THE SYNTHESIS AND ELECTROCATALYTIC ACTIVITY OF PbO 2 -POLYELECTROLYTE AND PbO 2 -SURFACTANT COMPOSITE COATINGS

The regularities of the PbO 2 -polyelectrolyte and PbO 2 -surfactant composite coatings deposition have been investigated. On a cyclic voltammogram (CV) several characteristic areas can be distinguished: at the anode region of the CV at potentials higher than 1.4 V, the anode current is growing exponentially due to the simultaneous reactions of Pb(II) oxidation and the evolution of oxygen. At the cathodic branch of a CV, a current maximum is observed at potentials of 1.0–1.2 V, corresponding to the reaction of the reduction of lead dioxide. If the polyaminoguanidine hydrochloride is present in the electrolyte, the electrodeposition of lead dioxide is inhibited. If the anionic polymer additive Nafion ® is in the electrolyte, one can see an increase in the peak of cathodic reduction of lead dioxide, which indicates an increase in the formation rate of PbO 2 . The addition to the electrolyte of anionic surfactants leads to a slight inhibition of the process of deposition of PbO 2 . As one can see from the experimental data, the adsorption of anionic surfactants is described satisfactorily by the Langmuir isotherm. Values of the limiting adsorption and the adsorption equilibrium constant were calculated. According to the results obtained, the anionic surfactants, cationic polyelectrolyte polyaminoguanidine hydrochloride and anionic polyelectrolyte Nafion ® can be used as additives to the electrolyte during the deposition of the lead dioxide. It has been established, that they are being included into the growing deposit by forming the composite coatings with different composition and various electrocatalytic activity in the reaction of the evolution of oxygen. The content of organic compound in the oxide can vary from 2 to 16 w.%, a surfactant–oxide and polyelectrolyte–oxide composite coating is formed. The overpotential of the oxygen evolution decreases in the line C 4 F 9 SO 3 K> C 12 H 25 O 4 SNa> C 16 H 29 O 6 SNa. It should be noted that the energy of adsorption on PbO 2 increases in the same line. Установлено, что они включаются в растущий осадок, образуя композиционные покрытия с различным составом и электрокаталитической активностью в реакции выделения кислорода. Содержание органического соединения в оксиде может варьироваться от 2 до 16 мас.%, образуя композиционное покрытие ПАВ-оксид и полиэлектролит-оксид. Перенапряжение выделения кислорода уменьшается в ряду Следует отметить, что энергия адсорбционного с PbO увеличивается в Ключевые слова: полиэлектролит; вещество; диоксид свинца; нитратный


Introduction
Lead dioxide electrodes are of considerable interest in their use at high anodic polarizations [1][2][3]. They can be obtained by electrodeposition from aqueous solutions and are characterized by high electrical conductivity, low cost, suitable corrosion resistance and high electrocatalytic activity in reactions proceeding at high anodic potentials with the participation of oxygencontaining radicals [1; 4; 5]. At the same time, lead dioxide-based materials have not yet had wide practical application, because of unsuitable mechanical properties, which are manifested when the composite metal-based dimensionallystable anodes are being created, such as the bad adhesion of the active layer to the substrate, in particular. In this case, the rate of mechanical destruction of the oxide layer of a dimensionallystable anode is proportional to the intensity of the evolution of oxygen. One of the ways to solve this problem is the surface treatment, the creation of a transition layer, the deposition of a catalyst on the another type of substrate, for example Ebonex ® [6]. Another way is to create a composite material that would not be inferior in its properties to the base oxide, but at the same time will have lower internal stresses [7][8][9][10][11].
It is widely recognized, that the presence in the solutions for the deposition of various additives of ions and oxides of other metals has a significant effect on the composition, physico-chemical properties and electrocatalytic activity of the obtained lead dioxide based materials [7; 12-15]. At the same time, in contrast to metal coatings, the effect of organic additives on the regularities of PbO2 electrodeposition and the properties of resulting materials remains virtually uninvestigated. Since the electrodeposition of lead dioxide is carried out at high anodic potentials, at which most organic compounds oxidize at a sufficiently high rate, the choice of additives is a significant problem. Based on the general considerations, water-soluble polymers (polyelectrolytes) may have value for its usage as an additive to the deposition electrolyte. The long polymer chain and high molecular weight of the polyelectrolytes would ensure the stability of this class of compounds in the range of PbO2 electrodeposition potentials. In this paper, we investigated the effect of a polymer additive on the electrodeposition of lead dioxide, the composition and the properties of the materials obtained.

