2.0 *0.2 V vs. SCE . c 1.0-c 300 400 SO0 600 Wavelength/nm Fig. 4. Action spectra of TiO2-free polypyrrole film (a) and TiO2incorporated polypyrrole films (b, c) taken in 0.5M KCI under electrolysis at 0.2 and -0.4V vs. SCE. The polypyrrole films given by (a), (b), and (c) are the same as those shown in Fig. 3.role takes place with the involvement of electrolyte cations. This implies that the anion-adsorbed TiO2 in the film acts as a fixed dopant as in the case of Nafion-polypyrrole composite (15), and electrolyte cations work as the charge compensators in the redox process of the polymer films.

Whether the influence of magnetic field B on the diffusion process during an electrochemical reaction is now well known, the magnetic field effects on the electrochemical kinetics are still a subject of controversy. We have investigated, by means of electrochemical impedance spectroscopy, different electrochemical systems, mass transport controlled, kinetically controlled, and mixed systems, under superimposed B. All results led to the conclusion that a homogeneous B parallel to the electrode surface and varying up to 1 T had no effect on the charge-transfer coefficient.

The effects of a superimposed magnetic field B on the structure of nickel electrodeposits prepared from a quiescent Watts solution were studied by scanning electron microscope and transmission electron microscopy investigations. It was observed that B can induce a change of the surface morphology and of the preferential growth direction of the nickel grains. In the absence of organic inhibitor, it is shown that these effects result from an inhibition of nickel electrocrystallization in relation to the mass-transport enhancement of W ions promoted by the magnetic field. The phenomenon is much more important in the presence of a strong inhibitor such as 2-butyne-1,4-diol, the activity of which is also under mass-transport limitation. The presence of the magnetically induced convective flow is directly illustrated by the perturbations of the relief of the deposit in the vicinity of attached bubbles. These results demonstrate that most structural modifications of nickel electrodeposits observed in the presence of a magnetic field are consequences of convection phenomena induced by a magnetohydrodynamic effect.

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