Energetic performance optimization of a capacitive deionization system operating with transient cycles and brackish water

In recent years, more efforts have been made to improve new and more efficient non-membrane-based methods for water desalination. Capacitive deionization (CDI), a novel technique for water desalination using an electric field to adsorb ions from a solution to a high-porous media, has the capability to recover a fraction of the energy consumed for the desalination during the regeneration process, which happens to be its most prominent characteristic among other desalination methods. This paper introduces a new desalination method that aims improving the performance of traditional CDI systems. The proposed process consists of an array of CDI cells connected in series with buffer containers in between them. Each buffer, serve two purposes: 1) average the outlet solution from the preceding cell, and 2) secure a continuous water supply to the following cell. Initial evaluation of the proposed CDI system architecture was made by comparing a two-cell-one-buffer assembly with a two cascaded cells array. Concentration of the intermediate solution buffer was the minimum averaged concentration attained at the outlet of the first CDI cell, under a steady state condition. The obtained results show that proposed CDI system with intermediate solution had better performance in terms of desalination percentage. This publication opens new opportunities to improve the performance of CDI systems and implement this technology on industrial applications.

In recent years, more efforts have been made to improve new and more efficient nonmembrane-based methods for water desalination. Capacitive deionization (CDI), a novel technique for water desalination using an electric field to adsorb ions from a solution to a high-porous media, has the capability to recover a fraction of the energy consumed for the desalination during the regeneration process, which happens to be its most prominent characteristic among other desalination methods. This paper introduces a new desalination method that aims at improving the performance of traditional CDI systems. The proposed process consists of an array of CDI cells connected in series with buffer containers in between them. Each buffer serves two purposes: (1) averaging the outlet solution from the preceding cell and (2) securing a continuous water supply to the following cell. Initial evaluation of the proposed CDI system architecture was made by comparing a two-cell-one-buffer assembly with a two cascaded cells array. Concentration of the intermediate solution buffer was the minimum averaged concentration attained at the outlet of the first CDI cell, under a steady-state condition. The obtained results show that the proposed CDI system with intermediate solution had better performance in terms of desalination percentage. This publication opens new opportunities to improve the performance of CDI systems and implement this technology on industrial applications.

Due to the increasing demand for clean and potable water stemming from population growth and exacerbated by the scarcity of fresh water resources, more attention has been drawn to different and innovative methods for water desalination. Capacitive deionization (CDI) is a relatively new, low maintenance, and energy efficient technique for desalinating brackish water. In this technique, an electrical field is employed to adsorb ions into a high-porous media. After the saturation of the porous electrodes, their adsorption capacity can be restored through a regeneration process. Various parameters affect the overall performance of CDI. The flow rate at which water is purified in CDI plays an essential role in its ultimate performance. Many studies have shown that desalination percentage decreases as flow rate increases in CDI, since the advection of ions in the flow becomes more dominant than their diffusion toward the electrodes. However, herein, based on a physical model previously developed, we conjecture that for a given amount of time and volume of water, multiple desalination cycles in a high flow rate regime will outperform desalinating in a single cycle at a low flow rate. Moreover, splitting a CDI unit into two sub-units, with the same total length, will lead to higher desalination. Based on these premises, we introduce a new approach aimed at enhancing the overall performance of CDI. An array of CDI cells are sequentially connected to each other with intermediate solutions placed in between them. These intermediate solutions act as buffers to homogenize the outlet concentration of the preceding cell and maintain a constant inlet concentration for the following cell. Desalination tests were conducted to compare the performance of the proposed system, consisting of two CDI units and one intermediate solution buffer, with a two-cascaded-CDI unit system with no intermediate solution. Desalination tests were performed in a high flow rate regime with a low salinity initial solution of NaCl in water. In the buffered arrangement, the concentration of the solution buffer was set at the minimum average outlet concentration of the first CDI test. Experimental data demonstrated the improved performance of the buffered system over the non-buffered system, in terms of desalination percentage and energy consumption. Increasing the number of CDI units and solution buffers in a buffered system, the new proposed method will lead to lower amount of energy consumed per unit volume of the desalinated water.

