Unraveling the effect of CDI electrode characteristics on Cs removal from the perspective of ion transfer and energy composition

Capacitive deionization (CDI) is surprisingly efficient to remove the aqueous Cs ion due to its small hydrated size and low hydration energy. But current experimental techniques fail in investigating deeply into the influence of some key electrode characteristics due to the difficulty in experimentally fabricating the electrodes as desired. This work presents a dynamic transport model of salt ions in a flow-by CDI cell. By using this model, the electrode thickness, macro- and micro-porosity are investigated to evaluate Cs ion removal efficiency and energy efficiency particularly from the aspect of ion transfer by the approach of decomposing energy contribution. The results indicate that the thick electrode coupled with the high current could greatly improve the effluent quality, but reduce the salt adsorption capacity (SAC). The increasement of the current density from 3 A/m² to 6 A/m² greatly decreases the SAC from 4.0 mg/g to 0.8 mg/g. Lower current could prolong the charging period, leading to more ions stored in the micropore. Not all the electrical energy is consumed for separating ions from the feed as desired, but some are used for driving ions diffusing in the electrodes. Consequently charging efficiency will be reduced especially when the electrodes are characterized with high porosity. It is highlighted that future work is required to further consider the complex details of porous structure and pore connectivity.

Controlled fabrication of nitrogen-doped porous carbon foam with refined hierarchical architectures for desalination via capacitive deionization

Porous carbon materials have been regarded as a promising alternative to activated carbon for desalination via capacitive deionization (CDI) due to refined architectures and functionalities. However, it is still challenging to obtain a controlled hierarchical pore structure and considerable nitrogen-doped content by convenient method. Herein, nitrogen-doped hierarchical porous carbon foams (NHCFs) with different microstructural features, nitrogen contents and nitrogen species were successfully fabricated via a stepwise pyrolysis carbonization strategy using easily available melamine foam. Due to the synergistic effect of hierarchical porous structure and doped nitrogen, the optimized NHCF sample carbonized at 800℃ (NHCF-800) exhibited a maximum desalination capacity of 30.1 mg g^-1 at the optimal operating parameters (500 mg/L NaCl solution, 1.2 V) and an excellent regeneration performance after 50 continuous adsorption-desorption cycles. Furthermore, density functional theory (DFT) was also conducted to elaborate the disparity of sodium adsorption energy among the nitrogen species for in-depth understanding, and it mainly benefits from the ascendency of the pyrrolic-N and pyridinic-N over the graphitic-N dopant. This work paves the way of rational regulation of nitrogen-doped process and hierarchical porous structure carbon as CDI electrode materials for desalination.

Optimizing the Micro/Mesoporous Structure of Hierarchical Porous Carbon Synthesized from Petroleum Pitch Using the Solvent-Free Method for Ultra-Fast Capacitive Deionization

In this work, a solvent-free ZnO-template method is used to synthesize hierarchical porous carbons (denoted as HPC-X; X = 1, 1.5, 2, and 4 g of ZnO) via the pyrolysis of petroleum industrial-residual pitch with ZnO. The proposed method allows precise control of the micro/meso/macroporous structure of the HPC by adjusting the amount of ZnO. The results show that the average pore size of HPCs prominently increases from 2.4 to 3.7 nm with the increase in the ZnO/pitch ratio. In addition, it is shown that HPCs have a high surface area between 1141 and 1469 m² g^-1, a wide-range pore size distribution (micro-, meso-, and macropores), and a tap density ranging from 0.2 to 0.57 g cm^-3. The capacitive deionization performances of HPCs for sodium and chloride ions are investigated. The results show that HPC-2 exhibits the highest electrosorption capacity of 9.94 mg g^-1 within 10.0 min and a maximum electrosorption capacity of 10.62 mg g^-1 at 1.2 V in a 5.0 mM NaCl solution. Hence, HPC-2 is a highly promising candidate as an electrode material for rapid deionization.

