Mesopore-enhanced graphene electrodes with modified hydrophilicity for ultrahigh capacitive deionization

Recent progress in graphene-based and ion-intercalation electrode materials for capacitive deionization

Construction and evaluation of a novel three-electrode capacitive deionization system with high desalination performance

ZiF-8 induced carbon electrodes for selective lithium recovery from aqueous feed water by employing capacitive deionization system

Capacitive deionization (CDI) technology is a challenge on an economical and effective electrosorption desalination method. The paper analyses the CDI current status and progress of carbon electrode materials, and describes the types of CDI and its performances of testing materials. The electrosorption capacities are summarized on the carbon electrode materials and the current hurdles. The reported numbers from the literature vary in a wide range between 0.25 and 26.42 mg/g of both electrodes CDI cell, we suggest that the CDI electrodes should have an adsorption of at least 9.0 mg/g NaCl when the applyed voltage is 2.0 V. The potential capacitive deionization technologies are proposed here.

Prussian blue analogues (PBAs) are considered as promising cathode materials for capacitive deionization (CDI) technology due to their 3D open‐frame structure and tunable redox active sites. However, the inevitably high content of [Fe(CN)6] vacancies in the crystal structure results in a low salt adsorption capacity (SAC) and poor recycling performance. Herein, a high‐salt nano‐reaction system is established by mechanochemical ball milling, enabling the preparation of a variety of highly crystallized PBAs (metal hexacyanoferrate, MHCF‐B‐170, M = Ni, Co, or Cu) with low vacancies (0.05–0.06 per formula unit). The reduction of vacancies in the PBAs lattice not only strengthens the conductivity and promotes the rapid transfer of electrons, but also reduces the migration energy barrier and accelerates the fast and reversible diffusion of Na+ ions. The structural characterization method and theoretical simulation demonstrates the excellent reversibility and crystal structure stability of MHCF‐B‐170 during the CDI process. Impressively, the NiHCF‐B‐170 exhibits excellent CDI performance, characterized by an exceptionally high SAC of up to 101.4 mg g−1 at 1.2 V, and demonstrates remarkable cycle stability with no significant degradation observed even after 100 cycles. This PBAs with low Fe(CN)6 vacancies are expected to be a competitive candidate material for CDI electrodes.

Capacitive deionisation (CDI) electrodes with identical composition were prepared using three deposition methods: (1) slurry infiltration by calendering (SIC), (2) ink infiltration dropwise (IID), and (3) ink deposition by spray coating (IDSC). The SIC method clearly showed favourable establishment of an electrode with superior desalination capacity. Desalination results showed that electrodes produced from slurries mixed longer than 30 min displayed a significant reduction in the maximum salt adsorption capacity, due to the agglomeration of carbon black. The electrodes were then thermally treated at 130, 250, and 350 °C. Polyvinylidene difluoride (PVDF) decomposition was observed when the electrodes were treated at temperatures higher than 180 °C. The electrodes treated at 350 °C showed contact angles of θ = 0°. The optimised electrodes showed a salt adsorption capacity value of 24.8 mg/g (130 °C). All CDI electrodes were analysed using specific surface area by N2 adsorption, contact angle measurements, conductivity by the four-point probe method and salt adsorption/desorption experiments. Selected reagents and CDI electrodes were characterised using thermogravimetric analysis coupled with mass spectrometry (TGA-MS) and differential scanning calorimetry (DSC), as well as scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDS). Electrode structure and the development of the critical balance between ion- and electron- conductive pathways were found to be a function of the electrode slurry mixing procedure, slurry deposition technique and thermal treatment of the electrodes.

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, contact resistance, and fixed charge density 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 the sensitivity analysis and technology comparison were used to evaluate the current effectiveness of CDI technologies as well as near-term projections for improvements in material performance. Through this analysis we are able to set targets for performance and material properties necessary for each CDI configuration to favorably operate.

Capacitive deionization (CDI) is a potential technique for water treatments such as brackish water desalination, seawater desalination, wastewater remediation, and water softening. CDI has more advantages over other desalinating processes: (i) the technique works by the process of electroabsorption, and is even more energy-efficient, as it does not require any high-pressure pumps; (ii) as the module works in 0.8–2.0 V. An ideal electrode for CDI system requires the following characteristics: (i) large surface area; (ii) high porosity (both meso and micropores); (iii) high electrical conductivity; (iv) electrochemical stability; (v) bio-inertness; (vi) fast adsorption-desorption kinetics; (vii) good wetting behavior; (viii) low cost; and (ix) scalability. In Vietnam, coconut shell-derived activated carbon is produced by a calcinated process under nitrogen gas, which exhibits a high surface area (800-1000 m2/g). In this work, we prepared the CDI composite electrode based on coconut shell-derived activated carbon and high conductive carbon like CNTs or carbon black in the pilot scale. CDI systems with a capacity of 1 m3 per day were fabricated and monitored the desalination performance in Mekong Delta.

