Cathodlc Reactivity of Platinum and Pallachum in Electrolytes in Superdry Conditions THE NATURE OF THICK LAYERS ELECTROCHEMICALLY FORMED AT THE METAUELECTROLYTE INTERFACE

By Charles Cougnon and Jacques Sirnonet' Laboratoire d'Electrochimie Moleculaire et Macromoleculaire, UMR 651 0, Campus de Beaulieu, Universite de Rennes, 35042 Rennes Cedex, France, 'E-mail: jacquessimonetQuniv-rennesl .fr

Platinum and palladium behave in an unexpected manner when cathodicall?, polarised in the presence of electrolytes dissolved in carefi1l.v dried polar organic solvents, such as

N,N-dimethyljormamide. A reductive layer is formed on the metal, the thickness of which depends on the amount of electricity consumed during the course ofthe electrolyses. Although this reaction seems to be o f a general character with most ofthe common electrolytes. in this paper we will focus on results obtained with a large palette of tetraalk,vlammonium salts and alkali metal iodides.

The reaction of the electropositive alkali metals with a large number of more electronegative main group 'meta-metals' (such as Pb, Si or Ge) to form the so-called Zintl phases (1-3) was discovered by Eduard Zintl at the beginning of the twentieth cen- tury. Zintl phases are electronically positioned between intermetallics and insulating compounds and are semiconductors. They form compounds where the heavier metal forms clusters in polyan- ionic units, surrounded by the lighter alkali metal cations - and have a large range of structures. Such materials may possess covalent, metallic and ionic bonding (4, 5).

The synthesis of Zintl phases (6, 7) generally involves heating up a mixture of the elements in closed tantalum or niobium containers. Another method is the reduction of post-transition metals (or more commonly of a salt of these metals) in the presence of sodium (Na) in liquid ammonia. Thus, Zintl was able to follow the reduction by potentio- metric titration of Na ions (8). These experiments allowed the composition of the generated phase to be specified. On the other hand, by an electro- chemical technique, cathodically-polarised post- transition metals, such as lead, could be used as working electrodes, into which the non-electroac- tive cations, such as potassium (K), could be inserted, as shown next (9, 10):

4K+ + 4.- + 9Pb (cathode) ++ [4K+, Pbg'] Until now, platinum (Pt) and palladium (Pd)

were considered to be totally inactive towards the alkali metals and also electrochemically inert. The latter property has allowed Pt and Pd to be widely used as cathode materials. However, their weak hydrogen overvoltage obviously reduces the usable cathodic range (in non-aqueous solvents without careful prior-dqmg treatment, the usable cathodic range is limited to about -2.0 V vs. a saturated calomel electrode (SCE)). If the residual moisture in the polar aprotic solvent is drastically reduced (for example by an in sihc solid non-electroactive drier, such as neutral alumina) down to 50 ppm, hydrogen evolution almost vanishes. What are the electrode reactions then? This field seems to be totally unexplored and the aim of the present work, devoted only to Pt and Pd, is to demonstrate that it is of importance.

Earlier observations of the cathodic behaviour of Pt in an electrolyte of dry N,iV-dimethylfor- mamide (TlMF) in the presence of tetraalkyl- ammonium tetrafluoroborate, %NBF+ were the first to show the electrochemical construction of a thick layer on the Pt surface comprising both W' cations and the salt itself (11). The slow degrada- tion of these layers by air led to the restoration of the Pt metal surface. However, the surface had

Phtinum Mefah Rey., 2002,46, (3), 96105 94

undergone tremendous structural changes (such as formation of dendrites, channels and/or grain boundaries) together with the visible emergence of the absorbed salt from the Pt bulk.

This paper describes results obtained for Pd and Pt cathodes in the presence of a wide range of tetraalkylammonium salts, WX, as well as alkali halides (iodides because of their greater solubility) (12,13). Experiments allowed us to determine the nature of the layers formed. Methods such as coulometry, the electrochemical quartz crystal microbalance technique, chronopotentiometry, SEM analysis and impedance spectroscopy were used. Until now the rather poor chemical stability of the layers did not allow X-ray characterisation of their structure.

Experimental Procedure Salts and Solvent

In most of the experiments, electrolyte concen- tration was 0.1 M. Potassium, lithim (Li), Na and caesium iodides were used and the tetraalkylm- monium salts were of > 99.7% purity (puriss grade). All salts were used without fbrther purifi- cation after being thoroughly dried under vacuum at 100°C for 48 hours. The DMF was checked (by the Karl Fischer method) to ascertain it contained less than 50 ppm of water, having been stored over neutral alumina, previously activated under vacuum at 300°C for 4 hours. All experiments were performed in a carefully dried argon atmos- phere. Electrolyte solutions were maintained in the electrochemical cell over activated alumina.

