Advanced Characterization in Clean Water Technologies

Carbon-based electrode materials with ion exchange membranes for enhanced membrane capacitive deionization

Capacitive deionization (CDI) is a class of electrochemical desalination technologies which desalinate via ion storage in electric double-layers. CDI has received renewed attention in recent years due to the ability to couple energy storage with salt separation. During galvanostatic operation, a current is applied between porous carbon electrodes until a limiting voltage is reach. The cell is then discharged by applying a reverse current, generating brine and recovering stored charge. However, CDI desalination and energy efficiency can be limited by parasitic side reactions, and selective adsorption of counter-ions (anions at the positive electrode, and cations at the negative electrode). Several material additions and electrode configurations have been proposed to overcome these limitations, with the most prominent being the addition of ion exchange membranes (IEMs) promote counter ion flux out of the desalination chamber and incorporation of carbon slurry electrodes to increase system adsorption capacity. Likewise, functionalization of carbon electrodes has been studied to improve counter-ion adsorption within EDL micropores. While the incorporation of functionalized carbon, IEMs in membrane capacitive deionization (MCDI), and the use of slurry electrodes in flow capacitive deionization (FCDI) have successfully reduced energy consumption or increased ion adsorption capacity in CDI systems, these modifications are often evaluated under limited conditions on the basis of specific performance enhancements. Additional clarity is necessary to evaluate the associated performance and cost tradeoffs across the design space. In this study, an equivalent circuit (M)CDI model, with porous electrode sub-models, was used to measure the sensitivity of CDI performance to material selection, design, and operating choices. In order to investigate the performance of FCDI, pulse-flowed electrodes of high capacity and electronic resistances were incorporated into the existing model. Similarly, fixed charge in the anodic and cathodic micropores was studied to investigate functionalized carbon. Constrained system parameters were randomly selected via latin hypercube sampling (LHS) across multiple electrode geometries and influent salt concentrations. The resultant model outputs were then correlated with input values to quantify parameter sensitivity. Our sensitivity analysis shows that operating current density, electrode specific capacitance, and contact resistance were the parameters which most significantly dictated (M)CDI performance. These parameters where then used to construct an operational space for CDI, MCDI, and FCDI. The results of our operational space were then used to develop a simplified, operational model for both capacitive and faradic materials in CDI. Using this model we conducted a techno-economic analysis (TEA) of proposed improvement to CDI. From the TEA we are able more directly set operating parameters under which CDI might favorably compete with the primary technology for desalination, reverse osmosis (RO). We are able to evaluate materials lifetimes and costs necessary to economically operate CDI. Lastly, goals for operating parameters highlighted in the sensitivity analysis (specific capacitance, voltage limits, charge efficiency, and cell resistance) were set. These benchmarks will provide target for the continued development of CDI towards and economic and viable alternative to RO for brackish water desalination.

Modeling technologies for desalination of brackish water — toward a sustainable water supply

Capacitive deionization (CDI) is a developing technology for water desalination applications with benefits over current methods such as reverse osmosis (RO) and distillation due to its operation at lower temperatures and pressure. CDI uses small applied potentials (<2.0 V) to selectively separate ions from feed water streams by reversibly adsorbing these ions in the electric double layer of conductive, high surface area, and porous carbon electrodes. This reversible absorption process can be used in a variety of configurations including flow-by, flow-through, and flowable electrode arrangements.1 Classic CDI systems have charge efficiency (equivalent charge of salt adsorbed per electronic charge input) values well below 100% due to parasitic discharge processes and Faradaic reactions.2,3 Therefore, ion-exchange membranes have been added into these systems to increase the effective adsorption capacity of the electrodes and limit the extent of parasitic processes. Overall, CDI systems are considered to be more efficient than RO membranes for many lower concentration salt streams (<5000 ppm) due to differences in its separation mechanism.4 Shown in Figure 1 is an energy cost comparison of CDI and RO technologies as a function of salt concentration. CDI is shown with energy recovery values of 0, 25, and 50%. As the concentration of NaCl decreases, there is a distinct region where the energy cost of this separation can best RO. The more recent addition of ion-exchange membranes in a membrane capacitive deionization (MCDI) process can further increase the charge efficiency to over 100% in certain instances and aid in further decreasing energy costs.5 The efficiency of both the CDI and MCDI process make them attractive options for many larger scale applications where energy cost becomes a dominant factor in choosing the water treatment system. In this talk, a comparison between the energy costs of RO and CDI/MCDI will be given. The use of recent surface chemistries to increase the charge efficiency will be shown to decrease the energy cost of CDI/MCDI processes further. Finally, applications will be reviewed where CDI and MCDI can be used as a standalone option as well as interface with existing membrane processes. Acknowledgements The authors are grateful to the U.S. - China Clean Energy Research Center, U.S. Department of Energy for project funding (No. DE-PI0000017).

