Desalination Performance Of Capacitive Deionization Research Articles

Simultaneously improving surface area and hydrophilicity of biomass activated carbon for achieving superior desalination performance in CDI

Super activated carbon is a promising electrode material for capacitive deionization (CDI), an emerging sustainable desalination technology. Increasing its CDI performance requires improving hydrophilicity while maintaining high specific surface area. Traditionally this is challenging, but modifying super activated carbon with earth-abundant metal ions at room temperature can achieve both aims through a low-energy process aligned with sustainability. The modified super activated carbon showed higher specific surface area, improved hydrophilicity, and enhanced desalination performance. Particularly, the salt adsorption capacity of Cu2+-modified super activated carbon in 500 mg/L NaCl solution at 1.2 V increased from 35.4 mg/g to 49.3 mg/g, among the highest for sustainable carbon materials. The capacity increase was enabled by simultaneously increasing specific surface area and hydrophilicity through themetal modification process. Along with high charge efficiency (93.4%), fast adsorption rate, and good cycling stability (80% capacity retention over 50 cycles), the modified super activated carbon displayed superior overall desalination performance. This study exemplifies how sustainable design principles can be applied to synthesize high-performance CDI electrode materials, through green chemistry and engineering for optimized efficiency.

Ultrathin nitrogen-doped carbon Ti3C2Tx-TiN heterostructure derived from ZIF-8 nanoparticles sandwiched MXene for high-performance capacitive deionization

Rational design of high-performance electrode materials is crucial for enhancing desalination performance of capacitive deionization (CDI). Here, ultrathin nitrogen-doped carbon/Ti3C2Tx-TiN (NC/MX-TiN) heterostructure was developed by pyrolyzing zeolite imidazolate framework-8 (ZIF-8) nanoparticles sandwiched MXene (ZSM), which were formed by assembling ultrafine ZIF-8 nanoparticles with size of 20 nm on both sides of MXene nanosheets. The introduction of ultrasmall ZIF-8 particles allowed for in situ nitridation of the MXene during pyrolysis, forming consecutive TiN layers tightly connected to the internal MXene. The two-dimensional (2D) heterostructure exhibited remarkable properties, including high specific surface area and excellent conductivity. Additionally, the resulting TiN demonstrated exceptional redox capability, which significantly enhanced the performance of CDI and ensured cycling stability. Benefiting from these advantages, the NC/MX-TiN exhibited a maximum adsorption capacity of 45.6 mg g−1 and a steady cycling performance in oxygenated saline water over 50 cycles. This work explores the rational design and construction of MXene-based 2D heterostructure and broadens new horizons for the development of novel CDI electrode materials.

Two-dimensional bimetallic cobalt-copper metal organic framework for improved desalination performance of capacitive deionization

Capacitive deionization (CDI) technology has recently attracted much attention due to its distinct advantages, which include energy efficiency, eco-friendly method, and simple process. Capacitive materials, such as activated carbon, carbon fiber, and carbon aerogel, have mostly been used. However, the conventional carbon-based materials still suffer limitations, such as low electrosorption capacity, slow desalination rate, and insufficient desalination capacity for high saline concentration. Herein, the 2 Dimensional-Cobalt-Copper sulfur linker-based MOF material was prepared by a solvothermal method to develop the performance of electrode material, namely 2D-CoCu sMOF. We applied the synthesized material with porous carbon material (AC) as an electrode material in the CDI desalination system. Owing to the fast and efficient electron transfer in the electrode layer and the lower interfacial resistance between electrode surface and saline electrolyte originating from the redox reactions of MOF structure, 2D − CoCu sMOF exhibited a much higher desalination performance, compared to the pristine AC. CDI experimental results showed that the salt adsorption capacity (SAC) of electrode using 2D − CoCu sMOF was significantly improved from 4.18 to 7.55 mg/g without any changes in the long-term stability test, which was over an 80% increase in desalination performance. This approach provides an effective and simple method for the preparation of MOF-based CDI electrode material, which could have potential for high-performance green technology including desalination applications.

