Sonoelectrochemistry in aqueous electrolyte: A new type of sonoelectroreactor

The use of non-cavitating coupling fluids for intensifying sonoelectrochemical processes

For the first time, we have investigated the beneficial effects of non-cavitating coupling fluids and their moderate overpressures in enhancing mass-transfer and acoustic energy transfer in a double cell micro-sonoreactor. Silicon and engine oils of different viscosities were used as non-cavitating coupling fluids. A formulated monoethylene glycol (FMG), which is a regular cooling fluid, was also used as reference. It was found that silicon oil yielded a maximum acoustic energy transfer (3.05 W/cm²) from the double jacketed cell to the inner cell volume, at 1 bar of coupling fluid overpressure which was 2.5 times higher than the regular FMG cooling fluid. It was also found that the low viscosity engine oil had a higher acoustic energy value than that of the high viscosity engine oil. In addition, linear sweep voltammograms (LSV) were recorded for the quasi-reversible Fe^2+/Fe³⁺ redox couple (equimolar, 5 × 10^-3 M) on a Pt electrode in order to determine the mass-transport limited current density (j_lim) and the dimensionless Sherwood number (Sh). From the LSV data, a statistical analysis was performed in order to determine the contribution of acoustic cavitation in the current density variation |Δj|_average. It was found that silicon oil at 1 bar exhibited a maximum current density variation, |Δj|_average of ~2 mA/cm² whereas in the absence of overpressure, the high viscosity engine oil led to a maximum |Δj|_average which decreased gradually with increasing coupling fluid overpressure. High viscosity engine oil gave a maximum Sh number even without any overpressure which decreased gradually with increasing overpressure. The Sh number for silicon oil increased with increasing overpressure and reached a maximum at 1 bar of overpressure. For any sonoelectrochemical processes, if the aim is to achieve high mass-transfer and acoustic energy transfer, then silicon oil at 1 bar of overpressure is a suitable candidate to be used as a coupling fluid.

Sonoelectrochemistry for energy and environmental applications

Sonoelectrochemistry is the study of the effects and applications of ultrasonic waves on electrochemical processes. The integration of ultrasound and electrochemistry offers many advantages: fast reaction rates, enhanced surface activation, and increased mass transport at an electrode. Significant progress has been made in advancing basic and applied aspects of sonoelectrochemical techniques, which are herein reviewed by addressing the development and applications of sonoelectrochemical processes in energy and environmental areas. This review examines the experimental procedures that are used in various sonoelectrochemical techniques generally used for the synthesis of energy related materials (e.g., fuel cell electrocatalysts and materials for hydrogen production) and for the degradation of various organic compounds/pollutants. The challenges that remain for the sonoelectrochemical production of energy materials, the degradation of organic pollutants, and their associated reaction pathway mechanism(s) are also discussed. This review also highlights the significant improvements made to date. The provided information in this review may be helpful to scientists working in the research areas of environmental remediation, energy exploitation and exploration, as well as synthetic process-oriented research.

Recent developments in the sonoelectrochemical synthesis of nanomaterials

In recent years, the synthesis and use of nanoparticles have been of special interest among the scientific communities due to their unique properties and applications in various advanced technologies. The production of these materials at industrial scale can be difficult to achieve due to high cost, intense labour and use of hazardous solvents that are often required by traditional chemical synthetic methods. Sonoelectrochemistry is a hybrid technique that combines ultrasound and electrochemistry in a specially designed electrochemical setup. This technique can be used to produce nanomaterials with controlled sizes and shapes. The production of nanoparticles by sonoelectrochemistry as a technique offers many advantages: (i) a great enhancement in mass transport near the electrode, thereby altering the rate, and sometimes the mechanism of the electrochemical reactions, (ii) a modification of surface morphology through cavitation jets at the electrode-electrolyte interface, usually causing an increase of the surface area and (iii) a thinning of the electrode diffusion layer thickness and therefore ion depletion. The scalability of sonoelectrochemistry for producing nanomaterials at industrial scale is also very plausible due to its "one-pot" synthetic approach. Recent advancements in sonoelectrochemistry for producing various types of nanomaterials are briefly reviewed in this article. It is with hope that the presentation of these studies therein can generate more interest in the field to "catalyze" future investigations in novel nanomaterial development and industrial scale-up studies.

