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6 Charge–Discharge Processes of Platinized Platinum Layers

6 Charge–Discharge Processes of Platinized Platinum Layers

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Reduction of Platinum under Superdry Conditions

0.1 µm), the use of a superdry solvent–electrolyte allows one to obtain a quite pure

quasi-reversible step. For example, platinized platinum in 0.1 mol.L−1 TMAClO4

exhibits a pair of broad peaks whose half-peak potentials are E0.5pc = –1.90 V and

E0.5pa = –1.74 V, respectively, versus SCE. At scan rates greater than 50 mV s–1, the

areas of these two peaks are roughly the same. Coulometry, for the charge and

discharge, shows that the charge amounts are equivalent. At potentials more negative than –2.2 V versus SCE, the cathodic rise in current does not appear; reaching these very negative potentials revealed a process that strongly resembles a

self-inhibition phenomenon. It was proposed that this pure charging–discharging

process is specific to the platinized layer. Possibly, the compactness associated

with a structural change in the platinized layer creates a further control on the

movement of species taking part in this charging phenomenon. For times longer

than the duration of voltammetric experiments, the charging process depends on

the thickness of plating. With thicknesses of the order of 0.1 to 2 µm, the system

becomes quasi-reversible, with current efficiencies smaller than 60%. Figure 2.25

displays the two branches of chronocoulometric responses of platinized platinum

in 0.1 mol.L−1 TMABF4 in DMF; the very sharp slope at the beginning of the

discharge process is worth noting.



EV –2.5












20 t/s


Figure  2.25  Voltammetric responses of a platinum electrode in superdry DMF

containing 0.1 mol.L −1 TMAClO 4. Scan rate: 0.1 Vs−1. Apparent electrode area: 0.8

10 −2 cm 2. Potentials are referred to the SCE electrode. Platinized electrode (area of

platinum substrate: 0.8 10 −2 cm 2) with a plating of average thickness of δ = 0.1 μm. In

the insert, chronoamperometric curve with a reduction at –2.6 V (10 s) followed by an

oxidation at −0.5 V is shown. (Simonet, J., 2006, J. Electroanal. Chem. 93: 3. Used

with permission.)

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Electroanalytical Chemistry: A Series of Advances

Coulometric and ECQM measurements have revealed that tetramethylammonium salts behave quite differently from other TAAX salts. In fact, the obtained

phases appeared to be the following when the associated anions are ClO4−, or

BF4−: [Pt4−, TMA+, TMAX]. Under these conditions, the amount of charge inside

Pt would be significantly smaller than with the other TAAX.

2.6.2 Swelling of Platinized Layers upon Electric Charge

It was previously stressed that the insertion of ions in bulk platinum leads to

impressive changes of the surface morphology. Platinized layers deposited onto

a conducting material are of interest owing to a better permeability and a larger

specific surface than those of typical smooth platinum interfaces. Consequently,

it is relevant here to show the swelling of such platinum layers and briefly discuss

the morphology changes according the nature of the salts and the conditions of the

charge–discharge reaction. As a typical example, Figure 2.26 features the changes

of a platinum layer structure upon charging. The cathodic charge of the layer, in

particular by bulky ammonium cations, leads after oxidation (here by air) to the

formation of spherical nanostructures. In the case where the layer is thin enough,

the produced spheres are not always adjacent. Therefore, such changes show that

there is a profound modification via the reduced phase (sites of swelling) and also

discharge of these spheres. This is schematically shown in Figure 2.27.

Why a quasi-uniform size of spheres? For that, it may be proposed that these

structures grow under thermodynamic conditions. This assumption is supported

by the quasi-reversibility of the electrochemical process as depicted in Figure 2.25

with TMAX salts. The total conformational energy per sphere, Esp, may be split

into two terms corresponding to the bulk (ρv) and surface (ρs) energies, respectively, as given in the following equation:

Esp = (4/3) πR3ρv − 4πR2ρs

where R is the radius of each sphere. At equilibrium, the equation can be written as

(dE/dR)eq = 4πR2ρv − 8πRρs

Therefore, the equilibrium radius, Req, of spheres is given by

Req = 2ρs/ρv

Thus, the radii of the spheres depend only on energy factors that are specific to

the experimental conditions (such as the nature of the salt used, its concentration,

its level of dissociation, and also the amount of platinum available in the whole

deposited layer). The intriguing formation of contiguous spheres of very similar

volume suggests that the ionometallic layer (at the quasi-maximum of the charge)

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Reduction of Platinum under Superdry Conditions




Figure  2.26  Morphology change in the course of a charge–discharge process.

