poonb,*

pp

ex

sitiheionctivhi

33

ceiv

e Ldeposition on different kinds of carbon inside Nafion polymer mem-branes. In deposition from an aqueous PtCl6

22 solution, the phe-nomenon was strongly limited by anion diffusion across the Nafionmembrane, which is a cation exchange membrane. In this case, thedeposition generated dendritic Pt needles which grew from the car-bon surface across the Nafion membrane. Indeed, the Donanexclusion36 prevents the entry of anions inside the Nafion and hencelimits the deposition rate; the use of a supporting salt in the aqueouselectrolyte only allows a partial decrease of the Donan exclusioninside the Nafion membrane. Finally deposition from an aqueousPtCl6

22 solution produces numerous, small Pt particles ~2-5 nm!,but the Pt loading and Pt mass fraction @Pt/~Pt1C!# cannot behigher than a few micrograms per square centimeter and a few per-cent, respectively. Some researchers26,28 have used an aqueousPt~NH3)421 solution to perform Pt deposition under the same con-ditions. In this case, ionic species are cationic and can diffuse moreeasily across the Nafion membrane. Electrochemical deposition islimited by a first chemical step ~the loss of one or more ligand byPt~NH3)421! and requires a temperature increase. However, the

sen for these experiments, because they are often used in PEMFCtechnology. The H2SO4 electrolyte was prepared from Merck Supra-pur concentrated sulfuric acid and ultrapure water ~Millipore superQ system: resistivity .18 MV cm!. High purity nitrogen gas wasused.

Electrodes.For the study, the electrode support ~a 0.2 cm2glassy carbon rod from Carbone Lorraine, imbedded in polytet-rafluorethylene! was successively polished using 9, 3, and 1 mmdiamond pastes, ultrasonically cleaned in acetone, ethanol ~RP Nor-mapur Prolabo!, and ultrapure water for 15 min, and dried prior tocoating with a thin active layer ~ca. 1 mm!.

Active layer coating procedure.Carbon was activated underCO2 at 930C for 1 h, i.e., until a 27-30% mass loss. Well-definedmasses of carbon and H2PtCl6 ~calculated for a final Pt mass fraction@Pt/~Pt 1 C!#!, and ethanol ~or propanol! were mixed. The mixturewas homogenized, first with 1 h of ultrasound and then mechani-cally for 12 h ~in the dark!. This treatment allowed a good impreg-nation of the carbon by the hexachloroplatinic acid. A Nafion solu-tion ~carbon/Nafion volume ratio 5 1! was added to the mixture,In Situ Electrochemical Deon Carbon and Inside NafiOlivier Antoinea,z and Robert DurandaDepartement de Chimie Minerale, Analytique et A4, SwitzerlandbCNRS/ENSEEG, 38402 Saint Martin dHe`res Ced

The paper describes an in situ electrochemical method for depoproton exchange membrane fuel cell active layer conditions. TH2PtCl6 and electrochemical deposition. It guarantees goodplatinum mass fractions ~20 to 40% Pt/~Pt 1 C!), i.e., smaller aa narrow nanoparticle size distribution ~2-4 nm!. This providesmass activities for the oxygen reduction reaction. 2001 The Electrochemical Society. @DOI: 10.1149/1.13612

Manuscript submitted November 7, 2000; revised manuscript re

Common ex situ chemical techniques for the preparation of elec-trocatalysts supported on carbon are impregnation, ion exchange,and adsorption of colloidal particles.1 Impregnation and ion ex-change techniques are based on adsorption or the exchange of ametallic precursor on carbon followed by its chemical reduction.The technique of adsorption of colloidal particles is based on thechemical reduction of colloidal particles in solution followed bytheir adsorption on carbon. In the case of platinum, these ex situdeposition techniques are useful and industrially used. However,there are some difficulties in obtaining high Pt mass fractions withsmall nanoparticle sizes and good ionic and electronic percolations.Therefore, research into other methods is still continuing.

