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ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL No 2 2020 ISSN 0005-2531 (Print)
UDC 541.13.544.65
ELECTROREDUCTION OF THIOSULPHATE IONS FROM NON-AQUEOUS
SOLUTIONS
V.A.Majidzade
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
Received 23.05.2019 Accepted 19.09.2019
Sulphide semiconductor materials have attracted much attention due to their excellent photoelectrochemical and high photocatalytic activities. Sulphides of transition metals are semiconductor materials with band gaps in the range of 1-2 eV which these find application in optoelectronics and as quantum dots. The research work is devoted to the electrochemical reduction of thiosulphate ions from ethylene glycol. By recording cyclic and linear polarization curves on Pt and Ni substrates, the kinetics and mechanism of the process and the influence of various factors on the electroreduction of thiosulphate ions have been studied. Polarization curves show that the electroreduction process proceeds in one-stage within a potential interval of (0.5-(-1.0)) V. According to the obtained data, the effective activation energy was calculated. The calculation results show that the electroreduction process of thiosulphate ions in ethylene gly-col is accompanied by concentration polarization.
Keywords: polarization, thiosulphate ions, electrochemical reduction, semiconductors, ethylene glycol.
doi.org/10.32737/0005-2531-2020-2-61-66 Introduction
Metal chalcogenides are the main material in the manufacture of modern thin-film solar cells. As is known, depending on the band gap, these semiconductor thin films exhibit different properties - thermoelectric, photocatalytic, photoelectrochemical, and so on. Therefore, they are widely used as catalysts for photogalvanic cells, photoresistors, laser materials, optical recording devices and in other technical fields [1-7].
Semiconductors with S and Se indicate a great band gap among the other chalcogenides.
To obtain thin films of semiconductor materials by co-deposition, as is known, the first step is to study the kinetics and mechanism of the electroreduction process of the components separately. Therefore, our work aims to study the kinetics and mechanism of the electro-reduction process of thiosulphate ions from eth-ylene glycol, to determine the potentials region at which sulfur is deposited and there is a deep reduction, for obtaining thin sulfide films.
The electrodeposition of elemental sulfur has not been widely studied due to its utilization mainly in the form of compounds, and also because of its high specific electrical resistance [8-14].
In [8] the authors actualized an electrore-duction process of thiosulfate ions on Multi-Walled Carbon Nano Tubes Paste (MWCNT) electrodes in aqueous electrolytes. The results show that at higher concentrations of thiosul-phate ions on the surface of the electrode, the process proceeds more slowly than at lower concentrations this oceurs because that thiosul-phate dissociates in an acidic medium, forming colloidal sulfur and sulfite ions, which can be electroreduced into sulfide ions. It was found that the electrochemical reduction of thiosul-phate at lower pH values differs from those at higher pH values. The activation energy of the thiosulphate was calculated at various cathodic potentials, and it was found that the activation energy of the studied electrochemical reaction is of relatively a small value of the effective activation energy (~20.8 kJ/mol), that is specific of the typical diffusion process.
The authors of [9] recording cyclic current-voltage curves studied the deposition of sulfur from an electrolyte containing 0.10 M Na2S4+0.10 M KClO4 in dimethyl sulfoxide (DMSO) on the Au surface, ITO (indium-tinoxide) and glassy carbon electrodes. In this case, anodic sulfur peaks are observed on all
three substrates, but thick and adhered to electrode films were obtained only on Au electrodes. Films with a high percentage of sulfur S42- are obtained by adding Na2S and S to the electrolyte in a molar ratio of 1:3. Also, the electrolyte was boiled at 1600C for 80 hours with recooling it. Anode electrodeposition of sulfur on Au electrodes within 48 hours shows that at a potential of +0.46 V relative to Ag/Ag+, a thin film with a thickness of about 10 p,m is obtained. Elemental analysis of EDX suggests that these thin films contain only sulfur. The current density for electrodeposition of anode sulfur in LiClO4 is 60% higher than in electrolytes containing KClO4, and more compact sulfur -deposits are obtained. A higher current density for LiClO4-containing electrolytes is at least partially related to a higher rate of electron transfer in LiClO4 in comparison with KClO4-containing electrolytes, that demonstrated by electrochemical impedance spectroscopy (EIS).
