2005_Rao

Coordination Chemistry Reviews 249 (2005) 613631

ReviewChemical and electrochemical depositions of platinum group

metals and their applicationsChepuri R.K. Rao, D.C. Trivedi

Electrochemical Materials Science Division, Central Electrochemical Research Institute, Karaikudi 630006, IndiaReceived 12 July 2004; accepted 12 August 2004

Available online 14 November 2004

Contents1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6142. Basic aspects of chemical and electrochemical depositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 614

2.1. The role of coordination chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6153. Chemical (electroless) depositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 617

3.1. Choice of reducing agents and the mechanism of chemical deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6173.2. Electroless palladium depositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6183.3. Electroless platinum depositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6193.4. Electroless deposition of other PGMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 620

4. Electrochemical deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6204.1. Platinum electrodeposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6214.2. Palladium electrodeposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6224.3. Rhodium electrodeposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6244.4. Electrodeposition of ruthenium, iridium and osmium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6244.5. Analysis of electroplating baths: NMR and other spectroscopic techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 625

5. Platinum group metals in fuel cell technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6265.1. Developments in direct methanol fuel cell electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6265.2. Developments in PEM fuel cell electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6285.3. Precursor concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 628

6. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 629Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 629References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 629

Abstract

This paper reviews the chemical and electrochemical depositions of platinum group metals (PGMs) from aqueous solutions. With a briefintroduction on the fundamental aspects of chemical/electrochemical depositions, the review describes recent advances in chemical and

Abbreviations: CD, cathode current density expressed as amperes per cm2; CE, cathode current efficiency; dc, direct current; DMFC, direct methanol fuelcell; DNS, dinitrosulfatoplatinate; Electroforming, deposition of thick metallic coating which is later removed from cathode substrate, used for preparing thickmetallic foils, halograms, duplicating complicated structures etc.; NHE, normal hydrogen electrode; PCB, printed circuit board; PGM, platinum group metal;PEMFC, polymer electrolytemembrane fuel cell; PET, polyethylene terphthalate; Pt-P-salt, cis-diammineplatinum(II)nitrite; PWB, printedwiring board; Strike,very thin metallic coating that is given prior to the main PGM coating for changing the character of base metal; SCE, saturated calomel electrode Corresponding author. Tel.: +91 4565 227550; fax: +91 4565 227779/227713.E-mail address: trivedi [email protected] (D.C. Trivedi).

0010-8545/$ see front matter 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.ccr.2004.08.015

614 C.R.K. Rao, D.C. Trivedi / Coordination Chemistry Reviews 249 (2005) 613631

electrochemical deposition technologies. It discusses the properties and applications of the deposits. The review also discusses the applicationsand recent progress of PGMs as fuel cell catalysts. 2004 Elsevier B.V. All rights reserved.

Keywords: Platinum group metals; Electroless deposition; Electrodeposition; Fuel cell; Electro catalysts; Corrosion resistance; Stability constant

1. Introduction

The deposition of precious metals either chemically orelectrochemically plays an important role in the develop-ment of technologies where these metals are used. Partic-ularly this is true in the area of electrodeposition as eachmethod with different operating parameters such as temper-ature, pH and current density is likely to produce differentkinds of deposit structures. As the platinum group metals(PGMs) are known to be good catalysts for various chemicaland electrochemical reactions, the production of such cat-alytic surfaces with a range of particle sizes and surface areof prime importance. The reduction of precious metal saltsto the metallic state has become a focus in material sciencedealing with nanoparticles. Metal nano- or micro-particlesserve as useful electrocatalysts in certain chemical reactions.For example nanoparticles of Pt0.5Ru0.5 are effective cata-lysts for the oxidation of methanol in direct methanol fuelcell (DMFC); platinum nanoparticles are useful as catalystsfor oxidizing a variety of molecules such as oxygen, photocatalysts for splittingwater on semi conducting TiO2 surfacesetc.The cathodic electroreduction of oxygen has been a ma-

jor concern in the electrochemical kinetics due to its impor-tance in energy conversion systems such as batteries and fuelcells [1,2]. The requirement for oxygen reduction is that theprocess proceed at low overpotential. Platinum group met-als satisfies this condition and there have been many reportsdealing with the construction of the platinum electrodes indispersed form on conductive supports. Nanoscaled metalparticles have usually been synthesized by impregnation-reduction method; the electrodeposition method is seldomemployed.However, recently the electrodeposition techniquehas gained momentum and there has been a significant num-

Fig. 1. Cell used for electrodeposition.

ber of reports on the syntheses of such nanosized electrocatalysts.There is no review available in the literature on the recent

developments in electrodeposition of all six platinum groupmetals following the first review by Reid on the electrodepo-sition of PGMs in 1963 [3]. Moreover this review discussedonly the electrochemicalmethod of deposition and did not ad-dress chemical depositions. The present review describes thedevelopments in the deposition of all PGMs both by chemicaland electrochemical methods. The review also focuses on thevarious uses of the deposited coatings with special emphasison the functional uses in electrochemical technology of fuelcells.

2. Basic aspects of chemical and electrochemicaldepositions

Electrochemical deposition is a versatile technique bywhich a thin desired metallic coating can be obtained onto the surface of another metal by simple electrolysis of anaqueous solution containing the desired metal ion or its com-plex (Fig. 1). The basic concepts of electrodeposition and theelectrode reactions involved are best described in the recenttutorial papers by Walsh et al. [46]. Electroless depositionis a method of obtaining a desired coating by chemically re-ducing the metal ion or its complex on to the substrate in acontrolled fashion. Table 1 compares the nature of reactionsoccurring in these two processes.The two processes distinctly differ in their reduction ap-

proaches. In the electrochemical method, reduction takesplace by supplying current externally and the sites for theanodic and cathodic reactions are separate. For the chemi-cal deposition method, electrons required for the reductionare supplied by a reducing agent and the anodic/cathodic re-actions are on the inseparable work piece. More over thesereactions proceed only on catalytically active surfaces, i.e.,the newly coated metallic surface should be catalytically ac-tive enough to promote the redox reactions. All PGMs arecatalytically active and can be deposited. Deposits obtainedfromboth chemical and electrochemical processes havemanyapplications. The first and foremost application is resistanceagainst corrosion for the underlying layers. The other ap-plications include wear resistance for the surfaces, decora-tive coating to enhance the aesthetic appeal for the objectsand functional applications such as to offer low resistancefor the electrical contacts, catalytic surfaces for the elec-trodes for chemical reactions. Electrodeposition is the onlytechnique by which metals with high melting points (e.g.,

C.R.K. Rao, D.C. Trivedi / Coordination Chemistry Reviews 249 (2005) 613631 615

Table 1Nature of reactions and their sites occurring in chemical and electrochemical depositionsProperty Electrochemical deposition Chemical deposition1. Driving force External power supply Reducing agent [RA] and auto-catalytic property of the deposited metal2. Cathode reaction Mn+ + ne M Mn+ + RAM3. Anodic reaction M ne Mn+ RA ne [RA]Oxidized form4. Overall reaction Manode Mcathode Mn+ + RAM + [RA]Oxidized form5. Anodic site Anode itself Work piece6. Cathodic site Work piece Work piece

platinum, rhodium) can be deposited. Electrodeposits havefine structure and have valuable physical properties such ashigh hardness, high reflectivity etc. A great advantage of theelectrochemical deposition is that the thickness of the layercan be controlled to a fraction of a micron.Metallic coatings can be classified as anodic or cathodic

according to the nature of the protection they offer. Only zincand cadmium are anodically protecting on iron or iron alloys;on the other hand the platinum group metals cathodicallyprotect such base metals.The reduction mechanism for an electrochemical deposi-

tion of a simple solvated metal salt is shown in Scheme 1 [7]and this mechanism can be extended to other ligand coordi-nated metal systems. Moreover the metal need not necessar-ily be four-coordinated but can be six coordinated also. Thesolvated metal ion present in the electrolyte arrives at thecathode under the influence of the imposed electrical fieldas well as by diffusion and convection (a). At the cathode itenters the diffusion layer. The field strength in the diffusionlayer is not sufficiently strong to liberate the free metal ionfrom the solvated state but the solvated water molecules arealigned by the field (b). Themetal ion then passes through thediffuse part of the double layer. As the field strength of thedouble layer is high (of the order of 107 V/cm), the solvatedwater molecules are removed leaving the free ion (c). Thenthe metal ion is reduced and deposited at the cathode via anad-atom mechanism [7].In electrodeposition baths, supporting electrolytes are

added to increase the conductivity of the solutions. Whena ligand is used as a complexing agent, it is always addedin many fold excess relative to the stoichiometric reactioninvolved in the metal complex formation. Hence for simplic-ity, the concentrations are expressed in grams per liter (g/l).This is primarily because, after formation of the required

Scheme 1. Electroreduction mechanism for solvated metal ion.

electroactive complex species, it should be available contin-uously throughout the deposition process. The presence ofexcess of ligand enables the continuous supply of the elec-tro active species and also ensures the rapid formation of theelectro active complex from the Mn+ ions that are broughtinto the solution by the anodic reaction [M Mn+]. Withfew exceptions, many of these PGM plating baths give brightdeposits and hence no additive is needed. However, in somecases, additives to relieve stresses are needed when higherthickness are plated.