Defluorination technology is crucial for ensuring the safety of accessible water. The application of capacitive deionization (CDI) technology faces challenges due to competitive adsorption of fluoride ions within complex natural fluoride-rich brackish water matrices, which often contain high levels of dissolved inorganic carbon (DIC) species (mainly HCO3– and CO32–). These DIC species are pH-dependent, playing a significant role in the selective removal of fluoride by the CDI process. Thus, there is a knowledge gap in understanding the effects of membranes in membrane capacitive deionization (MCDI) on fluoride removal. In this study, we examined the key operating parameters in CDI and MCDI, including applied constant voltages and different types of anion-exchange membranes (AEMs), on the desalination performance in F- and dissolved inorganic carbon water matrices. The application of AEMs significantly improve the salt adsorption capacity (SAC) for both F- and DIC species, and reduced energy consumption. However, it simultaneously results in a notable decrease in F- selectivity as membranes control mass transfer. Higher applied voltages enhance the SAC performance for F- and DIC species, but also induce more severe Faradaic reactions, leading to increased energy consumption and lower energy efficiency. Additionally, ion species and pH changes during CDI and MCDI processes are interrelated, indicating that stability tests of CDI electrodes in batch mode are not reliable when using the same testing solution repeatedly. The diverse valence states of ions in the solution impact pH variations under different voltages in the CDI/MCDI process. These findings provide valuable insights into the development of water purification and desalination technology, particularly for the application and further advancement of selective fluoride removal by the CDI process.

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).

The desalination of seawater is one of the most established techniques in the world. In the middle of the 20th century this was achieved using water evaporation systems, later with reverse osmosis membranes and nowadays with the possibility of capacitive deionization membranes. Capacitive deionization and membrane capacitive deionization are an emerging technology that make it possible to obtain drinking water with an efficiency of 95%. This technology is in the development stage and consists of porous activated carbon electrodes, which have great potential for saving energy in the water desalination process and can be used for desalination using an innovative technology called capacitive deionization (CDI), or membrane capacitive deionization (MCDI) if an anion and cation membrane exchange is used. In this paper is proposed and designed a characterization system prototype for CDI and MCDI that can operate with constant current charging and discharging (galvanostatic method). Adequate precision has been achieved, as can be seen in the results obtained. These results were obtained from the performance of typical characterization tests with electrochemical double layer capacitors (EDLC), since they are electrochemical devices that behave similarly to MCDI, from the point of view of the electrical variables of the processes that take place in MCDI. A philosophy of using free software with open-source code has been followed, with software such as the Arduino and Processing programming editors (IDE), as well as the Arduino Nano board (ATmega328), the analogical-digital converter (ADC1115) and the digital-analogical converter (MCP4725). Moreover, a low-cost system has been developed. A robust and versatile system has been designed for water treatment, and a flexible system has been obtained for the specifications established, as it is shown in the results section.

Can capacitive deionization outperform reverse osmosis for brackish water desalination?

