Let’s Not Ignore the Ultrasonic Effects on the Preparationof Fuel Cell Materials

Bruno G. Pollet

# Springer Science+Business Media New York 2014

Abstract This article is a follow-up paper recently publishedentitled ‘The importance of ultrasonic parameters in the prep-aration of fuel cell catalyst inks’ [1] describing the effect oflow-frequency high-power ultrasound on the catalyst inkcomposition used for the fabrication of fuel cell electrodes.In this paper, it was shown that care should be taken whenusing low-frequency ultrasound whereby (i) the ultrasonicparameters such as frequency, power and duration may affectthe final ink composition and rheology and therefore its elec-trochemical performance, (ii) the ultrasonic equipment (andmake), frequencies, powers, durations and the distance be-tween the vibrating source and the reaction vessel should bereported, (iii) the catalyst ink temperature should bemonitoredand regulated during the course of the experiment, (iv) im-mersing the ultrasonic probe into the solution may lead tocontamination (arising from the erosion of the titanium alloyvibrating tip) and (v) high-shear mixing of the catalyst inksusing rotor-stator mixers at high rotation speed in silent con-ditions should be performed, analysed and compared toultrasonicated samples for consistency and comparison pur-poses between studies. A careful and systematic approachshould be adopted due to the fact that low-frequency ultra-sound is known to be an intensification technology offeringremarkable advantages: (a) an increase in fluid degasification,de-agglomeration (and particle size reduction), dispersion,homogenisation, emulsification, atomisation, molecular deg-radation and chemical rates and yields and (b) an improve-ment of surfaces due to very efficient cleaning (mainly ero-sion). These ultrasonic effects are known to be caused by (a)an increase in mass transfer and heat transfer induced by

extreme solution ‘mixing’ and (b) the production of cavitationbubbles undergoing very short and violent collapse within thefluid generating local ‘hotspots’ of high energy (temperaturesof up to 5,000 K and pressures of up to 2,000 atms), leading to(i) radicals formation and (ii) jets of liquid of high velocity (upto 200 m s−1) near surfaces.

Keywords Ultrasound . Sonochemistry . Pt/C/Nafion® .

Catalyst ink . Fuel cells

Introduction

Literature search revealed that most of low-temperature fuelcell catalyst ink preparations use ultrasound (usually labora-tory ultrasonic cleaning baths) for the efficient dispersion andhomogenisation of the catalyst inks (Pt/C/Nafion®) prior tofuel cell electrode fabrication. In many cases, the literaturedoes not report the following: (i) the ultrasound source typeand make, the ultrasonic frequency, power, intensity andirradiation time and (ii) the solution temperature (which isoften not controlled and regulated during the course of theexperiment). It is well known that low-frequency ultrasoundyields rapid temperature rises with ΔTs of up to~+50 °C inshort exposure times starting from room temperature. In othercases, ultrasonic irradiation time is mentioned and varies from5-min to 24-h ultrasonication from one study to another. Thus,a careful and systematic approach should be adopted whenultrasonicating fuel cell materials due to the fact that low-frequency ultrasound is known to affect the solutionproperties.

Before discussing the effect of ultrasound on Nafion® andfuel cell catalyst inks, the review will first focus on the effectof power ultrasound in chemistry (sonochemistry), cavitationphenomena, ultrasonic equipment and chemical cells used insonochemistry and ultrasonic power determination methods.

B. G. Pollet (*)Faculty of Natural Sciences, South African Institute for AdvancedMaterials Chemistry (SAIAMC), University of the Western Cape,Robert Sobukwe Road, Bellville, Cape Town 7535, South Africae-mail: bgpollet@uwc.ac.za

ElectrocatalysisDOI 10.1007/s12678-014-0211-4

Many of us are familiar with the use of ultrasound, forexample, in biochemistry, ultrasound is commonly used forthe disruption of cells and living tissues in order to extracteffectively important constituents. Ultrasound is also used as amedical imaging tool (e.g., prenatal image scanning) and as adiagnostic tool (e.g. non-destructive testing ofmaterials) in thefrequency range of 2–10 MHz, but recently, there has been anupsurge of interest in the application of low-frequency high-energy power ultrasound (20 kHz to 2MHz). First observed inthe nineteenth century with the discovery of the piezoelectriceffect by Curie and the ultrasonic whistle by Galton, ultra-sound is usually defined as a sound wave with a frequencyabove 16 kHz (16,000 Hz or 16,000 cycles per second) withthe upper limit usually taken to be 5 MHz for gases and500 MHz for liquids and solids (Fig. 1).

The application of ultrasound in chemical, physical andbiological sciences can be divided into two main groups: (i)low-frequency or power ultrasound (20–100 kHz), alsoknown in the 1950s as ‘Macrosonics’ and (ii) high-frequency or diagnostic ultrasound (2–10 MHz).

Diagnostic ultrasound is often used in chemical analysisand medical scanning and in the study of relaxation phenom-ena [2, 3]. Low-amplitude waves are used to determine thevelocity and absorption coefficient of the sound wave by themedium, i.e. the effect of the medium on the ultrasonic wave.However, power ultrasound can be regarded as the effect ofthe sound wave on the medium. Power ultrasound used inliquid systems causes (i) a zone of extreme ‘mixing’ close tothe ultrasonic source (i.e. the ultrasonic transducer), (ii)degassing, (iii) surface cleaning and pitting (erosion) and(iv) an increase in bulk temperature (Fig. 2). This is for thesereasons that low-frequency and high-energy waves are used inultrasonic cleaning, drilling, soldering, chemical processesand emulsification [1].

Ultrasonic cleaning is probably the most common andknown application of ultrasound; however, there are severalareas where power ultrasound has been successfullyemployed (Fig. 3) such as: water and soil remediation (e.g.

destruction of bacteria and organics, heavy metal removaletc), manufacturing of food ingredients and products (e.g.emulsification of oil-/water-based fluids, flavouring and vita-min extraction etc), impregnation of various materials,crystallisation and precipitation of organic and inorganic com-pounds, nanosized materials production, polymerisation, dril-ling, soldering, cutting, plastic welding, surface treatment andpreparation (for activation and modification) prior to platingand electroplating, metal finishing and precision engineering(e.g. the aerospace industry) [2, 3].