2.1. The role of coordination chemistry

The platinum group metals, namely, Ru, Rh, Pd, Os, Irand Pt are noble in their character and placed at the bottomof the emf series. The emf values (Table 2) show that it isextremely easy to reduce their ions and as a consequence,they try to remain in the metallic state. Hence these elementsare often found in the earths crust as metals or alloys suchas osmoiridium, siserskite.Finely divided metals are obtained when acidic solutions

of salts or complexes are reduced by Mg, Zn, H2 or evenby citrates, oxalic or formic acids [8]. Palladium and Plat-inum are more reactive than other members of the group.The corrosion resistance of these PGMs stems from their no-ble character and when given as coating on other metals, theyoffer cathodic protection to the basis metal. PGMs are rela-tively inert with respect to chemical attack by oxygen or bymany acids. The chemical reactivity is greatly affected bythe size of the particles. Thus while palladium in the formof a sponge is dissolved by all mineral acids, the compactform is attacked only by acids in hot conditions. Rhodiumis attacked by boiling sulfuric acid or hydrobromic acid andnot dissolved by aqua regia. Iridium, ruthenium and osmium


Table 2Physical data for the PGMsProperty/metal Ru Rh Pd Os Ir PtAt. no. 44 45 46 76 77 78At. wt. 101.07 102.9 106.4 190.2 192.2 195.09Ionic radii (A)/(ox. state) 0.82 (+3) 0.81 (+3) 0.78 (+2) 0.78 (+4) 0.82 (+3) 0.74 (+2)Density at 20 C (g/cm3) 12.45 12.41 12.02 22.61 21.65 21.45Hardness (kg/mm2)(i) Metal (annealed) 200350 120140 3740 300500 200240 3742(ii) Electrodeposited (kg/mm2) 9001000 800900 200400 Unknown 900 200400

Crystal structure hcp fcc fcc hcp fcc fccElectrical resistivity (106 ohm cm, 0 C) 6.8 4.33 9.9 8.12 4.7 9.85emf (M/M2+) (vs. NHE) +0.680a +0.758b +0.951 +0.85 +1.156a +1.188a Calculated from thermodynamic data for M/M3+ reaction RuCl3 + 3e Ru + 3Cl.b For the couple M/M3+.

Scheme 2. Synthetic routes to some of PGM metal complexes used in electrodeposition.


in compact form are practically unattacked by hot mineralacids or aqua regia. Platinum is attacked only by aqua regia.The important property of the platinum group metal com-plexes is the formation of intermediate hydride complexeswith MH bond by insertion reactions. Among PGMs, pal-ladium is known to absorb large volumes of molecular hy-drogen [9] followed by platinum, the property which greatlydepends on the physical condition of the metal. Hence thesetwo metals are known to be the best hydrogenation catalysts.However, this property proves to be a disadvantage (knownas hydrogen embrittlement) in electrodeposition processesleading to high internal stress in the deposits. The complexesof these metals are predominantly halide ions, [e.g., PtCl62]ammine or nitrite ligands [e.g. Pt(NH3)42+, Pt(NO2)42] andthese simple coordination complexes, owing to their stability,are good sources of metals for the large number of electro-plating baths. Scheme 2 shows syntheses of some of the PGMcomplexes used in plating baths.The electrodeposition of platinumgroupmetals is not sim-

ply an electrochemical process but it is also associated withthe coordination chemistry of the depositing metal ion. Be-cause of their nobility, the stability of simple salt solutions ispoor and hence they are suitably coordinated by variety of lig-ands. The suitability is determined by the formation constant!n of the complex, expressed by the Eq. (1).

Mz+ + nL MLn, n = [MLn][Mz+][L]n (1)

For example cyanide ion forms a strong complex withsome of the PGMs. If Pd2+ reacts with cyanide, square pla-nar Pd(CN)42 is formed where 4 for the reaction Pd2++ 4CN Pd(CN)42 is about 1051. This value indicatesthe high stability of the complex and the reduction poten-tial of this complex species is more negative than the onsetof hydrogen evolution. Thus on electrolysis only hydrogenis liberated at the cathode. This seems to be true with allthe PGM cyano complexes. For this reason, high tempera-ture electrolysis (molten state) is used if electrodepositionis desired from Pd(CN)42 eliminating complications aris-ing from water as solvent. On the other hand ligands likeammonia form moderately stable tetra-ammine complexesM(NH3)42+ with palladium or platinum ions with 4 1030and 1035, respectively and electroreduction is feasible in wa-ter. Therefore the selection of the ligand should be such thatthe metalligand interaction is not too strong, but just enoughto allow the discharge of metal ion at the cathode. But stillhydrogen liberation and the resulting embrittlement is a topicof major concern when such complex baths are operated atlow pH conditions in PGM electrodeposition.Another important benefit arising from complexation is

as follows. Because of positive values of E.M.F., the solu-tions of PGMs show tendency for displacement plating (im-mersion plating) when metals like copper are immersed forplating, resulting in loose, non-adherent deposits even beforethe current is applied to the electroplating cell. This leads tocontamination of the electroplating bath by copper ions. This

is avoided by shifting the reduction potential of the PGM ionto more negative by complexation. On the other hand, attackon (copper like) cathode materials by the acidic PGM platingsolutions is prevented by applying a very thin coating (calledstrike) of gold or nickel from non-corrosive baths such asgold cyanide or nickel strike bath prior to PGM plating.

3. Chemical (electroless) depositions

3.1. Choice of reducing agents and the mechanism ofchemical deposition

Electroless metal deposition is the term first coined byBrenner and Riddell [10,11] as early as 1946 and is definedas an autocatalytic process of depositing a metal in the ab-sence of an external source of electric current. The depositionis achieved by the incorporation of a reducing agent in thebath. The process is autocatalytic and proceeds on the newlyformed catalytically active surface. Electroless deposition ofmetals has significant practical importance in modern tech-nologies, especially in the production of new materials forapplications in electronics, wear and corrosion resistant ma-terials, medical applications, battery technologies. As willbe noted in the present review, with the exception of plat-inum and palladium, little progress has been made in the de-velopment of electroless processes for other platinum groupmetals.Table 3 gives the properties of various reducing agents

used in electroless plating [12]. The solutions for electrolessdeposition essentially contain hypophosphite, borohydride,alkylamine boranes or hydrazine as reducing agents and thesource of the metal to be deposited. The amine-boranes areaddition compounds of amine and boron hydride of generalformula R3NBH3. While the use of borohydride is limitedto highly alkaline medium, the amine-boranes are used formildly alkaline, neutral or mildly acidic solutions. Metalssuch as silver and copper that are non-catalytic in hypophos-phite are efficiently catalytic to initiate deposition sponta-neously from DMAB baths.Ohno et al. [13] have studied the catalytic activity of some

metals including palladium and platinum for the anodic ox-idation of reductants such as formaldehyde, borohydride,

Table 3Properties of the reducing agentsReducing agent (no. ofelectrons available)

Representative chemicalequation in the text

Redox potential(vs. NHE) (V)

Sodium hypophosphite(2)

5 1.40

Hydrazine (4) 2 (or 3) 1.16Dimethyl amine borane(DMAB) (6)

10 1.20

Diethyl amine borane(DEAB) (6)

10 1.10

Sodium borohydride (8) 8 1.20


hypophosphite, DMAB and hydrazine in electroless baths.Their results can serve as a basis for choosing reducing agentsfor metal deposition.The use of boron and phosphorus based reducing agents

leads to amorphous deposits along with incorporation of el-emental phosphorus or boron. These incorporations in metaldeposits are strictly prohibited in some applications of im-plantable medical devices used in defibrillation, pacing andcardiomyoplasty where a pure metal such as platinum isneeded. In such cases the best option is to use hydrazine asreducing agent wherein deposits are 9799% pure with thebalance consisting of nitrogen and oxygen with other traceelements [14].Hydrazine is a powerful reducing agent that works both in

acid and alkaline media. It can reduce the higher valent metalions to lower valent one or to the zero valent state dependingon the conditions of the reaction [15,16].N2H4 + 4OH N2 + 4H2O+ 4e (2)N2H5+ N2 + 5H+ + 4e (3)Comparing Gibbs free energy "G0 of the reactions 2 and3, [447.7 and 88.7 kJ/mol, respectively] hydrazine is astronger reducing agent in alkaline medium compared toacidic medium. Some hydrazine will also be oxidized to am-monia as shown by the Eq. (4).N2H4 + 2H2O+ 2e 2NH3 + 2OH (4)When hypophosphite is used as reducing agent the follow-ing chemical reaction takes place with a limited utilizationefficiency of 35% (Eq. (5)). Side reactions to give elementalP and molecular hydrogen will also take place (Eqs. (6) and(7))2H2PO2 + 2H2O 2HPO3 + 4H+ + 2Hads + 2e (5)Hads + H2PO2 P+ OH + H2O (6)2H2O+ 2e H2 + 2OH (7)When borohydride and dimethyl (or diethyl)amine boraneare used, the following reactions take place (Eqs. (8)(10))with formation of elemental boron as a side product in theformer case.BH4 + 8OH B(OH)4 + 4H2O+ 8e (8)B(OH)4 + 3e B+ 4OH (9)

R2NH BH3 + 3H2O R2NH2+ + H3BO3 + 5H+ + 6e (10)

The final step in the electroless deposition mechanism is thereduction of free metal cation species or complex species byelectrons facilitated by the reducing agent, to pure metal Mor alloy of phosphorus or boron Mx(P or B)y.

