RESEARCH INTO EFFECT OF PROPIONIC AND ACRYLIC ACIDS ON THE ELECTRODEPOSITION OF NICKEL

Electrolytic nickel plating is a widespread process in galvanotechnics. This is predetermined by the presence of a complex of valuable properties of nickel-based galvanic precipitation. Thus, machine-building industry constantly demands corrosion-resistant, hard, and wear-resistant coatings. Modern electrotechnical production requires plastic, non-strained protective coatings, which can be soldered. Development of hydrogen energy generation initiates improvement of the technologies for obtaining catalytically-active materials. High decorative characteristics of nickel coatings render leading positions to nickel plating in finishing the fittings, accessories, car emblems, jewelry production. Nickel dispersions are used in the manufacture of complex metal parts using 3D printers. One of the directions of development of electrochemical nickel plating is the use of electrolytes containing carboxylic acids. High buffer properties of carboxylic acids damp changes in pH of the electrolyte near the electrode. This enables enhancement of the working density of nickel deposition current. In addition, in the presence of carboxylic acids in the electrolyte, nickel ions bind in complexes. The latter is used to control the speed of the charge and crystallization conditions of the cathode sediment. Thus, there is no doubt about the relevance of studying special features of chemical and electrochemical stagRESEARCH INTO EFFECT OF PROPIONIC AND ACRYLIC ACIDS ON THE ELECTRODEPOSITION OF NICKEL


Introduction
Electrolytic nickel plating is a widespread process in galvanotechnics. This is predetermined by the presence of a complex of valuable properties of nickel-based galvanic precipitation. Thus, machine-building industry constantly demands corrosion-resistant, hard, and wear-resistant coatings. Modern electrotechnical production requires plastic, non-strained protective coatings, which can be soldered. Development of hydrogen energy generation initiates improvement of the technologies for obtaining catalytically-active materials. High decorative characteristics of nickel coatings render leading positions to nickel plating in finishing the fittings, accessories, car emblems, jew-elry production. Nickel dispersions are used in the manufacture of complex metal parts using 3D printers.
One of the directions of development of electrochemical nickel plating is the use of electrolytes containing carboxylic acids. High buffer properties of carboxylic acids damp changes in pH of the electrolyte near the electrode. This enables enhancement of the working density of nickel deposition current. In addition, in the presence of carboxylic acids in the electrolyte, nickel ions bind in complexes. The latter is used to control the speed of the charge and crystallization conditions of the cathode sediment.
Thus, there is no doubt about the relevance of studying special features of chemical and electrochemical stag- es of electroreduction of monosubstituted forms of nickel aqua-complexes with carboxylic acids as the basic component of the complex nickel plating electrolytes.

