Nb electrodissolution in aqueous alkali: dependence on the alkali metal

Nb electrodissolution is studied in solutions of alkali metal hydroxides MOH (M = Li, Na, K, Rb and Cs). The j-E curves show in all cases a dissolution/passivation peak followed by a current plateau. The current shows a marked dependence oil the cation M: the peak current density j(pk) goes through a maximum for M = K and the plateau current density j(pl) increases monotonically with the atomic number of M. Electrochemical impedance spectroscopy indicates that in the plateau region, a similar barrier oxide is present over the metal substrate in all solutions. XPS analyses show that polarisation causes. in certain cases, formation of a porous film of alkali metal niobates or mixed oxides, probably by a mechanism of dissolution/precipitation. It is proposed that the peak current is controlled by both the solution basicity and the solubility of this layer, whereas the dependence of the plateau current on M reflects the variations in solution basicity and in the specific ability of the metal cations to promote dissolution of the barrier oxide. (C) 2003 Elsevier Science B.V. All rights reserved.