3.2. Electroless palladium depositions

Electroless palladium deposition is the most extensivelystudied subject followed by platinum among the PGMs.Electrolessly deposited palladium has many applications inelectronics as a barrier layer, conductive film, corrosion re-sistance non-porous deposits and for increasing the surfacehardness of the components. Inorganic membranes coveredwith a thin electroless Pd film are used for hydrogena-tion/dehydrogenation reactions. Pure palladiumfilms are use-ful for hydrogen separations. Palladium is largely consumedin electronics as components, in multilayered ceramic capac-itors and smaller amounts in integrated circuits and plating.The next largest application is in the area of catalysis.The general equation for the reduction of palladium is,

Pd2+ + reducer Pd0. The reaction does not have any sig-nificance as Palladium is obtained as a black powder. But ifthe same reaction is carried under controlled fashion in pres-ence of a complexing agent (e.g. ammonia) and a catalyticsurface (Eq. (11)), useful bright and adherent palladium lay-ers are obtained on the surface; such reactions are basis forthe palladium electroless plating processes.

2Pd(NH3)42+ + N2H4 + 4OH 2Pd0 + 8NH3 + N2 + 4H2O (11)Various solutions for electroless deposition of palladium

have been used in which reduction with hydrazine and hy-pophosphite in alkalinemedium is generally practiced. Pearl-stein andWeightman reported the first attempt to electrolesslydeposit palladium with hypophosphite in 1969 [17a]. Rhodareported the development of several electroless palladium so-lutions with hydrazine as a reducing agent [17bd]. Themainsource of the palladium metal is Pd(NH3)4Cl2. The rate ofmetal deposition was found to increase with rise in temper-ature, Pd concentration and hydrazine concentration in thebath.Deposition rates fall appreciably after several hours of use

of plating bath in hydrazine based baths. This leaves muchof the starting material unused. The main reason for this isthe catalytic decomposition of hydrazine aided by freshly de-posited palladium as shown in Eq. (4). In such situations freshhydrazine can be added. There are reports [18,19] where thisproblem is rectified by proper modification of the bath. Thelatter reference [19] not only deals with chemical deposi-tion of Pd on Zr metal, but also highlights the importance ofcomplexing the metal ion in the deposition process. Only im-mersion plating took place when a Zr electrode was dipped inPdCl2HCl solution. The coating is discontinuous and poorlyadherent to the substrate. To avoid this, Pd2+ is complexed byammonia to form the tetraammine complex. When this com-plex is reduced with hypophosphite at 50 C, an adherent Pdcoating of 5"m thick was obtained in 3 h.The mechanism of palladium reduction by hypophosphite

can be described in the following separate anodic and ca-thodic reactions.


Anodic processes:H2PO2 HPO2 + H (12)HPO2 + OH H2PO3 + e (13)H + H H2 (14)H + OH H2O+ e (15)Cathodic processes:Pd2+ + 2e Pd (16)2H2O+ 2e H2 + 2OH (17)H2PO2 + e P+ 2OH (18)For hydrazine, the reducing mechanism is given by

N2H4 + 4OH N2 + 4H2O+ 4e (anodic process)(19)

2Pd(NH3)42+ + 4e 2Pd0 + 8NH3 (cathodic process)(20)

The overall reaction is given by Eq. (11)A stable electroless palladium plating bath containing

EDTA and sodium hypophosphite as reducing agent hasbeen patented by Sergienko [20]. Pearlstein and Weight-man reported the use of hypophosphite [17a] based bathconsisting of palladium-ammine complex and ammoniumchloride as stabilizer. Studies on hadrazine and hypophos-phite baths showed that baths based on hypophosphite asreducing agent performed better in terms of bath stabilityand deposit quality. Thus a practical palladium electrolessbath contains hypophoshite as reducing agent and in generalpalladium tetraammine as metal source. Ammonium chlo-ride is most often used as stabilizer [21,22]. The stabiliza-tion is achieved for the bath by keeping the concentration ofPd(NH3)42+ species at maximum during high working tem-perature by the presence of excess ammonium chloride in thebath.Pd(NH3)42+ Pd(NH3)4x2+ + xNH3 +xNH4Cl

Pd(NH3)42+ (21)EDTAalso serves as stabilizer formany electroless palladiumsolutions where it is believed to form simple square planar(N2O2) based chelatewith Pd(NH3)42+ by ligand substitutionas shown. This needs slightly higher temperature for reduc-tion compared to ammonia based baths, owing to its higherstability.2Pd(NH3)42+ + EDTA4 (NH3)2Pd(OCOCH2)2NCH2CH2N(CH2COO)2Pd((NH3)2

(22)A brief review on electroless deposition of palladium and

platinum is informative [23]. Recently there has been much

progressmade in electroless deposition of palladium reportedin a series of patents [21,24,25]. These literature includespatents by Abys [21] and Hough et al. [24,25]. The patentby Abys et al. described electrolyte systems that are basedon palladium salt with organic acids. Only a narrow class ofreducing agents is used. The process yielded plating rates ofabout 6"in./min. Patents by Hough et al. [21,22] describedelectrolyte systems based on divalent palladium complexedby ammonia or amine with other thio-organic compounds,iminonitriles as stabilizers. Essentially an alloy containing13% amorphous boron is obtained. A strong laminate canbe formed when plated on electroless nickel. Table 4 givesseveral bath formulations and their operating conditions forelectroless deposition of palladium.Another important area where an electroless palladium

process used is for metallising non-metallic substrates forplating. This process makes the non-conducting substrateconducting and further electroless deposition of the desiredmetal is continued. Essentially, metallisation using palladiumchloride involves two steps. The substrate is suitably etchedfor anchoring the tiny deposits. Then the substrate is dippedin SnCl2 solution of particular concentration followed byPdCl2HCl solutions. This leads to chemical reduction ofPd2+ ions to Pd0 [Pd2+ + Sn2+ Pd0 + Sn4+] which an-chors on to the etched surface making it conducting. Litera-ture shows that the palladium particle size ranges frommicroto a nanometer.

3.3. Electroless platinum depositions

Table 4 gives electroless bath formulations used for plat-inum deposition. The first noted patent was given by Oster inearly 1969 [30] based on platinum sulfate and borohydridesystem. A process employing hydrazine (1 g/l) as reducingagent and sodium hexahydroxy platinate (10 g/l) was givenby Rhoda and Vines in late 1969 [31]. The process involvesaddition of hydrazine to the bath either continuously or in por-tions as the hydrate solution or as hydrazine salts dissolvedin water to initiate platinum deposition. The process works atroom temperature with a deposition rate of 12"m/h. Appli-cation of this process was suggested to be on nickel powderand graphite powder compacts and protective coatings forcopper, nickel, iron, titanium and molybdenum.A more efficient electroless plating bath was described

by Leeman et al. in 1972 [32] based on hydrazine as a re-ducing agent and hexachloroplatinic salts as two differentprocesses working one in alkaline and another in acidic pHrange. The acidic process is limited to plate on gold, pre-cious metal alloys, ABS plastics while the alkaline processcan be used for depositions on metals like copper and alsopolypropylene. We have recently shown that platinum canbe deposited from the acidic process on titanium panels. Athickness of 3"m can be achieved in 3 h using solutions con-taining 1 g/l of platinum metal [33]. It was also shown thattitanium powder and PET can be homogeneously platinisedusing this process. The acidic process uses hexachloroplatinic


Table 4Data on electroless plating systems for palladium and platinum metalsMetal source Concentration of metal

salt or complex (g/l)Other main electrolytes in (g/l) Medium Reducing agent (g/l) Temperature

(C)Reference

PdCl2 2.7 (i) Ammonium hydroxide, 390ml, B Formaldehyde, 1.5ml 64 [26](ii) Na2 EDTA, 20

2.0 (i) NH4OH, 150ml B Sodium hypophosphite 60 [27]1.7 (i) Ethylenediamine, 4.8 A Sodium phosphite 60 [28]

Pd(NH3)4(NO3)2 2.5 Ammonium hydroxide, 200mlNa2EDTA, 20

B Hydrazine, 0.8mol 64 [26]

Pt(NH3)2(NO2)2 2.0 Acetic acid, pH 3 A Hydrazine hydrate, 3 5080 [29]A = acidic, B = basic, N = neutral.

acid and hydrochloric acid at 6070 C and the reduction toplatinum metal by hadrazine is given by Eq. (19).H2PtCl6 + N2H4 2HCl 8HCl+ Pt+ N2 (23)In alkaline process (NH4)2PtCl6 and hydrazine are used.

