Enhancing the Ion-Size-Based Selectivity of Capacitive Deionization Electrodes.

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.

Enhanced desalination efficiency in capacitive deionization with an ion-selective membrane

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.

Maxing Out Water Desalination with MXenes

Complementary surface charge for enhanced capacitive deionization

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.

Extreme Monovalent Ion Selectivity Via Capacitive Ion Exchange

Capacitive deionization (CDI) continues to emerge as a viable option for the treatment of saline streams with up to brackish levels of ionic contaminants. CDI functions similar to a capacitor, storing ions from a liquid stream onto highly porous electrodes with the aid of an applied electric field. When the pores become saturated, the electric field can be reduced or reversed to discharge the previously stored ions and form a concentrate stream [1]. At the same time, the electronic energy input to the cell during ion storage can be recovered to power auxiliary devices or coupled CDI units. The coupling of CDI cells offers not only the ability to lower the energy requirements for cell operation, but also the possibility of semi-continuous CDI operation. With much effort devoted to materials development and performance enhancements, there have been few reports on energy recovery in CDI literature. Of these, discharging a charged CDI cell over a resistive load, direct connection of CDI cells, and the use of buck-boost DC-DC converters have been demonstrated. In this work, parallel CDI cells are integrated with a DC-DC converter based on a Ćuk topology (Figure 1) to show active energy recovery from a discharging cell and subsequent deionization with a secondary cell using energy stored in the initial charged cell. Unlike buck-boost, the Ćuk topology incorporates a power supply that compensates for energy losses during the electron transfer process, thereby allowing the system to simultaneously transfer energy and meet its desalination goals. Inverted capacitive deionization (iCDI) was recently demonstrated where deionization was accomplished by a polarity difference between chemical surface charges on electrode pairs that facilitates adsorption of ions from solution [2]. Conversely to classical CDI, cell discharge is accomplished by applying an external electric field. We have improved upon the iCDI cell architecture by incorporating ion-selective membranes to form a new inverted membrane capacitive deionization cell (iMCDI). We will demonstrate energy recovery results for both the iCDI and iMCDI cells, and explore the influence of converter operation and cell architecture on energy recovery.

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).

Ion-exchange membrane technology has shown a great potential to enhance the desalting efficiency. Ion-exchange membranes are placed in front of the electrodes so that the charged ions can be selectively passed through the membrane layer and captured by the oppositely charged electrode more quickly, so as to increase the efficiency of desalination. In this research, carbon electrodes have been synthesized from an activated carbon (700 – 1400 m2/g) and polyvinyl alcohol (PVA) as a binder using freezing thawing method. A solution with 180 µS/cm NaCl was pumped to the capacitive deionization (CDI) cell using a Boyu Submersible pump (model SP-601) at a flow rate of 25 mL/min and the voltage was set at 2 V. The result showed that the CDI cell with ion-exchange membrane (MCDI) has the salt removal efficiency greater than the CDI cell without ion-exchange membrane. The salt-removal percentage of MCDI was achieved at 66.36%, meanwhile the CDI cell without ion-exchange membrane resulted in 54.4%.

The demand for potable water is continuously increasing. Therefore energy-efficient water desalination methods are the focus of intensive research1–3. Capacitive deionization (CDI) is an energy-efficient water desalination technology. The presentation will address the problematic corrosion which develops in CDI cells during periodic water desalination processes.4–8 Cells comprising identical pairs of activated carbon electrodes, which are initially symmetric, lose their symmetry upon cycling as illustrated in figure 14,6. The positive electrodes are gradually oxidized and their pore systems are damaged. In order to understand corrosion processes of the positive electrodes in CDI cells and to learn how to avoid it, our research focused on electrodes oxidation in CDI cells in three different aspects4. The first one is the effect of the geometry of the cells and the flow regimes. We compared CDI cells that were operated in flow-through and flow-by modes. We were able to conclude that the geometry has an impact on the electro-oxidation of the positively polarized electrodes. Flow-by cells were found to be more stable than flow-through cells. It is explained by a local basic environment developed at the negative electrodes which affects badly the stability of the positively polarized electrodes. Such an inter-electrodes impact is relevant only to flow-through cells. The second aspect that was examined was operation by alternating application of potential on flow-through CDI cells: the electrodes were polarized periodically to negative and positive potentials. It was clearly demonstrated that such a mode of operation pronouncedly extends the stability of the electrodes by mitigating their oxidation due to the positive polarization. Finally, to better understand the effect of the dissolved oxygen in the solution on the electrodes’ oxidation, we operated flow-through CDI cells under nitrogen atmosphere. Operation under nitrogen did not avoid oxidation of the positive electrodes, when 0.9V were applied to the cells. However, the stability of the cells were extended by three fold, compared to operation under air. It is clear from these experiments that dissolved oxygen has a very detrimental effect on the stability of CDI cells, because trace oxygen reduction at the negative electrodes leads to asymmetric potential application, in which most of the potential applied falls on the positive electrodes. Hence, their oxidation is accelerated. In any event, it is impossible to avoid the presence of trace oxygen in a practical CDI operation. Based on the results that will be presented, it is clear that alternating potential application, using flow-by mode, further optimization of the potential applied and electrodes modifications that will make them more resistive to oxidation, can help to extend remarkably the stability of CDI cells in desalination processes and promote their practical use. Fig. 1. (A) Illustration of the operational parameters: potential applied, potential dependent concentration of adsorbed and desorbed ions (counter-ions and co-ions), of initially symmetric CDI cells. The electrodes are polarized upon charging (desalination step) and are shorted to 0 V upon discharge (extraction of waste, salty solution). (B) The situation when the inversion point is reached: illustration of the unsymmetrical situation developed with CDI cells, which positive electrodes are oxidized and their Point of Zero Charge (PZC) is sifted to positive potentials (compared to that of pristine electrodes). (C) Illustration of the operation of unsymmetrical CDI cells, which positive electrodes are oxidized and have high resistivity and low surface area because of detrimental oxidation processes. The electrodes’ PZC and the potential of cell shorting, E0, vs. a virtual reference electrode, are marked.

