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AZERBAIJAN CHEMICAL JOURNAL № 2 2023 ISSN 2522-1841 (Online)
ISSN 0005-2531 (Print)
UDC 541.13.544.65
ELECTROCHEMICAL REDUCTION OF CADMIUM IONS FROM ETHYLENE
GLYCOL ELECTROLYTE
N.Sh.Soltanova, A.Sh.Aliyev, B.A.Ismailova, R.G.Huseynova
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, Ministry of Science and Education
of the Republic of Azerbaijan
Received 04.11.2022 Accepted 11.01.2023
The work is devoted to the study of the process of cathodic reduction of Cd ions from anhydrous ethylene glycol electrolyte. The mechanism of the process of cadmium reduction was studied by taking linear and cyclic potentiodynamic polarization curves. The influence of cadmium chloride concentration, potential sweep rate and temperature on the process of electrochemical deposition of cadmium ions was studied. With an increase in the concentration of cadmium in the electrolyte, the rate of its release at the cathode increases. Precipitates of good quality are obtained at current densities of 25 mA/sm2 It has been established that the cadmium deposition potential almost does not change with increasing potential sweep rate. The dependence of cadmium electrodeposition from ethylene glycol electrolyte on temperature was studied, which made it possible to establish the optimal deposition temperature, which was 363-373 K, and to conclude that the deposition process is controlled by electrochemical polarization. The diffusion coefficient of cadmium ions to the cathode surface has been calculated.
Keywords: electroreduction, cadmium, potential, polarization, ethylene glycol.
doi.org/10.32737/0005-2531-2023-2-40-46
Introduction
Recently, there has been a need to develop methods for obtaining new generations of solar cells used as anode materials in the form of thin films and nanostructures based on cadmium. Analysis of the literature revealed a number of works devoted to the electrolytic reduction of cadmium and the preparation of its chalco-genides from various electrolytes [1, 2, 8-13].
The authors of [1] studied the electro-reduction of cadmium ions in a sulfate electrolyte in the presence of N-methylpyrrolidone. It has been established that N-methylpyrrolidone plays the role of an inhibitor in the course of Cd electrodeposition over the entire range of concentrations under study. However, in sulfate electrolytes, during the deposition process, cadmium ions form complexes with N-methyl-pyrrolidone, that block the cathode surface. Research results show that the inclusion of N-methylpyrrolidone in the composition of electrolytic deposits improves their tribotechnical properties, which prolongs the service life of parts subjected to friction.
Electrodeposition of cadmium from wa-ter-methylpyrrolidone electrolytes with a predominant content of the water phase was studied using diagnostic criteria commonly used in chronopotentiometry and chronovoltammetry
[2]. The obtained chronopotentiograms had a single wave of cadmium ion reduction at all studied current densities and concentrations of the non-aqueous component. The presence of linear (Tafel) sections on the AEn - lgi dependences indicates that the electrochemical reaction of the discharge is the stage that determines the speed of the electrode process at the initial moments. It is established that the electrochemical process is quasi-stationary.
The electrochemical reduction of cadmium from thiosulfate electrolytes is studied in
[3]. The electrodeposition of cadmium has been found to occur in the potential range of -0.85-(-0.92) V, while its nanoparticles were distributed in all directions between and along the nanotubes, and their diameter varied in the range of 20-70 nm.
Electrodeposition of Cd from sulfate electrolyte in the temperature range of 20-700C
is studied in works [4, 5]. By recording cyclic potentiodynamic and chronoamperometric curves has been found that the electrodeposition of cadmium is a process controlled by diffusion. Optimum conditions of electrodeposition of cadmium from sulfuric acid electrolyte are determined: 0.01 M Cd2+, pH = 2.5, t = 400C. It was found that the rate of electrodeposition increased with an increase in temperature, cadmium concentration and a decrease in pH of the solution. At the same time, the number of nu-cleation centers, i.e. the rate of nucleation, also increased. The effective activation energy of the process was calculated, which confirmed that the deposition of cadmium from the sulfate electrolyte is accompanied by diffusion polarization.
Electrochemical synthesis of CdS, CdTe nanowires with high photoefficiency suitable for creating new generation solar cells is described in work [6]. The paper also includes research on the optical-physical parameters of these nanofilms and on the photoelectrocatalytic conversion of alcohols.