The following chemical reactions take place using a bathconsisting of chloroplatinic acid, ammonium hydroxide andhydrazine at 7075 C.H2PtCl6 + 2NH4OH (NH4)2PtCl6 + 2H2O (24)N2H4 + 4NH3 4NH4+ + 4e + N2 (25)(NH4)2PtCl6 + 4e 2NH4Cl+ Pt+ 4Cl (26)Koslov et al. [29] have patented an autocatalytic plating

process from Pt-DNP and the hydrazine systemwhich is ableto deposit platinum on alloys such as Co-super alloy, Inconel,pure Al, AlTi alloy, graphite; several other applications sug-gested are in batteries, fuel cells and capacitors.The electroless deposition of platinum metal on to poly-

mers is finding applications in the medical field. Platinumbeing biologically inert, is one of the metals used for coatingimplantable electrodes. As the metal deposition takes placeonly on a conducting surface, it is necessary to metallize orseed the non-conducting polymer for electroless deposition.In commercial electroless platinum deposition, a tin sensi-tizer and PdCl2 activator are used to provide catalytic centers.But tin is toxic and therefore is not suitable for use in med-ical implants. Recently there is a new process avoiding us-age of tin for electroless deposition of platinum on PET [34].This involves dipping the etched PET film in a PdCl2DMSOcomplex solution followedby a dip in hydrazine solution.Hy-drazine reduces PdDMSO complex to metallic particles onwhich further deposition of platinum continues

3.4. Electroless deposition of other PGMs

Very few reports are available on the electroless depositionof other platinum group metals. Noted systems for iridiumare based on hydrazine complexes [35,36]. The main speciesinvolved in the process is H[Ir(N2H5Cl)Cl4]. The bath worksat a temperature of 6090 C and pH of about 2. The bath isself reducing in this temperature range and is used for the pro-

duction of iridium coatings on cation exchange membranesused for water electrolysis. It can be used to coat Ir on met-als such as Cu, Fe, Ni and valve metals such as Ti, Ta andNb. Rhodium can be deposited using sodium borohydride(0.11M) as reducing agent from the electrolyte containingRhCl34H2O (0.01M), ethylenediamine (0.8M), dimethyl-glyoxime (0.025M) and sodium hydroxide (1.5M) at 90 C[37]. Recently a rhodium electroless bath based on pyridine-2,6-dicarboxylic acid (pda) was reported by Lothar Weber etal. [38]. The bath contains Rh(III)[pda]2, hydrazine and op-erates at 8090 C. The process is suitable for coatingmetals,ceramics and their powders. Okuno et al. [39] have demon-strated the chemical deposition of ruthenium using hydrazinehydrate in basic pH range 12.613.6 at 5565 C.The processis useful for coating ruthenium on electronic materials.The only report available on the electroless deposition of

osmium is on silicon using osmium tetroxide and a sodiumhypophosphite bath around pH 10 at a temperature of 85 C[40]. The coating is amorphous. While there are no furtherinvestigations to understand the species formed in the bath,it is believed that an anionic complex of Os is formed withsulfamic acid.

4. Electrochemical deposition

Because PGMs are costly, electrodeposition of a PGMmetal is undertaken under special circumstances; when pro-tection is needed against high temperature corrosion or forparticular functional applications like catalysts in chemicaltransformations, low resistance contacts etc.Platinum is extensively plated on titanium for use as an-

odes in the plating of precious metals. Thick platinum layersare required to protect the refractory metals from oxidationat high temperatures when used as anodes. The elements Ru,Rh and Pd are lighter elements with density approximately12 g/cm3 and the other three elements are heavier with den-sity approximately 22 g/cm3. The difference in the densitiesof these two group metals can be exploited by choosing alight metal (e.g. ruthenium, which is also cheap) as coatingmaterial so that it covers nearly the double the area for agiven thickness. The hardness values provide indication of


strength and ductility of the metals (Table 2). Ru and Os arehard and strong with some brittleness. Pd and Pt are soft, lowstrength but ductile. In general electrodeposits are harder thanthe bulk materials because of fine grain structure formed inthe low temperature electrodeposition processes compared tohigh temperature processed cast or wrought metals. Internalstress which leads to cracking of the deposit is a major prob-lem in PGMplating and this is considerably reduced by usingaddition agents.

4.1. Platinum electrodeposition

Electrochemical deposition of platinum is practiced fromchloride, ammine, sulfate-nitrite and hydroxy complexes[41]. The pioneeringwork on electrochemical depositionwasdone by Elkington more than 100 years ago in 1837 [42]. Ingeneral high current density during plating cannot be used asit leads to evolution of hydrogen due good catalytic activityof freshly deposited platinumThe chloride based electrolytes contain platinum com-

plexed in the +4 state (usually H2PtCl6). These electrolytessuffer from a disadvantage that the substrates made up fromcopper like metals are sometimes corroded. In these elec-trolyte systems higher cathode current densities in the range2.53.5A/dm2 can be employed. However, in these systemsthe cathode current efficiencies are found to be poor in therange 1520%. When (NH4)2PtCl6 is used as source for Ptwith sodium citrate-ammonium chloride as supporting elec-trolyte the current efficiencywas found to be high (70%)withlow applied cathode current density. Crack free, crystallinelayers of Pt can be deposited from these baths up to a thick-ness of 20"m. However, these electrolytes are corrosive andhence most base metals require a protective layer of gold,silver or palladium. The electroplating bath works at a tem-perature range 4590 C. The platinum is self-replenished bydissolution of platinum anode as a high concentration of HClis used.Another widely studied system is an electrolyte sys-

tem based on the complex, Pt(NO2)2(NH3)2 by Keitel andZschegher [41]. These electrolyte system allows a maximumcurrent density of 5A/dm2, highest known in platinum elec-trodeposition systems. The preparation of this complex isstraight forward. Addition of excess of sodium or potas-sium nitrite to chloro platinic acid (IV) leads to reductionof Pt (IV) to Pt (II) state to form the square planar complexK2Pt(NO2)4 with evolution of nitric oxides; and additionof stoichiometric amounts of ammonia leads to the pre-cipitation of crystalline cis-Pt(NH3)2(NO2)2 (or Pt-P-salt).The other supporting electrolyte includes ammonium nitrate,sodium nitrate and ammonia. The electrolyte systems suf-fer from a disadvantage that there will be changes in nitriteconcentration which leads to irregularity in plating. The re-plenishment of platinum is achieved by constant addition ofPt-P-salt. Reports showed these systems give good depositsup to 7.5"m. The current efficiency varies from 10 to 40%depending on supporting electrolytes, the maximum being

achieved with sulfuric acid. A high working temperature inthe range 7090 C is needed.A similar electroplating bath is based on [Pt(NO2)2

(SO4)]2 (DiNitroSulfato,DNS) byHopkin andWilson [41].This can be used for plating on many metals such as Ti, Cueven at room temperature. This platinum complex is obtainedfrom Pt(NO2)2(NH3)2 by reacting with H2SO4 stoichiomet-rically (Scheme 2). The supporting electrolyte is 1N sulfuricacid for maintaining pH 2.Apart from chloride electrolytes, electroplating baths

based on Pt(IV) species are alkali salts of platinum hy-droxides and platinum tetrachloridephosphate electrolytes[41]. In the former system, a hydroxide complex of platinumPt(OH)62 is the main electrolyte with sodium or potassiumions as counter ions. The efficiency reported is in the range80100% with working temperature in the range 6590 C.Insoluble anodes of Ni or stainless steel (SS) are used. De-spite the easy preparation of the electrolyte, the disadvantageof low stability also comes into the picture; decompositionof Na2Pt(OH)6 to N2OPtO2 takes place. Another problemis that it absorbs CO2 from air to form carbonates. Improve-ments in stability were also reported by addition of oxalates,sulfates or acetates. A recent report showed that stainlesssteel can also be crack-free plated up to 5"m from theseelectrolytes [43].Recently, significant developments in platinum electrode-

position have been made by JohnsonMatthey Ltd. and others[4447]. Johnson Matthey Ltd. described a new bath formu-lation based on platinumtetraammine complex in phosphatebufferwith higher cathode current efficiency [4446] anduse-ful as commercial electroplating bath where high rates andthickness are required. Pletcher et al. [4852] studied the fun-damental and applied aspects of this new bath. The authorsproposed a mechanism for reduction of this complex. Hightemperatures are needed for the reduction. The mechanismwas shown to be stepwise replacement of ammonia ligands bywater molecules. The high temperature is essential to drivethe slow ligand displacement reaction to a reasonable rate.The following reaction is believed to occur during the elec-trolysis.Pt(NH3)42+ + xH2O Pt(NH3)4x(H2O)x2+ + xNH3 Pt+ (4 x)NH3 + xH2O (27)

A report on a plating system based on Pt(H2O)42+ hasappeared [49,52]. Basic electrochemical studies were per-formed on this complex by synthesizing the complex fromPtCl42 and Pt(NO2)42 complexes. It was shown that it isdifficult to remove the fourth NO2 ligand from Pt(NO2)42and a reasonable deposition can be achieved from the triaqua complex species Pt(H2O)3(NO2)+ with current den-sity range 115mA/cm2. However, the current efficiencywasonly 1015%. A study of the hydrolysis of PtCl4 showed thatPt(H2O)42+ is stable at very low pH in 1M perchloric acidand this formed a basis for a room temperature plating bath[44].


