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Review

Synthesis methods of low-Pt-loading electrocatalysts for proton exchangemembrane fuel cell systems

A. Esmaeilifar a, S. Rowshanzamir a,b,*, M.H. Eikani c, E. Ghazanfari a

a Fuel Cell Laboratory, Green Research Center, Iran University of Science and Technology, Tehran, Iranb School of Chemical Engineering, Iran University of Science and Technology, Narmak, Tehran 16846-13114, IrancDepartment of Chemical Industries, Iranian Research Organization for Science and Technology (IROST), Tehran, Iran

a r t i c l e i n f o

Article history:Received 18 November 2009Received in revised form5 May 2010Accepted 5 June 2010Available online 8 July 2010

Keywords:Pt loadingProton exchange membrane fuel cellElectrocatalyst synthesisCarbon nanotube

a b s t r a c t

While the use of a high level of platinum (Pt) loading in proton exchange membrane fuel cells (PEMFCs)can amplify the trade off towards higher performance and longer lifespan for these PEMFCs, the devel-opment of PEMFC electrocatalysts with low-Pt-loadings and high-Pt-utilization is critical and the limitedsupply and high cost of the Pt used in PEMFC electrocatalysts necessitate a reduction in the Pt level. Inorder tomake such electrocatalysts commercially feasible, cost-effective and innovative, catalyst synthesismethods are needed for Pt loading reduction and performance optimization. Since a Pt-deposited carbonnanotube (CNT) shows higher performance than a commercial Pt-deposited carbon black (CB) withreducing 60% Pt load per electrode area in PEMFCs, use of CNTs in preparing electrocatalysts becomesconsiderable. This paper reviews the literature on the synthesis methods of carbon-supported Ptelectrocatalysts for PEMFC catalyst loading reduction through the improvement of catalyst utilization andactivity. The features of electroless deposition (ED) method, deposition on sonochemically treatedCNTs, polyol process, electrodeposition method, sputter-deposition technique, g-irradiation method,microemulsion method, aerosol assisted deposition (AAD) method, Pechini method, supercriticaldeposition technique, hydrothermal method and colloid method are discussed and characteristics of eachone are considered.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Among the various types of fuel cells, proton exchangemembrane fuel cells (PEMFCs) possess a series of highly advanta-geous features such as a low-operating temperature, sustainedoperation at high-current density, 10- to 100-fold reduction in theplatinum loading in electrode by using nanosize electrocatalystparticles [1], development of catalytic materials for hydrogengeneration [2], membrane electrode assembly manufacturingoptions and synthesis process for many of the membranes and forthe gas diffusion layers [3], progresses in water and thermalmanagement [4], low weight, compactness, potential for low costand volume, long stack life, fast start-ups and suitability fordiscontinuous operation [5,6]. These features have elevatedPEMFCs as the most promising and attractive candidate for a widevariety of power applications ranging from portable and stationarypower supplies to transportation, e.g. city buses [7] and cars [8].

Therefore, fuel cell and automotive companies over the past fewyears have announced several new technologies like catalystdevelopment for fuel cell powered vehicles [9], commercializationof zero-emission vehicles [10,11], or prototype vehicles adoptingPEMFCs [12]. Recently, PEMFCs have begun to move from thedemonstration phase to commercialization due to the impressiveresearch effort in recent years. Nevertheless, several outstandingcost reduction problems and technological challenges remain to besolved [4]. The electrode fabrication cost can be reduced throughseveral approaches such as (i) reducing the platinum (Pt) loadingson both electrocatalysts by improving electrode preparationmethods with higher control for platinum particle deposition, (ii)achieving a more effective membrane electrolyte assembly (MEA),(iii) the search for high temperature tolerant membranes topromote electrocatalysis and hence reduce electrocatalystrequirements and (iv) the search for new electrocatalyst materialsother than platinum [13].

The limited supply and high cost of the Pt used in PEMFCelectrocatalysts necessitate a reduction in the Pt level [14,15]. Inaddition, the U.S. Department of Energy has set long-term goals forPEMFC performance in a 50 kW stack that included operation withcathode loadings of 0.05 mg cm�2 or less [16].

* Corresponding author. Fuel Cell Laboratory, Green Research Center, IranUniversity of Science and Technology, Tehran, Iran. Tel./fax: þ98 21 77491242.

E-mail address: rowshanzamir@iust.ac.ir (S. Rowshanzamir).

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However, recent research has suggested that the lifespan orstability of PEMFCs is the most important factor, irrespective of thePt loading per area. In other words, reducing the Pt loading in theelectrocatalyst is not essential if the high-Pt-loading acts toincrease the fuel cell lifespan, stability and effectiveness.

Nonetheless, the development of electrocatalysts with low-Pt-loadings remains important. The reduction of Pt loadings inelectrocatalysts can be achieved through an enhancement of the Ptutilization by increasing the active Pt sites, thinning the active layerthickness (�25 mm) [17,18] and introducing smaller, carbon-supported, nanometer-sized, Pt particles (<10 nm) [19]. In addition,the electrode represents 40%, including 1.7% of Pt, of the total cost ofPEMFCs [20]. Therefore such steps can substantially lower the MEAcost and reduce the weight and volume of PEMFCs [21]. The size ofPEMFC is more important when PEMFCs are applied to micro-system technology.

Therefore, much of the research on the development of MEA hasbeen devoted to improving the utilization efficiency of Pt electro-catalyst via new structure development [22], improvement of thincatalyst layer deposition on membrane electrolyte [23], reducingthe Pt loadings [24] and study on optimum Nafion content in thecatalyst layers [25].

In this review, we introduce and discuss some of the latestresearches of reducing the Pt loadings while increasing the Ptutilization in PEMFCs. The present review discusses the methods of

synthesizing low-Pt-loading electrocatalyst at both electrodes withemphasis on advantages and disadvantages of each method.

2. Synthesis methods of the Pt-loaded electrocatalysts

2.1. Electroless deposition (ED) method

One promising method of synthesis that offers the potential forimprovement in each of the above shortcomings of electrocatalysisis ED. ED is a catalytic or auto-catalytic process whereby a chemicalreducing agent reduces a metallic salt onto specific sites of a cata-lytic surface which can either be an active substrate or an inertsubstrate seeded with a catalytically active metal [26]. ED has beentypically used as an alternative to electroplating of substrates and isalso referred to as electroless plating. Plating generally involvescreating thin (on the order of several microns) homogeneous metallayers. Electroless plating has many applications in various fieldssuch as electronics, wear and corrosion resistant materials, medicaldevices, and battery technology [27]. Electrolessly depositedplatinum has become commonplace in the plating industry and isused in many situations where conventional electrical deposition isnot precise enough in its uniformity of deposition [28e30]. Indeed,this process was initially referred to as electroless plating [31].However, selective and controlled deposition, as opposed to

Fig. 1. Micrographs of PteRh/XC-72 catalysts. Pt on Rh/C catalysts made at a temperature of 80 BC, pH of 11, DMAB/citrate/PtCl6�2 mole ratio¼ 5:5:1, time of deposition is 60 min.(a) 5.5% Pt on 0.4% Rh/XC-72; (b) 6.8% Pt on 2.2% Rh/XC-72; (c) 6.8% Pt on 4.4% Rh/XC-72; and (d) 20% Pt/XC-72 (E-TEK) (reprinted with permission from [34]).

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continuous plating, hasmany applications in catalysis that have justrecently been explored [32,33].

The preparation of the bimetallic compositions by ED wasconducted by Beard et al. [34] in two steps. Firstly, 2.0 g of carbonsupport (Vulcan XC-72), previously washed in nitric acid to removeimpurities, was seeded with Rh particles by the wet impregnationof the appropriate amount of Rh4(CO)12 dissolved in 100 mL ofdichloromethane.

