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ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 2 2022 lv
ISSN 0005-2531 (Print) 3
UDC 541.13.544.65
ELECTRODEPOSITION OF COBALT FROM ALKALINE GLYCINE ELECTROLYTE N.R.Abishova, U.M.Gurbanova, R.G.Huseynova, A.Sh.Aliyev
M.Nagiyev Institute of Catalysis and Inorganic Chemistry,NAS of Azerbaijan
Received 08.12.2021 Accepted 27.12.2021
This article presents a study of the electrochemical reduction of cobalt ions from an alkaline glycine electrolyte by measuring cyclic and linear potentiodynamic polar curves. In the study, a cadmium salt (CoCl2) dissolved in a 1.5 M glycine solution was used as a source of cadmium ions and the pH of the electrolyte was adjusted to the required value by adding NH4OH to the solution. By taking cyclic polarized curves, the range of potentials at which the reduction of cobalt ions occurs was established. The effect of cobalt concentration and temperature on the reduction process of its ions from a studied electrolyte has been studied. Finding a linear relationship between ip and u1/2 established that the process of electrodeposition of cobalt ions from an alkaline glycine electrolyte is controlled by diffusion polarization, which was confirmed by the curves obtained when plotting the lglk - 1/T dependence.
Keywords: cobalt, electrodeposition, glycine, polarization, alkaline electrolyte.
doi.org/10.32737/0005-2531-2022-2-113-120
Introduction
It is known that when studying the codeposition of two or more metals, it is necessary to first study the deposition of each of the components that make up the alloy separately under the same conditions. The ultimate goal of the study is to obtain thin Co-Mo-P films from al-kali-glycine electrolytes. In this paper, we present studies related to the cobalt deposition separately from the same electrolyte. The electro-deposition of cobalt has been studied by many researchers due to its ferromagnetic nature and potential applications in information storage devices and magnetic sensors [1, 2], and cobalt-based alloys attract researchers with their magnetic properties [5-8]. Electrodeposited cobalt and its alloys are widely used in the computer industry as recording media both on a magnetic drum and on a magnetic tape memory system [3, 4]. Dennis and others produced cobalt coatings and Co alloys with tungsten and molybdenum by electrodeposition from an acidic Watts type of electrolyte [9, 10]. In the works, the structure and some properties of cobalt-molybdenum and cobalt-tungsten alloys were studied, it was found that these deposits are fine-grained, hard and relatively brittle with a low coefficient of friction.
However, compared to other elements, very little research has been devoted to the elec-trodeposition of cobalt. Thus, the kinetics of the initial stages of cobalt electrodeposition on glassy carbon from a chloride bath containing citrate was studied in [11] using various electrochemical methods. Thin films of cobalt were deposited on the surface of glassy carbon from a chloride electrolyte containing citrate ions by both galvanostatic and potentiostatic methods. The dependence of the physical properties and morphology of the deposited films on the composition and pH of the electrolyte, temperature, deposition potential, and current density was studied. It was concluded that the electrodeposi-tion of Co from citrate electrolytes occurs via instantaneous nucleation followed by three-dimensional growth.
Some researchers argue that the electro-deposition of cobalt from acidic electrolytes occurs similarly to the electrodeposition of other metals of the iron group in two stages with stepwise electron transfer [12], the first stage including the formation of an intermediate adsorbed ion. The authors of [13] found that, depending on the pH of the electrolyte and the applied potential or current, the electrodeposition of cobalt occurs simultaneously with the reaction of hydrogen evolution. Cobalt films were obtained from an electrolyte containing 0.01 M
CoCl2 and 0.1 M NaCl without any additives, and glassy carbon electrodes were used as an anode and cathode, and a saturated calomel electrode was used as a reference electrode [14]. The studies were carried out by the methods of cyclic voltammetry, chronoamperometry, and by the removal of linear polarization curves. It turned out that at potentials from 0.0 to -0.3 V, cobalt precipitation does not occur, and in the range from -0.3 to -0.9 V, cobalt precipitation occurs simultaneously with hydrogen evolution, while the process of hydrogen evolution prevailed. But at a potential of -1.1 V, according to the polarization curves, the deposition of cobalt was predominant with a current efficiency of 96%. The linear relationship between the square root of the scan rate and the cathodic current peak indicates that the cobalt deposition process was controlled by diffusion polarization.