Electrochemical Instrumentation and Procedures

Cyclic voltammetric investgations were carried out in a standard three-electrode cell using a pAUTOLAB potentiostat connected to a comput- er equipped with standard electrochemical system software. For analytical purposes the working elec- trode was a disk of Pt or Pd (area 8 x cm? and the counter electrode was a glassy carbon rod. All potentials given here refer to the aqueous SCE. However, the SCE was not used as the reference electrode in the cell (due to possible water diffu- sion). As reference electrode, the Ag/AgI/O.l M

INBu, system (in DMF) was used, and potentials were corrected afterwards.

Prior to the experiments the Pt and Pd working electrodes were carefully polished with silicon car- bide paper of successively smaller partide size (18 to 5 pm), then by diamond powder (6 and 3 pn). Finally, the working electrode was rinsed with ethanol and acetone and dried. Between each scan the electrode surface was repolished with diamond powder (3 p).

For macroelectrolysis investigations, Pt sheets (99.99Yo purity, area 1 cm’, thickness 0.05 mm) and Pd sheets (99.95% purity, area 1 cm’, thick- ness 0.1 mm) were used. They were used once only for SEM analysis without further treatment.

Chronocoulometric Investigations on a Thin Metallic Layer

Coulometric experiments were carried out on Pt and Pd film electrodes prepared, respectively, by depositing the metals from solutions of 10 g 1-’ H2PtCI, in 0.1 M HCl and 10 g 1-’ PdCl, in 0.1 M HCl onto polished gold disks (2 x cm?. The plating was carried out in a galvanostatic mode (current 10” A cm-7. All the experiments were performed with gold substrates but with different thicknesses of deposited metal. The gold substrate could be a gold microelectrode, polished before each deposition, or (for EQCM experiments) a larger electrode of gold-coated quartz crystals.

Electrochemical Quartz Crystal Microbalance (EQCM) Instrumentation

Simultaneous voltammetric and mass balance experiments were carried out with an oscillator module quartz crystal analyser connected to a potentiostat. This device was computer controlled. In the experiments, 9 MHz AT-cut gold-coated quartz crystals were plated electrochemically with a thin film of Pt or Pd. Plating was achieved by the chronocoulometric procedure. The deposited mass was checked by the EQCM. The EQCM measurements were performed in a Teflon cell equipped with a glassy carbon counter electrode, a reference electrode and the Pt- or Pd-plated quartz crystal working electrode. The apparent area of the quartz crystal was about 0.2 cm’.

Pkztinum Metah Reu., 2002, 46, (3) 95

10-3

Table I

Potentials ( f p c & Epa) and Peak Currents (Ipc & Ips) Obtained by Cyclic Voltammetry at a Palladium Cathode Relative to 0.1 M Alkali Halide Salts and to Tetra-n-butylammonium Salt Solutions in Dry DMF

Lil Nal KI Csl B u ~ N I BurNBFr

I Bu4NC'0s

Electrolyte, 0.1 M

Results Voltammetric Data

All the monovalent cations (Li', Na+, K', Cs' as well as R N - regardless of the n-alkyl chain

4 c I

V

-3.07 -2.07 -2.12 -2.22 -2.84 -2.87 -2.82

28 16.8 15

8.2 13 17 28

IoglpJlog V'a)

0.54 0.39 0.45 0.42 0.52 0.45 0.56

€pa,

V

-2.20 -1.13 -0.82 -0.78 -0.77 -0.72 -0.70

19.0 14.0 8.0 8.2 2.3 3.0 4.5

0.68 0.83 0.53 1 .oo 0.18 0.1 7 0.16

Epc - €pa,

V

0.87 0.94 1.30 1.44 1.51 2.15 2.12

Voltammetric data are relative to a stationary palladium electrode of areu 8 x 10 ' cm2 Potentials are referred to the SCE. The sweep rate, v = dV/dt is 200 m V s I . w The slope for sweep rates Jiom 0.02 to 5 V sC'

Microgravimetric data, reported in terms of mass change, were calculated using the Sauerbrey equation, which links the resonant frequency and mass change. For most cases it was found that the mass discharge of the Pt (or Pd) lilm at 0 V was not completely reversible and thus, a small excess of mass remained at the start of each experiment. However, the amount of extra mass gained during the charge process remained the same. These experiments suggested that the EQCM technique was an accurate method for quantifymg the charge/discharge process as there was no notice- able loss of mass (for example, occurring by mechanical degradation of the layer in the course of the charge process).