In [10] the kinetics and mechanism of electrode processes of sulfur redox reactions, which occur on an electroconductive sulfur-graphite electrode in an alkaline medium have been studied by the potentiodynamic method. Anodic-cathodic and cathodic-anodic cyclic polarization curves, and also anodic polarization curves have been recorded to clarify the mechanism of electrode processes that occur during polarization by the alternating current on a sulfur-graphite electrode. According to polarization measurements, the kinetic parameters were calculated: charge transfer coefficients, diffusion coefficients (D), heterogeneous rate constants of the electrode process (k), and effective activation energy of the process (E). An analysis of the obtained results and the calculated kinetic parameters of the electrode processes showed that the discharge-ionization of sulfur in alkaline solutions proceeds in two successive stages and is a quasi-reversible process.
The authors of [11] were firstly studied the electrochemical behavior of an electrocon-ductive sulfur electrode in a sulfate medium. Based on the current-voltage studies, it was shown that the concentration of sulfate solution, the electrolyte temperature, and the potential
sweep speed significantly affect on the electrode processes of sulfur-electrode.
In [12], it was investigated the electrochemical behavior of sulfite, thiosulfate, and selenosulfate ions in alkaline solutions on various solid electrodes. According to the results, the electrode material significantly affects the cathodic and anodic behavior of the above indicated ions. It was established that SO I' ions
exhibit electrochemical activity on platinum, nickel and copper anodes.
The work [13] is devoted to the electrochemical reduction of thiosulfate ions on a Mo electrode. The nature of the polarization and the influence of some factors on the electroreduction process have been investigated by the authors. The results show that the electroreduction of thi-osulfate ions on the Pt cathode occurs at a potential of -0.5 V by electrochemical polarization.
The authors of [14] also studied the effect of temperature on the kinetics of sulfur deposition from sulfuric acid solutions. The nature of polarization in particular sections of the cathod-ic process was determined, and the areas of appearing electrochemical and concentration polarization were revealed.
The voltamperometric method was used to study the electrochemical reduction of sulfite ions on a Pt electrode from tartaric solutions [15]. When studying the kinetics and mechanism of the process referring to cyclic and linear polarization curves, it was achieved that the process of electroreduction is accompanied by electrochemical polarization. The results of all the performed experiments show that the concentration of sulfite ions, temperature, and pH of the solution effect on electroreduction process of sulfite ions from tartaric electrolytes. Using these studies, the optimal mode (T = 298 K, pH = 1.6-2.3) and the electrolyte composition, mol/L (0.1 Na2SO3 + 0.007 C4H6O6) were chosen for the electroreduction of sulfite ions.
According the literature data in the it is clear that the kinetics and mechanism of the electrochemical reduction of thiosulfate ions from aqueous and non-aqueous electrolytes have been studied in different ways. Therefore, our work aims to determine a certain potential
region by studying the kinetics and mechanism of the electroreduction process of thiosulfate ions till sulfide ions in non-aqueous medium (as an example of ethylene glycol).
Experimental part
The electrolyte composition used in the experiments for non-aqueous electrolytes is as follows: 1.0 M Na2S2O35H2O was dissolved in 100 ml ethylene glycol at a temperature of 313-323 K.
Polarization curves were recorded at the IVIUMSTAT Electrochemical Interface poten-tiostat. An electrochemical three-electrode glass cell was used in this case. A Pt electrode with an area of 0.4 cm and a Ni electrode with an area of 2 cm2 operated as a working electrode. The silver chloride electrode served as the reference electrode, and the platinum plate with an area of 4cm2 was used as the counter electrode. To control the temperature in the electrolyzer, we used the UTU - 4 universal ultra-thermostat.
For the actualization of experiments, platinum electrodes need periodic cleaning. At the start of the experiments, Pt electrodes were purified in concentrated nitric acid, and then washed with bidistilled water. Then they must be kept for 30 minutes in boiling nitric acid which contains a small amount of ferric chloride. Next, they should be thoroughly washed with tap water, and then with distilled water, and finally rinsed with alcohol or acetone.
Results and discussion
Kinetics and mechanism of electrochemical reduction process of thiosulphate ions from ethylene glycol have been determined by the potentiodynamic method (Figure 1).
As seen from the figure, the electroreduc-tion process carries out in one-stage, in more negative potentials of (0.5-(-1.0))V. In our opinion, within a potential interval of 0.2-(-0.5) V, adsorption of thiosulphate ions happens, but after -0.5 V they are reducted till S2- ions.