More recently, power ultrasound has been successfullydemonstrated as an effective industrial process intensification(PI) technique to treat water effluents and produce pharma-ceutical materials. The systems are used as non-chemicalprocesses to either (i) control and eradicate microbes andbacteria (microbial activity) in contaminated soils and waters(in other words, power ultrasound acting as a powerful bacte-ricide) mainly in the water treatment industry or (ii) producemicro and nanosized pharmaceutical ingredients (processknown as Sonocrystallisation) in the fine chemicals and phar-maceutical industries. The systems are safe, robust and proventechnologies.

Sonochemistry

Over the past few years, the use of power ultrasound has foundwide applications in the chemical and processing industrieswhere it is used to enhance both synthetic and catalytic pro-cesses and to generate new products. This area of research hasbeen termed sonochemistry, which mainly concerns reactionsinvolving a liquid leading to an increase in reaction rates,product yields and erosion of surfaces [2, 3]. However, themain reason for most of the observed effects of ultrasound onsurfaces and chemical reactions is recognised as being due to‘cavitation’ effect which occurs as a secondary effect when anultrasonic wave passes through a liquid medium.

Cavitation was first reported in 1895 by Thornycroft andBarnaby [4] when they observed that the propeller of a sub-marine eroded over short operating times, caused by collaps-ing bubbles induced by hydrodynamic cavitation in turnsgenerating intense pressure and temperature gradients locally.In the late 1920s, Lord Rayleigh [5] published the first math-ematical model describing cavitation in incompressible fluids.It is not until 1927 that the use of ultrasound on chemical andbiological systems was first observed and recognised as auseful tool by Richard and Loomis [6].

As an ultrasonic wave passes through a liquid, fluctuatingpressures are rapidly set up as a result of the alternate periodsof compression and rarefaction associated with the wave[1–3]. During the compression cycle, the liquid is subjectedto a sufficiently positive pressure which pushes the moleculesof the liquid together, whilst during the following rarefactionFig. 1 The frequency ranges of (ultra)sound [2, 3]

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cycle, the liquid is subjected to an equal but negative pressurewhich pulls the molecules of the liquid away from each other.Increasing the amplitude of the wave leads to an increase inthe magnitude of the positive and negative pressures. If theliquid is subjected to a sufficiently large negative pressureduring the rarefaction cycle, the molecules are torn away fromeach other producing very small cavities, called‘microbubbles’. In other words, these negative pressures arestrong enough to overcome the intermolecular forces bindingthe liquid. The process of tearing the liquid apart is known ascavitation and the microbubbles are called ‘cavitation bub-bles’ (Fig. 4) [1–3]. The cavitation threshold is the limit ofsound intensity below which cavitation does not occur in aliquid.

Cavitation phenomenon is well known to cause erosion,emulsification, molecular degradation, sonoluminescence andsonochemical enhancements of reactivity solely attributed to

the collapse of cavitation bubbles [1–3]. It is now well accept-ed in the field that the cavitation bubble collapse leads to nearadiabatic heating of the vapour that is inside the bubble,creating the so-called hotspot in the fluid, where:

1) High temperatures (ca. 5,000 K) and high pressures (ca.2,000 atms/200 MPa) are generated with a collision den-sity of 1.5 kg cm−2 and pressure gradients of 2 TPa cm−1,with lifetimes shorter than 0.1 μs and cooling rates above109–10 K s−1 during the collapsing of cavitation bubblesare observed. Here, water vapour is ‘pyrolysed’ intohydrogen radicals (H·) and hydroxyl radicals (OH·),known as water sonolysis (Fig. 5). Note that the exacttemperatures and pressures generated during cavity im-plosion are difficult both to calculate theoretically and todetermine experimentally.

Fig. 2 The effect of ultrasound ina liquid and near a surface

Fig. 3 The use of ultrasound in industrial applications Fig. 4 Representation of a cavitation bubble imploding in a liquid

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2) The interfacial region between the cavitation bubbles andthe bulk solution is paramount. The temperature is lowerin the interior of the bubbles than the exterior but highenough for thermal decomposition of the solutes to takeplace with greater local hydroxyl radical concentrations inthis region.

3) The reactions of solute molecules with hydrogen atomsand hydroxyl radicals occur in the bulk solution at ambi-ent temperature.

Because of the ultrasound’s ‘extraordinary’ effects, exten-sive work has been carried out in which high-power ultra-sound (20 kHz to 2 MHz) was applied to various chemicalprocesses leading to several industrial applications and manypublications over a wide range of subject areas [2, 3]. It hasbeen shown that the effects of high-intensity ultrasonic irradi-ation on chemical processes lead to both chemical and phys-ical effects, for example, mass-transport enhancement, surfacecleaning and radical formation (HO•,HO2

• and O•) due tohomolytic cleavage via sonolysis due to cavitation phenomena(Fig. 5) [2, 3].

Although, the use of ultrasound in chemistry went througha period of neglect until the 1980s when laboratory equipmentin the form of cleaning baths and biological cell disrupters(ultrasonic probes, see ‘The Ultrasonic Probes’) became moreavailable. Consequently, interest has been revived and nowa-days, ultrasound is applied to a wide range of subject areaswithin chemistry such as nanochemistry, analytical chemistry,organometallic chemistry, organic chemistry, inorganic chem-istry, electrochemistry, material science, environmental chem-istry and polymer science [3].

Cavitation Bubble Formation

There are two kinds of cavitation—transient and stable cav-itation [2, 7]. Transient cavitation bubbles are those whichexist for one, or at most a few, acoustic cycle expanding to aradius of at least double their initial size before collapsingviolently into several smaller bubbles. These tiny bubbles mayact as nuclei for other bubbles. Transient bubbles occur mainlyin liquids subjected to ultrasound intensities which are greater

than 10 W cm−2. It was thought initially that the effectssuch as erosion, emulsification, molecular degradation,sonoluminescence and sonochemical enhancement of re-activity were solely attributable to the collapse of tran-sient cavities.

