Ion-Exchange Materials for Membrane Capacitive Deionization

The scarcity of clean water is a problem affecting large parts of the world. In fact, the World Health Organization/UNICEF Joint Monitoring Programme for Water Supply, Sanitation and Hygiene (2019) estimates that up to 2.2 billion people lack access to safely managed drinking water services. To address this, desalination techniques such as reverse osmosis, flash distillation, and electrodialysis have been utilized to convert the plentiful amounts of salt water into consumable water supplies for the general population. In the past 15 years, membrane capacitive deionization (MCDI) has emerged as an alternative desalination technique that has since received extensive research attention. MCDI has sought to challenge benchmark methods such as reverse osmosis, removing salt by application of a voltage between two electrodes covered with ion-exchange membranes, all under ambient conditions. The incorporation of ion-exchange materials over electrodes in MCDI has been shown to maximize the desalination performance in terms of salt removal and energy efficiency. This review provides a comprehensive assessment of the developments relating to ion-exchange materials in MCDI. The fabrication and characterization methods of the materials have been outlined and compared with those of commercially available ion-exchange membranes where possible. A critical comparison of the ion-exchange materials has been conducted, and the commercial viability of the technologies has been evaluated. In light of the findings of the review, the authors have indicated future directions and action points the field should look to address in the coming years. It is hoped that the findings of this review can contribute to the large-scale commercialization and application of MCDI, which can improve aspects of water treatment and quality, contaminant removal, and sanitation on a global scale.