CC BY
21
ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)
AZERBAIJAN CHEMICAL JOURNAL № 4 2022
27
UDC 541.13.544.65
INVESTIGATION OF ELECTROCATALYTIC ACTIVITY OF Ni-Mo THIN FILMS FOR WATER ELECTROLYSIS
U.M.Gurbanova, R.G.Huseynova, N.R.Abishova, E.F.Ismaylova Orudjeva, M.T.Abbasov, Ya.A.Nuriyev, A.Sh.Aliyev, D.B.Tagiyev
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
Received 14.07.2022 Accepted 03.08.2022
The paper presents data on the study of the electrocatalytic properties of Ni-Mo thin films in neutral and alkaline media, obtained as a result of electrochemical synthesis. Comparative characteristics of the electrocatalytic properties of deposited Ni-Mo films on various substrates and different compositions with the catalytic activity of Ni, Pt and St-3 were determined by the method of recording linear polarization curves and determining the slope of the Tafel curves. The highest catalytic activity was exhibited by Ni73 5Mo13.3O13.2 thin films on a nickel substrate that were not annealed. The dependence of the micro-hardness of the films on their composition was determined, and Ni-Mo films with a Mo content of 38% had the highest microhardness. It has been established that the corrosion resistance of films depends on the content of molybdenum in them, and an increase in its content in alloys increases their corrosion resistance, but the catalytic activity of the films decreases.
Keywords: electrocatalysis, electrodeposition, molybdenum, nickel, hydrogen.
doi.org/10.32 73 7/0005-2531-2022-4-27-32
At present, due to the shortage of fossil fuels, the production of hydrogen as a "clean" energy carrier is especially important, since it can be obtained directly from renewable energy sources by decomposing water [1, 2]. The search for an environmentally friendly, highly efficient and cheap energy carrier, which would be an inexhaustible and easily accessible resource, is currently relevant, since there is a gradual depletion of the usual energy resources in the bowels of the earth and a constant rise in their cost. Hydrogen reserves on our planet are inexhaustible and renewable. The main source of hydrogen on planet Earth is water, during the electrolysis of which oxygen and hydrogen are released. In addition to water, sources of hydrogen can be natural gas, coal, and plant and waste biomass.
The review articles [3, 4] give the entire prehistory - development, crisis and significance, past, present, prospects and future associated with hydrogen production. It is well known that the most active materials as electrodes for the hydrogen evolution reaction (HER) are noble metals such as platinum, gold, silver, palladium and ruthenium [5]. This is due to the fact that the rate of adsorption of protons on their surface is almost equal to the rate of desorption or recombination with the formation of gaseous hydrogen. However, their high cost
does not allow them to be widely used to produce hydrogen on an industrial scale. To provide the economy with environmentally friendly and sustainable hydrogen energy, it is necessary to obtain hydrogen at a low price. To obtain cheaper hydrogen by electrolysis of water, it is necessary to develop new electrode materials with increased electrocatalytic activity for the reaction of hydrogen evolution. An ideal elec-trocatalyst at industrial current densities should have a low hydrogen overvoltage value with a constant potential value that does not change with time. The electrocatalyst should have chemical and electrochemical resistance, long service life, not emit harmful substances during electrolysis, have good adhesion to the substrate surface, do not contain impurities that reduce the sensitivity of the electrodes, should not be sensitive to voltage drops, be environmentally friendly, and its production should be easy and low cost [6-9]. The productivity of electrocatalysts can be increased either by increasing their real area, or by the electrochemical method, reducing the overvoltage of hydrogen during its release. The cost of noble metal electrodes can be reduced by alloying them with other metals, synthesizing alloys based on them, or developing new electrode materials based on cheaper elements.
