CC BY
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ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)
AZERBAIJAN CHEMICAL JOURNAL № 3 2019
25
UDC 541.13.544.65
ELECTRODEPOSITION OF Ni-Mo ALLOYS FROM AMMONIUM ELECTROLYTES U.M.Gurbanova, R.G.Huseynova, H.M.Tahirli, S.D.Dadashova, A.Sh.Aliyev, D.B.Tagiyev
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
[email protected] Received 13.03.2019
The influence of various factors current density, concentration of electrolyte components of stirring, and temperature on the composition and quality of Ni-Mo thin films, obtained by electrochemical deposition has been studied. It is established that the composition of deposited compounds strongly depends on the concentration of electrolyte components and electrolysis conditions. Increasing the current density increases the molybdenum content, and increasing the temperature increases the nickel content in the sediments. Stirring affects the electrodeposition process slightly. For obtaining high quality deposits, optimal conditions and electrolyte composition have been determined.
Keywords: molybdenum, nickel, electrodeposition, electrocatalyst, ammonium hydroxide. https://doi.org/10.32737/0005-2531-2019-3-25-31
Introduction
Hydrogen is considered the most promising substitute for traditional fossil fuels [1-3]. The main problem in the production of hydrogen is its massive and cheap production. The hydrogen evolution reaction (HER) is relatively a simple reaction, because it goes through a limited number of stages with a single type of intermediate product. Among pure metals, the best catalysts for HER are precious metals. They are electrocatalytically active metals for the hydrogen evolution reaction; however, the deficiency and high cost don't allow them to be widely used in industry [4-7]. Therefore, to find a cheap, highly efficient and stable electrocata-lyst for the process of decomposition of water is an actual problem.
In accordance with the theory of electro-catalysis, electrocatalytic activity depends on the heat of adsorption of the intermediate compound deposited on the electrode surface, which is well known as the "volcano" curve [8, 9]. It is known that doping two or more metals from the two branches of the "volcano" can lead to increasing activity for HER [10].
Ni-Mo alloys are considered highly active catalysts for the hydrogen evolution reaction in alkaline electrolytes. The study of the kinetics and mechanism of HER on electrode-posited Ni-Mo alloys has been carried out by many scientists [11-17]. In works [18, 19], dedicated to the electrodeposition of Ni-Mo alloys, the mechanism of induced codeposition is discussed. Many works were devoted to the study
of the morphology and phase composition of elec-trodeposited Ni-Mo films. These studies showed that Ni-Mo alloys were almost amorphous and contained some oxide phases [20, 21].
New alloys of metals of the iron group with refractory metals, synthesized by the electrochemical method, exhibit electrocatalytic activity in the hydrogen evolution reaction that is not exceeding the electroactivity of noble metals [16-17]. As is known that Ni is perfect corrosion - resistant in hot concentrated alkaline solutions [22, 23]. Since nickel and its alloys in alkaline media are more resistant than other transition metals, such as Fe or Co, they arouse great interest among researchers as electrodes over the aqueous electrolysis reaction (HER) [24].
Ni-Mo alloys can be obtained by several methods: by metallurgy of easy oxidation at a high melting point of molybdenum, by method of powder metallurgy and mechanical doping [25], spark plasma sintering [26], laser cladding [27] and by electrochemical method [28].
Alloys on the basis Mo of over the past few years have been obtained either by electroplating or by thermal alloying [29, 30]. Comparing with the electrochemical method, all the other above - mentioned methods of the obtaining of Ni-Mo alloys are costly and labor-consuming. The electrochemical method is simple to perform, does not require complex equipment, it is possible to easily control the composition of the deposited alloy changing the electrolysis conditions and the composition of the electrolyte [16, 17].
We obtained with electrochemically method thin films of Ni-Mo on platinum, nickel, and steel substrates of ammonium electrolytes and investigated their electrocatalytic properties in alkaline and neutral solutions. It has been established that in neutral solutions the catalytic activity of precipitated Ni-Mo thin films is an order of magnitude higher than the activity of the platinum cathode, and in alkaline solutions this electrode can be successfully used as an anode.