Recently a fundamental study on platinum depositionfrom H3Pt(SO3)2OH solutions obtained from Na6Pt(SO3)4onto a glassy carbon electrode has appeared [47]. The de-posits were characterized by cyclic voltammetry and SEM.A few new patents dealt with the process of platinum electro-plating working on Pt-P-salt [53], [Pt(OH)6]2 [54] and incombination with sulfamic acids [55]. Strangman et al. [53]have patented the bath formulations based on P-salt and alkalimetal carbonates. The operating temperature is in the range1598 Cwith current density 0.057A/dm2. It contained theplatinum salt up to 320 g/l. Kitida et al. [54] have shown thatbrass can be platinum plated up to as thick as 150"m usingformulations based on H2Pt(OH)6. Kuhn et al. [55] obtaineda patent on an electroplating bath that contains a Pt-ammine-sulfamate complex that gives crack-free, bright platinum lay-ers up to 100"m. Some of these systems [54,55] can be usedfor electroforming applications. For example work by Kuhnet al. showed that 120"m thick platinum foil can be obtainedby electroforming.Platinumiridium alloy coatings from amidosulfuric acid

solutions have been electrodeposited on nickel-base singlecrystal superalloy TMS-75 [third generation super alloy con-taining Al (6wt.%), Co (12wt.%), Cr (3wt.%), Ta (6wt.%),Mo (2wt.%),W (6wt.%) and Re (5wt.%) and rest Nickel] byelectrodeposition [56]. The effects of electrolyte temperature,current density and mole concentration ratios of [Ir3+]/[Ir3+]+ [PtCl62] on the deposition rate, composition and crystal-lographic structures of PtIr alloy coatings were investigated.With increasing electrolyte temperature, the deposition rateand Ir content increases, whereas the grain size of PtIr alloycoatings decreases. Smooth and dense PtIr alloy coatingscan be obtained at 1A/dm2 and 353K. PtIr alloy coat-ings with expected compositions can be readily fabricatedby controlling the mole concentration ratios of [Ir3+]/[Ir3+]+ [PtCl62] in the electrolyte. A detailed investigation ofthe structure and morphology of electrodeposited PtIr al-loy coatings is also presented in this report. XRD analysisrevealed that all the coated PtIr alloys have a single phasewith fcc structure, and the lattice parameters of the coatingsdecrease linearly with increasing Ir content. The coatingsare useful in increasing the performance of Pt-modified alu-minide coatings. The electrochemical reductions while de-positing the alloy are as follows:PtCl62(aq)+ 4e Pt (s)+ 6Cl (aq), E0 = 0.744V

(28)

Ir3+(aq)+ 3e Ir(s), E0 = 1.156V (29)Comparing the above reduction potentials, iridium can bepreferentially reduced to platinum and is indeed observedin this investigation. The process is categorized as normalcodeposition.Farndon et al. have studied the electrodeposition of

platinum onto a conducting ceramic, Ebonex from Pt 5Qelectroplating solution at 368K [57]. Ebonex is a ceramic

consisting of a mixture of Magneli phase titanium oxides,largely Ti5O9 but with some Ti4O7 and Ti6O11, with a typi-cal conductivity of 103#1 cm1. It is very stable in a widerange of environments including acidic fluoride solutions andit does not react with hydrogen.A fundamental electrochemical study on platinum depo-

sition on HOPG (highly oriented pyrolytic graphite) andtungsten was under taken by Glaoguen et al. [58] and Ke-laidopoulou et al. [59]. Dimiter Stoychev et al. [60] studiedfundamental aspects for platinum electrodeposition on sev-eral materials including tungsten, titanium, rhenium, zirco-nium, stainless steel, glassy carbon (GC), polyaniline, andpoly(2-hydroxy-3-aminophenazine). Efforts were made toestablish experimental conditions under which the very ini-tial, nucleation stage of the platinum electrodeposition couldbe studied. It was found that tungsten, titanium, and GCweresuitable substrates for nucleation and growth studies in aque-ous 0.1M HClO4 solution containing K2PtCl6.The other very useful area is platinizing the semiconductor

silicon surfaces which are useful as solar cells. Photovoltaicconversion using solar cells is a most promising method forthe utilization of solar energy. Photoelctrochemical (PEC)solar cells use n-type silicon electrode materials modifiedwith ultra fine platinum particles. Garrido et al. [61] and Yaeet al. [62] have shown that it is possible to deposit dispersedplatinum coatings on silicon wafers of varying particle sizesfrom hexachloroplatinic acid electrolyte.Platinum is the best choice among the PGMs to be used

in many biosensors for its biocompatibility. De Haro et al.[63] studied electrochemical platinum coatings for improv-ing performance of implantable microelectrode arrays. Theformation and properties of electrodeposited platinum coat-ings grown on contacts contained in implantable flexible mi-croelectrodes were investigated. The platinum deposits wereobtained by the cyclic voltammetry method from baths con-taining chloroplatinic acid. The benefits of this process areascribed to higher corrosion resistance, lower impedance andimproved adhesion to the deposited platinum. These im-provements can make the application of this electrochem-ical technique highly useful for increasing the lifetime ofimplantable microelectrode arrays, such as cuff structures.These medical devices, obtained by semiconductor technol-ogy could be used for selective stimulation of nerve fascicles.Table 5 lists electroplating systems for platinum metal.

4.2. Palladium electrodeposition

There is a vast amount of literature available on the sub-ject (Table 6). The electrodeposition and material propertiesof palladium are greatly affected by deposition conditionslike temperature and pH as these two parameters decide theamount of co-deposited hydrogen.Palladium coatings have technological importance. Palla-

dium exhibits many desirable characteristics like excellenttarnishing, wear and corrosion resistance with low electri-cal contact resistance and hence has found applications in


Table 5Data on electroplating systems for platinum metalPlatinum source Concentration as

Pt metal (g/l)Other main electrolytein (g/l)

Medium Current density(A/dm2)

Temperature(C)

Reference

H2PtCl6 525 HCl, 180300ml A 2.53.5 4590 [41](NH4)2PtCl6 6 Sodium citrate, 100 N 0.51.0 8090 [41]PtCl4 3 Na2HPO4, 100 B 0.31.0 7090 [41]K2Pt(OH)6 10 K2SO4, 40 B 0.31.0 7090 [43]Pt(NH3)2(NO2) 510 Ammonia, 50 B 0.32.0 9095 [41]H2Pt(NO2)2(SO4) 5.7 H2SO4, to pH 2 A 2.5 3070 [41]A = acidic, B = basic, N = neutral.

P.W.B fingers. Palladium coatings with thin gold flash areused as contact materials. Pd can be readily soldered usingconventional soft solders and hence is good candidate in theelectrical industry and PCBs. It is also used in semiconductorpackaging due to their wire bonding and solderability prop-erties.Basic electrochemical studies on palladium dissolu-

tion and deposition were conducted by Harrison et al.[64,65]. Palladium is deposited frommany bath compositionsbased on simple salt PdCl2 and also from complexes suchas Pd(NH3)2(NO2)2, Pd(NH3)2Br2, H2PdCl4, Pd(NO3)2,Na2Pd(NO3)4, Pd(NH3)4(NO3)2, Pd(NH3)2(NO3)2, Pd-(NH3)4Cl2. Ammonia is themost suitable complexing ligandfor palladium electrodeposition and most of the available lit-erature is on ammonia complexed to palladium solely or incombination with other ligands (Table 6). Palladium read-ily forms square planar complexes with ammonia or amineswhich show intermediate degree of stability as evident from4 (Section 2.1). Pd2+ forms an anionic complex in the pres-ence of HCl withmoderate stability (4 =1011) and is suscep-tible to easy electrochemical reduction. This is the basis forpalladiumdepositionbasedonPdCl2-HClbaths.The stabilityconstants and half-cell potentials for anionic halo complexesare given below [66].

