Pseudocapacitive Behaviors of Polypyrrole Grafted Activated Carbon and MnO2 Electrodes to Enable Fast and Efficient Membrane-Free Capacitive Deionization

Enhanced phosphate recovery in R-MCDI systems: Synergistic effects of modified electrodes and membrane-electrode-current collector assembly

Efficient phosphorus (P) recovery is critical for sustainable wastewater management and resource reuse. This study optimized a reservoir of membrane capacitive deionization (R-MCDI) system by integrating acid-modified activated carbon cloth (ACC) electrodes and a membrane-electrode-current collector assembly (MECA) configuration. Acid modification enhanced the electrode's specific surface area, microporosity, and carboxyl group content, while reducing charge transfer resistance, significantly improving P recovery and selectivity. The ACC-42 electrode achieved optimal performance, achieving a 52% P recovery efficiency and low energy consumption of 8.8 kWh/kg P. The MECA configuration further amplified P recovery by optimizing electric field distribution and maximizing electrode utilization, achieving a fourfold higher recovery rate (0.081 μmol·cm-2·min-1) while reducing energy consumption by 59% compared to alternative setups. Multi-cycle operations validated the system's robustness, with P concentrations reaching 397 mg/L in the electrode chamber and a nearly 15-fold increase in selectivity for P over sulfate. This study highlights the synergistic effects of electrode modification and assembly configuration in enhancing R-MCDI performance, providing a scalable and energy-efficient solution for nutrient recovery in wastewater treatment.

High-efficiency electrochemical desalination enabled by nanosheet-structured redox polymer for sustainable and versatile water treatment

Hybrid capacitive deionization (HCDI) has emerged as a promising desalination technology, but its development is hindered by the lack of high-performance organic electrodes with abundant redox-active sites and robust structural stability. To address this challenge, we design a novel two-dimensional π-conjugated polymer (SFPBI) through molecular engineering, integrating multiple redox-active moieties with aromatic reinforcement strategies. Comprehensive electrochemical analysis, supported by in-situ spectroscopic characterization and theoretical calculations, reveals that SFPBI polymer, enriched with C=N and C=O functional groups, enables efficient pseudocapacitive ion adsorption. Its rigid backbone and extensive electron delocalization, characterized by a narrow HOMO-LUMO gap (2.99 eV), ensure exceptional structural stability and electrochemical activity. A HCDI device incorporating the SFPBI electrode achieves a remarkable salt removal capacity of 79.43 mg g-1, a rapid average removal rate of 2.65 mg g-1 min-1, and excellent regeneration stability (∼92.01 % retention over 500 cycles), outperforming reported organic electrodes. As a proof of concept, we develop an integrated solar-powered desalination system using interconnected HCDI devices, which not only produces desalinated water meeting human consumption standards but also efficiently removes organic dyes and recovers energy. This study demonstrates a breakthrough in organic electrode design for HCDI, offering a scalable and energy-efficient solution for water desalination and purification.

A novel composite electrode with multiple pore structures for efficient treatment of heavy metal ions in capacitive deionization

Multiple porous carbon materials have great promise and potential in the capacitive deionization (CDI) field. Specific surface area (SSA), pore size distribution, and preparation method of CDI electrode materials are essential for the treatment of heavy metal ions. In this work, PPy composited porous carbon electrodes (hypercrosslinked polymers/polypyrrole, HCPs/PPy) were obtained by one-step crosslinked carbonization preparation and electro-deposition. The diverse pore structure gives the composite electrode a large SSA and excellent adsorption performance. HCPs/PPy-4 gives a high SSA of 251.26 m2/g. In the CDI process, the adsorption capacity of HCPs/PPy-4 for Fe3+, Cu2+, Pb2+, and Ag+ is 20.69 mg/g, 37.81 mg/g, 26.86 mg/g, and 40.95 mg/g. The negative electrode recoveries for the adsorption of the four ions were reached 81.2%, 89.2%, 85.5%, and 100%, respectively. It indicates that HCPs/PPy is a novel and potentially porous carbon electrode for high-performance CDI.

A highly stable full-polymer electrochemical deionization system: dopant engineering & mechanism study

Electrochemical deionization (ECDI) has emerged as a promising technology for water treatment, with faradaic ECDI systems garnering significant attention due to their enhanced performance potential. This study focuses on the development of a highly stable and efficient, full-polymer (polypyrrole, PPy) ECDI system based on two key strategies. Firstly, dopant engineering, involving the design of dopants with a high charge/molecular weight (MW) ratio and structural complexity, facilitating their effective integration into the polymer backbone. This ensures sustained contribution of strong negative charges, enhancing system performance, while the bulky dopant structure promotes stability during extended operation cycles. Secondly, operating the system with well-balanced charges between deionization and concentration processes significantly reduces irreversible reactions on the polymer, thereby mitigating dopant leakage. Implementing these strategies, the PPy(PSS)//PPy(ClO4) (PSS: polystyrene sulfonate) system achieves a high salt removal capacity (SRC) of 48 mg g-1, an ultra-low energy consumption (EC) of 0.167 kW h kgNaCl-1, and remarkable stability, with 96% SRC retention after 104 cycles of operation. Additionally, this study provides a detailed degradation mechanism based on pre- and post-cycling analyses, offering valuable insights for the construction of highly stable ECDI systems with superior performance in water treatment applications.