Stable cavities are those which contain gas and vapour andare known to be generated at low ultrasonic intensities (1–2 W cm−2). These cavities oscillate non-linearly about anequilibrium size over a number of acoustic cycles. Stablecavities are capable of being transformed into transient cavi-ties and are known now to be responsible for numerouschemical effects.

Transient Cavitation Bubble

As the name implies, transient cavitation bubbles have a shortlifetime (ca. 10−5 s in a 20-kHz ultrasonic field) before theycollapse violently on compression and disintegrate into small-er bubbles. These smaller bubbles may act as nuclei for furtherbubbles, or if they are of sufficiently small radius, they candissolve into the bulk of the solution. During the short lifetimeof the transient bubble, it is assumed that little or no diffusionof dissolved gas can take place from the solution bulk into thecavity, whereas condensation and evaporation of liquid isassumed to take place freely. Since there is no gas present inthe bubble to act as a cushion, the implosion leads to a veryviolent collapse.

It was assumed by many authors, such as Noltingk andNeppiras [8], that adiabatic collapse of the bubbles wouldallow for a calculation of the temperature or pressure withinthe bubble (Eqs. (1) and (2)). For example, if it is assumed thatthe vibrations of the bubbles occur so very fast that little heatexchange can occur with the surrounding liquid environment,then the vapour inside the bubble is heated during the com-pression cycle and one may deduce, under these adiabaticconditions, that the maximum pressure, Pmax, and tempera-ture, Tmax, can be given as follows:

Pmax ¼ PPm γ−1ð Þ

P

� � γγ−1

ð1Þ

Fig. 5 Schematic of watersonolysis

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Tmax ¼ T o Pmγ−1ð ÞP

� �ð2Þ

where To is the ambient temperature, γ is the ratio of thespecific heat capacities of the gas (or gas vapour) mixture, Pis the pressure in the bubble at its maximum size (usuallyassumed to be equal to the vapour pressure, Pv, of the liquid)and Pm is the pressure in the liquid at the moment of transientcollapse.

At ambient temperature and pressure, Noltingk andNeppiras [8] were able to deduce that extremely high temper-atures (Tmax) and pressures (Pmax) were produced in the finalphase of implosion equivalent to 5,000 K and 2,000 atms. Therelease of the pressure as a shock wave is a factor which hasbeen used to explain increased chemical reactivity, due toincreased molecular collision, and polymer degradation; thehigh temperatures within the bubble have formed the basis forthe explanation of radical production and sonoluminescence[3].

Stable Cavitation Bubble

A stable bubble is one of which is thought to exist for manycycles. Stable bubbles contain mainly gas and vapour. Theyare produced at low intensities (1–3 W cm−2) and oscillateabout an equilibrium size for several acoustic cycles [3, 7].The timescale over which they exist is long enough for bothmass transfer and thermal diffusion to occur freely. Thisprocess occurs as follows:

In the rarefaction phase of the sound wave, gas diffusesfrom the liquid into the bubble causing the bubble to expand,whilst in the compression phase, gas diffuses out of the bubbleinto the liquid. The increased surface area increases the rate ofdiffusion of gas and vapour into the bubble, and as asucceeding compression wave passes through the liquid, thebubble is compressed and gas and vapour diffuses out. As aresult, the rate of inward diffusion will become greater thanthe rate of diffusion back to the liquid and this will lead to anoverall growth of the bubble. As the bubble grows, the bubblewill become more compressible due to changes in the acous-tical and environmental conditions of the medium.

The stable bubble may be transformed to a transient bubbleand undergo collapse, the violence of the collapse being lessthan that of a vapour-filled transient bubble since the gaspresent cushions the implosion. There are two possible fatesfor stable cavitation bubbles. They may either grow sufficient-ly large to be capable of rising to the surface of the liquid—this is the process of ultrasonic degassing—they can becomeunstable due to differences between the resonant frequencyand the driving frequency (i.e. the frequency from the trans-ducer) and be transformed into transient bubbles.

It was also found that the maximum temperatures andpressures of these cavitation bubbles generated upon col-lapse are less than for transient cavitation bubbles due tothe ‘cushioning’ effect [3, 7]. However, it was shown thatcontracted bubbles cause temperature rise within the bub-ble and changes in local hydrostatic pressure [3, 7]. Cal-culations of the local pressures due to these resonancevibrations have resulted in values which exceed the hy-drostatic pressure by a factor of 150,000. There is nodoubt that the intense local strains in the vicinity of theresonating bubble are the cause of the many disruptivemechanical effects of ultrasound.

In summary, cavitation bubble formation is a three-stepprocess consisting of (1) nucleation, (2) bubble growth and(3) collapse of gas vapour-filled bubbles in a liquid phase.These bubbles transform the low-energy density of a soundfield into a high-energy density by absorbing energy from thesound waves over one or several cycles and releasing it duringvery short intervals. During rarefaction cycles, negative pres-sures developed by the high-power ultrasound are strongenough to overcome the intermolecular forces binding thefluid. The succeeding compression cycles can cause themicrobubbles to collapse almost instantaneously with therelease of a large amount of energy. When the cavitationbubble collapses close to or on a solid surface, it can only doso asymmetrically due to the surface which leads to a microjetof liquid being directed towards the surface of the material atspeeds of up to 200 m s−1 [7, 9].

Physical Aspects of Ultrasound

Sound-Induced Vibration

When a liquid is subjected to an acoustic field, the sonicvibrations create an acoustic pressure, Pa. This pressure mustbe added to the ambient hydrostatic pressure (Ph) alreadypresent in the medium. As with displacement, the acousticpressure at any instant is time (t) and frequency (f ) dependent:

Pa tð Þ ¼ PAsin 2πftð Þ ð3Þ

where f is the frequency of the ultrasonic wave and PA is themaximum pressure amplitude of the wave.

