Cathodic Reactivity of Platinum and Palladium
`in Electrolytes in Superdry Conditions
`
`THE NATURE OF THICK LAYERS ELECTROCHEMICALLY FORMED
`AT THE METAL/ELECTROLYTE INTERFACE
`
`By Charles Cougnon and Jacques Simonet"
`Laboratoire d’Electrochimie Mol~culaire et Macromel~culaire, UMR 6510, Campus de Beaulieu, Universit~ de Rennes,
`35042 Rennes Cedex, France, "E-mail: jacques.simonet@univ-rennesl.fr
`
`Platinum and palladium behave in an unexpected manner when cathodically polar&ed in
`
`the presence of electrolytes dissolved in carefully dried polar organic solvents, such as
`
`N,N-dimethylJbrmamide. A reductive layer is formed on the metal, the thickness of which
`
`depends on the amount of electricity consumed during the course of the electrolyses. Although
`
`this reaction seems to be of a general character with most of the common electrolytes, in
`
`this paper we will focus on results obtained with a large palette o.f tetraalkylammonium
`
`salts and alkali metal iodides.
`
`The reaction of the electropositive alkali metals
`
`with a large number of more electronegative main
`
`group "recta-metals’ (such as Pb, Si or Ge) to form
`
`the so-called Zinfl phases (1-3) was discovered by
`
`Eduard Zinfl at the beginning of the twentieth cen-
`
`tury. Zinfl 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 Zinfl 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,
`
`Zind 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-dectroac-
`
`rive cations, such as potassium (K), could be
`
`inserted, as shown next (9, 10):
`
`4K* + ate- + 9Pb (cathode) <--> [4K+, Pb,~*-]
`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 widdy
`used as cathode materials. However, thei~ weak
`hydrogert overvoltage obviously reduces the usable
`cathodic range (in non-aqueous solvents without
`careful prior-drying treatment, the usable cathodic
`range is limited to about -2.0 V vs. a saturated
`calomd dectrode (SCE)). If the residual moisture
`in the polar aprotic solvent is drastically reduced
`(for example by an in sit~ 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 behaviout
`of Pt in an electrolyte of dry N,N-dimethylfor-
`mamide (DMF) in the presence of tetraalkyl-
`ammonium tetrafluoroborate, RaNBFa, were the
`first to show the electrochemical construction of a
`thick layer on the Pt surface comprising both R4N÷
`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
`
`Platinum Metals !Lev., 2002, 46, (3), 94-105
`
`94
`
`Lupin Ex. 1055 (Page 1 of 12)
`
`