Knowledge and Technology Used in Capacitive Deionization of Water

The demand for water and energy in today’s developing world is enormous and has become the key to the progress of societies. Many methods have been developed to desalinate water, but energy and environmental constraints have slowed or stopped the growth of many. Capacitive Deionization (CDI) is a very new method that uses porous carbon electrodes with significant potential for low energy desalination. This process is known as deionization by applying a very low voltage of 1.2 volts and removing charged ions and molecules. Using capacitive principles in this method, the absorption phenomenon is facilitated, which is known as capacitive deionization. In the capacitive deionization method, unlike other methods in which water is separated from salt, in this technology, salt, which is a smaller part of this compound, is separated from water and salt solution, which in turn causes less energy consumption. With the advancement of science and the introduction of new porous materials, the use of this method of deionization has increased greatly. Due to the limitations of other methods of desalination, this method has been very popular among researchers and the water desalination industry and needs more scientific research to become more commercial.

Layered double hydroxide coated electrospun carbon nanofibers as the chloride capturing electrode for ultrafast electrochemical deionization

Slow desalination kinetics and poor durability of the electrodes are two key limitations of electrochemical deionization (EDI) that are considered to be the next generation of capacitive desalination (CDI). Herein, we report the design of a high-efficiency chloride removal electrode material for accelerating the desalination kinetics and concurrently improving the durability of EDI, which is based on coating NiMn-Cl layered double hydroxides (LDHs) on the surface of electrospun carbon nanofibers (CNFs@LDHs). The salient features of the as-developed CNFs@LDHs are that applying layer-structured LDHs with abundant redox-active sites to accelerate the pseudo-capacitive ion storage via fast ion intercalation/deintercalation, and leveraging the rigid CNF backbone to strengthen its durability by preventing the potential aggregation of LDHs. As expected, the CNFs@LDH based EDI system displays an ultrafast desalination rate of 0.51 mg g^-1 s^-1 and outstanding long-term stability (only 10.66 % desalination capacity reduction after 35 cycles), which is achieved without sacrificing its excellent desalination capacity (72.04 mg g^-1). This work could be inspirational for the future design of ultrafast yet durable EDI approaching industrial desalination applications.

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.

Enhanced electrochemical and capacitive deionization performance of metal organic framework/holey graphene composite electrodes

In this paper, we designed and prepared a novel metal organic framework (MOF)/holey graphene (HG) composites as electrode materials for electrochemistry and capacitive deionization (CDI). The MOF nanoparticles were attached to the surface of the HG sheets to form layered porous structure, which promoted the transport of ions and electrons in the electrode/electrolyte interfaces. Additionally, the synergistic effect of these composite electrodes, which combined pseudocapacitance performance of MOF and the high conductivity of graphene, contributed to enhancing the performance of electrochemistry and CDI. The MOF/HG-2 exhibited high capacitances of 526 F g^-1 at current rates of 0.1 A g^-1, low charge transfer resistance of 0.53 Ω, and excellent cycling stability (retention of about 90.3% after 5000 cycles at 2 A g^-1). As electrode materials for CDI, the MOF/HG-2 displayed a remarkable electrosorption capacity of 39.6 mg g^-1 with initial salt concentration of 800 mg L^-1, and there was no obvious attenuation after 20 CDI regeneration cycles. These results confirmed that MOF/HG was a promising electrode material for the actual application of CDI.

Modification of Metal-Organic Framework-Derived Nanocarbons for Enhanced Capacitive Deionization Performance: A Mini-Review

Capacitive deionization (CDI) is a promising electrochemical water treatment technology. Development of new electrode materials with higher performance is key to improve the desalination efficiency of CDI. Carbon nanomaterials derived from metal-organic frameworks (MOFs) have attracted wide attention for their porous nanostructures and large specific surface areas. The desalination capacity and cycling stability of MOF-derived carbons (MOFCs) have been greatly improved by means of morphology control, heteroatom doping, Faradaic material modification, etc. Despite progress has been made to improve their CDI performance, quite a lot of MOFCs are too costly to be applied in a large scale. It remains crucial to develop MOFCs with both high desalination efficiency and low cost. In this review, we summarized three modification methods of MOFCs, namely morphology control, heteroatom doping, and Faradaic material doping, and put forward some constructive advice on how to enhance the desalination performance of MOFCs effectively at a low cost. We hope that more efforts could be devoted to the industrialization of MOFCs for CDI.

