Capacitive deionization (CDI) is an emerging technology for the facile removal of charged ionic species from aqueous solutions, and is currently being widely explored for water desalination applications. The technology is based on ion electrosorption at the surface of a pair of electrically charged electrodes, commonly composed of highly porous carbon materials. The CDI community has grown exponentially over the past decade, driving tremendous advances via new cell architectures and system designs, the implementation of ion exchange membranes, and alternative concepts such as flowable carbon electrodes and hybrid systems employing a Faradaic (battery) electrode. Also, vast improvements have been made towards unraveling the complex processes inherent to interfacial electrochemistry, including the modelling of kinetic and equilibrium aspects of the desalination process. In our perspective, we critically review and evaluate the current state-of-the-art of CDI technology and provide definitions and performance metric nomenclature in an effort to unify the fast-growing CDI community. We also provide an outlook on the emerging trends in CDI and propose future research and development directions.

/pdf/water-desalination-via-capacitive-deionization-what-is-it-p45le9ivut.pdf

Water desalination via capacitive deionization : What is it and what can we expect from it?

Faradaic reactions in capacitive deionization (CDI) - problems and possibilities: A review.

The last five decades have witnessed the rapid development of capacitive deionization (CDI) as a novel, low-cost and environment-friendly desalination technology. During the CDI process, salt ions are sequestered by the porous electrodes once exposed to an electric field. These electrodes, acting as an ion storage container, play a vital role during desalination. In this review, various carbon-based composite electrode materials, including carbon–carbon composites, carbon–metal oxide composites, carbon–polymer composites and carbon–polymer–metal oxide composites, are systematically presented. Applications of these carbon-based composite materials for the removal of the salt ions from solution are demonstrated and they exhibit improved CDI performances compared with pristine carbon electrodes.

/pdf/review-on-carbon-based-composite-materials-for-capacitive-3azscw01cb.pdf

Review on carbon-based composite materials for capacitive deionization

Reversible electrochemical processes are a promising technology for energy-efficient water treatment. Electrochemical desalination is based on the compensation of electric charge by ionic species, through which the ions are immobilized and, thereby, removed from a feed-water stream flowing through a desalination cell. For decades, electrochemical desalination has focused on the use of carbon electrodes, but their salt-removal ability is limited by the mechanism of ion electrosorption at low molar concentrations and low charge-storage capacity. Recently, charge-transfer materials, often found in batteries, have demonstrated much larger charge-storage capacities and energy-efficient desalination at both low and high molar strengths. In this Review, we assess electrochemical-desalination mechanisms and materials, including ion electrosorption and charge-transfer processes, namely, ion binding with redox-active polymers, ion insertion, conversion reactions and redox-active electrolytes. Furthermore, we discuss performance metrics and cell architectures, which we decouple from the nature of the electrode material and the underlying mechanism to show the versatility of cell-design concepts. These charge-transfer processes enable a wealth of environmental applications, ranging from potable-water generation and industrial-water remediation to lithium recovery and heavy-metal-ion removal. Electrochemical processes enable energy-efficient desalination of water and the separation and recovery of elements. In this Review, we discuss the mechanisms and materials of this emerging generation of water-remediation technology.

Charge-transfer materials for electrochemical water desalination, ion separation and the recovery of elements

There is an increased global demand for activated carbon (AC) in application of water treatment and purification. Water pollutants that have exhibited a greater removal efficiency by AC included but not limited to heavy metals, pharmaceuticals, pesticides, natural organic matter, disinfection by-products, and microplastics. Granular activated carbon (GAC) is mostly used in aqueous solutions and adsorption columns for water treatment. Commercial AC is not only costly, but also obtained from non-renewable sources. This has prompted the search for alternative renewable materials for AC production. Biomass wastes present a great potential of such materials because of their availability and carbonaceous nature. This in turn can reduce on the adverse environmental effects caused by poor disposal of these wastes. The challenges associated with biomass waste based GAC are their low strength and attrition resistance which make them easily disintegrate under aqueous phase. This paper provides a comprehensive review on recent advances in production of biomass waste based GAC for water treatment and highlights future research directions. Production parameters such as granulation conditions, use of binders, carbonization, activation methods, and their effect on textural properties are discussed. Factors influencing the adsorption capacities of the derived GACs, adsorption models, adsorption mechanisms, and their regeneration potentials are reviewed. The literature reveals that biomass waste materials can produce GAC for use in water treatment with possibilities of being regenerated. Nonetheless, there is a need to explore 1) the effect of preparation pathways on the adsorptive properties of biomass derived GAC, 2) sustainable production of biomass derived GAC based on life cycle assessment and techno-economic analysis, and 3) adsorption mechanisms of GAC for removal of contaminants of emerging concerns such as microplastics and unregulated disinfection by-products.

Synthesis and application of Granular activated carbon from biomass waste materials for water treatment: A review

Capacitive deionization (CDI) is an electrochemical method for water desalination using porous carbon electrodes. A key parameter in CDI is the charge efficiency, Λ, which is the ratio of salt adsorption over charge in a CDI-cycle. Values for Λ in CDI are typically around 0.5-0.8, significantly less than the theoretical maximum of unity, due to the fact that not only counterions are adsorbed into the pores of the carbon electrodes, but at the same time coions are released. To enhance Λ, ion-exchange membranes (IEMs) can be implemented. With membranes, Λ can be close to unity because the membranes only allow passage for the counterions. Enhancing the value of Λ is advantageous as this implies a lower electrical current and (at a fixed charging voltage) a reduced energy use. We demonstrate how, without the need to include IEMs, the charge efficiency can be increased to values close to the theoretical maximum of unity, by increasing the cell voltage during discharge, with only a small loss of salt adsorption capacity per cycle. In separate constant-current CDI experiments, where after some time the effluent salt concentration reaches a stable value, this value is reached earlier with increased discharge voltage. We compare the experimental results with predictions of porous electrode theory which includes an equilibrium Donnan electrical double layer model for salt adsorption in carbon micropores. Our results highlight the potential of modified operational schemes in CDI to increase charge efficiency and reduce energy use of water desalination.

Enhanced charge efficiency and reduced energy use in capacitive deionization by increasing the discharge voltage

Capacitive deionization (CDI) as a class of electrochemical desalination has attracted fast-growing research interest in recent years. A significant part of this growing interest is arguably attributable to the premise that CDI is energy efficient and has the potential to outcompete other conventional desalination technologies. In this review, systematic evaluation of literature data reveals that while the absolute energy consumption of CDI is in general low, most existing CDI systems achieve limited energy efficiency from a thermodynamic perspective. We also analyze the causes for the relatively low energy efficiency and discuss factors that may lead to enhanced energy efficiency for CDI.

Energy Efficiency of Capacitive Deionization

Theory of pH changes in water desalination by capacitive deionization.

Resistance identification and rational process design in Capacitive Deionization

The use of carbon flow electrodes has significantly impacted electrochemical energy storage and capacitive deionization (CDI), but device performance is limited as these electrodes cannot surpass ∼20 wt% carbon while maintaining flowability. We here introduce flowable fluidized bed electrodes which achieve up to 35 wt%, and apply these to water desalination by CDI.

J.E. Dykstra

Papers

Enhanced charge efficiency and reduced energy use in capacitive deionization by increasing the discharge voltage

Energy Efficiency of Capacitive Deionization

Theory of pH changes in water desalination by capacitive deionization.

Resistance identification and rational process design in Capacitive Deionization

Fluidized bed electrodes with high carbon loading for water desalination by capacitive deionization