Ultrasound in electrochemistry

A classical and long-established electrochemical synthesis is the formation of alkanes (R-R) from the dimerisation of radicals generated via electro-decarboxylation of carboxylate salts known as the Kolbe reaction. The radical species are produced through a single electron process and 

The application of ultrasound on electrochemical polymerisation of conducting polymers has also been studied. In particular the electrochemical polymerisation of thiophene has been carried out both in the presence and absence of ultrasonic waves [38, 39]. In the absence of ultrasound the anode potential increased with increasing current density. However, polymerisation at high potential gave a lower polymer yield and produced macroscopically heterogeneous films. Ultrasonic irradiation 

Some unusual compounds have been synthesised efficiently through sonoelectrochemistry. The compound digermane Ge(R R R )2 has been manufactured by the electrode reaction of Ge(R R R )X (R = H, alkyl, aryl, alkoxy or amino group) in an aprotic solvent containing Mg, Cu, or A1 as an anode and perchlorate as a supporting electrolyte with ultrasound. This methodology is claimed to be safe and non-toxic with minimal pollution [40]. 

The effects of ultrasound on electrochemical processes remain a largely unexplored area but research suggests significant benefits. These include modifications to the chemistry of reactions at the electrode and greatly increased current efficiencies. The applications of sonoelectrochemistry in environmental control is also a developing area as will be seen in the next section. 

One major result of these studies could be that industrial electrochemistry may become a more attractive proposition. 

---

Thus in summary, some of the particular advantages which accrue from the use of ultrasound in electrochemistry include ... 

Ultrasound may influence the outcome of competing reactions thus, for example, on electrolyzing cyclohexanecarboxylate at a platinum electrode in methanol, a switch from one-electron Kolbe products to two-electron non-Kolbe products was observed. Reviews of the use of ultrasound in electrochemistry have recently appeared [18]. 

The use of ultrasounds in electrochemistry has provided a powerful electroanalytical tool. Thus, ultrasound-enhanced voltammetry possesses some attractive features such as the following ... 

The effect of ultrasound in electrochemistry, i.e. that the application of ultrasonic energy can increase the rate of electrolytic water cleavage, was discovered as early... 

Ultrasound and electrochemistry provide a powerful combination for several reasons. Ultrasound is well known for its capacity to promote heterogeneous reactions, mainly through increased mass-transport, interfacial cleaning, and thermal effects. Effects of ultrasound in electrochemistry may be divided into several important branches (1) Ultrasound greatly enhances mass transport, thereby altering the rate, and sometimes the mechanism, of the electrochemical reactions. 

The distinction in previous sections of electroanalysis, inorganic electrochemistry (particularly metal systems), and electroorganic synthesis leaves out a number of other electrochemical systems. Ultrasound has been applied to many of these, to interesting effect, and this section concerns a number of such systems. There is, of course, overlap in any attempt at compartmentalization, and here some studies on batteries, electrochemiluminescence, and micellar systems could be considered as contributing to electroanalysis, while other multiphase electrolyses might be considered as electrosynthesis. In addition, most multiphase electrolysis is directed to the destruction of haloorganics and is aimed at waste treatment. There are also one-off applications of ultrasound in electrochemistry, which are collected at the end of this section. 

The major thrust of this work has, in particular, concerned the electrolytic dechlorination of polychlorinated biphenyls and similar species for environmental control of pollutants. However, there is a wider potential for this technology in the use of aqueous solvent systems for syntheses involving otherwise water-immiscible organic compounds. Water is of course the cheapest, most widely available and environmentally friendly solvent and with modem concerns over ecology and the search for clean technologies there is considerable opportunity for this particular application of ultrasound in electrochemistry. 

The best established effect of ultrasound in electrochemistry is the diminution of the diffusion layer and the enhanced limiting currents so produced. This is, per se, of benefit towards sensitivity improvement in electrochemical sensors, and is also the origin of many sonoelectrochemical phenomena. Ultrasound also affects electrode surfaces, which has been exploited as a pretreatment protocol, and has a beneficial effect during electrolysis. 

The simultaneous application of ultrasonic irradiation to an electrochemical reaction which has been termed sonoelectrochemistry has been shown to produce a variety of benefits in almost any electrochemical process. These include enhanced chemical yield in electrosynthesis and the control of product distribution improved electrochemical efficiency in terms of power consumption, improved mixing, and diffusion in the cell minimization of electrode fouling accelerated degassing and often a reduction in the amount of process-enhancing additives required. In a major chapter devoted to this topic, Suki Phull and Dave Walton have attempted to cover the majority of applications of ultrasound in electrochemistry including electrochemical synthesis, electroanalytical chemistry, battery technology, electrocrystallization, electroinitiated polymerization, and electroplating. 