The study of cobalt deposition from an electrolyte containing 0.5 M sodium sulfate, 0.5 M boric acid, and CoSO47H2O of various concentrations was carried out using a rotating glassy carbon disk [15]. The rotation speed of the disk electrode was varied from 400 to 2500 rpm, platinum served as an anode, and a saturated calomel electrode (SCE) was used as a reference electrode. Sulfuric acid or sodium hydroxide was used to achieve the desired pH (pH=6). It has been established that in a sulfate electrolyte, the process of deposition and dissolution of a thin cobalt film is a surface reaction that occurs without significant overvoltage. The rate of hydrogen reduction, a reaction that competes with the process of cobalt deposition, depends on the pH of the electrolyte. At low pH values, intense hydrogen evolution occurs, which leads to the formation of a hydrogen-rich cobalt phase. As the pH of the solution increases, this effect is minimized and a pure cobalt phase is formed.
The authors of [16] studied the process of cobalt electrodeposition from an electrolyte containing 0.1 M K2SO4+1 mM H2SO4+10 mM CoSO4 in the presence and absence of chloride ions at pH=3. A platinum plate was used as a cathode. Hydrogen evolution was the main obstacle to Co2+ deposition, which reduced the current efficiency of the main product to 63 %. It was found that
thicker films with a highly ordered, even structure were deposited from sulfate electrolytes.
Cobalt was deposited on various substrates, such as nickel, glassy carbon, and copper, and from different electrolytes containing aqueous solutions of chloride or sulfate [17-19], citrate solution [20], triethylenediamine, cobalt (III) chloride in 30 % KOH, solutions of cobalt (II) thiocyanate in N,N-dimethyl formamide [21, 22], Co (II) in aqueous solutions of ammonium chloride [23]. More recently, the crystallographic structures of an electrodeposited Co film have been obtained from aqueous solutions with five organic solvents (ethanol, formamide, N-methyl-formamide, N, N-dimethylformamide, and dimethyl sulfoxide) in baths [24].
Electrodeposition of cobalt on a steel substrate was carried out from solutions containing cobalt sulfate, boric acid, and sodium gluconate [20]. The effect of the electrolyte composition, current density, pH, and temperature on the cobalt deposition process was studied, and the efficiency of the cathode current and the scattering ability of the electrolyte was determined. It has been established that the microhardness of cobalt electrodeposited from gluconate electrolytes is higher than that of cobalt deposited under similar conditions from sulfate, chloride, bromide, and acetate electrolytes.
To avoid coprecipitation of impurities, some researchers have considered the possibility of using organic solutions to obtain pure cobalt [25-29].
Ethylene glycol electrolytes containing chloride salts of cobalt have been studied in the electrodeposition of thin films and nanowires [30]. The electrochemical deposition of 0.5 M Co (II) chloride solution at 700C was investigated by cyclic voltammetry on Pt substrate. It was shown that the process of Co reduction is irreversible with a high Faraday efficiency (85-90 %). The electrodeposition process was also studied on the copper substrate at various cathode potentials from -0.75 V to -0.95 V.
The present research aims to develop new electrolytes for the deposition of good quality cobalt from more environmentally friendly electrolytes than traditional ones, given that the process of cobalt deposition from gly-
cine electrolytes has not been studied. In addition, the study is aimed at shedding light on the mechanism of cobalt electrodeposition from alkaline glycine electrolytes and elucidating the influence of factors such as cobalt ion concentration and electrolyte temperature in this process.
Experimental part
When studying the electrochemical deposition of cobalt from an alkaline glycine electrolyte, the electrolyte was prepared by introducing the calculated amount of cobalt chloride Coa2-6H2O (Coa2-6H2O of the Indian firm, Qualikems Fine Chem Pvt. Ltd.) into a 1.5 M glycine solution, the pH value of the electrolyte was adjusted to 10 by adding NH4OH to it. Linear and cyclic polarization curves were recorded on an IVIUMSTAT Electrochemical Interface potentiostat using a three-electrode cell with a capacity of 100 ml, equipped with a water jacket. A platinum wire with an area of 3-10-3 dm2 served as a working electrode, a platinum plate with an area of 4-10-2 dm2 served as an auxiliary electrode, and a silver chloride electrode served as a reference electrode. When studying the effect of temperature on the reduction of cobalt ions during the recording of polarization curves, the temperature in the cell was maintained constant using a universal ultrathermostat UTU-4.
Results and discussion
The stationary potential of a platinum electrode in an alkaline glycine electrolyte is from 0.1 V c.s.e. regardless of the concentration of cobalt ions in the electrolyte. To study the mechanism of cobalt deposition from an alkaline glycine electrolyte, linear and cyclic polarization curves of the reduction of cobalt ions were taken depending on the temperature and concentration of cobalt in the electrolyte.