Space stability by incorporating iron atoms into the cobalt structure enabling capacitive deionization stability possible

Exploring and designing high performance pseudocapacitance electrode materials is of great significance in enhancing the desalination performance of capacitive deionization (CDI). Transition metal cobalt is considered a prospective electrode material for CDI because of its abundance and high theoretical capacity, which can produce a higher salt removal capacity than pure capacitive materials in the CDI process. However, due to the collapse and loss of the active electrode crystal matrix, the cobalt electrode exhibits poor electrochemical performance. In this work, we used a second metal element, Fe, introduced by ion exchange pyrolysis to regulate the leaching effect and optimize the ion matching for cobalt to protect the electrode structure. In addition, the interconnected external carbon layers form a robust matrix for enhanced electrical conductivity with ion transport. Thanks to the synergistic effect of the cobalt‑iron (Co3Fe7@C) electrode structure and surface engineering, the adsorption capacity reached 72.29 mg·g−1, achieving 110 % capacity retention in 100 cycles of chlorine removal. As a result, the carbon-coated cobalt‑iron bimetallic electrode exhibits excellent dechlorination capabilities in capacity, cycle life and rate capability. This study provides a promising avenue for the design of stable electrode materials for capacitive deionization electrodes.

Magnetic-assisted strategy for performance enhancement of flow-by capacitive deionization

Capacitive deionization is an environment friendly electro-sorption desalination technology that has broad application prospects in high-efficiency seawater desalination and brackish water purification. However, the insufficient ion separation performance and desalination capacity restrict the large-scale application of capacitive deionization with flow-by structure. Since the separation of cations and anions is intensified via the synergetic manipulation of the electric field force and Lorentz force, a magnetic-assisted strategy was proposed to enhance the performance of capacitive deionization. The effects of magnetic field strength and layout on the desalination performance of capacitive deionization were experimentally investigated. The results confirmed the feasibility of performance enhancement via the effect of the magnetic field. The performance of capacitive deionization under a vertical magnet layout was increased by 25.2 % at a magnetic field strength of 0.33 T, compared to that without a magnetic field. However, the deionization performance decreased when the magnetic field strength was further increased to 0.45 T, which indicates the existence of a critical magnetic field strength. Benefitting from the superposition of magnetic fields, the horizontal layout of magnets can provide a stronger average magnetic field than the vertical layout. This study provides insights into a new design of flow-by capacitive deionization.

Three‐dimensional graphene/MWCNT-MnO2 nanocomposites for high‐performance capacitive deionization (CDI) application

Surface area and the conductivity of the electrode materials are crucial in achieving high desalination performance in capacitive deionization (CDI) applications. In this study, we have successfully fabricated 3D Graphene/MWCNT-MnO2 nanocomposites by self-assembling negatively charged manganese dioxide (MnO2) decorated with multiwall carbon nanotube (MWCNT) and positive graphene oxide (GO). The MWCNT-MnO2 was prepared by the uniform decoration of MnO2 nanoparticles on the MWCNT surface, and positive GO was prepared by functionalizing the GO surface with ethylenediamine. The 3D composite was obtained by self-assembling positive GO and negative MWCNT-MnO2 via electrostatic co-precipitation method followed by hydrothermal treatment. The structure and morphology of the 3D composites materials are thoroughly studied and correlated with the electrochemical performance. Cyclic voltammetry (CV) curves exhibit high specific capacitances and depict a quasi-rectangular shape, suggesting a high pseudo-capacitive behavior resulting from electrical double-layer (EDL) at the electrode-solution interface and the presence of MnO2. The salt electrosorption capacity of the 3D composites investigated using 600 ppm NaCl solution and 1.2 V showed the highest salt adsorption capacity of 65.1mgNaClg-1. The excellent performance of the 3D composite materials as the electrode is attributed to the three-dimensional macroporous architectures and a high pseudo-capacitive behavior. Moreover, the study suggests that the electrode performance can be altered by choosing suitable anode materials.

