(Invited) Using Intercalation Compounds for Electrochemical Water Desalination

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 characteristics of compressed granular activated carbon

Capacitive deionization (CDI) is an effective method for removing salt from brackish water. In this work, we systematically investigated the effect of temperature and impregnation route on the synthesis of activated carbon from natural anthracite, and its impact on salt removal efficiency (η), salt adsorption capacity (SAC), charge efficiency (Λ), and energy consumption in a symmetric CDI cell. Furthermore, the physical properties of the resulting activated carbon samples were identified by several analysis techniques including X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS) and N 2 sorption‐desorption. In addition, the desalination performances of the electrodes material were assessed by single capacitive deionization (CDI) cell using batch mode in a 50 ppm of NaCl solution at 1.2 V. The sample synthesis with solid impregnation at 900 °C demonstrated a superior performance compared to other activated carbon samples, with a high specific surface area of 3909 m 2 . g −1 , good salt adsorption capacity at 11.15 mg . g −1 and low energy consumption at 185.45 kJ . mol −1 . Notably, the good salt adsorption capacity (SAC) is a direct result of the high surface area, which is achieved through a high proportion of micropores generated during the activation process.

Prussian blue analogs, e.g., nickel hexacyanoferrate, NiFe(CN)6 or NiHCF, are promising candidates as low-cost and high-rate intercalation materials for secondary batteries.1–4 Recently, this material class has been shown to possess tremendous potential for a novel energy-efficient water desalination approach.5–8 Rising water demands are exacerbating water scarcity in many world regions. It is estimated that 60% of the global population will face severe water scarcity by 2025.9 The growing water demand necessitates new desalination technologies with high energy efficiency, low capital and operating cost and high freshwater output. In this work, we assess the performance and lifetime of electrochemical water desalination cells based on sodium intercalation into nickel hexacyanoferrate.10–12 The battery desalination cells feature a symmetric design, with two NiHCF electrodes at opposite state-of-charge (SOC), capable of intercalating Na+-ions into their crystal structure. The electrodes are separated by an anion exchange membrane, a porous functionalized polyether ether ketone (PEEK) membrane, that only permits negatively charged ions, e.g., Cl--ions, to pass. Two feed water streams with 20 mM NaCl enter the symmetric cell on either side (see Figure 1a). During charge of the symmetric cell, incoming Na+-ions are removed from one water stream and intercalated into the NiHCF electrode at low SOC. Simultaneously, Na+-ions are deintercalated from the opposite NiHCF electrode at high SOC. In order to maintain charge neutrality, Cl--ions cross the anion exchange membrane. Thus, during every charge/discharge cycle, one water stream is desalinated forming a freshwater stream, while the other is enriched in NaCl forming a brine waste stream (see Figure 1b).In order to quantify performance and lifetime of the novel battery-type water desalination cells, we define and measure objective metrics. We see that energy consumption (Wh/l) and productivity (l/h/m2) of NiHCF/NiHCF cells are superior to cells based on membrane capacity deionization (mCDI). Stable charge/discharge cycling of NiHCF/NiHCF cells can be achieved for over 500 cycles with NaCl feed water, but rapid aging is observed with CaCl2 feeds. Synchrotron-based characterization of NiHCF/NiHCF cells is used to elucidate the reason for capacity fade. X-ray absorption spectroscopy and X-ray fluorescence spectroscopy reveal Fe dissolution from the NiHCF active material as a primary aging mode with CaCl2 water feeds. References Wessells, C. D., Peddada, S. V., Huggins, R. A. & Cui, Y. Nickel hexacyanoferrate nanoparticle electrodes for aqueous sodium and potassium ion batteries. Nano Lett. 11, 5421–5425 (2011).Wessells, C. D. et al. Tunable reaction potentials in open framework nanoparticle battery electrodes for grid-scale energy storage. ACS Nano 6, 1688–1694 (2012).Pasta, M. et al. Full open-framework batteries for stationary energy storage. Nat. Commun. 5, 1–9 (2014).Firouzi, A. et al. Monovalent manganese based anodes and co-solvent electrolyte for stable low-cost high-rate sodium-ion batteries. Nat. Commun. 9, (2018).Pasta, M., Wessells, C. D., Cui, Y. & La Mantia, F. A desalination battery. Nano Lett. 12, 839–843 (2012).Lee, J., Kim, S. & Yoon, J. Rocking Chair Desalination Battery Based on Prussian Blue Electrodes. ACS Omega 2, 1653–1659 (2017).Kim, T., Gorski, C. A. & Logan, B. E. Low Energy Desalination Using Battery Electrode Deionization. Environ. Sci. Technol. Lett. 4, 444–449 (2017).Porada, S., Shrivastava, A., Bukowska, P., Biesheuvel, P. M. & Smith, K. C. Nickel Hexacyanoferrate Electrodes for Continuous Cation Intercalation Desalination of Brackish Water. Electrochim. Acta 255, 369–378 (2017).Jones, E., Qadir, M., van Vliet, M. T. H., Smakhtin, V. & Kang, S. mu. The state of desalination and brine production: A global outlook. Sci. Total Environ. 657, 1343–1356 (2019).Metzger, M. et al. Techno-economic analysis of capacitive and intercalative water deionization. Energy Environ. Sci. 13, 1544–1560 (2020).Sebti, E. et al. Removal of Na+ and Ca2+ with Prussian blue analogue electrodes for brackish water desalination. Desalination 487, (2020).Besli, M. M. et al. Performance and lifetime of intercalative water deionization cells for mono- and divalent ion removal. Desalination 517, 115218 (2021). Figure 1. (a) Battery-type water desalination approach in symmetric NiHCF/NiHCF cells with two salt water streams entering the cell and a brine stream and freshwater stream exiting the cell. (b) During galvanostatic charge/discharge cycling the salt concentrations of brine and freshwater stream can be monitored with microfluidic operando conductivity probes to determine important performance metrics. Figure 1

