Sonoelectrochemistry of molecular and colloidal redox systems at carbon nanofiber-ceramic composite electrodes

An electrically conducting composite electrode consisting of a ceramic paper substrate, which has been densely coated with carbon nanofibers of ca. 100 nm diameter, is employed in the presence of 24 kHz (8 W cm^-2) power ultrasound emitted from a glass horn system. The carbon composite electrode remains stable during prolonged use in the presence of ultrasound. Sonovoltammetric limiting currents for the reduction of Ru(NH₃) ₆³⁺, the oxidation of hydroquinone, and the reduction of colloidal hydrous iron oxide are reported. A comparison of sonovoltammograms obtained at a porous carbon nanofiber-ceramic composite electrode with those obtained at a conventional glassy carbon electrode shows that (i) the average mass transport limited current density at carbon nanofiber-ceramic composite electrodes is increased by approximately one order of magnitude, and (ii) due to the porous topography of the electrode surface, molecules remain resident within the diffusion layer for a longer period of time. Colloidal hydrous iron oxide is not reduced at conventional glassy carbon electrodes but can be reduced at the porous carbon nanofiber-ceramic composite electrode, presumably due to a more effective particle-carbon nanofiber electrode surface interaction.