New and more efficient water desalination technologies have been a topic of incipient research over the past few decades. Although the focus has been placed on the improvement of membrane-based desalination methods such as reverse osmosis, the development of new high surface area carbon-based-electrode materials have brought substantial interest towards capacitive deionization (CDI), a novel technique that uses an electric field to separate the ionic species from the water. Part of the new interest on CDI is its ability to store and return a fraction of the energy used in the desalination process. This characteristic is not common to other electric-field-based desalination methods such as electro-deionization and electrodialysis reversal where none of the input energy is recoverable. This paper presents work conducted to analyze the energy recovery, thermodynamic efficiency, and ionic adsorption/desorption rates in a CDI cell using different salt concentration solutions and various flow rates. Voltage and electrical current measurements are conducted during the desalination and electrode regeneration processes and used to evaluate the energy recovery ratio. Salinity measurements of the inflow and outflow stream concentrations using conductivity probes, alongside the current measurements, are used to calculate ion adsorption efficiency. Two analytical species transport models are developed to estimate the net ionic adsorption rates in a steady-state and nonsaturated porous electrode scenario. Finally, the convective and electrokinetic transport times are compared and their effect on desalination performance is presented. Steady test results for outlet to inlet concentration ratio show a strong dependence on flow rate and concentration independence for dilute solutions. In addition, transient test results indicate that the net electrical energy requirement is dependent on the number of carbon electrode regeneration cycles, which is thought to be due to imperfect regeneration. The energy requirements and adsorption/desorption rate analyses conducted for this water-desalination process could be extended to other ion-adsorption applications such as the reprocessing of lubricants or spent nuclear fuels in a near future.

New and more efficient water desalination technologies have been a topic of incipient research over the past few decades. Although much of the attention and efforts have focused on the improvement of membrane-based desalination methods such as reverse osmosis, the development of new high-surface area carbon-based-electrode materials have brought substantial interest towards capacitive deionization (CDI), a novel technique that uses electric fields to separate the ionic species from the water. Part of the new interest on CDI is its ability to store and return a fraction of the energy used in the desalination process. This characteristic is not common to other electric-field-based desalination methods such as electro-deionization (EDI) and electro-dialysis reversal (EDR) where none of the input energy is recoverable. This paper presents work conducted to analyze the energy recovery, thermodynamic efficiency, and ionic adsorption/desorption rates in a CDI cell using different salt concentration solutions and various flow-rates. Voltage and electrical current measurements are conducted during the desalination and porous electrode regeneration processes and used to evaluate the percentage of energy recovery.. Salinity measurements of the inflow and outflow stream concentrations using conductivity probes, alongside the current measurements, are used to calculate ion adsorption/desorption efficiencies. Correlation of these measurements with an analytical species transport model provides information about the net ionic adsorption/desorption rates in non-saturated-carbon-electrode scenarios. The results show a strong dependence of the net electrical energy requirements with the number of carbon electrodes regeneration cycles. Finally, a non-dimensional number that compares the convective and electro-kinetic transport times is presented. The energy requirements and adsorption/desorption rates analyses conducted for this water-desalination process could be extended to other ion-adsorption applications such as the re-process of spent nuclear fuels in a near future.

Flow capacitive deionization is a water desalination technique that uses liquid carbon-based electrodes to recover fresh water from brackish or seawater. This is a potential second-generation water desalination process, however it is limited by parameters such as feed electrode conductivity, interfacial resistance, viscosity, and so on. In this study, titanium oxide nanofibers (TiO2NF) were manufactured using an electrospinning process and then blended with commercial activated carbon (AC) to create a well distributed flow electrode in this study. Field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray (EDX) were used to characterize the morphology, crystal structure, and chemical moieties of the as-synthesized composites. Notably, the flow electrode containing 1 wt.% TiO2NF (ACTiO2NF 1 wt.%) had the highest capacitance and the best salt removal rate (0.033 mg/min·cm2) of all the composites. The improvement in cell performance at this ratio indicates that the nanofibers are uniformly distributed over the electrode’s surface, preventing electrode passivation, and nanofiber agglomeration, which could impede ion flow to the electrode’s pores. This research suggests that the physical mixture could be used as a flow electrode in capacitive deionization.