In the works dedicated to the co-precipitation of nickel with molybdenum from aqueous electrolytes, the molybdenum content in deposition did not exceed 38-40% [31, 32]. This paper discusses the conditions under which is possible to obtain deposition with molybdenum content up to 80% and the influence of various factors on the electrochemical production of Ni-Mo alloys suitable as cathodes for HER.
Experimental methods
Ni-Mo deposits were obtained by the galvanostatic method in an electrolytic water jacketed cell.
To prepare the electrolyte, sodium mo-lybdate (Na2MoO4-2H2O) and nickel sulphate (№SO^7H2O) were dissolved in 7M ammonia, boric acid was also added to the electrolyte as a buffer additive to keep of molybdenum pH of the electrolyte constant and NiCl26H2O - to the electrolyte to prevent the formation oxygen compounds.
Platinum, nickel, and steel (st. 3) electrodes have been used as cathodes. According to the method [33] nickel electrodes were subjected to electrochemical polishing before using them. When determining the chemical composition of the alloys, the deposition was carried out on platinum plates; in both cases, a platinum plate was used as the anode. To keep the temperature constant, a universal ultrathermostat UTU-4 was applied. A quantitative analysis of the deposited alloys was carried out using the photocoloric method according to [34] and using a scanning electron microscope (SEM) manufactured by "Carel Zeiss Sigma", Germany. The thickness of the deposited Ni-Mo films was determined by microinterferometer of Linnic MII-4.
Discussion of the results
As is known that three stable phases MoNi(S), MoNi3(y), MoNi4(P) and solid solutions were formed on the basis of Mo and Ni in the Ni-Mo system [35]. The investigation of codeposition of Ni with Mo indicated that the constitution of cathodic depositions and their qualities depend on factors such electrolyte content, concentrations of main components in electrolyte, current density, stirring and temperature.
The Figure 1 illustrates the dependence of deposit content on current density at 298K temperature on a platinum electrode. The studies have been carried (0.05 ^ 0.13) M concentration of Na2MoO4-2H2O.
From curves of Figure 1 it is seen that with increasing current density, the amount of molybdenum in deposition content increases. The dependence of the molybdenum content in the composition of the films on the current density is nonlinear, but the character of the curves at different concentrations of Mo is same. With an increase in the current density from 0.5 to 2.5 A/dm2, the content of molybdenum in deposition almost doubles. Ni-Mo thin films had a strong adhesion to the substrate, but at current densities above 2.5 A/dm , the adhesion of the film to the substrate deteriorated, and part of the sediment crumbled from the electrode surface to the bottom of the electrolyzer. The optimum current density for obtaining high-quality Ni-Mo deposition with good adhesion to the substrate surface and high catalytic activity is 1.0-2.5 A/dm2.
The influence of the concentration of the main components of the electrolyte on the composition of the deposited films have been also investigated. Figure 2 illustrates the dependence of the composition of deposits on the concentration of Mo in the electrolyte at various current densities on the nickel electrode.
It can be seen from the Figure 2 with an increase in the concentration of Mo ions in the electrolyte, its content in deposits increase, and some increase in the concentration of molybdate ions in the electrolyte leads to a significant increase in the molybdenum content in the alloy.
So at the current density of 1.5A /dm2 an increase in the concentration of Mo ions from 0.03 to 0.124 M leads to an increase from 35 to 68% in its content in deposition.
Mo %'
80 70 60 50 40 30
0,5
—i—
1,5
—i—
2,5
Fig.1. The dependence of Mo content in the Ni-Mo deposition on current density at various molybdenum concentration in the electrolyte (composition, M: NiSO4-7H2O -0.107, H3BO3 - 0.1, NiCl2Cl6H2O - 0.13, NH4OH - 7, Na2MoO4-2H2O - 0.124): 1 -0.05, 2 - 0.08, 3 - 0.124, 4 -0.13; pH = 11.2, T=298 K.
ik A/dm"
Mo % 80 70 60 50 40 30
0.03 0.06
0.09
Fig.2. The dependence of the composition of Ni-Mo deposits on the concentration of mo-lybdate ions in the electrolyte (composition M: NiS04-7H20-0. 107, H3BO3 - 0.1, NiCl26H20 - 0.13, NH4OH - 7), a various current density: 1 - 1.0, 2 - 1.5, 3 - 2.5, (cathode - nickel, anode - platinum; pH = 11.2, 7=298 K.
