High desalination performance by contribution of pseudocapacitive behaviors in hierarchical activated carbon mesoporous electrodes derived from rubber seed shells for membrane capacitive deionization (MCDI): Optimization and electrosorption mechanism

Machine learning models guided optimal control toward membrane capacitive deionization system with exceptional energy recovery rate and desalination rate

ABSTRACT Electrochemical CO 2 capture offers a tunable, low‐temperature alternative to thermal methods. Among available strategies, bipolar membrane electrodialysis (BPMED) and capacitive deionization (CDI) are notable for their distinct mechanisms. BPMED induces pH swings via water dissociation, while CDI concentrates CO 2 ‐related ions through electric double‐layer adsorption. This review provides a comparative evaluation of BPMED and CDI in terms of working principles, energy performance, system integration, and application scenarios, including direct air capture (DAC), carbon capture from industrial flue gas, and direct ocean capture (DOC). BPMED demonstrates high‐capture rates and compatibility with in situ mineralization, whereas CDI offers lower energy demand and modular flexibility. Their respective strengths suggest potential complementarity—CDI may be better suited to treat liquid phase systems derived from point‐source emissions, in which dissolved inorganic carbon species dominate the ionic composition and the background of competing ions is relatively controllable; BPMED may be better suited for treating environmental carbon sources with large volumes, low concentrations or high ionic strength. This framework offers potential insights for developing scalable electrochemical CO 2 capture systems.

Preparation of porous SPES/PES cation exchange membrane with interconnected spongy morphology for membrane capacitive deionization

Chemically and physically modified PVA/CA: High-performance sustainable material for membrane capacitive deionization

A significant advantage of membrane capacitive deionization (MCDI) lies in its ability to achieve medium to high water recovery rates. A prototype of MCDI unit demonstrated a recovery around 68% while consistently achieving salt removal efficiencies of ≥ 90% from feed water with a total dissolved solids (TDS) concentration of 1490 mg/L. However, the presence of coagulant-derived multivalent ions, particularly Fe2+, Fe3+, and Al3+, poses a challenge to long-term salt rejection efficiency. When Fe3+ or Al3+ was present at concentrations near 10 mg/L in feed water with a TDS of ~400 mg/L, the residual iron or aluminum concentration in the treated water exceeded the permissible limits defined by drinking water standards. Despite high removal efficiencies (> 90%) for key cations including Na+, Ca2+, Mg2+, Al3+, Fe2+, and Fe3+, regeneration studies revealed a distinct desorption trend: Mg2+ > Na+ > Ca2+ > Al3+ > Fe2+ ≈ Fe3+. This trend indicates that Fe3+ and Fe2+ are the most strongly retained and thus the most scale-forming ion in MCDI systems, followed by Al3+. Salt adsorption capacity of NaCl is 0.66-4.14 mg/g and modeled using the modified Donnan model effectively described the nonlinear adsorption behavior and also for all other systems with and without coagulant ions. Due to the presence of divalent ions, Donnan potential decreased compared to NaCl system without coagulant ions. The presence of coagulant ions further decreased the Donnan potential. Energy consumed 68.2-78.6 kT/ion and mostly increased to 60.6-101.3 kT/ion during partially choked condition. Post-operational surface analyses using x-ray photoelectron spectroscopy (XPS) and energy-dispersive spectroscopy (EDS) confirmed the accumulation of these metal ions on the carbon electrode surfaces. The observed deposition of oxide and hydroxide of coagulant ions significantly impacts long-term MCDI performance, underscoring the need for pretreatment strategies and electrode material optimization to enhance the sustainability and effectiveness of MCDI in domestic water purification applications.

