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120 AZERBAIJAN CHEMICAL JOURNAL № 4 2023 ISSN 2522-1841 (Online)
ISSN 0005-2531 (Print)
UDC 541.13.544.65
ELECTROCHEMICAL SYNTESIS OF A CATHODE MATERIAL BASED ON Co-Se
ALLOY
K.I.Hajiyeva, Y.E.Alizade, A.G.Aliyeva, S.P.Javadova
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, Ministry of Science and Education of the
Republic of Azerbaijan
Received 16.01.2023 Accepted 06.04.2023
The present work is devoted to the electrochemical deposition of thin CoSe films and study of the influence of various factors on the composition of the resulting film. It was established that during the deposition of both individual components and in their joint presence, the electroreduction potentials (Se, Co and CoSe) are respectively equal to 0.35, -0.2, 0.23 V. Anodic dissolution of CoSe is observed at a potential of 0.75 V. The electrochemical reduction of selenium and cobalt occurs in 2 stages. The results demonstrated that both the temperature and the electrode material have a significant effect on the kinetics of CoSe electrodeposition. At temperatures above 600C, the quality of the resulting films deteriorates. High-quality crystalline films with high adhesion surface are obtained in the temperature range of 25-600 C. Pt, Ni were taken as a cathode material and Pt served as the anode. The optimal electrolyte composition and electrolysis mode for obtaining thin CoSe films close to the stoichiometric composition were also determined. 0.01 M H2SeO3, 0.1 M CoCl2, 100 ml H2O, i=0.2-0.4 mA/sm2, T=25-600C, Cathode - Pt, Ni. anode Pt.
Keywords: CoSe, thin coatings, electrochemical deposition, alloys, current density.
doi.org/10.32737/0005-2531-2023-4-120-127 Introduction
Electrolysis of water is regarded as an attractive and feasible way for producing hydrogen. So far, various non-noble metal nanomaterials have been reported as excellent electrocatalysts for hydrogen evolution reaction. Especially, due to the low cost, earth-abundance and tunable properties, transition metal selenides with different compositions, sizes and structures have been explored broadly as efficient catalysts with the relatively high activities, high stabilities and high efficiencies in full pH range of electrolyte for electrochemical hydrogen evolution reaction [1].
The electrode material selected should have good corrosion resistance, high conductivity, high catalytic effect, and low price [2].
The more current passes through one unit of surface area, the more hydrogen is released; therefore, it is necessary to take into account the properties of the surface and the structure of the selected cathode material. CoSe has just such a structure. Thus, the number of surface-active sites on the surface of CoSe is large, and these active sites have catalytic properties. Co, as a
monatomic element, is used as a catalyst in hydrogen evolution during water electrolysis, however, the catalytic properties of cobalt chalco-genide solutions are 15-20 times higher than that of monatomic Co.
Transition metal chalcogenides, comprised of earth-abundant elements, have served as effective alternatives for expensive, noble metal catalysts for the hydrogen evolution reaction (HER), and the oxygen evolution reaction (OER). Especially, cobalt selenides have attracted much attention due to their favorable attributes such as good electrical conductivity, optimal bandgap (~1.5 eV) in terms of match with the solar spectrum, and a high optical absorption coefficient. Cobalt-based compounds including metallic cobalt, oxides, hydroxides, carbides, nitrides, phosphides, sulfides, selenides, tellurides, and so on have been explored as alternative catalysts for electrochemical water splitting because of the natural abundance, low cost, and simple chemistry to produce compounds with various valence states [3-7].
As one of the advanced cobalt-based materials, cobalt sulfides with novel architecture have attracted huge interest due to the low cost, easy availability, and promising bifunctional activity for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), which is essential for next-generation energy storage devices [8].
Cobalt selenide has a variety of compounds such as CoSe2, CoSe, Co0.85Se, Co3Se4 and Co2Se3. Cobalt selenide nanomaterials have been prepared by diverse methods including hydrothermal method, solvothermal method, microwave-assisted pol-yol process, mechanical alloying, and potentiostat-ic electrodeposition route [9].