(a) Galvanostatically deposited platinum layer onto platinum substrate. (b) The same layer

after a charge process in 0.1 M tetra-n-hexyl ammonium bromide in DMF until saturation

and oxidation by air (Simonet, J., Unpublished results).

turns out to be very mobile on the conducting substrate. Thus, a model taking

into account the growth of swollen structures from randomly distributed activated

centers onto the substrate surface is certainly wrong. Experiments allow visualizing the occurrence of the platinum swelling. The degree of swelling in the case

of TMAX could be estimated from the phase formula [Pt4−, TMA+, TMAX]. The

example of TMAI could be considered as representative. From the values of ionic

radii for the phase constituents (Pt = 0.80 Å, I− = 2.2 Å, and TMA+ = 3.01 Å),

it is possible to assess the degree of expansion of the platinum layer. A rapid

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Electroanalytical Chemistry: A Series of Advances

Ions + Electrolyte

Pt substrate















Figure  2.27  Schematic representation of the morphology change of a Pt–Pt layer

(shown in a) during the charging process by TMAX (processes b and c), followed by

air oxidation (see d). The ionometallic layer (adjacent spheres) leads to a totally reduced

form (shown in d). The oxidation by dioxygen affords smaller spongy spheres, of which

the shrinking factor here is totally arbitrary. (Cougnon, C., and J. Simonet, 2002,

J. Electroanal. Chem. 531: 179. Used with permission.)

calculation (assumption done for closely adjacent ions in the resulting structure)

leads to a volume increase of about 30 times, which is considerable. Similar calculations confirm the swelling factors for tetrafluoroborate and perchlorate ions.

By contrast, the swelling limited to the insertion of cation alone (without that of

the salt) would appear smaller but remains theoretically important (14 times with

the iodide).

Similarly, the discharge process through chemical (in the presence of dioxygen with the concomitant diffusion of the superoxide) or electrochemical (withdrawing of inserted electrons in the structure and the simultaneous ejection of

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Reduction of Platinum under Superdry Conditions


( II )







1 µm

Figure 2.28  Progressive formation of smooth platinum surface by oxidation by air (I) or

anodic oxidation (II) of a totally reduced layer. In general, several reduction–oxidation

cycles are necessary to obtain an almost perfect annealing of the original surface. The

image (bottom) clearly shows the crushing of spheres or their corrosion (top) in the course

of the first anodic oxidation. Conditions: TMAClO4 in DMF; reduction at −2.5 V versus

SCE; oxidation at 0 V. (Cougnon, C., and J. Simonet, 2002, J. Electroanal. Chem. 531:

179. Used with permission.)

salt ions) processes is also intriguing. Indeed, the structure in layers of spheres

is maintained. It is expected that produced spheres will be affected by the

discharge due to the regeneration of the platinum metal as becoming spongy

or of smaller size. As seen in Figure  2.28, they totally collapse upon repetitive charge–discharge processes until a quasi-smooth Pt surface is obtained.

Among the remarkable morphologic changes observed with TMAX salts, one

could observe the formation of holes or caldera-like profiles. Such structures

are provoked by a sudden collapse of the spheres due to a concomitant ejection of the salt and cations (TMA+) during the reoxidation of the layer through

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Electroanalytical Chemistry: A Series of Advances

Ions Motion



charge process




500 nm


1 µm

Figure 2.29  Schematic presentation of the swelling of a platinum layer (E = −2.5 V

versus SCE) of platinized platinum in 0.1 mol.L−1 TMAI. (a) Initial surface. (b) During

the charge process. Thickness of platinum deposit: 0.2 µm. In (c) the quantity of charge

corresponds to about half of the maximum charge, and in (d) the full charge process

was achieved, which permits then to get the “cauliflower”-like structure. Oxidation by air

(Simonet, J., Unpublished results).

injection of electrons from the substrate as in the case of a chemical oxidation

(see Figures 2.29 and 2.30).

