A review of recent advances in electrode materials and applications for flow-electrode desalination systems
Graphical abstract
Introduction
Water pollution arising from global industrialization and overpopulation has pushed the demand for safe and clean water to extremely high level in the 21st century, which is resulting in the development of numerous techniques for obtaining fresh water [1]. So far, the most common methods for producing freshwater from various water sources, such as brackish water and seawater, have been reverse osmosis (RO), multistage flash distillation (MSF), nanofiltration (NF), and electrodialysis (ED) [2], [3]. Although they have revolutionized the water treatment technology and have seen significant improvement since their first emergence, these techniques still have limitations of high energy consumption, cost-intensive facility, and maintenance cost [4], [5]. Capacitive deionization (CDI) is an emerging and promising alternative technology for water treatment [6]. The principles for CDI process are based on the elimination of charged ions existing in water by electrochemical ion removal mechanisms [7], [8]. A typical CDI cell consists of a pair of current collectors, electrodes (positive and negative), and a center channel for feedwater stream. When a potential or current is applied to the cell, an electric field is generated, and pulls the ions in the feed stream to the oppositely charged electrode. Electric double layers (EDLs) are formed onto the electrode/electrolyte interfaces, and the salt ions are adsorbed within the EDLs. During this process, the saline feedwater turns into deionized water, producing freshwater. When the electrodes are fully saturated with adsorbed ions, a discharge process has to be carried out to release the ions adsorbed during charging, and refresh the capacity of the electrode in the discharging process [9]. Compared to traditional desalination processes, including RO and MSF, CDI is highly energy efficient, because it removes the salt ions that are a minor part of the electrolyte, whereas RO and MSF must remove the water molecules that are the major part of the electrolyte [10]. However, the salt removal capacity of typical CDI desalination is restricted, due to the limited surface area, and the amount of electrodes used [11]. To overcome this issue, various CDI desalination systems have been intensively studied in recent years. From the viewpoint of CDI cell configuration, one of the significant innovations in CDI desalination is flow-electrode capacitive deionization (FCDI) [12], [13], [14]. This differs from the conventional CDI in that the liquid state electrodes are constantly supplied into the cell. The size and capacity of the electrode in a FCDI cell can vary greatly, since the amount of electrode, which determines the total salt removal capacity, is not confined within the cell, unlike the conventional CDI system that uses fixed solid electrodes. This unique feature allows FCDI to have much higher salt removal capacity and be more suitable for the treatment of highly saline water sources, such as seawater. Moreover, it allows the FCDI cell to run continuously, without the need for discharging process. Also, the liquid electrode of the FCDI system can be pumped out, and replaced with a new one, without the need to disassemble the cell, as in the CDI with solid electrodes. This feature is exceptionally important when considering the integration into industrial-size system for the timesaving and continuity of operation [12], [13], [15].
In both CDI and FCDI system, the most commonly used electrode materials are carbon and their variants. Those materials utilize their high surface area to store salt ions. The advantages of such materials include high electrochemical and pH stability, simple working mechanism, and cost-effectiveness. However, they also have drawbacks, such as low salt removal rate, low conductivity, and limited applications besides desalination. From the electrode materials viewpoint, redox-active materials have the potential to provide higher salt removal rate and lower energy consumption, owing to the redox-coupling reaction between electrode and electrolyte ions. Also, each redox material has unique chemical reactions with different types of ion species, which allow them to have specific affinity (or selectivity) of certain ions in electrolyte. This characteristic is highly valuable when applying for the removal of toxic ions and recovery of valuable materials. Nevertheless, redox materials also have some notable drawbacks that include electrochemical instability, complex mechanism of their reactions, higher cost compared to capacitive materials, and the potential generation of toxic byproducts. In comparison with capacitive electrodes, the redox active electrode for FCDI systems employs relatively recently emerged materials. Therefore, their full potential has yet to be fully discovered.
