Electrochemical Growth of Surface Oxides on Nickel. Part 3: Formation of β-NiOOH in Relation to the Polarization Potential, Polarization Time, and Temperature

Electro-oxidation of surface beta-Ni(OH)(2) residing on metallic Ni to beta-NiOOH was studied in 0.5 M aqueous KOH at 277 K <= T <= 318 K by means of cyclic voltammetry (CV) and chrono-amperometry (CA). The process is accompanied by a diffusion of H+ within the surface oxide phase. The formation of beta-NiOOH gives rise to an anodic peak in CV profiles, the potential of which depends on the scan rate (s). An analysis of the relation between the anodic peak current density (j (peak, AN)) and s indicates that the growth of beta-NiOOH is controlled by the diffusion of H+ and its modelling leads to the determination of the diffusion coefficient of H+ (D(H+)). In the case of 277 K <= T <= 318 K, the values of D(H+) are of the order of 10(-11) cm(2) s(-1), when calculated with respect to the electrode's geometric surface area (A (geom)), and of the order of 10(-12) cm(2) s(-1), when calculated with respect to the electrochemically active surface area (A (ecsa)). The activation Gibbs energy of H+ diffusion (Delta(diff) G (not equal)(H+)) is in the 19.5-22.6 kJ mol(-1) range. Chrono-amperometry transients for the formation of beta-NiOOH are analyzed on the basis of finite-space diffusion, with the assumption that beta-NiOOH can be formed through three mechanistic pathways. The values of D(H+) determined for both A (geom) and A (ecsa) using this approach are of the order of 10(-12) cm(2) s(-1). They are smaller than the analogous values of D(H+) determined on the basis of CV measurements but the values of Delta(diff) G (not equal)(H+) obtained using these two experimental approaches are comparable.