Literature review and problem statement
Kinetic characteristics of nickel ions electroreduction and the properties of the obtained precipitation are determined, first of all, by the composition of the nickel plating electrolyte. The need to intensify the process of nickel deposition led the scientific community to explore the new types of electrolytes. One of the variants to modify the process is to use complex electrolytes. During alloys electrodeposition, the presence of complexes is required for the convergence of stationary potentials of the separation of components of the alloy [1,2]. The introduction of carboxylic acids to the nickel plating electrolyte affects the kinetics of electroreduction of nickel ions. The process of deposition of nickel-based coatings is largely dependent on the buffer properties of electrolyte [3][4][5] and the stability of complex ion compounds [6].
The electrodeposition of nickel from aqueous solutions is accompanied by a parallel course of hydrogen evolution reaction. The result is an increase in the pH of a near-electrode layer. The elevated pH values may lead to the formation of insoluble hydroxide nickel compounds [7]. The introduction of such compounds to the cathode sediment has a negative impact on the quality of nickel coatings. Authors of [8] showed that the use of sodium citrate as a buffer supplement makes it possible to obtain fine-crystalline compact nickel precipitation at high current output values. Authors of [9] proposed sodium gluconate as the buffer additive. To increase the limit current density of the nickel deposition, authors of [10] introduced amber acid to the nickel plating electrolyte. In the amber-acidic electrolyte almost all acid is associated with a nickel complex [NiHSucc] + . High buffer capacity of solutions is provided by the dissociation of these compounds.
One of carboxylic acids that are often used in electrolytes for the electrodeposition of metals and alloys is glycine (НGІу) [11][12][13][14]. In [15], authors studied buffer properties of acetate, sulfate and chloride glycine-containing nickel plating electrolytes. It was shown that different forms of amino acid and background anions act in a solution as two buffer systems. Buffer properties of the electrolyte are the additive magnitude of the action of these systems.
Authors of [16] note that in addition to the buffer action, carboxylic acids affect the kinetics of electrodeposition of nickel through complex-formation. It was established that all glycinate complexes in the nickel plating electrolyte are reduced at the same time. The reduction of these systems proceeds irreversibly with a dominant control over the stage of charge transfer. The process is complicated by the adsorption of reagents and a preliminary chemical reaction. An analysis of partial voltammograms of nickel isolation from a glycine-containing sulfate electrolyte showed [17] that the limiting electrochemical stage is the transfer of the first electron. This stage is complicated by the adsorption of glycine.
An analysis of the scientific literature [15][16][17] indicates that the multifactority of the examined systems does not make it possible to substantiate optimization of the electrolyte composition based on empirical data only. Given this, it would be expedient to explore the influence of carboxylic acids on electrochemical properties of monosubstituted nick-el aqua-complexes using a quantum-chemical simulation. Propionic and acrylic acids, close in buffer properties, were selected for the research.

The aim and objectives of the study
The aim of present work was to establish influence of the nature of carboxylic acid on the chemical and electrochemical stages of the process of electroreduction of monosubstituted nickel(II) aqua-complexes. This will make it possible to conduct predictable optimization of compositions of the complex nickel plating electrolytes in order to ensure high performance of the process of nickel electrodeposition.
To achieve the set aim, the following tasks have been solved: -using a quantum-chemical modeling, to identify the nature of intermediates and to determine thermodynamically possible pathways of electroreduction of monosubstituted nickel aqua-complexes with propionic and acrylic acids; -to establish kinetic peculiarities of nickel electrodeposition in the presence of propionic and acrylic acids, and to compare empirical data to the results of quantum-chemical calculations.

Materials and methods for research into nickel electrodeposition from the complex electrolytes
Quantum-chemical simulation was performed using non-empirical methods of the software WinGAMESS [18]. Cluster systems were calculated using the spin-unlimited Hartree-Fock method. The central atom of a metal was described by the basis 6-31G**, atoms of the ligands -by the basis 6-311G. We employed a hybrid B3LYP method from the density functional theory, which includes five functionals: exchange functionals by Becke, Slater, Hartree-Fock, as well as LYP and VWN5 correlation functionals [19]. During modelling, we optimized the examined ions surrounded by a first solvate shell and estimated energies of the optimized complexes. Next, the energies were refined taking into consideration the solvation using a polarization continuum model [20].
Using the DFT theory, in particular the hybrid potential B3LYP, significantly improves the convergence of results [20]. The application of the heavier full-electron basis 6-31G of the central atom markedly worsens the correlation convergence of results. More accurate results were obtained when using a central atom of the basis CRENBL ECP. For Ni + cation, we calculated energies in the high-spin and lowspin states.
Polarization measurements were carried out using the potentiostat PI-50-1 (Belarus) in a set with the programmer PR-8 (Belarus). Electrochemical studies were conducted in a three-electrode glass cell. We used the USB-oscilloscope (Ukraine) as a recording device, connected to a Pentium Celeron computer (USA). We used a gold working electrode during research. The end of the gold rod, pressed in Teflon, with a cross-sectional area of 0.4 cm 2 served as the electrode. The preparation of the electrode before the research involved polishing by a suspension of magnesium oxide and washing with bidistilled water.
Empirical studies were conducted in the electrolytes containing, as a background, a single molar solution of sodium perchlorate. The source of nickel ions was nickel perchlorate. To preparation the solutions we used chlorine acid (60 %) and nickel carbonate of the qualification (p. a.). Propionic (НPr) and acrylic (НAk) acids were in conformity with the qualification (pur.).
PH values of the solutions were brought to the preset value with a solution of sodium hydroxide. Acidity of the solutions was controlled using the universal ionomer EV-74 (Belarus).

