Carbon fiber sheets coated with thin-films of SiO2 and γ-Al2O3 as electrodes in capacitive deionization: Relationship between properties of the oxide films and electrode performance

Abstract

Nanoporous thin-film coatings of SiO2 and γ-Al2O3 on low surface area carbon fiber sheets may provide an alternative electrode material for capacitive deionization (CDI). In this paper, composite electrodes were fabricated by dipping a low surface area carbon fiber support (less than 2m2/g) into metal oxide sols of either acidic SiO2 (pH 2.9), basic SiO2 (pH 8.3) or γ-AlOOH. Furthermore, the number of times the supports were dipped into the respective sols was varied in an effort to deposit different quantities of the metal oxide. This resulted in composite electrodes with metal oxide contents ranging from 1.5 to 6.7% by weight and specific surface areas from 6 to 35m2/g. When these electrodes were submitted to CDI testing, the rate, as well as the quantity of Ca2+ removed, exceeded that of the uncoated carbon support, independent of the metal oxide coating utilized. Moreover, after 30min of removal, all of the composite electrodes removed >90% of the Ca2+ from a 1.0mM CaCl2 solution – an increase of over 30% as compared to that of the uncoated carbon. In addition, the rate of Ca2+ removal appeared to be dependent upon the nature of the metal oxide coating (SiO2>γ-Al2O3) on the carbon support. The ion removal observed in this work may be driven by electrical double layer formation, specific adsorption, and/or faradaic reactions, such as the oxidation of Cl− to ClO3−. During the regeneration of the electrode, it was shown that the application of an optimal electric potential increased the quantity of Ca2+ desorbed after 30min by approximately 60% for an uncoated pair of electrodes and 90% for a coated pair as compared with electrode regeneration performed under no applied potential. Furthermore, the coated electrode pair was able to withstand a higher voltage for regeneration than an uncoated pair before adsorption of counter-ions exceeded that of co-ions desorption.