0.12 0.15 C
Na2Mo04*2H20g(MI
Fig.3. The effect of nickel ion concentration in the electrolyte (composition, M: Na2Mo04-2H20 -0.124, H3B03 - 0.1, NiCl26H20 - 0.13, NH40H - 7) on the composition of Ni-Mo deposition, at various current density: 1 - 1.0, 2 - 1.5, 3 - 2.5 A/dm2, pH = 11.2, 7=298 K.
It seems also that by changing the electrolyte composition and electrolysis conditions at room temperature (298 K), it is possible to obtain deposits with 80% of Mo content.
The effect of nickel ion concentration on the composition of the obtained deposits is shown in Figure 3. Increasing the concentration of nickel ions in solution leads to a slight decrease in the molybdenum content in the sediments. The nickel content in the sediments almost does not change at concentration above 0.06 M values of the NiSO4-7H2O electrolyte. Increasing the nickel concentration from 0.02 to
0.06 M reduces the molybdenum content at a current density of 2.5 A/dm2 by only 10%. With an increase in the concentration of NiS04-7H20 from 0.06 to 0.14 M, the content of molybdenum almost does not change.
A study was made of the effect of electrolyte stirring on the composition of cathode deposits. Nickel was used as the cathode. From Figure 4 it can be seen that, in the same electrolyte composition and with the same electrolysis mode, the stirring of the electrolyte slightly increases the molybdenum content. At a current density of 2.5 A/dm2, stirring the electrolyte
28
with 100 rpm rate increases the content of molybdenum in thin Ni-Mo films by only 2.2%. This is explained by the fact that during the electrodeposition of molybdenum together with nickel, diffusion restrictions are almost absent.
Increasing the temperature slightly increases the nickel content in the sediments; however, this occurs up to a certain temperature. An increase in the electrolyte temperature above 318 K effects slightly on the composition of Ni-Mo deposits. At the electrolyte temperature above 308 K, a strong odor of ammonia is revealed; probably ammonium hydroxide is decomposed at high temperatures (Figure 5).
Increasing in the concentration of boric acid slightly decreases the molybdenum content in the sediments. The increase in H3BO3 concentration from 0.05 to 0.2 M, the content of molybdenum in deposition decreases by only 16%.
The analysis of deposits made on a scanning electron microscope (SEM), showed that oxygen is also present in their composition be-
sides nickel and molybdenum. This is due to the formation of oxygen compounds of molybdenum and nickel, which enter partly into deposition. The presence of boric acid in the electrolyte reduces the formation of oxygen compounds of nickel in the sediments, as well as the buffering additive. It was experimentally established that the optimal concentration of H3BO3 is 0.1M. The presence of oxygen in the sediments is explained by the fact that nickel deposits together with molybdenum oxides [31, 32]. In [16], it was suggested that the deposition of the Ni-Mo alloy usually occurs through the formation of an intermediate complex of molybdenum and nickel, where the divalent nickel ion plays catalyst role. A porous film of a mixture of molybdenum and nickel oxides was found on the cathode surface. It is assumed that this film is a product of intermediate step of the reaction, necessary for the reduction of molybdenum to the metal.
Fig.4. The dependence of the precipitates composition on the current density (electrolyte,s composition, M: NiSO4-7H2O - 0.107, Na2MoO4-2H2O - 0.08, H3BO3 - 0.1, NiCl26H2O - 0.13, NH4OH - 7) at different string rates, rpm: 1 - without stirring, 2 - 100 rpm, 3 - 200 rpm; cathode - St.3, anode -platinum; pH = 11.2, T= 298 K.
Mo % 1 k
90807060-
Fig.5. The effect of temperature on the composition of Ni-Mo deposition electrolyte,s composition M: NiSO4-7H2O - 0.107, Na2MoO4-2H2O - 0.124, H3BO3 - 0.1, NiCl26H2O -0.13, NH4OH - 7) at T, K: 1 -298, 2 - 308, 3 - 318; cathode and anode -platinum; pH = 11.2, ik - 2.5 A/dm2.