The excess dichloromethanewas removed by rotary evaporation(50 mm pressure and 40 BC). The Rh-impregnated carbon supportwas reduced under flowing H2 at 100 BC for 1e2 h to reduce anyresidual, oxidized Rh species. Following preparation of the Rh/carbon substrate, ED of Pt on the Rh seed sites was conducted. TheED bath consisted of a reducible platinum salt, chloroplatinic acid,a chemical reducing agent, dimethylamine borane (DMAB), anda stabilizing agent, sodium citrate to help maintain the platinumsalt in the ED bath (prevent thermal reduction of the Pt salt by theDMAB). The chloroplatinic salt, at an initial concentration of0.00014 M, sodium citrate of variable concentration ranging from0.00014 M to 0.00112 M, and de-ionized water were combined;sodium hydroxidewas used to fix the initial pH at the desired value.Lastly, the temperature of the solution was adjusted and main-tained at 80 BC using a hot water bath. At this point, DMAB and theRh-seeded carbon support were added simultaneously undervigorous agitation. The total deposition period was 1 h; however, tomeasure the kinetics of Pt4þ deposition, a syringe with a filter tipwas used to remove small aliquots of liquid from the ED bath atdifferent time intervals during ED. These aliquots were analyzed by

atomic absorption to determine, by subtraction, the amount of Ptthat had been electrolessly deposited on the seeded carbon supportduring that time interval. At the completion of the ED experiment,the final catalyst was separated by filtration; after thoroughwashing with distilled water to remove water-soluble, unreducedPt salts, the sample was vacuum dried and given a final reductionstep in flowing H2 at 100 BC for 1 h.

In some cases, the final catalyst was digested in hot aqua regia toquantitatively remove the Pt and Rh components, and the solutionwas then analyzed by AA to determine the final composition of thecatalyst. Results showed that for similar Pt weight loadings, the Ptparticle diameters decreased with increasing Rh loadings becauseequal amounts of Pt were deposited on greater numbers of Rh seedparticles (Figs. 1 and 2).

The polarization curves for fuel cell electrodes prepared usingsynthesized catalyst, at various temperatures ranging from 50 to80 �C under an operating pressure of 15 psi are given in Fig. 3 [32].As shown in Fig. 3, at current density of 500 mA cm�2, the fabri-cated electrode gives a potential of 680 mV, which is equal to thereported value for commercial 20% Pt on C catalyst (E-TEK).

Due to the inert property of the CNT surface, the surfaceoxidation treatment in HNO3 or HNO3þH2SO4 to obtain the surfacefunctional groups such as hydroxyl, carboxyl and carbonyl isnecessary for effective deposition of Pt nanoparticles on the CNTsurfaces. The created functional groups, which have a strong effecton the formation of the favorable morphology of catalysts, canserve as the anchoring sites for Pt ions. Rajalakshmi et al. [32]confirmed that the Pt/CNT with a smaller Pt particle size andmore uniform particle distribution has higher oxygen reductionreaction (ORR) activity compared to that catalyzed by a non-oxidized CNT-supported Pt catalyst.

2.2. Deposition on sonochemically treated carbon nanotubes (CNTs)

The conventional ED method can produce highly dispersed Ptnanoparticles with small particle sizes. The mean particle size is inthe range of 1e10 nm and the Pt content in Pt/CNT catalyst is below20 wt%. In 2004, a sonochemical process was developed by Xing[35] to treat CNTs in nitric and sulfuric acids to create surfacefunctional groups for metal nanoparticle deposition.

2.2.1. Sonochemical treatment of multiwalled CNTsMWCNTs (10.0 mg, 95% purity) were weighed and placed in

a 25 mL Pyrex glass flask; 9.4 mL of HNO3 (69%), 8.0 mL of H2SO4(96.2%), and 0.6 mL of deionized H2O were added to the flask. Thismakes an acidic solution of 8.0 M HNO3 and 8.0 M H2SO4 witha total volume of about 18.0 mL. The solution, with the as-purchased CNTs in it, was first stirred using a vortexmixer for about1 min, and thenwas put in a laboratory ultrasonic bath for 5 min toallow dispersion of the CNTs. This mixing and dispersion processwas repeated twice to break big CNT aggregates. The flask was thenput in the ultrasonic bath, and the bath temperature was raised to60 �C and remained the same throughout the treatment experi-ment. The treatment process lasted for 2 h. After the surfacetreatment, the CNTs were separated from the acids in a centrifugeat 4500 rpm and washed with 15.0 mL of deionized water fivetimes. The sonochemically treated CNTs were then ready for use inthe deposition of Pt nanoparticles.

2.2.2. Deposition of Pt nanoparticles on MWCNTsDeposition of Pt nanoparticles on MWCNTs was achieved by

reducing the Pt salt precursor, K2PtCl4, in an ethylene glycolewatersolution. In a typical experiment, the sonochemically treated CNTswere put in a 25 mL flask. A 15.0 mL aliquot of ethylene glyco-lewater solution (2:1 volume ratio) was added to the flask. The

Fig. 2. Particle size distribution for Pt on Rh/C (reprinted with permission from [34]).

Fig. 3. Polarization curves of PEMFC performance at various temperatures and pres-sures for the catalyst synthesized by electroless deposition (ED) method (reprintedwith permission from [32]).

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metal precursor is a 0.01 M aqueous solution, and depending on thedesirable nanoparticle loading, an appropriate amount of theprecursor was added to the flask using a pipette. The flask was thenput on a hot plate with magnetic stirring. The reduction reactionswere performed under reflux conditions (ca. 125 �C) for 2 h. TheCNTs with Pt nanoparticles on them were then separated fromthe ethylene glycol solution in the centrifuge and washed withdeionized water for five times.

Two hours was long enough to reduce all Pt precursors on theCNTs, as inductively coupled plasma-mass spectroscopy resultsshowed a negligible amount of Pt left in solutions after thereduction reaction. Thus, a catalyst, i.e., Pt nanoparticles supportedonMWCNTs, with a desirablemetal loading is obtained and is readyfor physical property and electrochemical characterization.

Fig. 4 shows typical transmission electron microscope (TEM)images of the Pt nanoparticles supported on sonochemicallytreatedMWCNTs with three metal loadings, i.e., 10, 20, and 30 wt %.It can be seen from the images that aggregation of the Pt nano-particles is minimal.

The Pt nanoparticles are highly dispersed on the CNTs and theirdispersion is much better than those obtained previously by otherworkers. The nanoparticle sizes can be clearly seen to increase from

the 10wt % catalyst to the 30wt % catalyst. The Pt nanoparticle sizesobtained from image analysis have a relatively narrow size distri-bution, as shown in a typical histogram in Fig. 5 for the 30 wt %catalyst. The mean nanoparticle sizes (diameter) were found to be2.78� 0.86, 3.57� 0.78, and 4.46�1.33 nm for the 10, 20, and 30wt % catalysts, respectively.

Fig. 6 shows the lowest loading catalyst has the highest activity(w0.96). Catalyst activity decreases with the increase of Pt loadingfor the catalysts.

2.3. Polyol process

It is well known that the performance of catalysts can beimproved byachieving nanosizedparticles, uniformdistribution andhigh loading of catalysts over large surface area carbons [22,36,37].Conventional preparation techniques used for the preparation ofsupported catalysts are based on the wet impregnation followed byreduction in a hydrogen atmosphere at high temperatures or thechemical reduction of the metal precursors using reducing agents.However, thesemethods do not provide adequate control of particlesize and distribution. Many studies have shown the difficulty of highmetal loadings without a significant increase in the particle size[38,39]. Extensive investigations have been carried out to developalternate routes for preparing supported Pt catalysts by the colloidalmethod using diverse stabilizing agents. In those attempts, a stabi-lizing agent was used to prevent the aggregation of metal particles

Fig. 4. TEM images of Pt nanoparticles deposited on the sonochemically treated carbon nanotubes (CNTs) with Pt loading of 10, 20, and 30 wt %, respectively, from left to right. Thescale bar in the middle image is applied to all images (reprinted with permission from [35]).

Fig. 5. Size distribution of Pt nanoparticles on CNTs with 30 wt % Pt loading (reprintedwith permission from [35]). Fig. 6. Catalyst activity vs. of Pt loading (reprinted with permission from [35]).