At any time, the pressure (Pm) within the mediumthrough which the wave passes will be the sum of thepressure already present within the fluid (Ph) and theapplied acoustic pressure, i.e.:

Pm ¼ Pa þ Ph ð4Þ

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[The pressure within the medium (Ph) is usually taken to beambient or atmospheric pressure]

As sound is a form of energy, the molecules of the mediumpossess kinetic energy and one can deduce the ultrasonicintensity, Y , as

Y ¼ P2A

2ρcð5Þ

where ρ is the density of the medium and c is the velocity ofsound in that medium. If ρ and c can be regarded as constant,then the sound intensity is proportional to the square of theacoustic amplitude.

Sound Attenuation in a Liquid Medium

The intensity of sound decreases as it progresses through amedium. As the molecules of a liquid vibrate under the actionof the sound wave, they experience viscous interactions whichdegrade the acoustic intensity and some energy is lost in theform of heat. Heating will occur at the sites of compression,and cooling at the sites of rarefaction. However, because of thelow compressibility, there will be little appreciable heating ofthe bulk medium. The small-bulk heating effect which occurson passing high-power ultrasound through a liquid medium isdue to absorption of degraded acoustic energy. The energyloss is represented by Eq. (6):

Y ¼ Ψ oexp −2adð Þ ð6Þ

whereY o is the initial ultrasonic intensity,Y is the ultrasonicintensity at some distances d from the ultrasonic source and αis the absorption coefficient.

Factors Affecting Cavitation

In order to have a further understanding of cavitation in a‘real’ system, it is necessary to discuss the effect of ultrasonicfrequency, solvent, temperature, gas type, external-appliedpressure and ultrasonic intensity on cavitation.

Ultrasonic Frequency

It is well known in the field that as the ultrasonic frequency (f )is increased, the production of cavitation in a liquid decreases.One explanation is that at very high frequencies, where therarefaction (and compression) cycles are very short, the timeof the rarefaction may be too short to permit a bubble to growto a sufficient size to cause disruption of the liquid. However,it can also be argued that even if a bubble was produced during

the rarefaction phase, the compression phase may not be longenough to collapse the bubble.

Higher frequencies require more power for an equivalentamount of chemical work since the higher frequencies resultin greater power losses. Ten times more power is required tomake water cavitate at 400 kHz than at 10 kHz. Because ofthis, most sonochemical work is performed at frequenciesbetween 20 and 50 kHz.

Solvent

For pure solvents, the most important factor is the vapourpressure (Pv); the effect of which can be seen from Eqs. (1)and (2), where an increase in vapour pressure leads to adecrease in Tmax and Pmax. In other words, solvents with highvapour pressure undergo less intense cavitational effects, i.e.the bubble collapse is less violent.

Temperature

The vapour pressure of the mediumwill rise as its temperatureis increased. This will result in a lowering of both Tmax andPmax for the reasons given above. Increasing the temperaturei.e. vapour pressure will produce a larger number ofcavitational bubbles, and these bubbles may act as a ‘cushion’to attenuate the ultrasonic energy as it passes through theliquid.

Gas Type and Content

The importance of the γ value of a gas can be seen fromEqs. (1) and (2). Since the value γ of a monoatomic gas isgreater than of a diatomic gas, monoatomic gases give a moreviolent collapse. It is for this reason that they are preferred todiatomic gases. The extent of the sonochemical effects willalso depend upon the thermal conductivity of the gas. Thegreater the thermal conduction of the gas, the more heat willbe dissipated to the surrounding liquid, effectively decreasingTmax. Increasing the gas content of a liquid leads to loweringof both the cavitational threshold and the intensity of the shockwave released on the collapse of the bubble. The threshold islowered as a consequence of the increased number of gasnuclei present in the liquid, whilst the cavitational intensityis decreased as a result of the greater cushioning effect pro-vided by the gas present in the bubble itself.

External Pressure

By examining Eq. (4), if Ph is increased, then the pressure inthe medium Pm is also increased. Thus, the intensity of theapplied wave must be increased to give a sufficient pressuredrop to cause cavitation. Equation (4) shows that an increasein Ph leads to an increase in Pm and from Eqs. (1) and (2), then

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Tmax and Pmax must also increase. In other words, increasingthe external pressure (Ph) leads to both an increase in thecavitation threshold and the intensity of cavity collapse.

Ultrasonic Intensity

Increases in ultrasonic intensity (Y ) will bring about an in-crease in the sonochemical effects. However, sonochemicaleffect cannot be increased indefinitely by increasing intensity.If an increase in PA takes place (Y α PA

2, Eq. (5)), the bubblemay grow so large on rarefaction that the time available forcollapse is insufficient. Alternatively, if too many bubbles areproduced, they may attenuate the ultrasonic energy enteringthe system by acting as a cushion. In order to transmit energyfrom source to liquid, the source must remain in contact withliquid (i.e. coupled). If a large number of bubbles are producedat the transducer/liquid interface, this will reduce coupling andthus efficiency. Care must therefore be taken when working athigh intensity that decoupling of the system does not occur.

Effect of Temperature and Intense Stirring

It should be emphasised that the improved processes inducedby ultrasound may only be due to intense stirring and increasein bulk temperature but not purely to ultrasonication. Thus,comparison of the validity of a sonochemical process with thatof a conventional chemical process, the stirring andthermostattic conditions for silent processes must be studied.In the absence of such information, the effect of ultrasoundcould be overemphasised and would lead to the false conclu-sion that the overall effect is due to ultrasound, whereas itoriginates partially from a stirring and a bulk temperatureeffect.

