Electrochemical characterization of thin passive films on Nb electrodes in H3PO4 solutions

The electrical and semiconducting properties of thin anodic passive films potentiostatically formed (1 V less than or equal to E-f less than or equal to 5 V vs. see) on polycrystalline niobium electrodes in aqueous 0.5 mol/L H3PO4 solutions (pH 1.3) were studied, at room temperature, using electrochemical impedance spectroscopy. The data were analysed with a transfer function using a non-linear fitting routine, assuming that the resistance of the film is coupled in series with the faradaic impedance of the Nb(O) --> Nb(V) reaction, and these in parallel with the capacitance of the passive film/electrolyte interface. The relative permittivity of the films was estimated as about 44. The number concentration of donors (N-D) in the films was found to decrease with E-f (i.e., with increasing film thickness). A flat band potential value of -0.72 V was also obtained from Mott-Schottky plots.