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during the nucleation and growth steps. Boennemann et al.developed organoaluminum-stabilized colloids with a particle sizesmaller than 2 nm at room temperature [40]. Organic stabilizerssuch as polyvinyl pirrolidone (PVP) and the surfactant dodecyldi-methyl (3-sulfo-propyl) ammonium hydroxide (SB12) are widelyused in the preparation of metal colloids [41e43]. The intrinsicproblem underlying this process is that the stabilizing organicmaterial remains on the surface of metal colloids. This should beremoved prior to the application of metal particles for electro-catalysis. Removal of the organic material is important as it hindersthe access of fuel to the catalyst sites. In general, the removal ofstabilizer involves heat treatment. Consequently, due to the sinteringeffect, the phase separation and the distribution of metal particlesare affected, resulting in lowered catalytic performance. In thisrespect, preparation via the polyol process is preferred due to severaladvantages. The polyol process is a technique inwhich a polyol suchas ethylene glycol is used as both solvent and reducing agent. Thepolyol method has been used for the preparation of nanometalpowder [44,45] and nanowires [46,47]. A unique property of thepolyol process is that it does not require any type of polymerstabilizer. In the polyol process using ethylene glycol, metal ions arereduced to form a metal colloid by receiving the electrons from theoxidationof ethylene glycol to glycolic acid. Glycolic acid is present inits deprotonated form as glycolate anion in alkaline solution. It isbelieved that the glycolate anion acts as a stabilizer by adsorbing themetal colloids [48]. Furthermore, removal of these organics on themetal surface by heat treatment below 160 BC has been reported,which is low enough to avoid the deleterious effects associated withheat treatment. However, no information is available on metalloading as a function of solution pH and different gas environment.To prepare Pt/C catalyst by the polyol process a measured amount ofPtCl4 and NaOH were dissolved in 25 ml of ethylene glycol undervigorous stirring for 30 min. NaOH was introduced to adjust pH ofsolution. Since the pH is one of the crucial operating parameters in

the polyol process it was precisely controlled and recorded at everystep. After recording the initial pH of solution, the appropriateamount of carbon black (CB)was added to solution to produce 40wt% of Pt/C. The resulting suspension was stirred for 1 h at roomtemperature followed by heating under reflux at 160 BC for 3 h. Thesolutionwas allowed to cool down to room temperature and left for12 h with continuous stirring. The pH of the solution was measuredagain and accepted as the final pH. The Pt/C particles in the solutionwere then filtrated and thoroughly washed with water. This carbon-supported Pt catalyst was dried in air for 1 h at 160 BC and a mortarwas used to homogeneously grind the Pt/C catalyst material topowder. During each step of experiment, different gases (N2, air andO2) were supplied to create different atmospheres [49].

The mean Pt particle sizes calculated from Scherrer’s formulabased on Pt (111) peak are listed in Table 1. With increasing NaOHconcentration, the diffraction peaks of Pt become broad whichindicates a decrease in particle size. The particle size of Pt wasremained constant after pH of 6 which was also observed from TEManalysis. These results strongly support the role of glycolate anionas a stabilizing agent (Fig. 7)

By the optimization of the gas environment during the reaction,it was possible to obtain high loading of 39.5 wt% with a 2.8 nm sizeof Pt particle. From the results of Fig. 8, it is found that the electrodeprepared at 0.075 M of NaOH with N2 followed by open air shows

Table 1Effect of NaOH concentration on the resulting Pt particles size and pH (reprintedwith permission from [49]).

NaOH(mol L�1)

Initial pH Final pH Particle size (nm)in XRD

Particle size (nm) inHR-TEM image

0.0 0.72 1.45 5.2 8.50.05 11.01 3.82 3.1 3.80.075 11.73 4.33 2.1 3.10.1 11.66 6.03 1.7 2.50.15 11.75 8.50 1.7 2.30.2 11.71 9.39 1.6 2.2

Fig. 7. Dependence of glycolate anion concentration as a function of pH (reprintedwith permission from [49]).

Fig. 8. Comparison of MEA performance between the electrodes prepared at differentNaOH concentrations and gas atmospheres (reprinted with permission from [49]).

Fig. 9. Schematic diagram of the three-step electrochemical synthesis of Pt nano-particles on CNTs (reprinted with permission from [59]).

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much better performance in comparison to the electrode with 0 Mof NaOHwith open air. This is attributed to the difference in particlesize of Pt.

Results of single cell test (as shown in Fig. 8) demonstrate thatoperating in ambient O2 at 70 �C can deliver high performance ofmore than 0.6 V at 1.44 A cm�2.

2.4. Electrodeposition method

The electrodeposition method used for Pt particle attachmentonto the carbon supports has been developed by many researchgroups to improve the Pt utilization and reduce Pt loading [50e55].Taylor et al. explored this method and successfully fabricatedelectrodes with a very low-Pt-loading [54,55]. In their method, thePt was first electrodeposited on the Nafion coated carbon supportsin a plating bath. In this way, the Pt ions could diffuse through thesurface of the Nafion coating into the carbon support surface toform Pt particles. Smaller Pt particles with diameters of less than4e5 nm can be deposited on the carbon surface to form a Pt/Carboncatalyst [56]. It was suggested that the ionic path channels insidethe Nafion coating could serve as the diffusion path for Pt ions. ThePt particle size of 2e3.5 nm at a Pt loading of less than 0.05 mg cm�2

was also obtained. Electrochemical tests showed that a 10-foldincrease in mass activity for ORR was achieved in comparison with

a conventional electrode catalyzed by an E-TEK catalyst. This resultcould be attributed to the increase in Pt utilization. However, thisapproach is strongly limited by the diffusion of Pt ions througha Nafion ionic channel. In order to overcome this limitation, Antoineet al. first impregnated carbon support with H2PtCl6, then coatedthe carbon support with Nafion [53]. In this way, the Pt ions do notneed to diffuse across the recast Nafion layer from the externalaqueous electrolyte to the carbon support surface, because they arealready present on the carbon support surface. A Pt content over 20wt% in the Pt/C catalyst was achieved with a narrow Pt particle sizedistribution, i.e., about 2e4 nm. It was expected that this methodcould increase the Pt content up to 40wt%with the same Pt particlesize distribution. Several groups have reported the synthesis of Pt/CNTcatalysts by this electrodepositionmethod [57e59].Wang et al.successfully employed this method to prepare Pt/MWCNT catalysts

Table 2Pt:Ru atomic ratio and mean particle sizes of the PtRu/C electrocatalysts preparedwith different water/ethylene glycol ratios (20 wt%, Pt:Ru atomic ratio 50:50, doserate of 0.5 kGy h�1 and total dose of 12.2 kGy) (reprintedwith permission from [71]).

Water/ethyleneglycol (v/v)

Color of reaction mediumafter g-irradiation

Pt:Ru atomicratio, EDX

Particlesize (nm)

100/0 Brown e e

90/10 Clear yellow 83:17 3.075/25 Clear yellow 81:19 2.550/50 Clear yellow 85:15 2.825/75 Clear yellow 94:6 2.70/100 Green e e

Fig. 10. TEM micrograph of PtRu/C electrocatalyst prepared with water/ethylene glycolratio of 75/25 (v/v) (reprinted with permission from [71]).

Fig. 11. Polarization curves of four MEAs with microemulsion electrocatalysts (40% Pt/C), H2/O2 at 60 BC and 3 bar (reprinted with permission from [73]).

Fig. 12. Schematic diagram of the reactor used for the synthesis of the Pt nanoparticles(reprinted with permission from [82]).

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[57]. In their work, the MWCNT was directly fabricated on thecarbon paper through a chemical vapor deposition process (CVD).An electrodeposited Co catalyst on the carbon paper was employedfor MWCNT growth. The main function of the Co catalyst was toimprove the electrical contact between the MWCNTs and thecarbon paper. Following the growth ofMWCNTs on carbon paper, Ptwas then electrodeposited on the MWCNTs by an electrochemicalmethod in H2SO4þH2PtCl6 solution. The deposition potential used

was 0 V (vs. SCE) and the loading of Pt could be controlled by thetotally passed charge. Although the deposition process wassuccessful with a Pt loading of 0.2 mg cm�2 on the MWCNT surface,the average diameter of the Pt particle was too large, i.e. w25 nmcompared to the commercially available Pt/C catalyst.