Fabrication of an efficient vanadium redox flow battery electrode using a free-standing carbon-loaded electrospun nanofibrous composite

Vanadium redox flow batteries (VRFBs) are considered as promising electrochemical energy storage systems due to their efficiency, flexibility and scalability to meet our needs in renewable energy applications. Unfortunately, the low electrochemical performance of the available carbon-based electrodes hinders their commercial viability. Herein, novel free-standing electrospun nanofibrous carbon-loaded composites with textile-like characteristics have been constructed and employed as efficient electrodes for VRFBs. In this work, polyacrylonitrile-based electrospun nanofibers loaded with different types of carbon black (CB) were electrospun providing a robust free-standing network. Incorporation of CBs (14% and 50% weight ratio) resulted in fibers with rough surface and increased mean diameter. It provided higher BET surface area of 83.8 m² g^-1 for as-spun and 356.7 m² g^-1 for carbonized fibers compared to the commercial carbon felt (0.6 m² g^-1). These loaded CB-fibers also had better thermal stability and showed higher electrochemical activity for VRFBs than a commercial felt electrode.

Trace-Fe-Enhanced Capacitive Deionization of Saline Water by Boosting Electron Transfer of Electro-Adsorption Sites

Capacitive deionization (CDI) is a promising water purification technology. However, the current ion adsorption capacity of CDI electrode materials is still an issue, which cannot meet the rapid demand of clean water from saline water. Herein, trace-Fe-enhanced removal of ions from saline water via CDI is presented. The ion adsorption capacity of CDI electrodes is up to 36.25 mg g^-1 in a 500 mg L^-1 NaCl media at 1.2 V together with stable regeneration property. In situ Raman and ex situ XPS measurements unravel the removal mechanism of ions from saline water, and the reinforced adsorption of ions is due to the introduction of trace Fe boosting electron transfer of electro-adsorption sites during the CDI process. This work presents a promising solution to highly efficient capacitive deionization for saline water.

Advances in Manufacturing Composite Carbon Nanofiber-Based Aerogels

This article provides an overview on manufacturing composite carbon nanofiber-based aerogels through freeze casting technology. As known, freeze casting is a relatively new manufacturing technique for generating highly porous structures. During the process, deep cooling is used first to rapidly solidify a well-dispersed slurry. Then, vacuum drying is conducted to sublimate the solvent. This allows the creation of highly porous materials. Although the freeze casting technique was initially developed for porous ceramics processing, it has found various applications, especially for making aerogels. Aerogels are highly porous materials with extremely high volume of free spaces, which contributes to the characteristics of high porosity, ultralight, large specific surface area, huge interface area, and in addition, super low thermal conductivity. Recently, carbon nanofiber aerogels have been studied to achieve exceptional properties of high stiffness, flame-retardant and thermal-insulating. The freeze casting technology has been reported for preparing carbon nanofiber composite aerogels for energy storage, energy conversion, water purification, catalysis, fire prevention etc. This review deals with freeze casting carbon nanofiber composite materials consisting of functional nanoparticles with exceptional properties. The content of this review article is organized as follows. The first part will introduce the general freeze casting manufacturing technology of aerogels with the emphasis on how to use the technology to make nanoparticle-containing composite carbon nanofiber aerogels. Then, modeling and characterization of the freeze cast particle-containing carbon nanofibers will be presented with an emphasis on modeling the thermal conductivity and electrical conductivity of the carbon nanofiber network aerogels. After that, the applications of the carbon nanofiber aerogels will be described. Examples of energy converters, supercapacitors, secondary battery electrodes, dye absorbents, sensors, and catalysts made from composite carbon nanofiber aerogels will be shown. Finally, the perspectives to future work will be presented.

Towards Electrochemical Water Desalination Techniques: A Review on Capacitive Deionization, Membrane Capacitive Deionization and Flow Capacitive Deionization

Electrochemical water desalination has been a major research area since the 1960s with the development of capacitive deionization technique. For the latter, its modus operandi lies in temporary salt ion adsorption when a simple potential difference (1.0-1.4 V) of about 1.2 V is supplied to the system to temporarily create an electric field that drives the ions to their different polarized poles and subsequently desorb these solvated ions when potential is switched off. Capacitive deionization targets/extracts the solutes instead of the solvent and thus consumes less energy and is highly effective for brackish water. This paper reviews Capacitive Deionization (mechanism of operation, sustainability, optimization processes, and shortcomings) with extension to its counterparts (Membrane Capacitive Deionization and Flow Capacitive Deionization).

