Electrochemical dissolution, solid electrodes

The quantum efficiency for solid-state devices, e.g. solar cells, is always below unity. For n-type silicon electrodes anodized in aqueous or non-aqueous HF electrolytes, quantum efficiencies above unity are observed because one or more electrons are injected into the electrode when a photogenerated hole enters the electrolyte. Note that energy conservation is not violated, due to the enthalpy of the electrochemical dissolution reaction of the electrode. 

Abrasive stripping voltammetry — Technique where traces of solid particles are abrasively transferred onto the surface of an -> electrode, followed by an electrochemical dissolution (anodic or cathodic dissolution) that is recorded as a current-voltage curve [i]. It allows qualitative and quantitative analysis of metals, alloys, minerals, etc. The technique is a variant of - voltammetry of immobilized particles [ii]. 

ELECTROCHEMICAL DISSOLUTION OF SOLID ELECTRODES 22.5.1 Anodic Dissolution of Metal Electrodes... 

Yang YJ, He LY (2006) Dissolution of lead electrode and preparation of rod-shaped PbS crystals in a novel galvanic ceU. J Solid State Electrochem 10 430-433... 

Solaliendres MO, ManzoU A, Salazar-BandaGR, Egmluz KIB, Tanimoto ST, Machado SAS (2008) The processes involved in the Se electrodeposition and dissolution on Au electrode the H2Se formation. J Solid State Electrochem 12 679-686... 

Theoretical model porous electrode solid reagent dissolution electrochemical reaction crystallization polarization characteristic chloranile carbon black. 

Herein, we consider the case when a porous conducting matrix with inclusion of active solid reagents represents the electrode. It is supposed, that both the reagent and the product are nonconductive. The conversion of the solid reagents is assumed to proceed via a liquid-phase mechanism in the following way dissolution - electrochemical reaction - crystallization. Figure 1 shows the structure of the electrode and its model. The model has been developed on the bases of several assumptions. 

A common feature of all electrochemical pore formation processes in solid-state electrodes of a homogeneous chemical composition is the remarkable difference in dissolution rate between pore tip and pore wall. This is usually discussed in terms of an active-passive transition between the pore tip interface and the pore wall interface. But this still leaves the question open as to what quality of the pores makes their tips active and the remaining surface passive. On a basic level the active-passive transition has been ascribed to three distinct causes [Le31] ... 

As a result of that reductive process, a deposit of copper metal (denoted in Eq. 2.2 by s for solid ) is formed on the carbon electrode surface. The prominent anodic peak recorded in the reverse scan corresponds to the oxidative dissolution of the deposit of copper metal previously formed. The reason for the very intense anodic peak current is that the copper deposit is dissolved in a very small time range (i.e., potential range) because, in the dissolution of the thin copper layer, practically no diffusion limitations are involved, whereas in the deposition process (i.e., the cathodic peak), the copper ions have to diffuse through the expanding diffusion layer from the solution to the electrode surface. These processes, labeled as stripping processes, are typical of electrochemically deposited metals such as cadmium, copper, lead, mercury, zinc, etc., and are used for trace analysis in solution [84]. Remarkably, the peak profile is rather symmetrical because no solution-like diffusive behavior is observed. 

In this equation, aua represents the product of the coefficient of electron transfer (a) by the number of electrons (na) involved in the rate-determining step, n the total number of electrons involved in the electrochemical reaction, k the heterogeneous electrochemical rate constant at the zero potential, D the coefficient of diffusion of the electroactive species, and c the concentration of the same in the bulk of the solution. The initial potential is E/ and G represents a numerical constant. This equation predicts a linear variation of the logarithm of the current. In/, on the applied potential, E, which can easily be compared with experimental current-potential curves in linear potential scan and cyclic voltammetries. This type of dependence between current and potential does not apply to electron transfer processes with coupled chemical reactions [186]. In several cases, however, linear In/ vs. E plots can be approached in the rising portion of voltammetric curves for the solid-state electron transfer processes involving species immobilized on the electrode surface [131, 187-191], reductive/oxidative dissolution of metallic deposits [79], and reductive/oxidative dissolution of insulating compounds [147,148]. Thus, linear potential scan voltammograms for surface-confined electroactive species verify [79]... 

Ratieuville Y, Wu BE, Lincot D, Vedel J, Yu LT (1999) Voltammetric and electrogravimetric study of manganese dioxide thin film electrodes. J Electrochem Soc 146 S-1-17/23. Bakardjieva S, Bezdicka P, Grygar T, Vorm P (2000) Reductive dissolution of microparticulate manganese oxides. J Solid State Electrochem 4 306-313. 

Electrolytes are used in electrochemistry to ensure the current passage in -> electrochemical cells. In many cases the electrolyte itself is -> electroactive, e.g., in copper refining, the copper(II) sulfate solution provides the ionic conductivity and the copper(II) ions are reduced at the - cathode simultaneous to a copper dissolution at the - anode. In other cases of -> electrosynthesis or - electroanalysis, or in case of - sensors, electrolytes have to be added or interfaces between the electrodes, as, e.g., in case of the -> Lambda probe, a high-temperature solid electrolyte. 

The transport steps may be controlling either in bringing reactant material to the reaction site (the metal-solution interface) from either phase, or in removing products from the site into the liquid phase,. Accepting the fact that metal reactions in conducting solutions are electrochemical in nature, it follows that control may reside in transport with respect to the cathodic process, or with respect to the anodic process, or with respect to both simultaneously a much less likely possibility. In metal dissolution reactions, the steps can be described still less equivocally for the first case, the transfer of reducible species from the solution to the electrode is involved for the second case, the removal of oxidized species from the electrode is involved. In the latter instance complications are usually caused by the formation of solid reaction products. 

The logarithmic dependence of the dissolution rate on the electrode potential can be explained in the case of ionic crystals under the following assumptions (a) The dissolution process is far from equilibrium, (b) The passage of cations and anions can be treated as independent electrochemical reactions. The rate of each of them depends logarithmically on the electrode potential and on the chemical potential of the species in question in the solid phase, (c) According to the... 

The first equation represents the equilibrium between hydrated Ag+ ions and Ag atoms in a single-crystal configuration. Alternatively, we may say that there is a heterogeneous thermodynamic equilibrium between Ag+ ions in the solid phase (where they are stabilized by the gas of free electrons) and Ag+ ions in the liquid phase (stabilized by interaction with water molecules). The forward reaction step corresponds to the anodic dissolution of a silver crystal. On an atomic level, one may say that a Ag" " core ion is transferred from the metallic phase to the liquid water phase. In an electrochemical cell, an electron flows from the Ag electrode (the working electrode) to the counter electrode each time that one Ag+ ion is transferred from the solid to the liquid phase across the electrochemical double layer. Although the electron flow is measured in the external circuit between the working... 

FIGURE 5.58. Voltage versus time curve (solid line) for an n -type silicon electrode anodized with a constant current density in NH4F. The thickness of the anodic oxide was measured by ellipsometry (open circles, dashed line fitted as a guide to the eye). The OCP dissolution time of the anodic oxide in the electrolyte was measured (values above arrows) at different points of the oscillation. The bar graph visualizes the proposed oscillation mechanism. After Lehmann. (Reproduced by permission of The Electrochemical Society, Inc.)... 

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