The fuel cell performance catalyzed by a synthesized Pt/MWCNTcatalyst was also lower than that catalyzed by conventional catalystat a comparable Pt loading. Guo et al. employed a three-stepprocess including the electrodeposition method to prepare SWNT-supported Pt catalysts for methanol oxidation [59]. As shown inFig. 9, the first step is to create the surface functional groups. TheMWCNT paste, which was mixed with mineral oil and packed intoa cavity of Teflon tubing was electrochemically activated bypotential cycling at a potential scan rate of 200 mV s�1 fromþ1.8 to�0.4 V (vs. SCE) in 0.5 M Na2SO4 for 10 min. Such an electro-chemical activation treatment could produce various oxidefunctional groups such as carbonyl, carboxylate and hydroxide atthe defect sites such as the tube ends and/or the sidewalls ofSWNTs. The second step is to form surface octahedral Pt(IV)complexes which are used as the precursors. This step was carriedout in a 2.5 mM K2PtCl4þ 0.1 M K2SO4 aqueous solution bypotential cycling from þ0.3 to þ1.3 V (vs. SCE). The lower limit ofthe potential scan was restricted to þ0.3 V in order to prevent theelectrodeposition of Pt(IV). The oxygen atom of the functionalgroups, such as carboxylate, can serve as one of the two axialligands when the planar complex of Pt(II) is oxidized to form theoctahedral complex of Pt(IV). In the final step, the octahedralcomplexes of Pt(IV) on the SWNT surfaces are electrochemicallyreduced to Pt metal through potential cycling from þ1.6 to �2.5 V(vs. SCE) in a 0.1 M H2SO4 solution. The Pt/MWCNT catalysts

Fig. 13. Comparison of the average particle and grain size obtained by analysis bySEM and XRD, respectively, for two different Ar flow rates (reprinted with permissionfrom [82]).

Fig. 14. Top view SEM images of the samples deposited on Si (100) substrates for various Ar flow rates and deposition times. All samples were deposited at 460 �C temperature and0.01 Mprecursor concentration. As theflowrate increased tomore than 1000 sccm the Pt nanoparticles started to agglomerate forming solidfilms (reprintedwithpermission from [82]).

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obtained by this three-step process have a Pt nanoparticle size of4e6 nm. The TEM images for this Pt/SWNT catalyst showed thepartial aggregation of Pt nanoparticles on the thicker bundlesrather than on the thinner bundles or individual tube. Such anaggregation of Pt nanoparticles could be understood based onthe assumption that the number of carboxylic acid groups on thethicker bundles was greater than on the thinner bundles. ThePteSWNT composites show excellent electro-catalytic activity formethanol oxidation and good stability. This method is not limitedto Pt; it may be used to prepare a variety of metal nanoparticles onSWNT surfaces for catalysis applications [59].

2.5. Sputter-deposition technique

A sputtering-deposition method was recently explored toprepare the PEM fuel cell cathode catalysts, aiming at Pt loadingreduction and Pt utilization improvement [18,60]. Mukerjee et al.investigated the oxygen reduction kinetic parameters ona 0.4 mg cm�2 Pt-loaded electrode on top of which a sputter-deposited Pt filmwas formed with a load of 0.05 mg cm�2 [18]. ThePt sputtered electrode showed an active surface that was 2 timeslarger and a current density at 0.9 V (vs. RHE) that was 4 timeslarger than that of Pt unsputtered electrode. Hirano et al. prepareda low-Pt-loading catalyst layer (0.1 mg cm�2) on an uncatalyzed

E-TEK electrode using the sputter-deposition technique [60]. Thethickness of the sputtered catalyst layer could be decreased toabout 1 mm. When compared to a 32 mm E-TEK electrode with a Ptloading of 0.4 mg cm�2, this sputtered electrode showed slightlylower cathode potential and exchange current density at a lowcurrent density region than that of the E-TEK electrode. This couldbe attributed to the lower active surface areas. However, at a high-current density region, the sputtered electrode could give a higherORR potential than that of E-TEK electrode, which could be due tothe low mass transport overpotential. Based on their results, thesputter-deposition technique was proposed as a novel electrodesynthesis method for a low-Pt-loading cathode. More recently, thesputter-deposition technique has been further employed to preparea CNT-supported Pt catalyst by several research groups. In theefforts of Chen’s group [61], the CNTs were first fabricated on thecarbon cloth on which the Pt particles were then sputtered. Theyused a bias-assisted microwave-plasma-enhanced CVD method toprepare CNTs, which is a slightly different process than thatemployed by Sun et al. [62] and Wang et al. [57]. A Fe catalyst anda CH4þH2 gas mixture were used for the direct fabrication of CNTson the carbon cloth at 300 W microwave power and 10 Torr ofworking pressure. In the process, the length of CNTs could beincreased with the increase of negative bias voltage. A bias voltageof �100 V was found to be an optimum condition for Pt sputteringdeposition. The employed sputtering current and time were 10 mAand 30 s, respectively. For comparison, the ED method was alsoused to deposit Pt particles on the same CNTs. TEM results showedthat the sputtering method could generate highly uniformed Ptnanoparticles on CNTs compared to that produced by the EDmethod. The Pt particle sizes deposited by the sputtering methodwere in the range below 2 nm, while those deposited by electrolessmethod produced a particle size range of 2e5 nm on the sameCNTs. Sun et al. [63] tried to deposit Pt nanoparticles on nitrogen-containing CNTs (CNx NTs) for mDMFC application. The CNx NTswere grown on Si substrate through microwave-plasma-enhancedchemical vapor deposition (MPECVD) using CH4, N2 and H2 gases.For Pt deposition, a DC sputtering technique was employed. Somewell-separated Pt nanoparticles were formed with an averagediameter of 2 nm on CNx NTs while a continuous Pt thin film wasobserved on the bare Si substrate. Cyclic voltammogram showedthat Pt/CNx NTs catalyst had electrochemical activity towardsmethanol oxidation. Their results suggest that the sputter-deposi-tion technique is a better way to deposit small and uniform Ptnanoparticles. This method can also generate a thinner catalyst

Fig. 15. Plot of the average grain size calculated by XRD vs. Ar flow rate for fourdifferent deposition times (reprinted with permission from [82]).

Fig. 16. Typical oxygen reduction reaction (ORR) half-cell experiment obtained fromthe Pt nanoparticles vs. a commercial fuel cell electrode (reprinted with permissionfrom [82]).

Fig. 17. X-ray diffraction analysis of the platinum thin film (3 painted layers) onto Tisubstrate. Precursor solution 1:12:96 (Pt:AC:EG) thermally treated at 600 �C. A: Ti,and þ: Pt (reprinted with permission from [91]).

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layer that could give a higher fuel cell cathode performance and, atthe same time, reduce the Pt loading considerably. However, withrespect to the electrode mass production, the sputter-depositiontechnique may face some technical challenges.

2.6. g-irradiation method

Delcourt and co-workers [64] prepared platinum nanoparticlessubmitting a K2PtCl4 salt dissolved in a CO-saturated water/2-propanol solvent to g-irradiation. The reduction of platinum ionsoccurred by a combined effect of CO and radicals produced byradiolysis, leading to the formation of platinum nanoparticles of2e3 nm that were further impregnated on the carbon support.These catalysts were found to be efficient for methanol or hydrogenelectro-oxidation. Recently, Spinacé et al. [65] prepared in a singlestep carbon-supported PtRu nanoparticles submitting water/2-propanol solutions containing Pt(IV) and Ru(III) ions and thecarbon support to g-irradiation. However, the obtained PtRu/Celectrocatalysts showed inferior performance compared tocommercial PtRu/C E-TEK, which is considered as a reference formethanol electro-oxidation [66]. This inferior performance wasattributed principally due to fact that the obtained nanoparticlessurfaces were enriched in Ru atoms and to a bigger nanoparticlessize than the commercial catalyst [65]. In this work, PtRu/C elec-trocatalysts were prepared using different water/ethylene glycolratios (v/v) as a reaction medium and tested for methanol electro-oxidation aiming fuel cell application. Ethylene glycol was used

because, like 2-propanol, it acts as a radical scavenger of hydroxylradicals leading to formation of reactive radicals with reducingproperties [67,68]. Also, it could act as a stabilizing agent preventingthe growth of the nanoparticles [69]. The colloidal noble metalnanoparticles can be prepared by g-irradiation-induced reductionof noble metal ions without the addition of a chemical reductionagent. Also, the g-irradiation can generate the functional groups onthe CNT surface, such as hydroxyl and carboxyl groups. Oh andco-workers dispersed Ag, Pd, and PteRu alloy nanoparticles in poly(vinylpyrrodione) (PVP) and on single-walled carbon nanotubes(SWCNTs) as supporting materials by g-irradiation induced reduc-tion of metal ions at room temperature. The photographs fromAtomic ForceMicroscopy (AFM) inform that Ag, Pd, and PteRu alloynanoparticleswere dispersedwith the particle sizes of about 25 nm,0.6e1.5 nm, and 15 nm, respectively, in the aqueous dispersions[70]. The following procedurewas used by Silva et al. [71] to preparePtRu/C electrocatalysts (20 wt%, Pt:Ru atomic ratio of 50:50).

