Asymmetric Capacitive Deionization Utilizing Low Surface Area Carbon Electrodes Coated with Nanoporous Thin-Films of Al₂O₃and SiO₂

Abstract

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−.

Evaluation of operational parameters for a capacitive deionization reactor employing asymmetric electrodes

“…The carbon support used in this work was chosen because of its high surface area, relatively cheap cost and availability. Thus, this material, unlike other metal-oxide coated composites described elsewhere [15][16][17][18], has a surface area over 1200 m 2 g À1 . Finally, most of the work in CDI has focused on desalination/brackish water treatment [22,23], making NaCl solutions the most studied systems [7][8][9]14,21,24,25].…”

“…The specific surface area was calculated from the BET equation using relative pressures in the data range between 0.06 and 0.2. The PSD was determined by fitting theoretical adsorption isotherms based on the DFT model, which produced the fit with the smallest standard deviation [17]. The quantity of metal oxide that had been deposited on the substrate was determined by thermo-gravimetric analysis using a Mettler Toledo TGA/SDTA851e device, following a methodology previously described [17].…”

“…This study was based on an asymmetric CDI configuration [2,[15][16][17][18], where the positive and negative electrodes in the system are not the same. The negative electrode is often optimized for cation electrosorption and/or anion rejection, whereas the positive electrode would be improved for anion sorption and/or cation rejection [2,19].…”

“…Other benefits such as an increase in surface area and wettability have also been observed [16], all leading to improved electrosorption capacity. However, studies in these asymmetric oxide-coated electrodes have been limited to small-scale batch experiments [15,17,18]. Therefore, performance issues associated with operating a CDI system furnished with these electrodes, handling larger processing volumes, need to be understood in order to asses the commercial feasibility of this technology.…”

Capacitive deionization with asymmetric electrodes: Electrode capacitance vs electrode surface area

“…These imbalances have been used to selectively remove ions from solution [21,[28][29][30], but could adversely affect the equilibrium salt adsorption capacity of a CDI cell. A recent study reported that the effect of asymmetry in anode and cathode geometric ratios (where anode was 2x and 3x times larger than the cathode and vice-versa) led to an increased salt adsorption capacity, which was stated to be dependent on ionic sizes and governed by the requirement to maintain charge neutrality in solution.…”