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Capacitive deionization (CDI) has recently gained some degree of popularity as a water treatment technology. This paper demonstrates the benefits of coating low surface area carbon electrodes with a sub-micron nanoporous layer of either γ-Al2O3 or SiO2. As shown by cyclic voltammetry, the oxide layer, having pores sizes ranging from larger micro to meso, substantially increased capacitance for ion removal while still maintaining reasonably high ohmic conductivity. The amount of Ca2+ removed in a CDI cell comprised of a SiO2 coated carbon as the negative and γ-Al2O3 carbon as the positive electrode was nearly five times higher than a cell with uncoated electrodes. Removal of Ca2+ followed pseudo first order kinetics, while Cl- removal was better described by a first order kinetic equation. The highest value for Ca2+ adsorption measured was 0.17 mEq/g of electrode material. This compares favorably with previously reported literature values obtained with high surface area carbons. The removal of Ca2+ and Cl− proved to be asymmetric (mEq Ca2+ > mEq Cl− removed). Consequently, to maintain electroneutrality, pH decreased during the removal process. Furthermore, some of the asymmetry was found to be due to a parasitic reaction, that of Cl− ions being oxidized to ClO3−.

Capacitive deionization (CDI) is a rapidly emerging desalination technology that promises to deliver clean water while storing energy in the electrical double layer (EDL) near a charged surface in a capacitive format. Whereas most research in this subject area has been devoted to using CDI for removing salts, little attention has been paid to the energy storage aspect of the technology. However, it is energy storage that would allow this technology to compete with other desalination processes if this energy could be stored and reused efficiently. This requires that the operational aspects of CDI be optimized with respect to energy used both during the removal of ions as well as during the regeneration cycle. This translates into the fact that currents applied during deionization (charging the EDL) will be different from those used in regeneration (discharge). This paper provides a mechanistic analysis of CDI in terms of energy consumption and energy efficiencies during the charging and discharging of the system under several scenarios. In a previous study, we proposed an operational buffer mode in which an effective separation of deionization and regeneration steps would allow one to better define the energy balance of this CDI process. This paper reports on using this concept, for optimizing energy efficiency, as well as to improve upon the electro-adsorption of ions and system lifetime. Results obtained indicate that real-world operational modes of running CDI systems promote the development of new and unexpected behavior not previously found, mainly associated with the inhomogeneous distribution of ions across the structure of the electrodes.

Activated carbons prepared using polyaniline (PAni), a N-containing precursor, doped with different anions were successfully employed in this work as electrode materials for capacitive deionization. The aim of this research was to investigate the effect of chloride (Cl À ), p-toluenesulfonate (PTS À ), dodecylbenze-sulfonate (DBS À ) and polystirenesulfonate (PSS À ) as PAni dopants on the textural and electrochemical properties of PAni activate carbon (PAC) and evaluate their performance for desalination. It was demonstrated that textural PAC properties such as microporosity could be properly tuned, resulting in a suitable proportion of micro-and mesoporosity by using different doping anions. Furthermore, it was observed that the higher the oxygen content the higher the electrode hidrophilicity due to introduction of surface polar groups, as identified by XPS. These groups were found to be the most important variable influencing on the PAC electrosorption capacity and energy efficiency. The highest specific adsorption capacity (14.9 mg g À1 ), along with the lowest specific energy consumption, was obtained using the PTS-doped PAC electrode. Considering its high capacity, low-cost and ease of synthesis, PAC/PTS seems to be a promising electrode for CDI.

Low surface area carbon fiber sheets coated with nanoporous thin-films of SiO 2 and γ-Al 2 O 3 may provide an alternative to high surface area carbon electrodes for electrosorption and electrical double layer processes. These composites were fabricated using a dip-coating sol-gel process. Studies of this fabrication process demonstrate that sol concentration, number of times dipped and capillary porosity of the carbon sheets were major variables controlling the metal oxide deposition. SEM images of the composites show that a major fraction of the metal oxide deposited appears as a thin-film surrounding the carbon fibers. The BET surface areas of the composites were largely increased, up to 25 times, compared to that of the uncoated carbon sheet. Furthermore, the specific surface area of the composites for a given oxide is directly proportional to the oxide content. Carbon sheets coated with acidic SiO 2 or γ-Al 2 O 3 showed a pore size distribution reflecting that of the parent metal oxide gel. However, those coated with basic SiO 2 produced smaller pore sizes than the parent oxide. Cyclic voltammograms displayed an increase in capacitance with the coatings, but the magnitude of the increase was dependent upon the nature of the metal oxide deposited and in some cases the quantity deposited.

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A novel current collector-free electrode for capacitive deionization to treat brackish water, with low energy consumption and exceptional desalination properties directly linked to the nanoparticle metal oxide/carbon nanotube fibre network morphology.

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