Ex situ electrochemical deposition techniques theoretically con-trol the surface concentration and size of nanoparticles by pulseshape and number. They have been the subject of numerous papersand patents.2-14 However, although these techniques are easily per-formed on bulk carbon, they are difficult to perform on carbon pow-der. Furthermore, platinized carbon powders need to be impregnatedby proton exchange polymer electrolyte ~i.e., Nafion! before use asproton exchange membrane fuel cell ~PEMFC! electrodes. Conse-quently, they do not guarantee a good ionic percolation. Finally,these techniques are not better than chemical ones and are oftenmore complicated, hence more expensive.

In situ Pt electrochemical deposition methods have been investi-gated in various polymers.15-24 Some researchers25-35 performed Pt

Electrochemical and Solid-Stat1099-0062/2001/4~5!/A55/4/$7.00* Electrochemical Society Active Member.z E-mail: Olivier.Antoine@chiam.unige.chsition of Pt Nanoparticles

liquee, Sciences II, Universite de Gene`ve, 1211 Geneve

, France

ng platinum nanoparticles on carbon and inside Nafion, i.e., inmethod is based on the association of carbon impregnation byic and electronic percolations, i.e., small ohmic drops, highe layer thickness and hence smaller diffusional limitations, and

gh efficiency factors and results in high values of specific and

# All rights reserved.

ed February 5, 2001. Available electronically March 20, 2001.

main limitation comes from the small amount of Pt cations presentin the Nafion thin active layer ~case of standard in situ deposition!and comes from ion exchange of the Pt cations with the protons ofthe carboxylic groups on the strongly oxidized carbon surface ~caseof ex situ and in situ methods!. Therefore, the Pt loading and Ptmass fraction cannot be superior to approximately 50 mg cm22 and5%, respectively. This method does not allow the preparation ofoxygen cathodes, but remains useful for the preparation of hydrogenanodes.

To avoid these limitations, the platinum precursor (PtCl622)must be present at carbon/Nafion interface when electrochemicalpulses are applied. A newly patented method37 consists of the asso-ciation of the impregnation and in situ electrochemical reductionsteps. The deposition is performed directly in a ~H2PtCl6 impreg-nated carbon! Nafion active layer. This technique is described hereand preliminary results are given for the in situ electrochemicaldeposition of Pt nanoparticles within thin active layers on a glassycarbon support in aqueous electrolyte. Applications to porous gas-eous electrodes and an X-ray absorption spectroscopy ~XAS! studyof the Pt deposition mechanism are currently underway.38

Experimental

Reagents.Hexachloroplatinic acid ~^H2PtCl6,6H2O& from Ald-rich!, a black carbon powder ~Vulcan XC-72 from Cabot!, and aNafion solution ~5 w/w 1100 EW solution from DuPont! were cho-

etters, 4 ~5! A55-A58 ~2001!The Electrochemical Society, Inc.

A55which was then further homogenized with ultrasound. A measuredvolume of this mixture was dropped on a glassy carbon support toobtain an active layer. This procedure gave a thin active layer, ho-mogeneous in thickness and composition, with known thickness

e L~about 1 mm! and Pt loading. Low boiling point solvents wereevaporated at room temperature, followed by a heat-treatment at160C. It was necessary to heat to above the glass transition tem-perature of Nafion ~140C!, to obtain a well recast ionomer.39 Theresulting active layer was dense ~without pores! and thin. Electro-chemical reduction experiments were performed directly on thiskind of active layer on glassy carbon support.

Electrochemical instrumentation.Classical electrochemicalequipment included a EG&G PAR 273 potentiostat coupled via aNational Instrument NI-488 interface to a computer. A Pyrex glasscell was used. The active layer was in contact with a 1 M H2SO4solution used as external aqueous electrolyte. A saturated calomelelectrode ~SCE! reference was connected to the active layer via aLuggin capillary. Potentials are reported vs. the reversible potentialtaken by the Pt/C interface when the Nafion phase was saturatedwith H2.

