Impedance spectroscopy study of nickel electrodeposits
H. S. Karayiannia, , , G. S. Patermarakisa and J. C. Papaioannoub
a Section of Materials Science and Engineering, Department of Chemical Engineering, National Technical University of Athens, Iroon Polytechniou 9, Zografou 157 80, Athens, Greece
b Laboratory of Physical Chemistry, Department of Chemistry, University of Athens, Panepistimioupolis 157 71, Athens, Greece
Received 28 August 2000; revised 19 June 2001; accepted 21 June 2001. Available online 30 January 2002.
Nickel electrodeposits prepared in NiSO4 and NiCl2 electrolytes with thicknesses of 43–49.4 m were examined by the impedance spectroscopy (IS) method for a–c frequencies 1 Hz–100 kHz. The Nyquist diagrams were single, almost perfect semicircles, suggesting the applicability of the in parallel combination of the Ra–Ca elements characterised by a single relaxation time, 8.27×10-6 s, combined in series with another Rb element, Rb<<Ra. This relaxation time predicted that the incorporated hydrogen atoms are involved in the process of conduction by a hopping/diffusion mechanism, which is differentiated in the cases of NiSO4–Ni and NiCl2–Ni deposits. The diffusion coefficient of hydrogen, 1.25×10-11 cm2 s-1, was determined. The microstructure of Ni electrodeposits was also studied and characterised by electron microscopy. Finally, a general model for the microstructure of Ni electrodeposits, consistent with the results of impedance spectroscopy, was formulated.
Author Keywords: Nickel electrodeposits; Impedance spectroscopy; Hydrogen hopping/diffusion conduction mechanism; Microstructure model