Although much progress was made in light emitting devices, the ability to electrically control their spectral emission remains limited. We will present a novel approach and experimental results for dynamic color control, by electrically modulating the non-radiative Forster resonance energy transfer (FRET) efficiency between donor and acceptor dyes in a solution. FRET efficiency depends on the 6th power of the distance between donor and acceptor dye molecules, and thus, it is sensitive to variations in acceptor's concentration. Controlled acceptor concentrations could be achieved by attracting or repelling ionic dyes from the electrodes using a capacitive deionization (CDI) cell, with high surface area porous electrodes. This approach to dynamic color control may open new directions in 100% fill-factor displays, and can be expanded to energy saving applications such as controlling building’s external wall emissivity. We studied the modulation of a single dye emission using a CDI cell with negatively charged Fluorescein Sodium Salt in aquatic solution. Photoluminescence was measured along few charging-discharging CDI cycles and showed the ability to control extensive optical response through CDI. We experimented with two types of FRET-pair dyes: a) anion-cation, where the acceptor and the donor ions are oppositely charged, and b) zwitterion and ion, where the donor is neutral. We found that electrical control on FRET in aquatic solution is weak, due to hydrophobic attractive interaction between the acceptor and the donor. In order to avoid this effect, we are experimenting FRET control in organic solvents. These results will be presented in the talk.

Performance comparison and energy consumption analysis of capacitive deionization and membrane capacitive deionization processes

Theory of water treatment by capacitive deionization with redox active porous electrodes

Wastewater treatment and freshwater production are integral to sustaining an increasing global population. Presently, techniques like reverse osmosis (RO), and multi- stage distillation (MSD), which are extensively used for water cleanup are cost and energy intensive due to an operational focus on removing a majority constituent water from salts. Capacitive de-ionization (CDI) is an emerging technique for water treatment whereby dissolved ionic content in the bulk solution is stored in highly porous electrodes upon the application of an electric field, i.e., removing salts from water. When the potential is released, a concentrated waste stream is produced (1, 2). A lot of research has been devoted to optimizing the electrodes, usually carbon to possess high surface area, large porosity, and high conductivity for use in CDI. Other key research areas include membrane assisted capacitive deionization and chemically functionalized electrodes. While CDI has been touted as an efficient process for brackish water desalination, there remains intensive energy consumption to keep the traditional CDI process from becoming a transformational technology, and reverse osmosis continues to be more economical when desalinating higher concentration streams such as sea water. Many authors have discussed the possibility of parallel CDI operation with energy recovery to reduce energy requirements. However, beyond discharging a charged CDI cell into a resistive load, there has been little demonstration of CDI to CDI energy recovery. The interfacing of CDI cells offers not only the ability to lower the energy requirements, but also the possibility of semi-continuous CDI operation. In this work, parallel CDI cells are integrated with a multi-input bi-directional DC/DC converter to show active energy recovery from a discharging cell and subsequent deionization of a secondary cell using energy stored in the initial charged cell. Deionization experiments are conducted by closed-loop recirculation of a saline stream through the CDI cell to achieve a batch-mode configuration. The test system is equipped with inline conductivity probes for continuous monitoring of concentration. Along with the converter, the deionization system is connected to a power supply to compensate for losses and maintain operation. Initial testing was done by charging a primary cell at 1.2 V (Figure 1), and then directly connecting to a secondary cell. There were 12 and 7 µS/cm drops in concentration for the primary and secondary cells respectively. Future studies will track inter-cell energy transfer and show recovery of more than 50 % of the energy stored in the initial cell using this integrated CDI-converter setup.AcknowledgementsThe authors are thankful for the support of the State of Wyoming Advanced Conversion Technologies Task Force for supporting this research.