Asymmetric Capacitive Deionization Utilizing Low Surface Area Carbon Electrodes Coated with Nanoporous Thin-Films of Al₂O₃ and SiO₂

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⁻.