ABSTRACT Advanced desalination technologies are crucial for addressing the growing global freshwater shortage crisis. Capacitive deionization (CDI) is a relatively new energy‐efficient desalination technology that has emerged in recent years. Advanced electrode materials are crucial for CDI. While research predominantly targets cathode materials for Na + capture, the equally critical challenge of chloride ion Cl − removal remains relatively underexplored, and electrode materials specifically designed for efficient chloride ion removal in CDI have received relatively little attention, limiting the progress of CDI‐based desalination technology. Here, we developed a viologen‐based cationic covalent organic frameworks (COFs)‐based active anode material exhibiting unique pseudocapacitive behavior and excellent chemical stability. Experiments demonstrate that an asymmetric electrode based on the TAPT‐BDB‐COF(Cl − )//MnO 2 (Na + ) system exhibits a low specific energy consumption of 80.70 kJ mol − 1 NaCl and maintains high capacitance retention after 500 cycles. A membrane capacitive deionization (MCDI) system assembled based on this material achieves a specific adsorption capacity of 87.2 mg g − 1 at 1.5 V and a low salt concentration of 500 mg L − 1 , increasing to 124.57 mg g − 1 at a high salt concentration of 3000 mg L − 1 . Performance remains stable after 20 desalination/regeneration cycles. This material serves as a valuable reference for the development of efficient electrode materials for capacitive deionization and chlorine removal technologies.

In-situ triboelectric nanogenerator for energy self-cycling electrochemical systems in wastewater treatment.

Integration of transition metal onto alginate activated carbon (M-AAC) for enhancing molybdate electro-adsorption.

Self-N-doped hierarchical porous carbon electrodes derived from shrimp shells for high-performance membrane capacitive deionization

Membrane capacitive deionization by spent coffee grounds electrodes for lithium recovery

Liquid nitrogen quenching treatment stabilizes high-spin V ions in V2C/V1.156Se2 heterostructure for enhanced sodium ions capture in membrane capacitive deionization

Lignin-free wood pulp-derived graphenic carbon electrodes: Unlocking superior electrosorption capacity and application in membrane capacitive deionization

Effective removal and concentration of perfluoroalkyl substances (PFAS) using asymmetric membrane capacitive deionization with stop–flow operation

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

Faradaic deionization (FDI) using intercalation materials with cation-blocking membranes shows promise for energy-efficient desalination. Recently, we demonstrated for the first time the use of a symmetric FDI cell comprising two nickel hexacyanoferrate electrodes separated by an anion-exchange membrane to desalination feeds with seawater salinity down to near freshwater salinity at low energy consumption (Energy Environ. Sci., 2023, 16, 3025). However, reliance on ion-exchange membranes (IEMs) greatly increases the capital cost of FDI, limiting widespread adoption and commercialization. Herein, we investigate the desalination performance of a symmetric FDI cell that uses nanofiltration membranes as separators. The dynamic salt depletion/enrichment mechanism of the cell renders a salt removal that depends on a characteristic time constant and a Damköhler number, which represents the ratio of the rate of salt depletion caused by cation intercalation to the rate of salt diffusion through the NF membrane. This theory predicts that such an IEM-free symmetric FDI cell can produce freshwater from brackish water, though not from seawater. Experimental validation demonstrates the ability of the cell to desalinate 5 g/L NaCl and 3.2 g/L of Instant OceanÒ containing multiple salts to freshwater (< 1 g/L) and drinkable water (< 0.5 g/L), respectively. The cell exhibits a volumetric specific energy consumption of 1.4 – 2.2 kWh/m3, which is comparable to brackish water reverse osmosis, electrodialysis, or membrane capacitive deionization. Controlled experiments using IEMs reveal that salt diffusion through the NF membrane consumed 35 – 50% of the total charge passed to the cell, decreasing with increasing currents, whereas 25 – 40% of the charge is lost by other processes, which increases with currents. Unexpectedly, water recovery (WR) is shown in IEM-free SFDI to increase with the amount of charge transferred during batch-type experiments, contrasting with the usual decrease of WR that accompanies using IEMs.