298 308 318 328
T,K
Mo %f
5040-1-1-1-1-►
8 9 10 11 pH
Fig. 6. The effect of electrolyte pH on the composition Ni-Mo deposition (electrolyte,s composition, M: NiS04-7H20 - 0.107, Na2Mo04-2H20 - 0.124, NiCl2 6H20 - 0.13, NH40H - 7, H3B03 - 0.1, ik - 2.5 A/dm2, T -298 K.
The amount of the formed molybdenum oxides is some what reduced in the presence of chlorine ions in solution. In any case, the main cathodic process, apparently, is the reduction of heptamolybdate ions to molybdenum dioxide, and only after this molybdenum is formed from molybdenum dioxide. To prevent the release of molybdenum in the form of dioxide, NH4Cl was added to the electrolyte, which significantly reduces the formation of molybdenum oxygen compounds in the electrolyte.
The effect of electrolyte pH on the process of co-deposition of nickel together with molybdenum within the pH range of 8-11.2 was also investigated. It is established that with increasing pH, the content of molybdenum in deposition increases. Figure 6 illustrates the dependence of the molybdenum content on the pH of the electrolyte. As the pH value increases from 8.2 to 11.2, the molybdenum content in deposition increases from 55.3% to 82.1%.
From the above, we can conclude that high-quality deposition with high adhesion and catalytic activity can be obtained by changing the composition of the electrolyte and the conditions of electrolysis. The thickness of the electrodeposited films depends on the duration of the electrolysis, it was measured using an MII-4 microscopes, and amounted to 2-6 p,m.
Thus, as a result of the study, to obtain high quality films, the following electrolyte composition was proposed M: 0.107, NiSO4-7H2O, 0.124, Na2MoO4-2H2O, 0.1 H3BO3, 0.13 NiCh 6H2O and 7 NH4OH. Electrolysis mode: current density 1.0-2.5A/dm2, electrolyte temperature 298 K, electrolyte pH - 11.2, electrolysis duration - 16 hours, anode - platinum, cathode -( St.3 steel), platinum and nickel.
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AMMONYAK ELEKTROLiTLORiNDON Ni-Mo ORiNTiLORiNiN ELEKTROKiMYOVi
USULLA COKDURULMOSI
U.M.Qurbanova, R.H.Huseynova, H.M.Tahirli, S.D.Dada^ova, A.§.Oliyev, D.B.Tagiyev
Elektrokimyavi gokma usulu ila alinan Ni-Mo nazik tabaqalarinin tarkibina va keyfiyyatina muxtalif faktorlann -carayan sixligi, elektrolitdaki komponentlarin qatiliginin, qan§dinlma va temperaturun tasiri tadqiq edilmi§dir. Muayyan edilmi§dir ki, gokuntularin tarkibi elektrolitdaki komponentlarin qatiligi va elektroliz §araitindan gox asilidir. Carayan sixliginin artmasi gokuntularin tarkibinda molibdenin miqdarinin, temperaturun artmasi isa nikelin miqdanmn artmasina sabab olur. Elektrolitin qan§dinlmasi gokma prosesina ahamiyyatli tasir gostarmir. Yuksak keyfiyyatli gokuntulann alinmasi ugun optimal elektroliz §araiti va elektrolit tarkibi muayyan edilmi§dir.
Agar sozlar: molibden, nikel, elektrogokm3, elektrokatalizator, ammonium hidroksid.
ЭЛЕКТРООСАЖДЕНИЕ СПЛАВОВ Ni-Mo ИЗ АММИАЧНЫХ ЭЛЕКТРОЛИТОВ
У.М.Курбанова, Р.Г.Гусейнова, Г.М.Тахирли, С.Д.Дадашева, А.Ш.Алиев, Д.Б.Тагиев
Изучено влияние различных факторов: плотности тока, концентрации компонентов электролита, перемешивания и температуры на состав и качество тонких пленок Ni-Mo, полученных методом электрохимического осаждения. Установлено, что состав осадков сильно зависит от концентрации компонентов электролита и условий электролиза. Увеличение плотности тока повышает содержание молибдена, а рост температуры увеличивает содержание никеля в осадках. Перемешивание влияет на процесс электроосаждения незначительно. Определены оптимальные условия процесса и состав электролита для получения качественных осадков.
Ключевые слова: молибден, никель, электроосаждение, электрокатализатор, гидроксид аммония.