Sonoelectrochemical one-pot synthesis of Pt - Carbon black nanocomposite PEMFC electrocatalyst

Simultaneous electrocatalytic Pt-nanoparticle synthesis and decoration of Vulcan XC-72 carbon black substrate was achieved in a novel one-step-process, combining galvanostatic pulsed electrodeposition and pulsed ultrasonication with high power, low-frequency (20kHz) ultrasound. Aqueous chloroplatinic acid precursor baths, as well as carbon black suspensions in the former, were examined and decoration was proven by a combination of characterization methods, namely: dynamic light scattering, transmission electron microscopy, scanning electron microscopy with EDX-analysis and cyclic voltammetry. In particular, PVP was shown to have a beneficial stabilizing effect against free nanoparticle aggregation, ensuring narrow size distributions of the nanoparticles synthesized, but is also postulated to prevent the establishment of a strong metal-substrate interaction. Current pulse amplitude was identified as the most critical nanoparticle size-determining parameters, while only small size particles, under 10nm, appeared to be attached to carbon black.

Sonochemistry in the service of SOFC research

Decoration of SOFC anode cermets with metal nanoparticles (NPs) enchance their ability and stability in natural gas to hydrogen reform. A novel sonoelectrochemical approach of Au-NPs synthesis (mean 12.31±2.69nm) is suggested, according to which the sonication is held constant while the electrochemical activity is either pulsed or continuous. The gold colloidal solution is cosonicated with state of the art cermet powder to yield particles decorated with Au-NPs. Nevertheless sonochemical routes of mixed molybdenum, rhenium or tungsten mixed oxides synthesis are utilized in order to decorate SOFC anode cermets. The decoration loading achieved spanned from 0.1 to 10.0wt.%.

Production of Metal Nanopowders by Sonoelectrochemistry

Metal powders with a diameter ranging from 20 to 50 micrometers are currently produced by electrolysis at high current density in aqueous electrolytes. A lot of research is then devoted to produce by electrolysis and with a high yield, ultrafine metal powders. Two main techniques are actually developed: strong electrolyte stirring and pulsed currents in order to obtain high current densities and accordingly, high nucleation rates and small nuclei diameters. By combining pulsed current and pulsed ultrasound on the same electrode surface, it has been possible to produce by electrolysis, with a yield ranging from 80 to 95%, crystalline metal powders with a sharp diameter distribution around 100nm. Possible explanations of these results are presented. This new technique, called sonoelectrochemistry, is illustrated by metal powders analysed by HIRTEM, X-ray diffraction and laser diffusion. The perspectives of sonoelectrochemistry for the production of metal or ceramic nanopowders are studied.

Production of Magnetic Nanopowders by Pulsed Sonoelectrochemistry

Sonoelectrochemistry, or pulsed electrodeposition at high current density in presence of high intensity ultrasound, is used to produce magnetic powders with a mean diameter in the range of 100 nm. Pure iron, cobalt and nickel powders are produced together with their binary and ternary alloys. The powders are crystalline and homogeneous as observed by scanning and transmission electron microscopy, electron diffraction and X-ray fluorescence. The compositions of the binary and ternary alloy powders reproduce strictly the iron, nickel and cobalt compositions of the starting electrolytes except for the nickel-cobalt alloys.

Sono and electrochemical synthesis and characterization of copper core-silver shell nanoparticles

Cu-Ag core-shell nanopowders have been prepared by ultrasound-assisted electrochemistry followed by a displacement reaction. The composition of the particles has been determined by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX). The XRD patterns versus time displacement show that higher are the silver peaks intensities, weaker are the copper ones. That exhibits the progressive recovering of copper by silver. EDX results and quartz crystal microbalance results indicate that various reaction mechanisms are implied in this process. Transmission electron microscopy (TEM) points out variable nanometric diameter grain and some small agglomerates. Elemental mapping obtained by electron energy-loss spectroscopy (EELS) underlines the core-shell structure.