Experimental and Methods
All chemicals were of the class of reagents. Lead dioxide was electrodeposited from the nitrate electrolytes which contained 0.1 M HNO3, 0.1 M Pb(NO3)2.
Platinized titanium was used as a sheet. It was treated as described in [7] before the platinum layer was deposited. The content of organic compounds varied within: 1·10 -6 to 0.01 M, depending on the purposes of the experiment. Electrolyte compositions and conditions of the deposition of composite coatings were selected in such a way that in all cases the current efficiency of lead dioxide deposition was about 100%.
Electrodeposition regularities of doped lead dioxide were studied on a Pt rotating disk electrode (Pt-RDE, 0.19 cm 2 ) by steady-state voltammetry, chronoamperometry. For the RDE experiments the voltammetry system SVA-1BM was used. The rate of the scanning potential was varied within 1÷100 mV/s depending on purposes of the experiments. Before an each experiment, the electrode surface was treated with freshly prepared mixture (1:1) of concentrated H2SO4 and H2O2 [7]. This preliminary treatment technique permits to stabilize the electrode surface, which is oxidized to a certain state under the action of strong oxidizing medium (the phase and chemical composition of the oxides on the surface is defined), which determines the favourable reproducibility in obtaining the cyclic voltammograms in the background electrolyte (0.1 M HNO3). Voltammetry measurements were carried out in a standard temperature-controlled three-electrode cell.
The reaction of the evolution of oxygen was investigated by steady-state polarization on a computer controlled EG & G Princeton Applied Research potentiostat model 273A in 1M H2SO4. All potentials were recorded and reported vs. Ag / AgCl / KCl (sat.).
Adsorption measurements were carried out in 0.5 g of PbO2 powder in 0.1 M HCl solutions containing various amounts of additive. It should be noted that the measurements were carried out with the presence of the indifferent electrolyte (0.1 M KCl), which shielded the electrostatic field of the oxide surface. The time for establishing the adsorption equilibrium was 24 hours. The content of organic substance was determined by the photocolorimetrical measurements after the extraction of ionic associate with chloroform.

Results and Discussion
The development of a technology sets the task of creating new composite polymer coatings or films that combine both metal and / or oxide properties and polymer properties (high elasticity and strength). The electrodeposition method has been well studied for the preparation of polymer films [16][17][18][19]. Electrodeposition is also an effective method in the manufacture of ceramic materials, as well as organoceramic films [20][21][22].
Electrodeposition of ceramic materials can be performed both in cathodic and anodic ways. At the same time the nanostructured ceramic films are formed. The main component of the formation of films is one that is used for the electrophoresis -a process in which the charged ceramic particles, which are dispersed in a solution, migrate in an electric field to the electrode. The formation of the composite occurs by coagulation of particles on the surface of the electrode. For a more successful deposition of ceramic films the electrolytes include various additives, such as polymers, which influence the formation of colloidal particles [23].
It should be noted that there are no data in the literature discribing the effect of polyelectrolytes on the regularities of electrodeposition and the physico-chemical properties of oxide materials, lead dioxide in particular.
In some cases, small amounts of ionic additives are included in the growing oxide, forming the micromodified materials based on lead dioxide [7; 12]. On this basis, it should be assumed that the addition of organic substances will also affect the electrodeposition, composition and the properties of the oxide materials.
At the beginning of investigation, polyelectrolytes and surfactants of various classes were selected.
Since the organic additives must be adsorbed on the surface of the oxide, the adsorprion of the compounds included in the PbO2 has been investigated.
It was established that the adsorption of Nafion ® (Fig. 1) and polyaminoguanidine hydrochloride (Fig. 2) on the surface of lead dioxide is described by an isotherm, similar to the Langmuir isotherm. The nature of the adsorption isotherm shows the high affinity of the adsorbate to the surface. Even at low concentrations, the magnitude of adsorption increases sharply and quickly reaches an almost constant value. As a result of potentiometric measurements, it was established that the adsorption of Nafion ® on PbO2 is accompanied by a shift of the zero charge point of the oxide to a region of higher pH. This suggests that the adsorption of polyelectrolyte at pH ˂ pHo is caused by not only the electrostatic attraction of the polyanion to the positively charged surface of lead dioxide, but also, apparently, as a result of some specific interaction. This assumption is wellsupported by the data on the adsorption material balance [24]. It was established that the adsorption of the polymer is accompanied by the release of OHions. If we assume that the adsorption is carried out with the participation of the SO3 -groups of the polymer, the release of hydroxide ions can be explained by the coordinating interaction of the polyanion with the lead surface cations on the surface of PbO2: With a decrease of the pH of the solution, the adsorption of Nafion ® on PbO2 increases significantly (see Fig. 1), which can be associated with both the increasing role of the electrostatic factor, caused by the increase in the positive charge of the oxide, and the transition of the polymer to the protonated form. The adsorption of Magnafloc is also specific, and its basic regularities are similar to those previously described for Nafion ® .
As one can see from the experimental data, the adsorption of anionic surfactants is described satisfactorily by the Langmuir isotherm (Fig. 3). In coordinates А -1 =f (C -1 ), a linear dependence is observed (correlation factor 0.998), from which the values of the limiting adsorption and the adsorption equilibrium constant were calculated (Table 1).

