catalysts

Review The Use of Power Ultrasound for the Production of PEMFC and PEMWE Catalysts and Low-Pt Loading and High-Performing Electrodes

Bruno G. Pollet

Hydrogen Energy and Research Group, Department of Energy and Process Engineering, Faculty of Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway; [email protected]

 Received: 30 January 2019; Accepted: 5 March 2019; Published: 7 March 2019 

Abstract: This short review paper highlights some of the research works undertaken over the years by Pollet’s research groups in Birmingham, Cape Town, and Trondheim, in the use of power ultrasound for the fabrication of low temperature fuel cell and electrolyzer catalysts and electrodes. Since the publication of ‘The use of ultrasound for the fabrication of fuel cell materials’ in 2010, there has been an upsurge of international interest in the use of power ultrasound, sonochemistry, and sonoelectrochemistry for the production of low temperature fuel cell and electrolyzer materials. This is because power ultrasound offers many advantages over traditional synthetic methods. The attraction of power ultrasound is the ability to create localized transient high temperatures and pressures, as a result of cavitation, in solutions at room temperature.

Keywords: PEMFC; PEMWE; PGM; ultrasound; sonochemistry

1. Introduction The world is unanimous in the attempt at reducing carbon emissions by 30–40% by 2030 and completely by 2050 whilst increasing energy efficiency. energy is one of the many solutions for reducing greenhouse gases (GHG) and particulate emissions as long as it is produced from renewable energy via, for example, the use of cost-effective and efficient electrolyzers [1]. Fuel cells and electrolyzers, and particularly Proton Exchange Membrane Fuel Cells (PEMFCs) and Proton Exchange Membrane Water Electrolyzers (PEMWEs), are currently evolving to be one answer to consumer demand for energy and fuel replacement, increased power, and current environmental issues [1]. A PEMFC is an electrochemical device that transforms the chemical energy of a fuel (hydrogen) into electrical energy, when reacting with an oxidant (air or oxygen) in the presence of a catalyst (usually a Platinum Group Metal—PGM); whilst a PEMWE breaks down water into hydrogen (cathode) and oxygen (anode) in the presence of PGMs. The heart of a PEMFC and a PEMWE—the Membrane Electrode Assembly (MEA)—is an essential component for the functioning of the electrochemical devices as it is where reactions occur at the two electrodes (anode and cathode). It is currently accepted in the hydrogen and fuel cell technology (HFCT) market that PEMFCs and PEMWEs will play an important role in the sustainable growth of the global industries through transportation (passenger vehicles: cars, buses, trains, and airplanes), stationary (combined heat and power—CHP and uninterrupted power system—UPS) and portable (mobile phones, tablets, laptops, etc.) applications. Although PEMFCs and PEMWEs have been widely and extensively researched and developed throughout the last decade, there are many existing issues required to be overcome. For example, high costs of components, sub-components, and systems need to be reduced (ca. 30%) to achieve competitive price levels, increased reliability, and durability before they can be successfully

Catalysts 2019, 9, 246; doi:10.3390/catal9030246 www.mdpi.com/journal/catalysts Catalysts 2019, 9, 246 2 of 18 commercially deployed and publicly accepted. One of the major issues is due to the MEA being made of costly precious-metal catalysts (e.g., Ir, Pt, Rh, Ru, Pt-Ru, Pt-Rh, IrO2) and the relatively high cost of producing and processing MEAs. The use of precious or noble metal PGM as a catalyst in PEMFC and PEMWE systems leads to a significant increase in the overall cost of the system. Furthermore, there is possibility of PGM deposits becoming scarce in the years to come [1]. For clarification, in the PEMWE industry, Pt is no longer the critical PGM catalyst for the hydrogen evolution reaction (HER) and carbon is no longer a preferential catalyst support material, however in some industrial cases and for R&D purposes, Pt/Vulcan XC72 carbon is still employed [2–6]. In the PGM-based catalyst layer of a conventional PEMFC and PEMWE, the simultaneous access of the PGM nanoparticle by the electron conducting medium and the proton-conducting medium is achieved via a skillful blending of the PGM-supporting carbon or metal oxide particles and proton-conducting polymer [4–6]. However, even with the most advanced traditional electrode preparation, a significant portion of the PGM is isolated from the external circuit and/or the PEMFC and the PEMWE, resulting in the low PGM utilization. For example, PGM utilization in current commercially offered prototype low-temperature PEMFC remains very low (20–40%), although higher performance has been achieved in laboratory devices [2,3]. In the case of PEMFC, one of the many challenges is to overcome the kinetic limitations of the oxidation reduction reaction (ORR) in the cathode, which has led to three fundamental problems. Firstly, the substantial overpotential for the ORR at practical operating current densities reduces the theoretical thermal efficiency from ~80% down to ~40%. Secondly, an approximately 5- to 10-fold −2 reduction of the amount of Pt in current PEMFC stacks (0.4–0.5 mgPt cm MEA) is needed to meet the cost requirements for demanding applications. For example, the US Department of Energy (DoE) −2 target for the Pt loading in the cathode of MEAs for automotive applications is 0.1 mgPt.cm MEA. Finally, the dissolution and/or loss of Pt surface area in the cathode must be greatly reduced [2]. In the automotive sector, the major issue still remains the high intrinsic cost of Pt. Current state-of-the-art technology MEAs have a total Pt loading of about 0.40 mg Pt cm−2, i.e., to ∼0.05 and ∼0.35 mg Pt cm−2 of Pt catalyst is used on the anode (hydrogen oxidation reaction—HOR) and on the cathode (ORR) respectively. A typical Pt loading for an automotive 100 kW PEMFC stack is ∼30 g Pt per kW. In the case of MEAs for PEMWEs, the PGM loadings are typically 0.1–1 mg cm−2 at the cathode and 0.3–3 mg cm−2 at the anode [2,7–9]. It is therefore required to drastically reduce the amount of Pt to meet the Pt loadings in current automotive catalytic converters, i.e., 3–10 g, in other words, up to a 10-fold reduction. Thus, from a PGM point of view, it is therefore important to research and develop (i) Non-Precious Metal Catalysts (NPMC) for PEMFC and PEMWE or (ii) novel and cost-effective deposition processes for the fabrication of (ultra-)low PGM loading, low-cost, highly efficient, and durable Proton Exchange Membrane (PEM) fuel cell and PEM water electrolyzer MEAs, in turn lowering manufacturing costs [2–14]. There are many well-described methods detailing the catalyst ink (Pt/C mixed with a solubilized polymer electrolyte and a solvent) deposition onto the gas diffusion layer (GDL) and the polymer electrolyte membrane (PEM) yielding GDEs (gas diffusion electrodes) and CCMs (catalyst coated membranes), respectively. The most common methods used for the production of PEMFC and PEMWE electrodes are by either the decal, blade process, screen-printing, painting, spraying, electro-spraying, or electrophoretic methods [3]. However, the main disadvantages of these methods are as follows: (i) most of the time an oxidative treatment (e.g., heating) is required to ‘clean’ the PGM nanoparticles caused by chemical contamination, (ii) PGM nanoparticles’ surface structure and surface morphology are affected by the treatments, and (iii) Triple-Phase Boundary (TPB) often possesses inactive PGM sites for electrocatalysis [2,8,9,13]. For further information on the deposition techniques, the reader may be invited to consult the excellent overviews of fuel cell electrode fabrication methods by Litster and McLean [3] and Wee et al. [14]. One of the many routes to maximize PGM utilization in the catalyst layer is to employ highly efficient stirring or forced convection in the form of power ultrasound. Fairly recently, Catalysts 2019, 9, 246 3 of 18

Pollet showed [11] that ultrasound can be employed for producing precious metal catalysts as Catalysts 2019, 9, x FOR PEER REVIEW 3 of 19 low-temperature PEMFC and PEMWE materials. In his comprehensive review [15], Pollet clearly demonstratedthe ultrasonication, that the sonochemical, ultrasonication, and sonochemical,sonoelectrochemical and sonoelectrochemicalmethods can be employed methods for can the be employedproduction for of the efficient production catalyst of efficientnanoparticles, catalyst carbon-supported nanoparticles, electrocatalysts, carbon-supported and electrocatalysts, fuel cell and andelectrolyzer fuel cell and electrodes electrolyzer due to electrodes the ‘extraordinary’ due to the experimental ‘extraordinary’ conditions experimental due to conditionsthe enhanced due mass to the enhancedtransport mass phenomenon, transport phenomenon,cavitation, and cavitation,water sonolysis and water[15–17]. sonolysis [15–17].