Order-in-disordered ultrathin carbon nanostructure with nitrogen-rich defects bridged by pseudographitic domains for high-performance ion capture

Carbon materials with defect-rich structure are highly demanded for various electrochemical scenes, but encountering a conflict with the deteriorative intrinsic conductivity. Herein, we build a highway-mediated nanoarchitecture that consists of the ordered pseudographitic nanodomains among disordered highly nitrogen-doped segments through a supramolecular self-assembly strategy. The "order-in-disorder" nanosheet-like carbon obtained at 800 °C (O/D NSLC-800) achieves a tradeoff with high defect degree (21.9 at% of doped nitrogen) and compensated electrical conductivity simultaneously. As expected, symmetrical O/D NSLC-800 electrodes exhibit superior capacitive deionization (CDI) performance, including brackish water desalination (≈82 mgNaCl g-1 at a cell voltage of 1.6 V in a 1000 mg L-1 NaCl solution) and reusage of actual refining circulating cooling water, outperforming most of the reported state-of-the-art CDI electrodes. The implanted pseudographitic nanodomains lower the resistance and activation energy of charge transfer, which motivates the synergy of hosting sites of multiple nitrogen configurations. Our findings shed light on electrically conductive nanoarchitecture design of defect-rich materials for advanced electrochemical applications based on molecular-level modulation.

Carbon dioxide-activated mesoporous date palm fronds carbon integrated with MnO2/polyaniline for highly efficient capacitive deionization of water

The continuous population growth and drying up the freshwater reservoirs around the world are increasing the demand for fresh water. Therefore, there is an urgent need to explore newer technologies able to purify water on large scales for human usage. Capacitive deionization is one of the most promising approaches to generate fresh water by the removal of salt ions from brackish water. In this work, we prepared three different capacitive deionization electrodes using carbonized palm tree fronds (PFC). These PFC activation was achieved using CO2 at 900°C. To generate the deionization electrodes, PFC activated carbon was combined with either polyaniline (PANI), MnO2, or both (PFC-PANI, PFC-MnO2, and PFC-MnO2-PANI). The MnO2 and PANI provided additional functionality and enhanced electrical conductivity, which resulted in much higher Na+ and Cl- ions adsorption. The BET surface area of PFC-MnO2-PANI was estimated to be 208.56 m2/g, which is approximately three times that of PCF-PANI and PFC-MnO2 alone. The morphological analysis showed that the PANI and MnO2 nanorods were well dispersed throughout the PFC network. Although PANI and MnO2 is largely embedded inside the PFC network, some remnants are visible on the surface of the electrodes. The cyclic voltammetry (CV) curves showed capacitive behavior of all electrodes in which PFC-MnO2-PANI showed highest specific capacitance of 84 F/g, while the PFC-MnO2 and PFC-PANI showed 42 and 43 F/g, respectively. Owing to its enhanced functionality and CV characteristics, the PFC-MnO2-PANI showed maximum salt adsorption capacity of 10.5 mg/g in contrast to 3.72 and 5.64 mg/g for PFC-MnO2 and PFC-PANI, respectively. Moreover, the measured contact angle for PFC-MnO2-PANI was ~51°, which indicates the hydrophilic nature of electrode that improved ions adsorption. PRACTITIONER POINTS: Date tree fronds were converted into mesopores carbon using CO2 as activation agent. Three composites were prepared with PANI, MnO2, and date palm fronds activated carbon (PFC-MnO2, PFC-MnO2-PANI, and PFC-PANI). Surface area, pore profile, surface morphology, electrochemical behavior, desalination performance, and hydrophilicity of all the electrodes were investigated. The PFC-MnO2-PANI showed maximum salt adsorption capacity of 10.5 mg/g in contrast to 3.72 and 5.64 mg/g for PFC-MnO2 and PFC-PANI, respectively.

Capacitive removal of Pb ions via electrosorption on novel willow biochar-manganese dioxide composites

Biochar derived from lignocellulosic biomass has been used as a low-cost adsorbent in wastewater treatment applications. Due to its rich porous structure and good electrical conductivity, biochar can be used as a cost-effective electrode material for capacitive deionization of water. In this work, willow biochar was prepared through carbonization of shrub willow chips, activated with potassium hydroxide, and loaded with manganese dioxide (WBC-K-MnO2 nanocomposite). The prepared materials were used to electrochemically adsorb Pb2+ from aqueous solutions. Under the applied potential of 1.0 V, the WBC-K-MnO2 electrode exhibited a high Pb2+ specific electrosorption capacity (23.3 mg/g) as compared to raw willow biochar (4.0 mg/g) and activated willow biochar (9.2 mg/g). KOH activation followed by MnO2 loading on the surface of raw biochar enhanced its BET surface area (178.7 m2/g) and mesoporous volume ratio (42.1%). Moreover, the WBC-K-MnO2 nanocomposite exhibited the highest specific capacitance value of 234.3 F/g at a scan rate of 5 mV/s. The electrosorption isotherms and kinetic data were well explained by the Freundlich and pseudo-second order models, respectively. The WBC-K-MnO2 electrode demonstrated excellent reusability with a Pb2+ electrosorption efficiency of 76.3% after 15 cycles. Thus, the WBC-K-MnO2 nanocomposite can serve as a promising candidate for capacitive deionization of heavy metal contaminated water.

