A one-dimensional model for water desalination by flow-through electrode capacitive deionization

Capacitive deionization (CDI) is an emerging water desalination technique. In CDI, pairs of porous electrode capacitors are electrically charged to remove salt from brackish water present between the electrodes. We here present a novel experimental technique allowing measurement of spatially and temporally resolved salt concentration between the CDI electrodes. Our technique measures the local fluorescence intensity of a neutrally charged fluorescent probe which is collisionally quenched by chloride ions. To our knowledge, our system is the first to measure in situ and spatially resolved chloride concentration in a laboratory CDI cell. We here demonstrate good agreement between our dynamic measurements of salt concentration in a charging, millimeter-scale CDI system to the results of a modified Donnan porous electrode transport model. Further, we utilize our dynamic measurements to demonstrate that salt removal between our charging CDI electrodes occurs on a longer time scale than the capacitive charging time scales of our CDI cell. Compared to typical measurements of CDI system performance (namely, measurements of outflow ionic conductivity), our technique can enable more advanced and better-controlled studies of ion transport in CDI systems, which can potentially catalyze future performance improvements.

Brackish water resources may be an attractive option for human consumption, agriculture, and industry if efficient water purification can be implemented. In the past few decades, research and development of various desalination technologies have been carried out, among which distillation, reverse osmosis, and electrodialysis are the most commonly known and widespread.1 Capacitive deionization (CDI) is an alternative, emerging, and energy-efficient technology for water desalination, which employs an electrochemical flow cell configured with polarized porous carbon electrodes to remove ionized salts in a stream with low molar concentration. Briefly, by regulating an external voltage to a CDI cell, ionized salts are electrostatically captured (or released) in the pores of the carbon electrodes, resulting in the stream being deionized (or the electrodes being regenerated).2-4 Recent studies have found that the salt adsorption capacity (SAC) could be substantially improved by using surface modified carbon electrodes resulting from nitric acid and ethylenediamine treatments.5 Combined with the modified Donnan model including a term of chemical surface charge, this improved SAC was accounted for by enhancement of the chemical charges immobilized in the carbon micropores, validating both enhanced CDI (e-CDI) and extended-voltage CDI (eV-CDI) effects in the CDI literature (Fig. 1).6In summary, it is considered that, for the carbon electrodes used in a CDI cell, an increase in the chemical surface charges makes the pores more readily available for salt adsorption under proper applied voltages. In addition to the surface modified carbon electrodes, immobilized chemical charges can be found in ion-exchange materials. For instance, a well-known cation-exchange polymer, Nafion, contains the negative chemical charges, -SO3 -, while an anion-exchange polymer typically holds positive chemical charges, e.g., NR4 + and NR3 +. As a consequence, together with the knowledge gained above, ion-exchange polymers coating were used in our current studies to explore new composite carbon electrodes for CDI cycling tests. As shown in an initial test (Fig. 2), the addition of ion-exchange polymers results in the SAC not only being increased but also being stabilized with operational time when NaCl solution was used. In this presentation, the preparation and characterizations of composite carbon electrodes will be detailed including comparisons to conventional CDI and membrane capacitive deionization cells. Furthermore, these composite electrodes will be configured into a CDI cell to investigate both e-CDI and eV-CDI effects in various salt solutions such as CaCl2, Na2SO4, and NH4NO3. In addition, the relevant charge efficiency and cycling longevity will be reported and discussed. Figure 1. Demonstration of both enhanced CDI (e-CDI) and extended voltage CDI (eV-CDI) effects using the modified Donnan model with the addition of chemical surface charge. The parameters used in the model can be found in ref. (5 and 6). Figure 2. Improved salt adsorption capacity and operational stability using cation- and anion-exchange polymers added to the carbon cathode and anode, respectively, in a CDI cell. The CDI cell was operated using 1 V charging and at 0 V discharging in ~31 L of ~7 mM deaerated NaCl solution.

