Analysis of Electrodes Displaying Frequency Dispersion in Mott-Schottky Tests

A procedure is proposed for using the Mott–Schottky approximation to obtain the electronic properties (flatband potential and charge carrier density) of passive films that are modeled by equivalent circuits that contain a constant phase element (CPE) rather than a capacitor. Capacitances are calculated from the equivalent circuit containing a CPE using expressions developed by Hsu and Mansfeld [ Corrosion , 57 , 747 (2001) ], and Brug et al. [ J. Electroanal. Chem. , 176 , 275 (1984) ]. Both expressions calculate similar flatband potentials for Ti and Ni, but the charge carrier densities are significantly different. The approach is applied to the analyses of passive films formed on titanium in and nickel in 1 M NaOH. The passive film formed on Ti was found to be n-type, have a flatband potential of −330 mV vs a sat. Ag/AgCl reference electrode, and a charge carrier density of . The film formed on Ni was p-type, had a flatband potential of 100 mV, and a charge carrier density of .