Constructing ion-transport blockchain by polypyrrole to link CoTi-ZIF-9 derived carbon materials for high-performance seawater desalination

The instability and poor electronic conductivity of carbon materials derived from bimetallic zeolite imidazolate frameworks (ZIFs) pose significant challenges for utilizing these carbon materials as direct electrodes for achieving rapid electron transfer and high-performance capacitive deionization (CDI). However, modifying ZIFs through conductive polymers is a wise tool to enhance the target characteristics of ZIFs. Herein, a strategy is proposed to use polypyrrole (PPy) to interlink the carbon units derived from CoTi-ZIF-9 to construct a blockchain network system with high capacity and fast electrochemical kinetics for high performance CDI. In this system, PPy serves as a branched link connecting each carbon unit, so that the ions in the electrolyte can achieve low barrier and fast transmission in the three-dimensional network structure between the unit structures. As expected, with the improved charge transfer efficiency between electrode materials and electrolyte, the CDI cell exhibits excellent desalination capacity (77.3 mg g-1). In addition, density functional theory calculations also indicate that the introduction of PPy results in a higher electron density near the fermi surface of carbon material, which is conducive to electron transport and reaction kinetics. This work may provide important concepts for the design of CDI electrodes with high-conductivity and high-performance.

Enhanced Desalination Performance Using Phosphate Buffer-Mediated Redox Reactions of Manganese Oxide Electrodes in a Multichannel System

Water desalination mediated by electrochemical reactions to directly capture and release salt at electrode materials offers a low-voltage method for producing freshwater. Developing new system designs has allowed electrode materials to maximize their capacity for salt separation, especially when a multichannel system is used to introduce a separate electrode rinse solution. Here, we show that the use of an additive can provide a new strategy for improving electrode capacity and, hence desalination performance, which so far has been limited to increasing the electrolyte concentration. A custom-built, 2/2-channel flow cell divided by two cation exchange membranes and an anion exchange membrane was fed with 50 mM NaCl as the feed (two inner channels) and 0.5 M NaCl containing up to 0.1 M phosphate as the electrode rinse (two outer channels). Using manganese oxide electrodes with phosphate buffer-mediated redox reactions exhibited an improved desalination capacity of 68.0 ± 5.2 mg g-1 (0.55 mA cm-2) and a rate of 5.6 ± 1.3 mg g-1 min-1 (0.96 mA cm-2). The improvement was attributed to the buffer that served as a proton donor for promoting the H+ insertion reaction of amorphous or poorly crystalline MnO2. Additionally, the buffering capacity against acidification and the creation of insoluble manganese phosphate on the electrode surface prevented the dissolution of Mn2+, which could otherwise occur at the anode due to a decrease in the local pH upon H+ deinsertion. Thus, the use of manganese oxide electrodes coupled with phosphate provides a new strategy of increasing electrode capacity for water desalination.

In situ regulation of selectivity and permeability by electrically tuning pore size in trans-membrane ion process

Regulating ion transport behavior through pore size variation is greatly attractive for membrane to meet the need for precise separation, but fabricating nanofiltration (NF) membranes with tunable pore size remains a huge challenge. Herein, a NF membrane with electrically tunable pores was fabricated by intercalating polypyrrole into reduced graphene oxide interlayers. As the potential switches from reduction to oxidation, the membrane pore size shrinks by 11%, resulting in a 16.2% increase in salt rejection. The membrane pore size expands/contracts at redox potentials due to the polypyrrole volume swelling/shrinking caused by the insertion/desertion of cations, respectively. In terms of the inserted cation, Na+ and K+ induce larger pore-size stretching range for the membrane than Ca2+ due to greater binding energy and larger doping amount. Such an electrical response characteristic remained stable after multiple cycles and enabled application in ion selective separation; e.g., the Na+/Mg2+ separation factor in the reduced state is increased by 41% compared to that in the oxide state. This work provides electrically tunable nanochannels for high-precision separation applications such as valuable substance purification and resource recovery from wastewater.

Soft-hard interface design in super-elastic conductive polymer hydrogel containing Prussian blue analogues to enable highly efficient electrochemical deionization

The poor cycling stability of faradaic materials owing to volume expansion and stress concentration during faradaic processes limits their use in large-scale electrochemical deionization (ECDI) applications. Herein, we developed a "soft-hard" interface by introducing conducting polymer hydrogels (CPHs), that is, polyvinyl alcohol/polypyrrole (PVA/PPy), to support the uniform distribution of Prussian blue analogues (e.g., copper hexacyanoferrate (CuHCF)). In this design, the soft buffer layer of the hydrogel effectively alleviates the stress concentration of CuHCF during the ion-intercalation process, and the conductive skeleton of the hydrogel provides charge-transfer pathways for the electrochemical process. Notably, the engineered CuHCF@PVA/PPy demonstrates an excellent salt-adsorption capacity of 22.7 mg g-1 at 10 mA g-1, fast salt-removal rate of 1.68 mg g-1 min-1 at 100 mA g-1, and low energy consumption of 0.49 kW h kg-1. More importantly, the material could maintain cycling stability with 90% capacity retention after 100 cycles, which is in good agreement with in situ X-ray diffraction tests and finite element simulations. This study provides a simple strategy to construct three-dimensional conductive polymer hydrogel structures to improve the desalination capacity and cycling stability of faradaic materials with universality and scalability, which promotes the development of high-performance electrodes for ECDI.