Scanning Electron Microscopy Experiments Surfaces treated electrochemically (samples

were rinsed using an alcohol/acetone mixture in an ultrasound bath for 2 h) were analysed by a scan- ning electron microscope.

peak (or wave in the case of ammonium ions) of current, I,,, which is always associated with an anodic step peak of current, Ips. The anodic step peak corresponds to the oxidation of the materi- al(~) formed while held at the level of the cathodic step (Tables I and II).

A Pd microcathode, with only CsI (0.1 MJ in dry DMF (carefully maintained over neutral alumi- na in -ri&) displayed an irreversible step, IF, at -2.22 V. Holding the potential at this level (-2.22 V) allowed the sharp anodic peak to increase, at Epa = -0.78V (Figure 1). In all cases, whatever the cath- ode material and electrolyte, the cathodic currents

rig. I Typical voltammetric behaviour of a 0.1 M CsUDMF solution in contact with a stationaw oalladium fluoroborate Or perchlorate) displayed, at the Pt

, 1

and Pd microelectrodes, an irreversible cathodic microcathode (sweep rate: 200 m V s ~ y

PLztinum Met& Rm, 2002,46, (3) 96

length) when associated to anions (halides, tetra-

Table II

Potentials (EPc and Epa) and Peak Currents (Ipc ) Obtained from Cyclic Voltammetry at a Platinum Cathode Relative to 0.1 M Salt Alkali Halide Salts and to Tetra-n-butylammonium Salt Solutions in Dry DMF

Electrolyte, 0.1 M

Li I Nal KI Csl BIJ~NI B u ~ N B F ~ BudNC104

Epc, Ipc'a), €pa, Epc - fpa,

V PA V V

-2.84 -5 -2.42 0.42 -2.07 -1 0 -1.72 0.35 -2.17 -6.4 -1.62 0.55 -2.18 -5.4 -1.77 0.41 -2.89 -1 1 -1.27 1.62 -2.91 -1 2.5 -1.09 1.82 -2.84 -25 -0.99 1.85

were found to vary linearly with the square root of the scan rate up to 5 V sd indicating a diffusion- controlled current. The small currents observed for these cathodic steps suggest that the limiting diffusion corresponds to ion insertion into the metallic bulk. It is also important to stress that such cathodic peaks do not diminish when a l h -

na is progressively added to the voltammettic cell, and cannot be due to moisture reduction. Their limit current was found to depend on both the concentration of the salt and on the nature of the cation. Thus, in the presence of alkali iodides, the currents follow the order:

Cs+ < Kt < Na+ << Lit

Surprisingly, tetraallcylammonium cations dis- played quite large peak currents (IF values in Tables I and Il). .

To fhd out what role any residual water played, the system was dosed with water in amounts over 20Ck500 ppm. The specific redox system disap- peared and was replaced by a cathodic wall (strong cathodic current) corresponding to water reduction.

SEM Analysis and Microelectrolyses Potentiostatic macroelectrolyses were per-

formed on as-received polycrystalline Pt and Pd sheets (see Figure 2(a) for as-received Pd sheet).

Fixed potentials were set very close to the corre- spondmg peak potentials or at the beginning of the plateau region for 'wave-shaped' steps. The amount of electricity involved was somewhat larg- er than 20 C cm". In all cases a chemical transformation of the metal surfaces was observed after the sheets had been removed from the elec- trolysis cells and carefully rinsed in DMF. The samples were exposed to air to see the change of structure caused by oxidation.

Using SEM analysis, particularly with tetra- alkylammonium salts, black zones were observed to decrease progressively, while white (or light grey) areas became progressively more dominant, with time (see Figure 2@)). The latter areas cor- respond to pure metal. The black zones were shown (by a suitable probe) to contain elements such as carbon and iodine (when Bu$rJI was the electrolyte, Figure 2(c)). Additionally, crystals of electrolyte were found to emerge from the disap- pearing black zones. This process has been explained by air oxidation of the cathodic layer at the metal surface.

After a long time in air (sometimes 10- than one day) the metal structure would change dramat- ically, see Figure 3(a), which shows very regular fraaals for Pd with angles very close to 45" and 90". This Pd surface corresponds to pure metal after the total oxidation of the electrochemically-built layer.

Phfimm Metah Rcv., 2002,46, (3) 97

Voltammetric data are relative to a stationary platinum cathode of area 8 x I0 cm'. Potentials are re/rred to the SCE The sweep rate, v, i s 0 2 V s-I.

The relatiomhip I, vs Y''~ was found to be linear in all cases in the range 0 02 to 5 V s-' (correlation coefficient is always 2 0 99)

I 600

400

200

C 0 ';" ........................................