The electrolyte composition, temperature, anodic oxidation potential, and others have been determined in order to create nanoporous templates based on anodic aluminum oxide (AAO), suitable for the synthesis of CdS and CdTe nanostructures and nanowires. The optimal mode of annealing and its duration after electrolysis, which enable to obtain nanostructures with the necessary properties, have been determined [7].
In [8], the optimal conditions for the electrodeposition of a CdSe film on n-Si are demonstrated. The structural and optical properties of the films were studied before and after annealing. In particular, the crystallinity of the samples was evaluated. After annealing at 4000C in a nitrogen atmosphere, an increase in the photoluminescence of the obtained thin films by almost an order of magnitude was observed.
The paper [9] describes methods for growing ([9]) thin-film CdS/CdTe solar cells. The authors have analyzed the methods of growing layers of electronic material, which are usually grown in three ways; layered growth
mode, layered and cluster growth mode and growth mode with the formation of clusters. Considering most of these methods in detail, the authors came to the conclusion that the most acceptable method of obtaining thin-film solar cells is electrodeposition due to its simplicity and low cost.
The paper [10] is devoted to the electro-deposition of cadmium telluride films by the potentiostatic method. Precipitation was carried out from an alkaline ionic liquid (IL), butyl me-thylimidazolium chloride (BmimCl) at 800C and an applied potential of -1.45. Papers [1113] are also devoted to the deposition of CdTe
The authors of works [14-18] devoted their research to the co-deposition of metal chalcogenides. Thus, in [14], photoactive composites containing nanofilms of cadmium selenide (CdSe) and poly(o-methoxyaniline) from an electrolyte containing CdSO4 8H2O, SeO2, o-anisidine, perchloric acid (HClO4), sodium sulfite (Na2SO3)were obtained by a simple deposition method. Thin films of CdSe were doped with zinc [15] and indium [16]. Works [18] are devoted to the study of the properties of Cd-based thin films obtained by electrochemical deposition.
It should be noted that the kinetics and mechanism of the cadmium electrodeposition from polar organic electrolytes have not been sufficiently studied, and the given data presented is quite contradictory.
The purpose of this work is to study the electrodeposition of thin cadmium films from polar organic solvents, in particular from eth-ylene glycol.
Experimental technique
During the research, the cadmium salt CdCl2-5H2O was dissolved in ethylene glycol (C2H6O2) at a temperature of 363K. Cyclic and linear polarization curves were taken on an IVIUMSTAT Electrochemical Interface poten-tiostat, which made it possible to determine the potential range at which cadmium ions are deposited and to study the kinetics and mechanism of the deposition process. Polarization studies were carried out in a three-electrode electroly-
zer, platinum served as a working electrode, surface 2 mm2, platinum served as an auxiliary electrode, surface 4 cm2, a silver chloride electrode served as a reference electrode, and all potential values given in the article are indicated in relation to this electrode. The concentration of cadmium in the study of its effect on the deposition process varied from 0.1 to 0.01 M. When studying the effect of temperature on the process of reduction of cadmium ions, the electrolyte temperature was changed in the range of 333-373 K, the electrolyte temperature during electrolysis was maintained using a UTU-4 thermostat. The choice of such a temperature range is due to the choice of electrolyte, at low temperatures in an electrolyte viscous with eth-ylene glycol, the diffusion of metal ions is rather low, which significantly slows down the process of electroreduction. The film thickness was measured using a Microinterferometer-4 microscope.
Results and discussion
To study the mechanism and kinetics of cadmium deposition from ethylene glycol electrolytes, cyclic and linear potentiodynamic polarization curves were taken. In Figure 1 the curve of electroreduction of cadmium from ethylene glycol electrolyte is presented, from which it can be seen that the process of deposition, in contrast to aqueous solutions, occurs at more negative potentials.
The stationary potential of cadmium in the ethylene glycol electrolyte was -0.2 V (c.s.e.). The first traces of cadmium appeared on the surface of the platinum electrode at a potential of -0.653 V, while the surface of the platinum electrode was covered with a gray-black cadmium precipitate.