In this review, we provide an understanding of the fundamental mechanism of FCDI systems, reviewing recent approaches in FCDI electrode materials, as well as categorizing their mechanism. Finally, we provide insights into the recent various applications of FCDI and CDI desalination and discuss the challenges for and future development of these systems.
Section snippets
FCDI mechanism
In CDI desalination operation, ion separation and storage simultaneously occur by two main electrochemical mechanisms: non-Faradaic (capacitive), and Faradaic reaction contribution (redox coupling reaction) between electrode and electrolyte ions [9], [16]. FCDI uses a similar ion removal mechanism to the conventional CDI system (Fig. 1a). Non-Faradaic ion removal has been the most common mechanism in the ion separation of CDI desalination. This mechanism normally occurs at an applied potential
Basic requirements for the ideal electrode materials
According to the ion removal mechanism, the electrode material used in FCDI desalination can be mainly divided into two types, which are capacitive, and redox-active electrode materials. Capacitive electrodes mainly store target ions in the EDLs of the electrode surface by physical adsorption (non-Faradaic contribution), whereas redox-active electrodes primarily store ions through redox reactions (Faradaic contribution). The combination of capacitive and redox-active electrodes is also being
Applications
Since its first introduction, CDI-based technologies have been investigated mostly for salt removal applications. However, recent studies showed that by modifying components, such as electrode materials or operating conditions, applications for CDI can vary a lot, from the selective removal of toxic ions, heavy metals, and capture of pollutants to the recovery of useful ions [3], [25], [164], [165], [166]. As described above, FCDI offers various advantages compared to the conventional CDI in
Conclusion and outlook
In this review, we have reviewed the basic principles of the FCDI desalination system, various cell configurations, choice of electrode materials, and their various applications. Recently, the FCDI system has received a lot of attention, owing to its distinct advantages over CDI, including unrestricted electrode amount, continuous desalination, and the ability to treat highly saline feed waters. In addition to the traditional configurations, many components in the FCDI cell can be modified or
Declaration of competing interest
The authors declare no competing financial interests.
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2021R1A2C1092184). This work was also supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20184030202130). This work was also supported by the Soonchunhyang University Research Fund.
References (248)
- et al.
Pressure driven membrane processes — the answer to the need of a growing world population for quality water supply and waste water disposal
Sep. Purif. Technol.
(2001) - et al.
Applications of capacitive deionization: desalination, softening, selective removal, and energy efficiency
Desalination
(2019) - et al.
A short review on reverse osmosis pretreatment technologies
Desalination
(2014) - et al.
Review on the science and technology of water desalination by capacitive deionization
Prog. Mater. Sci.
(2013) - et al.
Application of capacitive deionisation in water desalination: a review
Desalination
(2014) - et al.
Improvement of desalination efficiency in capacitive deionization using a carbon electrode coated with an ion-exchange polymer
Water Res.
(2010) - et al.
Faradaic reactions in capacitive deionization (CDI) - problems and possibilities: a review
Water Res.
(2018) - et al.
Batch mode and continuous desalination of water using flowing carbon deionization (FCDI) technology
Electrochem. Commun.
(2014) - et al.
Improving the feasibility and applicability of flow-electrode capacitive deionization (FCDI): review of process optimization and energy efficiency
Desalination
(2021) - et al.
Single module flow-electrode capacitive deionization for continuous water desalination
Electrochem. Commun.
(2015)
A direct comparison of flow-by and flow-through capacitive deionization
Desalination
Electrode reactions and adsorption/desorption performance related to the applied potential in a capacitive deionization process
Desalination
Capacitive deionization (CDI) for desalination and water treatment—past, present and future (a review)
Desalination
Comparison of faradaic reactions in capacitive deionization (CDI) and membrane capacitive deionization (MCDI) water treatment processes
Water Res.
