• Energy Research

The electrodissolution of p-Ge in aqueous NaOH solutions (pH 12-13) has been investigated with steady-state and impedance techniques. In the potential region where the process is under kinetic control, the impedance patterns are characterised by a high frequency capacitive loop, associated with the space charge, largely merged with another due to charge transfer resistance and double layer capacitance, and two low frequency loops changing from inductive to capacitive upon increasing the potential. At larger potential, under mixed control, the same high frequency features are observed, while a convection diffusion impedance dominates the low frequency response. The data relevant to pure kinetic control are interpreted with a simplified kinetic model, based on the classical scheme of Beck and Gerischer [Z. Electrochem., Ber. Bunsenges. Physik. Chem. 63 (1959) 500], neglecting transport effects and assuming only three electrochemical steps: (i) oxidation, involving water molecules, of Ge(0) to dihydroxylated Ge(II) by two holes; (ii) oxidation by one hole of Ge(II)-Ge(III) in a step requiring OH-; (iii) oxidative dissolution of Ge(III) proceeding either by hole capture or electron injection, leaving behind a surface Ge(0) species. Diagrams calculated for the limiting cases of pure depletion layer control reproduce the essential features of the experimental ones.

Two different mathematical formulas for estimating effective capacitance from Constant-Phase-Element (CPE) parameters, taken from the literature, are associated unambiguously with either surface or normal time-constant distributions. Application to different systems are used to illustrate the importance of using the correct formula that corresponds to a given type of distribution. Experiments and simulations are used to show that the effective capacitance obtained for a normal distribution yields correct values for the film thickness under conditions where the local resistivity does not vary significantly. When the local resistivity varies considerably over the thickness of a film, the experimental frequency range may preclude observation of the capacitance contribution of a portion of the film, resulting in under prediction of the film thickness.

The electrodissolution of Ti and p-Si electrodes in acidic fluoride solutions has been investigated by electrochemical impedance spectroscopy as a function of potential E, fluoride concentration, pH and angular speed of the electrode, with the aim of characterizing the oxide layers covering both materials. The impedance diagrams have been analyzed to obtain the low frequency capacitance C lf, the high frequency capacitance C hf and the product R hfI (where R hf is the high frequency resistance and I the steady-state current). In the potential domain where the oxide thickness x is proportional to E, the formation ratio dx/dE has been computed from C lf, C hf and R hfI. The calculated values have been found to be in good mutual agreement and close to the ones found in the literature.

The impedance response of ITO electrodes with different sheet resistance R sh (/sq) immersed in electrolytes of different conductivity is investigated under blocking conditions. The pattern observed when the electrode is much more resistive than the electrolyte resembles that typical of porous metal electrodes - in the hf limit a line forming an angle of 45 with the real axis, in the lf limit a vertical line corresponding to constant values of resistance and capacitance - and is discussed with reference to the classical theory of de Levie. The pattern observed when electrode and electrolyte resistances are comparable depart from the above limiting behavior and may be reproduced by a finite elements calculations able to account for current distribution.

In the anodisation of Nb in acidic fluoride solutions or warm aqueous alkali, and in that of Mo in concentrated phosphoric acid, impedance spectroscopy provides evidence that 3D film growth proceeds already during the active–passive transition. Indeed, linear potential dependences of both the inverse of the high-frequency capacitance and of the product of the high-frequency resistance times the steady-state current are observed already below the peak potential. Simultaneously, a pseudo-inductive loop at intermediate frequencies is detected in the impedance spectra. In the present paper, we propose a kinetic model for the interpretation of these data. The model assumes that the metal is covered with a non-stoichiometric oxide containing at least two oxidation states of the cation. The processes of oxidative dissolution of the lower valence cations and their transformation to cations of higher valence, leading to passivation, explain the shape of the I–E curve. These processes are limited by both charge transfer at the film–solution (F/S) interface and transport of cation vacancies through the film. Thickening of the oxide film is assumed to proceed simultaneously, and is limited by transport of oxygen vacancies accelerated by an interfacial charge of cation vacancies.

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.

The anodic dissolution and passivation of Si in aqueous hydrazine have been studied, with special emphasis on the effects of hydrodynamic conditions, by using a submerged impinging jet cell. Experiments, performed in the temperature range 20-50°C, show an anomalous current decrease on increasing mass-transfer rate, more pronounced at lower temperatures and observed both in the presence and in the absence of dissolved oxygen. It is shown that data may be interpreted by a mechanism similar to that previously proposed for Si dissolution in NaOH. As a result of electrode processes and of the electroneutrality constraints in a binary electrolyte, the equal surface concentrations of the two ionic species, OH- and N2H5+, increase above the bulk value; one of these species, most probably OH-, has a catalytic action on the dissolution steps and its local buildup promotes a larger current; increased mass transport, by reducing the surface concentration excess, reduces the current. The model explains the main experimental data, including the marked decrease of the mass-transport effect upon addition to the solution of significant concentrations of either NaOH or NaCl.