The reduction of cadmium ions occurs in one step at a potential of -0.7 V according to the reaction:
Cd2 + + 2e = Cd0.
The peak on the reverse curve of the cyclic voltammogram at a potential of -0.58 V corresponds to the dissolution of cadmium from the platinum surface. The restoration and oxidation of cadmium ions occur at a barely noticeable polarization, and the difference between the potentials of the cathode and anode peaks was 0.12V, which indicates the reversibility of the electrode process.
The influence of various factors on the process of cadmium deposition was studied, which made it possible to choose the optimal conditions for the deposition of qualitative sediments in the galvanostatic mode. One of the most important factors influencing the electrochemical reduction process is the concentration of cadmium ions in the electrolyte. Figure 2 shows the polarization curves of cadmium recovery depending on its concentration, which varied in the range of 0.01^0.1M.
^A
a
ë s U
50.
-50.
-1— -0.8
—I—
-0.6
—I—
-0.4
—I— -0.2
V
Potential
Fig.1. Polarization curves of cadmium reduction from ethylene glycol electrolyte on Pt-electrode. Electrolyte composition (M): 0.1 CdCl2-5H2O+ C2H6O2. T=363K, Ev=- 0.005 V/s.
The upper limit of cadmium concentration is determined by its limited solubility in ethylene glycol. It can be seen from Figure 2 that with an increase in the concentration of cadmium in the electrolyte, its amount in the double electric layer increases and its reduction occurs at more positive potentials, if at a cadmium concentration of 0.01M its reduction occurs at a potential of -0.74V, then at a concentration of 0.1M - at -0.7V. Qualitative deposits of cadmium up to
7-10 microns thick are obtained at its concentration in the electrolyte 0.08 ^ 0.1M.
To study the kinetics and mechanism of cadmium deposition, polarization curves of its deposition were recorded depending on the potential sweep, which varied in the range of 0.005-0.12 V/s. Figure 3 shows cyclic poten-tiodynamic curves of cadmium deposition from ethylene glycol depending on the potential sweep.
Fig.2. Influence of the concentration of cadmium ions on the process of its cathodic reduction, cadmium concentration (M): 1- 0.01; 2-0.03; 3-0.05; 4-0.08; 5-0.1; T =363K; EV=- 0.005 V / s.
^A
13
C
Fig.3. Effect of potential sweep on electroreduction of cadmium ions. Electrolyte (M): 0.1 CdCl2 + C2H6O2, T = 298K. EV= (V / s): 1- 0.001; 2-0.02; 3-0.04; 4-0.06; 5-0.08;
Although the potential of cadmium deposition does not change significantly with an increase in the potential sweep (if, with a potential sweep of 0.005 V/s, the electroreduction of cadmium occurs at a potential of -0.653V, then with a sweep of 0.12 V/s, the process occurs at -0.648V, i.e., it practically does not change).
In order to obtain additional information about the nature of the polarization of cadmium during its deposition, the polarization curves of the dependence of the electrochemical reduction of cadmium from the ethylene glycol electrolyte on temperature were recorded (Figure 4). Figure 4 shows that with an increase in temperature from 333K to 363K, the discharge of cadmium ions on platinum becomes easier. A further increase in temperature to 373 K almost does not
affect the discharge rate of cadmium ions and at the same time leads to a deterioration in the quality of the deposited coatings, the degree of their adhesion to the substrate decreases, and the deposited films are not uniform in thickness. Therefore, the optimal temperature range for cadmium deposition from ethylene glycol electrolyte is 353-363 K.
Based on the data shown in Figure 4, the dependence of lgik on 1/T was constructed for various values of polarization E. In this case, straight lines with different angles of inclination were obtained.
Figure 5 shows that the straight line No 1, corresponding to E, which is equal to 0.5 mV, differs in slope from other straight lines, the slope of which is approximately the same.
HA ioo_
c
a
3 -100_ U
-200_
-300_
~1-'-1—
-0.4 -0.2
Potential
0.2
V
Fig.4. Influence of temperature on the process of reduction of cadmium ions. Electrolyte (M): 0.1 CdCl2 + C2H6Û2;T (K): 1 - 333; 2 -353; 3 - 363; 4 - 373; EV= - 0.005 V/s.