Boosted brackish water desalination and water softening by facilely designed MnO2/hierarchical porous carbon as capacitive deionization electrode

Capacitive deionization (CDI) is a promising technique for brackish water desalination. However, its salt electrosorption capacity is insufficient for practical application yet, and little information is available on hardness ion (Mg²⁺, Ca²⁺) removal in CDI. Herein, hierarchical porous carbon (HPC) was prepared from low-cost and renewable microalgae via a simple one-pot approach, and both MnO₂/HPC and polyaniline/HPC (PANI/HPC) composites were then synthesized using a facile, one-step hydrothermal method. Compared with the MnO₂ electrode, the MnO₂/HPC electrode presented an improved hydrophilicity, higher specific capacitance, and lower electrode resistance. The electrodes exhibited pseudocapacitive behaviors, and the maximum salt electrosorption capacities of MnO₂/HPC-PANI/HPC CDI cell was up to 0.65 mmol g^-1 NaCl, 0.71 mmol g^-1 MgCl₂, and 0.76 mmol g^-1 CaCl₂, respectively, which were comparable and even higher than those of the previously reported CDI cells. Additionally, the MnO₂/HPC electrode presented a selectivity order of Ca²⁺ ≥ Mg²⁺ > Na⁺, and the divalent cation selectivity was found to be attributed to their stronger binding strength in the cavity of MnO_2. Multiscale simulations further reveal that the MnO₂/HPC electrodes with the unique luminal configuration of MnO₂ and HPC as supportive framework could offer a great intercalation selectivity of the divalent cations and exhibit a great promise in hardness ion removal.

Interlayer Structure and Chemistry Engineering of MXene-Based Anode for Effective Capture of Chloride Anions in Asymmetric Capacitive Deionization

Negatively charged surfaces and readily oxidizabile characteristics fundamentally restrict the use of MXene building blocks as anodes for anion intercalation. Herein, by embedding bacterial cellulose nanofibers with conformal polypyrrole coating (BC@PPy) and populating them between MXene (Ti₃C₂T_x) interlayers, we enable the fabricated MXene/BC@PPy (MBP) composite films to be highly efficient anodes for Cl^--capturing in asymmetric capacitive deionization (CDI) systems. Performance gains are realized due to the surface electronegativity of MXene nanosheets becoming compensated by positively charged BC@PPy nanofibers, alleviating electrostatic repulsion, thus realizing reversible Cl^- intercalation. More crucially, the anodization voltage of MBP is effectively enhanced as a result of the increase of the Ti valence state in MXene nanosheets with the addition of the BC@PPy spacer. Furthermore, BC@PPy nanopillars effectively enlarge the interlayer space for facile Cl^- de-/intercalation, improve the vertical electron transfer between loosely deposited MXene nanosheets, and perform as additional active materials for Cl^--capturing. Consequently, the MBP anode exhibits a promising desalination capacity of up to 17.56 mg g^-1 at 1.2 V with a high capacity retention of 94.6% after 30 cycles in an asymmetric CDI system. This work offers a simple and effective strategy to unlock the application potential of MXene building blocks as anodes for Cl^--capturing in electrochemical desalination.

In Situ Continuous Measurement of Salinity in Estuarine and Coastal Sediments by All-Solid Potentiometric Sensors

Salinity is crucial for understanding the environmental and ecological processes in estuarine and coastal sediments. In situ measurements in sediments are scarce due to the low water content and particulate adsorption. Here, a new potentiometric sensor principle is proposed for the real-time in situ measurement of salinity in sediments. The sensor system is based on paper sampling and two all-solid electrodes, a cation-selective electrode (copper hexacyanoferrate, CuHCF) and an anion-selective electrode (Ag/AgCl). The spontaneous aqueous electrolyte extraction and redox reaction can produce a Nernstian response on both electrodes that is directly related to salinity. This potentiometric sensor allows for salinity acquisition in a wide salinity range (1-50 ppt), with high resolution (<1 ppt), and at a low water content (<30%), and it has been applied for the in situ measurement of salinity and the interpretation of cycling processes of metals in estuarine and coastal sediments. Moreover, the sensor coupled to a wireless monitoring system exhibited remote-sensing capability and successfully captured the salinity dynamic processes of the overlying water and pore water during the tidal period. This sensor with its low cost, versatility, and applicability represents a valuable tool to advance the comprehension of salinity and the salinity-driven dissolved-matter variations in estuarine and coastal sediments.