Long term stability of capacitive de-ionization processes for water desalination: the challenge of positive electrodes corrosion
Electrochim. Acta
Comparison of faradaic reactions in flow-through and flow-by capacitive deionization (CDI) systems
Electrochim. Acta
Flow-electrode capacitive deionization (FCDI) scale-up using a membrane stack configuration
Water Res.
Membrane-spacer assembly for flow-electrode capacitive deionization
Appl. Surf. Sci.
Analysis of the desalting performance of flow-electrode capacitive deionization under short-circuited closed cycle operation
Desalination
Scale-up desalination: membrane-current collector assembly in flow-electrode capacitive deionization system
Water Res.
Theory of membrane capacitive deionization including the effect of the electrode pore space
J. Colloid Interface Sci.
Analysis of capacitive and electrodialytic contributions to water desalination by flow-electrode CDI
Water Res.
Recovering ammonia from municipal wastewater by flow-electrode capacitive deionization
Chem. Eng. J.
Desalination behavior and performance of flow-electrode capacitive deionization under various operational modes
Chem. Eng. J.
A review on free-standing electrodes for energy-effective desalination: recent advances and perspectives in capacitive deionization
Desalination
Investigation of fine activated carbon as a viable flow electrode in capacitive deionization
Desalination
Capacitive neutralization deionization with flow electrodes
Electrochim. Acta
Optimized desalination performance of high voltage flow-electrode capacitive deionization by adding carbon black in flow-electrode
Desalination
Flow-electrode capacitive deionization with highly enhanced salt removal performance utilizing high-aspect ratio functionalized carbon nanotubes
Water Res.
Enhanced desalination performance utilizing sulfonated carbon nanotube in the flow-electrode capacitive deionization process
Sep. Purif. Technol.
Phosphate selective recovery by magnetic iron oxide impregnated carbon flow-electrode capacitive deionization (FCDI)
Water Res.
Carbon electrodes for capacitive technologies
Energy Storage Mater.
Carbon-based slurry electrodes for energy storage and power supply systems
Energy Storage Mater.
Recent progress on metal-organic framework-derived porous carbon and its composite for pollutant adsorption from liquid phase
Chem. Eng. J.
Enhanced salt removal performance of flow electrode capacitive deionization with high cell operational potential
Sep. Purif. Technol.
Carbon aerogel electrode for excellent dephosphorization via flow capacitive deionization
Desalination
Electrochemical characterization of electrolyte-filled porous carbon materials for electrosorption process
J. Electroanal. Chem.
Anion-/cationic compounds enhance the dispersion of flow electrodes to obtain high capacitive deionization performance
Desalination
Enhanced capacitive deionization using a biochar-integrated novel flow-electrode
Desalination
Use of mesoporous conductive carbon black to enhance performance of activated carbon electrodes in capacitive deionization technology
Desalination
Hierarchical meso–macro structure porous carbon black as electrode materials in Li–air battery
J. Power Sources
Effects of carbon black on the electrochemical performances of SiOx anode for lithium-ion capacitors
J. Power Sources
Carbon nanotube-and graphene-based nanomaterials and applications in high-voltage supercapacitor: a review
Carbon
Development of sulfonated-carbon nanotubes/graphene three-dimensional conductive spongy framework with ion-selective effect as cathode in high-performance lithium-sulfur batteries
Chem. Eng. J.
RGO wrapped tungsten trioxide hydrate on CNT-modified carbon cloth as self-supported high-rate lithium-ion battery electrode
Electrochim. Acta
Zinc-based energy storage with functionalized carbon nanotube/polyaniline nanocomposite cathodes
Chem. Eng. J.
Carbon nanotubes/activated carbon hybrid as a high-performance suspension electrode for the electrochemical desalination of wastewater
Desalination
Porous carbon derived from metal–organic framework (MOF) for capacitive deionization electrode
Electrochim. Acta
Metal-organic framework derived carbon nanoarchitectures for highly efficient flow-electrode CDI desalination
Environ. Res.
The future of seawater desalination: energy, technology, and the environment
Science
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These authors contributed equally to this work.