Microstructure, surface chemistry and electrochemical response of Ag|AgCl sensors in alkaline media

Abstract

Characterization of the Ag/AgCl electrode is a necessary step toward its
application as a chloride sensor in a highly alkaline medium, such as concrete.
The nucleation and growth of AgCl on Ag in 0.1 M HCl was verified through
cyclic voltammetry. Ag anodization was performed at current densities, determined
by potentiodynamic polarization in the same (0.1 M HCl) medium. The
morphology and microstructure of the AgCl layers were evaluated via electron
microscopy, while surface chemistry was studied through energy-dispersive
spectroscopy and X-ray photoelectron spectroscopy. At current density above
2 mA/cm2, the thickness and heterogeneity of the AgCl layer increased. In this
condition, small AgCl particles formed in the immediate vicinity of the Ag
substrate, subsequently weakening the bond strength of the Ag/AgCl interface.
Silver oxide-based or carbon-based impurities were present on the surface of the
sensor in amounts proportional to the thickness and heterogeneity of the AgCl
layer. It is concluded that a well-defined link exists between the properties of the
AgCl layer, the applied current density and the recorded overpotential during
Ag anodization. The results can be used as a recommendation for preparation of
chloride sensors with stable performance in cementitious materials.