Thus, the goal of modern electrocatalysis
is the complete replacement of Pt with inexpensive and active catalytic materials. In nature, there are active metals such as Fe, Ni or Co, which are much cheaper than noble metals, but under the conditions of the water decomposition reaction, they are subject to corrosion and passivation [10]. An increase in the corrosion resistance and activity of these metals can be achieved by alloying them with other metals or by changing the method of their synthesis. Composite coatings based on nickel are widely used in electrochemistry due to their high catalytic activity in the electrolysis of water and the corrosion resistance of nickel in aggressive media [11]. In industry, nickel electrodes are mainly used, which are highly resistant to corrosion even in hot concentrated alkaline solutions [12, 13], but at the same time, the release of hydrogen on their surface is accompanied by a high overvoltage, which leads to high energy consumption. Due to the low catalytic activity and instability of nickel during long-term electrolysis, its alloys are most often used, the catalytic activity of which is higher.
To improve the electrocatalytic properties of nickel, it is alloyed with various metals or metal oxides, such as Mo, MoO2, MoO3 [10, 14, 15], or nickel alloys with refractory metals are used [9, 16]. Recently, molybdenum has attracted great interest due to its high corrosion resistance and low hydrogen overvoltage, which is an important factor in the water decomposition reaction [17, 18]. The purpose of this work is to study the electrocatalytic and other properties of Ni-Mo thin films obtained by the electrochemical method, depending on their composition and annealing temperature.
Experimental part
Thin films of Ni-Mo obtained by electrochemical method in the galvanostatic mode from an alkaline electrolyte containing NiSO4-7H2O from the Indian company Central drug house (P) Ltd, Na2MoO4-2H2O from the Indian company Qualikems Fine Chem Pvt were studied. Ltd. Both salts were dissolved in NH4OH, boric acid was introduced into the electrolyte as a buffering additive, and NiCl2-6H2O (Central drug house (P) Ltd) was introduced into the electrolyte to reduce the formation of metal oxides, the pH of
the electrolyte was 11.2. The current density varied within R2.5 A/dm2, the electrolysis was carried out at room temperature, the electrolysis duration was 12-15 hours.
Corrosion studies were carried out under laboratory conditions by the gravimetric method in a solution of 0.5 M Na2SO4. Before testing, the samples were dried to a constant weight, after testing they were thoroughly washed, dried and weighed with an accuracy of 2-10-4 g. After determining the mass loss due to corrosion, its rate was calculated by the formula:
K=ti (1)
where, K - the corrosion rate, Am - the mass loss, t - the time of the experiment, S - the geometric area of the sample surface.
The microhardness of the samples was studied using a PMT-3 hardness tester with a diamond rhombic pyramid.
The film thickness was measured using an MII-4 microinterferometer, and at an electrolysis duration of 14 hours and a current density of 2.5 A/dm2, the film thickness varied within 2-3 ^m.
The catalytic activity of thin films and substrates was determined by recording polarization curves in a neutral electrolyte (0.5 M Na2SO4) on an IVIUMSTAT Electrochemical Interface potentiostat. To compare the catalytic activities of different substrates, a platinum plate
3 2
with a surface of 4-10" dm , a 1x1 nickel elec-
2 2
trode with a surface of 2-10" dm , and an electrode made of Steel-3 with a surface of 7.6-10" dm2 were used as working electrodes. The polarization curves were recorded in a three-electrode
2 2
cell, a platinum plate with a surface of 4-10" dm was used as an auxiliary electrode, and a silver chloride electrode (Ag/AgCl) served as a reference electrode. Before electrolysis, the nickel substrates were electrochemically polished and then cleaned with alcohol. The platinum electrode was cleaned in nitric acid, and then immersed in an H2SO4:H2O2 solution for a few minutes and then subjected to electrochemical triangular pulse cleaning. Steel electrodes were polished mechanically, and then cleaned with alcohol. The morphology and composition of the films were studied by scanning electron micros-
copy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction analysis (X-ray). X-ray phase analysis was done on a Bruker D2 Phazer (Cu Ka; Ni filter). Thin Ni-Mo films were obtained from an alkaline ammonia electrolyte with the following composition: 0.107 M NiSO4, 0.124 M Na2MoO4, 0.1 M H3BO3, 0.13 M NiCl2, 7 M NH4OH, at current densities ik=1-2.5 A/dm and an electrolyte temperature of 298 K, detailed studies are given in our previous works [19].
