Electrochemical Preparation and Characterization of a Pt – Polytyramine Composite Material with Electrocatalytic Properties

Electrochemical deposition of polytyramine (PTy) was performed by cyclic voltammetry and it was observed that, in acidic media, 20 to 50 consecutive deposition cycles allowed us to obtain polymer coatings with good conductivity and high porosity. The polytyramine layers were used as a matrix for electrochemical deposition of platinum and the electrocatalytic activity for methanol oxidation of the composite material thus obtained was put in evidence.

Over the past several years, there have been numerous works devoted to the study of the polymerization of various aromatic compounds, to obtain electrodes modified with conductive, semiconductive, or insulating polymer layers [1][2][3][4], for several practical applications as sensors [5,6] or catalysts [7][8][9].Among the investigated polymerization methods, the most promising seems to be the electrodeposition because it is a straightforward method, which allows obtaining polymeric matrices with aimed and reproducible properties, with no limitation from the surface size or the geometry of the electrodes [10,11].In this respect, there are many data in the literature concerning the preparation, the characterization and the possible applications of electrodes modified with conductive polymers, including polypyrrole, polyaniline, polyacetylene, polyindole, or polythiophene [11][12][13].
The results have shown that the polymerization of monomers containing aromatic groups directly linked to an oxygen atom occurs more readily, resulting in polymer coatings with higher chemical and mechanical stability.Tyramine (4-(2-aminoethyl)phenol) also belongs to this class of compounds and its use for biosensors applications is very promising, mainly due to the fact that the presence of free amino groups allows the immobilization (by peptidic links) of various bioactive chemical species [14].These findings triggered increasing interest for the study of the mechanism of tyramine polymerization [15], the electrodes thus modified being successfully used for biosensors [16,17] and immunosensors [18] applications.Furthermore, due to its specific features (high electrical conductivity, reasonably good chemical, electrochemical and mechanical stability) there are reasons to believe that PTy could be a suitable matrix for the immobilization of micro and nanoparticles of metals and oxides with electrocatalytic properties.Such composite materials will ensure more efficient use of noble metals (or noble metal oxides) because they enable obtaining electrodes with large surface area and higher electrocatalytic activity resulting from a better dispersion of the catalyst in the polymer matrix.
The present work was aimed at studying the electrochemical deposition of polytyramine coatings on graphite substrate, with an eye to Pt/PTy composite material preparation for electrocatalytic applications [19].

Experimental part
The electrochemical experiments were performed with a PAR 273A potentiostat in a conventional three-electrode cell, by using a platinum auxiliary electrode (~ 3 cm 2 ) and an Ag/AgCl reference electrode, linked to the main compartment of the cell by means of a Vycor glass junction.For the electrochemical impedance spectroscopy (EIS) measurements a PAR-FRD100 frequency response detector was used, together with a ZSimpWin 3.21 software.
The PTy layers were deposited on graphite substrate (surface area, 0.12 cm 2 ) by cyclic voltammetry from a 0.1 M HClO 4 solution with a concentration of tyramine (Aldrich) of 15 mM.The study of the effect of the pH on the anodic oxidation of tyramine (Ty) was performed at a boron-doped diamond (BDD) electrode (0.5 cm 2 ) in a 0.04 M Britton-Robinson buffer solution (boric acid + phosphoric acid + acetic acid) containing appropriate amounts of NaOH.All the substances were analytical-reagent grade, and all the solutions were prepared using bidistilled water.