Results of research into nickel electrodeposition from the electrolytes containing propionic and acrylic acids
To establish the role of complexation in the process of nickel electrodeposition, we examined propionic and acrylic acids. Dissociation constants of these acids are 1.34·10 -5 and 5.53·10 -5 , respectively [21]. Therefore, the buffer properties of solutions of propionic and acrylic acids are close. Unification of the pH magnitude of a near-electrode layer makes it possible to run a comparative analysis of the impact of these acids on the kinetics of nickel electrodeposition from the standpoint of structure and electrochemical behavior of the corresponding complexes with nickel.
Results of the energy estimation of possible structures of monosubstituted nickel(II) aqua-complexes are shown in Fig. 1, a. The introduction of anions of propionic acid to the composition of an aqua-complex results in the offset of energy values towards negative side for the particles containing four molecules of water in the inner coordination sphere. Thus, the dominant form of the system is the pentaligand complexes of nickel. It should be noted that a decrease in the coordination number of energetically favorable form of nickel aqua-com-plexes is not the consequence of bidentate of the carboxyl group of the propionate ion (Fig. 2, a).  (Fig. 2, b). The quantum-chemical calculation shows that the total charge of water molecules changes from +0.120 to -0.427 (Table 1). Therefore, there occurs the acceptance of charge by water molecules. According to the estimated data (Table 1), the electron is localized on a nickel ion in the case of dehydration of the inner coordination sphere of transition particle [Ni 2+ (H 2 O) -4 (Pr -)] * . This process is energetically favorable and proceeds with heat release (Fig. 1, b): Also, energetically favorable is the process of reduction of the intrasphere water molecules of the transient particle: It is obvious that these reactions will compete with other. Coordination number in [Ni + (H 2 O) 3 (Pr -)] remains equal to 5 (Fig. 2, c). This is predetermined by the activation of the second oxygen atom of the carboxyl group, which becomes a bidentate. The assimilation of the second electron by intermediate [Ni + (H 2 O) 3 (Pr -)] results in the propionate-anion becoming monodentate again.
The formed structure [Ni 0 (H 2 O) 3 (Pr -)] is unstable and will quickly disintegrate with the formation of a biligand complex and the release of a significant amount of heat (Fig. 1, b): Since in the biligand aggregate [Ni 0 (H 2 O)(Pr -)] a charge of nickel atoms and propionate-ions is negative (Table 1), it is susceptible to decomposition into its component parts: That is, the end product of the electroreduction of propionate complexes is the monohydrated nickel atoms.
Results of the energy calculation of complex structures of nickel with acrylic acid proved to be different from those for nickel with propionic acid. Comparing the energy of the optimized clusters of complexes [Ni 2+ (H 2 O) n (Ak − )](H 2 O) 5-n revealed (Fig. 3, a) It is energetically possible to bind acrylic acid in molecular form into a complex: At pH 3-4, nickel plating electrolyte is dominated by the complexes of nickel with acrylate-ions (Fig. 4, a).
The transfer of the first electron to the external unoccupied orbital of acrylate complex [Ni 2+ (H 2 O) 5 (Ak − )] results in the following transformations. The bond of acrylate-ion with the carboxyl oxygen is strengthened (Ni-O distance is reduced from 2.032 Å to 1.886 Å). The energy of bond with one intrasphere water molecule is somewhat increased.
Transitional structure [Ni 2+ (H 2 O) 5 (Аk 2-)] * (Fig. 4, b) retains the geometry of the starting complex [Ni 2+ (H 2 O) 5 (Аk -)]. The assimilated electron is localized on the vinyl fragment of acrylate-ion. According to the energy chart (Fig. 3, b), a steady intermediate is particle [Ni + (H 2 O) 2 (Аk -)]. The cleavage of three molecules of water at its formation makes it possible for the electron to take one of the orbitals of Ni 2+ ion and to convert it into the reduced form of Ni + (Fig. 4, c).
The charge of the central atom of the formed intermediate [Ni + (H 2 O) 2 (Аk -)] reduces to +0.645, while the charge localized on acrylate-ion becomes equal to -0.54.
The process of restructuring the inner coordination sphere of the partially reduced nickel complexes is accompanied by an additional release of energy: When transferring the second electron to intermediate [Ni + (H 2 O) 2 (Ak -)], the charge is localized not on the orbitals of nickel, but on the vinyl fragment of acrylate-ion. An analysis of structures with a lower coordination number of nickel did not reveal more energetically favorable variants than [Ni 0 (H 2 O) 2 (Ak -)].