Figure 1 shows a cyclic current-voltage curve for the reduction of cobalt ions in an alkaline glycine electrolyte on a platinum electrode. The composition of the electrolyte included 0.1 M CoCl2, 1.5 M H2N-CH2-COOH, and the pH of the electrolyte was controlled by adding NH4OH to the electrolyte. It can be seen from the curve that the reduction of cobalt ions begins at a potential of ~0.0 V, and the first traces of deposited cobalt appear at this potential. At
the potential of -0.18 V a small plateau appears, and further, at potentials of -0.85 ^ -0.9 V the deposition of cobalt occurs already simultaneously with the release of hydrogen. By the location of the anodic and cathodic components of the polarization curve, it can be seen that the cathodic and anode curves are not symmetrical, which indicates the irreversibility of the process of cobalt deposition from alkaline glycine electrolytes, and the difference in the heights of the cathode and anode peaks, it is determined that the deposition occurs in two stages according to reactions 1 and 2:
Co2+ +e- ^ Co+ (1)
Co+ +e- ^ Co (2)
On the anode component of the polarization curve (Figure 1), can see a peek at a potential of -0.42 V and a small plateau at a potential of +0.11 V. Thus, the dissolution of cobalt precipitates occurs in two stages. Such behavior of electrodeposited thin cobalt films was also found by other authors during the deposition of cobalt from citrate electrolyte [13, 15, 31].
It is assumed that the first plateau at a potential of -0.42 V corresponds to the dissolution of the hydrogen-enriched cobalt phase deposited at higher potentials close to the hydrogen evolution potential. The second peak, at a potential of +0.11 V, corresponds to the dissolution of the pure cobalt phase. According to [15], these cobalt phases have different crystal structures, cobalt coatings containing hydrogen atoms had a hexagonal close-packed structure (hcp), while purely cobalt coatings had a face-centered cubic structure (fcc). It is the difference in crystal structures that explains the dissolution of cobalt layers at different anodic potentials, first the phase enriched in hydrogen is dissolved, and then the phase of pure cobalt.
Figure 2 shows the cyclic current-voltage curves of cobalt from solutions containing its various concentrations. The cobalt concentration in the electrolyte varied from 0.1M to 0.8M. It can be seen from the curves that with an increase in the concentration of cobalt ions in the electrolyte, the reduction potential of both pure cobalt and hydrogen-enriched cobalt shifts to the positive side, and the height of the peaks increases accordingly.
Fig.1. Cyclic volammogram of cobalt deposition from an electrolyte containing 0.1M CoCl2; 1.5M H2N-CH2-COOH, pH=10, u=0.1V/sec, T=298K.
Fig.2. Cyclic current-voltage curves for the reduction of cobalt ions from an alkaline glycine electrolyte at various concentrations of CoCl2 in the electrolyte. Electrolyte composition (M): 1.5 H2N-CH2-COOH, CoCl2 -1-0.1; 20.3; 3-0.5; 4-0.8. pH=10, Ev=0.1V/sec, T=298K.
With an increase in the concentration of cobalt in the electrolyte, the plateau corresponding to the reduction of cobalt ions in the cathode components of the polarization curves gradually disappears and is almost not noticeable at concentrations of 0.5 -^0.8 M. On the anodic dissolution curves of cobalt at low concentrations of its ions in the electrolyte (curves 1 and 2), the height of the peaks corresponding to the dissolution of pure cobalt is higher than at its high concentrations (curves 3 and 4). This suggests that at low concentrations of Co ions in the electrolyte, a greater amount of pure cobalt is deposited than at high concentrations, and more hydrogen-enriched cobalt is formed. This can
be judged by the height of the peaks corresponding to the dissolution of the hydrogen-enriched cobalt phase at potentials of -0.3 ^ -0.2 V, which also increase and shift towards more positive values.
To elucidate the nature of polarization and determine the rate-limiting stage in the electrodeposition of cobalt from alkaline glycine electrolytes, we studied the effect of the potential scan rate on the cathode process.
The current value noticeably increases with an increase in the potential sweep rate, (Figure 3), and the dependence between the current peak (ip) and u1/2 is linear (Figure 4.), which indicates the diffusion nature of the polarization during cobalt deposition.
Fig.3. Current-voltage curves of reduction of cobalt ions at different values of the potential sweep rate on the platinum electrode. Electrolyte composition (M): 0.1 - CoCl2; 1.5 - H2N-CH2-COOH, pH=10; T=298K; Ev - 1-0.01 V/s; 2-0.02V/sec; 3--0.04 V/s; 4-0.06V/sec; 5--0.08V/sec.
Fig.4. Dependencies ip and u
Potential v
Fig.5. Polarization curves of cobalt reduction depending on electrolyte temperature. Electrolyte composition (M): 0.1-CoCl2; 1.5-H2N-CH2-COOH, pH=10; Ev=0.1V/sec. T: 1-298K; 2-308K; 3-323K; 4-333K..
