, including variations such as electrodialysis reversal, electro-electrodialysis and bipolar membrane electrodialysis, electrodeionization, and Donnan dialysis.

Study of calcium carbonate scales by electrochemical impedance spectroscopy

Ion exchange membranes (IEMs) play a significant role in fields of energy and environment, for instance fuel cells, diffusion dialysis, electrodialysis, etc. The limited choice of commercially available IEMs has produced a strong demand of fabricating IEMs with improved properties via facile synthetic strategies over the past two decades. Poly(phenylene oxide) (PPO) is considered as a promising polymeric material for constructing practical IEMs, due to its advantages of good physicochemical properties, low manufacturing cost and easy post functionalization. In this review, we present the accumulated efforts in synthetic strategies towards diverse types of PPO-based IEMs. Relation between polymer structures and the resulted features is discussed in detail. Besides, applying IEMs from PPO and its derivatives in fuel cell, diffusion dialysis and electrodialysis is summarized and commented.

A comprehensive scientometric approach was adopted to study the research on ion exchange membranes. The statistical analysis was conducted based on 21 123 publications which were related to the topic of ion exchange membranes. Specifically, from 2001 to 2016, over 18 000 articles were published on ion exchange membranes, indicating researchers' great interest in this topic. Especially, compared to 2001, the number of articles published in 2016 increased approximately six-fold. This trend continued in 2017 since over 2000 articles were published in the year of 2017. Also, these articles were spread across over 1000 different journals, near 100 countries/regions and over 5000 research institutes, revealing the importance of ion exchange membrane as well as the broad research interest in this field. Besides, the properties and applications of ion exchange membranes were also discussed statistically. Furthermore, keywords analysis indicated that fuel cell and proton exchange membrane had the highest cooccurrence frequency. Finally, research areas analysis revealed that ion exchange membranes had a close relation with chemistry, energy and materials.

Application of ion exchange membranes to the recovery of acids by diffusion dialysis

Chapter 15 - Polymeric membranes in electrodialysis, electrodialysis reversal, and capacitive deionization technologies