Exceptional capacitive deionization rate and capacity by block copolymer-based porous carbon fibers

Capacitive deionization (CDI) is energetically favorable for desalinating low-salinity water. The bottlenecks of current carbon-based CDI materials are their limited desalination capacities and time-consuming cycles, caused by insufficient ion-accessible surfaces and retarded electron/ion transport. Here, we demonstrate porous carbon fibers (PCFs) derived from microphase-separated poly(methyl methacrylate)-block-polyacrylonitrile (PMMA-b-PAN) as an effective CDI material. PCF has abundant and uniform mesopores that are interconnected with micropores. This hierarchical porous structure renders PCF a large ion-accessible surface area and a high desalination capacity. In addition, the continuous carbon fibers and interconnected porous network enable fast electron/ion transport, and hence a high desalination rate. PCF shows desalination capacity of 30 mgNaCl g-1 PCF and maximal time-average desalination rate of 38.0 mgNaCl g-1 PCF min-1, which are about 3 and 40 times, respectively, those of typical porous carbons. Our work underlines the promise of block copolymer-based PCF for mutually high-capacity and high-rate CDI.

High adsorptivity and recycling performance activated carbon fibers for Cu(II) adsorption

In order to develop adsorbent materials with high Cu(II) adsorptivity and renewable recycling for Cu(II), nitric acid oxidation process is optimized and ameliorated by microwave and sonication to obtain an efficient modification and regeneration processes. Microwave-assisted nitric acid oxidation process has the most significant enhancement effect to the Cu(II) adsorptivity of activated carbon fiber felts (ACFFs), which can reach 23.13 mg/g and 4.55 times of pristine felts. It is due to this process can greatly increase the ultramicropore volume and polar oxygen-containing groups. In addition, sonication-assisted-pickling regeneration process achieves efficient regenerations and enhancements of Cu(II) adsorptivity for ACFFs. The Cu(II) adsorptivity and regeneration rate of ACFFs are still up at 25.51 mg/g and 379.59% after five times recycling by the process of sonication-assisted pickling regeneration process.

Enhanced Electrosorption Ability of Carbon Nanocages as an Advanced Electrode Material for Capacitive Deionization

The structure of an electrode material has an important impact on the performance of a capacitive deionization (CDI) device. However, it is still a challenge to design and synthesize electrode materials with a rational structure based on deep understanding of their structure-dependent CDI performance. Herein, we report the preparation of carbon nanocages (CNCs) with regulated shell thickness and a rich pore structure as an advanced material for high-performance CDI electrodes. The as-prepared CNC has a considerable specific capacitance of 149 F g^-1 at a scan rate of 5 mV s^-1. When used as CDI electrodes, the CNC shows an outstanding electrosorption ability of 17.5 mg g^-1 at 1.4 V at an initial concentration of 250 mg L^-1 NaCl solution. Furthermore, the CNC electrode displays high salt adsorption rate and good cyclic stability. Finite element simulations reveal that the superior structure of the CNC substantially promotes the ion transfer rate by shortening ion diffusion paths in the cavity of the electrode material. Also, both inner and outer walls of the CNC provide sufficient active sites for fast adsorption and desorption of salty ions. This work not only demonstrates that the CNC is a potential electrode material for CDI applications but also paves a way to design and prepare high-performance electrode materials based on a new perspective on their structure-performance relationship.

Sustainable lignin-derived hierarchically porous carbon for capacitive deionization applications

Cross-linked lignin with glyoxal leads to a support mesopore structure of lignin-based porous carbon with improved capacitive deionization performance.

3D Bombax-structured carbon nanotube sponge coupling with Ag3PO4 for tetracycline degradation under ultrasound and visible light irradiation