While Earth’s oceans contain about 97% of its water, their high salinity makes them unsuitable for human consumption [1]. In regions where freshwater is limited or unpredictable, seawater desalination can provide a substantial and dependable supply. Existing desalination technologies – reverse osmosis, thermal distillation, and electrodialysis – are effective but energy-intensive, relying on high pressure, thermal input, or electricity.Membrane capacitive deionization (MCDI) is a promising alternative due to its cost-effectiveness, energy efficiency, and environmentally friendly operation. MCDI uses electrochemical adsorption and desorption of salt ions for separation. It is modular, electrified, and does not require high-pressure piping or generate significant acoustic, thermal, or electromagnetic signatures. Flow-by MCDI, a commercialized design, removes ions using electrical energy and consists of two porous electrodes covered by ion-exchange membranes [2]. However, the salt adsorption capacity of carbon electrodes is limited to ~40 mg/g, making them ineffective for seawater desalination [3]. Achieving full electrode regeneration is essential to maximize salt removal.This work introduces an operational strategy using dilute NaCl regeneration solutions (0–5 g/L) to treat 35 g/L NaCl and a mixture of 30 g/L NaCl with 5 g/L MgSO₄. Additionally, surface patterning is explored as a method to enhance membrane performance by increasing the interfacial area and salt flux [4]. Patterned membranes have been shown to improve local hydrodynamics, enhance concentration polarization via secondary flows, and reduce boundary layer thickness and osmotic pressure.Poly(phenylene) alkylene ion-exchange membranes were fabricated with nanopatterns—hexagonal, double ring, octagonal, and rectangular—ranging from 100 to 300 nm using electron beam lithography. Silicon wafers were spin-coated with Zep 520A121 resist and anisole (1:1), baked at 180°C, and exposed using a RAITH EBPG 5200 system (150 nA, 600 µm aperture, 180 µC/cm²). Patterns were developed in n-Amyl acetate and 2-propanol, then dried. PDMS molds were formed by mixing Sylgard 184 elastomer and curing agent (10:1), degassing, pouring onto the patterned wafer, and curing at 65°C for 4 hours. Ionomers were drop-cast onto the molds.To further enhance performance, Prussian blue analogues (PBAs)—redox-active materials with open framework structures—were used as electrode modifiers to improve salt adsorption and charge redistribution [5][6]. A nickel PBA (NaNi[Fe(CN)₆]·nH₂O) was mixed with PVDF and conductive carbon (8:1:1) in N-methyl-2-pyrrolidone and coated on activated carbon cloth electrodes.Initial results show strong deionization across regeneration concentrations (0–5000 ppm NaCl), with minor ion accumulation at 5000 ppm. Compared to flat membranes, hexagonal nanopatterned membranes showed the greatest surface area enhancement, a ~500 mV reduction in cell voltage during chronopotentiometry, a 45 Ω·cm² decrease in area-specific resistance, and an 18.71 Ω·cm² reduction in capacitance during impedance spectroscopy.These findings show that combining dilute regeneration strategies, nanopatterned membranes, and PBA-modified electrodes enables MCDI to effectively manage higher salinity feeds, expanding its potential for seawater desalination. References Saline water and salinity. (n.d.). USGS. Retrieved March 25, 2025, from https://www.usgs.gov/special-topics/water-science-school/science/saline-water-and-saliniPalakkal, V. M., Rubio, J. E., Lin, Y. J., & Arges, C. G. (2018). Low-resistant ion-exchange membranes for energy efficient membrane capacitive deionization. ACS Sustainable Chemistry & Engineering, 6(11), 13778-13786.Tang, K., Kim, Y. H., Chang, J., Mayes, R. T., Gabitto, J., Yiacoumi, S., & Tsouris, C. (2019). Seawater desalination by over-potential membrane capacitive deionization: Opportunities and hurdles. Chemical engineering journal, 357, 103-111.Hasan, M., Shrimant, B., Waters, C. B., Gorski, C. A., & Arges, C. G. (2024). Reducing Ohmic Resistances in Membrane Capacitive Deionization Using Micropatterned Ion‐Exchange Membranes, Ionomer Infiltrated Electrodes, and Ionomer‐Coated Nylon Meshes. Small Structures, 5(9), 2400090.Zhang, X., & Dutta, J. (2021). X-Fe (X= Mn, Co, Cu) Prussian blue analogue-modified carbon cloth electrodes for capacitive deionization. ACS Applied Energy Materials, 4(8), 8275-8284.Pothanamkandathil, V., Boualavong, J., & Gorski, C. A. (2023). Open-circuit potential drift in intercalation electrodes: role of charge redistribution in a prussian blue analog. Journal of The Electrochemical Society, 170(11), 110503. Figure 1