Most ultrasonication experiments are performed for longerthan 30 min and consequently, the bulk solution ‘heats’ up. Itis well known that the use of power ultrasound leads to anincrease in bulk temperature, especially at lower ultrasonicfrequencies, i.e. higher ultrasonic amplitudes (e.g. 20 kHz).Although this may benefit endothermic reactions as well asthe kinetics of many reactions, some issues may arise withregard to the reproducibility of the data. It is thus paramountthat the bulk temperature is regulated and controlled duringthe ultrasonication in order to separate the effects of ultra-sound to temperature effect. For example, a fast temperaturerise may lead to volatilisation of the analyte (water, organicsolvent etc) in other words to molecular degradation. In addi-tion, as the temperature is increased, the physical characteris-tics of the analyte are affected and no cavitation is observed—this phenomenon is known as the ‘decoupling effect’.

There are three options to control the bulk temperature ofthe analyte when it is subjected to ultrasonication. For

example, the first and simplest is to use an ice bath (to ensurerapid heat dissipation), in which the reactor is inserted. Themain drawback is that the ultrasonic energy is absorbed by theice, and thus, the ice needs to be replaced if longer irradiationtimes are applied. The second option is to use specially de-signed reactor cells, e.g. water-jacketed cells (see later), andthe third one is to use the ‘pulse’mode of ultrasonication (onlyavailable in newer ultrasonic systems).

Ultrasonic Equipment and Chemical Cells

They are several forms of ultrasonic equipment commerciallyavailable including ultrasonic cleaning baths, ultrasonicprobes systems (also known as ultrasonic cell disrupters),ultrasonic submersible transducers, whistle and tube reactors.In the laboratory, there are generally two methods of generat-ing acoustic wave in any liquid media. They are (i) theultrasonic cleaning baths and (ii) the ultrasonic probe systems(Fig. 6).

The Ultrasonic Cleaning Bath

The ultrasonic bath is the most widely available and cheapestsource of ultrasonic irradiation in the chemical laboratory. Itusually consists of a stainless steel (304 or 316) tank ofrectangular cross-section with piezoelectric transducers at-tached underneath the flat base (Fig. 6c–e). The ultrasonicfrequency and power of an ultrasonic bath depends upon thetype and number of transducers used in its construction. Someultrasonic baths have adjustable power and a thermostattedheater. However, these are in a minority. The maximum ultra-sonic intensity varies from 1–5 W cm−2 depending on thefrequency used. Ultrasonic bath systems have the advantagesof the following: (i) a good energy distribution through thereaction vessel, (ii) they are widely used, (iii) they are inex-pensive piece of equipment, (iv) no adaptation of reactionvessel is needed and (v) a fairly good temperature controlwhen compared with probe systems. However, ultrasonicbaths have a few disadvantages, e.g. (i) a fairly low ultrasonicpower is transmitted into the reaction vessel, (ii) a fixedultrasonic frequency is used and (iii) the positioning of thereaction vessel in the bath affects the consistency on thegenerated data (this is related to the ultrasonic power—see‘Ultrasonic Power Determination’).

The Ultrasonic Probe

To overcome the disadvantages of the cleaning ultrasonicbath, workers in the field have turned to the ultrasonic probesystem. In order to increase the amount of ultrasonic poweravailable to a solution, it is necessary to introduce the energydirectly into the system rather than rely on its transfer through

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the water of a tank followed by transfer through the reactionvessel walls. The easiest method of achieving this is to employan ultrasonic probe (Fig. 7a, b).

The ultrasonic probe system consists of a specially de-signed length of metal rod attached to the end of a transducer.This rod extension is termed am ultrasonic horn or velocitytransformer. The probe, or more correctly the ultrasonic horn,involves direct immersion of the metal probe (titanium al-loy—Ti-6Al-4V) into the solution. The horn is driven by atransducer and ultrasound enters the fluid via the probe tip(made of Ti-6Al-4V—see Fig. 7). The intensity ofultrasonication and the vibrational amplitude of the tip canbe controlled by altering the power input to the transducer andall commercially available ultrasonicators have a power con-trol. There is another technique for controlling power input toa system, and this depends upon the type of probe used, e.g.stepped, linear taper and exponentional taper shape (Fig. 7a).Most modern systems are designed to operate with a range ofdetachable metal probes of differing tip diameters.

The essential parts of the ultrasonic probe are given below:

(a) The Generator. This is the source of alternating electricalfrequency which supplies the transducer. It enables theprobe system to be ‘tuned’ to optimise the performanceof the ultrasonic horn in order to ensure that the completesystem is in resonance. The generator may also have a

pulse facility. This consists of a timer which switches thepower to the probe ‘on’ and ‘off’ rapidly.

(b) The Transducer Element. This consists of a piezoelectricmaterial, such as quartz crystal or lead zirconate titanatecrystals (PZT), which are designed to convert electricalenergy into mechanical energy, i.e. high-frequency ultra-sound. The transducer element is protected by a casingwhich is perforated to allow cooling and thereby mini-mise the risk of overheating.

(c) The Upper Horn Element. This consists of a fixed robustaccurately machined piece of titanium alloy vibrating atmaximum amplitude. In the centre, there is a point ofzero motion or ‘null’ point where a screw thread islocated for the attachment of ancillary equipment.

(d) The Detachable Horn. This allows the vibration of thefixed horn to be transmitted through a further length ofmetal which may be necessary to allow easy access tolong-necked vessels. The horn design is a very importantaspect of ultrasonic engineering as the material used foracoustic horns should have excellent physicochemicalproperties (e.g. dynamic fatigue strength, low acousticloss, physical/chemical resistance etc). The most suitablematerial for this purpose is titanium alloy. However, tiperosion due to cavitation is a common problem withprobe systems (Fig. 7e). This problem may be temporar-ily alleviated by the use of screw-on tips which

Fig. 6 Ultrasonic equipment used in R&D laboratories. Ultrasonicprobe: (a) Vibra-Cell VCX750 ultrasound probe with an ultrasonic gen-erator/processor by Sonics & Materials Inc., f=20 kHz, net power out-put=750 W; (b) Vibra-Cell VCX130 ultrasound probe with an ultrasonicgenerator/processor and a series of detachable microtips by Sonics &Materials Inc. f=20 kHz, net power output=130 W. (b) Ultrasonic baths:

(c) QS12 by Ultrawave, f=32–38 kHz, ultrasonic power=200W; heatingpower=300 W); (d) 475H Sonomatic Ultrasonic Cleaner by LangfordUltrasonics, f=40 kHz, high-frequency peak power=300 W; heatingpower=200 W; (e) XUB18 by Grant Instruments, f=32–38 kHz, ultra-sonic power=300 W; heating power=450 W

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eliminates the replacement of the whole system. Further-more, for maximum acoustic efficiency, the length andthe shape (e.g. uniform cylinder, linear and exponentialtapers and stepped) of the horn should be carefullychosen when employing detachable horns, probes am-plify and radiate the ultrasonic energy into the sample(Fig. 8).

