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ULTRASOUND IN ELECTROCHEMISTRY: SONOELECTROCHEMISTRY
Historical Perspective
Nowadays we take it for granted that the effects of cavitation will strongly influence electrochemistry. It can be readily appreciated that there are general benefits with the application of ultrasound which include keeping the electrodes clean, degassing at the electrode surface, improved mass transport to the electrode and the disturbance of the diffusion layer resulting in minimal ion depletion. In the 1930’s there were two types of application that were initiated. The basic science of ultrasonic effects on electrode processes e.g. the work of Moriguchi who showed that ultrasound reduced the decomposition voltage of water at platinum [1] and the remarkable improvements reported by Young and Kersten when ultrasonic radiation was applied to electrodeposition [2]. Some 20 years later Yeager reviewed these effects in a general survey of the applications of ultrasonic waves in electrochemistry which were discussed in terms of (1) the effects of ultrasonic waves on electrode processes (2) electrokinetic phenomena involving ultrasonic waves and (3) ultrasonic waves as a tool in the study of the structure of electrolytic solutions [3]. In the same decade Rich published a paper which has proved to be quite fundamental in subsequent studies leading to modern ultrasonically assisted plating [4]. The first use of the term sonoelectrochemistry can be traced to 1990.
1. Morigushi, N., The effect of supersonic waves on chemical phenomena, (III).The effect on the concentration polarization J. Chem. Soc. Jpn,. 55: 749-750 (1934) 2. Young, W. and H. Kersten, Effect of ultrasonic radiation on electrodeposits. J Chem Phys,,. 4: 426-455. (1936) 3. Yeager, E. and F. Hovorka, Ultrasonic Waves and Electrochemistry. I. A Survey of the Electrochemical Applications of Ultrasonic Waves.. 25: 443-455. (1953) 4. Rich, R., Improvement in electroplating due to ultrasonics. Plating, 42: 1407-1411. (1955) 5. Mason, T.J., Lorimer, J.P. and and Walton, D.J., Sonoelectrochemistry, Ultrasonics, 28, 333‑337 (1990).
General Overview
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. 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.
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). Sonoelectrochemical (20 kHz) production of Co65Fe35 alloy nanoparticles from Aotani solutions, M. Dabala, B. G. Pollet, V. Zin, E. Campadello and T. J. Mason, J Appl Electrochem, 38, 395-402, 2008 Electrocrystallisation of lead dioxide: Analysis of the early stages of growth M.Y. Abyaneh, V. Saez, J. González-García and T.J. Mason, Electrochimica Acta, 55, 3572–3579, 2010
Review articles
Saez, V. and T.J. Mason, Sonoelectrochemical Synthesis of Nanoparticles. Molecules, 14(10): 4284-4299 (2009) Gonzalez-Garcia, J., et al., Current topics on sonoelectrochemistry. Ultrasonics, 50(2): 318-322. (2010) Cobley, A J; Mason, T J; Saez, V., Review of effect of ultrasound on electroless plating processes, Transactions of the Institute of Metal Finishing, 89 (6)
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