Introduction of new nanomaterials with conductivity, salt adsorption capacity (SAC) and rate (SAR) exceeding that of carbon electrodes may greatly improve capacitive deionization of water. However, those materials show a different electrochemical behavior, which must be studied and optimized for practical use. Here, effects of operating conditions on desalination performance of pre-conditioned Ti3C2Tx-MXene-based electrodes in a symmetric membrane capacitive deionization (MCDI) system were investigated. Specifically, influences of discharge potential, half-cycle length (HCL), and flow rate were systematically studied. Results showed different degrees of performance dependence on operating conditions. For instance, lower discharge potentials increased SAC and SAR by 152%. However, longer HCL increased SAC by 32% while decreasing SAR by 54%. Finally, faster flow rates decreased both SAC and SAR by 20%. Desalination performances of symmetric pre-conditioned MXene and activated carbon cloth (ACC) electrodes were gravimetrically and volumetrically compared in MCDI system. Pre-conditioned MXene electrodes gravimetrically performed 30% lower than ACC due to their notably higher density. Yet, pre-conditioned MXene electrodes volumetrically outperformed ACC by 162%. Results suggest that although MXenes offer high electrochemical activity and hydrophilicity, making them promising candidates for CDI applications, the strong dependence of desalination performance of MXenes on operating conditions requires in-depth understanding and warrants further research.

Capacitive deionization (CDI) is an emerging energy efficient, low-pressure and low-cost intensive desalination process that has recently attracted experts’ attention. The process is to explain that ions (cations and anions) can be separated by a pure electrostatic force imposed by a small bias potential. Even at a rather low voltage of 1.2 V, desalinated water can be produced. The process can be well operational by a professional cell design. Although various processes have been manufactured before, in this study, membrane was removed and a new unit was designed and manufactured (Using CFD Simulation). In this case, the combination of activated carbon powder (with an effective surface area of 2600 m2 per gram), carbon black, and polyvinyl alcohol with a ratio of 35/35/30 coated on carbon paper as electrode materials was considered for tests. The weight was 1.41 grams for each material, and the thickness was 0.44 mm. CDI system was tested, and the results of charge-discharge cycles, cyclic voltammetry, and impedance spectroscopy were evaluated. It can be implied that there is no need for a strong pump and, also, pressure drop can be reduced due to such a noticeable space between two electrodes. Preliminary experimental results showed high specific capacitance (2.1 Farad) and ultra-high salt adsorption capacity, compared with similar cases.

Prussian blue analogue based integrated membrane electrodes for desalination and selective removal of ammonium ions in a rocking-chair capacitive deionization

Capacitive deionization (CDI) is an electrochemical water treatment process that allows one to purify saline water by removing ionic species while storing energy simultaneously. The key to CDI is the adsorption of charged particles (ions) in the electrical double layer (EDL) of an electrode upon polarization by a direct current (DC) power source, the same as charging an electrochemical supercapacitor [1][2]. However, there are some differentiating aspects between CDI and supercapacitors that must be considered before this technology can be commercially competitive [3]. When a supercapacitor is charged, energy used is influenced by different parameters such as the current intensity, the electrode capacitance, and also by the current intensity of the previous step (discharge). If the capacitor operates under ideal conditions (i.e, high conductor electrolyte, low resistance connections), the influence of these parameters is practically negligible. However, if the system works under non-ideal conditions (greater electrolyte resistance) as in a CDI system, the influence of these parameters in subsequent stages may be relevant when the current intensity during charging is different than that used in discharge. Thus, a better understanding of the operational problems arising from the use of Supercapacitors is significantly important to aid their application as CDI in real-world devices including desalination, municipal drinking water, wastewater treatment and water-reuse projects [4]. In this work, a 310 Farads – 2.5 Volts supercapacitor cell and a CDI stack (70 Farads – 1.5 Volts) are tested under new operational cycling modes based on successively apply discharge rates with the same charge intensity, and similarly different charging rates over constant discharge intensity [5]. This approach allow to analyses comparatively the impact of applying charge and discharge at different rates on electrical performance (capacitance, resistance and round-trip energy efficiencies) of both technologies; supercapacitors and CDI.

