• 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.

The effects of anodic polarization of p-Si electrodes in alkaline medium have been investigated by the probe beam deflection (PBD) or 'mirage' technique and the optical cantilever or bending beam method (BBM). The PBD technique permits a monitoring of dissolution and passivation processes and provides an estimate of the oxide etchback times during open circuit corrosion. The BBM technique has been used to estimate the stress of the thin oxide layer, which appears to be in the order of 180 MPa for very thin oxide films (nm range). The result is discussed in comparison with literature properties of thermally generated oxides.

The behavior of Si electrodes in aqueous alkaline media has been investigated by voltammetric and impedance techniques. The flatband potential has been estimated on the basis of Mott-Schottky plots and photocurrent onset as EFB = -1.15 V (SCE) for p-Si and EFB = -0.45 V(SCE) for p-Si. A dissolution/passivation mechanism is proposed, assuming: (1) irreversible electrochemical oxidation of H-terminated Si to a monohydroxylated surface species, which may undergo either (2) irreversible electrochemical oxidation to a dihydroxylated species responsible of 2D surface blocking or (3) chemical dissolution as Si(II) regenerating an H-terminated surface; furthermore, (4) the blocking species may be dissolved chemically as Si(IV); (5) a chemical reaction with water generating monohydroxilated Si (and H2) is also considered. A kinetic model is developed for the case of n-Si, assuming Helmholtz-layer control. The model reproduces the essential experimental features of both J - E curves and impedance diagrams in the active dissolution and passivation regions but is not expected to be realistic in the passive region where 3D surface blocking should be considered.

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.

Pd-modified Ni foam electrodes were prepared by a spontaneous deposition method. Ni foam samples were immersed in acid PdCl 2 solutions for different durations (t SD). The Pd loading and the surface area of the Pd deposits were determined as a function of [PdCl 2] and t SD. SEM-EDX showed that Pd deposits were homogeneously formed on the walls of both the outer and the inner cells of the Ni foam. Pd-modified Ni foam electrodes were used as anodes for the oxidation of methanol, ethanol, ethylene glycol, and glycerol in basic media. Voltammetric curves for the oxidation of alcohols showed that the peak current increased with increasing Pd loading in a sub-linear way. For the lowest loading explored (ca. 1 mg of Pd in a 1 cm 3 foam volume), the peak current per unit Pd mass was of the order of 650 A g -1 for 0. 5 M methanol and ethanol, ca. 1,500 A g -1 for glycerol and higher than 2,000 A g -1 for ethylene glycol.