Fig.5. Dependence of lg ik on 1/T for different values of polarization E, v: 1 - 0.5, 2 - 0.55, 3 - 0.6, 4 - 0.65, 5 - 0.7.
The change in the activation energy with increasing potential is clearly visible in Figure 6.
From the dependence of the effective activation energy on the potential shown in Figure 6, according to the values of the effective activation energy according to [19], we can conclude that the deposition of cadmium from the ethylene glycol electrolyte is controlled by diffusion polarization, which coincides with the studies given in [4-6].
Conclusion
As a result of the research, the composition of the ethylene glycol electrolyte and the optimal electrolysis mode for the electro-deposition of cadmium from the ethylene glycol electrolyte were chosen. The electrolyte composition is proposed: CdCh- 0.08-01 M + ethylene glycol, electrolysis mode: ik - 2-510-3 A/dm2, temperature is 333-363K, electrolysis duration is 60-120 min. The resulting cadmium deposits had a gray-black color, a smooth surface, and strong adhesion to the substrate surface. For practical purposes, the deposition process in the galvanostatic mode was carried out on nickel and ITO substrates; the thickness of the resulting deposits was 10-15 microns.
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KADMIUM IONLARININ ETILENQLIKOL ELEKTROLITINDO ELEKTROKIMYOVI REDUKSIYASI
N.§.Soltanova, A.§.Oliyev, B.A.ismayilova, R.Q.Hüseynova
Taqdim olunan i§ etilenqlikol elektrolitindan Cd ionlannin katodda reduksiya prosesinin öyranilmasina hasr edilmi§dir. Xatti va tsiklik potensiodinamik polyarizasiya ayrilarini qeyd etmakla, etilenqlikol elektrolitindan kadmiumun reduksiyasi prosesinin mexanizmi öyranilmi§dir. Kadmium ionlarinin elektrokimyavi Qökma prosesina qatiligin, potensialin dayi§ma süratinin va temperaturun tasiri öyranilmiijdir. Elektrolitda kadmium konsentrasiyasinin artmasi ila onun katodda ayrilma sürati artir. Carayan sixligini 25 mA/sm2 qiymatinda keyfiyyatli tabaqalar alinir. Kadmiumun Qökma potensialinin artmasi potensialin dayi§ma süratini demak olar ki, dayi§mir. Etilenqlikol elektrolitindan kadmiumun elektrogökmasinin temperaturdan asililigi 363-373 K arasinda tadqiq edilmi§dir, Qökma prosesinin polarizasiyasi diffuziya tabiatli olmu§dur. Kadmium ionlarinin katod sathina diffuziya amsali hesablanmi§dir.
Agar sözlzr: elektroreduksiya, kadmium, potensial, polyarizasiya, etilenqlikol.
ЭЛЕКТРОХИМИЧЕСКОЕ ВОССТАНОВЛЕНИЕ ИОНОВ КАДМИЯ ИЗ ЭТИЛЕНГЛИКОЛЕВОГО
ЭЛЕКТРОЛИТА
Н.Ш.Солтанова, А.Ш.Алиев, Б.А.Исмаилова, Р.Г.Гусейнова
Работа посвящена изучению процесса катодного восстановления ионов Cd из безводного этиленгликолевого электролита. Снятием линейных и циклических потенциодинамических поляризационных кривых исследован механизм процесса восстановления кадмия из этиденгликолевого электролита. Исследовано влияние концентрации хлорида кадмия, скорости развертки потенциала и температуры на процесс электрохимического осаждения ионов кадмия. С повышением концентрации кадмия в электролите скорость его выделения на катоде увеличивается. Осадки хорошего качестве получаются при плотностях тока 25 мА/см2. Установлено, что потенциал осаждения кадмия с увеличением развертки потенциала почти не меняется. Исследована зависимость электроосаждения кадмия из этиленгликолевого электролита от температуры позволили установить оптимальную температуру осаждения, которая составила 363-373 К и прийти к выводу, что процесс осаждения контролируется диффузионной поляризацией. Вычислен коэффициент диффузии ионов кадмия к поверхности катода.
Ключевые слова: электровосстановление, кадмий, потенциал, поляризация, этиленгликоль.