Turning waste into valuables: In situ deposition of polypyrrole on the obsolete mask for Cr(VI) removal and desalination

The global mask consumption has been exacerbated because of the coronavirus disease 2019 (COVID-19) pandemic. Simultaneously, the traditional mask disposal methods (incineration and landfill) have caused serious environmental pollution and waste of resources. Herein, a simple and green mass-production method has been proposed to recycle carbon protective mask (CPM) into the carbon protective mask/polydopamine/polypyrrole (CPM/PDA/PPy) composite by in situ polymerization of PPy. The CPM/PDA/PPy composite was used for the removal of Cr(VI) and salt ions to produce clean water. The synergistic effect of PPy and the CPM improved the removal capability of Cr(VI). The CPM/PDA/PPy composite provided high adsorption capacity (358.68 mg g^-1) and economic value (811.42 mg $^-1). Consequently, the CPM/PDA/PPy (cathode) was combined with MnO₂ (anode) for desalination in CDI cells, demonstrated excellent desalination capacity (26.65 mg g^-1) and ultrafast salt adsorption rate (6.96 mg g^-1 min^-1), which was higher than conventional CDI cells. Our work proposes a new low-carbon strategy to recycle discarded masks and demonstrates their utilization in Cr(VI) removal and seawater desalination.

Preparation of MOF/polypyrrole and flower-like MnO2 electrodes by electrodeposition: High-performance materials for hybrid capacitive deionization defluorination

Fluorine pollution has become a global public health problem due to its adverse health effects. Adsorption is the primary method for removing fluoride from drinking water. However, the adsorption method has disadvantages such as difficulty in recovering the adsorbent, and the need to add additional chemicals for regeneration, thereby causing secondary pollution, which limits further industrial applications. Capacitive deionization (CDI), as an emerging water treatment technology, has attracted widespread attention due to its advantages of simple operation, low energy consumption and less environmental impact. In this study, a polypyrrole (PPy) film was prepared on a graphite substrate by electrodeposition, and then metal-organic framework Ce/Zn-BDC-NH₂ (CZBN) was deposited on the PPy film by electrophoretic deposition to obtain CZBN/PPy electrode was obtained. The CZBN/PPy anode was then coupled with the MnO₂ cathode for capacitive removal of fluoride in a CDI cell. Both CZBN/PPy and MnO₂ electrodes exhibit pseudocapacitive behavior, which can selectively and reversibly intercalate F^- (CZBN/PPy) and Na⁺ (MnO₂) ions. As expected, the CZBN/PPy-MnO₂ system exhibits excellent fluorine removal performance. In 1.2 V, 100 mg/L F^- solution, the F^- removal capacity can reach 55.12 mg/g. It has high F^- selectivity in the presence of some common anions, and can maintain high F^- removal ability even after five adsorption regeneration processes. The mechanism of F^- removal was studied by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). F^- was mainly removed by electrostatic interaction and ion exchange with hydroxyl. The excellent defluorination performance of the CZBN/PPy-MnO₂ system makes it have good practical application prospects.

Hierarchical MXene/Polypyrrole-Decorated Carbon Nanofibers for Asymmetrical Capacitive Deionization

Membrane capacitive deionization (MCDI) has emerged as a promising electric-field-driven technology for brackish water desalination and specific salt or charged ion separation. The use of carbon-based or pseudocapacitive materials alone for MCDI usually suffers from the drawbacks of low desalination capacity and poor cycling stability due to their limited accessible adsorption sites and obstructed charge-carrier diffusion pathways. Therefore, developing a hybrid electrode material with multiple charge storage mechanisms and continuous electron/ion transport pathways that can synergistically improve its specific capacitance and cycling durability has currently become one of the most critical technical demands. Herein, we developed a novel hierarchically architectured hybrid electrode by first spinning MXene into polyacrylonitrile (PAN)-based carbon nanofibers (MCNFs) to obtain a highly conductive carbon nanocomposite framework. The excellent spatial support structure can effectively prevent the dense packing of Cl^-- and DBS^--doped polypyrrole (PPy) molecular chains during the following electrodeposition process, which not only ensures the efficient transport of electrons in the continuous hybrid carbon nanofibrous skeleton but also provides abundant accessible sites for ion adsorption and insertion. The obtained self-supporting membrane electrodes (MCNF@PPy⁺Cl^- and MCNF@PPy⁺DBS^-) have the advantages of outstanding specific capacitance (318.4 and 153.9 F/g, respectively), low charge transfer resistance (10.0 and 5.3 Ω, respectively), and excellent cycling performance (78% and 90% capacitance retention ratios, respectively, after 250 electrochemical cycles). Furthermore, the asymmetrical membrane electrodes showed a superior desalination capacity of 91.2 mg g^-1 in 500 mg/L NaCl aqueous solution and obvious divalent ion (Ca²⁺, Mg²⁺) selective adsorption properties in high-salt water from the cooling towers of thermal power plants.

