(Invited) Sonoelectrochemistry: A Powerful Tool for Elucidating Mechanisms and for Accelerating Electrochemical Processes
Authors: Hihn Jean-Yves , Doche Marie-Laure , Hallez Loic , Et Taouil Abdeslam , Pollet Bruno G. 
Published: February 2020

Surface irradiation by power ultrasound has proven to offer beneficial effects not only for surface cleaning, but also for the modification of functional properties of metallic and organic coatings. However, the process scale-up has failed to match laboratory observations and data, in particular for the design of industrial sono-(electro)chemical reactor systems. To solve this major problem, electrochemical systems have been developed and employed as effective “sensors” for probing ultrasonic activity, in turns allowing numerous useful quantitative information to aid designing better sono-(electro)chemical reactor and process control. Indeed, it is possible to use electrochemistry as a tool to investigate phenomena occurring at the electrode surface and at a given location in the sono-(electro)chemical reactor. From a process accelerated by power ultrasound, electrochemistry becomes a very useful tool to quantify the ultrasonic energy (and power) scattered at the immediate working electrode surface vicinity. The electro-diffusional method is of great help, as it consists of measuring limiting currents generated by Linear Sweep Voltammetry (LSV) experiments. This approach has turned out to be extremely efficient and useful for the determination of acoustic intensity at various locations in the sono-(electro)chemical reactor [1]. By systematically moving a working electrode in an ultrasonic field, it is possible to map out the acoustic activity, especially in the zone close to the ultrasonic transducer where the most intense cavitation activity takes place. Then, to demonstrate the ‘portability’ of our generated data, i.e. to allow relevant comparisons between experiments performed using different ultrasonic and electrochemical equipment, we have proposed to convert the raw electrochemical values into equivalent velocities U, corresponding to normal flows directed towards the working electrode surface resulting in the same electrochemical signal than in the presence of power ultrasound [2]. Comparisons with ‘real fluid motion’ at a given location with the help of Particle Image Velocimetry (PIV) technique allows separation of the respective contributions of cavitational events occurring at the working electrode surface to the convection flow called ultrasonic wind [3]. This approach has been extended to several electrochemical systems. Moreover, new types of ultrasonic transducers (for example, focalized ultrasonic transducers) and new progress in the modulation of ultrasonic transducer excitation have pave the way to industrialization of sono-(electro)chemical reactors [4].

[1] Hihn J.-Y., Doche M.L, Mandroyan A., Hallez L. and Pollet B.G., Chapter 23 "Ultrasound and Better Reactor design" in Handbook on Applications of Ultrasound: Sonochemistry for sustainaibility by CRC Press Taylor & Francis, 2011, p599-622

[2] Pollet B.G., Hihn J.-Y., Doche M.L, Mandroyan A., Lorimer J.P., Mason T.J “Transport limited currents close to an ultrasonic horn: equivalent flow velocity determination”, Journal of Electrochemistry Society, 2007, 154(10), p E131-E138

[3] J.Y. Hihn, M.L. Doche, A. Mandroyan, L. Hallez, B.G. Pollet, "Respective contribution of cavitation and convective flow to local stirring in sonoreactors" Ultrasonics Sonochemistry 18(4), 881-887, (2011)

[4] Hallez L., Lee J., Touyeras F., Nevers A., Ashokkumar M., Hihn J.-Y., “Enhancement and Quenching of HIFU Cavitation Activity via Short Frequency Sweep Gaps”, Ultrasonics Sonochemistry, 2016, 29, p. 194-197

Figure 1