Commercially, two types of ultrasonic probes are available(Fig. 8):

(a) Probes With Replaceable Tips. Probes are made fromtitanium alloy and machined to specific sizes and shapes.The sample volume to process directly corresponds withthe tip diameter of the probe. Smaller tip diametersdeliver high-intensity ultrasonication, but the energy isfocused within a small concentrated area. On the otherend, larger tip diameters can process larger volumes, butoffer lower ultrasonic intensity.

(b) Microtip Probes. Microtips are thin, very high ultrasonicintensity probes which are designed for processingsmall-sample volumes.Microtips screw into the threadedend of a standard 1/2" (mm) probe.

As discussed above, a large maximum power density canbe achieved at the radiating tip. This can be of the order ofseveral hundred watts per square centimeter depending on theworking frequencies used. The ultrasonic frequency range

used in R&D labs is of the order of 20–80 kHz. Usually, allsonic horns are tuned in air and then in water by increasing thepower output until minimum deviation is observed. Accurateknowledge of tip area is necessary for many ultrasonic exper-iments, especially when dealing with the ultrasonic intensity(Y ). All tip areas are geometrically determined by using anelectronic micrometer.

Probe systems offer advantages and disadvantages overbath systems. The main advantages are as follows: (a) a muchhigher ultrasonic powers can be used since energy lossesduring the transfer of ultrasound through the electrolyte andthe electrochemical vessel walls are eliminated, (b) the systemcan be tuned to give optimum performance in the reaction cellfor a range of powers and (c) the ultrasonic intensity and sizeof the sample to be irradiated can be matched accurately foroptimum effect.

The main disadvantages are as follows: (a) tip erosionwhich can cause contamination by the released titaniumalloy particles during ultrasonication, (b) fixed ultrasonicfrequency, (c) difficulty in controlling the temperature, (d)high generation of radical species in the vicinity of thevibrating tip and (e) costly. Indeed the probe suffers fromhigh and fast temperature rises. This problem can bealleviated to some extent in modern instruments by theincorporation of a pulse mode of operation. This consistsof a timer attached to the amplifier which switches thepower delivered to the transducer on and off repeatedly.The off time allows the system to cool between the pulses

Fig. 7 Ultrasonic probes (a) typeof probes used, e.g. stepped,linear taper and exponentionaltaper shape. (b, c) Sonics &Materials Inc. detachable/replaceable sonic horns (steppedmicrotips and probes) of variouslengths. (d) Sonics & MaterialsInc. replaceable tips fabricatedfrom titanium alloy Ti-6Al-4V(and autoclavable). (e) Exampleof an eroded replaceable tip aftermany hours of operation

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of sonication. The on time is represented as a fraction ofthe total time involved in the cycle (about 1 s).

Chemical Cells for Ultrasonic Experiments

All the conventional cells employed have flat bottoms tomaximise energy transfer. For example, energy is radiatedvertically as ultrasound waves from the base of the bath andthrough the glass walls of the cell into the solution itself. Thus,it is more effective to employ a flat base for the cell allowing agreater transfer of ultrasonic energy. The base area of thesecells is of importance as it allowed deduction of the ultrasonicintensity (power per tip or transducer or cell base area–seelater) [1].

Conventional Sonochemical Cells

Usually, ultrasonic and sonochemical experiments are com-monly performed using an ultrasonic bath or a probe arrange-ment using a one-compartment pyrex cell (or beaker) ofvarious volumes. However, due to (i) temperature increases,(ii) possible contaminations and (iii) poor repeatability in thedata, specially designed reactions cells are usually employed.

The Besançon Ultrasonic Cell

This new ultrasonic cell was developed by Hihn et al. [10] foruse in aqueous and ‘exotic’ solvents (e.g. room-temperature

ionic liquids, deep eutectic solvents [11]) at various coolantpressures. This jacketed cooling ‘microsonoreactor’ (Fig. 9) isbased on a particular design consisting of offsetting the ultra-sonic probe out of the reaction volume (inner cell, Vic=10 cm3) in order to avoid any possible contamination, toregulate temperature and to ensure perfect electric insulationfrom the ultrasonic probe (important if the cell is used forelectrochemical experiments).

Ultrasonic Power Determination

Many workers have also investigated the distribution of ultra-sonic waves or energy in various reactors at low ultrasonicfrequencies and high ultrasonic powers in the range of 20–55 kHz. Several methods for such determination have beenproposed, e.g. the aluminium foil erosion, sonoluminescenceand chemical dosimetry, but the most useful methods havebeen found to be (i) the calorimetric and (ii) the electrochem-ical methods [1, 3].

Calorimetric Method

The ultrasonic power dissipated (PT) in the solution by theultrasonic equipment is often determined calorimetrically ac-cording to the procedure ofMason et al. [12, 13] andMarguliset al. [14]. In these experiments, temperature (T) is recorded

Fig. 8 Technical specificationson stepped microtips and probes(Sonics & Materials Inc.). Withthe amplitude control set at 100%(i.e. the amplitude at the convertertip is 20 μm=0.0008 in.)