Results and discussion
For kinetic studies of the process of hydrogen evolution, the following methods are most often used: polarization curves (Tafel) [3, 7], potential stepwise charging [10], galvanostatic pulse method [8, 9], potential relaxation in an open circuit [17, 20], and impedance method [5].
The reaction of hydrogen evolution is one of the multistage processes and, depending on the conditions (environment, electrode material, temperature), can proceed according to various mechanisms. For example, on a mercury electrode, the limiting stage is the stage of the discharge of hydronium ions with the formation of adsorbed hydrogen, which is subsequently removed through a fast stage of electrochemical desorption (Volmer-Heyrovsky mechanism). The reaction mechanism of hydrogen evolution in alkaline and neutral media consists of the following three stages:
I) water electrolysis, hydrogen adsorption; Volmer reaction (equation (2))
II) electrochemical desorption of hydrogen; Heyrovsky reaction (equation (3))
III) chemical desorption; Tafel reaction (equation (4)) [17, 21].
M + H2O + e- = MHads + OH- (2)
MHads + H2O + e- = M + H2 + OH- (3)
2MHads = 2M + H2 (4)
Thin Ni-Mo films with molybdenum content in precipitates from 12.3 to 64.85 wt% were studied. It should be noted that, in the deposits subjected to annealing, the content of molybdenum decreased from 64.85 to 42.8 wt%; the molybdenum content after annealing decreased by ~22.05%. This is most likely due to the fact that the formation of Ni-Mo alloys occurs
through the stage of formation of MoO2 under the catalytic action of nickel, then MoO2 is reduced to Mo with the formation of an alloy. An analysis of the alloy before annealing showed the presence of Mo oxides in its composition, some of which apparently oxidize during annealing at a temperature of 773 K, reducing the amount of molybdenum in the precipitation composition and increasing the oxygen content in them, which is consistent with the literature data [10].
The X-ray pattern of the films subjected to annealing confirmed an increase in the oxygen content in the composition of the Ni-Mo films [22].
Presumably, the coprecipitation of nickel with molybdenum occurs through the stage of formation of oxides of these metals and, as a result, a solid solution is formed. During annealing of the deposited films, a solid-phase reaction occurs between nickel and molybdenum oxides, as a result of which 3 phases appear on the diffraction pattern - NiMoO4, MoO3 and Ni [22].
The MoO3 phase is most likely formed as a result of induced precipitation from molyb-date ions according to reaction (5):
MoO4-2+Ni2++H2O+2e-^MoO3+Ni+2OH- (5)
Figure 1 shows the polarization curves of the reaction of water decomposition in a neutral medium (0.5 M Na2SO4) for various electrodes. For a comparative characterization of the catalytic activity of the synthesized Ni-Mo alloys, electrodes made of platinum, nickel, steel grade St-3 and electrolytically obtained Ni-Mo films subjected and not subjected to heat treatment with a nickel content of 68.2 mass% and 73.5 wt%, respectively. It can be seen from the figure that thin Ni-Mo films not subjected to annealing with a composition of 73.5 wt% Ni, 13.3 wt% Mo, and 13.2% O2 had the best catalytic properties; the catalytic activity of these films was ~1.5 times higher than the catalytic activity of platinum.
It can be seen from the curves that Ni-Mo thin films obtained as a result of electrochemical synthesis effectively operate in a neutral medium as a cathode. The catalytic activity of thin Ni-Mo films was studied depending on
their composition. According to the data presented in the Table, it can be seen that the films not subjected to annealing with a nickel content of 73.2% had the smallest Tafel slope (curve 1).