Characterization of Pt/C.Following the electrochemical reduc-tion, cyclic voltammograms ~0.05 to 1.3 V; 0.01 V s21! were re-corded for an active layer in contact with the same external aqueouselectrolyte saturated by nitrogen. The electrochemically active Ptsurface area (SPt) of the active layer was measured by hydrogenadsorption-desorption coulometry between 0.05 and 0.4 V vs. refer-ence hydrogen electrode ~RHE!, assuming that the charge related toa hydrogen monolayer adsorption on Pt is equivalent to 210 mCcm22 Pt. Measurements of Pt surface area in contact with Nafionand available for electrochemical reactions allowed us to evaluatethe roughness ratio g: platinum real area/geometric area. Becausethe Pt loading of the active layer was known, it was possible toestimate the specific catalyst area S ~m2 kg21! and the average par-ticle size d (m) by assuming spherical particles: d 5 6/(214003 S), where 21400 kg m23 is the platinum density. These d valueswere compared with those obtained by transmission electron micros-copy ~TEM!. A complete reduction of PtCl6

22 species and goodelectronic and ionic percolations for all Pt particles would result inproviding similar d values for the two methods.

Results and DiscussionThe combination of impregnation and in situ electrochemical re-

duction eliminates diffusional limitations. PtCl622 anions do not

have to diffuse from the external aqueous electrolyte to the carbonsurface across recast Nafion membrane, because they are alreadypresent on the carbon surface. Therefore, there is no preferentialdeposition near the interface between the Nafion membrane and theaqueous electrolyte. Moreover, particles are homogeneous in size ~ifPtCl6

22 anions and crystallization sites are evenly dispersed on car-bon surface! and small ~if number of sites is sufficient!. The Nafionmembrane prevents diffusion of PtCl6

22 from the carbon surfaceand expulsion into the external aqueous electrolyte ~there was nevermore than a few percent of total Pt in the electrolyte followingexperiments!. The active layer must be correctly hydrated ~hydrationtime around 2 min! before deposition to allow for a good ionicconductivity during deposition. Under these conditions, Pt nanopar-ticles are mainly deposited where ionic and electronic percolationsare effective.

There are two common electrochemical deposition techniques,potentiostatic and galvanostatic depositions. The first allows for thestudy of the electrocrystallization mechanism and reduction kinetics.The latter is useful for industrial applications because it is less ex-pensive and can be used for large surface area PEMFC electrodes.

Potentiostatic deposition.Two-pulse techniques are used inclassical electrocrystallization experiments ~i.e., with the active spe-cies in the aqueous electrolyte!, because it allows for studies of both

Electrochemical and Solid-StatA56nucleation and growth processes of the platinum nanoparticles.10,40In the present applied research, there are almost no active species inthe external aqueous electrolyte. Furthermore, the carbon surfacegeometry is complex ~specific area around 300 m2 g21! and thedeposition overvoltages are high ~in absolute values! to get a largenumber of small particles. Therefore, standard electrocrystallizationconcepts can only provide qualitative guidelines. Experimentalcurves ~see Fig. 1 and 2! decrease from the beginning and presentcurrent densities much higher than those of a simple double layercharging ~on nonimpregnated carbon for example!. Therefore, depo-sition takes place at short times ~nucleation time !1021 s! and nu-clei growth is masked by the surface depletion around each nucleus~in spite of a probable fast surface diffusion phenomenon!.

When the deposition potential is higher than 0 V vs. RHE, thecurrent density ~c.d.! of the second potential pulse is lower than thec.d. of the first one ~Fig. 1!. At this potential, there is no hydrogenevolution reaction ~HER! and the current is mainly due to electro-chemical Pt deposition. Preferential reduction of PtCl6

22 and par-ticle growth take place during the second potential pulse on Pt nano-particles produced during the first potential pulse, but the surfaceconcentration depletion around each nucleus implies a small c.d. atthe end of every pulse.