2. Power2. Power Ultrasound, Ultrasound, Sonochemistry, Sonochemistry, and and SonoelectrochemistrySonoelectrochemistry

2.1.2.1. Power Power Ultrasound Ultrasound InIn , biochemistry, ultrasound ultrasound is commonly is commonly employed employed for the for disruption the disruption of living of tissues living (includingtissues cells)(including and natural cells) plantsand natural in order plants to extractin order effectively to extract effectively important important constituents. constituents. Ultrasound Ultrasound is usually definedis usually as a defined sonic wave as a sonic with wave a frequency with a frequency above 16 above kHz 16 with kHz the with upper the upper limit limit usually usually taken taken to be 5 MHzto befor 5 MHz gases for and gases 500 MHzand 500 for MHz liquids for and liquids solids, and as solids, shown as in shown Figure 1in[16 Figure,17]. Power 1 [16,17]. ultrasound Power canultrasound be regarded can as be the regarded effect ofas the effect sonic of wave the so onnic the wave medium, on the i.e.,medium, the application i.e., the application of low-frequency of low- high-energyfrequency ultrasoundhigh-energy in ultrasound the range 20in kHz–2the range MHz. 20 kHz–2 When powerMHz. When ultrasound power is ultrasound used in liquid is used systems, in it causesliquid systems, (i) an area it causes of extreme (i) an area ‘mixing’ of extreme close ‘mixing’ to the ultrasonic close to the transducer ultrasonic transducer (source); (ii)(source); degassing; (ii) (iii)degassing; surface cleaning, (iii) surface pitting, cleaning, and erosion; pitting, and and (iv) aerosion; substantial and increase(iv) a substantial in bulk temperature, increase in as bulk shown in Figuretemperature,2[16,17 as]. shown in Figure 2 [16,17].

FigureFigure 1. 1.(Ultra)sonic (Ultra)sonic frequency range. range.

Catalysts 2019, 9, 246 4 of 18 Catalysts 2019, 9, x FOR PEER REVIEW 4 of 19

Figure 2. Ultrasonic effects in a liquid and near a solid surface [15]. Figure 2. Ultrasonic effects in a liquid and near a solid surface [15]. 2.2. Sonochemistry 2.2. Sonochemistry Power ultrasound has been used in many applications such as the chemical and processing industriesPower ultrasound where it is employedhas been toused improve in many both catalyticapplications and synthetic such as processes the chemical and to produceand processing new industrieschemicals. where This it technologyis employed area to has improve been termed both sonochemistrycatalytic and, which synthetic mainly processes focusses and on chemical to produce new reactionschemicals. in solutions This technology yielding an increasearea has in erosionbeen termed of catalytic sonochemistry surfaces, reaction, which kinetics, mainly and focusses product on yields [12,13]. Moreover, the most observed effects of power ultrasound on solid surfaces and chemical chemical reactions in solutions yielding an increase in erosion of catalytic surfaces, reaction kinetics, reactions is known to being due to the ‘cavitation’ effect occurring as a secondary effect when an and product yields [12,13]. Moreover, the most observed effects of power ultrasound on solid surfaces ultrasonic wave passes through a liquid medium. This process of tearing the liquid apart (i.e., during and chemicalthe continuous reactions compression is known and to rarefaction being due cycles) to the is ‘cavitation’ known as ‘cavitation’ effect occurring and the as microbubbles a secondary are effect whencalled an ultrasonic ‘cavitation wave bubbles’, passes as shown through in Figure a liquid3[ 16 medium.,17]. Most This of the process time, the of cavitation tearing the threshold liquid isapart (i.e., takenduring as thethe limit continuous of sonic intensitycompression below whichand rare cavitationfaction doescycles) not occuris known in a liquid. as ‘cavitation’ and the microbubblesDuring are the called continuous ‘cavitation compression bubbles’, and rarefaction as shown cycles, in Figure these cavitation 3 [16,17]. bubbles Most grow,of the become time, the cavitationunstable, threshold and then is implodetaken as by the forming limit of jets sonic of liquid intensity reaching below velocities which up cavitation to 200 m· sdoes−1 leading not occur to in a liquid.mass-transport enhancement, surface cleaning, and surface erosion. Moreover, it has been shown that theDuring cavitation the continuous bubble collapse compression also leads to and sonolysis, rarefa i.e.,ction the cycles, homolytic these cleavage cavitation of water bubbles yielding grow, • • • becomeradical unstable, formation and HO then, HO implode2 , and O by, as forming shown in jets Figure of 4liquid[15–18 ].reaching velocities up to 200 m·s−1 leading to mass-transport enhancement, surface cleaning, and surface erosion. Moreover, it has been shown that the cavitation bubble collapse also leads to sonolysis, i.e., the homolytic cleavage of water • • • yielding radical formation HO , HO2 , and O , as shown in Figure 4 [15–18].

Catalysts 2019, 9, 246 5 of 18 Catalysts 2019, 9, x FOR PEER REVIEW 5 of 19 Catalysts 2019, 9, x FOR PEER REVIEW 5 of 19

FigureFigure 3. AA cavitation cavitation bubble bubble imploding imploding onto onto a surfacea surface [16]. [16]. [16 ].

TheThe cavitation cavitationcavitation phenomenon phenomenonphenomenon is wellisis well knownwell knownknown to cause toto erosion, causecause erosion, emulsification,erosion, emulsification, emulsification, molecular molecular degradation, molecular degradation,sonoluminescence,degradation, sonoluminescence, sonoluminescence, and sonochemical and sonochemicalsonochemical enhancements e nhancementsenhancements of reactivity of solely of reactivity reactivity attributed solely solely to attributed the attributed collapse to to of thecavitationthe collapse collapse bubbles of of cavitation cavitation [15–18 ].bubblesbubbles [15–18].

Figure 4. 4. SchematicSchematic of of water water sonolysis sonolysis [15]. [15 ]. Figure 4. Schematic of water sonolysis [15]. 2.3.2.3. Sonoelectrochemistry Sonoelectrochemistry 2.3. Sonoelectrochemistry SonoelectrochemistrySonoelectrochemistry isis the the study study of theof combinationthe combination of ultrasound of ultrasound and electrochemical and electrochemical processes andprocesses itsSonoelectrochemistry applications and its [applications19]. This is areathe [19]. ofstudy research This ofarea coversthe of co research variousmbination studiescovers of fromultrasoundvarious organic studies and syntheses, fromelectrochemical organic polymeric processesmaterialssyntheses, syntheses,and polymeric its applications production materials syntheses,[19]. of nanomaterials, This producarea oftion environmental research of nanomaterials, covers (soil various and environmental water) studies treatments, from (soil organicand water syntheses,disinfection,water) treatments, polymeric corrosion water materials of disinfection, metals, syntheses, analytical corrosion produc procedures, of metals,tion ofanalytical filmnanomaterials, and procedures, membrane environmental film preparations and membrane (soil to and the water)elucidationpreparations treatments, of to electrochemical the water elucidation disinfection, mechanismsof electrochemical corrosion in various of me metals,chanisms ‘exotic’ analytical in solvents various procedures, ‘exotic’ (e.g., room solvents film temperature and (e.g., membrane room ionic preparationsliquidstemperature and deep to ionic the eutectic liquids elucidation and solvents). deep of electrochemical eutectic It is well-known solvents). me Itchanisms is in well-known the area in various that in thethe ‘exotic’area main that effects solvents the main of ultrasound(e.g., effects room temperatureinof ultrasound ionic in liquidselectrochemistry are the and increase deep areeutectic in the mass increase solvents). transfer in Itandmass is well-known heat transfer transfer and in induced theheat area transfer bythat extreme the induced main solution effectsby extreme solution ‘mixing’, a substantial thinning of the diffusion layer thickness (δ) at the electrode ‘mixing’,of ultrasound a substantial in electrochemistry thinning of the are diffusion the increase layer thicknessin mass transfer (δ) at the and electrode heat transfer surface, induced as shown by in surface, as shown in Figure 5, a noticeable decrease in overall cell voltage (Vcell) and electrode extremeFigure5, solution a noticeable ‘mixing’, decrease a substantial in overall thinning cell voltage of the ( V diffusion) and electrodelayer thickness overpotential (δ) at the (η electrode), and an overpotential (η), and an observable surface modificationcell due to erosion and pitting. These surface,observable as surfaceshown modificationin Figure 5, duea noticeable to erosion decrease and pitting. in overall These observationscell voltage are(Vcell mainly) and causedelectrode by observations are mainly caused by the implosion of high-energy cavitation bubbles in turn generating overpotentialthe implosion of(η high-energy), and an observable cavitation bubblessurface in modification turn generating due high to velocityerosion microjets and pitting. of liquid These (up high velocity microjets of liquid (up to 200 m·s−1) hitting the electrode surface, as shown in Figure 2, observationsto 200 m·s−1) are hitting mainly the caused electrode by the surface, implosion as shown of high-energy in Figure2 ,cavitation and by acoustic bubbles streaming in turn generating together high velocity microjets of liquid (up to 200 m·s−1) hitting the electrode surface, as shown in Figure 2,

Catalysts 2019, 9, x FOR PEER REVIEW 6 of 19 CatalystsCatalysts2019 2019, 9,, 246 9, x FOR PEER REVIEW 6 of6 of 18 19 and by acoustic streaming together with microstreaming at or near the electrode surface [17]. Figure 6 showsand by a acoustic short summary streaming of togetherthe benefits with of microstreaming ultrasound in electrochemistry at or near the electrode [17]. surface [17]. Figure with6 shows microstreaming a short summary at or near of the the benefits electrode of ultrasound surface [17 ].in Figureelectrochemistry6 shows a [17]. short summary of the benefits of ultrasound in electrochemistry [17].