Comparison of energy consumption in desalination by capacitive deionization and reverse osmosis

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.

Electro-driven technologies are viewed as a potential alternative to the current state-of-the-art technology, reverse osmosis, for the desalination of brackish waters. Capacitive deionization (CDI), based on the principle of electrosorption, has been intensively researched under the premise of being energy efficient. However, electrodialysis (ED), despite being a more mature electro-driven technology, has yet to be extensively compared to CDI in terms of energetic performance. In this study, we utilize Nernst-Planck based models for continuous flow ED and constant-current membrane capacitive deionization (MCDI) to systematically evaluate the energy consumption of the two processes. By ensuring equivalently sized ED and MCDI systems-in addition to using the same feed salinity, salt removal, water recovery, and productivity across the two technologies-energy consumption is appropriately compared. We find that ED consumes less energy (has higher energy efficiency) than MCDI for all investigated conditions. Notably, our results indicate that the performance gap between ED and MCDI is substantial for typical brackish water desalination conditions (e.g., 3 g L-1 feed salinity, 0.5 g L-1 product water, 80% water recovery, and 15 L m-2 h-1 productivity), with the energy efficiency of ED often exceeding 30% and being nearly an order of magnitude greater than MCDI. We provide further insights into the inherent limitations of each technology by comparing their respective components of energy consumption, and explain why MCDI is unable to attain the performance of ED, even with ideal and optimized operation.

Desalination technologies are expected to play an important role in producing clean water in the future, resulting a surge in the research and development of energy efficient and cost effective technologies for desalination of seawater and brackish water. Membrane-based desalination technologies such as reverse osmosis (RO) are the most commercially used desalination methods for seawater and treatment of agricultural water, but are highly energy intensive and the future development of desalination is dependent on finding more energy efficient technologies. Capacitive deionization (CDI) is an emerging technology for water desalination, and is based on the phenomenon of ion electrosorption. Simple CDI is an energy efficient technology and its applications for desalination of low molar concentration streams, like brackish water, is demonstrated. However, to expand the applications, research is focused on improving the efficiency and salt removal capacity of CDI systems. To this end, the important requirements are finding electrode materials with higher capacities and CDI systems with higher efficiencies. Also, the large-scale application of CDI for personal and industrial applications is dependent on designing CDI systems with potential to be implemented at different capacities and length scales. In this work, we have studied the CDI for removal of salt and nutrients from agricultural water. Various high surface area electrode materials were considered and the efficiency of CDI for removal is measured via downstream analyses on batch experiments and reported, allowing development of preliminary parameters for optimizing CDI systems for high-strength agricultural wastewaters.

Water and energy shortages came due to rapid population growth, living standards and rapid development in the agriculture and industrial sectors. Desalination tends to be one of the most promising water solutions; however, it is a process of intense energy. Membrane Capacitive Deionization (MCDI) has received considerable interest as a promising desalination technology, and MCDI research has increased significantly over the last 10 years. In addition, there are no guidelines for the design of Capacitive Deionization (CDI) implementation strategies for individual applications. This study, therefore; provides an alternative of CDI’s recent application developments, with emphasis placed on hybrid systems to address the technological needs of different relevant fields. The MCDI’s energy consumption is compared with the reverse osmosis literature data based on experimental data from laboratory-scale system. The study demonstrates that MCDI technology is a promising technology in the next few years with an extreme competition in water recovery, energy consumption and salt removal for reverse osmosis.

Seawater Use and Desalination Technology