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 ≤ Ef ≤ 5 V vs. sce) 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(0) → 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 (ND) in the films was found to decrease with Ef (i.e., with increasing film thickness). A flat band potential value of -0.72 V was also obtained from Mott-Schottky plots.