δUS Silent δUS Silent

Figure 5. Concentration profiles and the diffusion layer thickness (δ) in the presence of ultrasound Figure(USFigure) and 5. Concentration 5.in Concentrationthe absenceprofiles (silent profiles). and and the diffusionthe diffusion layer layer thickness thickness (δ) in the(δ) in presence the presence of ultrasound of ultrasound (US) and(US in) the and absence in the absence (silent). (silent). BENEFITS OF SONOELECTROCHEMISTRY BENEFITS OF SONOELECTROCHEMISTRYEnhanced electrochemical diffusion processes ImprovedEnhanced electrode electrochemical kinetics diffusion processes ImprovedImproved electrode electrode surface kinetics activation BetterImproved electrode electrode surface surface cleanliness activation LowerBetter electrode electrode overpotentials surface cleanliness SuppressedLower electrode electrode overpotentials fouling EfficientSuppressed degassing electrode at the fouling electrode surface EnhancedEfficient electrochemicaldegassing at the rates electrode and yields surface ImprovedEnhanced electrodeposit electrochemical quality rates and and properties yields BetterImproved electrochemical electrodeposit process quality efficiencies and properties  Better electrochemical process efficiencies Figure 6. Short summary of the benefits of ultrasound in electrochemistry [19].

3. Sonochemical and Sonoelectrochemical Fabrication of PEMFC and PEMWE Catalysts Figure 6. Short summary of the benefits of ultrasound in electrochemistry [19]. PEMFC and PEMWE strongly depend upon the electrocatalyst material efficiency especially for Figure 6. Short summary of the benefits of ultrasound in electrochemistry [19]. the3. Sonochemical ORR, the OER and (oxygen Sonoelectrochemical evolution reaction), Fabrication the HOR, and of PEMFC the HER and [4,7 PEMWE]. The important Catalysts mechanism controlling3. Sonochemical the metallic and Sonoelectrochemical catalyst is nucleation Fabrication and growth, of depending PEMFC and strongly PEMWE upon Catalysts the synthetic routes.PEMFC Many and methods PEMWE exist strongly for the preparation depend upon of catalystthe electrocatalyst nanoparticles, material including efficiency alcohol especially reduction, for citratethe ORR, reduction,PEMFC the andOER polyol PEMWE (oxygen reduction, strongly evolution borohydride depend reaction), upon reduction, thethe electrocatalyst HOR, photolytic and the reduction, material HER efficiency[4,7]. radiolytic The especially reduction,important for lasermechanismthe ablation, ORR, controllingthe and OER metal (oxygen evaporationthe metallic evolution condensationcatalyst reaction), is nucleation [20]. the Fairly andHOR, recently, growth, and Pollet thedepending HER showed [4,7]. strongly that The it is upon possibleimportant the tosyntheticmechanism fabricate routes. catalyst controlling Many nanomaterials methods the metallic exist sonochemically catalystfor the preparat is nucleation byion simply of andcatalyst using growth, nanoparticles, power depending ultrasound including strongly in the alcoholupon form the ofreduction,synthetic ultrasonic routes.citrate probes Manyreduction, and bathsmethods topolyol reduce exist forreduction, metallic the preparat borohydride inion aqueous of catalyst reduction, solutions nanoparticles, inphotolytic a specially including reduction, designed alcohol experimentalradiolyticreduction, reduction, setupcitrate as laserreduction, shown ablation, in Figure polyol and7a,b metalreduction, [15]. evaporation borohydride condensation reduction, [20]. Fairlyphotolytic recently, reduction, Pollet showedradiolytic that reduction, it is possible laser to ablation,fabricate andcatalyst metal nanomaterials evaporation sonochemicallycondensation [20]. by simplyFairly recently, using power Pollet ultrasoundshowed that in the it is form possible of ultrasonic to fabricate probes catalyst and nanomaterialsbaths to reduce sonochemically metallic ions in by aqueous simply solutions using power in a speciallyultrasound designed in the form experimental of ultrasonic setup probes as shown and baths in Figure to reduce 7a,b [15]. metallic ions in aqueous solutions in a specially designed experimental setup as shown in Figure 7a,b [15].

Catalysts 2019, 9, 246 7 of 18 Catalysts 2019, 9, x FOR PEER REVIEW 7 of 19

Figure 7.FigureSonochemical 7. Sonochemical ((a) ultrasonic((a) ultrasonic probe probe setup; setup; ((bb) ultrasonic ultrasonic bath bath setup) setup) and andsonoelectrochemical sonoelectrochemical ((c) ultrasonic((c) ultrasonic probe probe used used as the as vibratingthe vibrating cathode) cathode) toto prepare Prot Protonon Exchange Exchange Membrane Membrane Fuel Cell Fuel Cell (PEMFC),(PEMFC), Direct Direct Methanol Methanol Fuel Fuel Cell Cell (DMFC), (DMFC), and and ProtonProton Exchange Exchange Membrane Membrane Water Water Electrolyzer Electrolyzer (PEMWE) electrocatalyst nanoparticles [15]. CE: Counter Electrode, RE: Reference Electrode, WE: (PEMWE) electrocatalyst nanoparticles [15]. CE: Counter Electrode, RE: Reference Electrode, WE: Working Electrode Working Electrode. 3.1. Sonochemical Production of PEMFC and PEMWE Catalysts 3.1. Sonochemical Production of PEMFC and PEMWE Catalysts In 2010, in his extensive review, Pollet showed that it is possible to employ power ultrasound In 2010,for the in preparation his extensive of PEMFC review, and Pollet PEMWE showed cataly thatsts it [15]. is possible The review to employ detailed power the ultrasonic, ultrasound for the preparationsonochemical, of PEMFC and sonoelectrochemical and PEMWE catalysts generation [15]. of The noble review metal detailed electrocatalysts the ultrasonic, with nanosizes sonochemical, of and sonoelectrochemical<10 nm with and without generation the addition of noble of metal surfactants electrocatalysts and alcohols, with as shown nanosizes in Table of <10 1, carbon nm with and withoutsupported the addition electrocatalysts, of surfactants fuel and cell alcohols, electrodes as, shownand membranes. in Table1 ,The carbon paper supported showed that electrocatalysts, (i) the production of these nanomaterials is mainly attributed to the presence of radical species induced by fuel cell electrodes,water sonolysis and (caused membranes. by cavitation), The paper (ii) the showed catalyst thatnanoparticle (i) the production size depends of strongly these nanomaterials upon the is mainly attributedultrasonic frequency to the presence and irradiation of radical time, species surfactant, induced alcohol, by and water atmospheric sonolysis gas types, (caused and by (iii) cavitation), the (ii) the catalystas-prepared nanoparticle catalyst on carbon size dependsnanomaterials strongly leads to upon excellent the electrocatalytic ultrasonic frequency activity due and to carbon irradiation time, surfactant,support surface alcohol, functionalization and atmospheric and effective gas dispersion types, and caused (iii) by the sonication.as-prepared catalyst on carbon nanomaterialsIn leadsthe same to excellentreview, Pollet electrocatalytic proposed a possible activity general due topathway carbon for support the sonochemical surface functionalization reduction of PGM salts (e.g., chloroplatinic acid) and proposed a mechanism under ultrasonication [)))] using and effective dispersion caused by sonication. a specially designed sonoreactor, as shown in Figure 8, as follows [15]: In the same review, Pollet proposed a possible general pathway for the sonochemical reduction of (1) PGM salts (e.g., chloroplatinic acid) and proposedHO •H•OH a mechanism under ultrasonication [)))] using a specially designed sonoreactor, as shown in Figure8, as follows [15]: RH • OH• H R•HOH (2)

𝑚M 𝑛𝑚H•→))) M𝑚𝑛𝑚H (3) H2O → •H + •OH (1) 𝑚M 𝑛𝑚R•→M𝑚𝑛𝑚R 𝑛𝑚H (4) ))) where RH denotes the radical’sRH scavenger,+ •OH( acting•H) → as protectiveR• + H2 Oagent(H2 (surfactant) or alcohol), R• is the (2) secondary radical, and R’ is the deprotonated form. During sonolysis, water dissociate into n+  0 + hydrogen radicals (H•) and hydroxmM yl+ radicalsnmH• (OH•) → M accordingm + nmto EquationH (1). R’ species are formed (3) by hydrogen abstraction from RH with OH• and H• (Equation (2)) and react with M cations (Mn+) to +   0 + form metal particles (M°)m Maccordingn + nm toR Equations• → M (3)0 mand+ (4).nm R + nmH (4) where RH denotes the radical’s scavenger, acting as protective agent (surfactant or alcohol), R• is the secondary radical, and R’ is the deprotonated form. During sonolysis, water molecules dissociate into hydrogen radicals (H•) and hydroxyl radicals (OH•) according to Equation (1). R’ species are formed by hydrogen abstraction from RH with OH• and H• (Equation (2)) and react with M cations (Mn+) to form metal particles (M◦) according to Equations (3) and (4). Catalysts 2019, 9, 246 8 of 18

Table 1. Sonochemical production of nanosize electrocatalyst materials at several ultrasonic frequencies and powers in the presence of several alcohols and surfactants (adapted from [15]).

