Formation of the electrochemical properties of nickel composite coatings containing molybdenum or tungsten
Magdalena Popczyk
Quarterly No. 3, 2010 pages 260-265
DOI:
keywords: composite coating, nickel, molybdenum, tungsten, hydrogen evolution reaction (HER), electrochemical impedance spectroscopy (EIS)
abstract Electrodeposition of Ni+Mo and Ni+W composite coatings was conducted with nickel-plating baths, containing additional composite component (Mo, W) in separate phase. The electrochemical activity of these coatings was studied in the process of hydrogen evolution reaction (HER) using steady-state polarization and electrochemical impedance spectroscopy (EIS) methods. These measurements were carried out in 5 M KOH solution, using Autolab® electrochemical system. The auxiliary electrode was a platinum mesh and the reference electrode was of the type Hg/HgO/6 M KOH. Complex plane plots were analysed using a modified version of a complex non-linear least-squares fitting program (CNLS), from which the experimental parameters of an electrical equivalent circuit were determined. It has been found that impedance behaviour may be well described by the one-CPE electrode model. Approximations of the experimental impedances permitted of determine the following parameters: Rct charge-transfer resistance, Rs solution resistance, T capacity parameter, coefficient of dispersion and in consequence the surface roughness factor Rf was estimated. Basing on the results of EIS measurements the rate constants of the HER as well as the surface coverage by adsorbed hydrogen were also determined. Intrinsic activity of the composite coatings is related to the real (per unit of real surface area) rate constants. In order to evaluate the true catalytic effect of the investigated coatings the values of the rate constants divided by the roughness factor. It was found that Ni+Mo composite coating is characterized by enhanced electrochemical activity towards hydrogen evolution as compared with Ni+W composite coating. High activity of this coating in the hydrogen evolution reaction results from property of phase boundary connected with different orientation of nickel crystallites at the embedded molybdenum grain in comparison with their orientation in the depth of nickel matrix. It was found that this fact has accurate relation with change of mechanism of hydrogen evolution reaction and change of limit stage, with Heyrovsky stage on Volmer stage. Thus obtained composite coating may be useful in application as electrode material for the hydrogen evolution reaction.