ULTRASOUND IN ELECTROCHEMISTRY:  SONOELECTROCHEMISTRY                

Recent studies have demonstrated that there are several aspects of ultrasound which recommend its use in conjunction with electrochemical processes

·                    Ultrasonic degassing limits gas bubble accumulation at the electrode.

·                    Ultrasonic agitation (via cavitation) disturbs the diffusion layer and stops the depletion of electroactive species.

·                    Ultrasonic agitation provides more even transport of ions across the electrode double layer.

·                    Ultrasonic irradiation continuously cleans and activates the electrode surfaces.

These improvements include enhanced diffusion processes, increased yields, increased current efficiencies, increased limiting currents, lower overpotentials and improved electrodeposition rates. Whilst there may be different origins for the variety of these effects, one well-characterized effect of ultrasonic irradiation is the generation and subsequent collapse of cavitation bubbles both within the electrolyte medium and near to the electrode surface of the electrochemical cell. The electrode surface causes asymmetrical collapse of a bubble which in turn leads to the formation of a high velocity jet of liquid which is directed toward the surface. This jetting is thought to lead to the destruction of the mass transfer boundary layer at the electrode. This improves the overall mass transfer of the system and, as a consequence, the reaction rates at the electrodes.

Electroplating

            Early research into the field of sonoelectrochemistry seems to have been carried out mainly by metallurgists concerned with improving the efficiency of electroplating. Using the simple method of directly sonicating the plating bath considerable savings are possible in processing costs through improvements via a shortening in process time, an increase in the deposition rate and a reduction in the plating current which occurs in conventional electroplating due to polarisation. Research in this domain continues towards improvements in both electroplating and electroless plating.

Electrosynthesis

            Investigations into the influence of ultrasound on electrode reactions and electrosynthesis are of more recent origin. For the reasons outlined above the interfacing ultrasound with electrochemistry appears to hold a lot of potential and the field of sonoelectrochemistry is set to make new strides. For example, the electrochemical oxidation of Fe²⁺ to Fe³⁺, Fe(CN)₆^4- to Fe(CN)₆³ and Cr³⁺ to Cr⁴⁺ have been investigated and the yields and current efficiencies for the reactions were studied at a current density of 0.25 A/mm². It was found that ultrasound accelerates the process and increases the current efficiency and also raises the limiting current density considerably, thus causing a reduction of the diffusion layer thickness resulting in the increased efficiency of the process.

            The application of ultrasound on electrochemical polymerisation of conducting polymers has also been studied. In the case of the electrochemical polymerisation of thiophene ultrasonic irradiation resulted in an improvement of the polymer yield and in a lowering of the anode potential during polymerisation. The polythiophene films that are produced using sonoelectrochemistry have been shown to be flexible and tough in contrast to the more brittle forms produced using conventional technology.

            The effects of ultrasound on electrochemical processes suggest significant benefits. These include modifications to the chemistry of reactions at the electrode and greatly increased current efficiencies.  One major result of these studies could be that, in the future, industrial electrochemistry might become a more attractive proposition.

1                    Sonoelectrochemistry, T.J. Mason, J.P. Lorimer and D.J. Walton, Ultrasonics, 28, 333‑337 (1990).

2                    The Applications of Ultrasound in Electroplating, J.P. Lorimer and T.J. Mason, Electrochemistry, 67, 924-930 (1999).

3                    The Effect of Ultrasonic Frequency and Intensity upon Electrode Kinetic Parameters for the Ag(S₂O₃)₂^3-/Ag Redox Couple, B. Pollet, J.P. Lorimer, S.S. Phull, T.J. Mason and D.J. Walton, The., J. Applied Electrochem, 29, 1359 (1999).

4                    Sonoelectrochemical Recovery of Silver from Photographic Processing Solutions, B. Pollet, J.P. Lorimer, S.S. Phull and J. Y. Hihn, , Ultrasonics Sonochemistry, 69, 7 (2000).

5                    Sonoelectrochemical effects in electro-organic systems, D.J. Walton, J. Iniesta, M. Plattes, T.J. Mason, J.P. Lorimer, S. Ryley, S.S. Phull, A. Chyla, J. Heptinstall, T. Thiemann, H. Fuji, S. Mataka and Y. Tanaka, Ultrasonics Sonochemistry 10,  209-216 (2003).

6                    The sonoelectrooxidation of thiophene s-oxides, J. Iniesta Valcarel, D.J. Walton, H. Fujii, T. Thiemann, Y. Tanaka, S.Mataka, T.J.Mason, J. P. Lorimer, Ultrasonics Sonochemistry 11,  227-232 (2004).

Examples of projects

“Ultrasonically assisted removal of metals from wastewater”

“The effect of ultrasound in combination with uv radiation and/or electrolysis for the biological decontamination of potable water”

“The effect of ultrasound on trivalent chrome plating”

“Preparation of a carbon-based composite electrode for the purpose of electrochemical degradation of chlorinated phenols”

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