Applications of ultrasound in electrochemistry

Silva-Martínez, Susana (1997) Applications of ultrasound in electrochemistry. University of Southampton, Doctoral Thesis.

Abstract

The effect of the ultrasound on electrochemical processes has been investigated employing a microelectrode within the cavitating media. Transient mass transport was strongly enhanced in the presence of ultrasound. High rates of mass transfer of up to 1.5 cm s^-1 were observed. These high rates of mass transfer were attributed to two cavitation processes. First, bubble collapse at or near the solid-liquid interface with the consequent formation of a high speed liquid microjet directed at the electrode surface. Second, bubble motion near or within the diffusion layer of the electrode.

High time resolution single current transients were also recorded. These single current transients were attributed to the short-time perturbation of the diffusion field of the microelectrode due to impacts of cavitation bubble collapse followed by a long time relaxation of the diffusion field back to the steady state. The influence of the ultrasonic source to electrode separation, temperature of the bulk solution, electrode potential and electrode size on the magnitude of current transients was also studied. All of these parameters affected markedly the magnitude of the current transients recorded at microelectrode in the presence of ultrasound.

An alternative approach is presented to characterise fast heterogeneous electron transfer reactions employing ultrasound as a mass transport enhancement tool. Two innovative techniques, sampled-current voltammetry and sampled-mean current voltammetry, were developed during the course of this thesis. The technique of sample-current voltammetry reported values of the standard rate constant of heterogeneous electron transfer of up to 1.2 cm s^-1 in the presence of ultrasound. This technique focuses on the electrochemical phenomena under investigation at the point of impact of the ultrasonic event, produced by asymmetric cavitation bubble collapse near the electrode surface.

Bubble dynamics was also examined under the experimental conditions employed in the present study. The bubble behaviour was observed from the numerical solution of the RPNNP equation that describes the motion of a gas filled bubble in a homogeneous phase. This equation was solved numerically employing the Four-order Runge-Kutta method.

Finally, a preliminary study of surface process in the presence of ultrasound is presented. High time resolution erosion-current events were recorded. These current events were attributed to the reoxidation of the exposed metal as a result of cavitation events. This study shows that surface erosion can be electrochemically investigated in-situ within a cavitating medium.

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