Solid State Phenomena Vol. 228

Abstract: The Ni+Mo composite coatings were prepared by electrodeposition under the galvanostatic conditions (j_dep = -300 mA cm^-2) from the nickel bath containing molybdenum powders of different granulation (3-7 μm, <150 μm, <100 nm). Thermal treatment of the obtained coatings was conducted in the argon atmosphere. The surface morphology of the coatings was studied using a scanning electron microscopy (SEM). Chemical composition of the electrodeposits was determined by X-ray fluorescence spectroscopy (XRF). Phase composition investigations were conducted by X-ray diffraction (XRD) method. Investigations of hydrogen evolution reaction (HER) and electrochemical corrosion resistance were carried out in 5 M KOH solution. It was found that for the Ni+Mo thermally treated coatings the decrease in activity towards the HER was observed. Simultaneously these coatings are more corrosion resistant than Ni+Mo as-deposited coatings. The reasons for the electrochemical properties of these coatings have been discussed.

Abstract: The Ni+MoS₂ composite coatings were prepared by electrodeposition under galvanostatic conditions from the Ni-plating bath containing suspended MoS₂ powder (100 mesh). Investigations of hydrogen evolution reaction (HER) were carried out using steady-state polarization measurements and electrochemical impedance spectroscopy (EIS) in 5 M KOH solution on the coatings obtained at 30, 40, and 50°C. It was found that the kinetics of the HER on the Ni+MoS₂ coatings decreases with the increase in the electrodeposition temperature of the coatings. This effect is attributed to decreasing content of MoS₂ (from 26.4 to 18.0 wt.%) embedded into the Ni matrix as composite crystalline component having the electrocatalytic properties towards the HER and/or surface development of the coatings. The higher amount of MoS₂ was embedded, the more porous electrodes containing pear-shape pores on the surface were produced what was detected by EIS.

Abstract: Porous Ni coatings were prepared by galvanostatic electrodeposition on the steel substrate from the Watts type bath at the deposition current density j_dep = -250 mA cm² at the temperature 40, 50 and 60°C. Investigations of hydrogen evolution reaction (HER) on the obtained Ni electrodes were carried out in 5 M KOH solution at room temperature. It was found that with the increase in the electrodeposition temperature of the coatings, the activity of the Ni electrocatalysts towards the HER decreased due to diminishing surface development of the Ni coatings.

Abstract: Porous Ni-P alloy coatings were prepared by galvanostatic electrodeposition at the deposition current density of j_dep = -250 mA cm^-2 from the nickel plating bath of the Watts type containing different content of sodium hypophosphite. Investigations of hydrogen evolution reaction (HER) were carried out in 5 M KOH solution at room temperature. Ac impedance behavior of the electrodes was described using electrical equivalent circuits containing two constant-phase elements (two-CPEs electrode model). The results obtained from the EIS and steady-state measurements allowed to determine the Volmer-Heyrovský mechanism and kinetics of the HER. It was found that for the Ni-P alloy coatings deposited from the galvanic baths after addition of sodium hypophosphite in the amount of 5, 10, 20 and 30 g dm^-3, the decrease in their catalytic activity towards the HER is observed due to diminishing of intrinsic activity of the electrodes.

Abstract: Kinetics of hydrogen evolution reaction (HER) was investigated in 1 M NaOH solution at room temperature on a polycrystalline Fe electrode material which was electrochemically activated and unactivated. Studies of the HER were carried out using steady-state polarization and electrochemical impedance spectroscopy (EIS) measurements. It was found that for the Fe electrode material after activation at j = -320 mA cm^-2 for 24 h, the increase in the catalytic activity towards the HER was observed in comparison with that on the unactivated iron electrode material. Ac impedance behavior of the Fe electrode changed from a typical for smooth electrodes before activation (one time constant in the circuit) to that being characteristic for porous electrodes after activation (two time constants in the circuit). The reason for that is formation of solid products of the iron corrosion in alkaline solution which can cause passivation of the electrode surface and catalyse the HER.

Abstract: The Ni+NiAl nanocomposite coatings were prepared by the electrodeposition under the galvanostatic conditions (j_dep = -250 mA cm^-2) from the nickel bath containing the suspension of NiAl nanopowder. For comparison, the Ni coatings were also obtained. The combined steady-state polarization and electrochemical impedance spectroscopy (EIS) investigations of hydrogen evolution reaction (HER) were carried out in 5 M KOH solution at room temperature. It was found that the Ni+NiAl nanocomposite coatings revealed higher apparent electrocataltic activity towards the HER as compared to the comparable Ni deposits due to the increase in the real surface area development and the intrinsic activity.

Abstract: The Ni+MoS₂+Mo composite coatings were obtained by the electrodeposition under the galvanostatic conditions (j_dep = -250 mA cm^-2) at the temperature of 30-60°C from the nickel bath containing a suspension of MoS₂ (<2 μm) and Mo (3-7 μm) powders. Studies of hydrogen evolution reaction (HER) were carried out in 5 M KOH solution at room temperature using steady-state polarization and electrochemical impedance spectroscopy (EIS) measurements. The decrease of electrochemical activity towards the HER was found for the Ni+MoS₂+Mo composite coatings with the increase in the temperature of their electrodeposition due to decrease of both the intrinsic activity and surface development.

Abstract: The Ni-Mo alloy coatings with a high content of Mo up to 44.5 at.%, were prepared by galvanostatic electrodeposition in the range of deposition current density, j_dep, from-30 to-240 mA cm^-2 from the nickel plating bath containing potassium pyrophosphate, nickel chloride, sodium molybdate, and sodium bicarbonate. Investigations of hydrogen evolution reaction (HER) were carried out in 5 M KOH solution at room temperature using steady-state polarization and electrochemical impedancy spectroscopy (EIS) measurements. It was found that for the Ni-Mo alloy coatings, the increase in their catalytic properties towards the HER with the increase in the value of j_dep of the coatings, was due to the intrinsic activity.

Abstract: The Ni+MoO₂ and Ni+Cr₂O₃ composite coatings were prepared by the electrodeposition under the galvanostatic conditions (j_dep = -200 mA cm^-2) from the nickel bath containing powder of MoO₂ or Cr₂O₃. Studies of hydrogen evolution reaction (HER) in 5 M KOH solution at room temperature were conducted using steady-state polarization and electrochemical impedance spectroscopy (EIS) methods. It was ascertained that the HER rate is higher for the Ni+MoO₂ composite electrode in comparison with the Ni+Cr₂O₃ composite electrode due to the presence of Mo in the nickel matrix.

Abstract: The Ni+Mo and Ni+MoO₂+Mo composite coatings were electrodeposited at the constant deposition current density of j_dep = -200 mA cm^-2 from the nickel plating bath containing powder of Mo or MoO₂ and Mo. The investigations of hydrogen evolution reaction (HER) were carried out in 5 M KOH solution at room temperature using steady-state polarization and electrochemical impedance spectroscopy (EIS) techniques. It was found that for the Ni+MoO₂+Mo composite coating the increase in the activity of HER was observed in comparison with Ni+Mo composite coating probably due to the more porous surface and presence of additional composite component (MoO₂) in the nickel matrix.