Capacitive deionization (CDI) is a water desalination technology in which ions are removed from water by creating a potential difference between two capacitive electrodes. Porous carbon has been extensively used as an electrode material in CDI. However, recent developments in the field of intercalation materials have led to their application in CDI due to their large ion storage capacity. One such intercalation material, nickel hexacyanoferrate (NiHCF), was used in this study as the electrode material. A symmetrical cell was assembled with two identical NiHCF electrodes separated by an anion-exchange membrane. The effect of operational parameters such as current density, feed concentration and flow rate on the desalination characteristics of the cell was investigated. The highest salt adsorption capacity of ≈ 35 mg/g was measured at a current density of 2.5 A/m2 in a 20 mM NaCl feed solution. Furthermore, a Nernst-Planck transport model was successfully used to predict the change in the outlet concentration and cell voltage of the symmetric CDI cell. Finally, performance of the symmetric NiHCF CDI cell was compared with an MCDI cell with porous carbon electrodes. The NiHCF cell, on average, consumed 2.5 times less energy than the carbon-based MCDI cell to achieve similar levels of salt removal from saline water in CDI.

Even though two-thirds of our world's surface is covered by water, less than 1% of that water can be directly consumed to satisfy the rapid growth in population, urbanization, and industrialization.[1] Water quality and scarcity have become some of the most important global challenges of our time. Current desalination technologies such as multi-stage flash distillation and reverse osmosis can be costly to implement and operate, requiring significant pretreatment and consistent maintenance procedures.[2] Thus, investigations into alternative desalination options are being explored toward building more sustainable water treatment systems.Capacitive deionization (CDI) is a desalination technology using highly porous carbon electrodes that can reversibly adsorb dissolved ions. By regulating applied voltages to a CDI cell, ionized salts are trapped in the electric double layers (EDLs) at carbon electrodes, thereafter desalinating water in the CDI cell.[3] CDI technology may have potential advantages over current desalination methods in that no heat treatment or high pressure is required, potentially leading to a significant decrease in the operational and energy costs compared to current desalination processes and aiding in the production of clean/fresh water.Since 2011, researchers from the University of Kentucky Center for Applied Energy Research (UK CAER) have contributed to ongoing efforts to advance CDI technology from theoretical studies to applied process research.[3-22] Works primarily include the improvement of desalination capacity, mitigation of performance degradation, and technology commercialization. In this talk, one of the presenters will provide key milestones of the CDI technology developed at UK CAER in honor of Prof. D. Noel Buckley for his 50-year experience in electrochemistry research.Ref:[1] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science, 333 (6043) (2011), pp. 712-717[2] J.-J. Yan, S.-F. Shao, J.-H. Wang, J.-P. Liu, Improvement of a multi-stage flash seawater desalination system for cogeneration power plants, Desalination, 217 (1) (2007), pp. 191-202[3] A. Omosebi, X. Gao, J. Rentschler, J. Landon, K.-K. Liu, Continuous operation of membrane capacitive deionization cells assembled with dissimilar potential of zero charge electrode pairs, J. Colloid Interf. Sci., 446 (2015), pp. 345-351[4] J. Landon, X. Gao, A. Omosebi, K. Liu, “Local pH Effects on Carbon Oxidation in Capacitive Deionization Architectures” Environmental Science: Water Research & Technology, 7, 861 – 869 (2021)[5] A. Omosebi, Z. Li, N. Holubowitch, X. Gao, J. Landon, A. Cramer, K. Liu, “Energy recovery in capacitive deionization systems with inverted operation characteristics”, Environmental Science: Water Research & Technology, 6, 321-330 (2020)[6] X. Gao, A. Omosebi, Z. Ma, F. Zhu, J. Landon, M. Ghorbanian, N. Kern, K. Liu, “Capacitive Deionization Using Symmetric Carbon Electrode Pairs”, Enviro. Sci.: Water Res. Tech., 5, 660-671 (2019).[7] J. Landon, X. Gao, A. Omosebi, K. Liu, “Progress and outlook for capacitive deionization technology”, Current Opinion in Chemical Engineering, 25, 1-8 (2019)[8] N. Holubowitch, A. Omosebi, X. Gao, J. Landon, K. Liu, “Membrane-Free Electrochemical Deoxygenation of Aqueous Solutions Using Symmetric Activated Carbon Electrodes in Flow-Through Cells”, Electrochim. Acta., 297, 163-172 (2019).[9] X. Gao, A. Omosebi, J. Landon, K. Liu, “Voltage-Based Stabilization of Microporous Carbon Electrodes for Inverted Capacitive Deionization”, J. Phys. Chem. C, 122, 1158-1168 (2018).[10] A. Omosebi, X. Gao, N. Holubowitch, Z. Li, J. Landon, K. Liu, “Anion Exchange Membrane Capacitive Deionization Cells”, J. Electrochem. Soc., 164, E242-E247 (2017).[11] N. Holubowitch, A. Omosebi, X. Gao, J. Landon, K. Liu, “Quasi-Steady-State Polarization Reveals the Interplay of Capacitive fand Faradaic Process in Capacitive Deionization”, ChemElectroChem, 4, 2404-2413 (2017).[12] X. Gao, A. Omosebi, N. Holubowitch, J. Landon, K. Liu, “Capacitive Deionization Using Alternating Polarization: Effect of Surface Charge on Salt Removal”, Electrochim. Acta, 233, 249-255 (2017).[13] X. Gao, A. Omosebi, N. Holubowitch, A. Liu, K. Ruh, J. Landon, K. Liu, “Polymer-Coated Composite Anodes for Efficient and Stable Capacitive Deionization”, Desalination, 399, 16-20 (2016).[14] X. Gao, S. Porada, A. Omosebi, K. Liu, P. M. Biesheuvel, J. Landon, “Complementary Surface Charge for Enhanced Capacitive Deionization”, Water Res., 92, 275-282 (2016).[15] X. Gao, A. Omosebi, J. Landon, K. Liu, “Enhanced Salt Removal in an Inverted Capacitive Deionization Cell Using Amine Modified Microporous Carbon Electrode”, Environ. Sci. Tech., 49, 10920 (2015).[16] X. Gao, A. Omosebi, J. Landon, K. Liu, “Surface Charge Enhanced Carbon Electrodes for Stable and Efficient Capacitive Deionization Using Inverted Adsorption-Desorption Behavior”, Energy Environ. Sci., 8, 897 (2015)[17] X. Gao, A. Omosebi, J. Landon, K. Liu, “Dependence of the Capacitive Deionization Performance of Potential of Zero Charge Shifting of Carbon Xerogel Electrodes during Long-Term Operation”, J. Electrochem. Soc., 161, E159 (2014).