Several methods have been attempted to prepare these selenides thin films: molecular beam epitaxy, metallorganic chemical vapor deposition, evaporation, chemical bath deposition, electrodeposition, and spray pyrolysis [10].
When obtaining thin CoSe films by other methods (hydrothermal, vacuum deposition, etc.), changes in the composition, morphology, and structure of the obtained films are observed, since the process proceeds at a high temperature. These methods require complex expensive equipment and high temperatures and pressures.
Recently, increased interest in production of nanocoatings of binary ternary alloys by electrodeposition [11-17].
Electrochemical techniques have an important role in the modern concept of nanotech-nology. Electrodeposition is used for controlling the structure, composition and properties during the process of fabrication of nanostructured materials. This in turn facilitates the preparation of novel materials with features unattainable by any other techniques or processes [18].
Based on the principle of electrolysis, it is a process that uses electrical current to reduce the cations of a desired material from an electrolyte and coat those materials as a thin film onto a conductive substrate surface. This is done to achieve the desired electrical and corrosion resistance, reduce wear and friction, improve heat tolerance and for decoration [19].
Electrochemical production of Co-Se thin films in various solutions (chloride, acid-
acid, organic solutions) is widely described in the literature [20-24]. In this paper, the main goal is to study and propose a CoSe thin film as a cathode material for water electrolysis.
As is known, during electrolysis, metal ions, before entering the crystal lattice of the cathode deposit, go through a series of successive stages, each of which proceeds at a certain rate. The slowest stage limits the speed of processes in general.
According to the literature data, the electrodeposition of cobalt selenide was carried out from various solutions.
The paper reports the results of electro-deposition of cobalt selenide on the surface of a Pt electrode from a 0.1 M Na2SO4 electrolyte solution containing 5 mM SeO2 and 5 mM Co(CH3COO)2 by the method of linear sweep voltammetry. Four cathodic waves were observed during the linear scans and the reactions corresponding to these waves were investigated with LSV and EQCM. Combined stripping voltammetry and EQCM showed that CoSe was electrodeposited via two routes: (1) Underpo-tential deposition of Se followed by deposition of cobalt as CoSe; and (2) Reaction of Co(II) with electrogenerated Se(-II) to result in CoSe. Compositional analyses revealed that the elec-trodeposited films contained CoSe and free Se, depending on the deposition potential. However, no cobalt was found in these films because of chemical (galvanic) instability of the cobalt film in the deposition bath [25].
According to the results of work cobalt selenide thin films have been prepared onto tin oxide glass substrates by electrodeposition po-tentiostatically from an aqueous acid bath containing H2SeO3 and Co(CH3COO)2 at 500C. The electrodeposition mechanism was investigated by cyclic voltammetry. The morphological, compositional, structural, and optical properties of the deposited films have been studied using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and optical absorption [26].
Co-Se layer was electrodeposited from the electrolyte consisted of SeO2, CoCl2 and NH4Cl. The concentration of SeO2, CoCl2 and
NH4O was 5 mM, 5mM and 0.1 M, respectively. Cyclic voltammograms indicate that Se is more easily deposited on Co-Se phases than Pt electrode. Both electrode material and electrolyte temperature have significant effects on the electrochemical kinetics of Co-Se electrodepo-sition. Another anodic peak appears on GC electrode, which is associated with the dissolution of Se. However, the electrolyte temperature has little effect on the diffusion and nucleation mechanisms of Co-Se. The Co-Se electrodeposi-tion is controlled by diffusion growth process and the Co-Se nucleation displays 3D progressive nucleation process at both 20 and 500C. The diffusion coefficient D for Co and Se increases with the increase of temperature, respectively. Porous Co-Se layer with nanopheres was elec-trodeposited on ITO glass at -0.9 V and 40 ~ 600C. The nanospheres generate agglomeration when the deposition potential negatively shifts to -1.1 V, however no significant change can be seen when the temperature is 40 ~600C [27].