For practical applications of such structures, it is likely that such layers composed of empty spheres could present a strong surface activation. For this purpose,

the surface increase provoked by the layer swelling was estimated by coulometry

of the anodic oxidation of the platinum external layer (coverage of a monolayer of

PtO). It was established that the surface increase is large, until three to five times,

when a few repetitive charge–discharge scans are achieved. It is also worth underlining that swellings described here appear to be very similar to the observations

described in the EC-AFM experiments (see Section 2.4) performed on microsized

platinum particles. In conclusion, the swelling process is produced only during

the charge process (see, for example, Figure  2.18) and not in the course of the

layer reoxidation.

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Reduction of Platinum under Superdry Conditions







Oxidation by air









1 µm

Figure 2.30  Schematic changes of morphology provoked by electrochemical reduction of the Pt–Pt layer followed by two kinds of oxidation processes. (a) Initial surface. (b)

A reduction. It is proposed here that the swelling into adjacent spheres occurs at activated

centers at the Pt surface. The oxidation may be achieved by air (leading to spongy spheres)

(c) or by anodic oxidation which implies a specific motion of electrons (d). This difference

would afford the formation of a caldera-like morphology, which progressively vanishes

in the course of recurrent reduction–oxidation cycles (case of TMA tetrafluoroborate).

(Simonet, J., 2006, J. Electroanal. Chem. 93: 3. Used with permission.)

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Electroanalytical Chemistry: A Series of Advances

2.7 Is Platinum Really Inert toward Electrogenerated

Organic Anions and π-Acceptor-Reduced Forms?

When a platinum phase of general formula [Ptn−, M+, MX] is formed in the presence of a salt MX under the experimental conditions described earlier, several

questions arise: What are the species that could react with the platinum phase?

Any kinds of anions, cations—organic or inorganic? These questions are fundamentally important. Such reduced phases of platinum prepared in the presence of

complex salts could be used to elegantly immobilize some special bulky oligomeric anions or cations at the surface that will be definitively blocked inside the

Pt matrix at this end of the reoxidation process. Therefore, it should be possible

to use the platinum charge for the immobilization of large ions in a way similar

to the numerous procedures where the conducting organic polymers (such as, for

example, polythiophene or polypyrrole) play the role of a conductive matrix. So

far, this preliminary study was done essentially on platinum and does not totally

answer all these questions. In the present chapter, we will focus on the role of

some π-acceptors (and their electrogenerated forms) toward unreduced and

reduced platinum.

2.7.1 Reactivity of Acceptors toward Platinum -Reduced Phases

An intriguing experiment was first done with reduced platinum (prepared in the

presence of CsI), which was placed in an acenaphthenoquinone (AcQ) (E° =

−0.31 V versus SCE) solution. No diffusion of the anion radical was observed

around the reduced Pt. On the contrary, an intense blue color stays at the platinum surface. One can expect the formation of an insoluble radical-anion–cesium

salt that covers the surface. However, after rinsing the sample with acetone and

water in an ultrasonic bath, it appears that the Pt surface is deeply attacked

[52]. This preliminary experiment suggests that AcQ is reduced by the cesium–

platinum phase and that the reduced form of the acceptor reacts with the still

unreacted platinum phase. In order to simply mimic this kind of chemical route,

the concomitant reduction of the acceptor at a platinum sheet was realized under

superdry conditions with a salt such as CsI. One may see a noticeable increase

of the mass of the electrode after a sufficient charge was passed, accompanied

by a huge modification of the surface as depicted by SEM images, exemplified

in Figure 2.31.

A close examination of the surface by the SEM technique shows the presence

of large crystals of AcQ, or of one of its reduced forms, clearly embedded in

the platinum plate. Thorough rinsing by ether and acetone leads to elimination

of most of the organic acceptor. Additionally, very thin platinum needle-shaped

crystals are also formed. Such an experiment is a striking example of the attack

of the electrode material; the polarized platinum is reactive toward an organic

acceptor! It is quite reasonable to believe that the concomitant reduction of platinum and AcQ produces another platinum phase at the interface including the

AcQ −• anion-radical salt. The organic anion radical could concomitantly react

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Reduction of Platinum under Superdry Conditions











Pt Pt Cs



















Figure 2.31  Reduction of platinum in the presence of acenaphthenoquinone (10 mmol.