Acid-functionalized single-walled carbon nanotube (a-SWCNT)-coated reticulated vitreous carbon (RVC) composite electrodes have been prepared and the use of these electrodes in capacitive deionization (CDI) cells for water desalination has been the focus of this study. The performance of these electrodes was tested based on the applied voltage, flow rate, bias potential and a-SWCNT loadings, and then evaluated by electrosorption dynamics. The effect of the feed stream directly through the electrodes, between the electrodes, and the distance between the electrodes in the CDI system on the performance of the electrodes has been investigated. The interaction of ions with the electrodes was tested through Langmuir and Freundlich isotherm models. A new CDI cell was developed, which shows an increase of 23.96% in electrosorption capacity compared to the basic CDI cells. Moreover, a comparison of our results with the published results reveals that RVC/a-SWCNT electrodes produce 16 times more pure water compared to the ones produced using only CNT-based electrodes. Finally, it can be inferred that RVC/a-SWCNT composite electrodes in newly-developed CDI cells can be effectively used in desalination technology for water purification.

Understanding degradation of capacitive deionization cells: Full–cell simulations with anode corrosion

High-performance nitrogen-doped porous carbon electrode materials for capacitive deionization of Industrial salt-contaminated wastewater

Numerical modeling of ion transport and adsorption in porous media: A pore-scale study for capacitive deionization desalination

The effect of the flow-regime, reversal of polarization, and oxygen on the long term stability in capacitive de-ionization processes

This paper presents mathematical modeling of capacitive de-ionization (CDI) system with activated carbon electrodes. In CDI, a voltage is applied across two oppositely placed porous electrodes that results in adsorption of ions from the saline water stream forcing anions to move towards anode and cations to move towards cathode under the effect of electric field, thus producing ion depleted product stream. The ions are stored in the electric double layer in the activated carbon electrodes. When the electrodes capacity is reached, voltage is reduced to zero or inverted releasing the ions from the electrodes to produce a highly concentrated salty stream (brine). A mathematical model of CDI is presented based on its electrical equivalent circuit that describes how the effluent salt concentration varies with respect to time in a CDI cell. The model also predicts the amount of charge captured by the CDI cell and the current in the CDI cell, both as a function of time. The effectiveness of the model is evaluated by comparing its results with the electrosorption experimental results of CDI unit from AQUA Electronic Water Purifier (EWP) [1], [2]. The model also evaluates the performance of the AQUA EWP CDI cell to different operational parameters i.e., the feed total dissolved solids concentration and flow rates. The model results are in good agreement with the experimental results.