Due to the increasing demand for clean and potable water stemming from population growth and exacerbated by the scarcity of fresh water resources, more attention has been drawn to innovative methods for water desalination. Capacitive deionization (CDI) is a low maintenance and energy efficient technique for desalinating brackish water, which employs an electrical field to adsorb ions into a high-porous media. After the saturation of the porous electrodes, their adsorption capacity can be restored through a regeneration process. Herein, based on a physical model previously developed, we conjecture that for a given amount of time and volume of water, multiple desalination cycles in a high flow rate regime will outperform desalinating in a single cycle at a low flow rate. Moreover, splitting a CDI unit into two subunits, with the same total length, will lead to higher desalination. Based on these premises, we introduce a new approach aimed at enhancing the overall performance of CDI. An array of CDI cells are sequentially connected to each other with intermediate solutions placed in between them. Desalination tests were conducted to compare the performance of the proposed system, consisting of two CDI units and one intermediate solution buffer, with a two-cascaded-CDI unit system with no intermediate solution. Experimental data demonstrated the improved performance of the buffered system over the nonbuffered system, in terms of desalination percentage and energy consumption. The new proposed method can lead to lower amount of energy consumed per unit volume of the desalinated water.

Capacitive deionization (CDI) is a water desalination technique in which salt ions are removed from brackish water by flowing through a spacer channel with porous electrodes on each side. Upon applying a small voltage difference between the two electrodes, cations move to and are accumulated in electrostatic double layers inside the negatively charged cathode and the anions are removed by the positively charged anode. Therefore, one of the advanced merits of CDI is the low driven energy by compared to other desalination technologies. Inspired this, we have performed the calculation on energy consumption of activated carbon based CDI in different operation conditions. The results show that the energy consumptions are significantly related to cell voltage as well as solution concentration. Furthermore, the round trip efficiency as a vital indication in terms of energy consumption have been introduced and discussed as well.

Desalination and disinfection of inland brackish ground water in a capacitive deionization cell using nanoporous activated carbon cloth electrodes

Maxing Out Water Desalination with MXenes

Research progress of sodium super ionic conductor electrode materials for capacitive deionization

Although current water desalination technologies are mature enough and advanced, the shortage of freshwater is still considered as one of the most pressing global issues. Therefore, there is a strong incentive to explore and investigate new potential methods with low energy consumption. It is well-known that polymer hydrogel network has the ability to absorb water via swelling. In the case of polyelectrolyte hydrogels, the charges localized on the polymer chains, which mainly drive the swelling pressure inside the hydrogel, can also separate added salt via charge-based selectivity (Donnan exclusion). When combining this material with a temperature-sensitive polymer, the heat generated by solar energy can trigger the desorption process via conformational change of polymer chains. Hence, hydrogels designed from both materials, polyelectrolyte and thermo-responsive polymer can reduce the salinity of water, such as brackish water by means of reversible thermally-induced absorption and desorption desalination processes. In addition, the desorption process can also be achieved based solely on a polyelectrolyte hydrogel system by altering the ionization of charges within the hydrogel via pH. In this thesis, hydrogel-based water desalination process were developed using acrylic acid (AAc)/sodium acrylate (SA)-based polyelectrolytes as the charge-based separation function, alone or with a combination of N-isopropylacrylamide (NIPAAm) as thermo-responsive comonomer. In the latter case, a series of chemically cross-linked polymeric hydrogels were synthesized via either free radical-initiated copolymerization or reversible addition-fragmentation chain transfer (RAFT) polymerization, thus realizing different macromolecular architecture. According to the nature of hydrogels, the reversible sorption/desorption state were triggered by either chemical stimulus (pH), or physical stimulus (heat) as the thermo-responsive polymer introduced into the hydrogels. In detail, the effect of hydrogel composition as well as the influence of the macromolecular architecture on the swelling/deswelling behavior for the synthesized hydrogels were studied. For this, their properties including their responses to external stimuli were investigated, and their ability to desalinate brackish water as well as the effciency of such desalination process were evaluated. Generally, the results demonstrated correlations between macromolecular architecture of the network structure and their performance in the proposed desalination process, such as salt rejection and desalination capacity. Moreover, the potential of the best performance materials for applications was also discussed.

