Platinum Metals Rev., 2002, 46, (3), 94
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
- Jacques Simonet i
- Laboratoire d’Electrochimie Moléculaire et Macromoléculaire, UMR 6510, Campus de Beaulieu, Université de Rennes, 35042 Rennes Cedex, France
Platinum and Palladium behave in an unexpected manner when cathodically polarised in the presence of electrolytes dissolved in carefully dried polar organic solvents, such as N,N-dimethylformamide. 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 of tetraalkylammonium 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 century. 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 electrochemical 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):
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-drying 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 situ 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,N -dimefhylfor-mamide (DMF) in the presence of tetraalkyl-ammonium tetrafluoroborate, R4NBF4, 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 degradation of these layers by air led to the restoration of the Pt metal surface. However, the surface had 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, R4NX, 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.
Salts and Solvent
In most of the experiments, electrolyte concentration was 0.1 M. Potassium, lithium (Li), Na and caesium iodides were used and the tetraalkylam-monium salts were of > 99.7% purity (puriss grade). All salts were used without further purification after being thoroughty dried under vacuum at 1000C 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 3000C for 4 hours. All experiments were performed in a carefully dried argon atmosphere. Electrolyte solutions were maintained in the electrochemical Cell over activated alumina.
Electrochemical Instrumentation and Procedures
cyclic voltammetric investigations were carried out in a standard three-electrode Cell using a μAUTOLAB potentiostat connected to a computer equipped with standard electrochemical system software. For analytical purposes the working electrode was a disk of Pt or Pd (area 8 x 10-3 cm2) 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 diffusion). As reference electrode, the Ag/AgI/0.1 M 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 carbide paper of successively smaller particle size (18 to 5 μm), then by diamond powder (6 and 3 μm). Finally, the working electrode was rinsed with ethanol and acetone and dried. Between each scan the electrode surface was repolished with diamond powder (3 μm).
For macroelectrolysis investigations, Pt sheets (99.99% purity, area 1 cm2, thickness 0.0.5 mm) and Pd sheets (99.95% purity, area 1 cm2, thickness 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 1-1 H2PtCl6 in 0.1 M HCl and 10 g l-1 PdCl2 in 0.1 M HCl onto polished gold disks (2 x 10-3 cm2). The plating was carried out in a galvanostatic mode (current 10-2 A cm-2). 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 cm2.
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) film 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 quantifying the charge/discharge process as there was no noticeable loss of mass (for example, occurring by mechanical degradation of the layer in the course of the charge process).
Scanning Electron Microscopy Experiments
All the monovalent cations (Li+, Na+, K+, Cs+ as well as R4N+ — 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 peak (or wave in the case of ammonium ions) of current, I pc, which is always associated with an anodic step peak of current, I pa. The anodic step peak corresponds to the oxidation of the materi-al(s) formed While held at the level of the cathodic step (Tables I and II).
|Electrolyte, 0.1 M||Epc, V||Ipc(a), μA||Epa, V||Epc -Epa, V|
A Pd mictocathode, with only CsI (0.1 M) in dry DMF (carefully maintained over neutral alumina in situ) displayed an irreversible step, I pc, at —2.22 V. Holding the potential at this level (-2.22 V) allowed the sharp anodic peak to increase, at E pa = –0.78V (Figure 1). In all cases, whatever the cathode material and electrolyte, the cathodic currents were found to vary linearly with the square root of the scan rate up to 5 V s-1 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 alumina 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 alkali iodides, the currents follow the order:
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 disappeared and was replaced by a cathodic wall (strong cathodic current) corresponding to water reduction.
SEM Analysis and Microelectrolyses
Potentiostatic macroelectrolyses were performed 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 corresponding 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-2. In all cases a chemical transformation of the metal surfaces was observed after the sheets had been removed from the electrolysis 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(b)). The latter areas correspond to pure metal,. The black zones were shown (by a suitable probe) to contain elements such as carbon and iodine (when Bu4NI was the electrolyte, Figure 2(c)). Additionally, crystals of electrolyte were found to emerge from the disappearing 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-ically, see Figure 3(a), which shows very regular fractals 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. Alkali metal iodides may also cause specific and dramatic changes to the Pd and Pt cathode surfaces, see Figures 4 and 5, respectively.