`

`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 desctibes results obtained for Pd
`
`and Pt cathodes in the presence of a wide range of
`
`tetraalk~lammonium salts, R,NX, 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
`
`rnicrobalance technique, chronopotentiometry,
`
`SEM analysis and impedance spectroscopy were
`
`used. Until now the rather poor chemical stability
`
`of the layers did not allow X-ray charactetisation
`
`of their structure.
`
`Experimental Procedure
`Salts and Solvent
`
`INBu4 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 ~m), then by diamond powder (6 and 3
`Finally, the working electrode was rinsed
`ethanol and acetone and dried. Between each scan
`the electrode surface was repolished with diamond
`powder (3
`For macroelectrolysis investigations, Pt sheets
`(99.99% purity, area 1 cm2, thickness 0.05 mm)
`and Pd sheets (99.95% purity, area 1 cm2, thick-
`ness 0.1 ram) were used. Theywere used once only
`for SEM analysis without further treatment.
`
`Chronocoulometric Investigations on a
`Thin Metallic Layer
`
`Coulometric experiments were carried out on
`
`In most of the experiments, electrolyte concen-
`
`Pt and Pd film dectrodes prepared, respectively,
`
`tration was 0.1 M. Potassium, lithium (Li), Na and
`caesium iodides were used and the tetraalkylam-
`
`by depositing the metals from solutions of 10 g 1-1
`
`H2PtC16 in 0.1 M HCI and 10 g 1-I PdCI2 in 0.1 M
`
`monium salts were of > 99.7% purity (puriss
`
`HC1 onto polished gold disks (2 x 10-3 cm2). The
`
`grade). All salts were used without further purifi-
`
`plating was carried out in a galvanostafic mode
`
`cation after being thoroughly dried urtder vacuu_m
`at 100°C for 48 hours. The DMF was checked (by
`
`(current 10-2 A cm-Z). All the experiments were
`
`performed with gold substrates but with different
`
`the Karl Fischer method) to ascertain it contained
`
`thicknesses of deposited metal. The gold substrate
`
`less than 50 ppm of water, having been stored
`
`could be a gold microelectrode, polished before
`
`over neutral alumina, previously activated under
`
`each deposition, or (for EQCM experiments) a
`
`vacuum at 300°C for 4 hours. All experiments
`
`larger electrode of gold-coated quartz crystals.
`
`were performed in a carefully dried argon atmos-
`
`phere. Electrolyte solutions were maintained in the
`electrochemical cell over activated alumina.
`
`Electrochemical Quartz Crystal
`
`Microbalance (EQCM) Instrumentation
`
`Simultaneous voltammetric and mass balance
`
`Electrochemical Instrumentation and
`
`experiments were carried out with an oscillator
`
`Procedures
`
`Cyclic voltammetric investigations were carried
`
`module quartz crystal analyser connected to a
`potentiostat. This device was computer controlled.
`
`out in a standard three-electrode cell usJ_txg a
`~AUTOLAB potendostat connected to a comput-
`
`In the experiments, 9 MJ-Iz AT-cut gold-coated
`
`quartz crystals were plated electrochemically with
`
`er equipped with standard electrochemical system
`software. For analytical purposes the working dec-
`
`a thin film of Pt or Pd. Plating was achieved by the
`
`chronocoulometric procedure. The deposited
`
`trode was a disk of Pt or Pd (area 8 × 10 s cm2) and
`
`mass was checked by the EQCM. The EQCM
`
`the counter electrode was a glassy carbon rod. All
`
`measurements were performed in a Teflon cell
`
`potentials given here refer to the aqueous SCE.
`
`equipped with a glassy carbon counter electrode, a
`
`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/0.1 M
`
`reference dectrode and the Pt- or Pd-plared quartz
`crystal working electrode. The apparent area of the
`
`quartz crystal was about 0.2 cmz.
`
`Platinum Metals Rev., 2002, 46, (3)
`
`95
`
`Lupin Ex. 1055 (Page 2 of 12)
`
`