H2PtCl6$6H2O andRuCl3$1.5H2O asmetal sources, were dissolvedin water/ethylene glycol solutions (v/v). After this, the CarbonVulcan� XC72R, used as support, was dispersed in the solution usingan ultrasonic bath. Argon was bubbled through the resultingmixtures for 15 min and they were submitted to g-irradiation atroom temperature under stirring. After irradiation the mixtureswere filtered and the solids (PtRu/C electrocatalysts) were washedwith water and dried. Table 2 shows Pt:Ru atomic ratio and meanparticle sizes of the prepared PtRu/C electrocatalysts.

Active PtRu/C electrocatalysts were prepared in a single stepusing g-irradiation. Using only water or ethylene glycol as a reac-tion medium the reduction of metal ions was not observed.

For PtRu/C electrocatalysts prepared with water/ethylene glycolratios of 90/10, 75/25, 50/50 and 25/75, the Pt:Ru atomic ratiosobtained were approximately 80:20, showing that not all of the Ru(III) ions were reduced, even if the irradiation time was increased.

The obtained PtRu/C electrocatalysts showed the typical fccstructure of platinum and platinum alloys, mean particle sizes of

Fig. 18. SEM micrographs of Ti/Pt thin film obtained using a precursor solution 1:12:96 (Pt:AC:EG) thermally treated at 600 �C (a) surface image (magnification 3000�) and(b) cross-sectional surface image, the measured thickness is inserted (magnification 2000�) (reprinted with permission from [91]).

Fig. 19. Experimental setup for supercritical deposition (reprinted with permissionfrom [100]).

Table 3Electrochemical and total surface area of the prepared and commercial catalysts(reprinted with permission from [100]).

Catalyst d(nm)

SAPt

(m2 g�1)ESAPt

(m2 g�1)Ptwt%

% Pt utilization(ESA/SA� 100)

Pt/C (E-TEK) 6 46.73 57 10 122Pt/VXR (scCO2) 1.2 233.64 173 9 74Pt/MWCNT (scCO2) 2 140.2 130 9.9 93Pt/BP2000 (scCO2) 1 280.4 102 47.5 36Pt/C (Tanaka) 2 140.2 128 46.5 91

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2.5e3.0 nm and were more active for methanol electro-oxidationthan the commercial PtRu/C electrocatalyst at ambient tempera-ture. TEM micrograph of prepared PtRu/C electrocatalyst is shownin Fig. 10. Further work is necessary to modify the catalyst prepa-ration methodology in order to obtain PtRu/C electrocatalyst withmore ruthenium content.

2.7. Microemulsion method

Themicroemulsionmethodwas used by Boutonnet et al., for thefirst time to prepare monodisperse particles (in the size range3e5 nm) of Pt, Pd, Rh and Ir by reduction of metal salts which aredissolved in the water pools of microemulsions with hydrogen orhydrazine. The method needs the following conditions to begenerally applicable: (1) the solubility of the salts should not be

limited by specific interactions with the solvent or the surfactantand (2) the reducing agent should react only with the salt. Underproperly chosen conditions the particles can be transferred tosupports without agglomeration [72]. It consists of incorporatingmetal salts in the aqueous core of the small aggregates that areformed by surfactant molecules in a non-polar solvent at certainconcentrations. Typically, themixture consists of 20 wt.% surfactant(Berol 050, Azko Nobel) in iso-octane and 5e10 wt.% water. In orderto prepare mixed PteRu metal particles and mixed PtePd metalparticles, aqueous solutions of chloroplatinic acid, rutheniumchloride and palladium chloride have been used in the preparationof the microemulsion solution. The reduction of the metal saltswith hydrazine at room temperature engenders the formation ofmetal and metal oxide particles. The obtained suspension of metalparticles is very stable. Therefore, tetrahydrofuranwas added to thesuspension and an ultrasonic treatment was used to destabilisethe suspension and to get the particles onto the charcoal support.The obtained electrocatalyst was washed several times withethanol in order to eliminate the organic residues and, then, heatedin a gas mixture of 50% N2 and 50% H2 at 300 BC during 2 h.

Escudero et al. [73] prepared electrocatalysts based on Pt, PteRuand PtePd with a low platinum load (0.37e0.50 mg/cm2) usingmicroemulsion method and showed that the MEAs with theelectrocatalysts prepared by microemulsion have a performancecomparable to that of the MEAs with commercial electrocatalysts.The characterization of the synthesized electrocatalysts (40% Pt/C)was performed in a single fuel cell and compared to the commercialones. The fuel cell performance was assessed by measuring thepolarization curves (voltage vs. current e Fig. 11) and internalresistance under different operation conditions. As Fig. 11 shows, atcurrent density of 220 mA cm�2, the achieved maximum potentialfor the MEA is 650 mV.

The satisfactory results obtained show that microemulsion isa promising method for the preparation of electrocatalysts for fuelcells. This method allows for a very narrow size distribution ofmetal particles, with an average size smaller than that of conven-tional electrocatalysts prepared by impregnation [74,75].

2.8. Aerosol assisted deposition (AAD) method

Properties of the nanoparticles differ from those of the bulksince quantum size effects dominate their characteristics. The

Fig. 20. HR-TEM image for Pt/VXR prepared catalyst (reprinted with permission from[100]).

Fig. 21. TEM images of 40% Pt/CNTs prepared by thermal method (reprinted with permission from [107]).

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synthesis of well-controlled shapes and sizes of colloidal particles iscritical for various applications, e.g., the catalytic activity of Ptelectrodes depends on the size of the nanoparticles [76]. AADmethod, as distinct from wet chemistry techniques (electrodepo-sition, solegel and hydrosol fluid, etc.), allows the direct depositionof active species on the substrate without any further treatment.This eliminates steps of impregnation, washing, drying, calcination,and reduction. Most drawbacks of classical synthesis of catalystsdue to the use of liquid solvents are avoided: redistribution of activesites during drying, surface poisoning during impregnation andmigration or agglomeration of particles during sintering in thesteps of preparation involving high temperatures. Moreover, thestructure and features of the substrate, in particular its porosity andits specific area, remain unaltered by the AAD process. Depositionmethods such as metal organic chemical vapor deposition ina fluidized bed and physical vapor deposition methods such assputtering and ebeam evaporation, are known to produce nano-particles at low substrate temperature range, however, they requirethe use of high vacuum and expensive vacuum equipment in orderto control the particle or film purity [77]. AAD process or similarspray pyrolysis techniques have been used before to create oxidefilms and powders [78e80] However, there are only a few reportsof synthesis of platinum nanoparticles using this method [81].

Paschos et al. [82] used a 200 diameter, non-vacuum, customdesigned quartz reactor shown in Fig. 12 to synthesize platinumnanoparticles. The reactor was equipped with an ultrasonic nebu-lizer operating at a frequency of 2.4 MHz and was located at thebottom of the vertical reactor. The nebulizer was employed togenerate precursor mist while the source vaporization temperaturewas kept constant at room temperature. A substrate chuck wasplaced in the reaction zone located in the middle of the quartz tubea couple of inches away from the stage of the nebulizer. Both thereaction zone and substrate chuck were heated by heating tapeswrapped outside the quartz tube. A thermocouple was used tocontinuously monitor the temperature during the deposition

process on the back of the substrate. In order to ensure precursormist delivery to the substrate, ultra-high purity (99.99%) argon (Ar)carrier gas flow was supplied through the lower part of the tube.

The Ar gas flow was controlled by a mass flow controller. In thereactor geometry employed, reaction by-products and excess gaswere removed from the top of the reactor. The design of the processchamber allowed well controlled synthesis of Pt nanoparticles overa 100 �100 substrate. This approach and reactor design allowed precisecontrol on the process stability, reproducibility, and purity of Pt.Chloroplatinic acid hexahydrate (H2PtCl6) was employed as the Ptprecursor because of its chemical simplicity and associated bondingenergy considerations. Its chemical structure is conducive to bonddissociation at low temperature, with recombination being avoidedin the presence of argon to yield Pt nanoparticles. Also, the PteClbond strength is w209.2�13 kJ/mol, as compared to 610.9� 33 kJ/mol and 347.3� 33 kJ/mol for, PteC and PteO, respectively [83],indicating that H2PtCl6 can decompose at significantly lowertemperature thanother Pt containing organic or inorganicmolecules.For all depositions, the precursor was dissolved in distilled water.

