Electrochemical dissolution and passivation of nickel powder randomly dispersed in a graphite + polypropylene matrix

Abstract

Nickel powder (10% in weight) randomly dispersed in a graphite (40%) + polypropylene (50%) matrix was studied as a composite electrode in a sulphuric-sulphate acid medium by means of voltammetry and Electrochemical Impedance Spectroscopy (EIS). Under voltammetric conditions, this thermoplastic material shows an electrochemical behaviour similar to nickel metal. However, under EIS measurement conditions the electrodissolution behaviour of the nickel powder is partially limited by the diffusion of the anions in the electrolyte in order to electroneutralize the excess of electrogenerated positive charge. The low frequency range of the measured impedance spectra can be interpreted as a parallel arrangement of a capacitive and a Warburg impedance, instead of a CPE element initially considered in the fitting. Values for CPE exponents lower than 0.85 are interpreted as a competition between a capacitive/transport contribution to the overall impedance response.

Growth of passive layers on nickel during their voltammetric anodic dissolution in a weakly acid medium

“…This value is in good agreement with the previously obtained one during nickel deposit potentiostatic passivation [38]. From this fitting procedure the following values for the electric field ε is obtained: ε = 2 × 10 5 V cm −1 .…”

“…γ 2 obtained values are between 0.38 and 0.28. Therefore, only a minor part of the total current leads to passive layer formation as observed in other experimental conditions [38,49,50]. The fact that only a minor part of the total current leads to passive layer formation is in agreement with the fact that the main charge carriers within passive layers on nickel are cation vacancies (see Eqs.…”

“…According to our previous obtained results, chemical dissolution of the electrogenerated passive layer does not play a significant role in these experimental conditions [33]. Moreover, according to our recent results in a similar pH, passive layer chemical dissolution begins to become important after 30 s of a potentiostatic transient [38], since Eq. (5) will not be considered from now on, since, by considering a scan rate of 20 mV s −1 , passive layer chemical dissolution will not have any influence in the explored potential interval.…”

“…3. It is also considered that the potential drop variations at the film/solution interface can be disregarded when they are compared with potential drops at the metal/film and within the film [ 18,35,38]. In a voltammetry experiment E = E 0i + νt, where ν is the scan rate and E 0i represents the initial potential.…”

“…Instantaneous mass/charge analysis proved useful in the characterization of the passive layer growth of Ni(OH) 2 generated at constant potential in a weakly acid sulphate medium [38]. This work is focused on the study of the usefulness of Fdm/dQ function during the active/passive transition observed on the anodic scan of cyclic voltammograms [39].…”