A direct comparison of flow-by and flow-through capacitive deionization

Capacitive deionization (CDI) is a promising technology for desalination of seawater and recovery of mineral resources. By applying a voltage to a pair of electrodes within the potential of water electrolysis, ions such as Li+ and Na+ can be recovered by electrosorption. In order to increase the salt adsorption capacity (SAC) of the electrodes, various metal oxides have been proposed.1 For example, manganese oxide, titanium oxide and ruthenium oxide show higher SAC than activated carbon, owing to the contribution from both electric double-layer capacitance and pseudocapacitance. In particular, the SAC of ruthenium oxide electrodeposited on activated carbon was ~4 times higher than that of pristine activated carbon due to the additional pseudocapacitance.2 Since hydrous RuO2 shows higher pseudocapacitance compared with anhydrous RuO2,3 it is anticipated that hydrous RuO2 would show higher SAC compared to anhydrous RuO2. Here we evaluate and discuss the pseudocapacitance and SAC capability of hydrous and anhydrous RuO2-based electrode materials. RuO2·0.5H2O or RuO2 were prepared by heating of RuO2·xH2O at 200°C or 450°C, respectively.3 The slurry, prepared by mixing RuO2·0.5H2O or RuO2, polyvinylidene fluoride and N, N-dimethylformamide, was pasted on a Ti plate and dried at 120oC to prepare electrodes (10-11 mg cm-2) for batch-mode CDI experiments. Activated carbon (MSP-20) electrode was prepared as a control sample. The CDI experiment was carried out with a voltage of 1.2 or 0 V, by circulating 100 mL NaCl feed solution (initial concentration: 5 mM NaCl) at a flow rate of 9.8 mL min-1 per electrode. The SAC was determined by measuring the change in the conductivity on the negative electrode side, using a calibration curve of conductivity and concentration of NaCl solution.The specific capacitance of RuO2·0.5H2O and RuO2 were 172 F g-1 and 13 F g-1, while the SAC was 7.4 mg g-1 and 4.0 mg g-1, respectively. This means that RuO2·0.5H2O shows 13.2 times higher capacitance and twice higher SAC. The results suggest that the increase in the specific capacitance by applying RuO2·0.5H2O might be attributed to the increase in the SAC value. On the other hand, the specific capacitance of RuO2·0.5H2O was higher than that of activated carbon, while the SAC of RuO2·0.5H2O was lower than that of activated carbon. There seems to be a difference in the effect of pseudocapacitance and electric double layer capacitance on the SAC value.This work was supported in part by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Challenging Research (Exploratory) Grant Number 17K19174. References K. Singh, S. Porada, H. D. de Gier, P. M. Biesheuvel and L. C. P. M. de Smet, Desalination, 455, 115 (2019).X. Ma, Y. Chen, K. Zhou, P. Wu and C. Hou, Electrochim. Acta, 295, 769 (2019).W. Sugimoto, H. Iwata, K. Yokoshima, Y. Murakami and Y. Takasu, J. Phys. Chem. B, 109, 7330 (2005). Figure 1

Capacitive deionization (CDI) technology’s potential for sustainable desalination is contingent on breakthroughs in electrode materials. Defect-rich titanium (sub)oxides (anatase TiO₂(A), Ti₂O₃, Ti₃O₅, Ti₄O₇) offer compelling advantages, including tunable capacity, low environmental impact, and robustness. This study provides a systematic comparison of their structural features, electrochemical responses, and desalination efficacy. Electrochemical characterization results show anatase TiO₂(A) leads in specific capacitance (252.5 F g⁻¹ at 0.3 A g⁻¹), while Ti₃O₅’s highest oxygen vacancy minimizes charge transfer resistance and maximizes ion migration rates. In desalination trials, TiO₂ delivers a maximum salt adsorption capacity of 37.1 mg g⁻¹ (500 mg L⁻¹ NaCl, 1.2 V). All electrodes exhibit outstanding cycling stability (capacity retentions > 83.5 %), affirming their practical potential. Correlation analysis discloses the oxygen vacancy-driven charge transfer mechanism inside these electrodes and establishes a structure-performance relationship. Overall, this work establishes fundamental structure-property relationships that underpin future electrode innovation via the oxygen vacancy-engineering strategy, which represents a promising pathway for advancing CDI performance boundaries.