Suppression of electrode reactions and enhancement of the desalination performance of capacitive deionization using a composite carbon electrode coated with an ion-exchange polymer

The suppression of electrode reactions at the composite carbon electrode (IEP-CCE) coated with an ion-exchange polymer (IEP) was studied. IEP-CCEs coated with a cation- and anion-exchange polymer on a carbon electrode (CE) were prepared. To compare the electrode reactions of coated and uncoated CEs, a capacitive deionization cell using the IEP-CCEs and a membrane capacitive deionization (MCDI) cell using uncoated CEs were manufactured. For each cell, the maximum allowable charge (MAC), which is the charge accumulated at the start of the electrode reactions, was measured. The MAC values of the uncoated CE and IEP-CCE were 58.8 and 79.8 C/g, respectively. It is thought that the IEP coating suppresses carbon oxidation, resulting in an increase in the MAC. Desalination experiments were conducted while varying the total charge (TCAd) during the adsorption process. When the TCAd was above the MAC, electrode reactions occurred in both cells, but adsorption–desorption proceeded stably without reaction below the MAC. In this case, the amount of adsorption in the IEP-CCE cell was 20.3 mg/g, which was approximately 47% higher than that in the MCDI cell (13.8 mg/g). The results showed that the IEP-CCE is very effective in improving the adsorption performance by suppressing electrode reactions.

Synergetic effects of different ion-doped polypyrrole layer coupled with β-cyclodextrin-derived hollow bottle-like carbon supporting framework for enhanced capacitive deionization performance

The structural characteristics of electrode material greatly affect desalination performance of capacitive deionization (CDI) process. Currently, there appears a new trend to develop novel materials with improved salt adsorption ability by designing reasonable hollow and robust spatial supporting structure. Herein, we report a novel hollow porous bottle-like carbon supporting framework (HPBC) derived from β-cyclodextrin for enhancing the CDI performance. The superior framework endows HPBC with large surface area, robust spatial supporting structure, optimized ion diffusion pathway, and ample ion storage space. By coupling ion-doped polypyrrole layer with HPBC, two types of composite materials (PPy-Cl/HPBC and PPy-DBS/HPBC) are developed with the integrated advantage of electrical-double layer ion storage process and ion-exchange pseudo-capacitive behavior. Electrochemical tests results indicate that both composite electrodes present considerable specific capacitances of 241.6 and 210.2 F g−1, fast charge transfer rate, and stable cycling performance. When PPy-Cl/HPBC and PPy-DBS/HPBC electrodes are assembled into the CDI cell (named cell Cl/C-DBS/C), the cell demonstrates the superior desalination ability of 34.5 mg g−1. Furthermore, the CDI cell Cl/C-DBS/C exhibits fast salt adsorption rate (4.0 mg g−1 min−1), prominent charge efficiency (77.6%), and excellent deionization stability after multi-regeneration cycles owing to the synergetic effect of ion-exchange ability of the PPy layer and the unique structural characteristics of HPBC. The CDI cell Cl/C-DBS/C also shows preferable adsorption of monovalent ions to divalent ions in the multi-salt solutions.

Enhanced Desalination Performance of Capacitive Deionization Using Nanoporous Carbon Derived from ZIF-67 Metal Organic Frameworks and CNTs.

Capacitive deionization (CDI) based on ion electrosorption has recently emerged as a promising desalination technology due to its low energy consumption and environmental friendliness compared to conventional purification technologies. Carbon-based materials, including activated carbon (AC), carbon aerogel, carbon cloth, and carbon fiber, have been mostly used in CDI electrodes due their high surface area, electrochemical stability, and abundance. However, the low electrical conductivity and non-regular pore shape and size distribution of carbon-based electrodes limits the maximization of the salt removal performance of a CDI desalination system using such electrodes. Metal-organic frameworks (MOFs) are novel porous materials with periodic three-dimensional structures consisting of metal center and organic ligands. MOFs have received substantial attention due to their high surface area, adjustable pore size, periodical unsaturated pores of metal center, and high thermal and chemical stabilities. In this study, we have synthesized ZIF-67 using CNTs as a substrate to fully utilize the unique advantages of both MOF and nanocarbon materials. Such synthesis of ZIF-67 carbon nanostructures was confirmed by TEM, SEM, and XRD. The results showed that the 3D-connected ZIF-67 nanostructures bridging by CNTs were successfully prepared. We applied this nanostructured ZIF-67@CNT to CDI electrodes for desalination. We found that the salt removal performance was significantly enhanced by 88% for 30% ZIF-67@CNTs-included electrodes as compared with pristine AC electrodes. This increase in salt removal behavior was analyzed by electrochemical analysis such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements, and the results indicate reduced electrical impedance and enhanced electrode capacitance in the presence of ZIF-67@CNTs.