In this report, we show that inexpensive and environmentally friendly manganese oxides can achieve high ion removal capacities in a hybrid capacitive deionization (HCDI) configuration and highlight the importance of understanding the relationship between material crystal structure and ion size in electrochemical water desalination. As population and pollution levels continue to rise and threaten fresh water sources, the scarce availability of potable water has become a critical issue for society. Therefore, it is imperative to develop efficient water desalination techniques, and capacitive deionization (CDI) has emerged as a low cost and low pressure deionization technique [1]. In a typical CDI process, brackish water is flown through a channel between two carbon electrodes, and a potential is applied across the electrodes. Ions from the water are then attracted to the electric double layer on the surface of the carbon materials and removed from solution, resulting in a desalinated water stream exiting the channel. When the electrodes are saturated, the potential can be reversed, releasing ions from the surface of the carbon electrodes and regenerating them for subsequent desalination. The ion removal capacity of carbon electrodes in CDI is limited by the surface area of the carbon since the mechanism of ion removal is based on the electric double layer formed on the carbon surfaces. Recently, an approach has emerged (HCDI) that utilizes one carbon electrode and a redox active material as the opposite electrode [2]. This approach has the potential for higher ion removal capacities since the redox active material can remove and release ions from solution via oxidation and reduction reactions with bulk of the material and is therefore not limited by the available surface area. A tunnel manganese oxide (TuMO), Na4Mn9O18, demonstrated the effectiveness of this approach by achieving a high capacity in NaCl solution while in an HCDI configuration [2]. Motivated by this past work, we study the behavior of other TuMOs in an HCDI configuration. Manganese oxides are of great interest for electrochemical water desalination due to their inherently low cost, environmentally friendliness, high electrochemically activity, and stability in aqueous environments. More specifically, TuMOs consist of MnO6 octahedra arranged in various tunnel configurations around stabilizing cations. The large, open tunnel structures provide ample volume to store ions removed from solution. Further, by varying synthesis conditions, the size, shape, and ionic content of the structural tunnels can be modified, thus making TuMOs an attractive materials system to investigate for the relationship between crystal structure and ion removal capacity. TuMOs can all be synthesized with a flexible nanowire morphology as well, which allows for excellent access of water and ions to the surface of the materials. In this work, we investigate the ion removal behavior of four TuMOs in an HCDI configuration in NaCl, KCl, and MgCl2 solutions. Two of the phases studied are well-known tunnel structures, α-MnO2 with 2x2 octahedra square tunnels and manganese oxide with todorokite crystal structure (Tod-MnO2) containing tunnels of 3x3 octahedra dimensions. The other two phases are novel TuMOs with ordered (2xn-MnO2) and disordered (Hybrid-MnO2) combinations of structural tunnels with multiple different dimensions. The ion removal capacities of the four TuMOs are shown in Figure 1, and it was found that each material demonstrated high ion removal capacities above 20 mg g-1 in all solutions tested. α-MnO2, with the smallest tunnels, demonstrated lower capacities in the solution containing the largest hydrated cation, MgCl2. The other three phases, which contain larger structural tunnels, were found to demonstrate superior removal of larger hydrated ions from solution. Extended ion removal experiments showed the TuMOs to be stable for over 20 ion removal/ion release cycles, indicating the efficacy of using TuMOs for repeated water desalination via HCDI. Moreover, ex-situ XRD and EDS show that the ion removal mechanism is a result of chemical reaction between the ions in solution and the TuMO electrode. In summary, this work demonstrates not only the efficacy and high ion removal capacities of TuMOs in an HDCI configuration, but also shows the importance of understanding the relationship between material crystal structure and the size of ions in solution for maximizing performance in electrochemical water desalination.

Brackish water desalination using reverse osmosis and capacitive deionization at the water-energy nexus

Capacitive deionization (CDI) is a promising water purification technology which works by removing salt ions or charged species from aqueous solutions. Currently, most of the research on CDI focuses on the desalination of water with low or moderate salt concentration due to the low salt adsorption capacity of the electrodes. The electrosorption capacity of CDI relies on the structural and textural characteristics of the electrode materials. The cost of electrode materials, the complicated synthesis methods, and the environmental concerns arising from material synthesis steps hinder the development of large-scale CDI units. By considering the good electrical conductivity, high specific surface area (SSA), porous structure, availability, mass production, and cost, porous carbon derived from biomass materials may be a promising CDI electrode material. This review presents an update on carbon nanomaterials derived from various biomasses for CDI electrodes. It covers different synthesis methods and the electrosorption performance of each material and discusses the impact of the SSA and porous structure of the materials on desalination. This review shows that a variety of biomass materials can be used to synthesize cost-effective CDI electrode materials with different structures and good desalination performance. It also shows that diverse precursors and synthesis routes have significant influences on the properties and performance of the resulting carbon electrodes. Additionally, the performance of CDI does not depend only on BET surface area and pore structure but also on the applied voltage, initial concentration of the feed solution, and mass, as well as the capacitance of the electrodes.