Electrochemical properties of carbon nanoparticles entrapped in a silica matrix

Carbon-based electrode materials have been widely used for many years for electrochemical charge storage, energy generation, and catalysis. We have developed an electrode material with high specific capacitance by entrapping graphite nanoparticles in a sol-gel network. Films from the resulting colloidal suspensions were porous due to the removal of the entrapped organic solvents from the sol-gel matrix giving rise to high Brunauer-Emmett-Teller specific surface areas (364 m(2)/g) and a high capacitance density (similar to 37 F/g). An exponential decrease of capacitance was observed with increasing scan rates in cyclic voltammetry studies on these films, suggesting the presence of pores ranging from micro- (< 2 nm) to mesopores. Nitrogen adsorption-desorption analysis and scanning electron microscope images of these films also confirmed the presence of the micropores as well as mesopores. A steep drop in the double-layer capacitance with polar electrolytes was observed when the films were rendered hydrophilic upon exposure to mild oxygen plasma. We propose a model whereby the microporous hydrophobic sol-gel matrix perturbs the hydration of ions which moves ions closer to the graphite nanoparticles, consequently increasing the capacitance of the film. (C) 2008 The Electrochemical Society.