Ultrasonic Noble Metals Ultrasonic Power Surfactant Solvent Particle Size/nm Frequency/kHz SDS, PEG40-MS, Ag 200 200 W Propan-2-ol 13 ± 3 Tween20 SDS, Tween20, Pd 200 6.0 W cm−2 - 6–110 PEG40-MS, PVP Pd 50 - PVP, EG - 3–6 Tween20, PEG-MS, Ag, Au, Pt, Rh 200 6.0 W cm−2 - 5 SDS, PVP Pt 200 6.0 W cm−2 SDS - 2.6 ± 0.9 1 (PEG-MS) SDS, DBS, PEG-MS, Pt 200 200 W - 3 (SDS) DTAC, DTAB 3 (DBS) Ethanol, Propan-1-ol, Pt 20 10.0 W cm−2 - 2.6 Pentan-1-ol 2 (N2)–Pd 3.6 (Ar)–Pd Pt, Pd 20 100 W PVP Propan-2-ol 3.2 (N2)–Pt 2.9 (Ar)–Pt 1.5(Xe)–Pt Ag 20 100 W cm−2 - Ethanol 20 (Ar–H2) Pt 20 22 & 27 W cm−2 PVP, EG Citrate < 1 6 (Pt) Pt, Au 42 - PVP - 10 (Au) Methanol, 7 ± 2 (methanol) Au 20 23–47 W cm−3 - Propan-2-ol, 6 ± 4 (propan-2-ol) n-pentanol 5 ± 3 (n-pentanol) Methanol, 9–25 (depending on Au 20 9 W cm−2 - 1-Propanol, Butanol alcohol concentrations) Au 20,213,358,647&1062 0.1 ± 0.01 W cm−3 - 1-Propanol 15–30 Ru 20,213,355,647&1056 - SDS 1-Propanol 10–20 Pt–Ru 20 17 W cm−2 - Methanol, THF 2.0 ± 0.1 Pt–Ru 213 110–125 W cm−3 SDS, PVP - 5–10 Pd–Sn 20 42 W cm−2 - Citric acid, ethanol 3–5 Au–Ru 355 - PEG - 15 8 [6 nm Au core & 1 nm Au (core)–Pd (shell) 200 200 W SDS - Pd shell] 8 [4 nm Au core & 4 nm Au (core)–Pd (shell) 40 100 W PVP, EG - Pd shell] Au (core)–Pt (shell) 200 4.2 W cm−2 SDS, PFG - 6.5 ± 1.5 to 10.1 ± 3.6 Au (core)–Ag (shell) 20 23–47 W cm−2 PEG, EG - 20 Catalysts 2019, 9, x FOR PEER REVIEW 9 of 19

FigureFigure 8. 8.Sonochemical Sonochemical reactorreactor or ‘Sonoreactor’ (Besançon (Besançon cell—Professor cell—Professor Jean-Yves Jean-Yves Hihn’s Hihn’s Group, Group, Université de Franche-Comté) to produce PEMFC and PEMWE catalysts. Université de Franche-Comté) to produce PEMFC and PEMWE catalysts.

3.2. Sonoelectrochemical Production of PEMFC and PEMWE Catalysts Dabalà et al. [21] demonstrated the production of nanoparticles by sonoelectrochemistry as the technique offers many advantages: (i) great enhancement in mass-transport near the electrode, thereby altering the rate, and sometimes the mechanism, of the electrochemical reactions; (ii) modification of surface morphology through cavitation jets at the electrode–electrolyte interface, usually causing an increase of the surface area; and (iii) reduction of the electrode diffusion layer thickness and therefore depletion. Zin, Pollet, and Dabalà [22] modified Reisse’s original setup by only using the vibrating tip of the ultrasonic probe as the cathode—‘sonoelectrode’, as shown in Figure 9. The setup exposes only the flat circular area at the end of the ultrasonic vibrating tip to the electrodeposition electroanalyte solution. The tip acts as both the cathode and the ultrasonic vibrating emitter. A short pulse of galvanostatic (electrolysis) current reduces many metallic ions (Mn+ e.g., Pt2+ or Pt4+) at the cathode, forming a high density of fine metallic nuclei and nanoparticles. This short electrodeposition current pulse is immediately followed by a very short burst of power ultrasound (e.g., 20 kHz) that removes the metallic nanoparticles from the vibrating cathode, cleans the cathode surface, and replenishes the double layer with fresh metallic ions by highly efficient stirring of the electroanalyte solution (see earlier).

Catalysts 2019, 9, 246 9 of 18

3.2. Sonoelectrochemical Production of PEMFC and PEMWE Catalysts Dabalà et al. [21] demonstrated the production of nanoparticles by sonoelectrochemistry as the technique offers many advantages: (i) great enhancement in mass-transport near the electrode, thereby altering the rate, and sometimes the mechanism, of the electrochemical reactions; (ii) modification of surface morphology through cavitation jets at the electrode–electrolyte interface, usually causing an increase of the surface area; and (iii) reduction of the electrode diffusion layer thickness and therefore ion depletion. Zin, Pollet, and Dabalà [22] modified Reisse’s original setup by only using the vibrating tip of the ultrasonic probe as the cathode—‘sonoelectrode’, as shown in Figure9. The setup exposes only the flat circular area at the end of the ultrasonic vibrating tip to the electrodeposition electroanalyte solution. The tip acts as both the cathode and the ultrasonic vibrating emitter. A short pulse of galvanostatic (electrolysis) current reduces many metallic ions (Mn+ e.g., Pt2+ or Pt4+) at the cathode, forming a high density of fine metallic nuclei and nanoparticles. This short electrodeposition current pulse is immediately followed by a very short burst of power ultrasound (e.g., 20 kHz) that removes the metallic nanoparticles from the vibrating cathode, cleans the cathode surface, and replenishes the double layer with fresh metallic ions by highly efficient stirring of the electroanalyte solutionCatalysts 2019 (see, 9 earlier)., x FOR PEER REVIEW 10 of 19

tON tUS (Electrolysis (Ultrasound Time) / ms Time) / ms 100 100 100 200 200 100 200 200 200 400 300 150

FigureFigure 9. 9.Sonoelectrochemical Sonoelectrochemical setup for for the the production production of of PEMFC PEMFC and and PEMWE PEMWE catalysts catalysts [17,21,22]. [17,21, 22]. GC: Glassy carbon. PTFE:GC: Polytetrafluoroethylene. Glassy carbon. PTFE: Polytetrafluoroethylene

TheThe workers workers found found that that (i) ultrasound(i) ultrasound had had the principalthe principal role in role causing in causing cavitation cavitation in the electrolytic in the solutionelectrolytic and solution in ablating and in the ablating metallic the nuclei metallic and nuclei nanoparticles and nanoparticles from the from vibrating the vibrating cathodic cathodic surface, (ii)surface, the nuclei (ii) the and nuclei nanoparticles and nanoparticles formed formed on the vibratingon the vibrating GC (glassy GC (glassy carbon) carbon) electrode electrode surface surface during during the tON time were ejected into the electrolyte during tUS, (iii) all metallic nanoparticles (Pt) the tON time were ejected into the electrolyte during tUS, (iii) all metallic nanoparticles (Pt) showed singleshowed phase single Pt phase with aPt face with centered a face centered cubic ( fcccubic) structure, (fcc) structure, and (iv) and the (iv) metallic the metallic nanoparticles nanoparticles had a meanhad a average mean average size of size 14.06 of nm 14.06 with nm a with standard a standard deviation deviation of 2.78, of in2.78, excellent in excellent agreement agreement with with values obtainedvalues obtained from XRD from analyses, XRD analyses, as shown as shown in Figure in Figure10[22]. 10 [22].

Pt nanoparticles of average size of ca. 14.06 nm with a SD of 2.78

Figure 10. (Clockwise) TEM image, X-ray energy dispersive spectrometry, and XRD patterns of Pt nanoparticles [22].