Fig. 3. The Langmuir isotherm of SDS (a), SLES (b) and potassium nonafluoro-1-butanesulfonate (c) on PbO2 in linear coordinates
The values of the energy of adsorption interaction (ΔG) are changed in line C16H29O6SNa˃ C12H25O4SNa > C4F9O3SK and are 35.0; 34.1 and 31.5 kJ mol -1 , respectively. The obtained values indicate the electrostatic adsorption of anionic surfactants on the lead dioxide. This is also confirmed by a slight shift of the pH0 of the oxide, which is 0.71±0.1 V in 0.1 M HNO3 [7; 24]. Non-ionic and cationic surfactants are not adsorbed on the positively charged PbO2 surface.
The next step was to investigate the electrochemical stability of the organic additives which have an impact on the PbO2 deposition potentials.
Cyclic voltammograms (CV) were taken on a Ptelectrode in a 0.1 M HNO3 background electrolyte with the included additives (potential scan range 0-1.8 V). It was found that the cationic polyelectrolyte Magnafloc shows an electrochemical activity, forming a phase film on the electrode. In this regard, this polyelectrolyte is not of our interest in further research. The remaining additives, selected by the adsorption, did not show an electrochemical activity in the potential range under study. Thus, anionic polyelectrolyte Nafion ® , cationic polyelectrolyte polyaminoguanidine hydrochloride and anionic surfactants can be used as additives to the lead dioxide deposition electrolyte.
To determine the effect of organic additives on electrodeposition of lead dioxide qualitatively, the cyclic voltammograms were obtained on a platinum electrode. On the CV (potential scan range 0.8-1.6 V) several characteristic areas can be distinguished [7,25] (Fig. 4). At the anode region of the CV at potentials higher than 1.4 V, an anode current is growing exponentially due to the simultaneous reactions of Pb(II) oxidation and oxygen evolution. At the cathodic branch of the CV, a current maximum is observed at the potentials of 1.0-1.2 V, corresponding to the reaction of the reduction of lead dioxide: PbO2 + 4H + → Pb 2+ + 2H2O + 2e -(2) As it was established earlier [7; 25], the area and size of the cathode peak characterizes the amount of oxide formed on the electrode surface, that allows one to use these parameters for a qualitative assessment of the PbO2 electrodeposition rate.
The addition of polyelectrolytes into the deposition electrolyte affects the oxidation process of Pb 2+ (Fig. 4). At the same time, the nature of the substance plays a significant role. When polyaminoguanidine hydrochloride is present in the electrolyte, the electrodeposition of lead dioxide is inhibited. With the presence of anionic polymer additive Nafion ® in the electrolyte, one can see an increase in the peak of cathodic reduction of lead dioxide on the CV (see Fig. 4), which indicates an increase in the formation rate of PbO2. The addition to the deposition electrolyte of anionic surfactants leads to a slight inhibition of the process of deposition of PbO2 (Fig. 5).
It was established that the additives chosen for further studies (anionic surfactants, polyaminoguanidine hydrochloride and Nafion ® ) are being included in the growing lead dioxide. The content of organic compound in the oxide can vary from 2 to 16 w.%, the surfactant-oxide and polyelectrolyte-oxide composite coating is formed ( Table 2).  The inclusion of organic additives into the coating (like the ionic additives [12,25]) can have a significant impact on physico-chemical properties of obtained materials, for example, on their morphology and structure.
It should also be assumed that the composite oxide-surfactant and oxide-polyelectrolyte materials will differ significantly in their electrocatalytic activity from lead dioxide. Since the vast majority of electrochemical processes at high anodic potentials occur with the participation of oxygen-containing particles which are adsorbed on the electrode (so-called oxygen transfer reactions), it is convenient to evaluate the nature of effects by changing the reaction rate of the evolution of the oxygen.
The rate of the process of the evolution of oxygen on composite materials depends on both the nature and the content of the additive in the oxide (Fig. 6). Thus, the overpotential for the evolution of oxygen decreases in the line C4F9SO3K> C12H25O4SNa> C16H29O6SNa. It should be noted that the adsorption energy on PbO2 increases in the same row. The nature and content of polyelectrolytes in lead dioxide also have a significant effect on the reaction of the evolution of oxygen (Fig. 7). .

Conclusions
Thus, according to the results obtained, anionic surfactants, cationic polyelectrolyte polyaminoguanidine hydrochloride and anionic polyelectrolyte Nafion ® can be used as additives to the electrolyte during the deposition of the lead dioxide. It has been established, that they are included into the growing deposit and form the composite coatings with different composition and various electrocatalytic activity in the reaction of the evolution of oxygen. The content of organic compound in the oxide can vary from 2 to 16 w.% and the composite surfactant-oxide and polyelectrolyte-oxide coating is formed. The overpotential for the evolution of oxygen decreases in the line C4F9SO3K> C12H25O4SNa> C16H29O6SNa. It should be noted that the adsorption energy on PbO2 increases in the same line.