Pd2+(aq)+ 4X PdX42(aq), X = Cl (4 = 1011),Br (4 = 1015), I (4 = 1025) (30)

Table 6Data on electroplating systems for palladium metalPalladium source Concentration as

Pd metal (g/l)Other main electrolytein (g/l)

Medium Current density(A/dm2)

Temperature(C)

Reference

PdCl2 140 Potassium phosphate, 50100 B 0.150 4070 [67,70]13 Sodium chloride, 1060 A 0.55.0 2545 [75]

H2PdCl4 5 Sodium chloride, 40 A 0.41.0 50 [76]Pd(NH3)4Cl2 20 Ammonium sulfate, 20 NB 0.51.5 2040 [77,78]

525 Ammonium chloride, 90 NB >2.5 3050 [79]Pd(NH3)2(NO3)2 4 Sodium nitrate, 1011 NB 0.14.0 4055 [76]Pd(NH3)4(NO3)2 10 Ammonium nitrate, 90100 B 0.52.0 6080 [76]Pd(NO3)2 215 Sulfuric acid, 98 A 0.58.0 2035 [76]A = acidic, B = basic and N = neutral.

PdX42 (aq)+ 2e Pd (aq)+ 4X (aq),X = Cl (E = 0.62V), Br(E = 0.49V),I(E = 0.18V) (31)In the reduction mechanism of PdCl4, it is believed that

the complex dissociates sequentially to form normal PdCl2and then reduction occurs by step wise one electron transfers.PdCl42 PdCl3 PdCl2 (32)PdCl2 + e PdCl2 (slow and rate determining step)(33)PdCl2 + e Pd+ 2Cl (fast) (34)In Pd-ammonia baths, the high concentrations of ammonia

pose problems while in operation such as evaporation and pHchanges. For these reasons other non-volatile amine basedsystems are preferred. The most practical and thoroughlystudied system is propylenediamine based [67,68] Pd(pn)Cl2by Abys et al. A good reference book for palladium deposi-tion is given by him [68]. The deposits from this bath aresmooth and brightness can be controlled by operating condi-tions like temperature, pH and current density and are foundsuitable for connector applications. Lai et al. [69] have stud-ied the basic aspects of palladium deposition from Pd(en)Cl2baths by pulse electrodeposition method. It is believed thathydrogen adsorption is lowered in this technique and betterquality deposits are obtained compared to normal dc elec-trodeposition. Pd-tetraammine based electroplating systems


were studied [70]. These baths work at near neutral pH condi-tions. The electroreduction of these tetraammine complexesis easier compared to PdCl2HCl due to complexation byammonia. Electrodeposits from Pd(NH3)4Cl2 are dull andgray and a brightener additive is necessary. Class I or class IIbrightener can be used in the above baths. Class I brighten-ers are generally unsaturated sulfonic compounds where theunsaturation is # or ! with respect to the sulfonic group withgeneral formula ASO3B. A is an aryl or alkene group andB may be OH, OR, OM, NH2, NH, H, R (where R is alkalimetal and R is an alkyl group with less than six carbons) [68].The class II organic brighteners are unsaturated or carbonylcompounds. Examples are C O, C C , C N, C N ,C C , N N . These additives are added in small quan-tities. The additives are deposited along with the metal togive specular reflections to the surface and are seen as brightdeposits.Pletcher et al. have studied the electrodeposition behavior

of Pd(NH3)X2 type systems [71,72]. While depositing pal-ladium from these baths, competitive reductions take placebetween palladium ions and other electroactive species.Pd(NH3)42+ + 2e Pd+ 4NH3 (35)2H2O+ Pd+ 2e Pd(H2)+ 2OH (36)2H2O+ 2e H2 + 2OH (37)Hence experimental parametersmust be carefully selected

to minimize these undesired reactions. It was also shown thata complexing agent is necessary to stabilize Pd2+.At the sametime the presence of strong complexing ligandwould shift thePd2+/Pd couple to negative regions where PdH is formed.Further, the study indicated Pd2+ existed as Pd(NH3)42+ butnot as Pd(NH3)22+.The first and simplest Pd plating bath that operates in the

acidic pH range is PdCl2HCl system where H2PdCl4 com-plex is readily formed. The stability constant 4 is only 1012hence some immersion plating may take place. High concen-trations of palladium chloride are used; the rate of deposi-tion is high with low internal stress. It is suitable for plat-ing high thickness coatings and also for electroforming. Thesystem was thoroughly investigated by Raub in 1977 [73].Another plating system that works in the acidic pH range isa Pd(NO3)2-sulfuric acid bath [74]. This report [74] showsthat addition of sulfite is necessary to bring down the concen-tration of free palladium to get satisfactory deposits. Sulfideincorporation is also found in the deposits due to reductionof sulfites.

4.3. Rhodium electrodeposition

The most stable oxidation state of Rh is +3 which has agreat tendency to form complex ions in aqueous solutions. Rhis used as coating material on scientific, surgical instruments,contact materials in rf circuits. The high reflectivity makesRh for use in instrument mirrors.

Rhodium is usually electroplated from sulfate, phosphateor sulfatephosphate baths. Reports are also available onsulfamate, perchlorate, fluoborate and nitrate electrolyte sys-tems [80].Rhodiummetal complexbathswere reported basedon citric, tartaric, lactic, boric acids, alkaline phosphate andaminonitrates [80]. In rhodium sulfate solutions the speciespresent are mostly Rh(H2O)63+ and some times species likeRh(SO4)33 are also observed. The difficulty in prepara-tion of water soluble salts of rhodium resulted in preparationof concentrated rhodium sulfate solutions containing even100 g/l of rhodium metal. These concentrates upon additionof required amounts of water and sulfuric acid give platablesolutions. Briefly the preparation of this concentrate involvesdissolving rhodium black (obtained by reducing rhodiumchloride) in sulfuric acid. Rhodium is precipitated as rhodiumhydroxide with ammonium hydroxide. After thorough wash-ing, it is again re-dissolved in sulfuric acid and stored asconcentrate. Rhodium is plated using insoluble platinum an-odes.For decorative plating of rhodium, sulfate or sulfate

phosphate [80] containing Rh as sulfate 2 g/l, 2030ml/l ofsulfuric acidwith current density of 14A/dm2 at low temper-atures of 3040 C are used. For heavy deposits Rh concen-tration is increased to 10 g/l with 50ml/l of sulfuric acid. Thedeposits are usually cracked and current efficiency is about75%. In sulfatephosphate baths the current efficiency is lowat room temperatures and increases to 70% at 70 C where amarked decrease in cracking is observed. It was shown thatcracking can be controlled by the addition of selenic acid[81]. The deposits are softer than with the sulfate bath. Fora thick plating of Rh, the sulfate bath has advantages thatderive from higher current efficiency, low internal stress andhigh hardness. Replenishment is done by manual addition ofrhodium sulfate to the bath.Recent work [82,83] by Pletcher et al. on Rh deposition

from chlorides and sulfates in acidic media threw light onthe fundamental chemistry and electrochemistry of electrol-ysis. The studies showed that strong acid medium, as com-mercially practiced now, which leads to decrease in currentefficiency, is not necessary. It is possible to achieve 100%current efficiency keeping the pH in the range 24 duringelectrodeposition.

4.4. Electrodeposition of ruthenium, iridium andosmium

Ruthenium is the least expensive PGM metal and is aneconomical alternative to both rhodium and gold in contactfinish applications. Electrical cathodes may be protected byusing Ru coating as they offer excellent arc resistance. Theapplications of iridium coatings include for inert electrodes,sliding contacts, reflectors, mirrors, vacuum tube elements,lab wares. Electrodeposits from osmium are hard and wearresistant and its high melting point suggests usage in reedswitch contacts as an arc-resistant coating alternative to lowermelting rhodium coatings. The work function of osmium is


high and hence it is used in few thermoionic devices. It is usedas an OsIr alloy known as osmiridium for the hard wearingtip of gold fountain pen nibs. Osmiridium occurs in smallamounts as native metal with a composition of Os 27%, Ir55%, Pt 10% (and rest) Ru and Rh.The most useful starting material for iridium deposition is

IrBr3. Good deposits can be obtained from these electrolytes[84] at a current density 0.10.2A/dm2, pH around 1.8 withcurrent efficiency of 3065%.This bath enables deposition oncopper, titanium, brass, nickel and mild steel up to thicknessof 10"m at the rate of nearly 1"m/h.The IrCl3/sulfamic acid system [85] works at low and

higher temperatures with current efficiency range 663% atcurrent density of 0.1A/dm2. Recent literature on the elec-trodeposition of iridium includes fundamental and appliedareas [85]. This study [85] focused on the electrochemistryof iridium deposited from (NH3)4IrCl6 on titanium. The effi-ciency of electrodeposition process was enhanced at elevatedtemperatures and a good coating was achieved at 343K.An early ruthenium plating bath used ruthenium as

rutheniumnitrosylsulfamate in sulfamic acid at 70 C withcurrent density of 4A/dm2. A poor current efficiency of1020% was observed and the formation of rutheniumtetraoxide at the anode was a major problem. The intro-duction and use of electrolytes based on bridged dinuclearcomplexes [K or NH4]3[Ru2NCl8(H2O)2] by Reddy et al. in1969 [86] was a major achievement. An aqueous solution ofRuCl3 which was previously activated in concentrated hy-drochloric acid reacted with sulfamic acid for long periods atreflux and resulted in a red crystalline product. The structurewas shown to be two Ru(IV)Cl4 units bridged by nitrogenatoms as shown in Scheme 2. This bath produced smoothand bright deposits with a fairly good current efficiency ofabove 90% for a cathode current density of approximately1A/dm2. These coatings are crack free below 2"m.The literature on the electrodeposition of osmium is

scant. An article by Jones [87] gives information on someelectrodeposition processes. A bath based on hexachloroos-mate was proposed by Notley [87]. This contains chloroos-mate, works at a temperature of 70 C, low pH and has arather low current efficiency of about 30%. The deposits arecrack-free up to only 1.5"m. The plating rate was foundto be 4"m/h. Other processes cited in the literature areby Greenspan in 1972 from OsO4 [88] and Crossby in1976 from a nitrosyl complex derived from K2OsCl6 [89].A study by Greenspan showed that coherent deposits of

