Faradaic Electrodes Open a New Era for Capacitive Deionization

Abstract

Capacitive deionization (CDI) is an emerging desalination technology for effective removal of ionic species from aqueous solutions. Compared to conventional CDI, which is based on carbon electrodes and struggles with high salinity streams due to a limited salt removal capacity by ion electrosorption and excessive co-ion expulsion, the emerging Faradaic electrodes provide unique opportunities to upgrade the CDI performance, i.e., achieving much higher salt removal capacities and energy-efficient desalination for high salinity streams, due to the Faradaic reaction for ion capture. This article presents a comprehensive overview on the current developments of Faradaic electrode materials for CDI. Here, the fundamentals of Faradaic electrode-based CDI are first introduced in detail, including novel CDI cell architectures, key CDI performance metrics, ion capture mechanisms, and the design principles of Faradaic electrode materials. Three main categories of Faradaic electrode materials are summarized and discussed regarding their crystal structure, physicochemical characteristics, and desalination performance. In particular, the ion capture mechanisms in Faradaic electrode materials are highlighted to obtain a better understanding of the CDI process. Moreover, novel tailored applications, including selective ion removal and contaminant removal, are specifically introduced. Finally, the remaining challenges and research directions are also outlined to provide guidelines for future research.

Electro-Driven Materials and Processes for Lithium Recovery—A Review

“…Electro-active adsorbent materials, such as those used in CDI, mostly involve physisorption at the solid–liquid interface to support ultra-selective extraction of resources [ 90 , 92 , 94 , 95 , 96 ]. Typical CDI configurations are shown in Figure 4 .…”

“… Two type of ions accumulation in ( a ) activated carbon according with EDLs mechanism and ( b ) intercalation and de-intercalation ions according with Faradaic reactions. Reprinted based on the open access license from [ 95 ]. …”

Carbon‐Based Capacitive Deionization Electrodes: Development Techniques and its Influence on Electrode Properties

“…Faradaic CDI uses redox materials on both anode and cathode whereas hybrid CDI pairs a redox active cathode with a carbon anode. The use of faradaic electrodes and cell architectures resulted to remarkably high salt adsorption capacities, which made it a growing area of interest for CDI [34–35] …”

“…Faradaic electrodes, on the other hand, rely on redox interactions to adsorb ions. Their mechanism can be further divided into three: (1) ion intercalation in the crystal lattice, (2) conversion reactions to form new compounds, and (3) ion‐redox ion interaction [34] . Depending on the type of charge transfer, faradaic electrodes can be described as pseudocapacitors or batteries.…”

Highly Efficient Capacitive Deionization Enabled by NiCo4MnO8.5 Electrodes

“…Recently, the Faradaic reactions electrodes have demonstrated remarkable salt removal capacity and current efficiency in CDI research since they can store ions within the crystal structures. [ 15 , 16 , 17 , 18 , 19 , 20 ] Among all Faradaic electrodes, the transition metal oxides have drawn great attentions in energy storage research field due to their suitability in aqueous solution, [ 15 , 16 , 21 ] i.e., NiO, Co 3 O 4 , and MnO 2 . [ 22 ] Nevertheless, the low conductivity and ion diffusion rate associated with transition metal oxides are harmful to their electrochemical activity.…”