Discussion of results of research into nickel electrodeposition from the electrolytes containing propionic and acrylic acids
Experimental data on nickel electrodeposition in the presence of propionic and acrylic acids indicate exponential dependence of the potential on current density. Consequently, discharging of nickel ions in the investigated range of potentials proceeds in accordance with the equation of a delayed discharge, which is typical for the electrodeposition of nickel [3]. Voltammograms, linearized in semi-logarithmic coordinates, are shown in Fig. 5. The introduction of the examined carboxylic acids to the electrolyte containing nickel ions leads to a decrease in the kinetic constraints for the isolation of nickel.
A possible reason is a varying degree of blocking the surface of cathode with hydroxide nickel compounds in the examined electrolytes. This process is most pronounced in the electrolyte, which does not contain any buffer additives. The difference in the kinetics of nickel isolation, observed in the presence of propionic and acrylic acids, can be explained in the following way. During nickel electrodeposition from the acrylate complexes, the electroreduction of intrasphere water molecules is not energetically favorable. Therefore, in this case, a change in the pH of a near-electrode layer will be smaller. The specified comparison seems justified because buffer properties of the examined acids are close.
Thus, to interpret experimental data on the kinetics of nickel electrodeposition from the electrolytes containing carboxylic acids, we applied results of the quantumchemical calculations. We noted a satisfactory correlation between the effects observed in the experiment and those expected based on the simulation. The advantage of using quantum-chemical calculations is the possibility to establish the nature of intermediate particles whose detection in situ is difficult. At the same time, reliability of the simulation results depends on taking into consideration the maximum quantity of influencing factors. Consequently, empirical and estimated data are complementary.

Conclusions
1. By using quantum-chemical calculations, it was established that the coordination number of the nickel acrylate complex is six. For the case of the propionate nickel complex, an energetically favorable form is the pentaligand complex.
2. It was found that during electroreduction of the propionate nickel complex the charge is localized on the water molecules of an intermediate particle. This can lead to the electroreduction of an intrasphere molecule of water, which is accompanied by an increase in the pH of the electrolyte. In the acrylate complex, the localization of a charge occurs on the vinyl fragment of acrylate-ion. Electrochemical reaction of reduction of the coordinated water molecules in such a particle is not energetically favorable.
3. Voltammetric studies have shown that the isolation of nickel from the electrolyte, not containing carboxylic acids, is very difficult. This is possibly due to the blocking of the cathode surface with insoluble hydroxide compounds of nickel. In the absence of a buffer additive, pH of the near-electrode layer increases due to the occurring reaction of hydrogen evolution. The introduction to the electrolyte of the examined carboxylic acids partly reduces this effect. Acrylic acid contributes to the greater activation of nickel isolation than propionic acid. Because the buffer properties of these acids are close, the observed difference in the kinetics of nickel electrodeposition is explained by the peculiarities of electron structure of the transient particles containing acrylate-and propionate-ions.