No abstract

We have studied the reduction mechanism and electronic conduction during electrodeposition of tantalum in various TaCI5-alkali chloride melts at temperatures up to I100 K. This has been performed by simultaneous measurements of impedance and Raman spectra together with cyclic voltammograms. A detailed analysis of etectrochemical and spectroscopic results yields the following predominant reduction steps during electrodeposition in pure alkali chloride melts: Ta(V) -Ta(IV) -Ta(O). The equilibrium potentials of the corresponding redox reactions are separated by 40 mV which is clearly resolved in impedance measurements. In LiCl/KCl-eutectic melts a three step mechanism is found with Ta(1II) being the intermediate species. In evaluating the impedance data we found indications for electronic contributions which are most prominent with added oxide impurities to the molten electrolyte. Direct evidence for an appreciable electronic conductivity has been obtained by electrochemical polarization measurements. These are reported here the first time for fused KCITaCl5/Ta at 1050 K, where electronic conductivities up to 0.5 W ' cm-' have been measured, i.e. of same order of magnitude as ionic conductivities.

The corrosion behavior of the high-Tc-superconductor YBa2Cu3O7 in aqueous solutions was studied at room temperature by electrochemical dc and ac methods and surface analysis. The oxidic ceramics is found to be more stable in alkaline than in acid media. The surface stability decreases with the time of contact to the electrolyte and depends strongly on the polarization conditions. Segregation and decomposition effects are observed. Ageing effects are related to an increase of the ohmic resistance of the surface layers. A relatively positive reduction peak in cyclic voltammograms is correlated to the reduction of a formal Cu(III)-oxide species, which may be better explained in terms of a Cu(II)-peroxide species as [Cu2+ -O−]+ . The high-Tc-superconductor YBa2Cu3O7 does not act as a special electrocatalyst for the anodic oxygen evolution reaction. The superconductivity of the samples is a bulk property and remains even after electrochemical surface treatment.

The mechanism of electrochemical reduction of TaCl5 dissolved in molten alkalihalides or their mixtures has been investigated by impedance spectroscopy in the frequency range 10 mHz≤ω≤100 kHz at various cathodic and anodic potentials and temperatures up to 1100 K. From a quantitative interpretation of the diffusion controlled impedance various reduction steps are resolved to better than ±10 mV. This is demonstrated here for a few selected examples. Besides this, the most important observation reported here for the first time is that electronic contributions can become dominant. Only with appropriate consideration of the electronic conduction a quantitative description of the impedance spectra over the whole potential range is found.

Electrochemical dc and transient measurements in the system YBa 2 Cu 3 O 7/ acetonitrile + tetrabutylammonium-perchlorate (as a supporting electrolyte) at T = 298 K showed a significant photoeffect on single crystal electrodes after irradiation with chopped white or monochromatic light under cathodic polarization conditions. The effect could not be observed on polycrystalline electrodes and on single crystals under anodic polarization conditions. Under potentiostatic control, the photoreduction current increased nearly exponentially with decreasing electrode potential. Obviously, an enhanced reduction of copper oxide components at the surface takes place during illumination. The photoeffect was found to depend on both the photon energy and the adjustment of the (twinned) single crystals and may be explained assuming a significant band gap in c-direction and an intrinsic photo-excitation of electrons from the valence band to the conduction band of the p-type conducting material.