0 2 4 6 8

I L

0 0 6 8

c PS

-- 0' 1 , . . . . . . . . . . . . . . . . . . . . . . . . . . 0 2 4 6 8

ENERGY, keV

Alkali metal iodides may also cause specific and dramatic changes to the Pd and Pt cathode SUT- faces, see Figures 4 and 5, respectively.

Coulometric Data The voltammetric results and the instability of

the electrochemically-formed layer strongly sug gest that a charge/discharge process is taking place. This assumption can be checked by coulom-

etry since a large part of the electricity stored dur- ing the cathodic processes should be able to be anodically restored. However, the charge/dis- charge phenomenon was not found to be totally reversible since the solvent could not be made totally anhydrous and free of acidic impurities - some hydrogen evolution occurred (in yields of 65 to 75% depending on the salt used).

It has been suggested that the electric charge

Pkztinum Met& Rev., 2002, 46, (3) 98

fig. 2 SEMSs of palladium sheet at sheet

two magnifications:

(b) cathodically treatd in 0.1 M 0.1 M mfiBu4 NI in DMF by potentiostaticet electrolyiss. Potential: - 2.6 V1 2.6 V

amount of electricity: 130 C cm2. cm2(c) Probe of image (b) (treated treated

palladium). The white zoines (top (topscan) shows only palladium metalat sheet while the dark grey zones (lowe lowerr

fig. 2 SEMSs of palladium sheet at sheet fig. 2 SEMSs of palladium sheet at sheet

scan) alkso exhibit the presencefig. 2 SEMSs of palladium sheet at sheet

throught the dark zones sones

emergence of Bu4NI pure crystals crystals

of carbon and iodine. NOte the the

(a) as-reciedved

Fig. 3 SEMs of'u pulludiiim slieet (a) aJ?er cathodic treaiment iri DMF coniaining 0. I M Bir~NC1,followed by long cofituct with air: Reduction poteniial: -2.8 l! Amoiirit qf'f'elecirici[i3: 250 C em '. For comparison, (b) .show the striicture qf the Pd sheet before reduction. Both images lime the same magnificaiion

Fig. 4 SEMs of'pulludiiim s1iec.t.s ufier cathodic tre~itiiieii/ in 0. I M alkali iodide in DMI; and then coiiiaci with air,for at least 2 hoiir.s. The iniuges liave i/ie .same magn;ficatiori. (a) Aspect of as-received coninicwiul pulladiimi slieri (,tithoiit mi)' cutliodic ireatmuit). (h) Microderidrite.f~riiiati(~ii ufrer polurisation Cf'the palludiiini sheet at -2.3 V in C d (anioririt of'electrici~~: 25 C em '). (c) Srirface structure reorganisation !tier catliodic ireatnieiit in Nul ut -2. I V (anioiint of electricity: 24 C cnii'). (4 Afrer cathodic yolarisciiion at -3 V iii Lil (amount of'e1ectricit.v: 27 C cni')

Pkztinum Metah Rm, 2002, 46, (3) 99

Fig. 5 Structural changes, after cathodic treatment of the platinum sheets in DMF-containing alkali iodides. revealed by SEM analysis after ensuing contact with air The image magni9cations are all similar (a) As-received commercial platinum sheet. @) Structural reorganisation after cathodic polarisation at -2.3 V with 0.1 M CsI (amount of electricity: 27 C cm (c) Morphological changes of a platinum sheet electrolysed at -2.1 V with 0.1 M Nal (amount of electricity: 28 C cm ). The surface exhibits grain boundaries and tunnel ends (in black) from which Nal microcrystals emerge. (d) Special feature (platinum at the inte$ace air/electrolyte) obtained by cathodic polarisation at -2.8 V in 0. I M Lil (amount of electricity: 19 C cm-2)

'I.

stored in the material (Qa in Figure 6(a)) can be measured accurately during the discharge process, assumed to be specific to the process (oxidation at 0 V). Coulometric measurements can therefore give information on the nature of the electrochem- ical reactions involving Pt and Pd.

In order to calculate the exact number of metal atoms dung part in the reduction process, Pd and Pt were electrochemically deposited onto elec- troinactive substrates, such as gold or glassy carbon. When gold was the substrate, the accuracy of the mass of Pt or Pd deposited was also checked by the EQCM technique. After electrolysis, for

a length of time needed to ensure a total sample saturation upon charging, Qa was found to be proportional to the number of Pt or Pd atoms in the layer deposited on the substrate (Figure 6b). The number of metal atoms, x, involved in the electron transfer was equal to 2 in most cases. However the value x = 4 was obtained in the pres- ence of salts with soft anions (such as BFA- and

From the ratio Qa/Qc (ratio of amounts of elec- mcity involved in the charge/discharge process) the yield of the charge/discharge process was found to be from 55 to 70%. Another way to

ClO,?.