L−1) superdry DMF + 0.1 molL−1 CsI. Reduction potential: −2.1 V versus SCE electrode.

Amount of electricity: 25 C cm−2. Surface structure after oxidation by air and rinsing

with water and alcohol. EDS spectra show differences between the dark zones (essentially

carbon and oxygen) and light zones (only platinum needles). One notes the embedding

of organic crystals inside the platinum bulk. (Cougnon, C., Ph.D. thesis, Université de

Rennes 1, 2002).

with the polarized platinum to yield a new phase or simply give an exchange with

the iodide anion inside the Pt/CsI phase:

Pt + e– + 2 CsI

[Pt–2 , Cs+, CsI]


or Pt + e– + 2 AcQ–• Cs+


[Pt–n , Cs+, (AcQ Cs+)m]



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Electroanalytical Chemistry: A Series of Advances














1 µm

Figure  2.32  Electrochemical reduction of platinum in the presence of an organic

π-acceptor. Superdry conditions. (a) Electrolysis of 2.5 µg of Pt deposited onto a goldfilmed quartz. EQCM analysis upon time in 0.1 molL−1 in DMF at –2.1 V versus SCE.

After 100 s (obtaining of [Pt2−, Na+, NaI]), 1,4-diacetyl benzene (to lead in total to a solution

of 10 mmol.L−1) is suddenly added to the solution. (b) SEM image of the resulting surface

after an electrolysis at –2.1 V/SCE until 5 C cm−2, and the oxidation by air followed by

a thorough rinsing with alcohol and acetone. (Cougnon, C., Ph.D. thesis, Université de

Rennes 1, 2002).

Thus, the oxidation process could restore in volume the platinum metal and AcQ.

In order to verify the validity of the mechanism given in Scheme 2.7, which supposes the concomitant insertion of AcQ via its reduced form, EQCM experiments

have been performed. For example, as displayed in Figure 2.32, the addition of

a π-acceptor to the saturated phase of platinum provides a new mass increase.

Moreover, it was established that the mass increase specifically due to the

π-acceptor, is proportional to the initial amount of platinum.

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Reduction of Platinum under Superdry Conditions






1,4 Diacethylbenzene








Figure  2.33  Charge–discharge process of platinum under superdry conditions in

the presence of π-acceptors. Solvent–electrolyte: DMF/0.1 mmol.L−1 NaI. Concentration

of organic compounds: 10 mmol.L−1. Values of Qa (see definition in Section 2.3) as a

function of the amount of deposited Pt onto a gold surface. Potentials of charge and discharge: −2.1 V and −0.5 V versus SCE, respectively. (Cougnon, C., and J. Simonet, 2002,

J. Electroanal. Chem. 531: 179. Used with permission.)

In Figure 2.32, after a careful rinsing, the appearance of a very porous platinum surface is noticeable. The structure is apparently produced by the dissolution

of the π-acceptor by means of an appropriate solvent.



Phase [Ptn−, M+, (π-acceptor−, M+)] → [Pt, π-acceptor] → divided Pt

Attempts to estimate the stoichiometry of the new inserted phases involving

π-acceptors were achieved. To this purpose, coulometry and ECQM experiments were performed. With several π-acceptors, it was additionally shown

(Figure  2.33) that the stored charge Qa is proportional to the thickness of the

platinum layer; the ratio of stored charge toward the number of available platinum atoms is now equal to one-fourth. Thus, the soft-donor effect of the anion

implied in the phase structure could significantly decrease the global acceptor

capacity of platinum. If we assume that the organic substrate is involved in the

form of its anion radical salt, the global formula of these new organometallic

phases (determined thanks to a series of EQCM data) fits well with the following

formula: [Pt4−, M+, π-acceptor•−, M+].

2.7.2 Reactivity of Reduced Organic Acceptors toward Platinum

In the first part of Section 2.7, it was shown that the reduced platinum phase

could specifically react with acceptors. In this connection, is pure platinum able

to provide an electron exchange with the reduced forms of acceptors, essentially

organic? An intriguing observation concerns the reduction of some π-acceptors

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