Low Surface Area Carbon Fiber Electrodes Coated with Nanoporous Thin-Films of γ–Al 2 O 3 and SiO 2 : Relationship between Coating Conditions, Microstructure and Double Layer Capacitance

Abstract

Low surface area carbon fiber sheets coated with nanoporous thin-films of SiO2 and gamma-Al2O3 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 SiO2 or gamma-Al2O3 showed a pore size distribution reflecting that of the parent metal oxide gel. However, those coated with basic SiO2 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. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.056208jes] All rights reserved.