Polyaniline coated MOF-derived Mn2O3 nanorods for efficient hybrid capacitive deionization

Mn-based oxides are efficient pseudocapacitive electrode materials and have been investigated for capacitive deionization (CDI). However, their poor conductivity seriously affects their desalination performance. In this work, polyaniline coated Mn₂O₃ nanorods (PANI/Mn₂O₃) are synthesized by oxidizing a Mn-based metal organic framework (MOF) and subsequent in-situ chemical polymerization. The polyaniline not only acts as a conductive network for faradaic reactions of Mn₂O₃, but also enhances the desalination rate. PANI/Mn₂O₃ has a specific capacitance of 87 F g^-1 (at 1 A g^-1), superior to that of Mn₂O₃ nanorod (52 F g^-1 at 1 A g^-1). The hybrid CDI cell constructed with a PANI/Mn₂O₃ cathode and an active carbon anode shows a high desalination capacity of 21.6 mg g^-1, superior recyclability with only 11.3% desalination capacity decay after 30 desalination cycles and fast desalination rate of 2.2 mg g^-1 min^-1. PANI/Mn₂O₃ is a potential candidate for high performance CDI applications.

Hierarchical MXene/transition metal oxide heterostructures for rechargeable batteries, capacitors, and capacitive deionization

2D MXenes have attracted considerable attention due to their high electronic conductivity, tunable metal compositions, functional termination groups, low ion diffusion barriers, and abundant active sites. However, MXenes suffer from sheet stacking and partial surface oxidation, limiting their energy storage and water treatment development. To solve these problems and enhance the performance of MXenes in practical applications, various hierarchical MXene/transition metal oxide (MXene/TMO) heterostructures are rationally designed and constructed. The hierarchical MXene/TMO heterostructures can not only prevent the stacking of MXene sheets and improve the electronic conductivity and buffer the volume change of TMOs during the electrochemical reaction process. The synergistic effect of conductive MXenes and active TMOs also makes MXene/TMO heterostructures promising electrode materials for energy storage and seawater desalination. This review mainly introduces and discusses the recent research progress in MXene/TMO heterostructures, focusing on their synthetic strategies, heterointerface engineering, and applications in rechargeable batteries, capacitors, and capacitive deionization (CDI). Finally, the key challenges and prospects for the future development of the MXene/TMO heterostructures in rechargeable batteries, capacitors, and CDI are proposed.

Highly Selective Recovery of Phosphorus from Wastewater via Capacitive Deionization Enabled by Ferrocene-polyaniline-Functionalized Carbon Nanotube Electrodes

While capacitive deionization (CDI) is a promising technology for the recovery of nutrients from wastewater, a selective recovery of phosphate from the wastewater containing high concentrations of competing ions is still a huge challenge. Herein, we reported a ferrocene-polyaniline-functionalized carbon nanotube (Fc-PANI/CNT) electrode prepared through amidation reaction and chemical oxidation polymerization, aiming for a highly selective recovery of phosphorus from wastewater. The Fc-PANI/CNT electrode with a unique structure and high conductivity could efficiently adsorb phosphate ions from complex synthetic wastewater with a nearly 100% selectivity, mainly because the integration of ferrocene and an amide bond in Fc-PANI resulted in an enhanced charge transfer (Faradaic reactions) and a strong hydrogen bonding interaction with phosphate ions in its oxidized state. Density functional theory calculations showed that the binding energies of the oxidized Fc-PANI with HPO₄^2- and H₂PO₄^- were much greater than those of the oxidized Fc-PANI with other competing anions. The affinity of Fc-PANI/CNTs with phosphate can be controlled electrochemically based on the synergetic effects of Faradaic reactions and hydrogen bonding, enabling a selective recovery of phosphate through charging/discharging cycles. The phosphate adsorption capacity reached up to 35 mg PO₄^3- g^-1 in a NaCl/Na₂SO₄/NaNO₃/NaH₂PO₄ complex mixture at 1.2 V, outperforming most of the other reported CDI systems. The Fc-PANI/CNT electrode also exhibited a decent regeneration ability and durability during repeated CDI tests, demonstrating a great potential for the application of selective recovery and enrichment of phosphate from wastewater.

Recent progress in materials and architectures for capacitive deionization: A comprehensive review

Capacitive deionization is an emerging and rapidly developing electrochemical technique for water desalination across the globe with exponential growth in publications. There are various architectures and materials being explored to obtain utmost electrosorption performance. The symmetric architectures consist of the same material on both electrodes, while asymmetric architectures have electrodes loaded with different materials. Asymmetric architectures possess higher electrosorption performance as compared with that of symmetric architectures owing to the inclusion of either faradaic materials, redox-active electrolytes, or ion specific pre-intercalation material. With the materials perspective, faradaic materials have higher electrosorption performance than carbon-based materials owing to the occurrence of faradaic reactions for electrosorption. Moreover, the architecture and material may be tailored in order to obtain desired selectivity of the target component and heavy metal present in feed water. In this review, we describe recent developments in architectures and materials for capacitive deionization and summarize the characteristics and salt removal performances. Further, we discuss recently reported architectures and materials for the removal of heavy metals and radioactive materials. The factors that affect the electrosorption performance including the synthesis procedure for electrode materials, incorporation of additives, operational modes, and organic foulants are further illustrated. This review concludes with several perspectives to provide directions for further development in the subject of capacitive deionization. PRACTITIONER POINTS: Capacitive deionization (CDI) is a rapidly developing electrochemical water desalination technique with exponential growth in publications. Faradaic materials have higher salt removal capacity (SAC) because of reversible redox reactions or ion-intercalation processes. Combination of CDI with other techniques exhibits improved selectivity and removal of heavy metals. Operational parameters and materials properties affect SAC. In future, comprehensive experimentation is needed to have better understanding of the performance of CDI architectures and materials.