E,V
Fig.1. Catalytic activity of various electrodes in a neutral medium: 1 - Ni73.5Mo13.3O13.2 (on Ni) without annealing, 2 - Ni83 4Mo21O14.5 (on Pt), 3 -Ni68.2Mon.5O20.3 (on Ni) after annealing, 4 - Ni, 5 -Pt, 6 - Ni70.12Mo14.59O15.29 on St-3, 7 - St-3, 8 -
Ni13.79Mo64.85O2! .36
Table 1. Dependence of the slope of the polarization curves of various electrodes on the substrate material and the composition of Ni-Mo alloys_
№ Electrode dE/dlgi (mV)
1 Pt platinum 164
Ni83.4Mo21014.5 110
nickel 136
Ni73.5Mo13.3013.2 105
2 Ni without annealing
Ni68.2Mo11.5020.3 after annealing 123
Ni13.79Mo64.85021.36 220
3 Steel grade Steel St-3 190
St-3 Ni70.12M°14.59°15.29 183
After annealing, the nickel content in these films decreased to 68.2% and the slope of the Tafel curve increased (curve 3). The decrease in the catalytic activity of thin films after annealing is associated with a change in the structure of the films. It is known that the catalytic activity of amorphous alloys is always higher than the activity of crystalline alloys, which is explained by the large real surface of amorphous alloys [23]. The difference in the values of the hydrogen evolution
overvoltage of the Pt, Ni, and Ni73.sMo13.3O13.2 electrodes upon reaching the current values of 10 and 100 mA/cm2 was 0.376 V, 0.254 V, and 0.183 V, respectively.
Corrosion of catalysts leads to a decrease in its initial amount. Experience shows that, contrary to popular belief about the resistance of precious metals to corrosion, noticeable corrosion occurs even when using platinum metals. In order to establish the corrosion resistance of the obtained films, thin films of Ni13.79Mo64.85O2136 containing 64.85 wt % Mo and Ni73.5Mo13.3O13.2 with a Mo content of 13.3 wt % were studied. Corrosion resistance was studied in a solution of
0.5 M Na2SO4 for 50 hours, the surface of the
2 2
nickel cathode was equal to 2-10" dm , by the
difference in the mass of the sample before and
after testing, inserting the obtained data into
formula (1), the values of corrosion coefficients
for alloys were calculated Ni73.5Mo13.3O13.2 and
Ni13.79Mo64.85O21.36, which were equal to 0.0005 22 g/m •h and 0.0003 g/m •h, respectively, i.e. The
corrosion rate of thin films with a high molybdenum content was lower than that of films with a low molybdenum content.
The electrodes used in the water decomposition reaction must also have high wear resistance. In this regard, studies were carried out on the influence of the composition of Ni-Mo thin films and the annealing temperature on their microhardness.
Table 2. Dependence of the microhardness of the Ni68.2Mo11.5O20.3 film on the annealing temperature
t0 annealing Microhardness MPa
- 934.2
100 985.3
200 1050.5
500 1120.8
The value of microhardness of thin Ni-Mo films before and after annealing at a temperature of 773 K for 1 hour was established, its value was 934.2 MPa and 1120.8 MPa, respectively. The dependence of the microhardness of the films on the annealing temperature is given in Table 2. It can be seen from the table that an increase in the annealing temperature leads to an increase in the microhardness of the films.
MPa"
800
¡0 20 30 40 50 60 70 Mrf^
Fig.2. Dependence of the microhardness of Ni-Mo thin films on the mass content of Mo
Figure 2 shows the dependence of the microhardness of thin Ni-Mo films not subjected to annealing on the mass content of molybdenum in them. It can be seen from the figure that the microhardness of the films increases with an increase in the content of molybdenum in them and reaches a maximum at a molybdenum content of about 38 mass%, then the mi-crohardness gradually decreases. Such dependence in the form of a curved line is characteristic of alloys, during the formation of which a solid solution is formed between the alloy components. This is explained by the fact that even minor additions of impurities to a pure component change the value of the microhardness of the latter, since the impurity forms a solid solution with the pure metal [24].