There are various preparation methods for carbon (and other conductive support materials [16]) supported PGM nanoparticles, e.g., impregnation, sputtering and electrodeposition, the colloidal method, and the ion-exchange method [20]. In most cases, decoration of support substrates has usually been achieved through impregnation via wet . Recently, Karousos et al. [23]

Catalysts 2019, 9, x FOR PEER REVIEW 10 of 19

tON tUS (Electrolysis (Ultrasound Time) / ms Time) / ms 100 100 100 200 200 100 200 200 200 400 300 150

Figure 9. Sonoelectrochemical setup for the production of PEMFC and PEMWE catalysts [17,21,22]. GC: Glassy carbon. PTFE: Polytetrafluoroethylene

The workers found that (i) ultrasound had the principal role in causing cavitation in the electrolytic solution and in ablating the metallic nuclei and nanoparticles from the vibrating cathodic surface, (ii) the nuclei and nanoparticles formed on the vibrating GC (glassy carbon) electrode surface during the tON time were ejected into the electrolyte during tUS, (iii) all metallic nanoparticles (Pt) showed single phase Pt with a face centered cubic (fcc) structure, and (iv) the metallic nanoparticles Catalysts 2019, 9, 246 10 of 18 had a mean average size of 14.06 nm with a standard deviation of 2.78, in excellent agreement with values obtained from XRD analyses, as shown in Figure 10 [22].

Pt nanoparticles of average size of ca. 14.06 nm with a SD of 2.78

FigureFigure 10. (Clockwise) 10. (Clockwise) TEM TEM image, image, X-ray X-ray energyenergy dispersi dispersiveve spectrometry, spectrometry, and XRD and patterns XRD patterns of Pt of Pt nanoparticles [22]. nanoparticles [22]. There are various preparation methods for carbon (and other conductive support materials [16]) Theresupported are various PGM nanoparticles, preparation e.g., methods impregnation, for carbon sputtering (and other and electrodeposition, conductive support the colloidal materials [16]) supportedmethod, PGM and nanoparticles, the ion-exchange e.g., method impregnation, [20]. In most sputtering cases, decoration and electrodeposition, of support substrates the has colloidal method,usually and the been ion-exchange achieved through method impregnation [20]. In most via cases,. decoration Recently, of support Karousos substrates et al. [23] has usually been achievedCatalysts 2019 through, 9, x FOR impregnation PEER REVIEW via wet chemistry. Recently, Karousos et al. [23] showed11 of that19 Pt/C can be produced by simultaneous Pt nanoparticle synthesis and decoration of Vulcan XC72 carbon black substrate,showed as shown that Pt/C in Figure can be 11 produced, in a one-step-process by simultaneocombiningus Pt nanoparticle galvanostatic synthesis pulsed and decoration electrodeposition of and pulsedVulcan ultrasound XC72 carbon (20 kHz).black Thissubstrate, was achievedas shown byin usingFigure carbon11, in a black one-step-process suspensions combining in an aqueous galvanostatic pulsed electrodeposition and pulsed ultrasound (20 kHz). This was achieved by using solution containing chloroplatinic acid precursor baths and PVP (polyvinylpyrrolidone) as surfactant carbon black suspensions in an aqueous solution containing chloroplatinic acid precursor baths and (or stabilizerPVP (polyvinylpyrrolidone) against free nanoparticle as surfactant aggregation). (or stabilizer The against workers free foundnanoparticle that PVP-capped aggregation). The octahedral -2 Pt nanoparticlesworkers found with that (111) PVP-capped facets and octahedral an average Pt diameternanoparticles of ~5with nm (111) (obtained facets and at 1 an mA average cm current pulses, followeddiameter of by ~5 20nm kHz(obtained ultrasonic at 1 mA pulses)cm-2 current were pulses, ‘strongly’ followed attached by 20 kHz onto ultrasonic the Vulcan pulses) XC72 were carbon black substrate.‘strongly’ Theyattached also onto observed the Vulcan that XC72 partial carbon formation black substrate. of larger They Pt nanoparticles also observed (NPs)that partial in the range 20–100 nmformation were of formed. larger Pt Innanoparticles order toeffectively (NPs) in the caprange the 20–100 Pt NPs, nm were high formed. PVP In concentrations order to effectively were used, cap the Pt NPs, high PVP concentrations were used, however this strategy had a deteriorating effect howeveron this the strategyestablishment had a of deteriorating a sufficiently effectstrong onmetal-substrate the establishment binding, of leaving a sufficiently part of strongthe formed metal-substrate NPs binding,unbound leaving to part the ofVulcan the formed XC72 carbon NPs unboundblack substrate to the [23]. Vulcan XC72 carbon black substrate [23].

Figure 11.FigureRepresentative 11. Representative TEM TEM images images of of (a (a)) Pt Pt nanoparticles,nanoparticles, and and (b) (Ptb )nanoparticles Pt nanoparticles formed formed directly directly sonoelectrochemicallysonoelectrochemically onto onto the carbonthe carbon particles. particles. TheThe particle particle size size distribution distribution of the of carbon the carbon supported supported Pt nanoparticles is also shown in (c). Modified from [23]. Pt nanoparticles is also shown in (c). Modified from [23]. 4. Ultrasonic, Sonochemical, and Sonoelectrochemical Production of PEMWE and PEMWE Electrodes

4.1. Ultrasonic Inclusion of Nano-Electrocatalytic Materials on Substrates A few researchers successfully demonstrated that ultrasound can be employed for the ‘doping’ of nanoparticles in various polymeric and ceramics materials [20]. For example, Gedanken et al. [24] showed that ultrasound can be used for the insertion of amorphous catalyst nanomaterials into the ceramic mesopores and polymerics, and the deposition of various nanomaterials (metals, metal oxides, semi-conductors, etc.) on the surfaces of ceramic and polymeric materials [20]. They concluded that when compared to other methods, such as thermal spreading or impregnation, ultrasound yielded better material properties. Early investigations by Pollet et al. [25] demonstrated that the insertion of catalyst nanoparticles in PEM materials increased the performance of PEMFCs. For the first time, Pollet et al. [25] showed that the incorporation of nano-electrocatalysts in polymeric membranes, e.g., Nafion® for fuel cells using power ultrasound, improved PEMFC performance, however, the system was not optimized. Only a few studies in the literature focus on the use of ultrasonic, sonochemical, and sonoelectrochemical methods to fabricate PEMFC and PEMWE electrodes. At present, there are various techniques for producing PEMFC, DMFC (Direct Methanol Fuel Cell), and PEMWE catalyst

Catalysts 2019, 9, 246 11 of 18

4. Ultrasonic, Sonochemical, and Sonoelectrochemical Production of PEMWE and PEMWE Electrodes

4.1. Ultrasonic Inclusion of Nano-Electrocatalytic Materials on Substrates A few researchers successfully demonstrated that ultrasound can be employed for the ‘doping’ of nanoparticles in various polymeric and ceramics materials [20]. For example, Gedanken et al. [24] showed that ultrasound can be used for the insertion of amorphous catalyst nanomaterials into the ceramic mesopores and polymerics, and the deposition of various nanomaterials (metals, metal oxides, semi-conductors, etc.) on the surfaces of ceramic and polymeric materials [20]. They concluded that when compared to other methods, such as thermal spreading or impregnation, ultrasound yielded better material properties. Early investigations by Pollet et al. [25] demonstrated that the insertion of catalyst nanoparticles in PEM materials increased the performance of PEMFCs. For the first time, Pollet et al. [25] showed that the incorporation of nano-electrocatalysts in polymeric membranes, e.g., Nafion® for fuel cells using power ultrasound, improved PEMFC performance, however, the system was not optimized. Only a few studies in the literature focus on the use of ultrasonic, sonochemical, and sonoelectrochemicalCatalysts 2019, 9, x FOR PEER methods REVIEW to fabricate PEMFC and PEMWE electrodes. At present, there12 of are 19 various techniques for producing PEMFC, DMFC (Direct Methanol Fuel Cell), and PEMWE catalyst ‘inks’,‘inks’, i.e., i.e., forfor the mixing mixing and and milling milling the the carbon carbon support support with with the the carbon-support carbon-support electrocatalyst electrocatalyst and andthe ionomer the ionomer either either by forced by forced magnetic magnetic stirring, stirring, ultrasonic ultrasonic stirring, stirring, or ball-milling, or ball-milling, as shown as shown in Figure in Figure12. 12.