In Figure 2 the electrochemical reduction of thiosulphate ions in the non-aqueous electrolytes on the Ni-electrode was shown. As seen from the figure, the process on the Ni-electrode corresponds to the potential interval of 0.0-(-1.2) V. Within -0.2-(-1.2) V potential interval,
the thiosulphate ions are reduced till sulhide ions according to the following reaction:
S2O32- + 6H+ + 8e = 2S2- + 3H2O.
Our results correspond with reference data [16].
1.0 -0.5 0.0 0.5 V
Potential
Fig.1. The polarization curve of the electroreduction process of thiosulphate ions on Pt-electrode in nonaqueous solution. Electrolyte composition: 0.1 M Na2S2O3+CH2OH-CH2OH. Ev=0.02 (V/s), 7=293 K.
■1.0 -0.5 ' 0.0 ' CX5
Potential V
Fig.2. The potentiodynamic curve of the electroreduction process of thiosulphate ions on the Ni-electrode in non-aqueous electrolytes. Electrolyte composition: 0.1 M Na2S2O3+CH2OH-CH2OH. Ev=0.02 (V/s.), T=293 K.
Further, the Gorbachev method [17] was used to determine the kinetics of the process. To this end, the influence of the temperature on the electroreduction process of thiosulfate ions was studied within the intervals of 288-358 K by the potentiodynamic method (Figure 3). The results show that with an increase in the temperature of the electrolyte, the electroreduction
potential of thiosulfate ions shifts to a more positive side by about 0.2 V.
Fig. 3. The influence of temperature on electrore-duction process of thiosulphate ions on Pt-electrode. 1 - 288 К, 2 - 298 К, 3 - 30 8К, 4 - 318 К, 5 - 328 К, 6 - 338 К, 7 - 348 К. Electrolyte (mol/L): 0.1 М Na2S2Ü3 + CH2OH-CH2OH, Ev- 0.02 V/s.
osulfate ions from non-aqueous electrolytes is accompanied by concentration polarization (Figure 5), since, in the range of potential values of -0.95-(-1.0) V, the effective activation energy slightly depends on the potential.
As can be seen from Figure 3, at a temperature of 288 K and 348 K, the beginning of a noticeable electroreduction occurs in -1.1 V and -0.85 V potentials respectively. Using these polarization curves, the dependence logik-1/r was plotted for various potentials (Figure 4).
3.0 3.2 3.4 l/T10',Kt
-0.4-0.6 ■ \
5 4
3
2 I
Fig.4. The dependes lgik-1/r. potentials (V): 1 - (-0.95), 2 - (-1.0), 3 - (-1.05), 4 - (-1.1), 5 - (-1.15).
The tga value is calculated from the obtained lines. The of the effective activation energy obtained from Aef = 2.3Rtga equation, shows that the electrochemical reduction of thi-
But, with a further shift of the potential to the negative direction, it decreases and after -1.1 V practically does not change, i.e. a decrease in the concentration of discharging ions at the cathode surface because of a further increase in the potential is independent of the potential.
The effect of the concentration of thiosulfate ions on the process is first studied (Figure 6) for explaining the influence of various factors on the electrochemical reduction of these ions.
mA
1.5 -1.0 -0.5 0.0
Potential
Fig. 6. The effect of concentration of thiosuphate ions on electroreduction process on Pt electrode. Electrolyte (mol/L): Na2S2Os + CH2OH-CH2OH. Concentration (mol/L): 1 - 0.005, 2 - 0.05, 3 -0.1, 4 - 0.15, 5 - 0.2. T = 298 K. EV= 0.02 V/s.
As can be seen, the polarization curves of the electroreduction process with increasing concentration of thiosulfate ions in the solution also shift to the positive direction. Since, in the concentration of thiosulfate ions of 0.005 and 0.2 mol/L, electroreduction begins at potential values of -1.2 V and -0.95 V, respectively.
It has been also investigated the influence of scanrate on the electroreduction process. Figure 7 shows the polarization curves of the electroreduction process of thiosulphate ions taken linearly. As can be seen from the figure, with increasing in the scanrate, an increase in current spent on electroreduction process happens. Since, the current in 0.005V/sec. and 0.1V/sec. is equal to -3.72-10-4 A and -5.23-10-4 A, respectively.
-0.5 Potential
Fig. 7. The influence of scanrate on electroreduction process of thiosulphate ions on Pt electrode. Electrolyte (mol/L): 0.1 M Na2S2O3 + CH2OH-CH2OH. Potential sweep (V/s.): 1 -0.005, 2 - 0.02, 3 - 0.04, 4 - 0.06, 5 - 0.08, 6 -0.1. T = 298 K.