Phtinnm Metah Rev., 2002,46, (3) 100

check electron storage in the metal was achieved with Pt in the presence of NaI or CsI. Pt sheets and wires were charged in a standard three-elec- trode cell with 3 C 5 0 coulombs of electricity. After careful rinsing in dry DMF, the metal pieces were dipped into distilled water. Large jumps in pH, of up to 5 pH units, were noticed and gas was observed to be evolved simultaneously at the metal surface. If we assume that one stored elec- tron is equivalent to a basic charge, the reaction with water should be:

ept- + HzO + '/zH*f + OH-

OH- species in the solution were titrated with lo-' M HC1 in the presence of phenolphthalein.

0 025-

J 0050- E

0.075-

: 0 100-

5 0 125-

0 150- , 1: , 0175, .

0 25 50 75 100 125 1 5 0 175 TIME, s

"0 I 4 1-1 0.12-

010- V E 008- I ' 0.06-

0.04.

/ 0.02i/' , . . , ,

20 40 60 80 100 120 THICKNESS, 6, n m

0.12-

010- V E 008- I ' 0.06-

0.04.

0 02.

20 40 60 80 100 120 THICKNESS, 6, n m

Fig. 6 Cathodic reactivi@ o f a palladiuni ,film on a gold substrate at -2.3 l! (a) Chronocoulometric response (charge during 60 s at -2.3 V and then dischar.ge at 0 recorded in 0. I M CsI + DMF at a palladium .film cathode electrodeposirrd onto a polished gold electrode (total palladium mass deposited: 0.13 pg). (b) Anioirrit of electrical dischar.yr, corresponding to the procedure ahove. for different thicknesses (6) oftha palladiuni lyvei: For each experiment the average palladium thickness wus calculated frani the mass of electrodeposited palladium, (lie apparent area of the electrode and the metal density

100 260 300 400 500 TIME, s

Fig. 7 Stoichiometi?, of CsI/Pd modified lavers using the EQCM technique. (a) Mass increase during polarisation ofi~alladium (electrodeposited onto a gold-coated quartz crystal), for diflirent umounts o f m p , l at -2.3 V in 0.1 M CsI f DMF; nip+. A blank; B 5 pg: C 7 pg: D 10.5 pg. (b) Erpcv-iniental relationship henveeri niaximurii niass increuse (.wturation ofthe layer) arid amoitrit of palladiuin deposited on the gold-coated quartz

Experimental values for the charge stored in the material were found to be in quite good agreement with the corresponding coulometric data.

EQCM Experiments The use of a small thin quartz crystal carefully

covered with a layer of Pt or Pd of known mass allowed us to follow the cathodic insertion of both cations and anions. Such experiments are comple- mentary to those already performed by coulometry. In coulometry, metal layers were elec- trolysed while being held at the level of the cathodic peak potential until a limit to the mass increase was obtained, see Figure 7(a). The blank experiment (curve A) shows that the gold sub- strate does not react at all with the electrolyte.

Therefore, the existence, during the charge/ dischaige cycles, of the experimental plateaux was

Pkatinm~ Metalr REX, 2002,46, (3) 101

Table 111

Stoichiometry Determination of Reduced Palladium Phases in the Presence of Electrolytes, NIX, for Thin Palladium Layers Electrodeposited onto Gold Substrates

Electrolyte, 0.1 M

Lil Nal KI Csl Bu~NI Bu~NBF~ B~4NC104

Applied potential, V

-3.02 -2.12 -2.22 -2.32 -2.80 -2.80 -2.80

x Value for Pd;M+

1.95 f 0.05 2.00 f 0.02 1.99 f 0.12 2.08 t 0.05 2.10 0.15 3.87 2 0.1 1 4.05 f 0.08

y Value for (PdTM', yMX)

1.01 f 0.02 0.91 f 0.07 1.02 t 0.03 0.97 f 0.05 0.92 f 0.04 0.97 f 0.06 1.02 f 0.09

c o n b e d , although a small excess mass was noticed at 0 V at the end of each discharge process. This 'mass remanence' may be due to the very slow rate of diffusion of electrolyte out of the metal at the end of the discharge. However, no loss in mass was observed and it was concluded that no degra- dation of the metal layer had occurred. In addition we checked that the saturation mass was not affected by repetitive charge/discharge processes. Figure 7(b) shows that the increase in mass is pro- portional to the mass of metal deposited onto the gold substrate. Thus the coulometiic and EQCM experiments allow us to conclude that both cations and anions of the electrolyte h4X are involved in the charging process. The electrogenerated plat- inum phase and palladium phase should have the respective stoichiometiies:

Ptx-, M+, y o 1 and Pdx-, M+, y o 1

The values of x and y, assumed to be whole num- bers, were determined in a large number of experiments, and obtained with relative error of 5 per cent - probably of the same order of accuracy as the EQCM technique, see Tables I11 and IV. In all cases, it was found that two cations were inserr- ed for each anion.