By this method, Pt nanoparticles with sizes ranging from 4 nmto 78 nm were synthesized on Si, silicon dioxide coated Sisubstrates and CNTs. The size and density of the nanoparticles werefound to depend strongly on the precursor carrier gas flow rate anddeposition time.

The results of the study showed that the Pt particle size wasincreased as the deposition timewas increased (Figs.13 and 14), butthe same was not observed for the argon flow rate, where theparticle size seemed to have a maximum for a given flow rate anddecreased after that as shown in Fig 15.

The samples showed no presence of impurity elements asanalyzed by XPS, indicating that themethod results in the completedecomposition of the platinum precursor and the production ofhighly crystalline, metallic nanoparticles. The method developedcan deposit uniform coatings of highly oriented, pure Pt nano-particles without the need of any substrate pretreatment such assurface functionalization, deposition of seed layer for electrode-position on insulating or semiconducting substrates, and withoutthe use of expensive vacuum equipment.

Fig. 16 shows a typical ORR obtained from the platinum nano-particles, compared to a commercially available fuel cell electrode,demonstrating their feasibility to be used as a catalyst for fuel cells.

2.9. Pechini method

Noble metal electrodes have wide applicability in electro-chemistry, since they are valuable tools in electroanalytical chem-istry, electrocatalysis, fuel cells, electrosynthesis, and fundamentalstudies on metal surfaces. The important properties of noble metalelectrodes are (1) their inertness at extremely positive potentialsand (2) their ability to chemisorb intermediate species and therebycatalyze electrode process. Platinum is a very expensive material,therefore, it is interesting to manufacture Pt electrodes on a cheap

Fig. 22. Size distribution of Pt nanoparticles on CNTs (reprinted with permission from[107]).

Table 4Average crystallite size of Pt nanoparticles, active specific area, peak current density and specific current density for methanol oxidation reaction of various Pt/MWCNTsnanocomposites (reprinted with permission from [108]).

Pt/MWCNTs nanocomposites d (nm) QH (mC) SEL (cm2) AEL (m2 g�1) PCD (mA cm�2) SCD (mA cm�2)

Pt/MWCNTs-1 3.0 1.28 4.6 95.8 43 0.66Pt/MWCNTs-2 4.2 1.14 4.1 85.4 33 0.57Pt/MWCNTs-3 9.1 0.30 1.1 22.3 4 0.26Pt/MWCNTs-4 6.7 0.58 2.1 43.8 21 0.71

d: the average size of Pt nanoparticles was obtained by XRD analysis; QH: the amount of charges exchanged during the electroadsorption of hydrogen on Pt nanoparticles; SEL:the surface area of Pt nanoparticles obtained electrochemically; AEL: the specific surface area of Pt nanoparticles obtained electrochemically; PCD: peak current density at0.64 V for methanol oxidation reaction which was normalized to the geometric area of electrode; SCD: specific current density at 0.64 V for methanol oxidation reaction interms of the real surface area of electrode.

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substrate as long as they have the same electrochemical andchemical properties observed in the bulk material. In this way,a platinum film deposited onto titanium substrate can be a goodalternative to replace platinum bulk metal electrodes because theformer are cheaper than the latter ones. The method used tomanufacture the platinum film electrodes is the polymericprecursor method (PPM) [84,85] (also called Pechini method).Among the solegel methods [86,87] the PPM has the mainadvantages, the manipulation easiness and the fact that it is notsensitive to the water presence, which makes it simpler than thesolegel method based on metallic alkoxides. Besides there aremany preparation variables that can be controlled to lead todifferent defect densities or even phase segregations in theobtained material. In the PPM, the metallic salts are dissolved ina mixture of (EG) and (CA) giving rise to a polyester networkcontaining the metallic ions homogeneously distributed. Thepolymeric solution is applied onto the support and, generally, themetal oxide thin film is obtained by the calcination at adequatetemperatures. Another advantage of the PPM, which appears also inthe direct decomposition of (NH3)4PtCl2 [88] is the easiness of thesynthesis, the salt solubility at room temperature and the lowdecomposition temperature [89]. Also, it is not necessary to usea reducing agent such as 10% H2 in either N2 or Ar [88,89] or sodiumborohydride [90] to obtain the metallic film. Freitas et al. useda 10� 5� 0.5 mm titanium plate as support to prepare a Pt film onit. The supports were treated by sandblasting followed by a chem-ical treatment in hot oxalic acid 10% (w/w) for 30 min. At the end of

the chemical treatment, the supports were washed with Millie-Qwater and dried at 130 �C. The precursor solution was prepareddissolving citric acid in ethylene glycol at 60 �C. Subsequently,H2PtCl6 was added into this solution at different molar ratios Pt:CA:EG. The precursor solutions were painted with a brush onto thesupport (Ti) and the material was thermally treated at 130 �C for30 min to eliminate water and then at 300 or 600 �C for 10 min toeliminate the organic portion leading to the formation of themetallic film. This procedure was repeated three times.

In the third thermal cycle, a cooling rate of 5 �Cmin�1 was used.All the materials were obtained in static air atmosphere [91]. The Ptloading for each electrode produced is 0.24 mg cm�2. This loadingis lower than the Pt loading for an electrode used in a PEMFC [92]using the electronspray technique (0.5 mg cm�2). The X-raydiffraction pattern of a Ti/Pt thin film prepared using the Pechinimethod is presented in Fig. 17 as titanium peaks were observed athigher intensities rather than Pt ones. The scanning electronmicroscope (SEM) images for the same sample described in Fig. 17are presented in Fig. 18.

From Fig. 18a, a continuous Pt surface on the Ti substrate can beseen. The apparent thickness of the film was measured usinga cross-sectional image as presented in Fig. 18b (6.7 mm). Using thismethod, low cost production of small Pt loading electrodes with thesame electrochemical characteristics of the bulk platinum, withoutthe necessity of a further step related to Pt reduction in the films bya reducing agent can be obtained [91].

2.10. Supercritical deposition technique

Supercritical deposition is an alternative and promising way toprepare electrocatalysts. This process involves the dissolution ofa metallic precursor in a supercritical fluid and the exposure ofa porous support to the solution. After adsorption of the precursoron the support, themetallic precursor is converted to its metal formby chemical or thermal reduction. Using a supercritical fluid (SCF)as the processing medium for synthesis of electrocatalysts hasmany advantages which are directly related to the special proper-ties of the SCFs. Properties of a SCF are different from those ofordinary liquids and gases and are tunable simply by changing thepressure and temperature. In particular, density and viscositychange drastically at conditions close to the critical point. Sincefluid densities can approach or even exceed those of liquids, variousSCFs are good solvents for a wide range of organic and organo-metallic compounds. Compared with conventional liquid solvents,high diffusivities in SCFs combined with their low viscosities resultin enhanced mass transfer characteristics. The low surface tension

Fig. 23. TEM images of SWNHs nanostructure (a) and Pt/SWNH catalyst (b) (reprinted with permission from [109,110]).

Fig. 24. Comparison of the fuel cell performance at room temperature and 1 atm(reprinted with permission from [109]).

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of SCFs permits better penetration and wetting of pores than liquidsolvents do.

Among the SCFs, supercritical carbon dioxide (scCO2), readilyaccessible with a Tc of 31 BC and a Pc of 7.38 MPa, is particularlyattractive since it is abundant, inexpensive, nonflammable,nontoxic, environmentally benign and leaves no residue on thetreated medium [93]. This promising catalyst preparation tech-nique results in small particle sizes and homogeneous dispersions[94e97]. An additional advantage of this technique is the ability tothermodynamically control [98,99] the metal loading. Bayrakçekenet al. [100] used supercritical deposition technique to preparePt-based electrocatalysts for polymer electrolyte membrane fuelcells. In this way, they employed carbon supports, VXR and BP2000and MWCNT (i.d.¼1e3 nm, o.d.¼ 3e10 nm, L¼ 0.1e10 mm, >90%)were impregnated with Pt using the scCO2 deposition technique.