`

`Table I
`
`Potentials (Epc & Epa) and Peak Currents (/pc & /pa) 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
`
`Electrolyte,
`
`0.1 M
`
`Lil
`Nal
`KI
`Csl
`Bu4NI
`Bu,NBF4
`Bu4NCI04
`
`Epc,
`
`V
`
`-3.O7
`-2.07
`-2.12
`-2.22
`-2.84
`-2.87
`-2.82
`
`Ipc,
`pA
`
`28
`16.8
`15
`8.2
`13
`17
`28
`
`log/pc/log v la)
`
`0.54
`0.39
`0.45
`0.42
`0.52
`0.45
`0.56
`
`Epa,
`
`V
`
`-2.2O
`-1.13
`-0.82
`-0.78
`-0.77
`-0.72
`-0.70
`
`/pa,
`pA
`
`19.0
`14.0
`8.0
`8.2
`2.3
`3.0
`4.5
`
`Ipa/]pc
`
`Epc - Epa,
`
`V
`
`0.87
`0.94
`1.30
`1.44
`1.51
`2.15
`2.12
`
`0.68
`0.83
`0.53
`1.00
`0.18
`0.17
`0.16
`
`Voltammetric
`data are relative to a stationary palladium electrode of area 8 × 10 3 cm2.
`
`Potentials are r~[brred to the SCE. The sweep rate, v = dV/dt is 200 m V s 1.
`The slope for sweep rates lhom 0.02 to 5 V s-l
`
`Microgravimetfic data, reported in terms of
`
`mass change, were calculated using the Sauerbrey
`
`peak (or wave in the case of ammonium ions) of
`current, !pc, which is always associated with an
`
`equation, which links the resonant frequency and
`
`anodic step peak of current, Ipa. The anodic step
`
`mass change. Fox most cases it was found that the
`
`peak corresponds to the oxidation of the mated-
`
`mass discharge of the Pt (or Pd) film at 0 V was
`
`al(s) formed while held at the level of the cathodic
`
`not completely reversible and thus, a small excess
`
`step O?ables I and Ii).
`
`of mass remained at the start of each experiment.
`
`A Pd microcathode, with only CsI (0A M’) in
`
`However, the amount of extra mass gained during
`
`dry DMF (carefully maintained over neutral alumi-
`
`the charge process remained the same. These
`
`na in si~) displayed an irreversible step, lr~, at -2.22
`
`experiments suggested that the EQCM technique
`
`V. Holding the potential at this level (-2.22 V)
`
`was an accurate method for quanti~ing the
`
`allowed the sharp anodic peak to increase, at Er~ =
`
`charge/discharge process as there was no notice-
`
`-0.78V (Figure 1). In all cases, whatever the cath-
`
`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 ~5_~ased using an alcohol/acetone mixture in an
`ultrasound bath for 2 h) were analysed by a scan-
`
`ning electron microscope.
`
`Results
`Voltammetric Data
`
`A.I1 the monovalent cations (Li+, Na+, K+, Cs+ as
`
`well as R.N+ - regardless of the n-alkyl chain
`
`length) when associated to anions (halides, tetra-
`
`fluoroborate or perchlorate) displayed, at the Pt
`
`and Pd microelectrodes, an irreversible cathodic
`
`ode matet~l and electrolyte, the cathodic currents
`
`E~voRS -2
`
`Fig. 1 Typical voltammetric behaviour of a O. 1 M
`Csl/DMF solution in contact with a stationary palladium
`microcathode (sweep rate. 200 m V s-t)
`
`Platinum Metals R~., 2002, 46, (3)
`
`96
`
`Lupin Ex. 1055 (Page 3 of 12)
`
`

`

`Table II
`Potentials (Epc and Epa) and Peak Currents (/pc) 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
`
`Lil
`Nal
`KI
`Csl
`Bu4NI
`Bu4NBF4
`Bu~NClO,
`
`E~c,
`V
`
`-2.84
`-2.07
`-2.17
`-2.18
`-2.89
`-2.91
`-2.84
`
`/~c*a),
`pA
`
`-5
`-10
`-6.4
`-5.4
`-11
`-12.5
`-25
`
`Epa,
`V
`
`-2.42
`-1.72
`-1.62
`-1.77
`-1.27
`-1.09
`-0.99
`
`E,c - Epa,
`V
`
`0.42
`0.35
`0.55
`0.41
`1.62
`1.82
`1.85
`
`Voltammetrie
`
`data are relative to a stationary platinum cathode 9f area 8 x 10 3 cm2.
`
`"
`The sweep rate, v, ts 0.2 V s .
`Potentials are referred to the SCE.
`ta) The relationship lpc vs. v1/2 was found to be linear in all cases in the range 0.02 to 5 V s-! (correlation coefficient is always _> 0.99)
`
`were found to vary linearly with the square root of
`the scan rate up to 5 V s ! indicad_,zg 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 alumi-
`na is progressively added to the voltammetric 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 all,,~li iodides, the
`currents follow the order:
`
`Cs÷ < K+ < Na+ << Li+
`
`Surprisingly, tetraalkylammonium cations dis-
`played quite large peak currents (Ipc values in
`Tables I and II).
`To find out what role any residual Water played,
`the system was dosed with water in amounts over
`200-500 ppm. The specific redox system disap-
`peared and was replaced by a cathodic wall
`(strong cathodic current) corresponding to water
`reduction.
`
`Fixed potentials were set very dose to the corre-
`sponding peak potentials or at the beginrdng of
`the plateau region for ~,ave-shaped’ steps. The
`amount of electricity involved was somewhat larg-
`
`er than 20 C cm-2. 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 ~hite (or light
`
`grey) areas became progressively more dominant,
`
`with time (see Figure 2(b)). 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 Bu2qI 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 longer than
`
`one day) the metal structure would change dramat-
`
`SEM Analysis and Microelectrolyses
`
`ically, see Figure 3(a), which shows very regular
`
`Potentiostatic macroelectrolyses were per-
`
`formed on as-received polyctyst~lline Pt and Pd
`
`sheets (see Figure 2(a) for as-received Pd sheet).
`
`fractals for Pd with angles very dose to 45° and 90°.
`This Pd surface corresponds to pure metal aRer the
`total oxidation of the electrochemically-built layer.
`
`PlaZin~t~ Metals Rtv., 2002, 46, (3)
`
`97
`
`Lupin Ex. 1055 (Page 4 of 12)
`
`