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every 5 s over a period of 1 min (t) using a thermocouple fittedto a digital thermometer. Experimental temperature (T in K) isplotted against time (t in s) and a curve fitting analysis isperformed. In some conditions, T vs. t plots are found to bea sixth-order polynomial in the form of:

T ¼ aþ bt þ ct2 þ dt3 þ et4 þ f t5 þ gt6 ð7Þ

Differentiation of Eq. (7) yields Eq. (8)

dT=dtð Þ ¼ bþ 2ct þ 3dt2 þ 4et3 þ 5f t4 þ 6gt5 ð8Þ

for which at t=0, Eq. (5) gives

dT=dtð Þt¼0 ¼ b ð9Þ

The ultrasonic power (PT) is then determined as:

PT ¼ m� Cp � dT=dtð Þt¼0 ð10Þ

where PT is the transmitted ultrasonic power in W, m is themass of the solution (in g), CP is the specific heat capacity of

the solution (in J g−1 K−1), (dT/dt)t=0 is the slope (b) at t=0 (inK s−1).

If the solution is mainly made of water, the specificheat capacity of water is often taken as 4.184 J g−1 K−1

[12–14] and ultrasonic powers (PT) are quoted as W(Watts). The ultrasonic intensity (Y ) is calculated as theultrasonic power transmitted (PT, W) divided by the ul-trasonic horn tip area (Auht, cm2) (or the chemical cellbase area if immersed in an ultrasonic bath), i.e.:

ψ ¼ PT=Auht ð11Þ

Electrochemical Method

Pollet et al. [7, 9] showed, with the aid of mathematicalmodels based on mass-balance equations and using thequasi-reversible redox couple Fe(CN)6

3−/Fe(CN)64− as an

electrochemical model, that a Levich-like equation relatingthe limiting current, the inverse square root of the elec-trode radius, the inverse square root of the electrode-horndistance and the square root of the transmitted ultrasonicintensity (and thus the transmitted ultrasonic power) maybe generated for ultrasonic frequencies of 20 and 40 kHz(probe systems only and at 298 K) using Eq. (9) alsoknown as the Pollet equation [1, 2, 7, 9]:

I lim ¼ 0:84nFADo2=3v−1=6re

−1=2d−1=2C � ψ1=2 ð12Þ

Fig. 9 The Besançon Ultrasonic/Sonochemical Cell [10, 11]

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Inserting Eq. (11) into Eq. (12) yields:

I lim ¼ 0:84nFADo2=3v−1=6re

−1=2Auht−1=2d−1=2C � PT

1=2 ð13Þ

where Ilim is the limiting current (A), n is the number ofelectrons transferred during the electrochemical process, F isthe Faraday constant (C.mol−1), A is the electrode area (cm2),Do is the diffusion coefficient (cm2.s−1) of the electroactivespecies, d is the ultrasonic horn-electrode distance (cm), ν isthe kinematic viscosity (cm2.s−1), re is the working electroderadius (cm), Auht is the ultrasonic horn tip area (cm

2),C* is thebulk concentration of the electroactive species (mol.cm−3) andPT is the ultrasonic power transmitted (W).

From Eq. (13), experimentally limiting current values andassuming that all the above parameters are known, the trans-mitted ultrasonic power (PT) can be calculated.

The Effect of Ultrasound on Nafion®

It is well known in the field that ultrasound can be used toeither form polymers (from monomers) or degrade them[15–21] depending upon the experimental conditionsemployed. Generally, free-radical polymerisation consists offour steps: (1) initiation, (2) propagation, (3) chain-transferand (4) termination. It was found that ultrasound can beemployed to initiate polymerisation due to the formation ofradicals induced by sonolysis. It was also reported [20, 21]that organic initiator can be replaced by ultrasound due to itssource of free radicals in the emulsion polymerisation ofstyrene. There are several possible explanations to this find-ing: (i) H·and OH·radicals produced by sonolysis may re-combine to form hydrogen peroxide and thus oxidise impuri-ties which could have acted as inhibitors, (ii) ultrasonicdegassing may remove absorbed oxygen in the solution, (iii)the initiator breakdown may be accelerated by ultrasonicationand (iv) an increase in mixing of the components produces afiner emulsion.

However, it has been shown that ultrasound can also en-hance the degradation/decomposition of some polymers[16–19]. Ultrasound has been regarded as a useful methodfor the depolymerisation ofmacromolecules, usually observedin the reduction of the polymers’ molecular weights (RMM),mainly caused by cavitation phenomena. It was also foundthat in the presence of ultrasound, the degradation of polymers(i) increases with decreasing ultrasonic frequency, due to thelower frequency providing a longer time for bubble growthand then collapse; (ii) decreases in the presence of a volatilesolvent, attributed to a lower cavitational pressure due to anincrease in vapour pressure and (iii) increases in deareatedsolutions, caused by a lowering of the cavitation threshold and

thus an increase of cavitation bubbles. For all cases, long-timeultrasonic irradiation of the polymer leads to a permanentreduction in viscosity and thus to irreversible reactions[19–21].

In the case of Nafion®, it was recently shown (preliminarystudies) that its ultrasonication over various irradiation timesrevealed a decrease in viscosity [1, 22]. However, it was foundthat at a minimum ultrasonic time and a fixed ultrasonicfrequency, an increase in Nafion® polymer viscosity was alsoobserved. This observation was mainly attributed to the factthat depolymerisation caused by ultrasound supplies newchain carriers for polymerisation; in other words, under care-fully chosen conditions, ultrasound may initiate polymerisa-tion as previously observed in other studies using variouspolymers [15–20].

The Effect of Ultrasound on Fuel Cell Catalyst Inks

There are numerous well-documented methods describing thepreparation of Polymer Exchange Membrane Fuel Cell(PEMFC) and Direct Methanol Fuel Cell (DMFC) catalystinks. For example, Litster and McLean [23] and Wee et al.[24] give excellent overviews of PEMFC catalyst ink andelectrode fabrication methods. As described by Takashi andKocha [25], it is common for catalyst inks to be mixedultrasonically for a few minutes or hours (typically by immer-sion in laboratory ultrasonic cleaning baths in the range of 38–40 kHz) in order to produce a homogeneous mixture ofcarbon-supported Pt catalyst and ionomer binder, which isessential in order to maximise catalyst utilisation at the‘three-phase reaction zone’ (also known as the triple phaseboundary—TPB).