A novel photocatalytic carbon nanotube sponge with three-dimensional Bombax-structure was fabricated by a facile chemical vapor deposition followed by in situ ion-exchange approach. The as-prepared sponge achieved both high-efficiency adsorption and photocatalysis towards antibiotics, which can remove up to 90% of tetracycline within an hour. The morphology and mechanism of the photocatalytic CNT sponge were explored by multiple measures. Results show the functional groups and high specific surface area of CNT sponge play vital roles in preparing this Bombax-structured Ag₃PO₄/CNT sponge, the band gap of which can be tuned by varying the ration between Ag₃PO₄ and CNT. The photodegradation experiments of tetracycline with the assistance of ultrasound irradiation were performed, Ag₃PO₄/CNT sponge exhibits preferable photocatalytic activity, which can be attributed to both the enhancement of specific surface area of Ag₃PO₄ and the cavitation effect on CNT surface. The efficiency contributed by ultrasound could account for more than half of the degradation efficiency when the ultrasound power was 100 W. The improvement in transfer efficiency and the delay in charge recombination of Ag₃PO₄/CNT sponge were further verified by Electrochemical impedance spectra (EIS) and Photoluminescence tests (PL). Reactive free-radical species were detected by the Electron Spin Resonance (ESR). The intermediates and possible pathway were analyzed by gas chromatography-mass spectrometer (GC-MS) technique.

Enhanced Hybrid Capacitive Deionization Performance by Sodium Titanium Phosphate/Reduced Porous Graphene Oxide Composites

In this study, sodium titanium phosphate/reduced porous graphene oxide (NTP/rPGO) composites are used as novel electrode materials for hybrid capacitive deionization (HCDI). The composites are synthesized through assembling the NaTi₂(PO₄)₃ precursor with etched graphene oxide under hydrothermal condition. The NTP/rPGO composites demonstrate a porous hierarchical structure, where uniformly dispersed NaTi₂(PO₄)₃ particles are attached on the rPGO sheets, which provide abundant adsorption sites, highly conductive networks, and short diffusion lengths for salt ions. Benefiting from the redox reaction of the NTP and electrical double-layer capacity of the rPGO, the NTP/rPGO composite containing 77 wt % NaTi₂(PO₄)₃ presents a high specific capacity of 396.42 F g^-1 and a high electrosorption capacity of 33.25 mg g^-1 at the voltage of 1.4 V with the initial salt conductivity of 1600 μS cm^-1 (786 mg L^-1). Further, it also shows excellent recycling stability and rapid desalination rate of 0.30 mg g^-1 s^-1 (100 times as fast as the bare graphene electrode). Therefore, the NTP/rPGO composites exhibit a promising prospect for desalination application in the HCDI system.

Electrospun Nanomaterials for Energy Applications: Recent Advances

Electrospinning is a simple, versatile, cost-effective, and scalable technique for the growth of highly porous nanofibers. These nanostructures, featured by high aspect ratio, may exhibit a large variety of different sizes, morphologies, composition, and physicochemical properties. By proper post-spinning heat treatment(s), self-standing fibrous mats can also be produced. Large surface area and high porosity make electrospun nanomaterials (both fibers and three-dimensional fiber networks) particularly suitable to numerous energy-related applications. Relevant results and recent advances achieved by their use in rechargeable lithium- and sodium-ion batteries, redox flow batteries, metal-air batteries, supercapacitors, reactors for water desalination via capacitive deionization and for hydrogen production by water splitting, as well as nanogenerators for energy harvesting, and textiles for energy saving will be presented and the future prospects for the large-scale application of electrospun nanomaterials will be discussed.

Fabrication of a 1D Mn3O4 nano-rod electrode for aqueous asymmetric supercapacitors and capacitive deionization

The Mn₃O₄ nano-rods & rGO system realizes the utilization of energy effectively for asymmetric supercapacitors and capacitive deionization.

Efficient Capacitive Deionization Using Natural Basswood-Derived, Freestanding, Hierarchically Porous Carbon Electrodes

Carbon electrodes are of great importance in constructing high-performance capacitive deionization (CDI) devices. However, the use of conventional carbon electrodes for CDI is limited because of their poor mechanical stability and low mass loading. Herein, we report a binder-free, freestanding, robust, and ultrathick carbon electrode derived from a wood carbon framework (WCF) for CDI applications. The WCF inherits the unique structure of natural basswood, containing straightly aligned channels interconnected with highly ordered, open, and hierarchical pores. A CDI device based on thick WCF electrodes (1200 μm, equal to a mass loading of 50 mg cm^-2) exhibits a remarkable areal salt adsorption capacity (SAC) of 0.3 mg cm^-2, a high volumetric SAC of 2.4 mg cm^-3, and a competitive gravimetric SAC of 5.7 mg g^-1. Also, the good mechanical strength and water tolerance of the WCF electrodes improve the cycling stability of the CDI device. Finite element simulations of ion transport behavior indicate that the unique structure of the WCF substantially facilitates ion transport within the ultrathick CDI electrodes. This work provides a viable route to the rational design of freestanding and ultrathick electrodes for CDI applications and offers insights into the structure-performance relationship of CDI electrodes.

