Low Surface Area Carbon Fiber Electrodes Coated with Nanoporous Thin-Films of γ–Al₂O₃and SiO₂: Relationship between Coating Conditions, Microstructure and Double Layer Capacitance

Abstract

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.

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].…”

“…However, re-adsorption would be counteracted by the repulsive effect of positively charged alumina nanoparticles [18]. Both coatings were prepared by sol-gel methods described elsewhere [16]. Briefly, a silica sol was made by mixing TEOS and acidified water (pH 3) in a volumetric ratio of 3:20.…”

“…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].…”

“…Finally, doping and/or coating electrodes with non-carbon materials may enhance such asymmetry and the overall CDI performance. In particular, carbons coated with metal oxide nanoparticles have shown to be effective to provide the substrates with surface charges of opposite sign at circumneutral pH [15,16]. Other benefits such as an increase in surface area and wettability have also been observed [16], all leading to improved electrosorption capacity.…”

“…In particular, carbons coated with metal oxide nanoparticles have shown to be effective to provide the substrates with surface charges of opposite sign at circumneutral pH [15,16]. 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].…”