The only difference between all the experi- ments lies in the stoichiometry of the metal atom. In most cases x = 2, except for the electrolytes with large, bulky anions, such as BF4- and C10,

for which x = 4 (surprisingly, PF, did not display any noticeable cathodic reactivity with Pd or Pt electrodes). On the contrary, very bulky cations, such as tetra-n-hexyl- and tetra-n-octylammonium, unexpectedly exhibited reactivity with the same stoichiomeuy as tetramethyl- and tetra-n-butyl- ammonium cations.

Impedance Spectroscopy Present work, using the impedance spec-

troscopy technique (14) with a Pt cathode, has led us to expect that there are four zones of progres- sive reactivity from the bulk of the metal to the liquid interface corresponding to: [i] pure Pt [ii] a zone of electrical resistance due to the inser- tion of immobilised cations [iii] a strongly perturbed layer through which the ions may migrate and [iv] a very porous layer with pores and channels of high ionic capacity through which ions can move. These preliminary results should, of course, be ver- ified by using a larger range of electrolytes, but they undoubtedly confirm the progressive chemical change of the Pt interface during the reduction.

Electrogenerated Phases as Reducing Reagents The new phases obtained with Pt and Pd were

shown to be reducing reagents, acting by electron transfer. For example, the phase obtained in the

Pkatiinum Metah b., 2002, 46, (3) 102

were obtained qfiev total satiir-ution ojthe elect~iii~i~positrrt and correspond in each case to at least 5 experiments(seetextIqwr- The average thickness o/di/fi?r.enf Pd 1uver.s 11'u.v 10 iini < d , 200 nm . Values of x (coulometric analysis and y (EQCM technique technique

Table IV

Platinum Cathodes in the Presence of 0.1 M Electrolyte MX/DMF Solutions.

Electrolyte, MX

Lil Nal KI Csl Bu~NI B u ~ N B F ~ Bu4NC104

Applied potential, V

-2.82 -2.12 -2.22 -2.32 -2.80 -2.80 -2.80

x Value y Value

2.05 i 0.1 1 2.07 i 0.07 1.87 i 0.05 2.05 f 0.07 2.06 i 0.09 4.04 f 0.05 3.96 ?r 0.06

1.06 f 0.06 0.98 i 0.08 0.97 2 0.08 0.88 2 0.05 0.95 f 0.06 1.05 f 0.09 1.06 f 0.05

presence of N d on Pt could efficiently reduce ex anion. For example, 2,4-dinitrotoluene could be ~ 7 2 ~ a large range of 7c-acceptors. When the elec- reduced by ptz-, Na', NaI]. Thus, in Figure 8 a tron transfer rate is high enough (E" of a dark blue-green radical anion progressively x-acceptor > -1.5 V), a specific strong colour appears. ESR experiments confirmed that this appears at the metal interface due to the radical paramagnetic species had been obtained.

Simulations confmed that in most cases radical anions were produced.

Discussion As shown in prior work on chemical and elec-

trochemical reduction of meta-metals, such as Pb, Sn, Sb ... (15,16), alkaline cations and tetraalkylam- monium cations can react cathodically with transition metals, such as Pt and Pd (17, 18). However the reactions described here for Pt and Pd appear to be totally original since the elec- trolyte itself (its anion probably acting as a donor) is specifically involved in building well-organised phases. Thus, with electrolyte MX, cathodic inser- tion of M' and X- can occur together and the nature of the insertion process has been firmly established as corresponding to phases of general formula:

In most cases, n = m = 2, except for salts hav- ing a 'lar@' anion with diffuse charge, such as BF4-

Fig. 8 Eleciron trunS/2r reductiott of 2,4-ditiit,otoluette ( I 0 ' M t') di~sssolved in deaerated DMF (progressive appearance of a radical onion) bv cottiact with platinzrm h+rehand cathodirallv modi/ied in the pre.yence o/' Nal. Total amount qj' f'electricih~ u ~ a s 45 coulomlis (u/ter electrolwis. the platinum was ccrrqfitlli~ rinsed in

and C104-. Surprisingly, PF6 (bipyramidal struc- m e ) did not show any insertion - its rate of

dio,~l.gen-/bcv DMF) insertion would probably be too slow to be

Phtinum Metah Rev., 2002,46, (3) 103

The average thickness o/di/fi?r.enf Pt layers was 5 nm < d <200 nm. Coulometric (parameter x) and Eqcm data (parametery)reelated to electrogenerated phases [Pt-2, M+, yMX] formed byt cathodic reaction of MX after totlal redction (see text) rIqwr-