Biobased polyporphyrin derived porous carbon electrodes for highly efficient capacitive deionization

Recently, capacitive deionization (CDI) has attracted considerable interest as a potential desalination technique for seawater. It is thus desirable to develop low-cost, sustainable, and efficient electrode materials for desalination. In this study, the polyporphyrin was prepared by a one-pot reaction from biobased furan derivative, followed by activation to manufacture nitrogen-doped polyporphyrin derived porous carbons (NPPCs) for efficient capacitive deionization. In the presence of KOH as a pore activator, NPPCs exhibited cross-linked interconnected nanosphere chain-like structures inherited from the polyporphyrin backbone with coexisting mesopores and micropores, leading to extremely high specific surface area (2979.3 m² g^-1) and large pore volume (2.22 cm³ g^-1). The electrochemical measurements revealed good conductivity, outstanding stability, and extraordinary specific capacitance (328.7 F g^-1) of NPPCs, which can be ascribed to rich nitrogen content (8.0 at%) and high Pyrrolic nitrogen ratio. Due to their superior hierarchical porous structure and excellent electrochemical performance, the NPPC-800 electrodes presented a high salt adsorption capacity (SAC) of 35.7 mg g^-1 and outstanding cycling stability in 10 mM NaCl solution at 1.2 V during the desalination tests. This work demonstrates the utilization of biobased porous carbon material will pave a prospective way in sustainable and potential applications for CDI technique.

Recent Advances in Faradic Electrochemical Deionization: System Architectures versus Electrode Materials

Capacitive deionization (CDI) is an energy-efficient desalination technique. However, the maximum desalination capacity of conventional carbon-based CDI systems is approximately 20 mg g^-1, which is too low for practical applications. Therefore, the focus of research on CDI has shifted to the development of faradic electrochemical deionization systems using electrodes based on faradic materials which have a significantly higher ion-storage capacity than carbon-based electrodes. In addition to the common symmetrical CDI system, there has also been extensive research on innovative systems to maximize the performance of faradic electrode materials. Research has focused primarily on faradic reactions and faradic electrode materials. However, the correlation between faradic electrode materials and the various electrochemical deionization system architectures, i.e., hybrid capacitive deionization, rocking-chair capacitive deionization, and dual-ion intercalation electrochemical desalination, remains relatively unexplored. This has inhibited the design of specific faradic electrode materials based on the characteristics of individual faradic electrochemical desalination systems. In this review, we have characterized faradic electrode materials based on both their material category and the electrochemical desalination system in which they were utilized. We expect that the detailed analysis of the properties, advantages, and challenges of the individual systems will establish a fundamental correlation between CDI systems and electrode materials that will facilitate future developments in this field.

Hybrid capacitive deionization of NaCl and toxic heavy metal ions using faradic electrodes of silver nanospheres decorated pomegranate peel-derived activated carbon

Capacitive deionization (CDI) is an evolving technology for eradicating salt and toxic heavy metal ions from brackish wastewater. However, traditional CDI electrodes have lower salt adsorption capacity and inadequate adsorption of selective metal ions for long-term operations. Herein, Ag nanospheres incorporated pomegranate peel-derived activated carbon (Ag/P-AC) was prepared and implied to the CDI process for removing NaCl, toxic mono-, di-, and trivalent metal ions. Morphological analysis revealed that the 80-100 nm-sized Ag nanospheres were uniformly decorated on the surfaces of P-AC nanosheets. The Ag/P-AC has a higher specific surface area (640 m² g^-1), superior specific capacitance (180 F g^-1 at 50 mV s^-1) and a lower charge transfer resistance (0.5 Ω cm²). CDI device was fabricated by Ag/P-AC as an anode, which adsorbed anions and P-AC as cathode for adsorption of positively charged ions at 1.2 V in an initial salt concentration of 1000 mg L^-1. An asymmetric Ag/P-AC//P-AC exhibited a maximum NaCl adsorption capacity of 36 mg g^-1 than symmetric P-AC//P-AC electrodes (22.7 mg g^-1). Furthermore, Pb(II), Cd(II), F^-, and As(III) ions were successfully removed from simulated wastewater by using Ag/P-AC//P-AC based CDI system. These asymmetric CDI-electrodes have an excellent prospect for the removal of salt and toxic contaminants in industrial wastewater.

Selective Capacitive Removal of Heavy Metal Ions from Wastewater over Lewis Base Sites of S-Doped Fe-N-C Cathodes via an Electro-Adsorption Process

The pollution of toxic heavy metals is becoming an increasingly important issue in environmental remediation because these metals are harmful to the ecological environment and human health. Highly efficient selective removal of heavy metal ions is a huge challenge for wastewater purification. Here, highly efficient selective capacitive removal (SCR) of heavy metal ions from complex wastewater over Lewis base sites of S-doped Fe-N-C cathodes was originally performed via an electro-adsorption process. The SCR efficiency of heavy metal ions can reach 99% in a binary mixed solution [NaCl (100 ppm) and metal nitrate (10 ppm)]. Even the SCR efficiency of heavy metal ions in a mixed solution containing NaCl (100 ppm) and multicomponent metal nitrates (10 ppm for each) can approach 99%. Meanwhile, the electrode also demonstrated excellent cycle performance. It has been demonstrated that the doping of S can not only enhance the activity of Fe-N sites and improve the removal ability of heavy metal ions but also combine with heavy metal ions by forming covalent bonds of S^- clusters on Lewis bases. This work demonstrates a prospective way for the selective removal of heavy metal ions in wastewater.