Prussian blue analogs (PBAs) represent a crucial class of intercalation electrode materials for electrochemical water desalination. It is shown here that structural/compositional tailoring of PBAs, the nickel hexacyanoferrate (NiHCF) electrodes in particular, can efficiently modulate their capacitive deionization (CDI) performance (e.g., desalination capacity, cyclability, selectivity, etc.). Both the desalination capacity and the cyclability of NiHCF electrodes are highly dependent on their structural/compositional features such as crystallinity, morphology, hierarchy, and coatings. It is demonstrated that the CDI cell with hierarchically structured NiHCF nanoframe (NiHCF-NF) electrode exhibits a superior desalination capacity of 121.38mg g-1 , a high charge efficiency of up to 82%, and a large capacity retention of 88% after 40 cycles intercalation/deintercalation. In addition, it is discovered that coating of carbon (C) film over NiHCF can lower its desalination capacity owing to the partial blockage of diffusion openings by the coated C film. Moreover, the hierarchical NiHCF-NF electrode also demonstrates a superior selectivity toward monovalent sodiumions (Na+ ) over divalent calcium (Ca2+ ) and magnesim (Mg2+ ) ions, allowing it to be a promising platform for preferential capturing Na+ ions from brines. Overall, the structural/compositional tailoring strategies would offer a viable option for the rational design of other intercalation electrode materials applied in CDI techniques.

Selective capture of ammonium ions from municipal wastewater treatment plant effluent with a nickel hexacyanoferrate electrode

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

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

Enhanced performance stability of carbon/titania hybrid electrodes during capacitive deionization of oxygen saturated saline water

Capacitive Deionization (CDI) is one of the most relevant emerging desalination technologies. CDI is based on the electrosorption of salts using a pair of electrically charged porous electrodes. In this way the CDI process consists on charging (ion removal)/discharging (ion desorption) cycles using the same operational principles of the supercapacitors. Therefore, CDI cells might potentially allow to recover part of the energy consumed in the desalination process while delivering drinking water. One of the key elements in this technology are, as in the supercapacitor field, the electrodes. In accordance with this, intensive research has been conducted in order to find the best electrode candidates in terms of specific surface area (SSA), porosity, electrical conductivity, surface chemistry and optimized chemical surface charge of the electrode. The combination of these properties are expected to provide higher values of salt adsorption capacity (SAC) and average salt adsorption rate (ASAR). Moreover, it should be also remarked that one of the critical requirements in order to make CDI competitive with current desalination technologies (such as reverse osmosis or electrodialysis) is the cost of the electrode materials. In order to address this challenge, the preparation of carbon electrodes from abundant biowaste precursors was studied as a low cost and green alternative to more innovative carbon composite complex materials.In this work the authors decided to evaluate different activation treatments of a biowaste precursor in order to obtain suitable activated carbons for capacitive deionization. The different structural properties (BET SSA, pore size distribution, chemical surface groups) of the synthesized AC’s were correlated not only with their electrochemical response (specific capacitance) but also with the CDI performance (SAC, electrosorption kinetics, charge efficiency and the energy consumption).

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

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

Graphite felt 3D framework composites as an easy to scale capacitive deionization electrode for brackish water desalination

In parallel to the depletion of potable water reservoirs, novel technologies have been developed for seawater softening, as it is the most abundant source for generating deionized water. Although salt removal at subosmotic pressures and ambient temperatures by applying low-operating potentials with high energy efficiency made capacitive deionization (CDI) an advantageous water-softening process, its practical application is limited by insufficient ion removal capacity and low concentration influent. The performance of a CDI system is in progress with engineering the electrode active materials, also facilitating the advance design in highly saline- and seawater study. Herein, an innovative strategy was developed to provide high-performance CDI systems based on efficient and electrochemical ion-uptake active materials with a simple initial preparation. Nitrogen-doped porous carbons (N-pCs) received benefits from a high specific surface area and good surface wettability. The N-pCs were modified with molybdenum oxide/sulfide intercalative array and developed as CDI electrode active materials for desalination of both low/medium saline- and seawater. The MoS2/S,N-pC electrode materials exhibited perfect optimized salt adsorption capacity (SACs) of 47.9 mg g-1 when compared to N-pC (37.9 mg g-1) and MoO3/N-pC (39.6 mg g-1) counterparts at 1.4 V in a 750 ppm NaCl solution. In addition, the assembled CDI cells exhibited reasonable cycle stability and retained 96.7% of their initial SAC in continuous CDI cycles for 128,000 s. The fabricated CDI cell rendered an excellent salt removal efficiency (SRE, %) of 13.34% from the real seawater sample at 1.2 V. In detail, the SRE % of the NaCl, KCl, MgCl2, and CaCl2 soluble salts with respect to seawater sample exhibited a remarkable SRE % of 30.8%, 36%, 32.6%, and 19.3%, respectively. These SRE % values (>13.34%) provide convincing evidence on the reasonable ion uptake capability of the fabricated CDI cells for removing Na+, K+, Mg2+, and Ca2+ ions compared to other soluble component. The advanced cell design parallel to the promising outcomes provided herein makes these CDI systems immensely propitious for efficient water softening.

Carbon electrodes with ionic functional groups for enhanced capacitive deionization performance