Results and discussion
It is widely known that anodic oxidation of phenolic compounds usually results in the fouling of conventional electrodes (e.g.graphite, glassy carbon, platinum) due to the adsorption of reaction products or intermediates.In order to avoid that, the study of the effect of the pH variation on the tyramine oxidation process was performed at a BDD electrode which ensures more stable and reproducible voltammetric responses, due to its low susceptibility to adsorption [20].The measurements were carried out within the pH range 2.5 ÷ 12.0 in a Britton-Robinson buffer solution with a content of Ty of 85 µM, at a sweep rate of 50 mV s -1 .
It was observed that, under these experimental conditions, tyramine oxidation overall process is irreversible within the potential range 0.0 ÷ 1.5 V as indicated by the absence of any cathodic peak during the reverse scan.As figure 1 shows, the increase of the pH facilitates Ty anodic oxidation and the corresponding voltammetric peak shifts from ca. 1.4 V to ca. 0.7 V, as the pH increases from 3.5 to 9.4, respectively (curves 1 and 3 from fig. 1).This behavior is obviously resulting from the fact that in alkaline media the equilibrium phenol/ phenolate: lies far to the right and phenolate species are more readily oxidized due to a higher electronic density [21].
-0.40 ÷ 1.25 V, in a 0.1 M HClO 4 fresh solution.Figure 3 illustrates the voltammetric behaviour observed for the electrodes loaded with increasing amounts of PTy, up to 100 deposition cycles.It was found that by increasing the number of the deposition cycles the active surface of the electrodes increases, as observed from the variation of the overall reversible voltammetric charge.This behaviour suggests that the deposited polymer layers have not only a high roughness but also a good electrical conductivity, which recommend the PTy coatings thus obtained to be used as matrices for electrocatalysts immobilization.In strong alkaline solutions (curve 4 from fig. 1), although the Ty oxidation process starts at lower potential values, the anodic current is limited within the range 0.5 ÷ 1.0 V as the electrode surface is blocked by reaction products.This observation is in line with previous data from the literature concerning the low electrical conductivity of the PTy layers obtained in alkaline media [21].In order to be used as a matrix for electrocatalyst the polymer coating must exhibit good electrical conductivity and this is why PTy deposition should be carried out in acidic media, at potential values higher than ca.0.9 V (curve 1 from fig. 1).
Based upon these results, the polytyramine layers were deposited on the graphite substrate by cyclic voltammetry (sweep rate, 50 mV s -1 ) within the potential range -0.40 ÷ 1.25 V, from a 0.1 M HClO 4 solution containing 15 mM tyramine.Between two consecutive deposition cycles, the electrode was kept for one minute under open circuit conditions in order to allow the equilibration of the Ty concentration at the interface.
Figure 2 shows cyclic voltammetric curves recorded during Ty polymerization and it can be observed that the increase of the PTy loading results in the increase of the corresponding reversible voltammetric charge in the potential range from 0.0 to 0.8 V.As the overall polymerization process proceeds, the voltammograms exhibit two well-defined peaks, corresponding to reversible redox processes characteristic to PTy formation [21].During the first scan, the anodic peak ascribed to tyramine oxidation is located at ca. 1.05 V, while after ca.20 consecutive cycles the same peak occurs at a potential value of ca.0.95 V.This behaviour suggests that, in the early stage of the process, the polymerization takes place by a different mechanism, in agreement with previous elipsometric results reported in the literature [15].
In order to better put into evidence the properties of the PTy coatings thus obtained, after a certain number of consecutive deposition cycles the electrodes were taken out from the tyramine solution, thoroughly rinsed with bidistilled water and then cyclic voltammograms were recorded (sweep rate, 50 mV s -1 ) within the potential range It was observed, however, that the increase of the thickness of the PTy layer results in a slight increase of the distance between the anodic and the cathodic peaks characteristic for the electronic structure of the polymer.A possible explanation for this behaviour is provided by assuming an increase of the electrical resistivity of the thick PTy films.The presence of an anodic peak located at ca. 0.95 V (ascribed to Ty oxidation) on the voltammograms from figure 3 indicates that, due to the higher porosity, thick PTy films can retain small amounts of tyramine even if the electrodes are rinsed with water.The electrical charge associated to the voltammograms from figure 3 was integrated within the potential range -0.1 ÷ 1.0 V and the average of the anodic and cathodic charges (Qm) was used as a measure of the active surface of the electrodes.The variation of the average charge as a function of the number of deposition cycles (see the inset from fig. 3) shows that during the first ca.50 cycles the active surface of the Deposition cycles electrodes increases almost linearly, while further increase of the PTy amount results in a less significant effect, probably due to a decrease of the polymer porosity.Taking into account the fact that thick PTy layers could also exhibit a high ohmic drop, one may draw the conclusion that polytyramine matrices obtained after ca.50 consecutive deposition cycles are well-suited for the immobilization of electrocatalyst particles.