Current topics on sonoelectrochemistry

Sonoelectrochemistry is undergoing a reemerging activity in the last years with an increasing number of papers appearing in a wide range of peer review journals. Applied studies which cover environmental treatments, synthesis or characterization of nanostructures, polymeric materials synthesis, analytical procedures, films preparations, membrane preparations among other interesting applications have been reviewed. The revised fundamental analyses trying to elucidate the mechanism of the interactions between the ultrasound and electrical fields, are focused on test electrochemical processes, on the use of unconventional solvents and combination with other techniques. After the review of the achievements and faults of sonoelectrochemistry, future research lines are suggested.

Synthesis and characterization of copper oxide (I) nanoparticles produced by pulsed sonoelectrochemistry

Cu(2)O nanopowders have been prepared by ultrasound-assisted electrochemistry with a potentiostatic set-up. Their composition has been determined by X-ray diffraction and energy dispersive X-ray spectroscopy. Transmission electron microscopy and centrifugation analyses indicate that the nanopowders consist of agglomerates of variable nanometric diameter grain. Most of particles have a diameter of 8 nm whatever the electrodeposition potential. The influence of the parameters of electrochemical and ultrasonic pulses on the particle diameter was also studied. The specific surface areas determined by Brunauer-Emmet-Teller (BET) model are very high with a value close to 2000 m(2)g(-1).

Pulsed sonoelectrochemical synthesis of size-controlled copper nanoparticles stabilized by poly(N-vinylpyrrolidone)

A novel sonoelectrochemical method for the size-controlled synthesis of spherical copper nanoparticles in an aqueous phase was developed. In this study, poly(N-vinylpyrrolidone) (PVP) was used as the stabilizer for the copper clusters. The copper nanoparticles were characterized by XRD, UV-vis, IR, DLS, TEM, and HRTEM. The PVP was found to greatly promote the formation rate of copper particles and to significantly reduce the copper deposition rate, thereby making monodispersed copper nanoparticles. We could control the particle size by adjusting various parameters such as current density, deposition, temperature, and sonic power, and improve the homogeneity of the copper particles. The results also showed that the transfer rate of PVP-stabilized copper clusters from the cathodic vicinity to the bulk solution played an important role in the preparation of the monodispersed nanoparticles.

Microelectrode study of single cavitational bubbles induced by 500 kHz ultrasound

Insight is gained into about the processes governing cavitational activity and acoustic streaming induced by high frequency (500 kHz) ultrasound by the use of microelectrodes with short time resolution electrochemical equipment to allow monitoring of the activity of single cavitating bubbles. Current transients are interpreted as showing the flux of solution towards the electrode surface due to microstreaming. In order to explain the current amplitude, a simplified model is produced. Important parameters such as bubble size and shape on the surface as well as the boundary layer thickness for microstreaming are taken into account. This model leads to the amplitude of the oscillations of the cavitating bubble. Introducing realistic bubble sizes, this amplitude is found to be in the order of 1 micron. The conclusions arising from this work allow a further interpretation of previous observations at millimeter scale electrodes.

Ultrasound in photoelectrochemistry: a new approach to the enhancement of the efficiency of semiconductor electrode processes

A novel photoelectrochemical experiment which simultaneously allows the illumination of a TiO2 semiconductor electrode surface and the application of power ultrasound emission is described. The horn probe of an immersion horn transducer is modified by an oxide coated titanium tip and placed in a conventional three electrode electrochemical cell which allows light from a monochromated source to be focussed onto the electrode surface. Well-defined photocurrents are observed in aqueous media and for the photoinduced oxidation of water in acetonitrile and of 2.4-dichlorophenol in acetonitrile. The effect of ultrasound is to shift the observed photocurrent responses to more negative potentials and therefore to enhance the observed processes. Several possible interpretations associated with the complex effects induced by ultrasound are considered and a model based mainly on the extreme change of mass transport at the semiconductor/solution interface is suggested. Considerably enhanced performance for non-Nernstian processes, such as those observed in photoelectrochemical reactions at semiconductor electrodes, can be achieved in the presence of ultrasound.