Prior to impregnation all carbon supports were heat treated ina pyrolysis oven at 423 K for 4 h in N2 (99.999%) atmosphere. In thissynthesis, dimethyl (cyclooctadiene) platinum(II) (PtMe2COD)(99.9%) was used as the Pt precursor. A schematic of the super-critical deposition setup is given in Fig. 19. The heat-treated carbonsupport was placed into a pouch made of a filter paper and placedinto the vessel together with a certain amount of PtMe2CODprecursor, and a stirring bar. The vessel was sealed and heated to343 K using a circulating heater/cooler apparatus and then chargedslowly with carbon dioxide (99.998%) to a pressure of 24.2 MPausing a syringe pump. These conditions were maintained fora period of 6 h which was enough for the system to reach equi-librium and then the vessel was depressurized. After allowing thevessel to cool, the pouch was removed and the impregnated carbonsupport was weighed to determine the amount of precursor

Table 6Properties of electrocatalysts synthesized by different methods.

Catalyst synthesis method Mean Pt particlesize (nm)

Pt content(wt%)

Pt utilization(%)

Catalyst performance Reference

ED method 4 19.6 44 676 mV at 500 mA cm�2 (PEMFC test at 80 BC, 1 bar) [32]Deposition on sonochemically treated CNTs 5 30 63 e [35]Polyol process 2.8 39.5 e 600 mV at 1440 mA cm�2 (PEMFC test at 70 BC, 1 bar) [36,49]Electrodeposition method 5.3 e e e [59]Sputter-deposition technique 2 e e e [63]g-irradiation method 2.7 e e e [71]Microemulsion method 4 40 e 650 mV at 220 mA cm�2 (PEMFC test at 60 BC, 3 bar) [73]Aerosol assisted deposition method 4 e e Typical ORR obtained from the synthesized catalyst is

comparable to a commercially available fuel cell electrode.[82]

Pechini method Ti/Pt film witha thickness of6.71 mm

e e e [91]

Supercritical deposition technique 2 10 93 ESA three times larger than that of the commercial E-TEKcatalyst; the performance is substantially higher than thecommercial electrocatalysts.

[100]

Hydrothermal method 3 20 e 640 mV at 660 mA cm�2 (for methanol oxidation) [108]Colloid method 2 30 e 500 mV at 200 mA cm�2 (PEMFC test at 25 BC, 1 bar) [109]

Table 5Summarization of electrocatalyst synthesis methods.

Catalyst synthesis method Advantages Drawbacks References

Electroless deposition (ED)method

Effective for Pt/Ru binary electrocatalyst. High fabrication cost; broad particle sizedistribution (caused by agglomerationwhich results in higher particle sizes).

[32,34]

Deposition on sonochemicallytreated carbon nanotubes (CNT)

Ability to synthesize high loading Pt on CNTs (specially forthe cathode).

e [35]

Polyol process Possibility to obtain high loading of Pt with narrow sizedistribution.

Removal of the organic material by sinteringcauses lowered catalytic performance.

[36,49]

Electrodeposition method Relatively high-Pt-utilization; same narrow Pt particle sizedistribution at high Pt content.

Cl� ions produced in process can poisonPt and reduce the catalytic activity.

[18]

Sputter-deposition technique Good way to deposit small and uniform Pt nanoparticles;also can generate a thin catalyst layer.

May face some technical challenges. [62,63]

g-irradiation method Effective to disperse PteRu alloy particles on carbonsubstrate.

Partial aggregation was observed; Furtherwork is necessary to obtain PteRu/Celectrocatalyst with more ruthenium content.

[71]

Microemulsion method Very narrow size distribution of metal particles, witha small average size.

Removing the surfactant may be problematic. [73e75]

Aerosol assisted depositionmethod

Deposition of uniform coatings of highly oriented, pure Ptnanoparticles without the need of any substratepretreatment. No need to use of expensive vacuumequipment.

There are only a few reports of synthesis ofplatinum nanoparticles using AAD methodand further work is nessecary to optimizethe process.

[82]

Pechini method Easy manipulation; low cost production. Insensitive towater presence.

e [91]

Supercritical depositiontechnique

Uniformly dispersed particles on the carbon supports; thebest performance for electro-oxidation and hydrogenreduction; Strong control on particle size by adjustingoperating condition.

Costly operating conditions. [100]

Hydrothermal method Production of highly dispersed and size-controlled Ptnanoparticles. Process simplicity and low environmental load.

e [107,108]

Colloid method Effective to disperse homogenously small-sized Pt particleon the SWNH.

There are only a few reports of synthesis ofplatinum nanoparticles using colloid method.

[109]

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adsorbed using an analytical balance accurate to �0.1 mg. Subse-quently, the carbon support was placed in an alumina process tubein a tube furnace, and the precursor was reduced thermally underflowing N2 (100 cm3min�1) for 4 h at 473 K. The size of the Ptparticles, calculated surface area (SA) and electrochemical surfacearea (ESA) are given in Table 3.

As given in Table 3, the ESAs of the prepared Pt/VXR and Pt/MWCNT catalysts are about three times larger than that of thecommercial E-TEK catalyst for similar (10 wt% Pt) loading. By usingthis method Pt nanoparticles, about 1e2 nm in diameter, weredispersed uniformly on the carbon supports (Fig. 20). The Pt/VXRcatalyst prepared by supercritical deposition showed the bestperformance for electro-oxidation and hydrogen reduction andthe electrocatalytic activity was substantially higher than thecommercial Pt/VXR. Depending on the carbon support used, theESAs and the Pt utilizations changed, likely due to the differentmicroporous and meso/macroporous structures of the supportsthat affect the accessibility of the electrolyte to the metal.

2.11. Hydrothermal method

Hydrothermal processing can be defined as any heterogeneousreaction in the presence of aqueous solvents or mineralizers underhigh pressure and temperature conditions to dissolve and recrys-tallize (recover) materials that are relatively insoluble underordinary conditions [101]. In recent years noblemetal particles (likeAu, Ag, Pt, etc.), magnetic metals (like Co, Ni and Fe), metal alloys(like FePt, CoPt) and multilayers (like Cu/Co, Co/Pt), etc. haveattracted the attention of researchers owing to their new inter-esting fundamental properties and potential applications asadvanced materials with electronic, magnetic, optical, thermal andcatalytic properties [102e105]. As mentioned before, the intrinsicproperties of noble metal nanoparticles strongly depend upon theirmorphology and structure. The synthesis and study of these metalshave implications for the fundamental study of the crystal growthprocess and shape control. Majority of the nanostructures of thesemetals alloys and multilayers form under far-from-equilibriumconditions [106]. Wang et al. [107] and Chen et al. [108] used thismethod to synthesize Pt nanoparticles on CNTs. For this purpose,CNTs (0.02 g) were dispersed in 200 ml of distilled water bystrongly stirring for 4 h. The suspension for subsequent use wasdesignated as A. H2PtCl6$2H2O (0.0183 g) were put into a 10 mlbeaker after being weighed on an electronic balance. TheH2PtCl6$2H2O was dissolved by adding some water. Some NaOHsolutionwas put into the beaker to ensure the pH being seven afterthe following chemical reaction:

H2PtCl6þ 2H2Oþ 2HCHO/ 2HCOOHþ PtYþ 6HCl

HCOOHþHClþ 2NaOH/HCOONaþNaClþ 2H2O

At the same time, the addition of NaOH can make the reactionbalance transfer to the right. The redox reaction proceeds morecompletely than before. HCHO (4 ml) solution was put into another10 ml beaker. The two beakers with the solutions were taken intoa high-pressure vessel. After that, the sealed high-pressure vesselwas set in a stove at different temperatures. The high pressurevessel was upset to blend the H2PtCl6$2H2O and HCHO solutions atdifferent temperatures for different reaction times. When thetemperature of the high-pressure vessel reached the designatedtemperature with the different reduction reaction times, the high-pressure vessel was taken out at once after the scheduled time wasover. The mixture solution containing Pt nanoparticles was pouredinto suspension A. The whole solution was followed by strongly

stirring for 5 h, filtering and drying. Fig. 21 shows TEM image of theprepared 40% Pt/CNTs catalyst.

The average size of the Pt nanoparticles is less than 1.94 nmwitha homogenous dispersion and narrow distribution (Fig. 22).

ESAs of various Pt/MWCNTs nanocomposites are listed inTable 4. The specific SAs of Pt nanoparticles are 95.3, 85.4 and22.3 m2 g�1 for pHs 13, 12 and 10, respectively. It shows that thereal SA of Pt nanoparticles increases with the increase of synthesissolution pH. This is not surprising in view of the decrease of themetallic particle size with the pH increasing [108].