In order to better characterize the PTy-modified electrodes, electrochemical impedance spectroscopy measurements were also performed within the frequency range 10 kHz ÷ 3 mHz, in a 0.1 M KNO 3 solution with a content of 5mM of the redox couple K 3 [Fe(CN) 6 ]/ K 4 Fe(CN) 6 ].The EIS experiments were carried out both for bare and for PTy-modified (20, 40, 60 and 100 deposition cycles) electrodes at an applied potential of 0.4 V and an amplitude of the alternating signal of 10 mV.An equivalent circuit was used as a model for the electrode/electrolyte interface, the parameters of which were estimated by means of an appropriate software.For bare graphite electrodes a Randles type circuit was used that takes into account the resistance of the solution (Rs), the double layer capacity (Csd), the charge transfer resistance (Rts), and a Warburg impedance element (W) that simulates semiinfinite linear diffusion to a plane surface.For PTy-modified electrodes, two additional components describing the polymer behaviour were added: Rp, the resistance of the layer (mainly controlled by the size and the geometry of the pores) and the capacity of the PTy coating, Cp.
Figure 4 illustrates the results of the EIS measurements (in the Bode representation) together with simulated data, based upon the corresponding equivalent circuits (see the insets), both before polymer deposition (curve 1) and after 20 and 100 deposition cycles (curves 2 and 3, respectively).In figure 4, the data concerning the behaviour of PTy layers obtained as a result of 40 and 60 deposition cycles, as well as the variation of the phase angle as a function of the frequency were omitted for simplicity.graphite surface and on the thin PTy layer, previously deposited.It is worthy to note that these findings are in line with the results of the voltammetric charge measurements (see the inset in fig.3).It was also found that during the first 20 deposition cycles the capacity of the double layer increases from about 20 µF (for bare graphite) to ca. 4 mF, due to the enhancement of the active area of the electrode.After ca.50 consecutive deposition cycles the capacity slowly decreases, presumably as a result of the decrease of the PTy coating porosity.This behaviour was not evidenced by the voltammetric measurements which suggests the fact that the integrated reversible charge includes a pseudocapacitive contribution.The results thus far indicate that PTy layers deposited by 20 to 50 consecutive cycles exhibit the most promising features for being used as matrix for electrocatalysts immobilization.
In order to assess the practical application of the procedure described above, after 20 deposition cycles the PTy-modified electrode was immersed for 15 min in a 46 mM H 2 PtCl 6 solution, thoroughly washed with bidistilled water and then immersed in a 0.1 M HClO 4 fresh solution.The reduction of the Pt(IV) species adsorbed into the PTy film pores was carried out by applying a constant potential of -0.1 V, and a value of 390 mC cm -2 was found for the electric charge corresponding to the platinum deposition process (Q Pt ).Because at potential values higher than ca.-0.2 V the hydrogen evolution is negligible, a current efficiency close to 100 % is expected for platinum electrodeposition.The amount of deposited platinum (W in µg cm -2 ) can be calculated as: where M = 195.1 g mol -1 is the atomic weight of Pt, z = 4 the number of exchanged electrons and F = 96500 C mol - 1 , the faradaic constant.Thus, a platinum loading of 197 µg cm -2 can be calculated.
After platinum deposition, the electrodes were kept for 15 minutes in bidistilled water in order to remove traces of Pt(IV), and then the catalytic activity was checked by cyclic voltammetry in a 0.1 M HClO 4 + 3.5 M CH 3 OH solution.Figure 6 shows cyclic voltammograms recorded at a sweep rate of 10 mV s -1 , both in the absence (curve 1) and in the presence (curve 2) of methanol.It can be observed that in the absence of CH 3 OH, the shape of the voltammogram is characteristic for platinum behaviour in acidic media, the hydrogen evolution process at potential values lower than -0.2 V being well-evidenced.
The results illustrated in fig.6 are noteworthy because they show a good electrocatalytic activity for methanol oxidation of the Pt/PTy composite material, even though the amount of platinum is very small.This behaviour demonstrates that the Pt particles are well-distributed into (2) after 20 deposition cycles; (3) after 100 deposition cycles Among the parameters of the equivalent circuits, particularly significant for the characterization of the PTy coatings are the double layer capacity (as a measure of the electrochemically active area) and the resistance Rp. Figure 5 shows the variation of the double layer capacity (curve 1) and that of the resistance of the polymer layer (curve 2) as a function of the number of deposition cycles.
A more significant increase of the resistance during the first ca.40 cycles can be observed, indicating that the efficiency of the deposition process is higher on the bare the pores of the PTy coating, being accessible not only for protons but also for methanol molecules.

Conclusions
The study of the electrochemical deposition of polytyramine showed that, in acidic media, the use of cyclic voltammetry enables obtaining PTy coatings with rather high electrical conductivity and good chemical and electrochemical stability.It was observed that the polytyramine film obtained as a result of 20 to 50 consecutive deposition cycles can be used as a matrix for platinum immobilization, ensuring good dispersion of the electrocatalyst particles and therefore high catalytic activity of the electrodes.Even with platinum loadings as low as 200 µg cm -2 , the composite material Pt/PTy can be successfully used to activate graphite electrodes in order to achieve methanol anodic oxidation.

Fig. 2 .Fig. 3 .
Fig. 2. Cyclic voltammograms recorded during polytyramine deposition on graphite substrate.(The number of the corresponding deposition cycle is indicated on each curve.)

Fig. 5 .Fig. 6 .
Fig. 5.The variation of the double layer capacity (1) and of the PTy film resistance (2) as a function of the number of deposition cycles Deposition cycles