Hierarchically porous biocarbons prepared by microwave-aided carbonization and activation for capacitive deionization

Capacitive deionization (CDI) is an emerging technology for the energy-efficient and cost-effective removal of ions from salt water by electrosorption. To further enhance the capacitive deionization performance, sugarcane bagasse biowastes are used as raw materials to synthesize biomass-derived electrodes via microwave carbonization and activation assisted with potassium hydroxide in the flows of carbon dioxide, which has the advantages of reducing preparation time and saving energy. Experimentally, the effect of microwave power (500–800 W) has been studied on the morphology, the ratios of mesoporosity in the biocarbons and their corresponding desalination performance of CDI. Accordingly, the biocarbons which are activated at 700 W of microwave irradiation under carbon dioxide atmosphere (denoted as SB-CO2–700) possess the higher fraction of mesopore to total pore volume (ca. 64.1%) with surface area (764 m2 g−1). By using cyclic voltammetry, the specific capacitance of SB-CO2–700 is calculated to be ca. 123 F g−1 at 5 mV s−1. With the superior properties (greater fraction of mesopore to total pore volume, reasonable surface area and hierarchical porosity), the electrosorption capacity of SB-CO2–700 samples is estimated to be 11.4 mg g−1 in 10 mM of NaCl solution at 1.2 V. These low-cost biowastes-derived hierarchically porous carbons prepared by time and energy saving route may provide a potential electrode for CDI applications.

Desalination Performance and Fouling Mechanism of Capacitive Deionization: Effects of Natural Organic Matter

This article focuses on the capacitive deionization (CDI) performance and fouling mechanism during individual/combined fouling with natural organic matter (NOM). The desalination performance of CDI was significantly reduced in the presence of NOM and decreased as the NOM concentration increased. This was attributed to the fact that NOM deposition changed the adsorption site, resistance and capacitance characteristics of the electrode. The migration behaviors of NOM in the CDI system were studied by mass conservation, and the deposition of humic acid (HA) on the electrode was greater than that of bovine serum albumin (BSA). However, BSA induced more severe CDI fouling and caused a 40% reduction in the adsorption capacity of the CDI system, due to the different deposited morphologies on the electrode surface. Scanning electron microscopy (SEM) showed that HA was mainly adsorbed in the pores of the electrode, while BSA was deposited on the electrode surface in the form of obvious contamination spots due to agglomeration, resulting in a further decrease in the adsorption sites for CDI. In addition, compared to HA, the resistance of the CDI system fouled by BSA was increased by 25%, while the capacitance was reduced by 15%.

Enhancing the Desalination Performance of Capacitive Deionization Using a Layered Double Hydroxide Coated Activated Carbon Electrode

Capacitive deionization (CDI) is a promising desalination technology because of its simple, high energy efficient, and eco-friendly process. Among several factors that can affect the desalination capacitance of CDI, wettability of the electrode is considered one of the important parameters. However, various carbon materials commonly have a hydrophobic behavior that disturbs the ion transfer between the bulk solution and the surface of the electrode. In this study, we fabricated a layered double hydroxide (LDH) coated activated carbon electrode using an in-situ growth method to enhance the wettability of the surface of the carbon electrode. The well-oriented and porous LDH layer resulted in a better wettability of the activated carbon electrode, attributing to an enhanced capacitance compared with that of the uncoated activated carbon electrode. Furthermore, from the desalination tests of the CDI system, the LDH coated carbon electrode showed a higher salt adsorption capacity (13.9 mg/g) than the uncoated carbon electrode (11.7 mg/g). Thus, this enhanced desalination performance suggests that the improvement in the wettability of the carbon electrode by the LDH coating provides facile ion transfer between the electrode and electrolyte.