Capacitive deionization (CDI) is a technology in which water is desalinated by ion electrosorption into the electric double layers (EDLs) of charging porous electrodes. In recent years significant advances have been made in modeling the charge and salt dynamics in a CDI cell, but the possible effect of surface transport within diffuse EDLs on these dynamics has not been investigated. We here present theory which includes surface transport in describing the dynamics of a charging CDI cell. Through our numerical solution to the presented models, the possible effect of surface transport on the CDI process is elucidated. While at some model conditions surface transport enhances the rate of CDI cell charging, counter-intuitively this additional transport pathway is found to slow down cell charging at other model conditions.

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.

All efforts to obtain, reuse or purify water are extremely significant for society. Recently, researchers have begun to delve in an idea born decades ago: the desalination of water using highly porous electrodes. It is based on a fundamental aspect of electrical double layers, namely, their huge capacitance. The ions of a solution can be partially removed under the application of an electric field when the solution fills the space between porous electrodes, either bare (CDI, or capacitive deionization), coated with ionic exchange membranes (MCDI) or chemically treated (inverted-CDI or i-CDI). One of the challenges of the last years was to explore new materials and arrangements to improve the efficiency of the system. In this work, we propose a new approach inspired in the electrokinetics of soft particles: a layer of polyelectrolyte (cationic on one electrode, anionic on the opposite one) coats the carbon electrodes, converting them in a sort of "soft" electrode pair. We present a theoretical model and a set of experiments showing how soft electrodes can be successfully employed in capacitive deionization.

Capacitive deionization (CDI) is an emerging approach to water treatment that makes use of common porous carbon materials to electrostatically remove ions from a solution.(1-3) Desalination using CDI is achieved in a flow cell consisted of a minimum of a pair of porous carbon electrodes separated by a water channel as sketched in Fig. 1(a). Often, constant-voltage operation is adopted to investigate ion transport of a CDI cell with a charging voltage (V ch) for ion adsorption and a discharge voltage (V dis) for ion desorption.(2)Details are illustrated in Fig. 1(b) by using an ionic charge curve plotted in a potential distribution diagram, where the ionic charge curve is defined by the modified Donnan (mD) model with chemical surface charge for a single carbon electrode according to reference (4, 5). Ion adsorption-desorption by a CDI cell depends upon the placement of the potential of zero charge (E PZC) versusE oin a potential distribution diagram,(6)where E PZC commonly defines a potential when the electrode has a minimum in ion adsorption,(7)as depicted at the lowest point on the ionic charge curves in Fig. 1(b). In this work, by modifying carbon surface chemistry using carbon oxidation, scenarios of different E PZCversus E o are created to investigate the effect of carbon surface charge on ion adsorption-desorption of a CDI cell. A comprehensive study will be performed to demonstrate the importance of the E PZCparameter in a potential distribution diagram on resulting adsorption-desorption characteristics, primarily including real-time measurements of the potential drops at the CDI electrodes during charging and discharging, and developments of a new approach to estimate the E PZCaccording to the mD model with chemical surface charge. References J. Landon, X. Gao, B. Kulengowski, J. K. Neathery and K. Liu, J. Electrochem. Soc., 159, A1861 (2012). X. Gao, J. Landon, J. K. Neathery and K. Liu, J. Electrochem.Soc., 160, E106 (2013). A. Omosebi, X. Gao, J. Landon and K. Liu, ACS Appl. Mater. Interf., 6, 12640 (2014). X. Gao, A. Omosebi, J. Landon and K. Liu, J. Phys. Chem. C, 122, 1158 (2018). P. M. Biesheuvel, H. Hamelers and M. Suss, Colloids Interf. Sci. Commun., 9, 1 (2015). E. Avraham, M. Noked, I. Cohen, A. Soffer and D. Aurbach, J. Electrochem. Soc., 158, P168 (2011). A. J. Bard and L. R. Faulkner, Electrochemical Methods - Fundamentals and Applications, John Wiley & Sons, New York (2001). Acknowledgement This work is supported by the Crosscutting Research, National Energy Technology Laboratory, U.S. Department of Energy (DE-FE0031555). Figure 1

Activated carbon recycled from bitter-tea and palm shell wastes for capacitive desalination of salt water