Conclusions
As a result of the studies of the comparative electrocatalytic characteristics of various electrodes, it was found that unannealed Ni73.5Mo13.3O13.2 thin films electrochemically deposited from an alkaline electrolyte on a Ni substrate had the best catalytic activity in the reaction of hydrogen evolution in a neutral medium. The microhardness of the deposited films depends on the temperature of their annealing and on the composition of thin films; films with a Mo content of 38 wt% had a high microhard-ness. It has been established that an increase in the content of molybdenum in the composition of the films contributes to an increase in the corrosion resistance of thin Ni-Mo films, but their catalytic activity decreases.
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SUYUN ELEKTROLiZi PROSESiNDO Ni-Mo NAZiK TOBOQOLORlNiN ELEKTROKATALiTiK
AKTiVLiYiNiN TODQiQi
Ü.M.Qurbanova, R.Q.Hüseynova, N.RAbi^ova, E.F.ismayilova Orucova, M.T.Abbasov, Y.O.Nuriyev,
A.§.Oliyev, D.B.Tagiyev
Maqalada elektrokimyavi sintez naticasinda alda edilmi§ neytral va qalavi mühitlarda Ni-Mo nazik tabaqalarinin elektrokatalitik xassalarinin öyranilmasina dair malumatlar taqdim olunmu§dur. Müxtalif elektrodlarda (Ni, Pt, Polad-3) va müxtalif tarkiblarda gökdürülmü;? Ni-Mo nazik tabaqalarinin elektrokatalitik xassalarinin müqayisali xarakteristikalari xatti polyarizasiya ayrilarinin gakilmasi va Tafel meylinin hesablanmasi ila müayyan edilmi§dir. On yüksak katalitik aktivlik termiki emal edilmami§ nikel elektrodda gökdürülmü§ Ni735Moi33Oi32 tarkibli nazik tabaqalarda alda edilmi§dir. Tabaqalarin mikrobarkliyinin onlarin tarkibindan asililigi müayyan edilmi§ va tarkibinda 38% Mo olan Ni-Mo nazik tabaqalari an yüksak mikrobarkliya malik olmu§dur. Müayyan edilmi§dir ki, tabaqalarin korroziyaya davamliligi onlarin tarkibindaki molibdenin tarkibindan asilidir va arintilarda onun miqdarinin artmasi onlarin korroziyaya davamliligini artirir, lakin tabaqalarin katalitik aktivliyi azalir.
Agar sözlzr: elektrokataliz, elektrogökm3, molibden, nikel, hidrogen.
ИССЛЕДОВАНИЕ ЭЛЕКТРОКАТАЛИТИЧЕСКОЙ АКТИВНОСТИ ТОНКИХ ПЛЕНОК Ni-Mo
ПРИ ЭЛЕКТРОЛИЗЕ ВОДЫ
У.М.Курбанова, Р.Г.Гусейнова, Н.Р.Абышова, Э.Ф.Исмайлова Оруджева, М.Т.Аббасов, ЯА.Нуриев,
А.Ш.Алиев, Д.Б.Тагиев
В работе приведены данные по исследованию электрокаталитических свойств тонких пленок Ni-Mo в нейтральной и щелочной средах, полученных в результате синтеза электрохимическим способом. Методом снятия линейных поляризационных кривых и определения наклона тафелевких кривых определены сравнительные характеристики электрокаталитических свойств осажденных Ni-Mo пленок на различных субстратах и различного состава с каталитической активностью Ni, Pt и Ст- 3. Самой высокой каталитической активностью обладали тонкие пленки Ni73.5Moi33Oi32 на никелевой подложке, не подвергнутые отжигу. Определена зависимость микротвердости пленок от их состава и самой высокой микротвердостью обладали пленок Ni-Mo с содержанием Мо 38%. Установлено, что коррозионная стойкость пленок зависит от содержания молибдена в них и увеличение его содержания в сплавах повышает их коррозионную стойкость, но каталитическая активность пленок при этом снижается.
Ключевые слова: электрокатализ, электроосаждение, молибден, никель, водород
