• 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.

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Electrolysis of suspensions of Co3O4 particles in Pb2+-containing electrolytes has been used for depositing PbO2 + Co3O4 composite layers on Ni rotating disc anodes. A sufficiently high angular speed of the electrode is necessary to obtain layers of homogeneous thickness and Co3O4 concentration. The volume fraction of Co3O4 particles in the deposit ? reaches a limiting value of ca. 0.1 when the volume fraction of particles in suspension C exceeds 0.008. The current density j has little effect on ? as long as it is in the range 1 to 20 mA cm-2; if j increases further, ? decreases. PbO2 + Co3O4 composite layers have been studied as electrode materials for the oxygen evolution reaction (mainly in NaOH solution). The overpotential and Tafel slope decrease upon increasing ?. At a fixed potential, j is roughly proportional to OH- concentration. The PbO2 + Co3O4 electrode performance is fairly stable at 25°C but declines with time at higher temperature. © 1997 Elsevier Science S.A.

Electrocatalysts for methanol oxidation were prepared throughatw ostage deposition process: porous cobalt layers were deposited, by cathodic reduction of Co2+ ions, and then modified by spontaneous deposition of Pd. A basic sulphate solution and amildly acid chloride solution were compared as media for