`

`Fig. 2 SEMs qipalladium sheet at
`two magn(fications:
`(a) as-received
`(b) cathodica!~v trealed in O. I M
`Btt: NI in DMF Ov potentiostatic
`electrolysis. Potentiah 2.6 E
`mnotmt ~tfelectriciO’: 130 C cm :.
`(c) Probe ~/’mtage (b; (treated
`palladium). The white :ones (top
`scan) shows on(v p.lladium metal
`while the dark gr~v 2ottes Uower
`scan) also t~rhibit the presettce
`~[carbon and iodine. Note the
`
`th/vt/g]t the thtrk zo!tes"
`
`cps
`
`aO0!
`
`Pd
`
`200"~ [
`
`0
`
`2
`
`4
`
`6
`
`8
`
`30(
`
`c
`
`10,
`
`Pd
`
`I
`
`0
`
`2
`
`4
`ENERGY, keY
`
`Alkali metal iodides may also cause specific and
`
`etry since a large part of the electricity stored dur-
`
`dramadc changes to the Pd and Pt cathode sur-
`
`hag the cathodic processes should be able to be
`
`faces, see Figures 4 and 5, respectively.
`
`anodically restored. However, the charge/~s-
`
`Coulometric Data
`
`charge phenomenon was not found to be totally
`reversible since the solvent could not be made
`
`The voltammetric results and the instability of
`
`totally anhydrous and free of acidic impurities -
`
`the electrochemically-formed layer strongly sug-
`
`some hydrogen evolution occurred (in yields of 65
`
`gest that a charge/discharge process is taking
`
`to 75% depending on the salt used).
`
`place. This assumption can be checked by coulom
`
`it has been suggested that the electric charge
`
`Platinum Metals Rev., 2002, 46, (3)
`
`98
`
`Lupin Ex. 1055 (Page 5 of 12)
`
`

`

`Fig. 3 SEMs qla palladium sheet (a) qfier cathodic treatment in DMF containing O. 1 M Btt4NCl.[bllowed by long
`contact with air. Redttction potential." -2.8 E Amount o.]electricity: 250 C em " For comparison, (b) shows the
`strttcture ~f the Pd sheet before reduction. Both images have the same magn([?cation
`
`Fig. 4 SEMs ~f palladium sheets ~([~er cathodic treatment in O. 1 M alkali iodide in DMF and then contact with air for
`at least 2 hours. The images have the same magn~]Tcation.
`(a) Aspect of as-received commer.cial palladittm sheet (without art), cathodic treatmenO.
`(b) Microdendr.ite,[m’mation c~fter polarisation of the palladium sheet at -2.3 V in C~I (amotmt e?]electriciO’: 25 C cm ’),
`(c) Stoface strttctttr.e reot~ganisation c~]~er cathodic tr.eatment in Nal at -2.1 V (amount of electrici(v: 24 C cm-").
`(d) After cathodic polarisation at -3 V in Lil (amount ¢~[’electricity: 27 C cm-’)
`
`Platinum Metal~ Rev., 2002, 46, (3)
`
`99
`
`Lupin Ex. 1055 (Page 6 of 12)
`
`

