Electrolytic production and characterization of nickel composite and alloy coatings containing tungsten
Magdalena Popczyk
Quarterly No. 1, 2009 pages 62-66
DOI:
keywords: composite and alloy coatings, hydrogen evolution reaction (HER), electrochemical impedance spectroscopy (EIS)
abstract Ni+W composite coating was obtained by electrodeposition from the following electrolyte (concentrations in g dm−3): NiSO4 • 7H2O - 84, H3BO3 - 8, CH3COONa - 10, C6H5O7Na3 • 2H2O - 30, NH4Cl - 10 + 50 g dm−3 of tungsten powder (< 150 μm, Aldrich). Ni-W alloy coating was obtained by electrodeposition from the following electrolyte (concentrations in g dm−3): NiSO4 • 7H2O - 13, Na2WO4 • 2H2O - 68, NH4Cl - 50, C6H5O7Na3 • 2H2O - 200. Ni+W and Ni-W coatings were prepared under the galvanostatic conditions in such manner, that the coatings contain the same quantity of tungsten. The surface morphology of the coatings was studied using a scanning microscope (JEOL JSM – 6480). Chemical composition of obtained coatings was determined by the X-ray fluorescence spectroscopy (XRF) with a special attachment to the X-ray generator TUR-M62 with flat LiF crystal. The surface of the Ni-W alloy coating is relatively regular of an island character and the surface of the Ni+W composite coating is covered by a well visible velvet-like dark grey tarnish. This composite coating has a matt, rough surface with visible grains of incorporated powder. Introduction of tungsten powder into the nickel matrix causes obtained compo-site coating of very developed and rough surface. Ni+W and Ni-W coatings contain almost identical quantity of tungsten and therefore was determined influence of manner of tungsten introducing to the nickel matrix on electrochemical properties of these coatings. The electrochemical activity of these coatings was studied in the process of hydrogen evolution reaction (HER) from 5 M KOH solution using steady-state polarization and electrochemical impedance spectroscopy (EIS) methods. Appro-ximations of the experimental impedances permitted determinations of the following parameters: Rct, Rs, T, and in consequence the surface roughness factor Rf was estimated. It was assumed, that surface roughness factor, is connected with the value of electrochemical active surface. The values of Rf, obtained for the Ni+W composite coating are higher, than that for the Ni-W alloy coating. The reason of this could be increase of real surface area for the Ni+W composite coating. Basing on the results of EIS measurements the rate constants of the HER were determined. It was found that Ni+W composite coating is characterized by enhanced electrochemical activity towards hydrogen evolution as compared with Ni-W alloy coating. An increase in the electrochemical activity may be attributed both to the synergetic effect of tungsten dispersed in the nickel matrix and to the increase in the real surface area resulting from the composite character of coating. Thus obtained composite coating may be useful in application as electrode material for the hydrogen evolution reaction.