The kinetics of silver bromide film formation on the silver anode

Abstract

The anodic potentiodynamic formation of an AgBr film on an Ag rotating disc electrode was studied in aqueous bromide solutions. As the electron transfer step (1) is intrinsically fast, Ag + Br ^AgBr + c",and as the film remains porous throughout its growth, the rate of film growth is limited by physical parameters such as ionic diffusion and migration in the solution.The anodic E/I curves for AgBr film formation were calculated quantitatively by computer on the basis of the following model of film growth. Film formation occurs initially by the nucleation of islands of film to a critical thickness, and then these islands spread laterally until only small pores remain between them. As these pores become small, the resistance of the solution within them becomes rate limiting (at the anodic current peak). After the peak, the concentration of bromide ions at the pore base falls to zero and current is then controlled by the diffusion of bromide ions into the lengthening pores of the film.This lateral spreading mechanism and the subsequent retention of the porous film morphology has been substantiated by comprehensive Scanning Electron Microscope investigations.

Investigating mechanisms of anodic film formation by electrochemical probe beam deflection (EPBD)

“…4 that for a potential step to +250 mV, the kinetics of film formation is controlled by the diffusion of bromide ions to the electrode, as suggested by Birss and Wright [27]. However, although the concentration gradient given in Eq.…”

“…This negative deflection angle was assigned to the production of soluble Ag + (aq) . Birss and Wright observed a sharp increase of the anodic current at highly anodic limits during the oxidation of silver in solutions containing low concentrations of bromide [27]. This current increase was also determined to be due to the dissolution of silver as Ag + (aq) [27].…”

“…3, to standard diffusion models. It has been demonstrated from potentiodynamic methods using a rotating disk electrode (RDE) that the diffusion of bromide ions to the silver electrode is the rate-limiting step for AgBr (s) growth [27]. Therefore, the EPBD response to a potential step can be quantitatively evaluated by obtaining the concentration profile of bromide ions in the vicinity of the electrode surface for these conditions.…”

“…Considering the time scale of the cyclic voltammogram, the similar values of the cathodic and anodic charges can be interpreted as the formation of a insoluble film at the electrode surface. In fact, the oxidation of silver in medium containing bromide ions is known to lead to the formation of AgBr (s) [27,30]. The CD curve in Fig.…”

“…The objective is to demonstrate that valuable insights into the mechanism of complex electrochemical problems, such as film formation and reduction, can be readily obtained by EPBD. The electrochemical behaviour of silver in halide-containing media is well known [25][26][27], providing a very convenient model system for this type of investigation. We have recently investigated the electrochemical oxidation of silver in 0.1 M KClO 4 solutions containing several concentrations of KCl by cyclic voltammetry (CV) and EPBD [24].…”