Advancements in Materials & Membrane Engineering in Membrane Capacitive Deionization for Water Desalination

Abstract

Water scarcity is an urgent issue which is affecting global health and sanitation, with up to 2 billion people estimated to live in water scarce regions. Membrane capacitive deionization (MCDI) has emerged as a water purification technique which has gained interest in fields such as desalination, water softening and heavy metal removal. The principle of MCDI desalination involves the removal of ions by application of an electric field between two oppositely polarized electrodes, covered by ion-exchange membranes (IEMs). The low voltages, ambient conditions and regenerative nature of MCDI has led to its exploration as a potentially low energy route to clean water. Despite this, there remains work to be done to improve operational factors such as membrane properties, desalination performance and process sustainability for the technology to challenge traditional desalination techniques. The current state of the field of membrane capacitive deionization was assessed, detailing novel materials, preparation methods, performance and cost analysis. The limitations and industrial potential of the materials were presented, as well as current challenges and future directions for the evolving field. Innovative anion-exchange membranes (AEMs) based on a blend of polymers were fabricated for MCDI application. The selection of polymers with an abundance of quaternary nitrogen groups produced membranes with enhanced electrochemical properties compared to commercial membranes. These quaternized polymer blend membranes improved the desalination performance by over three times that of conventional capacitive deionization without membranes. With the aim of increasing the process sustainability and commercial feasibility of MCDI, the elimination of toxic solvents and persistent binders from electrode processing was investigated. A systematic study was conducted to determine the feasibility of the inclusion of various bio-derived/fluorine-free binders and green solvents within electrode processing. Electrodes produced using sustainable binders and solvents exhibited preferable physicochemical and electrochemical properties for desalination application, producing improved performance compared to electrodes prepared using traditional materials. The incorporation of tailored nanomaterials into a polymer matrix was able to enhance the properties of polymer anion-exchange membranes beyond those of the pristine polymer, and similar commercial membranes. The embedding of ionic covalent organic nanosheets into a quaternized polybenzimidazole matrix achieved enhanced desalination performance, due to the increase of ion-exchange capacity and ion transport channels donated by the predesigned nanomaterials. The research presented herein outlines advancements in materials, membrane engineering, process sustainability and performance within membrane capacitive deionization technology.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Krishna Persaud (Supervisor) & Robert Dryfe (Supervisor)