engleski

Electrochemmical Behaviour of Bismuth as Valve Metal

In this work we report a study of the kinetics of a surface oxide film nucleation and growth on a bismuth electrode and the impedance characteristics of the system Bi/Bi_2O_3/electrolyte. Experimental: Spectroscopic pure Bi disc (Johnson-Matthey), with the roughness factor 1.8, was used as the working electrode. All experiments were carried out in a Na-borate buffer solution pH 9.2 using the electrochemical set up consisted of an EGG&PAR potentiostat/galvanostat Model 273A, and an EGG&PAR Lock-in Amplifier Model 5301A, controlled by an IBM PC. Impedance measurements were performed in the frequency range from 50 kHz to 50 mHz with the AC voltage amplitude ą5 mV. Results and discussion: The nucleation and growth mechanism of bismuth oxide films on bismuth has been studied using cyclic voltammetry, potentiostatic and galvanostatic techniques. Cyclic voltammetry results showed the evidence of Bi primary passivation in dynamic conditions. The process of oxide film formation is under an ohmic resistance control. The change in ohmic resistance is caused by the nucleation and spreading of an oxide as a layer on the metal surface. The oxide monolayer film is formed by the diffusion-controlled 3D nucleation mechanism. Growth of a bulk film on the top of this monolayer occurs by activation-controlled ionic conductivity under the high electric field, according to the exponential law. The kinetic parameters of the film growth have been determined. The behavior of Bi during anodic oxidation resembles in many respects the kinetics of anodization of typical valve metals (Nb, Ta, Zr, Al). AC impedance and photopolarization in-situ methods have been employed to investigate the electric, dielectric and electronic properties of the Bi_2O_3 films under the conditions in which film growth was insignificant and the current passing through the film was entirely electronic. The impedance of the Bi_2O_3 anodic oxide film formed under potentiostatic conditions can be represented by a simple equivalent circuit which at high frequencies corresponds to the resistance of the oxide film (related simultaneously to the geometric capacitance of the film) and at low frequencies to the leaking capacitor.

bismuth; oxide films; valve metals; ionic conductivity; high field law; galvanostatic anodization; cyclic voltammetry; photopolarization

nije evidentirano