Facile Designed Manganese Oxide/Biochar for Efficient Salinity Gradient Energy Recovery in Concentration Flow Cells and Influences of Mono/Multivalent Ions

Development of effective, environmentally friendly, facile large-scale processing, and low-cost materials is critical for renewable energy production. Here, MnO_x/biochar composites were synthesized by a simple pyrolysis method and showed high performance for salinity gradient (SG) energy harvest in concentration flow cells (CFCs). The peak power density of CFCs with MnO_x/biochar electrodes was up to 5.67 W m^-2 (ave. = 0.91 W m^-2) and stabilized for 500 cycles when using 1 and 30 g L^-1 NaCl, which was attributed to their high specific capacitances and low electrode resistances. This power output was higher than all other reported MnO₂ electrodes for SG energy harvest due to the synergistic effects between MnO_x and biochar. When using a mixture with a molar fraction of 90% NaCl and 10% KCl (or Na₂SO₄, MgCl₂, MgSO₄, and CaCl₂) in both feed solutions, the peak power density decreased by 2.3-40.1% compared to 100% NaCl solution with Ca²⁺ and Mg²⁺ showing the most pronounced negative effects. Our results demonstrated that the facile designed MnO_x/biochar composite can be used for efficient SG energy recovery in CFCs with good stability, low cost, and less environmental impacts. When using natural waters as the feed solutions, pretreatment would be needed.

Nano-manganese oxide and reduced graphene oxide-incorporated polyacrylonitrile fiber mats as an electrode material for capacitive deionization (CDI) technology

Capacitive deionization (CDI) is a trending water desalination method during which the impurity ions in water can be removed by electrosorption. In this study, nano-manganese dioxide (MnO2) and reduced graphene oxide (rGO)-doped polyacrylonitrile (PAN) composite fibers are fabricated using an electrospinning technique. The incorporation of both MnO2 and rGO nanomaterials in the synthesized fibers was confirmed by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The electrochemical characteristics of electrode materials were examined using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and constant current charge-discharge cycles (CCCDs). The specific capacitance of the PAN electrode increased with increasing MnO2 and rGO contents as well as when thermally treated at 280 °C. Thermally treated composite fibers with 17% (w/w) MnO2 and 1% (w/w) rGO (C-rGOMnPAN) were observed to have the best electrochemical performance, with a specific capacitance of 244 F g-1 at a 10 mV s-1 scan rate. The electrode system was used to study the removal of sodium chloride (NaCl), cadmium (Cd2+) and lead (Pb2+) ions. Results indicated that NaCl showed the highest electrosorption (20 472 C g-1) compared to two heavy metal salts (14 260 C g-1 for Pb2+ and 6265 C g-1 for Cd2+), which is most likely to be due to the ease of mass transfer of lighter Na+ and Cl- ions; When compared, Pb2+ ions tend to show more electrosorption on these fibers than Cd2+ ions. Also, the C-rGOMnPAN electrode system is shown to work with 95% regeneration efficiency when 100 ppm NaCl is used as the electrolyte. Hence, it is clear that the novel binder-free, electrospun C-rGOMnPAN electrodes have the potential to be used in salt removal and also for the heavy metal removal applications of water purification.

Selective Capacitive Removal of Pb2+ from Wastewater over Redox-Active Electrodes

Water pollution has become an environmental hazard. Diverse metal cations exist in wastewater; lead is the most common heavy metal pollutant among them. Selective removal of highly toxic and ultradiluted lead ions from wastewater is a major challenge for water purification. Here, selective capacitive removal (SCR) of lead ions from wastewater over redox-active molybdenum dioxide/carbon (MoO₂/C) electrodes was developed by an environment-friendly asymmetric capacitive deionization (CDI) method. The MoO₂/C spheres act as cathodes of an asymmetric CDI device and effectively reduce the concentration of Pb²⁺ from 50 ppm to <0.21 ppb. Moreover, the SCR efficiency of lead ions over redox-active MoO₂/C electrodes is >99% in mixtures of 100 ppm Pb(NO₃)₂ and 100 ppm NaCl solutions. In addition, the electrodes exhibit high regeneration performance in mixtures of NaCl and Pb(NO₃)₂ and high SCR efficiency for lead ions from mixtures of heavy metal ions. The tetrahedral structure of the [MoO₄] lattice is shown to be more favorable for the intercalation of lead ions. In situ Raman spectroscopy further shows that the transition of the crystal interface between [MoO₆] and [MoO₄] cluster lattice could be electrochemically controlled during SCR. Therefore, this study provides a new direction for the SCR of lead ions from wastewater.