`

`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 magnifications are all similar
`(a) As-reeeived commercial platinum sheet,
`
`(b) Structural reorganisation after cathodic polarisation at 2. 3 V with O, 1 M Csl (amount of electricity: 27 C cm ~.
`(c) Morphological changes of a platinum sheet electrolysed at -2.1 V with O. 1 M Nal (amount of electrici(y." 28 C cm ").
`The surface exhibits grain boundaries and tunnel ends (in black)from which Nal mieroerystals emerge.
`(d) Special feature (platinum at the interface air/electrolyte) obtained by cathodic polarisation at -2.8 V in O. 1 M Lil
`(amount of electricity: 19 C cm-z)
`
`stored in the material (Q= 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 taking part in the reduction process, Pd and
`Pt were electrochemically deposited onto dec-
`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, Q~ 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 BF4 and
`C104=).
`From the ratio Q~/Qc (ratio of amounts of elec-
`tricity involved in the charge/discharge process)
`the yidd of the charge/discharge process was
`found to be from 55 to 70%. Another way to
`
`Piatingra Metals Re~., 2002, 46, (3)
`
`100
`
`Lupin Ex. 1055 (Page 7 of 12)
`
`

`

`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-dee-
`
`r_rode cell with 30-50 coulombs of electricity.
`
`16"
`
`After careful rinsing in clty 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 dec-
`
`tron is equivalent to a basic charge, the reaction
`
`TIME, s
`
`with water should be:
`
`ePt- + H20 -~ ½Hz? + OFF
`
`OH- species in the solution were titrated with 10-z
`
`M HCI in the presence of phenolphthalein.
`
`¢
`
`B
`
`rapt deposited~ Pg
`
`Fig. 7 StoichiometO’ of CsllPd modified layers using the
`EQCM technique.
`(a) Mass increase during polarisation of palladium
`(electmdeposited onto a gold-coated qaartz co,stuD for
`d(~brent amounts of mpa at-2.3 V in 0.1 M Csl + DMF,"
`mpa: A blank: B 5 pg: C 7 pg; D 10.5 pg.
`(b) Ea~erimental relationship be~veen maximum mass
`inctzase (saturation of the lqveO and amount ¢?/"
`palladium 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 carefially
`
`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. ]at coulometty, metal layers were dec-
`
`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/
`
`discharge cycles, of the experimental plateaux was
`
`20
`
`BO
`~0
`40
`THICKNESS~ 5, nn’,
`
`100
`
`120
`
`Fig. 6 Cathodic reactivity of a palladium .film on a gold
`subsn’ate at 2.3 V.
`(a) Chronocoulometrie response (charge during 60 s at
`~. 3 V and then dischmge at 0 V) recorded in O. 1 M Csl
`+ DMF at a palladium.film cathode electrodeposited
`onto a polished gold electrode (total palladium mass
`deposited: O. 13 pg).
`(h) Amount of electrical discharge, con’esponding to
`the pmeedure above..[br d~l]kreat thicknesses (6) qf the
`palladium ta.vet: For each experiment the average
`palladium thickness was calculated from the mass
`electrodeposited palladium, the apparent area of the
`electrode and the metal densi&
`
`Platinura Metals Rev., 2002, 46, (3)
`
`10t
`
`Lupin Ex. 1055 (Page 8 of 12)
`
`