Standard Catalyst Preparation Pt/C Preparation

Pt/C supported Nafion® HSA Carbon Support** catalyst* dispersion 250 – 1,270 m2 g-1 - Alcohol reduction - Citrate reduction - Polyol reduction + -Borohydridereduction + - Photolytic reduction 0 - Radiolytic reduction Pt (II) Pt -Laser ablation - Magnetic stirring Solvent -Metal evaporation - Ball-milling )))) -Metal condensation - Ultrasound SONO(ELECTRO)CHEMICAL REDUCTION Nafion® + dispersion As-prepared Pt/C )))) Solvent supported catalyst

*Commercial: **High Surface Area (HSA) Johnson Matthey, Tanaka, HySA etc Commercial: Cabot, Akzo Nobel etc

Figure 12. Various routes for preparing PEMFC, DMFC, and PEMWE catalyst ‘inks’. Figure 12. Various routes for preparing PEMFC, DMFC, and PEMWE catalyst ‘inks’. The catalyst ink may be either deposited to the gas diffusion layer (GDL) to yield a gas diffusion The catalyst ink may be either deposited to the gas diffusion layer (GDL) to yield a gas diffusion electrode (GDE) (or also known as catalyst coated substrate (CCS)) or the polymeric proton exchange electrode (GDE) (or also known as catalyst coated substrate (CCS)) or the polymeric proton exchange membrane to form a catalyst coated membrane (CCM). The GDEs are usually prepared by screen membrane to form a catalyst coated membrane (CCM). The GDEs are usually prepared by screen printing, hand painting, ink-jetting, spreading, spraying (air and ultrasonic), (electro)deposition, printing, hand painting, ink-jetting, spreading, spraying (air and ultrasonic), (electro)deposition, ionomer impregnation, and sputtering. In the case of CCMs, they are produced by decaling, screen ionomer impregnation, and sputtering. In the case of CCMs, they are produced by decaling, screen printing, hand painting, spraying (air and ultrasonic), impregnation reduction, evaporation deposition, sputtering, and dry spraying, as shown in Figure 13. For further details on PEMFC manufacturing, the reader is invited to consult the reviews detailed by Mehta and Cooper [26] and Litster and McLean [3].

Catalysts 2019, 9, 246 12 of 18 printing, hand painting, spraying (air and ultrasonic), impregnation reduction, evaporation deposition, sputtering, and dry spraying, as shown in Figure 13. For further details on PEMFC manufacturing, the reader is invited to consult the reviews detailed by Mehta and Cooper [26] and Litster and McLean [3]. Catalysts 2019, 9, x FOR PEER REVIEW 13 of 19

Two types of Membrane Electrode Assemblies

Proton Exchange Gas Diffusion Layer Membrane (GDL) (PEM)

 Decal  Blade process  + Pt/C Catalyst Ink Screen-printing + Pt/C Catalyst Ink  Painting  Spraying  Impregnation  Evaporation  Sputtering

Gas Diffusion Electrode Coated Catalyst Membrane (GDE) (CCM)

Figure 13. VariousVarious preparation preparation methods methods for for gas gas diffusion electrodes (GDE) and catalyst coated membranes (CCM).

4.2. Sonochemical Sonochemical Production of PEMFC and PEMWE Electrodes In 2011, PolletPollet etet al.al. [[27]27] showed showed that that GDEs GDEs for for PEMFCs PEMFCs may may be be produced produced by by ultrasonic-spray ultrasonic-spray for forusing using the the commercially commercially available availableSono-Tek Sono-Teksystem, system, as shown as shown in Figure in Figure 14. 14. In this study, various types (five) (five) of GDL were deposited with catalyst ink of known PGM loading by by the ultrasonic-spray technique technique and and thei theirr performances performances (and (and dura durability)bility) were were compared. compared. It was observed that (i) SGL 10BC exhibited the best performance in in an MEA when the GDL was ultrasonically spray coated and, (ii) SGL 34BC ex exhibitedhibited mass-transport issues. The workers showed that the ultrasonic-spray technique led led to better MEA performances compared to the hand-painted method especiallyespecially atat low low PGM PGM loadings, loadings, as as shown shown in Figurein Figure 15. Moreover,15. Moreover, a 50% a increase50% increase in peak in power peak powerratings ratings was found was at found ultra-low at ultra-low PGM loadings PGM loadin for thegs ultrasonically for the ultrasonically sprayed GDEs sprayed when GDEs compared when comparedto the hand-painted to the hand-painted ones, as shown ones, in as Figure shown 15 .in It Fi wasgure also 15. found It was that also the found ultrasonic-spray that the ultrasonic- method spraydistributed method the distributed catalyst ink the more catalyst evenly ink ontomore the evenly GDL, onto asshown the GDL, in Figureas shown 16, in yielding Figure better16, yielding PGM betterutilization PGM compared utilization to compared the hand-painted to the hand-painted method. This method. was also This further was also evident further at evident lower platinum at lower platinumloadings, loadings, as shown as in shown Figure 17in [Figure27]. 17 [27].

Catalysts 2019, 9, 246 13 of 18 Catalysts 2019, 9, x FOR PEER REVIEW 14 of 19

Catalysts 2019, 9, x FOR PEER REVIEW 14 of 19

FigureFigure 14. (14.a) Sono-Tek(a) Sono-TekUltrasonic Ultrasonic Spray Spray system—‘ExactaCoat’; system—‘ExactaCoat’; (b (b) )representation representation of ofthe the vibrating vibrating Figure 14. (a) Sono-Tek Ultrasonic Spray system—‘ExactaCoat’; (b) representation of the vibrating nozzle cross-section; (c) mist formation of the liquid schematic; (d) CCMs for PEMFC, DMFC, and nozzle cross-section;nozzle cross-section; (c) mist(c) mist formation formation of of the the liquid liquid schematic; schematic; (d) CCMs (d) for CCMs PEMFC, for DMFC, PEMFC, and DMFC, and PEMWEPEMWEPEMWE manufactured manufactured by by by the thethe Sono-Tek Sono-TekSono-Tek system;system;system; (e ())e representationrepresentation) representation of of nanoparticle ofnanoparticle nanoparticle de-agglomeration de-agglomeration de-agglomeration via thevia ultrasonic-sprayviathe theultrasonic-spray ultrasonic-spray method method method vs. vs. vs. the the the air air air spray sprayspray methodmethod [27]. [[27].27].

-2-2 ≈ ≈ A:A: 0.05 0.05 mg mgPtPtcmcm 50%50% -2-2 C: C:0.15 0.15 mg mgPtPtcmcm

Figure 15. Polarization and power density curves comparing ultrasonic-sprayed and hand-painted SGL 10BC GDLs. Anode (A) = 50%; cathode (C) = 50%. The loadings for the MEAs were; ultrasonic sprayed: A = 0.05 mg cm−2, C = 0.15 mg cm−2 and hand-painted: A = 0.09 mg cm−2, C = 0.18 mg cm−2 [27].

CatalystsCatalysts 2019 2019, 9, ,9 x, xFOR FOR PEER PEER REVIEW REVIEW 1515 of of 19 19

FigureFigure 15. 15. Polarization Polarization and and power power density density curves curves compar comparinging ultrasonic-sprayed ultrasonic-sprayed and and hand-painted hand-painted SGLSGL 10BC 10BC GDLs. GDLs. Anode Anode (A) (A) = =50%; 50%; cathode cathode (C) (C) = =50%. 50%. The The loadings loadings for for the the MEAs MEAs were; were; ultrasonic ultrasonic Catalysts 2019, 9, 246 −2 −2 −2 −2 14 of 18 sprayed:sprayed: A A = =0.05 0.05 mg mg cm cm−2, ,C C = =0.15 0.15 mg mg cm cm−2 and and hand-painted: hand-painted: A A = =0.09 0.09 mg mg cm cm−2, ,C C = =0.18 0.18 mg mg cm cm−2 [27].[27].

-2 0.40 mg cm-2-2 0.05 mgPt cm-2 0.40 mgPtPtcm 0.05 mgPt cm

USUS 454µm 454µm USUS 413µm 413µm -2 0.15 mg cm-2-2 0.40 mgPt cm-2 0.15 mgPtPtcm 0.40 mgPt cm

USUS 427µm 427µm HPHP

FigureFigure 16. 16.16. SEM SEMSEM images: images: images: ( a()a ) (top atop) topview view view and and ( and b(b) )cross-section cross-section (b) cross-section of of the the catalyst ofcatalyst the catalystlayer layer prepared prepared layer preparedby by the the by the −2 −2 ultrasonic-sprayultrasonic-spray method. method. method. Pt Pt loading: loading: Pt loading: 0.40 0.40 mg mg 0.40 cm cm− mg2 on on Sigracet cm Sigraceton 10BC 10BC Sigracet GDL, GDL, (10BC c()c )top top view GDL, view and and (c) ( d top(d) )cross- cross- view and (d) −2 −2 cross-sectionsectionsection of of catalyst catalyst of catalyst layer layer (CL) (CL) layer prepared prepared (CL) prepared by by the the US US bytechnique. technique. the US technique.Pt Pt loading: loading: 0.15 Pt0.15 loading: mg mg cm cm−2 on 0.15 on Sigracet Sigracet mg cm 10BC 10BCon Sigracet −2 10BCGDL,GDL, ( GDL,e()e )top top (view eview) top and and view ( f()f )cross-section cross-section and (f) cross-section of of CL CL prepared prepared of CL by by preparedUS. US. Pt Pt loading loading by US. is is 0.05 0.05 Pt loadingmg mg cm cm−2 on is on 0.05Sigracet Sigracet mg cm−2 on Sigracet10BC10BC GDL, GDL, 10BC ( g(g) )top top GDL, view view ( gand )and top ( h(h) view)cross-section cross-section and (h )of of cross-section catalyst catalyst layer layer prepared of prepared catalyst by by layerhand-painting. hand-painting. prepared Pt Pt by loading: loading: hand-painting. −2 0.40 mg cm−2 on Sigracet 34BC GDL [27]. Pt0.40 loading: mg cm 0.40on Sigracet mg cm 34BC−2 on GDL Sigracet [27]. 34BC GDL [27].

