Comprehensive Study of the Electrochemical Formation of Thin Oxide Layers on Nickel and the Electrochemical Reduction of Monolayer Oxides on Platinum
Alsabet, Mohammad H.
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The anodic polarization of Ni electrode in 0.5 M aqueous KOH solution at various polarization potential (Ep), time (tp) and temperature (T) values leads to the formation of β-Ni(OH)2 films. The growth of the hydroxide layers are irreversible and cannot be reduced electrochemically to metallic Ni. The hydroxide layer becomes thicker at higher values of Ep and/or tp and/or T. The thickness of β-Ni(OH)2 hydroxide were determined using ex–situ XPS and depth–profile techniques. Application of the oxide growth theories to our data indicate that the development of the β-Ni(OH)2 layer follows inverse logarithmic growth kinetics. The driving force of the process is the strong electric field that is established across the oxide layer. The strength of electric field is in the range of 0.015 – 0.197 x 109 V m–1. The oxidation mechanism of the Ni(II) surface compound to Ni(III) is electrochemically irreversible and the process is treated according to Randles–Sevcik equation. A linear relation was determined between the peak current density (jp) and the square root of the potential scan rate (v1/2) for the entire range of Ep, tp and T. The diffusion coefficient (D) values calculated for anodic and cathodic processes are 8.1 ± 0.2 x 10–12 and 4.3 ± 0.2 x 10–12 cm2 s–1, respectively. The activation energy (Ea) values for the diffusion process are 23 ± 2 kJ mol–1 (anodic) and 26 ± 2 kJ mol–1 (cathodic). The D and Ea values calculated from chronoamperometry measurements are comparable with those calculated from jp vs. v1/2 plots. The electro–reduction of PtO electrochemically pre–formed on Pt electrode in 0.5 M aqueous H2SO4 solution was also investigated. A well–controlled reduction conditions (Er, tr and T) were applied to determine the amount of the reduced PtO oxide. The reduction of the PtO requires much less time once ca. 1 monolayer (ML) of the oxide has been removed (ca. 1 ML of PtO remains). As expected, the longer tr and/or lower Er values, the greater the amount of the reduced oxide and consequently the smaller the amount of the remaining PtO oxide.