To clarify the kinetics and mechanism of the electroreduction process of thiosulfate ions,
we also plotted a relationship between ip and
1/2
v (Figure 8). As can be seen from the Figure specified dependency is straightforward. That is, with increase in the scanrate, ip also increas-es.is with increase the speed of scanrate, the rate of the cathodic process increases.
Conclusions
The voltamperometric method was used to study the electrochemical reduction of thiosulfate ions on Pt and Ni electrodes from non-aqueous solutions.
Fig. 8. Dependence of the peak current density dependence on the square root of scanrate. Electrolyte (mol/L): 0,1 M Na2S2O3 + CH2OH-CH2OH. Scanrate (V/sec.): 1 - 0.005, 2 - 0.02, 3 - 0.04, 4 - 0.06. 5 - 0.08, 6 - 0.1. T = 298K.
Polarization curves show that the electro-reduction process proceeds in one-stage within a potential interval of (0.5 - (-1.0)) V. The effect of concentration of thiosulfate ions, temperature, the nature of the electrode, etc. on the electroreduction process of thiosulfate ions in non-aqueous electrolytes was also studied. Using these studies, the optimal mode and composition of electrolyte were selected for the electroreduction process of thiosulfate ions in non-aqueous electrolytes. At studying the kinetics and mechanism of the process by use of polarization curves, it was revealed that the electrore-duction, in this case, is accompanied by concentration polarization.
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SUSUZ MOHLULLARDAN TIOSULFAT iONLARININ ELEKTROKlMYOVi REDUKSlYASI
V.A.Macidzad3
Kükürdlü yarimkegirici materiallar yüksak fotoelektrokimyavi va fotokatalitik aktivliklarina göra diqqati calb edirlar. Kegid metallarinin sulfidlari, optoelektronikada va kvant texnikasinda tatbiq olunan 1-2 eV qadagan olunmu§ zölaga malik yarimkegirici materiallardir. Taqdim edilan i§ etilenqlikol elektrolitindan tiosulfat ionlarinin elektrokimyavi üsulla reduksiyasina hasr edilmi§dir. Tsiklik va xatti polyarizasiya ayrilarinin gakilmasi ila Pt na Ni elektrodlan üzarinda tiosulfat ionlannin elektrokimyavi reduksiya prosesinin kinetika va mexanizmi öyranilmi§, müxtalif amillarin reduksiya prosesina tasiri tadqiq edilmi§dir. Polyarizasiya ayrilari göstarir ki, elektroreduksiya prosesi bir marhalali olub, (0.5-(-1.0)) V potensial intervalinda ba§ verir. Alinan naticalara asasan prosesin effektiv aktivla§ma enerjisi hesablanmi§dir. Hesablamalar göstarir ki, etilenqlikol elektrolitindan tiosulfat ionlannin elektroreduksiya prosesi qatiliq polyarizasiya ila mü§ayiat olunur.
Agar sözlar: polyarizasiya, tiosulfat ionu, elektrokimyavi reduksiya, yarimkegirici, etilenqlikol.
ЭЛЕКТРОВОССТАНОВЛЕНИЕ ТИОСУЛЬФАТ ИОНОВ ИЗ НЕВОДНЫХ РАСТВОРОВ
В.А.Меджидзаде
Сульфидные полупроводниковые материалы привлекли большое внимание благодаря их превосходной фотоэлектрохимической и фотокаталитической активности. Сульфиды переходных металлов представляют собой полупроводниковые материалы с шириной запрещенной зоны в диапазоне 1-2 эВ, которые находят применение в оптоэлектронике и в качестве квантовых точек. Работа посвящена электрохимическому восстановлению тиосульфат ионов из этиленгликоля. Снятием циклических и линейных поляризационных кривых на Pt- и Ni- подложках изучены кинетика, механизм процесса и влияние различных факторов на процесс электровосстановления тиосульфат- ионов. Поляризационные кривые показывают, что процесс электровосстановления протекает одностадийно при интервалах потенциалов (0.5-(-1.0)) В. По полученным данным рассчитан эффективная энергия активации. Результаты вычисления показывают, что процесс электровосстановления тиосульфат-ионов в этиленгликоле сопровождается концентрационной поляризацией.
Ключевые слова: поляризация, тиосульфат-ионы, электрохимическое восстановление, полупроводники, эти-ленгликоль.
