Electrochemical Dynamics and Structure of the Ag/AgCl Interface in Chloride-containing Aqueous Solutions

In this paper we report on electrochemical and structural properties of the Ag/AgCl interface in contact with aqueous solutions containing Cl−. Electrochemical oscillations are connected with the anodic formation of AgCl films on polycrystalline Ag and Ag(111) electrodes in contact with Cl− solutions. The effects of chemical and electrochemical variables on the oscillatory behaviour are taken into account. Persistent oscillations are found for pH values ranging from acidic to alkaline. Oscillatory behaviour is shown to be typical of high-Cl− and low current density conditions. The polycrystalline vs single-crystal nature of the Ag substrate, affecting the AgCl film structure, is shown to have a quantitative bearing on the oscillating behaviour. The electrochemical results are complemented with electron microscopic observations and X-ray diffractometry. Coherent Ag/AgCl interfaces are found after prolonged oscillation experiments and grain-boundary type pore structures are found; lack of porosity in the structure correlates with the absence of potential oscillations. A simple linear dynamic model is devised for the rationalisation of the oscillatory behaviour, based on reaction-diffusion processes within the pores of the anodic AgCl film. Qualitative predictions of the model match experimental evidence in terms of: oscillation amplitude, frequency, phase and conditions for the onset of the oscillations.