Kumbhat S (2000) Potentialities of power ultrasound in electrochemistry an overview. Bull Electrochem 16 29-32... 

Klima J (2011) Applicatitm of ultrasound in electrochemistry. An overview of mechanisms and design of experimental arrangement. Ultrasonics 51 202-209... 

In organic synthesis, sonication has a long tradition. Ultrasound in electrochemistry has been introduced as a means to enhance convection in coulometric cells [286, 287]. Most important application became Anal Chem [288, 289]. Very popular was electrochemical stripping analysis [290], since in this case the action of ultrasound can be restricted to the electrochemical trace accumulation, where no interaction between power sound and sensitive signal measurement has to be suspected. 

Lorimer P, Mason TJ (1999) Sonoelectrochemisry. The application of ultrasound in electroplating. Electrochemistry 67 924—930... 

The advantages of using ultrasound in the electrochemistry of organic compounds has been reviewed elsewhere [40]. The combination has been found to be particularly useful in the destruction of chemical pollutants. It has been applied to the destruction of phenols by electrochemical oxidation. Ultrasound (25 kHz, lO W m ) when applied... 

This section deals with the use of ultrasound in the general area of Inorganic Electrochemistry , and the areas that are reviewed here include ... 

Since the mid-1970s there has been a considerable amount of material published on the influence of ultrasound upon the electrochemistry of metal systems. Most of this work was carried out in former Eastern block countries and concentrated on such electrochemical processes as corrosion, electrodeposition, and electrochemical dissolution. Recently there has been an upsurge in the interest shown in sonoelectrochemical processes using both non-metal and metal systems worldwide. There have been a considerable number of publications in the employment of ultrasound in areas as diverse as semiconductor production to sono-electrochemical machining and metal finishing. A review by R. Walker [27] into the use of ultrasound in metal deposition systems, provides an introduction into the fundamental effects of ultrasound in plating and metal finishing. 

Ultrasound recently has been studied as a new and novel technique in the removal and breakdown of both chemical and biological pollutants, and there are several articles in the literature dealing with this subject [243,244]. Over the past few years there has been considerable interest shown in combining the two techniques of ultrasound and electrochemistry for the treatment of a variety of pollutants, both biological and chemical. Some of the most recent and important papers in this field are reviewed below. 

With electrochemistry The advantages of using ultrasound in conjunction with electrochemistry have already been referred to above (Table 10.6). This combination has been particularly beneficial in the destruction of phenols by electrochemical oxidation. Ultrasound (25 kHz, lO" W m" ) when applied to a solution containing phenol (100 g 1 ) and NaCl (2 g 1 ) achieves better than 80% oxidation to maleic acid [55]. In the absence of ultrasound only 50% decomposition was obtained under the same conditions. In a more recent study Berlan et al. have achieved almost complete sonochemical destruction of phenol in saline solution at pH 6 in 10 min at a current density of 170 A m [56]. The reaction was shown to proceed via intermediate chlorinated phenols. 

A recent approach to this end has been the combination of ultrasound with electrochemistry. Perhaps the most striking influence of ultrasound concerns heterogeneous systems as shown in previous chapters, and a typical heterogeneous interface is that between an electrode surface and an electrolyte. 

An example of enhancement in mass transfer by acoustic cavitation is the increase in the limiting current density in electrolysis [79], The electrochemistry with ultrasound is called sonoelectrochemistry. Another example is ultrasonic cleaning [80], Soluble contaminants on a solid surface dissolve into the liquid faster with acoustic cavitation. Insoluble contaminants are also removed from a solid surface with ultrasound. This is also induced by acoustic cavitation in many cases, but in some other cases it is by acoustic streaming [81-85],... 

Marken F, Compton RG (1996) Electrochemistry in the presence of ultrasound the need for bipotentiostatic control in sonovoltammetric experiments. Ultrason Sonochem 3 S131 —S134... 

Sonoelectrochemistry can be considered as the interaction of sound (hence SONO) with electrochemistry which is itself the interconversion of electrical and chemical energies. Whilst this chapter will concentrate on the application of ultrasound to important industrial processes such electrodeposition (or electroplating) and electo-or-ganic synthesis, it is important to first introduce the concept of electrochemistry, for those who are unfamiliar, so that we will have a better understanding as to what precisely happens in an electrochemical or electroplating process and how the application of ultrasound will be of benefit. 

---