The electrocatalysts were labeled as Pt/MWCNTs-1, Pt/MWCNTs-2 and Pt/MWCNTs-3 for pHs 13, 12 and 10, respectively.The Pt/MWCNTs nanocomposite prepared from the ethylene glycolsolution of H2PtCl6 with pH 13 was referred to as Pt/MWCNTs-4.

2.12. Colloid method

Yoshitake et al. reported the colloid method for the synthesis ofPt/CNT catalysts in their PEM fuel cell cathode preparation usingsingle-wall carbon nanohorns (SWNHs) as the support [109]. TheSWHN is a new type of CNT with a horn-shaped sheath of single-wall graphene sheets. Fig. 23(a) shows the structure of a SWNH. TheSWNHs were prepared using CO2 laser ablation. The Pt catalyst wassupported on the SWNHs by colloidal method. In the preparation,NaHSO3 and H2O2 were added into a H2PtCl6 solution to form Ptoxide colloids. Then the SWNH powder was added into the Pt oxidecolloid solution where the Pt oxide colloid was adsorbed on theSWNH surface. After eliminating Cl, Na and S ions, the sampleswere dried and reduced by H2 gas.

The produced Pt/SWNH catalyst showed very homogeneousdispersion of Pt nanoparticles with an average size of 2 nm, asshown in Fig. 23(b).

The Pt content in the Pt/SWNH composite was determined to be20e40 wt% from thermogravimetry measurements. The loading ofPt nanoparticle dispersion on the surface of SWNH was less thanhalf of that supported on the conventional CB (Vulcan). The PEMfuel cell catalyzed by Pt/SWNH catalysts showed a 20% higherperformance (500 mV at 200 mA cm�2) than that catalyzed byconventional Pt/C catalysts. Fig. 24 compares the fuel cell perfor-mance of the SWNHs and the CB. The improvement of the fuel cellperformance may be attributed to the homogenously dispersedsmall-sized Pt particle on the SWNH.

3. Conclusions

During the last two decades, there have been many pioneeringworks that focused on the synthesis of Pt/C catalysts for PEM fuelcell catalysis. These works have revealed that the morphology andcatalytic activity of Pt/C catalysts are strongly affected by theirsynthesis methods. For the synthesis of carbon-supported Pt cata-lysts, several catalyst synthesis methods such as ED, deposition onsonochemically treated CNTs, polyol process, electrodeposition,sputter-deposition technique, g-irradiation technique, micro-emulsion, aerosol assisted deposition, Pechini, supercritical depo-sition, hydrothermal, and the colloid method are reviewed in thispaper.

In particular, due to the inert property of the CNT surface, thesurface oxidation treatment in HNO3 or HNO3þH2SO4 to obtain thesurface functional groups such as hydroxyl, carboxyl and carbonylis necessary for effective deposition of Pt nanoparticles on the CNTsurfaces. The created functional groups, which have a strong effecton the formation of the favorable morphology of catalysts, canserve as the anchoring sites for Pt ions. Rajalakshmi et al. [32]confirmed that the Pt/CNT with a smaller Pt particle size andmore uniform particle distribution has higher ORR activity

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compared to that catalyzed by a non-oxidized CNT-supported Ptcatalyst. The sonochemical technique can also provide an effectiveway of allowing for CNT surface functional site increases during theoxidation treatment in acidic solution [35]. This technique couldincrease the Pt content up to 30 wt% in the Pt/CNT catalyst throughincreasing the surface functional groups. By the optimization of thegas environment during the Polyol process, it was possible toobtain high loading of 39.5 wt% with a 2.8 nm size of Pt particle[49]. The PteSWNT composites synthesized by electrodepositionmethod show excellent electro-catalytic activity for methanoloxidation and good stability [59]. This may be attributed to thesmall particle size and high dispersion of platinum and the natureof the SWNT supports. This method is not limited to Pt; it may beused to prepare a variety of metal nanoparticles on SWNT surfacesfor catalysis applications. Using sputter-deposition method, it wasfound that well-separated Pt nanoparticles would form with anaverage diameter of 2 nm on the arrayed CNTs while a continuousPt thin filmwas observed on the bare Si substrate [63]. The averagePt particle size of 2.7 nmwith a narrow particle size distribution of3.2� 0.9 nm was obtained by g-irradiation technique [71]. Thismethod was also employed to create surface functional groups onCNTs without surface oxidation treatment and the addition ofa chemical reduction agent [70]. The microemulsion technique hasbeen applied to prepare new electrocatalysts based on Pt, PteRuand PtePd with low platinum (0.37e0.50 mg/cm2) which arecomparable with the commercial electrocatalysts [73]. Aerosolassisted deposition process has been developed to yield pure singlecrystal Pt particle coatings with size in the range of 4e78 nm. Theparticles showed a catalytic performance for the reduction ofoxygen and therefore the process can be used to coat nano-structures as electrodes for fuel cell applications [82]. Using Pechinimethod, Pt electrodes can be produced with a low cost with thesame electrochemical characteristics of the bulk platinum, withsmall Pt loadings and without the necessity of a further step relatedto Pt reduction in the films by a reducing agent [91].

By using supercritical deposition technique Pt nanoparticles,about 1e2 nm in diameter, were dispersed uniformly on the carbonsupports. Depending on the carbon support used, the ESAs and thePt utilizations changed, likely due to the different microporous andmeso/macroporous structures of the supports that affect theaccessibility of the electrolyte to the metal [100]. Also, Pt nano-particles were deposited onMWCNTs withmean size of 3.0 nm andgood dispersion during a facile hydrothermal method [108]. In thecolloid method, SWNHs were used as a Pt catalyst support inelectrodes of PEM fuel cells. The Pt particles were homogeneouslydispersed on the SWNHs, and their particle size was about 2 nm.Table 5 summarizes the advantages and drawbacks of the reviewedsynthesis methods.

In summary, the literature reported information about mean Ptparticle size, Pt content, Pt utilization and catalyst performance formentioned methods are given in Table 6. As seen, all the methodscan produce small particles (>6 nm). The microemulsion methodand polyol process have the highest Pt content (40 wt%) but thecatalyst performance of the polyol process is considerably higher(600 mV at 1440 mA cm�2) which can be attributed to smallparticle size and uniform dispersion of the Pt particles. Possessinghigh Pt content and small particle size with uniform dispersionsimultaneously in polyol process is very considerable.

ED and hydrothermalmethod have the same Pt content (20wt%)while the catalyst performance for hydrothermal method is almost30% higher (640 mV at 660 mA cm�2).

Supercritical deposition technique possesses the least Pt content(10 wt%) and the highest Pt utilization (93%). Besides, the catalystperformance of this method is substantially higher than thecommercial electrocatalysts.

Despite of the high Pt content (30 wt%) and small particle size(2 nm), the colloid method does not give high catalyst performance(500 mV at 200 mA cm�2).

While a high-Pt-loading gives PEMFCs the advantages of longerlifespan with more stability and effectiveness, the development ofelectrocatalysts with low-Pt-loadings remains fundamentallyimportant because such development will substantially lower theMEA cost and reduces the PEMFC weight and volume. In addition,the U.S. Department of Energy has set long-term goals for PEMFCperformance in a 50 kW stack that included operationwith cathodeloadings of 0.05 mg cm�2 or less [16]. Furthermore, higher Ptloadings do not necessarily mean higher power densities. The low-Pt-loadings and high-Pt-utilization strongly depend on the elec-trocatalyst synthesis technique, substrate loading, and electrodestructure. However, recent research has suggested that the lifespanor stability of PEMFCs is the most important factor, irrespective ofthe Pt loading per area. In other words, reducing the Pt loading inthe electrocatalyst is not essential if the high-Pt-loading acts toincrease the fuel cell lifespan, stability and effectiveness. Therefore,according to the various fields of PEMFC application, it is importantto make an optimization between long lifespan and cost reductionto select the suitable synthesis methods and related factors whichhold the promise of increasing the efficiency.

Also, the lack of information about Pt utilization and catalystperformance, especially for deposition on sonochemically treatedCNT method, electrodeposition method, sputter-deposition tech-nique, g-irradiation method and Pechini method can be used as aninstructor to direct the future research activities.

Acknowledgment

The authors are grateful for the financial support of theRenewable Energy Initiative Council of Iran (REIC).

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