14"m are formed from electrolytes obtained from reac-tion of sulfamic acid with osmium tetraoxide. The reaction isbelieved to form an anionic complex. The operating temper-ature is 6575 C with a current density range 0.21A/dm2and a cathode efficiency of 4080%. Crossby studied thedeposition of osmium from osmium(II)nitrosyl complexK2[Os(NO)(OH)(NO2)4] [89]. The electrolyte gave brightand adherent deposits. However, the current efficiency waspoor, in the range 812% with a plating rate of 23"m/h.Optimum plating performance was obtained from an alka-line solution fromwhichmetal can be plated directly onto thebase metal without an undercoat. It was noticed that thoughthe starting material is a nitrosyl complex, its identity waslost during plating. Table 7 lists electroplating systems forthese metals.

4.5. Analysis of electroplating baths: NMR and otherspectroscopic techniques

Nuclear magnetic resonance (NMR) is a most versatileand promising spectroscopic method for the elucidation ofstructure and concentrations of coordination species presentin the electroplating solutions. It also allows us to study thecourse of complex forming reactions in such baths. NMRspectroscopy is reasonably sensitive in that a number of iso-topes are measurable in concentrations of about 1% or less.The sensitivity varies with the magnetic properties of the iso-tope and under some conditions it is limited by isotope envi-ronments. The technique is not suitable for trace analysis ofppm ranged species. Precision and accuracy of NMR mea-surements are governed by environment factors. NMR utilityis limited by the natural abundance and resonance character-istics of the observable isotope. In general only those isotopeswhich contain an odd number of protons in their nuclei areNMR active (with an exception of Be). Isotopes with evennumber of protons and neutrons do not have a magnetic mo-ment and are NMR inactive. Table 8 lists magnetically activePGM nuclei with their relative abundances.An article by Pletcher et al. [91] discussed the application

of 195Pt NMR in the analysis of the platinum plating bath. Insome of his papers [48,50,52,83] 195Pt and 103Rh NMR areused as a probe for the characterization of the different speciespresent in the electrolyte. When K2Pt(NO2)4 is dissolved inmethane sulfonic acid, it was shown by the this technique thatnitrite ligands are sequentially displaced by water moleculesto form Pt[(NO2)(H2O)3]+ while the final water molecule is

Table 7Data on Electroplating systems for Ru, Ir, Os and Rh metalsSalt/complex Conc. as metal (g/l) Other electrolytes (g/l) Medium Current density (A/dm2) Temperature (C) ReferenceK2[Ru2O(H2PO4)] 610 Phosphoric acid, 2mol A [90](NH4)2 IrCl6 68 Sulfuric acid, 0.8mol A 0.1 1825 [85]IrCl3 515 Sulfamic acid, 2550 A 0.10.2 5080 [85]K2OsCl6 10 Potassium bisulfate, 60 A 14 70 [89]Rh as sulfate concentrate 2 Sulfuric acid, 20ml A 4 4045 [82,83]A = acidic.


Table 8Data on magnetically active nuclei of PGMsIsotope Natural abundance

(%)Magnetic moment(N) (nuclearmagnetrons)

Spin (I)(h/2pi units)

99Ru 12.81 0.630 5/2101Ru 16.98 0.690 5/2103Rh 100.0 0.087 1/2105Pd 22.23 0.570 5/2189Os 16.1 0.650 3/2191Ir 38.5 0.160 3/2193Ir 61.5 0.170 3/2195Pt 33.7 0.600 1/2

Table 9Approximate electronic absorption bands of some PGM complexesComplex Absorption maximum (cm1)RuF63 10,000; 20,000RuCl63 16,500Ru(H2O)63+ 16,670; 25,510; 30,000Ru(NH3)63+ 23,000; 30,000RhF62 11,600; 16,000; 19,200; 21,200RhCl63 14,700; 19,300; 24,300RhBr63 18,100; 22,200Rh(en)3+ 33,200; 39,600Rh(NH3)63+ 32,800; 39,200Pd(NH3)42+ 1188PdCl44 18,800PdCl46 20,800Pd(NH3)2Cl2 14,640; 15,440IrCl62 17,400IrCl63 16,300; 17,900; 24,100; 28,100IrBrl63 16,800; 22,400; 25,800Ir(NH3)63+ 31,800; 39,800; 46,800t-Ir(en)Cl2+ 24,000; 29,200; 36,000

difficult to displace.While there are no other reports availableon the use of 103Rh NMR for the analysis of electroplatingbaths, basic work by Mann [92,93] significantly contributesto the understanding of solution speciation in electroplatingbaths.Another useful tool for the analysis is UVvis spec-

troscopy as many of the PGM complexes have measurabledd absorptions. The dd bands are specific to each com-plex species. This technique is also useful to estimate theamount of organic additive in the bath. Table 9 give absorp-tion maxima (max) of some of the metal complexes that areencountered in the electrodeposition of the PGMs. The maxvalues listed [94] are measured in different solvents and havedifferent values. These values will be found red or blueshifted for the complexes in the plating solutions because ofinterference with other ionic species. Thus standard solutionsare to be prepared first and recorded in terms of max and values. The freshly prepared, unused plating bath can serve asstandard solution. After several turn overs, the solutions arerecorded for their particular max values and are accordinglyrectified.

5. Platinum group metals in fuel cell technologies

The possibility of producing electrical energy by continu-ously feeding electrochemically active materials to a suitablecell has attracted the scientific community from an early dateleading to the fuel cell based on Groves pioneering work onthe gas battery [95]. The fuel cell concept did not develop un-til the 1950s when a high energy/density system was neededfor the space programme. Now the focus is on the designof platinum or platinum alloy catalysts for fuel cell applica-tions.Fuel cells are considered to be alternatives to our present

power sources because of their high operational efficien-cies and environment-friendly working characteristics. Con-struction and operating costs are crucial for the successfulcommercialization of fuel cell technologies and hence recentdevelopments are focussed on these two factors. Construc-tion costs can be lowered by using much lower noble metalcatalyst loading without loss of performance. The operatingcosts can be lowered by using hydrogen from other sourcesavoiding costly electrolytic hydrogen and the use of air inplace of pure oxygen.

5.1. Developments in direct methanol fuel cell electrodes

Two of the most advanced low temperature fuel cellsare the proton exchange membrane fuel cell (PEMFC) andthe direct methanol fuel cell (DMFC). Both contain similarmembrane electrode assemblies (MEA), but show a differentdegree of performance. The DMFC has a maximum thermo-dynamic voltage of 1.18V at 25 C and the PEMFC a maxi-mum voltage of 1.23V at 25 C. In practice, the cell voltagesare much less than these values. Therefore, the power den-sity and efficiency are considerably higher in the PEMFCthan in DMFC. Both types of cells are limited by the poorelectrochemical activity of their cathodes. In the case of theDMFC anode, there is kinetic loss arising from poisoning byCOmolecules [96]. The anode and cathode reactions for boththe DMFC and PEMFC are given in Table 10. The overallcell reaction in DMFC is oxidation of methanol moleculesto produce CO2 and H2O and for PEMFC the reaction of H2and O2 to give water with tapping of energy in both cases. In-stead of pure hydrogen in the PEMFC, methanol, gasoline ornatural gas is converted into reformate which is a hydrogen-rich gas stream with small percentages of carbon dioxide andcarbon monoxide. The drawback for the usage of reformateis poisoning of electrodes with carbon monoxide.The platinum metal is usually dispersed on carbon sup-

ports by chemical or electrochemical reduction of plat-inum salts (Table 11). Platinum complexes like Na2PtCl6,Pt(NH3)42+, Na6Pt(SO3)4 are used for this purpose. Chem-ical reduction of platinum on to the carbon support forexample, Vulcan XC-72 is brought about by dispersingchloroplatinic acid or its sodium salt inwaterwith appropriateorganic solvent by the use of ultrasonics or any other efficientstirring method, neutralizing the resulting suspension to pH


Table 10Anodic and cathodic reactions of direct methanol and proton exchange membrane fuel cellsDMFC PEMFCAt anode: CH3OH + H2O CO2 + 6H+ + 6e (E0a = 0.046V) 2H2 4H+ + 4e (E0a = 0.00V)At cathode: 3/2O2 + 6H+ + 6e 3H2O (E0c = 1.230V) O2 + 4 H+ + 4 e 2 H2O (E0c = 1.230V)Overall: CH3OH + H2O + 3/2O2 CO2 + 3H2O (E0cell = 1.180V) 2H2 + O2 2 H2O (E0cell = 1.230V)