When deposition potential is lower than 0 V vs. RHE, the c.d. ofthe second potential pulse is higher than the c.d. of the first one ~Fig.2!. At this potential, the HER can compete with electrochemical

Figure 1. Chronoamperometries ~carbon impregnated by PtCl622; Pt massratio: 20 % Pt/~Pt 1 C!; active layer thickness: 2 mm; geometrical area ;0.2cm2; hydration time ;2 min; two successive identical pulses a andb: 120 mV vs. RHE during 0.5 s; pulses interval ;1 min; 20C; 1 M H2SO4;final g 5 29.1; dcalculated 5 4.8 nm!.

Figure 2. Chronoamperometries ~carbon impregnated by PtCl622; Pt mass

etters, 4 ~5! A55-A58 ~2001!ratio: 20 % Pt/~Pt 1 C!; active layer thickness: 2 mm; geometrical area ;0.2cm2; hydration time ;2 min; two successive identical pulses a and b: 250mV vs. RHE during 0.5 s; pulses interval ;1 min; 20C; 1 M H2SO4; finalg 5 45.9; dcalculated 5 3.05 nm!.

e Lreduction of PtCl622 on the first produced Pt nuclei since the first

pulse: the c.d. are not equal to zero at the pulse ends.Platinum surface areas ~and roughness values g, see figure cap-

tions! are higher when deposition is performed at a potential lowerthan 0 V vs. RHE, i.e., in presence of HER. The most probableexplanation is that at lower pulse potentials the number of nuclei islarger and, therefore, the surface region to deplete around eachnucleus is smaller and the final particles are smaller. However, wecannot exclude the possibility of a complementary effect of the HER~for example, a PtCl6

22 reduction by H2!. Therefore, a study on theH2 effect is underway in the laboratory.38

Galvanostatic deposition.The perfect deposition process 4,10,40theoretically consists of a first short, strong pulse, which produces ahigh number of nuclei, and a second long, weak pulse, which in-duces a growth of nuclei with a narrow size distribution. In ourexperimental curves ~Fig. 3a and b!, the classical nucleation peakand growth zone did not appear. During the first pulse, a potentialplateau, below 2300 mV vs. RHE, is reached only after about 15ms. Therefore, the plateau potential is assumed to mainly depend onthe HER. During the second pulse, at lower c.d., the plateau poten-tial is closer to 0 V vs. RHE and it is not possible to discriminatebetween the two reactions ~HER or electrochemical Pt reduction!.This experiment produces a final active layer ~2 mm thick! with aroughness, g, equal to 51.5 ~calculated from cyclic voltammogramunder an inert gas!. Assuming a total reduction of the PtCl6

22 ~less

Figure 3. Chronopotentiometries ~carbon impregnated by PtCl622 ; Pt mass

Electrochemical and Solid-Statratio: 20% Pt/~Pt 1 C!; active layer thickness: 2 mm; geometrical area ;0.2cm2; hydration time ;2 min; two successive pulses: ~a! 2100 mA/0.1 s and~b! 210 mA/1 s; pulses interval ;1 min; 20C; 1 M H2SO4; final g5 51.5; dcalculated 5 2.7 nm!.than 5% of the initial mass of Pt is released into the external aque-ous electrolyte! and knowing the initial Pt loading, the calculatedmean nanoparticle diameter is equal to about 3 nm. TEM observa-tions ~Fig. 4! show a Pt nanoparticle size distribution between 2 and4 nm. These mean values are close enough to assume that the over-all surface area of the catalyst that is dispersed in the active layer isavailable.