`

`Table III
`
`Stoichiometry Determination of Reduced Palladium Phases in the Presence of Electrolytes, MX,
`for Thin Palladium Layers Electrodeposited onto Gold Substrates
`
`Electrolyte,
`
`0.1M
`
`Lil
`Nal
`KI
`Csl
`BIhNI
`Bu4NBF.
`Bu4NCI04
`
`Applied potential,
`V
`
`-3.02
`-2.12
`-2.22
`-2.32
`-2.80
`-2.80
`-2.80
`
`x Value
`
`for Pd×-M~
`
`1.95 _+ 0,05
`2.00 _+ 0.02
`1.99 ± 0.12
`2.08 ± 0.05
`2.10 ± 0.15
`3.87 _+ 0,11
`4.05 ± 0.08
`
`y Value
`
`for (Pd2-M÷, yMX)
`
`1.01 ± 0,02
`0.91 ± 0.07
`1.02 ± 0.03
`0.97 +_ 0.05
`0.92 + 0.04
`0.97 _+ 0.06
`1.02 _+ 0.09
`
`The average thickness of d([]&ent Pd layers was I0 nm -< ~ <- 200 ran. Values qf x (coutontetric amt~vsis) ~md y (EQCM technique)
`were obtained after total saturation of the electrodeposited h(ver attd cot’re,vmod in eoch ~’ase to at least 5 experiment.~ (see t~t)
`
`confirmed, although a small excess mass was
`
`for which x = 4 (surprisingly, PF6 did not display
`
`noticed at 0 V at the end of each discharge process.
`
`any noticeable cathodic reactivity with Pd or Pt
`
`This ’mass remanence’ may be due to the very slow
`
`electrodes). On the contrary, very bulky cations,
`
`rate of diffusion of electrolyte out of the metal at
`
`such as tetra-n-hexyl- and tetra-n-octylammonium,
`
`the end of the discharge. However, no loss in mass
`
`tmexpectediy exhibited reactivity with the same
`
`was observed and it was concluded that no degra-
`
`stoichiometry as tetramethyl and tetra-n-butyl-
`
`dation of the metal layer had occurred. In addition
`
`ammonium cations.
`
`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-
`
`Impedance Spectroscopy
`Present work, using the impedance spec-
`
`portional to the mass of metal deposited onto the
`
`troscopy technique (14) with a Pt cathode, has led
`
`gold substrate. Thus the coulometric and EQCM
`
`us to expect that there are four zones of progres-
`
`experiments allow us to conclude that both cations
`
`sive reactivity from the bulk of the metal to the
`
`and anions of the electrolyte MX are involved in
`
`liquid interface correspondiag to:
`
`the charging process. The electrogenerated plat-
`
`[i] pure Pt
`
`inum phase and palladium phase should have the
`
`[h’] a zone of electrical resistance due to the inser-
`
`respective stoichiometries:
`
`tion of immobilised cations
`
`[iii] a strongly perturbed layer through which the
`
`ions may migrate and
`
`The values of x and y, assumed to be whole num-
`
`[iv] a very porous layer with pores and channels of
`
`bers, were determined in a large number of
`
`high ionic capacity through which ions can move.
`
`expetfments, and obtained with reladve error of 5
`
`These preliminary results should, of course, be ver-
`
`per cent - probably of the same order of accuracy
`
`ified by using a larger range of electrolytes, but they
`
`as the EQCM tecb_nique, see Tables llI and IV. In
`all cases, it was found that two cations were insert-
`
`undoubtedly confirm the progressive chemical
`
`change of the Pt interface during the reduction.
`
`ed for each anion.
`
`The only difference between all the experi-
`ments lies in the stoichiometry of the metal atom.
`
`Electrogenerated Phases as Reducing Reagents
`
`The new phases obtained with Pt and Pd were
`
`In most cases x = 2, except for the electrolytes
`
`with large, bulky anions, such as BFg and C10,~
`
`shown to be reducing reagents, acting by electron
`transfer. For example, the phase obtained in the
`
`Platinum Metals Rev., 2002, 46, (3)
`
`102
`
`Lupin Ex. 1055 (Page 9 of 12)
`
`

