• Energy Research

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 on the cation M: the peak current density jpk goes through a maximum for M=K and the plateau current density jpl 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.

Abstract AsSb alloys were deposited electrochemically onto flat, smooth Ni-plated Cu sheets from aqueous citric acid solutions and analysed by X-ray photoelectron spectroscopy (XPS) and angle-resolved (AR) XPS. Samples prepared and manipulated in air showed marked surface oxidation of both alloy elements, present as As 2 O 3 and Sb 2 O 3 . Oxide-free samples were obtained when electrochemical synthesis and transfer to the ultrahigh vacuum XPS chamber were carried out in an inert atmosphere. ARXPS analysis of oxide-free alloy samples obtained at large negative potential showed an increase in intensity of the As(2p 3 2 ) peak at grazing take off angles. This result, showing As surface enrichment, supports a previously proposed kinetic model of AsSb alloy electrodeposition.