Membrane capacitive deionization (MCDI) offers energy‐efficient seawater desalination but is limited at high salinity by membrane resistance and incomplete electrode regeneration. Nanopatterned ion‐exchange membranes, dilute regeneration protocols, and Prussian blue analog (PBA)‐functionalized electrodes are combined in a flow‐by‐MCDI cell. Nanopatterned ion‐exchange membranes (hexagonal, octagonal, double‐ring, rectangular) enhance interfacial ion transport, with hexagonal geometry delivering ≈12.5% greater surface area and the best performance. PBA‐functionalized electrodes increase salt adsorption and charge‐transfer kinetic rates. The integrated system lowers the area‐specific resistance by 45 Ω cm2, resulting in a 500 mV reduction in the cell voltage for a current density of 2 mA cm−2 for a 35 000 ppm NaCl feed. This improves the energy‐normalized salt adsorption six fold (64–382 mmol J−1). Low salinity (2000 ppm) and mixed‐salt regeneration sustains a ≈39% water recovery and stable performance for at least seven cycles. Overall, combining nanopatterned membranes, which promote confinement‐enhanced ion mobility, and PBA electrodes, which enhance salt adsorption, improved the energy efficiency of MCDI.

Selective capture of nitrate is a critical process for water purification and resource circularity.1 The major challenge of developing selective adsorbent and membrane materials towards nitrate has arisen from similar physical properties between nitrate and competing anions such as chloride.2 Redox active polymers with nitrogen-containing functional groups such as polyaniline (PANI) have shown promising nitrate adsorption capacities (1.13 mmol/g polymer) but with limited nitrate selectivity (αNO3/Cl = 3.2).3 To extend beyond the nitrate selectivity of the conventional redox polymers, we propose a novel material design to separate nitrate based on their solvation properties. In this work, we design functional polyaniline redox-polymers as highly selective electrosorbents towards nitrate by controlling the polymer surface hydrophobicity through synthetic functionalization. We elucidated the mechanisms behind the exceptional nitrate selectivity through a combination of ab initio molecular dynamics (AIMD) and in-situ electrochemical quartz crystal microbalance (EQCM) studies. The hydrophobicity of the alkylated PANIs reduces chloride binding, thus enhancing electrosorptive selectivity towards nitrate. Through technoeconomic analysis (TEA), we report a 50% lower estimated nitrate removal costs using PNMA electrode compared to the scenario using non-functional PANI due to enhancement of selectivity and uptake, and a corresponding decrease in energy consumption per nitrate ion removed. Cho, K.-H.; Chen, C.-Y.; Aguda, A.; Fournier, M. J.; Su, X., Toward sustainable electrochemically mediated separations driven by renewable energy. Joule 2024.Tsai, S.-W.; Wu, M.-C.; Ng, H. Y.; Hou, C.-H., Advancing the Electrosorption Selectivity of Nitrate Through Fine-Tuning Hydrophobic Ammonium Functional Groups in Anion Exchange Membranes for Membrane Capacitive Deionization. ACS ES&T Water 2024, 4 (12), 5598-5607.Kim, K.; Zagalskaya, A.; Ng, J. L.; Hong, J.; Alexandrov, V.; Pham, T. A.; Su, X., Coupling nitrate capture with ammonia production through bifunctional redox-electrodes. Nature Communications 2023, 14 (1).

Vacancy and defect engineering in N, S co-doped carbon Nanosheets: From waste valorization to superior desalination via membrane capacitive deionization

This study compares the operational performance of membrane capacitive deionization (MCDI) and circulation-MCDI (C-MCDI) under symmetric (2/2, 3/3, 4/4 min) and asymmetric (5/2, 5/3, 5/4 min) adsorption/desorption cycles to identify efficient operating conditions at the pilot scale. A pilot system was tested with a NaCl solution of about 1000 mg/L, and 15 consecutive cycles were conducted to evaluate removal efficiency, specific energy consumption (SEC), and stability. MCDI consistently achieved over 90% removal efficiency with SEC below 0.6 kWh/m3 across all modes, maintaining stable performance over 15 cycles. The 2/2 condition provided the shortest cycle time and the highest treated water productivity, making it the most efficient condition for the pilot-scale MCDI tested. C-MCDI showed stronger dependence on operating conditions, with the number of stable cycles ranging from 3 to 7 depending on desorption duration. Nevertheless, the 5/2 condition achieved about 91% removal efficiency with 0.58 kWh/m3 SEC, and its extended adsorption period yielded about 2.5 times more treated water per cycle than the 2/2 case. Overall, this work provides a comparative pilot-scale evaluation of MCDI and C-MCDI, highlighting their advantages, limitations, and potential applications, and offering practical insights for energy-efficient and sustainable desalination strategies.