Effect of Hydrophilicity of Activated Carbon Electrodes on Desalination Performance in Membrane Capacitive Deionization

Membrane capacitive deionization (MCDI) is a modification of capacitive deionization (CDI) using ion-exchange membranes (IEM) in front of the electrodes. Electrode properties, especially the specific surface area, are known to be strongly related with desalination performance in CDI, but the effects of other properties in MCDI are not fully understood. The objective of this study was to investigate the effect of hydrophilicity in activated carbon electrodes on desalination performance in MCDI. Two types of activated carbon (P60 and YS-2) whose specific surface areas were similar were used as electrode materials, but they had different hydrophilicity (i.e., P60 was originally hydrophobic and YS-2 was relatively hydrophilic due to its nitrogen-containing surface chemistry). These hydrophilic electrodes (either the electrode itself or modified with polydopamine (PDA)) led to an increase in the salt adsorption capacity (SAC) in MCDI because they facilitated the access of both ions and water molecules into the electrode pores. In particular, the SAC of the P60 electrode displayed a large increase to almost reach that of the YS-2 electrode due to the improved hydrophilicity with PDA modification and the insignificant effects of PDA modification on an already hydrophilic YS-2 electrode. Additionally, PDA-modified IEM in MCDI reduced the SAC as a result of the additional insulating PDA layer with little changes in hydrophilicity.

Adsorption behavior of powdered activated carbon to control capacitive deionization fouling of organic matter

Capacitive deionization (CDI) is a promising desalination technology, but it suffers from serious irreversible fouling caused by the presence of organic matter, which severely reduces the desalination performance. In this research, powdered activated carbon (PAC) was used to reduce the fouling caused by the presence of humic acid (HA). Due to the effective removal of HA by PAC adsorption, HA deposited onto the CDI electrode was reduced by about 40%. Desalination performance of CDI was improved when PAC adsorbed HA entered into the CDI system, resulting in the removal of HA under the action of an electric field in the CDI system, thereby alleviating the irreversible CDI fouling caused by HA. Alternatively, PAC entering the CDI system improved electrical conductivity due to the formation of electrical double layers (EDLs) in the flow path, thereby reducing the internal resistance of CDI system to increase the capacitance of CDI system. The presence of PAC in the CDI system was responsible to reduce the deposition of organic matter onto the electrode as well as the resistance of CDI system, resulting in optimized specific surface area and conductivity to improve the CDI performance.

Comparative study on electrosorptive behavior of NH4HF2-etched Ti3C2 and HF-etched Ti3C2 for capacitive deionization

In this work, the MXene Ti3C2 with excellent electrical conductivity is obtained by etching Ti3AlC2 with NH4HF2 solution. The electrosorption performance, the electrochemical and physicochemical properties of as-prepared Ti3C2 are characterized. The results demonstrate that NH4HF2-etched Ti3C2 possesses the better intercalation pseudo-capacity and the specific capacitance is up to 78 F∙g−1, which is an increase of 34% than HF-etched Ti3C2. In addition, the desalination capacity of NH4HF2-etched Ti3C2 enhances 35.5% than that of HF-etched Ti3C2. When the initial conductivity of NaCl is ~ 1000 μS/cm, the desalination capacity of NH4HF2-etched Ti2C3 electrode is 12.1 mg∙g−1, and the pseudo-first-order model can well describe the electrosorption kinetics of NH4HF2-etched Ti3C2 electrode. Furthermore, the regeneration performance of the NH4HF2-etched Ti3C2 electrode is excellent. This study indicates a new kind of effective electrode material to improve the desalination performance of capacitive deionization (CDI) technology.

Characteristics of Nitric Acid‐Modified Carbon Nanotubes and Desalination Performance in Capacitive Deionization

AbstractMultiwalled carbon nanotubes (MWCNTs) were treated by nitric acid to probe the effect of nitric acid modification on their properties and desalination performance in capacitive deionization (CDI). The nitric acid modification exerts a slight influence on the morphologies, specific surface area, and pore properties of the MWCNTs but can increase the amount of oxygen‐containing functional groups and then obviously enhance the specific capacitance of the electrodes. Thus, the desalination efficiency can be significantly improved as the modified MWCNTs serve as electrodes in CDI. The adsorption of salt ions on the original and modified MWCNT electrode is fitted well by the Freundlich isotherm other than the Langmuir isotherm.

Hollow ZIFs-derived nanoporous carbon for efficient capacitive deionization

Desalination performance of capacitive deionization (CDI), a promising deionization technology, depends significantly upon the compositions and internal structures of electrode materials. Herein, hollow ZIFs-derived nanoporous carbons (HZCs) were prepared via chemical etching method and subsequent pyrolysis. Particularly, the tannic acid as an etching agent was employed to tailor hollow structure. The resultant HZCs possess high surface area, wide pore size distribution, obvious hollow cavity and high nitrogen content. Moreover, HZCs electrodes exhibited a significantly improved CDI performance over the solid ZIFs-derived nanoporous carbons (SZCs) electrodes. The improved electrosorption rate and capacity (15.31 mg g-1 at 1.2 V) of HZCs electrode imply that hollow structure facilitates the transportation for both ions and mass on CDI process. Besides, the further cycle experiment exhibits a great regeneration stability of HZCs electrode over 20 adsorption-desorption cycles. All these results indicate that HZCs should be a promising candidate for CDI application.