To verify the correctness of this conclusion, polarization curves were taken depending on the electrolyte temperature. When studying this dependence, the electrolyte temperature varied in the range from 298 K to 333 K.
Figure 5 shows that an increase in temperature also increases the reduction rate of cobalt ions. Based on the temperature dependence curves of the polarization curves, the dependence of lgik on 1/T was plotted for various potential values; the results obtained are shown in Figure 6.
It can be seen from the figure that for all values of the potential, the dependence lgik - 1/T is rectilinear, and the slope of lines 1-4 is almost the same, the slope of the line corresponding to the potential -0.725 V is somewhat less, and the largest slope of line 6, corresponding to
the potential -0.75 V. From the slope of these straight lines, the values of tga were determined for each straight line and the values of the effective activation energy Aeff were calculated using the formula:
Aeff= -2.3Rtga (3)
From Figure 7 and according to [32], we can conclude that the process of reduction of cobalt ions from an alkaline glycine electrolyte is controlled by diffusion polarization.
Thus, it can be concluded that the reduction of cobalt ions from an alkaline glycine electrolyte occurs in two stages. First, pure cobalt is deposited, then hydrogen-enriched cobalt is. Increasing the temperature and the rate of potential sweep increases the rate of the cathod-ic process, and the deposition process itself is initially controlled by diffusion polarization.
Fig.6. The dependence of the current density on the temperature at various potentials Ec(V): 1-(-0.6); 2-(-0.65); 3-(-0.675); 4-(-0.7); 5-(-0.725); 6-(-0.75).
-0.75 -0.7 -0.65 -0.6 E,V Fig.7. The dependence of the effective activation energy on the potential.
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QOLOVÍ QLÍSÍN ELEKTROLÍTÍNDON KOBALTIN ELEKTROÇÔKDÛRÛLMOSi
N.R.Abi$ova, Ü.M.Qurbanova, R.Q.Hüseynova, A.§.0liyev
Maqalada tsiklik va xatti potensiodinamik polyarlaçma ayrilarinin çakilmasi yolu ila qalavi qlisin elektrolitindan kobalt ionlarinin elektrokimyavi reduksiyasina dair tadqiqatlar taqdim olunur. Kobalt ionlarinin manbalarinin keyfiyyatini ôyranmak ûçûn kobalt duzu (CoCl2) 1.5 M qlisin mahlulunda hall edilmiç va elektrolitin pH-ni lazimi qiymata çatdirmaq ûçûn mahlula NH4OH alava edilmiçdir. Tsiklik polyarizasiya ayrilari çakmakla kobalt ionlannin reduksiyasinin potensial diapazonu müayyan edilmiçdir. Tadqiq olunan elektrolitdan kobaltin elektroçôkma prosesina asas komponentlarin konsentrasiyasinin va temperaturun tasiri ôyranilmiçdir ip va u1/2 arasinda xatti asililiqdan gorünür ki, kobalt ionlannin qalavi qlisin elektrolitindan elektroçôkdûrûlma prosesi diffuziya polarizasiyasi ila idara olunur. Bu da lgik - 1/T asililigi ila sübut olunur.
Açar sozlar: kobalt, elektroçôkdûrûlma, qlisin, polarizasiya, qalavi elektrolit.
ЭЛЕКТРООСАЖДЕНИЕ КОБАЛЬТА ИЗ ЩЕЛОЧНОГО ГЛИЦИНОВОГО ЭЛЕКТРОЛИТА
Н.Р.Абышова, У.М.Курбанова, Р.К.Гусейнова, А.Ш.Алиев
В данной статье представлено исследование электрохимического восстановления ионов кобальта из щелочного глицинового электролита путем измерения циклических и линейных потенциодинамических поляризационных кривых. В качестве источников ионов кобальта использовали соль кобальта (СоС12), растворенную в 1,5 М растворе глицина, а рН электролита доводили до необходимого значения добавлением в раствор МН4ОН. Путем снятия циклических поляризованных кривых был устанавлен диапазон потенциалов, при котором происходит восстановление ионов кобальта. Исследовано влияние концентрации кобальта и температуры электролита на процесс восстановления его ионов из щелочного глицинового электролита. Линейная зависимость между ip и и12 позволяет сделать вывод, что процесс электроосаждения ионов кобальта из щелочного глицинового электролита контролируется диффузионной поляризацией, что было подтверждено кривыми, полученными при построении зависимости - 1/Т.
Ключевые слова: кобальт, электроосаждение, глицин, поляризация, щелочной электролит.