Toward higher Pt/(Pt 1 C) mass ratios.This method can beeasily used to produce in situ Pt nanoparticles deposition with a Ptmass fraction equal to 20% and a size distribution between 2 and 4nm. However, the method is especially interesting for the prepara-tion of the PEMFC oxygen cathode only if it allows the attainmentof higher Pt mass ratios ~necessary for the use of thin active layers!with the same size distribution ~necessary to reach high mass activi-ties!. Consequently, carbon was impregnated by more PtCl6

22 toobtain a final Pt mass fraction equal to 40%. The number and thetotal charge of the applied pulses were proportionally increased ~thecharge is around two times the theoretical minimum value, to takeinto account the presence of HER!. Unfortunately, in initial experi-ments, the higher Pt mass fraction deposit could not be reached witha good particle size distribution. We assumed that this limitationdepended on the PtCl6

22 impregnation quality onto the carbon andtherefore experimented with different alcohols ~ethanol, 1 and2-propanol! in the initial mixture to increase the surface accessibilityof the carbon ~wettability!. A higher Pt mass ratio ~40 % Pt/~Pt1 C!) with the same particle size distribution between 2 and 4 nmwas attained using 2-propanol ~Fig. 5!. This experiment produced afinal active layer which was 2 mm thick with a roughness, g, ofabout 140. 2-Propanol is assumed not only to act on surface acces-sibility of carbon, but also on the PtCl6

22 impregnation mechanism.Indeed, according to Teranishi et al.,41 PtCl6

22 can be chemicallyreduced by different alcohols in solution. They showed that thePtCl6

22 chemical reduction became faster in the order methanol,ethanol,propanol and Pt nanoparticle size becomes smaller in theorder methanol.ethanol.propanol. In this study, experimental con-ditions were different and PtCl6

22 is assumed not to be totally butonly partially chemically reduced by the 2-propanol prior to theelectrochemical reduction by pulses and/or the chemical reductionby H2. Experiments ~especially by XAS! on the effects of the alco-hol and the H2 are now in progress in the laboratory. It would appear

22

Figure 4. TEM observation on the electrode active layer associated withFig. 3.

etters, 4 ~5! A55-A58 ~2001! A57that the PtCl6 is partially chemically reduced by the 2-propanoland adsorbed onto the carbon and that chemical reduction by hydro-gen at room temperature is only effective with thin active layers andperfect PtCl6

22 impregnation onto carbon.38

3. R. D. Giles, J. A. Harrison, and H. R. Thirsk, J. Electroanal. Chem., 20, 47 ~1969!.4. K. Shimazu, D. Weisshaar, and T. Kuwana, J. Electroanal. Chem., 223, 223

~1987!.5. P. Allongue and E. Souteyrand, J. Electroanal. Chem., 286, 217 ~1990!.6. N. Georgolios, D. Jannakoudakis, and P. Karabinas, J. Electroanal. Chem., 264,

235 ~1989!.7. J. Lin-Cai and D. Pletcher, J. Electroanal. Chem., 149, 237 ~1983!.8. P. Bindra and E. Yeager, in Electrocrystallization, R. Weil and R. G. Barradas,

Editors, PV 81-6, p. 233, The Electrochemical Society Proceedings Series, Pen-nington, NJ ~1981!.

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10. B. Scharifker and G. Hills, Electrochim. Acta, 28, 879 ~1983!.11. J. L. Zubimendi, L. Va`zquez, P. Ocon, J. M. Vara, W. E. Triaca, R. C. Salvarezza,

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Electrochemical and Solid-State Letters, 4 ~5! A55-A58 ~2001!A58ConclusionsThis new method is based on a combination of impregnation and

in situ electrochemical reduction. It allows the in situ production ofPt nanoparticles, i.e., onto carbon and inside Nafion, with Pt massratios equal to 20% and a narrow size distribution in the range 2-4nm. HER is a side reaction and appears to have an effect on the Ptdeposition. The method can be used for PEMFC oxygen cathodepreparation, because it also allows the obtention of higher Pt massratios ~40%! with the same size distribution. In this case, the pres-ence of 2-propanol in the initial mixture is necessary to obtain anarrow size distribution. This method will soon be extended to thepreparation of PEMFC active layers, with a thickness in the 10 mmrange.

The University of Geneva assisted in meeting the publication costs of thisarticle.

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