Figure 17. Column histograms comparing (a) peak power densities and (b) peak power per Pt loading Figure 17. 17. ColumnColumn histograms histograms comparing comparing (a) peak (a) peakpower power densities densities and (b) andpeak ( bpower) peak per power Pt loading per Pt loading −2 −2 forfor both both hand-painted hand-painted (P) (P) and and ultrasonic ultrasonic-sprayed-sprayed (US) (US) SGL SGL 10BC 10BC GDLs: GDLs: 0.40 0.40 mg mg cm cm−2, ,0.15 0.15 mg− mg2 cm cm−2, , −2 for both hand-painted−2 (P) and ultrasonic-sprayed (US) SGL 10BC GDLs: 0.40 mg cm , 0.15 mg cm , 0.05 mg cm−2 [27]. 0.05 mg mg cm cm− [27].2 [27 ]. 4.3. Sonoelectrochemical Production of PEMFC and PEMWE Electrodes 4.3.4.3. Sonoelectrochemical Production Production of PEMFC of PEMFC and PEMWE and PEMWE Electrodes Electrodes PolletPollet [25] [25] showed showed for for the the first first time time that that the the sonoelectrochemical sonoelectrochemical techni techniqueque can can be be employed employed for for Pollet [25] showed for the first time that the sonoelectrochemical® technique can be employed for fabricating PEMFC electrodes whereby Pt loaded on Nafion® -bonded carbon anodes were produced fabricating PEMFC electrodes whereby Pt loaded on Nafion -bonded® carbon anodes were produced fabricating PEMFC2 electrodes4 whereby Pt loaded on Nafion -bonded carbon anodes were produced in inin thethe presencepresence KK2PtClPtCl4 inin solutionssolutions byby galvanostaticgalvanostatic pulsepulse electrodepositionelectrodeposition underunder silentsilent andand silent theultrasonicultrasonic presence conditions conditions K2PtCl 4(20 (20in kHz). solutionskHz). He He found found by galvanostatic that that th thee PEMFC PEMFC pulse electrodes electrodes electrodeposition generated generated sonoelectrochemically sonoelectrochemically under and ultrasonic conditionsexhibitedexhibited superior superior (20 kHz). performance performance He found compared compared that the to PEMFC to those those produced electrodesproduced by by generatedthe the galvanostatic galvanostatic sonoelectrochemically pulse pulse method method only only exhibited superiorunderunder silent silent performance conditions conditions and comparedand the the conventional conventional to those producedmethod method as as byshown shown the galvanostaticin in Figure Figure 18. 18. He He pulse found found method that that (i) (i) onlya a under silentsignificantsignificantconditions increase increase and in in Pt thePt active active conventional surface surface area area method at at a a given given as Pt shown Pt loading loading in was Figure was observed, observed, 18. He which foundwhich was was that possibly possibly (i) a significant increase in Pt active surface area at a given Pt loading was observed, which was possibly due to a decrease in Pt size, (ii) from SEM studies, ultrasound yields compact electrodeposit in turn decreasing the porosity and improving access of reactants to Pt sites leading to better Pt utilization and better fuel cell performance (a significant increase in power density was observed) [28]. Catalysts 2019, 9, x FOR PEER REVIEW 16 of 19

due to a decrease in Pt size, (ii) from SEM studies, ultrasound yields compact electrodeposit in turn decreasing the porosity and improving access of reactants to Pt sites leading to better Pt utilization Catalystsand better2019 ,fuel9, 246 cell performance (a significant increase in power density was observed) [28]. 15 of 18

985 mW cm-2 915 mW cm-2 860 mW cm-2

Figure 18.18. I–V and power curves for PEMFC electrodes pr preparedepared by (ii) the sonoelectrochemically, (ii)(ii) galvanostaticgalvanostatic oror PulsedPulsed CurrentCurrent (PC)(PC) methodmethod onlyonly (i.e.,(i.e., silentsilent conditions)conditions) andand (ii)(ii) conventionalconventional method. ModifiedModified from [[25].25].

In orderorder toto shedshed lightlight onon thethe observedobserved increaseincrease inin fuelfuel cellcell performanceperformance forfor MEAsMEAs preparedprepared sonochemically and and sonoelectrochemically, sonoelectrochemically, in in 2011, 2011, Pollet Pollet et et al. al. [28] [28 studied] studied the the electrodeposition electrodeposition of ofPt on Pt glassy on glassy carbon carbon (GC) (GC) and gas and diffusion gas diffusion layer (GDL) layer surfaces (GDL) surfaces in dilute in chloroplatinic dilute chloroplatinic acid solutions acid solutions (10 mM PtCl 2− in 0.5 M NaCl). The experiments were performed potentiodynamically (10 mM PtCl42− in 0.5 M4 NaCl). The experiments were performed potentiodynamically in the absence inand the presence absence of and ultrasound presence of(20 ultrasound kHz) at various (20 kHz) ultrasonic at various powers ultrasonic (up powersto 6 W). (up In to the 6 W). workers’ In the workers’conditions, conditions, it was found it was that found platinum that platinumelectrodeposition electrodeposition was an irreversible was an irreversible process which process required which requireda substantial a substantial overpotential overpotential to drive the to formation drive the formation of Pt nuclei of on Pt the nuclei GC onand the GDL GC surfaces. and GDL However, surfaces. However,under ultrasonication under ultrasonication Pt electrodeposition Pt electrodeposition became easier became due easierto lower due concentration to lower concentration and nucleation and nucleationoverpotentials overpotentials and overall and currents overall significantly currents significantly increased compared increased to compared silent conditions. to silent conditions.Moreover, Moreover,the specific the Electrochemical specific Electrochemical Surface Area Surface (ECSA Area) was ( ECSAsignificantly) was significantly affected for affectedPt/GC and for Pt/GDL Pt/GC andelectrodes Pt/GDL prepared electrodes in the prepared presence in theof rotation presence (i of.e.,rotation GC only) (i.e., and GC in only) the presence and in the of presenceultrasound of ultrasoundcompared to compared those prepared to those under prepared silent conditions. under silent Thisconditions. observation This was observation explained was to be explained due to both to belarger due and to both agglomerated larger and agglomeratedPt nanoparticles Pt nanoparticlesformed on the formed GC and on theGDL GC surface and GDL caused surface by intense caused byconvection. intense convection. It was also Itobserved was also that observed ultrasound that ultrasound generated larger generated Pt nanoparticles larger Pt nanoparticles on GC electrodes on GC electrodesthan thosethan on GDL those electrodes. on GDL electrodes. It was found It was that found the thatelectrodeposited the electrodeposited Pt on GDL Pt on samples GDL samples under underultrasound ultrasound led to ledsmaller to smaller Pt particle Pt particle sizes sizes(<200 (<200 nm) nm)compared compared to silent to silent conditionsconditions (ca. (ca. 0.9–10.9–1 µm),µm as), asshown shown in inFigure Figure 19. 19 These. These observations observations were were attribut attributeded to to the the implosion implosion of of cavitation cavitation bubbles at the GC andand GDL GDL surfaces surfaces enabling enabling the ‘deagglomeration’the ‘deagglomeration’ of Pt nanoparticles of Pt nanoparticles or/and activating or/and nucleationactivating sitesnucleation for Pt. sites for Pt.