[Pt;, M', hK] and [Pd,,-, M', MX]

Reduction of Thin Platinum Layers

noticed during the voltammetric studies and the charge/&scharge processes described in this review. However, phase stoichiometry does not depend on the size and nature of the cation since similar results have been observed whatever M’ is. Thus it remains astonishmg that cations of tetra-n- octylammonium and tetramethylammonium led experimentally to the same stoichiometry. All these new electrochemically generated phas-

es were obtained by slow (M’ = Li’, Na’, K’, Cst) and even extremely slow @I+ = Rs\T’) cathodic interface modification in a depth of at least several hundred run. At present, the intrinsic values of the Eos of the electtogenerated phases have not been precisely determined, but for alkaline salts are expected to be in the range -2.0 V < Eo < -1.5 V. This potential range has been roughly determined by the reactivity of the reduced phase toward n- acceptors of known E”. For this, the variation in the Qa value, determined by coulometry in the absence and presence of a n-acceptor, was of great help.

The reducing power of the phases, pt;, M’, Mx] and [Pd,,-, M+, Mx], towards dioxygen appears to be the main cause of their instability. Oxidation by air (as abundantly shown) could be the cause of the tremendous structural changes. These structural changes may be of limited nature (for example, greater roughness of the interface with microdendrites or well-formed fractals which may resemble a kind of ‘electrochemical recrystalli- sation’), or they could be of ‘earthquake-type’ change (a sudden large-scale decomposition of the phase). In the latter case, ‘collapse processes’ could force the inserted electrolyte to leave the material through the channels and grain boundaries offer- ing the hghest flux.

Preliminary data obtained by impedance spec- troscopy suggest that the cathodic ‘corrosion’ of Pt and Pd implies the growth of a layer which pos- sesses dramatically diminished electronic conductivity, compared to pure metal, and which brings specific inhibition phenomena towards reducible species when the metals are used as elec- trode materials.

Experimentally, layers with thicknesses up to a micrometre and tens of micrometres are expected

to be attained for large amounts of electricity (say - 20 to 200 C cm-’1. The increasing thickness of this ionically conducting layer could be responsible for the progressive slowing of purely interfacial reactions. More specifically such layers may be eas- ily used as electrode modifiers in which M+ and X- can be changed in a high variety.

Conclusion Electrochemistry is a powerful tool to specifi-

cally modify interfaces of noble metals, such as Pt and Pd, in the presence of salts. New types of phases (resembling those of Zintl but with a spe- cific insertion of electrolytes) are formed and their oxidation by &oxygen leads to tremendous changes in the metal surface. The reactivity of these reduced phases towards the n-acceptors affords new organometallic layers (19) allowing the the inbeddmg of organic species into the metallic phase. However, more experiments are necessary to achieve better understandmg and applications (ii the field of sensors and catalysis) of the strange behaviour of noble metals used as cathodes in super-dry conditions.

Acknowledgments The authors wish to thank the University of Rennes and the

CNRS for financial support. They ace particularly grateful to Mr Le Lannic for his efficient collaboration in the SEM expethenu.

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References J. D. Corbeq C h . b., 1985,85,383 H. Schifri and B. Eisenmann, Rw. I n q . Chem., 1981,3,29 “Chemistry, Structure, and Bonding of Zintl Phases and Ions”, ed. S. M. Kauzlarich, VCH Publishers, New York, 1996 R Nesper, Pmg. SolidSkrtc Cbsm., 1990,20, 1 H. Schafer, B. Eisenmann and W. Miiller, hp. Cbm., Ink Ed Engl., 1973,12,694 S. M. Kauzlatich, Comments I-. Cbsm., 1990,10,75 C. Belin and M. Tillard-Charbonnel, Coo& Cbm. Rm., 1998,178,529 E. Zintl, J. Gourbeau and W. Dullenkopf, Z. Pby. C h . , Abt. A, 1931,154, 1 J. B. Chlistunoff and J. J. Lagowski,J. PLp. Chm. B, 1997,101,2867 J. B. Chlistunoff and J. J. Lagowski,]. PLy. Cbm. B, 1998,102,5800 J. Simonet, Y. Astier and C. Dano, J. Elechund. Cbm., 1998,451,5