A comparison of multicomponent electrosorption in capacitive deionization and membrane capacitive deionization

In this study, the desalination performance of Capacitive Deionization (CDI) and Membrane Capacitive Deionization (MCDI) was studied for a wide range of salt compositions. The comprehensive data collection for monovalent and divalent ions used in this work enabled us to understand better the competitive electrosorption of these ions both with and without ion-exchange membranes (IEMs). As expected, MCDI showed an enhanced salt adsorption and charge efficiency in comparison with CDI. However, the different electrosorption behavior of the former reveals that ion transport through the IEMs is a significant rate-controlling step in the desalination process. A sharper desorption peak is observed for divalent ions in MCDI, which can be attributed to a portion of these ions being temporarily stored within the IEMs, thus they are the first to leave the cell upon discharge. In addition to salt concentration, we monitored the pH of the effluent stream in CDI and MCDI and discuss the potential causes of these fluctuations. The dramatic pH change over one adsorption and desorption cycle in CDI (pH range of 3.5–10.5) can be problematic in a feed water containing components prone to scaling. The pH change, however, was much more limited in the case of MCDI for all salts.

Nitrogen-Doped Hollow Mesoporous Carbon Spheres for Efficient Water Desalination by Capacitive Deionization

Water desalination performance of capacitive deionization (CDI) largely depends on electrode materials properties. Rational design and regulation of the structure and composition of electrode materials to acquire high CDI performance is of great significance. Herein, nitrogen-doped hollow mesoporous carbon spheres (N-HMCSs) were investigated as electrode material for CDI application. To understand the effect of structure and composition on CDI performance, another two CDI electrode materials, i.e., hollow mesoporous carbon spheres (HMCSs) and solid mesoporous carbon spheres (SMCSs) were prepared for comparison. The obtained N-HMCSs possessed unique hollow cavity and excellent nitrogen doping property, resulting in fast ion diffusion, good charge transfers ability and fine wettability. Compared with HMCSs and SMCSs electrodes, N-HMCSs electrode exhibited an improved electrosorption capacity and rate, demonstrating the dependence of CDI performance on the synergistic effect of hollow structure and nitrogen ...

Enhanced desalination performance of capacitive deionization using zirconium oxide nanoparticles-doped graphene oxide as a novel and effective electrode

Abstract Due to its eco-friendly and low energy technique for removing salt ions from saline water, capacitive deionization (CDI) is highly recommended as a desalination process. Based on its good features, large surface area and good electric conductivity, graphene oxide is a promising electrode in the CDI technology if the specific capacitance could be enhanced. In this study, to improve the electrochemical performances, novel ZrO 2 nanoparticles-incorporated graphene oxide nanosheets with different concentrations were successfully synthesized by hydrothermal treatment, their electrosorption characteristics in CDI unit were examined. The morphology, structure and electrochemical performance of the fabricated materials were investigated by scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), cyclic voltammetry and electrochemical impedance spectroscopy. The capacitive and electrosorption performances in NaCl solution were studied. Moreover, the role of ZrO 2 loading was investigated. The introduced ZrO 2 -doped graphene oxide showed a distinct improvement in the electrosorption capacity and revealed higher specific capacitance compared to the pristine graphene oxide. The obtained results indicated that the synthesized ZrO 2 -doped graphene oxide nanocomposite having 10 wt.% ZrO 2 displayed a significant increase in the specific capacitance as the corresponding value (452.06 F/g) was nine folds more than that of the pristine GO at 10 mV/s. Moreover, the same electrode exhibits great cycling stability, excellent salt removal efficiency (93.03%), and distinct electrosorptive capacity (4.55 mg/g). Overall, the proposed GO/ZrO 2 nanoparticle composite electrode is appropriate for utilizing as optimum electrodes for the CDI technique.