As can be seen from the literature (2027, 28, 29), 2 types of solvents (H2O+ inorganic, H2O+organic solvents) and a larger amount of reagents (Na2SO4 NH4Q CoCh SeO2 H2SeO3) were used to obtain thin CoSe films. But in this work, we used only 1 type of solvent (distilled water) and 2 reagents (CoCl2 and H2SeO3). Similarly to the literature data, in our work, the content of Se was low, because at a high content of Se, along with CoSe the formation of free Se is also observed.
The purpose of this work is the electrochemical synthesis of new nanomaterials based on the CoSe alloy on various substrates. For this purpose, a study of the cathodic processes of CoSe electrodeposition in an aqueous electrolyte on Pt and Ni electrodes was carried out. The studies were carried out in solutions of the following composition (mol/l): 0.01M H2SeO3 + 0.1M CoCl2 + H2O, T=250C. Ev =0.02 V/s.
Results and discussion
The working electrodes were platinum and nickel electrodes with the surface area of 0.07 cm . The three-electrode cell contained the electrode under study, an auxiliary platinum
electrode with an area of 4 cm2, and silver chloride reference electrode. To study the structure and composition the films were deposited on Pt and Ni substrates with a surface area of 2.0 cm2. The working temperature for electrodeposition was 25-600C, deposition time 60 min. The kinetics of the processes was controlled using measurements by the method of cyclic voltammetry on IVIUMSTAT. X-ray diffraction analysis of the obtained films was carried out on a DRON-5 using CuKa radiation. The films were obtained in the galvanostatic mode without electrolyte stirring. For analysis, the cathode deposit was dissolved on heating in concentrated HNO3 acid.
For the joint deposition of two metals on the cathode during electrolysis, it is necessary that the discharge potentials of each metal in the electrolyte be as close as possible. The discharge potential of each metal is determined by the value of its equilibrium potential and the magnitude of the cathode polarization.
In the case when the values of the equilibrium potentials of two metals differ markedly, but the cathodic polarization of one of them (more electronegative) is less than the polarization of the other (less electronegative), then joint deposition in this case is possible with an increase in current density.
Another way to achieve co-deposition on the cathode of two or more metals is to change their polarization during discharge. For this purpose, selective surfactants are introduced into the electrolyte, that is, additives that inhibit the discharge of ions of one of the components.
For a comprehensive study of the process of obtaining electrolytic alloys, it is necessary to have data on the kinetics of deposition of the alloy and individual components. The kinetics and mechanism of obtaining thin CoSe films by the electrochemical method were studied in an aqueous solution of CoCl2 and H2SeO3. By recording cyclic polarization curves, the range of deposition potentials was determined both for individual Co and Se components and for their joint presence of CoSe.
The cyclic polarization curve of selenium was recorded in the potentiodynamic mode on a Pt electrode in the potential range (-0.3^1.0V), at a potential sweep rate of EV=0.02V/s (Figure 1).
Fig. 1. Cyclic polarization curve of the electrochemical deposition of selenium from an aqueous electrolyte on a Pt electrode. Electrolyte composition: 0.01M H2SeO3 + H2O T=250C, EV=0.02V/s.
Starting from a stationary potential of 0.7 V in an aqueous solution of selenic acid 0.01 M H2SeO3, the process of selenium deposition proceeds in the potential range of (0.25 ^ 0.4 V). At a potential value of 0.35 V, selenium is deposited on the surface of the Pt electrode. Then the process continues. however, the deposition rate is reduced.
The electrochemical reduction of selenium occurs in 2 stages [28]: Se(1V)-Se(0) -Se(II)
H2SeO3 + 4H+ + 4e = Se + 3H2O H2SeO3 + 6H+ + 6e = H2Se + 3H2O H2SeO3 + 2H2Se = 3Se + 3H2O
The electrochemical reduction of selenium is confirmed by anodic dissolution at a potential of 0.9 V.