Catalysts 2019, 9, 246 16 of 18 Catalysts 2019, 9, x FOR PEER REVIEW 17 of 19

FigureFigure 19. 19. (C)) Particle Particle size size distribution distribution curves: curves: (a) no (a) ul notrasound, ultrasound, (b) 2.3 W, (b) (c) 2.3 3.2 W, W, (c) and 3.2 (d) W, 4.4 and W. (d) 4.4(D W.) SEM (D) pictures SEM pictures of Pt electrodeposited of Pt electrodeposited on GDL (a) on under GDL silent (a) under and (b)silent ultrasonicand (b) (4.4 ultrasonic W) conditions (4.4 W) conditions[28]. [28]. 5. Conclusions 5. Conclusions TheThe first first paper paper describing describing thethe generationgeneration of of Platinum Platinum Group Group Metal Metal (PGM) (PGM) nanoparticles nanoparticles from from aqueousaqueous solutions solutions (chloroplatinic (chloroplatinic acid) and in in the the absence absence of of surfactants surfactants using using sonoelectrochemistry sonoelectrochemistry waswas in in 2009. 2009. In In these these experiments, experiments, thethe fabricationfabrication of Pt nanoparticles (NPs (NPs ≈≈15 15nm) nm) was was performed performed galvanostaticallygalvanostatically at at room room temperaturetemperature using a a vibrating vibrating working working electrode electrode (sonoelectrode (sonoelectrode) producing) producing shortshort applied applied current current pulses pulses triggeredtriggered and followed followed immediately immediately by by short short ultrasonic ultrasonic pulses pulses (20 (20kHz). kHz). MoreMore recently, recently, it it was was successfully successfully shownshown that simultaneous electrocatalytic electrocatalytic Pt-NP Pt-NP synthesis synthesis and and decorationdecoration of of Vulcan Vulcan XC-72 XC-72 carbon carbon black black substrate substrate was achievedwas achieved in a novel in aone-step-process novel one-step-process, combining, galvanostaticcombining galvanostatic pulsed electrodeposition pulsed electrodeposition and pulsed an ultrasonicationd pulsed ultrasonication (20 kHz). Pollet(20 kHz). et al. Pollet also showedet al. thatalso power showed ultrasound that power (20 kHz)ultrasound can be (20 used kHz) effectively can be used for the effectivelyin-situ fabrication for the in-situ of PEMFC fabrication electrodes of leadingPEMFC to electrodes highly performing leading to highly electrodes performing containing electrodes (ultra)-low containing loading (ultra)-low Pt. loading In 2011, Pt. aIn research2011, collaborationa research collaboration with Sono-Tek with Corporation Sono-Tek Corporation (USA) showed (USA) thatshowed the that novel the ultrasonic-spray novel ultrasonic-spray (US)method (US) canmethod be used can for be preparing used for preparing GDEs on variousGDEs on commercial various commercial woven and woven non-woven and non-woven GDLs containing GDLs −2 (ultra)-lowcontaining Pt (ultra)-low loadings inPt the loadings range 0.40–0.05in the range mg cm0.40–0.05−2. It was mg foundcm . It that was the found GDEs that prepared the GDEs by this USprepared method exhibitedby this betterUS method performances exhibited compared better toperformances those prepared compared commercially, to those especially prepared at low commercially, especially at low Pt loadings. Pt loadings.

Catalysts 2019, 9, 246 17 of 18

Funding: This research received no external funding. Acknowledgments: The author would like to thank E.F. Valzer, O.J. Curnick, B. Millington, J.T.E. Goh, S. Du, V. Whipple, V. Zin, M. Dabalà, C. Argirusis, P.M. Sakkas and M.T.Y. Paul. Conflicts of Interest: The authors declare no conflicts of interest.

References

1. Pollet, B.G.; Staffell, I.; Shang, J.L. Current status of hybrid, battery and fuel cell electric vehicles: From electrochemistry to market prospects. Electrochim. Acta 2016, 84, 235–249. [CrossRef] 2. Pollet, B.G. Polymer Electrolyte Membrane and Direct Methanol Fuel Cell Technology—A review. Platinum Metals Rev. 2013, 57, 137–142. [CrossRef] 3. Litster, S.; McLean, G. Review: PEM fuel cell electrode. J. Power Sour. 2004, 130, 61–76. [CrossRef] 4. Carmo, M.; Fritz, D.L.; Mergel, J.; Stolten, D. A comprehensive review on PEM water electrolysis. Int. J. Hydrog. Energy 2013, 38, 4901–4934. [CrossRef] 5. Jorge, A.B.; Dedigama, I.; Miller, T.S.; Shearing, P.; Brett, D.J.L.; McMillan, P.F. Carbon Nitride Materials as Efficient Catalyst Supports for Proton Exchange Membrane Water Electrolyzers. Nanomaterials 2018, 8, 432. [CrossRef][PubMed] 6. Felix, C.; Bladergroen, B.; Linkov, V.; Pollet, B.G.; Pasupathi, S. Ex-situ electrochemical characterization of

IrO2 synthesized by a modified Adams fusion method for the oxygen evolution reaction. Catalysts 2019. under review. 7. Sharma, S.; Pollet, B.G. Support materials for PEMFC and DMFC electrocatalysts—A review. J. Power Sour. 2012, 208, 96–119. [CrossRef] 8. Du, S.; Pollet, B.G. Catalyst loading for Pt-nanowire thin film electrodes in PEFCs. Int. J. Hydrog. Energy 2012, 37, 17892–17898. [CrossRef] 9. Du, S.; Koenigsmann, C.; Sun, S. One-dimensional Nanostructures for PEM Fuel Cell Applications. In Hydrogen and Fuel Cells Primers Series; Pollet, B.G., Ed.; Academic Press: Cambridge, MA, USA, 2017. 10. Bessarabov, D.; Millet, P. PEM Water Electrolysis, 1st ed. In Hydrogen and Fuel Cells Primers Series; Pollet, B.G., Ed.; Academic Press: Cambridge, MA, USA, 2018; Volume 1. 11. Bessarabov, D.; Millet, P. PEM Water Electrolysis, 2nd ed. In Hydrogen and Fuel Cells Primers Series; Pollet, B.G., Ed.; Academic Press: Cambridge, MA, USA, 2018; Volume 2. 12. Felix, C.; Jao, T.-C.; Pasupathi, S.; Linkov, V.; Pollet, B.G. Fabrication of gas diffusion electrodes via electrophoretic deposition for high temperature polymer electrolyte membrane fuel cells. J. Power Sour. 2014, 258, 238–245. [CrossRef] 13. Curnick, O.J.; Mendes, P.M.; Pollet, B.G. Enhanced durability of a Pt/C electrocatalyst derived from Nafion®-stabilised colloidal platinum nanoparticles. Electrochem. Comm. 2010, 12, 1017–1020. [CrossRef] 14. Wee, J.-H.; Lee, K.-Y.; Kim, S.H. Review: Fabrication methods for low-Pt-loading electrocatalysts in proton exchange membrane fuel cell systems. J. Power Sour. 2007, 165, 667–677. [CrossRef] 15. Pollet, B.G. The use of ultrasound for the fabrication of fuel cell materials. Int. J. Hydrog. Energy 2010, 22, 1039–1059. [CrossRef] 16. Pollet, B.G.; Ashokkumar, M. Introduction to Ultrasound, Sonochemistry and Sonoelectrochemistry; Pollet, B.G., Ashokkumar, M., Eds.; SpringerBriefs: Berlin, Germany, 2019; in press. 17. Pollet, B.G. (Ed.) Power Ultrasound in Electrochemistry: From Versatile Laboratory Tool to Engineering Solution; John Wiley & Sons: Hoboken, NJ, USA, 2012. 18. Islam, Md.H.; Burheim, O.S.; Pollet, B.G. Review: Sonochemical and sonoelectrochemical production of hydrogen. Ultrason. Sonochem. 2019, 51, 533–555. [CrossRef][PubMed] 19. Pollet, B.G. A short introduction to Sonoelectrochemistry. Electrochem. Soc. Interface 2018, 27, 41–42. [CrossRef] 20. Bock, C.; Halvorsen, H.; MacDougall, B. Catalyst synthesis techniques. In PEM Fuel Cell Electrocatalysts and Catalyst Layers: Fundamentals and Applications; Zhang, J., Ed.; Springer: London, UK; Guildford, UK, 2008; 1137p. 21. Dabalá, M.; Pollet, B.G.; Zin, V.; Campadello, E.; Mason, T.J. Sonoelectrochemical (20 kHz) production of

Co65Fe35 alloy nanoparticles from Aotani solutions. J. Appl. Electrochem. 2008, 38, 395–402. [CrossRef] Catalysts 2019, 9, 246 18 of 18

22. Zin, V.; Pollet, B.G.; Dabalà, M. Sonoelectrochemical (20 kHz) production of platinum nanoparticles from aqueous solutions. Electrochim. Acta 2009, 54, 7201–7206. [CrossRef] 23. Karousos, D.S.; Desdenakis, K.I.; Sakkas, P.M.; Sourkouni, G.; Pollet, B.G. Argirusis. Sonoelectrochemical one-pot synthesis of Pt–Carbon black nanocomposite PEMFC electrocatalyst. Ultrason. Sonochem. 2017, 35, 591–597. [CrossRef][PubMed] 24. Gedanken, A. Doping nanoparticles into polymers and ceramics using ultrasound radiation. Ultrason. Sonochem. 2007, 14, 418–430. [CrossRef][PubMed] 25. Pollet, B.G. A novel method for preparing PEMFC electrodes by the ultrasonic and sonoelectrochemical techniques. Electrochem. Comm. 2009, 11, 1445–1448. [CrossRef] 26. Mehta, V.; Cooper, J.S. Review and analysis of PEM fuel cell design and manufacturing. J. Power Sour. 2003, 114, 32–53. [CrossRef] 27. Millington, B.; Whipple, V.; Pollet, B.G. A novel method for preparing proton exchange membrane fuel cell electrodes by the ultrasonic-spray technique. J. Power Sour. 2011, 196, 8500–8508. [CrossRef] 28. Pollet, B.G.; Valzer, E.F.; Curnick, O.J. Platinum sonoelectrodeposition on glassy carbon and gas diffusion layer electrodes. Int. J. Hydrog. Energy 2011, 36, 6248–6258. [CrossRef]

© 2019 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).