Plafznum Metals Rm., 2002.46, (3) 104

12 J. Simonet and C. Cougnon, J. Ekchanal. Cbem., 2001,507,226

13 J. Simonet and C. Cougnon, Ekctmcbem. Commun., 2002, in press

14 J. Simonet, J. Tanguy and C. Cougnon, unpublished research

15 E. Kariv-Miller, P. D. Christian and V. Svetlicic, Lngmuir, 1995,11, 1817

16 M. M. Filder, V. Svetlicic and E. Keriv-Miller, J. Ekcbvanal Cbem., 1993,360,221

17 J. Simonet and C. Cougnon, Ekcbvcbem. Commun., 2001,3,209

18 J. Simonet, E. Labaume and J. Rault-Berthelot,

19 J. Simonet and C. Cougnon, J. Ekcfmand. Cbem.,

The Authors Jacques Simonet is Professor of Electrochemisty at the Laboratoire d'Electrochimie Moleculaire et Macromoleculaire at the Universite de Rennes. His main interests are in organic synthesis, electrocatalysis, electron transfer and conducting organic and inorganic materials.

Charles Cougnon is a Ph.0. student in the same laboratory. His main interest is the cathodic behaviour of transition metals under super-dry conditions

Ehdmcbem. Commun., 1999,1,252

2002, in press

Nanocrystalline Ruthenium Supercapacitor Material Electrochemical supercapacitors (1) can be power

sources for a large range of equipment. They store large quantities of charge and can be repeatedly charged/discharged either quickly or slowly over tens of thousands of cycles. The high energy density is due to an electric double layer, while the rapid charge/dis- charge ability comes from being made of materials having either rectangular capacitive cyclic voltammo- g r a m s (CVs) or pseudocapacitive behaviour, such as h g h surface-area activated carbons or conducting polymers. Both can be used in aqueous or nonaque- ous media.

Ruthenium oxide has a very high charge storage capacity when used in aqueous solutions. In attempts to optimise its capacitive properties, prior work has looked at the hydration of Ru oxide, its crystallinity and particle size. As Ru is costly, other elements have often been mixed with it.

Now scientists ftom the Universiti du Quibec i Montrkal, INRS-Energie et Matiriaux and Institut de recherche #Hydro-Quebec, Canada, have produced nanocrystalline TixFeyRu70. powders by b d - m i h g , and have examined their structures and electrochem- ical characteristics (P. Soudan, J. Gaudet, D. Guay, D. Bdanger and R. Schulz, Gem. Muter., 2002, 14, (3). 1210-1215). When the O T i ratio was > 1, the Ru atoms were in an hexagonal phase. Electrodes made from the powders had an increase in capacitance from - 5 to - 50 F g-' on cycling in H2S04 or NaOH - due to the growth and modification of a surface layer, RUO~'XH~O. When 0:Ti < 1, Ru was in a cubic phase and after cycling or (preferably) leaving the electrode in 1 M NaOH, the maximum capacitance was near 50 to 60 F g-', also due to the growth and modification of a surface oxide layer.

The Ru material produced by ball-rmlling had a specific surface area of only a few m2 g-' and agglom- erated into larger grains, so reducing the electro- chemical surface area. However, a leaching process

(milling the nanocrystalline material with Al, then removing the Al) increased the specific area 10-fold. CVs of this nanocrystalline TizFeRuOz in NaOH had 110 F g-' capacitance, even when returned to HzS04.

It is concluded that the crucial factors for improv- ing the capacitance of Ru-containing materials are: the electrochemical behaviour of the mamces used to dis- solve the Ru and preventing the Ru agglomeration.

Reference 1 S. Trasatti and P. Kurmeil, P h h m Metah Rev.,

1994,38, (2), 46

3D Platinum Nanoparticle Networks Assemblmg nanosized metal particles from their

basic components, such as colloids or molecules, into ordered arrays is believed to offer ways of creating new nanostructured materials with properties differ- ent to the bulk material. For instance, adding a few hundred atoms or less to electronic or optical devices, allows a single particle to show quantum size effects. A number of nanocrystal superlattices and colloidal networks have already been produced, but the 'bot- tom-up' preparation of nanoparticles to give three- dimensional (3D) structures remains a challenge.

Now a team in Germany have produced nano- structured metal/organic networks by cross-linking Al-organic-stabilised Pt nanoparticles with bifunc- tional organic spacer molecules (H. BoMemaM, N. Waldofner, H.-G. Haubold and T. Vad, Cbem. Mufm., 2002,14, (3), 11 15-1120). Pt panicles were formed by reacting Pt(II) acetylacetonate with Al(CH& at 60°C in Ar, forming an air-sensitive Pt colloid with a Pt:Al ratio of - 1:2. The Pt is thought to be surrounded by a highly reactive protective shell which allows protonolytic chemical reactions to occur. The col- loidal panicles can be cross-linked by bifunctional alcohols. Incorporating spacer molecules increases the interparticle distance. This method may thus form the basis for preparing highly ordered networks.

Plarinrm Metalr Rev., 2002, 46, (3) 105