Table 11Data on preparation of platinum or platinum alloy fuel cell catalystsComplex Reducing method/agent Support Average Size of the particles (nm) ReferenceH2PtCl6 Borohydride [117,118]Na6Pt(SO3)4 H2 Carbon 1.51.8 [119]PtCl62 Electrochemical PEM 1.520 [98,103,104,120123]Pt(NH3)42+ Electrochemical PEMcarbon 2.03.0 [100102,125]H2PtCl6 + RuCl3 Sodium dithionite Mesocarbon microbeads [111]Stabilized PtCl2 + RuCl3 Alkali hydrotriorganoborates Carbon


According to the second theory, the role of ruthenium isto modify the electronic structure of the platinum surface byinteracting with the platinum conduction band so that weak-ening of the PtCO bond leads to easy oxidation of CO [107].Basic electrochemical studies for oxidation of methanol onplatinumbinary catalystswith Sn, Pd,Re,Au, Pd andRhhavebeen studied by various groups [107112]. Catalysts Pt + Sn,Pt + Pb, Pt + Re, Pt + Ru have been prepared by Beden et al.[108] by electrodeposition Results in this study showed thatonly the PtRu system exhibited enhanced activity whereasother systems showed enhancement only under limited ex-perimental conditions. Studies by Watanabe et al. [109] haveshown that AuPt and AuPd electrocatalysts showed highcatalytic activity in the oxidation of methanol and the rea-sons were explained in terms of a bifunctional theory putforward by them. Liu et al. [111] have studied mesocarbonmicrobed supported Pt-Ru catalysts as anode materials formethanol oxidation. Their results showed that these electro-catalysts exhibited lower polarization characteristics than thatwith carbon black support.

5.2. Developments in PEM fuel cell electrodes

Platinum is deposited into/onto the polymer electrolytemembrane (PEM) to produce Pt/PEM electrodes using theimpregnation-reduction method [113116] where reductionis effected chemically or electrochemically. The commonand efficient chemical reducer for this purpose is borohy-dride [117,118]. Another procedure for preparing a high sur-face area Pt dispersion carbon chemically is by the colloidalmethod known as the sulfito route [119]. In this methodchloroplatinic acid is converted into Na6Pt(SO3)4 by react-ing with excess sulfite salt thereby removing chlorides whichwould otherwise poison the catalyst; platinum oxide colloidsare formed from this procedure on to the carbon supports.The final step is chemical reduction of the colloid/carbon bymolecular hydrogen [119]. The average Pt particle size is1.51.8 nm.The electrodeposition method of preparing Pt/C is advan-

tageous in achieving greater selectivities in the placement ofthe platinum particles into the preformed electrode, but it alsohas a drawback. After initial formation of a platinum particle,further deposition continues on the same particle resulting inincrease of the size of the particle.Platinum particles of size 201.5 nm have been elec-

trochemically deposited from PtCl62 solutions for use inPEMFC electrodes [98,99,104,120123]. At present elec-trodeposition seems to be the only technique through whichloadings in microgram range [typically 10750"g Pt/cm2]are achievable [101,120122,124]. Taylor et al. have pro-duced 23 nm sized platinum particles on Nafion bound car-bon black by using Pt(NH3)42+ complex by electrodeposition[100102]. This complex species can be cation exchangedand reduced [125]. The authors of this report [125] discussedthe importance of the availability of active platinum sites tothe fuel in the PEM electrode. Though all the platinum parti-

cles in PEM electrode are capable of oxidizing hydrogen orreducing oxygen, only those particles which are located at theinterfacial region between the polymer electrolyte and car-bon, function as real electro catalysts. The electrode shouldbe suitably designed to increase the availability of this activethree-phase reaction zone to the fuel.The main disadvantage to PEMFC anodes is the poison-

ing of Pt. Even ppm levels of CO bind strongly and cover98% of Pt [126]. This prevents the dissociative electrosorp-tion of hydrogen H2 + 2Pt 2PtHad which lowers thecell potential produced by the MEA as much higher anodepotential is needed for the electrooxidation [127]. Studies[128131] showed that PtRu alloy electrocatalysts exhib-ited much more tolerance towards CO poisoning than purePt. A recent study by Johnson andMatthey [132,133] showeda significant increase of the cell potential by facilitating dis-sociative electrosorption of H2. The advantage seems to arisefrom the lower particle sizes of the alloy compared to the purePt metal particles.Studies on alloys of the type PtRu, PtRh, PtIr as electro-

catalysts for the anode were undertaken by General ElectricCo. in 1960 as unsupported metal blocks with high loadingsof 30mg/cm2 of Platinum. Recently these have been reexam-ined [134] on carbon supports with loadings of 0.5mg/cm2 Ptat temperatures below 100 C. However, the results showedinferior performances except for PtRu. Replacement of Ruby a 3d metal in this system (viz. PtV, PtCr, PtCo, PtNi, PtFe,PtMn) were also investigated on carbon supports [130] andall systems exhibited inferior CO tolerances compared withthe PtRu alloy system. Binary and ternary carbon supportedcatalysts of Pt alloyed with Ru, W, Mo, Sn synthesized byimpregnationreduction and the colloid method were testedfor their activity for the oxidation of H2 containing 150 ppmofCOandmethanol. ThePtRuW ternary systemwas foundto be superior [110]. Ramesh et al. have studied the hydro-gen oxidation reaction on PtRu, PtPd, PdRu deposits oncarbon and demonstrated that these bimetallic systems canperform better than the corresponding single metal electro-catalysts [135].

5.3. Precursor concept

Another approach for preparing nanosized catalyst par-ticles is given by Precursor concept. In the case of metalparticles with diameter approximately 2.5 nm, approximately60% atoms exist on the surface. Optimum functioning ofthe catalyst takes place at the ratio 60% surface atoms and40% volumetric atoms contained in the nanoparticle. Whencarbon-containing feeds are used, CO poisoning of the anodebecomes the problem, especially in low-temperature PEMand direct methanol fuel cells. One possible way to improvelow-temperature CO tolerances is by adding a second orthird metal. The precursor concept by Bonnemann et al.facilitates the synthesis of such multimetallicnanoparticlesystems via nanostructured metal colloids [136142]. Thismethod is useful to prepare colloids of metals of group 411


of the periodic table. Metal salts, usually halides, are usedwith alkali hydrotriorganoborates as reducing agents to ob-tain boron free powdered metals. The main advantages arethat (i) the process yields stable metal colloids which are easyto isolate as dry powder hence easy handling (ii) the parti-cle distribution is nearly monodisperse and (iii) bimetalliccolloids are easily accessible by co-reduction.Using this concept, a Pt50Ru50 bimetallic colloid catalyst

was prepared using the reducing agent N(Oct)4[BEt3H] andthe performance was found to be equal to the commerciallyavailable Pt/Ru electrocatalysts. The advantage of the pre-cursor concept compared with the salt-impregnation tech-nique lies in the fact that both the size and composition ofthe colloid metal precursors may be tailored independentlyof the support. Thus the precursor concept provides an excel-lent tool for the preparation of bimetallic electrocatalysts ofa wide range of PtM combinations.

6. Concluding remarks

This review has shown that there are new significant con-tributions in the chemical and electrochemical depositiontechnology of PGMs, particularlywith regard to platinumandpalladium. Among these, the development of very efficientplating systems for platinum based on Pt(NH3)4(HPO4) withever higher plating efficiency is noteworthy. Similarly thereconsiderable improvements in palladium deposition. Hydro-gen evolution and the resulting hydrogen embrittlement isstill a major problem in PGM electroplating.Amajor impediment for commercialization of PEMbased

fuel cell technologies is high costs associated with the plat-inum metal. There are attempts to counter this problem byfinding new electrode materials in the form of alloys (e.g.PtRu). The new precursor route to such nanosized alloysystems is a promising method for the design of effectivecatalysts. Apart from being cost-effective, these alloy sys-tems significantly modified the slow kinetics of pure plat-inum catalyst by oxidizing the adsorbed CO molecules. Theelectrochemical depositionmethod has an advantage over thechemical deposition method in achieving lower size particlesfor constructing a fuel cell catalyst. Though PtRu alloy par-ticles on carbon supports are currently the state-of-the-artcatalyst for reformate and direct methanol fuel cells, thereis a great need for further investigation for improvements intheir performance. For PEM fuel cells, the other main chal-lenge is to improve the utilization of all platinum particlesthat are loaded into the polymer electrolyte. Though all theplatinum contained in the electrode is active for the oxida-tion of hydrogen and reduction of oxygen reactions, the finalperformance of the electrode depends on the availability ofthe three phase reaction zones containing platinum particleto the fuel. Progress in the design of such electrodes whichcan improve the availability of reaction zones to the fuel isalso needed.

Acknowledgements

The authors are grateful to the referees for their helpfulcomments in revising the manuscript.

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