For the last 20 years, there have been a few reports on theuse of ultrasound for fabricating noble metal nanoparticles,catalysts and other fuel cell materials. Pollet [26] showed inhis comprehensive review that the ultrasonic, sonochemicaland sonoelectrochemical methods used for the preparation ofmono- and bi-metallic nanoparticles, carbon-supportedelectrocatalysts (via the ultrasonic functionalization of thecarbonaceous material), fuel cell electrodes and membranesoffer unique and often highly advantageous experimentalconditions by virtue of ultrasound-induced cavitation, watersonolysis and enhanced mass transport phenomena.

In a recent paper by Pollet and Goh [1], it was shown forthe first time a systematic study of the effects of ultrasound onthe performance of the catalyst ink. electrochemical surfaceareas (ECSA) were compared for catalyst inks prepared in theabsence and presence of ultrasound, at various ultrasonicfrequencies (20 and 40 kHz), powers and exposure times. Inthis study, two commercial carbon-supported Pt (Pt/C) cata-lysts (E-Tek and TKK) were used and dispersed in Nafion®ionomer. Catalyst ink samples prepared from Nafion®, IPA

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and water were either ultrasonicated (20 kHz up to 12.23 Wand 40 kHz at 1.82 W) or mechanically shear-mixed(19,000 rpm) for various durations (up to 120 min). Allcatalyst ink samples were characterised by XRD, BETand TEM, and electrochemical measurements were per-formed in liquid electrolytes. It was found that anoptimised ultrasonic treatment is required to improve thecatalytic ink activity, but longer irradiation is detrimental toits composition and morphology, mainly due to cavitation andsonolysis phenomena. This observation was ascribed to one ormore of the following effects: (i) a possible physical detach-ment of Pt nanoparticles from the carbon support underultrasonication induced by violent implosion of cavitationbubbles producing jets of liquid exceeding 20–200 m s−1

directed onto the carbon surface and Pt/carbon interfaces [1],(ii) a possible partial or complete sonochemical dissolution ofplatinummay occur caused by sonolysis and erosion followedby an increase in the rate of Pt particle growth via Ostwaldripening [1] and (iii) agglomeration of Pt nanoparticle causedby weakening of the metal-support interaction, along with theaction of Van der Waals forces. The authors also showed thatthe ultrasonication of catalyst inks may lead to simultaneousmechanisms as shown in Fig. 10.

Conclusions

The effect of ultrasound upon liquid media and chemicalreactions is mainly concerned with reactions which involvea liquid component within which cavitation can be induced. Ithas been shown by several authors in many fields of chemistryand engineering that the application of power ultrasound maylead to an increase in reaction rate and product yield. Theorigin of these enhancements is still not fully understood.However, they might be a result of a combination of thefollowing: (i) reactions within the cavitation bubble wheretemperatures and pressures are extremely high, (ii) reactionsas a result of secondary reactions occurring at the interfacebetween the bubble and the bulk solution, (iii) reaction as aresult of the pressures released on bubble collapse and (iv)reactions as a result of high mechanical effect of ultrasound.

In the case of the use of ultrasound in the preparation of fuelcell catalyst inks, precautions should be taken as ultrasound isnot only an effective homogeniser/disperser but also a veryefficient source of (i) high-velocity liquid jets which mayclean/erode/damage the catalyst surface and thus dislodgethe Pt catalyst from the carbon surface (due to poor adhesion)and (ii) highly reactive radicals which may damage molecular

Fig. 10 Schematic mechanism(s)for ultrasonic treatment of catalystinks (Pt/C/Nafion®)

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structures and Pt crystal structure and take part to undesirablereactions.

References

1. B.G. Pollet, J.T.E. Goh, Electrochim. Acta 128 (2014)2. B.G. Pollet (ed.), Power ultrasound in electrochemistry: from versa-

tile laboratory tool to engineering solution (John Wiley & Sons Ltd,Chichester, 2012)

3. T.J. Mason, J.P. Lorimer, Sonochemistry, theory, applications anduses of ultrasound in chemistry (Ellis Horwood, Chichester, 1998)

4. J. Thorneycroft, S.W. Barnaby, Inst. Civil Eng. 122, 51 (1895)5. L. Rayleigh, Philos. Mag. 34, 199–04 (1917)6. W.T. Richards, A.L. Loomis, J. Am. Chem. Soc. 49 (1927)7. Bruno G. Pollet, The effect of ultrasound upon electrochemical

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Isabelle Bisel, Philippe Moisy, Phys. Chem. Chem. Phys. 10 (2008)

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13. T.J. Mason, A.J. Cobley, J.E. Graves, D. Morgan, Ultrason.Sonochem. 18, 1 (2011)

14. M.A. Margulis, I.M. Margulis, Ultrason. Sonochem. 10 (2003)15. E.W. Flosdorf, L.A. Chambers, J. Am. Chem. Soc. 55 (1933)16. A. Weissler, J. App. Phys. 21, 2 (1950)17. P.A.R. Glynn, B.M.E. Van der Hoff, J. Macromol. Sci., Part A: Pure

Appl. Chem 8, 2 (1974)18. C.E. Gall, B.M.E. Van der Hoff, J. Macromol. Sci., Part A: Pure Appl

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20. G.J. Price, P.J. West; P.F. Smith, Ultrason. Sonochem. 1 (1994)21. K.S. Suslick, G.J. Price, Ann. Rev. Mater. Sci. 29 (1999)22. H.Momand, The effect of ultrasound on Nafion® Polymer Exchange

Membrane Fuel Cells (PEMFCs), The University of Birmingham(2013) (M.Res. Thesis)

23. S. Litster; G. McLean, J. Power Sources 130 (2004)24. J.-H. Wee; K.-Y. Lee; S.H. Kim, J. Power Sources 165 (2007)25. I. Takahashi, S.S. Kocha, J. Power Sources 195, 19 (2010)26. B.G. Pollet, Int. J. Hydrogen Energy 35, 21 (2010)

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