`

`Table iV
`
`Platinum Cathodes in the Presence of 0.1 M Electrolyte MX/DMF Solutions.
`Reduction of Thin Platinum Layers
`
`Electrolyte,
`
`MX
`
`Lil
`Nal
`KI
`Csl
`Bu4NI
`Bu4NBF4
`Bu4NCI04
`
`Applied potential,
`V
`
`x Value
`
`y Value
`
`-2.82
`-2.12
`-2,22
`-2.32
`-2.80
`-2.80
`-2.80
`
`2.05 ± 0.11
`2.07 ± 0.07
`1.87 _+ 0.05
`2.05 ± 0.07
`2.06 _+ 0,09
`4.04 _+ 0.05
`3.96 ± 0.06
`
`1.06 _+ 0.06
`0,98 ± 0.08
`0.97 ± 0.08
`0.88 _+ 0.05
`0.95 +_ 0.06
`1.05 + 0,09
`1.06 _+ 0.05
`
`The average thickne.ss o.f d~ffetvnt Pt la.w,tw wcis 5 nm < ~ < 200 nm, Uoulomutric ¢parmneter x) and EQCM data qmrameter.v)
`related to electrogenerated phase.~ [Pt~, M +, ~, M.¥] [brined by cathodic reaction qf MX after total ~vduetion (see tuxt)
`
`presence of NaI on Pt could efficiently reduce ex
`
`anion. For example, 2,4-dinitrotoluene could be
`
`situ a large range of ~-acceptors. When the elec-
`
`reduced by [Pt~-, Na÷, Nail. Thus, in Figure 8 a
`
`tron transfer rate is high enough (E° of a
`
`dark blue-green radical anion progressively
`
`~-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 conftrmed 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 tetxaalkylam-
`
`monium cations can react cathodicaliy 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:
`
`and [Pd,., M+, MX]
`
`Fig. 8 Electron tran~f’er reduction of 2, 4-dinitrotoluene
`(10 : M U) dissolved in deaerated DMF (progressive
`appearance of a radical anion) by contact with platinum
`be.forehand cathodically modifie~t in the presence
`NaI. Total amount of electricity was 45 coulombs (~[~er
`
`electrolysis, the platinum was carcf]blly rinsed in
`diox.vgenzfree DMF)
`
`In most cases, n = m = 2, except for salts haw
`
`ing a ’large’ anion with diffuse charge, such as BF4-
`
`and CIOo-. Surprisingly, PF~, (bipyramidal struc
`ture) did not show any insertion - its rate of
`
`insertion would probably be too slow to be
`
`Platinum Meta& Rt’u., 2002, 46, (3)
`
`103
`
`Lupin Ex. 1055 (Page 10 of 12)
`
`

`

`noticed during the voltammetric studies and the
`charge/discharge processes described in this
`review. However, phase stoichiometty does not
`depend on the size and nature of the cation since
`similar results have been obserced whatever M+ is.
`Thus it remains astonishing that cations of tetra-n-
`octylammonium a~d tetramethylammonium led
`experimentally to the same stoichiometry.
`All these new electrochemically generated phas-
`es were obtained by slow (M+ = Li÷, Na+, K+, Cs÷)
`and even extremely slow (M+ = R,N+) cathodic
`interface modification in a depth of at least several
`hundred nm. At present, the intrinsic values of the
`E°s of the eleetrogenerated phases have not been
`precisely determined, but for alkaline salts are
`expected to be in the range -2.0 V < E° < -1.5 V.
`This potential range has been roughly determined
`by the reactivity of the reduced phase toward ~-
`acceptors of knovm E°. For this, the variation in
`the Q, 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 [Pdd, M÷, Iv[X], 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-
`sadon3, or they could be of ’eardiquake-type"
`change (a sudden large-scale decomposition of the
`phase). In the htter case, ’collapse processes’ could
`force the inserted dectrolyte to leave the material
`through the channels and grain boundaries offer-
`mg the highest 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
`redudble species when the metals are used as dec-
`trode materials.
`Experimentally, layers with thicknesses up to a
`micrometre and tens of micrometres are expected
`
`to be attained for large amounts of electridty (say
`~ 20 to 200 C cm 2). The increasing thickness of
`this ionically conduct~tg layer could be responsible
`for the progressive slowing of purely interfaeial
`reactions. More spedfically 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 Zinfl but with a spe-
`cific insertion of electrolytes) are formed and their
`oxidation by dioxygen leads to tremendous
`changes in the metal surface. The reactivity of
`these reduced phases towards the ~r-acceptors
`affords new organometallic layers (19) allowing the
`the inbedding of organic species into the metallic
`phase. However, more experiments are necessary
`to achieve better understanding and applications
`(~m 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 are particularly grateful to Mr
`
`Le Lannic for his efficient collaboration in the SEM experiments.
`
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`
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`
`Lupin Ex. 1055 (Page 11 of 12)
`
`

`

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