The cyclic polarization curve of cobalt was recorded in the potentiodynamic mode on a Pt electrode in the potential range of (-0.2^ -0.8V), at Ev = 0.02 V/s, I = 10 mA (Figure 2). Deposition of cobalt from 0.1 M CoCl2 starting from a stationary potential of 0.5 V is observed in the potential range of (-0.15 ^ -0.2V). The electrochemical reduction of cobalt proceeds in 2 stages (29).
C0+2 + e = Co+ I Co+ +e = Co0 II
Fig. 2. Cyclic polarization curve of the electrochemical deposition of cobalt from an aqueous electrolyte on a Pt electrode. Electrolyte composition: 0.1M CoCl2 + H2O T=250C. EV=0.02V/s.
Anode dissolution of cobalt occurs at a potential value of -0.2 V.
The cyclic polarization curve of CoSe shown in Figure 3 was recorded in the potentio-dynamic mode on a Pt electrode in the potential range of (-0.2V - - 1.0V) in a solution of 0.01M H2SeO3 + 0.1M CoCl2 + H2O. The electrodeposition process of CoSe is observed in the potential range (0.15 - -0.3 V), starting from the stationary potential 0.7 V. At a potential value of 0.23 V, the electrode surface is completely covered with CoSe.
Anodic dissolution of CoSe occurs at 0.75 V, then at a potential value of 0.9 V a peak of free selenium dissolution is observed. With a large amount of selenium in solution, along with CoSe, free Se atoms are also formed on the
cathode. If the amount of selenium is small, free Se atoms are not observed.
The formation of CoSe proceeds according to the following mechanism: Se+xCo2++2xe =CoxSe H2 S e+xC o2+=C oxS e+2H+ H2SeO3+xCo 2+4H+(4+2x)x=C oxSe+3H20.
At different values of the potential sweep rate, the cyclic polarization curve of the electrochemical deposition of CoSe was recorded. As can be seen from the cyclic curve, with an increase in the potential sweep rate, the formation of CoSe increases. The current consumed on cathodic process also increases, and the peaks of anodic dissolution are observed more clearly (Figure 4).
Fig. 3. Cyclic polarization curve of the electrochemical deposition of CoSe from an aqueous electrolyte
on a Pt electrode. Electrolyte composition: 0.01 M H2Se03 + 0.1 M CoCl2, T=25°C, Ev = 0.02 V/s.
Potential
Fig. 4. Cyclic polarization curve of the electrochemical deposition of CoSe on a Pt electrode from an aqueous electrolyte at E1=0.04 V/sec and E2=0.005 V/sec. Electrolyte composition: 0.01M H2SeO3 + 0.1M CoCl2, T=250C.
The nature of the electrode material has a slightly different effect on the electrochemical deposition of CoSe. As can be seen from the figure, the deposition potentials on the cyclic polarization curves recorded on nickel (0.7 ^ -0.9 V) and platinum (0.23 V) electrodes differ (Figures 3 and 5).
Temperature has a significant effect on the production of a thin CoSe film by the electrochemical method. At temperatures above
60 C, the quality of the resulting film deterio-
rates. High-quality CoSe films with high adhesion to the surface are obtained in the temperature range of 25-600 C. Pt and Ni were taken as a cathode material, and Pt served as the anode. In experiments carried out on a nickel electrode in the galvanostatistical mode, at low current densities, Se precipitation is observed (red precipitate), at high current densities, Co precipitation (black precipitate) and at medium values (i = 0.2-0.4 mA/cm2), the formation of CoSe (black-brown) is observed.
Fig. 5. Cyclic polarization curve obtained for CoSe on a nickel electrode. Electrolyte composition: 0.01 M H2SeO3+0.1 M CoCl2+H2O, Ev=0.02 V/s T=250C.