In situ combined analysis of gases and electrochemical signals of an activated carbon-based supercapacitor at 2.7–4 V

An activated carbon (AC) electrode-based capacitor soft-pack was incorporated into a gas bag, and the gases, if produced during the charge and discharge of the capacitor, can be online analyzed by Gas chromatography. This method is effective to test the stability of capacitors using PC-based organic electrolytes up to 4 V. The kinetics of gas production was obtained and the components of the gases were analyzed quantitatively. Salts in the organic electrolyte were firstly decomposed, rather than the solvent, under the conditions of less water and AC with few functional groups. It provided new insight into understanding the stability of capacitors in high potential.

Charging supercapacitors with high purity activated carbon provided different gas information and suggested the decomposition of salt prior to the solvent (PC).

Concentration-Gradient Multichannel Flow-Stream Membrane Capacitive Deionization Cell for High Desalination Capacity of Carbon Electrodes

We present a novel multichannel membrane flow-stream capacitive deionization (MC-MCDI) concept with two flow streams to control the environment around the electrodes and a middle channel for water desalination. The introduction of side channels to our new cell design allows operation in a highly saline environment, while the feed water stream in the middle channel (conventional CDI channel) is separated from the electrodes with anion- and cation-exchange membranes. At a high salinity gradient between side (1000 mm) and middle (5 mm) channels, MC-MCDI exhibited an unprecedented salt-adsorption capacity (SAC) of 56 mg g^-1 in the middle channel with charge efficiency close to unity and low energy consumption. This excellent performance corresponds to a fourfold increase in desalination performance compared to the state-of-the-art in a conventional CDI cell. The enhancement originates from the enhanced specific capacitance in high-molar saline media in agreement with the Gouy-Chapman-Stern theory and from a double-ion desorption/adsorption process of MC-MCDI through voltage operation from -1.2 to +1.2 V.

Use of Soft Electrodes in Capacitive Deionization of Solutions

All efforts to obtain, reuse or purify water are extremely significant for society. Recently, researchers have begun to delve in an idea born decades ago: the desalination of water using highly porous electrodes. It is based on a fundamental aspect of electrical double layers, namely, their huge capacitance. The ions of a solution can be partially removed under the application of an electric field when the solution fills the space between porous electrodes, either bare (CDI, or capacitive deionization), coated with ionic exchange membranes (MCDI) or chemically treated (inverted-CDI or i-CDI). One of the challenges of the last years was to explore new materials and arrangements to improve the efficiency of the system. In this work, we propose a new approach inspired in the electrokinetics of soft particles: a layer of polyelectrolyte (cationic on one electrode, anionic on the opposite one) coats the carbon electrodes, converting them in a sort of "soft" electrode pair. We present a theoretical model and a set of experiments showing how soft electrodes can be successfully employed in capacitive deionization.

Study of the adsorption and electroadsorption process of Cu (II) ions within thermally and chemically modified activated carbon

The aim of this work is to modify the porous texture and superficial groups of a commercial activated carbon through chemical and thermal treatment and subsequently study the kinetics of adsorption and electroadsorption of Cu (II) ion for these carbons. Samples of three activated carbons were used. These were a commercial activated carbon, commercial activated carbon modified thermically (C-N₂-900) and finally commercial activated carbon modified chemically C-SO₂-H₂S-200. The activated carbons were characterized chemically and texturally and the electrical conductivity of them determined. Different kinetic models were applied. The kinetics of the adsorption and electroadsorption process of the Cu (II) ion fits a pseudo second order model and the most likely mechanism takes place in two stages. A first step through transfer of the metal mass through the boundary layer of the adsorbent and distribution of the Cu (II) on the external surface of the activated carbon and a second step that represents intraparticle diffusion and joining of the Cu (II) with the active centres of the activated carbon. Finally, the kinetics of the adsorption process are faster than the kinetics of the electroadsorption but the percentage of the Cu (II) ion retained is much higher in the electroadsorption process.