Exploring flow-electrode capacitive deionization: An overview and new insights

Amidst the escalating severity of global water scarcity, a diverse array of scientific undertakings is devoted to the advancement of technologies concerning water desalination. A notably promising advancement in this field is known as flow-electrode capacitive deionization (FCDI). This pioneering electrochemical methodology represents a derivation of capacitive deionization (CDI) technology, amalgamating ion-exchange membranes with flow particle-based electrodes. Over the preceding seven years, FCDI has garnered considerable interest due to its potential for energy-efficient, sustainable, and consistently high-quality production of fresh water. Additionally, it affords flexibility in the management of particle electrodes and concentrated streams. In this review, an all-encompassing survey of the recent strides in the realm of FCDI is provided. The focal point encompasses the fundamental principles of FCDI, diverse designs including cell architecture, operational modes, choices of flow electrodes and strategies for optimizing their performance in FCDI. This review thoroughly examines the complex mechanisms of mass and charge transfer within flow electrodes in FCDI. It also explores the impact of feed saline water and applied voltage, detailing their respective roles in the FCDI process. A detailed analysis of ion selectivity is provided, alongside discussions on the environmental applications of FCDI in areas such as water desalination, resource recovery, and the removal of waterborne pollutants. Clear definitions and performance metrics are established, and the review concludes with a forward-looking perspective on the future potential of FCDI technology, highlighting key areas requiring focused attention moving forward. • FCDI facilitates continuous operation by enabling material regeneration external to the cell. • The resistance and charge transport within the flow-electrode play crucial roles in determining its performance in FCDI. • The charge transfer mechanism is contingent upon the composition of the electrode materials and the operational mode. • Significantly improving electronic conductivity within the flow electrode is a key factor in enhancing the performance of FCDI. • Desalination, the recovery of heavy metals, and nutrient retrieval are research areas under investigation in the context of FCDI.