Fig. 6. X-ray pattern of CoSe taken from an aqueous solution of 0.1 CoCl2+0.01 H2SeO3+H2O on a Ni electrode.
According to the results of X-ray phase analysis, thin CoSe films obtained by electrochemical method on nickel electrode have an amorphous structure.
Deposited amorphous thin CoSe films were crystallized in an argon atmosphere at a temperature of 4000C for 1 hour. The X-ray phase study also confirms that thin CoSe films obtained by electrochemical method contain free Se atoms. Depending on the potential, the resulting films have n-type conductivity with a small content of Se and p-type conductivity with a large content of Se [30].
Based on the experimental results, the optimal electrolyte composition and the electrolysis mode for obtaining high-quality thin CoSe films were selected. 0.01 M H2 SeO3+0.1 M CoCl2, H2O, i=0.2-0.4 mA/sm2, T=25-600C, Katod - Pt, Ni . Anod - Pt.
Conclusion
By recording cyclic polarization curves, the electroreduction potentials were determined both for individual components Se, Co (0.35, -0.2) and in their joint presence (0.23 V). The nature of the electrode affects the process of CoSe electro-deposition in different ways. It was established that when recording cyclic polarization curves on Pt and Ni electrodes, CoSe films deposited at different deposition potentials. Depending on the electrode material the consumed current also differs. According to the results obtained, high-quality crystalline CoSe films are formed at a cathode potential of 0.23 V. Thus, based on experimental data, the following electrolyte composition (mol/l) is recommended for obtaining thin CoSe films: 0.01 M H2SeO3+0.1M CoCl2+H2O, i=0.2-0.4 mA/sm2, T=600C. Katod -Pt, Ni. Anod - Pt.
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Co-Se ORÎNTiSi OSASINDA KATOD MATERiALININ ELEKTROKiMYOVi ALINMASI
K.i.Haciyeva, Y.E.Olizada, A.Q.Oliyeva, S.P.Cavadova
Hazirki iç Co-Se nazik tabaqalarinin elektrokimyavi çôkdûrûlmasina va amala galan tabaqalarin tarkibina müxtalif amillarin tasirinin öyranilmasina hasr edilmiçdir. Malum olmuçdur ki, ham ayri-ayri komponentlarin çôkmasi zamani, ham da onlarin birga içtiraki zamani (Se, Co va CoSe) elektroreduksiya potensiallari müvafiq olaraq 0.35 V, -0.2 V, 0.23 V-a barabardir. CoSe-nin anod hall olmasi 0.75 V potensialinda mûçahida olunur. Selen va kobaltin elektrokimyavi reduksiyasi 2 marhalada baç verir. CoSe elektrokimyavi çôkmasinin kinetikasina ham temperatur, ham da elektrod materiali ahamiyyatli tasir göstarir. 600C-dan yuxari temperaturda alinan tabaqalarin keyfiyyati pislaçir. 25-600C temperatur intervalinda yüksak adgeziya qabiliyyatina malik keyfiyyatli kristal tabaqalar alinir. Katod materiali kimi Pt, Ni götürülmüijdür, anod - Pt. Eleketrokimyavi yolla alinmiç CoSe nazik tabaqalari Se-nin miqdari açagi olduqda n tipli keçiriciliya, Se-nin miqdari yüksik olduqda isa p tipli keçiriciliya malikdir. Bundan alava, elektrolitin optimal tarkibi va stexiometrik tarkiba yaxin nazik CoSe tabaqalarinin alinmasi ûçûn elektroliz rejimi müayyan edilmiçdir: 0.01M H2SeO3, 0.1М CoCl2, 100ml H2O, i=0.2-0.4 mA/sm2, Т=25-600С, Katod - Pt, Ni. Anod- Pt.
Açar sözlzr: CoSe, nazik örtüklsr, elektrokimyavi çôkms, arintilar, сзгзуап sixligi.