The voltammetric results and the instability of the electrochemically-formed layer strongly suggest that a charge/discharge process is taking place. This assumption can be checked by coulometry since a large part of the electricity stored during the cathodic processes should be able to be anodically restored. However, the charge/discharge 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 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 electrochemical 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 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 presence of salts with soft anions (such as BF4 - and ClO4-
From the ratio Qa/Qc (ratio of amounts of electricity involved in the charge/discharge process) the yield of the charge/discharge process was found to be from 55 to 70%. Another way to 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 30—50 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 electron is equivalent to a basic charge, the reaction with water should be:
OH- species in the solution were titrated with 10-2 M HCl in the presence of phenolphthalein. Experimental values for the charge stored in the material were found to be in quite good agreement with the corresponding coulometric data.
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 complementary to those already performed by coulometry. In coulometry, metal layers were electrolysed 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 substrate does not react at all with the electrolyte.
Therefore, the existence, during the charge/ discharge cycles, of the experimental plateaux was confirmed, 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 degradation 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 proportional to the mass of metal deposited onto the gold substrate. Thus the coulometric and EQCM experiments allow us to conclude that both cations and anions of the electrolyte MX are involved in the charging process. The electrogenerated Platinum phase and Palladium phase should have the respective stoichiometries:
The values of x and y, assumed to be whole numbers, 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 III and IV. In all cases, it was found that two cations were inserted for each anion.
The only difference between all the experiments 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 ClO4- for which x = 4 (surprisingly, PF6 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 stoichiometry as tetramethyl- and tetra-n-butyl-ammonium cations.
Present work, using the impedance spectroscopy technique (14) with a Pt cathode, has led us to expect that there are four zones of progressive reactivity from the bulk of the metal to the liquid interface corresponding to:
a zone of electrical resistance due to the insertion of immobilised cations
a strongly perturbed layer through which the ions may migrate and
a very porous layer with pores and channels of high ionic capacity through which ions can move. These preliminary results should, of course, be verified 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 presence of NaI on Pt could efficiently reduce ex situ a large range of π-acceptors. When the electron transfer rate is high enough (E° of a π-acceptor > —1.5 V), a specific strong colour appears at the metal interface due to the radical anion. For example, 2,4-dinitrotoluene could be reduced by [Pt2-, Na+, NaI]. Thus, in Figure 8 a dark blue-green radical anion progressively appears. ESR experiments confirmed that this paramagnetic species had been obtained. Simulations confirmed that in most cases radical anions were produced.
As shown in prior work on chemical and electrochemical 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 electrolyte itself (its anion probably acting as a donor) is specifically involved in building well-organised phases. Thus, with electrolyte MX, cathodic insertion 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 having a ‘large’ anion with diffuse charge, such as BF4-and ClO4-. Surprisingly, PF6 (bipyramidal struc ture) did not show any insertion - its rate of insertion would probably be too slow to be noticed during the voltammetric studies and the charge/discharge 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 astonishing that cations of tetra-n-octykmmonium and tetramethykmmoniuni led experimentally to the same stoichiometry.
All these new electrochemically generated phases were obtained by slow (M+ = Li+, Na+, K+, Cs+) and even extremely slow (M+ = R4N+) cathodic interface modification in a depth of at least several hundred nm. At present, the intrinsic values of the E°s of the electrogenerated 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 π-acceptors of known E°. For this, the variation in the Qa value, determined by coulometry in the absence and presence of a π-acceptor, was of great help.
The reducing power of the phases, [Ptn-, M+, MX] and [Pdm- 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 offering the highest flux.
Preliminary data obtained by impedance spectroscopy suggest that the cathodic ‘corrosion’ of Pt and Pd implies the growth of a layer which possesses dramatically diminished electronic conductivity, compared to pure metal and which brings specific inhibition phenomena towards reducible species when the metals are used as electrode 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-2). 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-Uy used as electrode modifiers in which M+ and X” can be changed in a high variety.
Electrochemistry is a powerful tool to specifically 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 specific 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 π-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 (in the field of sensors and catalysis) of the strange behaviour of noble metals used as cathodes in super-dry conditions.
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Jacques Simonet is Professor of Electrochemisty at the Laboratoire d'Electrochimie Moléculaire et Macromoleculaire at the Université de Rennes. His main interests are in organic synthesis, electrocatalysis, electron transfer and conducting organic and inorganic materials.
Charles Cougnon is a Ph.D. student in the same laboratory. His main interest is the cathodic behaviour of transition metals under super-dry conditions.